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Showing papers on "Dipole model of the Earth's magnetic field published in 1972"


Journal ArticleDOI
TL;DR: The correlation between the azimuthal direction of the interplanetary magnetic field and the most simple polar cap signatures is discussed in this article, where only the spatial distribution of the dawn-dusk polar cap field is considered.
Abstract: The correlations between the azimuthal direction of the interplanetary magnetic field and the most simple polar cap signatures are discussed. Only the spatial distribution of the dawn-dusk polar cap field is considered. For each OGO 6 traverse across the northern or southern polar cap, the simultaneous values of the interplanetary magnetic field in solar-equatorial coordinates were recorded by the Explorer 33 magnetometer. Histograms of these values are presented and are discussed. The high degree of correlation with the longitudinal angle indicates that the relative geometry of the interplanetary magnetic field and magnetospheric magnetic fields must be fundamental to explaining the distribution of polar cap electric fields. The sign of the solar-equatorial component perpendicular to the sun-earth line appears to be a more critical parameter than the sign of the component toward the sun. The Svalgaard-Mansurov correlation and the correspondence between fast convection and parallel magnetospheric and interplanetary magnetic fields are described.

427 citations


Journal ArticleDOI
TL;DR: In this article, a practical representation of magnetic fields in terms of a class of force-free magnetic fields is described, based on the physical consideration that in the chromosphere and lower corona, a quasistatic magnetic field must be nearly force free and that for the class of forcesafe magnetic fields, the magnetic field can be determined uniquely from the observed distribution of the vertical component of a magnetic field.
Abstract: Various manners of determination of a magnetic field are reviewed briefly from the standpoint of practicality and uniqueness. Then a practical representation of magnetic fields in terms of a class of force-free magnetic field is described. The proposed scheme is based on the physical consideration that in the chromosphere and lower corona a quasistatic magnetic field must be nearly force-free and that for the class of force-free magnetic field, i.e., ∇×B=αB with α = constant, the magnetic field can be determined uniquely from the observed distribution of the vertical component of a magnetic field. The applicability of the representation is demonstrated by examples and the limitations are discussed.

211 citations


Journal ArticleDOI
TL;DR: In this article, a comparison of pairs of Mount Wilson magnetograph recordings made in the two spectral lines Fei 5250 A and Fei 5233 A obtained on 26 different days was made for obtaining information about the unresolved filamentary structure of solar magnetic fields.
Abstract: A method is presented for obtaining information about the unresolved filamentary structure of solar magnetic fields. A comparison is made of pairs of Mount Wilson magnetograph recordings made in the two spectral lines Fei 5250 A and Fei 5233 A obtained on 26 different days. Due to line weakenings and saturation in the magnetic filaments, the apparent field strengths measured in the 5250 A line are too low, while the 5233 A line is expected to give essentially correct results. From a comparison between the apparent field strengths and fluxes and their center to limb variations, we draw the following tentative conclusions: (a) More than 90 % of the total flux seen with a 17 by 17 arc sec magnetograph aperture is channeled through narrow filaments with very high field strengths in plages and at the boundaries of supergranular cells. (b) An upper limit for the interfilamentary field strength integrated over the same aperture seems to be about 3 G. (c) The field lines in a filament are confined in a very small region in the photosphere but spread out very rapidly higher up in the atmosphere. (d) All earlier Mount Wilson magnetograph data should be multiplied by a factor that is about 1.8 at the center of the disk and decreased toward the limb in order to give the correct value of the longitudinal magnetic field averaged over the scanning aperture.

178 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of substorm activity, interplanetary magnetic field latitutde, and dipole tilt angle on high-latitude precipitation of 700 eV electrons were studied.
Abstract: Data from the auroral particles experiment on OGO-4 were used to study effects of substorm activity, interplanetary magnetic field latitutde, and dipole tilt angle on high-latitude precipitation of 700 eV electrons. It was found that: (1) The high-latitude zone of 700 eV electron precipitation in late evening and early morning hours moves equatorward by 5 to 10 deg during substorms. (2) The low-latitude boundary of polar cusp electron precipitation at 9 to 15 hours MLT also moves equatorward by several degrees during substorms and, in the absence of significant substorm activity, after a period of southward interplanetary magnetic field. (3) With times containing substorm activity or a southward interplanetary magnetic field eliminated, the low-latitude boundary of polar cusp electron precipitation is found to move by approximately 4 deg over the total yearly range of tilt angles. At maximum winter and summer conditions the invariant latitude of the boundary is shown to shift by approximately -3 deg and +1 deg respectively from its equinox location.

