Data on the horizontal component of the earth's magnetic field obtained during 60 days of counter electrojet events have been subjected to harmonic analysis for the stations of Trivandrum, Kodaikanal, Annamalainagar and Alibag. A similar analysis has been done for 60 quiet days also as control days with a view to find out the changes in the relative amplitudes and phases of the various harmonic components which contribute to the abnormal time variation of the geomagnetic field on counter electrojet days.
It is found that the amplitudes of the semi-diurnal and ter-diurnal components are greater on counter electrojet days compared to the quiet days, while a less marked trend of a decrease in the amplitude of the diurnal component is observed on the counter electrojet days. The phases of the diurnal, semi-diurnal and ter-diurnal components on counter electrojet days are found always to lead those of the respective components on the control days. During strong counter electrojet events, the phase lead is as much as 2 to 4 hours. The implications of the analytical results are discussed in the light of present theories on counter electrojet.

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This paper attempts to highlight progress in studies of magnetosphere-ionosphere coupling made during the last two years using a wide variety of techniques, covering a broad range of topics. Emphasis is placed on pointing out the key issues in the area that need substantial innovation in the near future. This includes the problem of differentiating between the substorm expansion and enhanced convection, and the importance of time-dependent treatment of the entire magnetosphere-ionosphere coupling system.

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After summarizing typical features of the plasma sheet boundary layer, this review will be addressed to that topic where a major advance was made during the last two years, namely understanding the role of the plasma sheet boundary layer in magnetospheric substorms. After a description of motivation and key features of the recently developed ‘boundary layer model’. I shall list several assumptions made in this model which do not seem explored deeply enough yet as well as observations which cannot be explained within the framework of this model. I will end with an update of the ‘near-earth neutral line model’, recent simulations of which now explicitly include boundary layer flows. I come to the conclusion that the near-earth neutral line model still explains the wealth of substorm phenomena more comprehensively, but that certain refinements are needed. The boundary layer model, on the other hand, describes certain substorm features quite well but needs to be considerably extended before it can serve as a coherent description of all substorm phenomena.

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Nous présentons une étude sur le mode d'acquisition et les propriétés de l'aimantation rémanente chimique (ARC) de champ faible (0, 1 à 0, 5mT) acquise, soit au cours de la réduction de l'hématite en magnétite par chauffe sous vide, soit au cours de la réaction inverse obtenue par chauffe à l'air de la magnétite. Précisons qu'était exclu de notre propos l'examen de la direction de l'ARC, notamment en cas d'acquisition en présence d'une rémanence préexistante. Les échantillons étaient constitués de poudres synthétiques à grains fins (<1μm) dispersées dans du kaolin. Le contrôle du taux d'oxydation ou de réduction a été effectué en suivant, au cours même du déroulement de différents paliers prolongés de température de plus en plus élevée, ou après chaque refroidissement consécutif, les variations des grandeurs magnétiques suivantes: acquisition de l'aimantation induite et rémanente, susceptibilité initiale et sa variation thermique, cycles d'hystérésis en champ fort, aimantations spécifiques à saturation et champs coercitifs. Un bon rendement de la réduction de l'hématite a pu être atteint en présence de poudre d'aluminium à 450°C. L'oxydation de la magnétite a été obtenue à 600°C grâce à l'adjonction de perchlorate de potassium. Nos résultats concernant l'intensité de l'ARC sont les suivants, ramenés à un champ de 0, 1mT: ARC (hématite, 600°C)=2, 9×10^-3Am²/kg; ARC (magnétite, 450°C)=110×10^-3Am²/kg. Pour une quantité identique de substance, l'ARV acquise par l'hématite sur une durée de l'ordre de grandeur de celles de nos expériences reste inférieure au tiers de l'ARC. Pour la magnétite ce rapport est de l'ordre de 0, 1 à 0, 2. L'ARC acquise par l'ensemble des grains de magnétite néoformée était dans nos expériences environ 25 fois plus faible que l'ATR totale qu'ils sont susceptibles d'acquérir. Ce rapport s'établissait à 1/3 dans le cas de l'hématite. L'ARC acquise par celle-ci s'est révélée un peu moins intense, et un peu moms résistante à l'attaque thermique, qu'une ATR partielle acquise par refroidissement eotre 652 et 600°C.
We present a study of the mode of acquisition and properties of the chemical remanent magnetization (CRM) in low fields (0, 1 to 0, 5mT) acquired, either during the reduction of hematite into magnetite by heating in vacuum, or during the inverse reaction obtained by heating magnetite in air. Let us specify that our work is not concerned by the direction of the CRM, namely when it is acquired in presence of a preexisting remanence. The samples were made of fine grained (<1μm) synthetic magnetic powder dispersed in kaolin. The control of the rate of oxidation or reduction has been made by following, during different prolonged temperature levels or after each consecutive cooling, the variations with time of the main magnetic parameters: acquisition of induced and remanent magnetizations, initial susceptibility and its thermal change, high fields hysteresis loops, specific saturation magnetizations and coercive fields. A good efficiency of the reduction of hematite was attained at 450°C in the presence of aluminium powder.

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A recent geomagnetic model containing 720 trend and trigonometric coefficients represents each spherical harmonic coefficient as a continuous function of time which exactly duplicates the IGRF coefficients at the nine IGRF epochs (1945 to 1985 at five year intervals) and allows a reasonable prediction five years beyond the last epoch. Use of this model permits time derivatives of magnetic elements and other derived quantities to be taken so that a detailed look at several geomagnetic parameters vs time can be obtained. It is shown that from 1945 to 1985 the quadrupole field contributed 2.6 times more power to the secular variation field on the surface of the Earth than the dipole field. It is also shown that the root-mean-squared surface field shows a strong periodicity which may be related to the solar cycle. This apparent solar-cycle effect diminishes with time as more satellite data are used indicating that an appreciable part of it may be caused by noisy data and a poor distribution of data for a couple of decades following World War II.

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