arXiv subjects

Debesh Bhattacharjee

Publications and source records attributed to Debesh Bhattacharjee.

10 recordsLinked to original sources

arXiv2026

Intermittent turbulent fluctuations in solar coronal mass ejections

Localised regions of high intensity fluctuations are known to be signatures of intermittency in fluid and plasma turbulence. We investigate such turbulent spots using near-Earth {\em in-situ} spacecraft observations of a sample of 125 solar coronal mass ejections (CMEs). We present statistical results which suggest that the intensity of the strongest turbulent spot and the turbulent spot occurrence rate are reliable indicators of the onset of the leading part of the CME event. Our findings also suggest that turbulent spots can be sites of enhanced proton heating. The findings of this study can enhance our understanding of intermittence in collisionless plasma turbulence and can improve CME/sheath-driven space weather impact prediction models.

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arXiv2025

Energy-Containing Electrons in Solar Flares: Improving Hard X-Ray and EUV Diagnostics

Solar flares effectively accelerate particles to non-thermal energies. These accelerated electrons are responsible for energy transport and subsequent emissions in HXR, radio, and UV/EUV radiation. Due to the steeply decreasing electron spectrum, the electron population and consequently the overall flare energetics, are predominantly influenced by low-energy non-thermal electrons. However, deducing the electron distribution in this energy-containing range remains a significant challenge. In this study, we apply the warm-target HXR emission model with kappa-form injected electrons to two well-observed GOES M-class flares. Moreover, we utilize EUV observations to constrain the flaring plasma properties, which enables us to determine the characteristics of accelerated electrons across a range from a few keV to tens of keV. We demonstrate that the warm-target model reliably constrains the properties of flare-associated electrons, even accounting for the uncertainties that had previously been unaddressed. The application of a kappa distribution for the accelerated electrons allows for meaningful comparisons with electron distributions inferred from EUV observations, specifically for energy ranges below the detection threshold of RHESSI. Our results indicate that the accelerated electrons constitute only a small fraction of the total electron population within the flaring region. Moreover, the physical parameters, such as electron escape time and acceleration time scale, inferred from both the warm-target model and the EUV observations further support the scenario in which electrons undergo thermalization within the corona. This study highlights the effectiveness of integrating the warm-target model with EUV observations to accurately characterize energy-containing electrons and their associated acceleration and transport processes.

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arXiv2025

Characterizing proton polytropic indices inside near-Earth magnetic clouds and ICME sheaths

The thermodynamics of interplanetary coronal mass ejections (ICMEs) is often described using a polytropic process. Estimating the polytopic index ($\gamma$) allows us to quantify the expansion or compression of the ICME plasma arising from changes in the plasma temperature. In this study, we estimate $\gamma$ for protons inside the magnetic clouds (MCs), their associated sheaths, and ambient solar wind for a large sample of well-observed events observed by the Wind spacecraft at 1 AU. We find that $\gamma$ shows a high ($\approx 1.6$) - low ($\approx 1.05$) - high ($\approx 1.2$) behavior inside the ambient solar wind, sheath, and MCs, respectively. We also find that the proton polytropic index is independent of small-scale density fluctuations. Furthermore, our results show that the stored energy inside MC plasma is not expended in expanding its cross-section at 1 AU. The sub-adiabatic nature of MC plasma implies external heating - possibly due to thermal conduction from the corona. We find that the heating gradient per unit mass from the corona to the protons of MC at 1 AU is $\approx 0.21$ erg cm$^{-1}$ g$^{-1}$ which is in agreement with the required proton heating budget.

