bluuebunny/crossref_metadata_2025 huggingface dataset:
satellite tori at Saturn
Authors: Richardson John D. , Eviatar A. , Siscoe G. L.
Published: February 2008

The inner satellites of Saturn are icy bodies imbedded in a plasma environment in which they are continuously bombarded by energetic ions, corotating plasma, and solar radiation. Laboratory sputtering experiments indicate that this should result in the injection of substantial amounts of neutral H, H2, OH, H2O, and O2 into the magnetosphere. We model the atomic processes affecting these neutrals and the neutrals and ions formed from them and calculate the steady state neutral and ion densities expected in the plasma tori of Enceladus, Dione‐Tethys, and Rhea. Comparison with observations shows that recombination can limit the Enceladus and Dione‐Tethys tori to the observed densities but that transport rates of at least 4×10−8 Rs²/s are required to limit torus densities at Rhea to the observed values.
Low‐latitude aurorae and storm time current systems
Authors: Tinsley B. A. , Rohrbaugh R. , Rassoul H. , Sahai Y. , Teixeira N. R. , Slater D.
Published: February 2008

An analysis is made of the vibrational development shown in N2+ 1 N (first negative) emissions in low‐latitude aurorae during the last half century. The data imply collisional excitation by energetic heavy particles (oxygen, helium, or hydrogen ions or neutrals) of velocity of the order of 107 cm s−1, and there may be additional pumping in some cases due to solar photons and low‐energy electrons. When low‐energy electrons are present as large fluxes, they produce bright red (O I 6300 Å) emission, with weaker emission from N I 5200 Å and other features of excitation potential only a few electron volts. From about 40° dip latitude to the auroral zone the decrease of N2+ 1 N vibrational development with increasing latitude is consistent with satellite observations of the increase with increasing latitude of the mean energy of precipitating O+ fluxes and the increasing fraction of H+ in the ion precipitation. From the equator to about 40° dip latitude the intensity and the latitudinal gradients of the emissions are consistent with energetic neutral precipitation as the primary source. The intensity variations of low‐latitude displays show large‐amplitude changes on a time scale of 0.1 hour, which are closely related to the magnetic signatures of the storm time current system. The brightest emissions occur when |Dst| is large and specifically when positive H excursions take place, with first positive then negative D excursions, as recorded on magnetograms from nearby observatories. The positive ΔH at low latitudes is accompanied by the negative ΔH of large substorms as recorded at higher latitude observatories. The 0.1‐hour time scale of these fluctuations implies that the ring current is incompletely shielded, allowing ionospheric currents and the injection of the ring current to unusually low latitudes.
Model predictions of subionospheric VLF signal perturbations resulting from localized, electron precipitation‐induced ionization enhancement regions
Authors: Tolstoy A. , Rosenberg T. J. , Inan U. S. , Carpenter D. L.
Published: February 2008

Whistler‐induced precipitation of energetic electrons produces transient ionospheric conductivity variations that perturb the amplitude and phase of VLF signals propagating in the earth‐ionosphere waveguide (i.e., Trimpi events). This study uses a waveguide mode theoretic propagation model to predict the effect of localized ionization enhancement (IE) regions on nighttime long‐path VLF signals from the U.S. Navy navigational transmitters NSS, NAA, NLK and NPM to Antarctic receiving stations, principally the U.S. station at Palmer. These predictions are then compared with the observed signal behavior in Trimpi events from which inferences can be drawn about an IE region's approximate location, size, and particle characteristics. The propagation model considers multiple modes, variable ground conductivity, and mode conversion. IE regions of variable length (typically 50–150 km) and distance from the receiver (up to 600 km) were considered. The conductivity change within an IE region was modeled by exponential electron density profiles modified by precipitated monoenergetic fluxes of 50‐ to 150‐keV electrons. Our calculations show that some signal paths are more sensitive than others to perturbation by IE regions and that the size of an IE region is less important than its distance from the receiver. For a specified flux, higher‐energy electrons produce larger effects than lower energy electrons; for a specified energy, larger fluxes will produce larger signal changes. For the particular case of 150‐keV electrons precipitating into an ionosphere with VLF reflection height of 87 km, the received signal was largely insensitive to increasing the electron flux above ∼10−5 erg cm−2 s−1. This flux is equivalent to ∼3×10−3 erg cm−2 s−1 for E > 40 keV and an exponential energy spectrum with e‐folding energy in the range 40–100 keV. The measured electron fluxes precipitated by lightning are of the same order. Higher saturation flux levels would apply in situations where the normal VLF reflection height was lower over that portion of a path where the whistler‐induced precipitation was occurring. This study also examines the effects of transmitter‐induced electron precipitation on the VLF signals. Such precipitation is known to occur and, as shown here, may on occasion have important effects on certain signals.
Structural changes in the currents and fields of the equatorial electrojet due to zonal and meridional winds
Authors: Anandarao B. G. , Raghavarao R.
Published: February 2008

