bluuebunny/crossref_metadata_2025 huggingface dataset:
Plasma stability of quiescent prominences
Authors: Migliuolo Stefano
Published: February 2008

The stability of the Kippenhan‐Schluter model prominence to ideal MHD perturbations is analyzed. It is shown that all modes with g · ▽v1 = 0, where g is the gravitational field and v1 the perturbed velocity, are stable. The analysis is done by self‐consistently taking into account the spatial variation of equilibrium quantities (density, magnetic field) and looking for bounded eigenmodes. Some consequences (of the derived stability of perturbations) are drawn with regards to observed prominence phenomena.
A three‐component model of the variability of the solar ultraviolet flux: 145–200 nM
Authors: Lean J. L. , White O. R. , Livingston W. C. , Heath D. F. , Donnelly R. F. , Skumanich A.
Published: February 2008

A three‐component model has been developed to examine the variation with solar activity of the far ultraviolet irradiance between 145 and 200 nm. This model is based on spatially resolved observations of the Call K chromosphere and includes the contributions to the full disk flux from both plage and active network emission. The 27‐day modulation of the ultraviolet flux is explained by the evolution and rotation of the plage regions on the solar disc. Over the longer time scale of the eleven‐year cycle it is essential that changes in the active network arising from the decay of plage regions also be considered as a source of ultraviolet flux variability. Only by using a three‐component model of the solar flux is it possible to simultaneously reproduce the 27‐day variability observed by the solar backscatter ultraviolet experiment on the Nimbus 7 satellite and the changes from the minimum to the maximum of the solar activity cycle observed by the rocket experiments of the Laboratory for Atmospheric and Space Physics and by the extreme ultraviolet spectrometer on the Atmospheric Explorer E satellite. It is shown that the AE‐E experiment measured a smaller solar cycle variability for the ultraviolet irradiances than is predicted by the model calculations because of the spatially restricted field of view of this instrument.
Contact discontinuities in a cold collision‐free two‐beam plasma
Authors: Kirkland K. B. , Sonnerup B. U. Ö
Published: February 2008

A contact discontinuity in a collision‐free magnetized plasma is a thin layer, possessing a nontrivial magnetic structure, across which no net plasma flow takes place (υn = 0) even though the magnetic field has a nonvanishing component (Bn ≠ 0) normal to it. This paper examines the structure of such discontinuities in a simple plasma model consisting of two oppositely directed cold ion beams and a background of cold massless electrons such that exact charge neutrality is maintained so that the electric field E ≡ 0. The basic equations describing self‐consistent equilibria are developed for the more general situation where a net flow across the layer takes place (υn ≠ 0) and where the magnetic field has two nonzero tangential components By and Bz but where E remains zero. These equations are then specialized to the case υn ≡ 0, Bz ≡ 0, and four different classes of sheets are obtained, all having thickness of the order of the ion inertial length: (1) layers separating two identical plasma and magnetic field regions, (2) an infinite array of parallel layers producing an undulated magnetic field, (3) layers containing trapped ions in closed orbits which separate two vacuum regions with uniform identical magnetic fields, and (4) layers which reflect a single plasma beam, leaving a vacuum with a reversed and compressed tangential field on the other side. Solutions for which υn = 0 but Bz ≠ 0 may also exist but have not been analyzed; rotational discontinuities are shown not to be possible in this model.
Comparison of gas dynamic model with steady solar wind flow around Venus
Authors: Mihalov J. D. , Spreiter J. R. , Stahara S. S.
Published: February 2008

A gas dynamic model for solar wind flow around Venus has been compared with Pioneer Venus orbiter plasma analyzer measurements from times when the solar wind flow seemed steadiest. The comparisons were made near the terminator. When the observed and model bow shock locations are matched, the model agrees fairly well with the observed parameters: the components of the flow velocity and magnetic field, and the proton number density and isotropic temperature. However, the Mach numbers required to fit the observed bow shock locations are less than 2/3 those estimated from the measured parameters of the free‐stream flow, because the bow shock locations (near the terminator) are farther from Venus than expected. Sometimes the measured flow speeds appear to be retarded near the ionopause.
Charged particle behavior in low‐frequency geomagnetic pulsations: 3. Spin phase dependence
Authors: Kivelson Margaret G. , Southwood David J.
Published: February 2008

