bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial distributions of magnetic field fluctuations in the dayside magnetosheath
Authors: Luhmann J. G. , Russell C. T. , Elphic R. C.
Published: February 2008

In a study that tests the hypothesis that magnetosheath magnetic fields are disturbed on plasma streamlines which are connected to the quasi‐parallel bow shock, magnetometer observations from the ISEE 2 and IMP 8 spacecraft are used to investigate the dayside spatial distributions of fluctuating magnetosheath fields for different interplanetary field orientations. The results suggest that although other sources such as Kelvin‐Helmholtz instabilities and flux transfer processes probably contribute to the fluctuations in the magnetosheath field, the quasi‐parallel shock source is an important contributor in the dayside region.
Field‐aligned electron precipitation at the edge of an arc
Authors: McFadden J. P. , Carlson C. W. , Boehm M. H.
Published: February 2008

Measurements of field‐aligned electrons at the edge of an arc are presented from a sounding rocket flight through a quiet afternoon auroral arc. High time resolution measurements show the evolution of the electron distribution function over the 2.3‐km width of field‐aligned precipitation. A nearly constant 1.2‐eV perpendicular temperature was found for these field‐aligned fluxes over a broad range of parallel energies (100–900 eV). The small perpendicular temperature indicates that the acceleration region is located at a low altitude. Two models of cold plasma convection into the edge of a V‐shaped potential structure are examined and found consistent with both the observed fluxes and spatial width of the edge precipitation. Both models predict an average source plasma density less than 130/cm³ to account for the observed field‐aligned fluxes and thus an acceleration region well above 1000 km.
Differential measurement and model calculations of cosmic ray latitudinal gradient with respect to the heliospheric current sheet
Authors: Christon S. P. , Cummings A. C. , Stone E. C. , Behannon K. W. , Burlaga L. F. , Jokipii J. R. , Kota J.
Published: February 2008

Simultaneous magnetic field and charged particle measurements from the Voyager spacecraft with heliographic latitude separations of >10° are used to investigate the distribution of ∼1‐GeV galactic cosmic ray protons with respect to the heliospheric current sheet in the outer solar system. By comparing the ratio of cosmic ray flux at Voyager 1 to that at Voyager 2 during periods of relatively quiet interplanetary conditions when the spacecraft are either both north or both south of the heliospheric current sheet, we derive an average latitude component of the gradient of the cosmic ray flux on opposite sides of the current sheet under restricted interplanetary conditions of −0.22±0.03%/deg, equivalent to a decrease of ∼1%/AU away from the current sheet at ∼12 AU. Our results for these limited periods are in qualitative agreement with propagation models incorporating particle drifts.
Absolute experimental differential and integral electron excitation cross sections for atomic oxygen: 1. The (³P → ³So) Transition (1304 Å) at 100 eV
Authors: Doering J. P. , Vaughan S. O.
Published: February 2008

The absolute differential electron scattering cross section for the atomic oxygen (³P → ³So) transition (1304 Å) has been measured at 100‐eV electron impact energy. The experimental apparatus was a variable angle and incident energy electron spectrometer coupled to a microwave discharge free supersonic jet expansion source. The source produced an intense jet containing O2, H, O, and He with an O/O2 ratio typically greater than 2.5. Relative densities of species in the jet were determined from the zero angle forward electron energy loss spectrum and the known optical oscillator strengths for transitions in each species. Measured relative differential cross sections (DCS) were put on an absolute scale by comparison with the known DCS for the He 2 ¹P transition. As a check on the atomic oxygen values a simultaneous determination was made of the DCS for the H (1s) → H (n = 2) transition. Results for this transition were in excellent agreement with previous experimental and theoretical results. An additional check on the DCS values for O and H was given by the extrapolation within experimental error of the generalized oscillator strengths calculated from the measured DCS values to the optical oscillator strengths at zero momentum change. The O ³So DCS is extremely forward peaked and decreases by almost 5 orders of magnitude from 0° to 90° scattering angle. The integral, direct excitation, cascade‐free cross section at 100 eV is 6.4 (±1.0) × 10−18 cm².
Transition region, corona, and solar wind in coronal holes
Authors: Hollweg Joseph V.
Published: February 2008

Previous wave‐driven solar wind models (Hollweg, 1978) have been extended by including a new hypothesis for the nonlinear wave dissipation. The hypothesis is that the waves dissipate via a turbulent cascade at the rate given by (1) and the waves evolve according to (16). A subhypothesis is that the relevant correlation length scales as the distance between magnetic field lines. This hypothesis allows us to treat the corona and the solar wind on an equal footing; unlike in previous wave‐driven models, we do not assume that the coronal heating takes place below the base of the model. The models exhibit the correct qualitative features, viz., a steep temperature rise (the transition region) to a maximum coronal temperature in excess of 106 K, and a substantial solar wind mass flux in excess of 3.5×108 cm−2 s−1 at 1 AU. However, the model fails in detail. Parameters that yield a high‐speed flow at 1 AU have base pressures that are too low; parameters that yield correct base pressures have low solar wind flow speeds. However, the model “comes close.” Thus although we have not shown that the initial hypothesis is consistent with available data, we feel that there are sufficient uncertainties both in the model and in the data to preclude outright rejection of the hypothesis altogether.
Average dimension and magnetic structure of the distant Venus magnetotail
Authors: Saunders M. A. , Russell C. T.
Published: February 2008

