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Substorm occurrence during quiet solar wind driving
Author(s) -
Pulkkinen T. I.,
Partamies N.,
Kilpua E. K. J.
Publication year - 2014
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2013ja019503
Subject(s) - substorm , electrojet , solar wind , interplanetary magnetic field , magnetometer , physics , quiet , earth's magnetic field , electric field , atmospheric sciences , environmental science , geophysics , magnetic field , astronomy , magnetosphere , quantum mechanics
We examine the OMNI database and International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer chain records to study the substorm occurrence and characteristics during quiet solar driving periods, especially during the solar minimum period in 2009. We define substorm‐like activations as periods where the hourly average AL is below −200 nT. Using the OMNI data set, we demonstrate that there are limiting solar wind speed, interplanetary magnetic field magnitude, and driving electric field values below which substorm‐like activations ( AL < 200 nT, intensification and decay of the electrojet) do not occur. These minimum parameter values are V < 266 km/s, B < 1.4 nT, and E < 0.025 mV/m; such low values are observed less than 1% of the time. We also show that for the same level of driving solar wind electric field, the electrojet intensity is smaller (by few tens of nT), and the electrojet resides farther poleward (by over 1°) during extended quiet solar driving in 2009 than during average solar activity conditions. During the solar minimum period in 2009, we demonstrate that substorm‐like activations can be identified from the IMAGE magnetometer chain observations during periods when the hourly average IL index is below −100 nT. When the hourly IL activity is smaller than that, which covers 87% of the nighttime observations, the electrojet does not show coherent behavior. We thus conclude that substorm recurrence time during very quiet solar wind driving conditions is about 5–8 h, which is almost double that of the average solar activity conditions.