bluuebunny/crossref_metadata_2025 huggingface dataset:
Simultaneous observations of H+ and O+; ions at two altitudes by the Akebono and Dynamics Explorer 1 satellites
Authors: Peterson W. K. , Yau A. W. , Whalen B. A.
Published: February 2008

We report simultaneous observations of H+ and O+ ions from the suprathermal ion mass spectrometer (SMS) on Akebono and the energetic ion composition spectrometer (EICS) on Dynamics Explorer 1. These observations were made simultaneously above and below regions of ion acceleration on auroral and polar cap magnetic field lines. The evening auroral zone data reported here directly confirm the existence of an ionospheric preenergization mechanism for oxygen ions. The preenergized oxygen ions have characteristic energies of the order of 10 eV and are transported to altitudes of several thousand kilometers where they acquire significant additional energy. Nearly simultaneous observations of O+ ions on polar cap field lines near the cusp/cleft region reveal two distinct populations. The first population is energized at altitudes of a few thousand kilometers and transported up and into the polar cap by the combination of the effects of the gradient in the magnetic field and the prevailing magnetospheric convection field. The second population has significantly more energy than the first. This second population could arise through several mechanisms, which are identified and discussed. The data presented illustrate that the creation (energization) and transport of streams of upflowing ions in the polar cap is more complex than previously anticipated and point to the importance and our limited knowledge of ionospheric processes responsible for the pre‐energization of oxygen ions.
On the interplanetary cosmic ray latitudinal gradient
Authors: Lockwood J. A. , Webber W. R.
Published: February 2008

We have measured the temporal history of the latitudinal and radial gradients of E >70‐MeV cosmic ray particles from 1983 to 1990 using data from the Voyager 2 and Pioneer 10 spacecraft near the heliographic equatorial plane and from Pioneer 11 and Voyager 1 at average latitudes of 16° and 30°, respectively. The 26‐day average integral latitudinal gradient reaches a maximum of ∼ −2.5%/AU in 1987 and essentially disappears when the average current sheet tilt angle exceeds the latitude of either Pioneer 11 or Voyager 1. Its magnitude shows a very strong correlation with the tilt angle of the neutral sheet. Using the data from Pioneer 11 and Voyager 1 at different latitudes, we are also able to deduce the latitude dependence of the 26‐day average differential latitudinal gradient which is found to be a strong function of latitude when the tilt angle approaches zero and becomes essentially independent of latitude for tilt angles > 30°. The relationship between the latitudinal and radial gradients has been used to estimate the perpendicular diffusion coefficient for >70‐MeV particles. From the slope of the regression line between the instantaneous integral latitudinal and radial gradients we deduce that the perpendicular diffusion coefficient at the Earth is about 9×1019 cm²/s or nearly 2 orders of magnitude less than the parallel diffusion coefficient. In the outer heliosphere where the wavy current sheet has large excursions from the heliographic equator such a ratio of the perpendicular to the parallel diffusion coefficient is sufficient to short circuit the neutral sheet when the tilt is >30°–40°, thus explaining the lack of correlation of the gradients with the current sheet tilt angle when the tilt angle becomes large. We also find excellent agreement between the observed temporal behavior of the magnitude of the latitudinal gradient and a detailed model of solar modulation which includes time dependent drifts and an outward propagating current sheet tilt Potgieter and LeRoux [1992].
Unexpected features of the ion precipitation in the so‐called cleft/low‐latitude boundary layer region: Association with sunward convection and occurrence on open field lines
Authors: Nishida A. , Mukai T. , Hayakawa H. , Matsuoka A. , Tsuruda K. , Kaya N. , Fukunishi H.
Published: February 2008

Ion precipitations along the boundary of the polar cap are studied using the Akebono (EXOS D) observations of plasma, electric field and magnetic field during the dawn‐dusk crossings across the dayside polar cap at the altitudes of 7500 km to 10,000 km. Only the cases where the convection streamlines consisted of two cells rather than four have been chosen for analysis, and hence the Kp index was moderate or high and the IMF polarity was southward in most of the intervals studied. It is found that the ion precipitations in morning and evening sectors along the polar cap boundary are associated with the sunward convection at the above heights, and judging from the presence of the polar rain the high‐latitude part of these precipitations occur in the region of open field lines. Association with the sunward convection and occurrence on open field lines contradict the view that these ions originate from the low‐latitude boundary layer (LLBL) on closed field lines. We propose to designate these ions as circumpolar ion precipitation (CPIP) in order to avoid questionable presumptions on their source region. The sunward convection in the morning and evening sectors of the CPIP is observed systematically, has speeds of about 1 km/s, and often extends over a few degrees or more in invariant latitude. The CPIP ions may have leaked from the boundary region of the plasma sheet, may have been generated in the distant neutral sheet, or may have entered from the magnetosheath. Only within a few hours of the noon meridian the CPIP ions are associated with the antisunward flow at the height range of the Akebono observations.
Funnel‐shaped, low‐frequency equatorial waves
Authors: Boardsen S. A. , Gallagher D. L. , Gurnett D. A. , Peterson W. K. , Green J. L.
Published: February 2008

