Showing papers on "Dipole model of the Earth's magnetic field published in 1983"

Effects of drift on the transport of cosmic rays. VI - A three-dimensional model including diffusion

Abstract: The first results are presented from a series of computer simulations of the solar modulation of galactic cosmic rays using a full three-dimensional model which incorporates all known important effects on particle transport, particle drifts, convection with the solar wind, energy loss, and anisotropic diffusion. The model is time-independent in the coordinate frame rotating with the sun, so corotating effects can be studied. Modulation in an interplanetary magnetic field model is considered in which the current sheet separating the northern and southern solar hemispheres is warped and corotating with the sun. The amplitude of the warp is varied to simulate possible solar cycle variation of the magnetic field. Substantial effects due to the warp of the current sheet are found. Comparison of the model results with various data is presented. Among other things, it is shown that the intensity may decrease away from the current sheet for both signs of the magnetic field, as suggested by recent observations, and in contrast with inferences from earlier, more approximate calculations.

The structure of the heliospheric current sheet: 1978–1982

Abstract: The structure of the heliospheric magnetic field changes substantially during the 11 year sunspot cycle. Its configuration for the period 1976 through 1982 using a potential field model was calculated. The structure during the rising phase, maximum, and early decline of sunspot cycle 21, from 1978 to 1982 is considered.

An empirical electric field model derived from Chatanika radar data

Abstract: Plasma convection velocity observations obtained with the Chatanika, Alaska incoherent scatter radar have been used to prepare a quantitative empirical model of the convection electric field at auroral latitudes between 58°Λ and 75°Λ. Vector averaging of the individual days' data and smoothing across 2.5 hours of local time were used to reduce substorm perturbations. Tables of the vector electric field components at 300-km altitude have been prepared for moderately disturbed (Kp = 3) summer conditions. Resolution of the model is 0.5° of invariant latitude and one half hour of magnetic local time. The model field is asymmetric with the afternoon convection cell dominant in magnitude and spatial extent and corresponds to conditions in which the cross polar cap potential is approximately 70 kV.

The Earth’s Ring Current: Causes, Generation, and Decay

Abstract: The development of currents due to arbitrary distributions of trapped particles in the geomagnetic field is described These currents form the Earth’s ring current and are responsible for world wide decreases of the surface magnetic field observed during magnetic storms It is shown that we do not yet know the relative abundances of the ions forming the ring current Because of this we do not understand how various sources mix to produce the ring current Several possible generation mechanisms are discussed Finally, the decay of the ring current is discussed and is shown to be due primarily to charge exchange with important secondary effects attributable to wave-particle interactions

Doubly diffusive magnetic buoyancy instability in the solar interior

Abstract: An investigation of the buoyancy of diffuse magnetic fields has shown that in the presence of rotation, static equilibrium configurations of the toroidal magnetic field and ambient plasma can exist In that case, the escape of toroidal magnetic flux from the solar interior may be determined by the growth of instabilities which the equilibrium configuration may be subject to In connection with the present investigation, it is assumed that in the region of toroidal magnetic flux amplification, the magnetic field has not as yet filamented into flux ropes, and is therefore 'diffuse' A study is conducted of the MHD stability of an electrically conducting and differentially rotating gas in the presence of a toroidal magnetic field, an external constant gravitational field, and radiance pressure The full dispersion relation for the magnetic buoyancy problem is developed, and the solutions of the dispersion relation are discussed

Radio studies of the main F region trough in Antarctica

Abstract: Case studies of the equatorward movement of the main F region trough are presented, using data recorded by a digital ionospheric sounder located at Halley Bay (76°S, 27°W; Λ = 618, L = 42) Three events were chosen to give a range of magnetic conditions and local times Comparison of the results, projected into the equatorial plane, with theoretical predictions of the simple ‘teardrop’ model of the plasmapause, shows remarkably good agreement for the two occasions when the level of magnetic activity remained approximately constant over the observing period Consequently, estimates of the strength of the dawn-dusk convection electric field in the magnetosphere have been made However, magnetic activity changed during the course of the other event and the comparison gave poor results The main F region trough results are also compared with the predictions of contemporary empirical equations which express the invariant latitude of the trough minimum as a function of local time and magnetic index, Kp, based on global satellite surveys It is shown that none of these equations adequately accounts for the observed equatorward motion of the trough for these three cases In particular, they contain insufficient allowance for the observed strong dependence of equatorward velocity upon magnetic activity Some suggestions upon possible improvements to the empirical models are made which, if implemented, would be of great value for improving the reliability of predictions for high frequency, transauroral radio wave communications

