bluuebunny/crossref_metadata_2025 huggingface dataset:
Collisionless reconnection: Effects of Hall current and electron pressure gradient
Authors: Wang Xiaogang , Bhattacharjee A. , Ma Z. W.
Published: February 2004

An analytical treatment is given of two‐dimensional, quasi‐steady collisionless reconnection on the basis of the generalized Ohm's law, including the effects of the Hall current and scalar electron pressure gradient. The equilibrium magnetic field configuration is of the form

, containing a neutral line at z = 0 and a constant guide field BT. The dispersion relations of the waves in the ideal region as well as the reconnection layer are discussed, including the effects of plasma beta. When BT = 0, the reconnection layer supports obliquely propagating Alfvén‐whistler waves, and the reconnection dynamics is controlled by the Hall current. When BT/BP0 ≥ 1, the reconnection layer supports kinetic/inertial Alfvén waves, and the reconnection dynamics is controlled by the electron pressure gradient. Analytical estimates are obtained for the nonlinear reconnection rate with and without the guide field from the generalized Ohm's law. A recent claim by Shay et al. (1999) that the reconnection rate is a “universal constant” is questioned. Although the leading‐order reconnection rate is independent of the mechanism that breaks field lines (resistivity or electron inertia) and the system size as the system size becomes large, it does depend on global conditions such as the boundary conditions driving reconnection. The analytical predictions are tested by means of Hall magnetohydrodynamics simulations. While some of the geometric features of the reconnection layer and the weak dependence of the reconnection rate on resistivity are reminiscent of Petschek's classical model, the underlying wave and particle dynamics mediating reconnection in the presence of the Hall current and electron pressure gradient are qualitatively different.
Interplanetary fast shocks and associated drivers observed through the 23rd solar minimum by Wind over its first 2.5 years
Authors: Berdichevsky Daniel B. , Szabo Adam , Lepping Ronald P. , Viñas Adolfo F. , Mariani Franco
Published: February 2004

A list of the interplanetary shocks observed by Wind from its launch (in Nov 1994) to May 1997 is presented. The magnetohydrodynamic nature of the shocks is investigated, and the associated shock parameters and their uncertainties are accurately computed using two techniques. These are: 1) a combination of the “preaveraged” magnetic‐coplanarity, velocity‐coplanarity, and the Abraham‐Schrauner‐mixed methods, and 2) the Viñas and Scudder [1986] technique for solving the nonlinear least squares Rankine‐Hugoniot equations. Within acceptable limits these two techniques generally gave the same results, with some exceptions. The reasons for the exceptions are discussed. The mean strength and rate of occurrence of the shocks appear to correlate with the solar cycle. Both showed a decrease in 1996 coincident with the time of the lowest ultraviolet solar radiance, indicative of solar minimum and the beginning of solar cycle 23. Eighteen shocks appeared to be associated with corotating interaction regions (CIRs). The shock normal distribution showed a mean direction peaking in the ecliptic plane and with a longitude of ∼ 200° (GSE coordinates). Another 16 shocks were determined to be driven by solar transients, including magnetic clouds. These had a broader distribution of normal directions than those of the CIR cases with a mean direction close to the Sun‐Eart line. Eight shocks of unknown origin had normal orientations far off the ecliptic plane. No shock propagated with longitude ϕn≥ 220±10°, i.e. against the average Parker spiral direction. Examination of the obliquity angle θBn(i.e., between the shock normal and the upstream interplanetary magnetic field) for the full set of shocks revealed that about 58% were quasi‐perpendicular, and about 32% of the shocks oblique, and the rest quasi‐parallel. Small uncertainty in the estimated angle θBnwas obtained for about 10 shocks with magnetosonic Mach numbers between 1 and 2.
Properties of coronal mass ejections: SOHO LASCO observations from January 1996 to June 1998
Authors: St. Cyr O. C. , Howard R. A. , Sheeley N. R. , Plunkett S. P. , Michels D. J. , Paswaters S. E. , Koomen M. J. , Simnett G. M. , Thompson B. J. , Gurman J. B. , Schwenn R. , Webb D. F. , Hildner E. , Lamy P. L.
Published: February 2004

We report the properties of all the 841 coronal mass ejections (CMEs) observed by the Solar and Heliospheric Observatory (SOHO) Large Angle Spectroscopic Coronagraph (LASCO) C2 and C3 white‐light coronagraphs from January 1996 through June 1998, and we compare those properties to previous observations by other similar instruments. Both the CME rate and the distribution of apparent locations of CMEs varied during this period as expected based on previous solar cycles. The distribution of apparent speeds and the fraction of CMEs showing acceleration were also in agreement with earlier reports. The pointing stability provided by an L‐1 orbit and the use of CCD detectors have resulted in superior brightness sensitivity for LASCO over earlier coronagraphs; however, we have not detected a significant population of fainter (i.e., low mass) CMEs. The general shape of the distribution of apparent sizes for LASCO CMEs is similar to those of earlier reports, but the average (median) apparent size of 72° (50°) is significantly larger. The larger average apparent size is predominantly the result of the detection of a population of partial and complete halo CMEs, at least some of which appear to be events with a significant longitudinal component directed along the Sun‐Earth line, either toward or away from the Earth. Using full disk solar images obtained by the Extreme ultraviolet Imaging Telescope (EIT) on SOHO, we found that 40 out of 92 of these events might have been directed toward the Earth, and we compared the timing of those with the Kp geomagnetic storm index in the days following the CME. Although the “false alarm” rate was high, we found that 15 out of 21 (71%) of the Kp ≥ 6 storms could be accounted for as SOHO LASCO/EIT frontside halo CMEs. If we eliminate three Kp storms that occurred following LASCO/EIT data gaps, then the possible association rate was 15 out of 18 (83%).
Seasonal effects on auroral particle acceleration and precipitation
Authors: Liou K. , Newell P. T. , Meng C.‐I
Published: February 2004

