bluuebunny/crossref_metadata_2025 huggingface dataset:
Low‐energy ion distribution functions on a magnetically quiet day at geostationary altitude (L = 7)
Authors: Singh N. , Raitt W. J. , Yasuhara F.
Published: February 2008

By using averaged data from ATS 6, ion energy and pitch angle distribution functions were examined for a magnetically quiet day (July 18, 1974). The data showed that for both field‐aligned and perpendicular fluxes, the populations had a mixture of characteristic energies. It was found that over three different energy bands in the range 3–600 eV the distribution functions could be fairly well approximated by Maxwellian distributions with temperatures in the ranges 3–10 eV, 30–50 eV, and approximately 70 eV in energy bands of 3–30 eV, 30–140 eV, and 140–600 eV, respectively. Pitch angle distributions were found to vary with the local time; strong field‐aligned particle fluxes were measured in the midnight and afternoon sectors, minor field‐aligned components persisted to some extent at all times, especially at low energies (E < 10 eV). The pitch angle distributions and energy distributions were used to compute total ion density, assuming that the proton was the dominant ion species. In the morning sector (0300–0600 LT) a change of field‐aligned fluxes to pancake distributions peaking at pitch angles near 90° was seen. By using the assumption that the plasma was corotating with the satellite, we have examined pitch angle scattering mechanisms responsible for the observed transformation of pitch angle distribution. It was found that a magnetic noise of power spectral density b²≲10−3 γ²/Hz belonging to electromagnetic ion cyclotron mode (L mode) near the ion cyclotron frequency could be very effective in trapping the field‐aligned fluxes by pitch angle scattering.
High spatial attenuation of the Siple transmitter signal and natural VLF chorus observed at ground‐based chain stations near Roberval, Quebec
Authors: Tsuruda K. , Machida S. , Terasawa T. , Nishida A. , Maezawa K.
Published: February 2008

The field intensity of the Siple transmitter signal was measured at four stations along the geomagnetic meridian passing about 100 km west of Roberval, Quebec, during about 1 month from July 10 to August 10, 1979. The distance between adjacent stations was 70–90 km, and the L values covered by the four stations were from 4.15 to 4.7. It is found that the field intensity of the Siple transmitter signal falls off very rapidly with distance. A typical rate of attenuation with distance is found to be as high as −7 db per 100 km, which is nearly 1 order of magnitude higher than the value expected from the earth‐ionosphere cavity propagation theory. The intensity of VLF chorus was measured at six stations along the same meridian on August 8, 1979. The two stations were added to the north of the four stations, and their L values were 4.85 and 5.1. The chorus intensity was found to fall off with distance almost with the same rate as the Siple signal.
Measurement of the profile of solar He I resonance lines
Authors: Phillips E. , Judge D. L. , Carlson R. W.
Published: February 2008

The profile of the helium resonance line at λ 584 Å from the full solar disk has been investigated through analysis of data obtained by sounding rockets flown in 1977 and in 1980. The instrument, a helium‐filled spectrometer, made use of a curve‐of‐growth technique. In the analysis, the solar He I line profile was represented by a Gaussian distribution. Between 6.5 and 13% of the Gaussian core area was found to be missing due to self‐reversal. Data from our 1980 experiment gave a line width of 101 ± 10 mÅ (full width at half maximum). The 1977 line width, obtained with less experimental reliability, was 128 ± 20 mÅ (full width at half maximum). Results obtained in a similar experiment in 1974, corrected for background radiation and for solar line self‐reversal, gave a revised 1974 line width of 103 mÅ (full width at half maximum). While the measured line width did vary, a comparison of the 1974, 1977, and 1980 data showed no consistent relationship between the measured widths and solar activity. An observation of the solar He I 537‐Å emissions during the 1980 sounding rocket flight provided a measurement of that line width, giving a value of 148 ± 27 mÅ (full width at half maximum).
Long period hydromagnetic wave inside the plasmasphere
Authors: Stuart W. F. , Lanzerotti L. J.
Published: February 2008

A long period (∼6 min) hydromagnetic wave was observed near L∼4 in the southern hemisphere at both Halley Bay and Siple, Antarctica, to have a substantially larger amplitude than that observed in the northern hemisphere conjugate areas. At Halley Bay, as well as at Siple, the wave appeared quite monochromatic. Data from a station array conjugate to Siple in the northern hemisphere showed the wave to be localized near L∼3.4. Whistler wave determinations of the equatorial electron densities show the wave to occur inside the plasmasphere. The wave period and localization region are not readily reconcilable with a cold, pure hydrogen plasma. The density of heavy ions that would be required within the plasmasphere to explain the observations appears excessive from present understanding of magnetosphere plasmas.
The thickness of the magnetopause current layer: ISEE 1 and 2 observations
Authors: Berchem Jean , Russell C. T.
Published: February 2008

