共查询到20条相似文献，搜索用时 718 毫秒
1.
《Planetary and Space Science》2007,55(13):1978-1989
The propagation of extremely low frequency (ELF) electromagnetic waves in the Earth's ionospheric cavity and the associated resonance phenomena have been extensively studied, in relation with lightning activity. We perform a similar investigation for Titan, the largest moon of Saturn. There are important differences between Earth and Titan, as far as the cavity geometry, the atmospheric electron density profile, and the surface conductivity are concerned. We present an improved 3D finite element model that provides an estimate of the lowest eigenfrequencies, associated quality factors (Q-factors), and ELF electric field spectra. The data collected by the electric antenna of the Permittivity, Waves, and Altimetry (PWA) instrument reveals the existence of a narrow-band signal at about 36 Hz during the entire descent of Huygens upon Titan. We assess the significance of these measurements against the model predictions, with due consideration to the experimental uncertainties. 相似文献
2.
S. S. De B. K. Sarkar B. Ghosh A. C. Sen S. Bandyopadhyay S. K. Adhikari 《Earth, Moon, and Planets》1991,53(1):1-10
Enhancement of conductivity tensor modification within the ionospheric plasma by electron density modulation through the temperature dependence of the recombination rates of different ionspecies caused due to electron temperature modulation during high-power wave propagation is studied. Variation of Hall and Pedersen conductivities of the ionosphere has been investigated in the height range between 85 km to 250 km. The generation of waves at the modulation frequency and its harmonics in the ELF, VLF bands in the process is discussed. 相似文献
3.
R. Grard J.J. López-Moreno I. Jernej F. Simões R. Trautner F. Ferri R. Rodrigo C. Béghin V.J.G. Brown J.M. Jeronimo T. Tokano 《Planetary and Space Science》2006,54(12):1124-1136
The permittivity, waves and altimetry (PWA) instrument was designed for the investigation of the electric properties and other related physical characteristics of the atmosphere of Titan, from an altitude around 140 km down to the surface. PWA carried sensors to measure the atmospheric conductivity, and record electromagnetic and acoustic waves up to frequencies of 11.5 and 6.7 kHz, respectively. PWA also measured the relief roughness during the descent and the permittivity of the surface after touchdown. The measurements and the results of the preliminary analysis are presented. An ionized layer is detected at altitudes above 50 km, using two independent techniques, and the presence of free electrons in the upper atmosphere is confirmed. An electric signal at around 36 Hz is observed throughout the descent, but it is not yet confirmed that this emission is unambiguously related to a resonance of the ionospheric cavity. The relative dielectric constant of Titan's surface material is nearly 2 and the electric conductivity 4×10−10 S m−1. The electric properties of the surface seem to evolve after touch-down, possibly due to a local warming of the landing site by the Huygens Probe body. 相似文献
4.
We model the global electromagnetic (Schumann) resonance in the atmosphere of Titan. Parameters of conductivity of the lower ionosphere were implemented taken from existing aeronomic models of Titan's atmosphere. Two exponential conductivity profiles were constructed: one of them suggests favorable conditions for Schumann resonance and the other models considerable attenuation in the ionospheric plasma. Peak frequencies and Q-factors of resonance were computed as well as resonance spectra for the signals arriving from individual vertical lightning discharges and from strokes uniformly distributed over the planet. The models show that detection of Schumann resonance on Titan is feasible, especially in favorable conditions. Possible applications of Schumann resonance in the studies of Titan's lightning activity are outlined. 相似文献
5.
Terrestrial lightning is generated by the separation of electric charge residing on water-ice particles in clouds, a few kilometers above the electrically conducting surface of the Earth. It is detected optically, electromagnetically, and aurally. The majority of discharges occur within or between clouds with about one third discharging to the surface of the Earth. Upward-propagating lightning also occurs with effects extending into the ionosphere. On Venus, the clouds are close to 50 km above the surface of the planet, where the temperatures and pressures are near those of Earth’s surface. In contrast the atmospheric pressure near the surface of Venus is nearly 100 times that of Earth. Thus, while intra- and inter-cloud lightning is expected to occur in a manner similar to that on Earth, we do not expect discharges from the clouds to the surface to occur. Upward-going lightning may be more frequent at Venus because the ionosphere is closer to the clouds. As at Earth, Venus lightning has been detected optically and electromagnetically from a variety of platforms. We find that some of the observed properties of lightning are different at the two planets. Many of the differences in the electromagnetic waves detected by spacecraft can be attributed to effects during ionospheric propagation to the spacecraft. We review the differences in the ionospheres of Earth and Venus and how they affect observations. We use both the Pioneer Venus electric antenna observations as well as the Venus Express magnetic measurements. 相似文献
6.
