Planet. Space Sci. 1972, Vol. 20, PP. 154.1to 1553. Pwxamon Press. Printed in Nonhcrn Ireland
NOISE IN THE GEOMAGNETIC
TAIL
CHRISTOPHER T. RUSSELL
Institute of Geophysics and Planetary Physics, University of California,
Los Angeles, California 90024, U.S.A.
Abstract-Present observations have revealed a variety of magnetic wave phenomena in the
tail, from ULF to ELF frequencies. However, only VLF measurements of electric
fields have been made. These measurements reveal that the tail is electrically quiet at VLF frequencies, except in the near Earth plasma sheet during substorm expansion phases. The magnetic waves observed iocIude: waves with periods of about 2 min which cause the plasma
sheet boundary position and the neutral sheet location to oscillate; waves from 10-l to 1 Hz
which occur throughout the plasma sheet during plasma sheet expansions; and ELF waves
which occur sporadically in the plasma sheet.
IlWRODUCTION
A detailed knowledge of the physical processes occurring in the Earth’s magnetotail is
essential to our understanding of the substorm process, because the energy released into the
night-time magnetosphere is for some period of time stored in the magnetotail. By studying
the fluctuations observed in the magnetic and electric fields in the tail, we should be able to
identify which instabilities arise there and what forces act on the particles. For example, if
we know the wave amplitudes and polarizations we can calculate the effective conductivity
due to wave-particle interactions. At the present time we have much information about
the magnetic field fluctuations but very little information about the electric field. This is
illustrated in Fig. 1.
This figure shows the frequency range of the various observations of magnetic and electric noise in the tail and the names of some of the researchers who have performed these
studies. Turning our attention first to the electric field observations, we see that very little
work has been done in this area. In fact, the only electric field measurements actually made
in the magnetotail are those of Scarf et al. (1971) on the OGO-5 satellite at VLF frequencies.
However, I have included Mozer and Carpenter because their observations of electric
fields in the night-time magnetosphere do reflect the variations of the electric field in the
tail. F. S. Mozer (1971) has measured these fields with instruments carried on balloons,
while D. L, Carpenter (1971) has calculated this field from the motion of whistler ducts.
The magnetic observations listed in the upper panel of this figure are more complete.
The work of the researchers in the first list, at periods of several hours, has shown that the
overall configuration of the magnetic field in the tail changes in response to substorms.
The plasma sheet, in which the field magnitude is reduced due to particle pressure, first
thins and then expands during substorms and the field direction changes to become more
dipole-like (Behannon, 1970; Hruska and Hruskova, 1969; 1970; Fairfield and Ness,
1970; Aubry and McPherron, 1971; Russell et al., 1971a; Meng et al., 1971).
Field fluctuations at periods of minutes have been revealed in studies of boundary
motions in the tail performed by Mihalov et ~1.(1970) and Russell et ~1. (1971b). Waves in
the tail with periods of minutes cause the magnetopause, the plasma sheet boundary and the
neutral sheet to make periodic crossings of satellites in the tail.
At higher frequencies noise is observed only within the plasma sheet. Noise at frequencies near 1 Hz, that is at ULF frequencies is seen whenever the plasma sheet expands and
1541
1542
CHRISTOPHER
T. RUSSELL
PERIOD
Behonnon
Hruska and Hruskava
FairfIeld and Ness
Aubry and MCPherron
RUSSBII et ‘31.
Meni; 01 01.
f:
2
Russell
et
ai.
EW0cly
0
lo-’
-
to-
to-
IO‘”
IO-’
IO’
FREQUENCY
FIG. 1.
ERED
b&‘ESTIGATORS
BY THE
FREQUENCY
WHO
HAVE
RANGE
MADE
IO‘
IO'
1
10’
(HERTZ)
OBSERVATIONS
OF THE OBSERVATXONS
ELECTRIC
IO'
OF NOISE IN THE TAZLL.
AND
WX-IETHER
THE
THE LIST
OBSERVATIONS
IS ORDWERE
OR MAGNETIC.
seldom at other times (Russell et al., 1971b). ELF noise bursts observed by Brody (1970)
occur even less frequently than this ULF noise.
In this review, I shall cover only the fluctuations with periods of shorter than several
minutes, since the lower frequency observations have already been discussed at this conference in the session on substorms.
Oscillations with periods of several minutes
C~culations of the eigenmode oscillations of the magneto~il by McCIay and Radoski
(1967) predict that the tail has natural resonant frequencies with periods of from 5 to 30 min.
