bluuebunny/crossref_metadata_2025 huggingface dataset:
Generation of high‐frequency broadband electrostatic noise: The role of cold electrons
Authors: Schriver David , Ashour‐Abdalla Maha
Published: February 2008

Broadband electrostatic noise (BEN) is commonly observed in the plasma sheet boundary layer in association with ion beams. Here we investigate the generation of these waves in a plasma consisting of an ion beam and a background of hot ions, hot electrons, and cold electrons. The cold electrons are of ionospheric origin. A complete, systematic study of electrostatic ion beam instabilities, including cold electrons, has been done, and it is shown that for the plasma configuration described, four instabilities can be excited. These are the (1) ion acoustic, (2) Buneman, (3) beam resonant, and (4) electron acoustic instabilities. A low and high beam temperature division is shown to exist that separates when different instabilities can be excited. For typically observed parameters in the plasma sheet boundary layer, the ion beams lie in the high‐temperature regime. In this regime, the beam resonant and electron acoustic instabilities are excited, and these instabilities can account for the high‐frequency (> 500 Hz), low‐power portion of the BEN spectrum. In the absence of cold electrons, no such wave growth occurs.
AMPTE IRM observations of waves associated with flux transfer events in the magnetosphere
Authors: LaBelle J. , Treumann R. A. , Haerendel G. , Bauer O. H. , Paschmann G. , Baumjohann W. , Lühr H. , Anderson R. R. , Koons H. C. , Holzworth R. H.
Published: February 2008

The AMPTE IRM wave instrument has been applied to the study of flux transfer events (FTE's). This initial investigation concentrates on FTE's observed in the magnetosphere during the fall of 1984. The wave morphology consists of four significant features: at frequencies below the ion gyrofrequency, magnetic fluctuations occur with amplitudes of the order of 1 nT; at frequencies from a few hertz to a few hundred hertz, electric field fluctuations are observed which have a broadband amplitude of a few millivolts per meter, which are perpendicularly polarized at the lowest frequencies, and which are partly electrostatic and partly electromagnetic; in the frequency range from 100 Hz to 10 kHz, broadband spikelike waves occur with a time‐averaged amplitude of about 0.1 mV/m; and near the electron plasma frequency, bursts of waves occur at the boundaries of FTE's. In none of the 25 events surveyed does the total broadband amplitude of all the waves exceed a few millivolts per meter, an amplitude far too small to provide the dissipation required by “traditional” reconnection models in which the dissipation region has a thickness the order of an ion gyroradius. Thus, either all of the FTE's are observed at some distance from the diffusion region, or the observable waves play no significant role in the diffusion process, or “traditional” reconnection models do not apply to reconnection in FTE's.
Linear transport theory of auroral proton precipitation: A comparison with observations
Authors: Basu B. , Jasperse J. R. , Robinson R. M. , Vondrak R. R. , Evans D. S.
Published: February 2008

In this paper we present nearly coincident Chatanika radar electron density measurements and NOAA 6 particle data for a continuous (diffuse) auroral E layer with a peak electron density of 1–2 × 105 cm−3 produced entirely by proton precipitation. The radar and particle data are analyzed using the Jasperse‐Basu transport theoretic method and the semiempirical, continuous slowing down method of Rees. Comparisons between the radar results for the electron density profile and the two theoretical results are given. We conclude that the transport theoretic method of Jasperse and Basu gives a more accurate result for the shape of the electron density profile and for the location of its peak than the semiempirical, continuous slowing down method of Rees. We also apply the transport theoretic method to derive a closed form expression for the energy deposition function and compare the transport theoretic energy deposition function with that used by Rees in order to explain the differences in the electron density profiles obtained by the two theoretical methods.
The normal modes of the thermosphere
Authors: Larsen M. F. , Mikkelsen I. S.
Published: February 2008

We show that the linearized momentum, energy, and continuity equations for the thermosphere can be reduced to a form that gives the vertical structure for each horizontal wave mode. The vertical structure equation can be described in terms of the normal modes, or eigenmodes, of the thermosphere. We solve for the latter using a 27‐layer model that includes a realistic temperature profile and the effects of the Lorentz force, viscosity, and heat conduction. The normal modes have one real eigenfrequency for every two complex conjugate eigenfrequency values. The real modes have a dominant rotational wind component and are nonpropagating. The complex modes have comparable divergent and rotational wind components. The complex eigenvalues give vertically propagating modes, primarily associated with the transient response to forcing, and are significantly affected by the dissipation in the upper E region and F region. Our results show that the rotational wind component dominates in the steady state when the forcing is due to the two‐cell convection pattern at high latitudes and that the normal modes explain the large shears and large winds speeds that are typically observed in the high‐latitude E region. We have also calculated the vertical energy flux for the normal modes. The results show that the flux is upward above 130 km but downward in the lower E region for the total solution. The downward energy flux is a contribution from the real eigenmode structure.
Thermospheric dynamics during the March 22, 1979, magnetic storm: 2. Comparisons of model predictions with observations
Authors: Forbes Jeffrey M. , Roble Raymond G. , Marcos Frank A.
Published: February 2008

