bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation between the He/H ratios in upstream particle events and in the solar wind
Authors: Ipavich F. M. , Gosling J. T. , Scholer M.
Published: February 2008

We have compared the alpha particle to proton ratio measured at a fixed energy per charge (∼30 keV/e) in 29 well‐developed diffuse ion events upstream of the earth's bow shock with the same ratio observed simultaneously in the solar wind. The data were obtained with the Max‐Planck‐Institut/University of Maryland energetic particle sensor and the Los Alamos National Laboratory/Max‐Planck‐Institut plasma instrument aboard ISEE 1. We have also correlated the energetic He+2 to H+ intensity ratio in these upstream events with the same intensity ratio measured in the ring current population close to the magnetopause during the same spacecraft orbit. The high correlation of the ratio in upstream events with the ratio in the solar wind and the much weaker correlation with that in the ring current population strongly suggests, independent of other arguments, that the solar wind is the primary source for diffuse upstream ions. We also find that the ratio of the He+2 to H+ intensities at ∼30 keV/e is on the average enhanced by a factor of 1.6 over the solar wind ratio, although the three events with the lowest Alfven Mach numbers show a depletion relative to the solar wind ratio. Our results are consistent with Fermi acceleration of a seed particle population, which is extracted from the solar wind, generally with little or no compositional bias, and then moderately enhanced in helium during the acceleration process.
The energy spectrum of 35‐ to 1600‐keV protons associated with interplanetary shocks
Authors: van Nes P. , Reinhard R. , Sanderson T. R. , Wenzel K.‐P. , Zwickl R. D.
Published: February 2008

We present the results of a statistical study on the proton energy spectra in the range of 35–1600 keV during the one‐hour interval centered on the time of arrival of the shock front at the spacecraft of 75 interplanetary shocks that cover the period from August 1978 until December 1980, using the low‐energy proton experiment on ISEE 3. The strength of the shocks was determined by calculating the ratio of the downstream to upstream plasma density by using the ion data obtained by the Los Alamos solar wind instrument. The shock events were sub‐divided into four different classes based on the behavior of their low‐energy (35–238 keV) spectral index. The signatures of the shock events and their spectral index‐time profiles in the different classes are (1) smooth profiles associated with oblique, strong, and fast shocks, which roughly corresponds with predictions following from diffusive shock acceleration theory; (2) irregular profiles mainly associated with quasi‐perpendicular shocks and (3) spikelike profiles associated with quasi‐perpendicular shock spike events, where the properties within both classes point at predominant shock drift acceleration; (4) flat profiles, mainly associated with weak shocks accompanied by little or no shock‐accelerated particles. Strong and fast oblique shocks are found to be the most effective particle accelerators. The spectrum at the shock can generally be described by two power laws with a breakpoint energy near 250 keV. For only 15% of the events the spectrum followed a power law over the full energy range. We found that the low‐energy spectral index, measured immediately downstream of shocks associated with clear flux enhancements, is related to the shock strength according to predictions from first‐order Fermi acceleration, irrespective of the assigned diffusive or drift character of the event.
Results on relative scattering cross section of 140‐MHz auroral backscatter
Authors: Haldoupis C. I. , Nielsen E.
Published: February 2008

Observations of power spectra and backscattered signal intensities associated with auroral electron density fluctuations with scale length of nearly 1 m have been made with the Scandinavian Twin Auroral Radar Experiment (STARE). The intensities of the backscattered signals, which are related to the radar scattering cross sections, have been analyzed to determine their angular distribution in the plane perpendicular to the magnetic field with respect to the direction of the electron drift velocity. In the analysis the Doppler spectral characteristics of the data are used in order to identify the type of irregularities. It is found that the primary irregularities, having narrow spectra centered at large Doppler shifts, have a highly anisotropic scattering cross section as compared with the secondary 1‐m waves. The strength of primaries is large and strongly dependent on flow angle θ; typically, the intensity decreases from 0.3 to 0.6 dB/deg with θ increasing. The intensities of the secondaries are relatively weak and rather insensitive to angular variations. We find the ac variation of the backscatter amplitudes associated with primary and secondary irregularities to be closely correlated. This indicates a common modulation mechanism to dominate for the two kinds of scatterers.
Comment on the observed plasma densities in Jupiter's distant magnetotail (wake): A possible explanation
Authors: Grzedzielski S. , Macek W.
Published: February 2008

It is shown that the average plasma density observed by Voyager 2 inside the distant Jovian magnetotail may be explained by assuming that the tail (wake) boundary is a rotational discontinuity.
ISEE 1 and 2 observations of an oscillating outward moving current sheet near midnight
Authors: Kelly T. J. , Russell C. T. , Walker R. J.
Published: February 2008

