Transition through the magnetosheath matters for space weather

Magnetic clouds are gigantic helical flux ropes in interplanetary space. They originate from the Sun as violent eruptions of plasma and magnetic field, called coronal mass ejections. Magnetic clouds are one of the key causes of significant space weather storms at the Earth as they often embed strong and southward magnetic fields. Before reaching the Earth’s magnetic environment, magnetic clouds  move past the bow shock and the magnetosheath.  This can alter significantly their magnetic structure and change their ability to drive geomagnetic disturbances. In our new Journal of Geophysical Research  paper we study the transition of 82 magnetic clouds from the solar wind to the Earth’s magnetosphere using a magnetohydrodynamic (MHD) model and observations from THEMIS, Double Star, GEOTAIL, and Interball. We found that the largest changes in their magnetic structure occurred when the bow shock was quasi-parallel, i.e., when kinetic processes dominate. In such a case the MHD model is not sufficient to capture the transition. Our results also emphasise the importance of the east-west magnetic field component in controlling the ability of a magnetic cloud to drive magnetospheric storms. This has important implications for forecasting space weather as all those spacecraft  that provide continuous solar wind measurements are  located upstream of the bow shock.

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