bluuebunny/crossref_metadata_2025 huggingface dataset:
Asymmetric time‐dependent and stationary magnetic reconnection at the dayside magnetopause
Authors: Scholer Manfred
Published: February 2008

Using a two‐dimensional compressible MHD code we have numerically studied the reconnection process at an interface where (1) the magnetic field is higher on one side than on the other and total pressure is in balance by a higher density on the weak field side, and where (2) the magnetic field is identical on both sides, but the temperature is higher on one side than on the other. Reconnection is caused by applying a localized resistivity in the center of the current sheet. The plasma is allowed to enter and exit freely at the boundaries. By using a system size with a large extension parallel to the current sheet, we are able to study time‐dependent as well as steady state effects. Denoting the weak magnetic field or low temperature region as magnetosheath and the high magnetic field or high temperature region as magnetosphere we find the following. Sudden onset of reconnection causes the development of a bulge in the current layer. The bulge moves with about 70% of the magnetosheath Alfvén speed along the magnetopause away from the reconnection region. The growth of the bulge normal to the magnetopause is proportional to the reconnection rate. For a ratio of magnetospheric to magnetosheath magnetic field larger than about 1.5 the bulge is almost exclusively on the magnetosheath side of the magnetopause current layer. The motion of the bulge along the magnetopause leads to signatures in the normal magnetic field component similar to the ones observed in flux transfer events. When the bulge has moved far enough from the reconnection site, stationary reconnection proceeds at the X line. If there exists initially a magnetic field difference across the magnetopause current layer, a slow mode shock is formed in the magnetosphere and possibly a strong intermediate shock is formed in the magnetosheath. The magnetosheath plasma is accelerated to about twice the magnetosheath Alfvén speed and constitutes a boundary layer on the magnetospheric side of the magnetopause current layer.
Auroral pulsations from ionospheric winds
Authors: Nakada M. P.
Published: February 2008

The possibility that auroral pulsations are due to oscillatory electrical circuits in the ionosphere that are driven by the negative resistance of jet stream winds is examined. For the condenser plates, the highly conducting surfaces above the edges of the jet stream are postulated. The dielectric constant of the plasma between the plates is quite large. The current that is driven perpendicular to and by the jet stream closes along the plates and through Pedersen currents in the F region above the stream. This closed loop gives the inductance and resistance for the circuit. Periods of oscillation for this circuit appear to be in the range of Pc 1 to Pc 3. In accord with observations, this circuit appears to be able to limit the brightness of pulsations.
Determination of comet Halley Gas emission characteristics from mass loading of the solar wind
Authors: Huddleston D. E. , Johnstone A. D. , Coates A. J.
Published: February 2008

We set out to fit the velocity profile of the solar wind during Giotto's approach to comet Halley with the mass loading produced by a simple model of the neutral particle distribution. The model assumes a constant gas production rate, Q, ionization rate, ν, and a radial expansion velocity, Ve, from the comet. It is used to calculate the implanted ion flux at Giotto for any given time and position along the spacecraft trajectory. Comparing the added flux with Giotto solar wind proton data from the inbound leg outside the bow shock, we compute the ratio of the total mass‐loaded ion flux to the solar wind flux at the spacecraft. Hence using equations given by Galeev et al. (1985) we infer values of the solar wind velocity, u∞, far upstream from the comet. Imposing the condition that u∞ should be as nearly as possible constant in time, we deduce values of Q and the ratio Ve/ν required to fit the model to our data. The values obtained are consistent with those derived by more direct methods. Finally, we use our Q, Ve and ν values to predict a possible innermost shock boundary.
Observation and theory of Pc 5 waves with harmonically related transverse and compressional components
Authors: Takahashi K. , Cheng C. Z. , McEntire R. W. , Kistler L. M.
Published: February 2008

The properties of 23 magnetic pulsation events observed by the AMPTE CCE spacecraft are studied. These events are selected on the basis of the field magnitude which oscillated at the second harmonic of a simultaneously present transverse oscillation. The events have a second harmonic period of 80–600 s (roughly the Pc 5 range), are observed in cluster in the dawn (0300–0800 magnetic local time, MLT) and dusk (1600–2100 MLT) sectors, and are localized near the magnetic equator. Although the azimuthal wave number, m, estimated from an ion finite Larmor radius effect, is generally large (|m| ∼ 50), there is a marked difference between the events observed in the dawn and dusk sectors. In the dawn sector the waves have low frequencies (1–5 mHz), indicate left‐hand polarization with respect to the ambient magnetic field, and propagate eastward with respect to the spacecraft. In the dusk sector the waves have high frequencies (5–15 mHz), indicate right‐hand polarization, and propagate westward. We suggest that the waves are all westward propagating in the plasma rest frame and that local‐time‐dependent Doppler shift is the reason for the local time dependence of the wave properties. The drift mirror instability is considered to be the mechanism for exciting the westward propagating waves. An analytical formula for the ion flux oscillations is derived on the basis of the nonlinear gyrokinetic theory. The observed correlation between the ion flux and the parallel magnetic field perturbation δB∥ can be adequately explained with this analytical formula.
Ion beam heating in the auroral zone
Authors: Schriver David , Ashour‐Abdalla Maha , Collin Henry , Lallande Nicole
Published: February 2008