173 citations


Journal ArticleDOI
TL;DR: In this article, the influence of the direction of the interplanetary magnetic field on the geomagnetic field at high latitudes is used to study the long-term behavior of the sector structure during nearly four solar cycles.
Abstract: The influence of the direction of the interplanetary magnetic field on the geomagnetic field at high latitudes is used to study the long-term behavior of the sector structure during nearly four solar cycles. It is found that the rotation period of the sector structure varies from about 28.5 days in the beginning of a solar cycle to 27.0 days in the end. Also it is shown that short-lived sectors rotate more slowly than long-lived ones.

165 citations


Journal ArticleDOI
TL;DR: A clear correspondence between the sector polarity of the interplanetary magnetic field and geomagnetic variations in the polarcap has been found to exist by Svalgaard (1968) and by Mansurov (1969) as discussed by the authors.
Abstract: .An observed influence of the interplanetary magnetic sectorstructure on the geomagnetic variations in the polar cap appearsto be due to the component of the interplanetary magnetic fieldnear the ecliptic perpendicular to the earth-sun direction. Thissuggests that the observed effect on the ground originates in thefront of the magnetosphere.A clear correspondence between the sector polarity of the inter­planetary magnetic field and the geomagnetic variations in the polarcap has been found to exist by Svalgaard (1968) and by Mansurov (1969).For a positive polarity of the interplanetary magnetic field (a direc­tion away from the sun) the vertical component, Z, of the magneticfield at Thule (INVL, invariant latitude, = 86.8°N) is depressedduring several hours around magnetic noon and the horizontal compo­nent,H, at Godhavn (INVL = 77.5 0 N) 1S increased. Correspondingly anegative sector polarity gives r1se to an increase of the Z-componentat Thule and a decrease of the H-component at Godhavn. This correlationbetween the interplanetary magnetic field and the geomagnetic field 1nthe polar cap is so distinct that it has been possible to infer thesector polarity from the geomagnetic field variations at the polar capobservatories Thule and Godhavn (Friis-Christensenet al., 1971),(Svalgaard, 1972).But a more detailed examination of the observed relationship (Friis­Christensen, 1971) revealed a few cases in which the geomagnetic var1a­tions at Thule and Godhavn apparently disagreed with the sector polarity.Especially May 29th and June 21st in 1968 showed evidence of beingsituated in a sector of opposite polarity to the one observed by space­craft (Wilcox and Colburn, 1970). As these days occurred around shiftsin the sector polarity a possibility of a time delay was at firstlooked for. The fact, however, that the shift in the type of dailyvariation of the geomagnetic field in the polar cap did occur nearly

159 citations


Journal ArticleDOI
TL;DR: In this paper, a two-fluid model of the solar wind with anisotropic proton temperature and allowing for extended coronal proton-heating is considered for the case of a purely radial and of a spiral magnetic field.
Abstract: A two-fluid model of the solar wind with anisotropic proton temperature and allowing for extended coronal proton-heating is considered for the case of a purely radial and of a spiral magnetic field. Proton-proton Coulomb-collisions together with a spiral interplanetary magnetic field are found to be sufficient to reduce the thermal anisotropy in the proton gas to a value in agreement with observations. Reasonable values are obtained for the flow-velocity, number density and the protontemperature near the orbit of the Earth.

