arXiv subjects

Anil Bhardwaj

Publications and source records attributed to Anil Bhardwaj.

At least 55 records · Page 3Linked to original sources

arXiv2019

Acceleration of Energetic Ions in Corotating Interaction Region near 1.5 au: Evidence from MAVEN

The dearth of observations between 1 au and 3 au limits our understanding of energetic particle acceleration processes in interplanetary space. We present the first-of-their-kind observations of the energetic particle acceleration in a Corotating Interaction Region (CIR) using data from two vantage points, 1 au (near Earth) and 1.5 au (near Mars). The CIR event of June 2015 was observed by the particle detectors aboard the Advanced Composition Explorer (ACE) satellite as well as the SEP (Solar Energetic Particle) instrument aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft situated near 1.5 au. We find that a CIR shock can accelerate a significant number of particles even at 1.5 au. During this event the acceleration by the shocks associated with the CIR could cause an enhancement of around two orders of magnitude in the SEP energetic ion fluxes in the ~500 keV to 2 MeV range when the observations near 1 and 1.5 au are compared. To demonstrate the differences between SEP acceleration in CIR and other impulsive events, we show the energetic ion flux observations during an intense CME period in March 2015, in which case the enhanced SEP fluxes are seen even at 1 au. These observations provide evidence that CIR shock can accelerate particles in the region between Earth and Mars, that is, only within the short heliocentric distance of 0.5 au, in interplanetary space.

physics.space-ph↗

arXiv2019

X-rays Studies of the Solar System

X-ray observatories contribute fundamental advances in Solar System studies by probing Sun-object interactions, developing planet and satellite surface composition maps, probing global magnetospheric dynamics, and tracking astrochemical reactions. Despite these crucial results, the technological limitations of current X-ray instruments hinder the overall scope and impact for broader scientific application of X-ray observations both now and in the coming decade. Implementation of modern advances in X-ray optics will provide improvements in effective area, spatial resolution, and spectral resolution for future instruments. These improvements will usher in a truly transformative era of Solar System science through the study of X-ray emission.

astro-ph.HE↗

arXiv2017

Radio Observation of Venus at Meter Wavelengths using the GMRT

The Venusian surface has been studied by measuring radar reflections and thermal radio emission over a wide spectral region of several centimeters to meter wavelengths from the Earth-based as well as orbiter platforms. The radiometric observations, in the decimeter (dcm) wavelength regime showed a decreasing trend in the observed brightness temperature (Tb) with increasing wavelength. The thermal emission models available at present have not been able to explain the radiometric observations at longer wavelength (dcm) to a satisfactory level. This paper reports the first interferometric imaging observations of Venus below 620 MHz. They were carried out at 606, 332.9 and 239.9 MHz using the Giant Meterwave Radio Telescope (GMRT). The Tb values derived at the respective frequencies are 526 K, 409 K and < 426 K, with errors of ~7% which are generally consistent with the reported Tb values at 608 MHz and 430 MHz by previous investigators, but are much lower than those derived from high-frequency observations at 1.38-22.46 GHz using the VLA.

astro-ph.EP↗

arXiv2016

Monte Carlo model for electron degradation in xenon gas

We have developed a Monte Carlo model for studying the local degradation of electrons in the energy range 9-10000 eV in xenon gas. Analytically fitted form of electron impact cross sections for elastic and various inelastic processes are fed as input data to the model. Two dimensional numerical yield spectrum, which gives information on the number of energy loss events occurring in a particular energy interval, is obtained as output of the model. Numerical yield spectrum is fitted analytically, thus obtaining analytical yield spectrum. The analytical yield spectrum can be used to calculate electron fluxes, which can be further employed for the calculation of volume production rates. Using yield spectrum, mean energy per ion pair and efficiencies of inelastic processes are calculated. The value for mean energy per ion pair for Xe is 22 eV at 10 keV. Ionization dominates for incident energies greater than 50 eV and is found to have an efficiency of 65% at 10 keV. The efficiency for the excitation process is 30% at 10 keV.

