bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron heating and phase space signatures at strong and weak quasi‐perpendicular shocks
Authors: Hull A. J. , Scudder J. D. , Frank L. A. , Paterson W. R. , Kivelson M. G.
Published: February 2004

The coherent effects implied by the magnetic field jump [B] and the deHoffmann‐Teller frame (HTF) potential jump [ΦHT] on electron heating and phase space signatures at shocks of different strengths are presented. Of particular interest is whether these coherent effects have sufficiently different signatures to explain the observed preferential transverse heating in weak shocks while still producing nearly isotropic heating for strong shocks. Vlasov‐Liouville mappings of an upstream electron core‐halo distribution function ƒ1, modified to reflect electron mirroring, are employed to determine the downstream electron distribution function ƒ2. Electrons within the shock are treated as a laminar Vlasov guiding center ordered fluid. These mappings demonstrate that the coherent effects play a major role in producing, at all pitch angles, characteristic electron distribution function signatures observed behind strong and weak shocks and thus significantly impact electron heating. A favorable test of the Vlasov‐Liouville mapping concept was performed using detailed upstream and downstream distribution functions at a weak bow shock observed by Galileo on the second Earth flyby. Especially noteworthy is that the Vlasov‐Liouville procedure recovers the anisotropic inflationary signatures of the observed downstream electron distribution function.
Generation of electromagnetic perturbations by acoustic waves in the ionosphere
Authors: Aburjania George D. , Machabeli George Z.
Published: February 2004

A new mechanism of transformation of long‐wavelength acoustic waves into electromagnetic in partially ionized plasmas is suggested. The range of geomagnetic pulsation frequencies has been studied. It is shown that in collective motion acoustic wave involves the charged particles of media by means of the collisions. Relative motion of charged particles excites the alternating current with arbitrary phase, and consequently, the parametric generation of electromagnetic fields occurs. The growth rates, excitement thresholds, spectra, and polarization of electromagnetic disturbances are determined. The suggested mechanism not only has generalphysical importance, but it can also explain the observed data and the prediction of expected character of atmospheric waveprocesses before the earthquake.
Evidence for recombination as a significant source of metastable helium
Authors: Noto J. , Kerr R. B. , Shea E. M. , Waldrop L. S. , Fisher G. , Rudy R. J. , Hecht J. H. , Gonzalez S. A. , Sulzer M. P. , Garcia R.
Published: February 2004

A Fabry‐Perot spectrometer optimized for maximum near infrared (NIR) transmission and featuring a germanium detector with a quantum efficiency of 86% at 10,830 Å is used to measure the brightness and spectral width of the metastable helium emission in the upper thermosphere. Observations are made at the optical facility associated with the Millstone Hill Incoherent Scatter Radar site in Massachusetts and at the optical facility associated with the Arecibo Observatory in Arecibo, Puerto Rico. Using a spectral resolution of 0.117 Å, these are the first observations to isolate the metastable helium triplet 2p3P0,1,2–2s3S1 from nearby OH airglow contamination. Measured 10,830 Å brightness decays with shadow in a manner consistent with an emission that is due to resonant scattering of the solar 10,830 Å emission. Although the technique we describe was conceived as a powerful tool to measure upper thermospheric and exospheric temperatures, the data indicate a surprising increase of temperature with shadow height to values much greater than commonly ascribed to the near solar minimum thermosphere. We assert that the broad 10,830 Å spectral line widths, retrieved when the shadow heights are above ∼300 km, are due to a nonthermal metastable helium population. This source of hot, metastable helium is very likely He+ recombination in the twilight. That conclusion is contrary to an historical understanding that the dominant source of metastable helium is the impact of photoelectrons with energies in excess of 19.8 eV with ground state helium.
Broadband plasma waves observed in the polar cap boundary layer: Polar
Authors: Tsurutani B. T. , Lakhina G. S. , Ho C. M. , Arballo J. K. , Galvan C. , Boonsiriseth A. , Pickett J. S. , Gurnett D. A. , Peterson W. K. , Thorne R. M.
Published: February 2004

