bluuebunny/crossref_metadata_2025 huggingface dataset:
Low‐energy particles at the bow shock, magnetopause, and outer magnetosphere of Saturn
Authors: Maclennan C. G. , Lanzerotti L. J. , Krimigis S. M. , Lepping R. P.
Published: February 2008

Low‐energy electrons (>22 keV) and protons (≳30 keV) measured by the Low‐Energy Charged Particle Experiment (LECP) during the encounters of the two Voyager spacecraft with Saturn are described. The characteristics of the dayside bow shock, magnetopause, and outer magnetosphere are emphasized. Only one crossing of the Saturnian bow shock was observed inbound during the Voyager 1 encounter, whereas five crossings of the bow shock were identified during the Voyager 2 approach to the planet. During several of these bow shock crossings, low energy protons were observed to be streaming from the direction of the dawnside of the magnetosphere. In the magnetosheath the protons were observed to be oriented primarily with pitch angles of ∼90°. Prior to the inbound magnetopause crossings (as defined by the magnetometer experiment on Voyager), the low‐energy protons and electrons were observed to increase in intensity. Further, during Voyager 2 encounter, an increase in the proton and electron fluxes accompanied a change in orientation of the magnetosheath magnetic field from one with a vertical component opposite to the planetary field to one with a vertical component in the direction of the planetary field. Examination of the flux distributions of the protons suggests that the magnetopause was moving inward with a lower limit speed of ∼10 km/s during the Voyager 2 approach to the planet. The observed average subsolar magnetopause position at the time of Voyager 2 encounter was 18.5 RS, whereas during the Voyager 1 encounter it was considerably more extended, at 23.5 RS. While the ratios of the low‐energy proton energy densities to the magnetic field energy densities (βh) during both encounters were ≳0.5 in the outer magnetosphere, the Voyager 2 measurements were made at higher latitudes; reasonable extrapolation suggests that βh was >1 in the equatorial plane at this time, suggesting that a significant hot plasma sheet probably existed at Saturn. Intervals of diamagnetic depression observed in the magnetic field energy density during the Voyager 2 encounter coincided very well with enhancements in the low‐energy proton energy density; during one 10‐min interval the ratio of the proton to magnetic field energy density βh was significantly greater than 1, even at the higher latitudes where the measurement was made.
Characteristics of eastward drifting omega bands in the morning sector of the auroral oval
Authors: Opgenoorth H. J. , Oksman J. , Kaila K. U. , Nielsen E. , Baumjohann W.
Published: February 2008

The STARE radars and the Scandinavian networks of magnetometers, all‐sky cameras, and riometers recorded during the night of October 21/22, 1979, the occurrence of a fairly regular sequence of auroral omega bands and associated magnetic and electric field variations. The combined two‐dimensional data are used to derive a realistic model for the three‐dimensional current flow associated with the auroral forms. In the model calculations the observed structure in the particle precipitation is accounted for by an inhomogeneous ionospheric conductivity distribution. The main resulting feature of the model current system is a sequence of east–west orientated pairs of upward and downward directed field‐aligned currents, associated with the bright and dark areas of the visual aurora, respectively. The major source of magnetic disturbances on the ground is a “meandering” ionospheric Hall current, composed of a westward background electrojet and circular Hall current vortices around the locations of the localized field‐aligned currents. The total magnetic disturbance observed on the ground during different events appears, however, to be strongly dependent on the Hall to Pedersen conductivity ratio and the degree of inhomogeneity in the conductivity distribution. The three‐dimensional current system associated with the auroral omega bands drifts eastward with a velocity comparable to an E×B drift within the general southward directed electric background field. However, complete agreement was not found at all times.
Comparison of measured and calculated low‐latitude ionospheric properties
Authors: Chandler M. O. , Behnke R. A. , Nagy A. F. , Fontheim E. G. , Richards P. G. , Torr D. G.
Published: February 2008

Measurements of ionospheric parameters above Arecibo, Puerto Rico, have been compared with a computer simulation for a variety of conditions. The simulation includes solutions to the energy, heat flow, continuity, and momentum equations for O+, H+, and electrons. Agreement was found between the measured and calculated electron concentration during geomagnetically quiet conditions. Comparisons for more active conditions indicate a significant upward flow of ionization during the mid‐afternoon. Calculated electron temperatures were found to be consistently lower than measured temperatures during the daytime. Calculated values of NmF2 and hmax agreed with measured results except during the post‐midnight period. Calculated values of the ion flux indicate a 24‐hour net flow of ionization from the northern to the southern hemisphere amounting to 12% and 6% of the equilibrium flux tube content above 1000 km for the winter solstice and equinox cases, respectively.
Cyclotron maser emission of auroral Z mode radiation
Authors: Hewitt R. G. , Melrose D. B. , Dulk G. A.
Published: February 2008

