bluuebunny/crossref_metadata_2025 huggingface dataset:
Bands of ions and angular V's: A conjugate manifestation of ionospheric ion acceleration
Authors: Winningham J. D. , Burch J. L. , Frahm R. A.
Published: February 2008

Data from the hot plasma instruments on the Dynamics Explorer 1 and 2 spacecraft have been used to study the injection, drift, and subsequent precipitation of suprathermal positive ions in the auroral zone. The observation at both high and low altitudes of electron inverted “V” events in the boundary plasma sheet (BPS) and of ion “bands” (energy decreasing with decreasing latitude) in the adjacent central plasma sheet (CPS) leads to the following ion injection model: upward‐moving energetic ion beams are injected onto BPS magnetic field lines by the electrostatic potential drops associated with electron inverted V's. As the ion beams move toward the equator and into the conjugate hemisphere they are convected to lower latitudes and into the CPS. The energy‐latitude dependence of the ion bands, coupled with concurrent ion convection measurements, indicate that the ion distributions are primarily O+, in agreement with their postulated ionospheric source.
A mechanism for the westward traveling surge during substorms
Authors: Kan J. R. , Williams R. L. , Akasofu S.‐I.
Published: February 2008

It is proposed that the westward traveling surge is formed as a result of a partial blockage of the substorm‐enhanced Hall current from closure in the magnetosphere. The partial blockage of the enhanced Hall current leads to a buildup of “excess” charge in the ionosphere that in turn produces a polarization electric field. We show that the resulting ionospheric electric field exhibits the characteristic distortion of the convection pattern associated with substorms. The distortion of the convection streamlines intrudes westward; the westward traveling surge is an optical manifestation of this intrusion. The speed of the westward traveling surge is identified with the speed of the charge spread, which depends on the time rate of change of the Hall current blockage and the propagation of charges across field lines via electrostatic ion acoustic waves in the ionosphere.
An association between discrete aurora and energetic particle boundaries
Authors: Lyons L. R. , Evans D. S.
Published: February 2008

NOAA 6 satellite observations at 850 km over the auroral zones have been examined. We find that regions of auroral electron percipitation (0.3–20 keV) with energy fluxes sufficient to produce discrete aurora are generally spatially associated with significant changes in the intensity of 30‐ to 800‐keV protons filling the atmospheric loss cone. The proton intensity changes often occur simultaneously with spatial changes in the intensity of energetic (>30 keV) electron precipitation at the high‐latitude boundary of the electron population. The velocity‐space distribution of the proton precipitation agrees with that observed for earthward streaming protons at the outer boundary of the plasma sheet and also with those calculated to result from current sheet energization. This agreement between the proton distributions implies that electron precipitation responsible for discrete aurora often occurs at the outer boundary of the plasma sheet in association with boundaries and other spatial structure in the streaming ion distributions. This furthermore implies that the upward directed, field‐aligned currents associated with discrete aurora are also often associated with the streaming ion structures. The comparisons with the distributions of ions ejected from the current sheet suggest that discrete aurora often occur on magnetic field lines which map along the outer boundary of the plasma sheet into the tail current sheet and that the cause of these discrete aurora is associated with current sheet processes.
Low‐altitude image striations associated with bottomside equatorial spread F: Observations and theory
Authors: Vickrey James F. , Kelley Michael C. , Pfaff Robert , Goldman S. Robert
Published: February 2008

Ionospheric plasma instabilities are usually discussed in terms of local parameters. However, because electric fields of scale size λ ≳ 1 km map along magnetic field lines, plasma populations far away from a locally unstable region may be affected by the instability process and vice versa. We present observations of electron density variations in the F1 region of the ionosphere at two locations near the magnetic equator. Oscillations in electron number density that were confined to a narrow wavelength regime were observed in a region of the ionosphere with a very weak vertical density gradient. Since magnetic flux tube interchange instabilities cannot create structure in such an environment we suggest that these are “images” of instabilities occurring elsewhere along the magnetic field line. A simple steady state theory of image formation is developed that is in good agreement with the observations. Moreover, this theory predicts a scale size dependent “effective diffusion” process in the F region that may dominate over classical cross‐field diffusion at kilometer scale sizes. Such a scale size dependent diffusion process is required to explain recent scintillation observations of decaying equatorial plumes.
On the acceleration of energetic ions in Jupiter's magnetosphere
Authors: Barbosa D. D. , Eviatar A. , Siscoe G. L.
Published: February 2008

A theoretical model and analysis of the acceleration of high‐energy ∼ MeV/nuc ions in Jupiter's magnetosphere are presented. All measurements conducted in this energy range have consistently pointed to the probable occurrence of local nonadiabatic acceleration above that obtainable by radial diffusion alone. We consider first the creation of fast 75 km/s neutral sulfur and oxygen atoms in the Io torus by charge exchange. This process will eject upward of 2×1028 neutrals per second which are recaptured in the magnetosphere by subsequent electron impact and charge exchange with the ambient plasma outside the torus as well as by solar UV photoionization. All processes together lead to an injection rate of greater than 4×1025 heavy ions recaptured per second with a gyrospeed comparable to the local plasma flow speed. We show that subsequent adiabatic radial diffusion cannot produce the observed spectrum or high energies required by observations. We then consider the stochastic acceleration of this seed population by magnetohydrodynamic (MHD) waves. It is demonstrated how the properties of the observed particle spectrum are related to those of the power spectrum of MHD fluctuations measured by the magnetometer.
Intense low‐energy ion populations at low equatorial altitudes
Authors: Williams D. J. , Frank L. A.
Published: February 2008

