摘要:

Computer simulation studies of the electric fields and currents in the global ionosphere produced by field‐aligned electric currents for quiet periods are conducted. The steady state equations for current conservation are solved numerically by assuming (1) several divided regions of the global earth (such as the polar cap, auroral zone, and middle‐low latitudes), (2) exponentially distributed anisotropic electric conductivities for each zone with a continuous change at the boundaries of the regions, and (3) exponentially distributed downward and upward field‐aligned current intensities in the auroral region, assumptions based on our current knowledge of auroral phenomena and geomagnetic variations as well as rocket and satellite measurements of field‐aligned currents. Resultant computer‐plotted diagrams include equipotential contours of the electric fields, vector distributions of the electric fields and currents, and electric current patterns equivalent to the magnetic field effect produced by the field‐aligned and real ionospheric currents. One of the merits of this simulation method is that the three‐dimensional current system can roughly be estimated from the equivalent current system obtained from ground‐based geomagnetic data alone. This paper also provides a foundation for a similar study of substorms. The following main results are obtained: (1) Conductivity inhomogeneity alters considerably the electric field pattern that has previously been obtained by assuming the uniform conductivity distribution. (2) Even a slight conductivity enhancement along the nightside auroral belt results in a large modification of the electric field. (3) The existence of the strong conductivity gradients and the field‐aligned currents in the equatorward half of the auroral oval reduces the electric field in the middle and low latitudes. This corresponds to the ‘shielding’ effect of the electric field inside the Alfvén layer in the magnetotail. (4) Seasonal changes in the polar cap conductivities cause surprisingly large effects on the electric fields and currents. (5) The equivalent ionospheric currents differ significantly from real ionospheric currents in both i

ISSN:0148-0227    

DOI:10.1029/JA084iA08p04083

年代:1979

数据来源: WILEY

摘要:

The computer simulation scheme for calculating the electric fields and currents in the global ionosphere in relation to field‐aligned currents for quiet periods that we previously developed is extended to disturbed conditions. In a series of numerical simulations representing various substorms we take into account our current knowledge of characteristics of auroral enhancement in the ionospheric electric conductivities as well as changes in the intensity and location of field‐aligned currents. The possible effects of these variable parameters in producing the observed complexity of the electric field and current patterns are studied, with the following main results: (1) The conductivity inhomogeneity in the auroral belt can reproduce the observed local time dependence of the electric fields and currents. (2) The amount of the ‘return’ currents from the auroral electrojets flowing within the ionosphere is small, an indication that the major part of the electrojets is connected directly to the field‐aligned currents. Therefore the ground magnetic perturbations in the polar cap and also in the middle and low latitudes during substorms can be ascribed mainly to the magnetic effects of the field‐aligned currents. (3) Even when the intensities of the field‐aligned currents and the associated auroral electrojets do increase considerably during substorms over their values of quiet times, the total potential difference across the polar cap does not increase very much, because of a simultaneous increase in the ionospheric conductivities along the nightside auroral oval. (4) No special field‐aligned current system in the Harang discontinuity region is required to reproduce this feature. (5) The distribution pattern of the equivalent ionospheric current differs significantly from that of the real ionospheric current vectors. The maximum electrojet intensity seen in the equivalent current system is weaker than that of the real auro

ISSN:0148-0227    

DOI:10.1029/JA084iA08p04099

年代:1979

数据来源: WILEY

摘要:

Search coil magnetometer data obtained directly in the magnetosphere on Ogo 5 and Ogo 6 have been studied extensively for possible evidence of man‐induced stimulation of the two dominant electromagnetic emissions, chorus and plasmaspheric hiss. Analysis of chorus obtained during the first year of operation of Ogo 5 exhibits considerable variability with geographic longitude. However, there is little or no correspondence with the longitudinal dependance of Ogo 3 chorus previously reported byLuette et al.(1977). This discrepancy is explained in terms of the persistent nature of chorus which can lead to oversampling of the data. When oversampling is eliminated, the longitudinal variability of Ogo 5 chorus is adequately explained by normal statistical fluctuations. A search was also made for enhancements, onsets or quantum jumps in chorus emissions at frequencies related to harmonics of the earth's transmission lines. No significant effect was found. An independent study of plasmaspheric hiss detected on the polar‐orbiting satellite Ogo 6 indicates that this emission is also essentially uniformly distributed in dipole longitude. The only exception to this was a modest and probably significant (4.5σ) enhancement in the occurrence of weak emissions in the 340°–360° dipole longitude region during very quiet geomagnetic conditions. The source of this enhancement could be man induced, or it might simply be related to the expected enhanced level of sferic activity associated with thunderstorm activity over the North American continent. However, because this feature is present only when ELF hiss is weak, it is clear that it will not contribute significantly to the non adiabatic dynamics of radiation belt electrons. We conclude that the totality of satellite data analyzed to date is consistent with a predominantly natural origin for chorus and plasmaspheric hiss. While we do not dispute the existence of triggered waves, it is difficult, in view of their low overall occurrence, to imagine that they play any major role in the non adiabatic dynamics of geomagnetically trapped e

