Reflected Solar Wind in the foreshock region of Venus

observed by Venus ExpressM. Yamauchi1, Y. Futaana1, R. Lundin1, S. Barabash1, M. Wieser1, A. Fedorov2, J.-A. Sauvaud2, T.L. Zhang3, W. Baumjohann3, J.D. Winningham4, R.A. Frahm4, E. Dubinin5, and M. Fraenz5.

(1) Swedish Institude of Space Physics, Kiruna, Sweden, (2) Centre d’Etude Spatiale des Rayonnements, Toulouse, France, (3) Space Research Institute, Graz, Austria(4) Southwest Research Institute, San Antonio, Texas, USA(5) Max-Planck-Institut fur Sonnensystemforschung, Katlenburg-Lindau, Germany

Characteristics of ions flowing upstream along IMF from the Venus bow shock (BS): Comparison to Earth and Mars

Venus Express / ASPERA-4 often observes nearly field-aligned beam-like protons (FAB) in the foreshock region of Venus (Fig. 1, Fig. 2) : (1) They travel away from the quasi-parallel bow shock of Venus, nearly along the interplanetary magnetic field (Fig. 2, Fig. 3). (2) About 2~6 times energy of the solar wind proton energy (Fig. 1). (3) The protons are classified into two beams: (a) field-aligned beam in the SW frame of reference, and (b) field-aligned beam in Venus frame of reference which gyrates in the SW frame (Fig. 3). The combination forms a pancake-like distribution.

They are similar to those at the terrestrial foreshock region (Fig. 4). On the other hand, Mars Express, with the same instrumentation a Venus Express, does not observe the FAB in the Martian foreshock region. We instead observe effective perpendicular acceleration (Fig. 4) along the shock surface. The difference must come from the different gyroradii of the ions.

Summary

Scan

=-45

°~45

°

Fig 1:e- (top) & H+ (rest) at different angle

SWSW SW

SW

BS BS BSBS

, ∑ 3

-min

sca

n

2006-6-18 2008-10-28

B

connected to BS

B B B

Beam = foreshock

Fig 2:

connected to BSSometimes no beam in foreshock

Top: VEX trajectory (both approaching Venus) and projected IMF, Middle: three components of IMF, Bottom: H+ spectrogram at sectors where FAB is observed (cf Fig. 1)

Fig 3: pitch angle distribution

SW SW SW

SW

FAB in the SW frameFAB in the Venus frame

(Cao et al., 2008)

V// V//

V V

cluster-3 cluster-1

SW SW

Earth ≈ Venus, but f ≈ Maxwellian

Fig 4: other planets

Mars ≠ Venus, but both // and accel. are clear

Fig 4b

Fig 3b

2006-6-20

BX, BY, BZ

not connected to BS

Y

X X

ZFig 2b