Theories for the origin of lunar magnetism

This paper reviews the major theories which have been proposed to explain the remanent magnetism found in the lunar crust. A total of nine different mechanisms for lunar magnetism are discussed and evaluated in the light of the theoretical and experimental constraints pertinent to lunar magnetism. We conclude that none of these theories in their present state of development satisfy all the known constraints. However, the theories which agree best with our present understanding of the Moon are meteorite impact magnetization, thermoelectric dynamo field generation, and an early solar wind field.     

月球磁场与月球演化

月球的磁场强度一般在1~10 nT（1 nT=10-9T）的范围内，最大磁场强度超过100 nT，强磁场区一般位于大的撞击盆地的对峙区域.月球样品的剩磁强度与铁的丰度呈负相关，在38~36亿年间形成的岩石样品剩磁强度最大.月球磁场的变化特征与月球的形成与演化有重要的关系，大碰撞学说来解释月球磁场的变化较合理.     

Textural remanence: A new model of lunar rock magnetism

In reexamining the accumulated magnetic data on lunar rocks, several common patterns of magnetic behavior are recognized. Their joint occurrence strongly suggests a new model of lunar rock magnetism, which appeals only to partial preferred textural alignment of the spontaneous moments of magnetic grains, without requiring the existence of ancient lunar magnetic fields. This magnetic fabric, mimetic to locally oriented petrofabric, gives rise to an apparent “textural remanent magnetization” (TXRM). In order to account for the observed intensity of “stable remanence” in lunar rocks, only a minute fraction (10^?3 to 10^?5) of the single-domain iron grains present need be preferentially aligned. Several mechanisms operating on the lunar surface, including shock and diurnal thermal cycling, appear adequate for producing the required type and degree of magnetic alignment in all lunar rock classes. The model is supported by a wide variety of direct and indirect evidence and its predictions (e.g. regarding anisotropic susceptibility and remanence acquisition) can be experimentally tested.     

Constraints on the origins of lunar magnetism from electron reflection measurements of surface magnetic fields

Apollo 15 and 16 subsatellite measurements of lunar surface magnetic fields by the electron reflection method are summarized. Patches of strong surface fields ranging from less than $\text{1}\text{4}$° to tens of degrees in size are found distributed over the lunar surface, but in general no obvious correlation is observed between field anomalies and surface geology. In lunar mare regions a positive statistical correlation is found between the surface field strength and the geologic age of the surface as determined from crater erosion studies. However, there is a lack of correlation of surface field with impact craters in the mare, implying that mare do not have a strong large-scale uniform magnetization as might be expected from an ancient lunar dynamo. This lack of correlation also indicates that mare impact processes do not generate strong magnetization coherent over ～ 10 km scale size. In the lunar highlands fields of >100 nT are found in a region of order 10 km wide and >300 km long centered on and paralleling the long linear rille, Rima Sirsalis. These fields imply that the rille has a strong magnetization (>5 × 10^?6 gauss cm³ gm^?1 associated with it, either in the form of intrusive, magnetized rock or as a gap in a uniformly magnetic layer of rock. However, a survey of seven lunar farside magnetic anomalies observed by the Apollo 16 subsatellite suggests a correlation with inner ejecta material from large impact basins. The implications of these results for the origin of lunar magnetism are discussed.     

在嫦娥一号探月工程中求定月球重力场

月球重力场制约着近月外空间物体的运动,同时环月飞行器的运动也反映了月球重力场的作用. 本文结合我国嫦娥一号探月工程,探讨了利用月球卫星的地面跟踪资料,求定月球重力场的基本理论和方法,分析了环月卫星的轨道高度、地面跟踪采样时间间隔和跟踪精度等对求定月球重力场的影响. 若单独利用我国嫦娥一号探月工程的地面跟踪数据,恢复30阶次左右的月球重力场模型是一个比较实际的目标. 地面跟踪最好能以75s的时间间隔进行采样,数据连续提供时间应不少于30个昼夜,月球卫星星下点的月面轨迹间距不大于110km.     

On the geomagnetic lunar variation at Istanbul

Summary The article describes lunar daily magnetic variation inH, D andZ components of the earth's magnetic field at Istanbul. Maximum occurs at 10.03 l.hr. (lunar hour) in theH component, 3.38 l.hr. in theD and 2.15 l.hr. in theZ component, during the period 1949 to 1968. Also, the seasonal variation of the lunar magnetic variation has been determined and it is seen that the variation of the phase inD andZ are opposite from the phase of the lunar variation inH, and the amplitudes of the lunar variation inH, D andZ are greatest during the northern solstice.     