Recent satellite observations at high altitudes (> 5000 km) in the auroral zone have shown the existence of hybrid or bimodal ion beam distributions that are evidence of both parallel and perpendicular ion acceleration. Acceleration parallel to the magnetic field is most likely due to quasi‐static electric fields (double layers) which can create outflowing ion beams; since ions of different mass will have different drift speeds due to this acceleration, a plasma configuration unstable to the ion‐ion two‐stream acoustic mode develops. When the net drift velocity (U) between the two ion species is greater than the sound speed (Cs), the ion‐ion instability has maximum growth at oblique wave propagation. To study the nonlinear effects of the ion‐ion instability in terms of plasma heating, a numerical simulation parametric study has been performed. It was found that the parallel acceleration that forms the ion beams occurs on a time scale faster than ion‐ion wave growth at low drifts; thus ion‐ion wave growth is expected to occur primarily for higher drift speeds (U > Cs), which results in strong oblique heating of the ions (both hydrogen and oxygen) forming elevated ion conics (sometimes called “bowl” distributions). Also, strong parallel electron heating in the direction of the ion beams can occur, and electrons near the top of the acceleration region may attain a net upward drift along with the elevated ion conics. Variation of the oxygen density greatly affects the ion heating due to the ion‐ion instability; as the oxygen density decreases, oxygen heating increases, in agreement with observations (Collin et al., 1987). Ion‐ion electrostatic wave properties and the plasma heating that results over a wide range of auroral zone parameters are included.
A resonance broadening kinetic theory of the modified two‐stream instability: Implications for radar auroral backscatter experiments
Authors: Robinson T. R. , Honary F.
Published: February 2008

The kinetic theory of the modified two‐stream instability (MTSI) is usually considered more accurate than the corresponding fluid theory, for the purpose of interpreting VHF and UHF coherent radar backscatter measurements. However, recent developments in the nonlinear theory of the MTSI have retained the fluid theory formalism and consequently may not be entirely valid in the short‐wavelength regime where VHF and UHF radars operate. In this paper, a nonlinear kinetic theory dispersion relation which takes account of the nonlinear resonance broadening effects is developed. With the aid of this dispersion relation, the phase speeds of the short wavelength plasma waves are calculated, as functions of wavelength, aspect angle, and flow angle. The results indicate that phase speeds tend to increase with increasing drift speed, at all wavelengths. Furthermore, under given flow conditions, the phase speeds are relatively insensitive to the flow angle and aspect angle but vary consideraby with altitude. However, unlike long‐wavelength fluid type waves, short‐wavelength MTSI waves are moderately dispersive, the shorter wavelengths having the larger phase speeds. Finally, these kinetic theory calculations are also used to estimate the form of the k spectrum of saturated MTSI waves, and the results are compared with previously published fluid theory predictions of spectral density.
Low‐ and mid‐latitude ionospheric electric fields during the January 1984 GISMOS Campaign
Authors: Fejer B. G. , Kelley M. C. , Senior C. , de la Beaujardiere O. , Holt J. A. , Tepley C. A. , Burnside R. , Abdu M. A. , Sobral J. H. A. , Woodman R. F. , Kamide Y. , Lepping R.
Published: February 2008

This paper examines in detail the electrical coupling between the high‐, middle‐, and low‐latitude ionospheres during January 17–19, 1984, using interplanetary and high‐latitude magnetic field data together with F region plasma drift measurements from the EISCAT, Sondre Stromfjord, Millstone Hill, Saint‐Santin, Arecibo, and Jicamarca incoherent scatter radars. We study the penetration of both the zonal and meridional electric field components of high‐latitude origin into the low‐latitude and the equatorial ionospheres. In the dusk sector, a large perturbation of the zonal equatorial electric field was observed in the absence of similar changes at low and middle latitudes in the same longitudinal sector. The observations in the postmidnight sector are used to compare the longitudinal variation of the zonal perturbation electric field with predictions made from global convection models. Our results show that the meridional electric field perturbations are considerably more attenuated with decreasing latitude than the zonal fluctuations. As a result, we conclude that variations in the meridional electric field at low latitudes are largely due to dynamo effects. These observations are used to show that the global convection models reproduce a number of characteristics of low‐latitude and equatorial electric fields associated with changes in the polar cap potential drop. In addition, we highlight several areas where there is still substantial disagreement between the electric field data and the theoretical results.
Electron collection enhancement arising from neutral gas jets on a charged vehicle in the ionosphere
Authors: Gilchrist Brian E. , Banks Peter M. , Neubert Torsten , Williamson P. Roger , Myers Neil B. , Raitt W. John , Sasaki Susumu
Published: February 2008