Christian Béghin Orélien Randriamboarison Michel Hamelin Erich Karkoschka Christophe Sotin Robert C. Whitten Jean-Jacques Berthelier Réjean Grard Fernando Simões 《Icarus》2012,218(2):1028-1042
This study presents an approximate model for the atypical Schumann resonance in Titan’s atmosphere that accounts for the observations of electromagnetic waves and the measurements of atmospheric conductivity performed with the Huygens Atmospheric Structure and Permittivity, Wave and Altimetry (HASI–PWA) instrumentation during the descent of the Huygens Probe through Titan’s atmosphere in January 2005. After many years of thorough analyses of the collected data, several arguments enable us to claim that the Extremely Low Frequency (ELF) wave observed at around 36 Hz displays all the characteristics of the second harmonic of a Schumann resonance. On Earth, this phenomenon is well known to be triggered by lightning activity. Given the lack of evidence of any thunderstorm activity on Titan, we proposed in early works a model based on an alternative powering mechanism involving the electric current sheets induced in Titan’s ionosphere by the Saturn’s magnetospheric plasma flow. The present study is a further step in improving the initial model and corroborating our preliminary assessments. We first develop an analytic theory of the guided modes that appear to be the most suitable for sustaining Schumann resonances in Titan’s atmosphere. We then introduce the characteristics of the Huygens electric field measurements in the equations, in order to constrain the physical parameters of the resonating cavity. The latter is assumed to be made of different structures distributed between an upper boundary, presumably made of a succession of thin ionized layers of stratospheric aerosols spread up to 150 km and a lower quasi-perfect conductive surface hidden beneath the non-conductive ground. The inner reflecting boundary is proposed to be a buried water–ammonia ocean lying at a likely depth of 55–80 km below a dielectric icy crust. Such estimate is found to comply with models suggesting that the internal heat could be transferred upwards by thermal conduction of the crust, while convective processes cannot be ruled out. 相似文献
7.
Employing a realistic ionospheric model and a suitable energetic electron spectra, detailed power calculations are carried out to confirm the generation mechanism of low-latitude VLF emissions observed both in the satellites and on the ground. Raypaths of the radiated waves are also calculated to account for the attenuation and spreading losses. It is shown that 100 eV?1 keV electrons radiating incoherently in the Cerenkov mode are the main sources of these emissions. 相似文献
8.
The gradient coupling within the ionospheric plasma in the presence of local variation of atmospheric heating and other modifications during high power radio wave propagation has been studied. The appropriate form of coupled equations for the modified situation is obtained. The electromagnetic and acoustic modes of wave propagation have been separated and solved, by the WKB method. Field quantities may be used to explore different properties of the medium under the modified condition. 相似文献
9.
《Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science》2000,25(4):353-357
The signals of Global Navigation Satellites have found a large number of uses in atmospheric and ionospheric research. Reception of the signals from a satellite in a Low Earth Orbit (LEO) leads regularly to occultation of the signals by the surface of the Earth. Before an occultation the signals traverse the ionosphere with rays with decreasing height of their perigees. Satellite electron content observed prior to ‘setting’ occultations or after `rising' occultations can be used as input data for inversion. The inversion procedure gives horizontally averaged height profiles of electron density.Assessment studies are needed to find out under which conditions the profiles from inversions are representative for ‘true’ electron density profiles above the Earth occultation point.A great number of such studies have been carried out using the ionosphere/plasmasphere model NeUOG-plas for forward and backward modelling. Different transmitter-receiver scenarios have been investigated.We describe the assessment procedure and report on results showing the most interesting cases and statistics. 相似文献
10.
We discuss the possibility of exciting whistler mode waves (WMWs) in the Earth's ionosphere, by using two high frequency beams of electromagnetic waves (f1f2) suitably orientated to the geomagnetic field Ho, so that a non-linear resonant interaction can take place in the natural ionospheric plasma, approximately at the altitude of the F2 maximum electron density. Within the limitations imposed by ionospheric inhomogeneities in the interaction region, it should be possible to excite a WMW which propagates along a predetermined direction, e.g. parallel to Ho.
If we assumef1 andf2 to be approx 30 MHz (i.e. well above the ionospheric plasma frequency), this method would make it possible to select and vary the frequency range of the excited WMW up to a few hundreds kHz without substantial alterations to the high frequency transmitting system.
Since the two beams should form an angle close to 90° to the direction of propagation of the WMW, this technique may prove particularly suitable for active wave experiments at low geomagnetic latitudes, where the geometry of the geomagnetic field limits the feasibility of direct wave injection experiments.
Using the results of theoretical calculations of the three wave coupling coefficients, it will be shown that the transmitters required to produce WMWs with field strengths comparable to that of naturally occurring strong whistlers are substantial, but feasible. 相似文献
11.
At the Low-Frequency Array (LOFAR)(Planet. Space Sci. (2004) these proceedings) frequencies (HF/VHF), extraterrestrial radiation experiences substantial propagation delay as it passes through the ionosphere. The adaptive calibration technique to be employed by LOFAR will use signals from many known bright radio sources in the sky to estimate and remove the effects of this delay. This technique will operate along many simultaneous lines of sight for each of the stations. Measurements will be made on time scales of seconds or shorter, and with accuracies corresponding to path length variations of 1 cm or less. Tomographic techniques can be used to invert the thousands of changing and independent total electron content (TEC) measurements produced by LOFAR into three-dimensional electron density specifications above the array. These specifications will measure spatial and time scales significantly smaller and faster than anything currently available. These specifications will be used to investigate small-scale ionospheric irregularities, equatorial plasma structures, and ionospheric waves. In addition, LOFAR will improve the understanding of the solar drivers of the ionosphere by simultaneously measuring the solar radio bursts and the TEC. Finally, LOFAR, which will be situated to observed the galactic plane, will make continuous, high-resolution observations of the low-latitude ionosphere, an important but under-observed region. This paper will look at LOFAR as an ionospheric probe including comparisons to other ionospheric probes as well as possible methods of operation to optimize ionospheric measurements. 相似文献
12.