Low‐frequency modulations of the fluxes of magnetospheric charged particles are often observed in conjunction with geomagnetic pulsations. In some cases, the flux modulations depend on detector look direction even after effects arising from pitch angle anisotropy have been removed. For a detector on a spinning spacecraft, the look direction dependence creates flux modulations ordered by the spin phase angle, so the term ‘spin phase dependence’ provides a convenient description of the phenomenon considered in this paper. Spin phase dependence occurs only when the gyroradius of the detected particles (charge, q; energy, W) is comparable with some other characteristic length of the problem. In particular, spin phase dependence occurs when the particle gyroradius aL is comparable with λ⊥, the wavelength transverse to B. ATS 6 and GEOS have provided examples of this type of ‘finite gyroradius’ effect. Wave‐related spin phase dependence may also arise with aL ≪ λ⊥. For example, a wave convecting a steep spatial gradient in the particle density n(W) back and forth creates spin phase dependent flux oscillations if aL ≃ n(W)/|▽n(W)|. This situation is described and related to ISEE boundary wave studies. Gyration acceleration, i.e., acceleration by the wave electric field E, leads to spin phase dependence of the flux of particles for which aL ≃ W/γqE, where γ is the power law coefficient of the differential particle flux, again without any restriction to short wavelengths. Some OGO 5 and ISEE wave analyses have relied on this type of spin phase dependence. Gyration acceleration has also been invoked to explain particle pulsations seen on the nonspinning ATS 6 spacecraft, albeit in circumstances where the spin dependence cannot be checked. Finally, spin phase dependences may be produced by large‐scale standing wave structure parallel to the ambient magnetic field, an effect which appears in some ISEE particle data.
Quiet‐time electron precipitation at L = 4 in the South Atlantic Anomaly
Authors: Benbrook J. R. , Bering E. A. , Leverenz H. , Roeder J. L. , Sheldon W. R.
Published: February 2008

On January 12, 1978, at 1000 UT, a Superarcas sounding rocket was launched from Siple Station, Antarctica, (L = 4.1, 76°S, 84°W) during a prolonged geomagnetically quiet period (Kp = 1+) with very low VLF activity. The parachuted payload observed a moderately intense charged particle flux that smoothly decreased as the payload descended through the atmosphere. These data have been carefully extrapolated to zero depth in the atmosphere for comparison with both low‐altitude satellite measurements and theoretical predictions of the precipitating flux in the region of the South Atlantic anomaly. The integrated flux was found to agree reasonably well with both the satellite measurements and prediction. The calculated spectrum was found to be much softer than typical trapped electron spectra reported from satellite measurements after extended periods of low magnetic activity. This result is consistent with experimental observations that the lifetime of outer belt particles increases with energy.
EXOS‐B/Siple Station VLF wave‐particle interaction experiments: 1. General description and wave‐particle correlations
Authors: Kimura I. , Matsumoto H. , Mukai T. , Hashimoto K. , Bell T. F. , Inan U. S. , Helliwell R. A. , Katsufrakis J. P.
Published: February 2008

In situ measurements of both energetic particles and VLF waves have been carried out in a joint program involving the Japanese satellite EXOS‐B and the Siple Station VLF transmitter. A general description of the experiment is given as well as some results concerning wave‐particle correlations. Detailed analysis of the observed wave characteristics is given in a companion paper. Correlations of electron flux and pitch angle anisotropy in the energy range from 85 eV to 6.9 keV with waves in a range from 300 Hz to 9 kHz are examined. These electrons sometimes have a pitch angle distribution with a peak flux at 90° pitch angle (so‐called pancake distribution). On five passes out of a total of 50 during the summer campaign in 1979, the energy of the electrons that showed a high pitch angle anisotropy shifted upward as the satellite moved into the plasmasphere, crossing the plasmapause in the equatorial region. In two cases out of five, strong Siple signals were observed in the geomagnetic equatorial region just outside the plasmapause, in association with such a pancake pitch angle distribution of electrons. The Siple signals are most likely amplified by the cyclotron instability due to the high pitch angle anisotropy (HPAA), although the flux of resonant electrons was relatively small. For three other cases of HPAA, the satellite location was so far away from the Siple meridian that the signal level, even if amplified, was too weak to be detected by the satellite. Emissions associated with Siple signals were detected on five (two equatorial and three high latitude) passes, which were all confined on 6 days after a large magnetic storm. On the days when the Siple triggered emissions were observed, the pitch angle anisotropy was low, but the electron flux at resonant energies in the equatorial region was four or five times larger than that on other non‐triggering days in all energy channels from 85 eV to 6.9 keV.
EXOS‐B/Siple Station VLF Wave‐Particle Interaction Experiments: 2. Transmitter signals and associated emissions
Authors: Bell T. F. , Inan U. S. , Kimura I. , Matsumoto H. , Mukai T. , Hashimoto K.
Published: February 2008

Interactions between coherent VLF waves and energetic particles in the magnetosphere have been studied in a joint program involving the Japanese high‐altitude satellite EXOS‐B and the Siple Station VLF transmitter. During the period July 15–September 7, 1979, transmissions to the EXOS‐B satellite were carried out on 50 separate occasions when the spacecraft was within ±60° longitude of the magnetic field lines linking Siple Station, Antarctica (76°S, 84°W geographic, L ∼ 4.1), with its conjugate station at Roberval, Canada. Of this total, 37 were carried out while the satellite was located in the 1000–1600 LT sector of the magnetosphere and 13 while in the 0300–0800 LT sector. The transmitter signals were detected on EXOS‐B on 50% of the occasions when transmissions were attempted, and on 5 occasions the transmitter signals were observed to have triggered VLF emissions somewhere along their ray path between the ionosphere and the satellite. All 5 triggering events occurred in a 6‐day period following a large magnetic storm that took place on August 13, 1979, with 2 events occurring in the 1130–1330 LT sector and 3 events occurring in the 0400–0600 LT sector. Analysis of the emission triggering events provided strong evidence that the triggering took place inside whistler‐mode ducts and that the emissions reached the satellite only after being scattered at one end of the ducts by ionospheric irregularities. On at least one day, emissions were triggered by ducted echoes of the transmitter signals but not by the direct ducted pulses themselves. On another day a new type of triggered emission was observed. We conclude that in the noon sector of the magnetosphere the amplitude of nonducted signals from the Siple transmitter is generally less than the threshold level necessary for triggering in the nonducted mode.
Linear theory of the E×B instability with an inhomogeneous electric field
Authors: Huba J. D. , Ossakow S. L. , Satyanarayana P. , Guzdar P. N.
Published: February 2008