physics.atom-ph↗

arXiv2016

Proton entry into the near-lunar plasma wake for magnetic field aligned flow

We report the first observation of protons in the near (100--200 km from surface) and deeper (near anti-subsolar point) lunar plasma wake when the interplanetary magnetic field (IMF) and solar wind velocity ($v_{sw}$) are parallel (aligned flow, angle between IMF and ($v_{sw} \le 10^\circ$). More than 98% of the observations during aligned flow condition showed the presence of protons in the wake. These observations are obtained by the SWIM sensor of the SARA experiment on Chandrayaan-1. The observation cannot be explained by the conventional fluid models for aligned flow. Back-tracing of the observed protons suggests that their source is the solar wind. The larger gyro-radii of the wake protons compared to that of solar wind suggest that they were part of the tail of the solar wind velocity distribution function. Such protons could enter the wake due to their large gyro-radii even when the flow is aligned to IMF. However, the wake boundary electric field may also play a role in the entry of the protons in to the wake.

physics.space-ph↗

arXiv2016

Characteristics of proton velocity distribution functions in the near-lunar wake from Chandrayaan-1/SWIM observations

Due to the high absorption of solar wind plasma on the lunar dayside, a large scale wake structure is formed downstream of the Moon. However, recent in-situ observations have revealed the presence of protons in the near-lunar wake (100 km to 200 km from the surface). The solar wind, either directly or after interaction with the lunar surface (including magnetic anomalies), is the source of these protons in the near-wake region. Using the entire data from the SWIM sensor of the SARA experiment onboard Chandrayaan-1, we analysed the velocity distribution of the protons observed in the near-lunar wake. The average velocity distribution functions, computed in the solar wind rest frame, were further separated based on the angle between the upstream solar wind velocity and the IMF. Several proton populations were identified from the velocity distribution and their possible entry mechanism were inferred based on the characteristics of the velocity distribution. These entry mechanisms include (i) diffusion of solar wind protons into the wake along IMF, (ii) the solar wind protons with finite gyro-radii that are aided by the wake boundary electric field, (iii) solar wind protons with gyro-radii larger than lunar radii from the tail of the solar wind velocity distribution, and (iv) scattering of solar wind protons from the dayside lunar surface or from magnetic anomalies. In order to gain more insight into the entry mechanisms associated with different populations, backtracing is carried out for each of these populations. For most of the populations, the source of the protons obtained from backtracing is found to be in agreement with that inferred from the velocity distribution. There are few populations that could not be explained by the known mechanisms and remain unknown.

physics.space-ph↗

arXiv2015

A new view on the solar wind interaction with the Moon

Characterised by a surface bound exosphere and localised crustal magnetic fields, the Moon was considered as a passive object when solar wind interacts with it. However, the neutral particle and plasma measurements around the Moon by recent dedicated lunar missions, such as Chandrayaan-1, Kaguya, Chang'E-1, LRO, and ARTEMIS, as well as IBEX have revealed a variety of phenomena around the Moon which results from the interaction with solar wind, such as backscattering of solar wind protons as energetic neutral atoms (ENA) from lunar surface, sputtering of atoms from the lunar surface, formation of a "mini-magnetosphere" around lunar magnetic anomaly regions, as well as several plasma populations around the Moon, including solar wind protons scattered from the lunar surface, from the magnetic anomalies, pick-up ions, protons in lunar wake and more. This paper provides a review of these recent findings and presents the interaction of solar wind with the Moon in a new perspective.

physics.space-ph↗

arXiv2015

Prediction of forbidden ultraviolet and visible emissions in comet 67P/Churyumov-Gerasimenko

