Searching for ulf signatures of the cusp: observations from search coil magnetometers and auroral imagers in svalbard

Spacecraft traveling through the cusp at a variety of altitudes have consistently found the cusp to be filled with intense, often irregular power in the upper ULF frequency range. Some ground-based studies have observed narrowband waves in this same frequency range in the vicinity of the cusp foot point, but it has not been possible with magnetometers alone to either confirm or deny a cusp source for these waves. We report here on the first simultaneous, collocated observations of a set of induction magnetometers installed at three near-cusp sites on Svalbard and an all-sky auroral imager located at Longyearbyen. Data during northern winters of 2006-2007 and 2007-2008, when the cusp foot point was in darkness, showed occasional broadband noise when energetic particle precipitation occurred overhead, but on most days no broadband ULF power was observed above the noise level near noon when only soft cusp precipitation or poleward moving auroral forms occurred overhead. However, on 3 days, including 15 January 2007, several bursts of band-limited Pc 1-2 waves were observed in association with regions of intense soft precipitation that peaked near the poleward edge of the cusp. Their properties are consistent with an origin in the plasma mantle, as observed in a recent satellite-ground study by Engebretson et al. (2005). These observations confirm that even intense cusp precipitation is not effective in generating ion cyclotron waves that penetrate to the ground, if it is embedded within the central regions of the cusp, whereas regions of enhanced precipitation at the poleward edge of the cusp are associated with observed waves.