共查询到20条相似文献，搜索用时 31 毫秒
1.
Intriligator DS Collard HR Mihalov JD Whitten RC Wolfe JH 《Science (New York, N.Y.)》1979,205(4401):116-119
Additional plasma measurements in the vicinity of Venus are presented which show that (i) there are three distinct plasma electron populations-solar wind electrons, ionosheath electrons, and nightside ionosphere electrons; (ii) the plasma ion flow pattern in the ionosheath is consistent with deflected flow around a blunt obstacle; (iii) the plasma ion flow velocities near the downstream wake may, at times, be consistent with the deflection of plasma into the tail, closing the solar wind cavity downstream from Venus at a relatively close distance (within 5 Venus radii) to the planet; (iv) there is a separation between the inner boundary of the downstream ionosheath and the upper boundary of the nightside ionosphere; and (v) during the first 4.5 months in orbit the measured solar wind plasma speed continued to vary, showing a number of high-speed, but generally nonrecurrent, streams. 相似文献
2.
Ness NF Behannon KW Lepping RP Whang YC Schatten KH 《Science (New York, N.Y.)》1974,183(4131):1301-1306
The NASA-GSFC magnetic field experiment on Mariner 10 is the first flight of a dual magnetometer system conceived to permit accurate measurements of weak magnetic fields in space in the presence of a significant and variable spacecraft magnetic field. Results from a preliminary analysis of a limted data set are summarized in this report, which is restricted primarily to Venus encounter. A detached bow shock wave that develops as the super Alfvénic solar wind interacts with the Venusian atmosphere has been observed. However, the unique coincidence of trajectory position and interplanetary field orientation at the time of bow shock crossing led to a very disturbed shock profile with considerably enhanced upstream magnetic fluctuations. At present it is not possible to ascertain the nature and characteristics of the obstacle responsible for deflecting the solar wind flow. Far downstream disturbances associated with the solar wind wake have been observed. 相似文献
3.
Kivelson MG Kennel CF McPherron RL Russell CT Southwood DJ Walker RJ Hammond CM Khurana KK Strangeway RJ Coleman PJ 《Science (New York, N.Y.)》1991,253(5027):1518-1522
During the 10 February 1990 flyby of Venus, the Galileo spacecraft skimmed the downstream flank of the planetary bow shock. This provided an opportunity to examine both the global and the local structure of the shock in an interval during which conditions in the solar wind plasma were quite steady. The data show that the cross section of the shock in planes transverse to the flow is smaller in directions aligned with the projection of the interplanetary magnetic field than in directions not so aligned. Ultralow-frequency waves were present in the unshocked solar wind, and their amplitude peaked when the spacecraft was downstream of the foreshock. At large distances down the tail, the Mach number of the flow normal to the shock is low, thus providing the opportunity to study repeated crossings of the collisionless shock in an interesting parameter regime. Some of the shock crossings reveal structure that comes close to the theoretically predicted form of intermediate shocks, whose existence in collisionless plasmas has not been confirmed. 相似文献
4.
Dyudina UA Ingersoll AP Ewald SP Vasavada AR West RA Del Genio AD Barbara JM Porco CC Achterberg RK Flasar FM Simon-Miller AA Fletcher LN 《Science (New York, N.Y.)》2008,319(5871):1801
The camera onboard the Cassini spacecraft has allowed us to observe many of Saturn's cloud features. We present observations of Saturn's south polar vortex (SPV) showing that it shares some properties with terrestrial hurricanes: cyclonic circulation, warm central region (the eye) surrounded by a ring of high clouds (the eye wall), and convective clouds outside the eye. The polar location and the absence of an ocean are major differences. It also shares properties with the polar vortices on Venus, such as polar location, cyclonic circulation, warm center, and long lifetime, but the Venus vortices have cold collars and are not associated with convective clouds. The SPV's combination of properties is unique among vortices in the solar system. 相似文献
5.
Wolfe J Intriligator DS Mihalov J Collard H McKibbin D Whitten R Barnes A 《Science (New York, N.Y.)》1979,203(4382):750-752
Initial results of observations of the solar wind interaction with Venus indicate that Venus has a well-defined, strong, standing bow shock wave. Downstream from the shock, an ionosheath is observed in which the compressed and heated postshock plasma evidently interacts directly with the Venus ionosphere. Plasma ion velocity deflections observed within the ionosheath are consistent with flow around the blunt shape of the ionopause. The ionopause boundary is observed and defined by this experiment as the location where the ionosheath ion flow is first excluded. The positions of the bow shock and ionopause are variable and appear to respond to changes in the external solar wind pressure. Near the terminator the bow shock was observed at altitudes of approximately 4600 to approximately 12,000 kilometers. The ionopause altitutde ranged fromn as low as approximately 450 to approximately 1950 kilometers. Within the Venus ionosphere low-energy ions (energy per untit charge < 30 volts) were detected and have been tentatively idtentified as nonflowing ionospheric ions incident from a direction along the spacecraft velocity vector. 相似文献
6.
