29th International Cosmic Ray Conference, Pune, India, 2005

Imaging Interplanetary Disturbances Causing Forbush Decreases

S.W. Kahler
Air Force Research Laboratory, Space Vehicles Directorate, Hanscom AFB, MA, USA

G.M. Simnett
School of Physics and Astronomy, Univ. of Birmingham, UK


Forbush decreases (FDs) in neutron monitor (NM) counting rates are caused by enhance magnetic fields in interplanetary shocks and solar ejecta that shield the Earth from galactic cosmic rays (GCRs). The solar origins of those ejecta can be observed as coronal mass ejections (CMEs) in coronagraphs, but their propagation through interplanetary space near of past the Earth has not been previously observable. The Solar Mass Ejection Image (SMEI), launched into polar Earth orbit in January 2003, now allows us to search for the white light signatures of interplanetary CMEs (ICMEs) responsible for FDs. SMEI is unique in that it can monitor the progress of CMEs through the inner heliosphere out to disturbances beyond 1 AU and distinguish those which hit the Earth from those that do not. For comparison with SMEI observations, we selected all FDs of >= 2% observed with the Oulu, Finland, NM. We find an excellent association of SMEI CMEs with those FDs and for each of the associated SMEI CMEs a good candidate associated LASCO CME was also found. The SMEI observations provide information on the approximate spatial locations and trajectories of large ICMEs that may result in FDs and hencee can be useful as a space weather tool.