The combined data from the ISEE 1 medium energy and plasma instruments on November 29, 1977, have identified the existence of surprising spectral structures in the trapped ion population at low equatorial altitudes, 2 ≲ L ≲ 4 and αeq ≳ 50°. By analyzing our instruments' responses at the limits of their capabilities, we are able to resolve a double peak structure just below the instruments' energy overlap region (20–40 keV). Two narrow ((ΔE/E) ∼ 0.4) energy peaks are observed. We conclude that for this specific case the higher energy (HE) peak is composed of protons and that the lower energy (LE) peak is made up of singly ionized ions (H+, He+, O+). At L ∼ 3 the HE peak is located at ∼17 keV and the LE peak at ∼12 keV. The energy of the peaks increases as L decreased and the intensity of the peaks is ∼2(10)5 ions/cm s sr keV. Data from the medium energy particles instrument, which responds to the higher energy portion of the observed peaks, have been surveyed through the period November 1977–September 1979. Orbit geometry and available data recordings allowed these ions to be observed only for the six‐month period November 1977 through April 1978 and over the local time range ∼0300–1500 hours LT. These characteristics plus charge exchange lifetimes pose severe boundary conditions on possible source and injection mechanisms. Of those considered, ion cyclotron acceleration of thermal ions in the presence of an anisotropic energetic population appears a likely source of these quasi‐monoenergetic peaks. Additional peaks in the energetic ion spectra are observed at energies ≳40 keV. Some of this higher energy structure may be related to cross L diffusion of multi‐component ion populations.
Direct access to plasma resonance in ionospheric radio experiments
Authors: Mjølhus Einar , FlÅ Tor
Published: February 2008

The concept of linear conversion of radio waves into electrostatic (ES) waves is adapted to ionospheric radio heating experiments. It is identified as access to the plasma resonance through the radio window. By means of existing heating facilities, large concentrations of electrostatic wave energy can be generated. The ES waves are confined to a restricted region in space, horizontally displaced in relation to the vertical over the heating transmitter along the magnetic meridian, the displacement being southward on the northern hemisphere. Ray‐tracing studies of the location of the resonance region relative to the transmitter are presented, and the horizontal dimension is estimated.
Energization of ions in the auroral plasma by broadband waves: Generation of ion conics
Authors: Singh Nagendra , Schunk R. W.
Published: February 2008

The ion conics provide an important mechanism for the transportation of ionospheric ions to the magnetosphere. The conics are generated by a transverse acceleration of the ions and by the subsequent upward repulsion by the magnetic mirror force. Here, the acceleration of ions by a broadband lower hybrid (LH) wave is considered. Since the broadband waves have a finite correlation time, the heating occurs even in the absence of a resonance between the ions and the waves. The LH waves excited by precipitating energetic electrons in the auroral ionosphere have characteristics such that ambient cold ions (<1 eV) cannot resonate with the waves. Thus the nonresonant heating mechanism discussed here may play a direct role in generating ion conics with energies of about a few hundred electron volts at altitudes below the auroral acceleration region where the lower hybrid waves have been observed. The nonresonant mechanism may also act to preheat the ions so that the resonant heating can follow. The extent of the energy gain of H+ ranges from a few electron volts to a few keV as the turbulence noise level varies from 10−9 to 10−6 V² m−2 s−1. The heavy ion gains comparably less energy. The calculations were done for turbulence regions that were both extended and limited in distance, the latter being particularly relevant to the generation of conics with very sharp pitch angle distributions.
Heliospheric current sheet displacements during the solar cycle evolution
Authors: Tritakis Vasilis P.
Published: February 2008

The purpose of the present study is to confirm the view of a heliospheric current sheet moving with respect to the solar equator, in relation to the solar cycle phases. For this reason, we have analyzed both observed and high‐accuracy inferred daily interplanetary magnetic field (IMF) polarities between the maxima of solar cycles 18 and 21 (1947–1980). Statistically significant north‐south and “away”‐“toward” differences of the IMF mean sector widths (MSW) have been detected around the solar activity minima while no significant differences have been observed around the maxima or the solar polarity reversals. This peculiar variability of the MSW differences in the solar activity extrema can be finally interpreted by assuming a variable placement of the heliospheric current sheet with respect to the solar equator and the solar cycle phases. Actually, in the epochs of the solar polarity reversal our observations are consistent with a current sheet located symmetrically to the solar equator, almost vertical to it and extending to high heliolatitudes. In the opposite, in the epochs of the solar minima the interpretation of our observations could be due to a current sheet which is southward depressed in the northern but symmetrical in the southern hemisphere. The various placements of the current sheet on both sides of the solar equator in relation to the solar cycle phases, are supposed to originate in north‐south asymmetric solar activity which has been found by several authors to predominate in the solar minima. The MSW differences detected in the last minimum (1975–1977) of the time span under consideration give reasons to venture the prediction that in the solar minima of the time period 1976–1998 the heliospheric current sheet will appear northward depressed in the southern hemisphere but symmetrical in the northern hemisphere. Systematic meridional and high‐latitude observations of both solar wind parameters and IMF patterns would contribute significantly in giving a final answer to the question of possible heliospheric current sheet displacements during the solar cycle evolution.
On the excitation mechanism of the low‐frequency upstream waves
Authors: Watanabe Yukio , Terasawa Toshio
Published: February 2008

