bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of simultaneous chatanika and Millstone Hill temperature measurements with ionospheric model predictions
Authors: Rasmussen C. E. , Sojka J. J. , Schunk R. W. , Wickwar V. B. , de la Beaujardiere O. , Foster J. , Holt J.
Published: February 2008

As part of the MITHRAS program, the Chatanika and Millstone Hill incoherent scatter radars made coordinated observations of the polar ionosphere on June 27 and 28, 1981. The temperature data obtained during these days were compared with predictions made by a high‐latitude ionospheric model. The comparison of the temperature measurements and the results of the ionospheric model depend on the assumptions made both in reducing the data and on the inputs that are needed by the model. The deduction of electron temperature from radar measurements depends upon a knowledge of the mean ion mass as a function of altitude. The model requires a knowledge of the heat flux at the upper boundary and the volume heating rate. The results of the model were compared with measurements for a variety of combinations of the required inputs. It was found that the best fits resulted with a heat flux of from 0 to −0.7×1010 eV cm−2 s−1 at the upper boundary and a relatively high volume heating rate. These results also required that the model predictions for the average ion mass be used in the reduction of the radar data. However, other combinations of assumptions also produced good fits. A systematic temperature difference of between 200 and 300 K was found between the Chatanika and Millstone Hill measurements of electron temperature at high altitudes.
Interplanetary magnetic field orientations associated with bidirectional electron heat fluxes detected at ISEE 3
Authors: Stansberry J. A. , Gosling J. T. , Thomsen M. F. , Bame S. J. , Smith E. J.
Published: February 2008

A statistical survey of interplanetary magnetic field orientations associated with bidirectional electron heat fluxes observed at ISEE 3 in orbit about the Sunward Lagrange point indicates that magnetic connection of the spacecraft to the Earth's bow shock was frequently the source of the bidirectionality. When the interplanetary magnetic field was oriented within 5° of the Earth‐spacecraft line, backstreaming electrons from the bow shock were clearly observed approximately 18% of the time, and connections apparently occurred for angles as large as ∼30°–35°.
Plasma line observations in the auroral oval
Authors: Valladares Cesar E. , Kelley Michael C. , Vickrey James F.
Published: February 2008

We report here a series of experiments conducted at the Sondre Stromfjord incoherent scatter radar, aimed at detecting enhanced plasma lines associated with midnight sector auroral arcs. Using different receivers, we detected both ion and plasma lines simultaneously. The plasma line signal was recorded with the use of a filter bank of eight frequencies. Plasma lines were found to originate mainly from the topside of the particle‐produced E layer. The enhanced plasma lines are sometimes a factor of 100 times larger than the thermal level. Our data show a rapid decay of the plasma lines, however. In some cases, only a 30‐s integration time was needed in order to unambiguously detect both upshifted and downshifted lines. The level of the plasma lines reaches values of, for the largest cases, up to 40°K above the noise temperature. These are considerably higher than results from prior auroral zone plasma line experiments. In situ observations of enhanced plasma waves in this same region are reported in a companion paper.
Magnetic clouds, geomagnetic disturbances, and cosmic ray decreases
Authors: Zhang G. , Burlaga L. F.
Published: February 2008

Nineteen magnetic clouds are identified in the years from 1978 through 1982 and studied using superposed epoch analysis. The magnetic field intensity, proton density, and proton temperature are enhanced ahead of magnetic clouds that are preceded by a shock, while strong magnetic field intensity and low proton temperature are observed within the clouds. A relatively large (∼2.5%) decrease in cosmic ray intensity is associated with magnetic clouds that are preceded by a shock, perhaps caused by the turbulent sheath behind an interplanetary shock ahead of the magnetic cloud, whereas only a small (∼0.5%) decrease in intensity is associated with the magnetic cloud itself. Magnetic clouds can produce geomagnetic activity with a decrease in Dst index of the order of 100 γ. The magnitude of the change in the Dst index for the case when southward fields arrive first is comparable to that for the case when northward fields arrive first, and the phase is such that geomagnetic activity is associated with southward fields.
Interplanetary magnetic field draping about fast coronal mass ejecta in the outer heliosphere
Authors: McComas D. J. , Gosling J. T. , Winterhalter D. , Smith E. J.
Published: February 2008

Considerable recent research on the draping of the interplanetary magnetic field (IMF) about the ionospheres of comets and Venus and about the Earth's magnetosphere, as well as draping of magnetospheric fields about Io's ionosphere and plasmoids, has indicated the fundamental and prevalent nature of this process. In this paper we consider the possibility of magnetic field draping about fast coronal mass ejections (CMEs) which propagate into the outer heliosphere through slower moving, quiescent solar wind. In particular, when this velocity difference is appreciably greater than the local Alfvén speed, draping should produce extended magnetotaillike configurations somewhat analogous to those observed behind Venus and comets. For CMEs, however, such tails would point sunward and form on scales comparable to these large ejecta. In the draped magnetotail regions sunward of fast CMEs in the outer heliosphere the IMF should be relatively radial, in contrast to the generally transverse orientation of the Parker spiral there. Low transverse flow velocities in the draped regions just upstream from fast CMEs suggest that swept‐up interplanetary magnetic flux is hung up on these large structures (∼1 AU in transverse dimension at 1 AU) for many days. We have searched for evidence of magnetic field draping about fast CMEs and the existence of large draped magnetotails in the outer heliosphere by examining the Pioneer 11 data set between 6.9 and 9.4 AU. Several events consistent with such structures have been found, and two are displayed in the study.
Short‐wavelength modes on expanding plasma clouds
Authors: Winske D.
Published: February 2008

