bluuebunny/crossref_metadata_2025 huggingface dataset:
Vertical wind disturbances in the afternoon of mid‐summer revealed by the MU radar
Authors: Sato Kaoru
Published: February 2008

This paper reports a kind of vertical wind disturbances which appear daily in the afternoon of mid‐summer in the troposphere. A detailed analysis was made of wind data monitored by the MU radar (35°N, 136°E, Shigaraki, Shiga, Japan) for about five days in July 1987 as a typical example. The disturbances propagate upward up to (sometimes over) the tropopause level with a speed of 0.4–1.8 m s−1 as a packet. The timing of generation and propagation suggests that they are convective motions of cumulus clouds generated by solar heating on the ground. A map of cloud‐top height with high horizontal resolution of about 1 km provided by a satellite of NOAA‐9 supports this inference. The structures near the tropopause and spectral characteristics are shown.
Initial investigation of the Landers, California, Earthquake of 28 June 1992 using TERRAscope
Authors: Kanamori Hiroo , Thio Hong‐Kie , Dreger Doug , Hauksson Egill , Heaton Tom
Published: February 2008

The 1992 Landers earthquake (Ms=7.5, Mw=7.3) was recorded at six TERRAscope stations in southern California. Peak accelerations ranged from 0.16 g at SVD (Δ=63 km) to 0.0092 g at ISA (Δ=245 km), decreasing with distance away from the fault zone. The peak velocity showed a different pattern reflecting the rupture directivity from south to north. The largest peak velocity, 19 cm/sec, was observed at GSC (Δ=125 km). Moment tensor inversion of long‐period surface waves yielded a mechanism with M0=1.1×1027 dyne‐cm (Mw=7.3), dip=74°, rake=−176°, and strike=340°. Inversion of teleseismic P and S waves revealed two distinct sub‐events of 6 and 8 sec duration and about 10 sec apart. The source parameters for the first and second events are: M0=1.9×1026 dyne‐cm, dip=83°, rake=179°, strike=359°; and M0=6.1×1026 dyne‐cm, dip=87°, rake=178°, strike=333°, respectively. The radiated wave energy, ES, was estimated as 4.3×1023 ergs. The ratio Es/M0=3.9×10−4 corresponds to a stress drop of 280 bars, and suggests that the Landers earthquake belongs to the group of high stress drop earthquakes, and occurred on a fault with a long recurrence time. The rupture directivity can be seen clearly in the records from PFO (Δ=68 km) located to the south and GSC located to the north of the epicenter. The maximum displacement at PFO is only 13% of that at GSC despite the shorter epicentral distance to PFO than to GSC. The slip distribution determined with the empirical Green's function method indicates that the Landers earthquake consists of two distinct sub‐events about 30 km apart, with the second sub‐event to the north being about twice as large as the first one. This slip distribution is consistent with the teleseismic data and the surface offsets mapped in the field.
System III variations in apparent distance of Io plasma torus from Jupiter
Authors: Dessler A. J. , Sandel B. R.
Published: February 2008

The dusk side of the Io plasma torus is brighter than the dawn side (the “east‐west effect”), which has been explained in terms of a uniform dawn‐to‐dusk cross‐tail magnetospheric electric field. Such a field would cause the torus to be offset so the dusk ansa is closer to Jupiter and the dawn ansa is farther, the two displacements being equal. We find the displacement of the two ansae to be unexpectedly complex. The displacements are unequal and both ansae are in motion with the motion of the approaching ansa being the lesser of the two. The radial motions, as measured from either the center of Jupiter or the offset‐tilted‐dipole, are of unequal magnitude and have the System III periodicity. We conclude that the cross‐tail electric field that causes these torus motions is concentrated on the dusk ansa, varies with the System III period, and shows magnetic‐anomaly phase control. Finally, we find that the dawn‐dusk asymmetry in brightness (the east‐west effect) is not explained simply by the cross‐tail electric field. We conclude that there is a heating mechanism, yet to be fully identified, that causes the dusk side of the Io plasma torus to be brighter than the dawn side.
Thermal history of chondrites: Hot accretion vs. metamorphic reheating
Authors: Haack Henning , Taylor G. J. , Scott E. R. D. , Keil Klaus
Published: February 2008

The different degrees of equilibration observed between different petrologic types of ordinary chondrites have been attributed to either hot accretion and a resulting slower cooling of the higher petrologic types, or to cold accretion followed by a separate metamorphic reheating event. We find that constraints on the thermal evolution of chondrules from primary heating in the nebula to thermal processing in chondrite parent bodies are inconsistent with accretion of hot (>800°C) chondrules: a) Cooling rate experiments suggest that chondrules cooled to below 800°C in less than a day. We find that the well‐mixed state of chondrules and matrix material in chondrites could not have been obtained in such a short period of time, b) Chondrule cooling rates indicate that they cooled in a hot, relatively slowly cooling environment, whereas the volatile contents of matrix material in chondrites indicate formation in a cold environment. Relatively slowly cooling chondrules could therefore not have coexisted with the matrix material in the nebula, as required by the “hot accretion” scenario, c) Our calculations show that retention of heat in the hot accretion scenario requires accretion rates of the order of 100 km/y, i.e., four orders of magnitude above the accretion rates inferred from theoretical studies for accretion of planetary objects. We therefore find it impossible to create a physical scenario, within the existing constraints, that would allow for hot accretion of chondrules and conclude that the petrologic types of ordinary chondrites were established by metamorphic reheating after cold accretion.
An empirical model of the plasma density distribution in the distant Earth magnetosheath
Authors: Steinberg J.‐L. , Lacombe C.
Published: February 2008

