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VOI. 67, pp. 1261-1266,1977.
[1901 K. C. Yeh and C.C. Yang, “Mean arrival time and mean pulsewidth of sign& propagating through a dispersive
and random
medium,” ZEEE Trans. Antennas Propagat., vol. AP-25, pp.
710-713, 1977.
[ 1 9 1 ] K. C. Yeh and C.H. Liu, “Ionospheric effects on radio
communication andranging pulses,”ZEEETrans.AntennasPropagat.,
VOI. AP-27, pp. 747-751, 1979.
[ 1921 C. H.Liu and K. C. Yeh, “Model computations of power spectra
forionosphericscintillationsatGHzfrequencies,”
J. A n o r
Terr. Phys., vol. 39, pp. 149-156, 1977.
[ 1 9 3 ) C.C. Yang and K. C. Yeh, “Temporal behavior of pulses after
propagating throughaturbulence
ionosphere,” t o appearin
Prm. 1981 Symp. on dhr Effect of the Ionosphere on Radiowave Systems held in 14-1 6 Apr. 1981.
[ 1941 C. H. Liu and K.C. Yeh, “Puke propagation in random medii,”
ZEEEZ’rans. AnteMPrPro?mgat.,vol.AP-26,pp.561-566,1978.
[ 1951 F. B. Hildbrand,IntrOduction to NumericalAndy&. New York:
McGraw-Hill, 1974,2nd ed.
[ 1961 A. Papoulis, The Fourier Zntegml and I t s Applications. New
York: McGraw-Hill, 1962.
[ 1 9 7 ] M. R. Tucker, “A deterministic study of puke propagation
in
an electron bubble medium,” M S . thesis, Department of Electrical Engineering, University of Illinois
at UrbanaChampaign,
1981.
[ 1981 D. L. Knepp, “Multiple phase-screen calculation of the temporal
behavior of stochastic waves,” presented at the NorthAmerican
Radio Science Meeting, paper B.9-6, Quebec, June 2-6, 1980.
[ 1991 R. L. Bogusch, F. W. Guigliano, D. L. Knepp, andA. H. Michelet,
“Frequencyselectivepropagationeffects
on spread-spectrum
receiver tracking,”Proc. ZEEE,vol. 69, pp. 787-796, 1981.
[ 2 0 0 ] R.C.Dixon,SpreadSpecfrumSy~tems. NewYork: Wiley, 1975.
I2011 C. L. R h o , V.H. Gonzalez,and A. R. Hesing, “Coherence
bandwidth loss intransionosphericradiopropagation,”Radio
Sci., vol. 16, pp. 245-255, 1981.
[ 2 0 2 ] R. K. Crane,“Ionosphericscintillation,”
Proc. ZEEE, vol. 65,
pp. 180-199, 1977.
[ 2 0 3 ] G.G.
Getmantsevand L. M. Eroukhimov,“Radiostarand
satellite scintillations,” Ann. ISQY, vol. 5 , paper 13, pp. 2 2 9 259,1967.
[ 2 0 4 ] J. P. McClure, W. B. Hanson, andJ. H. Hoffman, “Plasma
bubblesand
irregularities in theequatorialionosphere,”
J.
Geophys. Res.,vol. 82, pp. 2650-2656, 1977.
[ 2 0 5 ] M. C. Kelley and E. Ott, “Two-dimensionalturbulencein
euqatorialspread
F,” J, Ceophys.Res.,vol.
83, pp. 43694372,1978.
[ 2 0 6 ] C. M. Crain, H. G. Booker, and J.A. F e r g w n , “Use of refractive
scattering t o explain SHF scintillation,”Rudio Sa.., vol. 14,
pp. 125-134, 1979.
(2071 A. W. Wernik, C. H. Liu, and K.C. Yeh, “Model computations
of Radio Waves Scintillations Caused by Equatorial Ionospheric
B u b b l ~ , ” R a d i o S c i . , ~ ~15,
l . pp. 559-572, 1980.
(2081 S. Basu and M. C. Kelley, “A review of recent observations of
equatorial scintillations and their relationshipt o current theories
of F regionirregularitygeneration,”Radio
Sci., vol. 14, pp.
471-485,1979.
2091 S. L. Ossakow,“Ionosphericirregularities,”in
U.S. Nat.Rep.
1975-1978 on Papers in Solar Planetary Relations, 17th General
Assembly of International Union of Geodesy and Geophysics,
Canberra, Australia, pp. 521-533, Dec. 1979.
2101 H. E. Whitney and S. BSU, “The effect of ionospheric scintillation on VHF/UHF satellite communications,” Radio Sci., vol.
12, pp. 123-133, 1977.
2 1 1 ] M. C. Kelly and J. P. McClure, “Equatorial spread-F: A review
of recent experimental results,” J. Atmos. Terr. Phys., vol. 4 3 ,
PP. 427-436, 1981.
L. Rino,.V. H. Gonzalez, and A.R.Hessing,“Coherence
I2121
bandwidth loss in transionospheric radio propagation,” Radio
Sci.,vol. 16, 1981.
12131 P. E. Serafim,“Effectoftheshapeofplasmadensitypower
spectra on ionospheric scintillations,” Radio Sci., vol. 15, pp.
1031-1044,1931.
C.
Global Morphology of Ionospheric Scintillations
JLES AARONS,
FELLOW, IEEE
Invited Paper
Abmct-Starting with post World W u I1 studies of fading of radio
star sources and continuing with fading of satellite signals of Sputnik,
vast quantities of data have built up on the effect of ionospheric irregularities on signals from beyond the F layer. The review attempts to
organize the available amplitude and phase scintillationdata into equatorial, mid&, and high4atitude morphdogies The effect of magnetic
activity, solar sunspot cycle, and time of day is shown for eachof these
three latitudinal sectors.
The effect of the very high levels of solar flux during thepast sunspot
maximum of 1979-1981 is stressed During these years unusually hi@
levels of scintillation were noted near the peak of the Appleton quatorial anomaly (- +15” awayfrom the magnetic equator) as wen as
over polar latitudes. New data on phase fluctuations are summarized
for the auroralzone with its sheet-like irregularity structure.
Manuscript received October 19, 1981; revised February 2, 1982.
The author was with the Air Force Geophysics Laboratory,Hanscom
AFB, MA 01731. He is now withtheDepartment
of Astronomy,
Boston University, Boston, MA.
One m d is now availablewhich will yield amplitude and phase
predictions for varying sites and solar conditions. Other models, more
limited m their output and u
se,are also available. The models are outlined with their limitations and databases noted.
New advances m morphology and m understanding the physics of
irre%uity development in theequatorialandauroralregions
have
taken place. Questions and unknowns in morphology and in the physics
of heguhity developmentremain.Theseincludethe
origin of the
Beeding sources of equatorinl irreguluities, the physics of development
of auroral irresulority patches, and the morphdogyof F-layer irregularities at middle latitudes.
I. INTRODUCTION
A
RADIO WAVE traversing the upper and lower atmosphere of the earth suffersadistortion
of phaseand
amplitude.
When it traverses drifting
ionospheric
irregularities, the radio wave experiencesfadingand
phase
fluctuation which varywidely with frequency, magnetic and
U. S. Government work not protected by U. S. copyright
AARONS: GLOBAL MORPHOLOGY OF IONOSPHERIC SCINTILLATIONS
361
solar activity, time of day, season, and latitude. It is the purpose of this review paper to organize theexperimentaland
theoretical studies which have been brought to bear to isolate
the variables.
Experimentally,theadvent
of beacons ranging from lowaltitude satellites transmitting at 40 MHz to 3000 MHz and
synchronousandvery-highaltitudesatellitestransmitting
in
the UHF to microwaveregionhaveallowed
geophysicists to
take data over more than a solar cycle. The recent protracted
highsolarflux in 1979,1980,and1981hasshownunusual
activityin
the polarand theequatorial regions. We shall
attempt to sort out the geographical and geophysical effects.
On the theoretical side, conceptual advances in the instability
mechanisms which could fit the dataplus an extensive program
in simulation haveallowed physicists to developtheories of Fig. 1. Globaldepth of scintillationfading during low and moderate
solar activity.
the formation of equatorial and auroral irregularities. We shall
briefly touch on “accepted” concepts of the development of
SCINTILLATIONAH0 FADE DURATION ANALYSIS
irregularities. These irregularities in the ionosphere introduce
fading and enhancement of amplitude, phase fluctuation, and
angle of arrival variations; collectively the effect is ionospheric
5 GNAL
scintillation.
LEVEL
The irregularities producing scintillations are predominantly
in the F layer at altitudes ranging from 200 to 1000 km with
the primary disturbance region for high and equatorial latitude
irregularities between 250 and 400 k m . There are times when
E-layerirregularitiesin
the90- to 100-km region produce
scintillation, particularly sporadic E and auroral E ; we shall
refer to these in the appropriate sections.
Several techniques havebeenused
to study irregularities.
Theseinclude1)ground,airborne,andsatellite
based HF
swept frequency sounders studying electron density structure
Fig. 2. Sample of intensity fading produced by signal passing through
and observing both bottomide and topside F-layer irregularirregularities. Fade duration and cumulative probability density are
of
ities; 2) in-situ measurements by rocketsandsatellites
also shown.
electronandiondensityirregularities,electric
fields, and
electron and ion flux; 3) coherent radar backscatter-VHF to be illustrated. At theequatorthe
earth’smagnetic fieldis
microwave; and 4) the scintillation technique which measures
parallel to the earth’s surface and is oriented magnetic N-S.
directly the perturbations of the radio signal as it transits the At Thule, the magnetic field is directed vertically and electrons
ionosphere. While we shall attempt to bridge the gap between spiral along the lines of force.
sounders, radar backscatter, in-situ measurements, and scintillations we shall concentrate on scintillation morphology which B. Scintillation Examples
may differ considerably from the other data.
