bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct influence of ring current on auroral oval diameter
Authors: Schulz Michael
Published: February 2004

The inside diameter of the auroral oval (conceived as marking the boundary between closed and polar cap field lines) is known to vary with storm intensity, as measured by the Dst index. Part of this variability may result from storm‐associated changes in solar wind pressure and/or from storm‐associated changes in location of the nightside neutral line (inner edge of neutral sheet). The magnetospheric B field is also directly distorted (stretched outward) by the stormtime ring current so as to increase the amount of polar cap magnetic flux. The magnitude of this last effect on auroral oval diameter is estimated from a simple model in the present work for storms of various intensities. The prototype for an empty magnetosphere in this effort is Dungey's model (dipole plus uniform southward Bz ≈ −9.0 cos6Λ* μT, with Λ* being the invariant latitude of the quiettime auroral oval). Addition of a ring current field ΔBz(r0) with Dst < 0 moves the circular neutral line outward (to a larger radial distance r0 in the equatorial plane) but simultaneously increases the amount of polar cap magnetic flux. The consequent increase in auroral oval diameter can be calculated (in closed form for sufficiently simple ΔBz models) by magnetic flux bookkeeping. In the present model, this increase (≡ −2 ΔΛ*) is typically about 4° ± 1° for each −100 nT of Dst.
Relation of the magnetic field in the magnetosphere to the geomagnetic and solar wind activity
Authors: Ostapenko A. A. , Maltsev Y. P.
Published: February 2004

We have used more than 14,000 three‐component magnetic measurements from the database of Fairfield et al. [1994] in the magnetospheric domain with the inner boundary r = 3 RE and outer boundary (x2 + y2)1/2 = 10 RE, |z| = 7 RE for studying statistical connection of the external magnetic field with Dst and Kp indices as well as with the solar wind dynamic pressure and the z component of the IMF. The components of the external magnetic field in the magnetosphere have been presented as fourth‐order polynomials of the geocentric distance. The polynomials include 17 terms: six terms corresponding to the azimuthally symmetrical part of the field, six terms suggesting the day‐night asymmetry and five terms accounting for the dipole tilt. Each coefficient of the series has been found as a linear combination of the four above mentioned parameters. The least squares technique allowed us to obtain 85 coefficients. The normalized residual sum of squares of the model equals 0.20, which is smaller than other models yield. The coefficients of all six terms of the azimuthally symmetrical part depend mostly on Dst. The asymmetrical and dipole tilt parts depend on the four above mentioned parameters to a nearly equal extent. The effect of each parameter on the cusp position has been examined. Dst appeared to be the strongest factor affecting the cusp latitude.
Ionospheric convection during nonsteady interplanetary magnetic field conditions
Authors: Ridley A. J. , Lu Gang , Clauer C. R. , Papitashvili V. O.
Published: February 2004

We investigate six cases in which the interplanetary magnetic field (IMF) orientation is changing. The assimilative mapping of ionospheric electrodynamics (AMIE) technique is used to compute the ionospheric electric potential in a 5‐min time step over the nonsteady periods. For each period a steady convection pattern prior to the convection change is determined, and this pattern is then subtracted from each of the nonsteady convection patterns. From these residual potential patterns a number of parameters can be determined, such as the start and end time of the convection change, the amount of change and the location of the strongest changes. By further examining the residual patterns, we reach the following conclusions: (1) convection changes occur in a fixed region of space, while the magnitude of the convection change grows linearly with time; (2) the strength of the convection change and the rate of the change are linearly proportional to the change observed in the IMF By and Bz components; and (3) the merging region may not react to long duration changes in the IMF until the ratio of Bz/|By| passes through some threshold value. This can cause the ionospheric reconfiguration time to be less than the IMF reorientation time.
Critical issues for understanding particle acceleration in impulsive solar flares
Authors: Miller James A. , Cargill Peter J. , Emslie A. Gordon , Holman Gordon D. , Dennis Brian R. , LaRosa T. N. , Winglee Robert M. , Benka Stephen G. , Tsuneta S.
Published: February 2004

This paper, a review of the present status of existing models for particle acceleration during impulsive solar flares, was inspired by a week‐long workshop held in the Fall of 1993 at NASA Goddard Space Flight Center. Recent observations from Yohkoh and the Compton Gamma Ray Observatory, and a reanalysis of older observations from the Solar Maximum Mission, have led to important new results concerning the location, timing, and efficiency of particle acceleration in flares. These are summarized in the first part of the review. Particle acceleration processes are then discussed, with particular emphasis on new developments in stochastic acceleration by magnetohydrodynamic waves and direct electric field acceleration by both sub‐ and super‐Dreicer electric fields. Finally, issues that arise when these mechanisms are incorporated into the large‐scale flare structure are considered. Stochastic and super‐Dreicer acceleration may occur either in a single large coronal reconnection site or at multiple “fragmented” energy release sites. Sub‐Dreicer acceleration requires a highly filamented coronal current pattern. A particular issue that needs to be confronted by all theories is the apparent need for large magnetic field strengths in the flare energy release region.
Formation of density troughs embedded in the outer plasmasphere by subauroral ion drift events
Authors: Ober Daniel M. , Horwitz J. L. , Gallagher D. L.
Published: February 2004