It is proposed that the short‐wavelength (less than an ion gyroradius) flute modes observed on the surface of expanding plasma clouds, such as the Active Magnetospheric Particle Tracer Explorers (AMPTE) barium releases in the magnetotail, can be explained in terms of the lower hybrid drift instability, which in its general form includes an electron‐ion drift induced by the deceleration of the plasma across a magnetic field. Similar types of structure are also seen in laser and theta pinch experiments and in recent simulations modeling these phenomena. Analytical and numerical solutions of the linear dispersion relation are given for a number of parameters. The linear theory reproduces the growth rate scaling of Hassam and Huba in the long‐wavelength limit and indicates that maximum growth occurs at shorter wavelengths. The growth of the modes is illustrated by particle simulations showing that the instability is excited at early times, remains locked in at a fixed mode number, and because it continues to be driven by the deceleration, grows to amplitudes far larger than expected from usual ion trapping arguments. Issues related to the linear mode structure and the nonlinear evolution of the instability in the context of AMPTE are also discussed.
A search for lower hybrid drift turbulence in slow shocks
Authors: Coroniti F. V. , Scarf F. L. , Kennel C. F. , Tsurutani B. T. , Smith E. J.
Published: February 2008

The slow shocks which have been observed in the distant geomagnetic tail by ISEE 3 do not exhibit the large‐amplitude magnetic field rotations which are theoretically expected. A possible explanation is that the rotational trailing wave train is resistively damped by strong lower hybrid drift turbulence excited in the shock front. Magnetic wave spectra for three slow shocks are constructed from the ISEE 3 dc magnetometer and search coil experiments. The observed magnetic amplitudes near the lower hybrid frequency are below the level at which anomalous resistance would damp the slow shock magnetic rotations.
Noncoplanar magnetic fields at collisionless shocks: A test of a new approach
Authors: Gosling J. T. , Winske D. , Thomsen M. F.
Published: February 2008

Within the foot and ramp of a fast mode collisionless shock the magnetic field rotates out of the plane of coplanarity defined by the upstream magnetic field and the shock normal. As previously noted (Goodrich and Scudder, 1984), the sense of this rotation is such as to reduce the cross‐shock potential drop when measured in the deHoffman‐Teller frame relative to that measured in the normal incidence frame. From a consideration of the requirement that there be zero current in the coplanarity plane downstream of the shock, Jones and Ellison (1987) have argued that the field rotation and potential drop difference are a consequence of unequal ion and electron masses, and have derived an expression for the spatial integral of the noncoplanar field component in terms of the electron current within the shock layer. Moreover, by assuming that the ion current within the shock layer is negligible compared to the electron current, they derive equations which predict the magnitude of both the field rotation and the potential drop difference in terms of upstream quantities and the field jump at the shock. We have tested their equations with ISEE 1 and 2 plasma and field measurements at the Earth's bow shock and by means of numerical simulations. We find substantial support for their suggestion that the field rotation and thus also the frame dependence of the potential drop are fundamentally a consequence of unequal ion and electron masses. Further, for subcritical shocks (low Mach number) one can neglect the ion current to predict both the sign and the magnitude of the field rotation and potential drop difference. However, at supercritical shocks (high Mach numbers) the ion current associated with reflected, gyrating ions cannot be neglected, and the final equations of Jones and Ellison seriously underestimate the magnitude of the field rotation and the potential drop difference at these shocks.
Filamentation instability of electron and ion cyclotron waves in the ionosphere
Authors: Stenflo L. , Shukla P. K.
Published: February 2008

Filamentation instabilities of electron and ion cyclotron waves are investigated by taking into account electromagnetic wave driven stationary density ripples in a magnetized plasma. A general dispersion relation for the convective amplification of the radiation in a two‐component electron‐ion plasma is derived. Specific results for electron and ion cyclotron waves are presented. The relevance of our investigation to the ionosphere is pointed out.
Acceleration of 0.29‐ to 0.5‐MeV protons by interplanetary shocks
Authors: Kessel Ramona L.
Published: February 2008

