bluuebunny/crossref_metadata_2025 huggingface dataset:
Global geodesy using GPS without fiducial sites
Authors: Heflin Michael , Bertiger Willy , Blewitt Geoff , Freedman Adam , Hurst Ken , Lichten Steve , Lindqwister Ulf , Vigue Yvonne , Webb Frank , Yunck Tom , Zumberge James
Published: February 2008

Baseline lengths and geocentric radii have been determined from GPS data without the use of fiducial sites. Data from the first GPS experiment for the IERS and Geodynamics (GIG '91) have been analyzed with a no‐fiducial strategy. A baseline length daily repeatability of 2 mm + 4 parts per billion was obtained for baselines in the northern hemisphere. Comparison of baseline lengths from GPS and the global VLBI solution GLB659 (Caprette et al. 1990) show rms agreement of 2.1 parts per billion. The geocentric radius mean daily repeatability for all sites was 15 cm. Comparison of geocentric radii from GPS and SV5 (Murray et al. 1990) show rms agreement of 3.8 cm. Given n globally distributed stations, the n(n ‐ 1)/2 baseline lengths and n geocentric radii uniquely define a rigid closed polyhedron with a well‐defined center of mass. Geodetic information can be obtained by examining the structure of the polyhedron and its change with time.
Time resolved simulation of lightning by LIP*
Authors: Jebens D. S. , Lakkaraju H. S. , McKay C. P. , Borucki W. J.
Published: February 2008

In this letter, we report results of time resolved spectra of lightning simulation in air by laser‐induced plasma (LIP). The measured electron temperatures varied from (1.7±0.1)×104 K at 600 ns to (1.2±0.1)×104 K at 4 μs after the ignition of the plasma. Electron densities in the plasma varied from (7.0 ±1.4)×1017 /cm3 to (9.0±1.8)×10l6 /cm3 for the above times. We also present, for the first time, a comparison of state variables for LIP and natural lightning. We find that immediately following the ignition both natural lightning and LIP reach temperatures in excess of 16,000 K and relax to ambient pressure with different rates. They are expected to approach 2000 – 4000 K temperature range, which is important to chemical synthesis, in an asymptotically similar fashion.
First observations of mesospheric dynamics with a partial reflection radar in Hawaii (22°N, 160°W)
Authors: Fritts David C. , Isler Joseph R.
Published: February 2008

We present here an overview of the wind measurements made during the first year of operation of a partial reflection radar system in Hawaii. The system operates at 1.98 MHz and provides wind estimates between ∼60 and 100 km at 2‐km intervals. Our wind observations reveal a zonal mean motion that has several features in common with the mesopause semiannual oscillation (MSAO) seen at more equatorial latitudes, but which is influenced as well by the seasonal variations of the mesospheric jet structure at higher latitudes. Major features of the zonal mean wind structure include a downward progression of strong eastward and westward phases of the MSAO from ∼January to July, coinciding with the period during which the MSAO and the annual cycle of the zonal mean wind at higher latitudes are in phase. This results in an eastward maximum of ∼60 ms−1 near 80 km during January and February which descends rapidly and a westward maximum of ∼50 ms near 85 km during March and April which descends much more slowly. The second MSAO cycle is greatly suppressed relative to the first due to the reversal of the correlation between this and the annual cycle at higher latitudes from ∼July to December. Mean eastward motions occur primarily above ∼85 km early in this cycle and throughout the height range at later times. Planetary wave activity is seen to contribute substantial variability during times of eastward mean flow and to have a smaller role when zonal mean motions are westward. Tidal and 2‐day motions are found to be large and variable, with the maximum 2‐day amplitudes occurring during February and August. The largest diurnal tides were observed during October and November, March and April, and July and August, while the semi‐diurnal amplitudes exhibited a minimum from ∼April to June.
Stable isotope ratios in irrigation water can estimate rice crop evaporation
Authors: Simpson H. J. , Herczeg A. L. , Meyer W. S.
Published: February 2008

Irrigated crops provide about one third of world food production, and the total area under irrigation has increased by more than a factor of three since 1950 [Brown, 1988]. Possibilities for further geographical expansion are limited; therefore future production increases are likely to require higher efficiency of water use. Of the major grain crops, lowland rice requires the most water, with total demand per unit area at least twice that for wheat and maize. Stable isotope abundance changes in irrigation water can provide direct indication of integrated evaporation losses exclusive of transpiration and thus provide a new tool to monitor a key parameter relevant to water use efficiency. Large enrichments of deuterium and oxygen‐18 in rice field water compared to initial input water in a semi‐arid region of southeastern Australia indicate high evaporation rates (7 mm day−1) during the first month following flooding. This contrasts with semi‐mature rice crops which had small heavy isotope enrichments of field water, indicating low evaporation (<1 mm day−1), compared to transpiration (6 to 7 mm day−1). Over the entire rice‐cropping season, evaporation accounted for about 40 per cent of total losses to the atmosphere, with transpiration providing the remainder.
Comments on absolute and convective instabilities
Authors: Ram A. K. , Bers A.
Published: February 2008

