bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetospheric processes preceding the onset of an isolated substorm: A Case study of the March 31, 1978, substorm
Authors: Nishida A. , Kamide Y.
Published: February 2008

We examined in detail the effect of a southward turning of the interplanetary magnetic field (IMF) on the state of the magnetosphere, taking advantage of the availability of the data from IMS magnetometer meridian chains and from several spacecraft. A clear onset substorm occurred on March 31, 1978, when the magnetometer stations were located in the midnight to morning sector and the spacecraft were near the equatorial plane of the nightside magnetosphere. The onset time of the substorm expansion phase could be determined unambiguously in terms of both ground‐based magnetic and auroral signatures, and there was an interval lasting about 1 hour between the IMF southward turning and this onset. In this intervening interval the ionospheric current system of the DP 2 type developed. This enhancement of the ionospheric current was driven directly by the solar wind‐magnetosphere coupling. The onset of the expansion phase was then associated with the decrease in the magnetic field energy density in the tail, providing evidence that the substorm energy was supplied by the release (unloading) of energy from the tail. It is most likely that substorm energy dissipated in the auroral ionosphere throughout this relatively isolated and simple event was supplied by two components, ‘directly driven’ and ‘loading‐unloading’, the relative importance of which varied depending on the different substorm phases.
Latitudinal (Seasonal) Variations in the thermospheric midnight temperature maximum: A tidal analysis
Authors: Herrero F. A. , Mayr H. G. , Spencer N. W.
Published: February 2008

Recent observations of a seasonal oscillation in the time of occurrence of the thermospheric midnight temperature maximum suggest that even at low latitudes (<20°) there is a latitudinal (seasonal) variation in the amplitudes and phases of the higher‐order tidal modes. Temperature data from the Atmosphere Explorer E Neutral Atmosphere Temperature Experiment have been used to investigate this question, obtaining the temperature as a function of local time at two mean latitudes: −17.5° and 17.5°. The altitude range covered is 250–400 km. The 24‐hour temperature functions are obtained during the same time of the year in both hemispheres, and they show the midnight temperature maximum occurring earlier in the summer hemisphere than in the winter hemisphere. In addition, the daytime summer temperature variation is characterized by a gradual rise toward the afternoon maximum followed by a rapid decrease into the night, whereas the winter variation shows a rapid increase toward the afternoon maximum followed by a gradual decrease into midnight. Tidal analysis of the temperature function shows that the observed variations can be accounted for by the first three tidal harmonics, each harmonic showing a latitudinal (seasonal) variation. The most significant variation is observed in the semidiurnal component, which is always dominant in the winter hemisphere and therefore accounts for most of the winter nighttime temperature maximum. The terdiurnal component is as strong as the semidiurnal one in the summer hemisphere but is negligible in winter. The equinoxes are characterized by symmetric excitation of the tidal components with respect to the equator, with the semidiurnal larger than the terdiurnal. We interpret the latitudinal solstice variation of the semidiurnal component as a superposition of the semidiurnal tide induced by ion drag and the semidiurnal component excited by the EUV source. In summer the duration of the EUV source exceeds 12 hours, and the sign (phase) of the semidiurnal source component opposes that of the ion drag‐induced component. In winter, the source duration is less than 12 hours, and the phases of these two components are of the same sign, enforcing each other, thus producing the larger semidiurnal component observed.
Energetic particles in the vicinity of a possible neutral line in the plasma sheet
Authors: Möbius E. , Scholer M. , Hovestadt D. , Paschmann G. , Gloeckler G.
Published: February 2008

We have analyzed energetic protons in the energy range 30 to 500 keV and energetic electrons ≥75 keV obtained with the Max‐Planck‐Institut/University of Maryland sensor system on ISEE‐1 during a plasma sheet crossing on March 26, 1978. The behavior of protons with energies of more than ∼100 keV is very different from that of the ∼30 to ∼100 keV protons which represent the suprathermal tail of the plasma sheet distribution. The more energetic ions appear on a time scale of several minutes following a northward turning of the tail magnetic field. At about the same time the plasma measurements show a velocity of ∼200 km/s in the tailward direction. This velocity enhancement is first seen at ISEE‐1 and later on at ISEE‐2, which is earthward of ISEE‐1. The temporal sequence of the energetic particle, magnetic field, and plasma observations and the proton and electron anisotropies are discussed in terms of acceleration near a magnetic neutral line which occurs well within the plasma sheet. In this framework the magnetic neutral line would move earthward, followed by a magnetic island. The extent of the neutral line would be limited to the dusk side of the tail. No disruption of the plasma sheet is observed as compared to large‐scale substorm activity.
Intermediate‐term epochs in solar soft X ray emission
Authors: Bouwer S. D.
Published: February 2008