ISSN:0148-0227    

DOI:10.1029/JA084iA08p04116

年代:1979

数据来源: WILEY

摘要:

Data from the synchronous altitude satellite ATS 1 and near‐conjugate measurements of bremsstrahlung X rays and ground magnetic variations have been used to analyze an event of modulated auroral zone electron precipitation and magnetic pulsations in the Pc 3 range. Transverse, azimuthal, nearly linearly polarized waves, strongly peaked at ∼25‐s period were observed at ATS 1 from 0600 to 1000 LT on August 18, 1967, diminishing in intensity thereafter. Weak pulsations with periods ≲18 s and wave properties similar to the above followed in the interval from 1300 to 1700 LT. Ground magnetic pulsations at College, Alaska, were observed mainly in the transverseHandDcomponents with maximum power spectral density in the period range 25–40 s prior to 1300 LT and from 20 to 30 s thereafter. Four main intervals of modulated electron precipitation, centered roughly on local magnetic noon, were noted in the X ray data obtained near Fort Yukon, Alaska. The intervals, each lasting for ∼30 min, were separated by ∼90 min. Fluctuations of the 50‐ to 150‐keV trapped electrons at ATS 1 during the precipitation event remained typically within 1 or 2σ of the average counting rates and showed no clear association with pulsations in either the X ray or the magnetic data. The origin of the Pc 3 waves is attributed to local field line resonances induced by Kelvin‐Helmholtz instability at the magnetopause. The observed wave periods can be accounted for by a field line distribution of plasma density of the formr−4with equatorial values atr= 6.6REof order 1 cm−3. The wave resonance model can satisfactorily explain observed differences in the pulsation activity at the ground, balloon, and satellite if account is taken of the spatial sensitivities of the different techniques and the location of observing sites with respect to the probable location of resonant field lines. Although the event pertains to a disturbed period, there is insufficient evidence to associate gross temporal changes in the intensity of pulsation activity with the occurrence of specific substorms. However, the data suggest that electron precipitation pulsations will be found to correlate with Pc 3 magnetic pulsations when substorm injections coupled with azimuthal drift provide enhanced energetic particle fluxes within d

ISSN:0148-0227    

DOI:10.1029/JA084iA08p04125

年代:1979

数据来源: WILEY

摘要:

Although DMSP satellite data are widely used, there has been no reliable absolute calibration. Coordinated data with ground‐based photometers allow a calibration curve of film density versus 4728 N2+intensity to be derived. The DMSP satellites (5C series) record airglow and can detect auroral forms of intensities ≥50 R of 4278 N2+. It is estimated that the 5D series satellites are capable of detecting auroras with ∼25 R of 427

ISSN:0148-0227    

DOI:10.1029/JA084iA08p04134

年代:1979

数据来源: WILEY

摘要:

Plasma wave measurements using instruments on the Imp 6 and Hawkeye 1 satellites are utilized in a study of very intense electrostatic waves near the upper hybrid resonance frequency in the region just outside the plasmapause. These intense plasma wave events have electric field strengths of ∼1–20 mV m−1and are among the most intense waves seen in the earth's magnetosphere. Detailed studies of more than 140 of these intense electrostatic disturbances reveal that the events occur at all local times and at magnetic latitudes varying from the equator to as high as 50°. The polarization of these waves is such that the wave electric field vector is oriented perpendicular to the geomagnetic field. In most cases the center frequency of the intense waves appears to correspond to an (n+ ½)fg−harmonic near the upper hybrid resonance frequency. A survey of plasma measurements made simultaneously using the Hawkeye 1 Lepedea shows that the occurrence of the intense electrostatic waves is not strongly controlled by the intensities of ∼1‐ to 20‐keV electrons but that specific details of the hot electron distribution function are directly related to the wave turbulence. All events at magnetic latitudes less than about 10° show strong pitch angle anisotropy with the greatest intensities at α ≈ 90°. The hot distribution functionf(υ⊥, υ∥) is described for a few events showing two sources of free energy; a temperature anisotropy and a loss cone distribution. One event shown suggests that a bump on tail in υ⊥may also contribute free energy in some cases. A possible mechanism for producing intense waves near the upper hybrid resonance frequency is suggested which draws upon current theories applied to the generation of (n+ ½)fg−bands. Evidence is given which suggests that the intense electrostatic waves may be a source o

ISSN:0148-0227    

DOI:10.1029/JA084iA08p04145

年代:1979

数据来源: WILEY

摘要:

A model is presented to explain the nonresonant waves which give rise to the diffuse resonance observed near 3/2fHby the Alouette and Isis topside sounders, wherefHis the ambient electron cyclotron frequency. These waves are the result of plasma wave instabilities driven by anisotropic electron velocity distributions initiated by the high‐power (400 W) short‐duration (100 µs) sounder pulse. In a strictly linear analysis these instability‐driven waves will decay owing to Landau damping on a time scale much shorter than the observed time duration of the diffuse resonance. Calculations of the nonlinear wave‐particle coupling coefficients, however, indicate that the diffuse resonance wave can be maintained by nonlinear Landau damping of the sounder‐stimulated 2fHwave. This 2fHwave is always observed with a time duration longer than that of the diffuse resonance wave, a finding which indicates that it is available as an energy source for the proposed nonlinear mechanism. The time duration of the diffuse resonance is determined by the transit time of the instability‐generated and nonlinearly maintained diffuse resonance wave from the remote short‐lived hot region back to the antenna. The model is consistent with the Alouette/Isis observations and clearly demonstrates the existence of nonlinear wave‐particle interactions in the ionosphere. Such interactions have been observed in the laboratory and have long been speculated as being important in the energetics of the earth's magnetospher

ISSN:0148-0227    

DOI:10.1029/JA084iA08p04165

年代:1979

数据来源: WILEY

摘要:

Charged particle observations from several high‐altitude sounding rocket flights launched from Churchill Research Range into auroral events when magnetic‐field‐aligned electron precipitation was observed are presented. Large field‐aligned enhancements were observed near the edges of regions of intense electron precipitation (auroral arcs), while only small or no enhancements were observed inside these regions. Field alignment was spread over a large energy range, at times from 0.5 to 60 keV, and showed no enhancement near the characteristic peak in the energy spectrum. These and other observations are compared with predictions of models involving acceleration by quasi‐static parallel electric fields, including the possibility of beam‐plasma instability effects, and are shown to be inconsistent. Acceleration by broadband electrostatic waves is suggested as a possible energization mechanism for field‐alig

ISSN:0148-0227    

DOI:10.1029/JA084iA08p04175

年代:1979

数据来源: WILEY

摘要:

Large‐scale electric fields associated with storms or substorms are responsible for inward convection and energization of plasma sheet plasma. Calculations based on steady state convection theory show that the response to such electric fields qualitatively accounts for many features of the injected particle distribution, but quantitative agreement with the theory has not yet been obtained. It is known that the predictions can be improved by introducing the concept of convection in response to a time dependent electric field. On the other hand, time dependent calculations are sensitive to the choice of initial conditions, and most models have failed to incorporate these conditions in a realistic and self‐consistent manner. In this paper we present a more complete model consisting of realistic initial conditions and time dependent convection to explain a typical substorm‐associated electron injection event. We find very good agreement between the observed electron flux changes and those predicted by our

ISSN:0148-0227    

DOI:10.1029/JA084iA08p04183

年代:1979

数据来源: WILEY

摘要:

We present several examples of the ionospheric electron drift patterns and of the aurora near the Harang discontinuity. The drift patterns were detected with the Scandinavian Twin Auroral Radar Experiment, and the auroral features were determined by using all‐sky camera data. We find the behavior of the discontinuity to be strongly dependent on the level of magnetic activity as measured by theKpindex. During low activity the discontinuity remains at a fairly constant latitude for a considerable length of time. Usually, one to two auroral arcs are observed within the discontinuity. With increasing magnetic activity the Harang discontinuity tends to surge rapidly equatorward with mean speeds ranging from 150 to 600 m/s. This equatorward surge is observed at earlier local times as magnetic activity increases. We have found two cases in which an arc appears in the discontinuity at some point during its equatorward surge. The equatorward surge of the essentially east‐west‐aligned discontinuity appears to be associated with a limited region of strong equatorward electron flow which splits up into eastward and westward flow. This region may form the eastward termination of the discontinuity. The eastward termination of the Harang discontinuity during times of low magnetic activity appears to consist of an equatorward extension of the region of eastward electron flow within a spatially limited region near local magnetic midnight. We find the equatorward surges to be terminated by the sudden activation of an arc in the discontinuity, followed by a poleward expansion. Following these substorms the Harang discontinuity disappears from the field of view. At times, we have observed arcs to brighten and surge poleward in the region equatorward of the discontinuity. The discontinuity did not appear to be strongly affected by these substorms. We find in general that auroral arcs are located in or poleward of the discontinuity, even though active arcs at times appear south of the discontinuity. Diffuse aurora have been observed only equatorward of the disconti

ISSN:0148-0227    

DOI:10.1029/JA084iA08p04189

年代:1979

数据来源: WILEY