bluuebunny/crossref_metadata_2025 huggingface dataset:
Plasma waves in the polar cusp: Observations from Hawkeye 1
Authors: Gurnett D. A. , Frank L. A.
Published: February 2008

In this paper we investigate the characteristics of plasma waves observed by the Hawkeye 1 spacecraft in the vicinity of the polar cusp. The primary types of plasma waves associated with the polar cusp are (1) a band of ULF‐ELF magnetic noise extending from a few hertz to several hundred hertz, (2) a broad‐band electrostatic emission extending from a few hertz to about 30–100 kHz, with maximum intensities at about 10–50 Hz, (3) electrostatic electron cyclotron waves near the electron gyrofrequency, and (4) whistler mode auroral hiss emissions. Of these various types of waves, only the ULF‐ELF magnetic noise is uniquely associated with the cusp in the sense that the noise can be used as a reliable indicator of the polar cusp region. All of the other types of plasma waves occur in regions adjacent to the polar cusp as well as in the cusp itself. Spectrum measurements often show that the ULF‐ELF magnetic noise extends up to, but does not exceed, the local electron gyrofrequency fg−. This upper cutoff strongly suggests that the noise consists of whistler mode electromagnetic waves. The mechanism for generating these waves remains highly uncertain, however, since the electron angular distribution in the cusp is usually not sufficiently anisotropic to account for these waves by the well‐known whistler mode cyclotron resonance instability. Other mechanisms, such as turbulence generated by the Kelvin‐Helmholtz instability or by a drift wave instability, have also been suggested to generate this noise. The broad‐band electrostatic noise is believed to be caused by a current‐driven electrostatic instability (ion cyclotron or ion acoustic) of the type widely believed to occur in auroral field‐aligned current systems. The mechanisms for generating electron cyclotron waves and auroral hiss emissions are believed to be reasonably well understood, on the basis of previous studies of these emissions in other regions of the magnetosphere.
Initial results from a tracking receiver direction finder for whistler mode signals
Authors: Leavitt M. K. , Carpenter D. L. , Seely N. T. , Padden R. R. , Doolittle J. H.
Published: February 2008

A new apparatus combining a tracking receiver and a direction‐finding signal processor for observation of VLF whistler mode signals is described. The system is expected to achieve greater bearing accuracy than can be obtained from a conventional goniometer and is usable on a wider variety of signals than can be processed by most other techniques. Convenience in data output has been emphasized. In July–August 1975 the new tracker/direction finder (TR/DF) was successfully field tested at Roberval, Canada (L ≃ 4). Observations were made on natural whistlers and emissions and on signals from the Siple, Antarctica, VLF transmitter. As a test of accuracy on arrival bearing of whistler mode signals, the TR/DF output was compared with results from a goniometer operating from the same set of crossed loop and vertical antennas. The two systems agreed within ≃10° on several‐minute averages of bearings of both Siple transmitter pulses and natural signals. Effects of multipath propagation and polarization error have been identified. On the basis of the initial data sets it is found that whistler mode signal paths in the outer plasmasphere have ionospheric exit points that can frequently be resolved to within a region 30–40 km on a side. The apparent whistler mode signal path endpoints are relatively close to the ionospheric projections of the field lines along which the wave energy propagated, within 50–100 km (in the north‐south direction). Bearings of Siple transmitter signals show an apparent concentration to the southwest of the receiving station.
Generation mechanism of proton cyclotron echoes due to pulsed radio frequency waves in space plasma
Authors: Oya Hiroshi
Published: February 2008

Analyses of the proton cyclotron echoes observed on the Alouette 2 ionograms, in more than 800 examples, indicate that the echoes are produced in a range within ±25° width of the magnetic dipole latitude centered at the magnetic equator. The generation mechanism of these proton cyclotron echoes is clarified here as follows: (1) the transmitted RF pulse makes an electrostatic plasma wave that produces a fast electron stream in opposite directions with respect to the wave propagation direction during damping processes of the wave; (2) the RF pulse also produces a proton concentration surrounding the antenna due to negative excursion of the antenna potential; (3) the proton concentration produced is repeated each proton cyclotron period and makes a pseudo‐wave of the electric field; and (4) the encounter of the electron stream with the pseudo‐wave due to the proton concentration makes a plasma wave with frequency in the transmitted RF range. The place where the electron stream encounters the pseudo‐wave should be located at a position relatively close to the satellite for the observation of the RF echoes.
Whistler mode noise in Jupiter's inner magnetosphere
Authors: Sentman D. D. , Goertz C. K.
Published: February 2008

