The effect of gradient and curvature drifts on Forbush decreases

[1] Cause and general shape of Forbush decreases of cosmic rays are relatively well understood, however, the knowledge of their recovery times remains rather poor. Earlier results of theoretical and fragmentary statistical studies are in disagreement whether the recovery time does or does not depend on the energy of cosmic rays. A thorough empirical study of the recovery phase of strong isolated Forbush decreases is presented here, based on the ground based data from the World Neutron Monitor Network since 1964 and three ground based muon telescopes since 1973. In total 39 strong Forbush decreases, suitable for the analysis, have been identified for the period 1964-2006, 24 of them depicting a clear energy dependence of the recovery time and 15 consistent with no energy dependence. All analyzed Forbush decreases with magnitudes exceeding 10% demonstrate an energy dependence of the recovery time, while smaller events can be of either type. No apparent relation between the occurrence of energy dependent/ independent recovery and the IMF polarity has been found. This result provides an observational constraint for more detailed modeling of the propagation of interplanetary transients and their dynamic effects on cosmic ray transport.

Section: Introductionsupporting

confidence: 54%

Forbush decreases of cosmic rays: Energy dependence of the recovery phase

Usoskin

Braun

Gladysheva³

et al. 2008

“…The diffusion mechanism [ Laster et al , 1962; Nishida , 1982; Badruddin et al , 1986, 1991; Zhang and Burlaga , 1988; Venkatesan et al , 1992; Ananth and Venkatesan , 1993; Badruddin , 2002b] suggests that the decrease in GCR intensity is caused by interaction with the turbulent magnetic field of the IP shock sheath region rather than the strong magnetic field of the ICME. However, the drift mechanism [ Barouch and Burlaga , 1975; Cheng et al , 1990; Lockwood et al , 1991; Mulder and Moraal , 1986; Rana et al , 1996] implies that the FD is caused by the sweeping effect due to the enhanced magnetic field of the IP shock. Cane et al [1993] suggested a third view, that both enhanced turbulence and strength of the IMF in the shock sheath region cause the FD.…”

Section: Introductionmentioning

confidence: 99%

Globally nonsimultaneous Forbush decrease events and their implications

Kim

2008

[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.

“…An enhanced solar wind speed leads to increased advection, whereas variations in the magnetic field topology, strength, or/and irregularities lead to differences in the diffusion and drift rates. Some models have focused on enhanced drift [e.g., Cheng et al , 1990; Rana et al , 1996] while others have concentrated on diffusive or scattering models [e.g., Lockwood et al , 1986; Webber et al , 1986; Wibberenz and Cane , 2000; Chih and Lee , 1986; Badruddin , 2002]. For an overview, see le Roux and Potgieter [1991].…”

Section: Introductionmentioning

confidence: 99%

Decay of interplanetary coronal mass ejections and Forbush decrease recovery times

Penna

Quillen

2005

We investigate the relation between Forbush cosmic ray decrease recovery time and coronal mass ejection transit time between the Sun and Earth. We identify 17 Forbush decreases from ground‐based neutron count rates between 1978 and 2003 that occur during the same phase in the solar cycle and can be associated with single coronal mass ejections (CMEs) in the SOHO LASCO CME Catalog or previously published reports and with specific interplanetary coronal mass ejections (ICMEs) crossing the vicinity of Earth. We find an anticorrelation between Forbush recovery times and CME transit time that contradicts the predictions of simple cosmic ray diffusive barrier models. The anticorrelation suggests that the decay rate of ICMEs is anticorrelated with their travel speed. Forbush recovery times range from seven times the transit time for the fastest disturbance to a fifth the Sun‐Earth transit time for the slowest. To account for the large range of measured recovery times, we infer that the slowest disturbances must decay rapidly with radius, whereas the fastest ones must remain strong. The longest recovery times suggest that the fastest disturbances in our sample decayed less rapidly with radius than the ambient solar wind magnetic field strength.