bluuebunny/crossref_metadata_2025 huggingface dataset:
A time‐dependent gyro‐kinetic model of thermal ion upflows in the high‐latitude F region
Authors: Loranc M. , St. ‐Maurice J. ‐P.
Published: February 2004

Ample evidence supports the significance of the high‐latitude ionospheric contribution to magnetospheric plasma. Assuming flux conservation along a flux tube, the upward field‐aligned ion flows observed in the magnetosphere require high‐latitude ionospheric field‐aligned ion upflows of the order of 108 to 109 cm−2 s−1. Since radar and satellite observations of high‐latitude F region flows at times exceed this flux requirement by an order of magnitude, the thermal ionospheric upflows are not simply the ionospheric response to a magnetospheric flux requirement. Several ionospheric ion upflow mechanisms have been proposed, but simulations based on fluid theory do not reproduce all the observed features of ionospheric ion upflows. Certain asymmetries in the statistical morphology of high‐latitude F region ion upflows suggest that the ion upflows may be generated by ion‐neutral frictional heating. We developed a single‐component (O+), time‐dependent gyro‐kinetic model of the high‐latitude F region response to frictional heating in which the neutral exobase is a discontinuous boundary between fully collisional and collisionless plasmas. The concept of a discontinuous neutral exobase and the assumption of a constant and uniform polarization electric field reduce the ion guiding center motion in the frame of a convecting flux tube to simple one‐dimensional ballistic trajectories. We thus are able to analytically calculate a time and height‐dependent ion velocity distribution function, from which we can compute the ion density, parallel velocity, parallel and perpendicular temperature, and parallel flux. Using our model, we simulated the response of a convecting flux tube between 500 km and 2500 km to various frictional heating inputs; the results were both qualitatively and quantitatively different from fluid model results, which may indicate an inadequacy of the fluid theory approach. The gyro‐kinetic frictional heating model responses to the various simulations were qualitatively similar: (1) initial perturbations of all the modeled parameters propagated rapidly up the flux tube, (2) transient values of the ion parallel velocity, temperature, and flux exceeded 3 km s−1, 2 × 104 K, and 109 cm−2 s−1. respectively, (3) a second transient regime developed wherein the parallel temperature drops to very low values (a few hundred Kelvins), and (4) well after heating ceased, large parallel temperatures and large downward parallel velocities and fluxes developed as the flux tube slowly returned to diffusive equilibrium. The ion velocity distributions during the simulation are often non‐Maxwellian and are sometimes composed of two distinct ion populations.
Results of critical velocity experiments with barium, strontium, and calcium releases from CRRES satellite
Authors: Wescott E. M. , Stenbaek‐Nielsen H. C. , Hampton D. L. , Delamere P. A.
Published: February 2004

As part of the NASA CRRES chemical release program in September 1990, two Ba and also one each Sr and Ca canisters of a boron‐titanium thermite mixture, which vaporizes the element on ignition, were released near perigee after dusk in the South Pacific to study the critical velocity effect proposed by Alfvén. The critical velocities of these three elements are 2.7, 3.5, and 5.4 km/s respectively, all well below the orbital velocity of 9.4 km/s. On September 10, 1990, a Sr and Ba pair (G‐13, or critical ionization velocity (CIV) I) was released near Rarotonga at ∼515 km altitude in a background electron density of 3.4 × 106 cm−3. On September 14, 1990, G‐14 or CIV II released a Ca and Ba pair west of New Caledonia near 595 km at an electron density of 1.5 × 106 cm−3. Ions of all three elements were observed with low‐light level imagers from two aircraft after they had transited up the magnetic field lines into the sunlight. Emissions from the spherically expanding neutral gas shells below the solar terminator, observed with cameras filtered for the Ba+ ion line at 4554 Å and also in unfiltered imagers for ∼15 s after release, are probably due to excitation by hot electrons created in the CIV process. The ions created clearly lost much of their energy, which we now show can be explained by elastic collisions: Ba+ + O. Inventories of the observed ions indicate yields of 0.15% and 1.84% for Ba in the first and second experiments, 0.02% for Sr and 0.27% for Ca. Ionization from all the releases continued along the satellite trajectory much longer (>45 s) than expected for a CIV process. The ion production along the satellite track versus time typically shows a rapid rise to a peak in a few seconds followed by an exponential decrease to a level essentially constant rate. The characteristic distances for CIV I and II are 47 and 62 km, respectively. We interpret the early time rise and exponential fall to be due to CIV ionization, of 0.014% (CIV I) and 0.40% (CIV II) for the Ba releases. The later ions produced at a constant rate probably have origins from other such processes as stripping and associative ionization collisions with atmospheric constituents primarily O, and charge exchange with O+, He+, and H+. We suggest that the much larger Ba ionization rate in CIV II than CIV I is due to the fact that the release occurred in the peak Ca density where hot electrons were already present.
The relationship between ionospheric convection and magnetic activity
Authors: Shue J. ‐H. , Weimer D. R.
Published: February 2004

