Abstract
Abstract. We document the detailed spatio-temporal structure of the dayside aurora during intervals of ongoing dayside magnetopause reconnection, primarily during interplanetary magnetic field (IMF) Bz≤0 conditions. The present study is based on ground auroral observations in combination with particle precipitation data from a DMSP spacecraft. We describe auroral forms corresponding to the following particle precipitation regimes identified by Newell and Meng (1994): (i) central plasma sheet (CPS), (ii) precipitation void, (iii) dayside boundary plasma sheet (BPS), and (iv) cusp (LLBL/cusp/mantle). Two distinctly different auroral configurations are observed, corresponding to different regimes of the IMF clock angle (θ) and the ∣By∣/∣Bz∣ ratio. Two regimes are defined. In regime (I) θ lies within ∼ 90–135° and ∣By∣/∣Bz∣>1 (By-dominated), while in regime (II) θ is in the range 135°–180° and ∣By∣/∣Bz∣<1 (Bz-dominated). Within regime (I) the auroral response to reconnection events typically progresses from lower to higher latitudes in stages as indicated below: (A) equatorward boundary intensifications (EBIs): sequential brightenings of closely spaced, fragmented, rayed bands (BPS aurora) within the ∼08:00–15:00 MLT sector, each of which are moving noonward/sunward, (B) poleward moving auroral forms (PMAFs): forms expanding westward from the postnoon side (By>0) and later appearing as a poleward expanding form in the convection throat in the ∼09:00–12:00 MLT sector, with a fading phase in the regime of mantle precipitation. During strongly southward IMF conditions (regime II), the intense PMAF activity is replaced by a more latitudinally restricted, but longitudinally wide aurora of moderate intensity. The latter auroral state is accompanied by a 2-cell convection pattern which is rather symmetrical about noon. This state is very different from the convection/FAC configuration present during IMF regime (I), with its strong zonal flows (convection current), more intense FAC sheets and PMAF activity in the midday sector. The strong IMF regulation of the dayside BPS aurora, consisting of keV electrons, and its location with respect to the green line auroral gap (precipitation void), indicate that it is an important signature of the reconnection process, located on open boundary layer field lines. The observed longitudinal bifurcation of the auroral brightenings (EBIs) preceding PMAFs is consistent with antiparallel magnetopause reconnection. Key words. Magnetospheric physics (auroral phenomena magnetopause, cusp, and boundary layers: solar windmagnetosphere interactions) – Ionosphere (particle precipitation)
Highlights
Large statistical surveys have demonstrated that the various plasma populations of the dayside magnetosphere are manifested in corresponding particle precipitation regimes, as observed from satellites in polar orbit (Newell and Meng, 1988; Newell et al, 1989; Newell and Meng, 1994)
It consists of the following scenario in the ∼08:00–15:00 MLT/70-80◦ MLAT sector: (i) near-simultaneous activations of boundary plasma sheet (BPS) auroral forms in the pre- and postnoon sectors, each of which is moving noonward, followed by (ii), poleward moving auroral forms (PMAFs) which are moving northwest or northeast, depending on the By polarity, and (iii), a fading phase, characterized by predominantly poleward motion, within the regime of mantle precipitation
While meridian scanning photometer (MSP) data are shown in two wavelength channels (630.0 and 557.7 nm) for the whole interval, all-sky camera (ASC) data are shown for more limited intervals, which are selected with the purpose of illustrating main features of the 2-D auroral response
Summary
Large statistical surveys have demonstrated that the various plasma populations of the dayside magnetosphere are manifested in corresponding particle precipitation regimes, as observed from satellites in polar orbit (Newell and Meng, 1988; Newell et al, 1989; Newell and Meng, 1994). According to the observations reported in this paper, the most spectacular auroral response to reconnection events occurs for IMF clock angles in the intermediate regime (|By|/|Bz|≥1). It consists of the following scenario in the ∼08:00–15:00 MLT/70-80◦ MLAT sector: (i) near-simultaneous activations of BPS auroral forms in the pre- and postnoon sectors, each of which is moving noonward, followed by (ii), PMAFs which are moving northwest or northeast, depending on the By polarity, and (iii), a fading phase, characterized by predominantly poleward motion, within the regime of mantle precipitation
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