Theintensity fading and itscharacterization canbestbe
A . Global Morphology
characterized by the idealized example such as in Fig. 2. The
signal is modulated by the passage through the irregularities
From the global point of view there are three major sectors
of scintillation activity (Fig. 1).The equatorialregion comprises so that the level instantaneously both increases and decreases.
an area within *2O0 of the magnetic equator. The high-latitude In Fig. 2 the signal level at times is 3 dB above the mean sigregion, for the purposes of the scintillation description, com- nal level and at other times fades below the 6dB level. The
prises the area fromthe high-latitude edgeof thetrapped
number of fadesand the fade duration for
a typical 15-min
charged particleboundary
intothe polar region. We shall length of signal from a synchronous satelliteis shown in Fig. 2
alongwith the cumulativeprobabilitydensityfunction.
In
term all other regions “middle latitudes.”
this example 9 1.7 percent of time thesignal wasabove the 6dB
In all sectors,there is apronouncednighttimemaximum.
At the equator, activity begins only after sunset. Even in the fade level.
polar region, there appearst o be greater scintillation occurrence A slow speed recording of a transmission from Si Racha to
during the dark months than during the months of continuous Hong Kongvia satellite is shown in Fig. 3 [ 11. In this case
was 4 GHz. The
the uplink was 6 GHz andthedownlink
solar visibility.
To order the geophysical occurrence and intensity
of irregular- fadingreached 8 d B peak to peak in this example from the
ities, reliance must beplaced on amagneticpictureof
the disturbed equatorial region during a year of very high sunspot
earth. While the sun’s role is ordered along geographical lines, number.
the geophysicsof irregularities is dominated bythe tilted earth’s
magnetic field. Motions of ionizedparticlesare governed by C. Signal CharacteTistics
the earth’s magnetic field with its northern pole near Thule,
The amplitude, phase, and angleof arrival of a signal will
Greenland anditseccentricmagneticequator.
The magnetic
equator’s meanderings relative to the geographic equator will fluctuate during periods of scintillation. The intensity of the
PROCEEDINGS OF THE IEEE, VOL. IO, NO. 4, APRIL 1982
362
-
-3
4
1 :
3i
:z
-2
-4
I
I
I
I
I
I
2000
2 100
2 200
2300
0000
0100
LOCAL TIME
Fig. 3. Slow speed recording of a transmission from Si Racha t o Hong
Kong. Peak-to-peak fluctuations range t o 8 dB [ 1 ) .
scintillation is characterized by the variance in received power
with the S4 index commonly used for intensity scintillation
of the varianceofreceived
and defied as thesquareroot
power divided by the mean value of the received power [ 21.
An alternative, less rigorous but simple measure of scintillation
index has been adopted by many workers in the field [3] for
scaling long-term chart records.
The defiition is
SI =
then there is an effective X' dependence over the frequency
interval. When strongscatteringoccursbut
is notconstant
over the frequencyinterval,
the wavelength dependence is
difficult to determine.The
[6] observations also show that
the phase scintillationindex varies as A undermost condialso obtained by Crane [7] although at low
tions,aresult
frequenciesthis has notyet been shown.Phasefluctuations
do not experience a variation in frequency dependence in the
strong scatteringregion.
Pmax - Pmin
E. Fading Spectra
Radio waves fromsatellitesencountering
the ionospheric
where Pmax is the power level of the third peak down from
irregularities
undergo
spatial
phase
fluctuations.
Intensity
the maximum excursion of the scintillations and Pmin is the
as
the
wave
emerges
from
the
irregularity
fluctuations
develop
level of thethird
peak up from the minimumexcursion,
reachingtheir maximum intensity in the far field. Focusing
measured in decibels [ 31 ,
effects can further increase intensity fluctuations.
The equivalence of selected values of these indicesis indicated
The two-dimensional spatial spectrum of phase fluctuations
below.
is proportional to theintegration of the three-dimensional
if
irregularityspectrum
along the propagationpath.Thus
s4
dB
the power spectrum of the three-dimensional irregularity has a
1
0.075
power-law slope of index p , the spatial phase spectrum will
0.1 7
3
have
a power-law index of p-1 .
0.3
6
The
amplitude
scintillations
undergo
Fresnel
filtering.
0.45
10.
Amplitude scintillations do not fully develop after traversing
Scaling of the chart records is facilitated by simply measuring very large irregularities observed at
distances
very much
the decibelchange between the Pmax and Pmin levels. The shorterthanthe
Fresnelzoneradius
(XF =
where z
phase variationsarecharacterized
by thestandarddeviation
is the effectivedistancefrom thelayer.Irregularities smaller
of phase u4.
than the Fresnel zone distance, according to in-situ measureAttempts have been made to model the observed amplitude ments of the intensity of electron and ion irregularities [ 81,
PDF. Whitney et Q I . [ 41 and Crane [ 51 have constructed model [ 91 have low intensities with power-law behavior and therefore
distribution functions based upon the use of the Nakagami-m have a lesser effect.TheFresnelfilterfunctiontherefore
distribution (m = (S4)-2) and have shown that the empirical generatesmaximumintensity
at aspatial wavelength of the
models provide a reasonable approximation to the calculated Fresnel scale.
distribution functions. In addition, the Rayleigh PDF provided
For weak scattering the spatial spectrumof intensity flu'ctuaa good fit to the data under conditio? of very strong scintilla- tions is in effect a convolution of the phase spectrum with the
tion (S4 >> 0.9). The Nakagami-m distributionapproaches
Fresnel filter function.
A comparison of moderate scintillation levels (S, = 0.5) and
higher
the Rice distribution as m approachesunityfrom
values and equals the Rayleigh distribution for m = 1 (strong very high scintillationsindices (S4 = 0.94(close to Rayleigh
fading)) is shown in Fig. 4(a) and (b) [ 101. The low frequency
scintillation).
flat portion of the spectrum is extended in the strong scatterD. Frequency Dependence
ing case (Fig. 4(b));the slope of the falling portion of the
change significantly,keepingaspectral
Observations [6] employingtenfrequenciesbetween
138 spectrumdoesnot
MHz and 2.9 GHz transmitted from the same satellite, show a index of 3.
For the synchronous satellite the spectra essentially include
consistent
behavior of S4 for S4 less thanabout 0.6. The
the Fresnel wavefrequency dependence becomes less steep for stronger scintilla- the velocity of the ionosphericdriftsand
tion, as S4 approachesamaximum
valuenear unitywith a length. The spectra of phase scintillations however are not
few rare exceptions. When S4 exceeds 0.6 (peak-to-peak values affected by Fresnel fitering.
The intensity spectrum changes as a function of drift speed,
> 10 dB) thefrequency dependence exponent decreases. If
of scattering. Thusthe
two frequencies are being compared and both experience strong irregularityspectrum,andstrength
of these
scattering to theextentthat
each displaysRayleigh fading, morphology of spectra is inrealitytheinterplay
Pmax +Pmin
6)
AARONS: GLOBAL MORPHOLOGY OF IONOSPHERIC SCINTILLATIONS
-9
ANCON. PERU
LES
2 4 9 MHz
S4=050
dB
0111 U T
r
-
lot
-2cL
,
-601
60
I20
180
~ c - IO-'
~
T I M E - secs
,
,
,o0
FREOUENCY
10'
- Hz
(a)
ANCON, PERU
S
dB
r
LE5
~4 0:
0214 UT
d(z/ cos i) f($, @)
where
ionospheric
zenith
angle = angle between
radio
ray
and irregularity layer
$
propagation angle = angle between
the
radio
ray
and the magnetic field direction
q5
azimuth of the radioray in local coordinatesystem
of z axis along the magnetic field and y axis in the
magnetic east-west direction
f($, 9) = ay(y' cos' @ +sin2 @ +cosz $(cos' @ + y2 sin' @)
+ 'a sin' $/[yz cos' $ + 'a sin' $(y2 cos'
+ sin2 41 ' I 2
Z
reduced
slant
range to irregularity
layer
= z1(z2
- z1)/z2 where z1 = slant range to irregularity layer,
z2 = slant range to satellite
a
elongation of the
irregularities
along the magnetic
field lines
y
elongation of the irregularitiesin the magnetic eastwest direction.
i
,
'
60
when dealing with ionospheric irregularities represented by a
Gaussian power spectrum.
[ 151have
attempted to determinethe
hfikkelsen etal.
theoretical
scintillation
index
S4 when the irregularities
are described bya power-lawpower spectrumwithathreedimensional spectral index P = 4. This utilizes the coordinates
of the radio ray in the local coordinate system with set values
for the elongation of the irregularities along and perpendicular
to the magnetic field lines.
Mikkelsenassumed theapproximate dividing linebetween
weak andstrongscintillationis -9 dB, with SI< 9 dB denoting
the weak case. For this case, the geometric variation of S4 is
given by
249 MHz
= 0.94
- 201
0
-9
363
I20
I&
:::L
10-2
TIME- secs
1c-1
,oo
FREOUENCY
',
- Hz
@)
Fig. 4. Intensity scintillation and frequency spectrum forboth moderate
(a) and very high (b) scintillation indices.
factors. In each of the geophysical areas where intense activity
occurs, thethree
factorsmust
be utilized to estimatethe
We shall also refer to
spectra of the scintillationintensity.
modelcomputations of Wernik etal. [ 111 relative to nonstationary wedge-like electrondensitystructures.
In such
cases the intensity scintillations exhibit spiky temporal variations and fluctuations become nonstationary.
