CUSP LATITUDE MAGNETIC IMPULSE EVENTS .2. INTERPLANETARY MAGNETIC-FIELD AND SOLAR-WIND CONDITIONS

The interplanetary magnetic field (IMF) conditions and solar wind plas ma parameters prevailing during the magnetic impulse events identified by Lanzerotti et al. (1991) at the near cusp latitude stations Iqalui t, Northwest Territories, Canada, and South Pole Station, Antarctica, are examined. The impulse events are found to occur during periods of high IMF variability. The prevailing IMF orientation, averaged over 11 -min periods during the events, shows no distinct anisotropies. Howeve r, by examining the IMF orientation when the IMF variability is small a south-east anisotropy is found. Examining the IMF orientation on tim escales of less than 1 min reveals that the impulse events are charact erized by a more southward oriented IMF than is to be expected statist ically. The duration of the events is found to be controlled by both s olar wind speed and by the IMF B(y). The IMF B(y) control of the durat ion of nonconjugate events is in accordance with the effect expected f rom the presence of magnetic tension in a reconnection setting. The am plitude of the events is found to be strongly related to solar wind sp eed. These findings suggest that sporadic reconnection is among the me chanisms producing the events. On the basis of the amplitudes of the i mpulse events, it is argued that the fractional changes in dynamic pre ssure must be of the order of 1 to be able to associate the impulse ev ents with changes in the dynamic pressure. Such variability in the dyn amic pressure at the time of the magnetic impulse events is not observ ed. It is concluded that a change in the solar wind dynamic pressure, while not among the primary mechanisms generating the magnetic impulse events, may account for between 15% and 30% of them. On the other han d, reconnection processes are able to produce impulse events of the re quired magnitude. It is argued then that magnetic reconnection is like ly responsible for generating a minimum of 50%-70% to a maximum of 90% of the events. However, this conclusion leaves parts of the picture u nclear. Lanzerotti et al. (1991) found that magnetic impulse events wi th a unipolar deflection in the vertical component of the magnetic fie ld possessed a local time dependence; positive deflections occurred in local morning, while negative deflections occurred in local afternoon . This dependence can be explained by postulating that the magnetic im pulse events are associated with vortices generated by the Kelvin-Helm holtz instability in the low-latitude boundary layer. As such, it is s uggested that the Kelvin-Helmholtz instability may mediate the process by which sporadic reconnection produces a ground signature.