Dayside and nightside contributions to cross-polar cap potential variations: the 20 March 2001 ICME case

Abstract

Abstract. We investigate the association between temporalspatial structure of polar cap convection and auroral electrojet intensifications during a 5-h-long interval of strong forcing of the magnetosphere by an ICME/Magnetic cloud on 20 March 2001. We use data from coordinated ground-satellite observations in the 15:00-20:00 MLT sector. We take advantage of the good latitudinal coverage in the polar cap and in the auroral zone of the IMAGE chain of ground magnetometers in Svalbard -Scandinavia -Russia and the stable magnetic field conditions in ICMEs. The electrojet events are characterized by a sequence of 10 min-long AL excursions to −1000/−1500 nT followed by poleward expansions and auroral streamers. These events are superimposed on a high disturbance level when the AL index remains around −500 nT for several hours. These signatures are different from those appearing in classical substorms, most notably the absence of a complete recovery phase when AL usually reaches above −100 nT. We concentrate on polar cap convection in both hemispheres (DMSP F13 data) in relation to the ICME B y conditions, electrojet intensifications, and the global UV auroral configuration obtained from the IM-AGE spacecraft. The temporal evolution of convection properties such as the cross-polar cap potential (CPCP) drop and flow channels at the dawn/dusk polar cap (PC) boundaries around the time of the electrojet events are investigated. This approach allows us to distinguish between dayside (magnetopause reconnection) and nightside (magnetotail reconnection) sources of the PC convection events within the context of the expanding-contracting model of high-latitude convection in the Dungey cycle. Inter-hemispheric symmetries/asymmetries in the presence of newly-discovered convection channels at the dawn or dusk side PC boundaries are determined.

The pulsed nature of the nightside contribution to polar cap convection: repetitive substorm activity under steady interplanetary driving

“…We point out that this flow event is observed at the time of PBI/streamer activity in the same MLT sector in the Northern (summer) Hemisphere. This is an interesting observation which is consistent with the ideas and observations of Andalsvik et al (2011Andalsvik et al ( , 2012, but the relationship between these polar cap flow channel events and substorm activity (substorm onset, PBIs, streamers, and BBFs) is still uncertain and needs further documentation (see e.g., Lyons et al, 2012). Figure 1 shows interplanetary (IP) data from spacecraft Wind during passage of an ICME on 30 May 2005.…”

“…During winter conditions, when large conductivity gradients are present at the polar cap boundary, the condition of enhanced antisunward convection in the night sector of the polar cap is expected to give rise to flow channel FC 3 (see Andalsvik et al, 2011, their Fig. 1).…”

“…The relative contributions to the cross-polar cap potential (CPCP) from the dayside (CPCP/day) and nightside (CPCP/night) sources during intervals of substorm activity have been studied in recent years, applying different observational techniques (Bristow et al, 2004;Lockwood et al, 2009;Kullen et al, 2010;Gordeev et al, 2011;Andalsvik et al, 2011Andalsvik et al, , 2012. Fox et al (1999), Grocott et al (2002) and Provan et al (2004) estimated the convection response to isolated substorms by continuous observations, applying a ground-based radar technique.…”

M–I coupling across the auroral oval at dusk and midnight: repetitive substorm activity driven by interplanetary coronal mass ejections (CMEs)

“…However, during the expansion phase of certain substorm activities, including the category we discuss in this paper (see e.g. Cai et al, 2006;Henderson et al, 2006;and Andalsvik et al, 2011), the WEJ and the Harang reversal boundary expand westward across the 18:00 MLT meridian (Andalsvik et al, 2011), as we shall also demonstrate below. According to Nielsen and Greenwald (1979) the Harang discontinuity (our Harang reversal boundary) tended to be observed by the Stare radar in the 21:00-24:00 MLT sector, but it was observed as early as 18:30 MLT under very disturbed conditions.…”

“…The UT interval we study is determined by the fact that the IMAGE chain of magnetometers typically enter the Harang region at ∼ 14:30-16:00 UT (∼ 17:00-18:00 MLT) under the strong solar wind forcing conditions we study (Andalsvik et al, 2011Sandholt et al, 2012).…”