F. GeometricalConsiderations
Theintensity at whichscintillationsare observed depends
upon the position of the observer relative to the irregularities
in the ionosphere that cause the scintillation.Keeping both
the thickness of the irregularity region and A N , the electron
densitydeviation
of the irregularity,constant,geometrical
factors have to be considered to evaluate data and t o predict
scintillation effects at a particular location. Among these are:
a) Zenithdistance
of the irregularityat the ionospheric
layer. One study [ 121 found the intensity of scintillation may
be relatedapproximately tothezenithpath
valuesby the
secant of the zenith distances to 70'; below that an elevation
andthefirstpower
of
angle dependence ranging between
the zenith angles should be used.
b)Propagation angle relative to the earth's magneticfield.
Performing this calculation demands the use of an irregularity
configuration and the consideration of a Gaussian or a powerlaw model for the irregularities. Sheet-like irregularities with
forms of 10 : 10 : 1 have been found in recent auroral studies
[ 131. For equatorial latitudes, this elongation along the lines
of force may be of the order of SO to 100 [ 141.
c)The distancefrom the irregularity region to the source
and to the observer (near the irregularities, only phase fluctuations are developed). As noted in [ 5 I and [ 151 the theoretical
scintillation index can beexpressed in terms of the above factors
3
Using his irregularity formulation he found the Narssarssuaq
observations of the orbiting satellite, Nimbus-4,at an altitudeof
1000 km a best fit of irregularity configuration with2.5 : 1.3 : 1;
the first term is a,elongation of the irregularity along the lines
of force of the magnetic field, the second is y, orthogonal to
the elongation along the lines of force, being in the magnetic
is orthogonal to the
east-west dimension,andthelastterm
other two planes. At high latitudes this last term would lie in
the north-south meridian.
11. SPREADF AND SCINTILLATIONS
The term spread F is given to a type of F-layer backscatter
signal taken by a vertically directed sweeping H F sounder. The
returns from the F layer at each frequency are normally observed from that height at which the electron density reaches
a value where the ionosphere acts as a reflector. When the returns from the F layer are observed from a series of '%eights"
ratherthana
single altitude we have aspread F condition.
When a widerange of frequencies shows returns from many
ranges then the ionogram is said to exhibit "range spread F."
When the spread in range is predominantly at the high end of
the frequency sweep then the ionogram is said to beof the
"frequency spread F" type. The major morphological studies
of spreadF [ 16 I and [ 17I have used predominantly frequency
spread data to construct their maps of occurrence of spread F .
The evidence from the correlation of scintillation occurrence
and spread F [ 181 is that at equatorial and middle latitudes,
range spread is associated with strong scintillation activity and
frequencyspread is not. Thus the available spread F maps
PROCEEDINGS OF THE IEEE, VOL. 70, NO. 4, APRIL 1982
364
cannot be used for scintillation observations in these regions;
they are dramatically misleading in many cases. In the Mghlatitude region no statistical study has been made to correlate
types of spread F with scintillation activity.
It might be notedthat
evenrange spreadoccurrenceand
scintillation have important differences(EquatorialSection
111). Ionosondes only observe reflections from the bottomside,
fromaltitudes
of theionospherelowerthanthe
level of
maximum
ionization
density;
if sounders
are
in
satellites,
only
from the topside. The reduced data are in terms of occurrence
rather than amplitude of response or spread in range. In addition, there is no indication as to the thickness of the irregularities,
their
geometry,
rate
the
or
of fading. In their
present
form,
spread
F morphological
studies
are
not useful even for
indications of scintillation occurrence. Wright et al. (1 977)
have found a means for converting
ionograms
into (ANIN)
rms bymeasuring
the spread in frequencyon"frequency
spread"
'
0
0
> 1000 k m
-
S
THREE DIMENSIONAL
PATCH
MODEL
Fig. 5. A magnetic equatorcut through the g e n d form of the equate
rial patch with
typical
dimensions
shown.
-I
111. EQUATORIALSCINTILLATIONS
In their
intensity
and
their
effect
on transionospheric
propagation,equatorialF-layer
irregularities dwarf those of
the high-latitude regions.
Fluctuations
from
ionospheric
irregularities in the F layer have been reported at frequencies
as high as 7 GHz. Fang has reportedthat over periods of
time of theorder of half an hour and longer,peak-to-peak
fluctuations of 9 dB at 4 GHz may occur at elevation angles
above 10' [ l l .
A . Patch Characteristics
Through theoretical considerationsof instability mechanisms
and through radar backscatter and rocket and satellite
in-situ
measurements, it hasbeenestablished
thatnighttime ionospheric equatorial irregularityregions emerging after sunset develop from bottomside instabilities, probably of the RayleighTaylor type. The depleted density bubble rises into the region
above the peak of the F 2 layer. Steep gradients on the edges
of the hole help to generate the smaller scale irregularities within the patch which produces intense scintillation effects [ 191.
I ) Patch Development,Motion and Decay: A plume-like
irregularity region develops, fmally forming a patchof irregularities which has been likened to a banana or an orange segment.
A cut through the centerof the "banana" is shown in Fig. 5.
The characteristics of thepatchdevelopment,
motionand
decay can be summarized as follows:
1) A new patch forms after sunset by expanding westward
in the direction of the solar terminator withvelocities probably
similar to those of the terminator. It comes to an abrupt halt
after typically expanding to an east-west dimension of 100 to
several hundredkilometers.
It appears to have aminimum
size of -100 km.
2)It is composed of field-aligned elongated rodorsheet
irregularities. The vertical thickness of thepatch is 50 to
several hundred kilometers. The patch has maximum intensity
irregularitiesinaheight
region from225 to450 km, with
irregularities to over 1000 k m .
3) Its north-south dimensions are of the order of 2000 km
or greater.
4) Once formed,thepatchdriftseastwardwith
velocities
ranging from 100 to 200 m/s.
5) The patch durationas measured by scintillation techniques
L
401
I I'I\If
20
I f "
c
19
20
1
AFGL AIRCRAFT
i
L E S - 9 . 249YHz
Y.-^'.
23
00
4
1
N "I I
21
22
01 L S T
Fig. 6. Fading rates and scintillation observations made by the AFGL
aircraft on March 19-20, 1977 illustrating the slowing down of the
patch after 2350 LST.
is known to be greater than 2 1/2 h; individual patches have
been tracked by airglow techniques up to 3 h where they have
maintained their integrity [ 201. Effects have been seen over
8 h.
6 ) The life history of a few patches has been studied in years
of low and moderate solar flux [21]. The decayof patches
in the midnight time period was of the order of 1 h after local
midnight in years of low sunspot activity. Aarons e t al. [ 221
havealso shown weak 3-m size irregularitiesonbackscatter
contours coupled with low or no scintillation activity. Fig. 6
[ 221 gives evidence for slowdown of the velocity of the patch
The fading rate when the patch
bymeans of aircraftdata.
was decaying (2350-0050 LST) showed thesame rate whether
the aircraft wasflyingagainst the patch motion (W)or with
it (E), indicatingaslowdown
of irregularityvelocity at the
time when scintillation indiceswere low.
For an observer of synchronous satellites in the equatorial
region, the eastward
nighttime
plasma drift moves these
patches of irregularities through his beam. An encounter with
one of thesepatchesand the amplitude fading produced by
them can best be illustrated by the severe case shown in Fig. 3
wherean uplink signal from Si Racha at 6 GHzwas retransmitted to Hong Kong at 4 GHz [ 11. The resulting scintillation
activity is probably predominantlyat 4 GHz from the downlink
path.
365
AARONS: GLOBAL MORPHOLOGY OF IONOSPHERIC SCINTILLATIONS
NOS1
Fig. 7. Horizontal profde of ionospheric F-region plasma density indicated by electron (le)and ion ( l i ) currents on Rev. #2177 of S3/4.
Sl/I is relative irregularity intensity [ 231.
TEC
tx~'~at/rn'~
ISLAND
ASCENSION
26-27FC0.1970
SIR10SATELLITE
2000
2100
2200
2500
oooo UT
Fig. 8. Two depletions in total electron content from an assumed quiet
background observed on Feb. 26-27, 1978 at Ascension Island. The
close assodation with theOccurrence of amplitude scintillation should
be noted.
2 ) Patch In-Situ Measurements: In-situ measurements within
the F layer (at 225 km, for example) measure irregularity intensity as a function of electron density by measuring electron
andioncurrents.
In oneexample [231 (Fig. 7) the electron
and ion densities in the S3-4 data showed severe depletions.
Exactcorrespondence
has been found between the in-situ
depletionsandscintillationactivity
[24] andbetween
the
scintillationactivity and depletions as shownbyoptical airflow measurements [ 251 .
3 ) Polarization Fluctuations: Patches show both depletions
andpolarizationfluctuations,thelattereffect
is noted bya
variation in total electron content as seen on Faraday rotation
records [ 2 6 ] . While the total number of electronsdepleted
may be only of the order of 20 percent in some cases, the depletion at certain altitudes is on occasion of the order of one
or two magnitudes.
To illustrate Total Electron Content (TEC) and scintillation
observations, Klobuchar and Aarons [27] recorded these two
parameters at AscensionIsland
ata dip latitude of -16's
for the 350-km intersection point. Continuous measurements
of theFaradayrotation
havebeen converted to equivalent
vertical TEC inastandardmanner
using thelongitudinal
magnetic field intensity and zenith
angle at a mean height of
420 km. Fig. 8 shows an evening period when two clearly
evidentdepletionsinTECoccurred.
Notethat depletions
from an assumed quiet background TEC, indicated by a dashed
line, are up to 10-1 5 percent. In addition, the close association
with the occurrence of amplitude scintillation is indicated by
the start and stop times of amplitude scintillation. This figure
shows the intimate association of TEC fluctuations with rapid,
severe amplitudescintillations observed along the samepath
[281.
B. Variation of Scintillotion Activity
I ) LongitudinalVariations: Spread F measurements have
shown that there is a clear longitudinal difference in F-layer
irregularity occurrence as a function of day of the year. The
differences maybe due to the displacement of the magnetic
pole vis-&vis the geographical pole, to the seasonal pattern of
lower
atmospheric
triggering activity
(thunderstorms,
for
example) as a function of longitude, or to global wind systems.
