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Contact:Supplementary material to “Tackling Substorm Problems: New Observational and Modeling Capabilities”
R. Nakamura and W. Baumjohann, Space Research Institute, Austrian Academy of Sciences, Graz, Austria
Citation:
Nakamura, R., and W. Baumjohann (2008), New spaceborne and ground-based observational and modeling capabilities help scientists tackle substorm problems, Eos Trans. AGU, 89(35), 324. [Full Article (pdf)]
The Ninth International Conference on Substorms demonstrated the importance of new advanced observation capabilities both in space and on ground and strength of the theoretical and empirical modeling at different scales to tackle the complex multi-scale problems of substorms.
The Ninth International Conference on Substorms, May 5–8, 2008, Seggau Castle, near Graz, Austria
The Ninth International Conference on Substorms was held on May 5–9, 2008 at Seggau Castle, near Graz, Austria. It was organized by the Space Research Institute (IWF) supported by the Austrian Academy of Sciences, the Austrian Research Promotion Agency, the Association of Austrian Space Industries, and the Government of Styria. About 130 scientists attended more than 150 oral and poster presentations. The theme of the conference was "Key elements of substorms and their coupling". There were five topical sessions. Each session was concluded with an open review and discussion on what has been achieved and what still needs to be studied in that particular area.
The first session focused on the formation and instabilities of the substorm current sheet and plasma sheet. While there were many observational papers mainly from Cluster, THEMIS and Geotail, almost half of the presentations were theoretical and modeling studies enabling closer comparison between observation and theory. Formation of an embedded current sheet before the growth of the instability rather than thinning of the entire plasma sheet was concluded from Cluster observations and from theory. The evolution of instabilities leading to magnetic reconnection was reported by several theoretical studies, while characteristics of particles and fields in the reconnecting current sheet were seen in in-situ Cluster and THEMIS observations. How the different mode of relevant instabilities take place at what thickness of the current sheets still needs to be confirmed from observations. Different modes of current sheet oscillations were also reported both in observational and theoretical papers. Flapping current sheets are not necessarily relevant to substorms but rather a consequence of the fast flows. Ballooning modes occur both in the context of oscillations and instabilities leading to the dipolarization. Papers based on theory as well as measurements by Geotail and THEMIS together with auroral observations showed that instabilities such as ballooning modes near the region between tail-like and dipolar fields were needed to explain the initial auroral arc formation.
The second session was devoted the propagation and evolution of substorm disturbances. The papers presented in this session mainly discussed the propagation of reconnection related disturbances in the near tail, i.e. fast flows and plasmoids/flux ropes, dipolarization and energetic particle injections in the inner magnetosphere, and causal relationships among these disturbances in the context of substorms development. More than half of the papers presented in the sessions were based on recent multi-point observations by THEMIS, which demonstrated the strength of simultaneous monitoring these key regions in the magnetosphere together with ground observations. Particularly, the optimized spacecraft configuration allowed investigating the multi-onset properties (space localizations, multiple sources and impulsive development), detailed timing and propagation of disturbances. All possible disturbance propagation directions, i.e., tailward, Earthward, and azimuthal, of disturbances were reported, supporting both "In-Out" and "Out-In" substorm scenarios. One of the important open issues is how to specify the spacecraft location in 3D space, since the distance from the current sheet center affects the interpretation of propagation and mapping. Studies using extensive data sets from Geotail showed large-scale evolution particularly associated with fast flows. While no systematic relationship between Earthward flow observed by Geotail and dipolarization at geosynchronous orbit was obtained, statistical evidence was presented that tailward fast flows always exist in the near-tail region associated with auroral breakup when both observations were made at close local time region.
The topic of the third session was magnetosphere-ionosphere coupling. Many new ground-based network observations from all-sky auroral records, radar, photometer, and magnetometer observations were reported combined with multi-point magnetospheric observations by THEMIS and other spacecraft. Detailed observations around the onset were highlighted by several papers. It was reported that the initial auroral arc brightening occurred near the equatorward edge of plasma sheet electron precipitation, which maps to the region between tail-like and dipolar fields. This arc is not necessarily the most equatorward arc and it develops slowly for few minutes prior to auroral breakup. Both theoretical as well as observational studies hint to the importance of an Alfven auroral arc at substorm onset and fast flow-associated auroral precipitation. Further area of interest were the dynamics and magnetospheric processes corresponding to auroral bulge and poleward arc and the electrodynamics in the Harang reversal region.
Global mode of substorms was the topic of the fourth session. Solar-wind magnetosphere coupling was discussed in this session mainly from the viewpoint of magnetic flux transport, i.e., how is the magnetic flux opened on the dayside transported to the tail lobes to be closed and returned to the day side during different condition of IMF and solar wind dynamic pressure and their variations. New results from statistical studies using extensive data sets from the solar wind, global auroral images, and radar observations were presented providing quantitative measurement of global magnetospheric parameters with also help from the global MHD simulation. Although many substorms are triggered by IMF and solar wind pressure, the existence of a threshold in open flux and the importance of the loading-unloading process support the need for an internal trigger mechanism. While substorms occur when the magnetosphere is driven by short intervals of southward IMF, open magnetic flux closure in the magnetotail were shown to take place also during IMF Bz positive with IMF By and also with a strong solar wind driver when the level of open flux seems to be saturated. How the efficiency of magnetosphere-solar wind coupling changes with the state of the solar wind and the mode of response is still a key question.
The conference concluded with a fifth session to discuss substorms as a common process in the universe. Despite the fact that both the phenomena and processes of Earth magnetospheric substorms still contain many controversial problems and therefore the topic was quite challenging, an universal framework to study substorms was proposed to be the sequence of slow build-up followed by sudden release of energy in the magnetic field. Commonalities and differences were discussed widely from heliophysical as well as astrophysical perspectives, such as relativistic jets from a rotating black hole. Planetary substorm-like phenomena were presented in the context of magnetic flux transport in a system with a larger amount of open flux than Earth (Saturn), periodic magnetotail reconnection in an internally driven system (Jupiter), magnetospheric substorms without an ionosphere (Mercury), or reconnection in the nightside of the planet but on draped IMF field-lines (Venus). Pre-reconnection macroscale motion and importance of different scales of instabilities, although the causality might differ, were stressed to be common between coronal mass ejections and terrestrial substorms.
In summary the conference demonstrated that substorms are a rich field of space plasma physical problems dealing with different set of coupling processes at different scales. This became particularly clear from the recent space-borne and ground observational and computational capabilities.
Conference papers will be published in a special issue of Annales Geophysicae. More information about the conference can be found at http://ics9.oeaw.ac.at/.