bluuebunny/crossref_metadata_2025 huggingface dataset:
Substorm effects on the plasma sheet ion composition on March 22, 1979 (CDAW 6)
Authors: Lennartsson W. , Sharp R. D. , Zwickl R. D.
Published: February 2008

Data from the Plasma Composition Experiment on ISEE 1, covering the energy range 0.1–16 keV/e, show that a dramatic change took place in the plasma sheet ion composition in conjunction with the magnetic substorm activity on March 22, 1979. Beginning about 1124 UT the ion population at the ISEE 1 location changed from what appeared to be predominantly ions from the solar wind to a mixture of comparable numbers of solar wind and terrestrial ions. ISEE 1 was inbound in the predawn sector during this time, and the plasma composition experiment provided data from R ≃ 21 RE and LT ≃ 0130, down to R ≃ 3 RE and LT ≃ 0530. Prior to the substorm activity about 90–95% of the ion density was due to H+ and He++ ions, which appeared to be mostly of solar wind origin. The H+ and He++ components, each approximated by a Maxwell‐Boltzmann distribution, had a temperature ratio T(He++)/T(H+) ≃ 4 and a density ratio n(He++)/n(H+) ≃ 1.5–3%. Both values are consistent with measurements made concurrently in the solar wind by the plasma experiment on ISEE 3. The remaining 5–10% of the density was due mainly to O+ and He+ ions of ionospheric origin. All four ion populations had broad energy spectra with mean energies of several keV/e. About 25–30 min after the onset of the first substorm expansion at 1054 UT the H+ and He++ densities dropped sharply at ISEE 1 (at about 15 RE), and within the next 10 min the O+ density increased by almost an order of magnitude throughout the measured energy range, resulting in comparable amounts of O+ and H+. The O+ component of the plasma remained large or dominant through the remainder of the inbound pass. A significant portion of the O+ ions was contained within strongly collimated beams flowing tailward with energies mostly below 1 keV but occasionally reaching 6–7 keV. Similar beams of weaker intensity were also seen in the H+ and He+ components. The anticorrelation of ions from the two sources indicates that the particle flow pattern underwent a fundamental change in conjunction with the first substorm, possibly associated with a diversion of cross‐tail currents through the polar ionosphere. The strong increase in the O+ density may suggest that the O+ ions carried a substantial portion of an outward field‐aligned current connecting the auroral electrojet regions to the plasma sheet near ISEE 1 and beyond. The fact that the O+ and He+ ions had energies much higher than what is typical in the ionosphere can perhaps be explained in general terms by a large electric “potential difference” along the magnetic field, but the broad energy spectra and varying pitch angle distributions of these ions indicate that the acceleration process was also “diffusive” in nature.
A method to measure the bulk velocity of an energetic ion distribution in the presence of ion composition mixing
Authors: Daly P. W. , Sanderson T. R. , Wenzel K.‐P.
Published: February 2008

A method is presented for determining the bulk velocity of an energetic ion distribution containing a mixture of protons, alpha particles, and heavy ions, measured by a solid state detector system that does not distinguish different ion species. It is applied to the energetic particles anisotropy spectrometer on board ISEE 3 during its deep geotail mission. The measurements at 24 directions and six energy channels (35–619 keV) are fitted to the sum of three ion populations, all subjected to a common velocity transformation. Spectral shapes assumed are a combination of power law and exponential, based on measurements in the deep geomagnetic tail by composition experiments. Very good fits are achieved, and comparison of the result with those of composition experiments are in good agreement. The bulk ion velocities compare favorably to those of the plasma, at least in the plasma sheet. The error in the derived bulk velocity is estimated to be 100 km s−1. Wide differences are seen in the energetic ion boundary layer. Second‐order anisotropies are also determined in the moving frame of reference. These can be significantly different from those measured in the spacecraft frame because of the large transformations speeds.
Morphological study of energetic electron precipitation events using the satellite Bremsstrahlung X Ray Technique
Authors: Imhof W. L. , Kilner J. R. , Reagan J. B.
Published: February 2008

The precipitation of energetic electrons into the atmosphere is investigated with simultaneous measurements of bremsstrahlung X rays emitted from different local time sectors performed from the low‐altitude polar‐orbiting satellite P78‐1 with an array of X ray spectrometers (21–139 keV), each looking in a different direction. From the measurements spanning a broad interval in L, simultaneous profiles in local time are obtained of the intensities and energy spectra of the bremsstrahlung X rays which relate to the corresponding parameters for the precipitating electrons. From ∼0930 to ∼1400 MLT the average X ray intensity was found to decrease with increasing magnetic local time, whereas from ∼2200 to ∼0200 MLT, no clear trend with local time was evident. Within both of these local time intervals the average flux increased with increasing level of geomagnetic activity, and the data acquired near midnight were best organized by the AE index. The correlation between the fluxes of X rays emitted simultaneously at different magnetic local times decreased with increasing separation in local time, but a significant correlation still existed over separations as great as 3 hours. The energy spectrum was harder before midnight than after and harder before midnight than near noon. When a hard energy spectrum was observed in one local time sector, the spectra at other local times within 3 hours were significantly harder than average. From the entire set of data no evidence has been found for a distinct group of relativistic electron precipitation events.
A simple model for polar cap convection patterns and generation of θ auroras
Authors: Lyons L. R.
Published: February 2008

