bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical study of the spectrum of broadband electrostatic noise in the magnetotail
Authors: Grabbe Crockett L.
Published: February 2008

A numerical study is made of the spectrum of broadband electrostatic noise (BEN) generated by ion beams in the plasma sheet boundary layer (PSBL), as introduced by Grabbe and Eastman (1984). Two cases are analyzed. The first case has a single hot electrons species present. In that case, two modes are unstable. One is a beam‐acoustic mode, which drives instabilities over a broad frequency and angular range. This is a negative energy resistive medium instability in which inverse Landau damping causes growth and which occurs in a broad‐angle cone centered about the magnetic field. Another is an ion‐ion two‐stream instability which has a much narrower frequency and angle range and considerably greater growth rates. The angle of maximum growth of this instability is typically around 75° (as in the work by Grabbe (1985a, b)) and is found to vary from the high 60s to the low 80s. It is only unstable in the low‐frequency part of the BEN spectrum. The polarization, as measured in the lower part of the spectrum, is predicted to occur at an angle near 75° with respect to the magnetic field, in very good agreement with previous polarization measurements. The high‐frequency part of the spectrum is predicted to be more closely field aligned. A second case analyzed is that in which a second population of cold electrons (arising from diffusion from the lobe into the plasma sheet boundary layer) is present. This cold electron population significantly increases the upper frequency range of the unstable spectrum, and the associated growth rates are enhanced. Its primary influence is found to be on the field‐aligned component at small angles, but if the concentration of cold electrons is sufficiently large, it decreases the growth rates of the oblique ion‐ion instability, thus stabilizing the larger angles. The field‐aligned spectrum undergoes a transition from a negative energy to a positive energy two‐stream instability. This second electron population generates the very high frequency part of the observed BEN spectrum that is just below the plasma frequency and creates wave amplitudes that are as large as the one produced by the large‐angle instability. These results explain the gradual rise in the upper frequency range of BEN as the spacecraft approaches the plasma sheet from the lobe, and the sudden jump in that frequency up to near the plasma frequency upon crossing the PSBL. Also, it is predicted that the frequencies of field‐aligned waves close to the plasma frequency are produced in a region where the cold electrons diffuse in from the lobe, and the large angles of growth are produced outside this region in the PSBL.
Solar wind stream structure at large heliocentric distances: Pioneer observations
Authors: Gazis P. R.
Published: February 2008

Time profiles and histograms of plasma data from Pioneers 10 and 11 are examined for the period between 1975 and 1983. During this time, Pioneer 10 traveled between a heliocentric distance of 8.7 and 30.4 AU. The velocity structure of the solar wind at these heliocentric distances is found to have one of two distinct forms: Approximately 70% of the time the solar wind has a nearly flat velocity profile. Occasionally, this flat velocity profile is accompanied by quasi‐periodic variations in density and in thermal speed consistent with the concept that the “corotating interaction regions” which are produced by the interaction of high‐ and low‐speed streams at intermediate heliocentric distances are replaced by “pressure regions” in the outer heliosphere. The remaining 30% of the time the solar wind is marked by large (50–200 km/s) long‐term (30–120 day) shifts in the average solar wind velocity.
The dynamics of weakly charged dust: Motion through Jupiter's gravitational and magnetic fields
Authors: Schaffer Les , Burns Joseph A.
Published: February 2008

We derive perturbation equations for the motion of weakly charged dust grains (q/M1→0, where q is the grain's electric charge and M1 its mass) in planetary rings, with emphasis on the Jovian ring system. Our analysis includes the effects of planetary oblateness and an arbitrarily complex magnetic field expressed in terms of spherical harmonics. The perturbation solutions are shown to agree with numerical integrations for most initial conditions, and to provide a simple means of understanding the nature of charged grain trajectories in terms of forced oscillator models. We identify “Lorentz resonances” where a frequency of the perturbing Lorentz force matches a natural orbital frequency of the grain. The resonances come in pairs, with one on either side of the synchronous orbit rs; as rs is approached, resonances are more closely spaced, and their zones become narrower. Large vertical and horizontal excursions from planar circular orbits are predicted at these locales, and this is confirmed in numerical simulations. The amplitudes of the vertical oscillations at resonance can be 1–2 orders of magnitude larger than those outside such zones. The enhanced vertical excursions mean that for shallow viewing angles the optical depth will decrease as lines of sight approach the Lorentz resonance radii. This then provides a mechanism for explaining some apparent boundaries in dusty rings, such as those of the Jovian halo and, less likely, the outer edges of the Jovian main ring and the gossamer ring. For future use, we list the low‐order resonances that lie at or near several features in the Voyager images. Theoretical ring cross sections and overall ring topology are obtained from averages of the analytical solutions over initial conditions. We find that because of the forced response to perturbations, short time solutions predict a warping of the halo out of the equatorial plane that is not seen in the available images. We then point out the significance of including a slow evolutionary process, such as plasma drag, in the dynamical model. Since the large vertical excursions produced as grains drift through Lorentz resonances apparently survive over the radial evolution time scale, the natural response will dominate the motion in the halo region, and the halo will not be appreciably warped.
Fast shocks at the edges of hot diamagnetic cavities upstream from the Earth's bow shock
Authors: Fuselier S. A. , Thomsen M. F. , Gosling J. T. , Bame S. J. , Russell C. T. , Mellott M. M.
Published: February 2008

