bluuebunny/crossref_metadata_2025 huggingface dataset:
The 2‐ to 5‐eV energy spectra of thermospheric photoelectrons: Measurements in apparent conflict with theory
Authors: McMahon W. J. , Heroux L. J.
Published: February 2008

The existence of marked structure in the 2‐ to 5‐eV energy spectra of near‐earth ionospheric photoelectrons has been predicted by theory and observed by experiment. This structure, arising from resonant vibrational excitation of N2, is seen as a valleylike dip in the spectra near 2.5 eV and is predicted to be most prominent at low altitudes (110 km), then to diminish with increasing altitude until it disappears in the 200‐ to 250‐km region. All measurements to date indicate that the overall magnitude of this valley or dip is significantly less than that predicted by theory, but confirm a trend toward diminishing prominence with increasing altitude above about 150 km. Below 140–150 km, however, the valley is observed in the present data to also diminish with decreasing altitude, a reversal of the predicted trend. The present measurements include the only observations, to our knowledge, of this spectral structure below 140 km, and these indicate that its maximum prominence occurs in the 140‐ to 150‐km altitude region, below which it is consistently observed to decline rapidly in magnitude. Recent refinement of theory may lead, in part, to an explanation of these discrepancies. The experimental results are summarized and the difficulties peculiar to space vehicle measurements of very low energy ambient electrons are examined.
Latitudinal and field‐aligned cosmic ray gradients 2 to 5 AU Voyagers 1 and 2 and IMP 8
Authors: Roelof E. C. , Decker R. B. , Krimigis S. M.
Published: February 2008

Latitudinal and field‐aligned cosmic ray gradients have been measured separately by inter‐normalization of the similar integral response (greater than approximately 30 MeV/nucleon) and high counting rates of approximately 60 counts per second of the anticoincidence detectors of the LECP experiment on Voyagers 1 and 2 and the CPME experiment on IMP 8. The unambiguous separation of latitudinal from field‐aligned gradients in long‐lived (greater than 10 days) cosmic ray structures is possible because during their Earth‐Jupiter transits, the two Voyagers are never separated by more than a few degrees in latitude or longitude, or by more than 8% of the helioradius of either spacecraft, and because the Voyagers remain magnetically well‐connected to Earth, which allows a direct estimate of field‐aligned gradients. The IMF connection is estimated by using the solar wind velocities measured by the PLS experiment on Voyager 1. Latitudinal gradients of approximately 2 to 5%/deg are found in short‐lived (10 to 30 days) structures, while they are approximately 1 to 2%/deg in structures recurring over several solar rotations. Radial gradients, except in the onsets of Forbush decreases, are commonly approximately 2%/AU, although there are rotations on which neither a radial nor a latitudinal gradient is measurable above 1%/AU or 1%/deg. Application of diffusion‐convection theory to these gradients shows that if one assumes that diffusion dominates transport transverse to the magnetic field, one obtains an upper bound on the transverse mean free path for scattering λ⊥ much less than 10−3 AU at 1 GeV/nucleon, which is inconsistent with values predicted by diffusion theory. The contrary hypothesis of very weak scattering is theoretically self‐consistent and is also supported by the observation of the effects expected from small‐scale (a few degrees) latitudinal gradients upon the diurnal variation of high latitude neutron monitors.
Observations of the high‐latitude ionosphere with the Poker Flat MST radar: Analyses using simultaneous Chatanika radar measurements
Authors: Luhmann J. G. , Johnson R. M. , Baron M. J. , Balsley B. B. , Riddle A. C.
Published: February 2008

The NOAA MST (mesosphere‐stratosphere‐troposphere) VHF radar at Poker Flat, Alaska, provides nearly continuous high time resolution measurements of the velocity of 3‐m scale plasma density irregularities at altitudes ∼85 km during the summer and less continuous but lower (∼65 km) altitude observations during the winter. However, incomplete knowledge of the origin and behavior of the VHF scatterers limits the applications of these data. Observations from the nearby Chatanika incoherent scatter radar are used to investigate the effects of electron precipitation and magnetospheric convection electric fields on the VHF observations. It is shown that the backscattered VHF signal is usually enhanced when auroral particles increase the D region ionization. The magnetospheric convection electric fields do not generally affect the interpretation of the measured velocities of the 3‐m plasma irregularities as mesospheric winds. Combined neutral wind measurements from the two radars show some evidence for a continuous tide from the mesosphere through the thermosphere.
Fine‐scale structure of the Jovian magnetotail current sheet
Authors: Behannon K. W.
Published: February 2008

