bluuebunny/crossref_metadata_2025 huggingface dataset:
Thresholds for magnetic percolation through the magnetopause current layer in asymmetrical magnetic fields
Authors: Kuznetsova M. M. , Roth M.
Published: February 2004

The Vlasov kinetic approach is used to study the stability of magnetic surfaces with respect to spontaneous excitation of collisionless tearing perturbations within magnetopause current layers (MCLs) with asymmetrical magnetic field profiles. For the unperturbed configuration a “tractable” (that is, with a minimum number of free parameters) Vlasov equilibrium model describing a tangential discontinuity is developed. In this model, asymmetrical MCLs are not electrostatically equipotential configurations and their structure is only determined by the angle of the magnetic field rotation θ0 and the magnetic field asymmetry factor κB = (B2 ‐ B1)/B2, where B1 and B2 are the magnetic field intensities in the adjacent magnetosheath and magnetospheric regions, respectively. The stochastic percolation model by Galeev et al. (1986), based on the symmetrical charge‐neutral Harris equilibrium, is generalized for asymmetrical MCLs. Asymmetry in the B field profile strongly modifies the dependence of the marginal MCL thickness (below which the MCL is subjected to percolation) on the polarity of the interplanetary magnetic field (IMF). For a northward IMF (θ0 < 90°), the percolation is impossible when κB ≥ 0.4, while for moderate values of κB (0.15 ≤ κB ≤ 0.4) only thin MCLs can be percolated. When κB > 0.3, the maximum thickness of MCLs subjected to percolation is achieved for θ0 > 90°, that is, for a southward IMF. Assuming that the magnetopause should, on the average, be close to its stability threshold, realistic asymmetrical MCLs (with κB > 0.3) should be thinner for a northward IMF than for a southward IMF.
Ionospheric effects of the March 1990 Magnetic Storm: Comparison of theory and measurement
Authors: Richards P. G. , Torr D. G. , Buonsanto M. J. , Sipler D. P.
Published: February 2004

This paper presents a comparison of the measured and modeled ionospheric response to magnetic storms at Millstone Hill and Arecibo during March 16‐23, 1990. Magnetic activity was low until midday UT on day 18 when Kp reached 6, days 19 and 20 were quiet, but a large storm occurred around midnight UT on day 20 (Kp=7) and it was moderately disturbed (Kp=4) for the remainder of the study period. At Millstone Hill, the daytime peak electron density (NmF2) showed only a modest 30% decrease in response to the first storm and recovered to prestorm values before the onset of the second storm. The model reproduces the daytime peak electron density well for this period. However, the severe storm on March 20 caused a factor of 4 depletion in electron density, while the model densities were not greatly affected. The inclusion of vibrationally excited nitrogen (N2*) in the model was unable to account for the observed large electron density depletions afterward March 20. The storm did not appear to affect the overall magnitude of the electron density at Arecibo very much, but did cause unusual wavelike structure in the peak density and peak height following the storm. The model reproduces the daytime NmF2 very well for Arecibo, but after sunset the model densities decay too rapidly. This study indicates that successful modeling of severe ionospheric storms will require better definition of the storm time inputs, especially of the neutral atmosphere.
Differential flow between protons and alphas in the solar wind: Prognoz 7 observations
Authors: Yermolaev Y. I. , Zastenker G. N.
Published: February 2004

Solar wind data obtained with plasma experiment on the Prognoz 7 satellite have been reanalyzed to study the dependence of α particle and proton differential flow on both proton velocity Vp and the type of solar wind stream. It is found that velocity difference, ΔV = |Vα| − |Vp|, increases with increasing Vp in streams from coronal holes and decreases in high‐velocity streams behind interplanetary shocks and in coronal mass ejections. Thus the Prognoz 7 data do not contradict the results of earlier papers and confirm the conclusion that at 1 AU there is strong differential streaming in fast streams from coronal holes but not in fast streams from coronal mass ejections.
Subgrid modeling of convective turbulence in weakly ionized collisional plasma by renormalization group analysis
Authors: Hamza A. M. , Sudan R. N.
Published: February 2004

The equations governing the nonlinear evolution of density fluctuations in a low‐pressure weakly ionized plasma driven unstable by the E × B or gradient‐drift instability were derived by Sudan and Keskinen (1977, 1979) for addressing the electrostatic turbulence in the E and F regions of the Earth's ionosphere. We have developed a subgrid model suitable for the numerical simulation of these equations which is closely related to renormalized diffusion caused by small‐scale fluctuation spectrum. “Dynamical Renormalization Group” (RNG) methods are employed to obtain the renormalized diffusion. This procedure computes the long‐wavelength, long‐time behavior of density correlations generated by the evolution equation for the plasma stirred by a Gaussian random force characterized by a correlation function ∝ km where k is the wavenumber of the forcing function. The effect of small scales on the large‐scale dynamics in the limit k → 0 and infinite Reynolds number can be expressed in the form of renormalized coefficients; in our case, renormalized diffusion. If one assumes the power spectra to be given by the Kolmogorov argument of cascading of energy through k space then one can derive a subgrid model based on the results of RNG.
Growth rates of new parametric instabilities occurring in a plasma with streaming He++
Authors: Jayanti V. , Hollweg Joseph V.
Published: February 2004