Recently, several interesting events, described as hot expanding diamagnetic cavities, have been observed upstream from the earth's bow shock using the ISEE 1 and 2 spacecraft. It has been suggested that fast shocks may form at the edges of some of these events because of the rapid expansion of the cavities. We have examined plasma density, temperature, velocity, and total field changes across the edges of several events and find these changes to be qualitatively consistent with the presence of shocks there. The presence of flat‐topped electron distributions and occasional electron beams at and downstream from the edges provides additional evidence for shocks. Plasma wave observations also show shocklike electrostatic noise at the edges of several events. We conclude that the edges of diamagnetic cavity events are often shocks, with a range of shock strengths similar to that observed in the interplanetary medium. The range of shock strengths may be the result of different convection and/or expansion speeds of the cavities; these convection and/or expansion speeds in turn may ultimately be related to the age of the cavities.
Thermospheric dynamics during the March 22, 1979, magnetic storm: 1. Model simulations
Authors: Roble R. G. , Forbes J. M. , Marcos F. A.
Published: February 2008

The physical processes involved in the transfer of energy from the solar wind to the magnetosphere and its release associated with substorms on March 22, 1979, have been studied in detail by the Coordinated Data Analaysis Workshop 6 (CDAW 6). The information derived from the CDAW 6 study, as well as other information obtained from magnetospheric modeling, is used to prescribe the time‐dependent variations of the parameterizations for the auroral and magnetospheric convection models that are incorporated within the National Center for Atmospheric Research thermospheric general circulation model (TGCM). The period preceding the magnetic storm (March 21) was geomagnetically quiet, and the TGCM was run until a diurnally reproducible pattern was obtained. The time variations of auroral particle precipitation and enhanced magnetospheric convection on March 22 caused a considerable disturbance in the high‐latitude circulation, temperature, and composition during the storm period that began at about 1055 UT. Large‐ and medium‐scale disturbances were launched during the event that propagated to equatorial latitudes. The thermospheric response in the northern hemisphere was larger than that generated in the southern hemisphere, because the auroral oval and magnetospheric convection pattern in the northern hemisphere were in sunlight during the storm period whereas they were in darkness in the southern hemisphere. The storm response was also different in the upper and the lower thermosphere. In the upper thermosphere the winds generally followed the two‐cell pattern of magnetospheric convection with a lag of only ½ to 1 hour. In the lower thermosphere there was a pronounced asymmetry between the circulation cells on the dawnside and on the duskside of the polar cap. The circulation features in the lower thermosphere, once established, tended to persist several hours after the storm time forcings subsided. The TGCM‐calculated response to the storm is compared with predictions made by the mass spectrometer/incoherent scatter (MSIS‐83) empirical model. In a companion paper, TGCM predictions are compared with data obtained by a satellite electrostatic triaxial accelerometer system on board an orbiting satellite and with incoherent scatter radar data.
Joule heating in the mesosphere and thermosphere during the July 13, 1982, solar proton event
Authors: Roble R. G. , Emery B. A. , Killeen T. L. , Reid G. C. , Solomon S. , Garcia R. R. , Evans D. S. , Hays P. B. , Carignan G. R. , Heelis R. A. , Hanson W. B. , Winningham D. J. , Spencer N. W. , Brace L. H.
Published: February 2008

The solar proton event of July 13, 1982, produced considerable ionization in the polar cap mesosphere. Energetic solar proton fluxes were measured by the NOAA‐6 satellite. The Dynamics Explorer 2 satellite measured the low‐energy electrons, the ion drift velocity, and other atmospheric and ionospheric properties during the event. The measured solar proton flux is used in a proton degradation code to calculate the ionization rates below 100 km and the electron and ion number densities are calculated using an ionospheric D region code with the neutral atmosphere specified by a chemical/dynamic model of the middle atmosphere. Electron and ion densities above 100 km are calculated using a satellite track ionospheric code developed for the Dynamics Explorer satellite project. Electric fields at ionospheric heights map efficiently downward into the mesosphere where the Pedersen conductivity is enhanced. In the region of the measured maximum electric field (189 mV m−1 at 2215 UT near 60°N), a Joule heating rate of 1°–3°K/day is calculated betwen 70 and 80 km, exceeding the heating due to ozone absorption at noon in the summer hemisphere in that altitude range. The Joule heating rate above 90 km greatly exceeded 20°K/day. The calculated height integrated Joule heating rate above 100 km in the same region exceeded 400 ergs cm−2 s−1 and DE‐2 near 350 km measured neutral winds of nearly 1000 ms−1 and neutral gas temperatures of over 2000°K. The overall ionospheric structure calculated below the DE‐2 satellite is described.
Meridional transport of magnetic flux in the solar wind between 1 and 10 AU: A theoretical analysis
Authors: Pizzo V. J. , Goldstein B. E.
Published: February 2008

