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1.
Radio emissions from Uranus were detected by the Voyager 2 plasma wave instrument about 5 days before closest approach at frequencies of 31.1 and 56.2 kilohertz. About 10 hours before closest approach the bow shock was identified by an abrupt broadband burst of electrostatic turbulence at a radial distance of 23.5 Uranus radii. Once Voyager was inside the magnetosphere, strong whistler-mode hiss and chorus emissions were observed at radial distances less than about 8 Uranus radii, in the same region where the energetic particle instruments detected intense fluxes of energetic electrons. Various other plasma waves were also observed in this same region. At the ring plane crossing, the plasma wave instrument detected a large number of impulsive events that are interpreted as impacts of micrometer-sized dust particles on the spacecraft. The maximum impact rate was about 30 to 50 impacts per second, and the north-south thickness of the impact region was about 4000 kilometers.  相似文献   

2.
The first inbound Voyager 2 crossing of Saturn's bow shock [at 31.7 Saturn radii (RS), near local noon] and the last outbound crossing (at 87.4 RS, near local dawn) had similar plasma wave signatures. However, many other aspects of the plasma wave measurements differed considerably during the inbound and outbound passes, suggesting the presence of effects associated with significant north-south or noon-dawn asymmetries, or temporal variations. Within Saturn's magnetosphere, the plasma wave instrument detected electron plasma oscillations, upper hybrid resonance emissions, half-gyrofrequency harmonics, hiss and chorus, narrowband electromagnetic emissions and broadband Saturn radio noise, and noise bursts with characteristics of static. At the ring plane crossing, the plasma wave instrument also detected a large number of intense impulses that we interpret in terms of ring particle impacts on Voyager 2.  相似文献   

3.
The Voyager 1 plasma wave instrument detected many familiar types of plasma waves during the encounter with Saturn, including ion-acoustic waves and electron plasma oscillations upstream of the bow shock, an intense burst of electrostatic noise at the shock, and chorus, hiss, electrostatic electron cyclotron waves, and upper hybrid resonance emissions in the inner magnetosphere. A clocklike Saturn rotational control of low-frequency radio emissions was observed, and evidence was obtained of possible control by the moon Dione. Strong plasma wave emissions were detected at the Titan encounter indicating the presence of a turbulent sheath extending around Titan, and upper hybrid resonance measurements of the electron density show the existence of a dense plume of plasma being carried downstream of Titan by the interaction with the rapidly rotating magnetosphere of Saturn.  相似文献   

4.
The plasma wave instrument on the International Cometary Explorer (ICE) detected bursts of strong ion acoustic waves almost continuously when the spacecraft was within 2 million kilometers of the nucleus of comet Giacobini-Zinner. Electromagnetic whistlers and low-level electron plasma oscillations were also observed in this vast region that appears to be associated with heavy ion pickup. As ICE came closer to the anticipated location of the bow shock, the electromagnetic and electrostatic wave levels increased significantly, but even in the midst of this turbulence the wave instrument detected structures with familiar bow shock characteristics that were well correlated with observations of localized electron heating phenomena. Just beyond the visible coma, broadband waves with amplitudes as high as any ever detected by the ICE plasma wave instrument were recorded. These waves may account for the significant electron heating observed in this region by the ICE plasma probe, and these observations of strong wave-particle interactions may provide answers to longstanding questions concerning ionization processes in the vicinity of the coma. Near closest approach, the plasma wave instrument detected broadband electrostatic noise and a changing pattern of weak electron plasma oscillations that yielded a density profile for the outer layers of the cold plasma tail. Near the tail axis the plasma wave instrument also detected a nonuniform flux of dust impacts, and a preliminary profile of the Giacobini-Zinner dust distribution for micrometer-sized particles is presented.  相似文献   

