共查询到20条相似文献，搜索用时 31 毫秒
1.
David D. Morgan Donald A. Gurnett J. David Winningham David A. Brain Janet G. Luhmann Jared R. Espley Jeffrey J. Plaut 《Icarus》2010,206(1):95-103
Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is a subsurface and topside ionosphere radar sounder aboard the European Space Agency spacecraft Mars Express, in orbit at Mars since 25 December 2003, and in operation since 17 June 2005. The ionospheric sounding mode of MARSIS is capable of detecting the reflection of the sounding wave from the martian surface. This ability has been used in previous work to show that the surface reflection is absorbed and disappears during periods when high fluxes of energetic particles are incident on the ionosphere of Mars. These absorption events are believed to be the result of increased collisional damping of the sounding wave, caused by increased electron density below the spacecraft, in turn caused by impact ionization from the impinging particles. In this work we identify two absorption events that were isolated during periods when the surface reflection is consistently visible and when Mars is nearly at opposition. The visibility of the surface reflection is viewed in conjunction with particle and photon measurements taken at both Mars and Earth. Both absorption events are found to coincide with Earth passing through solar wind speed and ion flux signatures indicative of a corotating interaction region (CIR). The two events are separated by an interval of approximately 27 days, corresponding to one solar rotation. The first of the two events coincides with abruptly enhanced particle fluxes seen in situ at Mars. Simultaneous with the particle enhancement there are an abrupt decrease in the intensity of electron oscillations, typically seen by the Mars Express particle instrument ASPERA-3 between the magnetic pileup boundary and the martian bow shock, and a sharp drop in the solar wind pressure, seen in the proxy quantity based on MGS magnetometer observations. The decrease in oscillation intensity is therefore the probable effect of a relaxation of the martian bow shock. The second absorption event does not show a particle enhancement and complete ASPERA-3 data during that time are unavailable. Other absorption events are the apparent result of solar X-ray and XUV enhancements. We conclude that surface reflection absorption events are sometimes caused by enhanced ionospheric ionization from high energy particles accelerated by the shocks associated with a CIR. A full statistical analysis of CIRs in relation to observed absorption events in conjunction with a quantitative analysis of the deposition of ionization during space weather events is needed for a complete understanding of this phenomenon. If such analyses can be carried out, radar sensing of the martian ionosphere might be useful as a space weather probe. 相似文献
2.
An initial study of the behaviour of He+ ions in the mid-latitude plasmasphere is carried out by solving the time-dependent equations of continuity and momentum. Starting with a low He+ tube content, results are obtained for a period of 8 days. In the topside ionosphere there is an upflow of He+ during the day and downflow at night, for the sunspot maximum conditions considered. The downflow at night differs from the behaviour of H+ for these atmospheric conditions. However, little of the He+ produced in the daytime is lost by recombination at night; it is suggested that the supply of He+ to the mid-latitude plasmasphere is, in effect, an escape process for neutral helium. 相似文献
3.
Wave normal directions of VLF signals propagating through the ionosphere can be determined by measuring Doppler frequency shift of the signals by means of rocket borne receivers. Two rockets were launched to detect the NWC signal of 22.3 kHz which was transmitted from Australia and propagated on two completely different paths, one being propagated through the Earth-ionosphere waveguide and up to the rocket, the other propagated down to the rocket by the whistler mode directly from the source in the opposite hemisphere. The wave normal directions of the latter mode were almost vertically downward in the ionosphere in the northern hemisphere, although substantial error was involved in the determination of the wave normal direction for a part of the upgoing flight of the rockets, due to the relative geometry of the directions of the rocket flight and the geomagnetic field. The effect of the horizontal gradients of the ionosphere on the above results were found to be not significant. From the experimental results it is concluded that field aligned ducts stretching down to the rocket altitudes did not exist, at least, during the rocket flights. 相似文献
4.
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
5.
《Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science》1999,24(4):393-399
The influence of ionosphere plasma holes on HF radio wave propagation in approximation of geometrical optics is studied. The results of HF radio wave propagation modeling on routes crossing ionosphere holes are presented. The natural and modeling ionosphere sections are used (we use experimental radiotomography sections on routes: Moscow-Arkhangelsk, Moscow-Murmansk (Russia) and Block Iland-Rabiwhol (USA)). It is shown that characteristics of HF radiowave trajectory (in coordinate: height-latitude) depend on geometrical sizes of hole and the direction, point, angle and frequencies of sounding. It is shown that the asymmetry of electron density distribution results in qualitative and quantitative distinctions of waves trajectories characteristics in dependencies on direction propagation. The characteristic modes of HF radio wave propagation through hole are emphasized — one hop, captured by channel E-F, captured by hole, trapezifrom and so on. The higher interest presents the family of trajectories captured by hole. On estimations, the longest time-delay for different conditions makes up to 23 ms (or range up to 7000 km!). It is possible to say, that the hole in ionosphere can be the trap of HF radiowave. It is discussed conditions of capture and keeping of radio wave by ionospheric hole. The fact, that holes in top ionosphere are long-living and large-scale formations, indicates that disregard of radiowave propagation effects, connected with holes, will result in data distortion or misunderstanding of ionosphere monitoring and control by radiosounding in HF radiowave range. 相似文献
6.
Position of the mid-latitude trough in the topside ionosphere as deduced from ESRO 4 observations 总被引：1，自引：0，他引：1
The position of the mid-latitude trough in the topside ionosphere is determined from electron density data of ESRO 4 during quiet solar conditions. An analysis of about 300 trough observations in the northern and southern hemispheres showed that the trough was generally seen only during the night from 19 to 05 hr LT. During this period, the invariant latitude of the minimum of the trough was found to decrease both with increasing geomagnetic disturbance represented by Kp and with increasing LT after sunset. 相似文献
7.
Quantitative estimates of ionization sources that maintain the night-time E- and F-region ionosphere are given. Starlight (stellar continuum radiation in the spectral inverval 911–1026 Å) and resonance scattering of solar Ly-β into the night sector are the most important sources in the E-region and are capable of maintaining observable electron densities of order (1–4) × 103 cm?3. Starlight ionization rates have substantial variations (factors of 2–4) with latitude and time of year since the brightest stars in the night sky occur in the southern Milky Way and Orion regions. In the lower F-region the major O+ source in the equatorial ionosphere is 910 Å radiation from the O+ recombination in the F2-region, whereas in the extratropical ionosphere interplanetary 584 Å radiation only exceeds resonance scattering of solar 584 and 304 Å radiation as the dominant O+ source during the month of December. 相似文献
8.
T. Nygrén 《Planetary and Space Science》1981,29(5):521-528
A simple method for obtaining the reflection and transmission coefficients and fields for an electromagnetic wave propagating vertically in a horizontally stratified ionosphere is introduced. The ionosphere is divided into a great number of thin layers, and the boundary conditions of the electric and magnetic fields are applied at all layer interfaces. A recursive formalism is derived, which gives the altitude dependence of the reflection and inverse transmission coefficient matrices starting from the top of the layer. It is then shown how these results can be used in calculating the height dependence of a wave corresponding to any incident polarization. Test results are also presented in order to demonstrate the applicability of the procedure. 相似文献
9.
The effects on the upper dayside Venus ionosphere of a slow increase in solar wind dynamic pressure are simulated numerically with a 1-dimensional (spherically symmetric) Lagrangian hydrodynamical code. The simulation is started with an extended ionosphere in pressure equilibrium with the solar wind at the ionopause. The pressure at the ionopause is gradually increased to five times the initial pressure with rise times of 5, 15, and 30 min. It is found that, for rise times greater than about 10 min, the compression of the ionopause is nearly adiabatic, with the ionopause moving downward at velocities of ～1?2 km/sec until it reaches a maximally compressed states, at which time the motion reverses. For short rise times the compression produces a shock wave similar to that occuring in the case of a sudden increase in pressure. The global implications of these processes are discussed within the context of Pioneer Venus observations and future theoretical work on this problem is outlined. 相似文献
10.