We describe calculations of the effects of zonal and meridional winds on the equatorial electrojet vis‐a‐vis the observed features by solving the electrodynamic equations. Vertical shears in the zonal winds can cause significant changes in the electrojet beyond about 2° latitude. At the magnetic eqator these changes are less than about 10%. The width (latitudinal) and the thickness (altitudinal) of the jet are changed by as much as 100% by these shears. When the width is decreased, the intensity of the current densities in meridional plane is increased. Steady (not varying with altitude) zonal winds are ineffective. These results are discussed in comparison with those of earlier authors. Meridional winds cause substantial cross‐equatorial currents leading to considerable asymmetry in the distribution of the electrojet current density as inferred by the ground‐based and rocket‐borne magnetometers. The electrojet center is shifted to about 0.5° south of the equator by a steady northward wind of speed 100 m/s. The deviations in the magnitude of the electrojet current produced by the meridional winds are significant only beyond ±2° off the equator. Structures in the vertical profiles of the eastward current density observed by rocket‐borne magnetometers a few degrees off the equator can be accounted for by the zonal wind shears.
On calculating ionospheric conductances from the flux and energy of precipitating electrons
Authors: Robinson R. M. , Vondrak R. R. , Miller K. , Dabbs T. , Hardy D.
Published: February 2008

Auroral zone conductances can be estimated from the energy flux and average energy of precipitating electrons. These estimates are based on the assumption that the conductances produced by the electrons are very similar to those produced by electrons with Maxwellian energy distributions having the same energy flux and average energy. There has been some confusion in the application of this method because for a Maxwellian the average energy is twice the characteristic energy or temperature. We present revised expressions that relate height‐integrated Hall and Pedersen conductance to the flux and average energy of a Maxwellian. We show that the accuracy of this method depends on the minimum and maximum energy within which the distribution is integrated to get the energy flux and average energy. We also confirm that the conductances produced by some of the more common auroral spectral distributions are similar to those produced by a Maxwellian with the same average energy and energy flux. The application of these results is demonstrated using precipitating electron measurements made by the Hilat satellite during a pass over Greenland.
Finite‐Larmor‐radius effect on field‐aligned currents in hydromagnetic waves
Authors: Cao F. , Kan J. R.
Published: February 2008

Within the MHD approximation, the Alfvén mode is the only hydromagnetic wave mode in which the field‐aligned current can be nonzero. By means of the two‐fluid equations, it is shown that the finite‐Larmor‐radius effect is responsible for the field‐aligned currents in the fast and slow modes and for the compressibility in the Alfvén mode. These results are useful in understanding the time‐dependent field‐aligned current sheets associated with auroral arcs with thickness down to the ion gyroradius scale.
Slow shocks and their transition to fast shocks in the inner solar wind
Authors: Whang Y. C.
Published: February 2008

The jump conditions of MHD shocks may be directly calculated as functions of three upstream conditions: the shock Alfven number based on the normal component of the relative shock speed, the shock angle, and the plasma β value. The shock Alfven number is less than 1 for a slow shock and greater than 1 for a fast shock. A traveling, forward shock can be a slow shock in coronal space, where the Alfven speed is of the order of 1000 km/s. The surface of a forward slow shock has a bow‐shaped geometry with its nose facing toward the sun. The decrease in the Alfven speed at increasing heliocentric distance causes the shock Alfven number of a forward slow shock to become greater than 1, and the shock eventually evolves from a slow shock into a fast shock. During the transition the shock system consists of a slow shock, a fast shock, and a rotational discontinuity. They intersect along a closed transition line. As the system moves outward from the sun, the area enclosed by the transition line expands, the fast shock grows stronger, and the slow shock becomes weaker. Eventually, the slow shock diminishes, and the entire shock system evolves into a forward fast shock.
Control of the ionospheric conductivities on large‐scale Birkeland current intensities under geomagnetic quiet conditions
Authors: Fujii Ryoichi , Iijima Takesi
Published: February 2008

In order to investigate the characteristics of generation mechanisms of large‐scale Birkeland currents from the standpoint of relative importance of a voltage source and a current source, we have determined quantitatively the relationships between large‐scale Birkeland current intensities and ionospheric conductivities under geomagnetic quiet conditions. The large‐scale Birkeland currents are the well‐defined region 1 and region 2 systems that were determined from transverse magnetic disturbances acquired with the Magsat satellite over the entire polar regions except for the midnight sector. The ionospheric conductivities were determined by solar EUV ionization. The principal results include the following: (1) The region 1 and region 2 systems exhibit different dependence on ionospheric conductivities. The region 1 current intensities are proportional linearly to the conductivities at all MLTs. The region 2 currents also show a dependence on ionospheric conductivities, but they have in general poorer correlations at all MLTs than the region 1 currents have. (2) The correlations with conductivities are improved by using the region 1 current density and the region 2 current latitudinal width. (3) The degree of the coupling between the region 1 system and the region 2 system seems to have no correlation with conductivities. (4) The above‐mentioned characteristics are common both in the northern and southern polar regions. These findings suggest that the large‐scale region 1 system is primarily driven by voltage generators in the magnetosphere. On the other hand, the region 2 system seems to be driven by a combination of voltage and current generators.
A critical comparison of ionospheric depletion chemicals
Authors: Bernhardt P. A.
Published: February 2008

