bluuebunny/crossref_metadata_2025 huggingface dataset:
Simultaneous density and electric field fluctuation spectra associated with velocity shears in the auroral oval
Authors: Basu Sunanda , Basu Santimay , MacKenzie E. , Fougere P. F. , Coley W. R. , Maynard N. C. , Winningham J. D. , Sugiura M. , Hanson W. B. , Hoegy W. R.
Published: February 2008

Simultaneous satellite in situ measurements of density (ΔN/N) and electric field fluctuation (ΔE) spectra in the high‐latitude ionosphere are presented using two orbits of Dynamics Explorer 2 (DE 2) data traversing, respectively, the F region at 350 km altitude and the topside ionosphere at 900 km altitude. The spectral study was primarily confined to large structured velocity regions in the auroral oval. By means of the very complete set of energetic particle, dc and ac electric field, field‐aligned current, thermal plasma density, and temperature measurements available from DE 2, we were able to identify two categories of spectra associated with velocity shears irrespective of the height of the satellite. The first category was observed in very intense velocity shear regions of shear frequencies ∼10 Hz in conjunction with large field‐aligned current densities. Under these conditions the spatial spectra of ΔN/N and ΔE had identical power law indices of −1.8±0.2 between scale lengths of approximately 10 km and 300 m. At scale lengths shorter than 300 m the ΔE spectra steepened to an index of −3±0.5 while the spectral index of ΔN/N remained close to its original value of approximately −1.8±0.2, with large power spectral densities observed down to 10 m scale lengths. The second category was observed in more moderate velocity shear regions of shear frequencies ∼ 1 Hz in conjunction with weak field‐aligned currents. In this case the slopes of the density spectra were essentially unchanged, while the ΔE spectra had a much steeper slope of −3±0.5 between 10 km and a few hundred meters. Other factors identifying the two categories are as follows. The first category of spectra was characterized by the existence of upward flowing ions with conic distributions energized to 30 eV and possibly O+ ion cyclotron waves and large electron temperature enhancements. The second category of spectra was associated with wave activity in the 4‐ to 16‐kHz range, most probably O+ lower hybrid waves, and occasionally large ion temperature enhancements. The observations of ΔN/N and ΔE spectral behavior are compared to recent work on two‐dimensional plasma turbulence theory and nonlinear simulations of the collisional Kelvin‐Helmholtz (KH) instability. In particular, the spectral behavior associated with the moderate velocity shear category agrees well with some recent computations of the spatial power spectra of the KH instability (Keskinen et al., 1988).
E layer ionospheric disturbances following the Coalinga earthquake
Authors: Fitzgerald T. Joseph , Wolcott John H.
Published: February 2008

In a previous report, Doppler shifts in HF radio transmissions through the ionospheric F region following the Coalinga, California earthquake of May 1983 were described. We report here on disturbances which appeared between 400 and 500 s after the earthquake as peaks or broad bands of enhanced power in the spectra of the HF transmissions and not as Doppler shifts. While the time delay (600–800 s) before the occurrence of the Doppler shifts corresponds to the acoustic travel time to the F region, the time delay for the spectral changes indicates a travel time to the ionospheric E region. We believe that the occurrence of the spectral changes represents the presence of temporary E‐region propagation modes induced by the passage of the acoustic waves responsible for the Doppler shifts observed at higher altitudes. Because the enhanced spectral power occurs at negative frequency relative to that of the F‐region propagation modes, the disturbances could be the result of reflections from electron density gradients induced by upwardly propagating acoustic waves.
The source location of Jovian millisecond radio bursts with respect to Jupiter's magnetic field
Authors: Genova Françoise , Calvert Wynne
Published: February 2008

As in the previous study of the smooth Io‐dependent radio emissions from Jupiter (the so‐called L bursts or L emissions) by Genova and Aubier (1985), the location of the source of the jovian S bursts was studied by comparing the high frequency limit of these emissions, recorded in Nançay, France, to the surface gyrofrequency at the foot of the magnetic field lines which intersect Io's orbit, according to the O4 magnetic field model. For this purpose, the statistical occurrence of the S bursts was examined, both in central meridian longitude versus Io phase and as a function of the relative phase of Io with respect to Jupiter. The S bursts and the Io‐dependent L emissions were found to originate from approximately the same locations at Jupiter, and probably under similar conditions of excitation by Io, although the beaming of these S emissions, which is indicated by the compactness of the occurrence patterns, was somewhat narrower than for the corresponding L emissions. Also, like the L emissions, an apparent delay of up to 70° (measured in the equatorial plane) was found to occur between the predicted instantaneous Io flux tube and the apparent source field line. The possible origin of this 70° delay is discussed.
Solar and magnetic declination control on the electrojet and distributed currents in the ionosphere over Jicamarca
Authors: Muralikrishna P. , Abdu M. A.
Published: February 2008

