共查询到20条相似文献，搜索用时 31 毫秒
1.
Photoelectron flux in the energy range 6–70 eV coming from the sunlight conjugate ionosphere has been measured directly by the rocket borne low energy electron spectrometer in the altitude region of 210–350 km. Pitch angle distribution of the measured flux is nearly isotropic, the flux decreasing slightly with pitch angle. The photoelectron fluxes measured at 350 km at the energies of 15 and 30 eV are 3 × 106 and 1 × 106 (cm2 s str eV)?1 respectively which decrease to 1 × 106 and 1 × 105 at 250 km at the same energies. These values are consistent with the vertical profile of the 630 nm airglow intensity measured simultaneously. The fluxes obtained near apogee show peaks in the range 20–30 eV which also appear in the daytime photoelectron flux, indicating reduced loss of electrons during the passage from the conjugate ionosphere through the plasmasphere at the low geomagnetic latitude where observation was made. Photoelectron fluxes observed below the apogee height are compared to the calculated fluxes to investigate the interaction of electrons with the atmospheric species during the passage in the ionosphere. Calculated fluxes obtained by using continuous slowing-down approximation and neglecting pitch angle scattering are in good agreement with the observations although there still remain disagreements in detailed comparison which may be ascribed to the assumptions inherent in the calculation and/or to the uncertainties of the input data for the calculation. 相似文献
2.
《Planetary and Space Science》2007,55(14):2164-2172
Both the MARSIS ionospheric sounder and the charged particle instrument package ASPERA-3 are experiments on board the Mars Express spacecraft. Joint observations have shown that events of intense ionospheric electron density enhancements occur in the lower ionosphere of magnetic cusp regions, and that these enhancements are not associated with precipitation of charged particles above a few hundred electron volts (<300 eV). To account for the enhancement by particle precipitation, electron fluxes are required with mean energy between 1 and 10 keV. No ionizing radiation, neither energetic particles nor X-rays, could be identified, which could produce the observed density enhancement only in the spatially limited cusp regions. Actually, no increase in ionizing radiation, localized or not, was observed during these events. It is argued that the process causing the increase in density is controlled mainly by convection of ionosphere plasma driven by the interaction between the solar wind and crustal magnetic field lines leading to excitation of two-stream plasma waves in the cusp ionosphere. The result is to heat the plasma, reduce the electron–ion recombination coefficient and thereby increase the equilibrium electron density. 相似文献
3.
The plasma oscillations that can be observed by the French incoherent scatter system have small phase velocities and are excited by low energy photoelectrons, typically 2–5 eV. Consequently, the method used to determine the energy photoelectron distribution from plasma line measurements made at other observatories (e.g. Cicerone, 1974) cannot be applied here: it is necessary to chose a model energy distribution with a small number of parameters. The energy shape of the flux is assumed Maxwellian and the angular shape is assumed linear with the cosine of the pitch angle. Total flux values and mean energies are obtained as a function of altitude, in agreement with other determinations, and the difference between upshifted and downshifted plasma line intensities lead to the determination of the anisotropy of the photoelectron flux. 相似文献
4.
J.H. Whitteker 《Planetary and Space Science》1977,25(8):773-786
A numerical time-dependent model of the topside and F-layer ionosphere is used to describe how the density of O+ ions and the plasma temperatures change as a result of transient electron precipitation with a soft energy spectrum (ca. 100 eV per electron). The response time for electron gas heating is about 2 min; for changes in topside scale height it is from 5 to 15 min, depending on altitude; and for changes in F-layer peak density, it is more than an hour. The low-density topside ion gas is thermally isolated on a short time scale; consequently the ion temperature responds almost adiabatically to volume changes. A transient precipitation event (of, say, 10 min duration) initiates a disturbance that propagates upward at approximately the sonic upeed in the plasma (ca. 2km/s), growing in amplitude with height. Such an event has little effect on the density at the peak of the F layer. An element of ionosphere that drifts horizontally in an antisunward direction through the magnetospheric cleft and into the polar cap recieves some ionization from the cleft, but not enough to be decisive in its survival. The collapse of the topside when heating is removed increases temporarily the density of the F layer. 相似文献
5.
Determination of low energy photoelectron distribution from plasma line measurements at Saint Santin
The plasma lines observed by the French incoherent scatter radar during the period 1973–1974 are studied. Two methods are used to determine the steady-state photoelectron flux from plasma line measurements; one using a Maxwellian model for the photoelectron distribution and the other by solving (numerically) the differential equation that is satisfied by the distribution.The direct numerical calculation of the photoelectron flux is used to obtain theoretical kTp values which are compared with those from the plasma line observation. The comparison leads to the conclusion that there must be a sharp increase of the photoelectron flux when the energy decreases below 4 ～ 5 eV.This result, in agreement with rocket and satellite measurements of the low energy photoelectron flux, is used to bear a new insight to the problem of the electron-gas heat balance: the problem is reduced to the need of an additional photoelectron flux production below 5 eV. 相似文献
6.
