bluuebunny/crossref_metadata_2025 huggingface dataset:
A search for the radio occultation flash at Jupiter
Authors: Martin J. M. , Tyler G. L. , Eshleman V. R. , Wood G. E. , Lindal G. F.
Published: February 2008

The ‘evolute flash,’, a focusing effect caused by the curvature of a planet's limb, was sought in the radio data taken during the occultation of Voyager 1 by Jupiter, using a modified matched‐filter technique. The expected frequency structure of the flash signal is double branched, while the intensity structure is highly localized in time. The search for the signal was carried out over a 6.4 s period. The signal parameters were varied to span the uncertainties introduced by imperfect knowledge of the orbit of the spacecraft and the shape of Jupiter. Several peaks at the 8 standard deviation level were present in the filter output. However, these peaks were separated in time by up to 3.3 s, and none could be identified as the flash. From this negative result a lower bound on the absorption along a ray with periapsis near the 4 bar level in Jupiter's atmosphere can be established at 25 dB. Employing the new Voyager results on the structure of the atmosphere of Jupiter and the mixing ratio of the absorbent ammonia, as well as the improved knowledge of flash characteristics resulting from this study, we estimate that the flash would have been detected if the distance behind the planet where the spacecraft trajectory crossed the evolute were at least 20 Jupiter radii, as compared with a value near 7 in the experiment. For focusing at this greater distance, the atmospheric pressure at the ray periapsis would be between 1.5 and 2 bar.
Near‐ultraviolet scattering properties of Jupiter
Authors: West Robert A. , Hord Charles W. , Simmons Karen E. , Coffeen David L. , Sato Makiko , Lane Arthur L.
Published: February 2008

The Voyager 2 photopolarimeter experiment obtained photometric observations of Jupiter at four phase angles between 20° and 80° and at various illumination angles from limb to terminator. These data were analyzed to determine the altitude distribution and scattering properties of the material responsible for Jupiter's low geometric albedo at 2400 Å. In polar regions the absorber must be higher than 40 mbar. In mid‐latitudes, center‐to‐limb measurements indicate that most of the absorbing material lies deeper than 100 mbar. Absolute reflectivity requires a considerable amount above 400 mbar. The maximum amount of solar UV flux below 2550 Å available for heating the stratosphere at 20° latitude is only 10−4 of the solar constant at Jupiter. The latitudinal distribution of stratospheric absorbers suggests that energy deposition from the magnetosphere is important in their formation. We examine the scattering properties of model atmospheres containing three distinct aerosol size distributions. Rayleigh's phase function fits our data at several phase angles better than any particle phase function. We derive an upper limit to the column abundance of particles larger than 0.05‐µm radius in the north tropical zone and north equatorial belt. These regions contain less than one extinction optical depth at 2400 Å of such particles above the 100‐mbar altitude. Rayleigh scattering from the top 100 mbar obscures effects due to scattering from the deeper absorbing particles over our range of phase angles. We are unable to determine whether the absorbers are particulate or molecular.
Parametric interaction and spatial collapse of beam‐driven Langmuir waves in the solar wind
Authors: Gurnett D. A. , Maggs J. E. , Gallagher D. L. , Kurth W. S. , Scarf F. L.
Published: February 2008

This paper presents observations of the parametric decay and spatial collapse of Langmuir waves driven by an electron beam streaming into the solar wind from the Jovian bow shock. High‐resolution frequency‐time spectrograms from Voyager 1 and 2 show that long wavelength Langmuir waves upstream of the bow shock are very effectively converted into short wavelength Langmuir waves which are no longer in resonance with the beam. This conversion is shown to be the result of a nonlinear interaction involving the beam‐driven pump, a sideband emission and a low level of ion‐acoustic turbulence which always appears to be present in the solar wind. The onset of the interaction occurs at about the time that the amplitude of the pump wave saturates, which indicates that parametric processes are probably playing an important role in stabilizing the electron beam. Detailed examination of the electric field waveforms shows that the beam‐driven Langmuir wave emission breaks up into a very complex sideband structure with both positive and negative Doppler shifts. Positive frequency shifts correspond to waves propagating away from the sun and negative frequency shifts correspond to waves propagating toward the sun. In some cases the sideband emissions consist of isolated wave packets with very short durations, sometimes lasting only a few msec. These short duration bursts, which are usually very intense, are thought to consist of envelope solitons which have collapsed down to spatial scales of only a few Debye lengths.
On the origin of the MeV energy nucleon flux associated with CIRs
Authors: Christon S. P.
Published: February 2008

