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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4.       Wave Occurrence Statistics

      In Figure 7a, 7b we show UADS data from the first three dayside periapsis seasons of the Pioneer Venus Orbiter (i.e., orbits 125-248, 345-475, and 570-700). In Figure 7a, 7b we have restricted the data to solar zenith angles (SZA) < 30°. The top panel of Figure 7a shows the peak wave amplitude binned as a function of altitude with respect to the OETP ionopause, using 25-km bins. For each channel the symbols indicate the median peak amplitude per bin, with the shaded area indicating the upper and lower quartile. Clearly, the largest signal is measured at 100 Hz, but both the 730-Hz and 5.4-kHz channels also observe a wave burst at the OETP ionopause. However, as noted earlier, the major signature associated with the OETP ionopause appears to be a change in the instrument background. For example, apart from the peak at the OETP ionopause, we would conclude that the 100-Hz background changed from 10 V m Hz within the ionosphere to 10 V m Hz within the magnetosheath. At the OETP ionopause the peak wave amplitude is 10 V m Hz.

Figure 7a, Figure 7b.     Wave and plasma variations as a function of altitude with respect to the OETP ionopause for solar zenith angles (SZA) <30°. In Figure 7a, 7b and subsequently (Figures 8a, 8b and 9a, 9b) we have used data from the Unified Abstract Data System database for the first three seasons of dayside periapsis. The wave data are peak amplitudes. The plasma data are from the Langmuir probe. In calculating the plasma beta we assume that the ion temperature (T) = T/1.8 for altitudes > 350 km, where T is the electron temperature. The top panel of Figure 7a shows the wave amplitude for the four wave channels, while the bottom panel shows magnetic field strength, electron density, and plasma beta. The symbols show the median values per 25-km-altitude bin, while the shaded regions mark the upper and lower quartiles per bin. In Figure 7b we show the angle the magnetic field makes with the presumed flow direction () and the angle the field makes with respect to the vertical (). Both angles have been folded into the range 0°- 90°. The bottom panel of Figure 7b shows the parallel and perpendicular current density, calculated assuming horizontal currents and neglecting the vertical component of the magnetic field.

      The bottom panel of Figure 7a shows the ambient magnetic field strength and plasma parameters. From the magnetic field profile we deduce that the OETP ionopause is probably within the magnetic barrier region; there is little change in field strength across the OETP ionopause.

      The plasma beta is deduced from the electron data, assuming that the ion temperature is related to the electron temperature. Miller et al. [1984] presented average profiles of the electron and ion temperature for the dayside ionosphere of Venus, and we have used a function attributable to J. L. Phillips to model the temperature ratio. For altitudes >350 km the ratio is constant, with the ion temperature (T) = T/1.8, where T is the electron temperature. The peak temperature ratio (T/T) is 4 at 200 km altitude. For reference, the electron temperature increases with increasing altitude, with T 0.1 eV at altitudes 160 km, 0.3 eV at 250 km, and then gradually increasing up to the OETP ionopause altitude, where T as measured by the Langmuir probe 1 eV. At lowest altitude the deduced ion temperature T 0.06 eV. Because of the enhanced T/T ratio at 200 km, T does not begin to increase until 250 km altitude. At 350 km altitude, T 0.2 eV, increasing to around 0.5 eV at the OETP ionopause. It should be remembered, however, that the ion temperature is deduced from the electron temperature and does not take into account the presence of superthermal ions often detected at or above the ionopause [Taylor et al., 1981] . Beta is generally <1 at the OETP ionopause.

      By definition the density at the OETP ionopause is 100 cm, and no electron data are included in the UADS data for lower values, when the spacecraft is in sunlight. However, electron data from higher altitudes may be included if the density is >100 cm. Thus the apparent ledge in the density profile could simply be an effect of the density threshold for the Langmuir probe.

