The ion mass loading rate at Io     

Joachim Saur  Darrell F. Strobel  Michael E. Summers 《Icarus》2003,163(2):456-468

The Io plasma torus, composed of mostly heavy ions of oxygen and sulfur, is sustained by an Iogenic mass loading rate of ∼10³⁰ amu s⁻¹ = 1.6 × 10²⁸ SO₂ s⁻¹ or approximately 10³ kg s⁻¹(A.L. Broadfoot et al., 1979, Science 204, 979-982). We argue on the basis of available power sources, reanalysis of F. Bagenal (1997, Geophys. Res. Lett. 24, 2111-2114), HST UV remote sensing, and detailed model calculations that at most 20% of this mass leaves Io in the form of ions, i.e., ≤3 × 10²⁷ × (n_e,0/3600 cm⁻³) ions s⁻¹, where n_e,0 is the average torus electron density. For the Galileo spacecraft Io pass in December 1995, the ion mass loading rate was ≤3 × 10²⁷ ions s⁻¹, whereas for the Voyager epoch with lower n_e,0 (=2000 cm⁻³), this rate would be ≤1.7 × 10²⁷ ions s⁻¹, consistent with the D.E. Shemansky (1980, Astrophys. J. 242, 1266-1277) mass loading limit of ≤1 × 10²⁷ ions s⁻¹. We investigate the processes that control Io’s large scale electrodynamic interaction and find that the elastic collision rate exceeds the ionization/pickup rate by at least a factor of 5 for all atmospheric column densities considered (10¹⁶-10²¹ m⁻²) and by a factor of ∼100 for the most realistic column density. Consequently, elastic collisions are mostly responsible for Io’s high conductances and thus generate Io’s large scale electrodynamic interaction such as the generation of Io’s electric current system and the slowing of the plasma flow. The electrodynamic part of Io’s interaction is thus best described as an ionosphere-like interaction rather than a comet-like interaction. An analytic expression for total electron impact rates is derived for Io’s atmosphere, which is independent of any particular model for the 3D interaction of torus electrons with its atmosphere.  相似文献   

The SO₂ atmosphere and ionosphere of Io: Ion chemistry,atmospheric escape,and models corresponding to the Pioneer 10 radio occultation measurements     

Shailendra Kumar 《Icarus》1985,61(1):101-123

Models of Io's ionosphere at the time of the Pioneer 10 encounter are constructed in the presence of an SO₂Na atmosphere on Io. The formation of the observed ionosphere on the downstream side requires precipitation of electrons; solar EUV alone is inadequate. Electron impact in the range 500–800 eV on an SO₂ atmosphere with a surface density of 14 × 10¹⁰ cm^?3 provides the best fit to the Pioneer 10 radio occultation entry data. The SO₂⁺, the major ion produced, is converted rapidly to SO⁺ and in turn to S⁺ by reactions with the dissociation products of SO₂. Ion chemistry leads to the formation of S⁺ as the dominant ion at and above the ionospheric peak. Na⁺ would dominate the ion composition near the surface, and it provides important constraints on the amount of Na allowed in the atmosphere. The relatively narrow energy range and flux required for incident electrons suggests that a fraction of torus plasma is accelerated in the wake region and penetrates deep into the atmosphere. On the upstream side the torus plasma compresses the ionosphere. These characteristics support the possible presence of a weak magnetic field associated with Io. S⁺ ions would escape from Io in the wake region at a rate of up to 10²⁶ sec^?1.  相似文献   

Galileo plasma wave observations of iogenic hydrogen     

《Planetary and Space Science》1999,47(10-11):1377-1387

The Galileo plasma wave instrument has detected intense electromagnetic wave emissions approximately centered on the second and fourth harmonics of the local proton gyrofrequency during the close equatorial flyby of Io on 7 December 1995. Their frequencies suggest these emissions are likely generated locally by an instability driven by non thermal protons. Given that this process occurs close to Io, we suggest that hydrogen-bearing compounds, escaping from Io, are broken up/ionized near this moon, thereby releasing protons. Newly-created protons are thus injected in the Jovian corotating plasma with the corotation velocity, leading to the formation of a ring in velocity space. Several electromagnetic wave–particle instabilities can be driven by a ring of newborn protons. Given that the corotating plasma is sub-Alfvénic relative to Io, the magnetosonic mode cannot be destabilized by this proton ring. The full dispersion relation is studied using the WHAMP program (Rönmark, 1982. Rep. 179. Kiruna Geophys. Inst., Kiruna, Sweden) as well as a new algorithm that allows us to fit the distribution function of newborn protons in a more realistic way. This improvement in the ring model is necessary to explain the relative narrowness of the observed spectral peaks. The measured E/B ratio is also used to identify the relevant instability and wave mode: this mode results from the coupling between the ion Bernstein and the ion cyclotron mode (IBCW). To our knowledge this mode has not yet been studied. From the instability threshold an estimate of the density of newborn protons around Io is thus given; at about 2 Io radii from the surface and 40°W longitude from the sub-Jupiter meridian, this density is found to be ≥0.5% of the local plasma density (∼4000 cm⁻³), namely ≥20 cm⁻³. Assuming a stationary pickup process and a r⁻ⁿ distribution of pickup protons within several Io radii of Io’s wake, this implies that more than 10²⁶ protons/s are created around Io. The ultimate origin of these protons is an open issue.  相似文献   

