bluuebunny/crossref_metadata_2025 huggingface dataset:
The Giacobini‐Zinner magnetotail: Tail configuration and current sheet
Authors: McComas D. J. , Gosling J. T. , Bame S. J. , Slavin J. A. , Smith E. J. , Steinberg J. L.
Published: February 2008

The high‐resolution plasma electron and magnetic field data sets from the ICE tail traversal of comet Giacobini‐Zinner have been combined to make a detailed study of the draped Giacobini‐Zinner magnetotail in general, and its field‐reversing current sheet in particular. The geometry of the magnetotail at the time of the ICE crossing is determined and is shown to be consistent with a circular tail cross section rotated 10.5° in the normal sense of aberration and 9.9° above the ecliptic plane, bisected by a cross‐tail current sheet which is rotated 43° out of the ecliptic about the solar wind velocity vector. MHD continuity, momentum, and energy equations are combined with the plasma and field observations to determine unmeasured plasma properties at ICE and upstream at the average point along each streamline where the cometary ions are picked up. The ion temperature, beta, and flow speed at ICE range from 1–1.5 × 106 K, 1–4, and −20 to −30 km s−1, respectively, in the draped lobes to ∼1.2 × 105 K, up to ∼40, and ∼−20 km s−1 in the current sheet. Upstream at the average pickup locations, the flow velocity, ion temperature, density, and ion source rates range from ∼−75 km s−1, ∼4 × 106 K, ∼20 cm−3, and ∼1.5 cm−3 s−1 in the regions upstream from the lobes to ∼ −12 km s−1, ∼1 × 105 K, 200–600 cm−3, and ∼3.6 cm−3 s−1 in the prime mass‐loading region upstream from the current sheet. Gradients in the plasma properties at the edges of this region are quite strong, and the diameter of the region is ∼1500 km, in good agreement with the expected size scale of the cometary ionopause; implications of our inferred plasma properties in this region are examined. The derived and measured flow velocity and measured plasma density at ICE are combined to calculate the transport rate of ions past the plane of the ICE encounter, and we calculate that ∼2.5–5 × 1028 ions per second (neutrals per second in the steady state) are produced by the comet with only ∼1% of these traveling down the magnetotail.
Hollowness of the observed auroral kilometric radiation pattern
Authors: Calvert W.
Published: February 2008

Presumably also generated by electron cyclotron emission, the earth's auroral kilometric radiation would be expected to exhibit a hollow pattern in the direction of the source magnetic field, similar to that reported for the comparable emissions from Jupiter. Although previously overlooked, such hollowness is clearly present in the new pattern measurements of Green and Gallagher (1985) at 56 kHz, occupying source‐centered latitudes of 30° to 45° and hence occurring exactly where it was predicted and previously observed. Being distributed in longitude and spanning the entire evening sector, presumably reflecting a similar longitudinal distribution of auroral zone sources, this hollowness is attributed to sources beamed preferentially in the poleward magnetic meridian.
A theory for the Langmuir waves in the electron foreshock
Authors: Cairns Iver H.
Published: February 2008

A theory for the Langmuir (L) waves observed in the electron foreshock is suggested. Free energy for the Langmuir wave growth is contained in cutoff distributions of energetic electrons streaming from the bow shock. These cutoff distributions drive Langmuir wave growth primarily by the kinetic version of the beam instability, and wave growth is limited by quasi‐linear relaxation. The observed bump‐on‐tail electron distributions are interpreted as the remnants of cutoff distributions after quasi‐linear relaxation has limited the wave growth. Only plausibility arguments for this theory are given since suitable treatments of quasi‐linear relaxation are not presently available. However, it is shown that the wave processes L ± S → L′ and L ± S → T (where S and T denote ion sound and transverse waves, respectively), refraction in steady‐state density structures, diffusion due to interactions with ion sound turbulence, and effects due to wave convection and spatial gradients in the beam velocity, are unable to suppress the beam instability. The theory leads to natural interpretations of the Langmuir electric field waveforms observed and of the decrease in the Langmuir wave electric fields with increasing distance from the foreshock boundary. The theory for the beam instability is reviewed, and previous analytic and numerical treatments of the beam instability are related. A difficulty in recent theoretical interpretations of broadband wave growth below the plasma frequency, relating to the version of the beam instability considered giving rise only to narrow‐band growth, is pointed out and suggestions for resolving this difficulty are made.
Large scale response of the magnetosphere to a southward turning of the interplanetary magnetic field
Authors: Sauvaud J. A. , Treilhou J. P. , Saint‐Marc A. , Dandouras J. , Rème H. , Korth A. , Kremser G. , Parks G. K. , Zaitzev A. N. , Petrov V. , Lazutine L. , Pellinen R.
Published: February 2008

