bluuebunny/crossref_metadata_2025 huggingface dataset:
A two‐fluid solar wind model with Alfven waves: Parameter study and application to observations
Authors: Esser Ruth , Leer Egil , Habbal Shadia R. , Withbroe George L.
Published: February 2008

The effects of Alfven waves from the inner corona on the solar wind density profile, flow velocity and on the random motion of protons are studied. Different base densities, temperatures, and wave velocity amplitudes, as well as different flow geometries, are considered. The model calculations are compared to simultaneous observations of the electron density profile and the resonantly scattered Lyman alpha line. Present observations, out to 4 solar radii, can be used to place limits on the coronal base density and temperature, and put an upper limit on the wave amplitude. It is pointed out that future observations of the electron density and the Lyman alpha line, out to larger heliocentric distances, and of lines from heavier elements, should be used to place more stringent constraints on the amplitudes of MHD waves in the corona.
Numerical simulation of spacecraft charging by impact‐induced plasmas during a cometary flyby
Authors: Thiemann H. , Singh N. , Schunk R. W. , Grard R.
Published: February 2008

The European Space Agency spacecraft Giotto will encounter comet Halley in March 1986 with a relative velocity of 69 km s−1. The fore section of the surface will be bombarded by neutral molecules which will produce secondary electrons and sputtered ions; a plasma cloud will form around the spacecraft, and its surface will be electrically charged. This paper summarizes the results which have been obtained from a numerical simulation of this phenomenon. It is found that a conductive spacecraft will reach a positive potential of the order of 10 V and that a positive potential barrier will develop in the close vicinity of the emitting surface, provided the flux of sputtered ions is sufficient. The charging time constant is approximately equal to the ion plasma period determined by the large ion density in front of the spacecraft. The electron distribution around the spacecraft is nearly isotropic, but the ion diffusion is limited to the forward direction. Thus a well‐defined ion wake forms behind the spacecraft. Some numerical aspects of the simulations are also discussed. In particular, it is found that the simulation of spacecraft charging by impact‐generated plasmas with the real ion masses is not prohibitively expensive using modern computers.
On the correlation of whistlers and associated subionospheric VLF/LF perturbations
Authors: Inan U. S. , Carpenter D. L.
Published: February 2008

Several periods of whistler‐associated subionospheric signal perturbations (i.e., Trimpi events) observed at Palmer, Antarctica (L ∼2.4) have been studied. In a case study of the time signature of the signal perturbations during a 10‐min recording period on March 30, 1983, the time delay between the whistler‐producing spheric and the onset of the change was found to be in the range 0.52–0.62 s, independent of the amplitude of the change. Event amplitude, as expected from previous work, was found to be well correlated with the associated whistler wave intensity. Other temporal features such as the rise and decay times of the perturbations were also found to be independent of the event amplitude. These results are consistent with a recent theoretical model of gyroresonant particle scattering interaction in the magnetosphere. The amplitudes of simultaneous perturbations on signals of different frequency and arrival bearing were well correlated in several cases, but exhibited case to case differences that appear to depend upon the spatial distribution of precipitation regions. Whistler occurrence times during two recording periods were found to approximately obey a Poisson distribution, while the statistics of Trimpi event occurrence in those periods showed significant deviations from Poisson behavior. The probability of random alignment in time of the whistlers subionospheric and perturbations is estimated to be < 10−14, in agreement with previous inferences of a cause and effect relation between whistlers and VLF/LF perturbations. Although nearly all of the whistlers observed originated in northern hemisphere lightning, the first example was found of a Trimpi event associated with a southern hemisphere source.
A case study of plasma processes in the dayside cleft
Authors: Baker K. B. , Greenwald R. A. , Walker A. D. M. , Bythrow P. F. , Zanetti L. J. , Potemra T. A. , Hardy D. A. , Rich F. J. , Rino C. L.
Published: February 2008

On November 28, 1983, between 1638 UT and 1642 UT, the HILAT satellite moving approximately along the noon meridian passed over northeastern Canada and encountered the dayside high latitude cleft. During the passage an upward current sheet bounded to the north and south by weaker downward current sheets was observed. The current sheets, particularly the upward current sheet, were immersed in a region of fairly intense low energy electron precipitation of the type commonly observed in the polar cleft. Simultaneous radar measurements were made in a region slightly to the east of the satellite track and in which beacon measurements between HILAT and a ground station at Sondre Stromfjord, Greenland, were being conducted. The radar indicated an extended, magnetic east‐west‐aligned region of small‐scale irregularities at the same magnetic latitude as the cleft‐related current sheets and precipitation. The HILAT beacon experiment showed the strongest scintillations within the same region. On the basis of the available data, it appears as if the convective flows from the dawn and dusk sectors were converging and rotating poleward within the observational volume. The irregularities producing the scintillations and radar backscatter may have been produced by the combined influence of the gradient drift and current convective plasma instabilities, the former process being excited by the poleward convection of plasma parallel to the precipitation‐induced density gradient and the latter process by the upward field‐aligned current. The apparent absence of irregularities further into the polar cap may be accounted for by the stabilizing effect of plasma drift antiparallel to the density gradient on the poleward edge of the cleft precipitation zone.
Finite parallel resistivity and uniform plasma motion across magnetic fields
Authors: Sperling J. L.
Published: February 2008

