bluuebunny/crossref_metadata_2025 huggingface dataset:
Interplanetary diffusion of solar cosmic rays: A new approximate analytic solution
Authors: Owens A. J.
Published: February 2008

A very simple approximate analytic solution, accurate to the order of rV/K, is given for the interplanetary propagation of solar cosmic rays. The solution includes effects of convection, diffusion, and adiabatic deceleration. Comparison with previous numerical and Monte Carlo calculations shows that the solution is accurate for K ≳ 5 × 1020 cm²/s. The model gives an exponential temporal decay for late times in long‐lived events. Fits to the November 15, 1960, and November 22, 1977, ground level events are given.
Atmospheric gamma ray angle and energy distributions from sea level to 3.5 g/cm² and 2 to 25 MeV
Authors: Ryan James M. , Jennings Mark C. , Radwin Martin D. , Zych Allen D. , White R. Stephen
Published: February 2008

Angle and energy distributions are reported here for gamma rays from ground level to 3.5 g/cm² residual atmosphere at energies of 2 to 25 MeV and zenith angles of 0° to 50° and 180° to 130°. They complement the previous results of Ryan et al. (1977) for altitudes above 100 g/cm². The observations were made with the University of California, Riverside (UCR), double Compton scatter telescope from a balloon launched from Palestine, Texas, geomagnetic cut‐off of 4.5 GV, on May 13, 1975. The ground level measurements were made at Riverside, California, at 1000‐g/cm² residual atmosphere, geomagnetic cut‐off of 5.4 GV, on July 21, 1977. Growth curves for downward‐moving gamma rays have pronounced maxima at 160±30 g/cm² residual atmosphere, while those for upward‐moving gamma rays are essentially constant at altitudes above the Pfotzer maximum. Below about 300 g/cm² both the downward and the upward fluxes decrease with average e‐folding depths of 192±15 and 179±12 g/cm², respectively. The ratio of the downward‐ to upward‐moving gamma rays near the vertical is 17±5, independent of depth below the Pfotzer maximum. The energy variation, E−1.1, of the downward‐moving gamma rays is flatter than the E−1.8 of the upward moving ones at all depths below 100 g/cm². All zenith angle distributions show peaks toward the horizon. Above the Pfotzer maximum the downward flux decreases more rapidly away from the horizon than the upward flux, while below the converse is true. The azimuth angle distributions at 4.0 g/cm² show northward anisotropies that increase with energy from 3.9±0.9% at 2–10 MeV to 7.0±2.3% at 10–25 MeV.
Residence lifetimes of 1.79‐ to 2.15‐MeV protons in the equatorial region of the Jovian magnetosphere
Authors: Northrop Theodore G.
Published: February 2008

The magnitude of the strong flow of 1.79–2.15 MeV protons away from the Jovian magnetic equator is used to estimate the residence lifetimes of these protons in the equatorial source region. The lifetime is approximately 1.2 × 105/(R − 5.8)² s, where R is distance in Jovian radii from Jupiter's center to the equatorial region of interest. At Europa and Ganymede (but not at Io) this lifetime is 1–2 orders of magnitude shorter than sweepup lifetimes by the moons. The fluxes of these protons, integrated over the observed range from 6 to 40 Jovian radii, amount to 3 × 1023 particles per second, per degree of longitude and carry away about 9 × 1017 ergs per second per degree.
Doppler shift effects on double‐probe‐measured electric field power spectra
Authors: Temerin Michael
Published: February 2008

Electric field power spectra of low‐frequency turbulence measured by satellite‐borne double probe detectors are affected by the orientation of the double probe with respect to the velocity of the satellite. The power spectrum from a spinning double probe will have a spin modulation that is dependent on the orientation of the spin axis and the intrinsic spectral index and dimensionality of the turbulence. The measured power spectrum is examined for various intrinsic spectral indices and dimensionalities as a function of the orientation of the double probe.
A history of Mars atmospheric opacity in the Southern Hemisphere during the Viking extended mission
Authors: Thorpe Thomas E.
Published: February 2008

A history of developing opacity in the Mars southern hemisphere during the Viking extended mission has been compiled using orbiter images obtained at high altitude. Observations of changing contrasts under similar viewing conditions have been modeled by multiple‐scattering intensity transfer equations that have produced a temporal description of changes in optical depth and particle‐scattering properties within a network of control points. The results are presented in the form of Mercator and perspective plots for various dates.
Plasma wave turbulence at the magnetopause: Observations from ISEE 1 and 2
Authors: Gurnett D. A. , Anderson R. R. , Tsurutani B. T. , Smith E. J. , Paschmann G. , Haerendel G. , Bame S. J. , Russell C. T.
Published: February 2008

In this paper we investigate plasma wave electric and magnetic fields in the vicinity of the magnetopause by using recent measurements from the ISEE 1 and 2 spacecraft. Strong electric and magnetic field turbulence is often observed at the magnetopause. The electric field spectrum of this turbulence typically extends over an extremely large frequency range, from less than a few hertz to above 100 kHz, and the magnetic field turbulence typically extends from a few hertz to about 1 kHz. The maximum intensities usually occur in the magnetopause current layer and plasma boundary layer. Somewhat similar turbulence spectra are also sometimes observed in association with flux transfer events and possible ‘inclusions’ of boundary layer plasma in the magnetosphere. In addition to the broad‐band electric and magnetic field turbulence, narrow‐band electrostatic emissions are occasionally observed near the electron plasma frequency in the vicinity of the magnetopause. Two possible plasma instabilities, the electrostatic ion‐cyclotron instability and the lower‐hybrid‐drift instability, are considered the primary candidates for explaining the broad‐band electric field turbulence. The narrow‐band electrostatic emissions near the local electron plasma frequency are believed to be either plasma oscillations or electrostatic waves near the upper‐hybrid‐resonance frequency.
Height dependence of spread F bubble drift velocities
Authors: Hudson Mark K. , Balsley Ben B.
Published: February 2008

