摘要:

An MHD time‐dependent numerical simulation, restricted to the solar equatorial plane, is used to demonstrate the interplanetary disturbances caused by several simplified coronal holes. Each ‘hole’ is assumed to have a configuration such that the higher solar wind velocity produced within their longitudinal extent is Gaussian over a 7‐day period at the inner boundary (0.3 AU) of the numerical simulation. A second, twin coronal hole is assumed to rotate on the solar disk behind its predecessor. It is shown that the first coronal hole‐produced interplanetary shock ensemble is overtaken by the second ensemble because of the higher velocity, lower density environment into which the latter propagates. A number of features predicted by MHD similarity theory are confirmed by the numerical simulation. These features include (1) strong azimuthal magnetic and plasma density compression, accompanied with average temperature depression, at the contact surface between forward and reverse shock ensembles, and (2) increasing spatial separation distance between forward and reverse shocks. The study is extended to a heliocentric distance

ISSN:0148-0227    

DOI:10.1029/JA081i031p05413

年代:1976

数据来源: WILEY

摘要:

The present theory of electron temperature in the daytime mid‐latitude ionosphere is tested by using Atmosphere Explorer‐C measurements. In the region below 300 km, where a balance is expected between electron heating by photoelectron impact and electron cooling to ions and neutrals, we find an imbalance in which the cooling rate is consistently higher than the heating rate. The shapes of the altitude profiles also differ substantially. The cooling rate has a sharp peak at about 220 km, while the heating rate exhibits a broad peak about 30 km lower. Improved agreement is achieved at higher altitudes by using an oxygen fine structure loss rate smaller by a factor of 2, based on more recent collision strength calculations. Although this improves the overall agreement of the heating and cooling rates, the shape discrepancy remains, and the new cooling rate falls consistently below the heating rate below 200 km. We conclude that the electron thermal balance is not adequately understood and that more theoretical and experimental work is needed on the atomic and perhaps the molecular cross sections for electron cool

ISSN:0148-0227    

DOI:10.1029/JA081i031p05421

年代:1976

数据来源: WILEY

摘要:

A strong interplanetary shock was detected by the Pioneer 8 magnetometer, plasma probe, and wave instrument at 0048 UT, on June 11, 1968. During the rest of this day the interplanetary medium was highly disturbed as seven well‐defined current layers orDsheets, as well as a number of other localized interaction regions, swept past the spacecraft. The local plasma characteristics that were best correlated with passage of theDsheets appeared to involve changes in the suprathermal electron population. Changes in the 400‐Hz wave levels were also detected near the discontinuities during periods when the local ion plasma frequency was near 400 Hz, but the limited measurement capability of the Pioneer 8 wave instrument does not allow an unambiguous identification of the wave‐particle interactions associated with these measure

ISSN:0148-0227    

DOI:10.1029/JA081i031p05431

年代:1976

数据来源: WILEY

摘要:

The action of strong dc electric fields at high latitudes combined with a predominance of ion‐neutral collisions has for some time been expected to create appreciable departures of the ion velocity distribution from the equilibrium Maxwellian configuration. We present new evidence gathered with the retarding potential analyzer on the AE‐C satellite that shows that the ion velocity distribution departs significantly from the Maxwellian shape at ion temperatures greater than 1500°K. From the analysis of more than 50 volt‐ampere characteristics we have determined the shape of the ion velocity distribution in the velocity plane perpendicular to the magnetic field direction. Theoretical analysis shows that the use of a relaxation model for the description of ion‐neutral collisions gives velocity distributions for both O+and molecular ions that are qualitatively correct. However, the departures from the Maxwellian configuration are always exaggerated by the theory. An immediate consequence is that the threshold for the onset of the Ott and Farley microinstability caused by the double‐hump character of the velocity distribution must be increased to at least 80 mV/m. Finally, if the ion temperature exceeds 2000°K, we find that the interpretation of the retarding potential analyzer data can be seriously affected by the use of the normally assumed Maxwellian ion velocity distribution in the data inversion process. For very strong electric fields, ion temperatures can be underestimated by more than 2000°K, and composition ratios can be affected by more than a factor of 2. Other ground‐based or in situ measurements should be simi

ISSN:0148-0227    

DOI:10.1029/JA081i031p05438

年代:1976

数据来源: WILEY

摘要:

Experimental results obtained with the Jicamarca radar and a new digital processing system during spreadFconditions are presented. The data consist of two‐dimensional maps showing backscatter power and samples of frequency spectra of the backscatter signals as a function of altitude and time. Almost simultaneous spreadFbackscatter power and incoherent scatter observations of electron density and vertical drifts are presented for one occasion. It is shown that spreadFcan occur at the bottomside, at the topside and the steep bottom of theFregion, and in the valley between theFandEregions when the electric field is either positive, negative, or null. The existence of plumelike structures extending hundreds of kilometers in altitude and physically connecting the spreadFon the topside with the bottomside is one of the highlights of the experimental results. They are interpreted as evidence of a Rayleigh‐Taylor instability. A mechanism involving ‘bubbles’ or low‐density plasma is proposed to extend the instability to the stable regions on the top. Other unstable processes are proposed for spreadFat other altitude ranges. The frequency spectra show a large variety of shapes. Simple or multiple peak spectra from a few hertz to a few hundred hertz wide are found. An interpretation of the spectral shapes is presented in terms of turbulent motions and the angular extent ofkvector angles of the fluctuation waves with respect to perpendicularity. A puzzling phenomenon, referred to as explosive spreadF, which consists of the simultaneous onset of short time enhancements in the backscatter power and involves selected heights in an altitude range of the order of 100 km, is

