摘要:

Nonlinear mixing of two electromagnetic waves, ω±andk±, in an inhomogeneous isotropic and collisionless plasma in the parameter range β2/3(KL)1/3≪ 1 is studied. Here β is the ratio of electron thermal velocity to speed of light,K= Ω/c, Ω = ω+− ω−, andLis a local characteristic length for electron density variation. Langmuir waves, Ω andkL, are found to be excited in two different ways. When (Δk/K)β2/3(KL)1/3≫ 1, where Δk=k+−k−, resonant wave mixing occurs in which the generated Langmuir waves satisfykL≈ Δk. When (Δk/K)β2/3(KL)1/3≪ 1, nonlinear conversion takes place at the critical surface where Ω equals the local plasma frequency, the Langmuir waves propagating antiparallel to the density gradient and not along Δk. Coherent scattering of electromagnetic waves off the density perturbations associated with the

ISSN:0148-0227    

DOI:10.1029/JA083iA05p01985

年代:1978

数据来源: WILEY

摘要:

Analyses of the proton cyclotron echoes observed on the Alouette 2 ionograms, in more than 800 examples, indicate that the echoes are produced in a range within ±25° width of the magnetic dipole latitude centered at the magnetic equator. The generation mechanism of these proton cyclotron echoes is clarified here as follows: (1) the transmitted RF pulse makes an electrostatic plasma wave that produces a fast electron stream in opposite directions with respect to the wave propagation direction during damping processes of the wave; (2) the RF pulse also produces a proton concentration surrounding the antenna due to negative excursion of the antenna potential; (3) the proton concentration produced is repeated each proton cyclotron period and makes a pseudo‐wave of the electric field; and (4) the encounter of the electron stream with the pseudo‐wave due to the proton concentration makes a plasma wave with frequency in the transmitted RF range. The place where the electron stream encounters the pseudo‐wave should be located at a position relatively close to the satellite for the observation of the RF

ISSN:0148-0227    

DOI:10.1029/JA083iA05p01991

年代:1978

数据来源: WILEY

摘要:

The delay in arrival times at the earth of solar cosmic rays has traditionally been explained in terms of diffusive transport within the corona to locations distant from the flare site and subsequent escape followed by interplanetary propagation more or less along the spiral field. The mechanism of such diffusive coronal transport has often been thought to be pitch angle scattering by irregularities in the coronal magnetic field. Recently, Ma‐Sung et al. have noted that the time delays attributed to the coronal portion of the propagation for electrons between 0.5 and 12 MeV and protons between 4 and 100 MeV are independent of rigidity and energy. It is demonstrated that the observed delays, their independence of rigidity over 2 orders of magnitude, and the absence of significant losses of particles in traveling large distances in the corona are inconsistent with propagation by continuous diffusion by pitch angle scattering for any reasonable topology of the coronal magnetic field. We propose an alternate mechanism for the coronal transport of solar cosmic rays: the bird cage model, in which particles are normally transferred between adjacent flux tubes by field line reconnection produced by the rearrangement of the field in the supergranulation network. The observed size of coronal flux loops and the rate of reorganization of flux in the supergranulation network lead to ‘diffusion’ rates which are both independent of rigidity and energy and consistent with the observed propagation

ISSN:0148-0227    

DOI:10.1029/JA083iA05p02009

年代:1978

数据来源: WILEY

摘要:

A statistical study of 42 solar proton and α particle events has been performed in the energy range of 9–36 MeV/nucleon (nuc) as measured by the Space Science Department/European Space Agency experiment on Heos 2 during 1972–1974. From one event to another there is a high variability of thep/α ratio at equal energy per nucleon ranging from about 4 to 1000 at 10 MeV/nuc. It is found that the lower value of thep/α ratios increases with azimuthal distance from the flare site. We show that this is not the result of rigidity‐dependent coronal processes but that the increase follows rather as a consequence of two observations: (1) thep/α ratio is correlated with the size of the particle event (Van Hollebeke, 1975) and (2) the number of observed particle events decreases to the east as is found by numerous authors. A model calculation based on these observations is in good agreement with the data. The conclusion of rigidity‐independent coronal propagation and escape also follows from other observations. First, thep/α ratios simultaneously measured by the University of Chicago experiment on board Pioneer 10 and 11, which have different connection longitudes, are generally the same as the Heos 2 values. Second, no systematic variation of thep/α ratio is found for the few cases of large individual events, when thep/α ratio can be followed over an extended time period, i.e., over an extended range of connection longitudes. Third, the proton and α particle spectral exponents show no systematic variation with heliolongitude. It is concluded that the observed large variations of thep/α ratio from event to event are essentially due to different acceleration or storage conditions a

ISSN:0148-0227    

DOI:10.1029/JA083iA05p02017

年代:1978

数据来源: WILEY

摘要:

Refraction and/or magnetic field curvature can lead to ‘geometrical mode coupling’ between the MHD modes. The effect is linear, but it is a finite‐wavelength effect. It is studied here for a simple configuration which is amenable to analysis and which illustrates the basic features of the coupling. In the solar wind the geometrical coupling may be operative only in the solar corona or in the interaction regions of high‐speed streams. In the latter case the geometrical coupling may provide an explanation for the non‐Alfvénic fluctuations but only for long‐period waves (greater than several hours) in small interaction regio

