bluuebunny/crossref_metadata_2025 huggingface dataset:
A two‐dimensional simulation of the radial and latitudinal evolution of a solar wind disturbance driven by a fast, high‐pressure coronal mass ejection
Authors: Riley Pete , Gosling J. T. , Pizzo V. J.
Published: February 2004

Using a hydrodynamic simulation, we have studied the two‐dimensional (symmetry in the azimuthal direction) evolution of a fast, high‐pressure coronal mass ejection (CME) ejected into a solar wind with latitudinal variations similar to those observed by Ulysses. Specifically, the latitudinal structure of the ambient solar wind in the meridional plane is approximated by two zones: At low latitudes (< 20°) the solar wind is slow and dense, while at higher latitudes the solar wind is fast and tenuous. The CME is introduced into this ambient wind as a bell‐shaped pressure pulse in time, spanning from the equator to 45° with a speed and temperature equal to that of the high‐latitude solar wind. We find that such an ejection profile produces radically different disturbance profiles at low and high latitudes. In particular, the low‐latitude portion of the ejecta material drives a highly asymmetric disturbance because of the relative difference in speed between the fast CME and slower ambient solar wind ahead. In contrast, the high‐latitude portion of the same ejecta material drives a much more radially symmetric disturbance because the relative difference in pressure between the CME and ambient background plasma dominates the dynamics. The simulations reveal a number of other interesting features. First, there is significant distortion of the CME in the interplanetary medium. By ∼ 1 AU the CME has effectively separated (in radius as well as latitude) into two pieces. The radial separation is due to the strong velocity shear between the slow and fast ambient solar wind. The latitudinal separation arises from pressure gradients associated with rarefaction regions that develop as the CME propagates outward. Second, there is significant poleward motion of the highest‐latitude portion of the CME and its associated disturbance. The main body of the CME expands poleward by ∼ 18°, while the forward and reverse waves (produced by the overexpanding portion of the CME) propagate all the way to the pole. Third, the simulations show that the high‐pressure region, which develops at low latitudes as the fast CME ploughs through the slow ambient solar wind, penetrates significantly (∼ 10°) into the high‐latitude fast solar wind. We compare the simulation results with a CME‐driven interplanetary disturbance observed at both low and high latitudes and find that the simulation reproduces many of the essential features of the observations.
Ionospheric structure produced during a rapid polar cap expansion
Authors: Idenden D. W. , Moffett R. J. , Quegan S.
Published: February 2004

We use results from a coupled thermosphere‐ionosphere‐plasmasphere model to illustrate a new mechanism for the formation of a large‐scale polar cap patch of ionization arising from a rapid polar cap expansion. The patch, having an ion density at the F2 peak twice that of the background plasma, is isolated within the polar cap by a depletion due to rapid flows in the cusp region. Such depletions caused by rapid flows are generally ascribed to increases in the temperature dependent rates of plasma loss reactions due to ion frictional heating. In this paper, we demonstrate, however, that the depletion at the F peak is caused primarily by large upward (field aligned) plasma flows. These flows also give rise to an increase in the height of the F peak in the depleted region. The morphology of the patch in the F region is subsequently modulated by the return flow of plasma down previously depleted flux tubes as they convect across the polar cap, so that the enhancement decreases rapidly in both magnitude and spatial extent. Time histories for the plasma density profiles above the EISCAT Svalbard radar and the optical observatory at Eureka are presented as examples of what ground‐based observers might be expected to measure under these circumstances.
Comparison of techniques for derivation of neutral meridional winds from ionospheric data
Authors: Buonsanto M. J. , Starks M. J. , Titheridge J. E. , Richards P. G. , Miller K. L.
Published: February 2004

We carry out a detailed comparison between winds derived from F2 peak heights and winds obtained from incoherent scatter radar (ISR) line‐of‐sight velocity measurements. A total of 34 incoherent scatter radar experiments at Millstone Hill spanning all seasons and levels of solar activity are included in this study. For two experiments we compare results from five different wind‐derivation techniques. According to work by Titheridge [1993, 1995a, b], neutral winds derived from the servo model are inaccurate during the sunrise and morning period because of a shift in the zero‐wind F peak downward from the balance height. To investigate this effect, we determine a correction factor cfac to be applied to the servo model c parameter as a function of time of day for summer, equinox, and winter at both solar maximum and solar minimum. Our results confirm that a sunrise decrease in cfac is necessary to bring about best agreement between the servo model winds and the winds derived from the ISR ion velocity data at Millstone Hill. However, the effect is not large, so that a constant cfac for each season/solar activity level usually introduces less error than other factors which may result in differences between the servo model and ISR winds. These factors include measurement errors in hmF2 and the ISR line‐of‐sight ion velocities, spatial variations in the wind field above the station, and the assumption that hmF2 is the peak in the O+ altitude profile.
Stability of resistive MHD tearing and ballooning modes in the tail current sheet
Authors: Sundaram A. K. , Fairfield D. H.
Published: February 2004

