bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial distribution of electron plasma oscillations in the Earth's foreshock at ISEE 3
Authors: Greenstadt E. W. , Crawford G. K. , Strangeway R. J. , Moses S. L. , Coroniti F. V.
Published: February 2004

Electric field oscillations recorded by the 10–56 kHz channels of TRW's plasma wave detector during parts of two of the ISEE 3 circumterrestrial orbits in 1983 have been used to make the first mapping of Earth's electron plasma wave foreshock. By combining data from the two trajectory segments, each of which provided relatively meager spatial sampling outside the bow shock, but high variation of interplanetary magnetic field (IMF) direction, a first‐order pattern of occurrence of electron plasma waves, hence also backstreaming electrons, has been determined. We depict the pattern with an adaptation of the mapping program previously used for the Venus electron foreshock. As at Venus, plasma wave activity was concentrated most densely along the IMF line tangent to the bow shock where energized electrons stream against the solar wind from the quasi‐perpendicular part of the shock. The size of the Earth's electron plasma wave foreshock, however, is vastly greater than that of Venus, implying that a foreshock's dimension scales with the size of its diamagnetic obstacle and associated bow shock. Our mappings with three additional ISEE 3 channels surrounding the local electron plasma frequency indicate a richer distribution of waves in the foreshock than the single electron frequency channel of Pioneer Venus Orbiter could detect around Venus.
Microscale anisotropy reduction and macroscale dynamics of the magnetotail
Authors: Birn Joachim , Gary S. Peter , Hesse Michael
Published: February 2004

Anisotropic magnetofluid simulations are used to investigate the influence of several models of anisotropy reduction on the dynamic evolution of the magnetotail. Two distinct magnetotail dynamic models are studied: a nonresistive simulation driven by an external electric field as a representation of the substorm growth phase, and a resistive simulation with a uniform enhanced resistivity as a representation of the substorm expansive phase. The plasma models include one which is fully isotropic, the double adiabatic model in which pressures parallel and perpendicular to the background magnetic field are completely uncoupled, and a model in which small‐scale instabilities impose an upper bound on the anisotropy. In this last model, anisotropies that exceed this bound are reduced on a short Alfvénic timescale. To investigate the transition to the fully isotropic model, a fourth plasma model is studied in which any anisotropy is reduced on the same short timescale. The growth phase magnetotail model is characterized by a localized increase in current density and by a reduction in the magnitude of BZ in the near‐Earth neutral sheet; both of these effects are more pronounced for the more isotropic plasma models. For the expansive phase magnetotail model the development of a generalized tearing instability occurs faster and the substorm wedge currents grow stronger when the plasma model is closer to isotropy. In both magnetotail models the plasma anisotropy model strongly influences the spatial variation of BZ in the neutral sheet and hence the location where reconnection occurs and a neutral line forms. Both the driven and the unstable anisotropic models develop mirror‐type anisotropies (p⊥ > p∥) in the (low beta or low current density) boundary regions of the plasma sheet and the adjacent lobes, while the (high beta or high current density) center region of the plasma/current sheet remains close to isotropy.
Conjugate occurrence of the electric field fluctuations in the nighttime midlatitude ionosphere
Authors: Saito A. , Iyemori T. , Sugiura M. , Maynard N. C. , Aggson T. L. , Brace L. H. , Takeda M. , Yamamoto M.
Published: February 2004

The DE 2 satellite observed electric field fluctuations on the topside of the nighttime midlatitude ionosphere. They extended several hundred kilometers in the latitudinal direction with wavelengths of several tens of kilometers, and their amplitudes were a few millivolts per meter. Such fluctuations were often observed at magnetically conjugate points in the northern and southern hemispheres. These electric field fluctuations are perpendicular to the geomagnetic field. They are not accompanied by any significant plasma depletion or electron temperature variations. Magnetic field fluctuations are sometimes observed simultaneously with electric field fluctuations. We interpret that these fluctuations are caused by field‐aligned currents which flow from the ionosphere in one hemisphere to the conjugate point in the other hemisphere. The power spectrum of these midlatitude electric field fluctuations follows a power law of the form Power α ƒ −n, with the spectral index n of 3.5 to 4.5, which is steeper than that of the electric field fluctuations in the high‐latitude ionosphere or in the equatorial ionosphere. This phenomenon may be related to other ionospheric phenomena, for example, the F region field‐aligned irregularities or spread‐F, observed by ground‐based methods such as the MU radar, but the relationship is not clear.
A new approach to low‐frequency “MHD‐like” waves in magnetospheric plasmas
Authors: Hurricane Omar A. , Pellat René , Coroniti Ferdinand V.
Published: February 2004

