# Observation of Suprathermal Flux from SROSS-C2 Data at Low Latitude

Authors: Rama Shankar Pandey; Krishan Datta Misra
Journal: Research Letters in Physics
 (2009)

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## Abstract

The studies  of suprathermal flux at equatorial latitude for low solar activity year of 1996 (from January to December) are reported. The retarding potential analyzer (RPA) data of Indian space research organization Stretched Rohini Satellite Series-C2 (SROSS-C2) are used. The suprathermal electron flux data are chosen in the height range of475±50 km. Seasional, latitudinal, and height variations of suprathermal flux having energy range above 30 eV along with comparisons in latitudinal variation of small energy superathermal flux of 5 eV are also reported. The data are selected during the period of local midnight to avoid contaminations from photoelectron flux and flux coming from coating of electrodes.


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## Body

## 1. Introduction

The  majority  of photoelectrons produced by
the ionization of atmospheric neutral  particles by solar ultraviolet radiations
have energies ranging from a few  eV up to about 60 eV [1–3]. Observations on suprathermal
electron flux and electron temperature at high latitudes were studied using
Explorer 33 for energy range of 5 eV to 200 eV [4]. Furthermore, they extended
their study to latitude passes at 1.2<L<2.5 [5].Although most of the photoelectrons are
produced below about 300 km and lose their energy locally through elastic and
inelastic collisions with neutrals, photoelectrons produced above that level
and those reaching that level from below can escape the local ionosphere and
travel along the geomagnetic field lines. Coates et al. [6] found that energy spectra of low-energy field-aligned
fluxes observed by suprathermal explorer (AE) satellite are at an altitude of
350 km at high latitudes. As the field-aligned fluxes are turned on at dawn, the conjugate hemisphere becomes sunlit. It was
further observed that field-aligned flows of electrons below 100 eV observed at geosynchronus orbit
were the photoelectrons, reaching from the ionosphere. Suprathermal electrons
were observed by tethered satellite system (TSS-1R) and moderate solar activity
(MSA) satellite in the range of 70 eV and 50 eV at corresponding height 20 km
away from earth and 200 km altitude, respectively [7, 8]. None has reported till
so far detail variations near equatorial latitudes.Recently,
the suprathermal flux was first seen in RPA data mounted on Stretched Rohini
Satellite Series-C2 (SROSS-C2) Satellite at equatorial and very low latitude
region of the ionosphere ranging in energy range of few eV to 50 eV. The data are obtained from the retarding
potential analyzer (RPA) on board of the Indian low-earth orbiting satellite
SROSS-C2 [9, 10]. An estimate of scintillation indices based on the in site
density measurements compares well with the ground-based measurements. The
bubble development on this night during a low solar activity period was
attributed to promote penetration event. Niranjan et al. [11] have investigated,
based on ionosonde 630.0 nm airglow photometer and SROSS-C2 satellite data
revealed, that seasonal variation of the occurrence and probable onset times of
the post-midnight spread-F during the summer solstice months of low solar
activity period both depend on the characteristics of the highly variable
Equatorial Mid night temperature Maximum Phenomena.Therefore, in this paper,
the measurements of
the suprathermal electron flux made with  the electron RPA onboard of the
SROSS-C2 satellite,
during low solar activity year of 1996, are being reported.
The high-energy tail in electron RPA, I-V characteristic, when the retardation
voltage is made more negative than about –1 V, is due to
the suprathermal electrons. During daytime, the suprathermal currents are at
least an order of magnitude higher than the nighttime values because of the
other contamination produced by photoelectron flux, which dominates the daytime
values. In the SROSS-C2 experiment, a retardation voltage of up to 32 eV has
been used. With a retardation voltage of 32 eV, the probe measures the integral
flux of suprathermal electrons of energies above 32 eV. In the present study,
only the midnight time data covering the period from January 1996 to December
1996 have been used. During this period, the satellite altitude remained
between 420 km to 620 km. Latitudinal, seasional, and altitude variations of
nighttime purely uncontaminated suprathermal flux are being reported at
equatorial latitudes.


## 2. Data

The SROSS-C2  satellite was of octagonal prismoid shape with eight body-mounted solar panels
on the eight sides of the prismoid. SROSS-C2 satellite data were processed for
the period of January 1996 to December 1996. This satellite has perigee, at 450 km and apogee, at 650 km with inclination of 45°. Retarding potential
analyzer is mounted on the satellite with magnetic angle, 130° and
sun angle 90°–120°.
In the SROSS-C2 experiment, a retardation voltage of up to 32 eV has been used.
Retarding potential analyzer planar ion traps are in the X and Z direction and
X axis is inclined to 20° to the velocity vector. The data
represents midnight position where only purely suprathermal electrons are recorded.
The period of year 1996 January to December is divided for seasonal variation
into winter, equinox, and summer months. The data were selected for small range
of values of longitudes, velocity vector, and local times suited to local
midnight period.