Observations of current collection enhancements due to cold nitrogen gas control jet emissions from a highly charged, isolated rocket payload in the ionosphere have been made during the cooperative high altitude rocket gun experiment (CHARGE) 2 using an electrically tethered mother/daughter payload system. The current collection enhancement was observed on a platform (daughter payload) located 100 to 400 m away from the main payload firing an energetic electron beam (mother payload). We interpret these results in terms of an electrical discharge forming in close proximity to the daughter vehicle during the short periods of gas emission. The results indicate that it is possible to enhance the electron current collection capability of positively charged vehicles by means of deliberate neutral gas releases into an otherwise undisturbed space plasma. Our results are also compared with recent laboratory observations of hollow cathode plasma contactors operating in the “ignited” mode.
Viking observations above a postnoon aurora
Authors: Brüning K. , Block L. P. , Marklund G. T. , Eliasson L. , Pottelette R. , Murphree J. S. , Potemra T. A. , Perraut S.
Published: February 2008

This is the first paper that combines observations of the electric and magnetic fields, particles, wave emissions and UV images taken at intermediate altitude (two Earth radii) above the acceleration region of a 14 MLT aurora. The observations were made by the Swedish Viking satellite. The aurora appeared in the region of large‐scale reversal of the perpendicular electric field. Strong fluctuations in the electric and magnetic field indicated that the aurora consisted of many arcs of varying latitudinal extent. Spin‐averaged data show that an upward current sheet was connected with the observed UV aurora, with an ionospheric current density of 4 µA/m² in the center of the sheet and 7 µA/m² near the equatorward edge. The field‐aligned potential drop V∥ was determined from the mean energy of the upward ion beams and the width of the electron loss cone. V∥ increased from several hundred volts near the equatorward edge of the current sheet to at least 3 kV in the center. Auroral kilometric radiation (AKR) and the widened electron loss cone indicated that the top of the acceleration region with V∥ ≥ 3 kV was located near one Earth radius. The critical field‐aligned potential drop, V∥c, which is strong enough to accelerate the bulk of the electrons into the loss cone, was derived from the characteristic electron energy T and the ratio of the magnetic field in the ionosphere to that at the top of the acceleration region, Bi/Bv. The source plasma parameters were typical for magnetosheath plasma with a characteristic electron energy of about 200 eV. This means that in the center of the current sheet V∥c was 1.4 kV, and thus the observed field‐aligned potential drop, V∥ ≥ 3 kV, was greater than V∥c, indicating a saturation current. At the edges V∥c was higher than the measured potential drop, indicating a linear relationship with current density. There was a density depletion in the region of saturation current, and upward ion conies were observed with energies above V∥c. The electron source density determined from (1) the particle data and (2) the local plasma frequency agrees fairly well with that calculated from the current density observed by the magnetometer, the characteristic electron energy, and the altitude of the top of the acceleration region. This fact provides further evidence for the existence of a saturation current. Comparisons with earlier work indicate that the observed values for T, Bi/Bv, and V∥ are not exceptions but rather typical for the postnoon sector. We therefore conclude that a saturation current may be a common phenomenon for intense postnoon aurora. Similarities with the current‐voltage relationship for strong laboratory double layers are discussed.
Theory and observations of electrostatic ion waves in the cold Io torus
Authors: Barbosa D. D. , Kurth W. S.
Published: February 2008

A study of the ELF plasma wave environment of the cold Io torus in Jupiter's magnetosphere is made. Voyager 1 data are presented which show three distinct types of electrostatic ion waves occurring there: the Buchsbaum ion‐ion mode just below the proton cyclotron frequency fcp, hydrogen Bernstein modes at (n+½)fcp, and lower hybrid waves near fLHR. The presence of these waves at their characteristic frequencies is consistent with a predominantly heavy ion plasma composed of singly ionized sulfur and oxygen ions along with a small admixture of protons. The hydrogen Bernstein modes are tightly confined to the magnetic equator, occurring within ±4° of it, while the Buchsbaum mode is localized to the dense heavy ion plasma of the cold torus near the centrifugal equator. A general theory for excitation of the waves based on the ion pickup process is developed. Newly ionized S+ and O+ ions created out of the extended neutral clouds of Io form a ring distribution in velocity space which is unstable to a large variety of plasma wave modes. The particular emissions found here are readily excited given the assumption that the heavy ions are unmagnetized. This condition allows for the propagation of strictly perpendicular wave modes free of electron Landau damping while sustaining large growth rates due to the pickup ions.
Electron dispersion events in the morningside auroral zone and their relationship with VLF emissions
Authors: Hardy David A. , Burke William J. , Villalon Elena
Published: February 2008