Funnel‐shaped, low‐frequency radiation, as observed in frequency time spectrograms, are frequently found at the Earth's magnetic equator. At the equator the radiation often extends from the proton cyclotron frequency up to the lower hybrid frequency. Ray‐tracing calculations can qualitatively reproduce the observed frequency‐time characteristics of these emissions if the waves are propagating in the fast magnetosonic mode starting with wave normal angles of ∼88° at the magnetic equator. The funnel‐shaped emissions are consistent with generation by protons with a ring‐type velocity space distribution. A ring‐shaped region of positive slope in the velocity space density distribution of protons is observed near the Alfvén velocity, indicating that the ring protons strongly interact with the waves. Ray‐tracing calculations show that for similar equatorial wave normal angles lower‐frequency fast magnetosonic waves are more closely confined to the magnetic equator than higher‐frequency fast magnetosonic waves. For waves refracted back toward the equator at similar magnetic latitudes, the lower‐frequency waves experience stronger damping in the vicinity of the equator than higher‐frequency waves. Also, wave growth is restricted to higher frequencies at larger magnetic latitudes. Wave damping at the equator and wave growth off the equator favors equatorial wave normal angle distributions which lead to the funnel‐shaped frequency time characteristic.
A nonlinear theory for large aspect angle echoes in the auroral E region
Authors: Hamza A. M.
Published: February 2008

It has been argued by a number of authors that existing conventional theories of small‐scale E region irregularities such as the linear and quasi‐linear theories cannot account for a number of physical features of E region irregularities revealed by the observations as well as the simulations. We present a fully nonlinear theory based on a two‐fluid model for the ions and electrons. We discuss two limits, that of the plasma within the electrojet where the ion collision frequency is larger than the ion gyrofrequency νi ≫ Ωi, and the case above the electrojet where νi ≪ Ωi. In both cases a set of coupled nonlinear equations governing the evolution of the density and electrostatic potential is derived. An argument regarding the nonlinear saturation of the turbulence is developed, and its consequences on the threshold drift velocity as well as the maximum aspect angle are discussed. By introducing both the effects of turbulence through frequency broadening and anomalous electron collision frequencies perpendicular and parallel to the magnetic field, we are able to predict the large aspect angles revealed by some of the radar observations. We then discuss two possible closure schemes, namely the “direct interaction approximation” and the “eddy damped quasi‐normal markovian” approximations, that allow us to evaluate self‐consistently the frequency broadening and the saturated spectra for both the density and the potential fluctuations.
Absolute differential and integral electron excitation cross sections for the O2 (a¹Δg ← X³Σg−) transition
Authors: Doering J. P.
Published: February 2008

Differential and integral electron excitation cross sections for the O2 a¹Δg ← X³Σg− transition at 0.97 eV have been measured as a function of incident electron energy from 2.6 to 28.6 eV. Differential cross sections (DCS) for the a¹Δg state were measured relative to elastic scattering as a function of scattering angle at each incident energy for the angular range 20°–134°. At incident energies between 7 and 18 eV, it was necessary to use a synthetic spectrum program to separate the a¹Δg ← X³Σg− band intensity from intense vibronic excitation of the O2 ground state. The relative DCS values were converted to absolute values using the elastic scattering cross‐section values of Trajmar et al. (1971) up to 90° and revised values at higher angles. The DCS results, which show the typical weak dependance on scattering angle of an optically forbidden transition, are in satisfactory agreement with previous measurements. The DCS were integrated to give integral cross sections at each incident energy. The results are in excellent agreement with previous measurements and help to establish the a¹Δg ← X³Σg− transition cross section as a valuable secondary standard for the measurement of electron excitation cross sections in unstable species such as atomic oxygen.
Plasma dynamics on current‐carrying magnetic flux tubes, 2, Low potential simulation
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 part of the flux tube is loaded with a hot, magnetically trapped population, and the two ends have a more dense, gravitationally bound population. A potential difference larger than the gravitational potential but less than the energy of the hot population is applied across the domain. The general result is that the potential change becomes distributed along the anode half of the domain, with negligible potential change on the cathode half. The potential is supported by the minor force of magnetically trapped particles. The simulations show a steady depletion of plasma on the anode side of the flux tube. The current steadily decreases on a time scale of an ion transit time. The results may provide an explanation for the observed plasma depletions on auroral field lines carrying upward currents.
Energetic electrons and ions in the magnetosheath at low and medium latitudes: Prognoz 10 data
Authors: Kudela K. , Sibeck D. G. , Slivka M. , Fischer S. , Lutsenko V. N. , Venkatesan D.
Published: February 2008