We use published data from several spacecraft and new data from the sounder on ISEE‐1 to construct a model of the normalized magnetotail magnetosheath plasma density distribution. The model assumes symmetry of revolution of the magnetosheath around the tail axis. It is valid in the range of aberrated GSE X' from 0 to − 200 RE with a rms fractional deviation from the data of 30%, equal to the relative uncertainty of the data themselves. As in gasdynamic models, the magnetosheath density gradient, perpendicular to the tail axis, increases when the distance to the bow shock decreases. For 0 > X′ > −12 RE, the density at the magnetopause is found to decrease with |X′| less rapidly than predicted by gasdynamic models.
A low‐altitude mechanism for mesoscale dynamics, structure, and current filamentation in the discrete aurora
Authors: Keskinen M. J. , Chaturvedi P. K. , Ossakow S. L.
Published: February 2008

The two‐dimensional nonlinear evolution of the ionization‐driven adiabatic auroral arc instability has been studied for the first time. We find: (1) the adiabatic auroral arc instability can fully develop on time scales of tens to hundreds of seconds and on spatial scales of tens to hundreds of kilometers, (2) the evolution of this instability leads to nonlinear “hook‐shaped” conductivity structures, (3) this instability can lead to parallel current filamentation over a wide range of scale sizes from kΛ0 < 1 to kΛ0 > 1 where k is the wavenumber and Λ0 is the initial arc width perpendicular to the geomagnetic field, and (4) the k‐spectra of the density, electric field, and parallel current develop into inverse power laws in agreement with satellite observations. Comparison with mesoscale auroral phenomenology and current filamentation structures is made
Anomalous propagation of Rayleigh waves along Izu‐Mariana Trench
Authors: Kato Mamoru , Hirahara Kazuro
Published: February 2008

Broad band records of Macquarie Ridge earthquake of May 1989 recorded in Japan show anomalous propagation of Rayleigh wave with frequency of 5 – 25 mHz along the back arc side of the Izu‐Mariana trench. Rayleigh waves at three stations in central Japan show different polarization, that is, while one station shows a normal Rayleigh wave, other two stations, only 290 and 510 km apart, show anomalous polarization with a large deviation from the radial direction and their temporal and gradual change. Possible causes of this anomaly include anisotropy as well as laterally heterogeneous velocity structure of the upper mantle in this active marginal basin region.
The effects of orbital and climatic variations on Martian surface heat flow
Authors: Mellon Michael T. , Jakosky Bruce M.
Published: February 2008

We have examined the effects of climate changes, induced by orbital oscillations, on Martian surface heat flow. It was found that the climatological component of the surface heat flow can be larger than the expected internal geothermal heat flow. We suggest that measurements of surface heat flow be targeted for equatorial and south polar regions to avoid climatic effects and that care be taken in interpreting measurements in mid‐latitude and north polar regions.
Rapid estimation of rupture directivity: Application to the 1992 Landers (MS = 7.4) and Cape Mendocino (MS = 7.2), California earthquakes
Authors: Ammon Charles J. , Velasco Aaron A. , Lay Thorne
Published: February 2008

Using empirical Green functions with regional and teleseismic surface waves, it is possible to resolve fault finiteness effects, in many cases uniquely defining the fault plane for relatively large earthquakes. The technique requires very little data processing and can be applied in near‐real time with the current distribution of seismic stations. The Landers strike‐slip earthquake was dominated by two sub‐events with predominantly north‐northwestward rupture. The second sub‐event was 1.5 times larger and rotated in strike by 12° counterclockwise relative to the first. The Cape Mendocino thrust event had a relatively smooth rupture that propagated to the southwest on a shallow dipping fault.
The dynamical evolution of small‐scale auroral arc phenomena due to a resistive instability
Authors: Otto A. , Birk G.
Published: February 2008

Discrete auroral arcs appear as bright structures along geomagnetic field lines, and they are associated with precipitating electrons and field‐aligned currents. Because of the magnetic shear produced by these field‐aligned currents, such configurations may be unstable with respect to the tearing mode instability. We suggest that the tearing instability and magnetic reconnection may cause the small‐scale filamentation of auroral arcs as it is observed in auroral spirals and folds. We derived a dispersion relation for the resistive tearing mode for systems with a neutral gas background which is present in the ionosphere. The dispersion relation provides onset criteria for the instability due to the strong damping of the mode for parameter regimes that involve a strong coupling of the plasma and the neutral gas component. For reasonable ionospheric parameters we solved the dispersion relation numerically. The growth time of the mode (approximately a few minutes) and the resulting spatial structures are in accordance with typical values observed for spirals and folds. By employing a new quasi‐neutral three‐fluid code, we carried out several runs in order to test the linear theory. The results confirm the linear theory for the considered fluid parameters and illustrate the evolution of a possible reconnection processes within arclike configurations where the plasma‐neutral gas interaction due to frictional forces is taken into account.
Interplanetary magnetic field connection to the Sun during electron heat flux dropouts in the solar wind
Authors: Lin R. P. , Kahler S. W.
Published: February 2008