Global auroral images acquired from the Polar ultraviolet imager in the Northern Hemisphere during the winter of 1996 and the summer of 1997 (4 weeks before and after solstice) are used to study seasonal effects on auroral acceleration and precipitation. The energy flux of precipitating electrons is inferred from auroral luminosity in the long‐wavelength bands (1600–1800 Å) of N2 Lyman‐Birge‐Hopfield (LBHl) auroral emissions, and the mean energy of precipitating electrons is inferred from the intensity ratio of LBHl to LBHs (1400–1600 Å, the shorter wavelength of LBH bands) auroral emissions. Results indicate that dayside and nightside regions of aurora reveal different seasonal effects: nightside (∼ 1900–0300 MLT) auroral power is suppressed in summer, while dayside auroral power is enhanced in summer and forms the so‐called postnoon auroral hot spots, all by a factor of ∼2. The average energy of precipitating electrons is higher in the dark than in the sunlit hemisphere, while the number flux is lower in the dark than in the sunlit hemisphere for all regions. These changes, up to a factor of ∼3, are local time and latitude dependent. The suppression of the nightside auroral power in summer is associated with a large decrease in the electron energy, whereas the enhancement of dayside aurora in summer is associated with a large increase in the electron number flux. The increase of dayside auroral power in summer may be associated with the large‐scale upward field‐aligned currents, which peak in summer. Results are also discussed in the context of a conductivity feedback instability and a cyclotron maser instability. The asymmetric seasonal effects on the dayside and nightside auroras suggest a voltage generator for the dayside magnetosphere and a current generator for the nightside magnetosphere.
A statistical study of variations in the near and middistant magnetotail associated with substorm onsets: GEOTAIL observations
Authors: Miyashita Y. , Machida S. , Mukai T. , Saito Y. , Tsuruda K. , Hayakawa H. , Sutcliffe P. R.
Published: February 2004

We have studied the three‐dimensional structure of substorm‐associated variations in the magnetotail with GEOTAIL data. For this study we selected 342 substorm events from the Pi2 pulsation and applied the method of superposed epoch analysis. We divided the data into those in the plasma sheet, the plasma sheet boundary layer, and the lobe by the ion β. It was found that the fast tailward flows start to develop in the premidnight plasma sheet around X ∼ −28 RE (GSM) about 0 ∼ 2 min before onset, associated with the plasmoid formation. Immediately after onset, the fast tailward flows develop further, and the magnetic field substantially increases southward. Simultaneously, the northward magnetic field increases around X ∼ −10 RE, corresponding to the dipolarization. In the lobe, the perpendicular plasma flow toward the plasma sheet, as well as the dawn‐dusk electric field, first starts to be enhanced around (X, Y) ∼ (−20,7) RE about O ∼ 2 min before onset and then in the surrounding regions successively. The total pressure decrease first occurs around (X, Y) ∼ (−18,7) RE about 0 ∼ 2 min before onset, and then propagates to the surrounding regions successively. The dawn‐dusk electric fields both calculated with E = −V × B and measured directly by the double probe, simultaneously develop around X ∼ −10 RE and X ∼ −28 RE immediately after onset, while those in the plasma sheet around.(X, Y) ∼ (−20,5) RE do not develop much even after onset. These observational results strongly suggest that an efficient magnetic reconnection takes place at least about 0 ∼ 2 min earlier than the Pi2 onset, that the substantial plasmoid evolution and the dipolarization occur simultaneously immediately after onset, and that, on average, the center of the energy release, where the near‐Earth neutral line (i.e., the diffusion region) is possibly created, is initially located around (X, Y) ∼ (−19,6) RE. These features are consistent with a thin‐current reconnection model.
Rapid acceleration of electrons in the magnetosphere by fast‐mode MHD waves
Authors: Summers Danny , Ma Chun‐yu
Published: February 2004

