bluuebunny/crossref_metadata_2025 huggingface dataset:
North‐south aligned equatorial airglow depletions
Authors: Weber E. J. , Buchau J. , Eather R. H. , Mende S. B.
Published: February 2008

A new instrument for all‐sky spectrophotometric imaging of aurora and airglow has been installed in the Air Force Geophysics Laboratory's Airborne ionospheric Observatory. Initial observations of equatorial and near‐equatorial 6300‐Å O I airglow show the existence of north‐south aligned regions of airglow depletion. These dark bands often extend more than 1200 km in the north‐south direction and 50–200 km in the east‐west direction. They are observed to drift toward the east during the evening‐midnight hours, with one observation of westward drift after local midnight. Airglow fine structure associated with the boundaries of the dark bands has been observed down to the 2.5‐km resolution limit of the instrument. Simultaneous airborne ionospheric soundings indicate that these regions of airglow depletion are characterized by an increase in the virtual height of the F layer. A simple model of field‐aligned electron density depletion in the bottomside of the F layer explains both the airglow observations and the ionospheric soundings.
Effect of the day night ionospheric conductivity gradient on polar cap convective flow
Authors: Atkinson G. , Hutchison D.
Published: February 2008

The electric field data in the polar cap do not display the expected mirror symmetry for positive and negative values of the solar magnetospheric y component of the interplanetary magnetic field, suggesting that an additional effect is squeezing the antisunward flow toward the dawnside of the polar cap. It is shown that a conductivity decrease toward the nightside will produce such an effect. Results are both for a model with a discontinuous decrease in density and for models with a smooth exponential decrease in conductivity with distance across the polar cap. Steeper conductivity gradients cause stronger squeezing of the flow toward the dawnside of the polar cap. Outside the polar cap the return flow towards the dayside spreads over a wider latitude range on the dawnside than on the duskside. This is a likely explanation of the observed asymmetry in the thickness of the auroral oval.
Electric field topology near the dayside magnetopause
Authors: Heikkila Walter J.
Published: February 2008

A direct test for the occurrence of the reconnection electric field ER tangential to the dayside magnetopause is possible by the use of in situ spacecraft observations of magnetic field and plasma parameters. By means of a simple local analysis it is shown that particle energization is given by ϵ = BMS|BMS ‐ BM|/μ0NMS, where the subscripts denote magnetic field strength and plasma density measured in the magnetosheath and the magnetosphere sides. One particular pass of Heos 2 on June 18, 1973, is discussed, although other published passes lead to similar conclusions, without exception. The expected energization is about 1 keV/particle, but none is observed, nor is there any evidence of an enhanced outflow as predicted by reconnection theories. An alternate appraisal, from a global viewpoint, shows that the effect of a reconnection electric field should be one of major proportions and could not be missed. The apparent conclusion is that no reconnection electric field tangential to the dayside magnetopause exists; thus the magnetopause must be very nearly an equipotential surface. The significance of this conclusion, in particular the role of the boundary layer inside the magnetopause, is discussed. This boundary layer must be polarized; it is thus in some respects a dipole sheet, which can transfer the external magnetosheath electric field into the magnetosphere. This may provide an explanation for the dependence of geomagnetic activity on the direction of the interplanetary electric field as an alternative to Dungey's reconnection model. The openness of the polar cap magnetic field lines is not necessarily in question, provided that an electric field component parallel to magnetic field lines is possible.
Compatibility of seasonal variations in mid‐latitude thermospheric models at solar maximum and low geomagnetic activity
Authors: Alcaydé D. , Bauer P. , Hedin A. , Salah J. E.
Published: February 2008

Satellite drag data, in situ mass spectrometer data, Fabry Perot interferometer data, and incoherent scatter data gathered above Saint‐Santin (44.6°N, 2.2°E) have been used to produce models of the concentration of atomic oxygen and molecular nitrogen in the thermosphere and of the exospheric temperature. A model of exospheric temperature has also been obtained from incoherent scatter observations conducted above Millstone Hill (42.6°N, 71.5°W). These various models are compared for the location of the ground stations (≃45°N). Good agreement is obtained for the annual variations of the atomic oxygen concentration at 400 km from the three techniques involved. The mean annual temperatures are within 20°K of each other for five of the six models involved. The remaining model, Ogo 6, gives a mean temperature which is 50°K larger. Four of the models are in good agreement concerning the annual temperature variation. However, the satellite drag model exhibits a smaller variation, and the Ogo 6 model exhibits a larger one. There is fair agreement between incoherent scatter models and mass spectrometer models for the N2 concentrations at 400 km, but there is disagreement between these models and the satellite drag model. It is shown that most disagreements can be resolved by changing both the vertical temperature profile and the lower‐thermosphere oxygen concentration in the direction indicated by previous incoherent scatter measurements.
Pitch angle dispersion of drifting energetic protons at synchronous orbit
Authors: Walker R. J. , Erickson K. N. , Winckler J. R.
Published: February 2008