Gosling (1975) and MacQueen (1980) pointed out that coronal mass ejections (CMEs) add significant magnetic flux which is connected to the Sun, to the interplanetary medium. Since the interplanetary magnetic field is observed to stay at a roughly constant level, some compensating removal of flux is necessary. McComas et al. (1989) suggested that magnetic reconnection in current sheets near the Sun removes flux by producing U‐shaped magnetic structures in the interplanetary medium which are disconnected from the Sun at both ends. Since solar wind halo electrons normally carry heat flux outward from the Sun's hot corona to the cool interplanetary medium, McComas et al. (1989) interpreted observations of dropouts of the solar wind electron heat flux as evidence for such disconnections. Here we use 2‐ to 8.5‐keV solar electrons observed by ISEE 3 as tracers of the magnetic topology for these heat flux dropout (HFD) events. These electrons are superior to the ∼0.1‐ to 1‐keV halo electrons as test particles since they have longer mean free paths, they are less affected by electric fields, and their transit times from the Sun to 1 AU are only ∼1–2 hours compared to 3–10 hours for the halo electrons. Furthermore, ≥2‐keV electrons are often accelerated in impulsive events, which are sometimes associated with solar flares, and they also generate type III solar radio bursts, which can be tracked from the Sun to 1 AU by the ISEE 3 radio experiment. In at least eight of the 25 HFDs we find strong streaming of the ≥2‐keV electrons outward from the Sun. In one HFD an impulsive solar electron event was observed with an associated type III radio burst which could be tracked from the Sun to ∼1 AU. We conclude that in many HFDs the interplanetary field is still connected to the Sun, and that some energy‐dependent process may produce HFDs without significantly perturbing electrons of higher energies. For two of the 25 HFDs, however, the ≥2‐keV electron observations exhibit all the characteristics of real magnetic disconnection events, including a depletion in the total ≥2‐keV fluxes.
Drift waves, magnetospheric interchange instability, and plasma transport in the magnetosphere of Jupiter
Authors: Fazakerley Andrew N. , Southwood David J.
Published: February 2008

There have been a number of attempts to describe plasma transport in Jupiter's magnetosphere in terms of interchange motions. Most of these have treated the magnetospheric plasma as a magnetohydrodynamic fluid but are unfortunately hard to reconcile with observational evidence. We therefore argue that a description of interchange of Jovian magnetospheric plasma which is compatible with observational evidence may have to take into account particle behavior that is not described in MHD models, such as drift motion. To this end we develop a linear stability analysis along the lines of earlier drift wave work. In contrast to that earlier work, which concentrates on stationary waves (implicitly an MHD‐like model), we consider traveling waves and show that an interchange instability can arise, driven by that fraction of the drifting particle population which is in resonance with the wave. An interesting consequence is the possibility that a plasma which is stable to MHD interchange motions may nevertheless be unstable to the traveling wave interchange motions. We suggest that a model based on traveling wave interchange motions may help us to understand plasma transport in the Jovian magnetosphere, and we sketch out such a model.
Scintillations of the Uranian kilometric radiation: Implications for the downstream magnetopause
Authors: Pedersen B. M. , Aubier M. G. , Desch M. D.
Published: February 2008

From January 27 to 30, 1986, while Voyager 2 was outbound from Uranus, the planetary radio astronomy experiment recorded strong (∼3–6 dB) modulations of the broadband smooth Uranian radio emission (Warwick et al., 1987a) at frequencies below 500 kHz. These fluctuations exhibited simultaneously two different time scales (∼100 s and ∼10 s) and were exclusively observed outside the Uranian magnetopause during six planetary rotations. The modulations were only present during a part of the total duration of each broadband smooth episode. This phenomenon is interpreted as being due to propagation effects through the magnetopause. The long‐period modulation is due to the influence of ∼8000‐km wavelength surface waves on Uranus' downstream magnetopause. Such waves have only been studied in detail for the inbound passage (Lepping et al., 1987). The short‐period oscillation might be the signature of a second magnetopause ripple period, similar to that measured by Aubry et al. (1971) for the Earth's magnetopause. There is strong evidence that the surface wave fluctuations causing the scintillations are induced by the solar wind interaction with the magnetosphere. We discuss why these modulations are confined to certain periods of time and to a certain frequency range.
Neptune's polar cusp region: Observations and magnetic field analysis
Authors: Lepping R. P. , Burlaga L. F. , Lazarus A. J. , Vasyliunas V. M. , Szabo A. , Steinberg J. , Ness N. F. , Krimigis S. M.
Published: February 2008

