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On the formation and origin of substorm growth phase/onset auroral arcs inferred from conjugate space‐ground observations
Author(s) -
Motoba T.,
Ohtani S.,
Anderson B. J.,
Korth H.,
Mitchell D.,
Lanzerotti L. J.,
Shiokawa K.,
Connors M.,
Kletzing C. A.,
Reeves G. D.
Publication year - 2015
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2015ja021676
Subject(s) - substorm , geophysics , physics , magnetosphere , ionosphere , conjugate points , arc (geometry) , phase (matter) , magnetic field , astrophysics , geology , geometry , mathematics , quantum mechanics
Magnetotail processes and structures related to substorm growth phase/onset auroral arcs remain poorly understood mostly due to the lack of adequate observations. In this study we make a comparison between ground‐based optical measurements of the premidnight growth phase/onset arcs at subauroral latitudes and magnetically conjugate measurements made by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) at ~780 km in altitude and by the Van Allen Probe B (RBSP‐B) spacecraft crossing L values of ~5.0–5.6 in the premidnight inner tail region. The conjugate observations offer a unique opportunity to examine the detailed features of the arc location relative to large‐scale Birkeland currents and of the magnetospheric counterpart. Our main findings include (1) at the early stage of the growth phase the quiet auroral arc emerged ~4.3° equatorward of the boundary between the downward Region 2 (R2) and upward Region 1 (R1) currents; (2) shortly before the auroral breakup (poleward auroral expansion) the latitudinal separation between the arc and the R1/R2 demarcation narrowed to ~1.0°; (3) RBSP‐B observed a magnetic field signature of a local upward field‐aligned current (FAC) connecting the arc with the near‐Earth tail when the spacecraft footprint was very close to the arc; and (4) the upward FAC signature was located on the tailward side of a local plasma pressure increase confined near L ~5.2–5.4. These findings strongly suggest that the premidnight arc is connected to highly localized pressure gradients embedded in the near‐tail R2 source region via the local upward FAC.