bluuebunny/crossref_metadata_2025 huggingface dataset:
Ion‐neutral momentum coupling near discrete high‐latitude ionospheric features
Authors: St.‐Maurice J.‐P. , Schunk R. W.
Published: February 2008

We have developed a two‐dimensional numerical model to study the momentum coupling between the ionosphere and neutral atmosphere in the vicinity of discrete high‐latitude features, such as convection channels and plasma density troughs. The model, which is based on generalized magnetohydrodynamic equations, takes account of global pressure gradients, viscous dissipation, ion drag, the Coriolis force, and electrodynamic drifts. From our initial steady state investigation, we have found the following: (1) in convection channels, significant shears and rotations of the thermospheric flow can occur below 200 km if a minimum in the electron density profile is present between the E and F regions; (2) in convection channels, the thermospheric wind decreases with height in the F region owing to the effects of horizontal viscosity; (3) at low altitudes, the boundaries of convection channels may produce Ekman spirals; (4) a convection channel acts to induce a thermospheric motion over a region that extends up to 1000 km on both sides of the convection channel owing to the effects of horizontal viscosity; (5) in general, thermospheric winds driven by large‐scale pressure gradients have an appreciable effect on the thermospheric flow in and near convection channels; (6) narrow channels of rapidly convecting, low density plasma produce a small thermospheric response; and (7) Joule heating is generally the dominant heat source inside convection channels, while viscous heating dominates outside. Frequently, however, viscous heating can be the dominant heat source both inside and outside of convection channels.
On rotational forces in the solar wind
Authors: Hollweg Joseph V. , Isenberg Philip A.
Published: February 2008

We present a new and simpler derivation of the rotational forces on minor ions in the solar wind. We show that the rotational forces can be interpreted as potential forces affecting all particles equally. As such, they do not involve interactions between different particle species and do not represent an equilibrating process.
The structure of Titan's wake from plasma wave observations
Authors: Gurnett D. A. , Scarf F. L. , Kurth W. S.
Published: February 2008

During the Voyager 1 flyby of Saturn's moon Titan, the plasma wave instrument detected several types of plasma wave emissions. On the inbound leg a broad region of intense low‐frequency noise was detected on the side of Titan facing away from Saturn. This noise has characteristics similar to the electric field turbulence observed in the magnetosheath at the earth and the ionosheath at Venus and is believed to be generated by newly created ions that are being accelerated in the vicinity of Titan by the corotational electric field. During the pass through the induced magnetic tail of Titan, a series of upper hybrid resonance emissions were observed. The electron density profile inferred from these emissions shows three distinct peaks with densities of ∼40 cm−3, the first peak corresponding to the entry into the magnetic tail, the second peak corresponding to the neutral sheet crossing from the northern to the southern tail lobe, and the third, somewhat smaller, peak corresponding to the outbound exit from the tail. Large depressions in the magnetic field strength are observed coincident with each of the density peaks. These effects indicate that a dense plume of plasma is being carried downstream of Titan by the interaction with the rapidly rotating magnetosphere of Saturn. By equating the magnetic field pressure in the tail lobe to the plasma pressure in the neutral sheet, the temperature of the plasma is estimated to be about 8600°K. This low temperature suggests that the plasma originates from the ionosphere of Titan, probably forming a plume of plasma with a θ or H cross section extending downstream from Titan. Within the tail lobes, a second type of low‐frequency electric field noise was observed, with characteristics very similar to a type of noise called broadband electrostatic noise, which is found in the earth's magnetic tail. As in the case of the earth, this noise is most intense near the outer boundary of the plasma sheet and is almost completely absent in the high‐density region near the neutral sheet.
Airglow measurement looking downward from orbit at selected darker fields of view
Authors: Croft Thomas A.
Published: February 2008

When airglow measurements are obtained from an orbiting satellite, instruments are typically directed either horizontally or upward in order that the glow may be seen against a nonreflecting background of low brightness. At many wavelengths, the brightest airglow is well below those altitudes at which orbit can be sustained, and so it would be desirable to look downward in many circumstances. The practicability of performing useful observations of airglow near nadir is herein examined by study of digital records from the DMSP visual scanner and comparable records from the VAE instrument on AE‐C. When looking downward the background may consist of erratically variable light because surface lights are often bright and the surface, reflecting moonlight and the glow itself, has variable albedo in some regions. The results indicate that on moonless nights, it may be feasible to devise machine algorithms that will isolate portions of the radiance measured near nadir from which the airglow intensity can be determined. These algorithms must discriminate in favor of the darker regions within the field of view and subsequently test for the uniformity of illumination that is expected of airglow from a geographically limited area. Some useful results may even be derived under weak moonlight. This is a provisional estimation because the data available for examination, while they were the best available, were lacking in several important respects.
The application of Dopplionograms to an understanding of sporadic E
Authors: Wright J. W.
Published: February 2008

The dopplionogram can be used to infer the vertical motion of plasma contours under some circumstances. In a sporadic E sequence, motions downward from above, and upward from below, are observed as the layer peak plasma density increases; this is interpreted as consistent with plasma convergence resulting from an east‐west wind shear. When divergent motions occur, the layer is observed to dissipate and recede rapidly.
Magnetopause modeling: Flux transfer events and magnetosheath quasi‐trapped distributions
Authors: Speiser T. W. , Williams D. J.
Published: February 2008