On the possibility of using lunar fines to determine the intensity of the ancient lunar magnetic field

The remanent magnetization of iron grains in lunar fines can be studied by allowing a sample of fines to acquire a depositional remanent magnetization (NRM) in the Thellier determination of an ancient lunar field intensity. Although a natural remanent magnetization (NRM) in the Thellier determination of an ancient lunar field intensity. Although a palaeointensity of ca. 1.0 Oe was determined by this method, several factors complicate the interpretation of the result. These include the origin and nature of the iron metal in lunar fines, and the time and acquisition process of its magnetization.     

On the interpretation of mantle discontinuities

Seismic discontinuities in the mantle are conventionally interpreted to be caused by phase transformations observed in the individual mineral species (including, to some extent, solid solution). Recent investigations, however, indicate that there are many difficulties in interpreting these discontinuities in terms of phase transformations alone. It is suggested that some of the seismic discontinuities in the mantle are due to phase transformations in the individual minerals, some due to interactions between the minerals, and still others due to compositional changes.     

On deducing the magnetization of the lunar surface from orbital surveys

A combination of orbital photographic, selenochemical and magnetic surveys may elucidate the mechanism by which the lunar surface became magnetized and possibly yield an estimate of the intensity of the ancient magnetizing field and its time variation. The determination of the size and shape of the magnetized regions requires the measurement of the altitude dependence of field, especially at low altitudes (< 100 km) and with a high enough sampling rate to resolve the profile at the edges of magnetized bodies. The planned Lunar Polar Orbiter may well provide the necessary data.     

On the lunar semidiurnal variation of theD andF2 layers

Summary The lunar semidiurnal variation of ionospheric absorption at Freiburg and of thef ₀ F2 at, Genova, Freiburg and Léopoldville has been deduced. The results are compared with those obtained by other Authors. Magnetic dip, rather than geomagnetic latitude, is found to be a parameter controlling the morphology of the lunar semidiurnal variation of theF2 layer. Finally, a graphical representation of the amplitude of lunar semidiurnal variation off ₀ F2 as function of the magnetic dip is also given.

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Riassunto Viene determinata la variazione lunare semidiurna dell'assorbimento ionosferico per Friburgo e della frequenza critica dello stratoF2 per Genova, Friburgo, Léopoldville. I risultati sono confrontati con quelli ottenuti da altri Autori per varie stazioni situate in diverse parti del globo. In particolare si trova che la inclinazione magnetica, meglio della latitudine geomagnetica, è il parametro che regola la morfologia della variazione lunare inF2. Infine si dà una rappresentazione grafica dell'ampiezza della variazione lunare dellaf ₀ F2 come funzione dell'inclinazione magnetica.

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This research was supported by a grant from the Committee on Lunar Variation (IAGA). The paper has been presented at the XII-Assembly of the I.G.G.U. in Helsinki (July–August 1960).     

On the seismic behaviour of monolithic lunar arches subjected to moonquakes

With a new era emerging in the field of lunar exploration and habitation, there is a need for research on structural forms made of local soil material (regolith), which will be able to endure the extreme conditions in harsh environments (e.g., extreme temperature fluctuations, solar and cosmic radiation, meteor showers, strong ground motions, etc.). The present work focuses on understanding the dynamic and seismic behaviour of certain structural typologies of monolithic arches by means of finite element analysis (FEA). These typologies were extensively investigated previously, using static analyses accounting for the reduced gravitational field on the moon, and proved to be of the optimum shape against certain loading scenarios. Specifically, these optimal monolithic arch forms (named enhanced varying-thickness arches – EVTAs) examined herewith, are described by varying-thickness geometry, properly enhanced at certain weak points for increasing their structural stability. Aiming at a fair comparison, the seismic behaviour of EVTAs is contrasted to that of their corresponding monolithic constant-thickness (CTAs) counterparts (having the same amount of structural material). After defining an appropriate damage state, the authors conduct preliminary pushover analyses to determine the structural capacity of the arches against lateral loading. Subsequently, the modal analysis of the EVTAs shows that the second/vertical mode exhibits a natural period almost equal to that of their first/translational mode and substantially longer than the corresponding second/vertical mode of their CTA counterparts, indicating a potential vulnerability along the vertical excitation. Furthermore, taking into account that shallow moonquakes are comparable to intraplate earthquakes in terms of hazard potential, the authors produce sets of stochastic seismic excitations used as time histories for seismic analyses. The probability of exceedance of the defined damage state as a function of the peak ground acceleration (PGA) is presented through indicative fragility curves, where the structural superiority of EVTAs against their CTA counterparts is demonstrated.     