In this paper we show that there is a significant relationship between magnetic activity and the high‐latitude ionospheric convection electric fields. In particular, when the westward electrojet is much stronger than the eastward electrojet, as occurs during magnetospheric substorms, the electric field patterns show subtle, but important, changes from the configurations that are obtained under more quiet conditions. The substorm differences are detected primarily near midnight, where there is an enhanced westward electric field associated with a penetration of the positive‐potential, dawn convection cell into the negative, dusk convection cell. The peak value of the positive potential also increases during substorms and its location shifts closer to midnight, while the negative cell remains relatively constant. These results were obtained through the use of a procedure that uses electric field measurements from the DE 2 satellite to derive maps of the locations of the convection reversal boundaries, and functions for the distribution of the electric potential around these boundaries. This was accomplished for several sets of data that were grouped according to the interplanetary magnetic field and geomagnetic activity. Further refinements in our analysis procedure are possible. The results could be used to produce more accurate maps of ionospheric convection. We also show that distortions of the electric field patterns near midnight are simply the result of polarization electric fields around conductivity enhancements. The distorted electric fields satisfy the condition that the ionospheric Hall current be divergence‐free.
Fine structure of low‐energy H+ in the nightside auroral region
Authors: Liu Chao , Perez J. D. , Moore T. E. , Chappell C. R. , Slavin J. A.
Published: February 2004

Low‐energy H+ data with 6‐s resolution from the retarding ion mass spectrometer instrument on DE 1 have been analyzed to reveal the fine structure at middle altitudes of the nightside auroral region. A new method for deconvolving the energy‐integrated count rate in the spin plane of the satellite has been used to derive the two‐dimensional phase space density. A detailed analysis reveals an alternating conic‐beam‐conic pattern with the observed conics correlated with large earthward currents in the auroral region. The strong downward current (>1 µA/m2 (equivalent value at ionosphere)) provides a free energy source for the perpendicular ion heating, that generates the ion conics with energies from several eV to tens of eV. The bowl shape distribution of the low‐energy H+ is caused by the extended perpendicular heating. The strong correlation between conics and large downward currents suggests that the current‐driven electrostatic ion cyclotron wave is an appropriate candidate for the transverse heating mechanism.
A resolution of the N2 Carroll‐Yoshino (c4′ ‐ X) band problem in the Earth's atmosphere
Authors: Stevens M. H. , Meier R. R. , Conway R. R. , Strobel D. F.
Published: February 2004

In the study of UV airglow from the Earth's atmosphere, the N2 Carroll‐Yoshino (CY) c4′ 1Σu+ ‐ X 1Σg+(0,0) and (0,1) Rydberg band emissions near 958 Å and 980 Å, respectively, are found to be weak relative to the c4′ (0) excitation rate. This result is surprising because laboratory measurements show that CY(0,0) and CY(0,1) are the brightest N2 emission features between 910‐1010 Å even under optically thick conditions [Zipf and McLaughlin, 1978]. In order to investigate the cause of this weak emission quantitatively, we have developed a resonant fluorescent scattering model for CY(0,0) and CY(0,1). The model is intended to be comprehensive, including multiple scattering, extinction, branching, escape to space, predissociation, and temperature effects. Results show CY(0,0) photons are radiatively trapped and undergo resonant fluorescent scattering accompanied by substantial loss in the atmosphere. Indeed, the model predicts weak CY(0,0) intensities, consistent with observations. We find that the most important loss processes for the CY(0,ν″) system in the Earth's dayglow are predissociation and branching to CY(0,1) followed by absorption by the overlapping, 100% predissociated Bridge‐Hopfield I (BH I) b1Πu(2) ‐ X1Σg+(0) band. Near solar minimum, model CY (0,1) and (0,2) dayglow zenith intensities between 160‐170 km range between 4‐9 R and 0.5‐1.5 R, respectively, where the lower number assumes 16.5% predissociation of the c4′(0) state and the higher number assumes 1% predissociation. These intensities are all consistent with observations reported by Morrison et al. [1990]. For the Earth's aurora, model CY(0,1) and (0,2) intensities averaged between 88°‐96° from the zenith at 118.5 km range between 60‐180 R and 150‐390 R, respectively, and CY(0,2) intensities at 170 km range between 200‐360 R. These results are consistent with upper limits from Feldman and Gentieu (1982) if the probability of c4′ (0) predissociation is at least 9%. We also present qualitative arguments to explain the relatively bright CY(0,ν″) emission on Titan and Triton.
Analytical self‐consistent model of the large‐scale convection electric field
Authors: del Pozo C. F. , Blanc M.
Published: February 2004

The large‐scale convection electric field results from the interaction between the solar wind and the outer Earth's magnetosphere and is modulated by the coupling between the ionosphere and the magnetosphere in the auroral and subauroral regions. The corresponding electrostatic potential distribution is basically controlled by two parameters: the cross‐polar cap potential drop imposed by the merging of solar wind and magnetospheric plasmas and magnetic fields and the latitudinal attenuation rate (shielding effect) of this potential, toward lower latitudes, by the coupling between the magnetospheric ring current and the conductive ionosphere, along the nearly equipotential geomagnetic field lines. The semiempirical Stern‐Volland model, which assumes a two‐cell convection pattern, gives a very simple parameterization of these effects, and it is used in the present study as the starting point for the development of a self‐consistent model of the large‐scale convection electric field. We derive what we have called the modified variable phase Stern‐Volland model which depends on the same parameters but includes the latitudinal variation of the local time of the maximum electrostatic potential, required by the self‐consistent treatment of the coupling between the magnetospheric and ionospheric transports. We also discuss the connection between the latitudinal attenuation rate of the potential and the resonant wave‐particle interactions at harmonics of the ion drift frequency in the ring current.
Vortex laws and field line invariants in polytropic field‐aligned MHD flow
Authors: Sonnerup B. U. Ö , Hau L. ‐N.
Published: February 2004