During the outbound leg of its passage through the Jovian magnetosphere in July 1979, the Voyager 2 spacecraft observed >50 traversals of the magnetotail current sheet during a 10‐day period at distances between 30 and 130 RJ. Detailed study of these and Voyager 1 outbound data show that the magnetic structure near and within the current sheet was variable with time and distance from Jupiter, but generally corresponded to one of the following four types: (1) simple rotation of field across the sheet, with either a southward or a northward directed field component normal to the sheet; (2) field having a southward component in a broad region near the sheet, but northward in a restricted region at the sheet itself; (3) a clear bipolar variation of the sheet‐normal field component as the sheet was crossed (i.e., the field became northward and then southward, or vice versa, in crossing the sheet); (4) large‐amplitude fluctuations in all field components near and in the sheet, with alternating northward and southward polarities. Considering Voyager 1 and 2 observations together, twice as many of type 1 signatures were seen as types 2 and 4, which occurred approximately in equal numbers, whereas type 3's were only half as frequent as the latter types. These magnetic structures are all morphologically similar to those observed at the current sheet in the earth's magnetotail at different times. While type 1 and 2 structures are indicative of a simple, static current sheet geometry, types 3 and 4 provide evidence for a more complex and dynamic internal sheet structure at the times of those traversals, suggesting a loop geometry and fine structure consistent with occurrence of the tearing mode instability. Except for one case, both type 1 signatures with southward components and type 2 signatures were found to occur exclusively in the near‐Jupiter region of the magnetotail. Beyond a downtail distance of ∼70 RJ, type 1 variations tended to have northward components, and types 3 and 4 were observed more frequently than other types. This suggests that there may have existed a quasi‐stationary crosstail neutral line near 70 RJ down the tail at the time of the Voyager 2 pass.
The collisionless macroscopic Kelvin‐Helmholtz instability: 1. Transverse electrostatic mode
Authors: Pritchett P. L. , Coroniti F. V.
Published: February 2008

The transverse electrostatic mode of the collisionless long‐wavelength Kelvin‐Helmholtz instability is investigated by means of plasma simulation. The configuration studied involves a sheared velocity flow perpendicular to a static magnetic field. Electrostatic particle simulations permit the determination of the linear growth rates for this mode and reveal the nonlinear evolution and saturation of the instability. The linear growth rates agree with MHD results for small values of the ion gyroradius. For larger values of ρi(ρi/a ≳ 0.5, where a is the velocity shear scale length) there is a clear reduction in the growth rate. These observations are compared with the results of a two‐fluid eigenvalue analysis with first‐order FLR corrections. The nonlinear stage of the instability is characterized by the presence of large vortices whose size is many times the initial shear length a. At saturation the simulation is dominated by the longest‐wavelength mode permitted in the system. A simple model is developed to explain these results. Implications of the numerical results regarding excitation of the Kelvin‐Helmholtz instability at the magnetopause boundary are discussed.
Thermospheric control of the auroral source of O+ ions for the magnetosphere
Authors: Lockwood M.
Published: February 2008

Linear theory, model ion‐density profiles and MSIS neutral thermospheric predictions are used to investigate the stability of the auroral, topside ionosphere to oxygen cyclotron waves: variations of the critical height, above which the plasma is unstable, with field‐aligned current, thermal ion density and exospheric temperature are considered. In addition, probabilities are assessed that interactions with neutral atomic gases prevent O+ ions from escaping into the magnetosphere after they have been transversely accelerated by these waves. The two studies are combined to give a rough estimate of the total O+ escape flux as a function of the field‐aligned current density for an assumed rise in the perpendicular ion temperature. Charge exchange with neutral oxygen, not hydrogen, is shown to be the principle limitation to the escape of O+ ions, which occurs when the waves are driven unstable down to low altitudes. It is found that the largest observed field‐aligned current densities can heat a maximum of about 5×1014 O+ ions m−2 to a threshold above which they are subsequently able to escape into the magnetosphere in the following 500s. Averaged over this period, this would constitute a flux of 1012 m−2 s−1 and in steady‐state the peak outflow would then be limited to about 1013 m−2 s−1 by frictional drag on thermal O+ at lower altitudes. Maximum escape is at low plasma density unless the O+ scale height is very large. The outflow decreases with decreasing field‐aligned current density and, to a lesser extent, with increasing exospheric temperature. Upward flowing ion events are evaluated as a source of O+ ions for the magnetosphere and as an explanation of the observed solar cycle variation of ring current O+ abundance.
A mechanism for the IPDP pulsations
Authors: Lee L. C. , Kwok Y. C.
Published: February 2008

The IPDP pulsations are assumed to be generated in the region where the low‐density energetic protons (10–100 keV) injected during substorms encounter the high‐density plasmasphere. The increase in the wave frequency during the IPDP events is proposed to be caused by the enhanced dispersion of the wave velocity due to the presence of the helium ions in the plasmasphere. In the presence of the helium ions, the variation in the traveling time of the unstable waves is found to be typically 5–20 min, which is comparable to the duration of some of the observed IPDP events.
Electron‐cyclotron maser emission: Relative growth and damping rates for different modes and harmonics
Authors: Melrose D. B. , Hewitt R. G. , Dulk G. A.
Published: February 2008

