Rapid and Asymmetric Response of the Earth＇s Bow Shock： Multipoint Observations

We analyze observations of three bow shock crossings which occurred during 2007.using upstream data from STEREO A/B.ACE and WIND,combined with multi-point THEMIS and Cluster data,and TC-1 data located near noon.During the crossing of 7 May 2007.we find that following a rapid reduction in solar wind ram pressure and subsequent pressure pulse seen by ACE and WIND upstream,the bow shock responds asymmetrically from dawn to dusk.Cluster data on the dawn-side suggest the bow shock is significantly flared and responds rapidly to the pulse arrival,while TC-1 at noon,and THEMIS on the dusk-side,are well matched to the model bow shock,but show a delayed response.The crossings observed on 21 May and 2 June show contrasting response matching the model boundary for northward Interplanetary Magnetic Field(IMF).The IMF and solar wind plasma data suggest that,the bow shock crossing at dawn-dusk side and subsolar point were mainly caused by large and smaller scale features of the solar wind ram pressure rise rather than the influence of IMF.