Z mode radiation has been observed in the auroral zones where the plasma frequency ωp is less than the electron cyclotron frequency Ωe. We explore the possibility that this radiation is generated in the same way as the X mode radiation of the auroral kilometric radiation (AKR), i.e., by cyclotron maser emission driven by a loss cone distribution, specifically by electrons reflected at a lower height and propagating upward. We calculate the growth rate for the Z mode by using a method developed for the X mode and the O mode. We find: (1) Growth occurs in a small crescent‐shaped region of ω‐θ space just outside a forbidden zone near θ = 90° with ω between Ωe and the upper hybrid frequency. (2) The temporal growth rate for the Z mode is less than that for the (unsuppressed) X mode but comparable with that of the O mode; for ωp/Ωe ≳ 0.3 the X mode is suppressed and the growth of the Z mode and the O mode compete for the available free energy. Because of the low group speed of the Z mode its spatial growth rate is higher than that of the O mode, giving it an advantage. (3) The product of the spatial growth rate and the bandwidth of the growing waves for the Z mode is comparable with that for the (unsuppressed) X mode and is much greater than that of the O mode. (4) Although all growing Z mode waves have slightly upward directed wave normals (θ ≳ 90°), most have downward directed rays, many at angles θg between 50° and 70°, and so can propagate toward regions where ω < Ωe. We argue that these properties suggest that loss cone driven cyclotron emission may be the mechanism generating the observed auroral Z mode radiation.
Some observational constraints on the global‐scale wind systems of Mars
Authors: Kahn Ralph
Published: February 2008

Wind direction measurements from a variety of indicators, taken over several Mars years and covering the entire planet, have been collected. These include observations of lee wave clouds not previously studied. The data exhibit a high degree of consistency and seasonal reproducibility, making it possible to interpret most of the observations in terms of the zonally symmetric circulation of Mars. We examine these data in the context of the constraints the observed wind directions place upon the thermal and polar cap mass flux fields which force the flows. Within the limitations of the data set, mid‐ and high‐latitude winds appear to be controlled by the thermal field during mid and late summer and winter, and in the northern hemisphere, during early spring and early summer. At other times, the cap flows dominate. At high latitudes, the strongest yearly winds are probably generated by cap formation in the north and cap recession in the south. The hemispheric asymmetry seems to result primarily from the effects of global dust storms on the north polar cap. Turning of the prograde wind from poleward to equatorward in midautumn is observed, consistent with reduced poleward mass flux and increased meridional temperature gradient at that time. The latitudinal extent of the equatorial Hadley cell, when observed, is probably larger than that predicted by nearly inviscid models of low‐latitude circulation. The cross‐equational Hadley cell, and high‐latitude poleward flows which occur in both hemispheres during the summer seasons, may set additional constraints on the thermal forcing mechanisms at these times.
A cause of the solar wind speed variations: An update
Authors: Hakamada Kazuyuki , Munakata Yoshinori
Published: February 2008

The relation between the solar wind speed (V) observed at the earth's distance and the latitudinal angular distance of the earth (λ) from the magnetic neutral line on the so‐called “source surface” of the sun, which is determined from the spherical harmonic analysis of the line‐of‐sight component of the photospheric magnetic field [Hoeksema et al., 1982], is examined. It is shown that the solar wind speed V increases as |λ| increases and that V does not depend on the heliographic latitude of the earth. This relation may be represented by V(km/s) = 408 + 473sin²λ during the period between May 1976 and August 1977. The presence of such a latitudinal gradient of V indicates that some of the time variations of V observed at the earth are caused by the spatial variations of V with respect to λ.
An attempt to measure the field‐aligned drift velocity of thermal electrons in the auroral ionosphere
Authors: Storey L. R. O. , Thiel J.
Published: February 2008

A medium‐frequency (0.1–1.5 MHz) mutual‐impedance probe was flown on three rockets in the auroral ionosphere, for the purpose of measuring the field‐aligned drift velocity of the thermal electrons. Detailed results are presented from one of these experiments, in which a sensitivity of the order of 1% of the electron thermal velocity was achieved. Downward electron drifts were indicated, but their high apparent velocities cast doubt on their authenticity. Data from this and the other two experiments suggest that the probe was being perturbed by the payload body. This problem needs to be solved, and the sensitivity improved by an order of magnitude, in order for the probe to become a useful measuring instrument.
New empirical models of the electron temperature and density in the Venus ionosphere with application to transterminator flow
Authors: Theis R. F. , Brace L. H. , Elphic R. C. , Mayr H. G.
Published: February 2008

The continuing rise of periapsis has allowed the Pioneer Venus orbiter (PVO) to encounter vast regions of the Venus ionosphere that were not sampled earlier when propulsion was being used to maintain periapsis deep in the ionosphere. We have employed the orbiter electron temperature probe measurements from the first seven Venus years (December 1978–December 1982) to devise new empirical models of electron temperature Te and density Ne. Departing from our earlier use of spherical harmonics to describe the solar zenith angle variations of Ne (Theis et al., 1980), we find that this more complete data set can be described better by error functions and exponentials in solar zenith angle and power law variations in altitude. This model represents a significant improvement over the previous models, as it allows the dawn and dusk sectors to be modeled separately, providing more realistic representations of the gradients near the terminators as well as the variation of those gradients with altitude. We employ these models to perform a two‐dimensional solution of the momentum equation for the nightward ion flow velocities believed to be largely responsible for the maintenance of the nightside ionosphere. The velocities at the terminator rise from the neutral atmospheric wind velocity (∼300 m/s) at 150 km to a peak velocity exceeding 2000 m/s above 500 km, in general agreement with PVO measurements of ion drift in that region.
The plasma sheet boundary layer
Authors: Eastman T. E. , Frank L. A. , Peterson W. K. , Lennartsson W.
Published: February 2008