We calculate and compare the temporal growth rate and the number of e‐folding growths for the following wave modes due to a loss‐cone‐driven cyclotron maser: fundamental x, o, and z modes and second harmonic x and o modes. The dominant mode of the maser should be the fastest growing mode for a saturated maser and should be the mode with the greatest number of e‐folding growths for an unsaturated maser; this mode is the fundamental x mode for ωp/Ωe ≲ 0.3, the z mode (or perhaps the fundamental o mode) for 0.3 ≲ ωp/Ωe ≲ 1.0, and the z mode (or perhaps the second harmonic x mode) for 1.0 ≲ ωp/Ωe ≲ 1.3. We discuss the effect of cyclotron damping by thermal electrons on the growth. Numerical calculations show that the effect is important only when the ratio of the mean energies of the thermal and maser emitting electrons exceeds 0.1–0.2. An analytic expression for the damping rate is derived and is used to show that some earlier treatments of cyclotron damping greatly overestimate the effect for loss‐cone‐driven maser emission. These results, when applied to AKR, imply that only either the fundamental x mode (for ωp/Ωe ≲ 0.3) or the z mode (for ωp/Ωe ≳ 0.3) is produced directly by maser emission. We suggest (1) that an o mode component in AKR might be due to partial reflection of x mode radiation incident onto sharp overdense plasma intrusions of the kind observed in the auroral cavity and (2) that a second harmonic component can be produced by coalescence of two z mode waves.
A search for the plasma processes associated with perpendicular ion heating
Authors: Kintner Paul M. , Gorney David J.
Published: February 2008

The S3‐3 data set has been examined for examples of perpendicular ion acceleration with simultaneous broadband plasma wave data. One event was found. Plasma waves in the frequency range for lower hybrid waves and in the frequency range for Doppler‐shifted O+ ion cyclotron waves modestly correlated with the perpendicular ion acceleration, although their electric field amplitudes were less than that assumed in present theories of ion acceleration: about 0.2–6 mV/m (rms) for the LHR waves and 4–9 mV/m (rms) for the possible Doppler‐shifted O+ cyclotron waves. Plasma waves propagating above the LHR frequency very briefly reached a value of 30 mV/m (rms), but during most of the event their amplitude was below 10 mV/m (rms). No evidence was found for the existence of H+ cyclotron waves during the period of perpendicular ion acceleration. The thermal or superthermal (Ee < 170 eV) electron field‐aligned current density, estimated by subtracting the current density measured by the electron spectrometer from that measured by the magnetometer, correlated very well with the period of perpendicular ion acceleration. However, the drift velocity was too small to excite instabilities locally.
Electron density and plasmapause characteristics at 6.6 RE: A statistical study of the GEOS 2 relaxation sounder data
Authors: Higel Bernard , Lei Wu
Published: February 2008

On board the geostationary GEOS 2, a relaxation sounding (RS) experiment using a pulsed active wave technique has been operated in the frequency range 0.3–77 kHz. In situ measurements of the equatorial magnetospheric electron density at L ≃ 6, obtained in the range from 0.1 to ∼100 cm−3 through on line data processing, are presented. The results from 4 months of routine operations (November 20, 1978–March 28, 1979) are statistically studied. Many features of the density distributions at 6.6 RE displayed from GEOS 2 RS data corroborate the morphological characteristics evidenced in past experimental data sets of magnetospheric density, like those from ground‐based whistler observation and from embarked particle experiments. In providing an improved accuracy—especially for the cold plasma density—and unique coverage in space and time, the GEOS 2 RS data set brings some extension of results. The average properties of different magnetospheric regions that can be sampled at 6.6 RE with respect to LT are focused: the dayside plasma trough, with a few cm−3 typically, the plasmaspheric bulge (a few tens of cm−3) and the associated plasmapause crossings (variations in cm−3 per minute), and the geosynchronous nightside where the density fluctuates in the lower range available (0.1–1 cm−3). The important contribution of cold plasma to the dayside magnetospheric density is reported. There are striking variabilities of the density distributions found from day to day, especially as far as the plasmaspheric bulge morphology is concerned. Relationships between geomagnetic activity and both the LT position and the LT width of the bulge are found. They confirm the role played by timing effects on the large scale convection patterns, which experimental and model studies performed earlier on the plasmapause dynamics have focused on.
Optical mapping of ionospheric conductance
Authors: Mende S. B. , Eather R. H. , Rees M. H. , Vondrak R. R. , Robinson R. M.
Published: February 2008

Ionospheric Hall and Pedersen conductances are derived from auroral spectroscopic measurements obtained with meridian scanning photometers. Conductances are also derived from height profiles of electron density obtained with the incoherent scatter radar at Chatanika in a coordinated experiment aimed at comparing the two techniques. While general agreement is obtained by using these basically different methods, the disagreements illustrate the advantages and limitations of each experimental technique. The radar yields accurate values of conductance with limited temporal and spatial resolution while the optical measurements provide less accurate values at high time resolution and over large regions of space.
Density distribution in looplike coronal transients: A comparison of observations and a theoretical model
Authors: Sime D. G. , MacQueen R. M. , Hundhausen A. J.
Published: February 2008

Examination of the intensity changes in five outer coronal “looplike” transients observed by the Skylab coronagraph shows general tendencies for (1) greatest concentration of material at the flanks rather than at the tops of the bright loops that characterize these transients, (2) presence of a large region of depleted density within these loops, and (3) development of bright legs that contain most of the material in the transient and that display very little lateral motion as the top of the bright loop moves radially outward through the outer corona. These properties of looplike coronal transients provide useful constraints on theoretical models of this phenomenon. In particular, direct comparison of the observed density distributions with those predicted by models of compressional waves initiated by an impulsive energy release in the low corona are a necessary test of these models. These models predict a maximum enhancement at the top of the loop, rather than at the flanks and “legs” that move laterally with a significant fraction of the propagation speed of the loop top, in conflict with the observed tendencies. If the observed loops are taken to have the geometry assumed in the compressive wave models (“toroidal symmetry” about a rotation axis passing through the center of the sun), the predicted density enhancements are several times larger than those inferred from the observations. Agreement cannot be achieved without use of a geometry which conflicts with that used for the model calculations.
An estimate of the H2 density in the atomic hydrogen cloud of Titan
Authors: Ip W.‐H.
Published: February 2008