The consequences of conservation of angular momentum in, single‐ or double‐polytropic, steady, compressible, magnetohydrodynamic (MHD) flow parallel to the magnetic field are examined, the principal result being the MHD counterparts of Crocco's theorem and Lord Kelvin's theorem, both expressed in terms of a generalized vorticity. Under special assumptions concerning geometry and/or homogeneity of the general field line invariants, these vortex laws lead to additional field line invariants which describe the conservation of generalized angular momentum in the plasma, taking into account torques produced by j×B forces and Coriolis forces.
Self‐consistent steady state model of the low‐latitude boundary layer
Authors: Drakou E. , Sonnerup B. U. Ö. , Lotko W.
Published: February 2004

Results are presented from a steady state numerical model of the low‐latitude boundary layer (LLBL) on closed field lines and its coupling to the dayside auroral ionosphere. In the model the boundary layer approximation is used, the result being that inertial forces are taken into account only in the main flow direction (−x) where they are balanced by pressure forces, j × B forces, and viscous forces. Motion in the transverse directions (y and z) is treated kinematically, the force balances in these two directions being purely static. Computationally, the model is two dimensional, describing the motion of plasma and frozen‐in magnetic field in the equatorial (xy) plane but allowing for lowest‐order polynomial variation of some quantities with the coordinate (z) perpendicular to that plane. The plasma expands and compresses isentropically; the magnetic field is calculated self‐consistently, which leads to approximately parabolic field line shape in planes parallel to the magnetopause (the xz plane), with maximum field curvature near the magnetopause edge of the LLBL. Coupling to the ionosphere via region 1 field‐aligned currents is included. The effects of the ionosphere are represented by two parallel resistive plates at fixed height above and below the equatorial plane. The model can be used to investigate the influence of various physical parameters, for example, viscous and magnetic Reynolds numbers, and of boundary conditions at the magnetopause and in the magnetosphere on the LLBL development in the −x direction. Special attention is given to viscous effects which, under suitable circumstances, lead to a region 1 current that first increases and then reduces with increasing longitude away from local noon. Asymptotic matching of the antisunward motion of the cool LLBL plasma to sunward convection of hot plasma in the magnetosphere is illustrated along with the entrainment of magnetospheric plasma by the antisunward LLBL flow.
Radial diffusion models of energetic electrons and Jupiter's synchrotron radiation: 2. Time variability
Authors: de Pater Imke , Goertz Christoph K.
Published: February 2004

We used a radial diffusion code for energetic electrons in Jupiter's magnetosphere to investigate variations in Jupiter's radio emission due to changes in the electron phase space density at L shells between 6 and 50, and due to changes in the radial diffusion parameters. We suggest that the observed variations in Jupiter's radio emission are likely caused by changes in the electron phase space density at some boundary L1 > 6, if the primary mode of transport of energetic electrons is radial diffusion driven by fluctuating electric and/or magnetic fields induced by upper atmospheric turbulence. We noticed an excellent empirical correlation, both in phase and relative amplitude, between changes in the solar wind ram pressure and Jupiter's synchrotron radiation if the electron phase space density at the boundary L1 (L1 ≃ 20‐50) varies linearly with the square root of the solar wind ram pressure, fnof; ∼ (Ns υs2)1/2. The calculations were carried out with a diffusion coefficient DLL = DnLn with n = 3. The diffusion coefficient which best fit the observed variations in Jupiter’s synchrotron radiation D3 = 1.3 ± 0.2 × 10−9 s−1 ≃ 0.041 yr−1, which corresponds to a lagtime of approximately 2 years. A comparison with previous estimates on diffusion coefficients in Jupiter's inner magnetosphere (L ≲ 10‐20) suggests that all estimates can be described by n = 2.5 and D2.5 = 4.5 × 10−9 s−1 ≃ 0.142 year−1. We further show that the observed short term (days‐weeks) variations in Jupiter’s radio emission cannot be explained adequately when radial diffusion is taken into account.
Variations of ionospheric ionization and related solar fluxes during an intense solar cycle
Authors: Balan N. , Bailey G. J. , Jenkins B. , Rao P. B. , Moffett R. J.
Published: February 2004

Variations of ionospheric ionization (represented by ionospheric electron content (IEC)) and related solar fluxes with the 10.7‐cm solar flux index (F10.7) are studied for the intense solar cycle 21 when F10.7 was as high as 367. The IEC data collected at several stations during 1980‐1985, the solar EUV (50‐1050 Å) fluxes obtained from the EUV91 solar EUV flux model, and the measured values of Lyman α (1216 Å) flux and He I (10,830 Å) equivalent width (EW) are used for the study. It is shown that daily values of diurnal maximum IEC (IECmax) saturate (remain constant) when F10.7 (or its 81‐day running average) exceeds a threshold (approximately 160‐200) which depends slightly on season and latitude. Variations of the model values of the solar EUV fluxes reveal that when F10.7 exceeds the threshold: (1) the integrated solar EUV (50‐1050 Å) flux increases at a very low rate, and (2) the fluxes of the important (for thermospheric heating) chromospheric lines and intervals generally saturate (remain constant), while those of the coronal lines and intervals increase at a reduced rate. Lyman α flux and He I EW, which are used as input data in the solar EUV flux model, also increase at a very low rate when F10.7 exceeds the threshold. The saturation of ionospheric ionization, observed for high values of F10.7 during the last three solar cycles 19‐21, is the result of the nonlinear variation of the solar EUV and Lyman α fluxes with F10.7. IECmax increases linearly with the integrated solar EUV flux, Lyman α flux and He I EW.
A test of source‐surface model predictions of heliospheric current sheet inclination
Authors: Burton M. E. , Crooker N. U. , Siscoe G. L. , Smith E. J.
Published: February 2004

