arXiv subjects

Sohom Roy

Publications and source records attributed to Sohom Roy.

13 recordsLinked to original sources

arXiv2025

A robust empirical relationship between speed and turbulence energy in the near-Earth solar wind

The connection between turbulence and solar-wind acceleration, long known in space physics, is further developed in this Letter by establishing a robust empirical law that relates the bulk-flow speed to the magnetohydrodynamic-scale fluctuation energy in the plasma. The model is based on analysis of twenty-five years of near-Earth observations by NASA's Advanced Composition Explorer. It provides a simple way to estimate turbulence energy from low-resolution speed data -- a practical approach that may be of utility when high-resolution measurements or advanced turbulence models are unavailable. Potential heliospheric applications include space-weather forecasting operations, remote imaging datasets, and energetic-particle transport models that require turbulence amplitudes to specify diffusion parameters.

physics.space-ph↗

arXiv2025

Characterization of the Trans-Alfv\'enic Region Using Observations from Parker Solar Probe

Close to Earth the solar wind is usually super-Alfv\'enic, i.e. the speed of the solar wind is much larger than the Alfv\'en speed. However, in the lower coronal regions, the solar wind is mostly sub-Alfv\'enic. With the Parker Solar Probe (PSP) crossing the boundary between the sub- and super-Alfv\'enic flow, Bandyopadhyay et al. (2022) performed a turbulence characterization of the sub-Alfv\'enic solar wind with initial data from encounters 8 and 9. In this study, we re-examine the turbulence properties such as turbulence amplitude, anisotropy of the magnetic field variance, intermittency and switchback strength extending with PSP data for encounters 8-19. The later orbits probe lower altitudes and experience sub-Alfv\'enic conditions more frequently providing a greater statistical coverage to contrast sub- and super-Alfv\'enic solar wind. Also, by isolating the intervals where the solar wind speed is approximately equal to the Alfv\'en speed, we explore the transition in more detail. We show that the amplitude of the normalized magnetic field fluctuation is smaller for the sub-Alfv\'enic samples. While solar wind turbulence in general is shown to be anisotropic, the sub-Alfv\'enic samples are more anisotropic than the super-Alfv\'enic samples, in general. Further, we show that the sub- and super-Alfv\'enic samples do not show much distinction in terms of intermittency strength. Finally, consistent with prior results, we find no evidence for polarity reversing > 90 degrees switchbacks in the sub-Alfv\'enic solar wind

astro-ph.SR↗

arXiv2025

von Karman - Howarth Similarity of Spatial Correlations and the Distribution of Correlation Lengths in Solar Photospheric Turbulence

Fluctuations in the Sun's photospheric magnetic field are the primary source of the turbulence that can heat and accelerate the solar atmosphere, and thus play an important role in the production and evolution of the solar wind that permeates the heliosphere. A key parameter that characterizes this turbulence is the correlation scale of fluctuations, which determines the injection of turbulent energy into the plasma and the diffusive transport of solar energetic particles. This study employs magnetogram data from the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory to characterize an ensemble of spatial autocorrelation functions (ACFs) of turbulence in the photosphere. It is shown that the two-point ACFs satisfy the similarity-decay hypothesis of von K\'arm\'an and Howarth, a fundamental property of turbulent systems: rescaling the ACFs by their respective energies and correlation lengths yields a quasi-universal exponential form. The probability distribution function of transverse correlation lengths ($\lambda$) is shown to be approximately log-normal, which is consistent with observations of turbulence in the solar wind. A ``mosaic'' of the spatial distribution of $\lambda$ over the photosphere is presented; the ``quiet Sun'' tends to have $\lambda\sim 1500$ km (albeit with a wide distribution), which is close to the scale of solar granulation; systematically longer lengths are associated with active regions. A positive correlation is observed between mean magnetic field magnitude and $\lambda$, and empirical fits quantify this relationship. These results improve our understanding of solar turbulence while providing observational constraints for models that describe turbulence transport from solar and stellar photospheres into their atmospheres.

astro-ph.SR↗

arXiv2025

Interplanetary magnetic correlation and low-frequency spectrum over many solar rotations

