摘要:

The ground‐level energetic solar particle event on 24 May 1990 has an extremely impulsive onset in the particle intensity that we attribute to a significant flux of direct solar neutrons impacting the top of the atmosphere. This is the largest solar neutron event reported to date with a 25.3% increase recorded by the Climax neutron monitor in the time interval 2050–2055 UT. At least 7 neutron monitors in North America recorded this increase simultaneously, the magnitude of which is ordered by the air mass along the line of sight to the sun at the time of the event. The initial solar neutrons arrive within the same minute as the maximum in both the soft X‐ray and the H‐alpha emission observed at the earth, whereas the onset of the world‐wide increase of solar protons occurred approximately 15 minu

ISSN:0094-8276    

DOI:10.1029/91GL02001

年代:1991

数据来源: WILEY

摘要:

A large (fivefold) increase in Mercury's potassium(K) column abundance on 14 October 1987, has been reported bySprague et al.[199O](SKH), who attributed the enhancement to diffusion through the surface in the Caloris Basin, from depths of order 10 km. The postulated source rate is much larger than any previously estimated diffusion rate, and if true certainly affects consideration of the origin of other atmospheric species. However,Killen et al.[1991] have pointed out that the claim is not supported by the published observations of K or sodium(Na) as a whole.Sprague et al.[1991] have responded by further hypothesizing the existence of several other sources of gas diffusing out of the regolith, all of which are time variable. In any case the SKH data indicate large variations in abundance, and it is important to understand the cause. With this issue in mind we have examined the available abundance estimates for correlation with possible controlling physical parameters. We have found a significant correlation between the average zenith K column abundance and indices of solar activity, although we are left with the intrinsic uncertainty of a small data set.

ISSN:0094-8276    

DOI:10.1029/91GL02000

年代:1991

数据来源: WILEY

摘要:

After its successful encounter with comet P/Halley and a four‐years hibernation period ESA's Giotto spacecraft has been reactivated in February 1990 and performed the first‐ever Earth gravity‐assisted maneuver on July 2, 1990 to be retargeted for comet P/Grigg‐Skjellerup. This swing‐by is of unique scientific interest due to Giotto's hyperbolic, high‐inclination orbit. Here, we shall report on scientific results of the Giotto magnetic field experiment. Due to the high fly‐by velocity and the relative quietness of the magnetosphere during the swingby period these measurements present a snapshot view of the Earth magnetosphere with clearly identified inbound and outbound bow shock and magnetopause crossings. The outbound crossings are of particular interest as surface waves at the polar magnetopause at a distance of 28 REas well as a strong quasi‐perpendicular bow shock at a distance of about 6

ISSN:0094-8276    

DOI:10.1029/91GL00550

年代:1991

数据来源: WILEY

摘要:

We have studied the propagation properties of perturbation energy fluxes of subsolar magnetosheath fluctuations. The Poynting flux, kinetic energy flux, and enthalpy flux are calculated using magnetic field and plasma measurements from the AMPTE IRM satellite during five intervals in 1984. The results are then compared with a disturbance indicator R of the magnetic field in the same magnetosheath region. It is shown that during disturbed periods with large transverse variations (low R level), the perturbation Poynting flux and the kinetic energy flux increase, and a significant portion of the fluxes consistently propagates toward the magnetopause. The Poynting flux of those fluctuations which consist of mainly compressional perturbations does not appear to propagate in any certain direction. The enthalpy flux of the perturbations does not propagate in any certain direction in any of the cases. The kinetic energy flux appears to be more important in exciting harmonic ULF waves in the dayside magnetosphere. What portion of this energy flux is transferred into the magnetosphere needs further investigation.

ISSN:0094-8276    

DOI:10.1029/91GL01849

年代:1991

数据来源: WILEY

摘要:

Periods of magnetic conjugacy between ISEE and the magnetometer array of the Institute for Geological Sciences have been examined to search for the simultaneous occurrence of Pc 3,4 magnetic pulsations. When compressional waves are seen in space, waves are also observed on the ground at the same frequency and with similar waveforms. The wave amplitude on the ground at midlatitudes is similar to that in space at ISEE but at high latitudes the amplitudes are larger than in space. The one occurrence of a purely transverse signal at ISEE was not observed on the ground. These results confirm that Pc 3,4. wave energy is most readily transported through the magnetosphere by compressional fluctuations.

