bluuebunny/crossref_metadata_2025 huggingface dataset:
High‐temperature rheology of enstatite: Implications for creep in the mantle
Authors: Mackwell Stephen J.
Published: February 2008

High‐temperature deformation experiments have been performed on oriented single crystals of enstatite at 1 atm under controlled conditions of oxygen fugacity and silica activity. The results indicate that creep on the weakest high temperature slip system in enstatite within the protoenstatite stability field is independent of oxygen fugacity and obeys a power law relation with a stress exponent of n = 3.8±0.5 and activation energy of Q = 820±80 kJ/mol. A decrease in creep rate with increasing aluminum and iron content suggests that aluminum or iron ions play an active role in maintaining charge neutrality within the crystals. The single crystal strengths are greater than those reported for polycrystalline enstatite deforming predominantly on the same slip system, suggesting that the polycrystalline samples may have been water weakened. A comparison of the single crystal results with those for olivine deformed at 1 atm indicates that enstatite is probably stronger than olivine under upper mantle conditions, unless the presence of water results in a greater weakening of enstatite than olivine.
Lunar surface composition and solar wind‐Induced secondary ion mass spectrometry
Authors: Elphic R. C. , Funsten H. O. , Barraclough B. L. , McComas D. J. , Paffett M. T. , Vaniman D. T. , Heiken G.
Published: February 2008

The Moon has no strong global magnetic field and only a tenuous atmosphere, so solar wind ions (∼95% H+, 5% He++) directly bombard the lunar surface, sputtering atoms and secondary ions from the exposed grains of the regolith. The secondary ions potentially provide surface composition information through secondary ion mass spectrometry (SIMS), a standard laboratory surface composition analysis technique. In this paper we report the results of laboratory SIMS experiments on lunar soil simulants using solar wind‐like ions. We find that H+ and He++, while not efficient sputterers, nevertheless produce significant fluxes of secondary lunar ions, including Na+, Mg+, Al+, Si+, K+, Ca+, Ti+, Mn+ and Fe+. We predict that lunar surface secondary‐ion fluxes range between ∼10 and 104 ions cm−2 s−1, depending on the species.
A field study of small raindrop oscillations
Authors: Beard Kenneth V. , Tokay Ali
Published: February 2008

Measurements were made at the ground in Hawaiian showers to detect oscillations of small raindrops as predicted by recent laboratory experiments. Oscillation frequencies and amplitudes were determined from photographs using backscattered light of the primary rainbow. A strobe provided highlights so that the fall speeds and corresponding sizes of each drop could be calculated. The drop‐size distributions were measured independently with a collocated disdrometer. The observations of 0.3 mm to 2.0 mm diameter raindrops clearly showed the onset of oscillations near 1 mm, at the same size where eddy shedding begins. Alternative causes of drop oscillations from drop collisions and wind shear were eliminated using estimates based on drop‐size distributions and winds. Our inferred oscillation sizes, modes and amplitudes were consistent with several previous measurements where small raindrops were found to be generally more spherical than the non‐oscillating shapes predicted by theory. This field study firmly establishes eddy shedding as the major cause of natural shape variations for small raindrops in precipitation—and provides further evidence that significant corrections are needed in estimating the scattering of microwaves in light to heavy rain showers (e.g., for interpreting polarization radar data and evaluating rain outages in communication links).
Radar detection of centimeter‐sized orbital debris: Preliminary Arecibo observations at 12.5‐CM wavelength
Authors: Thompson T. W. , Goldstein R. M. , Campbell D. B. , Stansbery E. G. , Potter A. E.
Published: February 2008

Orbital debris objects with sizes of 6 mm and larger were detected when they passed through the main beam of the high‐power, 12.6‐cm wavelength radar at the Arecibo Observatory. The number of objects detected at altitudes below 1000 km in the size range of 6 mm to 2 cm agreed with orbital debris model predictions to better than a factor of two. Radar detections of larger orbital debris (10‐cm diameter and above) were also compared with the objects in the U.S. Space Command catalog (the cataloged objects have UHF radar cross‐sections larger than 0.01 m2). This experiment demonstrated that the large, Earth‐based radar telescopes normally used for planetary studies can provide useful data about the near‐Earth orbital debris populations.
Electron microscope studies of Mt. Pinatubo aerosol layers over Laramie, Wyoming during summer 1991
Authors: Sheridan Patrick J. , Schnell Russell C. , Hofmann David J. , Deshler Terry
Published: February 2008

Stratospheric aerosol layers resulting from the June 1991 eruptions of Mt. Pinatubo were first observed over Laramie, Wyoming in July 1991. Atmospheric particles were collected from these layers during three balloon flights in July and August using cascade impactors. Analytical electron microscope analysis of the aerosol deposits indicated that a large majority (> 99%) of the fine particles in all three samples were collected as submicrometer aqueous H2SO4 droplets, which changed to (NH4)2SO4 particles over time. Other particles observed in the aerosol were larger, and consisted of supermicrometer sulfate particles and composite sulfate/crustal particles which ranged up to ∼10 μm in size. Peak aerosol concentrations for r > 0.15 μm diameter particles (determined by optical particle counters) in the layers were higher for the July flights than for the August sounding. This was reflected in the electron microscope results, which showed that the July impactor samples had particulate loadings on the fine particle stages which were 20–30% higher than those from the corresponding substrate from the August sample. A detailed analysis of the fine sulfate aerosol was performed to assess whether the sulfate particles contained small condensation nuclei. Nearly all analyzed sulfate particles showed no evidence of a solid or dissolved nucleus particle, which suggests that the volcanic H2SO4 aerosol formed through homogeneous nucleation processes. These data support heated‐inlet optical particle counter data from the balloon flights which suggest that 95–98% of the volcanic particles were aqueous H2SO4.
Differential SO2 column measurements of the Mt. Pinatubo volcanic plume
Authors: Hoff Raymond M.
Published: February 2008