Observations of the daily 1‐ to 8‐Å solar X ray emission from 1977 to 1981 are analyzed from a terrestrial viewpoint. A high‐resolution method of anharmonic frequency analysis is used to search for discrete frequencies in a finite time series and estimate the frequency, phase, and amplitude of each periodic component. The time series data are the broadband, full disk X ray measurements, primarily from the GOES 2 satellite, and are reduced from 3‐s data to a daily nonflare index representing background coronal emission. Intermediate‐term epochs are weakly stationary time intervals about 128 days long and correspond to major intervals of solar activity over about four and a half rotations. Time series of background X ray flux longer than about 128 days are nonstationary. Results suggest that near the boundary of an epoch the amplitude spectrum undergoes an episodic change in frequency, phase, and amplitude, after which a new spectrum appears that persists for the duration of the next epoch. Furthermore, those frequencies most closely corresponding to solar rotation are not normally distributed about a 27–28 day mean but occur at about 22,25, and 34 days.
Inferring electric fields and currents from ground magnetometer data: A test with theoretically derived inputs
Authors: Wolf R. A. , Kamide Y.
Published: February 2008

We report results of a “blind test” of the algorithm developed by Kamide et al. (1981) (KRM algorithm) for deriving ionospheric currents and electric fields using data from ground magnetometers. The “ground magnetometer data” used for the test came from the results of a substorm simulation performed with the Rice University Convection Model (Harel et al., 1981a). Pedersen, Hall, field‐aligned, and ring currents are included in the Rice Convection Model, but earth currents and polar cap currents are not. A Biot‐Savart routine was used to integrate over the model current system and compute vector magnetic fields
at 96 grid points on the ground. At the National Oceanic and Atmospheric Administration (NOAA), current
and electric‐field
distributions were derived by using the conductivity model and ground
values provided by Rice. Plots of
and
were then exchanged, one set giving the Rice‐computed
and
on which the “theoretical magnetometer data” were based and one set showing
and
computed at NOAA by inversion of the “magnetometer data.” At auroral latitudes, where currents and electric fields are strongest, the
and
plots are in encouragingly good agreement, with total polar cap potential drops agreeing to better than 10%. Total electrojet strengths estimated from the KRM inversion algorithm average about 0.84 of the electrojet strengths in the source‐current distribution. Overall, the test confirms the ability of the algorithm to deduce patterns of auroral ionospheric currents, Birkeland currents, and electric fields reasonably well from ground‐based magnetic data when the ionospheric conductivity is known.
Voyager photopolarimeter stellar occultation of Saturn's rings
Authors: Esposito L. W. , O'Callaghan M. , Simmons K. E. , Hord C. W. , West R. A. , Lane A. L. , Pomphrey R. B. , Coffeen D. L. , Sato M.
Published: February 2008

On August 25, 1981, the Voyager 2 photopolarimeter system observed a stellar occultation by Saturn's rings. We present a brief description of this experiment along with details of the data reduction. The occultation results are given in tabular and graphical form at a resolution of 60 km. Histograms of the frequency of optical depth show dominantly unimodal distributions in each of the classical ring elements. The frequency distribution of the entire ring system shows three modes at τ ≈ 0.08, τ ≈ 0.5, and τ ≳ 2.50.
Size estimates of Titan's aerosols based on Voyager high‐phase‐angle images
Authors: Rages K. , Pollack J. B. , Smith P. H.
Published: February 2008

Limits on the physical properties of the scattering haze near the top of Titan's atmosphere are derived from data obtained from seven high‐phase‐angle images from Voyager 1 and 2. From the ratio of the intensities observed at two different high phase angles, an estimate can be made of the forward scattering lobe of the single‐scattering phase function. Comparing the forward scattering estimate with diffraction lobes from particles of different radii, we conclude that the average radius of the particles found in the upper few tenths of an optical depth exceeds 0.19 μm. Judging from data observed at four different phase angles, the haze particles probably have a refractive index near 1.6 and a mean size of ∼0.5 μm, if the widths of their diffraction peaks are close to those for equal volume spheres. However, the highly polarizing nature of the particles over a broad wavelength band pass [Tomasko and Smith, 1982; West et al. this issue] combined with their forward scattering behavior makes it very unlikely that the particles are spherical. The nonsphericity contributes to the uncertainty about the radii of the particles, but it is thought that the average radius is several tenths of a micron.
Currents in Saturn's magnetosphere
Authors: Connerney J. E. P. , Acuña M. H. , Ness N. F.
Published: February 2008