We report the first major statistical investigation of the far wake of an unmagnetized object embedded in the solar wind. The investigation is based on Pioneer Venus Orbiter magnetometer data from 70 crossings of the Venus wake at altitudes between 5 and 11 Venus radii during reasonably steady IMF conditions. We find that Venus has a well‐developed tail, flaring with altitude and possibly broader in the direction parallel to the IMF cross‐flow component. Tail lobe field polarities and the direction of the cross‐tail field are consistent with tail accretion from the solar wind. Average values for the cross‐tail field (2 nT) and the distant tail flux (3 MWb) indicate that most distant tail field lines close across the center of the tail and are not rooted in the Venus ionosphere. We illustrate our findings in a three‐dimensional schematic.
The use of electrostatic noise to control high‐voltage differential charging of spacecraft
Authors: Hastings D. E.
Published: February 2008

High differential charging is known to occur on geostationary satellites between two electrically isolated pieces of the same material. This happens because current balance with the ambient space environment on each surface can be satisfied by different surface potentials. Three such potentials (two stable and one unstable) are found for the two‐Maxwellian plasmas considered in this study and in the absence of photoelectron emission. Thus two isolated surfaces can have significantly different potentials leading, possibly, to electrostatic discharges. It is shown that, in an electrostatically noisy environment, one of the surface potentials is more likely to occur than the others and thus both surfaces will achieve this most probable potential independent of their previous charging history. The time for this to happen, while dependent on the characteristics of the roots, can be relatively short and may limit this type of differential charging. This effect is examined in quantitative detail for the environments measured by the ATS‐5 and SCATHA satellites.
Production of flickering aurora and field‐aligned electron flux by electromagnetic ion cyclotron waves
Authors: Temerin M. , McFadden J. , Boehm M. , Carlson C. W. , Lotko W.
Published: February 2008

Recent observations have suggested that flickering aurora is produced by a modulation of the field‐aligned component of the electron flux within an auroral arc. It is proposed that a portion of the field‐aligned electrons are of ionospheric origin and that these electrons are accelerated and their flux modulated by electromagnetic ion cyclotron waves that occur below the main acceleration region on auroral arc field lines. A model of the electromagnetic ion cyclotron wave shows that the parallel phase velocity of the wave increases as the wave propagates toward the ionosphere. A test particle calculation shows that ionospheric electrons trapped or reflected by the wave are accelerated to energies of several keV and that their flux is modulated at the wave frequency. The relative amplitudes of the model wave electric fields are consistent with the observations of small‐scale low‐frequency ionospheric and magnetospheric electric fields near auroral arcs of approximately 10 mV/m and 100 mV/m, respectively. The large‐amplitude ion cyclotron waves also produce a ponderomotive force and a self‐consistent ambipolar electric field. Energy considerations show that the downward energy flux in the electromagnetic ion cyclotron wave can be several percent of the total downward auroral electron energy flux.
Fluid dynamic constraints of the Venus ionospheric flow
Authors: Pérez‐de‐Tejada H.
Published: February 2008

A critical analysis of the current interpretations of the antisolar motion of the Venus ionosphere is presented. It is argued that the pressure gradient forces present across the terminator are not sufficient to produce the observed acceleration of the plasma. A balance condition between the height‐integrated momentum flux of the ionospheric flow and the height‐integrated deficiency of momentum flux of the shocked solar wind in the vicinity of the terminator is formulated in terms of measurable quantities. It is found that the observed 2 to 4‐km/s flow velocities of the upper ionosphere can be readily inferred from such a relation. This result is consistent with the concept that an efficient transport of momentum takes place across the ionopause, and that the bulk of the kinetic energy required to accelerate the Venus ionosphere is derived from the shocked solar wind.
Multiple X line reconnection: 1. A criterion for the transition from a single X line to a multiple X line reconnection
Authors: Lee L. C. , Fu Z. F.
Published: February 2008

In an earlier paper we suggested that the multiple X line reconnection process may occur at the dayside magnetopause and lead to the occurrence of flux transfer events. In a series of papers we shall examine in detail various aspects of the multiple X line reconnection processes. In the present paper, the first of the series, we attempt to obtain a criterion for the transition from the classical single X line reconnection to the multiple X line reconnection based on a two‐dimensional simulation model. It is found that the development of tearing instability in the diffusion region of the reconnection configuration may lead to the formation of magnetic islands and hence to the occurrence of multiple X line reconnection. The criterion for the tearing instability is found to be l/δ > 7 coth (8/S0.5), where S is the Lundquist number based on the width of the diffusion region, l is the half‐length, and δ the half‐width of the diffusion region. Furthermore, a formula is obtained for the dependence of the ratio l/δ on the system length Lz, the imposed reconnection rate R0, and the resistivity η. The ratio l/δ is found to increase with an increasing system length Lz, an increasing imposed reconnection rate R0, and a decreasing resistivity η. Therefore the multiple X line reconnection process tends to occur when the system length is long, the reconnection rate is high, or the resistivity is small.
A model of impulsive acceleration and transport of energetic particles in Mercury's magnetosphere
Authors: Baker D. N. , Simpson J. A. , Eraker J. H.
Published: February 2008

