bluuebunny/crossref_metadata_2025 huggingface dataset:
Wide band attenuation of radio waves caused by powerful HF waves: Saturation and dependence on ionospheric variability
Authors: Stubbe P. , Kopka H. , Jones T. B. , Robinson T.
Published: February 2008

The phenomenon of wide band attenuation, i.e., anomalous absorption of an HF wave in the F region subjected to modification by a powerful HF wave, has been measured using the Tromsø heating facility. Two types of results were found. In a quiet F region situation, the attenuation amounted to approximately 15 dB and did not change when the effective radiated heating power was lowered from 160 to 20 MW. In a variable F region, indicated by a high fading rate of the diagnostic wave, the absorption was considerably less than 15 dB and showed a strong dependence on the heating power. An explanation for both types of results is offered by extending a recent theory of Das and Fejer.
Dissipation of ionospheric irregularities by wave‐particle and collisional interactions
Authors: Bernhardt P. A. , Pongratz M. B. , Gary S. Peter , Thomsen M. F.
Published: February 2008

The nonlinear dissipation of plasma irregularities aligned parallel to an ambient magnetic field is studied numerically using a model which employs both wave‐particle and collisional diffusion. A wave‐particle diffusion coefficient derived from a local theory of the universal drift instability is used. This coefficient is effective in regions of nonzero plasma gradients and produces triangular‐shaped irregularities with spectra which vary as f−4, where f is the spatial frequency. Collisional diffusion acts rapidly on the vertices of the irregularities to reduce their amplitude. The simultaneous action of the two dissipative processes is more efficient than collisions acting alone. In this model, wave‐particle diffusion mimics the forward cascade process of wave‐wave coupling.
Local time dependence of electron flux changes during substorms derived from multi‐satellite observation at synchronous orbit
Authors: Nagai Tsugunobu
Published: February 2008

Energetic electron (energy higher than 2 MeV) observations by a synchronous satellite chain (which consists of GOES 2, GOES 3, and GMS covering the local time extent of approximately 10 hr) have been used to study the large‐scale characteristics of the dynamic behavior in the near‐earth magnetosphere for substorms, in which low‐latitude positive bay aspects are clearly seen in the ground magnetic data. Simultaneous multi‐satellite observations have clearly demonstrated the local time dependence of electron flux changes during substorms and the longitudinal extent of electron flux variations. Before a ground substorm onset the energetic electron flux decreases in a wide longitudinal region of the nighttime and the flux decrease is seen even on the afternoonside. For the flux behavior associated with the onset of the substorm expansion phase, there exists a demarcation line, the LT position of which can be represented as LT = 24.3 ‐ 1.5 Kp. The flux shows a recovery to a normal level east of the demarcation line, and it shows a decrease west of the demarcation line. The region of the flux decrease during the expansion phase is restricted, and it is observed mainly on the afternoonside. The afternoonside flux decrease has a different characteristic from the nightside flux decrease preceding the expansion phase. The nightside flux decrease‐recovery sequence is observed in a wide region of more than 6 hr in the nighttime and the center of this variation exists in the premidnight region. It should be noted that the afternoonside flux decrease is not observed for every substorm and the nightside signature sometimes becomes a dominent feature even on the afternoonside.
The solar wind interaction with Mars as seen by the Viking retarding potential analyzers
Authors: Cragin B. L. , Hanson W. B. , Sanatani S.
Published: February 2008

In addition to the published retarding potential analyzer (RPA) data that were taken in the Martian ionosphere, there exist many Viking RPA measurements of low‐energy (<75 eV) electron fluxes out to 16,000 km above the Mars surface. Both energy spectra and periods of continuous monitoring of the total flux above 15 eV are available. The mean electron current at energies greater than 15 eV increases monotonically by nearly 2 orders of magnitude from about 9000 km down to 700 km on Viking 1, but no clear signature of the bow shock is seen. The total wave power in the 2‐s measurement intervals for this current does, however, show a broad peak near 1700‐km altitude. The preshock variations in the low‐energy electron fluxes can be related to whistler mode oscillations in the solar wind plasma. It is concluded that there may be a highly turbulent shock structure that masks a clear signature of the bow shock in the time‐averaged data. The interaction model that is consistent with the bow shock at 1700 km, coupled with the ionosphere measurements, indicates that Mars is likely to have a permanent magnetic field adequate to stand off the solar wind during the Viking 1 entry.
Pc 1–2 magnetic pulsation spectra and heavy ion effects at synchronous orbit: ATS 6 results
Authors: Fraser B. J. , McPherron R. L.
Published: February 2008

Two types of Pc 1–2 wave events have been identified in ATS 6 flux gate magnetometer data recorded approximately 12° off the geomagnetic equator. Left‐hand polarized waves observed in the afternoon are typical of IPDP ground events. Highly elliptical and linearly polarized waves occur around local noon. Spectra often show slots near the equatorial helium and oxygen cyclotron frequencies. The linear polarization may be interpreted in terms of propagation at a large wave normal angle or a crossover frequency that is a consequence of propagation in a multicomponent plasma. The spectral slots may be associated with either heavy ion cyclotron resonance pr propagation effects.
Energy coupling between regions 1 and 2 Birkeland current systems
Authors: Siscoe George L.
Published: February 2008