An outward moving current sheet is examined with the ISEE 1 and 2 magnetometers. It is found that the current sheet is moving outward with a velocity of about 17 km/s, a thickness of about 1300 km, and with the current flowing into the ionosphere. Oscillations observed in the magnetic field profile indicate the presence of a wave traveling along the current sheet from midnight toward the east with a velocity of 400 km/s. The oscillations of the plasma normal to the current sheet associated with this wave are sufficient to explain the amplitude of the electric field oscillations in the plane of the current sheet observed by the University of California‐Berkeley (UCB) electric field detector. In addition a strong electric field is observed normal to the sheet, corresponding to a flow along the sheet of 400 km/s, which is consistent with the velocity of the wave deduced from the intersatellite timing of the oscillations. Thus the wave is stationary in the frame of the current sheet plasma. This oscillating current sheet model explains the observed behavior of both the magnetic and electric field observations. The potential drop across the current sheet was very large at this time, 41±26 kV, and is comparable to the total normal potential drop across the polar cap.
Effects of oblique double layers on upgoing ion pitch angle and gyrophase
Authors: Greenspan M. E.
Published: February 2008

Observations of electrostatic shocks above the auroral zone, of upgoing ion beams that show evidence of both parallel and perpendicular acceleration, and of ion conics suggest that the shocks play a role in the energization of the beams and, perhaps, of certain conics. To investigate shock effects on ions, I follow the trajectories of many ions through a very simple model of an oblique double layer, a narrow region of strong electric field with components both parallel and perpendicular to the magnetic field. Acceleration through the model oblique double layer produces gyrophase bunching, and hence density oscillations. Although it arises from a model that is not self‐consistent, this result shows that a consistent oblique double layer model must include temporal variations and/or spatial oscillations in the density or electric field. The oblique double layer increases both the perpendicular and the parallel energy of ions. The amount of perpendicular energization is greater for O+ than for H+, in agreement with observations. I examine the parametric dependence of perpendicular energization on initial ion enery, the angle between the electric and magnetic field, and the width of the potential structure.
An energy principle for high‐latitude electrodynamics
Authors: Barbosa D. D.
Published: February 2008

A theoretical model for mid‐ and high‐latitude electric fields and currents is constructed using Fourier analysis methods. A two‐dimensional planar ionosphere with an enhanced conductivity auroral belt and field‐aligned currents at the edges is employed. The postulate that the electric field and currents adjust self‐consistently to minimize the global Joule dissipation rate defines a theoretical relation between the primary and secondary field‐aligned currents. This so‐called minimal dissipation configuration is examined using several input field‐aligned current models, and graphical solutions for the electric field and ionospheric current are shown. A detailed discussion and interpretation of the solutions with relation to diverse observations and high‐latitude phenomenology are included.
A two‐dimensional, high‐resolution, nested‐grid model of the thermosphere: 1. Neutral response to an electric field “spike”
Authors: Fuller‐Rowell T. J.
Published: February 2008

A two‐dimensional nested‐grid model of the thermosphere with high resolution in the horizontal direction has been developed to study a number of dynamic, electrodynamic, and chemical problems at high latitudes. The model solves the time‐dependent momentum, energy, continuity, and three‐constituent composition equation of the neutral gas self‐consistently, with the assumption of hydrostatic equilibrium. The grid network consists of a low‐resolution height/latitude slice stretching from the equator of one longitude sector, up to the pole, and down to the equator in the opposite sector. Imbedded within this mesh is a system of two nested grids to achieve a latitude resolution of 10 to 20 km. An initial simulation is presented to illustrate the response of the middle and upper neutral thermosphere to a narrow, 1° latitude, “spike” (100 mV m−1) of electric field in the vicinity of the auroral oval. The simulation revealed a number of points: (1) the magnitude of the dynamic response to the westward ion convection channel was 30% greater than that resulting from an identical eastward channel, due purely to the earth's rotation; (2) broadening of the latitudinal extent of the response, particularly on the equatorward edge of the channel, to the narrow primary sources of ion drag and Joule heating was most effected by horizontal advection; (3) horizontal heat conduction was important both within and near the edge of the convection channel; (4) in regions where advection was weak, particularly at the poleward edge of the channel, horizontal viscosity made a significant contribution to a broadening of the response, and viscous dissipation provided an additional heat source.
Longitudinal asymmetry in the Io plasma torus
Authors: Cheng A. F. , Paonessa M. T. , Maclennan C. G. , Lanzerotti L. J. , Armstrong T. P.
Published: February 2008

A clear inbound‐outbound asymmetry is found in Voyager 1 low‐energy charged particle (LECP) phase space densities near 6 RJ. The asymmetry is of the same sign and comparable magnitude for electrons and ions at several different energies. Ion phase space density profiles at three values of the first invariant are uniformly displaced to larger radius by ∼0.25 RJ during the inbound pass. Inbound‐outbound asymmetries which are very similar for electrons and ions at many different energies can be accounted for naturally by a convection electric field in the plasma torus. If the asymmetry is attributed to a convection field fixed in local time, then the field has a component of ∼4 mV/m pointing toward 0120 LT, roughly orthogonal to the dawn‐dusk electric field proposed to account for local time asymmetry in the plasma torus. On the other hand, the asymmetry can be attributed to a corotating convection field, such that plasma outflow occurs within 170° ≤ λIII ≤ 300° at a velocity of ∼2 × 10−6 RJ s−1 near L = 6. In this case the LECP observations would be consistent with the magnetic anomaly model. If asymmetric particle losses are responsible for the effect, then the ion loss rate was ∼25% greater during the inbound pass.
Generation of broadband electrostatic noise by ion beam instabilities in the magnetotail
Authors: Grabbe Crockett L. , Eastman Timothy E.
Published: February 2008

