摘要:

Data covering a complete solar cycle (1965–1976) have been analyzed in a comprehensive investigation of the nature of transient modulations of relativistic galactic cosmic rays associated with interplanetary magnetic field intensity excursions in the vicinity of the earth. It had been recognized earlier that the correlation coefficient between the cosmic ray intensity and the IMF magnitude is small. In fact, it remains less than 0.5 throughout the solar cycle. In order to understand the nature of field‐related modulations and to minimize the complications arising from the effects of other features of solar plasma, the data are here analyzed by the method of superposed epochs. Following newly developed procedures for evaluating the statistical significance of the results obtained with this type of analysis, it is demonstrated that, while the days characterized by high IMF magnitudeBare associated with intensity decreases, the IMF rarefactions (lowB) cause an increase in the flux. In contrast to the case of geomagnetic disturbances, which respond more rapidly, maximum cosmic ray intensity variations associated with both high and lowBoccur 1 day after the key days characterized by field departures from average values. This indicates that the modulation mechanism becomes efficient only when the plasma engulfment of the earth extends at least a gyroradius beyond 1 AU. Finally, while the geomagnetic indexApfollows the interplanetary magnetic field changes rather closely, the polar nucleonic intensity shows a prolonged recovery time (7–10 days) following magnetic enhancements as well as rarefactions. These results suggest that, on average, interplanetary magnetic field intensity variations which are presumably related to disturbances traveling from the sun are effective transient modulators of the relativistic particles and that the corotation of the modulating region plays a significant role in the recovery of the cosmic ray intensity. An investigation of the spectra of the cosmic ray intensity variations produced by interplanetary magnetic field enhancements and rarefactions, respectively, over the rigidity range covered by ground‐based nucleonic intensity detectors has revealed that a power law of the formKp−1fits the data in both cases. This result is relevant to further theoretical studies of cosmic ray modulation

ISSN:0148-0227    

DOI:10.1029/JA088iA04p02973

年代:1983

数据来源: WILEY

摘要:

Observations of solar wind ion velocity distributions made by the Helios spacecraft between 0.3 and 1 AU are used to study the radial evolution of the so‐called adiabatic invariants, for example, the ion magnetic moments. Significant differences between the parameters of protons and α particles have been found in dependence on the wind velocity. On the average, adiabaticity is observed to be violated. We interpret this violation of adiabatic invariance as evidence that protons are heated perpendicular to the field in fast streams and, with less statistical significance, that α particles are cooled more strongly than for adiabatic expansion parallel to the magnetic field. The contribution of the differential streaming energy to the total internal energy of the ions is briefly investigated. Also, average heliocentric radial profiles for the ion heat fluxes are presented, and the possible role of the ion heat flux in supplying thermal energy during the radial expansion of the wind is examined. Our findings suggest that wave‐particle interactions and (or) Coulomb collisions (or other yet unknown processes) have to be invoked in order to explain the thermal energy state of solar wind ions and their radial temperature pro

ISSN:0148-0227    

DOI:10.1029/JA088iA04p02982

年代:1983

数据来源: WILEY

摘要:

In this paper we examine the global characteristics of the magnetic flux ropes in the Venus ionosphere. These structures are the most common magnetic feature of the ionosphere, being observed on more than 70% of the orbits of the Pioneer Venus orbiter passing through the dayside ionosphere. The spatial occurrence of flux ropes maximizes at altitudes near 165 km; they may occupy more than 50% of the ionospheric volume at this altitude. At higher altitudes the fractional volume occupied by ropes decreases below 20%. Characteristic radii of the ropes vary between 6 km at 160 km altitude and 15 km at 500 km altitude, and scale roughly with both the local ion gyroradius and ion inertial length. Ropes above 200 km altitude in the low zenith angle regions (χ45°) tend to be horizontal above 300 km and randomly oriented below 300 km. Ropes may tend to be more tightly ‘twisted’ at low altitudes than at high, and more so in the χ<45° regions than nearer the terminator. Rope field strengths are highest near the altitudes where their occurrence maximizes, i.e., near 165 km, and scale with the square root of the ambient thermal pressure. Finally, the global polarities of flux rope field‐aligned currents appear to be random and do not clearly support a steady, nonturbulent global form

ISSN:0148-0227    

DOI:10.1029/JA088iA04p02993

年代:1983

数据来源: WILEY

摘要:

A persistent feature of the nightside Venus ionosphere is the existence of localized magnetic field aligned columns of depleted ionization—i.e., ‘holes’—in which the radial component of the field is enhanced. By analogy to the earth's auroral regions it has been inferred that parallel electric fields can rapidly deplete the ionization. Another, complementary plasma loss mechanism at night is downward transport into the lower ionosphere where chemical depletion occurs. A simple rectilinear two dimensional MHD model is used to explore the effects of field aligned plasma loss and cooling on a dense plasma convecting across a weak magnetic field. By parameterizing field aligned variations and explicitly solving for cross variations, it is shown that the abrupt horizontal enhancements of the vertical magnetic field as well as abrupt decreases of the plasma density to very low values—the two characteristics of a hole—can be produced in the presence of field aligned losses. The buildup of the magnetic field occurs in conjunction with a deceleration of the horizontal flow. The horizontal decrease in plasma density occurs abruptly when the magnetic field intensifies to such a level that the associated horizontal transport time becomes smaller than the field aligned loss time; this process is referred to as ‘magneti

ISSN:0148-0227    

DOI:10.1029/JA088iA04p03005

年代:1983

数据来源: WILEY

摘要:

