摘要:

Substorm onsets, identified by Pi 2 pulsations observed on the Air Force Geophysics Laboratory Magnetometer Network, are studied using magnetometer and electric field data from ISEE 1 as well as magnetometer data from the geosynchronous satellites GOES 2 and 3. The mid‐latitude magnetometer data provides the means of both timing and locating the substorm onset so that the spacecraft locations with respect to the substorm current systems are known. During two intervals, each containing several onsets or intensifications, ISEE 1 observed field‐aligned current signatures beginning simultaneously with the mid‐latitude Pi 2 pulsation. Close to the earth broadband bursts of wave noise were observed in the electric field data whenever field‐aligned currents were detected. One onset occurred when ISEE 1 and GOES 2 were on the same field line but in opposite hemispheres. During this onset ISEE 1 and GOES 2 saw magnetic signatures which appear to be due to conjugate field‐aligned currents flowing out of the western end of the westward auroral electrojets. The ISEE 1 signature is of a line current moving westward past the spacecraft. During the other interval, ISEE 1 was in the near‐tail region near the midnight meridian. Plasma data confirms that the plasma sheet thinned and subsequently expanded at onset. Electric field data shows that the plasma moved in the opposite direction to the plasma sheet boundary as the boundary expanded which implies that there must have been an abundant source of hot plasma present. The plasma motion was towards the center of the plasma sheet and earthwards and consisted of a series of pulses rather than a

ISSN:0148-0227    

DOI:10.1029/JA091iA01p00121

年代:1986

数据来源: WILEY

摘要:

Wave normal directions and wave distribution functions of ELF hiss emissions in a detached plasma region of the magnetosphere have been determined, using data from the geostationary satellite GEOS 2 located in the equatorial plane atL= 6.6. Three different methods of direction finding have been utilized: (1) Means's method based on the hypothesis of a single plane wave, (2) the maximum likelihood method, assuming a few plane waves, and (3) the maximum entropy method of determining the wave distribution function. An intercomparison of the results from those methods has proven that they provide very reliable and definite information on the wave normals. From analyses of two equatorial ELF hiss events it is found that the wave distribution function is composed of a single peak (i.e., the waves have a single propagation direction) and that the wave normals of the ELF hiss originating in the detached plasma region make very small angles with the Earth's magnetic field; also, we wore able to make an experimental estimate of the angular width of the unstable cone of the emissions. Both our initial direction finding results in the equatorial plane and the corresponding previous results at higher geomagnetic latitudes are compared with theoretical predictions, assuming that the waves are generated by the electron cyclotron instability due to medium‐energy (3–30 keV) electrons. It is concluded that all of the characteristics (morphology and direction‐finding results) are consistent with the electron cyclotron instab

ISSN:0148-0227    

DOI:10.1029/JA091iA01p00135

年代:1986

数据来源: WILEY

摘要:

The process of whistler mode sideband generation in the magnetosphere was studied by transmitting two monochromatic signals closely spaced in frequency (Δf= 5–45 Hz) from the experimental VLF transmitter at Siple Station, Antarctica. The signals were observed following ducted magnetospheric propagation to the conjugate station at Roberval, Quebec. Sidebands up to seventh order were generated extending to ±100 Hz with respect to the average frequency of the carriers. New frequencies were observed both under conditions of little or no growth of the input signals and when one of the input signals did grow significantly. At times the sideband amplitudes exceeded the intensity of either input signal. A small signal mechanism is proposed in which emissions are triggered by each beat between the input waves but are then suppressed by the following beat. The energy from the phase bunched particles is believed to feed preferentially into the sidebands rather than causing growth of the input waves themselves. In this model, phase trapping of resonant electrons by the wave is not required. The observed process of sideband generation provides a mechanism to break down the coherence of relatively narrowband waves in the magnetosphere and may thus account for existing evidence of the transformation of relatively coherent wave packets into broader more variable bands of noiselike sign

ISSN:0148-0227    

DOI:10.1029/JA091iA01p00143

年代:1986

数据来源: WILEY

摘要:

A new experimental technique is developed to simultaneously measure the wave propagation direction and the effective length of a small (L≪ λ) electric dipole antenna on a spin stabilized satellite in the magnetosphere. The technique relies on the near simultaneous measurement of single components of the electric and magnetic fields of a coherent VLF signal injected into the medium from a ground‐based source. The spin fading characteristics of the signal received by the electric dipole and the magnetic loop antenna permit the measurement of the wave normal direction assuming whistler‐mode propagation. In situ and remote measurements of the local cold plasma density are used to determine the refractive index. The wave electric field is then inferred from the wave magnetic field as measured on the loop antenna, the refractive index and the direction of propagation. Comparing this electric field with the measured voltage across the dipole antenna leads to the determination of the effective length of the receiving electric dipole. The technique is applied to data from the Dynamics Explorer 1 satellite observations of whistler mode signals injected into the magnetosphere from the Siple, Antarctica, VLF transmitter. In one case, with the measured background cold plasma density being 15 el cm−3, the effective length of the 200 m long electric dipole antenna is found to be 222±56 m, i.e., about twice the conventio

ISSN:0148-0227    

DOI:10.1029/JA091iA01p00154

年代:1986

数据来源: WILEY

摘要:

Measurements of field‐aligned currents (FAC's), convection electric fields, and precipitating electrons were made by the S3‐2 satellite in the midmorning time sectors of both the northern and southern high‐latitude ionospheres before and during a geomagnetic storm on June 24–25, 1976. The patterns of electric fields and FAC's did not match the standard configuration of two‐cell convection and the region 1/region 2 FAC's. In particular, near the beginning of the main phase of the storm an intense (242 mV/m) convection electric field and an extra FAC were observed just poleward of the region 1 current in the southern hemisphere. The observations, made over a period of at least 3 hours, were consistent with a quasi steady state three‐cell convection pattern with stresses from a strongBycomponent of the interplanetary magnetic field (IMF). An analysis of the particle and field data from within the region of the intense convection electric field is in sharp disagreement with the single‐particle motion model of Lyons (1980) and implies that a significant portion of the measured FAC was from ionospheric ions. Finally, large‐amplitude AC electric waves from broadband electrostatic noise (BEN) were present in the high‐altitude southern (or winter) passes but not in the northern passes at lower altitudes. In general, BEN has been observed occasionally by S3‐2 within the high‐altitude (1000–1500 km) winter auroral regions but has never been detected during other seasons or at lower altitudes. The implication of these measurements is that the low‐altitude limit for the region of field‐aligned potentials is determined

ISSN:0148-0227    

DOI:10.1029/JA091iA01p00165

年代:1986

数据来源: WILEY

摘要:

An attempt has been made to evaluate the direct contribution of the horizontal component of oblique field‐aligned currents (FAC's) to the surface magnetic field variations in the auroral region associated with a localized electric potential distribution on the horizontal plane, in comparison with that of the ionospheric eddy current. A relative importance of this direct contribution of oblique FAC's to that from the ionospheric eddy current is proportional to a product of two ratios, the Pedersen to Hall conductivities and the horizontal to vertical components of the local dipole magnetic field intensity. In addition, the shorter the horizontal scale length in the east‐west direction compared with that in the north‐south one, the larger the direct contribution of FAC's. For a localized perturbation with an isotropic horizontal structure at 60° geomagnetic latitude the direct effect of oblique FAC's to the ground magnetic field attains about 80% of the magnetic contribution of the associated ionospheric Hall current at the earth's surface. It is also shown that only the vertical component of the oblique FAC's connects with the irrotational part of ionospheric currents and that the effective conductance in the magnetosphere for three‐dimensional current systems is reduced to a smaller value than that for the case of vertical fie

ISSN:0148-0227    

DOI:10.1029/JA091iA01p00183

年代:1986

数据来源: WILEY

摘要:

The acceleration of electrons in a parallel electric field containing randomly distributed, micro double layers is investigated. The study is motivated by recent electric field observations in the auroral acceleration region. Its objective is to determine what effects stochastic versus static parallel electric fields may have on the spectral features of precipitating electrons. The average electron response as characterized by friction and diffusion coefficients is first analyzed. These coefficients are then used in a Fokker‐Planck equation for the altitudinal variation in the electron energy distribution. Electron sources at the top of the acceleration region (plasma sheet electrons) and at its latitudinal edges (ionospheric electrons) are included. It is shown that (1) the plasma sheet source gives rise to “inverted V” precipitation, as in quasi‐static models, and (2) the edge source is responsible for highly collimated, suprathermal precipitation. The spectral features of the former depend primarily on the average electric field and only weakly on its fluctuations. The properties of collimated precipitation are very sensitive to the microfield, however, owing to trapping phenomena. This analysis represents the first quantitative treatment of collimated electron precipitation; its predictions are in agreement with the observed intensity, pitch angle distribution, energy spectrum, and latitudinal width of edge region precip

ISSN:0148-0227    

DOI:10.1029/JA091iA01p00191

年代:1986

数据来源: WILEY

摘要:

A kinetic theory of the origins of the auroral arc scale spectrum is presented in this paper. The conceptual basis of the theory is current conservation in a turbulent plasma at the magnetospheric equatorial region in which a field‐aligned current is generated and the local electrostatic potential structure is forced to adjust to the presence of the field‐aligned current. This simple model uses an ad hoc Ohm's law relationship between the perpendicular current and the perpendicular electric field, but with a negative conductance in the generator region so thatJ⊥·E⊥<0. An exact solution of a simple model of the concept yields a bistatic auroral generator for which multiple‐arc formation is predicted if the field‐aligned current exceeds a critical value. The predicted scale spectrum is inversely proportional to the square root of the field‐aligned current str

ISSN:0148-0227    

DOI:10.1029/JA091iA01p00204

年代:1986

数据来源: WILEY

摘要:

The ratio of x to o mode radiation produced when an x mode wave strikes a steep density gradient is determined for wave and plasma parameters typical of those observed in the auroral kilometric radiation source region. We find that the waves reflected from or transmitted through the density gradient generally have an o mode component 20 dB or more weaker than the x mode component. This agrees with the ratio of x to o mode intensity observed in auroral kilometric radiation signals near the auroral plasma cavity by both the DE 1 and ISIS 1 spacecraft.

ISSN:0148-0227    

DOI:10.1029/JA091iA01p00211

年代:1986

数据来源: WILEY

摘要:

The results of a series of two‐dimensional numerical particle simulations and a linear analysis of the collisionless tearing mode instability are presented. Comparisons are made between codes which are run with (1) ion dynamics only and magnetic field confined to the plane of the simulation, (2) ion and electron dynamics, including electrostatic interaction, and magnetic field in the plane of the simulation, and (3) ions and electrons and all three components of the magnetic field, as well as the electrostatic field. It is found that the addition of electrons significantly reduces the linear growth and decreases the wavelength of the most linearly unstable mode. The growth rate and wavelength of the most linearly unstable mode are found to be consistent with linear analysis. The addition of the magnetic field component perpendicular to the plane of the simulation has a negligible effect on the linear growth rates or on subsequent nonlinear behavior. Significant electrostatic fields are generated in the nonlinear explosive phase. Somewhat larger potentials are generated in run 3 because of the finite component of the magnetic field normal to the plane of the simulatio

ISSN:0148-0227    

DOI:10.1029/JA091iA01p00219

年代:1986

数据来源: WILEY