The excitation mechanism of the upstream low‐frequency waves (0.01–0.05 Hz) is investigated based on the wave observations. We compared the observed wave frequencies with the frequencies expected from the ion (proton) beam‐cyclotron instability process. We estimated the beam velocity from the solar wind parameters using Sonnerup’s formula. Our analysis supports the hypothesis that these waves are excited by the resonant beam‐cyclotron instability. For the waves observed in the deep foreshock region, however, we found it necessary to include the solar wind convection effect on the wave propagation.
The spatial and seasonal distribution of Martian clouds and some meteorological implications
Authors: Kahn Ralph
Published: February 2008

The spatial and seasonal distributions of Martian atmospheric clouds, organized according to simple morphologic criteria, exhibit distinct patterns. When interpreted self‐consistently using analogies to terrestrial cloud forms and Martian temperature, wind direction, and water vapor measurements, patterns of cloud occurrence provide some loose constraints on global‐scale near‐surface wind speed, static stability, and humidity. We have examined all Mariner 9 and Viking Orbiter images for this study; we concentrate primarily on the larger Viking data set. Whenever observations are possible, relatively high near‐surface winds appear to follow the seasonal cap edges during early fall, late winter, and spring. Moderate to high winds are also inferred in mid‐latitudes for mid southern winter and in low latitudes, mainly in the steeply sloping areas of Tharsis, near the solstices. At other times, lower winds are suspected. Hemispheric asymmetries are traced to differences in atmospheric hydration state and to global dust storm related effects on the atmospheric temperature structure and the north polar cap recession schedule. Cloud occurrence data suggest that the near‐surface daytime static stability is low in the northern hemisphere during summer and at low latitudes during mid northern spring. Relatively high static stability is deduced near the winter poles and globally during the dust storm season. In spite of low atmospheric temperatures, the correspondingly low absolute humidity apparently precludes the formation of thick water ice clouds at high latitudes in mid to late autumn and winter in both hemispheres and in southern mid‐latitudes during early winter. During northern spring and summer, saturation conditions seem to be easily achieved at mid‐latitudes, and condensate clouds are abundant; the situation is complicated by atmospheric dust in southern spring and summer. Low latitudes generally appear to be farther from saturation than mid‐latitudes, and clouds generally form more easily in the northern hemisphere than in the south during corresponding seasons.
On the proper Mach number and ratio of specific heats for modeling the Venus bow shock
Authors: Tatrallyay M. , Russell C. T. , Luhmann J. G. , Barnes A. , Mihalov J. D.
Published: February 2008

When considering local solutions for the jump in plasma conditions across planetary bow shocks, a magnetohydrodynamic (MHD) formalism of the Rankine‐Hugoniot relations is used. However, when planetary shocks are modeled on a global scale, gas dynamic (GD) treatments are frequently employed which neglect the magnetic field effect on the shock structure and the flowing plasma. These treatments require the specification of a free‐stream Mach number and a ratio of specific heats. Herein observations of the Venus bow shock are used to illustrate how closely one can approximate the observed magnetic field jump with MHD and GD treatments if one chooses a particular Mach number and ratio of specific heats. As expected theoretically, it is found that the observed magnetosonic Mach number (and not the sonic Mach number) when used in the GD calculations gives the best approximate solutions to the MHD problem using reasonable value for the ratio of specific heats. At Venus we find that the best value for the ratio of specific heats (γ) is 1.85 (rather than 5/3 or 2), although this ratio depends on the Alfvenic Mach number MA and the angle between the shock normal and the upstream field, θBN. We cannot determine how well the GD solution reproduces the shock shape at Venus, because of the possible influence of both MHD and mass‐loading effects. In fact, the Venus bow shock is much farther away from the planet at the terminator than the GD model would predict for either γ = 5/3 or 2. Thus we must depend on terrestrial studies to determine how best to approximate shock shape with GD modes.
Ion‐neutral coupling in the high‐latitude F region: Evaluation of ion heating terms from Dynamics Explorer 2
Authors: Killeen T. L. , Hays P. B. , Carignan G. R. , Heelis R. A. , Hanson W. B. , Spencer N. W. , Brace L. H.
Published: February 2008