Vertical bubble velocities in equatorial spread F have been investigated analytically by Ott [1978], Ossakow and Chaturvedi [1978], and Hudson [1978], all of whom found a proportionality of the vertical velocity to bubble depletion density. If the depletion density increases with height for a convectively growing bubble formed at the bottom of the F layer, then the vertical bubble velocity should increase with height also. In this paper we present VHF radar observations of equatorial spread F that support the latter conclusion.
On the role of irregularities in plasma entry into the magnetosphere
Authors: Schindler K.
Published: February 2008

This paper deals with dynamic processes caused by irregularities close to the magnetopause as discussed by several authors in the recent literature. The problem of plasma entry is tackled theoretically by assuming individual filaments appearing at the magnetopause with an excess inward momentum. The conditions under which entry is possible are derived in a two‐dimensional model. With ideal MHD, entry is essentially confined to the cases of near magnetic field alignment. The consequences of nonideal MHD processes are discussed qualitatively. The results support the possibility of the presence of an irregular distribution of ‘merging patches’ on the magnetopause, giving better agreement with recent observations than the classical steady state reconnection picture. All entry mechanisms discussed are potential candidates for the processes that form the plasma boundary layer.
Instability of equatorial protons in Jupiter's mid‐magnetosphere
Authors: Northrop T.G. , Schardt A.W.
Published: February 2008

Two different models for the distribution function are fit to the Jovian protons seen by Pioneer 10 inbound. The models reproduce the observed energy and angular distributions. These models are then used to assess the collisionless mirror instability. Because of the pancake proton angular distributions in the equatorial ring current region, the ring current particle population appears to be mirror unstable at times, with instability growth rates of ∼10 min. Such a time is consistent with observed proton flux autocorrelation times. An instability such as this (there are other candidates) may be responsible for the previously established proton flux flowing parallel to the magnetic field away from the equatorial region.
Longitudinal position of polar cap plasma flow entry region
Authors: Primdahl F. , Primdahl Inge S. , Spangslev F.
Published: February 2008

The polar slant E condition (SEC) whose most prominent feature is the Ess ionogram trace is taken as evidence of an ion acoustic instability driven by strong electric fields and thus as evidence of strong plasma flow. The occurrence of SEC in the northern polar cusp region strongly suggests that the throat region of rotational plasma flow into the polar cap does not move in local time when the sign of the IMF By changes.
Some features of pararesonance (PR) whistlers
Authors: Morgan M.G.
Published: February 2008

Pararesonance (PR) whistlers observed in the topside ionosphere by the Dartmouth receiver on Ogo 6 are examined. The study extends that of Walter and Angerami (1969) to higher frequencies and shows that the upper cutoff frequency of PR whistlers closely follows a 1/L4 dependence from 6 to 100 kHz (at L=2.90 and 1.37, respectively). Most PR whistlers are attached to paralongitudinal (PL) whistlers due, presumably, to intermode coupling. The ‘walking trace,’ or unattached PR whistler, reported by Walter and Angerami is evidently unusual. The upper cutoff frequency follows 1/L4 whether attachment occurs or not. Rising sawtooth appendages starting at the upper cutoff frequency are frequently seen on PR whistlers. The required multivalued extensions, of the theoretical isofrequency curves plotted in travel time versus observing latitude by Walter and Angerami, to produce these appendages are given in the appendix, but their physical implication has not been investigated. The terms pararesonance (PR) and paralongitudinal (PL) are suggested as preferable to proresonance and prolongitudinal which have been previously used.
Methods for Monte Carlo simulation of the exospheres of the Moon and Mercury
Authors: Hodges R. Richard
Published: February 2008

A general form of the integral equation of exospheric transport on moon‐like bodies is derived in a form that permits arbitrary specification of time varying physical processes affecting atom creation and annihilation, atom‐regolith collisions, adsorption and desorption, and nonplanetocentric acceleration. Because these processes usually defy analytic representation, the Monte Carlo method of solution of the transport equation, the only viable alternative, is described in detail, with separate discussions of the methods of specification of physical processes as probabalistic functions. Proof of the validity of the Monte Carlo exosphere simulation method is provided in the form of a comparison of analytic and Monte Carlo solutions to three classical, and analytically tractable, exosphere problems. One of the key phenomena in moonlike exosphere simulations, the distribution of velocities of the atoms leaving a regolith, depends mainly on the nature of collisions of free atoms with rocks. It is shown that on the moon and Mercury, elastic collisions of helium atoms with a Maxwellian distribution of vibrating, bound atoms produce a nearly Maxwellian distribution of helium velocities, despite the absence of speeds in excess of escape in the impinging helium velocity distribution.
Type 1 radar echoes from the equatorial electrojet with double‐peaked Doppler spectra
Authors: Fejer B.G. , Farley D.T. , Johnston P. , Balsley B.B.
Published: February 2008

