bluuebunny/crossref_metadata_2025 huggingface dataset:
Downstream properties of magnetic flux transfer events
Authors: Sibeck D. G. , Siscoe G. L.
Published: February 2008

We consider the downstream evolution of the tubes of field lines known as flux transfer events (FTE's) formed by the localized, dayside, sporadic merging of magnetosheath and magnetosphere magnetic field lines. FTE's have magnetic field and plasma properties distinct from those of the nearby unmerged magnetosheath and magnetosphere field lines. Tailward advection stretches these earth‐rooted flux tubes and orients them parallel to the field in the magnetotail lobes. Following dayside merging, magnetosheath plasma flows into the magnetosphere part of the FTE. This flow is modeled by a self‐similar expansion of a plasma into a vacuum. The magnetosheath part of the FTE empties, and the magnetosphere part fills with time. After the FTE has moved 200 RE down the tail, the drained portion of the FTE reaches about 25 RE radially outward from the tail boundary. We suggest that most multiple crossings of the tail boundary observed by spacecraft are encounters with tailward moving FTE's. This interpretation explains fully the way boundary normals behave during multiple crossings. The previously favored interpretation, tailward moving boundary waves, only partly explains the known behavior. The model explains how the sign of the interplanetary magnetic field (IMF) causes the observed dawn‐dusk asymmetries in the thickness of the magnetotail boundary layer and in the pattern of “lobe filling.” It also predicts that multiple boundary crossings should be correlated with negative IMF Bz.
Magnetospheric energization by interaction between planetary spin and the solar wind
Authors: Isbell J. , Dessler A. J. , Waite J. H.
Published: February 2008

If the solar wind is capable of driving magnetospheric convection, then solar‐wind flow past any spinning, magnetized planet with a conducting ionosphere must cause the magnetic field lines in the outer part of its magnetospheric tail to be twisted into a helix. Such a magnetic field configuration requires magnetically field‐aligned (Birkeland) currents in the tail that flow in and near the magnetopause and close by driving Pedersen currents through the planetary ionosphere. The strength of the Birkeland currents (and, by current continuity, the Pedersen currents) is, to first order, independent of the angle between the planetary‐spin vector and the solar‐wind velocity vector. Rather, the total current is a function of the magnetic moment of the planet, the radius of the tail, the angular velocity of planetary spin, the conductivity of the ionosphere, and the solar wind speed. For Jupiter, Saturn, Uranus, and perhaps Neptune, the power these currents deliver to the ionosphere is significant with regard to magnetospheric dynamics such as the production of aurora and the generation of low‐frequency radio emissions. For Mercury, Venus, Earth, Mars, and probably Pluto, these currents are relatively small, although observable effects may be marginally detectable for the case of the Earth's magnetosphere.
Evolution of the Earth's distant magnetotail: ISEE 3 electron plasma results
Authors: Zwickl R. D. , Baker D. N. , Bame S. J. , Feldman W. C. , Gosling J. T. , Hones E. W. , McComas D. J. , Tsurutani B. T. , Slavin J. A.
Published: February 2008

Electron plasma measurements made by ISEE 3 from October 18, 1982, through April 19, 1983, were examined for signatures identifying the spatial evolution of the earth's magnetotail out to distances of ∼225 RE. Nearly 91,000 individual electron spectra were combined into a data set in which each point was first labelled as being magnetosheath, lobe, or plasma sheet and then separated into one of four radial‐distance bins. The lobe density distribution shows two peaks near earth (≤ 60 RE), a narrow peak at ∼10−2 cm−3 representing quiet lobe, and a second broader peak above 10−1 cm−3 representing boundary layer phenomena. At larger distances the lower density peak disappears and the remaining density peak (at ∼10−1 cm−3) becomes broader and contains a high density tail which extends well above 1 cm−3. The increase in lobe density at large distances is believed to be due to plasma which crosses the magnetopause on open field lines along its flanks and drifts inward toward the plasma sheet. The plasma sheet density distribution shows only a slight decrease in density at larger radial distances. The temperature of the plasma sheet and lobe electron plasma decreases with increasing distance, with the plasma sheet temperature decreasing at a faster rate. Evolution in the density and temperature of the electron plasma in the magnetotail results in a situation wherein it is often difficult to separate plasma sheet from lobe plasma at large radial distances by these two parameters alone. The bulk flow velocity of the plasma sheet and lobe plasma increases dramatically at larger radial distances. The electron bulk velocity observed in the plasma sheet beyond 180 RE often exceeds the normal solar wind velocity range (300–700 km/s). The most striking result of this study is the observed evolution of the electron bulk velocity flow angle from a bimodal distribution near earth to a single tailward directed peak beyond 180 RE. The decreasing amount of earthward flow with increasing distance directly implies that the location of the distant neutral line is usually within 120 RE of earth, and rarely beyond 180 RE. These observations also imply that the location of the neutral line is variable and dependent upon local interplanetary and geomagnetic conditions.
Characteristics of coronal mass ejections associated with solar frontside and backside metric type II bursts
Authors: Kahler S. W. , Cliver E. W. , Sheeley N. R. , Howard R. A. , Koomen M. J. , Michels D. J.
Published: February 2008

