bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparison of coronal and interplanetary current sheet inclinations
Authors: Behannon K. W. , Burlaga L. F. , Hundhausen A. J.
Published: February 2008

HAO white‐light K‐coronameter observations show that the inclination of the heliospheric current sheet at the base of the corona can be both large (nearly vertical with respect to the solar equator) or small during Carrington rotations 1660–1666 and even on a single solar rotation. We discuss Voyager 1 and 2 magnetic field observations of crossings of the heliospheric current sheet at distances from the sun of 1.4 and 2.8 AU. Two cases are considered: one in which the corresponding coronameter data indicate a nearly vertical (north–south) current sheet and another in which a nearly horizontal near‐equatorial current sheet is indicated. For the crossings of the vertical current sheet a variance analysis based on hour averages of the magnetic field data gave a minimum variance direction consistent with a steep inclination. The horizontal current sheet was observed by Voyager as a region of mixed polarity and low speeds lasting several days, consistent with multiple crossings of a horizontal but irregular and fluctuating current sheet at 1.4 AU. However, variance analysis of individual current sheet crossings in this interval, using 1.92‐s averages, did not give minimum variance directions consistent with a horizontal current sheet. We conclude that one cannot assume that the minimum variance direction will be the same as the normal to the current sheet when the analysis results are likely to be influenced by small‐scale variations or curvatures within or near the sheet proper. This influence may be more pronounced when the sheet is locally nearly horizontal.
Large‐scale counterstreaming of H+ and He+ along plasmaspheric flux tubes
Authors: Richards P. G. , Schunk R. W. , Sojka J. J.
Published: February 2008

An interhemispheric plasma transport model has been used to study the flow characteristics of H+, He+, and O+ along closed geomagnetic field lines for solstice conditions at noon local time. The calculations were carried out for the flux tube that passes through Millstone Hill. Both symmetric and asymmetric neutral winds in the conjugate ionospheres were considered. Initially, the flux tube content was partially depleted, and the subsequent refilling was studied until a steady state flow was established between the winter and summer hemispheres. The main conclusion to be drawn from this study is that H+‐He+ counterstreaming can be expected along a large segment of a plasmaspheric flux tube at solstice. For both symmetric and asymmetric wind patterns, the He+ flow is from the winter to the summer ionosphere not only in the steady state, but during flux tube refilling owing to the winter helium bulge and the depletion of N2. For symmetric poleward neutral winds in both hemispheres, H+ and O+ flow up from the conjugate ionospheres during flux tube refilling, which leads to large‐scale H+‐He+ counterstreaming in the summer hemisphere. In the steady state, both H+ and He+ flow from the winter to the summer ionosphere and no light‐ion counterstreaming occurs. When the neutral wind in the summer hemisphere is set to zero, which acts to reduce the F region ‘winter anomaly,’ H+‐He+ counterstreaming occurs in the summer hemisphere during refilling and along the entire flux tube in the steady state. The H+ and He+ counterstreaming velocities obtained are too small to excite plasma instabilities, but large enough to be measured.
On the origin of plasmaspheric hiss: Ray path integrated amplification
Authors: Church S. R. , Thorne R. M.
Published: February 2008

An investigation has been made of processes responsible for the growth, damping, and propagation characteristics of whistler mode emissions within the earth's plasmasphere. Three‐dimensional ray path calculations of the integrated wave gain in a realistic model plasmasphere have provided an explicit confirmation of the anisotropic electron cyclotron resonant generation mechanism for the maintenance of plasmaspheric hiss. Peak wave amplification occurs for field‐aligned waves in the outer plasmasphere. The majority of unstable low‐frequency waves follow ray trajectories that internally reflect at the plasmapause and are thus trapped within the plasmasphere. This reflection allows waves to subsequently propagate into the locally stable inner and middle plasmasphere and also migrate substantially in longitude. The combination of internal reflection at the plasmapause and magnetospheric reflection at high latitude provide an important class of wave trajectories that are recycled back to the equatorial growth region with sufficiently small wave normal angles to allow further cyclotron resonant amplification. It is unlikely that wave growth can occur from the natural incoherent emissivity level within the plasmasphere since the required gains (≈100 dB) would mandate unreasonably high energetic electron flux. An alternate, as yet unidentified, embryonic source is therefore required to initiate the growth process. But once established the observed spectrum of broad band plasmaspheric hiss can be maintained by the modest net gains anticipated along ray trajectories that are recycled back to the favorable equatorial growth region. A simulation of the spectral properties of waves arriving at selected observation points following several equatorial transits shows that the largest accumulated gain occurs for recycled waves that reflect from the plasmapause following each equatorial crossing. Furthermore, the peak gain is relatively insensitive to the location of the observation point, and waves are expected to arrive over a broad solid angle in k space consistant with observations. The ability of waves to gain access to regions far from the source is a direct result of the relatively weaker Landau damping of oblique waves along the high latitude portion of the recycled ray path in comparison to the equatorial cyclotron resonant growth. The hiss upper frequency cutoff can result from the limited anisotropy of the cyclotron resonant electrons, the more severe Landau damping at higher frequencies, or in part, from the decreased ability of the higher frequency waves to recycle and thus experience multitransit growth. The characteristic lower frequency cutoff can be due to either the decrease in flux of energetic electrons that cyclotron resonate with the lower frequency waves or from thermal electron Landau damping should the plasmaspheric electron temperature exceed several electron volts.
Anomalous resistivity due to low‐frequency turbulence
Authors: Rowland H. L. , Palmadesso P. J.
Published: February 2008