Energy/time dispersion events have been observed in the precipitating electron data in the energy range from 630 eV to 20 keV recorded by the J sensor on the low‐altitude, polar‐orbiting HILAT satellite. The dispersions are such that the higher‐energy electrons are observed earlier in time than the lower‐energy electrons. The time interval for a single dispersion event is from 1 to 2 s. Within an auroral pass in which such energy/time dispersion events are observed, there are typically several such events, and they can be spaced within the pass in either a periodic or aperiodic manner. The events are typically observed within and toward the equatorward edge of the region of diffuse auroral electron precipitation. During a given pass the events can be observed over a wide range of L shells. The occurrence of these events maximizes in the interval 0600–1200 hours MLT. The energy/time dispersion is generally consistent with the electrons originating from a common source. The events are seen at L shells from 3.7 to greater than 15. The source distance for the electrons is inferred to be generally beyond the equator for events at L shells less than approximately 8 and before the equator for events at higher L shells. Because of the low energies at which the dispersions are observed, it is unlikely that their occurrence can be explained by resonant interaction with VLF waves. Based on circumstantial evidence from other reported observations common to the morning sector, an alternative theoretical explanation is presented. According to this model the dispersion events result from impulsive interactions of the electrons with intense, asymmetric packets of VLF waves via the nonlinear, ponderomotive force.
Observation of an antenna‐plasma instability
Authors: Kellogg P. J. , Monson S. J. , Whalen Brian A.
Published: February 2008

During a rocket flight to high altitude (585 km) a narrow band signal on an electric antenna was sometimes observed, whose frequency varied as the rocket turned. Such a signal cannot be natural, but apparently must be generated by the interaction of the rocket‐antenna system with the ambient plasma. Conditions for development of the instability are investigated. Maximum oscillation amplitude occurs when the antenna is aligned with the Earth's magnetic field. Similar observations have been reported by Gurnett and Mosier [1969]. Several attempts are made to understand the nature of this interaction, but without success. The instability treated by Fiala, due to the interaction of an inductive antenna impedance with stray capacitance to a phase shifted point in the preamplifier, can be ruled out. It appears that a negative antenna resistance due to interaction with waves Doppler‐shifted through zero is an unlikely explanation. The rocket velocity seems too small to give such an anomalous Doppler shift, and even if the observations of plasma density and temperature are stretched, the positive sheath resistance is larger than calculated negative resistances. Ion transit time instability in the sheath would only work at 10 times higher frequency. Interaction of the flowing plasma with sheath waves around the antenna is suggested and appears promising but the theory is not sufficiently developed for meaningful comparison.
Simulation studies of plasma waves in the electron foreshock: The generation of Langmuir waves by a gentle bump‐on‐tail electron distribution
Authors: Dum C. T.
Published: February 2008

The generation of Langmuir waves by a gentle bump‐on‐tail electron distribution is analyzed. It is shown that with appropriately designed simulation experiments, quasi‐linear theory can be quantitatively verified for parameters corresponding to the electron foreshock. The distribution function develops a plateau by resonant diffusion, and changes outside this velocity range are negligible, except for the contribution of nonresonant diffusion to acceleration of bulk electrons. The dispersion relation is solved for the evolving distribution function and exhibits the dynamics of wave growth and changes in real frequency. The integral of the quasi‐linear equations is also used to relate the evolution of distribution function and wave spectrum and gives agreement with the simulations. Even in extremely long simulation runs there is practically no evolution in wave energy or the distribution function, once a plateau has been formed. The saturated field levels are much lower than the estimates that are generally used to assess the importance of additional weak or strong turbulence effects. These effects cannot prevent plateau formation and are only noticeable if ions are also included in the model. They then lead to a redistribution of the spectrum toward low wave number modes which propagate mainly opposite to the beam. This occurs long after plateau formation and plays no significant role in the overall system dynamics or energy balance. One will have to live with quasi‐linear theory as a key ingredient for a global model of foreshock wave phenomena.
A study of the undisturbed mid‐latitude ionosphere using simultaneous multiple site ionosonde measurements during the Sundial‐86 Campaign
Authors: Sica R. J. , Schunk R. W. , Wilkinson P. J.
Published: February 2008

The Sundial‐86 campaign obtained simultaneous ionosonde measurements of NmF2 from 41 mid‐latitude sites geographically dispersed around the world from September 21 to October 5, 1986. A three‐dimensional, time‐dependent model of Earth's ionosphere has been used to fit the variations in NmF2 obtained from these sites. The model successfully reproduced the measurements in both the northern and southern hemispheres using the vertical plasma drift as a free parameter. During geomagnetically quiet conditions this drift is primarily due to the meridional neutral wind. The vertical plasma drift required to fit the data is consistent from location to location and the deduced meridional neutral wind is also consistent with previous neutral wind measurements and models. However, it is shown that a lack of knowledge of the variation of the O+ flux at the top of the atmosphere introduces a large uncertainty in the deduced thermospheric winds.
The magnetopause for large magnetic shear: Analysis of convection electric fields from AMPTE/IRM
Authors: Sonnerup B. U. Ö. , Papamastorakis I. , Paschmann G. , Lühr H.
Published: February 2008

