Statistical studies of the relationship between the amplitude of positive magnetic bays at mid latitudes, geomagnetic activity and solar wind parameters

During the expansion phase of the substorm, the poleward jump of the aurora (breakup) and the expansion of the auroral bulge are observed. The expansion is accompanied by a negative magnetic bay under the aurora and a positive magnetic bay at the middle latitudes. The amplitude of the negative bay is characterized by the auroral AL-index. To characterize the positive bay, the MPB-index (Mid-latitude Positive Bay index) was previously proposed. The paper examines the statistical relationship of the MPB-index with the geomagnetic activity at different latitudes and with the parameters of the solar wind and the interplanetary magnetic field. It is shown that all extremely large values of the MPB-index (above 10.000 nT2) are observed during strong geomagnetic storms (when the Dst-index drops below –100 nT), and all extremely strong geomagnetic storms (when the Dst-index drops below –250 nT) accompanied by extremely high MPB-index values. Statistically, the MPB-index increases with the increasing of geomagnetic activity at any latitudes. The MPB-index, on average, increases with the increasing of the magnitude of the interplanetary magnetic field and any of its components. But for the Bz-component, large values of the MPB-index are observed by its southward direction. For plasma parameters of the solar wind, the MPB-index increases most strongly with the increasing of the solar wind speed. There is also the strong dependence on the dynamic pressure and on the magnitude of the EY-component of the solar wind electric field. However, the MPB-index weakly depends on solar wind density and temperature.