A set of coordinated measurements obtained on March 4, 1979, in the solar wind, in the magnetotail, at geostationary orbit, in the auroral ionosphere and on the ground allowed a detailed study of the overall magnetospheric response to a southward turning of the interplanetary magnetic field. This study shows that the magnetosphere responds without any appreciable delay to the change of the interplanetary magnetic field (IMF) direction. The intensity of the electrojets increases, and both westward electrojet and auroral forms move southward in the night sector. The magnetic field topology in the inner nightside magnetosphere (6.6 RE) becomes increasingly taillike, inducing the adiabatic earthward motion of the high‐energy electrons and the precipitation of those with small equatorial pitch angles. All these events coincide with the increase of the solar wind‐magnetosphere energy coupling function ε. The large set of available data shows that a series of weak, short‐lived substorms sporadically occurs during the period of southward directed IMF, before the main substorm onset. These small events are characterized by all or part of the following features: partial plasma sheet flux dropout, weak, short‐lived, dipolar reconfiguration of the B field at 6.6 RE, impulsive changes of the ionospheric electric field, limited motions of the westward electrojet boundaries, injection of energetic particles and Pi 2 bursts. Finally a well‐defined substorm occurs about 80 min after the IMF reversal and during the southward IMF episode. Magnetic and auroral activity increases again when the IMF begins to turn northward and subsides as soon as the IMF BZ component reaches positive values. This study suggests that, for the March 4 southward IMF episode, the bulk of the energy dissipated in the inner magnetosphere and auroral ionosphere before the main substorm onset was directly extracted from the solar wind.
Observations of plasma distributions during the coordinated data analysis workshop substorms of March 31 to April 1, 1979
Authors: Huang C. Y. , Frank L. A. , Eastman T. E.
Published: February 2008

On March 31 and April 1, 1979, a sequence of substorms was recorded on the ground. During the entire active period the ISEE 1 and 2 satellites were located in the magnetotail, between 22 RE and 12 RE from the earth. Observations of plasma distributions made at varying levels of activity during these substorms provide good examples of typical magnetotail responses. These measurements were obtained with the University of Iowa LEPEDEA on board the ISEE 1 spacecraft. While the entire magnetotail undergoes temporal variations during substorm activity, the most obvious effect seen is spatial motion of preexisting plasma regimes, for example, at substorm onset the central plasma sheet contracts. This can cause a spacecraft to move from the plasma sheet into the boundary layer. As the plasma sheet boundary layer almost always comprises streaming plasma, the high‐speed flows observed there are not necessarily due to the substorm process. We find that the essential nature of the main plasma regions of the magnetotail is relatively unchanged during high activity.
Observation of suprathermal electron fluxes during ionospheric modification experiments
Authors: Fejer J. A. , Sulzer M. P.
Published: February 2008

The temporal behavior of backscatter by ionospheric Langmuir waves was observed with the 430‐MHz radar at Arecibo while a powerful HF wave was cycled 2 s on, 3 s off. The time resolution was 0.1 s. Late at night, in the absence of photoelectrons, using an HF equivalent radiated power of 80 MW at 3.175 MHz, the initial enhancement of about 6% above system noise of the backscattered power with Doppler shifts between −3.75 and −3.85 MHz was reached about 0.25 s after switching on the HF transmitter. In the following second the enhancement gradually decreased to about 3% and remained there until switching off. During the late afternoon, in the presence of photoelectrons, using the same HF power at 5.1 MHz, an initial enhancement by 25% of the backscattered power with Doppler shifts between −5.25 and −5.35 MHz appeared within less than 0.1 s after switching on the HF transmitter. The incoherent backscatter by Langmuir waves enhanced by photoelectrons was already above system noise by a factor greatly in excess of 10 before switching on the HF transmitter; the 25% enhancement thus corresponds to an enhancement greatly in excess of 250% above system noise. The enhancement drops to less than one tenth of its original value in less than a second. The nighttime effect is attributed to multiple acceleration of electrons from the high‐energy tail of the Maxwellian distribution. The daytime effect is believed to be due to a modification in the distribution function of photoelectrons.
A comparative study of plasma heating by ion acoustic and modified two‐stream instabilities at subcritical quasi‐perpendicular shocks
Authors: Winske D. , Giacalone J. , Thomsen M. F. , Mellott M. M.
Published: February 2008

Plasma heating due to the ion acoustic instability and the modified two‐stream instability is examined for quasi‐perpendicular subcritical shocks. Electron and ion heating is investigated as a function of upstream electron to ion temperature ratio and plasma beta using second‐order heating rates. A simple shock model is employed in which the cross‐field electron‐ion drift speed is adjusted until the total (adiabatic plus anomalous) heating matches that required by the Rankine‐Hugoniot relations. Quantities such as the width of the shock and the maximum electric field fluctuations are also calculated, and the results are compared with the ISEE data set of subcritical bow shock crossings. The observed width of the shock, the amount of plasma heating, and the low‐frequency electric field intensity are in reasonably good agreement with the calculations for the modified two‐stream instability. On the other hand, the wave intensities at higher frequency are about 4 orders of magnitude smaller than those predicted for the ion acoustic instability at saturation, consistent with the fact that the measured shock widths imply cross‐field drift speeds that are below threshold for this instability. It is therefore concluded that the dissipation at these shocks is most likely due to the lower frequency, modified two‐stream instability.
A theory for low‐frequency waves observed at comet Giacobini‐Zinner
Authors: Goldstein M. L. , Wong H. K.
Published: February 2008