Upon entering Neptune's magnetosphere, Voyager 2 penetrated its distant polar cusp on August 24, 1989. At that time the planet's magnetic dipole axis was approximately parallel to the Sun‐planet line. Broadly speaking, the spacecraft on the day before closest approach traversed four plasma regions. These (and the “boundaries” separating them) are (1) solar wind (bow shock at 1438 UT), (2) magnetosheath (inner boundary of the magnetosheath, the magnetopause (MP), at 1800 UT), (3) cusp region (outer boundary of magnetosphere at 1930 UT), and (4) the magnetosphere. The MP is identified primarily by a 45° directional discontinuity in the magnetic field but also by a decrease in the flux of antisunward flowing plasma, and a change in flow direction toward the corotation direction; it is approximately where the global MP would be expected based on upstream conditions. It is shown that the MP boundary was a rotational discontinuity whose normal nMP was a severe ≈77° from the planet‐Sun line. Its thickness was 5.1×104 km (i.e., 24 proton gyroradii, consistent with MPs of the other magnetic planets). The boundary was moving at U = 148 km/s, relative to the Sun‐planet line, along nMP, in a sense opposite to the approximate rigid motion of the outer magnetosphere and being dominated by the speed of an Alfven wave moving into the magnetosphere. Plasma β drops from 1.21 to 0.73 across the MP, going from outside to inside. Total pressure balance was shown to exist across the boundary, and the normal momentum flux change was apparently balanced by forces related to the change in thermal anisotropy and the normal field according to MHD theory. Energetic (⪞28 keV) ion intensities peaked at ≈1800 UT and then decreased prior to a second onset at ≈2000 UT, while the electrons (⪞22 keV) showed only a marginal increase. The field in the cusp changed direction slowly and smoothly, approaching the magnetospheric field direction at its inner boundary. The plasma velocity and density changed very moderately as the spacecraft entered the cusp region. However, the quasi‐steadiness of these quantities in the sheath (on average) disappears as the cusp is entered, just as was true of the magnetic field. Similar magnetic field and plasma profiles in the vicinity of the Earth's distant polar cusp have been observed by Earth orbiting spacecraft.
Field and thermal plasma observations of ULF pulsations during a magnetically disturbed interval
Authors: Lin N. , Engebretson M. J. , Reinleitner L. A. , Olson J. V. , Gallagher D. L. , Cahill L. J. , Slavin J. A. , Persoon A. M.
Published: February 2008

ULF pulsations were observed by DE 1 between 1600 and 1830 UT, October 31, 1982, during a magnetically disturbed interval. Ground observations suggested that the pulsations were excited by a sudden increase in the solar wind velocity and pressure. During the pulsation interval DE 1 traveled near apogee from −55 to −20° geomagnetic latitude and from L ∼ 13 to L ∼ 4 at about 0900 LT. The waves observed were azimuthal oscillations preceded by gradually decaying long period compressional waves which lasted for more than 1 hour. Phase relations between magnetic and electric field oscillations and calculated Poynting flux indicate that in the outer magnetosphere (L > 8) DE 1 observed propagating waves which contained strong poloidal components, while the quasi‐sinusoidal toroidal waves seen later for L < 10.3 were standing along field lines. The toroidal waves appeared as four wave packets, each of which corresponded to a region with a distinct plasma distribution. The observed wave periods decreased with L over an extended magnetospheric region. The seemingly weak interaction between magnetic shells suggests that the source was a broadband one. Magnetometer data from several high latitude observatories located near the footpoints of the magnetic shells crossed by DE 1 were also examined. The magnetic pulsations on the ground contained many frequency components, and the waves seen most strongly in space were often not the strongest signals seen on the ground near the same field lines. The broadband nature of the ground pulsations indicates that the stations also detected oscillations of the adjacent field lines. The major frequencies seen at ground stations seemed to be roughly constant for about 2 hours but L dependent. This suggests that the changing periods seen in space by DE 1 were clearly L related and not temporally varying.
Auroral kilometric radiation from transpolar arcs
Authors: Pedersen B. M. , Pottelette R. , Eliasson L. , Murphree J. S. , Elphinstone R. D. , Bahnsen A. , Jespersen M.
Published: February 2008

Observations from the Swedish satellite Viking allow us to study the relationship between auroral kilometric radiation (AKR) and discrete auroral features. Previous work has shown that AKR generation is most often associated with nightside aurora. We present wave data which show that under certain circumstances the source regions may also occur on discrete features, identified as transpolar arcs. The wave spectrograms detected during crossings or closest approaches to such sources exhibit structures similar to those observed during nightside AKR source crossings. Also, the associated ion beams and trapped conical electron populations with enhanced upward directed loss cones peak at comparable energies (∼1 keV).
Motions of particle microstructures in the magnetopause boundary layer
Authors: Larson Nels R. , Parks George K.
Published: February 2008