From ISEE 1 magnetopause crossings on November 10, 1977, three‐dimensional distribution functions for energetic ions (24–120 keV) are studied in the magnetosphere, through the magnetopause, and in the magnetosheath. The particle distributions are particularly examined at and near the times that Russell and Elphic (1978) identified as flux transfer events (FTE). Using a simple, one‐dimensional, quasi‐static model, particle orbits are followed numerically, from the magnetosphere into the sheath. The inner, trapped, distribution initializes the distribution function. Liouville's theorem allows the inner distribution to be mapped into the sheath following the orbits. It is found that the modeled distribution function agrees quite well with that of the observed FTE's for an inward‐pointing, normal magnetic field component and the magnitude of any reconnection‐like tangential electric field must be less than about ½ mV/m. A tangential field up to about this limit may supplement gradient and curvature drifting, repopulating freshly ‘opened’ flux tubes. Electric fields associated with tangentially convecting sheath plasma are mapped along a connected flux tube, but do little to change the distribution function of the energetic ions. A quasi‐trapped population in the sheath usually seems to ‘sandwich’ the FTE distributions. These quasi‐trapped distributions are probably due to slow, large pitch angle, outward moving particles, left behind by the outward rush of the ions more field aligned at the time the flux tube was ‘opened.’ If this interpretation is correct, the spatial extent of ‘open’ field lines near the boundary is broader, not quite as localized as previously thought, at least for this particular data. The higher energy, outflowing particles with small to large pitch angles penetrate the magnetopause several thousand kilometers from the low energy particles. This result of the model, combined with 2 of the FTE observations, gives qualitative support to the suggestion that localized tangential electric fields above our upper limit may exist for this time period.
Radial variation of the solar wind speed between 1 and 15 AU
Authors: Collard Harold R. , Mihalov John D. , Wolfe John H.
Published: February 2008

Pioneer 10 and 11 solar wind speeds measured between 1.4 and 15.2 AU are compared with IMP 6, 7, and 8 solar wind speeds measured at 1 AU for six radial alignment periods between 1973 and 1978. The speed‐time profiles measured beyond 1 AU show a decreasing correlation with the 1‐AU profiles as the heliocentric distance increases. At 15‐AU distance the speed‐time profile is modified to such an extent that it is no longer correlated to any significant degree with the 1‐AU speed‐time profile. The linear correlation coefficient decreases from 0.83 at 1.8 AU to 0.06 at 14.8 AU. However, a stream structure is still evident at a distance as far out as 15 AU. Among various mechanisms which can modify the speed‐time profile are evolution of the streams during outward expansion and interactions between adjacent streams. The mean solar wind speed appears to vary little with heliocentric distance. The change in the mean solar wind speed between 1 and 15 AU is probably no greater than ±25 km/s. Both temporal changes and speed variations with heliographic latitude are too large to allow a more accurate determination. Speed variations become smaller with increasing radial distance from the sun. Both the highest and the lowest speeds disappear as the radial distance increases. An exponential fit to the ratios of the standard deviation of speeds at 1 AU to the standard deviation of speeds measured farther out at Pioneer 10 or 11 from all the alignments gives an e‐folding distance of 11±3 AU. If we assume a standard deviation at 1 AU of 100 km/s (the average measured standard deviation at 1 AU), this fit predicts a standard deviation of 5 km/s in the solar wind speeds at 33 AU. Therefore at distances from the sun greater than 30–40 AU the solar wind behavior may closely resemble that of a radially expanding constant speed plasma.
Improved Venus ionopause altitude calculation and comparison with measurement
Authors: Knudsen William C. , Miller Kent L. , Spenner Karl
Published: February 2008

We calculate the altitude of the Venus ionopause following the inviscid fluid approach initially introduced by Spreiter et al. (1970) and later modified by Spreiter and Stahara (1980) but incorporate several improved approximations. The calculated altitude is compared with median altitudes measured by the Pioneer Venus retarding potential analyzer. The calculated ionopause shape approximates the measured median shape closely in the solar zenith angle (SZA) range 0°–135° and is a definite improvement over previous calculations. Improved approximations include use of an ionospheric pressure field which varies with SZA in a manner consistent with measurements, use of the modified Newtonian approximation for the pressure exerted on the ionosphere by the ionosheath, use of the Prandtl‐Meyer expansion approximation for a short distance downstream from the terminator, and separation of the solar wind terminator from the solar extreme ultraviolet radiation terminator. The calculation is not expected to be accurate for solar zenith angles in excess of approximately 135°. Use of the improved approximations lowers the terminator ionopause altitude to approximately half that obtained with use of the usual Spreiter approximations. The calculated dawn ionopause is approximately 300 km higher in altitude than the dusk ionopause. Both ionopauses are close to their respective measured median values. The value of the upstream solar wind momentum flux required to fit the measured median ionopause altitudes is within 15% of the measured median solar wind momentum flux. We conclude that the median altitude of the ionopause between 0° and approximately 135° SZA may be adequately calculated without including a viscous interaction in the theory.
Growth rate calculations of auroral kilometric radiation using the relativistic resonance condition
Authors: Omidi N. , Gurnett D. A.
Published: February 2008