5.
Results of measurements of plasma electrons and poitive ions made during the Voyager 2 encounter with Saturn have been combined with measurements from Voyager 1 and Pioneer 11 to define more clearly the configuration of plasma in the Saturnian magnetosphere. The general morphology is well represented by four regions: (i) the shocked solar wind plasma in the magnetosheath, observed between about 30 and 22 Saturn radii (RS) near the noon meridian; (ii) a variable density region between approximately 17 RS and the magnetopause; (iii) an extended thick plasma sheet between approximately 17 and approximately 7 RS symmetrical with respect to Saturn's equatorial plane and rotation axis; and (iv) an inner plasma torus that probably originates from local sources and extends inward from L approximately 7 to less than L approximately 2.7 (L is the magnetic shell parameter). In general, the heavy ions, probably O(+), are more closely confined to the equatorial plane than H(+), so that the ratio of heavy to light ions varies along the trajectory according to the distance of the spacecraft from the equatorial plane. The general configuration of the plasma sheet at Saturn found by Voyager 1 is confirmed, with some notable differences and additions. The "extended plasma sheet," observed between L approximately 7 and L approximately 15 by Voyager 1 is considerably thicker as observed by Voyager 2. Inward of L approximately 4, the plasma sheet collapses to a thin region about the equatorial plane. At the ring plane crossing, L approximately 2.7, the observations are consistent with a density of O(+) of approximately 100 per cubic centimeter, with a temperature of approximately 10 electron volts. The location of the bow shock and magnetopause crossings were consistent with those previously observed. The entire magnetosphere was larger during the outbound passage of Voyager 2 than had been previously observed; however, a magnetosphere of this size or larger is expected approximately 3 percent of the time.  相似文献   

6.
The Voyager I plasma wave instrument detected low-frequency radio emissions, ion acoustic waves, and electron plasma oscillations for a period of months before encountering Jupiter's bow shock. In the outer magnetosphere, measurements of trapped radio waves were used to derive an electron density profile. Near and within the Io plasma torus the instrument detected high-frequency electrostatic waves, strong whistler mode turbulence, and discrete whistlers, apparently associated with lightning. Some strong emissions in the tail region and some impulsive signals have not yet been positively identified.  相似文献   

7.
Planetary radio astronomy measurements obtained by Voyager 2 near Saturn have added further evidence that Saturnian kilometric radiation is emitted by a strong dayside source at auroral latitudes in the northern hemisphere and by a weaker source at complementary latitudes in the southern hemisphere. These emissions are variable because of Saturn's rotation and, on longer time scales, probably because of influences of the solar wind and Dione. The electrostatic discharge bursts first discovered by Voyager 1 and attributed to emissions from the B ring were again observed with the same broadband spectral properties and an episodic recurrence period of about 10 hours, but their occurrence frequency was only about 30 percent of that detected by Voyager 1. While crossing the ring plane at a distance of 2.88 Saturn radii, the spacecraft detected an intense noise event extending to above 1 megahertz and lasting about 150 seconds. The event is interpreted to be a consequence of the impact, vaporization, and ionization of charged, micrometer-size G ring particles distributed over a vertical thickness of about 1500 kilometers.  相似文献   

8.
This report provides an initial survey of results from the plasma wave instrument on the Voyager 2 spacecraft, which flew by Jupiter on 9 July 1979. Measurements made during the approach to the planet show that low-frequency radio emissions from Jupiter have a strong latitudinal dependence, with a sharply defined shadow zone near the equatorial plane. At the magnetopause a new type of broadband electric field turbulence was detected, and strong electrostatic emissions near the upper hybrid resonance frequency were discovered near the low-frequency cutoff of the continuum radiation. Strong whistler-mode turbulence was again detected in the inner magnetosphere, although in this case extending out to substantially larger radial distances than for Voyager 1. In the predawn tail region, continuum radiation was observed extending down to extremely low frequencies, approximately 30 hertz, an indication that the spacecraft was entering a region of very low density, approximately 1.0 x 10(-5) per cubic centimeter, possibly similar to the lobes of Earth's magnetotail.  相似文献   

9.
Fluxes of high energy electrons and protons are found to be highly concentrated near the magnetic equatorial plane from distances of ~ 30 to ~ 100 Jovian radii (R(J)). The 10-hour period of planetary rotation is observed as an intensity variation, which indicates that the equatorial zone of high particle fluxes is inclined with respect to the rotation axis of the planet. At radial distances [unknown] 20 R(J) the synchrotron-radiation-producing electrons with energies greater, similar 3 million electron volts rise steeply to a maximum intensity of ~ 5 x 10(8) electrons per square centimeter per second near the periapsis at 2.8 R(J). The flux of protons with energies greater, similar 30 million electron volts reaches a maximum intensity of ~ 4 x 10(6) protons per square centimeter per second at ~ 3.5 R(J) with the intensity decreasing inside this radial distance. Only for radial distances [unknown] 20 R(J) does the radiation behave in a manner which is similar to that at the earth. Burst of electrons with energies up to 30 million electron volts, each lasting about 2 days, were observed in interplanetary space beginning approximately 1 month before encounter. This radiation appears to have escaped from the Jovian bow shock or magnetosphere.  相似文献   

