bluuebunny/crossref_metadata_2025 huggingface dataset:
Ion composition in sporadic E layers measured by the EISCAT UHF radar
Authors: Huuskonen A. , Nygrén T. , Jalonen L. , Bjørnå N. , Hansen T. L. , Brekke A. , Turunen T.
Published: February 2008

We present ion composition results for several sporadic E layers observed during a Norwegian special program carried out by the European Incoherent Scatter Facility (EISCAT) UHF radar in August 1984. A range resolution of 600 m was used. Eight sporadic E layers at altitudes 95–114 km were studied using various fitting strategies for obtaining the ion composition. A fixed temperature was assumed in some of the strategies; in others the temperature was fitted together with the composition. Heavy ions, most probably Fe+, were found to be present in most layers with relative abundances of 30–80%. In one case, the composition analysis shows that essentially no heavy ions are present. This layer is supposed to be composed of light metal ions, such as Mg+. In one particular event a layer is seen to separate into two sheets with a height difference of 2–3 km, and the ion compositions in the two parts are found to be different. Possible explanations of this phenomenon are discussed.
Ionospheric incoherent scatter measurements with the middle and upper atmosphere radar: Observations during the large magnetic storm of February 6‐8, 1986
Authors: Oliver W. L. , Fukao S. , Sato T. , Tsuda T. , Kato S. , Kimura I. , Ito A. , Saryou T. , Araki T.
Published: February 2008

The middle and upper atmosphere (MU) radar of Japan is a 46.5‐MHz, pulse‐modulated, monostatic, Doppler radar with an active phased‐array antenna which consists of 475 crossed yagis. This system has been used primarily, since its initial observations with a partial system in 1983, to observe the coherent backscatter from irregularities in the troposphere, stratosphere, and mesosphere (MST radar). However, this system was also designed to be able to observe the weak incoherent scatter (IS) from the free electrons of the ionosphere. We report here the MU radar IS observations made during the strong geomagnetic storm of 6–8, February 1986. During this period the MU radar observed the echo power (an approximate measure of electron density) simultaneously in four antenna beam positions. Coincident with the rise in the Kp index from 3+ to 6− near 1800 LT (0900 UT) on February 7, the MU radar detected the beginning of several hours of wave activity having a period of 100 min. By correlating the density observations in the different beams and at different altitudes, we were able to compute the phase speed (410 m/s), direction of travel (19° west of south), and horizontal (2500 km) and vertical (290–490 km) wavelengths of the wave. The F layer peak density and height oscillated during this nighttime period, and we consider several mechanisms that might cause these effects. We find that the density oscillation was too large to be caused solely by wave dynamics. We suggest that an influx of ionization from the plasmasphere, and a wave‐induced modulation of the assimilation of this plasma flux into the ambient density, may have been the cause of the observed density increases. During the following morning (February 8) the F layer density was observed to become increasingly large while the layer peak height dropped, and we also consider the cause for this effect.
The oblique behavior of low‐frequency electromagnetic waves excited by newborn cometary ions
Authors: Brinca Armando L. , Tsurutani Bruce T.
Published: February 2008

The free energy in oxygen or hydrogen ions freshly created in the solar wind stimulates low‐frequency electromagnetic waves whose growth does not always maximize at parallel propagation. Exploration of the wave vector plane discloses the frequent occurrence of islets of oblique growth unconnected to the unstable parallel modes. Contour plots of the growth rate, real frequency, polarization, and magnetic compression characterize the oblique wave behavior for large values (≥45°) of the initial pitch angle of the cometary particles. Although wave‐particle (Landau and cyclotron) resonances feed most of the surveyed oblique instabilities, some are seemingly fluidlike. The results, obtained from the numerical solution of the kinetic dispersion and wave equations, imply that newborn ions can easily excite significant oblique hydromagnetic wave activity. Cometary environments provide the adopted plasma model, but the study is helpful in the interpretation of other low‐frequency wave observations in space.
Low‐frequency plasma waves and ion pitch angle scattering at large distances (>3.5 × 105 km) from Giacobini‐Zinner: Interplanetary magnetic field α dependences
Authors: Tsurutani Bruce T. , Page D. Edgar , Smith Edward J. , Goldstein Bruce E. , Brinca Armando L. , Thorne Richard M. , Matsumoto Hiroshi , Richardson Ian G. , Sanderson Trevor R.
Published: February 2008

