Hydrogen Corona Research Articles

Atomic hydrogen corona of Uranus

We have analyzed Voyager 2 ultraviolet spectrometer observations of the H Lyman α emission from the atomic hydrogen corona of Uranus. The resulting estimated hydrogen density distribution has a radial profile ∝ 1/r2. This distribution implies a weakly bound or escaping population of hot (≳2 eV) hydrogen not thermalized with the 0.07 eV thermosphere/exosphere. The hot H density is enhanced on the sunward side of Uranus, with the maximum weakly associated with the dayside magnetic pole. The source of this hot hydrogen is presumably near the position we have found for the density maximum. Therefore we suggest that this source of hot H atoms may be associated either with electroglow or else with the dayside aurora.

Solar‐wind‐driven convection in the Uranian magnetosphere

We present an analytic, self‐consistent model of time‐dependent solar‐wind‐driven convection in the magnetosphere of Uranus. Because of the unusual orientation of the planetary rotation and magnetic dipole axes, magnetic merging on the dayside magnetopause varies as a function of planetary spin, in response to the changing orientation of the planetary magnetic field relative to the upstream interplanetary magnetic field, which is assumed to have a fixed direction for many planetary rotations. Therefore the magnitude of the solar‐wind driven convection electric field varies sinusoidally in time with the 17.2‐hour planetary spin period, even though the field direction is fixed in the corotating frame in a direction analogous to the dawn‐to‐dusk direction in the Earth's magnetosphere. We assume that the “hot” (keV) protons observed by the Voyager 2 plasma science instrument in the inner magnetosphere convect sunward from a source in the near tail and form a ring current shielding layer near L = 5. The shielding process requires a time‐dependent model because the convection timescale (∼20 days) is much larger than the 17‐hour period of variation of the convection field. The time‐averaged part of the imposed electric field is strongly attenuated inside the shielding layer, but the sinusoidally varying part of the imposed field penetrates the layer without significant attenuation because the shielding timescale (∼30 hours) is longer than the 17‐hour oscillation period. A fraction of the hot plasma is thereby “scattered” onto closed drift orbits to form a trapped ring current population. This trapped ring current population is sufficiently long‐lived to undergo charge exchange and inelastic collisions with the widely distributed neutral hydrogen corona, resulting in the energy degradation of the “hot” component and the simultaneous appearance of the “intermediate” (∼ 100 eV) and "warm" (∼ 10 eV) components evident in the Voyager 2 plasma science measurements between L = 5 and L = 7. The Birkeland current system is concentrated near the ring current shielding layer, consistent with the relatively low latitude of the auroral emissions observed by the Voyager 2 ultraviolet spectrometer.

Picked‐up protons near Mars: Phobos observations

The measurements carried out by the plasma spectrometer, ASPERA, onboard the PHOBOS‐2 spacecraft show that protons, originating in the extended hydrogen corona of Mars, were observed at altitudes ≤7500 km. The cyclotron instability of these pickup ions appears to generate Alfven waves observed by the MAGMA magnetometer. Analysis of the plasma data shows that weak pitch‐angle diffusion of a ring‐distribution of pickup protons occurs. The altitude profiles of pickup proton fluxes and number densities of the parent hydrogen atoms are derived.

Radial diffusion of low‐energy plasma ions in Saturn's magnetosphere

Analytic solutions to the steady state radial diffusion equation allowing for a distributed source and sink of ions are presented. The diffusion space has absorption boundaries at the ends and is divided into two regions separated by an interior boundary where a discontinuity in diffusion and loss parameters is allowed to exist. The most general solution consists of a collection of point sources with adjustable weights to approximate an arbitrary continuous distribution of ion sources. The theory is applied to the study of thermal ion diffusion in Saturn's magnetosphere. It is concluded that a relatively large transport rate is required to conform with low‐energy Voyager plasma measurements and a radial diffusion coefficient expressible as DLL = 4×10−7 s−1 (L/6)3–4 is indicated. In such a fast diffusion regime cool O+ ion densities at L = 2.8, 6, and 15 may be obtained from a single source in the form of the Dione‐Tethys neutral H2O cloud. Hot ion densities in the outer magnetosphere are obtained from ionization of Titan's neutral atomic clouds of hydrogen and nitrogen and ion pickup by the magnetospheric flow. The O+ thermal ion model is compatible with a bimodal distribution of neutral hydrogen in the magnetosphere with a Titan cloud [H] ≃ 20 cm−3 (L/20) in the outer magnetosphere and a low‐density [H] = 6 cm−3 (6/L)² hydrogen corona of either Saturnian origin or ring origin in the inner magnetosphere.