The time of arrival at ATS 6 of substorm‐associated energetic (>27 keV)proton flux enhancements has been examined as a function of both energy and pitch angle by using data from the University of Minnesota electron‐proton spectrometer. The protons exhibit energy dispersion with the particles with the highest energy arriving first. This energy dispersion generally has been interpreted as evidence that these particles have gradient‐ and curvature‐drifted to the spacecraft from an acceleration region on the nightside. In the evening quadrant, proton enhancements at 90° pitch angle arrived at the spacecraft prior to those at small pitch angles, as would be expected for particles drifting in a dipolelike magnetic field. However, on the dayside the increase occurs first at the smallest pitch angle (∼30°). The difference between the arrival times of the 30° and 90° protons increases for more westward local times. For the highest energy range observed (120–377 keV), drifting protons are rarely seen at large pitch angles. These observations place severe restrictions on the simple drift models. Since the pitch angle dependence of the particle drift velocity is a field geometric quantity, the observations require the modification of present magnetospheric magnetic field models. In particular, the models need to reproduce better the enhanced field observed on the dayside near synchronous orbit. The particle energy and pitch angle dispersion observations may provide a sensitive test of future models.
Photoelectrons and electron temperatures in the Venus ionosphere
Authors: Butler Dixon M. , Stolarski Richard S.
Published: February 2008

Solar wind induced magnetic fields may be present in the dayside ionosphere of Venus. A scale analysis and detailed calculations of the effects of different field line orientations on photoelectron transport and electron temperature show that horizontal magnetic fields considerably alter the profiles of electron heating and temperature and photoelectron flux. Direct injection of solar wind electrons has been considered, and the effects are minor. No evidence is found to support the suggestion that a sharp gradient in electron temperature is the cause of certain features in the Mariner 10 electron density profile.
Theory of Rayleigh‐Taylor bubbles in the equatorial ionosphere
Authors: Ott Edward
Published: February 2008

We present a theory of rising bubbles (or sharp density depletions) in the equatorial ionosphere. Both ion inertia and ion‐neutral collisions are included. In the collision‐dominated case the bubble velocity is independent of its size, while in the collisionless case it is proportional to the square root of the bubble size.
A unified view of substorm sequences
Authors: Kamide Y. , Matsushita S.
Published: February 2008

In an attempt to eliminate the ‘growth phase’ controversy a unified phenomenological model of substorm sequences is presented by taking recent findings into account. The following three fundamental requirements are important in synthesizing the vast amount of ground‐based and satellite observations: (1) substorms can occur regardless of the direction of the IMF (interplanetary magnetic field) except during a large prolonged northward IMF which yields a ‘ground’ state of the magnetosphere, (2) substorm occurrence probability increases with an increase of available energy in the magnetotail which exceeds the ground state energy, and (3) substorms are, in general, intense when the available energy is large.
Ion‐acoustic‐like instabilities in the solar wind
Authors: Gary S. Peter
Published: February 2008

The linear dispersion properties of k parallel to B electrostatic instabilities driven by an electron heat flux or an ion beam are studied by use of parameters appropriate to average values in the solar wind near 1 AU. These modes are also compared with field‐aligned electromagnetic instabilities driven by the same sources of free energy. The ion acoustic heat flux instability has a high threshold relative to the whistler heat flux instability, so the latter mode is the more likely cause of the heat flux limitation near 1 AU. However, at sufficiently large Te/Ti the electrostatic ion beam instability threshold is of the same order as that of the magnetosonic ion beam instability, implying that the former is the more likely source of ion‐acoustic‐like fluctuations observed near 1 AU.
Frequency and time dependence of the Jovian decametric radio emissions: A nineteen‐year high‐resolution study
Authors: Thieman J. R. , Smith A. G.
Published: February 2008

A nineteen‐year catalog of observations collected by the Universities of Florida and Texas has permitted detailed comparison of Jovian source characteristics at 10 frequencies from 5.6 to 30 MHz. The spectral morphology of Jupiter's decameter wavelength radio sources was studied by means of high‐resolution histograms of occurrence probability for intervals as small as 2° in Jovian system III longitude and the departure of Io from superior geocentric conjunction. Longitude histograms clearly exhibit a substantial shift in the source A and source B positions between 15 and 10 MHz. Similar diagrams isolate the dependence of each source on the position, of Io, illustrating changes with frequency in the structures and sizes of the main probability maxima and in the positions of surrounding secondary peaks. Individual frequency probability histograms for both longitude and Io phase reveal new phenomena, such as a bridge of radiation through source B longitudes connecting sources A and C at 15 MHz and spectral modification of portions of the Io‐related source B peak.
The O+(²P) emission at 7320 Å in twilight
Authors: Meriwether J. W. , Torr D. G. , Walker J. C. G. , Nier A. O.
Published: February 2008

We describe the measurement of the 7320‐ to 7330‐Å emissions by O+(²P) in the evening twilight over Arecibo. We show how contamination by scattered sunlight and by OH airglow can be subtracted from the measured spectrum to achieve an unambiguous identification of both doublets. The measured ratio of the brightnesses of the two features is in agreement with theory. The measured wavelength of the doublet at 7320 Å (7319.9 ± 0.1 Å) is different from the value usually quoted (7319.0 Å). We have assumed photoionization to be the major source, and we have compared our surface brightnesses measured as a function of solar depression angle with those derived theoretically from nearly coincident Atmosphere Explorer data on neutral composition and solar ultraviolet flux. Agreement is good. Since quenching of O+(²P) is negligible under the conditions of our experiment, it appears that ground‐based airglow measurements of the kind we describe can be used to monitor the variation of atomic oxygen density in the F region.
On the configuration of the magnetotail near midnight during quiet and weakly disturbed periods: State of the magnetosphere
Authors: West H. I. , Buck R. M. , Kivelson M. G.
Published: February 2008

