Showing papers on "Earth's magnetic field published in 1982"

Evidence for current sheet acceleration in the geomagnetic tail

Abstract: The existence of the current sheet and the dawn to dusk electric field in the geomagnetic tail implies there is particle energization in the tail current sheet of the order 2--10% of the total solar wind energy incident upon the dayside magnetopause. In this paper we determine that ion acceleration in a current sheet with a small magnetic field across the sheet, via single-particle motion which violates the guiding center approximation, can account for this large energization in the tail. We calculate the distribution of accelerated ions which result from the urrent sheet acceleration and compare the results with distributions of accelerated ions frequently observed flowing earthwards along the outer boundary of the plasma sheet. The comparison indicates that the observed earthward flowing ions result from current sheet acceleration. Comparison with measurements of auroral ion predictions at low precipitation at low altitudes implies that the accelerated ions ejected from the current sheet are also an important source of auroral ion precipitation. In addition, these acceletated ions may be an important source of plasma sheet ions.

Precipitating electron energy flux and auroral zone conductances - An empirical model

Abstract: Data from the low energy electron (LEE) experiments on the Atmosphere Explorer C and D satellites have been used to determine the average global distribution of the energy flux of precipitating auroral electrons and their average energy for different levels of geomagnetic activity. Measurements from the Atmosphere Explorer unified abstract file (15-s resolution) have been binned according to invariant latitude (in the range 50-90 deg), magnetic local time, and geomagnetic activity as measured by the Kp and auroral electrojet (AE) indices, separately. Bin-averaged values of precipitating energy flux and average energy have been calculated, and a smoothing algorithm used to reduce stochastic variations in the raw data. The results indicate that, for the parameters studied, the AE inces does a superior job of ordering the data with regard to geomagnetic activity. The global distribution of the auroral enhancement porition of the Pedersen and Hall conductances were inferred from the data by means of an empirical fit to detailed energy deposition calculations.

Global circulation and temperature structure of thermosphere with high‐latitude plasma convection

Abstract: This paper examines the effect of magnetospheric convection in modifying the diurnal neutral gas temperature distribution and circulation of the thermosphere for equinox conditions, using NCAR's thermospheric general circulation model. Numerical experiments are presented to illustrate the differences in temperature structure and circulation due to (1) solar heating alone, (2) solar heating plus plasma convection with coincident geographic and geomagnetic poles, and (3) solar heating plus plasma convection with displaced poles. The high-latitude plasma convection has an important influence on the global thermospheric structure and circulation. Plasma convection with displaced poles introduces a universal time dependence to the circulation and temperature structure; similar patterns occur in the northern and southern hemisphere, with a 12-hour time difference. Magnetospheric convection drives a largely rotational, nondivergent, double-vortex wind system at F region altitudes that can attain velocities greater than 500 m s−1 during moderate levels of geomagnetic activity. These vortices extend downward into the lower thermosphere. However, the cold low-pressure cyclonic circulation near the dawn terminator is much more pronounced than the warm high-pressure anticyclonic circulation in the evening sector.

A geomagnetic field spectrum

Abstract: A spherical harmonic model of the earth's internal magnetic field of degree and order 23 is derived from selected Magsat data, and its power spectrum, computed with terms developed by Mauersberger (1956) and Lowes (1974), is found to exhibit a change of a slope at n = 14 which is interpreted as an indication that the core field dominates at values lower than 13 while the crust field dominates above a value of 15. The representations of the two portions of the spectrum obtained can be used to establish order-of-magnitude inaccuracies due to both crustal fields and the inability to observe core field wavelengths beyond n = 13, at which point they are obscured by the crustal field, in core field models.

The SSC on July 29, 1977 and its propagation within the magnetosphere

Abstract: At 0027 UT on July 29, 1977 an interplanetary shock wave arrived at the front side magnetosphere and triggered substantial geomagnetic activity throughout the day. The propagation of the resulting MHD wave within the magnetosphere has been studied with measurements from a total of six satellites in (or near) the geostationary orbit and the interplanetary space and groundbased magnetometers. At the time of the SSC the European spacecraft GEOS 1 was located at R = 6.7 RE and 1300 LT providing accurate reference measurements for the hydromagnetic impulse spreading out in the magnetosphere. The signal transmission from the front side magnetopause down to the equatorial ionosphere corresponded to an average wave speed of υ = 600 km/s. A propagation speed of υ = 910 km/s was found for the signal transmission in the outer magnetosphere in and beyond the geostationary altitude. The results compared reasonably well with model calculations.