Using a relatively simple three‐dimensional model for a stratified plasma cloud surrounded by uniform ambient plasma, configurations of the density profile within the cloud are determined which permit uniform E × B motion of the cloud. The applied current density field which drives induced electrostatic potentials is permitted to have variation along the direction of the magnetic field and finite parallel resistivity is explicitly considered. The analysis and calculations suggest that exact uniform motion of the plasma cloud requires the plasma density within the cloud be smallest where the neutral wind is largest. Conversely, the plasma density is largest where the neutral wind is smallest. Quantitative examples suggest that the uniform motion of a long high‐density plasma cloud across the magnetic field is most likely to occur if the neutral wind has a component which is relatively uniform along the length of the plasma cloud. Qualitative application of the analytical and quantitative results is made to an assemblage of striations which are assumed to be a set of planar plasma sheets.
The shuttle environment: Effects of thruster firings on gas density and composition in the payload bay
Authors: Wulf E. , von Zahn U.
Published: February 2008

In order to study the gaseous environment of the STS orbiter a mass spectometer was operated aboard the SPAS‐01 subsatellite on the shuttle missions STS 7 and 41‐B. Data are reported on the impact of the operation of attitude control thrusters on the atmosphere in the payload bay. Firings of aft mounted thrusters have been observed to increase the total number density inside the payload bay to more than 1018 m−3 (equivalent to more than 7×10−5 mbar), whereas the operation of most of the forward mounted thrusters yields hardly a detectable signal. Nitric oxide is a major gaseous constituent inside the payload bay during the operation of aft mounted thrusters. Water desorbing from surfaces leads to a slowly decaying total number density following engine shutdown.
On ion acceleration in interplanetary quasi‐perpendicular shock waves
Authors: Pesses M. E. , Decker R. B.
Published: February 2008

The kinematic and pitch angle scattering constraints for multiple shock encounters to occur are investigated. The results suggest that (1) large particle anisotropies and a large number of shock encounters are not mutually exclusive and (2) solar wind ions are not directly injected into the nearly perpendicular interplanetary shock acceleration process. Also, the average number of shock encounters required to accelerate an ion between two energies is calculated. The results are consistent with the shock geometry dependence observed in interplanetary shock spike and corotating interaction region associated energetic ion events.
Quasi‐stagnant plasmoid in the middle tail: A new preexpansion phase phenomenon
Authors: Nishida A. , Scholer M. , Terasawa T. , Bame S. J. , Gloeckler G. , Smith E. J. , Zwickl R. D.
Published: February 2008

From the analysis of ISEE 3 data we have found that a plasmoid is sometimes formed in the middle tail outside the intervals of the substorm expansion phase. This plasmoid is produced by reconnection at the X‐type neutral line, which is located earthward of the distant neutral line but beyond the substorm‐associated near‐tail neutral line, and it is almost stagnant in that the associated flow speed is less than 300 km/s. The blocking effect of the distant neutral line is the most probable reason for the slow movement. The quasi‐stagnant plasmoid is observed at x = −60 to −100 RE for a duration of a few tens of minutes, and in about one half of the cases it is followed by the fast tailward streaming. The onset of this streaming tends to coincide with the onset of the substorm expansion phase, and this probably occurs when the reconnection at the middle‐tail neutral line comes close to processing the last closed field line. Intensification of the dawn‐to‐dusk electric field that causes the mantle plasma to reach the plasma sheet boundary closer to the earth is suggested as the reason for the formation of the middle‐tail neutral line earthward of the distant neutral line. The effects on the energetic particle flux and relation to the near‐tail reconnection are also discussed.
ELF and VLF signals radiated by the “polar electrojet antenna”: Experimental results
Authors: Barr R. , Stubbe P. , Rietveld M. T. , Kopka H.
Published: February 2008

The heating facility at Ramfjordmoen near Tromsø, Norway, has been used to modulate the auroral electrojet at frequencies in the range 223 Hz to 5.44 kHz. ELF/VLF signals have been received at Lycksele, Sweden, 554 km from the heating transmitter, over the whole frequency range with maximum amplitudes of ∼50 fT. Both azimuthal and radial magnetic field components were recorded and the ratio of these two components, commonly termed the polarization, was determined. The experimental results have been successfully modelled by using waveguide mode theory and assuming the heating source to be a point dipole located in the ionosphere at the height of the maximum ELF/VLF Hall current.
Meridional winds in the thermosphere derived from measurement of F2 layer height
Authors: Miller K. L. , Torr D. G. , Richards P. G.
Published: February 2008