Polar observations indicate the presence of intense broadband plasma waves nearly all of the time (96% occurrence frequency in this study) near the apogee of the Polar trajectory (∼6–8 RE). The region of wave activity bounds the dayside (0500 to 1800 LT) polar cap magnetic fields, and we thus call these waves polar cap boundary layer (PCBL) waves. The waves are spiky signals spanning a broad frequency range from ∼101 to 2 × 104 Hz. The waves have a rough power law spectral shape. The wave magnetic component has on average a ƒ−2.7 frequency dependence and appears to have an upper frequency cutoff of ∼(6–7) × 103 Hz, which is the electron cyclotron frequency. The electric component has on average a ƒ−2.2 frequency dependence and extends up to ∼2 × 104 Hz. The frequency dependences of the waves and the amplitude ratios of B′/E′ indicate a possible mixture of obliquely propagating electromagnetic whistler mode waves plus electrostatic waves. There are no clear intensity peaks in either the magnetic or electric spectra which can identify the plasma instability responsible for the generation of the PCBL waves. The wave character (spiky nature, frequency dependence and admixture of electromagnetic and electrostatic components) and intensity are quite similar to those of the low‐latitude boundary layer (LLBL) waves detected at and inside the low‐latitude dayside magnetopause. Because of the location of the PCBL waves just inside the polar cap magnetic field lines, it is natural to assume that these waves are occurring on the same magnetic field lines as the LLBL waves, but at lower altitudes. Because of the similar wave intensities at both locations and the occurrence at all local times, we rule out an ionospheric source. We also find a magnetosheath origin improbable. The most likely scenario is that the waves are locally generated by field‐aligned currents or current gradients. We find a strong relationship between the presence of ionospheric and magnetosheath ions and the waves near the noon sector. These waves may thus be responsible for ion heating observed near the cusp region. Antisunward convection of these freshly accelerated oxygen ions over the polar cap during intense wave events (occurring during southward Bz events) might lead to enhanced plasma sheet O+ population. For magnetic storm intervals this mechanism would lead to a natural delay between the main phase onset and the appearance of oxygen ions in the ring‐current.
A comparison of recurrent energetic ion enhancements observed at Ulysses and at 1 AU by IMP 8 and SAMPEX: Ulysses launch until following the first north polar passage
Authors: Richardson I. G. , Mazur J. E. , Mason G. M.
Published: February 2004

During the Ulysses mission, the Goddard Space Flight Center instrument on IMP 8 and the University of Maryland low energy ion composition analyzer instrument on SAMPEX observed recurrent MeV/amu ion enhancements in the ecliptic plane at 1 AU. We identify those IMP 8/SAMPEX events related to the recurrent events observed at Ulysses from its launch until mid‐1996, following the first passage over the north solar pole, and compare their properties. Two types of recurrent events were present at Earth. Recurrent enhancements of particles streaming sunward from CIRs in the outer heliosphere were observed most clearly at Earth after the Ulysses Jupiter encounter. Most of the CIRs/particle enhancements at Ulysses, even at high heliolatitudes, can be associated with recurrent events/high‐speed streams at Earth by considering simple corotation delays. This suggests that the events at high latitudes occurred poleward of equatorward extensions of the polar coronal holes which produced corotating high‐speed streams in the ecliptic. The association between events at Ulysses and Earth was less clear in 1996 when there were few major streams from polar coronal hole extensions present near the ecliptic. At the furthest distance of Ulysses from the Sun (5.4 AU), the intensity of ∼1 MeV/amu ions in CIR‐associated events was 1–2 orders of magnitude higher than at Earth. The intensity at Ulysses then declined as the spacecraft moved closer to the Sun and to high latitudes to reach < 0.1 % of that at Earth near polar passage. The observations confirm that the decrease in the intensity of corotating events at high latitudes was predominantly due to the changing spacecraft latitude rather than to a temporal decline in the intensity of corotating events throughout the heliosphere. We suggest that some of the north‐south asymmetries in the particle events observed by Ulysses at high latitudes may be associated with differences in the solar wind structure and energetic particle events at low latitudes during north and south polar passage. In contrast to these events, the prominent recurrent events observed at Ulysses and at Earth during the second half of 1991 were associated with solar activity which, over a period of several solar rotations, was largely confined to a limited range in Carrington longitude.
An initial look at the Iogenic SO2+ source during the Galileo fly by of Io
Authors: Smyth William H. , Marconi M. L.
Published: February 2004

Galileo in its December 7, 1995, encounter with Io flew downstream of Io and through the magnetospheric wake of the satellite with a closest approach altitude of ∼900 km. Magnetospheric instruments were therefore able to sample the Iogenic plasma source both outside as well as deeply within the Lagrange sphere of Io (∼5.81 satellite radii) where the gravity of Io dominates and where the plasma pickup processes are expected to be highly peaked about the satellite. The presence of both long‐lived atomic ions (H+, O+, O++, S+, S++) and short‐lived molecular ions (SO+, SO2+) was detected along the spacecraft trajectory. We have undertaken preliminary calculations for the density profile of SO2+. These calculations are compared with the SO2+ density profiles deduced from magnetic field fluctuations with periods of ∼2–3 s measured by the Galileo magnetometer and interpreted as ion cyclotron waves produced by fresh SO2+ Iogenic pickup ions created near Io. By matching the absolute SO2+ model density with the minimum ion density determined by Huddleston et al. [1997] in their analysis of the ion cyclotron waves, an SO2 source rate of ∼4 × 1027 molecules s−1 (425 kg s−1) at Io's exobase and a corresponding SO2+ source rate of 2.8 × 1026 ions s−1 (30 kg s−1) in the magnetosphere are determined. Most of the SO2 that undergoes interactions in the plasma torus is, however, rapidly dissociated primarily by electron impact, producing O, S, SO, and O2. These species subsequently undergo ionization and charge exchange reactions in the plasma torus, producing much larger mass and energy pickup plasma loading rates, including an SO+ source rate estimated to be somewhat smaller than the SO2+ source rate. Since the lifetime of SO2 is highly variable with Io's position in the plasma torus, it follows that the spatial profile for the amplitude of these magnetic fluctuations will also be highly space and time variable and will depend upon both Io System III longitude and Io geocentric phase angle.
Simultaneous 50‐MHz coherent backscatter and digital ionosonde observations in the midlatitude E region
Authors: Hussey G. C. , Schlegel K. , Haldoupis C.
Published: February 2004