Normal type 1 radar echoes obtained from relatively large zenith angles have a power spectrum with a single narrow peak whose Doppler shift corresponds approximately to the acoustic velocity in the medium. On some occasions, however, this single maximum splits into two distinct peaks, separated in phase velocity on one occasion by 270 m/s. This bifurcation is most easily observed at large zenith angles during daytime when a narrow antenna beam is used. It has also been seen in a daytime experiment in which radars at Jicamarca and Huancayo simultaneously probed the same region from two different radar zenith angles. The bifurcation has been observed so far only to the west of Jicamarca, over the Pacific Ocean. This spectral splitting could be caused by vertical electron density gradients, such as those associated with ‘blanketing’ sporadic E layers. A sufficiently sharp (scale lengths of a few hundred meters or less) positive gradient on the underside of the layer and negative gradient on the topside would cause the type 1 velocity to be decreased and increased, respectively, during the day, by amounts as large as those observed.
The effect of injection location on the spectrum of energetic magnetospheric particles
Authors: Birmingham Thomas J.
Published: February 2008

The role of local particle injections and accelerations in determining the radial profiles of fluxes of trapped magnetospheric particles is assessed. A simple, one‐dimensional, steady state model of radial transport in which diffusion is balanced against source‐sinks is adopted and solutions in two cases are contrasted. The first case is that of point injection near the magnetospheric boundary followed by loss‐ and source‐free, µ‐ and J‐conserving radial diffusion. In the second case, injection is going on over a band of radii in addition to radial diffusion, and the observation point is immersed in this injection region. In the case of point injection the observed spectrum is the injection spectrum suitably scaled in energy; in the case of banded injection, radial mixing results in a spectrum whose energy structure is determined by the radial dependencies of the diffusion coefficient, the injection, and the background magnetic field in addition to the input spectrum itself. The relevance to contemporary Jovian and terrestrial observations is pointed out.
Determination of the thermal rate coefficient, products, and branching ratios for the reaction of O+(²D) with N2
Authors: Torr D.G. , Torr M.R.
Published: February 2008

Measurements taken by the Atmosphere Explorer‐C satellite are used to determine rate coefficients for the reactions
. The following results are obtained: k1 = (1 −0.5+1) × 10−10 cm3 s−1, k2 = (3−2+7) × 10−11 cm3 s−1, and k3 < 5.5 × 10−11 cm3 s−1. Laboratory measurements of k1 and k3 have recently been published. These yielded k1 = 1.5 × 10−10 cm3 s−1 and k3 = 1.3 × 10−11 cm3 s−1, in agreement with the atmospheric determination.
Comment on ‘The annual and semiannual variations of the cosmic radiation’ by H. P. Khor, K. W. Kwok, and A. J. Owens
Authors: Antonucci Ester , Marocchi Daniela , Perona Giovanni E.
Published: February 2008

The phase changes of the annual and semiannual variations of the cosmic ray intensity through two solar cycles and its relationship to the 22‐year solar magnetic cycle have been observed and reported by us. The phase of the yearly cosmic ray modulation reverses at the reversals of the polarity of the solar polar magnetic fields. The significance of this effect cannot be dismissed on the basis of an analysis on an insufficient data set, in fact, during the period 1960–1975 used by Khor et al. [1979], the solar magnetic configuration is steadily the same except that for the 4 years 1972–1975, which, however, do correspond to a variable interplanetary magnetic dipolar configuration.
Modulation of terrestrial ion escape flux composition (by Low‐Altitude acceleration and charge exchange chemistry)
Authors: Moore Thomas E.
Published: February 2008

Motivated by recent observations of highly variable hot plasma composition in the magnetosphere, control of the ionospheric escape flux composition by low‐altitude particle dynamics and ion chemistry has been investigated for an e−, H+, O+ ionosphere. Theoretical polar wind results of Banks and Holzer and observations of highly nonthermal ion escape have been used as guides. It is found that the fraction of the steady state escape flux which is O+ can be controlled very sensitively by the occurrence of parallel or transverse ion acceleration at altitudes below the altitude where the neutral oxygen density falls rapidly below the neutral hydrogen density and the ionospheric source of O+ tends to be rapidly converted by charge exchange to H+. The acceleration is required both to overcome the gravitational confinement of O+ and to violate charge exchange equilibrium so that the neutral hydrogen atmosphere appears ‘optically’ thin to escaping O+. Constraints are placed on the acceleration processes, and it is shown that O+ escape is facilitated by observed ionospheric responses to magnetic activity.
Computer simulation of gradient drift instability processes in Operation Avefria
Authors: McDonald B.E. , Keskinen M.J. , Ossakow S.L. , Zalesak S.T.
Published: February 2008

We have solved numerically equations of motion for the evolution of a one‐level two‐dimensional F region ionospheric plasma cloud with five different ratios of cloud‐to‐background integrated Pedersen conductivity. The simulations show that (1) physically small clouds striate because of the gradient drift instability more rapidly than large clouds simulated previously and (2) there exists a minimum striation onset time as a function of the Pedersen conductivity ratio. Observation 1 is in agreement with the rapid striation development exhibited by the physically small Avefria barium cloud releases, relative to the large Secede or Stress type releases.
The propagation of Jovian electrons to Earth
Authors: Chenette D.L.
Published: February 2008

