Институт космических исследований Российской Академии наук

1. Институт космических исследованийРоссийской Академии наук О возможности моделирования изменения размеров и формы ионопаузы Венеры в цикле солнечной активности М.И. Веригин, Г.А. Котова Доклад на конференции “Физика плазмы в солнечной системе” 17-20 февраля 2009 г., ИКИ РАН GDRE ‘Cosmophysiqueworkshop, Toulouse, France, April 2-4, 2008

2. Venusian ionopauseobservations • A distinct upper boundary of the Venusian ionosphere, called ionopause, was first observed by lone radio occultation of Mariner 5 (Mariner Stanford Group,1967). • 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. • 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 13th Int.Symp.Space Tech.Sci. Tokyo, 1982, p.1533.

3. Venusian ionopauseobservations • 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

4. Venusian ionopauseobservations • Average ionopause height dependence on SZA as determined by different methods over PVO observations • Philips et al., JGR, 89, 10676-10684, 1984 • Very detailed study of Venusian ionopause by Phillips et al. (JGR, A5, 3927, 1988) included its ~ 800 PVO ionopause crossings with SZA < 90o and with upstream solar wind measurements

5. Venusian ionopauseobservations • Among all PVO ionopause observations, only 86 of its crossings took place under controlled solar V2 and EUV flux conditions. • All PVO ionopause crossings were observed in the initial period of observations under maximal solar activity conditions (SAmax) only • 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 ???

6. Ionopauseshape 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 pointscorresponds to period used in present analysis.

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

8. Ionopauseshape parameterization Ionotail diameter D comes fromnumerical integration of pressure balance equation with upper ionosphere pressure profile: Analytic approximation of D: with

9. Ionopauseshape 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, peq = 8.3 + 6.2(Feuv – 1.65) nP H1 = 11.9 + 9.9(Feuv – 1.65) km, H2 = 286 + 20(Feuv – 1.65) km

10. 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

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

12. 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 (?) con-nected 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

13. Выводы • Построена аналитическая модель ионопаузы Венеры, описывающая изменения ее положения и формы в зависимости от динамического давления солнечного ветра и потока УФ излучения Солнца. • Такая модель позволяет проследить вариации размеров и формы ионопаузы на протяжении солнечного цикла и, следовательно, может быть использована для начального периода измерений на спутнике Венера Экспресс, орбита которого не позволяла проводить прямые измерения в окрестности ионопаузы. • Другой областью возможного использования построенной модели ионопаузы является ее использование для моделирования околовенерианской ударной волны, что может обеспечить: • расширенные возможности для правильного переноса в подсолнечную точку и на терминатор ее наблюдавшихся положений, а также для их нормализации и мультифакторного анализа; • выяснение относительной роли различных факторов влияющих на необычно большую вариацию в солнечном цикле положения околовенерианской ударной волны на терминаторе.

14. Conclusions • Venusian analytical ionopause model is developed including variations of its position and shape separately from solar wind ram pressure and solar EUV flux. • 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. • 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.