Study of the development and mechanism of large amplitude decreases in cosmic ray intensity during geomagnetic disturbances in the magnetosphere

Abstract

We analyze the time-variation of cosmic ray intensity during the periods when large and sudden disturbances occur and last for several days. We consider periods when there are sudden decreases of ≥5% in the galactic cosmic ray (GCR) intensity records. For the analysis, in addition to cosmic-ray intensity data, we utilize several solar wind plasma and field parameters. Time variation of changes in GCR intensity have been compared with simultaneous changes in solar wind plasma and field parameters, in order to gain insight about the physical processes responsible for large-amplitude cosmic ray disturbances. We utilize not only data about changes in the magnitude of some of the plasma and magnetic field parameters but also the parameters which provide information about the turbulent or quite (non-turbulent) nature of the parameters. Moreover, we also utilize the magnitude of a number of plasma and field parameters and their various products, during cosmic ray decrease of various magnitudes. The magnitudes of various plasma and field parameters and their various products including some newly tried products are subjected to correlation analysis with the magnitude of cosmic-ray decreases. We identify plasma and field parameters and their products which best correlates with cosmic-ray decreases. We provide an empirical relation that can be useful to estimate the amplitude of Forbush decreases using interplanetary plasma and field observations. Our results provide further insight about the physical processes playing important role during large cosmic ray disturbances in the heliosphere. Our results further demonstrate that large-amplitude Forbush decreases in GCR intensity occur due to passage of enhanced and turbulent magnetic field structure, consistent with the model that Forbush decreases are mainly due to scattering of cosmic ray particles by enhanced turbulent magnetic field regions in the heliosphere.

Introduction

Disturbances in the geomagnetic field of the magnetosphere (Geomagnetic storms) and sudden disturbances (decreases) in the cosmic ray intensity (e.g., Forbush decreases) are observed together during the passage of interplanetary structures associated with the occurrence of coronal mass ejections (e.g., see, Badruddin et al., 1986, Venkatesan and Badruddin, 1990, Cane, 2000, Belov et al., 2001, Blanco et al., 2013, Arunbabu et al., 2013). However, the magnitudes of geomagnetic storms and Forbush decreases are not proportional to each other. Although the sources of the two phenomena may be the same, the generation mechanism of the two phenomena are different (e.g., Kudela and Brenkus, 2004, Kane, 2010, Badruddin and Kumar, 2015, Aslam and Badruddin, 2017).

Galactic cosmic rays (GCR) are high energy charged particles that enter the magnetic environment of the Sun (Heliosphere) from the outer space (Chowdhury et al., 2016, López-Comazzi and Blanco, 2020). The heliosphere is filled with solar plasma and the magnetic field carried outward by the solar wind blowing from the Sun. Travelling through this region from the outer heliosphere towards the Earth and Sun, these charged cosmic ray particles are affected by the presence of plasma and field in the interplanetary space. This result in variations of GCR intensity at various time scales, notably ~11-years and ~22-year periods (see, Potgieter, 2013, Aslam and Badruddin, 2015, Chowdhury and Kudela, 2018; and the references therein). However, in addition to these long-term variations, short-time variations (both periodic and non-periodic) are observed in GCR intensity (see, Dunzlaff et al., 2008, Singh et al., 2012, Kudela and Sabbah, 2016, Chowdhury et al., 2016, Poblet and Azpilicueta, 2018, López-Comazzi and Blanco, 2020, Oloketuyi et al., 2020). One of the prominent non-periodic short-time variations frequently observed is the so-called Forbush decrease; a sudden decrease in the GCR intensity of a few per cent magnitudes within a period of about one day followed a slow gradual recovery extending up to several days. These decreases are related to changes in plasma and field conditions in the interplanetary space. Changes in the plasma and field conditions observed in the interplanetary space are mainly due to two emissions of different nature; high-speed solar wind streams (HSS) from the coronal holes with open magnetic field lines and coronal mass ejections (CMEs) with closed magnetic field structures. Ejected from certain regions of the solar atmosphere and propagating in interplanetary space they evolve into two different types of large-scale interplanetary structures, corotating interaction regions (CIRs) and interplanetary manifestations of CMEs (ICMEs).

