bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy spectra of the major ion species in the ring current during geomagnetic storms
Authors: Kistler L. M. , Ipavich F. M. , Hamilton D. C. , Gloeckler G. , Wilken B. , Kremser G. , Stüdemann W.
Published: February 2008

Using the University of Maryland/Max‐Planck‐Institut für Aeronomie charge‐energy‐mass (CHEM) spectrometer on the AMPTE Charge Composition Explorer (CCE) spacecraft, we have examined the nearly equatorial storm time energy spectra of four major magnetospheric ions, H+, O+, He+, and He++, over the energy range 1–300 keV/e in the L range 3–6. The data were obtained during the main and early recovery phases of all geomagnetic storms with minimum Dst less than −50 nT in the time period September 1984 to November 1985. When the spectra are organized by local time, certain features emerge. In particular, there is a dip in the spectra of all ions at 5–20 keV/e in the dawn‐to‐noon sector, while in the noon‐to‐dusk sector the proton phase space density drops off sharply below ∼5 keV. We have compared these spectra with those predicted by a model of ion drift and loss in the magnetosphere. The model calculates the drift paths in a Volland‐Stern electric field and dipole magnetic field and determines the losses due to charge exchange and strong pitch angle diffusion along the paths. We find that the spectra are most consistent with a Volland‐Stern electric field with γ = 2 and with a rotation of the nominal dawn‐to‐dusk electric field eastward by 2 hours local time. Charge exchange is found to be the dominant loss process during the main phase of the storm, producing qualitative agreement with the observed spectra for all species. There are some quantitative disagreements, particularly in the prenoon sector, which may be explained either by an additional loss process or by a modified drift model.
Incoherent scattering of radar waves in the auroral ionosphere in the presence of high electric fields, and measurement problems with the EISCAT Facility
Authors: Hubert D. , Lathuillère C.
Published: February 2008

We have studied the effect of a distribution function of ions disturbed by an electric field perpendicular to the geomagnetic field in the lower F region, on the ionic part of the spectrum of incoherently scattered radar waves. The distortion of the spectra from the shape they would normally have, if the ion distribution were at local Maxwellian equilibrium, increases with the intensity of the electric field and with the angle of the direction of observation of the radar with respect to the magnetic field. We determined as a function of the ion population and of the direction of observation the range of electric field values where the assumption of a bi‐Maxwellian ion distribution is good enough to provide reliable measurements of the electron temperature and density and of the ion anisotropy temperature. Out of that range, the derived plasma parameters have large errors whose magnitude is related to the intensity of the electric field, to the ion composition, to the ratio of the electron temperature to the ion temperature, and to the direction of observation with respect to the magnetic field.
Field line resonances in a realistic magnetosphere
Authors: Mukherjee G. K. , Rajaram R.
Published: February 2008

An internally consistent theoretical framework is developed to study the field line oscillations in the realistic magnetospheric magnetic field using the Mead and Fairfield (1975) model. The nondipolar contributions are numerically computed for the fundamental period of the modes that would reduce to the localized toroidal and poloidal modes described by Cummings et al. (1969) in the dipole limit. It is shown that the nondipolar contributions are not significant at the geostationary orbit but become large further out in the magnetosphere. The nondipolar contributions are very different for the two modes. The situation becomes very much more complicated in the dawn/dusk region where a continuous range of periods exist depending on the orientation of the field line oscillation.
Polar wind ion bands after neutral sheet acceleration
Authors: Delcourt D. C. , Moore T. E. , Waite J. H. , Chappell C. R.
Published: February 2008

The transport of polar wind protons from their ionospheric source through the magnetosphere is examined, using three‐dimensional particle codes. The trajectory simulations demonstrate the escape of the high‐latitude polar wind in the boundary layers during the quiescent magnetosphere, and, in contrast, its trapping in the nightside plasma sheet at times of high Kp. It is shown that the equatorial curvature of the magnetic field lines plays the role of an acceleration filter which yields, during active conditions, an “ion band”‐like precipitation of the polar wind protons characterized by a decrease of the downflowing ion energy with latitude, from several hundred down to a few electron volts. A further comparison between data and computations tends to indicate that, during disturbed times, the dayside originating polar wind provides an important contribution to the hundreds of electron volt downflowing protons in the nightside sector.
Distribution function of continuously created newborn and pickup ions in outer cometary exospheres
Authors: Gaffey J. D. , Wu C. S.
Published: February 2008

The time evolution of the distribution function of newborn ions in the solar wind is investigated within the context of a quasi‐linear theory in which the level of intrinsic turbulence is assumed to be moderate and known. The initial distribution is taken to be a ring beam, which is approximated by delta functions in pitch angle and velocity, and it is assumed that the ions are created at a constant rate with a similar distribution. The long‐time asymptotic form of the distribution is obtained. This distribution is a mixture of ions created recently and ions generated throughout the entire process. The results obtained in the present analysis are found to be in good agreement with recent satellite observations. The time asymptotic distribution is also found to be unstable to low‐frequency hydromagnetic waves propagating parallel to the ambient magnetic field.
Electron acceleration to relativistic energies by traveling interplanetary shocks
Authors: Lopate C.
Published: February 2008

