bluuebunny/crossref_metadata_2025 huggingface dataset:
The role of field‐aligned ionization irregularities in the generation of the HF‐induced plasma line at Arecibo
Authors: Muldrew D. B.
Published: February 2008

By using the data from the July 1976 ionospheric modification experiments at Arecibo it was discovered that the Langmuir waves responsible for the enhanced plasma line are generated at the height in the ionosphere at which the Airy function, which describes the standing electric field of the high‐frequency (HF) modifying wave near reflection, has its main maximum. It would be impossible for the enhanced Langmuir waves which backscatter the 430‐MHz radar signal to exist at this height in a uniformly varying ionosphere. This implies that underdense ionization irregularities exist in the ionosphere that allow the Langmuir waves to be generated and to propagate to the appropriate ionization density for detection by the incoherent backscatter radar. This discovery puts a new light on the generation mechanism of the HF‐enhanced plasma line, since up until now it has been implicitly assumed in theoretical work that the parametric decay process occurs in a uniformly varying ionosphere, that is, an ionosphere without irregularities. The ionization irregularities responsible for the HF‐enhanced plasma line are probably field aligned with an electron density deviation decrease of about 4% or more and with a diameter between about 1 m and a few hundred meters. It is proposed that Langmuir waves are amplified parametrically as they propagate in the duct to where their wave normals are parallel to the radar beam. The observed frequency displacement from the radar frequency (430 MHz) of the parametric decay line at Arecibo is about fHF ‐ 3.5 kHz, where fHF is the frequency of the HF heating wave (4–10 MHz). Numerical calculations for typical daytime conditions show that the maximum coupling between the HF and Langmuir waves occurs for a frequency offset between the two waves of about 3.4 kHz for the field‐aligned irregularity or duct model, which is in good agreement with observations. The observed frequencies of the peaks in the HF‐enhanced plasma line spectrum, which are displaced from 430 MHz ± fHF by approximately odd harmonics of the ion acoustic or offset frequency, can be explained with the duct model. The ‘broad bump’ observed in some plasma line spectra probably results from cascading of intense Langmuir waves which are produced parametrically and propagate approximately along the axis of the duct. It is proposed that the background or ‘immature’ spectrum may result from a four‐wave decay process. The ‘overshoot’ phenomenon (intense plasma line signals occurring immediately after transmitter turn‐on) may be due to the effects of HF heating on the ducts and on the ambient plasma.
Adiabatic acceleration induced by convection in the plasma sheet
Authors: Galperin Yu. I. , Gladyshev V. A. , Jorjio N. V. , Kovrazhkin R. A. , Sinitsin V. M. , Cambou F. , Sauvaud J. A. , Crasnier J.
Published: February 2008

A study of the average energy and temperature of ions precipitated in the nocturnal auroral zone during periods of low magnetic activity is presented. On the basis of this study, carried out using the Aureole 1 satellite, it can be concluded that the characteristics of ions at auroral altitudes well reflect their convection in the distant plasma sheet and the betatron and Fermi type acceleration accompanying it.
Electric currents in a hot adiabatic plasma
Authors: Hruška A.
Published: February 2008

A large‐scale stationary and collisionless plasma with E × B drifts much smaller than the typical random velocities of particles is considered, using the Chew‐Goldberger‐Low fluid equations. Two mutually dependent but distinguishable currents may develop in the plasma. The magnetic field generated by the currents acquires a geometry in which one current flows in the direction of the binormal to the field lines, while the second current flows perpendicular to the binormal and consists of two inseparable components: magnetic field aligned current and the current flowing in the direction of the principal normal to B. A low‐β plasma immersed in the field of a magnetic dipole is analyzed, assuming the existence of nonzero gradients of the pressure components in the azimuthal direction. Current flowing in the meridional plane is directed almost perpendicular to the field lines near the equatorial plane and is directed mainly along the field lines at the high magnetic latitudes.
On the solar source of transient cosmic ray intensity variations
Authors: Duggal S. P. , Pomerantz M. A.
Published: February 2008

Studies of the mechanisms that produce transient cosmic ray intensity variations have been in progress for many years. It has long been recognized that classical nonrecurrent Forbush decreases (FD's), characterized by an abrupt reduction in the flux (typically over several hours) followed by a slow recovery (often lasting several weeks), are generally the result of processes initiated by solar flares. Thus, for example, it has been demonstrated that anisotropic cosmic ray flow patterns during FD's reflect the configuration of the flare‐associated interplanetary shocks that produce them [Duggal and Pomerantz, 1976].