We consider parametric instabilities of a circularly polarized pump Alfvén wave, which propagates parallel to the ambient magnetic field; the daughter waves are also parallel‐propagating. We follow Hollweg et al. (1993) and consider several new instabilities that owe their existence to the presence of streaming alpha particles. One of the new instabilities is similar to the familiar decay instability, but the daughter waves are a forward going alpha sound wave and a backward going Alfvén wave. The growth rate of this instability is usually small if the alpha abundance is small. The other three new instabilities occur at high frequencies and small wavelengths. We find that the new instability which involves the proton cyclotron wave and alpha sound (i.e., the (+f, αs) mode) can be the fastest growing instability if β ≈ 1. However, if β is small, then the instability which can compete with the decay instability is the (+f, − α) instability, which involves both the proton and alpha cyclotron resonances, but the pump wave must have low frequency and large amplitude. These instabilities may be a means of heating and accelerating alpha particles in the solar wind, but this claim is unproven until a fully kinetic study is carried out.
Spatial relationships between field‐aligned currents and suprathermal electron beams observed at the poleward boundary of the nightside auroral oval
Authors: Nagatsuma T. , Fukunishi H. , Mukai T.
Published: February 2004

Magnetic field and particle data obtained from the Akebono satellite in the period from December 1989 to February 1990 are used for examining the relationships between electron populations and field‐aligned currents in the poleward boundary region of the nightside auroral oval. It is found that suprathermal electron beams frequently observed at the poleward boundary of the auroral oval associated with a latitudinally narrow (∼ 1°) field‐aligned current system located at the poleward edge, which has been designated as “the boundary current system” by Fukunishi et al. (1993, p. 11,250). In addition, an unstructured band of plasma‐sheet‐like electrons with isotropic pitch angle distributions except for loss cone appears through the auroral oval including the boundary current region. As a result, the electron energy spectra at the poleward boundary are characterized by a superposition of two Maxwellian functions: the isotropic high‐temperature component and the field‐aligned low‐temperature component. From the Maxwellian fitting procedure, the temperature and the density of the high‐temperature component are estimated to be 0.3 ‐ 1.7 keV and ≤ 1.0 cm−3, respectively, and those of the low‐temperature component are estimated to be 10 ‐ 80 eV and 2 ‐ 16 cm−3, respectively. An important finding is that the temperature and the density of the low‐temperature component and the density of the high‐temperature component are significantly enhanced in the upward current region occupying the equatorward portion of the boundary current system, while the temperature of the high‐temperature component is nearly constant throughout this region. These characteristics strongly suggest that the high‐temperature component originates from plasma sheet electrons, while the low‐temperature component originates from ionospheric electrons. It is likely that ionospheric thermal electrons flow away from the polewardmost downward current region into the magnetosphere and are accelerated and heated by some wave‐particle interaction process during their inward motion due to E × B drift.
Injection and acceleration of interstellar pickup ions at the heliospheric termination shock
Authors: Kucharek H. , Scholer M.
Published: February 2004

We have used a one‐dimensional hybrid code (macro‐ions, massless electron fluid) in order to study the interaction of an interstellar pickup ion distribution with the heliospheric termination shock. The shock is generated by reflecting the solar plasma at a rigid wall. The interstellar pickup ions are additional populations modeled as spheres in velocity space with radii given by the solar wind speed, comoving with the solar wind. The pickup ions are located on the outer shell of the spheres. We have determined the reflection rates of the pickup ions, which have been defined as the ratio of the reflected to the incident pickup ions at the shock. The dependence of these reflection rates on the shock normal angle ΘBn and on an upstream imposed turbulence has been investigated. No backscattered pickup ions are found for ΘBn greater than 70° and the reflection rates decrease with increasing level of upstream imposed turbulence. The dependence of the reflection rate on the ratio of the upstream pickup proton density to the upstream solar wind density has been investigated. Since this ratio is proportional to the heliospheric distance of the termination shock we have been able to investigate the reflection rates of the pickup ions at different heliospheric locations of the termination shock. Based on these hybrid simulations a model for the acceleration of anomalous component of the cosmic rays has been developed which is able to explain the differential flux of anomalous helium at the termination shock needed in modulation calculations to fit observations in the inner heliosphere. In this model we have not included the influence of the anomalous component on the shock structure. From comparison with modulation calculations it is concluded that the location of the termination shock is at distances between 80 and 120 AU.
Density‐temperature correlation in solar wind magnetohydrodynamic fluctuations: A test for nearly incompressible models
Authors: Bavassano B. , Bruno R. , Klein L. W.
Published: February 2004

