bluuebunny/crossref_metadata_2025 huggingface dataset:
Satellite measurements through the center of a substorm surge
Authors: Weimer D. R. , Craven J. D. , Frank L. A. , Hanson W. B. , Maynard N. C. , Hoffman R. A. , Slavin J. A.
Published: February 2004

Measurements have been made of electric and magnetic fields, plasma drifts, and electron precipitation within a surge at the westward, leading edge of the auroral “bulge” at the peak of the substorm expansion phase. The trajectory of the DE 2 satellite over the auroral emissions is determined from nearly simultaneous observations with the imager on the DE 1 satellite at a higher altitude. The electric field and plasma drift measurements have enabled us to deduce the basic configuration of the ionospheric electric potential, or plasma convection, around the surge. The electric potential shows that the bulge is associated with a protrusion of the dawn convection cell into the dusk cell, poleward of the “Harang discontinuity”. This protrusion contains a westward electric field that strongly enhances the westward electrojet current by the creation of a “Cowling channel”. This westward electric field, and the associated Cowling current, appear to terminate within the surge, which contains an intense, upward field‐aligned current. The magnetic field measurements show that the region containing this field‐aligned current is shaped more like a cylinder rather than a long sheet. The total current is found to exceed one‐half million amperes.
Hybrid state of the tail magnetic configuration during steady convection events
Authors: Sergeev V. A. , Pulkkinen T. I. , Pellinen R. J. , Tsyganenko N. A.
Published: February 2004

Previous observations have shown that during periods of steady magnetospheric convection (SMC) a large amount of magnetic flux crosses the plasma sheet (corresponding to ∼10° wide auroral oval at the nightside) and that the magnetic configuration in the midtail is relaxed (the current sheet is thick and contains enhanced BZ). These signatures are typical for the substorm recovery phase. Using near‐geostationary magnetic field data, magnetic field modeling, and a novel diagnostic technique (isotropic boundary algorithm), we show that in the near‐Earth tail the magnetic configuration is very stretched during the SMC events. This stretching is caused by an intense, thin westward current. Because of the strongly depressed BZ, there is a large radial gradient in the near‐tail magnetic field. These signatures have been previously associated only with the substorm growth phase. Our results indicate that during the SMC periods the magnetic configuration is very peculiar, with co‐existing thin near‐Earth current sheet and thick midtail plasma sheet. The deep local minimum of the equatorial Bz that develops at R ∼ 12 RE is consistent with steady, adiabatic, Earthward convection in the midtail. These results impose constraints on the existing substorm theories, and call for an explanation of how such a stressed configuration can persist for such a long time without tail current disruptions that occur at the end of a substorm growth phase.
Fine structure in plasma waves and radiation near the plasma frequency in Earth's foreshock
Authors: Cairns Iver H.
Published: February 2004

Novel observations are presented of intrinsic fine structure in the frequency spectrum of electromagnetic (EM) radiation and plasma waves near the electron plasma frequency fp during a period of unusually high interplanetary magnetic field strength. Measured using the wideband receiver on the ISEE 1 spacecraft, fine‐structured emissions are observed both in the solar wind and the foreshock. The fine structure is shown to correspond to emissions spaced above fp near half harmonics of the electron cyclotron frequency fce, i.e., near fp + nfce/2. These appear to be the first space physics observations of emissions spaced by fce/2. Indirect but strong arguments are used to discriminate between EM and electrostatic (ES) signals, to identify whether ISEE 1 is in the solar wind or the foreshock, and to determine the relative frequencies of the emissions and the local fp. The data are consistent with generation of the ES and EM emissions in the foreshock, with subsequent propagation of the EM emissions into the solar wind. It remains possible that some emissions currently identified as ES have significant EM character. The ES and EM emissions often merge into one another with minimal changes in frequency, arguing that their source regions and generation mechanisms are related and imposing significant constraints on theories. The fce/2 ES and EM fine structures observed may be intrinsic to the emission mechanisms or to superposition of two series of signals with fce spacing that differ in starting frequency by fce/2. Present theories for nonlinear wave coupling processes, cyclotron maser emission, and other linear instability processes are all unable to explain multiple EM and/or ES components spaced by ∼fce/2 above fp for fp/fce ≫ 1 and typical foreshock beam parameters. Suitable avenues for further theoretical research are identified. Empirically, the observed fine structures appear very similar to those in split band and multiple‐lane type II solar radio bursts; interpretation of both these type II fine structures in terms of fce/2 splitting is suggested, thereby supporting and generalizing a suggestion by Wild (1950). A possible application to continuum radiation is mentioned. The ubiquity of these fine structures in the Earth's fp radiation and foreshock waves remains unknown. Only the ISEE 1 wideband receiver has sufficient frequency resolution (≲ 100 Hz) to perform a dedicated search. Further study of the ubiquity of these fine structures, of how reliably the splitting corresponds to fce/2 and of the other interpretations above is necessary.
Heating of the nighttime D region by very low frequency transmitters
Authors: Rodriguez Juan V. , Inan Umran S. , Bell Timothy F.
Published: February 2004

