Statistical Study of Whistler‐Mode Waves and Expected Pitch Angle Diffusion Rates During Dispersionless Electron Injections
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R. Ghaffari↗
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Relation Between Polar Plumes and Fine Structure in the Solar Wind from Ulysses High-Latitude Observations
Ulysses observations showed that pressure balance structures (PBSs) are a common feature in the high-latitude and high-speed solar winds near the solar minimum. On the other hand, coronal plumes are common in polar coronal holes. PBSs are believed to be remnants of coronal plumes and to be related to network activity such as magnetic reconnection in the photosphere from previous studies. This suggests that information on the magnetic structure of PBSs would help to confirm the relation between PBSs and polar plumes We have investigated the magnetic structures of the PBSs by applying a minimum variance analysis to Ulysses/Magnetometer data and by examining the pitch-angle distribution of energetic electrons measured with Ulysses/SWOOPS. We have found that PBSs have relatively more tangential discontinuities rather than rotational from the minimum variance analysis. Further, most PBSs also show bi-directional electron flux or isotropic pitch-angle distribution or the distribution expected in association with current-sheet structures from the analysis of high-energy electron data. In this, we find further evidence that PBSs are generated due by network activity at the base of polar plumes and their magnetic structures are like current sheets or plasmoids.
Yamauchi, Yohei↗
On the continuum radiation of gamma-ray bursts
I describe the qualitative features of a model for gamma-ray bursts assuming that they are produced by galactic neutron stars. The model is based on the assumption that electrons are continuously accelerated high up in the magnetosphere to relativistic energies and large pitch angles. These electrons emit a self-absorbed synchrotron spectrum at energies greater than mc-squared and, as a result, lose all of the energy associated with their motion perpendicular to the magnetic field. They thus slide along the field lines hitting the high density crust where by collision with ions acquire non-zero pitch angle (or are excited to higher Landau levels) and emit either cyclotron or bremsstrahlung photons.
Petrosian, Vahe↗
Angular dependent transport of auroral electrons in the upper atmosphere
The transport of auroral electrons through the upper atmosphere is analyzed. The transport equation is solved using a discrete-ordinate method, including elastic and inelastic scattering of electrons (resulting in changes of pitch angle) and degradation in energy as the electrons penetrate into the atmosphere. The transport equation is solved numerically for the electron intensity as a function of altitude, pitch angle, and energy. In situ measurements of the pitch angle and energy distribution of precipitating electrons over an auroral arc provide boundary conditions for the calculation. Model calculations were carried out with various different phase functions for elastic and inelastic collisions to attempt changing the angular scattering, but the observed pitch angle distributions remain unexplained. It is suggested that mechanisms other than collisional scattering influence the angular distribution of auroral electrons at or below 300 km altitude in the low-energy domain.
Lummerzheim, D.↗
Direct Observation of Electron Distributions Inside Millisecond Duration Electron Holes
Despite the importance of millisecond duration spatial structures [chorus wave nonlinearity or time domain structures (TDS)] to plasma dynamics, there have been no direct observations of the generation and interaction of these waves and TDS with electrons at the millisecond timescale required for their understanding. Through superposition of 0.195 ms Magnetospheric Multiscale Satellite electron measurements inside 37 superposed, millisecond duration electron holes, the first observations of electron spectra and pitch angle distributions on a submillisecond timescale have been obtained. They show that keV electrons inside the superposed electron hole are accelerated by several hundred volts and that the spectrum of electrons inside the electron hole contain several maxima and minima that are explained by a model of electron energy changes on entering the holes. We report the first observation of trapped electrons inside the TDS, in accordance with the theoretical requirement that such electrons must be present in order to form the phase space holes. Mechanisms of electron acceleration by electron holes (through perpendicular energy gain as the TDS moves into a converging magnetic field) and scattering (due to the perpendicular electric field) are discussed.