A dynamic global core plasma model (DGCPM) is used to investigate the effects of subauroral ion drift (SAID) events on the formation of trough density profiles in the outer plasmasphere during periods of high magnetic activity. The DGCPM includes the influences of convection on the changing flux tube volumes, as well as daytime refilling and nighttime draining of plasma, to calculate the plasma tube contents and equatorial plasma density distribution versus time throughout the magnetosphere. SAIDs are regions of latitudinally narrow westward flow of plasma equatorward of the auroral zone. We present DGCPM results for various presumed SAID locations and durations relative to enhanced substorm convection onset and decay, to parametrically elicit the formation of plasmaspheric density trough structures resulting from SAID effects. It is found that imposing a SAID event in the dusk‐evening sector for 30 min leads to the formation of a narrow (less than 1 RE near the equatorial plane) embedded plasma density trough in the dusk bulge region. The modeled plasmasphere density profiles with troughs generally resemble plasmasphere density profiles observed from DE 1 measurements.
Spacecraft transits across simulated field line resonance regions
Authors: Allan W. , Wright A. N. , McDiarmid D. R.
Published: February 2004

The field line resonance (FLR) mechanism has become a cornerstone of magnetohydrodynamic wave theory in the magnetosphere and the solar atmosphere. Evidence for the mechanism comes mainly from auroral radar and geosynchronous spacecraft observations. Spacecraft in elliptical near‐equatorial orbits (particularly Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE)) have seen frequency‐varying oscillations of magnetic field lines but have not seen much evidence of the large‐amplitude, narrowband FLRs seen by auroral radars. We simulate the resonance region of a large event observed at GOES 7 and also of a smaller‐amplitude event we term a “typical Scandinavian Twin Auroral Radar Experiment (STARE) event” with reference to the STARE auroral radar results. One set of simulated narrowband FLRs is driven by a magnetopause Kelvin‐Helmholtz wave. We add a fundamental continuum oscillation and analyze the resulting time series using the AMPTE CCE time‐frequency analysis. We find that at the observed milliHertz frequencies the narrowband FLRs appear as large or small enhancements in the continuum of the azimuthal magnetic field but are not obviously “monochromatic.” For large events, enhancements in the spectrum of the compressional magnetic component may suggest a narrowband driver. We also analyze FLRs driven by waveguide modes and find no monochromatic signatures in the compressional magnetic field.
On the “inward” component of the Alfvénic turbulence in the solar wind
Authors: Bruno R. , Bavassano B. , Pietropaolo E. , Carbone V. , Rosenbauer H.
Published: February 2004

Although the turbulent nature of solar wind fluctuations has been recognized almost 30 years ago, its governing mechanism is not yet completely understood. In situ observations by spacecraft and numerical simulations have all contributed to show that MHD turbulence observed within fast streams is mainly made of Alfvénic fluctuations whose spectral signature experiences a dramatic evolution during the wind expansion into the interplanetary space. Within the frame of incompressible MHD, this spectral evolution can be partly achieved only if we allow for nonlinear interactions between Alfvénic fluctuations with an “outward” (δZ+) and an “inward” (δZ−) sense of propagation with respect to the Sun. It has been shown that these fluctuations have a quite different radial evolution and it has been suggested that also their intrinsic nature might not be the same. This topic has been studied by several authors, but a definite answer has not been reached yet, because of the absence of multiple‐point observations from spacecraft is a major limitation to distinguish spatial from temporal events, that is, convected from propagating disturbances. The main goal of this paper is to bring additional evidence in favor of the Alfvénic nature of (δZ−) fluctuations at frequencies higher than a given limit which we estimate being around 1/30′ at about 60 solar radii.
Relationship of dayside auroral precipitations to the open‐closed separatrix and the pattern of convective flow
Authors: Lockwood M.
Published: February 2004

The implications are discussed of acceleration of magnetospheric ions by reflection off two magnetopause Alfvén waves, launched by the reconnection site into the inflow regions on both sides of the boundary. The effects of these waves on the ion populations, predicted using the model described by Lockwood et al. [1996], offer a physical interpretation of all the various widely used classifications of precipitation into the dayside ionosphere, namely, central plasma sheet, dayside boundary plasma sheet (BPS), void, low‐latitude boundary layer (LLBL), cusp, mantle, and polar cap. The location of the open‐closed boundary and the form of the convection flow pattern are discussed in relation to the regions in which these various precipitations are typically found. Specifically, the model predicts that both the LLBL and the dayside BPS precipitations are on newly opened field lines and places the convection reversal within the LLBL, as is often observed. It is shown that this offers solutions to a number of paradoxes and problems that arise if the LLBL and BPS precipitations are thought of as being on closed field lines. This model is also used to make quantitive predictions of the longitudinal extent and latitudinal width of the cusp, as a function of solar wind density.
Overexpanding coronal mass ejections at high heliographic latitudes: Observations and simulations
Authors: Gosling J. T. , Riley Pete , McComas D. J. , Pizzo V. J.
Published: February 2004