The availability of high time resolution spacecraft data has made possible in situ and detailed study of plasma processes in the interplanetary medium. One important process that has received a lot of attention is the energization of charged particles due to interaction with traveling interplanetary shock waves. The specific goal of this study is to make use of observed magnetic fields, plasma density and velocity, and initial particle trajectories calculated from real spacecraft orientations in a time‐reversed computer simulation which follows particles through a single complete interaction with a shock in order to predict the angular distribution of energetic protons (0.29‐0.5 MeV). This study is fully three‐dimensional and unique in that real conditions for specific shocks outside the Earth's magnetosphere are simulated, and the results are compared to observations for each particular shock. Energy gains and losses calculated in the simulation are used to predict the amount of enhancement in each sector, assuming an isotropic ambient medium and a relationship between energy and particle number that is based on a power law. This study is a test of single encounter shock drift acceleration for two real shocks: 1974 day 312 (θBn = 68°) and 1979 day 95 (θBn = 54°). The first is clearly a quasi‐perpendicular shock while the second has the long rise time associated with an energetic storm particle event with a shock spike just at the time of shock passage. In spite of the simplicity of the shock model used, we were able to reproduce the essential observed features, i.e., the observed angular distributions in the lowest‐energy proton channel of the IMP 8 spacecraft for sectors which interacted with the shock. The 1974 day 312 shock is an example of the model correctly predicting the observations, and the shock spike of the 1979 day 95 shock offers a good base for comparison with the day 312 shock.
Absolute differential and integral electron excitation cross sections for atomic oxygen, 6, the ³P → ³P and ³P → 5P transitions from 13.87 to 100 eV
Authors: Gulcicek E. E. , Doering J. P. , Vaughan S. O.
Published: February 2008

We report the absolute differential and integral cross sections of the forbidden atomic oxygen ³P → ³P, 10.99 eV, and ³P → 5P, 10.76 eV, transitions for incident electron impact energies of 13.87 to 100 eV. These measurements are an extension of excitation cross section results that were reported previously at 30 eV incident energy by Gulcicek and Doering (1987). The differential cross sections (DCS) for the ³P → ³P transition are forward peaked, although not as strongly as for an electric dipole allowed transition and became flatter at the lower incident energies. The DCS for the ³P → 5P transition is roughly isotropic with scattering angle at all incident energies. The agreement between the ³P → ³P direct integral excitation cross sections (ICS) and the experimental 8446 Å emission cross section of Zipf, adjusted to remove cascade, is good over the entire energy range studied. The agreement with some, but not all, previous theoretical calculations for both transitions is satisfactory. The worst disagreement, roughly a factor of 2, is at the lowest incident energy, 13.87 eV, where the experimental results suggest that the ICS has begun to decrease toward threshold while the theoretical calculations show a continued increase. The ratio of the ICS for the ³P → ³P to the ³P → 5P transition varies from >6 at energies >30 eV to 2.2 at 13.87 eV. These ratios, which correspond directly to the 8446 Å/7774 Å emission line ratio in the thermosphere, allow conclusions to be drawn about the exciting electron energy distribution from the emission intensity ratio.
Observations related to the acceleration, injection, and interplanetary propagation of energetic protons during the solar cosmic ray event on February 16, 1984
Authors: Debrunner H. , Flückiger E. , Grädel H. , Lockwood J. A. , McGuire R. E.
Published: February 2008

Data from the worldwide network of neutron monitors and from the cosmic ray telescopes on IMP 8 have been used to deduce the intensity‐time profiles, the anisotropy‐time profiles, the energy spectra, and the pitch angle distributions of the solar protons near Earth during the solar cosmic ray event on February 16, 1984. In addition, measurements by particle detectors on the GOES 5 and 6 and ICE spacecraft were used in the analysis. The pronounced anisotropy of the particle fluxes, lasting ∼40 min at relativistic energies and more than 2 hours at ∼75 MeV, shows that the propagation of the solar protons in the interplanetary magnetic field (IMF) was practically scatter‐free (mean free path λ ≳ 2 AU) and that the injection of the particles into the IMF at the Sun was time‐extended. Although the analysis of the onset times of the event as a function of the reciprocal of the proton speeds does not clearly exclude a δ‐like acceleration of the flare protons, we infer that the solar protons with 70‐ to 500‐MeV energy were injected into the IMF 4 ± 2 min earlier than the relativistic protons. Proton energy spectra at the Sun over a range from 25 MeV to 10 GeV, as constructed from the solar proton observations at Earth, evolved with time in both amplitude and energy. The time evolution of the energy spectra at the Sun also suggests that the lower‐energy protons were accelerated earliest. These observations are consistent with an acceleration of the protons by a coronal shock.
Downstream energetic proton and alpha particles during quasi‐parallel interplanetary shock events
Authors: Tan L. C. , Mason G. M. , Gloeckler G. , Ipavich F. M.
Published: February 2008

