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Journal ArticleDOI

Kelvin-Helmholtz instability and the semiannual variation of geomagnetic activity

01 Nov 1970-Journal of Geophysical Research (John Wiley & Sons, Ltd)-Vol. 75, Iss: 31, pp 6073-6084
TL;DR: Kelvin-Helmholtz instability at magnetopause, initiating semiannual variation of geomagnetic disturbances was first reported in the early 1970s by as discussed by the authors.
Abstract: Kelvin-Helmholtz instability at magnetopause, initiating semiannual variation of geomagnetic disturbances
Citations
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Journal ArticleDOI
TL;DR: In this article, a theory of long-period (Pc 3 to Pc 5) magnetic pulsations is presented based on the idea of a steady state oscillation of a resonant local field line that is excited by a monochromatic surface wave at the magnetosphere.
Abstract: A theory of long-period (Pc 3 to Pc 5) magnetic pulsations is presented based on the idea of a steady state oscillation of a resonant local field line that is excited by a monochromatic surface wave at the magnetosphere. A coupled wave equation between the shear Alfven wave representing the field line oscillation and the surface wave is derived and solved for the dipole coordinates. The theory gives the frequency, the sense of polarizations, orientation angle of the major axis, and the ellipticity as a function of magnetospheric parameters. It also clarifies some of the contradicting ideas and observations in relation to the sense of polarization and excitation mechanism. At lower latitude it is shown that the orientation angle rather than the sense of rotation is a more critical parameter in finding the direction of wave propagation in the azimuthal coordinate and hence in finding the evidence of wave excitation at the magnetospheric surface by the solar wind.

1,040 citations

Journal ArticleDOI
TL;DR: In this article, it was proposed that the semiannual variation in geomagnetic activity is caused by a semiannually variation in the effective southward component of the interplanetary field.
Abstract: It is proposed that the semiannual variation in geomagnetic activity is caused by a semiannual variation in the effective southward component of the interplanetary field. The southward field arises because the interplanetary field is ordered in the solar equatorial coordinate system, whereas the interaction with the magnetosphere is controlled by a magnetospheric system. Several simple models utilizing this effective modulation of the southward component of the interplanetary field are examined. One of these closely predicts the observed phase and amplitude of the semiannual variation. This model assumes that northward interplanetary fields are noninteracting and that the interaction with southward fields is ordered in solar magnetospheric coordinates. The prediction of the diurnal variation of the strength of the interaction at the magnetopause by this model, does not, however, match the diurnal variation of geomagnetic activity as derived from ground-based data.

943 citations

Book ChapterDOI
TL;DR: The magnetic field profiles across the magnetopause obtained by the ISEE-1 and -2 spacecraft separated by only a few hundred kilometers are examined for four passes as discussed by the authors, during which the magnetosheath field was northward, during one pass it was slightly southward, and in two it was strongly southward.
Abstract: The magnetic field profiles across the magnetopause obtained by the ISEE-1 and -2 spacecraft separated by only a few hundred kilometers are examined for four passes. During one of these passes the magnetosheath field was northward, during one it was slightly southward, and in two it was strongly southward. The velocity of the magnetopause is found to be highly irregular ranging from 4 to over 40 km s-1 and varying in less time than it takes for a spacecraft to cross the boundary. Thicknesses ranged from 500 to over 1000 km.

934 citations

Journal ArticleDOI
TL;DR: In this paper, it is proposed that the dynamo of the related current system is provided by the transfer of perpendicular momentum resulting from the plasma entry into the layer, which does not appear to be unobstructed, as has been claimed in the concept of a magnetospheric cleft.
Abstract: Heos 2 plasma and magnetic field measurements in the distant polar cusp region reveal the existence of a plasma layer on day side field lines just inside the magnetopause. Density and temperature in this layer are nearly the same as they are in the adjacent magnetosheath, but the flow lacks the order existing both in the magnetosheath and in the plasma mantle. Flow directions toward and away from the sun but, in general, parallel to the field lines have been found. The magnetopause (as defined by a sudden rotation of the magnetic field vector) mostly coincides with the transition to ordered magnetosheath flow. The inner boundary of the layer is located just within the outer boundary of the hot ring current plasma. In the region of overlap the hot electrons have the signature of trapped particles, though often at reduced intensity. The magnetic field is strongly fluctuating in magnitude, while its orientation is more stable, consistent with a connection to the earth, but is systematically distorted out of the meridian plane. The layer is thought to be a consequence of the entry of magnetosheath plasma, which does not appear to be unobstructed, as has been claimed in the concept ofmore » a magnetospheric cleft. The magnetopause has a cusplike indentation which is elongated in local time. The existence of field-aligned currents (total strength approx. =10/sup 6/ A) and their location of flow in the inner part of the entry layer (into the ionosphere before noon and out of it after noon) are inferred from the systematic bending of field lines. It is proposed that the dynamo of the related current system is provided by the transfer of perpendicular momentum resulting from the plasma entry into the layer. The essential features of the entry layer might be compatible with the model of plasma flow through the magnetopause of Levy et al. (1964) if a 'dam' effect caused by the cusp geometry were added. (AIP)« less

241 citations

References
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Journal ArticleDOI
TL;DR: In this article, it was found that a model with a southward interplanetary magnetic field leads to a natural explanation of the SD currents and speculative aspects of the problem as they appear at this time are discussed.
Abstract: : It was found that a model with a southward interplanetary magnetic field leads to a natural explanation of the SD currents. Speculative aspects of the problem as they appear at this time are discussed. It should be remembered that this problem is amenable to revolutionary progress by observations from rockets or satellites which go out more than a few earth's radii.