Measurements of recurrent enhancements of interplanetary nucleon flux in the MeV energy range, detected at widely separated points in the heliosphere by charged particle instruments on board the Pioneer 10 and 11, IMP 7 and 8, and Mariner 10 spacecraft, are presented and interpreted. These features recur at the solar rotation period in association with stream‐stream plasma interaction regions corotating with the sun (CIRs) and are not directly produced by solar flaring activity. At distances from the sun r < 1 AU, the maximum intensities of the hydrogen (H) and helium (He) components in these flux enhancements increase with increasing r at the same rate, usually between 300 %/AU and 600 %/AU. The nucleon abundance ratio H/He and the slopes of H and He differential energy spectra are relatively constant with r. One of the models proposed to explain these enhanced fluxes predicts the acceleration of nuclei from keV plasma energies to MeV energies via the transit time damping of magnetosonic waves as solar wind plasma flows from the sun. The predicted acceleration rates of H and He are different in this model. We compute numerical solutions of the transport equation to show that this model does not reproduce the radial variations of the H and He fluxes observed with nearly identical University of Chicago charged particle instruments. Flux associated with one of the relatively rare CIR shocks at 1 AU is shown to be similar to the flux in other corotating enhancements. Direct estimates of the radial diffusive mean free path λr ranging from 0.03 AU to 0.11 AU, are evaluated by using the sunward diffusive anisotropies obtained at 1 AU and the simultaneous increase of intensity with r. These values of λr are consistent with those derived in studies of solar flare related particle flux and other corotating nucleon flux enhancements at higher rigidities. The observations are therefore consistent with nucleon acceleration at the CIR shocks beyond 1 AU with subsequent diffusion inward toward the sun.
Upstream energetic ions and electrons: Bow shock‐associated or magnetospheric origin?
Authors: Scholer M. , Hovestadt D. , Ipavich F. M. , Gloeckler G.
Published: February 2008

We have analyzed 35 proton bursts observed with the Max‐Planck‐Institut/University of Maryland sensor system on ISEE 3 far upstream of the earth's bow shock. These upstream bursts fall into two distinctive groups: the first group is accompanied by energetic electrons (≳75 keV) and the proton spectrum extends up to energies ≳300 keV and higher and bends over towards lower energies (≲30 keV). The second group, which is unaccompanied by energetic electron bursts, exhibits spectra which can be represented very well by exponentials in energy with a mean e‐folding energy of ∼15 keV. We have supplemented these data with measurements obtained with an identical sensor system on ISEE 1 close to the bow shock or within the geomagnetic tail. Whenever ISEE 1 is upstream of the bow shock and ISEE 3 observes a burst of group 2, ISEE 1 observes diffuse upstream ions. When ISEE 3 observes bursts of group 1, ISEE 1 observes either no bursts, diffuse ions, or highly anisotropic distributions. It is suggested that group 1 is of magnetospheric origin, while group 2 is bow shock‐associated.
Generation of small‐scale field‐aligned irregularities in ionospheric heating experiments
Authors: Inhester B. , Das A. C. , Fejer J. A.
Published: February 2008

A mechanism for generating the small‐scale field‐aligned irregularities observed in ionospheric heating experiments is proposed. The Langmuir waves that result from the scattering of the electromagnetic pump wave of ordinary polarization by the irregularities, interact with the irregularities by secondary scattering processes. The differential dissipation of the resulting Langmuir waves causes the growth of the irregularities and of the Langmuir waves resulting from the secondary scattering. The instability is explosive in nature and initial irregularities whose fractional density perturbations exceed about 10−4, are needed for its occurrence. It is shown for a periodically varying field‐aligned density perturbation that although successive scattering of the Langmuir waves eventually becomes very intense and approaches an equipartition of the energy among the waves, it does not saturate the instability. It seems that the proposed mechanism is able to generate small‐scale field‐aligned irregularities that can reach the amplitudes observed in ionospheric heating experiments.
Pronounced proton core temperature anisotropy, ion differential speed, and simultaneous Alfvén wave activity in slow solar wind at 0.3 AU
Authors: Marsch E. , Mühlhäuser K.‐H. , Rosenbauer H. , Schwenn R. , Denskat K. U.
Published: February 2008

In the Helios 2 perihelion (0.3 AU) in May 1978, pronounced ion differential speeds Δvαp >0 and large proton temperature anisotropies T∥p/T⊥p <1 have been observed in slow solar wind, in distinct contrast to earlier observations during the phase of solar activity minimum. These features of ion distributions occurred simultaneously with high Alfvénic wave activity. Coulomb friction between the two ion species can be shown to play a minor role, thus obviously favouring the establishment of a large differential ion speed by preferentially accelerating wave forces. The observations may also indicate that strong waves sometimes only heat the protons without creating a fast solar wind.
The structure and dynamics of the heliospheric current sheet
Authors: Thomas Barry T. , Smith Edward J.
Published: February 2008

An analysis is presented of the Pioneer 10 and 11 magnetic field data, with regard to the sector structure in the interplanetary magnetic field. For the 3–4 years leading up to the last solar minimum, the data are shown to be consistent with the existence of a wavy equatorial current sheet in the heliosphere. Two sectors are shown to dominate the long term field structure, and each sector is shown to adopt a Parker spiral geometry in the outflowing solar wind. This observation is consistent with the existence of a tilted current sheet that is approximately flat near the sun but that develops a large scale wavy structure owing to the solar rotation. The recurrence pattern of the sector structure suggests an inertial rotation rate of 26.1 days (with a confidence limit of approximately 0.2 day) an observation which may indicate that the source of the low‐latitude solar wind is at mid‐latitudes on the sun. Corotating interaction regions are shown to be capable of modifying the structure of the current sheet. In particular it is suggested that they can absorb the current sheet by overtaking the nearest fold in its wavy structure.
Incoherent scatter radar studies of mesospheric temperatures and collision frequencies at Arecibo
Authors: Tepley C. A. , Mathews J. D. , Ganguly S.
Published: February 2008

