Statistical Studies Of Low-latitude Magnetic Disturbances During Geomagnetic Storms

The magnetic disturbance during geomagnetic storm plays an important role in the studies and predictions of the space weather.In this work,352 geomagnetic storms from 2000 to 2016are chosen for our investigation.First,the magnetic latitude locations of the ring current are investigated with the SuperMAG dataset during the main phases of 67 intense geomagnetic storms.Then the magnetic local time?MLT?distributions of the horizontal geomagnetic disturbance??H?are investigated with the SuperMAG and C/NOFS dataset during 330geomagnetic storms.Finally,the relationships between the MLT distribution of?H and the interplanetary condition are discussed with the models of Burton and Wang.The main results are as follows.The strongest?H is not always located in the magnetic equatorial plane,indicating that the ring current probably deviates from the magnetic equator plane during the main phases of geomagnetic storms.Tilts of the ring current in the day-night direction can be affected by the direction of solar wind which the magnetotail is stretched along.Tilts of the ring current in the dawn-dusk direction might be influenced by the y component of interplanetary magnetic field?IMF B_Y?.A possible interpretation might be the IMF B_Y-related twisting of the geomagnetic field.Besides,the seasonal variation of the latitudinal shift is characterized by a summer valley and a winter peak,which might indicate that the ring current is not centered on a single plane.Such variations are also probably related to the warping of the magnetotail and the hemispheric asymmetries of the disturbances caused by the magnetopause currents.The MLT distributions of?H are asymmetric with the asymmetry varying with storm phases.It is shown that the?H is positive at most MLTs and slightly stronger on the dayside during the initial phase.Such distribution might be related to the enhancement of the solar wind dynamic pressure?P?.In the main phase,the?H decreases prominently with the peak on the dusk side,especially for large solar wind electric field?E?.The region with?H larger than-20nT is concentrated on the dawn side for small E,which might be attributed to earthward bursty bulk flows.Besides,larger P might result in stronger disturbances under the same magnitude of E.In the recovery phase,similar dawn-dusk asymmetry occurs as that in the main phase for positive E,but the peak of strongest disturbance tends to weaken and move toward the dayside.The P and E play different roles on the changes of?H.The?H is affected by P in three ways as below.?1?The enhancement of P increases the magnetopause currents,which brings positive disturbances to?H.?2?Larger P might result in stronger disturbances under the same magnitude of E.?3?The decay time of the ring current shortens with the increase of P when E is negative,so the negative disturbances recover more quickly.The?H is affected by E in two ways as below.?1?As E increases,more ions are injected into the ring current region from the nightside.Ions on opened trajectories are transported toward the dayside magnetopause and eventually leaked into the magnetosheath via a reconnection process.Thus the?H decreases significantly with the strongest disturbance on the dusk side.?2?The decay time of the ring current shortens with the increase of E and shows obvious dawn-dusk asymmetry.It might be related to the ions of ring current escaping into the magnetosheath.The application of Wang model to?H at different MLT has two deficiencies,namely the model?H is smaller than the observed?H in the initial phase,but larger than the observed?H in the main phase.The processes of solving the parameters b and P₀ are modified.Then the shortcoming in the initial phase is improved a little,and that in the main phase is improved much.These results statistically show the evolution of?H in three storm phases and reflect the relationships between the distributions of?H and the interplanetary condition.The application of the models of Burton and Wang to the?H at different MLT provides a possible method to calculate the MLT distribution of?H,although it still needs some improvements.