bluuebunny/crossref_metadata_2025 huggingface dataset:
On the energy budget in the current disruption region
Authors: Hesse Michael , Birn Joachim
Published: February 2008

This study investigates the energy budget in the current disruption region of the magnetotail, coincident with a pre‐onset thin current sheet, around substorm onset time using published observational data and theoretical estimates. We find that the current disruption/dipolarization process typically requires energy inflow into the primary disruption region. The disruption dipolarization process is therefore endoenergetic, i.e., requires energy input to operate. Therefore we argue that some other simultaneously operating process, possibly a large scale magnetotail instability, is required to provide the necessary energy input into the current disruption region.
Thin current sheets in the deep geomagnetic tail
Authors: Pulkkinen T. I. , Baker D. N. , Owen C. J. , Gosling J. T. , Murphy N.
Published: February 2008

The ISEE‐3 magnetic field and plasma electron data from Jan ‐ March 1983 have been searched to study thin current sheets in the deep tail region. 33 events were selected where the spacecraft crossed through the current sheet from lobe to lobe within 15 minutes. The average thickness of the observed current sheets was 2.45RE, and in 24 cases the current sheet was thinner than 3.0RE; 6 very thin current sheets (thickness λ < 0.5RE) were found. The electron data show that the very thin current sheets are associated with considerable temperature anisotropy. On average, the electron gradient current was ∼ 17% of the total current, whereas the current arising from the electron temperature anisotropy varied between 8 – 45% of the total current determined from the lobe field magnitude.
Cooling of the Earth, Urey ratios, and the problem of potassium in the core
Authors: Breuer Doris , Spohn Tilman
Published: February 2008

The compatibility of mantle Urey ratios Um calculated from thermal evolution models with those calculated from geochemical models is considered. The initial heat generation rate in the mantle used for the thermal evolution models was derived from two geochemical models of the present day primitive mantle with concentrations of U of 20 ppb (2.7 times chondritic) and 26 ppb (3.5 times chondritic), respectively. The depletion of the mantle through continental crust production was allowed for; the effect of including the continental crust on Um is a reduction of its value by about 0.1. To balance the observed average value of the surface heat flow qs it was assumed that the core may contain K up to cosmochemically constrained quantities. For 26 ppb U in the present primitive mantle no core heat sources are required. The present value of Um for this model is 0.57 and the core heat flow is about 12% of qs. With 20 ppb U in the present primitive mantle, heat production equivalent to one to two silicate earth budgets of K is required in the core to balance qs and Um is between 0.35 and 0.4. The core heat flow is then between 25% and 33% of qs.
A geomagnetic polarity time scale for the period 0–17 Ma, based on 40Ar/39Ar plateau ages for selected field reversals
Authors: Baksi Ajoy K.
Published: February 2008

Utilizing 40Ar/39Ar plateau ages for selected field reversals at ∼ 1, 2, 10, 16 and 34 Ma, a geomagnetic polarity time scale (GPTS) is constructed, following the technique of Cande and Kent [1992]. This differs markedly from previous GPTS, especially in middle‐late Miocene time; the ages for the chrons in post‐Miocene time are in agreement with values derived by the astrochronological technique. Refinements in this GPTS await acquisition of 40Ar/39Ar plateau ages for field reversals in the time frames 3–8 Ma and 11–15 Ma.
The performance of a new instrument for in situ measurements of ClO in the lower stratosphere
Authors: Toohey D. W. , Avallone L. M. , Allen N. T. , Demusz J. N. , Hazen J. N. , Hazen N. L. , Anderson J. G.
Published: February 2008

Abundances of chlorine oxide (ClO) have been measured from 16 km to 30 km by a new balloon‐borne in situ instrument developed from an optical design flown previously on the NASA ER‐2 aircraft. This instrument, a prototype for one to be flown on the Perseus remotely piloted aircraft, was one‐third the weight of that on the ER‐2, yet retained the high precision and accuracy necessary for detailed photochemical studies of the lower stratosphere. In this paper we discuss the performance of the instrument during its first flight on March 31, 1991 over eastern New Mexico.
Wave breaking signatures in noctilucent clouds
Authors: Fritts David C. , Isler Joseph R. , Thomas Gary E. , Andreassen Øyvind
Published: February 2008

Results of a recent modeling study of gravity wave breaking in three dimensions by Andreassen et al. and Fritts et al. showed wave saturation to occur via a three‐dimensional instability oriented normal to the direction of wave propagation. The instability was found to occur at horizontal scales comparable to the depth of unstable regions within the wave field and to lead to substantial vertical displacements and tilting of isentropic surfaces. Because of strong similarities between the wave and instability structures in the simulation and the structure observed in noctilucent cloud layers near the summer mesopause, we have used these model results to compute the advective effects on cloud visibility and structure for a range of viewing angles and cloud layer widths. Our results show the gravity wave breaking signature to provide a plausible explanation of the observed structures and suggest that noctilucent cloud structures may be used in turn to infer qualitative properties of gravity wave scales, energy and momentum transports, and turbulence scales near the summer mesopause.
On the combination of the magnetotelluric and the geomagnetic depthsounding method for resolving an electrical conductivity increase at 400 km depth
Authors: Bahr Karsten , Olsen Nils , Shankland Thomas J.
Published: February 2008

