bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling the 6300‐Å low‐latitude nightglow
Authors: Fesen C. G. , Abreu V. J.
Published: February 2008

Observations of the 6300‐Å nightglow from the Visible Airglow Experiment (VAE) instrument on AE‐E are presented for spring equinox, solar cycle maximum conditions. The data comprise altitude profiles and integrated column brightness maps from ∼1800 to 0400 LT and within ±30° of the dip equator. The data clearly show near‐midnight enhancements of the 6300‐Å emission. Attempts to model the column brightness maps indicated that these enhancements are due to tidal effects: the enhancements were only reproduced in the theoretical calculations which included upward propagating tidal components in the neutral winds. Further, low equatorial intensities were observed by the VAE which could only be simulated by assuming that the phase of the E × B drift be shifted 1 hour LT; i.e., upward drift persists until 2000 LT instead of 1900 LT. The VAE observations could be reasonably simulated with the phase shift in the E × B drift and with the dip and geographic equators offset. The major discrepancy is in the magnitude of the nightglow maxima: the calculated intensities are a maximum of 2 times too large. Possible sources are uncertainties in the neutral densities, chemistry, and rate coefficients and in the neutral winds.
A study of MHD fluctuations upstream and downstream of quasi‐parallel interplanetary shocks
Authors: Luttrell A. H. , Richter A. K.
Published: February 2008

Applying power, coherence and magnetic and cross helicity spectral analysis of the solar wind plasma and magnetic field obtained by Helios‐1 and ‐2, we investigate the nature of MHD fluctuations occuring upstream and downstream of a parallel, supercritical, turbulent, of a quasi‐parallel, supercritical, turbulent, and of a quasi‐parallel, subcritical, laminar fast‐forward shock wave. The main results of our investigation are as follows: (1) The spectral slopes and powers vary significantly with the length of the data interval analyzed. The difference upstream of the parallel shock was such that the ratio of the axisymmetric spatial diffusion coefficients of a 1‐MeV proton, calculated from the 86.4‐min and 10.8‐min spectra using Morfill and Scholer's (1977) formula, is 11.82. (2) Counterstreaming Alfvén waves were identified immediately upstream and downstream of the quasi‐parallel, supercritical shock. (3) Fast magnetoacoustic waves were identified upstream of the quasi‐parallel, subcritical shock and far upstream of the quasi‐parallel, supercritical shock coinciding with the inclusion of a small transverse discontinuity in the spectral analysis. (4) Compressional turbulence which is not characteristic of either of the magnetoacoustic modes was observed downstream of the parallel, supercritical and quasi‐parallel subcritical shocks. As regards the determination of the parallel diffusion coefficient from power spectra of the observed magnetic field fluctuations, we may conclude from our results that this procedure is susceptible to major errors due to the following reasons: First, there is no complete theory for the diffusion of energetic particles in regions where the MHD fluctuations cannot be labeled Alfvénic or magnetoacoustic. Second, equations for particle motion in a field of counterstreaming Alfvén waves (second Fermi process) are also lacking. Third, even the “Alfvénic” fluctuations are never 100% Alfvénic; i.e., a non‐Alfvénic component is usually present. Such “mixing” is also not considered in the present theories of energetic particle propagation. Fourth and last, it is not at all clear how much data before/after the shock waves should be analyzed to compute values of the parallel diffusion coefficient (κ∥) or mean free path for scattering (λ∥,s) of an energetic particle. We show here that spectral analysis of the fluctuations 10.8 min., 43.2 min. and 86.4 min. upstream of the parallel shock yields reasonable, but very different, values of κ∥.
Decay of electrostatic hydrogen cyclotron waves into ion acoustic modes on auroral field lines
Authors: Bergmann R. , Hudson M. K.
Published: February 2008

The coherent three‐wave decay of a linearly unstable electrostatic hydrogen cyclotron (EHC) wave into stable EHC and ion acoustic modes is considered. Possible triads of parent and daughter waves are identified, and the coupling strength between the waves is calculated in two plasma models. The first model is the fluid limit of an electron‐hydrogen plasma, and the second is a three‐species kinetic model of the plasma in the evening auroral electron acceleration region. It is found that EHC waves commonly observed on auroral field lines are of sufficient amplitude to exceed the threshold for decay to weakly damped EHC modes. Further, frequency and wave number matching can be satisified only if the acoustic mode is oblique to the geomagnetic field. This decay is proposed to be a saturation mechanism for the linearly unstable coherent EHC mode. The temporal dependence of the amplitudes of the three interacting modes, as predicted by the four coupled wave kinetic equations, indicates that the coherent decay can act to saturate the parent wave. Further, it indicates that observed EHC waves may actually be linearly marginally stable daughter EHC modes.
Solar cycle variation of some mass dependent characteristics of upflowing beams of terrestrial ions
Authors: Collin H. L. , Peterson W. K. , Shelley E. G.
Published: February 2008

