5. SOLAR WIND INTERACTION

Venus ionosphere acts as an obstacle to the solar wind, and Figure 14 gives a schematic representation of this interaction The solar wind dynamic pressure PSW, is converted to magnetic pressure, PB, in the barrier and this in turn is balanced by the ionospheric thermal pressure, PP. This balance occurs at the ionopause, the region which is marked by a steep gradient in plasma density at the top of the ionosphere. The density here changes by a factor of about 100. The ionopause height adjusts to the solar wind pres-sure. As this pressure increases, a higher ionospheric pressure is needed to balance and the ionopause moves to a lower altitude, where the thermal pressure is higher and vice versa. For the same reason, the ionopause altitude increases with SZA, because effective PSW decreases in proportion to cos²x (x=SZA). The effect of PSW and SZA on ionopause altitude has been studied by several workers (e.g. Brace et al., 1980, 1983a). Figure 15 shows plots of plasma and magnetic pressure for three orbits with varying solar wind conditions: (a) low (b) moderate and (c) extremely high, PSW more than maximum plasma pressure. Time from periapsis on the x-axis provides the altitude of measurement.

Fig.14:A schematic of solar wind interaction at Venus showing the three major regions,
namely the solar wind region, the magnetosheath and the ionosphere. The region of the steep plasma gradient(above the top of the ionosphere)is the ionopause(from Mahajan, 1995).

The ionopause altitude falls rapidly with increasing PSW but levels off above about 4 nPa (Brace et al.,1980). This saturation has been explained by Mahajan et al. (1989) due to the ionopause being driven so deeply into the thermosphere that photo-ion production loads down the solar wind interaction (i.e. the photoionization of the neutral atmosphere replenishes the plasma that is swept away by the solar wind). In this process the height of the ionopause follows the height of the ionizable species, atomic oxygen, as will be evident from Figure 16. It can be noted that at low PSW, ionopause tracks the plasrna pressure while at high Psw it tracks the neutral gas pressure.