bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling the effects of an H2 gas release on the equatorial ionosphere
Authors: Anderson D. N. , Bernhardt P. A.
Published: February 2008

Release of a chemically reactive gas such as H2 into the ionospheric F region results in a depletion of the electron density. This is due to the high charge exchange rate between the dominant O+ ion and the H2 gas, with subsequent dissociative recombination of the product ions and electrons. We investigate the simulated response of the low‐latitude nighttime (dusk) ionosphere to a point release of H2 gas by solving the coupled set of time‐dependent continuity equations for O+ and the product ions OH+, H2O+, and H3O+, including the effects of production, loss, and transport of ionization. We find that a 5‐kg release at 300 km over the magnetic equator at 1930 LT produces a depleted electron density region 40 km wide 52 s after release. At the center the electron density is reduced 37% from its prerelease value. Releasing 20 kg at the same altitude produces a hole 60 km wide and a density reduction of 54%. Corresponding values for a 10‐kg release at 350 km are 65 km and a 51% reduction. We also calculate the flux tube electron content reduction and the Pedersen conductivity reduction capable of being produced by the chemical releases. A large depletion will produce a ‘bubble’ which will buoyantly rise through the ionosphere owing to a Rayleigh‐Taylor instability mechanism. For the three simulated releases of 5 kg at 300 km, 10 kg at 350 km, and 20 kg at 300 km the depletions in the electron content 276 s after release are 5.8%, 7.4%, and 11.8%, respectively. The corresponding reductions in the flux tube integrated Pedersen conductivity are 2.1%, 5.1%, and 4.3%. In addition, the H2 releases produce optical emissions at 6300 Å from O(¹D) and 3060 Å from OH(²Σ+) with intensities greater than 1 kR.
High‐altitude atomic nitrogen densities
Authors: Oran E. S. , Strobel D. F. , Mauersberger K.
Published: February 2008

Theoretical calculations of the seasonal and diurnal variations of atomic nitrogen are compared with measurements made by the open source neutral mass spectrometer on the AE‐C satellite. With the simultaneous measurements of molecular nitrogen and atomic oxygen densities as input, model calculations of odd nitrogen densities predict the same trends in atomic nitrogen as those observed. From these comparisons it is inferred that horizontal transport significantly reduces the diurnal variation of atomic nitrogen. Estimates are given of the sensitivity of atomic nitrogen densities to variations in the photoelectron flux, the neutral temperatures, and the neutral winds.
Plasma lines in the auroral E layer
Authors: Wickwar Vincent B.
Published: February 2008

The first successful measurements of plasma lines in the auroral E layer were made in January 1976 using the Chatanika incoherent scatter radar. Although they are considerably weaker than the signal from the ion component, the plasma line signals are readily detectable. Typical signal‐to‐noise ratios were about 4%. In this series of experiments, plasma lines were observed in the interval between 3.8 and 6.0 MHz, which corresponds to phase energies between 0.55 and 1.4 eV. They were found between 98 and 134 km; the derived plasma wave intensities varied between 0.06 and 1.0 eV, and the enhancements between 4 and 30 times the thermal level. There was a marked altitude dependence: the weakest waves were at the lowest altitudes, and the strongest were just below the maximum altitudes. There was no discernible frequency dependence at the lowest altitudes, but at the highest altitudes the lower‐frequency plasma waves were stronger. These variations are probably related to the altitude variations of the suprathermal electron spectrum and of the electron temperature.
Electron heating within interaction zones of simple high‐speed solar wind streams
Authors: Feldman W. C. , Asbridge J. R. , Bame S. J. , Gosling J. T. , Lemons D. S.
Published: February 2008

Electron heating within the high‐speed portions of three simple stream‐stream interaction zones has been studied in order to further our quantitative understanding of heat flux regulation in interplanetary space. The interaction zones were all characterized by enhanced values of (1) the temperature of the total electron population, Te, as well as the temperatures of the core (Tc) and halo (TH) components, (2) the total heat flux Qe, and (3) the interplanetary potential at 1 AU. The shapes of halo electron velocity distributions measured within the interaction zones were not consistent with conditions assumed for the Spitzer conductivity. Instead, temperature and density variations obeyed a polytropic relation with an index of p = 0.45 ± 0.05. If proton‐electron coupling and wave‐electron coupling are small, this relation implies a conduction law of the form Qe/(NU) ∝ Te, where N is the density and U is the bulk speed in a corotating coordinate frame. However, a conduction law of the form Qe ∝ Te fits the data better, and many other possibilities are consistent with the observations. Although a polytropic relation is simple, the microscopic physics is not. The data suggest that heat flux is not driven by a temperature gradient, since heat appears to conduct from lower to higher temperatures across the compression zones. This implausible occurrence may be possible in part because of the collisionless nature of the halo electrons which conduct most of the heat and because of the resultant macroscopic potential drop, which provides a one‐way barrier to the transport of most of the electron population.
Variations in ionospheric currents and electric fields in association with absorption spikes during the substorm expansion phase
Authors: Nielsen E. , Greenwald R. A.
Published: February 2008