VLF signals propagating in the Earth‐ionosphere waveguide are used to probe the heated nighttime D region over three U.S. Navy very low frequency (VLF, 3‐30 kHz) transmitters. Ionospheric cooling and heating are observed when a transmitter turns off and on in the course of normal operations. Heating by the 24.0‐kHz NAA transmitter in Cutler, Maine, (1000 kW radiated power) was observed by this method in 41 of 52 off/on episodes during December 1992, increasing the amplitude and retarding the phase of the 21.4‐kHz NSS probe wave propagating from Annapolis, Maryland, to Gander, Newfoundland, by as much as 0.84 dB and 5.3°, respectively. In 6 of these 41 episodes, the amplitude of the 28.5‐kHz NAU probe wave propagating from Puerto Rico to Gander was also perturbed by as much as 0.29 dB. The latter observations were unexpected due to the > 770 km distance between NAA and the NAU‐Gander great circle path. Heating by the NSS (21.4 kHz, 265 kW) and NLK (24.8 kHz, 850 kW) transmitters was observed serendipitously in data from earlier measurements of the amplitudes of VLF signals propagating in the Earth‐ionosphere waveguide. A three‐dimensional model of wave absorption and electron heating in a magnetized, weakly ionized plasma is used to calculate the extent and shape of the collision frequency (i.e., electron temperature) enhancement above a VLF transmitter. The enhancements are annular, with a geomagnetic north‐south asymmetry and a radius at the outer half‐maximum of the collision frequency enhancement of about 150 km. Heating by the NAA transmitter is predicted to increase the nighttime D region electron temperature by as much as a factor of 3. The calculated changes in the D region conductivity are used in a three‐dimensional model of propagation in the Earth‐ionosphere waveguide to predict the effect of the heated patch on a subionospheric VLF probe wave. The range of predicted scattered field amplitudes is in general consistent with the observed signal perturbations. Discrepancies in the predictions are attributed to lack of knowledge of the D region electron density profile along the probe wave great circle paths.
Coronal electron stream and langmuir wave detection inside a propagation channel at 4.3 AU
Authors: Buttighoffer A. , Pick M. , Roelof E. C. , Hoang S. , Mangeney A. , Lanzerotti L. J. , Forsyth R. J. , Phillips J. L.
Published: February 2004

Observations of an energetic interplanetary electron event associated with the production of Langmuir waves, both of which are identified at 4.3 AU by instruments on the Ulysses spacecraft, are presented in this paper. This electron event propagates inside a well‐defined magnetic structure. The existence of this structure is firmly established by joint particle and plasma observations made by Ulysses instruments. Its local estimated radial width is of the order of 2.3 × 107 km (0.15 AU). The electron beam is associated with a type III burst observed from Earth at high frequencies and at low frequencies from Ulysses in association with Langmuir waves detected inside the structure. The consistency of local (Ulysses) and remote (Earth) observations in terms of temporal and geometrical considerations establishes that the structure is anchored in the solar corona near the solar active region responsible for the observed type III emission and gives an accurate determination of the injection time for the observed electron beam. The width on the solar surface of the structure is estimated to be 6000 km. Propagation analysis of the electron event is presented. It is shown that this event is nearly scatter‐free. Ion plasma velocity variations inside the structure were very small in amplitude as well as in direction. The magnetic field inside this structure was also very quiet and organized. In order to quantify the magnetic field properties, a variance analysis has been performed and is presented in this paper. The analysis establishes that inside the structure the amount of magnetic energy involved in the fluctuations is less than 4% of the total magnetic energy; the minimal variance direction is well defined and in coincidence with the direction of the mean magnetic field. This configuration may produce conditions favorable for scatter free streaming of energetic electrons and/or Langmuir wave production. The results presented show that the magnetic field might play a role in stabilizing the coronal‐origin plasma structures and then preserving them to large, ∼ 4 AU, distances in the heliosphere.
DE 1 and Viking observations associated with electron conical distributions
Authors: Menietti J. D. , Weimer D. R. , Andre M. , Eliasson L.
Published: February 2004

Data from the electron detectors on board the Swedish Viking satellite launched during a period of low solar activity and from the DE 1 satellite launched during active solar conditions have been examined for the occurrence and location of electron conical distributions and several conclusions can be drawn. First, we note that most of the best examples of electron conics observed by the V‐3 experiment onboard Viking occurred in the afternoon sector in the range of magnetic local time 14 hours < MLT < 18 hours, at midaltitudes in the range 10,000 km < h < 13,500 km, with few occurring in the nightside auroral region, a region poorly sampled at altitudes greater than 5000 km. For the Viking data there is an association of electron conics with upper hybrid waves. DE 1 observations made by the high‐altitude plasma instrument (HAPI) indicate that electron conics were observed in the midmorning sector and the late evening sector, and as has been reported earlier, the correlation with upper hybrid waves was good. The HAPI did not sample the afternoon sector. The electron conics observed on both satellites occurred in the presence of at least a modest (several kilovolts) potential difference beneath the satellite with a maximum energy that was usually, but not always, equal to or greater than the maximum energy of the electron conics. Two independent sets of observations by DE 1 suggest two distinct production mechanisms for electron conics. Examination of DE 1 electric field measurements from the plasma wave instrument during the observation of electron conics show simultaneous parallel oscillations in the frequency range 0.2 Hz < f < 0.5 Hz during one and perhaps two of four events examined, and upper hybrid waves were observed on all four events. In addition, recent observations of “90‐deg” electron conics associated with auroral kilometric radiation source regions suggest a perpendicular heating mechanism produced by wave‐particle interaction. Such distributions may be observed as electron conics at higher altitudes. These results suggest more than one possible source mechanism may be responsible for electron conics.
Statistical study of hydromagnetic chorus events at very high latitudes
Authors: Anderson H. M. , Engebretson M. J. , Arnoldy R. L. , Cahill L. J. , Newell P. T.
Published: February 2004