Six chemicals, H2, H2O, CO2, SF6, CF3Br, and Ni(CO)4, are considered as ionospheric modification agents. Each of these species reacts in the F region to produce localized plasma depletions. The first three interact with O+ and yield polyatomic ions which dissociatively recombine with electrons to give neutrals. The last three dissociatively attach electrons to produce heavy negative ions which become mutually neutralized by reactions with O+. The effectiveness of these chemicals depends on the amount which goes into the vapor state upon release. Thermodynamic calculations show that H2O has the lowest vapor yield of about 20% from a heated, pressurized tank. Over 60% of the other substances should be vented in gaseous form. Based on estimates of plasma density reduction and airglow stimulation, nickel carbonyl is the most efficient of the six species for modifying the nighttime ionosphere. During the daytime, CF3Br and SF6 provide the largest depletions.
MSIS‐86 Thermospheric Model
Authors: Hedin Alan E.
Published: February 2008

The MSIS‐86 empirical model of thermospheric temperature, density and composition uses new temperature and composition data from the Dynamics Explorer satellite to improve the representation of polar region morphology over that in the MSIS‐83 model. Terms were added or changed to better represent seasonal variations in the polar regions under both quiet and magnetically disturbed conditions. Local time variations in the magnetic activity effect were added. In addition a new species, atomic nitrogen, was added to the previous list of N2, O2, He, O, H, and Ar covered by the model.
The ionosphere as a fully adequate source of plasma for the Earth's magnetosphere
Authors: Chappell C. R. , Moore T. E. , Waite J. H.
Published: February 2008

A series of recent measurements of the outflow of ionization from the ionosphere have further heightened our awareness of the strength of the ionospheric source of magnetospheric plasmas. In this paper the ionospheric contribution of the polar wind and cleft ion fountain at energies less than 10 eV has been added to the previously measured sources; this total ion outflow has then been used to calculate the resulting ion density in the different internal regions of the earth's magnetosphere: plasmasphere, plasma trough, plasma sheet, and magnetotail lobes. Using estimated volumes for these regions and an ion residence time characteristic of each region, we have found that the observed magnetospheric densities can be attained in all cases with no contribution from the solar wind plasma. In the case of the plasma sheet the ionospherically supplied density is more than enough to match the observations and even suggests an invisible component of low‐energy plasma (< 10 eV) which has never been observed. A detailed comparison between the calculated ionospheric source effects in the plasma sheet and those recently measured by ISEE shows excellent agreement and suggests a direct polar low‐energy ion source for the plasma sheet which has remained unmeasured because of spacecraft potential effects. Although the solar wind is clearly the earth's magnetospheric energy source and energetic solar wind ions are observed in the magnetosphere, these calculations suggest the possibility that the ionospheric source alone is sufficient to supply the entire magnetospheric plasma content under all geomagnetic conditions.
Interactions between neutral thermospheric composition and the polar ionosphere using a coupled ionosphere‐thermosphere model
Authors: Fuller‐Rowell T. J. , Rees D. , Quegan S. , Moffett R. J. , Bailey G. J.
Published: February 2008

A fully self‐consistent numerical model of the polar ionosphere and the global thermosphere has been developed to study the dynamical and chemical interactions between the neutral and ionospheric constituents of the polar regions. In the polar F region, the seasonal anomaly in ionospheric density is caused by the neutral thermospheric compositional response to the interhemispheric circulation cell near solstice, including the effect of the self‐consistent input from geomagnetic heating. A tongue of ionisation is carried by convection into the winter polar cap from the vicinity of the dayside (sunlit) polar cusp, while the auroral oval, at F region altitudes, is barely distinct from the surrounding lower‐latitude and polar regions. The subauroral trough of the winter hemisphere is caused by stagnation, and is thus dominated by characteristics of the polar convection pattern. A deep plasma trough is cut through the summer (southern) polar cap around 0600 UT, resulting from upwelling by geomagnetic forcing and solar insolation enhancement of molecular nitrogen.
Nonsteady boundary layer flow including ionospheric drag and parallel electric fields
Authors: Lotko W. , Sonnerup B. U. Ö , Lysak R. L.
Published: February 2008

A simple two‐dimensional model is developed for the perpendicular flow dynamics of boundary layers located on closed magnetic field lines in the equatorial magnetosphere. In addition to viscous damping in the equatorial region, nonlocal dissipation associated with the closure of field‐aligned currents in the ionosphere and field‐aligned potential drops is included and is shown to have two consequences: (1) coupling to the ionosphere causes large‐scale disturbances to decay anisotropically and (2) field‐aligned potential drops promote long‐lived fluctuations at scale lengths that can be comparable to the “inverted V” scale but which vary with the local Hartmann number (a measure of the ratio of ionospheric resistive friction to magnetospheric viscous friction). Depending on the degree of anisotropy, the combined effect is a tendency for the two‐dimensional flow to organize into either relatively isotropic eddies or essentially one‐dimensional striations. The limiting case of a one‐dimensional flow is considered with applications to the low‐latitude boundary layer and internal magnetospheric shear layers. A particular boundary layer equilibrium state is compared with observations of an ionospheric boundary layer region and is found to be in fair agreement with the observed properties for reasonable values of the model parameters. It is also shown, under certain conditions, that a monotonically varying internal shear layer can form paired oscillations in the electric field, which are characteristic of the so‐called “V shocks” that occur in and above the auroral acceleration region.
The effects of a magnetic By component on geomagnetic tail equilibria
Authors: Hilmer Robert V. , Voigt Gerd‐Hannes
Published: February 2008