We point out the misconceptions in the arguments put forth by Oscarsson and Rönnmark [1986] that question the validity and usefulness of the well‐known theory of absolute and convective instabilities. The solid basis of the well‐known theory is clarified.
A microwave measurement of high levels of thermospheric nitric oxide
Authors: Clancy R. Todd , Rusch David W. , Muhleman Duane O.
Published: February 2008

We report the first microwave detection of atmospheric NO, obtained through ground‐based observations of 250.796 GHz (1.2 mm) line emission from lower thermospheric NO. The observations were conducted with the National Radio Astronomy Observatory (NRAO) millimeter telescope at Kitt Peak, Arizona (32°N, 111°W) on April 1–3, 1991. The thermal linewidth of the measured NO spectrum corresponds to an average atmospheric temperature of 300 K which, combined with the observed lack of pressure broadening, places the origin of the NO emission at altitudes of 100–120 km. The thermospheric column (altitude >70 km) density of NO inferred from the observed line emission is

cm−2, indicating extraordinarily high abundances of lower thermosphere NO for the current solar maximum period. During the three days of observations, the NO line emission did not exhibit diurnal or daily variations to within the 10–15 % measurement uncertainties.
Travel‐time tomography in the northern Coachella Valley using aftershocks of the 1986 ML 5.9 North Palm Springs Earthquake
Authors: Nicholson Craig , Lees Jonathan M.
Published: February 2008

Tomographic inversion is applied to delay times from aftershocks of the 1986 ML 5.9 North Palm Springs (NPS) earthquake to image 3‐D velocity variations within the northern Coachella Valley. P‐wave arrival times from 1074 earthquakes, with depths ranging from 3 to 20 km, were used as sources recorded by 12 portable and 4 permanent stations. Preliminary results show well‐defined high‐ and low‐velocity anomalies (2–7%) that correlate with the rupture distribution of the 1986 mainshock. At depths less than 8 km, a low‐velocity anomaly predominates between the two NE‐dipping Banning and Mission Creek faults. From 8 to 12 km in depth, where the NPS mainshock and most of the aftershocks occur, a high‐velocity anomaly is observed. This high‐velocity feature is interpreted as imaging the asperity responsible for the 1986 rupture; and suggests that velocity information may help to define important elements, such as asperities, that control fault rupture, and thus, may help to predict the location and size of future events.
SO2 absorption cross‐section measurements from 197 nm to 240 nm
Authors: Martinez Richard D. , Joens Jeffrey A.
Published: February 2008

Absorption cross‐sections are reported for SO2 for wavelengths between 197 nm and 240 nm at a temperature of 300K and a spectral bandwidth of 0.10 nm. The results are in good agreement with those previously given by Warneck et al [1964] if a correction is applied to their wavelength scale. Differences between the present results and the high resolution cross‐sections reported by Freeman et al [1984] at 213K, degraded to a resolution of 0.10 nm, can be attributed to changes in the spectrum with temperature.
Magnetospherically reflected whistlers as a source of plasmaspheric hiss
Authors: Draganov A. B. , Inan U. S. , Sonwalkar V. S. , Bell T. F.
Published: February 2008

Ray‐tracing simulations and estimates of whistler wave damping show that magnetospherically reflected whistlers can persist for ∼102 s in a low frequency band (ƒ ∼1 kHz). The combined contribution from whistler rays produced by a single lightning flash but entering the magnetosphere at different points form a continuous hiss‐like signal, as observed at a fixed point. Estimates indicate that the total whistler wave energy input into the magnetosphere from lightning discharges may maintain experimentally observed levels of magnetospheric hiss.
Plasma motion in the Venus ionosphere: Transition to supersonic flow
Authors: Whitten R. C. , Barnes A. , McCormick P. T.
Published: February 2008

A remarkable feature of the ionosphere of Venus is the presence of nightward supersonic flows at high altitude near the terminator. In general the steady flow of an ideal gas admits a subsonic‐supersonic transition only in the presence of special conditions, such as a convergence of the flow followed by divergence, or external forces. In this paper we show that the relatively high pressure dayside plasma wells up slowly, and at high altitude it is accelerated horizontally through a relatively constricted region near the terminator toward the low‐density nightside. In effect, the plasma flows through a “nozzle” that is first converging, then diverging, permitting the transition to supersonic flow. Analysis of results from previously published models of the plasma flow in the upper ionosphere of Venus shows how such a “nozzle” is formed. The model plasma does indeed accelerate to supersonic speeds, reaching sonic speed just behind the terminator. The computed speeds prove to be close to those observed by the Pioneer Venus orbiter, and the ion transport rates are sufficient to produce and maintain the nightside ionosphere.
Turbulent Alfven boundary layer in the polar ionosphere: 1. Excitation conditions and energetics
Authors: Trakhtengerts V. Yu. , Feldstein A. Ya.
Published: February 2008