Differential SO2 column measurements of the Mt. Pinatubo volcanic plume were made from a NASA Electra aircraft flying in the Caribbean in July, 1991. The mean column content of the volcanic plume in the Caribbean was 25 μatm m (2.5 Dobson units) with periods of over 100 μatm m observed. The data indicate that an aerosol absorption or scattering effect occurs for long slant paths (low sun elevation angles) and 41 μatm m (an average obtained for 1–2 airmasses) may be more representative of a true column average. Dispersion of the initial cloud from Mt. Pinatubo and a one month SO2 to sulfate conversion time constant are in good agreement with the column contents found here.
Preliminary analysis of observations of the Pinatubo volcanic plume with a polarization‐sensitive lidar
Authors: Winker D. M. , Osborn M. T.
Published: February 2008

A dual‐polarization lidar aboard the NASA Electra aircraft was used in July 1991 to survey the stratospheric plume from the recent Mt. Pinatubo eruption. Both depolarizing and non‐depolarizing volcanic layers were observed, ranging from 17 to 26 km in altitude. Differences in the depolarization signatures of the layers indicates differences in the composition or physical state of the particles in the layers.
Profile of discontinuities beneath Hawaii from S to P converted seismic waves
Authors: Li Yingping , Thurber Clifford H. , Munson Clifford G.
Published: February 2008

We present new seismological evidence for the depth of depression of the volcano‐crust and crust‐mantle (Moho) boundaries and the nature of the volcano‐crust interface beneath the southeast flank of Mauna Loa volcano, Hawaii, from a profile of S to P converted waves from deep local earthquakes. Three‐component seismic array observations of these converted waves indicate the presence of a low velocity layer (LVL) at a depth of about 10 km, corresponding to the base of the volcanic pile. The thickness of the LVL is estimated to be approximately 1 km. The LVL and the Moho both deepen significantly from southwest to northeast beneath Mauna Loa's southeast flank. The observed polarity reversal of the LVL conversion compared to the Moho conversion strongly supports the existence of a buried sediment layer at the volcano‐ocean crust interface, at a depth that is consistent with the depth distribution of thrust earthquakes. Our results conflict with models of lithosphere flexure at Hawaii, indicating a depression of the lithosphere greater than that predicted by flexure models. The thickness of underlying oceanic crust is observed to be relatively constant, so this discrepancy cannot be accounted for by crustal underplating alone.
The Loma Prieta Earthquake, 1989 and Earth strain tidal amplitudes: An unsuccessful search for associated changes
Authors: Linde Alan T. , Gladwin Michael T. , Johnston Malcolm J. S.
Published: February 2008

The Loma Prieta earthquake of 1989 provided an opportunity for a sensitive test of suggestions that earthquakes may be preceded by variations in Earth tidal strain amplitudes. Such variations have been proposed as providing an advantageous technique for detecting precursory changes in elastic parameters in a seismogenic zone. We have analyzed data from two borehole strainmeters continuously operating within 40 km of the epicenter and within about 10 km of the southern end of the rupture. We are unable to identify any precursory changes in M2 and O1 tidal amplitudes and estimate that any large scale changes in Young's modulus must have been less than about 2%. If these results apply generally, we would conclude that variations in elastic material properties prior to earthquakes do not occur throughout substantial volumes of the subsequent hypocentral region.
A fast and accurate method of calculating particle diffusion: Application to the ionosphere
Authors: Karimabadi H. , Menyuk C. R.
Published: February 2008

Electron acceleration by a wave propagating obliquely in a magnetized plasma is considered. For wave amplitudes above the stochasticity threshold the motion of electrons is diffusive. A test of a simple diffusion formalism is presented. We reduce the diffusion equation to a one‐dimensional form by transforming to a coordinate system that has the Hamiltonian as one of the axes. We present a model for the diffusion coefficient and solve the resulting diffusion equation by means of the finite element technique. The results are compared with numerical solutions of the orbits. Finally, we apply our results to the problem of electron diffusion in the lower ionosphere.
ISEE 1 observations of electrostatic ion cyclotron waves in association with ion beams on auroral field lines from ∼2.5 to 4.5 RE
Authors: Cattell C. A. , Mozer F. S. , Roth I. , Anderson R. R. , Elphic R. C. , Lennartsson W. , Ungstrup E.
Published: February 2008