Energetic‐electron pitch angle data and magnetic field data obtained on inbound passes of Ogo 5 in 1968 near midnight during quiet periods have been studied. From observation of the vector field components along the orbit and knowledge of the satellite position when the pitch angle distributions changed from approximate isotropy (indicating non‐guiding center particle motion) to the butterfly distribution (corresponding to adiabatic bounce orbits) the configuration of the near‐earth tail field for individual orbits has been inferred. The details of the modeling are presented in a companion paper. We find that quasi‐steady field configurations near midnight can occur that are markedly different (ranging from taillike to fairly dipolelike) in different cases, even though the usual magnetic activity indicators (Kp, AE, auroral zone and mid‐latitude magnetograms, and BZGSM of the interplanetary magnetic field) differ but slightly from one case to another and indicate a quiet magnetosphere. The major differences in the data are that more intense lobe fields occur at the same distance down the tail for the more taillike field configurations (∼30 γ at 15 RE for taillike configurations and ∼15 γ at 15 RE for more dipolelike configurations). There appears to be evidence that the more taillike configuration is associated with an enhanced dynamic pressure from the solar wind. The data, viewed in the light of previously published data obtained by the present experimenters and others, argue strongly that during quiet times the plasma sheet, in its azimuthal variation, is at its thinnest in the region near midnight. These results affect current ideas of the substorm growth phase and the evolution of energetic‐electron pitch angle distributions in the postmidnight magnetosphere.
Influence of non‐Bi‐Maxwellian distribution function of solar wind protons on the ion cyclotron instability
Authors: Leubner M. P.
Published: February 2008

Satellite measurements of the thermal anisotropy of solar wind protons have shown that the particle velocity distributions at 1 AU are not adequately described by a bi‐Maxwellian distribution function. On the other hand, the stability criteria derived from a bi‐Maxwellian distribution require T⊥ > T∥, which is, however, hardly ever observed. A modified axisymmetric distribution function for solar wind protons is used here to calculate the stability and growth rate of the ion cyclotron instability in a warm slow speed solar plasma at 1 AU. Cyclotron wave growth is found also in the case T∥ > T⊥.
Electrostatic waves observed in an unstable polar cap ionosphere
Authors: Bahnsen A. , Ungstrup E. , Fälthammar C. ‐G. , Fahleson U. , Olesen J. K. , Primdahl F. , Spangslev F. , Pedersen A.
Published: February 2008

On an instrumented rocket payload flown through the polar ionosphere, large‐amplitude electrostatic ELF waves have been measured in the unstable E region. Using three‐axis instantaneous measurements of the electric vector of the wave field, it was possible to confirm the electrostatic nature of the waves and to make a direct determination of the full k vector as a function of altitude during upleg and downleg passages through the E region. Simultaneous measurements were made of electron density and temperature, dc electric field, and ac and dc magnetic field. This allows comparisons to be made with theories of the crossed field two‐stream instability, the conditions for which were well satisfied. The direction of propagation and the frequency range are in agreement with theory, but the phase velocity is lower than predicted, being only about one third of the electron drift velocity.
Differential photoelectric charging of nonconducting surfaces in space
Authors: Pelizzari Michael A. , Criswell David R.
Published: February 2008

Results of numerical calculations of photoelectric charging are presented for an infinitely long strip of sunlight across a nonconducting plane, the simplest geometry for which differential charging occurs. The model contains an electrical cutoff radius and therefore simulates charging of a sunlit area with dimensions equal to the strip's width, exposed to a plasma with a comparatively large Debye length. Uniform potential is quickly established on a uniformly sunlit strip as a result of charge redistribution by low‐energy photoelectrons. Results are consistent with a theoretical surface conductivity derived for photoelectron sheaths above highly charged sunlit areas. Conductorlike distribution of charge persists throughout electrification of the strip, accretion of photoelectrons by the dark plane, and contraction of the strip (simulation of solar occultation or sunset). The surface potential drops sharply across the sunlight‐shadow boundary, and if the boundary is diffuse, the potential in the penumbra falls substantially below the maximum strip potential only very near the penumbra's dark edge. These results demonstrate the generality of conductorlike behavior of sunlit areas and the consequent intensification of electrostatic fields at the sunlight‐shadow boundary.
Observations of large‐scale thermospheric waves during geomagnetic storms
Authors: Hernandez G. , Roble R. G.
Published: February 2008