We have analytically investigated the evolution of resistive MHD tearing and ballooning modes by assuming that the dissipation is anomalous in the current sheet region. A generalized technique is diplayed for obtaining the solutions for both these modes near the singular layer, where the Bx(z) field reverses sign. When the perturbation is limited to two dimensions, we have found that the stability of tearing modes is controlled by compressibility and the Lundquist number S, where S is the ratio of anomalous diffusion time to Alfvén time. For S ≪ 5 × 103, the fluid compressibility plays a significant destabilizing role while the normal component Bz contributes to a weak stabilization of tearing modes. In the three‐dimensional case with ky ≠ 0 (ky being the wavenumber), it is demonstrated that a linear coupling of the pressure gradient and the magnetic field curvature causes the excitation of a new class of unstable tearing and ballooning modes with their growth rates significantly dependent on anomalous resistivity and the Alfvénic frequency. The resistive ballooning modes, excited in the field reversal layer, are shown to enhance the current and the magnetic field gradients in the center of the plasma sheet and thereby provide the source for the excitation of tearing modes with a typical growth time of 5 s. Finally, the relevance of the newly excited modes to recent AMPTE/IRM, GEOS 2, and Geotail results is discussed.
Growth and decay of field line resonances
Authors: Fenrich F. R. , Samson J. C.
Published: February 2004

The apparent growth rates of ULF field line resonances seen by the SuperDARN HF radars show a striking correlation with the azimuthal wave number m of the resonance. The observed high‐m (m>17) resonances have amplitudes that increase with time, indicating positive growth rates, while the low‐m resonances have decreasing amplitudes, indicating negative growth rates. The resonance growth rates and latitudinal phase shifts, a decrease for low‐m modes and an increase for high‐m modes, are found to be determined by the direction of the Poynting flux in the system. In the case of the low‐m resonance the Poynting flux direction is such that the energy flows from the fast wave driver into the shear Alfvén field line resonance and down into the ionosphere. In the case of the high‐m resonance an internal driver couples to the system and reverses the direction of the Poynting flux such that energy flows from the internal driver into the shear Alfvén field line resonance and out into the fast wave. There is evidence to suggest that the internal driver is in the form of a wave‐particle interaction.
Central plasma sheet disruption and the formation of dayside poleward moving auroral events
Authors: Lyatsky Wladislav , Sibeck David
Published: February 2004

Although most dayside poleward moving auroral forms (PMAFs) seem to be reasonably explained as a result of magnetopause merging, a number of these events can hardly be explained in terms of reconnection. These events seem to appear on closed field lines, and their poleward movement seems to be controlled not only by convection due to the interplanetary magnetic field (IMF) but also to the polarization electric field appearing inside them. This type of PMAF may be observed not only for a negative IMF Bz component but also for the positive Bz and rather small By components when field line merging may be inefficient or may occur on lobe magnetic field lines, and the poleward propagation of these auroral forms may differ from the background magnetospheric convection. These events often appear at the equatorward edge of the red auroral band, and the spike structures in dayside electron precipitations which may be responsible for these auroral forms are often seen simultaneously with the plasma sheet population. Another possible mechanism for dayside poleward moving auroral events in a region of closed field lines is proposed. This mechanism is based on observations indicating that the central plasma sheet can be divided from the low‐latitude boundary layer and the magnetopause by a gap. This means that the dayside plasma sheet has an outer boundary inside the magnetosphere which is unstable with respect to the interchange (ballooning) instability. As a result, for some conditions, clouds of warm plasma may break out from the plasma sheet and move poleward because of the polarization electric field. Projection of these plasma sheet clouds on the ionosphere would have many features which are inherent for the dayside auroral events.
What caused the large geomagnetic storm of November 1978?
Authors: Cane H. V. , Richardson I. G.
Published: February 2004

We discuss energetic particle and solar wind plasma data for the period around the large geomagnetic storm of November 24‐25, 1978. The storm was clearly caused by an ejecta interacting with a corotating high‐speed stream. We conclude that there are no data to support the attribution of this storm to specific solar activity as previously suggested. This case study illustrates the important role of energetic particles in making correct associations between solar and interplanetary phenomena. Even if there had been an identifiable source region for the ejecta, the intensity of the geomagnetic storm resulted from the interplanetary interaction. Thus predictions of the strengths of such storms cannot be based on solar observations alone but also require knowledge of stream interactions.
Ulysses' rapid crossing of the polar coronal hole boundary
Authors: McComas D. J. , Riley P. , Gosling J. T. , Balogh A. , Forsyth R.
Published: February 2004

