Venusian ionopause observations v A distinct upper boundary

Venusian ionopause observations v A distinct upper boundary of the Venusian ionosphere, called ionopause, was first observed by lone radio occultation of Mariner 5 (Mariner Stanford Group, 1967). v Subsequent radio occultation of Mariner 10 found the ionopause at lower heght and not as sharp as in Mariner 5 case (Howard et al. , 1974), thus originating some uncertainty in this boundary. v The permanent existence of the Venusian ionopause became evident after multiple Venera 9 and 10 radio occultations (Aleksandrov et al. , 1976) Ionopause position in electron density profiles during Venera 9, 10 radiooccultations at different SZA Savich et al. , Proc 13 th Int. Symp. Space Tech. Sci. Tokyo, 1982, p. 1533.

Venusian ionopause observations v Pioneer Venus Orbiter (PVO) data provide large array of this boundary observations by different instruments at different solar zenith angles (SZA) and under various upstream solar wind and EUV flux (Feuv) conditions (see, e. g. , Elphic et al. , 1980, Brace et al. , 1983, Phillips et al. , 1988). PVO dayside ionopause altitude dependence on SW ram pressure (proxy) for three subsets of SZA Elphic et al. , GRL, 7, No. 8, 561, 1980

Venusian ionopause observations v Average ionopause height dependence on SZA as determined by different methods over PVO observations Philips et al. , JGR, 89, 10676 -10684, 1984 v Very detailed study of Venusian ionopause by Phillips et al. (JGR, A 5, 3927, 1988) included its ~ 800 PVO ionopause crossings with SZA < 90 o and with upstream solar wind measurements

Venusian ionopause observations v Among all PVO ionopause observations, only 86 of its crossings took place under controlled solar V 2 and EUV flux conditions. v All PVO ionopause crossings were observed in the initial period of observations under maximal solar activity conditions (SAmax) only v Latest Venus Express (VEX) orbiter enabled possibility of near solar minimum (SAmin) in situ studies of magnetic barrier region but not the ionopause, since it was seldom observed with VEX periapsis at 250 – 350 km (Zhang et al. , 2008) Hopeless situation ? ? ?

Ionopause shape long term variations deduction Fortunately range of EUV flux variations in initial PVO period was ~50% of its global value… Variation of Solar EUV flux during period of PVO measurements. Red points corresponds to period used in present analysis.

Ionopause shape parameterization Theoretical description: From pressure balance at ionopause : We will get shape of its nose as : with

Ionopause shape parameterization Ionotail diameter D comes from numerical integration of pressure balance equation with upper ionosphere pressure profile: Analytic approximation of D: with

Ionopause shape parameterization Ionopause shape analytic equation: 8 -D fitting of pressure profile parameters for minimizing of mean squared distance between observed ionopause positions and model ionopause surface req = Rv + 291 + 52(Feuv – 1. 65) km, H 1 = 11. 9 + 9. 9(Feuv – 1. 65) km, peq = 8. 3 + 6. 2(Feuv – 1. 65) n. P H 2 = 286 + 20(Feuv – 1. 65) km

Results of 8 -D ionopause fitting Region of EUV flux and ram pressure variations used in fitting Resulted scatter plot of external and internal pressures across the ionopause Variation of subsolar ionopause position or internal plasma pressure during the solar cycle

Results of 8 -D ionopause fitting Comparison of modeled and observed Venusian ionopause positions

Venusian bow shock: Search for Solar cycle variations Russell et al. , Adv. Space Res. , 10, 55, 1990 Zhang et al. , JGR, 95, 14961, 1990 Solar cycle variation of the Venusian terminator BS is the strongest and most reliably established This variation is mainly (? ) connected with solar EUV flux variations within solar cycle that influence neutral exosphere ionization rate Relative role of other factors, e. g. - solar wind Alfvenic Mach number systematic variation, - magnetic barrier shape variation …, yet to be defined by advance modeling

Conclusions v Venusian analytical ionopause model is developed including variations of its position and shape separately from solar wind ram pressure and solar EUV flux. v This model provides possibility to trace ionopause its position and shape variations within solar cycle and, thus, can be applied for the SAmin period of Venus Express observations, which orbit do not provides possibility for direct ionopause studies. v Another region of analytical ionopause model application is its use for the Venusian bow shock thus providing: • enhanced possibilities for reasonably correct mapping, normalization, and multifactor analysis of its crossings; • clearification of relative role of different factors of unusually large variation of the bow shock terminator position within solar cycle.