bluuebunny/crossref_metadata_2025 huggingface dataset:
Mirror waves driven by newborn ion distributions
Authors: Price Channon P.
Published: February 2008

Ring‐type ion distributions, such as are found in the vicinity of comets, are shown to drive mirror waves unstable. Major differences are found between mirror waves driven by anisotropic bi‐Maxwellian distributions and mirror waves driven by ring‐type distributions; in particular, for pure ring distributions the maximum mirror wave growth rate occurs for quasiparallel wave propagation. The growth rate for ring‐type driven mirror waves is approximated analytically and calculated numerically. For pure ring ion distributions, the analytic approximations are quite good. For ring‐beam distributions, the mirror waves appear with a real frequency approximately equal to the ring ion Doppler frequency; the analytic approximations agree with the numerical results when the real frequency is small compared to the proton gyrofrequency so that there is little proton Landau damping. The analytic approximations provide a simple explanation for the growth dependences on propagation angle and wavelength, and lead to a simple (approximate) expression for the most unstable wavelength. Resonant instabilities can also be driven by ring‐type distributions; growth rates for the nonresonant mirror wave instability are comparable to growth rates for resonant instabilities. In the presence of a ring‐type distribution, the mirror waves remain nonoscillatory (ωr = 0) when viewed in the frame of the newborn ions.
Energetic (>60 eV) atmospheric photoelectrons
Authors: Winningham J. D. , Decker D. T. , Kozyra J. U. , Jasperse J. R. , Nagy A. F.
Published: February 2008

Data from the low altitude plasma instrument (LAPI) on Dynamics Explorer 2 document a population of high‐energy (up to 800 eV) atmospheric photoelectrons that has not been reported in the published literature. The source of these photoelectrons is postulated to be the soft X ray portion of the whole Sun spectrum. This conclusion is supported by (1) sunrise‐sunset characteristics that track those of the classical (<60 eV) EUV‐produced photoelectrons and (2) theoretical results from two models that incorporate the soft X ray portion of the solar spectrum. The models include K shell ionization effects and predict peaks in the photoelectron spectrum due to Auger electrons emitted from oxygen and nitrogen. The peak for nitrogen is observed as predicted, but the peak for oxygen is barely observable. Excellent quantitative agreement is achieved between theory and experiment by using reasonable adjustments to the few published soft X ray spectra based on solar activity. The upflowing energetic photoelectrons provide a heretofore unknown source of electrons to the magnetosphere. They occur whenever and wherever the Sun is up, that is, at all invariant latitudes. Their density is low, but they are steady and ubiquitous. If scattering and trapping occur on closed field lines, then these photoelectrons could contribute as a significant particle source and thus represent a new facet of magnetosphere‐ionosphere coupling.
Spatial gradients in the heliospheric magnetic field: Pioneer 11 observations between 1 AU and 24 AU, and over solar cycle 21
Authors: Winterhalter Daniel , Smith Edward J. , Wolfe John H. , Slavin James A.
Published: February 2008

Magnetic field observations by Pioneer 11 and ISEE 3/ICE are used to investigate the large scale temporal behavior and spatial gradients in the heliospheric magnetic field. We extend and refine previous analyses by expanding the data set to include observations out to a radial distance of 24 AU and over a longer time interval which includes the recent solar minimum. The spatial gradients are investigated by removing the solar cycle variations with data obtained at 1 AU at the corresponding time. To first order, the gradients in the magnetic field magnitude and in the azimuthal component are found to agree with the Parker model. Furthermore, this analysis of the Pioneer data confirms previous reports of a magnetic field flux deficit of approximately 1%/AU, so that at 20 AU the relative deficit is of the order of 20%. The solar wind speed measured at 1 AU as well as at Pioneer 11 has been analyzed, but it is found that the small variations observed between the two locations affect this result only minimally. The average field strength at Pioneer 11 at a distance of 16 AU has been compared with a Voyager 2 average obtained at nearly the same radial distance and time interval but at a significantly different latitude. They agree very closely and both exhibit a deficit of ≈ 20% relative to the 1‐AU baseline. This agreement demonstrates that B did not vary significantly with latitude at this time (1984–1985 near solar minimum). Furthermore, the 1‐AU baseline has been adjusted to allow for an observed dependence of the field on latitude near solar maximum (1978–1982) inferred from a comparison of measurements in the inner heliosphere at 0.7 AU (PVO) and 1.0 AU (ISEE 3). This adjustment of the 1‐AU baseline did not significantly affect the deficit, again confirming that the Pioneer measurements were not strongly influenced by the latitude dependences. We thus argue that most of the magnetic field deficit is in the radial gradient.
A model of the energetic ion environment of Mars
Authors: Luhmann J. G. , Schwingenschuh K.
Published: February 2008