地磁房建设中的磁性检测

德都地震台从2006年4月25日开始进行台站的数字化改造,改造的主要内容是进行地磁绝对观测室的建设和地磁相对记录室的建设。众所周知,地磁观测用房建设难度大,而难度大主要来自地磁观测场地和地磁观测设施的磁性是否合格。本文就是对德都地震台建设地磁房期间有关磁性检测和检测结果的介绍。利用G856磁力仪和CTM-DI磁通门磁力仪,进行野外和台站的材料磁性检测,方便、快捷、准确,是保证地磁观测用房建筑材料磁性合格的好方法。     

On the interpretation of paleomagnetic data on moraines

Some methods of the interpretation and application of paleomagnetic data for the stratigraphic separation and correlation of Neopleistocene moraines, as well as the factors affecting the degree of ordering of magnetic moments of ferromagnetic particles in a moraine, are considered. A refined interpretation of the well-known Ryabushkin-Pevzner model of moraine magnetization is proposed. It is shown that the way of using an occasional coincidence of the direction of long axes of fragments with the direction of magnetization vectors, which is stated in some publications, has no grounds for its application for the stratigraphic separation of moraines related to the Dnieper and Moscow glaciations. However, it cannot be ruled out that this phenomenon may be used for the investigation of processes of glacial lithogenesis.     

On the interpretation of near-bottom water temperature anomalies

A positive water temperature anomaly of 0.11°C and an inverse gradient of potential temperature of 1.5 × 10^?2°C/m has been measured at the TAG hydrothermal field in the rift valley of the Mid-Atlantic Ridge at latitude 26°N by means of a thermistor array towed between 2 and 20 m above the seafloor. This anomaly appears to be associated with hydrothermal discharge from the oceanic crust. The temperature data are interpreted in terms of (1) a steady, turbulent thermal plume rising in a homogeneous, neutrally buoyant medium, and (2) turbulent diffusion in the ocean-bottom boundary layer. The calculations indicate that the thermal output of the TAG anomaly area is of the order of several megawatts, which is of the same order of magnitude as some continental geothermal systems. The thermal output from the TAG anomaly area represents a significant fraction of the total heat loss resulting from the generation of new lithosphere at the Mid-Atlantic Ridge at 26°N.     

On the automatic interpretation of direct current resistivity soundings

A method for the automatic inversion of resistivity soundings is presented. The procedure consists of two main stages. First, application of linear filters which transforms the apparent resistivity curve into the kernel function, and vice versa. In the second stage the first and second derivatives of the kernel function are calculated and used in a second-order modified Newton-Raphson iterative fitting procedure. The model obtained is optimal in the least squares sense. The method has been tried on some field examples and produced resistivity models which show a good agreement with the geological well logs.     

On the fast movements of the thermosphere: an interpretation

We examine the oscillations of the meridional neutral wind in the F region as seen by the EISCAT radar. We propose an interpretation in term of eddies (tourbillons) of typical size of a few tens to a few hundreds of kilometers. The observed rotation velocity is a few hundreds of meters per second. We suggest that the tourbillons are a common feature of thermospheric movements. We propose an optical experiment to check the validity of this assumption.     

On the interpretation of magnetic anomalies for strong remanent magnetizations

In this paper some aspects concerning the interpretation of magnetic anomalies are treated, particularly when the remanent magnetization intensity is strong. In this case, since total and induced magnetization vectors can have very different directions, a correct anomaly interpretation strictly depends on the knowledge of their declinations and inclinations.Thus, a specific procedure is described to determine such parameters from well-known semi-empirical techniques and vectorial relations.Furthermore, the classical definition of apparent susceptibility is shown to be inadequate to this problem and a more general formulation is suggested, which is not only related to the true susceptibility and to the Koenisberger ratio, but also to the declinations and inclinations of the induced and remanent magnetization vectors.The two apparent susceptibilities are then compared for some synthetic magnetic anomalies and significant differences are found.