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1.
The sputtering and decomposition of the surface of Europa by fast ions and electrons lead to the production of an atomosphere containing sodium and potassium atoms. Here time-of-flight energy distributions are measured for Na and K sputtered from a vapor-deposited ice by 200-eV electrons. These data are then used in a Monte Carlo simulation for alkalis in Europa's atmosphere. Na/K ratios versus distance from Europa are calculated and compared to the recent observations in the range 6 to 18 Europan radii from the surface. Normalizing to the observations, the Na/K ratio for the loss rates is ∼27 and the ratio for the average surface source rates is ∼20. These ratios are very different from the Na/K ratio at Io and are larger than the Na/K ratio suggested for Europa's putative subsurface ocean, consistent with fractionation on freezing and upwelling of ocean material. 相似文献
2.
The ambient photoelectron spectrum above 300 km has been measured for a sample of 500 AE-E orbits during the period 13 December 1975 to 24 February 1976 corresponding to solar minimum conditions. The 24 h average and maximum ΣKp were 19 and 35, respectively. The photoelectron flux above 300 km was found to have an intensity and energy spectrum characteristic of the 250–300 km production region only when there was a low plasma density at the satellite altitude. Data taken at local times up to 3 h after sunrise were of this type and the escaping flux was observed to extend to altitudes above 900 km with very little modification, as predicted by several theoretical calculations. The flux at high altitudes was found to be extremely variable throughout the rest of the day, probably as a result of attenuation and energy loss to thermal plasma along the path of the escaping photoelectrons. This attenuation was most pronounced where the photoelectrons passed through regions of high plasma density associated with the equatorial anomaly. At altitudes of 600 km, the photoelectron fluxes ranged from severely attenuated to essentially unaltered—depending on the specific conditions, Photoelectron fluxes from conjugate regions were often less attenuated than those observed arriving from the high density regions immediately below. Comparison of the observed attenuations, photoelectron line broadening, and energy loss due to coulomb scattering from the thermal plasma with rough calculations based on stopping power and transmission coefficients of thermal plasma for fast electrons yielded order of magnitude agreement—satisfactory in view of the large number of assumptions necessary for the calculations. Overall, the impression of the high altitude photoelectron flux which emerges from this work is that the fluxes are extremely variable as a consequence of interactions with the thermal plasma whose density is in turn affected by electrodynamic and neutral wind processes in the underlying F region. 相似文献
3.
Ambient thermal electrons are found to be heated to temperatures as high as 105 K by the passage of a field-aligned beam of suprathermal electrons through the ionosphere at altitudes over 660 km. These secondary electrons will increase the proportion of 630 nm emission, caused by the primary electron precipitation, and change the secondary electron spectrum observed at lower altitudes from that expected on the basis of atmospheric collisions alone. 相似文献
4.
Conventional ablation theory assumes that a meteoroid undergoes intensive heating during atmospheric flight and surface atoms
are liberated through thermal processes. Our research has indicated that physical sputtering could play a significant role
in meteoroid mass loss. Using a 4th order Runge-Kutta numerical integration technique, we tabulated the mass loss due to the
two ablation mechanisms and computed the fraction of total mass lost due to sputtering. We modeled cometary structure meteoroids
with masses ranging from 10−13 to 10−3 kg and velocities ranging from 11.2 to 71 km s−1. Our results indicate that a significant fraction of the mass loss for small, fast meteors is due to sputtering, particularly
in the early portion of the light curve. In the past 6 years evidence has emerged for meteor luminosity at heights greater
than can be explained by conventional ablation theory. We have applied our sputtering model and find excellent agreement with
these observations, and therefore suggest that sputtered material accounts for the new type of radiation found at great heights. 相似文献
5.
We have used a 2-D microphysics model to study the effects of atmospheric motions on the albedo of Titan's thick haze layer. We compare our results to the observed variations of Titan's brightness with season and latitude. We use two wind fields; the first is a simple pole-to-pole Hadley cell that reverses twice a year. The second is based on the results of a preliminary Titan GCM. Seasonally varying wind fields, with horizontal velocities of about 1 cm sec-1 at optical depth unity, are capable of producing the observed change in geometric albedo of about 10% over the Titan year. Neither of the two wind fields can adequately reproduce the latitudinal distribution of reflectivity seen by Voyager. At visible wavelengths, where only haze opacity is important, upwelling produces darkening by increasing the particle size at optical depth unity. This is due to the suspension of larger particles as well as the lateral removal of smaller particles from the top of the atmosphere. At UV wavelengths and at 0.89 micrometers the albedo is determined by the competing effects of the gas the haze material. Gas is bright in the UV and dark at 0.89 micrometers. Haze transport at high altitudes controls the UV albedo and transport at low altitude controls the 0.89 micrometers albedo. Comparisons between the hemispheric contrast at UV, visible, and IR wavelengths can be diagnostic of the vertical structure of the wind field on Titan. 相似文献
6.