The distinctive ‘hat shape’ of equatorial pitch angle distributions constructed from Pioneer 10 and 11 Jupiter observations of energetic electrons Ee > 21 MeV and Ee > 31 MeV at L = 3 is examined from the point of view of pitch angle diffusion by resonant interaction with a band‐limited spectrum of whistler mode noise. In this picture, the pitch angle profiles are consistent with whistler mode noise limited to frequencies below an upper cutoff frequency 2.8 ≲ fc ≲ 4.6 kHz. Equatorial linear growth rates of parallel propagating whistlers are evaluated in a fully relativistic manner for the inner region 3 < L < 10 using previously published models for the spatial distribution of thermal plasma and the energetic electron distributions. The maximum frequency for which wave growth is positive at L = 3 is roughly consistent with that implied by the 21‐ and 31‐MeV equatorial pitch angle profiles. Maximum growth rates computed from the respective energetic electron models span the range 0.1 ≲ γmax ≲ 10 s−1 throughout the inner region. The spectral extent of the whistler mode noise, defined by those frequencies for which γ > 0.5γmax, is approximately 2–10 kHz at L = 3 and falls smoothly to 0.2–2 kHz at L = 10. Raytracing of nonducted whistlers of characteristic frequencies 1–4 kHz generated at the magnetic equator and propagated through a finite temperature (100–400 eV) hydrogen plasma centrifugally confined near the magnetic equatorial plane shows that wave phase speeds decrease to near or below electron thermal speeds before high‐latitude reflection can occur. Wave growth in this case is limited to a disclike region centered about the magnetic equator. The amplitude growth of the most unstable whistlers propagating across the growth region parallel to the magnetic field is ∼exp (3–4). By equating energetic electron radial diffusion injection times to the pitch angle diffusion loss times of electrons in cyclotron resonance with waves of frequencies of maximum unstable growth, the power density of whistler mode noise is estimated to be 10−19±1 G² Hz−1. The balance equation for radial diffusive injection and pitch angle diffusive losses occurring in the presence of whistler mode noise of characteristic growths exp (3–4) yields upper limit estimates for the magnitude of fluctuations about the stably trapped limiting intensities in the region 3 < L < 10. This upper limit is approximately at the 10% level, in reasonable agreement with the smooth radial flux profiles observed during the Pioneer‐Jupiter encounters.
Electrostatic noise generated by the auroral electron beam
Authors: Maggs J. E.
Published: February 2008

Refraction of electrostatic noise in the whistler and upper hybrid modes out of the spatially limited amplification region of the auroral electron beam limits the amplitude of the noise produced. Power flux spectra are calculated by using the linear electrostatic growth rate and allowing for ray refraction in the direction perpendicular to the auroral arc. A graphical algorithm is developed for predicting the power flux spectra of whistler and upper hybrid noise for a wide range of ionospheric and beam parameters. Predicted power flux spectral shapes and intensities for the whistler mode are in good agreement with observations of VLF hiss. The power flux in the upper hybrid mode can exceed the whistler mode power flux in the region of the ionosphere where the electron plasma frequency is larger than the electron cyclotron frequency. A distinction based on the beam intensity is made between quiet and intense arcs. Quasi‐linear and nonlinear processes are not important in quiet arcs but may produce observable effects in intense arcs. In particular, intense electrostatic noise near the plasma frequency may produce deep density cavities that radiate electromagnetic kilometric radiation.
Nonlinear pitch angle scattering of energetic electrons by coherent VLF waves in the magnetosphere
Authors: Inan U. S. , Bell T. F. , Helliwell R. A.
Published: February 2008

A computer simulation approach is used to study the nonlinear cyclotron resonant interaction of energetic electrons and coherent VLF waves, with special emphasis on the pitch angle scattering of the particles. Complete equations of motion in an inhomogeneous magnetosphere are used, and the effects of various parameters are studied. Comparison is made with linear theory, and a quantitative and easy‐to‐use criterion to determine the applicability of linear theory under any given conditions is presented. For example, in the case of equatorial scattering by a 5‐kHz CW pulse near L = 4 it is found that linear theory begins to break down when the wave amplitude exceeds 3 mγ. The full distribution of particles is simulated by test electrons appropriately distributed in energy and pitch angle. By computing the scattering of these test particles and integrating over energy and pitch angle, the precipitated flux is obtained. The method used is quite general and can be used for any particle distribution. It is shown that significant particle fluxes are precipitated by waves of moderate intensity. For instance, energetic (1–2 keV) electron fluxes as high as ∼10−1 erg/cm² s can be precipitated by a CW wave of 10‐mγ amplitude and 5‐kHz frequency on the L ≃ 4 field lines. Such fluxes appear to be measurable with presently available instruments. Since the energy density of the precipitated flux is 50 dB above that of the wave, the leverage involved in the wave‐induced precipitation process is quite high. Our results indicate that coherent VLF waves can have a significant effect on the dynamics and lifetimes of energetic electrons trapped in the magnetosphere on magnetic shells illuminated by the waves.
Characteristics of sunward flowing proton and alpha particle fluxes of moderate intensity
Authors: Marshall F. E. , Stone E. C.
Published: February 2008

