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

Abstract

Abstract. In this paper we discuss counterstreaming electrons, electric field turbulence, HF radar spectral width enhancements, and field-aligned currents in the southward IMF cusp region. Electric field and particle observations from the FAST spacecraft are compared with CUTLASS Finland spectral width enhancements and ground-based optical data from Svalbard during a meridional crossing of the cusp. The observed 630 nm rayed arc (Type-1 cusp aurora) is associated with stepped cusp ion signatures. Simultaneous counterstreaming low-energy electrons on open magnetic field lines lead us to propose that such electrons may be an important source for rayed red arcs through pitch angle scattering in collisions with the upper atmosphere. The observed particle precipitation and electric field turbulence are found to be nearly collocated with the equatorward edge of the optical cusp, in a region where CUTLASS Finland also observed enhanced spectral width. The electric field turbulence is observed to extend far poleward of the optical cusp. The broad-band electric field turbulence corresponds to spatial scale lengths down to 5 m. Therefore, we suggest that electric field irregularities are directly responsible for the formation of HF radar backscatter targets and may also explain the observed wide spectra. FAST also encountered two narrow highly structured field-aligned current pairs flowing near the edges of cusp ion steps.

Multi-instrument mapping of the small-scale flow dynamics related to a cusp auroral transient

“…This is also confirmed by Stauning et al (2001), using high-resolution measurements from the Danish Ørsted satellite, which suggests that the region 1 current is located in the LLBL equatorward of the cusp (Woch et al, 1993). In a recent paper Oksavik et al (2004b) combined observations from the FAST spacecraft with ground-based optical images from Svalbard, and they found counterstreaming electrons associated with stepped cusp signatures and southward IMF cusp aurora. Furthermore, the cusp ion steps were associated with thin sheets of fine-scale field-aligned currents flowing near the edges of the ion steps, consistent with the predictions of Lockwood et al (2001) for pulsed reconnection during periods with large IMF B y .…”

Poleward moving auroral forms (PMAFs) revisited: responses of aurorae, plasma convection and Birkeland currents in the pre- and postnoon sectors under positive and negative IMF <I>B<sub>y</sub></I> conditions

“…9 are related to the nonzero divergence of magnetopause current at high latitudes on the dawn side as discussed by Taguchi et al (1993). The Birkeland current -aurora relationship for the B y >0 case has been documented in a previous study by during a traversal of spacecraft Polar along the 09:00 MLT meridian from the polar cap to plasma sheet latitudes, at mid-altitudes (see also Oksavik et al, 2004b;their Fig. 6).…”

“…9. The derivation of FACs in this perspective is made by recognizing the close link that exists between upward-directed FACs and discrete aurorae (Frey et al, 1996;McFadden et al, 1999) and by inferences from observations from satellites in polar orbit made previously in similar cases (Sandholt and Newell, 1992;Farrugia et al, , 2004bOksavik et al, 2004b). Plasma vorticity in the ionosphere (Reiff et al, 1978;Sofko et al, 1995) and at the magnetospheric level (see e.g.…”

Motion of the dayside polar cap boundary during substorm cycles: I. Observations of pulses in the magnetopause reconnection rate

“…One last feature to note is that the dispersion curve seen in the lowest energy sheath ions (the low-energy time-of-flight ion "cut-off") is not smooth but shows small fluctuations. Such features are often accompanied by poleward-moving ionospheric events, in which case they are caused by small, short-period reconnection rate variations (Cowley et al, 1991b, Lockwood andNewell and Meng, 1991;Escoubet et al, 1992;Lockwood et al, 1993a, Lockwood andDavis, 1995;1996;Pinnock et al, 1995;Farrugia et al, 1998;Morley and Lockwood, 2003;Oksavik et al, 2004). Cusp ion steps originating from purely spatial variations in reconnection rate are not accompanied by poleward-moving events (Lockwood, 1996).…”