摘要:

The direction of the interplanetary magnetic field appears to have a significant effect on the equilibrium size of the day side magnetosphere. The qualitative features of this effect are illustrated here by means of a simple vacuum model that ignores solar wind and ionospheric plasma densities as a first approximation. This idealized model produces earthward displacements of the subsolar magnetopause and equatorward displacements of the polar cusp associated with southward‐turning interplanetary fields that are about a factor of 2 greater than observed displacements. The nonzero solar wind plasma pressure and ionospheric conductivity produce large deviations from the simple vacuum configurations but do not change the qualitative conclusions regarding magnetopause and cusp displacements associated with the north‐south component of the interplanetary magnetic fi

ISSN:0148-0227    

DOI:10.1029/JA080i001p00001

年代:1975

数据来源: WILEY

摘要:

It is shown that proton streams reflected from a bow shock structure that partially organizes their distribution in gyrophase in such a way that the reflected beam distribution function has the formFb(υ⊥², υ∥, ϕ), where ϕ is the phase angle for motions perpendicular to the solar wind fieldBo, are capable of driving a left‐hand (Alfvén) wave at rest frame frequencies ω<Ωı. The driven wave amplitude becomes a sharply peaked function of ω/Ωı=x, and the locationx=xoand peak amplitude of the waveB(xo) depend critically on the ratio of beam speedVbto the Alfvén speedCAand on the value of solar wind β. Driven wave amplitudes are estimated to be up to several gammas for beam speeds (in the rest frame) ofVb∼ 2VSW. The observed frequency and polarization depend upon Doppler shifting due to solar wind flow, and observed polarization could be either left‐hand or right‐hand depending upon the vector properties of the solar wind vel

ISSN:0148-0227    

DOI:10.1029/JA080i001p00007

年代:1975

数据来源: WILEY

摘要:

The electric and magnetic field spectral densities of plasma waves in the earth's bow shock have been measured in the frequency range 20 Hz to 200 kHz by using two 16‐channel spectrum analyzers on the Imp 6 spacecraft. The electric field spectrum in the bow shock consists of two distinct components: one component has a broad peak typically centered between 200 and 800 Hz with an average (5.12‐s time constant) spectral density at the peak of about 10−9V² m−2Hz−1, and the other component increases monotonically with decreasing frequency approximately asf−(2.0±0.5)and has an average spectral density of about 3.0 × 10−9V² m−2Hz−1at 36.0 Hz. The magnetic field spectrum in the shock has only one component that increases monotonically with decreasing frequency approximately asf−(4.0±0.5)and has an upper cutoff frequency near the local electron gyrofrequency. This magnetic field spectrum appears to be associated with the monotonic component of the electric field spectrum. The electric to magnetic energy density ratio ϵE/ϵBof this noise is about 10−3to 10−4, which is consistent with the energy density ratio expected for electromagnetic whistler mode waves in the bow shock. The broad peak in the electric field spectrum between 200 and 800 Hz has a large electric to magnetic energy density ratio, ∼10² to 10³, indicating that this component consists of almost purely electrostatic waves. Electrostatic noise with a spectrum similar to the turbulence in the shock but with lower intensities is observed throughout the magnetosheath region downstream of the shock. This magnetosheath electric field turbulence often includes many bursts with a distinct ‘parabolic’ frequency‐time variation on a time scale of a few seconds. Spin modulation measurements of the electric field direction show that the electric field vectors in both the shock transition region and the magnetosheath region are preferentially oriented parallel to the static magnetic field direction. The electric field of upstream electron plasma oscillations also is orient

ISSN:0148-0227    

DOI:10.1029/JA080i001p00019

年代:1975

数据来源: WILEY

摘要:

Equations describing the magnetospheric flow in regions of equipotential convection are developed for the case of uniform ionospheric conductivity and zero cross‐field current inside the regions. The approach is equivalent to that of Vasyliunas, since the assumptions are those of equipotential convection. Flux tubes are regarded as fluid elements, and terms appear in the equations, corresponding to the stress due to the interaction of a flux tube with its neighbors and to stresses transmitted through the ends. The convective velocity is found to be expressible as the gradient of a two‐dimensional potential that obeys either Laplace's or Poisson's equation. The equations are applied to flow in the polar caps to determine what type of applied outer magnetospheric force produces the observed flows. It is required that flux tubes anchored near the outer boundary of the polar cap be subject to a shear force relative to other polar cap flux tubes to produce the narrow latitude‐limited flows often obs

ISSN:0148-0227    

DOI:10.1029/JA080i001p00032

年代:1975

数据来源: WILEY

摘要:

Simultaneous measurements of low‐energy precipitating electrons and magnetic fluctuations from the low‐altitude polar‐orbiting satellite Ogo 4 have been compared. Analysis of the two sets of experimental data for isolated events led to the classification of high‐latitude field‐aligned currents as purely temporal or purely spatial variations. Magnetic field disturbances calculated by using these simple current models and the measured particle fluxes were in good agreement with measured field values. Although fluxes of electrons of greater than 1 keV were detected primarily on the night side, magnetometer disturbances indicative of field‐aligned currents were seen at all local times, in both the visual auroral regions and the day side polar cusp. Thus electrons with energies of less than ∼1 keV are the prime charge carriers in high‐latitude day side field‐aligned currents. The satellite measurements are in good agreement with previously measured field‐aligned current values and with values predicted from several models involving magnetospheric fie