During major magnetic storms, enhanced fluxes of relativistic electrons in the inner magnetosphere have been observed to correlate with ULF waves. The enhancements can take place over a period of several hours. In order to account for such a rapid generation of relativistic electrons, we examine the mechanism of transit‐time acceleration of electrons by low‐frequency fast‐mode MHD waves, here the assumed form of ULF waves. Transit‐time damping refers to the resonant interaction of electrons with the compressive magnetic field component of the fast‐mode waves via the zero cyclotron harmonic. In terms of quasi‐linear theory, a kinetic equation for the electron distribution function is formulated incorporating a momentum diffusion coefficient representing transit‐time resonant interaction between electrons and a continuous broadband spectrum of oblique fast‐mode waves. Pitch angle scattering is assumed to be sufficiently rapid to maintain an isotropic electron distribution function. It is further assumed that there is a substorm‐produced population of electrons with energies of the order of 100 keV. Calculations of the acceleration timescales in the model show that fast‐mode waves in the Pc4 to Pc5 frequency range, with typically observed wave amplitudes (ΔB = 10–20 nT), can accelerate the seed electrons to energies of order MeV in a period of a few hours. It is therefore concluded that the mechanism examined in this paper, namely, transit‐time acceleration of electrons by fast‐mode MHD waves, may account for the rapid enhancements in relativistic electron fluxes in the inner magnetosphere that are associated with major storms.
High‐latitude currents and their energy exchange with the ionosphere‐thermosphere system
Authors: Thayer J. P.
Published: February 2004

The transfer of electrical energy at high latitudes between the magnetosphere and ionosphere‐thermosphere (IT) system is contained within the resulting currents and electric fields between the two systems. As such, the electrodynamic state of the IT system can influence how much of the electrical energy is transferred at any one time by modulating the currents that flow in the region. For this study, 1557 hours of detailed E region measurements by the Sondrestrom incoherent‐scatter radar from 1993 through 1998 were collected, of which 95 hours were used to determine the local electromagnetic energy transfer rates within the IT system and evaluate the role of the IT system in modifying this energy transfer. The measurements have been organized into 59 periods of enhanced electrical energy transfer and cover all universal times but are local to the region above Sondrestrom at 74.2° magnetic latitude. In many of the events studied it was found that the IT system acted to reduce the net electromagnetic energy transferred from the magnetosphere owing to the presence of neutral winds. The dawn sector proved to have on average greater passive energy deposition rates than in the dusk sector. The reduction of this rate by neutral winds also proved stronger in the dawn sector. This enhanced modulation caused the dawn‐dusk asymmetry in energy transfer to be less pronounced but still apparent in the electromagnetic energy transfer rate.
Magnetic structure of the slow solar wind: Constraints from composition data
Authors: Zurbuchen T. H. , Hefti S. , Fisk L. A. , Gloeckler G. , Schwadron N. A.
Published: February 2004

The elemental and ionic composition of the solar wind is determined very close to the Sun. Composition data therefore contain direct information about the structure of the corona and the processes which lead to the generation and acceleration of solar wind and can constrain processes occurring in the heliosphere. We use Advanced Composition Explorer (ACE)‐Solar Wind Ion Composition Spectrometer (SWICS) composition measurements of the solar wind to examine first the structure of the slow solar wind. We find that the statistical properties of the data suggest the presence of a large number of composition boundaries, where compositional patterns change abruptly. These discontinuities are interpreted here as a consequence of the structure of the solar wind source region. Using the experimental constraints of these observed composition boundaries, we derive direct constraints for turbulent diffusion of magnetic field lines in the heliosphere. We then discuss consequences for the propagation of energetic particles.
The ionospheric response to interplanetary magnetic field variations: Evidence for rapid global change and the role of preconditioning in the magnetosphere
Authors: Watanabe Masakazu , Sato Natsuo , Greenwald Raymond A. , Pinnock Michael , Hairston Marc R. , Rairden Richard L. , McEwen Don J.
Published: February 2004

We have found observational evidence for a rapid communication of interplanetary magnetic field (IMF) changes to the global ionosphere and evidence for the state of the magnetosphere in the previous hour conditioning this response. These conclusions are drawn from a case study of sunward flow bursts on the nightside polar cap boundary observed by geomagnetically conjugate HF radars. The flow burst excitation consists of two factors: (1) At the time of the flow burst, the magnetosphere still held a memory of the stable and northward IMF period that had persisted up until 1 hour before the flow burst (internal condition). During the northward IMF period a theta aurora associated with a sunward flow channel was formed in the polar cap. After that the IMF turned southward, and the transpolar arc decayed antisunward. However, by the time of the flow burst (i.e., 1 hour after the IMF southward turning), the Sun‐aligned arc had not yet completely vanished, and in the poleward expanded portion of the northern plasma sheet, there was still a remnant of the sunward flow channel susceptive to an external forcing. (2) One hour after the southward turning of the IMF a sharp IMF transition from southward to northward Bz impinged on the dayside magnetopause (external condition). On arriving at the dayside cusp ionosphere the Bz transition signal pervaded the entire polar cap ionosphere instantaneously (<1 min) and reached the nightside plasma sheet. There, the remnant of the sunward flow channel was reactivated by the Bz transition, and a sunward flow burst was observed first in the northern ionosphere and then in the southern ionosphere with a 7‐min time delay. Thus the sunward flow burst represents a rapid global response of the ionosphere starting 2–3 min after the IMF change at the subsolar magnetopause.
Observations of magnetic field dipolarization during auroral substorm onset
Authors: Frank L. A. , Paterson W. R. , Sigwarth J. B. , Kokubun S.
Published: February 2004

