Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024, U. S. A.
Originally published in:
Adv. Space Res., Vol. 2, No. 10, pp. 13-16, 1983.
When the solar wind dynamic pressure is high, the Venus ionosphere usually contains a belt of steady magnetic field at the very lowest altitudes to which Pioneer Venus probes. The current layer that flows on the high altitude side of this low altitude belt is centered at an altitude which ranges from 170 to 190 km with a most probable altitude of 182 km. This altitude is independent of solar zenith angle and hence the current system is flowing horizontally rather than vertically as proposed by Cloutier and co-workers. The lower edge of the magnetic belt was probed only on the lowest altitude passes of Pioneer Venus. This boundary is even more stable in location. The belt has decayed to 90% of its maximum strength usually by 162 km and to 50% of its maximum strength by 155 km. We interpret these data to indicate that the observed magnetic structure of the Venus ionosphere is a product of temporal evolution rather than of spacecraft motion through a spatially varying static structure.
Although the dayside ionosphere of Venus is often "field-free" except for fine scale features, large scale steady ionospheric magnetic fields with magnitudes exceeding 100 nT are occasionally observed in the Venus ionosphere at low altitudes . These fields are mainly horizontal and can assume any angle in the horizontal plane. With few exceptions the observations of these large scale fields occurs when periapsis of the Pioneer Venus spacecraft is at solar zenith angles of less than 50o. The occurrence of these large scale fields is coincident with the occurrence of periods of high solar wind dynamic pressure.
|Fig. 1. Altitude profiles of magnetic field strength on three inbound and outbound passes through the Venus ionopause when a low altitude magnetic belt was present.|
We have interpreted the occurrence of the low altitude belt in terms of a temporally varying model in which the ionosphere becomes magnetized at times of high solar wind dynamic pressure. The magnetic field decays when the dynamic pressure decreases leaving the low altitude magnetic belt in the region of slowest decay. However, an alternative model has been proposed in which the low altitude belt is a static feature driven by a low altitude extension of the ionospheric current system . In this model currents flow vertically in the ionosphere from the ionopause down to the lower ionosphere across the lower ionosphere and then up again closing on the ionopause. At low solar zenith angles and along a "belt" parallel to the magnetosheath magnetic field extending from the subsolar point in either direction, a magnetic field exists unattenuated from magnetosheath strengths. We will refer to this as the steady state electrodynamic model (SSEM).
A simple test of these models is to examine the geometry of the current sheets as best we can with the Pioneer Venus magnetic field observations . Since Pioneer Venus orbits Venus in a highly elliptical orbit we are restricted in the latitudinal width available for observation on each orbit. Thus we will concentrate on altitudinal profiles and examine these as a function of solar zenith angle.
In either our paradigm or that of the steady state electrodynamic model the lower border of the magnetic belt is essentially horizontal and near the bottom of the ionosphere. We have measured the peak field strength and the altitude at which the field strength dropped to 90%, 80%, 70% of its maximum value, etc., at altitudes below the peak for each Pioneer Venus orbit which passed low enough in the ionosphere when it was magnetized. There are only 15 such orbits in the 400 orbits studied to date.
|Fig. 2. The altitude of the current layer on the lower edge of the low altitude magnetic belt as a function of solar zenith angle. The various symbols show where the magnetic field reaches 90%, 80%, 70%, etc., of its peak value.|
The plasma flow in the Venus ionosphere is essentially horizontal and the gradients in the flow vertical so that the fastest velocities, about 2.5 km/see, occur at highest altitudes . Thus in our scenario, the erosion of the magnetized ionosphere by convective transport to the nightside as new ionosphere is created yields an essentially horizontal boundary, or current layer, on the upper edge of the low altitude belt. In the SSEM approach, the ionopause current system is diverted through the ionosphere vertically to close in the lower ionosphere. Thus, the current layer should be vertical rather than horizontal. Figure 3 shows the altitude of the mid-point of the current layer as a function of solar zenith angle. First, and most importantly, the current layer is found in a narrow altitude range. The most probable altitude of the layer is 182 km with 50% of the crossings within 6 km of this altitude. It is not crossed at a variety of altitudes dependent on the relative geometry of the orbit and the orientation of the magnetic field as would be predicted in the SSEM scenario. Second, there is no discernible change in altitude with solar zenith angle. The current layer is strictly horizontal.