The simultaneous use of two different passive electromagnetic (EM) sounding techniques for resolving the upper mantle electrical conductivity structure is emphasized: The geomagnetic (GM) method uses magnetic field variations at periods between 3 hours and 600 hours, yielding penetration depths between 300 km and 1000 km. The magnetotelluric (MT) method is applied to field variations at periods between a few seconds and 16 hours and yields penetration depths between a few km and 600 km. This is an extension of both methods, using additional DSt‐data for MT and additional Sq‐data for GM. In application to field data from Western Europe the resulting overlap of the target depths includes an electrical conductivity increase from 10−2 S/m at 300 km to 1 S/m at 700 km depth while a single conductivity jump at either 400, 500 or 700 km cannot explain the broadband data. Although the olivine to spinel transition is a possible explanation of the conductivity enhancement at 400 km depth, another mineralogy could also occur. This conductivity increase has not been resolved in other sub‐continental profiles and lateral heterogeneity is probable in this depth range.
Quantifying the transition between tectonic trend and meso‐scale texture in topographic data
Authors: Vergne Maxime , Souriau Marc
Published: February 2008

Topographic elevation is considered to be governed by a large‐scale trend generated by tectonics and by a meso‐scale texture described by fractal parameters. We investigate the scale threshold of this transition. A scalar parameter called vertical aspect ratio or mean slope r(d), where d is an horizontal distance, is shown to be sensitive to both components; r(d) is derived from the variograms. An application to selected windows in a Digital Elevation Model has detected a scale threshold called di1 such that one observes: (i) a strongly anisotropic trend for d > di1, (ii) for d < di1, a slight reversed anisotropic fractal signature or a complete isotropic signature when the mean topographic drift increases, (iii) a power law r(d) ∝ d−1 in the horizontal direction orthogonal to the trend, possibly related to a 1/k noise, k being wavenumber. The anisotropies detected reveal the relation of the vertical distribution of topography to the underlying drainage network. Depending on geographical location, we have observed 5 km < di1 < 25 km.
Evolution of the large‐scale temperature field in the Greenland Sea during 1988–89 from tomographic measurements
Authors: Worcester P. F. , Lynch J. F. , Morawitz W. M. L. , Pawlowicz R. , Sutton P. J. , Cornuelle B. D. , Johannessen O. M. , Munk W. H. , Owens W. B. , Shuchman R. , Spindel R. C.
Published: February 2008

The Greenland Sea Ocean Acoustic Tomography Experiment was conducted during 1988–89, as one component of the international Greenland Sea Project, to study deep water formation and the response of the gyre to variations in wind stress and ice cover. Six acoustic transceivers moored in an array 200‐km across transmitted to one another at four hour intervals. Near the end of February, 1989, a sub‐surface temperature maximum at several hundred meters depth disappeared over a surprisingly large area of the central Greenland Sea. While the water column was modified to about 1000 m depth over much of the gyre, the surface remained colder than the deeper water, contrary to what might be expected from simple models of convective renewal.
Rossby‐gravity waves in tropical total ozone
Authors: Standford J. L. , Ziemke J. R.
Published: February 2008

Randel [1992] has recently reported evidence for Rossby‐gravity waves in tropical data fields produced by the European Centre for Medium Range Weather Forecasts (ECMWF). The purpose of this paper is to show that similar features are observable in fields of total column ozone from the Total Ozone Mapping Spectrometer (TOMS) satellite instrument. The observed features are episodic, have zonal (east‐west) wavelengths of ∼ 6,000–10,000 km and oscillate with periods of 5–10 days. The modes exhibit westward phase progression and eastward group velocity. A simple linear model is used to estimate the magnitude of total ozone perturbations induced by Rossby‐gravity waves. The model is able to account for the magnitude of observed TOMS features as well as their asymmetry about the equator. The significance of finding Rossby‐gravity waves in total ozone fields is that (1) the report of similar features in ECMWF tropical fields is corroborated with an independent data set and (2) the TOMS data set is demonstrated to possess surprising versatility and sensitivity to relatively smaller scale tropical phenomena.
Interplanetary magnetic clouds: Topology and driving mechanism
Authors: Chen James , Garren David A.
Published: February 2008

A model is developed to study the origin and propagation of magnetic clouds. Starting with an equilibrium current loop embedded in an ambient plasma consistent with the solar corona, magnetic energy is injected by increasing the loop current. This causes the loop to rise, propelling plasma and magnetic field away from the Sun. Using a simple model of the interplanetary medium, the subsequent dynamics of the loop is calculated to 1 AU and beyond. The macroscopic properties of the resulting structures at 1 AU closely resemble those of observed magnetic clouds. Thermal effects indicate that clouds remain magnetically connected to the Sun in order to yield observed temperatures near 1 AU.
Determination of total ozone over Mauna Loa using very high resolution infrared solar spectra
Authors: David Shelle J. , Beaton Sheryl A. , Anderberg Mary H. , Murcray Frank J.
Published: February 2008

