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1.
《Planetary and Space Science》2006,54(13-14):1344-1351
Radio waves and optical flashes consistent with the lightning generation have been reported frequently at Venus. These observations point to the presence of electrical discharges in the sulfuric acid clouds of Venus. A particularly strong whistler-mode signal has been found propagating parallel to the magnetic field in the night ionosphere near 100 Hz by the Pioneer Venus spacecraft. At high (radio) frequencies, intermittent signals are also seen reminiscent of terrestrial lightning. However, these signals appear to be weaker than their terrestrial counterparts. On Venus Express, the magnetometer bandwidth is sufficient to record the lightning signals propagating in the whistler mode and will be used to map the occurrence of lightning across the nightside of the planet. 相似文献
2.
Harry A. Taylor Jr. Leonard Kramer Paul A. Cloutier Shannon S. Walker 《Earth, Moon, and Planets》1995,69(2):173-199
Plasma and field relationships observed across the nightside of Venus evidence a chaotic variety of interactions between the ionosphere and the combined effect of the solar wind and interplanetary magnetic field draped about the planet. Close examination of these data reveal within the chaos a number of repeatable signatures key to understanding fundamental field-plasma interactions. Observed from the Pioneer Venus Orbiter, (PVO), nightside conditions range from extensive, full-up ionospheres with little evidence of dynamic or energetic perturbations, to an almost full depletion, sometimes described as disappearing ionospheres. Between these extremes, the ionospheric structure is often irregular, sometimes exhibiting well-defined density troughs, at other times complex intervals of either abundant or minimal plasma concentration. Consistently, large B-fields (typically exceeding 5–10 nanoteslas) coincide with plasma decreases, whereas stable, abundant plasma distributions are associated with very low-level field. We examine hundreds of nightside orbits, identifying close correlations between regions of elevated magnetic fields featuring polarity reversals, and (a) exclusive low-frequency or distinctive broadband noise, or both, in the electric field data, (b) turbulent, superthermal behavior of the the ions and electrons. We review extensive studies of nightside fields to show that the correlations observed are consistent with theoretical arguments that the presence of strong magnetic fields within normal ionospheric heights indicates the intrusion of magnetosheath fields and plasma within such regions. We find abundant evidence that the ionosphere is frequently disrupted by such events, exhibiting a chaotic, auroral-like complexity appearing over a wide range of altitude and local time. We show that field-plasma disturbances, widely suggested to be similar to conditions in the Earth's auroral regions, are tightly linked to the electric field noise otherwise attributed to lightning. Owing to the coincidence inherent in this relationship, we suggest that natural, predictable plasma instabilities associated with the plasma gradients and current sheets evident within these events produce the E-field noise. The data relationships argue for a more detailed investigation of solar wind induced E-field noise mechanisms as the appropriate scientific procedure for invoking sources for the noise previously attributed to lightning. Consistent with these views, we note that independent analyses have offered alternative explanations of the noise as arising from ionospheric disturbances, that repeated searches for optical evidence of lightning have found no such evidence, and that no accepted theoretical work has yet surfaced to support the inference of lightning at Venus. 相似文献
3.
Photographic observations of the nightside of Jupiter by the Voyager 1 spacecraft show the presence of extensive lightning activity. Detection of whistlers by the plasma wave analyzer confirms the optical observations and implies that many flashes were not recorded by the Voyager camera because the intensity of the flashes was below the threshold sensitivity of the camera. Measurements of the optical energy radiated per flash indicate that the observed flashes had energies similar to that for terrestrial superbolts. The best estimate of the lightning energy dissipation rate of 0.4 × 10?3 W/m2 was derived from a consideration of the optical and radiofrequency measurements. The ratio of the energy dissipated by lightning compared to the convective energy flux is estimated to be between 0.27 × 10?4 and 0.5 × 10?4. The terrestrial value is 1 × 10?4. 相似文献
4.