The dynamical behavior of plasmas and magnetic fields in the vicinity of the equatorial crossing of magnetic field lines threading the onset auroral arc is examined for two substorms on November 26, 1997. The locations of the initial brightenings of the auroral arcs were determined with the cameras for visible and far‐ultraviolet wavelengths on board the Polar spacecraft. The equatorial positions of the field lines were in the range of radial distances of 8–12 RE as computed with models of Earth's global magnetic field. The radial distance of the Geotail spacecraft was 14 RE at a position in the premidnight sector that was 2 RE below the current sheet. This spacecraft was embedded in a low‐β plasma that was located adjacent to the central hot plasma sheet. For the first substorm, with onset at 1310 UT, no substantial effect was observed in the plasmas and magnetic fields, although the Geotail spacecraft was located only about 2 hours in magnetic local time from the field lines threading the on‐set auroral arc. For the second substorm onset, at 1354 UT, the spacecraft was positioned within tens of minutes in local time of the position of the magnetic field lines threading the onset auroral arc. This fortuitous spacecraft position in the relatively quiescent plasma and magnetic fields adjacent to the central plasma sheet and within several Earth radii of the position of the onset mechanism allowed determination of the beginning time of the dipolarization of the magnetic fields. This time was simultaneous with the onset brightening of the auroral arc within the approximately 1‐min time resolution of the auroral images. The simultaneity of the initial brightening of the auroral arc and of the initiation of the dipolarization of the magnetic field, presumably due to diversion of current from the equatorial current sheet to the ionosphere, provides an important guideline for global dynamical MHD models of Earth's magnetosphere.
Stormtime ring‐current formation: A comparison between single‐and double‐dip model storms with similar transport characteristics
Authors: Chen Margaret W. , Lyons Larry R. , Schulz Michael
Published: February 2004

Intense magnetic storms often develop in two stages such that a second ring current enhancement begins before the first ring current enhancement has recovered to the prestorm level. Since Dst traces of such storms exhibit two dips, we refer to these as double‐dip storms. Here we compare double‐ and single‐dip storms with similar convective and diffusive transport characteristics for effectiveness at forming the proton ring current. Our model storms consist of superposition of almost randomly occurring impulses in the convection electric field. We have synthesized a hypothetical double‐dip storm consisting of a moderate 6‐hour storm, followed by a 3‐hour quiet interval and then by a more intense 15‐hour storm, for a total duration of 24 hours. For comparison, we consider a single‐dip model storm with an unmodulated 24‐hour main phase during which the root‐mean‐square enhancement of the cross‐tail potential drop is made equal to the timeweighted rms enhancement for the double‐dip model storm. This leads to comparable timeaveraged diffusion coefficients for our single‐ and double‐dip model storms. The mean enhancement of the cross‐tail potential drop of the two storms are also comparable. When the stormtime proton plasma sheet distribution, the source of ring current protons, is left unchanged from its quiet (prestorm) level, we find little difference in proton energy content per unit R (which is geocentric distance normalized by RE) between our double‐ and single‐dip model storms. The protonenergy content of the magnetosphere is roughly increased by a factor of 2.5 by either model storm under this scenario, in which the overall amount of stormtime transport, whether convective or diffusive, is nearly the same for the double‐ and single‐dip model storms. As in our earlier work, we require here an enhanced stormtime plasma sheet population (in addition to enhanced particle transport) in order to achieve (for example) the 20‐fold increase in |Dst| characteristic of a large storm. Only when we invoke a two‐stage stormtime enhancement of the boundary (plasma sheet) phase space density in combination with the two‐stage enhancement in particle transport, our double‐dip model storm does show a much larger total energy content than our single‐dip model storm with a one‐stage enhancement of the boundary spectrum. This suggests that plasma sheet preconditioning may be important for the development of especially intense storms.
Enhanced current collection by a positively charged spacecraft
Authors: Singh Nagendra , Leung W. C. , Singh Gitanjali M.
Published: February 2004

The collection of electrons by a conducting spherical body at a high voltage Φo from a magnetized plasma is studied by means of a fully three‐dimensional particle‐in‐cell code. A relative motion between the plasma and the body is included to simulate the orbital motion of a spacecraft. The current‐voltage (I‐V) characteristic of the body as seen from the simulations is compared with that predicted by the theory giving the upper‐bound current Ipm [Parker and Murphy, 1967]. In agreement with measurements in the tethered satellite experiments TSS‐1 and TSS‐1(R), the simulations give currents I(Φo), which are higher than the corresponding currents Ipm(Φo). For sufficiently large values of Φo, Ipm∝Φo0.5, while from the simulations we find that Ipm∝Φo0.62, giving Isim/Ipm∝Φoδ with δ∼0.12. Thus our simulations predict a voltage‐dependent enhancement of the current although the dependence is weak. Despite this enhancement simulations show that the dependence of Isim on the body radius rs is the same as that given by Ipm(rs). In order to better understand these behaviors of the current collection, the distributions of potential and current in the plasma around the body are examined. When eΦo > Wo, the ram ion energy, the reflection of ram ions forms a virtual anode in front of the body in the ram region. The potential structure extends from the virtual anode along the magnetic field B0 and not along the magnetic shadow of the body. The current pattern in the plasma contributing to the collection of electrons reveal that when eΦo > Wo, the current flow is approximately magnetic‐field aligned at parallel distances |z|>rs in a cylindrical volume aligned with the magnetic field lines passing through the virtual anode. When |z|<rs, the parallel currents give way to azimuthal and radial currents in the ram volume in front of the body. The latter currents predominately contribute to the electron collection, which maximizes near the equatorial (z∼0) ram surface of the body. The current patterns are further studied by tracing electron trajectories in the potential distributions in the sheath of the body, showing that the E×B drifts of the electrons in the self‐consistently determined poteial distribution do indeed enable the collection of the electrons near the equatorial ram surface.
Large‐scale vortex dynamics in the evening and midnight auroral zone: Observations and simulations
Authors: Voronkov I. , Donovan E. F. , Jackel B. J. , Samson J. C.
Published: February 2004