Charged particle experiments on the Voyager spacecraft at Saturn can be used to provide some useful estimates on the charge exchange loss rate of magnetospheric particles in the atomic hydrogen cloud of Titan. The thermal plasma instrument measured the number density and ion and electron temperatures of the corotating plasma and thus the charge exchange loss time scale of the neutral gas; the low‐energy particle detectors measured the flux of the energetic neutrals generated by charge exchange recombination of the hot magnetospheric plasma. These observational results together with known reaction rate coefficients can be used to compute the total H + H2 density in the hydrogen torus. As the Voyager UV spectrometer experiment determined the average number density of hydrogen atoms independently, a limit on the H2 density in the neutral torus region can be estimated. This method leads to an H2 density value of ≲10 cm−3, considerably less than the limiting value for the ballistic motion of the neutral particles to be collisionally dominated.
Solar modulation of cosmic ray electrons 1978–1983
Authors: Evenson Paul , Meyer Peter
Published: February 2008

The ISEE 3 spacecraft has provided over 4 years of continuous data from nearby interplanetary space. By using these data together with prelaunch calibration data and precise electron spectra from balloon flights in 1979 and 1982 at Thompson, Manitoba, Canada, we monitor the flux level of approximately 1 GeV cosmic ray electrons with 1 month time resolution. The steplike onset of modulation observed for positive particles in 1979 is also seen in electrons, confirming that the dominant modulation mechanism is not sensitive to the charge of the particle. Hysteresis effects observed in the previous solar maximum by Burger and Swanenburg (1973) are not reproduced at the present solar maximum. A return to the relatively higher electron/proton ratios of the 1960's is a strong possibility. Conclusions must be drawn with caution, however, since the overall time structure of the present solar maximum is significantly different from that of the previous one.
Voyager observations of Jovian millisecond radio bursts
Authors: Alexander J. K. , Desch M. D.
Published: February 2008

Voyager planetary radio astronomy data collected over 30‐day intervals centered on the two close encounters with Jupiter have been utilized to study the characteristics of millisecond‐duration radio bursts (s bursts) at frequencies between 5 and 15 MHz. In this frequency range, s bursts are found to occur almost independently of central meridian longitude and to depend entirely on the phase of Io with respect to the observer's planetocentric line of sight. Individual bursts typically cover a total frequency range of about 1.5 to 3 MHz, and they are usually strongly circularly polarized. Most bursts in a particular s burst storm will exhibit the same polarization sense (either right‐hand or left‐hand), and there is some evidence for a systematic pattern in which one polarization sense is preferred over the other as a function of Io phase and central meridian longitude. These data are all suggestive of a radio source that is located along the instantaneous Io flux tube and that extends over a linear dimension of ∼5000 km along the field lines in both the northern and southern hemispheres.
Wave‐particle transport from density drift instabilities: A comparison of local and nonlocal theories
Authors: Bernhardt P. A.
Published: February 2008

A plasma density gradient perpendicular to an ambient magnetic field is unstable to the growth of electrostatic waves. The objective of this paper is to describe the rate at which these density drift instabilities transport plasma across the magnetic field lines. This objective is accomplished as follows: The plasma density irregularity is represented as a Fourier series. Using linear Vlasov theory, the nonlocal dispersion equation is derived, relating the Fourier amplitudes to the eigenvalues (i.e., wave frequencies and growth rates) for the electrostatic fluctuations. For each complex eigenvalue, an electrostatic eigenmode is computed. These eigenmodes grow to saturation and are summed, yielding the total electrostatic turbulence. The flux of plasma across the magnetic field lines is computed from the turbulence. The transport derived from the nonlocal theory is compared with that derived by using the less complicated local approximation. Wave‐particle transport based on the local approximation is shown to be valid only in regions where variations in the density gradient are small.
Radial gradient of cosmic ray intensity from a comparative study of data from Voyager 1 and 2 and IMP 8
Authors: Venkatesan D. , Decker R. B. , Krimigis S. M.
Published: February 2008