Large amplitude ion cyclotron waves have been observed on auroral field lines. In the presence of an electric field parallel to the ambient magnetic field these waves prevent the acceleration of the bulk of the plasma electrons leading to the formation of a runaway tail. It is shown that low‐frequency turbulence can also limit the acceleration of high‐velocity runaway electrons via pitch angle scattering at the anomalous Doppler resonance.
Dependence of auroral FUV emissions on the incident electron spectrum and neutral atmosphere
Authors: Strickland D. J. , Jasperse J. R. , Whalen J. A.
Published: February 2008

In this paper we examine the relationship among certain prominent auroral FUV emission features, the incident electron spectrum, and the model neutral atmosphere. Given the neutral atmosphere, we show that for simple models of the incident electron spectrum (Maxwellian and Gaussian in energy), satellite measurements of FUV emission features, in principle, determine the incident electron spectrum. We also discuss the relationship between the incident electron spectrum and the E region plasma density profile for the continuous (diffuse) aurora and for a stable arc.
An analytic solution for the response of the neutral atmosphere to the high‐latitude convection pattern
Authors: Mikkelsen I. S. , Larsen M. F.
Published: February 2008

We develop the analytic solutions to a two‐dimensional, shallow‐water model of the high‐latitude thermosphere. The equations are a linearized representation of the response of the neutral atmosphere to the momentum source due to the plasma convection pattern. Our results show how the neutral winds depend on the Pedersen and Hall ion drag coefficients and the horizontal wave number. The response depends on the dimensionless scale as it appears in the product of the Rossby radius of deformation and the wave number. The flow pattern is shifted in local time from the plasma flow pattern by an amount that depends on the average electron density. At the largest scales there is a 180° phase shift between the rotational flow components in the E and F regions. The model applies to quiet magnetic conditions and below 200 km of altitude.
The Voyager Mission: Encounters with Saturn
Authors: Stone E. C.
Published: February 2008

The following reports summarize a number of post‐Jupiter aspects of the first phase of the Voyager Mission, a major element in NASA's program of exploration of the outer planets. The objectives of this phase included comparative studies of the Jovian and Saturnian systems and studies of the inter‐planetary medium at increasing distances from the sun. Scientific reports on the mission through the Jupiter encounters may be found in the Journal of Geophysical Research (86, 8123–8841, 1981). With the completion of the mission through Saturn and with both spacecraft continuing to function well, a second mission phase was added with the objectives of exploring Uranus and investigating the interplanetary medium to beyond 20 AU. The Uranus flyby has been chosen so that a third phase can include the exploration of Neptune and extend the investigation of the interplanetary medium to distances beyond 40 AU and at distances of more than 20 AU above the ecliptic.
Voyager 2 observations of Saturn's northern mid‐latitude cloud features: Morphology, motions, and evolution
Authors: Sromovsky L. A. , Revercomb H. E. , Krauss R. J. , Suomi V. E.
Published: February 2008

Voyager 2 images provide a basis for detailed study of the morphology and circulation of Saturn's northern mid‐latitudes. Both Saturn's large‐scale cloud bands and the distribution of its local cloud features have a characteristic zonal organization. The region between 30°N and 45°N contains two oppositely directed jets in close proximity, with many bright, active features in the westward jet, and an unusual ribbon‐like wave feature encircling the planet in the eastward jet. Several of the smaller features within the westward jet do not remain at fixed latitudes and interact with each other. One group of v‐shaped features is found to have periods of high activity correlated with the passage of a cyclonic bright spot. The ribbon wave was Fourier analyzed to determine its spectral composition. The greatest power is near wave number 9, with significant additional peaks appearing at planetary wave numbers 19, 25–27, 35–38, and 47–51. The phase velocity increases with wave number but is not well described by a Rossby‐Haurwitz dispersion relation. The curvature of the mean wind profile obtained from cloud tracking indicates that the westward jet exceeds the standard barotropic instability condition, while the eastward jet marginally exceeds the deep‐circulation instability condition of Ingersoll and Pollard (1982). The rms eddy velocities on Saturn are less than half as large as those observed on Jupiter.
Saturn's upper atmosphere from the Voyager 2 Euv solar and stellar occultations
Authors: Smith Gerald R. , Shemansky D. E. , Holberg J. B. , Broadfoot A. L. , Sandel B. R. , McConnell John C.
Published: February 2008