The principle of ring current shieldding and the concept that in an electrical circuit analogy the ring current can be treated as a pseudo‐Hall current flowing in parallel with the ionospheric Hall current are used here to develop a simple model for the electrical coupling between the applied potential Φ associated with the Region 1 currents, and the ring current, associated with the Region 2 currents. The approach yields a formula for the radial position of the inner edge of the ring current as a function of Φ, which agrees with similar formulas derived previously on the basis of different approaches. The model can be expressed in the language of electrical circuit theory as an applied potential Φ driving a nonlinear RL circuit, in which the resistance R results from dissipation in the ionosphere and the inductance L from the energy of compression stored in the ring current. The formulation lends itself to a determination of the relative amounts of energy that are dissipated in the ionosphere and stored in the ring current, according to the applied potential model. If the shielding time scale L/R and the energization (i.e., substorm) time scale are comparable, as observations indicate, roughly equal amounts of energy are expended on both elements. The analysis shows that AE type geomagnetic indexes and the Dst index should bear different functional relationships to that combination of solar wind parameters that is primarily responsible for geomagnetic activity.
Observations of field‐aligned currents, particles, and plasma drift in the polar cusps near solstice
Authors: Bythrow P. F. , Potemra T. A. , Hoffman R. A.
Published: February 2008

Observations of particles, magnetic fields, and convective flow in the southern polar region by the AE‐C spacecraft in the spinning mode have provided a unique opportunity to study the polar cusp. Magnetic perturbations observed by TRIAD within 2 hours of the AE‐C pass provide field‐aligned current data from the same local time in the northern hemisphere. The spinning mode of AE‐C has enabled the utilization of the Z axis magnetometer for Birkeland current observations in conjunction with particle and drift measurements. This pass, AE‐C orbit 16723, occurred on January 15, 1977, at ∼0145 UT, 25 days after solstice. The 3‐hour Kp = 4‐ and the interplanetary magnetic field By was negative during this time. The average Bz = 1.9 nT and −1.1 nT during the first two hourly intervals on January 15, 1977. At this UT, the geomagnetic axis was at a near maximum inclination of ∼35° with the southern polar cusp directed toward the sun. Measurements from the low energy electron experiment (LEE) revealed intense fluxes of soft (100 eV) ‘cusp‐like’ (Maxwellian) electrons throughout the prenoon polar cap. AE‐C magnetometer and drift meter measurements detected three alternately directed current sheets associated with the ‘throat’ of the ion convection pattern and consistent with gradients in the ion drift velocities. The upward directed current can be identified as the dominant cusp current appropriate for By < 0. The downward directed current has the appropriate sign of a dayside region 1 current but is observed to lie entirely within westerly antisunward convecting plasma. TRIAD measurements revealed the presence of similar ‘cusp currents’ that persist in the northern hemisphere at ∼3° lower latitude than those detected by AE‐C.
Terrestrial kilometric radiation: The cyclotron theory
Authors: Melrose D. B. , Rönnmark K. G. , Hewitt R. G.
Published: February 2008

We examine the gyroresonance condition to determine which electrons can resonate with a specific electromagnetic wave (given ω and k∥) and find that for each gyroresonance harmonic the resonant electrons lie on an ellipse in v⊥ ‐ v∥ space. The growth rate for a given wave due to a given distribution f of electrons involves an integral around the ‘resonant ellipse.’ Using a contour plot of f in v⊥ ‐ v∥ space and a set of drawn ellipses, one can identify which waves grow fastest and which electrons contribute to the growth. We find that for ∂f/∂v⊥ > 0 at small v⊥ there always exist rapidly growing waves with k∥²c²/ω² = n²cos²θ ≪ 1. These ideas are applied to the interpretation of the terrestrial kilometric radiation (TKR). The growth rate for x mode radiation is calculated by using observations (from the ESRO 4 spacecraft) of the electron distribution in ‘inverted V’ events. A positive growth rate is found, providing strong support for the cyclotron theory of TKR. The calculated growth rate is smaller than required, but this may be partly a selection effect. It is argued that the specific features in the electron distribution that generate TKR should disappear in much less than a second and hence should not be observable in particle data averaged over more than a second. An analytic calculation of the maximum growth rate compatible with the data is at least an order of magnitude larger than that required to account for TKR. The basic feature in the electron distribution that gives rise to TKR is a one‐sided loss‐cone anisotropy in which upward‐moving electrons with small pitch angles are missing. We speculate that the particular features causing large growth rates might be due to the effects of the parallel electric field.
Sq currents: A comparison of quiet and active year behavior
Authors: Campbell W. H. , Matsushita S.
Published: February 2008

Spherical harmonic analyses (order 4, degree 12) for the 1965 and 1958 geomagnetic Sq fields are compared, using similar displays, to determine the external and internal current changes from extremely quiet to extremely active solar cycle conditions. For the middle latitudes the active year currents were found to be about 1.6 to 3.0 times as large as the quiet year currents with the larger ratio occurring in mid‐November and the smaller ratio occurring near the June solstice. The maximum in daily Sq currents seemed to happen about 1/2 hour earlier in the quiet year results. The daily range of quiet year Sq variations showed a dominant annual change that was largest from late July to early August, whereas the active year daily range of Sq exhibited a dominant semiannual change with maxima at the equinoxes.
Particle acceleration in time‐developing magnetic reconnection process
Authors: Sato Tetsuya , Matsumoto Hiroshi , Nagai Keisuke
Published: February 2008