Positive correlations of the magnetic y component of the IMF with those of both the magnetotail lobes and plasma sheet show a By component pervading the quiet magnetotail with enhanced strength near the neutral sheet (z = 0). A model constructed using linear solutions to the Grad‐Shafranov equation for two‐dimensional tail equilibria explains these and related observations in terms of MHD equilibrium theory. We conclude that the enhanced By values must be an essential part of the quiet magnetotail and do not result from a simple intrusion of the IMF. The By field consists of a constant background component plus a nonuniform field existing only in the plasma sheet, where it is dependent on the plasma parameter β and the strength of the magnetic Bz component. By is strongest at the neutral sheet and decreases monotonically in ± z direction, reaching a constant tail lobe value at the plasma sheet boundaries. Field lines are tilted about the tail axis with the angle of ascent in the y‐z plane being greatest at z = 0. This is also the region of greatest field line rotation in the x‐y plane, insuring that magnetic reversal always occurs near the neutral sheet. The maximum change in field direction between tail lobes is less than 180° with field lines most closely aligned with the tail axis at the plasma sheet boundaries. The presence of a significant positive By component produces currents, including field‐aligned currents, that flow through the equatorial plane and toward and away from earth in the northern and southern halves of the plasma sheet, respectively. A negative By component produces currents circulating in the opposite sense. The strength and location of maxima in current flowing along the tail axis depend strongly on the ratio [By(z = 0)/By (lobes)], growing and moving closer to the plasma sheet boundaries as this ratio increases.
Io's interaction with the plasma torus: A self‐consistent model
Authors: Wolf‐Gladrow Dieter A. , Neubauer F. M. , Lussem M.
Published: February 2008

The interaction between Io and the plasma torus is simulated by a three‐dimensional numerical model which allows the calculation of electric fields, current density distributions, and magnetic fields in Io's vicinity and in the Alfvén wings. The model is self‐consistent in the sense that the influence of magnetic field disturbances on the electric field is taken into account and vice versa. A two‐dimensional elliptic differential equation for the electric potential is derived by using Euler potentials. The current density in Io's vicinity is decomposed into poloidal and toroidal fields. This representation allows an effective integration to get the magnetic field disturbances. Pedersen, Hall, inertial (or polarization) and field‐aligned currents are calculated. We show results for different upstream plasma conditions and atmosphere/ionosphere models and discuss the consequences of variations in neutral density, Alfvénic Mach number, and other parameters. A series of simple spherically symmetric atmosphere/ionosphere models with neutral gas densities between 1016 m−3 and 1017 m−3 are investigated. By comparison with Voyager magnetic field measurement we get agreement for a column density of 7.5 × 1021 m−2 (surface density N0 = 5×1016 m−3, scale height H = 150 km).
Z mode radiation in Jupiter's magnetosphere
Authors: Kennel C. F. , Chen R. F. , Moses S. L. , Kurth W. S. , Coroniti F. V. , Scarf F. L. , Chen F. F.
Published: February 2008

The wideband plasma wave data acquired by Voyagers 1 and 2 in Jupiter's magnetosphere frequently contain a narrow‐band emission of variable intensity whose upper frequency is below the lower‐frequency cutoff of the continuum radiation. We have found examples in over 400 wideband frames taken outside the plasma sheet between 20 and 90 RJ from the planet in the dayside and nightside magnetosphere. The narrow‐band emissions do not appear to be electrostatic waves. If they are electromagnetic, they can be identified by relating their frequencies to the fundamental frequencies of the ambient plasma and the solutions to the cold plasma dispersion relation. Since the electron plasma frequency was not measured at the rate and accuracy we need, we attempt to infer it by testing two hypotheses about the lower‐frequency cutoff of the continuum. By analogy with terrestrial observations, we assume that the continuum cutoff is either at the plasma frequency or the right‐hand cutoff frequency at which the right‐hand polarized extraordinary mode cuts off. The measured electron cyclotron frequency then enables us to determine the wave mode of the narrow‐band emissions. More consistent results followed from assuming the continuum cut off at the right‐hand frequency. Under this assumption the narrow‐band emission falls between the left‐hand cutoff frequency and upper hybrid frequencies, which identifies the waves as the slow branch of the X mode, or the so called Z mode. This also implies that the continuum is comprised of R‐X mode waves where the Z mode is observed as a separate band, since the L‐O mode would fill the gap between the Z mode and continuum bands. As Voyager 1 approached the plasma sheet on March 8, 1979, the Z mode intensified and then disappeared on plasma sheet entry. We interpret this as evidence of local Z mode generation.
Vibrationally excited molecular hydrogen in the upper atmosphere of Jupiter
Authors: Cravens T. E.
Published: February 2008