Much confusion has resulted from the logically incorrect premise that every transient intensity modulation (TIM) is a Forbush decrease. Thus analyses based upon the subjective assignment of a large number of TIM's to their presumed initiating flares have yielded disparate conclusions [Iucci et al, 1977; Krivsky et al, 1977].
Nonlinear investigation of the spatial resonance effect in the nighttime equatorial F region
Authors: Klostermeyer J.
Published: February 2008

In the nighttime equatorial F region, zonal neutral air winds and gravity waves propagating across the magnetic field tend to drive the ionization as if there were no field. When the phase trace speed of a gravity wave equals the drift speed of the ionization, strong ionization perturbations may occur (Whitehead's spatial resonance effect). The resonance effect is described by a parametric differential equation. Without diffusion, it yields perturbations in the form of periodic δ functions. Therefore a coefficient of turbulent diffusion is introduced to limit the perturbation amplitude. Then the equation can be solved in terms of partial Fourier sums. Numerical calculations with reasonable F region parameters indicate that under spatial resonance conditions, gravity waves cause considerable secondary flows of ionization as a result of nonlinear effects. The contours of constant ionization density show steep or even breaking wave fronts similar to those observed by HF radar and radio propagation experiments in the large‐scale structure of the equatorial spread F. The spatial resonance mechanism produces field‐aligned tubes of decreased ionization density which may be the primary cause of ‘plasma bubbles’ detected by satellite and rocket probes.
Incoherent scatter radar observations of irregular structure in mid‐latitude sporadic E layers
Authors: Miller K. L. , Smith L. G.
Published: February 2008

The structure of mid‐latitude sporadic E layers has been investigated in a series of observations at the Arecibo Observatory. The basic experiment uses phase‐coded pulses to measure the electron density profiles with a range resolution of 600 m while scanning the radar beam. The data from incoherent scatter are compared with simultaneous ionosonde observations to determine the source of the partial transparency of sporadic E layers to radio waves at frequencies between fbEs and ftEs. It is concluded that sporadic E layers contain electron density irregularities of sufficient intensity to account for the partially transparent echoes observed by the ionosonde. A feature that becomes immediately obvious in examining the data taken with the incoherent scatter radar is that sporadic E layers are as variable in structure as they are unpredictable in occurrence. Some have simple vertical profiles and are uniform in the horizontal plane, consistent with rocket observations; others have complex vertical and horizontal structure, as has been implied by the radio experiments. Wavelike structure in the electron density is present in many of the layers observed. Evidence is shown of the limiting of peak ionization density in a layer by turbulence generated in a Kelvin‐Helmholtz instability. The possible proximity of a sporadic E layer to a critical level of a dominant gravity wave mode is discussed as a source of intense ionization irregularities. A possible irregular source of ionization from ablating meteors is also considered. It is shown that under specific conditions an ionized meteor trail can be converged by the action of the neutral wind into an irregularity of relatively small dimensions. It is concluded that the observations support the wind shear mechanism of formation of sporadic E layers when the combined effects of gravity waves, tides, and the irregular deposition of metallic ions are included.
A theory for chorus generation by energetic electrons during substorms
Authors: Curtis S. A.
Published: February 2008

Previous observations during magnetospheric substorms reveal the existence of relatively narrow band electromagnetic emissions occurring slightly above and below one half of the electron cyclotron frequency in the equatorial magnetosphere beyond the plasmapause. Between these two bands of emission there exists a gap of much lower emission intensity. In this paper an interpretation is presented for these chorus observations wherein the lower emission band is represented by whistler noise and the upper emission band is described by the lowest harmonic of the ordinary mode. These emissions are assumed to be locally generated at the observation point. Both emission bands are excited by power law‐distributed energetic electrons with a variable spectral index and a weak loss cone feature. The form of these energetic electrons is similar to the forms observed simultaneously with the chorus emissions. The calculated growth rates show a frequency distribution similar to that of the measured emissions. In addition, the same energetic electron distribution can also produce the electrostatic gyroharmonic emissions often seen by observers in conjunction with chorus emissions during substorms.
AE‐C observations of low‐energy particles and ionospheric temperatures in the turbulent polar cusp: Evidence for the Kelvin‐Helmholtz instability
Authors: Potemra T. A. , Doering J. P. , Peterson W. K. , Bostrom C. O. , Hoffman R. A. , Brace L. H.
Published: February 2008

The AE‐C spacecraft skimmed through the southern polar cusp over a wide range of magnetic local times (≈1030–1230 MLT) at approximately 71° invariant latitude during a large magnetic storm on May 16, 1975. The spacecraft was at a constant altitude of 283 km and was spinning at one revolution per 14.6 s. Low‐energy electrons (0–500 eV) were measured with the photoelectron spectrometer experiment, electrons and protons with energies between 200 eV and 25 keV were measured with the low‐energy electron experiment, and ionospheric temperatures with the cylindrical Langmuir probe experiment. The cusp was identified by the presence of intense fluxes of low‐energy electrons and protons with Maxwellian energy spectra similar to those acquired by previous experiments at higher altitudes. The remarkable feature apparent from the AE‐C observations reported here is the marked difference between the pitch angle distribution for the electrons and protons. The protons were observed to be flowing downward along the geomagnetic field lines in the cusp, whereas the pitch angles of the low‐energy electrons behaved in a peculiar fashion. During three revolutions of the spacecraft, intense fluxes of low‐energy electrons were observed at all pitch angles (i.e., nearly isotropic over the entire 4π sphere). High ionospheric electron temperatures (>3000°K) were observed in the cusp, and the highest temperatures (≈4000°K) occurred in the same region as the nearly 4π isotropic electron fluxes. These characteristics cannot be explained by field‐aligned potential drops caused by double layers or anomalous resistivity. The absence of electron fluxes with higher energies (> 1 keV) in this region would also eliminate various current‐driven instabilities as the cause. We suggest that the low‐energy electrons are scattered and that their pitch angles are isotropized by magnetic fluctuations (not measured on AE‐C, but often observed with other experiments) associated with the Kelvin‐Helmholtz instability caused by shear in the proton flow into the cusp. These observations provide the first evidence in the form of low‐energy particle measurements for an instability in the polar cusp at ionospheric altitudes, and they support the suggestion that the Kelvin‐Helmholtz instability plays an important role in this region.
Undisturbed trapping boundary for energetic electrons at low altitudes
Authors: Rossberg L.
Published: February 2008