The simple addition of a uniform interplanetary magnetic field and the Earth's dipole magnetic field is used to evaluate electric field convection patterns over the polar caps that result from solar wind flow across open geomagnetic field lines. This model is found to account for observed polar‐cap convection patterns as a function of the interplanetary magnetic field components By and Bz. In particular, the model offers an explanation for sunward and antisunward convection over the polar caps for Bz > 0. Observed field‐aligned current patterns within the polar cap and observed auroral arcs across the polar cap are also explained by the model. In addition, the model gives several predictions concerning the polar cap that should be testable. Effects of solar wind pressure and magnetospheric currents on magnetospheric electric and magnetic fields are neglected. That observed polar cap features are reproduced suggests that the neglected effects do not modify the large‐scale topology of magnetospheric electric and magnetic fields along open polar cap field lines. Of course, the neglected effects significantly modify the magnetic field geometry, so that the results of this paper are not quantitatively realistic and many details may be incorrect. Nevertheless, the model provides a simple explanation for many qualitative features of polar cap convection.
Coupled low‐energy ‐ ring current plasma diffusion in the Jovian magnetosphere
Authors: Summers Danny , Siscoe George L.
Published: February 2008

We derive a functional form of the diffusion coefficient appropriate to fully developed, steady state, centrifugally driven turbulence in Jupiter's magnetosphere, incorporating in the diffusion coefficient the effects of the pressure gradient of the ring current. We set up a steady state, coupled diffusion model to describe the simultaneous outwardly diffusing low‐energy (Iogenic) plasma and the inwardly diffusing ring current plasma. The model is a nonlinear, third‐order, ordinary differential system containing four parameters. In order to test the hypothesis that the pronounced reduction in the radial diffusion coefficient (implied by the existence of the Io plasma ramp) is caused by the pressure gradient of the ring current, we seek solutions to the model under the condition that a reduced form of the diffusion coefficient takes on a minimum value at some point within the spatial extent of the ramp, i.e., within the range 7 ≤ L ≤ 8, where L is radial distance in units of Jovian radius RJ. This condition leads to several necessary conditions on the model parameters and the values of the ring current density and density gradient at the minimum point. We find that high‐energy (0.5–1.05 MeV) ring current particles, which have been previously observed and whose density decreases inwardly by several orders of magnitude over the range 6 ≤ L ≤ 10, cannot cause a sufficient inhibition of Iogenic plasma diffusion to cause the ramp. Further, we show that for ring current pressure to cause the ramp there must exist a component of the ring current particle population with sufficiently low thermal energy and with sufficiently high density, e.g., corresponding to an Iogenic plasma flux of 2×1028 s−1 the bulk of the ring current particles must have thermal energies of less than 3.9 keV and corresponding ring current particle densities at, say, L = 7.5 must exceed by two orders of magnitude those observed for the high‐energy component. We infer the existence of this dense, low‐energy component of the ring current from an independent energy calculation. Our model predicts that the density of the dense, low‐energy ring current particles decreases inwardly only by about an order of magnitude over the range 6 ≤ L ≤ 10. We provide representative numerical solutions for the density profiles of the Iogenic and ring current plasmas.
Coupling of newborn ions to the solar wind by electromagnetic instabilities and their interaction with the bow shock
Authors: Winske D. , Wu C. S. , Li Y. Y. , Mou Z. Z. , Guo S. Y.
Published: February 2008

The process by which the solar wind assimilates newly ionized atoms is important for understanding the presence of planetary or interstellar helium in the solar wind, the dynamics of the Active Magnetospheric Particle Tracer Explorers (AMPTE) lithium releases in front of the earth's bow shock, and the formation of cometary tails. In this paper we examine how newborn ions can be coupled to the solar wind in the direction parallel to the magnetic field by means of electromagnetic instabilities driven by the distribution of newborn ions. The linear properties of three instabilities are analyzed and compared with numerical solutions of the linear dispersion equation, while their nonlinear behavior is followed by means of computer simulation to obtain the characteristic time for the pickup process. With a primary emphasis on the AMPTE lithium releases, various degrees of realism are introduced into the calculations to model the upstream conditions and the intersection of the lithium with the bow shock. It is shown that a time‐dependent shock model is needed to correctly reproduce the amount of lithium which is transmitted through the shock and that the resulting lithium ion distribution is still likely to be subject to the same type of instabilities in the magnetosheath. Application of these results to comets, in particular the artificial comet expected to be generated by the AMPTE barium release in the magnetosheath, is also briefly discussed.
Striation eigenmodes along the geomagnetic field and eigenvalues in the limit of strong ion‐neutral collisions
Authors: Sperling J. L. , Glassman A. J.
Published: February 2008

