Indian SROSS C2 Satellite Research Articles

Variability of ionospheric parameters during solar minimum and maximum activity and assessment of IRI model

The ionospheric parameters (electron and ion plasma temperatures (Te and Ti) and total ion density, Ni) as obtained by the Indian SROSS-C2 satellite (altitude ∼500km) have been investigated during low (year 1995, F10.7∼77 sfu) and high (year 2000, F10.7∼177 sfu) solar activity periods. The region under study spans over 5°S-30°N geomag. latitude and 60–100°E geog. longitude over the Indian sector. The observations are compared with the modelled values using IRI-2007 and IRI-2012 versions to assess model predictability. We found that minimum plasma temperatures (Te and Ti) in nighttime gets twice hotter whereas maximum temperatures in early morning gets reduced by half (cooler) when the solar flux gets doubled indicating a direct relation of Te and Ti with solar flux, F10.7 in nighttime but inverse in the morning hours. The ion density (Ni) exhibits solar activity dependence throughout the day and increases by one order when solar activity gets doubled. The modelled Te and Ti are found in agreement to the observed values for high solar activity over both the regions. Whereas the discrepancy exists during low solar activity period over both the regions with over-/under-estimated values in nighttime/morning and noontime respectively. The latest IRI-2012 model improves the nighttime Te and Ti whereas the modelled Ni is found in complete agreement to the observations.

Studies of electron and ion temperatures at 500 km altitude during sunrise using Indian SROSS C2 satellite

Solar dependence of electron and ion temperatures ( T e and T i) 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 ( T e enhances to ∼4000 K) during low-solar activity and to T e ∼ 3800 K, during higher solar activity. In summer, it is observed around 05:30 LT, but the rate of T e enhancement is higher during moderate solar activity (∼2700 K/hr) than the low-solar 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 T i 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, T i 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 T i is lower during high-solar activity than in low-solar activity.

THE ELECTROMAGNETIC ION-CYCLOTRON INSTABILITY IN THE PRESENCE OF A.C. ELECTRIC FIELD FOR LORENTZIAN KAPPA

Electromagnetic ion cyclotron (EMIC) waves have been studied in the presence of AC electric field perpendicular to ambient magnetic field in the ionosphere with observed superthermal electrons. The presence of 4 eV–50 eV superthermal electrons have been recently seen by Indian SROSS-C2 satellite, Lorentzian Kappa distribution has been used to derive dispersion relation and growth rate using method of characteristics and kinetic approach. The free energy source like anisotropy in temperature, AC electric field and presence of superthermal electrons affect the growth rate. Lorentzian kappa distribution plays important role in giving the wide spectrum range of emitted frequencies.

Theoretical simulation of electron density and temperature distribution at Indian equatorial and low latitude ionosphere

The electron density and temperature distribution of the equatorial and low latitude ionosphere in the Indian sector has been investigated by simultaneously solving the continuity, momentum and energy balance equations of ion and electron flux along geomagnetic field lines from the Northern to the Southern hemisphere. Model algorithm is presented and results are compared with the electron density and electron temperature measured in situ by Indian SROSS C2 satellite at an altitude of ∼500 km within 31°S–34°N and 75 ± 10°E that covers the Indian sector during a period of low solar activity. Equatorial Ionization Anomaly (EIA) observed in electron density, morning and afternoon enhancements, equatorial trough in electron temperature have been simulated by the model within reasonable limits of accuracy besides reproducing other normal diurnal features of density and temperature.

Spatial distribution of ionization in the equatorial and low‐latitude ionosphere of the Indian sector and its effect on the pierce point altitude for GPS applications during low solar activity periods

The equatorial and low‐latitude ionosphere is characterized by dynamic changes in the spatial distribution of F region electron density. The in situ density data from Indian SROSS C2 satellite was used in conjunction with the international reference ionosphere (IRI) to demonstrate how the median models have limitations to represent the spatial gradients in the equatorial and subtropical latitudes in the Indian sector. The ground‐based ionosonde data was used to derive the bottomside electron density profiles which were matched with IRI to derive the complete F region electron density profile for different times to show the variability in the centroid of ionization mass distribution due to the equatorial plasma dynamics associated with Appleton ionization anomaly. The study brings out that a uniform ionospheric pierce point altitude cannot be assumed for GPS applications in this sector. However, during low sunspot activity periods, the GPS‐derived total electron content (TEC) using 2 × 2 grid point data indicates that spatial gradients in TEC are not too significant in meridian transects and that the assumption of homogeneous distribution is valid for the conversion of slant to vertical TEC even in the equatorial latitudes during solar minimum. Keeping this in view, a qualitative study has been carried out on the possible variability in ionospheric pierce point altitude at representative subtropical latitude in the Indian sector and the results are discussed in light of diurnal and seasonal features of the equatorial ionosphere.

