bluuebunny/crossref_metadata_2025 huggingface dataset:
Kelvin‐Helmholtz Instability in the magnetopause‐boundary layer region
Authors: Lee L. C. , Albano R. K. , Kan J. R.
Published: February 2008

The Kelvin‐Helmholtz instability in the magnetopause‐boundary layer region is studied on the basis of an idealized model which consists of three uniform plasma regions: the magnetosheath, the boundary layer, and the magnetosphere. There are two unstable modes in the magnetopause‐boundary layer region: one is excited at the magnetopause (the magnetopause mode) and the other is excited at the inner surface of the boundary layer (the inner mode). The inner mode is found to be unstable most of the time, while the excitation of the magnetopause mode depends on the magnetic field in the magnetosheath. The observed variation of the boundary layer thickness can be attributed to the unstable inner mode. Possible relationships between the Pc 3–5 geomagnetic pulsations and the surface waves excited on the magnetospheric boundary are also discussed.
Empirical models of height integrated conductivities
Authors: Wallis D. D. , Budzinski E. E.
Published: February 2008

Two‐dimensional distributions of the height‐integrated Pedersen and Hall conductivities have been computed for latitudes poleward of 60° invariant representative of two activity levels: 0 ≤Kp ≤30 and 30 22 keV obtained by the energetic particle detector of Isis 2 during 1971–1974 are used as input to a Rees‐type computation. The assumption of equilibrium conditions and a recombination rate profile permit calculation of electron density profiles and conductivity profiles. Calculations are performed at 300 grid points, specifically 12 local times and 25 latitudes from 60° to 84° invariant latitude. The models include ionization due to galactic EUV and other background sources that produces base conductivities (ΣP ∼0.1, ΣH ∼0.2 mhos) as well as solar photon ionization through an empirical fit to Chatanika radar observations [Mehta, 1979]. Substantial modulation of the conductivities is found to result from longitudinal variation of the magnitude of the earth's magnetic field. The results show considerable spatial gradients and are expected to be of use in numerical modeling.
Time evolution and dynamics of equatorial backscatter plumes 1. Growth phase
Authors: Tsunoda Roland T.
Published: February 2008

Much of our understanding of equatorial spread‐F phenomena has resulted from VHF radar studies of backscatter ‘plumes’ i.e., patterns of backscatter that appear to extend from the bottomside into the topside of the F layer. By using ALTAIR, a fully steerable backscatter radar, to map selected plumes repeatedly, it was found that the dynamic behavior of plumes can be characterized by a growth and a decay phase. The growth‐phase characteristics described in this paper are found to be generally consistent with equatorial irregularity generation as predicted by the Rayleigh‐Taylor and gradient‐drift instabilities. In particular, (1) plumes are generated primarily in the local time around F layer sunset, (2) plumes develop from local perturbations in plasma density in the bottomside of the F layer, (3) plumes develop upward with growth velocities ranging from 125 to 350 m/s, and (4) backscatter strength increases during upward plume development and decreases when upward plume growth slows or ceases. However, certain characteristics, namely, an east‐west asymmetry in backscatter strength found in the bottomside F layer, have been found that suggest key roles played by the eastward neutral wind and by altitude modulation of the bottomside F layer in establishing the initial conditions for plume growth.
Solar ultraviolet irradiance at 40 kilometers in the stratosphere
Authors: Hall L. A.
Published: February 2008

The solar spectrum in the range 2000–3100 Å has been observed in 0.12‐Å resolution in the stratosphere by means of a balloon‐borne spectrometer. Two flights are reported, on April 21, 1977, and April 19, 1978, and the data from the two flights agree within 4% in the spectral regions of most accurate measurement. Absolute irradiance values at 40‐km altitude are presented. By extrapolation to zero optical depth, comparisons are made to two other recent measurements.
Multiple Alfven wave reflections excited by Io: Origin of the Jovian decametric arcs
Authors: Gurnett D. A. , Goertz C. K.
Published: February 2008

The recent Voyager radio astronomy measurements near Jupiter show that the Jovian decametric radiation consists of numerous discrete features called decametric arcs which are observed at all Jovian longitudes. It is generally believed that these arcs are produced by an interaction of Io with the Jovian magnetosphere. In this paper we propose that the large number of decametric arcs is caused by multiple reflections of a standing Alfven wave current system excited by Io. Estimates of the reflection coefficient at the ionosphere and other damping processes show that a large number of reflections can occur, with the Alfven wave current system possibly extending completely around Jupiter. This source geometry can account for a number of otherwise puzzling characteristics of the Jovian decametric radiation.
On the problems of the interaction between Pc 1/Pi 1 and Pc 4, 5 hydromagnetic waves
Authors: Gokhberg M. B. , Pilipenko V. A. , Pokhotelov O. A. , Troitskaya V. A.
Published: February 2008

We examine the influence of high‐frequency hydromagnetic noise in the range Pc 1 to Pi 1 on the stability of low‐frequency Alfvén and fast compressional waves in an inhomogeneous plasma imbedded in a curved magnetic field. Turbulent diffusion of hot protons caused by Pc 1/Pi 1 noise results in the appearance of an effective “collisional” viscosity, which in turn may induce the growth of drift instabilities, but only if the wave frequency is less than Larmor drift frequency. Using ATS 6 satellite data, we obtained some experimental evidence in support of the proposed plasma instability mechanism.
Equatorial counterelectrojet: Two distinct categories
Authors: Kane R. P.
Published: February 2008

It is shown that among the equatorial counterelectrojets there are two distinct categories, the fast one and the slow one, having different characteristics.