The analysis of the dual magnetic field profile obtained by the ISEE mission provides a unique opportunity to study the earth's magnetospheric boundaries. A survey of the magnetopause thickness over the dayside magnetosphere is carried out by using the magnetic field measurements from the twin ISEE 1 and ISEE 2 magnetometers. Since reliable measurements are possible only for small separations (<1000 km) between ISEE 1 and 2 our data set is restricted to only thirty magnetopause crossings in the range of local time from 0800 to 1700 and at GSM latitude from 2° to 35°. We find that the magnetopause is in constant rapid and irregular motion with velocities ranging, in 80% of the cases, from 10 km/s to 80 km/s and with the current sheet thickness ranging from 400 to 1000 km. The thickness seems much better ordered by dipole mangetic latitudes than by GSM latitude. Near the magnetic equator the magnetopause current sheet is thinnest, about 500 km on average. This observation suggests that reconnection is initiated in the equatorial regions rather than in the polar cusps.
Plasma and magnetic field characteristics of magnetic flux transfer events
Authors: Paschmann G. , Haerendel G. , Papamastorakis I. , Sckopke N. , Bame S. J. , Gosling J. T. , Russell C. T.
Published: February 2008

Plasma and magnetic field data from ISEE 1 and 2 are examined for 5 passes of the magnetopause region at 20°–40° northern latitudes and ∼0800 to 1215 hours local time. These intervals contained a total of 15 well‐defined magnetic flux transfer events, which occurred in the magnetosheath as well as the magnetosphere. In either case, flux transfer events are characterized by a mixture of magnetosheath and magnetospheric particles. This fact strongly supports the hypothesis that flux transfer events represent encounters of reconnected flux tubes. Inside all of the studied events, the magnetic field strength as well as the sum of plasma and magnetic pressures is strongly enhanced. This excess pressure appears to be balanced by the tension of the ambient magnetic field lines as they are draped around the reconnected flux tube. The different observed magnetic field signatures are consistent with expectations for encounters of the flux tubes at different relative locations. Only those events that appear to be crossings of the flux tube close to its magnetopause crossing show large (∼100 km s−1) plasma flow speed enhancements. These increased velocities are restricted to the trailing portion of the events and are directed at large angles with respect to the magnetic field. One hypothesis is that the increased flow speeds are caused by continued reconnection at the low‐latitude boundaries of the flux tubes.
Effect of dc electric field on whistler mode propagation
Authors: Das I. M. L. , Singh R. P.
Published: February 2008

The dispersion relation for parallel propagating whistler wave in an electromagneto plasma for both parallel and transverse orientation of a dc electric field has been derived. The electric field induced cyclotron instability has been studied for a wide range of parameters to show the relative importance of electric field strength, temperature anisotropy, parallel thermal velocity, and the ratio of particle pressure to magnetic pressure. It is found that the wave grows (or damps) in amplitude for a parallel (or antiparallel) electric field. In the case of the transverse dc electric field, the magnitude of growth rate and the width of the region of wave amplification in frequency domain increases with increasing temperature anisotropy. The contribution of the transverse electric field towards the wave growth is negligibly small as compared to temperature anisotropy.
ISEE 1 observations of VLF line radiation in the Earth's magnetosphere
Authors: Bell T. F. , Luette J. P. , Inan U. S.
Published: February 2008

A search has been carried out for magnetospheric line radiation in the VLF data acquired during the period October 1977–August 1979 by the Stanford University VLF receiver on the ISEE 1 satellite, while the satellite position varied between L = 2 and L = 8 and the satellite longitude lay in the range 50°W to 110°W. This magnetopsheric region encompassed the magnetic field lines linking three stations, Eights, Siple and Roberval, from which most ground‐based data on VLF line radiation has been obtained and also included one of the main regions in which the occurrence of VLF chorus has been actively linked to power line radiation. Line radiation was detected on only 5 of 90 orbits, and all examples occurred at frequencies below 4 kHz. The most clearly defined example of line radiation was acquired near L = 5 under conditions of very low magnetic activity during which the plasma‐pause was located at L ∼ 7. In this particular case, the strongest lines in the spectrum exhibited a frequency change of approximately 22 Hz/min over a 9‐min period, while weaker lines changed frequency as slowly as 6 Hz/min. The line radiation was detected during a period when whistler mode echoing was quite pronounced on lower L shells and may actually have been a scattered component of line radiation echoing between hemispheres on a magnetic shell of lower L value. In all instances the S/N ratio or the absolute intensity of the lines was quite low. Thus, in agreement with earlier work, we conclude that very little of the background VLF wave energy in the outer magnetosphere is contained directly in VLF line radiation. However, the catalytic role of line radiation in controlling VLF wave‐particle interactions remains to be assessed.
Dynamical features of moving double layers
Authors: Singh Nagendra , Schunk R. W.
Published: February 2008