Constraints on the neutral hydrogen corona at Uranus from its interaction with magnetospheric plasma

Models of collisional reactions between the hot (several keV) protons and neutral atomic hydrogen corona observed at Uranus by Voyager 2 are compared with the observations in order to deduce constraints on the coronal density. An upper limit is found for nH(5RU) of several tens of H/cm³ on the sunward side of Uranus and several H/cm³ on the dark side. The possibility that the hot plasma population might arise from photoionization and electron impact ionization of coronal and interstellar medium hydrogen in the magnetotail, followed by pickup and energization while convecting toward Uranus, is examined and rejected.

Voyager 2 plasma ion observations in the magnetosphere of Uranus

Positive ion measurements in the magnetosphere of Uranus have been made by the Voyager 2 plasma science experiment. We present an overview of the entire data set and a detailed analysis of the observations from the inner magnetosphere which complements and extends results reported elsewhere. Densities and temperatures are obtained from an analysis which incorporates details of the instrumental response. These results are then used to calculate flux tube particle and energy content to support the hypothesis that the plasma transport is controlled by a solar wind‐driven magnetospheric convection system. Variations in the flux tube content suggest both a local source of plasma, produced from the neutral hydrogen corona of Uranus, and a nonlocal source, convected inward and heated by adiabatic compression. In each case a proton composition is inferred. Sharp boundaries in the high‐energy (approximately 1 keV) plasma population are interpreted in terms of the spatial extent of the magnetospheric convection, with significant shielding of the convection electric field. The convection theory is also used in a simulation of the low‐energy (approximately 10 eV) ion component using the neutral hydrogen source, resulting in distribution functions which qualitatively agree with the observations.

Proton and oxygen plasmas at Uranus

Despite the presence of several large, icy moons within the Uranian magnetosphere, the Voyager 2 spacecraft did not detect any heavy ion plasma. This paper estimates the heavy ion density that would be consistent with a heavy neutral torus formed by photosputtering, charged particle sputtering, and micrometeoroid impact vaporization of icy surfaces on the moons, taking into account the large 60° angle between the satellite orbit plane and the magnetic equator. The expected heavy ion density is unobservably small. The observed proton plasma of the inner magnetosphere can be maintained by ionization of the atomic hydrogen corona and by the ionospheric proton source driven by photoelectron escape, for a plasma residence time of ∼30 days. These two proton sources are comparable near L = 5. The same 30‐day residence time for energetic protons implies an upper limit of 10−5 on the fraction of incident solar wind protons that enter the magnetosphere and penetrate to within L < 6 while conserving their first adiabatic invariants.

Survey of electrons in the Uranian magnetosphere: Voyager 2 observations

We present results of an analysis of the Voyager 2 plasma science experiment (PLS) electron measurements made during the Uranus encounter. The energy coverage is 10 eV ≤ E ≤ 5950 eV. The electron distribution functions are composed of a thermal (cold) and a non‐Maxwellian suprathermal (hot) component. Within the inner magnetosphere the electron density ne ranged between 0.02 cm−3 and 1.0 cm−3, while the electron temperatures Te ranged between ∼10 eV near the terminator and ∼100 eV within the nightside hemisphere. Thermal (suprathermal) electron temperatures Tc (Th) ranged from ∼7 to 10 eV (20–200 eV) near the terminator and from ∼10 to 30 eV (500 eV to 2 keV) within the nightside hemisphere. The observations revealed no significant electron fluxes above the detection limit at all observed energies in the dayside hemisphere, but electron fluxes orders of magnitude larger extended over the full PLS energy range in the nightside hemisphere. The large day‐night asymmetry together with the spin axis alignment with the solar direction and the large tilt ∼60° of the planetary magnetic dipole indicate that solar wind driven time dependent magnetospheric convection will be an important transport mechanism within the Uranian magnetosphere. The energy‐time evolution of the plasma boundary for ions and electrons near the terminator (L ∼ 6.7) at 1648 spacecraft event time (SCET) on January 24, 1986, is remarkably similar to that observed during substorms by spacecraft within the earth's magnetosphere at geosynchronous orbit in the dusk‐midnight quadrant. We show that Miranda cannot be the cause of the plasma sheet inner edge and that present “steady state” shielding models cannot account for the thinness of the “inner edge” (< 500 km) because of expected periodic merging (enhanced convection) every ∼17 hours on the dayside magnetopause. Resonant charge exchange collisions between protons and Uranus' hydrogen cloud are used to estimate an upper limit for the hydrogen corona density NH ∼200 cm−3 at L = 5. Using the local time asymmetry in the E ∼ 6 keV electron fluxes and the observation of E > 20 keV electron fluxes at all local times sampled by the spacecraft (Krimigis et al., 1986), we estimate that the steady state convection time τconv of the plasma is between 1 and 3 days. Finally, the local time asymmetry in the radio emissions from Uranus is consistent with the day‐night asymmetry in the keV electron fluxes observed by PLS which may provide the free energy for the radio emission.

