bluuebunny/crossref_metadata_2025 huggingface dataset:
A correlation between auroral kilometric radiation and field‐aligned currents
Authors: Green James Lauer , Saflekos Nicolaos A. , Gurnett Donald A. , Potemra Thomas A.
Published: February 2008

Simultaneous observations of field‐aligned currents (FAC) and auroral kilometric radiation (AKR) are compared from the polar‐orbiting satellites Triad and Hawkeye. The Triad observations were restricted to the evening‐to‐midnight local time sector (1900 to 0100 hours magnetic local time) in the northern hemisphere. This is the region in which the most intense storms of AKR are believed to originate. The Hawkeye observations were restricted to when the satellite was in the AKR ‘emission cone’ in the northern hemisphere and at radial distances ≥7 RE (earth radii) to avoid local propagation cutoff effects. A (R/7RE)² normalization to the power flux measurements of the kilometric radiation from Hawkeye is used to take into account the radial dependence of this radiation and to scale all intensity measurements so that they are independent of Hawkeye's position in the emission cone. Integrated field‐aligned current intensities from Triad are determined from the observed transverse magnetic field disturbances. There appears to be a weak correlation between the AKR intensity and the integrated current sheet intensity of field‐aligned currents. In general, as the intensity of auroral kilometric radiation increases so does the integrated auroral zone current sheet intensity increase. Statistically, the linear correlation coefficient between the log of the AKR power flux and the log of the current sheet intensity is 0.57. During weak AKR bursts (<10−18 W m−2 Hz−1), Triad always observed weak FAC’s (<0.4 A m−1), and when Triad observed large FAC's (≥0.6 A m−1), the AKR intensity from Hawkeye was moderately intense (10−15 to 10−14 W m−2 Hz−1) to intense (>10−14 W m−2 Hz−1). It is not clear from these preliminary results what the exact role is that auroral zone field‐aligned currents play in the generation or amplification of auroral kilometric radiation.
Pioneer 11 observations of trapped particle absorption by Amalthea
Authors: McKibben R. B. , Pyle K. R. , Simpson J. A.
Published: February 2008

During its flyby of Jupiter in 1974, Pioneer 11 passed through a region in which low‐energy (∼1 MeV) trapped protons had been exposed to absorption by Amalthea (L ≃ 2.5) only ∼5 hours earlier. In passing through this region, a brief (∼10 s duration) decrease was observed in the counting rate of ∼1‐MeV protons measured by the University of Chicago charged particle instrument. We interpret this decrease as a transient signature of the recent absorption of the proton flux by Amalthea. No such clear effects were observed in fluxes of electrons (E ≥ 3.4 MeV) or high‐energy nuclei (Z > 5, E ⪞ 70 MeV/nucleon) at any crossing of Amalthea's orbit. These observations place an upper limit on the radial diffusion coefficient, κ, for ∼1‐MeV protons at the orbit of Amalthea of 2×10−10 RJ²/s. By comparison with diffusion coefficients derived from absorption effects by Io (L ≃ 6), this limit requires that if the radial diffusion coefficient between L ≃ 2.5 and L ≃ 6 varies with L as Ln, then n ≥ 5, whereas diffusion driven by atmospheric neutral winds would predict κ ∝ L³ for low‐energy protons. Our results are consistent with diffusion driven by fluctuating electric (κ ∝ L7–10) or magnetic (κ ∝ L10) fields.
Convection electric fields and ionospheric currents derived from model field‐aligned currents at high latitudes
Authors: Rich F. J. , Kamide Y.
Published: February 2008

Computer simulations of convection electric fields and ionospheric currents driven by high‐latitude field‐aligned currents (FAC) have been conducted with an emphasis on field‐aligned currents in the dayside high‐latitude region. The particle‐induced conductivity is based upon average particle precipitation fluxes observed by Atmospheric Explorer. In order to examine conclusions drawn from satellite observations, the field‐aligned currents in the region of the dayside cusp are modeled in two ways. First, the cusp FAC are given as distinct currents poleward of the auroral oval field‐aligned currents. Their location and direction of current are determined by the By component of the interplanetary magnetic field. Second, the cusp FAC are given as an extension of the morning (or evening) region 1 current across the noon meridian and into the polar region for IMF By < 0 (or By > 0). The second, or overlapping, representation of FAC sheets near noon yields ionospheric currents that match the DPY current system better than the distinct cusp FAC model. The calculated convection electric fields do not show a drawing together of electric field equipotentials near noon to form a ‘throat’ of rapid convection. Variations of the model might yield a throatlike convection pattern on one side of noon but not both sides. The model results give a westward electrojet on the poleward edge of the evening region 1 as a natural part of the global ionospheric current system.
A relation between the altitude of the aurora and solar activity
Authors: Deehr Charles Sterling
Published: February 2008