Ulysses observations reveal that most coronal mass ejections (CMEs) observed in the solar wind far from the Sun at high heliographic latitudes have large radial widths and are still expanding as they pass the spacecraft. CME radial widths ranging between 0.5 and 2.5 AU have been observed at heliocentric distances between 1.4 and 4.6 AU and at latitudes greater than 22°. A CME may expand simply because it is ejected from the Sun with a leading edge speed that is greater than its trailing edge speed. Rarefaction waves produced by relative motion between a CME and the surrounding wind also can cause a CME to expand. Finally, a CME may expand because it is ejected into the wind with an internal pressure that is greater than that of the surrounding wind. In the latter case, which we have called “overexpansion,” the expansion tends to drive compressive waves into the surrounding solar wind; these waves commonly steepen into shocks at large distances from the Sun. The relative importance of these various expansion processes differs from event to event depending upon initial conditions within the CME and the surrounding wind. Using Ulysses observations and a simple one‐dimensional, gasdynamic code, we have explored how initial conditions affect the radial evolution of solar wind disturbances associated with overexpanding CMEs. We find good qualitative agreement between the results of our simulations and Ulysses observations of such disturbances.
Electron and proton radiation belt dynamic simulations during storm periods: A new asymmetric convection‐diffusion model
Authors: Bourdarie S. , Boscher D. , Beutier T. , Sauvaud J.‐A. , Blanc M.
Published: February 2004

Using a convection‐diffusion theory, we give the first results from a four‐dimensional model of electron and proton radiation belts. This work is based on the numerical solution of a convection‐diffusion equation taking into account (1) for protons, the deceleration of protons by the free and bounded thermospheric and ionospheric electrons, the charge exchange loss process, radial and azimuthal transports, and (2) for electrons, the deceleration of electrons by the free and bounded electrons of the medium, pitch angle diffusion by Coulomb and wave‐particle interactions, radial and azimuthal transport. This model allows for simulation of a magnetic storm effects by increasing convective electric field and injecting particles with keV range energies in the nightside region. Particles in the energy range 50 – 100 keV are “created” by acceleration of slower particles in the L = 4 region. Four hours are needed for ring current formation. The calculated particle distribution at 6.6 Earth radii as well as at low altitude are in good agreement with those deduced from ATS 6 measurements (the drift echo is well reproduced at this altitude) and from statistical studies of the precipitation by the DMSP satellites, respectively.
Nighttime thermospheric meridional winds at Cachoeira Paulista (23°S, 45°W): Evidence for effects of the equatorial midnight pressure bulge
Authors: Batista Inez S. , Sastri J. H. , de Medeiros R. T. , Abdu M. A.
Published: February 2004

We have studied the local time and seasonal variations of thermospheric nighttime meridional winds at the low‐latitude station, Cachoeira Paulista, Brazil, corresponding to quiet geomagnetic conditions of high solar activity (Fl0.7 > 150 units). The meridional winds are derived from F layer peak height (hmax) data using a modified form of the “servo” model wherein plasma transport due to electric field is taken into account. It is found that during the solstices the meridional winds exhibit the transequatorial neutral airflow from the summer to the winter hemisphere, i.e., the nighttime winds are primarily equatorward (poleward) during the December (June) solstice. It is shown that this neutral wind flow pattern is effectively modulated by winds related to equatorial midnight temperature maximum (MTM) and the associated midnight pressure bulge, especially in the December solstice (local summer). The seasonal visibility of the effect of MTM on low/equatorial latitude meridional winds seems to be different in the east Brazil (45°W) and Indian (75°E) sectors.
Modulated Langmuir waves: Observations from Freja and SCIFER
Authors: Bonnell J. , Kintner P. , Wahlund J.‐E. , Holtet J. A.
Published: February 2004

Modulated Langmuir waves were commonly observed using the HF waveform‐capture instruments on the Freja satellite and SCIFER sounding rocket in the terrestrial auroral zone. The modulation frequency of the Langmuir waves during several hundred events was estimated, and found to extend routinely to several tens of kilohertz. Using the linear dispersion relation for oblique Langmuir waves in the ƒpe < ƒce domain, these modulation band widths were found to correspond to interactions between waves at significantly larger angles to each other, and to B0, than typically treated in theoretical analyses of Langmuir wave modulations. A model of the modulation based on scattering via electrostatic whistler/lower hybrid waves is proposed as a viable explanation of the broad modulation bandwidth.
Equatorial plasma bubble evolution and its role in the generation of irregularities in the lower F region
Authors: Sekar R. , Sridharan R. , Raghavarao R.
Published: February 2004