We compare fast (υ ≥ 500 km s−1) coronal mass ejections (CME's) with reported metric type II bursts to study the properties of CME's associated with coronal shocks. We confirm an earlier report of fast frontside CME's with no associated metric type II bursts and calculate that 33±15% of all fast frontside CME's are not associated with such bursts. Faster CME's are more likely to be associated with type II bursts, as expected from the hypothesis of piston‐driven shocks. However, CME brightness and associated peak 3‐cm burst intensity are also important factors, as might be inferred from the Wagner and MacQueen (1983) view of type II shocks decoupled from associated CME's. We use the equal visibility of solar frontside and backside CME's to deduce the observability of backside type II bursts. We calculate that 23±7% of all backside type II bursts associated with fast CME's can be observed at the earth and that 13±4% of all type II bursts originate in backside flares. CME speed again is the most important factor in the observability of backside type II bursts.
The formation of a standing shock in a polytropic solar wind model within 1–10 Rs
Authors: Habbal Shadia R.
Published: February 2008

We show how a one‐fluid polytropic solar wind model exhibits properties similar to an isothermal wind when localized momentum addition and/or rapid area divergence produce multiple critical points in the flow. In particular, we show that when the sonic transition in the flow occurs closer to the coronal base, multiple steady solutions can exist. These multiple steady solutions consist of a continuous solution passing through the innermost critical point and other steady solutions involving a steady shock transition. By following the temporal evolution of the solar wind from a steady state with one critical point to a steady state with three critical points, we show that a standing shock solution is more likely to develop than a continuous solution when momentum deposition occurs close to the coronal base and the equation of motion admits multiple steady solutions. This result is particularly relevant to the solar wind when momentum deposition occurs as a result of a rapidly diverging coronal hole geometry.
Dynamics of intense upstream ion events
Authors: Wibberenz G. , Zöllich F. , Fischer H. M. , Keppler E.
Published: February 2008

We study temporal structures, energy spectra, and spatial gradients of 25–70 keV protons during four intense upstream ion events observed on December 3, 1977, by the medium‐energy particle telescope (KED) on ISEE 2. The strong role of the bow shock connection time in controlling the absolute intensity and spectral shape of the upstream ions is confirmed. The path along which the convected magnetic field is carried on the bow shock surface has no observable influence. During the plateau phases, we determine a field‐aligned gradient pointing toward the bow shock with an e‐folding distance L = 6.5±1.5 RE for ≈30‐keV protons. The combination with anisotropy data leads to a direct determination of the mean free path λ∥ = 2.6 ± 0.6 RE. A gradient perpendicular to the magnetic field points toward the nose of the bow shock with a north‐south component of about 6%/RE. Its conversion to a spatial scale allows to estimate the perpendicular diffusion coefficient. We conclude that lateral diffusion is not the main escape mechanism which determines the exponential energy spectrum. The control of the acceleration efficiency by local characteristics of the bow shock is suggested by various observations: (1) fluctuations on a temporal scale of about 20 min and with a peak‐to‐peak amplitude of about 50% superimposed on the plateau phases; (2) structured onsets of events during smoothly improving connection conditions; (3) strong intensity modulation during marginal acceleration conditions when the connection time is of the order 10 to 12 min; (4) convection of differently populated field lines across the observer. We tentatively relate the variable acceleration efficiency to changes in the normal angle (the angle between the shock normal and the local magnetic field direction). The hitherto unexplained existence of the limits in flux during the plateau phases is stressed.
The excitation of plasma waves by a current source moving in a magnetized plasma: The MHD approximation
Authors: Rasmussen C. E. , Banks P. M. , Harker K. J.
Published: February 2008

This is the first of a series of three papers which examine the excitation of low‐frequency plasma waves arising from a current source moving through a cold, magnetized plasma. In particular, the effects of motion of the source upon the radiation pattern are studied. In the present paper the general theory of wave excitation by a moving source is developed and specific approximations are made which are valid for frequencies much below the ion‐cyclotron frequency. The amplitude and pattern of shear‐Alvén waves excited by a current source are studied for sources whose size in the direction of motion is large in comparison to the size in other directions and also for sources whose size is large in the direction of current flow (also in comparison to the size of the source in other directions). It is shown that the radiation pattern of the shear waves arises naturally as a consequence of the motion of the source and that any modulation of the source current has little effect upon the waves, other than to reduce their amplitude.
Large‐scale solar modulation of >̃ 500 MeV/nucleon galactic cosmic rays seen from 1 to 30 AU
Authors: Fillius Walker , Axford Ian
Published: February 2008

