Penetration of low‐energy protons deep into the magnetosphere
TL;DR: In this paper, a simple model with constant convection electric field and a dipole magnetic field was used to calculate the plasma flow patterns with the convection fields as scale factors, which indicated that protons of a few hundred electron volts convected in from the tail to L = 3-4 could be responsible for the storm-time ring currents.
Abstract: By using a simple model with constant convection electric field and a dipole magnetic field, plasma flow patterns are calculated with the convection fields as scale factors. Unlike other particle trajectories, the flow patterns for protons with certain relative magnetic moments show double forbidden regions: one is composed of orbits that circle the earth; the other is composed of orbits that do not circle the earth. These protons can penetrate very close to the earth through the space between the two forbidden regions. The calculations based on the model of constant electric field with charge exchange as a loss mechanism indicate that protons of a few hundred electron volts convected in from the tail to L = 3–4 could be responsible for the storm-time ring currents.
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TL;DR: In this paper, the effects of Birkeland currents from the inner edge of a sheet of ions that moves under the influence of the computed electric fields were investigated, and 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.
Abstract: 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.
410 citations
TL;DR: In this article, a transition from trapped proton trajectories to open trajectories leading to the tail of the earth was shown to occur at about 10 kev, the precise value depending upon local time.
Abstract: A proton of low energy moving in the equatorial plane of the earth will experience drift motions due to both the magnetic field (magnetic gradient drift only, if the field is assumed to be that of a dipole) and the electric field The electric drift again separates into two parts - the drift due to the main electric field (or convection electric field) existing in the frame of the earth, and that due to the earth's rotation One result indicated by this work is that at distances of 4 - 6 earth radii, a transition from trapped proton orbits to open trajectories leading to the tail occurs at about 10 kev, the precise value depending upon local time Such a transition also seems to be indicated by particle observations using Explorer 45 The energy spectrum (at magnetically quiet times) of equatorial protons above this energy can be explained by charge exchange but increased flux observed below it seems to be related to the influx of particles on open orbits from the tail
365 citations
TL;DR: Many of the significant theoretical advances in understanding the origin and behaviour of low frequency hydromagnetic waves originating in the magnetosphere in the last decade are reviewed in this paper, including wave generation mechanisms, wave damping, effects of inhomogeneity, signal behaviour in the ionosphere and atmosphere.
Abstract: Many of the significant theoretical advances in understanding the origin and behaviour of low frequency hydromagnetic waves originating in the magnetosphere in the last decade are reviewed. Topics covered include wave generation mechanisms, wave damping, effects of inhomogeneity, signal behaviour in the ionosphere and atmosphere.
299 citations
TL;DR: In this article, the equations for conservation of ionospheric currents are used to deduce theoretical flow patterns, and the currents caused by the pressure of magnetospheric plasma are neglected.
Abstract: Convective flow of plasma in the magnetosphere is apparently driven by the interaction between the solar wind and the magnetosphere, but the flow pattern is regulated by the ionosphere and by pressure gradients in the magnetospheric plasma. The equations for conservation of ionospheric currents are used here to deduce theoretical flow patterns. The currents caused by the pressure of magnetospheric plasma are neglected. When all merging and friction processes are assumed to take place in the tail, and the dayside magnetopause is assumed to be an equipotential, computed trajectories of plasmasphere particles generally exhibit bulges in the dusk-to-midnight sector; if the conductivity model includes a sharp drop in conductivity at sunset, the computed bulge has a sharp onset near local sunset. When convection is assumed to be caused by merging or some other friction process operating near the nose of the magnetosphere, the outer-plasmasphere trajectories have pronounced bulges in the dusk-to-midnight sector, with sharp onset near local sunset, if the conductivity model includes a sharp drop in conductivity at local sunset and a band of substantially enhanced Pedersen and Hall conductivities in the auroral zone. Vasyliunas' observations indicate that the plasma sheet is generally well defined throughout the evening and afternoon sectors of the magnetosphere, to about 1300 LT. Comparing the observed plasma-sheet region with the regions of the computed flow patterns that are accessible to kilovolt electrons from the tail, we find agreement only in cases where the day-night asymmetry is included in the conductivity model, and only when a potential drop ≳35 kv is assumed to exist across the nose region of the magnetosphere. Throughout most of the afternoon and evening sectors, the computed shape of the inner edge of the plasma sheet is insensitive to the assumption about the rate of precipitation. The computed ionospheric current patterns resemble observed currents only if the auroral-zone Pedersen and Hall conductivities are assumed to be enhanced by an order of magnitude or more over the midlatitude nightside conductivities. When peak auroral-zone enhancements of the Pedersen and Hall conductivities are taken to be 3.3 and 10 mhos, respectively, current patterns computed assuming a large voltage drop across the nose of the magnetosphere are roughly consistent with observed DP2 fluctuations; currents computed assuming that all merging activity takes place in the tail then generally resemble DS currents.