We present a survey of Prognoz 10 energetic ion (>15 keV) and electron (>30 keV) observations at low and middle latitudes in the dayside magnetosheath. At low latitudes, peak fluxes are observed inside the magnetopause, whereas at middle latitudes the peak fluxes are generally observed in the magnetosheath at some distance from the magnetopause. Both electron and ion fluxes tend to be greater outside the dawnside magnetopause than outside the duskside magnetopause. The flux of energetic particles in the outer magnetosheath is almost invariably less than that within the inner magnetosheath. Although the particle flux exhibits a clear dependence upon geomagnetic activity, no such dependence upon the local magnetosheath magnetic field orientation was noted. We interpret the observations as indicating that leakage of magnetospheric particles is the dominant source of energetic particles in the magnetosheath, but do not rule out Fermi acceleration at the bow shock as a possible subsidiary contributor to the population of ions with energies of about 15 keV.
Ion tearing in a magnetotail configuration with an embedded thin current sheet
Authors: Burkhart G. R. , Drake J. F. , Dusenbery P. B. , Speiser T. W.
Published: February 2008

The ion tearing instability is investigated in a magnetotail configuration that consists of a diffuse plasma sheet current and an embedded, thin current sheet with a strong current. For historical reasons, the thin embedded current sheet will be called a “neutral sheet”, even though the normal component of the magnetic field, Bn, is nonzero. In particular, we assume that the current within the thin current sheet is due to the acceleration of “Speiserlike” ion trajectories by a cross‐tail electric field Ey. It is found that the strong current within the neutral sheet is essentially unimportant to the growth rate of the tearing instability, and that the growth rate scales as (λ0/Lz)², where Lz is the overall half thickness of the plasma sheet, λ0 is the ion inertial length, c/ωpi, and ωpi is the ion plasma frequency at the neutral sheet edge. In the absence of the current outside the neutral sheet, current filamentation is stable.
Electromagnetic modes driven by streaming proton populations
Authors: Basu B. , Coppi B.
Published: February 2008

The characteristics of electromagnetic modes that are driven by ion distributions with a drift relative to the electron distribution in the presence of a two‐component magnetic field are identified under finite‐β conditions for both the ion and the electron distribution. The rate of momentum transfer from the ion to the electron population, where a current is induced, is evaluated. The relevance of this mode to a model suggested for the interpretation of the structure of the Uranus bow shock is pointed out.
Theoretical interpretation of the electrostatic waves in the space shuttle induced plasma environment
Authors: Rivas D. R. , Hastings D. E.
Published: February 2008

The intense broadband electrostatic turbulence detected in the near vicinity of the shuttle during the OSS 1 and Spacelab 2 missions is interpreted in terms of the excitation of the beam‐arc plasma instability. This new instability is driven by the free energy involving the interaction of the beam‐arc (asymmetric ring) distribution function of the pickup water ions with the background ionospheric plasma. The results of a linear analysis show that the beam‐arc plasma instability has unique characteristics that can explain most details of the wave spectrum detected with the Plasma Diagnostic Package (PDP) within 10 m of the shuttle. However, an analysis on the effects of varying beam‐arc parameters such as the angular dependence and thermal spread shows that the spectrum peak near the lower hybrid frequency (ΩLH) is likely to be a result of nonlinear effects. This is because the heating of beam‐arc ions expected in the nonlinear regime should lead to a resonant type instability which according to linear theory does not involve significant growth near ΩLH. The linear beam‐arc plasma instability theory can also explain the enhancements of the low‐frequency component of the spectrum observed during water dumps. These enhancements are interpreted in terms of beam‐arc plasma waves propagating in directions which are consistent with the highly inhomogeneous characteristics of the water dump regions.
Determination of the quenching rate of the O(¹D) by O(³P) from rocket‐borne optical (630 nm) and electron density data
Authors: Sobral J. H. A. , Takahashi H. , Abdu M. A. , Muralikrishna P. , Sahai Y. , Zamlutti C. J. , de Paula E. R. , Batista P. P.
Published: February 2008

The quenching rate k7 of the O(¹D) by O(³P) in the nocturnal F region is evaluated on the basis of rocket‐borne photometric and electron density measurements that have been carried out aboard a Brazilian Sonda III rocket launched from Natal (geography 5.8°S, 35.2°W) on October 31, 1986, at 2259 Local Standard Time (LST). The nightglow emission and electron density vertical profiles were obtained respectively from a 630‐nm photometer and a high‐frequency capacitance probe. The quenching rate k7 was evaluated from model calculations of the OI 630‐nm emission rate, utilizing as input data the measured electron density. Assuming the value of 1.1 for the quantum yield factor f(1;D) a mean value estimated for k7 turned out to be 9.2×10−13 cm3; s−1, the upper and lower limits being 2.8×10−12 cm3 s−1 and zero, respectively. On the other hand, assuming Bates' [1990] height profile of Franck‐Condon f(1;D) for this same experiment, k7 resulted in 2.55×10−12 cm3 s−1. These results are discussed and compared with previous measurements from other authors.
Absolute differential and integral electron excitation cross sections for atomic oxygen: 9. Improved cross section for the ³P → ¹D transition from 4.0 to 30 eV
Authors: Doering J. P.
Published: February 2008