Using measurements of ≳500 MeV/nucleon cosmic rays obtained by Cerenkov counters on Pioneer 10 and Pioneer 11 and neutron monitor data from earth, we can observe the spatial and temporal development of cosmic ray modulation during the last solar maximum. The large‐scale features of modulation and recovery are similar at these three sites and thus appear rotationally symmetric near the ecliptic plane. Outward propagating features characterize the radial dependence. The decline of the old cosmic ray cycle is marked by steplike decreases that propagate outward at nearly the solar wind velocity, as pointed out by other investigators. During the start of the new cosmic ray cycle, recovery occurs first in the inner heliosphere and, after a lag comparable with that of the declining phase, appears later farther out. However, the direction of diffusive propagation is still inward because the gradient remains positive. Forbush decreases are common at all three sites and are evidently of great importance in understanding modulation. The largest decrease occurred during a short series of events in summer 1982 and had half the amplitude of the 11‐year cycle.
Voyager observations of ion phase space densities in the Jovian magnetosphere
Authors: Paonessa Mark
Published: February 2008

Data from the Voyager low‐energy charged particle experiment (LECP) were used to calculate ion phase space densities in Jupiter's magnetosphere. The calculation of F at constant µ and J2 requires the determination of particle fluxes at specific pitch angles and energies. It is shown that the greatest uncertainties in the determination of F from LECP data arise from the fits to the measured pitch angle distributions and differential energy spectra. An estimate is provided of this uncertainty and of others arising from model radial diffusion coefficients and magnetic fields. The general nature of the curves is consistent with inward diffusion of these energetic ions from the outer magnetosphere combined with losses near but starting beyond the orbit of Io. These losses are not consistent with simple satellite sweeping by Io alone and are probably due to an Io torus wave‐particle interaction. The lifetime against loss deduced from the data is ≈2×104 s near Io and is a value consistent with strong diffusion losses. The measured ion loss rate declines much more rapidly than the strong diffusion loss rate as L increases from 7 to 9, suggesting that the loss rate is well below the strong diffusion rate beyond 7 RJ for ions observed by the LECP.
Magnetotail plasma observations during the 1054 UT substorm on March 22, 1979 (CDAW 6)
Authors: Paschmann G. , Sckopke N. , Hones E. W.
Published: February 2008

The paper reports ISEE 1 and 2 plasma observations in the plasma sheet near 15 RE radial distance for the 1054 substorm on March 22, 1979. Noteworthy features are plasma sheet thinning and the brief appearance of rapid tailward flows after substorm onset, strong earthward flows (superimposed on dawnward convection) at the beginning of plasma sheet expansion and a quieting of all plasma properties at substorm recovery. Much of the time, however, plasma flows are small and switch directions. In particular, no strong flows are observed during neutral sheet crossings. Two‐dimensional velocity distribution functions show a complex behavior, with several plasma components often coexisting and at times streaming in opposite directions. The interpretation of the observations in terms of the formation of a neutral line earthward of and close to ISEE at substorm onset is tempting, but three‐dimensional spatial effects as well as rapid temporal changes have to be invoked to obtain consistency.
Estimation of electric fields and currents from international magnetospheric study magnetometer data for the CDAW 6 intervals: Implications for substorm dynamics
Authors: Kamide Y. , Baumjohann W.
Published: February 2008

Using a recently developed numerical scheme combined with International Magnetospheric Study magnetometer data and the Rice University ionospheric conductivity model as input, the global distribution of the key ionospheric parameters is estimated for the Coordinated Data Analysis Workshop (CDAW) 6 intervals. These outputs include ionospheric electric fields and currents, field‐aligned currents and Joule heat production rate at high latitudes, and are compiled in the form of a color movie film, which demonstrates dynamics of substorm changes of the three‐dimensional current system as well as of the associated potential pattern. The present paper gives, on the basis of the space‐time distribution of the key parameters, the substorm time frame that can be referenced to in terms of the substorm phases when discussing some other magnetospheric and ionospheric records. The distinction between “substorm expansion” and “enhanced convection” current systems is presented on the basis of the conventional equivalent current and potential patterns and “true” ionospheric currents. Although the auroral electrojets flow rather contiguously throughout the dark sector, there are several separate source regions of Joule heating from the electrojet currents. This indicates that the relative importance of the ionospheric conductivity and the electric field in the ionospheric currents varies considerably depending upon latitude and local time. A possible difference in the generation mechanisms of isolated and continuous substorm activity is also discussed to some extent in the light of the two CDAW 6 intervals.
Interplanetary magnetic field control of high‐latitude electric fields and currents determined from Greenland Magnetometer Data
Authors: Friis‐Christensen E. , Kamide Y. , Richmond A. D. , Matsushita S.
Published: February 2008