Hydromagnetic (HM) chorus events are ULF waves with typical frequencies from 0.2 to 0.6 Hz; their frequency‐time spectrum consists of a combination of band‐limited unstructured emissions and discrete elements. These waves are one of a class of short‐period ULF emissions (in the Pc 1 and 2 frequency range) that may be of value in ground‐based identification of the footpoint of magnetospheric boundary regions. As part of an extensive survey of Pc 1 and 2 waves at very high latitudes, we have identified all occurrences of HM chorus in nearly a full year's data from South Pole Station (−74.2° geomagnetic latitude, local noon ∼ 1530 UT) and McMurdo, Antarctica (−80.2° geomagnetic latitude, local noon ∼ 2030 UT) during 1990, and at McMurdo and Sondrestromfjord, Greenland (74.2° geomagnetic latitude, local noon ∼ 1330 UT) during 1988. In agreement with previous studies, these events tend to occur within a few hours of local noon. Earlier studies reported that HM chorus events are typically seen in ground records at auroral zone latitudes and suggested that they originated in the outer dayside magnetosphere. Study of DMSP particle boundary data indicated that the events reported here occurred during conditions of extremely contracted auroral ovals and are thus also consistent with an outer dayside magnetospheric source region. However, the occurrence of HM chorus events in this high‐latitude data set was limited to the months of October through May, with an occurrence peak in February, and no events were found in the months of July through September in either of the years studied. Since both hemispheres are covered, this suggests not a seasonal variation (with late winter minimum) but rather an apparent annual variation. We speculate that such an annual variation in very high latitude occurrence of HM chorus may be related to the MLT‐UT offset of the Earth's geomagnetic and geographic poles, in conjunction with wave propagation cutoffs at the high‐latitude magnetic field minima occurring on dayside magnetic field lines very near the magnetopause.
Occurrence probability, width and number of steps of cusp precipitation for fully pulsed reconnection at the dayside magnetopause
Authors: Lockwood M. , Davis C. J.
Published: February 2004

We discuss the characteristics of magnetosheath plasma precipitation in the “cusp” ionosphere for when the reconnection at the dayside magnetopause takes place only in a series of pulses. It is shown that even in this special case, the low‐altitude cusp precipitation is continuous, unless the intervals between the pulses are longer than observed intervals between magnetopause flux transfer event (FTE) signatures. We use FTE observation statistics to predict, for this case of entirely pulsed reconnection, the occurrence frequency, the distribution of latitudinal widths, and the number of ion dispersion steps of the cusp precipitation for a variety of locations of the reconnection site and a range of values of the local de‐Hoffman Teller velocity. It is found that the cusp occurrence frequency is comparable with observed values for virtually all possible locations of the reconnection site. The distribution of cusp width is also comparable with observations and is shown to be largely dependent on the distribution of the mean reconnection rate, but pulsing the reconnection does very slightly increase the width of that distribution compared with the steady state case. We conclude that neither cusp occurrence probability nor width can be used to evaluate the relative occurrence of reconnection behaviors that are entirely pulsed, pulsed but continuous and quasi‐steady. We show that the best test of the relative frequency of these three types of reconnection is to survey the distribution of steps in the cusp ion dispersion characteristics.
Anomalous ion mixing within an MHD scale Kelvin‐Helmholtz vortex: 2. Effects of inhomogeneity
Authors: Fujimoto M. , Terasawa T.
Published: February 2004

Hybrid code simulations of the MHD scale Kelvin‐Helmholtz (K‐H) instability have been conducted. In contrast to the homogeneous background case where the large‐scale ion mixing across the shear layer occurs inside the vortex obeying the hydrodynamic similarity law, the ion mixing is found to be reduced when the background magnetic field is inhomogeneous. This stabilization effect is twofold: it is an ion kinetic effect which reduces the saturation vortex size and so the mixing length when the shear layer width and the wavelength of the growing mode are small. The reduction effect still works even when the shear layer width/wavelength are large and a well defined vortical motion as in the fluid model is evident. The mixing is suppressed for this case because the ions on the lower magnetic field (magnetosheath) side cannot enter within the vortex island: The rolled‐up magnetopause current sheet block them out electrostatically. The geophysical implication of the present result is that the K‐H mixing alone may not be as efficient as to produce the large‐scale mixing blob of the low‐latitude boundary layer (LLBL) when the magnetic field intensity is highly different across the magnetopause.
Estimating the long‐term variations of the magnetotail pressure
Authors: Nakai H. , Kamide Y.
Published: February 2004