Examination of the S3‐3 and DE ion composition data spread over a solar cycle indicates that some characteristics of energetic upflowing terrestrial ion beams above the auroral zone show dependence on solar cycle. At solar maximum the different ion beam mass components have comparable mean energies, and O+ dominates the beam composition. The ion energies are consistent with having been acquired from the potential drop below the satellite inferred from the electron loss cone distributions. At solar minimum the beam composition is dominated by H+, but the O+ has a higher mean energy and is hotter than the H+ component. Also, the O+ has more energy than it could itself have acquired from the potential drop. These observations are qualitatively consistent with the ion beams having acquired their energies from a parallel electric field and being partially thermalized through the two‐stream instability between the two ion species, with this effect being modulated by the beam composition.
Theoretical study of the effect of ionospheric return currents on the electron temperature
Authors: Schunk R. W. , Sojka J. J. , Bowline M. D.
Published: February 2008

An electron heat flow can occur in a partially ionized plasma in response to either an electron temperature gradient (thermal conduction) or an electron current (thermoelectric heat flow). The former process has been extensively studied, while the latter process has received relatively little attention. Therefore a time‐dependent three‐dimensional model of the high‐latitude ionosphere was used to study the effect of field‐aligned ionospheric return currents on auroral electron temperatures for different seasonal and solar cycle conditions as well as for different upper boundary heat fluxes. The results of this study lead to the following conclusions: (1) The average, large‐scale, return current densities, which are a few microamps per square meter, are too small to affect auroral electron temperatures. (2) Current densities greater than about 10−5 A m−2 are needed for thermoelectric heat flow to be important. (3) The thermoelectric effect displays a marked solar cycle and seasonal dependence. (4) Thermoelectric heat transport corresponds to an upward flow of electron energy. (5) This energy flow can be either a source or sink of electron energy, depending on the altitude and geophysical conditions. (6) Thermoelectric heat transport is typically a sink above 300 km and acts to lower ambient electron temperatures by as much as 2000 K for field‐aligned return current densities of the order of 5 × 10 −5 A m−2. For this case, the electron temperature decreases with altitude above 300 km with a gradient that can exceed 1 K km−1. Also, the electron temperature can drop below both the ion and neutral temperatures in the upper F region owing to thermoelectric cooling. (7) A downward magnetospheric heat flux in combinations with an upward thermoelectric heat flux can produce steep positive electron temperature gradients in the topside ionosphere.
The plasma wave environment of an auroral arc: 3. VLF hiss
Authors: Bering E. A. , Maggs J. E. , Anderson H. R.
Published: February 2008

Intense auroral hiss was observed by a sounding rocket payload under circumstances that have allowed detailed comparison of the hiss spectrum with the predictions of a convective beam amplification hiss production model. The payload was launched from Poker Flat, Alaska at 0813 UT on March 9, 1978; it carried an electric field detector and electron detectors covering the energy range from 30 eV to 25 keV. A complete pitch angle scan was made every 0.2 s. The trajectory was close to the magnetic meridian and crossed the diffuse aurora, a dark region, and a quiet 40 kR auroral arc. Auroral hiss was first encountered at the poleward edge of the diffuse aurora, and was observed for the rest of the flight. A maximum intensity of 10−8(V/m)²/Hz at 30 kHz was seen in the dark region. Electron distribution functions were seen with δf/δv∥>0 and a large enough slope to have produced appreciable VLF amplification. These distributions were found in a region a few kilometers wide on the equatorward boundary of the arc. A model distribution function has been fitted to one of the observed distributions and used as input into an auroral beam noise generation model. Reasonable assumptions about the distribution of cold plasma and width of the unstable region yields a predicted VLF spectrum in good agreement with observation. Several secondary results were also obtained. The observed VLF hiss does not appear to have been produced by a payload‐plasma interaction. The intensity of the hiss measured in situ, above the ionosphere, was greater equatorward of the arc than within the arc, thus ruling out the ionospheric transmission model as an explanation for this intensity pattern. Quasi‐linear back reaction on the electron distribution was not found to be significant in this case. The correlation between 0.1 – 0.7 keV electron flux and hiss intensity was not better than the correlation with the flux of 5‐keV electrons. The data did not suggest the occurence of a 3 wave parametric decay. Finally, the possibility of a secondary source at ∼100 km altitude is not ruled out by these data.
Daytime lidar measurements of tidal winds in the mesospheric sodium layer at Urbana, Illinois
Authors: Kwon K. H. , Gardner C. S. , Senft D. C. , Roesler F. L. , Harlander J.
Published: February 2008

The first daytime lidar observations of the mesospheric Na layer at Urbana, Illinois (40°N, 88°W), were made in January and March 1986. The data show strong 12‐hour oscillations of the Na density and layer centroid height which appear to be caused by the semidiurnal tide. The peak‐to‐peak variations in column abundance and centroid height were of the order of 100% and 3 km, respectively. The vertical wind velocities were inferred from the temporal variations of the density gradients on the layer topside. The inferred wind amplitudes and vertical wavelengths for the semidiurnal tide were ∼30 cm s−1 and ∼50 km, respectively. The maximum upward vertical velocity occurs at about 1100 LST and 2300 LST.
Hydromagnetic wave excitation by ionized interstellar hydrogen and helium in the solar wind
Authors: Lee Martin A. , Ip W. ‐H
Published: February 2008

