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Studies of electron and ion temperatures at 500 km altitude during sunrise using Indian SROSS C2 satellite
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Advances in Space Research 43 (2009) 273–281 www.elsevier.com/locate/asr
Studies of electron and ion temperatures at 500 km altitude during sunrise using Indian SROSS C2 satellite Malini Aggarwal a,*, H.P. Joshi a, K.N. Iyer a,1, M.N. Jivani b b
a Department of Physics, Saurashtra University, Rajkot 360 005, India Department of Electronics, Saurashtra University, Rajkot 360 005, India
Received 28 February 2006; received in revised form 22 January 2008; accepted 13 April 2008
Abstract Solar dependence of electron and ion temperatures (Te and Ti) in the ionosphere is studied using RPA data onboard SROSS C2 at an altitude of 500 km and 77°E longitude during early morning hours (04:00–07:00 LT) for three solar activities: solar minimum, moderate and maximum during winter, summer and equinox months in 10°S–20°N geomagnetic latitude. In winter the morning overshoot phenomenon is observed around 06:00 LT (Te enhances to 4000 K) during low-solar activity and to Te 3800 K, during higher solar activity. In summer, it is observed around 05:30 LT, but the rate of Te enhancement is higher during moderate solar activity (2700 K/hr) than the lowsolar activity (1700 K/hr). During equinox, this phenomenon is delayed and is observed around 06:00 LT (4200 K) during all three activities. In winter, the Ti shows an enhancement (2300 from 700 K) around 06:00 LT during low-solar activity but during other activities, it raise smoothly with the progress of the morning. In summer, Ti increases at a rate of 260 K/hr during low and moderate activities and with half the rate (130 K/hr) during high-solar activity. And in equinox months, the Ti is lower during high-solar activity than in lowsolar activity. Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: SROSS C2; Electron temperature; Ion temperature; Morning overshoot; Solar activity
1. Introduction In the equatorial ionospheric F-region, plasma production and loss as well as transport and diffusion are important for the structure of plasma density and temperature. The heat gained by the ambient electron gas from the photoelectrons and from super elastic collisions with the neutrals acts to raise the electron temperature (Te) above the ion and neutral temperature (Ti and Tn). The hot ambient electrons then lose energy in coulomb collisions with the ambient ions and in elastic and inelastic collisions with *
Corresponding author. Current address: Physical Research Laboratory, Ahmedabad, India. Tel.: +91 9898237253. E-mail addresses: asmalini@rediffmail.com (M. Aggarwal), [email protected] (H.P. Joshi), [email protected] (K.N. Iyer). 1 Space Physics Laboratory, VSSC, Trivandrum, India.
the neutral atmosphere. The extent to which electron temperature is elevated depends on the relative importance of the various heating, cooling and energy flow processes, and this in turn is dependant upon altitude, latitude, local time, season and solar activity. The rapid rise in the electron temperatures before local sunrise was first noted by Carlson (1966) at Arecibo in mid-1960. It was then quickly established that the onset of this temperature rise occurs at a time that corresponds to sunrise in the magnetically conjugate ionosphere and that the heating is caused by photoelectrons arriving from the conjugate hemisphere. This phenomenon, known as the ‘predawn effect’ was extensively studied by Carlson (1966). The rapid increase of Te in the early morning period was first predicted by Dalgarno and McElory (1965). Da Rosa (1966) calculated the time-dependant behaviour of electron temperature (Te) during morning hours. Experimental evi-
0273-1177/$34.00 Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2008.04.006
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dence of the morning enhancement has been reported by many workers, Evans (1965), Mc Clure (1971) and Clark et al. (1972). Near sunrise the ratio Qe =N2e becomes large and electron temperature, Te might then attain values of several thousand degrees (Dalgarno and McElory, 1965). In reality, Te is limited by thermal conduction (Da Rosa, 1966), however, large values of Te are observed near the magnetic equator, where the gradient of Te is small in the direction of the geomagnetic field lines and conduction 0 is therefore, rather ineffective in cooling the electron gas (Farley et al., 1967). Oyama et al. (1996a) have observed the Te at the height of 600 km by the low-inclination satellite Hinotori in terms of local time, season, latitude, magnetic declination and solar flux intensity. The Te shows a steep rise in the early morning (well known as ‘‘morning overshoot”), a decrease after that and again an increase at 18 h (named as ‘‘evening overshoot”). The morning overshoot becomes more enhanced in the winter hemisphere and during higher solar flux. They have observed a slight difference between the morning and the evening overshoots in the 210°–285° and 285°–360° longitude sectors. This is thought to be due to the difference in magnetic declination of these two zones and the resulting difference in the effect of the zonal neutral wind on the thermal structure in the low-latitude ionosphere. Oyama et al. (1996b) found that the morning enhancement is strong during the northern summer months and grows with the increase in solar activity. Satellite – borne ionospheric experiments have been conducted in the past on many of the satellite missions, e.g. Atmospheric Explorer, Dynamic Explorer, ISIS etc. Even then the database over equatorial and low-latitudes is very less, especially over the Indian subcontinent. This is because the orbits of satellite are very high (500 up to 4000 km) or they are highly elliptical. However this discrepancy has been greatly reduced in the SROSS C2 mission where the orbit is just above the F-region peak (at an altitude of 400–500 km) and the orbit ellipticity is much less (Garg and Das, 1995). The in-situ measurements of Te and Ti were carried out using onboard RPA payload. The Te and Ti at the upper ionosphere depends on the solar activity level and is found to exhibit diurnal, seasonal, altitudinal and latitudinal variations (Kohnlein, 1986; Bilitza, 1991; Oyama et al., 1996b; Prabhakaran Nayar et al., 2004). A study on the characteristics of the Te and Ti helps in understanding the representative behaviour of the ionosphere both in the low and equatorial latitude regions at a height of 500 km. In this paper, the solar variation of electron and ion temperature using the SROSS C2 data for the low and equatorial geomagnetic latitude region about an altitude of 500 km during morning hours (04:00–07:00 LT) is studied. 2. Experimental details The Retarding potential analyzer (RPA) aeronomy experiment designed and developed at National Physical laboratory (NPL), New Delhi was sent in space onboard on Indian satellite SROSS C2 on May 4, 1994. A 114 kg