HEOS-2 has observed energetic electrons (> 40 keV) in the high latitude magnetosphere appearing as one or more peaks outside and often well separated from the trapping boundary. Most of the observations are between 70° and 80° invariant latitudes both in the day and nightside. The peaks are located in the dayside adjacent to the polar cusp and coincide in the nightside with the edge of the plasma sheet. The electron peak intensity on the nightside shows a clear correlation with AE. The electron peak intensities on the dayside exceed those on the nightside and are generally higher in the pre-noon than in the afternoon sector. Observations on the dayside in the distant cusp region and in the adjacent magnetosheath show high and fluctuating intensities of energetic electronswith an energy spectrum much harder than in the outermost trapping region.
This observational evidence suggests different source regions for these energetic electrons: one in the distant geomagnetic tail and another one around the dayside cusp indentation. 相似文献
5.
W.M. Farrell J.S. Halekas T.J. Stubbs G.T. Delory R.M. Killen R.E. Hartle M.R. Collier 《Icarus》2011,216(1):169-172
The non-condensing neutral helium exosphere is at its most concentrated levels on the cold lunar nightside. We show herein that these He atoms are susceptible to impact ionization from primary and secondary electrons flowing in the vicinity of the negatively-charged nightside lunar surface. The secondary electron beams are a relatively recent discovery and are found to be emitted from the nightside surface at energies consistent with the negative surface potential. The effect is to create an electron impact-created ionosphere in nightside regions, possibly especially potent within polar craters. 相似文献
6.
H.A. Taylor R.E. Hartle H.B. Niemann L.H. Brace R.E. Daniell S.J. Bauer A.J. Kliore 《Icarus》1982,51(2):283-295
Across the nightside of Venus, daily measurements from the PV Orbiter Ion Mass Spectrometer often indicate an ionosphere of relatively abundant concentration, with a composition characteristic of the dayside ionosphere. Such conditions are interspersed by other days on which the ionosphere appears to largely “disappear” down to about 200 km, with ion concentrations at lower heights also much reduced. These characteristics, coupled with observations of strong day to night flows of O+ in the upper ionosphere, support arguments that ion transport from the dayside is important for the maintenance of the nightside ionosphere. Also, U.S. and Soviet observations of nightside energetic electron fluxes have prompted consideration of impact ionization as an additional nightside ion source. The details of the ion and neutral composition at low altitudes on the nightside provide an important input for further analysis of the maintenance process. In the range 140–160 km, strong concentrations of O2+ and NO+ indicate that the ionization peak is at times composed of at least two prominent ion species. Nightside concentrations of O2+ and NO+ as large as 105 and 104/cm3, respectively, appear to require sources in addition to that provided by transport. The most probable sources are considered briefly, and no satisfactory explanation is yet found for the observed NO+ concentrations. Further analysis beyond the scope of this paper is required to resolve this issue. 相似文献
7.
T.E. Cravens L.H. Brace H.A. Taylor C.T. Russell W.L. Knudsen K.L. Miller A. Barnes J.D. Mihalov F.L. Scarf S.J. Quenon A.F. Nagy 《Icarus》1982,51(2):271-282
Instruments on the Pioneer Venus Orbiter have detected a substantial ionosphere on the nightside of Venus during most orbits. However, during some orbits the nightside ionosphere seems to have almost disappeared, existing only as irregular patches of low-density plasma. The solar wind dynamic pressure on these occasions is greater than average. We have correlated data from several instruments (Langmuir probe, ion mass spectrometer, retarding potential analyzer, magnetometer, and plasma analyzer) for a number of orbits during which the nightside ionosphere had disappeared. The magnetic field tends to be coherent, horizontal, and larger than usual, and the electron and ion temperatures are much larger than they usually are on the nightside. We suggest mechanisms which might explain the reasons for the disappearance of the ionosphere when the solar wind dynamic pressure is large. 相似文献
8.