Correlated particle and magnetic field observations of a large‐scale magnetic loop structure behind an interplanetary shock

Abstract: From a survey of observations on ISEE-3, an example of correlated particle and magnetic field observations of a large-scale magnetic loop structure is presented. Bidirectional proton fluxes were observed for a period of 40 hours in the energy range 35-1600 keV approximately 12 hours after the passage of the interplanetary shock of December 11, 1980, and directly after the passage of a discontinuity. For each of the eight logarithmically spaced energy channels, a three-dimensional anisotropy analysis reveals streaming along both directions of the magnetic field. The magnetic field rotated slowly but steadily through approximately 180 deg during this same 40-hour period; this is consistent with the existence of a large-scale loop with extent greater than 0.5 AU. The observations suggest that the particles are being injected into the loop sunward of the spacecraft; they appear as bidirectional fluxes in the outermost regions of the loop arising from a combination of focusing and near scatter-free transport.

Observations of hydromagnetic turbulence in the solar wind

Abstract: MHD turbulence is studied by analyzing magnetic field and plasma observations from Helios-1 and -2 at minimum solar activity. The steady conditions in the plasma flows and the magnetic field sector structure in 1975/1976 facilitate an investigation of the radial evolution of the turbulence from 0.29 to 1AU. In high speed streams the fluctuations in the solar wind velocity v and the magnetic field b are highly correlated (the correction coefficient almost being one), which indicates that the turbulence is mainly Alfvenic in high speed plasma. While some general fluctuation properties remain essentially unchanged from 0.29 to 1AU, power spectral analysis reveals a different frequency composition of the Alfvenic turbulence at different heliocentric distances. At 0.3AU much more 'high' frequency fluctuations contribute to the total power in the magnetic field and velocity fluctuations than at 1AU. The contributions of field magnitude fluctuations are found to be distance and frequency dependent. Magnetic field spectra with an extended frequency range up to 470Hz show certain frequency bands, where the steepness of the spectra is independent of the helicocentric distance.

Mass upflows in 'post'-flare loops

Abstract: A self-consistent numerical model of a reconnecting magnetic field configuration similar to that occurring during the main-phase of two-ribbon flares is used to estimate the upflow caused by the fast-mode expansion of the magnetic field moving into the reconnection region. Such an expansion creates a field-aligned pressure gradient which accelerates plasma upward from the chromospheric base of magnetic field lines in the region external to the loops. The numerical results imply that the amount of mass sucked up in this way is even smaller than was previously estimated by Kopp and Pneuman who used a kinematic model. Therefore, some indirect mechanism (such as evaporation), which would probably derive its motive power from the thermal energy generated by the reconnection, is required to explain the large mass upflows inferred from observations.

Equivalent Source Modeling of the Core Magnetic Field Using Magsat Data

Abstract: Experiments are carried out on fitting the main field using different numbers of equivalent sources arranged in equal area at fixed radii at and inside the core-mantle boundary. In fixing the radius for a given series of runs, the convergence problems that result from the extreme nonlinearity of the problem when dipole positions are allowed to vary are avoided. Results are presented from a comparison between this approach and the standard spherical harmonic approach for modeling the main field in terms of accuracy and computational efficiency. The modeling of the main field with an equivalent dipole representation is found to be comparable to the standard spherical harmonic approach in accuracy. The 32 deg dipole density (42 dipoles) corresponds approximately to an eleventh degree/order spherical harmonic expansion (143 parameters), whereas the 21 dipole density (92 dipoles) corresponds to approximately a seventeenth degree and order expansion (323 parameters). It is pointed out that fixing the dipole positions results in rapid convergence of the dipole solutions for single-epoch models.