10.
During the Galileo flyby of Venus the plasma wave instrument was used to search for impulsive radio signals from lightning and to investigate locally generated plasma waves. A total of nine events were detected in the frequency range from 100 kilohertz to 5.6 megahertz. Although the signals are weak, lightning is the only known source of these signals. Near the bow shock two types of locally generated plasma waves were observed, low-frequency electromagnetic waves from about 5 to 50 hertz and electron plasma oscillation at about 45 kilohertz. The plasma oscillations have considerable fine structure, possibly because of the formation of soliton-like wave packets.  相似文献   

11.
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   

12.
Lunar seismic data from artificial impacts recorded at three Apollo seismometers are interpreted to determine the structure of the moon's interior to a depth of about 100 kilomneters. In the Fra Mauro region of Oceanus Procellarum, the moon has a layered crust 65 kilometers thick. The seismic velocities in the upper 25 kilometers are consistent with those in lunar basalts. Between 25 and 65 kilometers, the nearly constant velocity (6.8 kilometers per second) corresponds to velocities in gabbroic and anorthositic rocks. The apparent velocity is high (about 9 kilometers per second) in the lunar mantle immediately below the crust.  相似文献   

13.
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   

14.
Coordinated observations of the earth's bow shock were made as Vela 3A and Explorer 33 passed within 6 earth radii of each other. Elapsed time measurements of shock motion give directly determined velocities in the range 1 to 10 kilometers per second and establish the existence of two regions, one of large amplitude magnetic "shock" oscillations and another of smaller, sunward, upstream oscillations. Each region is as thick as 1 earth radius, or more.  相似文献   

15.
The vector helium magnetometer on the International Cometary Explorer observed the magnetic fields induced by the interaction of comet Giacobini-Zinner with the solar wind. A magnetic tail was penetrated approximately 7800 kilometers downstream from the comet and was found to be 10(4) kilometers wide. It consisted of two lobes, containing oppositely directed fields with strengths up to 60 nanoteslas, separated by a plasma sheet approximately 10(3)kilometers thick containing a thin current sheet. The magnetotail was enclosed in an extended ionosheath characterized by intense hydromagnetic turbulene and interplanetary fields draped around the comet. A distant bow wave, which may or may not have been a bow shock, was observed at both edges of the ionosheath. Weak turbulence was observed well upstream of the bow wave.  相似文献   

16.
Voyager 1 radio occultation measurements of Titan's equatorial atmosphere successfully probed to the surface, which is provisionally placed at a radius of 2570 kilometers. Derived scale heights plus other experimental and theoretical results indicate that molecular nitrogen is the predominant atmospheric constituent. The surface pressure and temperature appear to be about 1.6 bars and 93 K, respectively. The main clouds are probably methane ice, although some condensation of nitrogen cannot be ruled out. Solar abundance arguments suggest and the measurements allow large quantities of surface methane near its triple-point temperature, so that the three phases of methane could play roles in the atmosphere and on the surface of Titan similar to those of water on Earth. Radio occultation measurements of Saturn's atmosphere near 75 degrees south latitude reached a maximum pressure of 1.4 bars, where the temperature is about 156 K. The minimum temperature is about 91 K near the 60-millibar pressure level. The measured part of the polar ionosphere of Saturn has a peak electron concentration of 2.3 x 10(4) per cubic centimeter at an altitude of 2500 kilometers above the 1-bar level in the atmosphere, and a plasma scale height at the top of the ionosphere of 560 kilometers. Attenuation of monochromatic radiation at a wavelength of 3.6 centimeters propagating obliquely through Saturn's rings is consistent with traditional values for the normal optical depth of the rings, but the near-forward scattering of this radiation by the rings indicates effective scattering particles with larger than expected diameters of 10, 8, and 2 meters in the A ring, the outer Cassini division, and the C ring, respectively. Preliminary analysis of the radio tracking data yields new values for the masses of Rhea and Titan of 4.4 +/- 0.3 x 10(-6) and 236.64 +/- 0.08 x 10(-6) times the mass of Saturn. Corresponding values for the mean densities of these objects are 1.33 +/- 0.10 and about 1.89 grams per cubic centimeter. The density of Rhea is consistent with a solar-composition mix of anhydrous rock and volatiles, while Titan is apparently enriched in silicates relative to the solar composition.  相似文献   