ISSN:0148-0227    

DOI:10.1029/JA083iA05p02030

年代:1978

数据来源: WILEY

摘要:

Enhancements of energetic protons (≳0.5 MeV) in association with forward‐reverse shock pairs have been observed at large heliocentric distances. An interpretation of the time profiles of these events is offered in terms of a model of solar wind stream structure. Persistent sweeping of energetic particles by each shock front and their banking‐up by reflection lead to the formation of a gradient along field lines upstream of the shocks. The rise (fall) of intensity before (after) the forward (reverse) shock is explained by a gradient in particle density directed along the upstream field lines toward the shock. A marked intensity decrease between the two shocks is interpreted in the same fashion as being due to parallel density gradients directed toward the shocks. First‐order Fermi acceleration upon reflection at each highly oblique shock front offsets loss by leakage through the shock front and sustains the particle intensity in the upstream region. Other acceleration mechanisms may not be required. A simple model is explored in which the gradient profile is primarily determined by a balance between shock sweeping and diffusive flow away from the shock (it is argued that acceleration by reflection and leakage through the shock cause a secondary modification of the profile). Profiles outside the compression region in 15 observations of corotating events suggest a radial mean free path λr= 0.021 → 0.64 AU (the mean is overr= 2.6–6 AU. The model predicts an anisotropy which upstream of each shock is lower in magnitude than observations by a factor of ≈ 2. The predicted direction in the upstream region is in the direction of corotation, i.e., perpendicular to the radius vector and counterclockwise in the ecliptic plane. Although net flows in this direction are observed, observations have not generally sought to distinguish upstream from downstream regions where net flows are expected to be approxim

ISSN:0148-0227    

DOI:10.1029/JA083iA05p02037

年代:1978

数据来源: WILEY

摘要:

The two Air Force Geophysics Laboratory low‐energy spherical electrostatic analyzers aboard Injun 5 have provided observations which are used in conjunction with simultaneous Lepedea observations to study the energetic and ambient thermal plasmas in the evening sector topside ionosphere. Significant thermal electron flux enhancements are observed in the vicinity of the inverted V structures which could be due to either auroral return currents or ionospheric scale height changes. Enhanced hyperthermal (E>28 eV) fluxes of positive ions, as well as vehicle potential modulations, are observed as the satellite passes through inverted V events. At 2500 km the polar cap electron density, temperature, and energy density were 89 ± 41 cm−3, 2234 ± 480°K, and 16.2 ± 5.8 eV/cm³, respectively. Higher energy densities were found during times of magnetic disturbance. Persistent fluxes of hyperthermal electrons are identified with the polar rain observed with Isis 2 experiments. Finally, evidence is cited for the existence of small‐scale (20 km) structures within the region of plasma sheet pre

ISSN:0148-0227    

DOI:10.1029/JA083iA05p02047

年代:1978

数据来源: WILEY

摘要:

Solar wind induced magnetic fields may be present in the dayside ionosphere of Venus. A scale analysis and detailed calculations of the effects of different field line orientations on photoelectron transport and electron temperature show that horizontal magnetic fields considerably alter the profiles of electron heating and temperature and photoelectron flux. Direct injection of solar wind electrons has been considered, and the effects are minor. No evidence is found to support the suggestion that a sharp gradient in electron temperature is the cause of certain features in the Mariner 10 electron density profile.

ISSN:0148-0227    

DOI:10.1029/JA083iA05p02057

年代:1978

数据来源: WILEY

摘要:

We present a theory of rising bubbles (or sharp density depletions) in the equatorial ionosphere. Both ion inertia and ion‐neutral collisions are included. In the collision‐dominated case the bubble velocity is independent of its size, while in the collisionless case it is proportional to the square root of the bubble s

ISSN:0148-0227    

DOI:10.1029/JA083iA05p02066

年代:1978

数据来源: WILEY

摘要:

Chatanika observations of latitudinal distributions of convection electric fields (E⊥) are compared with isointensity ΔHcontours in latitude and time from the Alaskan magnetometer chain and with the north‐south component of the interplanetary magnetic field (IMFBzm) from Imp‐J. As expected, northward electric fields were generally observed within latitude and time regions where ΔHwas positive, while southward electric fields were observed within negative ΔHregions. However, correlation between the magnitudes of the electric fields and of the ΔHperturbations was not strong, owing to variability in ionospheric conductivities produced by precipitation and solar illumination. In the midnight sector the northward‐to‐southward transition in the electric field and positive‐to‐negative ΔHtransition were roughly collocated (to within 1 hour in local time) as signatures of the Harang discontinuity. The most important findings are that (1) southward (northward) IMFBzmtransitions caused rapid equatorward (poleward) shifts of the electric field and ΔHpatterns and (2) southward IMFBztransitions, magnetospheric substorms, and local time transitions of the Harang discontinuity can all lead to northward‐to‐southward transitions of the electric field in the midnight sector. Due to the interlaced phasing of each of these three causal mechanisms a highly complex temporal pattern of

ISSN:0148-0227    

DOI:10.1029/JA083iA05p02071

年代:1978

数据来源: WILEY