Quasi‐monochromatic waves at ∼1.2 ƒcH, where ƒcH is the hydrogen cyclotron frequency, were observed as the ISEE 1 satellite traversed auroral field lines at radial distances of ∼2.5–4.5 RE near midnight on June 19, 1981. The waves were polarized perpendicular to the magnetic field (k∥sol; k⊥ ≲0.2). In addition, there were waves at slightly above both the helium and the oxygen cyclotron frequencies. The waves occurred within a region of reduced density (∼0.1–0.2/cm³) with the electron temperature greater than the ion temperature, upflowing hydrogen and oxygen beams, and weak field‐aligned currents bounded by electrostatic shocks. The wave characteristics and associations were similar to those observed at lower altitudes by the S3‐3 satellite. Comparisons were made between the observed H+ and O+ beams, field‐aligned currents, densities and temperatures, and linear theories of electrostatic ion cyclotron waves and H+‐O+ two‐stream instabilities. The features of the observed waves are most consistent with the current‐driven mode. In addition, numerical studies of the linear dispersion relation, using parameters based on the observations, showed that both the parallel and oblique two‐stream modes and the ion‐beam driven modes were stable while oblique current driven modes were unstable. The O+ and H+ distributions provide evidence for interactions with local electrostatic ion cyclotron waves and for the H+‐O+ two‐stream instability at altitudes below the satellite.
A time‐dependent, three‐dimensional MHD numerical study of interplanetary magnetic draping around plasmoids in the solar wind
Authors: Detman T. R. , Dryer M. , Yeh T. , Han S. M. , Wu S. T. , McComas D. J.
Published: February 2008

A spherical plasmoid is injected into a representative solar wind at 18 solar radii, which is chosen as the lower computational boundary of a 3‐dimensional MHD model. The field line topology of the injected plasmoid resembles the streamline topology of a spherical vortex. Evolution of the plasmoid and its surrounding interplanetary medium is described out to approximately 1 AU for three cases with different velocities imparted to the plasmoid. In the first case a plasmoid enters the lower boundary with a velocity of 250 km s−1 equal to the steady state background solar wind velocity at the lower boundary. In the second and third cases the plasmoid enters with peak velocities of twice and 3 times the background velocity. A number of interesting features are found. For instance, the evolving plasmoid retains its basic magnetic topology although the shape becomes distorted. As might be expected, the shape distortion increases with the injection velocity. Development of a bow shock occurs when the plasmoid is injected with a velocity greater than the sum of the local fast magnetosonic speed and the ambient solar wind velocity. The MHD simulation demonstrates magnetic draping around the plasmoid.
Numerical studies of electron dynamics in oblique quasi‐perpendicular collisionless shock waves
Authors: Liewer P. C. , Decyk V. K. , Dawson J. M. , Lembège B.
Published: February 2008

Linear and nonlinear electron damping of the whistler precursor wave train to low Mach number quasi‐perpendicular oblique shocks is studied using a one‐dimensional electromagnetic plasma simulation code with particle electrons and ions. In some parameter regimes, electrons are observed to trap along the magnetic field lines in the potential of the whistler precursor wave train. This trapping can lead to significant electron heating in front of the shock for low βe (∼10% or less). Use of the 64‐processor Caltech/JPL Mark IIIfp hypercube concurrent computer has enabled us to make long runs using realistic mass ratios (mi/me = 1600) in the full particle in‐cell code and thus simulate shock parameter regimes and phenomena not previously studied numerically.
On the possible source of the ionization in the nighttime Martian ionosphere: 1. Phobos 2 Harp Electron Spectrometer measurements
Authors: Verigin M. I. , Gringauz K. I. , Shutte N. M. , Haider S. A. , Szego K. , Kiraly P. , Nagy A. F. , Gombosi T. I.
Published: February 2008

The measurements of electron spectra in the Martian magnetosphere by the HARP instrument on board the Phobos 2 orbiter are presented. The energy of the electrons (a few tens of electron volts) is sufficient for the impact ionization of the planetary neutral gas, and the characteristic flux of electrons (∼108 cm−2 s−1) could produce the nightside ionospheric layer with a peak density of a few thousands of electrons per cubic centimeter, which corresponds to densities observed earlier during radio occultations of the Mars 4 and 5 and Viking 1 and 2 spacecraft. The possibility of magnetospheric electron precipitation into the nightside atmosphere of Mars is in agreement with the mainly induced nature of the magnetic field in the planetary magnetotail (as at Venus), while the variability of the Martian nightside ionosphere may be explained by the partial screening of the atmosphere by a weak intrinsic magnetic field of the planet.
On tidal variability induced by nonlinear interaction with planetary waves
Authors: Teitelbaum Hector , Vial François
Published: February 2008