Pioneer 10 observations suggest that the mean (longitudinally averaged) solar wind azimuthal field strength, Bϕ, near the ecliptic plane falls off more rapidly with heliocentric distance than would be expected in a classic Parker expansion, showing a deficit of 10‐20% (as compared to the projected 1‐AU value) by 10 AU. Though this observational interpretation has been challenged by subsequent analyses of Voyager data, it has nevertheless stimulated efforts to explain the inferred deficit on the basis of systematic north‐south magnetic pressure gradients generated by the differential spiral wrapping of magnetic field lines in interplanetary space. We reexamine this issue from the theoretical perspective using a three‐dimensional MHD nonlinear numerical model for steady, corotating flow. For realistic solar wind parameters we find that a purely axisymmetric expansion is capable of producing sizable magnetic flux deficits only when there are substantial meridional gradients in mean flow conditions localized about the ecliptic plane near the sun. Even then the match between plausible flow states and significant mean Bϕ deficit is achieved over such a limited parameter range that it is unlikely this mechanism alone can produce deficits of the magnitude inferred from the Pioneer data. We present calculations on three‐dimensional corotating flows which demonstrate that latitudinal transport of magnetic flux by stream interactions may be an important consideration. For streams having a tilted‐dipole geometry we find that deficits in the mean Bϕ of the order of 10% or more at 10 AU are readily produced over a broad range of tilt angles and flow conditions. Both the axisymmetric and stream interaction mechanisms should operate most efficiently and be most prominent observationally in the late declining and minimum phases of the solar cycle, when the solar wind flow is systematically organized with a narrow band of slow flow about the solar equator and fast flow toward the poles. However, it is unlikely that either mechanism would be capable of producing significant deficits under the global flow conditions of the solar cycle epoch (that centered about solar maximum) to which the published Pioneer and Voyager analyses refer. It is furthermore emphasized that deficits in Bϕ do not necessarily imply equivalent transport of magnetic flux and that a substantial portion of apparent Bϕ deficits may be due to the global, nonlinear averaging by stream interactions of the large‐scale, correlated variations in flow parameters near the sun.
Solar cycle evolution of solar wind speed structure between 1973 and 1985 observed with the interplanetary scintillation method
Authors: Kojima M. , Kakinuma T.
Published: February 2008

The solar cycle evolution of solar wind speed structure was studied for the years from 1973 to 1985 on a basis of interplanetary scintillation observations using a new method for mapping solar wind speed to the source surface. The major minimum‐speed regions are distributed along a neutral line through the whole period of a solar cycle: when solar activity is low, they are distributed on the wavy neutral line along the solar equator; in the active phase they also tend to be distributed along the neutral line, which has a large latitudinal amplitude. The minimum‐speed regions tend to be distributed not only along the neutral line but also at low magnetic intensity regions and/or coronal bright regions which do not correspond to the neutral line. As the polar high‐speed regions extend equatorward around the minimum phase, the latitudinal gradient of speed increases at the boundaries of the low‐speed region, and the width of the low‐speed region decreases. One or two years before the minimum of solar activity, two localized minimum‐speed regions appear on the neutral line, and their locations are longitudinally separated by 180°.
Ideal MHD equations in terms of compressive Elsässer variables
Authors: Marsch E. , Mangeney A.
Published: February 2008

By the help of the so‐called Elsässer variables, if adopted to the situation of a compressible plasma, the equations of ideal magnetohydrodynamics can be rewritten in a new form. This appears to be particularly suitable to describe the situation of compressive MHD turbulence. After having derived the basic equations of motion, the conservation equations for energy and cross‐helicity are rephrased in terms of Elsässer variables. The sinks and sources of cross‐helicity associated with the compressibility of the plasma are elucidated. A standard normal mode analysis, performed in terms of Elsässer variables, emphasizes their usefulness in treating hydrodynamic waves. In the conclusions some possible future, more sophisticated applications are outlined.
An extended study of the low‐latitude boundary layer on the dawn and dusk flanks of the magnetosphere
Authors: Mitchell D. G. , Kutchko F. , Williams D. J. , Eastman T. E. , Frank L. A. , Russell C. T.
Published: February 2008

We present a study of the low‐latitude boundary layer (LLBL) using ISEE 1 energetic particle, plasma, and magnetic field data obtained during numerous traversals of the LLBL that occurred on 66 ISEE 1 passes through the magnetospheric flank LLBL region. We use energetic particle distributions to determine dawn and dusk LLBL behavior and topology for varying orientations of the magnetosheath and/or interplanetary magnetic field (M/IMF), for different local times, and for changing levels of geomagnetic activity (Kp). This study corroborates and extends the earlier work of Williams et al. (1985) who presented a detailed study of two (dusk and dawn) ISEE 1 passes through the LLBL region for the case of northward M/IMF. We find that the dawn and dusk LLBL are on closed geomagnetic field lines for northward M/IMF but are on a combination of closed and open field lines for a southward M/IMF. The energetic particle distributions show that cases of reverse‐draped field lines in the LLBL are consistent with an open field line topology. In addition, we find that the LLBL is thicker (thinner) for northward (southward) M/IMF and becomes thicker with increasing distance from the subsolar point. LLBL electric fields nominally are in the few (3–5) millivolts per meter range and display an apparent maximum value of ∼10 mV/m. These electric fields capture magnetospherically drifting particles as they approach the LLBL and propel them tailward. In this way, the plasma sheet is the dominant source of energetic (≳10 keV) particles in the LLBL while the magnetosheath appears to be the dominant source for lower‐energy (≲10 keV) LLBL particles.
An MHD model for the complete open magnetotail boundary
Authors: Siscoe G. L. , Sanchez E.
Published: February 2008