This paper explores the consequences of enforcing the quasi‐neutrality (QN) condition on a magnetotail and auroral plasma. We show that this condition has never been handled correctly in the literature when the bounce motion of particles is involved, for either adiabatic or stochastic ion motion. Correct treatment yields a global electrostatic potential which is constant along the magnetic field line. We describe how the stability results of Hurricane et al. (1995) are modified. We show that the QN condition modifies the classical MHD analysis of symmetric interchange/ballooning modes with respect to low‐frequency MHD waves. An additional consequence of the QN condition on MHD‐like waves is a modification of the plasma equation of state. More precisely, we show that the plasma remains isothermal during such wave propagation. For antisymmetric modes there is no associated electrostatic potential, but we give the necessary corrections to the standard analysis upon entering a stochastic ion regime. Last, we show how the electrostatic potential couples to the ionosphere, revealing a new understanding of auroral arcs.
Simulation study of the Riemann problem associated with the magnetotail reconnection
Authors: Lin Y. , Lee L. C.
Published: February 2004

The structure of reconnection layer in the distant magnetotail is studied by solving the Riemann problem for the evolution of an initial current sheet using one‐dimensional MHD and hybrid simulations. Initially, the current sheet with total pressure balance separates the plasmas and fields in the two lobes. In the presence of a nonzero normal magnetic field, which is due to the magnetic reconnection, the initial current sheet evolves to form the reconnection layer, which consists of various MHD discontinuities. First, we study the symmetric case with equal magnetic field strengths, equal plasma densities, and exactly antiparallel magnetic fields (By=0) in the two lobes. The Petschek (1964) reconnection layer which contains two switch‐off slow shocks, whose intermediate Mach number MI=1, is obtained. In the hybrid simulation, the switch‐off shock possesses a large‐amplitude, left‐hand‐polarized rotational wave train of magnetic field in the downstream region. Each slow shock propagates to either lobe in the magnetotail. A strong temperature anisotropy with T∥>T⊥ appears in most region of the reconnection layer. This is due to the interpenetrating of ions between the two lobes and the backstreaming of ions from the downstream of each slow shock to the lobe. Second, the presence of a finite guide field in lobes (By≠0) is considered in the simulation. It is found that the slow shocks becomes nonswitch‐off shocks with an intermediate Mach number MI<1, and two rotational discontinuities are also present in the hybrid simulation to bound the reconnection layer from, respectively, the two lobes. In addition, our hybrid simulations show that the presence of the finite By destroys the coherent wave train in the reconnection layer if the lobe By0≥0.08Bx0. Finally, the simulations are extended to the asymmetric cases in which the plasma densities and/or the magnetic fields in the two lobes are unequal. For asymmetric cases with By=0 in the two lobes, it is found that an intermediate shock and a nonswitch‐off slow shock are present in the reconnection layer and that the large‐amplitude rotational wave structure does not exist in the entire reconnection layer if the density ratio between the two lobes N1/N0 ≥ 1.5, where N0 and N1 are the ion number densities in the two lobes, receptively. For general cases with both density asymmetry and By0≠0, two rotational discontinuities and two slow shocks are present in the reconnection layer, and the critical value of the density ratio, above which the coherent wave train does not exist, is reduced. The absence of a downstream wave train for the slow shocks observed in the magnetotail may be associated with the presence of a finite By0.
Altitude profiles of localized D region density disturbances produced in lightning‐induced electron precipitation events
Authors: Lev‐Tov Sean J. , Inan Umran S. , Bell Timothy F.
Published: February 2004

A three‐dimensional model of very low frequency (VLF) radio wave propagation in the Earth‐ionosphere waveguide in the presence of lower ionospheric disturbances is used to quantitatively interpret VLF signatures of lightning‐induced electron precipitation (LEP) events observed in two previously reported cases. One case is that of a 28.5‐kHz signal originating in Puerto Rico and propagating to a receiver in Lake Mistissini, Quebec. The other case involves a 24.0‐kHz signal originating in Cutler, Maine, and received at Stanford, California. In both cases, high‐resolution measurements of the VLF signals were made to accurately document characteristic signatures of LEP events (Inan et al., 1988b, 1990). The comparison of the model calculations with the data yields information about the altitude profiles of electron density of both the extra ionization produced by the LEP events and of the ambient ionospheric D region. The comparisons are carried out using generally accepted values of the spatial extent of the disturbed regions and the intensity of the particle flux constituting the LEP burst.
Suprathermal electrons associated with a plasma discharge on an active sounding rocket experiment
Authors: Bale S. D. , Kellogg P. J. , Monson S. J. , Anderson H. R. , Potter D. W.
Published: February 2004

Electrons with energies up to 600 eV are observed with the retarding potential analyzer (RPA) instrument aboard the Several Compatible Experiments (SCEX) III sounding rocket. The electrons are concomitant with high‐energy (2–6 keV) electron gun injections and also evidence themselves by luminosity observed with 3805 Å and 3914 Å photometers. Both the collected electron flux and luminosity measurements are strongly nonlinear with gun injection current. For a typical event, the electron distribution is similar to laboratory beam‐plasma discharge (BPD) distributions reported by Sharp (1982) and when backed by HF electric field observations (Goerke et al., 1992; Llobet et al., 1985), the BPD mechanism becomes a most likely explanation. Strong turbulence theories of BPD predict a power law tail in the electron distribution, and we compare our spectral index with some previous observations.
Evolution of Langmuir turbulence and stimulated electromagnetic emissions excited with a 3‐mHz pump wave at Arecibo
Authors: Thidé B. , Djuth F. T. , Leyser T. B. , Ierkic H. M.
Published: February 2004

