Cusp low‐energy ion cutoffs: A survey and implications for merging

Abstract

A fundamental prediction of merging theory is that recently merged field lines should exhibit sharp low‐energy ion cutoffs: If a field line merged at a distance d and a time t removed from the observation point, only ions with velocity above d/t should be observable. Two previous cases at the magnetopause and one in the midaltitude particle cusp have shown such effects. We studied the low‐altitude cusp for 22 quasi‐randomly selected DMSP F7 passes and determined that the low‐energy cutoff indeed clearly existed in most spectra for every pass. Of the individual spectra with cutoffs, 95% had the expected dispersion sense, with cutoff energy less than or equal to the next equatorward cutoff observed. Lockwood and Smith (1992) have pointed out that it is possible to reconstruct the time history of merging by examining d/t. The contribution to the cross‐polar cap potential from cusp merging could be determined by this means for 15 cases with reasonable accuracy; the value per unit length of merging line fell in the range 17–55 V/km, with a mean value of 31.5 V/km. We consider some of the ramifications for monitoring merging variability subject to the constraints of a detector with discrete energy levels. The grainy nature of the observations reduces the frequency of observed jumps in d/t (e.g., intervals without merging) while increasing the apparent size of small jumps. Longer intervals without merging can be either underestimated or overestimated. Despite these limitations, the present survey shows unambiguously that merging rarely ceases for as long as 1 min. Finally, a comparison between what the DMSP detector should observe under steady state merging with cutoffs actually observed in apparently smooth cusp crossings suggests that the merging rate is probably always fluctuating on timescales shorter than a few minutes.