The orientation of the heliospheric current sheet predicted from a source surface model is compared with the orientation determined from minimum‐variance analysis of ISEE 3 magnetic field data at 1 AU near solar maximum. Of the 37 cases analyzed, 28 have minimum variance normals that lie orthogonal to the predicted Parker spiral direction. For these cases, the correlation coefficient between the predicted and measured inclinations is 0.6. However, for the subset of 14 cases for which transient signatures (either interplanetary shocks or bidirectional electrons) are absent, the agreement in inclinations improves dramatically, with a correlation coefficient of 0.96. These results validate not only the use of the source surface model as a predictor but also the previously questioned usefulness of minimum variance analysis across complex sector boundaries. In addition, the results imply that interplanetary dynamics have little effect on current sheet inclination at 1 AU. The dependence of the correlation on transient occurrence suggests that the leading edge of a coronal mass ejection (CME), where transient signatures are detected, disrupts the heliospheric current sheet but that the sheet re‐forms between the trailing legs of the CME. In this way the global structure of the heliosphere, reflected both in the source surface maps and in the interplanetary sector structure, can be maintained even when the CME occurrence rate is high.
Variations of spectral width with aspect angle for type II VHF echoes from the eastward electrojet
Authors: Hall G. E. , Sofko G. J. , Koehler J. A.
Published: February 2004

Continuous wave radar data from the University of Saskatchewan summer 1981 radar auroral campaign were examined to study the variation of spectral width with aspect angle. In order to reduce the uncertainties caused by refraction from strong optical forms, the data were restricted to stable eastward electrojet events. A consistent and sharp increase of spectral width with aspect angle was observed, in qualitative agreement with theory.
Observations of earthward and tailward propagating flux rope plasmoids: Expanding the plasmoid model of geomagnetic substorms
Authors: Moldwin Mark B. , Hughes W. Jeffrey
Published: February 2004

A survey of IMP 8 magnetometer data for plasmoid signatures during magnetospheric intervals from 1981 through 1983 found 16 plasmoids and 37 traveling compression regions as well as two earthward propagating flux ropes and 19 south‐north bipolar lobe signatures. The properties of these relatively near‐Earth plasmoids, traveling compression regions, and earthward propagating flux ropes and a qualitative model for their formation are presented. The plasmoids have estimated sizes, durations, magnetic field signatures, downtail velocities, and substorm associations very similar to those of the plasmoids identified in ISEE 3 deep‐tail observations. The occurrence frequency of these near‐Earth plasma sheet plasmoids is significantly smaller than that of plasmoids found in the mid‐ and deep tail with ISEE 3. The earthward propagating flux ropes are characterized by a south‐north bipolar turning in the GSM Bz component, are localized near the noon‐midnight meridional plane, and are strongly correlated with interplanetary magnetic field Bz north and small isolated high latitude geomagnetic substorms. These events are also apparently very rare and/or spatially localized. We propose that these structures are “proto‐plasmoids,” i.e., plasmoids for which near‐Earth magnetic reconnection stopped before all the closed plasma sheet field lines were reconnected. The proto‐plasmoids are then “trapped” inside closed magnetic field lines and propagate earthward owing to the effect of the distant X‐line's earthward plasma flow. We suggest that the two different “types” of plasmoids are due to the different energy states of the magnetosphere during periods of southward and northward interplanetary magnetic field.
Global evolution of interplanetary sector structure, coronal holes, and solar wind streams During 1976‐1993: Stackplot displays based on solar magnetic observations
Authors: Wang Y. ‐M. , Sheeley N. R.
Published: February 2004

We use potential field calculations and solar magnetic observations during 1976‐1993 to infer the evolution of interplanetary sector structure, coronal holes, and solar wind streams at heliographic latitudes ranging from 80°S to 80°N. The results are presented in the form of stackplots, which show long‐lived patterns that rotate quasi‐rigidly at rates determined by the photospheric distribution of nonaxisymmetric magnetic flux. The fastest wind streams and their coronal hole sources form slowly rotating patterns near the poles just after sunspot maximum but migrate to lower latitudes and tend to rotate at near‐equatorial rates as sunspot activity declines.
Solar flare effects at Ebre: Regular and reversed solar flare effects, statistical analysis (1953 to 1985), A global case study and a model of elliptical ionospheric currents
Authors: Curto J. J. , Amory‐Mazaudier C. , Torta J. M. , Menvielle M.
Published: February 2004