We present the results of a comparative experimental study at Arecibo of the initial temporal development of stimulated electromagnetic emissions (SEE) and HF‐enhanced plasma lines (HFPLs) excited in the ionospheric F region by a high‐power O mode radio wave at 3.1745 MHz. These results show that the portion of the SEE spectrum which extends from approximately −30 kHz to +10 kHz in the sidebands around the injected HF wave, and which is usually referred to as the narrow continuum SEE component, exhibits an overshoot behavior very similar to that of the upshifted HFPL of the 430‐MHz radar. Over timescales of a few hundred milliseconds, the narrow continuum emission weakens at all frequencies and gradually changes shape to become more structured. At 3.1745 MHz the amplitude of the HFPL is significantly smaller than that observed at higher HF frequencies; after an overshoot period of 300–500 ms, the HFPL often disappears completely.
Mapping of the auroral oval and individual arcs during substorms
Authors: Pulkkinen T. I. , Baker D. N. , Pellinen R. J. , Murphree J. S. , Frank L. A.
Published: February 2004

Magnetospheric mappings of the auroral oval during the course of a substorm are presented for an event where conjugate auroral imaging was available (CDAW 9 event C on May 3, 1986). The mappings have been computed using time‐evolving magnetic field models developed for the substorm growth and recovery phases. We show that even though the models do not contain field‐aligned currents, conjugacy between images from the two hemispheres is good. Furthermore, the high time resolution Viking images show an accurate comparison with the east‐west spreading of the substorm current wedge observed by four spacecraft in the near‐Earth tail. We also show that the double oval configuration present during the substorm recovery phase maps mostly within the lunar distance. The void (weak luminosity) regions between the poleward and equatorward arc systems are connected to the region 20–30 RE down the tail. To complement the global view, mappings of individual growth phase auroral arcs were undertaken. These mappings did not result in a coherent pattern, suggesting that these arcs may be formed by low‐altitude acceleration region processes rather than plasma sheet instabilities.
Slow‐mode shocks in the magnetotail
Authors: Saito Yoshifumi , Mukai Toshifumi , Terasawa Toshio , Nishida Atsuhiro , Machida Shinobu , Hirahara Masafumi , Maezawa Kiyoshi , Kokubun Susumu , Yamamoto Tatsundo
Published: February 2004

We have identified slow‐mode shocks between the plasma sheet and lobe in the midtail to distant‐tail regions by using three‐dimensional magnetic field data and three‐dimensional plasma data including density, velocity, temperature, and heat flux of both ions and electrons observed by the GEOTAIL satellite. Analyzing the data obtained between September 14, 1993, and February 16, 1994, we have found 303 plasma sheet‐lobe boundary crossings at distances between ‐XGSE ∼ −30RE and XGSE ∼ −210RE. Thirty‐two out of these 303 boundaries are identified as slow‐mode shocks. We have found back streaming ions on the upstream side of the slow‐mode shocks, which may be important in understanding the dissipation mechanism of the slow shocks in collisionless plasma. We have also found acceleration of cold ions between the upstream and the downstream of the slow‐mode shocks. These cold ions are often observed in the lobe, and they are usually flowing tailward. Upon entering the plasma sheet, they are accelerated and rotate around the magnetic field and at times show ring‐shaped velocity distributions. These ions may reflect the kinetic structure of slow‐mode shocks. Slow shocks are at times observed also on the front side of plasmoids. These slow shocks on the front side of plasmoids have a different orientation from that of the ordinary slow shocks observed at the plasma sheet‐lobe boundaries, which suggests an existence of “heart” ‐shaped plasmoids predicted by a numerical simulation.
The relationship between pulsating auroras observed from the ground and energetic electrons and plasma density measured at geosynchronous orbit
Authors: Nemzek R. J. , Nakamura R. , Baker D. N. , Belian R. D. , McComas D. J. , Thomsen M. F. , Yamamoto T.
Published: February 2004

We present comparisons of ground‐based all‐sky video recordings of pulsating auroral events and magnetically conjugate geosynchronous orbit measurements of high‐energy electrons and low‐energy plasma. The large‐scale pulsation events were found to be closely tied to substorm electron injections. For most events, only upper limits were available for the plasma density, which was ≤1–2 cm−3. We were unable to find any clear dependence between the plasma density and the pulsations. There were, however, small, rapid fluctuations in the density which had apparent spatial scales similar to those of individual pulsating auroral patches. These simultaneous measurements of the electron flux, plasma density, and pulsation period should be of use in constraining theoretical treatments of the pulsating aurora problem. We find that the conditions for minimum plasma density and plasma density gradient predicted by Demekhov and Trakhtengerts (1994) may not be supported by the geosynchronous data but that their prediction for minimum energetic electron density is supported. Predictions by Davidson and Chiu (1987) of the minimum change in plasma density and electron flux also seem to be supported. The data indicate that the principal factor controlling the onset of pulsations over a particular area may be the arrival of substorm‐injected electrons at the conjugate equator. This conclusion could provide the basis for a visible magnetic mapping of the substorm injection region through observations of the pulsating aurora.
ULF pulsations driven by magnetopause motions: Azimuthal phase characteristics
Authors: Wright Andrew N. , Rickard Graham J.
Published: February 2004