Interstellar atoms penetrate the heliosphere, are ionized by solar UV radiation or charge exchange with solar wind ions, and are “picked up” by the solar wind onto a ring distribution in velocity space. The ring distribution is unstable to the generation of hydromagnetic waves with growth time scales which are small compared with those of the continual pickup process and convection with the solar wind. First, the growth rates for parallel propagation are derived, presented, and compared with previous work. Then the spatially homogeneous quasi‐linear equations describing the subsequent hydromagnetic wave excitation and pickup ion velocity diffusion in pitch angle and energy are presented under the assumption of wave propagation parallel to the ambient magnetic field. Neglecting ion energy changes, analytical expressions for the time‐asymptotic wave spectra accompanying the time‐asymptotic isotropic ion distribution are derived. The results indicate that pickup helium has a very small (unobservable) effect on the solar wind wave spectrum, but that pickup hydrogen results in substantial modifications at cyclotron resonant frequencies (∼10−2 Hz at ∼7 AU). Finally, based on the previous expressions, the radial evolution of the pickup‐hydrogen‐modified wave spectra for both polarizations and propagation directions is computed analytically including the degradation of the wave power in the divergent solar wind. The predicted modifications beyond ∼5 AU are substantial and could be observable at spacecraft frequencies greater than ∼5 × 10−3 Hz if not degraded by turbulent wave‐wave interactions or stochastic ion acceleration. The predicted enhancement by a factor of order 10 at ∼7 AU involves ∼10% of the power in the ambient field, is left‐polarized (Alfvénic as opposed to magnetosonic) in the solar wind frame, is unpolarized in the spacecraft frame, and includes an equal admixture of propagation directions.
Observational evidence of cavity modes in the Earth's magnetosphere
Authors: Crowley G. , Hughes W. J. , Jones T. B.
Published: February 2008

On November 30, 1982, two large‐amplitude Pc 5 pulsations were observed at Tromso, Norway, by both the European Incoherent Scatter (EISCAT) radar and ground‐based magnetometers. The pulsations were excited by a sudden impulse. Their amplitude subsequently decreased, allowing a damping rate to be determined. Estimates of the height‐integrated Pedersen conductivity obtained from the EISCAT data permit theoretical pulsation damping rates to be predicted. However, the theoretical damping rates are much greater than those actually measured, which indicates that energy was continuously fed into the field line resonance after its onset. The most likely source of this energy is coupling from a hydromagnetic cavity mode. If this is the case, the measured pulsation damping rate is controlled by the damping of the cavity mode rather than energy dissipation from the field lines resonance. An upper limit of γ/ω = 0.08 is obtained from the coupling efficiency between the two modes. The energy dissipated in the ionosphere by these pulsations was ∼4 × 1012 J. To store this energy in the cavity mode requires an initial cavity mode amplitude of only ∼0.4 nT. Long‐period magnetic perturbations of this amplitude would be difficult to detect in existing spaceborne magnetometer data sets.
Properties of low‐energy electron precipitation in the cleft during periods of unusually high ambient electron temperatures
Authors: Fontheim Ernest G. , Brace Larry H. , Winningham J. David
Published: February 2008

A recently developed automated fitting procedure for Dynamics Explorer 2 electron energy spectra represents each auroral electron spectrum as a superposition of several Maxwellian and several Gaussian components. The fitting parameters give a digital representation which uniquely describes each spectrum. It is shown that unusually high ambient electron temperatures (of the order of 10,000°K) in the cleft are strongly correlated with the intensity parameters of the lowest‐temperature Maxwellian components of the precipitation. The collisional heat input into the ambient electrons by these Maxwellian components of the precipitation fluxes is shown to be more than an order of magnitude higher in the high‐temperature storm time cases than it is in a control set of “normal temperature” quiet time cases. The heating effect of the solar flux is shown to be nearly the same for both sets of cases. As a result it is concluded that electron precipitation is the chief mechanism responsible for those high ambient temperatures.
Drawing inferences about solar wind acceleration from coronal minor ion observations
Authors: Esser Ruth , Holzer Thomas E. , Leer Egil
Published: February 2008

A parameter study is designed and carried out to illustrate the physical effects that can be studied through analysis and interpretation of coronal minor ion spectral line observations. It is shown that minor ion line width, together with the coronal Ly α line width and coronal white light observations, can yield important information concerning the transport and dissipation of energy carried outward from the coronal base by hydromaghetic waves. Although it is difficult to infer minor ion velocities through the Doppler dimming technique, the application of this technique using both radiatively and collisionally excited lines can provide constraints on the acceleration of coronal minor ions. It is concluded that the observation of coronal minor ion spectral lines represents an important component of a concerted observational approach to the solar wind acceleration problem. It must be emphasized, however, that the measurement of line widths is the most important coronal minor ion observation to obtain.
Solar cycle changes in the ionization sources of the nightside Venus ionosphere
Authors: Knudsen William C. , Kliore Arvydas J. , Whitten Robert C.
Published: February 2008