Fluctuations and structure across a wide range of spatial and temporal scales are frequently studied in the solar wind. The properties of the low-frequency fluctuations are of relevance to turbulent energy injection into the plasma and the transport of high-energy cosmic rays. Correlation analysis of decade-long intervals of interplanetary data permits study of fluctuations at time scales much longer than suitably defined correlation times, and therefore at frequencies well below those associated with the Kolmogorov inertial range of {\it in situ} turbulence. At the frequencies of interest, we study the familiar occurrence of the $1/f$ spectral signature. We also study point spectral features due to solar rotation and their relation with the $1/f$ signal. We report novel properties at timescales ranging from minutes up to years, using data selected by wind speed, phase of solar cycle, and cartesian components of the magnetic field. A surprising finding is that the power in solar rotation harmonics is consistent with an extension of the $1/f$ spectrum, down to frequencies as low as around $\unit[5 \times 10^{-7}]{Hz}$. The presence of a broadband $1/f$ spectrum across different wind types supports the interpretation that $1/f$ signals may be related to or even originate from the solar dynamo.

astro-ph.SR↗

arXiv2024

Observed Fluctuation Enhancement and Departure from WKB Theory in Sub-Alfv\'enic Solar Wind

Using Parker Solar Probe data from orbits 8 through 17, we examine fluctuation amplitudes throughout the critical region where the solar wind flow speed approaches and then exceeds the Alfv\'en wave speed, taking account of various exigencies of the plasma data. In contrast to WKB theory for non-interacting Alfv\'en waves streaming away from the Sun, the magnetic and kinetic fluctuation energies per unit volume are not monotonically decreasing. Instead, there is clear violation of conservation of standard WKB wave action, which is consistent with previous indications of strong in-situ fluctuation energy input in the solar wind near the Alfv\'en critical region. This points to strong violations of WKB theory due to nonlinearity (turbulence) and major energy input near the critical region, which we interpret as likely due to driving by large-scale coronal shear flows.

astro-ph.SR↗

arXiv2024

$1/f$ Noise in the Heliosphere: A Target for PUNCH Science

We present a broad review of 1/f noise observations in the heliosphere, and discuss and complement the theoretical background of generic 1/f models as relevant to NASA's PUNCH mission. First observed in the voltage fluctuations of vacuum tubes, the scale-invariant 1/f spectrum has since been identified across a wide array of natural and artificial systems, including heart rate fluctuations and loudness patterns in musical compositions. In the solar wind, the interplanetary magnetic field trace spectrum exhibits 1/f scaling within the frequency range from around 2e-6 Hz to around 1e-3 Hz at 1 au. One compelling mechanism for the generation of 1/f noise is the superposition principle, where a composite 1/f spectrum arises from the superposition of a collection of individual power-law spectra characterized by a scale-invariant distribution of correlation times. In the context of the solar wind, such a superposition could originate from scale-invariant reconnection processes in the corona. Further observations have detected 1/f signatures in the photosphere and corona at frequency ranges compatible with those observed at 1 au, suggesting an even lower altitude origin of 1/f spectrum in the solar dynamo itself. This hypothesis is bolstered by dynamo experiments and simulations that indicate inverse cascade activities, which can be linked to successive flux tube reconnections beneath the corona, and are known to generate 1/f noise possibly through nonlocal interactions at the largest scales. Conversely, models positing in situ generation of $1/f$ signals face causality issues in explaining the low-frequency portion of the 1/f spectrum. Understanding 1/f noise in the solar wind may inform central problems in heliospheric physics, such as the solar dynamo, coronal heating, the origin of the solar wind, and the nature of interplanetary turbulence.

astro-ph.SR↗

arXiv2024

The Alfv\'en Transition Zone observed by the Parker Solar Probe in Young Solar Wind -- Global Properties and Model Comparisons