We present numerical simulations of wave coupling in a cold, ideal, inhomogeneous plasma waveguide which approximates the flanks of the magnetosphere. Two types of driving conditions are investigated. The first corresponds to a displacement of the magnetopause over a fixed azimuthal range. The pulsations driven by this boundary condition have an azimuthal wavenumber and phase speed determined by the equilibrium structure of the magnetosphere. The second condition corresponds to a displacement pulse which runs antisunward along the magnetopause and produces pulsations with a phase velocity strictly equal to that of the boundary pulse. Our results are compared briefly with data and suggest that on some occasions the magnetosphere is driven in a fashion that is well approximated by a “running” pulse or wave on the magnetopause.
Effects of time‐dependent photoionization on interstellar pickup ions
Authors: Isenberg Philip A. , Lee Martin A.
Published: February 2004

We present an analytical model for the density variations of interstellar pickup ions in the solar wind due to a time‐dependent variation in the photoionization rate of the interstellar atoms. Following a sudden increase in the photoionization rate, our model predicts a pickup ion density enhancement lasting for a time of the order of the duration of the increase plus the solar wind convection time to the observation point. If the photoionization rate returns to its initial value, this enhancement is followed by a decreased pickup ion density resulting from a depleted interstellar neutral particle density. In the absence of further variations in the photoionization rate, the pickup ion density recovers on a time which scales as the radial position of the observation point divided by the inflow speed of the neutral particles. Gradual variations in the photoionization rate result in a pickup ion density which tends to track the ionization rate, though the density variations are smoothed and delayed in time due to the solar wind convection of ions picked up at points closer to the Sun.
Particle simulation in the Martian magnetotail
Authors: Lichtenegger H. , Schwingenschuh K. , Dubinin E. , Lundin R.
Published: February 2004

A gasdynamic model of the magnetosheath is extended into the tail region by incorporating a cometary tail field produced by mass loading. By means of test particle simulations, a picture of the martian pickup ion wake is described and contrasted with particle measurements of the Phobos 2 automatic space plasma experiment with a rotating analyzer (ASPERA). It is shown that the convection electric field alone is not sufficient to explain the observations. If magnetic shear stresses of the draped field are taken into account, the flux of low‐energy oxygen ions close to the central wake of Mars (a persistent observational feature in the particle data) is reproduced. Simulation results suggest that the low‐energy ion observations (E ≤ 350 eV) of the ASPERA particle instrument are due to particles picked up in a source region at lower altitude close to the terminator plane at low latitudes, while moderate energy ions are created at high areographic latitudes, depending on the orientation of the transverse interplanetary magnetic field component. High‐energy ions (E ≥ 2 keV) usually originate from more distant regions in the magnetosheath or in the solar wind above the dayside of Mars.
The latitudinal structure of the heliospheric current sheet during solar activity cycles 21 and 22
Authors: Valdés‐Galicia J. F. , Otaola J. A.
Published: February 2004

Source surface heliomagnetograms from the Wilcox Solar Observatory from 1976 to 1994 were used to determine the evolution of the heliospheric current sheet (HCS) latitudinal extent during solar cycles 21 and 22. We divided the data into four periods, two during the ascending (1976–1978, 1987–1989) and two during the descending phases (1982–84, 1991–94) of the activity cycles. Time intervals near solar maximum activity when the HCS cannot be uniquely determined for every day were not included. We observed a marked difference in the HCS temporal evolution for the ascending and descending periods. A different latitudinal distribution is also present in all four intervals used. For the descending phases of the cycles analysed a two‐sector structure is clearly predominant, whereas for one of the ascending phases (1976–1978) a four‐sector structure is more prominent. Results clearly demonstrate that the evolution of the HCS is far from being uniform during consecutive cycles, nor is it the same before and after the polar field reversals.
Interaction of the solar wind with the local interstellar medium
Authors: Pauls H. L. , Zank G. P. , Williams L. L.
Published: February 2004

First results from a two‐dimensional (axisymmetric) time‐dependent gas dynamic model for the interaction of the solar wind with the local interstellar medium are presented. The model includes the mutual influence of the interstellar and interplanetary plasma (protons and electrons) and the neutral interstellar hydrogen atoms. Neutrals created by charge exchange with the solar wind are ignored; this allows us to approximate the dynamical evolution of the system with the isotropic fluid equations. A supersonic interstellar wind is assumed. The global structure of the heliosphere and interaction region is described for both the plasma and neutral gas. The self‐consistent model is compared to a noninteracting gas dynamic model, and dramatic differences in the size of the heliosphere are found. The distribution of neutral hydrogen in the heliosphere is also discussed.
Simultaneous Global Positioning System and radar observations of equatorial spread F at Kwajalein
Authors: Kelley Michael C. , Kotsikopoulos Demetrios , Beach Theodore , Hysell David , Musman Steven
Published: February 2004