We argue, using new and previous experimental evidence that the height‐integrated flux of O+ ions transported from the dayside Venus ionosphere into the nightside ionosphere is strongly reduced (shut off) at solar cycle minimum (SCmin) and that the electron impact source becomes the predominant nightside ionization source during this phase of the solar cycle. The height‐integrated transterminator O+ flux, which is probably the dominant nightside ionization source at solar cycle maximum (SCmax), is shut off at SCmin by compression of the dayside ionopause to low altitude. We suggest that the ionopause is depressed because the dayside ionospheric kinetic pressure field is reduced by a factor of approximately 3 from its SCmax value and that the typical SCmin solar wind dynamic pressure, which is not less than the typical SCmax value, cannot be balanced by the ionospheric pressure field at altitudes above approximately 250 km.
Magnetic field drift shell splitting: Cause of unusual dayside particle pitch angle distributions during storms and substorms
Authors: Sibeck D. G. , McEntire R. W. , Lui A. T. Y. , Lopez R. E. , Krimigis S. M.
Published: February 2008

We present a magnetic field drift shell‐splitting model for the unusual butterfly and head‐and‐shoulder energetic (E > 25 keV) particle pitch angle distributions (PADs) which appear deep within the dayside magnetosphere during the course of storms and substorms. Drift shell splitting separates the high and low pitch angle particles in nightside injections as they move to the dayside magnetosphere, so that the higher pitch angle particles move radially away from Earth. Consequently, butterfly PADs with a surplus of low pitch angle particles form on the inner edge of the injection, but head‐and‐shoulder PADs with a surplus of high pitch angle particles form on the outer edge. A similar process removes high pitch angle particles from the inner dayside magnetosphere during storms, leaving the remaining lower pitch angle particles to form butterfly PADs on the inner edge of the ring current. A detailed case and statistical study of CCE/MEPA observations, as well as a review of previous work, shows most examples of unusual PADs to be consistent with the model.
Voyager 2 Photopolarimeter observations of the Uranian satellites
Authors: Nelson Robert M. , Buratti Bonnie J. , Wallis Brad D. , Lane Arthur L. , West Robert A. , Simmons Karen E. , Hord Charles W. , Esposito L. W.
Published: February 2008

We report the ultraviolet (0.25 µm) and infrared (0.75 µm) geometric albedos, phase curves, and phase coefficients for the large Uranian satellites using full disk photometric observations obtained by the photopolarimeter subsystem during the Voyager 2 encounter with the Uranian system. The phase coefficients and phase curves of the Uranian satellites are consistent with surfaces that have a loosely packed regolith with a heavily cratered terrain. Ariel, Titania, and Oberon exhibit a reddening with increasing phase angle of observation. We do not observe this effect for Umbriel. We have determined preliminary phase integrals for Ariel, Umbriel, Titania, and Oberon, and we have used this information and the geometric albedos reported herein and previously reported by the Voyager imaging subsystem to derive Bond albedos for the satellites. These are 0.22 ± 0.1, 0.07 ± 0.05, 0.16 ± 0.12, and 0.19 ± 0.22 for Ariel, Umbriel, Titania, and Oberon, respectively. We have compared the geometric albedos and color ratios of the Uranian satellites with comparable sets of data from the literature on other large satellites in the solar system, and we find that the Uranian satellites as a class are separate and distinct from the large Jovian and Saturnian satellites. This indicates a compositional difference between the three classes of large icy satellites. This may be due to different conditions at the time of formation and/or differences in the subsequent evolution of the surfaces of these objects. This finding is consistent with the hypothesis that a common surficial modification process exists for all the satellites in the Uranian system which is different from the processes which modify the surfaces of the Jovian and Saturnian satellites.
Evidence for change in particle excitation of Jupiter's atmosphere 1968–1979
Authors: Shemansky D. E. , Judge D. L.
Published: February 2008

Analysis of the Pioneer 10 and rocket observations of disk averaged emission from the sunlit atmosphere of Jupiter indicates that the spectrally integrated EUV brightness was reduced by at least a factor of 2 relative to Voyager spacecraft observations in 1979. Most of the variation is caused by the HLyα component in the spectrum, which was reduced ≈ 1 order of magnitude near the time of solar minimum in 1972‐1973. Although the analysis of the data does not produce entirely consistent results, the weight of evidence points to a factor of order ≈ 2 lower abundance of H in Jupiter's atmosphere in 1972–1973 relative to 1979. The low emission rate in HLyα near the time of solar minimum in this proposed scenario is caused by an electroglow energy deposition rate reduced by a factor of ≈ 3. The apparent reduced abundance of H implies a reduced thermospheric temperature, even under the assumption of a constant electroglow deposition rate.
Survey of low‐frequency electromagnetic waves stimulated by two coexisting newborn ion species
Authors: Brinca Armando L. , Tsurutani Bruce T.
Published: February 2008