SESCAT, a coherent backscatter radar system located in Crete, Greece, was operated for one summer together with a (CADI) digital ionosonde observing nearly the same scattering volume. The purpose of the experiment was to further investigate the origin of midlatitude E region VHF echoes which occur almost exclusively during summer nighttime. It was found that 50‐MHz midlatitude backscatter always occurs in association with sporadic E layers. A statistical analysis indicated significant correlations between SESCAT total echo power and Es characteristics such as the layer's top frequency ƒtEs (a measure of maximum Es electron density) and the apparent Es trace spread which results from range spreading due to oblique reflections from a nonuniform and horizontally inhomogeneous layer. Similar correlations were obtained for SESCAT spectral width and the same sporadic E characteristics. The experiment confirmed that the presence of an Es layer in the scattering volume, which could provide destabilizing electron density gradients perpendicular to the magnetic field, is necessary but not sufficient for the occurrence of 50‐MHz backscatter. We suggest that in addition there is need for an enhanced electric field to be present inside the layer as well, a notion that is in line with the observed correlation of backscatter with a dense but strongly inhomogeneous Es layer and a recently proposed mechanism for strong polarization fields at midlatitudes.
Multisatellite and ground‐based observations of a tailward propagating Pc5 magnetospheric waveguide mode
Authors: Mann Ian R. , Chisham Gareth , Bale Stuart D.
Published: February 2004

Using data from the Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer (CCE) satellite near local noon, and the AMPTE Ion Release Module (IRM) and United Kingdom Subsatellite (UKS) on the morning flank, we investigate the tailward propagation of a compressional Pc5 wave (380 s period) on October 28, 1984, and suggest that the observation represents a magnetospheric waveguide mode. The observed wave is a transient oscillation lasting about five wave cycles with an observed time of flight from CCE to IRM/UKS of 180 s. The first three or four cycles at the leading edge of the event display a remarkable wave packet coherence between CCE and IRM/UKS despite their large separation in the magnetosphere (∼ 6.2 RE). A time of flight analysis suggests a waveguide mode group speed ∼ 220 km s−1, being much less than the local Alfvén speed and comparable to the expected sub‐Alfvénic propagation speed of a waveguide mode. The ground‐based signature of the event is also observed near local noon by the European Incoherent Scatter magnetometer cross, displaying the same period and lasting for the same number of cycles. The ground‐based data show no evidence of a field line resonance; the event is monochromatic, and there is little variation of amplitude and polarization with latitude. An increase in solar wind ram pressure of ∼ 30% is observed by IMP 8 upstream in the solar wind just prior to the event, and this may have provided an impulsive energy source for the waveguide mode. This is the first time the signature of a particular waveguide mode harmonic has been observed by satellites which are widely spaced in the magnetosphere. Moreover, we present the first unambiguous observation of the downtail propagation of a waveguide mode. The observation also provides possible evidence of dispersion in the Earth's outer magnetospheric waveguide.
Numerical simulations of midlatitude ionospheric perturbations produced by gravity waves
Authors: Huang Chao‐Song , Sofko George J. , Kelley Michael C.
Published: February 2004

We have used a computer simulation to study ionospheric perturbations produced by gravity waves in the midlatitude F region. The change of amplitude and vertical wavelength of gravity waves with height is taken into account. The presence of plasma diffusion causes the height profile of the wave‐induced ionospheric perturbations to be quite different from the forcing gravity waves but does not lead to field alignment of the ionospheric structures. The attenuation of the amplitude of gravity waves with height does not cause the upturning of the phase surfaces of the wave‐induced ionospheric perturbations. The upturning of the phase surface of ionospheric perturbations produced by a gravity wave is caused solely by the increase with height of the vertical wavelength of the wave, regardless of whether the gravity wave amplitude increases or decreases with height. The amplitude of gravity‐wave‐induced ionospheric perturbations is smaller at higher altitudes than at lower altitudes because of the presence of plasma diffusion even if the gravity waves have larger amplitudes at high altitudes. Strong ionospheric density gradients can be produced in the altitude range of 200–400 km. The numerical results are compared with the values predicted by theory. If the gravity waves are uniform in space, the theory gives reasonable estimates of the relative ionospheric density fluctuations. Even when plasma diffusion is present, deviation of the numerical results from the theory is generally <10%. However, if a more realistic gravity wave model in which the amplitude and wavelength vary with height is used in the simulations, significant differences, as much as 50% for daytime ionospheric perturbations, arise between the numerical and theoretical results.
Statistical analysis of the plasmoid evolution with Geotail observations
Authors: Ieda A. , Machida S. , Mukai T. , Saito Y. , Yamamoto T. , Nishida A. , Terasawa T. , Kokubun S.
Published: February 2004

