bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic field and plasma flow structure near the magnetopause
Authors: Alexeev I. I. , Kalegaev V. V.
Published: February 2004

It has been experimentaly established that there exists a normal component of magnetic field on the magnetopause and that open field lines connect polar cap with interplanetary medium. To explain this information, it is necessary to develop a quantitative theory of an open magnetosphere and to study the structure of the magnetopause. The magnetosheath and the magnetosphere are considered as a unitary system. The dissipative MHD approach is used outside the magnetopause, and the magnetostatic approach is used inside. The frozen‐in condition is valid anywhere except at the magnetopause which is considered to be a thin dissipative boundary layer separating the magnetosheath and the magnetosphere. The magnetopause thickness is proportional to Rm−1/2 (where Rm is the magnetic Reynolds number) and is about 100 times less than stand‐off distance. The magnetic field near the magnetopause is formed due to mutual diffusion of the magnetospheric magnetic field in the magnetosheath and IMF into the magnetosphere. The magnetospheric magnetic field component normal to the magnetopause is proportional to Rm−1/2 and is about 1 nT at the dayside magnetopause. The related potential difference accross the open field lines is about 15 kV, which can be responsible for the background electric field measured in the polar caps. The portion of IMF which penetrates into the magnetosphere is proportional to Rm−1/4 and is about a factor of 10 less than the IMF. The potential difference accross the polar cap is about 100 kV for the southward IMF of −10 nT.
Atmospheric Response in Aurora experiment: Observations of E and F region neutral winds in a region of postmidnight diffuse aurora
Authors: Larsen M. F. , Marshall T. R. , Mikkelsen I. S. , Emery B. A. , Christensen A. , Kayser D. , Hecht J. , Lyons L. , Walterscheid R.
Published: February 2004

The goal of the Atmospheric Response in Aurora (ARIA) experiment carried out at Poker Flat, Alaska, on March 3, 1992, was to determine the response of the neutral atmosphere to the long‐lived, large‐scale forcing that is characteristic of the diffuse aurora in the postmidnight sector. A combination of chemical release rocket wind measurements, instrumented rocket composition measurements, and ground‐based optical measurements were used to characterize the response of the neutral atmosphere. The rocket measurements were made at the end of a 90‐min period of strong Joule heating. We focus on the neutral wind measurements made with the rocket. The forcing was determined by running the assimilated mapping of ionospheric electrodynamics (AMIE) analysis procedure developed at the National Center for Atmospheric Research. The winds expected at the latitude and longitude of the experiment were calculated using the spectral thermospheric general circulation model developed at the Danish Meteorological Institute. Comparisons of the observations and the model suggest that the neutral winds responded strongly in two height ranges. An eastward wind perturbation of ∼100 m s−1 developed between 140 and 200 km altitude with a peak near 160 km. A southwestward wind with peak magnitude of ∼150 m s−1 developed near 115 km altitude. The large amplitude winds at the lower altitude are particularly surprising. They appear to be associated with the upward propagating semidiurnal tide. However, the amplitude is much larger than predicted by any of the tidal models, and the shear found just below the peak in the winds was nominally unstable with a Richardson number of ∼0.08.
A storm time assimilative mapping of ionospheric electrodynamics analysis for the severe geomagnetic storm of November 8–9, 1991
Authors: Cooper M. L. , Clauer C. R. , Emery B. A. , Richmond A. D. , Winningham J. D.
Published: February 2004

Global parameters are obtained for the November 8–9, 1991 severe geomagnetic storm with the application of a slightly modified version of the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) data inversion model. Inputs to the model include ground magnetometer data, satellite electron precipitation data, satellite ion drift data, and ion drift velocities obtained from radar measurements. We compare two of the AMIE computed parameters, the total Joule heating and the polar cap potential, to both the AE(12) index and the AEAMIE index, an AMIE modeled AE index created specifically for severe storm times. The equation obtained from a linear fit to the total Joule heating versus the AEAMIE index closely resembles other equations found during much less disturbed times using the AE(12) index. Our results using the AE(12) index appear to saturate at a level around 1300 nT. However, this saturation is associated with the calculation of the AE(12) index and not a physical result. An apparent saturation can also be seen in our comparison of the polar cap potential with the AE(12) index, but this is not the case when using the AEAMIE index. The comparison between the polar cap potential and the AEAMIE index yields an approximate linear relationship with a correlation coefficient of 0.80. From our results, we find that the magnitude of the auroral electrojets and the polar cap potential drop do not show signs of saturation at the activity levels that were present during the November 8–9, 1991 severe storm.
High‐resolution spectra of 20‐300 Kev hard X‐rays from electron precipitation over Antarctica
Authors: Smith D. M. , Lin R. P. , Anderson K. A. , Hurley K. , Johns C. M.
Published: February 2004