This paper presents the results of the analysis of geomagnetic effects of solar flares (sfe) recorded at Ebre observatory (40.8° latitude N, 0.5° longitude E) during 33 years (1953‐1985). At Ebre, located near the focus latitude, two types of sfe can be observed: regular and reversed sfe. Regular sfe are those which have phase differences less than 90° with the regular diurnal magnetic variation of the day, SR. Reversed sfe are those which have phase differences greater than 90° with SR. From these 33 years, 140 sfe events were selected and a statistical study was performed. We found a local time dependence of the phase differences between the sfe and SR vectors. Morning hours have slightly positive values and afternoon hours have slightly negative ones. Reversed sfe, with a phase difference exceeding 90°, concentrate between 10 and 12 hours. Reversed sfe show a dominant equinoctial character. Also, a weaker correlation was found between solar activity with reversed sfe (r=0.47) than with regular sfe (r=0.68). Using data from 67 observatories, we performed a global study of a sfe case, seen at Ebre as reversed sfe. In this case, in the northern hemisphere, the sfe system was about 1 hour of local time eastward of the SR system and formed 4° higher in latitude. Finally, we present a model of two elliptical ionospheric equivalent current systems with focus offset about 1 hour in local time to explain the phase difference between the sfe and Sq magnetic vectors observed at Ebre. The parameters of this model have been fitted from the results of a previous statistical analysis from Ebre data. Spatial and temporal distribution of the sfe and Sq vector phases are calculated with this model, and conditions for reversed sfe occurrence are predicted.
Particle‐in‐cell simulations of the critical ionization velocity effect in finite size clouds
Authors: Moghaddam‐Taaheri E. , Lu G. , Goertz C. K. , Nishikawa K. ‐I.
Published: February 2004

The critical ionization velocity (CIV) mechanism in a finite size cloud is studied with a series of electrostatic particle‐in‐cell simulations. It is observed that an initial seed ionization, produced by non‐CIV mechanisms, generates a cross‐field ion beam which excites a modified beam‐plasma instability (MBPI) with frequency in the range of the lower hybrid frequency. The excited waves accelerate electrons along the magnetic field up to the ion drift energy that exceeds the ionization energy of the neutral atoms. The heated electrons in turn enhance the ion beam by electron‐neutral impact ionization, which establishes a positive feedback loop in maintaining the CIV process. It is also found that the efficiency of the CIV mechanism depends on the finite size of the gas cloud in the following ways: (1) Along the ambient magnetic field the finite size of the cloud, L∥, restricts the growth of the fastest growing mode, with a wavelength λm∥ of the MBPI. The parallel electron heating at wave saturation scales approximately as (L∥/λm∥)1/2. (2) Momentum coupling between the cloud and the ambient plasma via the Alfvén waves occurs as a result of the finite size of the cloud in the direction perpendicular to both the ambient magnetic field and the neutral drift. This reduces exponentially with time the relative drift between the ambient plasma and the neutrals. The timescale is inversely proportional to the Alfvén velocity. (3) The transverse charge separation field across the cloud was found to result in the modulation of the beam velocity which reduces the parallel heating of electrons and increases the transverse acceleration of electrons. (4) Some energetic electrons are lost from the cloud along the magnetic field at a rate characterized by the acoustic velocity, instead of the electron thermal velocity. The loss of energetic electrons from the cloud seems to be larger in the direction of plasma drift relative to the neutrals, where the loss rate is characterized by the neutral drift velocity. It is also shown that a factor of 4 increase in the ambient plasma density, increases the CIV ionization yield by almost 2 orders of magnitude at the end of a typical run. It is concluded that a larger ambient plasma density can result in a larger CIV yield because of (1) larger seed ion production by non‐CIV mechanisms, (2) smaller Alfvén velocity and hence weak momentum coupling, and (3) smaller ratio of the ion beam density to the ambient ion density, and therefore a weaker modulation of the beam velocity. The simulation results are used to interpret various chemical release experiments in space.
DMSP F8 observations of the mid‐latitude and low‐latitude topside ionosphere near solar minimum
Authors: Greenspan M. E. , Burke W. J. , Rich F. J. , Hughes W. J. , Heelis R. A.
Published: February 2004

The retarding potential analyzer on the DMSP F8 satellite measured ion density, composition, temperature, and ram flow velocity at 840‐km altitude near the dawn and dusk meridians close to solar minimum. Nine days of data were selected for study to represent the summer and winter solstices and the autumnal equinox under quiet, moderately active, and disturbed geomagnetic conditions. The observations revealed extensive regions of light‐ion dominance along both the dawn and dusk legs of the DMSP F8 orbit. These regions showed seasonal, longitudinal, and geomagnetic control, with light ions commonly predominating in places where the subsatellite ionosphere was relatively cold. Field‐aligned plasma flows also were detected. In the morning, ions flowed toward the equator from both sides. In the evening, DMSP F8 detected flows that either diverged away from the equator or were directed toward the northern hemisphere. The effects of diurnal variations in plasma pressure gradients in the ionosphere and plasmasphere, momentum coupling between neutral winds and ions at the feet of field lines, and E × B drifts qualitatively explain most features of these composition and velocity measurements.
Dynamics Explorer 2 observations of equatorial thermospheric winds and temperatures: Local time and longitudinal dependences
Authors: Wu Q. , Killeen T. L. , Spencer N. W.
Published: February 2004