Particle data from ISEE 1 sampled in the earth's magnetotail show the presence of energetic ion beams in the boundary layer of the plasma sheet. A theory of instabilities driven by the beams is developed and compared with wave data sampled simultaneous to the particle data. We conclude that the ion beams generate broadband electrostatic bursts of noise. The electrostatic noise correlates well with the occurrence of the beams, and the spectrum is consistent with that predicted from a negative energy beam instability under magnetotail conditions. The theory predicts that a spectrum of growing waves can be driven for frequencies from 0.001 ωpe up to ωpe, the electron plasma frequency, with a spectral peak typically near 0.01 ωpe or lower, in agreement with the wave data. Furthermore, as one moves away from the source region perpendicular to the magnetic field, the high‐frequency components of the observed wave spectra are predicted to disappear gradually, leaving the low‐frequency part of the spectrum, also as is observed. Evidence is given for significant pitch angle scattering of the beams by the broadband electrostatic noise leading to more isotropic ion distributions.
A statistical study of dayside magnetospheric electric field fluctuations with periods between 150 and 600 s
Authors: Junginger H. , Geiger G. , Haerendel G. , Melzner F. , Amata E. , Higel B.
Published: February 2008

One hundred eighty‐six days of electric field data from the GEOS 2 electron beam experiment have been used to study magnetospheric fluctuations at geostationary orbit with periods between 150 and 600 s. While fluctuations are nearly always present in the electric field data from the dayside magnetosphere with typical amplitudes between 0.2 and 0.5 mV/m, it is often hard to find well‐defined concurrent pulsations in the GEOS 2 magnetic field data. Most events occur near noon and have the same characteristics: They are toroidal and nearly linearly polarized, the sense of polarization and the orientation angles of the polarization ellipses change sign near noon, the instantaneous frequency of the fluctuations is correlated with the instantaneous electron density, and in a given sector of the magnetosphere the sense of polarization depends on the frequency. There is strong evidence that these fluctuations are fundamental mode eigenoscillations of field lines in the vicinity of the spacecraft which are generated in the inhomogeneous plasma of the magnetosphere by some kind of solar wind‐driven surface waves at the magnetopause or at the low‐latitude boundary layer.
Local time variation in the response of Pc 5 pulsations in the morning sector to substorm expansive phase onsets near midnight
Authors: Rostoker G. , Spadinger I. , Samson J. C.
Published: February 2008

Since substorm expansive phase activity in the midnight sector and Pc 4,5 pulsation activity in the morning sector are both colocated with the auroral oval, it is logical to assume that sudden changes in plasma and field parameters in the oval would cause perturbations in the activity level of both types of activity. In an earlier study, Samson and Rostoker [1981] demonstrated that the frequency of Pc 4,5 pulsations on the dayside increased at the onset of a substorm expansive phase in the night sector. In that study, only pulsations recorded near noon were investigated for their frequency content. In this paper we consider the response of pulsation activity across the entire morning sector to substorm onsets near midnight. Using superposed epoch analyses we demonstrate that the conclusions of Samson and Rostoker [1981] are valid across the morning sector. However close to dawn, Pc 5 activity appears to be initiated at substorm onset rather than decreasing by shifting frequency toward the Pc 5 band, as appears to be the case closer to noon. We present a model of Pc 4,5 activity to explain this phenomenon in which the origin of the pulsations is proposed to be a Kelvin‐Helmholtz instability at the interface between the low‐latitude boundary layer and the central plasma sheet.
Simultaneous observations of a pulsation event from the ground, with balloons and with a geostationary satellite on August 12, 1978
Authors: Iversen I. B. , Block L. P. , Brønstad K. , Grard R. , Haerendel G. , Junginger H. , Korth A. , Kremser G. , Madsen M. , Niskanen J. , Riedler W. , Tanskanen P. , Torkar K. M. , Ullaland S.
Published: February 2008

A pulsation event with a period of 6–8 min has been observed simultaneously by the geostationary satellite GEOS 2 and two balloons located in the area magnetically conjugate to the satellite. The balloons were equipped with electric field and X ray instruments. The pulsation event occurred in the magnetic local time sector 0600–1100 determined by ground‐based measurements. The satellite instruments observed the signature of a magnetosonic wave having a radially inward directed Poynting vector. The electric field associated with the wave was closely correlated with the electric field detected by the balloon instrument near the footprint of the magnetic field line through GEOS 2. The ground‐based observations show that the wave phase pattern was north‐south striated and that it moved from east to west. The balloon X ray measurements showed a strongly pulsating particle precipitation, in close correlation with the intensity of the energetic electron flux and the VLF electromagnetic radiation measured on the satellite, suggesting a wave‐particle interaction process. The pulsations seem to originate beyond the geosynchronous orbit. The study demonstrates the usefulness of simultaneous measurements with a geostationary satellite and instruments in the ground area magnetically conjugate to the satellite.
Ion propagation in the magnetosheath
Authors: Brinca Armando L.
Published: February 2008