As the ICE spacecraft approached comet Giacobini‐Zinner, low‐frequency waves in two frequency ranges were observed in the magnetic field data. We show that, under certain conditions, a gyrating beam of water group ions is capable of generating electromagnetic waves with periods near 100 s (in the spacecraft frame of reference) and shorter‐period whistler branch waves with periods near 3 s. The 100‐s wave can arise from either a nonresonant firehose instability or a resonant beam driven instability so long as the angle between the directions of the local magnetic field and solar wind velocity is less than 90°. Whenever that angle exceeds about 30°, the gyrating ring distribution formed by newly ionized water is also unstable to the generation of whistler waves with 3‐s periods.
Local time and cutoff rigidity dependences of storm time increase associated with geomagnetic storms
Authors: Kudo S. , Wada M. , Tanskanen P. , Kodama M.
Published: February 2008

The cosmic ray increases due to considerable depressions of cosmic ray cutoff rigidity during large geomagnetic storms are investigated. Data from a worldwide network of cosmic ray neutron monitors are analyzed for 17 geomagnetic storms which occurred in the quiet phase of the solar activity cycle during 1966–1978. As expected from the longitudinal asymmetry of the low‐latitude geomagnetic field during large geomagnetic storms, a significant local time dependence of the increment in the cosmic ray intensity is obtained. It is shown that the maximum phases of the local time dependence occur at around 1800 LT and that the amplitudes of the local time dependence are consistent with presently available theoretical estimates. The dependence of the increment on the cutoff rigidity is obtained for both the local time dependent part and the local time independent part of the storm time increase. The local time independent part, excluding the randomizing local time dependent part, shows a clear‐cut dependence on cutoff rigidity which is consistent with theoretical estimates.
Plasma wave evidence for reflected ions in front of subcritical shocks: ISEE 1 and 2 observations
Authors: Greenstadt E. W. , Mellott M. M.
Published: February 2008

Plasma wave profiles of bow shock crossings at ion acoustic frequencies exhibit “feet” or “pedestals,” just as magnetic field profiles do. These ion acoustic feet, consisting of increases in ion wave amplitude over background, are observed in front of virtually all quasi‐perpendicular shocks, regardless of Mach number, but are closely associated with magnetic feet and reflected ions circulating around the ambient upstream field at supercritical shocks. Observed gyroradii of the reflected ions have been found to be commensurate with the thicknesses of the magnetic feet, which consist of increases in average field magnitude just outside the main shock ramps in front of supercritical shocks. We now find that estimated scale lengths, that is, gyroradii, of reflected protons are proportional to the thicknesses of acoustic wave feet in all cases, indicating that such protons are probably universally present at subcritical as well as supercritical shocks, although the protons are observed only sporadically because of instrumental limitations. The mere presence or absence of reflected ions in front of a shock may therefore be an insufficient diagnostic of the shock's status with respect to criticality and ion dissipation.
Numerical modeling of hydromagnetic wave coupling in the magnetosphere
Authors: Inhester B.
Published: February 2008

We have developed a simple computer code that simulates the electromagnetic response of a simplified magnetosphere with respect to pressure variations at the magnetopause. The dependence of the magnetospheric response on the azimuthal wave number of the magnetopause perturbation and possible effects of a kinetic correction to the Alfvén wave dispersion are investigated. We find that the coupling of the cavity mode to Alfvén oscillations is most efficient for wave numbers m = 1 … 3. With increasing wave number, the fundamental cavity mode may lower its eigenfrequency, opposed to the behavior of the higher‐order modes. Kinetic effects cause a propagation of Alfvén wave energy away from the resonant field line.
The magnetosphere of Saturn
Authors: Beard David B. , Gast Mark A.
Published: February 2008

Pioneer 11 and Voyager 1 and 2 magnetic field measurements over the entire flyby of Saturn's magnetic field have been analyzed by fitting a magnetospheric dipole field (i.e., a dipole field plus the field due to currents in the magnetopause), higher moments of the internal field aligned with the dipole along the rotation axis, and the field due to an equatorial sheet current to the magnetic measurements. We find that a dipole moment of 21,431 nT Rs³, a quadrupole moment of 2403 nT Rs4, an octopole moment of 2173 nT Rs5, and an equatorial sheet current of half thickness 2.0 Rs from about 5 Rs to the solar edge of the magnetopause fit the measurements over the entire magnetosphere with a root mean square deviation of 3.2 nT. (Rs is the planet radius, 66,330 km). The primary feature of our analysis is the explicit inclusion of the calculated magnetopause current field, which reduces the overall root mean square deviation over the entire flyby from σ values of 4.7 and 5.9 nT using previous models to 3.2 nT using ours.
Magnetic signatures of precursors to substorm expansive phase onset
Authors: de Groot‐Hedlin C. D. , Rostoker G.
Published: February 2008

The expansive phase of magnetospheric substorms involves the development of auroral loops and surges with particular emphasis on the westward travelling surge which is identified with the western edge of the substorm current wedge. We use as a working hypothesis the contention that the wavelike auroral structures associated with the current wedge are a manifestation of the action of a Kelvin‐Helmholtz instability at the interface between the central plasma sheet (CPS) and the adjacent boundary layer plasmas in the deep magnetotail. Based on this concept, we used ground‐based magnetometer data in an attempt to identify the growth of ionospheric current systems which might be associated with the growth of the Kelvin‐Helmholtz instability. We have, in fact, found that the expansive phase is preceded by the growth of a weak substorm current wedge which strengthens explosively at the time of onset. We suggest that field‐aligned current flows out of the ionosphere as part of a process damping a growing wave at the CPS/boundary layer interface. When the field‐aligned current density exceeds ∼ 1 µA/m², an acceleration region at altitudes of ∼ 1 RE is activated marking the onset of the substorm expansive phase.
Ratio of oxygen to sulfur in the Io plasma torus
Authors: Shemansky Donald E.
Published: February 2008