The Ulysses spacecraft crossed from the slow dense solar wind characteristic of the solar streamer belt into the fast, less dense flow from the northern polar coronal hole over a very short interval (several days) in late March 1995. The spacecraft, which was at 1.35 AU and ∼19° north heliographic latitude, moving northward in its orbit, remained in the fast solar wind from then through summer 1996. This boundary crossing is unique in that the combination of the spacecraft motion and rotation of the structure past the spacecraft caused Ulysses to move smoothly and completely from one regime into the other. In this study we examine this crossing in detail. The crossing is marked by a region of enhanced pressure, typical of stream interaction regions, which extends ∼2 × 107 km across. We find that the transition between the slow and fast regimes occurs on several temporal, and hence spatial, scales. On the shortest scale (<8×104 km) the stream interface is a tangential discontinuity where the proton and core electron densities and ion and electron pressures all drop while the magnetic pressure jumps to maintain a rough pressure balance. The alpha to proton ratio also jumps across the stream interface to reach the comparatively constant polar hole value of ∼4.3%. On larger scales (a few ×106 km) the proton and alpha temperatures rise to their high‐speed wind values. Finally, on the largest scale (∼108 km) the solar wind speed ramps up from ∼400 km s−1 to ∼750 km s−1, typical of polar hole flows. While it seems likely that the stream interface maps back to a sharp boundary near the Sun, the large region of increasing flow speed suggests that there is also an extended gradient in solar wind source speed close to the Sun.
Measurements of artificial periodic inhomogeneities at HIPAS Observatory
Authors: Djuth F. T. , Groves K. M. , Elder J. H. , Shinn E. R. , Quinn J. M. , Villasenor J. , Wong A. Y.
Published: February 2004

Results are presented from recent experiments that employ high‐power, high‐frequency (HF) radio waves to probe the mesosphere and lower thermosphere. The measurements were made at the High‐Power Auroral Stimulation (HIPAS) Observatory located near Fairbanks, Alaska. One objective of the study was to determine the feasibility of using artificial electron density perturbations created in the auroral environment to measure the properties of the background neutral gas between ∼50 to ∼120 km altitude. The observing technique relies on the production of so‐called “artificial periodic inhomogeneities” (API) in the altitude region(s) of interest. These induced irregularities are believed to be horizontally stratified and conform to the standing wave pattern produced by the reflection of the powerful HF wave in the ionosphere. In the D region above HIPAS, API decay curves are strictly exponential and the phase histories are strictly linear. On occasion, echoes are detected at very low altitudes (∼45 km) in the vicinity of the polar stratopause. The API backscatter at HIPAS is often superimposed on regions of partial reflection, auroral E, and sporadic E. Information about ambipolar diffusion rates and electron attachment to O2 is obtained by measuring the relaxation time of the induced irregularities. In general, API phase velocities below ∼95 km altitude appear to be related to vertical neutral motions. However, detailed validation studies throughout the mesosphere have not yet been performed at HIPAS or any other API facility. At altitudes between ∼45 and ∼80 km, high‐resolution observations at HIPAS reveal the presence of sharply defined bands of API scatter 15–20 km in altitude extent. The existence of such bands and their fluctuation in altitude cannot be explained within the context of existing theory. Large variations in API backscatter power (10–20 dB) are typically observed in the D region over timescales of 30 s or less. Most likely this is caused by fading in the ionospherically reflected component of the standing wave pattern. Finally, power stepping studies reveal a roughly linear relationship between D region backscatter power and HF power. Fundamental questions related to the horizontal dimension of the API patch and its spatial structure remain to be addressed in future experiments.
Simultaneous observations of E region coherent radar echoes at 2‐m and 6‐m radio wavelengths at midlatitude
Authors: Koehler J. A. , Haldoupis C. , Schlegel K. , Virvilis V.
Published: February 2004

Two continuous wave Doppler radars, using scaled antenna arrays at the same receiving and transmitting sites in order to observe the same volume in the E region ionosphere, made simultaneous observations of coherent backscatter from the island of Crete, Greece. The radars operated at different frequencies, that is, 50 MHz and 144 MHz, which made it possible for the first time in midlatitudes to observe simultaneously aspect sensitive backscatter from about 3‐m and 1‐m irregularities, respectively. In this paper we introduce the experiment and present an overview of the observations. Both primary and secondary E region irregularities were observed at both radar frequencies, but the 144 MHz echoes were considerably weaker than those at 50 MHz. The observations demonstrated clearly the different character of E region type 1 and type 2 irregularities. The 144‐MHz type 2 echoes were completely absent during times of weak to moderately strong 50‐MHz backscatter activity and appeared only when the signal at 50 MHz became rather strong with relative intensities nearing and exceeding 20 dB above noise. The large differences in the observed signal levels between the 50‐ and 144‐MHz radar type 2 echoes suggest a very steep wavenumber spectrum in the irregularity wavelength range from 3‐m to 1‐m. On the other hand, and in sharp contrast to type 2 echoes, there was one to one correspondence in the occurrence of 50‐MHz and 144‐MHz type 1 echoes, even when the signal at 50 MHz was only a few decibels above noise. Although at times the observed velocities of the simultaneous 3‐m and 1‐m type 1 irregularities were the same, on the average the evidence favored a slightly higher threshold for excitation at 144 MHz than at 50 MHz. The results are in general agreement with what is anticipated from basic theory of E region meter‐scale plasma waves.
Three‐dimensional MHD simulation of the solar wind interaction with the ionosphere of Venus: Results of two‐component reacting plasma simulation
Authors: Tanaka T. , Murawski K.
Published: February 2004