A qualitative model of substorm processes in the Mercury magnetosphere is presented based on Mariner 10 observations obtained in 1974–1975. The model is predicated on close analogies observed with the terrestrial case. Particular emphasis is given to energetic particle phenomena as observed by Mariner on March 29, 1974. We support the suggestion that energetic particles up to ∼500 keV are produced by strong induced electric fields at 3 ∼ 6 RM in the Hermean tail in association with substorm neutral line formation. The bursts of energetic particles produced are, in this model, subsequently confined on closed field lines near Mercury and drift adiabatically on quasi‐trapped orbits for many tens of seconds. Such gradient and curvature drift of the particles can explain prominent periodicities of 5–10 s seen in the Mariner > 170‐keV electron flux profiles.
Electromagnetic ion beam instabilities: Comparison of one‐ and two‐dimensional simulations
Authors: Winske D. , Quest K. B.
Published: February 2008

Numerical simulations of electromagnetic instabilities, driven by a cool tenuous ion beam propagating along an ambient magnetic field, have been conducted in one and two spatial dimensions. The calculations employ particle ions, fluid electrons, and a predictor corrector scheme for solving the electromagnetic field in two dimensions that is described in some detail. While the principal features of the one‐dimensional calculations (which reproduce previous work) are retained, the two‐dimensional simulations show some reduction of the overall level of the magnetic field fluctuations. Enhancement of the heating of the beam ions at the expense of the core ions also occurs in the case where the beam density is sufficiently large that the right‐hand nonresonant instability dominates. Implications of the results for modeling of the ion foreshock and quasi‐parallel shocks are discussed.
On the influence of an external electric field on magnetotail reconnection
Authors: Birn J. , Schindler K.
Published: February 2008

We have studied magnetotail reconnection under the influence of a dawn to dusk electric field applied at the high‐latitude tail boundary using a two‐dimensional resistive MHD code. We found that the basic unstable sequence of tearing and plasmoid formation and ejection was not changed if the external electric field was fairly uniform. The externally forced compression, however, could shorten the onset time and the growth time of the evolution. The faster growth can be understood by the fact that the forced compression gradually alters the lobe field strength and the plasma sheet width and reduces thereby the relevant Alfvén time. In fact, if the evolution of the cases with imposed electric field is rescaled on the basis of these characteristics parameters, a good quantitative agreement with the case without external electric field is found.
Numerical simulations of atmospheric tides for solstice conditions
Authors: Vial François
Published: February 2008

Numerical simulations are made to study the behavior of atmospheric tides in an atmosphere with arbitrary distribution of mean temperature and zonal wind. Owing to their importance at meteor heights the parametrization of diffusive processes is discussed, and two kinds of models are used, depending on the particular mode studied. Specific attention is given to the (1, 1) mode of the diurnal tide because we want to take into account the modification of its latitudinal‐vertical structure due to the background atmosphere. Simulations of semidiurnal and diurnal tides are presented and compared with previous theoretical results and experimental data. However, our main goal is an improved theoretical description of the diurnal tide. In particular, (1, 1) mode “distortions” due to mean wind are shown to be important in accounting for the seasonal variation of the amplitude of the diurnal tide existing in experimental data. The action of turbulent diffusion on the diurnal tide in our numerical results is also analyzed.
Correlation of cusp latitude with Bz and AE (12) using nearly one year's data
Authors: Carbary J. F. , Meng C. I.
Published: February 2008

This investigation reports the statistical correlation of cusp latitude with the interplanetary magnetic field (IMF) Bz and the AE (12) index. Eleven months of data from 1979 were collected, subject only to the criteria that the IMP 8 spacecraft be outside the magnetopause to measure the IMF and that the daily average of AE (12) exceed 200 nT. In general, the latitudes of the poleward cusp boundaries display higher correlation coefficients than do the latitudes of the equatorward boundaries. The best correlations of cusp latitudes with AE involve no time delay, while the best correlations with Bz involve a 30‐min time delay. The strongest correlation (R = 0.77) occurs between the 30‐min time‐delayed Bz and the latitude of the poleward cusp boundary. The latitude of the equatorward cusp boundary correlates best with the time‐retarded southward Bz (R = 0.75). The best correlations of the cusp latitudes with AE (R = 0.68–0.71) are generally not as good as those with the time‐delayed Bz.
The magnetopause for large magnetic shear: AMPTE/IRM observations
Authors: Paschmann G. , Papamastorakis I. , Baumjohann W. , Sckopke N. , Carlson C. W. , Sonnerup B. U. Ö. , Lühr H.
Published: February 2008