We have studied the energetic proton and alpha particle populations during the extended downstream region of three quasi‐parallel interplanetary shock events. The observations were carried out using the University of Maryland/Max‐Planck‐Institut ISEE 3 ULECA (Ultra Low Energy Charge Analyzer) sensor, which unambiguously identifies protons and alphas using the electrostatic deflection versus residual energy technique. The downstream particles exhibit anisotropies due largely, but not entirely, to convection in the solar wind. Using a detailed technique to find the reference frame in which the particle distribution functions are isotropic, we find that this frame is moving with respect to the solar wind at speeds from tens to hundreds of kilometers/second, is usually within ∼20° of the magnetic field direction, and is directed toward the Sun. We identify this reference frame as the rest frame of the scattering centers which isotropize the particle distribution functions. In the solar wind frame the scattering center frame sometimes moves at the Alfvén velocity, but just as often it has significantly higher or lower velocity. The spectral indices of the proton and alpha distribution functions are found to be remarkably constant during the downstream period, being generally insensitive to changes in particle flux levels, magnetic field direction, and solar wind densities. In two of the three events the proton and alpha spectra are the same throughout the entire downstream period, in agreement with predictions of diffusive shock acceleration theory. We find that diffusive acceleration theory cannot predict the particle spectra using only solar wind data. However, using the scattering center velocities, we are able to correctly predict the particle spectral indices for these three events.
The ionopause transition and boundary layers at comet Halley from Giotto magnetic field observations
Authors: Neubauer F. M.
Published: February 2008

Giotto magnetic field observations at a time resolution of 28.24 vectors per second in the innermost part of the interaction region of comet Halley with the solar wind have yielded the following results: (1) The outer boundary of the cavity, the ionopause, has a thickness of 25 km only, both inbound and outbound, and magnetic field magnitude drops of 20 nT and 18.3 nT, respectively. Pressure balance arguments lead to an outward plasma pressure decrease of at least 1.5×10−10 Pa. Plasma densities reported by the Giotto mass spectrometers then lead to three alternatives: If no density jump occurs across the ionopause an electron temperature of at least 6000 K just inside the ionopause cannot be avoided. Smaller electron temperatures require an inward discontinuous increase in plasma density across the ionopause. Third, an additional hot ion population of sufficient pressure inside the cavity would make a high electron temperature unnecessary (D. T. Young, personal communication, 1987). (2) The ionopause boundary layers are characterized by (j × B) forces of 5.7×10−16 N m−3 and 7.3×10−16 N m−3 inbound and outbound, respectively. These magnetic forces must be balanced by the sum of the elastic frictional force, the forces due to massloading by photoions, due to ion molecule reactions and due to ionization by hot electrons. An electron pressure gradient may also contribute. (3) A magnetic field bulge of 80‐km length is interpreted as a weak, almost perpendicular, subcritical shock propagating radially outward generated by a temporary pressure increase in the cavity presumably due to a transient population of hot ions and electrons in the cavity. (4) The hot ions and particularly electrons and additional energy input suggested by these observations point to the importance of dynamic phenomena in the innermost part of the interaction region, such as magnetic field line merging in the tail.
A mechanism for the generation of cusp region hydromagnetic waves
Authors: Lee L. C. , Shi Y. , Lanzerotti L. J.
Published: February 2008

It is suggested that the existence of a sporadic multiple X line reconnection (MXR) process at the dayside magnetopause could be a source of energy to produce ULF hydromagnetic waves (ƒ ∼ 1–10 mHz) in the dayside magnetopause cusp region. The MXR process is characterized by the repeated formation and convection of magnetic islands and elongated plasma clouds, which compress and distort the adjacent closed geomagnetic field lines. This leads in a natural way to the generation of ULF waves with frequencies in the range 1–10 mHz along cusp geomagnetic field lines. These waves are expected to be appoximately linearly polarized, dominated by variations in the radial magnetic field component near the dayside magnetopause and by variations in the azimuthal magnetic field component near the resonant closed field lines in the magnetosphere. In addition, the field‐aligned currents associated with the MXR process may generate the impulsive magnetic field variations often observed to accompany the continuous variations in the ULF band. Hence the reconnection process may be an additional mechanism, together with the Kelvin‐Helmholtz instability and direct penetration of upstream waves, for the production of hydromagnetic energy in the dayside magnetosphere.
Cross‐B convection of artificially created, negative‐ion clouds and plasma depressions: Low‐speed flow
Authors: Bernhardt Paul A.
Published: February 2008

A negative‐ion, positive‐ion plasma produced by the release of an electron attachment chemical into the F region becomes electrically polarized by the collisions with neutrals moving across magnetic field lines. The resulting electric field causes E × B drift of the two ion species and the residual electrons. The cross‐field flow of the modified ionosphere is computed using a two‐dimensional numerical simulation which includes electron attachment and mutual neutralization chemistry, self‐consistent electric fields, and three‐species plasma transport. The velocity of the plasma is initially in the direction of the neutral wind because the negative‐ion cloud is a Pedersen conductivity enhancement. As the positive and negative ions react, the Pedersen conductivity becomes depressed below the ambient value and the velocity of the plasma reverses direction. A plasma hole remains after the positive and negative ions have mutually neutralized. The E × B gradient drift instability produces irregularities on the upwind edge of the hole. These processes may be observed experimentally with optical and backscatter‐radar diagnostics.
The role of interplanetary shocks in the longitude distribution of solar energetic particles
Authors: Cane H. V. , Reames D. V. , von Rosenvinge T. T.
Published: February 2008

