bluuebunny/crossref_metadata_2025 huggingface dataset:
Period doubling in the outer heliosphere
Authors: Burlaga L. F.
Published: February 2008

From approximately July 28 to November 26, 1984, IMP 8 at 1 AU observed quasi‐periodic interaction regions in the solar wind characterized by a peak every 13.4 days in the magnetic field strength, plasma density, and temperature, corresponding to an inertial period of 12.5 days. When the same solar wind reached Voyager 2 (which moved from 15.2 to 16.1 AU during the corresponding time interval, September 27, 1984, to January 27, 1985), the enhancements in the magnetic field strength and the plasma density and temperature recurred with a period of approximately 25 days. Thus the period of the large‐scale fluctuations in B, N, and T doubled between 1 AU and 15.2 AU. The magnetic field strength increased linearly with the density and the temperature in the Voyager 2 data. The tails of the distributions of B, N, and T in the Voyager 2 data were approximately exponential.
Magnetic clouds and the pinch effect
Authors: Suess S. T.
Published: February 2008

Magnetic clouds are characterized by a typical diameter of 0.25 AU, high magnetic field strength, and a magnetic field change predominantly by rotation of one component nearly parallel to a plane; i.e., the field in the cloud is nearly toroidal. They are a narrowly defined subclass of all interplanetary signatures of coronal mass ejections, whose geometry is thought to be quasi‐cylindrical or quasi‐spherical because of their magnetic field properties. The magnetic pressure is higher inside than the sum of the ion pressure and magnetic pressure outside clouds, leading to the suggestion that clouds are expanding at 1 AU. However, the suspected geometry of clouds is such that expansion need not occur because the imbalance can be counteracted by magnetic tension: the pinch effect. Pinch models that reproduce the observed properties of clouds are described here. The evidence for expansion of clouds at 1 AU is nevertheless quite strong, so another reason for its existence must be found. It is shown here that the expansion observations can be reproduced by taking into account the effects of geometrical distortion on the low plasma β clouds as they are advected away from the Sun. This hypothesis for the reason there is an increasing separation of the front and back edges of clouds with distance from the Sun has consequences that can be verified by coordinated spacecraft observations.
An unambiguous determination of the propagation of a compressional Pc 5 wave
Authors: Lin N. , McPherron R. L. , Kivelson M. G. , Williams D. J.
Published: February 2008

During a 3‐hour interval on August 21 and 22, 1978, a compressional Pc 5 event was observed by the ISEE‐1 magnetometer and medium energetic particle experiment instrument. During the event the ISEE spacecraft was inbound near the equator and near the dusk meridian at distances between 11 and 7 Re. The finite Larmor radii of the energetic protons allow us to determine unambiguously both the azimuthal and radial components of the phase velocity, and consequently the wave length and the azimuthal wave number. A 2nπ ambiguity in evaluating the phase velocity is removed by finding a consistent and physically reasonable solution for different energy channels. As the spacecraft approached the plasmapause, a quasi‐sinusoidal wave form having a 165‐s period was observed; it was found to be propagating azimuthally westward with a phase velocity of about 33 km/s and an azimuthal wave number of 60. In the outer magnetosphere, more irregular wave forms were observed. These waves also propagated azimuthally westward and may possibly have had a small earthward (radial) component of phase velocity. To examine the effect of heavy ions on the estimated wavelength, we have derived the perturbation of differential flux by calculating the first‐order perturbed phase space distribution of plasma consisting of two ion species. We found that, for the detector we used, the effect is very small even there is a significant fraction of heavy ions present. This is because for each energy channel, the detector response to the heavy ions is more sensitive to those with higher energy than those with the nominal proton energy, while the flux drops exponentially with the energy. In all cases the modulation amplitude of ion fluxes decreases with an increasing ratio of the ion Larmor radius to the wavelength.
Substorm‐associated VLF emissions with frequency drift observed in the premidnight sector
Authors: Hayakawa M. , Tanaka Y. , Sazhin S. S. , Tixier M. , Okada T.
Published: February 2008

Characteristics of premidnight substorm‐associated VLF emissions are examined on the basis of the data obtained by our VLF/ELF campaign during November 1978 to February 1979 by means of the direction‐finding measurements at two stations, Brorfelde in Denmark (L ∼ 3) and Chambon‐la‐Foret in France (L ∼ 2) as well as ISIS satellite measurements. Two remarkable events have been analyzed, and the following main properties have emerged: (1) the emissions observed are of hiss type and are excited mainly within the plasmapause, (2) the emission activity is preceded by development of two subsequent substorms, and (3) the emission frequency increases sharply at the first stage of the events and is followed by its subsequent gradual decrease. These properties of the temporal evolution of the wave spectrum are interpreted in terms of a quasi‐linear electron cyclotron instability model for wave excitation. The initial frequency increase is believed to be due to a combined effect of L shell drift of energetic electrons injected during the first substorm and decrease of a large‐scale convection electric field during the development of the events. The subsequent frequency decrease may be related to the additional injection of energetic electrons due to the development of the second substorm.
Thermospheric heating under proton precipitation
Authors: Srivastava V. , Singh Vir
Published: February 2008