The altitude of the nighttime aurora obtained by parallactic photography has been shown to rise with latitude poleward and equatorward of the auroral zone. Most remaining inconsistencies between various sets of height data taken between 1910 and 1970 are shown to be removed by ordering the data according to solar activity. The measure of solar activity used is the index of recurrence probability of geomagnetic activity developed by Sargent (1979). It is found that the altitude of the aurora is inversely proportional to the probability of recurrence of the geomagnetic storm. Remaining inconsistencies in the data are probably due to the disordered solar wind associated with high geomagnetic and solar activity. This suggests that the aurora is a more direct indicator of the condition of the solar wind than was previously suspected and that far better solar‐terrestrial correlations may be achieved by using measures of solar activity associated with the relatively high‐speed, ordered solar wind connected with solar coronal holes.
The atmospheric neutral sodium layer: 2. Diurnal variations
Authors: Kirchhoff V. W. J. H. , Clemesha B. R.
Published: February 2008

Using recently proposed chemistry which includes the NaOH molecule, static calculations simulating the nocturnal sodium layer have been shown to give results in good agreement with an average of sodium measurements. In order to further evaluate this Na layer model, a dynamic time dependent atmospheric photochemistry‐vertical diffusion model has been developed to calculate the diurnal variation of sodium and compare it with recent measurements. The diurnal calculations show that the Na model fails to reproduce the measured diurnal behavior of the Na layer, which, for example, shows little variation in abundance between day and night. The situation is further aggravated if we include the reaction NaO2 + O → NaO + O2. After a thorough review of the reaction rates it is shown that the major modification which needs to be introduced with respect to the previous model is the elimination of the reaction NaOH + H → Na + H2O. The production of the NaOH molecule is thus effectively transformed into a height dependent sink for Na. The sensitivity of the new model is tested for variations of the reaction rates and the eddy diffusion coefficient and the results are shown to be in much better agreement with experiment.
Studies of the self‐focusing instability at Arecibo
Authors: Farley Donald T. , LaHoz Cesar , Fejer Bela G.
Published: February 2008

Precisely simultaneous radar and satellite measurements at the altitude of reflection of a strong HF heating wave above the Arecibo Observatory were made on June 7, 1977. Parametric instabilities produce strong enhancements in the plasma line and ion line incoherent scatter radar echoes. These echoes also exhibit periodic deep fading that is attributed to a self‐focusing instability. This explanation was confirmed by the in situ observation of electron density fluctuations with peak‐to‐peak amplitudes reaching at least 3% and a spatial dependence that corresponded closely to the radar fading pattern, at least for irregularity wavelengths ranging from a few hundred meters to a few kilometers. The correspondence implies that the radar fading is associated with the convection of the density irregularities through the radar beam. The radar and satellite observations also provided values for the important parameters of the ambient ionosphere, making possible a quantitative comparison of the data with existing theories of the self‐focusing instability. In particular, the agreement with the theory of Cragin et al. (1977) is fairly good.
Computer simulation of whistler mode wave‐particle interactions using a free‐boundary encounter model
Authors: Hashimoto Kozo , Matsumoto Hiroshi , Serizawa Yoshizumi , Kimura Iwane
Published: February 2008

Computer simulations of whistler mode wave‐particle interactions have been performed by using a free‐boundary encounter model electromagnetic code. The free boundary is attained by adding a collision dominated damping region near one boundary to remove periodicities usually used in electromagnetic codes. In our model, a wave front encounters a resonant bi‐Maxwellian hot electron beam. Spatial amplitude modulations are observed in our simulations. The dips of the amplitude, however, sometimes decrease to zero levels. The null points of the wave magnetic field and the associated phase reversals of the wave cannot be described by the usual particle trapping theory. A component of the field contains a feature reminiscent of a double sideband (DSB) wave without a carrier. This is interpreted as a beat between an incident wave and a generated wave. This phenomenon occurs if the amplitudes of both the waves are almost equal. The results of these simulations provide a possible explanation for the N events observed by Dowden et al. during a VLF injection experiment.
S II and S III branching ratios in the 600‐ to 1200‐Å interval
Authors: Morrison M. D. , Cunningham A. J.
Published: February 2008

Branching ratios of singly and doubly ionized sulfur EUV emissions are presented. The S II transitions originated from the ²P, 4s′ ²D, and 4s ²P terms and terminated on the metastable states of the ion. The S III transitions originated from the 3d ³D0, 4s 3P0, 3p³ ³S0, 4s ¹P0, and 3s3p³ ¹P0 terms and terminated on the metastable and ground ionic states. The results for S III include branching ratios involving intercombination transitions which impact ongoing modeling of the energy budget of the Io plasma torus.
Remote sensing of a flux transfer event with energetic particles
Authors: Daly P. W. , Keppler E.
Published: February 2008