The atmospheric signature of Io's Prometheus plume and anti-jovian hemisphere: evidence for a sublimation atmosphere     

Kandis Lea Jessup  John R. Spencer  Robert R. Howell  Miquela Vigel 《Icarus》2004,169(1):197-215

Using the Hubble Space Telescope's Space Telescope Imaging Spectrograph we have obtained for the first time spatially resolved 2000-3000 Å spectra of Io's Prometheus plume and adjoining regions on Io's anti-jovian hemisphere in the latitude range 60° N-60° S, using a 0.1″ slit centered on Prometheus and tilted roughly 45° to the spin axis. The SO₂ column density peaked at 1.25×10¹⁷ cm⁻² near the equator, with an additional 5×10¹⁶ cm⁻² enhancement over Prometheus corresponding to a model volcanic SO₂ output of 10⁵ kg s⁻¹. Apart from the Prometheus peak, the SO₂ column density dropped fairly smoothly away from the subsolar point, even over regions that included potential volcanic sources. At latitudes less than ±30°, the dropoff rate was consistent with control by vapor pressure equilibrium with surface frost with subsolar temperature 117.3±0.6 K, though SO₂ abundance was higher than predicted by vapor pressure control at mid-latitudes, especially in the northern hemisphere. We conclude that, at least at low latitudes on the anti-jovian hemisphere where there are extensive deposits of optically-thick SO₂ frost, the atmosphere is probably primarily supported by sublimation of surface frost. Although the 45° tilt of our slit prevents us from separating the dependence of atmospheric density on solar zenith angle from its dependence on latitude, the pattern is consistent with a sublimation atmosphere regardless of which parameter is the dominant control. The observed drop in gas abundance towards higher latitudes is consistent with the interpretation of previous Lyman alpha images of Io as indicating an atmosphere concentrated at low latitudes. Comparison with previous disk-resolved UV spectroscopy, Lyman-alpha images, and mid-infrared spectroscopy suggests that Io's atmosphere is denser and more widespread on the anti-jovian hemisphere than at other longitudes. SO₂ gas temperatures were in the range of 150-250 K over the majority of the anti-jovian hemisphere, consistent with previous observations. SO was not definitively detected in our spectra, with upper limits to the SO/SO₂ ratio in the range 1-10%, roughly consistent with previous observations. S₂ gas was not seen anywhere, with an upper limit of 7.5×10¹⁴ cm⁻² for the Prometheus plume, confirming that this plume is significantly poorer in S₂ than the Pele plume (S₂ /SO₂<0.005, compared to 0.08-0.3 at Pele). In addition to the gas absorption signatures, we have observed continuum emission in the near ultraviolet (near 2800 Å) for the first time. The brightness of the observed emission was directly correlated with the SO₂ abundance, strongly peaking in the equatorial region over Prometheus. Emission brightness was modestly anti-correlated with the jovian magnetic latitude, decreasing when Io intersected the torus centrifugal equator.  相似文献   

Mid-infrared detection of large longitudinal asymmetries in Io's SO₂ atmosphere     

John R. Spencer  Emmanuel Lellouch  Miguel A. López-Valverde  Thomas K. Greathouse  Jean-Marie Flaud 《Icarus》2005,176(2):283-304