The plasma sheet boundary layer is a temporally variable transition region located between the magnetotail lobes and the central plasma sheet. We have made a survey of these regions by using particle spectra and three‐dimensional velocity‐space distributions sampled by the ISEE 1 LEPEDEA. Ion composition measurements obtained by the Lockheed ion mass spectrometers indicate that ionospheric ions play a crucial role in magnetotail dynamics. Eleven crossings from the lobes to the central plasma sheet taken at various local times and levels of geomagnetic activity are analyzed in detail. The average ratios of He+/H+, He++/H+, and O+/H+ are not significantly different between the plasma sheet boundary layer and central plasma sheet. Densities and temperatures intermediate between the central plasma sheet and lobes are observed in the plasma sheet boundary layer although bulk flow speeds there are typically enhanced. Counter‐streaming ion beams are often observed in the plasma sheet boundary layer at energies of ∼1 keV/q to >45 keV/q. Intense antisunward‐flowing beams of ionospheric origin at E/q of <1 kV are often seen in the tail lobes, the plasma sheet boundary layer, and, infrequently, in the central plasma sheet. Such beams are not commonly observed in the central plasma sheet, which is characterized by hotter and more isotropic ion and electron distributions. Our samples of ion distributions in the plasma sheet boundary layer frequently show an evolution of distribution functions from highly anisotropic single beams or counter‐streaming beams toward the more isotropic distributions typical of the hot component of the central plasma sheet. Provided that the acceleration process for these beams can be identified, we can then account for the transport and injection of hot plasma into the central plasma sheet. We conclude that the plasma sheet boundary layer is a primary transport region of the magnetotail.
Diffuse ions produced by electromagnetic ion beam instabilities
Authors: Winske D. , Leroy M. M.
Published: February 2008

The evolution of the elecromagnetic ion beam instability driven by the reflected ion component backstreaming away from the earth's bow shock into the foreshock region is studied by means of computer simulation. The linear and quasi‐linear stages of the instability are found to be in good agreement with known results for the resonant mode propagating parallel to the beam along the magnetic field and with theory developed in this paper for the nonresonant mode, which propagates antiparallel to the beam direction. The quasi‐linear stage, which produces large amplitude δB ∼ B, sinusoidal transverse waves and “intermediate” ion distributions, is terminated by a nonlinear phase in which strongly nonlinear, compressive waves and “diffuse” ion distributions are produced. Additional processes by which the diffuse ions are accelerated to observed high energies are not addressed. The results are discussed in terms of the ion distributions and hydromagnetic waves observed in the foreshock of the earth's bow shock and of interplanetary shocks.
Observation of an oscillating magnetic field shell at three locations
Authors: Cahill L. J. , Sugiura M. , Lin N. G. , Arnoldy R. L. , Shawhan S. D. , Engebretson M. J. , Ledley B. G.
Published: February 2008

On July 14, 1982, 1830–1930 UT, a complex magnetic pulsation event was observed near the magnetic shell L ∼4.5 in the local afternoon. A large magnetic storm was in progress and had proceeded to the early recovery phase. The event began, at 1832 UT, with a sudden decrease in |B| and a brief, 120‐s compressional pulsation. A transverse pulsation then developed with period near 180‐s and amplitude about 5 nT. Near 1850 UT this pulsation decreased abruptly and was replaced by a 44‐s transverse pulsation. By 1858 UT the 44‐s pulsation was fading and a 240‐s azimuthal pulsation developed rapidly. It slowly decreased in amplitude after 1910 UT and was gone by 1930 UT. The event was observed at the DE‐1 satellite near the equator in both electric and magnetic field components and at the ground magnetic observatories Siple, Antarctica, and Roberval, Quebec, 15° to 30° west of DE‐1 and at opposite ends of an L = 4.2 field line. The 180‐s and 240‐s pulsations were fundamental, toroidal, resonant oscillations of a magnetic field shell. Since the first 180‐s pulsation started and stopped simultaneously at all three locations and the corresponding magnetic pulsations remained in phase at all three locations, this is interpreted to be a large‐scale toroidal oscillation of one resonant field shell. In the 240‐s pulsation, DE‐1 was on one resonant shell and the ground stations, oscillating at 220 s period, were on another lower L value shell. The 44 s pulsation, also transverse at DE‐1, was weaker on the ground and poorly correlated with the DE‐1 measurements, suggesting that it was more localized in azimuth.
Long wavelength limit of the current convective instability
Authors: Huba J. D.
Published: February 2008

A linear theory is presented of the current convective instability in the long wavelength limit, i.e., kL ≪ 1 where k is the wave number and L is the scale length of the density inhomogeneity. A relatively simple dispersion equation is derived that describes the modes in this limit. Analytical solutions are presented in both the collisional (νin ≫ ω) and inertial (νin ≪ ω) limits where ω is the wave frequency and νin is the ion‐neutral collision frequency. It is shown that the growth rate scales as k in the collisional limit and as k2/3 in the inertial limit. The analytical solutions are compared to exact numerical solutions, and very good agreement is found. Applications to the auroral ionosphere are discussed.
Rayleigh‐Taylor instability in the presence of a stratified shear layer
Authors: Satyanarayana P. , Guzdar P. N. , Huba J. D. , Ossakow S. L.
Published: February 2008