The relativistic cyclotron resonance condition for right‐handed extraordinary mode waves defines an ellipse in velocity space. The position of the center and size of the semiminor axis of this ellipse are functions of the plasma frequency, gyrofrequency, wave frequency, and wave normal angle. The effect of varying these parameters on the position and size of the resonance contour is analyzed. The results show that as the wave normal angle decreases, the semiminor axis increases in size and as the plasma frequency to gyrofrequency ratio decreases, the minimum energy for resonating electrons decreases and the maximum wave normal angle allowed by the resonance condition increases. Also, as the wave frequency to gyrofrequency ratio increases, the center of the resonance ellipse moves away from the origin. The relativistic resonance condition and the electron distribution in velocity space obtained by the S3‐3 satellite are used to calculate numerically growth rates for the terrestrial auroral kilometric radiation. It is shown that the loss cone region of the electron distribution can give rise to growth rates for the extraordinary mode that are sufficiently large to account for the observed radio emission intensities.
Determination of photoionization branching ratios and total photoionization cross sections at 304 Å from experimental ionospheric photoelectron fluxes
Authors: Richards P. G. , Torr D. G. , Espy P. J.
Published: February 2008

High resolution measurements of the ionospheric photoelectron spectrum have been used to evaluate (1) the ratio of the molecular nitrogen density to the atomic oxygen density, (2) the sum of the total photoionization cross sections for N2 and O at 304 Å, (3) the ²P/4S and ²D/4S branching ratios for photoionization of atomic oxygen by 304 Å solar radiation, (4) the B/X and A/X branching ratios for the photoionization of molecular nitrogen by 304 Å solar radiation, (5) the ratios ²P/B, ²D/A, and 4S/X of the partial photoionization cross sections of O to the partial photoionization cross sections of N2, and (6) the ratio of the N2 total photoionization cross section to that of O at 304 Å.
Radial evolution of power spectra of interplanetary Alfvénic turbulence
Authors: Bavassano B. , Dobrowolny M. , Mariani F. , Ness N. F.
Published: February 2008

The radial evolution of the power spectra of the MHD turbulence within the trailing edge of high‐speed streams in the solar wind has been investigated with the magnetic field data of HELIOS 1 and 2 for heliocentric distances between 0.3 and 0.9 AU. In the analyzed frequency range 2.8 · 10−4 ‐ 8.3 · 10−2 Hz the computed spectra have, near the earth, values of the spectral index close to that predicted for an incompressible hydromagnetic turbulence in a stationary state. Approaching the sun, the spectral slope remains unchanged for frequencies f ≳ 10−2 Hz, whereas at lower frequencies we find a clear evolution toward a less steep falloff with frequency. The radial gradient of the power in Alfvénic fluctuations depends on frequency and it increases upon increasing frequency. For frequencies f ≳ 10−2 Hz, however, the radial gradient remains approximately the same. A discussion of possible theoretical implications of the observational features pointed out is given.
A study of whistlers correlated with bursts of electron precipitation near L = 2
Authors: Carpenter D. L. , LaBelle J. W.
Published: February 2008

The previously reported Trimpi effect involves observation of amplitude perturbations in subionospherically propagating fixed‐frequency VLF signals. The perturbations are coincident in time with magnetospherically propagating whistlers, and it has been inferred that the whistlers drive energetic electron precipitation of E > 100 keV in a manner such that a localized increase in ionization occurs near 80‐km altitude in the nighttime earth‐ionosphere waveguide. In the present work, VLF broadband, analog chart and DF data from Palmer Station, Antarctica (L ∼ 2.3), recorded in 1978 and 1979 are used to investigate additional features of the Trimpi process. Attention was concentrated on perturbations of the signals NSS (21.4 kHz) and NLK (18.6 kHz) that have arrival bearings at Palmer of 352° and 318°, respectively. In September–October 1978, amplitude perturbations on NSS and NLK at Palmer were as frequent as similar events previously reported from a station at L = 4. A large equinoctial peak in activity was found, as in the previous study. The perturbations on NSS and NLK were usually increases in the 10%–30% range; decreases on NSS were observed on one unusually disturbed day when the plasmapause was equatorward of Palmer. Case studies were performed on five days of high activity. Essentially, all of the 93 events involved exhibited a close time correlation between amplitude perturbation and received whistler. In some cases only a fraction of the observed whistlers appeared to correlate with fixed‐frequency signal changes. In one such case, correlated whistlers were found through DF analysis and frequency‐time pattern comparisons to contain a component not present in most other whistlers. Above a field strength of 13 µV/m, the amplitude of this component was found to be linearly related to NSS perturbation amplitude, while for cases of less than 13 µV/m, no NSS change was detected. It is concluded that the southern hemisphere region within which perturbations occurred for the five case studies extends 200 km east and west of Palmer and ∼400 km northward over the L range 2.4 to 2.1 (at 100‐km altitude). Within this area it is estimated that individual ionospheric perturbations were of order 100 km in the east‐west direction and that the perturbed regions were centered within ∼200 km of the affected great circle paths.
A model of energy deposition of energetic electrons and EUV emission in the Jovian and Saturnian atmospheres and implications
Authors: Gérard J.‐Cl. , Singh V.
Published: February 2008