For several years it has been known that traveling interplanetary shocks can accelerate ions to energies of ≥70 MeV/nucleon. Recently, a subset of these shocks has been found to accelerate electrons as well, sometimes to energies of 7 MeV. Between 1978 and 1985 the Pioneer 10/11 spacecraft have observed 11 traveling interplanetary shocks in the outer heliosphere, 7 to 28 AU, which accelerate protons and helium to energies >11–20 MeV/nucleon. Six of these shock events also accelerate electrons to the energy range 2–7 MeV. These are the only traveling interplanetary shocks seen by the Pioneer spacecraft where electrons are accelerated to relativistic energies. In this paper we give the results of analyses of the solar wind plasma, interplanetary magnetic field, and energetic particle data for the 11 shocks. Our analysis has determined that there appear to be three necessary conditions in order for a traveling interplanetary shock to simultaneously accelerate ions to at least 11–20 MeV/nucleon and electrons to relativistic energies: (1) that the upstream magnetic field have a precursor wave in the frequency range 0.003‐0.04 s−1; (2) that the shock has a compression ratio r (=ρ2/ρ1) of between 3 and 4, and has the scattering centers converge at |ΔV|≥50 km/s; and (3) that the relativistic electrons be accelerated via the Fermi mechanism. We propose a model to explain the simultaneous appearance of relativistic electrons and high‐energy ions at an outer heliospheric traveling shock. In this model the Fermi mechanism accelerates the electrons in two stages to relativistic energies, and the shock‐drift mechanism accelerates the ions to energies ≥11–20 MeV/nucleon. An analysis of the momentum spectra of various charged particle species leaves indeterminate the origin of the seed populations of both shocks which accelerate only ions and those which accelerate both ions and relativistic electrons.
The relationship between diffuse auroral and plasma sheet electron distributions near local midnight
Authors: Schumaker Timothy L. , Gussenhoven M. Susan , Hardy David A. , Carovillano Robert L.
Published: February 2008

A study of the relationship between diffuse auroral and plasma sheet electron distributions in the energy range from 50 eV to 20 keV in the midnight region was conducted using data from the P78‐1 and SCATHA satellites. From 1½ years of data, 14 events were found where the polar‐orbiting P78‐1 satellite and the near‐geosynchronous SCATHA satellite were approximately on the same magnetic field line simultaneously, with SCATHA in the plasma sheet and P78‐1 in the diffuse auroral region. For all cases the spectra from the two satellites are in good quantitative agreement. For 13 of the 14 events the pitch angle distribution measured at P78‐1 was isotropic for angles mapping into the loss cone at the SCATHA orbit. For one event the P78‐1 electron flux decreased with pitch angle toward the field line direction. At SCATHA the distributions outside the loss cone were most commonly butterfly or pancake, although distributions peaked toward the field line were sometimes observed at energies below 1 keV. Electron distributions, as measured where there is isotropy within the loss cone but anisotropy outside the loss cone, are inconsistent with current theories for the scattering of electrons by electrostatic waves. Using P78‐1 data to specify the pitch angle distribution in the loss cone for the distribution measured at SCATHA, the electron precipitation lifetimes were calculated for the 14 events. Because the distributions are anisotropic at pitch angles away from the loss cone, the calculated lifetimes significantly exceed the lifetimes in the limit when the flux is isotropic at all pitch angles. The computed precipitation lifetimes are found to be weakly dependent on magnetic activity. The average lifetimes exceed those for the case of isotropy at all pitch angles by a factor between 2 and 3 for Kp≤2 and approximately 1.5 for Kp>2.
The control of auroral zone dynamics and thermodynamics by the interplanetary magnetic field dawn‐dusk (Y) component
Authors: Sica R. J. , Hernandez G. , Emery B. A. , Roble R. G. , Smith R. W. , Rees M. H.
Published: February 2008

It is well known that ion drag momentum forcing is one of the primary drivers of the thermospheric wind at high latitudes. Previous theoretical and experimental studies have shown that the dawn‐dusk component of the interplanetary magnetic field (IMF By) expands the classical symmetric two‐cell convection pattern toward dusk (By negative) or toward dawn (By positive) in the northern hemisphere, altering the ion drag forcing on the neutral atmosphere. Measurements of the neutral dynamics associated with these convection patterns have been presented primarily at magnetic latitudes greater than 70° in the polar cap. In this study, nights with coincident IMF measurements have been selected from the extensive four‐year auroral zone thermospheric wind and temperature data set derived from Fabry‐Perot spectrometer measurements of the Doppler shifts and widths of the O(¹D) 15,867 cm−1 (630.0 nm) emission from College, Alaska. These nights were required to have (1) By values that do not change sign through the night, being either positive or negative, (2) Bx opposite the sign of By, (3) Bz less than or equal to 0, and (4) Ap less than 40 (case 1). The 13 nights which fit the above criteria have been averaged into By positive and By negative groups. Averages from 112 nights of measurements from College were also computed using a selection criterion that depended on the previous 2 hours of IMF measurements (case 2). This procedure yielded averages that differed at times from case 1. The wind and temperature averages for both cases show large variations with By in the auroral zone. The By negative (positive) winds exhibit stronger sunward zonal flow in the evening (morning) with corresponding neutral temperature enhancements in these regions. Furthermore, the zonal wind changes from westward to eastward earlier for By positive than for By negative, consistent with the Harang discontinuity shifting to earlier magnetic local times in the northern hemisphere as By becomes more positive. The wind averages for By negative and positive are compared with National Center for Atmospheric Research thermospheric general circulation model predictions that use a By‐dependent model of ionospheric convection. The results for By negative compare favorably with the averages, but there are significant differences between model calculations and averages for the By positive case. Possible causes for these discrepancies are discussed.
A model of global convection in Jupiter's magnetosphere
Authors: Cheng A. F. , Krimigis S. M.
Published: February 2008