The Arecibo 430 MHz incoherent scatter radar is used to measure the ion‐neutral collision frequency and temperature of the atmosphere in the 70 to 100 km altitude range. These parameters are determined over altitude segments that are comparable to the scale height of the mesosphere. The spectra scattered from the mesosphere are collision dominated, that is, for the Arecibo radar frequency, ψi > 1 at all altitudes observed. Analysis of the widths of these spectra show that both the temperature and collision frequency values agree quite favorably with the Cira model atmosphere for the summer months.
Nonlocal plasma turbulence associated with interplanetary shocks
Authors: Kennel C. F. , Scarf F. L. , Coroniti F. V. , Smith E. J. , Gurnett D. A.
Published: February 2008

The plasma wave instrument on ISEE 3 has detected regions of plasma turbulence that extend several tenths of an astronomical unit upstream or downstream of inteplanetary shocks. The plasma waves fall into four categories. Highly impulsive 1‐ 10‐kHz electric field bursts were found hours upstream of quasi‐parallel interplanetary shocks. On occasion their average and peak amplitudes increased monotonically until the shock crossing, at which time they were suppressed. A lower frequency electric field (0.1–1 kHz) component was enhanced at nearly all shocks and persisted downstream. Broadband low‐frequency (typically <178 Hz) magnetic fluctuations increased at, and persisted hours downstream of, every interplanetary shock in our sample. A smooth high‐frequency continuum, near and above the local electron plasma frequency, was enhanced at, and persisted well downstream of, every interplanetary shock we studied. Impulsive electron plasma wave bursts were occasionally found near the shocks. The shock‐associated plasma waves we found to extend over large spatial scales are similar to those found previously in local studies of interplanetary shocks. While no single interplanetary shock showed every effect, the ensemble of shocks contained at least one example of each type of plasma wave found upstream of the earth's bow shock. The 1‐ to 10‐kHz spectra upstream of interplanetary shocks and the earth's bow shock are similar. The low‐frequency electric and magnetic fluctuations downstream of interplanetary shocks and the bow shock have similar spectra. They seem to be ubiquitous features of flowing plasmas made turbulent by a shock.
A Sub‐Alfvénic solar wind: Interplanetary and magnetosheath observations
Authors: Gosling J. T. , Asbridge J. R. , Bame S. J. , Feldman W. C. , Zwickl R. D. , Paschmann G. , Sckopke N. , Russell C. T.
Published: February 2008

Many years of observation have established that the solar wind flow at 1 AU normally is both supersonic and super‐Alfvénic. However, for portions of an ∼5‐hour period on November 22, 1979, the solar wind flow speed (∼320 km s−1) observed at ISEE 3 was considerably less than the Alfvén speed (∼540 km s−1) that resulted from an abnormally low ion density (∼0.07 cm−3). The origin of this sub‐Alfvénic flow remains uncertain: the flow was not associated in any obvious way with the rarefaction region of a high‐speed stream or shock wave disturbance. Intermittent observations of the flow within the earth’s magnetosheath were made with instruments aboard ISEE 1 and 2 during the period when the solar wind flow was sub‐Alfvénic. The bulk of the magnetosheath plasma was compressed, heated, and slowed relative to the upstream solar wind plasma and was directed around the magnetosphere. Superposed on this flow was a hot, suprathermal ion beam with mean energy ∼1 keV directed along the magnetic field towards the magnetosphere. These observations suggest that the bow shock neither dissipated nor disappeared when the solar wind flow was sub‐Alfvénic.
Plasma pitch angle distributions near the substorm injection front
Authors: Moore T. E. , Arnoldy R. L.
Published: February 2008

Earthward propagation of substorm‐associated, abrupt plasma changes has been observed near synchronous orbit (Moore et al., 1981). We report data obtained from the University of New Hampshire hot plasma instrument on the ATS‐6 spacecraft during January 1980 that provide electron and ion pitch angle distributions in the vicinity of such an injection front. Evidence is found of symmetric atmospheric source cones for few 100‐eV electrons after front passage. This supports the concept of atmospheric electron degradation of the hot high altitude plasma, and the proposal that the injection front is a moving precipitation‐flow boundary between the hot plasma and the cooler plasma, which has become spectrally degraded via interaction with the atmosphere. In contrast, ion source cone distributions are more sporadic and not clearly associated with the injection front passage. The enhanced hot plasma electron intensities that appear in association with front passage exhibit a modest field‐aligned anisotropy with a minimum at 90° pitch angle and secondary minima at small and large pitch angles characteristic of symmetric loss cones. This is consistent with mirror compression of these electrons on inward‐collapsing field lines. Ions of comparable energies do not exhibit the field alignment, probably because their bounce periods are comparable to or longer than the time scale of collapse so that their second invariants are not conserved.
Transport theoretic solutions for auroral proton and H atom fluxes and related quantities
Authors: Jasperse J. R. , Basu B.
Published: February 2008

In this paper we show that linear transport theory may be used to solve for the auroral proton and H atom fluxes in plane‐parallel geometry in the forward scattering and average discrete energy loss approximations. In this approximation, inelastic scattering is taken into account, but elastic scattering drops out and the solutions are approximations to the fluxes in the downward hemisphere. Using the multiple scattering method, we obtain the particle fluxes as finite sums of analytic functions of altitude, energy, and pitch angle. Closed form expressions are also found for the hemispherically averaged fluxes and the energy deposition and ionization rates. The notion of pseudoparticles is discussed and used to approximate the sums that occur in the above formulae. Results are presented for incident isotropic‐Maxwellian proton fluxes.
Solitons and ionospheric heating
Authors: Weatherall J. C. , Sheerin J. P. , Nicholson D. R. , Payne G. L. , Goldman M. V. , Hansen P. J.
Published: February 2008