A total of 21 passes of AMPTE/IRM through the dayside (0800–1600 hours local time) low‐latitude magnetospheric boundary region have been examined, all of which were characterized by large magnetic shear across the magnetopause. The purpose of the study was to use the improved accuracy and time resolution of the IRM plasma measurements to reexamine the occurrence of high‐speed flows as signatures of the reconnection process at the magnetopause. A total of 12 of the 21 passes showed magnetopause crossings with high‐speed flows. The duration of these flows was sometimes as short as 10 s and rarely more than 30 s. The occurrence of high‐speed flows is inversely correlated with β, the ratio of plasma to magnetic pressure. For β < 2 the ratio, ΔV*, of observed to theoretically predicted flow velocity changes is high (ΔV* = 0.74 on average), while for β > 2 the average ratio is low (ΔV* = 0.18). We tentatively interpret this to indicate that reconnection may occur preferentially for low β values. A total of nine high‐speed flows were subjected to quantitative momentum and energy balance test. The agreement between observations and theoretical predictions was generally quite good. The signs of the normal magnetic field, Bn, deduced from the momentum balance agreed with the sense of magnetic connection inferred independently from the measured proton heat flux direction. In one case the observations suggest that the satellite may have traversed the diffusion region of the reconnection process. The momentum balance results indicate that at times the effective mass of the ions in the magnetopause and boundary layer was of the order of twice the proton mass.
Daytime high‐latitude rocket observations of the NO γ, δ, and ε bands
Authors: Cleary David D.
Published: February 2008

Observations of the Earth's thermospheric dayglow were obtained on August 10, 1982, from Poker Flat, Alaska, using a rocket‐borne spectrometer covering the wavelength range 1950 to 2170 Å. This part of the spectrum is dominated by emissions from the gamma, delta and epsilon bands of nitric oxide. Column densities of nitric oxide were determined by fitting these bands with synthetic spectra. At low altitudes the (1, 0) and (2, 0) gamma bands exhibit large self‐absorption. Three bands of the weak NO epsilon system, (0, 1), (0, 2), and (0, 3) were positively identified and used to determine NO densities between 105 and 125 km. The fluorescence efficiency of the NO delta bands was found to be ∼0.25 based on the fit to synthetic spectra. A one‐dimensional diffusive photochemical model of the earth's thermosphere was used for comparison with the rocket observations. The data was reproduced by the model using a 10 keV characteristic energy for the primary aurora electrons with a constant flux of 0.5 ergs cm−2 s−1. The production yield of N(²D) from the dissociation of N2 by electron impact used in this calculation was 0.6. An analysis was performed to show the sensitivity of the conclusions to the various model parameters.
A test of Lee's quasi‐linear theory of ion acceleration by interplanetary traveling shocks
Authors: Kennel C. F. , Coroniti F. V. , Scarf F. L. , Livesey W. A. , Russell C. T. , Smith E. J. , Wenzel K. P. , Scholer M.
Published: February 2008

We test Lee's (1983) quasi‐linear theory of ion acceleration using ISEE‐3 measurements of the November 12, 1978 quasi‐parallel interplanetary shock. His theory accounts with varying degrees of precision for the energetic proton spatial profiles; the dependence of the spectral index of the power law proton velocity distribution upon the shock compression ratio; the power law dependence of the upstream proton scalelength upon energy; the absolute magnitude of the upstream proton scale length; the behavior of the energetic proton anisotropy upstream and downstream of the shock; the behavior of the alpha‐particle proton ratio upstream; the equality of the spatial scale lengths at the shock of the upstream waves and of the protons that resonate with them; and the dependence of the integrated wave energy density upon the proton energy density at the shock. However, the trace magnetic field frequency spectra disagree with his theory in two ways. The part of the spectrum that can resonate with the observed protons via first‐order cyclotron resonance is flat, whereas Lee's theory predicts an f−7/4 frequency dependence for the November 12 shock. Higher frequency waves, which could not resonate with the observed upstream protons, increased in amplitude as the shock approached, suggesting that they too were generated by the shock.
Theoretical study of the electron temperature in the high‐latitude ionosphere for solar maximum and winter conditions
Authors: Schunk R. W. , Sojka J. J. , Bowline M. D.
Published: February 2008

The electron temperature (Te) variation in the high‐latitude ionosphere at altitudes between 120 and 800 km has been modeled for solar maximum, winter solstice, and strong magnetic activity conditions. The calculated electron temperatures are consistent with the plasma densities and ion temperatures computed from a time‐dependent ionospheric model. Heating rates for both solar EUV and auroral precipitation were included. In general, the predicted UT variation of the electron temperature that results from the displacement between the magnetic and geographic poles is only a few hundred degrees. However, in sunlit trough regions, Te hot spots develop, and these hot spots show a marked UT variation, by as much as 2500 K. The dominant parameter controlling the Te variation above 200 km is the magnetospheric heat flux into the ionosphere, which is essentially unknown. For realistic values of the magnetospheric heat flux, the maximum electron temperature ranges from 5000 to 10,000 K at 800 km. A magnetospheric heat flux is particularly effective in enhancing trough electron temperatures. In general, the electron heat flux at high altitudes is uniquely related to the electron temperature and gradient, except on auroral field lines where thermoelectric heat flow is important.
Equatorial F region vertical plasma drifts: Seasonal and longitudinal asymmetries in the American sector
Authors: Batista I. S. , Abdu M. A. , Bittencourt J. A.
Published: February 2008