Model calculations of the interaction of energetic protons with the thermosphere are performed to evaluate the heating efficiency. All energy channels which are responsible for the neutral gas heating are taken into account. The heating efficiency is evaluated for the Maxwellian and power law spectra of precipitating protons. It is shown that the altitude profile of heating efficiency is dependent on the proton spectrum. For a Maxwellian distribution with the characteristic energy α = 50 keV about 65% of the total energy input goes into heating, whereas for α = 2 keV only about 55% of the energy input goes into heating at the altitude where the energy deposition is maximum. For the power law spectrum the variation of heating efficiency with altitude is found to be quite different from that of Maxwellian distribution. The maximum value of heating efficiency is found to be 0.68 for the PCA event of March 21, 1969, which is considered in the model calculations for power law spectrum.
The precipitation of energetic heavy ions into the upper atmosphere of Jupiter
Authors: Horanyi M. , Cravens T. E. , Waite J. H.
Published: February 2008

Evidence for auroral particle precipitation at Jupiter was provided by the ultraviolet spectrometers on board the Voyager 1 and 2 spacecraft and by the International Ultraviolet Explorer (IUE). Magnetospheric measurements made by instruments on board the Voyager spacecraft indicate that energetic sulfur and oxygen ions are precipitating into the upper atmosphere of Jupiter. We have constructed a theoretical model describing the interaction of precipitating oxygen with the Jovian atmosphere. The auroral energy is deposited in the atmosphere by means of ionization, excitation, and dissociation and heating of the atmospheric gas. Energetic ion precipitation and electron precipitation are shown to have similar effects on the atmosphere and ionosphere of Jupiter.
Effect of pump‐induced density depletions on the spectrum of stimulated electromagnetic emissions
Authors: Leyser T. B. , Thidé B.
Published: February 2008

Analytical formulas describing the standing wave pattern in the ionospheric reflection region of a vertically propagating HF electromagnetic wave in the O mode are used to estimate density depletions due to the ponderomotive force of the successive wave maxima. The frequencies of Langmuir waves enhanced through the parametric decay instability in the created density wells are determined. It is suggested that a spectral feature of the stimulated electromagnetic emissions, downshifted by 1–2 kHz from the pump frequency, is an evidence of parametrically excited Langmuir waves which have been mode converted to electromagnetic waves. Stimulated radiation then provides a means for remote sensing of the peak electric field strength in the reflection region.
A description of electron heating with an electrostatic potential jump in a parallel, collisionless, fire hose shock
Authors: Ellison Donald C. , Jones Frank C.
Published: February 2008

We calculate the electron heating required if protons scatter elastically in a parallel, collisionless shock. Near elastic proton scattering off large amplitude background magnetic field fluctuations might be expected if the waves responsible for the shock dissipation are generated by the fire hose instability. The effects of an electrostatic potential jump in the shock layer are included by assuming that the energy lost by protons in traversing the potential jump is converted into electron thermal pressure. It is found that the electron temperature increase is a strong function of the potential jump. Comparison is made to the parallel shock plasma simulation of Quest (1987).
On magnetospheric hydromagnetic waves excited by energetic ring‐current particles
Authors: Chen Liu , Hasegawa Akira
Published: February 2008

A kinetic theory, allowing finite ion Larmor radii, general magnetic field geometries and plasma equilibria, has been developed for hydromagnetic Alfvén waves excited within the Earth's magnetosphere by the storm‐time energetic ring‐current particles. In particular, assuming adiabatically injected particles, it is found that the excitation mechanism is due to the magnetic drift‐bounce resonances of highly energetic protons and the various predicted instability properties are consistent with satellite observations.
Computer simulations of two‐pickup‐ion instabilities in a cometary environment
Authors: Gary S. Peter , Madland Christian D. , Omidi Nojan , Winske Dan
Published: February 2008

The velocity‐space evolution of newborn cometary ions is studied by means of one‐dimensional electromagnetic hybrid computer simulations of homogeneous plasmas. Newborn ions are injected into the simulations at a constant rate with velocity relative to the solar wind which is parallel to the ambient magnetic field. Both protons and oxygen ions are injected and both self‐consistently contribute to the growth of low‐frequency electromagnetic fluctuations. At weak injection rates, corresponding to relatively large distances from the cometary nucleus, the most important growing modes are two ion‐ion right‐hand resonant instabilities. The proton‐proton right‐hand mode grows more rapidly, with fluctuating magnetic field energy densities that exhibit linear temporal growth and that pitch angle scatter the protons to thin shelllike velocity distributions. The oxygen‐proton right‐hand mode has a smaller growth rate, but corresponds to a much larger free energy injection rate, so that its fluctuating magnetic field energy densities eventually exhibit exponential temporal growth to a peak amplitude, phase bunching, and magnetically trapping the oxygen ions. At times well after peak amplitude, the longer wavelength oxygen‐proton fluctuations stochastically accelerate some of the injected protons. An exponential energy distribution of accelerated ions is obtained such that the effective temperature of the high energy protons is larger than that of the heavier energetic ions. These simulations yield a scaling of the peak fluctuating field amplitudes as a function of injected beam properties and local injection rates. There is good agreement between this scaling and observations of magnetic field amplitudes at comet Giacobini‐Zinner.
Wave‐driven fluctuations in OH nightglow from an extended source region
Authors: Schubert G. , Walterscheid R. L.
Published: February 2008