Semiannual values of the dynamic pressure and the static pressure in the solar wind are calculated for 1965‐1988. Using the statistical relationship between the tail lobe magnetic field and solar wind parameters, variations of the magnetic field pressure in the magnetic lobe at X=−20 RE are predicted. This prediction is compared with various satellite observations of the total pressure, although systematic observations were not available on a continuous basis. It is predicted that the semiannual value of the magnetotail total pressure exhibits the variation with a period of ≈11 years in close conjunction with the solar cycles. The magnetotail total pressure tends to be minimum at the maximum epoch of the sunspot number.
The CRRES AA 2 Release: HF wave‐plasma interactions in a dense Ba+ cloud
Authors: Djuth F. T. , Sulzer M. P. , Elder J. H. , Groves K. M.
Published: February 2004

An ionospheric chemical release, designated AA 2, was performed on July 12, 1992, as part of the NASA Combined Release and Radiation Effects Satellite (CRRES) El Coqui rocket campaign. The purpose of the AA 2 experiment was to study the interaction between a powerful radio wave and a high ion mass (Ba+), “collisionless” plasma. Approximately 35 kg of Ba were explosively released near the center of the Arecibo high‐frequency (HF) beam at 253 km altitude. This was the largest Ba release of the CRRES experiments; it yielded a distinctive ionospheric layer having a maximum plasma frequency of 11 MHz. At early times (< 1 min after the release) the HF beam produced the strongest Langmuir waves ever detected with the Arecibo 430‐MHz radar. Resonantly enhanced Langmuir waves were observed to be excited principally at the upshifted plasma line (i.e., near 430 MHz + ƒHF, where ƒHF is the frequency of the modifying HF wave), and only weakly excited waves were apparent at the downshifted plasma line (430 MHz − ƒHF). The upshifted plasma‐line spectrum contained a dominant peak at the “decay line,” that is, at the frequency 430 MHz + ƒHF − δ, where δ is close to the Ba+ ionacoustic frequency (∼2 kHz). Downshifted plasma‐line echoes occurred at frequencies near 430 MHz − ƒHF and 430 MHz − ƒHF − 1 kHz and exhibited little or no signal strength at the decay line (430 MHz − ƒHF + δ). During an initial period of intense upshifted plasma‐line excitation, the power asymmetry between the upshifted and downshifted plasma lines was of the order of 105 at the decay line. The upshifted plasma line was accompanied by strong HF‐enhanced ion waves that were present only at the downshifted acoustic sideband. After geomagnetic field‐aligned irregularities formed in the plasma the amplitudes of the upshifted and downshifted plasma lines equalized, and each exhibited spectra characteristic of the parametric decay instability. At early times in the Ba+ plasma the symmetry of wave excitation anticipated for a parametric instability in a stationary, homogeneous plasma was absent. The experimental results indicate that the development of the parametric decay instability needs to be reexamined for a smooth plasma having a small (∼5 km) vertical scale length. Moreover, ion flow down geomagnetic field lines appears to suppress instabilities responsible for the formation of field‐aligned irregularities and may also have an impact on the way parametric instabilities are excited. New theoretical approaches are needed to resolve many of the issues raised by this experiment.
Electron structures in the cusp/cleft region observed by DE 2 satellite
Authors: Escoubet C. P. , Smith M. F. , Fung S. F. , Hoffman R. A. , Bosqued J. M.
Published: February 2004

Narrow enhancements of electron precipitation, with energy and flux well above typical values, have been observed with DE 2 in the cusp/cleft region. The electron flux in the energy range 0.2‐1 keV was 2 orders of magnitude higher in these structures than in the magnetosheath and were seen in approximately 80% of DE 2 cusp crossings at ionospheric altitudes. Typically, there was more than one electron structure in each cusp crossing. The position of these structures showed a systematic variation: for poleward ion dispersion (energy decreases with increasing latitude), electron structures were seen more often on the equatorial boundary of the cusp, while for equatorward ion dispersion (energy decreases with decreasing latitude), electron structures were more often seen on the poleward boundary. This suggests that the electron structures are associated with newly reconnected field lines. The electron spectra suggest that field‐aligned acceleration processes could produce the electron structures, first near the boundary of the cusp/cleft during the reconnection of field lines and then in the cusp/cleft during the motion of reconnected flux tubes through the polar ionosphere.
Study of dust in the vicinity of dione using the Voyager 1 Plasma Wave Instrument
Authors: Tsintikidis D. , Kurth W. S. , Gurnett D. A. , Barbosa D. D.
Published: February 2004

