Three-dimensional Spatial Structures of Mid-Latitude Type 1 Es Irregularities
Chien-Ya Wang 1 and Yen-Hsyang Chu2
1. Department of Physics, Chinese Culture University, Taipei, Taiwan, R.O.C.
2. Institute of Space Science/Center for Space and Remote Sensing Research,
National Central University, Chung-Li, Taiwan, R.O.C.
mechanisms
concurrent type 2 irregularities is also
reconstructed. The result strongly
suggests that it be a well-defined thin
involved in the generation of mid-latitude
type 1 sporadic E (Es) irregularities have
been suggested, rare observational
evidence is provided to validate the
proposed plasma structure associated
with the mid-latitude type 1 Es
irregularities. Concurrent type 1 and type
2 radar returns from sporadic E (Es)
irregularities detected by the Chung-Li
VHF radar are analyzed and discussed in
layer with thickness of 1 - 2 km and
horizontal extent of 9 – 17 km in E-W
direction. The observational evidences
that the slab structure has sharp edges in
lateral edges and the type 1 irregularities
drift in the meridional direction favor the
slab model proposed by Haldoupis et al.
(1996) to account for the excitation of the
mid-latitude type 1 irregularities.
However, the other observations,
this paper. For the present two cases the
mean Doppler velocities of the type 1
echoes are between about 220m/s and
302 m/s, smaller than those observed in
equatorial and auroral regions. The type
1 echoes are interferometrically analyzed
and the corresponding plasma structure
of the type 1 irregularities is
reconstructed. We find that the plasma
structure has sharp lateral and top and
including considerably large zonal extent
of type 1 irregularities in the plasma slab
and wide coverage of zonal polarization
electric field, are unfavorable to the slab
model.
Abstract
Although
the
plausible
Keywords:
type 1 Es irregularities、Chung-Li VHF
radar、interferometry technique
bottom boundaries with thickness of
about 1 – 2 km and horizontal extent of
about 12 - 3 km in E-W direction. Its
dimension in N-S direction cannot be
resolved by using interferometry
technique because of considerably
narrow width of expected echoing region
in elevation. The spatial structure of the
1. Introduction
Irrespective of large inclination
angle of geomagnetic field line and weak
dc electric field intensity, a number of
observational evidences in support of the
generation and existence of type 1
1
irregularities in mid-latitude sporadic E
Pedersen conductivity, and relatively
(Es) region have been provided recently
by various HF and VHF radars
(Haldoupis et al., 1996; Huang and Chu,
1998; Hysell and Burcham, 2000). In
general, the occurrence of mid-latitude
type 1 irregularies is very rare compared
with those in equatorial and auroral
regions. In addition, the durations of
mid-latitude type 1 echoes is short,
lasting from several seconds to a few
large ambient electric field be required to
explain the generation of mid-latitude
type 1 echoes. Recently, by using
numerical model of E region current
system in combination with a highly
elongated and patchy Es plasma structure
with axial ratio of major to minor axes
much greater than unity, Hysell and
Burcham (2000) further point out that
enhanced zonal polarized electric field
minutes, and their mean Doppler
velocities are relatively small, ranging
from about 220 m/s to 350 m/s, compared
cannot be sustained in the elongated Es
structure unless the minor axis of the
structure in zonal direction is much less
to nominal ion-acoustic wave speed
(around 360 m/s) in ionospheric Es
region. In order to explain the generation
of intense electric field required for the
excitation of two-stream instability,
Haldoupis et al. (1996) proposed a model
in relation to a plasma structure with
than 1 km.
sharp lateral boundaries. In this model, an
intense polarization electric field in the
zonal direction is induced due to charge
accumulation on the boundaries to
generate strong electron drift along the
radar beam direction at speed more than
enough to excite type 1 waves. Shalimov
et al. (1998) later improved the model of
Haldoupis et al. (1996) by including the
from 22:54 ~23:05. The 40 kW peak
power for each module is transmitted,
effects of field-aligned current and
considering anisotropic configuration of
elongated plasma structure with major
axis lied in meridional direction to
elucidate the generation and persistence
of polarization electric field. They
suggested that the stringent conditions of
highly elongated plasma structure, low
114.6 km to 150 km, in which 60 range
gates were sampled.