The transition from subAlfv\'enic to superAlfv\'enic flow in the solar atmosphere is examined by means of Parker Solar Probe (PSP) measurements during solar encounters 8 to 14. Around 220 subAlfv\'enic periods with a duration $\ge$ 10 minutes are identified. The distribution of their durations, heliocentric distances, and Alfv\'en Mach number are analyzed and compared with a global magnetohydrodynamic model of the solar corona and wind, which includes turbulence effects. The results are consistent with a patchy and fragmented morphology, and suggestive of a turbulent Alfv\'en zone within which the transition from subAlfv\'enic to superAlfv\'enic flow occurs over an extended range of helioradii. These results inform and establish context for detailed analyses of subAlfv\'enic coronal plasma that are expected to emerge from PSP's final mission phase, as well as for NASA's planned PUNCH mission.

astro-ph.SR↗

arXiv2024

Anisotropy of Density Fluctuations in the Solar Wind at 1 au

A well-known property of solar wind plasma turbulence is the observed anisotropy of the autocorrelations, or equivalently the spectra, of velocity and magnetic field fluctuations. Here we explore the related but apparently not well-studied issue of the anisotropy of plasma density fluctuations in the energy-containing and inertial ranges of solar wind turbulence. Using 10 years (1998-2008) of in situ data from the Advanced Composition Explorer (ACE) mission, we find that for all but the fastest wind category, the density correlation scale is slightly larger in directions quasi-parallel to the large-scale mean magnetic field as compared to quasi-perpendicular directions. The correlation scale in fast wind is consistent with isotropic. The anisotropy as a function of the level of correlation is also explored. We find at small correlation levels, i.e., at energy-containing scales and larger, the density fluctuations are close to isotropy for fast wind, and slightly favor more rapid decorrelation in perpendicular directions for slow and medium winds. At relatively smaller (inertial range) scales where the correlation values are larger, the sense of anisotropy is reversed in all speed ranges, implying a more "slab-like" structure, especially prominent in the fast wind samples. We contrast this finding with published results on velocity and magnetic field correlations.

physics.space-ph↗

arXiv2024

Low-cost, Lightweight Electronic Flow Regulators for Throttling Liquid Rocket Engines

For small-scale liquid rockets, pressure-fed systems are commonly favoured due to their simplicity and low weight. In such systems, accurate regulation of both tank and injector pressures over a wide range of upstream pressures is critical $-$ more accurate regulation allows for higher engine efficiency and minimal tank mass, thus improving flight performance. However, existing methods such as dome-loaded pressure regulators are inflexible, or require extensive characterization to function accurately. These methods also suffer from limited orifice size, droop, and slow reaction times, making them unsuitable for throttling by adjusting pressures in flight, which are increasingly important as propulsively landing rockets become more common. To overcome these challenges, we designed an electronic pressure regulator (eReg), a multi-input multi-output system utilising closed loop feedback to accurately control downstream pressures. Our design is simple, low-cost and robust: with a single ball valve actuated by a motor, we regulate both gaseous pressurant and cryogenic liquid propellant at high flow rates (1.14 kg/s of liquid; 0.39 kg/s of gas) and upstream pressures (310 bar). Using 2 eRegs to regulate propellant tank pressures, and 2 eRegs for regulating propellant flow to the engine, we demonstrated our system's ability, in a static fire test, to regulate pressures accurately (within 0.2 bar) while simultaneously throttling our engine. To the best of our knowledge, this is the first time any undergraduate team has successfully throttled a liquid bipropellant engine.

eess.SY↗

arXiv2023

Relaxation of the turbulent magnetosheath

In turbulence, nonlinear terms drive energy transfer from large-scale eddies into small scales through the so-called energy cascade. Turbulence often relaxes toward states that minimize energy; typically these states are considered globally. However, turbulence can also relax toward local quasi-equilibrium states, creating patches or cells where the magnitude of nonlinearity is reduced and energy cascade is impaired. We show, for the first time, compelling observational evidence that this ``cellularization'' of turbulence can occur due to local relaxation in a strongly turbulent natural environment such as the Earth's magnetosheath.

physics.flu-dyn↗

arXiv2022

Isotropization and Evolution of Energy-Containing Eddies in Solar Wind Turbulence: Parker Solar Probe, Helios 1, ACE, WIND, and Voyager 1

