arXiv subjects

Anil Bhardwaj

Publications and source records attributed to Anil Bhardwaj.

At least 73 records · Page 4Linked to original sources

arXiv2011

Impact of solar EUV flux on CO Cameron band and CO2+ UV doublet emissions in the dayglow of Mars

This study is aimed at making a calculation about the impact of the two most commonly used solar EUV flux models -- SOLAR2000 (S2K) of \cite{Tobiska04} and EUVAC model of \cite{Richards94} -- on photoelectron fluxes, volume emission rates, ion densities and CO Cameron and CO$_2^+$ UV doublet band dayglow emissions on Mars in three solar activity conditions: minimum, moderate, and maximum. Calculated limb intensities profiles are compared with SPICAM/Mars Express and Mariner observations. Analytical yield spectrum (AYS) approach has been used to calculate photoelectron fluxes in Martian upper atmosphere. Densities of prominent ions and CO molecule in excited triplet a$^3Π$ state are calculated using major ion-neutral reactions. Volume emission rates of CO Cameron and CO$_2^+$ UV doublet bands have been calculated for dif{}ferent observations (Viking condition, Mariner and Mars Express SPICAM observations) on Mars. For the low solar activity condition, dayglow intensities calculated using the S2K model are $\sim$40% higher than those calculated using the EUVAC model. During high solar activity, due to the higher EUV fluxes at wavelengths below 250 Å in the EUVAC model, intensities calculated using EUVAC model are slightly higher ($\sim$20%) than those calculated using S2K model. Irrespective of the solar activity condition, production of Cameron band due to photodissociative excitation of CO$_2$ is around 50% higher when S2K model is used. Altitude of peak limb brightness of CO Cameron and CO$_2^+$ UV doublet band is found to be independent of solar EUV flux models. Calculated limb intensities of CO Cameron and CO$_2^+$ UV doublet bands are on an average a factor of $\sim$2 and $\sim$1.5, respectively, higher than the SPICAM Mars Express observation, while they are consistent with the Mariner observations.

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arXiv2011

Calculations of N2 triplet states vibrational populations and band emissions in Venusian dayglow

A model for N2 triplet states band emissions in the Venusian dayglow has been developed for low and high solar activity conditions. Steady state photoelectron fluxes and volume excitation rates for N2 triplet states have been calculated using the Analytical Yield Spectra (AYS) technique. Model calculated photoelectron flux is in good agreement with Pioneer Venus Orbiter-observed electron flux. Since inter-state cascading is important for the triplet states of N2, populations of different levels of N2 triplet states are calculated under statistical equilibrium considering direct electron impact excitation, and cascading and quenching effects. Densities of all vibrational levels of each triplet state are calculated in the model. Height-integrated overhead intensities of N2 triplet band emissions are calculated, the values for Vegard-Kaplan (A^3Sigma_u^+ - X^1Pi_g^+), First Positive (B^3Pi_g - A^3Sigma_u^+), Second Positive (C^3Pi_u - B^3Pi_g), and Wu-Benesch (W^3Delta_u - B^3Pi_g) bands of N2, are 1.9 (3.2), 3 (6), 0.4 (0.8), and 0.5 (1.1) kR, respectively, for solar minimum (maximum) conditions. The intensities of the three strong Vegard-Kaplan bands (0, 5), (0, 6), and (0, 7) are 94 (160), 120 (204), and 114 (194) R, respectively, for solar minimum (maximum) conditions. Limb profiles are calculated for VK (0, 4), (0, 5), (0, 6) and (0, 7) bands. The calculated intensities on Venus are about a factor 10 higher than those on Mars. The present study provides a motivation for a search of N2 triplet band emissions in the dayglow of Venus.