86 citations


Journal ArticleDOI
TL;DR: In this paper, the authors measured electric fields in the frequency range from 10 to 500 Hz on the 400-km altitude polar orbiting OV1-17 satellite, which was more sensitive to ionospheric electromagnetic waves below 80 Hz than presently available search coil magnetometers and was free from in-orbit interference.
Abstract: Vector electric fields in the frequency range from 10 to 500 Hz have been measured on the 400-km altitude polar orbiting OV1-17 satellite. The detector had a broad band threshold level measured in flight to be 20 μv/m; it was more sensitive to ionospheric electromagnetic waves below 80 Hz than presently available search coil magnetometers and was free from in-orbit interference. Two distinct fluctuation modes have been observed on every orbit to be characteristic of the ionosphere at 400 km. The first of these modes, discussed in detail in this paper, has an electric field vector in the satellite frame of reference that is isotropic, that has a power law spectrum with an exponent of −0.73 ± 0.1, and that has a peak intensity at 12 Hz as great as 1 mv/m Hz1/2. This mode is observed throughout the high-latitude region between two sharp boundaries located at about 55° invariant latitude on the nightside and 75° on the dayside. The observed boundary for low-frequency electric field fluctuations is thus not coincident with the auroral oval at all local times, as had been reported earlier. These properties and additional information on the wavelength spectrum of the fluctuations are consistent with interpretation as electrostatic disturbances whose spatial variations along the trajectory were measured. The observation of strong electric field fluctuations parallel to the earth's magnetic field B0 implies that ionospheric plasma physics cannot be described by first-order magnetohydrodynamic theory in which the parallel electric field is identically equal to zero. If the fluctuations are due to waves, then under certain assumptions on the wave frequency, the signal is intense enough to cause wave-particle interactions which reduce the plasma conductivity parallel to B0 sufficiently to support parallel dc electric fields of the order of 10 mv/m. The region of occurrence of the turbulent electric field is in good agreement with that of ionospheric plasma turbulence as deduced from the scintillations of radio stars and measurements of spread F.

81 citations


Journal ArticleDOI
TL;DR: In this article, a selfconsistent calculation of the magnetic field and plasma distribution in the magnetotail has been undertaken for static conditions, and the authors find the best agreement with satellite in the tail for an isotropic particle pitch angle distribution, a slow decrease of magnetic field intensity as a function of distance from the Earth |x|−0.3, and a northward field in the equatorial plane of about 1 gamma at the position of the lunar orbit.

80 citations


Journal ArticleDOI
TL;DR: In this paper, the magnetic field within the magnetosphere has been obtained as a solution of the magnetostatics problem, summarised with the field of magnetopause currents induced by the internal sources.

71 citations


Journal ArticleDOI
TL;DR: In this article, a very regular pattern was observed in the solar radio spectrographic record between 220 and 320 MHz and the magnetic field strength range from 3 to 8 G, and can be determined very accurately.
Abstract: During a unique solar radio event on March 2, 1970, among very diverse features, a very regular pattern was observed in the solar radio spectrographic record between 220 and 320 MHz. The proposed explanation is emission by slightly relativistic electrons at the sum frequency of the plasma frequency (≈ 160 MHz) and the lower harmonics of the local gyrofrequency (n from 3 to 10), rather than emission at pure harmonics of the gyrofrequency (n from 15 to 20). The derived magnetic field strengths range from 3 to 8 G, and can be determined very accurately. From the simultaneously occurring pulsating structure in the type IV emission and the deduced magnetic field strength a typical size of the emitting region of 2–300 km is derived.

Journal ArticleDOI
TL;DR: Svalgaard and Mansurov as discussed by the authors showed that it is possible to infer the polarity of the interplanetary magnetic field quite reliably from observations of the diurnal variation of polar geomagnetic fields.
Abstract: Svalgaard (1968, 1972) and Mansurov (1969) have shown that it is possible to infer the polarity of the interplanetary magnetic field quite reliably from observations of the diurnal variation of polar geomagnetic fields. The effect is most prominent in the vertical component of geomagnetic observatories near the geomagnetic poles during several hours near noon. The interplanetary magnetic field observed with spacecraft near the earth is very similar to the mean solar magnetic field (i.e., the sun observed as though it were a star); thus the fact that observations of the polar geomagnetic field have existed without interruption since 1926 at the Danish Meteorological Institute station at Godhavn, Greenland, means that in effect the inferred solar magnetic field during five sunspot cycles is available for analysis.-