A second‐order linear differential equation is derived for determining the linear eigenmodes of striation instabilities along the geomagnetic field. Eigenvalues (i.e., temporal frequencies in complex space) are determined along with the eigenmodes. The differential equation includes the contribution of two significant physical effects, ion polarization currents and inductive electric fields. The numerical evaluation of the differential equation emphasizes the collisional limit appropriate to situations when the instabilities are driven at altitudes where the ion‐neutral collision frequency is much larger than the growth rate for the instability. The numerical results indicate that the instabilities can drive parallel electron currents in the magnetosphere. However, because of finite parallel resistivity, magnetic field lines generally cannot be considered to be equipotentials. It is suggested that the coefficient for ion diffusion resulting from ion‐neutral collisions plays an important role in determining the maximum perpendicular wave number for which bifurcation is possible. The coefficient for ion‐neutral diffusion results in a qualitatively and quantitatively meaningful diffusivity for application to the “U‐shaped” curve description of striation evolution.
Characteristic time constants and velocities of mid‐latitude Pi 2's
Authors: Samson J. C. , Harrold B. G. , Yeung K. L.
Published: February 2008

This study presents measurements of group delays, rise times, fall times, frequencies, and effective phase velocities of mid‐latitude (39°–55° geomagnetic latitude) Pi 2's recorded by the Air Force Geophysics Laboratory array of flux gate magnetometers. Measured group delays over the array are sometimes less than 5–10 s, indicating that in some instances Pi 2's can be used to give excellent timing of the onset of substorm expansive phases. Rise and fall times for the envelopes of Pi 2 oscillations are typically 60–100 s, bringing into doubt resonance theories for these types of waves. Most phase velocities indicate westward propagation, particularly near the center of the substorm current wedge, although there is some indication of eastward velocities far to the east of the center. We argue that these observations indicate that the magnetic fields associated with mid‐latitude Pi 2's are due to transient changes in the field‐aligned currents feeding the auroral electrojets at the beginning of the expansive phase of the substorm.
Eiscat multipulse technique and its contribution to auroral ionosphere and thermosphere description
Authors: Kofman Wlodek , Lathuillere Chantal
Published: February 2008

In this paper, we describe European Incoherent Scatter Scientific Association (EISCAT) capabilities to measure ionospheric parameters in the E and F regions simultaneously. We discuss especially the multipulse technique which we used for E region study. We show the behavior of the auroral ionosphere during a period with low energy input and its response to precipitation of electrons with wide energy spectrum. Finally, we show neutral density measurements in the lower thermosphere for two different days with different energy input.
Band‐limited ELF emission burst (auroral roar)
Authors: Sato Natsuo , Hayashi Kanji
Published: February 2008

A new type band‐limited ELF emission burst which is closely associated with impulsive auroral hiss and auroral breakup is identified on the ground at Syowa Station (L = 6.1) in Antarctica. The ELF emissions have a center frequency of 100–350 Hz and a duration of 10–150 s, in which band‐limited structures are found in frequency‐time displays. In many cases the band‐limited structures show falling tone with time. Most of the emissions are observed at the onset of the substorm expansion phase for the limited magnetic local time of 1700–2400 MLT and in the austral winter season of June–August. The new band‐limited ELF emission burst is termed “ELF auroral roar” because it is observed with a roaring sound through an audio speaker.
A major shock‐associated energetic storm particle event wherein the shock plays a minor role
Authors: van Nes P. , Roelof E. C. , Reinhard R. , Sanderson T. R. , Wenzel K.‐P.
Published: February 2008

The energetic particle event associated with the quasi‐perpendicular interplanetary shock which passed ISEE 3 on June 6, 1979, is characterized by persistent beamlike antisunward particle fluxes on both sides of the shock. We found that the shock has no significant nonadiabatic effects on the energetic particles near 1 AU. The lack of particles with pitch angles larger than 60° accounts for the absence of signatures of shock drift acceleration. The adiabatic behavior of the bulk of the particles at the shock offers a unique opportunity to understand the role of the postshock magnetic regime. A recently formed magnetic discontinuity just downstream from the shock forms an effective obstacle for particles, particularly those with a small gyroradius. From the spatial dependence of the particle population in front of the magnetic discontinuity we derived the escape probability for particles to cross the discontinuity. Strong anisotropic particle bursts are observed as intensity spikes (duration less than 1 min) both upstream and downstream from the shock. Velocity dispersion in some of these spikes is consistent with impulsive release at the magnetic discontinuity. We propose that the spikes are accomplished by a disturbance propagating along the discontinuity which develops a normal component perhaps accompanied by induced electric fields, thus enabling, in particular, low‐energy particles with their small gyroradii to cross and stream into the upstream region. The energetic particle fluxes in this event are among the highest we have observed at ISEE 3. These fluxes are not produced by shock acceleration near 1 AU, but originate somewhere else far downstream from the magnetic discontinuity. From what we have learned from the unambiguous particle population signatures in this event, we believe that most shock‐associated particle events cannot be understood on the basis of shock interactions alone; rather, the intensity history of energetic particles below 1 MeV is determined by the complete ensemble of magnetic structures embedded in the compressed plasma well behind the shock.
Correlated observations of substorm effects in the near‐Earth region and the deep magnetotail
Authors: Scholer M. , Baker D. N. , Bame S. J. , Baumjohann W. , Gloeckler G. , Ipavich F. M. , Smith E. J. , Tsurutani B. T.
Published: February 2008