Hydroxyl nightglow emissions arise from a region of the atmosphere several to 10 km thick centered at about the mesopause. The intensity of the nightglow 〈I〉 is therefore a superposition of contributions from individual thin emitting layers. The inferred rotational temperature 〈TI〉 is an intensity weighted average of the temperatures throughout the emitting region provided that the overall temperature variation of the region is small compared with the average temperature, and that the Boltzmann exponent in the rotational line intensity distribution is not much larger than unity. Acoustic‐gravity waves propagating through the emitting region produce fluctuations in the OH nightglow; intensity and temperature oscillations, 〈δI〉 and 〈δT〉, are conveniently related by the ratio
, where the overbar refers to time‐averaged quantities. When the vertical wavelength λυ of a disturbance is large compared to the vertical thickness of the emitting region, 〈η〉 is accurately given by
for a thin layer at the peak emission height. However, when λυ is comparable to or smaller than the emission region thickness, 〈η〉 values can vary widely from the peak emission height ratio η. When λυ is small, the magnitude of the normalized fluctuation in net intensity |〈δI〉/〈Ī〉| is considerably reduced from the large λυ value due to cancellation effects of out‐of‐phase layers in the emitting region. Therefore, there is an observational bias toward detection of nightglow fluctuations from waves with λυ greater than emission region thickness. Values of 〈η〉 versus period are reported for several horizontal wavelengths based on a dynamical‐chemical model that accounts for realistic OH chemistry and the dynamics of acoustic‐gravity waves propagating vertically through a height‐dependent basic state atmosphere.
Empirical global model of upper thermosphere winds based on Atmosphere and Dynamics Explorer satellite data
Authors: Hedin A. E. , Spencer N. W. , Killeen T. L.
Published: February 2008

Thermospheric wind data obtained from the Atmosphere Explorer E and Dynamics Explorer 2 satellites have been used to generate an empirical wind model for the upper thermosphere, analogous to the MSIS model for temperature and density, using a limited set of vector spherical harmonics. The model is limited to above approximately 220 km where the data coverage is best and wind variations with height are reduced by viscosity. The data base is not adequate to detect solar cycle (F10.7) effects at this time but does include magnetic activity effects. Mid‐ and low‐latitude data are reproduced quite well by the model and compare favorably with published ground‐based results. The polar vortices are present, but not to full detail.
Wavelike perturbations observed in the neutral thermosphere of Venus
Authors: Kasprzak W. T. , Hedin A. E. , Mayr H. G. , Niemann H. B.
Published: February 2008

Wavelike perturbations are evident in the neutral thermosphere measurements of He, N, O, N2, and CO2 by the Pioneer Venus orbiter neutral mass spectrometer. In the wavelength range from 100 to 600 km the amplitudes of the various species are comparable in magnitude, with CO2 having the largest amplitude, and with helium out of phase with respect to the heavier species. On the dayside the small scale variations of CO2 and N2 increase with increasing altitude from 170 to 210 km, with CO2 having the largest slope. Simple and complex wave structures are observed including wave trains and pulselike events. There is more activity during nighttime than during daytime and larger values of the activity occur in the vicinity of the predawn and postdusk terminator sectors. Below 160 km the standard deviation of the CO2 density on the nightside is 12%, about 3 times the daytime value. The data are consistent with the interpretation that the density perturbations are due to gravity waves propagating upward from the lower thermosphere.
Active current sheets near the Earth's bow shock
Authors: Schwartz Steven J. , Kessel Ramona L. , Brown Cassandra C. , Woolliscroft Les J. C. , Dunlop Malcolm W. , Farrugia Charles J. , Hall David S.
Published: February 2008

We present here an investigation of “active current sheets” observed by the AMPTE UK spacecraft near the Earth's bow shock, concentrating on their macroscopic features and geometry. Events selected primarily by flow directions which deviate substantially from the Sun‐Earth line show similar characteristics, including their association with an underlying macroscopic current sheet and a hot central region whose flow direction is organized, at least in part, by location relative to the inferred initial intersection point between the current sheet and the bow shock. This region is flanked by “edges” which, according to a Rankine‐Hugoniot analysis, are often fast shocks whose orientation is consistent with that expected if a “bulge” on the bow shock convected past the spacecraft. We have found the magnetosheath manifestations of these events which we study in detail. We suggest that these events are the direct result of the disruption and reformation of the bow shock by the passage of an interplanetary current sheet, most probably a tangential discontinuity.
Mapping of auroral kilometric radiation sources to the aurora
Authors: Huff R. L. , Calvert W. , Craven J. D. , Frank L. A. , Gurnett D. A.
Published: February 2008