Particle flux data acquired by the ISEE 1 and 2 spacecraft reveal many order of magnitude changes within the magnetopause boundary layer that occur during less than 1/4 s of time. These changes define small‐scale particle structures, or microstructures within the region, many of which are apparently inclusions of plasma from adjacent regions. Particle structures resolved by the high time resolution data include groups with periods as short as 0.375 s. An estimate is derived for the motion of the microstructures with respect to an oscillatory inner boundary for four afternoon sector boundary layer crossings by measuring interspacecraft time differences as the features propagate past the satellites. On average the microstructures move toward the tail of the magnetosphere in concert with the wavy large‐scale inner boundary. Furthermore, the orientations of the edges of the structures maintain an aspect similar to that of the boundary. These characteristics are expected for magnetohydrodynamic plasma flows that generate velocity due to velocity shear stresses.
Polarization of the Io‐C radio emission from Jupiter
Authors: Barrow C. H.
Published: February 2008

Old measurements of the Stokes polarization parameters for the Jovian decametric radio emission are reexamined for comparison with the broadband measurements for Io‐A and Io‐B emission, published recently by the Decametric Radio Astronomy Group at the Meudon Observatory. Records of an Io‐C source event, recorded in 1966 at 18 MHz, indicate average values of 0.63 and 0.72 for the degrees of circular and linear polarization, respectively, with a mean value of 0.92 for the total polarization fraction and 0.40 for the average axial ratio. These values, which have not previously been published for the Io‐C emission, complement and support the Meudon results. Burst‐by‐burst sequences of the polarization parameters reveal much diversity in the polarization and allow partial circular polarization to be distinguished from pure elliptical.
Semikinetic modeling of the outflow of ionospheric plasma through the topside collisional to collisionless transition region
Authors: Wilson G. R.
Published: February 2008

We apply a collisional semikinetic model to the study of the topside ionosphere transition region. In this study we consider the transition from (1) O+ to H+ dominance, (2) subsonic to supersonic H+ flow, and (3) collisional to collisionless plasma. We focus primarily on the H+ ions as they flow upward from a relatively low altitude of 500‐1000 km to a high altitude of 7000 km. These ions are subjected to collisions with O+ ions and self collisions, as well as the effects of macroscopic forces such as the magnetic mirror force, gravity, and the ambipolar electric force. We find that when supersonic outflow occurs, the H+ velocity distribution function shows a large departure from Maxwellian with a large positive and then negative heat flow, particularly in the region where the flow Mach number passes through one. In cases where subsonic outflow occurs and the H+ density is relatively low, distribution functions with two peaks can occur. In these situations the H+ ions have large parallel temperatures and large heat flows when the two peaks are of unequal height.
Nonlinear interaction of powerful radio waves with the plasma in the Earth's lower ionosphere
Authors: Shukla P. K. , Stenflo L. , Borisov N. D.
Published: February 2008

The nonlinear interaction of powerful radio waves with nonresonant density fluctuations in a nonuniform weakly ionized magnetoplasma is considered. It is shown that the interaction can generate a high‐frequency radio wave envelope, which generally is governed by a nonlinear Schrödinger equation. Accounting for the dependence of the attachment frequency on the radiation intensity, as well as the differential Joule heating nonlinearity, we derive the equations for the nonthermal electron density and temperature perturbations. The various nonlinear terms in the electron motion are compared. The implications of our investigation for the filamentation of radio waves in the lower part of the Earth's ionosphere are pointed out.
On the acceleration of alpha particles in the fast solar wind
Authors: Gomberoff L. , Hernández R.
Published: February 2008

Recently, Gomberoff and Elgueta (1991) showed that in a plasma composed of anisotropic protons and alpha particles drifting along an external magnetic field with a small velocity relative to the protons, strong left‐hand polarized electromagnetic ion cyclotron waves can be generated. These waves can accelerate the alpha particles to velocities well in excess of the proton bulk velocity. Here we assume a more realistic model of the solar wind by considering a double‐humped proton distribution. It is shown that the secondary proton beam has no important effects on the ion cyclotron waves for beam densities of the order of those observed in fast solar wind conditions. The fact that the alpha proton drift velocity is modulated by the Alfvén velocity remains unexplained.
Shapes of strong shock fronts propagating through the coronal streamer belt
Authors: Burton M. E. , Siscoe G. L. , Smith E. J.
Published: February 2008

A strong shock that would propagate spherically from the Sun were the solar wind homogeneous instead warps as it refracts and shears through inhomogeneities in solar wind density and velocity. The inhomogeneities can be disordered or ordered by quasi‐permanent coronal structures such as coronal holes and the coronal streamer belt. Distortions caused by interacting solar wind streams from coronal holes have been studied. Where colliding streams build density ridges, the shock slows, producing marked localized dimples (concave outward) in the shock front. Here we note that a similar dimple results from the high‐density, low‐velocity inhomogeneity that distinguishes the coronal streamer belt. That is, a concave outward dimple characterizes the part of the shock that crosses the belt. First, to illustrate this effect, we apply the linearized hydrodynamic model that reveals shock dimples at density ridges of colliding streams. Then, to test the effect, we present normals of the eight shocks we could independently assess to be within the expected range of influence of the belt. Six of the eight shocks showed the predicted distortion. The null hypothesis would give the same result in 1 out of 14 tries. Finally, we note that the dimple shape induces a postshock confluence in the center of the belt of material pushed centerward from the top and bottom of the belt. We suggest this confluence of material might drive field line reconnection at the heliospheric current sheet in the center of the belt, as MHD simulations have observed.
Three‐dimensional ionospheric plasma circulation
Authors: Heelis R. A. , Coley W. R. , Loranc M. , Hairston M. R.
Published: February 2008

