The coherence between the IMF and high-latitude ionospheric flows: The dayside magnetosphere–ionosphere low-pass filter |
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| Institution: | 1. Thayer School of Engineering, Dartmouth College, Hanover, NH, USA;2. Department of Astronomy, Boston University, Boston, MA, USA;1. Stanford University, Stanford, USA;2. Southwest Research Institute, San Antonio, USA;3. University of California, Los Angeles, USA;1. Built Environment Energy Systems Group (BEESG), Division of Solid State Physics, Department of Engineering Sciences, Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Sweden;2. Division of Electricity, Department of Engineering Sciences, Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Sweden;1. Center for Space Environment Modeling, University of Michigan, Ann Arbor, MI 48109, USA;2. K.U. Leuven, Belgium;3. KTH, Stockholm, Sweden;4. Los Alamos National Laboratory, USA;1. Institute for Risk and Disaster Reduction, University College London, London, UK;2. Department of Political and Social Studies, University of Bologna, Bologna, Italy |
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| Abstract: | This study seeks to establish a new system characteristic describing dayside convective flows in the coupled magnetosphere–ionosphere: the low-pass filter function through which interplanetary magnetic field (IMF) fluctuations are processed as they are communicated from the magnetopause to the high-latitude ionosphere near local noon. In doing so, this study confirms that variations in the ionospheric flows at high-latitudes near local noon are well correlated with variations in the IMF orientation and magnitude on short timescales. We construct the filter function by comparing time series of the ionospheric equivalent flows at a fixed location at magnetic local noon and 80° latitude with time series of the IMF. The coherence spectra of these two parameters—averaged over 330 h of comparison—indicate that there is a low-pass cutoff in the ionospheric response to IMF driving at a periods shorter than 20 min (frequencies higher than 0.8 mHz). When there is sufficient power in the IMF fluctuations, this cutoff is relatively sharp—the coherence drops by roughly a factor of three between the periods 32 and 21 min (0.5 and 0.8 mHz). The results also show that on average the coherence between the east–west component of the equivalent flows and IMF By tends to be less than the coherence between the north–south component of the equivalent flows and IMF Bz. |
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