摘要:

Plasmas on the surface of the plasma sheet at its interface with the high‐latitude lobes are studied at ∼30–40 earth radii (RE) in the tail of the earth's magnetosphere with Lepedea plasma instrumentation on board the earth‐orbiting Imp 7 and Imp 8 satellites. Protons at this interface are frequently found to be flowing toward earth with velocities greater than 400 km s−1. Proton bulk speeds measured near the plasma sheet surface are typically greater than those at locations deeper within the plasma sheet. Ten of the twenty cases studied exhibit plasma sheet cooling. Simultaneous magnetotail crossings by Imp 7 and Imp 8 occurred on October 31, 1975, during which the plasma sheet first engulfs Imp 7 and ∼74 min later is detected by Imp 8, located 7.2REfarther from the neutral sheet. Earthward‐streaming protons are detected at the boundary of the plasma sheet by both spacecraft. Typical plasma sheet conditions are observed to prevail deep within the plasma sheet coincident with the earthward streaming at the boundary of the plasma sheet. Simultaneous magnetic field measurements with Imp 8 are used to verify that the observed proton streaming is parallel to the magnetic field. TheAEindex indicates that a large magnetic substorm is nearing recovery as the plasma sheet expands past the two satellites. This event is interpreted as the buildup of the plasma sheet by the plasma flowing on its surface, and the velocity of this expansion of 11 km s−1, perpendicular to the neutral sheet, is provided by our two‐point measurements. The plasma sheet cools subsequent to passage of the expanding surface beyond each of the two observing spacecraft. Characteristic anisotropic proton velocity distributions are commonly measured for the earthward‐jetting plasma. A three‐parameter model is presented that explains these velocity distributions and that places a field‐aligned acceleration region at geocentric radial distances exceeding 30

ISSN:0148-0227    

DOI:10.1029/JA084iA09p05099

年代:1979

数据来源: WILEY

摘要:

Observations of very rapid phase advances of up to 8° for subionospheric VLF transmissions over a path length of 5745 km are found to be associated with whistlers. We show that the energetic electrons that precipitate in the D region and cause the phase advances interacted with the whistler on its second hop, which is consistent with amplification of whistlers by cyclotron resonance with electrons traveling in the opposite direction. No fast amplitude variations were detected. We propose a simple two‐mode model to describe the effects of nighttime D region disturbances on long‐distance subionospheric VLF propagation, and we show how this model can account for our observations and also the amplitude effects seen by Helliwell, Katsufrakis, and Tr

ISSN:0148-0227    

DOI:10.1029/JA084iA09p05122

年代:1979

数据来源: WILEY

摘要:

Whistlers observed on the ground with frequency cutoffs greater than one‐half the equatorial electron gyrofrequency along their field‐aligned propagation paths have been termed super whistlers. The rarity of the super whistler is a consequence of the relatively small number of ducts which allow their propagation. The upper cutoff frequency of the super whistler is directly related to the physical structure of the duct. In addition to the path location and equatorial electron density that can be deduced from ordinary whistlers, super whistlers also provide information on density gradients along the magnetic field lines near the equator. Analysis of one set of super whistlers shows that immediately after a magnetic disturbance the equatorial plasma nearL=4.5 is in an intermediate state between the collisionless and diffusive equilibrium distributions. The plasma distribution approaches that of diffusive equilibrium after a period of low geomagnetic activ

ISSN:0148-0227    

DOI:10.1029/JA084iA09p05131

年代:1979

数据来源: WILEY

摘要:

The three‐dimensional theory of the quiet magnetotail by Birn et al. (1977a) is revised and generalized. The general static solution is presented for the case of vanishing field‐aligned currents holding for quiet times with small bulk velocities and negligible pressure anisotropy. Since the knowledge of particle entry, transport, and loss and of the interaction of the magnetosphere with the solar wind is too small to determine the solution by appropriate initial and boundary conditions, the free functions of the general solution have been adjusted to the observational material inside the tail. It is shown that the quiet tail data can be fitted by a self‐consistent model without flux loss through the boundary (closed magnetosphere). Restrictions following from self‐consistency provide further (indirect) evidence for plasma sheet thickening towards the flanks. The effect of dipole tilt on the neutral sheet position and tail aberration can also be included in the model. It turns out that the curvature of the field lines in the x, y plane is connected with the occurence of field‐aligned

ISSN:0148-0227    

DOI:10.1029/JA084iA09p05143

年代:1979

数据来源: WILEY

摘要:

Geomagnetic micropulsations in the Pc 4 frequency band which are highly monochromatic and have amplitude‐modulated wave forms have been termed ‘giant pulsations’ and have been observed to reach amplitudes of several tens of nanoteslas. In this paper, four such events are analyzed by using meridian line and two‐dimensional magnetometer networks. It is found that these pulsations are highly localized in latitude and have a large azimuthal wave number k⊥, although the large magnitude of k⊥does not necessarily reflect the longitudinal extent of the disturbed region. While such events are extremely rare, they may occur on 2 or more consecutive days. The pulsations occur on quiet days and normally after a sustained period of low magnetospheric activity. The initiation of substorm activity appears to quench the giant pulsations, which may return after the substorm has subsided. We suggest that the pulsations accompany suddenly reduced levels of magnetospheric convection, and indeed the source region of the pulsations moves poleward during the course of the pulsational activity, suggesting a recovery phase relationship. It is suggested that the Pc 4 giant pulsations may be due to the occurrence of field line resonances at the plasmapause in the region where the electric field changes its azimuthal direction from westward