Spread F soundings havebeen separatedinto longitudinal
sectorsforpurposes of summarizingdata.Scintillationdata
taken at a common frequency for a common period and
reducedina
similar manner are, however, sparse. We shall
attempt to illustrate longitudinal differences with theavailable
data.
The dip latitude 8 used in this paper is based on the formula
tan 8 = 1/2 tan I where l i s an inclination or dipof the magnetic
fieldfrom thehorizontal. At the dip equatorthe magnetic
field is parallel to the earth's surface.
2 ) Data Comparison: Comparison was made of scintillation
activity at 250 MHz atavariety
of observatorieswithdata
taken over the sametimeperiod [29]. One set of data was
taken at Huancayo, Peru; Natal, Brazil;and Accra, Ghana with
all observationsmade
at elevationangles greater than 20'
366
PROCEEDINGS OF THE IEEE, VOL. 70, NO. 4, APRIL 1982
GEOGRAPHIC
LATITUDE
30.N
L~:N----------
'>
0
GUAM
YAG. EP.
-----
HUANCAYO
15.5
1k.E
)
60.W
3O.W
0.
LONGITUDE
Fig. 9. Map of equatorial regions using the 1975 epoch of the DMA
magnetic indination map. X marks subionospheric intersection.
and with distance between the most separated stations about
70' of longitude; a map of both geographical andmagnetic
coordinates is shown on the right side of Fig. 9.
The occurrence percentages are shown in Fig. 10(a) and (b).
For this longitude region (-0-7OoW) the lowest scintillation
occurrence takes place from May to July. The period August
to October shows similar occurrence rates at all observatories
with little dependenceon magnetic activity.
It might be noted that Accra and Natal, though both south
of the dip equator, are almost equidistant from the geographic
equator, one north and the other south
so that Juneis the center
of summer for Accra and winter for Natal
(and Huancayo).
Therefore, local summer and winter at a station do notplay a
role in scintillation occurrence. The intersection point of the
Huancayo path was north of the dip equator; that of the Natal
propagation path south of the dip equator, but the patterns
were similar.
A second comparison of data at 250 MHz was made between
observationsfromHuancayoandfromGuam.
Thedataare
shown in fig. 1 1 ;activity minima occur from May-July in Huancayo and from November-January in Guam. The conclusion
is that the occurrence patternsare longitudinally controlled.
Guam viewingofMARISAT was slightly north of the dip
equator as was the intersection point of the Huancayo path,
yet their patterns differed considerably.
While localsummer
attheobservationsite
cannot be a
factor as shown by the similar patterns of Huancayoand
Accra in May, June, and July (each on opposite sides of the
geographic equator) the pattern of seasonal electron density
variations at the ends of the equatorial field-aligned patches
may play a role.
It should be noted that in general maximum intensity occurs
in the equinoctial months. This can best be illustrated by the
occurrence of L-band 1500-MHz activity at Huancayo, Peru.
That evidence is shown in Fig. 12 [30]. L-bandactivity at
Huancayo does notsufferfromstrongscatteringorfrom
saturation (as do 136-MHz and 250-MHz data on occasion);
the data show clear equinoctial maxima.
3 ) GeomagneticControl of Scintillations: From available
data it appears as if geomagnetic control of the occurrence of
scintillation differed with longitude. The generalization can be
madethat
increased magneticactivityinhibitsscintillation
activitybeforemidnight-exceptduringthose
monthswith
very low scintillationactivity (May-July forthe region (-!
7OoW) and November-January in the Pacific longitudes (135 180'E)). After midnight the scintillation activity in general inThe
creases slightly with the presence of magneticstorms.
data shown in Fig. 1O(a) and (b) are for ayear's observation in
each case. The
complexities
of the magnetic
control
of
scintillationoccurrence
are illustrated by the variations in
the curvesof occurrence at each station in each season. For
further details see Mullen [3 11.
C. In-Situ Data
The larger data base of continuous observations from ground
station measurements has been utilized to establish the features
of the major m a t i o n regions. However, this is uneven in
longitudinal coverage and unavailableover
ocean surfaces.
Satellites carryingout in-situ observations of irregularity parameters such as electron density variations do providea mapping
technique.
One example of data collected and organized [32] is shown
in Fig. 13. It should be pointed out that this figure was o b
tained over aperiod of two months (November,December
1969), for a relatively high level of sunspotactivity,and is
valid for the timeperiod 19-23 LT. It is illustrative of mapping
which can be done at various altitudes. Scintillation intensity
is a function of both AN and the thickness of irregularity
layer. In-situ measurements do not measure thickness and its
variationsororientation
of the irregularities. Therefore,a
model must be developed to utilize these data.
Basu and Basu [33] have developedamodel from in-situ,
theoretical,andscintillationstudies.
In theirmorphological
model of scintillations, measurements of irregularity amplitude
h N / N as computed from T seconds of data are utilized in conjunction with simultaneous measurementof electron densityN .
A combination of ANIN and N data provides the required A N
parameter as afunction of positionandtime.
In case the
satellite altitude is much lower than the height of maximum
ionization, proper allowance should be made in deriving AN
estimates. The in-situ measurements of irregularityspectrum
and phase scintillation measurements with the 1000-km high
inclination Wideband satellite indicate that the outerscale at F
region heights is large, probably on the order of tens of kilometers. In view of this, the spatial length corresponding to Tseconds time intervalwhen projected in the direction
of shortest
361
AARONS: GLOBAL MORPHOLOGY OF IONOSPHERIC SCINTILLATIONS
___ K.0-I'
K = 3-9
FEE).
;
60
- APR
i
257MH2
ACCRA, GHANA
40
M
20
HUAMCAVO. PERU
20
20
(a)
___
-
"1
___
-KK :=O3'--9I '
K=O-I*
I(
i
3-9
YAY
60
AUG - 0 C T
- JULY
60
2 5 7 MHZ
257MHZ
ACCRA, GHANA
20
I2
m
(D
6
9
_j
1
,
~
I8
24
,
-
18
24
ACCRA,GHAN;
6
I 2 LT
,
12
NATAL, BRAZIL
2
12
6
12 LT
I2 LT
NATAL. BRAZIL
,
2 m
Y
0
6
/?i
60r
,~
24
18
42
2
24
18
6
12 L T
w
x
D
6or
r
HUANCAYO. PERU 4 0 1
pk
A
HUANCAYO. PERU
(b)
Fig. 10. (a) Seasonal patterns of occurrence o f scintillationactivity
> 6 dB (S, = 0.3) for very quiet (Kp = 0 - 1') and for disturbed (Kp =
3+ - 9) magnetic conditions for Nov.-Apr. (b) Seasonal patterns of
occurrence of scintillation activity >6 dB (S, = 0.3) for very quiet
(Kp = 0 - 1') and for disturbed ( K p = 3' - 9) magnetic conditions
for May-Oct.
PROCEEDINGS OF THE IEEE, VOL. 70, NO. 4, APRIL 1982
368
Fig. 11. Comparison of seasonal patterns of occurrence of scintillation
activity >10 dB for Guam and Huancayounderveryquiet
(Kp=
0 - 1') and disturbed (Kp= 3* - 9) magnetic conditions.
PERCENT OCCURREWE OREATER THAU 2d0 IS, = ,131
SUNSET
t
l3Mkml
SVNRlSE
I
i
I
I
4
SEP
I
I
1
I
IS
21
1
03
HUANCAYO I 5 4 G H z
APRIL 76
OCT 77
09
15
LT
Fig. 12. Percentage occurrence of 1.5-GHz scintillation 3 2 dB during
Apr. 1976-0ct. 1977.
correlation distance of electron density deviation sets the
apparent outerscale length 4 0 . The outer scale wavenumber is,
therefore, K O = 2n/qo. For the equatorial scintillation model
that they developed from the OGO-6 in-situ observations, the
time interval was T = 3 s and the outer scale length was considered to be 20 km corresponding to an outer scale wavenumber of K O = 0.3 km-'.
D. Sunspot Cycle Dependence
From the viewpoint of electron density variations the equatorial region around the magnetic equator displays a complex
pattern. During the day an increase in maximumelectron
densityoccursaway from the equator. Theelectrondensity
contours display a distinct trough of electron density in the
bottomside and topside ionosphere at themagnetic dip equator
with crests of ionizationat f1S0-200 north and south dip
latitudes; this is the Appleton anomaly with the region within
*So dip latitude of the magnetic equator termed the electrojet
region.
Fromthe solar cycle minimum in1974and maximum in
1969-1970, Aarons 1341 found that there was a higher occurrence of deep scintillations during a yearof high solar flux than
duringa year with low solar flux for observations atboth
Accra, Ghana and Huancayo, Peru.
Recentobservations
of L-bandscintillationsduring
the
period of maximum solar flux (1979-1981) [37] haverevealed
that scintillation intensities maximize in the Appletonanomaly
region rather than near the magnetic equator.
At Calcutta, India, which is situated close t o the northern
crest of the Appleton anomalyin the Indian longitudinal sector,
a remarkable increase in the Occurrence of VHF scintillation
was observed between 1977 and 1980 when solar flux increased [35].
The contrast between scintillation levels with the path t o the
satellite in the electrojet region and with thepathinthe
anomaly region can best be seen with the aid of the map in
Fig. 9 and the contrast indata between Natal, Brazil and Ascension Island, both observing the L-band beacon of MARISAT at
approximately the same longitude.
Natal
data
show
no
incidence of scintillationsbeyond
8 dB, Ascension Island
records show scintillation activity of the type shown in Fig. 14,
i.e., peak-to-peak fades of 27 dB for hours. Fig. 15 illustrates
the percentage Occurrence for a two month period during this
year of very high solar flux.