Jovian electron flux increases are observed by the University of Chicago experiment on the earth‐orbiting satellite Imp 8 throughout five ∼13‐month Jovian synodic years during the period from launch of the satellite in late 1973 to 1979. A detailed analysis of these data is presented to define the characteristics of Jovian electron propagation to earth. Corotating interaction regions (CIR) that form at the leading edges of fast solar wind streams continue to modulate the propagation of MeV electrons from Jupiter to the orbit of earth to produce ∼27‐day recurrent variations in the Jovian electron flux intensity. Between these CIR's, both the spatial distribution of Jovian electrons around earth's orbit and the time‐intensity profiles of the Jovian electron flux induced by the CIR modulation effect are shown to be consistent with the predictions of a simple diffusion model calculation, assuming that Jupiter's magnetosphere is a ‘point’ source of electrons. A new and significant result of this study is that these time‐intensity profiles are more accurately described not by the assumption that Jupiter is a constant source of electrons, but rather by assuming that electron emission is initiated with each passage of a CIR by Jupiter with the emission continuing for several days only. A measurement of the spatial gradient of these electrons demonstrates that there is a significant reduction in the Jovian electron flux between Jupiter and earth even when earth is aligned with Jupiter along the average direction of the interplanetary magnetic field. This observation is inconsistent with ‘scatter‐free’ propagation.
Observations of large fluxes of He+ in the solar wind following an interplanetary shock
Authors: Gosling J.T. , Asbridge J.R. , Bame S.J. , Feldman W.C. , Zwickl R.D.
Published: February 2008

Los Alamos Scientific Laboratory instrumentation on Imp 7 has detected large fluxes of He+ within that volume of solar wind plasma believed to be the solar ejecta driving the interplanetary shock wave disturbance of July 29, 1977. The very high He+/He++ abundance ratio of 0.3 measured during this event suggests that this was solar prominence material only partially ionized by its passage through the corona.
The chemistry of excited NO+ in an aurora
Authors: Young E.R. , Torr D.G. , Clark K.C.
Published: February 2008

The six most significant production and seven most significant loss mechanisms are identified for NO+(a³Σ) in the aurora. Rate constants are given for these reactions as well as for the 14 most important reactions competing with the production of NO+(a). Using available data and certain crucial deductions on chemical reaction rates to simulate a number of altitude profiles, we have concluded that the two predominant sources of NO+(a3Σ) are probably N+ + O2 → NO+(a) + O and N2+ + NO → NO+(a) + N2. Radiative decay is not the primary loss mechanism but can dominate above 150 km. Destruction via charge exchange with N2 and O are most important below 150 km. Total radiation in the a3Σ–X¹Σ transition is estimated to be between 35 R and 350 R for a model aurora of 78 ergs cm−2 s−1 energy input, which is nearly IBC III in intensity.
A laboratory study on the dissociative recombination of vibrationally excited O2+ ions
Authors: Zipf Edward C.
Published: February 2008

The production of metastable O(¹S) atoms by the dissociative recombination of O2+ ions has been studied in a laboratory plasma spectroscopy experiment. The specific dissociative recombination coefficient for O(1S) formation has a value of 2.1 × 10−8 cm3 s−1, corresponding to an O(¹S) quantum yield of 10% at an apparent temperature of 296°K, in agreement with the earlier but less comprehensive measurements of Zipf [1970]. By using a variety of gas mixtures in order to modify the vibrational population of the O2+ ions and laser‐induced photofluorescence techniques it was found that the laboratory O2+ plasmas are vibrationally excited and that O2+ ions in the υ′=4–12 vibrational levels are the chief source of the observed O(1S) atoms. The specific O(1S) dissociative recombination coefficient for O2+(2πg) ions in the lowest vibrational levels (υ′ ≤3) is much smaller (4.2 × 10−9 cm3 s−1). The implications of the quantum yield of 9.9% derived from Atmosphere Explorer data are discussed in the light of these findings. Preliminary electron‐heating experiments indicate that the specific O(¹S) recombination coefficient decreases with increasing electron temperature.
Implication of Z variations during a near‐noon equatorial counterelectrojet on March 6, 1967 in the Indian Zone
Authors: Kane R.P. , Trivedi N.B.
Published: February 2008

Daily variations of geomagnetic H, D, and Z were studied for an average of five international quiet days in March 1967, and for one particular day (March 6, 1967) when a spectacular near‐noon counterelectrojet occurred at Trivandrum (about 30 km south of the dip equator) and Kodaikanal and Annamalainagar (about 170 and 290 km north of the dip equator) in the Indian zone. For the average quiet days the electrojet seemed to be centered near the dip equator in the morning, but shifted steadily southward over Trivandrum and beyond by evening. For the near‐noon counterelectrojet day, single current systems flowing more than 100 km to the north of the dip equator and/or double current systems, one normal near the dip equator and another in reverse, about 100 km or more to the north were indicated.
Winter enhancement of atomic lithium and sodium in the polar upper atmosphere
Authors: Henriksen K. , Sivjee G.G. , Deehr C.S.
Published: February 2008