We have studied electromagnetic wave instabilities fed by coexisting newborn (cometary) hydrogen and oxygen ions, illustrating the modifications encountered in the unstable dispersion topology as the relative orientation of the interplanetary magnetic field and the solar wind velocity is varied, discussing the free energy sources, and assessing the influence of the newborn particle density and temperature. Utilization of generalized Brillouin diagrams clarifies the physical nature of new left‐hand polarized, costreaming modes excited by the newborn heavy ions and nonoscillatory, purely growing structures (unrelated to the mirror wave) generated by the two newborn ion species. Although the newborn protons stimulate resonant instabilities more efficiently in ion‐rich environments, the hierarchy of wave growth changes as the ion densities decrease (increasing distance from the comet). Larger growth rates for modes associated with the heavy‐ion beam may then occur. The interaction between the two coexisting ions is generally weak: each beam excites resonant instabilities without undue influence from the other newborn ion species in most dispersion domains. Cometary environments suggest the parameters of the adopted model, but the results are helpful in the interpretation of other observations of low‐frequency wave activity in space.
Latitudinal electron precipitation patterns during large and small IMF magnitudes for northward IMF conditions
Authors: Makita K. , Meng C.‐I. , Akasofu S.‐I.
Published: February 2008

We demonstrate that there are distinct differences in the electron precipitation patterns (or the polar cap size), geomagnetic activity, and field‐aligned currents in the highest‐latitude region for small and large IMF Bz values when the IMF Bz component is positive. First, during periods of weakly northward IMF, there is a distinct area in the highest‐latitude region in which the electron precipitation is absent except for the polar rain. By contrast, during strongly northward IMF, the entire polar region is often filled with burst‐type soft electron precipitations. Second, geomagnetic disturbances and field‐aligned‐current intensitites in the highest‐latitude region (>80° in geomagnetic latitude) are less during a weak IMF Bz condition than those during a strongly northward IMF Bz condition. Geomagnetic activity in the auroral zone for both conditions is absent or very weak.
Low‐energy bouncing ions in the magnetosphere: A three‐dimensional numerical study of Dynamics Explorer 1 data
Authors: Delcourt D. C. , Giles B. L. , Chappell C. R. , Moore T. E.
Published: February 2008

The low‐energy outflowing and bouncing ion distributions displayed by a statistical study of the retarding ion mass spectrometer data on DE 1 have been investigated using three‐dimensional simulations of ion trajectories in the magnetosphere. Because of the simultaneous azimuthal and latitudinal drifts imposed by the dawn‐dusk convection electric field, it is shown that the statistically observed ionospheric outflow evolves into the observed trapped ion population. The drift paths of these ions as well as the location of their “source” region strongly depend on their mass. It is demonstrated that the morning sector auroral zone in the nightside magnetosphere is the major source of low‐energy trapped ions. A comparison between the computed and observed magnitudes has been made for both light (H+) and heavy (O+) ions that reveals a better agreement at short flight times (about 2 hours) than at longer ones. This emphasizes the role of loss mechanisms and the dynamic aspects of the magrietospheric convection.
Observations of low‐latitude electron precipitation
Authors: Lieu R. , Watermann J. , Wilhelm K. , Quenby J. J. , Axford W. I.
Published: February 2008

Low‐latitude electron precipitation outside the well‐known region of the trapped Van Allen radiation belt has been known for some time, as has the occurrence of equatorial aurora. Observation by the Spacelab 1 electron spectrometer, working between 0.1 and 12.5 keV electron energy has revealed an unexpectedly high occurrence rate and flux intensity for these events. Data were analysed for 16 orbital passes, between ±30° latitude, outside the South Atlantic Anomaly. Many precipitation events with flux levels above 2×104 el cm−2 s−1 sr−1 were observed, each consisting of two separate electron populations (1) a low‐energy component (0.1–1 keV) with a power law spectrum, and (2) a high‐energy component (1–12.5 keV) with a distinct flattening of spectral slope, sometimes with a peak, and exhibiting a “flare‐up” behavior on a maximum time scale of 1.5 hours. Detailed study of the four orbital passes which detected the most intense fluxes (>3×105 el cm−2 s−1 sr−1;) of 1‐ to 12.5‐keV electrons showed intensity minima at the magnetic equator, with peaks on either side (L ∼ 1.12) and pitch angle distributions that indicate a significant nontrapped population. The observations place a lower limit of ∼0.1 Vm−1 on the peak electric fields responsible for acceleration of the electrons.
A theory of patchy and intermittent reconnections for magnetospheric flux transfer events
Authors: Kan J. R.
Published: February 2008

We propose that the elbow‐shaped flux transfer events (FTEs) of Russell and Elphic (1979) are initiated by a specific normal mode of three‐dimensional tearing of the magnetopause current sheet, resulting in a three‐dimensional component reconnection driven by the solar wind. The patchy nature of the proposed three‐dimensional tearing reconnection is determined by the structure of the three‐dimensional tearing on the dayside magnetopause, leading to isolated reconnection sites; the intermittent nature is governed externally by the fluctuation of the interplanetary magnetic field (IMF) and internally regulated by the ionospheric line‐tying effect. The proposed 3DT reconnection can occur simultaneously with the single x line intermittent or quasi‐steady reconnection if the IMF drapes over the dayside magnetopause, which takes place when the solar wind speed exceeds the MHD fast‐mode speed. Magnetic field signatures of the 3DT reconnection are consistent with the elbow‐shaped FTE signatures. Electric fields associated with the elbow‐shaped FTEs can be expected to be spiky, peaking near the center of the localized FTE reconnection sites. The spiky electric field of FTEs is expected to produce channels of enhanced magnetospheric convection. The observed multiple field‐aligned current sheets and auroral arcs fanning out from the cusp region along enhanced convection channels can be expected as consequences of the proposed 3DT reconnection on the dayside magnetopause. The polar cap potential due to the elbow‐shaped FTEs is predicted to have a dependence on the IMF polar angle somewhat different from that of the single x line reconnection.
Mapping electrodynamic features of the high‐latitude ionosphere from localized observations: Combined incoherent‐scatter radar and magnetometer measurements for January 18–19, 1984
Authors: Richmond A. D. , Kamide Y. , Ahn B. ‐H. , Akasofu S. ‐I. , Alcaydé D. , Blanc M. , de la Beaujardière O. , Evans D. S. , Foster J. C. , Friis ‐Christensen E. , Fuller ‐Rowell T. J. , Holt J. M. , Knipp D. , Kroehl H. W. , Lepping R. P. , Pellinen R. J. , Senior C. , Zaitzev A. N.
Published: February 2008