satellite SROSS C2 of Indian Space Research Organisation (ISRO) was launched from Sri Harikota Range (SHAR) having 46° inclination. This satellite carried an improvised version of RPA which is put in an elliptical orbit of 930 430 km by ASLV D–4 rocket. After two months of operation, the satellites apogee was brought down to 620 km and perigee of 430 km. The RPA experiment onboard the SROSS C2 consists of an electron RPA, ion RPA and a potential probe for making simultaneous measurements of plasma parameters. The Electron RPA makes measurements of the total electron concentration (Ne), irregularities in the Ne and the temperature of the electrons (Te). The Ion RPA makes measurements of the total ion density (Ni), irregularities in the Ni, temperature of the ions (Ti) and densities of the various ions (H+, He+, O+, Oþ 2 and NO+) present in the ionosphere. The variations in the satellite potential with respect to plasma potential during spin and motion of the satellite are measured with the help of the Potential probe. Temperature data are obtained by analyzing one complete electron and ion I–V plots through linear and non-linear curve fitting techniques, respectively. Each I–V plot is constructed by current collected by the sensors with 64 steps of retarding grid voltage applied to the sensor, which takes 1.408 s. During this time, satellite travels a distance of 10–11 km. Only those I–V curves that fall within the specified limits were analyzed for deriving Te and Ti. The data are sampled at every 22 ms which when translated into distance is 176 m taking the satellite velocity to be 8 kms1. The RPA experiment is switched on only during the satellite visibility over the ground station at Bangalore (12.6°N, 77.3°E geographic) and Lucknow (26.8°N, 80.8°E geographic). On an average, two overhead passes lasting 12 min are tracked. The spatial resolution of temperature measurements is about 95 km. 3. Method of analysis To study the variations of Te and Ti using RPA data in ascending phase of solar activity during early morning period, we have grouped the six years data into solar minimum (1995–96), moderate (1997–98) and solar maximum (1999– 2000) activities. The data for three seasons (winter, summer and equinox) in each solar activity period are acquired to get the variation of Te and Ti at referred geomagnetic latitude. The geographic latitude is converted to geomagnetic latitude at 77°E geographic longitude using the formula given by U ¼ ArcSinðSinaSin78:5 þ CosaCos78:5 Cosð690 þ bÞÞ where a is the geographic latitude, a is positive in North and negative in South, b is the geographic longitude, and is positive in the East and negative in the West. The observed Te and Ti are grouped into bins of 3° latitude intervals and ±15 min to obtain the variation without any longitudinal selection during early morning phase. If there were more than one satellite pass for a given local
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time in a season then the average Te or Ti is considered. The SROSS C2 payload data follow the universal time (UT), which is changed to local time (LT) to obtain a better picture of the observations. For the winter solstice – November, December, January and February: 1995–1996 for solar minimum, 1997–1998 for solar moderate and 1999–2000 for solar maximum; summer solstice—May, June, July and August: 1995–1996 for solar minimum, 1997–1998 for solar moderate and 1999–2000 for solar maximum; equinox season—September, October, March and April: 1995–1996 for solar minimum, 1997–1998 for solar moderate and 1999–2000 for solar maximum data were used in the investigation. The satellite’s data of 2 months, June 1998 and September 1998, were not available and hence are not included in the present study. And the seasonal variation of Te and Ti during each solar activity is investigated for the equatorial and low geomagnetic latitudes during early morning hours. The Te and Ti of different months falling in the same solstice is averaged in the period 04:00–07:00 LT in the geomagnetic latitude range of 10°S–20°N to get the variations during the morning hours. 4. Results and discussion Fig. 1 shows the Day-to-day variation of solar radio flux F10.7 cm (symbol) during the observational period of in-situ measurements of SROSS C2 satellite (1995–2000) and standard deviation of solar flux (shown by red-line) in winter, summer and equinox during solar minimum (min), moderate (mod) and maximum (max) activity. The F10.7 is minimum during 1995 and rises during 2000.
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4.1. Solar activity dependence of Te during winter solstice Fig. 2a–c represents the equatorial and low-latitudinal variation of electron temperature during the dawn period (04:00–07:00 LT) during different solar activities: minimum (F10.7 67), moderate (F10.7 90) and maximum (F10.7 146). Around 04:00 LT, the Te is 750 K, in 10°S 20°N latitude during all three activities. During solar minimum activity, there is a slight rise in temperature around 05:00 LT showing a crest over 10°N and 5°S, Te rises to 1400 K. After 05:00 LT, Te suddenly shoots to 4000K around 06:00 LT in the equatorial region. This enhancement during dawn period over the equator is termed as ‘‘morning overshoot”. The temperature then decreases in this region. But in 7–20°N, no such an enhancement is observed and the Te slowly increases as the photo – ionization process produces energetic photoelectrons. During moderate and high-solar activity also, such an enhancement is observed around 06:00 LT, but observed Te is less, 3800 K, and then decreases around 07:00 LT over the magnetic equator. Around 07:00 LT, during high activity, the Te again rises, and exhibits crests around ±5 degree geomagnetic latitudes. This can be due to the increased solar flux (F10.7 146) and the availability of fewer electrons at the equator to share the photo-absorption energy. At other latitudes, the Te increases slowly by 06:00 LT and then decreases as the more number of electrons are produced. Oyama et al. (1996a) have observed that morning overshoot becomes more enhanced in the winter hemisphere and during higher solar flux but our results show higher rise in Te during minimum solar activity. 4.2. Solar activity dependence of Te during summer solstice
Year 1995 Day No.
1996 365
1997 730
1998 1999 2000 1095 1460 1825 2190
350
300
F10.7
250
200 max
max
max
150 mod
100
mod mod
min 50 Winter
min Summer
min Equinox
Fig. 1. Day-to-day variation of solar radio flux F10.7 cm (symbol) during the observational period from year 1995–2000 and standard deviation of solar flux (shown by red-line) in winter, summer and equinox during solar minimum (min), moderate (mod) and maximum (max) activity. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3a–c shows the latitudinal variation of Te in summer solstice during minimum (F10.7 67), moderate (F10.7 86) and high (F10.7 160) solar activity. The observed Te around 04:00 LT is found to be the same as that of winter solstice and is 750 K during low and moderate solar activity. Around 04:30 LT, the Te starts increasing. This increase is higher during moderate solar activity; this can be due to the competitive result of incident solar flux and number of electrons present to share this energy. Around 05:00 LT, Te enhances considerably in the southern hemisphere, but in the N-hemisphere, Te is not much enhanced during all three activities showing a N–S asymmetry. Around 05:30 LT, the Te shoots up to 3800K in low, 4500 K in moderate and 3500 K in maximum solar activity in the equatorial region. And then Te decreases with the progress of the morning. 4.3. Solar activity dependence of Te during equinox Fig. 4a–c represents the early morning and latitudinal variation of Te in the equinox solstice. Around 04:00 LT, the observed Te is found to be 800 K during both: solar minimum (F10.7 66) and moderate activities (F10.7 86),
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a
4600 4000 3400 2800 2200 1600 1000
Electron temperature, Te
400 5200
b
4600 4000 3400 2800 2200 1600 1000 400 5200
c
4600 4000 3400 2800 2200 1600 1000 400 10S
5S
0
5N
10N
15N
20N
Geomagnetic latitude (degrees) 4.00LT 6.00LT
4.30LT 6.30LT
5.00LT 7.00LT
5.30LT
Fig. 2. Variation of electron temperature (K) during morning hours (04:00-07:00 LT) in winter solstice during (a) minimum (b) moderate and (c) maximum solar activity obtained by RPA data.