We use Canadian Auroral Network for the OPEN Program Unified Study All‐Sky Imager (ASI) and Meridional Scanning Photometer (MSP) data as the basis for a study of the dynamics of large‐scale (hundreds of kilometers) auroral vortices. We consider 28 events corresponding to a range of auroral activity levels. Three of these are presented in detail, one corresponding to growth phase, one to pseudo‐breakup and one to expansive phase onset. We show that vortex formation starts from a discrete arc with half thickness δ of the order of 20 km. This arc intensifies near the poleward boundary of enhanced proton aurora, as seen in the Hydrogen β (Hβ) MSP data and becomes azimuthally structured. This structuring is in the form of vortices with wavelength of the order of ∼ 2πδ. The vortices intensify and extend radially, leading to broadening of the initial arc. While the sizes and growth rates of the vortices vary, the overall scenario of vortex evolution is similar for all of the events. Structures that develop during the growth phase saturate at latitudes matching the poleward boundary of Hβ emissions and pseudo‐breakup structures saturate further poleward. Expansive phase onset vortices expand poleward in a similar fashion, but we do not observe any saturation stage, presumably due to limitations imposed by the ASI field of view. We present results of shear flow ballooning vortex modeling in which we used initial conditions and parameters consistent with our observations. On the basis of our model results, we speculate that all of these experimentally observed vortices are the result of shear flow ballooning instability in the hot proton region in the near‐Earth plasma sheet.
Return to Io by the Galileo spacecraft: Plasma observations
Authors: Frank L. A. , Paterson W. R.
Published: February 2004

On October 11, 1999, a series of high‐resolution plasma measurements were obtained during the second close flyby of Io of the Galileo mission. The closest approach to Io occurred at an altitude of 617 km along the flank of Jupiter's torus plasma flows past this moon. Energy/charge (E/Q) and mass/charge (M/Q) measurements with the plasma analyzer were used to identify the primary ions. At closest approach the ion number densities increased to their maximum values of 1200 cm−3 as calculated from the plasma moments. These ions were dominated by two thermal populations of torus ions with M/Q = 16, one with density and temperature of 800 cm−3 and kT= 40 eV and the other with 200 cm−3 and 160 eV. On the basis of the previous Voyager 1 remote measurements with a spectrometer, these two thermal ion plasmas are identified as O+ and S++, respectively. A substantial population of pickup ions was also observed, with densities of about 100 cm−3 each for S+ and SO+. The plasma bulk flows were strongly deflected around the body of Io, and the speed of the bulk flow increased to a factor of 1.3 greater than that for rigid corotational flow. The torus ion densities just outside of lo's orbit were about 800 cm−3. These densities at the immediate position of Io are smaller by a factor of about 4 relative to those during the first flyby on December 7, 1995, but should not be taken as indicative of a similar decrease in the bulk of the plasma torus. Two major plasma regimes were encountered before the closest approach to Io. The first was a region of hot ions located inside of lo's orbit which was previously identified as the “ribbon” of enhanced plasma densities with Voyager 1 plasma measurements and with ground‐based imagery. The ion composition in the ribbon included O+ and S++. The second region, closer to Io, exhibited a hot torus plasma which was mixed with pickup hydrogen ions. These hydrogen ions were detected at distances beginning at 7.8 RIo from this moon, which were generally inside the Hill (Lagrange) “sphere” for Io's atmosphere.
Geospace Environment Modeling magnetic reconnection challenge: Simulations with a full particle electromagnetic code
Authors: Pritchett P. L.
Published: February 2004

The objective of the Geospace Environment Modeling (GEM) magnetic reconnection challenge is to understand the collisionless physics that controls the rate of magnetic reconnection in a two‐dimensional configuration. The challenge involves investigating a standard model problem based on a simple Harris sheet configuration by means of a variety of physical models in order to isolate the essential physics. In the present work the challenge problem is modeled using an electromagnetic particle‐in‐cell code in which full particle dynamics are retained for both electrons and ions and Maxwell's equations are solved without approximation. The timescale for reconnection is of the order of
(where Ωi is the ion cyclotron frequency based on the asymptotic field B0), and the corresponding reconnection electric field is (c/vA)Ey/B0 ∼ 0.24. The diffusion region near the neutral line is observed to develop a multiscale structure based on the electron and ion inertial lengths c/ωpe and c/ωpi. The difference between the ion and electron dynamics in the diffusion region gives rise to in‐plane (Hall) currents which produce an out‐of‐plane By field with a quadrupolar structure. In the diffusion region the magnetic field is no longer frozen‐in to the electrons; the inductive Ey field is supported primarily by the off‐diagonal electron pressure terms in the generalized Ohm's law. The reconnection rate is found to be insensitive to electron inertia effects and to the presence of a moderate out‐of‐plane initial field component B0y ≲ B0. The results are consistent with the theory that the reconnection rate is independent of the mechanism which breaks the frozen‐in condition and is controlled by dynamics at length scales much greater than the electron dissipation region.
Secular variability of the geocoronal Balmer‐alpha brightness: magnetic activity and possible human influences
Authors: Kerr R. B. , Garcia R. , He X. , Noto J. , Lancaster R. S. , Tepley C. A. , González S. A. , Friedman J. , Doe R. A. , Lappen M. , McCormack B.
Published: February 2004

