Time-dependent magnetospheric electric fields have been computed, including the effects of Birkeland currents from the inner edge of a sheet of ions that moves under the influence of the computed electric fields. Ionospheric currents are also taken self-consistently into account with the use of a time-independent model of ionospheric conductivity that includes day-night asymmetry and auroral enhancements but neglects electric fields parallel to the magnetic field. Ion precipitation and neutral winds are also neglected. The behavior and effects of the ion sheet are studied in a series of model calculations, with the following results. (1) In agreement with the conclusions of E. T. Karlson, L. P. Block, V. M. Vasyliunas, and D. W. Swift, Birkeland currents from the Alfven layer (inner edge of the ion sheet) are found to reduce the electric field earthward of the Alfven layer to a small value by the time a steady state is reached; however, different parts of the electric field earthward of the Alfven layer are eliminated at different rates; one component of the nightside field relaxes to near its low asymptotic value in a few minutes, whereas the dayside field takes hours to relax. (2) If the ion sheet is brought in from the tail by a large cross-tail electric field that stays large, the inner edge of the sheet generally touches the magnetopause boundary layer; however, a decrease in the cross-tail field can cause the Alfven layer to contract and form a complete ring. (3) For the parameters used, a cross-tail potential of 134 kv will bring a sheet of ring current protons in to about L = 4; the results of the model calculations support the idea that convection electric fields bring low-energy ions in from the tail to form the storm time ring current; the minimum geocentric distance to which convection fields can bring the ions is found to be roughly proportional to (ημ/Φ0σρ)1/3, where η is the number of ions per unit magnetic flux, μ is the ion magnetic moment, Φ0 is the cross-tail potential, and σρ is an average height-integrated Pedersen conductivity on the dayside. (4) The nightside Alfven layer naturally produces a dividing line near local midnight, such that particles arriving west of the line drift to the west, whereas those arriving east of the line drift east; this characteristic is often observed in motions of barium clouds and auroral arcs.

Self‐consistent calculation of the motion of a sheet of ions in the magnetosphere

The electron–cyclotron maser is a process that generates coherent radiation from plasma. In the last two decades, it has gained increasing attention as a dominant mechanism of producing high-power radiation in natural high-temperature magnetized plasmas. Originally proposed as a somewhat exotic idea and subsequently applied to include non-relativistic plasmas, the electron–cyclotron maser was considered as an alternative to turbulent though coherent wave–wave interaction which results in radio emission. However, when it was recognized that weak relativistic corrections had to be taken into account in the radiation process, the importance of the electron–cyclotron maser rose to the recognition it deserves. Here we review the theory and application of the electron–cyclotron maser to the directly accessible plasmas in our immediate terrestrial and planetary environments. In situ access to the radiating plasmas has turned out to be crucial in identifying the conditions under which the electron–cyclotron maser mechanism is working. Under extreme astrophysical conditions, radiation from plasmas may provide a major energy loss; however, for generating the powerful radiation in which the electron–cyclotron maser mechanism is capable, the plasma must be in a state where release of susceptible amounts of energy in the form of radiation is favorable. Such conditions are realized when the plasma is unable to digest the available free energy that is imposed from outside and stored in its particle distribution. The lack of dissipative processes is a common property of collisionless plasmas. When, in addition, the plasma density becomes so low that the amount of free energy per particle is large, direct emission becomes favorable. This can be expressed as negative absorption of the plasma which, like in conventional masers, leads to coherent emission even though no quantum correlations are involved. The physical basis of this formal analogy between a quantum maser and the electron–cyclotron maser is that in the electron–cyclotron maser the free-space radiation modes can be amplified directly. Several models have been proposed for such a process. The most famous one is the so-called loss-cone maser. However, as argued in this review, the loss-cone maser is rather inefficient. Available in situ measurements indicate that the loss-cone maser plays only a minor role. Instead, the main source for any strong electron–cyclotron maser is found in the presence of a magnetic-field-aligned electric potential drop which has several effects: (1) it dilutes the local plasma to such an extent that the plasma enters the regime in which the electron–cyclotron maser becomes effective; (2) it generates energetic relativistic electron beams and field-aligned currents; (3) it deforms, together with the magnetic mirror force, the electron distribution function, thereby mimicking a high energy level sufficiently far above the Maxwellian ground state of an equilibrium plasma; (4) it favors emission in the free-space RX mode in a direction roughly perpendicular to the ambient magnetic field; (5) this emission is the most intense, since it implies the coherent resonant contribution of a maximum number of electrons in the distribution function to the radiation (i.e., to the generation of negative absorption); (6) it generates a large number of electron holes via the two-stream instability, and ion holes via the current-driven ion-acoustic instability which manifest themselves as subtle fine structures moving across the radiation spectrum and being typical for the electron–cyclotron maser emission process. These fine structures can thus be taken as the ultimate identifier of the electron–cyclotron maser. The auroral kilometric radiation of Earth is taken here as the paradigm for other manifestations of intense radio emissions such as the radiation from other planets in the solar system, from exoplanets, the Sun and other astrophysical objects.