A study of solar proton events with well‐identified sources has been carried out using data from Goddard particle experiments on IMPs 4, 5, 7, and 8 and ISEE 3. The experiments cover the energy range from about 1 to 300 MeV. The 235 events of our study represent approximately 70% of all increases above 10−3 particles cm−2 sr−1 s−1 MeVminus;1 at energies > 20 MeV detected in a 19.7‐year period commencing mid‐May 1967. It is shown that intensity‐time profiles of solar proton events display an organization with respect to heliolongitude. Whilst it has been known for many years that the profile of a proton event depends on the longitude of the solar event relative to the observer, we suggest that the major controlling agent is the existence of an interplanetary (IP) shock. Furthermore, we explain the change in shape as a function of heliolongitude within the framework of a recently derived model for the large‐scale structure of IP shocks. In particular, the long delay to maximum intensity for far eastern events (a property previously ascribed to coronal processes) and the overall extended duration can be accounted for by IP shock acceleration and continued magnetic connection to the shock even after it has propagated beyond 1 AU.
A three‐dimensional MHD simulation of the interaction of the solar wind with comet Halley
Authors: Ogino Tatsuki , Walker Raymond J. , Ashour‐Abdalla Maha
Published: February 2008

A three‐dimensional time‐dependent magnetohydrodynamic (MHD) simulation was used to model the interaction between the solar wind and the plasmas produced from cometary gases. The model, which includes cometary mass loading, reproduced several features observed by the recent missions to comet Halley: (1) a weak bow shock was formed 3.2×105 km in front of the comet, (2) the magnetic field increased by a factor of 3.7 across this weak bow shock and continued to increase up to the contact surface (Bmax/BIMF = ∼6 ‐ 8), and (3) the plasma temperature increased across the bow shock but decreased nearer to the comet. In addition, interplanetary magnetic field (IMF) lines were hung up on the comet and formed a long tail in which the lobe magnetic field was quite strong (Blobe/BIMF ∼ 5). A cold dense plasma sheet oriented normal to the IMF direction formed in the tail.
Plasma waves in the range of the lower hybrid frequency: ISEE 1 and 2 observations at the Earth's bow shock
Authors: Mellott M. M. , Greenstadt E. W.
Published: February 2008

This report presents a characterization of plasma wave noise in the range of the lower hybrid frequency associated with 65 crossings of Earth's bow shock observed by the ISEE 1 and 2 satellites. Wave growth generally becomes detectable at the upstream edge of the shock foot, increases at the upstream edge of the shock ramp, peaks within the ramp, and then quickly decays to steady downstream values. The upstream extent of the noise is on the same order as that of specularly reflected gyrating ions. Similar profiles were observed in subcritical and supercritical shocks, and no special behavior was associated with the first critical Mach number. Spectra in the foot and ramp were similar in shape, although the noise was 1 to 2 orders of magnitude more intense in the shock ramps than in the feet. Low frequency electric field spectra are rather flat, with spectral indices ∼ −2 to −1. Magnetic spectra are steeper, with spectral indices ∼ −4. Electric field intensities are positively correlated with solar wind speed and inversely related to electron beta and Mach number. Magnetic components are positively correlated with Mach number and beta. These results are generally consistent with suggestions that the noise consists of lower hybrid waves driven by reflected gyrating ions in the foot, and by additional instabilities, such as the cross‐field current, in the shock ramp.
Measurements of the thermospheric meridional wind from the S85‐1 spacecraft
Authors: Kayser D. C.
Published: February 2008

The upper atmosphere composition spectrometer (UACS) was flown on the Department of Defense Space Test Program mission S85‐1. UACS was designed to measure neutral composition, temperature, and the in‐track wind component in the lower thermosphere. In the high inclination orbit of S85‐1, UACS measured the meridional component of wind at all but the highest latitudes. While techniques to measure cross‐track neutral wind have been successfully demonstrated by other investigators, in situ measurements of the in‐track wind component have not been published before. UACS obtained a small but significant set of wind data for 2 days in July 1984 at altitudes between 200 and 235 km, at local solar times of 1030 and 2230 hours. The measured winds compare favorably with published results from ground‐based incoherent scatter radar and Fabry‐Perot interferometer experiments and with satellite‐based cross‐track measurements of the meridional wind.
Double‐probe potential measurements near the Spacelab 2 electron beam
Authors: Steinberg J. T. , Gurnett D. A. , Banks P. M. , Raitt W. J.
Published: February 2008