The transport of test ions through the magnetosheath depends upon the macroscopic structure defined by the average magnetic and flow fields, B0 and U, and the fluctuating fields, δE and δB, encountered by the particles. In this paper an approximate model is used to generate the macroscopic configuration: it satisfies the jump conditions at the bow shock, treats the magnetopause as a tangential discontinuity (B0 and U tangent to the magnetospheric boundary), and uses intermediate field amplitudes and orientations that resemble the more precise results of gas dynamic simulations. Equations of motion for a fictitious ensemble average particle describe the ion propagation, with the effects of the inhomogeneous turbulence phenomenologically incorporated into a friction tensor whose components reflect the coupling of the ions to the bulk plasma along and across the magnetic field B0. Application to the solar wind releases of lithium ions by the Active Magnetospheric Particle Tracer Explorers mission illustrates the approach: we follow the guiding center trajectory of the ensemble average “particle” for several upstream solar wind and interplanetary magnetic field conditions, different locations of the release site, and quiet and disturbed magnetosheath turbulence regimes. Implications of the results for the release strategy are discussed.
A reconciliation of propagation modes of auroral kilometric radiation
Authors: Hashimoto Kozo
Published: February 2008

The propagation modes of auroral kilometric radiation (AKR) are theoretically investigated by three‐dimensional ray tracing using an electron density model based on the auroral plasma cavity. Both right‐hand extraordinary (R‐X) mode and left‐hand ordinary (L‐O) mode observations are explained in a consistent manner. The following assumptions, which are based on observations near the source, are used for the ray tracing. (1) The radiation is generated in both R‐X and L‐O modes. (2) The ratios of the wave frequency to the R‐X mode cutoff frequency f/fR range from 1.01 to 1.1. (3) AKR waves are generated at wave normal angles almost perpendicular to the geomagnetic field. (4) AKR is generated in the auroral plasma cavity along a nightside field line with an invariant latitude of 70°. L‐O mode waves can propagate within a wider region than R‐X mode waves. Observed characteristics of AKR can be well explained under the following two additional assumptions, which are consistent with the cyclotron maser mechanism. (1) The R‐X mode is stronger than the L‐O mode. (2) Predominantly upgoing waves are generated at the source. The R‐X mode is dominant at high latitudes because it is stronger. The L‐O mode is received at lower latitudes which the R‐X mode waves cannot reach because of refraction and reflection mainly in the vicinity of the source and the plasmapause. Previous papers which claim the R‐X mode radiation are reconsidered. The present interpretation consistently explains the results of their observations without contradiction with the existence of the L‐O mode. It is clarified that they observed both modes.
Line width of the solar EUV He I resonance emissions at 584 and 537 Å
Authors: Ogawa Howard S. , Phillips E. , Judge Darrell L.
Published: February 2008

In this brief report we present our latest line width values for the solar He I lines at 584 and 537 Å determined from a curve‐of‐growth technique. The measurements were conducted with a rocket‐borne spectrometer viewing the full disk of the sun on August 10, 1982, at a zenith angle of ∼30° and along a trajectory for which the peak altitude was ∼325 km. The average of the 11 measurements obtained during the flight gave a full width at half maximum of 97±15 mÅ and 132±30 mÅ for the 584− and 537‐Å lines respectively. The solar activity indices for the day of the flight were F10·7 = 209.5 and RI = 155, and were significantly higher than they were during our previous flights on November 27, 1974, September 29, 1977, and September 30, 1980. The solar flux at 584 Å was also measured and changed considerably from flight to flight in accordance with solar indices. Comparison of solar line width data may indicate two possible trends although the measurement errors do overlap and improved statistics are required. The suggested trends are that (1) the helium line widths may decrease with increasing solar activity and (2) the n = 3 ‐ 1 line of He I may be broader than the n = 2 ‐ 1 line.
The Z3 model of Saturn's magnetic field and the Pioneer 11 vector helium magnetometer observations
Authors: Connerney J. E. P. , Acuña M. H. , Ness N. F.
Published: February 2008

Magnetic field observations obtained by the Pioneer 11 vector helium magnetometer are compared with the Z3 model magnetic field. These Pioneer 11 observations, obtained at close‐in radial distances, constitute an important and independent test of the Z3 zonal harmonic model, which was derived from Voyager 1 and Voyager 2 flux gate magnetometer observations. Differences between the Pioneer 11 magnetometer and the Z3 model field are found to be small (∼1%) and quantitatively consistent with the expected instrumental accuracy. A detailed examination of these differences in spacecraft payload coordinates shows that they are uniquely associated with the instrument frame of reference and operation. A much improved fit to the Pioneer 11 observations is obtained by rotation of the instrument coordinate system about the spacecraft spin axis by 1.4°. With this adjustment, possibly associated with an instrumental phase lag or roll attitude error, the Pioneer 11 vector helium magnetometer observations are fully consistent with the Voyager Z3 model. No evidence is found for any significant departure from axisymmetry of Saturn's internal magnetic field.
The ring atmosphere of Saturn: Monte Carlo simulation of ring source models
Authors: Ip W.‐H.
Published: February 2008

