title: The Contribution of Magnetospheric Currents to Ground Magnetic Perturbation during Geomagnetic Storms
Creators: name: Patra, Swadesh
Publisher: name: Utah State University
Year: 2013

description: A geomagnetic storm is triggered in response to a disturbance in the solar wind. The earth's ring current gets energized during a geomagnetic storm, which leads to a decrease in the horizontal component of the geomagnetic field on the earth's surface. The Disturbance Storm Time (Dst) index, which is a measure of the intensity of the ring current, is calculated by taking the average of this decrease in the horizontal intensity across four low latitude magnetometer stations and removing the quiet time secular variations. The rate of decrease of the Dst index is an indicator of the deenergization of the ring current particles. But there are several issues with the Dst measurement as a proxy of the ring current energy. In particular, the percentage contribution of the tail current effect to the Dst index is still debated. In this work, an effort has been made to separate and quantify the possible contribution of the tail current to the Dst index. The relative contribution for a selected set of storms for which the interplanetary magnetic field turned northward abruptly after the peak in Dst was observed is estimated. The WINDMI model of the nightside magnetosphere is used to investigate the contributions of ring current, magnetotail current, and magnetopause current on the observed two-phase decay of the Dst index. The role of different solar wind magnetosphere coupling functions on the Dst index calculated by the WINDMI model is also investigated. The performance of four other coupling functions in addition to the rectified vBs is evaluated. These coupling functions emphasize different physical mechanisms to explain the energy transfer into the magnetosphere due to solar wind velocity, dynamic pressure, magnetic field, and Mach number. One coupling function is due to Siscoe, another by Borovsky, and two by Newell. The results indicate that for a majority of cases, at most only vx, By, and Bz are needed to sufficiently account for the supply of energy to the ring current and geotail current components that contribute to the Dst index. The capabilities of the WINDMI model to reliably determine the state of the global magnetosphere are improved by employing the the Magnetotail (MT) index as a measurement constraint during large geomagnetic storms. The MT index is used as a proxy for the strength of the magnetotail current in the magnetosphere. The inclusion of the MT index as an optimization constraint in turn increases our confidence that the ring current contribution to the Dst index calculated by the WINDMI model is correct during large geomagnetic storms. To improve the models prediction of AL index, we also modify the ionospheric conductivity and fit to two substorms. The rate of reduction of convection in the magnetotail for some of these storms is numerically simulated by using inner magnetospheric models like the Fok Ring Current (FRC) and the Rice Convection Model along with the global BATSRUS model at the community coordinated modeling center. Model results are compared against magnetometer data by creating movie maps from several low-latitude magnetometer stations. The results indicate the contribution from the tail current to the Dst is important. In addition, the reduction of the cross-tail current during substorm dipolarization is predicted by the measured isotropic boundary locations. Several well known phenomena are identified in the magnetometer movie maps.