Showing papers on "Earth's magnetic field published in 1976"

Field‐aligned currents in the dayside cusp observed by Triad

Abstract: The characteristics of field-aligned currents at an altitude of 800 km in the dayside high-latitude region over the northern hemisphere were determined from the Triad satellite magnetometer data recorded at College, Alaska, from January 1973 to October 1974. The field-aligned currents discussed here are located poleward of the large-scale field-aligned currents reported earlier and referred to as 'region 1 field-aligned currents' by the authors (Iijima and Potemra, 1976). These high-latitude field-aligned currents are most often observed in the dayside sector between 0930 and 1430 MLT and are statistically distributed between 78/sup 0/ and 80/sup 0/ invariant latitude during weakly disturbed conditions as indicated by westward electrojet activity (abs. value of AL < 100..gamma..). Although these high-latitude field-aligned currents show complicated variations, they generally flow away from the ionosphere in the forenoon hours (0930--1200 MLT) and into the ionosphere in the afternoon hours (1200--1430 MLT). These flow directions are opposite to the quasi-permanent region 1 field-aligned currents related to the S/sub q//sup p/ currents previously discussed by the authors. The directions and spatial distribution of these field-aligned currents are consistent with the antisolarward equivalent ionospheric current near 1200 MLT deduced from simultaneous ground-based magnetograms at approx.81/sup 0/ invariant latitude. The intensitymore » of these high-latitude field-aligned currents increases as the interplanetary magnetic field increases in the southward direction. These field-aligned currents are located within the region associated with the dayside magnetospheric cusp, and their relationship to geomagnetic activity, especially interplanetary magnetic field variations, suggests that they may play an important role in the coupling between the interplanetary medium and the magnetosphere. (AIP)« less

Depletion of solar wind plasma near a planetary boundary

Abstract: A mathematical model is presented that describes the squeezing of solar wind plasma out along interplanetary magnetic field lines in the region between the bow shock and the effective planetary boundary (in the case of the earth, the magnetopause). In the absence of local magnetic merging the squeezing process should create a 'depletion layer', a region of very low plasma density just outside the magnetopause. Numerical solutions are obtained for the dimensionless magnetohydrodynamic equations describing this depletion process for the case where the solar wind magnetic field is perpendicular to the solar wind flow direction. For the case of the earth, the theory predicts that the density should be reduced by a factor exceeding 2 in a layer about 700-1300 km thick if the Alfven Mach number in the solar wind, is equal to 8. Scaling of the model calculations to Venus and Mars suggests layer thicknesses about 1/10 and 1/15 those of the earth, respectively, neglecting diffusion and ionospheric effects.

Magnetospheric convection induced by the positive and negative Z components of the interplanetary magnetic field: Quantitative analysis using polar cap magnetic records

Abstract: The dependence of the polar cap magnetic disturbance on the polarity and magnitude of the Z component of the interplanetary magnetic field is investigated by regression analysis using hourly values The Svalgaard-Mansurov effect has been eliminated by assuming a linear dependence on the Y component of the interplanetary field It is shown that as the northward component of the interplanetary magnetic field increases, a characteristic current system appears in the polar cap This current system is composed of two current vortices in the day side polar cap: one in the prenoon sector and the other in the afternoon sector The current direction is antisunward in the central polar cap, suggesting that sunward plasma convection is induced in the polar cap Current intensity is strongest at фM ∼ 84° around the noon meridian We propose that the tail field lines are reconnected with the northward interplanetary field on the polar side of the day side polar cusp and, as a result, that plasma convection is induced which is closed within the high-latitude magnetosphere On the other hand, when the interplanetary magnetic field is directed southward, a transpolar current sheet appears, covering the whole polar cap (фM ≥ 775°) The characteristics of this transpolar current sheet are as follows: (1) On the day side, especially around noon, the direction of the current is roughly consistent with the Hall current direction expected from the dawn-to-dusk electric field, while on the night side the direction of the current is considerably skewed from the noon-midnight meridian Skewing of the current direction can be explained by the effect of the currents external to the ionosphere (2) The strength of the current is almost linearly dependent on Bz when the interplanetary magnetic field is directed southward (θ < −45°) However, the current intensity is also a function of the magnitude of By, this being apparent when Bz ∼ 0 This indicates that the day side reconnection rate is a function of |By| as well as of Bz, and information is derived about the applicability of the three-dimensional reconnection model in the presence of finite |By|

