摘要:

The latitudinal distribution of the equatorial height‐integrated current density has been obtained from available experimental data and correlated with the latitudinal distribution of the horizontal component of the daily geomagnetic variations to determine the importance of the internal part of the electrojet field. A simple method is introduced that makes it possible to analyze the importance of the internal electrojet field and simultaneously to separate the planetary field. Contradicting what has been assumed in recent works and confirming earlier results, it has been found that the current induced in the earth by the electrojet contributes significantly to daily geomagnetic variations at South American longitude

ISSN:0148-0227    

DOI:10.1029/JA084iA05p01849

年代:1979

数据来源: WILEY

摘要:

Observations of hydrogen Balmer β and He+4686‐Å emission made at Huancayo, Peru, during two magnetic storms are consistent with the expectations of energetic neutral atom precipitation resulting from charge exchange loss of ring current ions and support the view that charge exchange is the major loss process for larger geomagnetic storms. The intensities are consistent with previous satellite observations of the emission (called the equatorial aurora) and when translated into ionization rates for the upperEregion give production rates an order of magnitude larger than normal nighttime levels. Such ionization enhancements have previously been measured by ionosondes and incoherent scatter at low latitudes and attributed to electron precipitation. New calculations of the latitude variation correct earlier work and show that for a ring current with pitch angle distribution isotropic to the loss cone, located on shells ofLvalue 2 to 6, the maximum influx rate of precipitating neutrals is found at magnetic latitudes 25° to 50°. Most of the energetic neutrals are lost to interplanetary space, and the fraction impacting the thermosphere has been recalculated to range from 11 to 2.2% forLvalues 2 to 6. For a typical magnetic storm with energy loss rate due to charge exchange, the equivalent to aDstrate of change of 20 nT/h, the energy input into the thermosphere at the latitude of maximum is calculated to be 0.15 to 0.05 mW/m² fromLshells 2 to 6. The ionization production can be of the order of 10 ions cm−3s−1at 140 km, and optical emission, of the order of 1 rayleigh (R), both varying according to the species and energy of the impacting neutrals (i.e., the former ring current ions). The latitude distribution shrinks toward the equator after injection has ceased, as the magnetospheric pitch angle distribution evolves toward 90°, on a time scale (for protons<30 keV atL= 3) of the order

ISSN:0148-0227    

DOI:10.1029/JA084iA05p01855

年代:1979

数据来源: WILEY

摘要:

The propagation characteristics of plane acoustic‐gravity waves in an atmosphere in diffusive equilibrium are studied by using a two‐fluid model which takes into account the collisional transfer of both momentum and energy between species. At wave periods of less than the shortest characteristic diffusion time for the minor gas, significant amplitude and phase differences between the wave‐induced density fluctuations of individual gases occur because of the different scale heights of the gases. At longer periods, diffusion induced by the wave acts to eliminate these amplitude and phase differences and restore the perturbed fluid to diffusive equilibrium. Vertical diffusion is most important at large scale sizes, but horizontal diffusion dominates for horizontal wavelengths of less than several hundred kilometers. As a result of wave‐induced diffusion, AE‐C satellite measurements of neutral density fluctuations of thermospheric constituents at 215‐km altitude are only compatible with internal gravity waves with periods of ∼15–30 min, horizontal wavelengths of ≃150–400 km, and downward phase propagation. Diffusion increases in importance with altitude, but is significant for periods of ≳30 min even at 150‐km altitude. Velocity and temperature differences for acoustic‐gravity waves are greatest at periods near the mean collision time and diffusion time. Vertical velocity differences between species can be as large as 40% in amplitude and 17° in phase at 215‐km altitude. Temperature perturbation differences are much smaller but can reach 17% in amplitude and 6.5° in phase at 300‐km altitude. Two other solutions to the governing equations, which may be called diffusion waves, have properties drastically different from the acoustic‐gravity wave solutions, including order‐of‐magnitude amplitude differences and close to 180° phase differences between the density, velocity, and temperature perturbations of the two species at most periods. Because of the velocity and temperature differences between species, dissipation of wave energy will occur. Damping of acoustic‐gravity waves is most significant at periods comparable to the mean collision and diffusion times and can be as large as 50–60% per wave cycle. This effectively filters out those waves with periods long enough to be affected by diffusion except when the observation is near the wave source. The predictions of the theory are consistent with a high‐latitude, lower‐altitude source like Joule heating or particle precipitation for the medium‐scale waves and a more localized random sou