We examine the radial evolution of correlation lengths perpendicular ($\lambda_C^{\perp}$) and parallel ($\lambda_C^{\parallel}$) to the magnetic-field direction, computed from solar wind magnetic-field data measured by Parker Solar Probe (PSP) during its first eight orbits, Helios 1, Advanced Composition Explorer (ACE), WIND, and Voyager 1 spacecraft. Correlation lengths are grouped by an interval's alignment angle; the angle between the magnetic-field and solar wind velocity vectors ($\Theta_{\rm BV}$). Parallel and perpendicular angular channels correspond to angles $0^{\circ}~<~\Theta_{\rm BV}~<~40^{\circ}$ and $50^{\circ}~<~\Theta_{\rm BV}~<~90^{\circ}$, respectively. We observe an anisotropy in the inner heliosphere within 0.40~au, with $\lambda_C^{\parallel} / \lambda_C^{\perp} \approx 0.75$ at 0.10~au. This anisotropy reduces with increasing heliocentric distance and the correlation lengths roughly isotropize within 1~au. Results from ACE and WIND support a reversal of the anisotropy, such that $\lambda_C^{\parallel} /\lambda_C^{\perp} \approx 1.29$ at 1~au. The ratio does not appear to change significantly beyond 1~au, although the small number of parallel intervals in the Voyager dataset precludes unambiguous conclusions from being drawn. This study provides insights regarding the radial evolution of the large, most energetic interacting turbulent fluctuations in the heliosphere. We also emphasize the importance of tracking the changes in sampling direction in PSP measurements as the spacecraft approaches the Sun, when using these data to study the radial evolution of turbulence. This can prove to be vital in understanding the more complex dynamics of the solar wind in the inner heliosphere and can assist in improving related simulations.

astro-ph.SR↗

arXiv2022

Pressure-Strain Interaction as the Energy Dissipation Estimate in Collisionless Plasma

The dissipative mechanism in weakly collisional plasma is a topic that pervades decades of studies without a consensus solution. We compare several energy dissipation estimates based on energy transfer processes in plasma turbulence and provide justification for the pressure-strain interaction as a direct estimate of the energy dissipation rate. The global and scale-by-scale energy balances are examined in 2.5D and 3D kinetic simulations. We show that the global internal energy increase and the temperature enhancement of each species are directly tracked by the pressure-strain interaction. The incompressive part of the pressure-strain interaction dominates over its compressive part in all simulations considered. The scale-by-scale energy balance is quantified by scale filtered Vlasov-Maxwell equations, a kinetic plasma approach, and the lag dependent von K\'arm\'an-Howarth equation, an approach based on fluid models. We find that the energy balance is exactly satisfied across all scales, but the lack of a well-defined inertial range influences the distribution of the energy budget among different terms in the inertial range. Therefore, the widespread use of the Yaglom relation to estimating dissipation rate is questionable in some cases, especially when the scale separation in the system is not clearly defined. In contrast, the pressure-strain interaction balances exactly the dissipation rate at kinetic scales regardless of the scale separation.

physics.space-ph↗

arXiv2021

A Study on Herd Behavior Using Sentiment Analysis in Online Social Network

Social media platforms are thriving nowadays, so a huge volume of data is produced. As it includes brief and clear statements, millions of people post their thoughts on microblogging sites every day. This paper represents and analyze the capacity of diverse strategies to volumetric, delicate, and social networks to predict critical opinions from online social networking sites. In the exploration of certain searching for relevant, the thoughts of people play a crucial role. Social media becomes a good outlet since the last decades to share the opinions globally. Sentiment analysis as well as opinion mining is a tool that is used to extract the opinions or thoughts of the common public. An occurrence in one place, be it economic, political, or social, may trigger large-scale chain public reaction across many other sites in an increasingly interconnected world. This study demonstrates the evaluation of sentiment analysis techniques using social media contents and creating the association between subjectivity with herd behavior and clustering coefficient as well as tries to predict the election result (2021 election in West Bengal). This is an implementation of sentiment analysis targeted at estimating the results of an upcoming election by assessing the public's opinion across social media. This paper also has a short discussion section on the usefulness of the idea in other fields.

cs.SI↗