arXiv subjects

Geeta Vichare

Publications and source records attributed to Geeta Vichare.

14 recordsLinked to original sources

arXiv2026

Ground Level Enhancement (GLE#77) in the gamma-ray component: First observation from Arctic and Antarctic stations

This article presents the observations of the extreme ground-level enhancement (GLE #77) of Solar Cycle 25 that occurred on 11 November 2025, using ground-based NaI(Tl) gamma-ray detectors deployed at Arctic and Antarctic stations, together with neutron monitor data and particle measurements from the GOES-18 satellite. The event was associated with an intense X-class solar flare and a strong solar energetic proton event. This paper reports the first ground-based detection of a GLE using gamma-ray detectors operating simultaneously in both polar regions, which are concurrent with increases in neutron monitor counts. Thus highlights the capability of polar gamma-ray detectors to complement traditional neutron monitor observations during extreme solar proton events. A detailed analysis revealed distinct prompt and delayed responses during the event evolution. Interestingly, the signature of the prompt peak of GLE#77 (at 10:38 UT) was observed up to high-rigidity neutron monitors (low latitudes). However, the delayed peak (at 13:08 UT) was not seen at the stations with rigidity > 6 GV. The timing of the prompt and delayed peaks coincided with the proton flux peaks observed by the GOES-18 satellite at energies > 150 MeV and 12-99 MeV, respectively. It is observed that the GLE amplitude has a strong dependence on geomagnetic cutoff rigidity and has a weak solar zenith angle dependence.

astro-ph.SR↗

arXiv2024

Low-Latitude Auroras: Insights from 23 April 2023 Solar Storm

In April 2023, low-latitude aurora observation by the all-sky camera at Hanle, Ladakh, India ($33^{\circ} {} N $ geographic latitude (GGLat)) was reported, which stimulated a lot of discussion among scientists as well as masses across the globe. The reported observation was intriguing as the solar storm that triggered this aurora was moderate and the first such observation from Indian region in the space-era. In this communication, we investigate such a unique modern-day observation of low-latitude auroral sighting occurring during the passage of sheath-region of Interplanetary-Coronal-Mass-Ejection, utilizing in situ multi-spacecraft particle measurements along with geomagnetic-field observations by ground and satellite-based magnetometers. Auroral observations at Hanle coincided with the intense substorm occurrences. It is unequivocally found that the aurora didnt reach India, rather the equatorward boundary of the aurora was beyond $ 50^{\circ} {}N $ GGLat. The multi-instrumental observations enabled us to estimate the altitude of the red auroral emissions accurately. The increased flux of low-energy electrons ($<$100 eV) precipitating at $\sim 54^{\circ}N$ GGLat causing red-light emissions at higher altitudes ($\sim$700-950 km) can be visible from Hanle. The observed low-latitude red aurora from India resulted from two factors: emissions at higher altitudes in the auroral oval and a slight expansion of the auroral oval towards the equator. The precipitating low-energy particles responsible for red auroral emissions mostly originate from the plasma sheet. These particles precipitate due to wave-particle interactions enhanced by strong compression of the magnetosphere during high solar wind pressure. This study using multi-point observations holds immense importance in providing a better understanding of low-latitude auroras.

physics.space-ph↗

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

Effect of the brightest gamma-ray burst (GRB 221009A) on low energy gamma-ray counts at sea level

A gamma-ray burst, named GRB 221009A, occurred on 9 October 2022 and is the brightest ever observed GRB, whose frequency is now estimated as once in 10,000 years. This GRB was reported to be observed from many space missions, VLF receivers, and ground observations in optical and radio data. Additionally, a strikingly large number of very high energy (VHE) photons associated with this GRB were observed by the gamma-ray and cosmic ray observatory LHAASO. Though gamma rays of cosmic origin usually tend to be absorbed by the atmosphere, the high fluence of this GRB, along with the observation of more than 5000 VHE photons (0.5 to 18 TeV) by LHAASO from the ground, emphasises the need to explore other possible ground observations of this GRB. The present paper examines the effect of this GRB using gamma-ray data in a low energy range (0.2-6) MeV obtained using NaI (Tl) detectors located at Tirunelveli (Geographic coordinates: 8.71{\deg}N, 77.76{\deg}E), India. With RA = 288.3{\deg} and Dec = 19.8{\deg}, the exceptionally bright fluence of this GRB was geographically centred on India. We report no significant change in the observations associated with GRB 221009A. We discuss the extent of attenuation of gamma-rays in the atmosphere that could explain the reported observations. Further, we investigate the likelihood of ground observation of gamma-rays ($<$10 MeV) for a much more intense hypothetical GRB, and estimate the parameters, such as distance, fluence, and isotropic energy of such a GRB.