Charged particle behavior in low-frequency geomagnetic pulsations, 2. Graphical approach

Abstract: The behavior of charged particles in an ultralow frequency magnetospheric pulsation with standing wave structure along the magnetic field is interpreted by using a graphical approach. Attention is directed principally to the way in which particles are accelerated as they bounce and drift through a wave with which they are in resonance, but nonresonant particles are also discussed. Under appropriate conditions, passage through the wave leads to modulation of particle energy. Applications emphasize the necessity of considering the large-scale structure of a wave in interpreting the particle behavior seen at a particular point in space.

Evidence for quasi-stationary reconnection at the dayside magnetopause

Abstract: The paper investigates several highly unusual encounters with the earth's magnetopause, that occurred during an approximately 5-hour period on November 22-23, 1979, when the ISEE 1 and 2 were near orbit apogee. A large decrease in the dynamic pressure exerted by the solar wind resulted in an expansion of the magnetosphere to and beyond the apogee of the ISEE 1 and 2 orbit, and the subsolar magnetopause of about 20.4 earth radii is farther than normal in geocentric distance by a factor of about 2. Field rotations varying from about 80 to 120 deg were involved in the transition from the magnetosheath to the magnetosphere, and hodograms of the tangential component of the magnetic field vector suggest that the magnetopause was a rotational discontinuity. These observations indicate that on occasion reconnection at the dayside magnetopause can be a quasi-stationary process.

Helium abundance enhancements in the solar wind

Abstract: Evidence for a link between helium enhancements at 1 AU and transient coronal mass ejections is provided by the statistical analysis of 73 large helium abundance enhancement observations made by IMPs 6, 7 and 8 over 1972-1978. These events, in which helium abundance enhancement is greater than about 10%, are sporadic, sometimes clustered in time, occur approximately in phase with the solar cycle, and nearly 50% of them are associated with interplanetary shocks and/or geomagnetic activity sudden commencements. The plasma pattern associated with them is nevertheless independent of shock occurrence, and features high magnetic field strength, low alpha-proton velocity difference, and low proton temperature, suggesting that the enhancement is embedded in a closed, magnetically dominated structure that expands adiabatically. Evidence of an association between helium enhancement at 1 AU and type II and IV radio bursts in the corona is presented.

Self‐consistent theory of time‐dependent convection in the Earth's magnetotail

Abstract: A self-consistent theory of time-dependent convection in the earth's magnetotail is developed. In contrast to earlier convection models our approach takes into account that the plasma sheet particles are effectively trapped on closed geomagnetic field lines. The results demonstrate that steady state convection, although possible in principle, is unlikely to occur in the earth's magnetotail. In particular, if particle or energy losses are sufficiently small and if the outer lobe field lines have convex shape, a steady state is impossible in the framework of the present polytropic model. Quantitative models for time-dependent convection are constructed. The time dependence introduces important consequences for energy storage, stability, and spatial dependence of the convection electric field. The results are consistent with a quasi-periodic evolution of the tail, where periods of quasi-static compressional convection are followed by phases of dynamic evolution.

Simulation of three-dimensional solar wind disturbances and resulting geomagnetic storms