A model of the interaction between incident electron precipitation and H2 atmospheres is described. The local degraded primary and secondary electron energy distributions are calculated by using the continuous slowing down approximation. The altitude distribution of the ionization rate and various H and EUV H2 emissions are calculated for four different incident electron spectra. A total EUV H2 emission efficiency of 10.6 kR/incident erg cm−2 s−1 is obtained for a pure H2 atmosphere. Comparison with the Voyager Jupiter observations indicates that an incident energy flux of about 8 ergs cm−2 s−1 was present at the time of the encounter if the emission is located in an H2‐dominated region. The local thermospheric heating rate was about 4 ergs cm−2 s−1 for Jupiter and of the order of 0.1 erg cm−2 s−1 for Saturn. A globally averaged atomic hydrogen production rate of ∼1×1010 atoms/cm² s−1 is induced by the Jovian auroral electron precipitation, largely exceeding the solar EUV dissociation rate.
Pc 1 emissions in the afternoon sector prior to the July 29, 1977 sudden commencement
Authors: Olson John V.
Published: February 2008

A series of three structured Pc 1 events were observed prior to the July 29, 1977, sudden commencement by magnetometer stations at College, Alaska, MacQuarie Island, Australia, and Vostok, Antarctica. Signal characteristics of the events lead us to infer that the near‐conjugate College and MacQuarie Island stations were near the source field lines for the second of these events. From the growth rates for ion cyclotron waves calculated from linearized equations for an anisotropic, drifting plasma we have inferred the presence of freshly injected plasma in the afternoon magnetosphere after 2330 UT at L = 5.4. This is consistent with the increased convection to be expected with the observed southward turning of the interplanetary magnetic field prior to the sudden commencement.
Field line topology determined by energetic particles during a possible magnetopause reconnection event
Authors: Scholer M. , Daly P. W. , Paschmann G. , Fritz T. A.
Published: February 2008

The September 8, 1978, crossing of the magnetopause by ISEE 1 and 2 has been studied in the light of energetic ion and electron data in order to derive the magnetic field topology. The plasma and magnetic field observations during this crossing provided evidence for the occurrence of plasma acceleration intrinsic to reconnection between interplanetary and magnetospheric magnetic field lines. The energetic particle observations result in two different pictures of the open and closed field lines within the magnetosphere, depending on either the ion or electron observations. According to the ion measurements, the high‐speed flow can be placed exclusively on open field lines. The electrons, however, exhibit trapped distributions during these times. This apparent dilemma can be resolved by invoking magnetic mirroring of the electrons off the increased magnetosheath field, as shown in a companion paper by Daly and Fritz (1982).
Trapped electron distributions on open magnetic field lines
Authors: Daly P. W. , Fritz T. A.
Published: February 2008

In a previous paper on the September 8, 1978, magnetopause crossing by the ISEE spacecraft, observations of trapped‐like electron distributions coexisting with streaming ion distributions were reported. These observations were made for particles in the energy range >20 keV, inside the magnetopause, and near a possible reconnection event. In this paper we have investigated the idea that the electrons could be trapped by magnetic mirroring from the field in the magnetosheath. The loss cone angle predicted from the instantaneously measured magnetic field and the observed maximum field in the magnetosheath compares very favorably with the observed loss cone angles. The measured loss cone is always slightly wider than the predicted one, which can result from superimposing the magnetospheric source distribution onto the loss cone, but follows the magnetic field strength variations as the predicted value does. We argue that the ions are not similarly reflected because they fail to meet the adiabatic condition during the passage through the magnetopause. Having thus resolved the conflicting predictions about field line topology based on electrons and ions, we can be much more confident that the September 8, 1978, crossing of the magnetopause by ISEE is indeed a reconnection event.
A large‐amplitude traveling ionospheric disturbance produced by the May 18, 1980, explosion of Mount St. Helens
Authors: Roberts David H. , Klobuchar John A. , Fougere Paul F. , Hendrickson David H.
Published: February 2008

A remarkable long‐lived, large‐scale traveling ionospheric disturbance (TID), excited by the May 18, 1980, explosion of Mount St. Helens, has been detected in total electron content monitor data. Oscillatory perturbations in the electron column density of the ionosphere with amplitudes about 10% of the nominal daytime content were detected at three stations whose ionospheric penetration points lie between 1610 and 1890 km from Mount St. Helens. Smaller perturbations were detected at five of six additional stations between 3760 and 4950 km away. The period of the TID increased linearly with great‐circle distance from Mount St. Helens, ranging from ≈37 min at the nearest station to ≈116 min at the most distant one. The TID persisted for at least four cycles at the three close stations and three cycles at the more distant stations and was qualitatively similar to TIDs produced by the low‐altitude thermonuclear detonations of the 1960's. The disturbance front of this TID accelerated from an average velocity of ≈350 m/s between Mt. St. Helens and the close stations to an average velocity of ≈550 m/s to the more distant ones. In analogy with the bomb‐excited disturbances, this TID is interpreted as the ionospheric fluctuations induced by a gravity wave, excited by the explosion of Mount St. Helens, propagating in the neutral atmosphere. A model based on the free wave response of an isothermal atmosphere to a point disturbance provides a good fit to the data at the three closest stations, but no such model can account for all of the data. Modeling of the long‐distance behavior of the Mount St. Helens TID in terms of upper‐atmosphere guided gravity waves is complicated by the requirement of exciting them by a ground‐level explosion. A comprehensive explanation would probably include the propagation of energy to ionospheric heights in free modes and the excitation thereby of guided modes. There was no evidence for a strong supersonic shock wave in the ionosphere. As a result, the Mount St. Helens disturbance may prove to be a cleaner test of detailed theories of the point excitation and propagation of gravity waves in a realistic atmosphere than were TIDs excited by thermonuclear weapons.
Spectroscopy of the O I 989‐ and 7990‐Å multiplets in the dayglow and aurora
Authors: Meier R. R.
Published: February 2008