but during high-solar activity (F10.7 144), the Te is higher (1200 K) in all latitudes. Around 05:00 LT, the Te begins to rise, as the photo production of the particles begins. Around 05:30 LT, the Te suddenly rises to 4200 K in the equatorial region (±5 degrees) during all the three solar fluxes, and after 06:00 LT, it decreases. Around 07:00 LT, the observed Te is less during high-solar activity than the lower activities; the Te is 2600 K during high and 3400 K during lower activities. This may be because the rate of production of electrons is high during high-solar activity and hence, the energy will be shared by more number of electrons. And hence the energy loss by collisions will also be more, resulting in the less energy possessed by the electrons. 4.4. Solar activity dependence of Ti during winter Fig. 5a–c shows the latitudinal ion temperature (Ti) variation obtained at 500 km, using the RPA data during dawn period during the winter solstice. Around 04:00 LT,
during low-solar activity the Ti is 700 K, which is almost equal to the electron temperature (Te) at the same time, location and the season. The Ti becomes steady up to 05:00 LT during all the three activities. After 05:00 LT, as the morning progresses, the Ti shows variations with the solar activities. During low-solar activity, after 05:00 LT, the Ti start intensifying and enhances to 1100 K at the equator and exhibits two crests at 13°N and 7°S around 05:30 LT. The Ti is 1700 K over 13°N and then again increases with the progress of the morning. But over 7°S, Ti rises higher to 2300 K (around 05:30 LT) from 700 K and then drops. This may be associated with the heating process of photoelectrons produced at the magnetic conjugate point. In the equatorial region (0–3°N), the Ti rises to 2000 K around 06:00 LT from 800 K, around 05:30 LT and then decreases. The Ti increases with the progress of the morning at other geomagnetic latitudes. During moderate solar activity (F10.7 90), the Ti smoothly increases with the time and becomes 2300 K
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a
4600 4000 3400 2800 2200 1600 1000
Electron temperature, Te
400 5200
b
4600 4000 3400 2800 2200 1600 1000 400 5200
c
4600 4000 3400 2800 2200 1600 1000 400 10S
5S
0
5N
10N
15N
20N
Geomagnetic latitude (degrees) 4.00LT 6.00LT
4.30LT 6.30LT
5.00LT 7.00LT
5.30LT
Fig. 3. Variation of electron temperature (K) during morning hours (04:00-07:00 LT) in summer solstice during (a) minimum (b) moderate and (c) maximum solar activity obtained by RPA data.
around 07:00 LT in the equatorial region and 2400 K around 06:30 LT around 15°N latitude. But during highsolar activity, the observed Ti increases at a lesser rate with the progress of the morning and is very less compared to lower solar activity. Ti is maximum around 07:00 LT and is 1300 K. This can be because the Te is also less during the high solar activity. 4.5. Solar activity dependence of Ti during summer Fig. 6a–c shows the latitudinal variation of Ti during summer, in varying solar activities. Around 04:00 LT, the Ti is 700K during all three activities. As the morning progresses, the Ti also increases, but with a reduced rate (260 K/hr), and rises to 1500 K around 07:00 LT during low and moderate activities. But, during high-solar activity, the Ti is increased to 1100 K, at much slower rate, 130 K/hr, which is half of that during low-solar activities, around 07:00 LT.
4.6. Solar activity dependence of Ti during equinox Fig. 7a–c represents the equinox months during solar minimum, moderate and high activities. Around 04:00 LT, the Ti shows a small crest over 3°N during low-solar activity, and the observed Ti is 1000 K. But at other latitudes, the Ti is 700 K during both low and moderate and is high (900 K) during higher solar activity. Around 05:00 LT, over 12°N geomagnetic latitude, the Ti gets enhanced to 1800 from 700 K and then decreases with time during solar minimum. This peak shifts to 7°N around 05:30 LT and then decreases with the progress of the morning. The observed Te also shows the enhancement at this time and the location as represented in Fig. 4a. At other latitudes, the maximum Ti is observed around 07:00 LT, and is higher in the southern latitudes (2100 K) than the northern latitudes. During moderate solar activity, the Ti rises with the progress of morning. Around 05:30 LT, the Ti shows two peaks; one over
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a
4600 4000 3400 2800 2200 1600 1000
Electron temperature, Te
400 5200
b
4600 4000 3400 2800 2200 1600 1000 400 5200
c
4600 4000 3400 2800 2200 1600 1000 400 10S
5S
0
5N
10N
15N
20N
Geomagnetic latitude (degrees) 4.00LT 6.00LT
4.30LT 6.30LT
5.00LT 7.00LT
5.30LT
Fig. 4. Variation of electron temperature (K) during morning hours (04:00-07:00 LT) in equinox season during (a) minimum (b) moderate and (c) maximum solar activity obtained by RPA data.
10°N (2200 K) and the other at 15°N (1900 K). Around 06:30 LT, the Ti enhances to 2500 from 1400 K and then decreases over the equator. The electron temperature also exhibits such a morning enhancement during the same time and location during equinox months in moderate solar activities. During high-solar activity, the observed Ti is less compared to the low-solar activities, and exhibits the highest temperature (1300 K) around 06:00 LT over the equator and then decreases. Fig. 4c also shows the high electron temperature around 06:00 LT over the equator. The electron temperature is strongly affected by the electron density. In general the temperature increases or decreases as the density decreases and increases, respectively. This relationship between the electron density and temperature is due to the heating losses exceeding the heat gains with increasing electron density and the heating dominating with decreasing electron density (Banks and Kockrats, 1973). The photoelectron production begins in the ionosphere through the ionization of neutral particles at sunrise. These
photoelectrons share their high energy with the ambient electrons, as a result the Te increases. This rise is rapid in the early morning hours due to the availability of fewer electrons. With the progress of the morning, more and more electrons are produced and the energy share for each electron decreases. And hence the Te after reaching a maximum, decreases and attains a steady value by the end of the morning. This rapid increase of Te in the early morning hours is the well known Te phenomenon called the ‘‘morning overshoot” and was predicted by Dalgarno and McElory (1965). Experimental evidence of the morning enhancement has also been reported by Evans (1965), Mc Clure (1971), Clark et al. (1972), Oyama et al. (1996b) and Bhuyan et al. (2002). Oyama et al. (1996b) reported that the Te in the morning rises from about 1200 to 4000 K within ±30° magnetic latitude. They have shown that the intense morning enhancement of Te observed over the equator is due to reduction in electron density caused by the downward drift of the plasma, which usually occurs in morning hours. After sunrise, the ion temperature falls
M. Aggarwal et al. / Advances in Space Research 43 (2009) 273–281 2600
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2300 2000 1700 1400 1100 800
Ion temperature, Te (K)
500 2600
b
2300 2000 1700 1400 1100 800 500 2600
c
2300 2000 1700 1400 1100 800 500 10S
5S
0
5N
10N
15N
20N
Geomagnetic latitude (degrees) 4.00LT 6.00LT
4.30LT 6.30LT
5.00LT 7.00LT
5.30LT
Fig. 5. Variation of ion temperature (K) during morning hours (04:00–07:00 LT) in winter solstice during (a) minimum (b) moderate and (c) maximum solar activity obtained by RPA data.
