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1.
Several satellite experiments have measured the solar Lyman-α line, either in scattering from upper atmospheric atomic hydrogen (the Lyman-α airglow) or directly at line center (which determines the hydrogen column density along the line of sight). Recent analyses of data from the above experiments consistently reveal the presence of an atomic hydrogen depletion at high latitudes. In situ determinations of hydrogen at lower altitude show no evidence of such behaviour. This has led us to postulate two mechanisms which may be more effective in reducing the high-latitude density at the high altitudes of the exospheric measurements (500–2000 km). The first is the polar wind loss of protons, which depletes atomic hydrogen through a charge exchange reaction. The second is a high-latitude magnetospheric heating of protons, followed by charge exchange. Opposing the above loss mechanisms are the influences of ballistic lateral flow and mean meriodional winds. We have shown by means of a three-dimensional exospheric transport model that none of the above mechanisms can reconcile the disparate results in the two altitude regimes, nor can they provide the large outward hydrogen fluxes and the correct seasonal variations observed at high latitudes. 相似文献
2.
A mathematical model has been developed to calculate consistent values for the O+ and H+ concentrations and field-aligned velocities and for the O+, H+ and electron temperatures in the night-time equatorial topside ionosphere. Using the results of the model calculations a study is made to establish the ability of F-region neutral air winds to produce observed ion temperature distributions and to investigate the characteristics of ion temperature troughs as functions of altitude, latitude and ionospheric composition. Solar activity conditions that give exospheric neutral gas temperatures 600 K, 800 K and 1000 K are considered.It is shown that the O+-H+ transition height represents an altitude limit above which ion cooling due to adiabatic expansion of the plasma is extremely small. The neutral atmosphere imposes a lower altitude limit since the neutral atmosphere quenches any ion cooling which field-aligned transport tends to produce. The northern and southern edges of the ion temperature troughs are shown to be restricted to a range of dip latitudes, the limiting dip latitudes being determined by the magnetic field line geometry and by the functional form of the F-region neutral air wind velocity. Both these parameters considerably influence the interaction between the neutral air and the plasma within magnetic flux tubes. 相似文献
3.
Brian A. Tinsley 《Planetary and Space Science》1978,26(9):847-853
It is argued that there is a terrestrial loss of hydrogen as ions which includes the polar wind but extends effectively down to a latitude in the range 45–50° invariant. In daytime and for much of the night-time the flux is close to the limiting value for H+ flow through the topside ionosphere. It is argued that the flux decreases rapidly with increasing solar activity, following the decrease in neutral hydrogen concentration. It has been found that as solar activity increases the Jeans escape flux increases, and the charge exchange escape flux increases until moderate solar activity levels are reached. As solar activity increases from moderate to high levels, the charge exchange escape may decrease again. A new budget for terrestrial hydrogen loss over the solar cycle is given. The global flux of hydrogen ions outward from the ionosphere is comparable with estimates of the plasma sheet loss rates, and this flux, together with some solar wind plasma, is an attractive source for the plasma sheet.The energetic neutrals produced from the charge exchange of ring current ions with thermal-energy neutrals in the exosphere produce the optical emission of the equatorial aurora, which can be related to ion production rates near and above the E-region. The ionization production is adequate to explain the enhancements in ion production observed during magnetic storms at Arecibo. 相似文献
4.
Nonthermal escape processes responsible for the escape of hydrogen and deuterium from Venus are examined for present and past atmospheres. Three mechanisms are important for the escape of hydrogen from the present atmosphere: (a) charge exchange of plasmaspheric H+ with exospheric H, (b) impact of exospheric hot O atoms on H, and (c) ion molecule reactions involving O+ and H2. However, in the past when the H abundance was higher, the charge-exchange mechanism would be the strongest. The H escape flux increases rapidly with increasing hydrogen abundance in the upper atmosphere and saturates at a value of 1 × 1010 cm?2 sec?1 emerging primarily from the day side when the H mixing ratio at the homopause is 2 × 10?3. This corresponds to an H2O mixing ratio of 1 × 10?3 at the cold trap and ～15% at the surface. Deuterium would also escape by the charge-exchange mechanism and a D/H enrichment by a factor of ～1000 over the nonthermal escape regime is expected, which could have lasted over the last 3 billion years. Coincidentally, the onset of hydrodynamic flow leading to efficient H escape occurs just at the H2O mixing ratio at which the charge-exchange escape flux saturates. Thus it is possible that Venus has lost an Earth-equivalent ocean of water over geologic time. If so, either the D/H enrichment has been kept low by modest outgassing of juvenile water or Venus started out with a D/H ratio of ～4.0 × 10?6. 相似文献
5.