The Spectroscopy of the 2p4 ³P‐2p³3s′ ³D° (989 Å) transition of atomic oxygen and the intermediate branch transition 2p³3p ³P‐2p³3s′ ³D° (7990 Å) has been modelled for an optically thick thermosphere using Monte Carlo simulation. The model reproduces rocket observations of 989 Å in the dayglow and aurora if the e + O electron impact excitation cross section is 8.4×10−18 cm² at 25 eV. The individual 989‐Å multiplet strengths agree with dayglow observations which partially resolve the line structure. The 7990‐Å singlet to doublet ratio observed in auroras is also consistent with the model. No evidence is found for selective absorption of individual triplet and doublet lines of the 989‐Å transition through accidental resonances with rotational lines of N2. This is compatible with the latest wavelength data which indicate that the O I and N2 lines are off resonance by two Doppler widths. The 7990/989 intensity ratio in an aurora is sensitive to the abundance of atomic oxygen and the height distribution of the initial excitation rate and, to a lesser extent, to absorption by O2. Ratios of the 7990‐Å multiplets primarily depend on the O abundance with second‐order dependence on O2 absorption and the characteristic energy of the aurora. Accurate ground‐based observations of the 7990‐Å multiplet ratios may yield the atomic oxygen abundance in auroras to within a factor of 2.
Heating at 430 MHz in the lower ionosphere at Arecibo
Authors: Coco D. , Ganguly S.
Published: February 2008

The 430‐MHz radar at Arecibo has been used both to produce and to measure electron temperature increases in the lower ionosphere. The 430‐MHz transmitter produced heating pulses from 0.2 to 6.0 ms in length having a maximum power flux of 12 W/m² at 100 km. For the first time, both the spectrum and the total power of the incoherent scatter signals were used to measure the electron temperature increases in the 65 to 110‐km altitude region. When the measured electron temperature increases are compared to the results of an ohmic heating model, measured increases are found to be significantly smaller than the predicted increases. Possible explanations for this lack of agreement, such as a non‐Maxwellian electron temperature distribution or increased electron cooling rates, are discussed.
Anomalous auroral electron distributions due to an artificial ion beam in the ionosphere
Authors: Moore T. E. , Arnoldy R. L. , Kaufmann R. L. , Cahill L. J. , Kintner P. M. , Walker D. N.
Published: February 2008

We report the results of an active experiment in which the auroral ionosphere was perturbed by the operation of an ion gun, which injected approximately 100 mA of 25 eV Ar+ ions at upgoing pitch angles over a discrete auroral arc. The gun was operated during five periods on the downleg of the flight as the payload descended from 220 km to 120 km altitude along the local magnetic field lines. A brief (< 100 ms) negative payload potential excursion was observed at the onset of each gun operation, followed by quasi‐steady conditions for the remainder of each period. The major steady effects were (1) the excitation of intense broad band electric field fluctuations, particularly at low frequencies 0‐10 kHz, and (2) the appearance of streaming and isotropic heating in different parts of superthermal electron velocity space. We have explored a scenario that appeals to such a wave spectrum as an obstacle to auroral parallel current and predicts this type of electron behavior. The waves are viewed as traps for the thermal electrons and scatterers of the faster electrons. In this scenario, electron runaway or streaming is expected between the trapping speed and the critical velocity for cyclotron interactions with the waves. These streaming electrons carry the current that would be carried by thermals or energetic electrons in the absence of the waves. Isotropic heating is expected near the critical speed where Doppler resonance occurs and is found near 12000 km/s (∼50 υte, where υte is the electron thermal speed). It may be inferred that waves are excited primarily at wavelengths of several meters and longer. A current of ∼1 µA m−2 is carried by the streaming electrons. Energetic considerations suggest that the heated electrons derive their energy from an external source rather than from the ion beam itself; the auroral generator appears to be more than adequate. The gun‐associated electrons were anomalous in the sense that their anisotropy was the opposite of that observed in the natural aurora, where electrons are isotropic at low energy and often field‐aligned at higher energy.
The plumes of Titan
Authors: Eviatar Aharon , Siscoe George L. , Scudder J. D. , Sittler E. C. , Sullivan James D.
Published: February 2008