Small‐scale auroral absorption events (‘spikes’) have been associated with the onset of an auroral substorm. In this paper we present detailed observations of ionospheric currents, electric fields, and aurora at the time of two spikes observed on March 1, 1977. The distribution of electric fields in the E layer around the time of occurrence of such absorption events has been investigated using the Stare auroral radar system. We find that the poleward motion of a substorm‐activated arc cannot be explained by E × B drift in the ionosphere. We furthermore find a predominantly southward electric field equatorward of the active arc, while it can be either northward or southward pointing poleward of the arc. Thus a poleward expanding arc leaves a westward current in its wake. We also detect an increase in the westward field component south of the arc. The only region where the westward field dominates is within an auroral structure which moves westward near the beginning of one of the substorms investigated. An example is shown of the reduction of the electric field over a large area including that of the riometer site, lasting for a few minutes shortly after the occurrence of an absorption spike.
Observations of counterstreaming between plasma and energetic particles in the magnetotail
Authors: Sarris E. T. , Williams D. J. , Krimigis S. M.
Published: February 2008

We present fine time resolution (10–20 s) measurements of the energy and angular distributions of three bursts of energetic particles in the magnetotail at distances of approximately 32 RE. These measurements were made near the onset of a magnetic substorm on October 16, 1973, by the NOAA/APL (50 ≤ Ep ≤ 220 keV) and JHU/APL (Ep ≥ 290 keV, Ee ≥ 220 kev) experiments on Imp 7 and the JHU/APL (210 ≤ Ep ≤ 560 keV) experiment on Imp 6. These energetic particle measurements are also correlated with plasma observations made by the University of Iowa Lepedea on Imp 7. These data reveal the presence of two distinct particle populations: (1) an earthward flowing hot plasma usually dominating the particle intensities up to an energy of ∼50 keV and (2) tailward streaming, nonthermal ions and electrons dominating the energy range 200 ≤ Ep ≤ 1 MeV. Nonthermal in this context signifies an energetic particle population bearing no direct relationship with the observed earthward flowing ambient hot magnetotail plasma. A careful examination of the angular distribution of the ions in the transition region (50 ≤ Ep ≤ 220 keV) has allowed us for the first time to separate the above two particle regimes. Although these initial results apply to a limited set of these events, inspection of the NOAA/APL energetic particle experiment data shows counterstreaming to be a common feature of magnetotail energetic particle events. We have been able to obtain, again for the first time, the energy spectrum of the nonthermal magnetospheric burst particles for energies ≥50 keV. These observations place a number of requirements on various theories which attempt to explain the dynamical processes in the magnetotail. No published theory to date seems to be in satisfactory agreement with all aspects of our observations.
The effects of fluctuation and noise on the neutron monitor diurnal anisotropy, 3. Total vector anisotropy
Authors: Owens A. J.
Published: February 2008

The day‐to‐day variations of the total vector anisotropy observed by neutron monitors are discussed. A model is developed to explain these variations in terms of counting‐statistics noise, convection in the solar wind, scattering of cosmic rays in the interplanetary magnetic field, and particle drifts. Both the amplitude and the phase variations of the Deep River neutron monitor diurnal anisotropy are shown to agree with the model's predictions. A novel prediction, confirmed by the observations, is the tendency for afternoon hour anisotropies to be larger than evening hour anisotropies.
The impact of gas‐surface reactions on mass spectrometric measurements of atomic nitrogen
Authors: Engebretson M. J. , Mauersberger K.
Published: February 2008

Recently, atomic nitrogen has been measured in the upper thermosphere with a mass spectrometer carried on the Atmosphere Explorer satellites. Only a small fraction of N atoms are directly measured by the mass spectrometer, however. The majority appear as NO, formed within the ion source. Occasionally a relatively large NO2 signal has also been observed, showing a strong dependence on the ion source surface temperature. Comparison of a numerical simulation of the reactions leading to NO2 with data obtained in circular orbits on AE‐C provides insight into the pertinent chemical reactions. It demonstrates that the techniques used in open source neutral mass spectrometer operation—warmup and outgassing of the source before measurements—were successful in minimizing the effects of this contaminant.
Occurrence of AE and Dst geomagnetic index levels and the selection of the quietest days in a year
Authors: Campbell Wallace H.
Published: February 2008

Three groups of indices were studied as indicators for quiet day geomagnetic field level determinations: one was the AE index, and the other two were the positive and negative value groups of the Dst index. A comparison of the behaviors of these three indices for 17 years showed their solar cycle changes and the differences in distribution of index levels between the times of high and low sunspot numbers. Two selections of the quietest days were made to provide at least 19 and 30 chosen days per year. These selections required the correspondence of equivalent fractional portions of the distribution of days in which all hourly values of indices were below specified levels. Lists of these days are given for the years 1958–1974. A comparison of these lists with the 5 days per month selection derived from the geomagnetic activity index Kp shows that for the same total number of quiet days per year only about one third to one half of the low‐Kp days would be quiet by AE and Dst index standards.
Observations of particles precipitating into detached arcs and patches equatorward of the auroral oval
Authors: Wallis D. D. , Burrows J. R. , Moshupi M. C. , Anger C. D. , Murphree J. S.
Published: February 2008