《Planetary and Space Science》2006,54(13-14):1482-1495
Venus has no internal magnetic dynamo and thus its ionosphere and hot oxygen exosphere dominate the interaction with the solar wind. The solar wind at 0.72 AU has a dynamic pressure that ranges from 4.5 nPa (at solar max) to 6.6 nPa (at solar min), and its flow past the planet produces a shock of typical magnetosonic Mach number 5 at the subsolar point. At solar maximum the pressure in the ionospheric plasma is sufficient to hold off the solar wind at an altitude of 400 km above the surface at the subsolar point, and 1000 km above the terminators. The deflection of the solar wind occurs through the formation of a magnetic barrier on the inner edge of the magnetosheath, or shocked solar wind. Under typical solar wind conditions the time scale for diffusion of the magnetic field into the ionosphere is so long that the ionosphere remains field free and the barrier deflects almost all the incoming solar wind. Any neutral atoms of the hot oxygen exosphere that reach the altitude of the magnetosheath are accelerated by the electric field of the flowing magnetized plasma and swept along cycloidal paths in the antisolar direction. This pickup process, while important for the loss of the Venus atmosphere, plays a minor role in the deceleration and deflection of the solar wind. Like at magnetized planets, the Venus shock and magnetosheath generate hot electrons and ions that flow back along magnetic field lines into the solar wind to form a foreshock. A magnetic tail is created by the magnetic flux that is slowed in the interaction and becomes mass-loaded with thermal ions.The structure of the ionosphere is very much dependent on solar activity and the dynamic pressure of the solar wind. At solar maximum under typical solar wind conditions, the ionosphere is unmagnetized except for the presence of thin magnetic flux ropes. The ionospheric plasma flows freely to the nightside forming a well-developed night ionosphere. When the solar wind pressure dominates over the ionospheric pressure the ionosphere becomes completely magnetized, the flow to the nightside diminishes, and the night ionosphere weakens. Even at solar maximum the night ionosphere has a very irregular density structure. The electromagnetic environment of Venus has not been well surveyed. At ELF and VLF frequencies there is noise generated in the foreshock and shock. At low altitude in the night ionosphere noise, presumably generated by lightning, can be detected. This paper reviews the plasma environment at Venus and the physics of the solar wind interaction on the threshold of a new series of Venus exploration missions. 相似文献
11.
A study is made of the hydromagnetic wave activity observed on the ground during the sub-auroral red (SAR) arc event of 17–18 December 1971. The available wave energy flux in the magnetosphere, inferred from the observed wave amplitude on the ground using the present understandings of wave localization and ionosphere wave attenuation is sufficient to produce the SAR arc. This finding supports kinetic Alfvén wave heating as a production mechanism for SAR arc optical emissions. 相似文献
12.
Parametric excitation of plasma oscillations in the ionosphere by an electromagnetic wave near the reflection level has been considered. The spectrum of plasma waves forms as a result of action of the source (a pump wave), non-linear transfer towards large scales and damping. The results of the theory are in satisfactory agreement with the experiment. 相似文献
13.
Atmospheric ozone number densities have been determined over the altitude range 30–75 km by measuring the absorption of lunar u.v. radiation in a number of wavelength bands between 2400 Å and 2900 Å. The measurements were made from rockets fired at night at times close to full Moon and show significant variations in ozone densities particularly at the higher altitudes. Comparison with other observations indicates that above 60 km the ozone densities at night are markedly greater than they are during the day. 相似文献
14.
The expression for nonlinear shift of a wave number of a whistler wave propagating through the ionosphere has been derived and the results have been discussed. It is seen that nonlinear shift of a wave number of a whistler is significant in some physical situations. From numerical estimations it is observed that wave number shifts of a whistler for both the LCP and RCP waves become significant when the frequency of the waves are nearly equal to the ion-cyclotron frequency. 相似文献
15.
F-lacuna event is a typical phenomenon of the high latitude ionosphere occurring during summer days. It consists in a disappearance of echoes from the F-layer on ionograms and a simultaneous extra absorption of about 0.1 to 0.4 dB on 30 MHz cosmic waves. This paper, based on data from the Dumont d'Urville station, describes the properties of this phenomenon: correlation with magnetic activity, convection electric field, interplanetary magnetic field, absorption in the lower ionosphere and electron density in the F-layer. A tentative model of interpretation in terms of large scale electron density irregularities in the F-layer is suggested. 相似文献
16.