Auroral kilometric radiation (AKR) and optical auroral emissions are observed simultaneously using plasma wave instrumentation and auroral imaging photometers carried on the DE 1 spacecraft. The DE 1 plasma wave instrument measures the relative phase of signals from orthogonal electric dipole antennas, and from these measurements, apparent source directions can be determined with a high degree of precision. Wave data are analyzed for several strong AKR events, and source directions are determined for several emission frequencies. By assuming that the AKR originates at cyclotron resonant altitudes, a candidate source field line is identified. When the selected source field line is traced down to auroral altitudes on the concurrent DE 1 auroral image, a striking correspondence between the AKR source field line and localized auroral features is produced. The magnetic mapping study provides strong evidence that AKR sources occur on field lines associated with discrete auroral arcs, and it provides confirmation that AKR is generated near the electron cyclotron frequency.
Subionospheric VLF signatures of nighttime D region perturbations in the vicinity of lightning discharges
Authors: Inan U. S. , Shafer D. C. , Yip W. Y. , Orville R. E.
Published: February 2008

A 12‐hour sequence of perturbations of subionospheric VLF signals observed in association with lightning provided preliminary evidence that the ionospheric regions perturbed in these events may be confined to within ∼150 km of the lightning discharges, and that intracloud flashes as well as cloud‐to‐ground lightning may be important in producing the perturbations. High‐resolution analysis of event signatures indicated the presence of two different classes of events. For one set of events, observed during the most active central 6 hours of the observation period, a ∼0.6‐s delay between the causative lightning and VLF event onset and a ∼1‐s onset duration was observed, consistent with previously suggested models of the gyroresonant whistler‐particle interaction that leads to particle precipitation and perturbation of the Earth‐ionosphere waveguide. However, another set of events, observed during the first 2 hours of the observation period, exhibited a very different temporal signature, characterized by a much smaller (<50 ms) delay and sometimes also very short (<50 ms) rise times. Such events are possibly related to previously reported cases of similarly early/fast events and may involve a more direct coupling between the lightning discharge and the lower ionosphere.
Observations of intense ULF pulsation activity near the geomagnetic equator during quiet times
Authors: Engebretson M. J. , Zanetti L. J. , Potemra T. A. , Klumpar D. M. , Strangeway R. J. , Acuña M. H.
Published: February 2008

The Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE satellite frequently observed ULF pulsations in the Pc 3‐4 period range as it traveled from L = 3 to beyond L = 8 near local noon. In addition to the azimuthally polarized, harmonically structured pulsations that occurred frequently at all dayside magnetic latitudes sampled by the satellite (±16°), a distinctive class of pulsations appeared occasionally during quiet times when the satellite was within 1° or 2° of the geomagnetic equator. These pulsations, with increased amplitude (up to 12 nT peak to peak) and without multiple harmonic structure, were also transverse but with dominant power in the radial component. They were observed during magnetically quiet periods in regions characterized by intense fluxes of warm, strongly trapped light ions, predominantly H+, and often with streaming low‐energy plasma. We interpret the strong latitudinal localization of these pulsations to be due to equatorial mass loading or to partial reflection of Alfvén wave energy by latitudinal gradients in plasma density. In most cases we have identified a recently injected energetic particle population at the satellite's location as the probable source of wave energy. The similarity in occurrence between these events and the “giant” pulsations occasionally observed on the ground suggests that latitudinally localized plasma structures may be a necessary but not sufficient condition for the onset of these unusually long‐lived, highly localized pulsations.
The gamma ray north‐south effect
Authors: White R. S. , O' T. J. , Tumer O. T. , Zych A. D.
Published: February 2008

We present theoretical calculations to explain the atmospheric gamma ray north‐south ratios measured by us and other observers. The north‐south effect is caused by the higher cosmic ray flux that strikes the Earth's atmosphere near the poles than near the equator. Primary and secondary cosmic ray integral energy distributions that vary as E−1.65 produce the gamma rays of 1–20 MeV of interest here. Gamma rays from distances north and south of the observer Compton scatter from the atmospheric nitrogen and oxygen into gamma ray telescopes flown on balloons at typical heights of 4 g/cm² residual atmosphere. The gamma rays that originate at the longest distances from the telescopes give the largest north‐south ratios. The north‐south ratios, between zenith angles of 15° and 60°, are greatest for the largest zenith angles and between 1.5 and 20 MeV for the largest gamma ray energies. Comparisons are made to experimental north‐south ratios measured on balloons launched from Palestine, Texas, and from Alice Springs, Australia. Predictions are made for ratios at other geomagnetic latitudes and longitudes. Observers who measure backgrounds for celestial sources 180° in azimuthal angle from the source direction may be misled unless they correct for the north‐south effect. Since neutrons of a few MeV are produced by cosmic ray collisions in the atmosphere in comparable numbers to gamma rays and their scatter path lengths are similar, we expect a neutron north‐south effect comparable to that for gamma rays.
Magnetic pulses with durations near the local proton cyclotron period: Comet Giacobini‐Zinner
Authors: Tsurutani Bruce T. , Brinca Armando L. , Buti B. , Smith Edward J. , Thorne Richard M. , Matsumoto Hiroshi
Published: February 2008