In a recent communication [Kane, 1978] it was shown that there is no clear‐cut relationship between equatorial counterelectrojet and interplanetary magnetic field (IMF). In the communications Rastogi and Patel [1975] and Patel [1978], sharp decreases in the equatorial H field during daytime (also termed as counterelectrojets by these authors) are shown to be associated with rapid fluctuations of the latitude θ of the IMF. From private conversations and correspondence, it seems that many people consider the latter results as contradiction of the former one. Contradictory opinions in scientific matters are neither uncommon nor unwelcome, as these aid the free spirit of scientific enquiry. However, in the present case, no contradiction whatsoever is involved, as is explained below. There are two distinct categories of equatorial counterelectrojets, and their characteristics are described in the sections that follow.
A statistical study on characteristics of high latitude Pi 3 pulsations
Authors: Suzuki A. , Nagano H. , Kim J. S. , Sugiura M.
Published: February 2008

Using magnetic data from the high‐latitude North American IMS network, Pi 3 pulsations having periods 6–25 min are analyzed. The latitude and magnetic local time distributions of the sense of polarization and the position of maximum intensity for Pi 3 pulsations are studied. These characteristics are compared with those of other pulsations such as Pc 5 and Pi 2. The following characteristics are found: (1) In the Xm‐Ym plane in dipole coordinates, regions of right‐handed (R) and left‐handed (L) polarizations are clearly separated during daytime; in the morning the R region is at higher latitudes than the L region, and in the afternoon the relative positions are reversed. The dividing line of the two regions is approximately at 70° dipole latitude at dawn and dusk and is about 75° latitude near noon. Between 2100 and 0500 MLT there are small R or L regions and a large mixed region. (2) For the polarization in the Xm‐Z plane the distribution is rather simple; namely it shows predominantly L polarization on the dayside and R polarization on the nightside, where the sense of polarization is defined by viewing the Xm‐Z plane from its west side. (3) The gross daytime distributions of the Pi 3 pulsations are similar to those for Pc 5, while the nightside distributions resemble those for Pi 2. (4) The maximum amplitude of Pi 3 occurs near the auroral electrojet region around midnight, while it is at much higher latitudes during late morning hours. For each of the three components, Xm, Ym, and Z, the locus of maximum amplitude forms an oval shape, but the oval does not coincide with Feldstein's auroral oval. On the basis of the morphological characteristics of Pi 3 pulsations found by the present analysis, we believe that dayside Pi 3 pulsations have a different generation mechanism from that for nightside Pi 3 events. The Kelvin‐Helmholtz instability on the magnetopause is likely to be the primary excitation source for the daytime Pi 3 events. For the cause of the nightside Pi 3 pulsations, oscillations of a magnetospheric‐ionospheric current system appear to be a more probable source.
Bursts of MeV Jovian protons observed in interplanetary space
Authors: Pesses M. E. , Tsurutani B. T. , Van Allen J. A. , Smith E. J.
Published: February 2008

Bursts of protons (0.6 <EP <3.4 MeV) were observed sunward of Jupiter's bow shock out to 200 RJ radial distance by the University of Iowa detector G on Pioneer 11. These observations supplement earlier reports by the University of Chicago and Goddard Space Flight Center groups and strengthen their suggestion of a Jovian origin by virtue of our findings that the protons exhibit a very anisotropic, field‐aligned pitch angle distribution directed away from the planet and that the bursts are not associated with interplanetary disturbances. Power spectra of the proton counting rates show no significant power at Jupiter's rotational period.
Pancake pitch angle distributions in warm ions observed with ISEE 1
Authors: Horwitz J. L. , Baugher C. R. , Chappell C. R. , Shelley E. G. , Young D. T.
Published: February 2008

Observations of pancake (peak flux near 90° pitch angle) distributions of low‐energy (≲100 eV) ions are reported. Pancake distributions occur often in H+ and He+ simultaneously while O+ fluxes are either undetectable or field‐aligned. These H+ and He+ pancake distributions typically display characteristic energies of the order of 10 eV and are frequently mixed with higher density, colder (κT≲3 eV), isotropic, quasi‐Maxwellian components. They appear often within the outer regions of the plasmasphere, and seem to occur most frequently on the dayside and near the magnetic equator.
A magnetospheric signature of some F layer positive storms
Authors: Miller N. J. , Mayr H. G. , Grebowsky J. M. , Harris I. , Tulunay Y. K.
Published: February 2008