Measurements of the geocoronal Balmer‐alpha (Hα) brightness taken between 1983 and 1994 at the Arecibo Observatory (18.35° N, 66.75° W) occasionally display aperiodic brightness variations that we attribute to a local response of exospheric hydrogen abundance to geomagnetic activity. Approximately twofold Hα brightness enhancements in the days following moderate storm onset conditions are demonstrated, and the absolute brightness during these events is significantly greater than modeled brightness based on quiet geomagnetic conditions. Although there are no direct measurements of the line center solar Lyman beta (Ly β) flux that pumps Hα during these events, the magnitude of the brightness enhancements and the temporal reproducibility of the phenomena (following onset of all storms in our data) make it likely that the enhanced Hα brightness is due to enhanced column abundances of hydrogen in the exosphere above Arecibo and not to sporadic increases in the solar Ly β flux. Increased hydrogen abundance due to thermal expansion of the midlatitude atmosphere following propagation of the auroral heating event cannot account for the entire enhancement. The data also suggest that Hα. brightness has become systematically stronger from 1984 to 1994, independent of solar cycle variability or variations due to geomagnetic activity. An increase of ∼ 3% per year, may be attributable to real change in the geocoronal hydrogen column abundance during the period, and is possibly a consequence of increasing hydrogenous species concentrations in the lower atmosphere. The enhancement of geocoronal Hα brightness between 1984 and 1994 may be the consequence of increasing methane in the troposphere and may signal that the effects of methane deposition in the lower atmosphere have propagated throughout the atmosphere.
Photoemission current‐spacecraft voltage relation: Key to routine, quantitative low‐energy plasma measurements
Authors: Scudder J. D. , Cao Xuejun , Mozer F. S.
Published: February 2004

Measurements of the floating potential and plasma return current on the GGS‐Polar spacecraft are used to determine the equilibrium photoemission current, Jħv, as a function of the spacecraft's (S/C) “floating potential,” ΔΦS/C. The photoemission current function is found to be time independent using nearly 10 months of GGS‐Polar data from April 1996 to March 1997 including 1.6 million separate spectra from the Hydra (hot plasma) and EFI (electric field) instruments. The photoemission current density leaving the spacecraft at positive floating potentials ΔΦS/C < 50 V is well fit by a sum of two exponentials of the form: Jħv(μA/m2) = A exp ( −ΔΦS/C/B) + C exp (−ΔΦS/C/D), where best fit estimates A ≃ 152 ± 57 μA/m2, B ≃ 1.7 ± 0.2 eV, C ≃ 0.86 ± 0.29 μA/m2, and D ≃ 9.5 ± 1.0 eV. In equilibrium this photoemission current density is determined from the ratio of sunlit to spacecraft areas and the plasma current density, JRC, collected by the spacecraft. For the Polar spacecraft this ratio of areas is not constant in time, and the observed return current voltage relation is time dependent. After correction for the orbitally induced time‐dependent ratio of areas, the photoemission curve reported above is obtained and is essentially time independent. After correcting for the different procedures used the present results are illustrated to be consistent with early results with less resolution reported by Pedersen [1995]. When the sensing of the floating potential by EFI on the spacecraft is interrupted, the photoemission‐voltage relationship is essential for the assignment of the ambient kinetic energy of the detected particle fluxes. We demonstrate a method using the plasma data and the statistical return current relationship that recovers the floating potential of the spacecraft with a typical precision that is the larger of 0.5 V and 0.1ΔΦS/C We also demonstrate that the ion and electron densities determined by numerical integration over distributions corrected with opposite energy shifts implied by the potential enhances their routine agreement, a further check on the absolute precision of the potential inferences. The “systematic” departures of 40% of the data from the statistically defined Jħv(ΔΦS/C) curve reported above are time varying and organized in radius and with L shell. These data are inferred to be the signature of missing ambient plasma currents to the spacecraft that are not directly detected by the Hydra instrumentation (cf. X. Cao et al., Properties of very cold (Te ≃ 0.1 eV) electrons within the magnetosphere, submitted to Journal of Geophysical Research, 1999).
Plasma sheet ion injections into the auroral bulge: Correlative study of spacecraft and ground observations
Authors: Sergeev V. A. , Sauvaud J.‐A. , Popescu D. , Kovrazhkin R. A. , Lutsenko V. N. , Zelenyi L. M. , Syrjasuo M. , Viljanen A. , Pulkkinen T. I. , Kudela K. , Kokubun S. , Mukai T.
Published: February 2004