P. R. Arendt 《Planetary and Space Science》1969,17(12):1993-1995
During the initial phase of magnetic storms with steep onset, the topside ionosphere shows enhancement of ionization above and depletion below a zone of unchanged ionization. During storms with a smoothly initiated disturbance, no enhancement of ionization is observed and depletion takes place at all altitudes. 相似文献
7.
George P. Mantas 《Planetary and Space Science》1981,29(12):1319-1323
The range of applicability of the transport equation for photoelectrons in the ionosphere (Mantas, 1975), is extended to thermal energies. The extension enables one to calculate the photoelectron distribution in the important low energy region (i.e. < 3 eV), where most of the thermal electron gas heating takes place, and thereby to obtain more accurate excitation rates for the low energy: electronic, vibrational and rotational states of the ionospheric constituents, than was formerly possible. 相似文献
8.
Steady-state calculations are performed for the daytime equatorial F2-region and topside ionosphere. Values are calculated of the electron and ion temperatures and the concentrations and field-aligned velocities of the ions O+, H+ and He+. Account is taken of upward drift, a summer-winter horizontal neutral air wind and heating of the electron gas by thermalization of fast photoelectrons.The calculated plasma temperatures are in accord with experiment: at the equator there is an isothermal region from about 400–550 km altitude, with temperatures of about 2400 K around 800 km altitude. The transequatorial O+ breeze flux from summer to winter in the topside ionosphere is not greatly affected by the elevated plasma temperatures. The field-aligned velocities of H+ and He+ depend strongly on the O+ field-aligned velocity and on the presence of large temperature gradients. For the minor ions, ion-ion drag with O+ cannot be neglected for the topside ionosphere. 相似文献
9.
The thermal response of the Earth's ionospheric plasma is calculated for various suddenly applied electron and ion heat sources. The time-dependent coupled electron and ion energy equations are solved by a semi-automatic computational scheme that employs Newton's method for coupled vector systems of non-linear parabolic (second order) partial differential equations in one spatial dimension. First, the electron and composite ion energy equations along a geomagnetic field line are solved with respect to a variety of ionospheric heat sources that include: thermal conduction in the daytime ionosphere; heating by electric fields acting perpendicular to the geomagnetic field line; and heating within a stable auroral red are (SAR-arc). The energy equations are then extended to resolve differential temperature profiles, first for two separate ion species (H+, O+) and then for four separate ion species (H+, He+, N+, O+) in addition to the electron temperature. The electron and individual ion temperatures are calculated for conditions within a night-time SAR-arc excited by heat flowing from the magnetosphere into the ionosphere, and also for typical midlatitude daytime ionospheric conditions. It is shown that in the lower ionosphere all ion species have the same temperature; however, in the topside ionosphere above about 400 km, ion species can display differential temperatures depending upon the balance between thermal conduction, heating by collision with electrons, cooling by collisions with the neutrals, and energy transfer by inter-ion collisions. Both the time evolution and steady-state distribution of such ion temperature differentials are discussed.The results show that below 300km both the electrons and ions respond rapidly (<30s) to variations in direct thermal forcing. Above 600 km the electrons and ions display quite different times to reach steady state, depending on the electron density: when the electron density is low the electrons reach steady state temperatures in 30 s, but typically require 700 s when the density is high; the ions, on the other hand, reach steady state in 700 s when the density is high, and 1500–2500 s when the density is low. Between 300 and 600 km, a variety of thermal structures can exist, depending upon the electron density and the type of thermal forcing; however steady state is generally reached in 200–1000 s. 相似文献
10.