Fang [ 1 ] has presented similar results of high scintillation
intensity observing from Hong Kong. He recorded fluctuations
to 9 dB on the 4-GHz COMSAT downlink with paths through
369
AARONS: GLOBAL MORPHOLOGY OF IONOSPHERIC SCINTILLATIONS
KWAJELEIN
HUlWCA.70
ACCRA
GHANA
PERU
THHUIIRA
INDIA
H0N6
KON6
QUAM
GEOGRAPHIC
LONGITUDE
Fig. 13. Percentage occurrence of scintillations >4.5 dB at 140 MHz
(19-23 LMT, Nov.-Dec. 1969. 1970,A p < 12) using scintillation data
and O W - 6 obser~ations.
A S C E N S I O NI S L A N D
26 DEC 1979
Fig. 14. Sample of both UHF and Lband data recorded at Ascension
Island during December1979-January1980.Note
excursions on
both UHF andL-band channels.
1) The equatorial anomaly has considerably higher electron
density values in high sunspot number years than in years of
low solar activity.
2) The occurrenceof maximum electron density for anomaly
latitudes is near sunset in the
years of high sunspot number and
in the afternoon in years of low solar activity. Thus the post
sunset irregularity patches form high m l e v e l s in the years of
high solar flux. Data from ionosondes and from total electron
content measurement corroborated the extremely high
levels
of electrondensityandthelateness
of the appearance of a
maximum of electron density during 1979 and 1980.
IV. MIDDLE-LATITUDE
SCINTILLATION
ASCENSION ISLAND
JAN FEB 1980
1541 MHz
-
LOCAL
MIDNIGHT
Fig. 15. Percentageoccurrence
of L-band scintillations 2 2 0 dBat
Ascension Island during Jan.-Feb. 1980. Observations are segmented
into quiet (Kp = 0-3) and disturbed (Kp = 3+-9) magnetic conditions.
the anomaly region. Recordings havebeen shownearlier in
Fig. 3.
Olderdata
have been reviewed [ 3 6 ] , i.e., resultsfrom
AscensionIsland
on an S-bandtransponderonthemoon.
Scintillations as large as 20 and 25 dB on the two-way path,
ground to transponder and return were noted. Canary Island
observations also throughtheanomalytakensimultaneously
between November 1969 and June1970, aperiod ofhigh
solar flux, showed similar scintillation activity.
The conclusion in the study [371 is that the intensescintillationactivity duringyears of high solarfluxaredue
to two
factors:
The middle-latitude scintillation activity is not as intense as
thatencountered at equatorial,auroral,orpolarlatitudes.
For the engineer, however, activity may reach levels, primarily
at VHF and UHF, which will increase error rates of systems
with low fademargins.
The difficultywithdescribingmiddle-latitudescintillation
activity is that at times what takes place
at middle latitudes
is an extension of phenomena at equatorial and auroral latitudes. For example, scintillation activity in 1979-1 98 1, years
of high sunspot number, was observed to be high in data from
Hawaii and from Japan; the effects
werepossiblycaused by
equatorial phenomena during years of high sunspot number.
The depletion regions which originate at equatorial latitudes
do move to higher altitudes but these irregularities would have
to be >2000-kmaltitude.Theperturbingeffects
of these
regions and the higher electron densities during high sunspot
number years might combine to provide effects along the lines
of forcethusextendingequatorialactivity
to the ''lower''
middle latitudes.
At high latitudes, there is a motion of the irregularity boundary equatorwards during years
of high sunspot number and
duringmagneticstorms.Auroras
havebeen
notedinthe
southern U.S., for example,along the 70°Wmeridian.Scintillation activity is present at these times at these lower latitudes where optical aurora areseen.
A second complicating factor in middle-latitude scintillation
morphology is the effect of sporadic E. Several studies have
shown thatintensesporadic
E producesscintillation.
The
behavior of sporadic E is totally different from the morphology
PROCEEDINGS OF THE IEEE, VOL. 70, NO. 4, APRIL 1982
3 70
+
SCINTILLATION
KOKUBUNJI
TIME OF MAXIMUM AMPLITUDE
I"
NOV - DEC
1976
1977
I1
II
GUAM
MAR-APR
+
i
I case
YAY-JUN
O
J
F
M
A
M
J
J
A
S
O
N
D
JUL - A M
1979
Fig. 16. (a) Percentage Occurrence of scintillation 3 3 dB seen at Kokobunji at midnight in 1977-1978. (b) Scintillation Occurrence >10 dB
in hours seen at Guam during 1979.
SEP
- DCT
16 2 0 2 2 0 2 4 6
of F-layer irregularities. Thus two independent variables produce thefading phenomena. .At middle latitudes, thereis a high
Occurrence of daytime sporadic E resulting in a second maximum of scintillation. Nighttime sporadic E adds to the effects
of F-layer irregularities.
A . Results from Longitudes in the Western Pacific
Measurements of scintillationactivity have been taken in
Japan primarily from Tokyo which observes asynchronous
satellite at its longitude through a 350-km ionospheric intersection of 36'N, a dip latitude of 27'N.
At the VHF frequency of 136 MHz, observing ETS2, Sinno
and Kan [38] found a maximum of scintillationactivity at
night and in the May-July time period. We have reconstructed
their data to show the percentage of occurrence of 3 d B scintillation in 1977 and 1978 (Fig. 16(a)). We have also placed
in Fig. 16(b) the occurrence of scintillation activity in Guam
for the following year [391 to allow the comparison of various
months of the year. Themonthlypattern of the Japanese
data follows somewhat the pattern of equatorial scintillation
except for August and September. The lack of exact correspondence of observation dates makes the comparison tentative.
By observing ETS-2 from Taiwan at 25'N, Huang [40]
found similar results, i.e., the same nighttime maximum in the
May-July period and a summer daytime
maximum of lower
level fluctuations.
Observations of severe ionospheric scintillations primarily in
the 4-GHz range have been reported by Tanaka (198 1) [80]
for paths primarily at higher latitudesthantheequatorial
anomaly region. For periods of time of 30 min t o a few hours,
on a few occasions, scintillations of the order of a 2-4 dB
were notedafter sunset. Thehypothesis advanced is that
duringionospheric stormsthe positive phase produces high
electron densities tolatitudes above the anomaly. The disturbing wave traveling from the equator to higher latitudes,
triggers plasma instabilities which affect the ambient high
electron densities during this phase of the storm.
LT
Fig. 17. Histograms of the times of maximumscintillationindexfor
each 2-month period throughout the observations made at 136 MHz
from Ramey, Puerto Rico.
B. Results from Longitudes in the Americas
With transionospheric propagation data taken in 1976 from
sites in Puerto Rico and Florida, Kersley et al. [42] found that
scintillation activity at Ramey, h e r t o Rico occurred between
2100 and 0230 LT with maximum levels in the post midnight
period (Fig. 17).
The general level of scintillation at 136 MHz was in the 2-8
dB peak-to-peak range with occasional increases to 12dB peak
to peak. For theseobservationsmaximumoccurrence
was
noted in July with minima in the equinoxes. The seasonal
pattern along with other factorsindicated that the low-latitude
scintillation activity was not related to equatorial irregularities.
By performing simultaneous incoherent scatter radar measurements and scintillation observations Basu et al. [43] demonstrated that the scintillation maximum is associated with the
midnight descent or collapse of the F region.
The general pattern of what might be termed "upper" middle
latitudes, i.e., from 30"-45' dip latitude is that two diurnal
maxima exist, one at midday and the other at midnight. The
midday maxima are associated withsporadic E and appear
primgily during the summer. The nighttime maxima appear
in all seasons and are predominantly associated with spread F
although high values of foE, were noted during nights of high
scintillation activity [44]. MacDougall, in observationsmade
from southern Ontarioin 1977-1 978 (privatecommunication),
showed a midday maximum in the summer.
C. Effect ofMagnetic Index on Middle-Latitude
Scintillation
At latitudes below the auroral oval, various sets of data have
yielded behavior indicating little correlationwith magnetic
conditions. Evans [45] found no correlation of the ionospheric
AARONS: GLOBAL MORPHOLOGY OF IONOSPHERIC SCINTILLATIONS
600 krn
.
AURORAL
.AR FI-u x
FLUX
C O R R E C T E DG E O M A G N E T I C
LATITUDE
Fig. 18. Depiction of high-latitude irregularities -22-02 LT. Sheetlike irregularities are seen in the auroral oval,rod irregularities at
higher and lower latitudes.
scintillation of 400-MHz radar signals withmagnetic index
when targets were south of their station at 56' invariant latitude. Aarons and Martin [46] found that during the August
4-10, 1972 magnetic storms there was a negative correlation
of scintillationand magnetic indexfor Athens, Greece and
Camp Parks, California and little correlation for the 45' intersection of Aberystwyth, Wales. Bramley [47]foundthat
exceptforthe
December 1971 magnetic storm (when the
irregularity region probably encompassed theintersection
point of -45"), there was no correlationbetweenmagnetic
activity and scintillations.
This type of data essentially corroboratesthe earlyradio
star observations in the U.K. which foundlittle correlation
with magnetic index except in paths to the north (with the
exception of some intense magnetic storms).
V. THE HIGH-LATITUDE REGION
Breaking the high-latitude region into zones that differ in
theirmorphology
and physics will allow the user of this
information t o isolate his interests. Fig. 18 depicts the intensity of scintillation in a very broad manner for the period of
time around midnight. It also attempts t o depict the form of
the irregularities and their angle with the vertical; all the structures are along the lines of force of the earth's field. The next
section will describe the detailed irregularity behavior in each
of the high-latitude regions. For high latitudes we have used
the corrected geomagnetic coordinate system. In this system
80"N and 80"W is the position of the north magnetic pole
(Hakura, 1965). The calculations give the displacement of the
"landing points" of geomagnetic field lines.