Short‐time variability of the atmospheric tides is frequently observed in the meteor region but is not yet fully explained in terms of production mechanisms. This is probably due to the existence of several such mechanisms acting together or separately. In this paper we show that many observations can be explained by nonlinear interactions between tides and planetary waves having periods corresponding to those of the observed tidal amplitude modulations. These nonlinear interactions generate two secondary waves whose frequencies are the sum and difference of frequencies of the primary waves. These two waves beat with the tide, modulating its amplitude with the planetary wave period. A numerical model is used to demonstrate that with primary waves of reasonable amplitudes the nonlinear interactions can be quite large. This is because the importance of nonlinearity depends essentially on the amplitude of the induced fluid velocity in the direction of wave propagation compared to the wave propagation velocity. When two waves propagate simultaneously, the fluid velocity can have a large component in the direction of propagation of one of the waves, and advective (nonlinear) terms can be large. This point is further illustrated in the case of two gravity waves interacting together. Finally, some observational campaigns carried out above Garchy (45°N) are analyzed using a nonparametric method. The results indicate that nonlinear interactions between tides and planetary waves really take place in the upper mesosphere and lower thermosphere.
Nonlinear Bidimensional evolution of ion beam driven electrostatic instabilities in the auroral region
Authors: Mottez Fabrice , Chanteur Gérard
Published: February 2008

A two‐dimensional (in configuration space), three‐dimensional (in velocity space) electrostatic explicit particle code is used to investigate the interaction between an ion beam flowing along the magnetic field and a highly magnetized plasma (ωce/ωpe = 2, 4, 8). The beam drift velocity Vdi is larger than Vte, the electron thermal velocity (Vdi/Vte = 1.2, 2.5), and much larger than the thermal velocities of the core and beam ions (Vdi/Vti core = 12, 25). Two instabilities are found to develop. First, the interaction between the ion beam and electrons leads to the rapid growth of parallel modes and to the fast diffusion of electrons along the magnetic field, predominantly in the ion beam direction. This ion‐electron instability is rapidly quenched by electron heating. Second, and ion‐ion instability develops, which involves oblique modes leading to a selective heating of ions in a direction oblique to B. It is shown that the heating via the electron‐ion instability is a necessary step for the development of the ion‐ion instability. Finally, it is shown that the nonlinear development of the ion‐ion instability gives rise to purely perpendicular modes that are not linearly unstable.
The solar wind interaction with Mars: Consideration of Phobos 2 mission observations of an ion composition boundary on the dayside
Authors: Breus T. K. , Krymskii A. M. , Lundin R. , Dubinin E. M. , Luhmann J. G. , Yeroshenko Ye G. , Barabash S. V. , Mitnitskii V. Ya. , Pissarenko N. F. , Styashkin V. A.
Published: February 2008

This paper describes the features of the boundary in the plasma ion composition near Mars which separates the region dominated by the solar wind protons from the plasma of planetary origin. This boundary was detected by the ASPERA experiment on Phobos 2. It is argued that the features of this boundary seem to be similar to those of other composition boundaries detected elsewhere: the cometopause near comet Halley, and a boundary in the ion composition which appears near Venus during periods of high solar wind dynamic pressure. Numerical modeling of the solar wind interaction with Mars supports the idea that during solar maximum the interaction of the Martian neutral atmosphere with the solar wind can result in a composition transition from solar wind to planetary ions in the low‐altitude magnetosheath. This transition occurs because of charge exchange of solar wind protons with the neutral atmosphere and photoionization.
Plasmoids as magnetic flux ropes
Authors: Moldwin Mark B. , Hughes W. Jeffrey
Published: February 2008

Observational constraints on the magnetic topology and orientation of plasmoids is examined using a magnetic field model. We develop a magnetic flux rope model to examine whether principal axis analysis (PAA) of magnetometer signatures from a single satellite pass is sufficient to determine the magnetic topology of plasmoids and if plasmoid observations are best explained by the flux rope, closed loop, or large‐amplitude wave picture. Satellite data are simulated by extracting the magnetic field along a path through our model of a magnetic flux rope. We then examine the results using PAA. We find that the principal axis directions (and therefore the interpretation of structure orientation) is highly dependent on several parameters including the satellite trajectory through the structure. Because of this we conclude that PAA of magnetometer data from a single satellite pass is insufficient to differentiate between magnetic closed loop and flux rope models. We also compare our model results to ISEE 3 magnetometer data of plasmoid events in various coordinate frames including principal axis and geocentric solar magnetospheric. We find that previously identified plasmoid events that have been explained as closed loop structures can also be modeled as flux ropes. We also searched the literature for previously reported flux rope and closed loop plasmoid events to examine if these structures had any similarities and/or differences. The results of our modeling efforts and examination of both flux rope and plasmoid events lead us to favor the flux rope model of plasmoid formation, as it is better able to unify the observations of various magnetic structures observed by ISEE 3.
Out‐of‐ecliptic tests of the inverse correlation between solar wind speed and coronal expansion factor
Authors: Sheeley N. R. , Swanson E. T. , Wang Y.‐M.
Published: February 2008

In this paper we address the question of whether out‐of‐ecliptic measurements satisfy the inverse correlation between wind speed at 1 AU and flux tube divergence in the corona, already found from measurements in the ecliptic. Using the in‐ecliptic calibration, we derive out‐of‐ecliptic speeds from coronal expansion factors determined from global observations of photospheric field and their current‐free coronal extension. These derived speeds are compared with speeds inferred from interplanetary scintillation measurements during 1972–1988 and with in situ speeds measured by the Pioneer 11 spacecraft at 16°N latitude during 1984–1988. These three sets of wind speed show the same overall variation with latitude and time during the sunspot cycle, with higher latitudes having more years of fast wind than lower latitudes and all latitudes having slow wind at sunspot maximum. Although some detailed discrepancies are also present, the overall agreement is comparable to that achieved in the ecliptic plane.
Naturally enhanced ion acoustic waves in the auroral ionosphere observed with the EISCAT 933‐MHz radar
Authors: Rietveld M. T. , Collis P. N. , St.‐Maurice J.‐P.
Published: February 2008

