*************************** ** THE GEM MESSENGER ** *************************** Volume 5, Number 23 September 25, 1995 ---------------------------------------------------------- Report on GEM Snowmass Meeting, June 26-30, 1995 - Part VI ---------------------------------------------------------- TAIL/SUBSTORM CAMPAIGN WG 2: SUBSTORM/TAIL PHENOMENOLOGY - OBSERVATIONS AND MODELS Co-Chairs: Harlan Spence and Terry Onsager Working group 2 has been focusing on the combination of models and data in order to facilitate the quantitative testing of current models and to promote discussion of model capabilities and limitations. The topics emphasized at this summer's meeting were: 1) Model Predictions and Data Comparisons and 2) Substorm Triggers: Theoretical Predictions and Observational Constraints. *************************************************************************** 1) Model Predictions and Data Comparisons The goal of this workshop session was to identify specific predictions of models involving magnetotail structure or dynamics and to identify observations that could be used to test the models and to differentiate between features of similar models. Some of the participants presented comparisons of their model results with data, while others made predictions that we expect could be tested in the near future. The models that were presented are listed below. Those who presented models are encouraged to refine further their quantitative tests and to present their results at the 1996 GEM Snowmass summer meeting. Anyone who has observational evidence that is relevant to the model predictions is encouraged to collaborate with the investigator to provide tests that will be as comprehensive as possible. The models presented at the summer meeting are summarized below: ----------------------------------------------------------------------------- Presenter: John Lyon Model: Global 3-D MHD John presented predictions of the temporal evolution of the near-Earth magnetotail during a substorm. He showed a timing chart, beginning with a southward turning of the IMF, that included current disruption, transient electric field signatures, and a variety of additional effects. Although approximate timing of specific features was presented, John made the point that because the substorm involves a global loss of equilibrium, a comparison of events detected by spatially separated spacecraft will be difficult to compare quantitatively with the simulations. The challenge now will be to define better the specific features seen in the simulation results and to obtain data for comparison. Presenter: Jim Raeder Model: Global 3-D MHD Jimmy showed a comprehensive set of results from his simulations, including predictions of AU and AL in the ionosphere, Bz, Vx, Ey, n, and T in the near and distant tail, and signatures of plasmoids and flux ropes. The simulation results had a close quantitative correspondence with data obtained from the Galileo spacecraft. As with the results presented by John Lyon, many of the signatures associated with substorm onset occurred nearly simultaneously in the near-Earth region. Further study will be required to determine which processes associated with substorm onset precede or perhaps cause others. Presenter: Joachim Birn Model: 3-D MHD Model of the Magnetotail Joachim's simulation was also designed to investigate substorm onset. The results indicated the intensification of the cross-tail electric field, intensification of the cross-tail current, and intensification of the total Region 1 field-aligned currents associated with substorm onset, with timing similar to that seen in the global MHD simulations. A specific prediction of this model is that the cross-tail current diversion occurs along sheets located roughly in x-z planes with large local-time extent, rather than at the local-time edges of the current-diversion region. Presenter: Michael Hesse Model: 2-D Hybrid Code Michael described the formation of thin current sheets by imposing a convection electric field at the simulation boundaries. His two main points were that only a small fraction of the electric field penetrates into the central region of the simulation where the thin current sheets form, and that the current within the thin sheets is carried predominately by the electrons. Presenter: Robert Winglee Model: Global 3-D MHD and Global 3-D MHD with 2-fluid Effects Robert used both the MHD and the 2-fluid codes to investigate the location of auroral currents and the equatorward edge of the evening auroral oval as a function of IMF and solar wind dynamic pressure. One specific prediction is that cusp currents form in the 2-fluid simulation that are not present in MHD simulations. Presenter: Ron Elsen: Model: Global 3-D MHD Ron presented a comparison of the magnetopause position predicted by his MHD code and observed crossings. Ron demonstrated that the magnetopause locations obtained from his code for a wide range of solar wind parameters agreed well with the average locations presented by Petrenic and Russell and by Roeloff and Sibeck. Presenter: Tony Lui Model: Theory of a Current Disruption Mechanism for Substorms Tony presented a number of features of his current disruption scenario for substorms. He provided some observations tests of his ideas. One testable prediction is the presence of broadband whistler waves in the current sheet at the time of current disruption. The whistler waves are also predicted to shift in frequency as current disruption