Институт космических исследованийИнститут космических исследований

Российской Академии наукРоссийской Академии наук

О возможности моделирования изменения О возможности моделирования изменения размеров и формы ионопаузы Венеры размеров и формы ионопаузы Венеры

в цикле солнечной активностив цикле солнечной активности

GDRE ‘Cosmophysique workshop, Toulouse, France, April 2-4, 2008

М.И. Веригин, Г.А. КотоваМ.И. Веригин, Г.А. Котова

Доклад на конференции“Физика плазмы в солнечной системе”

17-20 февраля 2009 г., ИКИ РАН

Venusian ionopauseVenusian ionopause observationsobservations

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.

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)

Venusian ionopauseVenusian ionopause observationsobservations

PVO dayside ionopause altitude dependence on SW ram pressure (proxy) for three subsets of SZA

Elphic et al., GRL, 7,No.8, 561,1980

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).

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

Venusian ionopauseVenusian ionopause observationsobservations

Average ionopause height dependence on SZA as determined by different methods over PVO observations

Philips et al., JGR, 89, 10676-10684, 1984

All PVO ionopause crossings were observed in the initial period of observations under maximal solar activity conditions (SAmax) only

Among all PVO ionopause observations, only 86 of its crossings took place under controlled solar V2 and EUV flux conditions.

Hopeless situation ???Hopeless situation ???

Venusian ionopauseVenusian ionopause observationsobservations

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)

IonopauseIonopause shape long term shape long term variations deductionvariations deduction

Fortunately range of EUV flux variations in initial PVO period was Fortunately range of EUV flux variations in initial PVO period was ~50% of its global value… ~50% of its global value…

Variation of Solar EUV flux during period of PVO measurements. Red points corresponds to period used in present analysis.

0 500 1000 1500 2000 2500PVO orb it

0 .8

1.2

1.6

2

Fe

uv

, 10

-12

phot

ons.

cm-2

. s-1

100

200

300

F1

0.7

, 10

-22

W. m

-2. s

-1

1979 1980 1981 1982 1983 1984 1985 1986years

SAm ax

S Am in

We will get shape of its nose as :

From pressure balance at ionopause :

IonopauseIonopause shape parameterizationshape parameterization

with

p(r))(dx/dy)/(1VkαcosVk 22vn

22

...8R

yb

2R

yrx(y)

30

4

00

2

0

)(rpr

)8p(r11

2

rR

00

000

)2p(r)R(rp

4

1r1

)(rp)(rp

)(rp)2p(r

1b

0

00

00

0

0

0

0

)p(rVk 02

Theoretical description:

Ionotail diameter D comes from numerical integration of pressure balance equation

with upper ionosphere pressure profile:

IonopauseIonopause shape parameterizationshape parameterization

Analytic approximation of D:

with

2

eq

1

eqeq

H

rrexp

H

rrexp

2

)p(rp(r)

),,(

),,(/tanh1

2

),(),(),(),,(

021

0210010202021 rHHB

rHHArrrHDrHDrHDrHHD eq

2/3

0

7/4

0

7/5

000 5

4

3

4

3

172),(

r

H

r

H

r

HrrHD

16

15

300

1

1

2

7/6

0

2021

5/4

0

2

31

321),,(

r

H

H

H

r

HrHHA

14

13

1100

1

1

2

0

2021

1

0

2

123

50),,(

r

H

H

H

r

HrHHB

p(r))(dx/dy)/(1Vk 22

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

IonopauseIonopause shape parameterizationshape parameterization

km,H

rrexp

H

rrexp

)p(rp(r) eqeqeq 154

22

21

2

Ionopause shape analytic equation:

2

2

30

22

00

2

0

41ln

322)(

D

y

R

Dyb

R

yryx

Results of 8-D ionopause fittingResults 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

0 5 10 15 20so lar w ind ram pressure , nP

1.2

1.4

1.6

1.8

EU

V

flux,

10

12 p

hot

on

s.cm

-2s-1

0.1 1 10 100kV 2 or ionospheric pressure, nP

100

300

500

700

he

igh

t, k

m

F EU V=1.65.10 12 ph.cm -2s -1

F EUV=0.94.10 12 ph.cm -2s -1

0.1 1 10therm al + m agnetic pressure , nP

0.1

1

10

V 2

. Co

s2

vn ,

nP

Results of 8-D ionopause fittingResults of 8-D ionopause fitting

0 5 10 15 20so lar w ind ram pressure, nP

1.2

1.4

1.6

1.8

EU

V f

lux,

1012

ph

oton

s.cm

-2s-1

Comparison of modeled and observed Venusian ionopause positions

-6000 -2000 2000 6000

-6000 -2000 2000 6000

-6000 -2000 2000 6000

-6000 -2000 2000 6000

X ase,km

Y 2as e+Z2

as e , km

v2 = 3 .75 nP

2000

4000

2000

4000

14 nP

2000

4000

2000

4000

X ase,km

F EUV=1.35.10 12 ph.cm -2s -1

FE

UV

=1

.50.

101

2 p

h.cm

-2s-1

V

2 =

7.5

nP

1.68.10 12 ph.cm -2s -1

Y 2as e+Z2

as e , km

This variation is mainly (?) con-nected with solar EUV flux variations within solar cycle that influence neutral exosphere ionization rate

Solar cycle variation of the Venusian terminator BS is the strongest and most reliably established

Venusian bow shock: Search for Venusian bow shock: Search for Solar cycle variationsSolar cycle variations

Russell et al., Adv. Space Res., 10, 55, 1990Zhang et al., JGR, 95, 14961, 1990

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

ВыводыВыводы

Другой областью возможного использования построенной модели ионопаузы является ее использование для моделирования околовенерианской ударной волны, что может обеспечить:

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.

• 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.

ConclusionsConclusions

Another region of analytical ionopause model application is its use for the Venusian bow shock thus providing:

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.

• 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.