Model calculations of collisional diffusive equilibrium in the Io plasma torus using the most recent estimates of reaction rates and electron excitation collision strengths indicate that oxygen and sulfur ions are partitioned roughly equally as major species, in conformity with Voyager EUV measurements of O II, S II, and S III emissions, and with recent rocket observations of O I and S I emissions.
Long‐period polar rain variations, solar wind and hemispherically symmetric polar rain
Authors: Makita K. , Meng C. ‐I.
Published: February 2008

On the basis of electron data obtained by the Defense Meteorological Satellite Program (DMSP) F2 satellite the long‐period variations of the polar rain flux are examined for four consecutive solar rotations. It is clearly demonstrated that the asymmetric enhancement of the polar rain flux is strongly controlled by the sector structure of the interplanetary magnetic field (IMF). However, the orbit‐to‐orbit and day‐to‐day variations of the polar rain flux are detected even during a very stable sector period, and the polar rain flux does not have any clear relationship to the magnitude of the IMF Bx or By. Thus the polarity of Bx controls only the accessibility of a polar region. It is also noticed that the intensity of polar rain fluxes does not show any relationship to the density of the solar wind, suggesting that the origin of the polar rain electrons is different from the commonly observed part of the solar wind electron distribution function. In addition to the asymmetric polar rain distribution, increasing polar rain fluxes of similar high intensity are sometimes detected over both polar caps. An examination of more than 1 year's data from the DMSP F2 and F4 satellites shows that simultaneous intense uniform precipitations (> 107 electrons/cm² s sr) over both polar caps are not coincidental; it also shows that the spectra are similar. The occurrence of hemispherically symmetric events is not common. They generally are observed after an IMF sector transition period, during unstable periods in the sector structure, and while the solar wind density is high.
Global excitation of wave phenomena in a dissipative multiconstituent medium: 3. Response characteristics for different sources in the Earth's thermosphere
Authors: Mayr H. G. , Harris I. , Varosi F. , Herrero F. A.
Published: February 2008

Using a transfer function model for the earth's thermosphere, we have previously discussed the properties of different wave modes and impulsive perturbations excited by Joule heating. The thermospheric response showed the expected propagating waves, but a significant part of the perturbation, the trapped component, remained confined to the source region. In the present paper we expand the analysis to include the electric field momentum source. We discuss the effects of these excitation mechanisms on the trapped component confined to the source region and on the propagating gravity waves emanating from the source. Our analysis leads to the following conclusions: (1) for Joule/particle heating, the temperature and density perturbations contain a relatively large trapped component which has the property of a low‐pass filter, with slow decay after the source is turned off. The decay time is sensitive to the altitude of energy deposition and is significantly reduced as one moves the source peak only from 125 to 150 km. (2) For electric field momentum coupling, the trapped components in the temperature and density perturbations are relatively small. In the curl field of the velocity, however, the trapped component dominates, but compared with the temperature (and density), its decay time is much shorter. (3) Outside the source region the form of excitation is of secondary importance for the generation of the various propagating gravity wave modes (direct, reflected, and ducted). Different atmospheric variables, however, respond differently. The model has been extended to allow for incoherence in the excitation sources, which is necessary to simulate the stochastic nature of auroral processes, and a numerical experiment is presented elucidating some salient features of the thermospheric response. Observations at times show long temperature and density decay times (order of days) in the source region, yet at other times the decay time is much shorter (order of hours). Differences in the excitation mechanism studied, e.g., the altitude of energy deposition, may be responsible for the apparent disparity.
The plasma environment during particle beam injection into space plasmas: 2. Charge‐neutral beams
Authors: Winglee R. M. , Pritchett P. L.
Published: February 2008

Charge‐neutral beams consisting of ions and electrons provide an alternate means to electron beams for active probing of the ionosphere and magnetosphere. Two‐dimensional (three‐velocity component) simulations are used to determine the properties of the beam and the surrounding plasma during continuous injection. It is shown that the beam electrons act as if they are tied to the field lines, whereas the beam ions are essentially unmagnetized for time scales much smaller than the ion‐cyclotron period. As a result, charge separation occurs for injection across the field lines. The resultant space‐charge fields cause the beam ions to be drawn back into the beam electrons on a time scale of a few ion plasma periods when the beam density is greater than the plasma density. If the mass of the beam ions is also greater than about that of the plasma ions, the plasma ions can be accelerated perpendicular to the magnetic field to energies comparable to the beam energy. Strong heating of the plasma electrons occurs irrespective of the ion beam mass. This electron heating occurs preferentially parallel to the magnetic field for strong magnetic fields. Perpendicular electron heating occurs as the magnetic field is decreased.
Estimates of absolute scattering coefficients of radar aurora
Authors: Moorcroft D. R.
Published: February 2008