A very high resolution infrared Fourier transform spectrometer has been used to record solar spectra at the Mauna Loa Observatory in Hawaii. Spectra are normally taken one day a week at sunrise. These spectra have been analyzed for ozone and N2O total columns, and the ozone column compared with the value reported by the Mauna Loa Dobson spectrophotometer. Aside from the FTIR reporting about 5% lower values than the Dobson (which may be due to a systematic difference in the treatment of tropospheric ozone), the FTIR and Dobson agree to within 2.7% (RMS) during this period.
Formation of thin auroral arcs by current striation
Authors: Otto A. , Birk G. T.
Published: February 2008

A model for the formation of auroral arcs is presented which provides an explanation for the extraordinary small thickness of auroral arcs and other morphological properties. The presence of field‐aligned current sheets allows for patchy (three‐dimensional) reconnection processes. Due to the presence of magnetic shear, the associated field‐aligned electric fields map to thin elongated regions at low altitudes. The ionospheric foot points of newly reconnected field lines can be interpreted as the locations of the auroral display. The process can occur in a cascading manner at varying altitudes depending on the local current densities and plasma conditions. The proposed properties are confirmed by results of three‐dimensional magnetohydrodynamik computations.
The relative quantum yields of O2(a¹Δg) from the photolysis of ozone in the wavelength range 270nm ≤ λ ≤ 329nm
Authors: Ball S. M. , Hancock G. , Murphy I. J. , Rayner S. P.
Published: February 2008

The relative quantum yield of O2(a¹Δg) produced by the photolysis of ozone has been measured over the wavelength range 270nm ≤ λ ≤ 329nm. The O2(a¹Δg) fragment was detected by [2+1] resonance enhanced multiple photon ionisation (REMPI). The quantum yield of O2(a¹Δg) is compared to that recommended for O(¹D) and is found to follow it closely except at the longer wavelengths where O2(a¹Δg) is still produced with a yield of 10% or more.
ELF harmonic radiation from the Tromsø Heating Facility
Authors: Barr Richard , Stubbe Peter
Published: February 2008

Using a new experimental technique the response of the auroral ionosphere to a very powerful HF wave amplitude modulated by an ELF square wave has been determined independent of the obfuscating effects of earth‐ionosphere‐waveguide resonances. It has been found that when resonance effects are compensated for, the odd harmonic amplitude ratios are predominantly those one would expect for a square wave, slightly modified by the non‐linear relationship between generated ELF signal and HF radiated power. The even harmonic amplitude ratios are found to be much lower than those observed in earlier experiments and have been modelled by assuming unequal electron heating and cooling time constants and hence different rise and fall times for the ionospheric currents.
Associations between temporal velocity changes and induced seismicity
Authors: Maxwell S. C. , Young R. P.
Published: February 2008

A new technique to image temporal velocity changes is proposed and applied to mining‐induced microseismic data recorded at Falconbridge's Strathcona Mine, Sudbury, Canada. The motivation for this work is to back analyze temporal changes in the velocity structure, which may shed light upon observed correlations between seismicity and velocity structure. The sequential imaging method involves cross‐correlating waveforms of event doublets and computing the relative event location from the time delays. Time delays, which are corrected for the travel time delay due to the differences in locations, are then used to compute images of temporal velocity differences between the recording periods of the doublets. Controlled blast data was used to examine the accuracy of relative locations, which was found to be 2.8 m compared to 13.5 m for the absolute locations. Corrected blast doublet time delays were inverted to test the sensitivity of the method, which confirmed the occurrence of no significant velocity changes. A case study of imaging velocity differences with event doublets recorded at different times, demonstrated a correlation between velocity decreases and both the location of induced microseismic events and a mN 2.6 tremor. The velocity decreases were attributed to fracture unclamping due to stress field rotations associated either with mining or the tremor. The events were found to locate in zones of anomalously high velocity in the static velocity image, corroborating similar observations made in both mining‐induced and natural seismicity studies.
The magnetic state of the lower ionosphere during Pioneer Venus entry phase
Authors: Russell C. T. , Strangeway R. J. , Luhmann J. G. , Brace L. H.
Published: February 2008

During the entry phase of the Pioneer Venus Orbiter, defined as that period at the end of mission in 1992 when the periapsis fell below 185 km, the magnetometer made repeated measurements throughout the post midnight ionosphere until about 0430 LT. In this region the magnetic field is generally stronger at comparable altitudes than it was earlier at times of higher solar activity. This increase combined with a decrease in electron density causes the ratio of the magnetic pressure to thermal pressure to approach unity at altitudes above 200 km, whereas it was much lower than unity at these altitudes during solar maximum. From 160–200 km the magnetic field pressure exceeds that of the ionospheric plasma quite unlike the usual conditions seen at the beginning of the mission. At lowest altitudes below 150 km, however, the field becomes weaker and hence no evidence for a planetary magnetic field is found.
Wintertime measurements of stratospheric HNO3 as part of the ISY Polar Ozone Project
Authors: Fast H. , McElroy C. T. , Wardle D. I. , Rosen James M.
Published: February 2008