In the Coroniti‐Kennel MHD model for the open magnetotail boundary, a standing slow‐mode expansion fan turns the magnetosheath flow into the plasma mantle and converts the magnetosheath field into the tail lobe field. The expansion fan ‐ plasma mantle is also called the high‐latitude boundary layer. The Coroniti‐Kennel model applies to the noon‐midnight meridian plane and the case of a purely southward interplanetary magnetic field (IMF). This paper (1) quantitatively tests and verifies the model, and (2) describes how the relatively thick and rapidly propagating high‐latitude expansion fan automatically thins and slows at lower latitudes. It tapers down to a thin, non‐propagating tangential discontinuity representing the low‐latitude boundary layer on closed field lines. Thus the model shows how the dawn‐to‐dusk current sheet that bisects the tail horizontally can return to dawn over the tail's back uninterruptedly across a seamless juncture between the high‐ and low‐latitude boundary layers. Other important properties emerge from the complete boundary model. A rotational discontinuity preconditions the field to pass through the slow mode wave. It induces a secondary circulation in the cross‐sectional plane. This circulation sweeps plasma sheet plasma dawnward and duskward away from the tail's midplane and draws it up the tail's flanks, possibly accounting for the plasma sheet's butterfly‐shaped cross section. The along‐the‐tail currents flowing on the rotational discontinuity have the same IMF By dependence as the cusp currents, which they might feed. The By distortion of the secondary circulation tends to rotate the plasma sheet about the tail axis to face the IMF.
The plasma environment during particle beam injection into space plasmas: 1. Electron beams
Authors: Pritchett P. L. , Winglee R. M.
Published: February 2008

A new isolated‐system, two‐dimensional electrostatic simulation model is used to investigate the plasma environment in the vicinity of a spacecraft during the injection of electron beams. The propagation of the electron beam and the plasma response to the beam injection are found to vary dramatically depending on the ratio of the ambient plasma density to the beam density. When the ambient plasma density is low, most of the beam electrons are drawn back into the spacecraft by the electric field associated with the charging of the spacecraft. Return currents are associated with field‐aligned flow of the ambient electrons and result in regions of charge imbalance away from the spacecraft. The resulting space‐charge fields produce strong perpendicular heating of the ions on a time scale of the ion plasma period, resulting in conical distributions. As the ambient plasma density is increased, the response time of the plasma to the electric fields associated with the beam injection is decreased. The beam is then increasingly neutralized, and the interaction of the beam with the spacecraft is correspondingly reduced. The fraction of the beam particles that propagate away from the vicinity of the spacecraft is increased. When the ambient plasma density is much larger than the beam density, the beam is able to propagate freely into the plasma. Coherent wave structures are produced in the beam near the spacecraft, while further downstream the beam becomes increasingly turbulent.
Interpretation of electrostatic noise observed by Voyager 1 in Titan's wake
Authors: Ma T. Z. , Gurnett D. A. , Goertz C. K.
Published: February 2008

During the Voyager 1 spacecraft flyby of Titan on November 12, 1980, an intense band of low‐frequency electric field noise was observed during the inbound wake crossing. This analysis shows that the noise is generated by a beam‐plasma interaction between the corotating magnetosphere of Saturn and newly created ions from the atmosphere of Titan. The analysis is based on plasma and wave measurements from Voyager 1 and reasonable assumptions. The results agree quite well with the observation. The analysis shows that the instability only occurs where the density of the corotational ions is comparable to the density of ions originating from Titan's atmosphere. The growth rate is high enough to generate the observed noise, and the calculated and observed frequency ranges are in good agreement. The asymmetry between the intensities in the inbound and outbound wake are explained by a simple model for the trajectories of ions from Titan's atmosphere.
Rocket‐borne spectroscopic measurements in the ultraviolet aurora: The Lyman‐Birge‐Hopfield bands
Authors: Eastes Richard W. , Sharp William E.
Published: February 2008

Ultraviolet emissions from Earth's aurora were observed at wavelengths between 1675 and 2075 Å by a sounding rocket payload launched at Churchill, Canada, on March 28, 1980. The emissions from the Lyman‐Birge‐Hopfield (LBH) and Vegard‐Kaplan (VK) bands of N2 were observed and analyzed to determine the relative populations of the υ′ = 0–6 levels of the a¹πg state and the υ′ = 4, 6, 7, and 8 levels of the A³Σg+ state respectively. The relative population of higher vibrational levels of the A³Σg+ state are consistant with direct excitation and cascade. The relative populations of the vibrational levels of the a¹πg state peak at υ′ = 2. Such a distribution has not been observed previously in the aurora.
ISEE 3 observations of low‐energy proton bidirectional events and their relation to isolated interplanetary magnetic structures
Authors: Marsden R. G. , Sanderson T. R. , Tranquille C. , Wenzel K. ‐P. , Smith E. J.
Published: February 2008

