*************************** ** THE GEM MESSENGER ** *************************** Volume 4, Number 14 August 12, 1994 --------------------------------------------------------------------------- REPORT ON 1994 SNOWMASS WORKSHOP - MAGNETOTAIL/SUBSTORM CAMPAIGN -- Part II --------------------------------------------------------------------------- WG 2: Substorm/Tail Phenomenology - Observations and Models Co-chairs: Terry Onsager and Harlan Spence The inaugural meeting of WG2 occurred at the 1994 GEM Snowmass Workshop. We met in three sessions during the second half of the week and sponsored an invited talk relevant to WG2 during one plenary session. Our philosophy was to promote a workshop atmosphere where issues were motivated through a series of short presentations with plenty of opportunity for audience participation. We would like to thank both the presenters and the audience for so many successful interchanges. Our feedback to date has been generally quite positive; discussions were lively and several important issues were addressed in depth. We welcome an ongoing dialogue with the community regarding the direction and format and issues confronting WG2. The following is a summary of WG2 sessions including the session topic, speaker, approximate title, and a short synopsis. (Apologies to the presenters if the co-chairs have synopsized inaccurately!) *************************************************************************** (1) MHD versus Kinetic Aspects of Tail Plasmasheet Modeling (Thursday, June 30, 10:30AM - 12:30PM) *************************************************************************** * Speaker: Dick Wolf (Adam Usadi, Duane Pontius) Title: Macroscopic Effects of Chaotic and Speiser Orbits in the Plasma Sheet - A Status Report Synopsis: This talk addressed the question, "How bad is bounce-averaged drift in the context of chaotic/Speiser orbits?", and included the summary of 2 studies: (1) Wolf and Pontius, GRL, 20,1455, 1993 - 2-D B-field, E=0, look at average drifts to establish that approach is valid for (1/V)(dV/dx) << 1/rho_max rather than the more restrictive 1/R_c << 1/rho_max (2) Usadi and Wolf (very preliminary results) simple B-field model, plasma consistent with analytic model - look at average drift and dispersion relative to exact solution * Speaker: Grant Burkhart Title: Observational Consequences of Current Sheet Particle Motion Synopsis: This talk gave background and summarized recent work on the contributions of particle motion to Microscopic (e.g., non-Maxwellian source of free energy) and Macroscopic (anisotropic pressure, effective resistivity, vertical length scale) plasma effects * Speaker: Zerefsan Kaymaz (G. Siscoe, J. Fedder, J. Lyon) Title: Comparison of IMP-8 data with Fedder-Lyon MHD Model Synopsis: This talk linked observations with models by comparing tail cross-sections of the average B directions with snapshots of MHD model runs. They find that a simple vacuum merging is not evident in either data or models, however they find that models are qualitatively consistent with data. * Speaker: Gary Erickson Title: Limitations of Ideal MHD Synopsis: This talk provided an excellent overview of where ideal MHD works and where and when it doesn't. *************************************************************************** (2) Tail Geometry: IMF Control and Triggers (Thursday, June 30, 3:20AM - 5:30PM) *************************************************************************** * Speaker: Don Fairfield (Lou Frank, Kent Ackerson, Kokubun) Title: The Absence of a Nominal Tail under Prolonged, Strongly Northward IMF Synopsis: GEOTAIL observations were discussed that suggest an odd tail geometry (maybe only 10 RE radius!) may exist at large downtail distances under unusually persistent and large IMF +Bz; this observation was cast in the context of other recent similar observations. * Speaker: Vilen Mishin Title: New Evidence in Support of Substorm Scenario with Two Active Phases and Tail Stretching Instability Synopsis: A conceptual model was discussed that incorporates two "active" substorm phases: the first active phase is characterized by multiple onsets (tail lobe flux increases and tail stretches); the second phase is expansion phase onset (flux decreases and tail shortens). This model was used to attempt