A model of Saturn's magnetospheric magnetic field is obtained from the Voyager 1 and 2 observations. A representation consisting of the Z3 zonal harmonic model of Saturn's planetary magnetic field together with an explicit model of the equatorial ring current fits the observations well within r < 20 Rs, with the exception of data obtained during the Voyager 2 inbound pass. The exception is attributed to a time variation of Saturn's magnetosphere driven by a drop in solar wind ram pressure. The magnetohydrodynamic momentum equation is used to obtain, from the magnetic field model, estimates of the plasma pressure and mass density at radial distances of 8 < r < 16 Rs. These estimates are generally consistent with those obtained by the Pioneer 11 and Voyager plasma investigations. The Voyager 1 observations suggest the presence of a global field‐aligned current of ∼107 A flowing into the (southern) auroral zone in the evening sector. No evidence of such a field‐aligned current system is found in the Voyager 2 observations obtained a year later.
Structure and other properties of Jupiter's distant magnetotail
Authors: Lepping R. P. , Desch M. D. , Klein L. W. , Sittler E. C. , Sullivan J. D. , Kurth W. S. , Behannon K. W.
Published: February 2008

Analyses using data from the plasma wave, plasma science, planetary radio astronomy, and magnetometer experiments onboard Voyager 2 covering the period October 1980 to August 1981 are shown to provide compelling evidence for and characteristics of a Jovian magnetotail extending at least to 9000 Jovian radii from the planet. Voyager 1 magnetic field and wave data from the same time period indicate that it was very unlikely that the distant Jovian tail was observed at that spacecraft. During approximately (25‐day) periodic sightings of the tail by Voyager 2 the magnetic field tended to point radially towards or away from Jupiter, indicating preservation to large distances of the bipolar, lobelike structure observed near the planet, as in the earth's case. This periodicity, along with various properties of the solar wind at this time, indicates that the tail is apparently influenced by recurrent solar wind features. Anomalous magnetic fields, not aligned with the nominal tail axis, also exist within the tail, especially in the low‐density, “central” (core) region, indicating some complexity of internal structure. Approximately centered at each of the tail encounter periods we see broad‐scale plasma velocity, density, and field magnitude decreases; the latter are not expected for a quasi‐steady tail structure but probably indicate tail expansion to the position of the spacecraft. On a finer scale the field increases across the inbound boundary as expected for an outward pressure imbalance. We argue that a possible contributor to the internal pressure of the tail is a considerable amount of tailward flowing plasma, in addition to the expected internal field pressure. Magnetic field variance analyses provide quantitative evidence for field line draping around the boundary of the tail and also show that internal tail field transverse variations occur. We suggest that the Jovian tail has a quasi‐periodically variable width (resembling a string of sausages) on a very long‐length scale, tens of thousands of Jupiter radii, due to the influence of the ≈ 25‐day radial variation of the solar wind pressure. The tail is also apparently filamentary to some degree.
Electric field evidence for tailward flow at substorm onset
Authors: Nishida A. , Tulunay Y. K. , Mozer F. S. , Cattell C. A. , Hones E. W. , Birn J.
Published: February 2008

Electric field observations made near the neutral sheet of the magnetotail provide additional support for the view that reconnection occurs in the near‐earth region of the tail. Southward turnings of the magnetic field that start at, or shortly after, substorm onsets are accompanied by enhancements in the dawn‐to‐dusk electric field, resulting in a tailward E×B drift velocity. Both the magnetic and the electric fields in the tailward‐flowing plasma are nonuniform and vary with inferred spatial scales of several RE in the events examined in this paper. These non‐uniformities may be the consequence of the tearing‐mode process. The E×B flow was also towards the neutral sheet and away from midnight in the events studied.
S3‐2 measurements of the polar cap potential
Authors: Doyle M. A. , Burke W. J.
Published: February 2008

A total of 83 dawn‐dusk, high‐latitude passes of the S3‐2 satellite has been analyzed for the period August through December 1976 to determine correlations between the cross polar cap potential Φpc and various solar wind‐interplanetary magnetic field parameters. Hourly and 15‐s averaged values of the interplanetary magnetic field components and hourly averaged values of the solar wind velocity were available for 68 of the passes. In 41 (27) of the cases studied, the interplanetary magnetic field had a southward (northward) component. Measured potentials ranged from 170 kV during a substorm to 12 kV during two periods of northward interplanetary magnetic field. Least squares fits to the data were performed in order to determine the dependence of the polar cap potential on four different electric field models derived from reconnection theory and other traditionally correlated solar wind‐interplanetary magnetic field parameters. In each case, there is a residual polar cap potential of the order of 40 kV. This suggests a time delay in the relaxation of the potential when the interplanetary magnetic field turns northward and/or processes other than reconnection contribute to the polar cap potential. Total interplanetary magnetic field variance and solar wind speed were tested as filters on the data and found to have some significance. Comparisons with similar studies based on measurements from the Atmosphere Explorer and S3‐3 satellites are discussed.
Large‐amplitude nightglow OH (8–3) band intensity and rotational temperature variations during a 24‐hour period at 78°N
Authors: Myrabø H. K. , Deehr C. S. , Sivjee G. G.
Published: February 2008