A nonlocal theory of the Rayleigh‐Taylor instability, which includes the effect of a transverse velocity shear, is presented. A two‐fluid model is used to describe an inhomogeneous plasma under the influence of gravity and sheared equilibrium flow velocity and to derive a differential equation describing the generalized Rayleigh‐Taylor instability. An extensive parametric study is made in the collisionless and collisional regimes, and the corresponding dispersion curves are presented. The results are applied to the equatorial F region and to barium releases in the ionosphere.
Nonlinear theory of the E × B instability with an inhomogeneous electric field
Authors: Keskinen M. J.
Published: February 2008

Using analytical and numerical simulation techniques, the nonlinear evolution of the E × B instability with an inhomogeneous electric field has been studied. For the case where the electric field component parallel to the density gradient is inhomogeneous, we find that the inhomogeneous E × B instability in the nonlinear regime (1) evolves into large‐scale anisotropic fingerlike structures, (2) can be described by power laws P(kx) ∝ kx−nx, nx ≃ 2, P(ky) ∝ ky−ny, ny ≃ 2–3, where P(kx) and P(ky) are the one‐dimensional power spectra parallel and perpendicular to the initial density gradient, and (3) can be stabilized by quasi‐linear mechanisms in which the initial density gradient is modified by a finite‐amplitude wave spectrum. Application is made to naturally occurring plasma density fluctuations in the auroral ionosphere, but the results are applicable to plasma jet and striation formation in artificially produced ionospheric plasma clouds.
Chemistry and dynamics of SF6 injections into the F region
Authors: Bernhardt P. A.
Published: February 2008

The chemistry of SF6 vapor reacting in the F region is critically examined. The SF6 molecule dissociatively attaches an electron, producing the SF5− ion. Reaction between an ambient O+ ion and the SF5− yields mutual neutralization. During the neutralizaton process, an electronically excited oxygen atom, O(¹D), may be produced which will subsequently emit a 630.0‐nm photon. A release of 20 kg of SF6 into an ambient O+ ion concentration of 105 cm−3 at 300‐km altitude is considered. The 630.0‐nm airglow intensity may increase following the release. The negative ion plasma is shown to be marginally unstable to a gradient drift instability.
High‐latitude irregularities in the lower F region: Intensity and scale size distributions
Authors: Rodriguez P. , Szuszczewicz E. P.
Published: February 2008

A wide range of lower F region irregularity scale sizes (75 m to 150 km) at high northern latitudes during the spring and summer of 1978 has been studied. The morphology and intensity distribution of the irregularities suggest that particle precipitation is the primary source of the larger‐scale density fluctuations, with second‐order effects (e.g., plasma instabilities) contributing to the smaller scale sizes. Under conditions of moderate geomagnetic activity (Kp ∼ 2–3, AE ∼ 100), the most intense density enhancements (∼106 cm−3) occur in the auroral oval. Large‐ (L > 7.5 km) and small‐scale (7.5 km > L > 75 m) irregularities are most prominent in the dayside auroral oval, including the region of the polar cusp. Density enhancements within the polar cap have been detected principally toward the duskward side. These polar cap enhancements are similar to auroral oval enhancements, suggesting that polar cap precipitation is the source. The altitude range (170–200 km) of the observations implies that chemical recombination is the primary decay mode of the irregularities. The decay rate of the irregularities is sufficiently fast that convection cannot transport the observed irregularities far from their source regions.
An investigation of the consistency of the ionospheric measurements of the photoelectron flux and solar EUV flux
Authors: Richards P. G. , Torr D. G.
Published: February 2008

In this paper we demonstrate that there is an inconsistency between ionospheric measurements of the photoelectron flux and the solar EUV flux. The problem which occurs for EUV wavelengths near 250 Å or photoelectron energies near 35 eV has serious implications for theoretical photoelectron flux calculations and could help to explain up to 40% of the difference between calculated and measured photoelectron fluxes for electron energies below 35 eV. We also show that (1) only photons with wavelengths shorter than 310 Å are of importance in producing photoelectrons, (2) above 20 eV the photoelectron flux is controlled by the production of primary photoelectrons, while below 20 eV the flux is controlled by degraded photoelectrons, (3) the calculated cascading production rate is insensitive to the cross sections used for the calculation, and (4) the photoelectron flux is inversely proportional to the total inelastic cross sections.
Resonant acceleration and heating of solar wind ions: Anisotropy and dispersion
Authors: Isenberg Philip A.
Published: February 2008