In December 1990, a set of liquid‐nitrogen‐cooled germanium hard X‐ray and gamma‐ray spectrometers was flown aboard a high‐altitude balloon from McMurdo, Antarctica, for solar, astrophysical, and terrestrial observations. This flight was the first circumnavigation (∼9‐day duration) of the Antarctic continent by a large (800,000‐cubic‐meter) balloon. Bremsstrahlung hard X‐ray emission extending up to ∼300 keV, from the precipitation of high‐energy electrons, was observed on six separate occasions over the auroral zone, all during low geomagnetic activity (Kp ≤ 2+). All events were consistent with emission at the trapping boundary; observations over the polar cap showed no precipitation. We present the first high‐resolution (ΔE ∼2 keV/full width at half maximum (FWHM) spectra of this hard X‐ray emission in the energy range 20–300 keV. The observed count spectra are deconvolved by model‐independent techniques to photon spectra and then to the precipitating electron spectra. The spectral hardness shows an inverse relation with L as expected. Our results suggest that high‐resolution spectroscopy could be extremely effective in characterizing electron precipitation if coupled with imaging capability.
Solar wind electron parameters from quasi‐thermal noise spectroscopy and comparison with other measurements on Ulysses
Authors: Maksimovic M. , Hoang S. , Meyer‐Vernet N. , Moncuquet M. , Bougeret J.‐L. , Phillips J. L. , Canu P.
Published: February 2004

Plasma thermal noise spectroscopy was used for the first time on a large scale on the Ulysses radio receiver data to measure the solar wind electron density and temperature in the ecliptic plane. The validity and limitations of the results obtained with this method are discussed. Nearly simultaneous measurements of the electron density and temperature from the radio receiver, the sounder, and the electron analyzer on Ulysses are intercompared. The thermal noise measurements are found to compare quite well with the other measurements, apart from some discrepancies, which are discussed. The uncertainties on the core temperature, derived from a least squares model fitting of the radio data, are shown to be statistically consistent and significant.
Dispersive field line resonances on auroral field lines
Authors: Streltsov A. , Lotko W.
Published: February 2004

The formation of dispersive Alfvén resonance layers is investigated using a three‐dimensional, two‐fluid, magnetically incompressible model, including electron inertia and finite pressure. The equations are solved in “box” geometry with uniform magnetic field bounded by perfectly conducting ionospheres. Field line resonance (FLR) is stimulated within a density boundary layer with gradient transverse to ambient B; a parallel gradient in the Alfvén speed is also included. Numerical results show that the resonance amplitude is largest on the magnetic shell with eigenfrequency matching the frequency of the surface wave propagating on the density boundary layer. Efficient coupling between the resonant Alfvén wave and surface wave produces a relatively narrow FLR spectrum, even when the driver is broadbanded. Effective coupling to the external driver occurs only for long‐wavelength azimuthal modes. It is shown that the parallel inhomogeneity limits radiation of dispersive Alfvén waves by the FLR. The results provide new insights into low‐altitude satellite observations of auroral electromagnetic fields and the formation of discrete auroral arcs.
Interaction of ring current and radiation belt protons with ducted plasmaspheric hiss: 2. Time evolution of the distribution function
Authors: Kozyra J. U. , Rasmussen C. E. , Miller R. H. , Villalon E.
Published: February 2004

The evolution of the bounce‐averaged ring current/radiation belt proton distribution is simulated during resonant interactions with ducted plasmaspheric hiss. The plasmaspheric hiss is assumed to be generated by ring current electrons and to be damped by the energetic protons. Thus energy is transferred between energetic electrons and protons using the plasmaspheric hiss as a mediary. The problem is not solved self‐consistently. During the simulation period, interactions with ring current electrons (not represented in the model) are assumed to maintain the wave amplitudes in the presence of damping by the energetic protons, allowing the wave spectrum to be held fixed. Diffusion coefficients in pitch angle, cross pitch angle/energy, and energy were previously calculated by Kozyra et al. (1994) and are adopted for the present study. The simulation treats the energy range, E ≥ 80 keV, within which the wave diffusion operates on a shorter timescale than other proton loss processes (i.e., Coulomb drag and charge exchange). These other loss processes are not included in the simulation. An interesting result of the simulation is that energy diffusion maximizes at moderate pitch angles near the edge of the atmospheric loss cone. Over the simulation period, diffusion in energy creates an order of magnitude enhancement in the bounce‐averaged proton distribution function at moderate pitch angles. The loss cone is nearly empty because scattering of particles at small pitch angles is weak. The bounce‐averaged flux distribution, mapped to ionospheric heights, results in elevated locally mirroring proton fluxes. OGO 5 observed order of magnitude enhancements in locally mirroring energetic protons at altitudes between 350 and 1300 km and invariant latitudes between 50° and 60° (Lundblad and Soraas, 1978). The proton distributions were highly anisotropic in pitch angle with nearly empty loss cones. The similarity between the observed distributions and those resulting from this simulation raises the possibility that interactions with plasmaspheric hiss play a role in forming and maintaining the characteristic zones of anisotropic proton precipitation in the subauroral ionosphere. Further assessment of the importance of this process depends on knowledge of the distribution in space and time of ducted plasmaspheric hiss in the inner magnetosphere.
Hybrid modeling of collisionless reconnection in two‐dimensional current sheets: Simulations
Authors: Hesse Michael , Winske Dan , Kuznetsova Masha M.
Published: February 2004

