*************************** ** THE GEM MESSENGER ** *************************** Volume 7, Number 26 July 2, 1997 ---------------------------------------------------------------- REPORT ON 1997 SNOWMASS WORKSHOP - MAGNETOTAIL/SUBSTORM CAMPAIGN ---------------------------------------------------------------- From: Michael Hesse Tail/Substorm Working Group 3: Quantitative Tail and Substorm Models Co-chairs: W. Lotko, M. Hesse During the 1997 Snowmass workshop T/S WG3 addressed three themes: Connectivity (here between the magnetopause and the plasma sheet), Dynamics (of substorsm with an emphasis on timing and causality), and Scale-Interactive Processes (in particular, three-dimensional magnetic reconnection). In a plenary talk, the latest in our understanding of storm-substorm connection was presented. ------- Tutorial: Storm-substorm relation Bob McPherron Realizing the apparent lack of knowledge in the area of storm-substorm coupling, T/S WG3 decided to invite Bob McPherron to present to the audience his latest understanding of the connection between storms and substorms. He made the following points - The determination of Dst is, due to its complexity, very suceptible to errors. - About 90% of the Dst variance can be well predicted from solar wind data alone, without knowledge of substorm indices such as AL. - The convective surge model, which claims that Dst (and ring current) enhancements are produced by a sequence of convection surges, does not reproduce the Dst time scales. This was demonstrated via a comparison of concurrent AL and Dst traces. - In the data, there also appears to be no evidence that substorm expansion phase injections feed the Dst index. - Some recent studies suggest that a large fraction of the Dst variance might be explained by tail current variations, or the closure of tail currents on the dayside rather that the high-latitude magnetopause. This suggestion might be worth further investigations. - In summary, it appeared quite unlikely that a storm can be understood as the sum of several substorms, although storms are usually accompanied by substorms. ------- Theme 1: Connectivity Topic: Global views of the connection between magnetopause merging and the neutral sheet The first discussion leader was George Siscoe. He used the ISM code (developed by Mission Research) to make a scientific predicition based on a numerical simulation in controlled input conditions. The following points were made: - The ISM code is a finite-difference, global MHD model that includes two-fluid effects (the second fluid is a neutral component), extending to the ionosphere. - A new feature at the magnetopause was found. It consisted of a wedge-shaped low magnetic field region, which extends tailward from the cusp. It forms, with the plasma sheet a "sigmoid"-shaped structure. - Reconnection, plasma heating and entry is found all the way along this region from the dayside into the mid-tail. - This model prediction can be and has been verified by means of IMP-8 data which show very similar magnetic field signatures. The second discussion leader was Joel Fedder, who analyzed magnetopause and magnetotail reconnection the presence of an IMF By magnetic field component. His analysis showed: - The magnetopause essentially appears to look like a vacuum superposition of the geomagnetic and interplanetary field. - Reconnection at neutral points is important at the magnetopause. Neutral points are connected by singular lines, the so-called null-null lines. They can also be a site of magnetic reconnection. - The neutral point position in controlled by the IMF. - Magnetotail reconnection in the presence of a cross-tail magnetic field component involves complicated magnetic topologies, even in the vicinity of the reconnection region itself. This might make the analysis of the magnetic field structure quite difficult. The discussion during this session brought out two points: There are two interesting operational modes for global (and local) MHD: - basic science mode: study evolution for controlled input conditions - event driven mode: support data analysis and verify MHD model The "Siscoe Sigmoid" was also seen in Joel Fedder's simulations. This indicates a certain robustness of the MHD results. ------- Theme 2: Dynamics Topic: Global substorm simulations - physical processes, location, and timing The first discussion leader in this session was Joel Fedder, who used his global MHD model to study in higher resolution details of substorm expansion. He emphasized the following points: - A substorm in his investigation is triggered by enhanced magnetopause reconnection. - Tail reconnection begins with a slow rate in a localized region. It becomes faster after lobe reconnection, still in a localized region, is initiated. - These localized reconnections causes only localized effects, although they include dipolarizations and fast earthward and tailward flows. - A substorm was preceeded in the simulation by several of these localized reconnection events. - It