Magnetic pulses with durations near the local proton cyclotron period (in the spacecraft frame) have been detected during the ICE encounter with comet Giacobini‐Zinner. The pulses typically last only a single cycle (solitary wave), are transverse, noncompressional oscillations, and have peak‐to‐peak transverse amplitudes of 2‐3 nT. Occasional large pulses with amplitudes of 5‐7 nT have been detected. The waves typically propagate at angles 2°‐15° relative to B0 and are highly elliptically (linearly) polarized. The pulses have been detected when α, the angle between the ambient magnetic field and the solar wind velocity, is 90° ± 30° and when ICE was 350,000‐700,000 km from the comet. At the present time it is uncertain whether such pulses are generated exclusively under large α conditions or whether the presence of large‐amplitude heavy ion cyclotron waves during more moderate α conditions masks their presence. It is also unclear whether these pulses are superposed on top of cometary turbulence or simply a part of it. The above observations will be compared to recent theoretical predictions of cometary waves generated during large α conditions.
Correlation studies between solar wind parameters and the decimetric radio emission from Jupiter
Authors: Bolton S. J. , Gulkis S. , Klein M. J. , de Pater I. , Thompson T. J.
Published: February 2008

Results of a study comparing long‐term time variations (years) in Jupiter's synchrotron radio emission with a variety of solar wind parameters and the 10.7‐cm solar flux are reported. Data from 1963 through 1985 were analyzed, and the results suggest that many solar wind parameters are correlated with the intensity of the synchrotron emission produced by the relativistic electrons in the Jovian Van Allen radiation belts. Significant nonzero correlation coefficients appear to be associated with solar wind ion density, ram pressure, thermal pressure, flow velocity, momentum, and ion temperature. The highest correlation coefficients are obtained for solar wind ram pressure (NV²) and thermal pressure (NT). The correlation analysis suggests that the delay time between fluctuations in the solar wind and changes in the Jovian synchrotron emission is typically about 2 years. The delay time of the correlation places important constraints on the theoretical models describing the radiation belts. The implication of these results, if the correlations are real, is that the solar wind is influencing the supply and/or loss of electrons to Jupiter's inner magnetosphere. We note that the data for this work spans only about two periods of the solar activity cycle, and because of the long time scales of the observed variations, it is important to confirm these results with additional observations.
Coronal mass ejection rate and the evolution of the large‐scale K‐coronal density distribution
Authors: Sime D. G.
Published: February 2008

Recently reported occurrence rates of coronal mass ejections (CMEs) are compared with the time scale for the long‐term evolution of the global white light coronal density distribution. This time scale is estimated from the synoptic observations of the corona made from Mauna Loa, Hawaii, by a series of K‐coronameters. The data span a period of more than 20 years and show evolution rates which vary with time roughly in phase with the solar activity cycle. However, there are detailed differences between the sunspot number curve and the long‐term behavior of this quantity. When the occurrence rates of CMEs observed from orbiting coronagraphs, available mainly during the descending phase of the activity cycle, are compared with this evolution time, it is found that the two quantities are inversely proportional. From energy considerations, it is unlikely that there is a causal relationship between CMEs and this coronal evolution. Rather, the result indicates that the processes which lead to the global evolution are intimately related to those which give rise to CMEs, a hypothesis consistent with current theories that CMEs arise from preexisting magnetic structures which become stressed by the global magnetic field rearrangement to the point of instability.
The structure of reconnection layers: Application to the Earth's magnetopause
Authors: Biernat Helfried K. , Heyn Martin F. , Rijnbeek Richard P. , Semenov Vladimir S. , Farrugia Charles J.
Published: February 2008

Using ideal magnetohydrodynamics, the structure of the reconnection layer in the Petschek reconnection model has been analyzed as a function of inflow parameters by Heyn et al. (1988). We examine the application of this analysis to conditions which are appropriate to the Earth's magnetopause. For non‐antiparallel magnetic field configurations, the field rotation from one inflow region to the other is achieved through two Alfvén waves bounding the reconnection layer, although the rotation on the magnetosheath edge is much stronger. Slow shocks or slow expansion waves are needed in addition in order to form a match between the field strengths, and their appearance and strength depends on the degree of asymmetry of the inflow region parameters. Tangential velocities in the inflow regions give rise to a structure of the reconnection layer which is different above and below the reconnection line. In the presence of these tangential velocities, the Kelvin‐Helmholtz stability of the magnetopause needs to be considered. If a region of the magnetopause is Kelvin‐Helmholtz unstable, it is possible that Petschek‐type reconnection is prevented from occurring in this region. Thus we can place constraints on the occurrence of Petschek reconnection.
Electrostatic plasma waves and HF auroral hiss generated at low altitude
Authors: Beghin C. , Rauch J. L. , Bosqued J. M.
Published: February 2008