Signature of seismic activities in the F2 region ionospheric electron temperature

Ionospheric response to the seismic events has been studied in the present paper by analyzing the variation in electron temperature during the occurrence of earthquake. The ionospheric electron temperature data recorded by the RPA payload aboard the Indian SROSS-C2 satellite during the period from January 1995 to December 1996 were used. Six earthquake events recorded during this period were used to analyze the temperature anomalies related to earthquake events. The details of these events are obtained from USGS earthquake data information website. A careful quantitative analysis of electron temperature data recorded before and during the occurrence of earthquake shows the consistent enhancement in the ionospheric electron temperature. The emission of electromagnetic radiations from the epicenter and their subsequent propagation to the ionosphere possibly generates the temperature anomalies. These electromagnetic radiations from ULF to VLF frequency range reach to the F region ionosphere and may enhance the local electron temperature in the ionosphere.

An empirical model of electron temperature in the Indian topside ionosphere for solar minimum based on SROSS C2 RPA data

An empirical model of electron temperature ( T e) for Indian equatorial and low latitudes at the altitude of 500 km is presented. The model, applicable for low solar activity, is based on the observation that electron temperature and electron density ( N e) in the F region of the Indian equatorial and low latitude ionosphere are inversely correlated during the daylight hours (0600–1800 LT) at all latitudes and in all seasons. Nighttime (1900–0500 LT) T e was found to be independent of N e. Electron temperature and electron density measured by the Retarding Potential Analyzer (RPA) instrument on board the Indian SROSS C2 satellite during the solar minimum period January 1995 to December 1996 have been used to derive the relationship between T e and N e. The satellite observations correspond to an average altitude of 500 km. Regression coefficients were obtained from least square fits for all data during daylight hours from 10°S to 15°N magnetic latitudes at 5° intervals. Using the coefficients and corresponding N e data for summer, winter and equinox, T e was calculated for the daylight hours. Average nighttime T e was taken as the model value for these hours. Latitudinal profiles were obtained employing a system of associated Legendre polynomials. The modelled and observed electron temperature was compared and found to be in good agreement. The model is limited to fixed altitude and low solar activity conditions.

Diurnal, seasonal and latitudinal variations of ion temperature measured by the SROSS C2 satellite in the Indian zone equatorial and low latitude ionosphere and comparison with the IRI

The diurnal, seasonal and latitudinal variations of ion temperature ( T i) measured by the Indian SROSS C2 satellite at equatorial and low latitudes along 75°E during the solar minimum period of January 1995 to December 1996 are investigated. The satellite covered the latitude belt of 31°S–34°N within the longitude range of 40°–100°E at an average altitude of ∼500 km . Measured T i varies between 600 and 700 K during nighttime (20:00–04:00 LT) and between 1000 and 1100 K around noon. Enhancement in T i upto ∼3000 K occurs in the pre-sunrise hours (04:00–06:00 LT). An afternoon increase of about ∼200 K over the daytime value is observed between 15:00 and 17:00 LT in summer. Latitudinal gradients in T i have been observed during the period of morning and afternoon enhancements. The ion temperature also exhibits large variability. Comparison of observed and International Reference Ionosphere (IRI) predicted ion temperature reveals that the IRI overestimates T i at about all local times and latitudes except during the periods of enhanced T i.

Enhancement in ionospheric temperatures during thunderstorms

It has been realized in recent years that the ionospheric temperatures and ion densities are influenced by the lightning activity. The ionospheric temperature (electron and ion temperature) were measured by the retarded potential analyzer payload aboard the Indian SROSS-C2 satellite. The data at low latitudes falling in the Indian subcontinent in the height range of 425– 625 km for the period 1995–1998 were used for this study. The data on thunderstorm activity for the same period were obtained from India Meteorological Department. To see the effect of active thunderstorm the ionospheric electron and ion temperatures have been compared to the values on normal days. It has been found that the average electron temperature is enhanced during thunderstorm activity by 1.2–1.7 times over the normal days. A similar enhancement has been found in ion temperature by 1.1–1.5 times. It is suspected that the ULF to VLF radiations generated during lightning and sprites from thunderstorm (Geophys. Res. Lett. 18 (1991) 705; 23 (1996) 1067) may cause of the enhancement in electron and ion temperatures.

