Cusp latitude magnetic impulse events: 2. Interplanetary magnetic field and solar wind conditions

Abstract

Listen

Translate article

The interplanetary magnetic field (IMF) conditions and solar wind plasma parameters prevailing during the magnetic impulse events identified by Lanzerotti et al. (1991) at the near cusp latitude stations Iqaluit, 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. However, by examining the IMF orientation when the IMF variability is small a south‐east anisotropy is found. Examining the IMF orientation on timescales of less than 1 min reveals that the impulse events are characterized by a more southward oriented IMF than is to be expected statistically. The duration of the events is found to be controlled by both solar wind speed and by the IMF By. The IMF By control of the duration of nonconjugate events is in accordance with the effect expected from the presence of magnetic tension in a reconnection setting. The amplitude of the events is found to be strongly related to solar wind speed. These findings suggest that sporadic reconnection is among the mechanisms producing the events. On the basis of the amplitudes of the impulse events, it is argued that the fractional changes in dynamic pressure must be of the order of 1 to be able to associate the impulse events with changes in the dynamic pressure. Such variability in the dynamic pressure at the time of the magnetic impulse events is not observed. 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 hand, reconnection processes are able to produce impulse events of the required magnitude. It is argued then that magnetic reconnection is likely responsible for generating a minimum of 50%‐70% to a maximum of 90% of the events. However, this conclusion leaves parts of the picture unclear. Lanzerotti et al. (1991) found that magnetic impulse events with a unipolar deflection in the vertical component of the magnetic field 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 impulse events are associated with vortices generated by the Kelvin‐Helmholtz instability in the low‐latitude boundary layer. As such, it is suggested that the Kelvin‐Helmholtz instability may mediate the process by which sporadic reconnection produces a ground signature.