More recent calculations by Siscoe (1969) and by McKenzie (1970; 1971) predict somewhat
shorter periods ranging from about 30 set to 10 min. Although the amplitudes of these
waves are probably quite small, of the order of a gamma or less, they can have dramatic
effects in magnetic field data when a satellite is at a boundary across which the magnetic
field changes markedly. This is because these waves cause a significant periodic displacement of the boundary. Multiple boundary crossings of the magnetopause, plasma sheet
and neutral sheet have been observed. Figure 2 shows multiple neutral sheet crossings
observed by Mihalov et al. (1970) with the Explorer 33 Ames Research Center magnetometer. Explorer 33 was 73 R, behind the Earth at this time. This figure shows the three
vector components of the field expressed in the solar magnetospheric coordinate system for
a time period of 1 hr. The neutral sheet crossings are marked by reversals in the polarity of
the X-component of the field. The irregular pattern of field variations shown here, of
course, cannot be explained by a simple single frequency sinusoidal oscillation of the
position of the neutral sheet, but it does show that there are significant oscillations with
periods of minutes.
1543
NOISE IN THE GEOMAGNETIC TAIL
F1o.2.THETEZRXBSOUR
MAGNETOSP~~RICVEC~~ORCO~~~ONTHBMAO~CF~W)
INGA~RO~NGO~THE~~~LS~~Y
EXPLORRR~~SPACIKXWT 73 R, BEHIND
(MIHALOV et al.,1970).
DURTHE EARTH
Figure 3 shows a histogram of the occurrence of time intervals between successive
neutral sheet crossings for this and other neutral sheet crossings studied by Mihalov et al.
We see that most oscillations of the neutral sheet have periods of from 4 to 10 min with a
peak in occurrence about 2 min which is in the range predicted by recent theory.
Figure 4 shows the magnetic signature of a multiple crossing of the boundary of the
plasma sheet as observed with the University of California, Los Angeles, fluxgate magnetometer carried on board the OGO-5 spacecraft. The satellite was 125 Re behind the Earth,
within 2 R, of the midnight meridian and 2 R, above the expected position of the neutral
sheet at this time. The data are split into two sections in the upper and lower panels each
covering a period of 6 min. The total field and the three solar magnetospheric vector
components are shown. The entry into the plasma sheet causes a reduction of the strength
of the magnetic field. This appears most clearly in the field strength and the X-component
which is parallel to the tail axis. I have shaded the region of diamagnetic depression on the
1544
CHRISTOPHER
T. RUSSELL
IOmin
I hr
I
I
n
1
I03
I02
n
Iin,
IO4
INTERVAL. set
%Z. 3. A HISTOGRAM
SHOWING
NUMBER
VALSBETWEENNEUTR.~~
SHEET CROSSINGS
OF
OCCURRENCES
OBSERVED
BY
OF
DlFFERENT
SEPARATION
THE EXPLORER33 SPACECRAFT
INTER-
(MIHALOV
et al., 1970).
X-component to illustrate the crossings more clearly. We interpret these crossings as
follows: The plasma sheet is expanding, probably due to a substorm and the border of the
plasma sheet crosses the satellite. Superimposed on this expansion there is an oscillation
of the boundary. As the plasma sheet expands, the satellite spends a longer and longer
time within the plasma sheet during each oscillation until finally at 0827 the satellite remains
in the plasma sheet. The period of this motion is about 2 min in this example, in close
accord with the calculations of Siscoe and of McKenzie, and the observations of Mihalov
et al.
Examining the magnetic field data in this figure more closely, we can see that, while the
field outside the plasma sheet as shown in the upper panel is relatively quiet, the field in the
plasma sheet is highly irregular. Before examining the spectral characteristics of this noise,
we will first discuss when and where it occurs.
Oscillations with periods of seconds
Figure 5 shows 1 min averages of the magnetic fieid obtained by OGO-5 on a pass
inwards towards the Earth near the midnight meridian. During this pass the satellite
remained above the neutral sheet and only slowly approached it. The format of these data
is quite different from the previous figure. The top panel is the measured field strength
minus the dipole field of the Earth. The next panel down is the inclination of the field line,
that is the angle between the magnetic field and the local horizontal. If the magnetic field
were radial and pointing towards the Earth this angle would be 90”. The declination is the
angle of the field projected into the horizontal plane at the satellite; it is an ~muthal
angle about the radius vector from the Earth. It is zero for northward pointing fields.