Winter twilight spectrophotometric observations of alkali metal resonance line emissions over Svalbard yield column densities of 1.9 × 107 Li atoms cm−2 and 1.1 × 1010 Na atoms cm−2 near the polar mesopause for November 1978 through January 1979. The latter density is high but consistent with observations elsewhere. The former is at least twice as large as either the sporadic increases or the seasonal peak observed at lower latitudes and was not significantly diminished according to similar observations in December 1977 and November 1979. Other polar observations nearer the equinox indicate lower Na abundances, implying the existence of a December–January polar seasonal maximum.
The energetic charged particle absorption signature of Mimas
Authors: Van Allen J. A. , Thomsen M. F. , Randall B. A.
Published: February 2008

Data are presented for a 1‐min dip in electron intensity that was observed coherently by four different University of Iowa detectors at 1546∶51 earth received time on day 244 of 1979 as Pioneer 11 crossed the orbit of Mimas inbound during its encounter with the Saturn system. By a detailed analysis we show that (1) this absorption microsignature in electron intensity is plausibly attributable to the particle sweeping effect of Mimas; (2) the radial width of the signature is caused primarily by energy dispersion in the longitudinal drift rate of electrons and not by radial diffusion; (3) the spectrum of trapped electrons at Mimas is nearly monoenergetic, centered at kinetic energy E=1.59 MeV with a spread δE ∼0.1 MeV; (4) this narrow spectrum, which is unique in magnetospheric physics, is caused by the ‘band‐pass filtering’ action of the successive inner satellites, most importantly Enceladus, on an inward diffusing population of electrons; and (5) the radial diffusion coefficient D of electrons E= 1.0 MeV is 1.0 × 10−10 Rs² s−1 at L=4 and probably in the range 3.7 × 10−11 to 8.3 × 10−12 Rs² s−1 at L=3.
Corotation of Saturn's magnetosphere: Evidence from energetic proton anisotropies
Authors: Thomsen M. F. , Northrop T. G. , Schardt A. W. , Van Allen J. A.
Published: February 2008

The theory and technique of Northrop and Thomsen [1980] are applied to observations of energy spectra and directional anisotropies of 0.61‐ to 3.41‐MeV protons in Saturn's magnetosphere. The observations were made by the Goddard Space Flight Center/University of New Hampshire and University of Iowa instruments aboard Pioneer 11 during the Pioneer encounter with Saturn in August–September 1979. Fourier fits to 15‐min intervals of data are combined with spectral indices to yield information about the E × B convection velocity and temporal changes in the particle population. There is a fundamental inability to distinguish unambiguously between the two, but if one can be assumed, the other then follows from these calculations. It is found that although these data do not by themselves allow an unambiguous determination of the extent of corotation in Saturn's outer magnetosphere, they are consistent with exact corotation at the nominal rotation period in the presence of significant but not unreasonable temporal variations in the energetic proton population.
High‐energy trapped radiation penetrating the rings of Saturn
Authors: Chenette D. L. , Cooper J. F. , Eraker J. H. , Pyle K. R. , Simpson J. A.
Published: February 2008

Electrons and protons in the energy ranges 2–25 MeV and >67 MeV, respectively, have been discovered throughout the entire equatorial region inward from the outer edge of the A ring at L=2.3 to the periapsis of the Pioneer trajectory at L ∼1.3. The trapped radiation which populates Saturn's magnetosphere beyond L=2.3 is totally absent in this region. The electron measurements include (1) a differential energy spectrum ∝ E−0.6, (2) an L dependence consistent with L2.8; and (3) an intensity of ∼0.05 el/cm² s sr near L=2 for the energy range 7–17 MeV (a factor of 5 times the interplanetary quiettime flux in this energy range). The proton measurements display an L dependence of L² with a flux level of ∼6 × 10−2 protons/cm² s sr above 67 MeV, just inside the edge of the A ring. The pitch angle distributions of both the electrons and protons are consistent with isotropy in the dipole magnetic field. It is argued from these results that the electrons and protons are trapped and thus penetrate the A‐B‐C rings. However, from the above experimental evidence it is concluded that this trapped radiation is not remnant radiation from the trapped radiation region beyond L=2.3. We find that these measurements are consistent with a model for splash albedo production of electrons and protons resulting from the bombardment of the atmosphere and/or rings of Saturn by cosmic ray protons with energies above the Störmer cut‐off at the magnetic latitudes of production. These secondary particles are then observed as trapped radiation propagating along the appropriate field lines crossing the ring plane. We also show that electron production may occur through π± → µ± → e± decay chain which yields an L dependence of L2.8 for pions. Our measurements and calculations are in reasonable agreement with extrapolations of electron albedo measurements from earth's upper atmosphere to the physical conditions at Saturn. We conclude that the absorption of the magnetospheric trapped radiation by Saturn's A, B, and C rings provides a set of physical conditions not present in other planetary magnetospheres for investigating the splash and reentrant albedo problems which can then be applied in the near‐earth space.
Observations of extreme ultraviolet emissions from the Saturnian plasmasphere
Authors: Wu F. M. , Judge D. L. , Carlson R. W.
Published: February 2008

Emission signals from the Saturnian plasmasphere at wavelengths shortward of 800 Å have been detected by the Pioneer ultraviolet photometer. The surface brightness of the emissions is about 0.3±0.2 R. These short‐wavelength emissions are interpreted as arising primarily from the radiative decay of electron excited atomic oxygen ions (O++), in the region between 5 and 7 Rs from Saturn. The total power radiated by the Saturnian plasma inferred from these ultraviolet measurements is about ∼2 × 1016 erg s−1, consistent with in situ plasma measurements. From the observed energy loss rate it is estimated that the ions are introduced into the plasma torus at a rate of 8 × 1025 ions/s, possibly through the sputtering of water ice on the surface of Tethys and Dione by particle impact.
Io and its plasma environment
Authors: Southwood David J. , Kivelson Margaret G. , Walker Raymond J. , Slavin James A.
Published: February 2008

