摘要:

We present a satellite observation of the spectrum of gamma radiation from the Earth's atmosphere in the energy interval from 300 keV to 8.5 MeV. The data were accumulated by the gamma ray spectrometer on the Solar Maximum Mission over 3½ years, from 1980 to 1983. The excellent statistical accuracy of the data allows 20 atmospheric line features to be identified. The features are superimposed on a continuum background which is modeled using a power law with index −1.16. Many of these features contain a blend of more than one nuclear line. All of these lines (with the exception of the 511‐keV annihilation line) are Doppler broadened. Line energies and intensities are consistent with production by secondary neutrons interacting with atmospheric14N and16O. Although we find no evidence for other production mechanisms, we cannot rule out significant contributions from direct excitation or spallation by primary cosmic ray protons. The relative intensities of the observed line features are in fair agreement with theoretical models; however, existing models are limited by the availability of neutron cross sections, especially at high energies. The intensity and spectrum of photons at energies below the 511‐keV line, in excess of a power law continuum, can be explained by Compton scattering of the annihilation line photons in traversing an average of ∼21g cm−2of

ISSN:0148-0227    

DOI:10.1029/JA094iA02p01211

年代:1989

数据来源: WILEY

摘要:

The response of a conducting fluid containing an embedded magnetic field (with β<1) to the sudden injection of material along the field lines is investigating using results from wave theory and numerical simulations of the nonlinear magnetohydrodynamic equations. A primary interest is the possible generation of, and the role played by, slow shocks in the medium response to the ejecta. We show that slow shocks may be produced; however, in contrast to previous suggestions for a slow shock preceding an impermeable ejecta, the slow shock configuration deduced in the present computations is concave toward the ejecta “driver.” Fast‐mode waves, which have not steepened into shocks, precede the slow shock and significantly alter the ambient medium (provided β is not too low). The slow waves are shown to play an important role in accommodating the driven ejecta. At low β the fast mode becomes primarily a transverse wave for parallel propagation, whereas the slow wave approaches a longitudinal, or sound, wave. The compressive effects of the motion then are principally transmitted by the slow shock, while the fast wave primarily just reorients the magnet

ISSN:0148-0227    

DOI:10.1029/JA094iA02p01222

年代:1989

数据来源: WILEY

摘要:

The variation of interplanetary parameters at Earth orbit has previously been studied by superposedepoch analysis of the spacecraft data obtained during solar cycle 20. It was shown that the magnetosonic speed possesses a quasi‐steady, but slightly drifting, distribution in Carrington longitude during all phases of solar cycle 20. Other magnetic parameters such as the Alfvénic speed and ratios of magnetic and total pressure to the kinetic energy density also reflect this same organization in Carrington longitude in both the maximum and minimum years. We extend the study of the quasi‐steady corotating solar‐terrestrial variation to include the long‐time series of geomagnetic indexC9 data covering solar cycles 13–20. Using the superposed‐epoch method in conjunction with digital filter and harmonic analysis, we find that during solar minima the corotating variation has a quite steady structure of distribution in the Carrington rotation, with the phase of the second harmonic remaining nearly the same in each of the even or odd cycles and shifting a large angle, probably reversing, from one to the next. Analysis of seasonal trends of the distribution suggests each of the two main peaks maintains the same magnetic polarity during the even cycles, and it is likely that their source regions are located in the northern and southern solar hemispheres, respectively, for the first and second peaks within each rotation. In addition, the locations of the peaks in the even cycle minima show only a small systematic drift in heliospheric longitude (about 6 Carrington days, net) from 1913 to 1974. The implication is that the stable high‐speed solar wind streams originate from a deeply rooted configuration of the general solar magnetic field which persists for at least fou

ISSN:0148-0227    

DOI:10.1029/JA094iA02p01235

年代:1989

数据来源: WILEY

摘要:

The magnetic sector polarity at the Sun and in interplanetary space has been surveyed during approximately three quarters of sunspot cycle 21 using photospheric magnetic field observations from the Wilcox Solar Observatory at Stanford University and spacecraft observations by Voyagers 1 and 2, Pioneer Venus Orbiter, and, to a lesser extent, Helios A, IMP 8, and ISEE 3. During the more than 8‐year period of the study, late 1977–1985, the Voyagers measured the interplanetary magnetic field (IMF) over a heliocentric distance range of 1–25 AU and a heliolatitude range of −5°–27°. The photospheric data were used together with a potential field (PF) model to calculate the location of the heliospheric current sheet near the Sun throughout the same period, which included sunspot maximum and much of the declining phase. The large‐scale magnetic polarity structure was found to be in almost continuous evolution throughout the period of study. This included an alternation between two‐sector and four‐sector patterns, with a mean duration in each case of approximately seven solar rotations. Although the azimuth of the IMF was found by the Voyager spacecraft to be oriented more than 60° from the theoretical Parker spiral angle 20% of the time, application of smoothing techniques produced IMF polarity patterns which could be compared with the pattern derived at the Sun. Such comparisons revealed very good agreement (82%) between solar and IMF sector patterns observed by Pioneer Venus in the inner heliosphere and significantly poorer agreement (61–64%) between solar and outer heliosphere patterns observed by Voyagers 1 and 2. This may be the result of dynamical processes in the solar wind which alter the position and shape of the current sheet beyond a few astronomical units. Sector pattern stability was found to decrease with increasing heliocentric distance, also. Evidence was found for the more frequent occurrence of four‐sector structure near the Sun than in the outer heliosphere. In 1985, when Voyager 1 reached a heliolatitude of 25°, it observed the disappearance of the sector structure, consistent with the tilt of the current sheet deduced from photospheric magn

ISSN:0148-0227    

DOI:10.1029/JA094iA02p01245

年代:1989

数据来源: WILEY

摘要:

Cometary protons picked up by the solar wind were detected by the high energy range spectrometer of the Giotto ion mass spectrometer starting at a cometocentric distance of ∼12×106km. On the average, the density of cometary protons varied approximately as the inverse square of the cometocentric distance, reaching a value of 0.11 cm−3just outside the bow shock. The data can be successfully fit to models that include substantial amounts of both slow (∼1 km/s) and fast (≥8 km/s) H atoms beyond the bow shock. Large local variations in the density of picked‐up protons can be explained on the basis of variations in the direction of the interplanetary magnetic field in upstream regions where pitch angle scatterin

ISSN:0148-0227    

DOI:10.1029/JA094iA02p01261

年代:1989

数据来源: WILEY

摘要:

A statistical study of the broadband ELF‐VLF plasma waves at the magnetopause has been performed using ISEE 1 plasma wave data. It is found that enhanced wave intensities are detected at 85% of all magnetopause crossings. Although wave amplitudes are highly variable from event to event and even within an event (particularly during high‐intensity events), the wave spectra averaged over many passes are remarkably similar at dawn, noon, and dusk local hours. Thus to first order, the average wave intensity and spectral shape are independent of local time. At select frequencies, the intensities are 10−8, 10−13, and 3 × 10−17V2/m2Hz at 101, 103, and 105Hz and 3 × 10−4and 5 × 10−8nT2/Hz at 101and 102Hz, respectively. The average wave intensity is independent of latitude, magnetosheath field strength, and magnetopause position. The only parameter found to be correlated with wave intensity is the magnitude of theZcomponent of the magnetosheath magnetic field. The broadband wave intensities increase with increasing negativeBz. These observational results put strong constraints on any proposed generation mechanism for the broadband magnetopause boundary layer waves. Reasonable mechanisms should be able to explain the lack of local time, latitude, and interplanetary parameter dependences. The dependence of IMFBzand possible correlation with magnetic reconnection should be a principal feature in the model. These observations support the conjecture that cross‐field diffusion of magnetosheath plasma by wave particle interaction with these waves (Tsurutani and Thorne, 1982) is the steady state source of the low‐latitude boundary layer, and that pitch angle scattering and the consequential particle precipitation into the ionosphere is the mechanism for the dayside aurora. For cyclotron resonant interaction withE>100 eV electrons during southward IMFBzevents, a total precipitation rate of 1.0 to 1.2 erg

ISSN:0148-0227    

DOI:10.1029/JA094iA02p01270

年代:1989

数据来源: WILEY

摘要:

A simple two‐dimensional, steady state, viscous model of the dawnside and duskside low‐latitude boundary layer (LLBL) has been developed. It incorporates coupling to the ionosphere via field‐aligned currents and associated field‐aligned potential drops, governed by a simple conductance law, and it describes boundary layer currents, magnetic fields, and plasma flow in a self‐consistent manner. Slab geometry is assumed, with no variations along the flow direction −xand with the layer on closed field lines. The currents in the layer are regulated by coupling to the ionosphere. The magnetic field induced by these currents leads to two effects: (1) a diamagnetic depression of the magnetic field in the equatorial region and (2) bending of the field lines into parabolas in thexzplane with their vertices in the equatorial plane, atz= 0, and pointing in the flow direction, i.e., tailward. Both effects are strongest at the magnetopause edge of the boundary layer and vanish at the magnetospheric edge. The diamagnetic depression corresponds to an excess of plasma pressure in the equatorial boundary layer near the magnetopause. This pressure drops off both with increasing distancezfrom the equatorial plane and with increasing distanceyfrom the magnetopause. It reaches the magnetospheric level forz= ±Has well as fory→ ∞. The boundary layer structure is governed by a fourth‐order, nonlinear, ordinary differential equation in which one nondimensional parameter, the Hartmann numberM, appears. A second parameter, introduced via the boundary conditions, is a nondimensional flow velocity at the magnetopause. It is shown that for largeMvalues the coupling to the ionosphere is weak; in that limit, or when is small, the model reduces to that discussed by Lotko et al. (1987) in which induced magnetic fields are neglected. Numerical results from the model are presented and the possible use of observations to determine the model parameters is discussed. The general predictions of the model in terms of region 1 currents and associated ionospheric signatures are similar to those obtained earlier by Lotko et al. and by Sonnerup (1980). Those predictions are in qualitative and approximately quantitative agreement with a number of observations. The main new contribution of the study is to provide a better description of the field and plasma configuration in the LLBL itself and to clarify in quantitative terms the circumstances in which induced magnetic fields become important. In particular, it appears that for the low values of the field‐aligned conductance expected on the duskside of the magnetosphere, these fields may remain relatively unimportant, at least in the noon to dusk

ISSN:0148-0227    

DOI:10.1029/JA094iA02p01281

年代:1989

数据来源: WILEY

摘要:

The topological properties of a magnetic field are interpreted in terms of magnetic helicity. The total helicity of a collection of flux tubes arises from the linking of flux tubes with one another (mutual helicity) and the internal magnetic structure of each flux tube (self‐helicity). Reconnection changes the topology and magnetic connectivity of flux tubes. This can also be viewed as a redistribution of self‐ and mutual helicities. If total magnetic helicity is approximately conserved, it is possible to put quantitative limits upon the changes in self‐ and mutual helicities. This can be interpreted as the change in magnetic flux tube linkage (due to reconnection) and amount of twist present in the reconnected flux tubes. The implications for reconnection in the terrestrial magnetosphere are also disc

ISSN:0148-0227    

DOI:10.1029/JA094iA02p01295

年代:1989

数据来源: WILEY

摘要:

We present a two‐dimensional, force‐balanced magnetic field model in which flux tubes have constantpVγthroughout an extended region of the nightside plasma sheet, between approximately 36REgeocentric distance and the region of the inner edge of the plasma sheet. We have thus demonstrated the theoretical existence of a steady state magnetic field configuration that is force‐balanced and also consistent with slow, lossless, adiabatic, earthward convection within the limit of the ideal MHD (isotropic pressure, perfect conductivity). The numerical solution was constructed for a two‐dimensional magnetosphere with a rectangular magnetopause and nonflaring tail. The primary characteristics of our steady state convection solution are (1) a pressure maximum just tailward of the inner edge of the plasma sheet and (2) a deep, broad minimum in equatorial magnetic field strengthBze, also just tailward of the inner edge. Our results are consistent with Erickson's (1985) convection time sequences, which exhibited analogous pressure peaks andBzeminima. Observations do not indicate the existence of aBzeminimum, on the average. We suggest that the configurations with such deep minima inBzemay be tearing‐mode unstable, thus leading to substorm onset in the inner pl

ISSN:0148-0227    

DOI:10.1029/JA094iA02p01303

年代:1989

数据来源: WILEY

摘要:

The linear global eigenmodes of the gradient drift instability in the daytime equatorial electrojet are investigated. A main feature of the analysis is the inclusion of ion‐neutral and electron‐neutral collision frequencies dependent on altitude. It is found that the basic characteristics and localization of the unstable modes are determined mainly by the profiles of the Pedersen and Hall mobilities, which are derived from the Cowling conductivity model and experimental data. The equilibrium density profile is parabolic, which is fairly representative of the actual measurements. The unstable modes are sensitive not to the details of this profile, but only to the average value of the gradient. The results are obtained from a direct numerical integration of the nonlocal linearized equations. They are further analyzed through an eikonal analysis, which provides both an interpretation of the transient modes observed by Fu et al. (1986) and some additional physical insight into the linear evolution of the global unstable modes. Finally, it is shown that the previously reported short‐wavelength stabilization effect due to velocity shear may be overshadowed by the presence of regions in which the transient modes can develop into absolute instabil

ISSN:0148-0227    

DOI:10.1029/JA094iA02p01317

年代:1989

数据来源: WILEY