Plasmoids in the Earth's magnetotail were studied statistically, using low energy particle (LEP) and magnetic field (MGF) data from the Geotail spacecraft. Their evolution along the tail axis from XGSM′ = −16 to −210 RE was investigated with 824 plasmoid events. Their dependence on YGSM′ was studied as well to derive the three‐dimensional structure of the plasmoids. (The coordinates are aberrated to remove the average effects of the orbital velocity of the Earth about the Sun.) We defined a plasmoid as a structure with rotating magnetic fields and enhanced total pressure. In the near tail (XGSM′ ≥ −50 RE), there was a tendency for the plasmoids to be observed in the premidnight sector around the tail axis (|YGSM′ − 3| ≤ 10 RE), while they were observed widely (|YGSM′ ≤ 20 RE) in the middle tail (−50 > XGSM′ ≥ −100 RE) and in the distant tail (−100 RE > XGSM′). The plasmoids expanded in the ±YGSM′ direction with typical velocities of ±130 ± 100 km/s in the near tail. This strongly supports the view that plasmoids are initially formed at the near‐Earth neutral line which has a limited extent in the YGSM′ direction. The plasmoids accelerated in the downtail direction from 400 ± 200 km/s to 700 ± 300 km/s from the near to the middle tail. Then, it is suggested that they decelerated to 600 ± 200 km/s as they traveled to the distant tail. The ion temperature inside plasmoids was 4.5 ± 2 keV in the near and middle tail, and then rapidly decreased to 2 ± 1 keV from the middle to the distant tail region. The ion temperature in the distant tail was 2 times higher than the values deduced previously. Typical plasmoid dimensions were estimated to be 10 RE (length) × 40 RE (width) × 10 RE (height) in the middle and distant tail. The energy carried by each plasmoid was ∼2 × 1014 J in the middle tail, half of which was lost from the middle to the distant tail. Inside plasmoids, the thermal energy flux exceeded the bulk energy flux and Poynting flux. The energy released tailward in the course of a substorm was estimated to be roughly 1015 J.
Centrifugally driven instability of a rotationally dominated magnetodisc
Authors: Liu W. William
Published: February 2004

The dynamics of a rorationally dominated magnetodisc is considered on the basis of linearized MHD. The treatment is prompted by the concern that the dominant theoretical view on this subject is based on the reduced framework of interchange motion, in which changes in magnetic field are suppressed. A realistic perturbation of a magnetodisc generally does not satisfy this constraint, and the changing magnetic field is expected to stabilize any small‐amplitude perturbation. We reexamine the problem in an idealized slab magnetodisc geometry but with dynamical terms fully installed in the MHD equations. We find that the traditional interchange instability, essentially a centrifugal buoyancy regulated by the ionosphere, is not a proper solution for a thin magnetodisc. For short‐wavelength modes the actual situation of instability comes to resemble what is known in the literature as magnetic buoyancy, i.e., a centrifugally driven motion moderated by the strong local magnetic field. Physically, the instability results as the rotational effects couple the Alfvén and slow modes; density perturbations formed by the latter are in turn driven radially by centrifugal buoyancy. The instability is manifest even for a perfectly uniform density distribution and for the characteristic parameters near the Io torus, dominates over the contribution from a radial density gradient. The theory of centrifugally driven radial diffusion is considered with this instability as the mechanism of eddy formation. This consideration gives a mass transport rate of ∼ 103 kg/s and a density profile n ∼ L−4 near the Io torus, both in good agreement with observations. Further computational works need to be carried out to compare our model with more detailed in situ observations.
Nitric oxide abundance in the mesosphere/lower thermosphere region: Roles of solar soft X rays, suprathermal N(4S) atoms, and vertical transport
Authors: Swaminathan P. K. , Strobel D. F. , Kupperman D. G. , Kumar C. Krishna , Acton L. , DeMajistre R. , Yee J.‐H. , Paxton L. , Anderson D. E. , Strickland D. J. , Duff J. W.
Published: February 2004

This paper carefully examines the inability of photochemical models to account for the large nitric oxide densities of ∼108 cm−3 at ∼105 km obtained from IR, UV, and microwave measurements. A detailed and up‐to‐date photochemical model is constructed that incorporates measured YOHKOH soft X ray fluxes, hot N atom chemistry with an energy dependent thermalization cross section and seven reaction sources, and laboratory‐constrained N(2D) yields. The resulting model which has well‐constrained chemistry compared to past models fails to generate high enough NO densities in comparison with the most reliable measurements of absolute NO concentrations in the lower thermosphere. The sensitivity of the model results and the known uncertainties in the inputs are used to identify where future efforts should be focused. A deficit remains despite an increase in the vertical mixing rates in the lower thermosphere from the very low Kzz profile used in our calculations and/or an increase in the N(2D) yield from electron impact dissociation of N2 from its nominal value of 0.54 to 0.62. The sensitivity of NO profiles to the nascent energy distributions of the atmospheric sources of suprathermal N atoms is illustrated by including the thermalization of suprathermal N atoms with an updated thermalization cross section. The diurnally averaged NO concentration at 105 km is enhanced by factors of 1.2 and 2.6 when the energy distributions of the N atoms from electron impact dissociation of N2 are chosen with peaks near 0.6 eV or 3–4 eV, but deficits of factors of ∼7 and ∼3, respectively, remain. There is higher sensitivity to vertical transport than to variations of chemistry within known uncertainties.
Electron density at the subsolar magnetopause for high magnetic shear: ISEE 1 and 2 observations
Authors: Hubert D. , Harvey C. C. , Roth M. , De Keyser J.
Published: February 2004