Data available in the literature and obtained with the Homer, Alaska 398 MHz phased‐array radar have been used to obtain estimates of absolute scattering coefficients of radar aurora over a frequency range of 30–1210 MHz, and to express them in terms of a wave‐packet scattering model. At any one frequency these scattering coefficients typically range over 4 or 5 orders of magnitude. Between 50 MHz and 850 MHz the frequency dependence of the strongest echoes seems to be described quite well by a scattering coefficient per unit volume of συ = 3 × 10−7 (k)−2.25 m−1, where k is the wave number of the fluctuations in the plasma. This expression is thought to describe scattering from primary instabilities whose propagation vectors are perpendicular to the magnetic field, and parallel to the electrojet current. There is evidence that at 30 MHz the largest scattering coefficients are significantly larger than predicted by this expression, while at 1210 MHz they are significantly smaller. At 50 MHz the strongest auroral scattering coefficient appears to be more than an order of magnitude greater than what is obtained in the equatorial electrojet. From the scattering coefficients, estimates of the mean fractional electron density fluctuation amplitude, δN/N, have been made and compared with previous results.
Faraday polarization fluctuations of VHF geostationary satellite signals near the geomagnetic equator
Authors: Bhattacharyya A. , Rastogi R. G.
Published: February 2008

A two‐slope power law spectrum for the equatorial F region irregularities has been introduced in the theory of Faraday polarization fluctuations (FPF) in place of a single‐slope power law spectrum. The break scale length, at which the transition from a shallow to a steep slope in the irregularity power spectrum occurs, is found to play an important role in determining the strength of FPF. Analysis of Faraday rotation data for 40‐ and 140‐MHz linearly polarized signals transmitted from the geostationary satellite ATS6 and received at the equatorial station Ootacamund (magnetic dip 6°N) yielded the following results: (1) FPF for the 140‐MHz signal is negligible (< |ΔΩ|²>1/2 « 0.1 rad) although the quadrature components from which the Faraday rotation angle Ω is derived show large fluctuations in the event of strong amplitude scintillations; (2) there is appreciable FPF (< |ΔΩ|²>1/2 ≥ 0.1 rad) for the 40‐MHz signal in these cases, which is explained on the basis of the two‐component nature of the irregularity power spectrum.
Cross‐polarized interferometry of a Jovian decametric radio storm
Authors: Phillips J. A. , Carr T. D. , Greenman W. B. , Levy J.
Published: February 2008

Observations of an 18‐MHz Io‐A storm have been conducted using a 46 km cross‐polarized interferometer at the University of Florida. The pre‐ and postdetection correlation properties of LH and RH elliptically polarized L bursts have been studied. Fringe visibility measurements suggest that two discrete and independent sources are sometimes simultaneously active, the polarization sense of one being LH and that of the other RH. A scintillation analysis supports the conclusion that two or more sources were active in such cases.
Spectral signatures of jumps and turbulence in interplanetary speed and magnetic field data
Authors: Roberts D. A. , Goldstein M. L.
Published: February 2008

The low‐frequency power spectra of the magnitude of the magnetic and velocity fields in the solar wind frequently have a somewhat steeper than f−2 dependence, whereas the vector components of these quantities often have spectra characteristic of turbulence (f−5/3) with a break to a flatter spectrum at low frequencies. It is shown here that, consistent with a suggestion of Burlaga and Mish, the f−2 spectra in the magnitudes result from jumps due to various rapid changes in the time series for these quantities. If these jumps are removed from the data, the spectra of the resulting “difference” time series have the f−5/3 form. We conclude that f−2 spectra in the magnitudes of the magnetic field and velocity in the solar wind arise from phase coherent structures that can be distinguished clearly from incoherent turbulent fluctuations.
MHD turbulence generated by time‐varying field‐aligned currents
Authors: Cerisier J. C. , Machard C. , Pottelette R.
Published: February 2008

The MHD turbulence driven by time‐varying field‐aligned currents is calculated. It is shown that a current pulse with a parallel velocity of the order of or smaller than the Alfven velocity can generate kinetic Alfven waves. These results are applied to the interpretation of magnetic pulses observed in the auroral zone by low‐altitude polar satellites. They provide a challenge to the usual interpretation in terms of a crossing by the spacecraft of static structures of thin parallel‐current sheets. It must be stressed that the field‐aligned current densities necessary to explain the observed magnitude of the magnetic turbulence are smaller for a time‐dependent than for a static model.
Simultaneous measurements of energetic ion (≥50 keV) and electron (≥220 keV) activity upstream of Earth's bow shock and inside the plasma sheet: Magnetospheric source for the November 3 and December 3, 1977 upstream events
Authors: Sarris E. T. , Anagnostopoulos G. C. , Krimigis S. M.
Published: February 2008