We report results from a continuous 24‐hour measurement of the OH (8–3) band emission in the nightglow at 78.4°N. A mean temperature of 273 K and a mean band intensity of 596 R were observed. Extreme temperature variations were seen with amplitudes up to ±70 K from the mean. It is suggested that these variations are related to the passage of internal gravity waves. If so, the extreme amplitudes of the variations might imply that the OH emitting layer is situated above 90 km at this latitude in January. The deduced η values (ΔI/I/ΔT/T) favor the ozone mechanism to be responsible for the OH emission with the possibility of an additional mechanism contributing up to 5%.
Formation of shock‐spike events at quasi‐perpendicular shocks
Authors: Decker Robert B.
Published: February 2008

This paper is an analysis of how shock‐spike particle events are formed by single‐encounter “shock‐drift” acceleration at quasi‐perpendicular fast‐mode interplanetary shocks. A set of time‐reversed equations, valid in the upstream or downstream solar wind frames, express a particle's initial or pre‐shock‐interaction kinetic energy and pitch angle as functions of its final or post‐shock‐interaction kinetic energy and pitch angle. These equations and particular forms of the initial or ambient energy spectrum and angular distribution yield model‐predicted intensity enhancements, energy spectra, and pitch angle distributions for comparison with spacecraft observations. It is shown that the final energy spectrum can be harder than, softer than, or equal to the ambient spectrum, depending upon, for example, the point along the final spectrum one chooses to examine, whether one looks at the upstream of downstream spectrum, and how rapidly the ambient spectrum decreases with increasing energy. It is shown that for a given form of the ambient energy spectrum, an upstream ambient angular distribution peaked toward (away from) the shock along the magnetic field yields a final energy spectrum that is harder (softer) in both the upstream and downstream regions than the spectrum produced for an isotropic ambient distribution. The single‐encounter scheme is shown to account for many features observed in the short‐lived, impulsive shock‐spike events. In particular, the existence of the high‐intensity spike in close temporal association (of the order of minutes) with the shock passage is consistent with a kinematical accumulation of particles accelerated and transmitted downstream of the shock. The single‐encounter acceleration process is also consistent with some features of energetic storm particle events observed at radial distances of 14–22 AU from the sun.
Second‐order effects related to a model for a parallel shock
Authors: Jackson R. W.
Published: February 2008

Second‐order effects are calculated for a low‐frequency electromagnetic instability due to an ion beam in a plasma. This instability is a characteristic of a parallel shock model that includes an ion reflecting electrostatic subshock. The analysis is similar to that done by S. P. Gary, but a counterstreaming configuration is chosen that is homogeneous in time and that has specially growing modes. Among other effects, energy is transferred from the incoming main plasma ions to the beam ions. Electron pressure effects are calculated and are small for low frequencies. The application of this model to the earth's bow shock is discussed.
Electric field measurements at the magnetopause: 1. Observation of large convective velocities at rotational magnetopause discontinuities
Authors: Aggson T. L. , Gambardella P. J. , Maynard N. C.
Published: February 2008

Large convective electric fields of the order of 10 mV/m (sometimes as high as 22 mV/m) are observed at rotational magnetopause discontinuities. These observations were made with the long cylindrical (179‐m base line) probes carried on the ISEE 1 satellite. These electric field observations yield convective velocity magnitudes, V* = |E×B/B²|, of the order of 150 km/s. In this V* format some of these observations are similar to the high speed plasma velocity observations that were made at the magnetopause with the plasma experiment carried on the ISEE 1 satellite. It is shown that, for many of these magnetopause crossings, there exists a special moving coordinate system where the observed electric fields vanish. Such a unique reference system is often used in theoretical studies of magnetic discontinuities. This special coordinate system does not move at the local plasma velocity but moves instead at a velocity intermediate between the convective velocity and the local Alfvén velocity. It is used here as a diagnostic tool for the experimental investigation of rotational discontinuities at the magnetopause.
A theoretical model study of observed correlations between whistler mode waves and energetic electron precipitation events in the magnetosphere
Authors: Chang H. C. , Inan U. S.
Published: February 2008

A recently extended test particle computer model of the gyroresonance wave‐particle interaction in the magnetosphere is applied to previously reported cases of observed correlations between whistler mode waves and ionospheric responses to particle precipitation. Three different ionospheric effects, X ray bursts, photoemissions, and D region perturbations, all correlated with VLF waves and believed to be caused by precipitated particles, are considered. The precipitation flux level, the pulse shape, and the associated time delays are computed for the parameters relevant to each case and are compared with values deduced from the data. The results demonstrate that the existing theoretical model can be useful for interpreting experimental results of this kind. Furthermore, the model results and observations, used together, provide a basis for additional diagnostics of the various parameters of the cold and energetic particle distributions in the magnetosphere. For example, when applied to the observed photoemission case (Helliwell et al., 1980) the model results imply that the trapped energetic particle distribution function at the time could be modeled as proportional to E−n/2 with n ≃ 3.5 to 6, where E is the particle energy.
Emissions of second‐harmonic auroral kilometric radiation
Authors: Wu C. S. , Qiu X. M.
Published: February 2008

