Plasma Waves Observed Above the Dayside Venus Ionopause

R. J. Strangeway and C. T. Russell
Institute of Geophysics and Planetary Physics
University of California at Los Angeles

G. K. Crawford
SRI International

Venus II Conference, January 1995

1. Introduction

Plasma waves observed above the dayside ionopause of Venus by the Pioneer Venus Orbiter Electric Field Detector (OEFD) were initially interpreted as whistler-mode waves [e.g., Scarf et al., J. Geophys. Res., 85, 7599-7612, 1980]. It was argued that these waves could supply heat to the ionosphere. However, the ambient magnetic field is draped over the dayside ionosphere, and so whistler-mode waves, which have Poynting flux parallel to the field, cannot transport energy into the ionosphere. Instead, Szego et al. [Geophys. Res. Lett., 18, 2305-2308, 1991] suggested that the waves are lower hybrid resonance (LHR) waves, generated by the relative flow between the solar wind and newly created planetary ions.

Szego et al. argued that these waves could transport energy into the ionosphere, but Strangeway and Crawford [Geophys. Res. Lett., 20, 1211-1214, 1993] noted that the parallel Poynting flux also dominates for these waves. Furthermore, thermal plasma effects, such as electron Landau damping, were not considered by Szego et al. Consequently, Strangeway and Crawford deduced that the waves were more probably ion acoustic waves, as originally suggested by H. A. Taylor et al. [Adv. Space Res., 1, 247-258, 1981].

Huba [Geophys. Res. Lett., 20, 1751-1754, 1993] followed up on this suggestion, presenting a theory for an acoustic mode driven by the relative flow between newly created oxygen ions and the magnetosheath electrons. However, Crawford et al. [in Plasma Environments of Non-Magnetic Planets, pp. 253-258, 1993] found that the waves appeared to be associated with field-aligned currents within the plasma mantle. The plasma mantle (see Figure 1) is a region of mixed magnetosheath and ionospheric plasma above the ionopause.

In this poster we follow up on the initial work of Crawford et al., using data from the first three seasons of dayside periapses. Our primary goal is to determine where the waves occur, and what properties of the underlying plasma control their occurrence. Through such a study we hope to shed light on possible generation mechanisms, and also address the role these waves play in the interaction of the solar wind with the ionosphere of Venus.

2. Examples of Waves Above the Ionopause:

PVO Orbit 169, Outbound

Figure 2a shows high resolution plasma wave data. Data to the right of the vertical line in the figure have had sun-synchronous noise removed using the method described by Higuchi et al. [Annals Inst. Stat. Math., 46, 405-428, 1994]. The vertical line also roughly marks the location of the "Brace Ionopause", where the Langmuir probe measures densities of 100 cm-3. All four channels of the OEFD show wave activity at this location. However, the most striking feature in the 100 Hz data is the abrupt decrease in noise to the left of the vertical line. Although initially interpreted as evidence for whistler-mode absorption, we believe instead that this signal is due to a change in plasma Debye length. Above the ionosphere, where the Debye length is several meters (c.f. 0.76 m, the OEFD antenna length), the wave instrument picks up what appears to be shot noise due to photo-electron emission. Within the ionosphere, where the Debye length is only a few cm, the antenna is effectively shielded from this noise.

Figure 2b shows the associated magnetic field data, cast into radial-east-north (REN) coordinates. The radial component of the magnetic field is very small, and the magnetic field is essentially draped over the ionosphere. There is a strong deflection of the field around 2104 UT, where the wave bursts occur in Figure 2a. This deflection without an associated depression of the field is strongly indicative of field-aligned currents. Perpendicular currents occur at lower altitudes, where the magnetic field is shielded from the lower ionosphere.

Data such as those shown in Figure 2a, 2b are the basis for stating that the plasma waves appear to be associated with field- aligned currents. In order to understand why such currents are flowing within the mantle we have used a new coordinates system, known as radial-clock-azimuthal, to explore the magnetic field geometry on both a case-study and statistical basis.

3. "Radial-Clock-Azimuthal" Coordinates

Radial-clock-azimuthal (RCA) coordinates are similar to REN coordinates, apart from a rotation about the radius vector. This rotation is such that the clock direction always points away from the sub-solar point in a locally horizontal plane (see Figure 3). If the ionopause is at constant altitude and impenetrable to the solar wind flow, and MHD effects do not influence the basic magnetosheath flow field, then the magnetosheath flow is nominally along the clock vector. Clearly this is only an approximation, the ionopause is not at constant altitude, nor is it an impenetrable barrier. Lastly MHD effects are important in breaking the assumed symmetry of the flow.