A . The Plasmapause, the Trough, and the Aurora
In ionospheric physics the plasmasphere is the region where
themotion of ions and electrons is trapped bythe earth's
magnetic field. It extendstoapproximately
60" Corrected
Geomagnetic Latitude (CGL) at night. Atlatitudes higher
than this electrons are not trapped; the edge of this region is
the plasmapause. At higher latitudes of the order of 60"-65'
CGL at nightatrough
or a region of lowelectrondensity
exists. This region is essentially below the auroral oval.
The present evidence is thatthere is a boundaryat high
latitudes where weak irregularities commence. It is probably
equatorwards of the plasmapause, between 45'-55'CGL.
This boundary of irregularities is observed on scintillation data
[48 I , on in-situ measurements of irregularities inthermal
371
plasma [49], and on in-situ measurements using anelectrostatic analyzer [ 8 I.
In the auroral and polar regions energetic electron precipitation and current systemsare dominant factors in producing
both the normal ionospheric layers and the irregularities.
If the ionosphere is perturbed on a percentage basis, AN in
the trough will be small since N is low; scintillations will then
be low. The data of Clark and Raitt [49] show a plateau of
irregularities in the trough region at midnight at a height of
800-1000 km in their observations of thermal electron irregularities. All observers of irregularities at higher auroral latitudes then see a dramatic change in irregularities at the auroral
oval, at the poleward edge of the trough. In the auroral oval,
the intensity of scintillations is a function of local magnetic
activity and is frequently correlated with auroral images as
shown by the optical sensors of the Defense Meteorological
Satellite Program satellites. Poleward of the aurora there may
again be a lowering of scintillation activity until the observing
path transits the polar region [SO] .
Basu [ 5 1 ]using simultaneous scintillation and TEC observations during a magnetic stormfrom a single middle-latitude
station suggested the existence of two regions of scintillations;
one on the equatorward wall of the trough and the second on
the poleward wall of the trough which then extended into the
auroral oval.
Houminer et al. [ 521 examining a series of magnetic storms
with data from twostations andwith corroborating in-situ
observations have shown the existence of maxima onthe
equatorward and poleward sides of the trough with the trough
region at night showing a somewhat lower level of scintillation
activity than the plasmapause region and a much lower level
compared to auroral scintillations.
B. Auroral Scintillations
The aurora is seen by the eye primarily at E-layer heights of
100 km but by other optical techniques can be observed to
extend t o well above 400 km. From studies of radio star and
low altitude satellite scintillations, a series of height measurements have pointed to F-layer heights as the primary seat of
the irregularities producing the signal fading. The irregularities
are found predominantly at altitudes of 250-500 km with a
mean of 400 f 50 km [53]. In-situ measurements [8], [54]
indicate that the irregularities may exist t o the relatively high
altitudes of 1000 km. Crane [7]notedthat during one intense magnetic storm the irregularities responsible for scintillation were at times at Mayerheights.
Maximum irregularity intensity appearsabove the region
showing maximum intensity aurora [ 551. Vickrey et al. [ 561
have shown that there is a collocation of scintillation patches
in the auroral oval and F-region ionization enhancements.
The mechanism recently advanced (Kelley et al. 1982 [ 821 )
for auroral region scintillations is that precipitation of low
energy electrons at F-layer heights initially produce the F-layer
irregularities. These electronsprobablyoriginatenear
the
poleward edge of the nightauroral oval. The irregularities
then last a very longtime andconvect equatorwards and
possibly polewards. Large scale irregularities (tens of kilometers) have long lifetimes and convect to great distances,
continuously producing smaller scale irregularities.
1 ) Morphology in Two Longitudinal Sectors: Perhaps the
most consistent studies of long-term behavior of scintillations
372
PROCEEDINGS OF THE IEEE, VOL. 70, NO. 4, APRIL 1982
MAV T H R U JULY
65.
Y
4
a
I
15-
-.
SAGAMORE HILL
i
1
4
K T
00
06
12
24
I8
MEAN S l ( d B )
06
2 7 PTS
I2
18
24 LT
Kp: 4 - 9
Fig. 21. Variation of mean seasonal index during the northern solstice
in decibels at 137 MHz with local time and invariant latitude derived
from hourly data at the 3 stations under disturbed magnetic conditions (Kp = 4 -9).
0-
+
I
I
I5
,
I
21
I
cdw
I
I
,
1
03
09
MEANSCINTILLATIONINDEX
1
15
LT
(dB1
NARSSARSSUAP 1968- 1974 Kp
0-3
Fig. 19.Contours ofmonthlymean scintillationindexindecibelsat
137 MHz as a function of local time for quiet magnetic conditions
(Kp= 0 - 3 ) obtained at Narssarssuaq during 1968-1974.
WINTER
K p e 6
Fig. 22. Comparison of Wand and EYPR~~QO-MHZ
winter data for the
disturbed magn&etase--(potid curves) with NarssarsPuaq and Goose
Bay model data (dashed curves) corrected for the ATS-3 intersection
point and converted t o 4 0 0 MHz.
I
15
I
21
I
C
/
h
I
1
03
I
09
I
I
15
LT
(dB1
NARSSARSSUAP 1968-1974 Kp=4-9
MEAN SCINTILLATION INDEX
Fig. 20. Contoursofmonthlymeanscintillationindexindecibelsat
137 MHz as a function of local time for disturbed (Kp= 4-9) magnetic conditions obtained at Narssarssuaq during 1968-1974.
at high latitudes have been in the auroral zone, at Alaskan
longitudes, and along the 70"W meridian. In this region there
are irregularities of some intensity on all nights. However,
a seasonal pattern exists (at certain longitudes) and magnetic
control is apparent.
Both the diurnal pattern of scintillationactivityand
the
seasonal behavior as observed from one site can be noted in
Figs. 19 and 20. The data used for this long-term study [571
were taken over a period of 6 years from Narssarssuaq by observing 137-MHz scintillations of the ATS-3 beacon; the propagationpath traversed the ionosphere at -63OCGL.
March,
April, and May were months of high scintillation activity even
when magnetic conditions were quiet. The same months
showed maximum activity during magnetically active periods.
October, November, and December showed both lower activity
and a less pronounced diurnal pattern.
Basu [581 established that the seasonal behavior of scintillations during quiet times was in close agreement with variations
of the auroral electrojet index A L in the same sector of the
auroral oval. It was proposed by Basu that the varying geometry of the plasma sheet with the dipole tilt angle of the earth's
magnetic field may cause a seasonal modulation of particle
precipitation andhence of scintillation. The hypothesis predicted that no such marked seasonal variation should be observed in the Alaskan and Scandanavian sectors of the auroral
oval. Recent VHF observationsmade in Alaska [59] have
indeed failed to show any pronounced seasonal variation.
During magnetically active periods (defied as K p = 4-9)
auroral related effects dominate the high-latitude region.
The long-term study used for Fig. 21incorporated
data
from three observatories (Narssarssuaq, Greenland; Goose Bay,
Labrador; and Sagamore Hill, Massachusetts). Thecontours
of reduced datafor one season (May-July) and for magnetically active periods of time are shown in Fig. 2 1 [57].
The boundary of active scintillation is pushed equatorwards
extendinginto what was the quiet trough and plasmapause
latitudes. Thus
during
magnetic
storms scintillations
and
opticalaurora can be notedfarthersouththan
55". In the
70"W longitude region thisextends below thelatitude of
Boston.
Another series of measurements of scintillations was made
with the 400-MHz radar of Millstone Hill, Massachusetts [ 121.
Theirwinter contoursfor K p = 4-6 are shownin Fig. 22.
Scintillationindex is in terms of S4. Using asuitablefrequency dependence, correcting for geometry, and converting
scintillation indices the dashed values at 137 MHz show the
similar forms and levels of activity as the 400-MHz data.
2 ) Geometry and Enhancement: Conceptually it is a p
AARONS: GLOBAL MORPHOLOGY OF IONOSPHERIC SCINTILLATIONS
373
OEOYETRICAL ENHANCEYENT FACTOR
FOR rms PHASE (8:8:1)
t
1
8-
6-
G
-
4-
-1
w
180.
Fig. 23. Plot ofcorrectionfactorsforviewinga1100-kmorbiting
satellitefrom Natssarssuaq under assumptionsofellipticalcolumn
irregularity model and a power-law irregularity power spectrum.
parent that if one observes a point source along a tube or a
rod, an enhancement would result from the long path length
to the samestructure.
relative to observationsorthogonal
Several studies [ 151, [ 551, and morerecently [ 591 have
shownthatduringthenight,
sheet-like irregularitiesare in
evidence in the auroral oval. Rino and Owen [60] found that
the sheets are aligned along the lines of force of the earth's
field with 8 : 8 : 1dimensions, Along the lines of forceand
in the magnetic east-west direction they are 8 times what they
are orthogonal to these planes.
[ 151
The generalized equation shown in the Introduction
for geometrical enhancement can be applied for a specific set
of parameters (a= 5,7= 2, ro = 1).
For determining the relationship of the scintillation index
S4 at various azimuths and zenith angles to its value at zenith
for measurements at Narssarssuaq, Greenland, Fig. 23 was
constructed; the multiplication factors are valid only for weak
scattering and for the parametersgiven.
The most dramatic variation takes place when phase fluctuations are observed since phase fluctuations increase in general
linearly with irregularity intensity and do not show any saturation effect.
For two sites in Alaska,Rino and Owen [ 601 constructed the
geometrical enhancement factor for rms phase fluctuations for
an 8 : 8 : 1irregularity (Fig. 24).Data for one year [59] are
seen in Fig. 25(a) for the Poker Flat station. The amplitude
enhancement, less dramatic but present, is also shown in Fig.