We have observed about 16 absorption lines of the ν₂ SO₂ vibrational band on Io, in disk-integrated 19-μm spectra taken with the TEXES high spectral resolution mid-infrared spectrograph at the NASA Infrared Telescope Facility in November 2001, December 2002, and January 2004. These are the first ground-based infrared observations of Io's sunlit atmosphere, and provide a new window on the atmosphere that allows better longitudinal and temporal monitoring than previous techniques. Dramatic variations in band strength with longitude are seen that are stable over at least a 2 year period. The depth of the strongest feature, a blend of lines centered at 530.42 cm⁻¹, varies from about 7% near longitude 180° to about 1% near longitude 315° W, as measured at a spectral resolution of 57,000. Interpretation of the spectra requires modeling of surface temperatures and atmospheric density across Io's disk, and the variation in non-LTE ν₂ vibrational temperature with altitude, and depends on the assumed atmospheric and surface temperature structure. About half of Io's 19-μm radiation comes from the Sun-heated surface, and half from volcanic hot spots with temperatures primarily between 150 and 200 K, which occupy about 8% of the surface. The observations are thus weighted towards the atmosphere over these low-temperature hot spots. If we assume that the atmosphere over the hot spots is representative of the atmosphere elsewhere, and that the atmospheric density is a function of latitude, the most plausible interpretation of the data is that the equatorial atmospheric column density varies from about 1.5×¹⁰¹⁷ cm⁻² near longitude 180° W to about 1.5×¹⁰¹⁶ cm⁻² near longitude 300° W, roughly consistent with HST UV spectroscopy and Lyman-α imaging. The inferred atmospheric kinetic temperature is less than about 150 K, at least on the anti-Jupiter hemisphere where the bands are strongest, somewhat colder than inferred from HST UV spectroscopy and millimeter-wavelength spectroscopy. This longitudinal variability in atmospheric density correlates with the longitudinal variability in the abundance of optically thick, near-UV bright SO₂ frost. However it is not clear whether the correlation results from volcanic control (regions of large frost abundance result from greater condensation of atmospheric gases supported by more vigorous volcanic activity in these regions) or sublimation control (regions of large frost abundance produce a more extensive atmosphere due to more extensive sublimation). Comparison of data taken in 2001, 2002, and 2004 shows that with the possible exception of longitudes near 180° W between 2001 and 2002, Io's atmospheric density does not appear to decrease as Io recedes from the Sun, as would be expected if the atmosphere were supported by the sublimation of surface frost, suggesting that the atmosphere is dominantly supported by direct volcanic supply rather than by frost sublimation. However, other evidence such as the smooth variation in atmospheric abundance with latitude, and atmospheric changes during eclipse, suggest that sublimation support is more important than volcanic support, leaving the question of the dominant atmospheric support mechanism still unresolved.  相似文献   

Sputtering of sulfur by kiloelectronvolt ions: Application to the magnetospheric plasma interaction with Io     

《Icarus》1987,70(1):111-123

Measurements of total yields, temperature dependences, mass spectra, and energy spectra of molecules sputtered from condensed sulfur (S₈) at low temperatures by keV ions are reported and results are given for Jovian plasma ion bombardment of Io. A change in the reflectance of the sulfur, which can be removed by annealing, is produced by the most penetrating ions and may be connected with the darker, colder polar regions on Io. The measured sputtering yields are much lower than those estimated earlier for room temperature sulfur films but are comparable to previous measurements of keV ion sputtering of SO₂ at low temperatures. The corrected mass spectrum indicates that ≈66% of the total yield corresponds to S₂ ejection while only 5 and 16% correspond to S and S₃, respectively. Therefore, if ions reach the surface of Io its atmosphere will have a non-negligible sulfur component of primarily S₂. The ejection of S and S₂ is temperature independent for temperatures characteristic of most of the surface of Io. The energy spectrum for S has an approximate 1/E² dependence at high ejection energies, whereas S₂ and S₃ fall off more rapidly. Assuming 50% coverage of both sulfur and SO₂ and a thin atmosphere (e.g., nightside and polar region) the direct sputter injection of sulfur atoms and molecules into the Jovian plasma torus and the indirect injection due to coronal processes are estimated. These injection rates for sulfur are compared to those for SO₂ showing that injection from sulfur deposits contributes 13% to the total mass injection rate of ∼2–3 × 10²⁹ amu/sec.  相似文献   

Monte Carlo modeling of Io’s [OI] 6300 Å and [SII] 6716 Å auroral emission in eclipse     

C. Moore  K. Miki  K. Stapelfeldt  P.L. Varghese  R.W. Evans 《Icarus》2010,207(2):810-833

We present a Monte Carlo (MC) model of [OI] 6300 Å and [SII] 6716 Å emission from Io entering eclipse. The simulation accounts for the 3-D distribution of SO₂, O, SO, S, and O₂ in Io’s atmosphere, several volcanic plumes, and the magnetic field around Io. Thermal electrons from the jovian plasma torus are input along the simulation domain boundaries and move along the magnetic field lines distorted by Io, occasionally participating in collisions with neutrals. We find that the atmospheric asymmetry resulting from varying degrees of atmospheric collapse across Io (due to eclipse ingress) and the presence of volcanoes contributes significantly to the unique morphology of the [OI] 6300 Å emission. The [OI] radiation lifetime of ∼134 s limits the emission to regions that have a sufficiently low neutral density so that intermolecular collisions are rare. We find that at low altitudes (typically <40 km) and in volcanic plumes (Pele, Prometheus, etc.) the number density is large enough (>4 × 10⁹ cm⁻³) to collisionally quench nearly all (>95%) of the excited oxygen for reasonable quenching efficiencies. Upstream (relative to the plasma flow), Io’s perturbation of the jovian magnetic field mirrors electrons with high pitch angles, while downstream collisions can trap the electrons. This magnetic field perturbation is one of the main physical mechanisms that results in the upstream/downstream brightness asymmetry in [OI] emission seen in the observation by Trauger et al. (Trauger, J.T., Stapelfeldt, K.R., Ballester, G.E., Clarke, J.I., 1997. HST observations of [OI] emissions from Io in eclipse. AAS-DPS Abstract (1997DPS29.1802T)). There are two other main causes for the observed brightness asymmetry. First, the observation’s viewing geometry of the wake spot crosses the dayside atmosphere and therefore the wake’s observational field of view includes higher oxygen column density than the upstream side. Second, the phased entry into eclipse results in less atmospheric collapse and thus higher collisional quenching on the upstream side relative to the wake. We compute a location (both in altitude and latitude) for the intense wake emission feature that agrees reasonably well with this observation. Furthermore, the peak intensity of the simulated wake feature is less than that observed by a factor of ∼3, most likely because our model does not include direct dissociation-excitation of SO₂ and SO. We find that the latitudinal location of the emission feature depends not so much on the tilt of the magnetic field as on the relative north/south flux tube depletion that occurs due to Io’s changing magnetic latitude in the plasma torus. From 1-D simulations, we also find that the intensity of [SII] 6716 and 6731 Å emission is much weaker than that of [OI] even if the [SII] excitation cross section is 10³ times larger than excitation to [OI]. This is because the density of S⁺ is much less than that of O and because the Einstein-A coefficient of the [SII] emission is a factor of ∼10 smaller than that of [OI].  相似文献   

