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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While the altitude profiles of current density and magnetic field orientation indicate that the magnetosheath magnetic field rotates at or near the OETP ionopause, the data shown in Figures 7a, 7b, 8a, 8b, and 9a, 9b do not indicate the sense of the rotation. In Figure 10 we show the orientation and the rotation of the magnetic field near the OETP ionopause. The magnetic field orientation just below the OETP ionopause is shown in the top panel of Figure 10. The data are binned in 10° bins, with the solid circles giving the median angle and the error bars marking the upper and lower quartile per bin. Noting that < 90° means that the field is pointing to higher altitude, Figure 10 shows that when the field is pointing away from the subsolar point (-90° < < 90°) the field is also pointing to higher altitude. The reverse is the case when the field points toward the subsolar point.
Figure 10. Magnetic field orientation and rotation at the OETP ionopause. The solid circles indicate the median, while the error bars mark the upper and lower quartile.
The bottom panel of Figure 10 shows the median and quartiles of the field rotation on passing through the OETP ionopause from above. The angle has been folded into the range 0°-90°, while the angle is defined as <0 if the net orientation of the field is more flow-aligned at lower altitude. Thus, for the majority of the data, the field rotates to a more flow-aligned orientation at lower altitude.
The magnetic field orientation and rotation shown in Figure 10 can be explained in terms of velocity shear across the OETP ionopause. Near the subsolar point the magnetosheath flow tends to transport magnetic flux to lower altitude, where the field is "hung up" on the obstacle, presumably because of the presence of ionospheric plasma. A flux tube that has been convected by the magnetosheath flow will be at lower altitude near the subsolar point and at higher altitude away from the subsolar point where the magnetosheath flow is more nearly tangential to the obstacle. Because the flow at higher altitudes is faster, while the subsolar portion of the flux tube moves more slowly, the flux tube will tend to be aligned along the flow as it is convected away from the subsolar region. This "weathervaning," discussed by Law and Cloutier  and modeled by Luhmann , is shown schematically in Figure 11.
Figure 11. Schematic of the magnetic field geometry in the mantle and upper ionosphere.
In Figure 11 the light gray surface marks a layer in which field-aligned currents flow. The solid arrows show the magnetosheath flow above this layer. The dark gray curves are magnetosheath flux tubes. When the flux tubes pass through the current layer, indicated by becoming darker gray, they encounter a region of reduced flow. As the flux tubes are transported away from the subsolar region, the shear in the flow causes the bend in the field to become larger, and the flux tube immediately below the current layer tends to be flow-aligned. In passing, we note that a similar process has been invoked for the formation of flux ropes [e.g., Luhmann and Cravens, 1991, and references therein], but flux rope formation occurs at lower altitude. Flux ropes are observed deep within the ionosphere below the magnetic barrier.
We can explain the field-aligned current layer as a shear Alfvén wave standing in the magnetosheath flow. This is in many ways analogous to the slow-mode standing wave observed in the terrestrial magnetosheath, as reported by Song et al. , and discussed theoretically by Southwood and Kivelson . For a shear Alfvén wave the flow perturbation (u) is related to the magnetic field perturbation (b) by
where B is the ambient magnetic field, V is the Alfvén speed = B/(), is the permeability of free space, is the mass density, and is the angle between the wave vector (k) and B. Since the wave is standing in the magnetosheath flow, k points upward, and . From Figure 10, if B points away from the subsolar point, then B also has an upward component. Thus > 0, and u is antiparallel to b. We illustrate the geometry of this situation in the two left-hand panels of Figure 12 for two orthogonal planes: the plane defined by the radial direction and the flow vector (top) and the horizontal plane (bottom). Since the horizontal component of the flow velocity (v) is also away from the subsolar point, > 0. The direction of u is such that the horizontal flow velocity is reduced below the field-aligned current layer. Taking primed vectors as being below the current layer, then , , and to first order = - . Since < 0, > , and the field is more flow-aligned below the current layer. We note that the closer alignment occurs with a change in both the field and the flow. When the magnetic field points toward the subsolar point, < 0 and < , but < 0, as illustrated in the two panels on the right-hand side of Figure 12. Again, the field is more flow-aligned below the current layer, and again, both field and flow rotate to accomplish this alignment.
Figure 12. The relationship between the magnetic field perturbation and velocity perturbation for a shear Alfvén wave standing in the magnetosheath flow. Figure 12 shows (a) magnetic field pointing away from the subsolar point, and (b) magnetic field pointing toward the subsolar point. The unprimed vectors are above the current layer, while the primed vectors are below. For each case the upper plot shows the projection in the radial-flow plane, with the lower plot showing the projection in the horizontal plane.
Figure 10 shows that the median rotation of the field is 10°, and hence b/B 0.18. From Figure 7a, B 100 nT at the OETP ionopause, while the plasma density is 100 cm. If protons are the dominant ion, then V 220 km s. Thus u 40 km s, which should be compared with a nominal sheath flow speed of 100 km s, and the flow velocity can be significantly reduced across the current layer. Increasing the amount of O present reduces V and hence u. However, an increase in O density would suggest that some slowing of the magnetosheath flow will have already occurred, and the flow velocity above the shear layer could be lower than 100 km s. Nevertheless, even if the flow is reduced because of mass loading at higher altitudes, our discussion here indicates that there is a marked reduction in flow below the OETP ionopause, presumably because of higher planetary ion densities.
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Text and figures by R. J. Strangeway
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Last modified: Sept. 19, 1996