GPS satellites broadcast at two frequencies (Ll of 1575.42 MHz and L2 of 1227.6 MHz). The dispersive property of the ionosphere is frequently used to correct positional measurements for ionospheric effects. Independent measurements at the two frequencies can also be combined to form a relative ionospheric delay and a measure of the total electron content (TEC) which is uncertain by an additive constant. In a previous paper (Musman et al., 1990) estimates of this offset were utilized in constructing models of the time history of the equivalent zenith delay at Westford, Massachusetts. An ionospheric model composed of uniform shells whose electron density changes slowly in a typical diurnal pattern would produce relative ionospheric delays with a simple u‐shaped or j‐shaped curve. Most of the change in delay would be a result of changes in geometry between the observer and the satellite. Departures from a simple pattern are indicative of ionospheric disturbances and the influence of the protonosphere. From GPS data alone, it is ambiguous whether these disturbances are due to spatial structures, temporal changes, or some combination of the two. Equatorial spread F (ESF) refers to a variety of equatorial ionospheric disturbances, some of which are associated with rising plasma plumes having low electron density and a high degree of turbulence. This phenomenon occurs primarily between local sunset and local midnight at sites within about 15° of the magnetic equator. In some seasons, disturbances can occur during two out of three evenings, while at other times it can be much quieter. GPS observations at Kwajalein (9°N latitude) reported here for August 14, 1990, show severe ionospheric disruption. Two independent and simultaneous sets of radar observations confirm the presence of ESF and reveal quite a bit about the spatial and temporal conditions which affect the system. GPS observations on August 15, 1990, when no ESF was present are much quieter. We find that tens of minute variations in the TEC correspond to the motion of large scale features across the GPS field of view. More severe GPS effects are seen to be collocated with turbulent low density plumes which rise rapidly to high altitudes and drift west to east across the GPS line of sight. Severe disruption can occur in moderately sophisticated GPS systems during such events, at least near solar maximum.
Impulsive bursts of energetic particles in the high‐latitude duskside magnetosphere of Jupiter
Authors: Zhang M. , McKibben R. B. , Simpson J. A. , Cowley S. W. H. , Staines K. , Anglin J. D. , Marsden R. G. , Sanderson T. R. , Wenzel K.‐P.
Published: February 2004

Recent studies using the entire set of proton and electron measurements of the cosmic and solar particle investigations (COSPIN) on Ulysses revealed that the impulsive, sometimes quasi‐periodic bursts discovered during the Ulysses' pass through Jupiter's duskside magnetosphere consisted not only of relativistic electrons and radio wave emissions but also protons and helium nuclei of energies from ∼0.7 to ∼10 MeV/nucleon. In this paper we present a detailed analysis of observations of the proton and helium bursts and of their correlation with relativistic electron bursts. Using an automated computer algorithm, we found 37 nucleonic bursts, all of which occurred in the duskside magnetosphere. Typically, the nucleonic bursts lasted only ∼20 s to ∼2 min and had a very rapid onset and shutoff, showing a spike‐like profile. The flux during the bursts often increased by a factor >10 above the background and had a very strong unidirectional anisotropy directed along the magnetic fields outward from the southern polar region of the planet. The half width of the pitch angle distributions for protons in the bursts was usually less than a few degrees. In the bursts, particles with highest energies were detected first, followed by lower‐energy particles, consistent with the velocity dispersion expected for particles arriving from a source at a significant distance. From this velocity dispersion relation and the O4CS magnetic field model we found that the source of the bursts was located a few Jovian radii above the surface over the southern high‐latitude polar region. Of the 37 nucleonic bursts, about half were associated with an electron burst originating at the same source and at the same time. Energy spectra for protons in the bursts often showed a peak at ∼1 MeV, and above this energy there was spectral hardening relative to preburst and postburst background spectra. These observations suggest that particle acceleration occurs in the high‐latitude polar inner magnetosphere of Jupiter and may be similar to those phenomena observed in the auroral region of Earth's magnetosphere. If the same acceleration process operates on both Earth and Jupiter, these observations imply that Jovian aurorae are probably associated with the high‐latitude boundaries of the polar cap rather than with the Io torus.
On the azimuthal variation of core plasma in the equatorial magnetosphere
Authors: Gallagher D. L. , Craven P. D. , Comfort R. H. , Moore T. E.
Published: February 2004