We have used the medium energy particle spectrometer on board ISEE 2 to perform an analysis of the azimuthal asymmetry at the onset and end of a flux transfer event (FTE) on the dayside magnetopause. The event chosen was on November 8, 1977, at 0212 UT, during a period of high bitrate. The resulting angular resolution (32 sectors) permitted a very detailed analysis. We demonstrate that it is possible to describe the ion intensity as a function only of pitch angle and distance of the gyro‐center from a moving planar surface separating the particle‐rich and empty regions of the FTE. The intensity changes sharply within a fraction of a gyro‐radius. In order to match the location of the step at different pitch angles, it is necessary to define the particle boundary with respect to the gyro‐center itself, i.e., the particle intensity changes sharply when the gyro‐center crosses the boundary, and this boundary is the same for all pitch angles. The FTE boundary velocities that are derived from the analysis are in the range 10–50 km/s. When the results are transformed into the magnetopause normal coordinate system (LMN), the FTE is seen to sweep over the spacecraft from the magnetopause side, to reverse and to return in the same direction. At both the beginning and end, the FTE boundary motion has a northward component. The event appears as a flux tube filled with streaming particles (primarily ions) moving northward in the plane of the magnetopause with a speed of 50–100 km/s.
Satellite and ground observations of a pre‐substorm phase on May 4, 1977
Authors: Ranta Aarne , Ranta Hilkka
Published: February 2008

On May 4, 1977, a polar orbiting low‐altitude DMSP satellite passed close to the Finnish riometer stations at the same time as an absorption bay preceding an onset of substorm was observed. The combined ground and satellite measurements show that the absorption bay was characterized by intense precipitation of energetic electrons at the inner border of the compressed plasma sheet. The precipitation occurred in a narrow band of latitude, perhaps less than 50 km wide. We suggest that the pre‐bay precipitation was due to strong pitch angle diffusion and/or scatter of energetic electrons at the inner border of the plasma sheet, probably from the outer radiation zone.
Effect of an electron beam on the current‐convective instability
Authors: Chaturvedi P. K. , Ossakow S. L.
Published: February 2008

We consider the possible effects of an electron beam on the current‐convective instability in a weakly ionized plasma, with application to the diffuse auroral situation. A linear stability analysis including these effects is presented. It is found that the presence of an electron beam has negligible effects on the instability growth rate for the diffuse auroral ionospheric application.
Voyager 2 observations of energetic particle variations in the Ganymede wake region: A possible acceleration mechanism
Authors: Tariq G. F. , Armstrong T. P. , Collison T. H.
Published: February 2008

Voyager 2's passage through the downstream corotation region of Ganymede found disturbances in the field and particle environment. Large fluctuations in the intensities and energy spectra of ions in the 0.1‐ to 4.0‐MeV interval were also observed with the low energy charged particle experiment. All ion species were evidently affected, up through medium (C, N, O) nuclei. Electrons intensities while disturbed, did not behave similarly to the ions. These effects are probably associated with the wake, produced by the absorption of Jovian magnetospheric corotating plasma by Ganymede. We propose a two‐dimensional theoretical model for perturbed field in the wake region and show by direct numerical simulation of exact particle trajectories that such perturbations are capable of modulating the charged particle energies to about the observed amount for the observed magnetic and plasma parameters. Analysis shows that the energization process is dependent on the pitch angles of the energetic ions and the process itself is non‐adiabatic (the Larmor radius is approximately the size of the disturbance region).
Electron current disruption and parallel electric fields associated with electrostatic ion cyclotron waves
Authors: Lang Alan , Boehmer Heinrich
Published: February 2008

Laboratory observations of electron flux disruption by the potential well of a large‐amplitude ion cyclotron wave, which can be interpreted as electron trapping, are reported for the first time. Associated with trapping of more than 50% of the electron distribution is the existence of a weak electric field parallel to the external magnetic field. The significance of these observations, particularly to magnetospheric phenomena, is discussed.
Drift Wave Model for geomagnetic pulsations in a high β plasma
Authors: Patel V. L. , Ng P. H. , Cahill L. J.
Published: February 2008

A dispersion relation for the stability of the coupled drift compressional, drift mirror, and shear Alfvén waves in the magnetospheric plasma is analyzed by numerical method. The analysis is suitable for the storm time plasma conditions in the magnetosphere which usually has gradients in plasma density, temperature, magnetic field, and anisotropy in temperature. The effect of high β which is characteristic of storm time plasma is included in the model. For a given appropriate set of plasma parameters, and wave parameters of a mode, relative wave amplitudes are calculated from the model. A comparison is made of model generated and observed relative wave amplitudes by using two sample wave events observed by Explorer 45 during the August 4–6, 1972, magnetic storm. The quantitative analysis shows good agreement between theory and observations for a gradient‐driven Alfvénlike instability; however, growth rates are very small for this mode. The anisotropy‐driven drift mirror instability has large growth rates but does not show correlation between theory and observations of relative wave amplitudes.
Radio emission signature of Saturn immersions in Jupiter's magnetic tail
Authors: Desch Michael D.
Published: February 2008

During the interval from about May through August 1981, when Voyager 2 was inbound to Saturn, the planetary radio astronomy instrument measured repeated, dramatic decreases in the intensity of the Saturn kilometric radiation (SKR). The emission dropouts averaged two orders of magnitude below mean energy levels and varied from about 1 to 10 Saturn rotations in duration. Comparison with pre‐Saturn encounter Voyager 1 observations (June to November 1980) shows that the SKR dropouts were unique to the Voyager 2 observing interval, consistent with the closer proximity of Saturn to Jupiter's distant magnetotail in 1981. Further, the dropouts occurred on the average at times when Voyager 2 is known to have been within or near Jupiter's magnetic tail. Interpretation of these events as the radio signatures of successive Saturn immersions into Jupiter's tail or wake region is consistent with the independent evidence that the SKR radio source is driven externally by the solar wind ram pressure. The dropouts are identical to the one observed just after Voyager 2 closest approach to Saturn, providing the best evidence that Saturn was within Jupiter's tail at the time of the encounter. The sequence of events during this Saturn tail encounter is deduced.
The distribution of auroral electrostatic shocks below 8000‐km altitude
Authors: Bennett E. L. , Temerin M. , Mozer F. S.
Published: February 2008

