摘要:

It has been widely held that the steep temperature gradient in the lower thermosphere can support one or more surface modes of long‐period gravity wave propagation, thereby acting as a ducting mechanism. This paper shows that such a ducting mechanism in fact does not operate in the earth's thermosphere. It is shown that the thermospheric gravity modes computed by Francis [1973] can instead be explained by internal waves undergoing total reflections in the lower thermosphere and weak partial reflections in the upper thermosphere due to viscosity and heat conduction effects. This ducting mechanism is, however, so weak that the usefulness of the concept of ducted thermospheric gravity modes appears to be limited. An alternative concept of freely propagating waves appears to have more practical application

ISSN:0148-0227    

DOI:10.1029/JA083iA04p01385

年代:1978

数据来源: WILEY

摘要:

Relative values of thermospheric photoelectron energy distributions between 2 and 100 eV were measured over the altitude range 100–220 km by using a 127° cylindrical electrostatic deflection analyzer flown on a rocket. The measured photoelectron spectra display distinct features that are theoretically predicted (Victor et al., 1976; Jasperse, 1977) and are similar to those obtained in recent satellite experiments (Doering et al., 1975, 1976). The valleylike structure in the energy range 2–5 eV attributed to resonant‐vibrational excitation of N2was observed in more detail and over a greater altitude range than that measured previously. The prominence of this low‐energy valley decreases with increasing altitude above about 150 km and disappears above 200 km, in general agreement with theory. However, in contrast to theoretical predictions, the low‐energy valley also appears to become less prominent as altitude decreases below about 140 km, down to the lowest observations

ISSN:0148-0227    

DOI:10.1029/JA083iA04p01390

年代:1978

数据来源: WILEY

摘要:

The hydromagnetic Rankine‐Hugoniot relations are extended to allow for separate ion and electron temperatures and arbitrary degrees of freedom for ion and electron heating; when the degrees of freedom are unequal, this comprises an unclosed set of equations. The system is reduced to one equation in two unknowns containing the usual one‐fluid model as a special case. Closure is achieved by means of empirical arguments for both subcritical and large Mach number shocks, and a general parametric solution is also presented. Analytic expressions for the fluid and field jumps are obtained and compared with various experimental observati

ISSN:0148-0227    

DOI:10.1029/JA083iA04p01395

年代:1978

数据来源: WILEY

摘要:

Measurements with Los Alamos Scientific Laboratory instrumentation aboard Imp 6, 7, and 8 reveal that approximately one third of all high‐speed solar wind streams observed at 1 AU contain a sharp boundary (of thickness less than ∼4 × 104km) near their leading edge, called a stream interface, which separates plasma of distinctly different properties and origins. Identified as discontinuities across which the density drops abruptly, the proton temperature increases abruptly, and the speed rises, stream interfaces are remarkably similar in character from one stream to the next. A superposed epoch analysis of plasma data has been performed for 23 discontinuous stream interfaces observed during the interval March 1971 through August 1974. Among the results of this analysis are the following: (1) a stream interface separates what was originally thick (i.e., dense) slow gas from what was originally thin (i.e., rare) fast gas; (2) the interface is the site of a discontinuous shear in the solar wind flow in a frame of reference corotating with the sun; (3) stream interfaces occur at speeds less than 450 km s−1and close to or at the maximum of the pressure ridge at the leading edges of high‐speed streams; (4) a discontinuous rise by ∼40% in electron temperature occurs at the interface; and (5) discontinuous changes (usually rises) in alpha particle abundance and flow speed relative to the protons occur at the interface. Stream interfaces do not generally recur on successive solar rotations, even though the streams in which they are embedded often do. At distances beyond several astronomical units, stream interfaces should be bounded by forward‐reverse shock pairs; three of four reverse shocks observed at 1 AU during 1971–1974 were preceded within ∼1 day by stream interfaces. Although stream interfaces often occur in close proximity to reversals in direction of the interplanetary magnetic field, the field reversals generally precede the interfaces by 1½ hours to 1½ days. Approximately 40% of stream interfaces at 1 AU produce si−in the geomagnetic field. It has been suggested previously that stream interfaces are the result of the nonlinear evolution of high‐speed streams produced by a smoothly varying temperature elevation in the solar envelope. Our measurements appear to contradict this thesis: many streams do not appear to be smoothly varying close to the sun, and a temperature elevation in the solar envelope is not required to explain the abrupt jumps in plasma properties which occur at stream interfaces. Our observations suggest that many streams close to the sun are bounded on all sides by large radial velocity shears separating rapidly expanding plasma from more slowly expanding plasma. The shear at the leading edge of such streams becomes the stream interface observed at 1 AU; however, momentum transfer across the interface reduces the magnitude of the speed jump across the shear with increasing distance from the sun. The large abrupt increases in electron and proton temperature observed at interfaces are primarily a consequence of the fact that interfaces separate what was originally thick (i.e., dense) slow gas from what was originally thin

ISSN:0148-0227    

DOI:10.1029/JA083iA04p01401

年代:1978

数据来源: WILEY

摘要:

The Spatial spectrum of electron density in the solar wind is estimated over the frequency range 2 × 10−3to 3 × 10−2km−1by using 74 MHz observations of interplanetary scintillation. The inversion technique by which the spatial spectrum is estimated from the observed temporal spectrum requires weak scattering. Hence at 74 MHz it can only be applied for solar elongations greater than 40°. The shape of the spectrum is best described by a two‐component power law (P(q) ∝ q−α) where α ≅ 2.7 at low wave numbers and α ≅ 3.2 at higher wave numbers. Occasionally, a single α ≅ 3.0 gives a satisfactory description. The shape of the spectrum varies significantly from day to day, but no mean variation with solar di