astro-ph.HE↗

arXiv2020

A possible case of sporadic aurora observed at Rio de Janeiro

Being footprints of major magnetic storms and hence major solar eruptions, mid- to low-latitude aurorae have been one of the pathways to understand solar-terrestrial environments. However, it has been reported that aurorae are also occasionally observed at low latitudes under low or even quiet magnetic conditions. Such phenomena are known as "sporadic aurorae". We report on a historical event observed by a scientist of the Brazilian Empire in Rio de Janeiro on 15 February 1875. We analyze this event on the basis of its spectroscopic observations, along with its visual structure and coloration, to suggest this event was a possible case of sporadic aurorae. Given the absence of worldwide aurora observations on that day as a consequence of low magnetic activity recorded on the days preceding the observation, in addition to a detailed description, the event observed can most likely be classified as a sporadic aurora. We discuss the geographic and magnetic conditions of that event. Thus, we add a possible case of sporadic aurora in the South American sector.

physics.space-ph↗

arXiv2020

Barometric Pressure Correction to Gamma-ray Observations and its Energy Dependence

Cosmic rays (CRs) have been studied extensively in the last century to understand the processes in the universe as well as in the solar system. In today's satellite era, although many observations are made from space, CR observations from the ground are still viewed as an important tool. These observations, however, mostly detect the secondary cosmic rays (SCRs) such as neutron, muon, and gamma. It is well known that the atmospheric pressure plays an important role in the SCR flux observed on the ground. Barometric pressure correction is standard practice for neutron monitor (NM) data. However, no such correction is applied to gamma-ray, being massless. But the pressure affects the particles such as $e^{\pm}$, $μ^{\pm}$, which produce gamma rays in the cascade. Subsequently, the indirect pressure dependence of the gamma-ray flux can be anticipated. We examine this aspect in detail by studying the gamma-ray counts detected by the NaI (Tl) detector. The present study confirms that there is no correlation between the atmospheric pressure and the total gamma-ray counts collected from all energies. However, the scenario differs when the gamma-ray fluxes of different energies are investigated separately. The gamma rays of energy below $\sim$3 MeV are primarily due to the radioactivity originating from the ground, whereas gamma rays above 3 MeV are mainly produced in the CR cascade. It is observed that the counts of energy above 3 MeV are well anti-correlated with the atmospheric pressure and hence need to be corrected. It is demonstrated that applying the barometric correction formula successfully removes the pressure dependence in the gamma-ray flux above 3 MeV. Therefore, we suggest that the gamma-ray data above 3 MeV needs to be corrected for the local atmospheric pressure variations.

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

Torsional Alfven wave embedded ICME magnetic cloud and corresponding geomagnetic storm

The energy transfer during the interaction of large-scale solar wind structure and the Earth's magnetosphere is the chronic issue in space-weather studies. To understand this, researchers widely studied the geomagnetic storms and sub-storms phenomena. The present understanding suggests that long duration of southward interplanetary magnetic field component is the most important parameter for the geomagnetic storm. Such long duration strong southward magnetic field is often associated with ICMEs, torsional Alfven fluctuations superposed co-rotating interacting regions (CIRs) and fast solar wind streams. Torsional Alfven fluctuations embedded CIRs have been known for a long, however magnetic cloud embedded with such fluctuations are rarely observed. The presence of Alfven waves in the ICME/MC and influence of these waves on the storm evolution remains an interesting topic of study. The present work confirms the torsional Alfven waves in a magnetic cloud associated with a CME launched on 15th February which impacted the Earth's magnetosphere on February 18, 2011. Further, observations indicate that these waves inject energy into the magnetosphere during the storm and contribute to the long recovery time of geomagnetic storms. Our study suggests that presence of torsional Alfven waves significantly controls the storm dynamics.

physics.space-ph↗

arXiv2017

Prediction of SYMH and ASYH indices for geomagnetic storms of solar cycle 24 including recent St. Patrick's day, 2015 storm using NARX neural network