We present simultaneous observations of energetic particle measurements from the geosynchronous satellite 1982–019 and magnetic field, electron plasma, and energetic proton and electron measurements obtained with ISEE 3 in the deep tail. The data are supplemented by ground magnetograms. A substorm occurred on March 22, 1983, close to 0300 UT as identified in the ground magnetograms and by a particle injection at geosynchronous orbit. About 10 min later, ISEE 3 observed (at a distance of ∼130 RE in the deep tail) magnetic field, plasma, and energetic particle signatures consistent with the passage of a plasmoid. After the passage of the plasmoid the satellite enters shortly into a lobelike environment, in which an energetic proton beam is observed. High‐resolution magnetic field data are indicative of small‐scale structures in the postplasmoid plasma sheet. From the plasma sheet flow speed during the plasmoid's passage it is concluded that the 0300 UT substorm is responsible for its origin. This allows an approximate timing of the plasmoid release at a near‐earth neutral line and of the plasma sheet recovery after substorm onset, and it indicates a close relationship between processes in the near‐earth plasma sheet and the deep tail during substorms.
DE 1 observations of siple transmitter signals and associated sidebands
Authors: Rastani K. , Inan U. S. , Helliwell R. A.
Published: February 2008

VLF signals from the Siple Station, Antarctica, transmitter received on the DE 1 spacecraft provide new information on whistler mode signal propagation paths in the magnetosphere. In two case studies, the measured group delay in conjunction with in situ density measurements and ray tracing analysis are used to distinguish between direct nonducted propagation and a hybrid mode consisting of one‐hop propagation in a duct followed after ionospheric reflection by nonducted propagation. The extent of the observations both in space and time indicates that such a hybrid propagation mode may be an important means by which whistler mode signals generated or amplified in ducts can populate the magnetosphere. Computed group time delays based on a diffusive equilibrium model for the density distribution along the field lines provide good agreement with measurements over a wide range of magnetic latitudes (50°S–25°N). In one case, observed Siple signals are associated with sidebands of ±30 Hz spacing, and emissions are occasionally triggered. Sidebands are associated with signal components that are believed to have propagated on direct nonducted paths as well as in a hybrid (ducted/nonducted) mode with the sideband spacing being equal in both cases. The sideband spacing is found to be independent of the carrier frequency and amplitude and the satellite location with respect to the magnetic equator.
Universal time dependence of nighttime F region densities at high latitudes
Authors: de la Beaujardière O. , Wickwar V. B. , Caudal G. , Holt J. M. , Craven J. D. , Frank L. A. , Brace L. H. , Evans D. S. , Winningham J. D. , Heelis R. A.
Published: February 2008

Coordinated EISCAT, Chatanika, and Millstone Hill incoherent scatter radar observations have revealed that in the auroral zone, the nighttime F region densities vary substantially with the longitude of the observing site: EISCAT's densities are the largest and Millstone Hill's are the lowest. The nighttime F region densities measured by the individual radars are not uniform: the regions where the densities are maximum are the so‐called “blobs” or “patches” that have been reported previously. The observations are consistent with the hypothesis that the nighttime densities are produced in significant amounts not by particle precipitation, but by solar EUV radiation, and that they have been transported across the polar cap. The observed differences can be explained by the offset of the geographic and geomagnetic poles. A larger portion of the magnetospheric convection pattern is sunlit when EISCAT is in the midnight sector than when Chatanika is. In winter, when Millstone Hill is in the midnight sector, almost all the auroral oval is in darkness. This universal time effect, which was observed on all coordinated three‐radar experiments (September 1981 to February 1982), is illustrated using two periods of coincident radar and satellite observations: November 18–19, and December 15–16, 1981. These two periods were selected because they corresponded to relatively steady conditions. Dynamics Explorer (DE) measurements are used to aid in interpreting the radar observations. DE 1 auroral images show what portion of the oval was sunlit. DE 2 data are used to measure the ion drift across the polar cap. Because the altitude of the ionization peak was high, the decay time of the F region density was substantially longer than the transit time across the polar cap. The southward meridional wind that was observed coincidentally with the ionization patches at Chatanika and EISCAT contributed to the maintenance of the F region by raising the altitude of the peak. DE 2 Langmuir probe measurements of electron density clearly showed a UT dependence, the same as that in the radar measurements.
Solar activity and modulation of the cosmic ray intensity
Authors: Akasofu S.‐I. , Olmsted C. , Lockwood J. A.
Published: February 2008

A transfer function F between a measure of the solar activity S and the 11‐year cosmic ray intensity modulation Io‐I is determined for the two sunspot cycles 19 and 20. For S we used the monthly occurrence of major flares, all flares, sunspots and ssc's. In agreement with D. J. Hatton [1980] and G. A. Bowe and D. J. Hatton [1982], we find a well‐behaved transfer function for both major flares and all flares. It is a damped pulse with duration of 16 and 13 months for the major flares and all flares, respectively. These figures are significantly longer than that of 10 months obtained by Bowe and Hatton.
Broadband electrostatic noise in the magnetotail: Its relation to plasma sheet dynamics
Authors: Nishida A. , Hada T. , Anderson K. A. , Anderson R. R. , Bame S. J. , Hones E. W.
Published: February 2008