17.
The magnetometer and electron reflectometer experiment on the Lunar Prospector spacecraft has obtained maps of lunar crustal magnetic fields and observed the interaction between the solar wind and regions of strong crustal magnetic fields at high selenographic latitude (30 degreesS to 80 degreesS) and low ( approximately 100 kilometers) altitude. Electron reflection maps of the regions antipodal to the Imbrium and Serenitatis impact basins, extending to 80 degreesS latitude, show that crustal magnetic fields fill most of the antipodal zones of those basins. This finding provides further evidence for the hypothesis that basin-forming impacts result in magnetization of the lunar crust at their antipodes. The crustal magnetic fields of the Imbrium antipode region are strong enough to deflect the solar wind and form a miniature (100 to several hundred kilometers across) magnetosphere, magnetosheath, and bow shock system.  相似文献   

18.
Thermal noise spectroscopy was used to measure the density and temperature of the main (cold) electron plasma population during 2 hours (1.5x10(5) kilometers perpendicular to the tail axis) around the point of closest approach of the International Cometary Explorer (ICE) to Comet Giacobini-Zinner. The time resolution was 18 seconds (370 kilometers) in the plasma tail and 54 seconds (1100 kilometers) elsewhere. Near the tail axis, the maximum plasma density was 670 per cubic centimeter and the temperature slightly above 1 electron volt. Away from the axis, the plasma density dropped to 100 per cubic centimeter (temperature, 2x 10(4) K) over 2000 kilometers, then decreased to 10 (1.5x 10(5)K) over 15,000 kilometers; outside that region (plasma tail), the density fluctuated between 10 and 30 per cubic centimeter and the temperature between 1x 10(5) and 4 x10(5) K. The relative density of the hot population rarely exceeded a few percent. The tail was highly asymmetrical and showed much structure. On the other antenna, shot noise was recorded from the plasma particle impacts on the spacecraft body. No evidence was found of grain impacts on the antennas or spacecraft in the plasma tail. This yields an upper limit for the dust flux or particle mass, indicating either fluxes or masses in the tail smaller than implied by the models or an anomalous grain structure. This seems to support earlier suggestions that these grains are featherlike. Outside the tail, and particularly near 10(5) kilometers from its axis, impulsive noises indicating plasma turbulence were observed.  相似文献   

19.
Analysis of the radio-tracking data from Mariner 10 yields 6,023,600 +/- 600 for the ratio of the mass of the sun to that of Mercury, in very good agreement with values determined earlier from radar data alone. Occultation measurements yielded values for the radius of Mercury of 2440 +/- 2 and 2438 +/- 2 kilometers at laditudes of 2 degrees N and 68 degrees N, respectively, again in close agreement with the average equatorial radius of 2439 +/- 1 kilometers determined from radar data. The mean density of 5.44 grams per cubic centimeter deduced for Mercury from Mariner 10 data thus virtually coincides with the prior determination. No evidence of either an ionosphere or an atmosphere was found, with the data yielding upper bounds on the electron density of about 1500 and 4000 electrons per cubic centimeter on the dayside and nightside, respectively, and an inferred upper bound on the surface pressure of 10(-8) millibar.  相似文献   

20.
Data from solar and stellar occultations of Uranus indicate a temperature of about 750 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane and acetylene in the lower levels. The ultraviolet spectrum of the sunlit hemisphere is dominated by emissions from atomic and molecular hydrogen, which are kmown as electroglow emissions. The energy source for these emissions is unknown, but the spectrum implies excitation by low-energy electrons (modeled with a 3-electron-volt Maxwellian energy distribution). The major energy sink for the electrons is dissociation of molecular hydrogen, producing hydrogen atoms at a rate of 10(29) per second. Approximately half the atoms have energies higher than the escape energy. The high temperature of the atmosphere, the small size of Uranus, and the number density of hydrogen atoms in the thermosphere imply an extensive thermal hydrogen corona that reduces the orbital lifetime of ring particles and biases the size distribution toward larger particles. This corona is augmented by the nonthermal hydrogen atoms associated with the electroglow. An aurora near the magnetic pole in the dark hemisphere arises from excitation of molecular hydrogen at the level where its vertical column abundance is about 10(20) per square centimeter with input power comparable to that of the sunlit electroglow (approximately 2x10(11) watts). An initial estimate of the acetylene volume mixing ratio, as judged from measurements of the far ultraviolet albedo, is about 2 x 10(-7) at a vertical column abundance of molecular hydrogen of 10(23) per square centimeter (pressure, approximately 0.3 millibar). Carbon emissions from the Uranian atmosphere were also detected.  相似文献   

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