To determine the effects of the interplanetary magnetic field (IMF) on the electric potential as well as on ionospheric and field‐aligned currents, a recently available numerical algorithm is applied to an empirical model of high‐latitude magnetic perturbations, parameterized in terms of the By and Bz components of the IMF. The empirical model is derived from 20‐min average magnetometer data observed during summer at the chain on the west coast of Greenland and the corresponding IMF information from the HEOS 2 satellite. The calculated results reproduce fairly well overall features of the influence of the IMF on high‐latitude electric fields which have been reported on the basis of more direct measurements. This confirms the validity of the numerical method and the conductivity distribution models. In addition, our results indicate that the system of ionospheric and Birkeland currents near the polar cusp, which has been shown to depend strongly on By, exists independently of the system of region 1 and region 2 field‐aligned currents, which, on the other hand, depends strongly on Bz. The direction of the field‐aligned currents in the dayside polar cap is uniquely controlled by the sign of the By component of the IMF, namely upward currents for By > 0 in the northern polar cap and oppositely directed for By < 0. At the dayside boundary of the polar cap the DPY ionospheric Hall current is sandwiched between the polar cap field‐aligned currents and an oppositely directed field‐aligned current sheet on the equatorward side. For Bz >0 and By small the ionospheric and field‐aligned currents are localized near the dayside polar cusp, and the electric field has a dusk‐dawn component in a narrow region near magnetic local noon in agreement with reported satellite measurements. The associated distribution of field‐aligned currents consists of the region 1 current system and an additional pair of oppositely directed currents located poleward of the region 1 currents.
Electric currents above Saint‐Santin: 3. A preliminary study of disturbances: June 6, 1978; March 22, 1979; March 23, 1979
Authors: Mazaudier Christine
Published: February 2008

This paper presents three case studies of ionospheric disturbances in electric fields, currents, and winds during periods of geomagnetic storms. These disturbances are detected by the Saint‐Santin incoherent scatter radar. The disturbances are shown to originate from two distinct physical mechanism: (1) penetration of electric fields to lower latitudes during times of rapid change in magnetospheric convection; and (2) the action of the disturbed ionospheric dynamo driven by storm‐induced wind disturbances. The storm of June 6, 1978, shows a simple illustration of penetrative convection electric fields. The storm of March 22, 1979, gives additional examples of this effect both when the Bz component of the interplanetary fields turns southward and northward. The observed events on March 23 are clearly identifiable as the delayed response of the disturbance ionospheric dynamo.
The August 28, 1978, storm: 1. GEOS 2 observations of the initial magnetopause crossings and STARE observations near the ionospheric convection reversal
Authors: Sofko G. J. , Korth A. , Kremser G.
Published: February 2008

During the postdawn period on August 28, 1978, from ∼0750 to 0830 magnetic local time, the magnetopause moved several earth radii inward to less than 6.6 RE, where it remained until ∼1120 MLT. As the magnetopause approached the earth, the poleward boundary of the westward auroral electrojet moved southward to geomagnetic latitudes less than 66°. Fortuitously, the geostationary satellite GEOS 2 was located in this morning sector during this entire period, and experienced a series of magnetopause crossings. In quiet periods GEOS 2 was conjugate to that area of the lower E region which constituted the field‐of‐view of the Scandinavian Twin Auroral Radar Experiment (STARE) radars. This paper compares the magnetospheric satellite and ionospheric radar observations. The combined observations suggest that, near the dawnside magnetopause, large‐scale merging was occurring, while in the ionosphere, the convection reversal region and poleward portion of the auroral zone underwent major equatorward shifts and featured prominent sunward moving ULF pulsations. Between the magnetopause and ionosphere a large increase in the ring current accompanied the equatorward shift of the convection reversal.
Electrostatic wave observation during a space simulation beam‐plasma discharge
Authors: Walker D. N. , Szuszczewicz E. P.
Published: February 2008

We have studied ELF waves which were observed during beam‐plasma discharge in the large vacuum chamber at Johnson Space Center. Phase delays as a function of radius (obtained from cross‐correlation measurements of density fluctuations) along with measurements of frequency and plasma potential, density, and temperature have been compared to a zero‐order slab model of nonlocal azimuthal drift wave propagation. The inferred wave phase velocity in the plasma frame after Doppler correction is found to be near one half the electron diamagnetic drift velocity. Although our measurements do not uniquely define a propagation mode, a model of azimuthal drift wave propagation is found to be consistent with our observations.
A laboratory investigation of the high‐frequency Farley‐Buneman instability
Authors: Kustom B. , D'Angelo N. , Merlino R. L.
Published: February 2008

A laboratory investigation of the high‐frequency Farley‐Buneman instability is described. The instability was studied theoretically by Lee et al. (1971) and is predicted to occur in the low E region of the ionosphere when the E/B drift velocity of the electrons relative to the ions is several times Cs, the ion acoustic speed. In our experiments an increase of the electric field well above the Lee et al. “threshold” merely enhances the general power level of the fluctuations but does not affect appreciably their spectral shape. The observed frequency spectra fall off very rapidly with increasing frequency, with a spectral shape of the type P(f) ∝ f−3.5. This result agrees well with recent ionospheric observations and suggests that a recently proposed mechanism of anomalous wave electron heating in the lower E region should not be linked to the presence of short‐wavelength (10–30 cm) irregularities.
Spectral broadening of VLF transmitter signals observed on DE 1: A quasi‐electrostatic phenomenon?
Authors: Inan U. S. , Bell T. F.
Published: February 2008