The “planetary wind” model, which holds that corotation must break down outside some Alfvén critical radius and that a centrifugally driven wind outflow must then develop, does not agree with Voyager observations at Jupiter. The discrepancies are most evident in the distant compositional signatures observed by Voyager that showed the plasma to be mostly of indigenous (i.e., Io) composition to ∼60 RJ, solar wind‐like to ∼150 RJ, and again mostly indigenous at ≳150 RJ. Throughout most of the Jovian magnetodisk plasma sheet, the corotational energy density exceeds the magnetic energy density, but corotation‐dominated flow is maintained. Furthermore, the composition of the energetic particles observed by Voyager changed significantly when the spacecraft left the rotating plasma sheet and entered the outflowing magnetospheric wind, contrary to the prediction of the planetary wind model. A new global convection model for the Jovian magnetosphere is proposed to explain these observations, drawing upon models of quasi‐stationary plasma convection in Earth's magnetosphere. The model predicts a substantial dawn‐dusk asymmetry in the structure, dynamics, and plasma composition of the magnetopause and magnetosheath, as well as a region of cross‐tail flow (dusk‐to‐dawn) in the nightside plasma sheet extending from ∼60–150 RJ and containing a substantial admixture of solar wind plasma.
Fast Fermi and gradient drift acceleration of electrons at nearly perpendicular collisionless shocks
Authors: Krauss‐Varban D. , Wu C. S.
Published: February 2008

Electrons can be reflected and effectively energized at quasi‐perpendicular shocks. At a curved shock this process is most prominent where the upstream magnetic field and the shock surface are tangent. The energization process has been observed at the Earth's bow shock but is also considered to occur elsewhere in space. A theoretical explanation of the underlying physical mechanism in terms of fast Fermi acceleration has been proposed in the literature. The theory uses properties of the de Hoffmann‐Teller frame and assumes conservation of magnetic moment in a static, simplified shock profile. Here we extend the discussion in order to clarify certain aspects of the reflection process and to pinpoint the physical mechanisms that are operative and dominant from the viewpoint of the normal incidence frame. By reducing the analysis to the essential physical content and solving the pertinent energy integral equation, the equivalence of fast Fermi and gradient drift acceleration is shown.
Radial diffusion models of energetic electrons and Jupiter's synchrotron radiation: 1. Steady state solution
Authors: de Pater Imke , Goertz Christoph K.
Published: February 2008

The results of a computer code modeling the radial diffusion of equatorially confined energetic electrons in Jupiter's inner magnetosphere are compared with spacecraft as well as ground‐based radio (synchrotron radiation) data. We find that the synchrotron radiation spectrum cannot be reproduced without a significant hardening of the electron spectrum between L = 3 and L = 1.5. This hardening may be due to energy degradation by Jupiter's ring particles. Our calculations also suggest that there may be larger‐sized material outside Jupiter's ring up to L ∼ 4 or Io's orbit.
Midday auroral breakup events and related energy and momentum transfer from the magnetosheath
Authors: Sandholt P. E. , Lockwood M. , Oguti T. , Cowley S. W. H. , Freeman K. S. C. , Lybekk B. , Egeland A. , Willis D. M.
Published: February 2008