Nevertheless, the RCA coordinate system is useful for exploring the underlying field geometry. This can be seen in Figure 4), where we show the UADS data for the same time sequence shown in Figures 2a, 2b). We use the UADS data in carrying out the statistical studies presented later. The 100 Hz wave amplitude peaks around 2104 UT, and at that time there is a strong rotation of the magnetic field. This rotation is such as to remove the azimuthal component of the field, and the field is solely in the clock direction. Thus at altitudes below the location of the wave peak the field has rotated to an almost completely flow-aligned orientation.

4. Statistical Results for Dayside Periapsis Seasons 1-3

In performing the statistical analysis of the UADS data we rotate the magnetic field data into instantaneous RCA coordinates, and bin the data as a function of solar zenith angle (SZA) and altitude with respect to the Brace Ionopause (data courtesy L. H. Brace).

The top panels of Figures 5a, 6a, 7a show the median and upper and lower quartiles of the wave amplitude for each of the four frequency channels observed by PVO, for three different SZA ranges. The bottom panels of these figures show the total magnetic field, the electron density and the plasma beta (assuming Ti = 1.8Te). It should be noted that at higher altitudes the 100 cm-3 cut-off in the Langmuir probe data biases the medians to higher densities. The waves show a sharp peak at the Brace Ionopause.

The top panels of Figures 5b, 6b, 7b show the median and quartiles of the magnetic field orientation. Phi-bv is the angle the field makes with respect to the clock (or nominal flow) direction, and has been folded into the range 0 deg - 90 deg. Theta-br is the angle the field makes with respect to the radius vector, again folded into the range 0 deg - 90 deg. Theta-br = 90 deg corresponds to purely horizontal field. The bottom panels of Figures 5b, 6b, 7b show the parallel and perpendicular current density, assuming the currents and field are both purely horizontal, and any changes in the field are due to vertical gradients. Except for the lowest SZA range, the field is strongly flow-aligned below the Brace Ionopause. The field-aligned current is stronger at the Brace Ionopause, as was deduced in Figure 4.

5. Magnetic Field Geometry

To further specify the magnetic field geometry and the nature of the field deflection associated with field-aligned currents in the mantle, we first plot the field orientation for all SZA in Figure 8. In determining the field orientation we have averaged the magnetic field for an interval restricted to 100 km below the Brace Ionopause for each orbit, inbound and outbound. The symbols in Figure 8 give the median theta-br angle with the range being specified by the upper and lower quartiles. The figure shows that the flux tubes are at lowest altitude at the sub-solar point. When phi-bv = 0 deg (tangential field pointing away from the sub-solar point), theta-br < 90 deg (vertical component of the field points away from the planet). The opposite is the case for phi-bv = 180 deg.

In Figure 9 we show the deflection of the field in passing through the Brace Ionopause from above. In specifying the field direction above the Brace Ionopause we have averaged the magnetic field data for an interval < 100 km above the Brace ionopause. The median deflection is negative, i.e., the field rotates to a more flow- aligned direction below the Brace Ionopause, as deduced earlier.

6. Conclusions

The plasma waves have a strong association with the Brace Ionopause, where the electron density = 100 cm-3. The marked reduction in apparent wave amplitude on entering the ionosphere is due to reduction in instrument noise for shorter Debye lengths.

There is often a strong deflection of the magnetic field at the Brace Ionopause. This deflection is such as to rotate the field into a more flow-aligned direction at lower altitudes.

The magnetic field orientation below the Brace Ionopause is such that flux tubes are at lowest altitude at the sub-solar point. This indicates that the flux tubes are "hung up" at lower altitudes through either diffusion or mass-loading (see, e.g., Luhmann and Cravens, Space Sci. Rev., 55, 201-274, 1991).

We conclude that the Brace Ionopause marks a transition from relatively unperturbed magnetosheath field and flow to a region of field and flow modified through interaction with the ionosphere. Field-aligned currents flow at this transition, which is within the mantle. It is possible that this transition also corresponds to a composition change within the plasma, and pick-up ions may be present. However, the field-aligned currents should be considered as an alternative source for the plasma waves.

Whatever the source of the waves, it appears that the waves do not transport energy directly into the ionosphere. Instead they provide means for either momentum coupling between pick-up ions and solar wind, or they heat the plasma and possibly result in anomalous resistivity, depending on the nature of the instability responsible for the waves. As such the waves are an important constituent of the processes occurring within the mantle.


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