arXiv subjects

Anil Raghav

Publications and source records attributed to Anil Raghav.

At least 19 recordsLinked to original sources

arXiv2026

Evidence of energy conversion in weakly collisional plasma during an interplanetary coronal mass ejection

Intervals of enhanced turbulent fluctuations are typically less frequent within the magnetic cloud region of an interplanetary coronal mass ejection (ICME). We investigate two such intervals inside an ICME observed by the \textit{Wind} spacecraft on 8--9 June 2000 and characterize their associated wave populations. We focus on spectral analysis and plasma instability analysis, using ion-scale normalized magnetic helicity and polarization properties with respect to the background magnetic field $B_0$. In the first interval, the ion-scale normalized magnetic helicity shows a left-handed circularly polarized signature. In the second interval, the left-handed signature persists and an additional high-frequency right-handed population appears. The propagation is approximately parallel to $B_0$. The left-handed fluctuations are compatible with Alfv\'en ion-cyclotron (AIC) waves, while the right-handed fluctuations are consistent with fast magnetosonic/whistler (FM/W) waves. The ICME plasma accesses resonance conditions that support multiple ion-scale wave modes. Evolving anisotropies in the plasma and the approach to marginal stability allow the coexistence of AIC-like and fast-magnetosonic/whistler-like fluctuations, with enhanced electron heating favoring the growth of the FM/W contribution and strengthening the density--magnetic-field magnitude correlation.

astro-ph.SR↗

arXiv2025

Pinching of ICME Flux Rope: Unprecedented Multipoint Observations of Internal Magnetic Reconnection during Gannon's Superstorm

The extreme solar storm of May 10, 2024, during the 25th solar cycle, which recorded a symmetric H component index (Sym-H) reaching -500 nT, was the strongest since the 2003 Halloween storm. This event offered a unique opportunity for unprecedented multipoint observation of the complex interaction of Interplanetary Coronal Mass Ejections (ICME) from different vantage points. Utilizing NASA's Wind, ACE, DSCOVR, THEMIS-C, STEREO-A, MMS, and ISRO's recently launched Aditya-L1 spacecraft, we comprehensively investigated the spatio-temporal variations in interplanetary plasma and magnetic field parameters. Our study reveals large-scale quasi-steady magnetic reconnection within the interior of the ICME flux rope, possibly triggered by interactions between multiple ICMEs. A current sheet (CS) forms within the flux rope, enabling internal magnetic reconnection between concentric magnetic surfaces, which leads to a sharp reversal of the IMF By component, as observed at the L1 point. Concurrently, reconnection exhaust and enhanced electron and ion fluxes were detected with the CS, extending over 200 RE (1.3 million km) along the GSE-y direction. This finding sheds new light on the role of internal reconnection in ICME evolution, highlighting its pivotal role in modifying the morphology of the ICME magnetic structure and exerting severe space weather effects on Earth.

astro-ph.SR↗

arXiv2025

Magnetic Moment-Field Interactions, A Universal Mechanism for Particle Energization

Magnetic reconnection is a pivotal mechanisms in the energization and heating of cosmic plasmas, yet the exact process of energy transfer during these events remain elusive. Traditional models, which focus on acoustic and magnetohydrodynamic waves and micro/nano-flares, fall short of explaining the extreme heating of the solar corona and the origins of the supersonic solar wind. In this study, we provide compelling observational evidence from Wind spacecraft data supporting the Raghav effect, a mechanism where interactions between the magnetic moments of charged particles and dynamic magnetic fields result in abrupt kinetic energy changes. Our analysis demonstrates that the observed proton plasma heating is consistent with theoretical predictions, establishing the Raghav effect as a universal mechanism for particle energization. This discovery offers a unified framework for understanding energy dynamics across a wide range of astrophysical magnetised plasma environments.

astro-ph.SR↗

arXiv2024

AuroraMag: Twin Explorer of Asymmetry in Aurora and Solar Wind-Magnetosphere Coupling