Intense broadband electrostatic noise is often observed in the magnetotail when the fast tailward flow and the southward polarity of the magnetic field indicate the progress of reconnection. We compare features of the noise with simultaneous observations of the magnetic field, plasma and energetic electrons. The noise intensity seems to maximize when the plasma density minimizes and the energetic electron flux rises in a spikelike fasion, appreciably later than the onset of the fast tailward flow associated with the southward magnetic polarity. Spectral characteristics of the noise suggest that in its high frequency
part at least the noise does not belong to normal modes of plasma waves. Several possibilities are considered for what this noise might be, including quasi‐thermal noise in the non‐Maxwellian plasma, artificial noise generated by spacecraft interaction with the medium, or electrostatic noise with wavelengths less than a Debye length.
Electron impact measurement of oscillator strengths for dipole‐allowed transitions of atomic oxygen
Authors: Doering J. P. , Gulcicek E. E. , Vaughan S. O.
Published: February 2008

Optical oscillator strengths for the seven most intense dipole‐allowed transitions in the 100‐eV incident energy electron scattering spectrum of atomic oxygen have been measured from forward scattering spectra. An apparatus was used in which a microwave discharge source, producing atomic oxygen seeded in a helium beam, was coupled to an electron spectrometer. The O ¹D population in the target beam was less than 1.5% of O ³P, and transitions from the O ³P ground electronic state only were observed. The performance of the apparatus was checked by measurement of optical oscillator strengths for helium and atomic hydrogen. Agreement with the National Bureau of Standards (NBS) values for a single spectrum of better than 10% was obtained for three He transitions and 13% for the atomic hydrogen n = 3 transitions when the He 1¹S ‐ 2¹P and H n = 1 ‐ n = 2 Lyman series transitions were normalized to the NBS values. The atomic oxygen 2p4 ³P ‐ 2p³3s ³S° (1304 Å) transition was normalized to an average experimental value of f = 0.048. Optical oscillator strengths obtained for the other transitions were 2p4 ³P ‐ 2p³4s ³S° (1040 Å), 0.010±0.002; 2p4 ³P ‐ 2p³3d ³D° (1026 Å), 0.019±0.001; 2p4 ³P ‐ 2p³3s′ ³D° (989 Å), 0.061±0.006; 2p4 ³P ‐ 2p³4d ³D° (972 Å), 0.016±0.006; 2p4 ³P ‐ 2p³3s″ ³P° (878 Å), 0.086±0.006; 2p4 ³P ‐; 2s2p5 ³P° (792 Å), 0.070±0.004. (All values have ±5% uncertainty in addition to the experimental scatter as indicated.) The electric dipole‐forbidden transition 2p4 ³P ‐ 2p³3p ³P was also observed as a weak process.
Nonisotropic coronal atmosphere on Io
Authors: Sieveka E. M. , Johnson R. E.
Published: February 2008

A model is presented for calculating nonisotropic coronal atmospheres. This is tested by comparison with the analytic results for an isotropic atmosphere. It is then used to consider differences between sublimated and sputtered corona on Io with reference to the ion and electron bombardment of such coronae when the exobase is at or near the surface.
A possible mechanism for the observed streaming of O+ and H+ ions at nearly equal speeds in the distant magnetotail
Authors: Singh Nagendra , Schunk R. W.
Published: February 2008

Several observations have been reported of O+ and H+ ions streaming away from the earth with nearly equal velocities in the distant magnetotail region. In these events, the O+ ions have an energy in the range 320–3200 eV, while the H+ ions are less energetic by a factor which is approximately equal to the O+/H+ mass ratio. A possible explanation for the creation of such streaming events is presented. The mechanism involves the acceleration of O+ and H+ ions through their interaction with electrostatic hydrogen cyclotron (EHC) waves on auroral field lines at altitudes above parallel potential drops. The H+ and O+ ions are accelerated sequentially in the following manner. When an upward flowing H+ ion beam interacts with a cold background H+ ion plasma, EHC waves can be strongly excited. These waves heat the cold background H+ ions to an average energy (W(H+)) in the range of from a few tens to a few hundreds of electron volts. The resulting pitch angle distribution of the energized H+ ions peaks at angles near 110°, which leads to an upward flowing ion cloud in the northern hemisphere. When a fresh H+ ion beam permeates this warm H+ ion cloud, weak EHC waves can be excited if the beam energy is near that needed for marginal instability. If the H+ ion beam is accompanied by an O+ ion beam, part of the O+ beam can interact with the weak EHC waves, producing O+ ions with energies W(O+) ≃ 16 W(H+), and the pitch angle distribution of these O+ ions matches that of the warm H+ ions. When such a synchronism is achieved, the O+ and H+ ions stream together along the field lines away from the source region. In the distant magnetotail, their pitch angle distribution becomes field‐aligned owing to the magnetic mirror force.
Characterization of the sources of VLF hiss and chorus observed on Geos 1
Authors: Lefeuvre F. , Helliwell R. A.
Published: February 2008

