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.
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.
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.
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.
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.
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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Created by R. J. Strangeway
Last modified: March 23rd, 1995.