We study a set of south (summer) polar passes from the Dynamics Explorer 2 data base that provide measurements of ion and neutral velocities along the track of the spacecraft for various geomagnetic conditions. The ion velocities are obtained from the ion drift meter and the retarding potential analyzer instruments; the neutral velocities are obtained from the Fabry‐Perot interferometer and the wind and temperature spectrometer instruments. These data enable a direct determination of the in situ ion‐neutral difference velocity. Simultaneous measurements of atmospheric composition using the neutral atmosphere composition spectrometer and of electron temperatures and densities using the Langmuir probe enable the ion heating and cooling rates to be determined. The rates are discussed in relationship to values for the electron, ion, and neutral kinetic temperatures. The comprehensive nature of the data set allows for a quantitative study of ion‐neutral coupling and, in particular, affords a more rigorous test of the common assumptions used in the ion energy equation than has been previously possible. From our study we find the following: (1) The ion‐neutral difference velocities, which drive the Joule heating, have a complex morphology that depends on the stability of the high‐latitude ion convection and the inertia of the neutral gas. (2) Ionospheric “hot spots” are observed to be related to large perpendicular electric fields that give rise to frictional (Joule) heating. (3) These events are often seen to occur in conjunction with troughs in the electron density and with decreases in the ratio of atomic oxygen to molecular nitrogen. In such cases, momentum transfer to the neutral gas is reduced, and the ion‐neutral velocity difference and associated Joule heating can be maintained over longer time periods. (4) An approximate balance between local frictional heating of the ions and local cooling of the ions to the neutrals is seen in regions of strong ion convection. The commonly used approximate form of the ion energy equation involving only local processes appears at times, however, to be inadequate for large regions of the summer polar cap. The ion temperatures observed in these regions are higher than expected from simple local heating/local cooling arguments. Additional source(s) of ion heating would be required to provide energy balance.
Generation of ordinary mode electromagnetic radiation near the upper hybrid frequency in the magnetosphere
Authors: Ashour‐Abdalla M. , Okuda H.
Published: February 2008

It is shown by means of plasma numerical simulations that long‐wavelength ordinary mode electromagnetic radiation can be generated from short‐wavelength electrostatic waves near the upper hybrid resonance frequency in an inhomogeneous plasma. A possible relation of this process to nonthermal continuum radiation in the magnetosphere is discussed.
Effect of hot electrons on the polar wind
Authors: Barakat A. R. , Schunk R. W.
Published: February 2008

A semikinetic model was used to study the effect that hot electron populations have on the polar wind. The model was used to describe the steady state plasma flow along diverging geomagnetic field lines in the collisionless regime at high altitudes. The plasma contained O+ and H+ ions and both hot and cold electron populations. Several hot electron populations were considered, including the polar rain, polar showers, and polar squall. Estimates of hot electron parameters based on characteristic energy and flux measurements indicate that the hot/cold electron temperature ratio varies from 10 to 104 and that the percentage of hot electrons varies from 0.1% to 10% at 4500 km. For ratios at the lower ends of these ranges the polar wind solutions with hot electrons are similar to those obtained previously for supersonic H+ outflow without hot electrons. For higher hot electron temperatures and a greater percentage of hot electrons, there is a discontinuity in the kinetic solution, which indicates the presence of a sharp transition. This transition corresponds to a contact surface between the hot and cold electrons. Along this surface, a double‐layer potential barrier exists which reflects the cold ionospheric electrons and prevents their escape. The presence of the hot electrons acts to increase the supersonic H+ outflow velocity and H+ energy but does not affect the already saturated H+ escape flux. The H+ energy gain may be as large as 1 to 2 keV. With regard to O+, the hot electrons act to reduce the potential barrier, thereby allowing more O+ ions to escape. A significant enhancement in the O+ escape flux can occur depending on the hot electron density and temperature.
Resonant harmonic Alfvén waves in the magnetosphere: A case study
Authors: Junginger Hans , Baumjohann Wolfgang
Published: February 2008

Strong dayside long‐period electric and magnetic field fluctuations were measured on board the geostationary satellite GEOS 2 on April 11, 1979. The electric and magnetic field spectra obtained throughout this day clearly exhibit discrete frequency bands, lasting simultaneously for about 5 hours. The lower‐frequency pulsation with a typical period of 260 s is only present in the electric field data (with typical amplitudes of 0.3 mV/m), whereas the higher‐frequency oscillation dominates the transverse magnetic field spectra (typical period 95 s, typical amplitude 1 nT). The periods of both pulsations increase toward the dusk sector. The observed periods of the pulsations in conjunction with the measured equatorial electron densities, the ratio of the electric and magnetic field amplitudes for both frequency bands, and the constancy of the period ratio yield strong evidence that the two frequency bands represent fundamental and second harmonic mode eigenoscillations of geomagnetic field lines. The mean ratio between the shorter and longer periods derived from the measurements (about 0.35) is slightly different from theoretically obtained ratios, thus indicating shortcomings of currently available models. The phase relationship between the electric and magnetic field fluctuations of the higher‐frequency pulsation, the direction of the wave Poynting vector, and the occurrence of the event after the spring equinox indicate that the major part of the wave energy is fed into the oscillation in the southern hemisphere and not close to the equatorial plane.
The effects of dipole tilt on the structure of the magnetosphere
Authors: Wu C. C.
Published: February 2008