The properties of 50‐ to 100‐s low‐frequency plasma waves and the parent 65‐ to 95‐keV H2O group ion pitch angle distributions are presented for spatial regions distant (3.5‐7.0 × 105 km) from comet Giacobini‐Zinner where the cometary ion densities are low and the solar wind interaction is weak. The presence of interplanetary discontinuities in this interval causes the interplanetary magnetic field angle α
to vary over a range from ∼35° to 90°, allowing the possible generation of several new wave modes which are theoretically expected, but as yet undetected. It is determined that when x = 45° ± 20° (Parker spiral orientations), the waves are typically elliptically polarized and propagate at 20°‐50° relative to B0. The wave properties are, however, not limited to the above range. They can exhibit even more extreme polarizations and propagation directions. The waves have periods near the H2O group ion cyclotron period and are left‐hand polarized in the spacecraft frame. In the relatively rare instances when wave steepening is detected, the steepened edge is near the trailing portion of the wave. The wave period, sense of polarization, and steepening are consistent with the properties of waves detected closer to the comet and identify them as being generated by the right‐hand resonant instability. The high degree of ellipticity is in sharp contrast to waves detected closer to the comet, however. The energetic ions are not isotropic in pitch angle (in the plasma frame), in general agreement with recent numerical modeling. When α increases to 70°‐90°, the right‐hand resonant mode is observed to cut off, as theoretical studies indicate. However, the left‐hand cyclotron resonant mode is not detected. Although the detected energetic 65‐ to 95‐keV ion fluxes are enhanced during these intervals (because of the instantaneous pickup by the solar wind), particle pitch angle scattering away from the initial ring distribution is very weak, consistent with the observed absence of resonant cyclotron waves.
PC 1 micropulsations observed by Magsat in the ionospheric F region
Authors: Iyemori Toshihiko , Hayashi Kanji
Published: February 2008

Large‐amplitude (5‐30 nT) Pc 1 micropulsations were detected near the plasmapause by a low‐altitude satellite, Magsat, in the ionospheric F region. Both left‐ and right‐hand‐polarized waves were observed in a latitudinally narrow (<100 km) region. The sense of polarization tends to switch around the amplitude peak, suggesting the coupling of fast mode and transverse mode waves and the propagation of the incident wave not only along the ambient magnetic field but also in the east‐west direction across the magnetic field. On the ground, periodic hm emissions having nearly the same frequencies as those at Magsat were observed at the stations 1000‐3000 km from Magsat orbit, and the amplitude on the ground was more than 2 orders of magnitude smaller than that observed by Magsat.
On propagation mechanisms of localized field‐aligned currents in the magnetosphere
Authors: Nakamura Tadas , Tamao Tsutomu
Published: February 2008

Two different mechanisms for field‐aligned current propagation, i.e., the Alfvén wave and the ballistic motion of energetic electrons associated with a localized source, are examined with linear kinetic theory. The characteristics of these two mechanisms and the relationship between them are investigated as an initial value problem. It is shown that in addition to the shear Alfvén waves, the ballistic motion of electrons can also carry currents with the perpendicular scale comparable to the electron inertia length. The propagation speeds of these currents are independent of the Alfvén speed.
Analysis and interpretation of observations of airglow at 297 nm in the Venus thermosphere
Authors: LeCompte Malcolm A. , Paxton Larry J. , Stewart A. Ian F.
Published: February 2008

Limb profiles obtained by the Pioneer Venus orbiter ultraviolet spectrometer (PVOUVS) are analyzed. The analysis is confined to those data obtained when the spacecraft was near periapsis (∼150‐200 km altitude) and the PVOUVS grating is set to a center‐of‐band‐pass wavelength of 297 nm. Sources are identified which give rise to the observed signal as they fall within the 1.3‐nm instrumental band pass: Rayleigh scattering, [O I] 2972 Å, and C I 2967 Å. Additionally, contamination from off‐axis scattering and the cosmic ray background are evaluated. Below 175 km, O(¹S) is produced through the photo‐dissociative excitation of CO2. Above 175 km, O(¹S) produced through the dissociative recombination of O2+ and photoelectron impact excitation of C(5S°) become important sources of airglow emissions. Structure in the [O I] 2972‐Å emission profile in the lower thermosphere is explained in terms of large temperature excursions which alter the rate of quenching of O(¹S) by CO2. This phenomenon suggests a new technique for remotely sensing Venus lower thermosphere temperatures. The deduced temperature profile is consistent with in situ measurements which have been interpreted as evidence of the propagation of atmospheric gravity waves.
Heating of upflowing auroral H+ and O+ Beams: Results from quasi‐linear theory
Authors: Ludlow G. R. , Kaufmann R. L.
Published: February 2008

Particle measurements at 3 RE in the auroral zone revealed the existence of simultaneous upgoing hydrogen and oxygen beams on auroral field lines. The observed beam distributions were broad, indicating that significant heating took place near or below the satellite. The most interesting features that were noted in the distribution functions included extended parallel O+ high energy and H+ low energy tails, and a perpendicular broadening of the O+ distribution. The purpose of the present work was to carry out an analysis of the mechanisms that could produce such heating, and to base this analysis as fully as possible on plasma and wave parameters measured by the Dynamics Explorer 1 (DE 1) satellite. An analysis of Maxwellian fits to the observed distribution functions showed that an oblique slow hydrogen acoustic mode was weakly unstable at frequencies between the oxygen and hydrogen gyrofrequencies. The location of wave‐particle resonant and nonresonant regions in velocity space suggested qualitatively that H+ undergoes weak parallel heating on the low velocity side of the beam and that O+ is heated in the perpendicular direction on the high velocity side of the beam at satellite altitude. Energy is transferred from parallel motion of H+ to perpendicular motion of O+ during this process. Calculations of diffusion time scales for both H+ and O+ showed quantitatively that resonant diffusion within 1 RE of the satellite is sufficiently fast to explain the perpendicular O+ heating and a substantial portion of the parallel H+ heating. Some additional parallel diffusion of both species must have taken place below the satellite. Nonresonant diffusion and interactions of ions with a parallel H+ acoustic wave, which may be unstable below the satellite, could be responsible for this additional heating. These processes transfer energy from parallel motion of H+ to parallel motion of O+. The presence of a third ion species also could have modified the parallel heating rate. These latter possibilities could not be studied in detail with the available data.
Heavy ion beam‐ionosphere interactions: Charging and neutralizing the payload
Authors: Kaufmann R. L. , Walker D. N. , Holmes J. C. , Pollock C. J. , Arnoldy R. L. , Cahill L. J. , Kintner P. M.
Published: February 2008