Brace LH Theis RF Krehbiel JP Nagy AF Donahue TM McElroy MB Pedersen A 《Science (New York, N.Y.)》1979,203(4382):763-765
Altitude profiles of electron temperature and density in the ionosphere of Venus have been obtained by the Pioneer Venus orbiter electron temperatutre probe. Elevated temperatutres observed at times of low solar wind flux exhibit height profiles that are consistent with a model in which less than 5 percent of the solar wind energy is deposited at the ionopause and is conducted downward through an unmagnetized ionosphere to the region below 200 kilomneters where electron cooling to the neutral atmosphere proceeds rapidly. When solar wind fluxes are higher, the electron temperatures and densities are highly structured and the ionopause moves to lower altitudes. The ionopause height in the late afternoon sector observed thus far varies so widely from day to (day that any height variation with solar zenith angle is not apparent in the observations. In the neighborhood of the ionopause, measuremnents of plasma temperatures and densities and magnetic field strength indicate that an induced magnetic barrier plays an important role in the pressure transfer between the solar wind and the ionosphere. The bow, shock is marked by a distinct increase in electron current collected by the instrument, a featutre that provides a convenient identification of the bow shock location. 相似文献
7.
Bridge HS Lazarus AJ Snyder CW Smith EJ Davis L Coleman PJ Jones DE 《Science (New York, N.Y.)》1967,158(3809):1669-1673
Abrupt changes in the amplitude of the magnetic fluctuations, in the field strength, and in the plasma properties, were observed with Mariner V near Venus. They provide clear evidence for the presence of a bow shock around the planet, similar to, but much smaller than, that observed at Earth. The observations appear consistent with an interaction of the solar wind with the ionosphere of Venus. No planetary field could be detected, but a steady radial field and very low plasma density were found 10,000 to 20,000 kilometers behind Venus and 8,000 to 12,000 kilometers from the Sun-Venus line. These observations may be interpreted as relating to an expansion wave tending to fill the cavity produced by Venus in the solar wind. The upper limit to the magnetic dipole moment of Venus is estimated to be within a factor of 2 of 10(-3) items that of Earth. 相似文献
8.
Mariner Stanford Group 《Science (New York, N.Y.)》1967,158(3809):1678-1683
Venus has daytime and nighttime ionospheres at the positions probed by radio occulation. The main layers are thin by terrestrial standards, with the nighttime peak concentration of electrons being about two orders of magnitude below that of the daytime peak. Above the nighttime peak were several scale-height regimes extending to a radius of at least 7500, and probably to 9700, kilometers from the center of Venus. Helium and hydrogen at plasma temperatures of 600 degrees to 1100 degrees K seem indicated in the regimes from 6300 to 7500 kilometers, with cooler molecular ions in lower regions. Above the daytime peak a sharp plasmapause was discovered, marking a sudden transition from appreciable ionization concentrations near Venus to the tenuous conditions of the solar wind. This may be indicative of a kind of interaction of the magnetized solar wind with a planetary body that differs from the two different kinds of interaction characterized by Earth and by Moon. For Venus and probably for Mars, the magnetic field of the solar wind may pile up in front of the conducting ionosphere, form an induced magnetosphere that ends at the plasmapause, above which any ionosphere that tends to form is swept away by the shocked solar wind that flows between the stand-off bow-shock and the magnetopause. The neutral atmosphere was also probed and a surface reflection may have been detected, but the data have not yet been studied in detail. Results are consistent with a super-refractive atmosphere, as expected from Soviet measurements near the surface. Thus, two unusual features of Venus can be described in terms of a light trap in the lower atmosphere, and a magnetic trap in the conducting ionosphere. 相似文献
9.
Frank LA Paterson WR Ackerson KL Coroniti FV Vasyliunas VM 《Science (New York, N.Y.)》1991,253(5027):1528-1531
Plasma measurements were obtained with the Galileo spacecraft during an approximately 3.5-hour interval in the vicinity of Venus on 10 February 1990. Several crossings of the bow shock in the local dawn sector were recorded before the spacecraft passed into the solar wind upstream from this planet. Although observations of ions of the solar wind and the postshock magnetosheath plasmas were not possible owing to the presence of a sunshade for thermal protection of the instrument, solar wind densities and bulk speeds were determined from the electron velocity distributions. A magnetic field-aligned distribution of hotter electrons or ;;strahl' was also found in the solar wind. Ions streaming into the solar wind from the bow shock were detected. Electron heating at the bow shock, 相似文献
10.