Particle trajectories are numerically followed in the presence of space‐ and time‐varying electric and magnetic fields that are obtained by the previous two‐dimensional magnetohydrodynamic (MHD) simulation of X type magnetic reconnection process (Hayashi and Sato, 1978). Two conclusions are notable. First, those particles that encounter a slow shock layer associated with the reconnection process suffer an abrupt and large orbit modification whereby they gain an energy roughly by one order larger than that corresponding to the Alfven speed, i.e., several electron volts for electrons and 10 keV for protons. Second, particles that are driven toward the neutral sheet by the E × B drift are accelerated almost instantaneously up to an energy range of auroral particles (1–10 keV for electrons and 10–100 keV for protons) when they reach near the X type neutral line, some leaving the neutral sheet immediately and others executing meandering motions for some time before leave. An important finding is that a meandering particle can leave the neutral sheet along a reconnected field line because reconnection proceeds and fields change in time. However, their pitch angles are usually larger than the loss cone angle so that they would not be able to be auroral particles in a direct way.
A systematic study of ULF Waves Above FH+ from GEOS 1 and 2 Measurements and Their Relationships with proton ring distributions
Authors: Perraut Sylvaine , Roux Alain , Robert Patrick , Gendrin Roger , Sauvaud Jean‐André , Bosqued Jean‐Michel , Kremser Gerhard , Korth Axel
Published: February 2008

A detailed analysis of the ULF noise observed on the GEOS magnetic antennas in the frequency range ∼0.2'12 Hz has revealed the properties of structured emissions occurring just above the proton gyrofrequency whose existence was reported by Russell et al. (1970) and Gurnett (1976). These waves are observed in the vicinity of the geomagnetic equator at all L values between ∼4 and ∼8. They propagate in a direction perpendicular to the dc magnetic field. The waves consist of harmonically related monochromatic emissions. The fundamental frequency is generally of the order of the local proton gyrofrequency. Sometimes the fundamental and first harmonics are missing. If there is more than one fundamental frequency present, nonlinear coupling often occurs between the different emissions. The amplitudes of individual events vary from some tens of milligammas to some hundreds. Their duration ranges from some tens of minutes to some hours. Within the range of sensitivity of the detectors (10−2 γ Hz−1/2 at 1 Hz, 10−3 γ Hz−1/2) at 8 Hz the average probability of emission occurrence during a given hour is 12%, this number increases to ∼30% during the afternoon and in the pre‐midnight sectors. Simultaneous observations of proton fluxes, as obtained from the two GEOS particle experiments show that these waves are often associated with distribution functions peaking at some energy (5 ≲ E ≲ 30 keV) for 90° pitch angle particles. This ring‐like distribution provides the energy source for the excitation of non‐resonant (k∥ = 0) instabilities near nFH+ (n = running number). A theoretical model is presented that qualitatively explains the main characteristics of the observed waves.
Global dynamic responses of the atmosphere to the eruption of Mount St. Helens on May 18, 1980
Authors: Liu C. H. , Klostermeyer J. , Yeh K. C. , Jones T. B. , Robinson T. , Holt O. , Leitinger R. , Ogawa Tadahiko , Sinno Kenji , Kato Susumu , Ogawa T. , Bedard A. J. , Kersley L.
Published: February 2008

We present evidence that shows some aspects of the global atmospheric dynamic responses to the eruption of Mount St. Helens on May 18, 1980. Although events such as volcanic eruptions may excite a number of acoustic‐gravity wave modes in the atmosphere, the observed surface pressure perturbations and distant ionospheric perturbations can be explained only in terms of propagation of Lamb modes with a horizontal propagation velocity slightly above 300 m/s. Results from model computations show good agreements with the observational data. Ground level pressure perturbations created by this event are only slightly smaller than those created by the historical Great Siberian Meteor.
High‐resolution dayglow O I (1304 Å) and O I (989 Å) rocket observations
Authors: Christensen A. B. , Eastes R. W. , Feldman P. D. , Gentieu E. P.
Published: February 2008

High‐resolution (∼1 Å) dayglow observations of the O I(1304 Å) and O I(989 Å) multiplets were made from an Astrobee‐F rocket payload (25.046 CE) launched from White Sands, New Mexico at local noon on June 27, 1980 to an apogee of 260 km. Three components of the O I(1304 Å) multiplet were measured at zenith angles of approximately 50° and 140° at 1.1 Å resolution. Over the entire altitude range of observation, i.e., 100–260 km, the three components were found to be equal to within ±15%. The absolute intensity profile of the total O I(1304 Å) multiplet was approximately 30% greater than that obtained on an earlier rocket flight (25.029 GA) launched January 9, 1978 from White Sands, when the solar activity and the elevation angle of the sun were both lower. The shape of the O I(989 Å) multiplet distribution observed on the flight at 1.3‐Å resolution was indistinguishable from an optically thin source with the energy levels of the excited state populated according to their statistical weights in spite of the large optical depths of the O I(989 Å) multiplet at rocket altitudes. Experiments with a laboratory UV source also showed the insensitivity of the multiplet distribution to the optical depth of the source. These results are entirely consistent with a recently developed PFR radiative transport code (Meier, this issue) in the absence of selective absorption by rotational lines of N2.
On Jupiter's whistler emission
Authors: Leubner M. P.
Published: February 2008