The argon release controlled studies (ARCS 1‐3) rocket flights carried ion generators to altitudes of 400‐500 km in the nighttime auroral ionosphere. Three distinct electrical charging and neutralization processes were seen on the payloads during gun operation: steady or dc vehicle charging, brief charging at gun turn‐on, and extended oscillatory sequences. Many of the unexpected consequences of gun firings are attributed to these payload charging and neutralization processes. Electrical charging is regulated by the rate at which low‐energy electrons escape from the generator, which in turn is dependent on magnetic field geometry. Each ion generator produced a dipolar magnetic field which merged with the Earth's field near the rocket. The resulting local magnetic field guided electrons back to the rocket for certain gun orientations, thereby inhibiting neutralization. Transient charging was attributed to the formation of an electron cloud around at least some vehicles, while dc charging altered the rocket's surroundings until the electron escape rate balanced the ion beam flux. We concluded that during oscillatory events the entire environment of a payload could alternate between hot electron and cold electron configurations at very high rates, possibly exceeding 10 kHz. These changes in the plasma environment did not produce substantial electric field perturbations at the dc or ac high impedance electric field sensors, so were not seen in data from typical wave detectors. However, changes in plasma density and temperature produced dramatic effects on low impedance electric current sensors such as Langmuir probes. These observations illustrate the importance of carrying both low and high impedance detectors on vehicles involved in active experiments. A number of possible future experiments are suggested to test and extend our understanding of the proposed charging and neutralization mechanisms.
Waves in low‐β plasmas: Slow shocks
Authors: Steinolfson R. S. , Hundhausen A. J.
Published: February 2008

The response of a conducting fluid containing an embedded magnetic field (with β<1) to the sudden injection of material along the field lines is investigating using results from wave theory and numerical simulations of the nonlinear magnetohydrodynamic equations. A primary interest is the possible generation of, and the role played by, slow shocks in the medium response to the ejecta. We show that slow shocks may be produced; however, in contrast to previous suggestions for a slow shock preceding an impermeable ejecta, the slow shock configuration deduced in the present computations is concave toward the ejecta “driver.” Fast‐mode waves, which have not steepened into shocks, precede the slow shock and significantly alter the ambient medium (provided β is not too low). The slow waves are shown to play an important role in accommodating the driven ejecta. At low β the fast mode becomes primarily a transverse wave for parallel propagation, whereas the slow wave approaches a longitudinal, or sound, wave. The compressive effects of the motion then are principally transmitted by the slow shock, while the fast wave primarily just reorients the magnetic field.
Excitation of low‐frequency waves by auroral electron beams
Authors: Lin C. S. , Wong H. K. , Koga J. , Burch J. L.
Published: February 2008

The electron distribution functions measured by DE 1 at high altitudes during an auroral pass on day 318, 1981, are used to conduct a linear instability analysis of low‐frequency electromagnetic and electrostatic waves near and below the hydrogen gyrofrequency. The free energy sources examined are electron beams in inverted‐V structures and in the central plasma sheet (CPS) region. It is found that inverted‐V electron beams with energies of several keV can excite electromagnetic ion cyclotron (EMIC) waves propagating obliquely with frequencies below the hydrogen gyrofrequency. When an oxygen ion is the dominant component, energetic electron beams can also excite electromagnetic oxygen cyclotron waves over a broad frequency range. We suggest that the oxygen and hydrogen EMIC waves excited by energetic electron beams might explain low‐frequency electric and magnetic noise (<100 Hz) in the auroral zone. Throughout the CPS, DE 1 measured energetic (>1 keV) trapped electrons, upward and downward streaming electrons with a beam energy less than 100 eV, and conic ions at energies less than 300 eV. Integration of the plasma data for several energy and pitch angle ranges shows that the conic ion number flux in the CPS correlates best with the number flux of streaming electrons with energies less than 235 eV, but not with the energetic trapped electron fluxes. Due to the high density (∼6 cm−3), the CPS electron beams can excite hydrogen and oxygen electrostatic ion cyclotron (EIC) waves with large growth rates. Although the CPS beam energy is too low to excite oblique kinetic Alfvén waves or EMIC waves through Landau resonance, the CPS electron beams can excite reactively left‐hand Alfvén waves propagating parallel to the magnetic field at frequencies below the O+ gyrofrequency. Based on the instability analysis, a possible interpretation for the correlation between the streaming electron and conic ion fluxes is that upward and downward streaming electrons throughout the CPS provide the free energy for heating oxygen ions through oxygen EIC waves. However, EMIC waves propagating from the inverted‐V structure into the CPS as Alfvén waves can also heat ions through electromagnetic ion cyclotron resonance. Detailed modeling of the observed conic ion distributions needs to include both oxygen EIC and Alfvén waves to resolve the question of oxygen heating in the CPS.
Energy spectra and pitch angle distributions of lightning‐induced electron Precipitation: Analysis of an event observed on the S81‐1(SEEP) satellite
Authors: Inan U. S. , Walt M. , Voss H. D. , Imhof W. L.
Published: February 2008

