摘要:

A line at 1280 Å, due to Raman scattering of solar Lyman alpha (Ly‐α) in the atmosphere of Uranus, has been detected by the Voyager Ultraviolet Spectrometer. The measured intensity of 40 ± 20 R implies that 200 R to 500 R of the measured 1500 R Ly‐α intensity at the sub‐solar point is due to Rayleigh scattering of the solar line. The presence of Rayleigh and Raman scattering at 1216 Å suggests that the Uranian atmosphere is largely devoid of absorbing hydrocarbons above the 0.5 mbar level. The most natural explanation of this depletion is very weak vertical mixing equivalent to an eddy coefficient on the order of 200 cm² s−1between 0.5 mbar

ISSN:0094-8276    

DOI:10.1029/GL014i005p00483

年代:1987

数据来源: WILEY

摘要:

The two‐dimensional structure of the solar wind near the sun is analyzed by the method of MHD simulation. The helmet‐streamer configuration with a sharp boundary between the closed and open field regions has been reproduced. Due to the pressure‐driven current in the sheet, the magnetic field, which has a dipole configuration at the photosphere, is found to behave like R−2at a large distance, R, from

ISSN:0094-8276    

DOI:10.1029/GL014i005p00487

年代:1987

数据来源: WILEY

摘要:

In 1980 and 1983 the asteroid 2201 Oljato passed inside the orbit of Venus, respectively 65 and 21 days prior to the passage of Venus, apparently causing disturbances in the interplanetary magnetic field, possibly through the interaction of its debris trail with the solar wind. In July 1986 the asteroid passed through the inner solar system 25 days after Venus and we expected to find similar disturbances if the debris trail extended in front of the asteroid. The expected disturbances were observed. All data from the longitude range of the disturbances have been examined for all Venus years as well as from a control period over the same solar longitudes but at different ecliptic longitudes. No enhancements in the rate of occurrence of events were seen in the control periods during 1980, 1983 and 1986, and the rate was uniformly low at all other times. The new data reaffirm the association of solar wind disturbances with the asteroidal passage and imply a debris trail that extends perhaps 1 AU both in front of and behind the asteroid.

ISSN:0094-8276    

DOI:10.1029/GL014i005p00491

年代:1987

数据来源: WILEY

摘要:

Newborn ions acquire highly unstable velocity distributions that, in the solar wind frame, might range from beam‐ to ring‐like. Solution of the kinetic dispersion equation shows that heavy pickup ions with large perpendicular energies excite a new hydromagnetic wave mode with maximum growth, high (magnetic field and mass density) compression ratios, and almost linear polarization coexisting slightly away (6° ‐ 15°) from parallel propagation. The mode arises from the coupling of the heavy ion beam to the modified left‐hand wave dispersion caused by the multi‐ion nature of the medium. The ion beam can also bring about nonoscillatory, purely growing structures distinct from the mirror wave through the coupling of a counter‐streaming left‐hand mode to a co‐streaming right‐hand wave. The free energy source parameters are varied to determine their relative influence and define the domain of existence of the instability. Its characteristics may prove helpful in the interpretation of recent cometary (Halley and Giacobini‐Zinner) and artificial release

ISSN:0094-8276    

DOI:10.1029/GL014i005p00495

年代:1987

数据来源: WILEY

摘要:

Recent observations at comet Halley show that the region within which cometary ions become the dominant component lies outside of the magnetic field‐free cavity. This behavior resembles that found at Venus under conditions where the incident solar wind dynamic pressure exceeds the ionospheric pressure. On these occasions the magnetosheath magnetic field is found well inside of the region where planetary ions are observed. Although scaling and the details of formation of the inner boundary of the magnetic field are different for these two objects, the processes by which the interplanetary magnetic field penetrates into the ionospheres at Venus and at Comet Halley are in many ways analogou

ISSN:0094-8276    

DOI:10.1029/GL014i005p00499

年代:1987

数据来源: WILEY

摘要:

A numerical gas dynamic model, which includes the effects of mass loading of the shocked solar wind, was used to calculate the density and magnetic field variations in the magnetosheath of Venus. These calculations were carried out for conditions corresponding to a specific orbit of the Pioneer Venus Orbiter (PVO orbit 582). A comparison of the model predictions and the measured shock position, density and magnetic field values showed a reasonable agreement, indicating that a gas dynamic model that includes the effects of mass loading can be used to predict these parameters.

ISSN:0094-8276    

DOI:10.1029/GL014i005p00503

年代:1987

数据来源: WILEY

摘要:

The variation of the average low altitude dayside ionospheric magnetisation with solar wind dynamic pressure is studied using the magnetometer measurements from the Pioneer Venus Orbiter. The data analysed, includes the first three dayside periapsis passes by the orbiter Passes with even low magnetisation values have been included in the analysis. The results indicate a statistically linear relationship between the ionospheric magnetisation and the solar wind dynamic pressure even below the critical pressure for magnetisation adopted earlier by Phillips et al. (1984). Also similar features in the low altitude magnetic field structure are seen to persist for the whole range of solar wind pressures. These observations point to a continuous rather than time evolving mechanism of generation of ionospheric magnetic fields on Venus.

ISSN:0094-8276    

DOI:10.1029/GL014i005p00507

年代:1987

数据来源: WILEY

摘要:

For two VIKING orbits crossing the night oval source region of Auroral Kilometric Radiation (AKR), we derive the AKR polarization from the cutoff‐frequencies and from the spin modulated pattern of the emission. The R‐X mode is predominant, but can be accompanied by L‐O mode and Z mode band. The Z mode is observed continuously from inside the plasmasphere to the AKR source region. An offset in the spin modulation pattern near the AKR source region during one of the orbits shows that steep gradients of the plasma density at the edge of the source region can play an important role for the orientation of the wave polarization plane or for the mode conve

ISSN:0094-8276    

DOI:10.1029/GL014i005p00511

年代:1987

数据来源: WILEY

摘要:

The source regions of Auroral Kilometric Radiation (AKR), as observed by the VIKING satellite, are usually characterized by steep gradients of the thermal electron density. On the polar and equatorial edges of these thermal density depletions, very intense bursts of broadband electrostatic noise are detected in a frequency range extending from the low frequency cutoff of the high frequency wave receiver (10 kHz) up to the neighborhood of the local electron gyrofrequency. The intensity of these bursts is at least of the same order of magnitude as the maximum intensity of AKR, which takes place in the center of the cavity. A possible link between the presence of the electrostatic bursts and the AKR generation mechanisms is investigated.

ISSN:0094-8276    

DOI:10.1029/GL014i005p00515

年代:1987

数据来源: WILEY

摘要:

Electrodynamic tethers are wires deployed across the earth's geomagnetic field through which a current is flowing. The radiation impedance of a tether with end connectors carrying an AC current is computed from classical antenna theory. This simulates the use of a tether on a space structure. It is shown that the current flow pattern at the tether connector is critical to determining the overall radiation impedance. If the tether makes direct electrical contact with the ionosphere then radiation impedances of the order of several thousand Ohms can be expected. If the only electrical contact is through the end connectors then the impedance is only a few Ohms for a DC current rising to several tens of Ohms for an AC current with frequencies in the whistler range.

ISSN:0094-8276    

DOI:10.1029/GL014i005p00519

年代:1987

数据来源: WILEY