Simultaneous observations of energetic ions (≳50 keV) and electrons (≳220 keV) by the IMP 7 and 8 spacecraft, carrying identical instruments and located within the distant (∼37 RE) magnetotail and upstream from the bow shock, have been employed to separate temporal variations from spatial variations during the upstream ion events observed on December 3, 1977 and November 2–3, 1977, in order to determine the source of these particles. The IMP data, when compared with those from ISEE 1 and 2, have also made possible the determination of field‐aligned and flux‐tube intensity gradients in the upstream region, thus enabling the test of specific predictions of the Fermi acceleration model for such events. The analysis of these three‐spacecraft observations and comparison with theory have revealed the following: (1) For each of the observed upstream enhancements, energetic ions and electrons were simultaneously present inside the plasma sheet—successive increases were in excellent time coincidence with substorm injection events seen on ground‐based magnetograms; (2) The low‐energy (≳50 keV) ion intensity profile inside the plasma sheet was relatively flat, while at higher (≳300 keV) energies there was considerable variability, with one case exhibiting an “inverse velocity dispersion” profile; (3) Relativistic electron bursts were seen inside the plasma sheet and also upstream of the shock but at substantially reduced intensities; (4) The ion energy spectrum for the December 3 event, extended to energies ∼2 MeV, was identical in form within the plasma sheet and upstream of the shock and can be described well by dj/dE ∝ E−5.5; (5) Ion anisotropies exhibited typically large dawn‐dusk or dusk‐dawn gradients, depending on spacecraft location, and showed large (up to 20∶1) field‐aligned streaming away from the bow shock; (6) Comparison of IMP and ISEE 2 intensities at ∼60 keV for identical magnetic connections to the bow shock during a period when the interplanetary magnetic field (IMF) was radial showed the IMP 7 intensity to be higher by a factor ∼10, even though IMP 7 was ∼20 RE further from the bow shock, but located toward the dusk side of the earth‐sun line; and (7) Comparison of unidirectional intensities of IMP 8 and ISEE 1 along the same radial IMF line showed the field‐aligned component to be identical at both spacecraft, while the omnidirectional intensities give a gradient of ∼2.5%/RE. The observations are compared with predictions of the Fermi model, and it is found that they are inconsistent with the principal features of the model, including the energy extent, form of the spectrum, anisotropies, scale lengths of acceleration region, IMF connection times, and the presence of electrons.
Delayed entry of solar protons (Ep > 0.3 MeV) into the Earth's magnetotail
Authors: Van Allen J. A. , Saflekos N. A. , Sarris E. T.
Published: February 2008

The delayed entry of solar protons (Ep > 0.3 MeV) into the Earth's magnetotail has been studied by simultaneous observation with three Earth‐orbiting spacecraft of a sequence of specially favorable events during 1968. Principal findings are that (1) such protons have immediate access to the magnetosheath; (2) delay times increase discontinuously from essentially zero in the magnetosheath to tens of minutes as an observing point moves inward across the magnetopause; (3) well‐defined delay times in the range 30–160 min occur for points interior to the magnetotail; (4) the inferred entry points lie at downstream distances of 80–500 RE; and (5) there is no discernible relationship between the direction and sense of the interplanetary magnetic field and the entry times in the northern and southern lobes of the magnetotail or the entry times at various distances from its central axis, the XGSM axis. This data set is compatible with previously published work but adds significantly to observational knowledge of the subject. In broad terms, the results favor some form of an open model of the magnetotail and are inconsistent with diffusive entry of protons across a closed magnetotail field with a time constant substantially greater than a few minutes.
Magnetopause properties from AMPTE/IRM observations of the convection electric field: Method development
Authors: Sonnerup B. U. Ö , Papamastorakis I. , Paschmann G. , Lühr H.
Published: February 2008

The possibility is examined of obtaining information about the orientation and state of motion of the magnetopause current layer from the convection electric field, Ec = −v × B, calculated from measured plasma velocities, v, and magnetic fields, B. Using AMPTE/IRM data from one particular traversal of a magnetopause rotational discontinuity as a guide, a new method has been developed for predicting the magnetopause normal vector, n. In its simplest form, the method consists of performing variance analysis on Ec data, or modifications thereof which take into account the motion of the magnetopause. The procedure is similar to the minimum variance analysis commonly performed on B data, except that the direction of maximum variance of Ec, rather than minimum variance of B is used as a predictor for n. In its most developed form, the method depends on the existence of a deHoffmann‐Teller (HT) frame in which the electric field vanishes or nearly vanishes. Procedures have been developed for the determination of the initial velocity, vHTo, and acceleration aHT, assumed constant over the analysis interval, of that frame. When these effects are taken into account, an accurate n vector may often be obtained: in the trial case, error analysis and consistency tests indicate uncertainties of the order of 2°–3°, even though the variance in the normal magnetic field, B·n, in this crossing was large enough to render the minimum variance direction of B useless as a predictor of n. In general, it is not possible to determine the initial magnetopause normal speed, uno from the electric field data but the analysis places certain, sometimes narrow, bounds on uno and on the normal magnetic field, B·n, the tangential electric field, Ec × n, and the distance, h, along n traversed during the data interval, all of which are found to depend linearly on uno. For the trial case, the following results were obtained: −97 < uno < −88 km/s; 0 < B·n < 2.4 nT; 0 < |Ec × n| < 1.1 mV/m; 3036 < h < 3643 km (the actual magnetopause thickness was much less than h). The acceleration aHT in general has both a normal and a tangential component: for the test case (which had a duration of 69 s) both were of the order of 1 km/s², the former corresponding to a deceleration of the inward magnetopause motion, the latter to a decrease and slight rotation of the tangential component of vHT, an effect that is shown to have a significant influence on the tangential stress balance.
Pitch angle distributions of low‐energy ions in the near‐Earth magnetosphere
Authors: Sagawa E. , Yau A. W. , Whalen B. A. , Peterson W. K.
Published: February 2008