As part of the Spacelab 2 mission the plasma diagnostics package (PDP) was released from the shuttle as a free‐flying satellite. The PDP carried a quasi‐static electric field instrument which made differential voltage measurements between two floating probes. At various times during the free flight, an electron beam was ejected from the shuttle. Large differential voltages between the double probes were recorded in association with the electron beam. However, analysis indicates that these large signals are probably not caused by ambient electric fields. Instead, they can be explained by considering three effects: shadowing of the probes from streaming electrons by the PDP chassis, crossing of the PDP wake by the probes, and spatial gradients in the fluxes of energetic electrons reaching the probes. Plasma measurements on the PDP show that energetic electrons exist in a region 20 m wide and up to at least 170 m downstream from the electron beam. At 80 or more meters downstream from the beam, the double probe measurements show that the energetic electron flux is opposite to the injection direction, as would be expected for a secondary returning electron beam produced by scattering of the primary electron beam.
Transverse ion energization and low‐frequency plasma waves in the mid‐altitude auroral zone: A case study
Authors: Peterson W. K. , Shelley E. G. , Boardsen S. A. , Gurnett D. A. , Ledley B. G. , Sugiura M. , Moore T. E. , Waite J. H.
Published: February 2008

The transport of ions from the ionosphere to the magnetosphere requires that ions acquire significant energy in directions both transverse and parallel to the magnetic field. There is a considerable body of experimental evidence that shows that transverse energization occurs over a wide range of altitudes on auroral field lines. Many recent analytical and simulation studies have addressed the microphysics involved in transverse ion energization. There are, however, remarkably few published high‐resolution plasma and plasma wave observations obtained in the mid‐altitude auroral region available to compare with the analytical and simulation studies. Several hundred hours of high‐resolution plasma data obtained from the Dynamics Explorer 1 satellite have been surveyed. A wide variety of plasma environments that are difficult to simply characterize were found. We present here a comprehensive set of high‐sensitivity, high‐resolution plasma wave, ion, and magnetometer data obtained from an evening auroral zone crossing at r/RE∼3. The total density, thermal structure, and composition of the plasma in this representative interval varied rapidly, as did the character (mode) of low‐frequency plasma waves observed. We did not find an unambiguous particle and wave signature of local transverse ion energization, but we did frequently find intervals where local transverse ion heating was consistent with the observations. We also found a downward flowing ion distribution that occurred simultaneously with a region of intense plasma wave emissions primarily below the lower hybrid resonance frequency.
A predicted energetic ion signature of a neutral line in the geomagnetic tail
Authors: Martin R. F. , Speiser T. W.
Published: February 2008

Under certain assumptions one can predict the formation of a magnetic neutral line on theoretical grounds, generally as a result of an unstable current sheet. Neutral points also seem to form spontaneously in a wide variety of plasma simulations, from MHD to fully kinetic. Hence such magnetic null regions are central to some models of magnetotail energization and magnetic substorms. Yet the observational evidence for such regions remains controversial. In this paper we present a new signature of a neutral line: the field‐aligned ion beam characteristic of current sheet acceleration becomes dispersed into a “ridge” in the velocity space distribution function as a neutral line is approached. The results were obtained using single‐particle simulations in a fixed magnetic field model. The origin of the ridge is shown to be due to initially earthward going ions which pass through the neutral line region. These ions come from a smaller part of the initially antiearthward flowing distribution, generating the depletion of the distribution function below the ridge.
Phase and amplitude perturbations on subionospheric signals explained in terms of echoes from lightning‐induced electron precipitation ionization patches
Authors: Dowden R. L. , Adams C. D. D.
Published: February 2008

Phase and amplitude perturbations (trimpis) on VLF signals propagating in the earth‐ionosphere waveguide are usually explained in terms of precipitation‐induced modification along the great circle path as though it were a one‐dimensional (1‐D) waveguide. The alternative 2‐D view presented here is that precipitation‐induced ionization patches, laterally displaced from the great circle path, produce echoes at the receiver slightly delayed relative to the direct signal. Interference between the direct and echo signals can produce both phase and amplitude trimpis of both signs. Single‐mode propagation is assumed here, since it is likely that the second mode amplitude exceeded that of the first by ∼12 dB. We apply this theory to a sequence of trimpis observed on the North West Cape (NWC)‐Dunedin path on July 10, 1987. This indicates a north–south drift of the precipitation region consistent with drift speeds of whistler ducts. The theory is also applied to events published in the literature.
A general association between discrete auroras and ion precipitation from the tail
Authors: Lyons L. R. , Fennell J. F. , Vampola A. L.
Published: February 2008