Mozer, F. S.↗
Electron loss from the Van Allen zones due to pitch angle scattering by electromagnetic disturbances
Fokker-Planck equation used to describe process of electron loss from Van Allen zones due to pitch angle scattering by electromagnetic disturbances
Roberts, C. S.↗
Ion and electron velocity distributions within flux transfer events
The detailed nature of the thermal and suprathermal ion and electron distributions within magnetic flux transfer events (FTEs) is examined. Examples of both magnetosheath FTEs and magnetospheric FTEs are discussed. The detailed distributions confirm that FTEs contain a mixture of magnetosheath and magnetospheric plasmas. To lowest order, the distributions are consistent with a simple superposition of the two interpenetrating populations, with no strong interactions between them. To first order, some interesting differences appear, especially in the electron distributions, suggesting that considerable pitch angle scattering and some electron energy diffusion are also occurring. These observations should provide a useful test of analytical and numerical studies of interpenetrating plasmas.
Thomsen, M. F.↗
Energetic electrons in the Jovian magnetosphere
Results are reported for a detailed analysis of Pioneer 10 data on energetic particle species in the magnetodisk region of Jupiter's magnetosphere. It is shown that the observed counting rates in the magnetodisk (beyond 20 Jupiter radii) were caused primarily by electrons with energies exceeding 0.06 MeV. Absolute omnidirectional electron intensities in the magnetodisk are presented for five integral energy ranges, and a model electron differential energy spectrum is found to fit the intensities throughout most of the encounter trajectory. It is suggested that the observed spectral shape results from losses of high-energy electrons by pitch-angle scattering. Observed equatorial energy spectra are used to compute distribution functions for several values of the first adiabatic invariant, mu. The radial profiles of the functions are found to have maxima at about 50 Jupiter radii inbound as well as at about 90 radii outbound and to diminish strongly for lesser radii. The large decreases in density are shown to require strong losses, and resonant electron whistler-mode pitch-angle scattering is suggested as a loss mechanism.
Baker, D. N.↗
Induced emission of Jupiter's decametric radiation by Io-accelerated electrons
A source mechanism for the Io-modulated component of the Jovian decametric radiation is proposed on the basis of the model where electrons in the Io flux tube (IFT) can be accelerated by Io's sheath. It is suggested that a significant fraction of the Io-sheath-accelerated electrons can have pitch angles greater than the atmospheric loss cone and therefore become trapped in the IFT. These electrons have flat helical orbits near their mirror points and can give rise to induced emission of extraordinary-mode radiation with frequencies close to the local electron gyrofrequency. The excitation mechanism is primarily due to the interaction of electromagnetic waves with the electrons via a relativistic gyroresonance which arises because of the momentum dependence of the gyrofrequency. Emphasis is put on the case of nearly perpendicular propagation because it is consistent with the observed source regions and beaming pattern of the decametric emissions. The energy requirement is satisfied within the context of the theory. It is also shown that emission occurs in regions close to the planet above the ionosphere.
Wu, C. S.↗
Rocket-borne measurements of auroral electrons
Rocket measurements of the pitch-angle and energy distributions of precipitated electrons in the 1- to 20-keV range show a consistent and relatively simple pattern. Features such as 'monoenergetic peaks' and pitch-angle distributions showing intense field-aligned components, rather than being anomalous, are characteristic of bright well-defined auroral forms. Presently available observations do not allow the identification of specific acceleration mechanisms, although they are consistent with several possibilities.
Boyd, J. S.↗
Investigation of S3-2 satellite data for local time variation of energetic electron precipitation
Data on precipitating electrons from the S3-2 satellite were investigated for local time variation at four L = 4 stations in the southern hemisphere. The equatorial pitch angles of electrons mirroring at 100 km, assumed to be the edge of the bounce loss cone, are calculated for L = 4 using the International Geomagnetic Reference Field for the epoch of the S3-2 data, along with the variation in mirror altitude per degree of equatorial pitch angle. The largest obstacle to the investigation was uneven sampling in terms of local time for all of the stations. However, this situation was improved upon by the use of S3-2 measurements at the conjugate locations of the four stations which provided additional data on electrons in the southern hemisphere bounce loss cone. Evidence for an effect of the dawn-to-dusk geoelectric field was found at two of the stations, Halley Bay and Siple, in the form of a minimum in electron precipitation at dusk. However, the present study does not completely resolve the question of local time modulation of electron precipitation at L = 4 in the southern hemisphere. Furthermore, while the average precipitation was lowest at the Kerguelen site, as would be expected on the basis of drift loss cone (DLC) theories, the intensity at that site exceeds the level that is expected on the basis of these DLC theories.