Nighttime thermospheric winds and temperatures have been measured over Fritz Peak Observatory, Colorado (39.9°N, 105.5°W), with a high‐resolution Fabry‐Perot spectrometer. The winds and temperatures are obtained from the Doppler shifts and line profiles of the (O I) 15,867 K (6300 Å) line emission. Measurements made during three large geomagnetic storms, when Kp exceeded 8, have shown large‐scale thermospheric waves that are probably generated by impulsive heating at high latitudes. These waves are observed to propagate equatorward over the observatory, and on one occasion a poleward traveling disturbance was observed several hours later. The characteristic features of the observed waves during the three storm periods are described, and the global properties of these waves are examined by using a numerical model of the zonally symmetric thermosphere. The model calculations show reasonable agreement with the observations over Fritz Peak Observatory, thus enabling an estimate to be made of the high‐latitude energy input that launched the waves.
Thermal electron density in the Jovian magnetosphere
Authors: Mekler Yuri , Eviatar Aharon
Published: February 2008

Observations of the portions of Io's sodium cloud which are far from Io are presented. We demonstrate that the size of the sodium cloud is highly variable and relate this variability to fluctuations in the state of the thermal electron plasma in the magnetosphere of Jupiter. We show that estimates of the plasma density based on the assumption that the main sodium sink is electron impact ionization are consistent with those based on the intensities of the forbidden doublet of ionized sulfur which is excited by electron collisions. We discuss briefly the implications of our results for the magnetosphere of Jupiter.
Further observational support for the lossy radial diffusion model of the inner Jovian magnetosphere
Authors: Goertz C. K. , Van Allen J. A. , Thomsen M. F.
Published: February 2008

A mathematical model describing radial diffusion, with violation of the third adiabatic invariant and with local losses, is applied to the Pioneer 10 and 11 observations of omnidirectional integral intensities of electrons with energy E >21 MeV. Local losses outside L=3 are much stronger than synchrotron losses but weaker than the losses one would expect for strong pitch angle diffusion. If radial diffusion is driven by ionospheric winds, as was suggested by Brice and McDonough (1973), we find that the theoretical solution which best fits the data requires that the radial diffusion coefficient D ≈ 3 × 10−9 L³ s−1 and the lifetime against local losses, τ, is approximately a few million seconds.
A semidiurnal tide in the meridional wind at F region heights at low latitudes
Authors: Harper R. M.
Published: February 2008

Arecibo data for equinox conditions at solar cycle minimum are presented which show the existence of an important semidiurnal meridional wind oscillation at F region heights at low latitudes. The inferred structure of the semidiurnal tide is in reasonable agreement with theoretical predictions based on in situ EUV and ion drag forcing and tidal energy propagating upward from the lower atmosphere. The post‐midnight descent of the F layer that is observed at low latitudes is shown to be the result of the semidiurnal winds. The height‐varying semidiurnal winds cause gradients in the vertical ion velocity which reach 2 × 10−4 s−1. Thus the semidiurnal winds have important effects not only on the F layer height but also on the layer shape and peak density.
Nonlinear stabilization of the E × B gradient drift instability in ionospheric plasma clouds
Authors: Chaturvedi P. K. , Ossakow S. L.
Published: February 2008

We present a two‐dimensional nonlinear stabilization mechanism for the E × B instability occurring in striated F region ionospheric plasma clouds. A simple model shows that the saturated power spectrum for striations is proportional to k−2. Comparisons with experimental data from Stress barium cloud experiments sponsored by the Defense Nuclear Agency and the results of numerical simulations are given.
Secular variations in the production of cosmogenic isotopes in the Earth's atmosphere
Authors: O'Brien K.
Published: February 2008

The dependence of cosmogenic isotope production on solar modulation and on geomagnetic field intensity has been calculated, and spallation yields based on the Silberberg‐Tsao formalism have been determined. The stratospheric inventory of 7Be during the period 1970–1974 has been calculated to be about 4.5 mCi, in good agreement with measured values. The calculated radiocarbon reservoir of 1.75 cme−2 s−1 (where results are integrated over the whole earth) is in good agreement with the experimental value of 1.8. Geomagnetic and solar effects were taken into account.
On the structure of auroral arcs: The results of numerical simulations
Authors: Swift Daniel W.
Published: February 2008

The subject of this paper is the structure of discrete auroral arcs and bands. The mechanism for acceleration of auroral electrons is taken to be the V‐shock structures characterized by electric fields oblique to the magnetic field. The starting point in the analysis is that the accelerating voltages are generated by electrostatic MHD turbulence. A numerical simulation is used to illustrate how the large potential differences, necessary for acceleration of auroral electrons, can be generated across magnetic fields by the turbulent cascade. It is shown that this generation mechanism requires a flow of enstrophy to short wavelengths as well as a flow of energy to long wavelengths. The most important result is that it is shown that the limitation of upward field‐aligned current by the V shocks can result in a clustering of precipitation regions and the imposition of a long‐range order on patterns of auroral precipitation. It is argued that this effect is likely responsible for the multiple, regular auroral bands observed during quiet conditions. When the auroral electric fields become sufficiently intense, turbulent cascade is initiated which breaks up the well‐ordered structure. It is concluded that the regions of energy input into the cascade must be loosely coupled to the region where the electrons are accelerated.
Auroral O(¹S) production and loss processes: Ground‐based measurements of the artificial auroral experiment precede
Authors: O'Neil R. R. , Lee E. T. P. , Huppi E. R.
Published: February 2008