Energetic ion and electron phase space densities in the magnetosphere of Uranus

Voyager 2 low‐energy charged particle (LECP) data from the magnetosphere of Uranus have been analyzed to obtain proton and electron phase space density profiles. The Uranus proton profiles show an approximately exponential decline with decreasing radius for L ≲ 9 in a relatively dense thermal plasma region with intense plasma wave activity. An analogy with the magnetospheres of Earth, Jupiter, and Saturn suggests a plasmasphere at Uranus. The ion flux tube content in the Uranian radiation belt is less than that in the other three cases. Proton and electron profiles ≲ 100 MeV/G show evidence of an absorption signature near the minimum L of Ariel. Proton profiles ≳ 200 MeV/G show evidence of injection events, suggesting substorm processes at Uranus. Electron profiles ≳ 100 MeV/G show clear minima at the minimum L values of Miranda, Ariel, and perhaps Umbriel, separated by broad maxima. These profiles are interpreted as implying local injection of energetic electrons. Distributed loss mechanisms at Uranus include satellite sweeping, wave‐particle interactions, and charge exchange of protons with an extended hydrogen corona. If the net loss rates of protons and electrons are estimated by calculated satellite sweeping rates, the radial diffusion coefficient is estimated near L = 7.5 to be 10−7 s−1 to 10−6 s−1. If the diffusion coefficient is ≲ 10−6 s−1, the power available from inward radial diffusion of energetic protons and electrons is ≲ 5 × 108 W and is less than the 4 × 1010 W needed to maintain the Uranian ultraviolet aurora. Local injection of electrons above 100 MeV/G between the minimum L values of Miranda and Ariel also requires a power of at least 8 × 107. The magnetosphere of Uranus is the third planetary magnetosphere for which evidence of substorm activity has been adduced, after those of Earth and Mercury.

Plasma Observations Near Uranus: Initial Results from Voyager 2

Extensive measurements of low-energy positive ions and electrons in the vicinity of Uranus have revealed a fully developed magnetosphere. The magnetospheric plasma has a warm component with a temperature of 4 to 50 electron volts and a peak density of roughly 2 protons per cubic centimeter, and a hot component, with a temperature of a few kiloelectron volts and a peak density of roughly 0.1 proton per cubic centimeter. The warm component is observed both inside and outside of L = 5, whereas the hot component is excluded from the region inside of that L shell. Possible sources of the plasma in the magnetosphere are the extended hydrogen corona, the solar wind, and the ionosphere. The Uranian moons do not appear to be a significant plasma source. The boundary of the hot plasma component at L = 5 may be associated either with Miranda or with the inner limit of a deeply penetrating, solar wind-driven magnetospheric convection system. The Voyager 2 spacecraft repeatedly encountered the plasma sheet in the magnetotail at locations that are consistent with a geometric model for the plasma sheet similar to that at Earth.

