Dual E~\texttimes~B flow responses in the dayside ionosphere to a sudden IMF By rotation

<p>We report for the first time a dual transition state in the dayside ionosphere following a sudden rotation of the interplanetary magnetic field (IMF) in the upstream magnetosheath from IMF <em>B</em>_<em>y</em>\&nbsp;\&lt;\&nbsp;0 to <em>B</em>_<em>y</em>\&nbsp;\&gt;\&nbsp;0 during <em>B</em>_<em>z</em>\&nbsp;\&lt;\&nbsp;0. <strong>E</strong>\&nbsp;\texttimes\&nbsp;<strong>B</strong> drifts respond with different time delays in the dayside auroral zone and high-latitude polar cap with an initial 11\&nbsp;min transition state of oppositely directed <strong>E</strong>\&nbsp;\texttimes\&nbsp;<strong>B</strong> drifts coexisting at these locations. This is followed by a 6\textendash8\&nbsp;min rotation of lower latitude <strong>E</strong>\&nbsp;\texttimes\&nbsp;<strong>B</strong> flow from dusk to dawn. We propose that this sequence of events is consistent with two separate X lines coexisting on the subsolar and lobe magnetopause. Time delays are proposed for merged flux of the draped preceding IMF to exit the subsolar region before the new IMF may be processed along a newly reconfigured component reconnection X line. Finally, a strong direct correlation is observed between magnetosheath plasma density and auroral zone <strong>E</strong>\&nbsp;\texttimes\&nbsp;<strong>B</strong> speeds.</p>