The next panel is the solar magnetospheric 2: component of the field; that is the component
perpendicular to the average neutral sheet. The bottom panel is the root-mean-square
amplitude of fluctuations with periods less than 15 sec. Six hr of data are shown.
This figure illustrates the typical long period variations in the magnetic field encountered
near the plasma sheet and referred to in the introduction. These variations usually have
the form of rapid decreases of the field strength with time scales of about 5 min or less
accompanied by rapid rotations of the field. These rapid changes are followed by slow
recoveries of the field strength accompanied by slow rotations of the field. Two of these
sequences of changes are shown in this figure. The first one begins at 1700 UT and the
IRI
0[
10
_
_
__-_
UNWERSAL
---_e-
TIME
__
AUGUST
20.
IS@
.--I-I‘---J”---
I
The depressions
of the X-component
upon entry into the plasma sheet have been shaded for easy identification.
FIG.~. THF, THREE SOLAR MAGNETOSPHERIC VECTOR COMPONENTS OF THE MAGNETIC FIELD AND THE TOTAL FIELD
DURING A CROSSING OF THE PLASMA SHEET BOUNDARY BY THE OGO-5 SPACECRADT.
0t
10
20
30
40
81
CHRISTOPHER
1546
;j
z
5
:
40
20
g
5
3
T. RUSSELL
0
V. ,
-20
-40
t
I..‘..l..~‘.lr,l.~l,.‘~,‘~“,~‘,,’*~’
AUGUST
9.
Fnx 2%ONE
4
1968
UNIVERSAL
MINUTE
MAGNETOMETER
2200
2100
2000
1900
1600
1700
1600
AVERAGES
OF THE MAGNETIC
ON A PASS INWARDS
TIME
FIELD OBTAINED
IN THE MIDNIGHT
APPROACHED
MERIDIAN
THE NEUTRAL
BY THE
OGO-5
FLUXGATR
AS THE SATELLITE ONLY
SLOWLY
SHEET.
AB is the observed field minus the dipole field. Bz is the solar magnetosphericZcompooent
the field and 6 is the r.m.s. amplitude of waves with periods t15 sec.
of
second about 2010 UT. We interpret these field decreases and increases as expansions and
contractions of the plasma sheet across the satellite. These in general are associated with
substorms. The increase in the solar magnetosphe~c 2 component during these events
shows that the field is becoming more dipolar.
Examining the bottom panel we see that the sudden changes in field magnitude and
inclination are accompanied by high frequency turbulence whereas there is little high
frequency noise at the time of slow changes in the field magnitude and inclination. Some of
the major changes in declination are also accompanied by noise but of a much smaller
amplitude.
In order to determine the average ampIitude of the noise occurring during these sudden
changes in field magnitude and the variation in amplitude with distance down the tail, we
have plotted in Fig. 6 the peak amplitude observed at or near these sudden changes. We
NOISE IN THE GEOMAGNETIC
0.11
-6
I
-6
I
-10
XOSM
FIG. 6. THE PEAK
AMP-E
I
-12
(EARTH
OF NOISE AT PERIODS
AS A FUNCTION
t
I
-14
-16
1547
TAIL
1
-I6
I
-20
RADII)
t15
OF RADIAL
SEC DURING
PLASMA
SHEET
EXPANSION
DISTANCE.
dots indicate the plasma sheet expansions caused a decrease in the field. The crosses
indicate the field suddenly increased. The horizontal lines are the medians of the observed
amplitudes for 2 Re intervals.
The
have restricted our study to regions within 5 R, of the midnight meridian and to changes of
the field greater than 3 y occurring in less than 7 min.
The vertical scale is the amplitude of the noise. The horizontal scale is distance parallel
to the Earth-Sun line. Crosses indicate a sudden recovery of the field strength and dots
indicate a sudden decrease in the field strength. The horizontal bars represent the median
of the data every two earth radii. We see that over the whole range of distances the amplitude of this peak noise during an event varies by an order of magnitude about the median
values. On the other hand, the median values do show a decrease with distance down the
tail of about a factor of two from 9 to 19 R,. However, we should be cautious about the
reality of this trend. Not only is there a lot of scatter in these points, but also, the points
at large radial distances were obtained on the average further from the neutral sheet.