The structure of the magnetosphere as a function of dipole tilt was studied by using a global magneto‐hydrodynamics (MHD) model. We show here that the cusp current sheet in the MHD model is related to the tail current sheet and that the calculated shape and positions of the tail cuurrent sheet are consistent with observations. In addition, we discuss the difference between the MHD model and the Chapman‐Ferraro type model which manifests as a function of tilt angle.
Shock acceleration of diffuse ions at the Earth's bow shock: Acceleration efficiency and A/Z enhancement
Authors: Ellison Donald C.
Published: February 2008

Observations of particle spectra, intensity, and enhancement of alpha particles over protons at diffuse ion events at the quasi‐parallel earth bow shock are compared to a Monte Carlo simulation of diffusive shock acceleration. The simulation includes the back reaction of accelerated particles on the shock structure, particle escape at an upstream free escape boundary, and a low energy per nucleon threshold for thermal leakage of downstream, shock‐heated particles into the upstream region. The simulation assumes that the same scattering operator that gives rise to shock acceleration can also describe a viscous shock governed by hydrodynamic turbulence. This implies that accelerated ions can be drawn directly from the thermal solar wind with no separate superthermal seed population. Good agreement between the simulation and observations made during nearly radial magnetic field configurations lends support to thermal leakage of downstream, shock‐heated ions as the mode of injection for diffuse ion events.
A simple model for the formation of “reflected,” “intermediate,” and “diffuse” ion distributions upstream of Earth's bow shock
Authors: Lee Martin A. , Skadron George
Published: February 2008

We present a simple model unifying the distinct energetic ion populations and their associated low‐frequency hydromagnetic waves within earth's ion foreshock. Ions initially injected onto magnetic field lines at the shock excite hydromagnetic waves which pitch angle scatter the ions back toward the shock. The ions are represented by inhomogeneous inward (toward the shock) and outward traveling beams, and the transition rate between beams is determined by an effective quasi‐linear pitch angle diffusion coefficient for the transition. The intensities of waves resonant with the beams are calculated from wave kinetic equations utilizing linear wave growth rates which in turn are determined by the instantaneous, local beam densities. The coupled equations for the spatial and temporal evolution of the ion densities and wave intensities along a given magnetic field line are solved numerically assuming steady injection of ions at the shock following the initial magnetic connection of the field line to the bow shock. The initial interplanetary waves are assumed to be unpolarized on average and to propagate predominantly away from the sun relative to the solar wind. We find that (1) the ion anisotropy near the shock decreases slowly during the initial minutes of magnetic contact, but then makes a rapid transition to a steady, “diffuse” distribution; (2) the diffuse ion density declines steeply away from the shock with a scale length of ∼5 RE, forms a broad minimum, and then increases with increasing anisotropy to form an “intermediate” ion distribution; (3) upstream of the intermediate ions the density forms a broad “reflected” ion distribution with high anisotropy extending to the foreshock boundary; (4) the reflected and intermediate ion distributions are confined to spatial bands which are aligned with the foreshock boundary and have ∼5‐RE widths; and (5) the waves associated with intermediate ions are strongly right‐hand polarized in the solar wind frame whereas the waves associated with diffuse ions exhibit a net polarization within a range about zero.
The propagation and growth of whistler mode waves generated by electron beams in Earth's bow shock
Authors: Tokar Robert L. , Gurnett Donald A.
Published: February 2008

In this study the propagation and growth of whistler mode waves generated by electron beams within the earth's bow shock are investigated using a planar model for the bow shock and a model electron distribution function. Within the shock, the model electron distribution function possesses a field‐aligned T⊥ > T∥ beam that is directed toward the magnetosheath. Waves with frequencies between about 1 and 100 Hz with a wide range of wave normal angles are generated by the beam via Landau and anomalous cyclotron resonances. However, because the growth rate is small and because the wave packets traverse the shock quickly, these waves do not attain large amplitudes. Waves with frequencies between about 30 and 150 Hz with a wide range of wave normal angles are generated by the beam via the normal cyclotron resonance. The ray paths for most of these waves are directed toward the solar wind, although some wave packets, because of plasma convection, travel transverse to the shock normal. These wave packets grow to large amplitudes, because they spend a long time in the growth region. The results suggest that whistler mode noise within the shock should increase in amplitude with increasing upstream θBn. The study provides an explanation for the origin of much of the whistler mode turbulence observed at the bow shock.
A simplified model for timing the arrival of solar flare‐initiated shocks
Authors: Smart D. F. , Shea M. A.
Published: February 2008