We have analyzed a large database of Dynamics Explorer 2 (DE 2) measurements of equatorial (<±9° latitude) thermospheric neutral temperatures and horizontal wind components in the altitude range from 200 to 400 km for quiet and active geomagnetic conditions (Kp < 3 and Kp > 3, respectively). Local time variations and longitudinal dependences on the duskside, dayside, and nightside have been separately examined. The dayside hourly averaged neutral zonal winds are relatively insensitive to variations in geomagnetic activity and appear to show an inverse dependence (lower zonal wind speed for higher Kp) in the midnight‐dawn local time sector. The duskside neutral temperatures show a region of enhancement (by ∼60 K) over the South Atlantic magnetic anomaly. The quiet time zonal winds are weaker than the corresponding active time zonal winds on the dayside near the South Atlantic magnetic anomaly. Enhanced ionization in the region, which increases the ion drag on the neutral gas, is suggested as the causal mechanism for these effects. The DE 2 averaged measurements have also been compared with MSIS‐86/HWM90 and VSH models. The data and models are in good general agreement, though, the MSIS‐86/HWM90 model predicts stronger dayside zonal winds than are observed under active conditions and the VSH model appears to underestimate the magnitude of the dayside zonal winds for both high and low Kp cases.
A two‐dimensional particle‐in‐cell model of a dusty plasma
Authors: Young B. , Cravens T. E. , Armstrong T. P. , Friauf R. J.
Published: February 2004

Dusty plasmas are present in comets, in the ring systems of the outer planets, and in the interstellar medium. A two‐dimensional particle‐in‐cell (PIC) model of a dusty plasma is presented in this paper. The PIC code is best suited for modeling the plasma‐dust interaction for large grains, with diameters of the order of a centimeter. We have modeled the charging process for an individual dust grain and the associated potential pattern in the surrounding plasma. We have also considered the case of a large number of grains in a plasma, with intergrain separations of the order of the Debye length, and have shown that the plasma becomes depleted and the charge on a dust grain is reduced, as other workers in this field have predicted (cf. C. K. Goertz, 1989). We examine the electron and ion distribution functions in the vicinity of a charged grain and demonstrate that the ions near a grain have clearly been accelerated by the electrostatic potential.
O+ phase bunching and auroral arc structure
Authors: Rothwell Paul L. , Silevitch Michael B. , Block Lars P. , Fälthammar Carl‐Gunne
Published: February 2004

The equations of motion are solved for ions moving in a model electric field that corresponds to the nightside equatorial region of the magnetosphere. The model represents the poleward region of the Harang discontinuity mapped to the magnetosphere. Within this region the model electric field has a constant earthward gradient superimposed on a constant dawn‐to‐dusk electric field. In combination with the earthward drift motion due to the dawn‐to‐dusk field, the electric field gradient introduces an earthward inertia drift, which is proportional to the ion mass and therefore faster for O+ ions than for H+ ions or electrons. It is also found that the entry of the ions into the gradient region causes phase bunching and as a result ion density striations form. The striations are enhanced for more abrupt changes in the electric field gradient, a weaker magnetic field, a stronger cross‐tail electric field and colder O+ ions. The first two conditions apply during the growth phase of a substorm. Using the Tsyganenko (1987) model a minimum electric field gradient value of 1 × 10−9 V/m2 ((1 mV/m)/1000 km) at L = 6‐7 is found. Charge neutrality requires coupling with the ionosphere through electrons moving along magnetic field lines, and such electrons may be the cause of multiple auroral arcs.
Field‐aligned current model for ultralow‐frequency pulsations in the auroral region
Authors: Elias C. , de Assis A. S. , de Azevedo C. A. , Shigueoka H. , de Azevedo M. T. , Sakanaka P. H.
Published: February 2004

We have modeled the auroral magnetized plasma as an inhomogeneous current‐carrying plasma and have shown that some features of the magnetic pulsations in that region not yet explained might be clarified by the discrete Alfvén eigenmode propagation in that plasma. This mode has the frequency outside of the shear Alfvén continuum, and it is a global mode rather then a localized Alfvén noneigenmode in inhomogeneous media. Discrete Alfvén waves are also described by the two‐fluid model finite ω/ωci, where (ωci) is the ion cyclotron frequency. In this paper we show that the current‐carrying magnetic field tube in magnetosphere can sustain discrete Alfvén modes whose frequency is compatible with the ultralow‐frequency pulsations observed.
Ulysses near‐ecliptic observations of differential flow between protons and alphas in the solar wind
Authors: Neugebauer M. , Goldstein B. E. , Bame S. J. , Feldman W. C.
Published: February 2004

The evolution of differential streaming between protons and alpha particles in the solar wind was observed with the solar wind plasma experiment on the Ulysses spacecraft over the solar range of 1.15 to 5.40 AU between November 18, 1990, and May 5, 1992. The correlation of the difference in ion speeds, ΔV = |V−| ‐ |Vp|, with the proton speed Vp observed by other spacecraft at solar distances ≤1 AU disappeared at ∼2 AU. At solar distances ≥2.85 AU, the largest values of both Vαp = |Vαp| = |Vα − Vp| and |ΔV| were found in the interaction regions on the leading edges of high‐speed streams. The differential streaming was typically enhanced just downstream of strong forward and reverse shocks, and large negative values of ΔV were frequently encountered in the interaction regions. A correlation between Vαp and the ratio τc/τe of Coulomb collision time to expansion time was observed at all distances, but it is suggested that at the larger values of τc/τe the observed correlation may arise from enhanced production of differential streaming by processes that also increase the entropy of the solar wind protons.
A modified lower hybrid drift instability as a possible mechanism for the generation of magnetic noise bursts in the magnetotail neutral sheet
Authors: Kuznetsova M. M. , Nikutowski B.
Published: February 2004

