bluuebunny/crossref_metadata_2025 huggingface dataset:
Further investigation of auroral roar fine structure
Authors: Shepherd S. G. , LaBelle J. , Trimpi M. L.
Published: February 2004

In April 1996, a downconverting receiver was operated in Churchill, Manitoba, Canada, to increase the statistics about the recently discovered fine structure of auroral roar emissions. Auroral roar is found to be both structured and unstructured. A wide variety of previously unknown tonal features drifting in a complicated manner were recorded. These structured features can be classified according to their duration, frequency drift, and grouping with like features. Typically, 95% of the structured features last less than 1 s. The slope of drifting features is more commonly negative than positive with a magnitude typically less than a few kHz s−1 and a maximum of ∼ 800 kHz s−1. The minimum bandwidth of features is 6 Hz or less, and typical separation between similar features is ∼ 400 Hz. These measurements form a basis for reviewing proposed generation mechanisms of auroral roar including a localized source model and laser cavity mechanism.
A new model to study galactic cosmic ray modulation in an asymmetrically bounded heliosphere
Authors: Haasbroek L. J. , Potgieter M. S.
Published: February 2004

We developed a two‐dimensional steady state numerical model that can describe the modulation of galactic cosmic rays in an arbitrarily shaped heliospheric outer modulation boundary. The model is based on the well‐known Parker transport equation for cosmic rays and incorporates: (1) gradient and curvature drift effects including drift along a wavy neutral sheet, (2) adiabatic energy changes, (3) diffusion parallel and anisotropic diffusion perpendicular to the heliospheric magnetic field, and (4) outward convection due to a latitude‐dependent expanding solar wind plasma. The model is described together with results illustrating the effects of an asymmetric heliospheric boundary on the spatial and energy variations of galactic cosmic ray protons throughout the heliosphere. The cosmic ray distribution in the inner heliosphere was only marginally altered, but significant changes were found beyond ∼30 AU, especially in the upwind direction. Concerning the energy dependence of these effects the maximum cosmic ray redistribution occurred at ∼50 MeV. At energies of > 5 GeV the effects became insignificant. Assuming a highly asymmetric heliosphere, we calculated a longitudinal gradient of 0.4 % deg−1 for 1 GV protons at 50 AU, which may be considered an upper limit.
The relativistic electron response at geosynchronous orbit during the January 1997 magnetic storm
Authors: Reeves G. D. , Friedel R. H. W. , Belian R. D. , Meier M. M. , Henderson M. G. , Onsager T. , Singer H. J. , Baker D. N. , Li X. , Blake J. B.
Published: February 2004

The first geomagnetic storm of 1997 began on January 10. It is of particular interest because it was exceptionally well observed by the full complement of International Solar Terrestrial Physics (ISTP) satellites and because of its possible association with the catastrophic failure of the Telstar 401 telecommunications satellite. Here we report on the energetic electron environment observed by five geosynchronous satellites. In part one of this paper we examine the magnetospheric response to the magnetic cloud. The interval of southward IMF drove strong substorm activity while the interval of northward IMF and high solar wind density strongly compressed the magnetosphere. At energies above a few hundred keV, two distinct electron enhancements were observed at geosynchronous orbit. The first enhancement began and ended suddenly, lasted for approximately 1 day, and is associated with the strong compression of the magnetosphere. The second enhancement showed a more characteristic time delay, peaking on January 15. Both enhancements may be due to transport of electrons from the same initial acceleration event at a location inside geosynchronous orbit but the first enhancement was due to a temporary, quasi‐adiabatic transport associated with the compression of the magnetosphere while the second enhancement was due to slower diffusive processes. In the second part of the paper we compare the relativistic electron fluxes measured simultaneously at different local times. We find that the >2‐MeV electron fluxes increased first at noon followed by dusk and then dawn and that there can be difference of two orders of magnitude in the fluxes observed at different local times. Finally, we discuss the development of data‐driven models of the relativistic electron belts for space weather applications. By interpolating fluxes between satellites we produced a model that gives the >2‐MeV electron fluxes at all local times as a function of universal time. In a first application of this model we show that, at least in this case, magnetopause shadowing does not contribute noticeably to relativistic electron dropouts.
Charts of joint Kelvin‐Helmholtz and Rayleigh‐Taylor instabilites at the dayside magnetopause for strongly northward interplanetary magnetic field
Authors: Farrugia C. J. , Gratton F. T. , Bender L. , Biernat H. K. , Erkaev N. V. , Quinn J. M. , Torbert R. B. , Dennisenko V.
Published: February 2004

