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Altitude-extended equatorial spread Fobserved near sunrise
terminator over Indonesia
S. Fukao, Y. Ozawa,
1
and M. Yamamoto
Radio Science Center for Space and Atmosphere, Kyoto University, Kyoto, Japan
R. T. Tsunoda
Center for Geospace Studies, SRI International, Menlo Park, California, USA
Received 11 August 2003; revised 8 October 2003; accepted 22 October 2003; published 18 November 2003.
[1] Spatial structure of a post-sunrise backscatter plume
associated with a plasma bubble has been observed for the
first time with the 47 –MHz Equatorial Atmosphere Radar
(EAR) in West Sumatra, Indonesia (0.20S, 100.32E; dip
latitude 10.36N). This plume is likely associated with a
geomagnetic storm. It extended from what appears to be
the base of the Flayer into the topside ionosphere and
differed from all plumes previously observed. It was also
extended in longitude (i.e., in the east-west direction), and
appeared to involve two spatially separated regions. The
plume was first observed around sunrise, close to 200 –
250 km altitude, but at a time when the Eregion was not yet
sunlit. Spatial maps of the line-of-sight Doppler velocity
show that there was upward development and westward
drift of backscatter regions, indicative of daytime drift
conditions. The observations remain puzzling because the
plume continued for approximately two hours after Eregion
sunrise. INDEX TERMS:6969 Radio Science: Remote sensing;
2415 Ionosphere: Equatorial ionosphere; 2439 Ionosphere:
Ionospheric irregularities; 2494 Ionosphere: Instruments and
techniques. Citation: Fukao, S., Y. Ozawa, M. Yamamoto, and
R. T. Tsunoda, Altitude-extended equatorial spread Fobserved
near sunrise terminator over Indonesia, Geophys. Res. Lett.,30(22),
2137, doi:10.1029/2003GL018383, 2003.
1. Introduction
[2] Upward-developing plumes from upwelling of the
bottom-side Flayer have been observed to occur during
Equatorial Spread F(ESF) events, by using sensitive VHF/
UHF/Lband backscatter radars located near the geomag-
netic equator; readers are referred to Kelley [1989] and
Fejer and Kelley [1980] for reviews on this topic. Radar
backscatter is caused by Bragg scatter from field-aligned
irregularities (FAI) with a spatial scale of one half the radar
wavelength, typically 1–3 m [e.g., Woodman and LaHoz,
1976]. The intense plume-like backscatter associated with
ESF is collocated with regions of depleted plasma density
called plasma bubbles [e.g., Tsunoda and Towle, 1979;
Tsunoda, 1980]. Consequently, such regions of intense
backscatter can be used as tracers of plasma bubbles,
especially during growth phase [Tsunoda, 1981], represent-
ing their activity and spatial structure. Recently, similar
observations have been conducted at a higher dip latitude,
6.46N in Gadanki, India (geographically, 13.5N, 79.2E)
[Rao et al., 1997].
[3] In general, ESF is a nighttime phenomenon, and only
several events of daytime ESF have been reported. For
example, Dyson [1977] presented the percentage occurrence
of topside spread Ffrom Alouette I ionograms and showed
that there is a small but significant occurrence of topside
spread Fbetween sunrise and local noon. Results obtained
with the Jicamarca radar included four such events, all
detected in the topside ionosphere, between 600 and 1400 km
altitude and during the spring equinox [Woodman et al.,
1985; Chau and Woodman, 2001]. The results differ in that
the Jicamarca results were from the afternoon sector, whereas
Dyson’s results were from the morning sector. Farges and
Blanc [2002] found evidence for weak irregularities
embedded in the bottomside of the daytime equatorial
Flayer, but these differ from the altitude-extended echoes
observed by the Jicamarca radar. The nature of the topside
irregularities in Alouette I data was not described by
Dyson [1977].
[4] The Equatorial Atmosphere Radar (EAR), located in
West Sumatra, Indonesia (0.20S, 100.32E; dip latitude
10.36N), has been used intermittently for observations of
ESF since October 2001 [Fukao et al., 2003b]. Observa-
tions of daytime ESF with the EAR have been rare. Except
for a few weak occurrences during 50 days of EAR
observation, all ESF occurrences were at night after passage
of the sunset terminator. In this paper, a post-sunrise plume,
extending from what appears to be the base of the Flayer
into the topside, detected with this radar is described.