Bidirectional, field‐aligned flows of low‐energy particles in interplanetary space have been proposed as a possible signature of large‐scale, looplike magnetic structures. We present the results of a survey of bidirectional anisotropy observations using data from the low‐energy proton and magnetometer experiments on board ISEE 3 covering a 45‐month period corresponding to the last solar maximum. During this period, 66 bidirectional flow (BDF) events have been identified, of which 48 could positively be associated with isolated magnetic field structures. We suggest that these structures are an interplanetary manifestation of coronal mass ejection (CME) events, and are able to group the BDF events into five classes according to the field signature of the related magnetic structure and the association or otherwise with an interplanetary shock. We conclude that the BDF events are an interplanetary signature not only of energetic, flare‐associated transients but also low energy, nonflare CMEs. From a comparison of anisotropy signatures at 35 and 620 keV we conclude that our observations are most consistent with the transient magnetic structures being detached bubbles comprising closed loops or tightly wound cylindrical helices rather than extended tonguelike loops attached to the Sun at the time of observation.
Electron/ion whistler instabilities and magnetic noise bursts
Authors: Akimoto K. , Gary S. Peter , Omidi N.
Published: February 2008

Two whistler instabilities are investigated by means of the linear Vlasov dispersion equation. They are called the electron/ion parallel and oblique whistler instabilities and are driven by electron/ion relative drifts along the magnetic field. We demonstrate that the enhanced fluctuations from these instabilities can explain several properties of magnetic noise bursts in and near the plasma sheet in the presence of ion beams and/or field‐aligned currents. At sufficiently high plasma β these instabilities may affect the current system in the magnetotail.
Critical angles of incidence for transmission of magnetohydrodynamic waves across shock surfaces
Authors: Whang Y. C. , Wei Fengsi , Du Heng
Published: February 2008

When an MHD wave impinges on a fast shock from its upstream side, in general it may excite six diverging waves on the downstream side when the angle of incidence, the angle between the normal direction of the incident wave and the shock normal, is less than a critical angle. The boundary conditions across the shock surface determine the perturbations excited on the downstream side of the shock. We introduce a wave function approach to study the interaction of an MHD wave with an fast shock. When the angles of incidence are greater than certain critical values, less than six diverging waves can be excited; the flow becomes unstable on the downstream side of the shock. The existence of critical angles of incidence infers that the conclusion obtained on the stability of fast MHD shock with respect to normal incidence of MHD disturbances does not hold true for oblique incidence of disturbances at large angles of incidence. We investigate the critical angles of incidence, which are calculated as functions of the mode of the incident wave and three upstream conditions of the shock: the fast Mach number, the plasma β value and the shock angle. Our numerical results show that the critical angles of incidence are of the order of 60°.
Low‐frequency electrostatic noise due to velocity shear instabilities in the regions of magnetospheric flow boundaries
Authors: Lakhina G. S.
Published: February 2008

The velocity shear of an ion beam interacting with a background plasma is shown to excite low‐frequency electrostatic Kelvin‐Helmholtz instabilities and ion cyclotron instability propagating nearly transverse to the magnetic field. In the plasma sheet boundary layer region, the noise generated by velocity shear instabilities is broadband with frequencies from 10−4 Hz to 4 Hz and the transverse wavelengths ranging from 30 km to 2400 km. The low‐frequency noise can efficiently scatter electrons and the beam ions and is expected to affect significantly the stability of the plasma sheet against ion tearing mode instability. In the polar cusp regions, the velocity shear instabilities can generate a low‐frequency broadband electrostatic noise from 2 Hz to 35 Hz provided the temperature of background electrons is ∼100 eV.
Large‐scale fluctuations between 13 AU and 25 AU and their effects on cosmic rays
Authors: Burlaga L. F. , Ness N. F. , McDonald F. B.
Published: February 2008

Measurements made by Voyagers 1 and 2 between 13 AU and 25 AU show that large‐scale fluctuations in the strength of the magnetic field were quasiperiodic and had large amplitudes in 1984, when large recurrent merged interaction regions were present, and the fluctuations were aperiodic and had small amplitudes in 1985. Both near the solar equatorial plane and at latitudes between 22° and 27° the spectra of low‐frequency fluctuations (periods between ∼5 days and 26 days) were ƒ−2 during 1984 and significantly flatter than ƒ−2 during 1985. Near the equator, the power at 26 days during 1984 was an order of magnitude greater than that during 1985. The spectrum of intermediate frequency fluctuations (periods from several hours to approximately 5 days) near the equator was ƒ−2 for both 1984 and 1985, but the spectrum at latitudes near 24° was ƒ−5/3 for both 1984 and 1985. The magnetic field strength distribution was skewed owing to the presence of an exponential tail, which was larger in 1984 than in 1985. During 1984 the tail was larger near the equator than near 23°, but during 1985 there was no latitudinal gradient in the tail. The ratio of magnetic field intensity near 23° to that near the ecliptic was approximately 0.8 in 1984, and the ratio of the magnetic field strength measured near 25° to that measured near the solar equatorial plane was 0.7 in 1985. The ratio of the magnetic field strength near the equatorial plane in 1985 to that in 1984 was 0.8. The model of Burlaga et al. (1985a) provides good first approximations to the intensity profiles of cosmic rays > 75 MeV in 1984 and 1985.
Cusp width and Bz: Observations and a conceptual model
Authors: Newell Patrick T. , Meng Ching‐I.
Published: February 2008