Coronal holes, solar wind streams, and recurrent geomagnetic disturbances - 1973-1976

Abstract: Observations of coronal holes, solar-wind streams, and geomagnetic disturbances during 1973-1976 are compared in a 27-day pictorial format which shows their long-term evolution. The results leave little doubt that coronal holes are related to the high-speed streams and their associated recurrent geomagnetic disturbances. In particular, these observations strongly support the hypothesis that coronal holes are the solar origin of the high-speed streams observed in the solar wind near the ecliptic plane.

On hot tenuous plasmas, fireballs, and boundary layers in the Earth's magnetotail

Abstract: Intensive correlative studies of magnetic fields and plasmas within the earth's magnetotail at geocentric radial distances of ∼23–46 RE during March-October 1974 have revealed striking new features. The instruments employed in this survey were the University of Iowa Lepedea and the Goddard Space Flight Center magnetometer. The hot tenuous plasmas within the plasma sheet were found to be in a state of almost continual flow and were threaded with northward, or closed, geomagnetic field lines. The magnetic field is referred to as ‘northward’ or ‘southward’ if the component Bz is positive or negative, respectively, regardless of the magnitudes of the other components. Proton bulk speeds were in the range 50–500 km s−1. The magnetic fields are directed northward, irrespective of whether the plasma flows are tailward or earthward. These observations show the demand for a strong persistent source of magnetic flux and hot plasmas for the plasma sheet. No characteristic proton bulk flows, e.g., strong dawn-to-dusk motions, were evident during crossings of the magnetic neutral sheet. Occasionally, the satellite encountered the region of acceleration in the magnetotail, the ‘fireball.’ This spectacular phenomenon exhibits strong jetting of plasmas in excess of 1000 km s−1, proton temperatures of ∼107 °K (kT ∼ 1 keV), disordered magnetic fields, southward magnetic fields during tailward jetting of plasmas, and northward magnetic fields for fast plasma flows toward earth. Observations of electrons are used to demonstrate that earthward plasma flows within the fireball are threaded with closed geomagnetic field lines, and open magnetic field lines are embedded in the tailward jetting plasmas. The magnetosheathlike plasmas within the boundary layers which are positioned contiguously to the plasma sheet display striking evidences of plasma heating, great changes in bulk flow velocities and acceleration of energetic electrons with E > 45 keV. Typical temperatures and bulk flow speeds within the boundary layer plasmas are ∼106 °K and ∼400 km s−1, respectively. Persistent zones of southward magnetic fields are detected, which are often positioned adjacent to the plasma sheet and within the boundary layer plasmas. Rotations of the magnetic fields from southward to northward, or vice versa, in these boundary layers are accompanied by large enhancements of energetic electron intensities, substantial heating of the low-energy electron distributions, and strong perturbations of the proton velocity distribution functions. During periods for northward directed fields the proton bulk flows decelerate to speeds of ≲200 km s−1, densities decrease, and the proton velocity distributions can be fitted with two Maxwellians with temperatures of ∼2 × 105 °K and ∼ 107 to 5 × 107 °K for specific examples. The boundary layer plasmas appear to be the primal source of hot plasmas and closed geomagnetic field lines for the plasma sheet. Qualitatively, all of the major results of the present study are consistent with macroscopic features which are expected from merging of magnetic field lines in the earth's magnetotail or, more specifically, in the magnetosheathlike boundary layer plasmas.

An illustration of modification of geomagnetic pulsation structure by the ionosphere

Abstract: In this paper we illustrate the results of a companion paper (Hughes and Southwood, 1976) in which we describe how geomagnetic pulsations are screened by the atmosphere and ionosphere. Here, using Fourier synthesis, we map down to the earth's surface the fields predicted in the vicinity of a resonating magnetospheric field line by Southwood (1975a). The results demonstrate that the ionosphere-atmosphere system screens out rapid horizontal variation and also provide an illustration of the ionospheric polarization rotation phenomenon. Comparison with recent experimental work is made.