The stability of thin current sheets, such as commonly occur in space plasmas, is investigated in the presence of a magnetic normal component. The investigation is based on hybrid simulations using a suitably adapted electron pressure model. In order to properly represent the effects of the electron cyclotron motion in the magnetic normal component, we include the cyclotron part of the electron pressure evolution in an implicit manner. The results of a number of these simulations are presented with emphasis on a comparison with the linear analytical theory (Kuznetsova et al., this issue), and on the nonlinear evolution of the tearing instability. We find, in agreement with linear theory, a transition to stability for electron mass below a certain threshold, which depends on the initial configuration. The results suggest that the onset of fast substorm evolution in the magnetotail may be a two‐step process, where another presently not included mechanism leads to the destabilization of the tearing instability followed by the nonlinear growth of a large scale instability.
Morphology of O++ ions and their maintenance in the nightside Venus ionosphere
Authors: Ghosh Suchita , Mahajan K. K. , Grebowsky J. M. , Nath N.
Published: February 2004

In this paper, measurements made by the Pioneer Venus orbiter ion mass spectrometer are used to study the temporal and spatial distribution of O++ ion concentrations in the Venus ionosphere. The diurnal analysis shows an enhancement of O++ around midnight. On the nightside the O++ densities are highly variable but show a positive correlation with variations in the O+ densities. Model calculations of O++ concentrations in the nightside are made assuming that impact ionization by precipitating electrons and transport of ions from the dayside were the sources for O++ ions. We find that in situ ionization by precipitating electrons contributes very little, whereas a downward O++ flux in the range of 105 to 106 cm−2 s−1 can account for the measured values of O++ densities near midnight. The high abundance of O++ near midnight is related to the special characteristics of multiply charged ions. Because of the double charge, thermal diffusion and the polarization electric field are particularly found to be signifiicant factors in the momentum equations for O++, and therefore the downward velocity for O++ is small compared with that of other ions. High O++ densities are observed to occur under low solar wind dynamic pressure conditions, consistent with the transterminator transport mechanism as the source of O++ ions in the antisolar region.
Dawn‐dusk magnetic field effects on ions accelerated in the current sheet
Authors: Baek Seok‐Chul , Choi Duk‐In , Horton Wendell
Published: February 2004

The effects of a By field on ions in unbounded transient orbits in the current sheet of the geomagnetic tail are studied. The trajectories and the energization of ions injected at the north edge of the current sheet (z = +L) are investigated varying the dawn to dusk magnetic field strength By. The current sheet with +By produces less energetic ions than that with −By. At By = 0 the exit positions of ions after acceleration from the current sheet are scattered almost equally above and below the current sheet. Finite By introduces a helicity to the magnetic tail structure that breaks this symmetry. When the By field reaches 3 nT, almost all of the tailward streaming ions injected with initial velocities (0 < V0 < 2000 km/s and a given initial kinetic energy ½mV02) are ejected above the current sheet after acceleration. However, a current sheet with a negative dawn‐dusk magnetic field ejects all of them below itself even for small negative By(−1 nT) values. The nonzero EyBy produces E‖ = EyBy/B with E · B being a Lorentz scalar invariant. The finite E‖ makes the electrons move so as to create a cancelling electrostatic potential difference along the magnetic field line. On the basis of the ion orbits found here for the standard model E and B fields we estimate the self‐consistent electrostatic field expected from the difference in the large ion orbits and the well‐magnetized electrons. We suggest that the onset of geomagnetic activity may be correlated with the By field and the associated electrostatic field in the central plasma sheet.
Reconnection of antiparallel magnetic flux tubes
Authors: Dahlburg R. B. , Antiochos S. K.
Published: February 2004

Many examples of solar activity, such as large two‐ribbon flares and prominence eruptions, are widely believed to involve the fast reconnection of magnetic flux tubes. Because of the difficulties associated with calculating the evolution of three‐dimensional (3‐D) flux tubes, however, the details of the energy‐release process are poorly understood. In this paper we describe our first attempts to shed light on this important process. We describe the results of 3‐D numerical simulations of initially distinct magnetic flux tubes interacting via magnetic reconnection. As a typical case, we consider an initial magnetic field given by a compact support function distribution so that the initial topology consists of two antiparallel flux tubes. We then impose an initial velocity field on this system which causes the flux tubes to move toward each other. As a result of this initial velocity, the tubes first flatten against each other and an electric current sheet begins to develop at the interface between them. After approximately 10 Alfven times we observe a burst of reconnection. The turbulent kinetic energy rises dramatically as two reconnection jets form, which are aligned parallel to the initial field. The reconnection phase lasts for approximately 20 Alfven times, by which time the central region of the initial tubes has been completely dissipated so that the system now consists of four tubes that are relatively widely separated and hence stop interacting. We find that the excitation of small‐scale spatial structure in the flow field depends critically on the value of the Lundquist numbers. Compressible effects are insignificant for this particular case of flux tube reconnection. The numerical simulations are carried out using a three‐dimensional explicit Fourier collocation algorithm for solving the viscoresistive equations of compressible magnetohydrodynamics. We also report on the performance of a new parallelized version of the code.
On the seasonal response of the thermosphere and ionosphere to geomagnetic storms
Authors: Fuller‐Rowell T. J. , Codrescu M. V. , Rishbeth H. , Moffett R. J. , Quegan S.
Published: February 2004