The degree of correlation existing between density and temperature fluctuations at MHD scales in the inner solar wind (0.3 to 1 AU) has been investigated as a test for nearly incompressible models. Correlation is evaluated over time intervals of 45 and 180 min for a period of about 3 months in conditions of low solar activity. The number of cases with significant correlation (or anticorrelation) comes out to be quite small. Anticorrelations are mostly observed in slow wind, while correlations do not appreciably depend on flow speed. Relative density fluctuations for intervals with significant correlation or anticorrelation do not scale with turbulent Mach number as expected on the basis of nearly incompressible theory. These findings raise doubts about a general and extended applicability of such theory to the solar wind fluctuations in the inner heliosphere. However, a more comprehensive data set, for instance, one including different solar activity conditions, may be required for definite conclusions to be drawn.
Topological structure of the magnetotail as a function of interplanetary magnetic field direction
Authors: Fedder J. A. , Lyon J. G. , Slinker S. P. , Mobarry C. M.
Published: February 2004

Magnetic reconnection between the interplanetary magnetic field (IMF) and the geomagnetic field is thought to play a major role in the transfer of solar wind momentum and energy to the magnetosphere. As the angle between the IMF and the geomagnetic field is changed at the bow of the magnetosphere, the topological record of the location of the reconnection region should be recorded in the magnetosheath and on the magnetopause along the flanks of the tail, because the super fast flow freezes strong magnetic gradients formed in the bow reconnection regions into the plasma downstream. In this report, we present results from a three‐dimensional, magnetohydrodynamic (MHD), global numerical simulation code for the location of the separatrix between unconnected IMF magnetosheath field lines and reconnected field lines which penetrate the magnetopause and connect to the polar ionosphere. The angle between the IMF direction and the line where the separatrix crosses the magnetopause is shown to be a sensitive function of the IMF clock angle. We also explain how this behavior can be used to derive an approximate relation for the dependence of the cross‐polar voltage on the IMF clock angle. We conclude with a note of caution concerning the importance of physical boundary conditions in magnetoplasma simulations.
A theoretical study on the broad symmetric structure in the stimulated electromagnetic emission spectrum
Authors: Huang J. , Kuo S. P. , Zhou H. L.
Published: February 2004

A modulational instability of the electron Bernstein wave is studied. The process involves decay of the pump electron Bernstein wave into both a Stokes electron Bernstein sideband and an anti‐Stokes electron Bernstein sideband, together with a lower‐hybrid decay mode. The threshold field of the proposed instability for third‐harmonic resonance is found to be around 1 V/m in a height region sufficiently below the upper‐hybrid resonance layer and increases with the harmonic number l. It is then proposed that scattering of the anti‐Stokes and Stokes sidebands of this modulational instability off field‐aligned density irregularities produces frequency‐upshifted and frequency‐downshifted sidebands constituting the broad symmetric structure in the stimulated electromagnetic emission spectrum observed in ionospheric heating experiments.
Synoptic maps of solar wind parameters from in situ spacecraft observations
Authors: Gazis P. R.
Published: February 2004

Solar wind observations from the IMP 8 and Pioneer Venus Orbiter (PVO) spacecraft from 1982 until 1988 are combined to construct synoptic maps of solar wind parameters near 1 AU. Each map consists of 6 months of hourly averaged solar wind data, binned by heliographic latitude and Carrington longitude and projected back to the Sun. These maps show the structure and time evolution of solar wind streams near 1 AU in the heliographic latitudes of ±7.25° and provide an explicit picture of several phenomena, such as gradients, changes in the inclination of the heliospheric current sheet, and the relative positions of various structures in the inner heliosphere, that is difficult to obtain from single‐spacecraft observations. The stream structure varied significantly during the last solar cycle. Between 1982 and early 1985, solar wind parameters did not depend strongly on heliographic latitude. During the last solar minimum, the solar wind developed significant latitudinal structure, and high‐speed streams were excluded from the vicinity of the solar equator. The interplanetary magnetic field was strongly correlated with the coronal field, and the current sheet tended to coincide with the coronal neutral line. The solar wind speed showed the expected correlations with temperature, interplanetary magnetic field, and distance from the current sheet. The solar wind speed was anticorrelated with density, but the regions of highest density occurred east of the heliospheric current sheet and the regions of lowest solar wind speed. This is consistent with compression at the leading edge of high‐speed streams.
Locating the polar cap boundary from observations of 6300 Å auroral emission
Authors: Blanchard G. T. , Lyons L. R. , Samson J. C. , Rich F. J.
Published: February 2004