## 3. Result and Discussion

Figure1(a) shows the variation of
suprathermal flux with latitude near midnight UT 17:40~19:40 and local time
22:32~23:30 in the height range 425~500 km and longitude range 64°~88°,
also θ  =  4°~18°during summer,
winter, and equinox where as other details are shown in Table 1.  The
suprathermal flux shows the seasonal variation with latitude in Figure 1(a). It
is maximum during equinox month and minimum during summer month and in winter
slightly less than equinox. Figure 1(b) shows the latitudinal profiles of 5 eV
range of suprathermal flux during summer, winter, and equinox months. The other
fixed parameters are given in Table 2. The suprathermal flux at 5 eV is one
order of magnitude less than that at 32 eV. At 5 eV, the flux is more in equinox
and winter months than in summer. This aspect conforms the observation of Wrenn et al. [12], showing
increase at 32 eV and decrease of suprathermal flux at 5 eV. Suprathermal electrons created in the
ionosphere below 250 km lose their energy locally due to high-density neutral
particles bringing the distribution function close to two component
distribution. The cold component of electrons <100 eV has its greatest temperature
near local midnight, which
gradually decreases in the morning sector. Furthermore, it has also been
reported [13] that the sharp ionization peak in photoionization in the energy
rang 25–30 eV and
characteristic troughs at energies less than 4 eV are associated with large
cross-section of the vibrational excitation of molecular nitrogen. The
suprathermal fluxes are having a positive corelation with equatorial anomaly,
normally observed in density variation. The electron production rate dependens on
concentration ratio of O+/O2, whereas loss rate is controlled by ratio of
O+/N2. This ratio was reported at 300 km to be three
times higher in winter months than in summer months [14, 15]. The change in
composition has been attributed to the pattern of global circulation in
thermosphere. In this case, SROSS-C2 data, also shows the small increase in
winter than in summer. For equinox period, it is two times higher than summer
months. In Figure 2, the height variation of suprathermal flux obtained from
SROSS-C2 satellite between latitude range (0°–5°)
and longitude range 60°–78° and for other fixed parameters as shown in Table 3 are given for winter summer
and equinox months. The suprathermal flux is more in summer and decreases with height
except during the month of equinox where suprathermal flux shows minimum
increase with height. The change and increase in superathermal flux may be due
to the principle of electrodynamics lifting at equator. The E×B field at the equator may enhance not
only the density flux but also suprethermal fluxes.  Furthermore, this may also be the remnant
part of suprathermal flux, even after photoionization, still not reaching to
thermal equilibrium due to horizontal transport mechanism reaching from dayside
to nightside from mid latitude to low equatorial latitude [3, 13]. The fountain
effect reported shows
higher density at equator and decreases further away from equator. Similar variations in
suprathermal flux show their maximum at equator and further decrease away from
equator [16]. The height variation of suprathermal at latitude range of (5°–10°)
in Figure 3 and fixed parameters are given in Table 4  showing similar decreasing trend from
equinox, summer, and winter away from equator. This decreasing aspect is
further extended up to (10°–15°)
latitude as shown in Figure 4 and Table 5, this might be associated with
equatorial anomaly.Table 1ParametersSummerWinterEquinoxUT17:28–19:5817:21–19:4717:1–19:41Height447 km–510 km429 km–482 km428 km–460 kmTheta4°–18°4°–19°1°–18°Longitude62°–91°69°–91°66°–91°Table 2ParametersSummerWinterEquinoxUT17:52–18:1017:0–17:4617:19–18:41Height428 km–469 km465 km–480 km429 km–500 kmTheta5°–18°8°–16°3°–18°Longitude69°–84°90°–93°71°–90°Table 3ParametersSummerWinterEquinoxUT18:54–19:3917:22–19:2217:0–19:29Latitude0°–5°0°–5°0°–5°Theta3°–19°4°–19°5°–18°Longitude62°–78°70°–88°71°–82°Table 4ParametersSummerWinterEquinoxUT17:30–19:5317:53–19:4617:0–17:32Latitude5°–10°5°–10°5°–10°Theta4°–17°2°–19°12°–18°Longitude71°–91°70°–90°75°–87°Table 5ParametersSummerWinterEquinoxUT17:44–19:5117:53–19:1317:1–18:56Latitude10°–15°10°–15°10°–15°Theta4°–18°4°–14°12°–19°Longitude70°–89°72°–82°71°–89°Seasional variations of superathermal flux with latitudes (a) at energy 32 eV
and (b) at energy 5 eV. The others fixed parameters are shown in Table1.
(a)(b)Figure 2
Seasional variation of superathermal flux with height at energy
32 eV, latitude range (0°–5°). The
others fixed parameters are shown in Table 3.Figure 3
Seasional variation of superathermal flux with height at energy
32 eV, latitude range (5°–10°). The
others fixed parameters are shown in Table 4.Figure 4
Seasional variation of superathermal flux with height at energy
32 eV, latitude range (10°−15°). The others fixed parameters
are shown in Table 5.


## 4. Conclusion

The suprathermal flux is more at equator and decreases away from
equator. The flux is maximum during equinox months than summer months. The
electrodynamics lifting and fountain effects are seen and are conforming to the
present data. The flux is an order of magnitude higher at 32 eV than those at
5 eV.



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*Source: 748104-2009-02-15.xml*