Temporal and spectral signatures of a lightning‐induced electron precipitation (LEP) burst observed on the S81‐1 (SEEP) satellite are analyzed and compared with the predictions of a test particle model of the gyroresonant whistler‐particle interaction in the magnetosphere. The flux to be detected by specific detectors on the low altitude (∼220 km) satellite at L ≃ 2.24 is calculated in terms of the integral counting rate as a function of time and in terms of the dynamic energy spectra during the initial ∼300‐ms precipitation pulse. For a whistler wave packet with frequency range 500 Hz to 6 kHz the dynamic energy spectra are found to depend sensitively on the electron angular distribution in the vicinity of the loss cone. In the case of a whistler wave originating in northern hemisphere lightning the maximum whistler‐induced pitch angle scattering of electrons occurs near ∼10°S geomagnetic latitude. However, scattering occurring over the latitude range of ∼20°N to ∼20°S is found to be significant and contributes to the observed LEP pulse. The dynamic energy spectra of the LEP pulse and the temporal profile of the integral counting rate are consistent with the predictions of a test particle model of the gyroresonant scattering of the electrons by a whistler wave having an equatorial intensity at 6 kHz of ∼200 pT. The measured LEP pulse pitch angle distribution is wider than that estimated on the basis of the test particle model.
EISCAT as a tristatic auroral radar
Authors: Schlegel Kristian , Moorcroft D. R.
Published: February 2008

We have used the European Incoherent Scatter radar (EISCAT) in a mode which allows us to use it as a tristatic auroral radar. Observing at an elevation of less than 10° with the Tromsø beam, we achieved magnetic aspect angles between 4° and 6° at 105 km altitude and recorded coherent echoes simultaneously from all three sites. The backscattered power for these echoes is up to 3 orders of magnitude higher than typical incoherent scatter echoes. Contrary to most existing auroral radars, we can calibrate the coherent echo strength and thus determine absolute values of the coherent backscatter cross section. Moreover, switching the common volume in short time intervals from E to F region heights, permits the determination of the E × B drift vector almost simultaneously with the E region coherent scatter measurements. This opens unique possibilities to study the E region plasma instabilities and their driving force. The main aim of this paper is to describe the capabilities of EISCAT as an auroral radar and to present and discuss results in terms of coherent backscatter cross sections, coherent spectra shape, irregularity phase velocities, and aspect angle dependence. In forthcoming papers several of these topics will be explored in more detail.
Stability of H+ beams in the polar wind
Authors: Barakat A. R. , Schunk R. W.
Published: February 2008

In the classical picture of the polar wind, the H+ flow becomes supersonic and collisionless, and the H+ velocity distribution becomes anisotropic and asymmetric at altitudes above about 3000 km. Previously, the stability of the classical polar wind was studied and found to be stable for a wide range of electron temperatures. However, this classical picture of the polar wind results from steady state models that neglect horizontal plasma convection. The plasma convection at high latitudes can significantly affect the characteristics of the polar wind. For example, ions can be energized in the cusp region and then convected into the polar cap, which subsequently results in energetic ion beams and/or conics passing through the classical polar wind. The effect of energetic H+ beams on the stability of the polar wind was studied with regard to the excitation of electrostatic waves. The cases considered covered a wide range of electron‐to‐background ion temperature ratios (Te/Ti = 0.1, 1, 10) and beam‐to‐background ion density ratios (nb/(ni + nb) = 0.1, 0.5, 0.9). A relatively cold beam was assumed (Tb = 0.1Ti). A combination of the Nyquist technique and a direct solution of the plasma dispersion equation was used to find the minimum beam drift velocity required to destabilize the plasma. The following results were obtained: (1) The plasma can be destabilized for relative drift energy ≲1 eV, (2) The plasma is more unstable for large electron temperatures and for comparable ion and beam densities, (3) For large electron temperatures the parallel propagating ion/ion plasma‐acoustic instability is triggered first, while the obliquely propagating ion/ion cyclotron instability tends to trigger first for low Te.
Modal effects on amplitude perturbations on subionospheric signals (Trimpis) deduced from two‐frequency measurements
Authors: Dowden R. L. , Adams C. D. D.
Published: February 2008

Interference between the first two modes of Earth‐ionosphere waveguide propagation at the high end of the VLF band (>18kHz) increases with distance from the transmitter out to very large distances and can add amplitude perturbations to the phase perturbations (trimpis) produced by lightning‐induced electron precipitation (LEP) on the great circle path. Since the two modes have slightly different phase velocities, an interference pattern or standing wave is formed which is shifted slightly along the propagation path by the LEP‐induced change in differential phase velocity. The modal effect at the receiver depends on the local gradient (along the great circle path) of amplitude with respect to the differential phase. Since this differential or mode beat phase varies with frequency, measurement of the resultant amplitude at two close frequencies enables an estimation of the modal effects. In this study, measurements were made at Dunedin at the two MSK frequencies, 22250 Hz and 22350 Hz, of the transmitter NWC, during a night of frequent “one‐dimensional” trimpis (i.e., those produced by large‐area LEP occurring close to the great circle path) and of strong and varying modal interference. Modal generation or modification of trimpi amplitude was related to the local gradient of amplitude as expected. From these results it was deduced that modal modification of “echo trimpis” (those produced by small area LEP occurring well off the great circle path), even under extreme conditions, is insignificant.
A fully analytic, low‐ and middle‐latitude ionospheric model
Authors: Anderson D. N. , Forbes J. M. , Codrescu M.
Published: February 2008