Observations of strongly enhanced ion acoustic shoulders of the incoherent scatter spectrum at 933 MHz at altitudes from 138 to 587 km have been obtained with the European Incoherent Scatter UHF radar. The enhancements can be up to 1 or 2 orders of magnitude in total backscattered power and can occur at either one or both of the ion acoustic shoulders. They show a variation of frequency with height of about 2 to 1, the same as the normal ion line spectral width and the ion temperature. These unusual spectra appear in two preferred height regions having different characteristics, one below 200 km and one above about 300 km. The enhancements are associated with geomagnetic disturbance, high electron temperatures, auroral arcs, and red aurora in the F region. The observations, which are mainly along the magnetic field direction, indicate that field‐aligned thermal electron drifts are destabilizing the ion acoustic waves. They confirm and extend the one other publication reporting on similar echoes. We suggest that field‐aligned flows of soft electrons depositing their energy at horizontally poor conducting F region heights are the cause of parallel electric fields in the ionosphere. These fields then produce the thermal electron motions that we argue have to be the cause of the observations.
Ion inertia effect on the Kelvin‐Helmholtz instability
Authors: Fujimoto M. , Terasawa T.
Published: February 2008

The Kelvin‐Helmholtz instability in a thin velocity shear layer is analyzed in a two‐fluid plasma. In the system treated here, electrons are treated as a massless, charge neutralizing fluid, and the ion inertia effects appearing through the Hall term and the electron pressure term in the generalized Ohm's law are added to the usual MHD system of equations. Linearized equations are derived and solved numerically for the parallel (V0 ∥ B0) and the perpendicular (V0 ⊥ B0) geometries. Although the growth rates are only slightly affected by the ion inertia effect for both configurations, the structures of the eigenmodes become highly different when the modes have spatial dependences along the background magnetic field. The implication of the present results for the magnetopause boundary layer is discussed.
Guided Z mode propagation observed in the OEDIPUS A Tethered Rocket Experiment
Authors: James H. G.
Published: February 2008

The tethered sounding rocket payload OEDIPUS A conducted bistatic propagation experiments on plasma waves in the auroral ionosphere. Synchronized sweeps of the frequency range 0–5 MHz by the 2‐W transmitter high‐frequency exciter (HEX) on the upper end of the tether and its associated receiver for exciter (REX) on the lower end have produced signatures of quasi‐electrostatic waves guided along field‐aligned depletions of ambient density. The propagation is in the slow Z mode, between the plasma frequency ƒp and the upper hybrid resonance frequency ƒuhr when ƒp is greater than the cyclotron frequency. The mode identification is based on payload measurements of ƒp. These waves have signal delays of about 1 ms. The delays are much greater than expected for free‐space propagation over the transmitter‐receiver separation distance which varies up to 960 m during the flight. The transmitted pulses typically appear inside a frequency bandwidth of about 100 kHz just above the plasma frequency, but occasionally occupy most of the available bandwidth, ≃300 kHz, between ƒp and ƒuhr. The observed delays and the stretching by a factor of 3 of the transmitted 300‐μs pulses are accounted for with two‐dimensional ray tracing using a complete electromagnetic solution of the hot plasma dispersion relation. Delayed Z mode pulses appear in about 20% of the ionograms. Given the weakness of the HEX transmitter and the abundance of examples obtained during the flight, guiding of natural Z mode emissions in the auroral ionosphere may be efficient and widespread.
Generation of Pc 1 waves by the ion temperature anisotropy associated with fast shocks caused by sudden impulses
Authors: Mandt M. E. , Lee L. C.
Published: February 2008

Observations have reported on the high correlation of Pc 1 events with magnetospheric compressions. A number of mechanisms have been suggested for the generation of the Pc 1 waves. In this paper, we propose a new mechanism which leads to the generation of Pc 1 waves. The interaction of a dynamic pressure pulse (Δρυ²) with the Earth's bow shock leads to the formation of a weak fast mode shock propagating into the magnetosheath. The shock wave can pass right through a tangential discontinuity (magnetopause) and into the magnetosphere, without disturbing either of the structures. In a quasi‐perpendicular geometry, the shock wave exhibits anisotropic heating with T⊥ > T∥. This anisotropy drives unstable ion cyclotron waves which we believe can contribute to the generation of the Pc 1 waves which are detected. The viability of the mechanism is demonstrated with simulations. This mechanism could explain peak in the occurrence of observed Pc 1 waves in the postnoon sector where a field‐aligned discontinuity in the solar wind would most often be parallel to the magnetopause surface due to the average Parker spiral magnetic field configuration.
Simulation of the nonlinear evolution of electron plasma waves
Authors: Nishikawa K.‐I. , Cairns I. H.
Published: February 2008