Combined observations by meridian‐scanning photometers, all‐sky auroral TV camera and the EISCAT radar permitted a detailed analysis of the temporal and spatial development of the midday auroral breakup phenomenon and the related ionospheric ion flow pattern within the 71°–75° invariant latitude radar field of view. The radar data revealed dominating northward and westward ion drifts, of magnitudes close to the corresponding velocities of the discrete, transient auroral forms, during the two different events reported here, characterized by IMF |BY/BZ| < 1 and > 2, respectively (IMF BZ between −8 and −3 nT and BY > 0). The spatial scales of the discrete optical events were ∼50 km in latitude by ∼500 km in longitude, and their lifetimes were less than 10 min. Electric potential enhancements with peak values in the 30–50 kV range are inferred along the discrete arc in the IMF |BY/BZ| < 1 case from the optical data and across the latitudinal extent of the radar field of view in the |BY/BZ| > 2 case. Joule heat dissipation rates in the maximum phase of the discrete structures of ∼ 100 ergs cm−2 s−1 (0.1 W m−2) are estimated from the photometer intensities and the ion drift data. These observations combined with the additional characteristics of the events, documented here and in several recent studies (i.e., their quasi‐periodic nature, their motion pattern relative to the persistent cusp or cleft auroral arc, the strong relationship with the interplanetary magnetic field and the associated ion drift/E field events and ground magnetic signatures), are considered to be strong evidence in favour of a transient, intermittent reconnection process at the dayside magnetopause and associated energy and momentum transfer to the ionosphere in the polar cusp and cleft regions. The filamentary spatial structure and the spectral characteristics of the optical signature indicate associated localized ˜1‐kV potential drops between the magnetopause and the ionosphere during the most intense auroral events. The duration of the events compares well with the predicted characteristic times of momentum transfer to the ionosphere associated with the flux transfer event‐related current tubes. It is suggested that, after this 2–10 min interval, the sheath particles can no longer reach the ionosphere down the open flux tube, due to the subsequent super‐Alfvénic flow along the magnetopause, conductivities are lower and much less momentum is extracted from the solar wind by the ionosphere. The recurrence time (3–15 min) and the local time distribution (∼0900–1500 MLT) of the dayside auroral breakup events, combined with the above information, indicate the important roles of transient magnetopause reconnection and the polar cusp and cleft regions in the transfer of momentum and energy between the solar wind and the magnetosphere.
Transition probabilities and Franck‐Condon factors for the second negative band system of O2+
Authors: Fox J. L. , Dalgarno A.
Published: February 2008

Transition probabilities for the second negative band system of O2+ are computed using the dipole transition moment presented by Wetmore et al. [1984]. Vibrational levels υ″ = 0 − 54 of the X²Πg ground state and υ′ = 0 − 33 of the excited A²Πu state are included. Franck‐Condon factors for ionization‐excitation of O2 (X³Σg−; υ = 0 − 25) to O2+(A²Πu;υ′ = 0 − 33) are also presented.
The magnetohydrodynamic response of the magnetospheric cavity to changes in solar wind pressure
Authors: Southwood David J. , Kivelson Margaret G.
Published: February 2008

The magnetohydrodynamic excitation of a cold magnetized plasma in a cavity by externally imposed pressure perturbations is examined through use of various simple models. The purpose is to outline the possible responses triggered in the terrestrial magnetosphere by the changes in the solar wind dynamic pressure or other sources of pressure perturbation at the magnetopause. Although the source is compressional, the impulse response of a uniform plasma in a cavity is found to contain both fast and transverse mode oscillations. When nonuniformity is introduced in the model, the fast and transverse signals couple and field line resonances and damped global eigenmodes appear in the response. Field aligned currents are a necessary component of any ionospheric response to compressional perturbations but arise only in models that allow for the nonuniformity of the system. The field‐aligned currents drive ionospheric motions similar to those predicted as signatures of flux transfer events on the magnetopause. The models qualitatively fit with the recent space, ground‐based and ionospheric radar measurements of magnetospheric response to solar wind changes.
Resonant decay of global MHD modes at “thick” interfaces
Authors: Hollweg Joseph V.
Published: February 2008

A global surface mode can exist on a “surface” which is not a true discontinuity, but without sufficient dissipation it is not a normal mode and it decays in time via resonance absorption. The decay rate can be calculated analytically when the “surface” is thin. The goal of this paper is to numerically estimate the decay rate when the surface is not thin. We consider a cold plasma in a uniform magnetic field, and we take the density to vary linearly across the “surface.” In our linearized calculation, the global surface mode is driven in steady state by an antenna located in one of the uniform regions external to the surface. The frequency is a free parameter, and resonance curves are computed numerically without approximations. The widths of the resonance curves are used to estimate the free decay times of undriven surface modes, via the uncertainty principle, equation (1). This procedure agrees with the analytical results for a thin surface. The thin surface results are found to break down when kT ≈ 0.3, where T is the surface thickness and k is the wave number along the surface. When the angle between k and B0 exceeds about 40 degrees, the decay rates show distinct maxima at kT ≈ 0.5–1.0. When applied to the active solar corona, the decay rates are large enough to account for the coronal heating, but it should be kept in mind that the role of nonlinearity in resonance absorption is still undetermined.
Effects of the solar cycle on the fractal and statistical properties of deep underground muons
Authors: Bergamasco L. , Provenzale A. , Castagnoli G. Cini , Serio M. , Kudrjavtsev V. A. , Kuznetsov V. A. , Ryazhskaya O. G.
Published: February 2008

We investigate the fractal and statistical properties of underground muons recorded at 570 hg/cm² and study their response to solar activity variations in the second part of solar cycle 21. A constant feature detected throughout the whole period considered is the self‐affine nature of the underground muon fluctuations which display a scaling range extending approximately from a few hours to more than 2 months. An important result of our study is that the scaling and spectral exponents are positively correlated with the sunspot cycle. In addition, the intensity distribution shows a more marked presence of short‐ and long‐term fluctuations near solar maximum. The presence of solar modulation in the muon record suggests dimensions of the heliosphere at least as large as a few scattering mean free paths of the 1.8‐TV primaries.
A statistical study of high‐altitude electric fields measured on the Viking satellite
Authors: Lindqvist P.‐A. , Marklund G. T.
Published: February 2008