This paper is an extension of recent work by Isenberg and Hollweg dealing with the preferential acceleration and heating of solar wind heavy ions by a resonant cyclotron interaction with a turbulent spectrum of ion‐cyclotron waves. Two major approximations of the previous work are eliminated: that of isotropic ion distributions, and that of nondispersive waves. The wave action flux of finite‐amplitude Alfvén waves in a multi‐ion, differentially flowing, gyrotropic plasma is derived. This quantity is incorporated into the wave‐driven solar wind model of Isenberg and Hollweg, which is then applied to gyrotropic protons and alpha particles. It is found that allowing for nonisotropic ions yields enhanced preferential acceleration of alpha particles. However, when dispersion is included in the resonant interaction, the preferential effects are again reduced. These more realistic calculations show that the observations of solar wind alpha particles cannot be explained by a resonant cyclotron interaction.
Multispacecraft observations of the harmonic structure of Pc 3–4 magnetic pulsations
Authors: Takahashi Kazue , McPherron Robert L. , Hughes W. Jeffrey
Published: February 2008

Harmonically related Pc 3–4 pulsations (7–100 mHz) are observed simultaneously by the three geosynchronous satellites ATS 6, SMS 1, and SMS 2, which are separated 20° from one another. At a given instance the frequency of the same harmonic is different from one spacecraft to another and each spacecraft observes a decrease in the fundamental frequency as it moves from morning (15 mHz) to afternoon (10 mHz). This frequency behavior is explained in terms of standing Alfvén waves, for which the frequency is determined by the local magnetic field and plasma density. Occurrence of harmonic Pc 3–4 waves only during daytime hours (0400–2000 LT) and their frequency characteristics suggest a broadband energy source located on the dayside. Possible azimuthal wave number m and azimuthal phase velocity Vϕ of the second through fourth harmonics are determined from an unusual interval during which identical harmonic frequencies were observed at SMS 1 and ATS 6. Under the assumption of tailward propagation of constant‐phase fronts at the same velocity for all these harmonics, we obtain Vϕ ∼ 1700 km/s.
Ohmic dissipation of Pedersen current as the cause of high‐latitude F region ionospheric irregularities
Authors: Lee M. C.
Published: February 2008

When the quasi‐dc electric field exceeds about 25 mV/m in the ionosphere, a thermal instability that is caused by the Ohmic dissipation of Pedersen current in the electron gas can produce ionospheric irregularities in the F region within a few minutes. The threshold fields and scale lengths of the instability are determined by the electron energy loss due to electron‐ion collisions, through which, as an intermediate step, energy is transferred from electrons to neutrals as energy sinks. The excited ionospheric irregularities have two scale length regimes: one is from tens of meters up to hundreds of meters, and the other one is from hundreds of meters up to a few tens of kilometers. This thermal instability is effective in generating high‐latitude ionospheric irregularities, especially in the polar cap, where large quasi‐dc electric fields have been observed even during quiet ionospheric conditions.
Ducted propagation of daytime whistlers at low latitudes as deduced from ground‐based direction findings
Authors: Ohta K. , Hayakawa M. , Tanaka Y.
Published: February 2008

The purpose of the present paper is to promote our understanding of the propagation characteristics of daytime whistlers at low latitudes on the basis of the data obtained by a newly developed automatic direction finding measurement at Yamaoka (geomagnetic latitude 25°N) and the synoptic routine data from the multistation network of Kagoshima (20°), Sakushima (24°), and Moshiri (34°). Two events of daytime whistlers with extremely enhanced occurrences are found on January 16 and 15, 1982, and they are examined in detail. It is found that the ionospheric exit points determined by the direction finder at Yamaoka remain very stable for less than 2 hours, so that the diameter of their distribution is less than 60 km. This experimental fact is likely to be strongly indicative of ducted propagation. An intercomparison of the whistler spectra at the four stations has been made for these events. Then it is shown that the whistlers that emerged near Yamaoka and Sakushima tend to propagate in the earth‐ionosphere waveguide toward higher latitudes, being observed at Moshiri with their frequency component cut off at frequencies below the tweek cutoff frequency, while heavily attenuated corresponding whistlers are simultaneous at Kagoshima. This phenomenon is again satisfactorily explained in terms of ducted propagation.
Correlated low‐frequency electric and magnetic noise along the auroral field lines
Authors: Gurnett D. A. , Huff R. L. , Menietti J. D. , Burch J. L. , Winningham J. D. , Shawhan S. D.
Published: February 2008