Io’s atmosphere: Constraints on sublimation support from density variations on seasonal timescales using NASA IRTF/TEXES observations from 2001 to 2010     

Constantine C.C. Tsang  John R. Spencer  Emmanuel Lellouch  Miguel A. López-Valverde  Matthew J. Richter  Thomas K. Greathouse 《Icarus》2012,217(1):277-296

We present an analysis of 19 μm spectra of Io’s SO₂ atmosphere from the TEXES mid-infrared high spectral resolution spectrograph on NASA’s Infrared Telescope Facility, incorporating new data taken between January 2005 and June 2010 and a re-analysis of earlier data taken from November 2001 to January 2004. This is the longest set of contiguous observations of Io’s atmosphere using the same instrument and technique thus far. We have fitted all 16 detected blended absorption lines of the ν₂ SO₂ vibrational band to retrieve the subsolar values of SO₂ column abundance and the gas kinetic temperature. By incorporating an existing model of Io’s surface temperatures and atmosphere, we retrieve sub-solar column densities from the disk-integrated data. Spectra from all years are best fit by atmospheric temperatures <150 K. Best-fit gas kinetic temperatures on the anti-Jupiter hemisphere, where SO₂ gas abundance is highest, are low and stable, with a mean of 108 (±18) K. The sub-solar SO₂ column density between longitudes of 90–220° varies from a low of 0.61 (±0.145) × 10^?17 cm^?2, near aphelion in 2004, to a high of 1.51 (±0.215) × 10¹⁷ cm^?2 in 2010 when Jupiter was approaching its early 2011 perihelion. No correlation in the gas temperature was seen with the increasing SO₂ column densities outside the errors.Assuming that any volcanic component of the atmosphere is constant with time, the correlation of increasing SO₂ abundance with decreasing heliocentric distance provides good evidence that the atmosphere is at least partially supported by frost sublimation. The SO₂ frost thermal inertias and albedos that fit the variation in atmospheric density best are between 150–1250 W m^?2 s^?1/2 K^?1 and 0.613–0.425 respectively. Photometric evidence favors albedos near the upper end of this range, corresponding to thermal inertias near the lower end. This relatively low frost thermal inertia produces larger amplitude seasonal variations than are observed, which in turn implies a substantial additional volcanic atmospheric component to moderate the amplitude of the seasonal variations of the total atmosphere on the anti-Jupiter hemisphere. The seasonal thermal inertia we measure is unique both because it refers exclusively to the SO₂ frost surface component, and also because it refers to relatively deep subsurface layers (few meters) due to the timescales of many years, while previous studies have determined thermal inertias at shallower levels (few centimeters), relevant for timescales of ～2 h (eclipse) or ～2 days (diurnal curves).  相似文献   

H2O on Io? IR spectra of SO2/H2O mixed ices in the 5000-450 cm−1 region     

R. Dahmani  R. K. Khanna 《Astrophysics and Space Science》1996,236(1):125-133

Laboratory transmission IR spectra of relatively thick films (up to 500 m) of mixed H₂O and SO₂ ices were measured at several temperatures between 10 and 130 K in the range 5000-450 cm^–1. In addition to the strong features due to crystalline SO₂ the spectra reveal bands at 3668 cm^–1, 3634 cm^–1 (with some structure) and 3300 cm^–1 which are identified with H₂O in SO₂ environment. Also, there is no overlap between any of the H₂O bands with the 3584 cm^–1 band of SO₂ at any temperature in the above range. The implication of this result is that H₂O, if present on Io, must be far less than 1 part in 10⁵ SO₂.  相似文献   

The Dual Sources of Io's Sodium Clouds     

Jody K. WilsonMichael Mendillo  Jeffrey BaumgardnerNicholas M. Schneider  John T. TraugerBrian Flynn 《Icarus》2002,157(2):476-489