This paper examines the distributions and characteristics of electrostatic shocks as observed by the S3‐3 polar‐orbiting satellite. There is auroral oval coverage by the satellite at all magnetic local times and at all altitudes between 240 and 8000 km. Electrostatic shocks are reasonably uniformly distributed in magnetic local time, with a slight increase in the probability of occurrence in the cusp region and a decrease in the post‐midnight region (0–6 hours magnetic local time). The occurrence of shocks increases sharply with altitude up to 5000 km, and at a slower rate above 5000‐km altitude. Larger shocks tend to occur above 5000‐km altitude and between 16 and 22 hours magnetic local time. Electrostatic shocks occur in or near the latitudes of the auroral oval, with few shocks occurring inside the polar cap. Only during periods of high Kp do shocks occur at low latitudes. Shocks occur with greater probability at low altitudes (2000–4000 km) during winter periods than during summer periods. In addition, a comparison of upward flowing ion beams with electrostatic shocks showed that energetic ion beams are associated with shocks but low energy ion beams are not, indicating a possible potential threshold for shocks in ion beams regions. Also, a search for upward flowing ion beams near electrostatic shocks at altitudes below 4000 km showed that upward flowing ion beams occur between 3000–4000 km altitude and between 15–24 hours magnetic local time with at least a 10% probability.
Lower thermospheric structure from Millstone Hill Incoherent Scatter Radar Measurements: 2. Semidiurnal temperature component
Authors: Wand Ronald H.
Published: February 2008

Daytime temperature determinations from 151 days of incoherent scatter radar measurements at Millstone Hill (42°N) from 1970 to 1975 were analyzed to characterize the semidiurnal temperature oscillation in the lower thermosphere (105–125 km). An analytical model fitted to the measured temperatures contained terms to specify the dependencies of the semidiurnal term on day of year (annual and semiannual terms), solar activity, and geomagnetic activity. The model representation, whose coefficients are tabulated, showed that seasonal effects were larger than effects associated with the other parameters. The annual mean semidiurnal oscillation
had a maximum amplitude of 28 K at 115 km and a vertical wavelength of 43 km. Variations associated with season were large, for example, ±17 K in amplitude and ± 1.2 hours in phase at 115 km when referred to the annual mean semidiurnal vector. Generally, the altitude of maximum was lowest at the solstices, and the longest vertical wavelength occurred in winter. Semidiurnal temperature measurements from Saint Santin showed good agreement with the Millstone Hill model results in winter, but some significant amplitude and phase differences were apparent in other seasons. Theoretical predictions indicated that the observed semidiurnal oscillation at Millstone Hill is primarily the upward propagating tide rather than an in situ tide. Comparisons with thermospheric Hough mode extensions indicated a temperature structure best matched by the S2,4 mode, whereas previously reported wind measurements were found to be best matched by the S2,2 mode. Because of their different vertical structures it was postulated that it might be possible to reproduce both the observed temperature and wind measurements by a suitable synthesis of the S2,2 and S2,4 modes. The observed solar flux effects could be reasonably attributed to changes in tidal dissipation in the lower temperature produced by changes in mean density and temperature with solar cycle.
Electron impact excitation of λ7990‐Å multiplet
Authors: Erdman P. W. , Zipf E. C.
Published: February 2008

We have investigated the dissociative excitation of the atomic oxygen multiplet O I(3s′ ³D0 → 3p ³P; λ7990 Å) due to electron impact on O2 and NO. With O2 as a target, no lines of this multiplet could be identified with a cross section greater than 2.4×10−23 cm² at 100 eV. Similar results were obtained for the excitation of NO, yielding an upper limit cross section of 1.1×10−23 cm². These results are consistent with a λ7990/λ989 branching ratio of ∼2.5×10−5 that is at least an order of magnitude smaller than either the previous experimental upper limits or a recent theoretical calculation. This finding complicates the study of (3s′ ³D0 → 2p4 ³P; λ989 Å) radiation entrapment in the thermosphere and increases the potential importance of selective absorption of the λ989‐Å multiplet by N2. Although the latter process may help to explain some aspects of the behavior of λ7990‐Å emission in aurorae, it is nonetheless difficult, in the light of the present results, to account for the observed absolute intensity of this infrared cascade transition using current entrapment models.
Whistler induced charged particle precipitation and distortion of geomagnetic field
Authors: Singh R. N. , Prasad R.
Published: February 2008