Longitudinal and seasonal asymmetries in the evening ionospheric F region plasma vertical drift (Vz) enhancements, between two longitudinally separated stations situated along the magnetic equator, in the American sector, are investigated under solar maximum conditions, based on results obtained from the analysis of ionosonde data for these stations. The two stations are Huancayo, Peru, and Fortaleza, Brazil, which have markedly different magnetic declination angles. The observed asymmetries are interpreted using a detailed numerical simulation of the E and F region electrodynamic coupling process that takes into account also its asymmetry about the magnetic equator arising from the finite magnetic declination angle. The results of the simulation show, in agreement with observations, that the occurrence time of the evening F region vertical drift prereversal peak and its seasonal variation at a station are controlled by the magnetic declination angle at that station, which determines the seasonal variation of the sunset times (and hence the integrated Pedersen conductivity longitudinal gradient) at its magnetic conjugate E layers. The amplitude of the prereversal peak, on the other hand, undergoes the influence of the magnetic declination angle as well as of the thermospheric zonal wind component.
Fine structure and pitch angle dependence of synchronous orbit electron injections
Authors: Arnoldy R. L.
Published: February 2008

For time resolution on the order of seconds, electron substorm‐related injections at synchronous orbit still show energy dispersion and have considerable structure. The structure is at Pi 2 periods and can be characterized as the successive heating and cooling of a Maxwellian distribution. This study lends support to the concept that synchronous orbit injections are due to the passage of hot tail plasma over the spacecraft on an injection front. In this injection front model one might understand Pi 2 periodicities as an inherent periodicity in the flow velocity of the driving gas of the injection front causing the front to oscillate as it envelops the spacecraft. One might view the injection process as a series of mini‐injections and relaxations which continually move the spacecraft into hotter plasma. The pitch angle dependence of the plasma on the injection front either is isotropic or has a loss cone at high energies (several keV and greater) and is field aligned at the very low energies (less than few keV), suggesting an ionospheric source for this population. The arrival of electrons of selected energies after substorm onset but before passage of the injection front over the spacecraft indicates that the injection front is of limited spatial size. Even after an analysis of the most rapid (least energy dispersed) electron injection measured over a span of 3 years of data it is not certain that a synchronous orbit spacecraft is ever observing within the acceleration region. If electrons less than a few hundred electron volts are depleted in the newly accelerated electron spectrum, then their field‐aligned enhancement as ionospheric secondaries might identify magnetic lines of force with energetic electron precipitation and perhaps the acceleration region.
The effect of Coulomb friction and Alfven waves on minor ions in the low corona
Authors: McKenzie J. F. , Bond R. A. B. , Dougherty M. K.
Published: February 2008

The properties of minor ions in the low solar corona are studied on the basis of a quasi‐hydrostatic equilibrium model suitably modified to account for the combined effects of Coulomb friction and Alfven wave acceleration in dragging minor ions out of the corona against the binding solar gravitational force. The analysis therefore generalizes the work of Hollweg (1981) to include the effect of waves which become important in the upper corona and the results, which must be treated with some caution, indicate that provided the proton flux is sufficiently large, the ions are evicted from the corona into the wind at the same speed as the protons with relative abundances which reflect their flux relative to protons in the low corona.
Collisionless coupling in the AMPTE artificial comet
Authors: Papadopoulos K. , Huba J. D. , Lui A. T. Y.
Published: February 2008

Analysis of previously reported observations of the solar wind‐barium interaction associated with the AMPTE artificial cornet release of December 27, 1984, is presented. On the basis of these results we argue that the solar wind couples momentum (and energy) to the barium ions through both laminar and turbulent processes. The laminar forces acting on the particles are the laminar electric and magnetic fields; the turbulent forces are associated with the intense electrostatic wave activity. This wave activity is shown to be caused by a cross‐field solar wind proton‐barium ion streaming instability. The observed wave frequencies and saturated amplitudes are consistent with our theoretical analysis.
Spectral studies of the sources of ionospheric electric fields
Authors: Earle G. D. , Kelley M. C.
Published: February 2008