We present maximum growth rate charts of the Kelvin‐Helmholtz (KH) and Rayleigh‐Taylor (RT) instabilities at the dayside magnetopause (MP), considering two orientations of the interplanetary magnetic field (IMF) (due north and 30° west of north). We input parameters in the plasma depletion layer calculated from an MHD code. We study both a sharp MP transition and an MP with an attached boundary layer (“thin” and “thick” approximations, respectively). Our analysis applies to wavelengths (λ) from ∼2 × 103 km to ≤9 RE. Thin model results are as follows: For a stationary MP and due north IMF, the off‐noon, low‐latitude MP is very low shear (≤10°) and is substantially KH active. With an IMF inclined to north, extremely low shear, KH‐active regions are confined to two strips, one in each hemisphere, where short λ perturbations are generated, which propagate as surface ripples on the high‐latitude, duskside MP. For a sunward accelerating magnetopause and IMF north, a large part of the MP is unstable. With an inclined IMF, the KH+RT unstable strips are broader and growth rates are higher. Thick model results are as follows: For IMF due north and a stationary MP, the middle‐ to high‐latitude MP is stable. At middle to low latitudes, the inner edge of the boundary layer (IEBL) is active, except for a 2‐hour local time band on either side of noon. For the inclined IMF, the MP is stable for long λ, with activity for short λ confined to two strips, as before, with slightly reduced growth rates. For the IEBL, a clear dawn‐dusk asymmetry in KH activity is evident. When the MP accelerates sunward and the IMF points north, we have to consider also the λ of the perturbation. For short λ, growth rates are enhanced with respect to stationarity at both the MP and the IEBL. While there are extensive regions of negligible growth at the MP, the entire IEBL is RT+KH unstable. We give an example of a long λ perturbation where both interfaces are coupled and oscillate together. Finally, for an inclined IMF, we have at the MP unstable strips which are wider and have higher growth rates. The IEBL, by contrast, is completely destabilized, with larger growth rates than under stationary conditions.
Comment on “Global magnetohydrodynamic simulation of a comet crossing the heliospheric current sheet” by Y. Yi et al.
Authors: Wegmann Rudolf
Published: February 2004

Des Wissens bar,

doch des Wunsches voll.

Richard Wagner, Twilight of the Gods
Radial evolution of ejecta characteristics and transient shocks: Ulysses in‐ecliptic observations
Authors: González‐Esparza J. A. , Neugebauer M. , Smith E. J. , Phillips J. L.
Published: February 2004

During its travel to Jupiter, Ulysses detected 25 ejecta and 32 transient forward shocks (TFS), and in the first 3 months after the Jupiter flyby Ulysses detected 9 ejecta and 5 TFS. Half of these ejecta (17) were associated with TFS. We identified the ejecta using bidirectional streaming of suprathermal electrons accompanied by other plasma cloud signatures. This data set of 34 ejecta at low latitudes (±10°) complements previous statistical studies of ejecta and TFS within 1 AU and allows us to study their heliocentric evolution from 1 to 5.4 AU. We used solar wind plasma data to analyze dynamic aspects of these events. In general, ejecta associated with TFS and ejecta without shocks had similar characteristics. However, ejecta associated with TFS had fronts propagating faster than the ambient solar wind, while ejecta not associated with shocks had fronts propagating at about the same speed as the ambient solar wind. The ejecta radial width did not present a clear statistical tendency to expand with heliocentric distance, and the ejecta mean flux density did not decrease more quickly than the square of the distance from the Sun. These two results might suggest that the rate of expansion of ejecta from 1 to 5 AU is less than the rate of expansion within 1 AU. However, a limitation of this analysis is the ambiguity of comparing ejecta parameters that can vary depending on différent aspects and with heliocentric distance. Only about 40% of the ejecta associated with TFS had leading edges propagating supermagnetosonically with respect to the ambient solar wind. Ejecta with large radial widths were faster than small ejecta. Stronger TFS (higher Mach numbers) were followed by larger ejecta.
A statistical study of the ionospheric convection response to changing interplanetary magnetic field conditions using the assimilative mapping of ionospheric electrodynamics technique
Authors: Ridley A. J. , Lu Gang , Clauer C. R. , Papitashvili V. O.
Published: February 2004

We examine 65 ionospheric convection changes associated with changes in the Y and Z components of the interplanetary magnetic field (IMF). We measure the IMF reorientations (for all but six of the events) at the Wind satellite. For 22 of the events the IMF reorientation is clearly observed by both Wind and IMP 8. Various methods are used to estimate the propagation time of the IMF between the two satellites. We find that using the magnetic field before the IMF orientation change gives the smallest error in the expected propagation time. The IMF is then propagated to the magnetopause. The communication time between when the IMF encounters the magnetopause and the start of the convection change is estimated to be 8.4 (±8.2) min. The resulting change in the ionospheric potential is examined by subtracting a base potential pattern from the changing potential patterns. From these residual patterns, a number of conclusions are made: (1) the location of the change in convection is stationary, implying that the change in convection is broadcast from the cusp region to the rest of the ionosphere in a matter of seconds and that the elctric field mapped down the cusp controls the entire dayside ionospheric convection pattern; (2) the shape of the change in the ionospheric convection is dependent on the IMF component that changes, which is indicative of the change in the merging rate on the dayside magnetopause; (3) 62% of the events change linearly form one state to another, while 11% of the events change asymptotically; (4) the change in the ionospheric potential is linearly related to the magnitude of the IMF orientation, with Bz changes having a larger proportionality constant than By changes; (5) the ionospheric convection takes, on average, 13 min to completely reconfigure; and (6) some of the ionospheric convection changes occur on a timescale shorter than that of the corresponding IMF reorientation, possibly as a result of thresholding in the dayside merging region.
Observation and simulations of winds and temperatures in the Antarctic thermosphere for August 2–10, 1992
Authors: Smith R. W. , Hernandez G. , Roble R. G. , Dyson P. L. , Conde M. , Crickmore R. , Jarvis M.
Published: February 2004

