摘要:

The identification and explanation of short wavelength antenna interference effects observed in spacecraft plasma wave data have provided an important new method of determining limits on the wavelength, direction of propagation, and Doppler shift of short wavelength electrostatic waves. By using the ISEE‐1 wide‐band electric field data, antenna interference effects have been identified in the ion waves upstream of the earth's bow shock. This identification implies that wavelengths of the upstream ion waves are shorter than the antenna length. The interference effects also provide new measurements of the direction of propagation of the ion waves. The new measurements show that the wave vectors of the ion waves are not parallel to the interplanetary magnetic field (IMF) as previously reported. The direction of propagation does not appear to be controlled by the IMF. In addition, analysis of the Doppler shift of the short wavelength ion waves has provided a measurement of the dispersion relation, ω0(k). The upper limit on the rest frame frequency was found to be on the order of the ion plasma frequency. At this frequency, the wavelength is on the order of a few times the Debye length. The results of this study now provide strong evidence that the ion waves in the upstream region are Doppler‐shifted ion acoustic

ISSN:0148-0227    

DOI:10.1029/JA089iA01p00091

年代:1984

数据来源: WILEY

摘要:

In this study, the Landau and cyclotron growth rates of whistler mode waves in earth's bow shock are calculated by using electron distribution functions obtained with the fast plasma experiment on ISEE 2. Three electron distribution functions measured within the transition region of the shock are analyzed. An important feature of these electron distribution functions is the presence of a field‐aligned beam with aT⊥>T∥anisotropy. The beam velocity vector is directed toward the magnetosheath. The calculations show that the electron distribution functions spontaneously generate whistler mode waves with plasma rest frame frequencies between about 0.1 and 100 Hz. The wave normal angles of the generated whistlers range from 0° to the resonance cone angle. Electromagnetic Landau resonance and/or cyclotron resonance contribute to wave growth over the range of velocity distributions observed. The waves that are generated by the normal cyclotron resonance have wave vectors directed toward the solar wind, while those that are generated by the Landau and the anomalous cyclotron resonances have wave vectors directed toward the magnetosheath. The predictions of the study are in qualitative agreement with observations of whistler mode waves near earth's bow

ISSN:0148-0227    

DOI:10.1029/JA089iA01p00105

年代:1984

数据来源: WILEY

摘要:

The evolution in velocity and configuration spaces of ions injected into a flowing turbulent magnetoplasma is studied from a test particle viewpoint. Numerical integration of the equations of motion for several types of fluctuations and injection velocities permits assessing the effectiveness of the turbulence in forcing the ensemble average velocity of the ions to approach the bulk flow velocity. The choice of parameters models the geomagnetosheath, and the downstream ions eventually decoupled from the solar wind flow while crossing the bow shock, but the envisaged conditions are relevant to several space plasma situations. The observed power spectra of the fluctuations in the sheath suggest that, though infrequently, the coupling of the injected ions to the bulk plasma motion may be attained within a few gyroperiods, strongly modifying the dynamics imposed by the average magnetic and convection electric fields.

ISSN:0148-0227    

DOI:10.1029/JA089iA01p00115

年代:1984

数据来源: WILEY

摘要:

Three passes of the ISEE 1 and 2 satellites through the dayside terrestrial magnetopause are discussed where the magnetopause is identified as a tangential discontinuity. This identification is based primarily on the failure of the plasma and magnetic field data to satisfy the conditions for a rotational discontinuity. In all these cases the interplanetary magnetic field was directed strongly southward and the angles between the fields on the two sides of the magnetopause ranged between 136° and 170°. As this is precisely the field geometry thought to be most conducive for reconnection, one would have expected the magnetopause to be a rotational discontinuity. The simplest explanation of this result would appear to be that the magnetic field orientation is not the only factor controlling the onset of reconnection. However, as our identification of the discontinuity applies only locally, it cannot be excluded that for the magnetic field conditions investigated here, different portions of the magnetopause can be described as tangential and rotational discontinuities simultaneousl

ISSN:0148-0227    

DOI:10.1029/JA089iA01p00127

年代:1984

数据来源: WILEY

摘要:

Since the magnetosheath plasma is highly turbulent, reconnection at the dayside magnetopause is likely to be temporally unsteady. The tearing mode can be viewed as a model for the unsteady development of a reconnecting magnetic topology. Magnetopause tearing occurs in the guide‐field limit and has a wave packet spatial structure in the east‐west direction. We solve for the nonlinear evolution of a single wavelength guide‐field tearing mode including the effects of finite transit time on the Landau resonant electrons. Short wavelength modes evolve algebraically in time with perturbation amplitudes proportional tot². Long wavelength modes are fully nonlinear, and the amplitude grows linearly i

ISSN:0148-0227    

DOI:10.1029/JA089iA01p00137

年代:1984

数据来源: WILEY

摘要:

Approximate analytic solutions exist for particle motion in a one‐dimensional current sheet with a constant normal magnetic field component. These solutions are tested against precise numerical calculations, and a range of validity of the analytic solutions is inferred. For example, in the geomagnetic tail neutral sheet, for a dawn‐dusk electric field of 0.1–1 mV/m, lobe field of 10–40 nT, and sheet thickness of 1000 km, the analytic solutions serve as a good predictor of particle motion when the normal magnetic field component is less than 3 or 4 nT. By using the analytic solutions, initial distribution functions are mapped into final (accelerated) distributions, and the analytic mappings are compared with numerical m

ISSN:0148-0227    

DOI:10.1029/JA089iA01p00147

年代:1984

数据来源: WILEY

摘要:

Simultaneous measurement of ELF emissions and bremsstrahlung X rays were made by using a balloon borne experiment flow from Fairbanks, Alaska, on September 30, 1976. Several intense auroral X ray bursts of 2–3 minute duration were detected that have been interpreted to be the result of relativistic electron precipitation (REP). Simultaneously, the intensity of ELF emissions increased and was also observed to have periodicities similar to those in the X ray flux. These combined measurements are evidence of magnetospheric wave‐particle processes contributing to electron precipitation. The presence of pulsations in the precipitating flux is the only report of this nature during relativistic precipitation. The data have been compared with precipitation and modulation theories, and we note that the data agree with a model in which ULF modulation of the emission growth rate occurs. It is postulated that this REP event resulted from ELF wave and electron interaction, possibly triggered by the ULF occurrence rather than directly by ULF electron interacti

ISSN:0148-0227    

DOI:10.1029/JA089iA01p00159

年代:1984

数据来源: WILEY

摘要:

The transverse electrostatic mode of the collisionless long‐wavelength Kelvin‐Helmholtz instability is investigated by means of plasma simulation. The configuration studied involves a sheared velocity flow perpendicular to a static magnetic field. Electrostatic particle simulations permit the determination of the linear growth rates for this mode and reveal the nonlinear evolution and saturation of the instability. The linear growth rates agree with MHD results for small values of the ion gyroradius. For larger values of ρi(ρi/a≳ 0.5, whereais the velocity shear scale length) there is a clear reduction in the growth rate. These observations are compared with the results of a two‐fluid eigenvalue analysis with first‐order FLR corrections. The nonlinear stage of the instability is characterized by the presence of large vortices whose size is many times the initial shear lengtha. At saturation the simulation is dominated by the longest‐wavelength mode permitted in the system. A simple model is developed to explain these results. Implications of the numerical results regarding excitation of the Kelvin‐Helmholtz instability at the magnetopause boundar

ISSN:0148-0227    

DOI:10.1029/JA089iA01p00168

年代:1984

数据来源: WILEY

摘要:

This paper describes the linear kinetic theory of electrostatic instabilities driven by a density gradient drift and a magnetic‐field‐aligned current in a plasma with weak charged neutral collisions. The configuration is that of a uniform magnetic field , a weak, uniform density gradient in the x direction and a weak, uniform electric field in the z direction. Collisions are represented by the BGK model. The transition from the (kinetic) universal density drift instability to the (fluidlike) current convective instability is studied in detail, and the short wavelength properties of the latter mode are investiga

ISSN:0148-0227    

DOI:10.1029/JA089iA01p00179

年代:1984

数据来源: WILEY

摘要:

Auroral backscatter radar observations were made from Ithaca, New York, at 50 MHz during the early morning of April 1, 1976, a period of high magnetic disturbance (Kp∼ 8). The backscattered power showed large rapid (time scale of a few minutes or less) variations, characteristic of discrete radar aurora, fromL= 3.5–4. Doppler spectra of waves propagating in nearly the north‐south direction from up to 28 different ranges were obtained simultaneously with good spatial (7.5 km) and temporal (2 s) resolution. Some unusual spectra with very narrow peaks at Doppler shifts between about 70 and 90 Hz were observed and were apparently associated with sheared flows of the auroral plasma. These echoes were observed at the southern edge of the scattering region, which moved poleward with a velocity which sometimes exceeded 800 m/s. There is evidence that the echoes were generated above about 140 km. Present electrojet instability theories cannot explain these narrow spectra, but the Doppler shift suggests that they might be caused by ion cyclotron waves generated by field‐aligned currents (electron drifts). However, the excitation of ion cyclotron waves requires currents considerably stronger than typically quoted mean values (∼10−5µA/m²). Furthermore, the most easily excited ion cyclotron waves have wavelengths longer than 3 m and do not propagate in quite the right direction for detection by our radar. The threshold for the waves that we observe is considerably higher than the minimum. Intense localized currents, such as those known to be associated with auroral curls and vortices (J≳ 10−4µA/m²), appear to be sufficient to excite the longer (∼10–20 m) ion cyclotron waves in the upperEregion; whether or not the field‐aligned electron drifts are ever large enough to excite directly the 3‐m waves we observe is an open question, but there is at least some evidence which suggests that such drifts do exist in very localized regions during

ISSN:0148-0227    

DOI:10.1029/JA089iA01p00187

年代:1984

数据来源: WILEY