25(a).Daytimescintillationdoes
notshowthe
sheet-like
structure-at least as observed from Alaska and Fig. 25(b) illustrates the daytimeincrease with increasing latitude.
C. Polar Scintillations
There is a scarcity of direct scintillation data at polar latitudes. This is due to avarietyof reasonsranging fromthe
difficulty of instrumenting and maintaining thequality of
long-term recordings at polar sites t o the fact that viewing of
synchronous satellites from polar latitudes is usually through
very low angles of elevation. Low angles of elevation jumble
troposphericeffects,refraction,multipathandionospheric
scintillation through long andvaried paths.
Theauroral oval maximum is quite clear inits behavior,
principallyitsexpansionandintensificationwithmagnetic
2-
I
I
I
5s
60
65
DIP LAT
- deq
I
70
(RINO AND OWEN,
I
75
80
1980)
Fig. 24. Model computations of phase geometrical enhancement factor
for sheet-like structures with an 8 :8 :1 anisotropy. Because of the
meridional pass trajectory, the location of the enhancement is independent of the pass elevation.
activity. Polewards of the oval, however, sparse data show a
shallow minimumwithasecondpeaklocatednear
the corrected geomagnetic pole [611. MeasurementsbyFrihagen
[53] indicateda small troughpoleward of the auroral oval
with some increase in intensity levels across the polar latitudes
with increasing magnetic activity. However the low frequency
and the saturation levels of the equipment gave only general
indications of the polar morphology.
1 ) Solar Flux Variations: A long-termconsistent series of
measurements has been taken at Thule, Greenland with observations at250 MHz [SO]. Thescintillations forthisstudy
ranged from very low values of 3-6 dB peak to peak on occasionduringaperiod
of lowsunspotnumber
to saturation
fading of 28 dB peak to peak for hours during winter months
of years of high sunspot number.
Oneset
of measurements was takenbetweenApriland
October 1975. During this period of low solar activity, there
wasan
absence of strongscintillationactivity
to suchan
extent that only the occurrence of scintillationgreater than
6 dB could be plotted. Only one period of 15 minutes showed
a single peak-to-peak fluctuation of 10 dB in the 1975 study.
Fig. 26 shows thecontrastbetweenthe1975period
when
solar flux was low(10.7-cmflux
was-75)
and the same
months in 1979 when the solar flux was high (-150-225).
The occurrence levels for both low and high magnetic activity
are shown in order
to separate the components due to magneticstormsfromthosedue
to variationsin the solarflux
forcingfunction.It
is clear that even withlowmagnetic
activity the year of high solar flux shows a dramatic increase
in the intensity and in the occurrenceof ionospheric irregularities which produce scintillations on a satellite to ground path.
2) DiurnalandSeasonalVariation:
A contour plot of the
percent occurrence of scintillation index greater than 10 dB
is shownin Fig. .27. Theplot wasdeveloped fromhourly
average values of the 15-min SI for each month for low magnetic activity ( K p = 0-3). Two patterns emerge:
1) Maximum
occurrence of activity takes place in the months of little or
no sunlight at F-regionheights.Much
lowerscintillation
occurrencetakes place in the sunlitmonths. 2) The diurnal
variation is weak, and apparent only during the winter months.
PROCEEDINGS O F THE IEEE, VOL. 70, NO. 4, APRIL 1982
3 74
1 60
1 40
1.20
130
0.80
OQ
0 60
0.40
0 20
0
50
65
I
I
55
60
1
1
I
I
70
75
80
0
as
DIP L A T I T U D E
(a)
1977 - 1978
's
'Os0
1_--/5
DIP L A T - d q
(b)
Fig. 25. (a) RMS phase and S, at SO-percent exceedance level versus
magnetic latitudefornighttimedata
during1977-1978.(b)
S, at
SO-percent exceedance levelversus magnetic latitude for daytime data
during 1977-1978.
PERCENTOCCURRENCEGREATER T H A N I O d 0
I
I
-
c
NOV
- DEC
- JAN
WINTER
-
I
FE0
4 MAR
SPRING
- APR
7 MAY
30t
-
JUN
SUMMER
-
JUL
-'nut
201
1
Fig. 26. Percentage occurrence of scintillation greater than 6 dB for low
solar flux period April-October 1975 is contrasted with that for high
solar flux periodApril-October1979forbothquietand
&tubed
magnetic conditions.
3 ) Evidence of Two Irregularity Components: Auroral arcs
in the polar cap are approximately aligned withthe noonmidnight magnetic meridian [62]. These arcs generally drift
in the dawn to dusk direction [ 631 ; however, reversals have
been noted[64],[65].
Recently Weber and Buchau [65]
described theorientation and motion of subvisual Player
( A = 6300-A O I ) polar cap arcs. Kilometer-size irregularities
within the arcs produced intense (saturated) amplitude scintillation at 250 MHz as the arcs drifted through asatellite to
ground ray path.Outside
the arcs, scintillation frequently
persisted at a lower level (SI 6 dB).
-
sEP FALL
Fig. 27. Contourplot of diurnalpattern of monthlypercentoccurrence of scintillationgreaterthan
10 dB for low magneticactivity
(Kp= 0-3). Observations were taken during Mar. 1979-Feb. 1980.
A three antenna spaced receiver experiment ofA. Johnson
measured the irregularity drift velocity [SO]. The irregularity
drift pattern, transformed t o CGGlocal time is shown in Fig.
28. The low intensity level irregularities showaconsistent
pattern of anti-sunward drift, with speeds ranging from 300500 m/s. This pattern is consistent with expected E X B
plasma drifts (assuming thattheionaspheric
irregularities
move with the background plasma) obtained from sunlit polar
capelectric field measurements [ 661, and empirical models
[671.
Apictorialrepresentation
of both the small-scale anti-sun-
AARONS: GLOBAL MORPHOLOGY OF IONOSPHERIC SCINTILLATIONS
ducting E layer may tend to short circuit irregularities which
can otherwise persist in the F layer. During the summer the
short circuiting by the E layer could account for the low level
of activity.
Polar scintillation activity is far from being fully explored.
Phase data are needed; amplitude data that are not saturated
are also needed. Indications are that a large range of frequencies must be used to sample the enormous changes of intensity
as a function of solar flux and of season. The cited data merely
give the outlines of morphology.
NXN (03)
CMRECTED G E W A G N E T I C LATITUDE I L K A L TIME
26
-
29
R R C H 1930
UIONIGHT
IONOSPHERIC IRREXILMITI
DRIFT I11 POLARCAP
Fig. 28. Ionospheric irregularity flow in the Arctic in correctedgeomagnetic latitude/local time determined from a spaced receiver experiment at Thule in March 1980.
-
375
c
D A W N - D U S KA R CD R I F T
Fig. 29. Conception of small-scale anti-sunward irregularity drift and
the patch motion.
ward irregularity drift and the patch motion (predominantly
dawn to dusk) is shownin Fig. 29 (E. Weber, private communication). Results point to two irregularity components in
the polarcap; antisunward drifting irregularitieswhich produce a background level of weak to moderate scintillation and
intense irregularities within F-layer polar cap arcs which
produce more discrete (-1-h duration) intense scintillation events
as the arcs drift through the ray path.
Thesquare of scintillation index (S4)’ is proportional to
( ( A N ) 2 ) ,the meansquareelectrondensitydeviation
of the
small scale irregularities responsible for fading and the thickness of the irregularity layer. If during years of high solar flux
the ambient electron density is high, a small disturbance (for
example, 10 percent) in electron density would produce a high
value of AN andtherefore(dependingonlayerthickness)
intensescintillationactivity.Correspondingly
the samepercentageperturbationduringyearswhenelectrondensity
is
low would produce lower scintillation levels.
Monthlymedian values of f o F 2 at Thuleduring1957(a
year ofhigh solar flux) and 1963 (a year of low solar flux)
support thishypothesis.
However the seasonalvariation of
scintillation is not explainedby
the seasonalvariation of
foF2.
The seasonal variation of scintillationmayberelated
to
E-layerconductivity changescaused by the presence or absenceof sunlight t o 100 km. As proposed by Heppner [66]
to explainfluctuating E fieldsin thewinterpolarcapand
more regular variationsin thesummerpolarcap,the
con-
VI. EMPIRICAL
MODEL OF GLOBALSCINTILLATION
BEHAVIOR
A . WBMOD
Over a period of years, starting fromavailable data and from
of scintillationtermed
weak scintillation theory, amodel
WBMOD has been developed by Fremouw andotherswith
attempts to satisfy propagation theory and incorporate a v d ableobservations [81,[681,[691,
1731, 1751, [761, 1781.
Theprogram provides for phase and amplitude information.
Input user parameters include frequency, location, local time,
sunspot number, and planetary magnetic index K p . The user
also mustspecify the longesttime the systemneeds phase
stability. Scintillation indices are the output. A model of the
irregularity drift velocity is contained in the program.
Program WBMOD permitsa user to specify his operating
p for powerscenario. Thecodereturnsthespectralindex
lawphase
scintillation, the spectralstrengthparameter
T,
the standard deviation u+, of phase, and the intensity scintillation index S4, as functions of a changing independent variable chosen by the user.
The theory employed in WBMOD is based on the equivalent
phase-screen representation of Booker,Ratcliffe,andShinn
[701,formulated to accountforthree
dimensionally anisotropic irregularities [ 71] described bya power-law spatial
spectrum. The formulation employed
was developed by Rino
[72] in the infinite outer-scale limit, but a means for dealing
with the effect of a finite outer scale on phase scintillation has
been incorporatedin WBMOD. Similarly, ameans hasbeen
provided for accomodatingmultiple-scattereffects on intensity scintillation that should suffice for practical applications.
is basedon
numerous
The descriptiveirregularitymodel
observations [68], [73], butmost particularly on observations
of phase scintillation performed in the DNA Wideband SatelliteExperiment [8 1. Themostsignificant caveat about use
of WBMOD, however, is that it has been calibrated quantitatively against Wideband data from only a single station in the
northern auroralzone(PokerFlat,
Alaska). Thedescriptive
model wasdevelopedby
iterativecomparisonwithmost
of
the Wideband data population from Poker Flat, with a portion
of the population reserved for finalcomparative tests.
