ULF cusp pulsations: Diurnal variations and interplanetary magnetic field correlations with ground‐based observations

In this paper we establish the Pc 5 magnetic pulsation signatures of the cusp and boundary regions for the high‐latitude dayside cusp region. These signatures were determined by comparing spectrograms of the magnetic pulsations with optical observations of particle precipitation regions observed at the cusp. The ULF pulsations have a diurnal variation, and a cusp discriminant is proposed using a particular narrow‐band feature in the pulsation spectrograms. The statistical distribution of this pattern over a 253‐day period resembles the statistical cusp description using particle precipitation data from the Defense Meteorological Satellite Program (DMSP). The distribution of the ground‐based cusp discriminant is found to peak 1 hour earlier than the DMSP cusp distribution. This offset is due to the interplanetary magnetic field (IMF) being predominantly negative By for the period when the data were collected. We find the diurnal variations so repeatable that only three main categories of pulsation spectra explain 86 percent of all data observed. We find that these categories have statistically different IMF distributions. The identification of the signatures in the magnetic spectrograms of the boundary regions and central cusp allows the spectrogram to be used as a “time line” that shows when the station passed under different regions of the dayside oval.