Characteristics of high‐altitude data from the Viking electric field instrument are presented in a statistical study based on 109 Viking orbits. The study is focused in particular on the signatures of and relationships between various parameters measured by the electric field instrument, such as the parallel and transverse (to B) components of the electric field, the satellite potential, and the electric field variability. A major goal of the Viking mission was to investigate the occurrence and properties of parallel electric fields and their role in the auroral acceleration process. The results in this paper on the altitude distribution of the electric field variability confirm earlier findings on the distribution of small‐scale electric fields and indicate the presence of parallel fields up to about 11,000 km altitude. The directly measured parallel electric field is also investigated in some detail. It is in general directed upward with an average value of 1 mV/m, but depends on, for example, altitude and plasma density. Possible sources of error in the measurement of the parallel field are also considered and accounted for.
On the importance of high‐altitude low‐frequency electric fluctuations for the escape of ionospheric ions
Authors: Lundin R. , Gustafsson G. , Eriksson A. I. , Marklund G.
Published: February 2008

Simultaneous measurements of electric fields and upward moving ionospheric ions from the Viking satellite show that upward escape of ionospheric ions in the tens of eV to tens of keV energy range occurs together with low‐frequency electric fluctuations, LEFs (frequencies less than ≈1 Hz). Upward flowing ions are observed when the electric field fluctuations are sufficiently strong in the altitude range ≈ 2000–13,500 km covered by Viking. Furthermore, the temperature of the upflowing ions is found to be well correlated with the power spectral density (PSD) of the LEFs. This suggests a conversion of electric field energy into ion thermal energy. A good correlation between the ion temperature and the wave power in the electrostatic ion cyclotron (EIC) frequency range is also found. However, the heating/acceleration and the corresponding escape of ionospheric ions is believed to be primarily caused by LEFs, first because of the much higher power in the LEFs than the power in the EIC frequency range and second because the PSD clearly peaks below ≈ 1 Hz while the frequency range of the EIC mostly appears as a monotonic tail or “background” of an overall PSD increase at low frequencies. Electric fluctuations, and the associated escape of ionospheric ions, are mainly observed inside or above the acceleration region. Below the acceleration region, predominantly downward accelerated electrons are observed. This can be understood as a short‐circuiting of electric fluctuations by the plasma in the topside ionosphere where the plasma is gaining kinetic energy by the electric field. Thus the electric energy from the source/dynamo in the outer magnetosphere is dissipated in the topside layer of the ionosphere, causing an acceleration and escape of ionospheric plasma. Ion conies with an ion heating up to several keV are associated with the strongest electric fluctuations (up to ≈ 10³ (mV/m)²/Hz), while low‐energy ion beams in general are connected with somewhat smaller electric fluctuations. Spatial or temporal electric fluctuations are expected to increase the magnetic moment of the local plasma via a transverse equal velocity gain acceleration process. This is consistent with the observed electric field power spectrum usually peaking below both the O+ and H+ local gyrofrequency.
Viking magnetic and electric field observations of Pc 1 waves at high latitudes
Authors: Erlandson R. E. , Zanetti L. J. , Potemra T. A. , Block L. P. , Holmgren G.
Published: February 2008

Magnetic and electric field fluctuations in the Pc 1 frequency range (0.2–5 Hz) have been observed by the polar‐orbiting Viking satellite. The fluctuations, interpreted here as electromagnetic ion cyclotron (EMIC) waves, were observed during 21 of 450 orbits surveyed between 0900 and 1400 MLT, near 3 RE geocentric altitude, and at invariant latitudes from 59° to 77°. The frequency structure of the waves is investigated, using spectral analysis and by determining the distribution of the wave frequency as a function of invariant latitude. At invariant latitudes from 59° to 72°, EMIC waves were observed in the frequency range below the equatorial He+ gyrofrequency, while from 70° to 77° invariant latitude, EMIC waves were observed in the frequency range above the equatorial He+ gyrofrequency. This latitude structure of the wave frequency is discussed in terms of the linear growth rate dependence of the waves on the heavy ion density, ion anisotropy, and ion energy. The propagation characteristics of these waves were also investigated, using minimum variance analysis and polarization analysis, and by estimating the Poynting flux based on the observed magnetic and electric field. The waves had Poynting vectors directed downward toward Earth and reached magnitudes between 0.01 and 0.1 erg/cm² s. The polarization of the waves was found to vary between linear, left‐hand, and right‐hand as a function of time or latitude. This variation is interpreted as the structure of spatially localized Pc 1 waves at high latitudes above the ionosphere.
Distribution of auroral precipitation at midnight during a magnetic storm
Authors: Sandahl I. , Eliasson L. , Pellinen‐Wannberg A. , Rostoker G. , Block L. P. , Erlandson R. E. , Friis‐Christensen E. , Jacobsen B. , Lühr H. , Murphree J. S.
Published: February 2008