Dawn‐dusk traversals of the auroral zone by the German polar orbiting satellite Azur are used to reexamine the previously reported dawn‐dusk asymmetry of the boundary position of the 40‐keV electrons. This asymmetry, i.e., higher latitudes of the boundary positions in the dawn sector than in the dusk sector, disappears during prolonged periods of low electrojet activity. Symmetric boundary positions, i.e., significantly lower than average boundary positions in the dawn sector, are found whenever the electron flux decreases smoothly to the cosmic ray background over the polar cap and when there is no enhanced precipitation. Similar, undisturbed flux profiles observed by Ogo 6 at E > 30 keV are used to extend the study to all local times. The resulting average boundary contour has an almost circular shape, centered at 2° away from the magnetic pole on the midnight meridian and is compared with the average position of quiet auroral arcs and also with the average pattern of field‐aligned currents for AL < 100 nT. The average boundary contour of undisturbed 30‐ and 40‐keV electron flux profiles is interpreted as the high‐latitude limit of stable trapping in the undisturbed magnetosphere.
Ring current electron trajectories associated with VLF emissions
Authors: Maeda Kaichi , Bewtra N. K. , Smith Paul H.
Published: February 2008

Intensity enhancements of the ring current electrons associated with the VLF emissions during the geomagnetic storms and substorms, which have been observed by the equatorially orbiting S³‐A satellite (Explorer 45), are so far limited only to electrons of energies below the order of 10 keV. Furthermore, onsets of the enhancement are detected in the lowest observable energy channel first, then shifting toward higher energies with time, a feature, which is opposite to the well‐known energy dependence expected from the drift motion of electrons in the earth's dipole magnetic field. In order to explain these findings on the ring current electron enhancements, trajectories of electrons injected into the nightside of the magnetosphere from the geomagnetic tail are calculated by modifying Ejiri's calculations which were used for the interpretation of the nose events of the ring current protons observed also by the S³‐A satellite. The results indicate that the electron intensity enhancements are limited to electrons below the order of 10 keV simply because of the location of the observations which are initially confined to the dusk‐midnight sector outside the plasmasphere. If the observations are made in the morning sector, where many VLF emissions such as chorus are observed, the enhancement should not be limited only to electrons below the order of 10 keV but should extend to electrons up to the order of 100 keV. The apparent inverse dispersion, i.e., the appearance of the enhancement at lower energies first, is also due to the location of the observations. Namely, if the observations are made in the morning sector outside the plasmasphere, the normal dispersion with the first enhancement by high‐energy electrons should be seen. Since the adiabatic energy increase of charged particles during cross‐magnetic field inward motion in the dipole field is largest for the 90° pitch angle particles, the energy at the source is lowest for 90° pitch angle electrons for the same energy at the observing location. The smaller enhancement of the small pitch angle electron intensity can therefore be explained by the energy spectra of source electrons in the geomagnetic tail.
Evidence for a magnetospheric effect on mid‐latitude electric fields
Authors: Gonzales C. A. , Kelley M. C. , Carpenter L. A. , Holzworth R. H.
Published: February 2008

Electric field measurements at Millstone Hill, which is located at a geographic latitude of 42.6°N and a geomagnetic latitude of 54.1°N, have been made during a time of extremely quiet geomagnetic activity in the 1800–0100 LT interval. The measured north‐south electric field component was primarily southward (eastward F region drift) in this data set, whereas in previously published Millstone Hill data it was northward (westward drift). Furthermore, the quiet time data are in agreement with incoherent scatter and barium cloud results at similar geographic latitudes in the European sector which are at lower geomagnetic latitudes. We thus conclude that the electrical structure over Millstone Hill is almost always affected by magnetospheric effects and that only under very quiet conditions is the classical neutral wind dynamo evident. Several mechanisms for producing the magnetospheric effect are discussed.
Diurnal variation of cosmic ray intensity, 1. Two approaches to the study
Authors: Sari J. W. , Venkatesan D. , Lanzerotti L. J. , Maclennan C. G.
Published: February 2008