Figure 1 presents range time
intensity contour plots of Es echoes
observed by the Chung-Li VHF radar on
24 January 1997 from 22:27:58LT to
22:46:30LT. As indicated, the Es echoes
appear in the range extent from 125 to
2. Results and Discussion
The data used for current analysis were
taken by the Chung-Li VHF radar on 24
January 1997 from 22:27 ~22:46 and
and the 28 s pulse length with phase
coding of 7-bit Barker code is employed
to improve the signal-to-noise ratio of the
radar returns from Es irregularities. The
inter-pulse period was 2 ms and two
times of coherent integration was made.
The radar probing range was set from
2
140 km. The range variations of Doppler
The effective antenna beam pattern
spectra of the Es echoes in the periods
from 22:32:22–22:37:42 LT are plotted
and depicted in Figure 2. Obviously, the
type 1 echoes with very narrow spectral
width primarily occur in the range from
136 km to 141 km and type 2 echoes with
relatively broad spectral width appear in
a very wide range from 125 km to 142
km. Fig.2 also reveals that the mean
Doppler velocities of type 1 echoes are
for the observations of highly
field-aligned Es irregularities is fan-like
for Chung-Li radar, that is, substantially
narrow in elevation and relatively wide
in azimuth directions. Obviously, the
effective
antenna
beam
pattern
incorporating with the spatial structure
of Es irregularities determine the echo
patterns of the Es irregularities projected
in mutually orthogonal planes. If the
between about 250 m/s and 302 m/s, and
those of type 2 echoes are between about
-5 m/s and 165 m/s. Clearly, the phase
spatial structure of field-aligned Es
irregularities is in a form of horizontally
stratified thin layer, the echo pattern
velocities of type 1 irregularities shown
here are much smaller than the nominal
ion acoustic speed in the E region (about
360 m/s).
Fig.3 shows selected spatial
distributions of Es echoes projected on
three mutually orthogonal planes, in
projected in azimuth plane (formed by
vertical and E-W axes) will be expected
to be a horizontal striation with width
equivalent to the vertical extent of the
layer and the projection of the echo
pattern on horizontal plane (formed by
E-W and N-S axes) will be a slant
which type 1 (marked with open circle)
and type 2 echoes (marked with cross)
are shown separately. The panels in the
top rows of Fig.3 display the Es echoes
projected in vertical planes with the axes
along vertical and north-south directions.
The panels in the middle row of Fig.3
display the projections of the echoes in
azimuth planes with axes along vertical
striation at a specific angle with respect
to boresight direction of antenna beam.
Fig.4 presents the schematic diagram
showing the expected echoing region
(shaded parallelepiped) of field-aligned
Es irregularities assembled in a
horizontally stratified thin layer and its
projections (shaded parallelograms) on
the horizontal, azimuth and vertical
and east-west directions, while the panels
in the third row represent the projections
of the Es echoes on horizontal planes.
The most striking feature revealed in
Fig.3 is the appearance of striation-like
echo patterns after 22:31:17 LT, when the
type 1 echoes are observed and coexist
with type 2 echoes.
planes, respectively. We note that the
expected ratio of LNS to LEW on the
horizontal plane for the Chung-Li VHF
radar is about 0.62, where LNS and LEW
are the dimensions of the slant
parallelogram in N-S and E-W
directions with respect to boresight of
radar beam.
3
In order to verify the radar returns
with aspect angle less than 0.25. Perfect
agreement between expected and
observed echoing regions also suggests
that ionospheric refraction effect on radar
wave propagation for the Chung-Li VHF
radar is insignificant and can be
neglected.