A comparison is made between simulations from the National Center for Atmospheric Research thermospheric general circulation model (TGCM), satellite electrostatic triaxial accelerometer measurements of neutral winds and total mass densities between 170 and 240 km, and mid‐latitude Thomson scatter measurements of neutral exospheric temperatures, for the isolated magnetic disturbance occurring on March 22, 1979, commonly referred to as the CDAW 6 interval. Points of consistency between theory and observation include (1) a localized region of daytime density enhancement (60% at 180 km) near the throat of the plasma convection pattern, (2) more structured density perturbations on the nightside propagating toward the equator, and (3) polar region winds of 250–600 m s−1 generally consistent with antisunward and return flows characteristic of a two‐cell pattern which persists for at least 6 hours after the magnetic disturbance has died away. The major points of discrepancy are (1) the excessive (factor of 2) TGCM estimates of the Millstone Hill exospheric temperature response, (2) relatedly, the comparatively pronounced equatorward penetration of the model's high‐latitute temperature and density enhancements in general, and (3) the measured reversals to eastward winds of 200 m s−1 near 50°–60° latitude, whereas the TGCM maintains a westward flow down to at least 40° latitude in the prenoon and premidnight sectors. We speculate that these differences may be caused by one or more of the following: (1) a more complex (structured) ion convection forcing than that parameterized within the TGCM model, (2) neglect of By‐induced variations in the ion convection pattern, (3) an incorrect positioning or orientation of the driving plasma convection pattern, or (4) inadequate modeling of high‐latitude ion densities in the TGCM, including the possible need for a coupled solution to the neutral wind and plasma distributions.
Space charge effects during the injection of dense electron beams into space plasmas
Authors: Winglee R. M. , Pritchett P. L.
Published: February 2008

One‐dimensional electrostatic particle simulations are used to investigate the injection and propagation of intense electron beams and the plasma response to the beam injection. Beam densities greater than about the plasma density are considered. It is shown that if the injection is continuous, most of the beam electrons are drawn back into the spacecraft because of the buildup of positive charge on the spacecraft. Those electrons which are able to propagate away from the spacecraft are emitted periodically because of space charge oscillations induced by electric fields associated with the beam. A substantial increase in the fraction and average energy of electrons which can propagate away can be obtained if the injection is changed to periodic pulses having width and period matched to the induced space charge oscillations.
Ion distributions in the dayside magnetosheaths of Jupiter and Saturn
Authors: Richardson John D.
Published: February 2008

Data from the Voyager 1 and 2 Plasma Science experiment in the dayside magnetosheaths of Jupiter and Saturn are analyzed. The ion distributions throughout the dayside magnetosheaths of both planets are well modeled by a two‐temperature proton distribution. The two proton populations have comparable densities, and temperatures of 100 and 1000 eV. Similar two‐temperature proton distributions are occasionally observed in earth's magnetosheath. Ion temperatures, densities, and bulk velocities for the four magnetosheath crossings are presented, confirming that sunward flow of up to 100 km/s occurs when the magnetosphere is expanding.
On the He2+ to H+ temperature ratio in slow solar wind
Authors: Hernández R. , Livi S. , Marsch E.
Published: February 2008

On the basis of a three‐fluid model with energy exchange by Coulomb collisions and heat flux degradation the ratio T of alpha to proton temperatures is investigated. For various relative ion densities and plasma parameters the collisional evolution of T is modeled and compared with Helios in situ measurements. Good agreement between data and theory is found.
Effect of He+ temperature and thermal anisotropy on the electromagnetic ion cyclotron instability in the geosynchronous region of GEOS‐2
Authors: Gomberoff L. , Vega P.
Published: February 2008

The existence of strong ULF electromagnetic ion cyclotron waves in the magnetosphere have been firmly established through several observations. Some recent studies have shown that an energetic heavy ion component may have a drastic influence on the ion cyclotron instability. By considering several possible sources of He+ thermal and anisotropy effects, it is shown that, within the linear theory and uncertainties of the available data, none of these effects have a major influence on the ion cyclotron instablity in the geostationary region of GEOS‐2. The analysis is rigorous for He+ parallel temperatures ranging between a few electron volts and a few keV, with a similar tendency for higher temperatures.
The O I 3d ³D° ‐ 2p4 ³P Transition at 1026 Å in the Day Airglow
Authors: Meier R. R. , Anderson D. E. , Paxton L. J. , McCoy R. P. , Chakrabarti Supriya
Published: February 2008

An emission feature at 1026‐Å observed at 600 km in the dayglow with a spectrometer on board the STP 78‐1 spacecraft cannot be explained by geocoronal hydrogen Lyman β. An accidental resonance of some lines of the O I multiplet at 1026‐Å with the broad solar Lyman β line can in fact account for the observed emission rate, based on analysis with a Monte Carlo radiative transport model. Photoelectron excitation of atomic oxygen is negligible compared with resonant scattering. These airglow results support a ratio of 66 to 1 for the solar Lyman α to Lyman β line center fluxes.
A dynamical‐chemical model of tidally driven fluctuations in the OH nightglow
Authors: Walterscheid R. L. , Schubert G.
Published: February 2008