A general linear theory of the E×B instability is developed which considers an ambient electric field that is at an arbitrary angle to the density gradient and allows the electric field component parallel to the density gradient to be inhomogeneous. A differential equation is derived which describes the mode structure of the unstable waves in the direction of the inhomogeneities. The theory (1) includes ion inertia effects, (2) allows for arbitrary density and electric field profiles, and (3) is valid in the long‐wavelength regime, i.e., kyL < 1, where L is the width of the boundary layer. The main results of the analysis are as follows. First, the inhomogeneous velocity flow caused by the inhomogeneous electric field can stabilize the instability. Second, short‐wavelength modes are preferentially stabilized over longer‐wavelength modes. Third, the stabilization mechanism is associated with the velocity shear due to an x‐dependent resonance [ω − kyVy(x)]−1, where Vy(x) = −cEx(x)/B, and not velocity shear terms explicitly proportional to ∂Vy/∂x or ∂²Vy/∂x². Fourth, the marginal stability criterion is weakly dependent on the magnitude of νin/ω. Applications of these results to ionospheric phenomena are discussed, viz., barium cloud striations and high‐latitude F region irregularities.
Spatial structure of solar wind in 1976
Authors: Zhao Xue‐Pu , Hundhausen A. J.
Published: February 2008

Interplanetary scintillation measurements of the solar wind speed from 1976 show the expected trends toward high speed at either high heliographic latitudes or at large angular displacements from an interplanetary neutral sheet deduced from coronal observations. However, detailed examinations of the speed measurements when the neutral sheet is displaced from the solar equator reveals a lack of equatorial symmetry and suggests a minimum speed near the neutral sheet. For this epoch, the average solar wind speed u varies with angular displacement λ from the neutral sheet as u = 350 km s−1 + 800 sin² λ km s−1 for |λ| ≤ 35° and is approximately constant at 600 km s−1 for |λ| > 35°.
Transmittance of the atmosphere in the (8–0) and (9–0) Schumann‐Runge bands of oxygen
Authors: Gibson S. T. , Gies H. P. F. , Blake A. J. , McCoy D. G.
Published: February 2008

Dissociation rate constants for absorption by atmospheric oxygen in the Schumann‐Runge bands are usually calculated by making line by line calculations of the absorption cross section. The accuracy of the cross sections used in these calculations needs to be tested against experimental data. A transmittance spectrum computed by using a detailed model cross section is compared with a spectrum recorded in the atmosphere and another recorded in the laboratory. It is concluded that details of the atmospheric absorption spectrum can be reproduced with excellent accuracy by the computed cross section.
Heavy ions in the outer Kronian magnetosphere
Authors: Eviatar Aharon , McNutt Ralph L. , Siscoe George L. , Sullivan James D.
Published: February 2008

Data from the plasma science experiment on Voyager 1 show the existence of a cold heavy ion component in the plasma populating the outer Kronian magnetosphere (L ≈ 15). The mass resolution is not sufficient to determine unambiguously whether the ions are oxygen (O+ mass 16) or nitrogen (N+ mass 14). If the former, their most probable origin is sputtering off the rings and icy satellites, which lie well inside the site of observation. Outward transport is therefore required. If the latter, the most probable source is the plume of Titan, and inward transport to the observing site is required. The near‐resonant charge exchange reaction between oxygen ions and hydrogen atoms in the neutral hydrogen torus of Titan makes it most probable that the heavy ions are nitrogen from Titan. Mass loading of the outer magnetosphere by ions from Titan impedes corotation but to a somewhat lesser extent than would be expected on the basis of the inferred source strength of Titan and the Pedersen conductance of the ionosphere of Saturn. However, the disagreement falls within the combined reasonable uncertainties in the governing parameters. The presence of heavy ions tends to exclude hydrogen ions from the equator, since lighter ions ‘float’ to the top in the centrifugal potential. We speculate that depletion of heavy ions near L = 14 by charge exchange with neutral hydrogen may be the cause of the observed absence of hydrogen ions off the equator in this region.
A theory of the Io phase asymmetry of the Jovian decametric radiation
Authors: Hashimoto Kozo , Goldstein Melvyn L.
Published: February 2008