The O I (³P → ¹D) transition cross section has been remeasured for a new set of incident energies. In addition, a new O2 (a ¹Δg ← X ³Σg−) reference cross section set has been used to normalize the relative measurements. The previous results of Doering and Gulcicek (1989) have been revised to reflect the improved reference cross section set. The new results combined with the revised Doering and Gulcicek (1989) values show that theory and experiment are in excellent agreement at incident energies greater than 9 eV. However, at lower incident energies, the experimental cross section rises to a sharp peak near 6 eV. The peak in the experimental cross section is 5.4 × 10−17 cm² (±35%) at 6 eV, approximately a factor of 2 larger than the theoretical cross section. The shape of this cross section vs. incident energy has many implications for aeronomy.
Low‐latitude plasma drifts from a simulation of the global atmospheric dynamo
Authors: Crain D. J. , Heelis R. A. , Bailey G. J. , Richmond A. D.
Published: February 2008

The results of a simulation of the global atmospheric dynamo are presented and compared to observations of the ion drift at Jicamarca and the drifts calculated from previous dynamo models. This simulation produces a global plasma distribution which is self‐consistent with the global potential distribution. We examine the dynamo‐generated potential distribution and the associated E × B drifts induced upon the plasma for two F region winds and a (1,‐2) tide in the E region. By using simple diurnal winds and tides and making the plasma and potential distributions self‐consistent, the agreement of the calculated E × B with the observed drifts at Jicamarca is improved. The use of an F region wind derived from the DE 2 satellite (Herrero and Mayr, 1986) can further improve the agreement between the calculated E × B drifts and the observed low‐latitude plasma drifts. These results lead to the conclusion that the F region dynamo is a very important source of electric fields at low latitudes at all local times. Based on these results, we propose that the postsunset enhancement of the vertical ion drift at Jicamarca is primarily due to a local time gradient in the F region zonal wind in conjunction with the conductivity gradient at the dusk terminator.
Modeling the solar cycle change in nitric oxide in the thermosphere and upper mesosphere
Authors: Fuller‐Rowell T. J.
Published: February 2008

Measurements from the Solar Mesosphere Explorer (SME) satellite have shown that low‐latitude nitric oxide densities at 110 km decrease by about a factor of 8 from January 1982 to April 1985. This time period corresponds to the descending phase of the last solar cycle where the monthly smoothed sunspot number decreased from more than 150 to less than 25. In addition, nitric oxide was observed to vary by a factor of 2 over a solar rotation, during high solar activity. A one‐dimensional, globally averaged model of the thermosphere and upper mesosphere has been used to study the height distribution of nitric oxide (NO) and its response to changes in the solar extreme ultraviolet radiation (EUV) through the solar cycle and over a solar rotation. The reference spectra for EUV have been extended, to wavelengths down to 0.1 nm, to include all radiation likely to affect the region above 70 km altitude, the lower boundary of the model. The primary source of nitric oxide is the reaction of excited atomic nitrogen, N(²D), with molecular oxygen. The atomic nitrogen is created by a number of ion‐neutral reactions and by direct dissociation of molecular nitrogen by photons and photoelectrons. The occurrence of the peak nitric oxide density at or below 115 km is a direct consequence of ionization and dissociation of molecular nitrogen by photoelectrons, which are produced by the solar flux below 30.0 nm (XUV). Nitric oxide is shown to vary over the solar cycle by a factor of 7 at low latitudes in the lower thermosphere E region, due to the estimated change in the solar EUV flux, in good agreement with the SME satellite observations. The NO density is shown to be strongly dependent on the temperature profile in the lower thermosphere and accounts for the difference between the current model and previous work. Wavelengths less than 1.8 nm have little impact on the NO profile. A factor of 3 change in solar flux below 5.0 nm at high solar activity produced a factor of 2 change in the peak NO density, consistent with SME observations over a solar rotation; this change also lowered the peak to 100 km, consistent with rocket data.
Ion acceleration in the Martian tail: Phobos observations
Authors: Dubinin E. , Lundin R. , Norberg O. , Pissarenko N.
Published: February 2008

The measurements carried out on the spacecraft Phobos‐2 have revealed that the plasma sheet of the Martian magnetosphere consists mainly of ions of planetary origin, accelerated up to ∼ 1 keV/q. Such an acceleration may result from the action of magnetic shear stresses of the draped field, the ion energy increasing toward the center of the tail where magnetic stresses are stronger. The energy gained by heavy ions does not depend on their mass and are proportional to the ion charge. The mechanism of the ion acceleration is related with the generation of a charge separation electric field, which extracts ions from “ray” structures in the Martian tail.
Collisionless parallel shocks
Authors: Khabibrakhmanov I. Kh. , Galeev A. A. , Galinsky V. L.
Published: February 2008

