Anil Raghav - Academia.edu (original) (raw)
Papers by Anil Raghav
Astrophysical journal/The Astrophysical journal, Apr 1, 2024
The Astrophysical Journal, Nov 30, 2023
The Astrophysical Journal, Oct 24, 2023
Journal of Atmospheric and Solar-Terrestrial Physics
The Forbush decrease (FD) and Geomagnetic storm (GS) are the two distinct space weather events ha... more 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.
arXiv (Cornell University), May 6, 2023
Interplanetary Coronal Mass Ejections (ICMEs) are prominent drivers of space weather disturbances... more 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 first two-step extreme geomagnetic storm (Sym− H ∼ −231 nT) of 25 th solar cycle. Our analysis demonstrates the transformation of ICME sheath into quasi-planar magnetic structures (PMS). The study corroborates our earlier reported finding that the less adiabatic expansion in possible 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 extreme storm. We found that the magnetosphere stand-off distance reduced to < 6.6R E , and it impacts geosynchronous satellites moving close to 6R E .
The Astrophysical Journal, Jul 28, 2021
This paper reports, for the first time on a statistical basis, on the key role played by the Alfv... more This paper reports, for the first time on a statistical basis, on the key role played by the Alfvénic fluctuations in modulating the recovery phase of the geomagnetic storms, slowing down the restoration of the magnetosphere toward its pre-storm equilibrium state. Using interplanetary and geomagnetic measurements collected over more than one solar cycle, a high correlation between the durations of Alfvénic streams and concurrent recovery phases is found, pointing to a clear coupling between Alfvénic turbulence and magnetospheric ring current dynamics. By exploiting current solar wind models, these observations also provide space weather opportunities of predicting the total duration of any geomagnetic storm induced by any solar driver provided that it is followed by an Alfvénic stream, a crucial piece of information for ground technologies and infrastructures that are affected by time-integrated effects throughout the duration of the storm.
Monthly Notices of the Royal Astronomical Society: Letters, Jun 18, 2018
Alfvén waves are primal and pervasive in space plasmas and significantly contributes to microscal... more Alfvén waves are primal and pervasive in space plasmas and significantly contributes to microscale fluctuations in the solar wind and some heliospheric processes. Here, we demonstrate the first observable distinct feature of Alfvén wave while propagating from magnetic cloud to trailing solar wind. The Walén test is used to confirm their presence in selected regions. The amplitude ratio of inward to outward Alfvén waves is employed to establish their flow direction. The dominant inward flow is observed in magnetic cloud whereas trailing solar wind shows the dominant outward flow of Alfvén waves. The observed reduction in Walén slope and correlation coefficient within magnetic cloud suggest (i) the simultaneous presence of an inward & outward Alfvén waves and/or (ii) a possibility of magnetic reconnection and/or (iii) development of thermal anisotropy and/or (iv) dissipation of Alfvénic fluctuations. The study implies that either the Alfvén waves dissipate in the magnetic cloud or its presence can lead to disruption of the magnetic cloud structure.
Monthly Notices of the Royal Astronomical Society, Oct 31, 2019
Generally, interplanetary coronal mass ejection (ICME) triggers intense and strong geomagnetic st... more Generally, interplanetary coronal mass ejection (ICME) triggers intense and strong geomagnetic storms. It has been established that the ICME sheath-moulded planar magnetic structure enhances the amplitude of the storms. Alfvén waves embedded in ICME magnetic clouds or high solar streams including corotating interacting regions (CIRs) in turn extend the recovery phase of the storm. Here, we investigate a geomagnetic storm with a very complex temporal profile with multiple decreasing and recovery phases. We examine the role of planar magnetic structure (PMS) and Alfvén waves in the various phases of the storm. We find that fast decrease and fast recovery phases are evident during transit of PMS regions, whereas a slight decrease or recovery is found during the transit of regions embedded with Alfvénic fluctuations.
Monthly Notices of the Royal Astronomical Society: Letters, Feb 13, 2018
The first insitu observation of torsional Alfvén waves during the interaction of large-scale magn... more The first insitu observation of torsional Alfvén waves during the interaction of large-scale magnetic clouds.