A theory is presented to explain tidally induced oscillations in the emission intensity I and rotational temperature T of the OH nightglow. The theory includes photochemical reactions among H, O, O3, OH, and HO2 and the complete dynamics of tides in an isothermal, uniformly rotating atmosphere. The ratio refers to a perturbation quantity and the overbar signifies an average) depends on basic state atmospheric structure, minor constituent scale heights, especially that of O, and altitude of OH emission. For both semidiurnal and diurnal migrating modes, η has an amplitude in the approximate range 1–2.5 and a phase between about 10° and 30°.
Auroral precipitation caused by auroral kilometric radiation
Authors: Calvert W.
Published: February 2008

If the auroral kilometric radiation were generated by loss cone lasing on closed field lines, as has been proposed, then it should cause substantial auroral precipitation by the pitch angle scattering of energetic electrons into the loss cone. A rough estimate for this precipitation, based upon the observed auroral kilometric radiation (AKR) amplitudes, would imply a flux of at least 2 × 108 el/cm² s over the projected ionospheric footprint of an individual laser and, if most of the AKR radio lasers occupied the same electron drift an L shell, an arc of 8 km width with a minimum average flux of roughly 109 el/cm² s. It is believed that this will account for auroral arcs and other aspects of auroral electron precipitation.
Finite Larmor radius effect on ion pickup at Venus
Authors: Phillips J. L. , Luhmann J. G. , Russell C. T. , Moore K. R.
Published: February 2008

The interaction of the solar wind with Venus is influenced by the pickup of newly born exospheric oxygen ions by the convecting magnetosheath plasma. The flow and field configuration of the magnetosheath plasma, together with the large gyroradius of the pickup ions, cause mass loading to occur preferentially on one side of the magnetosheath. The observed hemispherical asymmetry in the magnetic field in the near‐planet magnetosheath, attributed to this pickup process, is confirmed by direct observation of the picked‐up planetary particles. Test particle calculations show that a current system created by ion pickup has the appropriate location and magnitude to account for the magnetic field asymmetry. The results indicate that a fluid treatment of the Venus mass‐loading problem is not entirely appropriate; a hybrid or kinetic model is necessary to incorporate the finite Larmor radius of the pickup particles which produces the observed asymmetry.
The nature and evolution of magnetohydrodynamic fluctuations in the solar wind: Voyager observations
Authors: Roberts D. A. , Klein L. W. , Goldstein M. L. , Matthaeus W. H.
Published: February 2008

The magnetic field and plasma data collected by the Voyager spacecraft between 1 and 11 AU are used to study the properties of interplanetary MHD fluctuations and to attempt to answer several related questions about the Alfvénicity of solar wind fluctuations: First, to what extent are magnetic and velocity fluctuations Alfvénic? Second, does the dominant propagation direction of Alfvénic fluctuations evolve with heliocentric distance? Third, is the presence of Alfvénic fluctuations correlated with large‐scale structures, such as stream interaction regions? In addition, we investigated the contributions of compressive modes to the interplanetary fluctuations. We find that near 1 AU at most 15% of the fluctuations at the scale of a few hours or less are purely Alfvénic and these usually propagate outward from the Sun. The propagation direction becomes more inward on average with increasing heliocentric distance. Although it is commonly supposed that compression regions are not generally Alfvénic, we found that the wave propagation direction is only slightly more mixed in compression regions than in the corresponding rarefaction regions. Moreover, the evolution in propagation direction is not directly due to the growth of large‐scale compression regions. There is a tendency for magnetic fluctuations to be larger than velocity fluctuations at scales less than a day, while the reverse is true, due to the dominance of stream energy, at larger scales. While smaller‐scale density fluctuations are uniformly small (<δn/n> ≃ 0.1), they are usually negatively correlated with field magnitude variations even on time scales that preclude contributions from tangential discontinuities. In general the solar wind cannot be in a static state of superposed large‐amplitude waves. Fluctuations must be produced outside the Alfvénic critical point, perhaps generated by stream shear with subsequent nonlinear evolution.
Electric field measurements at subcritical, oblique bow shock crossings
Authors: Wygant J. R. , Bensadoun M. , Mozer F. S.
Published: February 2008

Electric field measurements at oblique, subcritical bow shock crossings are presented from the ISEE 1 University of California, Berkeley, double‐probe electric field experiment. The measurements averaged over the 3‐s spin period of the spacecraft provide the first observations of the large‐scale (100 km) laminar oscillations in the longitudinal component of the electric field associated with the whistler precursor which is characteristic of these dispersive shocks. The amplitude of the oscillations increases from ∼0.5 mV/m to a maximum of 6 mV/m across the magnetic ramp of the shock (directed along the shock normal). The calculated electric potential drops across the shocks varied from 340 to 550 volts, which is 40–60% of the observed loss of kinetic energy associated with the bulk flow of the ions. These measurements suggest that at these shocks the additional deceleration of incident ions is due to the Lorentz force. The contributions to the normal component of the large‐scale electric field at the shock due to the parallel and perpendicular components (relative to the magnetic field) of the electric field are evaluated. It is shown that the perpendicular component of the electric field dominates, accounting for most of the cross‐shock potential, but that there is a nonnegligible parallel component. This large‐scale parallel component has a magnitude of 1–2 mV/m which sometimes results in a potential well for electrons with a depth of ≈150 eV. It is experimentally demonstrated that the dominance of the perpendicular over the parallel component of the electric field resulted in a correlation between the longitudinal component of the large‐scale electric field and the fluctuations in the magnetic field component perpendicular to the coplanarity plane. This paper also presents, for the first time, high time resolution (dc to 32 Hz) measurements of the electric field waveform in the current layer of a collisionless shock. The measurements in this region reveal the presence of intense electric field spikes with amplitudes ranging up to 100 mV/m lasting about 0.1 s superimposed on the large‐scale laminar electric field wave train described earlier. The period of these spikes is consistent with lower hybrid or ion acoustic waves which have been strongly Doppler shifted as a consequence of the solar wind motion. These electric field structures can have significant components along the shock normal and may play a variety of roles in the microphysics of shock dissipation.
Beam production at plasma boundaries by kinetic Alfvén waves
Authors: Hasegawa Akira
Published: February 2008