Spectrally broadened VLF transmitter signals are observed on the DE 1 satellite using alternatively both electric and magnetic field sensors. It is found that at times when the electric field component undergoes significant bandwidth expansion (up to ∼110 Hz) the magnetic field component has a bandwidth of less than 10 Hz. The results support the theory that the off‐carrier components are quasi‐electrostatic in nature. Measurement of the absolute E and B field magnitudes of the broadened signals are used to determine the wave Poynting vector. It is found that the observed power levels can be understood without invoking any strong amplification process that operates in conjunction with the spectral broadening. The implications of this finding in distinguishing among the various possible mechanisms for spectral broadening are discussed.
The diffraction of VLF radio waves by a patch of ionosphere illuminated by a powerful HF transmitter
Authors: Barr R. , Rietveld M. T. , Stubbe P. , Kopka H.
Published: February 2008

At Skibotn in northern Norway, measurements have been made of the effect of powerful HF waves, from the heating facility at Ramfjordmoen, on the propagation of VLF waves from the Omega transmitter (12.1 kHz) near Aldra and on the 16.4‐kHz transmissions (JXN) at Helgeland. The HF beam was deflectable in a north‐south direction, which enabled it to intersect the VLF propagation path. The VLF amplitude and phase perturbations were found to oscillate about zero as the beam was scanned across the propagation path, and typical daytime amplitude and phase perturbations had maximum magnitudes of ∼0.1 dB and 0.5°. The maximum values appeared when the heated beam was symmetrically located across the VLF path. A study of the temporal variability of the amplitude and phase perturbations at a fixed‐beam deflection angle (38°S) showed that large enhancements of up to two orders of magnitude occurred in the amplitude and phase perturbations at times when the total received VLF field strength passed through a minimum. Maximum amplitude and phase perturbations of 6 dB and 50° were recorded during one such minimum in the total received signal. Theoretical computations are presented which evaluate the diffracting effect of a heated patch of auroral ionosphere on multimode propagation in the earth‐ionosphere waveguide. Computations of propagation under a series of ionospheres ranging from typical daytime to nighttime have been made, and the normal and enhanced amplitude and phase perturbations have been successfully modeled. Attempts at reproducing the perturbing effects of a mobile heated patch have not been nearly so successful, especially at large deflection angles.
Trapping of ion conics by downward parallel electric fields
Authors: Gorney D. J. , Chiu Y. T. , Croley D. R.
Published: February 2008

Several examples of upflowing field‐aligned electrons at altitudes between 1000 and 15000 km at high latitudes have been reported recently. The velocity‐space distributions of these electrons suggest that they were accelerated out of the ionosphere by downward parallel electric fields, and parallel potential drops of a few tens to a few hundred volts over the altitude range 1000–6000 km are implied. The electron beams are associated with ion conics. Data from electrostatic analyzers onboard the S3‐3 satellite show that ion conics can be trapped at low altitudes by the downward electric field. Evidence for the parallel electric field is obtained not only from observations of upflowing electron beams but also from observations of retardation of downflowing electrons and local acceleration of downflowing ions. The inferred parallel electric fields have magnitudes large enough to locally balance the magnetic mirror force on perpendicular ion conics in the few hundred electron volt energy range. This electrostatic trapping has important consequences for wave heating of ion conics since it extends the residence time of ions in the heating region. It is demonstrated that this multipass heating mechanism leads to perpendicular ion heating far in excess of that predicted by present theories of ion conic heating.
Mapping of electric field structures from the equatorial F region to the underlying E region
Authors: LaBelle J.
Published: February 2008

The theories of electrostatic fields in the equatorial ionosphere attributed to Farley [1960] and Spreiter and Briggs [1961] are reviewed, and a hybrid of these theories is applied to investigate how spread F irregularities map down to the underlying E region. It is found that structures larger than 1 km map easily through the E region, while smaller structures do not. This implies that in general, subkilometer F region irregularities cannot directly generate image striations in the underlying E region, a result which affects previous investigations of the role of images in equatorial spread F.
Cusp altitudinal electron temperature gradient: Dynamics Explorer 2 implications for heating mechanisms
Authors: Curtis S. A. , Hoegy W. R. , Brace L. H. , Winningham J. D.
Published: February 2008

Observations in the cusp from the Dynamics Explorer 2 (DE 2) satellite show an increase in upper F region electron temperature with altitude. Examination of precipitating cusp electron spectra suggests that this heat is introduced at higher altitudes as the result of wave‐particle interactions. Ionospheric heating is restricted to higher altitudes than DE 2 (300–1000 km) because the ratio of magnetosheath field‐aligned flowing electrons to thermal ionospheric electrons is sufficient there. The growth rates for instabilities of interest are proportional to this ratio; hence the spatial and temporal growth rates are too small in the ionosphere itself. Thus the upper F region cusp is heated primarily by electron heat transport from above rather than locally by either the impact of soft electrons precipitating in the cusp region or by plasma instabilities. The emerging picture is that (1) local plasma turbulence gives rise to anomalous heat conduction that may play an important role in maintaining the temperature gradient at higher levels than expected from classical transport properties of the ionosphere alone and (2) plasma instabilities at higher altitudes than DE 2 provide the coupling of the suprathermal electron beam free energy to the thermal cusp electron plasma and hence give rise to the nonlocal nature of the heat source itself.
Pioneer Venus Orbiter ultraviolet spectrometer limb observations: Analysis and interpretation of the 166‐ and 156‐nm data
Authors: Paxton L. J.
Published: February 2008