The spatial distributions of hydrogen atoms emitted from the rings of Saturn with a range of emission speeds are simulated by a Monte Carlo technique. Basically two issues, one concerning the source strength and the other concerning the geometrical configuration (which is specifically discussed in the present work), have to be dealt with. For example, among the several possibilities, photosputtering might be discarded because of its low production yield of H atoms and the absence of time variation of the H Ly α emission over the years. Interplanetary meteoroid impact could fail by a factor of 10 in providing the required emission rate of the ring hydrogen atmosphere if the conservative estimate for the impact vapor production rate is used; otherwise, it is an adequate source. Exospheric ejection of dissociated hydrogen fragments from the dayside disk is also promising, but we show that the global distribution of the hydrogen atoms in ballistic trajectories appears not to be in agreement with the reported Voyager UVS observations. Charge exchange interaction with the magnetospheric plasma has been found to be a significant loss mechanism of the ring atmosphere. In turn, the pitch angle distribution and thermal energy distribution of the corotating plasma could be strongly modified by the neutral atoms. The present study indicates that the loss rate of neutral particles via charge exchange could vary between 4×1027 s−1 and 7×1028 s−1.
Rocket observations of the ultraviolet airglow during morning twilight
Authors: Cebula R. P. , Feldman P. D.
Published: February 2008

The nitric oxide γ and δ bands, O II λ2470‐, O I λλ356‐ and 1304‐Å lines, and N2 Lyman‐Birge‐Hopfield and O2 Herzberg band emissions were observed during morning twilight between 90 and 246 km on September 24, 1979, with two rocket‐borne scanning monochromators viewing successively at 82° and 90° to the zenith near the sun‐earth‐rocket plane. The solar zenith angle was 110°. The relative intensitites of the two nitric oxide band systems suggest that the observed emission is primarily due to resonance fluorescence along the sunlit portion of the rocket line of sight, with the chemiluminescence contribution being greatly reduced from that observed during evening twilight. The densities at 200 km derived from the observed emission rates are 4×105 cm−3 for NO and 5×106 cm−3 for N(4S), consistent with recent odd nitrogen models which predict a much greater nighttime decrease of N(4S) than of NO. The atomic and ionic oxygen emissions are modeled in a self‐consistent way based on direct photoionization and photoelectron impact ionization. The atomic oxygen density near 100 km is derived from O2 Herzberg I band emission observed near the end of the flight.
The relation of the cusp precipitating electron flux to the solar wind and interplanetary magnetic field
Authors: Candidi M. , Meng C.‐I.
Published: February 2008

The temporal variation of the precipitating electron flux in the 47‐eV to 1‐keV energy range, detected in the low‐altitude cusp, is studied as a function of the solar‐wind parameters and the Bz component of the interplanetary magnetic field (IMF). It is found that the electron flux intensity in the polar cusp region depends nonlinearly on the density of the solar‐wind plasma and that it is higher in the negative IMF Bz period than in the positive IMF Bz period. Both these facts are consistent with the predictions of the existing models of magnetic field‐line merging at the magnetopause; however, the possibility that non‐linear diffusion processes predominate cannot be eliminated.
Spatial correlation of the ionospheric total electron content at the equatorial anomaly crest
Authors: Huang Yinn‐Nien
Published: February 2008

The spatial correlation of the ionospheric total electron content (TEC) at the equatorial anomaly crest was studied by recording Faraday rotation angle of the ETS‐II geostationary satellite at Lunping and Kaohsiung whose subionospheric points are located at 23.0°N, 121.9°E and 20.9°N, 121.1°E, respectively, and are about 280 km apart. The results show that the spatial correlation of TEC at the equatorial crest region is smaller than that at other places. The day‐to‐day variabilities of TEC differences between two subionospheric points are quite large. The day‐to‐day variabilities of the fountain effect seem to play an important role.
Relationship of dusk‐sector electric field to energy dispersion at the inner edge of the electron plasma sheet for nonequatorially mirroring electrons
Authors: Horwitz J. L.
Published: February 2008

The steady state relationships between the convection electric field and electron boundary energy dispersion in the dusk sector are obtained for off‐equatorially mirroring electrons in dipole and dipole plus current sheet magnetic field configurations. Comparison of these predictions with dusk‐sector ionospheric satellite measurements of electron boundaries and electric fields shows reasonable agreement in magnitude and general increasing trend with latitude, particularly when the dipole‐plus‐current sheet magnetic field configuration is used.
The role of finite parallel length on the stability of barium clouds
Authors: Sperling J. L. , Drake J. F. , Zalesak S. T. , Huba J. D.
Published: February 2008

A simple model is used to show that the finite parallel length of ionospheric plasma clouds can affect the growth rate of striation instabilities (e.g., gradient drift). The finite parallel length of plasma clouds tends to favor the growth of striations with short perpendicular wavelengths.
First observations of 140‐MHZ plasma line backscatter during heating experiments at Tromsø
Authors: Hedberg Å. , Thidé B. , Boström R. , Derblom H. , Kopka H. , Stubbe P.
Published: February 2008