The cloud-level atmosphere of Venus takes little more than 4 days to complete one rotation, whereas the solid planet below has a 243-day period. Computer simulations of the circulation of the Venus middle atmosphere between 40 and 85 kilometers, as driven by solar radiation absorbed in the clouds, reproduce (i) the observed cloud-level rotation rate, (ii) strong vertical shears above and below the cloud tops, and (iii) midlatitude jets and strong poleward flow on the day side. Simulated circulations converge to yield nearly the same zonal winds when initialized with both stronger or weaker rotation rates. These results support the hypothesis that the observed cloud-top rotation rate is maintained by statistical balance between fluxes of momentum by thermal tides and momentum advection by mean meridional circulation. 相似文献
11.
Knudsen WC Spenner K Whitten RC Spreiter JR Miller KL Novak V 《Science (New York, N.Y.)》1979,203(4382):757-763
Thermal plasma quantities measured by, the retarding potential analyzer (RPA) are, together with companion Pioneer Venus measurements, the first in situ measurements of the Venus ionosphere. High ionospheric ion and electron temperatures imply significant solar wind heating of the ionosphere. Comparison of the measured altitude profiles of the dominant ions with an initial modlel indicates that the ionosphere is close to diffusive equilibrium. The ionopause height was observed to vary from 400 to 1000 kilometers in early orbits. The ionospheric particle pressure at the ionopause is apparently balanced at a solar zenith angle of about 70 degrees by the magnetic field pressure with little contribution from energetic solar wind particles. The measured ratio of ionospheric scale height to ionopause radius is consistent with that inferred from previously measured bow shock positions. 相似文献
12.
Greeley R Schubert G Limonadi D Bender KC Newman WI Thomas PE Weitz CM Wall SD 《Science (New York, N.Y.)》1994,263(5145):358-361
Magellan images reveal surface features on Venus attributed to wind processes. Sand dunes, wind-sculpted hills, and more than 5830 wind streaks have been identified. The streaks serve as local "wind vanes," representing wind direction at the time of streak formation and allowing the first global mapping of near-surface wind patterns on Venus. Wind streaks are oriented both toward the equator and toward the west. When streaks associated with local transient events, such as impact cratering, are deleted, the westward component is mostly lost but the equatorward component remains. This pattern is consistent with a Hadley circulation of the lower atmosphere. 相似文献
13.
The Pioneer Venus electric field detector observes significant effects of the interaction of the solar wind with the ionosphere of Venus all along the orbiter trajectory. Information is obtained on plasma oscillations emitted by suprathermal electrons beyond the bows shock, on sharp and diffuse shock structures, and on waveparticle interaction phenomena that are important near the boundary of the dayside ionosphere. 相似文献
14.
Luz D Berry DL Piccioni G Drossart P Politi R Wilson CF Erard S Nuccilli F 《Science (New York, N.Y.)》2011,332(6029):577-580
Initial images of Venus's south pole by the Venus Express mission have shown the presence of a bright, highly variable vortex, similar to that at the planet's north pole. Using high-resolution infrared measurements of polar winds from the Venus Express Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument, we show the vortex to have a constantly varying internal structure, with a center of rotation displaced from the geographic south pole by ~3 degrees of latitude and that drifts around the pole with a period of 5 to 10 Earth days. This is indicative of a nonsymmetric and varying precession of the polar atmospheric circulation with respect to the planetary axis. 相似文献
15.
Taylor HA Brinton HC Bauer SJ Hartle RE Cloutier PA Michel FC Daniell RE Donahue TM Maehl RC 《Science (New York, N.Y.)》1979,203(4382):755-757
Bennett radio-frequency ion mass spectrometers have returned the first in situ measurements of the Venus dayside ion composition, including evidence of pronounced structural variability resulting from a dynamic interaction with the solar wind. The ionospheric envelope, dominated above 200 kilometers by O(+), responds dramatically to variations in the solar wind pressure, Which is observed to compress the thermal ion distributions from heights as great as 1800 kilometers inward to 280 kilometers. At the thermal ion boundary, or ionopause, the ambient ions are swept away by the solar wind, such that a zone of accelerated suprathermnal plasma is encountered. At higher altitudes, extending outward on some orbits for thousands of kilometers to the bows shock, energetic ion currents are detected, apparently originating from the shocked solar wind plasma. Within the ionosphere, observations of pass-to-pass differences in the ion scale heights are indicative of the effects of ion convection stimlulated by the solar wind interaction. 相似文献
16.