We present the first 3-dimensional self-consistent calculations of the response of Saturn's global thermosphere to different sources of external heating, giving local time and latitudinal changes of temperatures, winds and composition at equinox and solstice. Our calculations confirm the well-known finding that solar EUV heating alone is insufficient to produce Saturn's observed low latitude thermospheric temperatures of 420 K. We therefore carry out a sensitivity study to investigate the thermosphere's response to two additional external sources of energy, (1) auroral Joule heating and (2) empirical wave heating in the lower thermosphere. Solar EUV heating alone produces horizontal temperature variations of below 20 K, which drive horizontal winds of less than 20 m/s and negligible horizontal changes in composition. In contrast, Joule heating produces a strong dynamical response with westward winds comparable to the sound speed on Saturn. Joule heating alone, at a total rate of 9.8 TW, raises polar temperatures to around 1200 K, but values equatorward of 30° latitude, where observations were made, remain below 200 K due to inefficient meridional energy transport in a fast rotating atmosphere. The primarily zonal wind flow driven by strong Coriolis forces implies that energy from high latitudes is transported equatorward mainly by vertical winds through adiabatic processes, and an additional 0.29-0.44 mW/m2 thermal energy are needed at low latitudes to obtain the observed temperature values. Strong upwelling increases the H2 abundances at high latitudes, which in turn affects the H+3 densities. Downwelling at low latitudes helps increase atomic hydrogen abundances there. 相似文献
6.
The temporal response of ion and neutral densities to a geomagnetic storm has been investigated on a global scale with data from consecutive orbits of OGO-6 (>400km) for 4 days covering both magnetically quiet and disturbed conditions. The first response of the neutral atmosphere to the storm takes place in the H and He densities which start to decrease near the time of the storm sudden commencement. The maximum decreases in H and He were more than 40% of the normal density at high latitudes. A subsequent increase in O and N2 densities occurs about 8 hours later than the change in H and He densities, while the relative O and N2 density changes indicate a depletion of atomic oxygen in the lower thermosphere by more than a factor of two. The overall features of the change in the neutral atmosphere, especially the patterns of change for individual species, strongly support the physical picture that energy is deposited primarily at high latitudes during the storm, and the thermosphere structure changes through (1) heating of the lower thermosphere and (2) generation of large scale circulation in the atmosphere with upwelling at high latitudes and subsidence at the equator. The storm-time response of H+ occurs in two distinct regions separated by the low latitude boundary of the light ion trough. While on the poleward side of the boundary the H+ density decreases in a similar manner to the decrease in H density, on the equatorward side of the boundary the H+ decrease occurs about half a day later. It is shown that the decrease of H+ density is principally caused by the decrease in H density for both regions. The difference in H+ response between the two regions is interpreted as the difference in H+ dynamics outside and inside the plasmasphere. The O+ density shows an increase, the pattern of which is rather similar to that for O. Two possibilities for explaining the observed change in O+ density are suggested. One attributes the observed increase in O+ density to an increase in the plasma temperature during the storm. The other possibility is that the increase in the production rate of O+ due to an increase in O density exceeds the increase in the loss rate of O+ due to an increase in N2 density, especially around the time of sunrise. Hence the change in O+ density in the F-region may actually be controlled by the change in O density. 相似文献
7.
