Birth and life of auroral arcs embedded in the evening auroral oval convection: A critical comparison of observations with theory

Abstract

[1] We present and analyze data on auroral arcs obtained during a pass of the FAST satellite over the field-of-view of the all-sky camera at Ft. Simpson (Canada), supported by ground-based magnetometer and SuperDARN radar data, and plasma data from THEMIS-A near the source region of the auroral currents. The auroral event took place at 19:00 MLT during substorm activity further east. Active auroral arcs were present over six degrees in latitude moving equatorward with significant changes in brightness and structure. New arcs were forming continuously at the polar border of the auroral oval which was marked by an Alfvénic arc. The data analysis revealed that the equatorward drift of the arcs was in part due to convective motion of the plasma frame but was rather dominated by proper motions of the arcs. Interpretation of these findings in the framework of theoretical work by one of the authors reproduces quantitatively the observed proper motion as a consequence of the progressive erosion of magnetic shear stresses. Most important was the possibility to deduce the interaction time scale between arc and source region. On average it corresponded to about six to eight transit times of an Alfvén wave between arc and source plasma or two fundamental eigenperiods of toroidal mode or azimuthally polarized Alfvén waves. However, large variations of the interaction times and corresponding proper motions were found. They are attributed to temporal and spatial variations of the energy input from the source plasma. The more remarkable is the fact that analysis on the basis of a quasistationary model produces consistent results. The progressive release of shear stresses during the equatorward motion of the arcs leads to the conclusion that they are dying after having reached the maximum of the poleward Pedersen current.

Substorm simulation: Quiet and N‐S arcs preceding auroral breakup

“…One of such auroral activity is the quiet and homogeneous arc [ Akasofu , ] (hereinafter, referred to as a quiet arc). The quiet arc tends to lie longitudinally, and multiple quiet arcs are also observed to form within the auroral oval before the onset of substorm expansion [e.g., Akasofu , ; Haerendel et al ., ]. These quiet arcs have a thickness of around 100 km [ Johnson et al ., ] and move equatorward at a speed of 2–6 km min −1 , which is consistent in part with convective motion [ Lassen et al ., ; Haerendel et al ., ].…”

“…The quiet arc tends to lie longitudinally, and multiple quiet arcs are also observed to form within the auroral oval before the onset of substorm expansion [e.g., Akasofu , ; Haerendel et al ., ]. These quiet arcs have a thickness of around 100 km [ Johnson et al ., ] and move equatorward at a speed of 2–6 km min −1 , which is consistent in part with convective motion [ Lassen et al ., ; Haerendel et al ., ]. The electron precipitation has a sharp poleward boundary, and the boundary moves equatorward when the IMF is turned southward [ Hoffman and Burch , ].…”

“…The electron precipitation has a sharp poleward boundary, and the boundary moves equatorward when the IMF is turned southward [ Hoffman and Burch , ]. The quiet arcs are caused by precipitating electrons with a peak energy of approximately 1–5 keV, so that they are most likely accelerated by electric fields parallel to magnetic field lines [ Meng , ; Lassen et al ., ; Haerendel et al ., ]. Lessard et al .…”

M-I coupling scales and energy dumping

“…First, it should be noted that widths of the order 100 km or more are quite possible with the dynamic coupling scale λ arc or λ t as defined above. Values of K , as low as 10 −10 mho/m 2 [ Haerendel , ], and integral values of the Alfvén wave impedance, Q , of the order of 0.25 ohm [ Haerendel et al ., ] are not unusual. These numbers correspond to a width of 140 km.…”

Frozen-in condition for ions and electrons: implication on magnetic flux transport by dipolarizing flux bundles

“…In contrast to the above viewpoint, there are several proposed processes in which substorm dipolarization in the near-Earth region is produced close to the Earth (e.g., Lui et al 1988;Lopez et al 1988Lopez et al , 1989Roux et al 1991;Lui 1991Lui , 2011Lui , 2013Cheng and Lui 1998;Henderson 1994Henderson , 2009Liu 1997;Cheng 2004;Liu et al 2012;Haerendel et al 2012;Haerendel and Frey 2014;Akasofu 2013Akasofu , 2017. For comparison between these two viewpoints, it is important to address how effective are DFBs in transporting magnetic flux from the mid-tail region to the near-Earth region.…”