A gridded spherical electrostatic analyzer aboard Injun 5 has been used to measure fluxes of thermal and hyperthermal electrons at subauroral latitudes in the midnight sector of the northern ionosphere between altitudes of 2500 and 850 km. Due to the offset between the geomagnetic and geographic poles hyperthermal fluxes, consisting of energetic photoelectrons that have escaped from the sunlit southern hemisphere are observed along orbits over the Atlantic Ocean and North America but not over Asia. The ambient electron temperatures (Te) near 2500 km have their highest values at trough latitudes for all longitudes. At altitudes near 1000 km elevated electron temperatures in the trough were not a consistent feature of the data. Equatorward of the trough, in the longitude sector to which conjugate photoelectrons have access, Te ～ 4000 K at 2500 km and ～ 3000 K at 1000 km. For regions with the conjugate point in darkness Te ? 2300 K over the 1000–2500 km altitude range. The effective thermal characteristics of conjugate photoelectrons are studied as functions of altitude and latitude. The observations indicate that (1) at trough latitudes elevated electron temperatures in the topside ionosphere are mostly produced by sources other than conjugate photoelectrons, and (2) at subtrough latitudes, in the Alantic Ocean-North American longitude sector, conjugate photoelectrons contribute significantly to the heating of topside electrons. Much of the conjugate photoelectron energy is deposited at altitudes >2500 km then conducted along magnetic field lines into the ionosphere. 相似文献
11.
The ambient photoelectron spectrum below 300 km has been studied for a sample of 500 AE-E orbits taken during the period 13 December 1975 to 24 February 1976. During this solar minimum period, the average and maximum Σ Kp were 19 and 35 respectively. The agreement between the measured spectral shape and several recent calculations is extremely good. The daytime photoelectron spectrum below 300 km from 1 to 100 eV is illustrated by a number of spectra. Detailed 0–32 eV spectra are presented at various altitudes and solar zenith angles. High resolution 10–32eV spectra show the widths of the photoelectron lines in the spectrum and the variation of the linewidth and intensity with altitude. Data from the entire 500 orbit sample are combined into plots of the average flux over a number of altitude ranges up to 300 km at various local times and solar zenith angles. The data show that the photoelectron flux below 300 km is remarkably constant (typical variation less than ±50%) over a period of several months. The photoelectron lines between 20 and 30 eV are extremely sharp when the total plasma density is low but broaden significantly at high altitudes as the plasma density builds up during the day. The N2 vibration-rotation excitation dip at 2.3 eV is strongest at the lowest altitudes and decreases with increasing altitude and plasma density. The absolute accuracy of the experiment is discussed in detail and a correction factor for previously published AE-E fluxes is given. 相似文献
12.
The total ion current probe on the satellite ESRO-4 monitored thermal plasma density variations in the range ± 30% of ambient density with a spatial resolution of about 1.5 km. Latitudinal, diurnal, and altitudinal characteristics of density irregularities in the topside ionosphere have been investigated using the 2 × 108 total ion current values recorded during the lifetime of the satellite. Dominating the morphology of topside irregularities is the high-latitude zone evident throughout the day, with the appearance of a distinct sub-auroral zone at night. Significant mid-latitude irregularity occurs at low altitudes during the night. The results reported here provide the most comprehensive study of topside ionospheric irregularities from direct probe measurements, and reveal new evidence on possible irregularity production mechanisms. 相似文献
13.
Michael W. Liemohn R.A. Frahm Y. Ma R. Lundin A.F. Nagy J. Bell D. Mitchell M. Holmström M. Yamauchi S. McKenna-Lawler J.R. Scherrer A.J. Coates D.O. Kataria H. Koskinen T. Säles W. Schmidt D. Williams C.C. Curtis B.R. Sandel M. Carter A. Fedorov S. Orsini M. Maggi P. Bochsler J. Woch K. Asamura 《Icarus》2006,182(2):383-395
The Electron Spectrometer (ELS) instrument of the ASPERA-3 package on the Mars Express satellite has recorded photoelectron energy spectra up to apoapsis (∼10,000 km altitude). The characteristic photoelectron shape of the spectrum is sometimes seen well above the ionosphere in the evening sector across a wide range of near-equatorial latitudes. Two numerical models are used to analyze the characteristics of these high-altitude photoelectrons. The first is a global, multi-species MHD code that produces a 3-D representation of the magnetic field and bulk plasma parameters around Mars. It is used here to examine the possibility of magnetic connectivity between the high-altitude flanks of the martian ionosheath and the subsolar ionosphere. It is shown that some field lines in this region are draped interplanetary magnetic lines while others are open field lines (connected to both the IMF and the crustal magnetic field sources). The second model is a kinetic electron transport model that calculates the electron velocity space distribution along a selected, non-uniform, magnetic field line. It is used here to simulate the high-altitude ELS measurements. It is shown that the photoelectrons are essentially confined to the source cone, as governed by magnetic field inhomogeneity along the field line. Reasonable agreement is shown between the data and the model results, and a method is demonstrated for inferring properties of the local and photoelectron source region magnetic field from the ELS measurements. Specifically, the number of sectors in which photoelectrons are measured is a function of the magnetic field intensity ratio and the field's angle with respect to the detector plane. In addition, the sector of the photoelectron flux peak is a function of the magnetic field azimuthal angle in the detector plane. 相似文献
14.