We analyze the angular structure of the 21-cm interstellar neutral hydrogen emission at six and seven declinations in the northern (published previously) and southern polar caps of the Galaxy (Galactic latitudes from ?40° to ?90°), respectively, with an extent of 90° in right ascension. The RATAN-600 radio telescope has a beam width averaged over these regions of 2.′0×30′. One-dimensional power spectra for the angular distribution of interstellar neutral hydrogen emission were computed in each 6.3-km s?1-wide spectral channel by using the standard Fast Fourier Transform (FFT) code and were smoothed over 1h in right ascension. The Galactic latitude dependence of the mean parameters for the sky distribution of H I line emission at high latitudes was found to correspond to the distribution of gas in the form of a flat layer only in the northern region, while in the southern cap, the gas distribution is much less regular. In addition, the mean H I radial velocities are negative everywhere (?3.7±3.0 km s?1 in the north and ?6.0±2.4 km s?1 in the south). The power spectra of the angular fluctuations in the range of angular periods from 10′ to 6° appear as power laws. However, the spectral indices change greatly over the sky: from ?3 to ?1.2; on average, as the Galactic latitude increases and the H I column density decreases, the fluctuation spectrum of the interstellar gas emission becomes flatter. In the northern polar region, this behavior is much more pronounced, which probably stems from the fact that the gas column density in the south is generally a factor of 2 or 3 higher than that in the north. Therefore, the spectra are, on average, also steeper in the south, but the dependence on Galactic latitude is weaker. Using simulations, we show that the observed power-law spectrum of the H I emission distribution can be obtained in terms of not only a turbulent, but also a cloud model of interstellar gas if we use our previous spectra of the diameters and masses of H I clouds. 相似文献
8.
A three-dimensional, time-dependent model of thermospheric dynamics has been used to interpret recent experimental measurements of high altitude winds by rocket-borne and ground-based techniques. The model is global and includes a self-consistent treatment of the non-linear, Coriolis and viscosity terms. The solar u.v. and e.u.v. energy input provides the major energy source for the thermosphere. Solar u.v. and e.u.v. heating appear to be inadequate to explain observed thermospheric temperatures if e.u.v. heating efficiency (ε) lies in the range 0.3 < ε < 0.35. If the recent solar e.u.v. data are correct, then a value of ε between 0.4 and 0.45 would bring fluxes and observed temperatures into agreement. The Heppner (1977) and Volland (1978) models of high-latitude electric field are used to provide sources of both momentum (via ion drag) and energy (via Joule heating). We find that the Heppner Model CO (equivalent to Volland Model 1) is most appropriate for very quiet geomagnetic conditions (Kp ? 2) while Model A (equivalent to Volland Model 2) provides the necessary enhancement at high latitudes for conditions of moderate activity (Kp ～ 4). Even with the addition of a polar electric field, there still appears to be a shortage of high-latitude energy input in that model winds tend to be 10 m s?1 poleward of observed winds under quiet or average geomagnetic conditions. This extra energy cannot be provided by enhancing the polar electric fields since the extra momentum would cause disagreement with the observed high latitude winds. High latitude particulate sources of relatively low energies, ~100 eV, seem the most likely candidates depositing their energy above about 200km. Relatively modest amounts of energy are then required, < 1010W global, to bring the model into agreement with both high- and mid-latitude neutral wind results. 相似文献
9.
M. Lockwood 《Planetary and Space Science》1982,30(6):595-609
Topside ionospheric profiles are used to study the upward field-aligned flow of thermal O+ at high latitudes. On the majority of the field lines outside the plasmasphere, the mean flux is approximately equal to the mean polar wind measured by spacecraft at greater altitudes. This is consistent with the theory of thermal light ion escape supported, via charge exchange, by upward O+ flow at lower heights. Events of larger O+ flow are detected at auroral latitudes and their occurrence is found to agree with that of transversely accelerated ions within the topside ionosphere and the magnetosphere. The effects of low altitude heating of O+ by oxygen cyclotron waves, driven by downward field-aligned currents, are considered as a possible common cause of these two types of event. 相似文献
10.
Current theoretical models do not satisfactorily explain observed variations of the global exospheric atomic hydrogen density distribution. Differences between the mesospheric upward flux and the Jeans escape mechanism indicate that other escape fluxes affect hydrogen density. Observations of latitudinal depletions and an early morning trough in the exosphere can be attributed to the influence of additional escape mechanisms. These variations of the exospheric hydrogen density distribution seem to be correlated with other observed variations in the atmosphere; however, no straightforward explanation has been proposed to date. 相似文献
11.