ISSN:0148-0227    

DOI:10.1029/JA083iA04p01413

年代:1978

数据来源: WILEY

摘要:

Simultaneous ground‐based photometric measurements of auroral O I (6300 Ă), O I (7774 Ă), O I (8446 Ă), and N2+(4278 Ă) emissions and model calculations of their intensities are presented. Detailed examination of the variations in the observed brightness of these emissions indicates a decrease in theI(8446)/I(7774) ratio as the red (I(6300)) to blue (I(4278)) ratio becomes smaller. This is contrary to the constantI(8446)/I(7774) ratio expected from electron impact excitation of the O I (7774 Ă) and O I (8446 Ă) multiplets, which are not quenched at auroral altitudes. The observed relation between the intensities of the O I (6300 Ă) and O I (8446 Ă) emissions suggests that the dominant mechanism for the production of O I (8446 Ă) is electron impact excitation of atomic oxygen. An additional excitation mechanism for O I (7774 Ă) operating at relatively low altitudes in aurora is required to explain both the measured changes in theI(8446)/I(7774) ratio and the O I (7774

ISSN:0148-0227    

DOI:10.1029/JA083iA04p01421

年代:1978

数据来源: WILEY

摘要:

A model for the field‐aligned propagation of transverse electric fields and associated field‐aligned sheet currents is presented which takes into account the wave nature of the process. The model is applied to the separate cases of ionospheric and magnetospheric sources, and the resulting ionospheric electric field to field‐aligned sheet current ratios are determined for comparison with experimental observations. It is found that the magnetospheric wave ‘conductivity’ for shear mode Alfvén waves is small with respect to typical values of the height‐integrated ionospheric Pedersen conductivity. For plasma convecting across a stationary disturbance a dynamic equilibrium is achieved in which field‐aligned currents flow continuously away from the source on convecting field lines. Consistency with typical ionospheric electric fields requires that the field‐aligned sheet currents are limited to around 0.1 A/m for ionospheric polarization sources, while magnetospheric sources are easily capable

ISSN:0148-0227    

DOI:10.1029/JA083iA04p01426

年代:1978

数据来源: WILEY

摘要:

We study the motion of electrons trapped in the potential troughs of a monochromatic whistler wave propagating along geomagnetic field lines. Because of the coupling between inhomogeneity and trapping effects the distribution function of electrons at the termination of this triggering pulse can be very different from the distribution at its front provided that the triggering wave amplitude and duration are large enough to trap electrons over more than one trapping length. Particles which are detrapped at the triggering wave termination have been phase organized and may then act coherently for a while, giving rise to an emission with either rising or falling tones, depending on the sign of the inhomogeneity variation. The spatially averaged part of the distribution function of these suddenly detrapped electrons is shown to generate an instability which amplifies previously emitted waves. These results are compared with existing data. Finally, we show that emitted waves can easily trap electrons, thus explaining how the process of emission can be self‐sustaine

ISSN:0148-0227    

DOI:10.1029/JA083iA04p01433

年代:1978

数据来源: WILEY

摘要:

Special runs of the Arecibo incoherent scatter radar were made to provide high spatial resolution (8.7 km) and high temporal resolution (about 1 min) electron density data at discrete heights in the altitude range 200–500 km. The observed electron density fluctuations were spectrally analyzed and interpreted in terms of gravity wave induced traveling ionospheric disturbances. In many records, two kinds of persistent spectral dips for all heights were obtained. The first dip, occurring in the frequency range 0.025–0.055 min−1(18‐ to 40‐min period), is shown to be caused by the absence of ionospheric response when the gravity wave normal is perpendicular to the earth's magnetic field and therefore is of geomagnetic origin. The second dip, occurring in the frequency range 0.09–0.12 min−1(8‐ to 11‐min period), is shown to be related to high attenuation of those waves whose frequencies are slightly above the Brunt

ISSN:0148-0227    

DOI:10.1029/JA083iA04p01442

年代:1978

数据来源: WILEY

摘要:

In this paper we investigate the characteristics of plasma waves observed by the Hawkeye 1 spacecraft in the vicinity of the polar cusp. The primary types of plasma waves associated with the polar cusp are (1) a band of ULF‐ELF magnetic noise extending from a few hertz to several hundred hertz, (2) a broad‐band electrostatic emission extending from a few hertz to about 30–100 kHz, with maximum intensities at about 10–50 Hz, (3) electrostatic electron cyclotron waves near the electron gyrofrequency, and (4) whistler mode auroral hiss emissions. Of these various types of waves, only the ULF‐ELF magnetic noise is uniquely associated with the cusp in the sense that the noise can be used as a reliable indicator of the polar cusp region. All of the other types of plasma waves occur in regions adjacent to the polar cusp as well as in the cusp itself. Spectrum measurements often show that the ULF‐ELF magnetic noise extends up to, but does not exceed, the local electron gyrofrequencyfg−. This upper cutoff strongly suggests that the noise consists of whistler mode electromagnetic waves. The mechanism for generating these waves remains highly uncertain, however, since the electron angular distribution in the cusp is usually not sufficiently anisotropic to account for these waves by the well‐known whistler mode cyclotron resonance instability. Other mechanisms, such as turbulence generated by the Kelvin‐Helmholtz instability or by a drift wave instability, have also been suggested to generate this noise. The broad‐band electrostatic noise is believed to be caused by a current‐driven electrostatic instability (ion cyclotron or ion acoustic) of the type widely believed to occur in auroral field‐aligned current systems. The mechanisms for generating electron cyclotron waves and auroral hiss emissions are believed to be reasonably well understood, on the basis of previous studies of these emissions in other regio

ISSN:0148-0227    

DOI:10.1029/JA083iA04p01447

年代:1978

数据来源: WILEY