as the electron temperature also falls, due to the sharing of energy with more electrons. The morning enhancement is associated with the photoelectron heating of the morning low-density electrons which does not occur in summer because of the high morning electron density in this season (Otsuka et al., 1998). The morning enhancements are in general weak in Te and nearly absent in Ti at altitudes below 500 km. The morning enhancement in Te and Ti have larger values around the dip equator. This may be associated with the E B drift and the decrease in electron density as explained by Oyama et al. (1996a). According to Oyama et al. (1996a), the zonal wind blows in westward during morning in the northern summer. This causes an upward drift of the plasma in the northern hemisphere and downward drift in the southern hemisphere. The meridional wind blows from the northern (summer) to southern (winter) hemisphere causing a upward plasma motion along magnetic field line in the northern hemisphere and a downward plasma drift in the southern hemisphere. The effect of these two winds, therefore, strengthens each other. For northern winter conditions, the meridional wind effect reverses from the northern summer case but still the zonal wind effect is the
same, which results in the reduced wind effect. They have also observed this asymmetry between longitude zone of 210°–285° and 285°–360°. These zones correspond to positive (5°) and negative (20°) magnetic declination at the magnetic equator respectively. In the longitude zone of 285°–360°, the temperature of the morning overshoot in the northern (summer) hemisphere is higher than in the longitude zone of 210°–285°. The electron energy is transferred to the ions by elastic electron–ion collisions resulting in the increase in Ti (Banks, 1967). Hanson et al. (1973) showed that the electron and the ion temperatures are lower on the summer side of the magnetic equator than on the winter side at night. The Hinotori satellite indicates also that the electron temperature is higher in the winter hemisphere than in the summer hemisphere at day. The cooling rate of electron by the electron-ion collision is small in the winter hemisphere because of the low-plasma density. Oyama et al. (1996b) have found that the morning overshoot in Te becomes very strong around the equator when a downward drift of plasma during the morning hours is included in the theoretical Sheffield University Plasmasphere Ionosphere model (SUPIM).
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a
2300 2000 1700 1400 1100 800
Ion temperature, Te (K)
500 2600
b
2300 2000 1700 1400 1100 800 500 2600
c
2300 2000 1700 1400 1100 800 500 10S
5S
0
5N
10N
15N
20N
Geomagnetic latitude (degrees) 4.00LT 6.00LT
4.30LT 6.30LT
5.00LT 7.00LT
5.30LT
Fig. 6. Variation of ion temperature (K) during morning hours (04:00–07:00 LT) in summer solstice during (a) minimum (b) moderate and (c) maximum solar activity obtained by RPA data.
5. Conclusions Plasma temperature measurements were carried out with the SROSS C2 satellite during the rising phase of solar activity from 1995–2000 in the 77°E Indian longitude sector during early morning hours (04:00–07:00 LT) at 500 km. Oyama et al. (1996a) have observed that morning overshoot (Te) becomes more enhanced in the winter hemisphere and during higher solar flux but our results show higher rise in Te during minimum solar activity. The observed Te (750 k) is same in all three seasons around 04:00 LT during low and moderate solar activity. And it is found to be higher 1000 K during maximum solar activity around the same time. In winter the morning overshoot phenomenon is observed around 06:00 LT during all three solar activity periods. In summer, the Te shoots to 3800 K in low, 4500 K in moderate and 3500 K in maximum solar activity in the equatorial region around 05:30 LT. And then Te decreases with the progress of the morning. During equinox, after 05:30 LT, the Te shoots up to 4200 K in the equatorial region (±5 degrees) during all the three solar activity period. The observed Te (2600 K) is less during high-solar activity than the lower
activities (Te 3400 K) around 07:00 LT. The morning enhancement in Te depicts a strong dependence on solar activity level with minimum around the maxima of the solar cycle. The higher electron densities and lower electron temperatures are characteristics of Te at the mornings around the solar maximum. During winter, in the equatorial region (0–3°N), the Ti rises to 2000 K around 06:00 LT from 800 K, and then decreases. The Ti increases with the progress of the morning at other geomagnetic latitudes. In summer, as the morning progresses, the Ti also increases, but with a reduced rate (260 K/hr), and rises to 1500 K around 07:00 LT during low and moderate activities. But, during high-solar activity, the Ti is increased to 1100 K, at even much slower rate, 130 K/hr, which is half of that during low-solar activities, around 07:00 LT. And in equinox months, the Ti is lower during high-solar activity than in low-solar activity. Ti is minimum in higher solar activities during early morning hours in all seasons. During morning hours, solar activity dependence is observed in winter and equinox solstice, where Ti is decreased with the increase in solar activity. But in summer, no such dependence is found.
M. Aggarwal et al. / Advances in Space Research 43 (2009) 273–281 2600
281
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Ion temperature, Te (K)
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b
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c
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5S
0
5N
10N
15N
20N
Geomagnetic latitude (degrees) 4.00LT 6.00LT
4.30LT 6.30LT
5.00LT 7.00LT
5.30LT
Fig. 7. Variation of ion temperature (K) during morning hours (04:00–07:00 LT) in equinox season during (a) minimum (b) moderate and (c) maximum solar activity obtained by RPA data.
Acknowledgements One of the authors Malini Aggarwal is grateful to ISRO for providing her fellowship during the course of work. We are also grateful to Dr. S.C. Garg, P. Subrahmanyam and Parvati Chopra of National Physical Laboratory, New Delhi, for providing us the SROSS C2 satellite data. We thank anonymous reviewers for the depth of their analysis and commentary leading to the present version of this manuscript. References Banks, P.M. Ion temperature in the upper atmosphere. J. Geophys. Res. 72, 15323–15330, 1967. Banks, P.M., Kockrats, G. (Eds.). Aeronomy. Academic Press, New York, 1973. Bhuyan, P.K., Chamua, M., Subrahmanyam, P., Garg, S.C. Diurnal, seasonal and latitudinal variations of electron temperature measured by the SROSS C2 satellite at 500 km altitude and comparison with the IRI. Ann. Geophysicae 20, 807–815, 2002. Bilitza, D. International reference ionosphere update Eos. Trans. Am. Geophys. Union 72 (30), 317, 1991. Carlson, H.C. Ionospheric heating by magnetic conjugate point photoelectrons. J. Geophys. Res. 71, 195, 1966. Clark, D.H., Raitt, W.J., Willmore, A.P. The global morphology of electron temperature in the topside ionosphere as measured by an a.c. Langmuir probe. J. Atmos. Terr. Phys. 34, 1865–1880, 1972.