T.J. Fuller-Rowell D. Rees S. Quegan G.J. Bailey R.J. Moffett 《Planetary and Space Science》1984,32(4):469-480
The dynamics of the high latitude thermosphere are dominated by the ion circulation pattern driven by magnetospheric convection. The reaction of the neutral thermosphere is influenced by both the magnitude of the ion convection velocity and by the conductivity of the thermosphere. Using a threedimensional, time-dependent, thermospheric, neutral model together with different ionospheric models, the effect of changes in conductivity can be assessed. The ion density is described by two models: the first is the empirical model of Chiu (1975) appropriate for very quiet geomagnetic conditions, and the second is a modified version of the theoretical model of Quegan et al. (1982). The differences in the neutral circulation resulting from the use of these two ionospheric models emphasizes the need for realistic high latitude conductivities when attempting to model average or disturbed geomagnetic conditions, and a requirement that models should couple realistically the ionosphere and the neutral thermosphere. An attempt is made to qualitatively interpret some of the features of the neutral circulation produced at high latitudes by magnetospheric processes. 相似文献
12.
During moderate magnetic storms, changes in the neutral composition suggest that energy is deposited in the auroral zones. This results in thermal expansion (enhancement in N2, Ar) and consequent redistribution of the lighter species O and He such that their densities decrease at high latitudes and increase at low latitudes. From measurements obtained by the ESRO 4 gas analyzer during a major storm in late February 1973 (Kp = 7+) these typical high latitude characteristics were observed in the southern hemisphere and at certain longitudes to extend toward mid and low latitudes as far as ?20° invariant latitude. Further examination of these data for latitudes across the equator up to +20° latitude, however, shows evidence for an enhancement zone in He and O which is clearly displaced into the northern hemisphere thus suggesting a pronounced spherical asymmetry. Ground based observations on the state of the ionosphere between ±50° latitude confirm this asymmetry and suggest that the center of this enhancement zone occurs at about +15° invariant latitude. Adopting a suitable energy distribution in both hemispheres the magnetic storm response in the neutral composition is simulated with a circulation model. From this analysis it is concluded that for some longitudes a difference of a factor of two or more between the heating rates of the northern and southern hemispheres is required to match the ESRO-4 data. 相似文献
13.
A survey of metallic ions detected by the Bennett Ion Mass Spectrometers flown on the Atmosphere Explorer satellites, including both circular and eccentric orbital configurations, shows that patches of these ions of meteoric origin are frequently present during magnetically active periods on the bottomside of the F-layer at middle and high latitudes. In particular the F-region metals statistically tend to appear at night in the vicinity of the main ionospheric trough (in a band of invariant latitudes approx. 10 degrees wide) and on the day side of the polar cap. These distributions were previously associated with the expected dynamics of ions in the F-region above 140 km where meridional neutral wind drag and convection electric fields are the dominant ion transport mechanisms. However, the main meteor deposition layer—the presumed source region of the metals—is located below 100 km where these transport mechanisms do not prevail. It is demonstrated that the Pedersen ion drifts driven by intense electric fields such as those associated with sub-auroral ion drifts (SAID) are sufficient to transport the long-lived metallic ions upward from the main meteor layer to altitudes where the drag of equatorial directed neutral winds and electric field convection can support them against the downward pull of gravity and transport them to other locations. The spatial and temporal distribution of the middle and high latitude F-region metals are consistent with the known characteristics of the electric fields and with the expected F-region ion dynamics. 相似文献
14.
Brian A. Tinsley 《Planetary and Space Science》1973,21(4):686-691
This note critically examines the relative importance of several effects which influence the diurnal variation of atomic hydrogen abundance near the critical level.It is pointed out that the neglect of exospheric hydrogen in a recent theoretical treatment causes an overestimation of the diurnal variation at high exospheric temperatures, and an underestimation at low exospheric temperatures. The fluxes due to lateral flow are large compared to other fluxes only to the extent that the actual diurnal variation is very different from the diurnal variation corresponding to zero net lateral flow, which does not seem to be the case in the real atmosphere. Two effects which are probably important are charge exchange reactions with thermal oxygen ions, resulting in a diurnal exchange with the plasmasphere; and charge exchange reactions with high velocity protons, resulting in enhanced escape and diurnal variation. 相似文献
15.