Journal ArticleDOI
TL;DR: In this paper, the two-fluid model for the solar wind of Hollweg (1970) is reconsidered with the inclusion of the spiral structure of the interplanetary magnetic field.
Abstract: The two-fluid model for the solar wind of Hollweg (1970) is reconsidered with the inclusion of the spiral structure of the interplanetary magnetic field. In the present model, the protons are assumed to become collisionless beyond 0.1 AU from the sun, whereas the electrons are treated hydrodynamically and the electron temperature is supposed to obey the polytropic law. The electric field established from the charge separation, which is assumed to be derivable from a potential, tends to enhance the velocity of the solar wind at 1 AU to a value over 300 km/sec. The proton thermal anisotrophy T∥/T⊥ at the orbit of the earth is reduced from the value of 50 in the model with the radial magnetic field to the value of 11 in the present model.

01 Jan 1972
TL;DR: In this article, the authors examined possible noise sources in the measurement of the power spectrum of fluctuations in the interplanetary magnetic field and found that most measurements by fluxgate magnetometers are limited by digitization noise whereas the search coil magnetometer is limited by instrument noise.
Abstract: Examination of possible noise sources in the measurement of the power spectrum of fluctuations in the interplanetary magnetic field shows that most measurements by fluxgate magnetometers are limited by digitization noise whereas the search coil magnetometer is limited by instrument noise The folding of power about the Nyquist frequency or aliasing can be a serious problem at times for many magnetometers, but it is not serious during typical solar wind conditions except near the Nyquist frequency Waves in the solar wind associated with the presence of the earth's bow shock can contaminate the interplanetary spectrum in the vicinity of the earth However, at times the spectrum in this region is the same as far from the earth Doppler shifting caused by the convection of waves by the solar wind makes the interpretation of interplanetary spectra difficult

Journal ArticleDOI
TL;DR: In this paper, a generalization of the perturbation method is applied to the problem of the radial and non-radial oscillations of a gaseous star which is distorted by a magnetic field.
Abstract: A generalization of the perturbation method is applied to the problem of the radial and the non-radial oscillations of a gaseous star which is distorted by a magnetic field. An expression is derived for the perturbation of the oscillation frequencies due to the presence of a weak magnetic field when the equilibrium configuration is a spheroid. The particular application to the homogeneous model with a purely poloidal field inside, due to a current distribution proportional to the distance from the axis of symmetry, and a dipole type field outside is considered in detail. The main result is that the magnetic field has a large and almost stabilizing effect on unstableg-modes, particularly on higher order modes. With the considered magnetic field the surface layers appear to have a large weight.

Journal ArticleDOI
TL;DR: In this article, the structure and dynamics of neutral sheets in the solar wind are examined, and the internal magnetic topology of the sheet is argued to be that of thin magnetic tongues greatly distended outward by the expansion inside the sheet.
Abstract: The structure and dynamics of neutral sheets in the solar wind is examined. The internal magnetic topology of the sheet is argued to be that of thin magnetic tongues greatly distended outward by the expansion inside the sheet. Due to finite conductivity effects, outward flow takes place across field lines but is retarded relative to the ambient solar wind by the reverse J×B force. The sheet thickness as well as the internal transverse magnetic field are found to be proportional to the electrical conductivity to the inverse one third power. Estimating a conductivity appropriate for a current carried largely by the ions perpendicular to the magnetic field, we find sheet dimensions of the order of 500km representative for the inner solar corona. For a radial field of strength 1/2G at 2R⊙, the transverse field there is about 2 × 10−3G and decreases outward rapidly.

01 Jan 1972
TL;DR: In this paper, large-scale properties of the interplanetary magnetic field as determined by the solar wind velocity structure are examined, and the dominant role of high and low velocity solar wind streams that persist, with fluctuations and evolution, for weeks or months is emphasized.
Abstract: Large-scale properties of the interplanetary magnetic field as determined by the solar wind velocity structure are examined. The various ways in which magnetic fields affect phenomena in the solar wind are summarized. The dominant role of high and low velocity solar wind streams that persist, with fluctuations and evolution, for weeks or months is emphasized. It is suggested that for most purposes the sector structure is better identified with the stream structure than with the magnetic polarity and that the polarity does not necessarily change from one velocity sector to the next. Several mechanisms that might produce the stream structure are considered. The interaction of the high and low velocity streams is analyzed in a model that is steady state when viewed in a frame that corotates with the sun.