3,818 citations

Journal ArticleDOI
TL;DR: In this paper, the initial results of the detailed measurements of the interplanetary magnetic field and the interaction of the solar wind with the geomagnetic field are presented, with extreme values as low as 1 and as high as 10 γ.
Abstract: The interplanetary monitoring platform Imp 1, or Explorer 18, launched on November 27, 1963, has provided the first accurate measurements of interplanetary magnetic fields. The initial apogee of the satellite was 197,616 km on the sunlit side of the earth, with an apogee-earth-sun angle of 26°. This paper presents the initial results of the detailed measurements of the interplanetary magnetic field and the interaction of the solar wind with the geomagnetic field. The strength of the interplanetary magnetic field is found to vary between 4 and 7 γ, with extreme values as low as 1 and as high as 10 γ. The magnitude, however, is extremely stable over times of hours, although changes of direction are significant. The average direction of the interplanetary magnetic field is slightly below the plane of the ecliptic and approximately along the streaming angle predicted for a steady-state solar wind. A significant feature of the magnetic field measurements is the discovery of fields pointed diametrically opposite the streaming angle, indicating filamentary structure of the interplanetary field. Associated with the fields of opposite direction are null surfaces between the filaments and in the over-all field structure. The complex interaction of the solar wind and the geomagnetic field shows a variety of magnetic field fluctuations and transition characteristics. The detection of the collisionless magnetohydrodynamic shock wave at 13.4Re at the stagnation point associated with the super Alfvenic flow of solar plasma is one of the major results of this experiment. Details of the fluctuations are discussed, as well as the gross structure and shape of the magnetospheric surface (10.2Re at the subsolar point) and the shock wave from the subsolar point to the nighttime geomagnetic tail. The transition region between the shock wave and the magnetopause is one of high turbulence in the magnetic field. A unique aspect of the magnetic field data is the detection of the magnetohydrodynamic wake of the moon during the fifth orbit, when the satellite was eclipsed by the moon's magnetosphere while in interplanetary space. The implications of this experimental discovery are discussed.

451 citations

Journal ArticleDOI
TL;DR: In this paper, the hydromagnetic Kelvin-Helmholtz stability problem is studied for an infinite plane interface between compressible infinitely conducting fluids and the critical value of the relative streaming velocity for stability is studied by use of the equations for marginal stability.

357 citations

Journal ArticleDOI
TL;DR: In this paper, a homogeneous series of monthly means of terrestrial-magnetic activity for the years 1872 to 1930 is derived and extended backward, in annual means, to 1835.
Abstract: A homogeneous series of monthly means of terrestrial-magnetic activity for the years 1872 to 1930 is derived and extended backward, in annual means, to 1835. The annual variation of magnetic activity and of the relative sunspot-numbers is discussed by means of new tests for periods. Only the semi-annual wave in magnetic activity is recognized as physically significant. Its maxima prefer the times when the Sun is in the celestial equator, and not, as has been suggested, the times when the Sun's axis is most inclined towards the ecliptic. This view is supported by tests using the harmonic dial and the probable-error circle, and several independent considerations. The close relations between sunspot-numbers and terrestrial-magnetic activity in the annual and monthly means are discussed. Some general statistical aspects are given for the treatment of the correlation between such series with after-effects, for which both solar activity and terrestrial-magnetic activity are typical. The homogeneity of the whole available series for relative sunspot-numbers and for areas of sunspots and faculae is tested; some inhomogeneities are found, apart from a general lag of terrestrial-magnetic activity that has occurred in some sunspot-cycles. A break in the homogeneity of the international magnetic character-figures in recent years is discovered. The individual 27-day recurrences in terrestrial-magnetic activity during 1906–31, and their relations to solar activity are discussed with the help of a graphical day-by-day record. They indicate the existence of persistent active areas on the Sun's surface, called M-regions, which, in many cases, cannot be coordinated to such solar phenomena as are observable by direct astrophysical methods. This holds in particular for the new solar indices which are available for the years 1928–30, and which are found so closely correlated to sunspot-numbers, that they fail to improve the correlation between solar activity and terrestrial-magnetic activity. Observations of terrestrial-magnetic activity yield therefore not only information about geophysical influences of such solar phenomena that may be traced in astrophysical observations, but supplement these direct observations themselves.

314 citations