An artificial auroral experiment, Precede, was performed in the 80‐ to 120‐km altitude range above the White Sands Missile Range, New Mexico, in October 1974. A 2‐kW rocket‐borne electron accelerator, square wave modulated at 0.5 Hz, was activated at 95 km on payload ascent, was pulsed continuously through apogee (120 km) to a descent altitude of approximately 80 km, and provided a total of 90 pulses of a 2.5‐kV 0.8‐A electron beam over a period of 180 s. A ground‐based dual channel telephotometer recorded the time‐dependent photon emission rate of the N2+(B²Σu+→X²Σg+) first negative (0–0) band at 3914 Å and the O(¹S → ¹D) transition at 5577 Å induced in the night atmosphere by the pulsed electron source. An electron‐induced luminous efficiency of (4.5±0.4) × 10−3 was determined for the N2+ 1N (0–0) transition at 3914 Å in the 80‐ to 100‐km altitude range. The photon emission rates of several bright stars were measured to calibrate the telephotometer and to correct for the effects of atmospheric extinction. The time‐dependent O(¹S) 5577‐Å photon emission rate has been fitted with a model calculation providing insight into O(¹S) production and loss processes resulting from the deposition of energetic electrons in the 90‐ to 116‐km altitude range. At altitudes in excess of approximately 110 km the O(¹S) time‐dependent photon emission profiles indicate that consecutive reactions involving energy transfer from N2(A³Σu+) to O(³P) is the dominant O(¹S) production process. A rate coefficient of 5.7 × 10−12 cm³ s−1 representing an O(¹S) yield of 0.29 has been inferred from the data for the reaction of N2(A³Σu+) and O(³P). The dissociative recombination of O2+ has been established as the dominant O(¹S) production process at altitudes less than 96 km with an O(¹S) yield of 4.5–6.0% per dissociation. Other processes account for approximately 20% or less of the total O(¹S) production at 90 km with smaller contributions indicated at higher altitudes. Collisional deactivation by O(³P) accounts for approximately 50% of the total O(¹S) depopulation rate in the 95‐ to 116‐km altitude range with a rate coefficient of 6.0 × 10−11 e−305/T cm³ s−1. Quenching by O2 dominates as an O(¹S) loss mechanism at altitudes less than 94 km with an estimated rate coefficient of 1.2 × 10−11 e−850/T cm³ s−1. The rate coefficients determined for O(¹S) produced by the reaction of N2(A³Σu+) and O and the collisional deactivation processes have a probable error of approximately 25% if it is assumed that the model atmosphere used in the analysis contributes no significant uncertainty.
VLF transmitter‐induced quiet bands: A quantitative interpretation
Authors: Cornilleau‐Wehrlin Nicole , Gendrin Roger
Published: February 2008

Raghuram et al. (1977) have shown that a long‐duration monochromatic wave generated by a high‐power VLF transmitter may quench natural magnetospheric hiss emissions over a frequency range Δf of the order of 50–150 Hz below the transmitter frequency. We show that quasi‐linear diffusion in an inhomogeneous medium (Ashour‐Abdalla, 1972; Welti et al., 1973) cannot be invoked to explain these results. On the contrary, by using a formalism similar to the one introduced by Roux and Pellat (1976), i.e., nonlinear trapping and detrapping of natural particles, we are able to interpret the variation of Δf with both the field intensity and the transmitter frequency. The time constants for the establishment and disappearance of the quiet band are also qualitatively interpreted. Numerical agreement with experimental data is obtained if the field intensity induced by the Siple transmitter in the equatorial region at L = 3.5 is of the order of 4 mγ (1 mγ=1 pT (picotesla)).
Magnetosheath electrostatic turbulence
Authors: Rodriguez Paul
Published: February 2008

By using measurements with the University of Iowa plasma wave experiment on the Imp 6 satellite a study has been conducted of the spectrum of electrostatic plasma waves in the terrestrial magnetosheath. Electrostatic plasma wave turbulence is almost continuously present throughout the magnetosheath with broadband (20 Hz to 70 kHz) rms field intensities typically 0.01–1.0 mV m−1. Peak intensities of about 1.0 mV m−1 near the electron plasma frequency (30–60 kHz) have been detected occasionally. Two or three components can usually be identified in the spectrum of magnetosheath electrostatic turbulence: a high‐frequency (≥30 kHz) component peaking at the electron plasma frequency fpe, a low‐frequency component with a broad intensity maximum below the nominal ion plasma frequency fpi (∼fpe/43), and a less well defined intermediate component in the range fpi <f < fpe. The intensity distribution of magnetosheath electrostatic turbulence clearly shows that the low‐frequency component is associated with the bow shock, suggesting that the ion heating begun at the shock continues into the downstream magnetosheath. Electrostatic waves below 1 kHz are polarized along the magnetic field direction, a result consistent with the polarization of electrostatic waves at the shock. The high‐ and intermediate‐frequency components are features of the magnetosheath spectrum which are not characteristic of the shock spectrum but are often detected in the upstream solar wind. The intensity distribution of electrostatic turbulence at the magnetosheath plasma frequency has no apparent correlation with the shock, indicating that electron plasma oscillations are a general feature of the magnetosheath. The plasma wave noise shows a tendency to decrease toward the dawn and dusk regions, consistent with a general decrease in turbulence away from the subsolar magnetosheath. Occasionally, intense electrostatic bursts occur which are correlated with magnetic field gradients and fluctuations, possibly indicating the presence of strong current systems such as can occur at the magnetopause.
Large‐amplitude gravity wave energy production and dissipation in the thermosphere
Authors: Richmond A. D.
Published: February 2008