ISSN:0148-0227    

DOI:10.1029/JA080i001p00037

年代:1975

数据来源: WILEY

摘要:

A rapid reconfiguration of the dusk side magnetosphere was observed near the magnetic equator by particle and field experiments aboard Explorer 45 at 1715 UT on June 17, 1972. Following the onset of an 800‐γ negative bay in the auroral zone, Explorer 45, inbound at 1700 MLT andL∼ 5, observed a sharp dropout of outer zone electrons (35–560 keV). Simultaneously, the plasma density dropped from a level typical of the plasmasphere to one typical of the ion trough, and plasma sheet proton fluxes (3.5–138.5 keV) increased, producing a 100‐γ depression inB, in a shift to a taillike configuration. Outer zone electrons with pitch angles near 45° returned with delay times consistent with injection near midnight at the time of dropout and eastward drift to the dusk meridian. The equatorial electrons were not observed, since shell splitting apparently placed them on pseudotrapped orbits. Approximately 45 min after the reconfiguration, the satellite reentered the plasmasphere, and the electron fluxes returned to previous levels. The injected protons extended well into the plasmasphere. In the next orbit, approximately 6 hours later, a region of isolated plasma was detected near apogee at 1400 MLT. There is therefore circumstantial evidence that the type of magnetospheric reconfiguration observed here is responsible for the detachment of regions of cold plasma from the

ISSN:0148-0227    

DOI:10.1029/JA080i001p00047

年代:1975

数据来源: WILEY

摘要:

The local time variations of particle distributions, which can be produced by the sudden enhancement of a cross‐tail electric field, are examined by using adiabatic theory. The strong coupling between acceleration, inward motion, and east‐west drift is emphasized. The model is used to interpret recent measurements of delay times of newly accelerated electrons at different local times at geostationary orbit. Observations of the asymmetric storm time ‘ring current’ are interpreted in terms of the model without invoking partic

ISSN:0148-0227    

DOI:10.1029/JA080i001p00056

年代:1975

数据来源: WILEY

摘要:

Data from the energetic particle detectors on board the low‐altitude polar‐orbiting satellite Isis 2 are presented to describe several features of the low‐energy (0.15–9.7 keV) electrons in ‘inverted V’ substructures. The measurements show that, frequently, the fluxes of electrons in the inverted V events peak at ∼90° pitch angle. In addition to this principal peak the electron fluxes also have a tendency to once again increase at smaller pitch angles (usually ≲ 30°). This tendency becomes more significant at lower electron energies. On the basis of the measurements it is suggested that acceleration processes other than parallel electric fields are mainly responsible for the formation of at least some of the observed inverted V substructures and that such inverted V events probably occur on closed field lines in

ISSN:0148-0227    

DOI:10.1029/JA080i001p00066

年代:1975

数据来源: WILEY

摘要:

Data from about 1100 passes of the polar orbiting satellite Isis 2 have been used to determine average intensities and energies of electrons as functions of magnetic local time and invariant latitude for energies above 150 eV. It is found that at any local time the average energy of electrons (in the range 150 eV to 9.6 keV) is a minimum at or near the latitude of the average position of the trapping boundary for 35‐keV electrons but that the deepest minimums occur at local times of 0300 and 1600. It is also found that the highest average fluxes of 150‐eV electrons occur at 1500 MLT at the boundary and at 0300 MLT inside the boundary. On the basis of these measurements it is suggested that magnetoshenth particles enter the closed field region of the magnetosphere at all local times and that typically there is some energization associated with this entry. Entry is least likely at local times before noon, and energization is greatest at local times before midnight. It is further suggested that at least two processes, convection and possibly a diffusive mechanism, are involved in the entry of magnetosheath plasma to the magnetosph

ISSN:0148-0227    

DOI:10.1029/JA080i001p00073

年代:1975

数据来源: WILEY

摘要:

Energetic helium ions trapped in the radiation belt can get lost through charge exchange by collisions with neutral exospheric hydrogen. Since the cross section for conversion of singly ionized He to neutral He is much greater than the cross section for conversion of doubly ionized He to neutral He, the energetic He flux can be considered to originate from the He+population of the radiation belt. At low altitudes the energetic neutral helium atoms are again converted to ionized helium by collisions with the neutral oxygen atoms of the atmosphere. When a model for the flux distribution based on all available measurements of equatorially mirroring α particles is assumed, the expected flux of ionized He at low altitudes near the geomagnetic equator is computed according to an extension of the equilibrium model by Moritz (1972). It is assumed for the model calculations that the total ionized He particle distribution consists of singly charged He ions. A comparison of the results of the model calculations with measurements made in two different time periods with a proton‐α particle telescope on board the low‐altitude satellite Esro 4 below the radiation belt yields values of 6 and 23% for the percentage of singly charged helium in the trapped ionized helium at energies between 2.5 and 8 MeV. Accuracy of the results from the model calculations is limited by uncertainties in the trapped α particle characteristics, in the charge exchange cross sections above 1.5 MeV, and in the exospheric neutral hydrogen density. The different values obtained for the two periods is an indication of the variability of the He+radiation belt content and/or of the exospheric neutral hydrogen d

ISSN:0148-0227    

DOI:10.1029/JA080i001p00080

年代:1975

数据来源: WILEY