Io's sodium clouds result mostly from a combination of two atmospheric escape processes at Io. Neutralization of Na⁺ and/or NaX⁺ pickup ions produces the “stream” and the “jet” and results in a rectangular-shaped sodium nebula around Jupiter. Atmospheric sputtering of Na by plasma torus ions produces the “banana cloud” near Io and a diamond-shaped sodium nebula. Charge exchange of thermal Na⁺ with Na in Io's atmosphere does not appear to be a major atmospheric ejection process. The total ejection rate of sodium from Io varied from 3×10²⁶ to 25×10²⁶ atoms/s over seven years of observations. Our results provide further evidence that Io's atmospheric escape is driven from collisionally thick regions of the atmosphere rather than from the exosphere.  相似文献   

A massive, dusty toroid with large grains in the pre-planetary nebula IRAS22036+5306     

R. Sahai  K. Young  N. Patel  C. Sánchez Contreras  M. Morris 《Astrophysics and Space Science》2008,313(1-3):241-244

Using the Submillimeter Array (SMA), we have obtained high angular-resolution (∼1″) interferometric maps of the submillimeter (0.88 mm) continuum and CO J=3–2 line from IRAS 22036+5306 (I 22036), a bipolar pre-planetary nebula (PPN) with knotty jets discovered in our HST SNAPshot survey of young PPNe. In addition, we have obtained supporting lower-resolution (∼10″) 2.6 mm continuum and CO, ¹³CO J=1–0 observations with the Owens Valley Radio Observatory (OVRO) interferometer. We find an unresolved source of submillimeter (and millimeter-wave) continuum emission in I 22036, implying a very substantial mass (0.02–0.04M _⊙) of large (i.e., radius ≳1 mm), cold (≲50 K) dust grains associated with I 22036’s toroidal waist. The CO J=3–2 observations show the presence of a very fast (∼220 km s⁻¹), highly collimated, massive (0.03M _⊙) bipolar outflow with a very large scalar momentum (about 10³⁹ g cm s⁻¹), and the characteristic spatio-kinematic structure of bow-shocks at the tips of this outflow. The fast outflow in I 22036, as in most PPNe, cannot be driven by radiation pressure. The large mass of the torus suggests that it has most likely resulted from common-envelope evolution in a binary, however it remains to be seen whether or not the time-scales required for the growth of grains to millimeter sizes in the torus are commensurate with such a formation scenario. The presence of the torus should facilitate the formation of the accretion disk needed to launch the jet. We also find that the ¹³C/¹²C ratio in I 22036 is very high (0.16), close to the maximum value achieved in equilibrium CNO-nucleosynthesis (0.33). The combination of the high circumstellar mass (i.e., in the torus and an extended dust shell inferred from ISO far-infrared spectra) and the high ¹³C/¹²C ratio in I 22036 provides strong support for this object having evolved from a massive (≳4M _⊙) progenitor in which hot-bottom-burning has occurred.  相似文献   

Europa's atmosphere, gas tori, and magnetospheric implications     

William H. Smyth  Max L. Marconi 《Icarus》2006,181(2):510-526

A two-dimensional kinetic model calculation for the water group species (H₂O, H₂, O₂, OH, O, H) in Europa's atmosphere is undertaken to determine its basic compositional structure, gas escape rates, and velocity distribution information to initialize neutral cloud model calculations for the most important gas tori. The dominant atmospheric species is O₂ at low altitudes and H₂ at higher altitudes with average day-night column densities of 4.5×¹⁰¹⁴ and 7.7×¹⁰¹³ cm⁻², respectively. H₂ forms the most important gas torus with an escape rate of ∼2×¹⁰²⁷ s⁻¹ followed by O with an escape rate of ∼5×¹⁰²⁶ s⁻¹, created primarily as exothermic O products from O₂ dissociation by magnetospheric electrons. The circumplanetary distributions of H₂ and O are highly peaked about the satellite location and asymmetrically distributed near Europa's orbit about Jupiter, have substantial forward clouds extending radially inward to Io's orbit, and have spatially integrated cloud populations of 4.2×¹⁰³³ molecules for H₂ and 4.0×¹⁰³² atoms for O that are larger than their corresponding populations in Europa's local atmosphere by a factor of ∼200 and ∼1000, respectively. The cloud population for H₂ is a factor of ∼3 times larger than that for the combined cloud population of Io's O and S neutral clouds and provides the dominant neutral population beyond the so-called ramp region at 7.4-7.8 RJ in the plasma torus. The calculated brightness of Europa's O cloud on the sky plane is very dim at the sub-Rayleigh level. The H₂ and O tori provide a new source of europagenic molecular and atomic pickup ions for the thermal plasma and introduce a neutral barrier in which new plasma sinks are created for the cooler iogenic plasma as it is transported radially outward and in which new sinks are created to alter the population and pitch angle distribution of the energetic plasma as it is transported radially inward. The europagenic instantaneous pickup ion rates are peaked at Europa's orbit, dominate the iogenic pickup ion rates beyond the ramp region, and introduce new secondary plasma source peaks in the solution of the plasma transport problem. The H₂ torus is identified as the unknown Europa gas torus that creates both the observed loss of energetic H⁺ ions at Europa's orbit and the corresponding measured ENA production rate for H.  相似文献   