The diffusive streaming of 1.3‐ to 2.3‐MeV per nucleon protons has been found to be predominantly toward the sun during periods between prompt solar particle events. This sunward streaming occurs for essentially all proton intensities from <0.012 to 1.2 (cm² s sr MeV)−1 and for all solar wind velocities. The average radial component (16% ± 3%) of the diffusive anisotropy of 1.4‐ to 2.4‐MeV per nucleon alpha particles is very similar to that observed for protons (14% ± 1%), a finding suggesting a common origin. These periods are characterized by a limited variance in the proton intensities (10−1.2±1.1 cm−2 s−1 sr−1 MeV−1), in the proton spectra (E−3.0±0.8), and in the α/p ratio (3% ± 2%). The sunward diffusion of protons and alpha particles indicates that a positive radial gradient is characteristic of these modestly enhanced fluxes. A steady state propagation model which includes adiabatic energy loss and a source of particles beyond 1 AU produces the average radial anisotropy and its dependence on the solar wind velocity for κrr ∼ 4 × 1020 cm² s−1. The direction of the diffusive anisotropy is strongly dependent on the magnetic field direction, a situation indicating κ⊥ < κ∥. However, the two directions are not identical, a condition indicating nonnegligible flow perpendicular to the average field direction when averaged over a 6‐hour interval.
Preliminary measurements of ion‐neutral collision frequencies and mean temperatures in the Arecibo 80‐ to 100‐km altitude region
Authors: Tepley C. A. , Mathews J. D.
Published: February 2008

The Arecibo 430‐MHz radar was used in October and November of 1974 to make preliminary measurements of collision‐dominated Thomson scatter spectra in the 80‐ to 100‐km altitude region. A technique for interpreting the altitude variations of the collision‐dominated spectra is presented. This technique employs the relaxable assumptions of an isothermal atmosphere and ion‐neutral collision frequency variation with altitude according to the Barometric Law. Within these assumptions, temperature, collision frequencies, and total neutral number densities are inferred. We found the winter tropical mesopause region to be slightly warmer and denser than is indicated in the Cira (1972) standard atmosphere.
Comparison of 74‐MHZ interplanetary scintillation and Imp 7 observations of the solar wind during 1973
Authors: Coles W. A. , Harmon J. K. , Lazarus A. J. , Sullivan J. D.
Published: February 2008

Solar wind velocities measured by earth‐orbiting spacecraft are compared with velocities determined from interplanetary scintillation (IPS) observations for 1973, a period when high‐velocity streams were prevalent. The spacecraft and IPS velocities agree well in the mean and are highly correlated. No simple model for the distribution of enhanced turbulence within streams is sufficient to explain the velocity comparison results for the entire year. Although a simple proportionality between density fluctuation level and bulk density is consistent with IPS velocities for some periods, some streams appear to have enhanced turbulence in the high‐velocity region, where the density is low.
Magnetosheath bursts of predominantly medium nuclei observed with Imp 8 on February 16, 1974
Authors: Armstrong T. P. , Krimigis S. M. , Lepping R. P.
Published: February 2008

This paper presents observations made with detectors aboard Imp 8 of a unique series of charged particle bursts that occurred on February 16, 1974, while the spacecraft was traversing the dawn magnetosheath region. This event was unlike any other observation of Imp 7 and 8 in 5½ years of operation in orbit, and, to our knowledge, no other observation of this type has been reported. The measurements are most directly interpreted as an intense, 6 × 10³ (cm² s sr MeV/nucleon)−1, highly collimated (≲5° width) beam flowing in the antisolar direction composed of medium (carbon, nitrogen, or oxygen) nuclei. Both the degree of collimation in arrival direction and the composition are unique to this event and have never been observed in the hundreds of magnetospheric bursts or solar and interplanetary events present at these distances (20–35 RE) from earth. We present this event as a possibly important signature of terrestrial O+ ions escaping from the magnetosphere. Since it is a unique observation, we have examined the instrument performance very carefully and have concluded that it is valid.
A three‐dimensional model of corotating streams in the solar wind, 1. Theoretical foundations
Authors: Pizzo V.
Published: February 2008

This paper is concerned with the development of the theoretical and mathematical background pertinent to the study of steady, corotating solar wind structure in all three spatial dimensions. The dynamical evolution of the plasma in interplanetary space (defined as the region beyond roughly 35 Rs where the flow is supersonic) is approximately described by the nonlinear, single‐fluid, polytropic magnetohydrodynamic or hydrodynamic equations. We outline efficient numerical techniques for solving this complex system of coupled, hyperbolic partial differential equations. The present formulation is inviscid and nonmagnetic, but our methods allow for the potential inclusion of both features with only modest modifications. We examine one simple, highly idealized hydrodynamic model stream to illustrate the fundamental processes involved in the three‐dimensional dynamics of stream evolution. We find that spatial variations in the rotational stream interaction mechanism produce small nonradial flows on a global scale that lead to the transport of mass, energy, and momentum away from regions of relative compression and into regions of relative rarefaction. The magnitude of this transport is small, but inside 1 AU the nonradial flow can significantly retard shock formation by allowing fluid in the compressions to slip laterally, thereby partially relieving the stresses built up in the stream interaction. Comparison with simpler models demonstrates the essential nonlinear, multidimensional nature of the interplanetary dynamics. A subsequent paper will be devoted to the investigation of a wide range of more realistic model streams.
Stability of the auroral plasma: Parallel and perpendicular propagation of electrostatic waves
Authors: Kaufmann R. L. , Dusenbery P. B. , Thomas B. J.
Published: February 2008