Second‐harmonic auroral kilometric radiation (AKR) is discussed within the context of the cyclotron maser theory by Wu and Lee. The results are in good agreement with the ISIS 1 observation reported by Benson. The major conclusions are as follows: (1) when ωe/Ωe < 0.3 (where ωe is the electron plasma frequency and Ωe is the electron gyrofrequency), the fundamental AKR prevails over the second‐harmonic AKR; (2) when ωe/Ωe ≳ 0.3, the fundamental AKR may become weaker than the second‐harmonic AKR; (3) when ωe/Ωe > 0.3, the fundamental AKR may diminish; and (4) the second harmonic AKR with frequencies below 2Ωe can propagate downward.
Multistation measurements of high‐latitude ionospheric convection
Authors: Heelis R. A. , Foster J. C. , de la Beaujardiere O. , Holt J.
Published: February 2008

Satellite and ground‐based observations of the ionospheric drift velocity taken during a MITHRAS campaign have been combined to determine instantaneous pictures of the high‐latitude convection pattern. These data, taken when the interplanetary magnetic field has a relatively stable southward/away orientation, show the existence of an asymmetric convection pattern under these conditions. A stability in the high latitude convection geometry can also be seen and changes in response to magnetic disturbances are inferred. Changes in the convection pattern as the interplanetary field turns northward possibly provide some information about the nature of the magnetosphere‐solar wind interaction.
An analysis of the spatial distribution of dayside cleft optical emissions
Authors: Link R. , McConnell J. C. , Shepherd G. G.
Published: February 2008

An analysis of measurements of the 4278‐Å, 5200‐Å, 5577‐Å, and 6300‐Å emissions obtained from a rocket launch through the postnoon dayside cleft into the polar cap ionosphere is reported. The 4278‐Å and 5577‐Å emissions show a clearly defined decrease in intensity at the poleward edge of the dayside particle precipitation, while the 5200‐Å emission extends well into the polar cap with only a slight decrease in intensity. The 6300‐Å emission displays a behavior intermediate between these extremes. The optical data are analysed by using a dynamical model of dayside cleft auroral processes incorporating transport by ion and neutral winds. It is concluded that neutral winds are responsible for modifying the spatial distribution of the 5200‐Å and 6300‐Å emission, while the ion drift component is negligible. In addition, it is found that (1) 80% of the 4278‐Å emission intensity is due to resonance fluorescence in the sunlit cleft ionosphere, (2) the reaction O2(c¹Σu−) + O → O2 + O(¹S) is consistent with the 5577‐Å data if the yield of O(¹S) from the dissociative recombination of O2+ is about 2%, and (3) the rate of the reaction N2+ + O → NO+ + N is apparently higher in the ionosphere than indicated by the laboratory measurements.
O(¹S) from dissociative recombination of O2+: Nonthermal line profile measurements from Dynamics Explorer
Authors: Killeen T. L. , Hays P. B.
Published: February 2008

Direct measurements at high spectral resolution of the O(¹S) emission line profile at 5577 Å made using the Fabry‐Perot interferometer on Dynamics Explorer are reported. The line profile is found to have marked nonthermal characteristics in the nightglow. The nighttime emission line profiles, measured in the equatorial region, have been analyzed using a simple collisional relaxation model. The branching ratio for the dissociative recombination of O2+ leading to O(¹S) is inferred. The result shows that the O(¹S) + O(¹D) channel is favored over the O(¹S) + O(³P) channel by a factor of 4, in disagreement with the ratio previously reported by Hernandez. This result is, however, consistent with the active channel for O(¹S) production being via the ¹Σu+ repulsive state of O2 as suggested by the theoretical calculations of Guberman. A value for the excitation exchange cross section for O(¹S) of 2.45±0.8×10−16 cm² is obtained.
A reexamination of ATS 6 magnetometer data for radially polarized Pc 3 magnetic pulsations
Authors: Takahashi Kazue , McPherron Robert L.
Published: February 2008