The temperature and composition of the upper atmosphere of Saturn have been inferred from Voyager 2 ultraviolet spectrometer occultation measurements made by observing the sun and the star δ‐Scorpii while they were being occulted by Saturn. The observations analyzed here provide atmospheric parameters from 2900 km down to 960 km above the 1‐bar level referred to the equator. The temperature in the model simulation of the data is 420±30 K down to about 1600 km. Below 1600 km the temperature decreases with a variable lapse rate down to 120±30 K near the methane homopause located at 1010±40 km. A constant temperature at 120 K is applied in the model for the region of the methane homopause down to 960 km where the present analysis terminates. Column amounts of H2 and H were measured from 2900 km down to about 1100 km, giving densities of [H2] ∼ 1.0×108 cm−3 and [H] ∼ 5.5×106 cm−3 near the exobase at 2500 km. Near the methane homopause the H2 density is [H2] = 1.2 × 1012 cm−3 with a CH4 number density mixing ratio of 6.0×10−5. The eddy diffusion coefficient in the vicinity of the methane homopause is estimated to be ∼5.0×106 cm² s−1. The atomic hydrogen density profile suggests a downward H flux of ∼1.8×109 cm−2 s−1 from near the exobase down to a terminal boundary of about 1200 km.
Saturn's small satellites: Voyager imaging results
Authors: Thomas P. , Veverka J. , Morrison D. , Davies M. , Johnson T. V.
Published: February 2008

Voyagers 1 and 2 provided images of sufficient resolution for morphologic and photometric studies of Saturn's small satellites. These objects, all very difficult to observe from earth, orbit Saturn at distances of 2.3 to 6.3 Rs (just outside the A ring to the orbit of Dione) and range in mean diameter from 22 to 188 km. All are irregularly shaped (long/short axis ratios range from 1.4 to 2.0) and probably heavily cratered. While impacts have apparently been important in shaping these objects, observed crater densities suggest that the present forms may have survived for about 4 billion years. Geometric albedos vary from 0.4 to at least 0.8. These albedos and the few color data available are similar to those of larger Saturn satellites which are known to have surfaces made predominantly of water ice. The range of observed albedos could be explained by minor variations in the amount of dark, opaque contaminants.
The Z3 zonal harmonic model of Saturn's magnetic field: Analyses and implications
Authors: Acuña M. H. , Connerney J. E. P. , Ness N. F.
Published: February 2008

The planetary magnetic field of Saturn has been studied by the spacecraft Pioneer 11 in 1979, Voyager 1 in 1980, and Voyager 2 in 1981. The field is found to be primarily dipolar and axially coincident with the rotation axis but with significant quadrupole and octupole moments. The harmonic terms are g10 = 21535 nT, g20 = 1642 nT, and g30 = 2743 nT. This model field, Z3, in conjunction with a model for an equatorial ring current represents very precisely the in situ magnetic field measurements and data on charged particle absorption by satellites and rings within 8 Rs of the planet. However, this axisymmetric model fails to explain the periodic modulation of Saturn's kilometric radiation (SKR) or Saturn's electrostatic discharges (SED). This enigma of Saturn's magnetosphere remains unsolved in spite of extensive reconsideration of all available data bearing on this issue.
A survey of electrostatic waves in Saturn's magnetosphere
Authors: Kurth W. S. , Scarf F. L. , Gurnett D. A. , Barbosa D. D.
Published: February 2008

The Voyager 1 and 2 plasma wave instruments have provided initial observations of electrostatic waves in Saturn's magnetosphere. In general, the emissions at Saturn are similar to those found at earth and Jupiter, although there are significant differences in some of the detailed characteristics. In this paper we present an overview of the various types of electrostatic waves in the Saturnian magnetosphere, including Langmuir waves and electron cyclotron harmonic emissions. We shall summarize the temporal and spectral character, amplitude, and regions of occurrence for the various classes of emissions. These characteristics are compared with those of the terrestrial and Jovian counterparts with the goal of understanding how major differences in the magnetospheric configuration might contribute to the observed differences. Finally, we use the theory of electron cyclotron harmonic emissions to try to gain an insight into the electron distributions and possible wave‐particle interactions in Saturn's magnetosphere.
Conductive damping of coronal motions
Authors: Suess Steven T.
Published: February 2008

Time‐dependent motions in the solar corona are subject to modification through conductive transport of thermal energy away from compression regions and into rarefaction regions. The evolution depends on how the energy is introduced—by an essentially thermal perturbation or by a density perturbation. The net effect of thermal conduction is to convert local kinetic energy in the transient into widely distributed thermal energy, which then may in turn be partially converted into kinetic energy of the bulk flow. Local motion in the transient is thereby damped. These effects are modeled here using a numerical solution of the time‐dependent solar wind equations for single‐fluid thermally conductive flow. The simulations of transient phenomena are made for ordinary collisional thermal conduction and compared with similar simulations done using polytropic flow. Between one and five solar radii, the gross character in the two cases is very similar. However, there are effects with thermal conduction that cannot be modeled with polytropic flow, illustrated here by temperature and velocity forerunners and a delayed and large‐amplitude trailing velocity low. If thermal conduction is artifically inhibited by simple reduction of the thermal conduction coefficient, relative velocity amplitudes remain larger in approximate proportion to the amount the coefficient is reduced.
Mechanisms that limit potentials on ionospheric satellites
Authors: Parks D. E. , Katz I.
Published: February 2008