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arXiv2025

Total Power and Low-energy Cut-off Time Evolution of Solar Flare Accelerated Electrons Using X-Ray Observations and Warm-Target Model

A primary characteristic of solar flares is the efficient acceleration of electrons to nonthermal deka-keV energies. While hard X-Ray (HXR) observation of bremsstrahlung emission serves as the key diagnostic of these electrons. In this study, we investigate the time evolution of flare-accelerated electrons using the warm-target model. This model, unlike the commonly used cold-target model, can determine the low-energy cut-off in the nonthermal electron distribution, so that the energetics of nonthermal electrons can be deduced more accurately. Here, we examine the time-evolution of nonthermal electrons in flares well-observed by the RHESSI and the Solar Orbiter (SolO, using the STIX instrument) spacecrafts. Using spectroscopic and imaging HXR observations, the time evolution of the low-energy cut-off of the accelerated electron distribution, the total power of nonthermal electrons, total rate of nonthermal electrons, and excess thermal emission measure from the nonthermal electrons, are investigated. We find that the time profile of the low-energy cut-off of the accelerated electron distribution shows a high-low-high trend around the HXR bursts of flares, while the time evolution of the total rate of injected electrons shows a low-high-low behavior. Although the total power of nonthermal electrons is sensitive to the cut-off energy, the temporal variation of the flare power follows the temporal variation of the acceleration rate. We further find that the highest contribution of the excess thermal emission measure coming from thermalization of injected electrons takes place around the hard X-ray peak.

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arXiv2025

Proton heating estimates from near-Earth observations of coronal mass ejections in solar cycle 24

As solar coronal mass ejections (CMEs) propagate through the heliosphere, they expend energy in heating protons to compensate for the cooling that occurs due to expansion. CME propagation models usually treat energy dissipation implicitly via a polytropic index ($\delta$). Here we calculate the power dissipation implied by a given $\delta$ and compare it with the power available in the turbulent velocity fluctuations. We make this comparison using near-Earth {\em in-situ} observations of 27 of the most geoeffective CMEs ($D_{\rm st} < -75$ nT) in solar cycle 24. For $\delta = 5/3$, the power in the turbulent velocity fluctuations is $\approx 54$\% smaller than what would be required to maintain the proton temperature at the observed values. If the power in the turbulent cascade is assumed to be fully expended in local proton heating, the most probable value for $\delta$ is 1.35. Our results contribute to a better understanding of CME energetics, and thereby to improved CME propagation models and estimates of Earth arrival times.

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arXiv2025

Turbulent power: a discriminator between sheaths and CMEs

Solar coronal mass ejections (CMEs) directed at the Earth often drive large geomagnetic storms. Here we use velocity, magnetic field and proton density data from 152 CMEs that were sampled in-situ at 1 AU by the WIND spacecraft. We Fourier analyze fluctuations of these quantities in the quiescent pre-CME solar wind, sheath and magnetic cloud. We quantify the extent by which the power in turbulent (magnetic field, velocity and density) fluctuations in the sheath exceeds that in the solar wind background and in the magnetic cloud. For instance, the mean value of the power per unit volume in magnetic field fluctuations in the sheath is 76.7 times that in the solar wind background, while the mean value of the power per unit mass in velocity fluctuations in the sheath is 9 times that in the magnetic cloud. Our detailed results show that the turbulent fluctuation power is a useful discriminator between the ambient solar wind background, sheaths and magnetic clouds and can serve as a useful input for space weather prediction.

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arXiv2024

Flare-accelerated Electrons in the Kappa Distribution from X-Ray Spectra with the Warm-Target Model

X-ray observations provide important and valuable insights into the acceleration and propagation of nonthermal electrons during solar flares. Improved X-ray spectral analysis requires a deeper understanding of the dynamics of energetic electrons. Previous studies have demonstrated that the dynamics of accelerated electrons with a few thermal speeds are more complex than those with significantly higher speeds. To better describe the energetic electrons after injection, a model considering energy diffusion and thermalization effects in flare conditions (the warm-target model) has recently been developed for spectral analysis of hard X-rays. This model has demonstrated how the low-energy cutoff, which can hardly be constrained in cold-target modeling, can be determined. However, the power-law form may not be the most suitable representation of injected electrons. The kappa distribution, which is proposed as a physical consequence of electron acceleration, has been applied successfully in RHESSI spectral analysis. In this study, we employ the kappa-form injected electrons in the warm-target model to analyze two M-class flares, observed by RHESSI and STIX, respectively. The best-fit results show that the kappa-form energetic electron spectrum generates lower nonthermal energy than the power-law form when producing a similar photon spectrum in the fit range. We also demonstrated that the fit parameters associated with the kappa-form electron spectrum can be well determined with small uncertainty. Further, the kappa distribution, which covers the entire electron energy range, enables the determination of key electron properties such as total electron number density and average energy at the flare site, providing valuable information on electron acceleration processes.