Dynamics of the neutral atmosphere play an important role in the altitude distribution and peak electron density of the ionospheric F region. In this paper we describe a method for deriving neutral meridional winds from ground‐based measurements of the height of the F2 layer (hmax). This method involves the use of an ionospheric model to develop the relationship between meridional neutral wind and hmax at specified location, time, and solar conditions. The meridional wind is derived from a comparison of the modeled layer height with the measured height. Good agreement is found when the results are compared with meridional winds from Fabry‐Perot interferometer and incoherent‐scatter radar measurements. This method can produce results comparable with other methods, and has the advantage that it can use as input F2 layer heights from true‐height analysis of ionosonde measurements, which are already being made systematically and on a global scale.
A simulation study of magnetic reconnection: Transition from a fast mode to a slow mode expansion
Authors: Lee L. C. , Fu Z. F.
Published: February 2008

Magnetic reconnection is studied based on a two‐dimensional magneto‐hydrodynamic simulation model. We observe in our simulation study both the magnetic reconnection with a fast mode expansion (Petschek model) and the reconnection with a slow mode expansion (Sonnerup‐Yeh‐Axford model) upstream of the field reversal region. It is found that the slow mode expansion tends to occur when the system length is long, the imposed reconnection rate is high, or the resistivity is large. On the other hand, the fast mode expansion becomes dominant for a system with a short length, a low reconnection rate, or a small resistivity.
The Condor Equatorial Spread F Campaign: Overview and results of the large‐scale measurements
Authors: Kelley M. C. , LaBelle J. , Kudeki E. , Fejer B. G. , Basu SA. , Basu Su. , Baker K. D. , Hanuise C. , Argo P. , Woodman R. F. , Swartz W. E. , Farley D. T. , Meriwether J. W.
Published: February 2008

During the Condor campaign a number of instruments were set up in Peru to support the rocket experiments. In this series of papers we report on the results of the experiments designed to study the equatorial F region. In this overview paper we summarize the main results as well as report upon the macroscopic developments of spread F as evidenced by data from backscatter radars, from scintillation observations, and from digital ionosonde meaurements. In this latter regard, we argue here that at least two factors other than the classical gravitational Rayleigh‐Taylor plasma instability process must operate to yield the longest‐scale horizontal organization of spread F structures. The horizontal scale typical of plume separation distances can be explained by invoking the effect of a shear in the plasma flow, although detailed comparison with theory seems to require shear frequencies a bit higher than observations indicate. On the other hand, the largest‐scale organization or modulation of the scattering layer cannot be explained by the shear theory and must be due to local time variations in the ionospheric drift or to gravity wave induced vertical motions. Using simultaneous rocket and radar data, we were also able to confirm the oft quoted hypothesis that rapid overhead height variations in the scattering region over Jicamarca are primarily spatial structures advecting overhead. The detailed rocket‐radar comparison verified several other earlier results and speculations, particularly those made in the PLUMEX experiments. In particular, companion papers discuss and extend some of the PLUMEX results to include the role of anomalous diffusion (LaBelle et al., this issue) in the theory of equatorial spread F and to shed light upon the shallow spectral form often observed in the intermediate‐scale regime (LaBelle and Kelley, this issue).
Strong electron bidirectional anisotropies in the distant tail: ISEE 3 observations of polar rain
Authors: Baker D. N. , Bame S. J. , Feldman W. C. , Gosling J. T. , Zwickl R. D. , Slavin J. A. , Smith E. J.
Published: February 2008

A detailed observational treatment of bidirectional electrons (∼50 to 500 eV) in the distant magnetotail (r ≳ 100 RE) is presented. It is found that electrons in this energy range commonly exhibit strong, field‐aligned anisotropies in the tail lobes. Because of large tail motions, the ISEE 3 data provide extensive sampling of both the north and south lobes in rapid succession. These data demonstrate directly the strong asymmetries that exist between the north and south lobes at any one time. The bidirectional fluxes are found to occur predominantly in the lobe directly connected to the sunward interplanetary magnetic field in the open magnetosphere model (north lobe for away sectors and south lobe for toward sectors). Electron anisotropy and magnetic field data are presented which show the transition from unidirectional (sheath) electron populations to bidirectional (lobe) populations. Thus we demonstrate the open nature of the distant magnetopause and show that the source of the higher‐energy, bidirectional lobe electrons is the tailward directed electron heat flux population in the distant magnetosheath. Taken together, the present evidence suggests that the bidirectional electrons that we observe in the distant tail are closely related to the polar rain electrons observed previously at lower altitudes. Furthermore, these data provide strong evidence that the distant tail is composed largely of open magnetic field lines in contradistinction to some recently advanced models.
Superposed epoch analysis of corotating interaction regions at 0.3 and 1.0 AU: A comparative study
Authors: Richter A. K. , Luttrell A. H.
Published: February 2008