Abstract: A kinematic method of representing the three-dimensional solar wind flow is devised by taking into account qualitatively the stream-stream interaction which leads to the formation of a shock pair. Solar wind particles move radially away from the Sun, satisfying the frozen-magnetic field condition. The uniqueness of the present approach is that one can incorporate both theoretical and observational results by adjusting the parameters involved and that a self-consistent data set can be simulated. One can then infer the three-dimensional structure of the solar wind which is vital in understanding the interaction between the solar wind and the magnetosphere, and it is for this reason that the present kinematic method is devised. In the first part of this paper, the present kinematic method is described in detail by demonstrating that the following solar wind features can be simulated: (i) Variations of the solar wind quantities (such as the solar wind speed, the density and the IMF vector), associated with the solar rotation, at the Earth; (ii) the solar wind flow pattern in the meridian planes; (iii) the three-dimensional structure of the corotating interaction region (CIR); and (iv) the three-dimensional structure of the warped solar current sheet. In Section 2, the three-dimensional structure of solar wind disturbances are studied by introducing a flare-generated high speed stream into the two-stream model of the solar wind developed in Section 1. The treatment of the stream-stream interaction is generalized to deal with a flare-generated high speed stream, yielding a shock pair. The shock pair causes three-dimensional distortion of the solar current sheet as it propagates outward from the Sun. It is shown that a set of characteristic time variations of the solar wind speed, density, the interplanetary magnetic field magnitude B and angles Θ (theta) and gf (phi) result at the time of the passage at the location of the Earth for a given set of flare conditions. These quantities allow us to compute the solar wind-magnetosphere energy coupling function ɛ. Time variations of the two geomagnetic indices AE and Dst are then estimated from ɛ. The simulated geomagnetic storms are compared with observed ones. In the third part, it is shown that recurrent geomagnetic storms can reasonably be reproduced, if fluctuating components of the interplanetary magnetic field (IMF) are superposed on the kinematic model of the solar wind developed in the first part. As an example, we simulate the fluctuating components by linearly polarized Alfven waves and by random variations of the IMF angle Θ (theta). Characteristics of the simulated and observed geomagnetic storms are discussed in terms of the simulated and observed AE and Dst indices. If the fluctuating components of the IMF can generally be identified as hydromagnetic waves, they may be an important cause for individual magnetospheric substorms, while the IMF magnitude B and the solar wind speed V modulate partially the intensity of magnetospheric substorms and storms.

Some new methods in geomagnetic field modeling applied to the 1960-1980 epoch.

Abstract: The utilization of satellite and surface data together permitted the incorporation of a solution for the anomaly field at each observatory. The residuals of the observatory measurements to such models is commensurate with the actual measurment accuracy. Incorporation of the anomaly estimation enabled the inclusion of stable time derivatives of the spherical harmonic coefficients up to the third derivative. A spherical harmonic model is derived with degree and order 13 in its constant and first time derivative terms, six in its second derivative terms and four in its third derivative terms.

Stable regions in the Earth's liquid core

Abstract: Summary. Slow cooling of the whole Earth can be responsible for the convection in the core that is required to generate the magnetic field. Previous studies have assumed the cooling rate to be high enough for the whole core to convect. Here we study the effects of a low rate of cooling by assuming the temperature at the base of the mantle to remain constant with an initially entirely molten, adiabatic core. We argue that, in such a situation, convection would stop at the top of the core, and calculate the consequent thermal evolution. A stable, density stratified layer grows downwards from the core mantle boundary reaching a thickness of 100-1000 km in a few thousands of millions of years. There is some geomagnetic evidence to support belief in the existence of such a stable layer.

Was the 1970 geomagnetic jerk of internal or external origin

Abstract: Annual mean data from worldwide magnetic observatories show that there was a jerk1 (step-change in the second time derivative) in the geomagnetic field, which took place over an interval of <2 yr around 1970. If such a short-lived phenomenon originated in the Earth's core, and were still detectable after passing through the mantle to the surface, this would imply a much lower conductivity for the lower mantle than had been believed previously. Here we show by an objective test that most of the jerk has an internal origin.

Coupling function between solar wind parameters and geomagnetic indices

Abstract: Results of past studies on the correlation of a function of solar wind parameters with a geomagnetic index are reviewed and examined to identify the most relevant coupling function among those hitherto proposed. For the AL index, three functions, BsV, BsV², and Akasofu's e, are compared, where Bs is the southward component of the interplanetary magnetic field and V is the solar wind velocity, and the importance of BsV² is then confirmed by a new presentation of the prediction of each function and the AL index. For the Dst index, validities of the above three functions are also compared, but a new function, BsVP1/3, where P is the dynamic pressure of the solar wind, is proposed as the one that surpasses all of the other three functions in predicting Dst. Implications of the two coupling functions are discussed, and possible directions for future work on this problem are suggested.