Results on the motion of double layers from numerical simulations are presented. It was observed that after their formation double layers move toward the high‐potential side. In simulations with sufficiently long plasmas, the moving double layer was found to occur along with a series of other plasma phenomena. These include electron beam plasma turbulence and resulting plasma heating, plasma evacuation, electron current and ion current interruptions and subsequent recoveries, and the acceleration of ‘trapped’ ions on the low‐potential side in an expanding plasma front moving with the double layer. Furthermore, we observed recurring formation and motion of double layers with the above plasma phenomena repeating periodically. In the largest simulated plasma of length d = 400 λD, where λD is the Debye length at the low‐potential end of the plasma, the double layers became supersonic with respect to the ion acoustic speed. The accelerated ‘trapped’ ions moving with the double layer and the free ions accelerated by the double layer created counterstreaming ions on the low‐potential side of the layer. The question of double layer motion in relation to current continuity across it is examined. The effect of ion current injection on the double layer motion is simulated. The motion of double layers as seen in our simulations are compared with results from other simulations and those from laboratory experiments. The relevance of the simulation results to space plasmas is discussed, demonstrating that several observed phenomena in space plasma are qualitatively similar to those seen in the simulations.
Coordinated balloon‐satellite observations of energetic particles at the onset of a magnetospheric substorm
Authors: Kremser G. , Bjordal J. , Block L. P. , Brønstad K. , Håvåg M. , Iversen I. B. , Kangas J. , Korth A. , Madsen M. M. , Niskanen J. , Riedler W. , Stadsnes J. , Tanskanen P. , Torkar K. M. , Ullaland S. L.
Published: February 2008

Coordinated observations of electron precipitation via X ray measurements from three simultaneously flown balloons, and of energetic particles at the geosynchronous satellite GEOS 2, were used to investigate variations of the distribution of energetic charged particles at the onset of a magnetospheric substorm on July 3, 1979. The electron precipitation started shortly after the onset of the substorm. It was accompanied by the injection of energetic electrons and ions at GEOS 2. The intensity of the injected particles increased without any observable energy dispersion. The electron pitch angle distributions were characterized by strong intensity increases toward the geomagnetic field direction. Azimuthal anisotropies of the ion intensity around the geomagnetic field lines occurred and were interpreted as spatial density gradients. The temporal variations of the gradients indicated that a region of intense ion fluxes was located near the satellite during the period of field‐aligned electron fluxes. It is concluded that the electron precipitation was directly related to the injection of electrons and that the injection took place well earthward of the near‐earth reconnection region postulated in some substorm models. A possible mechanism that would connect injection to reconnection is proposed. The influence of parallel electric fields on the particle distribution is discussed.
Energy deposition by precipitating particles and Joule dissipation in the auroral ionosphere
Authors: Vickrey James F. , Vondrak Richard R. , Matthews Stephen J.
Published: February 2008

Eight days of synoptic data from the Chatanika incoherent scatter radar have been analyzed in an attempt to determine the characteristic morphology of auroral zone energy deposition by Joule heating and precipitating particles. The observations cover invariant latitudes between ∼62° and 68°. The composite spatial morphology derived from these eight days of data shows that morning sector particle precipitation deposits energy into the thermosphere at a faster rate and at lower altitudes than evening sector precipitation. The Joule heating rate has the opposite asymmetry about midnight, i.e., more Joule heating results for a given premidnight eastward electrojet current than for the same morning sector westward electrojet current intensity. This complementary asymmetry about midnight between the Joule and particle precipitation heating rates is consistent with the changes in ionospheric conductivity implied by the local time variation of precipitating particle hardness. The Joule heating rate generally dominates particle energy deposition in the premidnight sector. However, the daily averages of the two energy sources are roughly equal.
Perturbation of the low‐latitude upper atmosphere during magnetic substorm activity
Authors: Prölss G. W.
Published: February 2008

By using ESRO 4 data, this study investigates low‐latitude density perturbations during magnetically active periods. It is found that (1) the delay time between substorm activity (AE index) and density perturbations may be of the order of 4 hours only, (2) an equatorial density hump may develop which at times is well separated from the higher latitude disturbance, and (3) thermal and nonthermal effects contribute to the density perturbation. As to the origin of the disturbance, direct energy injection by the ring current mechanism is only of secondary importance. Instead, traveling atmospheric disturbances and large‐scale winds both may contribute to the observed density increase.
Comparison of nonflare solar soft X ray flux with 10.7‐cm radio flux
Authors: Donnelly Richard F.
Published: February 2008

The similarities and differences of the nonflare solar 1‐ to 8‐Å X ray flux and the daily 10.7‐cm Ottawa solar radio flux are examined. The radio flux is shown to be much less sensitive than the soft X ray flux on the average to the coronal emission of active regions located near or beyond the solar chromospheric limb relative to regions near the center of the solar disk. This is caused by the solar soft X ray emission's being optically thin while much of the 10.7‐cm active region emission is from optical depths of τ ∼ 1. The radio flux includes a large quiet sun flux which is emitted mostly from the tenuous chromosphere‐corona transition region (T ∼ 104–106 °K) and partly from the cooler portions of the quiet corona T ∼ 1.5×106 °K. Conversely, the solar soft X ray flux has a very small quiet sun component.
Helium abundance enhancements in the solar wind
Authors: Borrini G. , Gosling J. T. , Bame S. J. , Feldman W. C.
Published: February 2008

