Browsing by Author "Crowley, G."
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Item Restricted On the origin of pre-reversal enhancement of the zonal equatorial electric field(European Geosciences Union, 2009-05-05) Kelley, M. C.; Ilma, R. R.; Crowley, G.In November 2004, a large and variable interplanetary electric field (IEF) was felt in the reference frame of the Earth. This electric field penetrated to the magnetic equator and, when the Jicamarca Radio Observatory (JRO) was in the dusk sector, resulted in a reversal of the normal zonal component of the field. In turn, this caused a counter-electrojet (CEJ), a westward current rather than the usual eastward current. At the time of the normal pre-reversal enhancement (PRE) of the eastward field, the Jicamarca incoherent scatter radar (ISR) observed that the westward component became even more westward. Two of the three current explanations for the PRE depend on the neutral wind patterns. However, this unique event was such that the neutral wind-driven dynamos could not have changed. The implication is that the Haerendel-Eccles mechanism, which involves partial closure of the equatorial electrojet (EEJ) after sunset, must be the dominant mechanism for the PRE.Item Restricted Simulation of the pre‐reversal enhancement in the low latitude vertical ion drifts(American Geophysical Union, 2000-07-01) Fesen, C. G.; Crowley, G.; Roble, R. G.; Richmond, A. D.; Fejer, B. G.Low latitude F region ion motions exhibit strong seasonal and solar cycle dependences. The pre‐reversal enhancement (PRE) in the vertical ion drifts is a particularly well‐known low latitude electrodynamic feature, exhibited as a sharp upward spike in the velocity shortly after local sunset, which remains poorly understood theoretically. The PRE has been successfully simulated for the first time by a general circulation model, the National Center for Atmospheric Research thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM). The TIEGCM reproduces the zonal and vertical plasma drifts for equinox, June, and December for low, medium, and high solar activity. The crucial parameter in the model to produce the PRE is the nighttime E region electron densities: densities ≥ 104 cm−3 preclude the PRE development by short‐circuiting the F region dynamo. The E region semidiurnal 2,2 tidal wave largely determines the magnitude and phase of the daytime F region drifts.Item Restricted Superposed epoch analysis of the dayside ionospheric response to four intense geomagnetic storms(American Geophysical Union, 2008-07-09) Mannucci, A. J.; Tsurutani, B. T.; Abdu, M. A.; Gonzalez, W. D.; Komjathy, A.; Echer, E.; Iijima, B. A.; Crowley, G.; Anderson, D.Prompt daytime ionospheric responses are presented for the following four intense geomagnetic storms: 29 October 2003, 30 October 2003, 20 November 2003, and 7 November 2004. We perform a superposed epoch analysis of the storms by defining the start time of the epoch when the Kan‐Lee interplanetary electric field (proportional to the reconnection electric field) first reaches 10 mV/m during a period of continuously southward Bz. Measurements from the GPS receiver onboard the CHAMP satellite at 400 km altitude indicate significant low‐ to middle‐latitude daytime total electron content (TEC) increases above the satellite within 1–2 h of the defined start time for three of the storms (∼1400 local solar time). The 20 November 2003 data follow a different pattern: the largest TEC increases appear several hours (∼5–7) following the interplanetary magnetic field Bz event onset. TEC data obtained from ground‐based GPS receivers for the November 2003 storm tend to confirm a “late” TEC increase for this storm at ∼1400 LT. Estimates of vertical plasma uplift near the equator at Jicamarca longitudes (∼281 E) using the dual‐magnetometer technique suggest that variability of the timing of the TEC response is associated with variability in the prompt penetration of electric fields to low latitudes. It is also found that for the November 2003 magnetic storm the cross‐correlation function between the SYM‐H index and the interplanetary electric field reached maximum correlation with a lag time of 4 h. Such a large lag time has never been noted before. The long delays of both the ionosphere and magnetosphere responses need to be better understood.