The Radio Recombination Line Intensities Of Atoms And Ions: Helium,Carbon And Oxygen     

N.?I.?RovenskayaEmail author 《Astrophysics and Space Science》2005,300(4):357-366

The radio recombination line intensities of heavy elements of helium, carbon and oxygen are calculated with accounting for dielectronic recombination. Dielectronic recombination rates are determined accurate to the second order of a perturbation theory and the rates are described as function of principal quantum number for helium-like atom or ion. Balance equations are solved for the departure coefficients from LTE b_n. The collision and spontaneous transition rates are accounted for the balance equations, in which non-equilibrium distribution source is dielectronic recombination. Non-equilibrium amplification coefficients are found as functions of a medium temperature, density and ion charge z = 1–3 for radio recombination lines. Optical depths are calculated for the heavy element low-frequency lines with the numbers 300 > n > 1200. For the chosen electronic temperatures and densities T_e = 0.8× 10⁴–10× 10⁴ K, N_e = 0.05–0.1 cm⁻³ the line optical depth is determined by the values τ_L∼ 0.1× 10⁻⁴–100× 10⁻⁴. Calculated for free-free transition rates, the optical depth is given by using the value τ_ff∼ 10⁻²τ_L.  相似文献   

Studies of proton-irradiated SO₂ at low temperatures: Implications for lo     

Marla H. Moore 《Icarus》1984,59(1):114-128

The infrared absorption spectrum from 3.3 to 27 μm (3030-370 cm^?) of SO₂ ice films has been measured at 20 and 88°K before and after 1-MeV proton irradiation. The radiation flux was chosen to simulate the estimated flux of Jovian magnetospheric 1-MeV protons incident on Io. After irradiation, SO₃ is identified as the dominant molecule synthesized in the SO₂ ice. This is also the case after irradiation of composite samples of SO₂ with sulfur, or disulfites. Darkening was observed in irradiated SO₂ ice and in irradiated S₈ pellets. Photometric and spectral measurements of the thermoluminescence of irradiated SO₂ have been made during warming. The spectrum appears as a broad band with a maximum at 4450 Å. Analysis of the luminescence data suggests that, at Ionian temperatures, irradiated SO₂ ice would not be a dominant contributor to posteclipse brightening phenomena. After warming to room temperature, a form of SO₃ remains along with a sulfate and S₈. Based on these experiments, it is reasonable to propose that small amounts of SO₃ may exist on the surface of Io as a result of irradiation synthesis in SO₂ frosts.  相似文献   

Gamma-Ray Line Observations of the 2000 July 14 Flare and SEP Impact on the Earth     

Share  G.H.  Murphy  R.J.  Tylka  A.J.  Schwartz  R.A.  Yoshimori  M.  Suga  K.  Nakayama  S.  Takeda  H. 《Solar physics》2001,204(1-2):41-53

The HXS and GRS detectors on Yohkoh observed the 14 July 2000, X5.7 flare, beginning at ∼ 10:20 UT, ∼ 4 min before the peak in soft X-rays. The hard X-rays and γ-rays peaked ∼ 3 min later at ∼ 10:27 UT. Solar γ-ray emission lasted until ∼ 10:40 UT. Impact of high-energy ions at the Sun is revealed by the γ-ray lines from neutron capture, annihilation radiation and de-excitation that are visible above the bremsstrahlung continuum. From measurement of these lines we find that the flare-averaged spectrum of accelerated protons is consistent with a power law ge10 MeV with index 3.14±0.15 and flux 1.1×10³² protons MeV⁻¹ at 10 MeV. We estimate that there were ∼1.5×10³⁰ erg in accelerated ions if the power law extended without a break down to 1 MeV; this is about 1% of the energy in electrons > 20 keV from measurements of the hard X-rays. We find no evidence for spectral hardening in the hard X-rays that has been suggested as a predictor for the occurrence of solar energetic particle (SEP) events. This was the third largest proton event above 10 MeV since 1976. The GRS and HXS also observed γ-ray lines and continuum produced by the impact of SEP on the Earth's atmosphere beginning about 13 UT on 14 July. These measurements show that the SEP spectrum softened considerably over the next 24 hours. We compare these measurements with proton measurements in space.  相似文献   