In order to see if this scatter could be in part due to a variation from one event to another
of the distance from the neutral sheet at the time of the observations, we show in Fig. 7
the peak amplitude versus distance from the expected neutral sheet position. In order to
remove the effect of the possible variation of amplitude with distance down the tail, we
have restricted ourselves to the region from 12 to 16 R, behind the Earth. Again, the median
values have been plotted for every 1 R, interval. Although it is not possible to predict with
any certainty the position of the neutral sheet to within 4~R,, we see some order in this
plot. The noise appears to be most intense about l-2 R, from the neutral sheet rather than
at the neutral sheet.
1548
CHRISTOPHER
I
T. RUSSELL
I
I
I
-55Y s5Re
_
-12tX>-16Re
I&
2.0 -
l
.
z
.
.
.-
-
2
3
.
I.0
8
:.
2 0.0 2
2 0.6 2
-*
x0.4
9
a
-
0.2
.
l
..
.
.
..
.
0
I
I
I
2
IZ’I
(EARTH
I
3
I
4
RADII)
FIG. 7. THE PEAK NOISE AS A FUNCTION OF DISTANCE FROM THE EXPECTED NEUTRAL SHEET
POSITIONFORTHERANGEFROM
12 TO 16 R, BEHINDTHEEARTH.
THESAMEFORMATAS
FIG. 6.
We have examined many neutral sheet crossings and have not found any evidence of
magnetic turbulence in this frequency range at the neutral sheet. Figure 8 illustrates this.
At the top of the figure is the solar magnetospheric Xposition of the satellite, and the expected distance from the neutral sheet. The top panel is the difference between the measured
field and the dipole field, the next panel is the amplitude of the rms deviations of the field
and the bottom three panels are the three solar magnetospheric coordinates of the field.
There are three neutral sheet crossings in the middle of a broad depression in the
field. We note that our prediction of the distance from the neutral sheet is in error by from
4 to 1 R, here. There is no noise above +$h of a gamma here at any of the crossings.
This brief examination of the location and occurrence of the high frequency turbulence
gives us some clues as to what this noise can and cannot do. Its absence in the plasma sheet
at quiet times means it plays no role in the maintenance of the quiet time plasma sheet such
as providing scattering centers to scatter magnetosheath plasma into the plasma sheet, etc.
Its absence during the thinning of the plasma sheet means it is not responsible for loss of
particles from the plasma sheet at this time. However, its presence during the plasma sheet
expansions means it may play a role in the rapid change in configuration of the tail. We
note that although absent near the neutral sheet at quiet times, this noise may be associated
with the dissipation process because in the reconnection model the lines of force through
the neutral line form the boundary of the plasma sheet. It is precisely just inside this
boundary that the noise is most intense.
Figure 9 shows the power spectra of the three solar magnetospheric vector components
for these three intervals. The spectra cover the frequency range from 0.03 to 3.4 Hz. The
NOISE IN THE GEOMAGNETIC
TAIL
1549
40
s
20
_/f-y””
1*1”““““”
/
-20
’
c&o
1
I
,
0903
iw0
!iW
UNIVERSAL TIME
1200
im
,400
AUGUST 6, 1969
FIG. 8. 1 MIN AVERAGES OF THE MAGNETIC FIRLD DURING A PASSAGE OF OGO-5 COMPLETELY
THROUGH THE PLASMA SHEET AND NEUTRAL SHEET AT QUIET TIMES.
The distances on the top two scales are the distance along the Earth-Sun-line (Xe& and
distance above the expected neutral sheet position. The top panel shows the measured
field strength minus the dipole field strength. The panel below it shows the r.m.s amplitude of
noise with periods < 15 sec. The bottom three panels show the three vector components of
the field in soiar magnetospheric coordinates.
scale is the logarithm of the power in gammas squared per hertz. The horizontal
scale is the logarithm of the frequency.
In the first panel, which shows the power 10 min before the entry into the plasma sheet,
the power is equal on all three axes within our statistical accuracy as indicated by the error
bars. At high frequencies the noise approaches our digital noise level. The second panel
shows the spectra of the three components just before the entry into the plasma sheet. Here
the two components transverse to the field have increased somewhat but there is little
change in the component along the field.
The last panel shows the spectra after the last entry into the plasma sheet. Here all three
components have the same power and this power is much greater than before the entry.
We note that the spectra are essentiaiiy featureless with little or no change at the proton
gyro frequency. Also the spectra roughly are proportional tof-2 orf-2’6.