is not entirely clear what causes the localization as well as what determines the locus itself of the reconnection processes. It is quite likely that the IMF direction plays an important role here. The second discussion here was led by Joachim Birn, who used his tail MHD model to study substorm injections by means of particle tracing. He found the following results: - The tail MHD model includes a dipolar magnetic field region. - Substorm electric fields are first enhanced at the X-line. Later, however, electric fields are strongest in the dipolarizing magnetic field region. These electric fields dominate by far the reconnection electric fields. - Strong field-aligned currents do not extend all the way into the plasma sheet. They are located on field lines inward of the open-closed field-line boundary. - Substorm injections can be understood by energetic particle acceleration in the electric fields associated with the dipolarizing magnetic field. - Particle tracing in the MHD fields reproduced the observed dispersion and timing at different local times, ranging from pure ion injections to a combination of ion and electron injections to pure electron injections as one moves from dusk to dawn. - The three-dimensional structure of electric and magnetic fields is essential if one wants to explain the distribution functions of injected particles. This session led to two new future tasks: A present puzzle are the ionospheric signatures of earthward convection associated with reconnection flows prior to their arrival in the inner magnetotail. This requires a detailed study. The relation between the local flow channels as seen by Joel Fedder and the global substorm instability merits further investigations. ------- Theme 3: Scale interactive processes Topic: Three-dimensional features of magnetic reconnection Phil Pritchett - Uses 2-D and 3-D fully kinetic models - A Harris neutral sheet configuration and a regional kinetic model of the near-earth tail with a dipolar-like magnetic field and attached current sheet were discussed. The results of the regional model serve to illustrate the potential importance of 3d kinetic effects in tail configurational instabilities, but due to the severe computational constraints in implementing kinetic simulations of large volume, inhomogeneous regions, it was noted that it is not clear that the regional simulations map correctly onto actual tail configurations. - During solar wind driving, ion-electron decoupling leads to polarization electric fields and embedded thin current sheets. The current in these sheets is carried by the electrons. - The thin current sheets are stable (with respect to instabilities with nonvanishing wave vector in the y direction) until reconnection starts. Therefore, the behavior is initially two-dimensional. - The north-south magnetic field component readily evolves to a value less than zero, unlike ideal MHD, where such an evolution is prohibited. - The three-dimensional evolution generally is much more complicated than in two dimensions. Additional instabilities such as (kinetic and MHD) kink instabilities as well as current sheet breakup effects by whistler dynamic may be important. Michael Hesse - Suggested that instabilites with wave vector in y direction might produce local electron flow velocity enhancements by Hall electric fields - Investigation of such instabilities by 2.5D Hall MHD produced either nothing (when the current was carried by the electrons) or a Kelvin-Helmholtz instability (when the current was carried by the ions). - Using a 2.5D modified hybrid model, strong growth of a lower-hybrid-drift instability was observed on both sides of the current sheet. The interaction between the two sides appeared to lead to a kinking signature in the cross-tail current. The main future task that arose from this session is to attempt to incorporate kinetic dissipation into large scale models. In recognition of the expected difficulties it was suggested that one might want to start by trying use simple parametrizations initially. ------- A last note: After four and five years of service as co-chairs we felt that it was time for us to step down and let two new people take the helm of our working group activities. Therefore, this is our last report. We would like to use this opportunity to express our thanks to all the presenters as well as to a higly motivated audience who rendered discussions lively and our work enjoyable. We both look forward to our new role as part of the audience in the Tail/Substorm Campaign. Michael Hesse and Bill Lotko +-------------------------------------------------------------------------+ |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 | | | |URL of GEM Home Page: http://igpp.ucla.edu/gem/Welcome.html | |Please update your e-mail address. | |CAUTION: Do not send messages to gem at igpp.ucla.edu unless you want | | your message to go to everyone in the GEM mailing list! | +-------------------------------------------------------------------------+