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arXiv2022

Turbulence and Anomalous Resistivity inside Near-Earth Magnetic Clouds

We use in-situ data from the Wind spacecraft to survey the amplitude of turbulent fluctuations in the proton density and total magnetic field inside a large sample of near-Earth magnetic clouds (MCs) associated with coronal mass ejections (CMEs) from the Sun. We find that the most probable value of the modulation index for proton density fluctuations ($\delta n_{p}/n_{p}$) inside MCs ranges from 0.13 to 0.16, while the most probable values for the modulation index of the total magnetic field fluctuations ($\delta B/B$) range from 0.04 to 0.05. We also find that the most probable value of the Mach number fluctuations ($\delta M$) inside MCs is $\approx 0.1$. The anomalous resistivity inside near-Earth MCs arising from electron scattering due to turbulent magnetic field fluctuations exceeds the (commonly used) Spitzer resistivity by a factor of $\approx 500-1000$. The enhanced Joule heating arising from this anomalous resistivity could impact our understanding of the energetics of CME propagation.

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arXiv2022

On the specific energy and pressure in near-Earth magnetic clouds

The pressure and energy density of the gas and magnetic field inside solar coronal mass ejections (in relation to that in the ambient solar wind) is thought to play an important role in determining their dynamics as they propagate through the heliosphere. We compare the specific energy (${\rm erg\,g^{-1}}$) [comprising kinetic ($H_{\rm k}$), thermal ($H_{\rm th }$) and magnetic field ($H_{\rm mag}$) contributions] inside MCs and the solar wind background. We examine if the excess thermal + magnetic pressure and specific energy inside MCs (relative to the background) is correlated with their propagation and internal expansion speeds. We ask if the excess thermal + magnetic specific energy inside MCs might make them resemble rigid bodies in the context of aerodynamic drag. We use near-Earth in-situ data from the WIND spacecraft to identify a sample of 152 well observed interplanetary coronal mass ejections and their MC counterparts. We compute various metrics using these data to address our questions. We find that the total specific energy ($H$) inside MCs is approximately equal to that in the background solar wind. We find that the the excess (thermal + magnetic) pressure and specific energy are not well correlated with the near-Earth propagation and expansion speeds. We find that the excess thermal+magnetic specific energy $\gtrsim$ the specific kinetic energy of the solar wind incident on 81--89 \% of the MCs we study. This might explain how MCs retain their structural integrity and resist deformation by the solar wind bulk flow.

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arXiv2022

On modeling ICME cross-sections as static MHD columns

Solar coronal mass ejections are well known to expand as they propagate through the heliosphere. Despite this, their cross-sections are usually modeled as static plasma columns within the magnetohydrodynamics (MHD) framework. We test the validity of this approach using in-situ plasma data from 151 magnetic clouds (MCs) observed by the WIND spacecraft and 45 observed by the Helios spacecrafts. We find that the most probable cross-section expansion speeds for the WIND events are only $\approx 0.06$ times the Alfv\'en speed inside the MCs while the most probable cross-section expansion speeds for the Helios events is $\approx 0.03$. MC cross-sections can thus be considered to be nearly static over an Alfv\'en crossing timescale. Using estimates of electrical conductivity arising from Coulomb collisions, we find that the Lundquist number inside MCs is high ($\approx 10^{13}$), suggesting that the MHD description is well justified. The Joule heating rates using our conductivity estimates are several orders of magnitude lower than the requirement for plasma heating inside MCs near the Earth. While the (low) heating rates we compute are consistent with the MHD description, the discrepancy with the heating requirement points to possible departures from MHD and the need for a better understanding of plasma heating in MCs.

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