Simulation of gas dynamics and radiation in volcanic plumes on Io     

J. Zhang  D.B. Goldstein  N.E. Gimelshein  D.A. Levin 《Icarus》2003,163(1):182-197

Modeling results of volcanic plumes on Jupiter’s moon Io are presented. Two types of low density axisymmetric SO₂ plume flows are modeled using the direct simulation Monte Carlo (DSMC) method. Thermal radiation from all three vibrational bands and overall rotational lines of SO₂ molecules is modeled. A high resolution computation of the flow in the vicinity of the vent was obtained by multidomain sequential calculation to improve the modeling of the radiation signature. The radiation features are examined both by calculating infrared emission spectra along different lines-of-sight through the plume and with the DSMC modeled emission images of the whole flow field. It is found that most of the radiation originates in the vicinity of the vent, and non-LTE (non-local-thermodynamic equilibrium) cooling by SO₂ rotation lines exceeds cooling in the v₂ vibrational band at high altitude.In addition to the general shape of the plumes, the calculated average SO₂ column density (∼10¹⁶ cm⁻²) over a Pele-type plume and the related frost-deposition ring structure (at R ∼ 500 km from the vent) are in agreement with observations. These comparisons partially validate the modeling. It is suggested that an observation with spatial resolution of less than 30 km is needed to measure the large spatial variation of SO₂ near a Pele-type plume center. It is also found that an influx of 1.1 × 10²⁹ SO₂ s⁻¹ (or 1.1 × 10⁴ kg s⁻¹) is sufficient to reproduce the observed SO₂ column density at Pele. The simulation results also show some interesting features such as a multiple bounce shock structure around Prometheus-type plumes and the frost depletion by plume-induced erosion on the sunlit side of Io. The model predicts the existence of a canopy shock, a ballistic region inside the Pele-type plume, and the negligible effect of surface heating by plume emission.  相似文献   

Io's dayside SO₂ atmosphere     

Lori M. Feaga  Melissa McGrath 《Icarus》2009,201(2):570-1189

An extensive set of HI Lyman-α images obtained with the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) from 1997-2001 has been analyzed to provide information about the spatial and temporal character of Io's SO₂ atmosphere. An atmospheric distribution map derived from the observations reveals that the sunlit SO₂ atmosphere is temporally stable on a global scale, with only small local changes. An anti-/sub-jovian asymmetry in the SO₂ distribution is present in all 5 years of the observations. The average daytime atmosphere is densest on the anti-jovian hemisphere in the equatorial regions, with a maximum equatorial column density of 5.0×¹⁰¹⁶ cm−2 at 140° longitude. The SO₂ atmosphere also has greater latitudinal extent on the anti-jovian hemisphere as compared to the sub-jovian. The atmospheric distribution appears to be best correlated with the location of hot spots and known volcanic plumes, although small number statistics for the plumes limits the correlation.  相似文献   

Io's sodium emission cloud     

William W. Macy  Laurence M. Trafton 《Icarus》1975,25(3):432-438

Strong evidence that Io's sodium emission is due to resonant scattering is given by our observations which show a monotonic increase of emission intensity with residual solar intensity. In addition we detected no emission during three eclipse observations of Io. We propose a resonant scattering model with two spacial components comprising an optically thick atmosphere extending 10³ km above Io's surface surrounded by an optically thin cloud which forms a partial torus around Jupiter. In this model a flux of 10⁷ cm^?2 sec^?1 sodium atoms are sputtered from Io's surface by heavy energetic ions which are accelerated in a plasma sheath around Io. The atoms sputtered from the surface collide with atoms in Io's atmosphere so the equipartition of kinetic energy is established. The total sodium abundance is about 3 × 10¹³ cm^?2. During Io's day, sodium and other atmospheric constituents are ionized, giving rise to the ionosphere observed by Pioneer 10. Atoms escape by means of Jeans escape from the critical level, which is at the top of the atmosphere and the base of the cloud. We have observed sodium emission 6arcsec (6 Io diameters) above and below Io's orbital plane and 23arcsec toward Jupiter in Io's orbital plane. No emission was detected at maximum elongation 180° from Io. We interpret these results to mean that atoms escaping from Io form a partial torus whose thickness is about 12 arcsec and whose length is at least one-fifth of Io's orbital circumference.  相似文献   

Upper limits for an atmosphere on Io     

Bradford A. Smith 《Icarus》1972

The 14 May 1971 occultation β Scorpi C by Io was successfully observed in ultraviolet light near Kingston, Jamaica. Within the limits established by time resolution and the signal-to-noise ratio, both the disappearance and reappearance were found to be instantaneous. Upper limits for the surface pressure for N₂, CH₄, and H₂ atmospheres are 0.09, 0.13, and 44.ubar, respectively. The corresponding number densities are 6.2 × 10¹², 9.4 × 10¹², and 3.2 × 10¹⁵ cm⁻³. An isothermal atmosphere at a temperature of 100°K was assumed.  相似文献   