Journal ArticleDOI
30 Oct 1972-Nature
TL;DR: In this paper, the position angle of polarization is plotted against central meridian longitude, l, the curve departs significantly from the simple sinusoidal form predicted for a dipole field and previous workers have fitted harmonic series to the data, of the form illustrated in Figure 1.
Abstract: MEASUREMENTS of the linear polarization of the radio radiation have been widely used to study the magnetic field of the planet Jupiter and this method is believed to give the best estimate of the inclination and longitude of the magnetic axis1. When the position angle of polarization is plotted against central meridian longitude, l, the curve departs significantly from the simple sinusoidal form predicted for a dipole field and previous workers1, 2, 3 have fitted harmonic series to the data, of the form illustration Open image in new window

Journal ArticleDOI
TL;DR: In this paper, the effects of an interplanetary magnetic field on the propagation of flare-generated inter-planetary shock waves were investigated with an approximate analytical method, and it was found that the interplanolar magnetic field is relatively unimportant for strong shocks as far as the shock speed and transit time are concerned.
Abstract: The effects of an interplanetary magnetic field on the propagation of flare-generated interplanetary shock waves are investigated with an approximate analytical method. It is found that the interplanetary magnetic field is relatively unimportant for strong shocks as far as the shock speed and transit time are concerned. It has more significant effects for weak shocks. However, in all the situations examined, the error committed if the magnetic field is neglected is no more than 10%. It is suggested that a model without a magnetic field gives sufficiently accurate numerical results for the propagation of flare-generated shocks.

Journal ArticleDOI
TL;DR: In this article, a coordinate system for interpreting high-latitude energetic-particle data over the polar caps is proposed. But the coordinate system is not based on internal geomagnetic field components alone but also takes into account the currents flowing in the magnetopause and the neutral sheet.
Abstract: The coordinate system generally used to describe the structure of solar-particle profiles over the polar caps is based on invariant latitude and magnetic local time. By using trajectory integrations in a model geomagnetic field a new coordinate system is developed that is not based on internal geomagnetic-field components alone but also takes into account the currents flowing in the magnetopause and the neutral sheet. It is shown that the new coordinate system is more relevant for the interpretation of high-latitude energetic-particle data than the old system and that its use makes it possible to calculate reconnection rates and scattering mean free paths in the tail. In a companion paper the results of the calculations performed here are applied to observations first to decide which magnetosphere model (open or diffusive) is appropriate and second to use the solar-particle data for making measurements of important magnetospheric parameters.

Journal ArticleDOI
TL;DR: In this paper, the authors consider the effect of random, long-wavelength fluctuations in a turbulent magnetic field, showing that they can appreciably decrease the heat conductivity of a plasma along the magnetic field.
Abstract: Consideration of random, long-wavelength fluctuations in a turbulent magnetic field, showing that they can appreciably decrease the heat conductivity of a plasma along the magnetic field In simple cases of interest, the reduction along the average field is approximately by the factor (cos delta theta) squared, where delta theta is the angle of the local magnetic field relative to the average field Application to solar-wind electrons indicates that this reduction in heat conductivity due to observed fluctuations in the interplanetary magnetic field may be of the order of a factor of 2 This may help to explain recent measurements which indicate a rather low electron heat flux in the solar wind

01 Jan 1972
TL;DR: In this paper, the power spectra of the interplanetary magnetic field measured by near-earth satellites upstream from the earth's bow shock are free from terrestrial contamination provided the field at the satellite does not intersect the bow shock.
Abstract: Power spectra of the interplanetary magnetic field measured by near-earth satellites upstream from the earth's bow shock are free from terrestrial contamination provided the field at the satellite does not intersect the bow shock. Considerable spectral enhancement for the range of frequencies 0.01 to 1.00 Hz, due to turbulence caused by the shock, may occur if the field observed at the satellite intersects the shock. This turbulence occurs frequently in both the morning and afternoon quadrants. In the frequency band from 0.07 to 1 Hz, this noise decreases in amplitude with radial distance from the shock with an attenuation length of 4 R sub E.