Ultraviolet Spectrometer Observations of Uranus

Data from solar and stellar occultations of Uranus indicate a temperature of about 750 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane and acetylene in the lower levels. The ultraviolet spectrum of the sunlit hemisphere is dominated by emissions from atomic and molecular hydrogen, which are kmown as electroglow emissions. The energy source for these emissions is unknown, but the spectrum implies excitation by low-energy electrons (modeled with a 3-electron-volt Maxwellian energy distribution). The major energy sink for the electrons is dissociation of molecular hydrogen, producing hydrogen atoms at a rate of 10(29) per second. Approximately half the atoms have energies higher than the escape energy. The high temperature of the atmosphere, the small size of Uranus, and the number density of hydrogen atoms in the thermosphere imply an extensive thermal hydrogen corona that reduces the orbital lifetime of ring particles and biases the size distribution toward larger particles. This corona is augmented by the nonthermal hydrogen atoms associated with the electroglow. An aurora near the magnetic pole in the dark hemisphere arises from excitation of molecular hydrogen at the level where its vertical column abundance is about 10(20) per square centimeter with input power comparable to that of the sunlit electroglow (approximately 2x10(11) watts). An initial estimate of the acetylene volume mixing ratio, as judged from measurements of the far ultraviolet albedo, is about 2 x 10(-7) at a vertical column abundance of molecular hydrogen of 10(23) per square centimeter (pressure, approximately 0.3 millibar). Carbon emissions from the Uranian atmosphere were also detected.

Hydrogen production rate from comet Austin 1982g

Meaningful measurements with respect to the cometary Balmer-alpha (H-alpha) emission are difficult and require the use of special equipment. The first ground-based observations of H-alpha emission from a cometary hydrogen corona were conducted on comet Kohoutek 1973 XII with a large-aperture Fabry-Perot spectrometer installed at the McMath solar telescope at Kitt Peak National Observatory. The present investigation is concerned with the second ground-based observations of cometary H-alpha emission carried out during the apparition of comet Austin 1982g. A 150 mm dual-etalon Fabry-Perot spectrometer was employed in the experiment. Use was made of an observatory which is designed for the high spectral resolution study of faint extended sources such as interstellar and geocoronal emission lines. The investigation demonstrates that hydrogen production rates from comets as faint as about 7th magnitude can be routinely measured from the ground at minimal cost.

Hydrogen production rates from ground-based Fabry-Perot observations of comet Kohoutek

The only ground-based observations of a cometary hydrogen corona that have been obtained up to the present were carried out during the appearance of comet Kohoutek (1973 XII). Hydrogen Balmer alpha (H-alpha) emission from the gas cloud surrounding the comet was detected using a Fabry-Perot spectrometer at Kitt Peak National Observatory. These observations have been reexamined using (1) recently obtained solar full-disk Lyman beta emission line profiles, (2) a new calibration of the absolute sensitivity of the Fabry-Perot spectrometer based on comparison of NGC 7000 with standard stars and the planetary nebula NGC 7662, and (3) corrections for atmospheric extinction instead of the geocoronal H-alpha comparison method used previously to obtain comet H-alpha intensities. The new values for hydrogen production rates are in good agreement with results obtained from Lyman alpha observations of comet Kohoutek.

Hot hydrogen in the exosphere of Venus

Lyman-alpha measurements of the hydrogen corona of Venus by Mariners 5 and 10 have been shown to be consistent with a two-temperature component model. Bertaux et al. (1978) have successfully fitted the Venera 9 exospheric Lyman-alpha data to an elevated (500 K) single temperature. Various source mechanisms have been proposed to explain the 'hot' (1000 K) energetic component of the hydrogen corona. In the present paper recent results from the Pioneer Venus Orbiter are used to establish the major sources of this hot hydrogen population.

The Lyman-alpha observations of comet Kohoutek from Mariner 10

view Abstract Citations (4) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Lyman-alpha observations of comet Kohoutek from Mariner 10. Kumar, S. ; Holberg, J. ; Broadfoot, A. L. ; Belton, M. J. S. Abstract The paper presents the Lyman-alpha data from postperihelion observations of comet Kohoutek made with Mariner 10 UV spectrometer during the 1974 January 11-24 period when the comet distance from the sun varied from 0.521 AU to 0.860 AU. These data are analyzed to determine the hydrogen production rates. It is noted that observations against the low interplanetary background allowed a measurement of the hydrogen corona over 30 million km (or 15 minutes) as seen from the Mariner 10 spacecraft. The bulk of the observations can be fitted with a non-steady-state model assuming two velocity components (20 km/sec and 8 km/sec) of equal proportions. A 70% increase in the hydrogen production rate is found within 2 days from January 17 to 19 while the comet receded from the sun. Finally, it is noted that the postperihelion production rates from the Mariner 10 observations are comparable to the preperihelion production rates reported previously. Publication: The Astrophysical Journal Pub Date: September 1979 DOI: 10.1086/157320 Bibcode: 1979ApJ...232..616K Keywords: Kohoutek Comet; Lyman Alpha Radiation; Mariner 10 Space Probe; Spaceborne Astronomy; Ultraviolet Spectroscopy; Astronomical Models; Data Reduction; Hydrogen Production; Transient Response; KOHOUTEK; COMET; MARINER 10; ULTRAVIOLET SPECTROMETER; MODELS; HYDROGEN; COMAE; SPECTRUM; PRODUCTION RATES; OBSERVATIONS; DATA; BRIGHTNESS; LYMAN-ALPHA RADIATION; COMET TAILS; COMET HEADS; PARAMETERS; FLUX; VELOCITY; SOLAR RADIATION; CORONAS; Astrophysics; Comets; Comet 1973 XII Kohoutek; Comets:Hydrogen; Comets:Lyman Alpha full text sources ADS |