2. Significance of the EAR System and
Observations
[5] In general, radar observations of ESF events, except
for those made with the ALTAIR radar (8.8N, 167.5E; dip
latitude: 4.0N) by Tsunoda [1981], have been conducted
with fixed vertical beams. When fixed beams are used, only
altitude-time intensity (ATI) plots can be constructed, and
backscatter plumes (and plasma bubbles) must be inter-
preted as drifting from west-to-east without change in form
with time. However, Tsunoda [1981, 1983] showed that
plumes develop dynamically with time, sometimes rapidly,
especially during growth phase and along the west walls of
upwellings. Therefore, time sequences of spatial maps are
essential because much of the physics that remains to be
uncovered lies in the temporal development of the plasma
structure within plasma bubbles.
GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 22, 2137, doi:10.1029/2003GL018383, 2003
1
Now at IHI Aerospace Co. Ltd., Gunma, Japan.
Copyright 2003 by the American Geophysical Union.
0094-8276/03/2003GL018383
ASC 2 -- 1
[6] The EAR is a 47.0-MHz radar with peak and average
output power of 100 and 5 kW, respectively, and with a one-
way half-power antenna beamwidth of 3.4[Fukao et al.,
2003a]. The EAR consists of an active phased array antenna
system similar to the MU radar in Japan [Fukao et al.,
1985]; both have an antenna beam that can be steered on a
pulse-to-pulse basis. In comparison, the ALTAIR radar can
only be steered mechanically, at a rate much slower than
that of the EAR. This unique capability for viewing
different directions virtually instantaneously enables the
EAR to provide the spatial distribution of backscatter from
FAI that has not been available before.
[7] Figure 1 shows a fan-shaped range-azimuth sector (a
fan sector) that can be viewed with the EAR using eleven
beams, each separated by 10in azimuth from 130(Beam 1)
to 230(Beam 11). Each beam is directed perpendicular to
the geomagnetic field B. This arrangement covers approxi-
mately 600 km in east-west direction at 500-km altitude.
The altitude range over which the beam is perpendicular
to B, within the 3.4beamwidth, extends from approxi-
mately 200 km to more than 500 km. The antenna zenith
angle required to achieve this perpendicularity is 24.0
southward in the magnetic meridian, and increases
with azimuth, eastward and westward (34.6 and 36.2for
Beams 1 and 11), away from the meridian plane. The
magnetic field line located 500 km over the EAR reaches its
apex at 800 km over the magnetic dip equator.
[8] The time necessary to complete one fan sector scan is
approximately 140 sec. A 13-bit Barker code pulse with a
16-ms subpulse (or 2.4 km range resolution) is used.
3. Observational Results
3.1. The Overall Features
[9] Figure 2 shows an ordinary ATI plot of ESFback-
scatter observed in Beam 4 (150azimuth) from 0430 LT to
0730 LT on October 25, 2002. We note that, unlike radars
located on the magnetic dip equator, the altitudes are not
associated with locations along the dip equator. Instead,
these altitudes were calculated from ranges where the radar
beam is perpendicular to B. Therefore, higher altitudes
correspond to ranges further south from the EAR. A feature
of interest in this figure is the extension of the backscatter
region in altitude, from near the base of the F layer into the
topside ionosphere. This feature is quite different from those
seen in all previous observations during day. Another
notable finding is that the backscatter region appears to have
originated around the sunrise terminator, near 200 –250 km
altitude, and proceeded to move upward (and southward),
with backscatter intensity increasing with time. This event
occurred during the main phase of a geomagnetic storm
with a maximum Dst index of approximately 90, which
began around 0700 LT (LT = UT + 7 hrs) on the preceding
day but with no sudden commencement. The trigger of this
ESF is likely an eastward electric field that penetrated
promptly from high latitudes associated with this storm
[e.g., Fejer et al., 1979].