The flyby of Voyager 1 at Saturn yielded the detection of a large variety of plasma waves, for example, chorus, hiss, and electron cyclotron harmonics. Just before the outbound equator crossing, the Voyager 1 plasma wave instrument detected a strong, well‐defined low‐frequency enhancement in signal levels. Initially, it was thought that this enhancement was due to plasma waves, but more recently it was suggested that dust impacts might be at least partial contributors. In this report we present evidence that dust impacts are partly responsible for the low‐frequency enhancement. A new method of analysis which relies mainly on the 16‐channel spectrum analyzer has been used to derive the dust impact rate. The available wideband waveform observations (which have been used previously to study dust impacts) were useful for calibrating the impact rate from the spectrum analyzer data. The mass and hence size of the dust particles were also obtained by analyzing the response of the plasma wave spectrum analyzer. The results show that the region sampled by Voyager 1 is populated by dust particles that have rms masses of up to few times 10−11 g and sizes of up to a few microns. The dust particle number density is of the order of 10−3 m−3. The optical depth of the region sampled by the spacecraft is approximately 10−6. The particle population is centered at 2470(±150) km south of the equatorial plane and has a north‐south FWHM (full‐width, half‐maximum) thickness of 4130(±450) km. The dust may be part of the E ring or a localized ringlet associated with Dione.
Cosmic ray transverse gradient for a Hale Cycle
Authors: Ahluwalia H. S.
Published: February 2004

We have used neutron monitor data obtained at Deep River and Huancayo as well as the vertical underground muon telescope data obtained at Embudo to compute the cosmic ray transverse gradient (Gθ) for the 1965 to 1990 period. The computed mean value of Gθ, for a given qA epoch, changes sign with the change in the solar magnetic polarity in accordance with the predictions of some contemporary hypotheses. Also, we find that the magnitude of Gθ undergoes a significant time variation during a given polarity epoch. It has the largest value near the solar polar field reversal epoch boundaries, when magnetic polarity in the solar northern hemisphere is positive (qA>0). For each of the three solar polar field reversals, the change in the magnitude of Gθ is steeper across the epoch boundaries. There is a good correlation between Gθ and the tilt angle of the heliospheric neutral current sheet (HNCS). Our results are in agreement with the reported piecemeal measurements of the latitudinal gradient made by spacecrafts, at different times, at lower primary energies, at far off distances from Earth's orbit. As such, their global significance is appreciable. Even so, it is not clear yet how these findings reflect upon the drift hypothesis in its present form. However, they do affirm the important role played by the tilt angle of HNCS in the modulation processes.
Nonlinear decay of electromagnetic ion cyclotron waves in the magnetosphere
Authors: Gomberoff L. , Gratton F. T. , Gnavi G.
Published: February 2004

We study the parametric decays of left‐hand polarized electromagnetic ion cyclotron waves, propagating parallel to the external magnetic field, in the magnetosphere. We show that the presence of He + ions and a mixed population of thermal and hot protons give rise to new wave couplings. These couplings lead to a number of new instabilities. Some of the instabilities involve sound waves carried mainly by the He + ions, which can be very efficient in heating up the bulk of the He + ions via Landau damping. Other instabilities involve the branch of the left‐hand polarized electromagnetic ion cyclotron waves which has a resonance at the He + ion gyrofrequency. These instabilities can also play a role in the energy transfer from the pump wave to the He + ions through resonance absorption, preferably in the direction perpendicular to the external magnetic field. The new couplings give rise to several types of parametric instabilities such as ordinary decay instabilities, beat wave instabilities, and modulational instabilities. There are also couplings where the pump wave decays into two electromagnetic sideband waves.
Kinetic Kelvin‐Helmholtz instability at a finite sized object
Authors: Thomas V. A.
Published: February 2004

Two‐dimensional hybrid simulations with particle ions and fluid electrons are used to calculate the kinetic evolution of the self‐consistent flow around a two‐dimensional obstacle with zero intrinsic magnetic field. Plasma outflow from the obstacle is used to establish a boundary layer between the incoming solar wind and the outgoing plasma. Because the self‐consistent flow solution, a velocity shear is naturally set up at this interface, and since the magnetic field for these simulations is transverse to this flow, the Kelvin‐Helmholtz (K‐H) instability can be excited at low‐velocity shear. Simulations demonstrate the existence of the instability even near the subsolar location, which normally is thought to be stable to this instability. The apparent reason for this result is the overall time dependence at the boundary layer, which gives rise to a Rayleigh‐Taylor like instability which provides seed perturbations for the K‐H instability. These results are directly applicable to Venus, comets, artificial plasma releases, and laser target experiments. This result has potentially important ramifications for the interpretation of observational results as well as for an estimation of the cross‐field transport. The results suggest that the K‐H instability may play a role in dayside processes and the Venus ionopause, and may exist within the context of more general situations, for example, the Earth's magnetopause.
An empirical determination of the polytropic index for the free‐streaming solar wind using Helios 1 data
Authors: Totten T. L. , Freeman J. W. , Arya S.
Published: February 2004