Helium abundance (A(He)) enhancements observed with Los Alamos instruments on IMP 6, 7, and 8 during the 1972–1978 interval have been investigated. Statistical analysis of 73 large events with A(He) ≥ 10% (HAEs) provides evidence for a close link between helium enhancements at 1 A.U. and transient coronal mass ejections. HAE events are sporadic, sometimes clustered in time, and their frequency of occurrence is approximately in phase with the solar cycle. Nearly 50% of HAEs are associated with interplanetary shocks and/or geomagnetic activity sudden commencements, but the plasma pattern associated with A(He) enhancements is independent of shock occurrence. This pattern features high magnetic field strength, low alpha‐proton velocity difference, and low proton temperature. These plasma properties suggest that the enhancement is embedded in a ‘closed,’ magnetically dominated structure that expands adiabatically. In fact, HAEs are likely to be still evolving dynamically at 1 A.U. Evidence of a significant association between helium enhancements at 1 A.U. and type II and IV radio bursts in the corona is presented. We interpret these results to mean that most HAE events originate in transient coronal disturbances in which the magnetic field strength is enhanced.
Surface waves on solar wind tangential discontinuities
Authors: Hollweg Joseph V.
Published: February 2008

We demonstrate that (tangential) discontinuities in the magnetic field direction can support MHD surface waves. The surface waves resemble the usual Alfvén wave, but there are some important differences: (1) The surface waves exhibit a low‐frequency cutoff. (2) The velocity and magnetic field fluctuations are elliptically, and sometimes circularly, polarized. They may account for the solar wind helicity spectrum. (3) The surface waves are compressive, but there are special cases where they are noncompressive. (4) The wave vector k, the local normals to the surfaces of constant phase, and the magnetic minimum variance direction (mvd) do not all coincide. (5) There is a tendency for the mvd to align itself with the mean magnetic field direction. (6) The waves can be intrinsically nonplanar. (7) Equipartition between magnetic and kinetic energies is not obeyed locally. These properties of the surface waves lead us to believe that surface waves may be common in the solar wind.
Structure of the heliospheric current sheet in the early portion of Sunspot Cycle 21
Authors: Hoeksema J. Todd , Wilcox John M. , Scherrer Philip H.
Published: February 2008

The structure of the heliospheric current sheet on a spherical source surface of radius 2.35 Rs has been computed via the use of a potential field model during the first year and a half after the last sunspot minimum. The solar polar magnetic field that is not fully observed in conventional magnetograph scans was included in the computation. The computed heliospheric current sheet had a quasi‐stationary structure consisting of two northward and two southward maxima in latitude per solar rotation. The extent in latitude slowly increased from about 15° near the start of the interval to about 45° near the end. The magnetic field polarity (away from the sun or toward the sun) at the subterrestrial latitude on the source surface agreed with the interplanetary magnetic field polarity observed or inferred at the earth on 82% of the days. The interplanetary field structure observed at the earth at this time is finely tuned to the structure of low‐latitude fields on the source surface.
Observations of Jupiter's distant magnetotail and wake
Authors: Kurth W. S. , Sullivan J. D. , Gurnett D. A. , Scarf F. L. , Bridge H. S. , Sittler E. C.
Published: February 2008

In the period from mid‐1980 to August 1981 the Voyager 2 plasma wave instrument occasionally detected clear signatures of Jovian nonthermal continuum radiation while the spacecraft was in the general downstream direction from Jupiter at distances of 5000 < R < 9000 RJ (up to ∼4.5 AU). The magnitude and duration of the events increased as Voyager 2 approached the nominal aberrated tail position in the spring of 1981 and also indicated a periodicity suggesting some solar wind control. Each event shows characteristics suggestive of electromagnetic radiation trapped within a low‐density cavity. Supported with plasma observations, several of the events are characterized by a broad, moderately low density region surrounding a well‐defined, very low density core. We conclude that this series of continuum radiation events is best interpreted as the passage of Voyager 2 through the extended magnetotail and wake of Jupiter. Although the analysis of the events does not lead to a unique and unambiguous understanding of the structure of the tail, we can speculate on the various allowed tail configurations consistent with the observations. There are many cases in which there is a significant difference in the estimated duration of a given event as defined by the plasma wave or plasma observations, which may be explained by dynamical processes in the tail such as a pinch‐off or disconnection event similar to that observed in cometary plasma tails.
The proton concentration in the vicinity of the Io plasma torus
Authors: Tokar R. L. , Gurnett D. A. , Bagenal F.
Published: February 2008

This paper uses an improved model for the plasma distribution in Jupiter's magnetosphere to determine the light ion concentration in the vicinity of the Io plasma torus from whistler dispersion measurements. The model used assumes that the plasma is in diffusive equilibrium under the action of centrifugal, gravitational, and ambipolar electric field forces. The study provides an estimate of the plasma concentration at intermediate and high latitudes along field lines through the Io plasma torus. The method employed is to combine the Voyager 1 plasma wave instrument whistler observations with the Voyager 1 plasma instrument heavy ion charge concentrations throughout the Io torus to determine the light ion charge concentration along the whistler propagation paths. Because the light ion source is probably Jupiter's ionosphere and because Jupiter's atmosphere is primarily hydrogen, the light ions are taken to be protons. Whistlers at 13 L values between L = 5.25 and L = 5.85 are analyzed. Values of NL², the total number of ions per unit L multipled by L², are calculated and the ratio NL² (protons)/NL² (electrons) is found to have an average value of 0.2. This ratio is used to give a rough estimate for the ionospheric source strength.
Formation of Jovian decametric S bursts by modulated electron streams
Authors: Staelin D. H. , Rosenkranz P. W.
Published: February 2008