Daytime variation in the average east‐west drift velocity of electrons in the height range of 95‐110 km over Jicamarca (0.9°N, dip latitude) is compared with variation in the horizontal component of the geomagnetic field at the closeby Huancayo station (0.6°N, dip latitude). On magnetically quiet days, the ratio of ΔH to the electron drift velocity shows a local time variation asymmetric with respect to local noon, although one would expect a symmetric cos χ (where χ is the solar zenith angle) type of variation for the same, if, indeed, ΔH arises solely from the electrojet region in which the electron drift velocities are measured. The observed asymmetry indicates significant contribution of distributed currents from height regions above the electrojet in the integrated current represented by ΔH. Further, the observed seasonal variation in the degree of the asymmetry seems to point out the importance of magnetic and solar declination angles in controlling the relative contributions of the distributed (and electrojet) currents to the magnitude of the observed ΔH values.
Lyman α emission cross sections for low‐energy H and H+ collisions with N2 and O2
Authors: Van Zyl B. , Neumann H.
Published: February 2008

Absolute cross sections for emission of Lyman α radiation have been determined for low‐energy H and H+ impact on N2 and O2 targets. Data were obtained for projectile energies between about 0.04 and 2.5 keV. Using previously measured cross sections for Balmer α and Balmer β emission, we were able to evaluate the cascade contributions to the measured Lyman α signals and to deduce the cross sections for both direct 2p state excitation and total Lyman α emission. The data are compared with other results for projectile energies below 10 keV, and the problems associated with the application of such cross‐section data to proton auroral analyses are discussed.
A simple model for the initial phase of a water plasma cloud about a large structure in space
Authors: Hastings D. E. , Gatsonis N. A. , Mogstad T.
Published: February 2008

Large structures in the ionosphere will outgas or eject neutral water and perturb the ambient neutral environment. This water can undergo charge exchange with the ambient oxygen ions and form a water plasma cloud. Additionally, water dumps or thruster firings can create a water plasma cloud. A simple model for the evolution of a water plasma cloud about a large space structure is obtained. It is shown that if the electron density around a large space structure is substantially enhanced above the ambient density then the plasma cloud will move away from the structure. As the cloud moves away, it will become unstable and will eventually break up into filaments. A true steady state will exist only if the total electron density is unperturbed from the ambient density. When the water density is taken to be consistent with shuttle‐based observations, the cloud is found to slowly drift away on a time scale of many tens of milliseconds. This time is consistent with the shuttle observations.
Atmospheric neutrons at 8.5‐GV cutoff in the southern hemisphere
Authors: Ait‐Ouamer Farid , Zych Allen D. , White R. Stephen
Published: February 2008

We report here, for the first time, southern hemispheric measurements of the altitude, latitude, angle, and energy distributions of atmospheric neutrons from 5 to 80 MeV. The data were obtained with the University of California, Riverside, balloon‐borne telescope flown from Alice Springs, Australia, on November 10, 1981, at 29° south geomagnetic latitude (SGML). The efficiencies required to evaluate the fluxes for both the upward and the downward moving neutrons were obtained from a Monte Carlo program tested against neutron calibrations. The counting rates of the upward and the downward moving neutrons increase from a residual altitude of 4 g/cm² to a maximum at 100 g/cm² and then decrease rapidly. These variations agree well with the previous results of Preszler et al. (1974) at 41° north geomagnetic latitude (NGML). The escape current calculated at 29° SGML, 8.5‐GV cutoff, for neutron energies between 20 and 70 MeV, is 0.059 ± 0.008 neutrons/cm² s, adding another point to the latitude distribution, in good agreement with the theoretical predictions of Merker (1972). The angular distributions at all altitudes are similar, with dips near 60° and 130° from the zenith direction. The energy distributions decrease with energy at all altitudes but flatten at energies around 25 MeV. The albedo neutron current decreases as E−1.61 ± 0.28 at 4.1 g/cm². These values are compared with the experimental results of Preszler et al. (1974, 1976), Lockwood et al. (1976, 1979), and Bhatt (1983, 1986) and to the theoretical results of Armstrong et al. (1973) and Merker (1972, 1975).
Electrostatic waves stimulated by coherent VLF signals propagating in and near the inner radiation belt
Authors: Bell T. F. , Ngo H. D.
Published: February 2008

Recent ISEE 1 satellite observations indicate that electrostatic waves are commonly stimulated by coherent VLF electromagnetic whistler mode signals propagating on magnetic field lines in, and near, the inner radiation belt. Evidence for the existence of the electrostatic waves consists of apparent bandwidth increases of up to 300 Hz in the electric field component of 10 to 15 kHz signals from ground‐based VLF transmitters observed in the inner radiation belt. During the period July through December 1983, the effect was observed on 60% of the satellite orbits which penetrated the inner radiation belts (L ≤ 2), and in many cases the effect was observed continuously from the magnetic equator at L ∼ 2 down to perigee. The effect appeared in signals propagating from sources in both the northern and southern hemispheres. In general, the stimulated electrostatic waves were delayed with respect to the input signal by 50 ms to 500 ms, suggesting that the generation region was removed from the satellite location. The total apparent bandwidth Δω of the stimulated electrostatic waves observed on any given satellite pass was found to be roughly proportional to the local magnitude of the Earth's magnetic field, but the constant of proportionality varied over a 4‐to‐1 range for the complete data set. In general at any given time, signals of similar frequency and propagation direction exhibited similar Δω. However, signals of similar frequency propagating in opposite directions occasionally exhibited marked differences in Δω. Characteristics of the stimulated wave structure coincide closely with those reported in a recent study of low altitude satellite VLF wave data (Bell et al., 1983). In consonance with this earlier work it is hypothesized that the electrostatic waves are stimulated as the electromegnetic whistler‐mode input waves scatter from magnetic‐field‐aligned plasma density irregularities which exist within the inner radiation belt. It is further hypothesized that once generated, these short wavelength electrostatic waves may then interact with energetic radiation belt particles, producing particle pitch angle scattering and precipitation. The induced precipitation could result in ionospheric plasma density enhancements, and upward diffusion of cold plasma from these enhancements may produce new magnetic‐field‐aligned density irregularities. This feedback process may provide a mechanism for the creation and maintenance of magnetic‐field‐aligned plasma density irregularities in the inner radiation belts.
On the interplanetary transport of solar cosmic rays
Authors: Schlickeiser Reinhard
Published: February 2008

