Ion Velocities of Equatorial Plasma Bubbles Observed by ROCSAT-1
Chin S. Lin1 and Huey-Ching Yeh2
1
Southwest Research Institute, San Antonio, TX 78228
2
Institue of Space Science, National Central University, Chung-Li, Taiwan
Abstract
Plasma bubbles generated at equatorial
latitudes are important for understanding
ionospheric irregularities, F region plasma
instabilities, electric field dynamo, GPS
communications, and radio occultation in the
used for studying plasma bubbles.
Recently
we
used
simultaneous
observations of the equatorial ionosphere by
ROCSAT-1 and IMAGE satellites to study
plasma characteristics of equatorial plasma
ionosphere. ROCSAT-1 plasma measurements
have been analyzed to study periodic
structures of equatorial plasma bubbles.
It
is found that plasma bubbles are generated
bubbles [Lin et al., 2004].
IMAGE
Far-ultraviolet (FUV) nighttime images have
indicated signatures of depression in the
brightness of equatorial airglow arcs [Immel
when ion zonal velocity has a large gradient in
the 19-21 hour local time. The observed
plasma bubbles are probably maintained by
gravity acoustic waves with a wavelength of
700 km. It is suggested that the precision
orbit determination data of ROCSAT-3
et al., 2003]. Using the list of airglow
brightness depression events observed by
IMAGE, we surveyed ROCSAT-1 IPEI data
for simultaneous plasma observations in the
same local time.
Our investigation has
indicated that features of brightness
/COSMIC during Phase II orbit deployment
are processed to deduce neutral density. The
combined data set of electron and neutral air
density collected by the ROCSAT-3
/COSMIC micro-satellites could be useful for
understanding gravity acoustic waves and the
generation of plasma bubbles.
Keywords: Plasma Bubble; Ion Zonal Velocity
depression seen in FUV images were
correlated with equatorial plasma bubbles
detected by ROCSAT-1 at 600 km altitude.
In this paper, we examine the zonal
velocity of the background plasma when
plasma bubbles are observed and compare the
observed features with those in the absence of
plasma bubbles. The purpose is to determine
if generation of plasma bubbles depends on
the zonal velocity of the background plasma.
1. Introduction
Plasma bubbles generated at equatorial
latitudes are important for understanding
ionospheric irregularities, F region plasma
instabilities, electric field dynamo, GPS
communications, and radio occultation in the
ionosphere. We have analyzed ROCSAT-1
plasma measurements to learn how the
ionospheric
observations
by
the
ROCSAT-3/COSMIC constellation might be
2. IPEI Observations
Plasma bubbles are generally observed in
local time from 19 hour to 24 hour. When no
plasma bubble is detected in this local time
period the ion density profile at the
ROCSAT-1 altitude of 600 km indicates
features of equatorial anomaly with a local
maximum at northern and southern latitudes
about 10o. The density at the equatorial
anomaly peak is typically about 5 x 106 cm-3.
Figure 1 illustrates typical ROCSAT-1
plot the zonal velocity of the background
plasma as a function of local time by masking
out line plot of ion velocity inside plasma
measurements of density dropouts that were
detected on day 111, 2002.
We have
identified at least seven distinct plasma
bubbles in a series of density dropouts (top
panel, Figure 1). Their occurrence appears
to be periodic. We estimate the average elapse
time between plasma bubbles to be about 92
seconds, corresponding to a separation
distance of about 700 km. This separation
distance is in agreement with the typical
bubbles (top panel, Figure 2). Before plasma
bubbles were detected (20 hour LT), the
background plasma had a zonal velocity
increasing gradually from –50 m/s to about
+50 m/s.
When plasma bubbles were
detected, the zonal velocity of the background
plasma outside the bubbles increased from
about 50 m/s at 20 hour LT to a peak of about
120 m/s at 21 hour LT and then decreased
gradually to about 100 m/s at 23 hour LT (top
horizontal wavelength of atmospheric gravity
waves in the thermosphere, which is about
1000 km.
The ROCSAT-1 observations indicate that
panel, Figure 2).
The variation of ion zonal velocity in the
local time from 19 to 21 hour is of particular
interest because plasma bubbles are actively
three components of ion velocity had large
spikes associated with density dropouts
(Figure 1). The “radial” component of ion
velocity VM had positive spikes inside density
dropouts, indicating upward vertical velocity
inside the plasma bubbles (second panel,
growing during this local time zone. The
zonal velocity of the background plasma
varies appreciably at local time from 19 hour
to 21 hour. We estimate the gradient of the
zonal velocity to be about 70 m/s per hour
between 20 and 21 hour LT. After 21 hour
Figure 1). Large VM spikes are commonly
detected inside density dropouts detected
before 20.5 hour LT. Inside the density
dropouts, the zonal velocity VZ is negative
indicating ions were moving westward inside
the plasma bubbles (third panel, Figure 1).
