Globally nonsimultaneous Forbush decrease events and their implications

Abstract

[1] 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.

A multivariate study of Forbush decrease simultaneity

“…The difference in time of maximum decrease between stations represented by the same color code is about 1-3 h while the difference in time of detection between different color codes might be as long as a day. This indicates, as noted by Oh et al (2008), that globally nonsimultaneous Fds could be simultaneous at similar local times. It is believed that the differences in the time of observation could be attributed to changes in the directions of IMF passing over the Earth.…”

“…They agreed that the increase before the main phase onset time is dependent on local time. Recently, Oh et al (2008) and Oh and Yi (2009) investigated the global simultaneity of Fds by comparing the main phase of large and small Fds observed by three neutron monitors. Their definition of a simultaneous event was not based on the onset time or the time of maximum decrease in cosmic ray intensity.…”

Long-term Statistical Analysis of the Simultaneity of Forbush Decrease Events at Middle Latitudes

“…When magnetic clouds and IP shocks pass, the cosmic ray intensity decreases, and if the perturbation is strong, a sharp drop in cosmic ray intensity occurs. Oh et al (2008), and Oh & Yi (2009, 2011, classified FDs observed using the NMs located at high latitudes (Inuvik, Magadan, and Oulu) into simultaneous and nonsimultaneous FD events. They explained that the main phase of the cosmic-ray-intensity profile of simultaneous FD events observed at these three NM stations, overlap in universal time (UT), while the main phase of nonsimultaneous FD events overlap in local time (LT).…”

Simultaneity of Forbush decrease events observed at middle‐latitude neutron monitors

“…They can also be detected in muon signals. FD events are caused by magnetic‐field variations linked to the interplanetary shock‐sheath region [ Parker , ; Gosling et al ., ; Hundhausen , ; Kennel et al ., ; Lockwood et al ., ] and the magnetic cloud [ Burlaga et al ., , ; Klein and Burlaga , ; Sanderson et al ., ; Zhang and Burlaga , ; Badruddin , ; Oh et al ., ; Oh , ]. Such events act as interplanetary shock drivers.…”