Atmospheric infrared emission has been recorded with a scanning variable‐filter radiometer on eight stratospheric balloon flights at Alert, inside the arctic vortex, from December 1991 to March 1992. The radiometer and the data analysis for the determination of the altitude distribution of nitric acid vapour are described. The time development of the nitric acid altitude profile at Alert is presented. A sudden change in the profile and vertical column density appears to be associated with the mid‐January stratospheric warming. There is good overall agreement between our results and other arctic winter measurements.
Upper limits on the contribution of flux transfer events to ionospheric convection
Authors: Newell Patrick T. , Sibeck David G.
Published: February 2008

The possibility that merging might largely consist of bursts of subsolar merging repeating every 8 minutes (Flux Transfer Events — FTEs) has been raised. We examine the maximum possible contribution to the cross polar cap potential from FTEs. Assuming each candidate FTE actually transfers flux, we calculate the contribution from: (i) FTEs which fit statistical magnetopause observations; (ii) FTEs which fit typical ionospheric observations; (iii) The largest reported FTEs in case studies at low and high altitude. We find that FTEs account for less than 10% of a normal active time polar cap potential (∼100 kV); although some individual reported cases could, with optimistic assumptions, account for 20–33%. Two new limits on the size of FTEs are introduced. The requirement that FTEs form within ±6 Re of the subsolar point limits FTEs to producing about 38 kV average potential. An even more fundamental limit is imposed by coherence: to represent a merging burst which starts and stops in 2 minutes, an FTE size is limited by the fast mode wave velocity. The coherence limit is about 14.5 kV, meaning that larger pulses consist of independently active merging sites. Simulation results in which bursty merging dominates are reconsidered, and shown to have geometrical assumptions that may be applicable locally, but not globally. A burst of subsolar merging every 8 minutes therefore cannot contribute most of the cross polar cap potential.
Northern hemisphere nitrous oxide morphology during the 1989 AASE and the 1991–1992 AASE II campaigns
Authors: Podolske James R. , Loewenstein Max , Weaver Alex , Strahan Susan E. , Chan K. Roland
Published: February 2008

Nitrous oxide vertical profiles and latitudinal distributions for the 1989 AASE and 1992 AASE II northern polar winters are developed from the ATLAS N2O dataset, using both potential temperature and pressure as vertical coordinates. Morphologies show strong descent occurring poleward of the polar jet. The AASE II morphology shows a mid latitude “surf zone”, characterized by strong horizontal mixing, and a horizontal gradient south of 30°N due to the sub‐tropical jet. These features are similar to those produced by two‐dimensional photochemical models which include coupling between transport, radiation, and chemistry.
Post‐Pinatubo optical depth spectra vs. latitude and vortex structure: Airborne tracking sunphotometer measurements in AASE II
Authors: Russell P. B. , Livingston J. M. , Pueschel R. F. , Reagan J. A. , Browell E. V. , Toon G. C. , Newman P. A. , Schoeberl M. R. , Lait L. R. , Pfister L. , Gao Q. , Herman B. M.
Published: February 2008

In January and March 1992, DC‐8‐measured stratospheric particle optical depth spectra, τp(λ), peaked broadly at midvisible or longer wavelengths. At mid‐to‐high northern latitudes outside the vortex, τp(526 nm) above about 11 km was as large as 0.22 in both January and March, reflecting continued Pinatubo volcanic influence. In both months, in‐vortex τp(λ) above 11 km was smaller than outside‐vortex values by a factor of two or more, and in January a strong anticorrelation was observed between τp(λ) and HF column content (an indicator of vortex penetration). In late January at 18–20S, near the edge of the southern subtropical jet, τp(526 nm) above 12 km was only about 0.07–0.09, with a flatter spectral shape than northern mid‐ to high‐latitude measurements in both January and March. Occasional high‐latitude vertical profiles indicate 6–11‐km slab optical depths, Δτp(526 nm), of 0.05 to 0.1, which should be added to the above‐11‐km values to yield values above 6 km.
Altitude dependence of stratospheric ozone trends based on Nimbus 7 SBUV data
Authors: Hood Lon L. , McPeters Richard D. , McCormack John P. , Flynn Lawrence E. , Hollandsworth Stacey M. , Gleason James F.
Published: February 2008

A multiple regression statistical model is applied to estimate the altitude, latitude, and seasonal dependences of stratospheric ozone trends using 11.5 years of Nimbus 7 SBUV data for the period November 1978 to June 1990. In the upper stratosphere, the derived trends agree in both latitude dependence and approximate amplitude with published predictions from stratospheric models that consider gas‐phase chemical processes together with the observed ∼0.1 ppbV per year increase in tropospheric chlorine. The dominant contribution to column ozone trends occurs in the lower stratosphere where significant negative trends are present at latitudes >20° in both hemispheres. The observed latitude dependence is qualitatively consistent with model predictions that include the effects of heterogeneous chemical ozone losses on lower stratospheric aerosols.
Total ozone trends over low latitude Indian stations
Authors: Kundu Namita , Jain Meena
Published: February 2008