Experimental measurements of the electron distribution function are used to study the stability of plasma in the auroral ionosphere. Measurements were taken at altitudes of 200–250 km and cover the 25‐eV to 15‐keV energy range. The observed plasma is shown to be stable to electrostatic waves that propagate exactly parallel or perpendicular to the magnetic field. The measured distribution function has a secondary peak, but this peak is either too small or too limited in extent to drive parallel or perpendicular instabilities. A preliminary study of oblique electrostatic waves during one time interval indicates that the upper hybrid mode is stable for all propagation directions at the rocket location but becomes weakly unstable several hundred kilometers higher where the background plasma density drops below 3 × 104 el/cm³.
Testing Hooke's perturbation formula for the production of TID's by gravity waves
Authors: Morgan M. G. , Calderón C. H. J.
Published: February 2008

Hooke (1968) developed a formula for the perturbation of electron density in the F2 region of the ionosphere by any spectral component of a gravity wave, the perturbation being manifested as the corresponding spectral component of a traveling ionospheric disturbance (TID). We have used the phase portion of that formula to obtain the real part of the vertical component of the propagation constant of the causative gravity wave from that of a TID. With this we have compared the amplitude portion of Hooke's formula with the observed TID amplitude. Doing this over a range of height within a particular TID, we find relatively good agreement. It is suggested that further improvement would require using an electron density perturbation formula which takes account of ion diffusion.
Nonlinear equatorial spread F: Dependence on altitude of the F peak and bottomside background electron density gradient scale length
Authors: Ossakow S. L. , Zalesak S. T. , McDonald B. E. , Chaturvedi P. K.
Published: February 2008

Four different two‐dimensional (perpendicular to the ambient magnetic field) plasma fluid‐type numerical simulations following the nonlinear evolution of the collisional Rayleigh‐Taylor instability in the nighttime equatorial F region ionosphere have been performed. Realistic altitude dependent ion‐neutral collision frequencies, recombination rates, and ambient electron density profiles were used. In three cases (ESF 0, 1, 3) the electron density profile was kept constant, with a minimum bottomside background electron density gradient scale length L ∼ 10 km, but the altitude of the F peak was changed, with F peak altitudes at 340, 350, and 430 km. All cases resulted in bottomside growth of the instability (spread F) with dramatically different time scales for development. Plasma density depletions were produced on the bottomside with rise velocities, produced by nonlinear polarization E × B forces, of 2.5, 12, and 160 m/s and percentage depletions of 16, 40, and 85, respectively. In one case, ESF 0, the bubble did not rise to the topside, but in ESF 1 and 3, topside irregularities were produced by the bubbles (where linear theory predicts no irregularities). In these three cases, spread F could be described from weak to strong. In the fourth case (ESF 2) the altitude of the F peak was 350 km, but the minimum L on the bottomside was changed to 5 km. This resulted in a bubble rise velocity of ∼23 m/s and a 60% depletion with strong bottomside and moderate topside spread F and a time scale for development between ESF 1 and 3. Two other cases, ESF 0′ and 0″ with peaks at 330 and 300 km, respectively, and bottomside L ∼ 10 km, were investigated via linear theory. These cases resulted in extremely weak bottomside spread F and no spread F (entire bottomside linearly stable), respectively. These simulations show that under appropriate conditions, the collisional Rayleigh‐Taylor instability causes linear growth on the bottomside of the F region. This causes the formation of plasma density depletions (bubbles) which rise to the topside (under appropriate conditions) F region by polarization E × B motion. High altitude of the F peak, small bottomside electron density gradient scale lengths, and large percentage depletions yield large vertical bubble rise velocities, with the first two conditions favoring bottomside linear growth of the instability. The numerical simulation results are in good agreement with rocket and satellite in situ measurements and radar backscatter measurements, including some of the recent results from the August 1977 coordinated ground‐based measurement campaign conducted by Defense Nuclear Agency at Kwajalein.
Ion cyclotron waves generated by an ionospheric barium injection
Authors: Koons H. C. , Pongratz Morris B.
Published: February 2008

A shaped charge, barium, plasma injection experiment was performed in June 1976 at the ERDA Kauai test facility. The barium was injected perpendicular to the geomagnetic field at an altitude of 450 km. The second stage of the rocket, located approximately 1.2 km from the point of detonation, contained a payload to measure dc and ac electric fields using a dual spherical probe antenna. A variety of plasma waves were detected for approximately 20 s following the detonation. The initial impulse generated ion cyclotron waves at the Ba+, O2+, Al+, OH+, and O+ gyrofrequencies. No evidence for a Doppler shift was observed. This was followed by a time period during which ion cyclotron waves were observed in passbands between the barium gyrofrequency harmonics up to the oxygen gyrofrequency. At the high‐frequency end of this range the spectrum is essentially continuous, i.e., similar to an ion sound wave spectrum.
Plasma wave turbulence associated with an interplanetary shock
Authors: Gurnett D. A. , Neubauer F. M. , Schwenn R.
Published: February 2008