Because a southward turning of the interplanetary magnetic field Bz enhances day side merging, leading to an equatorward motion of the equatorward edge of the cusp, one might suppose that the cusp latitudinal width would be larger for a southward Bz condition. The Defense Meteorological Satellite Program F7 observations confirm two previous case studies showing that the cusp low‐altitude latitudinal extent is actually narrower when Bz is southward than when Bz is northward. The earlier studies dealt with intense geomagnetic storms; examples presented here show that it is routine to observe a narrowing of the cusp width following a southward turning of Bz. In addition, we present a simple conceptual model that shows how enhanced tailward convection of the newly opened field lines during periods of southward Bz can account for the observed narrowness.
Satellites of Uranus: Disk‐integrated photometry from Voyager imaging observations
Authors: Veverka J. , Thomas P. , Helfenstein P. , Brown R. H. , Johnson T. V.
Published: February 2008

Voyager 2 imaging observations over a wide range of phase angles are used to determine the fundamental photometric parameters for the five largest satellites of Uranus. Over the spectral range covered by Voyager cameras (approximately 0.35–0.60 µm) the disk‐averaged colors are moderately gray (no redder than the spectrum of Saturn's satellite Phoebe). Geometric albedos range from 0.19 for Umbriel to 0.40 for Ariel. Phase coefficients determined generally between phase angles of 10° and 60° vary from 0.021 magnitude per degree (mag/deg) for Ariel to 0.028 mag/deg for Miranda. Phase integrals lie in the range of 0.5–0.65. We estimate the following Bond albedos: about 0.1 for Umbriel and about 0.2 for the other satellites. The most complete phase angle coverage occurred in the case of Titania, for which data are available between 0.8° and 152.6°. The Titania phase curve shows a pronounced opposition surge, which can be modeled in detail by using Hapke's theory. A regolith texture more porous than that of the lunar surface is indicated. Observations at high phase angles (>139°) can be used to constrain the large‐scale roughness of the satellites. For Titania, Umbriel, and Oberon we find values of (the large‐scale roughness parameter in Hapke's model) comparable to those of the Moon. For Ariel a significantly rougher surface is suggested.
The helium abundance of Uranus from Voyager measurements
Authors: Conrath B. , Gautier D. , Hanel R. , Lindal G. , Marten A.
Published: February 2008

Voyager radio occultation and infrared spectroscopy measurements are combined to infer a helium mole fraction in the upper troposphere of Uranus of 0.152 ± 0.033; the corresponding mass fraction is Y = 0.262 ± 0.048. This value is in agreement with recent estimates of the solar helium abundance, suggesting that helium differentiation has not occurred on Uranus. Comparisons with values previously obtained for Jupiter and Saturn imply that migration of helium toward the core began long ago on Saturn and may also have recently begun on Jupiter. The protosolar helium abundance inferred from the Uranus measurements and from recent solar evolutionary models is used along with an assumed primordial helium mass fraction of 0.23–0.24 to estimate a 3–4% enrichment of helium in the interstellar medium between the big bang and the origin of the solar system. The result is in agreement with galactic chemical evolution models which include a substantial decrease in deuterium during the evolutionary process.
Vertical temperature gradients on Uranus: Implications for layered convection
Authors: Gierasch Peter J. , Conrath Barney J.
Published: February 2008

Analyses of Voyager infrared spectroscopy and radio occultation measurements confirm ground‐based observations that the deep lapse rate of Uranus exceeds that for an adiabat of molecular hydrogen in thermodynamic equilibrium. However, in approximately the same region of the atmosphere, Voyager infrared and ground‐based observations also indicate that the ratio of ortho to para hydrogen is near the equilibrium value. These two sets of observations can be reconciled by postulating the existence of rapid convective overturning within layers which are thin in comparison to a pressure scale height. Two forms of layered convection are examined. In one case, deposition of kinetic energy results in thin, rapidly overturning layers. Possible kinetic energy sources include breaking waves and local instabilities. In the second case considered, molecular weight discontinuities due to methane stratification stabilize the interfaces of thin layers. It is argued that both forms may be important in the convective portion of the Uranus atmosphere.
The sources of Uranus' Dominant nightside radio emissions
Authors: Kaiser M. L. , Desch M. D. , Curtis S. A.
Published: February 2008

We examine the broad bandwidth radio emission detected by Voyager 2 over the nightside of Uranus. We conclude that the source location of the smooth component is consistent with emission originating near the electron gyrofrequency from a small set of field lines whose foot points lie near the Uranomagnetic southern (dark) pole. The source centroid is at L = 11.5, and extends in latitude between L ≃ 8 and L ≃ 25. This deduced source region is primarily on closed field lines that pass through the outer radiation belt and have their opposite foot points near the Uranomagnetic northern pole near the present epoch terminator. The source location of the bursty component is less well defined but is consistent with the set of open field lines which map down to the region surrounding the planet's south magnetic dipole tip.
Energetic ion and electron phase space densities in the magnetosphere of Uranus
Authors: Cheng Andrew F. , Krimigis S. M. , Mauk B. H. , Keath E. P. , Maclennan C. G. , Lanzerotti L. J. , Paonessa M. T. , Armstrong T. P.
Published: February 2008