Applying the superposed epoch analysis technique to 16 and to 31 well‐defined, nonshock‐associated stream‐stream interaction regions observed by the Helios spacecraft in the distance ranges 0.3 to 0.4 AU and 0.9 to 1.0 AU, respectively, we obtain the average azimuthal variation in the solar wind density, velocity and temperature, in the magnetic field strength, and in the total proton plasma plus magnetic field pressure across CIRs at these two radial distances separately. For the radial evolution of these interaction regions we find by comparison: (1) due to compressional and rarefactional effects the amplitudes of all parameters in question taken along the leading as well as along the trailing part of the CIR are steadily increasing with the most pronounced increase in the pressure; (2) at the same time even the leading portion of the velocity profile steepens; (3) simultaneously, the positions in azimuth of the overall maximum values of the solar wind density and temperature, of the magnetic field strength and of the plasma plus magnetic field pressure are getting steadily lined up in longitude; (4) at the same time the leading portions of all profiles are steepening into discontinuous, shocklike structures. Thus, this analysis provides observational evidence for the following results obtained earlier from numerical simulation studies: Stream steepening does occur within 1 AU, and the probability of corotating shocks to form is, on average, much higher beyond than at or within 1 AU.
Precipitation of suprathermal ionospheric ions accelerated in the conjugate hemisphere
Authors: Bosqued J. M. , Sauvaud J. A. , Delcourt D. , Kovrazhkin R. A.
Published: February 2008

Low‐energy particle data from the AUREOL 3 satellite were examined in order to characterize latitude‐energy precipitation bands of low‐energy ions (<3 keV) sometimes detected embedded in the discrete auroral regions. A clear energy dispersion is observed, the less energetic ions being observed equatorward of higher‐energy precipitation. Generally the latitudinal extent of this structure is about ΔΛ ≃ 0.5°. Detailed ion energy spectra were used to infer that ionospheric ions are energized in the conjugate ionosphere, presumably in inverted‐V structures, and undergo velocity‐dependent electric drift effects during a single bounce. Model calculations and direct ion composition measurements clearly demonstrate that the H+ ions are predominant in these structures. Furthermore in several cases a double energy band is observed associated with each side of the conjugate V‐shaped electrostatic potential structure.
Excitation of MHD waves upstream of Jupiter by energetic sulfur or oxygen ions
Authors: Goldstein M. L. , Wong H. K. , Eviatar A.
Published: February 2008

Large fluxes of heavy ions have been reported upstream of Jupiter's bow shock as Voyager 1 approached the planet (Zwickl et al., 1981; Krimigis et al., 1985). Enhanced low‐frequency magnetic wave activity was also observed during the particle events. The fluctuations are left‐hand eliptically polarized in the plasma frame. The spectrum of these fluctuations contains a peak close to the Doppler‐shifted resonance frequency of a sulfur or oxygen beam with streaming energy of approximately 30 keV. These fluctuations are also present in the spectrum of the magnitude of the field. We conclude that the observations result from an instability driven by an energetic beam of either sulfur or oxygen. The wave observations can be described by a heavy ion distribution with both a streaming anisotropy and a temperature anisotropy (T⊥/T∥ ≃ 2). This class of heavy ion streaming instabilities may also play a role in wave‐particle interactions in the vicinity of comets.
Radar studies of long‐wavelength waves associated with mid‐latitude sporadic E layers
Authors: Riggin D. , Swartz W. E. , Providakes J. , Farley D. T.
Published: February 2008

The Cornell University Portable Radar Interferometer (CUPRI), a 50‐MHz Doppler radar system, was operated during May and August/September 1983 on the island of St. Croix (17.7°N, 64.8°W) to study the plasma instabilities associated with nighttime sporadic E layers. Two events, on May 7 and August 22, show evidence of large‐amplitude waves, with apparent horizontal wavelengths of 10–12 km and periods of 2–6 min. These apparent wavelengths are upper limits for the true wavelengths. The CUPRI beam was directed over Arecibo, Puerto Rico, and on May 7, concurrent electron density profiles within the CUPRI scattering volume were measured by the Arecibo Observatory's 430‐MHz radar. During the stronger event of August 22, radar echoes were received from several altitudes up to 130 km. Large mean Doppler velocities (at times exceeding 250 m/s) were observed during this event, and the power spectra closely resemble those obtained at the magnetic equator during type 1 conditions. We believe (1) that the mid‐latitude large‐scale waves are generated by the same gradient drift instability mechanism responsible for equatorial large‐scale waves and (2) that the type 1 3‐m waves can be generated at mid‐latitudes with drift velocities well below the sound speed because of the very sharp plasma density gradients associated with metallic ion sporadic E layers.
Coulomb collisional domains in the solar wind
Authors: Livi S. , Marsch E. , Rosenbauer H.
Published: February 2008

The role of Coulomb collisions in the solar wind, their number of occurrence for protons, and the correlation between collisional effects and the stream structure and large‐scale interplanetary magnetic field are investigated, Helios plasma data, displayed in Carrington rotation plots, clearly show that the relative number of collisions (ratio of expansion over collision time scale) maximizes near the heliospheric current sheet, which is revealed as the main collisional domain of the solar wind. Statistical results indicate that for about 20% of the time the low‐speed solar wind plasma is influenced strongly by Coulomb collisions.
Observations of the flow of H+ and He+ along magnetic field lines in the plasmasphere
Authors: Chandler M. O. , Chappell C. R.
Published: February 2008

