Simultaneous optical, CUTLASS HF radar, and FAST spacecraft observations: signatures of boundary layer processes in the cusp

摘要

In this paper we discuss counterstreaming electrons, electricfield turbulence, HF radar spectral width enhancements, andfield-aligned currents in the southward IMF cusp region. Electricfield and particle observations from the FAST spacecraft arecompared with CUTLASS Finland spectral width enhancements andground-based optical data from Svalbard during a meridionalcrossing of the cusp. The observed 630nm rayed arc (Type-1 cuspaurora) is associated with stepped cusp ion signatures.Simultaneous counterstreaming low-energy electrons on openmagnetic field lines lead us to propose that such electrons may bean important source for rayed red arcs through pitch anglescattering in collisions with the upper atmosphere. The observedparticle precipitation and electric field turbulence are found tobe nearly collocated with the equatorward edge of the opticalcusp, in a region where CUTLASS Finland also observed enhancedspectral width. The electric field turbulence is observed toextend far poleward of the optical cusp. The broad-band electricfield turbulence corresponds to spatial scale lengths down to 5m.Therefore, we suggest that electric field irregularities aredirectly responsible for the formation of HF radar backscattertargets and may also explain the observed wide spectra. FAST alsoencountered two narrow highly structured field-aligned currentpairs flowing near the edges of cusp ion steps.Key words. Ionosphere (electric fields and currents). Magnetospherephysics (magnetopause, cusp, and boundary layers;auroral phenomena)