Because Mars has a weak intrinsic magnetic field and a substantial atmosphere, instruments on orbiting spacecraft should detect a population of energetic heavy planetary ions which result from comet‐like ion pickup in the solar wind and magnetosheath convection electric fields, in addition to those that might result from processes internal to a Martian “magnetosphere.” Although this ion exosphere has been previously discussed in the literature, detailed predictions that might be directly applied to the interpretation of data are not available. Here a test particle model is used to construct a global picture of Martian pickup ions in the Mars environment. The model makes use of the recent Nagy and Cravens [1988] model of the Martian exosphere and Spreiter and Stahara's [1980] gas dynamic model of the magnetosheath. The pickup of ions originating at Phobos is also considered. Notable properties of the resulting ion distributions include their near‐monoenergetic spectra, pancake pitch angle distributions, and large gyroradii compared to the planetary scale.
Particle simulation of ion heating in the ring current
Authors: Qian S. , Hudson M. K. , Roth I.
Published: February 2008

Heating of heavy ions has been observed in the equatorial magnetosphere in GEOS 1 and 2 and ATS 6 data due to ion cyclotron waves generated by anisotropic hot ring current ions. A one‐dimensional hybrid‐Darwin code has been developed to study ion heating in the ring current. The mechanism for the heating of thermal hydrogen and helium by ion cyclotron waves has been studied previously in particle simulations of a bi‐Maxwellian plasma. Here, a strong instability and heating of thermal ions is investigated in a plasma with a loss cone distribution of hot ions. The linear growth rate calculation and particle simulations are conducted for cases with different loss cones and relative ion densities. The linear instability of the waves, the quasi‐linear heating of cold ions and dependence on the thermal H+/He+ density ratio are analyzed, as well as nonlinear parallel heating of thermal ions. Effects of thermal oxygen and hot oxygen are also studied.
Interplanetary Alfvén waves and auroral (substorm) activity: IMP 8
Authors: Tsurutani Bruce T. , Gould Tom , Goldstein Bruce E. , Gonzalez Walter D. , Sugiura Masahisa
Published: February 2008

Almost 1 year of IMP 8 interplanetary magnetic field and plasma data (days 1–312, 1979) have been examined to determine the interplanetary causes of geomagnetic AE activity. The nature of the interplanetary medium (Alfvénic or non‐Alfvénic) and the Bs correlation with AE were examined over 12‐hour increments throughout the study. It is found that Alfvénic wave intervals (defined as Vx−Bx cross‐correlation coefficients of >0.6) are present over 60% of the time and the southward component of the Alfvén waves is well correlated with AE (average peak correlation coefficient 0.62), with a median lag of 43 min. The most probable delay of AE from Bs is considerably shorter, about 20–25 min. Southward magnetic fields during non‐Alfvénic intervals (Vx−Bx cross‐correlation coefficients of <0.4) are equally effective in producing geomagnetic activity. Peak correlation coefficients and lags are similar to those of Alfvénic intervals. From this statistical study, no major differences in the magnetospheric response to Alfvénic and non‐Alfvénic intervals were obvious. The high‐intensity long‐duration continuous AE activity (HILDCAA) events discussed previously by Tsurutani and Gonzalez (1987) are demonstrated to be caused by the southward components of the Alfvén waves, presumably through the process of magnetic reconnection. The lag times of AE from Bs were variable from event to event (and at different times within the Alfvén wave train), ranging from 45 min to as little as 0 min. The cause of this variable delay is somewhat surprising and is not presently well understood.
Differences in the solar modulation of E > 60 MeV cosmic rays at Earth and 18 AU during a complete 11‐year cycle from 1977 to 1988
Authors: Lockwood J. A. , Webber W. R.
Published: February 2008

We have reconstructed the cosmic ray intensities for E > 60 MeV particles at a constant radius of 18 AU using data from the IMP, Voyager and Pioneer spacecraft. These intensities are compared with the intensities at Earth for a complete 11‐year cycle from 1977 to 1988. It is found that the variations at 1 AU and 18 AU track very closely during the decreasing portion of the cycle from 1977 to 1982 but during the recovery the intensity at 18 AU increases much less than at Earth with the result that in 1987 the intensity is only 82% of that at the previous minimum in 1977. At Earth this fraction is 96±1.5%. These results imply that the integral radial gradient between 1 and 18 AU is constant in time from 1977 to 1982 but decreases from 1982 to 1988. Thus, the amplitude of the 11‐year variation observed at a fixed energy is quite strongly radially dependent, with the amplitude becoming less at larger radii. We suggest that this asymmetry in the magnitude of the 11‐year variation is responsible for the appearance of “superfluxes” of H and He observed in 1976, but not in 1965 and 1987, and also for the time and energy dependent hysteresis effects observed in the recovery portion of the modulation cycle. The implications of this, which in effect is a 22‐year modulation cycle superimposed on the usual 11‐year modulation, are discussed in terms of the changing solar magnetic field polarity and modulation models such as the drift‐wavy current sheet model which predict significant alternate 11‐year modulation cycle effects.
Periodic auroral forms and geomagnetic field oscillations in the 1400 MLT region
Authors: Potemra T. A. , Vo H. , Venkatesan D. , Cogger L. L. , Erlandson R. E. , Zanetti L. J. , Bythrow P. F. , Anderson B. J.
Published: February 2008