The dynamics of earth's plasmasphere is affected by gradients in the plasma pressure, the convection electric field, and the corotation electric field. The Retarding Ion Mass Spectrometer (RIMS) on Dynamics Explorer 1 measures the pitch angles of this plasma from which the total plasma velocity vector can be determined. Using assumptions concerning the relative importance of convection and corotation, the velocity of ions along the magnetic field line can be determined. These results represent the first direct observations of ion velocities along magnetic field lines at high altitudes in the plasmasphere. Data from October 1981 through January 1982 have revealed several interesting aspects of the field‐aligned flow of H+ and He+. Downward directed velocities observed in the northern hemisphere within 30° of the equatorial plane are interpreted as interhemispheric flows. Typical velocities are of the order of a few hundred meters per second. Higher‐speed (up to 1 km s−1) flows observed in the outer plasmasphere appear to be associated with refilling. Counterstreaming of H+ and He+ has been observed in the autumn and winter hemisphere with He+ flowing upward and H+ flowing downward. Differences between these results and theoretical work can probably be attributed to differences in the season and the density level within the flux tube.
Slot region electron precipitation by lightning, VLF chorus, and plasmaspheric hiss
Authors: Imhof W. L. , Voss H. D. , Walt M. , Gaines E. E. , Mobilia J. , Datlowe D. W. , Reagan J. B.
Published: February 2008

Energetic electrons are precipitated from the slot region of the radiation belts by a variety of mechanisms, including short duration wave bursts associated with lightning and chorus and more slowly varying plasmaspheric hiss. Characteristics of the nightside short duration precipitation events, including their favored occurrence at certain longitudes in the northern hemisphere, indicate that they are predominantly associated with lightning. The dayside events seem to relate primarily to VLF chorus. Here a study is made of various characteristics of the short duration precipitation bursts, namely, the longitude and L shell variations, the day/night differences, the energies of spectral maxima, and the rapid spectral variations with time. In addition, the total loss rates of electrons from the radiation belts are obtained from the measured energy spectra and pitch angle distributions. An assessment is made of the relative importance of the bursts as a loss mechanism for slot region electrons in comparison to the more slowly varying precipitation processes. The assessment is based on a comparison of the energies of the peaks often observed in the energy spectra of both classes of precipitation. The slowly varying electron fluxes observed in the drift loss cone frequently display peaks in the energy spectra which indicate that they are precipitated by plasmaspheric hiss and that this process therefore represents a major loss mechanism. Well‐defined bursts, which appear to be associated with lightning at nighttime and perhaps chorus in the daytime, were observed on only about 2% of the satellite passes and are generally of lower energy than the general population in the drift loss cone, suggesting that the bursts are not the dominant loss mechanism. There is some overlap between the distributions in peak energy, and lightning may play a role in precipitating electrons from the slot region primarily at nighttime, with its greatest contribution at low L shells.
Electric and magnetic observations of the structure of standing waves in the magnetosphere
Authors: Cahill L. J. , Lin N. G. , Engebretson M. J. , Weimer D. R. , Sugiura M.
Published: February 2008

Electric and magnetic field instruments on the DE 1 spacecraft provided observations of toroidal standing wave oscillations of magnetic shells in 1981 and 1982. The amplitude and nodal structure of these waves, as a function of increasing magnetic latitude, was studied and compared with model calculations of these waves. Eight pulsation events were selected for study. The pulsations in these events ranged in period from 20 to 300 s. They were observed at L values between 2.8 and 9.7 and at magnetic latitudes from the equator to 48°. The duration of these pulsation events was 20 to 30 min, and they usually consisted of a 5‐ to 10‐min growth in amplitude, a 5‐ to 10‐min interval of fluctuating amplitude, and a 5‐ to 20‐min interval of decreasing amplitude. The relative E and B amplitudes and the phase relations between the E and B waves could be qualitatively explained by the nodal structure of the waves predicted by model calculations. Detailed quantitative comparisons of E and B amplitudes were in agreement with more recent model calculations. In a few cases, electron densities were also available as derived from plasma frequency observations; in these cases the standing wave periods calculated were reasonably close to, but somewhat less than, the observed periods.
Star of Lima: Overview and Optical Diagnostics of a Barium Alfvén Critical Velocity Experiment
Authors: Wescott E. M. , Stenbaek‐Nielsen H. C. , Hallinan T. , Föppl H. , Valenzuela A.
Published: February 2008

The Alfvén critical velocity mechanism for ionization of a neutral gas streaming across the magnetic field has been demonstrated in laboratory experiments. In March 1983, two rocket‐borne experiments with Ba and Sr tested the effect in the wall‐less laboratory of space from Punto Lobos, Peru, near 430 km altitude. “Star of Lima” used a conical Ba shaped charge aimed at an instrument payload about 2 km away. Because of rocket overperformance the detonation occurred in partial sunlight, so that less than 21.6% of the ionizing UV was present. As discussed in other papers in this series, particle and field measurements indicate the production of hot electrons and waves in the energy and frequency range respectively predicted to produce a cascade of ionization by the Alfvén mechanism. However, the ionization fluxes and wave energy density did not reach cascade levels, and optical observations indicate that only 2.5 to 5×1020 Ba ions were produced. A substantial portion and perhaps all of the ionization could have been produced by solar UV. The failure of the Alfvén process in this experiment is not well understood.
Sensitivity of the curve‐of‐growth technique utilized in rocket experiments to determine the line shape of solar He I resonance lines
Authors: Wu C. Y. Robert , Ogawa H. S.
Published: February 2008