Observations of solar wind proton temperatures indicate that the solar wind is heated as it moves outward toward the orbit of Earth. This heating, which may be the result of electron heat conduction and perhaps MHD waves, has proven difficult to quantify and hence is often neglected in MHD models of the solar wind. An alternate approach to finding explicit heating terms for the MHD energy equation is to use a polytropic approximation. This paper discusses the properties of the polytropic approximation and its application to the solar wind plasma. By using data from the Helios 1 spacecraft, an empirical value for the polytropic index of the free‐streaming solar wind is determined. Various corrections to the data are made to account for velocity gradients, nonuniformity in radial sampling, and stream interaction regions. The polytropic index, as derived from proton data, is found to be independent of speed state, within statistical error, and has an average value of 1.46. If magnetic pressure is included, the polytropic index has an average value of 1.58.
Sensitivity of the 6300 Å twilight airglow to neutral composition
Authors: Meléndez‐Alvira D. J. , Torr D. G. , Richards P. G. , Swift W. R. , Torr M. R. , Baldridge T. , Rassoul H.
Published: February 2004

The field line interhemispheric plasma (FLIP) model is used to study the 6300 Å line intensity measured during three morning twilights from the McDonald Observatory in Texas. The Imaging Spectrometric Observatory (ISO) measured the 6300 Å intensity during the winter of 1987 and the spring and summer of 1988. The FLIP model reproduces the measured intensity and its variation through the twilight well on each day using neutral densities from the MSIS‐86 empirical model. This is in spite of the fact that different component sources dominate the integrated volume emission rate on each of the days analyzed. The sensitivity of the intensity to neutral composition is computed by varying the N2, O2 and O densities in the FLIP model and comparing to the intensity computed with the unmodified MSIS‐86 densities. The ion densities change self‐consistently. Thus the change in neutral composition also changes the electron density. The F2 peak height is unchanged in the model runs for a given day. The intensity changes near 100° SZA are comparable to within 10% when either [O2], [N2] or [O] is changed, regardless of which component source is dominant. There is strong sensitivity to changes in [N2] when dissociative recombination is dominant, virtually no change in the nighttime (SZA ≥ 108°) intensity with [O2] doubled, and sensitivity of over 50% to doubling or halving [O] at night. When excitation by conjugate photoelectrons is the dominant nighttime component source, the relative intensity change with [O] doubled or halved is very small. This study shows the strong need for simultaneous measurements of electron density and of emissions proportional to photoelectron fluxes if the 6300 Å twilight airglow is to be used to retrieve neutral densities.
Dawn‐dusk asymmetries in the low‐latitude boundary layer arising from the Kelvin‐Helmholtz instability: A particle simulation
Authors: Wilber M. , Winglee R. M.
Published: February 2004

Along the low‐latitude boundary layer (LLBL), the Kelvin‐Helmholtz (K‐H) instability can provide a means for injection of solar wind plasma across closed field lines of the magnetosphere. A fully electromagnetic, two‐dimensional (three‐velocity) particle code is used to investigate dawn‐dusk asymmetries that can arise from velocity differences due to gradient drifts and electric field gradients, and ion acceleration across the narrow field transition layers in the dawn and dusk flanks. The model includes the dawn and dusk sides of the LLBL simultaneously in a slab geometry, incorporating seperate populations of ions and electrons for the magnetosphere and the magnetosheath plasmas. We report several effects: (1) asymmetries in the observed morphology of the turbulent structures, with familiar fluidlike vortex formation on the duskside, but tongues of magnetosheath plasma penetrating into the magnetosphere on the dawnside; (2) the formation of discrete current layers, characterized by strong currents and sharp gradients in the magnetic field, and discrete charge layers, having net charges and constant, weaker currents; (3) increasing asymmetry in dawn/dusk behavior with a decrease of initial boundary layer width; and (4) enhancement of the dawn‐to‐dusk electric field as magnetosheath particles and momenta are transported across the magnetopause.
Theory of 2ωpe radiation induced by the bow shock
Authors: Yoon Peter H. , Wu C. S. , F.‐Vinas A. , Reiner M. J. , Fainberg J. , Stone R. G.
Published: February 2004

A new radiation emission mechanism is proposed to explain electromagnetic radiation observed at twice the electron plasma frequency, 2ωpe, in the upstream region of the Earth's bow shock. This radiation has its origin at the electron foreshock boundary where energetic electron beams and intense narrow‐band Langmuir waves are observed. The proposed emission mechanism results from the interaction of the electron beam and Langmuir waves that are backscattered off thermal ions. This interaction is described by a nonlinear dispersion equation which incorporates an effect owing to electron trajectory modulation by the backscattered Langmuir waves. Subsequent analysis of the dispersion equation reveals two important consequences. First, a long‐wavelength electrostatic quasi‐mode with frequency at 2ωpe is excited, and second, the quasi‐mode and the electromagnetic mode are nonlinearly coupled. The implication is that, when the excited 2ωpe quasi‐mode propagates in an inhomogeneous medium with slightly decreasing density, the quasi‐mode can be converted directly into an electromagnetic mode. Hence the electromagnetic radiation at twice the plasma frequency is generated. Numerical solutions of the dispersion equation with the choice of parameters that describe physical characteristics of the electron foreshock are presented, which illustrates the viability of the new mechanism.
Early time evolution of a chemically produced electron depletion
Authors: Scales W. A. , Bernhardt P. A. , Ganguli G.
Published: February 2004