Dalgarno, A., McElory, M.B. The fluorescence of solar ionizing radiation. Planet. Space Sci. 13, 143–145, 1965. Da Rosa, A.V. The theoretical time-dependant thermal behaviour of the ionospheric electron gas. J. Geophys. Res. 71, 4107–4120, 1966. Evans, J.V. An F region eclipse. J. Geophys. Res. 70, 1175–1185, 1965. Farley, D.T., Mc Clure, J.P., Sterling, D.L., Green, J.L. Temperature and composition of the equatorial ionosphere. J. Geophys. Res. 72, 5837– 5846, 1967. Garg, S.C., Das, U.N. Aeronomy experiment on SROSS-C2. J. Spacecraft Technol. 5 (3), 11–15, 1995. Hanson, W.B., Nagy, A.F., Moffett, R.J. Ogo 6 measurements of super cooled plasma in the equatorial exosphere. J. Geophys. Res. 78, 751–756, 1973. Kohnlein, A. model of the electron and ion temperatures in the ionosphere. Planet. Space Sci. 34 (7), 609–630, 1986. Mc Clure, J.P. Thermospheric temperature variation inferred from incoherent scatter observations. J. Geophys. Res. 76, 3106–3115, 1971. Otsuka, Y., Kawamura, S., Balan, N., Fukao, S., Bailey, G.J. Plasma temperature variations in the ionosphere over the middle and upper atmosphere radar. J. Geophys. Res. 103, 20705–20713, 1998. Oyama, K.I., Watanabe, S., Su, Y., Takahashi, T., Hirao, K. Season, local time, and longitude variations of electron temperature at the height of 600 km in the low latitude region. Adv. Space Res. 18 (6), 269–278, 1996a. Oyama, K.I., Balan, N., Watanabe, S., Takahashi, T., Isoda, F., Bailey, G.J., Oya, H. Morning overshoot of Te enhanced by downward plasma drift in the equatorial topside ionosphere. J. Geomag. Geoelectr. 48, 959–966, 1996b. Prabhakaran Nayar, S.R., Alexnder, L.T., Radhika, V.N., Thomas, J., Subrahmanyam, P., Chopra, P., Bahl, M., Maini, H.K., Singh, V., Singh, D., Garg, S.C. Study of the evolution of Te and Ti at the lowlatitude upper ionosphere using SROSS C2 RPA observations. J. Atmos. Solar Terres. Phys. 66, 1075–1083, 2004.
Advances in Space Research 43 (2009) 273–281 www.elsevier.com/locate/asr
Studies of electron and ion temperatures at 500 km altitude during sunrise using Indian SROSS C2 satellite Malini Aggarwal a,*, H.P. Joshi a, K.N. Iyer a,1, M.N. Jivani b b
a Department of Physics, Saurashtra University, Rajkot 360 005, India Department of Electronics, Saurashtra University, Rajkot 360 005, India
Received 28 February 2006; received in revised form 22 January 2008; accepted 13 April 2008
Abstract Solar dependence of electron and ion temperatures (Te and Ti) in the ionosphere is studied using RPA data onboard SROSS C2 at an altitude of 500 km and 77°E longitude during early morning hours (04:00–07:00 LT) for three solar activities: solar minimum, moderate and maximum during winter, summer and equinox months in 10°S–20°N geomagnetic latitude. In winter the morning overshoot phenomenon is observed around 06:00 LT (Te enhances to 4000 K) during low-solar activity and to Te 3800 K, during higher solar activity. In summer, it is observed around 05:30 LT, but the rate of Te enhancement is higher during moderate solar activity (2700 K/hr) than the lowsolar activity (1700 K/hr). During equinox, this phenomenon is delayed and is observed around 06:00 LT (4200 K) during all three activities. In winter, the Ti shows an enhancement (2300 from 700 K) around 06:00 LT during low-solar activity but during other activities, it raise smoothly with the progress of the morning. In summer, Ti increases at a rate of 260 K/hr during low and moderate activities and with half the rate (130 K/hr) during high-solar activity. And in equinox months, the Ti is lower during high-solar activity than in lowsolar activity. Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: SROSS C2; Electron temperature; Ion temperature; Morning overshoot; Solar activity
1. Introduction In the equatorial ionospheric F-region, plasma production and loss as well as transport and diffusion are important for the structure of plasma density and temperature. The heat gained by the ambient electron gas from the photoelectrons and from super elastic collisions with the neutrals acts to raise the electron temperature (Te) above the ion and neutral temperature (Ti and Tn). The hot ambient electrons then lose energy in coulomb collisions with the ambient ions and in elastic and inelastic collisions with *
Corresponding author. Current address: Physical Research Laboratory, Ahmedabad, India. Tel.: +91 9898237253. E-mail addresses: asmalini@rediffmail.com (M. Aggarwal), [email protected] (H.P. Joshi), [email protected] (K.N. Iyer). 1 Space Physics Laboratory, VSSC, Trivandrum, India.