The discrepancy between the observed particle's mean free path against scattering λfit as derived from applying time‐dependent spatial diffusion models to solar particle events and the theoretical value λt from quasi‐linear theory can be resolved. A more complete quasi‐linear result for the pitch‐angle diffusion coefficient of particles in an uniform magnetic field with superposed parallel and antiparallel propagating transverse Alfvén waves yields finite and well‐behaved expressions for the mean free path λt even for rather large values of the spectral index q of the assumed power law spectrum of the power spectrum of magnetic irregularities. It is shown that values of q between 2.1 and 2.2 lead to remarkable good agreement of λt with the observed variation of λfit from solar cosmic ray events.
A synoptic study of Doppler scintillation transients in the solar wind
Authors: Woo Richard
Published: February 2008

This paper presents the first synoptic study of Doppler scintillation transients caused by the passage of interplanetary disturbances. It is based on 40,000 hours of 2.3‐GHz Doppler scintillation measurements surrounding 15 superior conjunctions of planetary spacecraft during 1979–1983, a period that includes solar maximum. A total of 148 separate transients covering a heliocentric distance range of 5–179 RS were detected: 122 by one spacecraft and 26 by more than one. The enchancement in scintillation level (termed EF for enhancement factor) of the transients varied from 2 to 100. The multispacecraft measurements of 22 disturbances showed that they attenuated and expanded as they propagated outward from the Sun with propagation speeds varying from 200 to 2000 km/s. The propagation speed was highly correlated with EF (correlation coefficient of 0.91), showing that EF serves as a good indicator of propagation speed, at least for large EF. The frequency of occurrence was highest near the Sun (within 40 RS), where it was 0.29 transients/d, and decreased with radial distance, a reflection of the radial evolution of the transients and the sensitivity of the Doppler scintillation measurements to transients. Since the Doppler scintillation measurements are most effective near the Sun, they ideally complement in situ spacecraft observations farther away. An earlier study of nine interplanetary shock waves based on scintillation, Solwind coronagraph, and Helios plasma measurements showed a significantly wider range of shock velocities close to the Sun than near 1 AU. The results of this synoptic study indicate that this is apparently a long‐term feature of the solar wind around solar maximum.
Average low‐latitude meridional electric fields from DE 2 during solar maximum
Authors: Maynard N. C. , Aggson T. L. , Herrero F. A. , Liebrecht M. C.
Published: February 2008

Electric field data from the double probe vector electric field instrument (VEFI) on the DE 2 spacecraft have been analyzed to determine the average meridional electric field (zonal ion flow) patterns in the region between ±30° magnetic latitude during solar maximum conditions. Over 300 passes were used to compile the data set. Data were projected to a constant 300‐km altitude and to the equatorial plane assuming that the electric field along the magnetic field was zero. The average data set displayed a rapid increase of the downward meridional electric field with local time near 1800 MLT with the higher latitudes seeing the change first. A secondary nighttime maximum of this electric field component was observed post midnight with the crossover to upward electric fields (westward ion flow) occurring between 0400 and 0500 MLT. A sharp return to near zero was observed between 1200 and 1300. Typical average amplitudes range between 3 and 6 mV/m. No consistent variations with magnetic activity were observed. Although the daily variation in the zonal ion flow is dominated by the diurnal term, a net superrotation is evident in the harmonic analysis. The superrotation is strongest near the equator and decreases with latitude, because of the disturbance dynamo. The higher order harmonics up through the quatrediurnal term are also of significant magnitude in the analysis of the shape of the daily variation. Close similarity is seen to the zonal neutral winds indicating that they form the principal driving force. The magnitudes of the electric field derived ion drift are somewhat higher than the average F region neutral wind values. This along with the higher order harmonic content argues for the need to develop fully coupled E and F region model depicting the ionosphere and thermosphere interactions in a self‐consistent fashion.
The global thermospheric mapping study
Authors: Oliver William L. , Salah Joseph E.
Published: February 2008