Journal ArticleDOI
TL;DR: In this article, the amplitudes and phases of the electric and magnetic fields along the surface of the bottom layer are calculated numerically and their dependence on the ratio of the conductivities of the two layers is investigated.
Abstract: The solutions for low-frequency fields of a horizontal magnetic dipole embedded within a two-layer conductor are derived. For convenience, the solutions are expressed in dimensionless form. The amplitudes and phases of the electric and magnetic fields along the surface of the bottom layer are calculated numerically and their dependence on the ratio of the conductivities of the two layers is investigated. Results indicate that, in general, the electric field induced by a subsurface horizontal magnetic dipole is more sensitive to the bottom-layer conductivity than is the magnetic field. Some of the results discussed in this paper are of interest in studying the seabed conductivity.

Journal ArticleDOI
TL;DR: In this article, the merits of a horizontal magnetic dipole in a source-location scheme were considered, and the results have possible applications in locating trapped miners following disasters, such as earthquakes and floods.
Abstract: The merits of a horizontal magnetic dipole in a source-location scheme are considered. The results have possible applications in locating trapped miners following disasters.

Journal ArticleDOI
01 Apr 1972
TL;DR: In this paper, boundary condition asymmetries inherent in the solar wind flow past the Moon are included in a cylindrical model of the interplanetary magnetic field -Moon interaction.
Abstract: Boundary condition asymmetries inherent in the solar wind flow past the Moon are included in a cylindrical model of the interplanetary magnetic field - Moon interaction. Numerical examinations of the sunward side response of this model are compared in the frequency domain with those of symmetrically excited spherical and cylindrical models and two characteristic differences are observed: the response of the asymmetric model is depressed at low frequencies due to magnetic diffusion around a conducting core, and is flattened at high frequencies because of the finite application time of the incident interplanetary magnetic field. The diffusion of field lines around the core is also evident in the time response of the model in the anti-solar cavity. The above features of the lunar response resulting from boundary condition asymmetries are shown to be evident in observational measurements.

Book ChapterDOI
01 Jan 1972
TL;DR: The strongest magnetic fields on the sun are those of sunspots as discussed by the authors, which are concentrated in bundles, small elements, showing twisting and vorticities, located inside a sunspot.
Abstract: The strongest magnetic fields on the sun are those of sunspots. Although their magnetic fields have been studied for a long time much remains unknown about the magnetism of these principal sources. At sufficiently large distances from a spot the magnetic field can, apparently, be well approximated as that of a dipole submerged below the solar photosphere, (classical Mt. Wilson observations of Hale, 1919; Bumba, 1960; Severny, 1964). However, a more detailed study of the distribution of magnetic fields inside the spot shows the fine structure of the field. The field is concentrated in bundles, small elements, showing twisting and vorticities, somewhere inside a sunspot (Severny, 1964; Stepanov and Gopasyuk, 1962). The field is also appreciably inhomogeneous, and has fine structure shown by a local maximum of the field strength at a certain depth (Guseeynov, 1969), the strong rotation of the transverse component of the magnetic field with depth (Severny, 1964), the fine structure in the pattern of vertical electric currents (Severny, 1964; Kotov, 1970).




01 Jan 1972
TL;DR: The positive polarity regular formations formed in the weak and old background magnetic fields seem to correlate well with geomagnetically enhanced periods of time (shifted for 4 days), which means that they seem to be the source of the quiet solar wind as discussed by the authors.
Abstract: Unlike the negative polarity solar magnetic field large-scale regular features that correlate with enhanced solar activity regions, the positive polarity regular formations formed in the weak and old background magnetic fields seem to correlate well with geomagnetically enhanced periods of time (shifted for 4 days), which means that they seem to be the source of the quiet solar wind. This behavior of the large intervals of heliographic longitude with prevailing positive polarity fields may be followed to the end of the 18th cycle, during the declining part of the 19th cycle, and during the first half of the present 20th cycle of solar activity.