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The annual average values of amplitudes and phases of first two harmonics of cosmic ray anisotropy have been derived by using the harmonic analysis technique for the period 1989 to 2004, which covers mostly the major period of solar cycles 22 and 23. In this paper we have taken the pressure corrected hourly data for Kiel neutron monitor station (cut off rigidity ≈ 2.29 GV) to derive the harmonic component of cosmic ray daily variation and compared with the data of Halekala neutron monitor (cut off rigidity ≈ 13.2 GV) for the period 1991 to 2004. From the analysis it has been concluded that the diurnal amplitude and phase of daily variation of cosmic rays have been found to be correlated with solar activity. However, the semi-diurnal amplitude and phase are inversely correlated with solar activity for both the stations

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Transient decrease in cosmic ray intensity following by a slow recovery typically lasting for several days is identified as Forbush decrease (Fd) event. As a result the geomagnetic index (Dst) decreased up to 300 nT, indicating a large geomagnetic storm and the percentage Fd decrease has gone to 16% giving rise a cosmic ray storm. Both events coincided with interplanetary conditions. Therefore, a systematic study has been performed to investigate the variation of cosmic ray intensity along with the interplanetary and geomagnetic disturbances. Results indicate a strong relationship between geomagnetic activity and Forbush decrease on short-term basis. Two types of interplanetary transient disturbances, namely magnetic cloud events and bidirectional events are analyzed to study the shortterm changes in the solar wind (SW) plasma components as well as in cosmic ray intensity.

[1] Often the interplanetary parameters used for the study of solar-terrestrial relationships are solar wind speed (V), the total interplanetary magnetic field (B), and the southward component of IMF (Bz). Both hourly and daily values of these parameters have usually been employed to associate with parameters defining terrestrial effects. For our study, we have used the daily values of the interplanetary indices (V, B, and Bz), as well as the daily values of the geomagnetic disturbance index Ap, for the years 1965-2007. The long-term averages have been calculated for the days when simultaneous data are available for all the four parameters (V, B, Bz, and Ap). These include their yearly averages, as well as the averages on the basis of the phases of the solar activity cycle. The statistical relationship between them has been investigated on these long-term averages, as well as on the day-to-day basis. These sets have been further divided and studied on the basis of days with Bz being <0 or !0. The results obtained can be summarized as follows: (1) the magnitudes of the values of Ap are observed to be always enhanced significantly for the case of Bz < 0, as compared to days with Bz ! 0, and (2) V versus Ap has always low values of correlation coefficient (r). (3) On the contrary, B versus Ap has high values of ''r''; (4) the product of V and B versus Ap always yields much better correlations than for V or B alone, either on an average basis or on a day-to-day basis, and (5) although, the long-term variations of V are not very significant, the variations of B follow solar activity cycle and has a continuously increasing trend during the solar cycles 20 and 21. The statistical results obtained here signify that the product of V and B is more significantly effective in producing large-scale geomagnetic disturbances.Citation: Charan Dwivedi, V., D. P. Tiwari, and S. P. Agrawal (2009), Study of the long-term variability of interplanetary plasma and fields as a link for solar-terrestrial relationships,