Rocket measurements from equatorial regions during equatorial spread F (ESF) consistently reveal the presence of plasma density irregularities in the lower F region (200–300 km) where the initial conditions including the vertical electron density gradient are not favorable for the development of linear instability. An investigation carried out using a nonlinear numerical simulation model and the results obtained from the lonization Hole campaign revealed that the altitude valuation of the recombination coefficient and the vertical polarization velocity due to fringe fields associated with the plasma bubble at the base of the F layer are responsible for changing the polarity of the vertical gradient in the plasma density profile in the lower F region, while the penetration of the fringe fields is mainly responsible for the development of zonal electron density gradient which plays a crucial role in the secondary plasma instabilities resulting in the generation of kilometer scale size irregularities in the vertical direction.
Injection and acceleration of thermal protons at quasi‐parallel shocks: A hybrid simulation parameter survey
Authors: Giacalone J. , Burgess D. , Schwartz S. J. , Ellison Donald C. , Bennett Lee
Published: February 2004

Collisionless shocks that propagate along the mean magnetic field are known to accelerate some fraction of the incident charged particles directly from the thermal pool to energies that are considerably higher than the energy at which the plasma rams into the shock. Using hybrid simulations, we address two issues: (1) the dependence of the injection/acceleration of thermal protons to energies much higher than the plasma ram energy on various shock parameters such as Mach number, plasma beta, etc., and (2) the effect of the high‐energy particles, accelerated directly from the thermal population by the shock, on the macroscopic properties of the shock, most notably, on the density compression. We find that for supercritical Mach numbers the acceleration of the thermal plasma is efficient enough that the back pressure due to the energetic particles can significantly increase the density compression across the shock, above the value expected from the simple Rankine‐Hugoniot prediction. Additionally, at low Alfvén Mach number, where the acceleration of the thermal plasma is inefficient, the density compression is smaller than the simple Rankine‐Hugoniot prediction owing to the nonresonant fire hose instability. The acceleration efficiency increases with Mach number except at very high Alfvén Mach numbers, where it begins to decrease for Mach numbers greater than ∼ 10. This is due to the presence of a fixed, free‐escape boundary that limits the size of the foreshock region measured in units of the mean‐free paths of the accelerated particles. Additionally, we find that regardless of the upstream plasma parameters, the acceleration efficiency increases with both the density compression ratio across the shock and the distance to the free‐escape boundary measured in units of the mean‐free path of the energetic particles. Both of these are consistent with analytic theory and numerical models that use a phenomenological scattering law.
Turbulence in coronal mass ejections
Authors: Ruzmaikin A. , Feynman J. , Smith E. J.
Published: February 2004

The magnetic fields measured by the ISEE 3 spacecraft are used to study MHD turbulence within coronal mass ejections (CMEs). The spectral indices of the turbulence inside CMEs are compared with spectral indices found in solar wind undisturbed by CMEs. Irrespective of the CME velocities, the spectra within CMEs are found to differ from those determined for the fast solar wind from coronal holes. Instead, the CME spectra more closely resemble those found in the slow solar wind. Since both CMEs and the plasma sheet are associated with closed magnetic structures in the solar corona, it appears that the turbulence in the solar wind arising from closed coronal regions is fundamentally different from the turbulence in wind from coronal holes. Helical properties of magnetic fields within CMEs at different scales are studied. Preliminary results indicate that, in small scales, helical fields are distributed in a random fashion with alternating signs of twisting structures. In addition however, CMEs have largescale helical structures. In the case of the magnetic cloud of day 302, 1978 this structure is interpreted as a part of the helical structure identified previously by fitting the data to a force‐free flux rope configuration.
Global patterns of heliospheric magnetic field polarities and elevation angles: 1990 through 1995
Authors: Burlaga L. F. , Ness N. F.
Published: February 2004

From 1990 through 1995, Voyager 1 (Vl) moved from 39.9 to 61.7 AU in the distant heliosphere at a latitude of ≈32 ± 1°N. During this period of declining solar activity, the latitudinal extent of the sector zone (the region containing sectors and the heliospheric current sheet) was decreasing. From 1990 through 1992, Vl observed both positive and negative magnetic polarities, indicating that it was in the sector zone. From 1993 through 1995, Vl observed primarily positive magnetic polarities, indicating that it was above the sector zone. The transition in the Vl polarity distribution at the beginning of 1993 implies that the maximum latitudinal extent of the sector zone was then ≈32°N. This transition is related to a decrease in the maximum latitudinal extent of the computed neutral line in the solar corona. The “classic model” of the neutral line provides a better description of the Vl polarities than the “radial model.” The latitudinal extent of the sector zone was nearly constant from the Sun to 50 AU. A similar transition in the polarity distribution was observed at ≈30°S by Ulysses in early 1993 within 5 AU, establishing the minimum latitudinal extent of the sector zone at that time. From 1990 through 1995, near‐Earth spacecraft (which were near the solar equatorial plane, in the middle of the sector zone) observed equal amounts of positive and negative polarities. The bimodal two‐polarity distributions observed by Voyager 2 showed an increasing fraction of negative polarities as it moved from 2.4°N to 15.1°S and from 31.1 to 47.7 AU. The distribution of elevation angles was gaussian both at Vl in the distant heliosphere and near 1 AU during the each of the years from 1990 through 1995. The width of the distribution did not change significantly at Vl from 1986 through 1995, consistent with little (<5°) or no variation with (1) radial distance from 40 to 62 AU, (2) latitude from 31° to 33°, and (3) the solar cycle. The width of the distribution of elevation angles at 1 AU did vary with solar activity, being 5° greater at solar maximum than at solar minimum, possibly owing to the extreme north‐south fields in many ejecta, which are more frequent near solar maximum.
MHD ballooning stability of a sheared plasma sheet
Authors: Hurricane Omar A.
Published: February 2004