The large‐scale electric potential patterns, describing ionospheric convection, are estimated for northern high latitudes during January 18‐19, 1984, from combined incoherent‐scatter radar and ground magnetometer observations, using the technique of Richmond and Kamide (this issue). The patterns usually have a dominant two‐cell characteristic, although the intensities, orientations and shapes of the cells undergo considerable changes with time. Often evident during substorm expansive phases is a “tongue” of low electric potential extending toward the east along the low‐latitude edge of the high potential cell at night. Time‐series plots of the maximum and minimum electric potentials show that they can respond rapidly to changes in the interplanetary magnetic field Bz component. Total estimated potential drops for this 2‐day period range from about 15 kV up to 108 kV. The influence of the different types of data on the resultant estimated electric potential patterns is analyzed. Where available, the direct electric field observations by the radars primarily control the characteristics of the estimated potential patterns, while the magnetometer data have their greatest influence in regions where direct electric field measurements are unavailable. We also employ the statistical electric potential model of Foster et al. (1986) to help fill in the patterns in datasparse regions. For the present study, data coverage is often good enough that the statistical model plays only a secondary role in determining the estimated convection patterns. The ionospheric electrical conductance observations from the Sondrestrom and EISCAT radars are very important in helping modify the statistical conductance model of Fuller‐Rowell and Evans (1987) to yield modified conductance distributions suitable for interrelating the electric fields, currents, and magnetic perturbations. Analysis of the statistical uncertainty in the estimated large‐scale electric field patterns shows the uncertainty to exceed 50% in the polar cap and sub auroral regions and to be less than 20% only in the vicinity of the radar electric field observations.
Comparative study of cross‐field and field‐aligned electron beams in active experiments
Authors: Winglee R. M. , Pritchett P. L.
Published: February 2008

Beam‐plasma interactions associated with the cross‐field injection of electron beams from spacecraft are investigated by means of two‐dimensional (three velocity component) electrostatic particle simulations and are compared with those for parallel injection. The degree of spacecraft charging and the nature of the plasma response depends on the ratio of the plasma response time trp to the beam stagnation time ts. In particular, when trp/ts ≲ 0.4 and the spacecraft size is comparable to the return current region, the spacecraft charging is reduced sufficiently so that the beam is able to escape from the near environment of the spacecraft. The parameters which determine the value of the ratio trp/ts are the density of the ambient plasma relative to the beam density, the beam width and energy and the spacecraft size. Thin energetic beams injected from wide spacecraft are most easily able to escape, with the spacecraft potential being proportional to the parallel beam energy rather than to the total energy. Strong space‐charge oscillations near the lower hybrid and upper hybrid frequencies are produced as the beam propagates into the plasma. These oscillations tend to cause beams injected across the field lines to lose their coherence after about one or two gyrations. The beam electrons are scattered into a hollow cylinder with radius equal to the beam electron gyroradius and thickness of about two beam Debye lengths. Parallel injected beams experience similar forces which cause the beam to expand to fill a solid cylinder of a comparable thickness. The return currents produced by both cross‐field and parallel beam injection have an extent across the field lines of the order of the gyroradius of a plasma ion accelerated up to about the spacecraft potential.
Solar cycle effects in intervals of pulsations of diminishing periods pulsation activity
Authors: Maltseva N. , Kangas J. , Pikkarainen T. , Olson J. V.
Published: February 2008

We report that intervals of pulsations of diminishing periods (IPDP) type plasma wave events which are associated with magnetospheric substorms are less frequent in the years of high solar activity than in years of low activity. This new result is based on two independent continuous data sets from 1974 to 1985. We show also that IPDP events occur throughout the recurrent magnetic storms in 1976, whereas they appear more sporadically in 1980, i.e., during high solar activity. It is concluded that several factors such as changes in plasmapause position, attenuation of horizontal ionospheric wave propagation, and heavy ion densities in the magnetosphere can contribute to the observed long‐term variability.
Determination of auroral electrostatic potentials using high‐ and low‐altitude particle distributions
Authors: Reiff P. H. , Collin H. L. , Craven J. D. , Burch J. L. , Winningham J. D. , Shelley E. G. , Frank L. A. , Friedman M. A.
Published: February 2008