The Global Thermospheric Mapping Study (GTMS) is a multitechnique experimental pilot study of the Earth's thermosphere designed to map simultaneously its spatial and temporal morphology. Two observation campaigns were conducted during June 26–28, 1984, and January 15–17, 1985, and involved the successful participation of 24 instruments worldwide. This paper provides the background for the study and presents the analysis techniques employed at Millstone Hill and results to date on thermospheric structure and dynamics. The first latitudinal‐temporal maps of exospheric temperature obtained from the incoherent scatter radar chain at 70°W meridian are presented for the two solstice periods, revealing substantial seasonal differences between them. The observed structure shows a relatively depressed temperature at high latitude in summer in contrast to the mass spectrometer /incoherent scatter 1983 [MSIS‐83] empirical model, which shows a maximum temperature at polar latitudes. The MSIS‐83 model predictions are in good agreement with the observed latitudinal‐temporal structure in winter. Comparison with the numerical predictions made for the June 26–28, 1984, period with the National Center for Atmospheric Research thermospheric general circulation model shows reasonable agreement in the latitudinal gradient but the observations indicate a cooler thermosphere by several hundred degrees. Neutral winds at mid‐latitudes are presented showing the expected strong southward winds at night, which are found to be consistent with the temperature gradients observed in the latitudinal maps. There is good agreement in the June winds between the available numerical model calculations and the observations. Work performed elsewhere on the GTMS data base is summarized for completeness.
Nighttime Na D emission observed from a polar‐orbiting DMSP satellite
Authors: Newman Alice L.
Published: February 2008

Limb scans of nighttime sodium doublet emissions at 589 nm were performed globally from a Sun‐synchronous satellite during July 1979. The observations have provided a global description of variation in the emitting layer with latitude and altitude. The limb scan data are consistent with the presence of a 10‐km‐thick emitting layer near the 90‐km peak of the sodium density in the northern mid‐latitude mesosphere in summer. At equatorial and southern latitudes, however, a narrower emission layer occurs 5–10 km lower, and the intensity is stronger by a factor of 3 or more. It appears likely that the shape of the Na D emission profile is influenced by the altitude distributions of both sodium and ozone, the reactants which are necessary to produce the emission. The observed variability may be explained by a systematic seasonal mismatch in the altitude of the mesospheric density maxima for sodium and ozone, combined with a known threefold wintertime increase in sodium density.
Proton beam generation of oblique whistler waves
Authors: Wong H. K. , Goldstein M. L.
Published: February 2008

It is known that ion beams are capable of generating whistler waves that propagate parallel to the mean magnetic field Bo. Such waves may have been observed both upstream of the Earth's bow shock and in the vicinity of comets. We extend previous analyses to include propagation oblique to Bo. The instability is generated by the perpendicular component of free energy in the ions; which can arise either via a temperature anisotropy (T⊥/T∥ ≫ 1) or via a gyrating distribution. In the former case the generation of whistler waves is confined to a fairly narrow cone of propagation directions centered about parallel propagation; while in the latter case, the maximum growth of the instability can occur at fairly large obliquities (θ ≅ 50°).
Ionospheric Hall conductivity deduced from ground‐based measurements
Authors: Nielsen E. , Senior C. , Lühr H.
Published: February 2008

The ionospheric height‐integrated Hall conductivity has been derived from two different techniques: first, by combining ground‐based magnetometer observations with measurements of the ionospheric electric field made by the Scandinavian Twin Auroral Radar Experiment (STARE), a coherent radar system, and, second, by combining electron density measurements made by the European incoherent scatter radar (EISCAT) with a neutral atmosphere model. For the events analyzed, EISCAT did not operate in a mode that allowed the electron and ion temperatures to be measured below 131 km. In the analysis these temperatures are therefore assumed to be of the same magnitude. For small electric fields (25 mV/m) we find the two conductivity estimates to be in good agreement. However, for large fields (75 mV/m) the former method yields a conductivity value larger by a factor of 1.5 than the latter method. We interpret this result as being due to an increase of the electron temperature above the ion temperature, possibly associated with the excitation of unstable short‐scale plasma waves in the ionospheric E region. The combination of STARE and magnetometer data is demonstrated to lead to realistic estimates of the Hall conductivity in the eastward electrojet.
Space shuttle charging or beam‐plasma discharge: What can electron spectrometer observations contribute to solving the question?
Authors: Watermann J. , Wilhelm K. , Torkar K. M. , Riedler W.
Published: February 2008

Several cooperative plasma experiments were carried out on board Spacelab‐1, the ninth payload of the Space Transportation System (STS‐9). Among them, the electron spectrometer 1ES019A was designed to observe 0.1–12.5 keV electron fluxes with high temporal and spatial resolution, while the SEPAC electron beam accelerator emitted electron beams with currents up to 280 mA and maximum energies of 5 keV. Since the question of orbiter charging to high voltages has controversially been discussed in several publications on STS‐3 and STS‐9 electron beam experiments, an attempt is made to relate information from the return electron flux observed during the SEPAC operations to the vehicle charging interpretation. A close examination reveals that most of our observations can be understood if the occurrence of a beam‐plasma discharge is assumed at least for electron beam intensities above 100 mA. This would provide a substantial return current capability. High orbiter charging effects during electron beam accelerator electron emissions are consequently not supported by our observations.
Motion of charged particles in planetary magnetospheres with nonelectromagnetic forces
Authors: Huang T. S. , Hill T. W. , Wolf R. A.
Published: February 2008