ISSN:0148-0227    

DOI:10.1029/JA081i031p05447

年代:1976

数据来源: WILEY

摘要:

Absolute intensity measurements of the (8, 6) OH band obtained during 10 flights of the December 1969 NASA Auroral Airborne Expedition are presented. Nightglow intensities higher by a factor of 2 than the usual values are recorded during flights 8, 14, and 15. The OH variations are compared with the evolution of the green line and O2(¹Δg) emissions measured by other experimenters on board the aircraft. Before sunrise the twilight variations of OH down to a solar depression angle of 5° show a rapid decrease. A theoretical prediction of the OH, O I 5577 Å, and O2(¹Δg) emissions is evaluated by means of an extensive time‐dependent oxygen‐hydrogen model of the 25‐ to 150‐km region. Twilight decrease of the OH emission is interpreted in terms of mesospheric ozone photodissociation. Nighttime variations of the emissions may be reproduced if modifications of the dynamic regime are introduced i

ISSN:0148-0227    

DOI:10.1029/JA081i031p05467

年代:1976

数据来源: WILEY

摘要:

Auroral photometric measurements were carried out at South Pole, Antarctica (74.5° invariant latitude), throughout the austral winter of 1972. Observed auroral emissions included 4278 Å N2+, 4861 Å Hβ, 5577 Å O I, and 6300 Å O I. A comprehensive study of auroral emissions and riometer absorption associated with the solar flare activity of August 1972 has been carried out. Energetic solar protons (>1 MeV) measured by satellite detectors over the polar caps were found to be responsible for the measured 30‐MHz absorption in both the northern and the southern hemispheres, except for two substorm‐related events. These protons account for only a fraction of the measured 4278 Å N2+with the remainder of the emissions excited by protons and electrons of lower energies. Simultaneous photometric measurements and riometer observations allow separation of precipitating particle species and characteristic energy, which in turn allows separation of substorm effects from energetic solar prot

ISSN:0148-0227    

DOI:10.1029/JA081i031p05479

年代:1976

数据来源: WILEY

摘要:

Power spectral, correlation, and coherency analyses have been made for selected periods of Pioneer 6 interplanetary magnetic field data for the frequency range of 4 × 10−4to 1.6 × 10−2Hz. The periods are chosen for times when the mean field was either radial or perpendicular to the heliocentric radius. At such times, some of the interpretive ambiguities which arise from the convection of the magnetic field by the solar wind are removed, and the field fluctuations can be understood in terms of the spatial dependence of wave or static structures. Results from the power spectral and coherency analyses for the radial field periods indicate the existence of constant‐magnitude Alfvén waves propagating along the field as well as magnetosonic waves propagating obliquely to the mean field. No evidence for a significant component of circularly polarized Alfvén waves is found. Our results also rule out some magnetic field models which have been widely used in cosmic ray studies. Differences in the spectral slopes and power levels for magnetic field fluctuations parallel and transverse to the mean field indicate that the radial periods cannot be approximated by isotropic magnetic field fluctuations while the perpendicular periods are not fit by the ‘slab’ model. All periods are consistent with a superposition of Alfvén waves and magnetosonic waves with less than one quarter of the power in the magnetosonic waves as in Alfvén waves. Finally, autocorrelation analysis for the radial periods indicates that the correlation length for transverse magnetic fluctuations between discontinuities

ISSN:0148-0227    

DOI:10.1029/JA081i031p05489

年代:1976

数据来源: WILEY

摘要:

Relativistic electron precipitation (REP) has been monitored by observing anomalous changes in phase or amplitude of ionospheric radio waves which propagate through the ionosphericDregion. Magnetospheric substorm activity appears to be a necessary though by no means sufficient condition for producing intense REP events. A general correlation is found between the onset of substorm activity and nocturnal REP events. In contrast, dayside REP events occur less frequently, are more restricted in local time, and are often delayed by up to several hours following the onset of substorm activity. The observational evidence supports the concept of parasitic strong diffusion scattering of trapped relativistic electrons by intense ion cyclotron turbulence which is generated following the injection of ring current ions into the outer radiation zone during the substorm expansive phase.

ISSN:0148-0227    

DOI:10.1029/JA081i031p05501

年代:1976

数据来源: WILEY

摘要:

Energy spectra of electrons between 1 and 500 eV have been obtained at altitudes mainly below 300 km over both north and south high‐latitude regions with the photoelectron spectrometer experiment on the Atmosphere Explorer C satellite. Although the high‐latitude fluxes of 1‐ to 500‐eV electrons are highly variable, it has been possible to organize the observed events into four major classes: (1) fluxes of electrons having a peak in energy below 100 eV, approximately isotropic over the upper 2π hemisphere, and located at high invariant latitudes on the dayside which apparently gain access to the ionosphere through the dayside magnetospheric cusps; (2) structured fluxes of electrons showing one or more discrete but variable peaks in energy, reminiscent of ‘inverted V’ phenomena at higher energies, observed at all magnetic local times; (3) fluxes which show a monotonically decreasing differential flux versus energy dependence proportional toE−α, whereE>30 eV and 1<α<2, characteristic of auroral secondary electrons; (4) intense fluxes of electrons with rapidly changing and random energy spectra, characteristic of passage through auroral arcs or breakup regions. Fluxes of types 3 and 4 depend on precipitation of high‐energy auroral primary particles, while those of types 1 and 2 appear to be a consequence of fundamental processes which couple the magnetosphere and the ionosphere. Detailed data are presented on classes 1 and 2, including occurrence and location as a function of

ISSN:0148-0227    

DOI:10.1029/JA081i031p05507

年代:1976

数据来源: WILEY