The polarization of Pc 3 (22–100 mHz) magnetic pulsations measured by the ATS 6 fluxgate magnetometer at synchronous orbit has been examined by using dynamic autospectral analysis. In contrast to the result obtained by Arthur et al. [1977] using the same data set, we find very few cases of radially polarized Pc 3 pulsations. We argue that satellite noise in the radial component, which depends on frequency f as 0.015/f (nT²/Hz), is responsible for this disagreement. In the presence of this type of noise, diagonalization of the spectral matrix can produce an erroneous major axis of polarization. Most Pc 3 pulsations classified as radially polarized by Arthur et al. appear to be a consequence of small‐amplitude azimuthal pulsations contaminated by satellite noise.
Short wavelength ion waves upstream of the Earth's bow shock
Authors: Fuselier S. A. , Gurnett D. A.
Published: February 2008

The identification and explanation of short wavelength antenna interference effects observed in spacecraft plasma wave data have provided an important new method of determining limits on the wavelength, direction of propagation, and Doppler shift of short wavelength electrostatic waves. By using the ISEE‐1 wide‐band electric field data, antenna interference effects have been identified in the ion waves upstream of the earth's bow shock. This identification implies that wavelengths of the upstream ion waves are shorter than the antenna length. The interference effects also provide new measurements of the direction of propagation of the ion waves. The new measurements show that the wave vectors of the ion waves are not parallel to the interplanetary magnetic field (IMF) as previously reported. The direction of propagation does not appear to be controlled by the IMF. In addition, analysis of the Doppler shift of the short wavelength ion waves has provided a measurement of the dispersion relation, ω0(k). The upper limit on the rest frame frequency was found to be on the order of the ion plasma frequency. At this frequency, the wavelength is on the order of a few times the Debye length. The results of this study now provide strong evidence that the ion waves in the upstream region are Doppler‐shifted ion acoustic waves.
ELF emissions and relativistic electron precipitation
Authors: West R. H. , Parks G. K.
Published: February 2008

Simultaneous measurement of ELF emissions and bremsstrahlung X rays were made by using a balloon borne experiment flow from Fairbanks, Alaska, on September 30, 1976. Several intense auroral X ray bursts of 2–3 minute duration were detected that have been interpreted to be the result of relativistic electron precipitation (REP). Simultaneously, the intensity of ELF emissions increased and was also observed to have periodicities similar to those in the X ray flux. These combined measurements are evidence of magnetospheric wave‐particle processes contributing to electron precipitation. The presence of pulsations in the precipitating flux is the only report of this nature during relativistic precipitation. The data have been compared with precipitation and modulation theories, and we note that the data agree with a model in which ULF modulation of the emission growth rate occurs. It is postulated that this REP event resulted from ELF wave and electron interaction, possibly triggered by the ULF occurrence rather than directly by ULF electron interactions.
Ion cyclotron waves as a possible source of resonant auroral radar echoes
Authors: Fejer B. G. , Reed R. W. , Farley D. T. , Swartz W. E. , Kelley M. C.
Published: February 2008

Auroral backscatter radar observations were made from Ithaca, New York, at 50 MHz during the early morning of April 1, 1976, a period of high magnetic disturbance (Kp ∼ 8). The backscattered power showed large rapid (time scale of a few minutes or less) variations, characteristic of discrete radar aurora, from L = 3.5–4. Doppler spectra of waves propagating in nearly the north‐south direction from up to 28 different ranges were obtained simultaneously with good spatial (7.5 km) and temporal (2 s) resolution. Some unusual spectra with very narrow peaks at Doppler shifts between about 70 and 90 Hz were observed and were apparently associated with sheared flows of the auroral plasma. These echoes were observed at the southern edge of the scattering region, which moved poleward with a velocity which sometimes exceeded 800 m/s. There is evidence that the echoes were generated above about 140 km. Present electrojet instability theories cannot explain these narrow spectra, but the Doppler shift suggests that they might be caused by ion cyclotron waves generated by field‐aligned currents (electron drifts). However, the excitation of ion cyclotron waves requires currents considerably stronger than typically quoted mean values (∼10−5 µA/m²). Furthermore, the most easily excited ion cyclotron waves have wavelengths longer than 3 m and do not propagate in quite the right direction for detection by our radar. The threshold for the waves that we observe is considerably higher than the minimum. Intense localized currents, such as those known to be associated with auroral curls and vortices (J ≳ 10−4 µA/m²), appear to be sufficient to excite the longer (∼10–20 m) ion cyclotron waves in the upper E region; whether or not the field‐aligned electron drifts are ever large enough to excite directly the 3‐m waves we observe is an open question, but there is at least some evidence which suggests that such drifts do exist in very localized regions during highly disturbed conditions.
Simplified derivation of an algorithm for nonlinear gravity waves
Authors: Weinstock J.
Published: February 2008

