Physics Department Seminar University of Alaska Fairbanks

J O U R N A L    C L U B

Storm-substorm relationships: The role of magnetic reconnection at the substorm expansion onset and during the ring current injection
Joe Kan
Geophysical Institute

Storms and substorms are both driven by earthward convection enhanced by magnetic reconnection in the plasma sheet. Substorm is driven by the M/DTXL (mid-to-distant-tail-X-line) beyond ~30 Re radial distance during the growth phase and by the NEXL (near-Earth-X-line) within ~30 Re during the expansion phase. Cowling current intensifies by the blockage of the northward Hall current due to lack of the radial currents or azimuthal pressure gradient in the midnight sector of the near-Earth plasma sheet. Interaction of intensified Cowling current loop with the near-Earth plasma sheet causes dipolarization. Dipolarization-induced thinning, tailward of the dipolarizing region, can cause the NEXL formation to drive the substorm expansion phase. Further intensifications of the Cowling current in the substorm current wedge, the tailward expansion of dipolarization region and the expansion of the auroral bulge are all driven by the NEXL during the expansion phase.

A ring current injection conjecture is proposed to order the storm intensity based on the X-line location. Ring current intensifies by convection driven by the NEXL during the substorm expansion phase to produce intense storms. On the other hand, ring current intensifies by the MDXL in the absence of the substorms to produce small storms. Results obtained previously with the Rice Convection Model coupled to an equilibrium magnetic-field solver indicate that the entropy function PV5/3 must be non-adiabatically reduced if plasma-sheet flux tubes are to be injected into the heart of the storm-time ring current. Magnetic reconnection in the plasma sheet reduces PV5/3, and the effect is stronger if the X-line is closer to the Earth. It is estimated that an X-line must form within ~25 RE of Earth in order for the reconnected flux tubes to penetrate deep into the heart of the ring current to produce major storms.

Friday, 6 October 2006
Globe Room, Elvey Building
3:45 PM