We propose an explanation of an asymmetry in the occurrence probability of the Io‐dependent Jovian decametric radiation. We find that this asymmetry arises because when Io is in the northern part of the torus, more intense Alfvén waves are generated propagating southward than northward. These waves then cause the excitation of decametric radiation in the northern ionosphere after reflection from the southern ionosphere. The asymmetry then results from the propagation time of the Alfvén wave and the bending of the magnetic field (Alfvén wing) along this trajectory. The ray paths of the decametric radiation are calculated using a three‐dimensional ray‐tracing program in the Jovian ionosphere. Variations in the expected probability plots are computed for two models of the Jovian ionosphere and global magnetic field, as well as for several choices of the ratio of the radiated frequency to the X mode cutoff frequency. The dependence of the occurrence probability on the Jovian declination of the earth is also discussed in the context of this model.
Observations of the HF‐enhanced plasma line with a 46.8‐MHz Radar and reinterpretation of previous observations with the 430‐MHz radar
Authors: Fejer J. A. , Ierkic H. M. , Woodman R. F. , Röttger J. , Sulzer M. , Behnke R. A. , Veldhuis A.
Published: February 2008

During 5.1‐MHz HF transmissions below the F region penetration frequency by a nearby ionospheric modification facility, radar echoes coming from F region heights at a frequency of 46.8 + 5.1 = 51.9 MHz were received while a VHF radar using the 305 m spherical reflector at Arecibo was transmitting pulses coherently on 46.8 MHz. The bandwidth of the echoes was less than 10 Hz. The so‐called decay line due to the parametric decay instability was expected at a frequency lower by about 300 Hz, but it was not observed. Density variations caused by the ponderomotive force resulting from the standing wave pattern at 5.1 MHz are tentatively invoked to explain the observations. Poorly understood aspects of some previous observations with the 430‐MHz radar can be similarly explained.
A theoretical study of the high latitude F region's response to magnetospheric storm inputs
Authors: Sojka J. J. , Schunk R. W.
Published: February 2008

The response of the polar ionosphere to magnetospheric storm inputs was modeled. During the storm the two major processes that couple the F region to the magnetosphere, namely the electric field distribution and the particle precipitation from the magnetosphere, undergo drastic modification on relatively short F region time scales. These time‐dependent changes are not simply related to the F region storm time dependent changes. The lower F region responds on a time scale of only minutes to the storm associated changes in the auroral precipitating electron flux, owing to the dominance of chemistry production‐loss mechanisms over transport processes. At higher altitudes in the vicinity of hmF2, the chemistry is balanced by both plasma diffusion along field lines and horizontal plasma convection, which acts to prolong the effect of the storm for many hours after it has ceased. The peak density responds only slowly to increased precipitation and may not reach its maximum enhanced value until over an hour after the storm main precipitation has passed. However, the F region peak can be drastically altered on a time scale of minutes if large vertical transport velocities are associated with the storm electric field distribution. In the topside ionosphere the density variations are not correlated with the morphology of the storm auroral precipitation or the temporal variation of the storm electric field pattern. Time delays of up to 3 or 4 hours occur at high altitudes for ‘peak’ densities to be reached after a storm, and the subsequent recovery is on the order of 5 hours. These long delays at altitudes above 400 km reflect the long time constants associated with plasma diffusion from low altitudes, where the plasma is created, to high altitudes.
Ionospheric electric field pulsations: A comparison between VLF results from an ionospheric heating experiment and STARE
Authors: Rietveld M. T. , Kopka H. , Nielsen E. , Stubbe P. , Dowden R. L.
Published: February 2008

Very low‐frequency radio waves can be generated in the lower ionosphere by periodically modulating the conductivity, and thus natural currents, using high power HF radio waves. In conjunction with the heating facility near Tromsø, Norway, such waves have been generated and detected on the ground using two orthogonal receiving antennas so that the polarization ellipse of the VLF signal could be reconstructed. Changes in the strength and direction of the electric field driving the modulated current are reflected in variations in the size and orientation of the VLF ellipse. During a natural Pc 5 pulsation event on October 16, 1981, a close correlation was found between the VLF signal and the electric field pulsations observed by the STARE auroral radars. This first comparison between the Heating and STARE experiments shows that VLF signals from modulated ionospheric heating experiments can provide a sensitive, high time resolution indicator of the ionospheric electric field.
Cosmic ray modulations related to the interplanetary magnetic field intensity
Authors: Duggal S. P. , Pomerantz M. A. , Schaefer R. K. , Tsao C. H.
Published: February 2008

Data covering a complete solar cycle (1965–1976) have been analyzed in a comprehensive investigation of the nature of transient modulations of relativistic galactic cosmic rays associated with interplanetary magnetic field intensity excursions in the vicinity of the earth. It had been recognized earlier that the correlation coefficient between the cosmic ray intensity and the IMF magnitude is small. In fact, it remains less than 0.5 throughout the solar cycle. In order to understand the nature of field‐related modulations and to minimize the complications arising from the effects of other features of solar plasma, the data are here analyzed by the method of superposed epochs. Following newly developed procedures for evaluating the statistical significance of the results obtained with this type of analysis, it is demonstrated that, while the days characterized by high IMF magnitude B are associated with intensity decreases, the IMF rarefactions (low B) cause an increase in the flux. In contrast to the case of geomagnetic disturbances, which respond more rapidly, maximum cosmic ray intensity variations associated with both high and low B occur 1 day after the key days characterized by field departures from average values. This indicates that the modulation mechanism becomes efficient only when the plasma engulfment of the earth extends at least a gyroradius beyond 1 AU. Finally, while the geomagnetic index Ap follows the interplanetary magnetic field changes rather closely, the polar nucleonic intensity shows a prolonged recovery time (7–10 days) following magnetic enhancements as well as rarefactions. These results suggest that, on average, interplanetary magnetic field intensity variations which are presumably related to disturbances traveling from the sun are effective transient modulators of the relativistic particles and that the corotation of the modulating region plays a significant role in the recovery of the cosmic ray intensity. An investigation of the spectra of the cosmic ray intensity variations produced by interplanetary magnetic field enhancements and rarefactions, respectively, over the rigidity range covered by ground‐based nucleonic intensity detectors has revealed that a power law of the form Kp−1 fits the data in both cases. This result is relevant to further theoretical studies of cosmic ray modulation processes.
A source of the backstreaming ion beams in the foreshock region
Authors: Tanaka Motohiko , Goodrich C. C. , Winske D. , Papadopoulos K.
Published: February 2008