The investigation of the daily variation of cosmic ray intensity has been carried out over the past two decades by either of two approaches: the traditional Fourier series method and the more recently introduced power spectral method. A comparison of the two approaches is essential to the proper understanding of the results derived from them. The present study, for the first time, adopts both approaches for investigating the data from the Sulphur Mountain super neutron monitor for the period of mid‐December 1965 to April 1966 (extending over five solar rotations), when interplanetary magnetic field data from Pioneer 6 were also available. Problems relating to the analyses of both data sets on a day‐to‐day basis and on a statistical basis over a number of days are discussed. The power spectral analysis method cannot provide information on the phase of the diurnal variation or information on the diurnal amplitude on a day‐to‐day basis. This method, however, provides excellent estimates of the diurnal anisotropy amplitude independent of any daily phase shifts in the anisotropy. In addition, this method provides a measure of the ambient anisotropy amplitude which, for the entire period studied, has a mean value of ∼0.1%. The Fourier series method can yield reliable measures of the amplitude and phase on a day‐to‐day basis, provided the time series is reasonably stationary. This method cannot estimate the ambient anisotropy amplitude which, for small amplitudes, contributes to large uncertainties in the Fourier coefficients. We find that there is a general agreement between the observed diurnal variation over each period considered and that predicted theoretically from parameters derived from the interplanetary data. However, for most of the periods examined the ratio of the perpendicular diffusion coefficient to the parallel diffusion coefficient is rather small (K⊥/K∥ ≲ 0.1). As such the diurnal variation amplitude is generally most sensitive to the interplanetary field direction and the solar wind velocity and not to the calculated values of the diffusion coefficients. Further tests of the theory will require analyses of other periods when perhaps the perpendicular diffusion is larger and/or measurements of cosmic rays outside the atmosphere are available in order to obtain better energy resolution on the detected particles.
Langmuir wave propagation and the enhanced plasma line in sporadic E
Authors: Muldrew D. B.
Published: February 2008

Observations of HF‐induced plasma lines in blanketing sporadic E at Arecibo have been reported in the literature. The purely growing instability was found to be the probable source of the plasma lines, although the parametric decay instability could not be ruled out. It was also observed that the upshifted plasma lines tend to be much stronger than the downshifted plasma lines. A calculation of the purely growing and decay thresholds, using typical ray paths of the type thought to be responsible for the Arecibo observations, indicates that the purely growing instability is most likely responsible for the observed Es plasma lines. The purely growing threshold is smallest when it is determined by electron collisions rather than by the density gradient and when the wave normal of the unstable waves is nearly parallel to the earth's magnetic field. It is thus likely that the unstable Langmuir waves are generated where these conditions are satisfied and subsequently propagate to where they are observed by the radar. In order that these waves not be highly damped, the density gradient must be greater than about 10° off the vertical. This indicates that these waves propagate in ionization irregularities embedded in the Es layer. For irregularities with an upward component of density gradient directed north of the antimagnetic field direction, upshifted Es plasma lines would occur; otherwise, downshifted Es plasma lines would result. Since the former case covers a much larger range of angles than the latter case, it is not surprising that normally the upshifted Es plasma line is stronger than the downshifted Es plasma line.
Measurements of the magnetic field orientation in the Jovian ionosphere deduced from Pioneer 10 and 11 scintillation observations
Authors: Woo Richard , Yang Fang‐Chou
Published: February 2008

In this paper we analyze the S band scintillations observed during the Pioneer 10 and 11 occultation measurements of Jupiter. We find that while the electron density irregularities are isotropic in the collision‐dominated lower ionosphere, they are anisotropic in the upper ionosphere because of alignment along the magnetic field. By using Rytov's approximation the frequency spectrum of the log‐amplitude scintillations is derived for a wave propagating in an anisotropic turbulent medium. It is shown that the spectrum depends to a large extent on the direction of anisotropy and is therefore useful for measuring the orientation of the magnetic field in regions that have not yet been probed by direct measurements. Applying this new technique to the Pioneer 10 and 11 observations, we deduce the first measurements of magnetic field orientation in the ionosphere of Jupiter.
Absolute cosmic ray ionization measurements in a 900‐liter chamber
Authors: Kyker Granvil C. , Liboff Abraham R.
Published: February 2008

We report a new measurement for the cosmic ray sea level ionization, 2.15 ± 0.05 in ion pairs/s cm³, in agreement with the previous determination by Shamos and Liboff (1966) (2.18 ± 0.05 ion pairs/s cm³) and the calculation by O'Brien (1970) for solar minimum (2.22 ion pairs/s cm³). In contrast with the 1966 measurement (which used CCl2F2), the present work was done in an air‐filled 906‐l aluminum chamber, with a wall thickness of ∼0.3 g/cm². Measurements were conducted over Lake St. Clair and Lake Huron during 1973 and 1974. Corrections for alpha and beta wall contamination were made in two independent ways, by an analysis of the ion current fluctuations and by operating the chamber in the low‐background Morton Salt Company Mine, located 600 m below ground in Painesville, Ohio.
Distortion of interplanetary pitch distributions by Earth's bow shock
Authors: Palmer I. D. , Craig G. S.
Published: February 2008