the neutral atmosphere. The extent to which electron temperature is elevated depends on the relative importance of the various heating, cooling and energy flow processes, and this in turn is dependant upon altitude, latitude, local time, season and solar activity. The rapid rise in the electron temperatures before local sunrise was first noted by Carlson (1966) at Arecibo in mid-1960. It was then quickly established that the onset of this temperature rise occurs at a time that corresponds to sunrise in the magnetically conjugate ionosphere and that the heating is caused by photoelectrons arriving from the conjugate hemisphere. This phenomenon, known as the ‘predawn effect’ was extensively studied by Carlson (1966). The rapid increase of Te in the early morning period was first predicted by Dalgarno and McElory (1965). Da Rosa (1966) calculated the time-dependant behaviour of electron temperature (Te) during morning hours. Experimental evi-
0273-1177/$34.00 Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2008.04.006
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dence of the morning enhancement has been reported by many workers, Evans (1965), Mc Clure (1971) and Clark et al. (1972). Near sunrise the ratio Qe =N2e becomes large and electron temperature, Te might then attain values of several thousand degrees (Dalgarno and McElory, 1965). In reality, Te is limited by thermal conduction (Da Rosa, 1966), however, large values of Te are observed near the magnetic equator, where the gradient of Te is small in the direction of the geomagnetic field lines and conduction 0 is therefore, rather ineffective in cooling the electron gas (Farley et al., 1967). Oyama et al. (1996a) have observed the Te at the height of 600 km by the low-inclination satellite Hinotori in terms of local time, season, latitude, magnetic declination and solar flux intensity. The Te shows a steep rise in the early morning (well known as ‘‘morning overshoot”), a decrease after that and again an increase at 18 h (named as ‘‘evening overshoot”). The morning overshoot becomes more enhanced in the winter hemisphere and during higher solar flux. They have observed a slight difference between the morning and the evening overshoots in the 210°–285° and 285°–360° longitude sectors. This is thought to be due to the difference in magnetic declination of these two zones and the resulting difference in the effect of the zonal neutral wind on the thermal structure in the low-latitude ionosphere. Oyama et al. (1996b) found that the morning enhancement is strong during the northern summer months and grows with the increase in solar activity. Satellite – borne ionospheric experiments have been conducted in the past on many of the satellite missions, e.g. Atmospheric Explorer, Dynamic Explorer, ISIS etc. Even then the database over equatorial and low-latitudes is very less, especially over the Indian subcontinent. This is because the orbits of satellite are very high (500 up to 4000 km) or they are highly elliptical. However this discrepancy has been greatly reduced in the SROSS C2 mission where the orbit is just above the F-region peak (at an altitude of 400–500 km) and the orbit ellipticity is much less (Garg and Das, 1995). The in-situ measurements of Te and Ti were carried out using onboard RPA payload. The Te and Ti at the upper ionosphere depends on the solar activity level and is found to exhibit diurnal, seasonal, altitudinal and latitudinal variations (Kohnlein, 1986; Bilitza, 1991; Oyama et al., 1996b; Prabhakaran Nayar et al., 2004). A study on the characteristics of the Te and Ti helps in understanding the representative behaviour of the ionosphere both in the low and equatorial latitude regions at a height of 500 km. In this paper, the solar variation of electron and ion temperature using the SROSS C2 data for the low and equatorial geomagnetic latitude region about an altitude of 500 km during morning hours (04:00–07:00 LT) is studied. 2. Experimental details The Retarding potential analyzer (RPA) aeronomy experiment designed and developed at National Physical laboratory (NPL), New Delhi was sent in space onboard on Indian satellite SROSS C2 on May 4, 1994. A 114 kg
satellite SROSS C2 of Indian Space Research Organisation (ISRO) was launched from Sri Harikota Range (SHAR) having 46° inclination. This satellite carried an improvised version of RPA which is put in an elliptical orbit of 930 430 km by ASLV D–4 rocket. After two months of operation, the satellites apogee was brought down to 620 km and perigee of 430 km. The RPA experiment onboard the SROSS C2 consists of an electron RPA, ion RPA and a potential probe for making simultaneous measurements of plasma parameters. The Electron RPA makes measurements of the total electron concentration (Ne), irregularities in the Ne and the temperature of the electrons (Te). The Ion RPA makes measurements of the total ion density (Ni), irregularities in the Ni, temperature of the ions (Ti) and densities of the various ions (H+, He+, O+, Oþ 2 and NO+) present in the ionosphere. The variations in the satellite potential with respect to plasma potential during spin and motion of the satellite are measured with the help of the Potential probe. Temperature data are obtained by analyzing one complete electron and ion I–V plots through linear and non-linear curve fitting techniques, respectively. Each I–V plot is constructed by current collected by the sensors with 64 steps of retarding grid voltage applied to the sensor, which takes 1.408 s. During this time, satellite travels a distance of 10–11 km. Only those I–V curves that fall within the specified limits were analyzed for deriving Te and Ti. The data are sampled at every 22 ms which when translated into distance is 176 m taking the satellite velocity to be 8 kms1. The RPA experiment is switched on only during the satellite visibility over the ground station at Bangalore (12.6°N, 77.3°E geographic) and Lucknow (26.8°N, 80.8°E geographic). On an average, two overhead passes lasting 12 min are tracked. The spatial resolution of temperature measurements is about 95 km. 3. Method of analysis To study the variations of Te and Ti using RPA data in ascending phase of solar activity during early morning period, we have grouped the six years data into solar minimum (1995–96), moderate (1997–98) and solar maximum (1999– 2000) activities. The data for three seasons (winter, summer and equinox) in each solar activity period are acquired to get the variation of Te and Ti at referred geomagnetic latitude. The geographic latitude is converted to geomagnetic latitude at 77°E geographic longitude using the formula given by U ¼ ArcSinðSinaSin78:5 þ CosaCos78:5 Cosð690 þ bÞÞ where a is the geographic latitude, a is positive in North and negative in South, b is the geographic longitude, and is positive in the East and negative in the West. The observed Te and Ti are grouped into bins of 3° latitude intervals and ±15 min to obtain the variation without any longitudinal selection during early morning phase. If there were more than one satellite pass for a given local
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time in a season then the average Te or Ti is considered. The SROSS C2 payload data follow the universal time (UT), which is changed to local time (LT) to obtain a better picture of the observations. For the winter solstice – November, December, January and February: 1995–1996 for solar minimum, 1997–1998 for solar moderate and 1999–2000 for solar maximum; summer solstice—May, June, July and August: 1995–1996 for solar minimum, 1997–1998 for solar moderate and 1999–2000 for solar maximum; equinox season—September, October, March and April: 1995–1996 for solar minimum, 1997–1998 for solar moderate and 1999–2000 for solar maximum data were used in the investigation. The satellite’s data of 2 months, June 1998 and September 1998, were not available and hence are not included in the present study. And the seasonal variation of Te and Ti during each solar activity is investigated for the equatorial and low geomagnetic latitudes during early morning hours. The Te and Ti of different months falling in the same solstice is averaged in the period 04:00–07:00 LT in the geomagnetic latitude range of 10°S–20°N to get the variations during the morning hours. 4. Results and discussion Fig. 1 shows the Day-to-day variation of solar radio flux F10.7 cm (symbol) during the observational period of in-situ measurements of SROSS C2 satellite (1995–2000) and standard deviation of solar flux (shown by red-line) in winter, summer and equinox during solar minimum (min), moderate (mod) and maximum (max) activity. The F10.7 is minimum during 1995 and rises during 2000.
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4.1. Solar activity dependence of Te during winter solstice Fig. 2a–c represents the equatorial and low-latitudinal variation of electron temperature during the dawn period (04:00–07:00 LT) during different solar activities: minimum (F10.7 67), moderate (F10.7 90) and maximum (F10.7 146). Around 04:00 LT, the Te is 750 K, in 10°S 20°N latitude during all three activities. During solar minimum activity, there is a slight rise in temperature around 05:00 LT showing a crest over 10°N and 5°S, Te rises to 1400 K. After 05:00 LT, Te suddenly shoots to 4000K around 06:00 LT in the equatorial region. This enhancement during dawn period over the equator is termed as ‘‘morning overshoot”. The temperature then decreases in this region. But in 7–20°N, no such an enhancement is observed and the Te slowly increases as the photo – ionization process produces energetic photoelectrons. During moderate and high-solar activity also, such an enhancement is observed around 06:00 LT, but observed Te is less, 3800 K, and then decreases around 07:00 LT over the magnetic equator. Around 07:00 LT, during high activity, the Te again rises, and exhibits crests around ±5 degree geomagnetic latitudes. This can be due to the increased solar flux (F10.7 146) and the availability of fewer electrons at the equator to share the photo-absorption energy. At other latitudes, the Te increases slowly by 06:00 LT and then decreases as the more number of electrons are produced. Oyama et al. (1996a) have observed that morning overshoot becomes more enhanced in the winter hemisphere and during higher solar flux but our results show higher rise in Te during minimum solar activity. 4.2. Solar activity dependence of Te during summer solstice
Year 1995 Day No.