A three magnetic field component sheared plasma sheet equilibrium model is developed in the context of ideal magnetohydrodynamics (MHD). It is argued that MHD stability theory may be sufficiently correct for a thin sheet if the shear in the magnetic field is large enough. Using the model magnetic field, the ballooning eigenvalue equation is solved for several different values of equilibrium parameters. It is shown that for sufficiently large values of the shear parameter, the model is unstable to symmetric ballooning modes with a growth rate on the order of one tenth the Alfvén rate.
A unified model of the magnetotail convection in geomagnetically quiet and active times
Authors: Nishida A. , Mukai T. , Yamamoto T. , Kokubun S. , Maezawa K.
Published: February 2004

Geotail observations have shown that during geomagnetically quiet times the northward magnetic field lines are convected tailward in the distant tail while they are convected earthward in the near‐Earth magnetotail. These seemingly incompatible observations when the interplanetary magnetic field (IMF) Bz is northward and |By| ≥ Bz can be explained by the magnetic reconnection which occurs in the distant tail where the neutral sheet is twisted by tens of degrees. According to this interpretation, the tailward convection of the northward field lines occurs beyond the reconnection line because the reconnected field lines that cross the neutral sheet from its northern side to the southern side can have the northward polarity everywhere by virtue of the twisting of the neutral sheet. Hence the reconnection can be the generic mechanism of the plasma acceleration and heating in the magnetotail in both geomagnetically active and quiet times, namely, under both southward and northward (with |By| ≥ Bz) IMF conditions. Both Geotail and DE 2 observations suggest that the closed field lines that are formed by the reconnection return to the dayside by convecting through the inner magnetosphere in geomagnetically quiet times as well as in active times. This suggests that the dayside reconnection with the northward IMF (with |By| ≥ Bz) on the magnetopause involves the closed field lines in addition to the open field lines as considered in the classical cusp reconnection model under the northward IMF.
Computer simulations on three‐dimensional plasmoid dynamics by the spontaneous fast reconnection model
Authors: Ugai M. , Wang W. B.
Published: February 2004

The present paper applies the spontaneous fast reconnection model to the three‐dimensional (3‐D) plasmoid dynamics by extending our 2‐D plasmoid study in the (x, y) plane [Ugai, 1995b] to the z (dawn‐dusk) direction. As the fast reconnection spontaneously develops, a large‐scale plasmoid evolves and propagates through a long current sheet system. There is a large‐scale vortex plasma flow around the plasmoid, so that the resulting plasmoid is significantly confined in a limited inner region (in the z direction) because of the converging flow toward the central z = 0 (i.e., (x, y)) plane in the backward of the plasmoid. Along the resulting plasmoid boundary layer, slow shocks and finite amplitude intermdediate waves simultaneously stand. A (pure) finite amplitude intermediate wave stands just ahead of the slow shock, so that there is a good correlation between the sheared field and flow components Bz and uz. The initial (one‐dimensional) sheet current is drastically disrupted in the inner fast reconnection and plasmoid regions, and the current flowing into the inner region is largely bifurcated and concentrated into the slow shock and plasmoid boundary layers. The 3‐D plasmoid structure is so complicated that the Bz (dawn‐dusk) field component may have a monopolar‐like or bipolar‐like change depending on the path through which the plasmoid is observed in the x direction. It is proposed that the sponaneous fast reconnection model can reasonably be applied to substorms.
Driving a physical ionospheric model with a magnetospheric MHD model
Authors: Sojka J. J. , Schunk R. W. , Bowline M. D. , Chen J. , Slinker S. , Fedder J.
Published: February 2004

This is the first study in which a physical ionospheric model (time‐dependent ionospheric model (TDIM)) has been driven through a substorm using self‐consistent magnetospheric convection electric field and auroral electron precipitation inputs. Both of these were generated from a simulation of a real substorm event using the MHD model [Fedder et al., 1995b]. Interplanetary magnetic field (IMF) data were available for 1.5 hours until the substorm breakup. Hence the substorm growth and expansion dynamics is captured in a 1.5‐hour time period. As a reference against which to compare this TDIM substorm simulation, a typical climatological TDIM simulation was carried out using standard statistical representations of the convection electric field and auroral oval. Note that these statistical representations are driven by the Kp index. This is a 3‐hour index, yet the substorm growth and expansion occurs in 1.5 hours. Hence a static convection electric field and auroral oval are used for the TDIM reference simulation. From the comparison of these two simulations, we find, as expected, the E region densities are different. However, these differences lead to factors of 2–4 differences in the integrated Hall and Pedersen conductivities. These conductivities, in turn, are crucial as an ionospheric boundary condition for magnetospheric MHD modeling. The F region spatial and temporal responses are complex and exhibit large differences, from tens of percents to factors of 4 in density and up to ±70 km in hmF2. These differences are all larger than typical experimental uncertainties. The day side and cusp variabilities are very sensitive to the convection pattern and are not well correlated to magnetic indices, such as the 3‐hourly Kp index. In the polar cap, the differences in the location of the tongues of ionization and the polar holes readily lead to factors of 2–4 in local density differences. Differences in the locations of “boundaries” in the plasma convection and auroral precipitation lead to large differences in the local F region densities and in the locations of strong density gradients, both of which are relevant to space weather applications.
Time variations of the cosmic ray distribution function during a solar proton event of September 29, 1989
Authors: Dvornikov V. M. , Sdobnov V. E.
Published: February 2004