Representation of magnetic fields in space

Abstract: Several methods by which a magnetic field in space can be represented are reviewed with particular attention to problems of the observed geomagnetic field. Time dependence is assumed to be negligible, and five main classes of representation are described by vector potential, scalar potential, orthogonal vectors, Euler potentials, and expanded magnetic field.

Magnetospheric electric fields and their variation with geomagnetic activity

Abstract: Diverse and independent measurements of the variation of particle convection boundaries with geomagnetic activity are used to obtain relations between the magnitude of a large scale electric field and the Kp index. The relations are compared with measured fields and with models and are found to be consistent with diverse observations and probably more reliable than results previously obtained.

Direct measurements of nighttime thermospheric winds and temperatures, 2. Geomagnetic storms

Abstract: A high-resolution Fabry-Perot spectrometer has been used at Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), to determine the nighttime thermospheric temperatures and winds during geomagnetic storm periods from the line profiles and Doppler shifts of the (O I) 15867-K (6300 A) line emission. Data obtained during four geomagnetic storm periods in 1974 are presented: March 20–22, October 15–20, June 29 to July 8, and April 16–27. The data are compared to temperatures determined from the Ogo 6 global empirical model of neutral composition and temperature and to winds calculated from a semiempirical dynamic model of the neutral thermosphere. The results show considerable variations in the dynamic character of the thermosphere which are also related to the intensity of geomagnetic activity. During geomagnetic storms the nighttime equatorward winds are greatly enhanced from their quiet time values with a maximum measured velocity of 640 m s−1 during a Kp = 9 storm. The zonal winds generally develop a westward component relative to the geomagnetic quiet zonal winds. During intense geomagnetic activity the zonal winds are westward at 100–200 m s−1 in the early evening hours, flowing in the direction of magnetospheric convection and opposite of model predictions. The meridional winds measured south of the site are generally smaller than those measured to the north. Sometimes the meridional winds measured north and south of the station flow in opposite directions. The nighttime neutral gas temperatures are observed to increase from their geomagnetic quiet values during the storm and are in general concurrence with the Ogo 6 model predictions.

Magnetic fields of the magnetosheath

Abstract: The magnetic field of the magnetosheath is most naturally discussed in terms of its steady state and its fluctuating components Theory of the steady state field is quite well developed and its essential features have been confirmed by observations The interplanetary field is convected through the bow shock where its magnitude is increased and its direction changed by the minimal amount necessary to preserve the normal component across the shock Convection within the magnetosheath usually increases the magnitude still further near the subsolar point and further distortes the direction until the field is aligned approximately tangent to the magnetopause Fluctuations of the magnetosheath field are very complex, variable and not well understood Spectral peaks are common features which occur at different frequencies at various times Perturbation vectors of hydromagnetic waves tend to be aligned with the shock and magnetopause surfaces Magnetosheath waves may be generated upstream, within the magnetosheath, at the bow shock, or at the magnetopause, but the relative importance of these sources is not known

The Lake Mungo geomagnetic excursion

Abstract: Archaeomagnetic studies have been made of prehistoric aboriginal fireplaces occurring along the ancient shore of Lake Mungo, a dried out lake in southeastern Australia. Directions of magnetization preserved in ovenstones and baked hearths show that wide departures of up to 120° from the axial dipole field direction occurred about 30000 years ago. The determination of the variation in geomagnetic field strength from the baked material is complicated by non-ideal behaviour during Thellier’s double heating method. The problem appears to arise from the subsequent (postfiring) formation of iron oxyhydroxides during a period in which the water level in the lake rose. During laboratory heatings these oxyhydroxides dehydrate causing the non-ideal behaviour observed. The ancient field strengths deduced are therefore probably minimum values. The geomagnetic excursion recorded between at least 30780 ±520 and 28140 ±370 years b.p. on the conventional radiocarbon time scale is associated with very high field strengths between 1 and 2 Oe (1 Oe » 79.6 A m_ 1 ). The field strength subsequently decreased to between 0.2 and 0.3 Oe after the excursion. This main excursion is referred to as the Lake Mungo geomagnetic excursion. There is evidence that a second excursion associated with low field strengths of 0.1-0.2 Oe occurred around 26000 years b.p. A review of geomagnetic excursions less than 40 000 years in age shows that it may be premature to assume that these are world-wide synchronous features. The range of ages and their groupings in different parts of the world may indicate they are temporary non-dipole features of continental extent. However, the duration of most excursions (order of 103 years) is very similar to that of polarity transitions and this could indicate they are aborted reversals.