Post midnight spread-F occurrence over Waltair (17.7° N, 83.3° E) during low and ascending phases of solar activity

Abstract. A study carried out on the occurrence of post midnight spread-F events at a low-latitude station, Waltair (17.7° N, 83.3° E), India revealed that its occurrence is maximum in the summer solstice months of the low solar activity period and decreases with an increase in the sunspot activity. The F-region virtual height variations show that 80% of these spread-F cases are associated with an increase in the F-region altitude. It is suggested with the support of the night airglow 6300 A zenith intensity data obtained with co-located ground-based night airglow photometer and electron temperature data from the Indian SROSS C2 satellite that the seasonal variation of the occurrence and probable onset times of the post midnight spread-F depend on the characteristics of the highly variable semipermanent equatorial Midnight Temperature Maximum (MTM).Key words. Ionosphere (ionospheric irregularities; ionosphere atmosphere interactions) Atmospheric composition and structure (airglow and Aurora)

Theoretical simulation of O<sup>+</sup> and H<sup>+</sup> densities in the Indian low latitude F-region and comparison with observations

Abstract. The O+ and H+ ion density distributions in the Indian low latitude F-region, within ± 15° magnetic latitudes, are simulated using a time dependent model developed on the basis of solution of the plasma continuity equation. The simulated ion densities for solar minimum June and December solstices are then compared with ionosonde observations from the period 1959–1979 and measurements made by the Indian SROSS C2 satellite during 1995–1996 at an altitude of ~ 500 km. The simulated O+ density has a minimum around pre-sunrise hours and a maximum during noontime. H+ density is higher at nighttime and lower during the day. The simulations reproduced the well-known equatorial ionization anomaly (EIA) observed in electron density at the peak of the F2-region in the Indian low latitude sector during solar minimum. In situ measurement of O+ density by the SROSS C2, however, showed a single peak of ionization around the equator.Key words. Ionosphere (equatorial ionosphere; modeling and forecasting)

Diurnal, seasonal and latitudinal variation of electron density in the topside F-region of the Indian zone ionosphere at solar minimum and comparison with the IRI

The diurnal, seasonal and latitudinal variation of electron density (Ne) measured by the Indian SROSS C2 satellite from December 1994 to January 1997 at low and equatorial latitudes along 75°E meridian are investigated. The satellite covered the latitude belt of 31°S to 34°N and longitude range of 40°E to 100°E at an average altitude of ∼500 km . Electron density is minimum before sunrise and reaches the diurnal maximum in the afternoon hours. A secondary enhancement is observed in the sunset hours of June solstice. The well-known equatorial ionization anomaly is found to be asymmetric at the altitude tracked by the satellite in this period of low solar activity (mean sunspot number=13.9). The location of the crest of the anomaly varies with season. There are seasonal and latitudinal variations of Ne. Further, electron density is found to be positively correlated with the strength of the equatorial electrojet. Comparison of measured density with that predicted by the International Reference Ionosphere (IRI) reveals that the IRI overestimates Ne at about all local times and in all seasons. The IRI also fails to produce the secondary enhancement of electron density.

Theoretical simulation of O + and H + concentrations obtained by the indian SROSS C2 satellite in the topside F-region and comparison with the IRI

A time dependent model is used to calculate the O + and H + ion density distribution in the Indian low latitude topside ionosphere within ±15° geomagnetic latitude. The calculations are performed along magnetic field lines from a base altitude of 120km in the Northern Hemisphere to the same altitude in the Southern Hemisphere for solar minimum equinoctial conditions. The simulated O + and H + values are compared with observations made from Indian SROSS C2 satellite at an altitude of ∼500km over India during solar minimum (1995–1996) equinoctial conditions and with predictions made by the IRI. It is found that the modelled O + and H + concentrations are in reasonable accord with SROSS C2 observations. The H + concentrations obtained from the IRI are comparatively higher than the observed and modelled values. However, in case of O + the IRI values are in conformity with the model and data.

Electron and ion temperature and electron density at ±10° magnetic latitudes from SROSS C2 measurements over India and comparison with the IRI

Diurnal and seasonal variation of electron and ion temperature and electron density at ∼500 km altitude measured by the Indian SROSS C2 satellite during 1995–96 are investigated. Two representative magnetic latitudes ±10° are considered. Electron temperature (Te) varies between 800°K and 900°K during nighttime, rises sharply at sunrise to 3000°K and then falls to a daytime average of 1500°K. A secondary enhancement of Te within 1500–1800 LT was observed in summer and equinox. Ion temperature (Ti) was found to be less than Te at both the locations and in all seasons. Electron density (Ne) shows a minimum before sunrise and a maximum in the afternoon. Seasonal as well as spatial differences in the diurnal behaviour were observed in case of all three parameters. Comparison of density and temperature measured by SROSS C2 with the IRI reveals that IRI underestimates Te during daytime and overestimates Ne at about all local times. Predicted and observed Ti also differ.