In the present-day context, small satellites and their constellations consisting of varying sizes (nano, micro, pico satellites) are being favored for remote sensing and in situ probing of the heliosphere and terrestrial magnetosphere-ionosphere system. We introduce a mission concept aimed at concurrently observing Earth's northern and southern auroral ovals while conducting in situ measurements of particles, fields, and temperature. The mission concept consists of two small satellites, each having an identical auroral X-ray imager, an in situ particle detector, a magnetometer pair, and an electron temperature analyzer onboard in an elliptical polar orbit (400X1000 km ). This mission would assist the space weather community in primarily answering important questions about the formation, morphology, and hemispherical asymmetries that we observe in the X-ray aurora, the fluxes of precipitating particles, Solar Energetic Particles, currents, and cusp dynamics. Once realized, this would be the first dedicated twin spacecraft mission of such kind to simultaneously study hemispheric asymmetries of solar-wind magnetosphere coupling. This study reveals the intricacies of the mission concept, encompassing orbital details, potential payloads, and its underlying scientific objectives. By leveraging the capabilities of small satellites, this mission concept is poised to make significant contributions to space weather monitoring and research.

physics.space-ph↗

arXiv2023

Quasi-planar ICME sheath: a cause of first two-step extreme geomagnetic storm of 25th solar cycle observed on 23 April 2023

Interplanetary Coronal Mass Ejections (ICMEs) are prominent drivers of space weather disturbances and mainly lead to intense or extreme geomagnetic storms. The reported studies suggested that the planar ICME sheath and planar magnetic clouds (MCs) cause extreme storms. Here, we investigated the severe two-step geomagnetic storm ($Dst \sim -187$ nT) of 25$^{th}$ solar cycle. Our analysis demonstrates flattened (pancaked) ICME structures, i.e., quasi-planar magnetic structures (PMS). The study corroborates our earlier reported finding that the less adiabatic expansion in quasi-PMS transformed ICME enhanced the strength of the southward magnetic field component. It contributes to the efficient transfer of plasma and energy in the Earth's magnetosphere to cause the observed severe storm.

physics.space-ph↗

arXiv2023

Observational Validation of Cosmic Ray Acceleration Hypothesis

Despite centuries of rigorous theoretical and observational research, the origin and acceleration mechanism of Galactic Cosmic Rays (GCRs) remain a mystery. In 1949, Fermi proposed a diffusive shock acceleration model that includes a prominent mechanism for GCR acceleration. However, observational evidence, on the other hand, remains elusive. Here, we provided the first apparent verification of GCR acceleration at 1 AU using measurements from the CRIS instrument onboard the ACE spacecraft.

astro-ph.HE↗

arXiv2022

First analysis of in-situ observation of surface Alfv\'en waves in ICME flux rope

Alfv\'en waves (AWs) are inevitable in space and astrophysical plasma. Their crucial role in various physical processes, occurring in plasma, has triggered intense research in solar-terrestrial physics. Simulation studies have proposed the generation of AWs along the surface of a cylindrical flux rope, referred to as Surface AWs (SAWs); however the observational verification of this distinct wave has been elusive to date. We report the first \textit{in-situ} observation of SAWs in an interplanetary coronal mass ejection flux rope. We apply the Wal\'en test to identify them. The Elsa\"sser variables are used to estimate the characterization of these SAWs. They may be excited by the movement of the flux rope's foot points or by instabilities along the plasma magnetic cloud's boundaries. Here, the change in plasma density or field strength in the surface-aligned magnetic field may trigger SAWs.

astro-ph.SR↗

arXiv2022

Distinct polytropic behavior of plasma during ICME-HSS Interaction

Interplanetary Coronal Mass Ejections (ICMEs) and High Speed Streams (HSSs) are noteworthy drivers of disturbance of interplanetary space. Interaction between them can cause several phenomena, such as; generation of waves, enhanced geo-effectiveness, particle acceleration, etc. However, how does thermodynamic properties vary during the ICME-HSS interaction remain an open problem. In this study, we investigated the polytropic behavior of plasma during an ICME-HSS interaction observed by STEREO and Wind spacecraft. We find that the ICME observed by the STEREO-A has polytropic index $\alpha = 1.0$, i.e., exhibiting isothermal process. Moreover, Wind spacecraft observed the HSS region, non-interacting ICME, and ICME-HSS interaction region. During each regions we found $\alpha$=1.8, $\alpha$=0.7, and $\alpha$=2.5, respectively. It implies that the HSS region exhibits a nearly adiabatic behaviour, ICME region is closely isothermal, and the ICME-HSS interaction region exhibits super-adiabatic behaviour. The insufficient expansion of the ICME due to the interaction with HSS triggers the system for heating and cooling mechanisms which dependent on the degrees of freedom of plasma components.