Jovian S bursts that are V shaped in the frequency‐time domain have been reported by several observers. These can arise if an outward moving ∼2.5‐keV electron beam in the Io flux tube is longitudinally modulated in velocity by ∼1 keV peak to peak, at a frequency of several hertz. Such modulation near the ionosphere could also explain the difference between the maximum observed frequencies of S burst and L burst emission. In order for this beam to produce the observed total S burst flux, a large part of the ∼5×106 A current observed in the Io flux tube must participate. In this case the observed duration of individual S bursts is consistent with magnetospheric ion densities greater than ∼15 cm−3. This ballistic model for S bursts also suggests an interpretation for the origin of L burst emission.
The origins of the plasma in the distant plasma sheet
Authors: Sharp R. D. , Lennartsson W. , Peterson W. K. , Shelley E. G.
Published: February 2008

A statistical study of the composition of the plasma sheet at geocentric radial distances between 10 and 23 RE has been conducted utilizing data from the energetic ion mass spectrometer experiment on ISEE 1. The composition of the ions in the energy per charge range 0.1 ≤ E/q ≤ 16 keV/e was found to be dependent on substorm activity. The O+/H+ and He+/H+ ratios of the ions increased and the He++/H+ ratio decreased with increasing AE. O+/H+ exhibited the largest changes, varying from about 2% during periods of extended quiet to about 40% shortly after a major substorm. H+ was typically the major constituent, but on some occasions O+ was dominant. Assuming that the composition of the ionospheric and solar wind supplied ions to the plasma sheet does not vary systematically from quiet to active times, inferences are drawn as to the relative importance of the ionosphere and solar wind as sources of the plasma sheet ions. The results suggest that during quiet times the plasma sheet is almost entirely of solar wind origin while during active times the ionosphere is of comparable importance to the solar wind. The average energies of the ions of solar wind origin appear to be significantly increased in association with substorm activity, while the average energies of the ions of ionospheric origin show little or no effect. The solar H+ component appears to be more strongly energized than does He++.
The anisotropy of high‐latitude nighttime F region irregularities
Authors: Livingston R. C. , Rino C. L. , Owen J. , Tsunoda R. T.
Published: February 2008

The anisotropy of intermediate‐scale, F region irregularities in the nighttime auroral zone is described. The study is based upon spaced‐receiver phase scintillation measurements made with the Wideband satellite at Poker Flat, Alaska. A systematic dependence of irregularity anisotropy with local time and magnetic latitude is observed, suggesting convective control. Sheetlike irregularities are confined to the zone of east‐west drift near the equatorward boundary of the auroral zone, and at the flow reversal, or Harang discontinuity, the cross‐field extension of the sheets is reduced. The extension of rodlike irregularities, which are observed poleward of the zonal convection boundary, also shows apparent convection dominance. Mechanisms for convection control of the anisotropy are discussed.
The adiabatic motion of charged dust grains in rotating magnetospheres
Authors: Northrop T. G. , Hill Jay Roderick
Published: February 2008

Dust grains in the ring systems and rapidly rotating magnetospheres of the outer planets such as Jupiter and Saturn may be sufficiently charged that the magnetic and electric forces on them are comparable with the gravitational force. The adiabatic theory of charged particle motion has previously been applied to electrons and atomic size particles. But it is also applicable to these charged dust grains in the micrometer and smaller size range. We derive here the guiding center equation of motion, drift velocity, and parallel equation of motion for these grains in a rotating magnetosphere. The effects of periodic grain charge‐discharge have not been treated previously and have been included in this analysis. Grain charge is affected by the surrounding plasma properties and by the grain plasma velocity (among other factors), both of which may vary over the gyrocircle. The resulting charge‐discharge process at the gyrofrequency destroys the invariance of the magnetic moment and causes a grain to move radially. The magnetic moment may increase or decrease, depending on the gyrophase of the charge variation. If it decreases, the motion is always toward synchronous radius for an equatorial grain. But the orbit becomes circular before the grain reaches synchronous radius, a conclusion that follows from an exact constant of the motion. This circularization can be viewed as a consequence of the gradual reduction in the magnetic moment. This circularization also suggests that dust grains leaving Io could not reach the region of the Jovian ring, but several effects could change that conclusion. Excellent qualitative and quantitative agreement is obtained between adiabatic theory and detailed numerical orbit integrations.
Solar wind flow about the terrestrial planets: 2. Comparison with gas dynamic theory and implications for solar‐planetary interactions
Authors: Slavin J. A. , Holzer R. E. , Spreiter J. R. , Stahara S. S. , Chaussee D. S.
Published: February 2008