A number of magnetopause crossings by the AMPTE/IRM spacecraft, which exhibited large shear in the magnetic field, were studied recently by Paschmann et al. (1986) in order to assess the presence or absence of reconnection processes at the dayside magnetopause. In the present article, eleven of these crossings are reanalyzed by use of methods described by Sonnerup et al. (1987). Although only eight of the cases lend themselves to this more detailed analysis procedure, the results derived when the analysis is successful are considerably more detailed and quantitative than those obtained previously. Four such cases are documented in detail in the paper. For these events it was possible to derive accurate vectors, n, normal to the magnetopause. It was also possible to place bounds on the values of average magnetic field and flow velocity components along n as well as on the average electric field tangential to the magnetopause and on the velocity of the magnetopause itself along n and therefore on the magnetopause thickness. It is particularly noteworthy that in two crossings (on September 4, 1984) it was possible to place lower bounds of 2.8 mV/m and 0.5 mV/m on the tangential (reconnection) electric field and corresponding lower bounds on normal flow and field. In a third crossing (September 8, 1984), located about 1 RE north of the subsolar point, evidence is deduced, indicating that the observed plasma acceleration along the magnetopause had a large temporal component, caused by a tangential gradient in the total pressure, in addition to the usual convective component caused by the Maxwell shear stresses. Finally, in the fourth crossing (October 19, 1984), large magnetic shear was present but, in agreement with a conclusion reached by Paschmann et al. (1986), the analysis shows that no evidence for reconnection was present and that the magnetopause had the properties of a tangential discontinuity. This case provides convincing evidence that the mere existence of a deHoffmann‐Teller frame of reference in which the flow is field aligned (an excellent fit of the data to such a frame could be performed in this case) does not guarantee that the magnetopause is a rotational discontinuity. The remaining events analyzed by use of the new procedure demonstrate that the methodology sometimes fails, or yields unconvincing results, even when there is considerable evidence, in the form of accelerated plasma flows, that some form of reconnection is in progress.
Longitudinal variation of mid‐latitude hiss from six long duration balloon flights
Authors: Dowden R. L. , Holzworth R. H.
Published: February 2008

The longitudinal variation of mid‐latitude hiss as measured on six balloons at latitudes of 35°S–55°S, shows a significant minimum at 70°E–80°E, about 1000 km east of the geomagnetic conjugate of the Soviet transmitter, UMS (17.1 kHz). It is suggested that the well documented pitch angle diffusion induced by the UMS signal removes the ability of the trapped electrons to maintain the amplification of the hiss to at least the reflection and re‐ducting losses (∼20 dB) at the ends of the echoing, ducted path. The eastward shift of the minimum from the conjugate of UMS is primarily caused by the time taken by the eastward drifting electrons to recover this ability, though the effect is enhanced by the east–west asymmetry in the propagation of hiss under the ionosphere.
A conjugate area study of Pc 3 pulsations near cusp latitudes
Authors: Wolfe A. , Venkatesan D. , Slawinski R. , Maclennan C. G.
Published: February 2008

Pc 3 pulsations are examined from conjugate studies at Iqaluit (Frobisher Bay), Northwest Territories, Canada, and at the geographic south pole, Antarctica (L ∼ 14). These Pc 3 (f ∼ 20–50 mHz) events are quantitatively selected from hourly power spectra of the H component magnetometer time series for local daytime hours. Above a cutoff frequency of 29 mHz we find that the frequency of these events has a linear relationship with hourly averages of the interplanetary magnetic field (IMF) magnitude as measured by the IMP 8 spacecraft while in the upstream region. These events are narrow‐banded and occur mostly in local morning and for IMF conditions with Bx > 0, By < 0, and Bz > 0. Further, the frequency of the selected events is found to be independent of the IMF below the 29‐mHz cutoff. These characteristics of Pc 3 agree well with our previously reported Pc 3 events observed at South Pole (Slawinski et al., 1988). However, few simultaneous events are observed at Iqaluit and South Pole, indicating a lack of conjugacy on an event by event basis. We conclude that upstream ion cyclotron waves are a source of these Pc 3 events above 29‐mHz frequencies in both hemispheres. We speculate that the lack of conjugacy may be due to the station(s) being on open field lines during these events or that field line resonance may not be excited at Pc 3 frequencies for such high latitudes.
The two‐tier structure of comet Giacobini‐Zinner's magnetic tail
Authors: Perez‐de‐Tejada H.
Published: February 2008

The structure of comet Giacobini Zinner's magnetic tail is interpreted in terms of a flow geometry similar to that proposed for the Venus wake. It is suggested that the two‐step shape of the magnetic field profile obtained from the ICE measurements across each lobe of the magnetic tail of that comet is due to the different entry of interplanetary magnetic fluxes into that region. The low (∼30 γ) intensity outer wings of the magnetic field profile result from magnetic fluxes convected by the mass‐loaded shocked solar wind that enters the tail from the polar boundary of the comet's magnetic cavity. The high (∼60 γ) intensity inner parts of the lobes result, on the other hand, from the compression that this flow produces on interplanetary magnetic field lines that have slipped past the polar boundary of the cavity after draping around the comet. The latter magnetic fluxes are not convected by the local plasma but forced to accumulate around the central plasma tail that extends behind the comet.
Spectral and spatial evolution of compressible turbulence in the inner solar wind
Authors: Marsch E. , Tu C.‐Y.
Published: February 2008

