EFFECT OF SOLAR EUV VARIATION ON THE NIGHTSIDE IONOSPHERE OF VENUS OBSERVED ON LANGMUIR PROBE AT SOLAR MAXIMUM

The antisunward ion flow across the terminator is believed to be the principal source of nightside ionosphere at solar cycle maximum. The controlling factor for the nightward flow is the altitude of the terminator ionopause which is determined by the combined effect of solar wind pressure (Psw) and solar EUV flux. The long-term effects of EUV flux are now known since the plasma density in the nightside ionosphere changes by an order of magnitude between solar maximum (SCmax) and solar minimum (SCmin). However, Brace et al. (1990) from Langmuir probe measurements on Pioneer Venus have observed a slight negative response of electron density (Ne) to solar EUV flux on the main nightside ionosphere at SCmax. By using the same measurements, we find that nightside ionosphere does exhibit the signature of day-to-day EUV variations, provided we can remove the effects of influencing parameters other than EUV flux. In our analysis we delete orbits which contain holes or have data gaps or correspond to high Psw conditions. We observe that main nightside ionosphere above 200 km shows positive response to variations in the solar EUV flux. This result supports the transterminator flow mechanism for the maintenance of the main nightside ionosphere. However, we also find that electron density below 200 km does not respond to solar rotation changes in EUV flux, a result consistent with Brace et al. (1990), who obtained a slight negative correlation. This indicates that the transterminator flow is not the sole source for the maintenance of the nightside ionosphere below 200 km.