The solar flare‐initiated shock can be considered to be an initially driven shock that converts into a blast wave as it propagates through the interplanetary medium. In our simplified shock modeling, to estimate the time of the shock arrival at a specified position in space we assume that the shock is initially driven from the flare position at the velocity indicated by the type II solar radio burst. We assume the shock is driven until the initiating solar flare energy is somewhat expended. After this initially driven phase, the shock front then propagates with the characteristic speed expected of the shock front of a blast wave. In the interplanetary medium with its 1/r² density dependence, the blast wave shock front speed should be proportional tor−0.5. We find that thisr−0.5speed dependence is a general characteristic of the shock front speed when the speeds are determined in the solar wind reference frame. The blast wave “rides over” the preexisting solar wind so that the disturbance speed of the shock front at any instant of time is the speed of the shock front added vectorially to the solar wind speed. Application of these principles enables a consistent “timing” of solar flare‐initiated shock waves that is event independent. We show that this concept is consistent with the time position profile derived from the analysis of kilometric type II events that are associated with interplanetary shocks.
Shock formation time and the viability of prompt MeV proton shock acceleration in solar flares
Authors: Smith Dean F. , Brecht Stephen H.
Published: February 2008

A critical analysis is made of a proposed mechanism to explain the ≲2‐ to 100‐s delays observed in some flares between γ rays due to several MeV protons and hard X rays due to ∼100‐keV electrons. The proposed mechanism is first‐order Fermi acceleration of protons bouncing between two shocks formed by the electrons causing the hard X rays. For the maximum possible flux for stable beam propagation the shock formation time is found to be ≳2 s and the shock velocity ≲1800 km s−1. The mechanism as proposed produces protons which gain only ∼3 MeV before they are transmitted through the shocks. The mechanism will work with additional pitch angle scattering, which also raises the possibility of multiple encounters with a single shock, a case not analyzed. For this reason the final energy gains of the proposed mechanism may be altered. The times during which this type of shock acceleration could be important and thus the range of observed delays which could be explained are given for a range of loop lengths and injected electron spectra. Alternate possibilities such as stochastic acceleration by hydromagnetic turbulence must be invoked to explain the shortest delays associated with this prompt proton acceleration.
Precipitation of radiation belt electrons by man‐made waves: A comparison between theory and measurement
Authors: Inan U. S. , Chang H. C. , Helliwell R. A. , Imhof W. L. , Reagan J. B. , Walt M.
Published: February 2008

The temporal and spectral shape and the absolute flux level of particle pulses precipitated by a VLF transmitter are examined from a theoretical point of view. A test‐particle model of the gyroresonant wave‐particle interaction is applied to the parameters of the observed cases for calculating the precipitation characteristics. The temporal shapes of the precipitation pulses are found to be controlled (1) by the pitch angle dependence of the particle distribution near the edge of the loss cone and (2) by the multiple interaction of the particles with the waves due to significant atmospheric backscatter.
Diurnal modulation of the quiet time penetrating electron flux
Authors: Sheldon W. R. , Benbrook J. R. , Gelpi C. G.
Published: February 2008

Three sounding rocket measurements of energetic electron precipitation in the 65 to 80 km altitude range above Siple Station, Antarctica suggest a possible diurnal modulation of the precipitation. Flights made on January 10 and 11, 1981, to investigate this possibility showed that no electrons were present after local noon, while a significant flux of penetrating electrons was observed the following day near local dawn, even though the level of magnetic activity was higher during the postnoon flight. A previous flight from Siple Station also observed a large flux of penetrating electrons near local dawn. Calculations of electron drift trajectories in a model magnetic field indicate that moderate values of the convection electric field could cause significant local time variations in the penetrating electron flux at the western edge of the South Atlantic anomaly. These calculations indicate that Siple Station is shadowed by eastern Siberia when it is at local noon, but not when it is at local dawn.
Solar wind control of magnetospheric pressure (CDAW 6)
Authors: Fairfield D. H.
Published: February 2008

The CDAW 6 data base is used to compare solar wind and magnetospheric pressures. The flaring angle of the tail magnetopause is determined by assuming that the component of solar wind pressure normal to the tail boundary is equal to the total pressure within the tail. Results indicate an increase in the tail flaring angle from 18° to 32° prior to the 1055 substorm onset and a decrease to 25° after the onset. This behavior supports the concept of tail energy storage before the substorm and subsequent release after the onset.
The stability of the oscillation motion of charged grains in the Saturnian ring system
Authors: Xu R. ‐L. , Houpis Harry L. F.
Published: February 2008

The stability of charged grains given a random initial velocity is determined by a perturbation approach for the gravito‐electrodynamic forces encountered in the corotating plasma environment of Saturn. Saturn's magnetic field is assumed to be a dipole with a small northward offset. If the equatorial component of the grain's initial velocity is given a specific value Vs0, the general results of two previous theories (Northrop and Hill, 1982; Mendis et al., 1982) are reproduced, including the critical radius for z instability at 1.616 Rs. However, for initial velocities other than Vs0 a large spectrum of new results occur. Here we discuss the implications of these results for the formation of the Saturnian ring system. In particular, we suggest that the marginal z stability radius at 1.5245 Rs for Kepler‐launched particles is due to an erosion process with ejecta of the order of 0.05 −0.5µm rather than that of plasma as previously suggested. The diffuseness of the Saturnian rings beyond the F ring is also explained in terms of instability, while a new critical radius for r instability is suggested for the optical depth feature at 1.72 Rs. Finally, the F ring is analyzed in detail.
Collisional tearing in a field‐reversed sheet pinch assuming nonparallel propagation
Authors: Quest Kevin B. , Coroniti Ferdinand V.
Published: February 2008