Two extreme ultraviolet (EUV) spectrophotometers flown in December 1978 on Venera 11 and Venera 12 measured the hydrogen Lyman α emission resonantly scattered in the atmosphere of Venus. Measurements were obtained across the dayside of the disk, and in the exosphere up to 50,000 km. They were analyzed with spherically symmetric models for which the radiative transfer equation was solved. The H content of the Venus atmosphere varies from optically thin to moderately thick regions. A shape fit at the bright limb allows one to determine the exospheric temperature Tc and the number density nc independently of the calibration of the instrument or the exact value of the solar flux. The dayside exospheric temperature was measured for the first time in the polar regions, with Tc = 300 ± 25°K for Venera 11 (79°S) and Tc = 275 ± 25°K (59°S) for Venera 12. At the same place, the density is nc = 4?2+3 × 104 atom.cm?3, and the integrated number density Nt from 250 to 110 km (the level of CO2 absorption) is 2.1 × 1012 atom.cm?2, a factor of 3 to 6 lower than that predicted in aeronomical models. This probably indicates that the models should be revised in the content of H-bearing molecules and should include the effect of dynamics. Across the disk the value of Nt decreases smoothly with a total variation of two from the morning side to the afternoon side. Alternately it could be a latitude effect, with less hydrogen in the polar regions. The nonthermal component if clearly seen up to 40,000 km of altitude. It is twice as abundant as at the time of Mariner 10 (solar minimum). Its radial distribution above 4000 km can be simulated by an exospheric distribution with T = 1030K and n = 103 atom.cm?3 at the exobase level. However, there are less hot atoms between 2000 and 4000 km than predicted by an ionospheric source. A by-product of the analysis is a determination of a very high solar Lyman α flux of 7.6 × 1011 photons (cm2 sec Å)?1 at line center (1 AU) in December 1978. 相似文献
16.
We have studied the escape of neutral helium from the terrestrial atmosphere through exothermic charge exchange reactions between He+ ions and the major atmospheric constituents N2, O2, and O. Elastic collisions with the neutral background particles were treated quantitatively using a recently developed kinetic theory approach. An interhemispheric plasma transport model was employed to provide a global distribution of He+ ions as a function of altitude, latitude and local solar time and for different levels of solar ionization. Combining these ion densities with neutral densities from an MSIS model and best estimates for the reaction rate coefficients of the charge exchange reactions, we computed the global distribution of the neutral He escape flux. The escape rates show large diurnal and latitudinal variations, while the global average does not vary by more than a factor of three over a solar cycle. We find that this escape mechanism is potentially important for the overall balance of helium in the Earth's atmosphere. However, more accurate values for the reaction rate coefficients of the charge exchange reactions are required to make a definitive assessment of its importance. 相似文献
17.
《Planetary and Space Science》2006,54(13-14):1457-1471
Observations of oxygen pickup ions by the plasma analyzer on the Pioneer Venus Orbiter (PVO) Mission arguably launched broad interest in solar wind erosion of unmagnetized planet atmospheres, and its potential evolutionary effects. Oxygen pickup ions may play key roles in the removal of the oxygen excess left behind from the photodissociation of water vapor by enabling direct escape, additional sputtering of oxygen when they impact the exobase, and escape as energetic neutrals produced in charge exchange reactions with the ambient exospheric oxygen and hydrogen. Although the PVO observations were compromised by an ∼8 keV energy limit for O+ detection, a lack of ion composition capability, and the limited sampling and data rate of the plasma analyzer which was designed for solar wind monitoring, these measurements provide our best information about the extended O+ exosphere and wake at Venus. Here we show the full picture of the spatial distribution and energies of the O+ ion observations collected by the plasma analyzer during PVO's ∼5000 orbit tour. A model of O+ test particles launched in the circum-Venus fields described by an MHD simulation of the solar wind interaction is used to help interpret the PVO observations and to anticipate the expanded view of Venus O+ escape that will be provided by the ASPERA-4 experiment on Venus Express. 相似文献
18.