In this paper we give a brief summary of the interplanetary shocks detected and analyzed to date from the Helios 1 and 2 spacecraft and present a detailed analysis of the plasma wave turbulence associated with one particular shock, on March 30, 1976. This event was selected because a very clearly defined burst of plasma wave turbulence occurs at the shock with otherwise quiet conditions in the solar wind upstream and downstream of the shock. The shock is an oblique shock and the upstream parameters are characterized by a low Mach number, a low beta, and an unusually large electron to ion temperature ratio. Three types of plasma waves are detected in association with this shock: (1) electron plasma oscillations, (2) electrostatic ion‐acoustic or Buneman mode turbulence from about 1 to 30 kHz, and (3) whistler‐mode magnetic noise. Because of the high velocity of the shock and the quiet conditions in the solar wind this event provides a particularly good determination of the scale size and characteristics of the precursor waves upstream of the shock and the electric and magnetic field turbulence in the wake behind the shock. The primary burst of electric and magnetic field noise at the shock occurs a few seconds after the jump in the magnetic field, with a broad maximum in the electric field intensities at about 3 kHz and a monotonically decreasing magnetic field spectrum below about 1 kHz. Many of the characteristics of this shock are found to be closely similar to, and are compared with, previous observations of plasma wave turbulence associated with the earth's bow shock.
Tides in the joint presence of friction and rotation: An f plane approximation
Authors: Forbes Jeffrey M. , Hagan Maura E.
Published: February 2008

A formalism is developed for approximating tidal structures in a spherical, rotating, viscous atmosphere by ‘equivalent gravity waves’ (EGW's) on an f plane. The frequency and zonal wave number of the EGW is chosen to match that of the simulated tidal mode. Then, for a given planar rotation rate f the meridional wave number of the EGW is determined by matching the vertical structure (equivalent depth) of the tidal mode in the absence of dissipation. Simulations are performed for the thermospheric extensions of the first two symmetric and antisymmetric semidiurnal tidal modes and compared to a full numerical treatment of the inseparable tidal equations in the thermosphere. Development of this technique forms the basis for investigation of a number of problems associated with thermospheric tides which might otherwise be precluded by computer limitations.
Dynamical effects on the global distribution of thermospheric atomic oxygen
Authors: Straus Joe M. , Christopher Lee A.
Published: February 2008

This numerical study of dynamical effects on the global distribution of atomic oxygen in the thermosphere makes use of a three‐component model of upper atmospheric dynamics which treats the gases N2, O2, and O coupled together by collisions. This model simultaneously treats diurnal and seasonal variations of the distributions of these gases. Specific comparisons of hydrostatic computations with those taking into account wind‐induced departures from hydrostatic equilibrium show that none of these gases can be considered to be in hydrostatic equilibrium below ∼200 km. Global scale winds are shown to cause the observed winter enhancement of atomic oxygen in the lower thermosphere. Comparison of the model results with observational data shows generally good agreement.
Measurements of spectrally integrated atmospheric transmittance in the O2 Schumann‐Runge bands and derived oxygen column densities: 76–102 km
Authors: Longmire M. S. , Bartoe J.‐D. F. , Brown C. M. , Brueckner G. E. , Tousey R.
Published: February 2008

Atmospheric transmittances integrated over wavelength intervals corresponding approximately to the (15–0) through (4–0) Schumann‐Runge bands of O2 have been determined from EUV solar spectra (1768 Å <λ <1948 Å) photographed at seven altitudes between 102 and 76 km with a rocket‐borne spectrograph having a resolution of 0.07 Å. The observed transmittances are compared with atmospheric transmittances predicted from three models of the O2 absorption cross section. The predicted transmittances have also been used to derive column densities of atmospheric O2 from the observations. The results are compared with values calculated from the U.S. Standard Atmosphere (1976) and with oxygen column densities determined by Prinz and Brueckner (1977) from EUV solar spectra of the Schumann‐Runge continuum (λ <1750 Å) and of the H‐Lyman alpha line (1216 Å) recorded on the same films used in the present research. The comparisons test the utility of the models for studies of atmospheric photochemistry, suggest which models may be best for this purpose, and indicate how the models can be improved.
HF Doppler observations associated with spread F
Authors: Cornelius D. W. , Essex E. A.
Published: February 2008

This paper discusses HF Doppler spectrum observations at mid‐latitudes when spread F was observed on local ionograms. The data indicate that Doppler broadening is related to spread F (they both commence simultaneously). The results can be simulated by using different ionospheric models. Calculations show that a rippled contour can cause HF Doppler broadening and range spreading on ionograms if the structure is traveling horizontally. Although this mechanism can lead to HF Doppler broadening, scattering at 4.5 and 7.5 MHz in an anisotropic medium from near overhead can also explain the experimental results. The observation of aspect‐sensitive scatter does, however, depend critically on the statistical description of the irregularity spectrum. It is suggested that an experiment designed to measure the Doppler shift as a function of zenith angle may enable some of the current ambiguities associated with spread F to be eliminated.
Temporal and spatial power spectrum studies of numerical simulations of Type II gradient drift irregularities in the equatorial electrojet
Authors: Keskinen M. J. , Sudan R. N. , Ferch R. L.
Published: February 2008