Examination of the ion drift velocity vector measured on the DE 2 spacecraft reveals the significance of ionospheric flows both perpendicular and parallel to the magnetic field at high latitudes. During periods of southward directed interplanetary magnetic field the familiar two‐cell convection pattern perpendicular to the magnetic field is associated with field‐aligned motion predominantly upward in the dayside auroral zone and cusp, and predominantly downward in the polar cap. Frictional heating by convection through the neutral gas and heating by energetic particle precipitation are believed to be responsible for the bulk of the upward flow with downward flows resulting from subsequent cooling of the plasma. Some of the upward flowing plasma is apparently given escape energy at altitudes above about 800 km. The average flow of ions across the entire high‐latitude region at 400 km is outward and comparable to the energetic ion outflow observed at much higher altitudes by DE 1.
Solar OI 1304‐Å triplet line profiles
Authors: Gladstone G. R.
Published: February 2008

Estimates are made for the full‐disk line profiles of the solar OI resonance triplet at 1304 Å using data from experiments on Solar Maximum Mission and Orbiting Solar Observatory 8. The observed profiles are considerably self‐reversed, and an accurate approximation to the actual line profiles using the sum of two equal and offset Gaussian profiles is presented. The set of observations, though sparse, cover the period of time from 1975 to 1985. The solar cycle variation of total OI 1304‐Å flux is ∼30% (as determined from rocket and Solar Mesophere Explorer observations), but no solar cycle variation is apparent in the line shapes above the level of day‐to‐day variability. In addition, the fraction of the total flux carried in each component line of the triplet was also approximately constant during the last solar cycle. The derived full‐disk line shape parameters may be used, when combined with an appropriate value for the line‐integrated full‐disk solar flux, in the analysis of planetary or cometary OI 1304‐Å observations under any solar conditions.
An assessment of the L shell fitting beam‐swinging technique for measuring ionospheric E region irregularity drift patterns
Authors: Yeoman T. K. , Mattin N. , Ruohoniemi J. M. , Lester M. , Pinnock M.
Published: February 2008

The majority of current ground‐based radar experiments measuring ionospheric convection flows derive the ionospheric flow vectors from line‐of‐sight (l‐o‐s) velocity measurements made from a single radar site. In order to deduce vector information from such a set of scalar measurements, assumptions have to be made about the spatial and temporal structure of the flow pattern. These assumptions involve an actual or effective beam‐swinging technique. In this study the flow direction estimates from the application of sophisticated beam‐swinging software, developed by the Applied Physics Laboratory of the Johns Hopkins University, to over 20,000 estimates of the l‐o‐s ionospheric E region mean irregularity drift velocity from the Wick radar of the Sweden And Britain Radar‐auroral Experiment (SABRE) system, are compared with merged velocity vectors derived from data from both SABRE radar sites. The application of the beam‐swinging software to VHF E region backscatter data is found to lead to flow direction estimations which are in error a significant amount of the time, with beam‐swinging‐derived drift velocities lying outside a cone of half width 30° around the merged SABRE drift velocities in 30% or more of cases. Both curvatures and gradients in the ionospheric flow pattern measured by SABRE cause distortions of the beam‐swinging‐evaluated flow patterns. Moreover, from an examination of the Wick l‐o‐s velocity data and the data fitting calculations alone, it is often not possible to distinguish between accurate and inaccurate beam‐swinging flow direction estimates. Criteria are discussed which can be employed to increase confidence in the beam swinging results. Although the present comparison is confined to E region irregularity drift velocity measurements, these criteria probably have implications for all monostatic experiments employing a beam‐swinging technique (e.g., coherent and incoherent radars and optical instruments). It is concluded that it is highly desirable for the next generation of ionospheric radars to be capable of making common‐volume measurements from two well‐separated sites.
Magnetic aspect angle effects in radar aurora at 48.5 MHz, corrected for refraction
Authors: Hall G. , Moorcroft D. R.
Published: February 2008

It is now widely recognized that at lower VHF frequencies, refraction by electron density structures in the auroral E region can greatly affect the character of radio auroral backscatter, making the quantitative study of magnetic aspect angle effects difficult or impossible. In the present study of data taken with the 48.5‐MHz Bistatic Auroral Radar System (BARS) in central Canada, these difficulties have been minimized by using only data obtained from spatially and temporally uniform events. After correcting for effects of refraction, it has been found that true large aspect angle backscatter occurs for magnetic aspect angles exceeding 6°. Doppler velocities from the smaller aspect angle Red Lake radar behaved as if proportional to the line‐of‐sight component of the convection drift velocity; the resulting flow directions were found to be in good agreement with estimates obtained from magnetometers located in the BARS field of view. The constant of proportionality between the radar velocity and the drift velocity component appears to decrease by a factor of approximately 2 as the aspect angle increases from 3° to 6°. At 3° aspect angle, the backscatter power decreased with increasing aspect angle by about 13 dB/deg (the aspect sensitivity), similar to what has been found in other studies at UHF. In common with those other studies this aspect sensitivity is found to decrease with increasing aspect angle, but more rapidly, falling to less than 4 dB/deg at aspect angles of 5°. In contrast to previous studies at small aspect angle, there is no evidence of a dependence of backscattered power on the flow angle, the angle between the radar line of sight and the direction of E × B drift.
A statistical characterization of local mid‐latitude total electron content
Authors: Gail W. B. , Prag A. B. , Coco D. S. , Coker C.
Published: February 2008