Mercury's neutral sodium exosphere is simulated using a comprehensive 3D Monte Carlo model following sodium atoms ejected from Mercury's surface by thermal desorption, photon stimulated desorption, micro-meteoroid vaporization and solar wind sputtering. The evolution of the sodium surface density with respect to Mercury's rotation and its motion around the Sun is taken into account by considering enrichment processes due to surface trapping of neutrals and ions and depletion of the sodium available for ejection from the surfaces of grains. The change in the sodium exosphere is calculated during one Mercury year taking into account the variations in the solar radiation pressure, the photo-ionization frequency, the solar wind density, the photon and meteoroid flux intensities, and the surface temperature. Line-of-sight column densities at different phase angles, the supply rate of new sodium, average neutral and ion losses over a Mercury year, surface density distribution and the importance of the different processes of ejection are discussed in this paper. The sodium surface density distribution is found to become significantly nonuniform from day to night sides, from low to high latitudes and from morning to afternoon because of rapid depletion of sodium atoms in the surfaces of grains mainly driven by thermal depletion. The shape of the exosphere, as it would be seen from the Earth, changes drastically with respect to Mercury's heliocentric position. High latitude column density maxima are related to maxima in the sodium surface concentration at high latitudes in Mercury's surface and are not necessarily due to solar wind sputtering. The ratio between the sodium column density on the morning side of Mercury's exosphere and the sodium column density on the afternoon side is consistent with the conclusions of Sprague et al. (1997, Icarus 129, 506-527). The model, which has no fitting parameters, shows surprisingly good agreement with recent observations of Potter et al. (2002, Meteor. Planet. Sci. 8, 3357-3374) successfully explaining their velocity and column density profiles vs. heliocentric distance. Comparison with this data allows us to constrain the supply rate of new sodium atoms to the surface. We also discuss the possible origins of the strong high latitude emissions (Potter and Morgan, 1990, Science 248, 835-838; 1997a, Adv. Space Res. 19, 1571-1576; 1997b, Planet. Space Sci. 45, 95-100; Sprague et al., 1998, Icarus 135, 60-68) and the strong variations of the total content of the sodium exosphere on short (Potter et al., 1999, Planet. Space Sci. 47, 1441-1449) and long time scales (Sprague et al., 1997, Icarus 129, 506-527). 相似文献
19.
Ion cyclotron waves are generated in the solar wind when it picks up freshly ionized planetary exospheric ions. These waves grow from the free energy of the highly anisotropic distribution of fresh pickup ions, and are observed in the spacecraft frame with left-handed polarization and a wave frequency near the ion’s gyrofrequency. At Mars and Venus and in the Earth’s polar cusp, the solar wind directly interacts with the planetary exospheres. Ion cyclotron waves with many similar properties are observed in these diverse plasma environments. The ion cyclotron waves at Mars indicate its hydrogen exosphere to be extensive and asymmetric in the direction of the interplanetary electric field. The production of fast neutrals plays an important role in forming an extended exosphere in the shape and size observed. At Venus, the region of exospheric proton cyclotron wave production may be restricted to the magnetosheath. The waves observed in the solar wind at Venus appear to be largely produced by the solar-wind-Venus interaction, with some waves at higher frequencies formed near the Sun and carried outward by the solar wind to Venus. These waves have some similarity to the expected properties of exospherically produced proton pickup waves but are characterized by magnetic connection to the bow shock or by a lack of correlation with local solar wind properties respectively. Any confusion of solar derived waves with exospherically derived ion pickup waves is not an issue at Mars because the solar-produced waves are generally at much higher frequencies than the local pickup waves and the solar waves should be mostly absorbed when convected to Mars distance as the proton cyclotron frequency in the plasma frame approaches the frequency of the solar-produced waves. In the Earth’s polar cusp, the wave properties of ion cyclotron waves are quite variable. Spatial gradients in the magnetic field may cause this variation as the background field changes between the regions in which the fast neutrals are produced and where they are re-ionized and picked up. While these waves were discovered early in the magnetospheric exploration, their generation was not understood until after we had observed similar waves in the exospheres of Mars and Venus. 相似文献
20.
Energetic ion (E ? 290 keV) and electron (Ee ? 220 keV) burst intensities were simultaneously monitored at various regions of the plasma sheet and magnetosheath by the CPME JHU/APL instruments on board the IMP-7 and 8 s/c during an extended period from day 250, 1975 to day 250, 1976 when the two spacecraft were closely trailing each other in crossing the geomagnetotail. The energy spectra of the energetic particle populations of different regions in the magnetotail were also computed and monitored simultaneously at the positions of the two spacecraft. The results indicate that the energetic particle intensities are higher and the energy spectra in general considerably softer inside the plasma sheet than the adjacent magnetosheath. The spectral index γ of a power law fit in the computed energy spectrum inside the plasma sheet occasionally exceeds γ > 10 for the ions and γ > 6 for the electrons. Furthermore simultaneous monitoring of particle intensities in the vicinity of the neutral sheet and the high latitude plasma sheet shows higher intensities in the former region. The observations suggest that the energetic particles escape to the magnetosheath from their source inside the plasma sheet by a rigidity dependent process. A dawn-dusk asymmetry in the particle acceleration and escape processes is implied in the observations and discussed in detail. 相似文献