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arXiv2011

Model calculation of N2 Vegard-Kaplan band emissions in Martian dayglow

A model for N2 Vegard-Kaplan (VK) band (A^3Sigma_u^+ - X^1Sigma_g^+) emissions in Martian dayglow has been developed to explain the recent observations made by the Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars (SPICAM) ultraviolet spectrograph aboard Mars Express. Steady state photoelectron fluxes and volume excitation rates have been calculated using the analytical yield spectra technique. Since interstate cascading is important for triplet states of N2, the population of any given level of N2 triplet states is calculated under statistical equilibrium considering direct excitation, cascading, and quenching effects. Relative population of all vibrational levels of each triplet state is calculated in the model. Line of sight intensities and height-integrated overhead intensities have been calculated for VK, first positive (B^3Pi_g - A^3Sigma_u^+), second positive (C^3Pi_u - B^3Pi_g), and Wu-Benesch (W^3Delta_u - B^3Pi_g) bands of N2. A reduction in the N2 density by a factor of 3 in the Mars thermospheric general circulation model is required to obtain agreement between calculated limb profiles of VK (0-6) and SPICAM observation. Calculations are carried out to asses the impact of model parameters, namely, electron impact cross sections, solar EUV flux, and model atmosphere, on the emission intensities. Constraining the N2/CO2 ratio by SPICAM observations, we suggest the N2/CO2 ratios to be in the range 1.1-1.4% at 120 km, 1.8-3.2% at 140 km, and 4-7% at 170 km. During high solar activity the overhead intensity of N2 VK band emissions would be ~2.5 times higher than that during low solar activity.

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arXiv2010

Extremely high reflection of solar wind protons as neutral hydrogen atoms from regolith in space

We report on measurements of extremely high reflection rates of solar wind particles from regolith-covered lunar surfaces. Measurements by the Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian Chandrayaan-1 spacecraft in orbit around the Moon show that up to 20% of the impinging solar wind protons are reflected from the lunar surface back to space as neutral hydrogen atoms. This finding, generally applicable to regolith-covered atmosphereless bodies, invalidates the widely accepted assumption that regolith almost completely absorbs the impinging solar wind.

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arXiv2010

Studying the Lunar-Solar Wind Interaction with the SARA Experiment aboard the Indian Lunar Mission Chandrayaan-1

The first Indian lunar mission Chandrayaan-1 was launched on 22 October 2008. The Sub-keV Atom Reflecting Analyzer (SARA) instrument onboard Chandrayaan-1 consists of an energetic neutral atom (ENA) imaging mass analyzer called CENA (Chandrayaan-1 Energetic Neutrals Analyzer), and an ion-mass analyzer called SWIM (Solar wind Monitor). CENA performed the first ever experiment to study the solar wind-planetary surface interaction via detection of sputtered neutral atoms and neutralized backscattered solar wind protons in the energy range ~0.01-3.0 keV. SWIM measures solar wind ions, magnetosheath and magnetotail ions, as well as ions scattered from lunar surface in the ~0.01-15 keV energy range. The neutral atom sensor uses conversion of the incoming neutrals to positive ions, which are then analyzed via surface interaction technique. The ion mass analyzer is based on similar principle. This paper presents the SARA instrument and the first results obtained by the SWIM and CENA sensors. SARA observations suggest that about 20% of the incident solar wind protons are backscattered as neutral hydrogen and ~1% as protons from the lunar surface. These findings have important implications for other airless bodies in the solar system.

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arXiv2010

The Sub-keV Atom Reflecting Analyzer (SARA) Experiment Aboard Chandrayaan-1 Mission: Instrument and Observations

SARA experiment aboard the first Indian lunar mission Chandrayaan-1 had the objective to explore the solar wind-lunar interaction using energetic neutral atoms (ENA) from the lunar surface as diagnostic tool. SARA consisted of an ENA imaging mass analyzer CENA (Chandrayaan-1 Energetic Neutral Analyzer) and an ion mass analyser SWIM (Solar Wind Monitor), along with a digital processing unit (DPU) which commands and controls the sensors and provides the interface to the spacecraft. Both sensors have provided excellent observational data. CENA has observed ENAs from the lunar surface and found that ~20% of the incident solar wind ions get backscattered as ENAs from the lunar surface. This is contrary to the previous assumptions of almost complete absorption of solar wind by the lunar surface. The observation is relevant for other airless bodies in the solar system.