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
13.
Extreme low frequency electromagnetic waves have been observed below the F peak in the equatorial ionosphere by instruments onboard OGO-6. Electrostatic wave observations indicate that the steep gradient was unstable to the process which causes equatorial spread F above the region where the electromagnetic waves were observed. The data are very similar to observations near the polar cusp and give further evidence that ELF waves are excluded from regions of rapid and irregular density increases. Low level electromagnetic waves with similar properties were occasionally observed on the nightside by the OVI-17 electric field sensor and may be plasmaspheric hiss which has propagated to low altitude. 相似文献
14.
The observations of electron inverted ‘V’ structures by the MGS and MEX spacecraft, their resemblance to similar events in the auroral regions of the Earth, and the discovery of strong localized magnetic field sources of the crustal origin on Mars, raised hypotheses on the existence of Martian aurora produced by electron acceleration in parallel electric fields. Following the theory of this type of structures on Earth we perform a scaling analysis to the Martian conditions. Similar to the Earth, upward field-aligned currents necessary for the generation of parallel potential drops and peaked electron distributions can arise, for example, on the boundary between ‘closed’ and ‘open’ crustal field lines due to shears of the flow velocity of the magnetosheath or magnetospheric plasmas. A steady-state configuration assumes a closure of these currents in the Martian ionosphere. Due to much smaller magnetic fields as compared to the Earth case, the ionospheric Pedersen conductivity is much higher on Mars and auroral field tubes with parallel potential drops and relatively small cross scales to be adjusted to the scales of the localized crustal patches may appear only if the magnetosphere and ionosphere are decoupled by a zone with a strong E∥. Another scenario suggests a periodic short-circuit of the magnetospheric electric fields by a coupling with the conducting ionosphere. 相似文献
15.
Using plasma parameters from a typical stormtime ionospheric energy balance model, we have investigated the effects of plasma turbulence on the auroral magnetoplasma. The turbulence is assumed to be comprised of electrostatic ion cyclotron waves. These waves have been driven to a nonthermal level by a geomagnetic field-aligned, current-driven instability. The evolution of this instability is shown to proceed in two stages and indicates an anomalous increase in field-aligned electrical resistivity and cross-field ion thermal conductivity as well as a decrease in electron thermal conductivity along the geomagnetic field. In addition, this turbulence heats ions perpendicular to the geomagnetic field and hence leads to a significant ion temperature anisotropy. 相似文献
16.
Owing to the effect of refraction on the propagation of electromagnetic waves in the terrestrial atmosphere, the direction of propagation is changed. The path of propagation path becomes a curve with an increased path-length so increasing the propagation time. A simplified spherically symmetric atmospheric model is adopted to calculate the delay caused by the path bending, for different zenith distances. 相似文献
17.
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
18.
This study results from a coordinated experiment involving ionospheric observations of Faraday rotation between a geostationary satellite and three ground based receivers at Aberystwyth and Bournemouth in the U.K. and Lannion, France, together with incoherent scatter observations at St. Santin-Nancay, France.Quasi-periodic variations of electron content observed simultaneously at the three stations are interpreted in terms of medium scale gravity waves travelling in the ionospheric F-region. Characteristics of these waves are derived by means of a cross-correlation technique.A reverse ray tracing computation, using data on the neutral atmosphere and neutral wind stratification from the incoherent scatter observations, has been used in an attempt to locate the sources of these waves.The results show that some of the waves are almost certainly generated above 100 km altitude, probably by auroral phenomena, while the others could be produced near ground level by meteorological sources. The reverse ray tracing indicates that the latter sources are in general located in a geographic area in the vicinity of a weather disturbance. A production mechanism for these waves is proposed involving ageostrophic perturbations of the neutral wind in a jet stream. 相似文献
19.
20.
The propagation features of extremely low frequency electromagnetic waves through the multicomponent ionospheric plasma are studied. It is shown that at relatively lower frequencies refractive index for right hand mode is higher than the left-hand mode, which is reversed at higher frequencies. The thermal temperature of plasma particle causes decrease in phase and group velocities of both right and left-hand modes. The crossover frequencies for different plasma models are computed and variation with ion concentration and thermal velocity is studied. Explicit expression for group velocity and travel time has been derived and studied numerically. Finally, we have presented simulation of the ion whistler spectrograms for Hydrogen, Helium and Oxygen ions present in the ionospheric plasma. The results are compared with the experimentally detected hydrogen and helium ion whistlers. The importance of the present study in the exploration of ionospheric plasma is illustrated. 相似文献