We have followed the transit of two active regions across the western solar limb during June 29 through July 2, 1980, as imaged in 3.5–16 keV X-rays by HXIS aboard the SMM. During frequent brightenings of large-scale coronal structures, hard X-ray emission in the 11–16 keV energy band was recorded up to altitudes of 76 000 km. Soft X-rays could be seen in excess of 250 000 km altitude above the photospheric active region. Many X-ray brightenings low in the corona in the active regions were followed by enhancements high in the corona in the large-scale coronal structures. Although subsequent enhancements rarely appeared in the same position, similar portions of the corona brightened intermittently, indicating that the general configuration of the coronal structures above the active regions did not change much, in spite of the frequent energy inputs. These inputs were of two kinds: nonthermal, with very fast response at high coronal altitudes within seconds or tens of seconds, and thermal, with a delay of several minutes. The nonthermal response is short-lived, reflecting the time profile of the primary source; the thermal response is more gradual and longer lasting than the primary source. In some enhancements of large-scale coronal structures both these kinds of response occur and can be clearly recognized. There are also active-region hrightenings without any response in the high corona and,vice versa, high-corona brightenings without any obvious primary source; in the latter case, it is likely that the source was hidden behind the limb. 相似文献
7.
Olga Popova 《Earth, Moon, and Planets》2004,95(1-4):303-319
The fate of entering meteoroids in atmosphere is determined by their size, velocity and substance properties. Material from
ablation of small-sized meteors (roughly R≤0.01–1 cm) is mostly deposited between 120 and 80 km altitudes. Larger bodies (up to meter sizes) penetrate deeper into the
atmosphere (down to 20 km altitude). Meteoroids of cometary origin typically have higher termination altitude due to substance
properties and higher entry velocity. Fast meteoroids (V>30–40 km/s) may lose a part of their material at higher altitudes due to sputtering. Local flow regime realized around the
falling body determines the heat transfer and mass loss processes. Classic approach to meteor interaction with atmosphere
allows describing two limiting cases: – large meteoroid at relatively low altitude, where shock wave is formed (hydrodynamical
models); – small meteoroid/or high altitudes – free molecule regime of interaction, which assumes no collisions between evaporated
meteoroid particles. These evaporated particles form initial train, which then spreads into an ambient air due to diffusion.
Ablation models should make it possible to describe physical conditions that occur around meteor body. Several self-consistent
hydrodynamical models are developed, but similar models for transition and free molecule regimes are still under study. This
paper reviews existing ablation models and discusses model boundaries. 相似文献
8.
High energy neutrinos play a very important role for the understanding of the origin and propagation of ultra high energy cosmic rays (UHECR). They can be produced as a consequence of the hadronic interactions suffered by the cosmic rays in the acceleration regions, as by products of the propagation of the UHECR in the radiation background and as a main product of the decay of super heavy relic particles. A new era of very large exposure space observatories, of which the JEM-EUSO mission is a prime example, is on the horizon which opens the possibility of neutrino detection in the highest energy region of the spectrum. In the present work we use a combination of the PYTHIA interaction code with the CONEX shower simulation package in order to produce fast one-dimensional simulations of neutrino initiated showers in air. We make a detail study of the structure of the corresponding longitudinal profiles, but focus our physical analysis mainly on the development of showers at mid and high altitudes, where they can be an interesting target for space fluorescence observatories. 相似文献
9.
Choosing an appropriate neutral-density model of martian atmosphere, the photoionization processes have been studied. Using relevant charge transfer and recombination reactions, the equilibrium electron-ion production rates have been computed for all species. The resultant electron-density profiles are compared with the ones measured experimentally by Viking 1 and 2. The theoretical electron-density profile compares well with the measured profiles at low altitudes, but shows rather large values at higher altitudes. The observed oscillation in the electrondensity profile at high altitudes is attributed to the variable nature of impact ionization produced by suprathermal charged particles of solar wind origin. 相似文献
10.
A.J. Baxter 《Planetary and Space Science》1975,23(6):973-983
A series of barium release experiments have been conducted at altitudes near 160 km from the R.A. Hebrides Range (L = 3.5). A predominantly westward neutral drift has been observed for all the releases conducted during evening twilight. The development of the ionized clouds are characterized, in this situation, by a lack of separation from the neutral release material and by a rapid onset of the formation of structure. The implications of these features are discussed and numerical modelling used to describe the steepening and striation onset in the ion clouds. It is concluded that the most likely striation sizes are those which are of sufficiently small scale to be electrostatically isolated from other regions of the ionosphere but not so small as to be diffusively dispersed. 相似文献
11.