Magnetic field and plasma data collected by the Helios spacecraft between 0.3 and 1 AU near the activity minimum of solar cycle 21 have been analyzed to establish spectral characteristics of compressive fluctuations in the inner solar wind and their relation to the morphology of the plasma flow and magnetic field. The compressive turbulence level is found to be closely related to the stream structure or latitudinal location with respect to the heliospheric current sheet. Compressive turbulence in low‐speed flows is more fully developed and intense. The spectra are radially invariant and come close to a −5/3 spectral law. In contrast, fast stream turbulence becomes increasingly compressive, in terms of radially growing amplitudes of δn/n and δB/B, with increasing heliocentric distance. The spectra reveal a flatter high‐frequency part, which gradually seems to get straightened out at large solar distances. Single case studies, as well as averaged spectra and their spectral features, are presented and discussed in the context of the theoretical turbulence literature.
Statistical analysis of upflowing ion beam and conic distributions at DE 1 altitudes
Authors: Kondo T. , Whalen B. A. , Yau A. W. , Peterson W. K.
Published: February 2008

Statistical analyses of energetic (0.01–17 keV) upflowing ions (UFI), beams, conies, and hybrid pitch angle distributions, have been made using the energetic ion composition spectrometer (EICS) data on the DE 1 satellite for the 8000‐km to 23,300‐km altitude range over a 5‐year period from September 1981 to June 1986. Hybrid types are defined here as combinations of lower‐energy beams and higher‐energy conics at three different energies (0.01–1, 1–4, and 4–17 keV). In general, it is found that (1) the source and transport mechanisms confine UFI mostly to auroral field lines; (2) although field‐aligned currents and UFI events have similar frequencies of occurrence, no obvious (one‐to‐one) relationship between current direction and UFI source characteristics was found; (3) although H+ and O+ UFI spatial (latitude, local time, and altitude) distributions are similar, O+ is more efficiently accelerated than H+ at higher values of Kp; and (4) the electric potential drop along auroral field lines, which would accelerate ions away from the ionosphere, is small (<1 kV) except perhaps in the dusk to midnight sectors where on occasion large values (1–10 kV) may be present.
Electron heating in quasi‐perpendicular shocks: A Monte Carlo Simulation
Authors: Veltri Pierluigi , Mangeney André , Scudder Jack D.
Published: February 2008

To study the problem of electron heating in quasi‐perpendicular shocks, under the combined effects of “reversible” motion, in the shock electric potential and magnetic field, and wave‐particle interactions we have derived a diffusion equation, in the drift (adiabatic) approximation and we have solved it, using a Monte Carlo method. The results show that most of the observations can be explained within this framework. Our simulation has also definitively shown that the electron parallel temperature is determined by the dc electromagnetic field and not by any wave particle induced heating. Wave‐particle interactions are effective in smoothing out the large gradients in phase space produced by the “reversible” motion of the electrons, thus producing a “cooling” of the electrons. Some constraints on the wave‐particle interaction process may be obtained from a detailed comparison between the simulation and observations. In particular, it appears that the adiabatic approximation must be violated in order to explain the observed evolution of the perpendicular temperature.
Relationship of solar wind parameters to continuous, dayside, high latitude traveling ionospheric convection vortices
Authors: McHenry Mark A. , Clauer C. Robert , Friis‐Christensen Eigil
Published: February 2008

In a companion paper we have shown that many continuous, dayside, high latitude magnetic pulsations are caused by steady, traveling ionospheric convection vortices (McHenry et al. this issue). A variety of evidence indicates that these vortices are the ionospheric signatures of the Kelvin‐Helmholtz instability at the inner edge of the magnetospheric boundary layer. In this paper we present the results of a statistical study of the occurrence of these vortices and the upstream solar wind parameters observed by the IMP 8 spacecraft. Surveying fifty days of Greenland west coast chain magnetometer data indicates this class of pulsations is most likely to be detected post local noon and when the solar wind speed is low. However, it is possible that observational factors significantly affect the detection of the vortices. The slow solar wind might create large, slow moving traveling vortices of steady strength which are easiest to identify. Little correlation is found between the average IMF and the probability of detecting the vortices. We also find a strong correlation between the frequency of dayside pulsations and the solar wind speed. This suggests that many pulsations are caused by traveling ionospheric current systems that map to the vicinity of the flows in the magnetospheric boundary layer. Periods also exist when the IMF is variable and large pulsations with 5 to 20 min period exist. These pulsations are not caused by traveling ionospheric vortices but are likely to be the result of rapid variations of the large scale field‐aligned cusp currents.
Differential memory in the trilinear model magnetotail
Authors: Chen James , Mitchell Horace G. , Palmadesso Peter J.
Published: February 2008

