摘要:

Ion and electron energy spectra (<30 keV) were recorded simultaneously by the AE‐C (at ∼400‐km altitude) and ATS 6 (at 6.63RE) spacecraft during the intense geomagnetic storm of October 28, 1977 (Kp→ 7+). Observations were acquired in the evening sector, when the two spacecraft were nearly on the sameLshell. The orbit of AE‐C provides wide longitudinal coverage of the auroral oval, whereas ATS 6 remains relatively stationary near the equatorial plane. An unusual particle injection event that was characterized by a series of periodic (∼12 min) bursts of electrons was observed by ATS 6. During this time a variety of phenomena could be identified in the AE‐C particle data, including a widespread low‐energy ‘inverted V’ that extended over 1‐1/2 hours of local time. Distinct from this region are precipitating electrons which have monoenergetic peaks of variable intensity. These electron spectra are similar to those normally observed coincident with discrete arc aurora. The coincident electron energy spectra acquired by AE‐C and ATS 6 match well above 5 keV. The spectra of electrons peak near 1 keV at ionospheric altitudes (typical of this event), whereas no peaks in energy appear

ISSN:0148-0227    

DOI:10.1029/JA085iA09p04563

年代:1980

数据来源: WILEY

摘要:

Two theories have been proposed that differently identify the frequencies of the low‐frequency cutoffs of nonthermal continuum radiation. The first of these theories states that the two low‐frequency cutoffs occur at the local plasma frequency andR=0 cutoff frequency, with the continuum radiation propagating in the ordinary mode between the cutoffs and a mixture of ordinary and extraordinary mode above the upper cutoff. The second theory suggests that the two low‐frequency cutoffs occur at the localL=0 cutoff frequency and plasma frequency, with the continuum radiation being generated by Cerenkov emission in theZmode between the local plasma frequency and upper hybrid resonance frequency. Mode coupling at the local plasma frequency was suggested to generate continuum radiation in the ordinary mode which freely propagates to remote regions of the magnetosphere. In this paper, several examples of continuum radiation observed in the outer magnetosphere by Imp 6 and ISEE 1 are analyzed in detail, and it is shown that these cutoff frequencies occur at the local plasma frequency andR=0 cutoff frequency. In addition, no substantive evidence is found in the outer magnetosphere for a component of continuum radiation propagating in the

ISSN:0148-0227    

DOI:10.1029/JA085iA09p04571

年代:1980

数据来源: WILEY

摘要:

We consider the diffusion of He+ions under the influence of ion cyclotron waves generated in a plasma consisting of three different ion populations: a thermal isotropic population containing both H+and He+ions and an energetic H+population, with a positive anisotropyA=T⊥/T∥‐1. We compute, in the velocity space υ∥, υ⊥, the diffusion curves that He+ions will follow in the presence of ion cyclotron waves propagating in such a medium. We show that for small concentrations of the He+ions, of the order of 1 to ∼10%, these ions can be energized by such a process up to and above suprathermal energies (E≳20 eV). On some occasions the He+ions may even reach energies of the order of the Alfvén energy of the cold plasma population:Ea≈mpVa²/2 ≈ 5 keV. Characteristic diffusion times, in pitch angle and energy, for both ion species, are evaluated. They are of the order of 2 to 20 min. These theoretical results are discussed in the frame of recent observations by Geos experimenters showing the close association that exists between the occurrence of ion cyclotron ULF waves and the presence of thermal or supra‐thermal He+ions in the equatorial region of the magnetosphe

ISSN:0148-0227    

DOI:10.1029/JA085iA09p04577

年代:1980

数据来源: WILEY

摘要:

The 6300‐Å auroral emission intensity at Kilpisjärvi, Finland, was monitored during the 1978–1979 winter, and comparisons made with data from the geostationary Geos 2 spacecraft located on the same magnetic field line. Optical 6300‐Å events with a rise time of a few minutes and a decay time of about 1 hour were regularly observed, and four events from the night of December 18, 1978, were analyzed in detail. All‐sky camera data showed that the events arose from poleward propagating auroras associated with auroral breakup. The Geos 2 plasma and field signatures for these events were remarkably well defined and consistent. About 1 hour prior to an event the tailward component of the magnetic field at Geos 2 begins to grow; it relaxes suddenly to a dipolar configuration at the event onset. At the same time there is a short‐lived electric field enhancement up to 15 mV/m and a burst of ULF waves (a short irregular pulsation event). There is a dropout inE20‐keV intensity, and an onset of VLF activity, all of which persists for roughly 1 hour, in parallel with the decaying 6300‐Å emission. The observed electric field values are comparable to the inductive electric field expected from the observed magnetic field changes and appear to play a crucial role in the processes. The energy release associated with the 6300‐Å emission is comparable to the energy stored in theE<500‐eV electrons prior to the onset. These events appear to correspond to injection events, occurring at the onset of the locally de

ISSN:0148-0227    

DOI:10.1029/JA085iA09p04587

年代:1980

数据来源: WILEY

摘要:

We have performed a statistical analysis of particles>30 keV/charge upstream of the earth's bow shock. The rate of occurrence of upstream particle events has been determined relative to the angle between the magnetic field and the shock normal at the shock intersection point as well as relative to the angle between the magnetic field and the radial direction (i.e., the sun‐earth line). The probability for upstream particle events is highest for small angles between the magnetic field and the radial direction, indicating that the occurrence is determined by the bow shock connection time of a field line convected with the solar wind. We have therefore determined for a model bow shock the occurrence rate of upstream particle events relative to the bow shock connection time. Assuming that the particles are accelerated by multiple reflections between upstream and downstream scattering centers, we can use the observed time constants in order to derive propagation parameters. We find a diffusion coefficient depending linearly on energy per charge, the value of the mean free path of a 30‐keV proton to be ∼4RE, and the region of strong scattering to be limited to ∼30REin front of the bo

ISSN:0148-0227    

DOI:10.1029/JA085iA09p04602

年代:1980

数据来源: WILEY

摘要:

Simultaneous data returned by the Apollo Suprathermal Ion Detector Experiments are combined to yield information on the properties of plasmas in the magnetosheath at lunar orbit. An interface between the magnetosheath and the plasma sheet has been identified. This layer is characterized by a systematic change in the magnetosheath flow properties in the vicinity of the magnetopause. The flow is observed to turn inward toward the plasma sheet and become thermalized, indicating that the magnetosheath particles can gain access to the plasma sheet at lunar distances. This layer, which we term the plasma sheet boundary layer (PSBL), is believed to be the extension of the magnetospheric boundary layer (also called plasma boundary layer) observed on the dayside. Rough order of magnitude calculation shows that the flux crossing the PSBL is capable of supplying the roughly 5 × 1025particles/s necessary for maintenance of the plasma sheet. Measurements of the thickness of the plasma sheet boundary layer, the magnetic field, and the plasma flow velocity allow an estimate of the potential drop across this layer of 4.2 kV. Since this is much less than the potential drop of opposite polarity across the balance of the tail, we conclude that the magnetosphere is open

ISSN:0148-0227    

DOI:10.1029/JA085iA09p04607

年代:1980

数据来源: WILEY

摘要:

The electric field along a magnetic neutral line of theOtype can be estimated by the method previously applied by Alfvén to neutral sheets, with the same result: the line integral of the electric field along the neutral line, multiplied by the elementary charge, is at most equal to twice the magnetic energy per particle. The same upper limit, within a numerical factor of order unity, is obtained from a hydromagnetic calculation where the plasma flow toward theOline is balanced by outflow within the narrow region where the hydromagnetic coupling between the flow and the electric field breaks down. The same upper limit can also be derived from nonsteady models with a nonzero curl of the electric field (noE × Binflow) or with a nonzero divergence of the mass flux density (no outflow), as well as from a model that involves anomalous resistivity when the current density exceeds a threshold value. The derived upper limit implies that the maximum energy that is gained by a particle that drifts along theOline is not significantly greater than the typical energy of a plasma sheet particle; hence simple acceleration by electric fields along magneticOlines cannot account for the energetic component of the charged particle population in the magnetotai