On the long‐period variations of the Earth's magnetic field from 2 months to 20 years

Abstract: Burg's maximum entropy spectral analysis is used in order to study monthly and annual mean values of absolute elements of the geomagnetic field at a number of world observatories. After removal of a parabolic trend, which accounts for most of the secular variation of internal origin, the residual variations clearly exhibit a worldwide character. A test study of monthly mean values at three sample observatories is first performed; earlier conclusions on both the origin and the geometry of the annual and semiannual variations are discussed. Annual mean values from approximately 30 observatories are next analyzed; most of the power found in the spectra, for periods between 2 and approximately 20 yr, is clearly related to the solar cycle (peaks around 10–12 yr and the first two harmonics). No peaks that can be related to phenomena of internal origin are found. The accuracy and resolving power of these data concerning the problem of the conductivity distribution in the deeper mantle are found to be rather disappointing.

The morphology of energetic O/+/ ions during two magnetic storms - Latitudinal variations

Abstract: Results are presented for a statistical analysis of the temporal variations of precipitating O(+) and H(+) ions in the energy range 0.7-12 keV during the magnetic storms on Dec. 16-18, 1971. Emphasis is on the temporal variations of parameters describing the intensity, average energy, and spatial location of the precipitation zones of the two ionic species. It is shown that the intensity of the precipitating O(+) ions correlate well with the geomagnetic indices which measure the strength of magnetospheric substorm activity and the strength of the storm time ring current, indicating that a previously unknown strong coupling mechanism existed between the magnetosphere and the ionosphere during the storm period. Correlations are found between the locations of the O(+) and H(+) precipitation zones and between the average energies of the two ionic species. Precipitation is shown to be an important loss mechanism for ring current ions with energy less than 12 keV during the magnetic storm period studied.

Downward mapping of high‐latitude ionospheric electric fields to the ground

Abstract: The main purpose of this paper is to draw attention to the important but long neglected problem of electrical coupling between the ionosphere and the lower atmosphere. A numerical technique is used to calculate electric fields and currents between 0- and 150-km altitude produced by large-scale horizontal electric fields known to exist in the polar cap and auroral ionosphere. A two-dimensional model assuming a flat earth and a vertical geomagnetic field is used. The results show that horizontal electric fields in the ionosphere map down to ∼ 10 km with little attenuation, in agreement with previous authors' results. In addition, these horizontal ionospheric electric fields should cause significant modulations of vertical electric fields down to the earth's surface. The effects of conductivity irregularities in the ionosphere, stratosphere, and troposphere are also examined. Localized conductivity enhancements associated with aurora are expected to produce a horizontal electric field of up to a few millivolts per meter at ∼30-km altitude in the absence of any horizontal field in the ionosphere. These conductivity enhancements have less than a 1% effect on the vertical electric field on the ground. The effects of stratospheric and tropospheric irregularities are negligible at large heights but become important below ∼20 km. Their magnitude depends on how the ratio between the local resistivity and the height-integrated columnar resistance is altered.

Low‐energy electron intensities at large distances over the Earth's polar cap

Abstract: The results of the character and temporal fluctuations study of electron intensities in the energy range of hundreds of electron volts, are reported which were measured at high latitudes and altitudes on geomagnetic field lines corresponding to those of the polar cap and magnetotail lobes. It is concluded that such electron intensities are diminutive relative to those found in other regions of the magnetosphere. Severe variations of intensities were found and the magnitudes of electron intensities appear to be strongly coupled to the directions of the interplanetary magnetic fields.