J. G. Doyle 《Solar physics》1983,89(1):115-131
A wavelength list is presented for the solar flare of 7 September, 1973 in the spectral range 1335 Å–380 Å. The ions observed suggest a range of temperatures in the flare plasma from 8 × 103 K to 107 K. This wavelength range contains many of the important electron density diagnostics lines for the solar transition zone and corona. The line list should also be of potential use in the identification and comparison with stellar spectra. 相似文献
15.
The fragmentation of SO2 following dissociative electron impact excitation has been studied under single collision conditions for incident electron energies up to 500 eV. The emission spectrum in the far v.u.v. spectral range (450–1100Å) shows many features arising from excited neutral oxygen and ionized oxygen and sulphur fragments. Absolute emission cross sections have been measured for the most intense lines and the maximum values were found to range from 1–12 × 10?19 cm2 with an uncertainty of approx. ± 35%. Dissociation mechanisms are discussed and in some cases the dissociation path could be uniquely identified. The striking differences between the v.u.v. emission spectrum produced by single step dissociation of SO2 and the spectra emitted by the plasma torus around Jupiter are discussed. 相似文献
16.
We have studied the extent to which certain transport processes affect ion composition and heat flow in the daytime, topside Venus ionosphere. Particular attention is given to the conditions that prevailed during the Mariner 5 measurements, at which time the topside Venus ionosphere appeared to be in a state of diffusive equilibrium. We have found that the ion composition is sensitive to the ion temperature, the ion temperature gradient, and to relative drifts between the ion species of a few . The electron density, on the other hand, is very insensitive to these parameters. As a consequence, ionospheric models of the topside Venus ionosphere are not likely to yield definitive information about the ion composition, the thermal structure or the flow conditions, since at present only electron density profiles are available for testing model predictions. We have also found that a relative drift between the ion species of a few induces an ion heat flow that is equivalent to a temperature gradient. This induced heat flow could influence the energy balance in the topside Venus ionosphere. 相似文献
17.
There is observational evidence showing that stellar and solar flares occur with a similar circumstance, although the former are usually much more energetic. It is expected that the bombardment by high-energy electrons is one of the chief heating processes of the flaring atmosphere. In this paper we study how a precipitating electron beam can influence the line profiles of Ly α , H α , Ca ii K and λ 8542. We use a model atmosphere of a dMe star and make non-LTE computations taking into account the non-thermal collisional rates owing to the electron beam. The results show that the four lines can be enhanced to different extents. The relative enhancement increases with increasing formation height of the lines. Varying the energy flux of the electron beam has different effects on the four lines. The wings of Ly α and H α become increasingly broad with the beam flux; change of the Ca ii K and λ 8542 lines, however, is most significant in the line centre. Varying the electron energy (i.e. the low-energy cut-off for a power-law beam) has a great influence on the Ly α line, but little on the H α and Ca ii lines. An electron beam of higher energy precipitates deeper, thus producing less enhancement of the Ly α line. The Ly α /H α flux ratio is thus sensitive to the electron energy. 相似文献
18.
R.G. Roble 《Planetary and Space Science》1975,23(7):1017-1033
A numerical model of current F-region theory is use to calculate the diurnal variation of the mid-latitude ionospheric F-region over Millstone Hill on 23–24 March 1970, during quiet geomagnetic conditions. From the solar EUV flux, the model calculates at each altitude and time step primary photoelectron spectra and ionization rates of various ion species. The photoelectron transport equation is solved for the secondary ionization rates, photoelectron spectra, and various airglow excitation rates. Five ion continuity equations that include the effects of transport by diffusion, magnetospheric-ionospheric plasma transport, electric fields, and neutral winds are solved for the ion composition and electron density. The electron and ion temperatures are also calculated using the heating rates determined from chemical reactions, photoelectron collisions, and magnetospheric-ionospheric energy transport. The calculations are performed for a diurnal cycle considering a stationary field tube co-rotating with the Earth; only the vertical plasma drift caused by electric fields perpendicular to the geomagnetic field line is allowed but not the horizontal drift. The boundary conditions used in the model are determined from the incoherent scatter radar measurements of Te, Ti and O+ flux at 800km over Millstone Hill (Evans, 1971a). The component of the neutral thermospheric winds along the geomagnetic field has an important influence on the overall ionospheric structure. It is determined from a separate dynamic model of the neutral thermosphere, using incoherent scatter radar measurements.The calculated diurnal variation of the ionospheric structure agrees well with the values measured by the incoherent scatter radar when certain restrictions are placed on the solar EUV flux and model neutral atmospheric compositions. Namely, the solar EUV fluxes of Hinteregger (1970) are doubled and an atomic oxygen concentration of at least at 120 km is required for the neutral model atmosphere. Calculations also show that the topside thermal structure of the ionosphere is primarily maintained by a flow of heat from the magnetosphere and the night-time F2-region is maintained in part by neutral winds, diffusion, electric fields, and plasma flow from the magnetosphere. The problem of maintaining the calculated night-time ionosphere at the observed values is also discussed. 相似文献
19.
