1 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA
2 Geophysics Program, University of Washington, Seattle, Washington, 98195-1650
3 Space Research Institute, Austrian Academy of Sciences, Infeldgasse 12, A-8010, Graz, Austria
4 Space Physics Res. Lab., University of Michigan, Dept. of Atmos., Oceanic & Space Sciences, 2455 Hayward, Ann Arbor, MI 48109-2143
The role of the foreshock in causing motions of the magnetopause is studied using both the number of crossings of the magnetopause found as ISEE passed through the expected region of the magnetopause on each orbit and by determining the minimum and maximum distances to the magnetopause on each pass. In the region downstream of the magnetosheath and bow shock that is free from foreshock effects, the afternoon side for typical interplanetary magnetic field (IMF) directions, the north-south component of the IMF controls these motions. On the morning side the foreshock plays a significant role in controlling the magnetopause motion for both northward and southward IMF. We interpret these increased motions as driven by the pressure variations in and behind the foreshock region rather than due to fluctuating IMF directions behind the foreshock. These results are consistent with other studies of foreshock effects on the magnetosphere which show little effect on erosion of the magnetopause or flux transfer event occurrence.
The solar wind interaction with the Earth's magnetosphere appears to be unsteady even when the solar wind conditions are steady. This is particularly evident when the IMF is southward. Magnetopause oscillations appear above the level expected for pressure variations in the solar wind [Song et al., 1988]. Flux transfer events arise even when the IMF is steady [Le et al., 1993]. In front of the bow shock is another unsteady region the foreshock [Greenstadt et al. 1970; Greenstadt and Russell, 1994] whose leading edge joins the bow shock where the shock normal of the IMF is about 45o and proceeds upstream as if it were carried by particles with about twice the solar wind velocity along the magnetic field direction [Le et al., 1992]. The foreshock is also a region of disturbances in the pressure of the solar wind and these pressure fluctuations clearly affect the magnetosphere [ Fairfield et al., 1990]. If fluctuations generated by the foreshock can cause fluctuations within the magnetosphere, they most certainly must affect the magnetopause itself. We can imagine several possibilities. The pressure fluctuations could directly move the boundary. The pressure fluctuations could also enhance the reconnection rate when the IMF was southward temporarily leading to erosion of the magnetopause and causing magnetopause oscillations. Finally, the magnetic waves themselves could lead to periodically enhanced reconnection and oscillations of the boundary. If flux transfer events are due to transient reconnection we might also expect that they too would be affected by the foreshock.
In this paper we will examine how magnetopause oscillations are affected by the foreshock and by the direction of the IMF. We find that both factors play a role in causing the magnetopause to oscillate.
In order to examine what factors control the oscillations of the magnetopause we will use the data set on magnetopause crossings observed by ISEE 1 and 2 developed by Song et al. . This data set provides two measures of the motion of the magnetopause: the number of crossings of the magnetopause observed on each pass through the magnetopause region and the amplitude of the motion of the magnetopause measured as the minimal and maximal radial distance of the magnetopause seen on each pass. We will use both measures. The study by Song et al.  used only the latter measure and did not attempt to determine foreshock effects. Merely it attempted to show what motions could be associated with the intrinsic pressure variations of the solar wind and which ones could not.
In order to be able to separate out foreshock effects we have selected only orbits when the foreshock was over the morning sector i.e. the Parker spiral angle of the IMF was within 45o of its expected direction. We have then examined each of 5 local time bins: dawn (4-8 LT), noon (10-14 LT), dusk (16-20 LT) and a morning and afternoon bin between these times. Further we separated the direction of the IMF into 5 bins according to the direction (clock angle) projected into the Y-Z GSM plane. Table 1 lists the bin ranges which overlap to avoid spatial aliasing of the data. The clock angle is zero for due northward fields and 90o for fields opposite the Earth's orbital motion projected into the plane perpendicular to the Earth-sun line. The sign of the Y-component of the magnetic field was not judged to be important in this study so that all directions on the "left" hand side of the clock dial were folded on to the "right" hand side. The right hand side corresponds to positive values of By GSM. Figure 1 shows the results of this process for the number of magnetopause crossings per pass. Each panel shows pie-slices for IMF orientations from northward to southward which represent the average number of magnetopause crossings seen at that local time and IMF direction. We recall that we have chosen only IMF directions
Figure 1. Average number of magnetopause crossings|
per pass of the ISEE-1 and 2 spacecraft as a function of
the clock angle of the IMF and local time. The outer
dashed circle indicates 10 magnetopause encounters per pass.
|Right Hand Range|
|Left Hand Range|
|0-36||0-54||-54 - 0|
|36-72||18-90||-90 - -18|
|72-108||54-126||-126 - -54|
|108-144||90-162||-62 - -90|
|144-180||126-180||-180 - -126|
that produce a foreshock on the dawn side of the Earth. The center panel shows that the fewest magnetopause crossings per orbit occur near noon, about 2 on average, and that there is little dependence of the number of crossings on the direction of the IMF. This suggests that at noon magnetopause motions are associated with the intrinsic pressure oscillations of the solar wind.