Effect of solar wind induced geomagnetic field distortion on whistler wave interaction with energetic charged particles has been studied. Precipitated charged particle influx has been found to increase with enhanced geomagnetic field distortions. It is envisaged that the changes in precipitated electron influx might carry the signatures of geomagnetic field distortions.
Voyager 2 photopolarimeter observations of Titan
Authors: West R. A. , Lane A. L. , Hart H. , Simmons K. E. , Hord C. W. , Coffeen D. L. , Esposito L. W. , Sato M. , Pomphrey R. B.
Published: February 2008

Observations of Titan's whole disk polarization at 2460 and 7500 Å are presented and analyzed in terms of model scattering atmospheres. If the Titan aerosols are spherical or nearly spherical, no single combination of refractive index and size distribution is able to fit data at both wavelengths. However, a vertically inhomogeneous distribution suggested by Tomasko and Smith (1980), characterized by a size gradient with altitude, fits the data at 2640 Å moderately well but must be modified at intermediate and large optical depths to fit the 7500‐Å data. Results for synthetic phase functions indicate that the single scattering polarization must be 70% or larger in the UV and 78% or larger in the near‐IR at 90° phase angle, depending on the phase function. If the correct phase function is similar to that for 0.5‐μm‐radius spheres, the UV single‐scattered polarization must be 84% and the near‐IR single‐scattered polarization must be over 90%. Such large polarizations are impossible for 0.5‐μm‐radius spheres but may be possible for nonspherical particles with effective radii near 0.5 µm, although the existence of nonspherical particles with the scattering properties required by these and other observations has not been demonstrated.
A mechanism for plasma waves at the harmonics of the plasma frequency in the electron foreshock boundary
Authors: Klimas Alexander J.
Published: February 2008

A bump‐on‐tail unstable reduced velocity distribution has been constructed from data obtained at the upstream boundary of the electron foreshock by the GSFC electron spectrometer experiment on the ISEE 1 satellite. This distribution is used as the initial plasma state for a numerical integration of the one‐dimensional Vlasov‐Maxwell system of equations. The integration is carried through the growth of the instability, beyond its saturation, and well into the stablized plasma regime. A power spectrum for the electric field of the stabilized plasma is computed. The spectrum is dominated by a narrow peak at the Bohm‐Gross frequency of the unstable field mode but it also contains significant power at the harmonics of the Bohm‐Gross frequency. The harmonic power is in sharp peaks which are split into closely spaced doublets. The fundamental peak at the Bohm‐Gross frequency is also split, in this case into a closely spaced triplet. The splitting is due to slow modulations of the stabilized electric field oscillations which, it is thought, are caused by wave‐particle trapping. The wave length of the mth harmonic of the Bohm‐Gross frequency is given by λu/m, where λu is the wavelength of the unstable mode. The mechanism for excitation of the second harmonic is shown to be second‐order wave‐wave coupling which takes place during that period in the evolution of the instability which would otherwise be called the linear growth phase. It is conjectured that the higher harmonics are excited by the same mechanism. It is further argued that harmonic excitation at the boundary of the electron foreshock should be a common occurrence.
A theoretical model of equatorial F region dynamics
Authors: Duhau S. , Louro A. A.
Published: February 2008

A simple dynamical model of the equatorial F region is presented in which this section of the ionosphere is coupled to the E region, which is represented by a thin horizontal current layer, and the vertical current density at the base of the F region is taken to be composed entirely of current normal to the geomagnetic field lines, according to a previous finding by the authors. The new boundary condition for the current density at the interface between the E and F layer simplifies the system of equations. The model is applied to calculate the zonal ion velocity and the vertical current at the boundary between the E and F regions.
Large amplitude MHD waves upstream of the Jovian bow shock
Authors: Goldstein Melvyn L. , Smith Charles W. , Matthaeus William H.
Published: February 2008

Observations of large amplitude MHD waves upstream of Jupiter's bow shock are analyzed. The waves are found to be right circularly polarized in the solar wind frame, which suggests that they are propagating in the fast magnetosonic mode. A complete spectral and minimum variance eigenvalue analysis of the data was performed. The power spectrum of the magnetic fluctuations contains several peaks. The fluctuations at 2.3 mHz have a direction of minimum variance antiparallel to the direction of the average magnetic field. Several harmonics at 6, 9, and 12 mHz are also present. The direction of minimum variance of these fluctuations lies at approximately 40° to the magnetic field. We argue that these fluctuations are waves excited by protons reflected off the Jovian bow shock. The inferred speed of the reflected protons is about two times the solar wind speed in the solar wind frame. A linear instability analysis is presented that suggests an explanation for many of the observed features of the observations. The fluctuations apparently contain a significant fraction of magnetic energy that is linearly polarized and in the Alfvén mode.
A Revised thermospheric model based on mass spectrometer and incoherent scatter data: MSIS‐83
Authors: Hedin A. E.
Published: February 2008