The integrated line‐of‐sight electron density within the ionosphere, known as the total electron content (TEC), is commonly used to quantify ionospheric propagation effects. In order to extrapolate single‐point measurements of TEC to other locations and times, some characterization of the TEC spatiotemporal variation must be available. Using a four‐channel receiver tracking coded signals from the NAVSTAR Global Positioning System satellites, estimates of both the mean variation and correlation coefficient have been made for the approximately 1200‐km or 1‐hour local time radius ionospheric region within view of a mid‐latitude station. Results were obtained for morning and midday over a 4‐week period near the autumnal equinox in 1989. The derived mean variation was found to be well characterized by linear functions of the local time and latitude separation between the ground site and the ionospheric penetration point of the signal. The correlation coefficient during midday was found to decrease linearly with latitude, longitude, and time separation, with values of about 0.91 for a 1000‐km separation and 0.98 for a 1‐hour separation. During morning hours the longitude and time coefficients were similar to the midday values, but the latitude coefficient was found to have a nonlinear dependence, with values as small as 0.70. The combined results suggest that the decorrelation is due primarily to longer term TEC fluctuations, such as day‐to‐day variation in the TEC spatial dependence, rather than to transient effects such as traveling ionospheric disturbances. The analysis provides a spatiotemporal characterization of TEC that can be used to extrapolate TEC values from single‐point measurements.
Energetic ion anisotropies in the geomagnetic tail: 2. Magnetic field and substorm characteristics
Authors: Kettmann Georg , Fritz Theodore A. , Hones Edward W. , Daly Patrick W.
Published: February 2008

In a companion paper we have reported results from a survey of strong energetic ion (E > 25 keV) anisotropies in the central geomagnetic tail based on 2 years of data from the ISEE 2 spacecraft. Here we extend this study with particular emphasis on the relation of high ion anisotropies to recent models of geomagnetic substorms and include magnetic field and ground‐based data. Beyond 16 RE downtail distance, we find a correlation between the magnetic field north‐south component (Bz) and the streaming direction of the energetic ions. Earthward ion streaming is observed almost exclusively when Bz is positive while tailward streaming tends to occur with negative Bz. Strong ion anisotropies are magnetic field aligned rather than perpendicular to it suggesting that most of them are observed at the plasma sheet boundary. However, they are also found in the central plasma sheet. A superposed epoch analysis using the AL index yields a relation between substorm phases and the observation of high ion anisotropies. Earthward streaming at larger distances from the Earth and/or the neutral sheet usually occurs during substorm recovery while earthward streaming closer to the Earth and tailward streaming events are related to the substorm expansion phase. Three events characterized by strong tailward ion and plasma streaming detected first changing to earthward streaming later on are presented in some detail. The overall features of two of these events strongly suggest the formation of a magnetic neutral line earthward of the satellite at substorm onset moving tailward around recovery. While most observations during the third event also indicate such a sequence, there are some puzzling details which do not match this interpretation. Although a certain By signature proposed by numerical simulations of near‐Earth reconnection could not be found, we conclude that the statistical properties of strong ion anisotropies can consistently be explained in terms of the near‐Earth neutral line model of substorms if the substorm neutral line sometimes forms well inside the ISEE orbit. Other recently published models of substorm onset such as the thermal catastrophe model and the current disruption model can also account for part of the observations but still require further refinement.
Waves in the cosmic ray north‐south anisotropy with periods of 27 days, 1 year, and 11 years
Authors: Swinson D. B. , Yasue S. ‐I.
Published: February 2008

We use data from 1971 to 1989 from the Nagoya cosmic ray muon telescope to look at waves in the cosmic ray north‐south anisotropy. The GG component of the Nagoya data is used because it is particularly sensitive to the north‐south anisotropy. The presence of waves with a period of 27 days is demonstrated through most of the period. For each 27‐day solar rotation the 27 daily values for the GG component have been harmonically analyzed to determine the amplitude of the best fit sine wave to the data. When these GG 27‐day amplitudes are plotted for each solar rotation during the 19‐year period, it becomes apparent that there is a very marked annual modulation of these amplitudes; the annual variation is greatest at solar maximum, and is also an 11‐year modulation of the effect. The cosmic ray north‐south anisotropy arises from the By × ▽NR, anisotropy resulting from the interaction of the y component of the interplanetary magnetic field (IMF), By and ▽NR, the radial heliocentric cosmic ray density gradient. We have also determined the daily average values for By from 1971 to 1987, and for each 27‐day solar rotation these daily values have also been harmonically analyzed to obtain the best fit sine wave of period 27 days. When the amplitudes of the 27‐day By waves are plotted for each solar rotation, an annual wave and an 11‐year wave are also found; the behavior is very similar in nature to the behavior of the waves in the Nagoya GG data. These two sets of data have been compared with the values of α, the tilt of the heliospheric neutral sheet, which is available for most of the solar rotations between 1971 and 1989. The largest annual waves in the amplitude of the 27‐day GG waves appear to coincide with relatively rapid excursions in the tilt of the neutral sheet. The annual mean of the amplitudes as well as the range (highest to lowest) in the amplitudes for the 27‐day waves in the GG component and in the By component have been determined for each year; in each case there is a periodicity of about 11 years comparable to the solar activity periodicity.
Particle dispersion at the nightside boundary of the polar cap
Authors: Sergeev V. A. , Bösinger T.
Published: February 2008