ISSN:0148-0227    

DOI:10.1029/JA085iA09p04616

年代:1980

数据来源: WILEY

摘要:

Using plasma data from the Isis 2 spacecraft and Arecibo radar, diffusive equilibrium models of the ionosphere were constructed for equinox conditions. These plasmaspheric models were combined with models of the neutral atmosphere to calculate the atomic hydrogen escape flux due to charge exchange between thermal protons and to calculate cooler hydrogen and oxygen atoms as a function of dipole latitude and local time. These calculations showed that the daytime escape flux increases as the absolute value of the dipole latitude decreases, reaching its maximum value at the magnetic equator. At 15 hours local time (LT) on March 23, 1972, the calculated escape flux varied from an insignificant amount at 55° dipole latitude, to 3 × 108atoms cm−2s−1at the magnetic equator. In addition, the relative importance to the total charge‐exchange escape flux of the component due to charge exchange between protons and atomic oxygen is discussed. The contribution to the escape flux of plasma at highLvalues, and the effect of newly produced hot hydrogen upon the neutral temperature are also di

ISSN:0148-0227    

DOI:10.1029/JA085iA09p04621

年代:1980

数据来源: WILEY

摘要:

Radio wave and optical experiments were conducted onboard a U.S. Air Force research aircraft in March 1977 and March 1978 at low magnetic latitudes to investigate the effects ofFregion electron density irregularities on transionospheric communications links. Imaging photometer, ionosonde, and satellite amplitude scintillation measurements were used to monitor the development and motion ofFregion 6300‐Å O I airglow depletions, spreadF, and scintillation producing irregularities that are all associated with low‐density bubbles in the postsunset equatorial ionosphere. The 6300‐Å airglow depletions are the bottomside signature of low plasma density within the bubbles. Examples of multiple airglow depletions and their relation to variations in theFlayer virtual height (h′F) and to the occurrence of amplitude scintillations on 250‐MHz satellite signals are described. Estimates of the average bottomside electron density, from simultaneous ionosonde measurements and 6300‐Å airglow intensities, show electron density decreases of ∼66% within the bubbles. These decreases are approximately the same for bubbles observed at the magnetic equator and near Ascension Island (18°S magnetic latitude). The measurements at Ascension Island show that airglow depletions extend away from the magnetic equator into the southern 6300‐Å intertropical arc. Variations in the maximum poleward extent of airglow depletions and of associated ionospheric irregularities that give rise to amplitude scintillations were observed. These latitudinal variations are interpreted, using field line mapping considerations, as variations in the maximum altitude of plasma bubbles over the magnetic equator. A north‐south flight confirms that the overall pattern of airglow depletions and associated ionospheric irregularities extends continuously across the magnetic equator to

ISSN:0148-0227    

DOI:10.1029/JA085iA09p04631

年代:1980

数据来源: WILEY

摘要:

Aeronomic systems such as the negative ion chemistry in theDregion are characterized by numerous reactions involving positive, negative, and neutral species. Classical solutions of such systems do not necessarily point out the role played by each parameter. When the chemical system is considered as a graph, it is possible to apply the signal flow graph technique, which allows a quantitative evaluation of the various paths and loops. A negative ion model is obtained for daytime conditions by using the signal flow graph technique, which is briefly described. This reference model is analyzed in terms of transmittances, path gains, and loop gains. An arbitrary variation of atomic oxygen and nitric oxide is introduced at a height of 65 km in order to show the induced effects on the negative ions and on the electron concentrations. Variations of the ratio λ between negative ions and electrons are also presented

ISSN:0148-0227    

DOI:10.1029/JA085iA09p04642

年代:1980

数据来源: WILEY