The greenhouse of Titan

Both non-gray radiative equilibrium and gray convective equilibrium calculations for Titan indicate that the discrepancy between the equilibrium temperature of an atmosphereless Titan and the observed infrared temperatures can be explained by a massive molecular hydrogen greenhouse effect. The convective calculations indicate a probable minimum optical depth of 14, corresponding to many tens of km-atm of H 2, and total pressures of ∼0.1 bar. The tropopause is several hundred km above the Titanian surface and at a temperature of about 90°K. Methane condensation is likely at this level. Such an atmosphere is unstable against atmospheric blow-off unless typical mesosphere scale heights are < 25km, an unlikely situation. Blow-off can also be circumvented by exospheric temperatures near the freezing point of hydrogen. It is considered more plausible that the present atmosphere is in equilibrium between outgassing and blow-off of the one hand and accretion from protons trapped in a hypothetical Saturnian magnetic field on the other; or exhibits uncompensated blow-off of outgassing products. To maintain the present blow-off rate without compensation for all of geological time requires an outgassing equivalent to the volatilization of a few km of subsurface ices. Photo-dissociation of these volatilized ices produces the observed high abundance of H 2 as well as large quantities of complex organic chromophores which may explain the reddish coloration of the Titanian cloud deck. An extensive circum-Titanian hydrogen corona is postulated. Surface temperatures as high as 200°K are not excluded. Because of its high temperatures and pressures and the probable large abundance of organic compounds, Titan is a prime target for spacecraft exploration in the outer solar system.

Determination of ion species in a hydrogen corona discharge between concentric hemispheres

A study has been made of the positive ion species emerging from the cathode of a corona discharge between concentric hemispheres. Hydrogen gas filling was used and a quadrupole mass spectrometer incorporated for determination of the relative concentration of the various species. H+, H3+ and H5+ ions were observed, and their concentration is given both as a function of the space-charge-corrected reduced electric field at the cathode and the gas filling pressure.

Mariner V Flight Past Venus

On 19 October 1967 the Mariner V spacecraft passed by Venus 10,151 kilometers from the center of the planet; the gravitational field, atmosphere, ionosphere, hydrogen corona, and interaction with the solar wind were observed.

Planetary coronae and atmospheric evaporation

Planetary coronae and atmospheric evaporation

The Distribution of Hydrogen in the Telluric Hydrogen Corona.

view Abstract Citations (29) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Distribution of Hydrogen in the Telluric Hydrogen Corona. Johnson, Francis S. Abstract The distribution of hydrogen in the telluric hydrogen corona is calculated by identifying the components of a fictitious hydrostatic distribution which are actually present in the real atmospheric distribution. The components which are clearly present are the captive ballistic orbits and the escaping ballistic orbits which originate near the base of the exosphere. The components which are clearly absent are the hyperbolic orbits which originate in space; these consist of two groups, those which do not intersect the base of the exosphere and which return to space and those which intersect the base of the exosphere and which are capture orbits. Trapped elliptic orbits, or satellite orbits, may or may not be present, but near the earth they are probably occupied. Each of these components can be evaluated as an explicit portion of a hydrostatic distribution, and the distribution calculated from the hydrostatic distribution in a straightforward manner. Calculations are presented for temperatures of 10000 and 15000 K. Publication: The Astrophysical Journal Pub Date: March 1961 DOI: 10.1086/147072 Bibcode: 1961ApJ...133..701J full text sources ADS |