The previously proposed concept of “differential memory” is quantitatively demonstrated using an idealized analytical model of particle dynamics in the magnetotail geometry. In this model (the “trilinear” tail model), the magnetotail is divided into three regions. The particle orbits are solved exactly in each region, thus reducing the orbit integration to an analytical mapping. It is shown that the trilinear model reproduces the essential phase space features of the earlier model (Chen and Palmadesso, 1986), possessing well‐defined entry and exit regions, and stochastic, integrable (regular), and transient orbits, occupying disjoint phase space regions. Different regions have widely separated characteristic time scales corresponding to different types of particle motion. Using the analytical model, the evolution of single‐particle distribution functions is calculated.
A procedure for the extraction of airglow features in the presence of strong background radiation
Authors: Swift W. R. , Torr D. G. , Torr M. R. , Hamilton C. , Dougani H. , Richards P. G. , Sivjee G. G.
Published: February 2008

Ground‐based measurements of terrestrial airglow are useful for many applications in aeronomy. However, in the past observations have been limited to times and circumstances when the relatively weak airglow features are not overwhelmed by bright sky background emission due mainly to Rayleigh scattering of sunlight. In this paper we report a procedure which can be used to automatically extract weak airglow emissions in the presence of strong background radiation. The computer code developed for this purpose is user friendly and provides, in addition to retrieved airglow intensities, quantitative diagnostics which include estimates of error of measurement. The procedure is designed to allow rapid bulk processing of data which has not been possible previously.
Development of pitch angle anisotropy and velocity diffusion of pickup ion shell distribution by solar wind turbulence
Authors: Yoon Peter H. , Ziebell L. F.
Published: February 2008

It is well known that intrinsic solar wind turbulence can cause rapid pitch angle diffusion of newborn ions and subsequently form a nearly isotropic shell distribution. If the initial shell distribution is subjected to a continuous influence of turbulence, the thickness of the shell distribution may increase as a result of diffusion in velocity space. Since the pitch angle diffusion rate is much larger than the velocity diffusion rate, it is often anticipated that the ion distribution will remain fairly isotropic. However, the velocity diffusion process is itself pitch angle dependent, and as a result, significant pitch angle anisotropy may develop at the early stage of evolution. This result can be of importance, particularly if the pickup ion density is sufficiently large, because the pitch angle anisotropy may excite collective instabilities that can further increase the wave level, thus accelerating the diffusion process. In this work, detailed analyses of the pitch angle and velocity diffusion processes of initial shell distribution subjected to a continuous influence of solar wind turbulence are presented.
An estimate of the maximum speed of the solar wind, 1938–1989
Authors: Cliver E. W. , Feynman J. , Garrett H. B.
Published: February 2008

Beginning with a list of severe geomagnetic storms, we obtain an estimate of the maximum solar wind flow speed at Earth during the past ∼50 years. We do this by (1) focusing on the subset of severe storms that followed major proton flares (since large proton events are strongly associated with fast coronal mass ejections), (2) calculating the average speed of the associated interplanetary shocks from the time intervals between the flares and the onsets of the storms, and (3) using an empirical relationship between the average shock speed and the maximum flow speed of the associated transient stream to infer a peak flow velocity for each event. We find no evidence for bulk flow velocities greater than the ≳2000‐km/s value deduced from Prognoz 2 and HEOS 2 in situ plasma measurements for the August 4, 1972, event. The ≳2000‐km/s speed for that event does not appear to be anomalously high, however; there are other credible events, occurring before 1960, with inferred flow speeds approaching this value. The ≳2000‐km/s value lies at the high‐speed edge of a reasonable single distribution of peak solar wind speeds for a representative sample of the most energetic solar‐terrestrial events observed from 1938 to 1989.
Magnetic reconnection in collisionless plasmas: Prescribed fields
Authors: Burkhart G. R. , Drake J. F. , Chen J.
Published: February 2008

The structure of the dissipation region during magnetic reconnection in collisionless plasma has been investigated. We consider a prescribed two‐dimensional magnetic x line configuration with an imposed reconnection electric field Ey. Particles are injected onto a computational grid, their orbits are integrated, and the moments of the distribution function are stored. The structure of the dissipation region depends on only two variables: a normalized reconnection electric field Ê and the opening angle θ of the separatrices of the magnetic field. An important conclusion of the work is that there is no linear relationship between the current sheet velocity νy and the electric field. For a small normalized electric field the maximum νy is localized away from the x line, is diamagnetic in origin (produced by local pressure gradients), and is independent of Ê as Ê → 0. For a large normalized electric field the effective life‐time of particles in the reconnection region scales as Ê−1/3 and νy, ∝ Ê2/3. In this limit, particles are ejected from the reconnection region as high‐velocity, gyrophase‐bunched beams. These beams produce an irregular filamentary current distribution in the outflow region. The beams which are ejected along the center of the outflow region are eventually trapped by the magnetic field while the beams ejected just downstream from the separatrix continue to move with high velocity out of the computational region. Analytic expressions for the current sheet velocity (νy), inflow and outflow velocities, and the scale size of the dissipation region are derived. Over the entire range of Ê, significant temperature anisotropies are produced, with T⊥ < T∥ in the inflow region and T⊥ ≫ T∥ in the outflow region. Implications of our results for understanding magnetic reconnection in magnetospheric plasma and the source of energetic particles measured in the plasma sheet boundary layer and in explaining recent observations of fine scale magnetic turbulence in the central plasma sheet are discussed.
A self‐consistent two‐dimensional resistive fluid theory of field‐aligned potential structures including charge separation and magnetic and velocity shear
Authors: Hesse Michael , Birn Joachim , Schindler Karl
Published: February 2008