For parameters characterizing the Platteville ionospheric heating facility, the Langmuir wave evolution at the exact reflection point of the heater wave involves an oscillating two‐stream instability followed by a collisionally damped three‐dimensional soliton collapse. The result provides an alternative explanation for certain experimental observations.
Dependence of electron beam instability growth rates on the beam‐plasma system parameters
Authors: Strangeway R. J.
Published: February 2008

Electron beam instabilities are studied by using a simple model for an electron beam streaming through a cold plasma, the beam being of finite width perpendicular to the ambient magnetic field. Through considerations of finite geometry and the coldness of the beam and background plasma, an instability similar to the two stream instability is assumed to be the means for wave growth in the system. Having found the maximum growth rate for one set of beam‐plasma system parameters, this maximum growth rate is traced as these parameters are varied. The parameters that describe the system are the beam velocity (υb), electron gyrofrequency to ambient electron plasma frequency ratio (Ωe/ωpe), the beam to background number density ratio (nb/na), and the beam half width (a). When Ωe/ωpe > 1, a mode with Ωe < ω < ωUHR is found to be unstable, where ω is the wave frequency and ωUHR is the upper hybrid resonance frequency. For low values of nb/na and Ωe < ωpe, this mode is still present with ωpe < ω < ωUHR. If the beam density is large, nb/na ≃ 1, the instability occurs for frequencies just above the electron gyrofrequency. This mode may well be that observed in laboratory plasmas before the system undergoes the beam‐plasma discharge. There is another instability present, which occurs for ω ≃ ωpe. The growth rates for this mode, which are generally larger than those found for the ω ≃ ωUHR mode, are only weakly dependent on Ωe/ωpe. That this mode is not always observed in the laboratory implies that some factors not considered in the present theory suppress this mode, specifically, finite beam length. Consequently, this mode should be unstable in space plasmas, and it is possible that the ω ≃ ωpe instability is of some significance for auroral zone electron beam instability. A more realistic electron distribution function should be employed to determine if a two‐stream instability exists for the auroral plasma, but since the beam width is found to be important only for a < c/ωpe, finite geometry can be neglected.
Titan's highly variable plasma environment
Authors: Wolf Dieter A. , Neubauer F. M.
Published: February 2008

Titan's plasma environment is variable due to two different causes. The variability of the solar wind has the consequence that Titan may be located in the outer magnetosphere, the magnetosheath, or the interplanetary medium around noon Saturnian local time. Additionally, there are local time variations in Saturn's magnetosphere. We have calculated the location of the stagnation point of Saturn's magnetosphere rst = 16‐26 RS assuming a terrestrial type magnetosphere. Typical plasma parameters along the orbit of Titan are shown for high solar wind pressure. The magnetoplasma incident on Titan will be generally be subsonic, subalfvénic in the tail, transsonic, transalfvénic in the outer magnetosphere, and subsonic, superalfvénic in the magnetosheath. During crossings of the Saturnian magnetopause or bow shock by Titan we expect abrupt changes of the flow direction and stagnation pressure and rapid associated changes in Titan's uppermost atmosphere.
Exospheric temperatures deduced from 7320‐ to 7330‐Å (O+(²P) ‐ O+(²D)) twilight observations
Authors: Yee J. H. , Abreu V. J.
Published: February 2008

A technique has been developed to deduce exospheric temperatures from the 7320‐ to 7330‐Å emission measured by the visible airglow experiment on board the AE‐E satellite. An excess emission in the measured 7320‐ to 7330‐Å brightness is noticed owing to the interaction between the spacecraft and the atmosphere. The observed brightnesses have been corrected for this effect. The galactic background emission has also been carefully subtracted. The deduced temperatures show a positive correlation with solar activity. It varied from ∼700°K in late 1976 up to ∼1700°K at the peak of this solar cycle. The presence of a nonthermal oxygen corona is inconclusive.
Photoabsorption into the ³Πu state of O2
Authors: Allison A. C. , Guberman S. L. , Dalgarno A.
Published: February 2008

Earlier calculations of the cross sections for absorption of ultraviolet radiation by molecular oxygen in the transition 1³Πu‐X³Σg− are corrected and extended to shorter wavelengths. The calculations confirm the experimental identification of strong absorption in the region of 1350 Å, attributable to the ³Πu state.
Thermal fluctuations and the diffuse electrostatic emissions
Authors: Sentman D. D.
Published: February 2008