The UV images obtained with the Viking satellite often show bright features which resemble “beads” or “pearls” aligned in the east–west direction between noon and 1800 MLT (Lui et al., 1989). Viking acquired a series of 25 UV images during a 28‐min period on July 29, 1986, which showed a distinctive series of periodic bright features in this region. Magnetic field and hot plasma measurements obtained by Viking confirm that the UV emissions are colocated with the field line projection of an upward‐flowing region 1 Birkeland current and precipitating energetic (∼200 eV) electrons. These observations support previous work (see the review by Evans, 1985), which concludes that this region is the ionospheric projection of the cusp and low‐latitude boundary layer. The magnetic field and electric field measurements show transverse oscillations (predominantly in the westward and southward components, respectively) with a nearly constant period of about 3.5 min from 67° invariant latitude equatorward up to the location of the large‐scale Birkeland current system near 76° invariant latitude. The electric field oscillations lead the magnetic field oscillations by about a quarter‐period. We interpret the observed oscillations as standing Alfvén waves driven at a frequency near the local resonance frequency by a large‐scale wave in the boundary layer. We propose that the energy flux of the precipitating low‐energy electrons in this afternoon region is modulated by this boundary wave and produces the periodic UV emission features. These observations may be interpreted in terms of a boundary wave drifting tailward with a speed of about 0.12 MLT hour/min (1.8° longitude/min or about 800 m/s in the ionosphere) and with a wavelength of about 0.35 MLT hour (5.3° of longitude or about 160 km in the ionosphere). The results of this study support the view that large‐scale oscillations of magnetospheric boundaries, possibly associated with the Kelvin‐Helmholtz instability, can modulate currents, particles, and auroral forms.
The role of magnetic‐field‐aligned electric fields in auroral acceleration
Authors: Block L. P. , Fälthammar C.‐G.
Published: February 2008

Electric field measurements on the Swedish satellite Viking have confirmed and extended earlier observations on S3‐3 and provided further evidence of the role of dc electric fields in auroral acceleration processes. On auroral magnetic field lines the electric field is strongly fluctuating both transverse and parallel to the magnetic field. The significance of these fluctuations for the auroral acceleration process is discussed. A definition of dc electric fields is given in terms of their effects on charged particles. Fluctuations below several hertz are experienced as dc by typical auroral electrons if the acceleration length is a few thousand kilometers. For ions the same is true below about 0.1 Hz. The magnetic‐field‐aligned (as well as the transverse) component of the electric field fluctuations has a maximum below 1 Hz, in a frequency range that appears as dc to the electrons but not to the ions. This allows it to cause a selective acceleration, which may be important in explaining some of the observed characteristics of auroral particle distributions. The electric field observations on Viking support the conclusion that magnetic‐field‐aligned potential drops play an important role in auroral acceleration, in good agreement with particle observations both on Viking and on the DE satellites. They also show that a large part, or even all, of the accelerating potential drop may be accounted for by numerous weak (about a volt) electric double layers, in agreement with earlier observations on the S3‐3 satellite and with an early theoretical suggestion by L. Block. The ratio of power densities in the transverse and parallel components of the fluctuating electric fields is also in agreement with this conclusion. It is suggested that the power density maximum below 1 Hz may be due to standing waves trapped within a region of high Alfvén velocity in the altitude range between about a half and a few Earth radii.
Solar EUV irradiance derived from a Sounding Rocket Experiment on November 10, 1988
Authors: Woods Thomas N. , Rottman Gary J.
Published: February 2008

The solar EUV irradiance from 30 to 100 nm was obtained from a sounding rocket experiment launched from the White Sands Missile Range, New Mexico, on November 10, 1988. This measurement will be used to cross calibrate the Airglow‐Solar Spectrometer Instrument on the San Marco D/L satellite. The solar EUV spectrograph had pre‐ and post‐flight photometric calibrations with an average uncertainty of 6% at the Synchrotron Ultraviolet Radiation Facility (SURF) at the National Institute of Standards and Technology. The observed solar EUV irradiance is about 20% less than the solar EUV flux from a proxy model based on the daily 10.7‐cm solar flux (F10.7) and its 81‐day mean () and the AE‐E solar EUV data taken in the 1970's. This measurement was obtained during the ascending phase of the solar cycle 22, while the AE‐E data were obtained during the ascending phase of solar cycle 21. Measurements of the solar Lyman α irradiance at 121.6 nm were also obtained with an nitric oxide ionization cell. The derived Lyman α flux is 3.30±0.5 (×1011 photons s−1 cm−2) as compared to 3.35±0.3 derived from the Solar Mesospheric Explorer solar data on November 10, 1988.
Temperatures of individual ion species and heating due to charge exchange in the ionosphere of Venus
Authors: Kim Jhoon , Nagy Andrew F. , Cravens Thomas E. , Shinagawa Hiroyuki
Published: February 2008

The coupled electron and multispecies ion energy equations were solved for daytime conditions in the Venus ionosphere. The heating rates due to charge exchange between hot oxygen atoms and thermal oxygen ions were calculated and incorporated into the energy equations. The combination of the traditional EUV heating and this hot oxygen energy source leads to calculated electron and individual ion temperatures significantly lower than the measured values during solar cycle maximum conditions. Calculations were also carried out for solar cycle minimum conditions, which led to considerably lower temperatures; no data are available which would allow direct comparisons of these results with measurements. In order to obtain calculated temperature values consistent with the observed ones, for solar cycle maximum conditions, topside heat inflows into the ion and electron gases have to be introduced or the thermal conductivity must be reduced by considering the effect of steady and fluctuating magnetic fields, as was done in previous studies. The addition of hot oxygen heating leads to minor increases in the calculated ion temperatures except for the case of reduced thermal conductivities. Separate temperatures were calculated for each ion species for a number of different conditions and in general the differences were found to be relatively small.
The dependence of modeled OI 1356 and N2 Lyman Birge Hopfield auroral emissions on the neutral atmosphere
Authors: Germany G. A. , Torr M. R. , Richards P. G. , Torr D. G.
Published: February 2008