The network of Dobson spectrometers in India have been measuring total ozone over several stations for the last three decades. Measurements over three stations Srinagar, Delhi and Kodaikanal are considered for trend analysis and its latitudinal variation. The data series extends over the period 1965–91 except for Srinagar where data is vailable only upto 1988. A statistical model having terms for different variabilities such as linear trend, six monthly, seasonal and biennial oscillations, as well as, long term solar variability is used. Since the data base is the monthly average ozone value, short term noise is eliminated. As the calculated and observed ozone data agree with each other the linear trend could be obtained from the model. The trend near the equator, though small, is positive, decreasing to negative values at higher latitudes. The analysis of 11 years TOMS data by other workers does not show the positive trend in the equatorial region. The trend of Dobson data near 30°N latitude is less negative than that obtained from TOMS data and agrees better with the theoretical model.
Sizes and locations of coronal mass ejections: SMM observations from 1980 and 1984‐1989
Authors: Hundhausen A. J.
Published: February 2008

The SMM coronagraph/polarimeter obtained images of the solar corona in 1980 and from 1984 to 1989. Approximately 1300 coronal mass ejections have been identified in this data set; accurate measurements of angular widths and apparent central latitudes have been made for 1209 of them. The distribution of observed angular widths is broad and slightly skewed toward large values; the average width is 47° (in position angle measured around the limb of the Sun), the median width is 44°. There is no evidence in this data set for any significant or systematic change in angular widths during the epoch of SMM observations. The distribution of apparent central latitudes for all 1209 measurements is roughly symmetric about the heliographic equator, with a root‐mean‐square average latitude of 35°. The latitude distributions for different calendar years show significant changes in the spread about the equator; mass ejections occurred over a wide range of latitudes at times of high solar activity but were largely confined to near‐equatorial latitudes at times of low activity. For example, the root‐mean‐square average latitude was 41° in 1980, 38° in 1989 (both years near maxima in sunspot number) but only 13° in 1986 (the year of minimum sunspot number). The changes in the distribution of mass ejection latitudes do not correspond to those for solar features or activity related to small‐scale magnetic structures such as sunspots, active regions, or Hα flares; they do resemble those of features related to large‐scale magnetic structures, such as prominences and bright coronal regions. In 1984, when the “quiet” or background corona suggested the presence of a magnetic dipole structure tilted at ∼30° with respect to the solar rotation axis, mass ejection latitudes were clumped about the tilted “heliomagnetic equator” rather than the heliographic equator. Approximately half of the mass ejections that occurred during 1984 were preceded by several days of brightening and spreading of the bright, background corona at the mass ejection site, and produced a conspicuous disruption of the preexisting structure. These observations strengthen the arguments for a close connection between mass ejections and large‐scale, closed magnetic structures in the corona.
Observations of optical aurora modulated by resonant Alfvén waves
Authors: Xu B. ‐L. , Samson J. C. , Liu W. W. , Creutzberg F. , Hughes T. J.
Published: February 2008

Optical aurora and magnetometer data from the Canadian Auroral Network for the OPEN Unified Study (CANOPUS) array in Canada are used in the study of the modulation of optical aurora in the 5577‐Å and 4709‐Å emission lines, by resonant shear Alfvén waves, in the frequency range of 1‐4 mHz. A total of four events, carefully chosen to represent different characteristics of timing and location, are analyzed. Typical of these events, the power spectra featured discrete spectral peaks usually near 1.3, 1.9, and 3.1 mHz. Furthermore, latitudinal phase shifts of about 180° were typically observed across the latitudinal maximum of a given frequency peak. These observations point irrevocably to the field line resonance as a major factor in the modulation of precipitation and possibly in the acceleration of electrons in forming auroral arcs. Our study demonstrates that the modulation phenomena are common features occurring in the auroral oval, observable in an extensive latitude range, from 66° to 73° invariant latitude. Of the events, one shows the modulation process accompanied by an inverted V structure of electron precipitation in the evening sector near 72°. Two events are observed in the equatorward region of the auroral oval just before substorms onset, and maybe related to the energetic electron arcs which are the precursor of substorm intensification. The fourth event is seen in the morning sector at an equatorward latitude and occurs in the recovery phase of a substorm. The diversity of these modulation events allows us to further infer that resonant Alfvén waves play a direct role in controlling the luminosity variation of the optical aurora.
Thermal structure of ions and electrons in Saturn's inner magnetosphere
Authors: Barbosa D. D.
Published: February 2008