This paper extends some of the analysis of gravity wave generation and dissipation in the thermosphere given in a previous paper [Richmond, 1978] by defining wave energy for gravity waves of arbitrary amplitude and by analyzing several numerical simulations. Wave energy as defined in this paper is closely related to the sum of kinetic energy and available potential energy of the atmosphere. In the absence of production or dissipation mechanisms, wave energy is approximately, but not strictly, conserved. Its dissipation is related to, but in general is not the same as, entropy generation. Numerical simulations substantiate the findings of Richmond (1978) that both the Lorentz force and the Joule heating of auroral currents can generate substantial gravity wave energy but that the waves produced by Joule heating are more capable of traveling to middle and low latitudes. The simulations also reveal that molecular dissipation of wave energy can be relatively less important, in comparison to daytime Joule dissipation, than the simple treatment of Richmond [1978] indicated.
Nighttime thermospheric winds at high latitudes
Authors: Hays P. B. , Meriwether J. W. , Roble R. G.
Published: February 2008

Nighttime thermospheric winds have been measured from Ester Dome, near College, Alaska (64.8°N, 147.8°W), using the high‐resolution Fabry‐Perot interferometer of the University of Michigan Airglow Observatory. The winds are determined from the Doppler shifts of the (0I) 15867 K (6300 Å) line emission. Measurements obtained during geomagnetic quiet times in January and February 1972 are averaged to obtain the characteristic nighttime behavior of both the zonal and the meridional neutral wind components at F‐region heights. Ion drifts during this period were measured by the Chatanika incoherent scatter radar facility. The measurements of the average nighttime zonal and meridional neutral wind components and of the average ion drift components are used with a three‐dimensional dynamic model of the neutral thermosphere to determine the effectiveness of various forces in controlling the winds at F‐region heights in the high‐latitude thermosphere. A least squares fit of the measured and calculated nighttime variations of the zonal and meridional winds, using the ion drag determined from the radar measurements, gives the Fourier coefficients of latitudinal and longitudinal pressure gradients that are necessary to drive the observed wind pattern. The results show that the ion drag force is essentially balanced by the pressure force in the meridional direction. However, in the zonal direction the pressure force does not balance the ion drag force. The pressure gradients, derived in terms of exospheric temperature variations, show a different nighttime variation from that predicted by the MSIS model for that latitude. The derived zonal winds are westward in the early evening hours, opposite of the winds calculated from a dynamic model that uses the pressure forces determined from the MSIS semiempirical model and ion drag.
Ion acoustic waves and related plasma observations in the solar wind
Authors: Gurnett D. A. , Marsch E. , Pilipp W. , Schwenn R. , Rosenbauer H.
Published: February 2008

This paper presents an investigation of solar wind ion acoustic waves and their relationship to the macroscopic and microscopic characteristics of the solar wind plasma. Comparisons with the overall solar wind corotational structure show that the most intense ion acoustic waves usually occur in the low‐velocity regions ahead of high‐speed solar wind streams. Of the detailed plasma parameters investigated, the ion acoustic wave intensities are found to be most closely correlated with the electron to proton temperature ratio Te/Tp and with the electron heat flux. Investigations of the detailed electron and proton distribution functions also show that the ion acoustic waves usually occur in regions with highly non‐Maxwellian distributions characteristic of double‐proton streams. The distribution functions for the double‐proton streams are usually not resolved into two clearly defined peaks, but rather they appear as a broad shoulder on the main proton distribution. Two main mechanisms, an electron heat flux instability and a double‐ion beam instability, are considered for generating the ion‐acoustic‐like waves observed in the solar wind. Both mechanisms have favorable and unfavorable features. The electron heat flux mechanism can account for the observed waves at moderate to large ratios of Te/Tp but has problems when Te/Tp is small, as sometimes occurs. The ion beam instability appears to provide more flexibility in the Te/Tp ratio; however, detailed comparisons using observed distribution functions indicate that the ion beam mode is usually stable. Possible resolutions of these difficulties are discussed.
Anomalous features in thermal radiance maps of Venus
Authors: Apt Jerome , Goody Richard
Published: February 2008

Seventeen thermal radiance maps of Venus were recorded in a 26‐day interval near the 1977 conjunction. An average symmetrical two‐dimensional function was removed from the data. Features remain near the east and west limbs which correspond in part to previously reported solar‐related effects but there are important differences, and we give reasons for questioning the interpretation of some of the earlier data. We conclude that these features cannot be solely under solar control. Features are also found at both poles, just inside the region covered by the ultraviolet polar hood. These are not inconsistent with disturbances rotating rapidly around the planet. There is weak evidence for a 5‐day repetition in some features.
Electron calibration of instrumentation for low energy, high intensity particle measurements at Mercury
Authors: Christon S.P. , Daly S.F. , Eraker J.H. , Perkins M.A. , Simpson J.A. , Tuzzolino A.J.
Published: February 2008

