Physics
Scientific paper
Aug 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001icrc....9.3784s&link_type=abstract
Proceedings of the 27th International Cosmic Ray Conference. 07-15 August, 2001. Hamburg, Germany. Under the auspices of the Int
Physics
3
Scientific paper
We analyze cosmic ray data as well as interplanetary magnetic field (IMF) data, to examine the relation and correlation between their 27-day variations during the time interval 1965-1995. The amplitude of the 27day variation of galactic cosmic rays is linearly correlated with: the IMF strength (B), the z-component (Bz) of the IMF vector and the product of the solar wind speed (V) times B (VB). It is well correlated with the heliospheric current sheet tiltangle.Thecross-correlationfunctionofthe27-daycosmic ray variation versus the solar wind speed shows a negative correlation. The solar wind speed leads the cosmic ray variation by 2 years. The 27-day variation of cosmic rays is correlated with the variation in both the xand y-components of the IMF, it lags with 3-5 years. 1. Introduction Galactic cosmic rays are modulated (modified) through their propagation in the heliosphere by the effect of the large scale structure of the interplanetary medium. A wavy structured neutralcurrentsheet(NCS) separatesthe heliosphereintotwo regions of opposite magnetic polarity. During positive magnetic phase, the interplanetary magnetic field (IMF) is directed away from the Sun above the NCS and toward the Sun south of it. During negative magnetic phase the IMF direction is reversed. The angle between the Sun's equatorial plane and the NCS is referred as the tilt angle R, of the neutral sheet. It exhibits a solar activity dependence, R is small near sunspot minimum and large near solar maximum. The 27-day variations of galactic cosmic rays have been related to the changing position of the interplanetary NCS (Swinson and Yasue, 1992; Valdes-Galicia and Dorman, 1997). Here we examine the effect of the interplanetary parameters upon the 27-day variation of galactic cosmic rays during the last three solar cycles. 2. Solar Cycle Dependance We used hourly averaged cosmic ray counts observed with neutron monitors at Deep River (DR) and Huancayo (HU) and muon surface telescope at Nagoya (NA). The amplitude of the 27-day variation has been calculated for each detector for every solar rotation during the time interval 1965 to 1995 (Sabbah, 2001). The 13-solar rotationrunning averages ofthe amplitude of the three detectors are displayed in the top panels (a & b) of Fig.1. We also used hourly averages values of IMF and plasma (OMNI data base form NSSDC) collected by variety of spacecraft near 1 AU during the same period in order to study their effect upon the 27-day cosmic ray variation. The 27-day averages of each solar rotation have been calculated for: field magnitude B, its components in the geocentric solar ecliptic coordinates (Bx, By, Bz), solar wind speed V and the product VB (Sabbah, 2000). These data cover three solar cycles (20, 21, 22). In panel (f), we plot the smoothed NCS tilt angle R. We see from the top panels (a & b) of Fig. 1 that the amplitude of the 27-day variation of cosmic ray exhibits an 11-year sunspot cycle variation, with minima occurring near sunspot minimum represented with dotted vertical lines at 1976 and 1986 and maxima occurring near sunspot maximum denoted with dashed vertical lines at 1968, 1979 and 1989. The values of the IMF magnitude B shown in (b), the product VB shown in (e) and the NCS tilt angle R, shown in (f), display separate solar cycle variation during cycles 21 and 22. Their values track the variation in the 27-day cosmic ray amplitude. The values of B and VB are enhanced right after solar activity maximum in 1979 and
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