progresses. Another test would be to investigate the stability of observed thin current sheets using Tony's local model of wave stability. Presenter: Dick Wolf Model: Magnetospheric Specification Model Dick proposed a test of the MSM by using measurements from GEOTAIL in the near-Earth region (~30 Re downtail) and from geosynchronous spacecraft. The plasma sheet at GEOTAIL would be convected to geosynchronous orbit using the MSM and then compared with particle data obtained by geosynchronous satellites. Presenter: Geoff Reeves: Model: Static Magnetic Field Models Geoff presented results of tests of static magnetic field models by combining DMSP particle data with geosynchronous satellite data. By comparing the particle spectra measured by DMSP as a function of latitude with data at geosynchronous orbit, the approximate conjugate locations were determined. These results were then compared with the field-line mapping given by many different static magnetic field models. Presenter: Ted Speiser Model: Particle Trajectory Calculations and Liouville Mapping Ted described his calculations that are used to model ion distribution functions in the current sheet. In one case, an x-line location of about X = -13 Re was predicted. One way to test these predictions would be to identify time periods when near-Earth satellites, such as AMPTE IRM or CLUSTER, measure ion distributions that suggest the presence of a near-Earth neutral line and compare with measurements made simultaneously by either IMP-8 or GEOTAIL farther downtail. Presenter: Harlan Spence Model: Analytic Model of Plasma Sheet Properties Harlan described calculations of plasma sheet density, pressure, and temperature based on a deep-tail plasma source and a low latitude boundary layer source in a bounce-averaged drift model. A specific comparison with Galileo measurements indicated quantitative agreement and predictions were made that about local time dependencies which are being explored with both magnetospheric and low-altitude spacecraft. *************************************************************************** 2) Substorm Triggers: Theoretical Predictions and Observational Constraints. This session was devoted to identifying measurements that could be used to quantitatively test specific predictions regarding substorm triggers. The presentations and discussion covered both theoretical and observational aspects. As expected, this session was filled with passionate discussion. In the case of theoretical presentations, an attempt was made to solicit not only a trigger scenario and its mechanism, but rather to identify an unambiguous observational signature predicted by the model. The main messages from each of the speakers is briefly noted below. ----------------------------------------------------------------------------- Presenter: Hengjin Cai Hengjin described recent theoretical work on an entropy function used to characterize the state of the magnetotail. The discussion included details about the relationship between the entropy function and the formation of thin current sheets. Data from geosynchronous and plasma sheet measurements could perhaps test these ideas. Presenter: Gerard Fasel Observational constraints were described using all-sky camera images and meridian scanning photometer measurements made near substorm onset. In particular, the sudden brightening of an electron signature preceding onset could be used as an indicator for timing studies. Presenter: Larry Lyons Larry discussed his premise that substorms are always triggered (perhaps) by a reduction in |Bz| or by a northward turning. This prediction was referred to as "The Strong Snowmass Conjecture." A need for further quantification of this conjecture and observational tests were stressed. Presenter: Shin Ohtani Shin described results from multiple spacecraft observations regarding the characteristic time scales of wave phenomena in the current-disruption region. These observations placed both temporal and spatial constraints on the substorm onset processes in the inner magnetotail region. Presenter: Victor Sergeev Victor showed results of timing studies of the propagation of isolated discontinuities in the solar wind. It was stressed that two satellites in the solar wind are necessary to determine properly the orientation of the discontinuities. Delays of up to 20 minutes are possible between when a discontinuity detected upstream actually reaches the magnetosphere and when it would be predicted based on an assumed discontinuity normal direction along the solar wind flow direction. Presenter: Gary Erickson Gary presented CRRES observations of particles and fields near substorm onset. These results suggested that periodic oscillations of the plasma sheet with a reversal in the electric field direction may act as a trigger. A possible connection to field-line resonances was considered. Presenter: Robert Lysak Bob proposed that substorm onset may involve a fast enhancement in ionospheric conductivity caused by a wave resonator effect. These ideas could be tested with measurements of PiB's, observations of structured contuctivity, observations of rapid auroral brightening, and pseudo-breakups. +-------------------------------------------------------------------------+ |To add name to the mailing list, send a message to: editor 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 (BL Campaign), 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. J. 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