      Figure 7b shows the magnetic field orientation and the current densities as a function of altitude with the respect to the OETP ionopause. From the top panel of Figure 7b we see that the field is mainly horizontal at and above the OETP ionopause. However, the orientation with respect to the presumed flow direction shows no preferred orientation as a function of altitude for this SZA range, with median value around 45° and the lower and upper quartiles at 25° and 65°. Because the top panel of Figure 7b does not show how the field changes direction along an individual orbit, we show the current density in the bottom panel of Figure 7b. As noted earlier, we assume only vertical gradients in the field. The field-aligned current indicates regions where the field rotates but does not change magnitude, while the perpendicular currents indicate where the field is shielded from the lower ionosphere. In presenting current densities in Figure 7b we have excluded data for which the vertical separation used to compute the current density is less than 20 km. This is done to exclude data near periapsis, where the spacecraft is traveling mainly horizontally, and the assumption that the observed gradients are vertical is probably not valid. At these low altitudes, fine-scale structures known as flux ropes tend to occur, further invalidating the assumption of vertical gradients.

      From Figures 7a and 7b we therefore conclude that the OETP ionopause orders the wave data, with the largest-amplitude wave bursts occurring at the OETP ionopause. Furthermore, the OETP ionopause marks a region where the magnetic field rotates and field-aligned currents flow. The causal relationship, if any, between the waves and the currents has yet to be determined, but at this stage we would conclude that the currents flow in response to the magnetic field geometry imposed by the different flow regimes within the magnetosheath and ionosphere.

      The peak current density in Figure 7b is 1 A m. This corresponds to a relative drift velocity of 60 km s for an electron density of 100 cm, which is the nominal plasma density at the OETP ionopause. This velocity is much smaller than the thermal velocity of the shocked solar wind protons (temperature 100 eV) and the ambient electrons, be they hot shocked solar wind (50 eV) or cold planetary (1 eV) electrons. In order to drive an instability, we expect the drift velocity of the current-carrying species to be larger than its thermal velocity. If the current is to be the source of an instability, this suggests that cold planetary electrons are the current carriers, and these electrons are a small fraction of the total density. If, for example, we assume that the electrons of planetary origin are only 3% of the total density, then the electron drift velocity 1800 km s, which is much larger than the cold electron thermal speed. This current could generate an obliquely propagating acoustic mode with phase speed of the order of the shocked solar wind sound speed but with parallel phase velocity of the order of the electron drift velocity. However, such an instability will be subject to Landau damping by the solar wind protons, unless the protons are cooler than the solar wind electrons. This is usually not thought to be the case, although Shapiro et al. [1995] do present an example of one orbit in which the electron temperature is 100 eV. Clearly, determining whether or not the currents are a source of the waves requires detailed knowledge of the ambient ion and electron populations.

      Figures 8a, 8b and 9a, 9b show plasma and field data for SZA in the range 30° < SZA < 60° and 60° < SZA < 90°, respectively. The format of Figures 8a, 8b and 9a, 9b is similar to Figure 7a, 7b, and again, current density data obtained for vertical separations less than 20 km have been excluded. As the SZA increases, the vertical scale of the plasma and field gradients also increases. Thus the current densities tend to be smaller at higher SZA. At higher SZA the change in orientation of the magnetic field becomes more clear, with the lowest occurring at the OETP ionopause. Figures 8a, 8b and 9a, 9b therefore reinforce the conclusions drawn from Figure 7a, 7b.

Figure 8a, Figure 8b.     Wave and plasma variations as a function of altitude with respect to the OETP ionopause for 30° < SZA < 60°. Similar in format to Figure 7a, 7b.

Figure 9a, Figure 9b.     Wave and plasma variations as a function of altitude with respect to the OETP ionopause for 60° < SZA < 90°. Similar in format to Figure 7a, 7b.


Next: 5. Field Rotation at the OETP ionopause
Previous: 3. Flow-Aligned Coordinates
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Text and figures by R. J. Strangeway
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Last modified: Sept. 17, 1996