Cosmic ray measurements obtained with integral detectors on Voyager 1 and 2 (Ep ≳ 70 MeV) moving toward the outer solar system and the earth‐orbiting IMP 8 satellite (Ep ≳ 35 MeV) over the period late 1977 through mid‐1982 are presented. During this period, Voyager 1 and 2 traversed the region from 1 to ∼13 AU and ∼10 AU, respectively, with little separation in heliolongitude; separation in heliolatitude was also small (≲2°) through the end of 1980, at which time the trajectory of Voyager 1 changed toward higher ecliptic latitudes. The following results have been obtained from comparisons of 27‐day averages of the data: (1) there exists a positive radial intensity gradient gr ∼ 2–4%/AU, on the average, during this period; (2) the decrease in cosmic ray intensity toward solar maximum in 1980–1981 for both ground‐based and spacecraft detectors proceeds in a stepwise fashion, with alternating periods of intensity “plateau” and “steep decrease” during which the gradient changes from ∼0%/AU to ∼8%/AU, respectively; (3) the cosmic ray minimum reached in late 1980/early 1981 appears nearly simultaneously (to within one solar rotation) at 1 AU and at ∼10 AU, with and without correction for propagation time delay effects between IMP 8 and the Voyager spacecraft; (4) recurrent intensity enhancements appearing at both Voyager 1 and 2 at 6–10 AU are generally absent at 1 AU; (5) the data are generally consistent with a heliolatitudinal gradient of 0±1%/deg, obtained from the periodic excursions of IMP 8 about the heliographic equator and the separation in latitude between Voyager 1 and IMP 8 of ∼15° in 1982. The value of the intensity gradient is found to be generally consistent with that obtained by Pioneer 10 at ∼23 AU around the same time but just prior to solar maximum. The implications of the results in the overall context of cosmic ray modulation theory are briefly pointed out.
Particle acceleration parallel and perpendicular to the magnetic field observed by DE‐2
Authors: Heelis R. A. , Winningham J. D. , Sugiura M. , Maynard N. C.
Published: February 2008

Observations by the instrument payload on DE‐2 have been used to study the plasma and electrodynamic properties of the ionosphere when field‐aligned currents approaching 100 µA m−2 flow at about 900‐km altitude. In such a situation it is found that the thermal ions may account for a substantial fraction of the current carriers and that the bulk of the energetic electron population has undergone a net acceleration of a few hundred electron volts. Large amplitude electrostatic waves that have a maximum near 100 Hz and an energetic ion population that shows evidence for both parallel and perpendicular acceleration to about 30 eV also accompany such events. The horizontal thermal ion drift (electric field) that accompanies these plasma phenomena has extremely large gradients over small spatial scales. Such a signature is consistent with a requirement for a large field‐aligned current and a limit to the available field‐aligned potential difference that may be applied to the plasma.
Relationships between a potential field‐source surface model of the coronal magnetic field and properties of the solar wind at 1 AU
Authors: Suess S. T. , Wilcox J. M. , Hoeksema J. T. , Henning H. , Dryer M.
Published: February 2008

A comparison is made between the properties at the source surface of a potential field model of the solar corona and solar wind properties observed with ISEE 3 at 1 AU and extrapolated back to the sun. This is done for three consecutive Carrington rotations in the summer of 1979. The already known properties of a velocity minimum at the interplanetary current sheet and a tendency for the average velocity to increase with longitudinal distance from the current sheet are recovered. A better correlation between source surface properties and velocity is demonstrated by using the field strength on the source surface rather than longitudinal distance from the current sheet. Applying a simple algorithm to distinguish between transient and corotating interplanetary variations substantially increases the correlation. However, given a reasonable estimate of the number of degrees of freedom for the sample, only the correlation between source surface field strength and flow speed after the transients have been eliminated is statistically significant at the 1% level.
Atmospheric gamma radiation: A comparison of experimental results at 4.5 GV and 11.7 GV
Authors: Lavigne J. M. , Niel M. , Vedrenne G. , Agrinier B.
Published: February 2008

“Agathe,” a joint Commissariat à l'Energie Atomique (Saclay) and Centre d'Etude Spatiale des Rayonnements (Toulouse) experiment is a spark chamber for gamma ray measurements in the E > 4 MeV energy range. It was used to measure the flux of atmospheric photons at λ = −23° geographic latitude (rigidity 11.7 GV) during two flights from Brazil, in the course of a campaign organized by the Centre National d'Etudes Spatiales, the French space agency. The results are compared to those obtained from flights of the engineering model of Agathe and a model for a time of flight system, tested to evaluate improvements for the Agathe experiment. The following results were obtained for the energy ranges 4–10 MeV, 10–25 MeV, and >25 MeV: (1) Determination of the atmospheric gamma ray fluxes or count rate as a function of pressure altitude; in these energy ranges the downward atmospheric flux is proportional to Pα with α ≃ 1. (2) The determination of the atmospheric spectrum at λ = −23° in Brazil and at λ = 40° in the northern hemisphere; in Brazil the downward atmospheric spectrum can be represented by Φ(E) = 2.4×10−3 E−1.4 photons/cm² s sr MeV g/cm² in the 4‐ to 100‐MeV energy range and Φ(E)′ = 2.3×10−2 E−1.9 photons/cm² s sr MeV g/cm² for E > 100 MeV energy range. (3) The determination of the ratio between the fluxes measured in Brazil and in the northern hemisphere; this ratio is 1.8±0.2. It has been shown that the fluxes of downward and upward moving gamma rays are equal at a pressure of about 40 mbar. The dependence of the downward moving fluxes as a function of the pressure is different from that published by Ling. But, if we assume that the atmospheric component is isotropic at 40 mbar, the experimental fluxes determined from the experiments described here agree with the measured fluxes taken into account by Ling in these calculations for the 4‐ to 10‐MeV energy range. Key words: gamma radiation and atmospheric component.
A new method for accurate ionospheric electron density measurements by incoherent scatter radar
Authors: Hagfors T. , Birkmayer W. , Sulzer M.
Published: February 2008