Instability of laminar magnetospheric convection with respect to the strongly anisotropic Alfven waves which are of small scale in the horizontal plane is examined. The waves prove to be trapped in the ionospheric Alfven resonator, bounded from below by the ionospheric E layer and from above by a zone of rapidly increasing Alfven velocity at altitudes of up to ∼104 km. The finite‐amplitude Alfven waves dissipate within a layer of anomalous resistance formed near the upper wall of the resonator. As a result, a high‐energy particle source appears in the upper ionosphere. Further evolution results in the transition of laminar convection to turbulent flow conditions and in the formation of a turbulent Alfven boundary layer in the polar ionosphere at altitudes from 10² to 104 km. The energy status of the turbulent Alfven boundary layer is calculated. It has been shown that the accelerated‐electron energy flux density can reach ∼100 ergs cm−2 s−1.
Observations of ion‐neutral collisional effects in the auroral E region
Authors: Watanabe S. , Whalen B. A. , Wallis D. D. , Pfaff R. F.
Published: February 2008

Thermal ion energy distribution functions and local electric and magnetic fields were directly measured for the first time in the ionospheric E region on board rockets which were launched as part of the E Region Rocket/Radar Instability Study (ERRRIS) from Esrange, Sweden, in 1988 and 1989 to investigate plasma instabilities in the auroral ionosphere. Measured ion distribution functions were fitted to shifted Maxwellian distributions, and their resulting ion drift velocities were compared with E×B/B² velocities from the double‐probe electric field observations. The results show that the ion drift direction rotates with respect to the local electric field direction and that the ratio of the magnitudes of the ion velocity to the E×B/B² velocity decreases with decreasing altitudes. Using these observations, the quiet time ion‐neutral collision frequencies and neutral wind velocities were estimated and found to be consistent with theoretical estimates. However, significant discrepancies between observations and theory are found in the disturbed E region near auroral particle precipitation regions. These data indicate that the auroral atmosphere is significantly perturbed due to Joule as well as particle heating effects.
Resonant acceleration of alpha particles by ion cyclotron waves in the solar wind
Authors: Gomberoff L. , Elgueta R.
Published: February 2008

Preferential acceleration of alpha particles interacting with left‐hand polarized ion cyclotron waves is studied. It is shown that a small positive drift velocity between alpha particles and protons can lead to alpha particle velocities well in excess of the proton bulk velocity. During the acceleration process, which is assumed to take place at heliocentric distances less than 0.3 AU, the alpha particle drift velocity should exceed the proton bulk velocity, and then the gap which exists around the alpha particle gyrofrequency should disappear. It is also shown that for proton thermal anisotropies of the order of those observed in fast solar wind, the waves either grow or are not damped excessively, so that the waves can exist and might thus lead to the observed differential speeds. However, the way in which the alpha particles exceed the proton velocity remains unexplained.
Magnetospheric cavity modes: Some nonlinear effects
Authors: Allan W. , Manuel J. R. , Poulter E. M.
Published: February 2008

Linear models of magnetospheric cavity modes are unlikely to be directly applicable to events generated by, for example, large solar wind pressure pulses. To determine basic nonlinear effects, we apply a two‐dimensional, nonlinear hydromagnetic computer code to the problem of pure fast mode cavity resonances in the simplest possible box model of the magnetosphere, stimulated by a magnetopause compression and relaxation with maximum velocity ν0, normalized against the Alfvén speed at the magnetopause boundary. The results show that significant nonlinear effects can occur for ν0 ∼ 0.2. The initial impulse develops into a relatively weak shock as it propagates into the magnetosphere. After cavity modes are set up, their temporal development is very different from the corresponding linear modes. In particular, the cavity mode frequencies are reduced, and the temporal structure of the modes is apparently not well represented by sinusoidal functions. Much of the change in behavior can be attributed to a large distortion of the background plasma mass density, which we consider to be an effect of the ponderomotive force (PMF). We develop expressions for the PMF as it applies to small but finite amplitude cavity modes in our box model and show that these expressions are consistent with the simulation results. For a cold plasma, extreme density enhancements occur at and near the magnetospheric equator. We consider such large enhancements to be unlikely, but enhancements of the order of 100% can occur in a warm plasma, accompanied by adiabatic heating within the enhancements, increasing the plasma temperature by a factor of about 2. The density enhancements are limited by the propagation of slow magnetosonic perturbations away from the positions of maximum density. This leads to quasi‐cyclic density structures as the perturbations reflect from the ionospheric boundaries, the quasi‐period depending on the sound speed and therefore the plasma parameter β. Complex density structures appear to evolve rapidly when β is of the order of 0.05. We consider that mass transport by the PMF is likely to occur in the magnetosphere when relatively large ULF wave fields exist, regardless of the exact driving mechanism.
The interaction of a very large interplanetary magnetic cloud with the magnetosphere and with cosmic rays
Authors: Lepping R. P. , Burlaga L. F. , Tsurutani B. T. , Ogilvie K. W. , Lazarus A. J. , Evans D. S. , Klein L. W.
Published: February 2008

