Exploring Magnetotail Structure and Dynamics with Magnetohydrodynamic Simulations

[摘要] The magnetotail, the region of stretched magnetic field lines on the night side of the Earth, is important in a number of space weather processes, in particular geomagnetic storms and substorms. These processes can cause effects such as geomagnetically induced currents on the ground, spacecraft charging, and communications outages, which cause damage to infrastructure and disruption to human activities. Better understanding of the magnetotail and its properties can help to understand and perhaps predict these phenomena. The vast size of the magnetotail, and combined with a limited number of satellites traversing it, mean that models and simulations play an important role in providing insights into the magnetotail’s structure and its involvement in geospace processes.This dissertation consists of four studies aimed at improving understanding of the magnetotail using MHD simulations performed using the SWMF. The first of these is a validation study which showed that SWMF was able to reproduce important characteristics of the observed distributions of the Kp, Sym-H, and AL indices, as well as cross-polar cap potential. The model’s ability to reproduce these quantities indicates that it accurately represents many aspects of the magnetospheric current system. However, a tendency to under-predict the strength of the most negative diversions of AL was also noted.The second study explores the ion IB, a feature within the ionosphere as a result of pitch angle scattering in the magnetosphere. One of the processes that can cause this is called CSS (current sheet scattering), the strength of which is controlled by the paramter K = Rc/rg, the ratio of the field line radius of curvature to the particle gyroradius. The study estimates K using SWMF and using several empirical models for quiet conditions on 13 February 2009, when CSS was expected to be the operative mechanism for IB formation. After applying correction factors based on in situ satellite observations from the magnetotail, K was shown to be less than 10 in a majority of cases, supporting the hypothesis that the IB’s were formed by CSS.The third study extends the second into storm conditions on 4-6 April, 2010,in which wave-particle interaction as well as CSS is expected to play a role in IB formation. K estimates from SWMF and from empirical models were used to estimate the fraction of IB observations that might have been associated with CSS during this interval. Based on the assumption that the threshold for CSS could fall between K = 8 and K = 12, we found that between 20% and 69% of the IB observations might have been associated with CSS. We also found that K did not vary with local time, suggesting that CSS played a significant role in a majority of the IB’s observed.The fourth study explores the ability of MHD to predict magnetospheric substorms. A new procedure is introduced for combining lists of substorms identified using several different techniques, and this procedure is applied to both MHD output and observational data. It is shown that the procedure reduces false positive identifications and helps to address gaps in observational data, and that the resulting substorm list is consistent with certain known characteristics of substorms. The MHD output is shown to reproduce the observed distribution of substorm waiting times, and is shown to have statistically significant skill in predicting substorm onset times.

[发布机构] University of Michigan

[效力级别] current sheet scattering

[关键词] magnetotail;current sheet scattering;magnetohydrodynamics;magnetosphere;model validation;Atmospheric;Oceanic and Space Sciences;Science;Atmospheric, Oceanic & Space Science