Wave‐induced transport can produce diffusion both in coordinate space and in velocity space with transport rates which are much closer than the corresponding rates for classical transport. In such a case the quasi‐linear diffusion in velocity space should be solved in conjunction with the diffusion in coordinate space. This leads to processes hitherto unknown, such as the generation of a large number of accelerated particles near the plasma‐vacuum boundary. As an example, the theory of a double diffusion caused by kinetic Alfvén wave turbulence is presented, and generation of ion beams is discussed.
Plasma and field observations of a compressional Pc 5 wave event
Authors: Baumjohann W. , Sckopke N. , LaBelle J. , Klecker B. , Lühr H. , Glassmeier K. H.
Published: February 2008

On October 24, 1984, the AMPTE/IRM satellite, on its inbound orbit in the 1300 LT sector, observed a strong compressional Pc 5 event lasting for about an hour. The use of data from the full complement of detectors aboard the spacecraft (magnetometer, three‐dimensional plasma instrument, wave experiment, and SULEICA time‐of‐flight analyzer including its electron sensor package) allowed for detailed measurements of field and particle oscillations, with the latter covering energies from a few electron volts up to tens of keV (electrons) or even 1 MeV (protons). Both energetic proton and electron fluxes were anticorrelated with the compressional magnetic field oscillations, indicating that the event belongs to the class of “in‐phase events.” But the energetic proton data also exhibited a new feature: Flux minima and maxima at low energies were observed somewhat later than those at higher energies (with time differences of ∼2 min between 3 and 40keV). The magnetic and plasma pressure oscillations satisfy the pressure balance equation for the drift mirror mode much better than that for drift compressional Alfvén waves. However, the classical criterion for the onset of the mirror instability is not satisfied. The low‐energy particles showed clear signatures of gradient convection due to the wave electric field with the protons additionally undergoing gyration acceleration. The period of the pulsation decreased while the satellite was moving inward, in agreement with the individual L shell resonance model. But in contrast to earlier observations the periods of the compressional and transverse oscillations differed significantly (by ∼25%). We interpret this as Doppler shift due to spacecraft motion since in the present event the transverse oscillations did not have the purely radial (poloidal) polarization common to other published cases. They had an additional azimuthal (toroidal) component, making it likely that the wave fronts of compressional and transverse oscillations were inclined with respect to each other.
Ion escape fluxes from the terrestrial high‐latitude ionosphere
Authors: Barakat A. R. , Schunk R. W. , Moore T. E. , Waite J. H.
Published: February 2008

The coupled continuity and momentum equations for H+, O+, and electrons were solved for the terrestrial ionosphere in order to determine the limiting ion escape fluxes at high latitudes. The effects of solar cycle, season, geomagnetic activity, and the altitude of the acceleration region on the ion escape fluxes were studied for average conditions. In addition, a systematic parameter study was conducted to determine the extent to which variations in ionospheric conditions (for example, electron temperature, ion temperature, induced vertical ion drifts, etc.) can affect the results. The main conclusions of the study are as follows: (1) as solar activity increases, the general trend is for an increase in the limiting O+ escape flux and a decrease in the limiting H+ escape flux; (2) in winter the limiting escape fluxes of both O+ and H+ are larger than those in summer, particularly for low geomagnetic activity; (3) the O+ content of the ion outflow increases with increasing “demand” imposed on the ionosphere by a high‐altitude acceleration process, with increasing solar activity, with increasing geomagnetic activity, with increasing solar elevation from winter to summer, and with a lowering of the altitude of the acceleration region; (4) when H+ is in a near‐diffusive equilibrium state and a selective mechanism accelerates O+, the limiting O+ escape flux is significantly reduced compared to that obtained when an H+ outflow also occurs; and (5) at a given time or location the general trends described above can be significantly modified or even reversed owing to natural variations of the ionospheric ion and electron temperatures, induced vertical ion drifts, etc. The general trends obtained for average conditions appear to mimic the qualitative behavior determined from statistically averaged data for comparable absolute escape flux magnitudes.
Proton beam generation of whistler waves in the Earth's foreshock
Authors: Wong H. K. , Goldstein M. L.
Published: February 2008