Temporal and spatial power spectra from computer simulations of the equations modeling type II irregularities in the equatorial electrojet are presented. The power spectra are computed using the code of Ferch and Sudan (1977), averaging over times as long as 20 s. The dependence of the average frequencies and line widths on wavelength and angle from the electrojet drift is studied. The mean frequencies obtained agree well with linear theory except at short wavelengths, while the line widths are greater than those predicted by the linear damping rates. The spatial power spectra were also computed and found to be approximately isotropic. Favorable comparisons are made with radar observations and the theory of Sudan and Keskinen (1977).
Characteristics of the magnetospheric boundary layer and magnetopause layer as observed by Imp 6
Authors: Eastman T. E. , Hones E. W.
Published: February 2008

Imp 6 observations of the low‐latitude magnetospheric boundary layer indicate that the plasma within it is supplied primarily by direct entry of magnetosheath plasma across the magnetopause layer. We define the magnetopause layer as the current layer (separating the magnetosheath from the boundary layer) through which the magnetic field shifts in direction. High temporal resolution (3‐s average) data reveal that in a majority of Imp 6 magnetopause crossings, no distinct changes in electron density or energy spectra are observed at the magnetopause layer. In all Imp 6 crossings, some magnetosheathlike plasma is observed earthward of the magnetopause layer, implying the existence of a boundary layer. Boundary layer electron energy spectra are often virtually indistinguishable from the adjacent magnetosheath spectra. Low‐latitude boundary layer bulk plasma flow as observed by Imp 6 almost always has an antisunward component and often has a significant cross‐field component. The boundary layer thickness is highly variable and is generally much larger than the magnetopause layer thickness. Energetic electron pitch angle distributions indicate that the low‐latitude boundary layer is normally on closed field lines. We conclude that diffusive as well as nondiffusive processes probably contribute to the entry of magnetosheath plasma into the boundary layer.
Comparisons of inferred total plasmasphere content with measured plasma density profiles
Authors: Lanzerotti L.J. , Maclennan C.G. , Webb D.C.
Published: February 2008

The first comparisons are reported between values of the total plasmasphere cold plasma density as deduced from whistler‐determined equatorial plasma profiles and as inferred from beacon satellite group delay methods. We find the two independently determined total content values to be equivalent to within about 25% for 3 days and to within a factor of <2 for 2 days, when data are presently available. We conclude that it is difficult to deduce the position of the plasmapause from group delay total content data without a separate measure of the plasma density at the base of the plasmasphere (∼2000‐km altitude).
Conductorlike behavior of a photoemitting dielectric surface
Authors: De Bibhas R.
Published: February 2008

We provide an analytical proof showing that a uniformly illuminated photoemitting dielectric surface where the photoelectron emission has an azimuthal symmetry becomes an equipotential surface in the steady state. This result supports the finding of an earlier numerical simulation study of the problem.
Stare auroral radar observations of Pc 5 geomagnetic pulsations
Authors: Walker A.D.M. , Greenwald R.A. , Stuart W.F. , Green C.A.
Published: February 2008

The Stare (Scandinavian twin auroral radar experiment) auroral radar system has been used to measure ionospheric electric fields associated with Pc 5 geomagnetic pulsations. With this system, electric fields are derived from the drift velocity of radar auroral irregularities. The spatial resolution is 20 km over a 200,000‐km² grid, and the temporal resolution is 20 s. It has been found that the oscillating electric field associated with a hydromagnetic field line resonance produces poleward moving, bandlike regions of radar aurora, which are aligned in the east‐west direction. The drift of the irregularities within these bands is alternately eastward and westward. The Stare electric field data have been used in conjunction with the Biot‐Savart Law and an assumed height‐integrated conductivity of 8–10 Ω−1 to calculate the ground magnetic disturbance. It has been found that the H and Z are well predicted, whereas D is generally underestimated. These results are consistent with a 90° rotation of the magnetic polarization ellipse in the ionosphere. By Fourier analyses of the Stare data it is found that the half‐power latitudinal width of the field line resonance is typically 100 km in the ionosphere. Moreover, the north‐south electric field undergoes a 180° phase shift about the resonance as predicted by theory. The data have been used to estimate equatorial plasma densities for 6 < L < 7, and values of the order of 20 cm−3 have been obtained. However, these determinations are strongly affected by distortions of the geomagnetic field from a dipolar geometry. In summary, (1) the experimental results strongly support the hydromagnetic field line resonance theory of pulsations; (2) the magnetic polarization ellipse is indeed rotated through 90° by the ionosphere; (3) the phenomena previously observed by auroral radar workers in association with Pc 5 pulsations were related to the electric field of the hydromagnetic wave near resonance; and (4) auroral radar measurements can be used to estimate the equatorial magnetospheric plasma density in the region 5 < L < 8.
Possible origin of transient dusk‐to‐dawn electric field in the nightside magnetosphere
Authors: Nishida A.
Published: February 2008

Development of the electromagnetic field in the magnetosphere‐ionosphere system is examined under the condition that a constant current generator supplies current to the system after t=0. It is shown that at high altitudes there are intervals when the electric field is transiently directed opposite to the direction of the supplied current. Namely, transient dusk‐to‐dawn electric field can be generated from ionospheric channeling of the dawn‐to‐dusk tail current during an early stage of the development of the tail‐ionosphere current system. Electric field with the opposite polarity is produced during intervals when the field reflected from the ionosphere dominates over the incident field.
Time dependent convection electric fields and plasma injection
Authors: Kaye Stanley M. , Kivelson Margaret G.
Published: February 2008