Persistent, weak‐banded electrostatic emissions with µV/m amplitudes have been observed in the magnetosphere at frequencies between the electron gyrofrequency ωce and upper hybrid frequency ωuh when ωce < ωuh. The suggestion that these low‐level emissions are thermal fluctuations is quantitatively explored by using a simple hot‐cold model for dayside electron distributions with density and temperature ratios nc/nh ≫ 1 and Th/Tc ≫ 1. We conclude that for typical dayside conditions the observed waves are weakly damped Bernstein‐Harris modes whose spectral density 〈E²(f)〉 ∼ 10−16V²/m²‐Hz and polarization perpendicular to the ambient magnetic field can be accounted for by the theory. The electrostatic power spectrum of the waves scales as ωce²Th/Tc3/2 for a Maxwellian hot component but may be enhanced considerably for the case of marginally stable or nonconvective modes. Wavelengths scale as the cold electron gyroradius ρc so that significant attenuation of the measured spectrum may occur when measurements are made with antennas of characteristic lengths l ∼ ρc. Representative fluctuation spectrums are computed for dayside sychronous orbit by using typical hot‐cold plasma parameters and hypothetical dipole antenna lengths of 50 and 100 m. It is suggested that the formalism developed here may be applicable to weak electron cyclotron emissions at Jupiter and Saturn and may be extended to lower‐frequency ion cyclotron fluctuations by modifying the theory to include ion contributions.
Application of Dopplionograms and Gonionograms to atmospheric gravity wave disturbances in the ionosphere
Authors: Wright J. W. , Pitteway M. L. V.
Published: February 2008

A sequence of digital ionograms is processed by dopplionogram and gonionogram methods. Together, these disclose a disturbance in the F region which descends in altitude with time. Two wavelike periods of the disturbance are evident. The Doppler and angle‐of‐arrival behavior are consistent with a semiquantitative model of the plasma perturbations caused by an internal atmospheric gravity wave.
The magnetopause as sensed by energetic particles, magnetic field, and plasma measurements on November 20, 1977
Authors: Fritz T. A. , Williams D. J. , Paschmann G. , Russell C. T. , Spjeldvik W. N.
Published: February 2008

The position of the trapping boundary for ≥24‐keV ions in the vicinity of the magnetopause has been determined using energetic particle three‐dimensional distributions observed by ISEE 1 and 2. These observations have been compared with simultaneous plasma magnetic field measurements for the 0130–0135 UT interval on November 20, 1977. The results show that for this interval the magnetosheath plasma penetration depth into the magnetosphere can be limited to a distance equal to the local plasma ion gryoradius of ≲100 km inside the position of the trapping boundary although penetration greater than 200 km was also observed. During this time the trapping boundary was in continual, rapid motion with velocities of up to ∼90 km/s. This motion could complicate the geometry and therefore explain the apparent deeper penetration of the magnetosheath plasma a few ion gyroradii inside the trapping region. If the trapping boundary, determined by this energetic particle sounding technique, is the last closed field line, then the observation of magnetosheath plasma a number of ion gyroradii earthward of the trapping boundary indicates that magnetosheath plasma is able to penetrate onto closed field lines.
The Global Pc 5 event of November 14–15, 1979
Authors: Higbie P. R. , Baker D. N. , Zwickl R. D. , Belian R. D. , Asbridge J. R. , Fennell J. F. , Wilken B. , Arthur C. W.
Published: February 2008

Large‐amplitude Pc 5 waves with 7–8 min quasi‐periodic variations were observed almost continuously for the 48 hours of November 14‐15, 1979, by several spacecraft in or near geostationary orbit on the dayside portions of their orbits. The waves were observed as large modulations in both the electron and ion fluxes over a wide range of energies (∼1 to 500 keV) by the spacecraft 1976–059, 1977–007, 1979–053, P78‐2, and GEOS 2 and as magnetic field peak‐to‐peak modulations of 15–25 γ by the P78‐2 and GOES 2 and 3 magnetometers. The remarkably long persistence of these waves contrasts substantially with observations of typical flux modulation events which usually last less than 1 hour and which typically show little modulation of the ≥150 keV proton fluxes. Data taken concurrently by the ISEE spacecraft in the solar wind and outer magnetosphere indicate that the solar wind also had unusual properties. ISEE 3 measurements indicate that the solar wind velocity (∼350 km/s) and density (∼2 cm−3) were simultaneously very low for this period. The alpha‐to‐proton ratio for the solar wind plasma attained an extremely low value (<1%) early in the event. These solar wind properties imply such a much reduced dynamic pressure on the magnetosphere during this period. Consequently, the ISEE 1 and 2 spacecraft passed through the magnetopause at the uncommonly large radial distance of 18 RE at ∼0830 local time where the typical magnetopause geocentric distance is 12 RE. The exceptional solar wind and outer magnetospheric conditions may have determined the unusual properties of the ULF event observed near geostationary orbit. Some candidate mechanisms for producing these oscillations are presented, but no definitive explanation for this event can be given at present.
Stimulated wave‐particle interactions during high‐latitude ELF wave injection experiments
Authors: Garnier M. , Girolami G. , Koons H. C. , Dazey M. H.
Published: February 2008