Optically derived upper thermospheric wind and temperature data, collected at Antarctic stations at South Pole (L = 14), Mawson (L = 9.3), and Halley (L = 4.6), and averaged over the low‐activity period August 2–10, 1992, have been interpreted with the help of simulation by the National Center for Atmospheric Research thermosphere ionosphere electrodynamic general circulation model (TIEGCM) with inputs matching the average conditions of observation. The simulation provides a global background context upon which the widely‐separated optical observations can be placed. The simulation shows three large‐scale structures in the polar wind field: the morning vortex, the evening vortex, and the cross‐polar wind jet. Each of these came within view of the group of observing stations during the diurnal cycle, providing arrival time observations and signatures which were examined relative to the TIEGCM simulation. Reasonable correspondence was found, indicating the capability of the model to agree simultaneously with observations at three widely spaced stations representative of the subauroral and auroral zones, as well as the polar cap. Simulated wind directions were in excellent agreement with observation, although wind magnitudes frequently exceeded measured values by up to 30%. Apparent divergent flows in the data from Halley and Mawson were explained as signatures of vortices from their presence in the simulated wind fields. Observed diurnal mean temperatures compared well with the simulation, confirming that heat inputs and the distribution of thermal energy in the model are, on average, reasonable. A significant and persistent difference between experimental and modeled temperatures was that the diurnal temperature variation observed at South Pole peaked at the nightside crossing of the jet and was minimum a few hours before noon magnetic local time, whereas the simulation indicated minimum temperatures on the nightside, in antiphase to the measurements. A simple calculation indicates that the observed temperature difference between the air parcels entering the polar cap, encountered on the dayside, and those leaving the polar cap on the nightside is reasonably matched to the heating due to the ion‐drag acceleration process. No explanation of the lack of this temperature rise in the TIEGCM simulation is presently available.
Preliminary retrieval of solar wind latitude distribution from Solar Wind Anisotropies/SOHO observations
Authors: Kyrölä E. , Summanen T. , Schmidt W. , Mäkinen T. , Quemerais E. , Bertaux J.‐L. , Lallement R. , Costa J.
Published: February 2004

The Solar Wind Anisotropies (SWAN) instrument on board the SOHO spacecraft measures Lyman alpha radiation emanating mainly from neutral hydrogen gas in the solar neighborhood. This gas is part of the local interstellar cloud in which the Sun and the heliosphere are immersed. Measurements of Lyman alpha can be used to infer the local cloud characteristics like the velocity and the direction of the flow, gas temperature, and density. The strong interaction between the Sun and the neutral hydrogen gas also makes possible investigations of solar characteristics by Lyman alpha measurements. In this work we will concentrate on deriving the latitudinal distribution of solar‐induced ionization from SWAN Lyman alpha maps measured in 1996 at a time of the solar minimum. From the ionization we derive the distribution of the solar wind mass flux. SWAN Lyman alpha data show that the ionization and the mass flux are nearly flat for all solar latitudes except the narrow belt from −20° to 20° around the solar equator. In this region, ionization and the solar wind mass flux show a definite increase, which can be seen as an intensity depression in the Lyman alpha data from directions near the ecliptic. These results confirm earlier in situ measurements by Ulysses during the present minimum and Lyman alpha measurements by Prognoz satellites 20 years ago.
Generalized kinetic description of a plasma in an arbitrary field‐aligned potential energy structure
Authors: Khazanov G. V. , Liemohn M. W. , Krivorutsky E. N. , Moore T. E.
Published: February 2004

We present a general solution to the collisionless Boltzmann (Vlasov) equation for a free‐flowing plasma along a magnetic field line using Liouville's theorem, allowing for an arbitrary field‐aligned potential energy structure including nonmonotonicities. The constraints of the existing collisionless kinetic transport models are explored, and the need for a more general approach to the problem of self‐consistent potential energy calculations is described. Then a technique that handles an arbitrary potential energy distribution along the field line is presented and discussed. For precipitation of magnetospherically trapped hot plasma, this model yields moment calculations that vary by up to a factor of 2 for various potential energy structures with the same total potential energy drop. The differences are much greater for the high‐latitude outflow scenario, giving order of magnitude variations depending on the shape of the potential energy distribution. Self‐consistent calculations for the photoelectron‐driven polar wind are compared with previous results, and it is shown that even a photoelectron concentration of 0.03% at the base of the simulation (500 km) will cause the potential energy distribution to violate the constraints of the existing models.
Levitation and dynamics of charged dust in the photoelectron sheath above surfaces in space
Authors: Nitter Tore , Havnes Ove , Melandsø Frank
Published: February 2004