Large‐scale electric fields associated with storms or substorms are responsible for inward convection and energization of plasma sheet plasma. Calculations based on steady state convection theory show that the response to such electric fields qualitatively accounts for many features of the injected particle distribution, but quantitative agreement with the theory has not yet been obtained. It is known that the predictions can be improved by introducing the concept of convection in response to a time dependent electric field. On the other hand, time dependent calculations are sensitive to the choice of initial conditions, and most models have failed to incorporate these conditions in a realistic and self‐consistent manner. In this paper we present a more complete model consisting of realistic initial conditions and time dependent convection to explain a typical substorm‐associated electron injection event. We find very good agreement between the observed electron flux changes and those predicted by our model.
Electron flow and visual aurora at the Harang discontinuity
Authors: Nielsen E. , Greenwald R.A.
Published: February 2008

We present several examples of the ionospheric electron drift patterns and of the aurora near the Harang discontinuity. The drift patterns were detected with the Scandinavian Twin Auroral Radar Experiment, and the auroral features were determined by using all‐sky camera data. We find the behavior of the discontinuity to be strongly dependent on the level of magnetic activity as measured by the Kp index. During low activity the discontinuity remains at a fairly constant latitude for a considerable length of time. Usually, one to two auroral arcs are observed within the discontinuity. With increasing magnetic activity the Harang discontinuity tends to surge rapidly equatorward with mean speeds ranging from 150 to 600 m/s. This equatorward surge is observed at earlier local times as magnetic activity increases. We have found two cases in which an arc appears in the discontinuity at some point during its equatorward surge. The equatorward surge of the essentially east‐west‐aligned discontinuity appears to be associated with a limited region of strong equatorward electron flow which splits up into eastward and westward flow. This region may form the eastward termination of the discontinuity. The eastward termination of the Harang discontinuity during times of low magnetic activity appears to consist of an equatorward extension of the region of eastward electron flow within a spatially limited region near local magnetic midnight. We find the equatorward surges to be terminated by the sudden activation of an arc in the discontinuity, followed by a poleward expansion. Following these substorms the Harang discontinuity disappears from the field of view. At times, we have observed arcs to brighten and surge poleward in the region equatorward of the discontinuity. The discontinuity did not appear to be strongly affected by these substorms. We find in general that auroral arcs are located in or poleward of the discontinuity, even though active arcs at times appear south of the discontinuity. Diffuse aurora have been observed only equatorward of the discontinuity.
Heating of ions to superthermal energies in the topside ionosphere by electrostatic ion cyclotron waves
Authors: Ungstrup Eigil , Klumpar David M. , Heikkila Walter J.
Published: February 2008

The soft particle spectrometer on the Isis 2 spacecraft, which is in a nearly circular orbit at 1400‐km altitude, frequently observes fluxes of upcoming ions with a conical pitch angle distribution and pitch angles usually in the range 90°–135°. The upcoming ions are usually observed near the boundaries of the auroral precipitation zone and primarily in the winter nighttime at the Isis 2 altitude. The observations are interpreted in terms of electrostatic ion cyclotron waves, which heat the ions to superthermal energies transverse to the earth's magnetic field. Subsequently, the ions are accelerated away from the earth by the earth's magnetic field.
Coherent generation of the auroral kilometric radiation by nonlinear beatings between electrostatic waves
Authors: Roux A. , Pellat R.
Published: February 2008

The propagation of electrostatic plasma waves in an inhomogeneous and magnetized plasma is studied analytically. These waves, which are driven unstable by auroral beams of electrons, are shown to suffer a further geometrical amplification while they propagate toward cut‐off. Simultaneously their group velocities tend to be aligned with the geomagnetic field. Then it is shown that the electrostatic energy tends to accumulate at or near ωLH and ωUH, the local lower and upper hybrid frequencies. Due to this process, large amplitude electrostatic waves with very narrow spectra should be observed near these frequencies at any place along the auroral field lines where intense beam driven instability takes place. These intense quasi‐monochromatic electrostatic waves are then shown to give rise by a coherent nonlinear three wave process to an intense electromagnetic radiation. Provided that the ratio ωpe/ωce tends to be smaller than unity, it is shown that the most intense radiation should be observed at 2ωUH in the extraordinary mode.
On the origin of plasmaspheric hiss: The importance of wave propagation and the plasmapause
Authors: Thorne R.M. , Church S.R. , Gorney D.J.
Published: February 2008