Spectral analyses have been performed upon a number of incoherent scatter radar data sets obtained at Jicamarca, Peru; Chatanika, Alaska; and Arecibo, Puerto Rico, with the goal of understanding the sources of electric fields with periods in the range of 1–10 hours. Two distinct sources are identified and studied in some detail. In quiet times, atmospheric gravity waves seem the most likely source of the ionospheric electric field. In fact, both in an average sense and in the single case study available the mesospheric winds measured at Poker Flat, Alaska, in this frequency range are remarkably similar in magnitude to the quiet time thermospheric plasma drifts measured overhead by the nearby Chatanika radar. Such drifts are driven by electric fields which, we argue, could easily be generated by the observed wind fields. Comparison with the spectra of electric field measurements at other latitudes suggests that such a source is worldwide and determines the geophysical noise level of low‐ and mid‐latitude electric field measurements. Turning to active times, we present a measure of the transfer function for electric field penetration between high‐ and low‐altitude L shells. At the very lowest frequencies (periods of ≥10 hours) the low‐altitude sites are well shielded, presumably by an Alfvén layer at the inner edge of the ring current. Higher frequency fluctuations penetrate very easily to low latitudes. A response peak seems to occur in the 3‐ to 5‐hour range of periods, with a lower response occurring at 1 cycle/hour, although this result must be viewed as preliminary for now. Between L = 5.5 and L = 1.4 the zonal electric field component as projected to the equatorial plane of the magnetosphere penetrates with little or no attenuation. At Jicamarca the field is reduced to about 75% of the high‐latitude source. Even so, the very high sensitivity of the Jicamarca measurement of zonal electric fields makes this component very easy to detect. We find that for fluctuation periods less than 5 hours the magnetospheric electric field source dominates atmospheric sources for Kp ≥ 3 at Jicamarca. In one remarkable event, six cycles of a 1‐ cycle/hour signal were recorded with nearly identical spectral features in the interplanetary medium, at auroral zone latitudes, and on both sides of the earth at equatorial ionospheric heights. The ratio of the relative magnitude of this oscillation at L = 5.5 to its value at L = 1.04 is in good agreement with the value of the average transfer function at 1 cycle/hour found in the 24‐hour runs.
Adiabatic cooling of the artificial porcupine plasma jet
Authors: Ruzhin Yu. Ya. , Treumann R. A. , Bauer O. H. , Moskalenko A. M.
Published: February 2008

Measurements of the plasma density obtained during the interaction of the artificial plasma jet, fired into the ionosphere, with the body of the Porcupine main payload have been analyzed for times when there was a well‐developed wake effect. Using wake theory, the maximum temperature of the quasi‐neutral xenon ion beam has been determined for an intermediate distance from the ion beam source when the beam has left the diamagnetic region but is still much denser than the ionospheric background plasma. The beam temperature is found about 4 times less than the temperature at injection. This observation is very well explained by adiabatic cooling of the beam during its initial diamagnetic and current‐buildup phases at distances r < 10 m. Outside this region the beam conserves the temperature achieved. The observation proves that the artificial plasma jet passes through an initial gas‐like diamagnetic phase restricted to the vicinity of the beam source where it expands adiabatically. Partial cooling also takes place outside the diamagnetic region where the beam current still builds up. The observations also support a recently developed current‐closure model of the quasi‐neutral ion beam.
Evolution of interstellar pickup ions in the solar wind
Authors: Isenberg Philip A.
Published: February 2008

A model is constructed for the evolution of an interstellar pickup ion distribution in the solar wind. The model assumes that the ions are immediately isotropized at ionization and follows the subsequent development of the distribution function as the particles are convected with the solar wind. The effects of energy diffusion in an ambient wave field with a power law spectrum, adiabatic deceleration in the expanding solar wind, and continual addition of newly ionized particles are all included in the model. An analytical expression describing the evolution of the distribution function in phase space velocity and heliocentric radius is obtained. The distribution quickly approaches an asymptotic shape in phase space which depends on the relative efficiency of the energy diffusion process compared to that of adiabatic deceleration. At large distances from the sun the density of pickup ions falls as r−1 in this model. The distribution function at large distances and for large particle speed υ has the form F ∼ υ−3/2 exp (−αυ³−γ), where γ is the index of the power law wave spectrum and α is a constant related to the energy diffusion coefficient. The asymptotic shape should describe the distribution of pickup ions in the outer heliosphere and could be used as an input distribution for a model of the anomalous component of cosmic rays. Comparison of this work with the recent observation of He+ at 1 AU implies that the energy diffusion process is very weak inside 1 AU.
Shock drift acceleration at low energies
Authors: Burgess David
Published: February 2008

Until now, studies of shock drift acceleration (SDA) at fast mode collisionless shocks have been confined to particles with initial energies greater than 10 keV. We present for the first time the results of test particle calculations of the interaction of low‐energy (1–8 keV) protons with both strong and weak oblique shocks. Since it is expected that the details of the shock structure will be important, we use a one‐dimensional hybrid, fully self‐consistent simulation to provide a realistic, time‐varying model of the shock fields. We follow the evolution, in these fields, of an isotropic, monoenergetic population of particles released just upstream of the shock. Our results indicate that SDA is a viable mechanism at oblique shocks for initial energies down to those normally classified as superthermal. We find that there is a minimum initial energy below which particles are not reflected; this threshold energy increases with both θBn (the shock normal angle) and MA (the Alfvén Mach number), and therefore implies that superthermal ions are more readily reflected as θBn and/or MA decreases. We suggest that the reflection of superthermal to very mildly energetic (around 1 keV) ions at the earth's bow shock could provide an explanation for the most energetic field‐aligned beams observed in the earth's foreshock. We have compared our results with those from a simplified model of a finite width shock transition, and we find that, statistically, it provides remarkably good agreement with the hybrid simulation results. The greatest differences between the two models appear for transmitted ions whose initial energy is low (1 keV).
Simulation of Kelvin‐Helmholtz instability at the magnetospheric boundary
Authors: Miura Akira
Published: February 2008