Ionosonde observations have provided the data to build a picture of the response of the midlatitude ionosphere to a geomagnetic storm. The particular characteristic of interest is the preference for “negative storms” (decrease in the peak electron density, NmF2) in summer and “positive storms” (increase in NmF2) in winter. A three‐dimensional, time‐dependent model of the coupled thermosphere and ionosphere is used to explain this dependence. During the driven phase of a geomagnetic storm the two main magnetospheric energy sources to the upper atmosphere (auroral precipitation and convective electric field) increase dramatically. Auroral precipitation increases the ion density and conductivity of the upper atmosphere; the electric field drives the ionosphere and, through collisions, forces the thermosphere into motion and then deposits heat via Joule dissipation. The global wind response is divergent at high latitudes in both hemispheres. Vertical winds are driven by the divergent wind field and carry molecule‐rich air to higher levels. Once created, the “composition bulge” of increased mean molecular mass is transported by both the storm‐induced and background wind fields. The storm winds imposed on the background circulation do not have a strong seasonal dependence, and this is not necessary to explain the observations. Numerical computations suggest that the prevailing summer‐to‐winter circulation at solstice transports the molecule‐rich gas to mid and low latitudes in the summer hemisphere over the day or two following the storm. In the winter hemisphere, poleward winds restrict the equatorward movement of composition. The altered neutral‐chemical environment in summer subsequently depletes the F region midlatitude ionosphere to produce a “negative storm”. In winter midlatitudes a decrease in molecular species, associated with downwelling, persists and produces the characteristic “positive storm”.
Radiation of whistlers by the ion‐acoustic turbulence in the ionosphere
Authors: Mishin E. V. , Fiala V.
Published: February 2004

The generation of whistlers in the ionosphere due to the nonlinear coalescence of ion‐acoustic waves is analyzed by means of the weak turbulence approach. The ion‐acoustic turbulence is supposed to be driven by field‐aligned currents or/and the nonlinear coupling of Langmuir waves generated in an unstable beam‐plasma system. The spectral power of whistlers radiated by the ion‐acoustic turbulence is evaluated with special regard to small‐scale auroral events, artificial injections of an electron beam, and the tethered satellite system. In particular, this mechanism of whistler radiation can contribute to the low‐frequency part of the auroral hiss.
The induced magnetosphere of comet Halley: 3. The plasma kinematics
Authors: Israelevich P. L. , Neubauer F. M. , Ershkovich A. I.
Published: February 2004

The plasma velocity in the magnetosphere of comet Halley is calculated by using the displacement of magnetic field lines during the Giotto flyby. The obtained profile is in good agreement with that found from the momentum balance condition. The radial component of plasma velocity measured in situ (Schwenn et al., 1987) correlates well with the spacecraft position in the coordinate system rotating with the interplanetary magnetic field. The radial velocity is shown to be larger in the plane perpendicular to the interplanetary magnetic field (IMF) than in the plane containing the IMF vector. The transverse (to the Giotto trajectory) velocity component decreases toward the nucleus, almost vanishing at the Halley ionopause. This fact favours our previous conclusion (Israelevich et al., 1992) that the conventionally used values of the gas production rate and/or ion‐neutral momentum exchange rate require reexamination. We arrived at the conclusion that the plasma dynamics in the comet Halley magnetosphere can be fairly well described by the frozen‐in magnetic field approximation; that is, the magnetic Reynolds number in the cometary ionosphere is high.
Pressure‐pulse driven surface waves at the magnetopause: A rebuttal
Authors: Sibeck D. G. , Newell P. T.
Published: February 2004

A variety of mechanisms have been proposed to account for transient events observed in the vicinity of the dayside magnetopause. Discriminating among the various proposed mechanisms is a key objective of magnetospheric physics but requires careful consideration of their predictions. Song et al. (1994) recently reported the results of an effort to determine the characteristics of transient events produced by variations in the solar wind dynamic pressure. Noting that the observed characteristics did not coincide with those they predicted, they concluded that transient events in the vicinity of the dayside magnetopause could not be caused by solar wind dynamic pressure variations. In this rebuttal, we call attention to our previous work concerning the expected characteristics of transient events produced by variations in the solar wind and foreshock pressure; that work uses more realistic parameters to conclude that the transient events are consistent with a solar wind/foreshock pressure mechanism.
Models of high‐latitude electric potentials derived with a least error fit of spherical harmonic coefficients
Authors: Weimer D. R.
Published: February 2004