Low‐energy (<1 keV) ions have been frequently observed to have some field alignment in their pitch angle distributions in the magnetosphere at geosynchronous orbit or near the equatorial plane. Using data from the energetic ion composition spectrometer on Dynamics Explorer 1, acquired from September 1981 to March 1985, we have examined the occurrence probabilities of various forms of ion pitch angle distributions, including field‐aligned, conical, and equatorially trapped ions. For both H+ and O+, bidirectional field‐aligned distributions at low energy (<1 keV), with pitch angle peaks parallel and antiparallel to the field line direction, were most common at L shells less than about 6 in local times from postmidnight to dawn, except for H+ near the equator, where the equatorially trapped (pancake) distribution had a comparable or dominant occurrence probability. The presence of O+ suggests that these bidirectional ions have a terrestrial origin. Upflowing ion events, which are believed to be a major source of magnetospheric hot ions, were observed most frequently at higher L shells and earlier local times than the bidirectional ions. Bidirectional H+ ions with local minima near the field line (bidirectional conies) were observed to dominate their field‐aligned counterparts in the morning local times. This is attributed to the charge exchange loss of the field‐aligned ions with atmospheric hydrogen in their course of eastward drift. Bidirectional ions were rarely observed in the afternoon local times. This is interpreted as a loss of ions whose drift paths crossed the magnetopause into the magnetosheath.
Earthward plasma flow during near‐Earth magnetotail reconnection: Numerical simulations
Authors: Scholer Manfred
Published: February 2008

Magnetic field configuration and flow pattern during near‐Earth reconnection are studied in a realistic two‐dimensional magnetotail with two different MHD simulations: (1) reconnection is forced at a given location by a nonuniform external inflow from the high‐latitude boundary and (2) reconnection is initiated at a given location by applying initially a small localized resistivity which is subsequently switched off. In both cases we found essentially the same sequence of events, namely, onset of reconnection, plasmoid formation and subsequent tailward motion of the plasmoid. We studied in detail the Earthward flow speed during the reconnection process. It is found that large and supersonic Earthward flows occur Earthward of the X point. However, these flows occur only in the central current sheet after the plasma sheet has been pinched off and ejected tailward as a plasmoid. At this time the plasma and field characteristics in the central current sheet are lobelike. The Earthward flow diverges around the plasma sheet and constitutes a boundary layer. The speed in the central part of the plasma sheet Earthward of the X point reaches maximum velocities up to 0.2 in dimensionless units for about 20 unit times. In dimensional units this corresponds to maximum velocities of about 70 km/s for about 6 min.
Boundary conditions for the neutral and ionospheric transport equations at the base of the equatorial F region
Authors: Duhau S. , Louro A. A.
Published: February 2008

In previous works, the authors used a simple model of the equatorial E and F regions to predict simultaneously zonal neutral and ion velocities and vertical currents at the base of the F region. In this work, the model is extended to include effects not considered previously: ion‐neutral drag at the F region base, the E region dynamo electric field, and the F region zonal electric field. The model is used to predict the variables mentioned above as functions of time and latitude. The results of the calculations show first that the ion velocity is quite sensitive to the combination of the three effects, and a substantial improvement over previous results is found upon comparison with measured values. The calculated zonal neutral wind also reveals the influence of ion drag through its latitudinal variations. Finally, the current density perpendicular to the geomagnetic field exhibits two pronounced peaks of the order of 10−7 A m−2 at sunrise and sunset at the dip equator.
Evidence for slow mode MHD turbulence in the solar wind: Post‐slow shock observations at 0.31 AU
Authors: Richter A. K. , Luttrell A. H.
Published: February 2008

First observational evidence is presented for low‐frequency slow mode MHD turbulence in the solar wind (ƒ ≤ 5 × 10−3 Hz, δB‐δN coherence greater than 0.6) by employing power and coherence spectral analysis of the Helios high time resolution solar wind plasma and magnetic field observations. This turbulence occurs directly behind an interplanetary slow forward shock wave at 0.31 AU. As the foreshock plasma conditions are such that steepening of slow waves is favored in comparison to Landau damping, the observations presented here do in addition provide unique support to the theoretical results of Hada and Kennel (1985). It is also shown that the turbulence upstream is predominantly Alfvénic (ƒ≲ 10−2 Hz, 10−2 Hz, δB‐δV coherence greater than 0.94) and propagating away from the Sun (shock). Thus we provide an example of an interplanetary slow shock where the turbulence upstream and downstream is completely different in nature. Moreover, the “evolutionary conditions” for slow shocks indicate that the downstream (upstream) turbulence could not have been generated by an interaction and wave mode conversion of the upstream (downstream) turbulence with and by the slow shock, respectively.
Radio science with Voyager 2 at Uranus: Results on masses and densities of the planet and five principal satellites
Authors: Anderson J. D. , Campbell J. K. , Jacobson R. A. , Sweetnam D. N. , Taylor A. H. , Prentice A. J. R. , Tyler G. L.
Published: February 2008