This study utilizes gas dynamic calculations in conjunction with observational bow shock models to investigate the solar wind flow patterns about the terrestrial planets. Average dayside bow shock position could be predicted for the earth by theory with an error of only ∼2%, given the observed shape and location of the magnetopause. Accordingly, our findings confirm the validity of the single‐fluid gas dynamic approximation for describing this major aspect of solar wind flow past the earth. Modeled using gas dynamic theory, the solar wind interactions with Venus and Mars exhibit very significant differences. At Mars the mean inferred altitude of the solar wind‐obstacle interface varies from 510 km at the stagnation point to almost 1000 km near the terminator. The effective magnetic moment required to produce a magnetosphere of this size for average solar wind dynamic pressures and terrestrial‐type internal current systems is 1.4 ± 0.6×1022 G cm³. Gas dynamic modeling of the January 21, 1972, Mars 3 and July 20, 1976, Viking 1 lander particles and fields observations supports the conclusion that the Martian obstacle to the solar wind lies at altitudes too high for it to be associated with only an ionospheric or atmospheric interaction. In contrast with Mars, our modeling of the Venus observations has found that the bow wave is closer to the planet than would be expected for a purely ionospheric obstacle. The subsolar width of the Venus ionosheath in the Venera and PVO measurements is only 60% and 90%, respectively, of that predicted by the gas dynamic model. This result is attributed to the presence of solar wind‐neutral atmosphere interactions in the lower ionosheath that are not included in the gas dynamic code.
Radial diffusion in Saturn's radiation belts: A modeling analysis assuming satellite and ring E absorption
Authors: Hood L. L.
Published: February 2008

Phase space density profiles derived by Pioneer 11 and Voyager 1 investigators for Saturnian radiation‐belt protons with zero second adiabatic invariants and first invariants μ of 600, 1100, 2500, 5000, 10,000, and 20,000 MeV/G have been analyzed for the purpose of inferring the amplitude and L dependence of the time‐averaged radial diffusion coefficient D(L) in the inner magnetosphere. Three models for solid‐body absorption outside of L = 3 were considered. The first included satellite absorption only with satellites represented as simple geometric absorbers in an axially symmetric planetary magnetic field. The remaining two models included additional absorption by Ring E using a representative optical depth profile derived from ground‐based telescopic measurements and two possible characteristic latitudinal ring thicknesses. Minimum Ring E encounter times were estimated by assuming that the mirror latitude appropriate for the sampled protons is less than the maximum magnetic latitude of Ring E. The Ring E encounter times are generally longer than the estimated satellite absorption times for protons of these energies. Additional Ring E loss processes such as charge exchange in a possible associated torus could significantly reduce the effective loss times but are not considered in the present work. A low‐order L dependence (D(L) ∝ L², L³, or L4) was selected via a minimum‐variance criterion for five of the six data profiles studied for either of the three assumed absorption models. The cumulative number of selection occurrences for given assumed L dependences suggests a diffusion coefficient proportional to L3±1. The preferred amplitude D0 obtained for a given set of experimental phase space densities varies by a factor of 2 or 3 depending on which solid‐body absorption model was assumed. For μ ≤ 2500 MeV/G, forms for D(L) ranging from 0.5×10−9 L3±1 to 1.5×10−9 L3±1 s−1 were obtained. For μ > 2500 MeV/G the inferred amplitudes increased approximately monotonically with increasing μ reaching a maximum near 10−8 L3±1 s−1 at μ = 20,000 MeV/G. The latter increase may be an artificial consequence either of inappropriateness of the higher‐energy proton data profiles for analysis assuming simple diffusive transport or of underestimating the satellite absorption times at the highest proton energies. The inferred form for D(L) is least consistent with ‘terrestrial‐type’ diffusion mechanisms including magnetic impulses (D(L) ∝ L10) and electrostatic impulses of magnetospheric origin (D(L) ∝ L6 for μ ≪ 20 L² MeV/G and D(L) ∼ L10/μ² for μ ≫ 20 L² MeV/G). Theoretical models for ‘Jovian‐type’ diffusion via the ionospheric dynamo mechanism or the centrifugal interchange instability in regions of strong negative radial plasma density gradients predict lower‐order L dependences that appear to be more consistent with the inferred form.
A Simple theoretical model for calculating and parameterizing the ionospheric photoelectron flux
Authors: Richards P. G. , Torr D. G.
Published: February 2008

We have developed a simple method for calculating the ionospheric photoelectron flux that uses the concept of average electron energy loss to simplify the calculation of the degraded electron spectrum. This method, which requires only a knowledge of the total inelastic electron impact cross sections, rather than a detailed knowledge of the many partial cross sections can be used for the calculation of all secondary ion and excited state production rates. We estimate that the use of an average energy loss reduces the computation time by a factor of 10 and considerably reduces storage requirements. The simple model is designed to provide ionospheric chemistry workers with a convenient means of obtaining photoelectron spectra similar to that which is currently available for obtaining neutral densities. It is shown that the ionospheric photoelectron flux in the local equilibrium region is directly proportional to the attenuated solar EUV flux, which is a function only of the total neutral column density, and is independent of the neutral density composition. Therefore, electron impact cross sections can be chosen so that the simple method may also be used to parameterize the measured ionospheric photoelectron fluxes.
Stability of electron distributions within the Earth's bow shock
Authors: Thomsen M. F. , Barr H. C. , Gary S. Peter , Feldman W. C. , Cole T. E.
Published: February 2008