For spinning satellites with dielectric coatings it is shown that the currents of rotating charge embedded in the dielectric surface can play a role in limiting the spacecraft ground potential of an ionospheric satellite relative to the surrounding plasma. Such currents, as well as small concentrations of H+, can account for previous discrepancies of more than a factor of 2 between measured and calculated potentials of satellites in the region of the ionosphere where O+ is dominant. More generally, dielectric conduction can also limit the satellite potential. The importance of such small currents as result from spin and conduction stems from the extremely small ion currents impinging on the wakeside of the satellite. Finally, theoretical estimates based on a newly described constant of the motion of a particle indicate that accounting for small concentrations of H+ remove the major discrepancy between calculated and measured currents on the wakeside of the AE‐C satellite.
Seasonal variations of lower thermospheric winds from the Millstone Hill Incoherent Scatter Radar
Authors: Wand Ronald H.
Published: February 2008

From July 1976 to November 1977 regular incoherent scatter observations of E and F region ion‐drifts were conducted at Millstone Hill (42.6°N) with the steerable (L band) and fixed (UHF) radars. Data were collected on a total of 46 days and analyzed for ionospheric electric fields and daytime neutral winds in the lower thermosphere at 105, 115, 125, and 135 km. The wind measurements characteristically exhibited the dominance of the semidiurnal oscillation with both wind components, showing a 12‐hour variation and an eastward wind maximum 3 hours ahead of a southward wind maximum. A model fit to the full set of wind measurements also allowed the smaller annual mean diurnal winds to be determined. The winds measured on 38 geomagnetically quiet days were grouped by day‐number to determine the seasonal dependence of the semidiurnal tide as well as the prevailing wind. The semidiurnal winds exhibited the following general characteristics. They typically grew in amplitude reaching a maximum in the vicinity of 125 km with larger values at the equinoxes than the solstices. The wind field at 105 km showed a significant polarization, with southward wind amplitudes larger than eastward amplitudes. Seasonal variations in tidal phase were generally less than one hour. The amplitude and phase of the measured diurnal wind were in reasonable agreement with theoretical expectations for the in situ tidal component, and there was no indication of the presence of an upward propagating diurnal component. On the other hand, the measured semidiurnal winds were consistent with being the result of the upward propagation of tidal energy from below 100 km rather than in situ tidal excitation. Attempts to match the measured vertical wind structure with that expected for the different semidiurnal Hough mode extensions in the lower thermosphere lead to no definite mode identification. The measured vertical wavelengths (70–100 km) and the altitude of maximum amplitude were in best agreement with the S2,2 mode, while the temperature/wind ratio suggested the S2,4 A mode. There is overall agreement of the Millstone Hill southward wind measurements with results from the French incoherent scatter radar at Saint Santin, but differences are noted as to the seasonal variation, the altitude of maximum wind, and the vertical wavelength. Some of these differences might be explained by a solar cycle dependence of tidal dissipation in the lower thermosphere.
Geomagnetic activity effects on semidiurnal winds in the lower thermosphere
Authors: Wand Ronald H.
Published: February 2008

Over the period from July 1976 to November 1977 regular incoherent scatter observations of E and F region ion‐drifts were conducted at Millstone Hill (42.6°N) by using the steerable (L band) and fixed (UHF) radars. Data were collected on a total of 46 days and analyzed to give the ionospheric electric field and daytime neutral winds in the lower thermosphere at 105, 115, 125, and 135 km. The wind measurements characteristically exhibited strong 12‐hour oscillations with the eastward wind component reaching a maximum 3 hours ahead of the southward wind component. These results imply that the neutral winds are being dominated by semidiurnal tides. The geomagnetic activity dependence of the semidiurnal winds was determined by dividing the observations into quiet and disturbed periods on the basis of the Kp index. During disturbed conditions the winds were found to be significantly altered from the quiet time behavior, with the semidiurnal amplitudes reduced by 20 to 50% at the lowest altitudes and the altitude of maximum wind increased to be above 125 km. The vertical wavelength also appeared to decrease under disturbed conditions. Some of these effects might be explained by geomagnetic‐activity induced changes in lower thermospheric temperature and density, which alter the dissipation of the upward propagating tidal component. Changes in the prevailing winds also appear to be associated with Kp increases, implying a modification of the general thermospheric circulation by high‐latitude heating. The incremental winds are small (∼5m/s) and eastward/equatorward at 105 km, becoming stronger ( ∼25m/s) and westward/equatorward at higher altitudes.
The coupling of shear Alfvén and compressional waves in high‐β magnetospheric plasma
Authors: Ng P. H. , Patel V. L.
Published: February 2008

