摘要:

The dominant forces resisting the transport of magnetic field into the inner coma of a comet are ion mass loading from and friction with the expanding neutral atmosphere. A magnetic cavity is thereby created. Close to it the frictional force is most important. Careful interpretation of the magnetic field profile measured during the Giotto flyby of comet P/Halley reveals the existence of an inward directed component of plasma flow of a few km/s, which drops to zero at the boundary of the cavity. The energy transferred from the neutral gas to the plasma by friction and mass loading is responsible for the strongly elevated ion temperatures outside the magnetic cavity. Fitting of the observed magnetic profile and ion temperature distribution yields quantitative determinations of some crucial parameters of the coma.

ISSN:0094-8276    

DOI:10.1029/GL014i007p00673

年代:1987

数据来源: WILEY

摘要:

The optical resolution of the Halley Multicolour Camera (HMC) is limited by its finite aperture, by minute mechanical degradations within the instrument, and by the high speed time delay and integration (TDI) illumination method adapted to the spin of the spacecraft. The sum of all implies a visible loss of image sharpness. The smearing effect has been calibrated using images of a star which can be considered as a point source. The images were superimposed in the frequency domain with subpixel accuracy. With these means the point spread function (PSF) has been determined and applied to the received images using Fourier methods. The effectiveness of the scheme is demonstrated on images of the earth and comet Halley.

ISSN:0094-8276    

DOI:10.1029/GL014i007p00677

年代:1987

数据来源: WILEY

摘要:

During two diffuse energetic ion events upstream of the Earth's bow shock with radial interplanetary magnetic field conditions comprehensive energy spectra of the protons and power spectra of the magnetic field fluctuations were obtained with the instrumentation on AMPTE/IRM. For diffuse ions a continuous distribution from ≈ 5 keV (starting above the solar wind energy) to the instrumental limit (230 keV/e) is found upstream and downstream of the shock. This strongly indicates a single source for the upstream diffuse ion population over the total energy range. Assuming that the ions are shock accelerated, an efficiency for the production of upstream energetic ions of ≈ 4 to 11% is found by comparing the energy flux densities of the energetic ions and of the incoming solar wind. The lower value is observed during the period with a low interplanetary magnetic field strength. The measured energy densities of the particles and the waves are compared with the self‐consistent model of Fermi acceleration by Lee (1982) which includes the coupled wave and particle behaviour. The theoretically predicted wave energy density is in excellent agreement with the observed wave energy de

ISSN:0094-8276    

DOI:10.1029/GL014i007p00681

年代:1987

数据来源: WILEY

摘要:

The Charge‐Energy‐Mass Spectrometer CHEM on the AMPTE/CCE spacecraft provides measurements of oxygen ions with charge states between 1 and 6 in the magnetosphere close to the equatorial plane. The first investigations of the spatial distribution of O3+, O4+, and O5+ions revealed that these ions are continuously present in the magnetosphere. Their L profiles exhibit broad maxima that are located at 6 ⩽ L ⩽ 7 for O3+and O4+and at 7 ⩽ L ⩽ 8 for O5+ions during periods of low to medium geomagnetic activity (Kp<4). With increasing Kp the flux versus L distributions extend further earthward and the maxima move to slightly lower L values. While the flux decrease earthward of the maximum is very steep, the gradual decrease outward of the maximum becomes flatter with increasing charge state. Comparison of these profiles to those of O+, O2+, and O6+ions suggests that O3+, O4+, and O5+ions are produced predominantly by charge exchange processes, and that the O4+and O5+ions are produced from the solar wind origin O6+. Our measurements of O3+indicate that both the solar wind O6+and the terrestrial ions O+and O2+contribute to the production of this

ISSN:0094-8276    

DOI:10.1029/GL014i007p00685

年代:1987

数据来源: WILEY

摘要:

Using magnetic field and precipitating particle data from the DMSP/F7 satellite, we have found that, during extended periods of geomagnetic quiet, the large‐scale field‐aligned current (FAC) system known as Region 1/Region 2 either does not exist or is too small to be observable. A clear dayside cleft FAC system is observed on the preferred side of local noon for the indicated by the IMF By component and the cleft electron precipitation extends far into the region normally considered to be the polar cap. These quiet periods represent times when energy input from the solar wind is low; the IMF is maintained at near zero or slightly northward levels and the solar wind speed is low. For such times, the Kp index is ≤ 1 for several consecutive 3 hour periods. In these cases, the auroral zone is marked by diffuse electron precipitation, and only small‐scale or fine‐scale FAC's. In the polar cap, electron precipitation is characterized by polar rain and there are no FAC systems (including the NBZ system). Some previous reports imply that one should always find this large‐scale Region 1/Region 2 system during quiet times. We suggest that the Region 1/Region 2 system vanishes 10's of mins to hrs after the IMF has turne

ISSN:0094-8276    

DOI:10.1029/GL014i007p00689

年代:1987

数据来源: WILEY

摘要:

Evidence of strong correlation between the counter electrojet and the behaviour of the topside ionosphere is presented here. The reality of the counter electrojet is established by the magnetic field variations of the H‐component and the bottom side ionograms at Huancayo. For the same event, the latitudinal plasma distributions of the topside ionosphere are studied. It is shown that the E‐region electric field associated with the counter electrojet pushed the plasma downwards even on the topside as high as 600 km. The well known crest‐trough formation is also shown to be absent on the counter electroje

ISSN:0094-8276    

DOI:10.1029/GL014i007p00693

年代:1987

数据来源: WILEY

摘要:

The stratospheric perturbation caused by the April 1982 eruption of the Mexican volcano El Chichón (17.3°N) passed through a maximum at northern midlatitudes ten months after the event. The following decay of the aerosol layer could be investigated until the end of 1985 when this period was terminated by renewed volcanism. Lidar backscatter measurements exhibit seasonal decay variations which can primarily be attributed to stratospheric column height variations. In addition, advection of particle loaded air masses contribute to this effect. With respect to the 1977/79 background situation the stratospheric aerosol layer decayed with an e‐folding lifetime of about 1 year, when only summer observations were conside

ISSN:0094-8276    

DOI:10.1029/GL014i007p00696

年代:1987

数据来源: WILEY

摘要:

The mesospheric sodium layer was measured for 42 nights during the period from April to October, 1985, at Syowa Station (69°00′ S, 39°35′ E), Antarctica, by using a dye laser radar (lidar) tuned to the D2line of sodium. This observation was made as part of the Middle Atmosphere Program (MAP). In contrast to the results obtained at mid‐latitudes in the northern hemisphere, the seasonal variation in abundance showed no significant winter maximum. Further, there was no apparent seasonal variation in the vertical distribution of sodium density. However, we observed oscillatory variations in abundance with a period of about 40 days. The nocturnal variations in the sodium density profile have a wavelike structure with a vertical wavelength of 10 to 16 km and a period of 3 to 8 hours, which suggests strongly that gravity waves are propagating upward through the sodium layer. The sodium lidar measurements during auroral activity give evidence that the layer is disturbed associated with the auroral breaku

ISSN:0094-8276    

DOI:10.1029/GL014i007p00700

年代:1987

数据来源: WILEY

摘要:

Physical conditions in the 10–20 km region of the Antarctic stratosphere make the [NO2]/[NO]ratio particularly sensitive to high chlorine levels in the form of ClO. According to simple known photochemical relationships between NO2, NO, ClO, and O3, high ClO levels of 1 ppbv over Antarctica must be accompanied by large values of the [NO2]/[NO]ratio. At high ClO abundances, the [NO2]/[NO]ratio is approximately proportional to the ClO concentration. It is proposed that in‐situ measurements of the [NO2]/[NO]ratio could be used to test the high chlorine hypothe

ISSN:0094-8276    

DOI:10.1029/GL014i007p00704

年代:1987

数据来源: WILEY

摘要:

The paper presents quantitative millimeter wave Doppler radar observations of radar reflectivity and air vertical velocity inside shallow cumuli whose vertical development is restricted by a capping inversion layer. The maximum observed radar reflectivity is −34 to −30 dBZ, which is equivalent to a cloud liquid water content of 0.1 to 0.25 g/m³ depending on cloud droplet size. The main updraft region, which coincides roughly with the maximum reflectivity region, occupies the center of the cloud and extends throughout the radar‐visible cloud depth. Downdrafts exhibit more complex structure and are found on the cloud

ISSN:0094-8276    

DOI:10.1029/GL014i007p00707

年代:1987

数据来源: WILEY