Remote observation of spectroscopic emissions is a potential tool for the identification and quantification of various species in comets. CO Cameron band (to trace \cod) and atomic oxygen emissions (to trace H$_2$O and/or CO$_2$, CO) have been used to probe neutral composition in the cometary coma. Using a coupled-chemistry emission model, various excitation processes controlling CO Cameron band and different atomic oxygen and atomic carbon have been modelled in comet 67P-Churyumov-Gerasimenko at 1.29~AU (perihelion) and at 3~AU heliocentric distances, which is being explored by ESA's Rosetta mission. The intensities of CO Cameron band, atomic oxygen and atomic carbon emission lines as a function of projected distance are calculated for different CO and CO$_2$ volume mixing ratios relative to water. Contributions of different excitation processes controlling these emissions are quantified. We assess how CO$_2$ and/or CO volume mixing ratios with respect to H$_2$O can be derived based on the observed intensities of CO Cameron band, atomic oxygen, and atomic carbon emission lines.The results presented in this work serve as base line calculations to understand the behaviour of low out-gassing cometary coma and compare them with the higher gas production rate cases (e.g. comet Halley). Quantitative analysis of different excitation processes governing the spectroscopic emissions is essential to study the chemistry of inner coma and to derive neutral gas composition.

astro-ph.EP↗

arXiv2015

Monte Carlo model for electron degradation in methane

We present a Monte Carlo model for degradation of 1-10,000 eV electrons in an atmosphere of methane. The electron impact cross sections for CH4 are compiled and analytical representations of these cross sections are used as input to the model.model.Yield spectra, which provides information about the number of inelastic events that have taken place in each energy bin, is used to calculate the yield (or population) of various inelastic processes. The numerical yield spectra, obtained from the Monte Carlo simulations, is represented analytically, thus generating the Analytical Yield Spectra (AYS). AYS is employed to obtain the mean energy per ion pair and efficiencies of various inelastic processes.Mean energy per ion pair for neutral CH4 is found to be 26 (27.8) eV at 10 (0.1) keV. Efficiency calculation showed that ionization is the dominant process at energies >50 eV, for which more than 50% of the incident electron energy is used. Above 25 eV, dissociation has an efficiency of 27%. Below 10 eV, vibrational excitation dominates. Contribution of emission is around 1.2% at 10 keV. Efficiency of attachment process is 0.1% at 8 eV and efficiency falls down to negligibly small values at energies greater than 15 eV. The efficiencies can be used to calculate volume production rate in planetary atmospheres by folding with electron production rate and integrating over energy

physics.space-ph↗

arXiv2014

Production of N2 Vegard-Kaplan and Lyman-Birge-Hopfield emissions on Pluto

We have developed a model to calculate the emission intensities of various vibrational transitions of N$_2$ triplet band and Lyman-Birge-Hopfield (LBH) band emissions in the dayglow of Pluto for solar minimum, moderate, and maximum conditions. The calculated overhead intensities of Vegard-Kaplan ($ A^3Σ_u^+ - X^1Σ^+_g $), First Positive ($ B^3Π_g - A^3Σ^+_u $), Second Positive ($ C^3Π_u - B^3Π_g $), Wu-Benesch ($W^3Δ_u - B^3Π_g$), Reverse First Positive, and LBH ($a^1Π_g$-- $X^1Σ^+_g$) bands of N$_2$ are 17 (74), 14.8 (64), 2.4 (10.8), 2.9 (12.7), 2.9 (12.5), and 2.3 (10) R, respectively, for solar minimum (maximum) condition. We have predicted the overhead and limb intensities of VK (150-190 nm) and LBH (120-190 nm) bands of N$_2$ on Pluto for the New Horizons (NH) flyby condition that can be observed by Alice: the ultraviolet imaging spectrograph also know as P-Alice. The predicted limb intensities of VK and LBH bands peak at radial distance of $ \sim$2000 km with the value of about 5 (13) and 9.5 (22) R for solar zenith angle 60$ ^\circ $ (0$ ^\circ $), respectively. We have also calculated overhead and limp intensities of few prominent transition of CO Fourth Positive bands for NH flyby condition.

astro-ph.EP↗

arXiv2014

Photochemistry of atomic oxygen green and red-doublet emissions in comets at larger heliocentric distances

