Pronounced diurnal variation in cosmic-ray intensity

Abstract

Enhanced diurnal variations during February 1–6, 1967; April 5–14, 1967; and February 22–28, 1968, have been examined by use of data from several super neutron monitors. It is found that during these days, cosmic‐ray intensity in a direction 40°–70° west of the sun‐earth line was lower with respect to other directions, in particular the polar and antigarden‐hose directions. This anisotropy, which resulted from a reduced intensity in the garden‐hose direction in the ecliptic plane, suggests that the entire diurnal variation observed in cosmic‐ray intensity cannot be attributed to a simple mechanism involving the corotation of cosmic‐ray gas, with the interplanetary field. Corotation of cosmic‐ray gas implies an enhancement of intensity from a direction roughly 90° east of the sun‐earth line (1800 hr direction). Highly variable north‐south asymmetries were also present during these periods. These anisotropies occurred without accompanying geomagnetic disturbances or Forbush decreases indicating that they were not due to solar activity on the visible side of the sun.

Solar modulation of galactic cosmic radiation

“…Figure 37 shows such examples of large and small amplitude wave trains observed by the Calgary neutron monitor. Mathews et al (1969) and Hashim and Thambyahpillai (1969) have independently concluded that during many days exhibiting large amplitude waves, the anisotropy was predominantly caused by a large decrease along the garden hose direction. The possible existence of such 'sinks' in the garden hose direction have been inferred earlier by Rao and Sarabhai (1964) from their study of the distribution of times of maxima and minima.…”

“…Recently attention has also been called to the wave trains with enhanced amplitudes caused by increased flux in the 18-21 hr directions (Kane, 1970;Rao et al, 1971b). Mathews et al, 1969).…”

“…The difference curve will therefore truly represent the local time variation of the cosmic ray flux at each hour, provided no strong asymmetries in the mean intensity are present during this period. In reality, strong and highly variable asymmetries have been observed (Mathews et al, 1969;Mercer et al, 1971) during such periods which makes the application of difference method to study such anisotropies questionable.…”

“…The analysis of the experimental observations, however, have yielded conflicting results. Whereas McDonald and Webber (1960), Webber (1962) and mo re recently Balasubrahmanyan and Venkatesan (1970), believe that the Forbush decreases and the long-term variation have essentially same rigidity dependence, McCracken (1960), Kane (1966, Kane and Winkler (1969) and Lockwood et al (1970) have concluded that the rigidity dependence of Forbush decrease is substantially different from the rigidity dependence of the 11 yr variation. Even though, in principle, extension of analysis to lower energies utilising the spacecraft data, should yield unambiguous results, the uncertainty in the background contribution to the low energy cosmic rays from local secondaries produced in the spacecraft and the difficulty in the exact determination of the solar contribution to the low energy cosmic radiation renders the interpretation derived from using the low energy cosmic radiation difficult.…”

The cosmic ray solar diurnal anisotropy

“…Thus, for example, the analysis of data recorded during the period February 1-6, 1967, and a few similar intervals at subsequent time (Lockwood, 1968;Mathews et al, …”