scostep 2010 (stp12) - Leibniz-Institut für Atmosphärenphysik an der ...

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STP12 Abstracts Berlin, 12 - 16 July 2010 SCOSTEP Symposium 2010 Quasi-equilibrium current sheet and the onset of impulsive bursty reconnection Skender Marina , Lapenta Giovanni Centrum voor Plasma Astrofysica, KU Leuven A two-dimensional reconnecting current sheet is studied numerically in the MHD approach. Different simulation setups are employed in order to follow the evolution of the formed current sheet in diverse configurations: Two types of initial equilibria, Harris and force-free, two types of boundary conditions, periodic and open, with uniform and non-uniform grid set, respectively. All the simulated cases are found to exhibit qualitatively the same behavior in which a current sheet evolves slowly through a series of quasi-equilibria; eventually it fragments and enters a phase of fast impulsive bursty reconnection. In order to gain more insight on the nature and characteristics of the instability taking place, physical characteristics of the simulated current sheet are related to its geometrical properties. The aspect ratio of the current sheet is observed to increase slowly in time up to a maximum value at which it fragments. Additional turbulence introduced to the system is shown to exhibit the same qualitative steps, but with the sooner onset of the fragmentation and at smaller aspect ratio. Comparison with observed solar current sheets is discussed.
STP12 Abstracts Berlin, 12 - 16 July 2010 SCOSTEP Symposium 2010 Reconstructing solar activity with applications in climate change. Solanki Sami K. Max-Planck-Institute for Solar System Research The Sun is a variable star, with all aspects of solar activity displaying a distinct cyclic variation as well as changes on other time scales. E.g. since 1978 measurements have revealed that the Sun varies in brightness on time scales from minutes to the solar cycles. With the current minimum a new aspect has been revealed: the fact that the solar brightness is not always the same at solar activity minimum, i.e. there irradiance also displays a secular variation. The magnetic field is most likely responsible for the variation of solar irradiance, just as it is for other aspects of solar activity. Models have been increasingly successful in reproducing the measured solar irradiance. In some respects, measurements have even lagged behind the models, a rarity in solar physics. Thus, models have been predicting for years that solar irradiance displays a significant secular variation. During the current activity minimum these predictions have finally been borne out by measurements. In order to detect the influence of solar variability on climate, longer time series than, e.g., the solar irradiance record are needed to complement studies on shorter time-scales. This requires models computing essential quantities, such as irradiance or open magnetic flux from sunspot numbers (available since 1610). At earlier times, even sunspot numbers or other measures of solar activity are not available and need to be reconstructed from cosmogenic isotopes. Finally, reconstructed solar activity/irradiance can be compared with climate records either directly, or via a GCM or other climate model. This talk will give an introduction to such activities.
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scostep 2010 (stp12) - Leibniz-Institut für Atmosphärenphysik an der ...

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