A new source mechanism is proposed for the ‘reflected’ ion beams observed in the foreshock region of the earth's bow shock. In our model the beams originate in the magnetosheath downstream of the quasi‐perpendicular portion of the shock. The quasi‐perpendicular shock transition is characterized by two downstream ion populations including high‐energy gyrating ions in addition to the directly transmitted anisotropic ions. We show by particle simulations that this highly anisotropic downstream ion distribution (T⊥/T∥ » 1) can excite electromagnetic ion cyclotron waves which, in turn, pitch angle scatter the gyrating ions in a few ion gyroperiods. As a result, some ions acquire large parallel velocities and move fast enough along the convecting downstream magnetic field to escape back across the bow shock into the upstream region. The distribution of escaping ions calculated by using the pitch‐angle‐scattered ions, as a source, becomes a beam with a large temperature anisotropy T⊥ ∼ 3–5 T∥ and a mean velocity along the magnetic field of about twice that of the solar wind velocity. A significant result is the presence of the maximum angle θnB = θc above which no ions can escape, where θnB is the angle between the shock normal and the interplanetary magnetic field. A wide peak of constant escaping ion flux is formed below θc whose number density is 1–2% of that of the solar wind. These results are in general agreement with the ISEE observations of the ‘reflected’ ions.
Measurements of the stability of energetic electron beams in the ionosphere
Authors: Duprat G. R. J. , Whalen B. A. , McNamara A. G. , Bernstein W.
Published: February 2008

A Nike Black Brant V rocket was launched from the Churchill Research Range (Manitoba) on December 3, 1979, into a bright east‐west oriented auroral arc. The rocket payload consisted of two separable sections, each containing its own telemetry and a full set of wave and charged particle detectors. An electron gun, carried in the main payload, produced a pulsed electron beam with energies of 1.9, 4, and 8 keV at 1, 10, and approximately 100 mA in a programed format. Charged particle observations from the flight are used to define the spatial distribution of perturbed volume surrounding the accelerator during gun firing. The radial dimensions of the perturbation were found to scale with the primary electron beam gyroradius and current and were also dependent on the beam injection angle. On magnetic field lines near the gun, the induced return electron energy spectrum is characterized by a monotonically decreasing intensity with increasing energy out to the approximate beam energy. At increasing distances across field lines the energy spectrum takes on a monoenergetic appearance peaked near the beam energy. All beam‐induced electron fluxes drop rapidly to background at the edge of the perturbed volume. The intense flux of low‐energy electrons observed on field lines near the rocket are shown to be accelerated ambients, whereas the particles at or near the beam energy and at large radial distances are presumably beam primaries. The ambient thermal ion plasma was not measurably affected by the beam while the local electron temperature increased during gun pulses. Results from this flight are compared with the corresponding observations made in a large vacuum tank simulation, and it is concluded that certain features in the data are consistent with the beam‐plasma instability observed in the laboratory.
N I [3466 Å] and N I [5200 Å] emissions from various, nighttime and daytime auroras
Authors: Sivjee G. G. , Marshall R. A.
Published: February 2008

N I [2p³ 2P → 4S, 3466 Å] and N I [2P³, ²D → 4S, ∼ 5200 Å] multiplet radiations were monitored simultaneously from various nighttime and daytime auroras by using two large through‐put co‐aligned spectrophotometers. Measurements were made around winter solstice periods at Longyearbyen (Λ = 75°N), Svalbard, where large (>20°) solar depression angles, even during local midday, permitted continuous round the clock observations. Average energy of particles precipitating in auroras sighted over Longyearbyen ranged from a few keV at night to less than a hundred eV in the midday cusp section of the auroral oval; the corresponding atmospheric heights of peak auroral‐energy deposition varied from around 110 km, where molecular species are the major constituents of the atmosphere, to above 200 km, where atomic species are more dominant. Because of the difference in radiative lifetimes of N I [²P] and [²D] states, as well as difference in atmospheric extinction around 3466 Å and 5200 Å, the intensity ratio I [5200 Å]/I [3466 Å] varied in dynamic auroras associated with large changes in auroral energy flux, and with the extent of clouds or haze in the sky. Using only measurements from clear sky during steady auroras, we find that in all auroras the average value of the intensity ratio I [5200 Å]/I [3466 Å] = 1.8±0.4.
Influence of the E region dynamo on equatorial spread F
Authors: Hanson W. B. , Sanatani S. , Patterson T. N. L.
Published: February 2008