Bodies in space subject to solar ultraviolet flux will emit photoelectrons. At steady state the current of escaping photoelectrons is balanced by an influx of particles from the surrounding plasma. When the photoelectrons dominate the space charge close to the surface, it has previously been shown that two steady state potential distributions can exist, one in which the potential decreases from the surface value to zero monotonically and one in which it decreases to a negative minimum and then increases to zero. It has been suggested that the nonmonotonic distribution is the stable one. By assuming planar geometry and a Maxwellian distribution of the emitted photoelectrons the charging of isolated dust particles in the plasma sheath is calculated for both the monotonic and nonmonotonic potential distribution. By increasing photoemission from the surface from zero a transition from an ordinary Debye sheath above a nonilluminated surface to a photoelectron sheath is simulated. Dynamical properties of the dust particles such as oscillations, damping, stability, and trapping are investigated. After being injected into the sheath or electrostatically levitated, dust may be stably suspended above illuminated surfaces in space, even in the case of zero gravitation. However, the smallest particles may escape completely from the body. For all sheath types an unstable layer exists close to the surface where dust cannot collect. The theory is applied to bodies in the solar wind and to the spokes of Saturn.
Global poloidal mode
Authors: Denton Richard E. , Vetoulis Giorgos
Published: February 2004

The poloidal mode is most often observed at the outer edge of the ring current during storm time enhancements. It is usually thought to be driven by the gradient in the hot particle pressure. Besides large azimuthal mode number m (∼100), these waves have periods of ≳100 s (Pc 4 and 5), dominant magnetic fluctuations in the radial (hence the name “poloidal”) or parallel direction, and second‐harmonic structure (single node at equator) for the azimuthal component of the electric field or the radial velocity component. One crucial question is what determines the radial extent of the mode. Ideal MHD predicts that the mode structure will be singular about the toroidal mode (azimuthal fluctuations in velocity and magnetic field) resonant surface. One theory that seeks to explain the radial structure of the poloidal mode (still within the framework of ideal MHD) is that of Vetoulis and Chen [1994, 1996]. They proposed that a finite width poloidal mode might exist in a region where there is a dip in the square of the second‐harmonic poloidal frequency ƒpol−2. The energy may leak out of the region of localization to be absorbed at a toroidal resonance, but if the dip in ƒpol−22 is large enough with the mode frequency down toward the minimum, the rate of leakage may be slow. In this paper, we examine their theory using two‐dimensional linear MHD simulations in dipole geometry. We find that the theory is mathematically correct; the poloidal mode can be localized in a region of depressed ƒpol−2. In order for this to happen, a special kind of pressure profile is required, one having a large second derivative with respect to L shell. Satellite data should be used to examine whether or not such pressure profiles occur during the time of observed high‐m poloidal modes.
EUVE measurement of the local interstellar wind and geocorona via resonance scattering of solar He I 584‐Å line emission
Authors: Flynn B. , Vallerga J. , Dalaudier F. , Gladstone G. R.
Published: February 2004

We present results from EUVE measurements obtained during the all‐sky survey of interplanetary and geocoronal He I 584‐Å emission. The data consist of count rates from the long wavelength spectrometer and the long wavelength photometric band (520–750 Å) of scanner C obtained over a 1‐year period from July 1992 to July 1993. During this period, EUVE was in survey mode so that the scanners made 5° × 360° sweeps of the sky in a plane perpendicular to the Sun‐Earth line, while the spectrometers were aligned with the antisolar direction. The interplanetary He I signal is morphologically consistent with previous observations with similar observing geometry, such as Prognoz 6 [Dalaudier et al., 1984]. However, unlike the Prognoz 6 data, the EUVE measurements were made from low Earth orbit (520 km) and so contain geocoronal emission as well. As a result, along sight lines where the relative speed between the interplanetary wind and the Earth is at a minimum, extinction of the interplanetary signal through resonance scattering by the He geocorona occurs. We believe this to be the first detection of line extinction of the local interstellar He wind emissions by the geocorona. We find that the geocoronal extinction signatures provide a new means of determining the interstellar He wind vector and emission line profile and add further constraints on the values of other interplanetary and solar He parameters and the morphology of the He geocorona. On the basis of model fits of the observed interplanetary emission and geocoronal extinction, we determine values for the interplanetary wind ecliptic longitude λ = 76.0 ± 0.4°, latitude ϕ = −5.4 ± 0.6° (downwind direction), speed υ0 = 26.4 ±1.5 km s−1, and temperature T0 = 6900 ± 600 K. In addition, assuming an interplanetary He density of 0.01 cm−3, we determine an average solar He I 584‐Å line center flux of 1.4 ± 0.3 × 1010 photons cm−2 s−1 Å−1 for the data analyzed here.
Concerning the origin of signatures in dayside equatorial ground magnetograms
Authors: Sibeck D. G. , Takahashi K. , Yumoto K. , Reeves G. D.
Published: February 2004

