Globally nonsimultaneous Forbush decrease events and their implications

Details Globally nonsimultaneous Forbush decrease events and their implications Globally nonsimultaneous Forbush decrease events and their implications

Jan 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008jgra..11301103o&link_type=abstract

Journal of Geophysical Research, Volume 113, Issue A1, CiteID A01103

Physics

3

Interplanetary Physics: Cosmic Rays, Interplanetary Physics: Interplanetary Shocks, Interplanetary Physics: Interplanetary Magnetic Fields, Interplanetary Physics: Heliosphere/Interstellar Medium Interactions, Magnetospheric Physics: Solar Wind/Magnetosphere Interactions

Scientific paper

Forbush decrease (FD) events are supposed to happen simultaneously over the globe of the Earth. However, there have been several reports on nonsimultaneous FD events. We investigate for the first time the properties of nonsimultaneous FD events and the solar wind conditions causing such events in detail in order to determine what solar wind conditions lead to global simultaneity of FD events. We examined the hourly data of the Oulu Neutron Monitor (NM) station from 1998 to 2002. We have selected 49 FD events that have greater than 3.5% intensity reductions. Global simultaneity was determined by comparing the time profiles of these FD events with those recorded by other NM stations at Inuvik and Magadan. These three NM stations are located at close magnetic latitude (cutoff rigidity) but fairly evenly spaced in longitudes. The solar wind parameters driving FD event main phase are studied. Most FD event onsets (37 out of 49) are observed simultaneously by each NM station in universal time (UT) regardless of the location of the station, whereas some other FD events are not simultaneously detected but are at similar local times (LT). The stronger FD events tend to be simultaneous events, but the weaker FD events are nonsimultaneous. The simultaneous FD events are caused by the solar wind of higher speed and stronger IMF, whereas the nonsimultaneous events are driven by relatively lower speed and weaker IMF solar wind. The nonsimultaneous FD event may occur only if the main phase of the FD is superposed in phase with the declining phase of diurnal variation. Our interpretation is that the simultaneous FD events might occur when the high-speed strong magnetic barrier (interplanetary shock sheath and magnetic cloud) overtakes the Earth, whereas the nonsimultaneous FD events may occur only when the slow-moving weak magnetic barrier passes by on the duskside of the Earth. The global simultaneity of FD events depends on speed and IMF strength of solar wind overtaking Earth's magnetosphere and its propagation direction. This model of FD simultaneity can be tested by the STEREO mission.

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