Modulation of Galactic Cosmic Rays Due to Magnetic Clouds and Associated Structures in the Interplanetary Space: 1996-2018

Astrophysics - We study the modulation of galactic cosmic rays due to magnetic clouds observed during solar cycles 23 and 24 (1996-2018). We utilize solar wind plasma and field data together with...     

Analysis of plasma and field conditions during some intensely geo-effective transient solar/interplanetary disturbances of solar cycle 23

The problem of solar wind-magnetosphere coupling is investigated for intense geomagnetic storms (Dst < -100nT) that occurred during solar cycle 23. For this purpose interplanetary plasma and field data during some intensely geo-effective transient solar/interplanetary disturbances have been analysed. A geomagnetic index that represents the intensity of planetary magnetic activity at subauroral latitude and the other that measures the ring current magnetic field, together with solar plasma and field parameters (V, B, Bz, σB, N, and T) and their various derivatives (BV,-BVz, BV², -BzV², B²V, Bz²V, NV²) have been analysed in an attempt to study mechanism and the cause of geo-effectiveness of interplanetary manifestations of transient solar events. Several functions of solar wind plasma and field parameters are tested for their ability to predict the magnitude of geomagnetic storm.     

Influence of magnetic clouds on cosmic ray intensity variation

The data from a high counting rate neutron monitor has been analysed to study the nature of galactic cosmic-ray transient modulation associated with three classes of magnetic clouds, i.e., clouds associated with shock, stream interface and cold magnetic enhancement.It is found that the decreases in cosmic-ray intensity which are associated with clouds preceded by a shock, are very high (Forbush-type) and these decreases start earlier than the arrival of the cloud at the Earth. From the study of the time profile of these decreases it is found that the onset time of a Forbush-type decrease produced by a shock-associated cloud starts nearly at the time of arrival of the shock front at the Earth and the recovery is almost complete within a week.The decreases in cosmic-ray intensity associated with clouds followed by a stream interface are smaller in magnitude and larger in duration. The depression starts on the day of the arrival of the cloud.The decreases associated with the third category of clouds, i.e., clouds associated with cold magnetic enhancement (a region in which plasma temperature is anomalously low and the magnetic field strength is enhanced) are of still smaller amplitude and duration. The decrease in this case starts on the day the cloud arrives at the Earth.It seems that the Forbush decrease modulating region consists of a shock front followed by a plasma sheath in which the field intensity is high and turbulent. The amplitude of decrease is related to the field magnitude and the speed of the cloud. Both shocked plasma and the magnetic cloud are influential in determining the time profile of these decreases. In our view it is not the magnetic field strength or the topology alone which is responsible for the cosmic-ray depression. The most likely additional effect is the increased degree of turbulence.     

Intensity variation of cosmic rays near the heliospheric current sheet

Cosmic ray intensity variations near the heliospheric current sheet—both above and below it—have been studied during 1964–1976. Superposed epoch analysis of the cosmic ray neutron monitor data with respect to sector boundaries (i.e., heliospheric current sheet crossings) has been performed. In this analysis we have used the data from neutron monitors well distributed in latitude over the Earth's surface. First, this study has been made during the two solar activity minimum periods 1964–1965 and 1975–1976, using the data from Thule (cut-off rigidity 0 GV), Deep River (cut-off rigidity 1.02 GV), Rome (cut-off rigidity 6.32 GV) and Huancayo (cut-off rigidity 13.45 GV) neutron monitors. We have also analyzed the data from Deep River, Rome and Huancayo neutron monitors, for whom we have the data for full period (1964–1976), by dividing the periods according to the changes in solar activity, interplanetary magnetic field polarity and coronal holes. All these studies have shown a negative gradient with respect to heliomagnetic latitude (current sheet). These results have been discussed in the light of theoretical and observational evidences. Suggestions have been given to overcome the discrepancy between the observational and theoretical results. Further, possible explanations for these observational results have been suggested.     

High-Speed Solar Wind Streams during 1996 – 2007: Sources,Statistical Distribution,and Plasma/Field Properties

We present a catalog of high-speed streams, along with their solar sources for solar cycle 23. We study their distribution during different years and different phases of solar cycle after classifying them into different groups based on their source(s), duration, and speed. We also study the average plasma/field properties of streams after dividing them into suitable groups on the basis of their source(s), duration and speed. It is expected that the catalog and statistical results presented in this work will further stimulate the space weather and solar-terrestrial studies involving high-speed streams.     