In comets the atomic oxygen green to red-doublet emission intensity ratio (G/R ratio) of 0.1 has been used to confirm H$_2$O as the parent species producing oxygen emission lines. The larger ($>$0.1) value of G/R ratio observed in a few comets is ascribed to the presence of higher CO$_2$ and CO relative abundances in the cometary coma. We aim to study the effect of CO$_2$ and CO relative abundances on the observed G/R ratio in comets observed at large ($>$2 au) heliocentric distances by accounting for important production and loss processes of O($^1$S) and O($^1$D) in the cometary coma. Recently we have developed a coupled chemistry-emission model to study photochemistry of O($^1$S) and O($^1$D) atoms and the production of green and red-doublet emissions in comets Hyakutake and Hale-Bopp. In the present work we applied the model to six comets where green and red-doublet emissions are observed when they are beyond 2 au from the Sun. In a water-dominated cometary coma and with significant ($>$10%) CO$_2$ relative abundance, photodissociation of H$_2$O mainly governs the red-doublet emission, whereas CO$_2$ controls the green line emission. If a comet has equal composition of CO$_2$ and H$_2$O, then ~50% of red-doublet emission intensity is controlled by the photodissociation of CO$_2$. The G/R ratio values and green and red-doublet line widths calculated by the model are consistent with the observation. Our model calculations suggest that in low gas production rate comets the G/R ratio greater than 0.1 can be used to constrain the upper limit of CO$_2$ relative abundance provided the slit-projected area on the coma is larger than the collisional zone. If a comet has equal abundances of CO$_2$ and H$_2$O, then the red-doublet emission is significantly ($\sim$50%) controlled by CO$_2$ photodissociation and thus the G/R ratio is not suitable for estimating CO$_2$ relative abundance.

astro-ph.EP↗

arXiv2013

The Hot and Energetic Universe: A White Paper presenting the science theme motivating the Athena+ mission

This White Paper, submitted to the recent ESA call for science themes to define its future large missions, advocates the need for a transformational leap in our understanding of two key questions in astrophysics: 1) How does ordinary matter assemble into the large scale structures that we see today? 2) How do black holes grow and shape the Universe? Hot gas in clusters, groups and the intergalactic medium dominates the baryonic content of the local Universe. To understand the astrophysical processes responsible for the formation and assembly of these large structures, it is necessary to measure their physical properties and evolution. This requires spatially resolved X-ray spectroscopy with a factor 10 increase in both telescope throughput and spatial resolving power compared to currently planned facilities. Feedback from supermassive black holes is an essential ingredient in this process and in most galaxy evolution models, but it is not well understood. X-ray observations can uniquely reveal the mechanisms launching winds close to black holes and determine the coupling of the energy and matter flows on larger scales. Due to the effects of feedback, a complete understanding of galaxy evolution requires knowledge of the obscured growth of supermassive black holes through cosmic time, out to the redshifts where the first galaxies form. X-ray emission is the most reliable way to reveal accreting black holes, but deep survey speed must improve by a factor ~100 over current facilities to perform a full census into the early Universe. The Advanced Telescope for High Energy Astrophysics (Athena+) mission provides the necessary performance (e.g. angular resolution, spectral resolution, survey grasp) to address these questions and revolutionize our understanding of the Hot and Energetic Universe. These capabilities will also provide a powerful observatory to be used in all areas of astrophysics.

astro-ph.HE↗

arXiv2013

CO Cameron band and CO2+ UV doublet emissions in the dayglow of Venus: Role of CO in the Cameron band production

Present study deals with the model calculations of CO Cameron band and CO2+ ultraviolet doublet emissions in the dayglow of Venus. The overhead and limb intensities of CO Cameron band and CO2+ UV doublet emissions are calculated for low, moderate, and high solar activity conditions. Using updated cross sections, the impact of dierent e-CO cross section for Cameron band production is estimated. The electron impact on CO is the major source mechanism of Cameron band, followed by electron and photon impact dissociation of CO2. The overhead intensities of CO Cameron band and CO2+ UV doublet emissions are about a factor of 2 higher in solar maximum than those in solar minimum condition. The effect of solar EUV flux models on the emission intensity is ~30-40% in solar minimum condition and ~2-10% in solar maximum condition. At the altitude of emission peak (135 km), the model predicted limb intensity of CO Cameron band and CO2+ UV doublet emissions in moderate (F10.7 = 130) solar activity condition is about 2400 and 300 kR, respectively, which is in agreement with the very recently published SPICAV/Venus Express observation. The model limb intensity profiles of CO Cameron band and CO2+ UV doublet are compared with SPICAV observation. We also calculated intensities of N2 Vegard-Kaplan UV bands and OI 2972 Å emissions during moderate and high solar activity conditions.