It is shown that the detectability of the plasma line in incoherent scatter radar experiments on the ionospheric plasma can be increased by factors in the range of 10–100 by chirping the transmitter frequency at a rate matching the rate of change of plasma frequency with height. The method is explained in detail, and some initial experiments carried out at the Arecibo Observatory are described. Plasma frequency versus height profiles with an accuracy better than one part in 10³ were established with 1‐s integration time per point. An example of quasi‐periodic oscillation of ionospheric electron density with a period of about 18 min is also shown. The chirped frequency observations will greatly facilitate plasma line studies of photoelectrons, allow for studies of small‐amplitude wavelike density perturbations, provide electron density profiles with high time resolution and unprecedented accuracy, make it possible to measure electron temperature independently of ion temperature, and probably allow for observations of vertical currents.
Analytical yield spectrum approach to photoelectron fluxes in the Earth’s atmosphere
Authors: Singhal R. P. , Haider S. A.
Published: February 2008

The analytical yield spectrum approach has been applied to calculate the photoelectron fluxes in the earth’s atmosphere. The results have been compared with the experimental data of Lee et al. (1980a, b). Good agreement is obtained at low altitudes. At high altitudes our theoretical results are lower than the experimental data in the energy region 18–23 eV by a factor of 1.5 and in the 35–45 eV by a factor of 2. Comparison with theoretical calculations performed by other workers shows that if the same set of inelastic electron impact cross sections had been used, then the photoelectron models of Strickland and Anderson (1983) and Richards and Torr(1983) would be in good agreement with our work and the experimental data. However, the photoelectron fluxes calculated by the model of Victor et al. (1976) is not consistent with the other theoretical models or the experimental data.
Sulfur and oxygen escape from Io and a lower limit to atmospheric SO2 at Voyager 1 encounter
Authors: Kumar Shailendra
Published: February 2008

It is proposed that the transport of material from Io to the plasma torus requires the presence of an atmosphere as an intermediate buffer. Direct escape of volcanic SO2 is expected to be negligible. The sputtering of surface SO2 frost by corotating ions (if they reach the surface) or by magnetospheric MeV ions would not provide sufficient escape flux. Instead, such sputtering process produces an ∼97% fraction with energy less than the escape energy; hence this process alone could produce an atmosphere thick enough to prevent the corotating ions from reaching the surface and thick enough to disallow even the energetic sputtered products created at the surface to escape. The escape from an atmosphere could take place by several mechanisms, of which secondary sputtering at the exobase is dominant. The escape rate from a thin atmosphere increases with increasing atmospheric SO2 content. By preparing photochemical models of SO2 atmospheres for varying total atmospheric pressure and calculating the escape rates, it is concluded that the globally averaged atmospheric SO2 pressure on Io at Voyager 1 encounter must have been ≳ 1.4×10−10 bar (surface density n0(SO2) ≳ 1.2×1010 cm−3, or column density of ≳ 1×1016 cm−2), as the escape rates in this limiting case are just adequate to populate the torus if the Brown et al. (1983) estimates of the supply rate are applicable. However, if the higher estimates of Pontius and Hill (1983) are appropriate, then more SO2 would be needed in the Io atmosphere.
Drift wave instabilities in a high β multispecies plasma
Authors: Patel V. L. , Ng P. H. , Ludlow G. R.
Published: February 2008

The dispersion relation for drift‐Alfvén waves in two‐component (cold and hot), high β inhomogeneous multispecies plasma containing protons, oxygen, and sulphur ions is solved numerically. The magnetic field is assumed to have a gradient in simple slab geometry configuration. The plasma ion composition consists of H, O, S, SO2, and Na ions. The effect of heavy ions and multiple charge states (Z > 1) on the growth rates has been numerically estimated for assumed ion compositions. The numerical calculations of growth rates also have been performed by using ion composition and plasma parameters based on Voyager spacecraft in the magnetospheres of Jupiter and Saturn.
Relationship between nighttime total electron content enhancements and VHF scintillations at the equator
Authors: Balan N. , Rao P. B.
Published: February 2008

The relationship between nighttime enhancements in total electron content (TEC) and VHF amplitude scintillations has been studied using the ATS 6 satellite data obtained from two closely located equatorial stations (Ootacamund and Thumba) for the period October 1975 to July 1976. The results show that the occurrence and various other characteristics of the TEC enhancements are correlated to corresponding characteristics of the scintillation index SI. The peak value of the SI index is found to increase linearly with that of the TEC enhancement until it reaches the saturation value of nearly 1 for the peak TEC enhancement of about 0.6×1017 m−2. Both phenomena start mostly prior to midnight with the TEC enhancement being delayed by 1 to 1½ hours with respect to the scintillations. The half‐amplitude duration of the scintillation enhancement is greater than that of the TEC by a factor of 1 to 2. The intensity and duration of both phenomena are found to be maximum at equinox and minimum in summer. From the results it is clear that the TEC enhancements cannot be regarded as a source for the small‐scale irregularities but rather they both seem to depend on the postsunset vertical drifts in the F layer.
Results of a comprehensive study of the photochemistry of N2+ in the ionosphere
Authors: Abdou Wedad A. , Torr D. G. , Richards P. G. , Torr M. R. , Breig E. L.
Published: February 2008