Multiple and sporadic time‐of‐flight velocity dispersed ion structures (TDIS) are systematically observed above the ionosphere at ∼3 Re altitude by Interball/Auroral spacecraft near the poleward edge of the auroral bulge. These events represent direct snapshots of the impulsive ion acceleration process in the equatorial plasma sheet which allow us to study the details of the connection between ionospheric and plasma sheet manifestations of the magnetospheric substorm. Two events are analyzed during which the spacecraft footpoints passed over the Scandinavian ground network. We found that the TDIS correlate with the intensifications of westward current and auroral activations at the poleward edge of the bulge, which confirms the association of these dispersed ion beams with the temporal evolution of impulsive reconnection in the tail. Furthermore, we present direct evidence of an active neutral line in the magnetotail during one of the events using plasma sheet measurements made concurrently by the Interball/Tail and Geotail spacecraft. The 2–3 min repetition period of these ∼1 min long activations indicates a fundamental time constant of the substorm instability. On the other hand, the estimated injection distances of the energy‐dispersed ions were inferred to be smaller than the estimated position of the reconnection region in the tail. We also found that the TDIS ion beams are released within the closed plasma flux tubes deep inside the plasma sheet, and yet they are synchronized with auroral activations at the poleward boundary. These facts imply that the ion beams are formed in a spatially extended region of the plasma sheet rather than in the close vicinity of the neutral line. We argue that braking of the reconnection‐induced fast flow bursts when they interact with the closed plasma flux tubes and the earthward propagating fast wave electric field generated in the braking region may be important in forming the observed multiple, sporadic, energy‐dispersed ion beams.
A numerical study of the evolution of the solar wind from Ulysses to Voyager 2
Authors: Wang C. , Richardson J. D. , Gosling J. T.
Published: February 2004

Voyager 2 continues to explore the outer heliosphere as Ulysses studies the latitudinal dependence of the solar wind. During the year 1991 these spacecraft were within 2° latitude and their radial separation was larger than 30 AU. This alignment presents a good opportunity to investigate the evolution of the solar wind and, in particular, the effect of pickup ions, which are an important component of the solar wind in the outer heliosphere. A numerical simulation study is used to model the evolution of solar wind structures from the location of Ulysses (1–5 AU) to Voyager 2 (33–36 AU) in 1991. The Ulysses observations were used as input into the numerical models and the theoretical predictions were compared with the Voyager 2 observations. The model produces profiles and magnitudes of the plasma parameters (e.g., flow speed and density) which are in reasonable agreement with the observations. The inclusion of pickup ions slows the solar wind, reduces the amplitudes of the speed variations and density spikes, and changes the speed of shock propagation. However, within 35 AU, pickup ions do not change the solar wind structures dramatically. Our calculations favor a low interstellar neutral hydrogen density, i.e., nH∞ = 0.05 cm−3, for this time period (1991).
Cold flowing O+ beams in the lobe/mantle at Geotail: Does FAST observe the source?
Authors: Seki K. , Elphic R. C. , Thomsen M. F. , Bonnell J. , Lund E. J. , Hirahara M. , Terasawa T. , Mukai T.
Published: February 2004

The Geotail spacecraft observed high‐energy (∼3–10 keV) cold O+ beams (COBs) streaming tail ward together with protons entering from the magnetosheath in the northern dusk lobe/mantle when the IMF (interplanetary magnetic field) By and Bz are steadily negative. He+ beams were also observed intermittently. During the same period the FAST satellite passed across the dayside northern polar regions from dawn to dusk at low altitudes (1200–3400 km) and observed O+ precipitation on both closed and open field lines. There are regions where the magnetosheath and dayside plasma sheet/ring current components coexist and are precipitating together. In the open field line regions the precipitating O+ seem continuous with the precipitation in the closed regions, while the H+ and He++ precipitations are denser and typically have lower energy than O+. The phase space density (PSD) of the precipitating ions is highly isotropic except for the loss cone in the upward direction. Utilizing Liouville's theorem, we have compared the PSD of locally mirroring O+ at FAST with the PSD of COBs observed at Geotail. This comparison shows that the PSD in the high‐energy precipitation region on closed field lines is comparable to that of the COBs. In regions where the magnetosheath and dayside magnetosphere ions coexist, the O+ PSD is typically smaller than that of the COBs, but at low latitudes it sometimes reaches values comparable to that of the COBs. These results suggest that the high‐energy O+ ions in the dayside magnetosphere are a promising candidate for the source of COBs in the lobe/mantle. The ion dynamics on reconnected flux tubes needs to be examined further to clarify the possible energization mechanisms and their effect on the O+ ions.
Cosmic ray spectral variations and anisotropy near Earth during the March 24, 1991, Forbush decrease
Authors: Hofer M. Y. , Flückiger E. O.
Published: February 2004

The cosmic ray spectral variations and anisotropy near Earth during the March 24, 1991, Forbush decrease were evaluated on the basis of neutron monitor data using the technique developed for the analysis of solar cosmic ray ground level enhancements. Evidence was found that the unusual short‐term increase just prior to the onset of the global decrease was due to an anisotropic flux of GV particles arriving from the local dusk‐midnight sector (GSE latitude: −1°, GSE longitude: 140°E) which are associated with the powerful interplanetary shock that struck the Earth's magnetosphere at 0342 UT on March 24, 1991. During the main phase of the event the cosmic ray anisotropy vectors exhibited a rotational behavior similar to the typical rotation of the magnetic field vector which is observed when a magnetic cloud is traversed. The study demonstrates the potential of neutron monitor data to provide information about complex transient structures in the near‐Earth interplanetary medium.
Simulation study on fundamental properties of the storm‐time ring current
Authors: Ebihara Yusuke , Ejiri Masaki
Published: February 2004