On the basis of cosmic ray (CR) ground‐based observations from the world stations network during the solar proton event of September 29, 1989, the method of spectrographic global survey was used to investigate variations of the particle distribution function in the rigidity range from a few to tens of gigavolts. By analyzing results obtained on variations of the angular distribution of different‐energy particles and time profiles of CR intensity, it is concluded that it is impossible to describe the process of solar CR propagation in terms of the simplest ideas of the motion of particles in the outer electromagnetic fields of the heliosphere. To explain the results obtained, it is speculated that a particle beam accelerated in the flare propagates along the heliospheric current sheet, as a result of which in interplanetary space polarization electric fields are produced, through which an energy exchange occurs between solar CR and background particles of solar wind plasma and galactic CR.
MHD simulation of an interaction of a shock wave with a magnetic cloud
Authors: Vandas M. , Fischer S. , Dryer M. , Smith Z. , Detman T. , Geranios A.
Published: February 2004

Interplanetary shock waves, propagating in the heliosphere faster than earlier‐emitted coronal ejecta, penetrate them and modify their parameters during this interaction. Using two and one half dimensional MHD simulations, we show how a magnetic cloud (flux rope) propagating with a speed 3 times higher than the ambient solar wind is affected by an even faster traveling shock wave overtaking the cloud. The magnetic field increases inside the cloud during the interaction as it is compressed in the radial direction and becomes very oblate. The cloud is also accelerated and moves faster, as a whole, while both shocks (driven by the cloud and the faster interplanetary shock) merge upstream of the cloud. This interaction may be a rather common phenomenon due to the frequency of coronal mass ejections and occurrence of shock waves during periods of high solar activity.
Shadowed particle distributions near the Moon
Authors: Bale S. D.
Published: February 2004

Recent Wind spacecraft observations of broadband ion acoustic noise and Langmuir waves near the lunar wake suggest that the electron dynamics play an important role in the solar wind‐Moon interaction. As the solar wind flows past the Moon, electrons and ions impacting the lunar surface are removed from their respective distribution functions. We extend a guiding center model of the solar wind velocity distribution into two dimensions and include separate electron and ion distributions. While the ion density shadow is predominantly in the antisunward direction, the electrons have density shadows that extend roughly along the direction of the interplanetary magnetic field lines tangent to the Moon. The charge imbalance created by the topological difference of these density distributions must induce space charge fields in the solar wind plasma; these may drive currents that generate the observed ion acoustic noise. Unstable positive slope features in the electron distribution function arise naturally in this scenario. Langmuir waves would then be generated by a time‐of‐flight cutoff distribution as in the terrestrial electron foreshock. This model is not self‐consistent, as it does not include space charge effects, but serves to point out the importance of considering a kinetic description of the solar wind‐Moon interaction.
Morphology of equatorial plasma bubbles
Authors: Singh Sardul , Bamgboye D. K. , McClure J. P. , Johnson F. S.
Published: February 2004

Some occurrence characteristics of plasma bubbles based on data from the ion drift meter aboard the Atmosphere Explorer E satellite are presented. Of 5200 bubbles detected between 250 and 475 km altitude, 65% had ion density depletions of less than 1 order of magnitude below the ambient ion density, and the occurrence frequency decreased with increasing depletion depth. Depletion depth decreased with local time, probably due to diffusive decay of bubbles. Bubble activity above 300 km altitude had its onset around 1900 and peaked around 2130 LT. Late, shallow bubbles appeared to be associated with frequency spread F, and early, deep bubbles with range spread F. Increase in magnetic activity inhibits bubble generation except near sunrise, at which time bubble activity has been found to increase with magnetic disturbance. Bubble activity is restricted largely to a belt within about ±20° dip latitude.
Proton temperature anisotropy upper bound
Authors: Gary S. Peter , Wang Joseph , Winske Dan , Fuselier Stephen A.
Published: February 2004

