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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6.       Conclusions

     Through analysis of data from individual orbits and also from statistical studies we find that the plasma wave data are well ordered by altitude with respect to the OETP ionopause, which is the altitude at which the Langmuir probe on PVO measures an electron density of 100 cm. Of the various definitions of the ionopause the OETP ionopause is usually at highest altitude. The dominant signal in the wave data is a change in the background noise of the instrument, which we attribute to changes in the plasma Debye length. At low altitudes, within the ionosphere, the wave instrument is shielded from noise due to photoelectron emission from the spacecraft. At higher altitudes, in the magnetosheath, the antenna is within the Debye sphere of the spacecraft and is probably more sensitive to photoelectron emission noise and other sources of noise on the spacecraft. Inspection of the high-resolution plasma wave data does suggest that the wave burst often observed near the OETP ionopause is due to naturally occurring waves, since the data are qualitatively different. However, some caution may be warranted since the Debye length is 0.74 m when n = 100 cm and T = 1 eV, this density and temperature being appropriate for the plasma at the OETP ionopause. Since the antenna separation is 0.76 m, a resonance between the antenna and the Debye sheath is possible.

      The statistical studies show that field-aligned currents flow at or perhaps slightly below the OETP ionopause. These field-aligned currents occur above the topside density gradient and well above the perpendicular currents which mark the bottom of the magnetic barrier. On comparison with the data presented by Spenner et al. [1980], this places the OETP ionopause and the field-aligned currents in the mantle.

      The field rotation associated with the field-aligned currents tends to align the field more closely with the magnetosheath flow. It appears that the flow alignment occurs because of a shear in the magnetosheath flow [see Luhmann, 1988; Law and Cloutier, 1995] which is probably due to mass loading by ionospheric plasma at lower altitudes. The magnetic field geometry is hence dictated by the field and flow boundary conditions imposed within the magnetosheath and the ionosphere, and the field-aligned currents occur in response to this imposed geometry. We have suggested that the field-aligned current layer is a shear Alfvén wave standing in the magnetosheath flow. The plasma waves observed at the OETP ionopause may be a consequence of the field-aligned currents but are certainly not a cause.

      Recently, Sauer et al. [1994] argued that a composition boundary should be present above the ionopause of Venus and Mars. Perhaps the OETP ionopause is this boundary. However, the two-dimensional simulations of Sauer et al. [1994] cannot explain the field rotation we have discussed here. This rotation in the field should only be present in three-dimensional simulations. We also note that the composition boundary discussed by Sauer et al. [1994], and observed at Mars by Dubinin and Lundin [1995], is associated with a decrease in the ambient magnetic field strength. At Venus the magnetic field strength tends to decrease at altitudes below the location of the field-aligned currents. The field rotation discussed here strongly suggests that the magnetosheath magnetic field passes into a region dominated by plasma of planetary origin (i.e., below the composition boundary), as the increased mass density would result in a velocity shear across the composition boundary.

      An additional complication in determining the relationship between the various signatures observed in the mantle is the "intermediate transition" (IT) [Pérez-de-Tejada et al., 1991, 1993, 1995]. The IT is usually observed above the ionopause near or behind the terminator and is often associated with both a reduction of the magnetic field strength and a rotation of the field to a more Venus-Sun-aligned orientation. Sauer et al. [1994] suggested that the IT is an example of the composition boundary found in their simulations. We suggest here that the transition within the field and plasma observed on the dayside evolves downstream, ultimately becoming the IT. Whether the OETP ionopause, which may be a plasma boundary, or the field-aligned current, which may mark a shear in the flow, evolves into the IT has yet to be determined.

      Indeed, the relationship between the OETP ionopause and the field-aligned current is unclear. Both occur within the mantle. The mantle provides the transition from magnetosheath to ionosphere which requires a change in magnetic field orientation, marked by the field-aligned current, and a change in plasma density and composition, perhaps corresponding to the OETP ionopause. Near the subsolar point we might expect these two transitions to be close together, since a change in the plasma mass density could introduce the velocity shear that results in the Alfvén wave. However, further downstream these two signatures could separate, since the shear is carried by a standing Alfvén wave, while a mass density change could be carried by a slow-mode wave.

      In conclusion, while the OETP ionopause clearly orders the magnetic field and plasma wave data, the relationship between each of these is not yet obvious. It is possible that the plasma waves are generated by the field-aligned currents which we attribute to a standing Alfvén wave associated with the shear in the plasma flow. On the other hand, if the OETP ionopause is a composition boundary, then we might expect pickup ion related instabilities to be present, be they lower hybrid [Szegö et al., 1991; Shapiro et al., 1995] or ion acoustic [Huba, 1993] . Alternatively, if the OETP ionopause corresponds to a density gradient, then perhaps gradient-drift instabilities generate the waves [cf. Huba, 1992]. Thus it is necessary to determine the nature of the OETP ionopause, which is an instrument-defined boundary, as being the altitude at which the Langmuir probe measures a density of 100 cm. Unfortunately, most of the plasma instrumentation on board the Pioneer Venus Orbiter was designed to operate optimally in a dense cold plasma, the ionosphere, or in a supersonic beam, the solar wind, but not in both. The possible exception is the orbiter retarding potential analyzer (ORPA). Regardless of the ultimate source of the waves, our analysis confirms that the waves occur within the plasma mantle and are hence not a direct source of heating for the topside ionosphere.


      We wish to thank L. H. Brace for kindly supplying the Langmuir probe data used in this study. We are also grateful to G. K. Crawford, whose initial efforts provided the basis for the present work. This is IGPP publication 4318 and was supported by NASA grants NAG2-485 and NAGW-3497.

      The Editor thanks K. Sáuer and J. D. Huba for their assistance in evaluating this paper.

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