Robert J. Stefant 《Planetary and Space Science》1985,33(3):333-349
The problem of the propagation of an electromagnetic wave originating for instance in a lightning flash through the ionospheric medium is analysed in order to understand the formation at high ionospheric altitudes of the so-called proton whistler. It is shown that the accessibility of the hydrodynamic (or kinetic) proton resonance at the satellite altitude requires that a mode conversion process must take place slightly above the transition region separating the one ion (O+) from the two ion (O+ + H+) component plasmas. Moreover, the transformation conditions in the wave conversion region imply that the magnetic field should be (almost) perpendicular to the density gradient. Otherwise, the incident electromagnetic wave will never reach the satellite altitude in the frequency range of the proton whistler. However, some former proton whistler theories have postulated that the signal is the result of simple ionospheric propagation effects, in contradiction with the above results. These former proton whistler theories are reviewed and it is shown that the basic flaw in these theories lies in that the incident electromagnetic wave has been supposed from the beginning to have reached the high ionospheric altitudes where is located the satellite without being influenced by the lower ionospheric layers. Some various aspects, like the high variability of the wave electric to magnetic field ratio and the harmonics bands as observed by Injun are analysed in the light of the obtained results. Finally, numerical solutions of the wave dispersion relation for both the fast hydrodynamic mode (the extraordinary mode) and the slow ion kinetic mode are presented which shows that a coupling process between the two modes may take place at various frequencies between the O+ and the H+ gyrofrequencies. 相似文献
12.
Observations from the high resolution spherical electrostatic analyzer experiment aboard ISIS 1 have been used to study large amplitude irregularities at low latitudes in the tipside ionosphere. The irregularities appeared as plasma depletions near the magnetic equator and were observed up to satellite apogee (3500 km). The altitude local time distribution of the depletions was such that those at altitudes greater than 2000 km were found only in the post-midnight sector. This result agrees with the predictions of a model for plasma bubbles drifting under the influence of gravity-buoyancy forces. Evidence is presented that the initial steep gradients observed at low altitudes are reduced by anomalous diffusion due to drift waves. 相似文献
13.
We report on observations of Neptune from the 10-meter W.M. Keck II Telescope on June 17-18 (UT) 2000 and August 2-3 (UT) 2002 using the adaptive optics (AO) system to obtain a spatial resolution of 0.06 arcseconds. With this spatial resolution we can obtain spectra of individual bright features on the disk of Neptune in a filter centered near 2 microns. The use of a gas-only, simple reflecting layer radiative transfer model allows us to estimate the best fit altitudes of 18 bright features seen on these 4 nights and to set a constraint on the fraction of hydrogen in ortho/para equilibrium. On these nights there were three main types of features observed: northern hemisphere features in the range from +30 to −45 degrees; southern hemisphere features in the range from −30 to −50 degrees; and small southern features at −70 degrees. We find that the altitudes of the northern features are in the range from 0.023-0.064 bar, which places them in Neptune's stratosphere. Southern features at −30 to −50 degrees are mainly at altitudes from 0.10 to 0.14 bars. The small features at −70 degrees are somewhat deeper in the upper troposphere, at 0.17 and 0.27 bars. This pattern of features located at higher altitudes in the northern hemisphere and lower altitudes in the south has also been noted by previous observers. The best fits for all the observed spectra give a value of 1.0 for the fraction of hydrogen in ortho/para equilibrium; the value of the helium fraction is less well constrained by the data at 0.24. We suggest that the southern mid-latitude features are methane haze circulated up from below, while the −70° features may be isolated areas of upwelling in a general area of subsidence. Northern bright features may be due to subsidence of stratospheric haze material rather than upwelling and condensation of methane gas. We suggest that convection efficiently transports methane ice clouds to the tropopause in the Southern mid latitudes and thus plays a key role in the stratospheric haze production cycle. 相似文献
14.
Orbiter ion mass spectrometer measurements, as available in the UADS data files are used to study the response of dayside Venus ions at various altitudes to solar wind dynamic pressure, P
sw. Ion densities below about 200 km are not affected by changes in P
sw. At altitudes above 200 km the ions get abruptly depleted with increase in P
sw, and this abrupt depletion occurs at lower altitudes when P
sw is high. At lower P
sw, the depletion occurs at higher altitudes. The effect is similar for all ions. These results are also compared with the empirical relationship observed by Brace et al. (1980) between the ionopause altitude and P
sw from electron density measurements on orbiter electron temperature probe. 相似文献
15.