The electromagnetic proton cyclotron instability and the mirror instability are driven by the proton temperature anisotropy T⊥p/T‖p > 1, where ⊥ and ‖ denote directions relative to the background magnetic field. Linear theory and one‐dimensional hybrid simulations imply that the former mode grows more rapidly over 0.05 ≤ β‖p ≤ 5 and that wave‐particle scattering by its enhanced fluctuations imposes an upper bound on the temperature anisotropy of the form

urn:x-wiley:01480227:media:jgra13669:jgra13669-math-0001

where
and B0 is the background magnetic field. Here Sp and αp are fitting parameters, and
. This paper describes results from more general two‐dimensional hybrid simulations, which permit both instabilities to grow simultaneously. These simulations confirm the one‐dimensional results on the initial domain
; enhanced fluctuations display the properties of the proton cyclotron instability and αp ≃ 0.4. On this domain the two‐dimensional simulations also yield an upper bound for the fluctuating field energy density of the form

urn:x-wiley:01480227:media:jgra13669:jgra13669-math-0005

with fitting parameter
. The simulations on the initial domain 10 ≤ β‖p≤100 show spectral characteristics of both instabilities and exhibit a more stringent bound on the proton anisotropy, in agreement with observations in the terrestrial magnetosheath.
Ion dynamics in magnetic reconnection: Comparison between numerical simulation and Geotail observations
Authors: Hoshino M. , Mukai T. , Yamamoto T. , Kokubun S.
Published: February 2004

We study the plasma thermalization and acceleration process in collisionless magnetic reconnection by using a two‐dimensional, particle‐in‐cell numerical simulation and discuss the plasma mixing process of cold lobe ions into the plasma sheet. We find that the plasma thermalization timescale is longer than the dynamic timescale of magnetic reconnection and that non‐Maxwellian ion velocity distribution functions produced during the evolution of magnetic reconnection play an important role on the dynamical structure of the plasma sheet. The ion velocity distribution functions are characterized by four class of distributions: (1) anisotropic, high‐speed ion beams along the magnetic field line in the plasma sheet boundary layer, (2) two counter‐streaming ions along the magnetic field line inside the plasmoid and around the edge of magnetic diffusion region, (3) nongyrotropic, dumbbell‐like ions near the X‐type region, and (4) the thermal distribution function downstream of slow shocks. We compare the behavior of those ion velocity distribution functions obtained by our reconnection simulation with the distribution functions observed by the Geotail satellite. Most of ion dynamics observed in the Earth's magnetotail can be well understood by our kinetic reconnection simulation. We find that the plasma mixing between the meandering ions accelerated around the X‐type region and the cold ions convected directly from the lobe without passing through the X‐type region plays a significant role on the formation of non‐Maxwellian ions.
Fully adiabatic changes in storm time relativistic electron fluxes
Authors: Kim Hee‐Jeong , Chan Anthony A.
Published: February 2004

It has been suggested that much of the drop and subsequent recovery of storm time relativistic electron fluxes at geosynchronous orbit can be explained in terms of a fully adiabatic response (all three adiabatic invariants conserved) to magnetic field changes. To calculate this effect, we assume a prestorm electron flux distribution constructed from CRRES satellite data, we use modular magnetospheric magnetic field models to represent the magnetic field configuration before and during the storm, and we use Liouville's theorem to evolve the prestorm electron flux. In this work we focus on the important special case of equatorially mirroring electrons. During the main phase of a storm with a Dst minimum of −100 nT we find that the fully adiabatic effect can cause a flux decrease of up to 2 orders of magnitude, consistent with observed flux decreases. We also find that the magnitude of the fully adiabatic flux decrease is larger for lower energies, again in agreement with observations. The contribution of prestorm electron fluxes to the recovery phase flux increase at synchronous orbit is expected to be small because of losses to the dawnside magnetopause. A comparison of fully adiabatic fluxes with measured electron fluxes for the November 2–5, 1993, storm indicates that for this event the fully adiabatic effect may be contributing to the observed decrease but that nonadiabatic effects are clearly important. Overall we conclude that the fully adiabatic effect can account for a significant fraction of observed flux decreases and that differences between the observed and the fully adiabatic fluxes help to clarify when and where additional loss and source mechanisms exist.
Ion dynamics at the earthward termination of the magnetotail current sheet
Authors: Delcourt D. C. , Belmont G.
Published: February 2004

We investigate the dynamics of plasma sheet ions upon interaction with the magnetotail current sheet. During this interaction, ions can be subjected to prominent bunching in gyration phase because of enhanced tailward oriented centrifugal pull. We focus on the inner part of the plasma sheet and examine the consequences of this centrifugally driven phase bunching effect. In this region of space, as their Larmor radius is locally comparable to or smaller than the field variation length scale, ions experience nonadiabatic motion in a manner which differs from that in the distant tail where particles meander about the field minimum. Within the limits of the calculations which do not treat the particle dynamics in a self‐consistent manner, we show that phase bunching of plasma sheet ions leads to a distinctive current distribution in the near‐Earth tail, which is not found further out. In the equatorial vicinity, this distribution is characterized by a striated pattern of current both in the dawn‐dusk and in the Earth‐tail directions. The latter current leads to field line inclination in the dawn‐dusk direction at the earthward edge of the magnetotail current sheet, which may be viewed as characteristic of this region of space. We also show that in this region, off‐diagonal elements of the pressure tensor appear of importance to achieve stress balance.
An empirical model of the substorm current wedge
Authors: Tsyganenko Nikolai A.
Published: February 2004