The Vll component shown in the bottom panel
of Figure 1 is the ion field-aligned velocity
defined as positive toward north. The Vll
component indicates that background ions
LT, plasma bubbles become stagnant.
were moving toward the southern hemisphere
along the field lines.
We compare the zonal velocity of the
background plasma with and without plasma
bubbles. In the absence of plasma bubbles,
the zonal velocity has little variation with
respect to local time in the period between 19
and 23 hour (bottom panel, Figure 2). We
[2002] interpreted the periodic structure as
caused by longitudinal propagation of gravity
acoustic waves. Gravity acoustic waves are
generally excited at lower altitude where
neutral density is higher.
The plasma
bubbles detected by ROCSAT-1 reflect the
higher altitude portion of equatorial plasma
bubbles generated at the topside F region.
3. Discussion
The ROCSAT-1 observations indicate
strikingly periodical structures of plasma
bubbles.
Similar periodic structures have
also been found in the brightness depression
of airglow arcs, which are believed to occur at
a couple hundred km altitude. Sakawa et al.
2
The close relationship between plasma
bubbles and gravity acoustic waves has been
observed before [Kelley et al., 1981; Hysell et
the ion drift velocity [Rishbeth, 1972]. This
implies that the neutral air velocity may have
a sharp gradient in the zonal direction as well.
al., 1990]. The present results imply that
neutral air density at a few hundred km has a
wavy structure due to gravity acoustic waves
with a wavelength of about 700 km.
Gravity acoustic waves can produce
strong ionization perturbations when the phase
speed of a gravity wave is equal to the drift
speed of ionization due to the spatial
resonance effect.
Figure 1 indicates that
large perturbation of ionization occurs when
Associated with the zonal velocity gradient,
neutral air may move under a longitudinal
pressure gradient and excite gravity acoustic
waves propagating longitudinally [Yeh and
Liu, 1974].
the ion zonal velocity is about 100 m/s. For
a wavelength of 700 km and a phase velocity
of 100 m/s, we estimate the wave period of
gravity acoustic waves to be 2 hours. The
dependent upon neutral air in the
thermosphere. Neutral wind may produce
ionized gas motion through polarization
electric field at equatorial latitudes during
estimated wave period is longer than the
typical wave period of atmospheric gravity
waves, which is about 1 hour.
Many studies have established that
equatorial plasma bubbles are produced by the
nonlinear evolution of the Rayleigh-Taylor
nighttime. The periodic structure of plasma
bubbles has been interpreted as evidence of
longitudinal propagation of gravity acoustic
waves. It is found that plasma bubbles are
generated when the ion zonal velocity has a
large gradient in the 19-21 hour local time.
instability.
A zonally propagating gravity
acoustic wave can initiate the Rayleigh-Taylor
instability in the bottomside F region [Huang
and Kelley, 1996].
The ROCSAT-1
observations support the assertion that neutral
air in the thermosphere is the driving force for
producing active plasma bubbles. Since the
Rayleigh-Taylor instability appears to be no
longer operative after 21 hour LT, gravity
acoustic waves play a major role in
We propose that thermosphere wind has a
large velocity gradient that might generate
gravity acoustic waves in the early evening
hours.
During the second phase of satellite
deployment, ROCSAT-3/COSMIC micro
-satellites will spend up to 12 months in a
circular orbit of 500 km before being raised to
the final orbit of 750 – 800 km. During this
phase, POD data and geodetic modeling might
maintaining structures of plasma bubbles.
The present study suggests that the sharp
gradient in the zonal velocity of the
background plasma might be responsible for
excitation of the seed gravity acoustic waves.
When ion density exceeds 2 x105 cm-3,
ion-drag acceleration is dominant, and the
thermospheric wind speed is comparable to
be used to deduce neutral density along the
satellite track. The neutral density data
together with electron density profiles
acquired from analysis of radio occultation of
GPS signals could be a useful data set for
understanding generation mechanisms of
plasma bubbles.
4. Summary
The ROCSAT-1 observations suggest that the
occurrence of plasma bubbles might be
3
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Figure 1.
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Figure 2. Comparison of zonal velocity of
the background plasma VZ in the evening
sector. The horizontal axis is local time.
Traces of ion velocity inside the bubbles are
masked out and replaced by dashed lines
between open circles, which mark the
boundary of plasma bubbles.
4