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arXiv2010

X-rays from solar system objects

During the last few years our knowledge about the X-ray emission from bodies within the solar system has significantly improved. Several new solar system objects are now known to shine in X-rays at energies below 2 keV. Apart from the Sun, the known X-ray emitters now include planets (Venus, Earth, Mars, Jupiter, and Saturn), planetary satellites (Moon, Io, Europa, and Ganymede), all active comets, the Io plasma torus (IPT), the rings of Saturn, the coronae (exospheres) of Earth and Mars, and the heliosphere. The advent of higher-resolution X-ray spectroscopy with the Chandra and XMM-Newton X-ray observatories has been of great benefit in advancing the field of planetary X-ray astronomy. Progress in modeling X-ray emission, laboratory studies of X-ray production, and theoretical calculations of cross-sections, have all contributed to our understanding of processes that produce X-rays from the solar system bodies. At Jupiter and Earth, both auroral and non-auroral disk X-ray emissions have been observed. X-rays have been detected from Saturn's disk, but no convincing evidence of an X-ray aurora has been observed. The first soft (0.1- 2 keV) X-ray observation of Earth's aurora by Chandra shows that it is highly variable. The non-auroral X-ray emissions from Jupiter, Saturn, and Earth, those from the disk of Mars, Venus, and Moon, and from the rings of Saturn, are mainly produced by scattering of solar X-rays. The spectral characteristics of X-ray emission from comets, the heliosphere, the geocorona, and the Martian halo are quite similar, but they appear to be quite different from those of Jovian auroral X-rays. X-rays from the Galilean satellites and the IPT are mostly driven by impact of Jovian magnetospheric particles. This paper reviews studies of the soft X-ray emission from the solar system bodies, excluding the Sun.

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arXiv2010

Model for Cameron band emission in comets: A case for EPOXI mission target comet 103P/Hartley 2

The CO2 production rate has been derived in comets using the Cameron band (a3Pi - X1Sigma) emission of CO molecule assuming that photodissociative excitation of CO2 is the main production mechanism of CO in a3Pi metastable state. We have devoloped a model for the production and loss of CO(a3Pi) which has been applied to comet 103P/Hartley 2: the target of EPOXI mission. Our model calculations show that photoelectron impact excitation of CO and dissociative excitation of CO2 can together contribute about 60-90% to the Cameron band emission. The modeled brightness of (0-0) Cameron band emission on comet Hartley 2 is consistent with Hubble Space Telescope observations for 3-5% CO2 (depending on model input solar flux) and 0.5% CO relative to water, where photoelectron impact contribution is about 50-75%. We suggest that estimation of CO2 abundances on comets using Cameron band emission may be reconsidered. We predict the height integrated column brightness of Cameron band of ~1300 R during EPOXI mission encounter period.

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arXiv2010

First observation of a mini-magnetosphere above a lunar magnetic anomaly using energetic neutral atoms

The Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian Chandrayaan-1 spacecraft has produced for the first time an image of a lunar magnetic anomaly in backscattered hydrogen atoms. The image shows that a partial void of the solar wind, a mini-magnetosphere, is formed above the strong magnetic anomaly near the Crisium antipode. The mini-magnetosphere is 360 km across at the surface and is surrounded by a 300-km-thick region of enhanced plasma flux that results from the solar wind flowing around the mini-magnetosphere. The mini-magnetosphere is visible only in hydrogen atoms with energy exceeding 150 eV. Fluxes with energies below 100 eV do not show corresponding spatial variations. While the high-energy atoms result from the backscattering process, the origin of the low-energy component is puzzling. These observations reveal a new class of objects, mini-magnetospheres, and demonstrate a new observational technique to study airless bodies, imaging in backscattered neutral atoms.

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arXiv2010

Monte Carlo model of electron energy degradation in a CO2 atmosphere

A Monte Carlo model has been developed to study the degradation of <1000 eV electrons in an atmosphere of CO2, which is one of the most abundant species in Mars' and Venus' atmospheres. The e-CO2 cross sections are presented in an assembled set along with their analytical representations. Monte Carlo simulations are carried out at several energies to calculate the "yield spectra", which embodied all the information related to electron degradation process and can be used to calculate "yield" (or population) for any inelastic process. The numerical yield spectra have been fitted analytically resulting in an analytical yield spectra (AYS). We have calculated the mean energy per ion pair and efficiencies for various inelastic processes, including the double and dissociative double ionization of \car\ and negative ion formation. The energy distribution of the secondary electrons produced per incident electron is also presented at few incident energies. The mean energy per ion pair for CO2 is 37.5 (35.8) eV at 200 (1000) eV, compared to experimental value 32.7 eV at high energies. Ionization is the dominant loss process at energies above 50 eV with contribution of ~50%. Among the excitation processes, 13.6 eV and 12.4 eV states are the dominant loss processes consuming ~28% energy above 200 eV. Around and below ionization threshold, 13.6 eV, 12.4 eV, and 11.1 eV, followed by 8.6 eV and 9.3 eV excitation states are important loss processes, while below 10 eV vibrational excitation dominates.