We describe and examine the interaction of Io with its plasma torus and the Jovian magnetic field in the context of several currently popular models. We address three specific matters. First, we discuss features implied by sub‐Alfvenic flow which must be common to all models. Next, we examine the magnetic signature observed near Io by the Goddard Space Flight Center Voyager 1 magnetometer and point out that the preliminary estimate of 5 × 106 A current may be an overestimate. We obtain good fits with alternative current distributions which yield currents as small as 7 × 105 A through Io. The best fits are obtained for an Alfven Mach number (MA) of 0.15, but good fits are also found for 0.1 <MA <0.25. Lastly, we point out the crucial role of charged particle data for probing the near Io interaction. We use published data from the low energy charged particle (LECP) detector to suggest that Io has an intrinsic magnetic field of magnitude comparable with earlier estimates. We make predictions which can test this picture as further data become available.
Microturbulence in solar wind streams
Authors: Ananthakrishnan S. , Coles W. A. , Kaufman J. J.
Published: February 2008

We present a comparison of spacecraft and IPS measurements of the solar wind velocity for 1973–75, when the large scale velocity structure was very stable. From this comparison we infer a prototype distribution across a high‐speed stream for the electron density fluctuations Δne in the spatial frequency range 10−4–10−2 km−1. This ‘microturbulence’ is more closely related to the velocity structure in a stream than to the bulk density np. Enhancements of Δne occur in the stream interaction region, but they are not as large as those of np and they peak 12 hours after the np peak. The distribution of Δne is similar to that of the large scale (106 km) magnetic field fluctuations σB, but Δne also shows a broad variation not present in σB. Overall, Δne is most closely matched to the positive velocity gradient at the leading edge of a stream.
Orthohelium emissions at 3889 and 5876 Å in the polar upper atmosphere
Authors: Sivjee G. G. , Henriksen K. , Deehr C. S.
Published: February 2008

Spectrophotometric observations of auroral emissions from Longyearbyen, Svalbard, show enhancements at 3889 and 5876 Å believed to originate from He I (3p³P → 2s³S) and (3d³D → 2p³P) transitions. The average intensity of each line is about 2±1 R, but enhancements are observed during some auroral activity, with the 3889‐Å emission reaching intensities as high as 13±3 R. Direct excitation of ambient He by auroral electron impact contributes less than 1 R at these wavelengths. Precipitating He+ ions interacting with air are most likely to produce He(3p³P, 3d³D). Lack of observable (∼3 Å) Doppler shift of the orthohelium lines in the aurora implies that the average energy of precipitating He+ must be less than 8 keV. Since the cross section for the production of He(3p³P, 3d³D) decreases precipitously at He+ energies below 1 keV, the sunlit terrestrial atmospheric region magnetically conjugate to Svalbard, which at most can provide He+ with energies of a few tens of electron volts, cannot be the source of the precipitating He+ ions. Hence the solar wind, known to contain ions with energies ranging from a few hundred electron volts to a few kiloelectron volts is the most likely source of He+ precipitating in the auroras over Svalbard and, consequently, of the observed enhancement in He I 3889‐Å emission.
On the role of Alfvénic fluctuations in the inner solar system
Authors: Villante U.
Published: February 2008

The radial evolution of the amplitude of the Alfvénic fluctuations associated with the recurrent high‐velocity solar wind streams has been usually considered to be consistent both with undamped fluctuations of solar origin and with waves which saturate at some constant value of the ratio between the wave energy density and the background magnetic field energy density. In the inner solar system the two physical situations can be tentatively distinguished: indeed, according to theoretical models the saturated waves are expected to exhibit a more rapid decrease than the undamped ones. The Helios 1 and Helios 2 spacecraft observed a stream front during March 1976, while they were coaligned along the streamlines. The radial evolution of the wave amplitudes might be considered to be more consistent with saturated waves propagating between 0.41 and 0.65 AU. If true, such a conclusion would lead to the prediction that the contribution of the Alfvénic fluctuations to the energetics of the solar wind expansion may be appreciably larger than was previously estimated.
Influence of magnetic shear on the current convective instability in the diffuse aurora
Authors: Huba J. D. , Ossakow S. L.
Published: February 2008

The influence of magnetic shear on the current convective instability is investigated for conditions typical of the high‐latitude F region during the diffuse aurora. It is found that magnetic shear (1) reduces the growth rate of the instability (although it does not stabilize the mode) and (2) substantially localizes the mode structure parallel to the density gradient.
Global observations of the composition and dynamics of the ionosphere of Venus: Implications for the solar wind interaction
Authors: Taylor H. A. , Brinton H. C. , Bauer S. J. , Hartle R. E. , Cloutier P. A. , Daniell R. E.
Published: February 2008

