Space Physics Seminar fall-2017

Magnetic activity on the Earth's surface is driven by the solar wind through the process of magnetic reconnection. A southward interplanetary magnetic field in the solar wind merges with the Earth's magnetic field and is transported over the poles and stretched into a long magnetic tail. The field eventually reconnects in the tail and returns in a convection system to the dayside. When the IMF is fluctuating this process takes about 3 hours and is called a magnetospheric substorm. If instead the IMF turns steadily southward and the solar wind speed is slow then the magnetosphere transitions from the substorm state to steady magnetospheric convection (SMC). In an SMC the reconnection rates at the front and rear of the magnetosphere are balanced so there are no changes in the configuration of the magnetosphere and no substorms occur after an initial substorm. If instead the IMF is strongly southward and the wind speed is high the magnetosphere enters a state of very large quasi-periodic substorms called sawtooth events. These three states are the fundamental modes of response of the magnetosphere to the solar wind. The magnetospheric substorm has three phases of growth, expansion, and recovery. The expansion phase is characterized by extremely active aurora near midnight that expands from the equatorward edge of the auroral oval to its poleward edge. Often a substorm will begin with a pseudo breakup which looks like it will become an expansion phase but then appears to be quenched. At any time but most often at the end of the expansion and in the recovery phase there will be intensifications of a band of aurora at the poleward edge of the oval that often develop into equatorward directed auroral streamers. These are called poleward boundary intensifications (PBI). The cause of the substorm expansion and the role of PBI in structuring activity are outstanding questions in magnetospheric physics and the subject of much controversy. This seminar will present examples of the three response modes and the two phenomenon which confuse the simple classification.