The spatial dispersion pattern of particle precipitation during an event of sustained moderate magnetic activity was studied using observations of energetic (>30 keV) and auroral (0.3–20 keV) particles made by two low‐altitude NOAA 6 and NOAA 7 spacecraft. The polar cap boundary (PCB, boundary between closed and open flux tubes) was identified on the nightside using the north–south asymmetry of solar energetic particle fluxes over the polar caps which were observed at that time. The energy and species dependency of the latitude of the boundary of energetic particles in the vicinity of the PCB was found to be consistent with a simple schema, being described by (1) the accelerator which provides the >30‐keV particle population operates at the outer boundary of the plasma sheet and (2) the particles that are transported across magnetic flux tubes equatorward (earthward) by the convection electric field during their flight over a distance of approximately 50 to 100 RE from the equatorial region down to the ionosphere. These findings are in tune with the concept of a far‐tail transient reconnection process which gives rise to suprathermal particles. The poleward boundary of the total energy flux of keV electrons (at a threshold of 0.01 erg cm−2 s−1 sr−1) was often encountered at significantly lower latitudes than the energetic proton boundary, however. The equatorward shift of the auroral electron boundary was as much as that expected for the 5‐ to 10‐keV protons, which suggests that there is not enough dense cold plasma in the outer tail plasma sheet to neutralize the electric space charge arising from the different convection trajectories of the earthward moving auroral protons and electrons, so that it appears that the auroral particles of both signs are forced to move together. The apparent convective displacement of auroral electrons (across the auroral zone) can be as large as 2° to 3° in latitude, which could shift their poleward boundary some 200–300 km equatorward of the actual PCB.
The propagation of low‐frequency whistler waves driven by ion beams in the magnetotail
Authors: Burinskaya T. , Schriver D. , Ashour‐Abdalla M.
Published: February 2008

The existence in the Earth's magnetotail of magnetic field and plasma density inhomogeneities can influence the propagation of electromagnetic waves in the lobe and plasma sheet. This is important in understanding the origin of magnetic noise bursts that are observed near the neutral sheet since the waves may not necessarily be generated locally but can be excited in other regions of the tail and propagate towards the central plasma sheet due to the presence of the global inhomogeneities. In this paper the generation and propagation of low‐frequency whistler waves excited by ion beams in the plasma sheet boundary layer is examined. The trajectories of waves that are generated inside the plasma sheet boundary layer are followed using standard ray tracing techniques assuming a Harris‐type neutral sheet magnetic field profile and with the density chosen such that the total pressure remains constant. The wave energy is calculated along the wave path taking into account growth due to the ion beam while the wave is in the plasma sheet boundary layer, and damping due to resonant interactions with the background plasma. Results show that the electromagnetic waves are guided towards the central plasma sheet by the plasma gradients but are damped out before reaching the neutral plane itself due to Cerenkov resonance with the hot central plasma sheet background ions. The amplitudes and final positions of the waves depend on the temperature of the background plasma, the ion beam drift speed, and the ion beam thermal speed.
Magnetic flux mapping considerations in the auroral oval and the Earth's magnetotail
Authors: Rostoker Gordon , Skone Susan
Published: February 2008

The prevalent theory describing the entry of solar wind energy into the magnetosphere involves the merging of solar wind and terrestrial magnetic field lines on the dayside magnetopause and the subsequent reconnection of those field lines across the neutral sheet in the Earth's magnetotail. The reconnection is thought to occur at a neutral line, whose location is often set at ∼100 Re behind the Earth. The location of this neutral line has not been verified experimentally to date, and in this paper we shall provide some constraints suggesting its average location to lie in the range −50>X>−80 Re. We shall use AMPTE, ISEE, and IMP 8 magnetic field data to evaluate the fall off of Bz at the position of the neutral sheet as one moves back into the tail. From these measurements we construct an analytical representation of Bz as a function of downtail position. Using the Coroniti and Kennel relationship for the flaring of the magnetotail, we evaluate the average position of the equatorward edge of the polar cap, which we then assume to represent the poleward edge of the average auroral oval. From flux conservation considerations, we then evaluate the average width of the auroral oval. We conclude the paper by discussing how the mapping from the magnetotail is reflected in the scale sizes of auroral features in the midnight sector of the auroral oval.