A large interplanetary magnetic cloud has been observed in the mid‐December 1982 data from ISEE 3. It is estimated to have a heliocentric radial extent of ≳0.4 AU, making it one of the largest magnetic clouds yet observed at 1 AU. The magnetic field measured throughout the main portion of the cloud was fairly tightly confined to a plane as it changed direction by 174° while varying only moderately in magnitude. Throughout nearly the entire duration of the cloud's passage, IMP 8 was located in the Earth's dawn magnetosheath providing observations of this cloud's interaction with the bow shock and magnetopause; the cloud is shown to maintain its solar wind characteristics during the interaction. Near the end of the cloud passage, at 0806 UT on December 17, ISEE 3 (and IMP 8 at nearly the same time) observed an oblique fast forward interplanetary shock closely coincident in time with a geomagnetic storm sudden commencement. The shock, moving much faster than the cloud (radial speeds of 700 and 390 km/s, respectively, on the average), was in the process of overtaking the cloud. The index Dst decreased monotonically by ≈130 nT during the 2‐day cloud passage by the Earth and was well correlated with the Bz component of the interplanetary magnetic field. There was no significant decrease in the cosmic ray intensity recorded by ground‐based neutron monitors at this time of rather strong, smoothly changing fields. However, a Forbush decrease did occur immediately after the interplanetary shock, during a period of significant field turbulence. Thus a large, smooth, interplanetary helical magnetic field configuration engulfing the Earth does not necessarily deflect cosmic rays sufficiently to cause a Forbush decrease, but there is a suggestion that such a decrease may be caused by particle scattering by turbulent magnetic fields. Finally, the field observations within the cloud are reasonably well described by a model developed by Burlaga (1988).
On the upward acceleration of electrons and ions by low‐frequency electric field fluctuations observed by Viking
Authors: Hultqvist Bengt , Vo Hien , Lundin Rickard , Aparicio Bruno , Lindqvist Per‐Arne , Gustafsson Georg , Holback Bengt
Published: February 2008

The dependence of the generation of upward electron beams in the topside ionosphere on the amplitude of the slow electric field fluctuations and the magnitude of the dc potential difference in the acceleration region is found to agree with the model involving the fluctuating electric field component presented in 1988 by Hultqvist. The ratio between the amplitude of the electric field fluctuations, Eƒ determined by the dipole electric field experiment, and the dc acceleration voltage Vacc, given by the energy of the upflowing ion beam, is much higher on average for satellite spins showing an upward electron beam than for the spins without. It is also shown that low plasma density favors the generation of electron beams.
On the influence of localized electric fields and field‐aligned currents associated with polar arcs on the global potential distribution
Authors: Marklund G. T. , Blomberg L. G.
Published: February 2008

The influence of localized field‐aligned current, associated with intense transpolar arcs mostly occuring during periods of northward interplanetary magnetic field (IMF), on the global electrodynamics has been investigated using a numerical simulation model. Idealized field‐aligned current distributions representing both the region 1/2 system of the auroral oval and the transpolar arc as well as a corresponding ionospheric conductivity distribution are fed into the model to calculate the potential distributions. The transpolar arc has been represented by a few alternative field‐aligned current distributions which are different in the way the downward return currents are distributed in the ionosphere. For the case with a single, upward current sheet the potential pattern assumes a form similar to that typical for IMF By positive conditions, namely a large dusk cell with sunward drift reaching very high latitudes and a crescent‐shaped dawn cell. If the conductivity of the main auroral oval is comparable to that of the polar arc the dusk cell will have two local potential minima and thus a region of weak antisunward convection in between. For the cases with two equal but oppositely directed current sheets the potential patterns are very similar to the symmetrical two‐cell reference pattern associated with solely the region 1/2 system with an exception for the immediate vicinity of the theta aurora. Depending on the direction of the polar arc current sheets the dawn‐dusk electric field will either be reversed (or weakened) or intensified at the location of the transpolar arc. The presence of a reversal depend, however, not only on the relative magnitude between the polar arc currents and those of the region 1/2 system but also on the characteristics of the acceleration region and of the conductivity distribution associated with the polar arc. Comparisons are made between the model results and Viking electric field data for a number of polar arc crossings to reveal the most common electrodynamical signatures of these auroral phenomena.
The low‐frequency spectral behavior of cosmic ray intensity
Authors: Kudela K. , Ananth A. G. , Venkatesan D.
Published: February 2008

Power spectral analysis of cosmic ray intensity recorded by neutron monitors at Calgary and Deep River, Alberta, was carried out over a wide range of frequencies from 3×10−9Hz to 6×10−6Hz (periodicities from 2 days to 11 years) during the epoch 1965–1989 and revealed different behavior of the power spectral density for the three ranges of frequency domains. At low frequencies corresponding to the periodicity T ≈ 20 months the power spectrum shows an abrupt change in the level and profile of power spectral density (PSD). This indicates that the processes responsible for the long‐period variations are different from the one which causes short‐period variations. At middle frequencies corresponding to periodicities between 6 and 18 months, the PSD indicates that the periodicities are not stable and show a transient character over the entire epoch of analysis. The PSD for periods T ≈ 27 days indicates an 11 year recurring tendency related to solar activity. The PSD for periods T > 27 days indicates a depression during the minimum of solar activity showing a 22 year recurring tendency and suggests that it is controlled by drift effects which are correlated to the solar polar field reversal. For higher frequencies f > 2×10−6Hz corresponding to periods T < 6 days, the slope of the PSD is found to be similar to that of the interplanetary magnetic field inhomogeneties measured at different radial distances between 1 and 30 AU.
Magnetic field reconnection patterns at the dayside magnetopause: An MHD simulation study
Authors: Shi Y. , Wu C. C. , Lee L. C.
Published: February 2008