Experiments on the Voyager 1 and 2 spacecraft and observations made by the International Ultraviolet Explorer (IUE) have provided evidence for the existence of energetic particle precipitation into the upper atmosphere of Jupiter from the magnetosphere. This auroral precipitation has been shown to generate large ionization and dissociation rates, to excite auroral emissions, and also to vibrationally excite molecular hydrogen. A theoretical model of vibrationally excited H2 in the upper atmosphere of Jupiter is presented in this paper. Models are considered for both the auroral region and also for lower latitudes, where H2 is vibrationally excited owing to processes associated with the absorption of solar ultraviolet radiation. The ground electronic state (X¹Σg+) of molecular hydrogen can be vibrationally excited in a variety of ways, such as by direct electron impact excitation, by dissociative recombination of H3+, and by excitation of the Lyman and Werner bands. Vibrationally excited H2 can react with either H2 or H via vibration‐translation (VT)interchange collisions, or via vibration‐vibration (VV) interchange collisions with other H2 molecules. Reaction with H+ ions is possible for vibrational levels with υ ≥ 4. Molecular and eddy diffusion of the vibrationally excited H2 is also taken into account in these model calculations. The reaction of H+ ions with vibrationally excited molecular hydrogen is shown to be a very important loss process for this ion and reduces the electron densities theoretically calculated for the auroral region by about a factor of 10. The theoretical ionosphere calculations are coupled with the vibrationally excited H2 calculations in a self‐consistent manner.
Modeled ground magnetic signatures of flux transfer events
Authors: McHenry Mark A. , Clauer C. Robert
Published: February 2008

The magnetic field on the ground due to a small (≤ 200 km scale size) localized field‐aligned current system interacting with the ionosphere is calculated in terms of an integral over the ionospheric distribution of field‐aligned current. Two different candidate current systems for flux transfer events (FTEs) are examined. The first system has current flowing down the center of a cylindrical flux tube with a return current uniformly distributed along the outside edge. The second system has upward current on one half of the perimeter of a cylindrical flux tube with downward current on the opposite half. The peak magnetic field on the ground is found to differ by a factor of 2 between the two systems, and the magnetic perturbations are in different directions depending on the observer's position. Assuming the current system moves in a constant linear poleward direction, we predict the ground magnetic field versus time which would be measured by a ground observatory. Using FTE detection statistics at the magnetopause, we estimate the detection rate of FTEs at a ground station under the dayside convection reversal to be between 12 and 60 min per sighting depending on which FTE current system is considered.
Particle orbits during the development of plasmoids
Authors: Scholer M. , Jamitzky F.
Published: February 2008

Single‐particle orbits are numerically followed in a time‐dependent electric and magnetic field obtained from a two‐dimensional magnetohydrodynamic simulation of bursty reconnection (Scholer and Roth, 1987). During the reconnection simulation the reconnection rate at the original X point decreases, a long current sheet develops, and reconnection sets in again at a new pair of X points. This leads to the development of a plasmoid. Particles injected with parameters appropriate for the plasma sheet at the time of the secondary tearing close to the new X points drift large distances parallel to the X line and gain high energies (up to ∼300 keV). These particles are subsequently trapped on closed field lines in the developing plasmoid. Since the X line has a limited extent, the acceleration mechanism leads to large dawn‐dusk asymmetries. The resulting distribution function within the plasmoid at various cross‐tail distances is computed. The distribution function is close to an exponential in velocity with a higher‐energy cutoff; with increasing distance toward the duskside the e‐folding velocity increases, and the high‐energy cutoff shifts to larger energies.
The low‐latitude auroral boundary: Steady state and time‐dependent representations
Authors: Gorney D. J. , Evans D. S.
Published: February 2008

Recently, extensive data sets have been acquired from charged‐particle detectors on low‐altitude satellites pertinent to the study of the low‐latitude boundary of the aurora, including both case studies and statistical treatments. The low‐latitude auroral boundary can be thought of as the low‐altitude manifestation of convection boundaries for magnetospheric particles of moderate energy, although the detailed shape of the boundary might be modified locally by spatial variations or temporal modulation of precipitation rates. Several attempts have been made to compare observed equatorial and low‐altitude boundaries with theoretical representations, with varying degrees of success. Here, the results of statistical analysis and case studies of the auroral boundary using precipitating particle data acquired by the National Oceanic and Atmospheric Administration spacecraft are presented. The ability of standard convection models to account for the observed boundary characteristics is discussed, with emphasis on the roles of convection strength and time dependence. It is demonstrated that within reasonable parameter limits, steady state convection patterns for moderate energy electrons generally fail to account for certain observed characteristics of boundary shapes. Further, it is demonstrated that time‐dependent transitions between various levels of activity can lead to characteristic boundary shapes that are quite dissimilar to steady state solutions and to some extent can provide a resolution between observed and computed boundary shapes for time scales pertinent to magnetospheric activity. The length of time after the onset of enhanced convection is found to be at least as important as the instantaneous level of activity as an ordering parameter for the shape of the low‐latitude auroral boundary.
Cratering history of the Uranian satellites: Umbriel, Titania, and Oberon
Authors: Plescia J. B.
Published: February 2008