A collisionless parallel shock model is presented which is based on solitary‐type solutions of the modified derivative nonlinear Schrödinger equation (MDNLS) for parallel Alfvén waves. We generalize the standard derivative nonlinear Schrödinger equation in order to include the possible anisotropy of the plasma distribution function and higher‐order Korteweg‐de Vies type dispersion. Stationary solutions of MDNLS are discussed. The new mechanism, which can be called “adiabatic,” of ion reflection from the magnetic mirror of the parallel shock structure is the natural and essential feature of the parallel shock that introduces the irreversible properties into the nonlinear wave structure and may significantly contribute to the plasma heating upstream as well as downstream of the shock. The anisotropic nature of “adiabatic” reflections leads to the asymmetric particle distribution in the upstream as well in the downstream regions of the shock. As a result, nonzero heat flux appears near the front of the shock. It is shown that this causes the stochastic behavior of the nonlinear waves which can significantly contribute to the shock thermalization. The number of adiabaticaly reflected ions define the threshold conditions of the fire‐hose and mirror type instabilities in the downstream and upstream regions and thus determine a parameter region in which the described laminar parallel shock structure can exist. The threshold conditions for the fire hose and mirror‐type instabilities in the downstream and upstream regions of the shock are defined by the number of reflected particles and thus determine a parameter region in which the described laminar parallel shock structure can exist.
A study of an expanding interplanetary magnetic cloud and its interaction with the Earth's magnetosphere: The interplanetary aspect
Authors: Farrugia C. J. , Burlaga L. F. , Osherovich V. A. , Richardson I. G. , Freeman M. P. , Lepping R. P. , Lazarus A. J.
Published: February 2008

In a series of three interlinked papers we present a study of an interplanetary magnetic cloud and its interaction with the Earth's magnetosphere on January 14/15, 1988. This first paper is divided into three parts describing the principal results concerning the magnetic cloud. First, by applying the cylindrically symmetric, magnetic flux rope model to the high time resolution magnetic field and plasma data obtained by the IMP‐8 spacecraft, we show that the axis of the magnetic cloud in question is approximately in the ecliptic and orthogonal to the Earth‐Sun line. We note the presence of pulsations of ∼5‐hour period in the bulk flow speed which are superimposed on an otherwise monotonically falling bulk speed profile. Second, we apply ideal MHD to model the self‐similar, radial expansion of a magnetic cloud of cylindrical geometry. As initial condition for the magnetic field we choose a constant‐α, force‐free magnetic configuration. We demonstrate that the theoretical velocity profile for the free expansion of a magnetic cloud is consistent with observations made during the January 14/15, 1988, magnetic cloud encounter. Comparing model with data, we infer that prior to the start of observations at 1 AU the magnetic cloud had been expanding for 65.4 hours; the radius of the magnetic cloud at the time it arrived at Earth was 0.18 AU; and its expansion speed at 1 AU was ∼114 km/s. Third, we discuss energetic (∼1 MeV) ion data, also from instrumentation on IMP‐8. We highlight the appearance of a sharp enhancement in the intensity of ∼0.5‐MeV ions while IMP‐8 was inside the cloud. These ions travel as a collimated, field‐aligned beam from the west of the Sun. This is an “impulsive” solar event in which particles accelerated at a magnetically well‐connected solar flare arrive promptly at the spacecraft. The observation of solar flare particles inside the cloud suggests that field lines within the magnetic cloud remained connected to the Sun. The observation is, however, inconsistent with the supposition that the cloud is formed of closed magnetic field loops disconnected from the Sun.
Effect of sudden solar wind dynamic pressure changes at subauroral latitudes: Change in magnetic field
Authors: Le G. , Russell C. T. , Petrinec S. M. , Ginskey M.
Published: February 2008

The observations obtained during the International Magnetospheric Study (IMS) from the magnetometers of the IGS network extending from Cambridge, England, to Tromso, Norway, are used to study the response of subauroral current systems to sudden changes in solar wind dynamic pressure. Observations show that the response is very strong at subauroral latitudes. The preliminary response in the H component is a brief, small increase in the dayside morning sector and a decrease in the afternoon and night sectors. The main response in the horizontal field (the H and D components) is toward the pole except in the dayside morning sector. The inferred ionospheric current is mainly a circulatory system flowing counterclockwise when viewed from the north pole everywhere at subauroral latitudes except the dayside morning sector.
Numerical simulation of the subsolar magnetopause current layer in the Sun‐Earth meridian plane
Authors: Okuda H.
Published: February 2008

A two‐dimensional electromagnetic particle simulation model was used to study formation and stability of the magnetopause current layer near the subsolar point in the Sun‐Earth Meridian plane. Interactions of the dipole magnetic field with the unmagnetized and magnetized solar wind for both the southward and northward pointing interplanetary magnetic fields (IMF) are considered in the simulations. For the case of zero IMF the simulation results show that the current layer remains stable and is essentially the same as in the one‐dimensional simulation. The width of the current layer is given by the electron‐ion hybrid gyroradius which is much smaller than the ion gyroradius. This is in sharp contrast to our earlier two‐dimensional simulations in the equatorial plane where the current layer is strongly unstable against the high‐frequency drift waves. The current layer is found to remain stable for the northward IMF as well. As in the one‐dimensional simulation, the jump of the magnetic field at the current layer for the northward IMF remains small. For the southward IMF, collisionless magnetic reconnection is found to develop, leading to formation of magnetic islands and density peaking within the current layer. At later times the coalescence of magnetic islands and the mixing of the plasma lead to a smooth density distribution in the magnetopause current layer, resulting in a current layer width of about 1 ion gyrodiameter.
The quiet sunrise E region: Enhancements at high latitudes in winter due to increased nitric oxide
Authors: Swider W. , Keneshea T. J.
Published: February 2008