A theory explaining Jupiter's banded whistler emission in particular low‐frequency hiss and chorus is proposed. It is shown that superthermal electrons described by a kappa distribution function cause whistler instability below one‐third of the electron gyrofrequency, whereas a two temperature Maxwellian leads to a band of unstable growing modes just below one‐half of the electron gyrofrequency. A superposition of both yields almost exactly the structure of whistler mode hiss and chorus detected by the Voyager 1 and 2 plasma wave instrument.
Quantitative tests of a steady state theory of solar wind electrons
Authors: Feldman W. C. , Asbridge J. R. , Bame S. J. , Gosling J. T.
Published: February 2008

IMP electron data are compared to several predictions of a steady state theory of solar wind electrons which assumes that transport is controlled by the macroscopic interplanetary electric and magnetic fields and elastic Coulomb collisions with the solar wind protons and thermal electrons. One of the predictions based on these assumptions is that the ratio of forward to backward phase space density of field‐aligned extrathermal electrons should be 6∶1. In contrast, measurements of electron distributions within the only steady state solar wind flow identified to date, the high‐speed solar wind, show this ratio to be typically about an order of magnitude larger. Based on a larger set of assumptions, the theory further predicts an anticorrelation between the solar wind bulk speed and (1) the fractional density of extrathermal electrons, (2) the component of the temperature of the extrathermal electrons parallel to the interplanetary magnetic field, and (3) the total heat flux. These anticorrelations are not found in the IMP electron data set. Good agreement between the predictions and the observations may require modification of a number of assumptions of the theory. Most important may be the need to include inelastic Coulomb and/or wave‐electron collisions.
Viscous dissipation at the Venus ionopause
Authors: Pérez‐de‐Tejada H.
Published: February 2008

A study of the effects of the transfer of momentum of the shocked solar wind to the Venus ionosphere near the terminator region is presented. It is shown that the deficiency of momentum flux within the velocity boundary layer in the ionosheath is comparable to the momentum flux present in the upper ionosphere near the terminator. Using the observed concentration of planetary O+ ions in the wake it is also shown that mass loading processes are not sufficient to account for the loss of momentum flux above the ionopause and that, instead, an efficient coupling must exist between the plasma above and below that boundary. With reasonable values of the thickness and length of the velocity boundary layer it is possible to estimate an effective viscosity coefficient of the flow and the energy released through viscous dissipation. The results indicate that a column integrated energy deposition of the order of 10−2 ergs cm−2 sec−1 can be predicted near the terminator. This number is comparable to that required to heat up the nightside ionosphere as assumed in the modeling studies of Knudsen et al (1979), Cravens et al (1980), Hoegy et al (1980), and Merritt and Thompson (1980).
The topside ionosphere above Arecibo during summer at sunspot minimum and the influence of an interhemispheric flow of thermal protons
Authors: Bailey G. J. , Vickrey J. F. , Swartz W. E.
Published: February 2008

Incoherent scatter measurements of the topside ionosphere above Arecibo on August 15–16, 1976, are presented and compared with a time‐dependent mathematical model of the L = 1.4 magnetic flux tube. In the mathematical model the ions are O+ and H+, and their concentrations and field‐aligned velocities are calculated from time dependent plasma continuity and momentum equations. As far as possible, the values of the atmospheric parameters used in the mathematical model are appropriate to August 15–16, 1976. However, to help explain the processes that occur in the topside ionosphere above Arecibo, calculations have been performed with some modified atmospheric parameters. The observed values of electron concentration and percentage H+ abundance can be reproduced by the mathematical model if we permit an interhemispheric flow of H+. There does not appear to be any other physically realistic way to achieve agreement. At times, the model indicates that interhemispheric flow of H+ is from the hemisphere conjugate to Arecibo, i.e., from the winter hemisphere to the summer hemisphere. However, the results of the calculations suggest a net 24‐hour transfer of H+ into the conjugate hemisphere, i.e., from the summer hemisphere to the winter hemisphere. Typically, it amounts to 55% of the 24‐hour mean value of the total plasma content of the L = 1.4 magnetic flux tube between an altitude of 963 km and the equatorial plane. This is in excellent agreement with the estimate of net integrated H+ flux given by Vickrey et al. (1979b), which was based on comparing Arecibo observations from summer and winter days. However, the model indicates that there is no simple relationship on a moment‐by‐moment basis between the magnitude and direction of the interhemispheric H+ flux and the magnitude and direction of the H+ field‐aligned flux at an altitude around 1000 km. We also show that provided the plasma concentration remains unchanged at F2‐region altitudes, the concentration of H+ in the topside ionosphere is more sensitive to changes in the neutral atomic hydrogen concentration, n(H), than to changes in the neutral atomic oxygen concentration. However, the concentration of O+ in the topside ionosphere is only marginally affected by n(H).
Generation of electron plasma waves in the upstream solar wind
Authors: Fung S. F. , Papadopoulos K. , Wu C. S.
Published: February 2008