A linear local kinetic analysis of a modified lower hybrid drift (LHD) instability near the neutral plane of a current sheet with a normal component of the magnetic field Bz is carried out. It is shown that in nearly drift‐free configurations the stabilizing effects of high plasma beta (cross‐field wave‐particle resonance) are significantly reduced in comparison with straight or current‐free magnetic field geometries. A parameter α0 characterizing the degree of local deviation of the system from the drift‐free configuration is introduced. The sign of this parameter characterizes the direction of the bounce averaged electron drift and depends on the value of the local gradient of the normal component of the magnetic field. It is shown that a modified LHD mode can be excited if this parameter exceeds a critical value αcr. For quiet time magnetostatic models of the magnetotail the parameter is below but very close to the critical value. It is suggested that in the course of magnetospheric convection the threshold condition for instability development could be locally satisfied which should result in generation of low‐frequency magnetic noise burst (fnof; < 1 Hz) at the neutral sheet.
Effect of high‐latitude ionospheric convection on Sun‐aligned polar caps
Authors: Sojka J. J. , Zhu L. , Crain D. J. , Schunk R. W.
Published: February 2004

A coupled magnetospheric‐ionospheric (M‐I) MHD model has been used to simulate the formation of Sun‐aligned polar cap arcs for a variety of interplanetary magnetic field (IMF) dependent polar cap convection fields. The formation process involves launching an Alfvén shear wave from the magnetosphere to the ionosphere where the ionospheric conductance can react self‐consistently to changes in the upward currents. We assume that the initial Alfvén shear wave is the result of solar wind‐magnetosphere interactions. The simulations show how the E region density is affected by the changes in the electron precipitation that are associated with the upward currents. These changes in conductance lead to both a modified Alfvén wave reflection at the ionosphere and the generation of secondary Alfvén waves in the ionosphere. The ensuing bouncing of the Alfvén waves between the ionosphere and magnetosphere is followed until an asymptotic solution is obtained. At the magnetosphere the Alfvén waves reflect at a fixed boundary. The coupled M‐I Sun‐aligned polar cap arc model of Zhu et al. (1993a) is used to carry out the simulations. This study focuses on the dependence of the polar cap arc formation on the background (global) convection pattern. Since the polar cap arcs occur for northward and strong By IMF conditions, a variety of background convection patterns can exist when the arcs are present. The study shows that polar cap arcs can be formed for all these convection patterns; however, the arc features are dramatically different for the different patterns. For weak sunward convection a relatively confined single pair of current sheets is associated with the imposed Alfvén shear wave structure. However, when the electric field exceeds a threshold, the arc structure intensifies, and the conductance increases as does the local Joule heating rate. These increases are faster than a linear dependence on the background electric field strength. Furthermore, above the threshold, the single current sheet pair splits into multiple current sheet pairs. For the fixed initial ionospheric and magnetospheric conditions used in this study, the separation distance between the current pairs was found to be almost independent of the background electric field strength. For either three‐cell or distorted two‐cell background convection patterns the arc formation favored the positive By case in the northern hemisphere.
Parallel inhomogeneity and the Alfvén resonance: 1. Open field lines
Authors: Hansen P. J. , Harrold B. G.
Published: February 2004

In light of a recent demonstration of the general nonexistence of a singularity at the Alfvén resonance in cold, ideal, linearized magnetohydrohynamics, we examine the effect of a small density gradient parallel to uniform, open ambient magnetic field lines. To lowest order, energy deposition is quantitatively unaffected but occurs continuously over a thickened layer. This effect is illustrated in a numerical analysis of a plasma sheet boundary layer model with perfectly absorbing boundary conditions. Consequences of the results are discussed, both for the open field line approximation and for the ensuing closed field line analysis.
Transport toward Earth of ions sputtered from the Moon's surface by the solar wind
Authors: Cladis J. B. , Francis W. E. , Vondrak R. R.
Published: February 2004

The transport of typical ions from the surface of the Moon to the vicinity of Earth was calculated using a test particle approach. It was assumed that the ions were sputtered from the surface by the solar wind, with fluxes in the range determined experimentally by Elphic et al. (1991), and were accelerated initially to 10 eV by the potential of the Moon on its sunlit side. Si+ and Ca+ ions were selected for this transport analysis because their masses are within two prominent ion mass groups that have high sputtering yields. In the solar wind the ion trajectories were traced in the following superimposed fields: (1) a steady magnetic field B0 at an angle of 45° to the solar wind velocity Vsw, (2) the motional electric field E0 = −Vsw × B0, and (3) turbulent magnetic and electric fields generated by hydromagnetic waves with a k‐space power spectrum of |k|−5/3 propagating along both directions of the magnetic field B0. Interactions with Earth's bow shock and magnetosphere were included. Case histories of the ions were recorded in the XGSM, YGSM plane and in various planes perpendicular to the E0 × B0 drift direction of the ions between the Moon and Earth. The number density, energy and angular distributions, and directional and omnidirectional fluxes of the ions were constructed from the case histories. It was found that the diffusion of the ions increases rapidly as the amplitude of the turbulence δBrms increases beyond the value 0.04B0. The directional fluxes of Si+ and Ca+ in the solar wind, upstream from the bow shock, were found to be of the order of tens of ions cm−2 s−1 sr−1 keV−1, with energies 35‐70 keV, for δBrms = 0.04B0, B0 = 5 nT, and Vsw = 400 km/s. The fluxes of Si+ and Ca+ ions that originate in the Moon's atmosphere were estimated to be smaller than the corresponding sputtered‐ion fluxes by about an order of magnitude. Recent measurements of lunar ions upstream of the bow shock by Hilchenbach et al. (1992) generally confirm the predicted behavior of the ions.
The Uranian aurora and its relationship to the magnetosphere
Authors: Herbert Floyd , Sandel B. R.
Published: February 2004