Ray‐tracing techniques are used to locate the apparent source regions of natural electromagnetic emissions observed on the GEOS 1 satellite and to determine the wave normal directions at this apparent source region. They are applied to VLF hiss, whose analysis in terms of the wave distribution function (WDF) has established that they often consisted of a sum of two or more very distinct wave packets at the point of observation. An example of a two‐peaked WDF chorus is also examined. A two‐dimensional ray‐tracing program is used. For the cases analyzed here, GEOS 1 data indicate that the earth's magnetic field is to be considered as dipolar, and a diffusive equilibrium model is used to describe the medium. It is shown theoretically that even if the exact location of the source region is difficult to estimate accurately, more precise information can still be obtained on the wave normal distribution at the source region. In the examples considered, it is pointed out that the VLF hiss was generated in the vicinity of the equator at high wave normal angles.
Temporal behavior of density perturbations in the polar wind
Authors: Singh Nagendra , Schunk R. W.
Published: February 2008

The temporal evolution of density perturbations in the supersonic, collisionless polar wind is presented by solving the time‐dependent hydrodynamic equations. When the perturbations occur in the form of extended density depletions, the temporal evolution shows some characteristics of expanding plasmas seen in numerical simulations of small‐scale plasmas, including ion acceleration. When the density depletion is relatively small, the expansion is headed by a shock. On the other hand, when the extended density depletion is very strong, the expansion is headed by a pair of forward and reverse shocks. Such shocks are common features of transients in the solar wind. The extent of ion acceleration also depends on the magnitude of the density depletions. Proton accelerations up to about 40 eV (∼90 km/s) have been seen. In a nonflowing plasma a rarefaction wave propagates in a direction opposite to that of the expansion. However, in the case of the polar wind the rarefaction front is convected along with the wind, and its traversal determines the time for the reestablishment of the steady state situation. Perturbations in the form of localized density enhancements and depletions have also been studied. Such perturbations are found to be propagated upward in the direction of the polar wind, but they undergo a considerable modification, evolving into forward‐reverse shock pairs. Soon after a localized density depletion is created, it breaks into two secondary holes which propagate as rarefaction waves. The forward‐reverse shock pair is sandwiched between these waves. In the case of density bumps, the forward shock precedes the front of accelerated ions, while the reverse shock precedes it in the case of a density hole. These results are relevant to expanding astrophysical and space plasmas as well as to plasma expansion in certain ionospheric modification experiments.
Flux transfer events at the Jovian magnetopause
Authors: Walker Raymond J. , Russell Christopher T.
Published: February 2008

Recent evidence indicates that magnetic reconnection at the earth's magnetopause may not be a steady process, but rather it is frequently impulsive and limited in spatial extent. These limited reconnection events are called flux transfer events (FTE's). We have searched the magnetic field observations at Jupiter from Pioneer 10 and 11 and Voyager 1 and 2 for evidence of FTE's and have found 14 possible events. The FTE's at Jupiter are associated with northward magnetosheath fields. The electric fields generated by Jovian FTE's are small in comparison with the corotation E field throughout much of the magnetosphere. Thus FTE's are probably not an important source of flow within the Jovian magnetosphere.
Plasma irregularities associated with a morning discrete auroral arc: Radar interferometer observations and theory
Authors: Providakes J. , Farley D. T. , Swartz W. E. , Riggin D.
Published: February 2008

During the early morning of February 7, 1983, with a VHF Doppler radar interferometer we observed a discrete auroral arc that was also photographed by an all‐sky camera (ASC) at Fort Churchill, Canada. We determined horizontal drift velocities and hence the vector electric fields from the interferometer data. We measured both a weak ambient and stronger arc‐associated polarization field (sometimes exceeding 100 mV/m) along the poleward boundary of the arc from where the radar echoes were received. This latter field was in the proper direction (roughly parallel to the electron density gradient) to excite the gradient drift plasma instability. Both the radar and the ASC observed large (20–50 km) structures, probably caused by a Kelvin‐Helmholtz instability, along this same poleward edge. The radar measured both type I and type II Doppler spectra, and in addition, we also saw a few examples of a very narrow resonant spectral peak. Because of the Doppler shift (∼60–90 Hz) of this feature and the fact that it was associated with velocity shears (which imply a horizontally divergent electric field and field‐aligned currents) we believe these latter echoes were from O+ electrostatic ion cyclotron (EIC) waves generated at an altitude of about 150 km. But linear kinetic theory indicates that such waves, with lengths as short as 3 m and propagating at the observed radar aspect angle (between k and B) of about 83°, could be directly excited only by an unreasonably large field‐aligned electron velocity (greater than the electron thermal speed). More realistic velocities could, however, excite 20‐m EIC waves propagating at this angle, and if these steepen due to some nonlinear process, they could perhaps explain our observations.
Lower hybrid parametric processes on auroral field lines in the topside ionosphere
Authors: Koskinen Hannu E. J.
Published: February 2008