Computer simulations of basic processes of coherent whistler wave‐particle interactions in the magnetosphere

Abstract: Basic processes of coherent whistler mode wave-particle interactions in the magnetosphere are studied by self-consistent computer simulations. The wave equations used in the simulation to determine the wave amplitude and frequency are discussed and physical interpretations of them are given. Nonlinear processes of the wave growth in a uniform magnetic field are examined in detail. Difference between uniform (periodic) and nonuniform (encounter) interactions is studied. Taking into account the inhomogeneity of the dipole geomagnetic filed, we found that untrapped resonant electrons as well as trapped electrons play significant roles in the wave evolution. The roles of trapped and untrapped electrons are different and exchange their roles depending on the sign of the geomagnetic gradient. In the hemisphere where the whistler wave propagates toward the equator, trapped electrons cause wave growth, while untrapped resonant electrons cause wave damping. In the opposite hemisphere, these roles are exchanged. Four simulations are carried out to demonstrate these differences separately.

STARE Observations of a Pc 5 pulsation with large azimuthal wave number

Abstract: In recent years the STARE system has been used to analyze Pc5 pulsations of several different types. In this paper a detailed description of a new type of pulsation is given. The event described occurred during local magnetic afternoon; it had small amplitude (Vertical BarEVertical Bar< or approx. =5mV m/sup -1/ in the ionosphere), a period which varied from 220 to 385 s during the event, and a large azimuthal wave number (mapprox.35) which varied such as to keep the azimuthal phase velocity approximately constant for a given geomagnetic latitude; propagation was geomagnetic westward. When maped into the equatorial plane, the properties of the wave strongly suggest that the wave phase velocity was determined by hot proton drift motions near the equatorial plane. The observational results are compared with theoretical properties of the drift mirror instability. While the results do not contradict the predicted properties, it is felt that further theoretical development is required.

New evidence of a meridional current system in the equatorial ionosphere

Abstract: An interesting variation in the geomagnetic D-component has been found by analysis of the Magsat data. Characteristics of this variation are: that it appears everyday on the low-latitude duskside and is antisymmetric about the dip equator (negative in the northern hemisphere); that the magnitude (from 5 to 25 nT) depends upon the geographic longitude and altitude; and that no corresponding change can be seen on the ground magnetic data. The change suggests a meridional current system in the equatorial ionosphere, produced in association with the electrojet current along the dip equator. This effect should be taken into account in the study of geomagnetic anomaly of internal origin with the Magsat data.

Ion Temperature Variations in the Daytime High-Latitude F Region

Abstract: The Schunk and Sojka (1981a, b) high latitude ionospheric model is improved through the inclusion of thermal conduction and diffusion terms in the ion energy equation, permitting the study of daytime, high latitude F layer temperature variations in a region poleward of the auroral oval. It is found that ion temperature variation with solar cycle, season and geomagnetic activity closely follows the neutral atomic oxygen variation, and that meridional electric fields of more than 40 mV/m can cause larger ion temperature changes than those due to solar cycle, seasonal or geomagnetic activity variations. In the presence of meridional electric fields, there is an upward flow of heat from the lower ionosphere that also acts to raise ion temperatures at high altitudes. Zonal electric fields affect ion temperature indirectly, through electron density changes.

Detransformation of the British geomagnetic secular variation record for Holocene times

Abstract: In this note we replot the British post-glacial secular variation curve in terms of actual values of declination and inclination, rather than the ‘transformed’ declination and inclination system used in our original paper (Turner & Thompson 1981). This should facilitate the comparison of our record with contemporaneous European archaeomagnetic data. Our composite record was obtained by averaging the dated records from 10 cores of lake sediment. The sequence of unit vectors describing the magnetic direction record of each core was rotated so as to place its mean direction (the mean over the section of core dated between 0 and 7000 BP, the time span of the shortest core) at zero transformed declination and inclination (dT = O”, IT = 0”). The transformed coordinate frame of each core is thus derived from its mean direction, assumed to represent the mean geomagnetic field direction over the 7000 yr period, rather than from the core tube and its orientation during coring. Hence the problem of not knowing the azimuthal orientation and inclination of the corer during core collection is avoided, and the 10 records can be averaged in this coordinate system. Transformed coordinates then, provide a convenient, generalised method of plotting averaged sediment records. However, in the archaeomagnetic method, sample orientation is straightforward, and the results can readily be plotted as absolute declination and inclination values. So for consistency when comparing the two, we have attempted here to ‘detransform’ our record. The following procedures, assumptions and approximations were taken to produce the detransformed magnetograms and Bauer-type plots of Figs 1 and 2. (1) Amplitude correction. Although the combination of 10 data sets produces a good estimate of the signature of the geomagnetic variations, the following factors contribute to an overall loss of amplitude in both declination and inclination. (a) The quality of magnetic recording varies widely between different sediments, depending on the porosity of the sediment, the size and shape distributions and the nature of the magnetic carrier (Verosub 1979). The Loch Lomond sediments carry a good, high amplitude pattern of remanent directions, of which the most recent closely follow the London observatory records. The much wetter and less compacted sediments of Llyn Geirionydd, on the