A theoretical model of thermal ion and electron temperatures in Saturn's inner magnetospheres is presented. The model is based on a fast mode of radial diffusive plasma transport with a diffusion time scale which varies with distance as τD = 2 × 106 s (6/L)³. Such a model provides a conceptual organization to the energy balance problem in that the plasma residence time in the region L > 4.5 is short compared to the time scale for Coulomb energy exchange and radiative losses, whereas the opposite is true in the region L < 4.5. This condition implies that in the Dione‐Tethys plasma torus the ion and electron temperatures reflect their initial values upon creation out of the neutral H2O cloud distributed throughout the region. Oxygen ions thus have a temperature corresponding to local pickup by the magnetic field and a large perpendicular temperature anisotropy, while electrons have a temperature corresponding to that of ionization secondaries created by electron impact dissociation/ionization of H2O by the ambient hot electron population. In the collisional L < 4.5 regime the ion temperature is controlled by local pickup from O+ ‐ O charge exchange, while the electron temperature is controlled by heating from thermal O+ ions against radiative losses from electron‐excited OII. A calculation of the off‐equatorial behavior of density and temperature in the Dione‐Tethys torus based on the assumption of a constant ion temperature anisotropy along field lines is also described. The model successfully reproduces the decrease with latitude in electron and O+ temperatures which occur as a result of the interaction of electrons with the ambipolar electric potential and O+ ions with the centrifugal potential.
Three‐dimensional steady compressible perturbations in the magnetohydrodynamic solar wind
Authors: Lou Yu‐Qing
Published: February 2008

We present a steady three‐dimensional magnetohydrodynamic (MHD) perturbation formulation for inhomogeneous solar wind stream structures. We derive analytical solutions for asymptotic behaviors of three‐dimensional compressible MHD perturbations at a large radial distance r in a background radial wind of spherical symmetry. There are three independent perturbation solutions at large r. In contrast with a three‐dimensional propagation of Alfvénic perturbations in the same kind of background wind, the magnetic field perturbation
associated with a steady compressible MHD perturbation does not dominate the background radial magnetic field at large r. Since the three independent compressible MHD perturbation solutions are related by the two regularity conditions at the MHD slow and fast critical points, the leading asymptotic radial scalings of the relevant compressible MHD perturbation variables are the same as those of the background variables at large r. In other words, while the radial variations of compressible MHD perturbations gradually merge into the background MHD wind at large r, the distinct angular variations of the perturbations persist throughout the wind. To complement these analytical analyses, we obtain complete numerical solutions in the entire radial range, which smoothly pass through the MHD slow and fast critical points. We provide numerical examples in which the perturbation increase (or decrease) of the radial wind speed can be as large as ∼ 100 km s−1 due to the perturbation increase (or decrease) of the temperature above the coronal base while the solar wind mass flux variation remains sufficiently small. This is possible because of our three‐dimensional approach and because of the regulation mechanism of the magnetic field in the wind. We also present an axisymmetric model for steady MHD solar wind structures in the presence of solar differential rotation.
A two‐dimensional array study of low‐latitude PC 5 geomagnetic pulsations
Authors: Ziesolleck C. W. S. , Chamalaun F. H.
Published: February 2008

Data obtained from the Australia Wide Array of Geomagnetic Stations (AWAGS) have provided a unique opportunity to investigate the spatial variation of long‐period (Pc 5) ULF wave signal characteristics as a result of plasma waves in low latitudes. Pure state filtering techniques were applied in order to detect highly polarized Pc5 events and derive the wave signal parameters, i.e., the wave frequency, amplitude, polarization, and phase. The results indicate that the Pc 5 signal frequency was virtually independent of latitude and longitude, while the amplitude decreased considerably toward lower latitudes. The spatial variation of the signal polarization was found to be small. However, a statistical analysis of the signal polarization revealed a systematic diurnal variation. Counterclockwise sense of rotation (viewed in the direction of the ambient magnetic field) with the major axis of the polarization ellipse in the northeast quadrant was observed in the local morning, while clockwise sense of rotation with the polarization major axis aligned mainly in the north‐south direction was seen in the local afternoon. Interstation phase analysis yielded small phase variations across the entire station array, indicating azimuthal wave numbers of 1<m<3. The signals appeared to propagate westward in the local morning sector, while eastward propagation prevailed in the local afternoon. The latitudinal phase variation was also very small, showing signals virtually in phase or with the lower‐latitude stations leading slightly. These observations appear to be consistent with the magnetic ground signature of global compressional modes or large scale‐cavity resonances trapped in the magnetosphere.
Generation of VLF saucer emissions observed by the Viking satellite
Authors: Lönnqvist H. , André M. , Matson L. , Bahnsen A. , Blomberg L. G. , Erlandson R. E.
Published: February 2008

Simultaneous observations by the Viking satellite of electric and magnetic fields as well as charged particles have been used to investigate V‐shaped wave phenomena. The intensity of these VLF and ELF emissions is V‐shaped when shown in a frequency versus time plot. Simultaneous observations of V‐shaped so‐called VLF saucer emissions, particles and field‐aligned currents strongly suggest, for the first time, that upgoing electrons with energies less than a few hundred electron volts can generate these waves. Broadband waves observed inside the saucer generation region, from frequencies much less than the ion cyclotron frequency up to the plasma frequency, may also be generated by these electrons. Viking observations of VLF saucers at altitudes between 4000 km and 13,500 km show that these emissions occur at higher altitudes than discussed in previous reports. The generation regions seem to be more extended at these higher altitudes than what has been reported at lower altitudes by other observers.
On the dispersion relations for parametric instabilities of parallel‐propagating Alfvén waves
Authors: Jayanti Venku , Hollweg Joseph V.
Published: February 2008