The large‐scale solar wind interaction with the Venusian ionosphere is numerically simulated in the framework of two‐component, three‐dimensional magnetohydrodynamics (MHD). The finite volume total variation diminishing scheme is used to solve this problem. The impinging solar wind is represented by H+ ions, and the ionosphere is assumed to consist of O+ ions produced by photoionization of atomic oxygen in the Venusian upper atmosphere and by charge exchange of CO2+ ions. The O+ ions are lost by charge exchange with carbon dioxide molecules. The numerical simulations are performed for an interplanetary magnetic field (IMF) perpendicular to the solar wind flow and for the solar wind parameters which correspond to maximum solar activity. Results of the calculation give the formation of the bow shock, the magnetic barrier, and the ionopause in the dayside region as a self‐consistent state of the interaction processes. The dynamical behavior of the dayside ionosphere under the influence of the impinging solar wind and the IMF slipping over the pole results in the formation of wing‐like bulges of the ionosphere and an accompanying poleward flow in the topside ionosphere. The model also reproduces several features of the nightside ionosphere that are predicted by earlier theories and observations, including complex structures such as a flattened ionotail, tail rays, and ionospheric holes, as a continuation of the wing‐like bulge. It is also shown that slow plasma flow in the ionotail and nonideal MHD process play important roles in the formation of the induced magnetotail of the planet.
Observation of He II emission from the plasmasphere by a newly developed EUV telescope on board sounding rocket S‐520‐19
Authors: Yoshikawa I. , Nakamura M. , Hirahara M. , Takizawa Y. , Yamashita K. , Kunieda H. , Yamazaki T. , Misaki K. , Yamaguchi A.
Published: February 2004

The helium emission monitor (HEM) is a newly developed extreme ultraviolet telescope adjusted to be sensitive to the He II emission line (304 Å). It is developed as a prototype for upcoming plasmaspheric imagery missions. The HEM is designed to be compact and simple using a multilayer coated mirror, a thin metal filter, and a microchannnel plate coated with CsI; it exhibits high sensitivity at 304 Å (∼100 cps/R) and high angular resolution (2.5°), and most importantly it is contamination free from other major emission lines, e.g., He I (584 Å), O II (834 Å), and H I (1216 Å). We report on the initial observation of He II emission from the plasmasphere by the HEM carried on board sounding rocket S‐520‐19. We compare the observation with the theoretical prediction using a diffusive equilibrium model, the MSIS‐86 model, and an equatorial electron density model. The model indicates a good agreement with the observations using values for the He+ number density at the topside ionosphere of 3700 cm−3 and for the equatorial temperature near the dawnside of the plasmapause of 8000 K.
Low‐latitude dusk flank magnetosheath, magnetopause, and boundary layer for low magnetic shear: Wind observations
Authors: Phan T. D. , Larson D. , McFadden J. , Lin R. P. , Carlson C. , Moyer M. , Paularena K. I. , McCarthy M. , Parks G. K. , Rème H. , Sanderson T. R. , Lepping R. P.
Published: February 2004

We have studied in detail a Wind spacecraft crossing of the low‐latitude dusk flank magnetosheath, magnetopause (MP), and the low‐latitude boundary layer (LLBL) when the local magnetic shear across the MP was low (<30°) and the interplanetary magnetic field (IMF) was northward. We find that the magnetosheath flow tangential to the MP slows down initially as one moves from the bow shock toward the MP. However, close to the MP this flow speeds up as the MP is approached. The source of flow acceleration is likely to be the magnetic force associated with draping of the field lines around the MP. Magnetic flux pile‐up and a plasma depletion layer are also observed next to the flank MP indicating that the level of magnetic flux transfer across the entire dayside low‐latitude MP via reconnection is low. The MP is characterized by changes in the plasma properties. The electron parallel temperature is enhanced across the MP and continues to increase across the LLBL, while the perpendicular temperature is constant across the MP. This constancy of the perpendicular temperature suggests that the transfer of plasma takes place across the local MP. In the LLBL, the ion and electron temperatures are well correlated with the density. In addition, the flow direction in a substantial portion of the LLBL is nearly aligned with that in the magnetosheath, and the flow speed tangential to the MP decreases gradually with decreasing LLBL density. The behavior of the particle distributions suggests that the entire LLBL was on closed field lines. In essence, our findings on the topology and on the LLBL plasma characteristics suggest that even in the absence of reconnection at the local low‐shear MP, the LLBL is locally coupled to the adjacent magnetosheath. The smooth variations of the plasma parameters with the density are consistent with the LLBL spatial profiles being gradual. This may suggest that diffusion processes play a role in the formation and dynamics of the LLBL. Finally, the magnetic field and the state of the plasma in the plasma sheet adjacent to the flank MP/LLBL appear to be functions of the IMF direction. Thus the IMF may control both the external (magnetosheath) and the internal (plasma sheet) boundary conditions for the flank MP processes.
Kinetic parameters for OH nightglow modeling consistent with recent laboratory measurements
Authors: Adler‐Golden Steven
Published: February 2004

