Abstract
Abstract. Foreshock ions are compared between Venus and Mars at energies of 0.6~20 keV using the same ion instrument, the Ion Mass Analyser, on board both Venus Express and Mars Express. Venus Express often observes accelerated protons (2~6 times the solar wind energy) that travel away from the Venus bow shock when the spacecraft location is magnetically connected to the bow shock. The observed ions have a large field-aligned velocity compared to the perpendicular velocity in the solar wind frame, and are similar to the field-aligned beams and intermediate gyrating component of the foreshock ions in the terrestrial upstream region. Mars Express does not observe similar foreshock ions as does Venus Express, indicating that the Martian foreshock does not possess the intermediate gyrating component in the upstream region on the dayside of the planet. Instead, two types of gyrating protons in the solar wind frame are observed very close to the Martian quasi-perpendicular bow shock within a proton gyroradius distance. The first type is observed only within the region which is about 400 km from the bow shock and flows tailward nearly along the bow shock with a similar velocity as the solar wind. The second type is observed up to about 700 km from the bow shock and has a bundled structure in the energy domain. A traversal on 12 July 2005, in which the energy-bunching came from bundling in the magnetic field direction, is further examined. The observed velocities of the latter population are consistent with multiple specular reflections of the solar wind at the bow shock, and the ions after the second reflection have a field-aligned velocity larger than that of the de Hoffman-Teller velocity frame, i.e., their guiding center has moved toward interplanetary space out from the bow shock. To account for the observed peculiarity of the Martian upstream region, finite gyroradius effects of the solar wind protons compared to the radius of the bow shock curvature and effects of cold ion abundance in the bow shock are discussed.
Highlights
Upstream of the terrestrial bow shock, back-streaming energetic protons are often found flowing nearly along the interplanetary magnetic field (IMF) away from the bow shock when the location is magnetically connected to the bow shock (Asbridge et al, 1968; Gosling et al, 1978; Paschmann et al, 1981; Sckopke et al, 1983; Fuselier et al, 1986; Meziane et al, 2004b; for review, Eastwood et al, 2005; Bale et al, 2005)
The Venus Express (VEX)/Ion Mass Analyser (IMA) has often observed a combination of field-aligned beams and a gyrating intermediate component traveling away from the Venus bow shock
The observed rather narrow δV cannot be explained by a velocity filter effect of ions leaking from the magnetosheath, while it is consistent with specular reflection of the solar wind
Summary
Upstream of the terrestrial bow shock, back-streaming energetic protons are often found flowing nearly along the interplanetary magnetic field (IMF) away from the bow shock when the location is magnetically connected to the bow shock (Asbridge et al, 1968; Gosling et al, 1978; Paschmann et al, 1981; Sckopke et al, 1983; Fuselier et al, 1986; Meziane et al, 2004b; for review, Eastwood et al, 2005; Bale et al, 2005). We call the last two categories as the “gyrating component” based on the distribution function because this paper deals with foreshocks of Mars and Venus, where the formation mechanisms of the foreshock ions can be different from those at the Earth Both the field-aligned beams and the intermediate component have lower alpha/proton ratios than that in the original solar wind or in the diffuse component (Fuselier and Thomsen, 1992), and their sources are considered to be either the reflected solar wind (e.g., Gosling et al, 1978; Paschmann et al, 1980; Mazelle et al, 2003; Kucharek et al, 2004; Meziane et al, 2004a), or heated magnetosheath ions that leak through the bow shock along the magnetic field (Tanaka et al, 1983). For details of the IMA instrument, see Barabash et al (2006), Fedorov et al (2006), and Yamauchi et al (2006)
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