E region electron concentrations derived from European Incoherent Scatter Facility observations at 70°N were found to be much larger than normal for high solar zenith angles during 23 of 32 days in winter. Electron concentrations computed with the Keneshea code are compatible with the observations if nitric oxide concentrations are about 109 cm−3 at 100‐110 km. For these altitudes near ground sunrise, it is shown that numerically, the nitric oxide concentration approximately equals the square of the electron concentration.
Nonlinear magnetogravity waves in a low‐β plasma
Authors: Inhester B.
Published: February 2008

We derive a set of nonlinear equations for magnetogravity waves based on a reduced set of fluid variables. The equations describe the self‐consistent time evolution of the density perturbation and the velocity and magnetic vector potential component in the direction along the horizontal background magnetic field. These reduced equations take account of the propagation of Alfvén waves along the background field, of gravity waves perpendicular to the background field and their mutual nonlinear interaction. The Strauss‐Boussinesq approximations are shown to be applicable if the vertical gravitational scale height H is much larger than the typical vertical wavelength λz under consideration and if the aspect ratio of the fluctuation wavelengths parallel to the background magnetic field to the perpendicular wavelengths is even larger than H0/λz. As an application of these equations, we investigate some typical nonlinear phenomena. A stability analysis of plane harmonic waves suggests that as a result of the parametric decay instability the energy of buoyancy fluctuations is eventually converted into magnetic fluctuation energy of Alfvén waves with large vertical wave number. Furthermore it is shown that modonlike, horizontally extended traveling vortices are a nontrivial solution to the Boussinesq‐Strauss equations. The vortex axis of these modons is inclined with respect to the background magnetic field and in a stably stratified plasma, a necessary condition for the existence of these solutions is an apparent phase velocity along the background magnetic field smaller than the Alfvén velocity.
A self‐consistent fully turbulent theory of auroral E region irregularities
Authors: Hamza A. M. , St‐Maurice J. ‐P.
Published: February 2008

We present a nonlinear theory of high‐latitude E region irregularities that is based solely on the effects of mode coupling on the turbulent state of the plasma. We have been using a nonlinear equation based on a two‐fluid model governing the evolution of ion acoustic turbulence in the absence of k∥ = k · B0/B0 effects. We have first shown that the linear eigenmodes cannot be used as a zeroth‐order approximation to describe the often‐seen strongly turbulent high‐latitude E region plasma waves. However, with the assumption of weakly growing or decaying nonlinear modes with broad frequency spectra (quasi‐steady state turbulence), we have been able to obtain a set of constraints on the mean frequency and spectral width of both linearly stable and linearly unstable modes. With the use of an “eddy damped quasi‐normal Markovian” closure scheme for the spectral density and a self‐consistent expression for the frequency broadening and the nonlinear growth rate, we have been able, as a result, to relate the Doppler width of radar spectra to the broadband density fluctuation level. Our estimates have led us to believe that strongly turbulent conditions are associated with broadband fluctuation levels of the order of 5 to 10%, no more, in the situations of interest. Our closure scheme also produces a specific set of nonlinear coupled equations that could be solved numerically in order to derive spectral laws pertinent to high‐latitude E region turbulence.
Geochemical and seismological investigations at Vulcano (Aeolian Islands) during 1978–1989
Authors: Chiodini Giovanni , Cioni Roberto , Falsaperla Susanna , Montalto Antonio , Guidi Massimo , Marini Luigi
Published: February 2008

Both geochemical and geophysical evidence indicates that the activity of the fumarolic system of the Fossa crater can be divided into two periods. From 1978 to 1983, such activity has been mainly controlled by two competing processes which affect the permeability of the fumarolic system at deep levels: (1) rock fracturing induced by faulting and (2) mineral deposition‐alteration. The latter, which causes a slow decrease of the permeability at deep levels through clogging of fractures and voids, prevailed at the end of this period, determining a fluid pressure increment in the deep parts of the system. From 1984 to 1989, fluid pressure at deep levels remained persistently high, triggering local microseismic swarms. Furthermore, PT conditions much higher than before were attained in the zones where hydrothermal fluids seep into the conduits from lateral aquifers, owing to an uprising of the isotherms, a deepening of these seeping zones, or both.
Magnetic modeling near selected areas of hydrothermal activity on the Mid‐Atlantic and Gorda Ridges
Authors: Wooldridge Anita L. , Harrison Christopher G. A. , Tivey Maurice A. , Rona Peter A. , Schouten Hans
Published: February 2008