292 citations
TL;DR: In this paper, a general approach to hydromagnetic instability in a magnetospheric geometry is given, and specific effects of the presence of finite ring current particles and cold particles of ionospheric origin are included.
Abstract: A very general approach to hydromagnetic instability in a magnetospheric geometry is given. The specific effects of the presence of finite ..beta.. ring current particles and cold particles of ionospheric origin are included. The generality of approach allows the theory to interrelate various approaches to low-frequency stability. It is concluded that resonant particle instabilities are likely to be the source of low-frequency oscillations that have been observed. Instability conditions and growth rates are estimated. Alfven waves may be excited with ..beta..<1, while for ..beta..approximately-greater-than1, compressional disturbances are expected to be generated. (AIP)
237 citations
References
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TL;DR: In this article, the occurrence at high latitudes of a large number of geophysical phenomena, including geomagnetic agitation and bay disturbances, aurorae, and various irregular distri...
Abstract: This paper is concerned with the occurrence at high latitudes of a large number of geophysical phenomena, including geomagnetic agitation and bay disturbances, aurorae, and various irregular distri...
1,320 citations
TL;DR: In this article, Ogo 3 measurements of proton and electron differential energy spectrums were used to detect charged particles of extraterrestrial ring current during geomagnetic storms, with Ogo
Abstract: Charged particles of extraterrestrial ring current during geomagnetic storms, with Ogo 3 measurements of proton and electron differential energy spectrums
411 citations
TL;DR: Magnetospheric plasma instabilities, discussing pitch angle diffusion instabilities and auroral precipitation boundary location, radial diffusion and maximum dissipation limit are discussed in this paper, where the authors also discuss radial diffusion.
Abstract: Magnetospheric plasma instabilities, discussing pitch angle diffusion instabilities, auroral precipitation boundary location, radial diffusion and maximum dissipation limit
407 citations
TL;DR: In this article, an upper atmosphere geophysical measurements are surveyed to determine to what extent the available data support the concept of bulk motion of the magnetosphere (convection) and the associated electric field.
Abstract: Fast magnetic field-line merging at the magnetospheric bow and in the tail are examined to determine their implications in regard to the concept of bulk motion of the magnetosphere (convection) and the associated electric field. Upper atmosphere geophysical measurements are surveyed to determine to what extent the available data support this concept. The dawn-dusk asymmetry in energetic particle fluxes, the movement of auroral ionization, ionospheric currents, and the location of the whistler knee (or ‘plasmapause’) are all consistent with this concept, the latter three all giving estimates of the electrostatic potential difference across the magnetosphere in the dawn-dusk meridian of a few tens of kilovolts. A ‘present best estimate’ of the flow pattern in the magnetosphere is derived, based primarily on the diurnal variation in the location of the whistler knee. The convective flow in from the tail appears to be stronger before midnight than after, which is consistent with the location of the maximum nighttime precipitation of energetic electrons. The derived flow is also qualitatively in agreement with that deduced from field-line merging. However, the Axford-Hines types of viscous drag will also give rise to convective flow, and the currently available data do not allow a definitive determination of whether field-line merging or viscous interaction is the principal driving force.
338 citations
TL;DR: Magnetosphere mathematical model with electric field representing plasma flow superimposed on geomagnetic field is presented in this article, where the electric field represents plasma flow and the magnetic field represents electric field.
Abstract: Magnetosphere mathematical model with electric field representing plasma flow superimposed on geomagnetic field
220 citations