In September 1983 VHF plasma line backscatter was obtained at Kiruna, Sweden, from the ionosphere illuminated by strong HF radiation from the German‐Norwegian heating facility located near Tromsø, Norway. The downshifted plasma line was observed during two periods when the heater operated on a frequency that was lower than critical. The plasma line was never observed earlier than about 15 s after heater turn‐on and the intensity was very variable in time. A plausible assumption is therefore that the plasma line scattering observed is caused by Langmuir waves generated by direct conversion of the heating wave due to interactions with striations built up slowly in the modified ionospheric region.
Anisotropy characteristics of geomagnetically trapped ions
Authors: Garcia Howard A. , Spjeldvik Walther N.
Published: February 2008

Radiation belt ions (protons plus heavier ions) are found to exhibit quiet time pitch angle anisotropies that vary significantly and nonmonotonically with L shell and energy in the range 20 keV to several MeV. Data from two spacecraft are reported: Explorer 45 measurements obtained in June 1972 and ISEE 1 measurements obtained from November 1977 through December 1978. Both data sets show that the radial distribution of pitch angle anisotropy contains one major minimum for most energies at some midrange L shell. The 1972 data collected over a 15‐day period in a localized region of the magnetosphere (between L = 2 and L = 5 in the afternoon local time sector) revealed an anisotropy profile for most energies that increased with lower L shells below L ∼ 2.5, minimized near L = 3, then attained a local maximum around L ∼ 4, and evolved to more isotropic distributions near the Explorer 45 orbit apogee at L = 5.2. The 1977–1978 pitch angle anisotropies observed by ISEE 1 were obtained over all local times from approximately L = 2.5 to the outer limit of trapping. These data in general tended to confirm the 1972 finding of nonmonotonic anisotropy distributions, but with some important modifications. The later quiet time distributions showed a broader region of low anisotropy and less regular L shell dependence than the 1972 data, and showed systematic trends in the anisotropy that were clearly dependent not only on L shell but also on energy. A single, major anisotropy minimum was observed at virtually every local time, the location of this minimum occurred at lower L shells with progressively increasing energy. Comparison of the two quiet time data sets separated by about 6 years suggests that there may also be an anisotropy dependence on the phase of the solar cycle, possible resulting from ion composition differences or from different wave‐particle interactions.
Electrical coupling effects on the temporal evolution of F layer plasma structure
Authors: Heelis R. A. , Vickrey J. F. , Walker N. B.
Published: February 2008

A time dependent model of F region structure decay by “classical” cross field diffusion and electrical coupling along magnetic field lines to the E region is examined. The temporal behavior of the ion concentration fluctuations is determined by the electric field in the coupled system as well as by the initial perturbation spectra and the E region recombination rate. The formation of image structure in the E region ion concentration affects the lifetime of F layer structure in a scale size dependent way. Once an image is formed, the image amplitude and the driving F region structure amplitude decay at the same rate. At large scale sizes, λ(λ=2π/k), this rate is proportional to k² and the ratio of the temperatures in each region. At small scale sizes it depends on the E region recombination rate and the temperatures of the two regions but is only very weakly dependent on k. The background E region concentration determines the wave number beyond which the structure amplitude decay rate is almost independent of its scale size.
High‐latitude indices of electric and magnetic variability during the CDAW 6 intervals
Authors: Kroehl H. W. , Kamide Y.
Published: February 2008

For the two Coordinated Data Analysis Workshop 6 intervals, we derived the global distribution above the north polar region of ionospheric currents, field‐aligned currents, electric fields and Joule heat from geomagnetic variations recorded at 107 stations and modeled values of ionospheric conductance (Kamide et al., 1983). In addition, we computed several scalar indices which summarize changes in the magnetic and electric fields at high latitudes. Since these indices were derived from the same data, namely ground magnetic variations, a high degree of similarity is expected to exist among their temporal variations, as the correlation coefficients lie between 0.41 and 0.98. From the cross correlation between the total auroral electrojet currents and the AU and AL indices of electrojet current density, we find that the latitudinal width of the westward electrojet expands (contracts) simultaneously with an increase (decrease) in current density. However, the lower correlation between AU and the total eastward current demonstrates the importance of changes of latitudinal width to the total eastward current. The relatively poor correlation between AU and AL highlights the differences between the two indices and raises doubts about the meaning of the AE index, which is a linear combination of the two. We notice that both electric potential and conductivity changes influence the electrojets. When the electric potential difference across the polar cap is large in relation to a given intensity of the total current, the “true” ionospheric current shows a maximum westward electrojet near dawn and a maximum eastward electrojet near dusk. When the two indices are relatively equal, the westward electrojet maximum moves toward midnight while the eastward electrojet stays near dusk.
Near‐conjugate observations of inverted‐V electron precipitation using DE 1 and DE 2
Authors: Lin C. S. , Barfield J. N. , Burch J. L. , Winningham J. D.
Published: February 2008

