Dayside aurorae and polar arcs under south-east IMF orientation

Abstract

Abstract. We document a characteristic spatial and temporal structure of the aurora in the postnoon sector present during a 10-h-long interval of very steady southeast IMF orientation (clock angle=135 • ) ending in a sharp south-to-north transition. Focus is placed on the detailed morphology of auroral forms/activities corresponding to merging and lobe convection cells obtained from SuperDARN convection data and Greenland magnetograms. The ground optical instruments at NyÅlesund, Svalbard (76 • MLAT) recorded different auroral forms/activities as the station moved to higher magnetic local times (MLTs) in the 13:00-17:00 MLT sector. Whereas the 13:00-15:00 MLT sector is characterized by classical poleward moving auroral forms (PMAFs) associated with merging cell transients, the aurora in the 15:00-17:00 MLT sector shows instead a characteristic latitudinal bifurcation consisting of standard oval forms and polar arcs, and a corresponding composite pattern of merging and lobe convection cells. The merging and lobe cells respond to the southward and northward IMF transitions by activation/fading and fading/activation, respectively. A sequence of brightening events is characterized by successive activations progressing in latitude from the merging cell regime to the lobe cell regime. Emphasis is placed on the association between polar arc brightenings and the activation of the channel of enhanced sunward flow in the lobe cell. The observations are discussed in relation to recent work on solar wind-magnetosphere-ionosphere interconnection topology.

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

“…These features constitute a central part of the duskcentered merging convection cell. A smaller and weaker lobe cell may be seen inside the merging cell, tailward of the 14:00 MLT meridian (see Sandholt et al, 2006, for further documentation and discussion).…”

Plasma flow channels at the dawn/dusk polar cap boundaries: momentum transfer on old open field lines and the roles of IMF <I>B<sub>y</sub></I> and conductivity gradients

“…We distinguish between the following regimes marked in the figure: (1) postnoon polar cap arcs associated with flow shear, i.e., flow shear arcs (Sandholt et al, 2006a), (2) polar rain and standard B y -related dawn-dusk asymmetrical polar cap convection, (3) C1-type Birkeland current, mantle precipitation and enhanced antisunward convection, (4) C2-type Birkeland current, mantle/LLBL precipitation, and enhanced (1.5 km/s) antisunward convection (FC 2), (5) convection reversal, (6) R2 Birkeland current, CPS precipitation and slow sunward convection.…”

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

“…"the midday gap aurora" (strongly attenuated emission intensity at noon), 2. different variants of PMAF activities appearing on both sides of "the midday gap aurora" at noon, i.e., four variants sorted by MLT sector (prenoon versus postnoon) and IMF B y polarity, 3. polar cap arcs appearing in the two variants NHdusk/B y > 0 (see Sandholt et al, 2006) and SHdusk/B y < 0 (see DMSP Figs. 3, 4, and 6) during B z < 0 conditions, 4. polar rain dropout in the SH-dusk sector of the polar cap where the Birkeland current (our L2 current) is inwarddirected in the IMF B y < 0 case (see the positive B z gradient in Fig.…”

“…Thus the present convection configuration is the composite pattern consisting of merging and convection cells which has been predicted and simulated during intervals of B y -dominated IMF orientations (Reiff and Burch, 1985;Crooker et al, 1998). The associated polar arcs in the lobe cell during intervals of similar IMF conditions has been documented in previous studies (Eriksson et al, 2003;Sandholt et al, 2006). For information on the L1-L2 Birkeland currents we refer to Eriksson et al (2002).…”

“…The outward-directed Birkeland current (L1) in the center of the lobe cell is accompanied by flow shear arcs (Reiff et al, 1978). These polar cap arcs are accelerated polar rain precipitation in the region of a converging electric field (see Carlson and Cowley, 2005;Sandholt et al, 2006).…”