Electron plasma waves, commonly called Langmuir waves, are often observed upstream from planetary bow shocks and are widely accepted as being generated by electron beams. We have studied the self‐consistent nonlinear evolution of electron plasma waves excited by electron beams streaming along the ambient magnetic field in a long simulation system Lx = 2048λe. Only electrostatic waves are treated by the simulation code. The simulation results show the presence of a process moving wave energy from frequencies and wavenumbers predicted by linear theory to Langmuirlike frequencies during saturation of the instability. An explanation in terms of the characteristics of the growing waves changing with beam temperature is suggested. A decay process producing backward propagating (backscattered) Langmuirlike waves is observed. Low‐frequency waves are also produced. However, the simulation results indicate that the backscattering process is not the conventional Langmuir wave decay. Electrostatic waves near multiples of the electron plasma frequency are generated by wave‐wave coupling during the nonlinear stage of the simulations, thereby confirming the suggestions of Klimas (1983). However, the spectral width of the pumping waves and beam density are found to significantly affect the levels of harmonic waves. Care must therefore be taken to prevent unrealistically high levels of harmonic waves by including sufficient numbers of linearly unstable modes in the simulation system.
A model for the frequency fine structure of auroral kilometric radiation
Authors: McKean M. E. , Winglee R. M.
Published: February 2008

One of the more interesting and as yet unexplained features of auroral kilometric radiation (AKR) is the frequency fine structure observed in wideband spectrograms. This fine structure consists of emission features that drift in frequency with time, and of banded emission structure. Both positive and negative drifts are observed. While the electron cyclotron maser instability is widely believed to be the source of AKR, no proposed explanation for the frequency fine structure seen in AKR has been confirmed. In this paper, we propose a model for AKR frequency fine structure in which the fine structure is a natural consequence of the emission of electron cyclotron maser radiation in a nonuniform magnetic field. This model is examined by using one‐dimensional, electromagnetic, particle‐in‐cell simulations. We find that maser radiation is emitted as individual wave packets that combine to form the drifting emission features that make up the fine structure. Rising frequency drifts are associated with wave packets emitted independently of one another. Wave packets that interact with one another produce features with falling frequency drift. Combinations of these wave packets can produce features that rise and fall. Both X mode radiation and O mode radiation produce fine structure with narrowband, drifting emissions. The emission features produced have a large spread in both positive and negative frequency drift rates. We find that the average positive drift rate of rapidly drifting features is approximately proportional to the maser growth rate. On the other hand, the average negative drift rate of rapidly drifting features remains approximately constant. For slowly drifting features, specifically, features with drifts that may be close to observed drift rates, both the positive and negative drift rates are approximately proportional to the magnetic field gradient.
Solar wind erosion of the Mars early atmosphere
Authors: Perez‐de‐Tejada H.
Published: February 2008

A calculation of the amount of volatiles that Mars has lost in the past through solar wind erosion is presented. The analysis is based on the examination of the ionospheric plasma flow produced by the transfer of momentum of the solar wind to the planet's upper ionosphere and is carried out by using loss rates suitable to a dense Venus‐like Martian early ionosphere. The results indicate that an amount of mass equivalent to that of a global water ocean at least 10 m deep was removed by this process over the planet's lifetime. Further calculations show that depending on the strength of the Martian early ionosphere the total mass eroded could have been the equivalent of a global ocean up to ∼30 m deep. These numbers are much larger than those derived from current planetary escape rates inferred from the recent Phobos measurements and represent up to about nearly one half of the amount of water which is believed was delivered to the Martian atmosphere through early volcanic activity.
On the motion of electrons in the slow electric field fluctuations observed by Viking
Authors: Hultqvist Bengt
Published: February 2008

Results are presented of calculations of the motion of electrons in slow, large‐amplitude fluctuations of the electric field, which have been observed by means of the Swedish satellite Viking. The E∥ component seen by the ionospheric electrons, entering the acceleration region from below, is assumed to vary along the path of the electrons along the magnetic field lines in the way that Viking recorded along its more or less horizontal path through, or above, the acceleration region. Although this is a simplified model, it is expected to illustrate the effect of the E∥ fluctuations on the “cold” electrons, which enter the acceleration region more realistically than in the earlier, highly simplified model used by Hultqvist (1988). The results of the calculations show that temporal variations of E∥ of the kind observed by Viking easily can bring the electrons to the top of an acceleration region, which extends 1000–10,000 km along the magnetic field lines, with energies in the range 100 eV to several keV, as have been observed.
Feautrier solution of the electron transport equation
Authors: Link R.
Published: February 2008

An efficient, accurate, and stable solution to the electron transport problem is presented. The two‐stream approximation is discussed in detail, but the solution is developed for the general multistream case. Applied to the photoelectron problem, it is shown that accurate escape fluxes are obtained using only 10 altitudes distributed over the region 100–1000 km. The local equilibrium, two‐stream, and multistream solutions to the photoelectron problem are compared. In addition, a recursion relation for the moments of the elastic angular scattering phase function is presented.
Alfvén waves in the auroral ionosphere: A numerical model compared with measurements
Authors: Knudsen D. J. , Kelley M. C. , Vickrey J. F.
Published: February 2008