An improved kinetic model for the Meinel bands of OH has been constructed from rate constants and Einstein A coefficients derived in recent laboratory experiments. Using a semiempirical parameterization of the state‐to‐state rate constants for OH(v) quenching by O2, the absolute OH(v) nightglow radiances are modeled to within the accuracies of the atmospheric constituent concentrations and the radiometric calibrations. Collisional quenching is found to be predominantly multiquantum at high v but single‐quantum at low v.
Force balance and substorm effects in the magnetotail
Authors: Kaufmann Richard L. , Larson Douglas J. , Kontodinas Ioannis D. , Ball Bryan M.
Published: February 2004

A model of the quiet time middle magnetotail is developed using a consistent orbit tracing technique. The momentum equation is used to calculate geocentric solar magnetospheric components of the particle and electromagnetic forces throughout the current sheet. Ions generate the dominant x and z force components. Electron and ion forces almost cancel in the y direction because the two species drift earthward at comparable speeds. The force viewpoint is applied to a study of some substorm processes. Generation of the rapid flows seen during substorm injection and bursty bulk flow events implies substantial force imbalances. The formation of a substorm diversion loop is one cause of changes in the magnetic field and therefore in the electromagnetic force. It is found that larger forces are produced when the cross‐tail current is diverted to the ionosphere than would be produced if the entire tail current system simply decreased. Plasma is accelerated while the forces are unbalanced resulting in field lines within a diversion loop becoming more dipolar. Field lines become more stretched and the plasma sheet becomes thinner outside a diversion loop. Mechanisms that require thin current sheets to produce current disruption then can create additional diversion loops in the newly thinned regions. This process may be important during multiple expansion substorms and in differentiating pseudoexpansions from full substorms. It is found that the tail field model used here can be generated by a variety of particle distribution functions. However, for a given energy distribution the mixture of particle mirror or reflection points is constrained by the consistency requirement. The study of uniqueness also leads to the development of a technique to select guiding center electrons that will produce charge neutrality all along a flux tube containing nonguiding center ions without the imposition of a parallel electric field.
Antisunward decay of polar cap arcs
Authors: Rodriguez J. V. , Valladares C. E. , Fukui K. , Gallagher H. A.
Published: February 2004

Data from an all‐sky imaging photometer (427.8 and 630.0 nm) in the central polar cap, the IMP 8 and ISEE 2 satellites in the solar wind, and the DMSP F6, F7, and F8 satellites in the topside ionosphere have been used to investigate ten cases of polar cap arcs decaying antisunward. In all cases, the decay occurs after the interplanetary magnetic field (IMF) turns southward; in eight cases, a period of Bz < 0 lasting ≥ 20 min follows the southward turning. The lags from the IMF turning to the arc decay are corrected for the travel time from IMP 8 or ISEE 2 to the cusp/cleft ionosphere. For eight of the arcs, the lags range from 15 to 30 min; while in the other two cases, the arcs first appear in the central polar cap 30–40 min after the southward turning, then decay antisunward during the expansion phase of a small substorm, 60–65 min after Bz turned negative. In all cases, the direction of dawn‐dusk drift is in the direction of By, and in the cases in which By also reversed, the corrected lags to a change in dawn‐dusk drift are < 5 min (in two cases) or 15–20 min (in two cases). In the five cases in which a DMSP satellite traverses the arc or its conjugate location, the precipitation data indicate a likely origin on open (closed) field lines in two (three) cases. The 15‐30 min Bz lags characterize both open and closed field line arcs, while the two arcs associated with 60–65 min lags are on closed field lines. Antisunward decay of polar cap arcs 15–30 min after Bz turns negative is consistent with the By‐dependent convection of new open flux tubes from the cusp to the center of the polar cap driven by dayside merging. The 60–65 min lags suggest an evolution in polar cap convection driven at first by dayside merging or viscous processes, then by the development of a substorm‐related DP 1 current pattern. If polar cap arcs map to the distant tail flanks, they may be associated with instabilities due to velocity shears between the plasma sheet and the low‐latitude boundary layer. Consequently, the dawn‐dusk drift and antisunward decay of polar cap arcs 15–30 min after the IMF turns southward suggest the stirring of the tail flanks by dayside‐driven merging.
Are particles detrapped by constant By in static magnetic reversals?
Authors: Chapman Sandra C. , Rowlands George
Published: February 2004

Single‐particle dynamics in simple models for the static reversing magnetic field of the geotail current sheet have been extensively studied in the case where the reversing field Bx(z) varies, the linking field Bz is constant, and the crosstail field By is zero (and some generalization to include time dependence has been achieved). More recently, numerically integrated trajectories in static reversals which include a constant shear By component have suggested some differences in the nature of the dynamics in this and the By = 0 case. The invariant of the z cross‐sheet motion for the By = 0 case is well known, here we find its equivalent for systems with constant By. Our results hold for reversals with general z dependence and arbitrary constant By. The form of this invariant suggests that it is still conserved for trapped particles, but for certain values of energy, By and Bz the invariant is destroyed and particles are detrapped. This corresponds to an increase in the volume in phase space available to current carrying particles that transit the sheet. For typical magnetotail parameters, both protons and electrons in an average 1 RE thick sheet will be detrapped, but in a thin ∼ 100 km sheet protons will not.
Global model of the corona with heat and momentum addition
Authors: Wang A. H. , Wu S. T. , Suess S. T. , Poletto G.
Published: February 2004

