Properties of localized, high latitude, dayside aurora

Abstract

[1] The FUV instrument on the IMAGE spacecraft frequently observes intense ultraviolet (UV) emission from a localized dayside region poleward of the general auroral oval location. One type of these emissions has been described as the signature of direct proton precipitation into the cusp after lobe reconnection during northward interplanetary magnetic field (IMF) and high solar wind dynamic pressure periods . Here we describe a completely different type of high latitude aurora, which does not show any signature of precipitating protons. It also occurs during northward IMF conditions however, only during periods of very low solar wind dynamic pressure. It occurs at a much higher geomagnetic latitude than the normal cusp location and only during periods of positive IMF B y . The intensity of the UV emission is somewhat anti-correlated with the solar wind dynamic pressure, much in contrast to the cusp emission. The brightness of the localized emission changes rapidly on time scales between 30 and 70 minutes without corresponding changes in solar wind properties. Coincident measurements by the FAST spacecraft verify that this is not the cusp, that ion precipitation is absent in these regions, and that strong precipitation of field-aligned accelerated electrons causes the aurora. We interpret this aurora as the optical signature of electron precipitation in the upward leg of a current system which closes the downward leg of the current system into the cusp in the ionosphere.

Polar cap convection/precipitation states during Earth passage of two ICMEs at solar minimum

“…The absence of polar arcs in case 2 (20 November 2007) may be partly due to the high plasma density density (10-20 cm −3 ) in this ICME case. This is consistent with a previous result indicating that polar arcs under the prevailing southwest IMF conditions are favoured by low solar wind density (Frey et al, 2003). This effect comes in addition to the negative B x component which is unfavourable for lobe reconnection to occur in the Southern Hemisphere.…”

The asymmetric geospace as displayed during the geomagnetic storm on August 17, 2001

“…Figure 13 shows IMAGE WIC images (northern hemisphere) from 16:19 UT to 16:49 UT, where this spot is seen clearly detached from the 15 auroral oval in all the ten images (see also the video, uploaded as auxiliary data). Frey et al (2003bFrey et al ( , 2004) explored statistically the characteristics of this high-latitude dayside aurora (HiLDA) and concluded that the major driving process was high-latitude (lobe) reconnection and that it occurred preferentially in the summer hemisphere.…”

The role of external triggers in flow shear arcs in the dayside aurora

“…From the direct observations of the electron precipitation flux (reaching 5 keV energy) on 4 December 1999 we infer that similar electron precipitation events, although mostly at lower energies, are present in the case of the type 3 PMAFs. Statistical observations from space indicate that such arcs show a maximum during the sunlit summer months (Frey et al, 2004). The present ground observations show that, given the appropriate interplanetary triggers, intense flow shear arcs are also present in winter.…”

“…This interpretation in terms of flow shear arcs (FSAs) is most easily applicable to the strongly distorted dusk-centered convection cells appearing during positive IMF B y conditions. According to Frey et al (2004) such high-latitude dayside arcs (HiLDAs) "shows a maximum in the Northern Hemisphere during sunlit summer months and an almost complete absence in the dark winter." Representative examples of winter PBIs are the 09:20 UT brightening arcs observed in both B y >0 cases reported here (3 December 1997 and4 December 1999).…”

“…Emphasis is placed on the role played by local ionospheric flow shears. It is well known that dayside aurorae and polar cap (sun-aligned) arcs may appear in regions of upward-directed Birkeland currents required by current continuity in regions of local flow shear (Reiff et al, 1978;Carlson et al, 1988;Sandholt et al, 1990;Frey et al, 2003;Oksavik et al, 2004Oksavik et al, , 2005. Here we shall investigate the role played by such flow shears in the excitation of the PMAF substructure we refer to as equatorward boundary intensifications (EBIs) and poleward boundary intensifications (PBIs).…”