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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3.       Flow-Aligned Coordinates

      Besides using the OETP ionopause as a reference altitude, we will also use a coordinate system for the magnetic field data that is based on the assumed flow direction within the magnetosheath. This coordinate system, which is similar to radial-east-north coordinates, is shown in Figure 2. In deriving the coordinate system we have assumed that the magnetosheath flow is symmetric about the Venus-Sun line and is mainly tangential to the obstacle, which we further assume is spherical in shape. Thus most of the flow is assumed to be along the direction labeled "flow," with very little or no radial component, except at the subsolar point. The direction labeled "perp" completes the triad and is perpendicular to both the radial and assumed flow direction. The transformation from the Venus solar orbital (VSO) coordinate system to the "radial-flow-perp" system can be obtained as follows: If the spacecraft position is given by R in VSO, then the perpendicular direction , where is the unit vector along the X axis. The flow direction is given by . The radial direction is of course given by . The dot product of any vector in VSO with the unit vectors , , and gives the vector in the radial-flow-perp coordinates. A similar coordinate system was employed by Law and Cloutier [1995] in their study of the magnetic field structure.

Figure 2.     Relationship between flow-aligned coordinates and Venus solar orbital (VSO) coordinates.

      The usefulness of this coordinate system can be seen in Figure 3, where we present high-resolution plasma wave and magnetic field data for the outbound portion of orbit 169. This orbit was also shown by Crawford et al. [1993]. The top four panels show the wave intensity for the four wave channels, while the bottom four channels show the magnetic field rotated into radial-flow-perp coordinates. The vertical line in each panel indicates the location of the OETP ionopause.

Figure 3.     High resolution plasma wave and magnetic field data from orbit 169 outbound. Fifteen minutes of data are shown, with the top four panels showing wave intensity for the four wave channels, using a logarithmic scale. The bottom four panels show the magnetic field, cast into radial-flow-perp components. The vertical line in Figure 3 marks the OETP ionopause.

      Before discussing the magnetic field data, we will repeat some of the points made by Crawford et al. [1993] concerning the plasma wave data. First, the dominant change in intensity, especially at 100 Hz, is the change in instrument background. This is due to the change in plasma Debye length, which is short (a few centimeters) within the ionosphere and long (a few meters) within the magnetosheath. The antenna length is 0.76 m [Scarf et al., 1980b] , while the spacecraft is 2.54 m in diameter. Hence the antenna is within the Debye sphere of the spacecraft when the spacecraft is in the magnetosheath, and the wave instrument is sensitive to noise caused by photoemission of electrons from the spacecraft and antenna elements. In the ionosphere the antenna is effectively shielded from photoemission noise. Thus any analysis of the wave data near the dayside ionopause should take into account the change in background. In particular, the decrease in wave intensity as the spacecraft enters the ionosphere cited by Scarf et al. [1980a] is not evidence for whistler mode absorption in the ionosphere but is simply the result of the change in instrument background. However, in addition to the change in background, Figure 3 shows a wave burst at all four channels at the OETP ionopause. This burst does not display any of the characteristics of the background noise and is probably due to plasma waves generated at or near the OETP ionopause.

      Turning now to the magnetic field data, Figure 3 shows that the field is quite large near the OETP ionopause. The magnetic barrier [Elphic et al., 1980] extends to altitudes well below the OETP ionopause. Throughout this region the radial component is small, and the field is mainly draped over the obstacle. However, the field direction changes just below the OETP ionopause. Above the OETP ionopause the field has large components both parallel and perpendicular to the assumed flow direction. Below the OETP ionopause the field rotates to a direction parallel to the assumed magnetosheath flow. Luhmann [1988] discussed the presence of a magnetic field rotation within the mantle region. In her model the rotation of the field was due to convection of the field to lower altitudes, where the horizontal plasma flow was reduced. This velocity shear causes the magnetic field to be more flow-aligned at lower altitudes, resulting in the "weathervaning" of the field described by Law and Cloutier [1995] in their analysis of magnetic field data from the dayside of Venus.

      Since there is little or no change in magnetic field magnitude, the current in the magnetic shear layer is mainly field-aligned. The magnetic field is mainly horizontal, and hence the field-aligned currents are horizontal. At lower altitudes the magnetic field is shielded from the ionosphere by perpendicular currents, which also flow horizontally. We therefore assume that there are only vertical gradients in the field, and the variation of the horizontal components of magnetic field as a function of altitude will be used to calculate the current density.

      Additional examples of high resolution wave and field data are given in Figures 4, 5, and 6. Each of Figures 4, 5, and 6 show a wave burst near the altitude of the OETP ionopause. In all the examples the burst is detected at 100 Hz, but sometimes the burst extends to higher frequencies. This argues against a lower hybrid wave mode and for an acoustic mode [Taylor et al., 1981; Strangeway and Crawford, 1993; Huba, 1993], since the lower hybrid resonance frequency is typically only a few tens of Hertz. Because lower hybrid waves are thought to be in resonance with ions of planetary origin, there would be little or no Doppler shift of the waves when observed in the spacecraft frame, which moves relatively slowly with respect to the planet. The spacecraft speed is 10 km s, while the shocked solar wind speed is 100 km s.

Figure 4.     Wave and magnetic field data for the inbound portion of orbit 157. Similar in format to Figure 3.

Figure 5.     Wave and magnetic field data for the periapsis portion of orbit 201. Similar in format to Figure 3. This orbit was also discussed by Luhmann [1988].

Figure 6.     Wave and magnetic field data for the outbound portion of orbit 171. Similar in format to Figure 3. This orbit was also discussed by Law and Cloutier [1995].

      In each of the examples shown in Figures 3, 4, 5, 6, the magnetic field deflects to a more flow-aligned direction at lower altitude. The tendency for the field to become more flow aligned below the OETP ionopause will be a conclusion of the statistical analysis presented in sections 4 and 5. However, it should be noted that not every orbit displays this behavior. There are orbits in which the field rotation is in the opposite sense, becoming transverse to the assumed flow direction at lower altitudes. This could be because the actual magnetosheath flow is not as assumed or because the interplanetary magnetic field changes direction. Clearly, these exceptions should be investigated, but we will defer such analysis to future studies.

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Text and figures by R. J. Strangeway
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Last modified: Sep. 2, 1996