摘要:

Seismic refraction data obtained at the Apollo 14, 16, and 17 landing sites permit a compressional wave velocity profile of the lunar near surface to be derived. Although the regolith is locally variable in thickness, it possesses surprisingly similar seismic characteristics. Beneath the regolith at the Apollo 14 Fra Mauro site and the Apollo 16 Descartes site is material with a seismic velocity of ∼300 m/s, believed to be brecciated material or impact‐derived debris. Considerable detail is known about the velocity structure at the Apollo 17 Taurus‐Littrow site. Seismic velocities of 100, 327, 495, 960, and 4700 m/s are observed. The depth to the top of the 4700‐m/s material is 1385 m, compatible with gravity estimates for the thickness of mare basaltic flows, which fill the Taurus‐Littrow valley. The observed magnitude of the velocity change with depth and the implied steep velocity‐depth gradient of>2 km/s/km are much larger than have been observed on compaction experiments on granular materials and preclude simple cold compaction of a fine‐grained rock powder to thicknesses of the order of kilometers. The large velocity change from 960 to 4700 m/s is more indicative of a compositional change than a change of physical properties alone. This high velocity is believed to be representative of the material that forms the lu

ISSN:8755-1209    

DOI:10.1029/RG012i003p00291

年代:1974

数据来源: WILEY

摘要:

Multiring impact basins, formed after solidification of the lunar crust, account for most or all premare regional deposits and structures expressed in the lunar landscape and for major topographic and gravity variations. A fresh basin has two or more concentric mountain rings, a lineated ejecta blanket, and secondary impact craters. Crackled material on the floor may be impact melt. The ejecta blanket was emplaced at least partly as a ground‐hugging flow and was probably hot. A suggested model of basin formation is that the center lifts up and the rings form by inward collapse during evisceration. The resulting basin is shallow and has a central uplift of the mantle. This results in a central gravity high and a ring low. Later flooding by mare basalt has since modified most near side basins. Highland deposits of plains, furrowed and pitted terrain, and various hills, domes, and craters that were interpreted before the Apollo missions as being volcanic can now be interpreted as being basin related. A province map of the whole moon shows that the relatively young Orientale and Imbrium basins imprinted and rejuvenated much of the moon's surface; older basins must have also. The most primitive cratered surface remaining is mostly on the far side, distant from Imbrium and Orientale and other large relatively young basins. All five lunar landings in the highlands sampled stratigraphic units probably related to basins. Several nearly obliterated basins have been discovered recently, including a deep one on the far side that is as wide as the moon's radius. The presence of these ancient basins suggests that the surface is effectively saturated by basins and that many others were completely destroyed by later impacts. Basin impacts may have churned the lunar crust to large depth

ISSN:8755-1209    

DOI:10.1029/RG012i003p00309

年代:1974

数据来源: WILEY

摘要:

Although solar activity cycles are in general remarkably similar, cycles 18, 19, and 20, with maximums in 1947.5, 1958.2, and 1968.9, respectively, differed in the heights of their maximums, in the patterns of their development, and to a certain extent in the character of their manifestations of solar activity. Of the 20 solar cycles since 1755, the maximums in sunspot numbers for cycles 18, 19, and 20 rank as 3, 1, and 10, respectively. Cycle 18, with a maximum smoothed monthly sunspot number of 151.8, was the cycle of ‘giant’ spots. Cycle 19, with the highest recorded maximum, 201.3, included many large spots but failed to produce spots with areas equal to those of cycle 18. The highest known daily 2800‐MHz flux occurred in cycle 18, not in cycle 19. Cycle 20, with a sunspot maximum of only 110.6, appears to have been close to the average cycle of the last 220 years. An attempt is made to compare the general levels of flare production in the three cycles. Cycle 19 is estimated to have been more flare‐rich than cycle 18 by a much smaller factor than the original data implied. In cycle 20, general flare production drops conspicuously. Two types of flare‐rich centers of activity are recognized, viz., those with extraordinarily high numbers of flares of Hα importance ⪖1 and those with unusually high frequency of flares with comprehensive indices of 11 or greater (great ionizing and radio frequency emission as well as Hα area). A list of the most flare‐rich regions in cycles 18, 19, and 20 is presented. In addition, the times and locations of the ∼100 greatest individual flares in the three cycles have been tabulated. North‐south asymmetry of sunspots and the shapes of the mean sunspot curves were not the same for the three cycles. The minimums between the cycles differed in duration and residual activity. Solar magnetic measurements exhibit the 22‐year period and make polarity patterns alike in cycles 18 and 20. Summarized coronal measurements from the Pic‐du‐Midi Observatory for 1944–1973 suggest that the relationships between coronal intensities and other measures of solar activity are complex. Emphasis on times of the greatest solar activity in cycles 18–20 leads to a certain sense of positive relationship between events on the sun and geophysical phenomena. Geophysical uncertainties abound, however, if consideration drops to just above average solar circumstances. Comments are made on certain problems in so