Particle observations made by the energetic particle detector on board Isis 2 are examined at times when the spacecraft intersects field lines mapping into regions of optical emission equatorward of the auroral oval. These emissions appear as arcs and patches and have been called detached arcs [Anger et al., 1978] and patches [Moshupi et al., 1977]. The forms are found at the feet of field lines having a trapped population of electrons 1 ≤Ee ≤10 keV as well as the usual higher‐energy radiation belt particles. Precipitation of these trapped particles at all energies up to 210 keV is observed above the optical forms. In two cases, detailed comparison confirmed that the precipitating electrons carries sufficient energy flux to account for the observed emission intensities. It is found that each of the detached arcs and all but one of the patches occur on field lines which have one end ‘sunlit’ and presumably carry photoelectrons. The geographic and local time distribution of field lines which are dark at one end and sunlit at the other is compatible with the observed distribution of detached arcs, although the role of photoelectrons in causing precipitation is not clear. The particle observations require that precipitation be caused by pitch angle scattering with scattering angle slightly larger than the loss cone. An empirical model is suggested in which a trapped, residual plasma sheet population drifting about the earth is precipitated when it enters scattering regions fixed in L‐MLT outside the plasmapause. The spatial confinement of the precipitation and its similarity to observed detached plasma regions suggest that the scattering regions have enhanced cold plasma densities. Electron cyclotron resonance is examined as the cause of the pitch angle scattering and found to be consistent with the observations.
Atmospheric neutron and gamma ray fluxes and energy spectra
Authors: Lockwood J. A. , Hsieh L. , Friling L. , Chen C. , Swartz D.
Published: February 2008

The energy spectra and fluxes of atmospheric neutrons and gamma rays from 8 to 60 MeV and from 2 to 10 MeV, respectively, have been measured with a time‐of‐flight (TOF) system using organic liquid scintillators at balloon altitudes (3–5 g/cm²) over Palestine, Texas (λ=42°N). The TOF detector consisted of two banks of six cylindrical NE213 liquid scintillators, each being 7.5 cm in diameter by 7.5 cm long, separated by 0.75 m. A TOF system was used to measure upward and downward moving neutrons and gamma rays. Pulse shape discrimination is included on each NE213 scintillator to discriminate between neutrons and gamma rays. The measured neutron leakage current is 1.9 × 10−3 from 8 ≤En <18 MeV, 1.5 × 10−3 from 18 ≤En <30 MeV, and 9.5 × 10−4 neutron/cm² s MeV from 30 ≤En ≤60 MeV. The total downward gamma ray flux from 2 to 6 MeV at 3.5 g cm−2 is given by 0.012E−1.5 photon/cm² s sr MeV. The upward moving gamma ray flux at 4 g cm−2 from 2 to 10 MeV is 0.063E−1.8 photon/cm² s sr Mev.
Radial diffusion of Io‐injected plasma
Authors: Goertz C. K. , Thomsen M. F.
Published: February 2008

We calculate the ion density observable by the plasma instrument on Pioneer 10 assuming that all ions are injected by Io within the radial range 5.8 <L <6.2 and are transported away by radial diffusion by flux tube interchange motion. To account for the observations of Frank et al. (1976), one can postulate an injection spectrum which contains a 400‐eV Maxwellian, a 5‐keV almost monoenergetic distribution, and a 13‐keV almost monoenergetic distribution. The 400‐eV contribution could be sodium or other heavy ions. The other two contributions would have to be due to some kind of acceleration mechanism. Io‐injected particles must have undergone pitch angle scattering in order to be detectable by the plasma instrument. An alternative explanation of the observations is that there is only injection by Io of the 5‐keV ions (or lower energy ions which are rapidly lost in the vicinity of Io), whereas the plasma inside and outside the Io L shell is due to an ionospheric source.
Average properties of cosmic ray diffusion in solar wind streams
Authors: Morfill G. , Richter A. K. , Scholer M.
Published: February 2008

Applying a superposed epoch analysis to the Mariner 5 plasma and magnetic field observations of 13 corotating high speed solar wind streams, we obtain the average azimuthal distribution of all relevant parameters of the background interplanetary medium, as well as those of superimposed Alfvén waves. Using these measurements in model calculations allows us to determine the radial and azimuthal variation of the background and fluctuation parameters between 1 and 5 AU, and thus to calculate the cosmic ray diffusion coefficient κ from the plasma and field properties. The calculation of κ assumes that quasi‐linear wave‐particle interaction theory is applicable, and that the Alfvén waves responsible for the scattering are propagating in the azimuthally varying solar wind according to geometrical optics. The consequences of these calculations regarding the occurrence of solar wind stream associated Forbush decreases are discussed.
Time‐varying photoelectron flux effects on spacecraft potential at geosynchronous orbit
Authors: Garrett Henry B. , DeForest Sherman E.
Published: February 2008