We solve a linear numerical model of Alfvén waves reflecting from the high‐latitude ionosphere, both to better understand the role of the ionosphere in the magnetosphere/ionosphere coupling process and to compare model results with in situ measurements. We use the model to compute the frequency‐dependent amplitude and phase relations between the meridional electric and the zonal magnetic fields due to Alfvén waves. These relations are compared with measurements taken by an auroral sounding rocket flown in the morningside oval and by the HILAT satellite traversing the oval at local noon. The sounding rocket's trajectory was mostly parallel to the auroral oval, and it measured enhanced fluctuating field energy in regions of electron precipitation. The rocket‐measured phase data are in excellent agreement with the Alfvén wave model, and the relation between the modeled and the measured amplitudes is fair, leading us to conclude that the rocket‐measured fields are dominated by interfering Alfvén waves, forming a standing wave pattern. The field amplitudes measured by HILAT are related by the height‐integrated Pedersen conductivity Σp, indicating that the measured field fluctuations were due mainly to structured field‐aligned current systems. A reason for the relative lack of Alfvén wave energy in the HILAT measurements could be the fact that the satellite traveled mostly perpendicular to the oval and therefore quickly traversed narrow regions of electron precipitation and associated wave activity. Alternatively, the lower velocity and eastward flight path of the rocket lead to Doppler‐shifted frequencies of L shell aligned current structures which are much smaller than HILAT would measure and which in fact may be below our range of interest (0.1–1 Hz), leaving Alfvén waves as the dominant source of fluctuating field energy in the rocket frame of reference.
Large‐scale HF‐induced ionospheric modifications: Experiments
Authors: Hansen J. D. , Morales G. J. , Duncan L. M. , Dimonte G.
Published: February 2008

Experimental investigations of large‐scale modifications created by a high‐power HF beam (effective radiated power of ∼100 MW, frequency of 3–5 MHz) in the nighttime ionosphere above Arecibo Observatory are presented. The modifications consist of field‐aligned temperature enhancements (δTe/Te0 ∼ 2–4) and density depletions (|δne|/ne0 ∼ 25–50%) and have length scales along the geomagnetic field of tens of kilometers. A two‐stage time evolution of the modifications is documented; they start from a broad and symmetric perturbation and develop into a narrow, northward shifted universal steady state on a time scale of 15–30 min. It is found that nonlinear refraction, i.e., the self‐consistent bending of the O mode HF beam across the geomagnetic field lines and the reorientation of the HF reflection surface to a geometry parallel to the geomagnetic field, is the key process involved in the generation of the large modifications. Preconditioning results showing the immediate onset of the narrow late‐stage behavior provide dramatic confirmation of the underlying nonlinear mechanism sustaining the large modifications. Highlights of quantitative comparisons of these observations with a transport model including nonlinear refraction have been given in a previous publication (Hansen et al., 1990).
Influence of a high‐power radio wave on velocity gradient driven instabilities in the auroral F region ionosphere
Authors: Shukla P. K. , Stenflo L.
Published: February 2008

It is shown that a powerful radio wave passing through the F region of the auroral ionosphere can modify the ion velocity gradient driven low‐frequency electrostatic fluctuation spectra due to parametric interaction processes. These are studied with the help of a nonlinear dispersion relation that includes the ponderomotive and differential Joule heating nonlinearities. Our results show how nonthermal fluctuations can be produced and controlled by a powerful radio wave in the auroral F region plasma.
The role of the whistler precursor during the cyclic reformation of a quasi‐parallel shock
Authors: Pantellini F. G. E. , Heron A. , Adam J. C. , Mangeney A.
Published: February 2008

It is known from observations in space plasmas and from numerical simulations that under certain conditions, obliquely propagating shocks generate a standing whistler precursor. A one‐dimensional electromagnetic implicit particle code is used to investigate its role during the cyclic reformation of quasi‐parallel shocks observed in hybrid simulations. It is shown that the precursor grows until a significant number of ions become trapped, at which time the shock ramp reforms near the leading edge of the whistler wave train. Electron inertial effects are found to be rather unimportant for the reformation mechanism, although some electrons are found to be accelerated to suprathermal velocities by the whistler precursor.
Focused transport of energetic particles along magnetic field lines draped around a coronal mass ejection
Authors: Tan L. C. , Mason G. M. , Lee M. A. , Klecker B. , Ipavich F. M.
Published: February 2008

The transport of particles in interplanetary space can be dominated by adiabatic focusing of particle pitch angle distributions under conditions when the ratio of the particle scattering mean free path to the magnetic field focusing length, λ/L, is ∼1. Since for the average Archimedean spiral field configuration, L ∼ 1 AU at Earth orbit, the only reported events dominated by focusing are those nearly “scatter‐free” events where λ ∼ 1 AU. However, if the interplanetary magnetic field is distorted so that the focusing length L is small, then even if the interplanetary scattering mean free path were small, focused transport would be expected to dominate, since λ/L would still be ∼1. We present evidence for such a event, obtained with observations of ∼0.1–1 MeV nucleon−1 protons and alpha particles obtained with the University of Maryland/Max‐Planck‐Institut experiment on the ISEE 3 spacecraft during the decay phase of the June 6, 1979, solar particle event. During the early portion of the decay phase of this event, interplanetary magnetic field lines were apparently draped around a coronal mass ejection, leading to a small focusing length L on the western flank where ISEE 3 was located. We observed a period of very slow decrease of particle intensity, along with large sunward anisotropy in the solar wind frame, which are inconsistent with predictions of the standard Fokker‐Planck equation models for diffusive transport; however, we are able to fit the observations assuming that focused transport dominates and that the particle pitch angle scattering is isotropic.
Plasma dynamics on current‐carrying magnetic flux tubes
Authors: Swift Daniel W.
Published: February 2008

