Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsm51b2096n&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SM51B-2096
Physics
[2704] Magnetospheric Physics / Auroral Phenomena, [2723] Magnetospheric Physics / Magnetic Reconnection, [2764] Magnetospheric Physics / Plasma Sheet, [2790] Magnetospheric Physics / Substorms
Scientific paper
The poleward portion of the auroral oval frequently shows intense, transient auroral disturbances, including poleward boundary intensifications (PBIs) forming along the poleward boundary of the oval and auroral streamers extending equatorward from PBIs. Such auroral activity is suggested to correspond to plasma sheet flow bursts, which play an important role in plasma transport in the magnetotail. Our recent analysis using THEMIS all-sky imager (ASI) data showed that such high-latitude aurora activity occurs near the end of the substorm growth phase and leads to substorm auroral onset. The precursor streamer sequence was also supported by in-situ spacecraft and radar measurements, suggesting that enhanced transient plasma flows toward the near-Earth plasma sheet are crucial to lead to substorm onset. However, the auroral sequence leads to an important question, which is the magnetotail source region of the PBI and pre-onset plasma sheet flow that initiate the precursor auroral sequence. The present study investigates the dominant magnetotail reconnection region (distant neutral line (DNL) or near-earth neutral line (NENL)) leading to the pre-onset PBIs and streamers, using the THEMIS/ARTEMIS spacecraft coordinated with the high-resolution THEMIS ASI array. Moreover, quiet-time, growth-phase and expansion-phase PBIs are analyzed to determine similarities and differences in the flow direction and properties for different magnetic conditions. Two events show distinct earthward flow bursts in the mid- and distant-tail associated with pre-onset PBIs. Such a flow direction with an equatorward-propagating streamer would suggest that pre-onset enhanced flow channels leading to precursor PBIs may originate from further downtail, namely DNL. Additional examples illustrate differences of the flow direction during post-onset and quiet times and the parameter related to the equatorward extent of streamers. In these examples, flows during post-onset PBIs are directed tailward, while the flows during quiet-time PBIs are directed earthward in a similar way to the pre-onset flows. Such a time evolution of the flow direction would suggest that the DNL prevails during quiet times until the pre-onset phase, and that the NENL becomes active during expansion phase and overcomes the DNL. These examples also suggest that streamers associated with lower entropy flow channels extend further equatorward than less-reduced entropy flows.
Angelopoulos Vassilis
Auster H.
Bonnell J. W.
Donovan Eric
Larson Davin E.
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