We interpret the plasma density enhancements observed during the Voyager 1 Titan encounter in terms of a single plume of plasma wrapped around Saturn by corotation. We consider the radial motions of the plume driven by fluctuations in solar wind pressure and the dispersal of the plasma by the centrifugal interchange instability, by heating and by azimuthal acceleration. We show that Saturn cannot readily impose corotation directly on the plume and incidentally, that the total dissipation associated with the Titan‐magnetosphere interaction is insufficient to supply the power to produce the observed Titan ultraviolet airglow. A pickup velocity of 8 km/s is inferred from the observed velocity and a standing wave model. We note that the Voyager plasma observations are consistent with the predictions of the model.
Satellite Bremsstrahlung X ray measurements at the onset of a magnetospheric substorm
Authors: Imhof W. L. , Stadsnes J. , Reagan J. B. , Kilner J. R. , Gaines E. E. , Datlowe D. W. , Mobilia J. , Nakano G. H.
Published: February 2008

Bremsstrahlung X ray (>21 keV) mappings and direct electron (>68 keV) measurements from the low‐altitude polar‐orbiting satellite P78‐1 were performed at critical times and locations near the onset of a magnetospheric substorm on July 3, 1979 at ∼2149 UT. The bremsstrahlung X ray intensities emitted from the atmosphere over a wide range of longitudes and L shells were negligible just before the substorm onset, then rose to a maximum within about a minute or less, and remained at an enhanced level for the next 9 minutes. The onset time for the total intensities of X rays observed from the P78‐1 satellite was nearly the same as for the X ray flux increase at one of the SBARMO‐79 balloons which were at fortuitous positions during the event and within the field of view of the satellite X ray detectors. The simultaneous X ray measurements from the SBARMO‐79 balloons and the P78‐1 satellite indicate that the electron precipitation started at L = 5–5.5 without a significant precipitation from higher L shells. In addition, the satellite data revealed a relatively sharp longitude decrease in electron precipitation at positions east of ∼30°E or at magnetic local times after ∼1.5 hours.
Wave‐particle interactions near ΩHe+ observed on board GEOS 1 and 2: 2. Generation of ion cyclotron waves and heating of He+ ions
Authors: Roux A. , Perraut S. , Rauch J. L. , de Villedary C. , Kremser G. , Korth A. , Young D. T.
Published: February 2008

This work is a continuation of paper 1 (Young et al., 1981) and is devoted to the generation process of ion cyclotron waves (ICWs) and the acceleration of He+ ions up to suprathermal energies. Simultaneous measurements are used from the ion composition experiment (0 < E < 16 keV), the energetic particle experiment (24 < E < 3 300 keV), and the ULF wave experiment (0.2–10 Hz) on board the GEOS 1 and GEOS 2 spacecraft. General characteristics of the local time distribution of ICWs will be presented and compared with those of the thermal anisotropy of energetic protons and the He+ abundance. Further calculations of the convective growth rate are conducted by applying two different methods, both of which are based upon the measured proton fluxes. The generation conditions of the ICWs in the presence of He+ ions will be investigated and three possible explanations will be discussed: (1) enhanced convection growth rates, (2) lowering of the threshold for absolute instabilities, and (3) change of the ICWs ray path (laser‐like effect). Finally, it is shown that the flux of suprathermal He+ ions is modulated at the ICW frequency. Owing to nonlinear effects, part of the energy of the energetic protons is transfered via the ICWs to the He+ ions that are essentially accelerated in the direction perpendicular to the static magnetic field. Then in the otherwise collisionless plasma the friction between energetic anisotropic protons and thermal He+ ions is achieved through the ICWs.
Ionospheric conductivity dependence of electric fields and currents estimated from ground magnetic observations
Authors: Kamide Y. , Richmond A. D.
Published: February 2008

Recent efforts in estimating the global distribution of the electric potential as well as of ionospheric and field‐aligned currents on the basis of ground magnetic records are reevaluated. For this purpose, we have repeated crucial tests for the algorithms by using a data base from the IMS Alaska chain of magnetic observatories along with completely different ionospheric conductivity models. It is found that the ionospheric current patterns are only weakly dependent on the choice of the conductivity, while the calculated field‐aligned currents are somewhat more sensitive and the electric field is quite sensitive to the assumed conductivity, as expected. A test is also conducted by using the calculated field‐aligned currents as inputs to an inverse calculation to attempt to reproduce the original ground magnetic perturbations. The results were quite satisfactory, indicating that the numerical accuracy of the algorithms is adequate. These tests increase our confidence that ground magnetic records from a close network can be used to study the extent to which the magnetosphere and ionosphere are electrically coupled.
Source regions deduced from attenuation bands in VLF saucers
Authors: Horita R. E. , James H. G.
Published: February 2008