We examine ground‐based observations of the meridional profile of 6300 Å atmospheric emission from 67.3° to 80.7° invariant latitude for the signature of the polar cap boundary, the ionospheric boundary between open and closed magnetic field lines. The open‐closed field line boundary is assumed to lie at the boundary between polar rain and plasma sheet precipitation. We assume that nonprecipitation‐dependent sources of 6300 Å emission cause a spatially uniform luminosity in the polar cap and that auroral zone luminosity is also spatially uniform. Therefore we determine the location of the polar cap boundary from the auroral emission data at each time by finding the best fit of the observations to a step function in latitude. Thus we produce a time series of the location of the polar cap boundary. We have developed criteria on the step function fit that identify when a reliable boundary identification has been obtained. Generally, where these criteria are not satisfied, the boundary is outside the latitudinal range of the optical observations. We compare the boundary identified from the emissions to the boundary in precipitating particle observations made by DMSP as it passes along a meridian within 1 1/2 hours of local time of the photometer. The latitudes of the two boundaries are highly correlated. During the expansion phase of substorms, however, there are large discrepancies apparently arising from longitudinal structure of the polar cap boundary associated with auroral surges. We conclude that 6300 Å emissions provide a good means for monitoring the polar cap boundary continuously with an estimated precision of ±0.9° invariant latitude.
Optical signatures of prenoon auroral precipitation: Sources and responses to solar wind variations
Authors: Jacobsen B. , Sandholt P. E. , Burke W. J. , Denig W. F. , Maynard N. C.
Published: February 2004

Ground‐based optical observations from Svalbard of 0900–1100 MLT auroral structures and dynamics are presented. Strong and stable red line emissions with weak, transient auroral activity were initially observed. The IMP 8 spacecraft, located outside the dawnside bow shock, detected large changes in interplanetary magnetic field (IMF) direction including a sharp transition from negative to positive IMF Bz and a subsequent decrease in the solar wind density. A poleward expansion of the auroral display was seen in response to these variations, with a several minutes time delay, consistent with the location of IMP 8 relative to the magnetopause. The initial ionospheric response to these changes involved an intense brightening and northwestward drifts of discrete auroral forms. These effects may be connected with the decrease of external pressure and associated magnetopause perturbations. During the next 2–3 hours, quasi‐periodic transient, discrete auroral forms were observed to expand in the antisunward direction. The morphology of the auroral forms detected in the prolonged period of northward IMF Bz are compared with in situ measurements taken during a DMSP pass to the west of Svalbard and discussed in terms of several possible generation mechanisms. These include variations in external pressure, magnetic merging poleward of the cusp, and dynamo processes in the plasma mantle and/or low‐latitude boundary layer, powered by intruding magnetosheath plasma elements. The optical measurements appear most consistent with expected effects of a magnetosheath plasma penetration of the plasma mantle.
Diffusion at the magnetopause: Hybrid simulations
Authors: Winske D. , Omidi N.
Published: February 2004

Electromagnetic wave generation and resulting cross‐field diffusion of plasma are considered at a tangential discontinuity, which characterizes the magnetopause for northward interplanetary magnetic field. Two‐dimensional hybrid (particle ions, massless fluid electrons) simulations, in which the tangential discontinuity is generated self‐consistently via a stream‐stream interaction, are used to show that wave growth occurs when the ambient magnetic field is predominantly perpendicular to the direction of the density gradient. Low‐frequency (≪ ion gyrofrequency) waves, with amplitudes δB/B≤0.2 and anticorrelated density fluctuations δn/n≤0.6, are generated at the discontinuity, resulting in cross‐field diffusion that is comparable to the Bohm rate. Both the fluctuation level and the lack of ion heating in the calculations are consistent with observations at the magnetopause. The magnitude of the diffusion is considered in the presence of numerical effects and in the context of the inferred diffusion rate at the magnetopause. The relation of the low‐frequency waves and their consequences to faster growing, short‐wavelength waves due to the lower hybrid drift instability is also addressed. The overall conclusion of this initial study is that diffusion due to low frequency waves is not likely to be a major effect at the magnetopause.
Evidence for magnetic disconnection of mass ejections in the corona
Authors: Webb David F. , Cliver Edward W.
Published: February 2004

Coronal mass ejections (CMEs) involve the expulsion of significant amounts of mass and magnetic flux into the heliosphere, a process which implies an unobserved continuous buildup of the net interplanetary magnetic flux. Some form of disconnection of the flux near the Sun, either directly associated with the CME or occurring elsewhere in the corona, appears to be required to prevent this buildup. Field line reconnection in the wake of CMEs is also a fundamental aspect of some types of magnetically driven eruptive flare/CME models. However, to date there have been very few reports of CMEs which exhibited evidence for disconnection, despite the detection of several thousand CMEs over nearly 2 decades of observations. We report on the results of a systematic search for candidate magnetic disconnection features, defined as transient large‐scale, concave‐outward bright regions usually following the CME leading edge, using both ground‐based solar eclipse and spaceborne coronagraph data. We conclude that ≥ 10% of all CMEs observed in the corona have possible disconnection structures. We propose a simple classification scheme for these features based on their morphology. The most common type of candidate disconnection feature (65% of all the features) had a circular or ovoid shape; 35% of the features consisted of concave‐outward partial arcs. The average leading edge speed and latitudinal span of these CMEs were slightly less than those of typical CMEs. The results are discussed in the context of recent studies of magnetic disconnection and reconnection in the corona.
Focusing of nonducted whistlers by the equatorial anomaly
Authors: Sonwalkar Vikas S. , Inan Umran S. , Aggson T. L. , Farrell W. M. , Pfaff R.
Published: February 2004