1996 365
1997 730
1998 1999 2000 1095 1460 1825 2190
350
300
F10.7
250
200 max
max
max
150 mod
100
mod mod
min 50 Winter
min Summer
min Equinox
Fig. 1. Day-to-day variation of solar radio flux F10.7 cm (symbol) during the observational period from year 1995–2000 and standard deviation of solar flux (shown by red-line) in winter, summer and equinox during solar minimum (min), moderate (mod) and maximum (max) activity. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3a–c shows the latitudinal variation of Te in summer solstice during minimum (F10.7 67), moderate (F10.7 86) and high (F10.7 160) solar activity. The observed Te around 04:00 LT is found to be the same as that of winter solstice and is 750 K during low and moderate solar activity. Around 04:30 LT, the Te starts increasing. This increase is higher during moderate solar activity; this can be due to the competitive result of incident solar flux and number of electrons present to share this energy. Around 05:00 LT, Te enhances considerably in the southern hemisphere, but in the N-hemisphere, Te is not much enhanced during all three activities showing a N–S asymmetry. Around 05:30 LT, the Te shoots up to 3800K in low, 4500 K in moderate and 3500 K in maximum solar activity in the equatorial region. And then Te decreases with the progress of the morning. 4.3. Solar activity dependence of Te during equinox Fig. 4a–c represents the early morning and latitudinal variation of Te in the equinox solstice. Around 04:00 LT, the observed Te is found to be 800 K during both: solar minimum (F10.7 66) and moderate activities (F10.7 86),
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c
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0
5N
10N
15N
20N
Geomagnetic latitude (degrees) 4.00LT 6.00LT
4.30LT 6.30LT
5.00LT 7.00LT
5.30LT
Fig. 2. Variation of electron temperature (K) during morning hours (04:00-07:00 LT) in winter solstice during (a) minimum (b) moderate and (c) maximum solar activity obtained by RPA data.
but during high-solar activity (F10.7 144), the Te is higher (1200 K) in all latitudes. Around 05:00 LT, the Te begins to rise, as the photo production of the particles begins. Around 05:30 LT, the Te suddenly rises to 4200 K in the equatorial region (±5 degrees) during all the three solar fluxes, and after 06:00 LT, it decreases. Around 07:00 LT, the observed Te is less during high-solar activity than the lower activities; the Te is 2600 K during high and 3400 K during lower activities. This may be because the rate of production of electrons is high during high-solar activity and hence, the energy will be shared by more number of electrons. And hence the energy loss by collisions will also be more, resulting in the less energy possessed by the electrons. 4.4. Solar activity dependence of Ti during winter Fig. 5a–c shows the latitudinal ion temperature (Ti) variation obtained at 500 km, using the RPA data during dawn period during the winter solstice. Around 04:00 LT,
during low-solar activity the Ti is 700 K, which is almost equal to the electron temperature (Te) at the same time, location and the season. The Ti becomes steady up to 05:00 LT during all the three activities. After 05:00 LT, as the morning progresses, the Ti shows variations with the solar activities. During low-solar activity, after 05:00 LT, the Ti start intensifying and enhances to 1100 K at the equator and exhibits two crests at 13°N and 7°S around 05:30 LT. The Ti is 1700 K over 13°N and then again increases with the progress of the morning. But over 7°S, Ti rises higher to 2300 K (around 05:30 LT) from 700 K and then drops. This may be associated with the heating process of photoelectrons produced at the magnetic conjugate point. In the equatorial region (0–3°N), the Ti rises to 2000 K around 06:00 LT from 800 K, around 05:30 LT and then decreases. The Ti increases with the progress of the morning at other geomagnetic latitudes. During moderate solar activity (F10.7 90), the Ti smoothly increases with the time and becomes 2300 K
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a
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400 5200
b
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c
4600 4000 3400 2800 2200 1600 1000 400 10S
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0
5N
10N
15N
20N
Geomagnetic latitude (degrees) 4.00LT 6.00LT
4.30LT 6.30LT
5.00LT 7.00LT
5.30LT
Fig. 3. Variation of electron temperature (K) during morning hours (04:00-07:00 LT) in summer solstice during (a) minimum (b) moderate and (c) maximum solar activity obtained by RPA data.
around 07:00 LT in the equatorial region and 2400 K around 06:30 LT around 15°N latitude. But during highsolar activity, the observed Ti increases at a lesser rate with the progress of the morning and is very less compared to lower solar activity. Ti is maximum around 07:00 LT and is 1300 K. This can be because the Te is also less during the high solar activity. 4.5. Solar activity dependence of Ti during summer Fig. 6a–c shows the latitudinal variation of Ti during summer, in varying solar activities. Around 04:00 LT, the Ti is 700K during all three activities. As the morning progresses, the Ti also increases, but with a reduced rate (260 K/hr), and rises to 1500 K around 07:00 LT during low and moderate activities. But, during high-solar activity, the Ti is increased to 1100 K, at much slower rate, 130 K/hr, which is half of that during low-solar activities, around 07:00 LT.
4.6. Solar activity dependence of Ti during equinox Fig. 7a–c represents the equinox months during solar minimum, moderate and high activities. Around 04:00 LT, the Ti shows a small crest over 3°N during low-solar activity, and the observed Ti is 1000 K. But at other latitudes, the Ti is 700 K during both low and moderate and is high (900 K) during higher solar activity. Around 05:00 LT, over 12°N geomagnetic latitude, the Ti gets enhanced to 1800 from 700 K and then decreases with time during solar minimum. This peak shifts to 7°N around 05:30 LT and then decreases with the progress of the morning. The observed Te also shows the enhancement at this time and the location as represented in Fig. 4a. At other latitudes, the maximum Ti is observed around 07:00 LT, and is higher in the southern latitudes (2100 K) than the northern latitudes. During moderate solar activity, the Ti rises with the progress of morning. Around 05:30 LT, the Ti shows two peaks; one over
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10N
15N
20N
Geomagnetic latitude (degrees) 4.00LT 6.00LT
4.30LT 6.30LT
5.00LT 7.00LT
5.30LT
Fig. 4. Variation of electron temperature (K) during morning hours (04:00-07:00 LT) in equinox season during (a) minimum (b) moderate and (c) maximum solar activity obtained by RPA data.