Pioneer Venus orbiter ultraviolet spectrometer (PVOUVS) limb observations at 166 and 156 nm have been analyzed, and emission mechanisms at these wavelengths have been modeled. The observations prove to be diagnostic of both the thermal carbon density and the O2 to CO2 mixing ratio. The photons seen by the PVOUVS at 156 and 166 nm arise from a variety of sources: photoelectron impact dissociative excitation of CO and CO2, photodissociative excitation of CO and CO2, photoelectron impact excitation of C, solar resonance scattering by C, and the underlying CO fourth‐positive bands (CO4PG). Solar resonant scattering by atomic carbon is an important contribution to the observed intensity and, furthermore, a diagnostic of the O2 to CO2 mixing ratio. An O2 to CO2 mixing ratio of 0.3% yielded the best agreement between the modeled and observed limb intensities for both the 156‐ and 166‐nm data sets. For that O2 to CO2 mixing ratio, the atomic carbon density reaches a maximum at 155 km of 5×106 cm−3.
Enhanced growth of whistlers due to bunching of untrapped electrons
Authors: Winglee R. M.
Published: February 2008

Man‐made signals propagating in the whistler mode in the magnetosphere have been observed to be amplified and to trigger other VLF emissions in the absence of other detectable magnetospheric signals in ground‐based recordings. The absence of other magnetospheric signals implies that the growth rate of the triggered wave is enhanced over background noise. It is shown that, for at least low amplitude triggering waves, the phase bunching of untrapped electrons can account for the observed enhanced growth. The phase bunching is initially produced by perturbations in the electron trajectories due to their interactions with the front of the wave. Due to the inhomogeneity of the geomagnetic field, the phase‐bunched electrons are able to gyroresonantly interact with a later portion of the triggering wave. Because the electrons are phase bunched, the electrons can lose energy to the wave faster than phase‐random electrons and are thereby able to produce enhanced growth. Plasma conditions for which this phase bunching is effective in producing enhanced growth are derived.
Finite temperature stabilization of the gradient drift instability in barium clouds
Authors: Drake J. F. , Huba J. D. , Zalesak S. T.
Published: February 2008

We present a relatively simple analysis of the gradient drift instability in barium clouds which includes the effects of both finite temperature and finite parallel length. It is found that short‐wavelength modes are stabilized as the electrons redistribute parallel to the magnetic field and neutralize the charge imbalance set up by the instability. An analytical expression for the critical wave number for stabilization is given as well as numerical results. We discuss the application of these results to the structuring of barium clouds.
Thermospheric hydrogen: The long‐term solar influence
Authors: Breig E. L. , Sanatani S. , Hanson W. B.
Published: February 2008

We describe an investigation of the long‐term behavior of neutral hydrogen in the thermosphere as influenced by changes associated with the solar cycle. Hydrogen concentrations are derived from application of charge exchange equilibrium to ionospheric measurements in the low‐latitude F region with the AE‐C and AE‐E satellites. The adopted data base spans the years, 1974–1979, of increasing solar activity and daily average exospheric temperatures ranging between 700° and 1300°K. Statistical analyses yield the diurnal relative variations and mean concentrations of hydrogen at a reference height of 300 km. These mean concentrations generally agree with other measurements over smaller temperature intervals, with minor differences at the higher temperatures. The associated daily average evaporative escape flux tends toward a constant value above ∼1000°K; this behavior is consistent with theory for the total vertical flux, but the magnitude, near 108 cm−2 s−1, is smaller by a factor of ∼3 than values deduced by other means. The observed diurnal variation has an average near 2.5 at the low temperatures, decreasing to about 2 as the temperature increases, in agreement with some but generally larger than most reported measurements. Current theoretical models require the presence of neutral winds and/or diurnally varying charge exchange fluxes into and out of the plasmasphere to provide diurnal ratios as large as those reported here.
Anomalously high plasma densities in the plasma sheet boundary layer
Authors: Etcheto J. , Saint‐Marc A.
Published: February 2008

We report the first observation of a high‐density (5 cm−3) plasma region with low perpendicular temperature (less than 30 eV) in the plasma sheet boundary layer. We used several experiments on board the ISEE spacecraft. The plasma density is determined by the relaxation sounder. The hot particle and magnetic field measurements show that high‐density regions are observed next to the region of high hot particle fluxes, and they also put an upper limit on the perpendicular temperature of the dense plasma population. Current sheets flowing primarily in the ± Y direction for this event are identified using their magnetic signature in the region of hot particle gradient.
Resonant instability near the two‐ion crossover frequency in the Io plasma torus
Authors: Thorne Richard M. , Moses Julie J.
Published: February 2008