The disagreement between model calculations of ionospheric N2+ concentrations and those measured by the Atmosphere Explorer (AE) satellite has been controversial for several years. The model calculations of N2+ require an additional loss of the order of ∼10² ions cm−3 s−1 at ∼250 km in order to account for the observed N2+ concentrations. After analysis of many orbits of AE data we have drawn the conclusion that such a loss rate could only be provided by either (1) enhancing the ionospheric dissociative recombination rate coefficient for N2+ by a factor of 2–3 over its currently accepted value if this is not precluded by laboratory data or (2) enhancing of the charge exchange of N2+ with atomic oxygen. This could be achieved by two mechanisms which either independently or combined can account for the observed N2+ concentrations. The first is the enhanced destruction of N2+ by charge exchange of vibrationally excited N2+ with O (enhancement of the branch to NO+ only transfers the problem to NO+). The second mechanism is the accidentally energetically resonant reaction O+(²D) + N2 ⇄ N2+(X)υ=5 + O. In spite of the valid theoretical arguments against the plausibility of these two mechanisms, without any viable alternatives we feel compelled to present our results. The best agreement between theory and data is obtained when these two mechanisms are combined with the rate coefficients for charge exchange of vibrationally excited N2+ with O increased by a factor of 50 and N2 set equal to 2.5×10−10 cm³ s−1 and where N2+(X)υ>0 is quenched by N2 at a rate of ∼5×10−10 cm³ s−1. An important aspect of the chemical scheme is negligible quenching of O+(²D) by O. We estimate an upper limit for the rate coefficient of this process to be ∼5×10−12 cm³ s−1.
The power spectrum of interplanetary Alfvénic fluctuations: Derivation of the governing equation and its solution
Authors: Tu Chuan‐Yi , Pu Zu‐Yin , Wei Feng‐Si
Published: February 2008

A model is presented to explain the radial evolution of the power spectra of interplanetary Alfvénic fluctuations found by Bavassano et al. (1982a) based on the magnetic field data of Helios 1 and 2. It is assumed in this model that Alfvénic fluctuations represent an asymmetric state of MHD turbulence in which most of the fluctuations are in the Alfvénic wave mode propagating outward from the sun, with a small part of the fluctuations in the wave mode propagating inward. There is weak nonlinear interaction between the two modes. It is also assumed that the turbulence will not become dissociated from the sources that create the waves propagating inward and that the weak nonlinear interaction will remain between 0.3 AU and 1 AU. The nonlinear interaction results in an energy cascading process. Both the effects of the cascading process and the effects of the slow variation of the solar wind on the outward propagating waves determine the radial evolution of the power spectra. Starting with the magnetohydrodynamic equations and deriving the equations governing fluctuations and correlation moments, we finally get an equation which describes the power spectrum. We present an analytic solution. The radial evolution of the power spectra of the fluctuations given by the analytical solution, from 0.29 AU to 0.87 AU, is in agreement with the observation of Helios 1 and 2 in the following aspects: (1) The spectral slope increases (in absolute value) for the frequency range lower than 2.5×10−3Hz while the slope remains almost unchanged for the frequency range higher than 10−1.5Hz. (2) The radial gradient of the power spectrum densities increases with increasing frequency. The dissipation length remains nearly unchanged for the frequency rangef≥ 10−2Hz. (3) The radial variation of 〈b²〉 is approximatelyr−3.56. (4) The ratio of 〈b²〉/Bo² is nearly a constant.
Spectral characteristics of Pc 3 and Pc 4/5 magnetic pulsation bands observed near L = 6
Authors: Tonegawa Y. , Fukunishi H. , Hirasawa T. , McPherron R. L. , Sakurai T. , Kato Y.
Published: February 2008

From dynamic spectral analysis of magnetic pulsations observed at auroral zone stations near L = 6, it has been found that there are two predominant spectral bands throughout the daytime, one in the Pc 3 range with frequencies of ∼20–80 mHz and the other in the Pc 4/5 range with frequencies of ∼3‐10 mHz. At synchronous orbit, corresponding spectral bands were seen in the azimuthal component with a harmonic structure when the satellite was located at the off‐magnetic equator. The result suggests that the 3‐ to 10‐mHz band is the fundamental mode of standing shear Alfvén waves and the 20‐ to 80‐mHz band is the higher harmonics. However, both bands were not always enhanced simultaneously. Switching of activity between both bands was frequently observed on magnetically quiet days. This result indicates that the Pc 3 and Pc 4/5 bands are excited by different external driving sources. It is likely that when the frequency range of the external driving source is in the Pc 3 range, this driving source excites the higher harmonics of standing Alfvén waves near L = 6, while when the frequency range is in the Pc 4/5 range, the external driving source excites the fundamental mode.
Unified theory of the power spectrum of intermediate wavelength ionospheric electron density fluctuations
Authors: Sudan R. N. , Keskinen M. J.
Published: February 2008

An extension of the Kolmogorov theory of fluid turbulence is used to develop a unified treatment of the power spectrum of intermediate wavelength electron density fluctuations in the equatorial and high‐latitude ionosphere. Favorable comparison is made between theory and experimental observations.
Growth and maintenance of large‐scale magnetic fields in the dayside Venus ionosphere
Authors: Phillips J. L. , Luhmann J. G. , Russell C. T.
Published: February 2008

