An interplanetary cause of large geomagnetic storms: Fast forward shock overtaking preceding magnetic cloud

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Abstract
In the event that occurred during October 3–6, 2000, at least one magnetosonic wave and one fast forward shock advanced into the preceding magnetic cloud (MC). By using the field and plasma data from the ACE and WIND spacecraft, we analyze the evolution of this event, including the characteristics and changes of the magnetic fields and plasma. At the rear part of the cloud, a large southward magnetic field is caused by a shock compression. The shock intensified a preexisting southward magnetic field. This increased the geoeffectiveness of this event and produced an intense geomagnetic storm with Dst = −175 nT. We also describe another event with a shock overtaking a MC on Nov. 6, 2001. A great geomagnetic storm of intensity Dst = 292 nT resulted. These observations are used to argue that shock compression of magnetic cloud fields is an important interplanetary cause of large geomagnetic storms. Our analyses suggest that the geoeffectiveness is related to the direction of preexisting magnetic fields, the intensity of overtaking shock, and the amount of shock penetration into the preceding MC.

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