The coupling of the compressional instabilities in a two‐component, hot and cold inhomogeneous plasma to the resonant shear Alfvén mode is studied in a slab geometry with straight field lines. The lowest order effect of the field inhomogeneity is incorporated into the dispersion relation by retaining the ▽B drift of the particles. It is found that the high‐phase velocity compressional mode with ω/k∥ > V∥i shows insignificant coupling to the shear Alfvén wave, while the low‐phase velocity branch with ω/k∥ < V∥i shows significant coupling. It is also shown that the coupling factor is of the form β∥i nc/nik⊥²rB², so that a more realistic field geometry should be used to model the magnetospheric field in the study of low‐frequency geomagnetic pulsations.
Incoherent scatter measurements of ion‐neutral collision frequencies and temperatures in the lower thermosphere of the auroral region
Authors: Lathuillere Chantal , Wickwar Vincent B. , Kofman Wlodek
Published: February 2008

Incoherent scatter observations performed in March and November 1978 at Chatanika have been used for studying the lower thermosphere in the auroral region. Neutral temperatures and densities have been found during periods without Joule heating. We present mean profiles of temperatures and collision frequencies (approximately proportional to neutral densities) for each month and profiles obtained during four specific nights. Between 93 km and 110 km the mean profiles of temperature are in good agreement with the Jacchia (1971) model, and the profiles of collision frequency are similar to those deduced from the model. Consistency checks between these collision frequencies and temperatures were performed, as were various simulations of the data. Neutral temperature profiles between 90 km and 140 km on the four specific nights show variations from one night to another that are not correlated to magnetic activity. However, there are systematic variations during each night that we suggest are due to atmospheric tides.
The dispersion of traveling ionospheric disturbances
Authors: Morgan M. G. , Tedd B. L.
Published: February 2008

The horizontal dispersion of relatively large daytime TID events seen by the Dartmouth rapid‐run ionosonde network is found to conform, at least qualitatively, to that predicted by previous theoretical treatments of freely propagating gravity waves in the presence of dissipation. The availability of TID vertical trace speeds, in addition to horizontal speeds, has allowed a direct comparison with gravity wave dispersion relations and dissipative criteria. Good agreement is found with theory for waves measured at the height of maximum gravity wave amplitude, but rather less convincing results are found at greater heights. The latter may be explained in part by the fact that ion diffusion was neglected in the inversion technique employed to derive the vertical component of the phase propagation vector of the gravity wave from that of the TID.
Ion cyclotron waves: Direct comparison between ground‐based measurements and observations in the source region
Authors: Perraut S. , Gendrin R. , Roux A. , de Villedary C.
Published: February 2008

Simultaneous measurements of ion cyclotron waves (ICW's) were performed on GEOS spacecraft and in the vicinity of their magnetic footprints with the French Mobile station. The detailed comparison between the two sets of data shows that while ICW's having F < FHe, the He+ gyrofrequency at the equator, generally propagate to the ground, only 50% of those generated above FHe can reach the ground station. It is shown that these results are in good agreement with the conclusions that Rauch and Roux [1982] drew on the basis of measurements reported by Young et al. [1981]. In an He+‐rich plasma, ICW's with F > FHe suffer a reflection where the frequency locally matches the local bi‐ion hybrid frequency. We extend the calculations of Rauch and Roux and calculate, as a function of the He+ concentration, the tunnelling of ICW's through the stopband induced by the presence of minor He+ ions. It is shown that the transmission coefficient strongly depends upon the wave frequency for a given He+ abundance ratio. The results obtained are shown to be supported by existing observations.
A statistical study of F region ion temperatures at high latitudes based on Atmosphere Explorer C data
Authors: St.‐Maurice J.‐P. , Hanson W. B.
Published: February 2008

We have used 40000 simultaneous ion temperature and drift measurements on Atmosphere Explorer C to study the ion energy balance in the high‐latitude F region. The large number of measurements allowed us to find how the mean ion temperature, and the average deviation from that mean, change as a function of ion drift speed during solar minimum conditions. We found that in spite of an apparent large scatter in the data, the measurements could always be reasonably explained in terms of fluctuations in the neutral dynamical properties, to which the ion energy is coupled. The results also provide evidence for increased neutral drifts as the ion E × B drift increases. Evidence for systematic ion drag effects can also be found, on the average, for dc electric fields stronger than 60 mV/m.
Simultaneous observations of ELF waves from an artificially modulated auroral electrojet in space and on the ground
Authors: James H. G. , Dowden R. L. , Rietveld M. T. , Stubbe P. , Kopka H.
Published: February 2008

