Abstract. Annual and seasonal variations in the lowlatitude topside ionosphere are investigated using observations made by the Hinotori satellite and the Sheeld University Plasmasphere Ionosphere Model (SUPIM). The observed electron densities at 600 km altitude show a strong annual anomaly at all longitudes. The average electron densities of conjugate latitudes within the latitude range AE25 are higher at the December solstice than at the June solstice by about 100% during daytime and 30% during night-time. Model calculations show that the annual variations in the neutral gas densities play important roles. The model values obtained from calculations with inputs for the neutral densities obtained from MSIS86 reproduce the general behaviour of the observed annual anomaly. However, the dierences in the modelled electron densities at the two solstices are only about 30% of that seen in the observed values. The model calculations suggest that while the dierences between the solstice values of neutral wind, resulting from the coupling of the neutral gas and plasma, may also make a signi®cant contribution to the daytime annual anomaly, the E Â B drift velocity may slightly weaken the annual anomaly during daytime and strengthen the anomaly during the post-sunset period. It is suggested that energy sources, other than those arising from the 6% dierence in the solar EUV¯uxes at the two solstices due to the change in the Sun-Earth distance, may contribute to the annual anomaly. Observations show strong seasonal variations at the solstices, with the electron density at 600 km altitude being higher in the summer hemisphere than in the winter hemisphere, contrary to the behaviour in NmF2. Model calculations con®rm that the seasonal behaviour results from eects caused by transequatorial component of the neutral wind in the direction summer hemisphere to winter hemisphere.

Abstract.The coupled, one-dimensional electron and ion energy equations, with a combination of small steady and fluctuating horizontal magnetic fields imposed, are solved for the Mars ionosphere, corresponding to conditions encountered during the Viking mission. A series of calculations with various boundary conditions and heat sources result in a range of electron and ion temperature profiles, which are compared with the results obtained by the RPA's carried aboard the Viking landers. It is shown that solar EUV heating alone does not lead to the observed temperature profiles and that assuming reasonable heat fluxes at the top result in good agreement. It is also found that the introduction of small steady and altitude dependent fluctuating horizontal magnetic fields, which modify the thermal conductivity, leads to electron temperatures in reasonably good agreement with the RPA data, but does not match the observed ion temperatures above about 240 kin. The effects of chemical and Joule heating are also examined and found not to be significant.

We measured electron density and temperature of the nighttime upper ionosphere for the several key intervals during the progress of the magnetic storm on 15 July 2000 with the Langmuir probe on Korea Multipurpose Satellite‐1 (KOMPSAT‐1). During the main phase of the storm KOMPSAT‐1 detected near 0°E a very deep and extensive trough of electron density centered around the geomagnetic equator. The electron density dropped sharply from ∼4 × 105cm−3 to less than 2 × 104cm−3, with the trough region extended over 1400 km along the satellite track. Later in the recovery phase, KOMPSAT‐1 observed severe distortion still persisted near 230°E, with enhanced density in the southern hemisphere. Together with DMSP observations, we estimate the size of the observed trough to be at least 5500 km in the longitudinal direction. DMSP data indicates that the trough was caused by the enhanced eastward electric field which lifted the equatorial F‐region ionosphere upward.

Electron temperature variations in the Earth's plasmasphere are studied using the Exos D satellite observations and the Sheffield University plasmasphere‐ionosphere model. The observations made during the years 1989–1994 are analyzed to investigate the local time and altitude (1000–8000 km) variations of the electron temperature at magnetic latitudes 0°–45°N. The observed electron temperature Te is almost constant during both day and night and is found to have large day‐to‐night differences that vary with altitude and latitude; the largest day‐to‐night ratio in Te (≈6500 K/2600 K) occurs at the highest altitude at equatorial latitudes and the smallest ratio (≈3300 K/2300 K) at the lowest altitude at midlatitudes. During daytime, Te increases rapidly with altitude in the lower plasmasphere (altitude <2500 km) and slowly in the upper plasmasphere with mean gradients of 1.33 and 0.22 K km−1, respectively. At night, on the other hand, the lower plasmasphere is in thermal equilibrium, and in the upper plasmasphere Te increases slowly with a mean gradient equal to the daytime value. The electron temperature shows maximum latitude variation at medium plasmaspheric altitudes (around 4000 km) and smaller variations at lower and higher altitudes, particularly during daytime. At the altitude of maximum latitude variation, Te increases by about 1200 K between the equator and 40°N during both day and night. The model reproduces the observations reasonably well and is used to explain the occurrence of a prominent morning peak in Te. The morning peak becomes prominent in the lower plasmasphere at low latitudes because of the vertical E × B drift in the equatorial F region, which increases Te during morning hours and decreases Te during daytime hours. The observations and model results also show the occurrence of an afternoon peak in Te, which becomes prominent with increasing altitude and latitude; the peak could be caused by the daytime poleward neutral wind.

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