We have analyzed radio Doppler data and star‐satellite imaging data from Voyager 2 at Uranus, along with 8 years of ground‐based astrometric data, and have obtained improved masses and densities for the satellites of Uranus as well as a new ratio of the mass of the Sun to the mass of the Uranian system of 22902.94 ± 0.04. The mean density of Uranus is 1.285 ± 0.001 g cm−3. The satellite densities are 1.25 ± 0.33 for Miranda, 1.55 ± 0.22 for Ariel, 1.58 ± 0.23 for Umbriel, 1.685 ± 0.068 for Titania, and 1.635 ± 0.060 for Oberon, all expressed in units of grams per cubic centimeter. The mean uncompressed density of all five satellites is 1.48 ± 0.06 g cm−3. This is 0.10 g cm−3 higher than the value expected for a homogeneous solar mix consisting of 34% anhydrous rock, 51% water ice, 7% ammonia ice, and 8% methane, present as clathrate hydrate. In order to reconcile this difference, we suggest that the Uranian moons contain roughly 15% by mass of pure graphite, in addition to a normal solar component of rocks and ices. If so, at least 50% of the carbon within the nebular gases from which Uranus and its satellites condensed was in the form of graphite, the remaining being in CH4. The high thermal conductivity of graphite ensures that the Uranian moons have remained cold and undifferentiated since the time of their formation, despite heating caused by the decay of radioactive nuclides. Apparently, an alternative cometary origin for the satellites is ruled out.
Ordinary mode radio emission from Uranus
Authors: Desch M. D. , Kaiser M. L.
Published: February 2008

On January 24, 1986, during the inbound trajectory to Uranus, the planetary radio astronomy instrument on board Voyager 2 detected left‐hand polarized emission for several hours at frequencies near 200 kHz. Unlike the dominant source observed on the nightside of the planet, only a single episode of this inbound emission was seen, and the emission was extremely weak. Generating less than 500 kW, it is probably the weakest freely propagating planetary radio emission thus far observed by Voyager. We associate this weak emission with a source region in the vicinity of Uranus' north (dayside) magnetic pole, where magnetic field lines point approximately in the direction of the spacecraft when the emission is observed. Therefore the wave normal angle relative to B is probably less than 90° in the generation region. We conclude that direct wave generation in the magnetoionic ordinary mode is most likely. The source location for this emission is consistent with a region magnetically conjugate to the dominant nightside source, which is located in the vicinity of the south magnetic pole. The ordinary mode emission is discussed in the context of the electron cyclotron maser mechanism.
Equatorial particle precipitation during magnetic storms and relationship to equatorial thermospheric heating
Authors: Tinsley B. A. , Sahai Y. , Biondi M. A. , Meriwether J. W.
Published: February 2008

We compare optical observations of ring current particle precipitation from a near equatorial site in Brazil with variations in the AE and Dst indices. Precipitation occurs during ring current injection when AE is large, but for AE large without ring current injection, little precipitation is seen. Comparisons are also made with 630‐nm observations of thermospheric heating made simultaneously in Peru. The thermospheric temperatures have been corrected for instrumental drift and temperature enhancements of several hundred kelvins are present. It is not possible to associate clearly the heating with direct precipitation nor in the short term with transport of heat from higher latitudes. On a longer time scale it is likely that both mid‐latitude ring current sources and high‐latitude Joule heating sources are involved.
The Alfvén wave equation for magnetospheric plasmas
Authors: Uberoi C.
Published: February 2008

It is shown that besides the continuous spectrum which damps away as inverse power of time, the coupled Alfvén wave equation, which gives coupling between a shear Alfvén wave and a surface wave, can also admit a well behaved harmonic solution in the closed form for a set of initial conditions. This solution, though valid for finite time intervals, points out that the Alfvén surface waves can have a band of frequency (instead of a monochromatic frequency for a nonsheared magnetic field) within which the local field line resonance frequency can lie, and thus can excite magnetic pulsations with latitude‐dependent frequency. By considering magnetic fields not only varying in magnitude but also in direction, it is shown that the time interval for the validity of the harmonic solution depend upon the angle between the magnetic field directions on either side of the magnetopause. For small values of the angle the time interval can become appreciably large.
Temperature gradients and velocity shears: Free energy sources for turbulence in the boundary layer
Authors: Migliuolo Stefano
Published: February 2008

The roles played by temperature gradients and velocity shears in causing turbulence in the plasma sheet boundary layer are examined. Both are sources of free energy that can be tapped by perturbations, which can then grow and result in turbulence, thus generating anomalous particle and energy transport. It is found that ion temperature gradients and ion velocity shear play parallel roles in destabilizing a unique low‐frequency mode, the ion acoustic wave which may be termed a Kelvin‐Helmholtz or temperature gradient instability, depending on which free energy source is dominant. There never is more than one unstable electrostatic mode present, driven by these two sources. A general expression for the critical velocity shear is presented, and the critical value(s) of the temperature gradient is (are) discussed. The finite‐β modifications to these instabilities of the ion acoustic wave and their quasi‐linear feedback on the plasma are discussed.