physics.space-ph↗

arXiv2022

In-situ Observation Of Alfv'en Waves In Icme Shock-Sheath Indicates Existence Of Alfv'enic Turbulence

The dynamic evolution of coronal mass ejection (CME) in interplanetary space generates highly turbulent, compressed, and heated shock-sheath. This region furnishes a unique environment to study the turbulent fluctuations at the small scales and serve an opportunity for unfolding the physical mechanisms by which the turbulence is dissipated and plasma is heated. How does the turbulence in the magnetized plasma control the energy transport process in space and astrophysical plasmas is an attractive and challenging open problem of the 21st century. For this, the literature discusses three types of magnetohydrodynamics (MHD) waves/ fluctuations in magnetized plasma as the magnetosonic (fast), Alfv'enic (intermediate), and sonic (slow). The magnetosonic type is most common in the interplanetary medium. However, Alfv'enic waves/fluctuations have not been identified to date in the ICME sheath. The steepening of the Alfv'en wave can form a rotational discontinuity that leads to an Alfv'enic shock. But, the questions were raised on their existence based on the theoretical ground. Here, we demonstrate the observable in-situ evidence of Alfv'en waves inside turbulent shock-sheath at 1 AU using three different methods desciribed in the literature. We also estimate Els"asser variables, normalized cross helicity, normalized residual energy and which indicate outward flow of Alfv'en waves. Power spectrum analysis of IMF indicates the existence of Alfv'enic turbulence in ICME shock-sheath. The study has strong implications in the domain of interplanetary space plasma, its interaction with planetary plasma, and astrophysical plasma.

astro-ph.SR↗

arXiv2022

Observation of Alfven wave in ICME-HSS interaction region

The Alfv\'en wave (AW) is the most common fluctuation present within the emitted solar wind from the Sun. Moreover, the interaction between interplanetary coronal mass ejection (ICME) and high-speed stream (HSS) has been observed on several occasions. However, can such interaction generate an AW? What will be the nature of AW in such a scenario remains an open question. To answer it, we have investigated an ICME-HSS interaction event observed on 21st October 1999 at 1 AU by Wind spacecraft. We have used the Wal\'en test to identify AW and estimated Elsasser variables to find the characteristics of the AWs. We explicitly find that ICME were dominant with Sunward AWs, whereas the trailing HSS has strong anti-Sunward AW. We suggest that the ICME-HSS interaction deforms the MC of the ICME, resulting in the AWs inside the MC. In addition, the existence of reconnection within the ICME early stage can also be the leading cause of the origin of AW within it.

astro-ph.SR↗

arXiv2021

The role of extreme geomagnetic storms in the Forbush decrease profile

The Forbush decrease (FD) and Geomagnetic storm (GS) are the two distinct space weather events having common causing agents like interplanetary coronal mass ejection (ICME) or corotating interacting region (CIR). Generally, an ICME causes high amplitude FDs and extreme GSs. However, the interlinks between extreme GS and strong FDs are poorly studied. Here, we demonstrate five ICME induced extreme storms and their effects on respective FD profiles. We observed the sudden storm commencement of the GS coincides with the FD onset. Interestingly, we also noted a gradual increase in neutron counts during the main and recovery phases of GS. The maximum enhancement in neutron counts coincides with the minimum value of the Sym-H index. The enhancement is visible primarily in all the neutron monitors but significantly pronounced in high-energy neutrons compared to low-energy neutrons. The weakening of Earth's magnetic shield due to ICME-Magnetosphere interaction allows more cosmic rays to reach the ground. Thus, we conclude that the geomagnetic storm conditions highly influence the FD profile along with the external causing agent. Therefore, it is essential to include the effect of geomagnetic field variation in the models that are used to reproduce the FD profile.

physics.space-ph↗

arXiv2019

Causes of the Diurnal Variation observed in $γ$-ray Spectrum using NaI (Tl) Detector

