FG13. Modes of Solar WInd-Magnetosphere Energy Transfer
Conveners: Larry Kepko <larry.kepko@ unh.edu> and Bob McPherron <rmcpherron@ igpp.ucla.edu>
The response of the magnetosphere to the solar wind is manifested in variety of ways. We used to think there were substorms and storms, and storms were simply a superposition of substorms. Today we know the situation is more complex. It is possible to identify at least three main modes: substorms, steady magnetospheric convection (SMC), and sawtooth injection events. In addition during these events we can identify pseudo breakups and poleward boundary intensifications (PBI). We are still in the process of identifying the characteristic behavior that identifies these various events as separate phenomena. We do not completely understand the solar wind conditions or internal state of the magnetospheric that allows a particular mode. We do not know what causes a transition from one to another, although solar wind velocity seems to play an important role. This focus group had two breakout sessions to discuss these phenomena. Most of the contributions dealt with sawtooth injection events, SMC, and PBI
This was the first year of the focus group. Speakers were particularly encouraged to address one or more aspects of the particular transport mode:
(1) The particular state of solar wind conditions associated with the response mode;
(2) The internal state of the magnetosphere during the response mode;
(3) What causes the transition into or out of a mode.
The focus group held two breakout sessions. The first session examined both large scale and long duration magnetospheric convection events. This includes steady magnetospheric convection, Sawtooth events, and High-Intensity, Long Duration, Continuous Auroral Activity (HILDCAA) events. The second session discussed small scale and/or short duration convective events, such as poleward boundary intensifications and pseudo breakups.
Mike Henderson presented a tutorial on sawtooth events. In the course of discussing the properties of sawtooth events, two points generated significant discussion. It was noted that the period of sawtooth events varies greatly, and that it was unknown how this period was established. Rather than a single period, sawtooth events exhibited a continuum of periods. Mike also discussed the commonly held belief that injections during sawtooth events were dispersionless over most local times. He pointed out that this wasn't the case; dispersion was observed away from midnight. The consensus opinion is that sawtooth injections penetrate deeply into the inner magnetosphere, and therefore appear less dispersed at different local times. That is, the commonly held belief that sawtooth events are global dispersionless injections is not correct.
The majority of the discussion centered on the primary question of what forces the magnetosphere into the quasi-continuous steady magnetospheric convection events or quasi-periodic sawtooth injections. Several speakers (Borovsky, DeJong, Cai) discussed the solar wind conditions during SMCs and sawtooth events, and noted that given the same solar wind IMF, the solar wind velocity dictates which of the two modes is dominant. It was not clear, however, what the physical mechanisms were that dictated the magnetospheric behavior. There was further discussion on what determined the period of sawtooth injections. It was generally agreed that the periodicity occurred over a continuum from approximately 1 to 4 hours, with a peak in the distribution near 2 hours. Most participants believed the period was determined by internal magnetospheric properties. Larry Lyons presented work suggesting the periodicity was not internal to the magnetosphere, and was driven by periodicities in the solar wind and IMF.
Near the end of the second breakout we discussed the role of global MHD simulations in answering the 2 main questions brought up during the discussions: (1) What are the solar wind conditions that determine the SMC vs. sawtooth mode of response; (2) What determines the periodicity of sawtooth injections? Both LFM and Open GGCM representatives agreed to simulate events for the next GEM meeting, in particular events observed by THEMIS. Furthermore, events lists compiled by different speakers (in particular McPherron and DeJong) will be collected and available to the GEM community for further studies.
The Focus Group leaders and Dr. Tung-Shin Hsu have organized a special session SM06 at the December 2008 meeting of the American Geophysical Union in San Francisco. The title of this session is "Modes of Solar Wind-Magnetosphere Energy Transfer". Thirty eight abstracts will be presented at this session.