Large‐amplitude (10 to 100 μV m−1 Hz−½) natural radio emissions in a wide frequency range (100 kHz up to 2 MHz) are frequently observed on board the AUREOL/ARCAD 3 satellite at high latitude and at altitudes between 400 and 2000 km. The simultaneous measurement of the local cold plasma density allows the identification of cutoff and resonance frequencies. Three different kinds of wave are observed: (1) electrostatic emissions near the local value of the plasma frequency (fp), (2) electromagnetic whistler mode emissions, sometimes associated with type (1) emissions, and (3) electromagnetic Z mode emissions, also associated with type 1 emissions, but occurring more rarely than the whistler mode emissions and then only when fp is greater than the electron cyclotron frequency (fce). These emissions are always associated with high levels of ELF electrostatic turbulence and a high flux of low‐energy precipitating electrons, extending in energy down to the lower limit of the detectors (∼100 eV). The statistical distribution of the emissions in geomagnetic coordinates shows an occurrence greater than 80% in the polar cusp region and between 25% and 60% in the nightside auroral zone. A generation mechanism for such emissions is proposed, based on the calculation of the growth rate of the kinetic Cherenkov instability, associated with a beamlike suprathermal tail in the parallel distribution of the bulk electron population. In particular, suprathermal, downward electron beams of about 100–200 eV energy, with a thermal spread of the same order, are found to be responsible for the generation of whistler mode and Z mode HF emissions in a source region extending down to 1000 km of altitude. It is suggested that such intense radio emissions should be considered as one of the energy dissipation processes resulting from magnetosphere‐ionosphere coupling, through interdependent electrodynamic mechanisms such as current systems, parallel anomalous resistivity, plasma turbulence, energy diffusion, and heating.
A test of magnetic field draping induced Bz perturbations ahead of fast coronal mass ejecta
Authors: McComas D. J. , Gosling J. T. , Bame S. J. , Smith E. J. , Cane H. V.
Published: February 2008

Recent studies have suggested that the ambient interplanetary magnetic field (IMF) drapes about fast coronal mass ejections (CMEs) as they plow out through slower moving, quiescent solar wind. In this study we examine ISEE 3 plasma and magnetic field data in order to look for observational evidence of IMF draping ahead of fast CMEs. Since the upstream IMF is excluded from CMEs, the ambient field must drape about these ejecta and connect back to the Sun around their edges. For example, for a purely radial, inward directed IMF, this would mean that a CME which extends into the ecliptic plane while being directed northward of it should give rise to a southward Bz perturbation in the ecliptic plane ahead of the CME. While purely radial IMF configurations are rare, the radial component of the IMF should also give rise to such Bz perturbations. In reality, it is difficult to make an unambiguous identification of a CME's source region, and the draped field sampled along a spacecraft trajectory ahead of a CME can be very complicated, including contributions from a wide range of heliolatitudes as well as heliolongitudes. In spite of these complications we set out to test the usefulness of the draping scenario for predicting the Bz perturbations (and hence associated geomagnetic activity) ahead of fast CMEs which produce interplanetary shocks using ISEE 3 plasma and magnetic field data and Cane's (1985) list of shocks associated with interplanetary type II radio bursts. Of Cane's 45 events, there were 26 events which were suitable for testing the draping hypothesis. However, for only 17 of these 26 events was there a substantial change in the average Bz between the regions upstream from the shocks and between the shocks and CMEs. Of these 17 events the simpleminded radial component predictor developed in this study correctly predicts the direction of the Bz perturbations for 13 events (76%). While this result is certainly not conclusive, we consider it to be supportive of the draping scenario. In terms of using the draping model for real‐time geomagnetic activity predictions, however, the outlook is not nearly so good since nine of the 26 events had no substantial change in the average Bz and since there is no a priori way to tell how substantial draping effects will be for a given case.
The long‐time evolution of a low‐density ion beam
Authors: Zachary Andrew L. , Cohen Bruce I. , Max Claire E. , Arons Jonathan
Published: February 2008

With a new, orbit‐averaged hybrid computer simulation code, we study a cold, fast low‐density ion beam which propagates along the ambient magnetic field as it interacts with a much denser fluid background. We examine the character of the interactions as we vary the ion beam density relative to the background density over the range 1×10−5 to 3×10−3. The low beam density simulations may not be directly observable upstream of the Earth's bow shock, but they are included to help develop an understanding of the results seen in the simulations with high‐beam density. However, our highest density simulation falls withn the range of solar wind data. All the simulations, regardless of the relative beam density, show three distinct phases: (1) an early or “linear” phase; (2) an intermediate or “trapping” phase; and (3) a late or “decorrelation” phase. In the early phase, the beam excites a nearly monochromatic Alfvén wave whose amplitude grows exponentially at a rate given by linear perturbation theory. The wave amplitude saturates when the linear growth rate is of the order of the trapping frequency. In the intermediate phase, the wave traps the beam, and the beam ions and the wave interchange energy at the trapped particle oscillation frequency. Views of particle phase space show that the trapped beam ions are also bunched in gyrophase. In the final phase, the beam ions decorrelate and fill the available phase space. When the Alfvén wave field is nearly monochromatic, as it is throughout each of the lower‐density beam simulations, the beam ion phase space orbits are constrained by a constant of the motion related to the helical symmetry of the entire system. The particle orbits are parabolas, rather than the circles of constant energy predicted by quasi‐linear theory. For these simulations, the final particle distribution superficially appears to be a drifting Maxwellian with a substantial thermal width, but in fact the distribution retains memory of its beamlike origin: the particles occupy a finite region of phase space bounded by two parabolas in (υ⊥, υ∥) space due to the constant of motion. In the highest density simulation, there appears to be sufficient parallel pressure to drive a conventional, linearly polarized firehose mode locally unstable. The presence of this additional large‐amplitude mode destroys the helical symmetry, and the particles scatter freely to form a hollow distribution in (υ⊥, υ∥) phase space.
The interplanetary magnetic field By‐dependent field‐aligned current in the dayside polar cap under quiet conditions
Authors: Yamauchi M. , Araki T.
Published: February 2008

