Polar observations of the time‐varying cusp

Abstract

We present a multi‐instrument case study of the signatures associated with the cusp observed by the Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE), Hot Plasma Analyzer (HYDRA) and Magnetic Fields Experiment (MFE) instruments onboard the Polar satellite between 0900 and 1300 UT on October 29, 1996. Characteristic signatures of the midaltitude cusp include ions of energies 102–104 eV, electrons of energies 10–103 eV, and a structured depression in the local magnetic field. During this cusp pass, ion fluxes in the 1–20 keV range were modulated quasi‐periodically on timescales of between 12 and 20 min, while energy‐dispersed ion count rates were observed close to the open‐closed field line boundary. The particle populations on the cusp field lines are dominated by H+ and He++ ions (characteristic of magnetosheath‐like plasma), and on closed field lines dominated by uniform fluxes of high‐energy (50–200keV) protons, which are not pulsed. The pulsed particle events during this case study occur during both southward and northward interplanetary magnetic field (IMF), but differ slightly in nature. After the IMF rotated northward, Polar still observed the pulsed particle events, but magnetosheath‐like and closed field line particles coexisted on the same magnetic flux tubes. It is not evident as to how this mix of plasma can exist on the same field line during northward IMF. HYDRA also observed higher‐frequency pulsing, hypothesized to be caused by low‐latitude transient magnetic reconnection. The simultaneous observation of two different frequency components has not been previously reported in spacecraft observations of the midaltitude cusp. Two further cusp passes during similar conditions are presented. The short‐period pulsed particle signatures were observed in all three examples, the longer‐period pulsing observed in two. The events differed in duration, most likely owing to the effect of IMF By on the location of the reconnection site, and hence the location of the polar cusp.