Magnetic field configurations associated with dayside reconnection are studied by a global two‐dimensional incompressible MHD simulation. Possible dayside reconnection patterns revealed in the simulation include magnetic field configurations associated with steady state single X line reconnection, impulsive multiple X line reconnection, and bursty single X line reconnection. Three physical parameters, the global magnetic Reynolds number Rm, the critical current density Jc for the resistivity enhancement, and the solar wind Alfvén Mach number MASW are found to be the most important parameters in determining the reconnection pattern. First, it is found that single X line reconnection tends to occur for small Rm (<100) and multiple X line reconnection takes place for large Rm (>200). Second, in the presence of a current‐dependent resistivity and large Rm various patterns of multiple X line reconnection are observed. Third, for a large MASW dayside reconnection tends to occur in the high‐latitude region. It is found in particular that multiple X line reconnection can indeed generate the primary flux transfer event signature, i.e., the bipolar pulse of normal magnetic field component. In addition, at the trailing edge of the bipolar pulses, enhanced plasma speed is observed in the simulation.
Deuterium and hydrogen flows in the thermosphere
Authors: Breig E. L. , Hanson W. B.
Published: February 2008

We describe an analysis of processes affecting the vertical distributions and flows of deuterium and hydrogen between 100 and 500 km in the daytime thermosphere. A steady state numerical model includes HD and H2 molecular flows, relevant chemistry, and transport by both molecular and eddy diffusion. The magnitude and height variation of the vertical fluxes are determined that satisfy preestablished conditions on concentrations at the two boundaries. The results confirm the suggestion of Breig et al. (1987) that the outward thermospheric fluxes and hence the planetary loss rates for deuterium must be large compared to evaporative escape, as is also the case for hydrogen. Using the observational hydrogen data in the mesosphere and lower thermosphere and AE‐C measurements of hydrogen and deuterium in the thermosphere, we derive nominal upward fluxes through the daytime thermosphere that exceed the corresponding Jeans evaporative escape rate by factors of ∼5 for H and in excess of 500 for D. We have examined the dependence of these results on several pertinent parameters; while the flows are especially sensitive to the total abundance of each species and the molecular diffusion rates in the lower thermosphere, no reasonable changes were found sufficient to reduce the outward thermospheric flux of either D or H to the level associated with evaporative escape. We must thus conclude that large vertical flows of both deuterium and hydrogen in the thermosphere and comparably large planetary losses are required to provide consistency between current theory and the available observational data.
Beam‐density gradient drift instabilities and heating in the plasma sheet boundary layer
Authors: Zhou G. C. , Zhu L. F.
Published: February 2008

The linear and second‐order theories of ion beam‐density gradient drift instabilities on the outer edge of the inhomogeneous plasma sheet boundary layer (PSBL), with warm current sheet ion beams and cold ionospheric ion beams drifting in opposite directions, are examined. It is shown that combined interactions of ion beams and plasma density gradient drifts can excite electrostatic ion beam‐density gradient drift instabilities. These instabilities combine to form the broadband maximum growth rate spectrum with the peak growth rate ∼ 0.34ωlhat the obliquely propagating low frequency, whereωlhis the lower hybrid frequency. These linear results can be used to explain the observed broadband electrostatic noise on the outer edge of the PSBL. The calculations for the second‐order heating rate show that three particle components in the model may be heated by electrostatic waves which result from ion beam‐density gradient drift instabilities. The second‐order heating rates for the cold ionospheric ion beams are the highest. The heating rate profiles with respect to the ambient magnetic field for each component are anisotropic and similar to the maximum growth rate profiles. The peak heating rate occurs at the obliquely propagating low frequency which corresponds to the peak growth rate. The second‐order heating considered here is due to resonance broadening of weak resonant components. These results may be used to understand transport processes on the outer edge of the PSBL.
Plasmapause morphology determined from an empirical ionospheric convection model
Authors: Doe Richard A. , Moldwin Mark B. , Mendillo Michael
Published: February 2008

The steady state electric potentials derived from empirical ionospheric convection models were mapped to the magnetospheric equatorial plane and added to the Earth's rotational field to examine the local time dependence of the plasmapause radius. The last closed equipotential line was assumed to be the plasmapause. The sensitivity of plasmapause shape and size derived in this way was studied by varying the z and y components of the interplanetary magnetic field (IMF) and the geomagnetic activity index Kp. Results show that the Heppner and Maynard model produces plasmasphere morphology that is broadly consistent with prior ground‐based and satellite measurements of plasmapause position. Our model results show that the largest change in the characteristic shape of the plasmapause is due to the direction of IMF Bz; for IMF Bz southward, the plasmapause is circular, while a broad oval plasmapause (with a bulge at 2100 UT) is found for IMF Bz northward. The ability of our model to reproduce the Lpp‐LT shape of the plasmapause as found by previous observations shows that empirical ionospheric convection models can be extended to provide a useful method to determine plasmapause location for a variety of steady geomagnetic conditions.
Computer simulations of field‐aligned currents generated by fast magnetic reconnection in three dimensions
Authors: Ugai M.
Published: February 2008