Previous results of plasmapause position surveys have been synthesized into a description of the underlying global distribution of plasmasphere‐like or core plasma densities unique to a steady state magnetosphere. Under these steady conditions, the boundary between high‐ and low‐density regions is taken to represent the boundary between diurnal near‐corotation and large‐scale circulation streamlines that traverse the entire magnetosphere. Results indicate a boundary that has a pronounced bulge in the dusk sector that is rotated westward and markedly reduced in size at increased levels of geomagnetic activity (and presumably magnetospheric convection). The derived profile is empirical confirmation of an underlying “tear drop” distribution of core plasma, which is valid only for prolonged steady conditions and is somewhat different from that associated with the simple superposition of sunward flow and corotation, both in its detailed shape and in its varying orientation. Variation away from the tear drop profile suggests that magnetospheric circulation departs from a uniform flow field, having a radial dependence with respect to the Earth that is qualitatively consistent with electrostatic shielding of the convection electric field and which is rotated westward at increased levels of geophysical activity.
Nonlinear evolution of equatorial spread F: 2. Gravity wave seeding of Rayleigh‐Taylor instability
Authors: Huang Chao‐Song , Kelley Michael C.
Published: February 2004

We have performed numerical simulations of nonlinear evolution of the Rayleigh‐Taylor instability in equatorial spread F (ESF) under different conditions. Our main purpose is to explain the generation of multiple plumes on the west wall of a plasma upwelling and the relationship between gravity waves and large‐scale ESF irregularities. We have studied ESF structures resulting from one‐dimensional or two‐dimensional initial density perturbations and initiated by gravity waves. It is found that although plasma bubbles may be produced in the all situations, the production of the plasma bubbles initiated by gravity waves takes a time much shorter than that resulting from two‐dimensional initial density perturbations. This is in agreement with a simple analytic nonlinear theory. We put our emphasis on the ESF structures growing from two different scale perturbations. The Rayleigh‐Taylor instability initiated by gravity waves produces a steep gradient on the west wall, which provides a favorable condition for excitation of smaller‐scale secondary instabilities. When both a seed gravity wave and a smaller‐scale initial density perturbation are used, it is observed that the large‐scale gravity wave determines the outer scale of ESF irregularities by seeding the Rayleigh‐Taylor instability and the smaller‐scale perturbation results in multiple plumes preferentially located on the west wall. The production, rise, and bifurcation of plasma bubbles are reproduced in the simulations.
Frequency variations of quasi‐periodic ELF‐VLF emissions: A possible new ground‐based diagnostic of the outer high‐latitude magnetosphere
Authors: Alford Jennifer , Engebretson Mark , Arnoldy Roger , Inan Umran
Published: February 2004

Magnetic pulsations and quasi‐periodic (QP) amplitude modulations of ELF‐VLF waves at Pc 3–4 frequencies (15–50 mHz) are commonly observed simultaneously in cusp‐latitude data. The naturally occurring ELF‐VLF emissions are believed to be modulated within the magnetosphere by the compressional component of geomagnetic pulsations formed external to the magnetosphere. We have examined data from South Pole Station (L ∼ 14) to determine the occurrence and characteristics of QP emissions. On the basis of 14 months of data during 1987 and 1988, we found that QP emissions typically appeared in both the 0.5–1 kHz and 1–2 kHz receiver channels at South Pole Station and occasionally in the 2–4 kHz channel. The QP emission frequency appeared to depend on solar wind parameters and interplanetary magnetic field (IMF) direction, and the months near fall equinox in both 1987 and 1988 showed a significant increase in the percentage of QP emissions only in the lowest‐frequency channel. We present a model consistent with these variations in which high‐latitude (nonequatorial) magnetic field minima near the magnetopause play a major role, because the field magnitude governs both the frequency of ELF‐VLF emissions and the whistler mode propagation cutoffs. Because the field in these regions will be strongly influenced by solar wind and IMF parameters, variations in the frequency of such emissions may be useful in providing ground‐based diagnostics of the outer high‐latitude magnetosphere.
Coronal heating, densities, and temperatures and solar wind acceleration
Authors: Hansteen Viggo H. , Leer Egil
Published: February 2004

The outflow of coronal plasma into interplanetary space is a consequence of the coronal heating process. Therefore the formation of the corona and the acceleration of the solar wind should be treated as a single problem. The deposition of energy into the corona through some “mechanical” energy flux is balanced by the various energy sinks available to the corona, and the sum of these processes determines the coronal structure, i.e., its temperature and density. The corona loses energy through heat conduction into the transition region and through the gravitational potential energy and kinetic energy put into the solar wind. We show from a series of models of the chromosphere‐transition region‐corona‐solar wind system that most of the energy deposited in a magnetically open region goes into the solar wind. The transition region pressures and the coronal density and temperature structure may vary considerably with the mode and location of energy deposition, but the solar wind mass flux is relatively insensitive to these variations; it is determined by the amplitude of the energy flux. In these models the transition region pressure decreases in accordance with the increasing coronal density scale height such that the solar wind mass loss is consistent with the energy flux deposited in the corona. On the basis of the present study we can conclude that the exponential increase of solar wind mass flux with coronal temperature, found in most thermally driven solar wind models, is a consequence of fixing the transition region pressure.
Relationship between the dayside auroral electrojets and the DPY current
Authors: Belehaki Anna , Rostoker Gordon
Published: February 2004

