On the importance of magnetotail untwisting

Abstract:

The magnetospheric response to a dawn-dusk component in the Interplanetary Magnetic Field (IMF) is known to lead to a number of asymmetries throughout the entire magnetosphere. We here present an event where simultaneous global imaging of the aurora from both hemispheres reveals a large longitudinal shift of the nightside aurora of around 3 hrs. This is interpreted as evidence of a highly twisted magnetotail due to the persistent positive IMF By before and during the event. At the same time, SuperDARN observes ionospheric flows in the nightside flow reversal region in both hemispheres. Our analysis indicates a stronger flow towards the dayside in the dusk cell in the southern hemisphere compared to its conjugate region. We interpret this as a signature of the process known as "magnetotail untwisting". We analyse this event with emphasis on the expected field-aligned currents (FAC) associated with the magnetotail untwisting mechanism. A more distinct arc is seen in the region of the untwisting fast flows compared to its conjugate location suggesting that the observed faster flows are associated with enhanced FACs as expected. To further investigate the influence of magnetotatil untwisting on FACs, we show average FAC maps from AMPERE (Active Magnetosphere and Planetary Electrodynamics Response Experiment) during IMF By dominated conditions. We show that suchIMF By dominated conditions are indeed characterized by significant asymmetries in FACs between the hemispheres, in good agreement with what is predicted from the untwisting mechanism. We therefore suggest that the process of magnetotail untwisting, controlled mainly by IMF By, likely plays an important role in driving asymmetric FACs in the two hemispheres.