Fourteen profiles of electron density in the ionosphere of Venus were obtainecd by the dual-frequency radio occulation method with the Pioneer Venus orbiter between 5 and 30 December 1978. The solar zenith angles for these measurements were between about 85 degrees and 92 degrees , and the latitudes ranged from about 81 degrees to 88 degrees (ecliptic north). In addition to the expected decreasein peak electron density from about 1.5 x 10(3) to 0.5 x 10(3) per cubic centimeter with increasing solar zenith angle, a region of almost constant electron density above about 250 kilometers was observed. The ionopause height varies from about 300 to 700 kilometers and seems to be influenced by diurnal changes in solar wind conditions. The structures of the profiles are consistent with models in which O(2)(+) dominates near the ionization peak and is replaced by O(+) at higher altitudes. 相似文献
17.
Brace LH Theis RF Niemann HB Mayr HG Hoegy WR Nagy AF 《Science (New York, N.Y.)》1979,205(4401):102-105
Empirical models of the electron temperature and electron density of the late afternoon and nightside Venus ionosphere have been derived from Pioneer Venus measurements acquired between 10 December 1978 and 23 March 1979. The models describe the average ionosphere conditions near 18 degrees N latitude between 150 and 700 kilometers altitude for solar zenith angles of 80 degrees to 180 degrees . The average index of solar flux was 200. A major feature of the density model is the factor of 10 decrease beyond 90 degrees followed by a very gradual decrease between 120 degrees and 180 degrees . The density at 150 degrees is about five times greater than observed by Venera 9 and 10 at solar minimum (solar flux approximately 80), a difference that is probably related to the effects of increased solar activity on the processes that maintain the nightside ionosphere. The nightside electron density profile from the model (above 150 kilometers) can be reproduced theoretically either by transport of 0(+) ions from the dayside or by precipitation of low-energy electrons. The ion transport process would require a horizontal flow velocity of about 300 meters per second, a value that is consistent with other Pioneer Venus observations. Although currently available energetic electron data do not yet permit the role of precipitation to be evaluated quantitatively, this process is clearly involved to some extent in the formation of the nightside ionosphere. Perhaps the most surprising feature of the temperature model is that the electron temperature remains high throughout the nightside ionosphere. These high nocturnal temperatures and the existence of a well-defined nightside ionopause suggest that energetic processes occur across the top of the entire nightside ionosphere, maintaining elevated temperatures. A heat flux of 2 x 10(10) electron volts per square centimeter per second, introduced at the ionopause, is consistent with the average electron temperature profile on the nightside at a solar zenith angle of 140 degrees . 相似文献
18.
Helium is removed at an average rate of 10(6) atoms per square centimeter per second from Venus's atmosphere by the solar wind following ionization above the plasmapause. The surface source of helium-4 on Venus is similar to that on Earth, suggesting comparable abundances of crustal uranium and thorium. 相似文献
19.
Pioneer Venus orbiter dual-frequency radio occultation measurements have produced many electron density profiles of the nightside ionosphere of Venus. Thirty-six of these profiles, measured at solar zenith angles (chi) from 90.60 degrees to 163.5 degrees , are discussed here. In the "deep" nightside ionosphere (chi > 110 degrees ), the structure and magnitude of the ionization peak are highly variable; the mean peak electron density is 16,700 +/- 7,200 (standard deviation) per cubic centimeter. In contrast, the altitude of the peak remains fairly constant with a mean of 142.2 +/- 4.1 kilometers, virtually identical to the altitude of the main peak of the dayside terminator ionosphere. The variations in the peak ionization are not directly related to contemporal variations in the solar wind speed. It is shown that electron density distributions similar to those observed in both magnitude and structure can be produced by the precipitation on the nightside of Venus of electron fluxes of about 108 per square centimeter per second with energies less than 100 electron volts. This mechanism could very likely be responsible for the maintenance of the persistent nightside ionosphere of Venus, although transport processes may also be important. 相似文献
20.
Initial observations by the Pioneer Venus mangnetometer in the sunlit ionosphere reveal a dynamic ionosphere, very responsive to external solar-wind conditions. The localtions of the bow shock and ionosphere are variable. The strength of the magnetic field just olutside the ionopause is in approximate pressure balance with the thermal plasma of the ionosphere and changes markedly from day, to day in response to changes in solar wind pressure. The field strength in the ionosphere is also variable from day to day. The field is often weak, at most a few gammas, but reaching many tens of gammas for periods of the order of seconds. These field enchantments are interpreted as due to the passage of spacecraft through flux ropes consisting of bundles of twisted field lines surrounded by the ionospheric plasma. The helicity of the flux varies through the flux tube, with lows pitch angles on the inside and very lage angles in the low-field outer edges of the ropes. These ropes may have external or internal sources. Consistent with previous results, the average position of the bow shock is much closer to the planet than would be expected if the solar wnd were completely deflected by the planet. In total, these observations indicate that the solar wind plays a significant role in the physics of the Venus ionosphere. 相似文献