Calculations using a self‐consistent model of the global thermosphere‐ionosphere system perturbed by high‐latitude thermospheric heating show that the resultant electron density disturbances within the mid‐latitude F layer can propagate upward along magnetic field lines to the equator. The F layer disturbances described by the model calculations correspond to the evolution of enhancements or reductions in electron density that is called the positive or negative phase of an F layer storm. We deduce that the positive phase of dayside F layer storms is initiated when high‐latitude thermospheric heating generates equatorward winds. These winds raise the mid‐latitude F layer along the geomagnetic field
through momentum transfer from neutral atoms to F layer ions that pull electrons with them. For L ∼3 or less the upward movement of ionospheric plasma results in ionization increases at all altitudes along
from the F2 maximum to the equator. An increase in the average magnitude of the equatorial dawn‐dusk magnetospheric electric field retards the dayside development of a positive storm phase by drifting plasma away from mid‐latitude field lines along which the electron density is increasing. During an F layer storm in June 1972, instruments on Explorer 45 and Ariel 4 detected dayside electron density enhancements simultaneously at 550 km over mid‐latitudes and near the equatorial plane in the magnetosphere. These in situ measurements support the model prediction that disturbances in the magnetospheric plasma near the equator can arise through interactions occuring at lower altitudes along a magnetic field line. Our study demonstrates that some storm time enhancements of dayside magnetospheric plasma near L ∼2–3 may be signatures of the positive phase of an F layer storm.
The auroral electron precipitation during extremely quiet geomagnetic conditions
Authors: Meng Ching‐I.
Published: February 2008

The electron precipitation over the polar regions during extremely quiet geomagnetic conditions is examined based on data from 5 years of DMSP observations. A total of 12 periods were selected for this study on the basis of prolonged, extremely low values of the Kp index which persisted for at least 2 consecutive days. The interesting electron precipitation features can be summarized as follows: (1) At all times, precipitation indeed occurred over both the northern and southern polar regions with significant intensity. The precipitating intensities were 1–2 orders of magnitude below the level for nominal, quiet (Kp ≲2) auroral oval precipitations. (2) The measured precipitating electrons were very soft, most having energies below 1 keV. The observed fluxes of low‐energy electrons between 50 eV to a few hundred eV were often merely the high‐energy tail of an extremely soft precipitation. (3) Electron precipitation with a most probable energy of a few keV (harder than that of the auroral oval) can form a band detached from and equatorward of the morning auroral oval. (4) A dramatic feature of the electron precipitation of the extremely quiet magnetosphere is the unexpectedly wide latitudinal extent of the low‐energy electron precipitation. It spreads into the high‐latitude polar regions to at least ∼85° geomagnetic latitude.
Generation of Langmuir waves by nonlinear wave‐wave interaction in the ionosphere
Authors: Cerisier J. C. , Lavergnat J. , Rihouey J. J. , Pellat R.
Published: February 2008

The nonlinear interaction between two parallel high‐frequency electromagnetic waves can generate either an electron plasma wave or an extraordinary wave, depending upon the angle of propagation with the external magnetic field (either θ ≃0 or θ≠0). When propagating in the inhomogeneous ionosphere the extraordinary mode couples to the plasma wave and the results obtained in both cases turn out to be identical.

The plasma wave can be detected from the ground through the enhancement of the amplitude of the plasma lines associated with the scattered spectrum of a third electromagnetic probing wave.

The amplitude of the electrostatic plasma wave, and of the scattered plasma lines, are calculated. The rendezvous conditions (in time and in space) between the plasma wave pulse and the probing pulse are evaluated and discussed for the Eiscat sounder.
Measurements of ionospheric electric field convection by the long‐line technique
Authors: MacDougall J. W.
Published: February 2008

Ionospheric E × B convections are measured by a new technique that uses satellite scintillations. The measurements are for an ionospheric region centred on 39°N, 82°W geographic or 53° invariant latitude. Results are presented for spring equinox 1980. The quiet condition eastward convection drift is approximately ViE=−12 + 43 cos (2π/24)(t ‐ 13.4). During disturbed conditions at nighttime the westward convection becomes large and highly variable. The northward perpendicular E × B convection is approximately ViN=−5 + 13 cos (2π/12)(t ‐ 10.5) + 6 cos (2π/24)(t ‐ 11.5) m/s during quiet conditions. During disturbed conditions the semidiurnal component of the northward convection increases by about a factor of 3 and becomes larger than the diurnal.
Factors controlling the occurrence of Pc 3 magnetic pulsations at synchronous orbit
Authors: Takahashi K. , McPherron R. L. , Greenstadt E. W. , Neeley Carlene Arthur
Published: February 2008

The dependence of the occurrence probability of Pc 3 pulsations (frequency of 0.022–0.100 Hz) at synchronous orbit on factors such as local time and solar wind parameters is studied. Power spectra calculated from the magnetic field observed at the ATS 6 satellite for a 1‐year period from June 1974 to May 1975 were examined to identify Pc 3 pulsation occurrence. Pc 3 pulsations are usually observed in the dayside magnetosphere. The average diurnal occurrence pattern is a maximum at 0900 LT, with the pattern shifting to earlier local times in winter and to later local times in summer. Solar wind velocity VSW and the direction of the interplanetary magnetic field with respect to the sun‐earth line, θXB, significantly affect the occurrence probability. Large VSW and small θXB are favorable for the occurrence of Pc 3 pulsations. The average occurrence probability over 0600–1600 LT changes from 10% for VSW ≲400 km/s and θXB ≳60° to 80% for VSW ≳600 km/s and θXB ≲20°. The result suggests that the solar wind controls Pc 3 occurrence. However, the study of the occurrence probability alone is not sufficient to distinguish between the most likely sources, that is, the magnetosheath turbulence and the Kelvin‐Helmholtz instability at the magnetopause.
Pc5 pulsations observed during the June 1972 geomagnetic storm
Authors: Engebretson Mark J. , Cahill Laurence J.
Published: February 2008