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arXiv2010

Protons in the near-lunar wake observed by the Sub-keV Atom Reflection Analyzer on board Chandrayaan-1

Significant proton fluxes were detected in the near wake region of the Moon by an ion mass spectrometer on board Chandrayaan-1. The energy of these nightside protons is slightly higher than the energy of the solar wind protons. The protons are detected close to the lunar equatorial plane at a $140^{\circ}$ solar zenith angle, i.e., ~50$^{\circ}$ behind the terminator at a height of 100 km. The protons come from just above the local horizon, and move along the magnetic field in the solar wind reference frame. We compared the observed proton flux with the predictions from analytical models of an electrostatic plasma expansion into a vacuum. The observed velocity was higher than the velocity predicted by analytical models by a factor of 2 to 3. The simple analytical models cannot explain the observed ion dynamics along the magnetic field in the vicinity of the Moon.

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arXiv2006

On the Solar EUV Deposition in the Inner Comae of Comets with Large Gas Production Rates

In this letter we have made a comparative study of degradation of solar EUV radiation and EUV-generated photoelectrons in the inner comae of comets having different gas production rates, Q, with values 1x10^28, 7x10^29, 1x10^31, and 1x10^32 s^-1. We found that in higher-Q comets the radial profile of H2O+ photo-production rate depicts a double-peak structure and that the differences in sunward and anti-sunward photoionization rates are pronounced. We show that photoelectron impact ionization is an order of magnitude larger than photoionization rate near the lower photoionization peak in comets with Q >~ 1x10^31 s^-1. The present study reveals the importance of photoelectrons relative to solar EUV as the ionization source in the inner coma of high-Q comets.

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arXiv2006

First Terrestrial Soft X-ray Auroral Observation by The Chandra X-ray Observatory

Northern auroral regions of Earth were imaged with energetic photons in the 0.1-10 keV range using the High-Resolution Camera (HRC-I) aboard the Chandra X-ray Observatory at 10 epochs (each ~20 min duration) between mid-December 2003 and mid-April 2004. These observations aimed at searching for Earth's soft (<2 keV) X-ray aurora in a comparative study with Jupiter's X-ray aurora, where a pulsating X-ray "hot-spot" has been previously observed by Chandra. The first Chandra soft X-ray observations of Earth's aurora show that it is highly variable (intense arcs, multiple arcs, diffuse patches, at times absent). In at least one of the observations an isolated blob of emission is observed near the expected cusp location. A fortuitous overflight of DMSP satellite F13 provided SSJ/4 energetic particle measurements above a bright arc seen by Chandra on 24 January 2004, 20:01-20:22 UT. A model of the emissions expected strongly suggests that the observed soft X-ray signal is bremsstrahlung and characteristic K-shell line emissions of nitrogen and oxygen in the atmosphere produced by electrons.

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arXiv2006

X-Ray Emission from Jupiter, Saturn, and Earth: A Short Review

Jupiter, Saturn, and Earth - the three planets having dense atmosphere and a well developed magnetosphere - are known to emit X-rays. Recently, Chandra X-ray Observatory has observed X-rays from these planets, and XMM-Newton has observed them from Jupiter and Saturn. These observations have provided improved morphological, temporal, and spectral characteristics of X-rays from these planets. Both auroral and non-auroral (low-latitude) 'disk' X-ray emissions have been observed on Earth and Jupiter. X-rays have been detected from Saturn's disk, but no convincing evidence for X-ray aurora on Saturn has been observed. The non-auroral disk X-ray emissions from Jupiter, Saturn, and Earth, are mostly produced due to scattering of solar X-rays. X-ray aurora on Earth is mainly generated via bremsstrahlung from precipitating electrons and on Jupiter via charge exchange of highlyionized energetic heavy ions precipitating into the polar atmosphere. Recent unpublished work suggests that at higher (>2 keV) energies electron bremsstrahlung also plays a role in Jupiter's X-ray aurora. This paper summarizes the recent results of X-ray observations on Jupiter, Saturn, and Earth mainly in the soft energy (~0.1-2.0 keV) band and provides a comparative overview.