Birkeland Currents: Present Understanding and Some Remaining Questions

Abstract: Kristian Birkeland determined that large-scale ionospheric currents were associated with the aurora. He deduced this from his surface magnetic field measurements in the polar regions at the beginning of the twentieth century. He suggested that these “auroral currents” originated far from the Earth and that they flowed into and away from the polar atmosphere along geomagnetic field lines. The existence of such field-aligned or Birkeland currents was widely disputed because it was not possible to unambiguously identify current systems that are field-aligned (Alfven, 1939; 1940) and those which are completely contained in the ionosphere (Vestine and Chapman, 1938) only from a study of surface magnetic field measurements. Observations acquired from a variety of rocket and satellite experiments have absolutely confirmed the presence of Birkeland currents and have demonstrated the important role that these intense currents (ranging between 106 and 107 amperes) play in the coupling of energy between interplanetary space and the lower atmosphere and ionosphere. The Birkeland currents are a critical ingredient in a variety of plasma processes on this planet (associated with aurora and radio emissions), on Jupiter (associated with Io-related radio emissions), and in our galaxy (related to comets and double radio sources, e. g., Alfven, 1981). Considerable effort has been directed, during the past number of years, in establishing the statistical characteristics of Birkeland currents. Some of this past work will be reviewed here and some major remaining questions will be posed.

Reflection of the solar wind ions at the Earth's bow shock: Energization

Abstract: The energies of the field-aligned proton beams observed upstream of the earth's bow shock are tested, on a statistical basis, against a simple reflection model. The comparison is carried out using both plasma and magnetic field data collected by the ISEE 2 spacecraft. The observations refer to the period from November 5 to December 20, 1977. According to this model, some of the solar wind protons incident upon the earth's shock front when reflected upstream gain energy by displacement parallel to the interplanetary electric field. The energy gained in the reflection can be described as a function of the angles between the interplanetary magnetic field, the solar wind bulk velocity, and the local shock normal. The task of finding these angles, i.e., the expected source point of the reflected ions at the earth's shock front, has been resolved using both the measured magnetic field direction and the actual beam trajectory. The latter method, which takes into account the ion drift velocity, leads to a better agreement between theory and observations when far from the shock. In particular, it allows us to check the energies of the field-aligned beams even when they are observed far from the earth's bow shock (at distances up to 10–15 RE). We confirm, on a statistical basis, the test of the model recently carried out using the Los Alamos National Laboratory/Max-Planck-extraterrestrische observations on ISEE 1 and 2. We infer that reflected beams can sometimes propagate far upstream of the earth's bow shock without changing their energy properties.

Role of the pressure anisotropy in the relativistic pulsar wind

Abstract: We generalize the hot relativistic MHD wind analysis to include the anisotropy of the pressure created in the pulsar wind by the strong magnetic field. Even with anisotropy the relativistic MHD equations integrate. In a very intense magnetic field, the motion of relativistic particles becomes rapidly one-dimensional in the direction of the field due to the very important radiative losses. Consequently, their distribution function becomes also one-dimensional and the component of the pressure, in the direction perpendicular to the magnetic field, decrease. In the limitP⊥≃0,P∥≠0 we obtain a solution for the fluid flow which, starting at the neutron star surface, reaches smoothly infinity.

On the Origin and Structure of Stellar Magnetic Fields

Abstract: Newly formed stars have magnetic fields provided by the compression of the interstellar field, and contrary to a widely accepted idea these fields are not destroyed by convective motions. For the same reason, the fallacy of ‘turbulent diffusion’, turbulent dynamo action is not possible in any star. Thus all stellar magnetic fields have a common origin, and persist throughout the lifetime of each star, including degenerate phases. This common origin, and a general similarity in stellar evolutionary processes, suggest that the fields may develop similar structural characteristics and MHD effects. This would open new possibilities of coordinating the studies of different types of stars and relating them to solar physics which has tended to become isolated from general stellar physics. As an initial step we consider three features of solar magnetic fields and their MHD effects. First, the solar magnetic field comprises two separate components: a poloidal field and a toroidal field. The former is a dipole field, permeating the entire Sun and closely aligned with the rotational axis; at the surface it is always concealed by much stronger elements of the toroidal field. The latter is probably wound from the former by differential rotation at latitudes below about 35°, where sections emerge through the solar surface and are then carried polewards. The second feature of solar magnetic fields is that all flux is concentrated into flux tubes of strength some kG, isolated within a much larger volume of non-magnetic plasma. The third feature is that the flux tubes are helically twisted into flux ropes (up to ≳1022Mx) and smaller elements ranging down to flux fibres (≲ 1018Mx). Some implications of similar features in other stars are discussed.