Characterizing Io’s Pele,Tvashtar and Pillan plumes: Lessons learned from Hubble     

Kandis Lea Jessup  John R. Spencer 《Icarus》2012,218(1):378-405

Hubble Space Telescope/Wide Field and Planetary Camera 2 (HST/WFPC2) images of Io obtained between 1995 and 2007 between 0.24 and 0.42 μm led to the detection of the Pele plume in reflected sunlight in 1995 and 1999; imaging of the Pele plume via absorption of jovian light in 1996 and 1999; detection of the Prometheus-type Pillan plume in reflected sunlight in 1997; and detection of the 2007 Pele-type Tvashtar plume eruption in reflected sunlight and via absorption of jovian light. Based on a detailed analysis of these observations we characterize and compare the gas and dust properties of each of the detected plumes. In each case, the brightness of the plumes in reflected sunlight is less at 0.26 μm than at 0.33 μm. Mie scattering analysis of the wavelength dependence of each plume’s reflectance signature suggests that range of particle sizes within the plumes is quite narrow. Assuming a normal distribution of particle sizes, the range of mean particle sizes is ～0.035–0.12 μm for the 1997 Pillan eruption, ～0.05–0.08 μm for the 1999 Pele and 2007 Tvasthar plumes, and ～0.05–0.11 μm for the 1995 Pele plume, and in each case the standard deviation in the particle size distribution is <15%. The Mie analysis also suggests that the 2007 Tvashtar eruption released ～10⁹ g of sulfur dust, the 1999 Pele eruption released ～10⁹ g of SO₂ dust, the 1997 Pillan eruption released ～10¹⁰ g of SO₂ dust, and the 1995 Pele plume may have released ～10¹⁰ g of SO₂ dust. Analysis of the plume absorption signatures recorded in the F255W filter bandpass (0.24–0.28 μm) indicates that the opacity of the 2007 Tvashtar plume was 2× that of the 1996 and 1999 Pele plume eruptions. While the sulfur dust density estimated for the Tvashtar from the reflected sunlight data could have produced 61% of the observed plume opacity, <10% of the 1999 Pele F255W plume opacity could have resulted from the SO₂ dust detected in the eruption. Accounting for the remaining F255W opacity level of the Pele and Tvasthar plumes based on SO₂ and S₂ gas absorption, the SO₂ and S₂ gas density inferred for each plume is almost equivalent corresponding to ～2–6 × 10¹⁶ cm^?2 and 3–5 × 10¹⁵ cm^?2, respectively, producing SO₂ and S₂ gas resurfacing rates ～0.04–0.2 cm yr^?1 and 0.007–0.01 cm yr^?1; and SO₂ and S₂ gas masses ～1–4 × 10¹⁰ g and ～2–3 × 10⁹ g; for a total dust to gas ratio in the plumes ～10^?1–10^?2. The 2007 Tvashtar plume was detected by HST at ～380 ± 40 km in both reflected sunlight and absorbed jovian light; in 1999, the detected Pele plume altitude was 500 km in absorbed jovian light, but in reflected sunlight the detected height was ～2× lower. Thus, for the 1999 Pele plume, similar to the 1979 Voyager Pele plume observations, the most efficient dust reflections occurred in the region closest to the plume vent. The 0.33–0.42 μm brightness of the 1997 Pillan plume was 10–20× greater than the Pele or Tvashtar plumes, exceeding by a factor of 3 the average brightness levels observed within 200 km of 1979 Loki eruption vent. But, the 0.26 μm brightness of the 1997 Pillan plume in reflected sunlight was significantly lower than would be predicted by the dust scattering model. Presuming that the 0.26 μm brightness of the 1997 Pillan plume was attenuated by the eruption plume’s gas component, then an SO₂ gas density ～3–6 × 10¹⁸ cm^?2 is inferred from the data (for S₂/SO₂ ratios ?4%), comparable to the 0.3–2 × 10¹⁸ cm^?2 SO₂ density detected at Loki in 1979 (Pearl, J.C. et al. [1979]. Nature 280, 755; Lellouch et al., 1992), and producing an SO₂ gas mass ～3–8 × 10¹¹ g and an SO₂ resurfacing rate ～8–23 cm yr^?1. These results confirm the connection between high (?10¹⁷ cm^?2) SO₂ gas content and plumes that scatter strongly at nearly blue wavelengths, and it validates the occurrence of high density SO₂ gas eruptions on Io. Noting that the SO₂ gas content inferred from a spectrum of the 2003 Pillan plume was significantly lower ～2 × 10¹⁶ cm^?2 (Jessup, K.L., Spencer, J., Yelle, R. [2007]. Icarus 192, 24–40); and that the Pillan caldera was flooded with fresh SO₂ frost/slush just prior to the 1997 Pillan plume eruption (Geissler, P., McEwen, A., Phillips, C., Keszthelyi, L., Spencer, J. [2004a]. Icarus 169, 29–64; Phillips, C.B. [2000]. Voyager and Galileo SSI Views of Volcanic Resurfacing on Io and the Search for Geologic Activity at Europa. Ph.D. Thesis, Univ. of Ariz., Tucson); we propose that the density of SO₂ gas released by this volcano is directly linked to the local SO₂ frost abundance at the time of eruption.  相似文献