We have been developing a series of global coronal models directed at a better simulation of empirical coronal hole and streamer properties. In a previous study, a volumetric heat source was used to produce a thin current sheet above streamers and high solar wind speed in the coronal hole. This improved the preexisting coronal structure for coronal mass ejection simulations even when not using a polytropic energy equation. Here we report on the addition of a momentum source to the model with volumetric heating and thermal conduction. Most theoretical acceleration models in coronal holes are driven either by thermal pressure or waves (magnetosonic, Alfvén, and sonic waves). In the thermal pressure driven models an artificially high effective temperature is assumed. In the wave driven models the force is generally not big enough to accelerate the solar wind as quickly as observed. In the present model, in comparison to earlier calculations [Suess et al., 1996], we reduce the heat source and add momentum. These changes appear to further improve the numerical simulation results in comparison to empirical properties. We have high solar wind speed in the hole without using unrealistic high plasma temperature. We also demonstrates that the deposition height of the momentum addition affects the mass flux. The model still predicts a slow‐speed solar wind source in the streamer and high plasma β at the top of the streamer.
A statistical study of the local time asymmetry of Pc 5 ULF wave characteristics observed at midlatitudes by SAMNET
Authors: Chisham G. , Orr D.
Published: February 2004

A total of 129 Pc 5 events were identified in more than a year of mid‐latitude magnetometer data from the United Kingdom Sub‐Auroral Magnetometer Network (SAMNET). The horizontal polarization and azimuthal wavenumbers associated with these events were estimated, and their variations with magnetic local time were studied. A marked local time asymmetry was apparent in the Pc 5 wave characteristics with distinctly different polarization and azimuthal wave characteristics occurring on either side of noon. The morning sector observations of predominantly clockwise polarization and westward phase propagation suggest that the majority of the morning sector Pc 5 waves are the midlatitude signature of solar wind driven field line resonances occurring at higher latitudes. It is suggested that the most likely energy sources for these waves are the Kelvin‐Helmholtz instability at the magnetopause boundary or cavity/waveguide modes prompted by impulsive variations in the solar wind. The large variation in the polarization azimuths (ellipse orientations) observed in the morning sector was interpreted as a result of the large ionospheric conductivity gradients that occur around and after dawn, which alter the ionospheric rotation of Alfvén waves from 90°. It is suggested that the afternoon sector Pc 5 waves are the ground signature of compressional waves in the magnetosphere, mainly cavity/waveguide modes. There is little evidence of field line resonances in the afternoon sector. The possibly dominant role of cavity/waveguide modes in the magnetosphere is heightened by the observation of preferential frequencies in both the morning and afternoon sectors.
Excitation of nonlinear Alfvén waves by an ion beam in a plasma: 1. Right‐hand polarized waves
Authors: Galinsky V. L. , Shevchenko V. I. , Ride S. K. , Baine M.
Published: February 2004

A self‐consistent theory of MHD wave excitation in the solar wind plasma by a cold ion beam moving along the ambient magnetic field is developed. Using analytical and numerical technics, we investigate the linear stage of the exponential growth of wave amplitudes, their saturation due to trapping or pitch angle diffusion of resonant ions and the steepening of the wave profile due to plasma nonlinearity. The derivative nonlinear Schrödinger equation was used to describe the nonlinear input of plasma particles, and a hybrid method of incomplete numerical simulation was used to investigate strongly nonlinear motion of resonant particles. We have studied the temporal behavior of wave energy for waves propagating in both positive and negative directions in relation to the magnetic field, directions and we have found that in cases under consideration the steepening profile developed only for right‐hand polarized waves. The physical mechanism for wave steepening is higher harmonic generation due to plasma nonlinearity.
Digital ionosonde measurements of the height variation of drift velocity in the southern polar cap ionosphere: Initial results
Authors: Parkinson M. L. , Monselesan D. P. , Smith P. R. , Dyson P. L. , Morris R. J.
Published: February 2004