Interactions in the magnetosphere between man‐made whistler mode waves and electrons can produce either an enhancement of the wave or the generation of waves at a different frequency. Furthermore, natural emissions can be frequency shifted or modified by a nearby transmitter signal. Injection of extremely low frequency waves into the outer magnetosphere was initiated in 1978 in Norway by both the University of Paris and The Aerospace Corporation. The antenna is a 10.6‐km power line tuned to the transmitted frequency. The University of Paris used a 1‐kW transmitter to drive the antenna with a maximum current of 8 A. The Aerospace Corporation used the transportable very low frequency transmitter with a typical antenna current between 20 and 45 A. Experiments were conducted at times when the GEOS 2 and SCATHA satellites were near the magnetic meridian of the transmitter. The transmissions consisted of either a keyed fixed frequency or continuous waves swept in frequency. Emissions correlated with the transmissions were detected by the satellite receivers on several dates. Although the satellites were within a few degrees of the magnetic meridian of the transmitter, emissions influenced by the transmissions are seen during only a small fraction (<10%) of the total transmission time. With the first type of transmission, signals similar to power line harmonic radiation were recorded. Artificially stimulated emissions are also likely to have been triggered. Both types of transmissions triggered or enhanced hiss at a constant frequency during fixed‐frequency transmissions and at a variable frequency during a swept‐frequency transmission. During swept‐frequency transmission there occurred two examples of natural emissions shifting in frequency by the man‐made signal. Electron cyclotron harmonic emissions also appear to have been shifted in frequency by the man‐made signal.
Electric and magnetic field characteristics of discrete arcs in the polar cap
Authors: Burke W. J. , Gussenhoven M. S. , Kelley M. C. , Hardy D. A. , Rich F. J.
Published: February 2008

Simultaneous electric field, magnetic field, and electron flux measurements from three dawn‐dusk passes of S3‐2 were analyzed for signatures of sun‐aligned arcs in the polar cap. Two of the passes were over the summer and one over the winter polar cap. In all three cases the IMF had a strong northward component. During the pass over the winter polar cap the electric fields were highly irregular but tracked with nearly one‐to‐one correspondence to the transverse magnetic fluctuations. Upward Birkeland currents were embedded in regions of negative space charge. The carriers for the upward Birkeland currents were weakly field aligned fluxes of electrons with a temperature of a few hundred eV that had been accelerated through a potential drop of ∼1 kV. The two summer polar cap passes differed from each other in their large‐scale electric field patterns. In one pass the electric field was irregular with the upward Birkeland currents in negative space charge regions. In the other summer hemisphere pass, convection was sunward in the central polar cap. Small‐scale Birkeland current systems, possibly signatures of arclike structures, were found in regions of both sunward and antisunward convection. In this pass, variations in the electric and magnetic field did not always show the strong correlations found in the other two cases, due to the effects of electrojet activity near the flanks of the polar cap. The measurements are found to be consistent with a ‘static’ model for the electrical coupling between the ionoshere and magnetosphere with the generator at the magnetopause.
Ion drift wave induced plasma convection at the magnetopause
Authors: Shukla P. K.
Published: February 2008

It is suggested that the ion drift wave could be one of the possible candidates for the low‐frequency electrostatic fluctuations at the magnetopause. Nonlinear mode coupling of ion drift waves can create purely growing convective cells. The latter may cause anomalous plasma diffusion across the ambient magnetic field.
Response of dayside net downward field‐aligned current to changes in the interplanetary magnetic field and to substorm perturbations
Authors: Rostoker Gordon , Mareschal Marianne , Samson John C.
Published: February 2008

We have studied the response of the auroral eastward and westward electrojets and of the downward field‐aligned currents across the noon sector which feed them to southward turnings of the interplanetary magnetic field (IMF) and to substorm onsets in the nightside magnetosphere. We have found that while southward turnings of the IMF lead to electrojet intensifications and low‐level substorm activity, northward turnings of the IMF appear capable of triggering large substorm disturbances. Both northward and southward turnings of the IMF cause the downward field‐aligned current across the noon sector, as well as the auroral electrojets, to strengthen within a few minutes of the impact of the changed IMF at the magnetopause. Since large substorms involve the sudden release of energy stored in the magnetotail, we conclude that the tail current system links directly to the overall magnetosphere‐ionosphere current system involving the auroral electrojets. In this context a substorm associated with a northward turning of the IMF reflects a reconfiguration of the magnetotail in response to a change in the level of the solar‐terrestrial interaction involving a decrease in the rate at which solar wind energy penetrates into the magnetosphere.
Standing hydromagnetic waves observed by ISEE 1 and 2: Radial extent and harmonic
Authors: Singer H. J. , Hughes W. J. , Russell C. T.
Published: February 2008

Previous observations of magnetic pulsations in space have not been able to distinguish temporal from spatial structure unambiguously. Using the ISEE 1 and 2 spacecraft, satellites closely spaced in identical orbits, we are able to determine whether the observed amplitude variations are due to the motion of the spacecraft through a time stationary structure or are due to temporal changes in the wave. We have examined four dayside Pc 4, 5 period pulsations, three of which are observed between L = 4 and 7 within 10° of the magnetospheric equator. The resonant region widths of these three events range from ∼0.2–1.6 L shells. These are the first direct measurements of resonant region widths in space. For two of the three near‐equatorial events, plasma density data was available and indicated that the events are associated with gradients in the electron number density at the plasmapause. In one event, although waves are observed continuously by ISEE 2 during a portion of its inbound trajectory, at ISEE 1 there is a gap in the waves at the same time as a depleted density region is encountered. The gap in the waves can be explained as either a temporal or spatial effect, although the latter seems more likely. In addition to these observations we are able to use two independent techniques to determine the harmonic of two pulsation events. The first method compares the observed wave period with the period predicted by standing wave theory. The second method, which previously has been applied only indirectly, is to compare the phase of the observed wave electric and magnetic field. Alone neither of these methods can unambiguously determine the resonance harmonic. Here, using both methods together for the first time, we find that both events are second harmonic oscillations. This suggests that second harmonic oscillations are perhaps more common than previously thought.
Modulation of energetic particle fluxes by a mixed mode of transverse and compressional waves
Authors: Lin C. S. , Parks G. K.
Published: February 2008