We have theoretically examined the sensitivity of the curve‐of‐growth (COG) technique utilized in rocket measurements to determine the line profiles of the solar He I resonance emissions. In the present work we are primarily interested in the possibility of determining the line core shape using this technique. We have selected the line at 584.334 Å as an illustration. Various possible source functions of the solar line have been assumed in the computation of the integrated transmitted intensity. A recent observational data set obtained by our group is used as the constraint of the computation. We have confirmed that the COG technique can indeed provide a good measurement of the solar line width. However, to obtain detailed knowledge of the solar profile at line center and in the core region, (1) it is necessary to be able to carry out relative solar flux measurements with a 1% or better precision, and (2) we must be able to measure the He gas pressure in the absorption cell to lower than 0.1 mtorr. While these numbers apply specifically to our own geometry, the results are readily scaled to other COG measurements using other experimental parameters.
Jovian 1.2‐kHz nonthermal continuum radiation
Authors: Leblanc Yolande , Jones Dyfrig , Rucker Helmut O.
Published: February 2008

Nonthermal continuum is observed at 1.2 kHz on Voyagers 1 and 2 within the Jovian magnetosphere. It is seen in the magnetospheric cavity on both the dayside and nightside, being most intense in the magnetotail lobes when Voyagers were above the plasma sheet. In these regions the radiation was distinctly left‐hand polarized. The observations are considered within the context of other plasma waves reported to exist in the Jovian magnetosphere and of analogous emissions observed at earth. Drawing in particular from our knowledge of continuum source regions within the terrestrial magnetosphere, it is suggested that the most likely main source of the 1.2‐kHz Jovian continuum is the morning/prenoon magnetopause.
Antisymmetric standing wave structure associated with the compressional Pc 5 pulsation of November 14, 1979
Authors: Takahashi Kazue , Higbie Paul R.
Published: February 2008

Model calculations are performed to explain ion and electron flux pulsations observed at geostationary orbit in association with the compressional Pc 5 magnetic pulsation of November 14, 1979. The amplitude of the flux perturbation is calculated based on the observed value of the wave frequency, the azimuthal wave number, and the average distribution f(W, α) of the particles, for assumed forms of a standing wave structure along the ambient magnetic field. A theory developed by Southwood and Kivelson is followed to calculate the changes in energy and the location of the guiding center, which are experienced by particles in a given wave, and the consequential modulation of particle fluxes. It is found that if the compressional magnetic field component b∥ of the wave has a symmetric structure about the equator, net acceleration of the particles contributes significantly to the total amplitude of flux pulsations; whereas if b∥ has an antisymmetric structure, the total energy is conserved with the exchange of perpendicular and parallel energy leading to the modulation of the pitch angle distribution as the dominant cause of the flux pulsation. For the latter case, local magnetic field compression and the pitch angle dependence of the average particle distribution determine the amplitude of the flux pulsation. We find that antisymmetric models of b∥ reproduce the observations better than symmetric models. This result implies that there exists a class of compressional Pc 5 waves that do not have a fundamental standing wave structure.
Waves generated during electron beam emissions from the space shuttle
Authors: Neubert T. , Taylor W. W. L. , Storey L. R. O. , Kawashima N. , Roberts W. T. , Reasoner D. L. , Banks P. M. , Gurnett D. A. , Williams R. L. , Burch J. L.
Published: February 2008

Some results from the SEPAC (Space Experiments with Particle Accelerators) experiment flown on the Spacelab 1 shuttle mission are presented. The active part of the experiment included the injection of electron beams at currents up to 300 mA, and energies up to 5 keV. The main focus of the paper is on the wave activity observed during these beam emissions. It is found that the noise in the 0.75‐ to 10‐kHz VLF band has a spectral shape that can be characterized by an f−n law, and that the VLF signal level depends on the beam angle to the magnetic field, the strongest emissions being observed for parallel beams. These features are found to be consistent with a drift wave instability. In the 0.1‐ to 8‐MHz frequency range, very little activity is detected, probably due to a too low gain setting. Reference is made to results from two other electron beam experiments performed from the space shuttle, namely PICPAB (Phenomena Induced by Charged Particle Beams), also flown on Spacelab 1, and FPEG (Fast Pulse Electron Gun) flown on STS 3. Some comments on the space shuttle as a platform for investigation of beam‐plasma interactions are provided.
Dissociative Excitation of the N+(5S) state by electron impact on N2: Excitation function and quenching
Authors: Erdman P. W. , Zipf E. C.
Published: February 2008