We report observations of large‐amplitude Pc5 magnetic pulsation events on three successive orbits of the Explorer 45 satellite near L=5 in the afternoon quadrant during the June 1972 geomagnetic storm. In each case, waves occur during a period of depressed magnetic field coinciding with a plasma injection event. The resulting waves have a significant compressional component and appear to be quite localized in longitude. Polarization analysis indicates that in two cases the satellite appears to have crossed the resonant field line of the observed waves. Variations in electron and proton energy density, where available, correlate well with the observed magnetic variations. The observations lend credence to earlier generalizations that storm time Pc5 pulsations are generated by newly injected hot plasma via local plasma instabilities and manifest themselves as coupled hydromagnetic waves with compressional and field line resonant transverse components.
Height and latitude structure of electric fields and currents due to local east‐west winds in the equatorial electrojet
Authors: Reddy C. A. , Devasia C. V.
Published: February 2008

Using the equivalent electrical circuit method for the theoretical treatment of the local wind effects on the equatorial electrojet, the height structures of electric fields and currents that are generated by height‐varying east‐west winds in the electrojet region have been calculated for theoretical model wind sturctures and for a variety of experimentally observed wind structures. The results bring out clearly the nature and extent of the local wind effects on the height and latitude structures of the equatorial electrojet.

The more important conclusions of the present study are: (a) The vertical wind shears associated with tidal winds and gravity waves can generate significantly large electric fields and east‐west currents in the equatorial electrojet (EEJ). (b) The wind‐generated electric fields and currents are characterized by large height gradients, latitudinal gradients, and reversals of direction. (c) At the magnetic equator, wind‐generated electric fields and currents are often small (10%–30%) in comparison with the eastward electric‐field‐generated polarization field and east‐west currents around noon hours. (d) At geomagnetic latitudes of 3° and beyond the wind‐generated currents and electric fields often exceed those caused by the eastward electric field. (e) Beyond 2° the neutral winds can drastically alter the intensity and latitudinal structure of the EEJ, depending upon the magnitude and height location of the wind shear. (f) The observed variabilities in the ‘width’ and latitude structure of the EEJ can arise from the effects of such east‐west winds that have been observed experimentally.
Stabilization of electrostatic, purely growing, finite ion gyroradius, flute instabilities in the ionosphere
Authors: Sperling J. L. , Krall N. A.
Published: February 2008

The ambient background plasma is shown to have an important stabilizing influence on the generation of electrostatic, purely growing, finite ion gyroradius, flute instabilities driven by ion loss cone distribution functions associated with shaped barium injection across the ambient magnetic field in the ionosphere. This stabilization implies that such short scale size instabilities and the striations they produce will only be present if the background plasma density is low enough. Consistent with the experimental observations of the striations, it is shown that for the parameters of the Buaro barium release experiment the background plasma density is insufficient to completely stabilize the finite ion gyroradius modes; however, background ion density does reduce the growth rate from the level predicted with a theory which does not include the background plasma dynamics.
Ionospheric scintillation observations at Natal
Authors: Yeh K. C. , Mullen J. P. , Medeiros J. R. , da Silva R. F. , Medeiros R. T.
Published: February 2008

We report ionospheric scintillation observations made at Natal, Brazil, during a period of 2½ years. The nighttime scintillations of satellite signals at 250 MHz commonly exceeded 20 dB peak‐to‐peak. Seasonally, it showed a typical behavior for the South American sector with a single peak centered around October–November months and the local evening hours of 2100–2300 LMT. This seasonal behavior was different from that of the African sector and very different from that of the Pacific sector. On a spacing of 278 m the eastward irregularity drift speed was measured by the method of similar fades. This, together with the determination of Fresnel frequency from the power spectrum analysis, was used to calculate the height of the irregularity patch. A value of 294 km was obtained. Multisatellite scintillation observations indicated that even though the initiation of an irregularity cloud commonly occurred earlier on signals transmitted by the eastern satellite, there were substantial cases when the reverse was observed. One case which was analyzed in detail suggested that the irregularity clouds formed independently along two radio paths separated by 520 km.
Generation of kilometer scale irregularities during the midnight collapse at Arecibo
Authors: Basu Sunanda , Basu Santimay , Ganguly S. , Klobuchar J. A.
Published: February 2008

Coordinated observations of the incoherent scatter radar at Arecibo, Puerto Rico, and total electron content/scintillation measurements made by using the 137‐MHz transmissions from the geostationary satellite SMS 1 from the nearby station of Ramey are studied to determine the background ionospheric conditions necessary for the generation of kilometer scale irregularities in the midlatitude ionosphere. It is found that the well‐known midnight descent or collapse of the F region known to occur in this part of the world is sometimes associated with increases in the bottomside electron content, as well as large scintillation events (∼10 dB). From a measurement of vector ion velocities and electron densities during such events, it is determined that the scintillations occur in a region of eastward and northward electric fields coupled with a northward directed density gradient caused by the northward propagation of the midnight collapse. The role of possible plasma instability mechanisms, specifically the E × B gradient drift instability and the Perkins instability, is discussed in the formation of the kilometer scale irregularities giving rise to VHF scintillations.
Studies of Jupiter's lower ionospheric layers
Authors: Hsin Chen Robert
Published: February 2008