[10] Temporal variations in the spatial distribution of FAI,
as represented in fan sector maps, such as presented in
Figure 3, are sometimes conspicuously different from those
which might be inferred from an ATI display, as shown in
Figure 2. Figure 3 shows signal-to-noise ratios of backscat-
ter echoes approximately every 14 min from 0502 LT to
0709 LT. The fan sector map at 0502 LT shows that ESF
echoes appeared almost simultaneously at both the east and
west edges of the fan sector, near 200–250 km altitude.
Considering that both echoes appeared from altitudes as low
as 200–250 km, it seems likely that they did not migrate
from outside of the fan sector but rather were generated
close to the edges at that instant.
[11] The echoing regions subsequently expanded upward
(and southward) in the 300 –550 km altitude range between
0516–0530 LT. The development of these regions was
likely caused by an eastward electric field which intensified
in the low latitude ionosphere near dawn by magnetic
storms, as suggested by Fejer and Scherliess [1997]. At
0530–0544 LT, the two regions merged and continued to
Figure 1. Beam directions of the Equatorial Atmosphere
Radar (EAR). The ordinate and abscissa are the latitudinal
and longitudinal distance from the EAR along the Earth’s
surface. Thick lines indicate the altitude range where
perpendicular intersection of the antenna beam with the
geomagnetic field Boccurs. The five chain lines shown
inside each map indicate perpendicular intersections with B
at altitudes from 100 km to 500 km at 100 km intervals.
Figure 2. Altitude-time intensity (ATI) plot of backscatter
from FAI observed in Beam 4 (150azimuth) from 0430 LT
to 0730 LT on October 25, 2002. The thick solid curve
shows the sunrise terminator.
ASC 2 -2 FUKAO ET AL.: POST SUNRISE EQUATORIAL SPREAD F
expand. Their extent exceeded the entire size of the fan
sector, but the altitude of the maximum echo intensity did
not change much. Finally, the echoes started to move
westward as the size and intensity of the echoing region
decreased from 0559 LT, and eventually disappeared
completely from the west edge at 0709 LT. The echoes
near the west edge did not move much to the east but rather
decayed in place with time. It took approximately two hours
for the backscatter to disappear completely from the fan
sector.
[12] Figure 4 contains contour plots of radial Doppler
velocity presented at approximately 28 min intervals for the
same period. The velocities were directed away from the
EAR and exceeded 150 m/s at 0516 LT, soon after the FAI
were generated. The large velocities are considered to be
caused by the eastward electric field intensified near the
sunrise terminator by the magnetic storm, as mentioned
above [Fejer and Scherliess, 1997]. This electric field might
collaborate with a large gradient of electron density in the
bottom-side ionosphere near dawn to generate the FAI.
However, the velocities became small (to less than one third
their original values) as the FAI started to expand at
0530 LT. In their mature phase from 0530 to 0550 LT, the
radial Doppler velocities, on average, were ranged between
20 m/s and 35 m/s, directed away from the EAR, while the
radial velocities weakened to approximately 5 m/s in their
decay phase after 0600 LT. The Doppler velocities are
consistent with the observed changes in range (range rate)
of the echoes.
3.2. Onset of FAIs
[13] Fan sector maps made rapidly in time are able to
differentiate between a time sequential event, such as the
passing of a sunrise terminator, and simultaneous growth of
two spatial structures. The time of sunrise at 200 – 250 km
altitude was 0501–0453 LT (0508 – 0502 LT) at the east
(west) edge of the fan sector on October 25, 2002. The
sunrise terminator at 100 km altitude is projected onto
the fan sector maps along the geomagnetic field lines in
Figure 5; this is similar to Figure 3 but shown every 140 s
between 0509 to 0521 LT. The shaded region indicates that
the ionosphere at 100-km altitude was not yet sunlit.
[14] The fan sector MAP at 0509 LT shows that both
echoes were generated in the region where the ionosphere at
100 km altitude was in darkness. There is almost no delay in
occurrence between the echoes spaced in east-west distance.
Simultaneous development of the spatially separated echoes
Figure 3. Fan sector maps of signal-to-noise ratio for
backscatter from FAI observed between 0502 LT and
0709 LT on October 25, 2002. The ordinate and abscissa are
the latitudinal and longitudinal distance from the EAR along
the Earth’s surface. The maps are displayed every 14 min
sequentially in time from top left to bottom right. The arc-
like appearance of the contours results from plotting in a
radial-tangential (polar) form with respect to the EAR.