During the late austral summer of 1995–1996 we operated an HF digital ionosonde located at Casey, Antarctica (66.3°S, 110.5°E, −80.8° corrected geomagnetic (CGM) latitude), in an experimental drift mode with the aim of resolving the height variation of drift velocity in the polar cap ionosphere. We devised control programs for a Digisonde Portable Sounder 4 to collect data at separate frequency‐range gates corresponding to the E and F regions to investigate the differences in their motions. During a 4‐day campaign commencing March 11, 1996, the mode values of the drift perpendicular to the magnetic field (V⊥) were 85 m s−1 in the E region and 485 m s−1 in the F region (using 10 m s−1 bins and echoes from all heights in each region). Vertical profiles of drift velocity were obtained by sorting echoes into 10‐km group‐height bins. For measurements obtained within ±3 hours of magnetic noon the average profile showed that in the lower E region V⊥ increased approximately exponentially with true height. The corresponding velocity scale height was <9.0 km at 105 km, where the gradient was >46.7 m s−1 km−1. The mean value of V⊥ leveled off to about 700 m s−1 above 120 km, where it remained up to the F region peak height. The vertical gradient was caused by the increase in collision frequencies at the lower heights. The F region field‐aligned component of drift (V‖) showed a strong diurnal variation, with mean values of −30 m s−1 near noon and +60 m s−1 during the night at a height of 180 km. The average over the whole day reveals a net upward drift of 30 m s−1. This behavior is attributed to the interaction between the meridional components of the generally antisunward neutral wind (UN) and perpendicular drift (V⊥s) moving plasma down the field lines during the day and up the field lines during the night, with UN and V⊥s having net equatorward values when averaged over all day. While the E region drift direction tended to be aligned with the basic antisunward convection which dominates the F region above Casey, it also tended to show greater temporal variability in direction, suggesting a smaller‐scale size and lifetime for the E region structures giving rise to the echoes. There were events lasting over 2 hours during which the drifts in the two regions were clearly resolved into different azimuths (by nearly 180° for two events). These transient directional shears show the time variability in the phase transition between an F region collisionless, magnetized plasma driven by the E × B/B2 convection to an E region collisional, unmagnetized plasma driven by E and irregular neutral winds.
Two substorm intensifications compared: Onset, expansion, and global consequences
Authors: Pulkkinen T. I. , Baker D. N. , Frank L. A. , Sigwarth J. B. , Opgenoorth H. J. , Greenwald R. , Friis‐Christensen E. , Mukai T. , Nakamura R. , Singer H. , Reeves G. D. , Lester M.
Published: February 2004

We present observations of two sequential substorm onsets on May 15, 1996. The first event occurred during persistently negative IMF BZ, whereas the second expansion followed a northward turning of the interplanetary magnetic field (IMF). While the first onset remained localized, the second event led to a major reconfiguration of the magnetotail. The two very different events are contrasted, and it is suggested that the IMF direction controls the evolution of the expansion phase after the initial onset. Magnetic field modeling and field‐aligned mappings are used to find the high‐altitude source region of the auroral features and currents giving rise to ground magnetic disturbances: It is shown that the auroral brightening is related to processes near the inner edge of the plasma sheet but that the initial field‐aligned currents couple to the midtail region. Ground magnetograms show an abrupt, large‐scale weakening of the electrojet during the recovery phase. This event is followed by eastward drifting omega bands in a double‐oval configuration. During that period, the Geotail plasma data show oscillations at <100 km/s amplitude. We argue that both these features are connected with the global tail evolution as the neutral line ceases to be active and reforms in the distant tail.
Modeling the high‐latitude equinoctial asymmetry
Authors: Aruliah A. L. , Schoendorf J. , Aylward A. D. , Wild M. N.
Published: February 2004

Fabry‐Perot Interferometer measurements of neutral winds and European Incoherent SCATter radar measurements of plasma velocities have shown a significant equinoctial asymmetry in the average behavior of the thermosphere and ionosphere above northern Scandinavia. Existing standard models of the upper atmosphere use forcing functions that are symmetric about the solstices, therefore these observations are unexpected. It is suggested that the asymmetry arises from the diurnal variation in the cross polar cap potential difference (CPCPD) because there is a 12 hour phase difference between the variations at the March and September equinoxes. The variation in the CPCPD is caused by an annual and diurnal variation in the orientation of the magnetosphere with respect to the interplanetary magnetic field. This is known as the Russell‐McPherron (R‐M) effect. The plausibility of this explanation of the equinoctial asymmmetry in thermospheric winds is supported by investigation of the effect of their geomagnetic history, i.e., the repercussions on the winds of the activity levels in the few hours prior to the observation. The consequences of the R‐M effect have been simulated in the University College London/Sheffield/Space Environment Laboratory coupled thermosphere‐ionosphere model by imposing a diurnally varying high‐latitude electric field pattern. The results are used to test the predictions, given in an earlier paper, of the average behavior expected at other high‐latitude sites. A corollary to the study is that the evidence presented here implies that the auroral oval may be smaller at solar minimum, which is also unexpected.
Effect of lower hybrid cavities on core plasma observed by Freja
Authors: Knudsen David J. , Dovner Per Ola , Eriksson Anders I. , Lynch Kristina A.
Published: February 2004

Sounding rocket experiments in the topside ionosphere have established previously a one‐to‐one relation between transversely heated ions and packets of lower hybrid waves trapped in localized density depletions (lower hybrid cavities (LHCs)). LHCs were observed routinely by the Freja wave instrument between 1000 and 1750 km; however, the effect of these LHCs on the core ion population is not as obvious as observed by sounding rockets. We have searched for signatures of LHC‐induced ion heating in data from the Freja cold plasma analyzer, which measures two‐dimensional integrated ion flux (0–20 eV) every 10 m along the satellite's trajectory. We identify a possible signature of localized ion heating in the form of isolated bursts and depletions of integral ion flux with amplitudes of the order of 108 cm−2 s−1 and lasting a few to tens of milliseconds or 30 to hundreds of meters along the trajectory when interpreted as spatial structures, with smaller features occurring more frequently. LHCs exist over a small subset of these scales, with a mean width of 54 m and a variance of only 15 m; in this case the spatial interpretation is supported by measurements of time delays between spatially separated density probes. Ion flux bursts are found in LHC‐containing regions; however, they also occur elsewhere. That is, the relation between flux bursts and LHCs is not one to one. Thus LHCs may cause some, but not all, ion flux bursts. If flux bursts are due at least in part to LHCs, there are several possible reasons why we observe no one‐to‐one relation between the two phenomena, including instrumental and sampling constraints and the fact that LHCs are not as intense as those observed at lower altitudes (several to tens versus hundreds of millivolts per meter peak to peak). Regardless of their cause, ion flux bursts provide an upper limit to the amount of heating caused by LHCs observed by Freja. We show that if flux bursts are caused by transverse heating of the core ion population, this heating cannot exceed ∼ 1 eV, although stronger heating of only a fraction of the ion population is also possible.
Cross‐bispectral analysis of the electromagnetic field in a beam‐plasma interaction
Authors: Rezeau L. , Belmont G. , Guéret B. , Lembège B.
Published: February 2004