Near‐conjugate plasma measurements from the DE 1 HAPI and the DE 2 LAPI instruments during nightside inverted‐V events are examined to determine quantitatively the energization of inverted‐V electrons. The plasma moments, including number flux, energy flux, and average energy, are computed. For the four near‐conjugate passes examined, the total downward energy flux at the DE 1 altitudes (8000–12,000 km) is generally several ergs per square centimeter per second. At the DE 2 altitudes (500–1000 km), the peak of the total downward energy flux is 3–15 times that measured by DE 1. Similar variation of the total downward energy fluxes with respect to invariant latitude is found at high and low altitudes. At the peaks of the total downward energy flux measured by DE 2 the energy flux diffused from magnetospheric to ionospheric electrons was generally negligible. However, appreciable diffused energy fluxes were sometimes found. According to the model that electrons are accelerated adiabatically by an electrostatic field, the downward energy flux at the DE 2 altitude is calculated from the DE 1 plasma data as a function of potential drop and compared with that measured by DE 2. When the modeled downward energy flux is assumed to equal the measured total downward energy flux minus the upward energy flux of secondary electrons, the required potential drop is shown to agree reasonably well with the potential drop inferred separately from the measured distribution function. This empirical result suggests that adiabatic acceleration of inverted‐V electrons by parallel dc electric field below the DE 1 altitudes could frequently account for the total downward energy flux measured at the DE 2 altitudes.
Revised ion temperatures for Voyager plasma measurements in the Io plasma torus
Authors: Bagenal Fran , McNutt Ralph L. , Belcher John W. , Bridge Herbert S. , Sullivan James D.
Published: February 2008

We have discovered that an error was made in computing ion temperatures for some of the results derived from the positive ion data from the plasma science (PLS) experiment on the Voyager spacecraft. The reported results directly affected by this error are the ion temperatures in the inner magnetosphere of Jupiter given by Bagenal et al. (1980) and Bagenal and Sullivan (1981) plus a single ion temperature calculated from Voyager 1 data obtained in Saturn's magnetosphere and quoted by Bridge et al. (1981). The ion temperatures quoted for the torus region in the original Jupiter Science article (Bridge et al., 1979) and all of the ion temperatures in the middle magnetosphere of Jupiter (McNutt et al., 1981) were derived independently and are unaffected by the error.
The temporal evolution of 3‐m striations in the modified ionosphere
Authors: Coster A. J. , Djuth F. T. , Jost R. J. , Gordon W. E.
Published: February 2008

Experiments were performed at Arecibo, Puerto Rico, to investigate the evolution times of 3‐m field‐aligned Striations produced in the ionosphere by powerful high‐frequency (HF) radio waves. The results of this investigation are now summarized. First, the striations' rise times are dependent on the HF electric field. The E region data suggest that this dependence is nonlinear. Second, the threshold value of the HF electric field required to produce detectable Striations was experimentally determined. At threshold the component of the HF electric field perpendicular to the geomagnetic field is calculated to be 0.09 V/m in the F region and 0.37 V/m in the E region. Third, both the E and the F region data verify theoretical predictions that the Striations' decay times are directly proportional to the electron diffusion across B. Finally, a one‐to‐one correspondence between the growth of the 3‐m Striations and the decline of the HF‐enhanced plasma line during overshoot is sometimes observed.
The altitude variation of the ionospheric photoelectron flux: A comparison of theory and measurement
Authors: Richards P. G. , Torr D. G.
Published: February 2008

We compare the 145 to 300 km altitude variation of the measured photoelectron flux in the 13 to 18eV, 28 to 34eV, and 50 to 55eV energy regions with the variations expected from theory. There is a strong linear relationship between the measured photoelectron flux and the attenuation of the solar EUV flux at these energies. Therefore, the photoelectron flux is sensitive to changes in the solar zenith angle, neutral density scale height, and total neutral density. However, contrary to previous assertions, the photoelectron flux at most energies is not sensitive to the relative densities of the neutral constituents. In addition, we obtain excellent agreement between theory and measurement. By using the concept of photoelectron production frequencies, we reduce the usually complex evaluation of the local equilibrium photoelectron flux to a trivial calculation so that the steps in the calculation can be readily verified.
A theory of diffuse emission bursts triggered by coherent VLF signals propagating parallel to the geomagnetic field
Authors: Matthews J. P.
Published: February 2008

Satellite and ground‐based studies have demonstrated that coherent VLF signals propagating in the terrestrial magnetosphere can trigger diffuse and structured emission bursts. A triggering mechanism for diffuse emissions which may operate when the coherent signal propagates parallel to the geomagnetic field is investigated in this note. The theory is based on the concept that trapping of energetic particles in the potential wells of the coherent signal alters the gradient of the particle distribution function with respect to parallel velocity. If, as a result of trapping, the gradient becomes sufficiently steep, broad band wave growth takes place as predicted by established theory. The resulting pitch angle diffusion of particles produces a more stable configuration in which the steep gradient is reduced towards that corresponding to the marginally stable distribution. For initially positive gradients in the energetic electron distribution function with respect to parallel velocity, wave growth takes place above the frequency of the causative coherent signal. The theory predicts that the bandwidth of emissions triggered in this way should rapidly increase with time and may reach maximum widths of several hundred hertz.
On the configuration of the polar cusps in Earth's magnetosphere
Authors: Voigt G.‐H. , Wolf R. A.
Published: February 2008

