Effects on the distant geomagnetic tail of a fivefold density drop in the inner sheath region of a magnetic cloud: A joint Wind-ACE study

Using a serendipitous configuration of the ACE and Wind spacecraft, we monitor the response of the distant geomagnetic tail (∼ −220 RE) to an abrupt, approx. fivefold pressure drop (from ∼19.0 to ∼3.5 nPa) at the front boundary of a magnetic cloud (MC) on November 20, 2003. The interplanetary data are from ACE in orbit around the L1 point. The far-tail observations are from Wind, which was nominally in the magnetosheath, separated from the Sun-Earth line by ∼40RE. The magnetic field in the innermost sheath region of the MC had a large By(∼30nT) and substantial and variable flows lateral to the Sun-Earth line. There was also a significant northward field (∼35 nT), unique in the vicinity of this MC. These extreme values are reached in a filament forming the earliest relic of material accreted by the MC en route to Earth. The effects resulting from these on the far geomagnetic tail are: (1) expansion, (2) tail twisting, and (3) tail tilting. These extreme conditions were in part responsible for a crossing by Wind of a neutral sheet which is tilted by ∼85° to the ecliptic. Further, Wind made two successive excursions deep into the geomagnetic tail, in the first of which a tailward flow burst of ∼1200 km/s was observed. The dayside part of the interaction of the sudden and large dynamic pressure drop with the bow shock is studied with a local 3D MHD simulation. This work is a contribution to the area ICME/MC-sheaths-magnetosheath interactions.