Abstract
Abstract. Substorm current wedge (SCW) formation is associated with global magnetic field reconfiguration during substorm expansion. We combine a two-loop model SCW (SCW2L) with a background magnetic field model to investigate distortion of the ionospheric footpoint pattern in response to changes of different SCW2L parameters. The SCW-related plasma sheet footprint shift results in formation of a pattern resembling an auroral bulge, the poleward expansion of which is controlled primarily by the total current in the region 1 sense current loop (I1). The magnitude of the footprint latitudinal shift may reach ∼ 10° corrected geomagnetic latitude (CGLat) during strong substorms (I1= 2 MA). A strong helical magnetic field around the field-aligned current generates a surge-like region with embedded spiral structures, associated with a westward traveling surge (WTS) at the western end of the SCW. The helical field may also contribute to rotation of the ionospheric projection of narrow plasma streams (auroral streamers). Other parameters, including the total current in the second (region 2 sense) loop, were found to be of secondary importance. Analyzing two consecutive dipolarizations on 17 March 2010, we used magnetic variation data obtained from a dense midlatitude ground network and several magnetospheric spacecraft, as well as the adaptive AM03 model, to specify SCW2L parameters, which allowed us to predict the magnitude of poleward auroral expansion. Auroral observations made during the two substorm activations demonstrate that the SCW2L combined with the AM03 model nicely describes the azimuthal progression and the observed magnitude of the auroral expansion. This finding indicates that the SCW-related distortions are responsible for much of the observed global development of bright auroras.
Highlights
Electric currents flowing along the magnetic field lines, known since K
We show two families of reference equatorial equidistant arcs mapped to the ionosphere using only the background model field as well as the background and the SCW2L field
Poleward expansion is clearly limited, and the latitudinal interval of intensified auroras is estimated to be roughly about ∼ 3◦ during the first activation. This is comparable to the ∼ 3.5◦ poleward expansion predicted by SCW2L the model
Summary
Electric currents flowing along the magnetic field lines (fieldaligned currents, FACs), known since K. The SCW concept was first introduced by McPherron et al (1973), who described an SCW as a 3-D current system, connecting the disrupted tail current with the ionosphere via the downward/upward R1 type FACs (at the dawn/dusk sides, respectively) Their single-loop SCW model qualitatively explained observed magnetotail field dipolarizations, and provided realistic magnitudes and polarity of bay-like magnetic field variations at midlatitudes. The quantitative evaluation of the role of FACs in variable patterns of field-line mapping during substorm-related dipolarization and auroral expansion, an interesting exercise, has been addressed in a few previous papers Those studies, utilized highly idealized SCW models with infinitely thin line currents flowing along either dipolar or early empirical model field lines (Vasilyev et al, 1986; Kaufmann and Larson, 1989; Tsyganenko, 1997), or used dipolarization effects to represent a global tail reconfiguration (Kubyshkina et al, 2011).
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