The integrated E region Pedersen conductivity can be an important parameter in determining whether the bottomside of the equatorial F layer will be stable against the Rayleigh‐Taylor gravitational instability. The F layer is observed to become unstable when it rises to great heights after sunset. One effect of this height rise is to decrease the stabilizing influence of ion‐neutral collisions at F region heights. It is shown here that the same eastward electric field that raises the F layer also decreases the Pedersen conductivity of the E region, which further destabilizes convective overturning. Because the conductivity of magnetic tubes that penetrate the main F layer is large compared to the E layer contribution, these effects are important only for the bottomside of the equatorial F layer.
Conjugate studies of an isolated equatorial irregularity region
Authors: Weber E. J. , Aarons J. , Johnson A. L.
Published: February 2008

Coordinated ground and airborne measurements were conducted at three points along the Ascension Island magnetic meridian to investigate the distribution of scintillation‐producing (km scale) ionospheric irregularities within an isolated equatorial plasma depletion. Airglow measurements near the northern Appleton anomaly show that irregularities are confined to the region within a 6300‐Å airglow depletion. The airglow measurements are then used to map the depletion region along magnetic field lines for comparison with scintillation measurements near the magnetic equator and near the conjugate point at Ascension Island. Results show that the irregularity region is magnetic flux tube aligned and conjugate over north‐south distances of ∼3000 km.
Day and night models of the Venus thermosphere
Authors: Massie S. T. , Hunten D. M. , Sowell D. R.
Published: February 2008

A model atmosphere of Venus for altitudes between 100 and 178 km is presented for the dayside and nightside. Densities of CO2, CO, O, N2, He, and O2 on the dayside, for 0800 and 1600 hours local time, are obtained by simultaneous solution of continuity equations. These equations couple ionospheric and neutral chemistry and the transport processes of molecular and eddy diffusion. Photodissociation and photoionization J coefficients are presented to facilitate the incorporation of chemistry into circulation models of the Venus atmosphere. Midnight densities of CO2, CO, O, N2, He, and N are derived from integration of the continuity equations, subject to specified fluxes. The nightside densities and fluxes are consistent with the observed airglow of NO and O2(¹Δ). The homopause of Venus is located near 133 km on both the dayside and nightside.
Response of nightside auroral‐oval boundaries to the interplanetary magnetic field
Authors: Nakai H. , Kamide Y.
Published: February 2008

The poleward and equatorward boundaries of the nightside auroral oval are scaled from DMSP auroral imagery and their response to the interplanetary magnetic field (IMF) is investigated. Although, as expected, the Bz component of the IMF is found to be the dominant factor in determining the latitude of the oval equatorward boundary, the correlation coefficients are not very high: the large variance results from the considerable scatter of points when the IMF is northward and large. By examining the oval boundaries from successive orbits, it is suggested that most of the expanded oval cases during such northward IMF periods occurred within several hours after periods of southward IMF. This indicates that the response of the oval to northward turnings (i.e., the contraction of the oval) takes place rather slowly, so that the contracted oval in its smallest size can be established only when the IMF has a northward component steadily for more than several hours. The observed oval equatorward boundary is mapped onto the equatorward plane of the magnetotail, and then plasma sheet processes are discussed with respect to present theories on the formation of the Alfvén layer as a result of the penetration of solar wind electric fields into the magnetotail.
Evidence for electrostatic shocks as the source of discrete auroral arcs
Authors: Kletzing C. , Cattell C. , Mozer F. S. , Akasofu S.‐I. , Makita K.
Published: February 2008

The first simultaneous observations of electrostatic shocks as inferred from their low altitude extensions, electron beams, and visual auroral arcs on the same magnetic field line are presented for three events in 1977. The events are the result of a comparison of electric field and particle data from the S3‐3 polar orbiting satellite with all‐sky camera data from stations in Alaska and Antarctica. All of the events were at low altitude (<4000 km), showed indications of regions of parallel electric field above the satellite, and were in agreement with potential models reported by other researchers that indicate that the electrostatic shock is one of the mechanisms of auroral particle acceleration.
The rotationally resolved 3400‐ to 3800‐Å terrestrial nightglow
Authors: Slanger Tom G. , Huestis David L.
Published: February 2008

Analysis and synthesis have been performed on portions of unpublished ground‐based nightglow spectra taken at ∼0.5 Å resolution by A. L. Broadfoot in 1968. The bands of the three O2 systems radiating in the 3400‐ to 3800‐Å region are rotationally resolved, and lines from three different c¹Σu− → X³Σg− bands, the system that dominates the visible Venusian nightglow, are clearly identified.
Multiple fluorescent scattering of N2 ultraviolet emissions in the atmospheres of the Earth and Titan
Authors: Conway Robert R.
Published: February 2008