We examine the linear stability properties of the collisional tearing mode in a reversed‐field sheet pinch assuming that the wave vector is not parallel to B, where B is the equilibrium magnetic field. We show that pressure balance in the direction of the equilibrium current requires a nonzero perturbed current component δJz that is driven toward the center of the pinch. At the center of the pinch, δJz goes to zero, and momentum is balanced by coupling to the ion‐acoustic mode. In order to achieve current closure, a large perturbed field‐aligned current is generated that is strongly localized about the dissipative tearing layer. The relation of this work to the collisionless case is discussed.
Azimuthal propagation and frequency characteristic of compressional Pc 5 waves observed at geostationary orbit
Authors: Takahashi Kazue , Higbie Paul R. , Baker Daniel N.
Published: February 2008

Energetic particle data from the 1977–007 and 1979–053 satellites and magnetic field data from the GOES 2 and 3 satellites have been used to study eight compressional Pc 5 wave events observed at geostationary orbit during 1979. All the events occurred on the dayside, and most of them were observed during the recovery phase of a geomagnetic storm. By using the data from two of the satellites which were close to each other, we measured the azimuthal phase velocity Vф and azimuthal wave number m for selected intervals. For all these intervals the waves propagated westward in the spacecraft frame, and we obtained |Vф| = 4–14 km/s and |m| = 40–120. In addition, harmonics of a local standing Alfvén wave were often present simultaneously with a compressional Pc 5 wave. The frequency of the compressional wave was typically 25% of that of the second harmonic of the Alfvén wave. These observed features are discussed in the light of existing theories of instabilities in the ring current plasma.
A preliminary spectroscopic assessment of the Spacelab 1/shuttle optical environment
Authors: Torr Marsha R. , Torr D. G.
Published: February 2008

The Spacelab 1/Shuttle mission which orbited for 10 days following launch on November 28, 1983, carried amongst the complement of instruments, an array of imaging spectrometers known as the Imaging Spectrometric Observatory. Because the instrument covers a broad wavelength range extending from the vaccum ultraviolet to the near infrared and has a relatively high spectral resolution (3–6 Å), it has provided some of the first spectral information on the Shuttle optical environment. Shuttle flights to date have shown that surfaces directed into the velocity vector develop a bright orange‐red glow. As there are extensive plans for optical studies from the Shuttle, as well as other orbiting structures such as the Space Telescope and Space Station, there has been considerable concern as to whether such glows and other sources of optical contamination might prevent or limit observations. In this paper we present spectral data from 1150 to 8000 Å measured with the instrument looking tangentially away from the earth and into the velocity vector. (Detailed quantitative studies are underway, but because of the numerous instrument design activities in process at this time which could benefit from this information, we present a preliminary assessment here.) We find that the spectrum in general contains the expected atmospheric features in addition to bright molecular bands between 6400 and 8000 Å. These bands appear to be part of the N2 First Positive system. Several other features of lesser significance have yet to be identified. If the bulk of the emission is due to the N2 first positive system, there appear to be two possible excitation mechanisms: direct collisional excitation of the local Shuttle atmosphere by the terrestrial atmosphere, or a surface interaction which generates metastable N2(A), which subsequently radiates via inverse first positive N2 transitions followed by first positive transitions.
Are observed broadband plasma wave amplitudes large enough to explain the enhanced electron temperatures of the high‐latitude E region?
Authors: St.‐Maurice J.‐P. , Laher Russ
Published: February 2008

We have studied the possibility that the electron heating that has been observed in the high‐latitude E region during disturbed periods is produced by low‐frequency broadband waves of the type that is measured on rockets. We have found that low‐frequency waves can be responsible for the observed electron temperature enhancements if a significant portion of the large amplitude waves does not have a wave vector exactly perpendicular to the magnetic field, but rather if their wavevector is a few (2–5) degrees away from perpendicularity. We show that this aspect angle constraint is present because in order to heat the electrons efficiently with low‐frequency waves in the magnetized E region we need a large enough component of the perturbed electric field in the magnetic field direction. While aspect angles that are larger than expected from classical linear theory have been detected at times, we propose that they should be limited to low‐frequency gradient drift waves. For such waves nonlocal effects are probably strong enough to force the waves to be less field aligned than originally expected. Finally, a direct implication of our findings is that a heating mechanism similar to the one discussed in the present paper may produce detectable effects in the equatorial E region during strong electrojet conditions.
Computer modeling of magnetotail convection
Authors: Birn J. , Schindler K.
Published: February 2008