New models of the high‐latitude electric potentials have been developed. These models show how the polar ionospheric electric field, or plasma convection, responds to changes in the interplanetary magnetic field (IMF) and other parameters, such as dipole tilt angle. These patterns were derived from measurements of the electric field on the DE 2 satellite, using all polar cap passes during which high‐resolution IMF data were available from the ISEE 3 or IMP 8 satellites. The data are sorted according to the angle of the IMF in the GSM Y‐Z plane. The measurements are further divided into different groups according to the magnitude of the IMF or the dipole tilt angle. All measurements in each group are then used to derive a model of the electric potential for the given conditions, using a new technique where the coefficients of a spherical harmonic expansion are found by least square error fits. The resulting convection patterns are very realistic and show consistent variations as the IMF BY/BZ angle rotates. For northward IMF (positive BZ) evidently there are four convection “cells,” rather than a distortion of the two‐cell pattern. Many other useful facts regarding the solar wind‐ionosphere coupling can be extracted from the derived patterns, such as the relationship between the polar cap potential drop and the IMF clock angle.
Study of the properties of the step decreases in galactic and anomalous cosmic rays over solar cycle 21
Authors: Fujii Z. , McDonald F. B.
Published: February 2004

The rigidity dependence of galactic and anomalous cosmic rays is studied for the four step decreases that define the long‐term modulation of solar cycle 21. The data are from cosmic ray experiments on IMP 8, ISEE 3, Voyager 1, and Pioneer 10. For the first three step decreases the diffusion coefficient κ is proportional to the particle rigidity R. For the fourth modulation event that occurs during the recovery phase of the long‐term modulation, κ is proportional to R0.4. For the three step decreases where simultaneous galactic cosmic ray ion and electron data are available at 1 AU, the same modulation function is obtained for both positively and negatively charged particles, suggesting that gradient and curvature drift effects are not an important element of these modulation events. The modulation functions for anomalous helium and oxygen from the Pioneer 10 and Voyager 1 experiments are identical to that of galactic cosmic rays and are consistent with the expected singly charged state of this component. At heliocentric distances beyond some 15 AU, κ⊥ is expected to dominate over κ∥. However, there is no significant change in the modulation function with increasing heliocentric distance suggesting that in general κ∥ and κ⊥ ∝ R.
One‐dimensional hybrid simulations of the diamagnetic cavity boundary region of comet Halley
Authors: Puhl‐Quinn P. , Cravens T. E.
Published: February 2004

A one‐dimensional hybrid technique (particle ions and fluid electrons) is used to study the cometary diamagnetic cavity surface (CS). This hybrid study is unique in that it includes the effects of binary ion‐ion Coulomb collisions, an important process in the dense inner coma. The equilibrium location of the CS is maintained by a force balance mainly between the ion‐neutral drag force and the magnetic pressure gradient force. However, the detailed structure of the CS layer also depends on properties of the plasma such as the thermal pressure. Significant variations of the ion density, ion flow speed, and magnetic field strength take place across the CS boundary layer. Our hybrid code description of the CS structure compares favorably with the data from experiments onboard the Giotto spacecraft. When compared to the magnetohydrodynamical (fluid) results of Cravens (1989), there is good agreement on the "core" width of the plasma density enhancement and on the width of the current layer associated with the magnetic field gradient, but a large discrepancy exists in the width of the ion flow speed transition because of the failure of the fluid model to discern particle effects. Related to this, the hybrid code ion density enhancement is not symmetric as a result of a magnetically reflected, backstreaming ion population within the cavity. The core width of this enhancement (Δn) is highly dependent upon the dissociative recombination rate coefficient, and the hybrid results agree to within 20 percent with the fluid model results of Cravens (1989). The width of the velocity transition (Δν), or the "tail" of the density enhancement, is determined by the collision time for the backstreaming ions. The effect of Coulomb collisions is to decrease (Δν) by a factor of 2. The magnetic field transition has a width (ΔB) that is of the order of a few ion gyroradii. Disrupting the ion gyration by including the effects of binary ion‐ion Coulomb collisions alters the role of the ions as the dominant current carriers within the current layer and slightly reduces ΔB.
Joule heating by ac electric fields in the ionosphere of Venus
Authors: Cole K. D. , Hoegy W. R.
Published: February 2004

It is shown that Joule heating from electron collision damping of ac electric fields could be a significant source of energy for electrons in the Venus ionosphere if they occur naturally on a large scale. On the assumption that the fields are due to electromagnetic waves on a planetary scale, we calculate heating rates implied by observations on the day side and compare them with the profiles of photoelectron heating rates used by modelers. It is found that sufficient amplitudes have been measured on Pioneer Venus Orbiter in the frequency channels of 730 and 100 Hz to cause greater local electron heating rates than that due to photoelectrons, up to 3 orders of magnitude in some cases. Nightside heating rates at low altitudes for the 100‐Hz signal would cause great heating of the ionosphere and thermosphère, airglow, and aurora. The difficulties produced by these findings would be relieved if the electric field disturbances were closely confined to the spacecraft or at least were much less than planetary in scale.
Large‐scale magnetic field and density distribution in the solar minimum corona
Authors: Gibson S. E. , Bagenal F.
Published: February 2004