Previous reports suggest that north/south interplanetary magnetic field (IMF) turnings are the predominant cause of north/south fluctuations in dayside equatorial ground magnetograms. We compare 210° Magnetic Meridian (MM) ground magnetograms with simultaneous IMP 8 solar wind and LANL geosynchronous energetic electron observations during 1993 and 1994 to search for this effect. About one third of the northward equatorial magnetic field enhancements can be associated with increases in the solar wind dynamic pressure, a second third with substorm onset as measured by energetic electron injections, and the final third with southward IMF turnings. An alternative explanation can be found for most enhancements associated with north/south IMF variations but not for enhancements associated with solar wind dynamic pressure variations or substorm onsets. Our results suggest that dayside equatorial ground magnetograms may be used to monitor both solar wind dynamic pressure variations and nightside substorm onsets.
A z mode electron‐cyclotron maser model for bottomside ionospheric harmonic radio emissions
Authors: Willes A. J. , Bale S. D. , Kuncic Z.
Published: February 2004

A model is proposed for recent ground‐based observations of auroral roar emissions, detected at 2Ωe and 3Ωe, where Ωe is the local electron cyclotron frequency in the source region, between 200 and 500 km above the Earth's surface. Electron cyclotron maser emission is a likely mechanism to account for these emissions because it naturally produces coherent radiation at harmonics of Ωe. A theory for auroral roar emissions has already been proposed, whereby maser‐generated second (X2) and third (X3) harmonic x mode radiation is amplified in the source region by multiple reflections off the walls of the density cavity in which they are produced. After many reflections the X2 and X3 waves propagate along the density cavity to a ground‐based observer. However, it is demonstrated here with ray‐tracing calculations that it is highly probable that maser‐generated X2 and X3 radiation is reabsorbed at lower altitudes and thus cannot be detected at the ground. An indirect maser mechanism is proposed instead, where maser‐generated z mode waves at Ωe grow to high levels in the source region and then undergo repeated nonlinear wave‐wave coalescence to produce second‐ and third‐harmonic waves that propagate directly to the ground. The z mode waves must satisfy the necessary kinematic constraints to produce observable second‐ and third‐harmonic radiation. The dependence of the z mode maser on the temperature and functional form of the unstable electron distribution is discussed, along with the conditions required for the coalescence processes to proceed and produce the observed levels of radiation.
Global configuration of the magnetotail current sheet as derived from Geotail, Wind, IMP 8 and ISEE 1/2 data
Authors: Tsyganenko N. A. , Karlsson S. B. P. , Kokubun S. , Yamamoto T. , Lazarus A. J. , Ogilvie K. W. , Russell C. T. , Slavin J. A.
Published: February 2004

Based on fine‐resolution Geotail magnetometer data, a set of 5‐min magnetic field averages was compiled for the period 1993–1997 and merged with 5‐min average solar wind parameters, measured by IMP 8 and Wind spacecraft. Using this data set, the shape of the tail current sheet was studied in the interval −100 < XGSM < − 10 RE as a function of the Earth's dipole tilt angle and of the By component of the IMF. The tilt‐related warping of the current sheet and its twisting around the magnetotail axis in response to the IMF were modeled by analytical functions, whose parameters were found by least squares fitting to the data, for several bins of XGSM. A similar modeling was also done for the near‐tail region −20 < XGSM < − 10 RE using a set of 5‐min ISEE 1/2 data, tagged by corresponding solar wind parameters from IMP 8, for the entire duration of the ISEE magnetometer experiment (1977–1987). The IMF‐related twisting steadily increases down the tail and is quite conspicuous even at close geocentric distances (−20 ≤ X ≤ −10 RE). A simple and flexible mathematical method is suggested, which allows quantitative modeling on a global scale of the IMF‐related deformation of the cross‐tail current by means of a “twist transformation” of the tail field. The method allows for a wide variety of possible geometries of the current sheet and keeps the total field confined within the magnetotail boundary. The results of the study are intended to be used in the development of an improved global model of the magnetospheric magnetic field, incorporating the effects of the IMF upon the magnetotail structure.
A simple model of magnetospheric trough total density
Authors: Gallagher D. L. , Craven P. D. , Comfort R. H.
Published: February 2004

Total electron densities in the magnetospheric trough are used to develop an empirical model of this region. Average trough filling from early morning through to afternoon local times is found to be 0.56±0.08 cm−3 h−1. Growing trough densities are found to change to falling trough densities at a magnetic local time (ΦTP) related to Kp, i.e., ΦTP = 0.145 Kp2 −2.63 Kp + 21.86. Trough densities are found to fall in the late afternoon and through evening local times with an average rate of −0.83±0.15 cm−3h−1. The derived model is smooth and continuous throughout all local times and may be used in a simulation or for defining a static description of typical total electron densities in the magnetospheric trough.
Large polarization electric fields associated with midlatitude sporadic E
Authors: Shalimov S. , Haldoupis C. , Schlegel K.
Published: February 2004