Modulation characteristics of particle fluxes in the presence of a mixed mode of compressional and transverse magnetic waves at hydromagnetic frequencies have been studied by means of kinetic perturbation of the distribution function. The magnetospheric medium in which the particles are modulated contains both the magnetic and pressure gradients. It is found that the modulation features are strongly dependent on the energy and pitch angle of the particles. Drifting particles can resonate with waves whose phase velocities are near their drift velocities. When this happens, the amplitude of the modulations become significantly large and large phase shifts will occur. Resonance is important for particles with mid pitch angles (40°–70°). The phase shift between the particle modulations and the magnetic field oscillations are strongly controlled by combined effects of transverse and compressional wave components and/or the occurrence of drift resonance. We have performed numerical calculations by using the dispersion relation of drift mirror Alfvén waves as an example of waves with both compressional and transverse components. The results derived in this study may be of importance in studying the relationship of particles and Pc 4–5 waves that are observed during magnetically disturbed times.
Use of the thin sheath approximation for obtaining ion temperatures from the ISEE 1 limited aperture RPA
Authors: Comfort R. H. , Baugher C. R. , Chappell C. R.
Published: February 2008

A procedure for analyzing low‐energy (≲100 eV) ion data from the plasma composition experiment on ISEE 1 is presented and examined in detail. The method is based on a derived analytic expression for particle flux to a limited aperture RPA in the thin sheath approximation, which takes into account some effects of a charged spacecraft on plasma particle trajectories. Calculations using simulated data are employed to test the efficacy and accuracy of the technique. On the basis of the analysis of these calculation results and the mathematical model, it appears that the method can generally be expected to provide accurate ion temperatures from all good plasmaspheric RPA data. Corresponding densities and spacecraft potentials should be accurate when spacecraft potentials are negative but are subject to error for positive spacecraft potentials, particularly when ion Mach numbers are low (≪1). Beyond the plasmasphere, the method is applicable only in special circumstances. Analysis of data from a representative ISEE 1 pass results in a plasmasphere temperature profile consistent in overall structure with previous observations.
A time dependent three‐dimensional simulation of the Earth's magnetosphere: Reconnection events
Authors: Brecht Stephen H. , Lyon John G. , Fedder Joel A. , Hain K.
Published: February 2008

This paper presents the results of three‐dimensional simulation of the earth's magnetosphere. The simulation region is 100 RE ≤ X ≤ 30 RE, |Y| ≤ 40 RE and |Z| ≤ 40 RE with solar wind parameters Bz = −2γ, density of 5 cm−3 and velocity of 400 km s−1. The results show the existence of multiple x points in the tail. Strong plasma flow is seen to exist in both earthward and tailward directions during formation of neutral lines. The exact magnetic field and plasma behavior is found to be very position dependent.
A comparison of the 0.1–17 keV/e ion composition in the near equatorial magnetosphere between quiet and disturbed conditions
Authors: Lennartsson W. , Sharp R. D.
Published: February 2008

Data from the plasma composition experiment on ISEE 1 were used to investigate the relative number densities and energy characteristics of H+, He++, He+, and O+ in the near‐equatorial magnetosphere at L ≤ 12 during magnetically quiet and disturbed conditions. The ‘quiet’ data were selected on the basis of both Dst and Kp over ∼5 day periods preceeding the data accumulations, and the ‘active’ data were from periods of Dst ≲ −100γ. The ions included in this study had energies in the range of 0.1 ≤ E/Q ≲ 17 keV/e. The data naturally subdivide into two largely different sets in terms of a dipole L parameter. (1) At L ≲ 5 the ion density is dominated by three species, H+, O+, and He+, appearing in variable ratios. The He++ is a small component and is generally obscured by background counts caused by penetrating MeV electrons. The O+ is the most abundant species in well over half of the plasma samples, whereas the H+ is more abundant in the remaining samples. No sample with statistically significant counts shows He+ to be the most abundant ion, although the He+ exceeds the H+ in density in several cases. The O+/H+ and He+/H+ ratios increase toward the inner edge of the ring current population and are, on the average, highest during quiet conditions. Both of these features are qualitatively consistent with a decay process dominated by charge exchange, but a comparison with published charge exchange rates seems to imply that other processes are also involved. All three ion species have a similar mean energy (∼2–7 keV) within the energy window of the experiment, and all three have a low‐energy component (0.1 ≤ E/Q ≲ 1 keV/e) that usually dominates the number density at the inner edge, at about L ∼ 3–4 during quiet conditions and L ∼ 2–3 during disturbed conditions. (2) At L > 5 all four species may be present simultaneously in highly variable ratios, but the H+ is usually the most abundant, particularly during quiet conditions. As a rule, the O+/H+ and He+/H+ ratios decrease with increasing L, whereas the He++/H+ ratio slowly increases. Typical values in the outer magnetosphere (L ∼ 10–12) are 1–3% for the He++/H+ and He+/H+ ratios during both quiet and disturbed conditions, whereas the O+/H+ ratio may range from below 1% to over 200%, depending on the magnetic activity level. The highest O+/H+ ratios are found here during the early main phase of magnetic storms, which is contrary to the conditions at L < 5. The H+, O+, and He+ all have a similar mean energy in this region (∼4–6 keV), whereas the He++ is 2‐3 times more energetic.
Volume emission rate profiles of the 6300‐Å tropical nightglow obtained from the AE‐E satellite: Latitudinal and seasonal variations
Authors: Abreu V. J. , Schmitt G. A. , Hays P. B. , Dachev T. P.
Published: February 2008

