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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2.       OETP Ionopause

      One possible source of confusion concerning the nature of the plasma waves observed on the dayside of Venus is due to different definitions of the ionopause. Phillips et al. [1988] show in their Figure 4 the altitude of the ionopause using different definitions. The four definitions discussed by Phillips et al. [1988] are (1) the altitude at which the PVO Langmuir probe (orbiter electron temperature probe, OETP) measures a density of 100 cm (the OETP ionopause); (2) the altitude at which the retarding potential analyzer (ORPA) measures 100 cm (the ORPA ledge); (3) the altitude where plasma thermal pressure equals magnetic pressure (the pressure balance ionopause); and (4) the altitude where the ORPA observes a break in the density profile (the ORPA top). These different "ionopauses" are usually encountered in the order given when passing from high to low altitude. At low solar zenith angles they are close together in altitude, but they can be hundreds of kilometers apart at the terminator.

      Crawford et al. [1993] noted that the wave bursts detected by the orbiter electric field detector tended to occur at or near the OETP ionopause. This can be seen clearly in Figure 1a, 1b. In Figure 1a, 1b we show the 100-Hz wave amplitude plotted using a gray-scale representation as a function of altitude and local time. The data are taken from the Unified Abstract Data System (UADS) database for orbits 125-248, with Figure 1a showing the inbound leg for each orbit and Figure 1b showing the outbound leg. UADS was originally designed as an on-line data system [Ferandin et al., 1980] , using a common 12-s format for all instrument data. Later, the on-line system was discontinued, and UADS data were subsequently submitted by experimenters to the National Space Science Data Center, as described in the appendix to Brace and Kliore [1991]. The circles in Figure 1a, 1b give the location of the OETP ionopause (L. H. Brace, personal communication, 1993) for each orbit.

Figure 1a, Figure 1b.     100 Hz peak amplitude as a function of altitude and local time for orbits 125-248. The data are shown for (a) inbound and (b) outbound portions of each orbit. The gray scale indicates the peak amplitude per 24-s interval, with 12-s spacing between samples. The circles indicate the altitude of the orbiter electron temperature probe (OETP) ionopause.

      In the UADS database the wave amplitude is given as both a peak and an average, and we have used the peak in Figure 1a, 1b. The peak amplitude allows us to more clearly discern the association between the waves and the underlying plasma structure, but some caution should be employed in interpreting the data. In particular, both the peaks and averages are calculated using a 24-s window with 12-s spacing. Thus there is a potential for aliassing of the data, especially in point-by-point comparisons. However, statistical studies will be less prone to aliassing, and Figure 1a, 1b shows that statistically the wave amplitude tends to peak at or near the OETP ionopause. Thus we will use altitude with respect to the OETP ionopause to order the data, rather than altitude from the surface of the planet. While subsequent analysis presented in this paper will show that the OETP ionopause is a useful reference altitude for ordering the data, this methodology is also supported by Crawford [1993]. He showed that for a sample of some 30 orbits the OETP ionopause altitude ordered the plasma wave data better than the = 1 altitude, which tended to occur always below the wave intensity peak altitude. Since the other ionopause definitions discussed earlier usually occur below the = 1 altitude, they will similarly be less useful in ordering the plasma wave data.

      In the introduction we stated that the waves are observed within the plasma mantle. Although we refer to the altitude at which the Langmuir probe measures a density of 100 cm as the OETP ionopause, we consider this as occurring within the plasma mantle. At higher altitudes the plasma is essentially shocked solar wind plasma. At much lower altitudes the plasma is ionospheric in origin. The OETP ionopause is therefore probably within a region of mixed plasmas. Whether or not the OETP ionopause corresponds to a sharp boundary, or is simply a point on a more gradual transition, requires further analysis.

      Before discussing the magnetic field and plasma wave signatures at the OETP ionopause in more detail, we note that the other striking feature in Figure 1a, 1b is the relatively low level of plasma wave noise at lower altitude. We will discuss this later. However, our main conclusion concerning the low level of wave noise is that this is mainly an instrument artifact associated with the changes in the plasma Debye length and is not due to the absorption of waves as postulated by Scarf et al. [1980a].

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