The first in situ measurements of the details of the global composition and dynamics of the ionosphere of Venus have been obtained from the Bennett ion mass spectrometer on the Pioneer Venus orbiter during the period December 1978 through August 1979. These results include observations of three related plasma regimes, including (1) the bowshock‐ionosheath region, (2) the thermal ionosphere, and (3) a superthermal flowing ion layer interfacing with the ionosphere at the ionopause and extending outward to variable heights above the planet. During quiet periods an abundant ionosphere is observed both on the dayside and on the nightside, generally dominated by O+ above 200 km (except for a predawn region where H+ exceeds O+) and by O2+ down to typical periapsis heights of about 160 km. A sampling of some of the less disturbed data exhibits strong day to night variations in the distributions of the most prominent ions, including O+, O2+, CO2+, C+, N+, CO+(N2+), NO+, H+, He+, O18+, O++, and H2+. Important features of the day‐night variation measured at 200 km and at a fixed latitude of about 8°N include asymmetric nightside bulges in the distributions of the light ions H+ and He+, peaking near dawn at 110° and 90° solar zenith angle, respectively. These asymmetries are associated with dawn‐dusk asymmetries in the distributions of O+, O2+, and other molecular ions, which exist in higher concentrations near dusk, relative to dawn. Associated observations of the bowshock‐ionosheath and the ionosphere‐superthermal plasma layer regions indicate close coupling among these plasma regimes. The ionopause, identified as the boundary between the thermal ionosphere and the superthermal flow layer, is encountered near 250–400 km near the subsolar point and extends at times to heights greater than 1000 km in the flanks and nightside regions. Under disturbed nightside conditions, particularly noticeable in the dusk region, the ionosphere may exhibit randomly spaced concentration gradients of an order of magnitude associated with complex patterns of ion flow with velocities up to 10 km/s encountered within the main body of the ionosphere.
Cloud morphology and motions from Pioneer Venus images
Authors: Rossow William B. , Del Genio Anthony D. , Limaye Sanjay S. , Travis Larry D. , Stone Peter H.
Published: February 2008

The orbiter cloud photopolarimeter on the Pioneer Venus spacecraft obtained over 300 images of Venus in the ultraviolet during a 3‐month period of extensive observations from January through March 1979. The planetary scale and small‐scale markings in the images provide information on the horizontal and vertical cloud structure, atmospheric waves, and wind speeds at the cloud top level. The images show evidence of long‐term evolution of cloud properties and the atmospheric dynamics in addition to rapid small‐scale changes in the cloud morphology. The polar regions are substantially brighter than at the time of Mariner 10 imaging. The planetary scale dark Y feature, previously identified in earth‐based and Mariner 10 images, rotates about the planet with a period of 4–5 days but often changes dramatically from one appearance to the next and sometimes disappears entirely. Bow‐shaped features and cellular features occur at all longitudes but are observed more easily near and downwind from the subsolar point. All of the cloud features exhibit a globally coordinated oscillation of their orientation relative to latitude circles. Retrograde zonal winds of ∼100 m s−1 near the equator, similar to wind speeds observed from Mariner 10, are inferred from tracking of small‐scale cloud features, but the two hemispheres exhibit significant differences. In the southern hemisphere the zonal wind speed decreases toward the poles at a rate similar to that for solid body rotation; the mid‐latitude jet stream observed by Mariner 10 is not present. In the northern hemisphere, two jet streams are present with the mid‐latitude jet stream weaker than that of the single northern hemisphere jet stream observed by Mariner 10. Meridional wind velocities are an order of magnitude weaker than the zonal velocities and are poleward in both hemispheres. The meridional wind speeds increase with increasing latitude up to mid‐latitudes, with those in the northern hemisphere attaining higher values.
Net radiation in the atmosphere of Venus: Measurements and interpretation
Authors: Suomi V. E. , Sromovsky L. A. , Revercomb H. E.
Published: February 2008

Net flux radiometer measurements in the atmosphere of Venus question the effectiveness of the greenhouse mechanism in maintaining the high surface temperature. To search for possible instrumental explanations, new spare instrument tests were conducted. These tests uncovered an error mechanism that is defined well enough in the upper atmosphere to make approximate corrections for the large transient errors initiated at the time of instrument deployment. No direct evidence for significant errors in the lower atmosphere was found. The observed net radiative energy loss near the surface is found not to violate the assumption of a steady state atmosphere. Comparisons of the corrected flux profiles with radiative transfer calculations support the following conclusions: a new source of infrared opacity in the upper cloud is required, possibly a submicron haze; the infrared opacity of the lower cloud is too small for the cloud particles to be concentrated sulfuric acid; the mean water vapor mixing ratio is of the order of several tenths of a per cent immediately below the clouds and of the order of a few parts per million near the surface; water vapor amounts vary greatly from probe to probe, by nearly two orders of magnitude at some levels; there are thin layers of opacity in the subadiabatic layer between 35 and 48 km; and, finally, the north probe flux profile depression near 60 km may be partly a result of temperature inversions immediately above.
Venus lower atmosphere heat balance
Authors: Ingersoll Andrew P. , Pechmann Judith B.
Published: February 2008

Pioneer Venus observations of temperatures and radiative fluxes are examined in an attempt to understand the thermal balance of the lower atmosphere. If all observations are correct and the probe sites are typical of the planet, the second law of thermodynamics requires that the bulk of the lower atmosphere heating must come from a source other than direct sunlight or a thermally driven atmospheric circulation. Neither the so‐called greenhouse models nor the mechanical heating models are consistent with this interpretation of the observations. One possible interpretation is that two out of the three probe sites are atypical of the planet. Additional lower atmosphere heat sources provide another possible interpretation. These include a planetary heat flux that is 250 times the earth's, a secular cooling of the atmosphere, and a chemically energetic rain carrying solar energy from the clouds to the surface. Other data make these interpretations seem unlikely, so measurement error remains a serious possibility.
Interaction between ELF‐VLF emissions and magnetic pulsations: Classification of quasi‐periodic ELF‐VLF emissions based on frequency‐time spectra
Authors: Sato Natsuo , Fukunishi Hiroshi
Published: February 2008