The ISEE radio wave propagation electron density experiment allowed the determination of the integrated electron density between the ISEE 1 and ISEE 2 satellites at the relatively high rate of 8 or 32 Hz. When the component of the spacecraft separation vector in the direction of the normal to the magnetopause is significantly smaller than the thickness of the current layer, this data set allows the internal structure of the magnetopause to be studied and compared with theoretical predictions. For a particular triple subsolar magnetopause crossing with high magnetic shear, an electron density overshoot is observed in the current layer adjacent to the magnetosheath. The similarity of the three crossings indicates that the internal structure of the magnetopause does not change dramatically during the time interval considered. A superposed epoch analysis of these crossings is consistent with the density profile obtained from kinetic simulations. The general relationship between magnetic field asymmetry, magnetic field rotation angle and electron density overshoot is discussed. It is concluded that a density overshoot could be a typical feature of the subsolar magnetopause with high magnetic shear. This conclusion is supported by two other dayside magnetopause crossings for which high time resolution electron density data are available.
Coronal mass ejections, magnetic clouds, and relativistic magnetospheric electron events: ISTP
Authors: Baker D. N. , Pulkkinen T. I. , Li X. , Kanekal S. G. , Blake J. B. , Selesnick R. S. , Henderson M. G. , Reeves G. D. , Spence H. E. , Rostoker G.
Published: February 2004

The role of high‐speed solar wind streams in driving relativistic electron acceleration within the Earth's magnetosphere during solar activity minimum conditions has been well documented. The rising phase of the new solar activity cycle (cycle 23) commenced in 1996, and there have recently been a number of coronal mass ejections (CMEs) and related “magnetic clouds” at 1 AU. As these CME/cloud systems interact with the Earth's magnetosphere, some events produce substantial enhancements in the magnetospheric energetic particle population while others do not. This paper compares and contrasts relativistic electron signatures observed by the POLAR, SAMPEX, Highly Elliptical Orbit, and geostationary orbit spacecraft during two magnetic cloud events: May 27–29, 1996, and January 10–11, 1997. Sequences were observed in each case in which the interplanetary magnetic field was first strongly southward and then rotated northward. In both cases, there were large solar wind density enhancements toward the end of the cloud passage at 1 AU. Strong energetic electron acceleration was observed in the January event, but not in the May event. The relative geoeffectiveness for these two cases is assessed, and it is concluded that large induced electric fields (∂B/∂t) caused in situ acceleration of electrons throughout the outer radiation zone during the January 1997 event.
Plasma entry from the flanks of the near‐Earth magnetotail: Geotail observations
Authors: Fujimoto M. , Terasawa T. , Mukai T. , Saito Y. , Yamamoto T. , Kokubun S.
Published: February 2004

Geotail observations of the low‐latitude boundary layer (LLBL) in the near–Earth tail flanks are reported. Cold‐dense stagnant ions, which are likely to be of the magnetosheath origin, are detected in this region of the magnetosphere. Charge neutrality is maintained by accompanying dense thermal (< 300 eV) electrons presumably also from the magnetosheath. Compared to the magnetosheath component, however, the electrons are anisotropically heated to have enhanced bidirectional flux along the field lines. The enhanced bidirectional flux is well balanced, and this fact, together with the slow convection, suggest the closed topology of the field lines. In addition to these common characteristics, a dawn‐dusk asymmetry is observed in data for several keV ions, which is attributed to the dawn‐to‐dusk cross tail magnetic drift of the plasma sheet ions. We also show a case that strongly suggests that this entry of cold‐dense plasma from the magnetosheath via near‐Earth tail flanks can be significant at times. In this case, the cold‐dense plasma is continuously detected as the spacecraft moves inward from the magnetospheric boundary to deep inside the magnetotail. By referring to the solar wind data showing little dynamic pressure variation during the interval, we interpret the long duration of the cold‐dense status as indicative of a large spatial extent of the region: The cold‐dense plasma is not spatially restricted to a thin layer attached to the magnetopause (LLBL) but constitutes an entity occupying a substantial part of the magnetotail, which we term as the cold‐dense plasma sheet. The continuity of the cold‐dense plasma all the way from the boundary region supports the idea that the magnetosheath plasma is directly supplied into the cold‐dense plasma sheet through the flank.
Three‐dimensional kinetic simulation of the nonlinear evolution of lower hybrid pump waves
Authors: Singh Nagendra , Wells B. Earl , Abdelrazek A. , Al‐Sharaeh S. , Leung W. C.
Published: February 2004