Theoretical calculations are made of the fluxes of heavy ions necessary to form the layers of enhanced ionization observed in the lower ionosphere of Jupiter. The results show that if the layers are formed from sodium or sulfur ions of Galilean satellite origin injected into the Jovian atmosphere then, in order to match the Pioneer 10 observations of the L6 layer, the Na+ flux must be about 3 × 104 cm−2 s−1 and the S+ flux must be about 4 × 10³ cm−2 s−1. From the shape of the ionization layers estimates of ion drift velocities and neutral wind speeds may be made. The vertical ion drift velocities range from a few centimeters per second to meters per second while the zonal wind shear ranges from 50 m/s westward to 200 m/s eastward over a 70 km altitude range and small (centimeters per second) meridional winds are sufficient to form the layers.
Ring current impoundment of the Io plasma torus
Authors: Siscoe G. L. , Eviatar A. , Thorne R. M. , Richardson J. D. , Bagenal F. , Sullivan J. D.
Published: February 2008

A newly discovered feature in the Io plasma formation that may be described as a ramp separating a high‐density plasma ledge on its Jupiterward side from the lower‐density radially distended Io plasma disc on its anti‐Jupiterward side is observed to coincide with a marked inward decrease in the ring current population. The spatial congruency of the counter‐directed maximal gradients in both plasma bodies reveals a profound coupling between them. The existence of the ramp requires a local order‐of‐magnitude reduction in the diffusion coefficient that governs radial mass transport. We demonstrate that the diminished diffusive efficiency there is caused by strong pressure gradient inhibition of the interchange instability that underlies mass transport. The Io plasma torus, which is defined as the region of strong ultraviolet emissions, is identified as the plasma ledge. The plasma density in the ledge is high and, incidentally therefore, able to emit strongly because it is impounded against rapid, centrifugal expulsion by the inwardly directed pressure of the ring current at its inner edge.
High resolution albedo measurements on Io from Voyager 1
Authors: Clancy R. Todd , Danielson G. Edward
Published: February 2008

The photometric properties of the surface of Io were investigated at high spatial resolution by a choice of 220 sample regions from the four‐color, 8 km/line pair resolution photomosaic of Io taken by Voyager 1. The mosaic longitudinal coverage extends from ∼200°W to 350°W (phase angle ∼10.5°). The regions were categorized on the basis of their visual color in the color print. Categories include: white, yellow, orange, red, brown (polar), and black regions. The photometrically corrected data were plotted as a function of intensity versus photometric angles for each of the color regions in all four filters (orange, blue, violet, UV) using a Minnaert function. The plots of these color regions show large scatter about the least squares fitted lines. The large scatter, particularly for the darker regions, indicates a continuous distribution of albedos on Io and gives evidence of compositional mixing. In all cases, limb‐darkening coefficients with useful error bounds (e.g., error < 0.2) are found only for the white (k = 0.6±0.1) and brown (k=0.8±0.1) regions. The larger coefficient found for the brown regions is biased upward due to polar darkening. Computed values of the limb darkening do not change significantly among filters. Color ratio plots of the reflectances for each of the regions were constructed (UV/blue and violet/blue versus orange/blue). The distributions of ratios of the various color regions are compared to laboratory measurements of solid SO2 (Nash et al., 1980) and various allotropes of sulfur (from J. Veverka and J. Gradie, unpublished data, 1979). These comparisons indicate that SO2 is the major component (photometrically) of the ‘white’ regions. Red and white sulfur are seen as variable components of these regions. Color ratios of ‘brown’ regions suggest a mixture of white and red sulfur. A significant portion of the ‘brown’ regions have color ratios near those of pure white sulfur. A component of SO2 is found for all ‘brown’ regions and appears to be latitude dependent. Red regions appear well described by a mixture of red and orange sulfur.
Enhanced cosmic ray anisotropies and the extended solar magnetic field
Authors: Swinson Derek B. , Saito Takao , Mori Satoru
Published: February 2008

Saito's two‐hemisphere model for the three‐dimensional magnetic structure of the inner heliomagnetosphere is used to determine the orientation of the two solar magnetic hemispheres. This orientation, as viewed from the earth, varies throughout the year. The orientations during 1974 are presented and are confirmed by satellite data for the interplanetary magnetic field. These data suggest a role for the field component perpendicular to the ecliptic plane Bz in giving rise to cosmic ray anisotropies detected at the earth. It is shown that an enhanced solar diurnal variation in cosmic ray intensity at the earth can arise from the constructive interference of three cosmic ray anisotropies, two of which depend on the direction of the interplanetary magnetic field. This is demonstrated by using cosmic ray data from the Nagoya muon telescope and underground muon telescopes in Bolivia, Embudo (New Mexico), and Socorro (New Mexico).
Stabilization of the electrostatic Kelvin‐Helmholtz instability in high β plasmas
Authors: Huba J. D.
Published: February 2008

