*************************** ** THE GEM MESSENGER ** *************************** Volume 5, Number 24 October 3, 1995 ----------------------------------------------------------- Report on GEM Snowmass Meeting, June 26-30, 1995 - Part VII ----------------------------------------------------------- TAIL/SUBSTORM CAMPAIGN WG 3: QUANTITATIVE TAIL AND SUBSTORM MODELS Bill Lotko and Michael Hesse, Co-Chairs Working Group 3 of the GEM Tail-Substorm Campaign is charged with the investigation and development of quantitative tail and substorm models. The complexity of the tail structure and dynamics necessitates an understanding of a variety of source and coupling mechanisms. Therefore, WG3 selected for the Snowmass 1995 workshop three overarching themes: Scale-Interactive Processes, Magnetotail Plasma Sources, and Coupling Processes. The WG3 tutorial dealt with data assimilation, a theme of high relevance to operational space weather forecasting models. Scale Interactive Processes The discussion here initiated with a presentation by Toni Lui on magnetotail current disruption mechanisms. Lui presented the theory of two micro-processes: The modified two-stream instability, and the ion Weibel mode. He pointed out that these kinetic processes can provide a means to limit the current density, which might be impossible in MHD. He suggested that these instabilities operate in thin current sheets, where ions become unmagnetized. The instabilities are presently treated only in local approximations. Finally, he elaborated on a current disruption scenario, in which a perpendicular current density becomes partially parallel as the current flows into a region of enhanced magnetic field, such as expected in the current disruption region. A lively discussion ensued during this presentation. While no general agreement on substorm onset mechanisms could be achieved, it was agreed, however, that substorm onset and expansion most likely involves more than one process operating simultaneously. Furthermore, the general current disruption mechanism proposed by Lui was widely accepted, and it was noted that similar conclusions could be drawn based on Vasyliunas' equation describing field aligned current generation. Lui's presentation was followed by a brief comment made by Joerg Buechner. Buechner pointed out that depending on geometry and dissipation it is possible to find earthward flows tailward of a reconnection region. The second major presentation of this section, on Generalized MHD and Multiple Scales, was given by Amitava Bhattacharjee. He began with a discussion of scale sizes relevant to magnetospheric dynamics. The large scales, of the order of the dimension of the system, are usually well represented by an MHD description. Other scales, on the other hand, figure most prominently in thin current sheets, where, as he pointed out, deviations in MHD can become important. The deviations can manifest themselves in one or more additional terms in Ohm's law, leading to a generalized Ohm's law. New effects beyond resistivity to consider here are: electron pressure gradients, Hall effects, and electron inertia. Effects like these can decouple ion and electron dynamics, potentially also involving parallel electric fields and parallel currents at different scales. At scale lengths characteristic of thin current sheets, finite Larmor radius effects also become important. Three related comments were made subsequently. First Jim Drake showed new results from his two-fluid model pertaining to the nonlinear stage of magnetic reconnection. He showed that ion-electron decoupling via Hall effects leads to reconnection processes dominated by the ions. The electrons form a thin layer with a thickness of the order of the collisionless skin depth, where they become unmagnetized. Next Robert Winglee showed a field-aligned current distribution which was modified by the effects of additional terms in Ohm's law, and finally, Frank Cheng discussed the use of a gyrokinetic model to improve MHD. Magnetotail Plasma Sources Tom Hill presented a comprehensive overview of the relevant sources and source mechanisms for the magnetotail plasma sheet. He distinguished 2 major source regions: The solar wind and the ionosphere. Averaging over losses, the plasma sheet typically requires an ion supply of about 10^26/s. The solar wind particle flux, multiplied by the magnetospheric cross section, yields a rate of 3 x 10^29/s. Clearly, not all of these particles enter the magnetosphere. The dominant entry mechanisms are - Flow across the magnetopause through the slow mode expansion fan. Hill estimates the loading rate of this process as 10^28/s. - Diffusion, estimated by Hill to lead to a rate of 10^27/s (x/100RE)^0.5. Here x is the effective length in GSM x of the diffusive region. A comparison of numbers shows that both mechanisms are by themselves sufficient to refill the plasma sheet loss. This does not imply, however, that the ionosphere is irrelevant. While the ionospheric supply rates are presently unknown, Hill noted that the plasma sheet ion population in the inner regions of the plasma sheet can contain up to 50% O+ ions during the recovery phase of substorms. Ionospheric ion supply mechanisms for the plasma sheet are: - the cleft ion fountain - upward ion acceleration - resistive ionospheric heating Inclusion of ionospheric plasma sources can thus constitute an important improvement to global MHD models. Following Hill's presentation, Joe Borovsky showed, using ISEE data, that the plasma sheet ion density seems to correlate well with the square of the solar wind ion density. Furthermore, he showed that very high plasma sheet densities can be found after sudden increases of Kp following long periods of low Kp values. Using orbit integrations, Joerg Buechner found that Cup ion distributions seen by GEOTAIL can be reproduced. The general discussion following these presentations led to recommendations to use POLAR and GEOTAIL to assess the