A fully analytic ionospheric model emphasizing the low‐latitude F region for various seasonal and solar cycle conditions is presented. Features such as the postsunset rise in the F layer peak height and the “equatorial anomaly” maxima in plasma density near ±15° geomagnetic latitude are designed to closely approximate those in the semiempirical low‐latitude ionospheric model (Anderson et al., 1987a). Significant improvements are thus obtained over previous comparable analytic models. Numerical computations are also presented which demonstrate the importance of these large‐scale plasma structures to the neutral dynamics of the low‐latitude thermosphere. Equally significant effects are anticipated for electrodynamic simulations of E region/F region coupling involving flux tube integrated electrical conductivities.
Absolute differential and integral electron excitation cross sections for atomic oxygen 7. The 3P → 1D and 3P → 1S transitions from 4.0 to 30 eV
Authors: Doering J. P. , Gulcicek E. E.
Published: February 2008

We report the absolute differential and integral cross sections of the forbidden atomic oxygen 3P → 1D( 1.97 eV), and 3P → 1S( 4.19 eV), transitions for incident electron impact energies of 4.0 to 30 eV. The 3P → 1D cross section was measured over the entire energy range while the 3P → 1S transition was measured above 7 eV. The differential cross section (DCS) for the 3P → 1D transition is strongly backward peaked at all incident energies, in agreement with theoretical calculations. The DCS for the 3P → 1S transition varies considerably in shape over the energy range 7–30 eV. At the lower energies, the transition shows an axially forbidden shape (no intensitiy in the forward or backward direction) in agreement with the Thomas and Nesbet (1975) calculations. At higher energies (20–30 eV), the DCS has a remarkable drop near 70°. The agreement between the DCS measured here and the previous measurements of Shyn and Sharp (1986) and Shyn et al. (1986) varies from fair to poor at different incident energies. The integral cross section (ICS) for the 3P → 1D transition agrees within experimental error with the measurements of Shyn and Sharp (1986) and the theoretical calculations of Henry et al. (1971) at 30 eV but is larger than the previous measurements by a factor of approximately 1.3 down to 15 eV and a factor of 2 near the 5 eV peak. The low‐energy peak in the ICS is shown to be between 5 and 7 eV incident energy and the 4 eV experimental measurement is definitely below the peak. No pronouced peak is observed for the 3P → 1S transition. The ICS is in good agreement with the calculations at 30 eV but is larger than the calculations by a factor of approximately 1.4 at lower energies down to 7 eV. The present results are systematically smaller in magnitude than those of Shyn et al. (1986) over the entire energy range studied. The difference varies from a factor of 1.3 at 10 eV to 2.5 at 30 eV. There is no pronounced peak in the 3P → 1S ICS, in agreement with the theoretical calculations.
Large‐scale fluctuations in underground muon time series
Authors: Bergamasco L. , Provenzale A. , Osborne A. R. , Castagnoli G. Cini , Kudryavtsev V. A. , Kuznetsov V. A. , Ryazhkaya O. G.
Published: February 2008

We study the self‐affine properties of intensity fluctuations of underground muons generated by primary cosmic rays of rigidity ≥1600 GV. The muon time series were recorded at a vertical depth of 570 Hg/cm² during the years 1981–1983. This time interval includes a period of intense solar activity in the summer of 1982 which was characterized by the creation of large solar wind shocks and associated production of major interplanetary disturbances. The purpose of our analysis is to obtain from the behavior of the muon intensity some insight into the large‐scale behavior of interplanetary magnetic field fluctuations and their effects on the propagation of high‐rigidity galactic cosmic rays. The results give good evidence for fractal behavior of large‐scale muon fluctuations in the range from a few hours to more than 10 days, with the scaling exponent H varying according to the time interval analyzed. We find that the large time scale muon fluctuations evolve from a preshock condition in late 1981 to early 1982 characterized by a scaling exponent H = 0.28 and power spectrum f−1.6 to a condition with H = 0.07 and an f−1.1 power spectrum during the most intense solar activity in 1982, followed by a slow return in 1983 to the original preshock conditions (H = 0.14, f−1.3).
Production of very energetic electrons in discrete aurora
Authors: Swift Daniel W. , Gorney David J.
Published: February 2008

Data are presented showing that acceleration of electrons to energies in excess of 100‐keV sometimes occurs on field lines associated with discrete auroral arcs. Velocity distributions suggest that acceleration is in directions primarily perpendicular to the magnetic field. The examples studied in the S3‐3 data set indicate that the process can occur in the altitude range from the topside ionosphere to 5500 km. Our favored explanation is that the field‐aligned beam excites upper hybrid waves when it interacts with the dense ionospheric plasma. The waves efficiently scatter the beam in both energy and pitch angle.
Optical observations of thermospheric neutral winds at Arecibo between 1980 and 1987
Authors: Burnside R. G. , Tepley C. A.
Published: February 2008