Frequency‐time (f‐t) spectra of quasi‐periodic (QP) ELF‐VLF emissions in the period range 5–200 s are examined by using a few hundred QP events observed at high latitudes. These f‐t spectra are classified into five types: nondispersive, rising tone, falling tone, mixed, and burst. Statistical characteristics of QP emissions in each type are examined by using f‐t spectra of ELF‐VLF emissions recorded continuously for 50 days in Iceland. A phenomenological model to explain the f‐t spectra is proposed; that is, the rising tone type of QP emission is generated by compressional mode Pc 3 magnetic pulsations which propagate in a radial direction toward the earth, while the nondispersive type is generated by standing oscillations of the magnetic fields which have effective compressional components.
A ground‐satellite study of wave‐particle correlations
Authors: Park C. G. , Lin C. S. , Parks G. K.
Published: February 2008

Very low frequency lpar;VLF) waves recorded at Siple, Antarctica (L ∼4; 84°W, geographic), are compared with low‐energy (<50 keV) electron data from the geostationary satellite ATS 6. During the period of this study (August 10–21, 1974) the satellite was anchored at 6.6 RE and 106.5°W, near the outer edge of the nominal ‘viewing area’ of the Siple VLF receiver, which is estimated to be L ∼2–6 and ±20° longitude. The results reveal two distinct types of chorus observed at Siple. One type is closely correlated with enhanced fluxes of >5‐keV electrons detected at the synchronous altitude, and its generation region is inferred to be outside the plasmapause. The chorus upper cutoff frequency increases with time in a characteristic manner, consistent with the expected adiabatic motion of injected electrons in cyclotron resonance with the waves. The second type of chorus, which we refer to as ‘plasmaspheric chorus,’ occurs inside the plasmapause, has no apparent relationship to particle injection at the synchronous altitude and shows clear evidence of being triggered by whistlers, power line radiation, and other signals. The two different types of chorus are readily distinguishable in frequency‐time spectrograms.
The airglow continuum at high latitudes—An estimate of the NO concentration
Authors: Witt G. , Rose J. , Llewellyn E. J.
Published: February 2008

A high‐latitude (68°N) rocket measurement of the NO2 continuum at 540 nm yielded an overhead intensity of 1.14 R/Å. The volume emission profile has been used with O densities, derived from measurements of the oxygen emissions made from the same rocket, to determine the NO profile for an air afterglow, which is excited via both two‐body and three‐body recombination paths. It is concluded that the NO concentration was 8 × 108 cm−3, with a factor 2 uncertainty at the peak, and that this enhancement from quiet ionospheric conditions is probably due to extended particle precipitation prior to the launching.
Evening (Harang) and morning discontinuities in ionospheric electron drifts and their dependence on the interplanetary magnetic field
Authors: Scourfield M. W. J. , Nielsen E.
Published: February 2008

Observations made with Scandinavian twin auroral radar experiment (Stare) have revealed the existence of a discontinuity in the electron drift pattern at high latitudes during the morning hours. The morning discontinuity appears to be equivalent to the evening (or Harang) discontinuity but has the region of westward drifting electrons located poleward of the eastward drifting electrons. The dependence of the discontinuity occurrence times on the sign of the azimuthal component, By, of the interplanetary magnetic field has been investigated. We find the evening discontinuity generally occurs earlier for By <0 than By >0. A similar behavior was not found for the morning discontinuity, possibly due to the effects of the high level of magnetic activity (Kp ≥5−) associated with observations of this discontinuity. The average time separation between evening and morning discontinuities was 12 h 30 min. It is tentatively proposed that the morning discontinuity is the footprint of the polar cleft in the ionosphere. This would imply that the cleft, during very disturbed magnetic conditions, can extend down to ∼65° geomagnetic latitude.
On the role of external and internal source in generating energy and pitch angle distributions of inner‐zone protons
Authors: Jentsch V.
Published: February 2008

We separate the steady‐state distribution function f of energetic inner‐zone protons into two parts: f=f1 + f2, where (1) f1 is a solution of the homogeneous radial diffusion equation, associated with the external (diffusive) source, and (2) f2 is a solution of the inhomogeneous diffusion equation, related to the internal (Crand)source. By making special assumptions about the energy dependence of source and diffusion processes, we are able to evaluate analytically f as a function of energy E, pitch angle α, and dipole shell parameter L. The analysis shows that the external source preferentially supplies protons of energies ≲30 MeV that mirror close to the equator, while the internal source is most important for protons having energies ≳50 MeV or pitch angles ≲50°. Agreement between measured and computed energy spectra and pitch angle distributions for L values from L= 1.2 to L1.7 requires diffusion coefficients DLL(e) ≈ 2 × 10−11 L5 E−1 s−1 or DLL(m) ≈ 3.75 × 10−12 L9 E−1 sin2.7 α s−1 (E in MeV), caused by electrostatic and magnetic fluctuations, respectively.