We derive expressions for the mirror point, the bounce period, the second adiabatic invariant, and the bounce‐averaged azimuthal drift velocity as functions of equatorial pitch angle for a charged particle in a dipole magnetic field in the presence of centrifugal, gravitational, and Coriolis forces. These expressions are evaluated numerically, and the results displayed graphically. We also evaluate the average azimuthal drift speed for a flux tube containing a thermal equilibrium plasma distribution.
High‐resolution bistatic CW interferometry for auroral ionospheric studies: Preliminary results
Authors: Haldoupis C. , Sofko G. J. , Koehler J. A. , Prikryl P. , McKibben M. J.
Published: February 2008

The monostatic coherent radar interferometer technique which has been applied in the past few years to studies of short‐wavelength plasma turbulence in the earth's ionosphere can provide, in addition to the Doppler spectrum, estimates of mean size, mean angular position, and velocity transverse to the viewing direction. Here we describe the conversion of an inexpensive bistatic 50‐MHz continuous wave (CW) Doppler system to an interferometer designed to investigate short‐term dynamics of highly localized scattering regions in the auroral ionosphere, with mean transverse sizes in the range from about 0.5 to 5.0 km. Preliminary analysis shows, at times, high‐coherence echoes, of type I or type III, to originate from localized, strongly unstable regions which are related to dynamic source regions of transient nature with mean azimuthal sizes possibly less than 1 km. Our observations suggest that the CW Doppler system, which provides high‐quality spectra and excellent temporal resolution, has its measurement capability enhanced significantly when operated as an interferometer, becoming a powerful tool for studying short‐term temporal and spatial variations of electrostatic turbulence in the auroral plasma.
Model calculations of O(¹S) and O(¹D) emissions under proton bombardment
Authors: Srivastava V. , Singh Vir
Published: February 2008

A theoretical model is developed to study the volume emission rate profiles of the O(¹S) and O(¹D) emissions under proton bombardment. All possible sources of these emissions are considered and recent experimental cross sections are used in the calculations. Emission intensities are computed by using Maxwellian and power law spectra for precipitating protons. The effects on the emission intensities due to various parameters involved in the proton spectra are studied. The results presented in this study can directly be used to study the O(¹S) and O(¹D) emissions in proton aurora as well as the solar proton events.
north‐south asymmetry of the interplanetary magnetic field spiral
Authors: Bieber John W.
Published: February 2008

An analysis of 20 years of interplanetary magnetic field (IMF) data reveals that the IMF spiral is more tightly wound north of the heliospheric current sheet than it is south of the current sheet, the difference in mean spiral angle being some 3.1° ± 1.1° at the orbit of Earth. A concurrent analysis of plasma data indicates that this asymmetry does not originate with a corresponding asymmetry in solar wind speed. Currently available information suggests that asymmetric solar rotation may give rise to part of the observed spiral asymmetry but probably not all of it. Whatever its origin, the asymmetry of the spiral may help explain north‐south differences seen in the solar modulation of galactic cosmic rays.
The altitude distribution of impulsive signals in the night ionosphere of Venus
Authors: Russell C. T. , von Dornum M. , Scarf F. L.
Published: February 2008

A new examination of burst like signals in the night ionosphere of Venus has been undertaken to determine the rate of occurrence of such signals. On average at all frequencies the bursts occur most often at lowest altitudes. The rates of occurrence of bursts at 0.73, 5.4 and 30 kHz fall off rapidly with altitude. At 350 km these bursts occur about an order of magnitude less often than at 150 km. At 100‐Hz the rate of falloff with altitude is less rapid. The occurrence rate decreases from about 50% at 150 km to between 20 and 40% at 500 km and then decreases more slowly above 500 km. This decrease suggests a wave source at lowest altitudes but high altitude sources also appear to be present. The rate of occurrence is quite variable both from season to season and from the inbound to the outbound portion of the orbit. This variability is consistent with either temporally or spatially varying source regions for these impulsive signals. Lightning in the Venus atmosphere could produce emissions consistent with the properties of those signals observed at lowest altitudes.
Simulation of electrostatic turbulence in the plasma sheet boundary layer with electron currents and bean‐shaped ion beams
Authors: Nishikawa K. ‐I. , Frank L. A. , Huang C. Y.
Published: February 2008

Plasma data from ISEE 1 show the presence of electron currents as well as energetic ion beams in the plasma sheet boundary layer. Broadband electrostatic noise and low‐frequency electromagnetic bursts are detected in the plasma sheet boundary layer, especially in the presence of strong ion flows, currents, and steep spacial gradients in the fluxes of few‐keV electrons and ions. Particle simulations have been performed to investigate electrostatic turbulence driven by a cold electron beam and/or ion beams with a bean‐shaped velocity distribution. The simulation results show that the counterstreaming ion beams as well as the counterstreaming of the cold electron beam and the ion beam excite ion acoustic waves with the Doppler‐shifted real frequency ω ≈ ± k∥ (Cs ‐ Vi∥). However, the effect of the bean‐shaped ion velocity distributions reduces the growth rates of ion acoustic instability. The simulation results also show that the slowing down of the ion bean is larger at the larger perpendicular velocity. The wave spectra of the electric fields at some points for simulations show turbulence generated by growing waves. The frequency of these spectra ranges from Ωi to ωpe, which is in qualitative agreement with the satellite data.
Optical measurements of winds in the lower thermosphere
Authors: Wiens R. H. , Shepherd G. G. , Gault W. A. , Kosteniuk P. R.
Published: February 2008