We seek a quantitative description of the large‐scale structure of magnetic field and density in the solar minimum corona that is consistent with observations of both white light intensity and the magnetic field at the photosphere. We use white light images from NASA's Solar Maximum Mission (SMM) Coronagraph/Polarimeter and the High‐Altitude Observatory Mark III (MkIII) K‐coronameter, along with photospheric field measurements from Stanford's Wilcox Solar Observatory (WSO), as constraints on the magnetostatic model of Bogdan and Low [1986] (B&L). We find a family of solutions to the B&L model that reproduce observations of white light quite well, each with a different magnetic field structure. We show that the observed photospheric field cannot be used as an exact boundary condition on the B&L model, but we can limit the white light solutions by matching the total observed photospheric magnetic flux. We find a set of model parameters that reproduces white light and photospheric field to within quantifiable model and observational limits and calculate the physical plasma properties corresponding to these parameters. We conclude that this fit represents a self‐consistent description of the solar minimum coronal magnetic field and density.
Ponderomotive acceleration in the auroral region: A kinetic simulation
Authors: Miller Ronald H. , Rasmussen Craig E. , Combi Michael R. , Gombosi Tamas I. , Winske Dan
Published: February 2004

Alfvén waves with frequencies of the order of several Hertz have been observed in the auroral region. These waves are thought to be responsible for the acceleration of ionospheric particles out of the ionosphere and into the magnetosphere. As an Alfvén wave propagates into an inhomogeneous ionosphere, the amplitude of the wave decreases resulting in a gradient in the electric field and a ponderomotive force. The nature of the ponderomotive force is to accelerate ions out of the ionosphere, while accelerating electrons into the ionosphere decreasing the ambipolar electric field. Ponderomotive acceleration of ions can penetrate to low enough altitudes affecting the production regions for O+ and H+ ions. To understand the influences of ponderomotive acceleration on the lower ionosphere, a one‐dimensional hybrid particle code is used. The simulation model allows for multiple species, atmospheric chemistry and photoionization, tilted dipolar coordinate system, corotation, and convection. The electrons are treated as a charge neutralizing fluid where the parallel and perpendicular temperatures and heat flows are given by the 16‐moment transport equations. Coulomb, ion‐neutral, and electron‐neutral collisions are included in the hybrid simulation using newly developed collision techniques applicable to kinetic simulations. The altitude dependence of an Alfvén wave propagating from 6000 to 200 km in an inhomogeneous ionosphere is determined by solving Maxwell's equations. The parallel and perpendicular envelopes are determined and the resulting nonlinear, non‐resonant ponderomotive acceleration of ionospheric ions is calculated. The ponderomotive force is included in the kinetic simulation in the same fashion as gravity or the self‐consistent electric field. The acceleration event lasts for 10 min, by which time the density and flow speed profiles have reached equilibrium. The density and flow speed at 2500 km after 10 min of acceleration increase from 4.3 cm−3 and 1.6 km/s to 34 cm−3 and 6.0 km/s for H+, while the same quantities for O+ increase from 1670 cm−3 and −0.0009 km/s to 4795 cm−3 and 7.0 km/s. Collisional coupling between the H+ and O+ contributes to enhancing the outflow of H+. The net response of the ionosphere to the nonlinear ponderomotive force is an enhancement in ionospheric outflow of ions into the magnetosphere.
Distributions of the intensity of ion temperature perturbations in the thermosphere
Authors: Gavrilov N. M.
Published: February 2004

The results are presented from the study of vertical, seasonal, and latitudinal variations of ion temperature perturbations in the altitude region 100–400 km using measurements at the network of IS radars over a multiyear period. The intensities of ion temperature perturbations having periods less than 6 hours are obtained using the least squares fit filtration of tides from the data. The intensities of the perturbations are maximized in the altitude region 150–180 km at low latitudes. At high latitudes the maxima become much smaller. Seasonal and latitudinal variations of the intensity have different structure below and above 150–180 km. At altitudes below 120–130 km the intensity of the ion temperature perturbations has a maximum in January and a minimum in July at high latitudes, and two maxima (in summer and winter) at low latitudes. Above 150–180 km the intensity of ion temperature perturbations has maxima at high and low latitudes in all seasons. The high‐latitude maxima can be attributed to the variations of electric fields and particle precipitation and to the atmospheric waves propagated from below and generated in the auroral region. The reasons for the low‐latitude maxima of the intensity are not clear enough. A possible reason may be the increased generation of internal gravity waves by tides in the equatorial upper atmosphere.
Energetic ring current particles generated by recurring substorm cycles
Authors: Liu W. W. , Rostoker G.
Published: February 2004

It is now generally accepted that substorm expansive phase activity involves a release of magnetotail energy, part of which is available for the acceleration of highly energetic particles (E > 100 keV) that play a role in ring current and radiation belt formation. However, the physical process whereby this acceleration takes place is not yet fully understood. In this paper we consider the physics of the acceleration process based on the tenet that acceleration of particles to E >100 keV occurs as a cumulative process involving a sequence of substorm expansive phases. In our view, nonadiabatic particle dynamics plays a key role in the energization process. We present both a qualitative description and quantitative treatment which starts with a consideration of a single particle and is then extended to track the evolution of the particle distribution function over several substorm expansive phase cycles. We stress two salient features of the evolution of particle distribution function, namely the rapid formation of a non‐Maxwellian high‐energy tail and the saturation of this tail distribution toward a kappa distribution in the asymptotic limit.
Fast formation of a plateau on the electron distribution function during the early phase of the ion‐ion stability
Authors: Mottez F.
Published: February 2004