The use of bicoherence has proved its efficiency in the analysis of nonlinear wave interactions. The method has been adapted to interpret the results of a simulation of the beam‐plasma interaction, in a case of broadband spectra, where many unstable modes can be suspected to couple. As the interacting waves exhibit different polarizations (electrostatic and electromagnetic), the bicoherence has been modified into a cross bicoherence in order to test the coupling of different field components. The visualization of the result has been designed in order to evidence the couplings in terms of efficiency (high bicoherence) and energy (high power). The application of this method to the simulation results shows that quadratic wave interactions are effective; new helpful information is stressed in order to interpret the observed growth of large‐scale magnetic fluctuations.
Ulysses observations of a “density hole” in the high‐speed solar wind
Authors: Riley Pete , Gosling J. T. , McComas D. J. , Forsyth R. J.
Published: February 2004

Ulysses observations at mid and high heliographic latitudes have revealed a solar wind devoid of the large variations in density, temperature, and speed that are commonly observed at low latitudes. One event, however, observed on May 1, 1996, while Ulysses was located at ∼ 3.7 AU and 38.5°, stands out in the plasma data set. The structure, which is unique in the Ulysses high‐latitude data set, is seen as a drop in proton density of almost an order of magnitude and a comparable rise in proton temperature. The event lasts ∼ 3½ hours giving the structure a size of ∼ 9.6 × 106 km (0.06 AU) along the spacecraft trajectory. Minimum variance analysis of this interval indicates that the angle between the average magnetic field direction and the minimum variance direction is ∼ 92°, suggesting that the “density hole” may be approximated by a series of planar slabs separated by several tangential discontinuities. We discuss several possible explanations for the origin of this structure, but ultimately the origin of the density hole remains unknown.
Simulation of the heliosphere: Model
Authors: McNutt R. L. , Lyon John , Goodrich Charles C.
Published: February 2004

The problem of the interaction of the solar wind with the very local interstellar medium (VLISM) is complicated by the role played by collisions between the plasma components of the heliosphere and VLISM and the neutral component of the VLISM. We outline the inherent approximations in fluid descriptions of the problem and give formulas for the near‐exact charge exchange and elastic collision transfer integrals for particles, momentum and energy for two drifting Maxwellians with different drift speeds and temperatures. To lowest order, all Boltzmann collision operators can be evaluated analytically in terms of exponentials and error functions. Analytic approximations that have relative errors of less than 2.62% compared with the exact expressions can be implemented in large simulation codes. Our formulation avoids approximations used previously by others and leads to a simplification of the formulation and increased accuracy for numerical simulations of the heliosphere/VLISM interaction.
Magnetic‐flux‐tube‐integrated evolution of equatorial ionospheric plasma bubbles
Authors: Keskinen M. J. , Ossakow S. L. , Basu Santimay , Sultan P. J.
Published: February 2004

The nonlinear evolution of plasma bubbles in the equatorial ionosphere has been computed using an inhomogeneous magnetic‐flux‐tube‐integrated model. We find that plasma bubbles develop in a fundamentally different manner with northern and southern hemispheric coupling as opposed to their evolution without coupling. Bubble density and electric field signatures derived from the numerical simulations are shown to be in qualitative agreement with recent satellite observations.
Direct observational evidence for a heliospheric magnetic field with large excursions in latitude
Authors: Zurbuchen T. H. , Schwadron N. A. , Fisk L. A.
Published: February 2004

Fisk has pointed out that the heliospheric magnetic field in fast solar wind, that is, at the higher heliographic latitudes, may undergo large excursions in heliographic latitudes, and thus that the field will deviate from the expected Archimedes spiral pattern. These excursions result from the interplay between the nonradial expansion of the solar wind in rigidly rotating coronal holes, and the differential rotation of the footpoints of the field lines anchored in the photosphere. In this paper magnetic field data from the Ulysses spacecraft in the southern solar hemisphere are analyzed and show that the field exhibits the systematic variations with latitude and longitude expected from the field configuration predicted by Fisk. We also show that variations about the predicted field should exhibit a unique periodicity of about 20 days at high latitudes and that the predicted periodicity is in fact observed. The observation conformation of the predicted field provides support for several other applications of this theory.