Radio waves at frequencies between 525 and 5850 Hz were observed simultaneously on the ground and in the VLF receiver of the ISIS 1 spacecraft during a flight over the Max‐Planck‐Institut für Aeronomie ionospheric heater near Tromsø, Norway, on December 9, 1981. The heater carrier frequency, 4.04 MHz, was amplitude modulated with a set of four frequencies: 525, 1725, 2925, and 4125 Hz. The satellite detected all these fundamental frequencies plus harmonics of the 525 Hz and 2925 Hz components, caused by the nonsinusoidal modulation of the carrier. Characteristics of the signal received along the satellite track are in approximate agreement with the results of three‐dimensional ray tracing applied to a model of the ionosphere around Tromsø. The time dispersion of the signals is best fitted with an ionospheric density model based on real time data with relatively low peak values. Observed signal levels have been transformed to power flux by using a plasma dipole theory and wave polarization information obtained from the three‐dimensional ray solutions. Through the use of a magnetoplasma dipole theory for the induced D/E region current, the observed levels are found to correspond to radiation created by effective dipole currents between about 1.0 and 100 A. The geometrical optics theory also predicts two solutions for rays reaching a given satellite point near the polar limit of the reception zone, and a northern limit corresponding to a caustic surface. The frequencies of signal fades observed in this region are also predicted by the geometrical optics theory, but the fade depths are not. Simultaneous ground observations correspond to source region currents of about 1 A or less. Also, the ELF waves observed on the ground have different harmonic signal ratios and much smaller fluctuations than those observed on ISIS.
The ion cyclotron dispersion relation in a proton‐alpha solar wind
Authors: Isenberg Philip A.
Published: February 2008

Recent models of preferential acceleration and heating of heavy ions in the solar wind have invoked a resonant interaction with parallel‐propagating ion cyclotron waves. However, these waves must have frequencies in the cold plasma stop band (Ωα < ω < ωc) to accelerate alpha particles to speeds faster than the bulk plasma. Such waves cannot exist in a plasma containing cold alpha particles. We investigate solutions to the WKB warm plasma dispersion relation in a proton‐alpha plasma to determine when parallel ion cyclotron waves can propagate at ω = Ωα. We present examples of the behavior of the dispersion curves in this resonant regime for several types of plasma states appropriate to the fast solar wind. We find that ion cyclotron waves can propagate at ω = Ωα for a bounded set of plasma parameters. However, use of the correct dispersion relation is likely to make resonant cyclotron acceleration of solar wind alpha particles less feasible as a mechanism to produce the observed preferential effects.
Collisionless dissipation in quasi‐perpendicular shocks
Authors: Forslund D. W. , Quest K. B. , Brackbill J. U. , Lee K.
Published: February 2008

Microscopic dissipation processes in quasi‐perpendicular shocks are studied by two‐dimensional plasma simulations in which electrons and ions are treated as particles moving in self‐consistent electric and magnetic fields. Cross‐field currents induce substantial turbulence at the shock front reducing the reflected ion fraction, increasing the bulk ion temperature behind the shock, doubling the average magnetic ramp thickness, and enhancing the upstream field aligned electron heat flow. The short scale length magnetic fluctuations observed in the bow shock are probably associated with this turbulence.
Planetary Mach cones: Theory and observation
Authors: Slavin J. A. , Holzer R. E. , Spreiter J. R. , Stahara S. S.
Published: February 2008

This study uses observations by a number of spacecraft to investigate the asymptotic behavior of planetary bow shocks. Toward this end a single standard method has been used to model distant bow shock position and shape. Mach cone angles of 13.9±2°, 11.4±3°, and 8.1±4° at Venus, Earth, and Mars, respectively, were determined from the observational shock models. These cone angles and their decrease with growing distance from the sun are consistent with downstream bow shock position being limited by the MHD fast mode Mach number. Gas dynamic solutions for solar wind flow about Venus, Earth, and Mars were computed up to 50 ROB (i.e., obstacle radii) behind each planet and compared with observed bow shock location. In each case the position of the shock was well predicted up to a certain distance downstream: −4 ROB at Venus, −6 ROB at Earth, and −10 ROB at Mars. Beyond this point the observed shock position lies farther from the aberrated sun‐planet line than the gas dynamic model with the discrepancy greatest at Venus and least at Mars. The better agreement between gas dynamic theory and observation with growing distance from the sun is attributed to an increase in the accuracy of the gas dynamic approximation with decreasing IMF strength.
The distribution of reconnection geometry in flux transfer events using energetic ion, plasma and magnetic data
Authors: Daly P. W. , Saunders M. A. , Rijnbeek R. P. , Sckopke N. , Russell C. T.
Published: February 2008

A survey of the locations of flux transfer events at the dayside magnetopause has been undertaken with emphasis on the direction of the energetic ion anisotropy along the magnetic field. The instrument used is the medium energy particle spectrometer on board ISEE 2. The events studied are from 1977 and 1978, giving coverage over the morning and afternoon sides within latitude range −10° to 40°. The anisotropy is an unambiguous indicator of the hemisphere from which the observed particles are escaping. Magnetic data from the flux gate magnetometer and plasma data from the fast plasma experiment have also been studied. The magnetic signature can be used to decide the direction of motion of the event with respect to the ambient field, while the change in plasma bulk velocity allows one to estimate the direction of magnetic tension in the reconnected field line. Apparent disagreements between the magnetic signature (positive‐negative indicating northward motion) and particle anisotropy (parallel to the field indicating southern connection) are resolved by the plasma data, which show that the bulk (northward) plasma speed exceeds the Alfvén speed, and thus these south connected events are blown northward faster than magnetic tension pulls them southward. The magnetic signature is therefore an unreliable indication of the hemisphere of connection. With this effect taken into consideration the overall pattern is that of an equatorial rather than a high‐latitude source for flux transfer events.
Distribution of solar wind angular momentum between particles and magnetic field: Inferences about the Alfvén critical point from Helios observations
Authors: Marsch E. , Richter A. K.
Published: February 2008