A theoretical study is made of the distortion of a given interplanetary pitch distribution when particles intercept the earth's bow shock. The behavior of energetic particles with gyroradius much larger than shock thickness, but smaller than a scale length for the magnetosheath geometry, is emphasized (this includes protons of energy 10 keV ≲ E ≲ 1 MeV). Scattering of pitch angles by small‐scale magnetic fluctuations is ignored. The distortion is studied at selected observing stations in both solar wind and magnetosheath. Substantial distortions occur when the interplanetary anisotropy is large. For a given ratio of maximum to minimum directional intensity in interplanetary space the distortion at certain stations depends on (1) the magnetic configuration and (2) the detailed shape of the interplanetary pitch distribution. Only when the bow shock is almost a perpendicular shock can the approximation of guiding center motion be employed with Liouville's theorem. Distorted pitch distributions in the magnetosheath are often characterized by a pronounced break, which can lie at positive, zero, or negative µ. Particle reflection from the bow shock sunward of earth is substantial. Some observations are presented which show trends that are consistent with the theoretical results. Over all, effects of the bow shock (at least sunward of earth) are serious, and particular care needs to be taken in inferring interplanetary pitch distributions and anisotropies from those measured by satellites near the earth.
Whistler observations of substorm electric fields in the nightside plasmasphere
Authors: Park C. G.
Published: February 2008

Whistlers recorded in the Antarctic were used to infer the east‐west component of electric field in the nightside plasmasphere between L ∼ 3 and 6 during moderate substorms. The results show eastward electric fields of ˜0.2 mV/m in the equatorial plane in the premidnight local time sector. Near midnight there is a sharp transition from these eastward fields to westward fields of ˜0.2–0.6 mV/m. The existence of westward substorm fields in the postmidnight sector has been reported in several previous papers as a prominent feature in the whistler data. These fields are distinct from quiet time fields that are directed eastward over the entire nightside and have an average magnitude of ˜0.05 mV/m. The substorm electric field pattern described here is consistent with previous ionosonde observations of large‐scale distortions of the nightside F layer showing an uplifting in the premidnight sector and a lowering in the postmidnight sector.
Tidal variations in total mass density as derived from the AE‐E Mesa Experiment
Authors: Forbes Jeffrey M. , Marcos Frank A.
Published: February 2008

Total mass density data from the miniature electrostatic accelerometer (Mesa) experiment on AE‐E are analyzed to determine diurnal and semidiurnal tidal variations in the lower thermosphere (150–245 km) at low latitudes (<20°). The local time density structure changes from predominantly semidiurnal below 180±5 km to predominantly diurnal above this transition height, with no strong dependence on season. However diurnal phases in summer occur 6 hours (at low heights) to 1½ hours (at higher heights) earlier than winter, whereas semidiurnal phases during summer occur from 1 hour (at low heights) to 6 hours (at higher heights) later than winter. Analyses of data from all seasons to determine an annual mean semidiurnal amplitude can therefore lead to unrealistically small semidiurnal amplitudes due to phase cancellation effects. These results are compared with analyses of AE‐E data by other investigators, and with recent empirical and theoretical models of thermospheric composition and density. Variations between equatorial and middle latitudes as predicted by theory are estimated to be of the order of 20–50% in amplitude and 3–6 hours in phase for the semidiurnal component, 0–20% in amplitude and 0–6 hours in phase for the diurnal component, and 15 km with respect to the transition height from semidiurnal to diurnal predominance. Therefore tidal variations in total mass density as derived from AE‐E measurements cannot be extrapolated to middle latitudes without incurring errors of these magnitudes.
Discrepancy in proton flux extrapolation along field lines in the middle Jovian magnetosphere
Authors: Schardt A. W. , Birmingham T. J.
Published: February 2008

Fluxes of energetic electrons and protons in Jupiter's outer magnetosphere were observed to be modulated with the 10‐hour rotation period of the planet. This modulation is due to the concentration of particles at the magnetic equator: the nonalignment of Jupiter's spin and rotation axes causes Pioneer 10 to oscillate between +2° and −19° magnetic latitude and hence between regions of stronger and weaker fluxes. In this paper we investigate the relationship between electron and proton fluxes observed off the magnetic equator with those measured at the equatorial crossing radii of the same flux tubes. Liouville's theorem is applied with the assumption that particles move conserving their magnetic moments. A magnetic model which matches the intensity and direction of the magnetic field along the Pioneer 10 trajectory is used for determining the positions of the equatorial crossings. Energetic electrons (1.3 MeV) compared in this way appear to be consistently described. Protons, on the other hand, show much weaker fluxes at the off‐equatorial points than would be predicted by this simple application of Liouville's theorem. Violation of the first adiabatic invariant is one explanation; other potential explanations depend on slow magnetic field fluctuations which are not included in the magnetic model and which conserve the first invariant or on a large asymmetry in equatorial proton flux as a function of system III longitude.
The solar proton event of April 16, 1970, 3. Evolution of pitch angle distribution as ≲1‐MeV protons propagate into the high‐latitude magnetotail
Authors: Palmer I. D. , Jackson J. C. , Hones E. W.
Published: February 2008