We show that proton beams, often observed upstream of the Earth's bow shock and associated with the generation of low‐frequency hydromagnetic fluctuations, are also capable of generating whistler waves. The waves can be excited by an instability driven by two‐temperature streaming Maxwellian proton distributions which have T⊥/T∥ ≫ 1. It can also be excited by gyrating proton beam distributions. These distributions generate whistler waves with frequencies ranging from 10 to 100 times the proton cyclotron frequency (in the solar wind reference frame) and provide another mechanism for generating the “1‐Hz” waves often seen in the Earth's foreshock.
An eastward propagating compressional Pc 5 wave observed by AMPTE/CCE in the postmidnight sector
Authors: Takahashi K. , Lopez R. E. , McEntire R. W. , Zanetti L. J. , Kistler L. M. , Ipavich F. M.
Published: February 2008

Data from three instruments, the magnetometer, the charge‐energy‐mass spectrometer, and the medium‐energy particle, analyzer onboard the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (CCE) spacecraft have been used to study a compressional Pc 5 wave observed at 1925–2200 UT on day 202 (July 21) of 1986 at a radial distance of ≃8 RE in the postmidnight sector near the beginning of minor geomagnetic activity. The wave exhibited harmonically related transverse and compressional magnetic oscillations, modulation of the flux of medium energy protons (E ≳ 10 keV), and a large azimuthal wave number (m ∼ 65). These properties are similar to those of compressional Pc5 waves observed previously at geostationary orbit. The unique observations associated with the CCE event are the occurrence in the postmidnight sector, the eastward (or sunward) propagation with respect to the spacecraft, and the left‐handed polarization of the perturbed magnetic field. These are opposite to previous geostationary observations. We propose that the unique propagation and polarization are caused by a dawn‐to‐dusk dc electric field which convects the plasma sunward. The wave, probably propagating westward in the plasma rest frame, appears to propagate eastward to the observer because the electric field drift velocity is larger than the wave phase velocity.
Simultaneous measurements of the magnetopause and flux transfer events at widely separated sites by AMPTE UKS and ISEE 1 and 2
Authors: Elphic R. C. , Southwood D. J.
Published: February 2008

On September 19, 1984, the ISEE 1 and 2 and AMPTE UKS and IRM spacecraft pairs crossed the dayside magnetopause at nearly the same universal time and magnetic local time but at much different latitudes. This fortuitous occurrence allows the magnetopause and flux transfer events (FTEs) to be studied at two widely separated sites simultaneously. FTEs are observed at both locations, those at UKS having standard normal component signatures, while those at ISEE have reverse signatures. The FTEs at UKS, closer to the equator, appear to have less helicity, or “twistedness” than those at ISEE far to the south. By identifying FTEs using the stringent Rijnbeek criterion for the bipolar Bn signature one might conclude that FTEs are not necessarily detected simultaneously at UKS and ISEE. However, careful analysis of the field strength behavior at ISEE does reveal evidence that FTEs are indeed observed at both sites simultaneously. While within the magnetosphere, AMPTE and ISEE both observe a coherent field rarefaction coupled with a tilt; we speculate that the signature is associated with time‐dependent dayside magnetopause reconnection.
Light hydrocarbons from plasma discharge in H2‐He‐CH4: First results and Uranian auroral chemistry
Authors: Thompson W. Reid , Henry Todd , Khare B. N. , Flynn Luke , Schwartz Joel , Sagan Carl
Published: February 2008

Voyager 2 found that the Uranian magnetosphere has a substantial flux of energetic charged particles, which becomes rich in higher energies at low magnetospheric L near the orbit of Miranda. The electrons precipitate to produce aurorae, which have been observed in the ultraviolet. The more energetic component of the precipitating electrons can initiate radiation chemistry in the methane‐poor stratosphere, near 0.1 mbar where the CH4 mole fraction XCH4 ≃ 10−5. We present laboratory results for cold plasma (glow) discharge in continuous‐flow H2‐He‐CH4 atmospheres with mol fractions XCH4 = 10−2 to 10−3 and total pressure p = 60 to 0.6 mbar. The yields of simple hydrocarbons in these experiments and an estimate of precipitating electron flux consistent with the Voyager ultraviolet spectroscopy results indicate that the globally averaged auroral processing rate of CH4 to higher hydrocarbons ≃ 3 × 106 C cm−2 s−1, comparable to the globally averaged photochemical production rate. The local rate ∼2 × 108 C cm−2 s−1 in the auroral zones (∼20° in diameter) at 15°S and 45°N latitude greatly exceeds the photochemical rate. Even at very low XCH4 ≃ 10−3 the yield (summed over all products) G ≳ 10−2 C/100 eV and the average slope α = 〈log10{η∑[CηHx]/(η ‐ 1)∑[Cη ‐ 1Hx]}〉 ≳ −0.4, where the summation is over all product molecules of a given carbon number η and the square brackets denote abundance. The yield therefore decreases slowly with molecular complexity: hydrocarbons through C7Hx should be present in auroral zones at abundances ≳ 10−2 of the simplest C2 hydrocarbons. Saturated hydrocarbons (C2H6, C3H8, C4H10, etc.) are mostly shielded from photodissocation by C2H2 and will therefore persist at the sunlit, as well as the currently dark, magnetic polar regions.
The upper atmosphere of Uranus: EUV occultations observed by Voyager 2
Authors: Herbert Floyd , Sandel B. R. , Yelle R. V. , Holberg J. B. , Broadfoot A. L. , Shemansky D. E. , Atreya S. K. , Romani P. N.
Published: February 2008