We first discuss theoretically the relative importance and the behavior of the two basic terms adding to the total angular momentum flux, the angular momentum of the particles (electrons, protons and alpha particles) and of the magnetic field stresses, respectively. Second, we analyze these two quantities with respect to their dependence on heliocentric distance by using the Helios 1 and 2 plasma and interplanetary magnetic field observations between 0.3 and 1 AU classified according to low‐speed (<400 km s−1), intermediate (400–600 km s−1), and high‐speed (>600 km s−1) solar wind for the 1975–1976 epoch. Applying now these results as well as various combinations of the constants of motion for the solar wind (such as the total angular momentum flux, the mass flux, and the magnetic flux) and their observational constraints, as deduced earlier by Marsch and Richter (1984), we finally present various methods (1) to derive the values of several characteristic solar wind plasma and magnetic field parameters at the Alfvén critical points, (2) to estimate their locations above the solar surface, and (3) to obtain the radial slope of the associated solar wind velocity profiles for the three solar wind classes separately.
A reexamination of rotational and tangential discontinuities in the solar wind
Authors: Neugebauer M. , Clay D. R. , Goldstein B. E. , Tsurutani B. T. , Zwickl R. D.
Published: February 2008

ISEE 3 magnetic field and proton data are used to study the properties of rotational and tangential discontinuities in the solar wind. A Sonnerup‐Cahill minimum variance analysis of the magnetic field data is used to determine the direction of the normal to each discontinuity. The discontinuities are then classified as rotational (RD), tangential (TD), or either (ED), depending on the relative values of the normal field component and the change of field magnitude across them. This process yields substantially more RD's than TD's, in agreement with earlier studies using this method of classification. Other field and plasma parameters are then examined for each of these three groups. The field magnitude passes through a local minimum while the field direction is changing for some TD's but not for RD's. The first and second adiabatic invariants for protons and the helium abundance are approximately conserved across RD's but not across TD's, although the helium abundance is observed to change at a small fraction of the RD's. The product of plasma density times the anisotropy factor tends to be conserved across all three types, The relative directions of the velocity and field changes across all three types of discontinuity are consistent with the propagation of RD's outward from the sun, even though no such relation is predicted for TD's. The magnitude of the velocity change at RD's is smaller than that predicted by MHD theory, and the use of a two‐stream fit to the proton data reduces, but does not remove, this discrepancy. The speeds of the alpha particles and the secondary proton beam relative to the primary proton beam result in little interaction between the alphas and RD's, while the primary and secondary proton beams flow through RD's in opposite directions and have oppositely directed velocity changes. The plasma conditions at ED's show a closer resemblance to RD's than to TD's.
Associations of geomagnetic activity with plasma sheet thinning and expansion: A statistical study
Authors: Hones E. W. , Pytte T. , West H. I.
Published: February 2008

Associations of geomagnetic activity in the auroral zone with thinnings and expansions of the magnetotail plasma sheet are examined statistically in this paper. We first identified many plasma sheet thinnings and expansions in plasma and particle data from VELA satellites and from OGO 5 without reference to ground magnetic data. These events were grouped according to the location of the detecting satellite in the magnetotail. For each such group the times of thinning or expansion were then used as fiducial times in a superposed‐epoch analysis of the geomagnetic AL index values that were recorded in 8‐hour intervals centered on the event times. The results show that many plasma sheet thinnings and expansions are related to discrete negative bay structures that are the classical signature of substorms. Furthermore, they support earlier findings that plasma sheet thinning and expansion at the VELA orbit (r ≈ 18 RE) tend to be associated with the onset of the auroral zone negative bay and the beginning of its subsidence, respectively. Earthward of r ≈ 13–15 RE, plasma sheet expansion occurs near the time of the onset of the negative bay, again in agreement with earlier findings. A large fraction of plasma sheet expansions to half thicknesses of ≳ 6 RE at the VELA orbit are associated not with a baylike geomagnetic disturbance but with subsidence of a prolonged interval of disturbance. The study also shows that many plasma sheet expansions are related simply to generally enhanced geomagnetic activity showing no baylike or other distinctive features.
Longitudinal structure in Pi 2 pulsations and the substorm current wedge
Authors: Lester Mark , Hughes W. Jeffrey , Singer Howard J.
Published: February 2008