A two‐dimensional magnetohydrodynamic simulation of Kelvin‐Helmholtz instability at the terrestrial magnetospheric boundary is performed by including gradients of plasma and magnetic field normal to the dayside low‐latitude magnetospheric boundary. A magnetopause current layer is corrugated highly nonlinearly by the instability, and a plasma blob is formed by an interchange motion associated with the instability. The magnetosheath plasma flow momentum is diffused into the magnetosphere by the anomalous tangential (Reynolds plus Maxwell) stresses associated with the instability, and a wide velocity boundary layer is formed just inside the magnetopause current layer, while the thickness of the magnetopause current layer remains almost constant during the evolution of the instability. The convection voltage drop (integral of the convection electric field) across the velocity shear layer is amplified several times by the anomalous momentum transport associated with the instability. The anomalous momentum flux into the magnetosphere (tangential stress) reaches 0.6 to a few percent of the magnetosheath flow momentum flux, and this anomalous momentum flux into the magnetosphere is sufficient for accounting for the observed tailward momentum flux in the low‐latitude boundary layer. The value of the anomalous viscosity νano depends importantly on the magnetosheath Alfvén Mach number MA, which is defined by a magnetosheath magnetic field component parallel to the magnetosheath flow velocity; for MA = 2.5,νano is equal to ∼ 0.014 × 2aV0, where 2a is the thickness of the initial velocity shear layer and V0 is the total jump of the flow velocity across the magnetospheric boundary, and it increases with MA, and for MA > 5.0 it is equal to ∼0.2 × 2aV0. For a reasonable set of parameters at the dayside magnetospheric boundary the anomalous viscosity obtained is just the right magnitude for driving a magnetospheric convection in the terrestrial magnetosphere.
The ionospheric signature of flux transfer events
Authors: Southwood D. J.
Published: February 2008

We examine the motion of an isolated flux tube connecting the interplanetary and terrestrial magnetic field that has been created by reconnection at the dayside magnetopause. Such tubes should create a distinct localized flow pattern at their feet in the ionosphere. Observational effects are discussed. It is argued that the delay time of the order of a minute or more corresponding to the time for propagation of information from the reconnection site to the ionosphere is important and could control the creation of flux transfer events. It is predicted that magnetic pulsations in the Pc 5 frequency band are likely to be set up on closed flux tubes immediately neighboring the newly connected tubes.
Low‐energy plasma observations in the magnetosphere of Uranus
Authors: McNutt Ralph L. , Selesnick Richard S. , Richardson John D.
Published: February 2008

The plasma science experiment on Voyager 2 detected a large magnetosphere at Uranus containing a low‐density plasma. All available evidence suggests the plasma is comprised principally of protons and electrons. The protons are subsonic and their motion is consistent with corotation with the planet. In addition, on a longer time scale, they are apparently transported from the nightside of the planet to the dayside by a convective electric field generated by the solar wind. This convective motion is not directly observable in the plasma data; however, there are conspicuous day/night asymmetries in the particle densities which are probably produced by the convection pattern. Features in the spatial distribution are probably explained by a convection boundary in the vicinity of L = 5 produced by this convective electric field. Inside of the boundary plasma moving sunward is excluded. Scaling from models which predict the location of such a boundary in the earth's magnetosphere, we obtain a value of 0.4 mho for the height‐integrated, ionospheric conductivity, consistent with pre‐encounter modeling of the Uranian ionosphere. The time for particles to convect through the Uranian magnetosphere is estimated to be about 1 week. Peak ion densities reach a few protons cm−3. The proton distribution functions are characterized by a “warm” (i.e., subsonic) core and a non‐Maxwellian tail which varies significantly along the spacecraft trajectory. The core temperature is ∼5 to 10 eV. The non‐Maxwellian tail carries most of the plasma energy density in the observed energy range (<6 keV), which is small compared to the energy density of the planetary magnetic field. The spacecraft potential was negative with respect to the ambient plasma for 2 hours while Voyager was outbound from the planet. Analysis of the data gathered in this charging region shows that the predominant proton population is subsonic, and there is no significant “hidden” population of cold, supersonic ions in the inner magnetosphere of Uranus. The core population is probably produced by a local source of protons, while the nonthermal component may be produced by injection of protons from the magnetotail or the auroral region of the ionosphere.
Plasma flow in the Mars magnetosphere
Authors: Pérez‐de‐Tejada H.
Published: February 2008