T.E. Cravens I.P. Robertson R.V. Yelle A.J. Coates K. Agren V. De La Haye F.M. Neubauer 《Icarus》2009,199(1):174-188
Solar and X-ray radiation and energetic plasma from Saturn's magnetosphere interact with the upper atmosphere producing an ionosphere at Titan. The highly coupled ionosphere and upper atmosphere system mediates the interaction between Titan and the external environment. A model of Titan's nightside ionosphere will be described and the results compared with data from the Ion and Neutral Mass Spectrometer (INMS) and the Langmuir probe (LP) part of the Radio and Plasma Wave (RPWS) experiment for the T5 and T21 nightside encounters of the Cassini Orbiter with Titan. Electron impact ionization associated with the precipitation of magnetospheric electrons into the upper atmosphere is assumed to be the source of the nightside ionosphere, at least for altitudes above 1000 km. Magnetospheric electron fluxes measured by the Cassini electron spectrometer (CAPS ELS) are used as an input for the model. The model is used to interpret the observed composition and structure of the T5 and T21 ionospheres. The densities of many ion species (e.g., CH+5 and C2H+5) measured during T5 exhibit temporal and/or spatial variations apparently associated with variations in the fluxes of energetic electrons that precipitate into the atmosphere from Saturn's magnetosphere. 相似文献
9.
R.C. Whitten P.T. McCormick D. Merritt K.W. Thompson R.R. Brynsvold C.J. Eich W.C. Knudsen K.L. Miller 《Icarus》1984,60(2):317-326
A two-dimensional model of the ionosphere of Venus which simulates ionospheric dynamics by self-consistently solving the plasma equations of motion, including the inertial term, in finite difference form has been constructed. The model, which is applied over the solar zenith angle range extending from 60 to 140° and the altitude range 100 to 480 km, simulates the measured horizontal velocity field quite satisfactorily. The ion density field is somewhat overestimated on the dayside because of the choice model neutral atmosphere and underestimated on the nightside because of setting the ionopause height at too low an altitude. It is concluded that solar photoionization on the dayside and ion recombination on the nightside are the processes mainly responsible for accelerating the plasma to the observed velocities. The plasma flow appears to be sufficient to maintain the nightside ionosphere at or near the observed median level of ion densities. 相似文献
10.
E. Kallio S. Barabash H. Gunell Y. Futaana T. Säles P. Riihelä H. Andersson A. Grigoriev R.A. Frahm J.R. Scherrer D.R. Linder J. Kozyra E. Roelof S. Livi C.C. Curtis B.R. Sandel M. Carter A. Fedorov S. McKenna-Lawler R. Cerulli-Irelli P. Wurz N. Krupp M. Fränz C. Dierker 《Icarus》2006,182(2):448-463
We have studied the interaction of fast solar wind hydrogen atoms with the martian atmosphere by a three-dimensional Monte Carlo simulation. These energetic neutral hydrogen atoms, H-ENAs, are formed upstream of the martian bow shock. Both H-ENAs scattered and non-scattered from the martian atmosphere/exosphere were studied. The colliding H-ENAs were found to scatter both to the dayside and nightside. On the dayside they contribute to the so-called H-ENA albedo. On the nightside the heated and scattered hydrogen atoms were found also in the martian wake. The density, the energy distribution function and the direction of the velocity of H-ENAs on the nightside are presented. The present study describes a novel “ENA sounding” technique in which energetic neutral atoms are used to derive information of the properties of planetary exosphere and atmosphere in a similar manner as the solar wind photons are used to derive atmospheric densities by measuring the scattered UV light. A detailed study of the direction and energy of the scattered and non-scattered H-ENAs suggest that the ENA sounding is a method to study the interaction between the planetary atmosphere and the solar wind and to monitor the density, and likely also the magnetization, of the planetary upper atmosphere. Already present-day ENA instrument should be capable to detect the analyzed particle fluxes. 相似文献
11.