We examine the effects of Io ejecta on the surface and environment of Europa. We find that the observed sulfur on the trailing side of Europa, when interpreted as a deposit in equilibrium between implanation of, and sputtering by, corotating Io ejecta, implies a slow loss of material from Europa by sputtering. From this we infer that the spectrum of particles sputtered from water ice is soft. The quantity of observed sulfur and its confinement to the trailing hemisphere appear to exclude significant implantation and sputtering by energetic heavy ions. We also conclude that the contribution from Europa to the magnetospheric plasma (even at Europa itself) is negligible compared to the matter ejected from Io. 相似文献
16.
17.
The mechanisms of the global circulation in the atmosphere of Venus have been studied with the use of numerical models. To calculate the heating/cooling of the atmosphere due to absorption/emission of electromagnetic radiation under initially weak and strong superrotation of the atmosphere, the complete system of gas dynamics equations in the relaxation approximation was considered. It has been shown that at sufficiently high rates of heating of the atmosphere by radiation on the day side and at sufficiently high rates of cooling on the night side, a thermal tide develops at altitudes of 40?C70 km, and its energy and impulse is transferred to the zonal superrotation of the atmosphere. Due to the interaction between the superrotation and the meridional transfer of the air mass through the polar region from the day side of the planet to the night side, near-polar vortices are formed at altitudes of 40?C70 km near the morning terminator. 相似文献
18.
The energization of charged particles, due to interaction with the ambient electromagnetic turbulence, has a significant influence
on the plasma transport in space. The effect of wave-particle interactions on the outflow characteristics of polar wind plasma
was investigated. The theoretical model included gravitational acceleration (g), polarization electrostatic field (Ep), and divergence of the geomagnetic field. Within the simulation region (1.7 to 10
earth radii, Re) the ions were assume to be collisionless and the electrons to obey a Boltzmann relation. Profiles of altitude-dependent
diffusion coefficients [D⊥ (O+) and D⊥ (H+)] were computed from the wave spectral density (S) observed by the Plasma Wave
Instrument (PWI) on board DE-1. The effects of WPI were introduced via a Monte Carlo technique, and an iterative approach
was used in order to converge to self-consistent results. The main conclusions of this study were the following. As a result
of perpendicular heating, the temperature anisotropy (T| /T⊥) was reduced and even reversed (T| < T⊥) at high altitudes. The
O+ velocity distribution function developed a conic shape at high altitudes. The altitudes above which the WPI influences
the O+ ions were lower than those for the H+ ions. The escape flux of O+ could be enhanced by more than an order of magnitude
while the H+ flux remains constant. The O+ ions are heated more efficiently than the H+ ions, especially at low altitudes
due to the 'pressure cooker' effect. As the ions are heated and move to higher altitudes, the ion's Larmor radius a
L
may become comparable to the perpendicular wavelength λ⊥. As the ratio aL /λ⊥ becomes > 1, the heating rate becomes self-limited
and the ion distribution displays toroidal features. This result is consistent with the observation of O+ toroidal distribution
in the high altitude ionosphere. Finally, the large variability in the wave spectral density S was studied. This variability
was found to change our results only in a quantitative manner, while our conclusions remained qualitatively unchanged.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
For application to studies of the high latitude ionosphere, we have calculated ion velocity distributions for a weekly-ionized plasma subjected to crossed electric and magnetic fields. An exact solution to Boltzmann's equation has been obtained by replacing the Boltzmann collision integral with a simple relaxation model. At altitudes above about 150 km, where the ion collision frequency is much less than the ion cyclotron frequency, the ion distribution takes the shape of a torus in velocity space for electric fields greater than 40 mV m?1. This shape persists for 1–2 hr after application of the electric field. At altitudes where the ion collision and cyclotron frequencies are approximately equal (about 120 km), the ion velocity distribution is shaped like a bean for large electric field strengths. This bean-shaped distribution persists throughout the lifetime of ionospheric electric fileds. These highly non-Maxwellian ion velocity distributions may have an appreciable affect on the interpretation of ion temperature measurements. 相似文献
20.
S. G.Coulson 《Monthly notices of the Royal Astronomical Society》2002,332(3):741-744
Earlier work on the resistance acting on a small sphere moving through a gas is reviewed. A model for the resistance encountered by a sphere, the surface molecules of which are sputtered off during collisions with the gas molecules, is derived and compared with the case of specular reflection. The sputtering model is applied to the case of small 10-μm radius meteoroids entering the Earth's atmosphere. A possible link between the results obtained and the recent discovery of unheated, organic grains at an altitude of 40 km in the Earth's atmosphere is considered. 相似文献