摘要:

Plasma electron observations made on board Mariner 10 during its three encounters with the planet Mercury show that the planet interacts with the solar wind to form a bow shock and a permanent magnetosphere. The observations provide a determination of the dimensions and properties of the magnetosphere, independently of and in general agreement with magnetometer observations. The magnetosphere of Mercury appears to be similar in shape to that of the earth but much smaller in relation to the size of the planet. The average distance from the center of Mercury to the subsolar point of the magnetopause is ∼ 1.4 planetary radii. Electron populations similar to those found in the earth's magnetotail, within the plasma sheet and adjacent regions, were observed at Mercury; both their spatial location and the electron energy spectra within them bear qualitative and quantitative resemblance to corresponding observations at the earth. In general, the magnetosphere of Mercury resembles to a marked degree a reduced version of that of the earth, there being no significant differences of structure revealed by the Mariner 10 observations. Quantities in the two magnetospheres are related by simple scaling laws. The size of Mercury relative to its magnetosphere precludes, however, the existence of stably trapped particle belts and of inner magnetosphere (L≲ 8 at the earth) phenomena generally. It is also expected that gradient‐curvature drift and loss cone effects should be relatively more important at Mercury, but no observations bearing on these points were obtained. Due to the limited shielding provided by its relatively weak magnetic dipole moment, the surface of Mercury is everywhere subject to bombardment by cosmic rays and solar energetic particles with energies greater than 1 MeV/nucleon. The region of potential precipitation by magnetospheric plasma particles is estimated to be a ring, roughly similar to the terrestrial auroral oval but broader and lying at lower latitudes, 50–57° on the noon meridian and 25–35° at midnight. The observed magnetospheric plasma densities are generally too high to be accounted for by any plausible ionospheric source, and a solar wind origin

ISSN:0148-0227    

DOI:10.1029/JA082i013p01807

年代:1977

数据来源: WILEY

摘要:

Measurements of the angular distribution of auroral kilometric radiation (AKR) are presented by using observations from the Hawkeye 1, Imp 6, and Imp 8 satellites. The University of Iowa plasma wave experiments on Hawkeye 1 and Imp 6 provide electric field measurements of AKR in narrow frequency bands centered at 178, 100, and 56.2 kHz, and the Imp 8 experiment provides measurements at 500 kHz. From a frequency of occurrence survey, at radial distances greater than 7RE(earth radii) it is shown that AKR is preferentially and instantaneously beamed into solid angles of approximately 3.5 sr at 178 kHz, 1.8 sr at 100 kHz, and 1.1 sr at 56.2 kHz, directed upward from the nighttime auroral zones. Simultaneous multiple satellite observations of AKR in the northern hemisphere show that the radiation occurs simultaneously throughout these solid angles and that the plasmapause acts as an abrupt propagation cutoff on the nightside of the earth. No comparable cutoff is observed at the plasmapause on the dayside of the earth.The results of computer ray tracing calculations for both the right‐hand (R‐X) and left‐hand (L‐O) polarized modes are also presented in an attempt to understand the propagation characteristics of the AKR. These calculations assume that a small source emits radiation at various frequencies along a magnetic field line at 70° invariant latitude near local midnight. The approximate altitude of the source can be determined for each of the two modes of propagation by adjusting the source altitude to give the best fit to the observed angular distributions. TheR‐Xmode is found to give the best agreement with the observed angular dis

ISSN:0148-0227    

DOI:10.1029/JA082i013p01825

年代:1977

数据来源: WILEY

摘要:

Ionospheric modification by intense radio waves is known to produce energetic tails on the electron velocity distribution function, resulting in enhanced airglow. We demonstrate that these energetic tails may be due to quasi‐linear velocity space diffusion in the presence of the background magnetic field. The waves responsible for the diffusion are produced by the parametric decay instability. The magnetic quasilinear diffusion considered here may often be more important than the unmagnetized resonance‐broadened diffusion considered by Weinstock; however, the two effects are additive, so that the predictions of either theory by itself are conservat

ISSN:0148-0227    

DOI:10.1029/JA082i013p01839

年代:1977

数据来源: WILEY

摘要:

The general dispersion relation for slow waves propagating in plasmas of arbitrary β is discussed in detail along with some possible applications to the magnetosphere. It is shown that the electromagnetic corrections to the quasi‐static approximation are important only at frequencies below the lower hybrid frequency when the parallel phase velocity is greater than the electron thermal speed. Next, a comparison between the properties of these slow waves and those of the fast electromagnetic ones is given. Adopting an ordering scheme in the frequency range close to the ion gyrofrequency appropriate to the physical parameters of interest, we show that the perpendicular phase velocity of the fast electromagnetic waves should be at least 1 order of magnitude greater than the thermal velocity, which imposes very stringent propagation conditions on the properties of the whistler and ion cyclotron waves close to the particle gyrofrequency. Applications to the magnetosphere are discussed. It is shown that it is almost impossible for the whistler mode or the electromagnetic ion cyclotron mode to be amplified by instabilities associated with the high perpendicular thermal velocities characteristic of the magnetospheric plasma. Finally, a new process concerning the ducting of the whistler mode along the geomagnetic field lines is propos

ISSN:0148-0227    

DOI:10.1029/JA082i013p01846

年代:1977

数据来源: WILEY

摘要:

The 0–1 band (4278 Å) of the first negative system of nitrogen has been measured in the spectrum of the night sky with a 1‐m Ebert‐Fastie spectrophotometer at the Arecibo Observatory. Integration periods of approximately 4 hours were needed to provide an adequate ratio of signal to noise. The measured spectra were corrected for blending with the 2–9 Herzberg band of oxygen at 4281 Å. The intensity of the 4278‐Å radiation measured on 6 nights in late March 1976 was about 0.1 R. Little variation in the intensity was observed from night to night. We suggest that most of the observed emission was produced not by charged particle bombardment of the atmosphere but by photoionization of N2by scattered solar radiation at 3

ISSN:0148-0227    

DOI:10.1029/JA082i013p01855

年代:1977

数据来源: WILEY

摘要:

From June 11 to September 16, 1974, the NOAA low‐energy proton detector on board the ATS 6 satellite observed 71 cases of ultralow‐frequency oscillations of proton flux intensities. The oscillation periods varied from 40 s to 6 min, and the events were observed most frequently during moderate geomagnetic conditions. The flux oscillations occurred at various local times, yet almost two thirds of the events were detected in the near‐dusk region of the magnetosphere. For a majority of the events in this set a substantial phase shift in flux oscillation was detected between different energy channels and/or between two oppositely oriented detector telescopes. The phase shift is mainly due to the finite gyroradius effect of the protons gyrating in the geomagnetic field. By examining this finite gyroradius effect on the perturbed particle distribution function associated with the wave in a nonuniform magnetic field we are able to determine the propagation direction of the wave from particle observations made by a single spacecraft. Although the type of wave and its excitation mechanism can only be conjectured at the present time, it is concluded that the wave propagates in the westward direction with a phase velocity of about 13 km/s. Furthermore, it also has a very small phase velocity of ∼0.15 km/s propagating toward the earth. If the wave had been traveling 1 hour or so before it was observed near the dusk magnetosphere, it might have originated in the dark magnetosphere in association with some changes in geophysical conditions. The statistical correlation between the times of the observed wave events and the onsets of the auroral magnetic bays indicates that although they seldom occurred simultaneously, 80% of the waves were observed within 1 hour of the bay onset. Therefore it is concluded that the condition of the magnetosphere after a substorm is favorable for the occurrence of the ring current proton ultralow‐freque

ISSN:0148-0227    

DOI:10.1029/JA082i013p01859

年代:1977

数据来源: WILEY

摘要:

Rocket observations of the lower ionosphere in the winter of 1971 at two locations show differences of electron density which are attributed to enhancements of nitric oxide and energetic electron fluxes precipitated into the mesosphere during the poststorm phase of a geomagnetic storm. Electron density distributions were observed above Wallops Island, Virginia, and Keweenaw, Michigan, larger values occurring at Keweenaw. Energetic electron fluxes were greater at Keweenaw (L= 3.9) than at Wallops Island (L= 2.5). While particle ionization was the dominant factor in establishing the electron density during one measurement at Keweenaw, particles were not present 2 days earlier, even though the electron density distribution was significantly larger than that observed at Wallops Island on both occasions. An accompanying ion composition profile measured at Keweenaw during the earlier flight showed NO+to be the dominant ion to 76 km, where the concentration of hydrated ions, H3O+· (H2O)n, exceeded that of NO+. This lowering of the transition height from NO+to hydrated species is in agreement with independent observations ofDregion ion composition during anomalous winter conditions

ISSN:0148-0227    

DOI:10.1029/JA082i013p01869

年代:1977

数据来源: WILEY

摘要:

Results are presented from low‐energy plasma analyzers (12 eV to 12 keV) carried on two rockets launched into the dayside cleft during January 1975. We conclude that (1) atmospheric interaction becomes important for2 keV) electrons intermingle with ‘magnetosheathlike’ plas

ISSN:0148-0227    

DOI:10.1029/JA082i013p01876

年代:1977

数据来源: WILEY

摘要:

The dependence of the growth rate and the threshold level of the whistler heat flux instability in the solar wind at 1 AU on the shape of the velocity distribution function and on the electron halo temperature anisotropy is examined by using a drifted bi‐Lorentzian distribution function. We find that for a given set of plasma parameters the maximum growth rate increases sharply with increasing slope of the halo electron velocity distribution and that the whistler instability threshold level increases sharply with increasing halo temperature anisotropy. In view of the sensitivity of the whistler instability threshold level to details of the shape of halo electron distribution functions it is not now possible to decide unambiguously if the whistler mode regulates the interplanetary heat flux some of the time at 1 AU, but it remains a strong candidat

ISSN:0148-0227    

DOI:10.1029/JA082i013p01889

年代:1977

数据来源: WILEY

摘要:

An examination is made of the impact of a layer of particulate matter, assumed to be ice crystals, on the albedo of the polar region. The model is time dependent, includes the growth of the layer, and incorporates the diffuse nature of radiation reflected from the surface and atmosphere. Although the magnitude of the effect is about an order of magnitude less than previous results, the impact is one of heating instead of cooling. It is also shown that ignoring the diffuse nature of the radiation reflected from the underlying earth‐atmosphere system, as has been done in many previous simple models, can result in overestimation of the climatological impact of aerosols in sign and magnitude by a factor of up to 4

ISSN:0148-0227    

DOI:10.1029/JA082i013p01893

年代:1977

数据来源: WILEY