astro-ph.EP↗

arXiv2013

Spatial heterogeneity in the radiogenic activity of the lunar interior: Inferences from CHACE and LLRI on Chandrayaan-1

In the past, clues on the potential radiogenic activity of the lunar interior have been obtained from the isotopic composition of noble gases like Argon. Excess Argon (40) relative to Argon (36), as compared to the solar wind composition, is generally ascribed to the radiogenic activity of the lunar interior. Almost all the previous estimates were based on, 'on-the-spot' measurements from the landing sites. Relative concentration of the isotopes of 40Ar and 36Ar along a meridian by the Chandra's Altitudinal Composition Explorer (CHACE) experiment, on the Moon Impact Probe (MIP) of India's first mission to Moon, has independently yielded clues on the possible spatial heterogeneity in the radiogenic activity of the lunar interior in addition to providing indicative 'antiquity' of the lunar surface along the ground track over the near side of the moon. These results are shown to broadly corroborate the independent topography measurements by the Lunar Laser Ranging Instrument (LLRI) in the main orbiter Chandrayaan-1. The unique combination of these experiments provided high spatial resolution data while indicating the possible close linkages between the lunar interior and the lunar ambience.

physics.space-ph↗

arXiv2012

Model for Atomic Oxygen Visible Line Emissions in Comet C/1995 O1 Hale-Bopp

We have recently developed a coupled chemistry-emission model for the green and red-doublet emissions of atomic oxygen on comet Hyakutake. In the present work we applied our model to comet Hale-Bopp, which had an order of magnitude higher H2O production rate than comet Hyakutake, to evaluate the photochemistry associated with the production and loss of O(1S) and O(1D) atoms and emission processes of green and red-doublet lines. We present the wavelength-dependent photo-attenuation rates for different photodissociation processes forming O(1S) and O(1D). The calculated radiative efficiency profiles of O(1S) and O(1D) atoms show that in comet Hale-Bopp the green and red-doublet emissions are emitted mostly above radial distances of 10^3 and 10^4 km, respectively. The model calculated [OI] 6300 A emission surface brightness and average intensity over the Fabry-P{é}rot spectrometer field of view are consistent with the observation of Morgenthaler et al. (2001), while the intensity ratio of green to red-doublet emission is in agreement with the observation of Zhang et al. (2001). In comet Hale-Bopp, for cometocentric distances less than 10^5 km, the intensity of [OI] 6300 A line is mainly governed by photodissociation of H2O. Beyond 10^5 km, O(1D) production is dominated by photodissociation of the water photochemical daughter product OH. Whereas the [OI] 5577 A emission line is controlled by photodissociation of both H2O and CO2. The calculated mean excess energy in various photodissociation processes show that the photodissociation of CO2 can produce O(1S) atoms with higher excess velocity compared to the photodissociation of H2O. Thus, our model calculations suggest that involvement of multiple sources in the formation of O(1S) could be a reason for the larger width of green line than that of red-doublet emission lines observed in several comets.

astro-ph.EP↗

arXiv2012

Coupled Chemistry-Emission Model for Atomic Oxygen Green and Red-doublet Emissions in Comet C/1996 B2 Hyakutake

The green (5577 Å) and red-doublet (6300, 6364 Å) lines are prompt emissions of metastable oxygen atoms in the $^1$S and $^1$D states, respectively, that have been observed in several comets. The value of intensity ratio of green to red-doublet (G/R ratio) of 0.1 has been used as a benchmark to identify the parent molecule of oxygen lines as H$_2$O. A coupled chemistry-emission model is developed to study the production and loss mechanisms of O($^1$S) and O($^1$D) atoms and the generation of red and green lines in the coma of C/1996 B2 Hyakutake. The G/R ratio depends not only on photochemistry, but also on the projected area observed for cometary coma, which is a function of the dimension of the slit used and geocentric distance of the comet. Calculations show that the contribution of photodissociation of H$_2$O to the green (red) line emission is 30 to 70% (60 to 90%), while CO$_2$ and CO are the next potential sources contributing 25 to 50% ($<$5%). The ratio of the photo-production rate of O($^1$S) to O($^1$D) would be around 0.03 ($\pm$ 0.01) if H$_2$O is the main source of oxygen lines, whereas it is $\sim$0.6 if the parent is CO$_2$. Our calculations suggest that the yield of O($^1$S) production in the photodissociation of H$_2$O cannot be larger than 1%. The model calculated radial brightness profiles of the red and green lines and G/R ratios are in good agreement with the observations made on comet Hyakutake in March 1996.