A tectonomagnetic effect observed before a magnitude 5.2 earthquake near Hollister California

Abstract: Simultaneous measurements of geomagnetic field with an array of seven proton precession magnetometers along the San Andreas fault show that the most significant local changes during 1974 were recorded at a site 11 km from a magnitude 5.2 earthquake that occurred on November 28, 1974. A systematic increase in magnetic field of 0.9 γ occurred at this site during the early part of 1974. A more dramatic increase of 1.5 γ occurred about 7 weeks before the earthquake, lasting about 2 weeks. Four weeks prior to the earthquake the magnetic field returned to approximately its initial value and remained at this value through April 1975. These data cannot be explained by ionospheric disturbances or telluric currents. The most probable source is a piezomagnetic effect, which implies that the magnetic field changes represent changes in stress in the rocks nearby the anomalous station.

Geomagnetic field variations

Abstract: In this review, an attempt has been made to describe the large variety of geomagnetic variations, both regular and irregular. After a brief description of the Earth and its environment, different types of quiet-day variations are described and present ideas regarding their possible mechanisms are discussed. In general, periodicities exceeding several tens of years can be attributed to changes in the interior of the Earth while periodicities of 22 years or less seem to be related to phenomena connected with the Sun, through the interaction of solar wind with the Earth's magnetosphere. The morphology of irregular storm-time variations and its relationship with interplanetary plasma parameters is discussed with particular reference to the orientations of interplanetary magnetic field, particularly the southward B z component which seems to play a crucial role. Various storm-time phenomena occurring in the polar, auroral, mid-latitude and equatorial regions and their interconnections are described. Theoretical models offering explanations of many of these phenomena are discussed, the unsolved problems are outlined, and the direction of the present effort in solving these is indicated.

Annual variation of the geomagnetic field

Abstract: Summary. Horizontal and vertical intensity data, obtained between 1957.0 and 1961.0 at 69 observatories, are analysed to determine the worldwide distribution of the annual variation of the geomagnetic field. Only data observed near local midnight are used, to avoid the small, but significant contamination from Sq. Over most of the world the variation is found to be small, with a clear dependence on latitude, but near the poles it is larger and more erratic. The non-polar variation is subjected to spherical harmonic analysis and separated into parts of internal and external origin. The polar variations are shown to be consistent with a north—south oscillation of the mean position of the auroral electrojets during the year. It is suggested that, with the exception of the polar effect, the annual variation is not due to ionospheric currents (as was hitherto believed), but results from an annual variation in the latitude of the ring current.

Transient ULF electric and magnetic fields following a lightning discharge

Abstract: A lightning discharge is modeled as an impulse current from cloud to ground which instantaneously removes a quantity of charge from a charge center originally screened by a neutralizing space charge. The resulting time-dependent electric and magnetic fields at the ground and in the upper ionosphere are examined for a time interval that is long in comparison to the duration of a typical discharge but short in comparison to the relaxation time of the screening space charge. The calculations are carried out for a nominal nighttime ionosphere with an assumed exponential conductivity profile and with anisotropy taken into account. First, analytic approximations are obtained for the various ULF transfer functions of the medium throughout the entire altitude range between the ground and the upper ionosphere. The transfer functions are then used to obtain analytic expressions for the ULF component of the time-dependent fields produced by the assumed source. At the ground the fields result from electric relaxation of the ionosphere and persist for times of a few seconds. The decay time of the electric field at the ground decreases with increasing distance from the source. The time dependence of the fields can be explained on the basis of a simple variable capacitor model of the medium. In the upper ionosphere there is an outgoing hydromagnetic pulse which arises from a resonant excitation of the ionosphere by the low-frequency components of the discharge itself. The duration of the pulse seems too short to excite geomagnetic pulsations in the magnetosphere.