Images of the entire auroral oval at carefully selected wavelengths contain information on the global energy influx due to energetic particles and some information on the characteristic energy of the precipitating particles. In this paper we investigate the sensitivity of selected auroral emissions to changes in the neutral atmosphere. In particular, we examine the behavior of OI 1356 Å and two Lyman Birge Hopfield (LBH) bands and their ratios to each other with changing atmospheric composition. The two LBH bands are selected so that one lies in the region of strong O2 absorption (1464 Å) and one lies at a wavelength where O2 absorption is effectively negligible (1838 Å). We find that for anticipated average uncertainties in the neutral atmosphere (factor of 2 at auroral altitudes), the resultant change in the modeled intensities is comparable to or less than the uncertainty in the neutral atmosphere. The smallest variations, for example, are for I 1838 (approximately 10 to 20%) while the largest variation is seen in the OI 1356 Å emission which is linear with [O] to within 20%. We have also investigated the dependence of these intensities, and their ratios, to much larger changes in the composition (i.e., [O]/[N2]) such as might be encountered in large magnetic storms, or over seasonal or solar cycle extremes. We find that the variation in the I 1356/I 1838 ratio over the equivalent of a solar cycle is less than 50%. The summer‐to‐winter changes are approximately a factor of 2. The I 1356/I 1838 ratio is a very sensitive indicator of the charactenstic energy, showing a change of 13 over the energy range 200 eV to 10 keV. The corresponding change in the LBH long‐to‐short wavelength ratio is much less (about a factor of 3). However, the latter is insensitive to changes in the neutral atmosphere (<20% changes in LBH emission ratio for large changes in N2). The three emissions therefore potentially provide a most valuable diagnostic of particle characteristic energy and energy flux.
A study of the daytime E‐F1 region ionosphere at mid‐latitudes
Authors: Buonsanto Michael J.
Published: February 2008

A photochemical equilibrium daytime model is used to study the ionosphere between 110 and 180 km at mid‐latitudes. The model includes the latest photoionization and photoabsorption cross sections, extreme ultraviolet (EUV) fluxes in 37 wavelength bands, and all reactions believed to be important in this region. Model results are compared with (1) noon‐time E layer critical frequency (foE) at Boulder and Wallops Island over a full solar cycle; (2) Millstone Hill incoherent scatter radar observations of electron density at 180 km (N180) for a wide variety of seasons and solar geophysical conditions; (3) selected Millstone Hill incoherent scatter profiles of electron density between 110 and 180 km which included E‐F1 valley minima; and (4) the ratio of the molecular ion concentration to the total ion concentration at 180 km for noon throughout the solar cycle as given by both the IRI‐86 ion composition model and the semiempirical ion composition model of Oliver. Best agreement between the photochemical model documented in this paper and the observations and ion composition models is generally obtained if (1) the EUV fluxes in the photochemical model are increased by 25–30% above values derived from published reference spectra; (2) neutral densities used in the photochemical model are decreased by 25% below those given by MSIS‐86 at equinox, with larger decreases in winter, and smaller or no decreases in summer. The results show that this region of the ionosphere can be modeled with reasonable success given our current state of knowledge. Modeling this region of the ionosphere is important for resolving ambiguities in true height analysis of ionograms and reduction of incoherent scatter spectra. Improved modeling requires more accurate values of aeronomical parameters, i.e., ionizing fluxes, cross sections, reaction rates, composition and temperature.
Conjugacy of daytime ELF‐VLF emission activities in the auroral zones
Authors: Sato Natsuo , Suzuki Hiroyuki , Maezawa Kiyoshi , Saemundsson Thorsteinnn
Published: February 2008

Statistical characteristics of emission occurrence are examined, using 1 year of digital data of 750‐Hz, 2‐kHz, and 4‐kHz intensity records. These waves were measured simultaneously at a conjugate pair of stations, namely Syowa Station in Antarctica and Husafell in Iceland. The following notable diurnal and seasonal variation and Kp dependence was found for the daytime emissions (04–14 MLT): (1) The 750‐Hz emissions were mostly observed during the daytime around noon in both conjugate regions. The emission occurrence reached a maximum 1–3 hours earlier at Syowa than at Husafell during the equinox season. The seasonal variation of 750‐Hz emission occurrence showed a maximum during local summer and a minimum during local winter at both stations. The ratio of the emission enhancement in summer to that at the equinox is higher at Husafell than at Syowa. The emissions mostly occurred during moderately disturbed conditions of Kp ∼ 2–4 at both stations. (2) The 2‐kHz emission occurrence reached a maximum around 13 MLT at Syowa and around 11 MLT at Husafell. Peaks of the emission occurrence during summer shifted to the afternoonside at Syowa and to the morningside at Husafell. The occurrences at Syowa reached a maximum during local summer and a minimum during winter. On the other hand, such a seasonal dependence was not clear at Husafell. The emissions showed the tendency to occur during disturbed magnetic conditions of Kp ≥ 4. (3) The occurrence of 4‐kHz emissions was much more frequent at Husafell than at Syowa. The emissions at Husafell occurred mostly in the morning (∼08 MLT) and in local winter, and the occurrences became more frequent with increasing magnetic activity. On the bases of these statistical characteristics, we discuss the effects of sunlight and geomagnetic activity which cause an asymmetry of wave propagation from the magnetosphere to the ionosphere in the two hemispheres.
On the structure of the tail magnetic field
Authors: Candidi M. , Orsini S. , Stokholm M. , Elphic R. C.
Published: February 2008