/pdf/the-electron-cyclotron-maser-for-astrophysical-application-d6r0ddgn6s.pdf

The electron–cyclotron maser for astrophysical application

The European Space Agency's Cluster programme is designed to study the small-scale spatial and temporal characteristics of the magnetospheric and near-Earth solar wind plasma. The programme is composed of four identical spacecraft which will be able to make physical measurements in three dimensions. The relative distance between the four spacecraft will be varied between 200 and 18000 km during the course of the mission. This paper provides a general overview of the scientific objectives, the configuration and the orbit of the four spacecraft and the relation of Cluster to other missions.

Cluster - Science and Mission Overview

A classification system based on dynamic power spectra of AMPTE/CCE magnetic field data has been developed which allows semiquantitative specification of pulsation activity at all times. Fifteen months of 6.24-s AMPTE/CCE magnetic field data have been analyzed using this scheme to generate a comprehensive data base of all pulsation activity observed from L = 5 to L = 9 at all local times in the frequency range 0-80 mHz. The pulsation divide naturally into five basic categories: toroidal fundamental mode resonances, toroidal harmonic resonances, compressional low-frequency waves, radially polarized waves, and disturbed intervals. Additional distinctions among members of these classes are made to arrive at 14 categories which describe 95 percent of the pulsations observed. Normal spatial distributions in local time and L shell are determined for each category. Magnetic latitude distributions are determined for four coherent categories of activity.

A statistical study of Pc 3–5 pulsations observed by the AMPTE/CCE Magnetic Fields Experiment, 1. Occurrence distributions

A model of auroral arcs is presented. It is assumed that the magnetic field lines into the auroral oval are loaded with kev electrons, that precipitating auroral electrons locally constitute a net field-aligned current, and that currents close in the outer magnetosphere by polarization currents. The model predicts that a flux tube convecting through the oval will undergo a highly nonlinear oscillation that produces standing waves, the auroral arcs. The conditions to be satisfied are (1) Ohm's law in the ionosphere, (2) electron number conservation in the ionosphere, (3) electric current continuity, (4) the tendency for electric fields to map between ionosphere and magnetosphere. The physics of condition 4 is not understood, but it is probable that arc-like solutions exist for several models. A specific model, including perfect mapping of electric fields, is studied. The solutions to this model closely resemble the arcs. Predicted in agreement with observation are arc thickness, inter-arc spacing, and the electric field behavior. The thickness and the spacing are functions of both the amplitude of the oscillation and the physical properties of the system. However, thickness times spacing is a function of the physical properties only, and the thickness divided by the spacing is a function of amplitude only. Presumably these and other quantitative relationships can be checked by observations.