Absorption and reemission by N2 of the Lyman‐Birge‐Hopfield (LBH) and Birge‐Hopfield (BH) band systems in the earth aurora and Titan dayglow has been calculated by using multiple scattering in a plane‐parallel radiative transfer model and a line‐by‐line synthesis of individual bands. The effects of rotational structure are included through the use of temperature‐dependent band escape functions and band transmission functions. Iteration through successive scatterings shows that fewer than ten scatterings are important, even for the thickest bands. The excitation rate caused by multiple scattering is sensitive to the vibrational population distribution of the ambient N2, especially for the BH system in the earth aurora. For a vibrational temperature of 2000 K, the fluorescent scattering contribution at 110 km is 15 times that of a vibrational temperature of 300 K. Comparison of theoretical intensities to rocket observations of the BH (1, 10) band indicates a vibrational temperature of 1000 K, but the corresponding (1, 3) band intensity is brighter than the observation. Scattering on Titan was modeled for both a high‐ and low‐altitude source and for various vibrational temperatures. The LBH system has an enhanced sensitivity to vibrational temperature on Titan because of the rapidly changing photoabsorption cross section of CH4 around 1400 Å. It is argued that the c4′ ¹Σu+ Rydberg bands should have an intensity profile similar to the BH bands. The c4′ (0, 0) band limb brightening inferred from the full‐disk and bright limb spectra observed by Voyager indicates a source around 4000 km, but this is in conflict with the observed intensity peak at 3660 km in the limb scan data.
The apparent spectral broadening of VLF transmitter signals during transionospheric propagation
Authors: Bell T. F. , James H. G. , Inan U. S. , Katsufrakis J. P.
Published: February 2008

VLF/ELF electric field wave data acquired on the ISIS 1, ISIS 2, and ISEE 1 satellites demonstrate the existence of a new phenomenon in which initially narrowband (∼1 Hz) upgoing signals from ground‐based VLF transmitters undergo a significant spectral broadening as they propagate through the ionosphere and protonosphere up to altitudes in the range 600–3800 km. For transmitter signals in the range 10–20 kHz, the spectral broadening can be as high as 10% of the nominal frequency of the input signal. Spectral broadening occurs only in the presence of impulsive VLF hiss and/or a lower hybrid resonance (LHR) noise band with an irregular lower cutoff frequency, and only for signals whose frequency exceeds the LHR frequency at the satellite location. It is often observed in association with a band of impulsive ELF hiss below 700 Hz. In many cases, the bandwidth of the spectrally broadened signals is a strong function of the electric dipole antenna orientation with respect to the local direction of the earth's magnetic field. Unusual dispersion in the components of the spectrally broadened pulses suggests that the spectral broadening may be due to a doppler shift effect in which the initial signals scatter from irregularities in the F region and couple into quasi‐electrostatic modes of short wave length. The large doppler shift associated with these short wavelength modes produces a significant increase in the bandwidth of the signal, as observed on a moving satellite. Since impulsive VLF hiss and irregular LHR noise bands have been linked to energetic (<1 keV) electron precipitation in the past, it is conjectured that the spectral broadening effect may be driven by precipitating electrons.
The extreme ultraviolet day airglow
Authors: Chakrabarti Supriya , Paresce Francesco , Bowyer Stuart , Kimble Randy , Kumar Shailendra
Published: February 2008

Satellite observations of the earth's extreme ultraviolet day airglow between 350 and 1400 Å are described. The atomic spectrum shows lines of O II (538–539, 555, 601, 617, 673, 718, 834), He I 584, O I (989, 1152, 1304, 1356), N II (916, 1085), N I (1134, 1200), and H I (1025, 1216, and possibly 973). Previously unobserved weak O II lines (515, 482, 470, 442) are observed below 530 Å. The Lyman‐Birge‐Hopfield (LBH) and Birge‐Hopfield (BH) bands of N2 between 900–1100 Å are the dominant molecular lines. Large scale high latitude and equatorial enhancements and hemispheric asymmetries are evident in the near zenith O II 834, O I 989 and N II 1085‐Å line intensities.
Stare radar observations of a PG pulsation
Authors: Poulter E. M. , Allan W. , Nielsen E. , Glassmeier K.‐H.
Published: February 2008

STARE radar observations of a Pg pulsation are used to study the spatial variation of the hydromagnetic wave electric fields in the ionosphere. Both the north‐south and east‐west electric fields appear to exhibit resonant behavior. The oscillation is confined to approximately 2° of latitude and both components have similar amplitude maxima of order 40 mV m−1. The longitudinal phase variations yield an azimuthal wave number m ∼ 17 and the corresponding ionospheric phase velocity is 8 km s−1 westward. Although apparently resonant, the magnitude of the east‐west electric field is considerably larger than for previously reported resonant Pc 5's thought to originate in the Kelvin‐Helmholtz instability on the magnetopause. It is suggested that the Pg may be an even mode oscillation generated by a bounce resonance instability.
Terrestrial versus Jovian VLF chorus; A comparative study
Authors: Inan U. S. , Helliwell R. A. , Kurth W. S.
Published: February 2008