10°N (2200 K) and the other at 15°N (1900 K). Around 06:30 LT, the Ti enhances to 2500 from 1400 K and then decreases over the equator. The electron temperature also exhibits such a morning enhancement during the same time and location during equinox months in moderate solar activities. During high-solar activity, the observed Ti is less compared to the low-solar activities, and exhibits the highest temperature (1300 K) around 06:00 LT over the equator and then decreases. Fig. 4c also shows the high electron temperature around 06:00 LT over the equator. The electron temperature is strongly affected by the electron density. In general the temperature increases or decreases as the density decreases and increases, respectively. This relationship between the electron density and temperature is due to the heating losses exceeding the heat gains with increasing electron density and the heating dominating with decreasing electron density (Banks and Kockrats, 1973). The photoelectron production begins in the ionosphere through the ionization of neutral particles at sunrise. These
photoelectrons share their high energy with the ambient electrons, as a result the Te increases. This rise is rapid in the early morning hours due to the availability of fewer electrons. With the progress of the morning, more and more electrons are produced and the energy share for each electron decreases. And hence the Te after reaching a maximum, decreases and attains a steady value by the end of the morning. This rapid increase of Te in the early morning hours is the well known Te phenomenon called the ‘‘morning overshoot” and was predicted by Dalgarno and McElory (1965). Experimental evidence of the morning enhancement has also been reported by Evans (1965), Mc Clure (1971), Clark et al. (1972), Oyama et al. (1996b) and Bhuyan et al. (2002). Oyama et al. (1996b) reported that the Te in the morning rises from about 1200 to 4000 K within ±30° magnetic latitude. They have shown that the intense morning enhancement of Te observed over the equator is due to reduction in electron density caused by the downward drift of the plasma, which usually occurs in morning hours. After sunrise, the ion temperature falls
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5N
10N
15N
20N
Geomagnetic latitude (degrees) 4.00LT 6.00LT
4.30LT 6.30LT
5.00LT 7.00LT
5.30LT
Fig. 5. Variation of ion temperature (K) during morning hours (04:00–07:00 LT) in winter solstice during (a) minimum (b) moderate and (c) maximum solar activity obtained by RPA data.
as the electron temperature also falls, due to the sharing of energy with more electrons. The morning enhancement is associated with the photoelectron heating of the morning low-density electrons which does not occur in summer because of the high morning electron density in this season (Otsuka et al., 1998). The morning enhancements are in general weak in Te and nearly absent in Ti at altitudes below 500 km. The morning enhancement in Te and Ti have larger values around the dip equator. This may be associated with the E B drift and the decrease in electron density as explained by Oyama et al. (1996a). According to Oyama et al. (1996a), the zonal wind blows in westward during morning in the northern summer. This causes an upward drift of the plasma in the northern hemisphere and downward drift in the southern hemisphere. The meridional wind blows from the northern (summer) to southern (winter) hemisphere causing a upward plasma motion along magnetic field line in the northern hemisphere and a downward plasma drift in the southern hemisphere. The effect of these two winds, therefore, strengthens each other. For northern winter conditions, the meridional wind effect reverses from the northern summer case but still the zonal wind effect is the
same, which results in the reduced wind effect. They have also observed this asymmetry between longitude zone of 210°–285° and 285°–360°. These zones correspond to positive (5°) and negative (20°) magnetic declination at the magnetic equator respectively. In the longitude zone of 285°–360°, the temperature of the morning overshoot in the northern (summer) hemisphere is higher than in the longitude zone of 210°–285°. The electron energy is transferred to the ions by elastic electron–ion collisions resulting in the increase in Ti (Banks, 1967). Hanson et al. (1973) showed that the electron and the ion temperatures are lower on the summer side of the magnetic equator than on the winter side at night. The Hinotori satellite indicates also that the electron temperature is higher in the winter hemisphere than in the summer hemisphere at day. The cooling rate of electron by the electron-ion collision is small in the winter hemisphere because of the low-plasma density. Oyama et al. (1996b) have found that the morning overshoot in Te becomes very strong around the equator when a downward drift of plasma during the morning hours is included in the theoretical Sheffield University Plasmasphere Ionosphere model (SUPIM).
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10N
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20N
Geomagnetic latitude (degrees) 4.00LT 6.00LT
4.30LT 6.30LT
5.00LT 7.00LT
5.30LT
Fig. 6. Variation of ion temperature (K) during morning hours (04:00–07:00 LT) in summer solstice during (a) minimum (b) moderate and (c) maximum solar activity obtained by RPA data.
5. Conclusions Plasma temperature measurements were carried out with the SROSS C2 satellite during the rising phase of solar activity from 1995–2000 in the 77°E Indian longitude sector during early morning hours (04:00–07:00 LT) at 500 km. Oyama et al. (1996a) have observed that morning overshoot (Te) becomes more enhanced in the winter hemisphere and during higher solar flux but our results show higher rise in Te during minimum solar activity. The observed Te (750 k) is same in all three seasons around 04:00 LT during low and moderate solar activity. And it is found to be higher 1000 K during maximum solar activity around the same time. In winter the morning overshoot phenomenon is observed around 06:00 LT during all three solar activity periods. In summer, the Te shoots to 3800 K in low, 4500 K in moderate and 3500 K in maximum solar activity in the equatorial region around 05:30 LT. And then Te decreases with the progress of the morning. During equinox, after 05:30 LT, the Te shoots up to 4200 K in the equatorial region (±5 degrees) during all the three solar activity period. The observed Te (2600 K) is less during high-solar activity than the lower
activities (Te 3400 K) around 07:00 LT. The morning enhancement in Te depicts a strong dependence on solar activity level with minimum around the maxima of the solar cycle. The higher electron densities and lower electron temperatures are characteristics of Te at the mornings around the solar maximum. During winter, in the equatorial region (0–3°N), the Ti rises to 2000 K around 06:00 LT from 800 K, and then decreases. The Ti increases with the progress of the morning at other geomagnetic latitudes. In summer, as the morning progresses, the Ti also increases, but with a reduced rate (260 K/hr), and rises to 1500 K around 07:00 LT during low and moderate activities. But, during high-solar activity, the Ti is increased to 1100 K, at even much slower rate, 130 K/hr, which is half of that during low-solar activities, around 07:00 LT. And in equinox months, the Ti is lower during high-solar activity than in low-solar activity. Ti is minimum in higher solar activities during early morning hours in all seasons. During morning hours, solar activity dependence is observed in winter and equinox solstice, where Ti is decreased with the increase in solar activity. But in summer, no such dependence is found.
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500 2600
b
2300 2000 1700 1400 1100 800 500 2600
c
2300 2000 1700 1400 1100 800 500 10S
5S
0
5N
10N
15N
20N
Geomagnetic latitude (degrees) 4.00LT 6.00LT
4.30LT 6.30LT
5.00LT 7.00LT
5.30LT
Fig. 7. Variation of ion temperature (K) during morning hours (04:00–07:00 LT) in equinox season during (a) minimum (b) moderate and (c) maximum solar activity obtained by RPA data.
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