ISSN:0148-0227    

DOI:10.1029/JA084iA05p01865

年代:1979

数据来源: WILEY

摘要:

This paper extends some of the analysis of gravity wave generation and dissipation in the thermosphere given in a previous paper [Richmond, 1978] by defining wave energy for gravity waves of arbitrary amplitude and by analyzing several numerical simulations. Wave energy as defined in this paper is closely related to the sum of kinetic energy and available potential energy of the atmosphere. In the absence of production or dissipation mechanisms, wave energy is approximately, but not strictly, conserved. Its dissipation is related to, but in general is not the same as, entropy generation. Numerical simulations substantiate the findings of Richmond (1978) that both the Lorentz force and the Joule heating of auroral currents can generate substantial gravity wave energy but that the waves produced by Joule heating are more capable of traveling to middle and low latitudes. The simulations also reveal that molecular dissipation of wave energy can be relatively less important, in comparison to daytime Joule dissipation, than the simple treatment of Richmond [1978]indicated.

ISSN:0148-0227    

DOI:10.1029/JA084iA05p01880

年代:1979

数据来源: WILEY

摘要:

The kinetic temperature and neutral composition data obtained from the Aeros B neutral atmosphere temperature experiment and the neutral and ion mass spectrometer show spatial structures characteristic of medium scale gravity waves with a wavelength in the range of several hundred kilometers. These waves are associated with auroral activity, and their spatial structure reflects the time history of the auroral electrojet. The medium scale gravity waves tend to propagate to mid‐latitudes on the nightside. On the dayside their range is limited to high latitudes. Gravity waves are carriers of auroral energy to middle and low latitudes where they may cause irreversible changes in temperature via viscous dissipation. Since auroral activity occurs frequently, it is suggested that this energy reaches the mid‐latitude region of the thermosphere much more frequently than is indicated by planetary magnetic indi

ISSN:0148-0227    

DOI:10.1029/JA084iA05p01891

年代:1979

数据来源: WILEY

摘要:

An active experiment has been conducted in space that allows a test for the theory of nonlinear development of striations in large barium clouds. The results are in excellent agreement with a computer simulation of theE × Binstability reported by Scannapieco et al. (1976). The power law irregularity spectrum predicted by the theory and verified by the experiment is shown here to be due to wave steepening and not to plasma turbulence. The barium cloud results are remarkably similar to bottomside equatorial spread. A possible role of theE × Binstability in bottomside equatorial spreadFis discussed as a supplement to the Rayleigh‐Taylor instability during the postsunset rise of theFlayer and during anomalous plasma uplifts which occur during geomagnetically active peri

ISSN:0148-0227    

DOI:10.1029/JA084iA05p01898

年代:1979

数据来源: WILEY

摘要:

Nighttime thermospheric winds have been measured from Ester Dome, near College, Alaska (64.8°N, 147.8°W), using the high‐resolution Fabry‐Perot interferometer of the University of Michigan Airglow Observatory. The winds are determined from the Doppler shifts of the (0I) 15867 K (6300 Å) line emission. Measurements obtained during geomagnetic quiet times in January and February 1972 are averaged to obtain the characteristic nighttime behavior of both the zonal and the meridional neutral wind components atF‐region heights. Ion drifts during this period were measured by the Chatanika incoherent scatter radar facility. The measurements of the average nighttime zonal and meridional neutral wind components and of the average ion drift components are used with a three‐dimensional dynamic model of the neutral thermosphere to determine the effectiveness of various forces in controlling the winds atF‐region heights in the high‐latitude thermosphere. A least squares fit of the measured and calculated nighttime variations of the zonal and meridional winds, using the ion drag determined from the radar measurements, gives the Fourier coefficients of latitudinal and longitudinal pressure gradients that are necessary to drive the observed wind pattern. The results show that the ion drag force is essentially balanced by the pressure force in the meridional direction. However, in the zonal direction the pressure force does not balance the ion drag force. The pressure gradients, derived in terms of exospheric temperature variations, show a different nighttime variation from that predicted by the MSIS model for that latitude. The derived zonal winds are westward in the early evening hours, opposite of the winds calculated from a dynamic model that uses the pressure forces determined from the MSIS semiempirical mode

ISSN:0148-0227    

DOI:10.1029/JA084iA05p01905

年代:1979

数据来源: WILEY

摘要:

The transport of solar 584‐A photons through the spherical, asymmetric helium geocorona is discussed in detail. The analysis shows that with a favorable geometry and a moderately optically thick atmosphere it is possible to determine the helium abundance from dayglow observations without knowing the incident solar flux or the instrument calibration. Comparison of theoretical intensities with 584‐A airglow observations made from the STP 72‐1 satellite shows that recent satellite‐drag and mass spectrometer‐incoherent scatter empirical models predict helium densities which have seasonal‐latitudinal dependences which approximate the airglow‐determined densities in shape, but in general are high by as much as a factor of 2 in absolute density. Airglow‐determined densities in the polar regions suggest a decrease in the exospheric helium density relative to the empirical

ISSN:0148-0227    

DOI:10.1029/JA084iA05p01914

年代:1979

数据来源: WILEY

摘要:

A study was made of the steady state loss coefficient ψ (=q/N²) deduced for quiet periods from previously published electron density height data. Near 75 km, ψ was found to remain fairly constant with diurnal changes of the solar zenith angle χ, while above 77 km, ψ decreased with decreasing χ. Since a seasonal variation in ψ was not clearly evident, observational data selected from more than 4 years of partial reflection observations at Ottawa, were analyzed for quiet intervals at the minimum solar zenith angle (χ≡68°) that could be followed in all seasons. This study revealed that for heights above 76 km, ψ was a minimum in winter and a maximum in summer. Near 75 km neither a seasonal nor a diurnal variation was found, and ψ remained fairly constant near 2 × 10−5cm³ s−1. These results are interpreted as indicating that for heights above 77 km, (1) as χ decreases, there is a gradual decrease in the number density of the larger hydrated positive ions thought to be responsible for the high ψ values and (2) the existence of these larger hydrate positive ions is temperature dependent, such that they are absent in the warm winter mesosphere and present in the co

ISSN:0148-0227    

DOI:10.1029/JA084iA05p01921

年代:1979

数据来源: WILEY

摘要:

The Nyquist or fluctuation‐dissipation theorem applies strictly only to linear systems in thermal equilibrium. However, it can also be used successfully to calculate fluctuating quantities in systems in quasi‐equilibrium, a familiar example being a circuit with elements at different temperatures. An analogous calculation can be carried out for a plasma, considering each particle species as a separate element of the system. One calculates the spontaneous force generated by each species (with its own temperature and mean drift velocity) and also a set of transfer functions which relate each of these forces to the electron perturbations which are responsible for the scattering. The important point to realize in the plasma case is that, with this approach, the electric field perturbations, which couple the species together, enter the calculation only through the transfer function and are not involved in the statistics of the ‘generalized forces.’ The results obtained apply for multiple ion species with unequal temperatures, unequal mean drift velocities, unequal charges (including negative), etc., and agree with similar calculations based on the dressed particle a

ISSN:0148-0227    

DOI:10.1029/JA084iA05p01930

年代:1979

数据来源: WILEY