Plasma wave and plasma measurements from the Dynamics Explorer 1 (DE 1) spacecraft are used to investigate an intense broadband spectrum of low‐frequency, < 100 Hz, electric and magnetic noise observed at low altitudes over the auroral zones. This noise is detected by DE 1 on essentially every low‐altitude pass over the auroral zone and occurs in regions of low‐energy, 100 ev to 10 keV, auroral electron precipitation and field‐aligned currents. The electric field is randomly polarized in a plane perpendicular to the static magnetic field. Correlation measurements between the electric and magnetic fields show that the perpendicular (∼ north‐south) electric field fluctuations are closely correlated with the perpendicular (east‐west) magnetic field fluctuations and that the Poynting flux is directed downward, toward the earth. The total electromagnetic power flow associated with the fluctuations is large, approximately 108 W. Two general interpretations of the low‐frequency noise are considered: first, that the noise is produced by static fields imbedded in the ionosphere and, second, that the noise is due to Alfven waves propagating along the auroral field lines. For the static interpretation the ratio of the magnetic to electric field strengths at the base of the ionosphere is determined by the Pedersen conductivity, B/(µ0E) = Σp, whereas for the Alfven wave interpretation it is determined by the Alfven index of refraction, cB/E = nA. Measurements show that the magnetic to electric field ratio decreases rapidly with increasing height. This height dependence is in strong disagreement with the static model if the magnetic field lines are assumed to be equipotentials (E∥ = 0). At present, no satisfactory model is available for comparison with the data if an electrostatic potential drop is assumed to exist along the magnetic field (E∥ ≠ 0). The Alfven wave model is in good agreement with the general form of the height dependence of the magnetic to electric field ratio but disagrees in certain details. The cB/E ratio tends to decrease with increasing frequency and is usually somewhat larger than the computed value of the Alfven index of refraction. Some of these difficulties could be accounted for by reflections at the base of the ionosphere or propagation at large angles to the magnetic field (kinetic Alfven waves). For both the static model and the Alfven wave model the source must be located at high altitudes, since the average Poynting flux is always directed downward, even at radial distances up to 2 RE.
High‐energy proton drift echoes: Multiple peak structure
Authors: Belian R. D. , Baker D. N. , Hones E. W. , Higbie P. R.
Published: February 2008

Many high‐energy proton drift echoes observed in association with magnetospheric substorms exhibit multiple fine‐structure peaks during the course of one or more of the echoes. Because of the difficulty in distinguishing between temporal, positional, pitch angle selection, and drift shell effects with instruments on a single satellite the identification of these as individual entities (as opposed to statistical variations in proton intensity or other effects) has not been convincingly established heretofore. We have examined energetic ion data from similar instruments on three widely spaced geosynchronous satellites, with the goal of establishing whether or not these multiple peaks are a persistent feature of the injection process. Our results suggest that multiple peaks are indeed a real feature of drift echoes. At least three quarters of the ion enhancements that result in drift echoes are made up of multiple‐peaked events. A very good correlation between the onset of the various peaks and detailed features observed in high‐latitude magnetograms is observed for some of the multiple peaked events. In fact, in their relation to the magnetograms, there seems to be a pairing of pulses; that is, with some dips in the magnetogram there appears to be two associated pulses. Various possible models for the production of multiple peaks, including the ion tearing mode instability in the plasma sheet, magnetic mirroring effects, temporal and/or positional effects of multiple neutral line formation, and effects due to different ion species are discussed.
Associations between coronal mass ejections and solar energetic proton events
Authors: Kahler S. W. , Sheeley N. R. , Howard R. A. , Koomen M. J. , Michels D. J. , McGuire R. E. , von Rosenvinge T. T. , Reames D. V.
Published: February 2008

We have used data from the Naval Research Laboratory (NRL) white light coronagraph on the P78‐1 spacecraft and energetic (E > 4 MeV) proton data from the Goddard Space Flight Center (GSFC) detectors on the IMP 8 and ISEE 3 spacecraft to investigate the association between proton events originating in flares and coronal mass ejections (CME's). The primary data were 50 prompt proton events observed between April 1979 and February 1982 for which reduced coronagraph data were available. H alpha flares could be confidently associated with 27 of these events, and in 26 of these 27 cases an associated CME was found, indicating a high but not perfect association of prompt proton events with CME's. Peak proton fluxes correlate with both the speeds and the angular sizes of the associated CME's. We show that the CME speeds do not significantly correlate with CME angular sizes, so that the peak proton fluxes are correlated with two independent CME parameters. With larger angular sizes, CME's are more likely to be loops and fans rather than jets and spikes and are more likely to intersect the ecliptic. Which of these factors is important to the peak proton flux correlation cannot be determined from the data. We find weak evidence that steeper proton spectra are associated with faster and wider CME's. Two of the 50 proton events of the study and two additional events, all with no associated CME's, share common characteristics: relatively short duration (∼1 day) proton events with low fluxes, parent flares with short (∼10 min) soft X ray duration, close magnetic connection to the earth, and gamma ray and metric type II emission.
Large‐scale flow in the dayside magnetosheath
Authors: Crooker N. U. , Siscoe G. L. , Eastman T. E. , Frank L. A. , Zwickl R. D.
Published: February 2008

The degree of control over plasma flow direction exerted by the compressed magnetic field in the dayside magnetosheath is examined by comparing ISEE 1 LEPEDEA data with hydrodynamic and magnetohydrodynamic predictions. Measured flow directions projected toward the subsolar region pass within ∼1 RE of the aberrated theoretical hydrodynamic stagnation point in 11 of 20 cases analyzed. The remaining nine cases pass within ∼2–3 RE of the stagnation point. One case with large deflection has been studied in detail with high‐time‐resolution plasma and magnetic field data both from ISEE 1 and from ISEE 3, in the role of a solar wind monitor. The deflected flow is persistent over a period of 1 1/2 hours, and its direction is consistent with a stagnation point displacement resulting from increased, asymmetric magnetic field pressure contributions during periods of low Alfven Mach number, as predicted by Russell et al. Of the other eight cases with large deflections, four are associated with flux transfer events identified independently by Berchem and Russell. The observed deflections in these cases are consistent with either the subsolar merging line or the antiparallel merging hypothesis, but not exclusively with one or the other. The results relating to the formation of a stagnation line rather than a stagnation point are inconclusive.
Relationship between the IMF magnitude and Pc 3 magnetic pulsations in the magnetosphere
Authors: Yumoto Kiyohumi , Saito Takao , Tsurutani Bruce T. , Smith Edward J. , Akasofu Syun‐Ichi
Published: February 2008