The relevant parameters of the magnetospheres of Jupiter and earth are studied from the point of view of wave‐particle resonant interactions that are believed to be responsible for the generation of VLF chorus emissions observed on Voyager‐1. Using existing models of the cold and energetic plasma distributions in the Jovian magnetosphere, expressions for the wave‐particle interaction length (LI) and the nonlinearity parameter (ρ) are derived. Values of these parameters are compared with those computed for the earth's magnetosphere. It is found that the typical interaction lengths are at least 2–5 times larger in the Jovian than in the terrestrial magnetosphere. Also, the wave intensity necessary to reach the threshold of nonlinearity in the Jovian magnetosphere was found to be up to 5–100 times lower. The Voyager 1 measurements show, however, that the inferred wave magnetic field intensities of the Jovian chorus are in the range of reported intensities for terrestrial chorus. This is attributed to fact that the fluxes of few keV resonant particles found in the Jovian magnetosphere were typically two orders of magnitude higher. In this case, it is predicted that the temporal growth rates of Jovian chorus bursts should be higher than for the earth. Growth rate measurements on Voyager 1 broadband wave data are used to confirm this hypothesis.
Longitudinal structure of substorm injections at synchronous orbit
Authors: Arnoldy R. L. , Moore T. E.
Published: February 2008

From multiple‐spacecraft measurements it is shown that the synchronous orbit manifestation of a substorm, i.e., plasma injection and magnetic field reconfiguration to dipolar, has an onset which expands both eastward and westward from a relatively narrow sector near midnight. For low‐to‐moderate geomagnetic activity the earliest onset sector at synchronous orbit is about 3 hours wide, skewed toward the evening side of midnight. Using the extensive International Magnetospheric Study ground magnetometer network beneath the satellites, it is found that simultaneous westward motion of electrojet intensification is seen in the ground data over a larger longitudinal range than the magnetospheric signatures. This might be explained in terms of distortion of the nightside magnetic field at synchronous orbit. Plasma that has already been injected near midnight at synchronous altitude undergoes no further change as a result of the westward and eastward motion of the borders of the plasma. This suggests that the expansion does not represent new substorm activation. One can interpret these results in terms of an injection front wedge which makes hot plasma accessible to the inner magnetosphere and which spatially expands or propagates with time. The origin of plasma behind the front is not addressed. Detailed pitch angle data do, however, show that strong precipitation would be expected from the front for about the first 10 min after the front passes over an observer. Finally, as a result of longitudinal expansion of plasma injections from midnight toward evening or morning and the continuation or even enhancement of field inflation in the evening or morning sectors, growth and expansive phase substorm signatures can occur simultaneously. Substorm expansion at midnight can thus be accompanied by continued growth signatures in evening or morning.
Ionospheric plasma bubble encounters or F region bottomside traversals?
Authors: Benson Robert F. , Brinton Henry C.
Published: February 2008

In situ AE‐C and AE‐E ion composition measurements, together with Manila and Huancayo ionosonde electron density profiles, are used in an attempt to distinguish between spacecraft encounters with equatorial plasma bubbles which have been pinched off from below, those still in the formation stage, and spacecraft excursions below the steep ionization gradient at the bottom edge of the postsunset F layer. Such excursions can result from quasi‐periodic oscillations of the altitude of the F layer—as deduced from the ion composition measurements during the circular orbit phase of the low‐inclination satellite AE‐E. It is found that depletion features that appear to be due to topside bubbles which have been pinched off from below seldom have ion concentration reductions as high as a factor of 10²; concentration drops associated with bubbles directly connected to bottomside plasma via vertical plasma density contours or those due to bottomside excursions, however, can be nearly a factor of 104. In the former case, O+ remains the dominant ion; in the latter case NO+ often becomes dominant. These results have important theoretical implications in that they are consistent with placing the bubble generation region on the steep density gradient of the lower ledge of the F region where O+ is the dominant ion.
Collisional modification to the exospheric theory of solar wind halo electron pitch angle distributions
Authors: Lemons Don S. , Feldman William C.
Published: February 2008

A theoretical model of suprathermal halo or strahl solar wind electrons, including both binary collisions and conservative force fields, is proposed. From this model we derive the collisionally modified electron pitch angle distribution and compare it with relevant measurements made in the solar wind at 1 AU by using the Los Alamos IMP 8 plasma analyzer. Although the collisionally modified distribution is more isotropic than that predicted by simple exospheric theory, it is not isotropic enough to describe the measurements.
Charged particle distributions in Jupiter's magnetosphere
Authors: Divine Neil , Garrett H. B.
Published: February 2008

A compact, quantitative model of the distribution of charged particles between 1 eV and several MeV in the Jovian magnetosphere is presented. The model is based primarily on in situ data returned by experiments on the Pioneer and Voyager spacecraft, supplemented where necessary by earth‐based observations and theoretical considerations. Thermal plasma parameters, notably convection speed, number density, and characteristic energy, are specified as functions of position for electrons and several ion species (H+, O+, O++, S+, S++, S+++, and Na+). At intermediate energy the electron and proton populations are modeled using kappa distributions, which join smoothly onto the radiation belt spectra at high energies (E > 100 keV). At these energies the radiation belt intensity spectra include angular distributions for energetic electrons inside 16 RJ and protons inside 12 RJ. Major features of the magnetic field, thermal plasma, and trapped particle distributions are modeled, such as ring and satellite absorption signatures and corotational flow within the Io plasma torus and the current disk. Within each plasma region the model results are compared with observed spectra, showing that the model represents the data typically to within a factor of 2±1 except where time variations, neglected in the model, are known to be significant. Several practical applications of the model to spacecraft near Jupiter are illustrated with sample results from radiation analyses and electrostatic charging calculations.