Influence of ancient solar-proton events on the evolution of life

Abstract: There is mounting evidence that past extinctions of faunal species have occurred in near coincidence with reversals in polarity of the geomagnetic field. Could the link lie in catastrophic depletions of stratospheric ozone caused by solar-proton irradiation over a reduced geomagnetic field?

Test of the dipolar nature of the geomagnetic field throughout Phanerozoic time

Abstract: THE morphology of the geomagnetic field throughout the Earth's history is of interest first, in a general way, to geomagnetists in terms of long term implications concerning the Earth's core, its thermal regime and the motions in it, and, second, in a very particular way, to palaeomagnetists since the geocentric dipole model is universally used in mapping observed field directions into palaeomagnetic poles. Runcorn1,2 has given strong reasons for supposing the mean geomagnetic field to be symmetrical about the Earth's spin axis. He has also argued3, on the basis of early palaeomagnetic data, that a dipolar field coupled with continental motion is preferable to higher order axial multipole fields. Here I discuss an observational test, based on the much larger body of palaeomagnetic data now available, to discriminate between a dipole field and those of higher order multipoles.

Radar measurements of the latitudinal variation of auroral ionization

Abstract: The Chatanika, Alaska, incoherent scatter radar has been used to measure the spatial variation of auroral ionization. A two-dimensional (altitude, latitude) cross-sectional map of electron densities in the ionosphere is produced by scanning in the geomagnetic meridian plane. The altitutde variation of ionization is used to infer the differential energy distribution of the incident auroral electrons. The latitudinal variation of this energy distribution and the total energy input are obtained by use of the meridian-scanning technique. Examples are shown of observations made during an active aurora.

Aeronomic effects of the South Atlantic Anomaly

Abstract: Experimental and theoretical values for precipitated electron and proton fluxes in the area of the South Atlantic geomagnetic anomaly are reviewed. Observations of airglow which have been reported in the literature suggest that there is no enhancement in the Brazilian region, though there may be some in the southeastern part of the Atlantic area. There is evidence from various types of observations that the electron density in the ionosphere is greater over the South Atlantic than at comparable places elsewhere, especially during magnetic disturbances, and similarly the ion density shows enhanced values. Electron and neutral temperatures may also be higher than normal in the anomalous region. Observations are required in all these fields, coordinated with particle flux measurements from rockets and satellites in the area, to confirm the reality of the effects and their relationship to particle precipitation.

The magnetic field of mars according to the data from the Mars 3 and Mars 5

Abstract: Experimental data obtained by the magnetometers on Mars 3 (1972) at heights of 1000 km on the day side and Mars 5 (1974) at heights up to 9000 km on the dusk and night sides during opposite polarities of the interplanetary magnetic field show that Mars has an intrinsic magnetic field. The magnetic moment of the Martian dipole is M ≈ 64Rm³ = 2.5·1022 G cm³, and the equatorial dipole field at the surface of the planet is 64 γ. The topology of lines of force on the day and night sides can be explained best if we assume that the magnetic dipole axis of Mars is tilted less than 15°–20° from the rotation axis. The bow shock position observed on the day side is in agreement with the hypothesis of an intrinsic Martian magnetic field.

Collisional ion cyclotron waves in the auroral ionosphere

Abstract: A nearly field-aligned current-driven ion cyclotron wave traveling obliquely to the earth's magnetic field becomes unstable in the auroral ionosphere owing to dissipative effects. Its relevance with regard to some recent observations of auroral irregularities is discussed. (AIP)

Day side reconnection between a dipolar geomagnetic field and a uniform interplanetary field

Abstract: Field line reconnection on the day side magnetopause is assumed to take place along the separator of field line connectivity in the magnetic topology resulting from the interpermeation of a dipolar geomagnetic field and a uniform interplanetary field. The induced voltage is calculated to show its dependence on the magnitude and direction of the incident magnetic field.

Plasma Observations in the Magnetotail

Abstract: Through the analysis of data returned from the three Rice Suprathermal Ion Detectors we have obtained information on the particle regimes present in the geomagnetic tail at lunar distances. We are able to distinguish the existence of two distinct particle populations and to quantify many of their spatial and plasma characteristics.