Present paper studies the $γ$-ray spectrum observed using NaI(Tl) scintillation detector over a year. The paper reports the presence of a distinct diurnal pattern in the total number of $γ$-ray counts detected by the NaI detector. The counts start decreasing after sunrise and show gradual recovery after sunset. The amplitude of this variation is quite significant ( > 10%) at the observation site Tirunelveli, South India. However, further investigation based on different energy ranges reveals that the mentioned diurnal pattern is actually present only in the energies related to the terrestrial background radioactivity. The study demonstrates that the pattern is associated with the radioactivity of isotopes of radon ($^{222}$Rn and $^{220}$Rn) and their daughter radionuclides. The amplitude of the diurnal variation is found to have seasonal dependence, with the lowest amplitude during June-October (~ 2%), and highest in April-May months (~ 14%). The dependence of the amplitude of the diurnal variation on various atmospheric parameters, namely temperature, relative humidity, and vertical wind is examined. The observed diurnal pattern is attributed to the generation of the inversion layer. The distribution of concentration of radon and thoron progenies in an air column changes with the vertical mixing and atmospheric boundary layer (ABL), resulting in the diurnal variation of $γ$-rays.

physics.ao-ph↗

arXiv2018

The first in-situ observation of torsional Alfven waves during the interaction of large-scale magnetic clouds

The large-scale magnetic cloud such as coronal mass ejections (CMEs) is the fundamental driver of the space weather. The interaction of the multiple CMEs in interplanetary space affects their dynamic evolution and geo-effectiveness. The complex and merged multiple magnetic clouds appear as the in-situ signature of the interacting CMEs. The Alfven waves are speculated to be one of the major possible energy exchange/dissipation mechanism during the interaction. However, no such observational evidence has been found in the literature. The case studies of CME-CME collision events suggest that the magnetic and thermal energy of the CME is converted into the kinetic energy. Moreover, the magnetic reconnection process is justified to be responsible for the merging of multiple magnetic clouds. Here, we present unambiguous evidence of sunward torsional Alfven waves in the interacting region after the super-elastic collision of multiple CMEs. The Walen relation is used to confirm the presence of Alfven waves in the interacting region of multiple CMEs/magnetic clouds. We conclude that Alfven waves and magnetic reconnection are the possible energy exchange/dissipation mechanisms during large-scale magnetic clouds collisions. The present study has significant implications not only in CME-magnetosphere interactions but also in the interstellar medium where interactions of large-scale magnetic clouds are possible.

physics.space-ph↗

arXiv2016

Presence of Turbulent and Ordered Local Structure within ICME shock-sheath and its Contribution in Forbush decrease

The transient interplanetary disturbances evoke short time cosmic ray flux decrease which is known as Forbush decrease. The traditional model and understanding of Forbush decrease suggest that the substructure of interplanetary counterpart of coronal mass ejection (ICME) independently contributes in cosmic ray flux decrease. These substructures, shock-sheath and magnetic cloud (MC) manifest as classical two-step Forbush decrease. The recent work by Raghav et. al. (2016a) has shown multi-step decreases and recoveries within shock-sheath. However, this can not be explained by ideal shock-sheath barrier model. Further, they suggested that the local structures within the ICME's sub-structure (MC and shock sheath) could explain this deviation of FD profile from the classical FD. Therefore, present study attempts to investigate the cause of multi-step cosmic ray flux decrease and their respective recovery within shock-sheath in detail. 3D-hodogram method has been utilized to get more details of the local structures within the shock-sheath. A 3D-hodogram method unambiguously suggests the formation of small scale local structures within the ICME (shock-sheath and even in MC). Moreover, the method could differentiate the turbulent and ordered Interplanetary Magnetic Field (IMF) regions within the substructures of ICME. The study explicitly suggests that the turbulent and ordered IMF regions within shock-sheath do influence cosmic-ray variations uniquely.