Recent 50 MHz E region coherent backscatter observations and in situ rocket measurements established the existence of enhanced electric fields in the midlatitude ionosphere that can become at times sufficiently large to excite the Farley‐Buneman instability. To understand the origin of these fields, we present a simple quantitative model that relates to a local polarization process acting inside spatially confined, nighttime sporadic E layers of dense ionization. By including the effects of field‐aligned currents in the current continuity equation we estimate the necessary conditions on the relative horizontal E layer extent and the ratio of integrated Pedersen conductivities above and inside the layer for the generation of both zonal and meridional polarization fields. We show that the polarization process can account for the elevated electric fields of several millivolts per meter, which are implied often from backscatter Doppler measurements during unstable E region conditions at midlatitude. The polarization process can become much more effective for dense and strongly elongated Es layers under the action of an enhanced ambient electric field. In this case, large polarization fields that may be capable of exciting Farley‐Buneman plasma waves can be sustained. The stringent requirements for strongly elongated sporadic E layers with sharp boundaries, low ionospheric Pedersen conductivities above the layer in relation to those inside, and relatively large ambient electric fields would explain why type 1 echoes are so rare in midlatitude E region backscatter.
Ionospheric ion acceleration by multiple electrostatic waves
Authors: Ram A. K. , Bers A. , Benisti D.
Published: February 2004

Observations by Topaz 3 show ionospheric O+ and H+ ions, with ambient energies of around 0.3 eV, to be transversely (to the geomagnetic field) energized, to around 10 eV, within lower hybrid structures composed of broadband large‐amplitude (100–200 mV/m) electrostatic waves. In this paper we show that the energization of O+ ions can be explained by a new nonlinear coherent interaction mechanism involving multiple electrostatic waves propagating across the magnetic field. Low‐energy ions, whose velocities are well below the phase velocities of the waves, are shown to gain energy monotonically increasing in time when averaged over a cyclotron orbit. We examine the properties of this coherent energization mechanism numerically and by an analytical, multiple timescale, analysis. We find, in accordance with observations, that the tail of the O+ distribution is most likely to be energized. The analysis provides the spatial extent, along the geomagnetic field, of the lower hybrid structures needed for the observed energization.
Formation of polar cap patches associated with north‐to‐south transitions of the interplanetary magnetic field
Authors: Valladares C. E. , Decker D. T. , Sheehan R. , Anderson D. N. , Bullett T. , Reinisch B. W.
Published: February 2004

On January 15, 1991, the Sondrestrom incoherent scatter radar probed the midday high‐latitude ionosphere to gather evidence for the formation and entry of polar cap patches. During the experiment the interplanetary magnetic field (IMF) BZ was positive and steady for few hours until 1548 UT when a short negative excursion of BZ occurred. Prior to the BZ excursion, and when this parameter was directed northward, the Sondrestrom radar detected a quasi‐stationary density feature situated near the poleward boundary of the oval. Convergent electric fields and slightly elevated Te values were seen accompanying the F region density feature. A numerical model of the high‐latitude ionosphere that uses a steady north BZ Heppner and Maynard convection pattern suggests that under these IMF conditions a tongue of ionization (TOI) can be formed near the midday sector, but it is confined to the poleward boundary of the auroral oval. It does not traverse into the polar cap. This simulated BZ northward TOI resembles the density feature seen in the radar data prior to the BZ negative excursion. When the BZ value was oriented southward, the radar detected the density feature moving poleward and then disappearing to the north of the radar field of view. At this time of BZ negative the radar data also displayed elevated Ti values and a new pattern of line‐of‐sight velocities. Nearly 34 min after the density feature departed from the field of view of the Sondrestrom radar, the Qaanaaq digisonde measured a factor of 2 increase in the ƒoF2 values. Similar enhancements are typically attributed to the passage of a patch. We also conducted a numerical simulation of the transit of the density feature from its initial location near the polar cap boundary up to its passage through the Qaanaaq station. The time that the density feature reaches Qaanaaq in our simulations is in good agreement with the actual time that the enhanced patch‐like number density was observed at Qaanaaq. The BZ switching mechanism does not dispute the validity of other patch formation mechanisms; it merely suggests that a BZ northward TOI can end up as a polar cap patch if a timely reversal of BZ occurs.
Parallel ion velocity shear driven electromagnetic fluctuations and associated particle transport in partially ionized plasmas
Authors: Shukla P. K. , Mirza Arshad M. , Faria R. T.
Published: February 2004

A nonuniform partially ionized magnetized plasma is shown to be unstable against electromagnetic perturbations. The source of free energy for the instability is sheared magnetic‐field‐aligned ion flow, which is coupled to the electromagnetic waves via charged particle‐neutral collisions. Analytical expressions for the growth rate and threshold are obtained. Numerical studies for the ionospheric parameters suggest that electromagnetic waves grow faster than their electrostatic counterparts. Furthermore, linearly excited electromagnetic waves in partially ionized collisional plasmas are shown to cause nonthermal cross‐field transport of plasma particles. The results can have relevance to the current filament at ion and the plasma diffusion in the ionosphere of the Earth, as well as in low‐temperature laboratory plasmas.
Study on neutral wind contribution to the electrodynamics in the polar ionosphere using EISCAT CP‐1 data
Authors: Fujii Ryoichi , Nozawa Satonori , Matuura Nobuo , Brekke Asgeir
Published: February 2004

