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We examine the Forbush decreases (FDs) associated with solar flares and coronal mass ejections (CMEs) from January 2008 to December 2019, covering the whole of Solar Cycle 24, and we compare them to our earlier study of Solar Cycle 23 (Bhatt and Chandra, Adv. Space Res. 71, 1098, 2023). On a sample of 56 events, we conducted a statistical analysis using solar and geomagnetic parameters to examine their effect on interplanetary space and the decrease in the galactic cosmic rays (GCR). Our research concentrated on an in-depth analysis of the two most significant events. The first event with an FD magnitude of 11.2% on 8 March 2012 and the second with an FD magnitude of 7.7% on 7 September 2017, respectively. We discovered that compared to other latitude bands, the northern 11 – 20° latitude band had the highest number of FD-associated flare events. We conducted a statistical analysis of FD magnitude, geomagnetic Dst-index, solar and interplanetary parameters. [ABSTRACT FROM AUTHOR]

We investigate the association of flare-CME events accompanied with DH type II radio bursts with interplanetary features such as magnetic field (IMF, B), geomagnetic storms (GSs), and cosmic ray intensity (CRI) from 2008 to 2019, covering the whole Solar Cycle 24th and compared this study with our previous study for the same in Solar Cycle 23rd (Chandra and Bhatt 2018). We have divided our selected data into two groups: in the first group, we have 6 events that lie under the CME span, and in the second group, 40 events that lie outside the CME span. In Solar Cycle 23rd, we considered a total of 114 events, out of which 43 were lying under the CME span and 71 were lying outside the CME span. We observed that activity in Solar Cycle 24th is less than in Solar Cycle 23rd. We observed that flare-CME accompanied with DH type II radio burst is inconsistent with CSHKP flare-CME and it agrees with our previous result for Solar Cycle 23rd. Applying the Chree analysis by the superposed epoch method, we found that under the CME span events, the maximum increase in IMF occurred one day before the events that occurred outside the CME span. Further, in Solar Cycle 24th, we observed that events in which flares lie outside the CME span are more geoeffective, whereas, in Solar Cycle 23rd, events in which flares lie under the CME span are more geoeffective. Thus, flare position with respect to the CME span plays a key role in the geo-effectiveness of the CME. [ABSTRACT FROM AUTHOR]

In the present study, we have selected 35 halo Coronal Mass Ejections (CMEs) associated with solar flares, Geomagnetic Storms (GSs) and Forbush decrease (Fd) chosen from 1st January 2000 to 31st December 2007 (i.e., the descending phase of solar cycle 23) observed by the Large Angle Spectrometric Coronagraph (LASCO) on board the SOHO spacecraft. Statistical analyses are performed to look at the distribution of solar flares associated with halo CMEs causing GSs and Fd and investigated the relationship between solar flare and halo CME parameters with GSs and Fd. Forbush decrease is the phenomenon of rapid decrease in cosmic ray intensity following the CME. Our analysis indicates that during 2000 to 2007 the northern region produced 44 % of solar flares associated with halo CMEs, GSs, and Fd, whereas 56 % solar flares associated with halo CMEs, GSs, and Fd were produced in the southern region. The northern and the southern hemispheres between 10° to 20° latitudinal belts are found to be more effective in producing events leading to Fd. From our selected events, we found that about 60 % of super-intense storms ( $\mbox{Dst} \leq -200~\mbox{nT}$ ) caused by halo CMEs are associated with X-class flares. Fast halo CMEs associated with X-class flares originating from 0° to 25° latitudes are better potential candidates in producing super-intense GSs than the slow halo CMEs associated with other classes of flares. [ABSTRACT FROM AUTHOR]

We investigate the association of the cosmic-ray intensity (CRI) with the interplanetary magnetic field (IMF, $B$) and geomagnetic storms (GS) for the period 1997 - 2006 (Solar Cycle 23). To do this, we conducted a Chree analysis by the superposed-epoch method. A transient decrease in CRI is found on the occurrence days of GS, and this decrease shows a similar pattern to that of the disturbance storm-time index (Dst). In addition, we show that the CRI decreases with the increase in IMF. The time lag between the decrease in CRI and increase in IMF is about one day or less. Furthermore, an increase in IMF $B$ is found with the decrease in Dst index. IMF and Dst index are highly anti-correlated to each other, while the sunspot number is not found to be correlated with IMF, Dst index, or CRI for the period studied. The IMF is found to be an effective parameter combination for producing GS and Forbush decrease. We also found two types of decrease in CRI for Solar Cycle 23: i) symmetric and ii) asymmetric decreases. The study of CRI decreases may be useful for studying space-weather effects. [ABSTRACT FROM AUTHOR]