Crater size‐frequency data for Umbriel, Titania, and Oberon are presented, and the implications of those data are discussed in terms of the geologic histories of these bodies and the populations of objects that have cratered them. The surfaces of Oberon and Umbriel are old and are inferred to date to a period early in their histories when the cratering rate was significantly higher than at present. No significant endogenic resurfacing appears to have occurred on either body after that inferred period of intense cratering. Titania exhibits the youngest surface of these three and appears to have undergone almost complete endogenic resurfacing. Among the Uranian satellites the surfaces of Oberon and Umbriel are interpreted to be the oldest, that of Titania intermediate, and those of Ariel and parts of Miranda the youngest. The size‐frequency distributions for these satellites have an average slope of about −3, indicative of a steep crater production function. The cumulative size‐frequency data for these Uranian satellites may be interpreted to indicate that parts of their surfaces are saturated with craters at small diameters.
Voyager infrared observations of Uranus' atmosphere: Thermal structure and dynamics
Authors: Flasar F. M. , Conrath B. J. , Gierasch P. J. , Pirraglia J. A.
Published: February 2008

Observations of Uranus by the Voyager infrared spectrometer at 325 cm−1 have been analyzed to obtain the variation with latitude of temperatures near the tropopause. The lowest temperatures occur near 25°S latitude, and a weaker minimum occurs near 40°N. The locations of the temperature minima and their relative amplitudes agree within measurement error with those reported earlier for temperatures two scale heights lower in altitude (Hanel et al., 1986). However, the meridional variation of the temperatures at the higher altitude is a factor of 3 greater in amplitude. The vertical shears of the zonal winds near the tropopause implied by geostrophy are at least ∼40 m s−1 per scale height increasing in the direction of the planetary rotation at low latitudes and up to 15 m s−1 per scale height in the opposite direction at mid‐latitudes (30°–40°). The derived temperature and zonal wind structure is interpreted in terms of a linear model of a zonally symmetric circulation with frictional and radiative damping in the absence of solar and condensation heating. In this interpretation the zonal winds decay with altitude in the upper troposphere and stratosphere, and at low latitudes the atmosphere is subrotating. For consistency between the observed tropopause temperatures and the mid‐ and high‐latitude zonal winds derived from Voyager images, the linear model requires that the ratio of the frictional damping time to the radiative damping time be ∼1‐2.
The damping of interplanetary Alfvénic fluctuations and the heating of the solar wind
Authors: Tu Chuan‐yi
Published: February 2008

The theoretical model of Tu et al. (1984) describing the radial variation of the power spectra of interplanetary Alfvénic fluctuations has been extended to explain the radial variations of Alfvénic fluctuations between 0.3 and 5 AU and of the solar wind proton temperature observed by Helios between 0.3 and 1 AU. The extended model predicts that the fluctuations have the following features. (1) If the lifetime of two wave packets in the fluctuations is less than their correlation time, their interaction time is determined by the lifetime. In this case, the spectral index in the high‐frequency range is close to −5/3. (2) The shape of the power spectrum evolves as the fluctuations propagate away from the Sun in the distance range between 0.3 and 1 AU; however, it remains approximately unchanged between 1 and 5 AU. The extended model also predicts that the proton magnetic moment increases with increasing heliocentric distance and that this increase for high‐speed streams from 0.35 to 0.95 AU is about 2×104K/nT. All these results are consistent with observations, indicating that the extended model proposed in the present paper can account for both the damping of Alfvénic fluctuations and the heating of the solar wind protons simultaneously. A relationship between the thermal speed of solar wind protons and the variance of the solar wind velocity components is also predicted for further comparison of the present model with Helios data.
The longitudinal and radial distribution of magnetic reconfigurations in the near‐Earth magnetotail as observed by AMPTE/CCE
Authors: Lopez R. E. , Lui A. T. Y. , Sibeck D. G. , McEntire R. W. , Zanetti L. J. , Potemra T. A. , Krimigis S. M.
Published: February 2008

Magnetic field and energetic particle data from the AMPTE/CCE have been used to identify 103 magnetic reconfiguration events in the near‐Earth (R ≤ 8.8 RE) nightside equatorial plane. The events involve a dipolarization of the magnetic field and increases in energetic particle fluxes typical of substorms. This study is the first survey of such signatures extended over both local time and radial distance. The events are distributed in magnetic local time (MLT) from 19.9 MLT to 4.7 MLT with the peak of the distribution near midnight. No events were observed inside of 6.4 RE, and the occurrence frequency increased with radial distance. For Kp ≤ 3−, the local time peak of the distribution is near 23 MLT, whereas for Kp ≥ 4+, the distribution center is near 2 MLT. Furthermore, the occurrence frequency of the events when Kp ≥ 4+ is 4 times greater than the occurrence frequency for events when Kp ≤ 3−. The results of this study suggest that (1) substorm reconfiguration of the magnetic field, presumably arising from disruption/diversion of the cross‐tail current, seldom reaches inside a radial distance of about 6.4 RE, and (2) the central meridian of the substorm current wedge tends to shift from the premidnight to the postmidnight sectors as the global geomagnetic activity increases.
Erupting prominences and the geometry of coronal mass ejections
Authors: Webb David F.
Published: February 2008