The basic calculations are made of two central quantities T
and p . Tis the spectral strength of phase at a fluctuation frequency of 1 Hz. p is the power4aw spectral index of phase.
T is highly variable, unlike p . The program calculates T and
p and the two commonly used indices of scintillation activity
based on them, one forphase u+,,and one for intensity S4.
In order to calculate T , p , 09, and S4, one must have values
for eight parameters describing ionospheric irregularites. They
are 1) the height h ; 2) vector drift velocity V , of the irregularities; 3) an outer scale a;4, 5, 6 , 7) four “shape” parameters
describing the irregularities’ three-dimensionalconfiguration
and spatial “sharpness,” a , b , 6, and v ; and 8) the height inteC,L. Program WBMOD contains
grated
spectral
strength
PROCEEDINGS OF THE IEEE, VOL. 70, NO. 4, APRIL 1982
3 76
models for the foregoing eight parameters, but the degree of
detail is very much less for some than for others.
The most variable and, probably, the most important of the
eight is theheight-integratedstrength C,L. Theirregularity
strength is modeled by
~ = E ( L , X T~, D,, R ) + M ( L , T ) + H ( L ,
Tm,KprR)
(1)
where
h, geomagnetic invariant latitude
geographic latitude
local meridian time
day of the year
R smoothed Zurich sunspot number
Tm geomagnetic time
KP planetary geomagnetic activity index.
The three terms in (1) respectivelydescribe the strength of
equatorial,middlelatitude,
and
high-latitude
irregularities.
The first two have not been tested extensively against Wideband data but H, the high-latitude termhas.
The high-latitude term is based on the observation that there
often is a more-or-less abruptboundary[741betweenthe
middle-latituderegion of relatively smoothionosphereand
the high-latitude scintillation region.
It is located, typically,
equatorward of discrete-arcaurorasin
the generalvicinity
of the. diffuse auroral boundary.
The underlying form of H
stems from the supposition that the instantaneous boundary
latitude is normally distrikted about a mean value X, for a
given set of Tm , K p , and R. This supposition, together with
other considerations to be discussed shortly, yields the following form forH:
h,
T
D
-
the system is sensitive. For instance, in the Wideband satellite
experiment with normal processing, f c was0.1 Hz [8] as set
by phase detrending.Inacoherentlyintegratingradar,
it
would be the reciprocal of the timeover which phase coherence
is required. For systems not
sensitive to phase instability in
thepropagation medium, f c is effectivelyinfinite,andthe
effective u$ is zero.
The scintillation index for intensity
is the ratio S4 of the
standard deviation of received signal power to the mean received power [21. Unlike u$, its relation to T is set not by a
system or anionosphericparameter,butbythediffraction
process that gives rise to intensity scintillation. For weak to
moderate levels of intensity scintillation, S: is very well a p
proximated [721, [751 by
t 3)
where C(v) is a normalization factor. The Fresnel filter factor
F(4, b , 6 , v ) describes the geometrical enhancement of intensityscintillation.It
also accountsfordiffraction,together
with the Fresnel-zone size
Z=
Xz seconds €3
4n
in which z is the effective “reduced height” (including correctionforwavefrontcurvatureandcurved-earthgeometry)
of
the irregularities. G describes the static geometrical factor.
While (3) is a weak-scintillation formula, it may be generalized for practical purposes, to include the well-known saturation of S4 at unity by writing
sf
= 1 - exp(-~$,)
which is exact for scintillating signals that obey Rice statistics
V61.
where the C’s are constants to be established by iterative testing against scintillationdata,andwheretheerrorfunction
arises from integration over the normal distribution of instantaneousboundarylocation,whichdistribution
has standard
deviation Ah [ 681.
The outer scale, height, spectral index, and drift velocity are
established by simple models in the program. The parameters
u , b , and 6 describe the threedimensional configuration of the
irregularities. As described previously, these have been established for the auroral zone by utilizing Wideband observations
at Poker Flat.
Once these eight parametershave been established, the model
will provide T , p, UG, and S4. Since our interest is in the scintillation indices, we will concentrate on them.
The scintillation index for phase is simply its standard deviation uG which may be calculated by integratingthephasescintillation temporal spectrum@$(f ) as follows:
B. Formulas in Atlantic Sector
Since WBMOD has been developedandcalibratedagainst
data from only one longitude sector (Alaska), it is appropriate
tonote empiricalformulaswhich,though
not as complex,
have been developed for another longitude sector, along the
7OoW meridian. These formulations have been made [771 for
Narssarssuaq, Greenland; Goose Bay, Labrador; and Sagamore
Hill, Massachusetts based on 3-7 yearsdata base of 15-min
scintillationindices.Theforcingfunctionsaretime
of day,
day of theyear,magneticindex,
and solarflux. However,
these individual models are much more limited than WBMOD
as 1) they are applicable only for the frequency
of the data
base, 137 MHz, 2) there is an equipment-based limited excurhave an implicit
sion of the scintillations, and 3) these data
dependenceonthegeometry
of theobservations,namely,
observing ATS-3 from the stations detailed above. This does
not permit other viewing geometries or taking into considerationtheconfiguration
of the irregularities unless correcting
factors are included.
With these caveats, the equations for each station are:
where
Narssarssuaq
fo = V,/Zncu
( V , being theeffectivevelocity
of the satellite).
[The outerscale a is measured in radians per meter in
the fieldnormal reference direction.]
In (2), f c is the lowest phase-fluctuation frequencyto which
SI(dB)=-6.4
*
*
+ 9.2(1
- 0.2FD) [ 1 + 0.23(1
- 0.3FD)
+ 2.0 + 0.34Kp) + 0.03
COS (2(HL - 0.6)) + 0.02 COS (3(HL
COS
+
(HL
3~~))12[0.14Kp(l+O.12FD)+O.09A~(1+1.76FD)]
377
AARONS: GLOBAL MORPHOLOGY O F IONOSPHERIC SCINTILLATIONS
FD = cos (DA + 15.6) + 0.56 cos (2(DA - 22.4))
Goose Bay
Sl(dB)=-1.3
COS
*
+ 1.1(1 - 0.77FD)
[ l + 0 . 5 ( 1 - 0.2FD)
(HL + 2.1 - 0.6Kp) + 0.06
cos (2(HL - 2.1)) + 0.02
+
COS
ACKNOWLEDGMENT
(3(HL
~.2))12[0.3K~(l+O.lFD)+0.8A~(l+1.2FD)]
FD = cos (DA + 0.5)+ 0.2 cos (2(DA - 99))
.
Sagamore Hill
Sl(dB) = 0.33
+ 0.02(1 + 0.2FD) [ 1 + 1.2(1 - 0.OlFD)
*
cos (HL - 0.4 - 0.1 5 K p ) + 0.3
*
cos (2(HL - 0.8)) - 0.1
+
lites of sufficient signal margins. Knowledge of morphology
will also help users to differentiate between fluctuations produced by ionospheric irregularities and those of equipment or
man-made origin.
COS
(3(HL
~.~))12[0~8Kp(l+0.3FD)+3.lAS(I-0.2FD)]
FD = cos ( D A + 56) + 0.7 cos (2(DA - 143)).
DA is day number, As = Sf/lOO, HL is local time (hours) at
subionospheric point (350 km), and Sf is solar flux at 2695
MHz in solar flux units.
In the reference cited [771 corrections for frequency dependence are given thus allowing higher frequency scintillationsto
beestimated.Inaddition,correctionsforgeometryarealso
given similar to those cited in Section I-F of this paper.
VII. CONCLUSIONS
The forms of morphology are now in
place but there are
many gaps. For example, in the equatorialregion there is little
known for the longitude region encompassed by India. There
is little information on the triggering mechanism for the generation of equatorial irregularities; this would allow us to fill in
the gaps in morphology in this latitude sector. The physics of
the instability mechanisms responsible for the strong irregularities and depletions appearsto be well in hand.
In middle latitudes there are vast amounts of data; little are
analyzed. The relatively small fluctuations produced by irregularities of sporadic E- and F-layer origin have little effect; the
signal margins for most equipment override the fading easily.
At auroral latitudes the Occurrence patterns are reasonably
well known although the absolute
values of the intensity of
thescintillation arepoorly known. During weak magnetic
activity in the auroral oval, low level scintillations have been
observed at 137 and 250 MHz. During magnetic storms, however, intense scintillations of saturation level have been noted
at the two frequencies used predominantly for making measurements.Onlyrarely
is scintillationactivitynotedatfrequencies of 1 GHz.
The polarregion has only recentlybeenexploredforits
levels of scintillationactivity. It showssaturated signals of
28-30 dB at 250 MHz during periods of very high solar flux.
The morphology is known only in its gross forms.
The outline of a working model for the community interested in theeffects of theionosphereontransionospheric
signals is in place. The detailed tuning of the model tofit data
from equatorial, middle-latitude, and polar regions has yet to
be done. It is of importance to keep revising the model.
Frequencies from 200-2000 MHz are being used for maritime satellite communications, for navigational purposes, and
foraircraft to ground(throughsatellites)communications.
Knowledge of the characteristics of scintillation will allow us
to develop models to minimize the fading problem and satel-
The author is parhculady indebted to E. MacKenzie for her
and technical participation in this review. S. Basu,
S. Basu, J. A. Klobuchar, and H. E. Whitney contributed greatly
in their critique of early drafts of this paper. A recent review
by Basu and Basu (1 981)1831 addresses many of the problems
of equatorial scintillations in a larger physical framework.
&stance
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