We report results from 6 days' continuous recording of pulsation activity on a chain of magnetometers aligned East-West at 56.5°N geomagnetic latitude. We find phase differences between stations are relatively small. Signal propagation sense and horizontal polarisation correlate as low frequency inhomogeneous plasma theory predicts. There is no evidence of the Kelvin-Helmholtz instability being the major source. The most highly coherent signals often occur near midnight. We suggest the nightside may be the source region for a significant amount of dayside pulsation activity. 相似文献
12.
The relationship between sudden geomagnetic field changes in the nightside cusp region and impulsive electron precipitation events in the auroral zone is investigated. The investigations are based on magnetic field measurements from the spacecraft Explorer 35, Explorer 33 and OGO-5 and on X-ray measurements with balloon-borne instruments from Kiruna/Sweden. The sudden field changes are characterized by a decrease of the field strength and a rotation of the field direction. The precipitation events represent strong flux increases within a few minutes. The field changes were accompanied by impulsive precipitation not only in the midnight sector but also on the dayside. They can be regarded as a manifestation of the unsteady magnetospheric processes during the expansion phase. Whereas both phenomena occurred simultaneously on the nightside, the increase of precipitation was delayed by ca. 5 min on the dayside. It is assumed that the simultaneous occurrence on the nightside can be related to the formation of a neutral line with a considerable length in dawn-dusk-direction. Mechanisms are also discussed which could be responsible for the time delay on the dayside. 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.
Sub-millimeter 12CO (346 GHz) and 13CO (330 GHz) line absorptions, formed within the mesospheric to lower thermospheric altitude (70–120 km) region of the Venus atmosphere, have been mapped across the nightside disk of Venus during 2001–2009 inferior conjunctions, employing the James Clerk Maxwell Telescope (JCMT). Radiative transfer analysis of these thermal line absorptions supports temperature and CO mixing profile retrievals, as described in a companion paper (Clancy et al., 2012). Here, we consider the analysis of the sharp line absorption cores of these CO spectra in terms of accurate Doppler wind profile measurements at 95–115 km altitudes versus local time (～8 pm–4 am) and latitude (～60N–60S). These Doppler wind measurements support determinations of the nightside zonal and subsolar-to-antisolar (SSAS) circulation components over a variety of timescales. The average behavior fitted from 21 retrieved maps of 12CO Doppler winds (obtained over hourly, daily, weekly, and interannual intervals) indicates stronger average zonal (85 m/s retrograde) versus SSAS (65 m/s) circulation at the 1 μbar pressure (108–110 km altitude) level. However, the absolute and relative magnitudes of these circulation components exhibit extreme variability over daily to weekly timescales. Furthermore, the individual Doppler wind measurements within each nightside mapping observation generally show significant deviations (20–50 m/s, averaged over 5000 km horizontal scales) from the simple zonal/SSAS solution, with distinct local time and latitudinal characters that are also time variable. These large scale residual circulations contribute 30–70% of the observed nightside Doppler winds at any given time, and may be most responsible for global variations in nightside lower thermospheric trace composition and temperatures, as coincidentally retrieved CO abundance and temperature distributions do not correlate with solution retrograde zonal and SSAS winds (see companion paper, Clancy et al., 2012). Limited comparisons of these nightside submillimeter results with dayside infrared Doppler wind measurements suggest distinct dayside versus nightside circulations, in terms of zonal winds in particular. Combined 12CO and 13CO Doppler wind mapping observations obtained since 2004 indicate that the average zonal and SSAS wind components increase by 50–100% between altitudes of 100 and 115 km. If gravity waves originating from the cloud levels are responsible for the extension of zonal winds into the thermosphere (Alexander, M.J. [1992]. Geophys. Res. Lett. 19, 2207–2210), such waves deposit substantial momentum (i.e., break) in the lower nightside thermosphere. 相似文献
15.