Three main electric current systems are known to flow in the high‐latitude ionosphere. These are the auroral electrojets, modulated by the interplanetary magnetic field (IMF) Bz; the DPY currents, modulated by the IMF By; and the Sq current system which is not modulated by the IMF. In the past several years, researchers have tried to establish whether or not there is a relationship between the DPY current and the auroral electrojets, with some researchers contending that the DPY is a separate current system while others believing that the DPY merely reflects a redistribution of the electrojet in a manner regulated by the IMF By. To further study this question, we inspect latitude profiles of the magnetic disturbance field recorded by the CANOPUS array in north central Canada and deal with subsets of these profiles during times of relative constant IMF Bz and positive and negative IMF By polarities. We find evidence for modulation of the auroral electrojets south of the cleft region by IMF By. Moreover, our results suggest that the DPY is part of a twin electrojet system consisting of two antiparallel Hall currents. Both these currents represent an extension across noon of the directly driven electrojets.
Measurement of the energy spectra of cosmic ray electron component and protons at ground level
Authors: Golden R. L. , Stephens S. A. , Stochaj S. J. , Webber W. R. , Brunetti M. T. , Codino A. , Grimani C. , Menichelli M. , Salvatori I. , De Pascale M. P. , Morselli A. , Picozza P. , Basini G. , Bongiorno F. , Brancaccio F. M. , Ricci M. , Ormes J. F. , Streitmatter R. E. , Papini P. , Piccardi S. , Spillantini P.
Published: February 2004

Using a superconducting magnet spectrometer, we have measured the energy spectra of electrons, positrons, and protons at ground level at an atmospheric depth of 945 g/cm². The differential energy spectrum of the electron component has been determined in the momentum interval between 0.1 and 2.0 GeV/c. This spectrum can be described by two power laws, one below 600 MeV with a spectral index of −1.8±0.1 and the other above this energy with an index of −2.9±0.2. The absolute flux values measured here are not in agreement with the earlier results. The fraction of positrons varies from a value of 0.45 at 200 MeV to about 0.5 above 1 GeV, which is consistent with the theoretical expectation. The momentum dependence of the e/µ ratio in the region between 0.25 and 2.0 GeV/c is proportional to p−2.2, and it appears that the soft component of the ionizing radiation might dominate at ground level at kinetic energies below about 70 MeV. The proton energy spectrum has been determined as a power law in kinetic energy between 2.9 and 19.1 GeV with a spectral index of −2.66±0.26. The p/µ ratio obtained from this experiment seems to have a steeper momentum dependence than from previous experiments.
A triple current sheet model for adjoining coronal helmet streamers
Authors: Dahlburg R. B. , Karpen J. T.
Published: February 2004

The highly structured magnetic field and plasma properties observed in the heliospheric extension of the coronal streamer belt have been interpreted as evidence for multiple current sheets originating at coronal helmet streamers. We explore the linear stability of a simple case: a triple current sheet, as would exist above two neighboring helmets of the same polarity. The behavior of the triple current sheet when perturbed by small disturbances can be described (in the incompressible limit) by MHD Orr‐Sommerfeld and Squire equations, which we solve with a Chebyshev‐τ method. We show the velocity and magnetic fields which characterize the three unstable modes and describe the modal dependence on fieldwise wavenumber and current sheet separation. At long wavelengths an unexpected phenomenon occurs: two modes degenerate into unstable traveling modes. We also explore the three‐dimensional behavior and the modal variation with both large and small values of the resistivity and viscosity. We conclude that the magnetic topology in closely packed streamers is susceptible to instabilities with growth times of the order of hours. Our predictions indicate that the resultant plasmoid structures should be observable with the large angle and spectrometric coronagraph (LASCO) and ultraviolet coronagraph spectrometer (UVCS) instruments on the upcoming Solar and Heliospheric Observatory (SOHO) mission.
Middle and upper atmosphere radar observations of the dispersion relation for ionospheric gravity waves
Authors: Oliver W. L. , Fukao S. , Sato M. , Otsuka Y. , Takami T. , Tsuda T.
Published: February 2004

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere radar, we have been able to track the passage of gravity waves and measure their propagation characteristics. Here we investigate the dispersion relation for these waves. This relation varies widely from day to day, even on two successive magnetically extremely quiet days. On average, however, we find that the wave horizontal phase trace speed is about 200 m/s for periods from 60 to 130 min with little variation with solar or magnetic activity. There is some indication that the speed increases slightly from equinox to winter.
A generalized transport model of plasma flow between Saturn's ionosphere and inner magnetosphere
Authors: Demars H. G.
Published: February 2004

