摘要:

We discuss the instability of transverse electromagnetic waves propagating parallel to the average magnetic field in an electron‐proton plasma, and we examine the question of whether the instabilities may occur in the solar wind. Our discussion is based on Stix' well‐known dispersion relation, but we depart from most previous analyses by assuming that the resonant protons lie near the peak of their distribution function. We find three instabilities, two of which are new. The first new instability is driven by anisotropic electrons in the same manner as is the firehose, and we believe that it represents an extension of the firehose instability to large wave numbers. The growth rates are large over a broad frequency range around Ωp. We suggest that the excitation of this instability results in a partial transfer of thermal energy from the electrons to the protons and, if it occurs in the solar wind, may therefore heat the solar‐wind protons and reduce their anisotropy. The second, new instability is driven by anisotropic protons withT⊥p⪞3T∥p; it may occur in local regions of the solar wind. The third instability is not new; it represents a continuation of the unstable whistler mode near Ωpto those cases where there are many resonant protons. In contrast to earlier papers, in which it is assumed that the number of resonant protons is small, our work demonstrates that this instability can in fact be effective in destroying proton anisotropies of the type observed in t

ISSN:0148-0227    

DOI:10.1029/JA075i028p05297

年代:1970

数据来源: WILEY

摘要:

The orientation of the interplanetary magnetic field has been studied by using magnetometer data from Mariners 2, 4, and 5. The magnetic fieldBand the solar wind velocityVwere measured in the right‐handed, solar equatorial coordinate system with unit vectorsrˆ,θˆ,andϕˆ. The tangent of the spiral angle, tan αB= ‐Bϕ/Br, is given by the Parker model as tan αp= Ωrsin θ/Vr, where Ω is the sun’s angular velocity andris the spacecraft‐sun distance. The results of the observations made with Mariners 2, 4, and 5 are that at ‘quiet’ periods of low solar activity, i.e., when ‘slow’ solar wind streams dominate the flow, tan αBtan αp. The time intervals during which these coherent deviations occur ranged from about 3 days to 27 days, and as much as several solar rotation periods. The difference between αBand αpmay possibly be explained by conservation of magnetic flux and by means similar to those used to describe fast‐mode blast waves. A portion of the theory presented deals with the nonzero value obtained whenBθ(‘north‐south’ component near the ecliptic) was averaged over 27 days for the + and − polarity fields. The nonzeroBθ, specificallyBθs= (Bθ+‐Bθ−)/2, is explained in terms of Alfvén shocks (bθ= ‐ (4πρ)1/2υθP, where ρ is the plasma density, andPis the field’s polar

ISSN:0148-0227    

DOI:10.1029/JA075i028p05310

年代:1970

数据来源: WILEY

摘要:

A comparison of the Explorer 35 solar wind plasma data with the data obtained by Pioneer 8 during a pass through the tail region at a geocentric distance of 500REsuggests the presence of a geomagnetic wake. The wake was characterized by a reduced density or increased temperature or both and by an almost unmodified flow speed as compared with upstream values. The change in density or temperature was of the order of 2 to 4. The wake is discussed in relation to the drag on the magnetosphere. A simple hydrodynamic calculation of wave drag effects gives approximate agreement with observed magnitudes, but the wake region appears to be somewhat larger than expected. Possible effects to account for this include heat diffusion out of the wake and a two‐fluid effect in which ions are heated more than electrons at the bow shock. The data do not provide evidence for a viscous boundary layer in the usual sense, but a magnetic surface drag is not excluded. Finally, the possibility of local heating at the boundary is shown to be incapable of accounting for all wake feature

ISSN:0148-0227    

DOI:10.1029/JA075i028p05319

年代:1970

数据来源: WILEY

摘要:

Dispersion equations are derived for hydromagnetic oscillations of the geomagnetic tail and plasma sheet by representing the tail as a long cylindrical current‐vortex sheet and the sheet as a slab of hot plasma surrounded by cold, magnetized plasma. It is shown that a cylindrical geomagnetic tail is stable if the speed of the hot gas flowing outside the cylinder is less than the Alfvén speed in the tail, and that the characteristic period is about 13 min for disturbances with wavelengths of the order of the diameter. The model of the plasma sheet yields periods of 6 min for the sausage mode, and 3 min for the kink mode, for wavelengths of the order of the thickness of the she

ISSN:0148-0227    

DOI:10.1029/JA075i028p05331

年代:1970

数据来源: WILEY

摘要:

The application of the virial theorem to the magnetosphere has been extended to the study of a closed magnetosphere. The result is used to express the field at the earth due to external current systems in terms of various self energies of the external systems. The special case of an isolated ring current reduces to the usual result. A new expression is obtained for the boundary current contribution and for the combined ring and boundary currents. Energy considerations are given that lead to an estimate of the size of the ring current effect on geomagnetic sudden impulses and sudden commencements.

ISSN:0148-0227    

DOI:10.1029/JA075i028p05340

年代:1970

数据来源: WILEY

摘要:

The near‐equatorial fluxes of outer‐zone electrons (E>0.5 Mev andE>1.9 Mev) measured by an instrument on the satellite Explorer 15 following the geomagnetic storm of December 17–18, 1962, are used to determine the electron radial diffusion coefficients and electron lifetimes as functions ofLfor selected values of the conserved first invariant µ. For each value of µ, the diffusion coefficient is assumed to be time‐independent and representable in the formD=DnLn. The diffusion coefficients and lifetimes are then simultaneously obtained by requiring that theL‐dependent reciprocal electron lifetime, as determined from the Fokker‐Planck equation, deviate minimally from a constant in time. Applied to the data, these few assumptions yield a value ofDthat is smaller by approximately a factor of 10 than the value recently found by Newkirk and Walt in a separate analysis of 1.6‐Mev electron data obtained during the same time period on another satellite. The electron lifetimes are found to be strong functions ofL, with 4‐ to 6‐day lifetimes observed at the high

ISSN:0148-0227    

DOI:10.1029/JA075i028p05351

年代:1970

数据来源: WILEY

摘要:

Proton differential fluxes are reported for 12 energies from 0.060 to 3.3 Mev at 6.6 earth radii for the time period of October 2 to 13, 1968. The proton energies were measured with a surface barrier semiconductor detector and a stacked discriminator pulse height analyzer on the Air Force Office of Aerospace Research Satellite OV2‐5 (1968‐81A). The typical quiettime fluxes decreased by 8 orders of magnitude over the measured energy interval. During this time the energy spectrum varied fromE−3.5toE−6.0. Below 200 kev the energy distributions flatten but never peak above the energy of the lowest channel, 68 kev. Energy spectra associated with the two magnetic storms on October 2 and 12 are presented. After the storms the fluxes were higher than before; this is particularly true at the lower energies measured. Quiettime pitch angle distributions near the local noon‐midnight meridian clearly show the east‐west effect caused by the radial gradient in the fluxes. The pitch angle distributions at local midnight show the ‘drift loss cone’ predicted by Roederer. This experiment provides the first confirmation of theRoederer[1967] model of shell splitting in the earth's magnetic field by measured proton angula

ISSN:0148-0227    

DOI:10.1029/JA075i028p05373

年代:1970

数据来源: WILEY

摘要:

The ϕ boundary was introduced in 1968 as a more meaningful concept of the high‐latitude limit to durably trapped ≥40‐kev electrons than the usual intensity cutoff. In the present study, this boundary is defined more precisely by using four different features of the latitude profile of the ϕ parameter. Two additional high‐latitude boundaries, the intensity cutoff (Λc) and the background boundary (Λbkg) for ≥40‐kev‐electrons are defined, and the average positions of each of these six ‘boundaries’ are investigated as a function of invariant latitude, magnetic local time, and magnetic activity. A frequency of occurrence of isotropic and near‐isotropic pitch‐angle distributions for ≥40‐kev electrons is also investigated as a function of magnetic local time and magnetic activity. The results of this study suggest that at least two separate mechanisms control the position of the high‐latitude boundary as a function of magnetic local time. One mechanism operates continuously near midnight, but its effectiveness decreases monotonically with increasing local time through dawn and noon, then reaches a minimum in effectiveness in the late afternoon and dusk hours. A second mechanism operates only near the noon meridian (0800 to 1400 hours) and is the dominant mechanism in this local‐time interval. There is also an indication that the orientation and symmetry axis of