astro-ph.EP↗

arXiv2012

Production of N2 Vegard-Kaplan and other triplet band emissions in the dayglow of Titan

Recently the Cassini Ultraviolet Imaging Spectrograph has revealed the presence of N2 Vegard-Kaplan band emissions in Titan's dayglow limb observation. We present model calculations for the production of various N2 triplet states in the upper atmosphere of Titan. The Analytical Yield Spectra technique is used to calculate steady state photoelectron fluxes in Titan's atmosphere, which are in agreement with those observed by the Cassini's CAPS instrument. Considering direct electron impact excitation, inter-state cascading, and quenching effects, the population of different levels of N2 triplet states are calculated under statistical equilibrium. Densities of all vibrational levels of each triplet state and volume production rates for various triplet states are calculated in the model. Vertically integrated overhead intensities for the same date and lighting conditions as the reported by UVIS observations for N2 VK, 1P, 2P, Wu-Benesch, and Reverse First Positive bands of N2 are found to be 132, 114, 19, 22, and 22 R, respectively. Overhead intensities are calculated for each vibrational transition of all the triplet band emissions of N2, which span a wider spectrum of wavelengths from ultraviolet to infrared. The calculated limb intensities of total and prominent transitions of VK band are presented. The model limb intensity of VK emission within the 150-190 nm wavelength region is in good agreement with the Cassini UVIS observed limb profile. An assessment of the impact of solar EUV flux on the N2 triplet band emission intensity has been made by using three different solar flux models, viz., Solar EUV Experiment, SOLAR2000 model of Tobiska (2004), and HEUVAC model of Richards et al, (2006). The calculated N2 VK band intensity at the peak of limb intensity due to S2K and HEUVAC solar flux models is a factor of 1.2 and 0.9, respectively, of that obtained using SEE solar EUV flux.

astro-ph.EP↗

arXiv2012

Model for the Production of CO Cameron band emission in Comet 1P/Halley

The abundance of CO2 in comets has been derived using CO Cameron band (a3pi --> X1Sigma+) emission assuming that photodissociative excitation of CO2 is the main production process of CO(a3pi). On comet 1P/Halley the Cameron (1-0) band has been observed by International Ultraviolet Explorer (IUE) on several days in March 1986. A coupled chemistry-emission model is developed for comet 1P/Halley to assess the importance of various production and loss mechanisms of CO(a3pi) and to calculate the intensity of Cameron band emission on different days of IUE observation. Two different solar EUV flux models, EUVAC of Richards et al. (1994) and SOLAR2000 of Tobiska (2004), and different relative abundances of CO and CO2, are used to evaluate the role of photon and photoelectron in producing CO molecule in a3pi state in the cometary coma. It is found that in comet 1P/Halley 60--70% of the total intensity of the Cameron band emission is contributed by electron impact excitation of CO and CO2, while the contribution from photodissociative excitation of CO2 is small (20--30%). Thus, in the comets where CO and CO2 relative abundances are comparable, the Cameron band emission is largely governed by electron impact excitation of CO, and not by the photodissociative excitation of CO2 as assumed earlier. Model calculated Cameron band 1-0 emission intensity (40 R) is consistent with the observed IUE slit-averaged brightness (37 +/- 6 R) using EUVAC model solar flux on 13 March 1986, and also on other days of observations. Since electron impact excitation is the major production mechanism, the Cameron emission can be used to derive photoelectron density in the inner coma rather than the CO2 abundance.

astro-ph.EP↗