WAMDII, the wide‐angle Michelson Doppler imaging interferometer, was used to measure the neutral wind in the lower thermosphere by the Doppler shift of the O I 557‐nm line. Observations were made at Saskatoon (60.5° N invariant) around the spring equinox of 1985 with WAMDII coupled to an all‐sky lens. With dopplergrams averaged over 3 to 30 min, no evidence was found for persistent highly localized winds on either of the two nights studied, one viewing only aurora and one viewing only air glow. The nocturnal variation was determined for both nights using average horizontal wind for the whole all‐sky image. The pattern for the auroral case shows winds parallel to the aurora orientation in the evening but substantial crosswinds near midnight. High latitude general circulation models seem to represent this case better than local auroral generation models. The airglow case showed eastward winds in the morning sector.
Polarization and amplitude characteristics of Pc 3 pulsations in conjugate area around L = 1.3–2.1
Authors: Yumoto K. , Saito T. , Tanaka Y. , Lynn K. J. W. , Menk F. W. , Fraser B. J.
Published: February 2008

In order to extend both theoretical and experimental investigations of low‐latitude Pc 3 magnetic pulsations, we conducted conjugate magnetic observations at Asahikawa (ASH; λ = 142.2°, Φ = 43.93°, L = 1.55) and Onagawa (ONW; 141.48°, 38.43°, 1.30) in Japan and Birdsville (BSV; 139.3°, −25.83°, 1.55), i.e., the conjugate point of ASH, Dalby (DAL; 151.2°, −27.18°, 1.56) and St. Kilda (SKD; 138.5°, −34.7°, 2.11) in Australia, during the period from July 20 to September 16, 1986. From the analysis of these conjugate‐area data, occurrence and polarization characteristics of low‐latitude Pc 3 pulsations can be summarized as follows: (1) The average amplitude at SKD (L = 2.1) is about twice that at BSV (1.55). (2) The H component amplitude at BSV in the winter hemisphere is larger than that at ASH in the summer hemisphere. (3) The polarizations at all stations switch statistically from left‐handed in the morning to right‐handed in the afternoon, where polarization senses are defined with reference to the direction of the main geomagnetic field. (4) However, the switch from L‐H to R‐H polarization at the conjugate stations (L = 1.55) does not always occur near local noon, but sometimes near 0500 and 1700 LT. (5) The abrupt changes of major axis orientations of Pc 3 polarization ellipses appear to coincide with the ionospheric conductivity enhancement and depression, respectively, associated with local sunrise and sunset. These observational facts suggest that a standing field line resonance in the Pc 3 frequency range is situated at L >2.1, and that at lower latitudes (L ≲ 1.5) Pc 3 magnetic pulsations could arise predominantly from compressional mode waves propagating transverse to the Earth's magnetic field through the low‐latitude ionosphere.
Doppler spectra of the UHF diffuse radio aurora
Authors: Hall G. E. , Moorcroft D. R.
Published: February 2008

A large number of radar backscatter Doppler spectra from the 398‐MHz phased array at Homer, Alaska, have been analyzed for representative periods of widespread, long‐lasting radar echoes in the premidnight and postmidnight sectors. Spectra corresponding to type 1 and type 2 echoes were observed. The type 1 spectra were most evident in the postmidnight data, where they appeared as velocity plateaus in azimuthal scans at the farther ranges. The type 2, low‐velocity spectra were generally observed only in the absence of type 1 echoes. The spectral widths of the type 2 spectra were consistent with decay rates estimated by linear theory, as might be expected for dissipative waves at the short‐wavelength end of a turbulent cascade. The velocity plateaus and other features of the azimuthal and range variation of Doppler velocity can be understood if the auroral electrojet is assumed to have moderate variations in direction on a scale of tens of kilometers. It was possible to obtain from the Doppler data qualitative estimates of the electric field, whose variation in time and latitude was in general agreement with the model electrojet configuration of Kamide and Vickrey (1983).
Wave normal direction and spectral properties of whistler mode hiss observed on the DE 1 satellite
Authors: Sonwalkar Vikas S. , Inan Umran S.
Published: February 2008