Voyager 2 low‐energy charged particle (LECP) data from the magnetosphere of Uranus have been analyzed to obtain proton and electron phase space density profiles. The Uranus proton profiles show an approximately exponential decline with decreasing radius for L ≲ 9 in a relatively dense thermal plasma region with intense plasma wave activity. An analogy with the magnetospheres of Earth, Jupiter, and Saturn suggests a plasmasphere at Uranus. The ion flux tube content in the Uranian radiation belt is less than that in the other three cases. Proton and electron profiles ≲ 100 MeV/G show evidence of an absorption signature near the minimum L of Ariel. Proton profiles ≳ 200 MeV/G show evidence of injection events, suggesting substorm processes at Uranus. Electron profiles ≳ 100 MeV/G show clear minima at the minimum L values of Miranda, Ariel, and perhaps Umbriel, separated by broad maxima. These profiles are interpreted as implying local injection of energetic electrons. Distributed loss mechanisms at Uranus include satellite sweeping, wave‐particle interactions, and charge exchange of protons with an extended hydrogen corona. If the net loss rates of protons and electrons are estimated by calculated satellite sweeping rates, the radial diffusion coefficient is estimated near L = 7.5 to be 10−7 s−1 to 10−6 s−1. If the diffusion coefficient is ≲ 10−6 s−1, the power available from inward radial diffusion of energetic protons and electrons is ≲ 5 × 108 W and is less than the 4 × 1010 W needed to maintain the Uranian ultraviolet aurora. Local injection of electrons above 100 MeV/G between the minimum L values of Miranda and Ariel also requires a power of at least 8 × 107. The magnetosphere of Uranus is the third planetary magnetosphere for which evidence of substorm activity has been adduced, after those of Earth and Mercury.
Magnetic field and current structures in the magnetosphere of Uranus
Authors: Voigt G. ‐H. , Behannon K. W. , Ness N. F.
Published: February 2008

We compare Voyager 2 magnetic field and plasma data with theoretical model calculations for the magnetosphere of Uranus in order to derive a global picture from the quite limited set of measurements. In two dimensions we use a tilt‐dependent linear MHD equilibrium model for the entire magnetosphere to calculate the plasma parameter β = 8πP/B² and the resulting B field in the tail plasma sheet. The three‐dimensional model satisfies the MHD equilibrium equations in an approximate fashion; it is used to calculate the large‐scale magnetic field structure, the tilt‐dependent shape and position of the Uranian magnetotail plasma sheet, and the influence of the interplanetary magnetic field (IMF). The main results of the model calculations are the following: (1) The actual solar wind pressure at Uranus was found to be about a factor of 5 higher than a calculated solar wind pressure derived from an adiabatically expanding quiet solar wind. This means that the Uranian magnetosphere must have experienced a major magnetic storm prior to the Voyager encounter. (2) The previous conjecture is supported by the fact that the asymptotic plasma beta parameter in the neutral sheet, β0 = β(|x| ≫ 1), relaxes from β0 = 37 to β0 = 14 between the first and second neutral sheet crossings. (3) The excellent agreement between measured and modeled tail lobe magnetic field values suggests that during the time period of the Voyager encounter the Uranian tail plasma sheet, measured in units of planetary radii, was thicker and less stretched than the average Earth's tail plasma sheet. (4) Despite sizable temporal variations suggested by the data and the model, the Uranian magnetosphere seems to reach the state of quasi‐static, i.e., slowly time‐dependent, MHD equilibrium. (5) In relation to a given IMF orientation the magnetosphere changes periodically every 8.62 hours (the Uranian day is 17.24 hours) from an “open” to a “closed” configuration and back. Thus the convection process, driven by the direct influence of the IMF, is periodically interrupted by the planet's rotation, such that the plasma sheet might not experience its full stretching. Consequently, the Uranian magnetosphere might not experience convectively driven, Earth‐type substorms. On the other hand, the possibility of substorms cannot be ruled out either.
Spatial extent of the plasma injection region in the cusp‐magnetosheath interface
Authors: Menietti J. D. , Burch J. L.
Published: February 2008

Using data from the high‐altitude plasma instrument on board the DE 1 spacecraft, we have estimated the perpendicular (to the magnetic field) extent of the region of ion injection into the cusp by measuring the energy dispersion of the ion V‐shaped signatures seen on energy‐time spectrograms. Three different passes were chosen based on the clarity of the V‐shaped ion signatures. These studies indicate that the injection region has a dimension perpendicular to the magnetic field in the range 1000 km < D < 6000 km at a radial distance of ∼ 10 RE, in good agreement with reported diameters of flux transfer events (FTEs). Similar measurement of the energy dispersion of an electron V‐shaped signature observed on one pass has yielded a parallel extent of the plasma injection region in the range 100 km < D∥ < 350 km. Our results lead to the suggestion that the plasma within an FTE travels down the field line as the field line convects antisunward, and may subsequently be observed in the polar cusp as a V‐shaped signature.
A comprehensive model of ion diffusion and charge exchange in the cold io torus
Authors: Barbosa D. D. , Moreno M. A.
Published: February 2008