to reconcile an assortment of substorm models (NENL, MIC, Lui, and RBSF models. Mishin also reported on the availability of "MIT" data and inversion techniques that could be brought to the GEM community. * Speaker: Bob McPherron Title: Northward Turnings of IMF as Substorm Trigger Synopsis: Examples of apparent triggering (good at five minute resolution but less certain at one minute resolution) were presented as well as a short review of previous studies. Statistics indicate that northward turnings occur about 45% at the time of sharp onsets (at five minute resolution) but that better determination of onset time and IMF data are needed for definitive study. * Speaker: Steve Petrinec (Chris Russell) Title: Sudden Impulse as Substorm Trigger Synopsis: A pressure balance magnetopause model (data driven) was presented. The magnetopause flare angle varies with IMF Bz (strong with -Bz, weak with +Bz) at fixed dynamic pressure. The model was used to investigate sudden impulses as substorm trigger. * Speaker: Dave Klumpar (presented work by Walter Lennartsson) Title: How Does One Populate the Inner Magnetotail Synopsis: Results from several recent papers were presented (principally based on JGR, 97, 19221,1992; 99, 2387, 1994) concerning the various sources of magnetotail plasmas. ISEE data were used to assess the relative importance of the solar wind source versus the terrestrial source using composition changes as a function of various mechanisms/parameters. *************************************************************************** (A) Plenary Session - WG2 Invited Presentation (Friday, July 1, 8:30AM - 9:30AM) *************************************************************************** * Speaker: Dan Baker Title: Plasmasheet Plasma Sources Synopsis: Tutorial overview of old/new, data/theory aspects of plasma sheet sources. This talk also cast the more recent Geotail observations in the context of the past literature. *************************************************************************** (3) Plasmasheet Plasma Sources & Tail Structure (Friday, July 1, 3:00PM - 4:30PM) *************************************************************************** * Speaker: Joachim Birn Title: Equilibrium Magnetotail Structure Synopsis: Reported on the use of equilibrium magnetostatics to construct 3D magnetic field tail structures. Key aspects of these models are the clear R1 FAC signatures and equatorial plasma sheet bulges near the flanks at large distance. * Speaker: Masaki Fujimoto Title: LLBL in the Magnetotail: GEOTAIL Observations Synopsis: Showed Geotail observations of B and plasma in the tail. They clearly identify mixing regions where magnetosheath and magnetospheric plasmas co-exist. * Speaker: Masato Nakamura Title: GEOTAIL Boundary Layer Identifications Synopsis: Again using B and plasma observations in the tail, they identify and quantify spatial regions by their field and plasma signatures. They report on large densities (1/cc) and low temperatures (100 eV) in the lobe. * Speaker: Harlan Spence (Margaret Kivelson) Title: Low Latitude Boundary Layer Entry into Plasmasheet Synopsis: Reported on recent work demonstrating the importance of dawnside LLBL plasma to inner magnetotail plasmas. Modeling results were compared favorably with plasma observations made by the Galileo s/c as it traversed the Earth's magnetotail. * Speaker: Terry Onsager (Elphic, Thomsen, Gosling) Title: Inferring X-line Location of PSBL Measurements Synopsis: Recent results of X-line location estimates were presented. From a time series of these estimates, the X-line did not appear to undergo a systematic tailward motion during substorm recovery, although there is considerable uncertainty in the estimates. +-------------------------------------------------------------------------+ |To add name to the mailing list, send a message to: guan at igpp.ucla.edu | |For message to whole GEM mailing list, send to: gem at igpp.ucla.edu | |For message to a specific working group, send to: | | gem_field at igpp.ucla.edu (WG1); gem_boundary at igpp.ucla.edu (WG2); | | gem_current at igpp.ucla.edu (WG3); gem_data at igpp.ucla.edu (WG4) | | gem_ggcm at igpp.ucla.edu (WG5) | | gem_chair at igpp.ucla.edu (WG chairs, Odile and W. 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