Sea surface magnetic and bathymetric data from two areas of observed high‐temperature hydrothermal activity in the Atlantic (trans‐Atlantic geotraverse, or TAG hydrothermal field) and Pacific (Sea Cliff hydrothermal field) and two areas of inferred hydrothermal activity in the Atlantic (Mid‐Atlantic Ridge (MAR) at 15°N and 17°N) were studied to determine the effect of vent activity on magnetization of the oceanic crust. All of the sites examined show a low in the magnetic anomaly in varying degrees. Magnetization lows within the axial marine magnetic anomaly have been associated with some areas of active and relict high‐temperature hydrothermal venting. A three‐dimensional magnetic inversion was performed on magnetic and bathymetric data from the TAG hydrothermal area on the Mid‐Atlantic Ridge (26°N), and two‐dimensional magnetic inversions were done on individual profiles from the other study sites. The map from the TAG area shows a “negative” in the crustal magnetization solution just to the east of the median valley axis which encompasses the active hydrothermal area. A low in magnetization continues up the eastern wall which suggests alteration of the crust. A distinct low in magnetization is present within the axial anomaly at the Sea Cliff hydrothermal field located on the eastern wall of the Gorda Ridge. The magnetization low becomes less pronounced farther south of the known vent field. Profiles from 15°N and 17°N on the Mid‐Atlantic Ridge show magnetization lows over areas of inferred hydrothermal activity. These lows are present over the the eastern and western walls of the median valley, respectively. The lows observed in the axial marine magnetic anomaly in these four study sites all occur slightly off axis, on crust that is at least 200,000 years old on the Mid‐Atlantic Ridge and at least 100,000 years old on the Gorda Ridge. On the basis of magnetic modeling and measurements of rock magnetic properties, the most probable cause of the low in the magnetization is alteration of the oceanic crust with other subsidiary contributing factors which vary from site to site. The magnetization low over TAG is most probably due to high‐temperature hydrothermal alteration of the crust and a thinner layer 2, with possible contributions from reversal events and magma chamber/Curie isotherm effects. The low near the Sea Cliff hydrothermal field on the Gorda Ridge is probably the product of tectonically controlled low‐temperature (nonhydrothermal) alteration, high‐temperature hydrothermal alteration and magnetic reversal events. At 15°N and 17°N on the MAR, the lows may be produced by alteration of the crust due to possible hydrothermal activity.
Time dependence of mantle plumes: Some simple theory
Authors: Sleep Norman H.
Published: February 2008

Dynamical calculations indicate that plate tectonics are likely to have a major effect on the initiation of plumes from the basal boundary layer in the mantle and their subsequent evolution. Plume start because the boundary layer is hotter than the overlying mantle and hence unstable to perturbations. The growth rate of perturbations is too weakly dependent on wavelength to control the geographic distribution of plumes. Instead, the distribution of plumes is strongly influenced by the shape of large perturbations associated with slabs and surface plates. Plumes evolve because downward return flow is needed to replace the material tapped from the boundary layer. This flow downwarps the top of the boundary layer several hundred kilometers from the plume and thus restricts further flow of hot material toward an active plume. The geometry of this downwarp is strongly influenced by the initial relief of the top of the boundary layer and hence by plate processes. The actual history of plates and slabs is thus needed in numerical models for comparison with real data on plume fluxes.
Reproducing Earth's kernel: Uncertainty of the shape of the core‐mantle boundary from PKP and PcP travel times
Authors: Stark Philip B. , Hengartner Nicolas W.
Published: February 2008

Within ray theory, travel times sample a finite set of points on the core‐mantle boundary (CMB). Only if the CMB is smooth can we estimate its shape from travel times. The assumed degree of smoothness affects the uncertainty of the estimate. We find lower bounds on the uncertainty of the CMB shape by assuming (1) travel time errors are independent and identically distributed with zero mean and known variance, (2) linearization does not compromise accuracy, (3) event locations are exact, (4) radial velocity follows the IASP91 model, and (5) the only three‐dimensional structure is a low‐degree spherical harmonic perturbation to the CMB. We compute confidence intervals and simultaneous confidence envelopes for the CMB with a new technique based on reproducing kernels. Simultaneous confidence envelopes have a specified probability of containing the entire CMB, rather than just one point on the CMB, and thus allow inferences about the CMB shape. Suppose the standard deviation of travel time errors is 0.25 s (it is probably closer to 1.0 s), and that the CMB has a degree 4 spherical harmonic expansion. Then the width of a 95% confidence envelope around the least squares estimate of the CMB using 776,958 PKP phases and 84,947 PcP phases ranges from 0.32 km under Europe and Tonga to a maximum of 1.06 km under the mid‐Pacific, with a median width of 0.58 km. If the CMB has structure up to degree 20, the envelope width ranges from 8.14 km to 245.0 km. It is wider than 40.5 km for 50% of the CMB, and wider than 67.3 km over 25% of the CMB. We construct a degree 20 CMB model extending almost 93 km into the lower mantle beneath the mid‐Pacific that changes travel times by only 0.1 s rms.
Beyond the Born and Rytov approximations: A nonlinear approach to electromagnetic scattering
Authors: Habashy Tarek M. , Groom Ross W. , Spies Brian R.
Published: February 2008