Since 1980, optical observations of nighttime thermospheric winds have been made using a Fabry‐Perot interferometer at the Arecibo Observatory, Puerto Rico. High‐resolution spectra of the O(¹D) airglow emission at 630.0 nm are obtained by observing at eight equally spaced azimuth positions and a fixed zenith angle of 60°. The neutral wind field is inferred by assuming that each component of the wind velocity vector may be expanded in a linear Taylor expansion about a point directly above the observatory. Both the zonal and meridional components of the thermospheric wind field are observed to have well‐defined seasonal and nocturnal variations. For each year between 1980 and 1987, eastward flow was observed in the evening hours, with an average peak velocity of about 100 m s−1 near 2200 AST. In the winter months, the zonal wind generally remains eastward throughout the night. However, in summer, a reversal to westward flow is usually observed after local midnight. In the meridional direction, the largest equatorward velocities are observed in summer. A reduction (or reversal) in the meridional wind velocity is most often observed after midnight in the summer and equinoctial months. We find that the nocturnal and seasonal variations in the neutral wind field at Arecibo are remarkably unaffected by changes in the solar cycle.
Three‐point magnetic field observations of substorms in the inner magnetotail
Authors: Fairfield D. H. , Zanetti L. J.
Published: February 2008

In 1984–1985 the GOES 5 and GOES 6 spacecraft monitored the Earth's magnetic field at two geosynchronous locations ∼30° apart while the eccentric orbiting AMPTE/CCE spacecraft made equatorial measurements out to 8.8 RE. After intercomparing GOES and CCE data at 6.6 RE on the dayside to reduce uncertainties in the spin axis magnetic field component at GOES, the data sets were studied in the nightside hemisphere. Comparing GOES data with CCE data as the latter spacecraft were passing through the synchronous orbit distance near midnight revealed north‐south (latitudinal) field gradients of the order 40–50 nT/RE in the range 0.5 to 1.3 RE north of the equatorial plane. Coherent variations at the various spacecraft predominate in the north‐south and radial components of the field and these variations are readily explained as the well‐known plasma sheet reconfigurations associated with substorms. The east‐west field component usually exhibits less coherence between the spacecraft. These differences in the east‐west component at various spacecraft are generally consistent with the current wedge model, but changes in the sign of this component between nearby spacecraft indicate that spacecraft ∼1 RE north of the equatorial plane are frequently immersed in or even north of the wedge currents. Little evidence was found for a high field injection front propagating inward from the tail, but the absence of such evidence may be due to the fact that our dual spacecraft measurements are 0.5–1.3 RE north of the geomagnetic equatorial plane in a strong field region where such injection‐front field increases are unlikely to occur.
Hot ion plasmas from the cloud of neutral gases surrounding the space shuttle
Authors: Paterson W. R. , Frank L. A.
Published: February 2008

Large intensities of hot positive ions are observed out to distances of several hundred meters from the space shuttle Challenger (Spacelab 2) with a plasma analyzer on board a small free‐flying satellite, the Plasma Diagnostics Package. This ion plasma is inferred to be generated by the charge exchange of ionospheric O+ ions with a large cloud of water molecules from the space shuttle. The measured ion density ranges from ∼30 to 104 H2O+ ions cm−3. The hot ion plasma forms an ion trail in the wake of the space shuttle. A model for the water vapor cloud provides the basis for density estimates as high as 109 H2O molecules cm−3 at a distance of 50 m from the space shuttle. Thus the space shuttle possesses a substantial coorbiting atmosphere.
The transmission of Alfven waves through the Io plasma torus
Authors: Wright Andrew N. , Schwartz Steven J.
Published: February 2008

In this paper we study the nature of Alfvén wave propagation through the Io plasma torus. A one‐dimensional model is used with uniform magnetic field and exponential density decrease to a constant value. The solution can be expanded near the center of the torus and far away from the torus to give propagating Alfvén waves. The time‐averaged Poynting flux is independent of position and, in the near and far limits, is equivalent to the sum of Poynting fluxes carried by individual wave trains. In this fashion it is possible to calculate the fraction of energy that is transmitted or reflected by the change in Alfvén speed through the torus, for a given wave train near Io. The solution is sensitive to the distant (or asymptotic) Alfvén speed. A lower limit for this speed can be found from the density decrease alone. Using this value we find, in accord with previous work, that there is negligible wave reflection. A more realistic asymptotic Alfvén speed takes into account the increase in field strength along the Io flux tube. This larger Alfvén speed value yields a significant reflection coefficient, and the result is in good agreement with our previous numerical solution. Our results imply that Io's Alfvén waves may not propagate completely through the plasma torus, and thus WKB theory and ray tracing may not provide meaningful estimates of the energy transport.
Ionization from soft electron precipitation in the auroral F region
Authors: Labelle J. , Sica R. J. , Kletzing C. , Earle G. D. , Kelley M. C. , Lummerzheim D. , Torbert R. B. , Baker K. D. , Berg G.
Published: February 2008