from Es irregularities presented in this
article are highly aspect sensitive, the
observed (dotted points) and expected
(crosses) echoing regions are compared
as shown in Fig.5, in which the expected
echoing region is computed from
IGRF95 model and the magnetic aspect
angle of 0.25 and the height range of
104.5 - 107 km are employed in the
computation. As indicated in Fig.5, the
3. Conclusion
configuration of the expected echoing
region is in a form of striation with width
intense type 1 echoes with relatively low
Doppler velocity (between about 250 and
302 m/s) were observed and investigated
By using the Chung-Li VHF radar,
of about 0.7 in elevation due to
exceedingly high aspect sensitivity of the
backscatter, which corresponds to
horizontal extent of about 2 km in
meridional direction. Fig.5 shows that the
angular extent of the expected echoing
region in elevation is a function of
in this article. Interferometry results show
that type 1 irregularities coexisting with
type 2 irregularities are confined in a
very thin layer with sharp boundaries not
only on top and bottom sides, but also on
lateral sides. The vertical extent of the
layer is about 2 km, and the horizontal
azimuth angle, ranging from 52 at
azimuth angle of 0 to 49 at azimuth
angle of –15. The corresponding
horizontal distances of the expected
echoing region are about 23.8 km in
east-west direction and about 7.2 km in
north-south direction. A comparison of
observed and expected echoing regions
presented in Fig.5 shows that the
observed echoing region is in excellent
extent of the type 1 irregularities in zonal
direction ranges between 12.7 km and 6.8
km. Although the meridional extent of
type 1 irregularities is very difficult to
obtain unambiguously because of the
limitation of the configuration of the
expected echoing region, data shows that
it is at least 5-7 km. Basically,
interferometry measurements presented
agreement with the expected one, where
the system phase bias has been adjusted
in interferometry equations (Wang and
Chu, 2001). In light of this, it is
concluded that the observed angular
positions of the Es irregularities in the
echoing region are correct and Es
irregularities are indeed field-aligned
in this article favor the slab model
proposed by Haldoupis et al. (1996) to
explain the formation of mid-latitude
type 1 irregularities. However, the slab
model cannot explain satisfactorily
observed phenomena, including large
zonal extent of type 1 irregularities and
sustenance of intense zonal polarization
4
observations of type 1 sporadic E
irregularities in the equatorial
anomaly region using the Chung-Li
radar, Geophys. Res. Lett., 25,
3779-3782, 1998.
Shalimov,
S.,
C.Haldoupis,
and
K.Schlegel,
Large
polarization
electric fields associated with
midlatitude sporadic E, J. Geophys.
Res., 103, 11617-11625, 1998
electric field over wide region. A
refinement of the existing slab model
such that it can satisfactorily account for
the observations is required. Furthermore,
the meridional dimension of the plasma
structure cannot be inferred from the
echo patterns due to the strict limitation
of tremendously high aspect sensitivity of
the backscatter in elevation. The
implementation of extra interferometers
separated from one another with
Figures
appropriate distances in N-S direction
should be required to validate the
proposed elongated plasma structure of
mid-latitude type 1 irregularities.
Reference
Chu Y. H., and C. Y. Wang,
Three-dimension spatial structures of
mid-latitude type 1 Es irregularities, J.
Figure 1. Range-time-intensity of Es
backscatter
Geophys.Res.,107(A8),1182,dio:10.1
029/2001JA000215, 2002.
Chu Y. H., and C. Y. Wang, Plasma
structures of 3-meter type 1 and type
2 irregularities in nighttime midlatude
sporadic E region, J. Geophys. Res.,
107(A12),1447,dio:10.1029/2002JA0
09318, 2002.
Haldoupis,
C.,
K.Schlegel,
and
D.T.Farley, An explanationfor type-1
echoes from the mid-latitude E –
region ionosphere, Geophys. Res.
Lett., 23, 97-100, 1996.
Hysell, D.L., and J.D.Burcham, The
30-MHz radar interferometer studies
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J. Geophys. Res., 105, 12797-12812,
2000
Huang, C. M., and Y. H. Chu, First
Figure 2. Range variations of type 1 and
type 2 radar spectra for the periods
22:32:22 – 22:37:42 LT
5
Figure
3.
Selected
echo
patterns
Figure 5.
comprised of type 1 (open circles) and/or
type 2 echoes (crosses) projected in
mutual orthogonal planes.
A comparison of observed
(dot) and modeled (asterisk) echoing
regions in elevation-azimuth plot, where
the modeled echoing region is calculated
from IGRF95 in the height range of
105-107 km with magnetic aspect angle
of 0.25.Note that 0 of azimuth angle
corresponds to the boresight direction of
antenna beam, which is toward north by
Zenith
Ra da r Be a m
west 17 geographically.
South
B
B
B
W e st
LEW
E a st
LNS
North
Figure 4.
Schematic figure showing
the expected echoing region (shaded
parallelepiped) of field-aligned Es
irregularities grouped in a form of
horizontally stratified thin layer and its
projection (shaded parallelograms) on
the horizontal plane.
6