Klaus Pinkau 《Experimental Astronomy》2009,25(1-3):157-171
Gamma-ray astronomy is devoted to study nuclear and elementary particle astrophysics and astronomical objects under extreme conditions of gravitational and electromagnetic forces, and temperature. Because signals from gamma rays below 1 TeV cannot be recorded on ground, observations from space are required. The photoelectric effect is dominant <100 keV, Compton scattering between 100 keV and 10 MeV, and electron–positron pair production at energies above 10 MeV. The sun and some gamma ray burst sources are the strongest gamma ray sources in the sky. For other sources, directionality is obtained by shielding / masks at low energies, by using the directional properties of the Compton effect, or of pair production at high energies. The power of angular resolution is low (fractions of a degree, depending on energy), but the gamma sky is not crowded and sometimes identification of sources is possible by time variation. The gamma ray astronomy time line lists Explorer XI in 1961, and the first discovery of gamma rays from the galactic plane with its successor OSO-3 in 1968. The first solar flare gamma ray lines were seen with OSO-7 in 1972. In the 1980’s, the Solar Maximum Mission observed a multitude of solar gamma ray phenomena for 9 years. Quite unexpectedly, gamma ray bursts were detected by the Vela-satellites in 1967. It was 30 years later, that the extragalactic nature of the gamma ray burst phenomenon was finally established by the Beppo–Sax satellite. Better telescopes were becoming available, by using spark chambers to record pair production at photon energies >30 MeV, and later by Compton telescopes for the 1–10 MeV range. In 1972, SAS-2 began to observe the Milky Way in high energy gamma rays, but, unfortunately, for a very brief observation time only due to a failure of tape recorders. COS-B from 1975 until 1982 with its wire spark chamber, and energy measurement by a total absorption counter, produced the first sky map, recording galactic continuum emission, mainly from interactions of cosmic rays with interstellar matter, and point sources (pulsars and unidentified objects). An integrated attempt at observing the gamma ray sky was launched with the Compton Observatory in 1991 which stayed in orbit for 9 years. This large shuttle-launched satellite carried a wire spark chamber “Energetic Gamma Ray Experiment Telescope” EGRET for energies >30 MeV which included a large Cesium Iodide crystal spectrometer, a “Compton Telescope” COMPTEL for the energy range 1–30 MeV, the gamma ray “Burst and Transient Source Experiment” BATSE, and the “Oriented Scintillation-Spectrometer Experiment” OSSE. The results from the “Compton Observatory” were further enlarged by the SIGMA mission, launched in 1989 with the aim to closely observe the galactic center in gamma rays, and INTEGRAL, launched in 2002. From these missions and their results, the major features of gamma ray astronomy are: Diffuse emission, i.e. interactions of cosmic rays with matter, and matter–antimatter annihilation; it is found, “...that a matter–antimatter symmetric universe is empirically excluded....” Nuclear lines, i.e. solar gamma rays, or lines from radioactive decay (nucleosynthesis), like the 1.809 MeV line of radioactive 26Al; Localized sources, i.e. pulsars, active galactic nuclei, gamma ray burst sources (compact relativistic sources), and unidentified sources. 相似文献
20.
Vertical profiles of electron density obtained in the vicinity of the plasmapause using the Alouette-II topside sounder have been analyzed to assess the presence of H+ flow in the topside ionosphere. The observations in the midnight sector show clearly the presence of the plasmapause; i.e. there is a sharp boundary separating the poleward regions of polar wind H+ flow and the more gentle conditions of the plasmasphere where light ions are present in abundance. In contrast, in the sunlit morning sector upwards H+ flow is deduced to be present to invariant latitudes as low as 48° (L = 2·2) in the regions normally known to be well inside the plasmasphere. The upwards H+ flux is sufficiently large (3 × 108 ions cm?2 sec?1) that the plasmapause cannot be seen in the latitudinal electron density contours of the topside ionosphere. The cause for this flow remains unknown but it may be a result of a diurnal refilling process. 相似文献