Robbe, S.↗
Auroral electron acceleration and atmospheric interactions - /1/ Rocket-borne observations and /2/ scattering calculations
Electron energy and pitch angle spectra are obtained by an instrumented rocket payload launched across a pre-midnight auroral arc. The interaction of the incident electron beam with the atmosphere is described through transport theory including inelastic and multiple-elastic scattering of electrons by neutrals. A two-stream approximation to the electron transport equation is used to compute the hemispherically averaged upcoming electron transport that resulted from the measured downcoming intensity. The model of Evans (1974) is employed to infer a primary electron spectrum within the discrete arc.
Pulliam, D. M.↗
The origin and propagation of chorus in the outer magnetosphere
Wave normals of chorus in the outer magnetosphere have been determined for the first time from data obtained with the Ogo 5 search coil magnetometer. These measurements combined with simultaneous information concerning geomagnetic field, plasma density, and the electron energy and pitch angle distributions provide a consistent picture of the generation, propagation, and subsequent damping of chorus in agreement with theory. Specifically, the data are consistent with chorus generation within 25 deg of the equatorial plane on the dayside and within 2 deg on the nightside. Chorus is generated by a Doppler-shifted cyclotron resonance with electrons between 5 and 150 keV but only when the pitch angle distribution is peaked at 90 deg to the local magnetic field and the anisotropy exceeds a critical value.
Burton, R. K.↗
Detailed spectral structure of magnetospheric electron bursts precipitated by lightning
The temporal structure of electron fluxes precipitated by lightning that was measured at night over Wallops Island, Virginia, during August 1984 has been analyzed. A component of the precipitating electrons is almost certainly due to equatorial wave-particle interactions involving equatorial electron cyclotron resonance (EECR) with whistlers which produce pitch angle scattering. Precipitated electron fluxes of equatorial origin are characterized by a very steep spectral power exponent of about -20 above 100 keV. A second electron distribution from some other origin is also found whose time-integrated energy flux appears to be about equal in magnitude to the fluxes identified with the EECR processes.
Goldberg, R. A.↗
Effects of Titan on trapped particles in Saturn's magnetosphere
Magnetic field data from Voyager 1 magnetometer experiment are used with angular distribution data from the same spacecraft's low energy charged particle experiment to investigate the influences of Titan on the magnetosphere energetic particle distributions, with attention to the pitch angle distributions of ions and electrons. Titan appears to disrupt the corotation motion of the ions, since, while the fluxes of the ions in the corotation direction are of diminished intensity, those observed in the equatorial plane perpendicular to this direction are almost unaffected. The resulting distribution and trajectory modeling conclusions suggest that the convection electric field is absent in the wake of Titan, and that the presence of Titan produces a reduction in the fluxes of the electrons and flatter distributions with pitch angle.
Maclennan, C. G.↗
Evidence that pitch angle scattering is an important loss mechanism for energetic electrons in the inner radiation belt of Jupiter
Analysis of data from the Pioneer 10 flyby discloses that pitch angle scattering plays an important part in determining the distribution of energetic electrons in the inner magnetosphere of Jupiter. Angular distributions measured by a Cerenkov detector reveal that redistribution takes place in pitch angle. Additionally, the radial profile of phase space density along the equator demands simultaneous particle losses. The loss rates are too high to be accounted for by synchrotron radiation loss, but are reasonably attributed to pitch angle scattering into the planetary loss cone.
Fillius, W.↗
Characteristics of pitch angle distributions of hundreds of keV electrons in the slot region and inner radiation belt
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H. Zhao↗
A fast Fermi process - Energetic electrons accelerated by a nearly perpendicular bow shock
A fast Fermi process which may explain how solar wind electrons are energized in the vicinity of the point of tangency of the IMF to the earth's bow shock is discussed. In the solar wind frame, the nearly perpendicular bow shock behaves as a rapidly moving magnetic mirror in such a manner that electrons with sufficiently large pitch angles can be reflected and accelerated. If the seed electrons have energies of several hundred electron volts, they can attain energies of several keV through the acceleration process.
Wu, C. S.↗