The MSIS‐83 empirical model of temperature, density and composition is a revision of the original MSIS model (including longitude/UT effects) and is based on temperature, density, and composition data from a comprehensive summary of rocket flights, seven satellites, and five incoherent scatter radars, including data from high solar activity. The model extends the previous description of neutral parameters to the base of the thermosphere in a continuous manner while maintaining the basic structure of the MSIS model at higher altitudes. As the altitude decreases, composition approaches lower atmosphere values, while yearly, and to a lesser extent daily, variations in temperature and density are in reasonable agreement with previous results for the lower thermosphere. An alternate description of magnetic storm variations is provided based on the three hour ap indices and an 8‐ to 10‐hour exponential decay in thermospheric density and temperature response after a heating event. Additional coefficients are included for the time independent and magnetic activity terms, including a longitudinally dependent seasonal magnetic activity effect. The molecular oxygen description is based on mass spectrometer and EUV absorption measurements rather than ion chemistry.
Long‐wavelength limit of the E×B instability
Authors: Huba J. D. , Zalesak S. T.
Published: February 2008

We present an analytical expression for the growth rate of the E×B instability in the long‐wavelength limit, i.e., kyLn ≪ 1, where ky is the wave number and Ln is the scale length of the density gradient. Specifically, we obtain results for the case of a single discontinuity in plasma density.
Ambipolar diffusion in the middle atmosphere
Authors: Tzur I. , Roble R. G.
Published: February 2008

In the middle atmosphere above 60 km the electron concentration increases with altitude, reaching values of 1010 m−3 in the daytime ionospheric E region near 100 km. The electrons are more mobile than the ions and diffuse more rapidly through the neutral atmosphere. The electron diffusion polarizes the medium, causing an electric field to develop that acts to retard the electron diffusion and enhance the conduction current of ions. We use a global zonally averaged numerical model of atmospheric electricity from the ground to 100 km to examine the effect of ambipolar diffusion and the earth's geomagnetic field on the currents and fields in the middle atmosphere. The results show that above about 65 km, ambipolar diffusion generates local electric fields and conduction currents that balance electron diffusion currents. The electric fields and conduction currents are a few orders of magnitude larger than the vertical fields and currents calculated from the downward mapping of the ionospheric potential without taking electron diffusion into account. Ambipolar diffusion does not alter the total current flowing in the global circuit. It is a local effect where enhanced conduction currents flow to balance the electron diffusion current.
A simple expression for kilometric radiation growth rates and analytical applications
Authors: Lyons L. R. , Dusenbery P. B.
Published: February 2008

A simple expression is derived for the linear growth rate of R‐X mode, auroral kilometric radiation under the assumptions of cold plasma wave dispersion and ωpe² ≪ Ωe², where ωpe and Ωe are the electron plasma frequency and gyrofrequency, respectively. The result is valid for all wave normal angles and all frequencies above the right‐hand cutoff frequency. With this expression, it is not necessary to evaluate the full dispersion tensor, and variations of the growth rate with plasma parameters are easily obtained. Corrections to a previously employed, equivalently simple, expression for the growth rate are also easily obtained, the previous expression being valid only for nearly perpendicular wave propagation and frequencies well above the cutoff. Applying the new expression, we find that kilometric radiation growth rates γ are strongly dependent upon the magnitude of the magnetic field‐aligned potential difference V∥, with γ ∝ V∥², and that significant growth of radiation at harmonics of the fundamental is unlikely in regions where ωpe² ≪ Ωe².
Three‐dimensional ray tracing of the Jovian magnetosphere in the low‐frequency range
Authors: Menietti J. D. , Green J. L. , Gulkis S. , Six F.
Published: February 2008

Three‐dimensional ray tracing of the Jovian DAM emission has been performed utilizing the O‐4 magnetic field model (Acuna and Ness, 1979) and a realistic plasma model. Minimal assumptions about the emission mechanism have been made that include radiation in the right‐hand extraordinary mode, propagating nearly perpendicular to the field line at source points located just above the RX cutoff frequency along Io flux tubes. We have performed ray tracing in the frequency range from 2–35 MHz from successive Io flux tubes separated by ten degrees of central meridian longitude for a full circumference of northern hemisphere sources. Our results show unusual complexity in model arc spectra that is displayed in a constant Io phase format with many similarities to the Voyager PRA data. The results suggest much of the variation in observed DAM spectral features is a result of propagation effects rather than emission process differences. We feel our work represents a comprehensive ray tracing investigation of DAM arc emission and presentation of the results in a unique format.
Magnetopause structure and the question of particle accessibility
Authors: Whipple Elden C. , Hill Jay R. , Nichols J. David
Published: February 2008

Previous Vlasov theories of the plane magnetopause have yielded nonunique solutions because particle distribution functions were assigned arbitrarily in different regions of phase space. We show that consideration of particle accessibility removes this nonuniqueness. In a restricted model consisting of a collisionless, steady state, plane model with no normal magnetic field component, particles are trapped at the magnetopause. Therefore these restrictions must be relaxed in order to allow for particle access to the magnetopause via drifts from source regions. The first adiabatic invariant for particle motion can be defined in such a way that it is preserved in regions of large field gradients, provided that the gradients are in a direction primarily perpendicular to the particle drifts. This generalized invariant provides an adiabatically conserved quantity that can be used to characterize particles as they move from a source region, through the drift region, and into the magnetopause itself, and therefore can be used to help resolve the accessibility question. We show how to obtain magnetopause structure based on location of the particle guiding points and give some preliminary results. However, a complete solution will probably require a time‐dependent determination of the particle drift paths.
Streaming reversal of energetic particles in the magnetotail during a substorm
Authors: Lui A. T. Y. , Williams D. J. , Eastman T. E. , Frank L. A. , Akasofu S.‐I.
Published: February 2008