About 32 h of Voyager Ultraviolet Spectrometer (UVS) observations of Uranus H2 band airglow emission (875 ≤ λ ≤ 1115 Å) have been analyzed using the singular value decomposition (SVD) approach to inversion, producing an intensity map showing aurora at both magnetic poles. An H Lyman α aurora may also be present but is difficult to separate from scattered solar and local interstellar medium components. SVD analysis of variance shows that the intensity estimate is significantly larger than the error estimate over both Uranographic poles and part of the equatorial region, fortuitously including both magnetic polar regions. The Goddard Space Flight Center Q3 magnetic field model correctly predicts that the aurora should be larger in area and emit more power at the weaker N magnetic pole than at the stronger S magnetic pole. However, the auroral emissions are quite localized in magnetic longitude and so do not form complete auroral ovals. The brightest auroral emission at each magnetic pole is confined to a range of ≈ 90° of magnetic longitude centered on the magnetotail direction, at moderate magnetic L parameter (5 ≤ L ≤ 10), but some emission at each pole is distributed over a range of more than 180° of longitude. The S polar auroral intensity maximum is coincident with the source of the broadband bursty and broadband smooth Uranian kilometric radio emission (UKR), while the N polar auroral intensity maximum may coincide with the dayside UKR source. The N and S auroral intensity maxima also lie at the conjugate magnetic footprints of the maximum intensities of whistler‐mode plasma wave emission and 22‐ to 35‐keV electron fluxes observed by Voyager. The magnetic longitudes of the aurora are completely inconsistent with the “windshield wiper” effect for either ions or electrons, indicating that some other effect, such as rapid depletion of the population of precipitating particles or highly localized strong pitch‐angle diffusion, may be acting to localize emission. The low apparent L of the precipitating particles indicates that their energies may be ≤ 10 keV. Hence magnetospheric convection is likely to be important, and thus particles exciting the aurora may not remain on constant L shells. The precipitating particles may be a relatively low‐energy population at high L that is heated to aurora‐exciting energy by adiabatic compression during convection to low L. We estimate that the total auroral power output at H Lyman α and shorter wavelengths is about 3 × 109 to 7 × 109 W, requiring about 10 times that much power for excitation.
Polar cap field‐aligned currents for southward interplanetary magnetic fields
Authors: Xu Dingan , Kivelson Margaret G.
Published: February 2004

It has been common to suppose that polar region field‐aligned currents for southward interplanetary magnetic fields (IMF) consist of two parts: region 1 and region 2 currents. It is often suggested that both of these current systems flow on closed field lines. In this pilot study the limited data available from the ISIS 2 satellite are used to examine region 1 currents with the objective of establishing whether or not they can exist partially on open field lines (i.e., inside the polar caps) for southward IMF. Magnetic field perturbations were used to identify the field‐aligned currents (FACs). Particle measurements from both the energetic particle detector and the soft particle detector on board the ISIS 2 satellite were used to distinguish between what we suggest are open and closed field lines. Although the identification is not unambiguous, ≳keV electrons are found principally on closed field lines. The absence of ≳keV electrons but the presence of ≲ 200 eV electrons in the polar cap or background polar rain is considered as the signature of open field lines. On some passes, region 1 sense FACs appear to be composed of two parts. The poleward part of the current signature is accompanied by electron fluxes at energies ≲ 200 eV or occasionally by fluxes at background levels while the equatorward part of the interval is accompanied by electron fluxes at energies both ≲ 200 eV and ≳keV. On other passes, region 1 sense currents are accompanied by both ≲ 200 eV and ≳keV electron fluxes during the entire pass. We propose that region 1 sense FACs flow on both closed and open field lines for the first situation and on closed field lines for the second situation. In seeking to understand why region 1 currents sometimes flow only on closed field lines and sometimes flow on open as well as closed field lines, we suggest a control by the IMF By. The IMF By may also shift the region 1 currents on open field lines to one side (dawn or dusk) of the polar cap like the convection cells. Such a shift provides a consistent model of the data taken on the dayside and we discuss why night side observations may be different. We suggest that in the presence of southward IMF, region 1 currents can be composed of two parts, one flowing on closed field lines and one flowing on open field lines. Because the portion of the region 1 currents inside the polar caps may be generated by different processes from the portion on closed field lines, we refer to the region 1 currents on open field lines as R1O currents. The region 1 currents on closed field lines may be called R1C currents.