The linear Vlasov stability of a class of electron velocity distributions which model those observed within the earth's bow shock is examined. The study is restricted to electrostatic waves which propagate parallel to the ambient magnetic field B. Two instabilities are identified which are driven by free energy in the direction parallel to B: an ion acoustic wave with real frequency below the ion plasma frequency and an electron acoustic wave with real frequency of several times the ion plasma frequency. The characteristics of the unstable waves are in accord with observations obtained within the bow shock, in particular, with the trend of the observed electrostatic waves to be polarized parallel to the magnetic field. The instabilities identified here may contribute to electron dissipation in collisionless shocks.
Electrostatic bursts generated by electrons in Landau Resonance with whistler mode chorus
Authors: Reinleitner Lee A. , Gurnett Donald A. , Eastman Timothy E.
Published: February 2008

Recent studies of wideband plasma wave data from the ISEE 1 and ISEE 2 spacecraft have revealed that whistler mode chorus emissions in the earth's outer magnetosphere are often accompanied by high‐frequency bursts of electrostatic waves with a frequency slightly below the electron plasma frequency. Investigations have shown that in some cases the electrostatic waves are modulated at the chorus frequency. Further studies using the plasma analyzer (LEPEDEA) data on ISEE 1 indicate that these bursts are produced by a ‘beam’ of electrons in Landau (longitudinal) resonance with the chorus wave and thus moving at the chorus phase velocity. A threshold exists in the chorus intensity below which the electrostatic bursts do not appear. The high‐frequency electrostatic waves are believed to be caused by a type of two‐stream instability called the resistive medium instability. The resistive medium instability is characterized by a reduction in the electrostatic burst frequency below the electron plasma frequency, The instability is applicable only in the regime where V0/VT is on the order of 1, where V0 is the velocity of the beam and VT is the average thermal velocity of the plasma electrons. Our derivation assumes cold ions but warm electrons. The instability requires Landau damping to operate. Thus the beam velocity must be in the region of steep slope on the electron distribution function rather than in the high‐velocity tail region. In the cases examined from the LEPEDEA data the electron thermal energies are on the order of a few hundred eV. The beam velocities in the observed cases were ≈ 400 eV and ≈ 630 eV, thus verifying that the electrostatic bursts are in the proper regime for the resistive medium instability.
Factors controlling the location of the Venus bow shock
Authors: Tatrallyay M. , Russell C. T. , Mihalov J. D. , Barnes A.
Published: February 2008

The location of the Venus bow shock determined from magnetic field measurements during the first and third years of Pioneer Venus orbiter operation is examined and compared with nearly simultaneously obtained interplanetary solar wind data to determine those factors that control the size of the Venus bow shock. The location of the intersection of the bow shock with the terminator that is best determined by the data does not vary significantly between years 1979 and 1981 and is only 16% more distant than the Venera 9 and 10 shock when account is taken of solar wind aberration. Alfvenic Mach number and magnetosonic Mach number affect the size of the bow shock significantly. Solar wind dynamic pressure has a lesser effect. No significant asymmetries in the shock shape were found either as a result of the orientation of the clock angle of the IMF in the terminator plane or the angle of the IMF relative to the shock normal.
Maps of the polarizations of high latitude Pi 2's
Authors: Samson J. C. , Harrold B. G.
Published: February 2008

This manuscript presents a study of the polarizations of Pi 2's recorded by the magmetometer array of the University of Alberta. To simplify the interpretation, all the data are plotted in substorm‐centered coordinates (θs‐latitude, λs‐longitude). We have determined the center of the substorm current wedge (θs = 0, λs = 0) by using magnetometer data from midlatitude observatories to identify the ΔD crossover and magnetometer data from high latitude stations to identify the latitude of the onset of the substorm enhanced, westward electrojet. Within the region of the westward travelling surge, WTS, the polarizations in the horizontal H‐D plane are clockwise, CW (viewed downward), whereas equatorward of this region all polarizations are counterclockwise, CC, for all λs. Far to the east and west of the WTS, the polarizations are CW, poleward of θs = 0°, whereas poleward of the WTS the polarizations are CC. The orientations of the polarization ellipses suggest the presence of two distinct regions of field‐aligned current, FAC, associated with the Pi 2 oscillations. One region is centered near θs = 0°, λs = 5°E, and the other, possibly associated with the WTS, at θs = 0°, λ = −10°E. Many of the features of the polarizations of the Pi 2's suggest that the overall morphology of the Pi 2 FAC, and ionospheric currents, are similar to that of substorm current systems, and that the elliptical polarizations observed on the ground are caused by azimuthal expansion of the FAC associated with the Pi 2. When these azimuthal motions occur, the latitudinally integrated FAC at a given longitude is phase‐shifted with respect to the latitudinally integrated ionospheric Hall current.
The effect of local perturbations of the geomagnetic field on cosmic ray cutoff rigidities at Jungfraujoch and Kiel
Authors: Flückiger E. O. , Smart D. F. , Shea M. A.
Published: February 2008

