Recurrent Forbush decreases and the relationship between active regions and M regions

Abstract

Recurrent Forbush decreases and recurrent geomagnetic disturbances have been attributed to the solar M regions, which are sources of high‐velocity solar plasma streams. A study of recurrent Forbush decreases for the period 1966‐1975 has been made to examine any possible relationship of M regions with solar active regions. It is shown that at the onset of the recurrent Forbush decrease at the earth there is a high probability of encountering a class of active regions at the central meridian of the sun which give rise to flares of importance ≥2B/3N. These active regions are found to be long lasting and to have large areas as well as high Hα intensities. Other active regions, producing flares of lower importance, are distributed randomly on the sun with respect to the onset of a recurrent Forbush decrease. By using the quasi‐radial hypervelocity approximation the base of the leading edge of the high‐velocity stream at the onset of a recurrent Forbush decrease at the earth is traced to the solar longitude about 40° west of the central meridian. From these results it is deduced that M regions are located preferentially to the west of long‐lasting magnetically complex active regions. Earlier studies of the identification of the M regions on the sun have been reexamined and shown to conform to this positional relationship. A possible mechanism of the development of an M region to the west of the long‐lasting magnetically complex active region is also discussed.

Corotating high‐speed solar‐wind streams and recurrent cosmic ray modulation

“…Recurrent cosmic ray modulation due to CIRs and high‐speed streams have been studied using ground‐based neutron monitors and spacecraft data [e.g., Vershell et al , 1975; Parker , 1976; Shah et al , 1978; Iucci et al , 1979; Duggal et al , 1981; Venkatesan et al , 1982; Tiwari et al , 1983; Burlaga et al , 1984; Mishra et al , 1990; Yadav et al , 1994; Shrivastava and Shukla , 1994; Kunow et al , 1995; Richardson et al , 1996, 1999; Badruddin , 1997; Zhang , 1997; Paizis et al , 1999; Alania et al , 2001; Reames and Ng , 2001; Gil et al , 2005, Gupta and Badruddin , 2005; Singh and Badruddin , 2005; Venkatesan and Badruddin , 1990; Simpson , 1998; McKibben et al , 1999; Richardson , 2004]. However, such depressions in cosmic ray intensity have been associated with enhanced convection by high‐speed solar wind [ Newkirk and Fisk , 1985; Iucci et al , 1979; Richardson et al , 1996], diffusion in enhanced/compressed field region [ Burlaga et al , 1984; Kota and Jokipii , 1991], and/or particle drifts in large‐scale heliospheric magnetic field [ Kota and Jokipii , 1991, 2001; Burger and Hitge , 2004].…”

Cosmic Ray and Geomagnetic Response to High Speed Solar Wind Streams

“…Short-term decrease in cosmic ray intensity observed by ground-based detectors are, in general broadly classified into two categories: (1) The classical (generally non-recurrent) Forbush decreases associated with transient on the Sun and (ii) the generally recurrent decreases associated with co rotating high speed solar wind streams from coronal holes (Venkatesan et al, 1978;Vershell et al, 1975 andShah et al, 1978). Space craft such as Voyagers and Pioneers have also observed some of these decreases to various distances from the Sun, with delays from those observed at 1 AU, depending on the solar wind speed.…”