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arXiv2005

Discovery of Oxygen Kalpha X-ray Emission from the Rings of Saturn

Using the Advanced CCD Imaging Spectrometer (ACIS), the Chandra X-ray Observatory (CXO) observed the Saturnian system for one rotation of the planet (~37 ks) on 20 January, 2004, and again on 26-27 January, 2004. In this letter we report the detection of X-ray emission from the rings of Saturn. The X-ray spectrum from the rings is dominated by emission in a narrow (~130 eV wide) energy band centered on the atomic oxygen K-alpha fluorescence line at 0.53 keV. The X-ray power emitted from the rings in the 0.49-0.62 keV band is 84 MW, which is about one-third of that emitted from Saturn disk in the photon energy range 0.24-2.0 keV. Our analysis also finds a clear detection of X-ray emission from the rings in the 0.49-0.62 keV band in an earlier (14-15 April, 2003) Chandra ACIS observation of Saturn. Fluorescent scattering of solar X-rays from oxygen atoms in the H2O icy ring material is the likely source mechanism for ring X-rays, consistent with the scenario of solar photo-production of a tenuous ring oxygen atmosphere and ionosphere recently discovered by Cassini.

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arXiv2005

Radial Distribution of Production Rates, Loss Rates and Densities Corresponding to Ion Masses <=40 amu in the Inner Coma of Comet Halley: Composition and Chemistry

In this paper we have studied the chemistry of C, H, N, O, and S compounds corresponding to ions of masses <=40 amu in the inner coma of the comet 1P/Halley. The production rates, loss rates, and ion mass densities are calculated using the Analytical Yield Spectrum approach and solving coupled continuity equation controlled by the steady state photochemical equilibrium condition. The primary ionization sources in the model are solar EUV photons, photoelectrons, and auroral electrons of the solar wind origin. The chemical model couples ion-neutral, electron-neutral, photon-neutral and electron-ion reactions among ions, neutrals, electrons, and photons through over 600 chemical reactions. Of the 46 ions considered in the model the chemistry of 24 important ions are discussed in this paper. The calculated ion mass densities are compared with the Giotto Ion Mass Spectrometer (IMS) and Neutral Mass Spectrometer (NMS) data at radial distances 1500, 3500, and 6000 km.

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arXiv2005

Solar Control on Jupiter's Equatorial X-ray Emissions: 26-29 November 2003 XMM-Newton Observation

During November 26-29, 2003 XMM-Newton observed soft (0.2-2 keV) X-ray emission from Jupiter for 69 hours. The low-latitude X-ray disk emission of Jupiter is observed to be almost uniform in intensity with brightness that is consistent with a solar-photon driven process. The simultaneous lightcurves of Jovian equatorial X-rays and solar X-rays (measured by the TIMED/SEE and GOES satellites) show similar day-to-day variability. A large solar X-ray flare occurring on the Jupiter-facing side of the Sun is found to have a corresponding feature in the Jovian X-rays. These results support the hypothesis that X-ray emission from Jovian low-latitudes are solar X-rays scattered from the planet's upper atmosphere, and suggest that the Sun directly controls the non-auroral X-rays from Jupiter's disk. Our study also suggests that Jovian equatorial X-rays can be used to monitor the solar X-ray flare activity on the hemisphere of the Sun that is invisible to space weather satellites.

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arXiv2005

Chandra Observation of an X-ray Flare at Saturn: Evidence for Direct Solar Control on Saturn's Disk X-ray Emissions

Saturn was observed by Chandra ACIS-S on 20 and 26-27 January 2004 for one full Saturn rotation (10.7 hr) at each epoch. We report here the first observation of an X-ray flare from Saturn's non-auroral (low-latitude) disk, which is seen in direct response to an M6-class flare emanating from a sunspot that was clearly visible from both Saturn and Earth. Saturn's disk X-ray emissions are found to be variable on time scales of hours to weeks to months, and correlated with solar F10.7 cm flux. Unlike Jupiter, X-rays from Saturn's polar (auroral) region have characteristics similar to those from its disk. This report, combined with earlier studies, establishes that disk X-ray emissions of the giant planets Saturn and Jupiter are directly regulated by processes happening on the Sun. We suggest that these emissions could be monitored to study X-ray flaring from solar active regions when they are on the far side and not visible to Near-Earth space weather satellites.

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