A fully self‐consistent theory of the electrostatic Kelvin‐Helmholtz instability in finite β plasmas is presented. The instability is driven by a parallel velocity flow which is sheared transverse to the magnetic field. The important features of our analysis are as follows: Vlasov theory is used so that finite Larmor radius effects and wave‐particle resonances are properly treated; the complete coupling of electrostatic and electromagnetic oscillations is considered (i.e., both the transverse and compressional magnetic fluctuations); and the ▽B orbit modifications (i.e., both resonant and nonresonant) are treated self‐consistently. The primary result is that the electrostatic Kelvin‐Helmholtz instability is stable in high β plasmas. The actual value of β for stabilization depends upon the parameters. The stabilization is mainly attributed to resonant and nonresonant ▽B effects. These results are applied to several space plasmas (i.e., polar cusp, cometary tails, magnetopause) and estimates of the anomalous diffusion coefficient associated with this instability are presented.
On the maintenance of the Venus nightside ionosphere: Electron precipitation and plasma transport
Authors: Spenner K. , Knudsen W. C. , Whitten R. C. , Michelson P. F. , Miller K. L. , Novak V.
Published: February 2008

Suprathermal integral electron spectra between 5 and 45 eV measured by the Pioneer Venus orbiter RPA are presented for the Venus nightside ionosphere. The observed integral electron flux is relatively constant with time and altitude. The simultaneously measured plasma density is much more variable and not correlated with the electron flux. For a typical electron spectrum the ionization rates and the ion density height profiles for O+ and O2+ are calculated for 10 and 90° magnetic dip angle. The O+ and O2+ ion density height profiles are also calculated for a downward flux of O+ ions at 10, 30, and 90° magnetic dip angle. Comparison of the numerical modeling results with median profiles of O+ and O2+ ions measured by the RPA reveals that a downward flux of O+ ions of between 1 and 2 × 108 cm−2 s−1 satisfactorily reproduces both the O+ and O2+ measured median nightside density profiles. The typical suprathermal electron spectrum produces an O2+ density profile with peak density approximately half that typically observed and produces an O+ profile which is about an order of magnitude too small. From the evidence presented in this paper, from previous measurements of adequate O+ transport across the Venus terminator, and from interpretation of ion thermal measurements, we conclude that transport of O+ ions from the dayside ionosphere is responsible for most of the ionization rate required to maintain the nightside ionosphere. Variation in the O+ transport mechanism is primarily responsible for the large variation of the nightside ionosphere density. Suprathermal electrons provide a relatively constant ionization rate which is of the order of one fourth that of O+ transport and which contributes principally to the O2+ peak.
Decay of fast ion structure in the ionosphere
Authors: Goldman S. R. , Sperling J. L.
Published: February 2008

Both proton auroral and downward flowing ion events have significant fast ion components. For these phenomena we demonstrate that charge exchange and collisional ionization processes can exert a strong effect on the imaging of ionized density structure to the lower ionosphere. This has value in relating ionized density spectral measurements at altitudes of the F region maximum and below to plasma behavior at higher altitudes, as well as providing initial conditions for locally driven plasma instabilities. It is shown that charge exchange of fast ions into fast neutrals results in the rapid attenuation of structure with neutral mean free paths much larger than the structure dimensions and the generation of structure of larger size. For structure with dimensions much larger than the neutral mean free path, fast neutral loss is diffusive rather than convective and attenuation of structure is less rapid.
Dominant configurations of scintillation‐producing irregularities in the auroral zone
Authors: Fremouw E. J. , Lansinger J. M.
Published: February 2008

Recent scintillation observations have disclosed the existence of sheetlike electron density irregularities aligned along L shells in the auroral zone ionosphere. In this paper we exploit the aspect sensitivity of phase scintillation to identify the dominant three‐dimensional configuration of irregularities in four latitude/time zones: (1) equatorward of the high‐latitude scintillation boundary on the nightside of the earth, (2) poleward of the nightside boundary, (3) equatorward of the boundary on the day side, and (4) poleward of the dayside boundary. We find the sheetlike irregularities to be confined to region 2, with few exceptions. The dominant configuration in the other zones, including zone 4, is that of axially symmetric rodlike irregularities aligned along the magnetic field.
Wave‐particle transport by weak electrostatic flow shear fluctuations
Authors: Gary S. Peter , Schwartz Steven J.
Published: February 2008

This paper presents calculations of transport in a Vlasov plasma due to weak electrostatic microfluctuations driven unstable by a shear in a bulk flow. The flow velocity is parallel to a uniform magnetic field and wave propagation is in the plane perpendicular to the shear direction. A consistent procedure is used to evaluate transport of the momentum and the kinetic energy tensor due to the electrostatic flow shear instability. The transport reduces the free energy driving the instability. The wave‐particle momentum loss rate scales as the cube of the inverse shear scale length, suggesting that it may be difficult to attain an asymptotically large Reynolds number for macroscopic turbulence in a collisionless plasma.
Solar cycle induced modulation of the 55‐MeV proton fluxes at low altitudes
Authors: Parsignault Daniel R. , Holeman Ernest , Filz Robert C.
Published: February 2008