The solar proton event of April 16, 1970 was monitored by Vela satellites, of orbit r=18 RE, in the solar wind and high‐latitude magnetotail (lobe). Intensity structure at ≲1 MeV indicates a delay of 85–102 min in access of protons to near the center of the north lobe, corresponding to entry points at 340–370 RE from the earth. In three sequential periods, of 16, 181, and 124 min duration, the average intensity in the north lobe was lower, higher, and lower, respectively, than that in interplanetary space by factors which varied from 2 to 5. These reversals were a consequence of reversals in field‐aligned anisotropy in interplanetary space, the interplanetary magnetic field remaining southward. Pitch angle distributions were measured in three dimensions in interplanetary space and in the north lobe. In the lobe the distributions were essentially isotropic at r=18 RE. Comparison is made with theoretical propagation of solar particles along field lines in an open tail model, under the following conditions along a trajectory: (1) adiabatic motion all the way (Liouville theorem)—the ‘adiabatic access model’ or (2) isotropization at the magnetopause followed by adiabatic motion—the ‘nonadiabatic access model.’ Neither mode of access explains the observations adequately. A hybrid mode is proposed, in which a minimal amount of scattering occurs as particles enter the tail, followed by amplification (attenuation) of intensity as the pitch distribution is transformed to near 18 RE in the favored (unfavored) lobe. In this mode a large part of the isotropization at Vela orbit is accomplished by the Liouville transformation, since particles entering the tail beyond ≈100 RE will see an increase in magnetic field by a factor of 3 as they propagate along the tail. The amount of scatter at the magnetopause is estimated to be Δµ(rms)=0.3, where µ is cosine of pitch angle. Intensity discrepancies later in the event are interpreted as (1) a brief adjustment in the topology of the magnetotail in which the field line through Vela 6A changed its exit point to ≈530 RE, compared to ≈350 RE before and ≈420 RE after and (2) interconnection between interplanetary and magnetotail fields determined by the radial component of the interplanetary field, as distinct from the southward component during the main portion of the event (the particle access window, however, remained at the same distance from the earth).
An alternative explanation of the Doppler spectra of current‐driven plasma instabilities
Authors: Greenwald Raymond A.
Published: February 2008

Radar observations of Doppler spectra from electron density irregularities in the auroral and equatorial electrojets often fall into two categories: narrow spectra with constant Doppler shifts and broad spectra with Doppler shifts dependent on the angle between the radar wave vector and the mean electron drift direction. Until now these spectra have been related to the two‐stream and gradient‐drift instabilities, respectively. In this paper we examine whether effects other than direct association of the instability type with the spectral type could reproduce the spectral observations. In particular, we make three assumptions: (1) both two‐stream and gradient‐drift irregularities have angle‐dependent Doppler shifts, (2) the electron drift direction within the scattering volume is broadened due to plasma turbulence or spatial structure, and (3) the scattering cross section (mean square density fluctuation) is dependent on the angle between the radar wave vector and the local electron drift direction. These assumptions are sufficient to reproduce many of the spectral observations, including Doppler velocity plateaus. The plateau velocities under these assumptions have magnitudes that are slightly less than the electron drift velocity.
Unstable electron cyclotron harmonic waves during the relaxation of a temperature anisotropy
Authors: Kiwamoto Y.
Published: February 2008

Electrostatic electron cyclotron harmonic waves are driven by an excess electron kinetic energy perpendicular to the magnetic field. If the anisotropy of the velocity distribution is transient, the propagation characteristics of the anisotropy‐driven waves change in time. The long‐time behavior of the waves is determined by the dispersion relation corresponding to the final velocity distribution. A model case is studied, within the framework of linear theory, for 3/2 electron cyclotron harmonic waves in the relaxation stage of a bi‐Maxwellian velocity distribution. It is shown that the dominant propagation angle changes from about 60° in an unstable state to about 80° in an isotropic state. Because of strong Landau damping, the most unstable wave in the instability cannot be a dominant wave after the relaxation. Since this result may be true for other velocity distribution models, it implies that great care should be taken in considering a wave model which requires a long lifetime for some of the waves involved.
A model of the auroral electric field
Authors: Kan J. R. , Akasofu S.‐I.
Published: February 2008

A model is constructed from a self‐consistent solution of the Vlasov‐Maxwell equations. The model has certain properties in common with auroral arcs. A pair of electric fields similar to the auroral electric fields is shown to be inherent to the electron current sheet in the model. The magnitude of the inherent fields is related to the width of the current sheet which in turn depends on the current density and the ion and electron energy densities. The potential energy associated with the inherent electric field is on the order of the ion thermal energy. These properties of the model are in qualitative agreement with the observed relationship between the width and the brightness of an auroral arc, and the relationship between the average energy of auroral electrons and the latitude.
F layer height bands in the nocturnal ionosphere over Arecibo
Authors: Behnke R.
Published: February 2008