Figure 4. Same as Figure 3 except for radial Doppler
velocities shown approximately every 28 min from 0516 LT
to 0641 LT.
Figure 5. Same as Figure 3, for the time period 0509 LT to
0521 LT. The shaded region indicates that the ionosphere at
100 km altitude was still dark.
FUKAO ET AL.: POST SUNRISE EQUATORIAL SPREAD FASC 2 -3
would be consistent with the presence of a wave-like
seed structure in the bottomside Flayer, and the
imposition of a widespread eastward electric field in that
sector [e.g., Kelley, 1989]. On the other hand, the east
echoes grew more rapidly than the west echoes as the
sunrise terminator moved westward. During 0509 –0514 LT,
the east echoes entered the 100-km sunlit ionosphere while
the west echoes remained in the night-side. Meanwhile, the
east echoes expanded to the west while the west echoes
expanded to the east with less speed. The eastward and
westward drifts are indicative of nighttime and daytime drift
conditions, respectively, following changes in sign of the
vertical electric field caused by the F-region dynamo
[Richmond et al., 1980]. The opposite east-west motions
of these two echo regions reflect these conditions.
4. Discussions and Concluding Remarks
[15] The spatial structure of a post-sunrise backscatter
plume associated with a plasma bubble was first observed
by the EAR through rapid antenna beam steering over a
wide range of azimuths. This plume extended from what
appeared to be the base of the Flayer into the topside, and
was different from those seen on all previous observations.
[16] The ordinary ATI format could easily be mistaken
for a plume which drifted through a fixed radar beam.
Instead, the spatial maps of backscatter intensity show that
the plume was also extended in longitude (in the east-west
direction) and appeared to involve two separate regions. The
plume was generated near 200 –250 km altitude during the
time of passage of the sunrise terminator, but at a time when
the Eregion ionosphere was still in darkness. Indications
are that plume generation was likely associated with the
geomagnetic storm, which occurred during that period.
Spatial maps of line-of-sight Doppler velocity show that
there was upward development and westward drift,
indicative of daytime drift conditions. The initial upward
motion may have been associated with a storm related
eastward electric field, such as suggested by Fejer and
Scherliess [1997].
[17] The observations remain puzzling because 3-m
irregularities are expected to disappear soon after the driver
of irregularity generation is turned off; the usual thinking is
that the daytime Eregion should cause the driver to be
damped. These results are also surprising because the
plume occurred below 600 km where cross-field dissipation
of irregularities should be faster than at altitudes above
600 km, where the Jicamarca radar has detected daytime
irregularities.
[18] One possible interpretation of the phenomena is as
follows: Bubble birth and growth starts at the base of the
Flayer where the field-line integrated Pedersen conductiv-
ity is low. Hence, development of a sunlit Elayer would
damp or prevent development of the plume because of the
much high conductivity of the Elayer. However, once
the bubble has developed by pushing into the peak of the
Flayer, it is possible that the field-line integrated
conductivity of the Flayer will still be more than that of
the Elayer. This would allow continued, though diminished
bubble growth, until the Elayer has developed further.
[19]Acknowledgments. The present project is partially supported by
Grant-in-Aid for Scientific Research on Priority Area – 764 of the Ministry
of Education, Culture, Sports, Science and Technology of Japan. We thank
W. L. Oliver (Boston University) for careful editing of revised version of
the manuscript. The operation of EAR is based upon the Agreement
between RASC and LAPAN signed on September 8, 2000.
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S. Fukao, Y. Ozawa, and M. Yamamoto, Radio Science Center for Space
and Atmosphere, Kyoto University, Uji, Kyoto 611-0011, Japan. (fukao@
kurasc.kyoto-u.ac.jp)
R. T. Tsunoda, Center for Geospace Studies, SRI International, 333
Ravenswood Ave., Menlo Park, CA 94025, USA.
ASC 2 -4 FUKAO ET AL.: POST SUNRISE EQUATORIAL SPREAD F