As we move to the afternoon and dusk sectors, the number of magnetopause crossings for northward and horizontal magnetic fields does not change. However, for southward IMF the number of crossings does increase. This is in accord with our expectations of the role of reconnection in eroding the magnetopause and with the study of Song et al. , that showed greater amplitudes of magnetopause motion for southward IMF.
As we move to the morning and dawn sectors, the number of magnetopause crossings increases at all clock angles. Moreover the increase for southward IMF directions is much greater (almost double) the increase in the afternoon and dusk sectors. Since we have constructed these bins to keep the foreshock above the morning/dawn sectors we must assume that the pressure fluctuations associated with the foreshock are responsible for this increase. Although the foreshock does produce directional fluctuations, the increase at all angles, north and south, suggests that the additional directional fluctuations have little effect on the magnetopause motion. If these directional fluctuations were effective in producing reconnection, they would have affected the magnetopause more greatly for southward IMF than northward, but they clearly are not effective at producing reconnection.
Figure 2 repeats the study but using the average amplitude of the magnetopause motion in each local time sector and clock angle bin. Again the smallest motions occur in the subsolar region with slightly smaller motions for southward than northward fields. As we move toward the dusk sector the amplitude of the motion for northward and horizontal fields remains roughly the same but the amplitude of magnetopause motions for southward fields grows rapidly. Again this is consistent with our expectations about the role of the IMF in eroding the magnetopause which add to the motions caused by the fluctuations in pressure in the solar wind.
Figure 2. Average amplitude of the motion of the |
magnetopause as observed by ISEE-1 and 2 as a function
of the clock angle of the IMF and local time. The
outer dashed circle indicates an amplitude of 1 Earth radius.
Shifting to the dawn side of Figure 2 we see that the amplitude of the motion of the magnetopause increases for all angles of the IMF, although it is clearly greatest for southward IMF. This again shows that the pressure fluctuations associated with the foreshock are responsible for the motion. Had it been the foreshock-associated directional changes in the magnetic field causing these motions, these should have modulated reconnection most for near-horizontal fields that could have been bent southward by the fluctuation. Northward and southward fields would not be changed significantly by these directional changes. Thus, we would have seen little additional motion for northward and southward IMF over that seen at dusk with most of the enhanced motion occurring for horizontal IMF.
It is clear that the foreshock plays more than just an incidental role in the solar wind interaction. Not only does it begin the deceleration process of the solar wind as it approaches the Earth [Bame et al., 1980; Bonifazi et al., 1981; Zhang et al., 1995] but it also increases the level of pressure fluctuations downstream that in turn can interact with the magnetopause [Fairfield et al., 1990]. The possible effect of such pressure fluctuations has engendered much controversy. In particular it has been suggested that flux transfer events [Russell and Elphic, 1978; 1979] could be driven by foreshock pressure pulses [Sibeck, 1992] but this suggestion has been challenged [Song et al., 1994]. This study together with some other recent papers help clarify where the foreshock affects the dynamics of the dayside of the magnetosphere and where it does not.
Concerning the issue of the origin of FTEs, the most compelling recent observations come from an exhaustive study of FTEs seen during the course of the entire ISEE-1 mission [Kawano et al., 1996]. They examine the local time distribution of FTE occurrence as we have done here for boundary motions but examining times of both normal and orthogonal spiral angles of the IMF. They found FTEs did not occur more frequently behind the foreshock. Rather FTEs occurred slightly more often in regions away from the foreshock.
Since the foreshock causes directional fluctuations of the IMF it might lead to enhanced reconnection when those fluctuations generated a southward IMF component. In turn this fluctuating southward IMF might lead to erosion of the magnetopause. This possibility was checked by Zhang et al.  who found no change in the magnetopause position when the subsolar magnetopause found itself downstream of the foreshock. Thus the foreshock does not appear to induce reconnection.
Although the foreshock does not appear to affect reconnection at the magnetopause, we do find one significant effect of the foreshock. As known for some time southward IMF leads to enhanced boundary motions and these motions get larger with distance away from the subsolar point. However, the foreshock plays an important role in these boundary motions. When the IMF is northward this is especially true, but even when the IMF is southward, the magnetopause oscillates more behind the foreshock than on the non-foreshock side. Because the foreshock does not seem to induce FTEs or erosion of the magnetopause, we attribute this additional motion not to periodic reconnection but to the pressure fluctuations induced in the foreshock region by the interaction of the backstreaming ions with the solar wind.
This study was supported by research grant NAGW-3948 from the National Aeronautics and Space Administration and ATM 94-13081 from the National Science Foundation.
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