Using two‐dimensional observations from the spinning, polar‐orbiting S3‐3 satellite, we have compared the locations of regions that we identify as discrete auroral arcs with regions of isotropic ion precipitation. Regions of discrete auroral arcs are defined to be regions containing particle distributions consistent with field‐aligned potential drops ≳0.5 kV. Polar cap arcs and local times near noon (0900‐1500 MLT) are excluded from this study. Our most important result is that the inferred locations of discrete auroral arcs are almost exclusively confined to the region of isotropic ion precipitation at all local times studied. In the rare exceptions, the ion pitch angle distributions approach isotropy. This association is what is expected if magnetic field lines containing discrete auroral arcs thread the tail current sheet at distances where ion motion violates the guiding center approximations. We have also found that discrete arc regions are generally associated with spatial structure and boundaries in the precipitating ions. Additionally, we occasionally observe upward going, return‐current electrons, and these are also associated with the structured, isotropic ion precipitation.
Dayside energetic electron precipitation over the South Pole (λ = 75°)
Authors: Matthews D. L. , Rosenberg T. J. , Benbrook J. R. , Bering E. A.
Published: February 2008

Balloon bremsstrahlung measurements of electron precipitation between 25 and 500 keV were made near South Pole station, 75° invariant latitude, between December 15, 1985 and January 15, 1986. On January 2 and 3, 1986, a period of low to moderate geomagnetic activity, precipitation with characteristic (average) energy E0 in the range 20‐200 keV was observed on the dayside, i.e., between ∼ 0500 and ∼ 1500 local magnetic time. In one event, with E0 initially 150 keV, 35‐keV electrons arrived 9 min later than 350‐keV ones; here the trapped energetic electron population may have been enhanced during an injection in the midnight sector and drifted eastward to undergo precipitation in the morning sector. In another event, E0 was 200 keV, but the least energetic electrons arrived within < 0.5 min of the most energetic, indicating a scattering process close to the station. These are the first published reports known to us of precipitating electrons at L > 10 with fluxes measured in the entire energy range from 20 keV to over 250 keV. Near noon on both days, effects were seen in the data that suggest passage of the cusp or polar cap boundary into the field of view of the balloon sensors.
Substorm variations in the magnitude of the magnetic field: AMPTE/CCE observations
Authors: Lopez R. E. , Sibeck D. G. , Lui A. T. Y. , Takahashi K. , McEntire R. W. , Potemra T. A. , Klumpar D.
Published: February 2008

By using AMPTE/CCE energetic particle data, 167 ion injections that are essentially dispersionless over a nominal energy range of 25 keV to 285 keV have been cataloged. The ion, electron, and magnetic field signatures of these events are the kind normally associated with substorms in the near‐Earth magnetotail. We have examined the total magnetic field strength variations over the course of these events to identify the dependence on dipole latitude. The change in
is poorly correlated with magnetic latitude. We interpret this result as indicating that during periods of substorm activity, the latitudinal position of the current sheet varies significantly within the 32° wedge centered on the dipole equator traversed by CCE. On the other hand, the changes in
are well correlated with the local field properties themselves. This study therefore indicates that even in the near‐Earth magnetotail out to 8.8 RE (CCE apogee), the local field measurements are a better guide to the determination of one's position relative to the current sheet during a substorm than the magnetic latitude. Statistical relationships between
Δ and both
and |Bx| (in GSM coordinates) are consistent with the changes in
that one would expect from a disruption/diversion of the cross‐tail current. The radial gradient of the reduction of the cross‐tail current during substorms appears to be negative, although the evidence is not conclusive. Finally, the local time variation of the statistical relationships is consistent with Birkeland current segments of the substorm current wedge which have a large extent in local time.
A diagnostic model for equatorial spread F, 1, Model description and application to electric field and neutral wind effects
Authors: Maruyama Takashi
Published: February 2008

Linear growth rates for the gravitational Rayleigh‐Taylor instability are calculated using the ionospheric electron density distribution obtained by solving the ion continuity equations including dynamics for O+, NO+, O2+, and N2+ ions. The instability model includes the contributions from plasma away from the magnetic equatorial plane. Effects of the transequatorial component of the thermospheric wind and the E × B drift are investigated. Thermospheric winds are based on theoretical calculations. The E × B drift velocities are modeled after the observations at Jicamarca and Fortaleza for different seasons. The results have shown that both effects are equally important to controlling the occurrence of equatorial spread F. Observed winter‐summer asymmetry of the occurrence probability at the Jicamarca longitude is explained by the characteristics of the E × B drift velocity model. However, in the case of Fortaleza, the observations cannot be explained strictly in terms of the E × B drift. Taking account of the E × B drift effect and the wind effect, we have obtained a qualitative agreement between the observations and calculations over all seasons for both Jicamarca and Fortaleza.
The source location and beaming of broadband bursty radio emissions from Uranus
Authors: Farrell W. M. , Calvert W.
Published: February 2008