Nonlinear evolution of lower hybrid (LH) waves is studied by means of a fully three‐dimensional parallel particle‐in‐cell (PPIC) code. The plasma is driven by a monochromatic LH pump wave, which drives secondary LH and low‐frequency waves having a broad frequency spectrum from
, to ω∼ωo >ωlh, where Ωi, ωo and ωlh are the ion cyclotron, pump and LH resonance frequencies, respectively. The temporal variations in the electric field components show both amplitude and phase modulations. In a plasma with equal electron and ion temperatures the dominant amplitude modulation occurs at the ion cyclotron timescale τci. The pondermotive force associated with the vector nonlinearity arising from theE ×B drift of electrons is seen to generate both density depletions and enhancements depending on the time‐varying phase difference between the orthogonal electric field componentsEx andEy transverse to the ambient magnetic fieldBo in thez direction. Despite the use of quite strong pump wave amplitude, wave collapse in density cavities alone is not seen; instead, equally strong density cavities (cavitons) and enhancements (pilons) occur quasi‐periodically both in time and space. The phase difference betweenEx andEy and its evolution yield a rotating transverse electric field vector with hodograms ofEx andEy changing with time. The temporal evolution of the parallel acceleration of electrons and transverse heating of ions are discussed. For relatively slow pumps the electron acceleration is predominantly unidirectional parallel to the pump phase velocityVp‖o. On the other hand, for sufficiently large pump phase velocities the acceleration becomes bidirectional. The parallel electron acceleration up toV‖max ∼ 2Vp‖o is common, and the transverse ion acceleration occurs up toV⊥max ≅(m/M)1/2Vp‖o ≅Vp⊥o, wherem andM are the electron and ion mass, respectively. The relevance of the above results to the observations on LH waves and their role in electron and ion accelerations is discussed.
A study of the effect of pitch angle and spatial diffusion on tearing instability using a new finite element based linear code
Authors: Brittnacher M. , Quest K. B. , Karimabadi H.
Published: February 2004

Although the collisionless tearing mode has been a leading candidate as the mechanism responsible for substorm expansion phase onset, it has not been established that the near‐Earth plasma sheet can become unstable to ion tearing owing to electron compression stabilization. The observation that the electron orbits become stochastic when the current sheet thins prior to substorm onset has given impetus to a view that pitch angle diffusion might overcome electron compression sufficiently to establish an ion mode. Theoretical attempts to demonstrate that intrinsic stochasticity in the electron orbits can destabilize the tearing mode have not been upheld without controversy. In order to examine the effect of pitch angle scattering and spatial diffusion on the tearing mode, we have developed a new linear code based on the finite element technique. This code incorporates numerical particle orbits and overcomes many of the usual difficulties associated with solving kinetic linear stability problems. We find that neither intrinsic stochasticity in the particle orbits nor externally imposed pitch angle scattering can destabilize the ion mode. The electron mode, however, can be reestablished by pitch angle diffusion, although only for values of the normal magnetic field component that are too small to be of physical significance. We also show that for the observed values of the normal magnetic field, the magnetotail is linearly stable to ion tearing even in the presence of a large spatial diffusion. The possibility of ion tearing due to nonlinear and/or three‐dimensional effects cannot, however, be ruled out at this time.
Thermal coupling of protons and neutral hydrogen in the fast solar wind
Authors: Allen Lorraine A. , Habbal Shadia R. , Hu You Qiu
Published: February 2004

We investigate the coupling between neutral hydrogen atoms and protons in the corona for a range of proton temperatures reaching a maximum of 6 × 106 K, as recently inferred from observations of the Ly α spectral line profiles by Kohl et al. [1996]. We adopt the approach used by Olsen et al. [1994], whereby the neutral hydrogen atoms are treated as test particles in a background electron‐proton solar wind. Charge exchange between neutrals and protons, radiative recombination, collisional ionization of the neutrals, and the effects of Alfvén waves, described by a single frequency or a spectrum, are included in the model. The computations show that an anisotropy in the neutral hydrogen temperature develops in the directions parallel and perpendicular to the magnetic field within 2–3 RS for solar wind conditions, consistent with observational constraints of the fast wind. Although exceeds in the inner corona, the anisotropy decreases as the peak proton temperature increases, with a temperature difference of <8 × 105 K when the protons reach 6 × 106 K. We find that the effective temperature , incorporating both random and wave motions of the neutral hydrogen, and the calculated Ly α line profile are independent of wave frequency. The dominant contribution to the profiles comes from at the point of closest approach to the Sun along the line of sight (LOS), although the widths of the profiles are significantly narrowed by non‐90° scattering along the LOS, broadened by outflow velocity, and narrowed or broadened by steep temperature gradients along the LOS. The proton and neutral hydrogen effective temperatures and velocities are comparable below ∼3 RS, thus implying that the measured Ly α profiles are equivalent to measurements of the velocity distribution of protons in that region. Beyond ∼3 RS, however, is found to be significantly lower than Tp(eff), and the measurement of provides a lower limit for Tp(eff) there.
Spectra of type‐B red lower borders
Authors: Hallinan T. J. , Kimball J. , Osborne D. , Deehr C. S.
Published: February 2004