Unique identification of the high intensity, impulsively accelerated charged particle fluxes discovered during Mariner 10's first encounter with Mercury (March 1974) requires a detailed knowledge of the responses of the two University of Chicago charged particle telescopes on the Mariner 10 spacecraft to low energy fluxes over a wide dynamic range of flux levels. The results of detailed analyses show that these telescopes can separate and identify unambiguously the presence of electron and proton fluxes for a wide range of electron spectra and intensities in the relevant overall energy range of ∼30 keV to ∼2 MeV. We reaffirm the detection of electrons with energies in the range 200–600 keV in portions of the burst events. Armstrong et al. (1975) argued that our reported low energy telescope (LET) proton fluxes which were observed during the most intense portions of the B and C events could be due to two‐ and three‐fold electron pile‐up resulting from electrons in the energy range ∼100–200 keV. Using proton measurements made with nearly identical LET's on Pioneer spacecraft near Jupiter and, in addition, using detailed laboratory calibrations, we find that their estimates are in error by orders of magnitude. Then, using our calibration results, we searched for those electron intensities and energy spectra which conceivably could simulate the charged particle telescope responses at Mercury I. From laboratory electron calibrations and calculations we find that assumed fluxes of electrons having power‐law differential energy spectra with spectral index ∼7–9 and intensities of ≈4 × 108 electrons/(cm² s sr) for energies greater than 35 keV, give rise to a calculated counting rate in our LET which is completely dominated by at least seven‐fold pulse pile‐up. These fluxes can simulate the observed Mercury I counting rates, but this imposes a rate of energy input of up to ≈30 ergs/(cm² s sr). All other electron spectra defined for energies >35 keV having a maximum differential intensity at an energy ≳50 keV fail to simulate simultaneously the Mercury 1 LET and main telescope (MT) responses. Thus the electron fluxes required to simulate our measurements of the Mercury I B and C events must be a factor ∼10²–10³ greater than the electron fluxes required to interpret those measurements as resulting from the simultaneous presence of electron and proton fluxes. We therefore favor an interpretation in terms of simultaneous electron and proton fluxes.
PC 4 giant pulsations in the morning sector
Authors: Rostoker Gordon , Lam Hing‐Lan , Olson John V.
Published: February 2008

Geomagnetic micropulsations in the Pc 4 frequency band which are highly monochromatic and have amplitude‐modulated wave forms have been termed ‘giant pulsations’ and have been observed to reach amplitudes of several tens of nanoteslas. In this paper, four such events are analyzed by using meridian line and two‐dimensional magnetometer networks. It is found that these pulsations are highly localized in latitude and have a large azimuthal wave number k⊥, although the large magnitude of k⊥ does not necessarily reflect the longitudinal extent of the disturbed region. While such events are extremely rare, they may occur on 2 or more consecutive days. The pulsations occur on quiet days and normally after a sustained period of low magnetospheric activity. The initiation of substorm activity appears to quench the giant pulsations, which may return after the substorm has subsided. We suggest that the pulsations accompany suddenly reduced levels of magnetospheric convection, and indeed the source region of the pulsations moves poleward during the course of the pulsational activity, suggesting a recovery phase relationship. It is suggested that the Pc 4 giant pulsations may be due to the occurrence of field line resonances at the plasmapause in the region where the electric field changes its azimuthal direction from westward to eastward.
Generation of long‐delay echoes
Authors: Muldrew D.B.
Published: February 2008

Long‐delay echoes (LDE), defined as echoes received from a fraction of a second to several seconds after a radio signal is transmitted, have been observed off and on for about 50 years. A variety of explanations has been proposed in the past but none is completely satisfactory. The following models are presently proposed for LDE: (1) Radio waves of frequency less than about 4 MHz can become trapped in magnetic field‐aligned ionization ducts with L values less than about 4. These waves after being trapped can propagate to the opposite hemisphere of the earth where they become reflected in the topside ionosphere. They can then return along the duct, leave it and propagate to the receiver. Delays of up to 0.4 s result and they probably account for most of the LDE at frequencies below 4 MHz with estimated delays of 1–2 s. (2) The signals from two separated transmitters T1 and T2, T2 transmitting a CW or quasi‐CW signal, interact nonlinearly in the ionosphere or magnetosphere. If the wave vector and frequency of the forced oscillation at the difference frequency of the two signals satisfies the dispersion relation for electrostatic waves, such a wave would exist and begin to propagate. This wave could grow in amplitude due to wave‐particle interaction. At a later time it could interact with the CW signal from T2 and if the wave vector and frequency of the forced oscillation at the difference frequency (frequency of T1) satisfy the dispersion relationship for electromagnetic waves, such a wave would exist and could propagate to T1, or some other receiving station tuned to the frequency of T1. Reasonable ionospheric and magnetospheric plasma parameters lead to delays of up to about 6 s with this model. (3) A large percentage of LDE have been reported with delays of tens of seconds. These delays could be explained if the model in (2) is applied to a magnetospheric ionization duct. Electrostatic waves could propagate for about 1000 km or more over the magnetic equator in such a duct and delays of about 40 s are possible. Dispersion for a finite frequency bandwidth would probably not be so large in cases (1) and (2) as to make voice unrecognizable. Dispersion in model (3) for delays greater than about 10 s would normally be too severe for voice modulation, but occasionally compensating effects might occur for which voice would be recognizable.
The morphology of equatorial Mg+ ion distribution deduced from 2800‐Å airglow observations
Authors: Gérard J.‐C. , Rusch D.W. , Hays P.B. , Fesen C.L.
Published: February 2008