Studies of the origins of coronal mass ejections (CMEs) show a strong association between CMEs and erupting prominences. “Loop” CMEs form the most common morphological class for both Skylab and Solar Maximum Mission (SMM) and the class most likely to be associated with erupting prominences. In the optically thin corona the geometry of such structures is interpreted as either planar (two‐dimensional) or shell‐like or bubblelike (three‐dimensional) structures. Trottet and MacQueen (1980) (hereinafter referred to as TM) first examined the orientations of prominences associated with Skylab CMEs and argued that looplike CMEs were planar structures in the plane of the sky. Following TM, I measured the initial orientations of prominences identified with SMM CMEs in 1980 in order to examine the overall geometry of CMEs. Contrary to TM's result, I found no preferred orientation for prominences associated with SMM CMEs nor any difference between that population and the population of all disk filaments. In addition, plots comparing CME/filament orientation angles and the angular widths of the CME envelopes show large scatter. Taken together, these results support a shell‐like rather than a planar geometry for at least looplike CMEs.
A monochromatic Pc 5 pulsation which exhibits a time‐varying Doppler‐shifted frequency
Authors: McDiarmid D. R. , Nielsen E.
Published: February 2008

Observations from the Sweden and Britain Radar Experiment and the Scandinavian Twin Auroral Radar Experiment are presented of a Pc 5 pulsation which was monochromatic in a reference frame moving in the north‐south direction with respect to the radar grid. This reference frame was fixed with respect to a boundary across which the latitudinal phase variation of the pulsation experienced a distinct discontinuity. The pulsation was embedded in westward or convection flow, and its azimuthal wave number was low. The data are reminiscent of those of a double resonance but nevertheless differ in significant ways. The event is discussed in terms of recent theoretical developments which, however, are not found to provide an adequate explanation.
Energy spectra of plasma sheet ions and electrons from ∼50 eV/e to ∼1 MeV during plasma temperature transitions
Authors: Christon S. P. , Mitchell D. G. , Williams D. J. , Frank L. A. , Huang C. Y. , Eastman T. E.
Published: February 2008

Measurements of charged particles in the plasma sheet by the low energy proton and electron differential energy analyzer (LEPEDEA) and medium energy particle instrument (MEPI) on ISEE 1 are combined to obtain ion and electron differential energy spectra for use in studying eight plasma sheet temperature transitions, periods of low plasma bulk velocity typically ∼1 hour in length during which the plasma thermal energy either increases or decreases steadily. Over the entire kinetic energy range sampled (50 eV/e ≲ E ≲ 1 MeV), the plasma and energetic ion and electron populations respond collectively as a single unified particle population during these temperature transitions. In order to test the hypothesis that the energy spectra of plasma sheet ions and electrons can be represented by a single functional form, the observed particle energy spectra have been visually compared to three model distribution functions: the Maxwellian (, where ET is the thermal energy), the kappa (ƒ ∼ [1 + E/κET]−κ −1, where κ is a constant), and the velocity exponential (ƒ ∼ e−( E/ε)1/2, where ε is constant). The kappa and velocity exponential distributions both provide reasonable fits above ∼200 eV, with the kappa distribution being more successful at the highest energies but less successful at the lowest energies. The Maxwellian does not provide an adequate fit for the overall distributions observed in the temperature transitions. At high energies (E ≫ κET) the observed spectra are more often similar to the kappa than to the velocity exponential; that is, a roughly power law form (E−κ) is in evidence. Although the value of the index varies from event to event, the particle distributions maintain their overall shape throughout a transition, during which the spectral index at high energies stays roughly constant. This could indicate either that the relaxation time of the plasma is short with respect to the time scale of the temperature transitions or that the spatial regions being sampled were all maintaining a stationary state plasma population, or both. Both temporal and spatial effects are evident in the temperature transitions studied. An indication of temporal dependence during the transitions is that on the average, ET increases with geomagnetic activity as indicated by the AE index at low to moderate levels (∼30 to 600 nT). However, a spatial effect is evident as well, since temperature increases (decreases) occurred as ISEE 1 was traveling toward (away from) the geocentric solar magnetospheric equator.
Time scales for formation and spreading of velocity shells of pickup ions in the solar wind
Authors: Gaffey J. D. , Winske D. , Wu C. S.
Published: February 2008