The early time evolution of an ionospheric electron depletion produced by a radially expanding electron attachment chemical release is studied with a two‐dimensional simulation model. The model includes electron attachment chemistry, incorporates fluid electrons, particle ions and neutrals, and considers the evolution in a plane perpendicular to the geomagnetic field for a low beta plasma. Timescales considered are of the order of or less than the cyclotron period of the negative ions that result as a by‐product of the electron attachment reaction. This corresponds to time periods of tenths of seconds during recent experiments. Simulation results show that a highly sheared azimuthal electron flow velocity develops in the radially expanding depletion boundary. This sheared electron flow velocity and the steep density gradients in the boundary give rise to small‐scale irregularities in the form of electron density cavities and spikes. The nonlinear evolution of these irregularities results in trapping and ultimately turbulent heating of the negative ions.
High‐latitude F region electron concentration measurements near noon: A case study
Authors: Pinnock M. , Rodger A. S. , Berkey F. T.
Published: February 2004

A case study is presented of a ridge of enhanced F region plasma in the afternoon sector near the equatorward edge of the auroral oval. The study for September 26, 1982, principally exploits data from three ionosondes near 62° invariant latitude in the southern hemisphere separated by 5½ hours of local time but only 3½ hours of magnetic time. These observations demonstrate that the ridge structure is better ordered in magnetic time than in local time. Each station observes the ridge for about 4 hours. The poleward part of the ridge is rich in small‐scale (1–10 km) ionospheric irregularities. It is suggested that the entire ridge is formed by the mechanism described by Foster (1993) in which plasma initially corotates past noon at lower latitudes, before being entrained in the convection pattern in the afternoon sector, and brought back toward noon. The plasma thus has an extended trajectory in sunlight leading to high concentrations. The irregularities probably result from the combined effects of high electric fields and energetic particle precipitation. The contribution which this plasma is likely to make to plasma structures in the polar cap is discussed briefly.
Electron temperature effects in the linear proton mirror instability
Authors: Pantellini F. G. E. , Schwartz S. J.
Published: February 2004

It is shown that when the electron temperature Te is of the same order of the proton temperature parallel to the background magnetic field, the growth rate of the proton mirror mode in the long‐wavelength limit is reduced by the presence of a longitudinal electric field. The field is due to the electron pressure gradient which builds up when Te ≠ 0, because the electrons are dragged by nonresonant protons which are mirror accelerated from regions of high into regions of low parallel magnetic field flux. In return, the longitudinal electric field causes the density of nonresonant protons with a perpendicular velocity smaller than a strongly Te‐dependent critical velocity υ⊥,crit to increase (decrease) at maxima (minima) of the parallel magnetic field flux. These nonresonant protons thus behave differently from the “circulating” protons described by Southwood and Kivelson (1993) in the cold electron limit. Although the instability threshold is only weakly affected by changes in Te, quantities like the growth rate, the compressibility, the polarization, and the angle between wave vector and magnetic field for the most unstable mode, as well as the structure of the perturbed proton distribution itself, are strongly modified by variations in the electron temperature. The predictions of the model are shown to agree well with numerical solutions of the full Vlasov dispersion relation, indicating that most long‐wavelength aspects of the proton mirror instability are included in the model.
The initiation of coronal mass ejections by newly emerging magnetic flux
Authors: Feynman J. , Martin S. F.
Published: February 2004

We present observational evidence that eruptions of quiescent filaments and associated coronal mass ejections (CMEs) occur as a consequence of the destabilization of large‐scale coronal arcades due to interactions between these structures and new and growing active regions. Both statistical and case studies have been carried out. In a case study of a “bugle” observed by the High‐Altitude Observatory Solar Maximum Mission coronagraph, the high‐resolution magnetograms from the Big Bear Solar Observatory show newly emerging and rapidly changing flux in the magnetic fields that apparently underlie the bugle. For other case studies and in the statistical work the eruption of major quiescent filaments was taken as a proxy for CME eruption. We have found that two thirds of the quiescent‐filament‐associated CMEs occurred after substantial amounts of new magnetic flux emerged in the vicinity of the filament. In addition, in a study of all major quiescent filaments and active regions appearing in a 2‐month period we found that 17 of the 22 filaments that were associated with new active regions erupted and 26 of the 31 filaments that were not associated with new flux did not erupt. In all cases in which the new flux was oriented favorably for reconnection with the preexisting large‐scale coronal arcades; the filament was observed to erupt. The appearance of the new flux in the form of new active regions begins a few days before the eruption and typically is still occurring at the time of the eruption. A CME initiation scenario taking account of these observational results is proposed.
Mapping ionospheric convection patterns to the magnetosphere
Authors: Maynard Nelson C. , Denig William F. , Burke William J.
Published: February 2004

