Plasma waves and field-aligned currents in the Venus plasma mantle

J. Geophys. Res., 101, 17,313-17,324, 1996
(Received October 30, 1995; revised March 15, 1996; accepted March 21, 1996)
Copyright 1996 by the American Geophysical Union.
Paper number 96JA00927.

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1.       Introduction

      The ionosphere forms the principal obstacle to the solar wind for an unmagnetized planet, allowing direct interaction between the solar wind and ionospheric plasma. At Venus, part of this interaction occurs in a region known as the plasma mantle [Spenner et al., 1980] , which lies above the dayside ionopause of Venus. The mantle contains a mixture of ionospheric and solar wind plasma. VLF and ELF plasma waves are observed in this region by the Pioneer Venus Orbiter (PVO) [Crawford et al., 1993] , and it appears likely that these waves are a consequence of the mixed plasmas. A similar region is found at Mars [Nagy et al., 1990; Sagdeev et al., 1990] , although it is referred to as the planetosphere rather than the mantle, and plasma waves are also observed within this region [Grard et al., 1991] .

      At Venus the VLF and ELF waves were originally thought to be whistler mode waves generated at the bow shock, propagating through the magnetosheath to be absorbed within the ionosphere [e.g., Scarf et al., 1979, 1980a; Taylor et al., 1979]. However, Szegö et al. [1991] pointed out that the magnetic field near the ionopause (in the mantle) is draped over the ionopause. Since whistler mode waves tend to carry energy parallel to the ambient magnetic field, very little wave energy is consequently transported to the ionosphere. Instead, Szegö et al. [1991] suggested that the waves were lower hybrid waves generated by the relative drift between ions of planetary and solar wind origin.

      Unfortunately, lower hybrid waves also carry most of their wave energy flux parallel to the ambient field [Strangeway and Crawford, 1993]. Moreover, thermal effects, such as ion and electron Landau damping, were neglected by Szegö et al. [1991]. Thus Strangeway and Crawford [1993] concluded that an ion acoustic-like instability [Taylor et al., 1981] may best explain the waves. Huba [1993] followed up on this suggestion and found that an ion-acoustic instability could be generated by the relative drift between shocked solar wind electrons and cold planetary oxygen ions. The group velocity of these waves is (nT/nm) 10 km s for typical mantle values, where n is the oxygen ion density, T is the magnetosheath electron temperature, n is the electron density, and m is the ion mass. Electrostatic ion acoustic waves have lower energy density than electromagnetic whistler mode waves for the same electric field amplitude. This, combined with the low group velocity, implies that ion acoustic waves would not transport significant energy from the mantle to the ionosphere.

      More recently, however, Shapiro et al. [1995] have argued that cold electrons of planetary origin would tend to quench the ion acoustic instability discussed by Huba [1993]. Instead, they suggested that the modified two-stream instability generates waves in two branches. One, the lower hybrid (or hydrodynamic) branch, is below the lower hybrid resonance frequency and is not detectable by the Pioneer Venus wave instrument. The other, which they refer to as the kinetic branch (absent in the earlier analysis of Szegö et al. [1991]) would be detectable by PVO. It should be noted, however, that this so-called kinetic branch appears to be simply the whistler mode resonance cone. Inspection of equation (11) and Figure 6 of Shapiro et al. [1995] shows that for this branch, (m/m)(k/k) cos, where is the wave frequency, m and m are the proton and electron masses, respectively, is the electron gyrofrequency, is the lower hybrid frequency (= (m/m)), k is the wave vector, is the angle of the wave vector with respect to the magnetic field, and k = kcos. For this mode the assumption that << is no longer valid, and kinetic effects due to gyroresonance, as well as Landau damping by the cold (planetary) electrons, should be included. The group velocity of both wave modes considered by Shapiro et al. [1995] is primarily along the magnetic field and hence mainly tangential to the ionopause.

      It therefore appears unlikely that the waves observed in the Venus plasma mantle are a significant source of energy for the dayside Venus ionosphere, regardless of the wave mode considered. However, the waves may play an important role within the mantle. If they are generated by the relative drift between magnetosheath and planetary ions, they may provide momentum coupling and also cause local heating of the planetary ions. On the other hand, Crawford et al. [1993] showed that the waves are often associated with field-aligned currents within the mantle region. It is therefore possible that the waves are generated by the field-aligned currents and may be a source of anomalous resistivity within the plasma. It is the purpose of this paper to determine, from a statistical viewpoint, where the waves occur within the mantle and what other phenomena are associated with the waves. This will hopefully provide an initial step in constraining the various hypotheses put forward to explain the waves.

      In section 2 we will discuss the different definitions of the ionopause at Venus. We will show that the ionopause as defined by the Langmuir probe (the orbiter electron temperature probe (OETP) ionopause) orders the plasma wave data. Of the various ionopause definitions the OETP ionopause is at highest altitude and generally lies within the plasma mantle, as defined by Spenner et al. [1980]. In section 3 we will describe a coordinate system used to order the magnetic field data. This coordinate system, which is aligned along the radial and assumed magnetosheath flow directions, allows us to show that the magnetic field is deflected above the ionosphere, in a manner consistent with mass loading at lower altitudes. In section 4 we will present a statistical analysis that further supports the results of sections 2 and 3, that the waves tend to occur at the OETP ionopause and that a field-aligned current is often observed near this ionopause. In section 5 we will study the observed magnetic field rotation in more detail, showing that the rotation can be explained as being due to field lines that are convected to lower altitudes in the subsolar region, where they are transported more slowly to the nightside than at higher altitudes. We will further show that the field rotation can be interpreted as a shear Alfvén wave standing in the magnetosheath flow. Last, in section 6 we will summarize our results. The OETP ionopause clearly orders the data, but it is not yet obvious what is the ultimate source for the waves. The next step appears to be determining the nature of the OETP ionopause.

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