Is the Current Disruption Region the Genesis Region for the Substorm X-Line?

Details Is the Current Disruption Region the Genesis Region for the Substorm X-Line? Is the Current Disruption Region the Genesis Region for the Substorm X-Line?

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmsm52b..08e&link_type=abstract

American Geophysical Union, Fall Meeting 2002, abstract #SM52B-08

Physics

2740 Magnetospheric Configuration And Dynamics, 2788 Storms And Substorms

Scientific paper

The nominal location for the substorm near-Earth X-line (NEXL) has been found to be outside but near 20RE in the tail. The modified Near-Earth Neutral Line (NENL) model postulates that braking of fast, earthward flows and pile up of magnetic flux accounts for the initiation of the substorm current wedge and dipolarization within 10RE, and its tailward expansion. Current disruption (CD) and CD-like magnetic activity accompanies dipolarization in the 8--12RE range and commences in close temporal proximity to auroral onset. We report here, based on Geotail observations, that 70% of CD-like activity in the 9 (perigee) to 12 RE range of the pre-midnight and midnight plasma sheet begins in the absence of earthward flow. In only 20% of the cases does CD-like activity start coincident with arrival of earthward flow. Indeed, in a like number of cases, CD-like activity starts coincident with a clear signal (tailward Poynting flux) arriving from nearer Earth. When auroral coverage is adequate, we have shown that these substorms proceed in two stages, with reconnection occurring during the second stage. But this is not the entire story. We note three pieces of evidence that lead us to suggest that the CD region is the genesis region for the NEXL. (1) In 10% of CD-like events, magnetic fluctuations commence like typical CD events, but rather than dipolarizing, the magnetic field diminishes. Whereas the distribution for the typical CD signature shows a strong peak near 10RE, these hybrid events are more uniformly distributed between 9 and 19 RE, and from 13--19RE represent 30% of all CD-like activity. (2) Signatures of a substorm NEXL earthward of Geotail can be found as near Earth as 13RE on occasion. (3) A minimum in equatorial magnetic field strength is believed to evolve during the substorm growth phase near 10RE. Hau and Wolf [JGR, 92, 4745, 1987] discuss how, in the presence of resistivity, the B-minimum structure diffuses tailward, and the minimum deepens, until a NEXL forms. Collocation of CD with the growth-phase minimum in B and the subsequent emergence of reconnection (NEXL genesis) removes some of the major observational difficulties associated with both the Near-Earth Current Disruption and the modified NENL models. We discuss the advantages of such unification.

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