Experiments flown on U.S. Air Force Satellites from 1961 until 1976 have measured the 55‐MeV proton flux at low altitudes, between 275 km and 600 km. The analysis of these data shows that in spite of all the uncertainties involved, the agreement obtained between the theoretical calculations and the data is quite good. We conclude that the major determining factors of the 55‐MeV proton fluxes in the inner zone are a nearly constant source coupled with a solar cycle varying atmospheric ionization loss process.
A study of the long‐term variation and radial gradient of cosmic rays out to 23 AU
Authors: Webber W. R. , Lockwood J. A.
Published: February 2008

An examination of the long‐term modulation of cosmic rays with E > 60 MeV from 1972–1980 using data from the Pioneer, Voyager and IMP spacecraft has revealed that (1) the long‐term modulation effects are propagated outward radially from the sun with a typical speed of 350–500 km/s, (2) the average integral radial gradient during this period between 2 and 23 AU is (2.85±0.5)%/AU, and (3) the boundary of the heliocentric modulation region in the ecliptic plane as derived from simple conventional modulation theory is >65 AU during this time period. The implications of these results to cosmic ray modulation theories are discussed.
Solar flare shocks in interplanetary space and solar flare particle events
Authors: Evenson Paul , Meyer Peter , Yanagita Shohei
Published: February 2008

We examine data on particle events resulting from three solar flares, two of which generated interplanetary shocks, to determine the effect of the shocks on the population of protons at energies ≳ 30 MeV. In both cases, the arrival of the shock marks the start of the decay phase of the particle event, and changes in particle anisotropies are related to features in the shock structure. We conclude that each shock is a source of protons with energies ≳ 30 MeV, which propagate along the interplanetary magnetic field lines and are observed at large distances (1 AU) away from the shock. A model in which these protons are accelerated by the shocks rather than being stored in or contained behind the shocks is shown to be consistent with the observations.
Faraday polarization fluctuations of transionospheric propagation
Authors: Lee M. C.
Published: February 2008

Rapid, intense Faraday polarization fluctuations of satellite HF and VHF beacon signals have been shown to be caused by wave scattering from density irregularities. Except in the case of HF signals (e.g., 20 MHz), irregularities with Gaussian spectra cannot affect the polarization of satellite signals as effectively as irregularities with power law spectra. It is adequate to use the theory of single scattering for describing the Faraday polarization fluctuations even in cases involving large density fluctuations, such as those of 41‐MHz signals near the magnetic equator and those of 136‐MHz signals near the equatorial anomaly region.
Chatanika Radar observations of the electrostatic potential distribution of an auroral arc
Authors: de la Beaujardière O. , Vondrak R.
Published: February 2008

Observations of a premidnight auroral arc were made with high temporal and spatial resolution, using the Chatanika incoherent scatter radar. The arc was a quiet east‐west aligned arc, about 40 km wide, that drifted slowly through the radar beam without appreciably changing in shape or luminosity. The distribution of ionospheric electric fields and currents associated with the arc were computed from the ion density and line of sight velocity. The electric field was 30 mv/m outside the arc and decreased to a few mv/m within the arc. The horizontal potential drop was only ∼ 0.6 kV across the arc. By deconvolving the electron density profiles, differential energy spectra and velocity distributions were obtained within the arc. From these the accelerating potential parallel to the magnetic field lines was estimated by several techniques. One technique makes use of an empirical relationship between the parallel potential drop and the total electron current. The accelerating potential reached 7 kV at the arc center. The parallel potential drop within the arc is combined with ionospheric electric field measurements to reconstruct the equipotential contours above the arc.
A global model of the neutral thermosphere in magnetic coordinates based on OGO 6 data
Authors: Stehle Carl G. , Nisbet John S. , Bleuler Ernst
Published: February 2008

Data from the OGO 6 satellite have been analyzed in magnetic latitude and magnetic local time coordinates for various seasons and magnetic activity levels. These measurements show considerable detail, particularly in the auroral regions where the energy inputs are well organized in this coordinate system. This detail is not readily observed in models based on geographic coordinates. Atomic oxygen and helium densities and a parameter related to the molecular nitrogen density were analyzed. The long‐term averages of these quantities are presented in graphical form and as analytic functions to provide models of these thermospheric parameters. The atomic oxygen and helium densities show minima at high latitudes in the postmidnight sector for nearly all seasons and magnetic activity levels. The exospheric temperature inferred from the N2 density increases toward high latitudes for all seasons and all magnetic activity levels. This inferred temperature is about 300° K higher at the summer magnetic pole than at the winter magnetic pole. This article contains supplementary material.
Evidence for quasi‐stationary reconnection at the dayside magnetopause
Authors: Gosling J. T. , Asbridge J. R. , Bame S. J. , Feldman W. C. , Paschmann G. , Sckopke N. , Russell C. T.
Published: February 2008

Several highly unusual encounters with the earth's magnetopause occurred during an ∼5‐hour period on November 22–23, 1979, when the ISEE 1 and 2 satellites were near orbit apogee (∼22.2 RE) at ∼0900 local time. These distant magnetopause crossings corresponded to a subsolar earth‐magnetopause distance of ∼20.4 RE and were associated with a drop in solar wind dynamic pressure to a value (∼1.4×10−10 dynes cm−2) ∼100 times lower than is typical. Accelerated plasma flows whose magnitude and direction were consistent with the predictions of reconnection theory were observed on each of several satellite encounters with the magnetopause and boundary layer during this 5‐hour period. Further, the field variations through the magnetopause layer suggest that the magnetopause had the structure of a rotational discontinuity as required by reconnection theory. These observations thus indicate that on occasion reconnection at the dayside magnetopause can be a quasi‐stationary process. Geomagnetic activity during this 5‐hour period, and for at least 9 hours thereafter, was extremely low. Thus dayside reconnection is certainly not a sufficient condition for enhanced geomagnetic activity.
Extremely high latitude auroras
Authors: Gussenhoven M. S.
Published: February 2008