Various parametric decay channels of lower hybrid waves (LHW) in a topside ionospheric plasma are considered in order to find out whether VLF waves generated by precipitating keV electrons could be converted into waves having larger perpendicular wave number and thus interacting more effectively with ionospheric ions. In dipole approximation it is found that the most promising decay channel is a decay into a LHW and a nonresonant quasi‐mode. However, use of the dipole approximation can be criticized, and a consideration of the wave vector of the pump suggests that also the decay into hot ion plasma waves or ion Bernstein waves and nonresonant quasi‐modes is of importance. It is also noticed that the theory of parametric processes could be applied in interpretation of certain sounding rocket data where artificially produced LHW's are detected together with various other wave modes.
Shock evolution in the outer heliosphere: Voyager and Pioneer observations
Authors: Gazis P. R. , Lazarus A. J. , Hester K.
Published: February 2008

We report observations of 35 interplanetary shocks detected at heliocentric distances between 6.5 and 9.4 AU in mid‐1980 by the Voyager 1, Voyager 2, and Pioneer 11 spacecraft. These shocks were all evidently generated by the interaction of corotating streams. Measurements of the pre‐ and postshock plasma parameters are used to determine the shock normals and speeds for each shock. Twelve of these 35 events are observed at all three spacecraft. The shock parameters at each spacecraft are compared to determine the time history for the 12 shocks. The single‐spacecraft determinations of shock normal and speed are compared with the results of several techniques for determining shock velocities using all three spacecraft. We find that shocks undergo significant evolution as they travel past the three spacecraft. Every shock seen by both end spacecraft was also seen by the middle spacecraft. Shock formation possibly may be occurring at a heliocentric distance as great as ∼7 AU. It appears that the shock surfaces have a complex three‐dimensional structure with “ripples” on a scale of 0.001–1.0 AU.
A Sun‐aligned arc observed by DMSP and AE‐C
Authors: Hoffman R. A. , Heelis R. A. , Prasad J. S.
Published: February 2008

On May 12, 1977, 8 minutes after a DMSP satellite photographed a sun‐aligned auroral arc in the southern polar cap, AE‐C crossed over the same arc. Precipitating electrons were observed with a peak energy flux of 0.94 erg cm−2 s−1 but with no clear monoenergetic beam, and coincident ion precipitation was measured at energies of a few keV. A very sharp ion convection reversal was found coincident with the particle precipitation and embedded in a region of constant antisunward flow. Magnetic field data indicate that the arc occurred during the recovery phase of a weak magnetic storm, with the IMF in a toward sector and the Z component nearly zero. The data are consistent with a source of particles at altitudes of the order of 5–8 RE on field lines containing the plasma sheet boundary layer. The electron spectra do not indicate the existence of an electrostatic potential along the magnetic field lines projecting from the arc. The electrodynamic properties associated with the arc appear to be consistent with a simple model in which field‐aligned currents are required along any boundary where the horizontal ionospheric current diverges.
Effects of Birkeland current limitation on high‐latitude convection patterns
Authors: Marklund G. T. , Raadu M. A. , Lindqvist P.‐A.
Published: February 2008

It is shown how the high‐latitude convection pattern may be modified by substorm‐enhanced polarization electric fields. These are generated whenever the flow of those Birkeland currents which are associated with ionospheric conductivity gradients is limited. Such Birkeland currents are fed mainly by the enhanced Pedersen current in the evening and morning sectors of the auroral oval and by the enhanced Hall current around local midnight. As the current limitation increases, the ionospheric potential, represented here by a symmetric two‐cell pattern, will rotate clockwise and deform, as will the associated Birkeland current distribution, which will attain roughly a symmetric shape relative to the noonmidnight magnetic meridian. The resulting patterns are shown to agree well with observations. A pronounced westward intrusion of the equipotential contours occurs in the auroral oval and may be associated with the westward traveling surge. This feature does not, however, require any assumed longitudinal conductivity gradients. Rather, it falls out naturally when the closure of the enhanced Pedersen current is also taken into account.
A split separator line merging model of the dayside magnetopause
Authors: Crooker N. U.
Published: February 2008

A new magnetic field configuration in the magnetopause of an open magnetosphere is constructed by superposing a uniform field and the idealized Chapman‐Ferraro field. A straightforward interpretation of the results synthesizes previously unrelated models and observations: antiparallel and subsolar merging, uniform‐field‐plus‐dipole models, merging by the tearing mode instability, the magnetic curl configuration in laboratory terella experiments, flux transfer events, and accelerated boundary layer flows.
The propagation of coronal mass ejection transients
Authors: MacQueen R. M. , Hundhausen A. J. , Conover Charles W.
Published: February 2008

Measurements of the direction of propagation of 29 coronal mass ejection events observed during the Skylab epoch (1973–1974) and 19 events observed during the SMM epoch (1980) reveal that the former undergo an average 2.2° equatorward deflection, while the latter do not deviate significantly from radial motion. No differences between eruptive prominence‐associated or flare‐associated events can be detected for either epoch. The results suggest that coronal mass ejection events are influenced by the background coronal magnetic and flow patterns; the nonradial forces affecting the Skylab epoch mass ejections arise from the large‐scale dipolar magnetic field and flow configuration present at that time.
Measurements of Siple transmitter signals on the DE 1 satellite: Wave normal direction and antenna effective length
Authors: Sonwalkar Vikas S. , Inan U. S.
Published: February 2008