On the night of November 4, 1986, a very complex precipitation pattern was observed by Viking in the magnetic midnight sector over Scandinavia and Svalbard. The pass took place during a magnetic storm, and during substorm recovery phase. Going from north to south, the satellite first encountered a plasma region of BPS‐type (name derived from boundary plasma sheet) and then a region of CPS type (derived from central plasma sheet). Then, however, a new region of BPS‐type was traversed. The quite intense, most equatorward aurora corresponded to a plasma region which was not of ordinary CPS type but contained sharp quasi‐monoenergetic peaks. The high‐latitude midnight sector was totally dominated by eastward convection. The Harang discontinuity had passed northern Scandinavia the first time as early as 17 to 20 MLT, more than three hours before the Viking pass. It is suggested that the particle precipitation pattern and the general shape of the aurora as observed by the Viking imager can be explained in a natural way by the convection pattern. The northernmost BPS‐ and CPS‐type regions originated in the morningside convection cell, while the more equatorward population of BPS type had drifted in from the eveningside. The interpretation is supported by ground‐based measurements by EISCAT and magnetometers.
Electron energy budget in the high‐latitude ionosphere during Viking/Eiscat coordinated measurements
Authors: Lilensten J. , Fontaine D. , Kofman W. , Eliasson L. , Lathuillere C. , Oran E. S.
Published: February 2008

The magnetospheric electron fluxes precipitating at the top of the auroral ionosphere contribute to the production of ionization, to the excitation of atmospheric constituents, and to the heating of the ambient electrons. This last process occurs essentially when the energy of the initial precipitated electrons and photoelectrons has been degraded to values lower than approximately 10 eV. The heated ambient electron gas loses this energy to the neutral gas and ambient ions. Finally, the temperature gradient produced in the ionospheric plasma induces a heat flux. In the absence of an electric field and for stationary conditions, the energy budget of ionospheric electrons results from the balance between these processes of heating, cooling, and heat conduction. The intensity of these different processes is quantitatively computed at each altitude in the ionosphere by combining simultaneous EISCAT and Viking in situ measurements, and by means of an electron transport model. The stationary electron flux, which leads to the heating rate, is computed, and remaining differences in the energy budget are discussed.
Currents between tethered electrodes in a magnetized laboratory plasma
Authors: Stenzel R. L. , Urrutia J. M.
Published: February 2008

Laboratory experiments on important plasma physics issues of electrodynamic tethers are performed. These include current propagation, formation of wave wings, limits of current collection, nonlinear effects and instabilities, charging phenomena, and characteristics of transmission lines in plasmas. The experiments are conducted in a large afterglow plasma (100 cm × 200 cm, ne ≤ 1012 cm−3, kTe ≤ 3 eV, Bo < 100 G, Ar, pn ≈ 3×10−4 Torr). The current system is established with a small electron‐emitting hot cathode tethered to an electron‐collecting anode, both movable across the magnetic field and energized by potential differences up to V ≈ 100 kTe/e. The total current density in space and time is obtained from complete measurements of the perturbed magnetic field, J = ▽ × B(r,t)/µo The fast spacecraft motion is reproduced in the laboratory by moving the tethered electrodes in small increments, applying delayed current pulses, and reconstructing the net field by a linear superposition of locally emitted wavelets. With this technique, the small‐amplitude dc current pattern is shown to form “whistler wings” at each electrode instead of the generally accepted “Alfvén wings.” For the beam electrode, the whistler wing separates from the field‐aligned beam which carries no net current. Large‐amplitude return currents to a stationary anode generate current‐driven microinstabilities, parallel electric fields, ion depletions, current disruptions, and time‐varying electrode charging. At appropriately high potentials and neutral densities, excess neutrals are ionized near the anode. The anode sheath emits high‐frequency electron transit‐time oscillations at the sheath‐plasma resonance. The beam generates Langmuir turbulence, ion sound turbulence, electron heating, space charge fields, and Hall currents. An insulated, perfectly conducting transmission line embedded in the plasma becomes lossy due to excitation of whistler waves and magnetic field diffusion effects. The implications of the laboratory observations on electrodynamic tethers in space are discussed.
Thermospheric meridional winds measured by the middle and upper atmosphere radar
Authors: Oliver William L. , Fukao Shoichiro , Takami Tomoyuki , Yamamoto Mamoru , Tsuda Toshitaka , Nakamura Takuji , Kato Susumu
Published: February 2008

We use full‐year and seasonal averages of F region electron density and ion velocity measurements made by the Japanese middle and upper atmosphere (MU) radar in Shigaraki, Shiga, Japan, to compute the neutral meridional winds for this location. The average winds are highly diurnal in nature. The all‐year average winds are poleward by day and equatorward by night. Maximum daytime winds are reached near 10 LT, and maximum nighttime winds are reached at midnight. This is consistent with the accepted picture of global circulation consisting of winds driven basically around the Earth from the dayside subsolar point toward the nightside antisubsolar point. Dependence on ion drag is seen in the form of larger nighttime winds and the prenoon daytime velocity maximum. In winter the winds are strongly northward for the entire daylight period, while in the summer the winds become northward for only a few hours before noon and then turn southward for the remainder of the day. The MU radar winds are consistent with the majority of other such wind results reported in the literature for other mid‐latitude locations concerning the phases and times of wind direction turnings. The amplitudes of the MU radar winds, however, are generally smaller than the amplitudes reported elsewhere by a factor of 1.5–2. We see no trace of an abatement in the southward wind at midnight as seen at lower latitudes or of large winds exceeding 200 m/s in the nighttime as reported for locations near the polar regions.
Intense keV energy polar rain
Authors: Newell Patrick T. , Meng Ching‐I.
Published: February 2008