We studied the dynamic evolution of the geomagnetic tail in response to a dawn‐to‐dusk electric field Ey applied at the high‐latitude boundary by means of a two‐dimensional nonlinear MHD code. We found that the electric field propagates inward through the lobes toward the plasma sheet. Only a small fraction, however, penetrates into the interior of the plasma sheet, and eventually an almost linear decrease from the boundary to the equatorial plane is assumed. There was no tendency to evolve toward a steady or nearly steady state which would require Ey ≈ const in a larger area. The resulting time‐dependent changes are consistent with quasi‐static analytic models of Schindler and Birn (1982) and Birn and Schindler (1983) and with observed substorm growth phase effects in the near tail. If the external electric field Ey is constant or varies only slightly along the tail, the convection speed within the plasma sheet remains small. A strongly nonuniform external electric field Ey leads to similar spatial and temporal variation of Ey. It does, however, produce a locally much stronger thinning which favors the onset of a tail instability such as the tearing mode. It can also produce stronger flows primarily parallel to the magnetic field.
Persistence of shocks to large distances in the solar wind
Authors: Kayser Susan E.
Published: February 2008

Present hydrodynamic models of solar wind streams predict that interactions will cause interplanetary shocks to decay and large‐scale structures to coalesce and smooth out, with a decay length of 10–15 AU for moderate or small‐amplitude shocks. The Pioneer 10 plasma data, extending 1–30 AU, are examined in the light of such predictions. It is found that coalescence of streams into a single stream per solar rotation does occur, in general, but that considerable structure remains by 30 AU. The leading edge of a stream often exhibits a velocity jump of >20 km/s; many of these may be shocks. There is a characteristic velocity‐density‐temperature signature of these distant streams which differs from the close‐in double‐shock signature. A unique transient was seen in July 1982, at 28 AU, with a velocity jump of about 235 km/s.
Analyses of experimental observations of electron temperatures in the near wake of a model in a laboratory‐simulated solar wind plasma
Authors: Intriligator D. S. , Steele G. R.
Published: February 2008

Laboratory experiments have been performed that show the effect on the electron temperature of inserting a spherical conducting model, larger than the Debye length, into a free‐streaming high‐energy (1 kv) unmagnetized hydrogen plasma. These experiments are the first electron temperature experiments conducted at energies and compositions directly relevant to solar wind and astrophysical plasma phenomena. The incident plasma parameters were held constant. A large number of axial profiles of the electron temperature ratios Tein/Teout behind the model downstream in the model wake are presented. A rigorous statistical approach is used in the analysis of the electron temperature ratio data in both our experimental laboratory data and in our reanalysis of the published data of others. The following new results are obtained: (1) In energetic plasma flow there is no overall temperature enhancement in the near wake since the best fit to the Tein/Teout data is a horizontal straight line having a mean value of 1.05; (2) No statistically significant electron temperature enhancement peaks or depressions exist in the near‐wake region behind a model at zero potential in a high‐energy plasma even at distances less than or equal to Ma, where M is the acoustic Mach number and a is the model radius. This implies a “filling in” of electrons in the wake region which may be due to the higher mobility of these energetic electrons. This mechanism may permit the solar wind electrons to significantly contribute to the maintenance of the nightside ionosphere at Venus.
Polar rain: Solar coronal electrons in the Earth's magnetosphere
Authors: Fairfield D. H. , Scudder J. D.
Published: February 2008

ISEE 1 low‐energy electron measurements have revealed the frequent presence of field‐aligned fluxes of a few hundred eV electrons in the geomagnetic tail lobes. In the northern tail lobe these electrons are most prominent when the interplanetary magnetic field is directed away from the sun. This characteristic helps identify the electrons as polar rain electrons that are usually identified by their uniform precipitation pattern over the earth's polar caps. Although tail lobe electrons have been seen previously at high altitudes, the ISEE data provide the first demonstration of their field‐aligned nature, and this characteristic places a stringent constraint on their origin. By mapping the tail lobe velocity distribution function into the solar wind, we have confirmed previous suggestions that the polar rain is indeed of solar wind origin and is due to the access of electrons to the magnetotail lobe. More specifically, however, it is demonstrated that the more energetic component of the polar rain is composed of electrons from the solar wind “strahl,” a field‐aligned component of the solar wind which is difficult to measure but which is thought to be caused by the collisionless transit of hundred eV electrons from the inner solar corona to 1 AU. The preferential entry of the strahl into that polar cap most directly connected magnetically to the sun explains a known north‐south asymmetry in polar rain intensity. Furthermore, it is suggested that cases of higher‐energy polar rain that have been thought to require a magnetospheric field‐aligned acceleration mechanism are more likely due to an enhanced intensity of the solar wind strahl caused by easier access of solar corona electrons to 1 AU and the polar caps. This idea is supported by the observation that previously reported cases of intense polar rain are invariably associated with tenuous or high‐speed solar wind that is expected to produce enhanced strahls. It is shown how polar rain measurements may provide a good measure of (1) the temperature and the state of statistical equilibrium in the solar corona and (2) the degree to which interplanetary field lines connect with those of the earth.