Analysis of plasma data from the geosynchronous satellites ATS 5 and 6 shows that the satellites vary rapidly in potential as they pass into and out of the earth's shadow. These variations result from the loss of photoelectrons as the sun is eclipsed. A model of this phenomenon which includes the effects of atmospheric attenuation of the solar flux between ∼1000 and ∼3000 Å and spacecraft charge buildup is used to predict the potential variations observed by ATS 5 and 6. The model predicts potentials within ±700 V of the observed potentials in the range 0 to 10,000 V.
Observations of significant magnetosheath antisolar energy flow
Authors: Williams Donald J.
Published: February 2008

Evidence for significant but sporadic magnetosheath energy flows in the antisolar direction is presented, using observations of 50 to 220‐keV ions made by the NOAA/Applied Physics Laboratory energetic particle experiment on the Imp 7 satellite. The observations have been made in the dawn and dusk magnetosheath at distances of ∼35 RE from the earth and cover a wide range of ZSM. Direct observations of 50‐ to 220‐keV ions yield an energy flow for the case studied of 2(10)−3−2(10)−2 erg/cm² s. By using a somewhat arbitrary but reasonable magnetosheath area based on 1 year of synoptic survey of such flows a total magnetosheath ≥50‐keV ion energy flow of ≳1018 ergs/s is obtained. Reasonable spectral extrapolations yield somewhat larger total flows. These values indicate that ions carry at least an equal portion of the energetic particle magnetosheath energy flow as the extrapolated >1‐keV electron flow obtained by Baker and Stone (1977). Possible sources of this energy flow include dissipation processes due to a magnetopause electric field, leakage of magnetospheric particles through the magnetopause into the magnetosheath, bow shock acceleration, and magnetosheath acceleration.
On the relationship of the polar cap current system to the north‐south component of the interplanetary magnetic field
Authors: Horwitz James L. , Akasofu S.‐I.
Published: February 2008

The daily magnetic variation at a polar cap station, Resolute Bay, is examined for its dependence on the north‐south component Bz of the interplanetary magnetic field (IMF). The magnetic perturbations are seen to be in approximately the same direction in the nightside polar cap for both northward and southward IMF, the amplitudes of the perturbations being larger for southward IMF. In the dayside perturbations, significant differences between the northward and southward IMF cases are observed. Examination of Thule perturbations shows the same serious discrepancy near noon. The observations are consistent with antisunward current (and sunward convection) in a central strip of the polar cap, as was first suggested by Maezawa (1976). Our tentative interpretation for the overall pattern of the polar ionospheric currents during northward IMF differs substantially from that of Maezawa, however. It is suggested that the overall pattern consists of two of the standard two‐cell current patterns placed in juxtaposition such that the interface between them is the site of the antisunward current (sunward convection). Within the centers of each of these two patterns the currents are sunward (convection is antisunward).
Detailed geometrical modeling of Jupiter's Io‐related decametric radiation
Authors: Thieman J.R. , Smith A.G.
Published: February 2008

By geometrical arguments it is possible to explain some of the details of the dependence of Jupiter's decametric radio sources on the position of the innermost Galilean moon Io and Jovian System III longitude. We have adopted the conical beaming model, first presented by Dulk, which is now common to many of the present decameter‐radiation theories. Detailed calculations of the beaming directions have been carried out assuming a source region located above the planetary surface along the Io‐affected magnetic flux tube at a point where the electron gyrofrequency is approximately equal to the radiated frequency. Agreement with observed morphological structure is critically sensitive to the magnetic field. The octupolar magnetic field models resulting from the Pioneer 11 measurements are the basis for the present analysis. An apparent displacement of the flux tube 20° ahead of the orbital position of Io produces good agreement between the calculated and observed positions of the sources. The longitudinal extents of the three main sources can be approximated by limiting radiation to points where the perpendicular angle between the magnetic field vector at the point of emission and the Zenographic equatorial plane is at or near minimum. Other source morphology appears to result from high‐frequency cutoff of emission when Jupiter's ionosphere interferes with the radiation process near the cloudtops. Further cutoffs are possible when the source region is occulted from an earth‐based viewpoint by the planetary surface. For lower frequencies the observations indicate that Io‐dependent radiation is emitted at all longitudes. The beaming model concurs with this finding when all cutoffs are eliminated.
Time structure of postmidnight energetic electron precipitation and the limit of stable trapping
Authors: Trefall H. , Williams D. J.
Published: February 2008