The characteristics of mid‐latitude Pi 2 pulsations are studied using a subauroral east‐west chain of magnetometers which spans over 4 hours local magnetic time. The mid‐latitude magnetic bays associated with the Pi 2 pulsations are used to define a longitudinal coordinate system based on the substorm current wedge model. The wave properties, frequency, polarization, and east‐west phase variations are examined in this coordinate system. This allows us to compare the wave characteristics at longitudes between the meridians of the two field‐aligned currents of the substorm current wedge with the characteristics both east and west of these meridians. In an attempt to minimize the influence of preexisting current systems on our estimate of the substorm current location we use only Pi 2 pulsations which follow a magnetically quiet interval and which start simultaneously with a magnetic bay. We find that the longitudinal pattern of the horizontal polarization ellipse azimuth found earlier extends beyond the meridians of the field‐aligned currents. The sense of wave horizontal polarization is predominantly counterclockwise at all longitudes, though the incidence of linear and clockwise polarization increases with distance from the field‐aligned current meridians. Estimates of signal phase differences between station pairs, if interpreted as azimuthal phase propagation, show that westward propagation dominates west of and within the field‐aligned current meridians but the eastward propagation dominates east of the current system. This latter observation fits a recent model introduced to explain the longitudinal variation in polarization azimuth. We also looked for variations in the signal frequency among stations and found some variation in about half the events studied but could find no systematic behavior.
Interplanetary flow systems associated with cosmic ray modulation in 1977–1980
Authors: Burlaga L. F. , McDonald F. B. , Ness N. F. , Schwenn R. , Lazarus A. J. , Mariani F.
Published: February 2008

The hydromagnetic flow configurations associated with the cosmic ray modulation in 1977–1980 were determined using solar wind plasma and magnetic field data from Voyager 1 and 2 and Helios 1. The modulation was related to two types of large‐scale systems of flows: one containing a number of transients such as shocks and postshock flows, the other consisting primarily of a series of quasi‐stationary flows following interaction regions containing a stream interface and often bounded by a forward‐reverse shock pair. Each of three major episodes of cosmic ray modulation was associated with the passage of a system of transient flows. Plateaus in the cosmic ray intensity‐time profile were associated with the passage of systems of corotating streams.
Detailed study of electron plasma waves upstream of the Earth’s bow shock
Authors: Etcheto J. , Faucheux M.
Published: February 2008

A detailed study of electron plasma waves observed upstream of the earth’s bow shock and of their relationships to the position of the satellite in the foreshock and to the electron measurements has been carried out. The wave characteristics depend on the position in the electron foreshock: a narrow‐band (a few percent) and intense (a few millivolts per meter) noise is observed at the plasma frequency at the edge of the foreshock while the spectrum widens (Δf/f ≃ 0.3) at the same time as the power decreases (hundreds of microvolts per meter) deeper (a few earth radii) inside the foreshock. Signals below the plasma frequency are also observed. These waves are polarized along the magnetic field, with long wavelengths below and at the plasma frequency and short wavelengths above it. They appear as short bursts, the duration of which depends on the frequency: longer close to the plasma frequency (50 ms), they shorten with increasing separation from the plasma frequency, the usual duration being 15 ms. While the correlation of the wave characteristics with the reflected electrons is good as the satellite moves inside the foreshock, no evolution is found with the distance to the bow shock, neither for the noise nor for the particles. These results are discussed in the frame of various mechanisms which have been proposed to explain these upstream waves but no satisfactory agreement is found with any of them.
The adiabatic energy change of plasma electrons and the frame dependence of the cross‐shock potential at collisionless magnetosonic shock waves
Authors: Goodrich C. C. , Scudder J. D.
Published: February 2008

In collisionless magnetosonic shock waves, ions are commonly thought to be decelerated by a dc electrostatic cross‐shock electric field along the shock normal
, where γ is the effective polytrope index for electrons. By observation this ratio is ∼1/10 at the earth’s bow shock. When viewed in the de Hoffman‐Teller frame, corresponding changes in the ion kinematics occur. Since the e[ϕ*HT] is an order of magnitude smaller than the ion energy, the ions are not significantly affected by the electrostatic force. They are instead primarily retarded in this frame by the magnetic force. Since this latter force is proportional to the component of B out of the coplanarity plane, infinitesimally thin shock models may not be realistic for the study of the ion and electron dynamics in the de Hoffman‐Teller frame.
Mapping the magnetosheath field between the magnetopause and the bow shock: Implications for magnetospheric particle leakage
Authors: Luhmann J. G. , Walker R. J. , Russell C. T. , Spreiter J. R. , Stahara S. S. , Williams D. J.
Published: February 2008

Observational studies have attributed some of the energetic (⪞50 keV) particles observed at various sites near the bow shock and in the magnetosheath to a magnetospheric source. A large‐scale picture of the spatial distribution of these particles in the magnetosheath is difficult to obtain from the data because of constantly changing interplanetary conditions. Here an approximate picture of the volumes occupied by particles that originate in the vicinity of the magnetopause is obtained by mapping magnetosheath magnetic field lines which drape over the magnetopause through the bow shock. Subsets of these field lines that connect to potential sites of magnetic merging on the magnetopause are also traced in the event that the particle leakage occurs preferentially where normal components of the field are present across that boundary. The results of this modeling exercise suggest that energetic magnetospheric particles which are not scattered by magnetosheath magnetic fluctuations are likely to exit the magnetosheath in the region of the quasi‐parallel shock.