ionospheric plasma source. Direct plasma composition measurements are possible with GEOTAIL, whereas POLAR can observe field-aligned current regions in conjunction with outflow and upward acceleration events. WG3 Tutorial An outlook into the future of space weather forecasting based on the past of weather forecasting was presented by George Siscoe in his tutorial on "Data Assimilation Techniques." Siscoe began with a historical overview of the development of weather forecasting. Milestones include: Discovery of Trade winds (Climatology) ~1700 First synoptic analysis of a storm 1859 US Weather Bureau opens (forecasting) 1871 In space science, two examples of synoptic modeling are the Akasofu ionospheric current systems, and the Tsyganenko magnetic field models. Siscoe went on to distinguish between subjective and objective forecasts. A subjective forecast assumes a subjective progression of present data, whereas the latter produces a forecast based on a quantitative, usually numerical model. Examples in the space arena of the former are the Heppner-Maynard ionospheric potential patterns, and of the latter the AMIE technique. The future of space weather forecasting has to provide means to assimilate data into forecasting models, similar to what is done presently in weather forecasting. Possible scenarios here are a combination of observational data with the AMIE, Rice Convection, and Tsyganenko models, or the Integrated Space Weather Prediction Model (ISM). An further advantage these models is their possible use as testbeds for substorm theories. Coupling The coupling between the magnetosphere and ionosphere plays a major role in magnetospheric dynamics. To explore the implementation of M-I coupling in global MHD (GMHD) simulations, Joachim Raeder provided a presentation on the UCLA efforts. Raeder discussed the ionospheric conductivity model used in his code, which depends on a model of EUV, and electron precipitation. The precipitation is modeled on the assumption that the particle flux depends on the field-aligned current density, which is taken to be proportional to the field-aligned potential drop. The ionospheric conductivity is found to control magnetospheric convection in the GMHD simulations, as well as substorm occurrence. The ionospheric potential and current patterns can also be compared to observations, such that individual event studies are possible. Further, Raeder pointed out that in his model region 1 type field aligned currents usually form inside the open- closed field line boundary during substorms. Raeder's presentation was followed by a brief statement by Joel Fedder, who mentioned the similarity of the Lyon/Fedder treatment of the ionosphere to Raeder's model. A further presentation on the subject of coupling was provided by Bill Lotko. Lotko concentrated on the coupling between the magnetosheath and the magnetotail for relatively special northward IMF conditions. Starting with a comparison of global MHD models for identical upstream solar wind conditions, Lotko showed that the Fedder/Lyon, and the Raeder model tended to give different results. While the former model predicted an entirely closed, tadpole-shaped magnetosphere of 165 earth radii length without any tail reconnection, Raeder's model predicts the existence of open lobe regions extending beyond 400RE with tail reconnection. Both models, however, show the presence of cusp reconnection, and the associated formation of a low-latitude boundary layer. While no conclusions regarding the source of the differences could be reached, it was surmised that they might be due to differences in the numerical models themselves. This question merits further investigation and perhaps a more detailed, intermodel benchmark study. Lotko continued with a review of the observational evidence on the structure of the nightside magnetosphere. He found the Richardson et al. ISEE data, which included varying IMF Bz directions, to support some aspects of the Raeder model results, whereas the Fairfield study, which took into account only intervals of northward IMF Bz, claims to support the Lyon/Fedder model results. The role of viscous coupling was also explored. Lotko mentioned Owen and Slavin's results indicating that about 6% momentum transfer from the magnetosheath are required to explain the tailward flows in the far magnetotail. He continued with a discussion of the Drakou et al. model of the low-latitude boundary layer, which includes viscosity in the ion momentum equation, coupling to the ionosphere, and a hot plasma sheet plasma source. Lotko needs only 10% of the Bohm diffusion limit to self-consistently explain the observed plasma sheet flows. However, he also noted that neither the tail current-sheet approximation employed in the Owen- Slavin study, nor the thin boundary layer approximation of the Drakou et al. model, are completely satisfactory for describing the apparently thick tailward boundary layer flows observed in the ``quiescent'' plasma sheet. Future Activities of WG3 During the Snowmass workshop, a number of key questions of relevance to tail structure, substorm onset, and expansion emerged from the discussion. These questions will receive particular attention in future WG3 symposia. These problems are: - Can MHD model substorm expansion without reconnection? - Can nonlocal theories and simulations of current driven instabilities be performed? - What is the large scale system feedback on local instabilities? - Can the ionospheric role as plasma source be included in large scale models? - What are the sources of differences in the GMHD results and what can be learned from these differences? - How can data be assimilated into (predictive) models? - What are verifiable and distinguishing data signatures of substorm theories and models? +-------------------------------------------------------------------------+ |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. 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