Spatial distribution and temporal variation of the interplanetary magnetic field (IMF) By‐dependent cusp region field‐aligned currents (FACs) during quiet periods were studied by use of magnetic data observed by Magsat. The analysis was made for 11 events (each event lasts more than one and a half days) when the IMF By component was steadily large and Bz was relatively small (|Bz|<|By|). Results of the analysis of total 62 half‐day periods for the IMF By‐dependent cusp region FAC are summarized as follows: (1) The IMF By‐dependent cusp region FAC is located at around 86°–87° invariant latitude local noon, which is more poleward than the location of the IMF Bz‐dependent cusp region FAC. (2) The current density of this FAC is greater than previous studies (≥4 µA/m² for IMF By = 6 nT). (3) There are two time scales for the IMF By‐dependent cusp region FAC to appear: the initial rise of the current is on a short time scale, ∼10 min, and it is followed by a gradual increase on a time scale of several hours to a half day. (4) The seasonal change of this FAC is greater than that of the nightside region 1 or region 2 FACs. (5) The IMF Bz‐dependent cusp region FAC is not well observed around the cusp when the IMF By‐dependent cusp region FAC is intense.
Jovian plasma sheet density profile from low‐frequency radio waves
Authors: Rucker H. O. , Ladreiter H. P. , Leblanc Y. , Jones D. , Kurth W. S.
Published: February 2008

By using planetary radio astronomy (PRA), plasma wave system (PWS), and Magnetometer (MAG) data from Voyager 1 and 2 (V1 and V2), essential features of the nightside Jovian plasma sheet are derived, and the density gradient of the corotating plasma structure in the middle Jovian magnetosphere is calculated. The PRA experiment gives information about the plasma wave polarization. To determine the density profile of the plasma sheet, we have derived the hinge point position of the plasma disc from MAG data and used the low‐frequency cutoffs observed at three frequencies (562 Hz, 1 kHz, and 1.78 kHz) from the PWS experiment. We show that the hinge point position varies with the solar wind ram pressure, and the plasma disc thickness decreases with distance up to about 60 RJ. The average thickness for an isodensity contour corresponding to 1 kHz is 3.29 RJ for V1 and 3.16 RJ for V2.
The motion of contaminant water plasma clouds about large active space structures
Authors: Hastings D. E. , Gatsonis N. A.
Published: February 2008

Large structures in low earth orbit will release neutral water or ions through outgassing, water dumps or thruster firings and thus perturb the ambient ionosphere. Neutrals and ions within the perturbed environment will undergo chemical reactions to form a contaminant cloud. It is assumed that the contaminant cloud consists of ions such as O+, H2O+, H3O+ and neutrals such as O, H, OH and H2O. A two‐dimensional model for the motion of the contaminant cloud perpendicular to the magnetic field lines is developed. Numerical solution of the derived model equations examines the effects of Alfvén wave coupling, neutral water density, ion temperature and initial conditions on the cloud motion. In low density neutral water clouds (≤109 cm−3) the shielding of the electric field is small. In neutral water clouds with densities of interest for shuttle conditions (∼1010 cm−3) the shielding is predicted to be of the order of the motional electric field and the ratios of the line averaged densities of the ions is consistent with experimental data. The effect of ion temperature is negligible for the range of densities and temperatures considered. It is predicted that for symmetric initial conditions, the drifting clouds will develop fingerlike instabilities. Plasma depletions are predicted in the wake of the moving structure with enhancements in the ram direction. Asymmetric initial conditions, such that could result from a thruster firing, rotate the plasma cloud which undergoes a highly asymmetric distortion.
Impulsive plasma waves observed by DE 1 in nightside magnetosphere
Authors: Ondoh T. , Nakamura Y. , Watanabe S. , Aikyo K.
Published: February 2008

Impulsive plasma waves with a frequency range of 1–9 kHz were found in the wide‐band electric field data (650 Hz to 10 kHz) from the DE 1 satellite received at the Kashima station in Japan. The impulsive plasma waves were often accompanied by a strong hiss band at about 1 kHz, but not by a chorus. The frequency‐time spectra of these waves are very similar to those of electrostatic bursts associated with ELF chorus in the outer dayside magnetosphere, but these impulsive waves were observed at low geomagnetic latitudes in nightside magnetospheric regions much nearer the Earth for geomagnetic quiet and disturbed times. Local plasma densities (∼1 cm−3) obtained from the characteristic frequency of VLF emissions associated with the impulsive plasma waves agree with those observed simultaneously by the DE 1 retarding ion mass spectrometer. It is inferred from the local plasma density and the empirical relation of the plasmapause position with Kp that the impulsive plasma waves are occurring just outside the plasmapause. The impulsive plasma waves are discussed in terms of a resistive medium instability caused by Landau resonant trapping of thermal electrons by the associated hiss band at about 1 kHz, since this mechanism seems to be plausible from the available data and the frequency‐time spectra are similar to electrostatic bursts identified previously. Some of the events do not seem to have a strong ELF hiss band accompanying the impulsive plasma waves. It is suggested that these events may be due to electrons bunched by an ELF hiss band in a nearby region or may be caused by space discharges in the vicinity of the long‐wire antenna on the DE 1 spacecraft.
Power spectrum of electrical discharges seen on Earth and at Saturn
Authors: Warwick James W.
Published: February 2008