physics.space-ph↗

arXiv2016

Cosmic ray acceleration via magnetic reconnection of magnetic islands/flux-ropes

The dynamic processes of magnetic reconnection and turbulence cause magnetic islands/flux-ropes generation. The in-situ observations suggest that the coalescence or/and contraction of magnetic islands are responsible to the charged particle acceleration (keV to MeV energy range). Numerical simulations also support this acceleration mechanism. However, the most fundamental question raise here is, does this mechanism contribute to the cosmic rays acceleration? To answer this, we report, in-situ evidence of flux-ropes formation, their magnetic re-connection and its manifestation as cosmic ray (GeV charged particle) acceleration in interplanetary counterpart of coronal mass ejection(ICME). Further, we propose that cosmic ray (high and/or ultra-high energy) acceleration by Fermi mechanism is valid not only through stochastic reflections of particles from the shock boundaries but also through the boundaries of contracting magnetic islands or/and their merging via magnetic re-connection. This has significant implications on cosmic ray origin and their acceleration process.

physics.space-ph↗

arXiv2016

Does Local Structure Within Shock-sheath and Magnetic Cloud Affect Cosmic Ray Decrease?

The sudden short duration decrease in cosmic ray flux is known as Forbush decrease which is mainly caused by interplanetary disturbances. Generally accepted view is that the first step of Forbush decrease is due to shock sheath and second step is due to Magnetic Cloud (MC) of Interplanetary Coronal Mass Ejection (ICME). However, there are few questions which needs detail investigation. (i) Does the complete (or part of) ICME shock or MC contribute in cosmic ray decrease? (ii) Is there any internal structure associated with the ICMEs shock/MC which precisely contribute in decrease? To address these questions we have studied a total of 18 large ($>=8 \%$) Forbush decrease events and associated ICMEs. Majority of events shows multi-steps Forbush decrease profile. This leads into re-classification of Forbush decrease events depending upon the number of steps observed in their respective profile. The present study clearly indicates that not only broad regions (shock-sheath and MC) contribute to Forbush decrease but also localized structures within shock-sheath and MC have very much significant importance in Forbush decrease profile.

astro-ph.HE↗

arXiv2014

Low energy secondary cosmic ray flux (gamma rays) monitoring and its constrains

Temporal variation of secondary cosmic rays (SCR) flux was measured during the several full and new moon and days close to them at Department of Physics, University of Mumbai, Mumbai (Geomagnetic latitude: 10.6 N), India. The measurements were done by using NaI (Tl) scintillation detector with energy threshold of 200 keV. The SCR flux shows sudden enhancement for approximately about 2 hour in counts during couple of events out of all experimental observations. The maximum Enhancement SCR flux is about 200% as compared to the diurnal trend of SCR temporal variations. Weather parameters (temperature and relative humidity) were continuously monitored during all observation. The influences of geomagnetic field, interplanetary parameters and tidal effect on SCR flux have been considered. Summed spectra corresponding to enhancement duration indicates appearance of atmospheric radioactivity which shows single gamma ray line. Detail investigation revealed the presence of radioactive Ar 41 . This measurements puts limitations on low energy SCR flux monitoring. This paper will help many researcher who want to measure SCR flux during eclipses and motivate to find unknown mechanism behind decrease/ increase during solar/lunar eclipse.

astro-ph.EP↗

arXiv2014

Understanding Forbush decrease drivers based on shock-only and CME-only models using global signature of February 14, 1978 event

We have studied Forbush decrease (FD) event occurred on February 14, 1978 using 43 neutron monitor observatories to understand the global signature of FD. We have studied rigidity dependence of shock amplitude and total FD amplitude. We have found almost the same power law index for both shock phase amplitude and total FD amplitude. Local time variation of shock phase amplitude and maximum depression time of FD have been investigated which indicate possible effect of shock/CME orientation. We have analyzed rigidity dependence of time constants of two phase recovery. Time constants of slow component of recovery phase show rigidity dependence and implies possible effect of diffusion. Solar wind speed was observed to be well correlated with slow component of FD recovery phase. This indicates solar wind speed as possible driver of recovery phase. To investigate the contribution of interplanetary drivers, shock and CME in FD, we have used shock-only and CME-only models. We have applied these models separately to shock phase and main phase amplitudes respectively. This confirms present accepted physical scenario that the first step of FD is due to propagating shock barrier and second step is due to flux rope of CME/magnetic cloud.

astro-ph.SR↗