Energy coupling between the thermosphere, ionosphere and magnetosphere is studied quantitatively through an analysis using the European Incoherent Scatter (EISCAT) Common Program (CP) −1 version H data obtained on May 3, 1988. A negative excursion of the H component in the Tromsø magnetogram occurred during the experiment period, which involved the following two features: (1) the electric potential across the polar cap was expected to be reduced abruptly in association with a sudden change of the interplanetary magnetic field (IMF) Bz polarity from southward to around null and (2) the negative excursion had a relatively long duration of development (about 4 hours), which may drive neutrals to move significantly through ion drag. In order to investigate the energy coupling between the thermosphere and ionosphere, we evaluate quantitatively the electromagnetic energy flux J·E, the Joule heating rate J·E′ (E′ = E + U × B), and the mechanical energy transfer rate U·(J × B), where U is the neutral wind velocity. The CP‐I‐H experiment provides directly or indirectly all quantities above at altitudes of 101 km, 109 km, 119 km, and 132 km. The results are summarized as follows. (1) The amplitude of the neutral wind related electric field U × B varied greatly with altitude, i.e., at altitudes above 119 km it often became larger than 50% of the amplitude of the observed electric field; (2) during the late recovery phase of the negative excursion of the H component of the Tromsø magnetic field, the neutral wind related electric field tended to be canceled with the observed electric field; (3) in the E region the neutral wind mechanical energy transfer rate U·(J × B) is not negligible but is comparable to the Joule heating rate J·E′; and (4) in particular, at higher altitudes (132 km high) the conversion from the neutral wind mechanical energy to the electromagnetic energy occasionally may occur.
A reexamination of the local time asymmetry of lobe encounters at geosynchronous orbit: CRRES, ATS 5, and LANL observations
Authors: Moldwin Mark B. , Fernandez Mark I. , Rassoul Hamid K. , Thomsen Michelle F. , Bame Samuel J. , McComas David J. , Fennell J. F.
Published: February 2004

The local time of occurrence of a geosynchronous plasma flux dropout or lobe encounter is dependent on the longitude of the observing spacecraft. Satellites located on or west of the International Geomagnetic Reference Field (IGRF) model magnetic dip prime meridian (magnetic dip meridian) observe almost exclusively (77%) dawnside lobe encounters, while satellites located east of the magnetic dip meridian observe mostly duskside lobe encounters (67%). However, the transition from observing dawnside or duskside lobe encounters is not symmetric about the magnetic dip meridian. Three satellites (ATS 5, 1991‐0180, and 1990‐095) located essentially at the magnetic dip meridian observed a majority (73%) of dawnside encounters. This study therefore concludes that there is a dawn‐dusk asymmetry of the synchronous orbit altitude magnetic equatorial plane during active geomagnetic periods with the dawnside magnetosphere experiencing more radical distortions than the duskside. A case study of near‐simultaneous lobe observations from CRRES and 1989‐046 supports this conclusion. Note that it is found that the IGRF model dip prime meridian better organizes the local time occurrence frequency than does the magnetic dipole magnetic meridian, which has been shown to organize the seasonal dependence of geosynchronous lobe encounters in previous studies (i.e., Thomsen et al., 1994). Hence this study emphasizes the importance of considering the relative position of satellites with respect to both the magnetic dipole equator and the geomagnetic equator when utilizing geosynchronous plasma, magnetic field, and energetic particle data as diagnostics of the energy state of the magnetosphere.
Error estimates for minimum variance analysis
Authors: Khrabrov A. V. , Sonnerup B. U. Ö.
Published: February 2004

Analytical estimates of errors, associated with random statistical noise in magnetic field and other data to which the minimum/maximum variance analysis technique is commonly applied, are derived from first principles. A systematic expansion procedure is used in which the expansion parameter is proportional to the noise amplitude and inversely proportional to the square root of the number of vector data samples, K. The two special cases where the signal‐to‐noise ratio is large and small are considered for arbitrary noise distributions. The ideal case of small errors and isotropic Gaussian noise allows determination of uncertainty cones of elliptic cross section for all three eigenvectors, x1, x2, x3, and uncertainty intervals for all three eigenvalues, λ1, λ2, λ3, of the variance matrix, all in terms of the eigenvalues themselves and the number of data points. Denoting the angular standard deviation (in radians) of vector xi toward or away from vector xj by Δϕij these results are Δϕij = ±[λ3(λi + λj − λ3)/(K − 1)(λi − λj)2]1/2 and Δλi = ±[2λ3(2λi − λ3)/(K − 1)]1/2 where λ3 is the smallest eigenvalue. The predictions from these formulas compare favorably with results from simple bootstrap numerical error analysis. Applications to maximum variance analysis of electric field data and to the study of anisotropic plasma pressure tensors are discussed as special cases.
Auroral activity associated with pressure pulses and substorms: A comparison between global fluid modeling and Viking UV imaging
Authors: Winglee R. M. , Menietti J. D.
Published: February 2004

