Abstract
Abstract. In this study, we perform a statistical investigation of magnetic impulse events identified in the Greenland magnetometer stations through the years 1995–2001. We focus on occurrence statistics that can be determined reliably with an automatic event identification procedure. Durin the first two years we observed almost 40% more events than in the following years. Season is not a significant factor in event occurrence. Event occurrence peaks near 12:00 UT, corresponding to approximately 10:00 magnetic local time (MLT) at the west coast of Greenland. More events occur prior to local noon than after. Event days are not distributed evenly. Large amplitude events, particularly, tend to appear on consecutive days. Events are observed at lower latitudes at earlier local times in a way consistent with the projection of the outer magnetospheric boundary into the ionosphere. Event latitude depends on dipole tilt angle in a manner similar to that reported for the cusp. Events occur during intervals of enhanced Kp. The main reason for this is that the events themselves contribute to the Kp index. Events exhibit a preference for high solar wind velocity. In particular, the large amplitude events occur during high-speed streams. A slight preference for lower density and more radial interplanetary magnetic fields, as compared to the nominal solar wind distribution, is also observed. However, both the nominal solar wind and event distribution exhibit large differences from year to year, indicating that events occur under a broad range of conditions. Key words. Ionosphere (ionospheric disturbances) Magnetospheric physics (magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)
Highlights
Transient (5–20 min) variations in the ground magnetic field at high latitudes are very common
A large portion of magnetic impulse events (MIEs) result as the magnetic signature of ionospheric traveling convection vortex (TCV) events as first demonstrated by Friis-Christensen et al (1988) and Glassmeier et al (1989)
Suggestions included bursty reconnection at the magnetopause (Glassmeier et al, 1984; Lanzerotti et al, 1986) and magnetopause motion driven by pressure pulses or abrupt changes in the interplanetary magnetic field (IMF) (Friis-Christensen et al, 1988; Sibeck et al, 1989)
Summary
Transient (5–20 min) variations in the ground magnetic field at high latitudes are very common. Murr and Hughes (2003) very convincingly demonstrate one-to-one correspondence between TCV events and foreshock cavity-like signatures for out of their events. This ties in well with statistical studies of MIEs that have shown them to occur more frequently prior to noon local time than after and to favor more radial IMF orientation (Glassmeier et al, 1989; Lanzerotti et al, 1991; Konik et al, 1994; Sibeck and Korotova, 1996; Zesta et al, 2002) as expected for the cavities (Sibeck et al, 2001). Many discrepancies exist between the earlier statistical studies causing confusion about their results regarding, for example, seasonal and local time occurrence rates (e.g. discussions in Sibeck and Korotova, 1996; Zesta et al, 2002)
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