Observations from the Pioneer Venus orbiter magnetometer reveal the presence of large‐scale magnetic fields in the dayside ionosphere during or after periods of high solar wind dynamic pressure. Various hypotheses have been proposed concerning the spatial and temporal evolution of these field structures. Here we examine the hypothesis that the field is produced by a diffusion/convection process rather than by currents driven by electric fields resulting from the solar wind interaction. Dynamic pressure variations occur on various time scales at Venus, producing transient and quasi‐steady magnetization features. A one‐dimensional diffusion/convection calculation is performed, using typical ionopause field and pressure values for a variety of altitudes. The Venus ionopause is considered to be the altitude at which ionospheric thermal pressure is equal to the magnetosheath magnetic pressure, which in turn is well correlated with the normal component of solar wind dynamic pressure. A subsolar model of downward plasma velocity in the 140–290 km range is used. The calculated growth of the ionospheric magnetic field, and the resulting quasi‐steady altitude profiles, compare favorably with the observed profiles. The majority of the observed magnetic structures are best explained as the quasi‐steady effects of the prevailing solar wind dynamic pressure, which determines the altitude and magnetic field strength of the ionopause boundary.
Production of high‐energy gamma rays by cosmic ray interactions in the atmosphere and lunar surface
Authors: Morris Daniel J.
Published: February 2008

The production of gamma rays above 10 MeV by the interaction of cosmic rays in the atmosphere and the lunar surface is simulated Using Monte Carlo methods. The calculation incorporates a new model of high‐energy nucleon‐nucleus interactions based on empirical fits to inclusive cross sections for the production of pions and nucleons. The atmospheric gamma ray flux is calculated as a function of direction, energy, and atmospheric depth. These calculations are compared with observations from balloons and from the SAS 2 satellite. Estimates of the flux of earth albedo electrons produced by cosmic ray interactions are presented. The lunar gamma ray albedo is calculated and compared with an upper limit based on SAS 2 measurements.
Tail field effects on drift mirror instability
Authors: Lin C. S. , Cheng C. Z.
Published: February 2008

The effects of geomagnetic tail fields on drift mirror instability are studied for a magnetic field model that contains a magnetic gradient and a tail field perpendicular to an ambient field. The magnetospheric plasma is assumed to consist of a cold component and a high β anisotropic inhomogeneous component. The gyrokinetic equations are used to derive the dispersion equation including the effects of magnetic gradient and curvature drifts. The dispersion equation is solved locally for wave frequencies less than the ion gyrofrequency. The numerical results indicate that the instability growth rate decreases when the tail field strength increases. The drift mirror instability is found to be stabilized when larger tail fields require a smaller pressure anisotropy and a smaller plasma β⊥ to maintain equilibrium. The theoretical results are used to explain the GOES satellite observations that storm time Pc 5 waves occur less frequently during periods of large tail fields.
Modulational instability and soliton formation during ionospheric heating
Authors: Payne G. L. , Nicholson D. R. , Downie R. M. , Sheerin J. P.
Published: February 2008

The most intense electric fields during ionospheric heating occur a fraction of a kilometer below the classical reflection point. At this location, the nonlinear evolution of Langmuir waves is studied within the context of the modified Zakharov equations. It is found that the modulational instability (oscillating two‐stream instability) is more important than the three‐wave parametric decay instability, leading to the rapid formation of solitons.
Plasma potential and grain charges in a dust cloud embedded in a plasma
Authors: Havnes O. , Morfill G. E. , Goertz C. K.
Published: February 2008

We have calculated the plasma potential and dust potential throughout the border region of semiinfinite dust clouds embedded in a plasma. A relevant ordering parameter is P = (N d0/N0)aμ (interior dust density/exterior plasma density) × dust radius (in microns). For comparatively high dust densities (P > 10−3) the cloud potential is high, screening out electrons, with low interior dust charges as a result. For decreasing P the cloud potential becomes less, with a corresponding increase in the dust charges. At P < 10−5 the cloud potential is practically zero and the dust particles have their maximum charge which is identical to that of a single dust particle in a plasma. For diffuse dust border regions with density scale length H ≫ λD (Debye length), the deviations from charge neutrality are small. When H ≲ λD, large deviations from charge neutrality occur in the border region and also some penetration of electric fields inside the border region.
Effects of electron pressure in quasi‐parallel collisionless shocks
Authors: Mandt M. E. , Kan J. R.
Published: February 2008

The effects of a finite electron pressure on quasi‐parallel collisionless shocks are examined by means of numerical simulations. The simulation is performed using a nonperiodic hybrid code in which the ion dynamics are followed exactly and the electrons are treated as a massless fluid of finite pressure. The results of the simulations show that the electron pressure significantly improves the stationarity of the shock structure. The finite electron pressure also leads to an electrostatic potential across the shock as expected. The potential structure is localized and stable at a low‐Alfvén Mach number (MA = 2), but for a higher Mach number (MA = 4) it is nonlocal and unstable. In either case the change in potential energy is on the order of the downstream electron parallel thermal energy. Power spectral analysis of the upstream waves shows that they are right‐hand polarized whistlers phase standing in the shock frame.