Over 1600 Defense Meteorological Satellite Program north polar images from the winters 1972–1973, 1974–1975 were studied for the occurrence of auroral arcs at corrected geomagnetic latitudes greater than 80°. Auroras at these latitudes are designated extremely high latitude auroras and are categorized by the local time sectors in which they occur and their relationship to discrete oval arcs. A data base of ∼300 cases of cap auroras was formed for correlation studies with geomagnetic activity indices and solar wind parameters. By far the greatest number of extremely high latitude auroras occur in the morning sector and appear to be expansions of the system of morning oval arcs poleward. The remaining categories include evening arcs expanding into the cap; single, sun‐aligned arcs; and midnight sector oval auroras expanding into the cap. Except for the last category the auroras occur for similar magnetospheric conditions: moderate values of Kp and Dst, but comparatively low values of AE. They also occur preferentially for high solar wind speed and for positive Bz. While far less clear, their location in the cap shows a dependence on By. Morning (evening) arcs occur with greater frequency for negative (positive) By.
The tailward magnetopause field beyond 10 RE
Authors: Beard David B. , Hirschi Dean , Propp Keith
Published: February 2008

The representation of the magnetopause field (i.e., the field contribution by magnetopause currents) in spherical harmonics fails at distances greater than the subsolar point distance because spherical harmonics are an inappropriate representation in the near earth tail. The truncation of the series expansion at order l = 7 or 8 in the spherical harmonics works very well near the earth (r ≤ 10 RE), but very many more terms are needed in the tail and a more appropriate, more rapidly converging expansion must be sought. For the case of the solar wind perpendicularly incident on the earth's dipole, we propose both a cylindrical harmonic expansion of eight terms or alternatively a divergence free function of r, θ, and ϕ for the r, θ, and ϕ components of the field. Both work very well. A more accurate set of spherical harmonic coefficients is also presented for r ≤ 10 RE since a fit to fields at larger r, which distorts the representation, is not included as in previous work. The total field is, of course, the field represented in this paper plus the dipole field plus the field contribution of the magnetotail currents (Beard, 1979) and to a lesser extent plus trapped plasma currents, Birkeland currents, and any other currents.
Generation of Alfvén waves by deceleration of magnetospheric convection and broadband PI pulsations
Authors: Kan J. R. , Lee L. C. , Chiu Y. T. , Longenecker D. U.
Published: February 2008

It is shown that Alfvén waves can be generated in the magnetosphere by the deceleration of magnetospheric convection due to the dynamo process in the magnetosphere‐ionosphere coupling. The growth rate of Alfvén wave packets bouncing between hemispheres is given by γ ≃ (2/µ0Σl) [αµ0²Σp²V0y² − 1] where α is a dimensionless constant (∼0.5), µ0 is the free space permeability, Σp is the height‐integrated Pedersen conductivity, l is the field line length between hemispheres, V0y is the convection speed in the magnetospheric dynamo region. Alfvén waves become unstable under disturbed conditions when both Σp and V0y are enhanced. The resulting unstable Alfvén waves can contribute to the Pi pulsations observed during substorms.
Observed magnetic substorm signatures at synchronous altitude
Authors: Nagai Tsugunobu
Published: February 2008

Magnetic field data from the geostationary satellites GOES 2 and GOES 3 are examined to study the development of substorm activity in the near‐earth nightside magnetosphere (around a radial distance of 6.6 RE). Substorm events are those in which a well‐defined single onset is seen at low latitudes on the ground. The field configuration change from more taillike to more dipolelike starts initially in a longitudinally localized region in association with the ground onset, and it develops westward and eastward, even when the simultaneous onset of the low‐latitude positive bay is recorded in a wide longitudinal region on the ground. It is also found that the variation caused by the field‐aligned current starts at the ground onset and reaches a peak at the time of the field change. The present results are consistent with the view that a substorm is associated with a disruption and subsequent conversion of the cross‐tail current to the field‐aligned current connected with the polar ionosphere. It is indicated that the cross‐tail current near synchronous orbit and its disruption are important in producing the field configuration change at synchronous orbit.
The effect of corotating interaction regions on the propagation of relativistic cosmic rays in the heliosphere
Authors: Thomas B. T. , Gall Ruth
Published: February 2008

This paper questions the validity of modulation theories that ignore the presence of large regions of enhanced magnetic field in the heliosphere. We simulate the scatter‐free and diffusive motion of relativistic cosmic rays in models of the heliospheric magnetic field which include compressed field structures representing the corotating interaction regions which are believed to be present in the interplanetary medium throughout most of the solar cycle. We find that the strong field gradients associated with these regions greatly perturb the drift pattern anticipated for simple Parkerian fields, and thus the expected streaming of cosmic rays from over the poles or along the current sheet, during consecutive solar cycles, no longer holds.