Fourteen VLF saucer events exhibiting attenuation bands related to the proton gyrofrequency were studied to check the usual assumption that saucers are due to upgoing whistler mode waves. It was found that most of the 14 saucers exhibited attenuation bands with proton gyrofrequencies smaller than the local proton gyrofrequency at the satellite. Assuming that the generation or interaction region with its characteristic proton gyrofrequency imprints its signature on the saucers originating at or propagating through that location, these observations suggest that the majority of the 14 saucer waves propagated down to the ISIS 2 satellite. The generation/interaction regions of these saucers exhibiting attenuation bands were also found to be located in the 2300–0100 local time sector between 60° and 70° geomagnetic latitude, with a tendency to be at lower latitudes for higher geomagnetic activity. Although the attenuation bands are usually horizontal, a new observation is the positively sloping attenuation bands, suggesting larger proton gyrofrequency values with time. When combined with already published data on particle precipitation, these observations imply that downgoing suprathermal electrons are also responsible for saucers.
Detailed correlations of magnetic field and riometer observations at L = 4.2 with pulsating aurora
Authors: Arnoldy R. L. , Dragoon K. , Cahill L. J. , Mende S. B. , Rosenberg T. J.
Published: February 2008

Pi 1 magnetic pulsations in the period range 5–15 s recorded on the ground at L = 4.2 during pulsating aurora are studied via the use of data collected at Siple Station, Antarctica, during 1979. Pulsating aurora exceeding 200 R of 4278‐Å emission occurred for approximately 5% of the 1400 hours of photometer operation and was always accompanied by ULF waves. A striking characteristic of the ULF waves is a strong westward component in dB/dt during the auroral light bursts. A detailed correlation of the ULF waves with riometer and flux gate magnetic field data reveal that the magnetic field variations were very closely correlated with D region cosmic noise absorption. Asymmetric pulsations result from the slow recovery of the magnetic field and occurred when the auroral light pulses were of short duration. We conclude from this study that the ground Pi 1 recorded during pulsating aurora at L = 4.2 were due to an overhead current system produced by the particle precipitation. The characteristic westward component in dB/dt could be interpreted as being due to an equatorially directed local ionospheric current driven by a polarization electric field. One must conclude from the results of this paper that the Pi 1 waves studied had no role in the mechanism that precipitated the particles responsible for the pulsating aurora at L = 4.2.
Ground magnetometer detection of a large‐M Pc 5 pulsation observed with the STARE radar
Authors: Allan W. , Poulter E. M. , Glassmeier K.‐H. , Nielson E.
Published: February 2008

Ground‐based magnetometer observations are used to confirm the properties of a large azimuthal wave number Pc 5 pulsation observed previously with the STARE radar. The magnetometer and STARE results allow an estimate to be made of the ionospheric height‐integrated Hall conductivity, as well as supporting theories that predict 90° rotation of polarization between ionosphere and ground. The properties of the observed vertical magnetic component at the ground are also found to be consistent with recent theory.
A new approach to pitch angle scattering in the magnetosphere
Authors: Retterer John M. , Jasperse J. R. , Chang T. S.
Published: February 2008

We develop a theory to describe the spatial variation of the particle pitch angle distribution function along a geomagnetic field line. Because spatial variation is most significant inside the loss cone, our theory is tailored to that part of velocity space. A boundary layer equation for the loss cone is derived and then solved. The description of the pitch angle distribution is completed by asymptotically matching the boundary‐layer solution to the distribution outside the loss cone, which can be obtained by using the bounce‐averaged diffusion equation. Calculated results for the particle distribution function in representative cases are presented. We apply our theory to ISIS 2 electron flux data for the continuous aurora to deduce the magnitude and the energy dependence of the pitch angle diffusion coefficient. We find that the diffusion coefficient is peaked at the invariant latitude at which the total energy flux into the atmosphere is largest, and that its energy dependence can be fitted with a power law E−n, with ½ ≲ n ≲ 1.
Titan's gas and plasma torus
Authors: Eviatar A. , Podolak M.
Published: February 2008

We assess the implications of the Voyager observations for a steady state model of a torus of hydrogen and nitrogen neutral gas and plasma. We constrain the nitrogen neutral density, the neutral hydrogen and nitrogen escape fluxes (from Titan), and the diffusion rate in terms of observed or inferred quantities. The results obtained are consistent with the Voyager observations.
On reconnection and plasmoids in the geomagnetic tail
Authors: Forbes T. G. , Priest E. R.
Published: February 2008

The nonlinear evolution of the collisional tearing mode is numerically determined for a two‐dimensional current sheet configuration whose magnetic field lines are tied at one end to a stationary surface. The configuration is analogous to that occurring in the geomagnetic tail at the start of a substorm. The numerical results suggest that the formation of a near‐earth neutral line at substorm onset is due to the asymmetric tearing that occurs because the field lines in the geomagnetic tail are partly line‐tied, or anchored, by the earth's ionosphere. The results also suggest that during substorm recovery the neutral line in the tail moves away from the earth at a speed on the order of the speed of the plasma flowing into the neutral line region. Overall, the solution is consistent with proposed reconnection models of the substorm but suggests that the recovery phase may be more complex than previously expected, due to the growth and coalescence of multiple magnetic islands (i.e., plasmoids).
Acceleration of hydrogen ions and conic formation along auroral field lines
Authors: Okuda H. , Ashour‐Abdalla M.
Published: February 2008