Rocket‐borne instrumentation, launched into the morning sector auroral zone from Sondre Stromfjord, Greenland, detects electron density enhancements correlated with enhancements in the flux of soft (less than 1 keV) downgoing electrons. These electron density enhancements seem most likely to have been generated by direct production of ionization at F region altitudes. Model calculations of the electron impact ionization rate, based on the measured electron spectrum, lend support to this hypothesis.
Tromsø Heating Experiments: Stimulated emission at HF pump harmonic and subharmonic frequencies
Authors: Derblom H. , Thidé B. , Leyser T. B. , Nordling J. A. , Hedberg Å , Stubbe P. , Kopka H. , Rietveld M.
Published: February 2008

Observations of electromagnetic emissions on the HF heater subharmonic and several harmonic frequencies are presented. Often the second harmonic emissions show characteristic amplitude variation, frequency shift, and spectral broadening that clearly have their origin in ionospheric plasma processes. The strongest candidate is a combined parametric decay and Raman up‐conversion process. The subharmonic emission is believed to be evidence of absolute stimulated Raman scattering and/or two plasmon decay processes which have not been observed before in ionospheric heating experiments.
Dynamics of a plasma expanding into a uniform magnetic field
Authors: Gisler Galen , Lemons Don S.
Published: February 2008

A heuristic model of an energetic plasma expanding into an initially uniform magnetic field is presented. In this model the plasma is cylindrically shaped, perfectly diamagnetic, and allowed to expand freely in the direction of the magnetic field. It results in a time evolution of the cylinder radius described by Airy functions and parameterized by the plasma β. For instance, the maximum extent of the radius scales as β1/3 while the maximum radial deceleration scales as β−1/3. Time evolution and scaling are compared both with other models and with two‐dimensional electromagnetic particle simulations. A spherically symmetric model is also found to agree well with the simulations and results in simpler expressions for the maximum radius and maximum radial deceleration. The larger deceleration obtained in these models, as compared with models that ignore the axial expansion, would shift toward longer wavelengths the peak growth of the instability thought to be responsible for the field‐aligned structures seen in the Active Magnetospheric Particle Tracer Explorers (AMPTE) magnetotail releases.
The composition, temperature, and density structure of cold ions in the quiet terrestrial plasmasphere: GEOS 1 results
Authors: Farrugia C. J. , Young D. T. , Geiss J. , Balsiger H.
Published: February 2008

We present a comprehensive study of the composition, and the density and temperature distributions of the thermal (energy≤110 eV) ion population in the terrestrial plasmasphere under quiet geomagnetic conditions. The data were collected by the Ion Composition Experiment (ICE) on board the European Space Agency's GEOS 1 satellite and cover the period from June 1977 to May 1978. For the data reduction we employ a method based on the modulation of the detector count rates by the rotation of the spacecraft. We find typical quiet time proton densities to vary smoothly between ∼10² cm−3 (L ≈ 6) and 2×10³ cm−3 (L ≈ 3). In the local time sector 1700–2200 the equatorial proton concentration obeys an inverse fourth power dependence with dipole L. He+ is a major ionic component in all L ranges investigated. Its concentration relative to H+ is highly variable, ranging from ∼1% to, on occasion, over 100%. The most frequent values we obtain lie in the range ∼2–6%. The averaged variation of He+ with dipole L in the 1700–2200 local time range shows a somewhat more rapid decrease with increasing L than does H+. The temperatures in the quiet plasmasphere are between 4×10³ K and 1.5×104 K and generally exhibit a slow increase with L value. The average radial temperature gradient near the equator is ∼0.15 K/km. The main ionic constituents are usually in thermal equilibrium throughout the plasmasphere. There are indications that the ionic component is in a state of thermal equilibration with the electronic component in the outer plasmasphere. Using alternate passes in the same region, we discuss the poststorm recovery of the plasmasphere on L shells >4 for both H+ and He+. Data are also presented on the minor ions O+, O++, D+, and He++. The O+ densities around L ≈ 3 are ≲1 cm−3, and the densities of O++ are comparable. Cases are shown where the mass/charge = 2 ion is predominantly D+ and others where it is mainly He++. Typical densities of this ion in the L range 2.6–3.6 are a few tenths per cubic centimeter. With one exception we found these minor ions to share a common temperature with the main constituents of the plasmasphere. The results of our survey are compared with previous studies and with theoretical modeling. In particular, the GEOS 1 H+ temperature structure is in substantial agreement with those from the Plasma Composition Experiment (PCE) on ISEE 1 and the retarding ion mass spectrometer (RIMS) on DE 1. We confirm the enhanced O++ to O+ density ratio in the equatorial plasmasphere with respect to values in the mid‐latitude ionosphere. Our observations on this ratio are compared with the predictions of three theoretical works.
A three‐dimensional numerical model of ionospheric plasma in the magnetosphere
Authors: Delcourt D. C. , Chappell C. R. , Moore T. E. , Waite J. H.
Published: February 2008