Thermospheric winds at high latitudes from chemical release observations

Abstract: The considered investigation takes into account wind measurements conducted with the aid of 39 sounding rocket chemical release payloads. Attention is given to the geomagnetic dependence, winds and ion convection patterns, subauroral dayside measurements, evening auroral belt measurements, evening polar cap measurements, postmidnight polar cap measurements, midnight and early morning auroral belt measurements, late evening auroral belt measurements, and anomalous winds. The prevailing theme in presenting and interpreting the wind measurements between 160 and 320 km has been that the gross characteristics of the high-latitude wind system can be readily understood in terms of the pattern distribution of electric fields when the inertial properties of the neutral atmosphere and Coriolis forces are taken into account.

Some New Methods in Geomagnetic Field Modeling Applied to the 1960-1980 Epoch

Abstract: The utilization of satellite and surface data together permitted the incorporation of a solution for the anomaly field at each observatory. The residuals of the observatory measurements to such models is commensurate with the actual measurment accuracy. Incorporation of the anomaly estimation enabled the inclusion of stable time derivatives of the spherical harmonic coefficients up to the third derivative. A spherical harmonic model is derived with degree and order 13 in its constant and first time derivative terms, six in its second derivative terms and four in its third derivative terms.

Proton temperature anisotropy in the polar wind

Abstract: The steady state flow of a fully ionized H(+)-O(+) electron plasma along geomagnetic field lines in the high-latitude topside ionosphere is studied. The theoretical formulation is based on a 13-moment system of transport equations, and allows for different species temperatures parallel and perpendicular to the geomagnetic field and nonclassical heat flows. For subsonic and supersonic flows, an appreciable H(+) temperature anisotropy occurs at all altitudes above 1500 km, and tends to be regulated at high altitudes. The direction of the temperature anisotropy is related to the direction of the H(+) heat flow to some extent; for supersonic flow an upward flow of heat from the lower ionosphere is required, while for subsonic flow solutions can be obtained with a downward H(+) heat flow. For subsonic flow, H(+)-H(+) collisions have an important effect on the H(+) stress and heat flow balance at all altitudes between 1500 and 12,000 km.

Helium resonance and dispersion effects on geostationary Alfven/ion cyclotron waves

Abstract: To understand observed structure and growth patterns of geostationary Alfven/ion cyclotron waves, the properties of the hot proton cyclotron instability within a helium rich plasma are explored here. This exploration proceeds with an examination of the net linear wave amplifications that result as a wave propagates through the magnetic gradients of a realistic magnetic field model (linearity is discussed and justified). By taking care in generalizing a single pass model to a multiple pass system, the following conclusions have been reached: (1) The basic structure of the frequency gap that is observed close to the helium cyclotron frequency can be explained by the ‘stop-gap’ dispersion effect; however, the helium cyclotron resonance effect contributes to the gap formation leading to He+ ion energization. (2) The presence of the magnetic gradients virtually insures that helium ion energization (by means of the low frequency wave branch only) is the inevitable consequence of the wave generation process. (3) The energized helium ions will sparsely populate a broad range of geomagnetic latitudes (±15 degrees) but will be concentrated strongly within several (2–4) degrees of the geomagnetic equator. (4) The presence of low percentages of helium ions is most likely to suppress the wave generation process (ATS 6 satellite observations support this conclusion for a portion of the wave spectrum).