Electrostatic ion cyclotron turbulence and the formation of ion conics at low altitudes (≈ 1500 km) along auroral field lines have been investigated analytically and by plasma numerical simulations. Ion cyclotron waves are assumed to be driven unstable by the upgoing cold ionospheric electrons associated with the downward auroral current. When the electron drift speed is comparable to the electron thermal speed, it was found that the large‐amplitude (eϕ/Te ≈ 1) coherent (ω = Ωi) ion cyclotron waves should exist along auroral field lines at low altitudes extending a few hundred kilometers. Ion conics are associated with ion cyclotron turbulence, and the ion bulk temperature is found to increase by a factor of 10 from the initial ionospheric temperature, while the temperature of the high‐energy tail can be as much as 100 times the ionospheric temperature. Theory and simulations are in good agreement.
High‐β theory of low‐frequency magnetic pulsations
Authors: Migliuolo Stefano
Published: February 2008

The theory of low‐frequency (compared to ion cyclotron) arbitrary‐β modes is developed for the following system: a two‐component (hot and cold) inhomogeneous plasma, and a straight inhomogeneous magnetic field. This system is taken to model the magnetosphere, near the geomagnetic equator. The stability properties of three modes are presented in detail: the drift‐compressional mode (driven by pressure gradients) the firehose mode (driven by T∥ > T⊥), and the drift mirror mode (driven by T⊥ > T∥). Comparisons to earlier models and to one observed event are also presented.
Global characteristics of magnetic flux ropes in the Venus ionosphere
Authors: Elphic R. C. , Russell C. T.
Published: February 2008

In this paper we examine the global characteristics of the magnetic flux ropes in the Venus ionosphere. These structures are the most common magnetic feature of the ionosphere, being observed on more than 70% of the orbits of the Pioneer Venus orbiter passing through the dayside ionosphere. The spatial occurrence of flux ropes maximizes at altitudes near 165 km; they may occupy more than 50% of the ionospheric volume at this altitude. At higher altitudes the fractional volume occupied by ropes decreases below 20%. Characteristic radii of the ropes vary between 6 km at 160 km altitude and 15 km at 500 km altitude, and scale roughly with both the local ion gyroradius and ion inertial length. Ropes above 200 km altitude in the low zenith angle regions (χ < 45°) appear to have quasi‐horizontal orientations, while those below 200 km tend to be quasi‐vertical. The high zenith angle cases (χ > 45°) tend to be horizontal above 300 km and randomly oriented below 300 km. Ropes may tend to be more tightly ‘twisted’ at low altitudes than at high, and more so in the χ < 45° regions than nearer the terminator. Rope field strengths are highest near the altitudes where their occurrence maximizes, i.e., near 165 km, and scale with the square root of the ambient thermal pressure. Finally, the global polarities of flux rope field‐aligned currents appear to be random and do not clearly support a steady, nonturbulent global formation mechanism.
On the nature of the interaction of the Jovian magnetosphere with the Icy Galilean Satellites
Authors: Wolff R. S. , Mendis D. A.
Published: February 2008

The nature of the interactions between the Jovian magnetosphere and Europa, Ganymede, and Callisto is examined. Effects of plasma and micrometeoroid bombardment of each satellite are considered in an attempt to understand the influences of these exogenic processes on the surface properties of each of the three moons. Earth‐based infrared observations of satellite surface temperatures are used as a basis to calculate (sublimated) H2O vapor pressures for each of the three satellites as a function of satellite latitude and local time. Sublimated and sputtered H2O leads to a net O2 atmosphere on each of the three moons, and the effectiveness of such an atmosphere in preventing the Jovian plasma from reaching each satellite surface is considered under a variety of magnetospheric conditions. The effects of the magnitude and orientation of possible satellite magnetic fields were also studied and used to estimate the properties of the satellite magnetospheres under conditions appropriate to each moon being alternately inside and outside the Jovian plasma sheet. In addition, the orbits of charged micrometeoroids in Jupiter's magnetosphere were examined to determine the relative flux of these particles over the surfaces of each of the three satellites. Finally, possible observational effects of continual (over geological time periods) plasma and micrometeoroid bombardment of the satellite surfaces are considered and compared with available laboratory simulations and earth‐ and space‐based observations of Europa, Ganymede, and Callisto. These efforts suggest that plasma and micrometeoroid bombardment of the icy Galilean satellites could act to substantially modify the surface properties of each of the three moons, resulting in ‘leading‐trailing’ asymmetries of the satellite surfaces not unlike those observed in the satellite spectral and polarization properties.
Saturn's E, G, and F rings: Modulated by the plasma sheet?
Authors: Morfill G. E. , Grün E. , Johnson T. V.
Published: February 2008

Saturn's broad E ring, the narrow G ring, and the structured and apparently time‐variable F ring(s) contain many micron and submicron‐sized particles, which make up the ‘visible’ component. These rings (or ring systems) are in direct contact with magnetospheric plasma. Fluctuations in the plasma density and/or mean energy, due to magnetospheric and solar wind processes, may induce stochastic charge variations on the dust particles, which in turn lead to an orbit perturbation and spatial diffusion. In addition, Coulomb drag forces may be important, in particular for the E ring. The possibility that electromagnetic effects may play a role in determining the F ring structure, and its possible time variations, is critically examined. Sputtering of micron‐sized dust particles in the E ring by magnetospheric ions yields lifetimes of 10² to 104 years. This effect as well as the plasma induced transport processes require an active source for the E ring, probably Enceladus.