Numerical simulations of the ion‐ion instability carried out with parameters suitable to the description of the plasma in the high‐altitude auroral region of the Earth show a very fast evolution of the electron distribution function that cannot be explained by the quasi‐linear theory. A detailed analysis of this phenomenon has been carried on. A model based on the reflection of electrons against potential structures created during the linear phase of the ion‐ion instability is discussed. The lifetime of the potential structures, which is longer than one ion gyroperiod, can be explained by the linear theory of the ion‐ion instability. A simple statistical model of these potential structures enables us to reproduce accurately the evolution of the electron distribution function.
Anisotropy and proton density in the Io plasma torus derived from whistler wave dispersion
Authors: Crary F. J. , Bagenal F. , Ansher J. A. , Gurnett D. A. , Kurth W. S.
Published: February 2004

During the Voyager 1 encounter with Jupiter, a large number of whistler waves were observed. Previous studies have examined the dispersion of these waves and made estimates of the electron and light ion (i.e., proton) densities. The current paper reexamines this data, taking into account the revised temperatures of the torus species the additional data on ion composition from the Voyager UVS instrument and the role of thermal anisotropy on the plasma densities. These refinements in the density model drastically alter the implications of the whistler wave data. Both the thermal and the nonthermal species must be anisotropic to fit the whistler dispersions. The thermal component must have T⊥/T‖ > 1.75 and the nonthermal component 3 < T⊥/T‖ < 10, The equatorial proton density is low, under 60 cm−3 in all cases. This results in a proton abundance (L shell proton content relative to the total ion content) of no more than 10%, approximately a factor of two lower than the conclusions of previous whistler analysis. At the high latitudes, the implied electron density results in a plasma frequency of under 20 kHz. Finally, it is evident from this analysis that not all of the whistler waves were propagating along the magnetic field lines, as was commonly assumed in previous work.
Prediction of solar and geomagnetic activity data using neural networks
Authors: Macpherson K. P. , Conway A. J. , Brown J. C.
Published: February 2004

Accurate predictions of the future behavior of the solar activity cycle have been sought for many years. Several classes of prediction approach have been proposed, with many variations in each class, and have achieved varying degrees of success. However, considerable room for improvement still remains. Artificial neural network models enjoyed a resurgence in popularity as prediction tools during the late 1980s, as a consequence of the discovery of the back propagation of errors learning algorithm. Initial investigations have been carried out into their potential for predicting solar activity (e.g., Koons and Gorney, 1990; Williams, 1991; Macpherson, 1993a, b). In this paper, we investigate in detail the effect different neural network architectures and learning parameters have on the prediction accuracy of various networks trained on smoothed monthly sunspot and solar 10.7‐cm flux data. The importance of obtaining the best generalization capability of a neural network is stressed. Prediction of the geomagnetic aa index is also considered. Finally, in order to validate the usefulness of this technique, the results are compared with a variant of the well‐established McNish and Lincoln method (McNish and Lincoln, 1949) and are found to be superior in terms of prediction accuracy.
Dispersion of electrostatic waves in the Io plasma torus and derived electron temperature
Authors: Moncuquet M. , Meyer‐Vernet N. , Hoang S.
Published: February 2004

We present a detailed analysis of the set of radio spectra acquired during the Ulysses spacecraft passage through the outer part of the Io plasma torus (at ∼ 8 RJ). Since Ulysses is the first spacecraft to explore the torus far from the equator and the onboard plasma analyzers were shut off, these wave data are the only ones providing an in situ plasma diagnostics outside the equatorial region. We present here two main results. First, by comparing the observed spin modulation of the signal measured between electron gyroharmonics to the theoretical modulation for electrostatic waves propagating roughly normal to B, we deduce experimental dispersion curves for these waves. These curves are very similar to Bernstein dispersion curves in a quasi‐thermal plasma. Second, by fitting these theoretical dispersion relations to the experimental ones, we are able to deduce the core electron temperature with about 20% uncertainty when the density is measured independently. Otherwise, we can get a rough evaluation of both the density and the temperature. The corresponding latitudinal variation of these parameters is analyzed in a related paper (N. Meyer‐Vernet, M. Moncuquet, and S. Hoang, 1995).
A generation mechanism for the downshifted peak in stimulated electromagnetic emission spectrum
Authors: Huang J. , Kuo S. P.
Published: February 2004