MHD simulations have studied the evolution of an electric current system associated with fast reconnection development in three dimensions. The initial equilibrium is a one‐dimensional antiparallel magnetic field with a current sheet (or plasma sheet) in the middle. Fast reconnection is initiated by a localized resistivity and spontaneously develops in a finite region of the current sheet, leading to an Alfvénic plasma jet. Ahead of the plasma jet the plasma sheet is notably compressed, forming a plasmoid which swells and propagates. In the fast reconnection region, notable plasma rarefaction occurs so that the plasma flow converges, whereas in the plasmoid, plasma is compressed so that the plasma flow diverges. The resulting velocity shear gives rise to a magnetic shear along the plasmoid boundary, which is directly related to a (positive) field‐aligned current along the zero‐order magnetic field. The field‐aligned current is thus formed like a tube extending along the plasmoid boundary, and it becomes larger as the plasma compression in the plasmoid becomes larger. The fast reconnection development in three dimensions causes a drastic change in the overall current system to generate a redistribution of current.
Convection of ion cyclotron waves to ion‐heating regions
Authors: Rönnmark Kjell , André Mats
Published: February 2008

Low‐frequency waves associated with ion conics have been observed in the central plasma sheet, in a region where there are no obvious sources of free energy that could destabilize these waves locally. We consider ion cyclotron waves generated in the equatorial plane by a proton temperature anisotropy and use computed growth rates to create a model wave distribution. Using ray tracing and conservation of the wave distribution function along phase space rays, we then map the wave intensities from the equatorial plane to the top of the ion‐heating region. We find that the spectral density at a geocentric distance of 2.8 RE will be about 10 times higher than that in the equatorial region. Thus, convection from the equatorial plane could explain the observed spectral density of 10−6 V2 m−2 Hz−1 and the associated oxygen ion heating.
Coordinated radar and optical measurements of stable auroral arcs at the polar cap boundary
Authors: Weber E. J. , Vickrey J. F. , Gallagher H. , Weiss L. , Heinselman C. J. , Heelis R. A. , Kelley M. C.
Published: February 2008

A specialized incoherent scatter radar scanning mode has been developed for use in conjunction with simultaneous real‐time all‐sky images. These complementary diagnostics are used to examine the aeronomy and electrodynamics of stable auroral arcs that delineate the boundary between the polar cap and the auroral oval. The first arc discussed, observed at 2000 MLT, represents the boundary between antisunward plasma flow in the polar cap and sunward return flow equatorward of the arc. The arc defined an equipotential in the high‐latitude convection pattern in that no plasma flowed across the arc. The radar line‐of‐sight velocity measurements also indicate that this arc is consistent with a convergent electric field and an associated weak upward field‐aligned current. The second arc was observed at 2330 MLT and was associated with a nightside gap or magnetic reconnection region. Strong antisunward flow was observed directly across the arc, although a velocity shear was superposed on this steady flow along the poleward edge of the arc. Detailed plasma density, temperature, and line‐of‐sight velocity measurements from the radar are presented for both arcs to define the electric field, horizontal and field‐aligned currents, and thermal plasma parameters associated with these arcs.
Venus ionospheric tail rays: Spatial distributions and interplanetary magnetic field control
Authors: Ong M. , Luhmann J. G. , Russell C. T. , Strangeway R. J. , Brace L. H.
Published: February 2008

Venus tail rays were first observed in the Pioneer Venus Langmuir probe data. These rays appear to be filaments of ionospheric plasma extending far downstream from the nightside ionosphere. The characteristic signatures of tail rays are in many ways similar to those of plasma “clouds” observed above the terminator ionopause. These signatures include a localized electron density enhancement, and occasional magnetic field reversal in the Bx component and/or enhanced electric field noise. Here, selected Pioneer Venus orbits from the interval between orbits 700 and 3500 during the period November 1980 to May 1988 are surveyed for tail ray structures as seen in electron density time series from the Orbiter Langmuir probe. After rotating the upstream transverse magnetic field (interplanetary magnetic field (IMF)) to point in the same direction, the resulting distribution of tail rays is found to be scattered rather than organized around a central “plasma sheet” in the plane perpendicular to the IMF. If tail rays originate from clouds, this lack of organization is consistent with the observed scatter of cloud locations around the planet in the terminator plane. On the other hand, statistical analyses indicate that tail ray formation does not appear to require an extraordinary degree of rotation of the IMF during the pass, unlike the clouds. The steadiness of the IMF during passes containing tail rays is further supported by gas dynamic magnetosheath field modeling. Only one upstream magnetic field vector is needed as input to simulate the magnetometer time series for the magnetosheath during orbits on which tail rays are seen. Thus, although tail rays and clouds exhibit similar plasma and field signatures, there is no conclusive evidence that tail rays evolve from clouds. The results of the present study suggest that tail rays are a normal feature of the steady solar wind interaction with Venus but are not generally associated with a central tail plasma sheet. Instead, a terminator source, or multiple sources, is suggested.
The quiet geomagnetic field at geosynchronous orbit and its dependence on solar wind dynamic pressure
Authors: Rufenach Clifford L. , McPherron Robert L. , Schaper Justin
Published: February 2008

