arXiv subjects

Yogesh

Publications and source records attributed to Yogesh.

32 records · Page 2Linked to original sources

arXiv2025

Evidence of Interaction between Ion-Scale Waves and Ion Velocity Distributions in the Solar Wind

Recent in situ observations from Parker Solar Probe (PSP) near perihelia reveal ion beams, temperature anisotropies, and kinetic wave activity. These features are likely linked to solar wind heating and acceleration. During PSP Encounter 17 (at $11.4R_s$) on Sep-26-2023, the PSP/FIELDS instrument detected enhanced ion-scale wave activity associated with deviations from local thermodynamic equilibrium in ion velocity distribution functions (VDFs) observed by the PSP/Solar Probe Analyzers-Ion (SPAN-I). Dense beams (secondary populations) were present in the proton VDFs during this wave activity. Using bi-Maxwellian fits to the proton VDFs, we found that the density of the proton beam population increased during the wave activity and, unexpectedly, surpassed the core population at certain intervals. Interestingly, the wave power was reduced during the intervals when the beam population density exceeded the core density. The drift velocity of the beams decreases from 0.9 to 0.7 of the Alfvén speed and the proton core shows a higher temperature anisotropy ($T_\perp/T_\parallel>2.5$) during these intervals. We conclude that the observations during these intervals are consistent with a reconnection event during a heliospheric current sheet crossing. During this event, $α$ particle parameters (density, velocity, and temperature anisotropy) remained nearly constant. Using linear analysis, we examined how the proton beam drives instability or wave dissipation. Furthermore, We investigated the nonlinear evolution of ion kinetic instabilities using hybrid kinetic simulations. This study provides direct clues about energy transfer between particles and waves in the young solar wind.

astro-ph.SR↗

arXiv2025

Modeling hot, anisotropic ion beams in the solar wind motivated by the Parker Solar Probe observations near perihelia

Recent observations of the solar wind ions by the SPAN-I instruments on board the Parker Solar Probe (PSP) spacecraft at solar perihelia (Encounters) 4 and closer find ample evidence of complex anisotropic non-Maxwellian velocity distributions that consist of core, beam, and `hammerhead' (i.e., anisotropic beam) populations. The proton core populations are anisotropic, with T_perp/T||>1, and the beams have super-Alfvenic speed relative to the core (we provide an example from Encounter 17). The alpha-particle population show similar features as the protons. These unstable VDFs are associated with enhanced, right-hand (RH) and left-hand (LH) polarized ion-scale kinetic wave activity, detected by the FIELDS instrument. Motivated by PSP observations, we employ nonlinear hybrid models to investigate the evolution of the anisotropic hot-beam VDFs and model the growth and the nonlinear stage of ion kinetic instabilities in several linearly unstable cases. The models are initialized with ion VDFs motivated by the observational parameters. We find rapidly growing (in terms of proton gyroperiods) combined ion-cyclotron (IC) and magnetosonic (MS) instabilities, which produce LH and RH ion-scale wave spectra, respectively. The modeled ion VDFs in the nonlinear stage of the evolution are qualitatively in agreement with PSP observations of the anisotropic core and `hammerhead' velocity distributions, quantifying the effect of the ion kinetic instabilities on wind plasma heating close to the Sun. We conclude that the wave-particle interactions play an important role in the energy transfer between the magnetic energy (waves) and random particle motion leading to anisotropic solar wind plasma heating.

astro-ph.SR↗

arXiv2025

Non-Standard Thermal History and Formation of Primordial Black Holes in Einstein-Gauss-Bonnet Gravity