Monthly Notices of the Royal Astronomical Society, 2019
The Astrophysical Journal, Oct 1, 2022
Various remote and in situ observations, along with several models, simulations, and kinetic stud... more Various remote and in situ observations, along with several models, simulations, and kinetic studies, have been proposed in recent years, suggesting that the morphology of an interplanetary coronal mass ejection (ICME) magnetic cloud can vary from cylindrical, elliptical, toroidal, flattened, pancaked, etc. Recently, Raghav et al. proposed for the first time a unique morphological characteristic of an ICME magnetic cloud at 1 au that showed characteristics of a planar magnetic structure, using in situ data from the ACE spacecraft. In this study, we statistically investigate the plasma properties of planar and nonplanar ICMEs from 1998–2017 at 1 au. The detailed study of 469 ICMEs suggests that 136 (∼29%) ICMEs are planar, whereas 333 (∼71%) are nonplanar. Furthermore, total interplanetary magnetic field strength, average plasma parameters, i.e., plasma density, beta, thermal pressure, and magnetic pressure in planar ICME, are significantly higher than in the nonplanar ICME. Also, we noticed that the thickness of planar ICMEs is less compared to nonplanar ICMEs. This analysis demonstrates that planar ICMEs are formed due to the high compression of ICME. Moreover, we also observed the southward/northward magnetic field component’s double strength during planar ICMEs compared to nonplanar ICMEs. It implies that planar ICMEs are more geoeffective than nonplanar ICMEs.
arXiv (Cornell University), Oct 13, 2017
Recently, a super-elastic collision of large-scale plasmoids i.e. solar coronal mass ejections (C... more Recently, a super-elastic collision of large-scale plasmoids i.e. solar coronal mass ejections (CMEs) has been observed and further supported by numerical simulations. However, the energy gain by the system in the collision process is not clear. In-fact during plasmoids collision process, the energy exchange mechanism is still a chronic issue. Here, we present conclusive in situ evidence of sunward torsional Alfven waves in the magnetic cloud after the super-elastic collision of the largest plasmoids in the heliosphere. We conclude that magnetic reconnection and Alfven waves are the possible energy exchange mechanism during plasmoids interaction.
Monthly Notices of the Royal Astronomical Society: Letters, Nov 21, 2022
ABSTRACT Alfvénic fluctuations are widespread and crucial in various physical processes of space ... more ABSTRACT Alfvénic fluctuations are widespread and crucial in various physical processes of space & astrophysical plasma. However, their role in heating and work done remains unexplored. Here, we have used Wind spacecraft’s data situated at 1 au distance to examine 12 distinct Alfvénic regions using polytropic analysis. The study finds an average polytropic index value α = 2.64, which is consistent with a superadiabatic behaviour for plasma particles with three effective degrees of freedom (f = 3). Moreover, this study examines several scenarios for plasma particles with different degrees of freedom. We noted that the investigated Alfvénic region could be adiabatic only for plasma particles with f = 1.26 degrees of freedom. In addition to this, for α = 2.64, the ratio of work done to the total heat supply within the system is fracdeltawdeltaq=−0.68\frac{\delta w}{\delta q} = -0.68fracdeltawdeltaq=−0.68, indicating that 68 per cent of the total supplied heat is utilized to accomplish work by the system on the surrounding (expansion phenomena), and the remaining is used to increase the internal energy of the system. As a result, we hypothesized that the Alfvénic plasma region is cooling more than the adiabatic expectation, resulting in supercooling phenomena. Thus, we propose that the discovered possible superadiabatic process would be critical in understanding the energy transfer from the Alfvénic zone to the surrounding plasma.
The Astrophysical Journal Letters
The sheath plasma of interplanetary coronal mass ejections (ICMEs) is highly compressed, heated, ... more The sheath plasma of interplanetary coronal mass ejections (ICMEs) is highly compressed, heated, turbulent, and magnetically intense relative to the ambient solar wind. In this Letter, we perform a detailed study of proton temperature anisotropy within the 333 ICME sheath regions observed on board the Wind spacecraft spanning the years 1995–2015. Our observations show that marginal stability thresholds of mirror mode and firehose instabilities predominantly constrain the proton temperature anisotropy within these sheath regions regardless of ICME sheath plasma speed. This is true even when the plasma beta values are less than 2, a parameter space that should have favored the prevalence of parallel firehose and proton cyclotron instabilities according to linear stability analysis. This investigation demonstrates the critical role played by distinct plasma instabilities in shaping the evolution of ICME sheath plasma compared to the broader solar wind environment.