A simplified proof is given that the linear dispersion relation for gravity waves is converted into a nonlinear dispersion relation by making the replacement (ωk ‐ k ·u0)→(ωk ‐ k ·u0 ‐ idw), where ωk is the wave frequency, k is the wave vector, u0 is the mean flow, and dw is a nonlinear damping rate explicitly determined by the rms wave velocity. This derivation differs from a previous one in its brevity as well as relative simplicity. The purpose is to present a derivation that does not require previous familiarity with the statistical theory of strong nonlinear interactions and to reveal more explicitly the underlying assumptions and approximations. A substance difference is that a spacetime average is used instead of an ensemble average, and the theory is no longer restricted to random phased waves. The physical significance of dw is then elucidated and related to recent experimental observations. Limitations of the derivation are to a quasi‐stationary, homogeneous state in which u0 and other average quantities vary only slowly, to wavelengths satisfying 2kzH ≫ 1, and to wave amplitudes that are too large to be treated by weak mode‐coupling theories.
Anomalous transport by magnetohydrodynamic Kelvin‐Helmholtz instabilities in the solar wind‐magnetosphere interaction
Authors: Miura Akira
Published: February 2008

A magnetohydrodynamic simulation of Kelvin‐Helmholtz instabilities in a compressible plasma has been performed for parallel (v0∥B0) and transverse (v0⊥B0) configurations, modeling high‐latitude (or downstream flanks) and dayside low‐latitude magnetospheric boundaries. In the parallel configuration, a super‐Alfvénic and transsonic shear flow (with 2 < MA = V0/υA < 4 and Ms = V0/cs = 1, where V0 is the total jump of the velocity across the velocity shear layer) leads to an oscillation of the velocity shear layer, which bends the initial uniform magnetic field. With a hyper‐Alfvénic shear flow (MA > 4), the instability develops into a more turbulent state and the initial parallel shear flow develops into small eddies, which strongly twist, compress, and hence amplify the magnetic field by a dynamo action with an amplification factor MA/2. In the nonlinear stage, however large the initial Alfvén Mach number MA may be, the magnetic field, amplified and twisted by the hydromagnetic flow vortices, eventually reacts back upon the flow evolution, and the flow vortices cascade into smaller scale structures. In the transverse configuration, for a fast magnetosonic Mach number Mf (= V0/(cs² + υA²)1/2) greater than a critical Mach number, the instability leads to the formation of a fast shock discontinuity from an initially subfast shear flow. Anomalous tangential stress in the transverse configuration reaches 0.01ρ0V0², and the energy flux across the boundary in the magnetospheric inertial frame reaches as much as 2% of the magnetosheath flow kinetic energy flux, ½ρV0³; this viscous tangential stress could account for the convection potential drop over the polar cap of 10–30 kV. The anomalous (eddy) viscosity νano becomes >10−2 ΔV0, where Δ is the thickness of the initial velocity shear layer, and becomes as large as or even larger than the Bohm diffusion for conditions typical at the magnetospheric boundary, thereby satisfying the requirements of the “viscouslike” interaction hypothesis by Axford and Hines (1961). In the parallel configuration, the slow rarefactive wave (or Alfvén wave in the incompressible limit) is excited by the instability and contributes to a strong anomalous diffusion of momentum or a dissipation of vorticity by the magnetic viscosity (Maxwell stress); this in turn gives rise to momentum and energy fluxes and an anomalous viscosity 2–3 times larger than those brought about by the hydrodynamic Reynolds stress in the transverse configuration and hence provides a very efficient viscous interaction at the magnetospheric boundary.
The inner edge of the plasma sheet and the diffuse aurora
Authors: Fairfield D. H. , Viñas A. F.
Published: February 2008

Three‐dimensional measurements from the ISEE 1 low energy electron spectrometer are used to map the location of the inner edge of the plasma sheet and study the anisotropies in the electron distribution function associated with this boundary. At lower energies the plasma sheet electrons have inner edges closer to the earth than higher energies with the separations at different energies being larger near dawn and after dusk than at midnight. Lowest energy inner edges are frequently located adjacent to the plasmapause in the dawn sector but are often separated from the average plasmapause in the dusk sector by a gap of at least several RE. The energy dispersion is minimal in the afternoon quadrant where the inner edge is near the magnetopause and frequently oscillating on a time scale of minutes. The location of the inner edge is probably determined primarily by the motion of electrons in the existing electric and magnetic fields rather than by strong diffusion, as has sometimes been supposed. Evidence against strong diffusion is (1) boundaries closer to the earth than would occur if strong diffusion were operating and (2) the frequent and persistent occurrence of anisotropies that would be rapidly smoothed if pitch angle scattering proceeded at the strong diffusion rate. These anisotropies include 90° pitch angle depletions at the inner edge and 90° enhancements in the surrounding regions. Both these anisotropies are predicted by calculations of single particle motion; the former are apparently due to more field‐aligned particles having inner edges slightly closer to the earth than 90° particles, and the latter are apparently due to the preferential adiabatic acceleration of 90° particles drifting in cross‐tail electric and dipolelike magnetic fields. These anisotropic distribution functions created by normal particle motion contain free energy that may be that necessary to drive the electrostatic electron cyclotron instabilities thought to be responsible for generating the waves that precipitate particles of the diffuse aurora. An additional cool component of the plasma is also observed that should also influence these instabilities.
Electrostatic ion (hydrogen) cyclotron and ion acoustic wave instabilities in regions of upward field‐aligned current and upward ion beams
Authors: Bergmann Rachelle
Published: February 2008