These two types of large-scale structures (CIRs and ICMEs) with distinct structures and plasma/field properties not only produce disturbances in GCR intensity (e.g., Lockwood, 1971, Mavromichalaki et al., 2003, Mavromichalaki et al., 2011, Modzelewska and Alania, 2012, Papaioannou et al., 2010, Papaioannou et al., 2020, Kumar and Badruddin, 2014, Badruddin and Kumar, 2016) but also generate disturbances in the geomagnetic activity in the near-Earth Space. The disturbances generated by CIRs in GCR intensity are in general, slowly varying of relatively smaller amplitudes and corotating in nature (see McKibben et al., 1999, Dumbović et al., 2012, Guo and Florinski, 2014, Luo et al., 2020). However, the ICME generated disturbances in GCR intensity are fast of larger amplitude and transient in nature (e.g., Iucci et al., 1989, Singh and Badruddin, 2007, Dorotovič et al., 2008, Richardson and Cane, 2011, Arunbabu et al., 2015, Belov et al., 2015, Bhaskar et al., 2016).

Individual transient decreases in GCR intensity, generally known as Forbush decreases (Forbush, 1937), are of different depth (magnitude of decrease) take different time to reach their minimum intensity level (different decrease duration) and take different time to reach normal intensity level (different recovery time). Study of Forbush decrease precursors along with interplanetary magnetic field and solar wind parameters can be useful to understand the relevance of Forbush decreases in space weather predictions, and also the physics of the transient variations (like FDs) in the GCR intensity (e.g., see, Kudela et al., 2000, Dorman, 2005, Chertok et al., 2018, Badruddin et al., 2019).

Although many studies in the past; both theoretical/modeling (Nishida, 1982, Kadokura and Nishida, 1986, Chih and Lee, 1986, le Roux and Potgieter, 1991, Luo et al., 2017) and observational (Lockwood et al., 1991, Badruddin et al., 1991, Belov et al., 2014, Belov et al., 2015, Badruddin et al., 2019) have attempted to understand this phenomenon of Forbush decrease (FD), opinions are still divided about the relative importance of different structures in ICMEs (shock/sheath/ejecta/magnetic clouds) in producing Forbush decreases (e.g., Zhang and Burlaga, 1988, Dasso et al., 2005, Reames et al., 2009, Yu et al., 2010, Petukhova et al., 2019). It is also to be established with certainty which interplanetary plasma/field parameter/function best correlates with the amplitude of Forbush decrease (Mavromichalaki et al., 2011, Melkumyan et al., 2019). Moreover, the physical process playing the dominant role in the phenomena of Forbush decreases still requires a general agreement (see, also Cane, 2000, Luo et al., 2017). It is also be established with certainty whether all the large amplitude Forbush decreases are due to shock associated ICMEs or some of them can be due to isolated ejecta/magnetic cloud only.

Recently, Fadaaq and Badruddin (2021) have studied the transient modulation of galactic cosmic rays. For this purpose, using superposed epoch analysis, they analyzed the cosmic ray intensity data and solar wind plasma/field parameters with reference to ICMEs associated/not-associated with shock and ICMEs having magnetic cloud/non-magnetic cloud structure. Their results are consistent with the hypothesis that transient decrease in cosmic ray intensity is mainly due to the turbulent shock/sheath region preceding the ICME and the decrease due to ICME without shock/sheath structure, if any, is small.

Present work is further attempt to understand the transient (Forbush) decrease phenomena better. For this purpose, we identified large-amplitude (≥5%) FDs with clear time profile. In this work, we use superposed epoch analysis to analyze the cosmic-ray neutron monitor data together with solar wind plasma/field data with reference to onset of selected Forbush decreases of different amplitude, decrease and recovery time. Moreover, for our analysis, we analyzed some additional solar wind plasma/field parameters (and their derivatives), compared to earlier studies (e.g., see Fadaaq and Badruddin, 2021 and the references therein).