We discuss the physical conditions that determine the configuration of the polar cusps in earth's magnetosphere. In the conventional picture, the cusp‐associated neutral points lie on the dayside magnetopause. An alternative picture has been suggested by C.‐C. Wu on the basis of his global magnetohydrodynamic (MHD) simulation: the polar cusps are swept back away from the sun, so that the magnetopause neutral points lie on the nightside of the earth. On the basis of two‐dimensional linear equilibrium calculations, we believe that the amount of plasma (i.e., the magnitude of the thermal plasma pressure) on low‐latitude polar cusp field lines is the most influential parameter that determines the cusp configuration: when the plasma pressure varies between zero (vacuum case) and the Harris sheet limit (zero Bz in the tail plasma sheet), the standard cusp geometry results with the two neutral points on the dayside magnetopause. When the plasma pressure exceeds the Harris sheet limit, the cusp field lines gradually shift to the nightside, until the neutral points are located tailward of the dipole. We considered three possible plasma supply mechanisms for polar cusp field lines: (1) Observed energetic ions flowing up from the ionosphere are shown to be clearly inadequate to create a Wu‐type cusp. (2) Adiabatic sunward drift of plasma sheet or ring current plasma from the nightside to the dayside is also clearly insufficient. (3) Particle diffusion through the dayside magnetopause could create only a thin layer of plasma on closed field lines, where the pressure might be sufficient to create a Wu‐type cusp. We suspect that unrealistically large numerical‐diffusion coefficients could cause MHD codes to predict unrealistically prominent closed‐field‐line boundary layers and corresponding Wu‐type cusp configurations.
A spectral search for Lyman‐Birge‐Hopfield band nightglow from Spacelab 1
Authors: Torr Marsha R. , Torr D. G. , Eun J. W.
Published: February 2008

The Lyman‐Birge‐Hopfield (LBH) system of N2 requires 8.5 eV for its excitation. Measurements were reported several years ago of significant intensities (≲1 kR) of this system at mid‐latitudes at night. It thus becomes important to establish whether such LBH nightglow is generally present in the nightglow in order to identify the source mechanism. In this paper we examine the ultraviolet spectra gathered at night from Spacelab 1 for further information on this phenomenon. We find that emission with the LBH spectral characteristics reported earlier are present in each of the five data sets analyzed. Each spectrum indicates an emission system arising from a vibrational distribution peaking at υ′ = 0, and the band ratios are convolved with O2 absorption. The intensities for the entire band range from 500 R to 5 kR in slant path intensity. The spectra studied include one obtained at northern mid‐latitudes (winter), one at equatorial latitudes, and three in southern mid‐latitudes (summer). In addition to these data obtained with the instrument field of view in fixed configurations with respect to the earth, we have also examined three limb scan sequences for information on the altitude distribution of the emission.
A theoretical study of the global F region for June solstice, solar maximum, and low magnetic activity
Authors: Sojka J. J. , Schunk R. W.
Published: February 2008

We constructed a time‐dependent, three‐dimensional, multi‐ion numerical model of the global ionosphere at F region altitudes. The model takes account of all the processes included in the existing regional models of the ionosphere. The inputs needed for our global model are the neutral temperature, composition, and wind; the magnetospheric and equatorial electric field distributions; the auroral precipitation pattern; the solar EUV spectrum; and a magnetic field model. The model produces ion (NO+, O2+, N2+, N+, O+, He+) density distributions as a function of time. For our first global study, we selected solar maximum, low geomagnetic activity, and June solstice conditions. From this study we found the following: (1) The global ionosphere exhibits an appreciable UT variation, with the largest variation occurring in the southern winter hemisphere; (2) At a given time, NmF2 varies by almost three orders of magnitude over the globe, with the largest densities (5×106 cm−3) occurring in the equatorial region and the lowest (7×10³ cm−3) in the southern hemisphere mid‐latitude trough; (3) Our Appleton peak characteristics differ slightly from those obtained in previous model studies owing to our adopted equatorial electric field distribution, but the existing data are not sufficient to resolve the differences between the models; (4) Interhemispheric flow has an appreciable effect on the F region below about 25° magnetic latitude; (5) In the southern winter hemisphere, the mid‐latitude trough nearly circles the globe. The dayside trough forms because there is a latitudinal gap of several degrees between the terminator and the dayside oval. In this gap, there is no strong ion production source, and the ionosphere decays; (6) For low geomagnetic activity, the effect of the auroral oval on the densities is not very apparent in the summer hemisphere, but is clearly evident in the winter hemisphere; (7) The densities in both the northern and southern polar caps exhibit a complex temporal variation owing to the competition between the various photochemical and transport processes.
Analysis of 16 plasma vortex events in the geomagnetic tail
Authors: Birn J. , Hones E. W. , Bame S. J. , Russell C. T.
Published: February 2008

The analysis of 16 plasma vortex occurrences in the magnetotail plasma sheet of Hones et al. (1983) is extended. We used two‐ and three‐dimensional plasma measurements and three‐dimensional magnetic field measurements to study phase relations, energy propagation, and polarization properties. The results point toward an interpretation as a slow strongly damped MHD eigenmode which is generated by tailward traveling perturbations at the low‐latitude interface between plasma sheet and magnetosheath.