The Visible Airglow Experiment on the Atmosphere Explorer E satellite has observed the resonantly scattered emission from Mg II at 2800 Å in the equatorial ionosphere. Altitude profiles of the Mg+ ion distribution have been obtained from the inversion of the surface brightness measurements made on spinning orbits. These data show a daytime metallic ion layer between 150 and 200 km developing in the early morning and reaching about 100 ions/cm³ in the afternoon. Mg+ ions are also seen in the F2 region mostly in the late afternoon hours within a few degrees of the dip equator. The study of the vertical column density measured in the despun mode indicates that the amount of Mg+ in the F region is most variable in the afternoon hours at low dip latitudes. These results can be explained in part by the diurnal variation of the E × B drift velocity which lifts the metallic ions up into the F region. The observations suggest that the vertical polarization electric field is not the primary transport mechanism extracting the Mg+ ions from the low‐altitude source layer.
Thermospheric heating in a magnetic storm: Dynamic transport of energy from high to low latitudes
Authors: Richmond A.D.
Published: February 2008

During a magnetic storm, energy can be transferred from high to low latitudes in the thermosphere by two dynamical processes: gravity waves and meridional circulation. This paper evaluates the relative importance of these two mechanisms with the aid of a numerical simulation. The appropriate auroral region conditions to be used as input parameters for such a simulation are a matter of considerable uncertainty, but even when these are chosen so as to be relatively favorable to gravity wave energy transfer, meridional circulation is found to be more important in heating the low‐latitude thermosphere.
On natural noises detected by antennas in plasmas
Authors: Meyer‐Vernet N.
Published: February 2008

A formal generalization of the Nyquist formula for an antenna in a possibly anisotropic equilibrium plasma is presented along with practically useful expressions derived from it. It is shown that this can explain some preliminary results of the recent three‐dimensional radio mapping experiment (SBH) on the ISEE 3 spacecraft.
Auroral kilometric radiation: Time‐averaged source location
Authors: Gallagher Dennis L. , Gurnett Donald A.
Published: February 2008

The location of the average generation region of auroral kilometric radiation is found by studying average electric field strengths as a function of spacecraft position in narrow frequency bands centered at 178, 100, and 56.2 kHz. A combined 5 years of data from the University of Iowa plasma wave experiments on satellites Hawkeye 1 and Imp 6 provide the basis for determining the average electric field strengths. Hawkeye 1 was in a highly elliptical, polar orbit with an apogee near 21 RE over the northern polar region, and Imp 6 was in a highly elliptical, near‐equatorial orbit with an apogee of 33 RE. Together these satellites provide extensive coverage from 3 to 21 RE in the northern hemisphere and inside of 3 RE in the southern hemisphere. Intense sources of auroral kilometric radiation are found in the northern and southern hemispheres. Their locations are near 65° invariant latitude in their respective hemispheres, between 22 and 24 hours magnetic local time, and near 2.5 RE. The total time‐averaged power generation is found to be about 107 W, assuming a spectral bandwidth of 200 kHz. Propagation effects limit the emission cone of auroral kilometric radiation in a given hemisphere to roughly 4.1 sr at 178 kHz, 2.2 sr at 100 kHz, and 1.5 sr at 56.2 kHz. Evidence that the polar cusp region is illuminated at distances as close as 4 RE suggests the possibility that previously observed polar cusp sources are the result of scattering from field‐aligned density irregularities.
Fluctuations of inverted V electron fluxes
Authors: Lin C.S. , Hoffman R.A.
Published: February 2008

Fluctuations of low‐energy electron fluxes were detected during an inverted V event around 1009 UT, November 5, 1975, by fixed‐energy electrostatic analyzers onboard the AE‐D satellite. The flux fluctuations were observed at energies of 0.72, 1.38, and 2.62 keV when these energies were below that of the monoenergetic beam, suggesting that the flux fluctuations might occur over a wide range of energies. The fluctuations were confined within a pitch angle of 15°, and the amplitude increased with decreasing pitch angles. The oscillation frequencies centered around 1.75 Hz. The fluctuations at two energies were generally correlated. From the variation of the distribution function with respect to the distribution function at 0.72 keV we deduce that secondary electrons were heated. Our interpretation of these observations is that the secondary electrons were heated anomalously by electron plasma wave turbulences and that the intensity of the wave turbulences was modulated by a low‐frequency wave. These results are consistent with the model that auroral precipitating electron beams enhance the anomalous resistivity at the topside ionosphere through parametric instabilities.