Advances in Space Research
Universe
Interplanetary coronal mass ejections (ICME) are large-scale eruptions from the Sun and prominent... more Interplanetary coronal mass ejections (ICME) are large-scale eruptions from the Sun and prominent drivers of space weather disturbances, especially intense/extreme geomagnetic storms. Recent studies by our group showed that ICME sheaths and/or magnetic clouds (MC) could be transformed into a planar magnetic structure (PMS) and speculate that these structures might be more geo-effective. Thus, we statistically investigated the geo-effectiveness of planar and non-planar ICME sheaths and MC regions. We analyzed 420 ICME events observed from 1998 to 2017, and we found that the number of intense (−100 to −200 nT) and extreme (<−200 nT) geomagnetic storms are large during planar ICMEs (almost double) compared to non-planar ICMEs. In fact, almost all the extreme storm events occur during PMS molded ICME crossover. The observations suggest that planar structures are more geo-effective than non-planar structures. Thus, the current study helps us to understand the energy transfer mechanism...
Advances in Space Research
arXiv (Cornell University), Mar 15, 2023
Despite centuries of rigorous theoretical and observational research, the origin and acceleration... more 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.
The Astrophysical Journal
Alfvén waves (AWs) are ubiquitous in space and astrophysical plasma. Their crucial role in variou... more Alfvén waves (AWs) are ubiquitous in space and astrophysical plasma. Their crucial role in various physical processes 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 in situ observation of SAWs in a flux rope of an interplanetary coronal mass ejection. We apply the Walén test to identify them. We have used Elsässer variables to estimate the characteristics of SAWs. They may be excited by the movement of the flux rope’s footpoints or by instabilities along the boundaries of the plasma magnetic cloud. Here, the change in plasma density or field strength in the surface-aligned magnetic field may trigger SAWs.
Astrophysical journal/The Astrophysical journal, Apr 1, 2024
The Astrophysical Journal, Nov 30, 2023
The Astrophysical Journal, Oct 24, 2023
Journal of Atmospheric and Solar-Terrestrial Physics
The Forbush decrease (FD) and Geomagnetic storm (GS) are the two distinct space weather events ha... more 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.
arXiv (Cornell University), May 6, 2023
Interplanetary Coronal Mass Ejections (ICMEs) are prominent drivers of space weather disturbances... more 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 first two-step extreme geomagnetic storm (Sym− H ∼ −231 nT) of 25 th solar cycle. Our analysis demonstrates the transformation of ICME sheath into quasi-planar magnetic structures (PMS). The study corroborates our earlier reported finding that the less adiabatic expansion in possible 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 extreme storm. We found that the magnetosphere stand-off distance reduced to < 6.6R E , and it impacts geosynchronous satellites moving close to 6R E .
The Astrophysical Journal, Jul 28, 2021
This paper reports, for the first time on a statistical basis, on the key role played by the Alfv... more This paper reports, for the first time on a statistical basis, on the key role played by the Alfvénic fluctuations in modulating the recovery phase of the geomagnetic storms, slowing down the restoration of the magnetosphere toward its pre-storm equilibrium state. Using interplanetary and geomagnetic measurements collected over more than one solar cycle, a high correlation between the durations of Alfvénic streams and concurrent recovery phases is found, pointing to a clear coupling between Alfvénic turbulence and magnetospheric ring current dynamics. By exploiting current solar wind models, these observations also provide space weather opportunities of predicting the total duration of any geomagnetic storm induced by any solar driver provided that it is followed by an Alfvénic stream, a crucial piece of information for ground technologies and infrastructures that are affected by time-integrated effects throughout the duration of the storm.