We consider the dispersion relation for the parametric instabilities of large‐amplitude, circularly polarized Alfvén waves, propagating parallel to the ambient magnetic field. A linear perturbation analysis is employed, and the perturbations are taken to propagate along the ambient field. The standard analysis which has been used previously assumes that density perturbations vary as exp [i(kz ‐ ωt)]; this defines the meaning of ω and k. However, the differential equations have periodic coefficients, implying that Floquet analysis should be used. We here present an analysis based on Floquet's theorem. The result is a hierarchy of dispersion relations. However, all the dispersion relations are found to be equivalent to the one obtained via the standard analysis; the differences between them are due only to how ω and k are defined. Thus we conclude that physically there is really only one dispersion relation, namely the “electrostatic dispersion relation,” which is in agreement with earlier works. However, we disagree with Viñas and Goldstein (1991b), who obtained additional dispersion relations which they have called the “electromagnetic dispersion relations.” Their additional dispersion relations are a consequence of first truncating the dispersion relation for obliquely propagating perturbations and then taking the limit of parallel‐propagating perturbations.
Magnetic structure of the low beta, quasi‐perpendicular shock
Authors: Farris M. H. , Russell C. T. , Thomsen M. F.
Published: February 2008

ISEE 1 and 2 magnetic field measurements are used to examine the structure of the low beta, quasi‐perpendicular shock. A shock crossing database consisting of ISEE 1 satellite crossings from the beginning of the mission in 1977 to the end of 1980 is utilized to identify shock crossings for this study. A set of 20 low beta, quasi‐perpendicular shock crossings are drawn from the database for study. Analysis of the shock overshoots indicates that the strength of the overshoot of low beta, quasi‐perpendicular shocks increases as the ratio of the Mach number to the first critical Mach number (or ratio of criticality) increases. There are subcritical crossings which have nonnegligible overshoots and other subcritical crossings which exhibit no overshoot. Wave analysis shows that the power of the downstream waves also increases as a function of this ratio of criticality. Upstream of the shock, large‐amplitude, low‐frequency whistler mode and higher‐frequency (f ∼ 1 Hz) whistler waves are evident for subcritical and marginally critical shocks. The lower‐frequency whistlers are right‐hand elliptically polarized and phase stand upstream of the shock, propagating along the shock normal direction. The thickness of the shock is found to be within a factor of 1 and 2 times greater than the wavelength of this precursor wave. This result is inconsistent with the conjecture that the shock is merely the last amplified cycle of the precursor wave, for if this were true, the thickness of the shock from minimum to maximum would be one half of the precursor wavelength. The 1‐Hz waves are right‐hand elliptically polarized and propagate upstream obliquely to the magnetic field direction. Downstream of the marginally critical and supercritical shock, left‐hand elliptically polarized waves are found to propagate along the magnetic field direction and have frequencies of about 0.2‐0.8 fci. These ion cyclotron waves appear to result from the excitation of the Alfvén ion cyclotron (AIC) instability. The AIC instability is driven by the Ti⊥>Ti∥ temperature anisotropy created in front of the shock by the reflection of solar wind ions. Ion cyclotron waves act to pitch angle scatter the ions downstream of the shock and remove the temperature anisotropy. A transitional behavior in the noncoplanar component of the magnetic field occurs at or about the first critical Mach number. Below the critical Mach number, the noncoplanar component is associated with the upstream whistler train. When the ratio of criticality is approximately unity, the noncoplanar component is isolated from any upstream or downstream wave activity. In the supercritical regime, this component of the field is associated with the downstream ion cyclotron wave train. For all ranges of criticality, the noncoplanar component is seen to lie within the shock ramp, and the transitional behavior of this component of the field indicates that it is an inherent part of the shock.
Solar wind conditions in the outer heliosphere and the distance to the termination shock
Authors: Belcher John W. , Lazarus Alan J. , McNutt Ralph L. , Gordon George S.
Published: February 2008