Occultation observations of the upper atmosphere of Uranus by the Voyager 2 ultraviolet spectrometer are analyzed. The measurements extend from 0.5 mbar to about 10−6 µbar using the EUV wavelengths 520 ≤ λ ≤ 1700 Å. H2 dominates the atmosphere (the approximately 15% He content deduced by the Voyager 2 infrared spectrometer cannot be seen in occultation at these wavelengths) up to the vicinity of the exobase near 1.25 RU, where atomic H becomes the major constituent. Apparently because of weak eddy mixing, the hydrocarbon mixing ratios are quite small in the measured pressure range, so that the atmosphere is more transparent than those of Jupiter and Saturn. Thus H2 Rayleigh scattering is the dominant source of opacity in the lower portion of the observed pressure range. The mixing ratio of C2H2 is on the order of 10−8 there, while only an upper limit (≤ 10−7) is available for CH4. Also, some evidence exists for the possible presence of C2H6 at a mixing ratio of several × 10−8. The value of the eddy diffusion coefficient at the homopause is much lower than at Jupiter and Saturn; the best fitting of several photochemical models which were matched to the observations assumed a value of 104 cm² s−1. This may represent an upper limit. In addition, the two high‐latitude occultations indicate little difference in upper atmospheric structure between the day and night hemispheres, despite the constancy of the illumination geometry over recent decades. The atmospheric temperature above about 0.01 to 0.001 µbar is 800 ± 100 K. Because of this high temperature the thermal component of the H exosphere extends to great altitude, with number densities of several hundred cm−3 at 2 RU. This high gas density has important implications for ring dynamics, possibly being responsible for the extreme narrowness and isolation of the visible Uranian rings. The extent and density of the H exosphere and the nonthermal corona (which has an even larger scale height) will also strongly affect the origin and maintenance of the unusual plasma populations observed at Uranus by the Voyager 2 plasma science and low‐energy charged particle experiments.
The excitation of the far ultraviolet electroglow emissions on Uranus, Saturn, and Jupiter
Authors: Clarke John T. , Hudson Mary K. , Yung Yuk L.
Published: February 2008

We propose that the diffuse FUV emissions of H and H2 in excess of photoelectron excitation observed from the sunlit atmospheres of Uranus, Saturn, and Jupiter are produced by electric field acceleration of photoelectrons and ions locally in the upper atmospheres. This in situ acceleration is required to satisfy the many observational constraints on the altitude distribution, exciting particle energy, and total input energy requirements of the electroglow mechanism. We further suggest that a primary mechanism leading to this acceleration is an ionospheric dynamo, which is created in the same manner as the Earth's dynamo. The calculated altitude of charge separation by the neutral wind drag on ions across magnetic field lines is consistent with the observed peaks in electroglow emissions from the Voyager ultraviolet spectrometer limb scan data on both Saturn (near the homopause) and Uranus (just above the homopause). This dynamo action therefore appears to initiate the acceleration process, which must have the form of field‐aligned potentials to accelerate the magnetized electrons. We propose that these field‐aligned potentials are due to anomalous resistivity, which results from sufficiently high field‐aligned currents in the ionosphere to generate plasma instabilities and therefore runaway electrons and ions above some critical lower initial energy. There are multiple candidate processes for inducing these currents, including polarization in the equivalent F regions and inner magnetospheric convection, and each of these processes should exhibit latitudinal structure. The acceleration of low‐energy electrons in an H2 atmosphere preferentially results in FUV radiation and further ionization, whereas electron acceleration in a nitrogen/oxygen atmosphere such as the Earth' is dominated by elastic scattering and thus results in electric currents. Individual electron and proton collisions with H2 molecules will result in excitation, ionization, and heating, so that considerable enhancement of the ionospheric density and heating of the upper atmosphere will accompany the FUV emission.
The main source of radio emission from the magnetosphere of Uranus
Authors: Gulkis Samuel , Carr Thomas D.
Published: February 2008

Observations of kilometric radiation from Uranus made with the planetary radio astronomy experiment on the Voyager 2 spacecraft are presented and discussed. Similarities between the auroral kilometric radiation from Earth and the observed Uranus emission are pointed out. A geometrical beaming model is developed in which a single distributed source is located above the darkside auroral region and emits in the extraordinary mode by the cyclotron maser process. The model can account for nearly all the Uranian kilometric radiation from the high‐frequency limit near 850 kHz down to about 150 kHz and for much of it down to the lower limit of 20 kHz.
Models of Uranian continuum radio emission
Authors: Romig Joseph H. , Evans David R. , Sawyer Constance B. , Schweitzer Andrea E. , Warwick James W.
Published: February 2008