Inflation provides a suitable environment for the formation of Primordial Black Holes(PBHs). In this article, we examine the formation of primordial black holes in the Mutated Hilltop inflation model coupled with the Einstein-Gauss-Bonnet term. A suitable choice of the coupling function with adjusted parameters can produce the USR regime during the inflationary phase, which lasts for some number of e-folds. The scalar field in this regime remains almost unchanged, and the first slow-roll parameter $ε_1$ drops dramatically, leading to a significant enhancement to the curvature power spectrum for small scales so that it grows up to order of $\mathcal{O}(0.01)$; a crucial feature for producing PBH and secondary gravitational waves (GWs). We investigate the formation of PBHs for different sets of parameters. By considering the behavior of the scalar power spectrum, it is realized that the peak in the scalar power spectrum occurs in different scales. Then, the presented model is capable of predicting PBH formation in a wide range mass, from $\mathcal{O}(10^{-14})M_{\odot}$ to $\mathcal{O}(10)M_{\odot}$, which are compatible with the LIGO-Virgo data. PBHs with mass $\mathcal{O}(10^{-5})M_{\odot}$ can account for the micro-lensing event in OGLE as well as the asteroid masses $\mathcal{O}(10^{-15})M_{\odot}-\mathcal{O}(10^{-12})M_{\odot}$ PBH can be attributed to $100\% $ dark matter present in the universe. The generated perturbations during inflation re-enter the horizons after the end of inflation, where the universe may be in a non-standard epoch rather than the radiation-dominant phase, with an equation of state $1/3< ω\leq 1$...

gr-qc↗

arXiv2024

Origins of Very Low Helium Abundance Streams Detected in the Solar Wind Plasma

The abundance of helium ($A_{He}$) in the solar wind exhibits variations typically in the range from 2-5% with respect to solar cycle activity and solar wind velocity. However, there are instances where the observed $A_{He}$ is exceptionally low ($<$ 1%). These low-$A_{He}$ occurrences are detected both near the Sun and at 1 AU. The low $A_{He}$ events are generally observed near the heliospheric current sheet. We analyzed 28 low-$A_{He}$ events observed by the Wind spacecraft and 4 by Parker Solar Probe (PSP) to understand their origin. In this work, we make use of the ADAPT-WSA model to derive the sources of our events at the base of the solar corona. The modeling suggests that the low-$A_{He}$ events originated from the boundaries of coronal holes, primarily from large quiescent helmet streamers. We argue that the cusp above the core of the streamer can produce such very low helium abundance events. The streamer core serves as an ideal location for gravitational settling to occur as demonstrated by previous models, leading to the release of this plasma through reconnection near the cusp, resulting in low $A_{He}$ events. Furthermore, observations from Ulysses provide direct evidence that these events originated from coronal streamers.

astro-ph.SR↗

arXiv2024

Understanding the variability of helium abundance in the solar corona using three-fluid modeling and UV observations

The variability of helium abundance in the solar corona and the solar wind is an important signature of solar activity, solar cycle, solar wind sources, as well as coronal heating processes. Motivated by recently reported remote sensing UV imaging observations by Helium Resonance Scattering in the Corona and Heliosphere (HERSCHEL) payload sounding rocket of helium abundance in inner corona on 14-Sep-2009 near solar minimum, we present the results of the first three-dimensional three-fluid (electrons, protons, alpha particles) model of tilted coronal streamer belt and slow solar wind that illustrate the various processes leading to helium abundance differentiation and variability. We find good qualitative agreement between the three-fluid model and the coronal helium abundances variability reported from UV observations of streamers, providing insight on the effects of the physical processes, such as heating, gravitational settling and inter-species Coulomb friction in the out-flowing solar wind that produce the observed features. The study impacts our understanding of the origins of the slow solar wind.

astro-ph.SR↗

arXiv2023

Is the NANOGrav detection evidence of resonant particle creation during inflation?

We show that the recently reported cosmic gravitational wave background by the NANOGrav 15-year collaboration may be the result of resonant particle creation during inflation. For the appropriate amplitude and particle mass an enhancement of the primordial scalar power spectrum could induce Secondary Induced Gravitational Waves (SIGW) which will appear on a scale corresponding to the frequency of the NANOGrav detection. Since the resonant creation will have an effect comparable to that of a delta function increment as studied by the NANOGrav 15-year collaboration, our study indicates that the low-frequency Pulsar Timing Array (PTA) data could reveal the aspects of the physics during inflation through the detection of a cosmic background of Gravitational Waves (GW).

astro-ph.CO↗

arXiv2023

New insights on the behaviour of solar wind protons and alphas in the Stream Interaction Region in solar cycle 23 and 24