Polar rain is usually a weak, diffuse precipitation of electrons with intensities below 0.1 erg/cm² s and temperatures around 80 eV. Occasionally, it can have much higher fluxes and temperatures. We carried out an investigation using an automated search of the DMSP F7 polar cap precipitation over a 1100‐day interval to find all the days with reasonably intense (>106 eV/cm² s sr) fluxes in one or more ≥1‐keV electron channels above 80 MLAT. After we eliminated precipitation associated with polar cap arcs, 17 such days were found. For 11 days, the precipitation was at least 0.4 erg/cm² s, and on 4 days it reached a few ergs/cm² s (for example, n = 0.85/cm³, and kT = 2000 eV). A fairly uniform polar rain over the polar caps at this intensity approximately equals the hemispherical energy flux over the entire auroral oval. Like ordinary polar rain, the keV precipitation occurs in only one hemisphere and is generally accompanied by an often high‐density polar rain at more typical energies. No particular association with storms or poststorm quieting was evident in our sample; instead, the results appear to be consistent with the model of Fairfield and Scudder, in that there is an apparent association between intense keV polar rain and low solar‐wind densities. The keV electrons were not observed within the cusp proper, further supporting the concept of polar rain as a special component of the solar‐wind electron population. A persistent gap between the poleward edge of auroral electron precipitation and the occurrence of polar rain is observed; the gap is largest at dusk, smaller at dawn, still smaller near midnight, and nonexistent on the dayside. We interpret the field lines poleward of the auroraloval precipitation and equatorward of polar rain as recently closed field lines.
Electron auroral excitation efficiencies and intensity ratios
Authors: Steele D. P. , McEwen D. J.
Published: February 2008

Ground‐based auroral observations with a meridian‐scanning photometer and all‐sky camera were made at Rabbit Lake, Saskatchewan, Canada, simultaneously with nearby HILAT satellite passes during six winter periods near new moon from 1984 to 1986. Eight passes were selected in which HILAT overflew auroras visible from Rabbit Lake. Auroral intensity measurements made at Rabbit Lake were corrected for proton aurora and nightglow and compared with HILAT electron flux measurements at 800 km. The excitation efficiency for the 4278‐Å auroral emission by electron precipitation was found to be (0.29±0.08) kR/(erg/cm² s). Excitation efficiencies for 5577‐Å emission of (1.73±0.51) kR/(erg/cm² s) and (1.23±0.44) kR/(erg/cm² s) were measured on different days in auroras excited by 1.8‐keV and 3.1‐keV Maxwellian precipitation during moderate geomagnetic disturbance. After small night‐to‐night variations in atmospheric composition indicated by the MSIS‐86 model atmosphere were accounted for, the magnetic zenith column intensity ratio 6300/4278 was found to depend on the electron characteristic energy Eo approximately as 6300/4278 ∼ 3.3 Eo−2.1 for characteristic energies of a few keV. The 5577/4278 ratio was found to decrease by about 40% from 1 to 5 keV characteristic energy. These Results pertain mostly to weak auroras (I(5577 Å) = 2–6 kR). Most of these new measurements agree fairly well with previous results of other workers, but the two 5577‐Å excitation efficiencies and the decreasing 5577/4278 intensity ratio show that the green line excitation efficiency varies appreciably even for small changes in the precipitating electron spectrum and atmospheric composition. Published calculations of the auroral 6300/4278 and 5577/4278 ratios versus characteristic energy are compared with our measurements. The magnitude and variation of the 5577/4278 ratios are consistent with roughly 60% of green line emission resulting from O(¹S) excitation by N2(A) energy transfer.
Nonlinear supersonic motions of magnetized plasma flow with mass loading
Authors: Khabibrakhmanov I. Kh. , Verheest Frank
Published: February 2008

A kinetic description of magnetized plasma flow with mass loading is developed and leads to a nonlinear wave equation for small scale dispersive magnetosonic plasma motions superimposed on large scale motions due to mass loading. The mass loading can drive quasi‐stationary, finite‐amplitude magnetosonic type waves in a supersonic plasma flow, even though small amplitude linear disturbances are damped, but only when the magnetosonic Mach number drops to 2 during the slowing down of the mass loaded plasma flow. Nonlinear motions with constant amplitude could be permanent features of a mass loaded plasma flow, as in the neighborhood of comets and planets with extended atmospheres but negligible magnetic fields. The relation between the wave amplitude and the local magnetosonic Mach number is computed and can easily be compared with in situ measurements in cometary plasma environments.
Radar and satellite observations of the storm time cleft
Authors: Yeh H.‐C. , Foster J. C. , Holt J. M. , Redus R. H. , Rich F. J.
Published: February 2008