The Uranian kilometric emissions detected by the Voyager 2 Planetary Radio Astronomy (PRA) experiment during the January 1986 encounter comprise two distinct types of emission: bursts and continuum. The bursts have been treated by Evans et al. (this issue). This paper considers the continuum emissions. The continuum emissions comprised four components: equatorial emissions, anomaly emissions, strong nightside emissions, and weak nightside emissions. These components are associated with different sources. In all cases the electron gyrofrequency and the extraordinary mode in the various source regions define the observed wave frequencies and polarizations. The equatorial emissions appeared most prominently during the days before closest approach and extended from 40 kHz or below to approximately 120 kHz. These emissions originate near the offset tilted dipole (OTD) magnetic equator. They are beamed perpendicularly to the local field lines. The anomaly emissions were seen about 12 hours before closest approach and extended to approximately 250 kHz. They originate within the “Q3 dayside polar anomaly” described by Connerney et al. (this issue). The strong and weak nightside emissions were seen near and after closest approach and extended from 40 to 865 kHz. These emissions are accurately modeled by sources in the OTD magnetic field. The strong nightside source is located on the nightside L‐5 shell, and the emission is beamed into a solid cone of half‐angle 10° to 30°, centered about the direction of the local magnetic field. The cone narrows monotonically with increasing frequency. The weak nightside source is located on high nightside L shells (L > 100), and the emission is beamed perpendicularly to the local magnetic field. The weak nightside emission may be associated with bursts. Miranda, orbiting Uranus in nearly a 2∶1 ratio to the 17.24‐hour PRA radio rotation period, may play a role in Uranian kilometric emissions. A coincidence between Miranda's phase and strong radio emission at 20.4 kHz, just after closest approach, suggests intense dynamic activity on the Miranda L shell.
Bursty radio emissions from Uranus
Authors: Evans David R. , Romig Joseph H. , Warwick James W.
Published: February 2008

Uranus emits a bursty form of radio emission in the range from 70 kHz to 1.2 MHz from a region near the negative magnetic pole. The emission is beamed into a flat equatorial sheet which appears to rotate with the magnetic field. The bursts typically have a duration of tens to hundreds of milliseconds, with a bandwidth of <20 kHz. A deep modulation of approximately 30 Hz is evident in the bursts.
Temporal variation of the Jovian H I Lyman alpha emission (1979–1986)
Authors: Skinner T. E. , DeLand M. T. , Ballester G. E. , Coplin K. A. , Feldman P. D. , Moos H. W.
Published: February 2008

Observations monitoring the long‐term spatial and temporal behavior of the Jovian H I Ly α emission have been made with the International Ultraviolet Explorer (IUE) observatory beginning in December 1978, and extending through December 1986. The hydrogen bulge region centered near λIII = 100° has been observed to be a persistent feature of the Jovian upper atmosphere throughout the period of the observations, which includes the maximum of solar cycle 21 and the current minimum in solar activity. New and previously unpublished measurements made since early 1982 show that the average brightness of both the bulge and nonbulge regions has varied with the long‐term solar Ly α output, which has decreased by a factor of ∼ 2 in this time frame. However, the bulge is ∼ 25% brighter on average and exhibits more short‐term variability than the nonbulge region. The factor of ∼ 2 change in Jupiter's Ly α brightness during the descending phase of solar cycle 21 is significantly less than the order of magnitude variability reported for the ascending phase of the same cycle. The IUE results are consistent with pure resonant scattering from an H column of ∼1017 cm−2 and indicate that no observable changes have taken place in the Jovian atomic hydrogen abundance or the average atmospheric conditions during the 8‐year period since December 1978. If the electroglow process recently proposed for the outer planets plays a significant role in the long‐term behavior of the Jovian Ly α emission, the electroglow must also follow the solar cycle at Ly α. Finally, as a result of revisions in both the IUE instrumental sensitivity at Ly α and the dimensions of the spectrograph large aperture, there is no significant difference between Voyager and IUE measurements of the Jovian Ly α brightness made within the same time frame and with approximately the same field of view.
Preshock region acceleration of implanted cometary H+ and O+
Authors: Gombosi Tamás I.
Published: February 2008

One of the most interesting results of the cometary flyby missions was the realization that magnetohydrodynamic turbulence generated by the pickup process of freshly ionized cometary particles results in fast pitch angle scattering of the newly implanted ions. This process rapidly modifies the newly implanted ion pitch angle distribution in the solar wind frame of reference, from the original pickup ring to a pickup shell distribution. It also appears that the implanted ions have an isotropic pitch angle distribution in the decelerating solar wind frame of reference. This paper presents a self‐consistent description of the multispecies plasma flow (composed of solar wind protons and cometary H+ and O+) in the unshocked upstream region. The solar wind is treated as a fluid, while the evolution of the implanted ion distributions is described by generalized transport equations. The ambient solar wind population is depleted by charge exchange, while implanted protons and oxygen ions are produced by photoionization and charge transfer and lost by charge exchange. The transport equations take into account the adiabatic velocity change due to the deceleration of the flow, velocity diffusion, charge exchange loss, and the pickup of newly created ions. The model equations were numerically solved for the comet Halley flyby conditions, and the results are compared with spacecraft observations. Our calculations imply that second‐order Fermi acceleration can explain the implanted spectra observed in the unshocked solar wind. The comparison of the measured and calculated distribution functions also indicates that spatial diffusion (and consequently first‐order Fermi acceleration) of implanted ions is likely to play an important role in forming the energetic particle environment in the vicinity of the shock.
Limiting energy spectrum of a saturated radiation belt
Authors: Schulz Michael , Davidson Gerald T.
Published: February 2008