High time‐resolution measurements of trapped and precipitated >30‐keV electrons from the low‐altitude polar‐orbiting satellite Ogo 6 in the postmidnight‐to‐dayside region of the auroral zone are presented. It is found that precipitation usually occurs in brief bursts of a few seconds duration or less, with normally a very abrupt transition between no precipitation and isotropic (full loss cone) precipitation. Thus if pitch angle scattering is the precipitation mechanism, it seems to be either very strong or nearly nonexistent, and intermediate‐strength pitch angle scattering modes do not usually seem to represent a stable precipitation regime in this part of the auroral zone. Sustained precipitation in time and/or space is found only at very high flux levels, whilst the duration as well as the frequency of the burstlike precipitation events observed at lower flux levels apparently decreases with decreasing flux of trapped electrons. This is consistent with theories which predict the existence of a critical flux level at which the electron population becomes unstable and strong pitch angle scattering takes place, though there remains the question of how the burstlike precipitation is generated. The action of a delayed negative feedback mechanism is suggested. The burstlike character of the precipitation is of great importance for the average lifetime of electrons drifting in the geomagnetic field, as it reduces the average loss rate to a level which obviates the need for any additional acceleration in the morning‐to‐dayside region to explain the ability of such electrons to gradient drift most of the way around the auroral zone.
An atmospheric absorption model for the Schumann‐Runge bands of oxygen
Authors: Blake A.J.
Published: February 2008

A detailed absorption cross section for molecular oxygen in the region of the Schumann‐Runge bands has been calculated using data about the molecular constants for oxygen, the band strengths and linewidths for the Schumann‐Runge system, and the absorption continua of oxygen in this region. The accuracy of the cross section is demonstrated by comparison with a range of experimental measurements. An absorption model for the mean transmission and effective local cross section for oxygen over 1.0‐nm and 0.2‐nm intervals in the wavelength region 175–205 nm has been based on the detailed cross section, and the coefficients of the model are given. The model incorporates an atmospheric temperature profile and is suitable for the calculation of photodissociation rates of oxygen and minor constituents in the atmosphere.
Dayside aurora and relevance to substorm current systems and dayside merging
Authors: Eather R.H. , Mende S.B. , Weber E.J.
Published: February 2008

Photometric keograms from South Pole Station are used to investigate dayside auroral precipitation. Identification of cusp auroral signatures allows cusp movements to be monitored and correlated with substorm activity and interplanetary magnetic field direction. We conclude that any erosion of dayside field is not a direct result of southward turning Bz. On the other hand, cusp position is intimately related to substorm timing and intensity. We suggest the associated dayside field reconfiguration results from diversion of near‐tail magnetopause currents through the nightside electrojet because of enhanced auroral ionosphere conductivity. This interpretation implies that substorms result from increased efficiency of the extraction of energy from the solar wind rather than from a release of internally stored magnetic energy in the tail.
High‐resolution spectrograms of ion acoustic waves in the solar wind
Authors: Kurth W.S. , Gurnett D.A. , Scarf F.L.
Published: February 2008

Ion acoustic waves, similar to those detected by the Helios spacecraft from 0.3 to 1.0 AU, have now been detected by the Voyager spacecraft in the solar wind out to heliocentric radial distances of 1.7 AU. High bit rate waveform measurements provide the first high resolution, frequency‐time spectrograms of these waves. The Voyager spectrograms show that the ion acoustic waves consist of narrowband bursts which last for a few seconds or less. The center frequency of the bursts can fluctuate rapidly in frequency, usually in the range between the electron and ion plasma frequency fp− and fp+ (These waves have been previously referred to as fp+ <f <fp− noise.) Comparisons of the high‐resolution spectrograms from the Voyager spacecraft with similar measurements from earth‐orbiting spacecraft show that spectra of the ion acoustic waves in the solar wind at 1.7 AU are identical to spectra of short wavelength ion‐acoustic waves detected upstream of the earth's magnetosphere. The latter waves have been associated with protons streaming into the solar wind from the bow shock. This close similarity suggests that the ion acoustic waves detected in the solar wind by Helios and Voyager may be driven by an ion beam instability, as has recently been suggested in a theoretical analysis by Gary.
Simulation studies of ionospheric electric fields and currents in relation to field‐aligned currents, 1. Quiet periods
Authors: Kamide Y. , Matsushita S.
Published: February 2008