The Born and Rytov approximations, widely used for solving scattering problems, are of limited utility for low‐frequency electromagnetic scattering in geophysical applications where conductivity can vary over many orders of magnitude. We present four new, relatively simple nonlinear estimators that can be used for rapid electromagnetic modeling. The first, termed the static localized nonlinear approximation, is designed specifically to correct the magnitude of the electric field internal to the scatterer. The second, termed the localized nonlinear approximation, improves the estimate of the phase of the scattered field and includes some of the cross‐polarization effects due to full wave scattering. Two further new estimators, based on the Rytov transformation (the localized nonlinear Rytov and the static localized nonlinear Rytov approximations) are designed to further improve the estimation of the phase of the scattered field, especially at high frequency and for larger size scatterers. Although these approximations are nonlinear functions in conductivity, they are generally much faster to compute than the full forward problem, and are almost as efficient as the Born or Rytov approximations. Moreover, the enhanced accuracy of the new estimators has made us optimistic about their application to low‐frequency three‐dimensional inverse problems in electromagnetics. The approximations developed in this paper will also be applicable to fields such as quantum mechanics, optics, ultrasonics, and seismology.
Recognition and analysis of precessing elliptical motion in paleomagnetic records
Authors: Darke Richard M. , Negrini Robert M.
Published: February 2008

Precessing elliptical motion (PEM) is an important possible trajectory of the virtual geomagnetic pole (VGP), an expected result of the superposition of dipole precession on nondipole elliptical motion as proposed and reported on in the literature. The complex plane spectral signature of a simple PEM combination is shown here to be bichromatic in general, consisting of two peaks at frequencies ωa and ωb. Associated with these two peaks is a frequency pair (‐ ωo, ωo) from which each of the peaks at ωa and ωb is shifted by an amount ωp. This shift corresponds to the frequency of precession of the axis of the elliptical motion modified by the precession. Shifts towards positive (negative) frequencies correspond to counterclockwise (clockwise) precessions. The complex plane spectrum associated with a PEM can be strongly asymmetrical and does not provide the investigator with a straightforward means of extracting the frequencies of the component precessional and elliptical motions which may belie processes fundamental to the geomagnetic field. To overcome this problem, an enhanced complex plane spectral analysis is developed which directly identifies both the frequencies of the component precessional and elliptical motions. Examples of the application of this method are given using discretely sampled, synthetic VGP time series.
Interannual variability of monthly Southern Ocean sea ice distributions
Authors: Parkinson Claire L.
Published: February 2008

The region of the Southern Ocean exhibiting interannual variability in monthly averaged sea ice distributions has been mapped and analyzed using data from the Nimbus 5 Electrically Scanning Microwave Radiometer (ESMR) and the Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR). Over the 13‐year record from the two instruments (1973–1976 and late 1978 through mid‐1987), interannual variability is particularly high for the early summer month of December, when 10 × 106 km2 of the ocean had ice‐covered conditions in some years and ice‐free conditions in other years. This area of December variability is almost three times the ocean area consistently having ice‐covered conditions in the 13 ESMR/SMMR Decembers. In contrast, in mid‐winter the area of consistent ice coverage far exceeds the area of variability, which, in spite of the much greater ice coverage overall, is not as large as the area of variability in December. The mid‐winter variability is confined to (1) a ring of variable width around the edge of the ice pack and (2) the location of the 1974–1976 Weddell polynya. Separation of the results to the ESMR and SMMR time periods shows that ice in the West Indian Ocean and the Ross Sea tended to be more expansive in the earlier, ESMR years than in the later, SMMR years, but that ice in the Weddell Sea tended to be less expansive in the ESMR years. In the Bellingshausen and Amundsen Seas, the ESMR years tended to have less summer ice than the SMMR years but not less winter ice. The images and plots provide a succinct summary of the range of spatial distributions of Southern Ocean sea ice during the ESMR and SMMR time periods. They can hence be used as a convenient base for comparisons with future sea ice distributions in the effort to identify occurrences of climate change.
Man‐induced salinity and temperature increases in western Mediterranean deep water
Authors: Rohling E. J. , Bryden Harry L.
Published: February 2008

The historical data base is used to study property changes in both the Western Mediterranean Deep Water (WMDW) and the Levantine Intermediate Water (LIW). Changes in WMDW properties during the past century have been described previously, although on a more limited data base. We are not aware of any previous study of changes in LIW properties. In the extensive data base we used, increases appear in both WMDW temperature and salinity, from 1909 to the present, which substantiate previously reported observations. In addition, we find that the density of WMDW seems to have increased as well, which disagrees with previous suggestions that it has remained constant. We observe that the WMDW temperature increase displays a distinct acceleration starting about 1955 and that a similar, although less conspicuous, acceleration occurs in the WMDW salinity increase. From our study of historical data on LIW properties, the LIW salinity also appears to have increased since 1909. We argue that the warming trend in WMDW may well be a response to the salinity increase, which seems to be imported from the eastern Mediterranean by LIW, and as such our observations endorse a recently published hypothesis. The increase in LIW salinity, in turn, is attributed to changes in the eastern Mediterranean freshwater budget, resulting from damming of major rivers that drain either directly or indirectly into the eastern Mediterranean. Finally, we demonstrate that the basin has not yet reached a new steady state after this freshwater disturbance and that the response time of the system seems to be of the order of 100 years.