We present a study comparing the magnetic field configuration in the magnetotail lobes with that in the plasma sheet boundary layer. The results indicate a systematic enhancement of the By component of the plasma sheet boundary layer field over that of the lobe. The value of such enhancement increases with distance from the midnight meridian plane and has the same sense in the northern and southern plasma sheet boundary layer. Such a By enhancement corresponds to a smaller rate of flaring of the magnetic field lines in the lobe with respect to the plasma sheet boundary layer. This observation is interpreted as a signature of field‐aligned currents at the boundary. The direction of the field‐aligned currents agrees with the nightside region 1 current system, which is the most poleward system in the nightside polar ionosphere at low altitude.
Plasmasphere‐ionosphere coupling: 2. Ion composition measurements at plasmaspheric and ionospheric altitudes and comparison with modeling results
Authors: Horwitz J. L. , Comfort R. H. , Richards P. G. , Chandler M. O. , Chappell C. R. , Anderson P. , Hanson W. B. , Brace L. H.
Published: February 2008

Observations of plasmasphere‐ionosphere coupling are made through thermal plasma measurements by the dual‐spacecraft mission Dynamics Explorer. DE 1 measurements of ion composition and temperatures at 1.4–3.5 RE in the plasmasphere are combined with DE 2 measurements of the ion composition and electron and ion temperatures in the upper F region/topside ionosphere closely spaced in universal and local time for cases in the period November 6–11, 1981. Our findings include the following: (1) In some cases, there is evidence that the light ion latitudinal density profiles in the upper ionosphere resemble those for the plasmasphere even though the dominant O+ density profile in the ionosphere is typically much smoother and flatter. (2) There is strong evidence that the O+ density profile variations within the plasmasphere correlate with the ion and electron temperature latitudinal variations in both the plasmasphere and ionosphere, but have very little correlation with the comparatively smooth ionospheric O+ density profile. (3) The He+/H+ density ratio is typically much greater than unity in the upper ionosphere sampled by DE 2, but is around 0.2 in the plasmasphere data taken by DE 1. (4) The latitudinal variations of ionospheric O+ temperature and plasmaspheric H+ temperature are broadly similar in that both show general increases with latitude and occasional correlated peaks in the outer plasmasphere, although the plasmaspheric temperatures are typically a factor of 3 or more larger than the ionospheric temperatures. The observations are compared directly with calculations of altitudinal ion density and temperature profiles from the field‐line interhemispheric plasma (FLIP) model. When the FLIP model permits fractional trapping of ionospheric photoelectrons and consequent plasmaspheric heating, generally quite good agreement with the observations is obtained.
Flow of oxygen ions in the solar wind acceleration region
Authors: Esser Ruth , Leer Egil
Published: February 2008

A solar wind model with protons, electrons, O VII and O VI ions is studied. It is found that ionization and recombination processes lead to an approximately constant density ratio of the oxygen states (nOVI/nOVII ≈ const ≪ 1) in the solar wind acceleration region. Although ionization and recombination have a significant effect on the flow speed of the O VI ions, these processes are not fast enough to bring the speed of O VI up to the flow speed of the O VII ions.
Auroral modeling of the 3371 Å emission rate: Dependence on characteristic electron energy
Authors: Richards P. G. , Torr D. G.
Published: February 2008

We have developed an efficient two‐stream auroral electron model to study the deposition of auroral energy and the dependence of auroral emission rates on characteristic energy. This model incorporates the concept of average energy loss to reduce the computation time. Our simple two‐stream model produces integrated emission rates that are in excellent agreement with the much more complex multistream model of Strickland et al. (1983) but disagrees with a recent study by Rees and Lummerzheim (1989) that indicates that the N2 second positive emission rate is a strongly decreasing function of the characteristic energy of the precipitating flux. Our calculations reveal that a 10 keV electron will undergo approximately 160 ionizing collisions with an average energy loss per collision of 62 eV before thermalizing. The secondary electrons are created with an average energy of 42 eV. When all processes including the backscattered escape fluxes are taken into account, the average energy loss per electron‐ion pair is 35 eV in good agreement with laboratory results.
Interplay between collective and collisional processes in critical velocity ionization
Authors: McNeil William J. , Lai Shu T. , Murad Edmond
Published: February 2008