During the magnetic storm of February 8–9, 1986, the region of strong ion convection in the vicinity of the dayside cusp expanded equatorward into the field of view of the Millstone Hill radar at lower mid‐latitudes. High‐speed (>1.5 km s−1) poleward ion flows were found at latitudes as low as 60° invariant latitude, at least 10° lower than the typical cleft/cusp position for moderately disturbed (Kp>4) magnetospheric conditions. The ion velocity pattern responded promptly to changes in the interplanetary magnetic field By direction. The large‐scale two‐dimensional convection pattern across the dayside was well resolved using radar azimuth scan data at Millstone Hill, thus enabling us to place the fine‐scale radar/satellite observations of the storm time cusp and cleft in the context of the large‐scale pattern. We present a detailed comparison of radar and DMSP F7 satellite observations in the prenoon sector during a period of Kp > 7, to examine the low‐altitude signatures of various plasma regions in the vicinity of the cusp. The combination of particle precipitation, magnetic field perturbation, radar measurements of ion heating, and convection consistently suggests the unusual low‐latitude position of cusp at 65° invariant latitude. Boundary plasma sheet particles were observed to coincide with a narrow region of magnetic‐field‐aligned currents, and with antisunward convection flows at the equatorward edge of the cleft. This may be an indication of the presence of a viscous cell. The radar and satellite observations indicate that the storm time cleft and cusp at ionospheric altitudes retain the general characteristics revealed in their average configuration. Particle and field signatures of the plasma sheet, plasma sheet boundary layer, low‐latitude boundary layer, cusp, and mantle are identified at unusually low magnetic latitude in the 9–10 magnetic local time sector during this event.
Collisionless tearing instability in magnetotail plasmas
Authors: Wang Xiaogang , Bhattacharjee A. , Lui A. T. Y.
Published: February 2008

The problem of the linear stability of collisionless tearing modes in the Earth's magnetotail is revisited. It is found that the collisionless tearing mode is linearly unstable with wavelengths of the order of 10 RE. It is shown that an important feature neglected in earlier theories is a nonzero equilibrium By‐field. The physics of the instability is elucidated in the context of a simple slab model with Bx ≠ 0, By ≠ 0, Bz = 0, and a sheared parabolic model which is representative of the magnetotail in which all three components of the magnetic field are nonzero. The dispersion equation for the instability is obtained by a boundary‐layer analysis. The implications of the theory for recent observations on current disruption and diversion during substorms is discussed.
Modification of fresh crater landforms: Evidence from the Moon and Mercury
Authors: Malin Michael C. , Dzurisin Daniel
Published: February 2008

The morphology of fresh lunar and mercurian craters provides insight into processes of crater formation and modification. Measurements determined for mercurian craters and compared to previously presented lunar data are depth/diameter, central peak and wall‐related mass movement frequencies as functions of diameter, crater rim wall width/rim diameter, rim diameter/floor diameter, and central peak height/rim diameter. Two important results are as follows: (1) there is no evidence for direct gravity scaling of crater morphology, although some slight (∼g1/4 to g1/8) scaling relationship may be indicated, and (2) mass movements are responsible for the change in depth/diameter relationship observed near 2‐km depth and 10‐km diameter. The latter result is helpful in explaining gravitational and topographic data which suggest low‐density regions beneath large, fresh craters.
Hydrostatic compression of the synthetic garnets pyrope and almandine
Authors: Sato Yosiko , Akaogi Masaki , Akimoto Syun‐Iti
Published: February 2008

Accurate measurements of the volume change of the synthetic garnets pyrope and almandine were performed under hydrostatic compression up to about 100 kbar by means of the X ray diffraction technique using a cubic anvil type of high‐pressure apparatus. A least squares calculation using the Birch‐Murnaghan equation gives the isothermal bulk modulus KT = 1.71 ± 0.03 Mbar for pyrope and KT = 1.75 ± 0.07 Mbar for almandine.
The relationship of source parameters of oceanic transform earthquakes to plate velocity and transform length
Authors: Burr Norman C. , Solomon Sean C.
Published: February 2008

The source parameters of large earthquakes on oceanic transform faults are closely related to the thermal and mechanical properties of oceanic lithosphere. Several characteristics of these earthquakes (including magnitude, moment, and apparent stress ηare synthesized according to local plate velocityV, ridge‐ridge offsetL, and average fault widthWestimated by Brune's method. Several relationships result: (1) the maximum momentM0on a transform decreases withV, (2) maximumM0appears to increase withLforL< 400 km and may decrease for greater offsets, (3) ηdoes not clearly depend on eitherVorL, (4) the maximum estimatedW(V) decreases withV, (5) the minimum estimatedW(L) increases withL, and (6) the largest earthquakes on long transforms occur near the transform center. Most of these relationships are consistent with the hypothesis that seismic failure occurs only at temperatures below a fixed value. An inversion of slip rate and magnitude data by transform supports this hypothesis and gives an estimate for the nominal temperature of the boundary separating stick slip and stable sliding. Though the actual thermal structure around oceanic transforms is not known, the idealized spreading plate models used in the inversion give a temperature range for the brittle to ductile boundary of 75°–150°C. If possible uncertainties in the thermal structure are allowed for, a range of 50°–300°C provides a conservative bound on the true limiting temperature. These temperature ranges are consistent with focal depths of transform earthquakes and with laboratory measurements of fault slip in rocks of compositions that are representative candidates for the material being faulted in oceanic transforms.