Surface brightnesses at 6300‐Å measured by the visible airglow experiment on board the AE‐E satellite have been inverted to obtain altitude profiles of volume emission rate. A morphological picture of the emission at low latitudes is developed and qualitatively interpreted in terms of the behavior of electric fields and neutral winds in the nighttime ionosphere. Global pictures of the emission are presented for solstice and equinox conditions between 1800 and 0400 LT. Observed regions of depleted and enhanced emission are in agreement with the emission pattern expected from the effect of E × B drifts on the ionization. Other features observed are interpreted in terms of a transequatorial wind and the presence of a pressure maximum near local midnight. These data suggest that the location of the pressure bulge changes seasonally with respect to the geographic equator. Average bottomside electron density altitude profiles are derived from the 6300‐Å volume emission profiles. The local time variation of the height of the peak electron density deduced from the airglow measurements is in agreement with incoherent scatter measurements made at Arecibo, Puerto Rico, during the same time period.
Momentum balance and flux conservation in model magnetospheric magnetic fields
Authors: Walker R. J. , Southwood D. J.
Published: February 2008

We have tested four recent quantitative magnetospheric magnetic field models to determine whether the magnetic field configuration is consistent with a balance of forces between the J × B force and the gradient of the pressure. A necessary condition for the models to be consistent with an isotropic pressure distribution is that curl (J × B) vanish. None of the models (Olson‐Pfitzer, Tsyganenko, Hedgecock‐Thomas, and Beard) meet this requirement. Our analysis indicates that the models are intrinsically inconsistent with momentum balance with any isotropic pressure distribution. We have also, for purposes of comparison, examined ▽ · B. In certain models this can be nonzero.
General concepts on the generation of auroral kilometric radiation
Authors: Dusenbery P. B. , Lyons L. R.
Published: February 2008

Auroral kilometric radiation (AKR) has a peak intensity at 250 kHz and is associated with discrete aurora in regions where ωp/ωc < 0.2. There are two high‐frequency electromagnetic wave modes in which AKR could propagate, the L mode and the R mode. Observations of AKR are inconsistent with the characteristics of L mode radiation. However, the characteristics of the upper frequency branch of the R‐X mode are consistent with AKR observations. We therefore look at the following free‐energy sources in the auroral electron distribution functions as possibilities for exciting the R‐X mode: (1) the accelerated, precipitating electrons, and (2) the upgoing loss cone associated with the electrostatically trapped electrons. Solving the relativistic resonance condition exactly gives contours in velocity space that are elliptical. Diffusion of precipitating electrons, which develop ∂f/∂υ⊥ > 0 because of the magnetic mirroring force, and diffusion of trapped electrons in the upgoing loss cone with R‐X waves are toward decreasing energies, so that wave growth is possible. To determine which frequency and wave vector (k, θ) is associated with each resonant contour, we solve the cold plasma dispersion relation for ωR, k, and θ, where ωR > ωx, the right‐hand cutoff frequency. Growing waves will be associated with those contours that pass through regions of velocity space where ∂f/∂υ⊥ > 0 is large. A simple criterion is given to show which R‐X waves will have large positive growth rates. Finally, we calculate the group velocity of R‐X waves and show that R‐X waves with large, positive growth rates also have small group velocities (Vg/c ≪ 1), implying a very small convective growth length ∼10 m. The intense wave generation should occur at wave frequencies just above the right‐hand cutoff frequency and have wave normal angles 75° < θ ≲ 105°. Wave growth maximizes in regions where ωp/ωc is small (≲0.06) and where there exists sufficient free energy (∂f/∂υ⊥ > 0 is large) associated with either the upgoing loss cone or the downgoing precipitating electrons that are undergoing mirroring.
Comparative observations of D region HF partial reflections at 2 and 6 MHz
Authors: Hocking W. K. , Vincent R. A.
Published: February 2008

Simultaneous comparisons of HF partial reflections from the ionospheric D region at 2 and 6 MHz have been made near Adelaide, Australia (35°S, 138°E). Echoes generally come from preferred heights, and these heights were the same on both frequencies. By accurately calibrating the receiving and transmitting systems used, it has been possible to compare effective voltage reflection coefficients at these two frequencies and hence provide further evidence that specular reflection processes are the primary means of scatter below about 75–80 km. At heights above 80 km, processes other than specular reflection appear to be important. Turbulent scatter may make a major contribution to the observed strengths, but this cannot yet be proved unambiguously. At 6 MHz, very little scatter was received from heights above 90 km, while strong echoes are usually observed at 2 MHz. This suggests that scales of 25 m are within the viscous range of turbulence at heights above 90 km.