Motion of the dayside polar cap boundary during substorm cycles: II. Generation of poleward-moving events and polar cap patches by pulses in the magnetopause reconnection rate

Abstract

Abstract. Using data from the EISCAT (European Incoherent Scatter) VHF and CUTLASS (Co-operative UK TwinLocated Auroral Sounding System) HF radars, we study the formation of ionospheric polar cap patches and their relationship to the magnetopause reconnection pulses identified in the companion paper by Lockwood et al. (2005). It is shown that the poleward-moving, high-concentration plasma patches observed in the ionosphere by EISCAT on 23 November 1999, as reported by Davies et al. (2002), were often associated with corresponding reconnection rate pulses. However, not all such pulses generated a patch and only within a limited MLT range (11:00-12:00 MLT) did a patch result from a reconnection pulse. Three proposed mechanisms for the production of patches, and of the concentration minima that separate them, are analysed and evaluated: (1) concentration enhancement within the patches by cusp/cleft precipitation; (2) plasma depletion in the minima between the patches by fast plasma flows; and (3) intermittent injection of photoionisation-enhanced plasma into the polar cap. We devise a test to distinguish between the effects of these mechanisms. Some of the events repeat too frequently to apply the test. Others have sufficiently long repeat periods and mechanism (3) is shown to be the only explanation of three of the longer-lived patches seen on this day. However, effect (2) also appears to contribute to some events. We conclude that plasma concentration gradients on the edges of the larger patches arise mainly from local time variations in the subauroral plasma, via the mechanism proposed by Lockwood et al. (2000).

Analysis of close conjunctions between dayside polar cap airglow patches and flow channels by all-sky imager and DMSP

“…5d, where fast flows of >~1500 m/s are predominantly seen during large |By| and small |Bz|. Under a large IMF By condition, it has been observed that mesoscale flows in the cusp region propagate poleward, and this has been suggested to be due to large tension force in dayside reconnection under large IMF |By| (Sandholt et al 2004;Lockwood et al 2005;Rinne et al 2010;Zhang et al 2011Zhang et al , 2013b. Combining with our observation, these mesoscale flows may move far away from the noon meridian and then propagate into the polar cap.…”

“…It is known that for large, negative By, the boundary between dawn and dusk convection cells is shifted toward dawn in the southern hemisphere (Reiff and Burch 1985;Lyons et al 1996) and toward dusk in the northern hemisphere (Weimer 2005). Additionally, the previous case studies have shown the IMF By-related formation of polar cap patches (Lockwood et al 2005;Zhang et al 2011Zhang et al , 2013b. In our study, we statistically showed that the departing points of these patches from the poleward boundary of auroral oval are associated with IMF By, and moreover, the airglow patches that depart at <9 MLT are with strong negative By.…”

“…Recently, Goodwin et al (2015) used SWARM data to document the intake of solar-EUV plasma through fast flow channels. Those fast flows are also associated with poleward moving auroral forms (PMAFs), which are considered to be an ionospheric signature of localized dayside magnetic reconnection and flux transfer events (FTEs) (Moen et al 1995;Provan et al 1998McWilliams et al 2000;Milan et al 2000;Sandholt and Farrugia 2003;Lockwood et al 2005;Carlson et al 2006;Zhang et al 2011Zhang et al , 2013b. PMAFs initiate near the poleward boundary of the dayside auroral oval and then propagate poleward before fading away (Lorentzen et al 2010).…”

EISCAT observations of plasma patches at sub-auroral cusp latitudes

“…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 in three families:…”

“…Several observation techniques have been used for patch studies, such as direct electron density measurements by ionosonde (Buchau et al, 1983;Dandekar and Bullett, 1999;Dandekar, 2002), incoherent scatter radar (e.g. Valladares et al, 1994Valladares et al, , 1996Valladares et al, , 1998Carlson et al, 2002Carlson et al, , 2004Carlson et al, , 2006Lockwood and Carlson, 1992;Lockwood et al, 2005aLockwood et al, , 2005bOksavik et al, 2006a), and ionospheric tomography (Walker et al, 1999;Pryse et al, 2004), 630.0-nm airglow intensity that is proportional to electron density (Weber et al, 1984;Harris, 1995, 1996;McEwen et al, , 2004Lorentzen et al, 2004), and more indirectly by HF coherent scatter radars of the SuperDARN network (Rodger et al, 1994;Ogawa et al, 1998;Rodger and Graham, 1996;Milan et al, 2002Milan et al, , 2005Oksavik et al, 2006a). The latter technique does not provide a measurement of the electron density but gives the backscatter power from irregularities on electron density gradient associated E×B drift instability (Tsunoda, 1988;Chaturvedi and Ossakow, 1981).…”