Electron plasma waves with amplitudes as high as 10 mV/m have been measured in the electron foreshock region of the earth's bow shock. It is shown that a population of stable suprathermal electrons of the type observed can produce the measured level of enhanced fluctuations without becoming unstable. The steady state wave level is given by the balancing between emission and absorption. The apparent association of fluctuation enhancements with sharp changes in electron fluxes can be understood in terms of modification of the electron distribution due to acceleration processes. The expected 2ωpe radiation from the model is consistent with the observations.
Precipitating electron energy flux and auroral zone conductances‐An empirical model
Authors: Spiro R. W. , Reiff P. H. , Maher L. J.
Published: February 2008

Data from the low energy electron (LEE) experiments on the Atmosphere Explorer C and D satellites have been used to determine the average global distribution of the energy flux of precipitating auroral electrons and their average energy for different levels of geomagnetic activity. Measurements from the Atmosphere Explorer unified abstract file (15‐s resolution) have been binned according to invariant latitude (in the range 50°–90°), magnetic local time, and geomagnetic activity as measured by the Kp and auroral electrojet (AE) indices, separately. Bin‐averaged values of precipitating energy flux and average energy have been calculated, and a smoothing algorithm used to reduce stochastic variations in the raw data. The results indicate that, for the parameters studied, the AE index does a superior job of ordering the data with regard to geomagnetic activity. The global distribution of the auroral enhancement portion of the Pedersen and Hall conductances were inferred from the data by means of an empirical fit to detailed energy deposition calculations.
A technique for establishing a reference potential on satellites in planetary ionospheres
Authors: Zuccaro D. R. , Holt B. J.
Published: February 2008

A simple, nearly passive technique is described that allows a spacecraft sensor to be driven to a potential close to that of a plasma where the ion concentration exceeds about 100 cm−3 even in the presence of a large vehicle potential. Such a technique may become increasingly useful in the event that compatibility constraints in the Space Shuttle demand a 28‐V negative ground spacecraft system.
Compression of the Venusian ionosphere on May 10, 1979, by the interplanetary shock generated by the solar eruption of May 8, 1979
Authors: Dryer M. , Pérez‐de‐Tejada H. , Taylor H. A. , Intriligator D. S. , Mihalov J. D. , Rompolt B.
Published: February 2008

An interplanetary shock wave that was produced by a solar eruption and its associated coronal transient on May 8, 1979, has been ‘tracked’ through interplanetary space to a rendezvous 2 days later with Venus. The interaction of the shock wave with the ionospheric obstacle at Venus produced a significant compression of the dayside ionosphere. It is believed that the tracking, as it were, was accomplished for the first time via the diagnostic observations provided by Hα and white light imagery near the sun and the plasma and field measurements of two, nearly radially aligned, spacecraft.
Stationarity of magnetohydrodynamic fluctuations in the solar wind
Authors: Matthaeus William H. , Goldstein Melvyn L.
Published: February 2008

Solar wind research and studies of charged particle propagation often assume that the interplanetary magnetic field represents a stationary random process. In this paper we investigate the extent to which ensemble averages of the solar wind magnetic fields follow the asymptotic behavior predicted by the ergodic theorem. Several time periods, including a span of nearly two years, are analyzed. Data intervals that span many solar rotations are found to satisfy the conditions of ‘weak’ stationarity if the effects of solar rotation are included in the asymptotic analysis. Shorter intervals, which include a small integral number of interplanetary sectors, also satisfy weak stationarity. The results are illustrated by using magnetometer data from the ISEE 3, Voyager and IMP spacecraft.
Magnetic field compression at the dayside magnetopause
Authors: Crooker N. U. , Siscoe G. L. , Mullen P. R. , Russell C. T. , Smith E. J.
Published: February 2008

The degree of compression sustained by the solar wind field as it convects to the magnetopause has been determined empirically with magnetometer data from ISEE 3 in the solar wind and ISEE 1 in the magnetosheath. At the stagnation point, the strength BSH of the magnetosheath field may be expressed as
which implies that BSH is equal to the geometric mean of the stagnation field strength BST and the shocked solar wind field, approximated by 4 BSWt For the usual spiral pattern of the solar wind field, BSH on the duskside of the magnetopause is about 20% stronger than it is on the dawnside. No dependence of BSH on the sign of the north‐south component of the solar wind field is apparent.
On ion harmonic structure in auroral zone waves: The effect of ion conic damping of auroral hiss
Authors: Gorney D. J. , Church S. R. , Mizera P. F.
Published: February 2008