ISSN:0094-8276    

DOI:10.1029/91GL01297

年代:1991

数据来源: WILEY

摘要:

Particle simulation is used to demonstrate the formation of ion holes and parallel potential structures in a background plasma driven by an ion beam. Recent satellite observations suggest that such potential structures which are observed along auroral field lines may form in a cool background ion population. We show that if such a background is present ion holes form in the background ion distribution and travel at the acoustic speed in the direction of the ion beam velocity. Linear stability analysis shows this system to be unstable to a beam‐acoustic mode involving the beam ions and the background electrons. These beam unstable modes nonlinearly pump the linearly stable ion acoustic modes in the background plasma seeding the formation of ion holes. The dynamics of these structures are essentially indistinguishable from the evolution of weak double layers seen in previous simulation

ISSN:0094-8276    

DOI:10.1029/91GL02089

年代:1991

数据来源: WILEY

摘要:

Measurements of upward electron beams and ion conics from the polar orbiting Viking satellite have been obtained in the cusp‐cleft region. The relative frequencies of occurrence have been calculated for studies of the MLT, Inv. Lat, and altitude distribution during the period March to June 1986. It was observed that the upward electron beams were more frequent in the dawn sector compared to the dusk sector. Similar observations have also been seen for ion conics.The Invariant Latitude dependence of upflowing electron beams shows a clear maximum around 78°. This agrees well with the maximum for ion conics.The altitude dependence shows a steep increase in frequency above 10,000 km up to the satellite apogee (≈ 13 500 km). This altitude dependence at differentAEindex was also studied giving information about the acceleration region and the altitude profiles at different levels of magnetic disturb

ISSN:0094-8276    

DOI:10.1029/90GL01579

年代:1991

数据来源: WILEY

摘要:

We investigate ion energization and ion conic production by broadband electric field fluctuations. The roles played by resonant fluctuations (which can satisfy the cyclotron resonance condition), and nonresonant fluctuations well removed from the ion gyrofrequency, are compared. We show that in the resonant case the mean ion energy increases essentially linearly with time, while in the nonresonant case this energy rapidly reaches a maximum. Using observed electric spectra we find that nonresonant energization due to high spectral densities at frequencies well below the ion gyrofrequency may be important for the generation of some heavy ion conics, but confirm that heating by resonant electric fluctuations is sufficient to explain most ion conics.

ISSN:0094-8276    

DOI:10.1029/91GL00169

年代:1991

数据来源: WILEY

摘要:

Theoretical profiles of cosmogenic7Be in the atmosphere above 100 km were computed in an effort to explain unexpectedly high accumulations of this radionuclide found on the leading surfaces of the Long Duration Exposure Facility (LDEF) spacecraft. Our diffusion calculations suggest that gravitational fractionation is sufficient to explain most, and possibly all, of the observed7Be enrichment at the recovery altitude of the spacecraft, provided only that the nuclide exists at and above the turbopause predominantly in the form of free atoms. Upward diffusion of7Be atoms through the turbopause appears to occur rapidly enough to largely offset losses at higher altitudes due to radioactive decay. The same model is used to compute cosmogenic10Be profiles in order to predict the probable outcome of planned measurements of10Be accumulations on LDEF surfaces.

ISSN:0094-8276    

DOI:10.1029/91GL02003

年代:1991

数据来源: WILEY

摘要:

A nonequilibrium kinetic model is used to calculate the energy distribution of translationally hot nitrogen atoms in the thermosphere. It is found that dissociation by solar EUV photons and photoelectrons provide an important source of non‐maxwellian high energy N(4S) atoms. We show that quenching of metastable N(2D) atoms by atomic oxygen also produces significant amounts of hot N(4S) atoms. A numerical simulation of the hot N(4S) atom kinetics show that these atoms are present in sufficient quantity in the thermosphere to play a role in the odd nitrogen chemistry and energetics of this atmospheric region. The steady state population of hot nitrogen is obtained by taking into account the relaxation by elastic collisions and high energy reaction with O2to form nitric oxide. It is found that the fraction of hot N atoms reacting with O2is about 15% of the production rate of atomic nitrogen by N2dissociation by photon and photoelectron impact and quenching of N(2D) metastable atoms by atomic oxyge

ISSN:0094-8276    

DOI:10.1029/91GL01566

年代:1991

数据来源: WILEY