Reversal from tailward streaming to earthward streaming of energetic ions at 0.29–0.50 MeV during a substorm on February 3, 1978, is studied with measurements of energetic particles, plasma, and magnetic field from the IMP 8 spacecraft near the dusk flank of the magnetotail. Four new features emerge when high time resolution data are examined in detail. The times of reversal from tailward to earthward streaming of energetic ions and from tailward to earthward plasma flow do not coincide. Second, the velocity distribution in the tailward flowing plasma has a crescent shape, whereas the velocity distribution in the earthward flowing plasma resembles a convecting Maxwellian. Third, tailward streaming of energetic ions is sometimes detected in northward magnetic field regions and conversely, earthward streaming in southward field environments. Fourth, energetic ions streaming earthward are occasionally present in conjunction with a strong tailward streaming population in the same energy range. These new features suggest that the streaming reversal of energetic ions and the plasma flow reversal in this event are due to the spacecraft traversing different plasma regions during the substorm‐associated configurational change of the plasma sheet and the magnetotail and is unrelated to the motion of an acceleration region such as an X type neutral line moving past the spacecraft.
Phase measurements of whistler mode signals from the Siple VLF transmitter
Authors: Paschal E. W. , Helliwell R. A.
Published: February 2008

A digital signal processing program has been developed to measure the phases of coherent VLF signals from analog tape recordings made in the field. The program uses a constant frequency pilot tone recorded with the VLF data to correct tape speed errors and reconstruct the signal phases. We analyze several examples of whistler mode signals from the VLF transmitter at Siple Station, Antarctica, as received at Roberval, Quebec. Pulses with temporal growth show a relative phase advance with time, and thus a positive frequency offset from the transmitted signal, often from the beginning of the pulse. Amplitude beating is often seen toward the end of a pulse, sometimes with phase cycle‐skipping as the emission becomes unlocked from the input signal. Current theories of wave‐particle interaction are reviewed and found to explain some of the observed signal features, though no theory predicts the initial frequency offset of a growing pulse.
Intensity and polarization characteristics of whistlers deduced from multi‐station observations
Authors: Machida S. , Tsuruda K.
Published: February 2008

Whistlers recorded at four stations along the geomagnetic meridian near Roberval, Canada, on July 23, 1979, have been analyzed. They showed a spatial attenuation rate as high as ∼6 dB/100 km, which is appreciably larger than previous theoretical predictions. The wave polarization of these whistlers was also analyzed; the wave polarization was a maximum (right handed) at the station where wave intensity was a maximum. The wave polarization was also right handed at a station distant from the wave exit region. The locations of the wave exit region inferred from the intensity maximum were compared with the L values of the whistler paths derived from the dispersion analysis. We obtained a consistent relation between them when uncertainties in the dipole field model are taken into account.
Observations of multiple ion beams in the magnetotail: Evidence for a double proton population
Authors: Candidi M. , Orsini S. , Ghielmetti A. G.
Published: February 2008

On day 143, 1978, 2230–2330 UT, the ISEE 2 plasma experiment detected the multiple population frequently found in the mantle and low‐latitude boundary layer regions of the magnetotail. This population consisted at this time of ionospheric O+ and H+ mixed with solar wind plasma (typically H+ and He++). A detailed analysis of the low‐energy population indicates that it can be further resolved into two distinguishable proton components. The ratio between the oxygen and the proton energies implies that one of the proton populations is of ionospheric origin and flows tailward together with the ionospheric oxygen. The angular distribution of the other proton population and of the He++ suggests a higher‐altitude origin, the presence of He++ ions suggesting that this is the solar wind proton component.
Ionospheric chemistry of NO+
Authors: Breig E. L. , Hanson W. B. , Hoffman J. H.
Published: February 2008

An investigation is described of the behavior of NO+ in the daytime F region, with basic ion concentration measurements from the Atmosphere Explorer C satellite. The data set was acquired along select orbits at low latitudes and exhibits substantial variations in the NO+ concentration, both along and between nearby orbits. An excellent consistency is demonstrated between these observations and current chemical equilibrium theory, in contrast to differences that have been reported for the related N2+ ion. Large variations in the concurrently observed electron temperature permit a relevant comparison between different laboratory determinations of the dissociative recombination rate coefficient. Contributions to the NO+ production from several secondary sources are also evaluated. Results strengthen the basis for the current theoretical ionospheric chemistry of NO+ and establish important constraints on resolution of the difficulties with N2+.
Excitation of the lower oblique resonance by an artificial plasma jet in the ionosphere
Authors: Thiel J. , Storey L. R. O. , Bauer O. H. , Jones D.
Published: February 2008