Several low‐altitude auroral wave observations exhibiting ion harmonic frequency structure have been reported. We present here recent S3‐3 wave and particle data that illustrate examples of ion harmonic structure in auroral hiss and the associated energetic ion distributions. Such harmonic features have previously been attributed to either hot ion absorption or emission processes. We model the observed ion conic distributions to evaluate their effect on auroral hiss near the lower hybrid resonance frequency where harmonic features are seen. The calculations indicate that resonant ion damping of auroral hiss can indeed account for this distinct class of observed harmonic feature. The concomitant heating of the ion tail, however, appears insufficient to significantly energize the ion distribution.
Upgoing ion beams: 1. Microscopic analysis
Authors: Kaufmann Richard L. , Kintner Paul M.
Published: February 2008

The stability of electrostatic waves with frequencies near the hydrogen cyclotron frequency is investigated for an auroral plasma containing an ion beam. The energetic portion of the model distribution function is based on measurements made by Aerospace ion and electron detectors on the S3‐3 satellite at an altitude of 1 RE. Parameters describing unmeasured cold background species are varied, and the resulting wave growth rates are compared with measurements from the Berkeley AC electric field experiment. The events studied in this report involve waves that peak just above the hydrogen cyclotron frequency. These waves are seen only when an upgoing ion beam is present. We conclude that of the wave modes investigated, only electrostatic hydrogen ion cyclotron waves can be driven by the existing plasma. The waves can be produced either by drifting electrons or by the ion beam. Although we cannot exclude the possibility that electrons are drifting and contributing to wave growth, we find that many features of the measurements are a natural consequence of instabilities generated by ion beams. In this model, ion beams are seen with their observed temperatures because they have evolved to a weakly unstable configuration in which the wave growth length is comparable to the width of the beam region. Waves are well confined to the beams because they are damped rapidly in the adjacent plasma. The mirror effect can maintain a weak instability over a considerable altitude range. The above explanation is consistent only if we make two assumptions concerning unmeasured particles. First, hydrogen ions that are too cold to be measured by the S3‐3 detectors must be present to sustain the hydrogen cyclotron waves and to produce a second peak in the ion distribution function. In addition, we require that almost no cold Maxwellian electrons are present because they would Landau damp the waves more rapidly than the ion beam can drive them. The measurements provide some indication that electrons have been heated to just above the temperature at which they no longer damp the waves. The above assumptions appear to be reasonable if the satellite is passing above a region containing parallel electric fields. The electric field accelerates beam ions up. Cold ionospheric electrons are reflected down so they cannot reach the satellite. In this environment, wave growth is balanced between a reasonance in the parallel ion beam velocity, where ∂f/∂υ∥ > 0, and ion cyclotron damping, which is also generated by the ion beam. That is, the apparent effect of the EHC (electrostatic hydrogen ion cyclotron) waves is to convert parallel ion beam energy to perpendicular ion beam energy. If this effect proves to be generally true, a source for strong pitch angle scattering has been found, and the nonexistence of downgoing ion beams has been explained.
Synthetic spectra for auroral studies: The N2 Vegard‐Kaplan Band System
Authors: Degen V.
Published: February 2008

Synthetic spectra of the N2 Vegard‐Kaplan system are given for general reference. The material contains the spectrum across the entire range of emission (1500–6500 Å) calculated for auroral conditions and three typical resolutions (3.5, 10, and 25 Å); also included are individual band profiles calculated for 300–5000°K rotational temperature and 0–5 Å spectral halfwidth. For the auroral conditions a rotational temperature of 800°K is assumed; the vibrational populations are inferred from matching synthetic spectra and various published data. The calculations suggest a weak Vegard‐Kaplan auroral contribution in the 1500–2000 Å region of the far ultraviolet where the N2 Lyman‐Birge‐Hopfield system is the dominant feature. Simulation of Vegard‐Kaplan emission from nitrogen trapped in a frozen argon matrix at 4.2°K is included as an example of a more unusual source.
The interaction of flowing plasmas with planetary ionospheres: A Titan‐Venus comparison
Authors: Kivelson Margaret G. , Russell Christopher T.
Published: February 2008

Titan and Venus are both unmagnetized bodies enveloped by induced magnetospheres created within the flowing plasmas in which they are embedded. When Saturn's magnetosphere extends beyond the position of Titan, as during the Voyager 1 flyby on November 12, 1980, the plasma incident at Titan contains a steady magnetic field approximately perpendicular to the plasma flow velocity. At Venus the incident plasma usually contains a time‐varying magnetic field, but occasionally, the interplanetary conditions remain steady for the few hours required for the Pioneer Venus orbiter to pass through the induced magnetosphere. In particular, on February 28, 1979, the interplanetary field appears to have been stable and transverse to the solar wind velocity during the wake pass which was selected for a comparative Venus‐Titan study. For both wakes, symmetry properties of the measured magnetic fields were used to infer the directions of ambient plasma flow. In each case the flow direction differed from nominal, with the flow at Titan aberrated about 27° radially inward of the corotation direction and the flow at Venus aberrated about 10° relative to the heliocentric radial direction. Corroborating evidence for the aberrated flows was provided by plasma observations. The wake field magnitudes were nonuniform, and in both cases, reduced magnitudes were found on field lines linked to the illuminated (dayside) ionospheres. Finally, both induced magnetotails contained regions in which the projection of the magnetic field onto the incident magnetic field was negative. This observation may indicate that in an induced magnetosphere the plasma flows toward the mid‐tail current sheet in the near wake, thus producing a field configuration that corresponds to a reconnection geometry.
Rotational discontinuities and the structure of the magnetopause
Authors: Swift Daniel W. , Lee Lou C.
Published: February 2008

