The two‐step Forbush decrease: An empirical model

Abstract

[1] The cosmic ray decreases and interplanetary disturbances observed at 1 AU have been investigated by using some cosmic ray hourly intensities recorded with groundbased monitors at Alert, Deep River, and Mount Washington as well as the interplanetary magnetic field (IMF) and the solar wind plasma bulk speed, density, and temperature in the near-Earth space. We studied a two-step Forbush decrease (FD)-type, with large cosmic ray intensity decreases, on 29-30 September 1978, 24-26 April 1979, 27-28 August 1978, 30 January 1978, 13-15 January 1967, 3-5 January 1978, 28-29 November 1989, and 29-30 December 1989, each of which appears to be produced by the structure within the shock and sheath preceding the interplanetary coronal mass ejection (ICME). There is a sheath upstream of the ICME led by a fast forward shock. The sheath material is slow solar wind that has been swept up by the ICME and is not part of the ICME itself. The sheath material is hot, dense, and turbulent and is most probably produced by the shock. The large IMF variations in the shocked plasma scatter the galactic cosmic rays and thus sweep away the cosmic ray particles. When the IMF variations in the sheath become too feeble (the lull region), the scattering of the galactic cosmic rays ceases, corresponding to the recovery phase of the first step of the FD; the particle scattering resumes when the high IMF variations in the sheath resume and the onset of the second decrease starts. There is a second and rear region inside the interplanetary macrostructure, the ICME/ magnetic cloud, where the galactic cosmic rays are also scattered away from the high magnetic field pressure in the magnetic cloud. A data set with fine time resolution (less than an hour) and good statistics is needed to provide an exact timing of the various phenomena.

Time variation analysis of the daily Forbush decrease indices

“…Belov et al (2001Belov et al ( , 2005 explained the Forbush effects as a storm in cosmic rays, which is a direct consequence of heliospheric storm and is very often observed simultaneously with a geomagnetic storm. All these perturbation disturbances in the solar wind, magnetosphere and cosmic rays are closely interrelated (Ifedili 2004) and are caused by the same active processes on the Sun. It is natural and useful to investigate them in common.…”

Magnetic clouds, cosmic ray decreases, and geomagnetic storms

“…The sheath is sufficiently turbulent as indicated by the large variations in both the field strength and direction as well as by the relatively high ( 10 nT) rms of the IMF components in the region; the sheath has a relatively high speed, is hot ( 10 5 K) and dense ( 10 cm −3 ) and was most probably produced by the shock propagation in space (Ifedili, 2004, and references therein).…”

“…• Magnetic Cloud: This is the region of IMF rotation (IMF direction changes smoothly from large southern (northern) direction to large northern (southern) direction) and elevated IMF intensity ( 10 nT), and relatively low SWP density and temperature ( 10 4 K) and relatively low ( 1 nT) rms of the IMF components (e.g., Burlaga et al, 1981;Klein and Burlaga, 1982;Zhang and Burlaga, 1988;Bavassano et al, 1989;Ifedili, 2004). Our estimated start and end times of the magnetic cloud in Fig.…”

“…These disturbances are the primary cause of geomagnetic storms (e.g., Gosling, 1993a, b). On the other hand, the two-step Forbush decrease (FD) in cosmic ray records seems to originate from the structure within the shock and sheath preceding the interplanetary coronal mass ejection (Ifedili, 2001(Ifedili, , 2004. Moreover, magnetic clouds pertain to a subset of CMEs Copyright c The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.…”

“…• Lull Region: This is the middle region of the sheath that coincides with the recovery phase of the first step of the two-step FD, and in which there are relatively low IMF variations as well as a relatively low ( 5 nT) rms of the IMF components (Ifedili, 2001(Ifedili, , 2004). …”