Monthly Notices of the Royal Astronomical Society: Letters, Jun 18, 2018
Alfvén waves are primal and pervasive in space plasmas and significantly contributes to microscal... more Alfvén waves are primal and pervasive in space plasmas and significantly contributes to microscale fluctuations in the solar wind and some heliospheric processes. Here, we demonstrate the first observable distinct feature of Alfvén wave while propagating from magnetic cloud to trailing solar wind. The Walén test is used to confirm their presence in selected regions. The amplitude ratio of inward to outward Alfvén waves is employed to establish their flow direction. The dominant inward flow is observed in magnetic cloud whereas trailing solar wind shows the dominant outward flow of Alfvén waves. The observed reduction in Walén slope and correlation coefficient within magnetic cloud suggest (i) the simultaneous presence of an inward & outward Alfvén waves and/or (ii) a possibility of magnetic reconnection and/or (iii) development of thermal anisotropy and/or (iv) dissipation of Alfvénic fluctuations. The study implies that either the Alfvén waves dissipate in the magnetic cloud or its presence can lead to disruption of the magnetic cloud structure.
Monthly Notices of the Royal Astronomical Society, Oct 31, 2019
Generally, interplanetary coronal mass ejection (ICME) triggers intense and strong geomagnetic st... more Generally, interplanetary coronal mass ejection (ICME) triggers intense and strong geomagnetic storms. It has been established that the ICME sheath-moulded planar magnetic structure enhances the amplitude of the storms. Alfvén waves embedded in ICME magnetic clouds or high solar streams including corotating interacting regions (CIRs) in turn extend the recovery phase of the storm. Here, we investigate a geomagnetic storm with a very complex temporal profile with multiple decreasing and recovery phases. We examine the role of planar magnetic structure (PMS) and Alfvén waves in the various phases of the storm. We find that fast decrease and fast recovery phases are evident during transit of PMS regions, whereas a slight decrease or recovery is found during the transit of regions embedded with Alfvénic fluctuations.
Monthly Notices of the Royal Astronomical Society: Letters, Feb 13, 2018
The first insitu observation of torsional Alfvén waves during the interaction of large-scale magn... more The first insitu observation of torsional Alfvén waves during the interaction of large-scale magnetic clouds.
Monthly Notices of the Royal Astronomical Society, 2019
The Astrophysical Journal, Oct 1, 2022
Various remote and in situ observations, along with several models, simulations, and kinetic stud... more Various remote and in situ observations, along with several models, simulations, and kinetic studies, have been proposed in recent years, suggesting that the morphology of an interplanetary coronal mass ejection (ICME) magnetic cloud can vary from cylindrical, elliptical, toroidal, flattened, pancaked, etc. Recently, Raghav et al. proposed for the first time a unique morphological characteristic of an ICME magnetic cloud at 1 au that showed characteristics of a planar magnetic structure, using in situ data from the ACE spacecraft. In this study, we statistically investigate the plasma properties of planar and nonplanar ICMEs from 1998–2017 at 1 au. The detailed study of 469 ICMEs suggests that 136 (∼29%) ICMEs are planar, whereas 333 (∼71%) are nonplanar. Furthermore, total interplanetary magnetic field strength, average plasma parameters, i.e., plasma density, beta, thermal pressure, and magnetic pressure in planar ICME, are significantly higher than in the nonplanar ICME. Also, we noticed that the thickness of planar ICMEs is less compared to nonplanar ICMEs. This analysis demonstrates that planar ICMEs are formed due to the high compression of ICME. Moreover, we also observed the southward/northward magnetic field component’s double strength during planar ICMEs compared to nonplanar ICMEs. It implies that planar ICMEs are more geoeffective than nonplanar ICMEs.
arXiv (Cornell University), Oct 13, 2017
Recently, a super-elastic collision of large-scale plasmoids i.e. solar coronal mass ejections (C... more Recently, a super-elastic collision of large-scale plasmoids i.e. solar coronal mass ejections (CMEs) has been observed and further supported by numerical simulations. However, the energy gain by the system in the collision process is not clear. In-fact during plasmoids collision process, the energy exchange mechanism is still a chronic issue. Here, we present conclusive in situ evidence of sunward torsional Alfven waves in the magnetic cloud after the super-elastic collision of the largest plasmoids in the heliosphere. We conclude that magnetic reconnection and Alfven waves are the possible energy exchange mechanism during plasmoids interaction.