Global three‐dimensional fluid simulations in conjunction with Viking UV observations are used to investigate the response of the auroral region to solar wind pressure pulses. The fluid simulations differ from ideal MHD in that the steady state electron momentum equation which incorporates the Hall and gradient pressure terms is used instead of the idealized Ohm's law. This treatment produces important corrections to the currents that map into the noon and midnight sectors of the auroral regions and can be important in determining magnetospheric activity during periods of changing interplanetary magnetic field (IMF). Two pressure pulses observed by IMP 8 in the solar wind are shown to produce transient (a few minute) variations in the auroral oval, starting with brightenings near noon which then quickly propagate into the nightside. Of the two pulses, the second pulse which is accompanied by southward IMF produces the larger nightside activity. The model shows that the observed UV brightenings are accompanied by large increases in the auroral field‐aligned currents (of the order of 1 MA) at approximately the same MLT as the UV brightenings. However, the current enhancements produced by each pulse are shown to develop along slightly different latitudes of the existing region 1 currents so that the current system can become filamented. The noon currents are also shown to be highly dependent on both By and Bz components of the IMF.
Rates and mechanisms of Fe‐Mg interdiffusion in olivine at 980°–1300°C
Authors: Chakraborty Sumit
Published: February 2004

Fe‐Mg interdiffusion rates have been measured in olivine solid solutions using the diffusion couple technique. Measured diffusion rates along [001] at fo2=10−12 bars and between 980°C and 1300°C are found to be slower by about 2 orders of magnitude, compared to previous studies with the exception of some isolated data points of Misener [1974]. There is no change in temperature dependence within this temperature interval for any composition studied. The temperature dependence at a composition of Fo86 is described by an activation energy of 226±18 kJ/mol and a preexponential factor of (5.38±0.89)×10−9 m2/s. The data are consistent internally as well as with well‐established theoretical models relating tracer and chemical diffusivities. It is shown that diffusion coefficients are a unique function of pressure, temperature, major element composition and oxygen fugacity for olivines and are not dependent on trace element contents of starting crystals. It is argued that a twofold classification of diffusion mechanisms into intrinsic and extrinsic is inadequate for Fe‐bearing silicates, and at least three categories need to be defined. Consideration of the data in combination with the point defect chemistry indicates that the observed diffusion in olivines occurs within a “transition‐metal extrinsic” regime. For low temperature extrapolations, significant changes in temperature dependence of diffusion rates are not anticipated, although the oxygen fugacity dependence may change. For calculations in natural systems the present data would yield higher closure temperatures, longer timescales, slower cooling rates, and shorter length scales of diffusion compared to those obtained using earlier diffusion data.
Postseismic deformation associated with the 1992 Mω=7.3 Landers earthquake, southern California
Authors: Savage J. C. , Svarc J. L.
Published: February 2004

Following the 1992 Mω=7.3 Landers earthquake, a linear array of 10 geodetic monuments at roughly 5‐km spacing was established across the Emerson fault segment of the Landers rupture. The array trends perpendicular to the local strike of the fault segment and extends about 30 km on either side of it. The array was surveyed by Global Positioning System 0.034, 0.048, 0.381, 1.27, 1.88, 2.60, and 3.42 years after the Landers earthquake to measure both the spatial and temporal character of the postearthquake relaxation. The temporal behavior is described roughly by a short‐term (decay time 84±23 days) exponential relaxation superimposed upon an apparently linear trend. Because the linear trend represents motions much more rapid than the observed preseismic motions, we attribute that trend to a slower (decay time greater than 5 years) postseismic relaxation, the curvature of which cannot be resolved in the short run (3.4 years) of postseismic data. About 100 mm of right‐lateral displacement and 50 mm of fault‐normal displacement accumulated across the geodetic array in the 3.4‐year interval covered by the postseismic surveys. Those displacements are attributed to postseismic, right‐lateral slip in the depth interval 10 to 30 km on the downward extension of the rupture trace. The right‐lateral slip amounted to about 1 m directly beneath the geodetic array, and the fault‐normal displacement is apparently primarily a consequence of the curvature of the rupture. These conclusions are based upon dislocation models fit to the observed deformation. However, no dislocation model was found with rms residuals as small as the expected observational error.
Upper mantle P wave velocity structure of the eastern Snake River Plain and its relationship to geodynamic models of the region
Authors: Saltzer Rebecca L. , Humphreys Eugene D.
Published: February 2004

Tomographic inversions of ∼5000 teleseismic P wave travel time residuals image a narrow, deep, low‐velocity region centered beneath the eastern Snake River Plain, Idaho. Aligned in aie direction of North American plate motion, the eastern Snake River Plain is the locus of time‐progressive volcanism leading to the Yellowstone hotspot. The low‐velocity anomaly extends to depths of at least 200 km and is flanked by high‐velocity mantle to either side. These results are inconsistent with standard mantle plume models which predict a shallow, wide, low‐velocity anomaly. By considering the effects of composition, partial melt, and temperature on both P wave velocities and upper mantle densities (assuming local isostasy), we conclude that the low‐velocity region is partially molten peridotite and the high‐velocity regions on either side are depleted residuum. Swell relief at hotspots has generally been attributed to thermal effects and is commonly used to estimate plume thermal buoyancy fluxes. However, we demonstrate that the compositional effects of mantle melting and depletion can account for at least one fifth and possibly all of the swell relief at Yellowstone. These results suggest that melt generation and segregation are dominant processes within the upper mantle of Yellowstone and allow for the possibility that the fundamental origin of the Yellowstone hotspot is something other than a deep‐seated mantle plume.