Although the enhancements in the alpha-proton ratio in the solar wind (expressed as $A_{He} = N_a/N_p*100$) in the Interplanetary Coronal Mass Ejections (ICMEs) have been studied in the past, $A_{He}$ enhancements at the stream interface region received very little attention so far. In this letter, by extensively analyzing the stream interaction region (SIR) events observed in solar cycle 23 and 24, we show that the stream interface of alphas starts separating out from that of protons from the minimum of solar cycle 23. The population of alpha particles are enhanced compared to protons at higher angles between bulk velocity and local magnetic field (henceforth, bulk velocity angle) in the fast wind region of SIRs if the background solar wind is taken as reference. The analysis of differential velocities between alphas and protons also reveals that the faster alpha particles accumulate near the fast wind side of the stream interface region leading to enhancement of $A_{He}$. The investigation brings out, for the first time, the salient changes in $A_{He}$ in SIRs for the two solar cycles and highlight the importances of bulk velocity angle and differential velocity in the fast wind region for the changes in $A_{He}$ in SIRs.

astro-ph.SR↗

arXiv2023

A Case Study of Small Field Inflationary Dynamics in the Einstein-Gauss Bonnet Framework in the Light of $GW170817$

We study two of the most theoretically promising models of inflation, namely the Natural inflation and the Mutated Hilltop inflation, in the Einstein-Gauss Bonnet(EGB) gravity framework. In this work, we try to explore these models in EGB framework, keeping the observations from $GW170817$ on the speed of gravitational wave to be equal to the speed of light. This has direct implication on the non-minimal coupling to the Gauss-Bonnet invariant in the action. Thus, the effective potential gets new features. We have not only analysed the inflationary dynamics, but also the reheating dynamics and finally the corresponding energy spectrum of the gravitational wave.

astro-ph.CO↗

arXiv2022

Production of Primordial Black Holes via Single Field Inflation and Observational Constraints

In a class of single field models of inflation, the idea of Primordial Black holes(PBHs) production is studied. In this case, the dynamics on small cosmological scales differs significantly from that of the large scales probed by the observations of cosmic microwave background(CMB). This difference becomes a virtue in producing correct physical ambiance for the seeds required to produce PBHs. Thus, once the perturbed scales renter the horizon of our Universe during the later epochs of radiation domination and subsequent matter domination, these seeds collapses to produce PBHs. We have shown, in this class of model, depending on the model parameters and the class defining set parameters, one can have PBHs formed for a vast mass ranges from $10^{-18}$ to $10^{-6}$ solar mass($\textup{M}_\odot$). We have also shown, for a particular class of model, the total dark matter density today can be attributed to the PBHs density. The vast range of the mass depending on the class parameter, gives ample opportunity to study enriched phenomenological implications associated with this model to probe the nascent Universe dynamics.

astro-ph.CO↗

arXiv2022

Lower tensor to scalar ratio in a SUGRA motivated inflationary potential

A scalar potential obtained from the $D$-term in the Supergravity models, which dominates over $F$ term and is mainly responsible for the inflationary phase in the early universe, is studied. The potential with canonical kinetic terms for scalar fields in the Lagrangian, has a very slow roll feature in comparison to various other plateau type inflationary potentials. In this case, a much lower tensor-to-scalar ratio ($r$) of $\mathcal{O}(10^{-3})$ is achievable. The requirement of slow roll condition for the inflation potential implies that the up type neutral scalar and the down type neutral scalar in Supergravity models are with equal field strength at the time of inflation. If this relationship holds down to the electroweak scale for the corresponding $vev$ values of these fields, then it will indicate a higher SUSY breaking scale around 100 TeV. The predicted values of the inflationary observables are well within the 1-$σ$ bounds of the recent constraints from {\it Planck'18} observations. The era of reheating after the inflationary phase, is also studied and the bounds on the reheating temperature ($T_{re}$) is calculated for a different equation of states during reheating ($w_{re}$) for the {\it Planck'18} allowed values of the scalar spectral index ($n_s$). For our model with $w_{re}=2/3$ and $w_{re}=1$, after satisfying all the bounds due to gravitino overproduction, we can have big parameter space for $T_{re}$ which is well inside {\it Planck'18} 1-$σ$ bound on $n_s$.

astro-ph.CO↗

arXiv2022

A holistic approach to understand Helium enrichment in Interplanetary coronal mass ejections: New insights