Data on the thermal structure of the nightside middle atmosphere of Venus, from 84 to 137 km altitude, have been obtained from analysis of deceleration measurements from the third Pioneer Venus small probe, the night probe, which entered the atmosphere near the midnight meridian at 27°S latitude. Comparison of the midnight sounding with the morning sounding at 31°S latitude indicates that the temperature structure is essentially diurnally invariant up to 100 km, above which the nightside structure diverges sharply from the dayside toward lower temperatures. Very large diurnal pressure differences develop above 100 km with dayside pressure ten times that on the nightside at 126 km altitude. This has major implications for upper atmospheric dynamics. The data are compared with the measurements of G. M. Keating, J. Y. Nicholson, and L. R. Lake (1980, J. Geophys. Res., 85, 7941–7956) above 140 km with theoretical thermal structure models of Dickinson, and with data obtained by Russian Venera spacecraft below 100 km. Midnight temperatures are ～ 130°K, somewhat warmer than those reported by Keating et al. 相似文献
16.
V.A. Sergeev E.M. Sazhina N.A. Tsyganenko J.Å. Lundblad F. Søraas 《Planetary and Space Science》1983,31(10):1147-1155
Characteristics of the nightside isotropic precipitation of energetic protons during a period of 4 quiet days has been studied using data from the ESRO 1A satellite. The observed features of the equatorward precipitation boundary (its thickness, energy dependence, dynamics, dependence of its latitudinal position on the magnetic field at the geosynchronous orbit, etc.) were found to be in good agreement with calculations based on recent magnetospheric magnetic field models. We argue that the mechanism of non-adiabatic pitchangle scattering in the equatorial current sheet is a dominant source of isotropic precipitation of energetic protons observed in the nightside auroral zone. Observations of the isotropic precipitation boundary can be used for monitoring the changes in the magnetotail current intensity. 相似文献
17.
V.G. Vorobjev G. Gustafsson G.V. Starkov Y.I. Feldstein N.F. Shevnina 《Planetary and Space Science》1975,23(2):269-278
The motion of auroral forms on the day- and nightside of the Earth has been studied during different substorm phases by means of all-sky camera films. A substorm is characterized by a shift of the luminescence region towards the equator at noon and mainly towards the pole at midnight. However, individual forms drift predominantly toward the pole on the dayside and towards the equator on the nightside. The velocity of the poleward motion at noon is largest during the expansive phase of a substorm and amounts on the average to 330 but even during relatively quiet magnetic conditions a poleward motion is observed. 相似文献
18.
19.
This paper reports some results of an attempt to simulate the large-scale changes of the internal structure of the magnetosphere during the magnetospheric substorm by assuming the growth of two current systems, one in the nightside and the other in the day-side. 相似文献
20.
Lunar surface potential and electric field 总被引：1，自引：0，他引：1
The Moon has no significant atmosphere, thus its surface is exposed to solar ultraviolet radiation and the solar wind. Photoemission and collection of the solar wind electrons and ions may result in lunar surface charging. On the dayside, the surface potential is mainly determined by photoelectrons, modulated by the solar wind;while the nightside surface potential is a function of the plasma distribution in the lunar wake. Taking the plasma observations in the lunar environment as inputs, the global potential distribution is calculated according to the plasma sheath theory, assuming Maxwellian distributions for the surface emitted photoelectrons and the solar wind electrons. Results show that the lunar surface potential and sheath scale length change versus the solar zenith angle, which implies that the electric field has a horizontal component in addition to the vertical one. By differentiating the potential vertically and horizontally, we obtain the global electric field. It is found that the vertical electric field component is strongest at the subsolar point,which has a magnitude of 1 V m-1. The horizontal component is much weaker, and mainly appears near the terminator and on the nightside, with a magnitude of several mV m-1. The horizontal electric field component on the nightside is rotationally symmetric around the wake axis and is strongly determined by the plasma parameters in the lunar wake. 相似文献