Computer simulation studies of the electric fields and currents in the global ionosphere produced by field‐aligned electric currents for quiet periods are conducted. The steady state equations for current conservation are solved numerically by assuming (1) several divided regions of the global earth (such as the polar cap, auroral zone, and middle‐low latitudes), (2) exponentially distributed anisotropic electric conductivities for each zone with a continuous change at the boundaries of the regions, and (3) exponentially distributed downward and upward field‐aligned current intensities in the auroral region, assumptions based on our current knowledge of auroral phenomena and geomagnetic variations as well as rocket and satellite measurements of field‐aligned currents. Resultant computer‐plotted diagrams include equipotential contours of the electric fields, vector distributions of the electric fields and currents, and electric current patterns equivalent to the magnetic field effect produced by the field‐aligned and real ionospheric currents. One of the merits of this simulation method is that the three‐dimensional current system can roughly be estimated from the equivalent current system obtained from ground‐based geomagnetic data alone. This paper also provides a foundation for a similar study of substorms. The following main results are obtained: (1) Conductivity inhomogeneity alters considerably the electric field pattern that has previously been obtained by assuming the uniform conductivity distribution. (2) Even a slight conductivity enhancement along the nightside auroral belt results in a large modification of the electric field. (3) The existence of the strong conductivity gradients and the field‐aligned currents in the equatorward half of the auroral oval reduces the electric field in the middle and low latitudes. This corresponds to the ‘shielding’ effect of the electric field inside the Alfvén layer in the magnetotail. (4) Seasonal changes in the polar cap conductivities cause surprisingly large effects on the electric fields and currents. (5) The equivalent ionospheric currents differ significantly from real ionospheric currents in both intensity and direction.
High‐altitude gas releases: transition from collisionless flow to diffusive flow in a nonuniform atmosphere
Authors: Bernhardt Paul A.
Published: February 2008

Vapors, released at high altitudes from rockets or the Space Shuttle, may travel many hundreds of kilometers before coming to rest. An understanding of the effects produced by these releases requires knowledge of the vapor flow. In the tenuous upper atmosphere, the injected gases expand from a collisionless to a collision‐dominated state. In this paper, this process is described by the Boltzmann equation with the Krook collision integral. The theory is applied to supersonic releases into a nonuniform atmosphere. The model predicts elongation and heating of the vapor trails due to collisions. The theory may be applied to a wide range of experiments whose objectives include the formation of ionospheric depletions, the triggering of instabilities, the generation of gravity waves, and the injection of neutrals as luminous tracers.
Relationship between the IMF, the midday gap, and auroral substorm activity
Authors: Dandekar B.S.
Published: February 2008

A study of auroral photographs from the satellites of the defense meteorological satellite program (DMSP) over the southern hemisphere for May–July 1974–1975 confirms previous findings and provides the statistical characteristics of the semi‐permanent gap in the day sector of the auroral oval. The gap morphology is related to the Bz component of the interplanetary magnetic field (IMF), and to the phase of the auroral substorm. When Bz is southward the equatorward movement of the gap is 0.6° CGL/unit Bz. The gap is relatively wider when Bz is positive than when Bz is negative. Inspite of trends observed to depend on By, the role of the By component on the morphology of the gap could not be established. The observations also confirm that the auroral activity in the night sector depends on the phase of the auroral substorm and that the phase of the auroral substorm is related to the variations of the Bz component of the IMF.
latitude Variation of Interplanetary Cosmic Ray Diffusion
Authors: Morfill G. E. , Völk H. J.
Published: February 2008

The radial diffusion coefficient for cosmic rays is calculated using a rudimentary model of the threedimensional solar cavity. Cosmic ray scattering is due to resonant interaction with Alfvén waves propagating outwards from the sun according to geometrical optics. In addition the latitude dependence of medium scale fluctuations caused by solar wind streams is considered. The latitudinal extent of the weak modulation region around the pole is found to be about 20°, although down to about 50° latitude significantly higher fluxes are expected. The results also give information on the perpendicular diffusion coefficient. It is quite small. The large intensity gradients in latitude nevertheless may lead to sizeable latitudinal diffusion, although a less pronounced one than in Fisk's (1976) model because of the absence of streams at high latitudes.
The nested variance power spectrum: A second look
Authors: Rickard James J.
Published: February 2008

The method of nested variances applied to the calculation of power spectra as discussed by Owens (1977) is examined. It is shown that the accuracy of the method is poor if the power spectrum decreases more rapidly than f−2 or when sharp features or cutoffs are present. The speed gain over fast Fourier transform algorithms is substantial, however. Coding changes are recommended which increase its speed even further.
Theory of the large‐amplitude plane magnetoacoustic wave propagating transverse to the magnetic field in a hot collisionless plasma
Authors: Barnes Aaron
Published: February 2008

We present the exact solution of the kinetic and electromagnetic equations for a large‐amplitude plane magnetoacoustic wave propagating transverse to the magnetic field in a hot collisionless plasma. The solution gives simple relations among the magnetic field strength, density, stress tensor and plasma velocity, all of which are measurable in the interplanetary plasma. These relations are independent of the electron and ion velocity distributions, subject to certain restrictions on ‘high‐velocity tails.’ The magnetic field of the wave is linearly polarized. The wave steepens to form a shock much as the analogous waves of MHD theory do.
Downward flowing ions and evidence for injection of ionospheric ions into the plasma sheet
Authors: Ghielmetti A.G. , Sharp R.D. , Shelley E.G. , Johnson R.G.
Published: February 2008