Symmetric and asymmetric rotational discontinuities are studied by means of a one‐dimensional computer simulation and by single‐particle trajectory calculations. The numerical simulations show the symmetric rotation to be stable for both ion and electron senses of rotation with a thickness of the order of a few ion gyroradii when the rotation angle of the tangential field is 180° or less. Larger rotation angles tend to be unstable. In an expansive discontinuity, when the magnetic field on the downstream side of the discontinuity is larger, an expanding transition layer separating the high‐field from a low‐field region develops on the downstream side, and a symmetric rotational discontinuity forms at the upstream edge. The implication of these results for magnetopause structure and energy flow through the magnetopause is described.
Quasi‐relativistic electron precipitation due to interactions with coherent VLF waves in the magnetosphere
Authors: Chang H. C. , Inan U. S.
Published: February 2008

The generalized equations of motion for the gyroresonance interaction between energetic electrons and coherent VLF waves in the magnetosphere are employed to study quasi‐relativistic electron precipitation. It is suggested that these exact equations be used for particle energies higher than 50 keV (corresponding to a 10% correction in average pitch angle scattering with respect to a nonrelativistic formulation). Based on these equations and the relativistic cyclotron resonance condition, a previous test particle simulation method (Inan, 1977; Inan et al., 1978, 1982) is extended to higher energies and is used to study the wave‐particle interactions involving the near‐loss‐cone quasi‐relativistic particles. The root mean square (rms) pitch angle scattering of the near‐loss‐cone particles and the corresponding precipitated energy spectrum are then calculated and compared. It is argued that the full width at half maximum (FWHM) of the rms scattering pattern would give an upper bound to that of the energy spectrum of the precipitated flux. Furthermore the peak widths are demonstrated to be within the upper limit values measured by recent satellite experiments. This agreement between the computed and measured spectral width lends credence to the suggestions that the observed narrow peaks in precipitated energy spectra may indeed be caused by monochromatic signals injected into the magnetosphere by VLF transmitters.
Particle and wave observations of low‐altitude ionospheric ion acceleration events
Authors: Yau A. W. , Whalen B. A. , McNamara A. G. , Kellogg P. J. , Bernstein W.
Published: February 2008

We report energetic ion and electron, wave, and ambient plasma observations from two sounding rockets which were launched from Churchill, Canada, into the expansive phases of two auroral substorms and which passed through source regions of transversely accelerated ionospheric ions (TAI). The two events were observed at low altitudes (400–600 km) and resulted in ion energization of hundreds of electron volts. In the acceleration region, the ionospheric ion velocity distribution function in the direction perpendicular to the local magnetic field (⊥B) displayed a distinct, non‐Maxwellian, high‐energy tail, suggesting ion cyclotron heating. The plasma density was lower than theoretical quiescent values, by as much as 2 decades in the stronger event. Strong thermal ion drift was observed in the perpendicular (⊥B) direction, but was absent in the parallel (∥B) direction. Large‐amplitude, low‐frequency fluctuations in plasma density were present, and a number of wave modes were observed, including upper hybrid and Langmuir waves, and whistler, ion acoustic, and ion cyclotron waves. No consistent correlation existed between the energetic particle precipitation and the TAI. However, strong field‐aligned electron enhancements were observed at times coincident with the TAI acceleration region. The characteristics of the TAI ion energy spectra were consistent with a simple model of ion cyclotron acceleration and energy loss due to ion‐neutral collisions. These observations are discussed in terms of current theories on electrostatic ion cyclotron acceleration and lower hybrid acceleration.
Method of remote sensing of horizontal stratification due to an ionospherically reflected powerful radio wave
Authors: Fejer J. A.
Published: February 2008

It is shown numerically that the group delay of a weak diagnostic radio wave is greatly reduced in the presence of a powerful radio wave of slightly lower frequency vertically incident on the ionosphere below the penetration frequency. The numerical results are qualitatively interpreted in terms of a narrow stop band for propagation in a spatially periodic medium, for wavelengths near twice the spatial period. Application to remote sensing of the horizontal density stratification in ionospheric modification experiments is discussed.
A theory of Jovian Decameter Radiation
Authors: Goldstein M. L. , Sharma R. R. , Ben‐Ari M. , Eviatar A. , Papadopoulos K.
Published: February 2008

A theory of the Jovian decameter radiation is presented based on the assumed existence of beams of energetic electrons in the inner Jovian magnetosphere. Beam‐like electron distributions are shown to be unstable to the growth of both upper hybrid and lower hybrid electrostatic waves. The upconversion of these waves to fast extraordinary mode electromagnetic radiation is calculated by using a fluid model. Two possibilities are considered. First, we make a random phase approximation which leads to a very conservative estimate of intensity that can be expected in decameter radiation. The alternative possibility is also considered, viz, that the upconversion process is coherent. A comparison of both processes suggests that an incoherent interaction may be adequate to account for the observed intensity of decametric radiation except perhaps near the peak of the spectrum (8 MHz). The coherent process is intrinsically more efficient and can easily produce the observed intensity near 8 MHz if only 0.01% of the energy in the beam is converted to electrostatic energy.
Kelvin:Helmholtz Instability at the magnetopause: Solution for compressible plasmas
Authors: Pu Zu‐Yin , Kivelson Margaret G.
Published: February 2008