Impulsive ELF/VLF electric field bursts observed by the vector electric field instrument (VEFI) on the Dynamics Explorer 2 (DE 2) satellite on almost every crossing of the geomagnetic equator in the evening hours are interpreted as originating in lightning discharges. These signals that peak in intensity near the magnetic equator are observed within 5‐20° latitude of the geomagnetic equator at altitudes of 300–500 km with amplitudes of the order of ∼ mV/m in the 512‐ to 1024‐Hz frequency band of the VEFI instrument. Whistler‐mode ELF/VLF wave propagation through a horizontally stratified ionosphere predicts strong attenuation of subionospheric signals reaching the equator at low altitudes. However, ray tracing analysis shows that the presence of the equatorial density anomaly, commonly observed in the upper ionosphere during evening hours, leads to the focusing of the wave energy from lightning near the geomagnetic equator at low altitudes, thus accounting for all observed aspects of the phenomenon. The observations presented here indicate that during certain hours in the evening, almost all the energy input from lightning discharges entering the ionosphere at <30° latitude remains confined to a small region (in altitude and latitude) near the geomagnetic equator. The net wideband electric field, extrapolated from the observed electric field values in the 512‐ to 1024‐Hz band, can be ∼10 mV/m or higher. These strong electric fields generated in the ionosphere by lightning at local evening times may be important for the equatorial electrodynamics of the ionosphere.
Coincident bursts of auroral kilometric radiation and VLF emissions associated with a type III solar radio noise event
Authors: Rosenberg T. J. , Singh S. , Wu C. S. , LaBelle J. , Treumann R. A. , Inan U. S. , Lanzerotti L. J.
Published: February 2004

This paper examines an isolated magnetospheric VLF/radio noise event that is highly suggestive of the triggering of terrestrial auroral kilometric radiation (AKR) by solar type III radio emission and of a close relation between AKR and broadband hiss. The solar type III burst was measured on polar HF riometers and was coincident with local dayside VLF/LF noise emission bursts at South Pole station. It was also coincident with AKR bursts detected on the AMPTE/IRM satellite, at the same magnetic local time as South Pole. On the basis of the close association of AKR and VLF bursts, and from geometrical considerations relating to wave propagation, it is likely that the AKR source was on the dayside and on field lines near South Pole station. The general level of geomagnetic activity was very low. However, an isolated magnetic impulse event (MIE) accompanied by a riometer absorption pulse was in progress when all of the VLF/radio noise bursts occurred. The very close association of the type III burst at HF with the AKR is consistent with external stimulation of the AKR, if a different, more immediate, triggering process than that implied by Calvert (1981) is invoked. It is suggested here that some of the HF solar radiant energy may decay into waves with frequencies comparable to those of the AKR by parametric excitation or some other process, thus providing the few background photons required for the generation of AKR by the Wu and Lee (1979) cyclotron maser instability. The AKR, perhaps by modifying the magnetospheric electron velocity distribution, might have produced the observed VLF emissions. Alternatively, the VLF emissions may have arisen from the same anisotropic and unstable electron distribution function responsible for the AKR. While the relationship of these emission features to the occurrence of the magnetic/absorption impulse may have been coincidental, the MIE could be a preconditioning agent for the ionosphere/magnetosphere plasma, making it susceptible to the external stimulation of the AKR.
Intermittent turbulence in solar wind from the south polar hole
Authors: Ruzmaikin A. A. , Feynman J. , Goldstein B. E. , Smith E. J. , Balogh A.
Published: February 2004

The magnetic fields measured by the Ulysses spacecraft are used to study solar wind turbulence in the fast solar wind from the south polar hole. The spacecraft was at about 46 deg south latitude and 3.9 AU. For a magnetic field with a Gaussian distribution the power spectrum (second‐order structure function) is sufficient to completely characterize the turbulence. However, the actual distribution is non‐Gaussian so that the effects of intermittency must be taken into account. The observed spectral exponents include effects of intermittency and cannot be directly compared with the standard second‐order spectral theories such as the Kolmogorov and Kraichnan theories. To permit a better comparison of the observations with the theoretical models, we study the structure characteristics of the data. We find the exponents of the second‐order structure functions (power spectra) and the higher‐order normalized structure functions for the components of the magnetic fields. We show that these sets of exponents can be approximately described by two basic numbers: the spectral exponent and the intermittency exponent. The intermittency exponent characterizes correlation properties of the energy cascade from large to small scales. Before comparing the observations to the theoretically expected values, a reduction must be made to the observed spectral exponent. The amount of the reduction depends on both the intermittency exponent and the model of the energy cascade assumed in the turbulence theory. We reduce the measured spectral indices according to a simple model for Alfvén turbulence that is described here. We then compare our reduced spectral indices with second‐order spectral theory. The reduced spectral indices for the period range of 1 min to about a half hour are remarkably constant and in good agreement with the value of 3/2. Thus our treatment is self‐consistent. Our tentative conclusion is that the high‐frequency turbulence appears to agree with the model of random‐phased Alfvén waves. This tentative conclusion must be tested by further theoretical and observational work.
Analysis of smooth high‐frequency emission at Neptune
Authors: Morgan D. D. , Menietti J. D.
Published: February 2004