Monthly Notices of the Royal Astronomical Society: Letters, Nov 21, 2022
ABSTRACT Alfvénic fluctuations are widespread and crucial in various physical processes of space ... more ABSTRACT Alfvénic fluctuations are widespread and crucial in various physical processes of space & astrophysical plasma. However, their role in heating and work done remains unexplored. Here, we have used Wind spacecraft’s data situated at 1 au distance to examine 12 distinct Alfvénic regions using polytropic analysis. The study finds an average polytropic index value α = 2.64, which is consistent with a superadiabatic behaviour for plasma particles with three effective degrees of freedom (f = 3). Moreover, this study examines several scenarios for plasma particles with different degrees of freedom. We noted that the investigated Alfvénic region could be adiabatic only for plasma particles with f = 1.26 degrees of freedom. In addition to this, for α = 2.64, the ratio of work done to the total heat supply within the system is fracdeltawdeltaq=−0.68\frac{\delta w}{\delta q} = -0.68fracdeltawdeltaq=−0.68, indicating that 68 per cent of the total supplied heat is utilized to accomplish work by the system on the surrounding (expansion phenomena), and the remaining is used to increase the internal energy of the system. As a result, we hypothesized that the Alfvénic plasma region is cooling more than the adiabatic expectation, resulting in supercooling phenomena. Thus, we propose that the discovered possible superadiabatic process would be critical in understanding the energy transfer from the Alfvénic zone to the surrounding plasma.
The Astrophysical Journal Letters
The sheath plasma of interplanetary coronal mass ejections (ICMEs) is highly compressed, heated, ... more The sheath plasma of interplanetary coronal mass ejections (ICMEs) is highly compressed, heated, turbulent, and magnetically intense relative to the ambient solar wind. In this Letter, we perform a detailed study of proton temperature anisotropy within the 333 ICME sheath regions observed on board the Wind spacecraft spanning the years 1995–2015. Our observations show that marginal stability thresholds of mirror mode and firehose instabilities predominantly constrain the proton temperature anisotropy within these sheath regions regardless of ICME sheath plasma speed. This is true even when the plasma beta values are less than 2, a parameter space that should have favored the prevalence of parallel firehose and proton cyclotron instabilities according to linear stability analysis. This investigation demonstrates the critical role played by distinct plasma instabilities in shaping the evolution of ICME sheath plasma compared to the broader solar wind environment.
Advances in Space Research
Universe
Interplanetary coronal mass ejections (ICME) are large-scale eruptions from the Sun and prominent... more Interplanetary coronal mass ejections (ICME) are large-scale eruptions from the Sun and prominent drivers of space weather disturbances, especially intense/extreme geomagnetic storms. Recent studies by our group showed that ICME sheaths and/or magnetic clouds (MC) could be transformed into a planar magnetic structure (PMS) and speculate that these structures might be more geo-effective. Thus, we statistically investigated the geo-effectiveness of planar and non-planar ICME sheaths and MC regions. We analyzed 420 ICME events observed from 1998 to 2017, and we found that the number of intense (−100 to −200 nT) and extreme (<−200 nT) geomagnetic storms are large during planar ICMEs (almost double) compared to non-planar ICMEs. In fact, almost all the extreme storm events occur during PMS molded ICME crossover. The observations suggest that planar structures are more geo-effective than non-planar structures. Thus, the current study helps us to understand the energy transfer mechanism...
Advances in Space Research
arXiv (Cornell University), Mar 15, 2023
Despite centuries of rigorous theoretical and observational research, the origin and acceleration... more 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.
The Astrophysical Journal
Alfvén waves (AWs) are ubiquitous in space and astrophysical plasma. Their crucial role in variou... more Alfvén waves (AWs) are ubiquitous in space and astrophysical plasma. Their crucial role in various physical processes 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 in situ observation of SAWs in a flux rope of an interplanetary coronal mass ejection. We apply the Walén test to identify them. We have used Elsässer variables to estimate the characteristics of SAWs. They may be excited by the movement of the flux rope’s footpoints or by instabilities along the boundaries of the plasma magnetic cloud. Here, the change in plasma density or field strength in the surface-aligned magnetic field may trigger SAWs.