The pickup of newborn ions in the solar wind by means of low‐frequency, electromagnetic waves, either due to the intrinsic turbulence of the solar wind or induced turbulence due to instabilities excited by the newborn ions, is discussed. The pickup process is envisioned to occur in three stages, formation of a ring‐beam distribution, rapid pitch angle scattering of this initial distribution into a thin shell, and slower velocity diffusion that spreads out the shell. The process of shell formation and evolution are studied by means of numerical simulations, first for the situation of relatively low levels of turbulence such as would occur naturally in the solar wind and then for higher levels of turbulence characteristic of those encountered near cometary bow shocks. The results of the numerical experiments are compared with theory. The initial velocity spreading and growth of electromagnetic fluctuations due to the interaction of a small initial density of newborn ions interacting with the solar wind in the absence of significant background turbulence is shown to be well described by linear‐ and second‐order theory. When the initial density of newborn ions is sufficiently small or the level of intrinsic turbulence is large enough so that there is no appreciable growth and self‐interaction of generated waves with the newborn ions, these ions pitch angle scatter to form a thin, partial shell distribution. The time scale for the initial scattering is shown to be related to the gyroperiod in the perturbed wave field. On a longer time scale that is consistent with a simplified quasi‐linear theory, the ions in this shell diffuse in velocity space to fill it out in pitch angle, as well as cause it to spread in energy. However, the diffusion process slows considerably, long before a filled‐in velocity sphere is formed. These results are then used to estimate the time scales for shell formation and diffusion in several situations of recent observational interest, the interstellar helium data obtained by AMPTE and cometary ion pickup distributions obtained by various spacecraft at comets Giacobini‐Zinner and Halley.
Dual periodicity of the Jovian magnetosphere
Authors: Sandel B. R. , Dessler A. J.
Published: February 2008

Jupiter's magnetic field, like that of the Sun, and perhaps Saturn, exhibits a clear, persistent dual periodicity, the two Jovian periods differing by almost exactly 3%. We offer a provisional definition of a new Jovian longitude system (which we call system IV) to organize magnetospheric data that are not stationary in system III. We show that available, independent data sets, covering a time interval of 4 years, which either drift in system III or show no particular organization in system III, fit mutually consistent patterns in system IV. All of the data sets covering several rotations of the planet that are presently available to us, including Voyager observations of ultraviolet and narrow‐band kilometric emissions and ground‐based optical observations, are organized in either system III, system IV, or both. Using these data, we derive provisional values for a transformation between systems III and IV: λIV = λIII + 338 − 25.486(t − 2443874.5) where t is the Julian day and fractional day of the observation. There are pronounced 14.1‐day variations in a number of Jovian magnetospheric phenomena. One possible interpretation of the system IV modulation is that it is a sideband resulting from the 14.1‐day amplitude modulation of system III phenomena. Alternately, the 14.1‐day period could be explained if we assume the existence of an active sector that is fixed in system IV but drifts approximately 25.5°/d relative to the active sector in system III. When the system III and system IV activity maxima are aligned, magnetospheric activity, such as radio emissions and torus asymmetries, is enhanced, and when the activity maxima are anti‐aligned, magnetospheric activity is subdued. The interval between alignments (or anti‐alignments) is 14.1 days. Finally, we note that, in developing system IV, we have utilized only a small number of data sets. System IV needs to be tested against additional data before its durability is assured.
General magnetic reconnection, parallel electric fields, and helicity
Authors: Schindler K. , Hesse M. , Birn J.
Published: February 2008

Analyzing the qualitative three‐dimensional magnetic structure of a plasmoid, we were led to reconsider the concept of magnetic reconnection from a general point of view. The properties of relatively simple magnetic field models provide a strong preference for one of two definitions of magnetic reconnection that exist in the literature. Any concept of magnetic reconnection defined in terms of magnetic topology seems naturally restricted to cases where the magnetic field vanishes somewhere in the nonideal (diffusion) region. The main part of this paper is concerned with magnetic reconnection in nonvanishing magnetic fields (finite‐B reconnection), which has attracted less attention in the past. We show that E∥ (the electric field component parallel to the magnetic field) plays a crucial physical role in finite‐B reconnection, and we present two theorems involving E∥. The first states a necessary and sufficient condition on E∥ for global reconnection to occur. Here the term ”global” means the generic case where the breakdown of magnetic connection occurs for plasma elements that stay outside the nonideal region. The second theorem relates the change of magnetic helicity to E∥ for cases where the electric field vanishes at large distances. That these results provide new insight into three‐dimensional reconnection processes is illustrated in terms of the plasmoid configuration, which was our starting point.
Magnetic clouds and force‐free fields with constant alpha
Authors: Burlaga L. F.
Published: February 2008

Magnetic clouds observed at 1 AU are modeled as cylindrically symmetric, constant alpha force‐free magnetic fields. The model satisfactorily explains the types of variations of the magnetic field direction that are observed as a magnetic cloud moves past a spacecraft in terms of the possible orientations of the axis of a magnetic cloud. The model also explains why the magnetic field strength is observed to be higher inside a magnetic cloud than near its boundaries. However, the model predicts that the magnetic field strength profile should be symmetric with respect to the axis of the magnetic cloud, whereas observations show that this is not generally the case.