The propagation characteristics of electromagnetic waves below the proton gyrofrequency are strongly influenced by the changing ion composition in the Io plasma torus. Obliquely propagating waves experience a natural reversal of their dominant sense of polarization when their frequency becomes equal to the crossover frequency. This dramatically modifies both the wave growth characteristics and their effect on scattering resonant particles. A numerical simulation of the path‐integrated gain of unducted waves in the torus has confirmed the importance of L mode ion cyclotron instability as the most likely mechanism for wave excitation. Whistler (R mode) instability is insignificant and it should be totally quenched by strong ion cyclotron damping. The favored region for wave excitation is at intermediate latitudes (λ ≥ 12°) on field lines passing through the outer torus (L ≥ 6). Unstable waves can subsequently propagate toward lower latitude following the natural polarization reversal (to R mode waves) at the wave crossover frequency. But the wave amplitudes observed in the low‐latitude region sampled by Voyager 1 are expected to be significantly lower those that in the source region due to strong ion cyclotron damping. This is consistent with recent observational evidence on the power spectral intensity of low‐frequency waves in the torus.
The ultraviolet dayglow at solar maximum: 2. Photometer observations of N2 second positive (0, 0) band emission
Authors: Conway R. R. , Christensen A. B.
Published: February 2008

The N2 second positive, C³Πu‐B³Πg, (0, 0) band dayglow emission at 3371 Å was observed during solar maximum with a rocket‐borne filter photometer, simultaneously with spectrometer observations of the far ultraviolet N2 and O dayglow spectrum. Modeling of the near ultraviolet dayglow spectrum indicates that the instrument also detected the N2 Vegard‐Kaplan, A³Σu+‐X¹Σg+, (0, 9) band. A spectral synthesis of these bands is used to derive emission intensities. The intensities are combined with the results of the analysis of the far UV observations reported by Meier et al. (this issue) to determine the topside neutral temperature and photoelectron flux. The effects of varying the reaction coefficient k0 for quenching of the υ′ = 0 level of the N2 A³Σu+ state are discussed. For k0 = 2.4×10−11 the derived g factor for C³Πu production is 4.4×10−8 s−1 at an altitude of 290 km, whereas for k0 = 2×10−10 it is 5.4×10−8 s−1. A synthetic spectrum of these two band systems in the wavelength region between 3000 and 4000 Å is presented.
CO2 impact ionization‐driven plasma instability observed by Pioneer Venus Orbiter at periapsis
Authors: Curtis S. A. , Brace L. H. , Niemann H. B. , Scarf F. L.
Published: February 2008

Observations of enhanced ac electric field noise about Pioneer Venus periapsis are shown to be related to spacecraft‐generated impact ionization of the ambient CO2. The frequency of the electric field noise is found to peak in the vicinity of the CO2+ ion plasma frequency and to closely follow the form of the neutral CO2 density profile. When the electric field noise in all channels is normalized by the square root of the CO2 number density, the ratio is constant. Since the impact electron density ne, measured by the Pioneer Venus Langmuir probe, is observed to scale directly with the neutral CO2, the growth of the electric field amplitude is found to be linear in time with a growth rate proportional to the CO2+ ion plasma frequency. On the basis of these results the impact ionization‐driven instability is shown to be the ion acoustic instability. This contrasts with the impact ionization process at earth where the lower beta plasma allows the modified two‐stream instability to develop rather than the ion acoustic instability. Implications for the lack of observations by Pioneer Venus of reflected‐O+‐driven instabilities, as have been proposed for the space shuttle, are discussed.
An ionization duct explanation of some plasma line observations with a 46.8‐MHz radar and with a 430‐MHz radar
Authors: Muldrew D. B.
Published: February 2008

Recently published HF‐enhanced plasma line observations using a 46.8‐MHz radar show that the upshifted decay line is absent. The upshifted spectrum consisted of a single “growing line” within a few hertz of 51.9 MHz (46.8 MHz plus the heating wave frequency of 5.1 MHz). The downshifted line was obscured by interference. The authors explain their observations by considering Langmuir wave propagation in the electron density depression generated by the ponderomotive force of the heating wave near reflection. Another approach, considered here, is to consider Langmuir wave propagation in magnetic field‐aligned ionization ducts which are generated by the HF heating wave. This latter approach appears to yield a more satisfactory explanation of the observations. Using the duct model, it is predicted that for a radar frequency near 50 MHz the downshifted decay line will have a higher intensity and occur at a lower altitude than the upshifted decay line. Some published observations showing an “early rise” time for the plasma line using a 430‐MHz radar can also be explained using the duct model.
Stream dynamics between 1 AU and 2 AU: A comparison of observations and theory
Authors: Burlaga L. F. , Pizzo V. , Lazarus A. , Gazis P.
Published: February 2008

A radial alignment study of three solar wind stream structures observed by IMP 7 and 8 (at 1.0 AU) and Voyager 1 and 2 (in the range 1.4–1.8 AU) in late 1977 is presented. It is demonstrated that several important aspects of the observed dynamical evolution can be both qualitatively and quantitatively described with a single‐fluid two‐dimensional MHD numerical model of quasi‐steady corotating flow, including accurate prediction of (1) the formation of a corotating shock pair at 1.75 AU in the case of a simple, quasi‐steady stream, (2) the coalescence of the thermodynamic and magnetic structures associated with the compression regions of a fast corotating stream interacting with a slower corotating stream, and (3) the dynamical “destruction” of a “small” (i.e., low velocity‐amplitude, short spatial scale) stream by its overtaking of a slower‐moving, high‐density region associated with a preceding shock and transient flow. The evolution of these flow systems is discussed in terms of the concepts of “filtering” and “entrainment,” which have appeared in the literature. It is concluded that this study provides support at a detailed, quantitative level for our general conceptual notions of stream evolution in the inner solar system.
Auroral zone electric fields from DE 1 and 2 at magnetic conjunctions
Authors: Weimer D. R. , Goertz C. K. , Gurnett D. A. , Maynard N. C. , Burch J. L.
Published: February 2008