A low‐light‐level television camera and a television spectrograph were used to study the variation of color with elevation angle in auroras having type‐B red lower borders. It was found that much of what is commonly described as type‐B aurora results from a simple separation between the prompt red and blue molecular nitrogen emissions on the one hand and the persistent green oxygen line on the other hand. The red and blue emissions combine to produce magenta which appears on the leading edge of rapidly moving auroral features. True type‐B aurora, not dependent on motion, was observed to result from a suppression of the green line at low altitudes, presumably due to quenching. While the quenching of the green is the predominant effect, there are, in addition, changes in the shade of magenta resulting from an enhanced red/blue ratio associated with the phenomenon of auroral hems (bands of enhanced luminosity along the lower borders of auroral curtains). In one case there was also evidence for a redistribution of the intensities of the nitrogen first positive bands, possibly by intercollisional transfer of energy from the W states to the B states as proposed by Benesch [1981]. In sum, evidence was found to support each of the prominent theories for type‐B auroras and that the various mechanisms can coexist.
Identification of mirror waves by the phase difference between perturbed magnetic field and plasmas
Authors: Lin C.‐H. , Chao J. K. , Lee L. C. , Wu D. J. , Li Y. , Wu B. H. , Song P.
Published: February 2004

A new approach is proposed to identify mirror waves based on the 90° phase difference between the magnetic field and plasma fluctuations. Magnetosheath observations, the linear MHD theory, and one‐dimensional hybrid simulations support that the 90° phase difference is a special character for mirror waves. This phase difference is independent of Doppler shift which means that our result remains the same in both the satellite (laboratory) and the plasma frames of reference. Therefore it is useful to distinguish mirror waves from slow waves which also have an anticorrelation (i.e., 180° out of phase) relationship between magnetic field strength and plasma density fluctuations. The time evolution in the later stage shows that the dominant perturbed components are the magnetic field fluctuation along the background magnetic field B0 and the velocity fluctuation perpendicular to B0 and the wave vector k. A higher coherence and a stable 90° phase difference between these two dominant components are found as time increases. The coherence between this velocity component and the plasma density is also high with a 90° phase difference. A tendency is found that the longer wavelengths are progressively generated when mirror waves grow in simulation.
Asymmetric propagation of flare‐generated shocks in the heliospheric equatorial plane
Authors: Hu Y. Q.
Published: February 2004

A numerical simulation study is presented in this paper in terms of a two‐dimensional, two‐component MHD model, concerning the propagation of flare‐generated shocks in the heliospheric equatorial plane. The numerical results show that the spiral interplanetary magnetic field and the heliospheric plasma sheet in the equatorial plane have a significant influence on the shock propagation. When the shock source lies east of the heliospheric current sheet (HCS) within a longitudinal distance of 50°∼70°, depending on the shock strength, the fastest propagation direction of the shock deflects eastward from the flare normal by an angle that increases with the initial shock strength and decreases with the distance between the shock source and the HCS. On the other hand, if the shock source lies west of the HCS within a longitudinal distance of 30°, the fastest propagation direction of the shock deflects westward from the flare normal by an angle that decreases with both the initial shock strength and the distance between the shock source and the HCS. For shocks whose source is far away from the HCS their fastest propagation direction always deflects westward from the flare normal, and the deflection angle decreases with the initial shock strength and does not depend on the source position. The theoretical predictions are compared with the conclusions reached by data interpretation of the propagation of flare‐generated shocks in the heliospheric equatorial plane.
Shock profile analysis using wavelet transform
Authors: Gedalin M. , Newbury J. A. , Russell C. T.
Published: February 2004

We study the fine structure of the collisionless shock front, paying particular attention to the large gradients and internal structure within the ramp and to quasiperiodic structures in the ramp vicinity. In order to separate random steep gradients with low amplitudes from steep large‐amplitude formations, we apply Gaussian wavelet transforms suitable for this task. We analyze several high Mach number shocks and show that ramp substructure is not uncommon and that the typical scales of the steepest gradient features are significantly smaller than the shock transition width (provided they are stationary in the shock frame). We apply the Morlet wavelet transform to the magnetic field data in order to identify quasiperiodic patterns which may last only several periods. It is likely that short quasiperiodic wave trains dominate in the wave activity. Such wave trains are not always associated with the ramp itself.
An explanation for the east–west asymmetry of the Io plasma torus
Authors: Smyth William H. , Marconi M. L.
Published: February 2004

One of the most interesting and unexplained spatial structures in the Io plasma torus located near Io's orbit about Jupiter is the east–west (or alternatively dawn‐dusk) asymmetry in the planetocentric distance of the so called plasma “ribbon,” the brightest and most prominently observed radial feature of the torus. The average radial position of the ribbon on the sky plane in both its ground‐based measured S+ optical emission (6716 Å, 6731 Å) and its Voyager measured S++ ultraviolet emission (685 A) is observed to be located closer to the planet and well within Io's orbit when it is west of Jupiter at the dusk (or receding) ansa and farther from the planet and very near Io's orbit when it is east of Jupiter at the dawn (or approaching) ansa. In addition, the ribbon is also observed to move about this average position as a function of its ansa System III longitude. It is shown that the location of this asymmetrical radial structure for the S+ ribbon arises naturally in the presence of an east–west electric field from a space and time dependent plasma source that is highly concentrated at Io's instantaneous orbital location (and hence initially located at a constant distance from the planet) and a plasma transport rate that increases radially outward. Model calculations reproduce both the observed average east–west asymmetry and the System III longitude dependence of the S+ ribbon location in the plasma torus. In the absence of an east–west electric field, however, the model‐calculated density peak for the S+ ribbon is located essentially symmetrically about Jupiter just inside Io's orbit and does not exhibit the observed east–west asymmetry. Since the S+ ribbon during its slow outward transport will undergo electron impact ionization, the radial location of the S++ ribbon can be expected to be created naturally with the same System III longitude dependence as for the S+ ribbon and at a position radially just beyond that of the S+ ribbon, as has been observed.
Formation of thin current sheets in space plasmas
Authors: Birn Joachim , Hesse Michael , Schindler Karl
Published: February 2004