Vector magnetic fields at geosynchronous orbit were measured during 1980‐1984 using the operational GOES 2, GOES 5, and GOES 6 spacecraft magnetometers. We corrected these spacecraft measurements for offsets due to spacecraft state and then used these field estimates to create a data base with 1‐min resolution. Hourly quiet field values were calculated for these years from this data base using the ground‐based geomagnetic index criteria AE < 120 nT and |Dst| < 20 nT. These quiet field components, rotated into dipole HVD coordinates, were approximated by the first two coefficients of a two‐dimensional Fourier series in time of day and season. These Fourier harmonics provide a compact method of approximating the quiet synchronous field at any time of day and any season. The quiet geosynchronous field components, to first order, are given by mean values of about 90 nT, −60 nT, and 5 nT; and sinusoidal diurnal amplitudes of about 21 nT, 5 nT, and 5 nT, respectively, for H, V, and D where the spacecraft magnetometer was located near the geomagnetic meridian. The second harmonic diurnal amplitudes and the first and second harmonic seasonal amplitudes are typically of the order of a few nanoteslas or less except for the D component, which exhibits a larger seasonal variation. Furthermore, a one‐dimensional Fourier series in time of day was used to study the quiet field dependence on solar wind dynamic pressure, Pd, by indexing the measurements into five pressure ranges during 1980. The H component of the quiet field increased 4.6 nT from 80.2 to 84.8 nT in its mean amplitude and 20.8 nT from 11.9 to 32.7 nT in its first harmonic amplitude for Pd increasing from 0.71 × 10−8 to 3.31 × 10−8 dyn/cm². These quiet H measurements, including the pressure dependence, are compared with a first‐order field model (Mead, 1964) superimposed with a tail current, resulting in magnetospheric currents (magnetopause and tail) in agreement with previous model values. The measured field pressure dependence and the Mead model suggest a tail current dependence on pressure.
Two encounters with the flank low‐latitude boundary layer: Further evidence for closed field topology and investigation of the internal structure
Authors: Traver D. P. , Mitchell D. G. , Williams D. J. , Frank L. A. , Huang C. Y.
Published: February 2008

We examine the structure of the flank low‐latitude boundary layer (LLBL) through differential energy spectra and particle angular anisotropies for traversals of the dawn flank (December 19, 1977) and dusk flank (July 7, 1978) during periods of predominantly northward magnetosheath field orientation. The LLBL during these crossings is known to consist of two regions with distinct energetic particle signatures, known in previous work (Williams et al., 1985) as the stagnation region and the LLBL (referred to in this paper as “outer LLBL”). We present spectra obtained from combined ISEE 1 low‐energy proton and electron differential energy analyzer (LEPEDEA) and medium energy particle instrument (MEPI) data extending over the 200 eV/q to 2 MeV energy range for the plasma sheet, stagnation region, outer LLBL and magnetosheath regions. The stagnation region is shown to be a unique region on the basis of abrupt changes observed in energy spectra, as well as the previously identified changes in bulk flow velocity, and flow azimuth upon entry into the region (Williams et al., 1985). This uniqueness is suggested to be the result of different convective histories (different source populations), but not the result of distinctly different physical mechanisms. The stagnation region and the outer LLBL are each a mixture of plasma sheet and magnetosheath populations, but the stagnation region contains a relatively higher fraction of plasma sheet particles, consistent with its placement earthward of the outer LLBL. No obvious ion heating was observed for any of the crossings of the magnetopause or of the LLBL for either event. Evidence for energization of thermal electrons (either magnetosheath or ionospheric in origin) appears during the dusk flank crossing. Bidirectional field‐aligned ion distributions are observed with typically 5‐to‐1 enhancement of the flux along the magnetic field during certain portions of the dusk flank crossing. Bidirectional field‐aligned electron distributions appear in the outer LLBL, stagnation region, and adjacent plasma sheet during both dawn and dusk flank crossings with typical ratios between field‐aligned and perpendicular flux on the order of 10‐to‐1. We find a quantitative flux balance in opposite field directions for these electrons in the stagnation region, outer LLBL, and adjacent plasma sheet on both dawn and dusk flanks, supporting the conclusion that the magnetic field is topologically closed during these crossings.
Linear and nonlinear dust drift waves
Authors: Shukla P. K. , Yu M. Y. , Bharuthram R.
Published: February 2008

The linear and nonlinear properties of low‐frequency motion in inhomogeneous, magnetized, dusty plasmas are investigated. The dynamics of the dust grains are taken into account by means of a fluid model. A new type of low‐frequency waves, the dust‐drift waves, is shown to exist. The nonlinear mode coupling equations are derived, and the possibility of propagating vortex structures is discussed. The dust‐drift waves may be relevant in astrophysical and cometary plasmas.
Mechanism by which merging at X lines causes discrete auroral arcs
Authors: Atkinson G.
Published: February 2008