Through a further application of the condition for magnetospheric wave growth in the presence of anisotropic charged‐particle distributions, the Kennel‐Petschek theory that traditionally imposes an upper bound on the integral flux of charged particles at energies above a certain threshold is extended to provide a limit on the differential flux at any energy above this threshold. Thus, a modest reformulation of the nonrelativistic Kennel‐Petschek problem for electrons and protons enables a limiting energy spectrum to be derived, such that (for specified pitch‐angle anisotropy s of the energetic particle population) electromagnetic‐cyclotron waves at each frequency less than a fraction s/(s + 1) of the equatorial gyrofrequency are marginally stable against spontaneous generation. The limiting spectrum is given in closed form for integer values of s (>0) and computed numerically or by analytical interpolation for non‐integer values of s. Asymptotic expansions for energies E barely above and much greater than the minimum resonant energy E* provide estimates of the limiting energy spectrum J4π*(E) in these extremes, regardless of whether s is an integer. A reconsideration of the original Kennel‐Petschek problem, in which the differential energy spectrum is not calculated but specified as a certain power law (J4π ∝ E1‐l), enables both the wave frequency ω*/2π corresponding to maximum spatial growth rate and the limiting integral flux I4π* above the minimum resonant energy E* to be calculated in closed form as functions of l and s.
An analysis of whistler mode radiation from the Spacelab 2 electron beam
Authors: Farrell W. M. , Gurnett D. A. , Banks P. M. , Bush R. I. , Raitt W. J.
Published: February 2008

During the shuttle's Spacelab 2 mission the University of Iowa's plasma diagnostics package (PDP) was released from the shuttle to free fly. At times during this free flight when the PDP was magnetically connected to the shuttle, Stanford's fast‐pulsed electron generator, located in the shuttle cargo bay, ejected a 1‐keV, 50‐mA electron beam. The plasma wave instrument on board the PDP detected intense whistler mode radiation during these beam ejections. This paper presents a study of a whistler mode emission detected during one particular continuous electron beam firing. Calculations indicate that the beam radiated approximately 1.6 mW in the whistler mode as the beam traversed the 200 m from the shuttle to the PDP. The emissivity also decreased by about a factor of 10 over this same distance. The measured wave powers are 107 greater than wave powers expected from incoherent Cerenkov radiation, verifying that the radiation is generated by a coherent process. Estimates of the emissivity based on measured electric field intensities in the beam indicate that the whistler mode noise is produced by radiation from electron bunches created by an electrostatic beam‐plasma instability.
Wave‐particle interactions in the foot of the Saturnian bow shock
Authors: Moses S. L. , Coroniti F. V. , Kennel C. F. , Scarf F. L.
Published: February 2008

Voyager 1 plasma wave data show that the quasi‐perpendicular, supercritical bow shock of Saturn exhibits the same plasma wave phenomenology observed near Jupiter. Using a quasi‐linear model developed for the Jovian bow shock, we show that plasma waves at Saturn can generate a significant portion of the total electron temperature jump measured across the shock. In this respect, Saturn's bow shock more closely resembles Jupiter's than the Earth's, where plasma waves contribute negligibly to the total electron temperature jump.
Electron density depletions in the nightside auroral zone
Authors: Persoon A. M. , Gurnett D. A. , Peterson W. K. , Waite J. H. , Burch J. L. , Green J. L.
Published: February 2008

Measurements from instruments on board Dynamics Explorer 1 are used to study regions of diminished electron density in the nightside auroral zone. Local electron densities are obtained from the electron plasma frequency cutoff of the whistler mode auroral hiss. Electron density profiles are highly variable through the nightside auroral zone. Sharply defined regions of low electron densities are a common feature of auroral zone crossings from the predusk hours until the early morning hours at all radial distances up to at least 4.6 RE. Electron densities in these regions are strongly depleted in relation to the adjacent polar cap and plasmaspheric densities, forming a low‐density cavity at 70° ± 5° invariant latitude. Minimum densities in the auroral cavity frequently fall to values below 0.3 cm−3and rarely exceed 3 cm−3 at radial distances of 2–4.6 RE. Within the cavity the electron density profile exhibits extreme variability, with variations of a factor of 2 or more on spatial scales of tens of kilometers. Electron plasma frequency to electron cyclotron frequency ratios of 0.02–0.4 are found inside the auroral cavity, consistent with the values required by theory for the generation of auroral kilometric radiation and Z mode radiation. A comparison of the electron density depletions with simultaneously measured low‐ and high‐energy particle measurements indicates a correspondence between low auroral plasma densities and upward directed ion beams and conics. Low‐energy (<1 keV) upflowing H+ ions are strongly correlated with the poleward edge of the auroral cavity. Although the energetic (>1 keV) precipitating auroral electrons do not correlate with the poleward edge of the auroral cavity, the peak energies of the precipitating electron population do occur inside the cavity interval. These correlations show that density depletions in the nightside auroral zone are directly associated with auroral acceleration processes.