ISSN:0148-0227    

DOI:10.1029/JA084iA09p05153

年代:1979

数据来源: WILEY

摘要:

By comparing Rae 1 and Imp 6 satellite measurements of Jupiter's radio emissions near 1 MHz with recent Voyager 1 and 2 observations in the same frequency range it is now possible to study the properties of the low frequency radiation pattern over a 10° range of latitudes with respect to the Jovian rotation equator. These observations, which cover a wider latitudinal range than is possible from the earth, are consistent with many aspects of earlier ground‐based measurements that have been used to infer a sharp beaming pattern for the decameter wavelength emissions. We find marked, systematic changes in the statistical occurrence probability distributions with system III central meridian longitude as the Jovigraphic latitude of the observer changes over this range. Moreover, simultaneous observations by the two Voyager spacecraft, which are separated by up to 3° in Jovigraphic latitude, suggest that the instantaneous beam width may be no more than a few degrees at times. The new hectometer wave results can be interpreted in terms of a narrow, curved sheet at a fixed magnetic latitude into which the emission is beamed to escape the pla

ISSN:0148-0227    

DOI:10.1029/JA084iA09p05167

年代:1979

数据来源: WILEY

摘要:

Using a simple, planar model of the high‐latitude ionosphere with an enhanced conductivity auroral ring, we compute the electric potential for a pair of field‐aligned current sheets inferred from observations. The relationship of various characteristics of the electric potential pattern to features of the field‐aligned current distribution are elucidated in the context of a generalized field‐aligned current Fourier analysis. On the basis of our analysis and observations to date we conclude that boundary layer dynamo action is the prevalent mechanism in the solar wind/magnetosphere/ionosphere inte

ISSN:0148-0227    

DOI:10.1029/JA084iA09p05175

年代:1979

数据来源: WILEY

摘要:

The Galileo satellite program includes the delivery of an entry probe to the Jovian equatorial atmosphere in mid‐1985. Optical and RF sensors for a lightning detection system are included as a part of the probe experimental payload. In this paper we present calculations of the RF wave propagation and reflection characteristics of the equatorial Jovian atmosphere and ionosphere for frequencies less than 100 kHz. We show that the wave propagation is limited to line‐of‐sight and one‐hop from the ionosphere. We also present results of a statistical treatment of the RF wave power densities for the case of a finite number of events and for the case of a uniformly distributed source. The results can be applied to specific RF experiment configurations concerned with establishing the statistical characteristics of Jovian li

ISSN:0148-0227    

DOI:10.1029/JA084iA09p05181

年代:1979

数据来源: WILEY

摘要:

The propagation of electrostatic plasma waves in an inhomogeneous and magnetized plasma is studied analytically. These waves, which are driven unstable by auroral beams of electrons, are shown to suffer a further geometrical amplification while they propagate toward cut‐off. Simultaneously their group velocities tend to be aligned with the geomagnetic field. Then it is shown that the electrostatic energy tends to accumulate at or near ωLHand ωUH, the local lower and upper hybrid frequencies. Due to this process, large amplitude electrostatic waves with very narrow spectra should be observed near these frequencies at any place along the auroral field lines where intense beam driven instability takes place. These intense quasi‐monochromatic electrostatic waves are then shown to give rise by a coherent nonlinear three wave process to an intense electromagnetic radiation. Provided that the ratio ωpe/ωcetends to be smaller than unity, it is shown that the most intense radiation should be observed at 2ωUHin the extraordin

ISSN:0148-0227    

DOI:10.1029/JA084iA09p05189

年代:1979

数据来源: WILEY

摘要:

Long‐delay echoes (LDE), defined as echoes received from a fraction of a second to several seconds after a radio signal is transmitted, have been observed off and on for about 50 years. A variety of explanations has been proposed in the past but none is completely satisfactory. The following models are presently proposed for LDE: (1) Radio waves of frequency less than about 4 MHz can become trapped in magnetic field‐aligned ionization ducts withLvalues less than about 4. These waves after being trapped can propagate to the opposite hemisphere of the earth where they become reflected in the topside ionosphere. They can then return along the duct, leave it and propagate to the receiver. Delays of up to 0.4 s result and they probably account for most of the LDE at frequencies below 4 MHz with estimated delays of 1–2 s. (2) The signals from two separated transmittersT1andT2,T2transmitting a CW or quasi‐CW signal, interact nonlinearly in the ionosphere or magnetosphere. If the wave vector and frequency of the forced oscillation at the difference frequency of the two signals satisfies the dispersion relation for electrostatic waves, such a wave would exist and begin to propagate. This wave could grow in amplitude due to wave‐particle interaction. At a later time it could interact with the CW signal fromT2and if the wave vector and frequency of the forced oscillation at the difference frequency (frequency ofT1) satisfy the dispersion relationship for electromagnetic waves, such a wave would exist and could propagate toT1, or some other receiving station tuned to the frequency ofT1. Reasonable ionospheric and magnetospheric plasma parameters lead to delays of up to about 6 s with this model. (3) A large percentage of LDE have been reported with delays of tens of seconds. These delays could be explained if the model in (2) is applied to a magnetospheric ionization duct. Electrostatic waves could propagate for about 1000 km or more over the magnetic equator in such a duct and delays of about 40 s are possible. Dispersion for a finite frequency bandwidth would probably not be so large in cases (1) and (2) as to make voice unrecognizable. Dispersion in model (3) for delays greater than about 10 s would normally be too severe for voice modulation, but occasionally compensating effects might occur for which voice would be rec

ISSN:0148-0227    

DOI:10.1029/JA084iA09p05199

年代:1979

数据来源: WILEY