The presence of a turbulent band of ELF hiss throughout the plasmasphere has been modeled by following unducted whistler mode waves from a restricted region of cyclotron resonant growth in the outer plasmasphere. The most unstable waves originate near the equatorial plane within a modest cone of wave normal angles aligned along the ambient magnetic field. As the waves propagate to higher latitude, their wave normal direction becomes progressively oblique, and ray paths generally migrate outward until they reach the plasmapause. A major fraction of the incident waves internally reflect at the steep plasmapause density gradient and subsequently propagate inward to populate most of the plasmasphere. Only a minor part of the wave energy is able to escape from the plasmasphere, primarily at high latitude. A detailed study of the internally trapped ray paths shows that certain waves can return to the equatorial growth region with field‐aligned propagation vectors and thus experience further amplification. This is particularly important for the maintenance of quiet time hiss when a single transit of the growth region is insufficient to amplify the background incoherent cyclotron noise to detectable levels. The model calculations demonstrate that the observed frequency spectrum and polarization of plasmaspheric hiss not only are controlled by the relative rates of cyclotron growth and Landau damping but also are strongly influenced by the wave propagation characteristics.
Drift boundary approximations in simple magnetospheric convection models
Authors: Southwood D.J. , Kaye S.M.
Published: February 2008

We analyze features of particle behavior in magnetospheric convection using simple analytic forms for the electrostatic potential which have been used in other recent works. We show that analytic approximations for the shape and position of surfaces delimiting closed and open particle orbits can be derived. These approximations are good for any pitch angle and at most energies. They break down only for the medium‐energy protons that are predicted by such models to penetrate far inside the plasmapause. The expressions we derive allow rapid tests of a variety of electric field models when analyzing particle data, and we illustrate this by intercomparing field models with several observations.
Propagation of two‐fluid interplanetary shock waves
Authors: Rosenau Philip
Published: February 2008

A two‐fluid time‐dependent analytical model of the perturbed solar wind is presented. The expansion of newly emitted material, caused, for instance, by the outburst of a solar flare, is simulated by a spherical piston. The electron collision time is phenomenologically decreased to account for a transit from a collisional (near the sun) to a collisionless regime (at 1 AU). This effectively inhibits thermal conductivity and enhances ion‐electron heat exchange. For a given thermal conductivity in the limit of strong coupling, one‐fluid flow in a thermally conducting medium is recovered. A pattern of flow which resembles one‐fluid flow in an adiabatic medium may be obtained if heat is removed from the perturbed plasma into the propelling plasma. The perturbed flow consists of a thermal precursor which is followed by a shock across which electrons are isothermal while protons are compressed and heated. For most of the postshock domain the electron temperature is considerably higher than the proton temperature. This may initiate ion acoustic instability, and as a result, the plasma may have anomalous resistivity. Finally, we show that the postshock rise and fall of density cannot be used to distinguish piston‐driven waves from blast waves.
A parametric study of electron multiharmonic instabilities in the magnetosphere
Authors: Ashour‐Abdalla M. , Kennel C.F. , Livesey W.
Published: February 2008

We present numerical calculations of spatial growth rates of multiharmonic electron cyclotron instabilities driven by a loss cone energetic distribution, in the presence of colder electrons. When the cold electron density is comparable to the hot, nonconvective instability is possible in harmonic bands below or including the cold upper hybrid frequency. When the cold electron density is larger than the hot, nonconvective instability is possible only in that band containing the cold upper hybrid frequency. Increasing the cold electron temperature in relation to the hot eventually removes all nonconvective behavior. Convective instability is still possible above the cold upper hybrid frequency.
A comment on the source region of VLF saucers
Authors: Temerin Michael
Published: February 2008

The source region of VLF saucers is consistent with a source extensive in longitude but narrowly confined in the other two dimensions.

James [1976] shows that the source region of VLF saucers is confined in all three dimensions. That is, the horizontal dimensions (the ones perpendicular to the magnetic field) of the source region are typically 0.5 km and the vertical dimension is of the order of 10 km. The source region is thus effectively a point. The purpose of this brief report is to show that there is an alternate geometry to the source region which (1) is consistent with the data presented by James, (2) explains the internal inconsistencies in the data mentioned by James, and (3) is physically more reasonable. The alternate geometry is that the source region is confined in the vertical and in one of the horizontal dimensions as mentioned by James but is extensive in the other, usually the longitudinal, dimension. That is, the source region is effectively a line. I will comment briefly on the three points mentioned above. I refer the reader to James for references, examples of data, and a discussion of the possible importance of VLF saucers to the physical concepts such as double layers. This report is essentially a footnote to that article.
Spacecraft radio scattering observations of the power spectrum of electron density fluctuations in the solar wind
Authors: Woo Richard , Armstrong J. W.
Published: February 2008

Solar wind electron density power spectra in the solar equatorial region are inferred from observations of phase scintillations and spectral broadening made with the Viking, Helios, and Pioneer spacecraft. The heliocentric distance range covered is 2–215 RS, and for some observations close to the sun the spectra extend to fluctuation frequencies as high as 100 Hz. For heliocentric distances ≳20 RS the equivalent spacecraft‐measured one‐dimensional density spectrum Vne is well modeled by a single power law (f−α) in the frequency range 10−4‐5 × 10−2 Hz. The mean spectral index α is 1.65, very close to the Kolmogorov value of 5/3. Under the assumption of constant solar wind speed, Vne varies as R−3.45, where R is heliocentric distance. Within 20 RS, Vne can still be modeled by a single power law over the frequency range 10−3‐10¹ Hz, but the spectral index becomes smaller, α ∼ 1.1. The flattening of the density spectrum within 20 RS is presumably associated with energy deposition in the near‐sun region and acceleration of the solar wind.