A new concept of data analysis has been attempted for studying the long/short term daily variations in cosmic ray (CR) intensity recorded with neutron monitors/meson telescopes. Fourier Technique has been applied on four different types of groups of days chosen according to their different geomagnetic conditions. The selected groups are 60 quietest days (60 QD), 120 quiet days (120 QD), continuous quiet days (CQD) and All Days (AD) in a year. CQD is a new set of days selected on the basis of Ap and Kp values. These are the days when transient magnetic variations are supposed to be regular and smooth continuously for a span of at least three days. The criteria of selection of CQD is based upon the mathematical manipulation with Ap index. The data of Deep River neutron monitoring station is used for the period 1985–1995, to investigate for a com-parative study of diurnal anisotropy in CR intensity on 60 QD, 120 QD, CQD and AD. It is observed that 60 QD are most suitable for the anisotropic studies on short/long term basis. The time/spatial variations in the amplitude and phase of the diurnal anisotropy become more pronounced for 60 QD for the period under investigation.

The present work deals with the study of first three harmonics of cosmic ray intensity on geo-magnetically quiet days over the period [1981][1982][1983][1984][1985][1986][1987][1988][1989][1990][1991][1992][1993] for Deep River and Inuvik neutron monitoring stations having mid and low cutoff rigidity. The amplitude of first harmonic remains high for Deep River having mid cutoff rigidity as compared to Inuvik neutron monitor having low cutoff rigidity on quiet days. The diurnal amplitude significantly decreases and phase shifts towards an earlier time during solar activity minimum years at both Deep River and Inuvik. The amplitude of second harmonic significantly enhanced during solar activity minimum as well as maximum at Deep River and remains low during solar activity maximum at Inuvik, whereas the phase shifts towards an earlier time during solar maximum for both the stations. The amplitude of the third harmonic significantly enhanced during solar activity minimum at Deep River and on solar activity minimum at Inuvik, whereas the phase does not show any significant characteristics and fluctuates quite frequently. The amplitude of semi/tri-diurnal anisotropy has a good positive correlation, while the others (i.e. amplitude and phase) have very weak correlation with solar wind velocity on quiet days at Deep River station during 1981-1993. The solar wind velocity significantly remains in the range 350 to 425 km/s i.e. being nearly average on quiet days. The amplitude and direction of the anisotropy on quiet days are weakly dependent on high-speed solar wind streams for two neutron monitoring station of mid and low cutoff rigidity threshold. The amplitude as well as direction of second harmonic has a good anti-correlation with IMF Bz and the product V x Bz on quiet days at Deep River station. However, the direction of second and third harmonic has a good anti-correlation with IMF Bz and the product V x Bz on quiet days at Inuvik station.

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has been presented so as to establish a possible correlation between them. We used the hourly averaged cosmic ray counts observed with the neutron monitor in Moscow. It is noteworthy that a significant negative correlation has been observed between the interplanetary magnetic field, product (V × B) and cosmic ray intensity during the solar cycles 21 and 22. The solar wind velocity has a good positive correlation with cosmic ray intensity during solar cycle 21, whereas it shows a weak correlation during cycles 20, 22 and 23. The interplanetary magnetic field shows a weak negative correlation with cosmic rays for solar cycle 20, and a good anti-correlation for solar cycles 21-23 with the cosmic ray intensity, which, in turn, shows a good positive correlation with disturbance time index (D st ) during solar cycles 21 and 22, and a weak correlation for cycles 20 and 23.

In the present work an analysis has been made of the extreme events occurring during July 2005. Specifically, a rather intense Forbush decrease was observed at different neutron monitors all over the world during 16 July 2005. An effort has been made to study the effect of this unusual event on cosmic ray intensity as well as various solar and interplanetary plasma parameters. It is noteworthy that during 11 to 18 July 2005 the solar activity ranged from low to very active. Especially low levels occurred on 11, 15, and 17 July whereas high levels took place on 14 and 16 July 2005. The Sun is observed to be active during 11 to 18 July 2005, the interplanetary magnetic field intensity lies within 15 nT, and solar wind velocity was limited to ~500 kms-1. The geomagnetic activity during this period remains very quiet, the Kp index did not exceed 5, the disturbance storm time Dst index remains ~-70 nT and no sudden storm commencement has been detected during this period. It is noted that for the majority of the hours, the north/south component of the interplanetary magnetic field, Bz, remains negative, and the cosmic ray intensity increases and shows good/high correlationwith Bz, as the polarity of Bz tends to shift from negative to positive values, the intensity decreases and shows good/high anti-correlation with Bz. The cosmic ray intensity tends to decrease with increase of interplanetary magnetic field strength (B) and shows anti-correlation for the majority of the days.