A mechanism is presented by which X lines cause auroral arcs. The mechanism should apply to most existing steady state merging models if the fluid is an electron‐ion plasma. Hall electric fields are produced in the plane perpendicular to the X line in order to equalize the flow of electrons and ions through the diffusion region despite their differences in inertia. It is suggested that these electric fields map along magnetic field lines to the auroral acceleration region at much lower altitudes. Since mapping along field lines occurs by oblique Alfvén waves, stationary in the convective flow, there are accompanying Birkeland currents. The currents and fields provide the upper boundary condition for the auroral electron acceleration region believed to exist above discrete arcs. A solution of a flow problem, similar to the merging situation, by Sonnerup and Wang is used to show that the voltage is sufficient for auroral arcs and that momentum transfer between the diffusion and exterior regions is consistent with the production of “paired electrostatic shocks” for a rigorous solution. An integration along a path through the diffusion region for the general case shows that auroral‐sized voltages are expected in both collisional and collisionless cases. Finally it is shown that the ion‐inertial scale length is appropriate for the diffusion region in the collisionless case, and maps to the ionosphere as ∼10 km.
Nitric oxide in the middle to upper thermosphere
Authors: Siskind David E. , Rusch David W.
Published: February 2008

We review the results of six rocket observations of thermospheric nitric oxide and attempt to reconcile them with the available laboratory photochemical data. Specifically, we assess the impact of the recently revised recommendation for the N(4S) + O2 rate coefficient on photochemical models. Use of the new rate coefficient leads to a significantly enhanced production of NO, particularly at F region altitudes during solar maximum conditions. A comparison of photochemical calculations with the rocket profiles indicates that the new rate coefficient introduces a significant discrepancy which can be resolved if the recombination reaction of N + NO is temperature dependent. Our best fit value for the N + NO rate coefficient at thermospheric temperatures is 1.6×10−10 exp (−(460 ± 60)/T). The temperature dependence of this rate coefficient disagrees with the current recommendation from the Jet Propulsion Laboratory, but it is in better agreement with another, earlier laboratory measurement. Calculations using our proposed rate coefficient predict the NO solar cycle variation at 180 km to be less than at 140 km which is also in agreement with the observations. It is likely that the use of these new rate coefficients will affect calculations of the thermal budget of the upper atmosphere as well as the downward transport of NO into the middle atmosphere.
Electron distributions upstream of the comet Halley bow shock: Evidence for adiabatic heating
Authors: Larson D. E. , Anderson K. A. , Lin R. P. , Carlson C. W. , Réme H. , Glassmeier K. ‐H. , Neubauer F. M.
Published: February 2008

We report on three‐dimensional plasma electron (22 eV to 30 keV) observations upstream of the comet Halley bow shock, obtained by the RPA‐1 COPERNIC (Rème Plasma Analyzer ‐ Complete Positive Ion, Electron and Ram Negative Ion Measurements near Comet Halley) experiment on the Giotto spacecraft. Besides electron distributions typical of the undisturbed solar wind and backstreaming electrons observed when the magnetic field line intersects the cometary bow shock, we find a new type of distribution characterized by enhanced low energy (<100 eV) flux which peaks at 90° pitch angles. These are most prominent when the spacecraft is on field lines which pass close to but are not connected to the bow shock. The 90° pitch angle electrons appear to have been adiabatically heated by the increase in the magnetic field strength resulting from the compression of the upstream solar wind plasma by the cometary mass loading. We present a model calculation of this effect which agrees qualitatively with the observed 90° flux enhancements.
The density minimum at the Earth's magnetic equator
Authors: Olsen R. C.
Published: February 2008

Observations of the density structure in the plasmapause region reveal the existence of a local minimum in the total electron density at the magnetic equator. Data from the plasma wave instrument and ion mass spectrometer on the Dynamics Explorer (DE) 1 satellite are used to study this phenomenon. The density depletion typically extends from ±5° to ±20° in latitude and is found at altitudes from 2 to 5 RE. Density depletions of 10‐70% are found in regions where the off‐equator density ranges from 10 to 1000 cm−3. This density structure is associated with equator crossings where the thermal plasma has been heated over normal plasmasphere values. The heated plasma is the equatorially trapped plasma previously reported from DE 1 and the SCATHA satellite. Within the plasmasphere, the drop in total (electron) density corresponds to a decrease in the cold‐ion density, in both H+ and He+. There is a rough pressure balance provided by the warm tail of the distribution, which is a few percent by density but 1–2 orders of magnitude higher in temperature.
Observation of dust shedding from material bodies in a plasma
Authors: Sheridan T. E. , Goree J. , Chiu Y. T. , Rairden R. L. , Kiessling J. A.
Published: February 2008

Exposure to a space plasma can cause a dusty body, such as a spacecraft or a boulder in Saturn's rings, to release dust into its environment. This is demonstrated in a laboratory experiment with an aluminum sphere covered with micrometer‐sized dust grains. The sphere was rotating and electrically floating like an object in space. Laser light scattering was used to detect dust falling from the body. When a low‐temperature nitrogen plasma was turned on, rapid dust shedding was observed, and when it was turned off, the shedding stopped. The rate of shedding increases with plasma density. The dust is not all released the instant the plasma is turned on but rather takes place over an extended period of time, with individual grains jumping off at random intervals with a certain probability per unit time.