A study of the nocturnal F layer over Arecibo, Puerto Rico, reveals that substantial spatial variations in the height of the layer are frequently present. The variations are deduced from electron density profiles measured by the incoherent radar in which the radar beam is offset 17° in zenith and rotated continuously in azimuth. This scanning procedure shows that the ionosphere above Arecibo often has well‐defined bands where the F region as a whole is alternately high and low. The transition between adjoining regions is very sharp. For example, electron density profiles taken at points only a few degrees apart in azimuth, which corresponds to a spatial separation of less than 10 km, can show variations in hmax of up to 70 km. It is suggested that these bands are manifestations of the so‐called ‘Perkins’ instability.
The relative intensity of the [N I] 1.04‐µ m multiplet as an indicator of the mean height of auroral forms
Authors: Jones A. Vallance , Gattinger R. L.
Published: February 2008

Quenching of N(²P) by atomic O has been included in the calculations used to estimate changes in mean emission heights of auroral forms previously studied by Vallance Jones and Gattinger (1978). Recent estimates of the production efficiency for N(²P) in electron impact dissociation of N2 permit a cross check on the estimated height from the intensity of the 1.04‐µ m [N I] multiplet. Satisfactory agreement is obtained if the atomic O concentration in the 110‐ to 120‐km region was about half that of the Jacchia (1971) model for a N(²P)‐O quenching rate coefficient of 10−11 cm³ s−1.
Characteristics of trough region auroral patches and detached arcs observed by Isis 2
Authors: Moshupi M. C. , Anger C. D. , Murphree J. S. , Wallis D. D. , Whitteker J. H. , Brace L. H.
Published: February 2008

Two distinct types of auroras have been observed in the trough region (almost exclusively in the northern hemisphere) equatorward of the diffuse auroral boundary by the Isis 2 satellite scanning auroral photometer. The first appears as east‐west‐aligned oval‐shaped ‘patches,’ often appearing in groups and corotating with the earth just poleward of the plasmapause. The second consists of isolated east‐west ‘detached arcs’ which decrease in latitude and intensity to the east and show no evidence of corotation. The arcs occur at higher latitudes than the patches near the poleward edge of the F layer trough. Both patches and arcs are evening sector phenomena, although patches can be found in the morning. Both also show a longitude dependence, with the arcs centered mainly over the Alaska sector and the patches over Canada. Patches are more intense than arcs, while both tend to be less intense than the diffuse aurora poleward of them, although they have a lower ratio of 5577‐Å intensity compared to 3914 Å than the diffuse aurora, a finding suggestive of greater particle energies. Patches occur at times when Dst is increasing following a magnetic storm, while detached arcs occur about 10–12 hours after a peak in the AE index and at a time when the (statistically averaged) interplanetary field component Bz is positive following an extended period when it was negative. These dependencies, together with the particle observations reported in a companion paper, suggest that the sources for the trough region auroras are plasma sheet particles injected during magnetic storms or substorms and left behind by a poleward retreat of the auroral oval. The similar latitude‐local time dependencies of ELF emission regions detected by Chan and Holzer suggest cyclotron resonance scattering of trapped particles as the likely mechanism for the precipitation of these particles. Two rather distinct types of cold plasma regions are then required in order to account for the two types of optical features.
Diurnal and seasonal variations of the steady state loss coefficient in the D region
Authors: Montbriand L. E. , Belrose J. S.
Published: February 2008

A study was made of the steady state loss coefficient ψ (= q/N²) deduced for quiet periods from previously published electron density height data. Near 75 km, ψ was found to remain fairly constant with diurnal changes of the solar zenith angle χ, while above 77 km, ψ decreased with decreasing χ. Since a seasonal variation in ψ was not clearly evident, observational data selected from more than 4 years of partial reflection observations at Ottawa, were analyzed for quiet intervals at the minimum solar zenith angle (χ≡68°) that could be followed in all seasons. This study revealed that for heights above 76 km, ψ was a minimum in winter and a maximum in summer. Near 75 km neither a seasonal nor a diurnal variation was found, and ψ remained fairly constant near 2 × 10−5 cm³ s−1. These results are interpreted as indicating that for heights above 77 km, (1) as χ decreases, there is a gradual decrease in the number density of the larger hydrated positive ions thought to be responsible for the high ψ values and (2) the existence of these larger hydrate positive ions is temperature dependent, such that they are absent in the warm winter mesosphere and present in the cool summer mesosphere.
Ultralow frequency oscillations of the high‐latitude ionospheric electric field
Authors: Gottlieb B.
Published: February 2008

Balloon observations of the horizontal electric field, from six Canadian sites during August 2, 3, 8, and 9 of 1969, have been analyzed for coherent oscillations: right, left, and linearly polarized in the 1–16 mHz range. On any particular day there are ‘calm’ periods where a number of frequencies in the 1–16 mHz range exist. The length of these periods may vary from a few hours to several hours. The ‘disturbed’ periods can occur at any time of the day. The length of disturbed periods, number of significant frequencies in the band, and their probability of occurrence show a tendency to increase with latitude up to the auroral zone, beyond which no significant increase has been seen. The regions of the disturbed electric field sometimes cover a wide range of latitudes (>10°) and longitudes in the auroral zone. Some of these regions are well defined and fixed in space and may remain unchanged for several hours to sometimes more than a day. For the frequency band studied, polarization of one kind (right or left) may dominate over the other for several hours after which the roles may interchange. These reversals do not show any clear‐cut latitudinal dependence. Instead, there is more similarity in their behavior. Preliminary study of power spectra shows the presence of several dominant frequencies. Comparison of the most dominant frequencies indicates a possible shift to higher frequency with increase in magnetic activity.
A search for geographic control on the occurrence of magnetospheric ELF emissions
Authors: Tsurutani B.T. , Church S.R. , Thorne R.M.
Published: February 2008