Among the radio emissions detected with Voyager 2 at Uranus was a so‐called broadband bursty or “b‐bursty” component at frequencies of 300‐700 kHz, which was first observed✓ just before closest approach and then during subsequent planetary rotations about 1‐6 hours before the more prominent smooth component. Although not obvious for individual rotations because of the bursty nature of these signals, a systematic occurrence pattern for these emissions has been detected in a new composite radio spectrogram where the bursts from different rotations were superimposed. Consisting of symmetrical emissions flanking a quiet region, with their peak frequencies occurring about an hour and a half on either side, this pattern is attributed to Voyager intercepting a hollow emission cone from the nightside of Uranus which tipped past its direction and back during successive planetary rotations. On the assumption that this cone was circular and aligned with the magnetic field at the cyclotron frequency, its source location and beam angles were determined by minimizing the apparent cone‐angle deviations at the highest frequencies. The source was thus found to occur at a southern Uranian latitude of 45°, west longitude 235°, and at a height of 1.43 RU, in excellent agreement with previous studies. The beam angle at the highest frequencies, with respect to the source magnetic field, was found to be about 85°, and that for the entire emission pattern about 80°‐87°.
Mapping of the solar wind electric field to the Earth's polar caps
Authors: Toffoletto F. R. , Hill T. W.
Published: February 2008

In this paper we describe a quantitative model of a magnetically interconnected (open) magnetosphere, developed as a perturbation to Voigt's closed magnetosphere model with a given magnetopause shape. The “interconnection” (perturbation) field is obtained as a solution to a Neumann boundary value problem, with the magnetopause normal component distribution as a boundary condition. The normal component at the magnetopause is required to be time independent, and is specified in accordance with one of two hypotheses: the subsolar point merging hypothesis and Crooker's antiparallel merging hypothesis. The resulting open magnetospheric configuration is used to map the magnetopause electric field down to the polar cap ionosphere. We present ionospheric convection patterns derived from three representative interplanetary magnetic field (IMF) orientations for each of the two dayside merging geometries. Both merging geometries reproduce the observed convergence of convection streamlines near noon in a convection “throat,” and the east‐west deflection of these streamlines in response to the east‐west IMF component. The major difference between the two dayside merging geometries occurs for nonsouthward IMF, and consists of a Sun‐aligned convection gap that bifurcates the polar cap in the case of the antiparallel merging geometry but not in the subsolar point merging geometry. This convection gap may plausibly be associated with the “theta aurora” structure observed when the IMF has a northward component.
Quasi‐steady corotating structure of interplanetary geomagnetic disturbances: A survey of solar cycles 13–20
Authors: Zhang Gongliang , Gao Yufen , Lu Chen , Feng Zhongxiao , Xu Yuanfang
Published: February 2008

The variation of interplanetary parameters at Earth orbit has previously been studied by superposedepoch analysis of the spacecraft data obtained during solar cycle 20. It was shown that the magnetosonic speed possesses a quasi‐steady, but slightly drifting, distribution in Carrington longitude during all phases of solar cycle 20. Other magnetic parameters such as the Alfvénic speed and ratios of magnetic and total pressure to the kinetic energy density also reflect this same organization in Carrington longitude in both the maximum and minimum years. We extend the study of the quasi‐steady corotating solar‐terrestrial variation to include the long‐time series of geomagnetic indexC9 data covering solar cycles 13–20. Using the superposed‐epoch method in conjunction with digital filter and harmonic analysis, we find that during solar minima the corotating variation has a quite steady structure of distribution in the Carrington rotation, with the phase of the second harmonic remaining nearly the same in each of the even or odd cycles and shifting a large angle, probably reversing, from one to the next. Analysis of seasonal trends of the distribution suggests each of the two main peaks maintains the same magnetic polarity during the even cycles, and it is likely that their source regions are located in the northern and southern solar hemispheres, respectively, for the first and second peaks within each rotation. In addition, the locations of the peaks in the even cycle minima show only a small systematic drift in heliospheric longitude (about 6 Carrington days, net) from 1913 to 1974. The implication is that the stable high‐speed solar wind streams originate from a deeply rooted configuration of the general solar magnetic field which persists for at least four solar cycles.
Imaging observations of lower thermospheric O(1S) and O2 airglow emissions from STS 9: Implications of height variations
Authors: Swenson G. R. , Mende S. B. , Llewellyn E. J.
Published: February 2008

Limb observations of the lower thermospheric nightglow in the southern hemisphere were made on several orbits during the shuttle orbiter Spacelab 1 mission (December 1983) with the Atmospheric Emissions Photometric Imager (AEPI). Sequential observations of emission from O(1S) at both 2972 and 5577 Å, O2(b1∑g+) at 7620 Å, OH (9–3), near 6300 Å, and the combined emission from the three upper states of O2(A3∑u+, c1∑u−, and A′3Δu) which lead to the Herzberg I, Herzberg II, and Chamberlain band emissions in the blue and near ultraviolet were obtained. Stellar image patterns extracted from the images were used to provide absolute calibration of the limb emission heights to ±1 km. The altitudes of peak emission heights were determined for portions of two orbits which covered a significant latitude range. The results show that the peak height of the emissions are not as constant with latitude as frequently assumed. The airglow limb height exhibited a variation of ∼8 km for the first of the two night passes in the mid‐latitude southern hemisphere. On the second of the two night passes the airglow emission layer was observed to become lower as the orbiter moved southward, away from the equator, in agreement with the OGO 6 observations of Wasser and Donahue (1979).