We present a detailed study of the highest‐frequency component of smooth radio emission observed during the Voyager 2 encounter with Neptune in August 1989. This emission occurs during three distinct periods on August 24 and 25, 1989, in the frequency range of 550 to 900 kHz. By assuming straight‐line propagation from sources of both fundamental and second harmonic gyroemission, we perform a detailed analysis of the observed polarization of the emission. The data are most consistent with an L‐O mode source in the north magnetic polar region, around 50°W, 50°N. A second possible source is in the north magnetic polar region, around 270°W, 50°N. This source must emit in the R‐X mode.
Vortex‐induced magnetic reconnection and single X line reconnection at the magnetopause
Authors: Fu S. Y. , Pu Z. Y. , Liu Z. X.
Published: February 2004

Vortex‐induced magnetic reconnection (VIMR) and bursty single X line reconnection (BSXR) at the magnetopause are studied using two‐dimensional MHD simulation. The VIMR and BSXR occur when the flow‐aligned Alfvén Mach number is greater or less, respectively, than a critical value which typically ranges from 0.3 to 0.6 for different anomalous resistance conditions. In the former case, the streamlines and magnetic field lines are in alignment and construct a concentric vortical pattern, whereas in the latter case, magnetic field lines do not form closed loops, and vortices occur around the X line. The topological structure of the asymptotic quasi‐steady state of 2‐D reconnection depends strongly upon the topological invariants such as the cross helicity. The different sites where VIMR and BSXR work at the magnetopause to form flux transfer events are discussed.
Linear and nonlinear properties of ULF waves driven by ring‐beam distribution functions
Authors: Killen K. , Omidi N. , Krauss‐Varban D. , Karimabadi H.
Published: February 2004

The problem of the excitation of obliquely propagating magnetosonic waves which can steepen up (also known as shocklets) is considered. Shocklets have been observed upstream of the Earth's bow shock and at comets Giacobini‐Zinner and Grigg‐Skjellerup. Linear theory as well as two‐dimensional (2‐D) hybrid (fluid electrons, particle ions) simulations are used to determine the properties of waves generated by ring‐beam velocity distributions in great detail. The effects of both proton and oxygen ring‐beams are considered. The study of instabilities excited by a proton ring‐beam is relevant to the region upstream of the Earth's bow shock, whereas the oxygen ring‐beam corresponds to cometary ions picked up by the solar wind. Linear theory has shown that for a ring‐beam, four instabilities are found, one on the nonresonant mode, one on the Alfvén mode, and two along the magnetosonic/whistler branch. The relative growth rate of these instabilities is a sensitive function of parameters. Although one of the magnetosonic instabilities has maximum growth along the magnetic field, the other has maximum growth in oblique directions. We have studied the competition of these instabilities in the nonlinear regime using 2‐D simulations. As in the linear limit, the nonlinear results are a function of beam density and distribution function. By performing the simulations as both initial value and driven systems, we have found that the outcome of the simulations can vary, suggesting that the latter type of simulations is needed to address the observations. A general conclusion of the simulation results is that field‐aligned beams do not result in the formation of shocklets, whereas ring‐beam distributions can.
Structure of the magnetopause inferred from one‐dimensional hybrid simulations
Authors: Omidi N. , Winske D.
Published: February 2004

The structure of the magnetopause is investigated by studying the interaction between two plasmas with solar wind and magnetospheric properties. Both Hall‐MHD and hybrid (particle ions and fluid electrons) simulations are performed to compare and contrast the nature of the solutions in the fluid and the kinetic limits. It is shown that, in accordance with previous studies, the fluid solutions consist of multiple discontinuities and waves such as slow shocks and waves, as well as contact and rotational discontinuities. In contrast, the kinetic solutions consist of fewer discontinuities and include non‐MHD boundaries. The difference between the two types of solutions are attributed to the absence of contact surfaces in collisionless plasmas and the possible Landau damping of slow waves. The kinetic solutions are found to be in a much better agreement with magnetopause observations, which have shown little evidence for the presence of slow shocks or contact discontinuities. The results of kinetic calculations suggest that the presence of a small but finite normal component of the magnetic field allows for the mixing of magnetosheath and magnetospheric plasmas and may, in part, be responsible for the formation of the boundary layer even during the periods of northward interplanetary magnetic field. These results also show that the necessary changes in the fields and the plasma parameters across the magnetopause do not occur simultaneously (i.e., in the same spatial location). As such, the magnetopause cannot be viewed as a single boundary but instead is a region which includes both abrupt and gradual changes in the fields and plasma parameters. For example, the jump from the magnetosheath to the magnetospheric magnetic field strength takes place at a narrow region (a few ion inertial lengths), which we refer to as the magnetic boundary. This boundary, which has no MHD counter part, is not associated with any abrupt change in the plasma density or temperature. Pile up of the magnetosheath mirror waves at the magnetic boundary can result in the compression and amplification of these waves. As a result, anticorrelated density and magnetic field fluctuations are present upstream of the magnetic boundary. Similar fluctuations have been observed at the magnetopause of the Earth and possibly at Jupiter and Saturn. Analysis of these waves at the Earth's magnetopause has shown that they have a finite frequency in the plasma rest frame. Here we show that this finite frequency is a direct consequence of the entry of the magnetosheath plasma into the boundary layer and may be used to determine the velocity of plasma penetration into the high magnetic field region.
Flux dropouts of plasma and energetic particles at geosynchronous orbit during large geomagnetic storms: Entry into the lobes
Authors: Moldwin Mark B. , Thomsen Michelle F. , Bame Samuel J. , McComas David J. , Birn Joachim , Reeves Geoffrey D. , Nemzek Robert , Belian Richard D.
Published: February 2004