The magnetospheric transport of terrestrial plasma is numerically investigated by means of three‐dimensional particle trajectory tracing in empirical models of the geoelectric and geomagnetic fields. Various ionospheric outflows (auroral, polar cap, cusp, and polar wind) are systematically examined using observational definitions of their respective locations and strengths, and assuming purely adiabatic motions under the effect of the large‐scale magnetospheric convection. Due to field model limitations, the simulations are limited in scope to the region within a geocentric radius of 17 RE. Consequently, much of the terrestrial H+ outflow cannot be accurately traced beyond the polar cap region, and the conclusions concerning the terrestrial contribution to plasma sheet H+ are necessarily limited. Many qualitative features of the plasma sheet are produced in the model by the ionospheric plasmas. The motions of terrestrial O+ outflow are well described within the assumptions of the calculation. It is found that, during quiet times, the observed ionospheric outflows provide a negligible contribution of energetic plasma sheet O+, in agreement with plasma sheet observations. In contrast, during high geomagnetic activity, the simulations demonstrate a substantial ionospheric supply of O+ to the energetic plasma sheet, primarily due to the convection of upwelling ions from the cleft ion fountain through the central plasma sheet. These O+ ions experience kiloelectron volt accelerations during such disturbed times and lead to earthward streaming ion distributions along surfaces which may be identified as the plasma sheet boundary layers. They are thus consistent with the observed characteristics of the active plasma sheet. At all levels of geomagnetic activity, the ionosphere yields significant quantities of low‐energy ions (0–100 eV) which provide potential plasma sources for the plasma sheet after nonadiabatic heating processes. Conclusions regarding the quantitative terrestrial H+ contribution to the plasma sheet must await development of field models adequate for trajectory tracing through the distant plasma sheet, and may require inclusion of nonadiabatic effects.
Plasma and field properties of suprathermal electron bursts
Authors: Robinson R. M. , Winningham J. D. , Sharber J. R. , Burch J. L. , Heelis R.
Published: February 2008

Electron spectrometer measurements from polar‐orbiting satellites frequently show narrow regions of enhanced fluxes of electrons in the energy range from a few electron volts to less than 1 keV. These suprathermal electron bursts are observed throughout the auroral zone even under quiet conditions. They are typically distributed in latitude for hundreds of kilometers on either side of an upward current region which sometimes also contains an inverted V event. Whereas in inverted Vs the electron spectrum shows a peak at an energy of a few keV, suprathermal electron bursts are characterized by enhanced fluxes at all energies below a cutoff energy that is usually less than 1 keV. We present observational evidence to show that suprathermal electron bursts are correlated with enhancements in the ac electric field at frequencies below 1 kHz. The wave spectrum above 1 kHz does not differ significantly within and outside a burst. These observations suggest that the electrons in suprathermal bursts are heated through interaction with waves, most likely ion cyclotron waves. The phenomenology of suprathermal bursts indicates that the waves are not generated on the same field line at which the heating is observed. The evidence suggests that the waves are generated in regions of upward field‐aligned currents and propagate obliquely away from the source region. Electrons with parallel velocities equal to the parallel phase velocity of the waves can be heated through Landau resonance. This accounts for the fact that the cutoff energy in the bursts is never higher than the peak energy of the associated inverted V precipitation, even though the inverted V precipitation may be latitudinally displaced by several hundred kilometers from the burst.
On the threshold electric field for the ∼1‐m auroral irregularity appearance
Authors: Kustov A. , Uspensky M. , Kangas J. , Huuskonen A. , Nielsen E.
Published: February 2008

We study the electric field threshold for 1‐m auroral electrojet irregularity excitation, using data from the special Finnish EISCAT experiment held on November 17, 1983. We find that at times irregularities can arise when the ionospheric electric field is 5–15 mV/m, i.e., well below the Farley‐Buneman instability threshold. The electron density fluctuations derived here are increased when the E field becomes more than 15 mV/m. As a result the dependence of electron density fluctuation amplitude on electric fields has a kind of a “soft” E field threshold. Possible reasons for such a dependence are discussed. We show that unresolved ionospheric microstructure in the radar scattering volume can cause this “threshold spreading.”
On the interpretation of measured ion streams in the wake of the Shuttle Orbiter in terms of plasma expansion processes
Authors: Singh N. , Wright K. H. , Stone N. H. , Samir U. , Hwang K. S.
Published: February 2008

Measurements of ion streams found to exist in the near wake of the shuttle Orbiter (Spacelab 2 mission) were used to examine the applicability of a one‐dimensional, time‐dependent plasma expansion model for wake filling. The results of the examinations show a good agreement between theory and experiment. This agreement implies that the measured streams can be interpreted in terms of collisionless plasma expansion.
Plasma density fluctuations observed during Space Shuttle Orbiter water releases
Authors: Pickett J. S. , D'Angelo N. , Kurth W. S.
Published: February 2008

Observations by the Langmuir probe on the Plasma Diagnostics Package flown as part of the Spacelab 2 mission in the summer of 1985 show a strong increase in the level of turbulence near the shuttle orbiter during operations in which liquid water is released. The spectrum of the plasma density fluctuations peaks at the lowest frequencies measured (a few hertz) and extends up to a few kilohertz, near the lower hybrid frequency. We suggest two potential mechanisms for generating the plasma turbulence which are both based on the production of water ions as a result of charge exchange with the ambient oxygen ions in the ionosphere. The first mechanism proposed is the ion‐plasma instability which arises from the drift of the contaminant ions with respect to the ambient oxygen ions. The other mechanism proposed is the Ott‐Farley instability which is a result of the ring distribution formed by the “pick‐up” water ions.