ISSN:0148-0227    

DOI:10.1029/JA075i028p05387

年代:1970

数据来源: WILEY

摘要:

Observations of electron intensities over the energy range ∼100 ev to 50 kev are presented forLvalues between 3 and 10RE(earth radii) near local midnight at low magnetic latitudes. These measurements were obtained over the period June 11 through July 23, 1966, with an array of electrostatic analyzers borne on the earth satellite Ogo 3. The earthward edge of the plasma sheet is characterized by severe decreases in electron energy densities with decreasing geocentric radial distance. These decreases have a distinct structure with fluxes of higher energy electrons decreasing further from the earth for the electron energy range ∼700 ev to 20 kev. A trough of ∼100‐ev electrons with number densities of ∼1(cm)−3is observed to fill the region between the earthward edge of the plasma sheet and the plasmasphere. The densities of electrons with energies greater than 700 ev are often less than 0.1 (cm)−3in this region. The observed separation between the inner edge of the plasma sheet and the plasmapause was ∼1 to 3REin June and ∼3 to 5REin July. Analysis of subsequent observations should determine whether this is a latitude or local time effect. The plasmapause is usually observed to be located at the minimum of the energy density profile for electrons with energies above 700 ev. At electron energies of ∼200 ev a plasmapause structure comprising a sharp radially outward boundary and a broader inward boundary is observed. The observed lifetimes of ∼10 days for lower energy electron (E∼1 kev) intensities in the plasmasphere that were enhanced during a moderate geomagnetic storm are similar to previously reported lifetimes for more energetic electron (E≥500 kev) intensities in the same region. Kilovolt electron intensities between the plasmasphere and the earthward edge of the plasma sheet decreased more rapidly and fell to typically quiescent values within the satellite’s orbital period of 48 hours. Simultaneous observations of the angular distributions of electron intensities at two pitch angles in the plasma sheet revealed that these angular distributions approached isotropy for the several plasma sheet crossings that were examined. Differential energy spectrums of electron intensities for the plasma sheet, the earthward edge, the electron ‘trough,’ and the

ISSN:0148-0227    

DOI:10.1029/JA075i028p05401

年代:1970

数据来源: WILEY

摘要:

A series of five Nike Tomahawk sounding rockets were launched from Fort Churchill into a variety of auroral conditions. Payload instrumentation included electric field meters, magnetometers, auroral photometers, and charged‐particle detectors. Horizontal dc electric fields were measured and were found to point southward during negative magnetic bays. This is consistent with the presence of westward‐flowing Hall currents. The relative absence of electric‐field variations with altitude indicates that field mapping along nearly equipotential magnetic field lines may be taking place. Perturbations measured by the rocket magnetometers and photometers suggest that the auroral electrojet often flows parallel to visible auroral forms and is comprised of a series of thin current filaments of varying intensity. According to the magnetometer records, horizontal electrojet currents of 104to 106amp existed during periods of visible auroral activity. An upper limit for a field‐aligned current in a zenith auroral display 10 km from the payload was calculated to be 2.5×105amp on the basis of flux‐gate magnetometer data. Results from a solid‐state particle detector, however, suggest that the current was probably of much smaller magnitude. Calculations using various current models indicate that it is often necessary to propose the existence of two or more horizontal current filaments to produce agreement between ground‐station and rocket ma

ISSN:0148-0227    

DOI:10.1029/JA075i028p05415

年代:1970

数据来源: WILEY