Signal Propagation from the Tail to the Ionosphere

Abstract:

The night side of magnetosphere is a very dynamic region, where substorms can release substantial amounts of energy in an explosive fashion. Even though many physical models explaining the substorm process have been proposed, the onset mechanism (initiation) of substorms is still an unsolved problem in Space Physics. It is presently not resolved whether the onset mechanism is triggered by current disruption (CD) process, by some other ideal MHD instability such as the ballooning mode, or by magnetic reconnection process (Rx). The former two processes would initiate a substorm close to Earth somewhere between 6 and 8 RE downtail, whereas a reconnection trigger is thought to occur around 20 RE. Recent observations by the THEMIS mission suggests that substorms are triggered by magnetic reconnection mechanism in mid-tail region (Angelopoulos et al., 2009 ), but those results are disputed (Lui et al., 2009). Distinguishing these processes depends critically on the correct timing of different signals in the plasma sheet and the ionosphere in order to establish a time sequence of events. This has been difficult to accomplish with data alone, since signals are sometimes ambiguous, or they have not been observed in the right locations. To investigate signal propagation paths and signal travel times, we use OpenGGCM global simulation. By launching wave pulses from various locations in the magnetosphere, we investigate the path taken by the waves and time it takes for different wave to reach the ionosphere. We find that the Tamao path is not generally preferred path for waves originating in the plasma sheet. We also find that a point source in the tail can lead to spread out signal in the ionosphere.