The Dynamics Explorer (DE) pair of spacecraft provide a unique opportunity to search for the presence of electric fields aligned parallel to magnetic field lines by sampling, nearly simultaneously, the velocity‐space distribution functions of ions and electrons at two points on auroral field lines: DE 1 at high altitudes (9000–15,000 km in this study) and DE 2 at low altitudes (400–800 km). Three independent techniques are used to infer the auroral electrostatic potential difference from the particle distributions: (1) the energy of the precipitating electrons at DE 2 (compared to that at DE 1), (2) the energy of the upgoing ions at DE 1, and (3) the widening of the loss cone for electrons at DE 1. The three estimates are in general agreement, confirming the long‐standing, but not fully accepted, hypothesis that parallel electrostatic fields of 1–10 kV potential drop at 1–2 RE altitude are an important source for auroral particle acceleration. The upflowing ion distribution typically can be characterized by a sharp peak and a falloff at high energies of the form exp‐{(E‐Epeak)/Eo}, with Epeak being the peak energy and Eo the characteristic energy. This is the functional dependence one expects if a Maxwellian of thermal energy Eo is accelerated upward by a parallel electric field with eΦ=Epeak. The fact that the peak energies and not the flow velocities of the various ion species are in agreement also lends strong credence to the parallel electric field hypothesis. The acceleration mechanism cannot be a simple parallel electric field, however, for two reasons: first, the characteristic energy Eo is considerably larger than the ionospheric thermal energy (Eo is typically hundreds of electron volts and 20–30% of Epeak), and second, the energy Epeak is typically 30–50% smaller than that inferred by the two other independent techniques. The distribution does appear to be consistent with an ionospheric source, heated within (or above) the acceleration region, since the ion average energy is comparable to eΦ. The average energies of O+ and He+ are comparable to, but typically somewhat larger than, that of H+, indicating that a two‐stream instability may be the heating mechanism. Electron heating is also detected within the auroral acceleration region, with gains in characteristic energies of 10–15% of eΦ. From the high‐altitude electron measurements, we can determine a minimum potential distribution as a function of altitude in order to overcome the mirror force. We find that in one case at least 9M V of the 2300‐V potential drop must occur above 7000 km, and at least 960 V above 2000 km.
Physical processes in the vicinity of the cometopause interpreted on the basis of plasma, magnetic field, and plasma wave data measured on board the Vega 2 spacecraft
Authors: Galeev A. A. , Gringauz K. I. , Klimov S. I. , Remizov A. P. , Sagdeev R. Z. , Savin S. P. , Sokolov A. Yu. , Verigin M. I. , Szegö K. , Tátrallyay M. , Grard R. , Yeroshenko Ye. G. , Mogilevsky M. , Riedler W. , Schwingenschuh K.
Published: February 2008

This paper presents the first step toward a comprehensive interpretation of the plasma, electric, and magnetic field measurements performed by four different instruments near the cometopause of Halley's comet. No flow parameter displays any drastic variation at this boundary which separates two environments with different chemical composition. A fire hose instability is possibly developing near the cometopause where the number of cometary ions significantly increases. The waves at the lower hybrid frequency and in the whistler range have similar amplitudes.
Origin of interplanetary southward magnetic fields responsible for major magnetic storms near solar maximum (1978–1979)
Authors: Tsurutani Bruce T. , Gonzalez Walter D. , Tang Frances , Akasofu Syun I. , Smith Edward J.
Published: February 2008

The origins of the interplanetary southward Bz which cause the 10 major (Dst < −100 nT) magnetic storms detected during the 500 days of study (August 16, 1978, to December 28, 1979) of the Gonzalez and Tsurutani (1987) work are examined in detail. A full complement of ISEE 3 plasma and field data, an 11‐station AE index and the near‐equatorial Dst index, are used in this analysis. It is found that the origins of the interplanetary southward Bz events are quite varied. If it is defined that the Bz event which leads to Dst < −100 nT is “the cause” of the storm, then one of the storm intensifications is caused by shock compression of preexisting southward interplanetary magnetic fields, four (or five) are related to driver gas magnetic fields, one (or two) are caused by shocked kinky heliospheric current sheets, two (or three) by turbulence or waves behind interplanetary shocks, and one possibly by draped fields associated with a noncompressive density enhancement event (without a shock or a high‐speed stream). In simplistic terms, four (or five) storms are caused by driver gas fields, four by shocked (sheath) fields, and one possibly by high‐intensity draped fields. In actuality, many of the interplanetary southward Bz and corresponding magnetic storm (Dst) structures are more complex than stated above. At least four of the interplanetary events have both major sheath and driver gas southward Bz events. In two storms, sheath southward Bz features led to Dst reaching levels of −90 nT prior to driver gas southward Bz features; the following driver gas fields then caused Dst to exceed our storm criteria of ≤ −100 nT. In two other cases, sheath Bz features led to magnetic storm onsets (Dst < −100 nT); the following driver gas southward Bz features cause further storm intensifications. The above magnetic storms therefore displayed two‐stage development characteristics. The results of this study indicate the equal importance of both sheath fields or draped fields and driver gas fields for the generation of major geomagnetic storms. Because of the importance of the sheath fields the intensity and duration of geomagnetic storms cannot be predicted by solar observations of active regions alone. Tang et al. (1988) will address this topic in detail.
Geometrical properties of three‐dimensional reconnecting magnetic fields with nulls
Authors: Greene John M.
Published: February 2008