A self‐consistent two‐fluid theory that includes the magnetic field and shear patterns therein is developed to model stationary electrostatic structures with field‐aligned potential drops. Shear flow is also included in the theory since this seems to be a prominent feature of the structures of interest. In addition, Ohmic dissipation, a Hall term and pressure gradients in a generalized Ohm's law, modified for cases without quasi‐neutrality are included. In the analytic theory, the electrostatic force is balanced by field‐aligned pressure gradients, i.e., thermal effects in the direction of the magnetic field, and by pressure gradients and magnetic stresses in the perpendicular direction. Within this theory simple examples of applications are presented to demonstrate the kind of solutions resulting from the model. The results show how the effects of charge separation and shear in the magnetic field and the velocity can be combined to form self‐consistent structures such as are found to exist above the aurora, suggested also in association with solar flares.
A numerical solution of the coupled proton‐H atom transport equations for the proton aurora
Authors: Basu B. , Jasperse J. R. , Grossbard N. J.
Published: February 2008

A numerical code has been developed to solve the coupled proton‐H atom linear transport equations for the proton aurora. The transport equations have been simplified by using plane‐parallel geometry and the forward‐scattering approximations only. Otherwise, the equations and their numerical solutions are exact. Results are presented for the particle fluxes and the energy deposition rates, and they are compared with the previous analytical results that were obtained by using additional simplifying approximations. It is found that although the analytical solutions for the particle fluxes differ somewhat from the numerical solutions, the energy deposition rates calculated by the two methods agree to within a few percent. The accurate particle fluxes given by the numerical code are useful for accurate calculation of the characteristic quantities of the proton aurora, such as the ionization rates and the emission rates.
Ion microprobe evidence for the presence of excess 26Mg in an Allende anorthite crystal
Authors: Bradley J. G. , Huneke J. C. , Wasserburg G. J.
Published: February 2008

An ion microprobe has been used to observe 26Mg/24Mg in excess of normal values in anorthite crystals from Allende refractory inclusions. In one crystal, 26Mg excesses up to 40% above normal, correlated with Al/Mg, were observed, These excesses are many times larger than those observed by other techniques. The correlation with Al/Mg indicated that the excess 26Mg in the Allende anorthite is uniformly distributed in the anorthite crystal. Analytical procedures and conditions were carefully controlled. Nevertheless, small but significant apparent 26Mg/24Mg anomalies in isotopically normal samples were observed in analyses by one ion microprobe. A repetition of the isotopic analyses using another ion probe produced normal ratios in the normal samples and confirmed large 26Mg excesses in the Allende anorthite.
The northwestern termination of the Himalayan Mountain Front: Active tectonics from microearthquakes
Authors: Armbruster J. , Seeber L. , Jacob K. H.
Published: February 2008

The seismicity of the northwestern Himalayan syntaxial bend and the geologically complex area to the west, between the Hazara thrust system (HTS) and the higher mountains of Indus‐Kohistan, is examined in a wider tectonic context by using data from about 1,800 microearthquakes. The microearthquake data were obtained from a telemetered seismic network in northern Pakistan centered at Tarbela dam on the Indus River and were collected during an 11‐month period prior to impounding of the Tarbela reservoir. The observed seismicity indicates that a branch of the main boundary thrust (MBT) traverses the region as a straight northwesterly extension of the Murree thrust, the mapped section of the MBT southeast of the syntaxial bend along the Kashmir Himalayas. Seismic release on this extension of the MBT, here named the Indus‐Kohistan seismic zone (IKSZ), is highest in the upper 25 km of the crust and correlates with a pronounced topographic step. Deeper activity on the IKSZ extends to a depth of 70 km. Seismicity in the lower crust defines a second lineation 100 km southwest of the IKSZ and parallel to it. The syntaxial bend and the eastern HTS, microseismically virtually inactive at present but associated with recent and historical microseismicity, overprint the two northwesterly seismic lineaments. The two tectonic regimes may be simultaneously active at different depths separated by an incompetent layer. A set of steeply dipping faults, either parallel or perpendicular to the IKSZ, is active in the region between the HTS and the IKSZ. Seventeen composite fault plane solutions show a predominant pattern of either reverse or strike slip faulting with the inferred slip vectors oriented such that north‐south compressional stresses are relieved. Such a stress field is compatible with the north‐south convergence between the Indian and Eurasian plates inferred from plate tectonics.