Journal of Atmospheric and Solar-Terrestrial Physics
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
Introduction
Measurement of thermal properties of the F-region of the ionosphere gives insight into the energy balance of the ionosphere thermosphere regime. Plasma temperatures in the ionosphere are usually greater than the temperature of the neutrals. This is due to the strong absorption of solar ultraviolet radiation in the F-region which leads to intense heating of the upper atmosphere. The photoelectrons rapidly thermalize with the ambient electron gas transferring in the process the large heat input from the sun. Energy is lost from ambient electrons to the neutral particles and ions by collisions and through nonlocal transport. The heat lost by the ambient electron acts as the heat source for the ions. Generally, the electron temperature is substantially higher than that of the neutral atmosphere and the ion temperature is an intermediate of electron and neutral temperature. Ion temperature in the topside F-region had been studied using satellite retarding potential analyzer (RPA) measurements by Hanson 1970, Hanson 1973, rockets and incoherent scatter radars (references in Whitten and Poppoff, 1971). McClure et al. (1973) compared electron and ion temperature measurements of OGO 6 satellite with those from incoherent scatter radars. Rishbeth et al. (1977) further studied the ion temperature measurements from OGO 6 satellite at equatorial and low latitudes. Similarly, electron temperature in the ionosphere has been studied using measurement from incoherent scatter radars (McClure 1969, McClure 1971; Mahajan, 1977; Oliver et al., 1991), rocket probes (Oyama et al., 1980) and satellite-based instruments (Brace 1967, Brace 1988; Oyama et al., 1996). Many special features of the equatorial and low latitude ionosphere such as the equatorial temperature and wind anomaly (Raghavarao et al., 1991), ion temperature trough (Hanson et al., 1973), equatorial plasma temperature anomaly (Balan et al., 1997), etc. have come to light from these studies. Kohnlein (1986) had presented empirical models of electron and ion temperature in the ionosphere while Moffett et al. (1993) and Watanabe et al. (1995) have theoretically investigated electron and ion temperature variations in the F-layer.
The equatorial and low latitude ionosphere exhibits many unique features in density and temperature, such as the equatorial ionization anomaly, the plasma fountain, equatorial electrojet, etc. The horizontal orientation of the geomagnetic field lines at the equator and the shift between the geographic and geomagnetic equator is known to be responsible for these observed features and their longitudinal variations. In the past, satellite missions such as the Atmospheric Explorer, Dynamic Explorer, ISIS, Aeros, OGO 6, etc. were used to measure ionospheric parameters in situ. However, the data over equatorial and low latitudes are sparse, particularly over the Indian subcontinent. The Japanese Hinotori satellite, which had a near circular orbit at , provided a good database for study of temporal and spatial variations of density and temperature in the topside ionosphere (Watanabe and Oyama, 1996; Su et al., 1996; Oyama et al., 1996). But the data are limited to periods of moderate and high solar activity, 150⩽F10.7⩽220. The SROSS C2 satellite provides an additional database for the study of electron, ion temperature and density variations at equatorial and low latitudes along 75°E for solar minimum to moderate solar activity periods. Bhuyan 2002a, Bhuyan 2002b, Bhuyan 2003 have presented temporal and spatial variations of electron temperature and electron density in the Indian zone equatorial and low latitudes from SROSS C2 measurements during the 1995–1996 solar minimum period. The aim of the present study is to investigate the diurnal, seasonal and latitudinal variations of ion temperature, Ti, measured by the SROSS C2 satellite during January 1995 to December 1996. The period of observation corresponds to the minimum of the solar cycle 23 (F10.7=74.5). The measurements are also used to assess the Ti predictions made by the International Reference Ionosphere (IRI) 1995 at altitude along 75°E meridian.
Section snippets
SROSS C2 satellite
The Indian satellite SROSS C2 was launched on May 4, 1994 into an orbit of 46° inclination and apogee with perigee. In July 1994, the orbit of the satellite was trimmed to . The satellite was spin stabilized with a spin rate of . Two RPA sensors were on board the satellite to measure electrons and ions separately. The RPA sensors mounted on the top deck moved in the cartwheel mode perpendicular to the spin axis of the spacecraft and collected data within 30° (for
Diurnal and seasonal variations
The diurnal variations of ion temperature measured by the SROSS C2 and predicted by the IRI over the geomagnetic equator for the low solar activity (F10.7=74.5) period of January 1995 to December 1996 are shown in Fig. 1. The data were selected within ±2.5° magnetic latitude and combined for all seasons. Measured Ti is at night and at noon. Enhancement in Ti up to occurs in the pre-sunrise hours (04:00–). An afternoon increase of about over the noontime value is
Discussion
The strong absorption of the solar EUV radiation in the F-region of the ionosphere leads to intense heating of the atmosphere. The photoelectrons produced by the solar EUV radiation heat the ambient electrons. The heat energy of the ambient electrons is lost to both the ions and neutral particles that surround the electrons. The balance between heating, cooling and energy flow processes determines the electron budget in the F-region. Most of the energy is deposited through ionization of O and N2
Summary
Ion temperature data obtained from the SROSS C2 satellite during the low solar activity period of 1995–1996 in the 75°E longitude sector have been used to investigate the diurnal, seasonal and latitudinal variation of this parameter at altitude over Indian low and equatorial latitudes. The average ion temperature at altitude is at night, increases to ∼1500– at sunrise and then settles to a daytime average value of within a couple of hours after sunrise. An
Acknowledgements
This work is partially supported by the Indian Space Research Organization through a grant (SROSS C2/ph2/99). The authors wish to thank the NASA, USA, for making the IRI-95 available through the Internet. The authors are also thankful to all associated with the SROSS C2 data acquisition program.
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