Interaction of magnetic clouds with the Earth’s environment: effects due to the terrestrial bow shock

Thursday, 18 December 2014
Dominique Fontaine, Lucile Turc and Philippe Savoini, LPP - CNRS - Ecole polytechnique - UPMC, Palaiseau, France
Magnetic clouds represent a sub-class of coronal mass ejection (CME) well identified in the solar wind by ACE or WIND with a magnetic structure described as a flux rope. They are known as very geoeffective events, capable to trigger strong storms in the terrestrial environment. Statistical studies that address their geoeffectiveness show dependence on their orientation in the interplanetary medium, on preceding or trailing events, … Before interacting with the magnetosphere, they cross the terrestrial bow shock and the magnetosheath. From CLUSTER observations in the magnetosheath, we show that their magnetic structure can be similar or modified downstream of the bow shock, and in some cases it presents a large rotation relative to upstream direction. The orientation of the magnetic field and in particular the orientation of the Bz component is crucial for the development of geomagnetic activity inside the magnetosphere. From observations and from simple modeling, we examine the configurations where the terrestrial bow shock is responsible for a modification of the clouds' magnetic field orientation and even for its reversal. We show that this depends on the configuration quasi-parallel/quasi-perpendicular at the shock and discuss the possible consequences for the geomagnetic activity.