Geosynchronous orbit spacecraft occasionally observe the magnetospheric tail lobe during disturbed geomagnetic conditions. These lobe intervals are characterized by an absence of detectable ion flux above 1 eV and an absence of electron flux above several hundred eV. This study is an extension of Thomsen et al. (1994) and examines geosynchronous orbit plasma measurements for lobe encounters during a 4‐year period involving three spacecraft, for a total of 85 spacecraft‐months of observations. During the interval surveyed, 160 lobe encounters or close encounters were observed. These events are divided into the following two classes: events observed near midnight and those observed along the flanks. Both classes of events were strongly associated with very active geomagnetic conditions as indicated by Kp (mean value of 5), Dst (mean value of −104 nT), and the Auroral Boundary Index, particularly the flank events. Fifty percent of the lobe encounters occurred within ±24 hours of a geosynchronous magnetopause crossing, which indicates that many of the lobe encounters occurred when there was severe compression and/or erosion of the magnetosphere. Most of the magnetopause‐crossing‐associated events were flank events. The flank‐lobe events were also generally associated with a large interplanetary magnetic field magnitude, with the By or Bz component being dominant. Dropouts in the energetic electron and ion fluxes were observed concurrently with the thermal ion plasma dropouts, though they often extended over a longer time interval. For most of the events a geosynchronous injection of energetic electrons and/or protons was observed following the beginning of the thermal ion dropout, often at a different satellite than the one observing the lobe. For a limited subset of the lobe events (17 events) in which a GOES satellite was nearby, the magnetic field data indicate that the field is often highly stretched or radial near the time of the lobe events. We conclude that the near‐midnight and flank‐lobe events are due to two different mechanisms as follows: the near‐midnight events are consistent with the growth phase picture of large geomagnetic substorms, while the flank events are due to a major reconfiguration of the magnetosphere during large geomagnetic storms which are triggered by the passage of a large solar wind disturbance such as a fast coronal mass ejection. These observations show that the magnetosphere can be highly distorted at synchronous orbit, even far from midnight.
Large aspect angles in auroral E region echoes: A self‐consistent turbulent fluid theory
Authors: Hamza A. M. , St.‐Maurice J. ‐P.
Published: February 2004

We use a recently formulated fluid theory of turbulent Farley‐Buneman waves to study the impact of anomalous transport coefficients on the aspect angles of large‐amplitude waves in the high‐latitude E region in the strong nonlinear regime. The theory is self‐consistent and contains all the nonlinearities of the system except for the generation of unstable secondary waves through a two‐step process, which has been studied elsewhere. Our study leads us to conclude that nonlinear processes are responsible for an increase in the aspect angles of large‐amplitude waves through an increase in the scattering of electrons in the direction parallel to the geomagnetic field in conjunction with a lack of any significant increase in the scattering of electrons by waves in the plane perpendicular to the geomagnetic field.
Collisionless reconnection in configurations with a minimum in the equatorial magnetic field and with magnetic shear
Authors: Pritchett P. L. , Büchner J.
Published: February 2004

Two‐dimensional (x, z) particle simulations are used to investigate the influence of a minimum in the equatorial magnetic field profile and of a By field component (sheared magnetic field) on the occurrence of collisionless reconnection in thin current sheets (Ri/L ∼ 1, where Ri is the ion gyroradius based on the lobe field Bo and L is the current sheet half thickness). It is shown that the pure ion tearing mode (ion to electron mass ratio Mi/me = 1) first produces an X line in a region of minimum average (over a tearing wavelength) normal field Bz(x,0) and in the region of minimum current sheet thickness; this need not coincide with the actual minimum in Bz. With a mass ratio Mi/me > 1 the tearing growth rate is strongly reduced as kρe → 1, where ρe is the electron gyroradius in the Bz field. This is an extension of the stabilization associated with electron confinement to a flux tube previously predicted and observed for a constant equatorial field profile. If the slow growth allowed for kρe ≳ 1 eventually produces an X line, then an “explosive” stage of reconnection occurs in which the electron dynamics no longer inhibits the tearing mode displacement, and the growth rate approaches that for the one‐dimensional neutral sheet. The addition of a uniform and static shear field By ≳ Bz is found not to remove the stabilizing effect associated with the electron dynamics. The significant differences between the parameter regime of the present simulations and that of analytic treatments of the effect of By on thick (Ri /L < 0.1) current sheets are discussed in detail.
Three‐Dimensional Ring Current Decay Model
Authors: Fok Mei‐Ching , Moore Thomas E. , Kozyra Janet U. , Ho George C. , Hamilton Douglas C.
Published: February 2004