Compressional waves in the Pc 3–4 frequency range (f = 16–100 mHz) detected at the synchronous orbit (GOES 2, L = 6.67) and low‐latitude pulsations detected on the ground (L = 1.3–1.8) are correlated with the strength of the interplanetary magnetic field (BIMF) observed by ISEE 3 and are expressed as f (mHz) ∼6.0 × BIMF (nT). The correlation between f and BIMF is better in the outer magnetosphere than at low latitudes on the ground, i.e., γ = 0.70 at GOES 2, 0.60 at L = 1.8, and 0.54 at L = 1.3. The correlation study suggests that the solar wind energy in the Pc 3–4 range, associated with the upstream waves of which frequencies are related to the ion cyclotron frequency and thus with the IMF magnitude, is ultimately transmitted across the ambient geomagnetic field into the deep magnetosphere. Scatter plots of pulsation frequencies observed in the magnetosphere versus the IMF magnitude show a remarkable distribution restricted within the forms f = 4.5 × BIMF and f = 7.5 × BIMF. The results are theoretically examined. If the restricted Pc 3–4 frequencies of the compressional waves at the synchronous orbit correspond to the Doppler‐shifted frequencies of magnetosonic upstream waves excited by the well‐known ion cyclotron resonance with reflected ion beams in the earth's foreshock, the resonant kinetic energy of the reflected ion beams can be inferred to be restricted within the 1–5 keV range in the spacecraft frame. The inference is consistent with the observational distributions of the reflected ion beams by the ISEE satellites in the earth's foreshock. It is likely that the magnetosonic upstream waves in the Pc 3–4 range in the earth's foreshock are convected through the magnetosheath to the magnetopause, transmitted into the deep magnetosphere without significant changes in spectra, and then couple with various hydromagnetic waves in the magnetosphere.
Acceleration of barium ions near 8000 km above an aurora
Authors: Stenbaek‐Nielsen H. C. , Hallinan T. J. , Wescott E. M. , Foeppl H.
Published: February 2008

A barium shaped charge, named Limerick, was released from a rocket launched from Poker Flat Research Range, Alaska, on March 30, 1982, at 1033 UT. The release took place in a small auroral breakup. The jet of ionized barium reached an altitude of 8100 km 14.5 min after release, indicating that there were no parallel electric fields below this altitude. At 8100 km the jet appeared to stop. Analysis shows that the barium at this altitude was effectively removed from the tip. We conclude that the barium was actually accelerated upward, resulting in a large decrease in the line‐of‐sight density and hence the optical intensity. The parallel electric potential in the acceleration region must have been greater than ∼1 kV over an altitude interval of less than ∼200 km. The acceleration region, although presumably auroral in origin, did not seem to be related to individual auroral structures, but appeared to be a large‐scale horizontal structure. The perpendicular electric field below, as deduced from the drift of the barium, was temporally and spatially very uniform and showed no variation related to individual auroral structures passing through.
The plasma wave environment of an auroral arc: 2. Ulf waves on an auroral arc boundary
Authors: Gelpi C. G. , Bering E. A.
Published: February 2008

On March 9, 1978, a sounding rocket launched from Poker Flat, Alaska, at 2200 LT, made a four‐component measurement of a 0.5 Hz hydromagnetic wave as the payload crossed the poleward boundary of a quiet homogeneous auroral arc. An energy flux of ∼10−6 W/m² was observed propagating upward with a left‐handed polarization within the arc, and a flux 6 times greater was observed propagating downward with a right‐handed polarization on the arc boundary. The waves were identified as shear mode Alfvén waves. Various models for the source of the free energy are discussed with the conclusion that the most likely production mechanism was either the electromagnetic or electrostatic Kelvin‐Helmholtz instability.
Interpretation and modeling of quasiperiodic diffraction patterns observed in equatorial VHF scintillation due to plasma bubbles
Authors: Franke S. J. , Liu C. H. , McClure J. P.
Published: February 2008

Quasiperiodic diffraction patterns have been observed in VHF scintillation recorded at Ascension Island in 1981. The patterns occur most often at the beginning or end of a scintillation patch and are shown to be consistent with those expected from irregularities having east‐west scale sizes of a few hundred meters that are associated with the walls (edges) of equatorial plasma bubbles. By adjusting the geophysical parameters in a computer simulation of the amplitude fluctuations expected simultaneously at VHF, L‐Band and C‐Band, we model a specific event in detail. The model contains information on the size and strength of the structures that cause the regular fading patterns. In addition, multi‐frequency diffraction patterns are computed for a “phase screen” model based on high resolution measurements of a structured plasma bubble wall made using the AE‐E satellite. The results are qualitatively very similar to the observations. Finally, implications of the model results for the extraction of information about the vertical structure of the irregularities are discussed.
Shock drift acceleration in the presence of waves
Authors: Decker R. B. , Vlahos L.
Published: February 2008

