Plasma structure within poleward-moving cusp/cleft auroral transients: EISCAT Svalbard radar observations and an explanation in terms of large local time extent of events

Abstract

Abstract. We report high-resolution observations of the southward-IMF cusp/cleft ionosphere made on December 16th 1998 by the EISCAT (European incoherent scatter) Svalbard radar (ESR), and compare them with observations of dayside auroral luminosity, as seen at a wavelength of 630 nm by a meridian scanning photometer at Ny A Ê lesund, and of plasma¯ows, as seen by the CUTLASS (co-operative UK twin location auroral sounding system) Finland HF radar. The optical data reveal a series of poleward-moving transient red-line (630 nm) enhancements, events that have been associated with bursts in the rate of magnetopause reconnection generating new open¯ux. The combined observations at this time have strong similarities to predictions of the e ects of soft electron precipitation modulated by pulsed reconnection, as made by Davis and Lockwood (1996); however, the e ects of rapid zonal¯ow in the ionosphere, caused by the magnetic curvature force on the newly opened ®eld lines, are found to be a signi®cant additional factor. In particular, it is shown how enhanced plasma loss rates induced by the rapid convection can explain two outstanding anomalies of the 630 nm transients, namely how minima in luminosity form between the poleward-moving events and how events can re-brighten as they move poleward. The observations show how cusp/cleft aurora and transient poleward-moving auroral forms appear in the ESR data and the conditions which cause enhanced 630 nm emission in the transients: they are an important ®rst step in enabling the ESR to identify these features away from the winter solstice when supporting auroral observations are not available.

On the importance of interplanetary magnetic field ∣By∣ on polar cap patch formation

“…Around the black vertical lines, the electron density showed clear enhancements, which matched well to the events seen propagating poleward in the low‐elevation beam, while the ion temperatures were also enhanced, indicating convection electric field enhancements [e.g., Pitout et al , 2002]. The electron temperature decreased at the time that the patches passed through the field‐aligned beam, consistent with previous observations of high electron density and low electron temperature in the polar cap patches reported by Lockwood et al [2000, 2005a, 2005b] and Moen et al [2004].…”

“…The early work of Elphic et al [1990] demonstrated that ionospheric flow bursts measured by European Incoherent Scatter (EISCAT) were associated with FTEs observed by the International Sun‐Earth Explorer (ISEE) and the first magnetically conjugate measurements of an FTE by Equator‐S and of ionospheric flow bursts by SuperDARN were presented by Neudegg et al [1999]. Poleward moving events or poleward moving flow channels, observed by the EISCAT radar, are widely accepted to be associated with bursts in the rate of dayside magnetopause reconnection generating new open flux [e.g., Davis and Lockwood , 1996; Lockwood et al , 1993a, 2000, 2001a, 2001b, 2005a, 2005b; Pitout et al , 2002; Oksavik et al , 2004, 2005; Rinne et al , 2010]. Poleward moving plasma concentration enhancements (sometimes called “polar cap patches”) [ Davies et al , 2002] are frequently observed in the F region of the polar ionosphere during periods of southward interplanetary magnetic field (IMF), and are also thought to be associated with bursty reconnection [e.g., Lockwood and Carlson , 1992; Carlson et al , 2002, 2004].…”

“…The difficulty with patch segmentation is to explain the low‐density regions between patches [ Lockwood et al , 2005b]. The mechanisms that have been suggested to segment the intake of cold solar EUV ionized plasma into the cusp throat can be divided into three families [ Lockwood et al , 2005b; Oksavik et al , 2006; Moen et al , 2006]: (1) IMF regulation of the cusp convection pattern, causing alternating intake of high‐ and low‐density plasma [ Anderson et al , 1988; Rodger et al , 1994; Milan et al , 2002]; (2) plasma depletion within flow burst channels due to enhanced recombination associated with newly opened magnetic flux tubes [ Rodger et al , 1994; Sojka et al , 1993; Valladares et al , 1994; Pitout and Blelly , 2003; Pitout et al , 2004]; and (3) plasma structuring by transient reconnection where the open closed boundary (OCB) leaps equatorward to a region of higher‐density plasma, followed by poleward relaxation of that boundary carrying with it the high‐density plasma accelerated into the polar flow [ Lockwood and Carlson , 1992; Lockwood et al , 2000; Carlson et al , 2002, 2004, 2006]. Sojka et al [1993] also suggested that the “tongue of ionization (TOI)” can provide a source of patches.…”

HF radar polar patch and its relation with the cusp during BY‐dominated IMF: Simultaneous observations at two altitudes

“…The fast plasma flow can cause a rapid depletion of ionization because of the increased O + + N 2 reaction rate associated with ion‐neutral frictional heating [ Schunk et al , 1975]. Another mechanism is the rapid transport of less dense plasma from other MLTs [e.g., Valladares et al , 1996; Lockwood et al , 2000].…”

Energetic particle sounding of the magnetopause: A contribution by Cluster/RAPID

“…Observations reported in this paper occurred during a period of extended small northward IMF Bz, with generally larger By and Bx. Opgenoorth et al [2001] have suggested a minor excursion of Bz toward small values between 1305 and 1312 UT which, together with a negative excursion of By, allowed for the occurrence of subsolar (Bz southern type) reconnection in the northern hemisphere prenoon region [ Lockwood et al , 2000].…”