Polar observations of the time-varying cusp

We present a multi-instrument case study of the signatures associated with the cusp observed by the Charge and Mass Magnetospheric. Ion Composition Ex periment (CAMMICE), Hot Plasma Analyzer (HYDRA) and Magnetic Fields Experim ent (MFE) instruments onboard the Polar satellite between 0900 and 1300 UT on October 29, 1996. Characteristic signatures of the midaltitude cusp incl ude ions of energies 10(2)-10(4) eV, electrons of energies 10-10(3) 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 tim escales of between 12 and 20 min, while energy-dispersed ion count rates we re observed close to the open-closed field line boundary. The particle popu lations on the cusp field lines are, dominated by H+ and He++ ions (charact eristic 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 southw ard and northward interplanetary magnetic field (IMF), but differ slightly in nature. After the IMF rotated northward, Polar still observed the pulsed particle events, but magneto sheath-like and closed field line particles c oexisted 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-la titude transient magnetic reconnection. The simultaneous observation of two different frequency components has not been previously reported in spacecr aft observations of the midaltitude cusp. Two further cusp passes during si milar 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 I MF B. on the location of the recolinection site, and hence the location of the polar cusp.