Theory and computer simulations of magnetospheric very low frequency emissions

Abstract: A theory of magnetospheric VLF emission must account for the following features: (a) the triggering of monochromatic emissions by signals of sufficient strength and duration, while the background noise and weak short signals are not amplified, and (b) the occurrence of frequency changes after the emissions have reached a sufficiently large amplitude. A nonlinear mechanism exhibiting these features, with fixed and varying frequencies, is examined analytically and by computer simulation techniques. This mechanism depends on a simultaneous propagation and amplification of wave packets along geomagnetic lines to maintain the nonuniformity ratio RproportionaldelB/sub 0//B/sub w/ in the regime Vertical BarRVertical Barroughly-equal0.5, corresponding to maximum amplitication. (B/sub 0/ is the geomagnetic field and B/sub w/ is the wave magnetic field.) For a constant frequency, this condition yields triggering thresholds which are related to the properties of the magnetosphere. For a varying frequency ..omega..(t), it yields the condition partial..omega../partialtproportional..omega../sup 2//sub t/ for the large-amplitude portion of the risers, where ..omega../sub t/proportionalB/sup 1/2//sub w/ denotes the trapping frequency of the wave.

Geomagnetic Variation and Field-Aligned Currents at Northern High-Latitudes, and their Relations to the Solar Wind Parameters

Abstract: By using statistical analysis to study the relation between the interplanetary and geomagnetic fields, geomagnetic variations have been detected that are controlled by different parameters of the solar wind. The equivalent current systems for these geomagnetic variations have been determined for the 1968 summer and winter seasons from data from 14 high latitude stations. It has been shown that the geomagnetic field at high latitudes ($\Phi \geq $ 60 degrees) can be represented by a sum of fields controlled by components of the Interplanetary Magnetic Field (I.M.F.) and by the velocity and density of the solar wind plasma. A correlation model is proposed by means of which hourly-mean values of the three components (X, Y, Z) of the geomagnetic field vector at high latitudes may be determined from hourly-mean values of the components B$\_{x}$, B$\_{y}$ and B$\_{z}$ of the I.M.F. The space--time distribution of field-aligned currents has been reconstructed from the horizontal component of the ground level geomagnetic field variation. The correlation model of field-aligned currents has been obtained for high latitudes ($\Phi \geq $ 60 degrees). According to the model the value and direction of the field-aligned current j$\_{\parallel}$ are determined by I.M.F. components and spatial coordinates. The proposed model of field-aligned currents enables the nature of the actual magnetic disturbance registered during a given pass of the TRIAD and ISIS-2 satellites through the high latitude region to be interpreted. Moreover it enables some discrepancies between observations and the models of field-aligned currents proposed in the literature to be understood.

Energy coupling between regions 1 and 2 Birkeland current systems

Abstract: The principle of ring current shieldding and the concept that in an electrical circuit analogy the ring current can be treated as a pseudo-Hall current flowing in parallel with the ionospheric Hall current are used here to develop a simple model for the electrical coupling between the applied potential Φ associated with the Region 1 currents, and the ring current, associated with the Region 2 currents. The approach yields a formula for the radial position of the inner edge of the ring current as a function of Φ, which agrees with similar formulas derived previously on the basis of different approaches. The model can be expressed in the language of electrical circuit theory as an applied potential Φ driving a nonlinear RL circuit, in which the resistance R results from dissipation in the ionosphere and the inductance L from the energy of compression stored in the ring current. The formulation lends itself to a determination of the relative amounts of energy that are dissipated in the ionosphere and stored in the ring current, according to the applied potential model. If the shielding time scale L/R and the energization (i.e., substorm) time scale are comparable, as observations indicate, roughly equal amounts of energy are expended on both elements. The analysis shows that AE type geomagnetic indexes and the Dst index should bear different functional relationships to that combination of solar wind parameters that is primarily responsible for geomagnetic activity.

A dynamic model of the magnetospheric magnetic and electric fields for July 29, 1977

Abstract: A quantitative vector representation of the magnetospheric field was developed specifically for the complex event of July 29, 1977. The magnetic field associated with each of the three major magnetospheric current systems was ‘controlled’ by solar wind and interplanetary field parameters and a magnetic index. This dynamic field model was compared with observed field data and found to be in good agreement. The procedures developed for quantitatively modeling time varying magnetospheric phenomena are general, and eventually it should be possible to use them to ‘predict’ some aspects of magnetospheric behavior.