The formation of thin current sheets is investigated on the basis of an adiabatic ideal MHD model. The primary applications are the substorm growth phase in Earth's magnetotail, prior to the dynamic expansion phase, and evolving structures in the solar corona which may similarly lead into an eruption such as a disappearing filament or a flare. A time‐dependent MHD code is used to study the evolution under the influence of an external electric field applied at the boundaries of the system. This electric field may be considered as the consequence of the coupling with the solar wind in Earth's case or as a representation of a converging footpoint motion in a bipolar region on the solar photosphere. Thin current sheets are found to form both in the high‐β and low‐β case (where β is the ratio of plasma pressure over magnetic pressure). They form in the closed field region even though the external electric field and the footpoint displacement are applied predominantly in the surrounding open field region. Within the limits of numerically stable solutions, the current sheet thickness and the maximum current density remain finite, scaling with a high power of the lobe field increase. Larger amplitudes of the current density and a stronger localization of the current sheet in the radial direction are found when the applied electric field decays more rapidly away from Earth or the solar surface. The value of γ, the polytropic index, has little influence on the evolution and the final current sheet structure. Force‐free configurations behave similarly as their equivalent high‐β current sheets.
Event study of deep energetic particle injections during substorm
Authors: Sergeev V. A. , Shukhtina M. A. , Rasinkangas R. , Korth A. , Reeves G. D. , Singer H. J. , Thomsen M. F. , Vagina L. I.
Published: February 2004

The origin of multiple energetic particle injections into the inner magnetosphere is addressed using a rare opportunity of measuring the energetic particle fluxes at different radial distances under known electric and magnetic fields. During a strong substorm on February 10, 1991, the CRRES spacecraft measured E and B fields and high‐energy particle fluxes near the magnetic equator at r ∼ 5 Re, whereas particle injections, their azimuthal locations, and some other parameters were simultaneously monitored by three geostationary spacecraft and ground networks. We show a multitude of impulsive short duration injection events which correlate with 1–2 min long pulses of dawn‐dusk electric field. The observations suggest that some E field pulses recorded deep in the inner magnetosphere were fast magnetosonic waves radiated by the current disruption region. This supports the concept of impulsive dissipation event as an elementary building block of substorm expansion. Furthermore, our modeling results indicate that most of the flux variations of energetic particles can be explained by the global convective transport and corresponding particle acceleration. However, we emphasize that, depending on particle spectra and radial flux gradient, one can observe either flux increase, or decrease, or no variation (often seen in different energy ranges simultaneously and at the same point) as a response to the electric field pulse. Both the cloud of injected particles and magnetic field dipolarization region had a sharp inner boundary (injection front) which propagated inward at the convection speed. We document the complicated structure of this front, consisting of a diamagnetic hot proton layer followed by the dipolarization front which contains enhanced energetic electron fluxes. Further study is required to understand how common this structure is and, if common, how it may be formed.
Precipitation of radiation belt electrons by magnetospherically reflected whistlers
Authors: Ristić‐Djurović J. L. , Bell T. F. , Inan U. S.
Published: February 2004

We use a test particle simulation model based on gyro‐averaged equations of motion to study the influence of oblique magnetospherically reflected (MR) whistlers on the near‐loss‐cone distribution function of radiation belt electrons. We find that MR whistlers originating in lightning can resonantly interact with radiation belt electrons over a broad range of L shells and precipitate higher energy electrons from lower L shells. Electrons in the energy‐range of 1 to 2.6 MeV are precipitated from L = 2, whereas from L = 4 the precipitated electron energy range is 150–220 keV. The precipitated differential electron flux, due to this interaction, is higher for higher L shells, and the maximum value ranges from ΦEprec(1.11 MeV) = 5.2 × 10−4 electrons cm−2 s−1 keV−1 at L = 2 to ΦEprec(173 keV) = 4.6 × 10−1 electrons cm−2 s−1 keV−1 at L = 4. The lifetimes of radiation belt electrons in a given magnetic flux tube around the L shell on which the interaction takes place are found to be of the order of several days, comparable to lifetimes corresponding to electron loss induced by hiss, which was heretofore assumed to be the dominant loss mechanism [Lyons et al., 1972], The minimum electron lifetimes vary from 2.4 days for 1.11 MeV electrons at L = 2 to 4.6 days for 173 keV electrons at L = 4.
Modification of the upper atmosphere over power lines: A geological effect
Authors: Farrell W. M. , Desch M. D. , Houser J. G.
Published: February 2004

Under certain circumstances, ELF radiation from power lines is able to very mildly heat the electrons in the upper atmosphere via ELF wave/electron interaction. The electron temperature increase forO‐mode heating is <1%. However, the significance of the heating is a function of the line construction and also the character of the underlying ground resistivity with the greatest heating effect occurring over a resistive earth.