We present a method for deriving the radio spectrum of electrical discharges from the properties of the time series of charges crossing the discharge gap. For the simple case considered, the resulting spectrum is the product of the power spectrum of emission from a single charge crossing the gap and of the power spectrum of the time series of charges crossing a given point in the gap. This result is applied to the observed spectra of both terrestrial lightning and Saturn electrical discharge(s) (SED). These observations are discussed, particularly in the case of SED. SED occurrence and power density are shown to have subtle, yet important, differences from these observables as they have been described in the last 5 years. It is demonstrated that throughout the episode of Voyager 1's (V1) closest approach to Saturn, SED probably occurred continuously in frequency upward at least from the upper limit of Saturn kilometric radiation at about 800 kHz. This is so despite the fact that in the dynamic spectra a strip in time and frequency in which SED do not occur extends in frequency from 1.3 MHz up to the oft‐discussed “lower limit” of SED in the leading edge of the episode of closest approach. This cutoff band can be well interpreted through the combined effects of lower receiver sensitivity resulting from spacecraft interference and the increasing efficiency of power transfer from a given small antenna as frequency increases up to about the quarter‐wave value (above 7 MHz for the planetary radio astronomy antennas). The greater power in SED that occurred after V1 closest approach is emphasized; it is shown to be consistent with the lower frequency of the maximum in their power spectra. This is a result of the greater length of the discharge gap as it generates SED after closest approach. The variable gap length factor is also invoked to explain the variable frequency cutoff in the range 5–15 MHz of the episodes before closest approach. The SED source moved along a single arc defining both preencounter and postencounter events. The discharge gap lengths were a continuous function of position along this arc, with the shortest gaps lying about 5° west (as seen from the spacecraft) of the noon meridian of Saturn and the longest gaps lying on the nightside of the planet.
Ultraviolet variability of the solar analogue star Alpha Centauri A
Authors: Sofia Ulysses J. , Bruhweiler Frederick C. , Sofia Sabatino
Published: February 2008

Fifty‐seven observations of the solar analogue Alpha Centauri A made by the International Ultraviolet Explorer over a 3‐year interval have been examined for variability. We find that the ultraviolet continuum flux of this star in the 1715‐ to 1915‐Å range varies by as much as 19% (with a standard deviation of ±1.3%). However, nearly all of this range in variability can occur within one stellar rotation period. Consequently, if Alpha Cen A undergoes a stellar activity cycle similar to that of the Sun, our results indicate that the amplitude of this modulation is small, contrary to recently proposed empirical (theoretical) models of solar activity.
Ionospheric parameters derived from heater‐enhanced plasma lines
Authors: Nordling Johan A.
Published: February 2008

A new radar method for the estimation of the electron number density, electron temperature, and ion temperature in a plasma which is modified by a strong electromagnetic wave is described. The method relies on the possibility of obtaining high resolution plasma line spectra containing the main parametric decay lines together with a few parametric decay cascade lines. Assuming that simple, linear wave kinematical arguments are valid it is shown that the frequency shifts between these lines under certain circumstances can be used to determine three of the most important plasma parameters in the nonlinear interaction region.
Low‐frequency electrostatic instabilities excited by injections of an electron beam in space
Authors: Hwang Y. S. , Okuda H.
Published: February 2008

One‐dimensional particle simulations have been carried out to study the low‐frequency broadband electrostatic noise that propagates almost perpendicularly from the magnetic field line when a nonrelativistic electron beam is injected into space from a spacecraft. For Te = Ti the electrostatic ion cyclotron waves appear as well as the waves near the lower hybrid frequency. When the magnetic field is reduced so that Ωe≪ωpe in a nonisothermal plasma, Te>Ti, oblique ion acoustic instabilities appear to propagate almost perpendicular to the magnetic field. In addition, a very low frequency mode at ω≪Ωi is found to be generated by the electrons flowing into the conductor. Both the injected beam electrons as well as the ambient electrons flowing into the spacecraft are responsible for generating those instabilities, which accelerate ions perpendicular to the magnetic field.
A quasi‐one‐dimensional model for the Giacobini‐Zinner plasma tail
Authors: Malara Francesco , Einaudi Giorgio , Mangeney Andrè
Published: February 2008

An assumption of quasi‐one‐dimensionality is used to derive a simple set of equations describing the comet Giacobini‐Zinner tail configuration. The MHD equations are expanded in terms of a parameter representing the ratio of the length scale in the direction perpendicular to the neutral sheet over the length scale in the direction parallel to the tail. It is shown that in this way it is possible to obtain much information on the structure of the tail and to fit reasonably well the observations made by the ICE spacecraft.