Energetic (keV) protons and 0+ ions with field aligned pitch angle distributions and peak fluxes up to ∼ 7 × 107 (cm² s sr keV)−1 have been observed streaming downward in the high altitude (∼ 1 RE) auroral ionosphere. In two examples described in detail, the correlations observed between the downward flowing and coincident trapped ion populations suggest a common origin. Downward flowing ion events are a much less frequent phenomenon than upward flowing ion events in the auroral region. The significant difference implies that the energetic upward flowing ions from the auroral ionosphere are commonly injected into the trapped population of the plasma sheet, and furthermore that parallel electric fields involving potential drops of ≳500 V are directed preferentially upward in the altitude range from 2000 km to ∼ 3 RE. Spatially localized regions of enhanced hot (∼keV) plasma density were frequently observed in the low L portion of the plasma sheet. The statistical location of these ‘plasma clouds’ correlates well with the substorm injection boundary near dusk inferred by McIlwain (1974) and with the upward flowing ion events observed at ∼ 1 RE altitude (Ghielmetti et al., 1978a, b). The downward flowing ion events occurred preferentially within such ‘plasma clouds.’ The results described in this paper suggest that upward flowing ions from the auroral acceleration regions are responsible for both the downward flowing ions and for at least some of the plasma clouds. A significant fraction of the equatorial substorm injected plasma clouds may thus result from this ionospheric source region.
Equatorial electric fields during magnetically disturbed conditions 2. Implications of simultaneous auroral and equatorial measurements
Authors: Gonzales C.A. , Kelley M.C. , Fejer B.G. , Vickrey J.F. , Woodman R.F.
Published: February 2008

Simultaneous auroral and equatorial electric field data are used along with magnetic field data to study anomalous electric field patterns during disturbed times. During some substorms, accompanied by ring current activity, the worldwide equatorial zonal electric field component reverses from the normal pattern. This is interpreted as a partial closure of high latitude field aligned currents in the dayside, low latitude ionosphere. These currents flow westward across the dayside. In several cases the zonal equatorial electric field component was nearly identical in form to the zonal auroral component, indicating the close electrical coupling between these regions. Less certain, but equally intriguing, is the evidence presented for a close relationship between the zonal equatorial electric field and the time derivative of the ring current induced magnetic field. Another class of events seems related to rapid changes of magnetospheric convection and a temporary imbalance between the field external to the plasmasphere and the shielding charges in the Alfven layer. Examples of both rapid increases and decreases are presented. The latter seems often to be related to a northward turning of the interplanetary magnetic field.
Magnetospheric electrostatic emissions and cold plasma densities
Authors: Hubbard R.F. , Birmingham T.J. , Hones E.W.
Published: February 2008

A synoptic study of electric wave, magnetometer, and plasma data from Imp 6 has been carried out for times when banded electrostatic waves are observed between harmonics of the electron gyrofrequency in the earth's outer magnetosphere. Four separate classes of such waves have previously been identified by us. The spatial and temporal occurrences of waves in each class are summarized here, as are correlations of occurrence with geomagnetic activity. Most importantly, associations between the observations of waves of different classes and the relative portions of cold and hot electrons present at the position of the spacecraft are established. The cold to hot ratio varies in accordance with the predictions of our previous theoretical work, which models the emission as arising unstably from a hot loss cone distribution existing simultaneously with a cold isotropic electron component. Finally, evidence for the signature of the loss cone is sought in the plasma data.
The dc and ac electric field, plasma density, plasma temperature, and field‐aligned current experiments on the S3‐3 satellite
Authors: Mozer F.S. , Cattell C.A. , Temerin M. , Torbert R.B. , Von Glinski S. , Woldorff M. , Wygant J.
Published: February 2008

Measurements of dc electric fields, field‐aligned currents, the plasma density, and wave electric fields and density fluctuations have been made for the first time at auroral zone altitudes between 1000 and 8000 km on the S3‐3 satellite. The design and operation of the various detectors that made these measurements is described, and some typical scientific results are presented. These include the observations that the high altitude auroral zone plasma density is typically 5–50 particles/cm³, that field‐aligned currents exceeding the electrostatic ion cyclotron instability threshold are often observed in regions of ion cyclotron wave emission and accelerated upgoing ions, that the auroral zone electric field penetrates the plasmapause to cause deviations from corotation of the plasmasphere, and that the energetic electron high latitude trapping boundary is sometimes equatorward of the last closed magnetic field line, as determined by the reversal in the poleward electric field component.
Polarization of high latitude ionospheric turbulence as determined by analysis of data from the OV1‐17 satellite
Authors: Temerin Michael
Published: February 2008

A reanalysis of OV1‐17 data shows that high latitude ionospheric electric field turbulence is polarized with an electric field perpendicular to the magnetic field.
On the orientation of the polar cap electric field
Authors: Nopper R.W. , Carovillano R.L.
Published: February 2008