Gurnett and Goertz (1981) proposed that the large number of discrete arcs observed in the Jovian decametric radio emission is caused by multiple reflections of Alfvén waves excited by Io. In this paper the plasma measurements that were made by the Voyager 1 plasma science experiment have been combined with a model of Jupiter's magnetic field to calculate the time an Alfvén wave takes to travel between Io and Jupiter's ionosphere and the period of subsequent bounces between the northern and southern hemispheres. The result is a wave pattern extending around Jupiter as the multiply reflected Alfvén waves are carried away from Io by the corotating magnetospheric plasma. Although the whole pattern continually changes over the 13 hours Io takes to move through 360° of Jovigraphic longitude, a general longitudinal structure is exhibited independent of the position of Io, due to the geometry of the magentic field and the distribution of plasma in the Io torus. If the Alfvén waves stimulate the decametric radio emission, then the wave pattern predicts specific properties of the decametric emission which can be compared with radio observat

ISSN:0148-0227    

DOI:10.1029/JA088iA04p03013

年代:1983

数据来源: WILEY

摘要:

The lower hybrid instability is studied in the perpendicular bow shock geometry including finite beta effects and an electron temperature gradient. The flute (k∥= 0) mode, which for constant electron temperature is stable at the shock, is destabilized for sufficiently large temperature gradient (εTρi≳ 1). Numerical solutions are presented for cases in which the ion distribution is either a drifting Maxwellian or consists of two Maxwellians to represent the effect of reflected ions at the shock. Implications of the results concerning ion and electron heating and electron acceleration at the bow shock are also disc

ISSN:0148-0227    

DOI:10.1029/JA088iA04p03026

年代:1983

数据来源: WILEY

摘要:

The linear Vlasov stability of a class of electron velocity distributions which model those observed within the earth's bow shock is examined. The study is restricted to electrostatic waves which propagate parallel to the ambient magnetic fieldB. Two instabilities are identified which are driven by free energy in the direction parallel toB: an ion acoustic wave with real frequency below the ion plasma frequency and an electron acoustic wave with real frequency of several times the ion plasma frequency. The characteristics of the unstable waves are in accord with observations obtained within the bow shock, in particular, with the trend of the observed electrostatic waves to be polarized parallel to the magnetic field. The instabilities identified here may contribute to electron dissipation in collisionless shocks.

ISSN:0148-0227    

DOI:10.1029/JA088iA04p03035

年代:1983

数据来源: WILEY

摘要:

A new source mechanism is proposed for the ‘reflected’ ion beams observed in the foreshock region of the earth's bow shock. In our model the beams originate in the magnetosheath downstream of the quasi‐perpendicular portion of the shock. The quasi‐perpendicular shock transition is characterized by two downstream ion populations including high‐energy gyrating ions in addition to the directly transmitted anisotropic ions. We show by particle simulations that this highly anisotropic downstream ion distribution (T⊥/T∥» 1) can excite electromagnetic ion cyclotron waves which, in turn, pitch angle scatter the gyrating ions in a few ion gyroperiods. As a result, some ions acquire large parallel velocities and move fast enough along the convecting downstream magnetic field to escape back across the bow shock into the upstream region. The distribution of escaping ions calculated by using the pitch‐angle‐scattered ions, as a source, becomes a beam with a large temperature anisotropyT⊥∼ 3–5T∥and a mean velocity along the magnetic field of about twice that of the solar wind velocity. A significant result is the presence of the maximum angle θnB= θcabove which no ions can escape, where θnBis the angle between the shock normal and the interplanetary magnetic field. A wide peak of constant escaping ion flux is formed below θcwhose number density is 1–2% of that of the solar wind. These results are in general agreement with the ISEE obse

ISSN:0148-0227    

DOI:10.1029/JA088iA04p03046

年代:1983

数据来源: WILEY

摘要:

To lend further support to the ‘injection boundary’ concept, this paper characterizes the details of geostationary particle signatures using a very simple‐minded analysis procedure. The signatures are generated using the time of flight effects which evolve from an initial sharply defined, double‐spiraled boundary configuration. By using only the most fundamental characteristics of standard convection configurations, the very complex and highly variable dispersion patterns frequently observed by geostationary satellites are successfully reproduced. In particular, seven distinctly different ion‐electron paired dispersion patterns on energy versus time spectrograms (1 eV to 100 KeV) are predicted, and all seven of these are observed on a regular basis by both the SCATHA satellite (in the near geostationary orbit) and the ATS‐6 satellite. Many of the details of the patterns have not been previously presented. It is concluded that most dynamical dispersion features (including energetic ion and electron echoes) can be mapped to the double‐spiral boundary without further ad hoc assumptions. It is shown further that the predicted and observed dispersion patterns have symmetries which are distinct from the symmetries generally associated with the quasistationary particle convec

ISSN:0148-0227    

DOI:10.1029/JA088iA04p03055

年代:1983

数据来源: WILEY

摘要:

Computer simulations of whistler mode wave‐particle interactions have been performed by using a free‐boundary encounter model electromagnetic code. The free boundary is attained by adding a collision dominated damping region near one boundary to remove periodicities usually used in electromagnetic codes. In our model, a wave front encounters a resonant bi‐Maxwellian hot electron beam. Spatial amplitude modulations are observed in our simulations. The dips of the amplitude, however, sometimes decrease to zero levels. The null points of the wave magnetic field and the associated phase reversals of the wave cannot be described by the usual particle trapping theory. A component of the field contains a feature reminiscent of a double sideband (DSB) wave without a carrier. This is interpreted as a beat between an incident wave and a generated wave. This phenomenon occurs if the amplitudes of both the waves are almost equal. The results of these simulations provide a possible explanation for the N events observed by Dowden et al. during a VLF injection exper

ISSN:0148-0227    

DOI:10.1029/JA088iA04p03072

年代:1983

数据来源: WILEY