Figure 10 shows these same three intervals arranged by component. This illustrates
that the turbulence along the X solar magnetosphe~c direction did not increase until the
vertical
CHRISTOPHER
T. RUSSELL
-7-y
0809-0814
0817-0822
E/20/68
E/20/68
log frequency
FIG.
9.
5?
'0
>I
POWER
Hz
SPECTRA OF THE MAONETIC
L
0
-I
log frequency
THE PLASMA
0827-0832
8/20/68
log
FIELD IN THE TAIL,
SHEET, JUST BEFORE THE ENTRY
-I
-2
Hz
AND
10 MIN BEFORE AN ENTRY
JUST AFTER
0
frequency
Hz
INTO
THE ENTRY.
The A’, Y and Z components are in the solar magnetospheric coordinate system. Some of the
data from which these spectra were computed are shown in Fig. 4.
l-0809
2 - 0817
TO 0814
3 -0827
TO 0822
XGSM
TO 0832
YGSM
ZGSM
2-
3\
3
2
2
‘I
I
c
‘\
\
~
~
-“!y-y-Tlog
FIG.
frequency
I
Hz
10. THE SAMEPOWER
+
-2
-I
log frequency
SPECTRA AS IN FIG.
9.
0
I
Hz
HERE EACH PANEL CONTAINS
AT EACH OF THB THREE TIMES.
2
-I
log
frequency
ONE CD~ONBNT
0
Hz
NOISE IN THE GEOMAGNETIC
1551
TAIL
entry into the plasma sheet although the power in the two transverse directions did begin
to increase before the entry.
To summarize the observations of this high frequency turbulence, it seldom occurs
except at timesof rapid changes in the field. The amplitude of this noise varies over a wide
range but appears to decrease only slightly, if at all, with distance from the Earth. The
peak noise is not encountered at the neutral sheet, but rather from about 1 to 2 R, from
the neutral sheet. No noise is found at neutral sheet crossings. Spectra of the turbulence
show that the noise in the plasma sheet is roughly isotropic and the spectra approximately
follow a power law off-2 to f-"'"
both inside and outside the plasma sheet.
We now turn to observations of ELF and VLF noise in the tail.
ELF AND VLF NOISE
Studies of ELF magnetic noise have been carried out by Brody (1970) with the OGO-1
triaxial search coil magnetometer. Figure 11 shows a typical record of some of these data.
The twelve traces on this plot are proportional to the square root of the power in frequency
bands at 10, 30, 100 and 300 Hz in each of three orthogonal directions. The amplitude
scale is in arbitrary units. The direction and frequency corresponding to each trace is
indicated by the letter and number to the left of the trace. 2 hr of data are shown, These
bursts are quite infrequent, occurring about once per hour when the satellite was in the tail
and have relatively low amplitude, about 50 milligammas. Figure 12 shows where some of
these bursts occurred. The two quantities shown are the amplitude and phase of the field
perpendicular to the OGO-1 spin axis. The arrows indicate the occurrence of bursts of
ELF noise. All the bursts occur in field depressions, that is in the plasma sheet.
A survey of VLF electric noise in the tail has been carried out by Scarf (personal communication, 1971), using the OGO-5 VLF electric field experiment. This survey revealed no
I
9/16/65
OGO-I
x300
Y300
2300
I
REV 142
I
Xl00
+----
YIOO
t------l
-I
+
I------l
ZIOO
x30
Y30
230
A
r.
----I
h
XI0
YIO
-.
il0
0500
0510
0520
0530
0540
0550
0600
0610
0630
0620
U
FIG.
11.
100
AND
THE TWELVE OUTPUTS OF THE OGO-1 SPECTRUhf
300 Hz FROM THE THREE ORTHOGONAL COILS
ANALYZERS
x,
0650
0640
.T.
AT THE FREQUENCIES l&30,
Y AND 2
SHOWING
TYPICAL
ELF
NOISE BURSTS IN THE TAIL.
The
vertical scale is in arbitrary
units which are a function
of frequency (Brady, 1970).
1552
CHRISTOPHER
T. RUSSELL
g/18/65
142
Rev
1
I 21
20
19
18
17
16
I5
14
13 ----tit
R. E.
Fro. 12. Tr-m PHASE AND
AMPLITUDE ON THE MAGNETIC FIELD IN THE SPIN PLANE
SATELLlTB
DURING
A PASS THROUGH
THE
NEAR
OF THE
OGO-1
TAIL.
The arrows indicate where ELF noise bursts were seen (Brody, 1970).
