摘要:

Ion temperature and ion drift velocity data from Atmosphere Explorer D have verified the existence of interhemispheric plasma transport and ion temperature troughs in the topside equatorial ionosphere. The data were taken during solar minimum conditions at night where the exospheric temperature was typically 700°K and the O+‐H+transition height was 520 km. Large field‐aligned ion velocities were observed above about 700 km, where the H+/O+number density ratio was about 3. Model calculations have shown that the ion temperature decrease is produced by quasi‐adiabatic expansion of the plasma, but the expansion cooling mechanism is less efficient at solar minimum than at solar maximum owing to the lower O+‐H+transition height. Ion temperature troughs greater than 400°K were not observed even when the field‐aligned ion velocity was greater than 700 m s−1. Most of the expansion occurs below the transition height, and during sunspot minimum, thermal coupling to the neutral atmosphere is much more effective in quenching the cooling which the field‐aligned transport t

ISSN:0148-0227    

DOI:10.1029/JA083iA08p03683

年代:1978

数据来源: WILEY

摘要:

A formalism is developed for computing tidal variations in thermospheric composition and is used to investigate diurnal and semidiurnal variations in O, O2, N2, Ar, He, and H at minimum and maximum levels of sunspot activity. The model formally accounts for tidal temperatures, horizontal and vertical tidal winds, photochemistry and ion chemistry, thermal diffusion, and exospheric transport in determining the tidal variations of these constituents, as well as deviations from diffusive equilibrium in the time average component due to photochemistry, Jeans escape, or background vertical winds. A procedure is described for using available measurements to calibrate the diurnal and semidiurnal wind and temperature fields, which are used as inputs to the model. Sunspot minimum calculations show excellent agreement with equatorial San Marco 3 Nace measurements in both amplitude and phase for diurnal and semidiurnal variations between 220 and 280 km. Major exceptions are the equatorial diurnal amplitude of Ar, which the model overestimates by 35%, and the equatorial diurnal amplitude of He, which the model underestimates by about 25%. Simulations for sunspot maximum conditions demonstrate substantial solar cycle differences in the vertical structures of amplitude and phase for each constituent and in the relative contribution of semidiurnal and diurnal components at different latitudes and heights. Consequently, a clear need is indicated for continued satellite mass spectrometer and accelerometer measurements over different levels of solar activity if comprehensive empirical models of thermosphere tides are to be developed.

ISSN:0148-0227    

DOI:10.1029/JA083iA08p03691

年代:1978

数据来源: WILEY

摘要:

Conventional dynamo theories start from wind fields which are considered as external forces independent of the ionospheric plasma. These dynamos are called kinematic dynamos. Coupling between the neutral wind and the ionospheric plasma in a self‐consistent manner results in a hydromagnetic dynamo. In this paper a theory of a hydromagnetic dynamo of the geomagnetic Sq current is developed assuming a thin shell model with constant elements of the electric conductivity tensor. Coupling between neutral gas and plasma is described by the Ampere forcej×Bo(jbeing electric current density andBogeomagnetic field) which depends not only on the velocity difference between neutrals and plasma (ion drag) but also on the electric polarization field. The electric polarization field includes a source free component which is not negligibly small at dynamo layer heights, so that the electric field cannot be represented by an electric potential. This theory leads to a modified Laplace equation of tidal theory which can be solved numerically. Calculations are presented for the diurnal and the semidiurnal tidal wave modes. Their equivallent depths, their vertical wavelengths, and their attenuation factors are discussed as functions of a plasma parameter δ which is proportional to a Cowling conductivity. The horizontal structures of pressure and electric current of the dominant symmetric tidal waves are determined for different values of δ representing the lower atmosphere (δ = 0), the dynamo region (δ ≃ 1), and F layer heights (δ ≃ 10). It is shown that waves with positive equivalent depths at δ = 0 are heavily attenuated at δ>1 and drastically change their horizontal structure there. On the other hand, waves with negative equivalent depths at δ = 0 are only weakly affected by the Ampere force. From our calculations we suggest that tidal wave theory at thermospheric heights should be reconsidered taking into account electric coupling between the modes. In particular, we predict that the semidiurnal (2, 2) wave so far considered as the dominant semidiurnal mode above about 200 km height may have lost its significance and may in fact be replaced by the symmetri

ISSN:0148-0227    

DOI:10.1029/JA083iA08p03699

年代:1978

数据来源: WILEY

摘要:

Magnetic field measurements made over a 21‐hour interval during the Mariner 10 encounter with Venus were used to study the downstream region of the solar wind‐Venus interaction over a distance of ≈ 100Rv(Venus radii). Mariner 10 encountered Venus on February 5, 1974, with closest approach at 1702 UT. For most of the day before closest approach the spacecraft was located in a sheathlike region which was apparently bounded by the planet's bow shock on the outer side and either a planetary ‘wake boundary’ or a transient boundarylike feature on the inner side. The spacecraft made multiple encounters with the wakelike boundary during the 21‐hour interval with an increasing frequency as it approached the planet. Each pass into the wake boundary from the sheath region was consistently characterized by a slight decrease in magnetic field magnitude, a marked increase in the frequency and amplitude of field fluctuations, and a systematic clockwise rotation of the field direction when viewed from above the plane of Venus' orbit. These boundary crossings were not accompanied strictly by hydromagnetic directional discontinuities, however, but occasionally (∼⅓ of the crossings) such a discontinuity was sufficiently close to the crossing zone to be considered part of the boundary transition. There were a significantly larger number of discontinuities in the overall 21‐hour period than were observed on average during other comparable periods both before and after encounter. A simple large‐scale draped field model in the sense of a magnetic ‘comet tail’ was found not to hold for the downstream region. The sporadic observation of the wake during the near‐encounter period may have been controlled by changes in the direction of

ISSN:0148-0227    

DOI:10.1029/JA083iA08p03709

年代:1978

数据来源: WILEY

摘要:

Preliminary results of ion and electron plasma measurements near Venus are presented and discussed. The data were obtained with wide‐angle plasma analyzers carried on the Venera 9 and 10 spacecraft. On the basis of 33 bow shock crossings the position of the shock is quite stable and agrees well with theoretical predictions of Spreiter et al. withH/r0= 0.01 and a stagnation point altitude of ∼500 km. This observation lends strong support to the assumption that the solar wind interacts with the upper ionosphere of Venus and not with a planetary magnetic field. These spacecraft are the first to explore the optical umbra of Venus. Close to the planet a stable population of electrons and an ill‐defined population of positive ions were found; this region is called the corpuscular umbra. The corpuscular umbra and the transition region are separated by a zone which contains both positive ions and electrons and is characterized by a flow velocity reduced in comparison with that of the transition region. This zone is called the corpuscular penumbra. The distribution of plasma density behind the bow shock (including the optical umbra of the planet) is given, and the existence of a Venusian plasma magnetic tail is rev

ISSN:0148-0227    

DOI:10.1029/JA083iA08p03721

年代:1978

数据来源: WILEY

摘要:

The two‐fluid solar wind equations, including inhibition of heat conduction by the spiral magnetic field, have been solved for steady radial flow, and the results are compared with those of our previous study of two‐fluid models with straight interplanetary field lines. The main effects of the spiral field conduction cutoff are to bottle up electron heat inside 1 AU and to produce adiabatic electron (and proton) temperature profiles at large heliocentric distances. Otherwise, the spiral field models are nearly identical with straight field models with the same temperatures and velocity at 1 AU, except for models associated with very low coronal base densities (n0∼ 106cm−3at 1Rs). Low base density spiral models give a nearly isothermal electron temperature profile over 50–100 AU together with high velocities and temperatures at 1 AU. In general, high‐velocity models do not agree well with observed high‐velocity streams; lower‐velocity states can be represented reasonably well at 1 AU, but only for very high proton temperatures (Tp∼ 2Te) at the coronal base. For spherically symmetric base conditions the straight field and spiral field models can be regarded, in lowest order, as approximations to the polar and equatorial three‐dimensional flows, respectively. This viewpoint suggests a pole to equator electron temperature gradient in the region 1‐10 AU, which would be associated with a meridional velocity of ∼0.5–1.0 km/s, diverging away from the equatorial plane. The formalism developed in this paper shows rather stringent limits to the mass loss rate for conductively driven winds and, in particular, illustrates that putative T Tauri outflows could n

ISSN:0148-0227    

DOI:10.1029/JA083iA08p03729

年代:1978

数据来源: WILEY

摘要:

Recurrent Forbush decreases and recurrent geomagnetic disturbances have been attributed to the solarMregions, which are sources of high‐velocity solar plasma streams. A study of recurrent Forbush decreases for the period 1966‐1975 has been made to examine any possible relationship ofMregions with solar active regions. It is shown that at the onset of the recurrent Forbush decrease at the earth there is a high probability of encountering a class of active regions at the central meridian of the sun which give rise to flares of importance ≥2B/3N. These active regions are found to be long lasting and to have large areas as well as high Hα intensities. Other active regions, producing flares of lower importance, are distributed randomly on the sun with respect to the onset of a recurrent Forbush decrease. By using the quasi‐radial hypervelocity approximation the base of the leading edge of the high‐velocity stream at the onset of a recurrent Forbush decrease at the earth is traced to the solar longitude about 40° west of the central meridian. From these results it is deduced thatMregions are located preferentially to the west of long‐lasting magnetically complex active regions. Earlier studies of the identification of theMregions on the sun have been reexamined and shown to conform to this positional relationship. A possible mechanism of the development of anMregion to the west of the long‐lasting magnetically complex active region is

ISSN:0148-0227    

DOI:10.1029/JA083iA08p03740

年代:1978

数据来源: WILEY

摘要:

Analytical expressions for the growth rates of unstable low‐frequency ‘fast’ modes are derived here in the low‐frequency, low‐βilimit for a plasma obeying classical linear transport theory. The results indicate that at the onset of instability (∼10Rsfrom the sun) the first growing mode in the solar wind has a frequency of ∼0.1 times the proton gyrofrequency and propagates at an appreciable angle to the background magnetic field. Substantial inhomogeneity is required for the growth to overcome competing processes. These modes may have consequences for thermal conduction, proton and helium heating, the observable wave spectrum, and other facets of solar wind theory, though considerable additional theory wi

ISSN:0148-0227    

DOI:10.1029/JA083iA08p03745

年代:1978

数据来源: WILEY

摘要:

In the nighttime equatorialFregion, zonal neutral air winds and gravity waves propagating across the magnetic field tend to drive the ionization as if there were no field. When the phase trace speed of a gravity wave equals the drift speed of the ionization, strong ionization perturbations may occur (Whitehead's spatial resonance effect). The resonance effect is described by a parametric differential equation. Without diffusion, it yields perturbations in the form of periodic δ functions. Therefore a coefficient of turbulent diffusion is introduced to limit the perturbation amplitude. Then the equation can be solved in terms of partial Fourier sums. Numerical calculations with reasonableFregion parameters indicate that under spatial resonance conditions, gravity waves cause considerable secondary flows of ionization as a result of nonlinear effects. The contours of constant ionization density show steep or even breaking wave fronts similar to those observed by HF radar and radio propagation experiments in the large‐scale structure of the equatorial spreadF. The spatial resonance mechanism produces field‐aligned tubes of decreased ionization density which may be the primary cause of ‘plasma bubbles’ detected by satellite and rocke

ISSN:0148-0227    

DOI:10.1029/JA083iA08p03753

年代:1978

数据来源: WILEY

摘要:

The structure of mid‐latitude sporadicElayers has been investigated in a series of observations at the Arecibo Observatory. The basic experiment uses phase‐coded pulses to measure the electron density profiles with a range resolution of 600 m while scanning the radar beam. The data from incoherent scatter are compared with simultaneous ionosonde observations to determine the source of the partial transparency of sporadicElayers to radio waves at frequencies betweenfbEsandftEs. It is concluded that sporadicElayers contain electron density irregularities of sufficient intensity to account for the partially transparent echoes observed by the ionosonde. A feature that becomes immediately obvious in examining the data taken with the incoherent scatter radar is that sporadicElayers are as variable in structure as they are unpredictable in occurrence. Some have simple vertical profiles and are uniform in the horizontal plane, consistent with rocket observations; others have complex vertical and horizontal structure, as has been implied by the radio experiments. Wavelike structure in the electron density is present in many of the layers observed. Evidence is shown of the limiting of peak ionization density in a layer by turbulence generated in a Kelvin‐Helmholtz instability. The possible proximity of a sporadicElayer to a critical level of a dominant gravity wave mode is discussed as a source of intense ionization irregularities. A possible irregular source of ionization from ablating meteors is also considered. It is shown that under specific conditions an ionized meteor trail can be converged by the action of the neutral wind into an irregularity of relatively small dimensions. It is concluded that the observations support the wind shear mechanism of formation of sporadicElayers when the combined effects of gravity waves, tides, and the irregular deposition of metallic ions are inc

ISSN:0148-0227    

DOI:10.1029/JA083iA08p03761

年代:1978

数据来源: WILEY