Browsing by Author "Anderson, B. J."
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- Field-aligned current reconfiguration and magnetospheric response to an impulse in the interplanetary magnetic field B-Y componentWilder, F. D.; Eriksson, S.; Korth, H.; Baker, Joseph B. H.; Hairston, M. R.; Heinselman, C.; Anderson, B. J. (American Geophysical Union, 2013-06-16)When the interplanetary magnetic field (IMF) is dawnward or duskward, magnetic merging between the IMF and the geomagnetic field occurs near the cusp on the dayside flanks of the magnetosphere. During these intervals, flow channels in the ionosphere with velocities in excess of 2km/s have been observed, which can deposit large amounts of energy into the high-latitude thermosphere. In this study, we analyze an interval on 5 April 2010 where there was a strong dawnward impulse in the IMF, followed by a gradual decay in IMF magnitude at constant clock angle. Data from the Sondrestrom incoherent scatter radar and the Defense Meteorological Satellite Program spacecraft were used to investigate ionospheric convection during this interval, and data from the Active Magnetospheric and Planetary Electrodynamics Response Experiment (AMPERE) were used to investigate the associated Field-Aligned Current (FAC) system. Additionally, data from AMPERE were used to investigate the time response of the dawnside FAC pair. We find there is a delay of approximately 1.25h between the arrival of the dawnward IMF impulse at the magnetopause and strength of the dawnward FAC pair, which is comparable to substorm growth and expansion time scales under southward IMF. Additionally, we find at the time of the peak FAC, there is evidence of a reconfiguring four-sheet FAC system in the morning local time sector of the ionosphere. Additionally, we find an inverse correlation between the dawn FAC strength and both the solar wind Alfvenic Mach number and the SYM-H index. No statistically significant correlation between the FAC strength and the solar wind dynamic pressure was found.
- On the influence of open magnetic flux on substorm intensity: Ground- and space-based observationsClausen, Lasse B. N.; Milan, Stephen E.; Baker, Joseph B. H.; Ruohoniemi, J. Michael; Glassmeier, K. H.; Coxon, J. C.; Anderson, B. J. (American Geophysical Union, 2003-06-01)Using the location of maximum region 1 current determined by the Active Magnetosphere and Planetary Electrodynamics Response Experiment as a proxy for the open/closed field line boundary, we monitor the evolution of the amount of open magnetic flux inside the magnetosphere during 772 substorms. We then divide all substorms into three classes, depending on the amount of open flux at expansion phase onset. Studying the temporal variations during the substorms of each class for a number of related geophysical parameters, we find that substorms occurring while the amount of open flux is large are generally more intense. By intense we mean that the auroral electrojet, region 1 current, auroral brightness, tail dipolarization and flow speed, ground magnetic signatures, Pi2 wave power, as well as the intensity and extent of the substorm current wedge (SCW) are all larger than during substorms that occur on a contracted polar cap. The SCW manifests itself as an intensification of the nightside R1 and R2 current system after onset. Our analysis shows that to dispose of large amounts of accumulated open magnetic flux, large substorms are triggered in the terrestrial magnetosphere.
- Temporal and spatial dynamics of the regions 1 and 2 Birkeland currents during substormsClausen, Lasse B. N.; Baker, Joseph B. H.; Ruohoniemi, J. Michael; Milan, Stephen E.; Coxon, J. C.; Wing, S.; Ohtani, S.; Anderson, B. J. (American Geophysical Union, 2013-06-01)We use current density data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) to identify the location of maximum region 1 current at all magnetic local times (MLTs). We term this location the R1 oval. Comparing the R1 oval location with particle precipitation boundaries identified in DMSP data, we find that the R1 oval is located on average within 1 degrees of particle signatures associated with the open/closed field line boundary (OCB) across dayside and nightside MLTs. We hence conclude that the R1 oval can be used as a proxy for the location of the OCB. Studying the amount of magnetic flux enclosed by the R1 oval during the substorm cycle, we find that the R1 oval flux is well organized by it: during the growth phase the R1 oval location moves equatorward as the amount of magnetic flux increases whereas after substorm expansion phase onset significant flux closure occurs as the R1 current location retreats to higher latitudes. For about 15 min after expansion phase onset, the amount of open magnetic flux continues to increase indicating that dayside reconnection dominates over nightside reconnection. In the current density data, we find evidence of the substorm current wedge and also show that the dayside R1 currents are stronger than their nightside counterpart during the substorm growth phase, whereas after expansion phase onset, the nightside R1 currents dominate. Our observations of the current distribution and OCB movement during the substorm cycle are in excellent agreement with the expanding/contracting polar cap paradigm.