Numerical simulations of critical ionization velocity (CIV) discharges have been performed, taking into account several collisional processes. The simulations are one‐dimensional with density variation in the direction of plasma‐neutral relative velocity. The heating of electrons by wave‐particle interactions results in an extended electron distribution with “hot tail” formation, which enables excitation and ionization of neutrals to occur. The tail formation is observed to persist with a maximum energy less than twice the beam energy. The collisional processes included in this simulation are ground state ionization, excitation and ionization of metastable states, ion‐neutral charge exchange, and electron‐neutral elastic collisions. It is found that near the critical velocity, metastable states participate strongly in ionization, while at high velocities the metastable effect becomes relatively unimportant. Electron‐neutral elastic collisions, within the approximations used in this model, appear to be unimportant. Charge exchange is found to be crucial in sustaining the discharge when the beam velocity is low but is unimportant when the beam velocity is high. Detailed results are shown for the case of neon, while threshold behavior is investigated for helium, xenon, and barium as well. The density dependence of ionization in neon and barium is investigated. Although we examine the effect of line excitation on thresholds for CIV in all species, xenon and barium are especially of interest because both species have been used or proposed to be used in space CIV experiments. We report that line excitation strongly drains energy for barium beams in space. As a result, despite a low ionization potential, the threshold velocity for critical ionization of barium is raised substantially. This suggests a reason why barium beam ejections, the most common method for investigating CIV in space, have so far achieved inconclusive results. We conclude that low ionization potential may not be the best criterion for choosing the optimal beams in space experiments on CIV.
Precipitating and trapped ions and electrons observed at 840 km during the Great Magnetic Storm of February 1986
Authors: Swider William
Published: February 2008

A detailed picture is presented of the equatorward boundaries of the auroral ovals at dawn, morning, dusk, and evening for the three most disturbed days of February 1986. North–south symmetry for the boundaries of keV particles was good, and the differences between the ion and electron boundaries agreed with statistics which show the ion edge slightly equatorward of the electron edge at dusk, with the reverse for dawn. The electron boundary was most equatorward of the ion boundary for morning. Best symmetry and least difference were for evening, the sector nearest the central plasma sheet. Ions with energies from thermal to several hundred electron volts penetrated inward to L = 1.2. Initial penetration was confined mainly, if not exclusively, to the dawn sector. The sudden appearance of low‐energy ions deep in the plasmasphere at dusk and evening after storm maximum suggests corotation from a plasmapause as low as L = 1.7 at dawn. Low‐energy electrons rarely advanced equatorward of the keV electrons. On the other hand, keV electrons occasionally were detected about the equator, apparently in relation to the inward convection of the radiation belt. Very energetic, MeV, particles occurred near 52° MLAT, mainly after storm maximum and often unsymmetrically, which may reflect a characteristic akin to the South Atlantic Anomaly.
Spatial coherence during pulsed injection of electron beams
Authors: Pritchett P. L.
Published: February 2008

A two‐dimensional, isolated‐system electrostatic simulation model is used to investigate the coherence properties of single and multiple pulses of electrons injected into the ionosphere from space vehicles. The initial helical structure of the beam is found to be destroyed on a time scale inversely proportional to the beam plasma frequency, resulting in a coherence time of the order of 1 μs. The subsequent interaction of a beam pulse with the ambient plasma produces an enhanced spectral density in the total parallel current extending up to wave numbers on the order ωpe/υxb, where ωpe is the ambient plasma frequency and υxb is the parallel beam injection speed. The magnitude of the spectrum in this range decreases roughly as k−1/4;. Pulsing the beam with a period comparable to the disruption time leads to an enhanced interaction between successive pulses.
Rocket observation of the magnetosphere‐ionosphere coupling processes in quiet and active arcs
Authors: Shiokawa K. , Fukunishi H. , Yamagishi H. , Miyaoka H. , Fujii R. , Tohyama F.
Published: February 2008

Magnetosphere‐ionosphere coupling processes in inverted‐V electron precipitation regions have been investigated using particle and magnetic field data obtained from two antarctic sounding rockets S‐310JA‐11, and S‐310JA‐12, launched into quiet and active auroral arcs, respectively. These data have suggested the presence of a linear relation between field‐aligned current and field‐aligned potential difference, i.e., j∥ = KV∥, given by Fridman and Lemaire [1980] and Lyons [1980]. The density n and thermal energy E0 of primary electrons have been obtained by fitting accelerated Maxwellian distribution functions to the observed electron energy spectra. It is found that the thermal energy E0 is larger inside the arcs than outside the arcs for both the quiet and active arcs observed, suggesting that precipitating electrons are heated during the field‐aligned acceleration. In contrast, the electron density n is enhanced outside the arcs for the quiet arc event, while it is enhanced inside the arcs for the active arc event. From an investigation of the relation between E0 and V∥ and also the relation between E0 and j∥, it is suggested that the heating occurs effectively when the current intensity exceeds a certain threshold and that the electron heating rate is 5–40% of the energy gain due to the field‐aligned acceleration. Assuming that, this heating is due to Joule heating through anomalous resistivity, it is also suggested that the total anomalous resistivity along the magnetic field line decreases with an increase of K.
A comparison of models of the Sun's extreme ultraviolet irradiance variations
Authors: Lean Judith
Published: February 2008