Search coil magnetometer data obtained directly in the magnetosphere on Ogo 5 and Ogo 6 have been studied extensively for possible evidence of man‐induced stimulation of the two dominant electromagnetic emissions, chorus and plasmaspheric hiss. Analysis of chorus obtained during the first year of operation of Ogo 5 exhibits considerable variability with geographic longitude. However, there is little or no correspondence with the longitudinal dependance of Ogo 3 chorus previously reported by Luette et al. (1977). This discrepancy is explained in terms of the persistent nature of chorus which can lead to oversampling of the data. When oversampling is eliminated, the longitudinal variability of Ogo 5 chorus is adequately explained by normal statistical fluctuations. A search was also made for enhancements, onsets or quantum jumps in chorus emissions at frequencies related to harmonics of the earth's transmission lines. No significant effect was found. An independent study of plasmaspheric hiss detected on the polar‐orbiting satellite Ogo 6 indicates that this emission is also essentially uniformly distributed in dipole longitude. The only exception to this was a modest and probably significant (4.5σ) enhancement in the occurrence of weak emissions in the 340°–360° dipole longitude region during very quiet geomagnetic conditions. The source of this enhancement could be man induced, or it might simply be related to the expected enhanced level of sferic activity associated with thunderstorm activity over the North American continent. However, because this feature is present only when ELF hiss is weak, it is clear that it will not contribute significantly to the non adiabatic dynamics of radiation belt electrons. We conclude that the totality of satellite data analyzed to date is consistent with a predominantly natural origin for chorus and plasmaspheric hiss. While we do not dispute the existence of triggered waves, it is difficult, in view of their low overall occurrence, to imagine that they play any major role in the non adiabatic dynamics of geomagnetically trapped electrons.
Transversely accelerated ions: An ionospheric source of hot magnetospheric ions
Authors: Klumpar D. M.
Published: February 2008

A source of hot magnetospheric ions has been discovered operating in the auroral topside ionosphere. A part of the cold ionospheric ion distribution is being transversely accelerated within a source region as low as 1000 km. Subsequent to their transverse acceleration the ions are driven upward into the magnetosphere by the gradient B mirror force. These ions are observed by the soft particle spectrometers on the Isis 1 and 2 satellites, with sharply defined pitch angle distributions centered between 90° and ∼120° and having energy distributions typically up to several hundred electron volts. Observations at 1400 km indicate the transversely accelerated ion (TAI) events are predominantly a winter nightside auroral zone phenomenon. However, in the summertime at altitudes above 2750 km, TAI events are observed on the dayside within the cleft. Probabilities of occurrence at some invariant latitude‐magnetic local time locations within the nightside auroral zone at 1400 km exceed 50%. Typical intensities of >108 ions/cm² s sr during TAI events suggest that they may be a significant source of hot ions for the magnetosphere.
Simultaneous multifrequency observations of Jovian S bursts
Authors: Flagg R.S. , Desch M.D.
Published: February 2008

The millisecond component of the Jovian decameter emission has been studied at high resolution, in order to examine the short‐term behavior of the S‐burst drift rates and to define the drift rate spectrum at high frequencies. By using dynamic spectra having 300‐µs and 3.3‐kHz resolution and covering discrete frequency bands in the range 26–33 MHz, large systematic changes in the magnitude of the S‐burst drift rates are observed to take place on a time scale of seconds to minutes. It is noted that the drift rate variability is of sufficient magnitude to prevent reliable evaluation of the drift rate dependence on frequency, unless simultaneous or near‐simultaneous multifrequency measurements are made. In presenting such observations we fail to observe the predicted drift rate turnover in the spectrum, in spite of measurements made at frequencies within 0.6 to 4.4 MHz of the electron gyrofrequency at the Jovian foot of the Io flux tube. This result is interpreted in terms of an ionospheric electron source, rather than within the conventional theoretical framework of trapped particles initially accelerated at Io. Low‐resolution intensity‐time tracings reveal a characteristic organization of S bursts into groups, each of several seconds duration and consisting of tens to hundreds of bursts. Abrupt changes in mean drift rates from group to group suggest that some of the group structuring may be a source‐related rather than a scintillation‐related phenomenon. Finally, some evidence exists for systematic variations in the group‐to‐group mean drift rates, which may be due to predictiable temporal changes in the magnetic field gradient on the Io flux tube. These temporal changes are due to motion of the Io‐associated magnetic flux tube in the Jovian magnetosphere.