Bi‐Maxwellian based 16‐moment transport theory has been used extensively during the last 10 years to describe plasma flows in the terrestrial ionosphere and plasmasphere and in the solar wind. Here it is applied to the field‐aligned flow of an electron‐proton plasma between Saturn's ionosphere and inner magnetosphere. Assuming steady state conditions, numerical solutions to the model have been obtained for a wide variety of flow conditions corresponding to both subsonic outflow and subsonic inflow of plasma. In particular, the effect of varying the electron and proton temperature gradients at the ionospheric boundary has been studied, assuming a wide range of boundary densities and temperatures. The resulting electron and proton thermal structures at high altitudes exhibit both similarities and differences when compared to 16‐moment theoretical predictions for the terrestrial ionosphere.
A stationary fluid model of field‐aligned electric fields and closure of Birkeland currents
Authors: Neukirch T. , Birk G. T. , Finger K. , Schindler K.
Published: February 2004

We introduce and analyze a two‐dimensional stationary self‐consistent model of the coupling of the magnetosphere and the ionosphere within the framework of fluid theory. This model contains localized field‐aligned electric fields as well as a steady state current circuit closing in the ionosphere. For the acceleration region above the ionosphere the magnetic field is determined by the solution of a Grad‐Shafranov equation, whereas the flow field and the electric field are determined by a set of integrals along the magnetic field lines. At ionospheric heights the dissipative interaction of the plasma with the neutral gas is included. We show that the problem may again be reduced, in this case to a coupled system of two nonlinear differential equations plus the continuity equation. Within this theory, simple examples of solutions are presented to demonstrate the capabilities of the model.
On energetic particle flux variations inside a magnetic cloud
Authors: Vandas M. , Fischer S. , Geranios A.
Published: February 2004

Helios 1 observed a magnetic cloud on the spacecraft's closest approach to the Sun (0.3 AU) on March 26, 1976. Fluxes of energetic particles detected by the spacecraft generally decrease during the passage of a cloud, but in this case the smooth intensity depression was interrupted by a sharp peak‐like increase coinciding with a deep drop of the magnetic field magnitude. This behavior is observed for protons and electrons in several energy bands. A possible explanation of this event is given by the spheroidal model of magnetic clouds, applicable for the plasmoid geometry (having field lines fully disconnected with the Sun).
An improved kinetic model for the polar outflow of a minor ion
Authors: Lie‐Svendsen Ø. , Rees M. H.
Published: February 2004

We have studied the escape of a minor ion from the terrestrial atmosphere by outflow along geomagnetic field lines that are open to the interplanetary medium. Coulomb collisions with the background ions are treated rigorously in the Fokker‐Planck transport equation which is adopted in the transition region between the collision‐dominated low‐altitude regime (treated by moment equations) and the collisionless regime at very high altitudes. Sample solutions are presented for both H+ and He+ ions in an O+ background. In the transition region and above, the distribution functions are not well represented by low‐order approximations, such as drifting Maxwellian distributions. For He+, while the densities and drift speeds are sensitive to the velocity distribution, the flux is a robust quantity that depends primarily on the altitude of the transition between the region of chemical equilibrium and no charge exchange loss.
Association of discrete hard X ray enhancements with eruption of Mount Pinatubo
Authors: Feldman W. C. , Symbalisty E. M. D. , Roussel‐Dupré R. A.
Published: February 2004

Hard X ray fluxes in the energy range between 50 keV and 140 keV measured using the Army Background Experiment are found to be associated with the injection by Mount Pinatubo of a massive aerosol into the stratosphere June 14–15, 1991. Discrete X ray enhancement events are observed to increase from an average of 1.8 per month to 20 during a 1‐month interval beginning 12 days after the Mount Pinatubo eruption. Systematics of these enhancements suggests a mechanism that requires a strong coupling between magnetospheric and atmospheric processes, perhaps associated with upward lightning.
Magnetopause crossings without a boundary layer
Authors: Eastman T. E. , Fuselier S. A. , Gosling J. T.
Published: February 2004

The microstructure of pristine magnetopause crossings has been analyzed by using high‐resolution particle and field data obtained by the Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer (CCE) and ISEE 2 spacecraft. These crossings are pristine in the sense that they exhibit no adjoining magnetospheric boundary layer, or, at most, a low‐density plateau. The CCE crossings include the low‐latitude near‐noon region not typically sampled by ISEE 2, which covers all other local time sectors in a complementary way. Magnetopause crossings without a boundary layer are found to occur to all local times, and such crossings constitute about 10% of all magnetopause crossings. Total pressure balance across the magnetopause is observed to within experimental errors; however, electron data, full‐energy composition measurements, and occasionally field stress are needed to fully evaluate pressure balance. The microstructure of the magnetopause current layer is also found to depend on local time. Crossings within about 1 hour local time of the noon meridian often exhibit very sharp density gradients on scale lengths down to a few plasma skin depths. These gradients are reduced for crossings farther from local noon such that, for cases near the dawn‐dusk meridian, the scale length for density gradients and the magnetopause current are roughly comparable. Magnetopause crossings without a boundary layer impose severe constraints on various theories of boundary layer formation. Pristine magnetopause crossings may be direct cuts through the diffusion region for reconnection. With this interpretation our results are in qualitative agreement with recent simulations of the diffusion region and associated turbulence by Drake et al. [1994], who propose the current convective instability as the dominant process for current transport at the magnetopause.