Dependence of the High-Latitude Ionospheric Fields and Plasma Characteristics on the Properties of the Interplanetary Medium

Abstract: It has now been 16 years since Fairfield and Cahill (1966) and Rostoker and Falthammar (1967) provided convincing evidence that, as predicted by Dungey (1961), the interplanetary magnetic field (IMF) modulated the transfer of energy from the solar wind to the magnetosphere. In particular, it was evident that the transfer function for the solar-terrestrial interaction was dependent on the direction of the IMF with respect to magnetopause field lines and, to some extent on the solar wind velocity. Rostoker and Falthammar (1967) suggested that the component of the interplanetary electric field in the ecliptic plane and normal to the sun-earth line (Eϕ = -vsBz where Vs is the solar wind velocity and Bz the component of the IMF parallel or antiparallel to magnetopause field lines) was the best quantitative indicator of the energy transfer and even at the present time it is favored by some researchers dealing with the solar-terrestrial interaction (e.g. Clauer, 1981). In recent years several researchers have attempted to utilize various combinations of solar wind parameters to predict magnetospheric activity levels (as quantified using ap and AE by Garrett (1973) and using AE by Murayama and Hakamada (1975) amongst others).

Simple models for magnetic flux tubes

Abstract: Known potential field solutions can be used to model the structure of magnetic fields in the solar photosphere. Several two-dimensional and axisymmetric solutions are compared. In the most satisfactory model the vertical component of the field is prescribed on a horizontal plane so as to be uniform within a finite disc and zero outside it. The resulting flux distribution provides a good description of small scale intergranular magnetic fields and of the observed field structure in a pore, but is inadequate for sunspots.

On the height scale of magnetic fields above sunspots derived from RATAN-600 observations

Abstract: Model calculations of the S-component are compared with observations of the RATAN-600 telescope at five discrete microwave frequencies referring to active region McMath No. 15974 on May 1, 1979. The spectral variations of source diameter, flux density, and degree of polarization are used to derive the height scale of the magnetic field in accordance with a magnetic dipole distribution under the assumption of advanced temperature and electron density distributions according to most recent EUV observations.

The non-axisymmetrical configuration of a large-scale solar and stellar magnetic field

Abstract: The non-axisymmetric and nonlinear solutions of the magnetostatic equations are given in three-dimensional space of spherical coordinates (r, θ, ϕ). These solutions are applied to the large-scale solar magnetic field. Their basic features are similar to a dipole field near the polar regions and the polarity reverses near the equator. These features agree with observations for the large-scale solar magnetic field. The solutions can also be applied to investigating the connection between the structure of the magnetic field and the density distribution of the corona. It is shown that the tops of the closed magnetic field associate with density enhancements.

Theory of quadrupolar sunspots and the active region of August, 1972

Abstract: It is pointed out that energy is stored when the force-free magnetic field in an active region departs from a potential field, the departure manifesting itself as a shear in the field. When the field untwists, energy will immediately be released to produce flares. On the basis of this idea, an analytical solution of the equation of a force-free field is derived under the assumption of a constant force-free factor, and expressions are found for seven important quantities for quadrupole sunspots. These are the magnetic energy of the twisted field, the magnetic energy of the potential field, the extractable free energy (delta M), the magnetic flux, the total current, the force-free factor and the field decay factor. The quantities are expressed in terms of three observables: the field intensity, the twist angle and the distance between two spots of the same polarity. The expression for delta M may have applicability in solar prediction work. For the active region of August 1972, delta M is found to be up to 6 x 10 to the 32nd erg, which is sufficient to supply the energy of the observed flare activity.

