Abstract
On July 24, 1986 narrow band magnetic field fluctuations were observed by VIKING near the apogee altitude of 13,500 km in the polar cusp region during a period of strong southward interplanetary magnetic field (IMF). The VIKING magnetic field experiment obtained vector magnetic field measurements up to nearly the proton cyclotron frequency (fH=28 Hz). Simultaneously, magnetosheath‐like electrons and ions as well as an upward‐flowing field‐aligned current were observed. The most intense flux of electrons and ions were colocated with a 35 nT depression in the total magnetic field strength of 1860 nT, interpreted as a diamagnetic effect. The density of magnetosheath‐like plasma, estimated from the diamagnetic depression, reached ∼500 cm−3. This extremely high density was also consistent with total plasma density estimates using high frequency waves. Magnetic field fluctuations were observed from 18 to 27 Hz and occurred when the magnetic field strength depression was maximum. These waves were left‐hand elliptically polarized with ellipticity between 0.2 and 0.3 and may propagate in the Alfvén‐ion cyclotron branch of the cold electromagnetic dispersion relation. Possible free energy sources included anisotropies in the electron and ion distributions.
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