ISSN:8755-1209    

DOI:10.1029/RG012i003p00329

年代:1974

数据来源: WILEY

摘要:

Solar phenomena produce cosmic ray intensity variations over a wide range of time scales. The observed flux is modulated and rendered anisotropic as the particles propagate in the solar wind, and it is occasionally enhanced by the sporadic emission of solar cosmic rays. Both quasi‐static phenomena (the long‐term omnidirectional intensity variation and the steady state diurnal anisotropy) and transient fluctuations (disturbed daily variation and Forbush decrease) as well as the spatial distribution of solar flare particles are represented by a theoretical model that prescribes the role of the several solar‐controlled parameters that characterize the electromagnetic properties of the interplanetary medium. Considerable information concerning the ambient conditions has been obtained with spacecraft. However, the in situ measurements are confined to a limited region near the ecliptic plane. Consequently, in some cases, theoretical predictions based upon them are not in accord with observations of cosmic ray intensity variations. Thus the modulations and anisotropies must be treated in a three‐dimensional framework. It is therefore reasonable to attempt to deduce the properties of the relevant inaccessible regions of the heliosphere from observations of the sun itself. These properties can be determined by relating the various cosmic ray phenomena to changes during a solar cycle and from one cycle to another. To this end, data covering at least two solar cycles are studied to determine the solar cycle dependence of the following effects: long‐term variations in the omnidirectional intensity, Forbush decreases, solar diurnal variations, and solar cosmic ray events (ground level enhancement and polar cap ab

ISSN:8755-1209    

DOI:10.1029/RG012i003p00343

年代:1974

数据来源: WILEY

摘要:

Magnetospheric effects associated with variations of the north‐south component of the interplanetary magnetic field (IMF) are examined in light of recent experimental and theoretical results. Although the occurrence of magnetospheric substorms is statistically related to periods of southward IMF, the details of the interaction are not understood. In particular, attempts to separate effects resulting directly from the interaction between interplanetary and geomagnetic fields from those associated with substorms have produced conflicting results. One can, however, say with some assurance that the transfer of magnetic flux from the day side to the night side magnetosphere, as evidenced by equatorward motion of the polar cusp and increases of the magnetic energy density in the lobes of the geomagnetic tail, is a direct consequence of the southward IMF. On the other hand, the formation of a macroscopicX‐type neutral line at tail distances less than 35REappears to be a substorm phenomenon. Although other plasma and field phenomena in the outer magnetosphere and at low altitudes may be directly associated with the southward IMF, the evidence is less convincing. The quantitative results that have been obtained are compared with current theoretical models of the reconnection proc

ISSN:8755-1209    

DOI:10.1029/RG012i003p00363

年代:1974

数据来源: WILEY

摘要:

The plasma sheet is a region of hot plasma (∼107°K) that surrounds the earth, filling the distant magnetosphere and separating the northern and southern lobes of the magnetospheric tail; its origin is one of the basic outstanding problems of magnetospheric physics. A critical review of the available observations indicates that the plasma sheet source must provide roughly 1025–1026particles/s to balance known loss mechanisms operating in the quiet time magnetosphere and that an adiabatic or quasi‐adiabatic (ΔE∝E) acceleration mechanism must provide most of the observed particle energy. These constraints together with auroral helium ion measurements indicate that the plasma sheet particles must originate in the solar wind rather than in the terrestrial ionosphere. The available hypotheses for a solar wind source are examined, and it is concluded that a two‐step process of magnetopause diffusion plus electrostatic acceleration is the simplest source mechanism that is consistent with the available o

ISSN:8755-1209    

DOI:10.1029/RG012i003p00379

年代:1974

数据来源: WILEY

摘要:

Models of the processes whereby a nuclear detonation emits a coherent electromagnetic pulse fall into three classes: those involving Compton electron currents produced by interaction of prompt γ radiation from the detonation with the environment, those involving photoelectron currents produced by the similar interaction of primary X radiation from the detonation, and those involving perturbation of the ambient magnetic field by the expanding plasma surrounding the detonation point. For each model considered the cause of the asymmetry in the current system necessary for the radiation of a signal is discussed. These causes include the earth‐atmosphere interface, the atmospheric density gradient, anisotropy of the environment by virtue of the presence of the earth's magnetic field, nonuniform emission of the energetic radiation (γ and X rays) by the detonation, and asymmetries of the delivery vehicle and device case. The available experimental data are then examined in the light of the models. These data suffice to establish the models as probably being correct in their identification of the principal processes whereby the nuclear electromagnetic pulse is generated, but they are inadequate for a quantitative assessment of the accuracy of the mod

ISSN:8755-1209    

DOI:10.1029/RG012i003p00389

年代:1974

数据来源: WILEY

摘要:

Arrays of about 25 portable magnetic variometers have recently been used to record the total varying geomagnetic field over an area 1000–2000 km square. The total varying field consists of a portion generated outside the solid earth (in the ionosphere and the magnetosphere) and a portion induced by this external field (in the material of varying conductivity in the crust and upper mantle of the earth). By plotting sets of magnetograms of the observed varying fields, Fourier spectra over the full range of periods available, or contours of Fourier amplitudes at specified periods, localized anomalies in the total field are identified. Careful examination of these various plots yields considerable qualitative information about the subsurface variations in conductivity that produce these anomalies. Formal methods of separating the observed field into components of internal and external origin exist and can be applied to this problem, but with questionable accuracy. Semiquantitative estimates of the separate components may be just as accurate. Quantitative interpretation of the anomalous internal fields in terms of inhomogeneities in conductivity is at present almost entirely based on numerical modeling techniques. Three such methods have been used so far, the one using finite differences being the most successful. Results of such modeling are neither unique nor very close to the observed field patterns. Another method of analysis is based on transfer functions and cross‐correlation spectra, and this method may turn out to be more use

ISSN:8755-1209    

DOI:10.1029/RG012i003p00401

年代:1974

数据来源: WILEY

摘要:

The earth's tidal deformations cause perturbations in the motions of close earth satellites, observations of which give estimates of the Love numberk2and phase lag δ. The contribution of the ocean tides has generally been considered unimportant, but this is not so. These ocean tides cause the same spectrum of orbital perturbations as the solid tide, and a complete separation, by analyzing satellite orbits of different elements, is not possible. Neglect of the ocean tide will introduce errors ink2of as much as 15% and in phase angle of as much as 10° depending on the orbit. This explains the low values fork2near 0.25 that have been obtained recently from orbital analyses. Comparison of several models for the principal lunar tide (M2) of the ocean indicates that these models are insufficient for making precise orbital corrections. For the other tidal frequencies that often have important effects on satellite orbits, no satisfactory tide models exist. The equilibrium theory does not suffice for these corrections, and improved values fork2and particularly for the phase lag δ can only come from a concomitant improvement in our knowledge of the ocean tides. It is possible to extract some information on the ocean tides from the orbital analyses, and this could be used as a constraint in numerical ocean tide computations. We have analyzed the orbit of the satellite Geos 2 for the lunarM2tide. After correcting for the ocean tides the value found fork2varies between 0.27 and 0.30, depending on which ocean tide model is used. The solutions for the phase angles are quite unsatisfactory, and this condition stems from insufficiently precise tracking data and inadequate coverage as well as from inadequate tide models. We have applied approximate ocean tide corrections to the tide parameters determined by several authors to give a mean value ofk2=0.31 and δ2=0.5°. This phase lag corresponds to a mantleQof abou

ISSN:8755-1209    

DOI:10.1029/RG012i003p00421

年代:1974

数据来源: WILEY

摘要:

Titan, a satellite of Saturn, is unique in that it is the only satellite in the solar system with an extensive atmosphere, possibly more massive than that of the earth. Methane and hydrogen have been discovered, and a greenhouse effect is apparent from infrared measurements. It may be enshrouded by clouds, and this intriguing body may, in time, provide clues to the evolution of the major planets and their satellites. At present, interpretation of the observational evidence concerning the properties of Titan is somewhat speculative and controversial. The status of the observations and theoretical studies is assessed.

ISSN:8755-1209    

DOI:10.1029/RG012i003p00435

年代:1974

数据来源: WILEY