This paper is concerned with the geometrical aspects of the reconnection of magnetic field lines. Reconnection is associated with discontinuities in the overall field line organization of the configuration in which it occurs. The nature of these discontinuities is that field lines that are immediately adjacent in one region can be widely separated elsewhere. Then infinitesimal field line motion can have a finite consequence. This is an essential aspect of the reconnection phenomenon and distinguishes it from diffusion. Such discontinuous behavior implies the existence of special magnetic structures, which is the subject discussed here. One such structure that is often studied theoretically is a field line that is a closed loop. A common variation is a two‐dimensional configuration in which certain lines are effectively closed by means of an implied periodicity or symmetry in the third direction. These will be called closed lines in this paper. Here, however, attention is focused on another special structure: isolated points where the magnetic field strength strictly vanishes. These points will be called magnetic nulls. Between magnetic nulls and closed field lines, the former are much more likely to be encountered in the uncontrolled extraterrestrial environment. Thus magnetic nulls are probably a crucial element of most reconnection events in the magnetosphere.
Observed rate of ionization in shaped‐charge releases of barium in the ionosphere
Authors: Hallinan Thomas J.
Published: February 2008

Video recordings of 36 shaped‐charge barium experiments were examined for indications that ionization might, under some circumstances, proceed at a faster rate than that known to result from solar ionization (28 s). Attention was focused on releases injected at an angle of less than 25° to the magnetic field. In these releases, the velocity perpendicular to B of much of the barium is less than Alfvén's velocity, but the kinetic energy nonetheless exceeds the ionization potential. In four of the experiments, the time constant for decay of the neutrals was measured using an ultrasensitive color TV camera and was found to have a value of 30±5s. In addition, it was shown that any impulse of ionization at the time of release must have been less than 4% of the barium. The qualitative appearance of most jets was found to be consistent with a slow process of ionization. Some releases, however, produced a thin confined jet that was suggestive of rapid ionization. The most extreme example of this type (Oosik) was analyzed by comparing intensity profiles through the jet to calculated profiles for various assumed time constants. The measured profile was consistent with any time constant in excess of 20 s and was inconsistent with burst ionization in excess of 1%. It is concluded that the barium is ionized primarily by sunlight and that other processes of ionization do not produce an effect large enough to measure in the presence of solar ionization.
On the persistence of unstable bump‐on‐tail electron velocity distributions in the Earth's foreshock
Authors: Klimas Alexander J. , Fitzenreiter Richard J.
Published: February 2008

Further evidence for the persistence of bump‐on‐tail unstable reduced velocity distributions in the Earth's electron foreshock is presented. This persistence contradicts our understanding of quasi‐linear saturation of the bump‐on‐tail instability; the distributions should be stabilized through velocity space diffusion too quickly to allow an observation of their unstable form. A modified theory for the saturation of the bump‐on‐tail instability in the Earth's foreshock is proposed and examined using numerical simulation and quasi‐linear theoretical techniques. It is argued that the mechanism due to Filbert and Kellogg and to Cairns which is responsible for the creation of the bump‐on‐tail velocity distribution in the foreshock is still operative during the evolution of the bump‐on‐tail instability. The saturated state of the plasma must represent a balance between this creation mechanism and velocity space diffusion; the saturated state is not determined by velocity space diffusion alone. Thus the velocity distribution of the saturated state may still appear bump‐on‐tail unstable to standard linear analysis which does not take the creation mechanism into account. The bump‐on‐tail velocity distributions observed in the foreshock would then represent the state of the plasma after saturation of the bump‐on‐tail instability, not before. Numerical simulations of two types are discussed. In the first, the particle distribution function and the field are assumed periodic over a spatial interval of several tens or hundreds of Debye lengths along the local magnetic field. In the second, nonperiodic boundary conditions are imposed on the distribution function to model the spatial gradients which are responsible for the creation of the bump‐on‐tail unstable distribution. This mechanism for creating the unstable distribution continues to drive the particle distribution unstable during the evolution of the bump‐on‐tail instability. The saturated velocity distributions of the periodic and nonperiodic simulated plasmas are compared. At saturation the periodic velocity distribution exhibits plateau formation while the nonperiodic distribution does not. A new quasi‐linear theory for the space‐averaged velocity distribution function is constructed. This theory includes the effects of nonperiodic boundary conditions; it can be used to compare the evolution of periodic and nonperiodic bump‐on‐tail unstable particle distributions. In addition, this theory is based on the filtered Vlasov‐Maxwell system of equations; velocity space filamentation has been removed from the distribution function. It is shown that this theory is applicable on a finite spatial interval within which the field has been expressed in terms of a discrete Fourier mode expansion. Thus direct comparisons between theory and simulation are possible. Some predictions of this new quasi‐linear theory are compared favorably to the results of the bump‐on‐tail simulations.