Despite helium abundance (AHe = nH/nHe) is ~ 8 % at the solar photospheric/chromospheric heights, AHe can be found to exceed 8% in interplanetary coronal mass ejections (ICMEs) on many occasions. Although various factors like interplanetary shocks, chromospheric evaporation and "sludge removal" have been separately invoked in the past to address the AHe enhancements in ICMEs, none of these processes could explain the variability of AHe in ICMEs comprehensively. Based on extensive analysis of 275 ICME events, we show that there is a solar activity variation of ICME averaged AHe values. The investigation also reveals that the first ionization potential effect as well as coronal temperature are not the major contributing factors for AHe enhancements in ICMEs. Investigation on concurrent solar flares and ICME events for 63 cases reveals that chromospheric evaporation in tandem with gravitational settling determine the AHe enhancements and variabilities beyond 8% in ICMEs. While chromospheric evaporation releases the helium from chromosphere into the corona, the gravitationally settled helium is thrown out during the ICMEs.We show that the intensity and timing of the preceding flares from the same active region from where the CME erupts are important factors to understand the AHe enhancements in ICMEs.

astro-ph.SR↗

arXiv2022

Study of Goldstone Inflation in the domain of Einstein-Gauss-Bonnet gravity

Realizing the inflationary epoch driven by a pseudo-Nambu Goldstone boson (pNGB) could ensure the coveted flatness, and the sub-Planckian scales related to the dynamics of the paradigm. In this work, we have taken the most general form of such a scenario: Goldstone inflation proposed in \cite{croon}, and studied the model in Einstein-Gauss-Bonnet gravity. Natural inflation is a limiting case of this model, is also studied here. The specific form of the EGB coupling gives ample opportunity to study the rich phenomenology associated with inflation as well as the reheating epoch. Predicted values of the inflationary observables, tensor to scalar ratio ($r$), and spectral index ($n_s$) are in good agreement with the recent observations from $Planck'18$ \cite{Planck2018}. Thus, in the framework of EGB one can resurrect the model, which otherwise needs quite a bit of fine tuning or diversion from the canonical domain as studied in \cite{Bhattacharya:2018xlw}, to survive in the standard cold inflationary scenario. Finally, the era of reheating is studied for different choice of model parameters.

astro-ph.CO↗

arXiv2021

Evidence for distinctive changes in the solar wind helium abundance in cycle 24

The relative abundance of alpha particles with respect to proton, usually expressed as $A_{He}$ = ($n_α/n_p$)*100, is known to respond to solar activity although changes in its behaviour in the last four solar cycles are not known. In this letter, by systematically analysing inter-calibrated $A_{He}$ data obtained from the first Lagrangian point of the Sun-Earth system, we show that $A_{He}$ variations are distinctively different in solar cycle 24 as compared to the last three cycles. The frequency of $A_{He}$ = 2-3% events is significantly higher in slow/intermediate solar winds in cycle 24 as opposed to the dominance of the typical $A_{He}$ = 4-5% events in the previous three cycles. Further, the occurrence of $A_{He}$ $\geq$ 10% events is significantly reduced in cycle 24. Not only that, the changes in delay of $A_{He}$ with respect to peak sunspot numbers are less sensitive to changes in solar wind velocity in cycle 24. The investigation suggests that the coronal magnetic field configuration started undergoing systematic changes starting from cycle 23 and this altered magnetic field configuration affected the way helium got processed and depleted in the solar atmosphere.

astro-ph.SR↗

arXiv2020

Power Law Plateau Inflation Potential In The RS $II$ Braneworld Evading Swampland Conjecture

In the recent time, inflationary cosmology is facing an existential crisis due to the proposed Swampland criterion which aims to evade any (meta-)stable de Sitter construction within the String landscape. It is been realised that a single field slow roll inflation is inconsistent with the Swampland criterion unless the inflationary model in realised in some non standard scenario such as Warm inflation or the Braneworld scenario. In \cite{Dimopoulos}, Dimopoulos and Owen introduced a new class of model of inflation dubbed as the power law plateau inflation in the standard cold inflationary scenario. But to realise this model in the standard scenario consistent with observation, they had to introduce a phase of thermal inflation. In this paper we have analysed this model in the braneworld scenario to show that for some choice of the parameters defining the model class, one can have an observationally consistent power law plateau without any phase of thermal inflation. We have also shown that, for the correct choice of model parameters, one can easily satisfy the Swampland criterion. Besides, for a particular choice of the potential one can also satisfy the recently proposed Trans-Planckian Censorship Conjecture.

astro-ph.CO↗