A wedge‐shaped current linking the nightside ionosphere and the plasma sheet is believed to be the principal cause of a major reconfiguration of the magnetospheric field on the nightside, observed at the onset of the substorm's explosive phase. No dynamical model of the magnetosphere is complete without this important component; however, no realistic representation for the field of the substorm current wedge has been available so far, in part due to the geometrical complexity of that current system and related computational problems. In this work, a simple and flexible analytical model is proposed for the magnetic field of substorm current wedge. The key element of the mathematical treatment is the vector potential of the field produced by a pair of current loops with a spread‐out volume current density. Applying appropriate shift, rotation, and a minor stretching deformation makes it possible to reproduce the desired geometry of the entire system, including field‐aligned currents. The current wedge has a variable longitudinal width, and the model is further generalized by including warping effects due to the tilt of Earth's dipole. This model is intended to be used in data‐based representations of the magnetic field dynamics in the near magnetosphere and will make it possible to reproduce the fast restructuring of the near‐Earth field during the explosive phase of a substorm.
Observations of auroral medium frequency bursts
Authors: LaBelle J. , Shepherd S. G. , Trimpi M. L.
Published: February 2004

Auroral medium frequency (MF) bursts are broadband impulsive radio emissions observed at ground level during the breakup phase of auroral substorms. Measurements made in northern Canada during 1995–1996 show the seasonal and local time dependencies of MF burst emissions, provide case‐study evidence for a null in the MF burst spectrum near twice the ionospheric electron gyrofrequency, and establish a correlation between MF burst and impulsive auroral hiss. High time resolution measurements reveal that MF bursts occur in 100–300 μs wave packets which sometimes appear periodic having a period close to the ionsopheric proton gyroperiod. The timescales of the MF burst wave packets are comparable to those of whistler solitary waves recently observed with the FAST satellite.
Generation of anomalous flows near the bow shock by its interaction with interplanetary discontinuities
Authors: Lin Y.
Published: February 2004

Two‐dimensional hybrid simulations using a curvilinear coordinate system are carried out to study the interaction of the Earth's bow shock (BS) with an interplanetary directional discontinuity. In particular, plasma flow patterns are examined. In the interaction of the bow shock with an interplanetary tangential discontinuity (TD), a bulge of magnetic field and plasma may be present near the intersection between the fronts of the BS and the TD. The bulge expands to the upstream and is embedded in the solar wind. High magnetic field and ion density are present in the boundary regions of the bulge, and the temperature in the bulge is significantly higher than that in the ambient solar wind. A core of low density and sometimes low field is present inside the bulge. The flow speed changes from supersonic in the solar wind to subsonic throughout the bulge. A strong sunward deflection in flow velocity is present both in the bulge and in the magnetosheath behind the bulge. The presence of such hot anomalous flows depends on the direction and symmetry condition of the upstream motional electric field. The formation of the bulge and the associated anomalous flows is found to be mainly due to (1) the reflected ions which are focused to TD under the proper electric field conditions, (2) the unbalanced pressure associated with the geometry change and reformation of the BS which causes the expansion of downstream and the sunward motion of ions, or (3) the occurrence of magnetic reconnection in the current sheet. In the interaction of the BS with an interplanetary rotational discontinuity (RD), the flow may be deflected sunward by the magnetic tension force associated with the resulting rotational discontinuities and slow shocks in the magnetosheath. It is suggested that the observed anomalous flow events upstream of the bow shock may be due to the BS/TD interaction, and both BS/RD and BS/TD interactions may generate strong sunward flow deflections in the magnetosheath.
Toward a consistent picture of the generation of electromagnetic ion cyclotron ELF waves on auroral field lines
Authors: Oscarsson T. , Vaivads A. , Rönnmark K. , Clemmons J. H. , de Feraudy H. , Holback B.
Published: February 2004

Simultanous Freja observations of precipitating keV electrons and electromagnetic emissions around half the local proton gyrofrequency are analyzed. The observed wave fields are used for reconstructing the energy density in wave vector space, the wave distribution function (WDF), and detailed observations of the electron distribution are used to analyze the linear stability of the plasma. We find that a local Landau resonance with precipitating electrons can generate electromagnetic waves having their wave vector (κ) pointing obliquely relative to the background magnetic field. However, the observed parallel electron drift energy suggests that the unstable frequencies are located above the peak in the observed power spectrum and that the κ spectrum predicted from the linear stability analysis does not overlap with the reconstructed WDF. This apparent inconsistency between the proposed instability mechanism and the available Freja data is resolved by placing the source region within 2400 km above Freja. By using a ray‐tracing mapping of an instability at an assumed generation altitude down to Freja altitudes, we show that an electron Landau instability occurring above Freja can be made consistent with the observed wave frequency as well as the reconstructed WDF. The agreement that we obtain between data and theory leads to consistent picture of the generation of electromagnetic ion cyclotron ELF waves on auroral field lines.