Artificial Neural Network (ANN) has proven to be very successful in forecasting variety of irregular magnetospheric/ionospheric processes like geomagnetic storms and substorms. SYMH and ASYH indices represent longitudinal symmetric and asymmetric component of the ring current. Here, an attempt is made to develop a prediction model for these indices using ANN. The ring current state depends on its past conditions therefore, it is necessary to consider its history for prediction. To account this effect Nonlinear Autoregressive Network with eXogenous inputs (NARX) is implemented. This network considers input history of 30 minutes and output feedback of 120 minutes. Solar wind parameters mainly velocity, density and interplanetary magnetic field are used as inputs. SYMH and ASYH indices during geomagnetic storms of 1998-2013, having minimum SYMH <-85 nT are used as the target for training two independent networks. We present the prediction of SYMH and ASYH indices during 9 geomagnetic storms of solar cycle 24 including the recent largest storm occurred on St. Patrick's day, 2015. The present prediction model reproduces the entire time profile of SYMH and ASYH indices along with small variations of ~ 10-30 minutes to good extent within noise level, indicating significant contribution of interplanetary sources and past state of the magnetosphere. Therefore, the developed networks can predict SYMH and ASYH indices about an hour before, provided, real-time upstream solar wind data is available. However, during the main phase of major storms, residuals (observed-modeled) are found to be large, suggesting influence of internal factors such as magnetospheric processes

physics.space-ph↗

arXiv2017

Quantitative assessment of drivers of recent climate variability: An information theoretic approach

Identification and quantification of possible drivers of recent climate variability remain a challenging task. This important issue is addressed adopting a non-parametric information theory technique, the Transfer Entropy and its normalized variant. It distinctly quantifies actual information exchanged along with the directional flow of information between any two variables with no bearing on their common history or inputs, unlike correlation, mutual information etc. Measurements of greenhouse gases, CO2, CH4, and N2O; volcanic aerosols; solar activity: UV radiation, total solar irradiance (TSI ) and cosmic ray flux (CR); El Nino Southern Oscillation (ENSO) and Global Mean Temperature Anomaly (GMTA) made during 1984-2005 are utilized to distinguish driving and responding climate signals. Estimates of their relative contributions reveal that CO 2 (~24%), CH 4 (~19%) and volcanic aerosols (~23%) are the primary contributors to the observed variations in GMTA. While, UV (~9%) and ENSO (~12%) act as secondary drivers of variations in the GMTA, the remaining play a marginal role in the observed recent climate variability. Interestingly, ENSO and GMTA mutually drive each other at varied time lags. This study assists future modelling efforts in climate science.

physics.ao-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↗

arXiv2016

Relative contribution of the magnetic field barrier and solar wind speed in ICME-associated Forbush decreases

We study 50 cosmic ray Forbush decreases (FDs) from the Oulu neutron monitor data during 1997-2005 that were associated with Earth-directed interplanetary coronal mass ejections (ICMEs). Such events are generally thought to arise due to the shielding of cosmic rays by a propagating diffusive barrier. The main processes at work are the diffusion of cosmic rays across the large-scale magnetic fields carried by the ICME and their advection by the solar wind. In an attempt to better understand the relative importance of these effects, we analyse the relationship between the FD profiles and those of the interplanetary magnetic field (B) and the solar wind speed (Vsw). Over the entire duration of a given FD, we find that the FD profile is generally well (anti)correlated with the B and Vsw profiles. This trend holds separately for the FD main and recovery phases too. For the recovery phases, however, the FD profile is highly anti-correlated with the Vsw profile, but not with the B profile. While the total duration of the FD profile is similar to that of the Vsw profile, it is significantly longer than that of the B profile.

physics.space-ph↗

arXiv2015

Spatial frequencies associated with the latitudinal structures of ionospheric currents seen by CHAMP satellite

The CHAMP magnetic field variations during international quiet days of low solar activity period 2008-2009 are investigated. The present paper reports the existence of frequency-peaks < 20 mHz in the compressional component of the magnetic field in almost all CHAMP passes. The magnetic field variations associated with these frequencies have amplitude of a few tens of nT during daytime. The geomagnetic activity and interplanetary magnetic field parameters were observed to be low during the period of study. The spectral powers of the observed frequencies show no dependence on solar wind velocity and cone angle; hence the reported frequencies are not related to the geomagnetic pulsations. For frequency-peaks <15 mHz, strong local-time dependence is observed with maximum power near noon and minimum at night. The longitudinal and seasonal variations of the powers of these frequency-peaks match well with those of the equator-to-middle latitude ionospheric currents derived by the earlier studies. As a polar Low-Earth-Orbiting (LEO) satellite spans the entire range of latitudes within few minutes, it monitors the geomagnetic field variations caused by the quiet-time ionospheric currents flowing at different latitudes. This can result in certain frequencies in the magnetic field recorded by LEO satellites. We demonstrate that the frequencies <10mHz are mainly due to the latitudinal structure of the equatorial electrojet. The observed frequencies in CHAMP data are therefore attributed to the latitudinal structures of the ionospheric currents that are monitored only by the polar LEO satellites and are found to alter the observations of geomagnetic pulsations (Pc4-5 and Pi2) significantly.

physics.space-ph↗

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↗