A new study of magnetospheric hiss as a spatially and temporally enduring phenomenon is undertaken using a recently developed formalism that allows the representation of hiss by a field distribution function (FDF). This formalism explicitly takes into account the whistler mode relationships and the linear and spin motion of the satellite, so that on a spin‐stabilized satellite, it becomes possible to measure the wave propagation direction(s) from the observed fading patterns in the received electromagnetic field data. We have analyzed hiss signals received by electric and magnetic field antennae aboard the DE 1 satellite during a ∼3‐hour period on September 23, 1983. A band of hiss at frequencies <2 kHz was observed continuously from 0236 UT to 0539 UT over a range of geomagnetic latitudes from λm = 45°N to λm = 20°S and L shells of L = 4.3 to L = 5.3. Electron density deduced from in situ and remote measurements indicates that during this time the DE 1 satellite was near the boundary of the plasmasphere. Observations can be summarized as follows: (1) The general character of the electric and magnetic field spectrum remained the same throughout the 3‐hour‐long period, exhibiting an intensity peak at 1550 Hz. (2) The intensities of both the electric and the magnetic field decreased during this interval by ∼30 dB. (3) Well‐defined fading patterns at half the spin period (3.04 s) were observed throughout this period. These patterns were stable over a time scale of ∼1 min. (4) In addition, a slow fading pattern with a time scale of 30 s was observed approximately 30% of the time. (5) The hiss intensity also showed fading over a time scale of ∼10‐15 min. The spin fading patterns led to the measurement of wave propagation direction(s). The results of our analysis can be summarized as follows: (1) Near the geomagnetic equator (within ∼ ±3° latitude) we observe a wave normal angle of 60° ± 5° with respect to the local geomagnetic field. (2) Away from the equator, wave normal directions range from 30° to 80° with respect to the local geomagnetic field. (3) All wave normal directions observed over the 3‐hour‐long period were within ∼ ±45° of the plane normal to the local magnetic meridional plane. Results indicate that the hiss source radiates with initial wave normal angles in the range 30° < θ0 < θg (Gendrin angle) and 90° < ϕ0 < 270°, contrary to the common assumption that the source emits with wave normal angles closely aligned with the geomagnetic field. The FDF formulation has also permitted elucidation of the statistical nature of hiss. The slow time fading (∼30 s) is interpreted in terms of a coherence bandwidth Δω of about 0.2 rad/s.
Interplanetary cosmic ray latitudinal gradient in 1984 to 1987 using IMP 8 and Voyager data
Authors: Lockwood J. A. , Webber W. R. , Hoeksema J. T.
Published: February 2008

We have used the cosmic ray particle measurements for E > 70 MeV from the Voyager 1 and 2 and IMP 8 detectors together with observations of the tilt of the heliospheric current sheet to determine the latitudinal gradient in 1984–1987 by a new method. We find that the cosmic ray intensity decreases with heliographic latitude and the changing latitudinal gradient observed during this time period can be directly attributed to the changes in the tilt of current sheet. For a radial gradient of 2.0%/AU between IMP 8 and Voyager 2 near the heliographic equator, the latitude gradient varies from ∼ −1.5%/deg to ∼ −0.9%/deg as the current sheet tilt decreases, and the latitudinal gradient that would be measured for a tilt of 0° is −1.5%/deg or −3.3%/AU. This is larger in magnitude than the radial gradient of 2.0%/AU existing at this time. The different methods used to determine the latitudinal gradient are discussed. The measured latitudinal gradients are then compared with the predictions of the drift models for cosmic ray propagation and the effects of the latitudinal variation of the solar wind upon cosmic rays. The measured gradients are generally much larger than predicted, especially in the case of the drift models. In all cases there is an important distinction between the measured average 26‐day gradient and the instantaneous or daily average gradient. The 26‐day variations of intensity seen at Voyager 1 during the period 1984–1987 are much smaller and more irregular than the predicted variations due to the wavy current sheet. It appears that neither the effects of drifts nor of the solar wind speed variations as a function of distance from the current sheet can fully explain the features of the latitudinal gradient reported here.
Auroral origin of a component of Jupiter's decametric radiation
Authors: Maeda K. , Carr T. D.
Published: February 2008

Simultaneous observations at about 22 MHz of an Io‐unrelated component of Jovian decametric radiation from Voyagers 1 and 2 and from a ground‐based observatory are analyzed to investigate the beam structure of the radiation and to locate the radio‐emitting region. It is concluded that the radiation of this component is emitted in quasi‐continuous beams that corotate with the Jovian inner magnetosphere and that each beam can be approximated by a sector of a hollow cone. The source locations are geometrically determined by fitting a hollow‐cone beam model, based on the O4 magnetic field model, to the observations. The result indicates that the radio sources lie above the northern hemisphere and on the field lines that are connected with the ultraviolet aurora of the so‐called active sector.
Cross correlation of high‐latitude upper mesospheric neutral winds with AE and Kp
Authors: Johnson R. M. , Luhmann J. G.
Published: February 2008

High‐latitude upper mesospheric neutral winds are investigated for evidence of a response to changes in the level of geomagnetic activity through a cross‐correlation analysis using AE and Kp. A consistent and significant causal response within several days of intervals of increased activity is not evident in the neutral wind data from 80 to 90 km altitude. Instead, through cross correlation with the neutral wind data set, the low‐frequency components of the AE and Kp data sets have been identified. These results, in conjunction with other recent studies, indicate that it is above 90 km, in the lower thermosphere, that magnetospheric forcing first becomes significant for affecting neutral dynamics at high latitudes.
X ray spectral images of energetic electrons precipitating in the auroral zone
Authors: Datlowe D. W. , Imhof W. L. , Voss H. D.
Published: February 2008