An analytic model of lossy radial diffusion of ions in the cold Io torus is developed. The model includes ion‐neutral charge exchange reactions with extended atomic and molecular clouds of S, O, SO, and SO2 which act as combined sources/sinks of ions that couple the species and their various charge states. The ion‐molecule reactions are crucial to the analysis; in particular, the exothermic reaction S+ + SO → S + SO+ followed by rapid dissociative recombination for the resultant SO+ ion is the only viable way of removing S+ from the cold torus in order that its density conform with the many independent measurements of it. Model calculations indicate optimum values for the neutral densities are [S] = 7 cm−3, [O] = 30 cm−3, [SO] = 10 cm−3, and [SO2] = 20 cm−3. Such a neutral cloud environment yields an excellent correspondence of hot torus ion partitioning models with that of the cold torus through lossy radial diffusion. The preferred model of the analysis for the hot torus composition is [S+] = 220 cm−3, [S2+] = 340 cm−3, [S3+] = 40 cm−3, [O+] = 800 cm−3, and [O2+] = 90 cm−3 at Voyager 1 epoch. These numbers imply not only that the plasma torus is SO2‐based but also that dissociation of the molecular SO2 cloud is the primary source of all other neutrals that are distributed throughout the Io torus region. A unique feature of the model is the production of a boundary layer between the cold and hot tori where cold S+ ions mix with hot plasma torus ions in a manner that reproduces the ribbonlike plasma structure described by Trauger (1984).
On the temporal fluctuations of pulsating auroral luminosity
Authors: Yamamoto Tatsundo
Published: February 2008

From a study of all‐sky TV records, it is shown that the luminosity fluctuations of pulsating auroras can be understood in terms of a series of pulses with rapid on‐off switchings in burstlike fashion and that the widths of successive pulses (pulsation “on” times) are fairly constant. This is common even when luminosity fluctuations consist of complex‐irregular variations, in contrast to the pulsation “off” time that is significantly variable. Complex‐irregular variations are found to be due to simultaneous appearance of more pulsating patches that exhibit movements eastward and westward over the site, and each of the patches shows primarily isolated luminosity pulses. Several examples are presented and described in detail. The pulsation “on” time is found to statistically concentrates in two small ranges (0.2–0.5 s for flash including 3‐Hz modulation and 2–6 s for pure‐expansion and streaming‐propagating modes), in spite of variations in appearance. On the other hand, the pulsation “off” time is widely scattered over a range from nearly zero to longer than 1 min. A natural consequence of these observations is that the classical concept of “period” does not mean much and the luminosity fluctuations should be treated as a series of individual isolated pulses where the pulsation “on” time is the most essential quantity. These characteristics are briefly discussed in relation to VLF/ELF wave‐particle interactions in the magnetosphere. Then a new interpretation of the nonlinear relaxation oscillation model is proposed, where the propagation effect of VLF/ELF waves in low energy plasma irregularities near the magnetospheric equatorial plane plays an essential role to produce rapid on‐off switchings of precipitating energetic electron fluxes. Both electromagnetic and electrostatic waves are possibly related to the precipitation pulsations.
Bursts of Pc 1‐2 near the ionospheric footprint of the cusp and their relationship to flux transfer events
Authors: Arnoldy Roger L. , Engebretson M. J. , Cahill L. J.
Published: February 2008

Ground‐based induction antennas at high latitudes measure a magnetic signature which can be modeled as the ground signature of a moving, large vortex of ionospheric Hall current. This Hall current is believed to be generated by the field‐aligned current in a helical flux tube that has been reconnected: a flux transfer event (FTE). These possible FTEs are seen predominantly by ground stations near the dayside cusp, although similar signatures can be found near the plasmapause and in the polar cap. A significant result obtained from the induction antennas is that for about 60% of the cusp FTEs there are coincident Pc 1–2 pulsations. If the FTE‐coincident Pc 1–2 pulsations are produced by ion cyclotron resonance, the energy of the streaming hot ion population can be estimated and is found to be consistent with midaltitude spacecraft measurements and sounding rocket data. Pc 1–2 pulsations measured near the plasmapause which occur simultaneously with the cusp Pc 1–2 coincident with FTEs pose some questions about the generation of the Pc 1–2 and whether the FTE signatures may not simply be the result of solar wind pressure impulses on the magnetopause.
Structure of VLF whistler mode sideband waves in the magnetosphere
Authors: Sá L. A. D. , Helliwell R. A.
Published: February 2008

An accurate determination is made of the sideband structure of ducted VLF whistler mode waves transmitted from Siple Station and observed at Lake Mistissini, Quebec. Single‐ and double‐frequency experiments are described. It is shown that sideband spectra can be explained by nonlinear interactions between two or more lines in the magnetosphere, the line intensities required for such interactions being low. If one of the lines is a transmitted carrier, the other line can be as much as 40 dB lower in amplitude, implying that power line radiation (PLR) can be an important factor in sideband generation. It is shown that “single‐line” sidebands are due to interactions between the input wave and harmonics of 60 Hz present within the duct (assumed to be PLR), and that double‐line spectra are affected by their position relative to the same 60‐Hz system of lines. A study of double‐line spectra as a function of line separation and line amplitude ratio is made.