While convection patterns in the high‐latitude ionosphere are usually presented in a corotating frame of reference, those of the magnetosphere are given in inertial coordinates. In the corotating representation the convection throat, which is frequently associated with the cusp, opens between 1000 and 1100 MLT. Cusp precipitation, however, centers about noon. We find that transforming the convection patterns of Heppner and Maynard (1987) (hereinafter H‐M) into inertial coordinates aligns the throat region with local noon. We present projections of the H‐M patterns to the magnetosphere in both corotating and inertial coordinates using the magnetic field model of Tsyganenko (1989). In inertial coordinates the mapped H‐M convection throat opens at noon. Consistent with predictions of the Rice convection model for magnetospheric electric fields late in the substorm cycle, only a small fraction of the equipotential contours penetrate to the subsolar region. This suggests that a significant portion of flux tube merging occurs on magnetic field lines whose equatorial mapping is on the flanks of the magnetosphere. Nonconjugacy between the mapping of H‐M patterns for both positive and negative interplanetary magnetic field BY, especially in the 1400‐1600 LT sector, may explain the BY dependence of the electron precipitation “hot spot” discovered by Evans (1985). A separate lobe cell is not required to explain the central, equipotential contours of the large convection cell.
The Earth's magnetosphere is 165 RE long: Self‐consistent currents, convection, magnetospheric structure, and processes for northward interplanetary magnetic field
Authors: Fedder J. A. , Lyon J. G.
Published: February 2004

The subject of this paper is a self‐consistent, magnetohydrodynamic numerical realization for the Earth's magnetosphere which is in a quasi‐steady dynamic equilibrium for a due northward interplanetary magnetic field (IMF). Although a few hours of steady northward IMF are required for this asymptotic state to be set up, it should still be of considerable theoretical interest because it constitutes a “ground state” for the solar wind‐magnetosphere interaction. Moreover, particular features of this ground state magnetosphere should be observable even under less extreme solar wind conditions. Certain characteristics of this magnetosphere, namely, NBZ Birkeland currents, four‐cell ionospheric convection, a relatively weak cross‐polar potential, and a prominent flow boundary layer, are widely expected. Other characteristics, such as no open tail lobes, no Earth‐connected magnetic flux beyond 155 RE downstream, magnetic merging in a closed topology at the cusps, and a “tadpole” shaped magnetospheric boundary, might not be expected. In this paper, we will present the evidence for this unusual but interesting magnetospheric equilibrium. We will also discuss our present understanding of this singular state.
Magnetic field changes at the neutral sheet associated with substorm expansion onset: A model prediction and observations
Authors: Nakai H. , Kamide Y.
Published: February 2004

Using a simple numerical model of the magnetotail current system, substorm‐associated changes in the large‐scale distribution of the neutral sheet current and the corresponding magnetic field at the neutral sheet are examined. In the current model it is assumed that the radial gradient of the neutral sheet current at the late expansion phase of substorms is steeper than that just prior to the expansion onset. It is shown that our current model can successfully reproduce the observed characteristics of the so‐called magnetic field dipolarization in the near‐Earth magnetotail. Comparing various satellite observations with what our current model predicts, the location of the substorm initiation and the propagation of the magnetic field dipolarization is discussed. It is discussed that there are at least three distinct types of the radial distribution of the neutral sheet current, depending on magnetospheric conditions: quiet times, just prior to substorm expansion onset, and near the end of the expansion phase (or the beginning of the recovery phase) of substorms.
Observations of traveling ionospheric disturbances with a satellite‐beacon radio interferometer: Seasonal and local time behavior
Authors: Jacobson Abram R. , Carlos Robert C. , Massey Robert S. , Wu Guanghui
Published: February 2004

We have operated a very long baseline interferometer array at a northern midlatitude site, illuminated by VHF radio beacons from two geosynchronous satellites, quasi‐continuously for over a year. The array can detect and measure the trace velocity of traveling ionospheric disturbances (TIDs) via their signatures in the line‐of‐sight total electron content (TEC). The system noise level is of the order of 1013 m−2 in the TEC, so that even very weak perturbations can be studied. We have used the year‐long TID detection/velocimetry data set to describe local time and seasonal dependences of the wave parameters. The most striking finding is that the preferred azimuths of TIDs in the data set tend to belong to either of two modes: The first mode, strongest at midday and in the early afternoon, particularly around winter equinox, propagates southward. The second mode, strongest in the evening, especially during summer solstice through autumn equinox, propagates west‐northwestward. The two modes are disposed in local time such as to suggest the agency of clockwise rotation of the TID preferred azimuths versus time, as expected by wind filtering in the thermospheric diurnal tide. However, there is a gap between the two modes' azimuth bands. Moreover, the two modes exist in all trace‐speed quartiles of the data set TIDs, a finding which is at variance with the hypothesis of wind filtering being the primary explanation of these modes.
Flow downstream of the heliospheric terminal shock: Magnetic field line topology and solar cycle imprint
Authors: Nerney Steven , Suess S. T. , Schmahl E. J.
Published: February 2004

The topology of the magnetic field in the heliosheath is illustrated using plots of the field lines. It is shown that the Archimedean spiral inside the terminal shock is rotated back in the heliosheath into nested spirals that are advected in the direction of the interstellar wind. The 22‐year solar magnetic cycle is imprinted onto these field lines in the form of unipolar magnetic envelopes surrounded by volumes of strongly mixed polarity. Each envelope is defined by the changing tilt of the heliospheric current sheet, which is in turn defined by the boundary of unipolar high‐latitude regions on the Sun that shrink to the pole at solar maximum and expand to the equator at solar minimum. The detailed shape of the envelopes is regulated by the solar wind velocity structure in the heliosheath.