Nearly simultaneous measurements of auroral zone electric fields are obtained by the Dynamics Explorer spacecraft at altitudes below 900 km and above 4500 km during magnetic conjunctions. The measured electric fields are usually nearly perpendicular to the magnetic field lines. The north‐south meridional electric fields are “projected” to a common altitude by a mapping function which accounts for the convergence of the magnetic field lines. When plotted as a function of invariant latitude, graphs of the projected electric fields measured by both DE 1 and DE 2 show that the large‐scale electric field is the same at both altitudes, as expected. Superimposed on the large‐scale fields, however, are small‐scale features with wavelengths of less than 100 km which are larger in magnitude at the higher altitude. Fourier transforms of the electric fields show that the magnitudes depend on wavelength. Outside of the auroral zone the electric field spectrums are nearly identical. But within the auroral zone the high‐ and low‐altitude electric fields have a ratio which increases with the reciprocal of the wavelength. The small‐scale electric field variations are associated with field‐aligned currents. These currents are measured with both a plasma instrument and magnetometer on DE 1. A Fourier transform of the east‐west magnetic field component measured on the high‐altitude satellite is found to be nearly identical to the Fourier transform of the north‐south electric field measured on the low‐altitude satellite, with a constant ratio. This ratio is proportional to the ionospheric conductivity. The experimental measurements are found to agree with a steady state theory which postulates that there are parallel potential drops associated with the variations in the perpendicular electric fields. It is assumed that there is a linear relationship between the field‐aligned current and the total parallel potential drop and that the field‐aligned currents close through Pedersen currents in the ionosphere. The theory predicts that the ratio between the low‐ and high‐ altitude electric fields varies with the wavelength. Below a “critical” wavelength the electric field is not effectively transmitted to low altitudes. Owing to the good agreement between the theory and observations, it is concluded that the linear relationship between the current density and potential drop is a valid approximation.
Geocoronal Balmer Alpha Line Profiles and implications for the exosphere
Authors: Yelle R. V. , Roesler F. L.
Published: February 2008

Ground‐based observations of geocoronal Balmer alpha line profiles are presented. Observations were made with a large‐aperture PEPSIOS spectrometer coupled to a flat mirror siderostat to eliminate contamination from nearby airglow lines and galactic emissions. Based on a single scattering Lyman beta excitation model, the observed line profiles are found to be narrower than predicted on the basis of Chamberlain's theory [1963, 1976] and also seem inconsistent with the Monte Carlo calculations of Hodges, et al. [1981]. Comparison with the OGO 5 Lyman alpha line width measurements [Bertaux, 1978] and Lyman alpha intensity measurements suggests that the narrow portion of the profiles may be due to emissions from satellite atoms with small eccentricities. The possibility that the narrow emissions are produced by cool hydrogen in the lower thermosphere is also discussed. The observed profiles are asymmetric, having an extended red wing greater than predicted by Chamberlain's theory [1963, 1976] but consistent with an enhanced escape flux due to charge exchange. The position of the red wing is also consistent with the presence of additional fine structure components possibly caused by a collisional excitation mechanism. Although some of the conclusions must be regarded as tentative until refined modeling including multiple Lyman beta scattering is completed, it seems clear that measurements of the type described can lead to improved understanding of geocoronal hydrogen.
Solar wind observations associated with a slow forward shock wave at 0.31 AU
Authors: Richter A. K. , Rosenbauer H. , Neubauer F. M. , Ptitsyna N. G.
Published: February 2008

A slow forward shock‐type discontinuity observed by Helios 1 on day 147, 1980, at 0.31 AU is analyzed in detail, employing high‐resolution (40.5 s) plasma and magnetic field measurements. It is shown within the bounds of uncertainties in the various parameters (1) that this event is not an MHD discontinuity, as the total pressure balance is violated and as the entropy increases across this structure even after inclusion of the alpha particles, (2) that the Hada‐Kennel relations, i.e., Ti/Te < 1 and β < 1, are satisfied upstream of this event even after inclusion of the alpha particles, indicating that steepening of MHD slow waves dominates Landau damping, (3) that the Rankine‐Hugoniot relations are fulfilled to an absolute difference of <10−5 by employing any possible combination of an upstream with a downstream time‐averaged data set, (4) that the MHD evolutionary conditions are satisfied for any solution to the jump conditions, and (5) that the shock speed, the amplitudes, and the relative amplitudes associated with this event are absolutely comparable to those derived earlier for fast mode shock waves within 1 AU. It is shown that this event is an almost quasi‐perpendicular slow forward MHD shock wave.