Parametric decay of an electron Bernstein pump wave into an electron Bernstein sideband wave and a nearly perpendicularly propagating ion acoustic (or electrostatic ion cyclotron) decay wave is studied. When incorporated with appropriate nonlinear scattering processes, this instability process along with its cascading is proposed to be the generation mechanism for the downshifted peak (DP) and 2DP features as well as the upshifted peak (UP) feature in the stimulated electromagnetic emission spectrum observed in ionospheric heating experiments. It is found that the instability can be excited most favorably when the heater wave frequency ƒ0 is in a very narrow frequency range near the third harmonic of the electron cyclotron frequency ƒc of the ambient ionosphere. Moreover, the frequencies associated with the low‐frequency decay mode decrease as ƒ0 is increased toward 3ƒc, consistent with experimental observations.
Field‐aligned currents associated with spatially periodic X ray structures in the morningside auroral oval
Authors: Robinson R. M. , Chenette D. L. , Datlowe D. W. , Schumaker T. L. , Vondrak R. R. , Bythrow P. F. , Potemra T. A. , Sharber J. R. , Winningham J. D.
Published: February 2004

On February 9, 1992, the Atmospheric X ray Imaging Spectrometer (AXIS) on the Upper Atmosphere Research Satellite (UARS) observed spatially periodic structures in X ray luminosity from the top of the atmosphere during a pass over the dawnside auroral oval. The patches of luminosity were several hundred kilometers in size and separated by a mean distance of about 700 km. As many as a dozen distinct patches could be identified extending from local times just after midnight to the noon meridian. The characteristics of the precipitating electrons responsible for the patches were determined from the measured X ray fluxes. Peak electron energy fluxes were between 4 and 10 ergs cm−2 s−1 with e‐folding energies of 10–25 keV. In situ magnetic field data from the onboard magnetometer were used to model the field‐aligned currents associated with two of the patches. The model that best fit the data had upward currents in the western portions of the patches and more intense downward currents confined to the eastern edges. The modeling also indicated that these small‐scale currents were embedded within the large‐scale region 2 field‐aligned current sheet. Data from a ground‐based magnetometer near the satellite ground track indicated the presence of Ps 6 pulsations. We have interpreted the observations in terms of eastward‐drifting current patch systems. Under this assumption, the patches moved at 1.7 km s−1, the drift speed of ∼30‐keV electrons. This motion, and the correlation with Ps 6 pulsations suggests that the X ray patches were associated with omega bands that are seen in the morning sector during geomagnetically active times.
Observations of electron conies by the Viking satellite
Authors: Eliasson Lars , André Mats , Lundin Rickard , Pottelette Raymond , Marklund Göran , Holmgren Gunnar
Published: February 2004

Electron angular distributions peaked at oblique angles to the magnetic field, electron conies, are frequently found in the Viking data at all magnetic local times, but with a maximum in the dusk sector. Several types of electron conies have been observed by Viking as well as by other satellites. One type is frequently seen below the acceleration region. It is caused by adiabatic motion of electrons with a field‐aligned distribution that has a broader angular width than the loss cone. The atmospheric loss of particles will result in a distribution peaked at pitch angles close to the loss cone. Another type is due to heating or acceleration processes at low altitudes and is seen above the acceleration region. Particle and wave characteristics and possible generation mechanisms for this type of electron conic are discussed. The ion arid electron angular distributions observed by Viking indicate that a parallel electric field is present below Viking during this type of event. Ion observations give important information on the processes that produce the electron conies. Almost 200 electron conic events have been detected in the Viking data, that is, in about every third orbit that was studied. Their frequency of occurrence in altitude maximizes in the upper part of the acceleration region, between 10,000 and 11,000 km. The number of events are found to decrease at higher altitudes. This might indicate that diffusion processes, associated with wave generation, remove the anisotropy in the electron distribution function in and above the acceleration region. Wave observations show the presence of both low‐frequency waves and waves close to the electron gyro frequency. Acceleration in a fluctuating (approximately 1 Hz) parallel electric field is suggested as a likely mechanism to create the electron conies observed by Viking. The period of the fluctuations is comparable to the travel time, below the acceleration region, of the electrons forming the conic distribution.
Propagation characteristics of whistler waves in the Jovian ionosphere and magnetosphere
Authors: Hobara Y. , Molchanov O. A. , Hayakawa M. , Ohta K.
Published: February 2004

The purpose of this paper is to carry out extensive reexamination of the propagation characteristics of unducted whistlers in the Jovian magnetosphere. In addition to the previous work on the dispersion of whistlers, our emphasis is placed on the detailed study of their amplitude variations in order to investigate the plasma structure of the magnetosphere and characteristics of causative lightning discharges. The two‐dimensional ray‐tracing computations are used as before to investigate magnetospheric whistler propagation paths, but with the first attempt of estimating the whole attenuation factors (polarization effect, focusing/defocusing and interactive damping (Landau and cyclotron)). The total attenuation due to three effects is estimated, which is compared with the amplitude of whistlers observed by the Voyager spacecraft. The following conclusions are reached. (1) Some bias in magnetospheric plasma density is required to explain the high‐frequency cutoff of whistlers, (2) the magnetospheric temperature is about 20 eV for the interpretation of low‐frequency cutoff of whistlers, and (3) estimated total radiated power per flash is about 4.5×106 W(0∼20 kHz) on the assumption of unducted propagation and nearly the same even in the case of ducted propagation.