The evolution of plasma in a current‐carrying magnetic flux tube of variable cross section is investigated using a one‐dimensional numerical simulation. The flux tube is narrow at the two ends and broad in the middle. The middle region of the flux tube is loaded with a hot plasma population, and the two ends have a much more dense gravitationally bound plasma. The result is that the number of particles per unit of flux tube length has a minimum near each end and a maximum in the middle where the flux tube has the largest cross section. A large potential difference, parallel to the magnetic field, is applied across the domain. The general result is that density minimum tends to deepen, primarily in the cathode end, and the entire potential drop becomes concentrated across the region of density minimum. The potential drop is maintained primarily by electron inertia. The evolution of the simulation shows some sensitivity to particle boundary conditions, but the simulations inevitably evolve into a final state with a nearly stationary double layer near the cathode end. The simulation results are at sufficient variance with observations that it appears unlikely that auroral electrons can be explained by a simple process of acceleration through a field‐aligned potential drop.
Electrostatic waves due to field‐aligned electron beams in the low‐latitude boundary layer
Authors: Peroomian V. , Ashour‐Abdalla M. , Fuselier S. A. , Schriver D. , Peterson W. K. , Strangeway R. J.
Published: February 2008

We have analyzed mass‐resolved ion, electron, and plasma wave data obtained from several low‐latitude boundary layer (LLBL) crossings by the AMPTE CCE satellite. The data clearly separate the LLBL from the adjacent magnetosheath and magnetosphere. The available plasma wave data were limited to high frequencies; therefore we focused our attention on wave‐particle interactions involving electrons. We found that electron beams are present in the LLBL during southward interplanetary magnetic field, along with a simultaneous enhancement of electrostatic waves with parallel polarization. Linear theory analysis shows that for plasma conditions in the LLBL, electron beams are unstable to electrostatic waves that propagate parallel to the local magnetic field, in agreement with observations. A numerical simulation study of the beam‐plasma interaction in the LLBL has also been undertaken to examine the saturation mechanism. The simulations show that the beam instability saturates by thermalization of the beam but that a beamlike structure can still remain in the electron distribution for certain initial parameters. The results suggest that peaks in the electron velocity distribution function may be found in the LLBL away from the beam source region.
Correlation function ratios and the identification of space plasma instabilities
Authors: Gary S. Peter , Winske Dan
Published: February 2008

Wave‐particle transport in a collisionless plasma is due to particle scattering by enhanced fluctuations associated with the growth of instabilities. In particular, relatively short wavelength kinetic instabilities are frequently invoked to explain many different types of plasma transport in space. Although there is an extensive theoretical and simulation literature describing the potential applications of many such instabilities, there are only a few cases of clear‐cut identification of kinetic modes in space. The research described in this paper uses linear Vlasov dispersion theory to study correlation functions and dimensionless correlation function ratios for fluctuations or instabilities in three space plasma regimes. This research shows that both the compressibility and the parallel compressibility are likely to be useful in distinguishing modes in the magnetosheath as well as in the plasma sheet boundary layer and that helicity remains a useful identifier of electromagnetic ion/ion instabilities in the foreshock.
Force balance near an X line along which E · J < 0
Authors: Lyons L. R. , Pridmore‐Brown D. C.
Published: February 2008

Observations suggest that plasma can occasionally be transferred from closed to open field lines in the geomagnetic tail during periods when the separatrix between open and closed field lines moves rapidly toward decreasing latitudes. Such transfer would require E · J < 0 along the distant X line (E being the electric field and J the current density), and it would give important information on the physical processes that occur near an X line. The condition E · J < 0 is not allowed in resistive reconnection theory. Collisionless reconnection theory, however, requires a balance between the electric force and a gyroviscous force at an X line but does not necessarily require that E · J > 0. We have recently shown that gyroviscosity can provide the force required to balance the electric force along an X line having E · J > 0. Here we extend this previous analysis of gyroviscosity to the case where E · J < 0 along an X line. We show how the balance between gyroviscosity and the electric force along an X line is maintained for any E · J. We conclude that observational evidence for the occasional existence of E · J < 0 along an X line provides support for the suggestion that collisionless gyroviscosity, rather than resistivity, balances the electric force along an X line. We also evaluate conditions for which particle motion in the vicinity of an X line can give a current in the direction appropriate for E · J < 0, and we find that there is a maximum electric field magnitude for particles to be able to carry a significant current. For parameters typical of the distant magnetotail the critical electric field magnitude was found to be ∼0.15 mV/m, which is of the order of, though somewhat less than, the potential electric field magnitudes expected in the magnetotail. This maximum allowable field magnitude is about the same for protons as it is for electrons in the magnetotail.