The day to night ratio in ionospheric radio wave absorption has been studied for College, Alaska using a narrow beam riometer array and computer assisted programming of the data. For the period of 6 November 1967 to 17 April 1968 no significant departure from a ratio of unity was found. 相似文献
17.
The day to night ratio in ionospheric radio wave absorption has been studied for College, Alaska using a narrow beam riometer array and computer assisted programming of the data. For the period of 6 November 1967 to 17 April 1968 no significant departure from a ratio of unity was found. 相似文献
18.
Liu Rui-yuan Hu Hong-qiao Liu Yong-hua Xu Zhong-hua Sato N. Fraser B.J. 《Solar physics》2001,204(1-2):305-313
Simultaneous observations at Zhongshan Station, Antarctica, are presented for the interval of 13–17 July 2000 to show responses
of the polar ionosphere to the Bastille Day (14 July 2000) solar event. The polar ionosphere was highly disturbed, as shown
by frequently large deviations of the geomagnetic H-component, large riometer absorption events and strong ULF waves. Associated
with the huge solar proton event produced by the X5/3B flare, a polar cap absorption (PCA) was observed. It began at ∼ 10:40 UT
on 14 July and ended at ∼ 19:40 UT on 17 July. Superposed on it, there was a large absorption event with a peak of 26 dB,
starting at ∼ 03:00 UT and ending at ∼ 11:10 UT on 15 July. This kind of absorption was probably produced by an intense `cloud
of energetic electrons' during an auroral substorm. The ULF waves were very intense during the main phase and the recovery
phase of the severe magnetic storm on 15 and 16 July. The ionospheric absorption was so strong that the digisonde signal was
blacked out most of the time. The ionosphere returned to normal in the afternoon on 17 July. 相似文献
19.
Hu Xiong Wu Xiao-cheng Gong Xiao-yan Wang Xin Xu Qin-chen 《Chinese Astronomy and Astrophysics》2010,34(1):100-112
The technical system of the Sino-Russian joint satellite-to-satellite Mars ionosphere occultation is analyzed and introduced. The analogue computation of the observed values of the radio waves of the ionosphere occultation event is carried out by adopting the three-dimensional ray tracking method and the electron density profile inversion is conducted by means of the simulated occultation observational data, with the result showing that the emulation algorithm is reliable. By taking advantage of the emulation method the case computation and analysis of the inversion errors caused by the observational error of the occultation radio wave phase and the satellite orbital error are respectively carried out, and it is obtained from the result that the effect of the phase measuring error of the 5% circle on the result of the daytime ionosphere occultation exploration may be neglected, while the absolute error of the night electron density measurement is less than 4 × 108 m?3, and the main effect of the satellite orbital error on the occultation leads to the lifting or falling of the ionospheric height. The result shows that the technical system of the Sino-Russian joint Mars ionosphere occultation exploration is advanced. It can be expected that the high accuracy electron density profile is obtained and the technical system can be applied to the exploration of the lunar ionospheric environment. 相似文献
20.
R.N. Srivastava 《Planetary and Space Science》1974,22(11):1545-1564
Comparison of the low altitude polar orbiting Injun 5 Satellite data with the ground VLF data has revealed that there is a definite scarcity of VLF/ELF emissions at the ground level compared with the extent to which they are present at or above the auroral altitudes. Reasons for this have been investigated by performing ray path computations for whistler mode VLF propagation in an inhomogeneous and anisotropic medium, such as the magnetosphere and the ionosphere. Based on wave normal computations in the lower ionosphere, it has been found that many of the near-auroral zone VLF/ELF events are frequently either reflected from, or heavily attenuated in, the lower ionosphere. Besides collisional loss, severe attenuation of VLF signals in the lower ionosphere is also caused by the divergence of ray paths from the vertical (spatial attenuation). Cone of wave normal angles for the wave, within which VLF/ ELF signals are permitted to reach the ground, has been established. Wave normals lying outside this transmission cone are reflected from the lower ionosphere and do not find exit to the Earth-ionosphere cavity. Computations for VLF signals produced at auroral zone distances in the equatorial plane of the magnetosphere indicates that these signals are more or less trapped in the magnetosphere at altitudes > 1RE. 相似文献