The Plasma Science experiment on the Voyager 2 spacecraft has measured to date the properties of solar wind protons from 1 to 40.4 AU. We use these observations to discuss the probable location and motion of the termination shock of the solar wind. A least squares fit of proton ram pressure to heliocentric distance R over this distance yields a ram pressure equal to (1.67 × 10−8 dynes cm−2) R−2.00 ± 0.02, where R is measured in astronomical units. Assuming that the interstellar pressure is due to a 5 µG magnetic field draped over the upstream face of the heliopause, this radial variation of ram pressure implies that the termination shock will be located at an average distance near 89 AU. This distance scales inversely as the assumed field strength, i.e., for a 7 µG field, the termination shock will be located on average at 64 AU. In addition to the global falloff with distance, there are large variations in ram pressure on relatively short time scales (tens of days), due primarily to large variations in solar wind density at a given radius. Such rapid changes in the solar wind ram pressure can cause large perturbations in the location of the termination shock. Using a simple kinematic model, we study the nonequilibrium location of the termination shock as it responds to these ram pressure changes. The results of this study suggest that the position of the termination shock can vary by as much as 10 AU in a single year, depending on the nature of variations in the ram pressure, and that multiple crossings of the termination shock by a given outer heliosphere spacecraft are likely. After the first crossing, such models of shock motion will be useful for predicting the timing of subsequent crossings.
A statistical survey of auroral solitary waves and weak double layers: 1. Occurrence and net voltage
Authors: Mälkki Anssi , Eriksson Anders I. , Dovner Per‐Ola , Boström Rolf , Holback Bengt , Holmgren Gunnar , Koskinen Hannu E. J.
Published: February 2008

Weak double layers have been proposed to contribute to the buildup of a quasi‐static potential drop along the geomagnetic field lines at the auroral latitudes. The discussions, however, have been based on very limited amounts of analyzed data. In this work we show results of a statistical study of the occurrence and average potential drop of weak double layers (WDLs) and solitary waves (SWs) in data from the Viking spacecraft. We conclude that these structures are found in connection with beams of moderate‐energy, upward‐flowing ions (UFI): Practically all observations are in UFI regions and these structures have been identified in approximately two thirds of the analyzed UFI beam regions. They are most typical at altitudes between 7000 and 11,000 km. The SWs/WDLs with largest amplitudes and net potential drops appear at the same altitude range where most structures are detected. The polarities and net voltages vary between different analyzed orbits and in a smaller scale also inside single ion beam regions. The structures tend to appear in groups with group sizes corresponding to a few kilometers extension perpendicular to the magnetic field. We conclude that the observed SW/WDLs are not sufficient as an explanation of parallel electrostatic fields with perpendicular extensions of hundreds of kilometers, but they may be important for structures with perpendicular sizes on the kilometer scale.
Inferring global characteristics of current sheet from local measurements
Authors: Lui A. T. Y.
Published: February 2008

A new procedure to extract global characteristics of a current sheet based on local measurements of magnetic field, plasma, and current density from a single spacecraft is described. By adopting suitable current density profiles, this method permits an estimate of the following parameters: (1) the asymptotic magnetic field strength outside the current sheet, (2) the north‐south thickness of the current sheet, (3) the spacecraft distance from the center of the current sheet, (4) the magnetic field line curvature at the center of the current sheet, (5) the volume current density at the center of the current sheet, (6) the integrated current density across the thickness of the current sheet, and (7) the κ parameter used by many researchers in the study of particle orbits in a current sheet geometry.
A model of the ionospheric tail rays of Venus
Authors: Luhmann J. G.
Published: February 2008

Distinctive structural patterns in the "tail rays" that form the outermost reaches of the Venus nightside ionosphere were observed on the Pioneer Venus Orbiter. The measurements further suggested that the tail rays are the vehicle for significant escape of atmospheric oxygen, but the manner in which they fit into the scenario of solar wind scavenging and the physics behind their formation remain unexplained. In this paper a model of tail ray generation and morphology is proposed which is based on knowledge of the magnetic field structure in the low‐altitude magnetosheath and near‐wake, where the tail rays are observed. The model applies specifically to those regions where the plasma pressure in the tail rays is less than the field pressure. It is shown that many of the observed structural patterns can be explained if it is assumed that initially low energy ionospheric O+ ions are picked up by an externally imposed convection electric field from a thin source region around the terminator. Gravity is found to play a significant role in determining the ion trajectories and hence the modeled tail ray structures. The model can produce single, double, triple, or quadruple thin tail rays, all of which are observed. The energies of the particles are also consistent with the available data. The reported tail ray dependence on solar EUV flux and solar wind dynamic pressure naturally fits into the proposed concept. The implication for ion escape is that no special mechanisms are required to explain Venus tail rays. They may be simply interpreted as the low‐altitude, low‐energy manifestation of the standard ion pickup process at a weakly magnetized planet.
Parametric instabilities of a large‐amplitude circularly polarized Alfvén wave: Linear growth in two‐dimensional geometries
Authors: Ghosh S. , Viñas A. F. , Goldstein M. L.
Published: February 2008

The growth of parametric instabilities, which may lead to the development of a turbulent cascade, is studied using a magnetohydrodynamic (MHD) code that permits nonlinear couplings in the parallel direction to the ambient magnetic field and one perpendicular direction. Compressibility is included in the form of a polytropic equation of state. Parametric instabilities associated with a parallel‐propagating decay instability are found to dominate the low‐beta case. An obliquely propagating filamentationlike instability dominates the high‐beta case. The nonlinear growth of the nth harmonic of a daughter wave growing as a factor of n times the fundamental's growth rate is found in both cases. Nonlinear saturation is caused by the parallel decay instability in the low‐beta case and by the oblique filamentationlike instability in the high‐beta case.