This work is an extension of a previous ring current decay model. In the previous work, a two‐dimensional kinetic model was constructed to study the temporal variations of the equatorially mirroring ring current ions, considering charge exchange and Coulomb drag losses along drift paths in a magnetic dipole field. In this work, particles with arbitrary pitch angle are considered. By bounce averaging the kinetic equation of the phase space density, information along magnetic field lines can be inferred from the equator. The three‐dimensional model is used to simulate the recovery phase of a model great magnetic storm, similar to that which occurred in early February 1986. The initial distribution of ring current ions (at the minimum Dst) is extrapolated to all local times from AMPTE/CCE spacecraft observations on the dawnside and duskside of the inner magnetosphere spanning the L value range L = 2.25 to 6.75. Observations by AMPTE/CCE of ring current distributions over subsequent orbits during the storm recovery phase are compared to model outputs. In general, the calculated ion fluxes are consistent with observations, except for H+ fluxes at tens of keV, which are always overestimated. A newly invented visualization idea, designated as a chromogram, is used to display the spatial and energy dependence of the ring current ion differential flux. Important features of storm time ring current, such as day‐night asymmetry during injection and drift hole on the dayside at low energies (<10 keV), are manifested in the chromogram representation. The pitch angle distribution is well fit by the function, jo(1 + Ayn), where y is sine of the equatorial pitch angle. The evolution of the index n is a combined effect of charge exchange loss and particle drift. At low energies (<30 keV), both drift dispersion and charge exchange are important in determining n.
Kinetic structure of rotational discontinuities: Implications for the magnetopause
Authors: Krauss‐Varban D. , Karimabadi H. , Omidi N.
Published: February 2004

Magnetic field rotations in the high ion beta magnetosheath that are part of the magnetopause structure are expected to have only a small normal component. We have studied the properties of rotational discontinuities (RDs) under these conditions, viewed as the limit of weak intermediate shocks (ISs), by performing hybrid simulations with a reflecting wall boundary condition (piston method). With this dynamic formation, the sense and size of rotation are not arbitrarily predetermined, but rather evolve from the given upstream (magnetosheath) and downstream (magnetospheric) boundary conditions, similar to what takes place at the magnetopause. This work focuses on several aspects: the observed minimum shear of RDs, their width, their internal signature, and their relation to ISs in isotropic plasmas. Our simulation results are in agreement with the minimum shear observations, that is, the RDs choose the sense of rotation that corresponds to the minimum angle between the upstream and downstream field vector. The RDs are stable, with a unique scale size. Typical gradient scale half widths are one to four ion inertial lengths with a total width up to ten times of that, in agreement with magnetopause observations. We develop a generalized fluid theory of RDs and discuss the characteristic internal signatures of the rotational layer, comparing the kinetic simulation results to predictions from the generalized fluid theory. The results show that ion inertia, anisotropic pressure, finite Larmor radius effects, nonzero ion heat flux, and reflected ions all contribute to the signatures of RDs on kinetic scales. The RDs may have upstream or downstream wave trains, which become weak for high ion beta and small normal components of the magnetic field. We explain the presence and direction of wave trains in terms of the kinetic properties of the Alfvén/ion‐cyclotron mode. Away from the RD limit there is a smooth transition to weak intermediate shocks, which have small jumps close to expected Rankine‐Hugoniot values. Apart from that, there are few kinetic plasma signatures that distinguish RDs from their neighboring ISs. However, noncoplanar ISs evolve in time into thin RDs. Using the properties of RDs and ISs, we make specific suggestions how these discontinuities can be distinguished observationally in the case of an isotropic plasma.
The middle ultraviolet dayglow spectrum
Authors: Cleary David D. , Gnanalingam S. , McCoy Robert P. , Dymond Ken F. , Eparvier F. G.
Published: February 2004

Spectroscopic measurements of the thermospheric dayglow in the wavelength range 1900 to 3400 Å are presented. These measurements were made during two rocket experiments conducted on March 30, 1990, and March 19, 1992, from White Sands Missile Range, New Mexico. The data are presented to provide reference spectra in the lower, middle, and upper thermosphere. The 1990 observations, which were made during high geomagnetic activity, showed considerably enhanced nitric oxide (NO) intensities. Self‐absorption theory is applied to the υ″ = 0 bands of the NO γ system. It is found that a recently published self‐absorption algorithm correctly accounts for the attenuation of the γ(1,0) bands. There is a small discrepancy between the theory and observation for the (2,0) band and the (0,0) band intensity. The fact that there is reasonable agreement for all three bands suggests that both the NO slant column density and oscillator strengths for these bands are correct.