Empirical models of solar EUV irradiance variations developed by Hinteregger et al. (1981) and Tobiska and Barth (1990) are compared over time scales of the 27‐day solar rotation and the 11‐year solar cycle. Relative to the Hinteregger et al. model, the Tobiska and Barth model predicts the variations of the longest‐wavelength EUV emissions (λ > 80 nm), which are entirely chromospheric, to be somewhat less during solar rotation and considerably less over the solar cycle. In contrast, emissions at the shortest EUV wavelengths (λ < 20 nm), which are predominantly of coronal origin, are predicted by the Tobiska and Barth model to vary considerably more during solar rotation and somewhat more during the solar cycle than by the Hinteregger et al. model. Within the wavelength region 20–80 nm, chromospheric emission variations are generally underpredicted on all time scales by the Tobiska and Barth model compared with that of Hinteregger et al., with differences between the predicted variations in the strongest chromospheric emission lines (He II 30.378 nm, He I 58.433 nm, and H I Lβ 102.572 nm) exceeding a factor of 2. Neither models nor measurements yet provide a consistent picture of long‐term variability in the EUV portion of the sun's spectrum. This is related to discrepancies of apparent instrumental origin which are identified between the rocket measurements of the EUV spectrum in August 1979 and in November 1988, to which the models are each independently tied. It is demonstrated that simple calculations of active region emission can provide useful constraints on empirical variability models, at least for chromospheric emissions.
Oblique broadband electron plasma waves above the plasma frequency in the electron foreshock: Linear theory and spacecraft observations
Authors: Canu Patrick
Published: February 2008

The linear dispersion relation of the beam‐plasma instability and the ISEE 1 wave observations in the electron foreshock are examined in order to study the existence of broadband emissions polarized in an oblique direction with respect to the direction of the electron beams reflected from the bow shock. The present work focuses on broadband emissions observed above the electron plasma frequency ωpe, in the frequency range 1 − 1.5 ωpe and above. The linear dispersion relation of waves is investigated numerically and the dependence of the unstable frequency bandwidth on the beam to plasma temperature ratio, Tb/Tp, and density ratio, Nb/Np, is studied. It is shown that electron plasma waves can initially grow in an oblique direction up to 2.5 ωpe and above, when the beam temperature is low, Tb/Tp below 10−2, and its density close to or above Nb/Np = 0.01. The unstable frequency bandwidth over which emissions can grow is shown to be highly controlled by the density ratio Nb/Np. The statistical study of the directivity of broadband emissions observed by ISEE 1 supports the existence of oblique emissions in the frequency range 1–1.5 ωpe, whereas the emissions with frequencies close to ωpe propagate mainly in the parallel direction.
Seasonal variations in the equatorial thermospheric wind measured at Arequipa, Peru
Authors: Biondi M. A. , Meriwether J. W. , Fejer B. G. , Gonzalez S. A.
Published: February 2008

Studies have been carried out at Arequipa, Peru, of the seasonal variations in the thermospheric winds at moderate solar flux levels and low geomagnetic activity. Fabry‐Perot interferometer measurements of the Doppler shifts in the 630.0 nm nightglow emission line from March to August 1983 and from April to October 1988 have yielded monthly‐average meridional winds that are nearly zero (<50 m/s) and possibly fluctuating in direction through much of the night but develop a southward flow at 50–100 m/s in the early and the late night from May onward. The average zonal winds are eastward throughout the night, reaching peak velocities before local midnight and then decreasing. The peak velocities increase to a maximum around the June solstice. The winds are generally stronger in 1988 than in 1983, even though the solar EUV fluxes are comparable for both years. Comparison of the present results with earlier satellite measurements, as embodied in the Horizontal Wind Model of Hedin et al., reveals generally satisfactory agreement at the equinox and June solstice, except for the June 1988 period. Killeen et al.'s vector spherical harmonic form of the NCAR Thermospheric General Circulation Model, calculated for similar solar flux levels, yields meridional and zonal wind variations which exhibit the same temporal behaviors but generally smaller values than the present measurements. The present wind patterns are also compared with those measured during the same years at Arecibo, Puerto Rico; one finds oppositely directed meridional flows and similar, eastward zonal flows at the two locations, as expected from a consideration of solar EUV heating as the principal driving force.
Fabry‐Perot imaging observations of OH(8‐3): Rotational temperatures and gravity waves
Authors: Swenson G. R. , Mende S. B. , Geller S. P.
Published: February 2008

Ground‐based observations of lower thermospheric emission originating from OH(8‐3) rotation lines P1,2 at 7316.4 Å and P2,3 at 7330 Å were made with an intensified imaging Fabry‐Perot interferometer from Santa Cruz, California, on September 25 and 26, 1987, during a CEDAR campaign. The rotational lines were imaged simultaneously from four different atmospheric volume elements which were separated by ∼15 km north to south and ∼30 km east to west. Two types of analysis are performed on the data. First, traditionally observed temporal structure in the rotational temperature showed acoustic gravity wave‐like periods which are compared to other experimental data. Spectral analysis on the data shows the traditional gravity wave characteristics for specific periods in four separated atmospheric volumes which were sampled simultaneously. Second, the simultaneous multiple‐point sample provides a measurement of horizontal phase velocity. Spectral components of temperature variation exhibited phase coherence in all four spatial regions, indicating wave phenomena of length scales ≫30 km. Gravity wave η versus period data are compared with the theories of Schubert and Walterscheid (1988) and Hickey (1988a, b).
A wave‐particle‐wave interaction mechanism as a cause of VLF triggered emissions
Authors: Sá L. A. D.
Published: February 2008