We have produced metastable N+(5S) ions in the laboratory by dissociative excitation of N2 with energetic electrons. The resulting radiative decay of the N+(5S) state was observed with sufficient resolution (<0.5 Å) to completely resolve the doublet from the nearby N2 molecular radiation. The excitation function was measured from threshold to 500 eV. The cross section peaks at a high electron energy (∼150 eV) and also exhibits a high threshold energy (∼37 eV), both of which are typical of dissociative excitation‐ionization processes. This finding complicates the explanation of electron impact on N2 as the mechanism for the source of the λ2145 Å “auroral mystery feature” by further increasing the required peak cross section. Preliminary results also indicate that quenching of the N+(5S) state by N2 is not an important factor in this laboratory experiment. It is suggested that the apparent N+(5S) quenching in auroras may be an artifact due to the softening of the electron energy spectrum in the auroral E region.
Ion tearing mode simulations with open boundary conditions
Authors: Price C. P. , Swift D. W.
Published: February 2008

The evolutionary dynamics of a magnetic neutral sheet are followed using a nonperiodic, two‐dimensional magnetoinductive particle‐in‐cell simulation. The purpose of this study is to observe the effects of particle and field boundary conditions. With open boundary conditions, particles can freely pass the simulation boundaries. Particles leaving the domain are lost forever, while other particles are continually reintroduced from the adjoining reservoirs. The statistical properties (density, flux, scalar pressure) of the reservoir particles, including gradients transverse to the boundaries, are specified. The field boundary conditions are similarly “open.” The effect of open boundary conditions on the ion tearing mode instability are studied by comparison with periodic simulations. The results show that there is little difference between the initial time evolution in the periodic and the open systems but that the boundary conditions can have a noticeable effect on the nonlinear phase of the tearing mode, especially on the properties of the magnetic islands. When there are no particles in the reservoir, particles at the ends of the simulation domain escape and an O‐type neutral line (magnetic island) is formed. When the reservoir particle distributions are maintained to give the same densities and pressures as existed in the initial distribution, the tearing instability leads to the ejection of all the particles from the simulation domain and the almost total collapse of the magnetic field.
Temporal evolution of the HF‐enhanced plasma line in the Arecibo F region
Authors: Djuth F. T. , Gonzales C. A. , Ierkic H. M.
Published: February 2008

The temporal development of the so‐called HF‐enhanced plasma line (HFPL) has been studied in detail using the high‐power high‐frequency (HF) facility located near Arecibo, Puerto Rico. Observations were made with the Arecibo 430‐MHz radar. In the current work, attention is focused on the evolution of the HFPL spectrum subsequent to the onset of the HF wave in the F region plasma. The development of the HFPL spectrum is examined for several different HF pulse lengths and HF duty cycles. In this regard, low duty cycles with long HF‐off periods tend to consistently produce a different spectral evolution than high duty cycles with short HF‐off periods. Broad, diffuse HFPL spectra are observed with low HF duty cycles, whereas sharply defined nonlinear Landau damping cascades develop at high duty cycles. Under a variety of observing conditions, the Arecibo HFPLs are found to develop dominant decay line peaks within a few milliseconds of HF turn‐on in the plasma. In addition, at early times during periods of slow HFPL growth, and also for very low HF power levels, weak flat‐top and/or double‐humped spectral features are observed that can be attributed to radio wave scattering off ion acoustic waves. During periods of strong HFPL excitation, weak spectral signatures are occasionally observed in the band 430 MHz ± (fHF + Δ), where fHF is the HF frequency and Δ = 10–100 kHz. The origin of this structure is currently not known. Finally, the temporal development of the 430‐MHz spectra is closely examined during the so‐called main plasma line overshoot. A diffuse spectral cascade is evident within a few milliseconds of the onset of the HF wave in the plasma, and dominant decay line peaks quickly develop thereafter. Processes involving direct conversion of the HF wave into Langmuir waves and/or soliton formation do not appear to be the dominant processes responsible for the 430‐MHz HFPLs observed in the Arecibo F region. However, the formation of solitons may influence the development of the HFPLs. The measured HFPL spectra are in qualitative agreement with the predictions of nonlinear Landau damping theory, but the experimental observations do not match the theoretical predictions in every detail. It is likely that HF‐induced small‐scale cavities and/or HF‐induced geomagnetic field‐aligned irregularities play a central role in determining the amplitude, spectral signature, and temporal development of strongly excited HFPLs at Arecibo.
Relationship between field‐aligned current density and potential difference of current‐driven double layer
Authors: Yamamoto Takashi , Kan J. R.
Published: February 2008

Relationship between field‐aligned current density and potential difference of current‐driven double layer is studied by one‐dimensional numerical simulations. The simulation domain is assumed to be located at a high altitude (≳ several earth radii) along the auroral field lines. The plasma sheet boundary supplies a plasma with number density n. Only the electrons injected from this boundary are allowed to have a drift velocity vd, which carries an upward field‐aligned current. The double layer potential is spontaneously produced by the space charge self‐consistently developed inside the simulation domain. For a given drift velocity vd, the relationship between the total current density J and the potential difference ϕ0 across the simulation domain is obtained by changing the number density n of the injection current. The results show that the double layer starts to be formed when the current density significantly exceeds the thermal current density of the ambient electrons and the potential difference ϕ0 increases with increasing current density J. It is also shown that at a given number density of the injection current the potential difference ϕ0 increases with increasing drift velocity vd.
Lidar studies of the nighttime sodium layer over Urbana, Illinois: 1. Seasonal and nocturnal variations
Authors: Gardner C. S. , Voelz D. G. , Sechrist C. F. , Segal A. C.
Published: February 2008