We have investigated the effect of local perturbations of the geomagnetic field on the vertical cosmic ray cutoff rigidities at Jungfraujoch and Kiel as representative mid‐latitude neutron monitor stations. The main, effective, and Störmer vertical cutoff rigidities and their changes were determined by utilizing the trajectory‐tracing technique in a magnetic field which is modeled as a simple dipole field to which the disturbance field is superposed. It was found that the cosmic ray cutoff rigidities are most sensitive to variations of the z component of the geomagnetic field at geomagnetic latitudes −20° < Λ < +30° and at longitudes within 90° to the east of these northern hemisphere stations. Furthermore, cutoff rigidity variations at Kiel are predominantly due to changes of the geomagnetic field within geocentric distances 2.5RE < r < 6RE, whereas at Jungfraujoch changes in cutoff rigidities are caused almost exclusively by magnetic disturbances within 1RE < r < 4.5RE. For both locations the dependence of the main, effective, and Störmer vertical cutoff rigidities on the radial, latitudinal and longitudinal structure of the magnetic perturbations is given explicitly. The results are discussed with respect to the theory by Treiman (1953) describing the effect of a ring current on cosmic ray cutoff rigidities. It is also shown that for the analysis of the characteristic properties of the correlation between cutoff rigidity variations and specific geomagnetic perturbations the rigidity corresponding to the first “discontinuity band” of the rigidity spectrum is an extremely useful parameter.
Energetic electron precipitation due to gyroresonant interactions in the magnetosphere Involving coherent VLF waves with slowly varying frequency
Authors: Chang H. C. , Inan U. S. , Bell T. F.
Published: February 2008

A test particle simulation model (Inan et al., 1982) of the gyroresonance wave‐particle interaction in the magnetosphere is extended to study the transient particle precipitation fluxes induced by various coherent VLF waves with slowly varying frequency propagating along the earth's magnetic field lines. For moderate wave intensities (Bw ≤ 10 pT at L = 4) a comparison between input signals with linearly rising and falling frequencies shows that they induce the same amount of precipitated particle energy deposition. However, when the wave growth near the principal interaction region is considered, the precipitation induced by a riser is considerably higher than that induced by a faller. The effect of the frequency‐time slope (df/dt) of the input signal on the induced precipitation energy flux is found to be small. Our results indicate, however, that the precipitation energy flux depends strongly on the falloff in energy of the trapped particle distribution function. The model is applied for computing the transient precipitation energy fluxes induced by a triggered rising emission, a hooklike signal and a one‐hop whistler. The total precipitated energy deposition for these various signals is found to be comparable to that induced by a linear ramp having the same input wave energy density.
A statistical study of large electric field events in the Earth's magnetotail
Authors: Levin S. , Whitley K. , Mozer F. S.
Published: February 2008

A statistical study of large electric field events, measured on ISEE 1 at geocentric distances of 3 to 23 RE, has been performed in order to determine the occurrence frequency and spatial distribution of such events. Large electric fields were seen at all altitudes, with the lowest altitude events occurring on auroral field lines and the higher altitude events occurring on the high latitude boundary of the plasma sheet. The largest events had amplitudes that were approximately proportional to the square root of the local magnetic field strength, in agreement with expectations of a simple mapping model. The apparent concentration of big field events near local midnight is caused, at least in part, by a nonuniform frequency of satellite crossings through the plasma sheet boundary as a function of local time. Thus the spatial distribution and amplitudes of the large field events are not inconsistent with the mapping of low‐altitude electrostatic shocks to the plasma sheet boundary along equipotential magnetic field lines.
The Fossil Theory of nighttime high latitude F region troughs
Authors: Evans J. V. , Holt J. M. , Oliver W. L. , Wand R. H.
Published: February 2008

Observations of F region electron density, electron and ion temperatures and ion drift velocity over the latitude interval 40° < Λ < 70° have been made from Millstone Hill by means of incoherent scatter radar. We conclude that the F region density trough that forms on the nightside of the earth may have more than one cause. Current computer models indicate that it can be produced as a consequence of the competing effects of convection and corotation that cause some flux tubes to remain for long periods in darkness in the premidnight sector, thereby allowing the ionisation in them to decay. Our observations show that troughs often are first seen in the late afternoon sector in regions of fast sunward convective flow and can move in latitude in association with substorms. We suggest that these troughs are produced by convection of low density regions from the nightside into daylight and the increased recombination rate that occurs in regions where the O+ ions are forced through the neutral air at high speed (∼1 km/s). Troughs created in the dusk sector by this mechanism during substorms can be left as the ionospheric signature of the substorm or “fossil” when the region of fast convective flow moves poleward following the substorm. These extended E‐W troughs then are carried across the nightside of the earth by corotation. Relative to the surrounding regions, the trough changes little with time and finally is destroyed only when it is carried into sunlight on the dawnside.