Dynamics of the Preflare Magnetic Field

Abstract: Many theories of the solar flare process invoke storage of energy in the active region magnetic field above the solar photosphere. Observational evidence relating to such storage is rather unsatisfactory owing to our inability to observe the three-dimensional structure of the magnetic field. Indirect evidence comes from changes in structures presumed to trace the magnetic field, from changes in the line-of-sight and transverse components of the photospheric magnetic field, from mass flow patterns observed by proper motions and line-of-sight Doppler shifts, and from radio observations. These data tend to confirm that energy sufficient to produce flares can be stored in active region magnetic fields with a characteristic time scale of hours but critical observation are not yet available.

Particle-Loss Reduction of Laser-Produced Anisotropic Plasmas by Means of a Uniform-Multipole Cusp Composite Magnetic Field Configuration

Abstract: In connection with the problems of filling a relatively large magnetic container with a clean laser-produced hot plasma, a plasma blob with initial spatial anisotropy is produced within a uniform as well as a uniform-multipole cusp composite magnetic field configuration, and its behavior is observed in some detail The results indicate that (1) there exist two separate physical mechanisms of particle escape across magnetic field, referred to as "plasmoid-type" and "drift-type," and (2) both types of particle losses may be drastically reduced or suppressed by the (local) employment of the composite field The results of approximate theoretical analysis on the basis of MHD-equations show good quantitative agreement with the experimental ones, and the magnetic-shear and/or spatial-gradient of magnetic field strength in the composite field are identified to be very effective for the reduction or suppression of these particle losses

Equatorial propagation of axisymmetric magnetogasdynamic shocks with increasing energy, I

Abstract: A self-similar solution, for the equatorial propagation of axisymmetric point explosion into an inhomogeneous ideal gas permeated by a current-free azimuthal magnetic field, are obtained. The model has been considered here in which the magnetic field is proportional tor−1, but the total energy of the wave is of the increasing order, not constant.

Influence of a dipole magnetic field on the topology of toroidal magnetic configurations around a gravitating body

Abstract: The topological structure of the toroidal magnetic field, which is affected by a dipole magnetic field, is studied. It is shown, that a dipole magnetic field is able to split the initial toroidal configuration into two toroids and, at a certain critical value of the magnetic dipole, can completely destroy the toroidal configuration. The dependence of the total mass excess on the dipole magnetic field is found.

Removal of velocity bias in the interpretation of measurements of the azimuthal component of the interplanetary magnetic field

Abstract: Parker's model for the interplanetary magnetic field predicts an r−1 dependence for the azimuthal component. Departures from this relation were indicated in an analysis of a subset of the Helios 1 data. As in a previous analysis of Pioneer 10 data, these departures can be traced to velocity bias. We examine the decrease in solar wind speed which occurred as Helios 1 approached the sun and thus simulated a steeper gradient of the azimuthal magnetic field component. A simple linear fit between flow speed and distance from the sun enables us to remove from the data most of the velocity bias present. This procedure substantially improves the agreement between Parke's model and the Helios data.

Bremsstrahlung Rates in Fully Ionized Gases in a Magnetic Field

Abstract: The bremsstrahlung rates in fully ionized gases in a magnetic field are theoretically investigated. Here it is assumed that the charged particles have the Maxwell velocity distributions, and that an external electric field does not exists. In the theoretical treatment, the followings are considered: When the Larmor radius is larger than the radius of the collisional cross section due to the bremsstrahlung, the radiation of energy during a short time at an instant of the collision is not affected by the magnetic field. However, according to the gyration, the relative velocity between an electron and a colliding ion during the free path is affected by the magnetic field strength. Numerical calculations show that the bremsstrahlung rate in the presence of the magnetic field is smaller than that in the absence of the magnetic field.

Photospheric Sources of Magnetic Field Aligned Currents

Abstract: Possible (small-scale) photospheric sources of coronal magnetic field aligned currents are discussed. Such currents are equivalent to local (small-scale) twists of the coronal magnetic field, and may cause field topologies that are (MHD or resistively) unstable, and thus contribute to the (small-scale) coronal activity.