X rays from the Earth's auroral zone are produced by the precipitation of electrons with kilovolt energies. We have used an imaging X ray spectrometer to observe this radiation over a ∼500‐km field of view divided into 16 pixels. The observations were made by an experiment on the S81‐1 satellite (SEEP: Stimulated Emission of Energetic Particles) during June 1982. Using high spectral resolution 4‐ to 40‐keV X ray data, we have calculated the flux and spectral slope of the precipitating electrons. We have conducted, for the first time, a systematic survey of the characteristics of electron spectra in a large number of individual X ray features. Most of the precipitation is confined to well‐defined locations, arcs or isolated patches, separated by regions where no X ray flux was detected. Narrow features are typical for energetic electrons above 5 keV, in contrast to lower‐energy electrons, which may produce relatively diffuse optical images. These X ray images have produced two‐dimensional maps of precipitation which show no trends of spectral hardness with position, while on the same spatial scales there are order of magnitude changes in the flux of precipitating electrons.
Magnetic field and density fluctuations at perpendicular supercritical collisionless shocks
Authors: Winske D. , Quest K. B.
Published: February 2008

Perpendicular collisionless shocks are studied by means of two‐dimensional hybrid (particle ion, fluid electron) simulations, with emphasis on the relaxation of the highly anisotropic ion distribution that arises primarily from reflection of some of the incident ions but also adiabatic compression of the other, directly transmitted ions and the related growth of low frequency electromagnetic waves. It is commonly assumed that the waves are due to the Alfven ion cyclotron instability that propagate parallel to the ambient magnetic field (k⊥ = 0) and that the isotropization of the ions due to pitch angle scattering by the waves and the corresponding modification of the wave spectrum is quasi‐linear. It is shown that this is indeed a reasonably good description downstream of the shock front, behind the magnetic overshoot. However, at the shock ramp there is a large discrepancy between the wavelengths measured in the simulations and those predicted by linear theory, and large density and magnetic field oscillations parallel to the ambient magnetic field are also seen. By comparing results for both high and low ion beta cases, it is shown that these effects can be understood in terms of obliquely propagating (k⊥ ≠ 0) modes, more likely due to the Alfven ion cyclotron instability instead of the (drift) mirror instability, although a more complete explanation awaits the derivation and analysis of an appropriate dispersion relation describing the growth and coupling of low‐frequency modes in the inhomogeneous high beta environment of the shock. The observational consequences of these results and their application to improving nonlocal leakage models for the ion foreshock are also discussed.
Auroral electron energy and flux from molecular nitrogen ultraviolet emissions observed by the S3‐4 satellite
Authors: Ishimoto M. , Meng C.‐I. , Romick G. J. , Huffman R. E.
Published: February 2008

The UV spectra over the southern hemisphere nightside auroral oval have been obtained from an AFGL spectral/photometric experiment on board the low‐altitude polar‐orbiting S3‐4 satellite. A detailed analysis of nightside auroral spectra from seven orbits between mid‐May and June 1978 was performed to estimate the average energy and total energy flux of incident electrons. This study was based on observations of the N2 LBH (3–10) (1928 Å) band and the N2 VK (0–5) (2604 Å) band emission intensities and the application of model calculations by Strickland et al. [1983] and Daniell and Strickland [1986]. Comparison of the estimated quantities with the statistical satellite measurement of incident particles by Hardy et al. [1985] indicates that the LBH (3–10) band emission intensity can be used to estimate the total energy flux of incident electrons, similar to the N2+ 1N (0‐0) (3914 Å) band emission intensity in the visible region. In addition, the ratio of the LBH (3–10) to the VK (0–5) band emission intensities indicates the average energy of incident auroral electrons in much the same way that the N2+ 1N (0–0) and O I (6300 Å) emission ratio does in the visible region. This study shows the use of different constituent emissions, model calculations, and synthetic spectra to infer the inherent possibilities in these types of studies.
Computer simulation of the Farley‐Buneman instability and anomalous electron heating in the auroral ionosphere
Authors: Machida S. , Goertz C. K.
Published: February 2008

We study the nonlinear saturation of the Farley‐Buneman instability in a collisional plasma by a 2 1/2 dimensional electrostatic particle simulation which includes inelastic and elastic collisions of electrons and elastic collision of ions with neutrals. In our simulation, a uniform convection electric field is applied externally so that the relative velocity between the electrons and ions is greater than the ion sound speed and destabilizes the instability. We find a nonlinear frequency shift from higher to lower frequencies and diffusion of the wave spectrum in two dimensional wave number space. We are especially interested in finding whether the saturated wave turbulence can account for the anomalous heating rates observed in the polar ionosphere by Schlegel and St.‐Maurice (1981). We find that the dominant mechanism for electron heating is due to an enhanced effective electron collision frequency and hence enhanced resistive heating as suggested by Primdahl (1986) and Robinson (1986) and not due to the heating of electrons by the electric field of the waves parallel to the magnetic field. For the ionospheric conditions discussed by Schlegel and St.‐Maurice (1981) we find an anomalous heating rate of about 4×10−7 W/m³.