Aboard the Porcupine rockets, bursts of noise were detected in the electron whistler range during the operation of a xenon plasma gun on a package ejected from the main payload. These observations can be interpreted in terms of excitation of the lower oblique resonance by instabilities associated with the motion of the xenon ion beam through the ionospheric plasma.
A second harmonic geomagnetic field line resonance at the inner edge of the plasma sheet: GEOS 1, ISEE 1, and ISEE 2 observations
Authors: Hughes W. J. , Grard R. J. L.
Published: February 2008

We describe observations made by GEOS 1, ISEE 1, and ISEE 2 of a hydromagnetic wave with a period ∼90 s observed near 0200 LT between L = 9 and L = 6, close to the measured inner boundary of the plasma sheet. The wave magnetic oscillations perpendicular to and along the ambient field had similar amplitudes. Using primarily the transverse magnetic components, we show that the wave is a second harmonic resonance of the local geomagnetic field lines. ISEE 1 and 2 observed the opposite sense of polarization for about 30 min, although the spacecraft were separated by only 9 min in their orbit; this remarkable feature cannot be explained by either a stationary spatial boundary or a simple temporal boundary but could result from a rapid movement of the resonant region. We argue that the most likely energy source is bounce resonance with medium energy (∼5 keV) ions. Calculations of the wave Poynting vector at ISEE 1 support this conclusion.
Propagation of plasma wave energy in the auroral E region
Authors: Moorcroft D. R.
Published: February 2008

The propagation of electrostatic plasma wave energy in the auroral E region has been studied using fluid theory. Typically, a wave travels at high speed nearly parallel to the magnetic field, except possibly for a small height interval where it is reflected upward; throughout, the direction of the propagation vector remains virtually unchanged and nearly perpendicular to the magnetic field. If k makes an angle of more than 0.25° with the magnetic field, the wave passes through the E region without reflection. Kinks in the magnetic field lines (due to auroral currents) may lead to trapping of waves in layers less than 1 km thick. Secondary irregularities may be limited in both horizontal and vertical extent because of their motion relative to the primary waves on which they depend for growth. The processes determining the amplitude of irregularities have also been considered in the light of this work. Several observed features of radio aurora may have explanations in terms of these properties of energy propagation.
Relationship of dusk sector radial electric field to energy dispersion at the inner edge of the electron plasma sheet
Authors: Horwitz J. L.
Published: February 2008

It is shown that, assuming conditions of approximate steady state and that the low to medium electrons forming the plasma sheet inner edge originate from a source in the magnetotail, the radial electric field in the dusk sector can be related to the radial energy dispersion of the plasma sheet inner boundary as Er, = −[(W⊥)e/e](d/dr)(ln B), where (W⊥)e represents the energy of the electron inner boundary. An increasing electric field component with radial distance is obtained from recent measurements of plasma sheet inner edge electron energy dispersion.
Spectral analysis of magnetohydrodynamic fluctuations near interplanetary shocks
Authors: Viñas Adolfo F. , Goldstein Melvyn L. , Acuña Mario H.
Published: February 2008

Evidence for the presence of relatively large amplitude right‐hand elliptically polarized MHD waves upstream and downstream of quasi‐parallel (θBn < 45°) forward and reverse interplanetary shocks is presented. The mode is observed with frequencies in the range of 0.025–0.17 Hz (in the spacecraft frame) and propagates along the magnetic field. The MHD modes have rest frame frequencies in the range 3.8–31 mHz, with typical wavelengths about 1.5×108 to 4.7×108 cm. The magnetic field power spectrum in the vicinity of these interplanetary shocks is much steeper than f−5/3 at high frequencies. The observed spectra have a high frequency dependence of f−2.5 to f−4. A peculiar feature of the fast mode identification in one event is the large correlation observed between |B| and proton density ρ for field‐aligned propagation. This appears to be a nonlinear effect, second order in the wave amplitude. The properties of the MHD waves can be understood in terms of the electromagnetic ion‐cyclotron instability. Both resonant and nonresonant interactions need be considered to account for the polarization and spectral content of the observed fluctuations. In association with one event, a distinct suprathermal component was observed in the solar wind proton distribution. The parameters of this distribution are adequate to drive a resonant instability with a maximum growth rate that coincides with the observed range of frequencies.
A computer simulation study of Hook‐induced electrostatic bursts observed in the magnetosphere by the ISEE satellite
Authors: Matsumoto H. , Ohashi M. , Omura Y.
Published: February 2008

Observations from the ISEE satellite have recently revealed interesting electrostatic bursts that are apparently hook‐induced emissions stimulated by the combined action of a coherent whistler‐mode wave and a simultaneously observed electron beam with an energy range of the order of 1 keV (Reinleitner et al., 1982). In order to seek a plausible generation mechanism and interpret this nonlinear phenomenon a computer simulation study was carried out. The code used is a 2½‐D electromagnetic particle code (EM2 code) following nonlinear motions of more than a half million particles in the 128 × 128 grid space under the self‐consistent fields. The results show an interesting two‐beam jetting as a result of a trapping and subsequent detrapping process, yielding a strong electrostatic emission with k vector parallel to the external magnetic field. The nonlinear evolutions of the wave spectra and of particle distribution functions and their phase‐space behavior are discussed in connection with the observed ES bursts.