VLF electric field oscillations in the tail with the exception of substorm associated waves in
the near tail within about 12 R, (Scarf et al., 1971). These signals increase in strength with
decreasing radial distances to about 8 R,. In this region, signals of up to 100 mV per meter
have been observed at 3 kHz.
The long term variations of the configuration of the tail in response to substorms indicate
a macroscopic instability of the tail. Waves at frequencies with periods less than several
seconds are seen in conjunction with these macroscopic changes, which indicate that
instabilities on a microscopic level are triggered at these times also. These instabilities have
not yet been identified.
The oscillations observed with periods of minutes may be generated by irregularities in
the solar wind but alternatively they may be the result of a Kelvin-Heimholtz instability
caused by the magnetosheath flow along the tail boundary.
In short, present observations have revealed a variety of magnetic wave phenomena in
the tail. However, low frequency electric fieId measurements have not been performed and
when they are, these too may reveal other sources of noise.
Acknowledgements-I
am indebted to D. 3. Southwood for agreeing to present this paper in my absence,
and for many stimulating discussions during his stay at UCLA. I would also like to acknowledge the
contributions of R. L. McPherron and P. J. Coleman, Jr., with whom much of the study of the OGO-5
data was carried out. The analysis of the OGO-5 fluxgate magnetometer was supported by the National
Aeronautics and Space Administration under contract NAS 5-9098.
AKI~RY,M. P. and MCPHERRON, R. L. (1971). Magnetotail changes in relation to the solar wind magnetic
field and magnetospheric substorms. J. geophys. Res. 76,438l.
B-ON,
K. W. (1970). Geometry of the geomagnetic tail. J. geophys. Res. 75, 743.
BRODY, K. I. (1970). A study of magneticnoise, magnetic field structure and magnetic bay generation in the
magnetotail. Ph.D. Thesis, Univ. of California.
NOISE IN THE GEOMAGNETIC
TAIL
1553
CARPENTER,
D, L. (1971). Plasmapause substorm-associated
variations. EOS Trans. Amer. geophys. Union
52,328.
FAIRFIELD,D. H. and NESS, N. F. (1970). Configuration of geomagnetic tail during substorms. J. geophys.
Res. 15,7032.
HRUXA, A. and HRUSKOVA,J. (1969). Long time-scale magnetodynamic noise in the geomagnetic tail.
Planet. Space Sci .17, 1497.
HRUSKA, A. and HRUSKOVA,J. (1970). Transverse structure of the Earth’s magnetotail and fluctuations
in the tail magnetic field. J. geophys. Res. 75,2449.
MCCLAY, J. F. and RADOSKI, H. R. (1967). Hydromagnetic propagation in a theta-model geomagnetic
tail. J. geophys. Res. 72,452s.
MCKENZIE, J. F. (1970). Hydromagnetic oscillations of the geomagnetic tail and plasma sheet. J.geophys.
Res. 75, 5331.
MCKENZIE,J. F. (1971). Hydromagnetic wave coupling between the solar wind and the plasma sheet.
J. geophys. Res. 76,29%X
MIHALOV,J. D., SONE~~, C. P. and COLBURN,D. S. (1970). Reconnection and noise in the geomagnetic
tail. Cosmic Electrodynamics 1, 178-204.
MENQ, C.-I., AKASOFU,S.-I., HONES,E. W. and KAWASAKI,K. (1971). Magnetospheric substorms in the
distant magnetotail observed by Imp-3. J. geophys. Res. 76,7584.
MOZ~R, F. S. (1971). Power spectra of the magnetospheric electric field. J. geophys. Res. 76,365l.
RUSSELL,C. T., MCPHERRON,R. L. and COLEMAN,P. J., JR. (1971a). Magnetic field variations in the near
geomagnetic tail associated with weak substorm activity. J. geophys. Res. 76, 1823-1829.
Rt~ssnu, C. T., MCPHERRON, R. L. and COLEMAN,P. J., Jr. (1971b). OGO-5 observations of magnetic
noise in the geomagnetic tail. EOS Trans. Amer. Geophys. Union 52,332.
SCARF, F. L., FREDRICKS,
R. W., KENNEL, C. F. and ~ORONKI, F. V. (1971). Satellite studies of magnetospheric substorms on August 15 1968: OGO-5 plasma wave observations.
TR Wpreprint 8023-71-58.
Srsco~, G. L. (1969). Resonant compressional waves in the geomagnetic tail. J. geophys. Res. 74,6482.
13