Similarities and Distinctions in Cosmic-Ray Modulation During Different Phases of Solar and Magnetic Activity Cycles

We study the solar-activity and solar-polarity dependence of galactic cosmic-ray intensity (CRI) on the solar and heliospheric parameters playing a significant role in solar modulation. We utilize the data for cosmic-ray intensity as measured by neutron monitors, solar activity as measured by sunspot number (SSN), interplanetary plasma/field parameters, solar-wind velocity [V] and magnetic field [B], as well as the tilt of the heliospheric current sheet [Λ], and we analyze these data for Solar Cycles 20?–?24 (1965?–?2011). We divide individual solar cycles into four phases, i.e. low, high, increasing, and decreasing solar activity. We perform regression analysis to calculate and compare the CRI-response to changes in different solar/interplanetary parameters during

1. different phases of solar activity and
2. similar activity phases but different polarity states.

We find that the CRI-response is different during negative (A<0) as compared to positive (A>0) polarity states not only with SSN and Λ but also with B and V. The relative CRI-response to changes in various parameters, in negative (A<0) as compared to positive (A>0) state, is solar-activity dependent; it is ≈?2 to 3 times higher in low solar activity, ≈?1.5 to 2 times higher in moderate (increasing/decreasing) activity, and it is nearly equal in high solar-activity conditions. Although our results can be ascribed to the preferential entry of charged particles via the equatorial/polar regions of the heliosphere as predicted by drift models, these results also suggest that we should look for any polarity-dependent response of solar-wind and transport parameters in modulating CRI in the heliosphere.     

Geoeffectiveness of magnetic cloud, shock/sheath, interaction region, high-speed stream and their combined occurrence

A subset of CMEs, called interplanetary magnetic clouds (MCs), are observed to have systematic rotation [northward to southward (NS) or southward to northward (SN)] in their field structures. These MCs identified in the heliospheric plasma and field data at 1 AU may have different features associated with them. These structures (NS/SN) may be isolated MC moving with the ambient solar wind. MCs (NS/SN) may also be associated with shock/sheath region, formed due to compression of the ambient plasma/field ahead of them. A fraction from each of these four types of MCs have additional features, being ‘pushed’ by fast solar wind streams from coronal holes, forming interaction region (IR) between MCs and high-speed solar wind streams (HSS). Using these different sets of MCs, we have done a detailed study of the geoeffectiveness of NS and SN turning MCs and their associated features (shock/sheath, IR and HSS). To study the process that produces the geomagnetic disturbances and influences its amplitude/duration, we have utilized the interplanetary plasma and field parameters, namely, plasma velocity, density, temperature, pressure, field strength and its north-south component, during the passage of these structures with different associated properties. Differences in the geoeffectiveness of MCs with different structural and dynamical properties have been identified. The possible role of high-speed stream in influencing the recovery time (and hence duration) of geomagnetic disturbance has also been investigated. A best-fit equation representing the relation between level of the geomagnetic activity (due to MCs) and interplanetary plasma/field parameter has been obtained.     

Geoeffectiveness of the interplanetary manifestations of coronal mass ejections and solar-wind stream–stream interactions

Coronal mass ejections and high-speed streams from the Sun, and related structures formed and evolved in interplanetary space, i.e. interplanetary manifestations of CMEs (ICMEs) and stream interaction regions (SIRs)/corotating interaction regions (CIRs), are mainly responsible for geomagnetic disturbances in the Earth’s magnetic environment. However, the presence or absence of associated/finer structures of ICMEs (e.g., shock/sheath, magnetic cloud) and SIRs/CIRs (forward and reverse shocks, stream interface) might influence their geoeffectiveness as these features within large-scale structures of ICMEs and SIRs display different and varying plasma and field characteristics. In this work, we analyze the solar-wind plasma and field parameters (plasma velocity, density and pressure, magnetic field, its north-south component and electric field) together with geomagnetic activity parameters (kp and Dst), applying the method of superposed epoch analysis. By systematically changing the time of passage of different features as epochs, e.g. discontinuities/shocks, CMEs/magnetic clouds in ICMEs and discontinuities/forward shocks in SIRs/CIRs, we study the relative geoeffectiveness of not only the large-scale structures (ICMEs/SIRs/CIRs), but of their finer features also. We critically analyze the differences in geoeffectiveness due to different structures and features, with distinct plasma/field characteristics, and we utilize these results to understand the mechanism during their interaction with geospace.     

Cosmic-Ray Modulation due to High-Speed Solar-Wind Streams of Different Sources,Speed, and Duration

We study the modulation of galactic cosmic rays (GCR) due to high-speed streams (HSS) identified in the solar wind. We compare the GCR modulation due to i) streams with different speed, ii) streams of different duration, and iii) streams from different solar sources. We apply the method of superposed-epoch analysis to analyze the interplanetary plasma and field parameters during the passage of streams with distinct plasma and field characteristics. We use the plasma/field characteristics to distinguish various features of solar sources and interplanetary structures, and discuss the observed differences in the cosmic-ray response. We study the influence of speed, duration, and solar sources of the streams on the GCR modulation. We discuss the relative importance of different solar-wind parameters in the modulation process.