Auroral arcs: Result of the interaction of a dynamic magnetosphere with the ionosphere

In April 1967 five Nike-Apache rockets were launched from Kiruna, Sweden, carrying barium release and electron concentration experiments. The component of the electric field transverse to the magnetic lines of force is derived from the motion of the ion clouds. The electron concentration measurements provide ionospheric conductivities. The magnitudes of the electric fields varied between 2 and 20 v/km. Their directions were northwestward for evening conditions and southwestward for morning conditions. The magnetic perturbations on the ground that would result from ionospheric currents derived on the basis of the measured fields and conductivities closely resemble the actual ground magnetometer data. The electric fields vary appreciably with space and time; they can even reverse their directions over distances of the order of 60 km.

Preliminary results of electric field measurements in the auroral zone

In July last year and in December 1984, barium clouds were injected into the solar wind from the AMPTE satellite. The clouds resembled those of natural comets in that a head and tail were clearly visible. Tail rays formed with speeds of a few tens of kilometres per second, and other structures separated at even higher speeds. The comet head, instead of moving slowly downstream, performed a lateral excursion of several hundred kilometres. In the first of a series of six articles, the properties of the ‘artificial comet’ are described.

The AMPTE artificial comet experiments

The Electron Drift Instrument (EDI) measures the drift of a weak beam of test electrons that, when emitted in certain directions, return to the spacecraft after one or more gyrations. This drift is related to the electric field and the gradient in the magnetic field, and these quantities can, by use of different electron energies, be determined separately. As a by-product, the magnetic field strength is also measured. The present paper describes the scientific objectives, the experimental method, and the technical realization of the various elements of the instrument.

The electron drift instrument for cluster

Using data from the GEOS 2 electron gun experiment, we have constructed average patterns of the convective flow of plasma in the equatorial plane at L = 6.6 between 0600 and 2100 LT and for Kp ranging between 0 and 4. During very quiet conditions (Kp = 0–1) the plasma flows predominantly westward with an outward component before noon and an inward component thereafter. The most likely dominant agent for this kind of convection is the atmospheric dynamo electric field. For moderate activity (Kp = 3–4) the plasma flows as predicted by a Volland-Stern-type dawn-to-dusk cross-tail electric field model (if the atmospheric dynamo effect is subtracted) with a shaping factor close to 3 and the axis of symmetry rotated toward earlier local times. The shielding of the cross-tail electric field from the inner magnetosphere seems to be somewhat weaker on the dayside than on the eveningside. During times of Kp = 2 sunward convection and atmospheric dynamo effect seem to compete, i.e., sunward convection prevails, but our average pattern apparently is still substantially influenced by the atmospheric dynamo field.

Magnetospheric convection observed between 0600 and 2100 LT: Variations with Kp

One hundred eighty-six days of electric field data from the GEOS 2 electron beam experiment have been used to study magnetospheric fluctuations at geostationary orbit with periods between 150 and 600 s. While fluctuations are nearly always present in the electric field data from the dayside magnetosphere with typical amplitudes between 0.2 and 0.5 mV/m, it is often hard to find well-defined concurrent pulsations in the GEOS 2 magnetic field data. Most events occur near noon and have the same characteristics: They are toroidal and nearly linearly polarized, the sense of polarization and the orientation angles of the polarization ellipses change sign near noon, the instantaneous frequency of the fluctuations is correlated with the instantaneous electron density, and in a given sector of the magnetosphere the sense of polarization depends on the frequency. There is strong evidence that these fluctuations are fundamental mode eigenoscillations of field lines in the vicinity of the spacecraft which are generated in the inhomogeneous plasma of the magnetosphere by some kind of solar wind-driven surface waves at the magnetopause or at the low-latitude boundary layer.

F. Melzner

Papers

Preliminary results of electric field measurements in the auroral zone

The AMPTE artificial comet experiments

The electron drift instrument for cluster

Magnetospheric convection observed between 0600 and 2100 LT: Variations with Kp

A statistical study of dayside magnetospheric electric field fluctuations with periods between 150 and 600 s