Charged particle acceleration via the shock drift mechanism at quasi‐perpendicular shocks has generally been analyzed by assuming uniform, time‐independent conditions at and near the shock. We present results from a model designed to study how the shock drift mechanism is modified when wave activity is included in the shock's upstream and downstream vicinities. The technique involves numerically following test particle trajectories in the wave‐shock system for predefined wave fields. In order to compare these results with those obtained in the scatter‐free (i.e., nonwave) case, we restricted particles to a single shock encounter, which is here defined as the period during which the particle remains within a gyrodiameter of the shock. As a particular example, we injected ensembles of ions into a system consisting of a quasi‐perpendicular shock moving through the interplanetary spectrum of ambient Alfvén waves. As compared with a single encounter in the scatter‐free limit, the inclusion of waves (1) increases particle transmission through the shock, (2) produces broader energy distributions for reflected and transmitted particles, with high‐energy tails at energies several times the maximum energy obtained in the scatter‐free case and (3) reduces anisotropies, particularly of reflected particles, but does not eliminate them. Also, for the range of energies studied, it was found that the approximate invariance of the magnetic moment for particle interactions with quasi‐perpendicular shocks is no longer valid when waves are present.
Lower hybrid waves in finite‐β plasmas, destabilized by electron beams
Authors: Migliuolo Stefano
Published: February 2008

The linear and quasilinear theories of lower hybrid waves in a finite‐β inhomogeneous plasma with electrons streaming parallel to the magnetic field are examined. These waves are found to be unstable for very low‐β and to be stabilized in finite‐β plasmas. The quasilinear feedback of unstable waves is shown to result in heating of ions perpendicular to the equilibrium magnetic field. Thus, unstable lower hybrid waves may play a role in producing ion conies in the auroral zone, and in heating ions in the solar plasma.
Polar cap inflation and deflation
Authors: Siscoe G. L. , Huang T. S.
Published: February 2008

Dayside and nightside merging rates usually differ; first one dominates and then the other to maintain long‐term flux balance. The polar cap, defined as the area in the ionosphere penetrated by open field lines, expands when closed field lines open during dayside merging and contracts when open field lines close during tail merging. The patterns of convection, electric field and current for polar cap inflation and deflation differ from those of steady state convection. This report models the flow and electrical parameters of polar cap inflation and deflation. We treat the idealized case of uniform conductivity, a planar ionosphere, and “pure Bz” merging (i.e., no By effect, although we note how the approach used here is readily modified to account for the By effect). For this idealized case, the potential associated with polar cap inflation and deflation is seen to be described by the same equations that govern the two‐dimensional hydrodynamic flow into and out of an expanding and contracting cylinder with a gap in its side. The gap corresponds to the breach in the polar cap boundary through which magnetic flux passes as a result of merging. The calculated potential pattern displays the usual two cell convection configuration, but the foci of the two cells are the edges of the flux gaps. Thus the cells are displaced sunward during dayside merging and tailward during tail merging.
Dynamics of the 1054 UT March 22, 1979, substorm event: CDAW 6
Authors: McPherron R. L. , Manka R. H.
Published: February 2008

The physical processes involved in the transfer of energy from the solar wind to the magnetosphere, and release associated with substorms, have been examined in a sequence of Coordinated Data Analysis Workshops (CDAW 6). Magnetic storms of March 22 and 31, 1979, were chosen to study the problem, using a data base from 13 spacecraft and about 130 ground‐based magnetometers. This paper describes the March 22 storm, in particular the large, isolated substorm at 1054 UT which followed an interval of magnetic calm. We summarize the observations in the solar wind, in various regions of the magnetosphere, and at the ground, synthesizing these observations into a description of the substorm development. We then give our interpretation of these observations and test their consistency with the reconnection model. The substorm appears to have been generated by a southward turning of the interplanetary magnetic field associated with a current sheet crossing. Models of ionospheric currents derived from ground data show the substorm had three phases of development. During the first phase, a two‐celled convection current system developed in the polar cap as synchronous spacecraft on the nightside recorded an increasingly taillike field and the ISEE measurements show that the near‐earth plasma sheet thinned. In the second phase, possibly triggered by sudden changes in the solar wind, a one‐celled current system was added to the first, enhancing the westward electrojet. During this phase the synchronous orbit field became more dipolar, and the plasma sheet magnetic field turned strongly southward as rapid tailward flow developed soon after expansion onset, suggesting that a neutral line formed in the near‐earth plasma sheet with subsequent plasmoid ejection. In the third phase, which occurred after interplanetary magnetic field turned northward, the magnetospheric current systems decayed as the plasma sheet expanded and the flow turned earthward. The data from this event support the notion that substorms include two major processes of energy dissipation: one directly driven by the solar wind and one driven by release of energy stored in the geomagnetic tail. The near‐tail data suggest that one or more neutral lines formed earthward of the ISEE spacecraft. However, all the signatures of plasma flow and energetic particles can be explained only by postulating a complex series of events. While this reconnection picture represents the interpretations of many of the CDAW 6 participants, it is not a consensus viewpoint.