The Kelvin‐Helmholtz (K‐H) instability of a tangential discontinuity in a compressible plasma is reexamined in the linear magnetohydrodynamic (MHD) approximation. For fixed plasma conditions, two different kinds of surface waves (labeled F for fast and S for slow) may exist simultaneously with different tangential wave vectors kt. The surface waves can be excited only for a limited range of U, the relative flow speed of the plasmas on the two sides of the interface. Thus the instability requires Ucm < U < Uum, where Ucm and Uum are the lower and upper critical velocities, respectively, and the subscript m distinguishes the fast and slow surface waves, with UcS < UcF. In the incompressible limit, there is only one surface wave with lower critical velocity, Uci, and no upper critical velocity. It has frequently been remarked that compressibility reduces the lower critical velocity. We show that the effect is small for the F wave but that UcS is considerably smaller than either UcF or Uci. However, for a given kt, the S wave is unstable only for a very small range of U. Growth rates, εm, of the surface waves are calculated and εF exceeds slightly the growth rate for the incompressible plasma (εi) when U ≳ Ui, but the slow wave growth rate is small in comparison with both εF and εi. Consequently, plasma compressibility is relatively ineffective in reducing the critical velocity for surface wave growth below that for an incompressible plasma. In particular, for the nominal conditions of the dayside equatorial magnetopause, S mode waves do not occur near the minimum UcS (i.e., with kt, perpendicular to the magnetospheric magnetic field) because εS → 0 and UuS = UcS. On the terrestrial magnetopause the F(S) surface waves can couple a quasi‐fast (quasi‐slow) MHD mode in the magnetospheric plasma to either a quasi‐fast or quasi‐slow MHD mode in the magnetosheath. These waves decay as they propagate into the bounding plasmas. Consideration of the limit U → UuF reveals the significance of the upper critical velocity. In this limit, the phase velocity of the fast surface wave approaches the MHD fast mode speed in both bounding plasmas. The imaginary part of normal component of k vanishes and the unstable surface waves change to stable MHD waves that propagate away from the boundary without damping. The above points are discussed in relation to previous work on the K‐H instability. Possible applications to observations in the terrestrial magnetosphere are mentioned.
Jovian modulation of interplanetary electrons as observed with Voyagers 1 and 2
Authors: Schardt A. W. , McDonald F. B. , Trainor J. H.
Published: February 2008

The release of magnetospheric electrons from Jupiter into interplanetary space is modulated by the Jovian rotation period. This effect was initially discovered by Pioneer 10, and the Voyager 1 and 2 observations permit a more detailed study of this modulation. It was found that the modulation period agrees on the average with the synodic period of Jupiter (9 h 55 min 33.12 s), but over intervals of weeks it can differ from the synodic period by several minutes. The lack of exact synchronization is attributed to changes of the plasma population in the Jovian magnetosphere. Such changes affect the magnetic field sweep‐back and departure from exact corotation. However, the magnetospheric asymmetry, which is responsible for the modulation, is always re‐established at the same longitude. Thus no long term departures occur from the synodic period. The Jovian modulation appears to be a persistent feature of the interaction between the solar wind and the magnetosphere, and the disappearance of the modulation away from Jupiter is attributed to interplanetary propagation conditions. Although modulation could be detected in interplanetary space out to only ∼108 km from Jupiter, it was still detectable at 3.8 AU behind Jupiter in the far magnetotail. This requires a mean free path in the tail ≥0.75 AU and good field connection along the tail to Jupiter. During two intervals, the electron spectrum was softer than the Jovian spectrum and was not modulated by the Jovian period. The time histories of these latter increases are similar to that of 30‐ to 60‐MeV protons measured simultaneously by the same detector system. They are associated with large solar cosmic ray events at 1 AU and appear to be of solar origin.
Density drift instabilities and weak collisions
Authors: Gary S. Peter , Bernhardt Paul A. , Cole Thomas E.
Published: February 2008

This paper describes the effects of weak collisions on the linear kinetic theory of electrostatic density drift instabilities. The model assumes a weak, uniform density gradient, a uniform magnetic field and the BGK collision operator with a modification of the local approximation to derive a dispersion equation valid at all frequencies and wave numbers. The properties of the universal and collisional density drift instabilities at maximum growth rates are presented in detail. Their thresholds in an ionospheric model, which includes ion‐neutral, electron‐neutral, and electron‐ion collisions, are discussed and compared with the threshold of the lower hybrid density drift instability. In particular, the ionospheric conditions (plasma density ≲105 cm−3, altitude ≳250 km) and maximum scale lengths necessary to excite the universal at wavelengths of the order of the ion gyroradius are presented. We conclude that the k−5 short wavelength density power spectra observed above 280 km in the PLUMEX experiment are due to the effects of the universal density drift instability.