This paper presents fundamental properties of the storm‐time ring current on the basis of the numerical simulations. This simulation model solves spatial and temporal evolution of the ion distribution in the magnetosphere by tracing the bounce‐averaged drift trajectories. The tracing is performed under the dipole magnetic field and the time‐dependent Volland‐Stern‐type convection field. After tracing particles, we calculate the differential flux, the plasma pressure, and the current density. The magnetic disturbance induced by the ring current is directly calculated from the Biot‐Savart integral over the whole three‐dimensional distribution of the calculated current density. We examined following subjects during the magnetic storms; the causes of the ring current buildup, the electric current distribution, the causes of the ring current decay, the energy composition of the plasma pressure, the response time of the plasma sheet density to the solar wind density, and the diamagnetic effect. This simulation suggests the following results: (1) The major variation of corrected Dst is mainly due to the changes in both the convection electric field and the plasma sheet density. (2) The Dessler‐Parker‐Sckopke relation overestimates the corrected Dst by a factor of 2.5–4. (3) The storm‐time ring current buildup is insensitive to the plasma sheet temperature for the temperature above 3 keV. (4) The ions with energies around 15–40 keV at L ∼4–6 in the dusk region mostly contribute to the perpendicular pressure. (5) The equatorial magnetic fields are dramatically distorted by the diamagnetic effect. The grad‐B drift trajectories under the distorted equatorial magnetic field can be classified into four patterns.
Magnetotelluric data in western Turkey: Dimensionality analysis using Mohr circles
Authors: Bayrak M. , Ilkişik O. M. , Kaya C. , Başokur A. T.
Published: February 2004

During the summer of 1996, wideband (0.003125–2000 s) magnetotelluric (MT) data were collected from 52 sites across western Turkey, with a site spacing of ∼5 km. The extensional regime is dominant as a result of the convergence of the African and Arabian plates against the Eurasian plate, and western Turkey is characterized with east‐west oriented, “continental escape” tectonics. To contribute to the knowledge of the geological structures along the profile, the dimensionality characteristics of the MT impedance tensors are computed, taking real and imaginary parts of the tensor elements separately. The rotationally invariant parameters of central impedance (d3) and anisotropy angle (λ) are also computed, the two being good dimensionality indicators. Pseudosections of these parameters reveal the presence of major geological structures in western Turkey, such as the zone between the Menderes Massif and the Bornova Flysch zone, the Izmir‐Ankara Suture zone, the western part of the North Anatolian fault zone, and grabens (Demirci, Gördes, and Bigadiç), which are the characteristic of the region. Interpretation of the full set of Mohr circles shows a strong anisotropy and high central impedance anomalies, while changes are shown in geological strike direction at estimated depths of about 7–8 km, 15–20 km, and 35–40 km along the profile from south to north. These anomalies are indicative of changes in the thickness of upper crust.
Fault‐slip directions in central and southern Greece measured from striated and corrugated fault planes: Comparison with focal mechanism and geodetic data
Authors: Roberts Gerald P. , Ganas Athanassios
Published: February 2004

Fault‐slip directions recorded by outcropping striated and corrugated fault planes in central and southern Greece have been measured for comparison with extension directions derived from focal mechanism and Global Positioning System (GPS) data for the last ∼100 years to test how far back in time velocity fields and deformation dynamics derived from the latter data sets can be extrapolated. The fault‐slip data have been collected from the basin‐bounding faults to Plio‐Pleistocene to recent extensional basins and include data from arrays of footwall faults formed during the early stages of fault growth. We show that the orientation of the inferred stress field varies along faults and earthquake ruptures, so we use only slip‐directions from the centers of faults, where dip‐slip motion occurs, to constrain regionally significant extension directions. The fault‐slip directions for the Peloponnese and Gulfs of Evia and Corinth are statistically different at the 99% confidence level but statistically the same as those implied by earthquake focal mechanisms for each region at the 99% confidence level; they are also qualitatively similar to the principal strain axes derived from GPS studies. Extension directions derived from fault‐slip data are 043–047° for the southern Peloponnese, 353° for the Gulf of Corinth, and 015–014° for the Gulf of Evia. Extension on active normal faults in the two latter areas appears to grade into strike‐slip along the North Anatolian Fault through a gradual change in fault‐slip directions and fault strikes. To reconcile the above with 5° Myr−1 clockwise rotations suggested for the area, we suggest that the faults considered formed during a single phase of extension. The deformation and formation of the normal fault systems examined must have been sufficiently rapid and recent for rotations about vertical axes to have been unable to disperse the fault‐slip directions from the extension directions implied by focal mechanisms and GPS data. Thus, in central and southern Greece the velocity fields derived from focal mechanism and GPS data may help explain the dynamics of the deformation over longer time periods than the ∼100 years over which they were measured; this may include the entire deformation history of the fault systems considered, a time period that may exceed 1–2 Myr.