Using realistic models of the ionospheric conductivity and the field‐aligned currents, we have determined how the distribution of the electric field in the polar cap ionosphere is controlled by the day‐night contrast in conductivity and by the relative strengths of the region 1 and region 2 field‐aligned currents. The sunward directed conductivity gradient acts to set up a space charge in the polar cap which crowds the equipotentials toward the dawn sector for current sources of both region 1 and region 2 polarity; this effectively shifts the polar cap convection pattern toward dawn. Our results show further that for a given conductivity distribution the orientation of the electric field in the central polar cap depends sensitively on the relative strengths of the Birkeland current pairs: for very weak region 2 currents (quiet times) the polar electric field is directed ≈ 60° east of noon, for equal region 1 and 2 currents (disturbed times) the direction is ≈ 10° east of noon, and for stronger region 2 than 1 currents (which may happen on occasion) the electric field points into the prenoon sector. These findings imply that the orientation of the polar cap electric field should serve as a measure or index of the ratio of region 1 to region 2 net current intensity and should possess correlations with geomagnetic activity similar to those of this ratio. This analysis does not include effects of possible source currents in the region of the polar cusp.
A model of substorm electric fields in the plasmasphere based on whistler data
Authors: Carpenter D.L. , Park C.G. , Miller T.R.
Published: February 2008

This paper presents a descriptive model of substorm‐associated east‐west electric fields with periods of ⪖30 min in the outer plasmasphere at L ≃4. The model is based on both published and previously unpublished data on cross‐L drifts of whistler paths. The whistler data were acquired on 34 observing days in the years 1963–1975. The recording stations were Eights (L ≃ 4), Byrd (L ≃ 7), and Siple (L ≃ 4), Antarctica. Substorm intervals were defined as those in which the AE index exceeded 200 γ. The rms value of 30‐min‐average fields on quiet days (AE < 200 γ) is found to be 0.09 mV/m, while for substorm periods the values are 0.17, 0.23, and 0.28 mV/m for AE = 200–300 γ, 300–500 γ, and >500 γ, respectively. The substorm model includes features such as (1) a reversal from eastward to westward fields (outward to inward drifts) at ∼23 MLT, (2) large westward fields on the nightside between 0000 and 0700 MLT mostly in the range 0.2–0.7 mV/m, (3) mostly eastward fields between 0700 and 2400 MLT in the range −0.1 to −0.2 mV/m, and (4) less efficient penetration of substorm electric fields near L = 4 near local noon than near dawn or dusk. A number of effects such as spatial structure in outflows near dusk are obscured by the averaging used in the model. There are important differences between temporally isolated and prolonged inward drift events in the postmidnight sector. The primary effect of the former is local modulation of the plasmasphere radius, while the latter tend to be less spatially localized and lead to significant reductions in overall plasmasphere size. Other topics briefly discussed include whistler evidence of relatively large values of the radial (north‐south) component of E in the dusk sector, as well as the question of the importance of the component of E induced by fluctuating plasma currents. While the model does not yet account for important temporal and spatial structure in substorm fields, it is believed to include the major spatial features of the east‐west field in the outer plasmasphere with periods of ⪖30 min.
Observations of equatorial ionospheric bubbles by the radio propagation method
Authors: Yeh K.C. , Soicher H. , Liu C.H.
Published: February 2008

Recent data collected near the magnetic equator depict one kind of ionospheric perturbation in the nighttime hours variously as bubbles or plumes. Theoretical studies show that the underside of the ionosphere is subjected to Rayleigh‐Taylor instabilities which, when they are triggered, will cause a region of depleted ionization to rise as bubbles. When such regions are traversed by a probing radio wave, the associated Faraday effect is expected to show depletions of the electron content. This paper describes some experimental results obtained at Natal, Brazil (35.23°W, 5.85°S, dip −9.6°), by monitoring radio signals transmitted by the geostationary satellites Marisat 1 and SMS 1. Using ionization depletions as indications of bubbles, statistical studies of occurrence, size, and magnitude of perturbations are carried out. The most probable depletions for the propagation path under study have values in the range 1–4 × 1016 el/m², but depletions as large as 1.2 × 1017 el/m² have also been observed. The average durations for each observed bubble may vary from less than 2 to over 30 min with an average of 8 min. The experimental data further show that the scintillation rate may increase suddenly when these bubbles either form along or drift across the propagation path.
Some properties of diffuse radio aurora at high aspect angles
Authors: Mareschal M. , McDiarmid D.R. , McNamara A.G.
Published: February 2008

It is established within experimental error that the relevant characteristics of diffuse radio aurora seen by the Thompson (48 MHz) radar at high aspect angles (8°–14°) are the same as those observed by others at low aspect angles. Specifically, during a study of radio aurora in the sunlit auroral oval it was found that the relationship between the intensity of high aspect angle diffuse radio aurora and the magnitude of the geomagnetic disturbance is consistent with that observed by others. McDiarmid and McNamara (1978) have already shown that the same radio aurora is related to field‐aligned currents in the way as was previously observed at low aspect angle. These results imply that more definitive work in the neighborhood of 50 MHz at these high aspect angles which occur at relatively high latitudes (Λ ≳ 70°) would lead to a significant increase in our understanding of auroral physics.