摘要:

Low‐frequency waves associated with ion conics have been observed in the central plasma sheet, in a region where there are no obvious sources of free energy that could destabilize these waves locally. We consider ion cyclotron waves generated in the equatorial plane by a proton temperature anisotropy and use computed growth rates to create a model wave distribution. Using ray tracing and conservation of the wave distribution function along phase space rays, we then map the wave intensities from the equatorial plane to the top of the ion‐heating region. We find that the spectral density at a geocentric distance of 2.8REwill be about 10 times higher than that in the equatorial region. Thus, convection from the equatorial plane could explain the observed spectral density of 10−6V2m−2Hz−1and the associated oxygen io

ISSN:0148-0227    

DOI:10.1029/91JA01793

年代:1991

数据来源: WILEY

摘要:

The linear and second‐order theories of ion beam‐density gradient drift instabilities on the outer edge of the inhomogeneous plasma sheet boundary layer (PSBL), with warm current sheet ion beams and cold ionospheric ion beams drifting in opposite directions, are examined. It is shown that combined interactions of ion beams and plasma density gradient drifts can excite electrostatic ion beam‐density gradient drift instabilities. These instabilities combine to form the broadband maximum growth rate spectrum with the peak growth rate ∼ 0.34ωlhat the obliquely propagating low frequency, whereωlhis the lower hybrid frequency. These linear results can be used to explain the observed broadband electrostatic noise on the outer edge of the PSBL. The calculations for the second‐order heating rate show that three particle components in the model may be heated by electrostatic waves which result from ion beam‐density gradient drift instabilities. The second‐order heating rates for the cold ionospheric ion beams are the highest. The heating rate profiles with respect to the ambient magnetic field for each component are anisotropic and similar to the maximum growth rate profiles. The peak heating rate occurs at the obliquely propagating low frequency which corresponds to the peak growth rate. The second‐order heating considered here is due to resonance broadening of weak resonant components. These results may be used to understand transport processes on the outer

ISSN:0148-0227    

DOI:10.1029/91JA01493

年代:1991

数据来源: WILEY

摘要:

In this paper the formulas for the ion distribution as well as the spectrum of radio waves scattered in a magnetized plasma with a strong electric field are derived. It is shown that the presence of the electric field in the ionosphere leads to an anisotropic ion velocity distribution and, therefore, to untypical incoherent scatter spectra for theFregion of the polar ionosphere which are caused by ion‐neutral together with ion‐ion collisions. The effect of ion‐ion collisions, which has not been taken into account so far, is to reduce the anisotropy of the ion velocity distribution. Estimates of the ion‐ion collision frequency derived from EISCAT measurements show that this may happen above about

ISSN:0148-0227    

DOI:10.1029/91JA01571

年代:1991

数据来源: WILEY

摘要:

The steepening mechanism of parallel propagating low‐frequency MHD‐like waves observed upstream of the Earth's quasi‐parallel bow shock has been investigated by means of electromagnetic hybrid simulations. It is shown that an ion beam through the resonant electromagnetic ion/ion instability excites large‐amplitude waves, which consequently pitch angle scatter, decelerate, and eventually magnetically trap beam ions in regions where the wave amplitudes are largest. As a result, the beam ions become bunched in both space and gyrophase. As these higher‐density, nongyrotropic beam segments are formed, the hydromagnetic waves rapidly steepen, resulting in magnetic pulsations, with properties generally in agreement with observations. This steepening process operates on the scale of the linear growth time of the resonant ion/ion instability. Many of the pulsations generated by this mechanism are left‐hand polarized in the space

ISSN:0148-0227    

DOI:10.1029/91JA01603

年代:1991

数据来源: WILEY

摘要:

We develop a general treatment of field‐aligned currents in quasi‐static adiabatic plasma. The formalism is an extension of an an earlier analysis (Heinemann, 1990) to include electric and gravitational fields. The assumption that the particle motions are adiabatic along the magnetic field leads to an expression for the total current density that is a generalization of expressions given by Grad (1964) and Vasyliunas (1970). The current density is a vector function of the gradients of the field line constants characterizing the plasma and the gradients of field line integrals of the partial derivatives of the parallel pressure with respect to the constants. The use of the expression as the current source in Ampere's law leads to an equation governing the equilibrium of the system of plasma and magnetic field. Examples based on bi‐Maxwellian distribution functions suggest that the effects of thermal anisotropy can be about as large as the currents due to isotropic plasma and that the effects of parallel electric fields are of the same order of magn

ISSN:0148-0227    

DOI:10.1029/91JA01825

年代:1991

数据来源: WILEY

摘要:

Magnetic field configurations associated with dayside reconnection are studied by a global two‐dimensional incompressible MHD simulation. Possible dayside reconnection patterns revealed in the simulation include magnetic field configurations associated with steady state single X line reconnection, impulsive multiple X line reconnection, and bursty single X line reconnection. Three physical parameters, the global magnetic Reynolds numberRm, the critical current densityJcfor the resistivity enhancement, and the solar wind Alfvén Mach numberMASWare found to be the most important parameters in determining the reconnection pattern. First, it is found that single X line reconnection tends to occur for smallRm(200). Second, in the presence of a current‐dependent resistivity and largeRmvarious patterns of multiple X line reconnection are observed. Third, for a largeMASWdayside reconnection tends to occur in the high‐latitude region. It is found in particular that multiple X line reconnection can indeed generate the primary flux transfer event signature, i.e., the bipolar pulse of normal magnetic field component. In addition, at the trailing edge of the bipolar pulses, enhanced plasma speed is observed in the simu

ISSN:0148-0227    

DOI:10.1029/91JA01423

年代:1991

数据来源: WILEY

摘要:

An old but rarely used resonance method is introduced and examined in order to study the very long term occurrence of Pc 1 micropulsations. By this method it is possible to detect Pc 1 pulsations with a frequency close to the eigenfrequency of the magnetometer system. We have analyzed the magnetograms of the LaCour‐type magnetometer used in a quick‐run operation at the high‐latitude station of Sodankylä, Finland. The eigenfrequencies of the three components covered the frequency range from about 0.3 to 0.5 Hz. Our analysis extends over the whole registration period from 1932 to 1944 and from 1953 to 1983, i.e., covering nearly four solar cycles. The results show a very strong negative correlation between the annual Pc 1 activity and the annual sunspot number which persists over the whole data period. According to the statistics obtained, it is, for example, very unlikely that the Pc 1 activity maximum at a high‐latitude station would occur during the declining phase of the sunspot cycle. Furthermore, as a result of the negative correlation, some of the differences between the various solar cycles can be seen in the corresponding cycles of annual Pc 1 activity. Our results also reveal some new features common for the long‐term variations of Pc 1 activity at high‐ and mid‐latitude stations and help in understanding the difference

ISSN:0148-0227    

DOI:10.1029/91JA01374

年代:1991

数据来源: WILEY

摘要:

The aim of this study is to find a quantitative description of both the magnetic field and the distribution of plasma in the lower corona that matches the white light images of the K‐corona. We use the magnetostatic model of Bogdan and Low (1986) and data obtained by the High Altitude Observatory Mark III K‐Coronameter stationed at Mauna Loa, Hawaii. To start with, we take the simplest, solar minimum case when the corona is approximately longitudinally symmetric. By varying parameters in the Bogdan and Low model we are able to quantitatively match the general characteristics of the lower corona at solar minimum: power law radial profiles of coronal brightness; enhanced brightness at the equator; uniform density depletion at the pole. Further, we find the set of parameters that best fit the data and investigate how well the model parameters are resolved by the d

ISSN:0148-0227    

DOI:10.1029/90JA02625

年代:1991

数据来源: WILEY

摘要:

Observations of some low Mach number collisionless shocks in space have revealed that their associated ion heating is greater than adiabatic, is very rapid, is primarily in the direction perpendicular to the magnetic field, and takes place in the bulk of the distribution. These features are very different from those encountered at crossings of supercritical shocks and not yet fully understood. In this paper we use a one‐dimensional hybrid kinetic simulation to study the details of the evolution of the incoming ions across a set of low Mach shocks. We find that the bulk of the ion velocity distribution becomes elongated along the direction perpendicular to the shock front as the ions traverse the shock ramp. The subsequent gyration of the ions in the steady field downstream gives rise to a temperature signature with features that correspond closely to those enumerated above. This suggests that these features may originate primarily from the kinetic behavior of the ions at the shock rather than from the operation of some instability. We also investigate the dependence of our results on a number of physical and numerical parameters in the simulatio

ISSN:0148-0227    

DOI:10.1029/91JA01646

年代:1991

数据来源: WILEY

摘要:

The macrophysics of a single‐fluid plasma at a hydrodynamic shock is characterized completely by the three Rankine‐Hugoniot conditions expressing conservation of particles, momentum flux, and energy flux across the shock. In a multifluid plasma containing an arbitrary number of ion species and electrons the Rankine‐Hugoniot conditions still characterize the behavior of the bulk plasma but not the properties of individual fluid constituents. Conservation laws for individual particle species are derived for parallel collisionless shocks. The shocks are assumed to be nonturbulent; i.e., the particles interact only via the electrostatic field. Given the plasma parameters upstream, the downstream solutions form a single‐parameter family; the free parameter can be taken to be the electrostatic potential jump across th

ISSN:0148-0227    

DOI:10.1029/91JA01811

年代:1991

数据来源: WILEY