A new experimental technique is developed to simultaneously measure the wave propagation direction and the effective length of a small (L ≪ λ) electric dipole antenna on a spin stabilized satellite in the magnetosphere. The technique relies on the near simultaneous measurement of single components of the electric and magnetic fields of a coherent VLF signal injected into the medium from a ground‐based source. The spin fading characteristics of the signal received by the electric dipole and the magnetic loop antenna permit the measurement of the wave normal direction assuming whistler‐mode propagation. In situ and remote measurements of the local cold plasma density are used to determine the refractive index. The wave electric field is then inferred from the wave magnetic field as measured on the loop antenna, the refractive index and the direction of propagation. Comparing this electric field with the measured voltage across the dipole antenna leads to the determination of the effective length of the receiving electric dipole. The technique is applied to data from the Dynamics Explorer 1 satellite observations of whistler mode signals injected into the magnetosphere from the Siple, Antarctica, VLF transmitter. In one case, with the measured background cold plasma density being 15 el cm−3, the effective length of the 200 m long electric dipole antenna is found to be 222±56 m, i.e., about twice the conventional value.
Diurnal variability of vibrationally excited mesospheric ozone as observed during the SPIRE Mission
Authors: Green B. D. , Rawlins W. T. , Nadile R. M.
Published: February 2008

Spectrally resolved observations of infrared fluorescence (9–12 µm) from mesospheric ozone in the earth limb have been analyzed to deduce local excited vibrational state number densities as functions of altitude and solar illumination between 70 and 105 km. The nighttime and insolated quiescent polar atmospheres were sampled near the dawn terminator by a rocket‐borne cryogenic scanning filter spectrometer. Both the total intensity and the spectral distribution of the ozone fluorescence were observed to change after sunrise. The deduced vibrational populations revealed that this change was primarily due to a daytime reduction in the population of O3 (υ = 1), which is excited directly from ground state O3 by resonance absorption of earth radiation. In contrast, higher vibrational levels, formed by three‐body recombination of O and O2, show diurnal variations only at the lower altitudes, where the atomic oxygen concentration is expected to increase upon insolation. Thus the O3 (υ) fluorescence appears to be closely coupled to the local photochemical behavior of O and O3 near the mesopause.
Synoptic study of a large injection event
Authors: Kling R. , Speiser T. W. , Fritz T. , Baker D. N. , Garcia H.
Published: February 2008

In 1977, Luhmann presented an explanation for particle pitch angle distributions observed in the magnetosphere based on pitch angle dependent drift speeds. This was a numerical model and was not supported by observations at the time. On November 10, 1977, a large injection event was observed at about 1300 UT with data from synchronous satellite SMS 2 located at about 0400 LT. The subsequent evolution of the injection event was studied here using data from several other satellites, namely, GOES 2, 1977‐007, 1976‐059, ATS 6, and ISEE 1. Comparing the SMS 2 data with observations from the other satellites, it is highly probable that the SMS 2 “ion response” was in reality due to energetic electrons. Using a simple model and drift velocity dispersion, we infer an injection time approximately equal to the observed substorm onset time and an injection source location spread over several hours of local time in the nightside magnetosphere. Injection‐associated “butterfly” distributions (flux minima near 90° pitch angle) in the dayside magnetosphere were detected by ISEE 1 and ATS 6. These distributions are qualitatively in agreement with pitch angle dependent drift rate models, such as Luhmann's.
A plasma simulation study of the transformation from a closed to an open magnetic configuration
Authors: Wagner J. S. , Gray P. C. , Tajima T. , Akasofu S.‐I.
Published: February 2008

A 2½‐dimensional fully electromagnetic particle simulation code is used to study the temporal evolution of a plasma magnetized by a dipole embedded in a uniform magnetic field, as the direction of the uniform field reverses direction. The magnetic field topology is observed to change from an open to a closed configuration. Our simulations show that as the magnetic field reversal propagates into the ambient plasma, a current sheet forms and moves toward the dipole at about the local Alfvén speed. The current sheet breaks into filaments, with each filament interacting with the dipole in a process that resembles coalescence, resulting in some particle heating. We find that the resulting magnetic field topology does not depend significantly on the initial plasma β for 1.0 > β > 0.05.
Changes in the energy spectrum of anomalous oxygen during 1977–1985
Authors: Cummings A. C. , Stone E. C. , Webber W. R.
Published: February 2008

We have used data from the cosmic ray experiments on the Voyager and Pioneer 10 spacecraft during the period 1977–1985 to measure the energy spectrum of oxygen with 4–125 MeV/nucleon, with particular emphasis on the energy interval from ∼4 to 30 MeV/nucleon, where the “anomalous” component of cosmic rays is dominant. This time period includes the sunspot minimum period at the beginning of solar cycle 21, the time of the solar magnetic field reversal in 1980, the maximum solar modulation period of 1981–1983, and part of the recovery of the solar cycle through early 1985. We find that the spectrum of anomalous oxygen changes dramatically after the middle of 1980, with the peak or plateau region of the differential spectrum shifting to a higher energy. This change does not appear to be due to the increasing radial distance of the spacecraft in this time period, nor does it appear to be simply correlated with the level of modulation. The energy shift also appears to be significantly greater in the ecliptic plane than at 24° north latitude (the heliographic position of Voyager 1 at the end of the time period).