Sodium lidar measurements obtained between December 1980 and May 1986 at Urbana, Illinois, are compared with other reported lidar measurements and with recently developed models of the layer. Sodium abundance at Urbana reaches a maximum value in November, December and January which is approximately 4.5 times larger than the June minimum value of 2.15×109 cm−2. Also in November and December, the layer centroid height is about 1.5 km lower than the yearly average value of 92 km. There is no significant seasonal variation of the rms layer width which has an average value of approximately 4.25 km. The peak‐to‐peak variations of the centroid height and rms width of the average nocturnal layer are 1 km and 600 m, respectively. The semidiurnal tide is primarily responsible for a 36% peak‐to‐peak abundance variation in the average nocturnal layer which is compatible with a 20 cm s−1 amplitude for the vertical wind velocity. The vertical phase velocity of the density perturbations is approximately 1 m s−1 which implies a 45 km vertical wavelength for the semidiurnal tide. Because of gravity waves and tides the structure of the nocturnal layer changes substantially throughout the night. Variations of over 200% in abundance, 2 km in centroid height, and 1 km in rms width have been observed within the time span of a few hours.
Observations of energetic ions on the nightside of Venus
Authors: Kasprzak W. T. , Niemann H. B. , Mahaffy P.
Published: February 2008

Ions with energy greater than 40 eV have been measured on the nightside of Venus by the Pioneer Venus orbiter neutral mass spectrometer (ONMS). For periapsis altitudes beyond 1500 km they are confined to solar zenith angles greater than 120°. The nightside energetic ion composition at 2000 km resembles the nightside thermal ion composition occurring at 1000 km with the dominant species being O+. Postflight laboratory calibrations performed on the ONMS backup unit have been used to estimate the energetic O+ ambient flux and density. The direction from which the energetic ions appear to be coming can be determined because the ONMS is mounted at an angle with respect to the spin axis of the spacecraft. Both tailward and antitailward components of the apparent ion flow are exhibited when projected into the ecliptic plane. The major component of the apparent ion flow is observed perpendicular to the ecliptic plane.
Physical nature of the diffuse plasma resonances in the ionosphere
Authors: Osherovich Vladimir A.
Published: February 2008

A recently developed theory of force‐free electromagnetic oscillations (Osherovich, 1985, 1986), is employed to interpret the diffuse plasma resonances stimulated by topside ionospheric sounders at the frequencies ƒDn. The main prediction of the theory, namely, that the spectrum of frequencies should be proportional to n1/2, n = 1, 2, 3, …, is checked against observations. It is found that ƒDn ≈ 0.95(ƒNƒH;)1/2n1/2 is a fair approximation for diffuse ionospheric resonances, where ƒN and ƒH are the plasma and electron gyrofrequencies, respectively.
Effects of non‐Maxwellian electron velocity distribution functions and nonspherical geometry on minor ions in the solar wind
Authors: Bürgi A.
Published: February 2008

A previous model has shown that in order to account for the charge state distribution in the low‐speed solar wind a high coronal temperature is necessary and that this temperature peak goes together with a peak of nα/np in the corona. In the present paper, one of the assumptions made previously, i.e., that coronal electrons are Maxwellian, is relaxed, and a much cooler model is presented, which could account for the same oxygen charge states in the solar wind due to the inclusion of non‐Maxwellian electrons. Also, due to a different choice of the coronal magnetic field geometry, this model would show no enhancement of the coronal nα/np. Results of the two models are then compared, and observational tests to distinguish between the two scenarios are proposed: comparison of directly measured coronal Te to charge state measurements in the solar wind, determination of the coronal nα/np, measurement of ion speeds in the acceleration region of the solar wind, and measurement of the frozen‐in silicon charge state distribution.
Unusual electron distribution functions in the solar wind derived from the Helios Plasma Experiment: Double‐Strahl distributions and distributions with an extremely anisotropic core
Authors: Pilipp W. G. , Miggenrieder H. , Montgomery M. D. , Mühlhäuser K. ‐H. , Rosenbauer H. , Schwenn R.
Published: February 2008

Electron distribution functions with unusual features, which have been observed on rare occasions in the solar wind by the Helios probes, are presented. Two examples show a strong symmetric bidirectional anisotropy in the energy regime of the halo up to particle energies of 800 eV (double‐strahl distributions). Another example shows an unusually strong bidirectional anisotropy in the energy regime of the core (below 150 eV). The infrequently observed double‐strahl distributions provide evidence that magnetic field loops can exist in the solar wind where electrons are trapped. In addition, they provide evidence that in the case of electrons trapped in closed magnetic field structures the break in the energy spectrum separating the core from the halo is produced only by collisions. On the other hand, the class of distribution functions with strongly anisotropic cores indicates that in the case of “open” magnetic field lines the break between core and halo is largely determined both by the interplanetary electrostatic potential and by collisions.