*************************** ** THE GEM MESSENGER ** *************************** Volume 10, Number 38 September 12, 2000 ----------------------------------------------------------------------- 1. Your Input Required for JGR Special Section on Substorm Onset Timing ----------------------------------------------------------------------- From: Shin Ohtani Substorm onset timing has been the main target of the GEM T/S observation working group (see the group report of this issue of the GEM messenger). We are currently considering a special section for JGR on this subject. According to Janet Luhmann, the senior editor of JGR, the review process of papers submitted to a special section is the same as that of regular papers, but they will be published under a special section title to gain higher publicity. Although the T/S observation working group has been focusing on satellite and ground measurements, the special section, if realized, will be open to all approaches including theory and model calculations. Contributions from outside of the group of regular attendees of the GEM workshop will also be welcome. It is, however, uncertain at present whether there exists sufficient interest for proposing this idea to the JGR editorial office. We would be most grateful if you could inform us of your interest at ohtani at jhuapl.edu. Mark Moldwin and Shin Ohtani (Co-chairs) -------------------------------------------------------------------------- 2. GEM Tail/Substorm Observation Working Group Report of the 2000 Snowmass Workshop -------------------------------------------------------------------------- From: Shin Ohtani The T/S observation workshop group had two sessions in Snowmass this year. In one session the working group discussed substorm onset timing focusing on the timing of the Pi2 onset relative to the auroral breakup. The other session was devoted to other substorm-related phenomena with an emphasis on near-Earth substorm processes. I. SUBSTORM ONSET TIMING Substorm onset timing has been the main subject of this working group. Whereas the Pi2 onset is often used as a substorm identifier, its timing relative to the auroral breakup has not been tested statistically until recently. Liou et al. [JGR, 105, p.2495, 2000; also presented at previous GEM workshops] reported that Pi2 onsets observed at a low-latitude ground station tend to be delayed from auroral breakups identified from Polar UVI images. In the session Kawano/Uozumi reported their model calculation of Pi2 propagation in the near-Earth tail. The propagation speed is significantly slower in the plasmasphere because of the high plasma density there, which affects the Pi2 onset time at low latitudes but not at high latitudes, providing an explanation for the delay of the low-latitude Pi2 onset. Lu reported an event observed by ground stations distributed globally, which indeed reveals a complicated spatial distribution of the onset time. Kepko proposed to use several stations at mid/low latitudes in the substorm onset sector for determining the onset time of Pi2. Whereas the onset determined in such a way presumably provides the earliest Pi2 onset, it is a separate issue whether or not that onset is coincident with the corresponding auroral breakup. Readers who are more interested in this issue are referred to Kepko and McPherron's comments on Liou et al. [2000] and the reply to the comments, which will appear in JGR in the near future. Kepko and Ohtani/Liou pointed out that it requires caution to using the FFT to determine the Pi2 onset. The Pi2 wave is often superposed to the positive bay. In such a case filtered data could identify a start time earlier than the actual onset. Whether it is filtered or not, the determination of the Pi2 onset is very often based on the visual inspection of data plots. It is a consensus of the working group that we need a quantitative criterion; an example was presented by Kepko. Whereas the delay of the Pi2 onset tends to be emphasized, another issue about the Pi2 wave is that according to Liou et al. [2000], very often Pi2 onsets cannot be associated with any clear auroral breakup, as pointed out by Ohtani/Liou. It is therefore suggested to avoid counting only on the Pi2 wave for identifying the substorm onset and to cross-examine other substorm identifiers, preferably auroral images. II. NEAR-EARTH SUBSTORM PROCESSES In the second session presentations were made about various subjects. Following are brief summaries: > Lyons suggested from the timing of the Geotail observation of plasma-sheet convection flows that a NENL is formed after a substorm onset. > Lu mapped FAC distributions deduced from ground magnetometer data to the equatorial region and suggested that the maximum FAC originates from the near-Earth (< 10 RE) region. The result also indicates the tailward expansion of the auroral source region. > Li reported geosynchronous injection events observed at storm time. Particle tracing he carried out suggests that in one particular event particles are accelerated on the dayside, rather than nightside, by a magnetospheric compression. > Nose' examined the enhancement of energetic O+ fluxes in the plasma sheet at substorm. He suggested that O+ ions are accelerated locally by inductive electric field associated with dipolarization. > Lessard reported auroral observations by CANOPUS & FAST during the growth phase and discussed data in terms of the M-I coupling. > Toivanen showed substorm signatures observed by the Polar satellite in the polar region and discussed the tail substorm dynamics by mapping them to the equatorial region. > Russell pointed out two types of auroral substorms, one triggered near the poleward edge of the oval and the other at the equatorward edge. He interpreted them in terms of distant and near-Earth neutral lines. > Ohtani reported a current disruption event observed by AMPTE/CCE and suggested from the anisotropy of energetic ion fluxes that the tail current is intensified explosively just prior to local dipolarization. +-------------------------------------------------------------------------+ |To add name to the mailing list or for a message to the GEM community | | please contact: editor at igpp.ucla.edu | | | |URL of GEM Home Page: http://www-ssc.igpp.ucla.edu/gem/Welcome.html | |Please update your e-mail address. | +-------------------------------------------------------------------------+