Cosmic Ray Modulation

Research Overview

My research investigates the short-term modulation of galactic cosmic rays by large-scale heliospheric disturbances, particularly interplanetary coronal mass ejections (ICMEs), their shock–sheath regions and magnetic clouds, and corotating interaction regions (CIRs). These structures modify the interplanetary magnetic field through compression, enhanced turbulence, and the formation or evolution of coherent magnetic structures, influencing the transport and observed intensity of galactic cosmic rays.

Using ground-based neutron-monitor observations together with in situ magnetic-field and plasma measurements from Wind, ACE, and other heliospheric datasets, I study the relationships between cosmic-ray intensity variations and the evolving magnetic configurations of interplanetary disturbances.

A central focus of my research is understanding the role of internal magnetic structures, including planar magnetic structures, small-scale flux ropes, and turbulent regions, in producing complex Forbush-decrease profiles. My observational studies examine why cosmic-ray intensities sometimes decrease or recover within individual ICME substructures, rather than following the conventional one- or two-step modulation pattern.

Key Scientific Questions

1. Cosmic-ray modulation by ICMEs: How do ICME shocks, turbulent sheath regions, and magnetic clouds contribute to the magnitude, timing, and recovery of Forbush decreases?

2. Role of internal magnetic structures: How do planar magnetic structures, small-scale flux ropes, and locally ordered or turbulent magnetic fields influence cosmic-ray intensity variations within ICME substructures?

3. Complex Forbush-decrease profiles: What physical conditions lead to multistep cosmic-ray decreases and partial recoveries during the passage of a single interplanetary disturbance?

4. Magnetic turbulence and cosmic-ray transport: How are magnetic-field fluctuations, compression, and coherent magnetic structures related to changes in cosmic-ray scattering and transport?

5. Modulation by stream interaction regions: How do CIRs and stream interaction regions modify cosmic-ray intensities, and what roles do magnetic compression and planar structures play in recurrent Forbush decreases?

6. Rigidity-dependent cosmic-ray response: How do particles sampled by neutron monitors with different geomagnetic cutoff rigidities respond to the same evolving heliospheric magnetic structures?

Selected Scientific Contributions

Turbulent and Ordered Magnetic Structures within ICME Sheaths

In Shaikh et al. (2017, ApJ), we investigated the contributions of turbulent and ordered magnetic structures within ICME shock–sheath regions to the temporal evolution of Forbush decreases. Combining neutron-monitor observations with interplanetary magnetic-field measurements and three-dimensional hodogram analysis, we identified distinct magnetic configurations associated with cosmic-ray intensity decreases and recoveries. The results demonstrated that the internal magnetic properties of the sheath, rather than its passage alone, are important for interpreting complex and multistep cosmic-ray variations.

Planar Magnetic Structures and Cosmic-Ray Intensity Variations

In Shaikh et al. (2018, ApJ), we investigated a Forbush decrease associated with an ICME containing a planar magnetic structure and a small-scale magnetic flux rope within its shock–sheath region. Using minimum variance analysis, magnetic-field hodograms, and neutron-monitor observations, we distinguished turbulent, planar, and flux-rope-like magnetic configurations. The observations showed that enhanced magnetic fluctuations and the planar structure coincided with cosmic-ray intensity decreases, whereas the passage of a relatively ordered small-scale flux rope was associated with recovery. These results provided further observational evidence that different internal magnetic structures can influence cosmic-ray intensity profiles within a single ICME disturbance.

Recurrent Cosmic-Ray Modulation by Stream Interaction Regions

In Shaikh et al. (2020, MNRAS), we investigated the evolution of planar magnetic structures within stream interaction regions and their relationship to recurrent Forbush decreases. The study identified planar magnetic configurations associated with compressed solar-wind regions and examined how their evolution corresponded to changes in cosmic-ray intensity. Our observations suggested that magnetic compression, enhanced field strength, and turbulent planar structures may contribute to reduced cosmic-ray transport across these regions, while the subsequent weakening of magnetic fields is associated with cosmic-ray recovery.

These findings extended our investigations of localized magnetic structures from ICME-driven disturbances to recurrent solar-wind interaction regions.

Representative Scientific Figures

Related Publications

1. Shaikh, Z., Raghav, A., & Bhaskar, A. (2017). The Presence of Turbulent and Ordered Local Structure within the ICME Shock-sheath and Its Contribution to Forbush Decrease. The Astrophysical Journal, 844, 121. DOI

2. Shaikh, Z. I., et al. (2018). The Identification of a Planar Magnetic Structure within the ICME Shock Sheath and Its Influence on Galactic Cosmic-Ray Flux. The Astrophysical Journal, 866, 118. DOI

3. Raghav, A., Shaikh, Z., Misal, D., Rajan, G., Mishra, W., Kasthurirangan, S., ... & Vichare, G. (2020). Exploring the common origins of the Forbush decrease phenomenon caused by the interplanetary counterpart of coronal mass ejections or corotating interaction regions. Physical Review D, 101(6), 062003. DOI

4. Shaikh, Z. I., Raghav, A. N., & Vichare, G. (2020). Evolution of Planar Magnetic Structure within the Stream Interaction Region and Its Connection with a Recurrent Forbush Decrease. Monthly Notices of the Royal Astronomical Society, 494, 5075–5080. DOI

Current and Future Research Directions

My future research aims to extend observational investigations of cosmic-ray modulation by combining ground-based particle measurements with high-resolution spacecraft observations and, where appropriate, physics-based cosmic-ray transport models developed through collaboration.

Cosmic-ray modulation by coherent magnetic structures: Investigating how planar magnetic structures, magnetic flux ropes, current sheets, and turbulent regions influence the temporal evolution of Forbush decreases.

Rigidity-dependent modulation: Comparing cosmic-ray intensity variations measured across different rigidity ranges to examine the response of energetic particles to evolving heliospheric disturbances.

Turbulence and particle transport: Studying relationships between magnetic turbulence, coherent structures, and cosmic-ray intensity variations to establish observational constraints on particle scattering and transport.

Multispacecraft heliospheric observations: Combining near-Earth measurements with observations from Parker Solar Probe, Solar Orbiter, and other heliospheric missions to investigate the magnetic environments associated with cosmic-ray modulation.

Observation-driven transport studies: Developing collaborations that connect neutron-monitor and high-rigidity cosmic-ray observations with particle-transport calculations, including future investigations using GRAPES-3 measurements and complementary spacecraft datasets.

These investigations aim to improve our understanding of how heliospheric magnetic structures influence the propagation and modulation of galactic cosmic rays across different temporal and spatial scales.