|Fig. 3. The altitude of the mid-point of the current layer on the upper edge of the low altitude magnetic belt as a function of solar zenith angle.|
In the model of the interaction that we sketched in the introduction, the magnetic field of the magnetosheath was imposed on the ionosphere when the ionopause moved to low altitudes and thickened. The thickening of the ionospheric current system is well documented in the Pioneer Venus data . By the time the ionopause reaches an altitude of about 220 km it is close to 100 km thick. If our model is correct then the low altitude ionosphere should become magnetized to a strength equal to or some fraction of the local magnetosheath field strength. Since the magnetosheath magnetic field has a well documented dependence of cosine of the solar zenith angle (SZA)  then we might expect the magnetic belt to have a solar zenith angle dependence also.
The strength of the magnetic field in the belt, when observed, is highly variable. We assume that this variability is caused in part by the variation in the compression of the ionosphere as the solar wind dynamic pressure increases and decreases both during and after the event that magnetizes the ionosphere. To account for this pressure variation we have first extrapolated the magnetosheath magnetic field strength measured just above the ionopause, from the point of observation to the subsolar point. When both inbound and outbound values were available we used both and averaged. Then on each pass we normalized the peak field value in the belt region by the magnitude of the subsolar magnetic field. This is shown in Figure 4 together with a plot of cos (SZA). There is much scatter but there is a clear suggestion of a cos (SZA) dependence to the data. The scatter may in part be due to the fact that the subflow point may deviate by 10 or more from the subsolar point. However, this does not explain the scatter at low solar zenith angles where one expects little variation in the field with solar zenith angle. Clearly, there are other processes acting to change the magnitude. Since the field values are all less than the instantaneous magnetosheath field, we attribute this reduction to the natural decay of the field in the belt due to convection and diffusion.
|Fig. 4. The strength of the magnetic field at the peak of the low altitude magnetic belt as a function of solar zenith angle.|
These observations suggest a qualitative model for the formation of the low altitude magnetic belt in the Venus ionosphere. In this model time variations are essential. We generally see the belt sometime after it has been created and has evolved. The ionosphere is magnetized at times of high solar wind dynamic pressure. When this pressure decreases, the ionopause moves out and the high altitude magnetized plasma is convected away to the nightside. The reason for this convective loss is simple. New plasma is created by photoionization on the existing field lines, weakening the field and setting up pressure gradients which lead to the convection of the plasma and field to the nightside. It is the same process that creates the flow field in the dayside ionosphere in the absence of a magnetic field. The magnetic field may or may not inhibit or accelerate the flow. That is not essential to the eventual loss of the plasma.
At low altitudes this does not occur. Plasma is being created at the same or greater rates by photoionization of the neutral atmosphere but it is also being lost through recombination here. In short, it is in or near photo-chemical equilibrium so that local loss balances formation and diffusion and convective transport are minimal. The strength of the belt will of course decrease in time. The lower current layer flows in a moderately resistive medium. The electric field necessary to drive this current can be supplied in our scenario by the decaying magnetic field. Furthermore, even though convective losses in the region of the low altitude belt are small, they certainly are not totally absent. Thus, we expect the low altitude belt to decay at some rate greater than the Cowling diffusion time for a slab of magnetic field in conducting plasma.
We feel that the observations we have reported here are consistent with the above picture: the location of the lower current layer, the location of the upper current layer and the strength of the peak field in the belt and their dependences on the solar zenith angle. One, perhaps subtle, but important, point should be made and that is that our picture is an attempt to describe the processes acting to create the ionospheric properties that we observe. We have not shown that the mechanism that Cloutier and co-workers  describe does not act to magnetize the ionosphere when the solar wind dynamic pressure reaches its peak, nor have we constructed a quantitative model. What we feel the observations show is that the ionosphere, when we measure it, is usually in the process of recovering from that magnetization and is no longer being directly driven. Only rarely, such as perhaps on orbits 176 and 190 as described by Elphic et al. , can a case be made for a directly driven current system during the observing period.
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