A narrowband‐filter spectral analysis is made of VLF emissions triggered by discrete waves transmitted from Siple Station, Antarctica and received at Lake Mistissini, Quebec, after ducted mode propagation. It is found that triggered emissions have a spectral structure similar to magnetospheric sidebands. As in the case of regular sidebands, their spectra and evolution are affected by the presence of power line radiation (PLR). Triggered emissions associated with monochromatic wave transmissions are explained as being caused by the interaction of the incoming wave with PLR. The issue of the importance of extremely weak signals in these phenomena is addressed, and it is shown that when a wave is perturbed by a weaker signal, sidebands and, consequently, triggered emissions can occur even if the amplitude ratio of the two signals is −50 dB. For such cases, the two waves should not be separated by a frequency larger than a few times the stronger wave trapping frequency. A mechanism, based on previous theoretical work, is presented to explain the creation and evolution of the triggered emissions studied in the paper.
Offset, tilted dipole models of uranian smooth high‐frequency radio emission
Authors: Schweitzer Andrea E. , Romig Joseph H. , Evans David R. , Sawyer Constance B. , Warwick James W.
Published: February 2008

During the Voyager 2 encounter with Uranus in January 1986, the Planetary Radio Astronomy (PRA) experiment detected a complex pattern of radio emissions. Two types of emissions were seen: smooth and bursty. The smooth emission has been divided into smooth high‐frequency (SHF) and smooth low‐frequency (SLF) components which are presumed to come from different sources because of their distinctly different characteristics. The SHF component is considered in this paper. The SHF emission has been modeled by many authors on OTD (offset, tilted dipole (Ness et al., 1986)) L shells ranging from 5 to 40. However, the bursts have been modeled at much higher L shells. We complete an OTD investigation of the SHF emission at high L shells within the range of the bursty source locations, and present a viable high L shell model. This model has fundamentally the same longitudinally symmetric net emission pattern in space as the L shell 5 model presented in Romig et al. (1987) and Barbosa (1988). However, we were unable to produce an acceptable model on intermediate L shells without restricting source longitude. We discuss the similarities and distinctions between our two models and the models of other authors. We believe that our high L shell model (and others similar to it) cannot account for the observed smoothness and periodicity of the SHF emissions because it has open field lines containing untrapped particles, which should produce more variable emission than that seen in the SHF data. Therefore, we prefer models at L shells less than 18, the boundary for closed field lines (Ness et al., 1986). We then discuss and contrast two models within this boundary: the L = 5 model and an L = 12 model by Kaiser et al. (1987) and Farrell and Calvert (1989b). The main distinction between these two models is the longitudinal extent of the source location. The L = 5 model has sources distributed 360° about the nightside OTD pole, while the L = 12 model has an asymmetric distribution with the sources confined to a small region, nearly a point. We propose several reasons why the symmetric distribution of the L = 5 model may be preferable. Finally, we suggest that Miranda, which reaches a minimum L shell at L = 5, may be related to the timing of several types of radio emissions.
Spaced direction finding of nighttime whistlers at low and equatorial latitudes and their propagation mechanism
Authors: Hayakawa M. , Ohta K. , Shimakura S.
Published: February 2008

The propagation mechanism of low‐latitude and equatorial latitude whistlers is investigated on the basis of spaced direction finding measurements in South China. Observations were made continuously in the local time (LT) interval from 0000h to 0400h during the period of January 5–11 1988 at the three stations (Zhanjiang (geomagnetic latitude 10.1°), Guilin (14.1°), and Wuchang (19.4°)) and two horizontal magnetic components and one vertical electric field component were simultaneously recorded over a wide frequency range to enable comprehensive direction finding. Two major occurrence peaks on January 5 and January 6 and two minor ones on January 9 and January 11 have been analyzed and the following experimental results have emerged. (1) The whistler occurrence at very low latitudes is generally very small compared with that at low latitude (geomagnetic latitude ≳20°), but once it occurs, the occurrence rate becomes comparable to that at low latitudes. (2) The whistler dispersion is single‐valued at any particular local time. (3) The ionospheric exit region of whistlers is very much restricted to the geomagnetic latitude range of 10°–14°, and there are no observed whistlers whose path latitude is between 14° and 20°. (4) The extent of their ionospheric exit region is very stable on different days, and the radius of the distribution is less than 40–50 km. (5) Surprisingly high occurrence of echo train whistlers is observed. (6) The propagation of whistlers in the Earth‐ionosphere waveguide after ionospheric transmission is more likely toward higher latitudes than toward the equator and the subionospheric propagation seems to exhibit a horizontal beaming around the magnetic meridian plane. We attempt to interpret these characteristics of the observations in terms of either nonducted or field‐aligned propagation, but discussion indicates that especially findings 4 and 5 are strongly indicative of field‐aligned propagation for very low latitude whistlers localized in geomagnetic latitudes of 10°–14°.