A study of the velocity, temperature and density of the shocked solar wind in the vicinity of the Mars magnetospheric tail is presented. It is argued that the deficiency of momentum flux seen in that region implies the existence of an antisolar magnetospheric flow with velocities much larger than the planet's escape velocity. It is further suggested that the planetary particle fluxes may extend across most of the cross‐sectional area of the magnetosphere near the terminator. The implications of the magnetospheric flow on the evolution of the Mars atmosphere are discussed. It is concluded that a dense Martian atmosphere could have been eroded by the solar wind in a time span of the order of 109 years.
A numerical study of suprathermal ionospheric ion trajectories in three‐dimensional electric and magnetic field models
Authors: Sauvaud J. A. , Delcourt D.
Published: February 2008

A numerical study of the magnetospheric trajectories of ions upflowing from the ionosphere has been performed using three‐dimensional magnetic and electric field models. Ion trajectories have been divided into three main types corresponding to (1) gravitational ions trapped between the magnetic and gravitational mirror points and drifting nearly along the local electric equipotential, (2) ions just escaping from the gravitational field and then convected to the magnetopause due to the loss of the main part of their kinetic energy, (3) trapped and pseudo‐trapped ions whose trajectories are mainly determined by the magnetic field topology (beams and conics) because of their large kinetic energy. Particles from conic distributions injected at inadequate local times and/or altitudes to gain enough energy to precipitate reach the dayside magnetopause after several bounce periods. An application of these numerical studies to ion beams is presented by Bosqued et al. (1986).
Heating of ionospheric O+ ions by shear Alfvén waves
Authors: Winglee R. M. , Ashour‐Abdalla M. , Sydora R. D.
Published: February 2008

Ionospheric ions, in particular O+ ions, which have been transversely heated, are often observed flowing upward along auroral field lines. A new mechanism, heating by current‐driven shear (or kinetic) Alfvén waves (SAW), is proposed. An electron current drives oblique SAWs unstable near ω ≃ Ω0, and these waves are in turn gyroresonantly absorbed by the ions. The mechanism is similar to ion heating by current‐driven electrostatic ion cyclotron waves (EICW). However, the SAW differs from the EICW in that as the perpendicular temperature of the ions increases, growth of the SAW can still occur, whereas growth of the EICW becomes suppressed. As a consequence the SAW is able to provide sustained perpendicular heating of ions with smaller currents being required for the heating than for heating via EICWs.
Iterative solution of the multistream electron transport equation: 1. Comparison with laboratory beam injection experiments
Authors: Porter H. S. , Varosi F. , Mayr H. G.
Published: February 2008

We describe a Neumann iteration method for solution of the electron transport equation with the independent variables of energy, pitch angle, and altitude. The method has been developed to study electron energy deposition in the high‐latitude thermosphere and mesosphere. It is fast and can compute numerical point response function solutions of the electron transport equation. Inelastic cross sections input to the model are empirically based but are constrained by theoretical consistency conditions. Angular elastic cross sections are also empirically based. Thus, the transport solutions obtained represent a test of compatibility between various sets of cross sections and energy deposition measurements. Energy coupling of the electron transport equation is treated using an efficient, discrete energy loss method that allows the use of a widely separated energy grid at high energy. However, there are limitations on the bin width that are due to physical considerations. This method allows separation of the electron transport equation into a system of single‐energy equations coupled in energy. The angular and spatial parts of each single‐energy equation can be solved by Neumann iteration. We employ an angular grid rather than an expansion of the angular dependence in terms of spherical harmonics. To obtain accurate phase function integrals at a low computational cost, we have developed a numerical quadrature based on the use of analytic phase function forms. We have also developed a quadrature for the spatial integration based upon the known analytic behavior expected from the solution of the transport equation. As an application of the model, we make the first published comparison between a direct solution—as opposed to a Monte Carlo method—using discrete energy loss from detailed atomic cross sections and monoenergetic, monodirectional electron beam injection experiments. Results obtained from the model for the practical range are within 10% of the observed range for electrons in N2 over the energies studied from 300 eV to 12 keV. The scaled energy deposition profiles computed by the model tend toward shape invariance at higher energies when plotted as a function of the fraction of the practical range as also seen in empirical studies. The computed shape is generally in good agreement with empirical determinations, being best at the highest incident energies.
Global convection electric field and current: Comparisons between model's predictions and data from STARE, Saint‐Santin, and magnetometers
Authors: Mazaudier C. , Senior C. , Nielsen E.
Published: February 2008

Ionospheric electric field and ground magnetic field measurements at high, middle and low latitudes are used to study the global pattern of the convection electric field on March 26, 1979. The IMF Bz component was southward and relatively steady over a prolonged period of time. The data set is thus representative of a steady magnetospheric convection. A semi‐analytical time‐dependent convection model is used to interpret the observations. During the daytime the model steady state results roughly reproduces the mean observed convection pattern. Fluctuations of the observed electric field from the model predictions are interpreted as resulting from (1) the transient responses of the ionosphere to small time variations of the IMF, (2) substorm effects, (3) neutral wind disturbances. Only the first type of fluctuations can be reproduced by the initial state prediction of the convection model.