An investigation is made into the stability of electrostatic hydrogen ion cyclotron and ion acoustic waves in a model plasma where an ion beam, population 2, and oppositely directed drifting electrons pass through a stationary ion background, population 1. The excited wave properties are then compared with the characteristics of the unstable modes observed on the S3‐3 satellite. Three temperature regimes are studied: (1) Te > Ti2 ≫ Ti1, (2) Ti2 > Te ≥ Ti1, and (3) Te ∼ Ti1 > Ti2. It is found that the ion beam acts as a free energy source only in regime 1. This regime is also highly unstable to the electrons as a free energy source. Unstable modes in regimes 2 and 3 seem to best satisfy the electrostatic hydrogen cyclotron wave (EHC) properties at 1RE. For these cases the electrons are the free energy source, the beam supplies damping.
Evidence for particle acceleration processes in the magnetotail
Authors: Kirsch E. , Pavlos G. P. , Sarris E. T.
Published: February 2008

Measurements of energetic protons (E ≥ 50 keV), electrons (E ≥ 30 keV), α particles (E ≥ 0.64 MeV) and Z ≥ 3 particles (E ≥ 2 MeV) in the distant magnetotail by the Energetic Particle Experiment (NOAA, Boulder) and the JHU/APL proton‐electron telescope on board the IMP 8 satellite for the period November 23–26, 1973, are used to investigate the particle acceleration process in the magnetotail. Evidence is presented that the plasma heating process starts 20–30 min before energetic particle bursts, which suggests that most probably the particle energization in this event takes place in two different phases: (1) up to E ∼ 50 keV in association with the plasma heating process and (2) explosive acceleration in excess of E ≳ 1 MeV energy.
Ion‐beam‐driven electrostatic hydrogen cyclotron waves on auroral field lines
Authors: Okuda H. , Nishikawa K.‐I.
Published: February 2008

It is shown analytically and by means of plasma numerical simulations that the large amplitude hydrogen cyclotron waves on auroral field lines near 1 RE altitude may be excited by upward flowing ion beams rather than by field‐aligned currents. The large amplitude electric field near ω ≈ 0 is identified as the ion gyro‐harmonic waves on the beam ions observed in the frame of background ions and, therefore, such a wave exists only when the instability is excited by ion beams. Both the theory and numerical simulations support the conclusion.
The 40‐keV electron durable trapping region
Authors: Feynman J. , Hardy D. A. , Mullen E. G.
Published: February 2008

The position and extent of the region in which electrons with energies less than 40‐keV are durably trapped in the nightside magnetosphere is found for both normal and disturbed geomagnetic conditions by using data from the P78‐2 (SCATHA) satellite. The region of the magnetosphere from 5.3 to 7.9 RE was studied. In this region neither solar‐magnetic nor geocentric‐solar magnetospheric coordinates order the data satisfactorily. A new coordinate system called composite coordinates is introduced. It takes account of the fact that this region of the magnetosphere is strongly influenced by both the earth's dipole field and the direction of the solar wind. In composite coordinates when Kp≤4+, 40‐keV electron fluxes were almost continuously present in a region centered on the equatorial plane and 1.2 RE in half width. At larger composite coordinate latitudes there is another region more than 1 RE thick within which 40‐keV electron fluxes routinely appear and disappear on time scales of one hour as the trapping boundary actively moves over the satellite. We have no evidence that SCATHA ever entered the tail lobes where no particles are trapped. When Kp≥6− the region in which 40‐keV electron fluxes were always present moved earthward and/or thinned but remained ordered in composite coordinates. We suggest that the new coordinate system will be useful for ordering other data sets taken in this region of the magnetosphere.
The zonally averaged circulation, temperature, and compositional structure of the lower thermosphere and variations with geomagnetic activity
Authors: Roble R. G. , Kasting J. F.
Published: February 2008

A zonally averaged chemical‐dynamical model of the thermosphere is used to examine the effect of high‐latitude particle and Joule heating on the neutral composition, temperature, and winds at solstice for solar minimum conditions. The meridional circulation forced by solar heating alone is a summer‐to‐winter flow, with a winter enhancement in atomic oxygen. The high‐latitude heat sources drive mean circulation cells that reinforce the solar‐driven circulation in the summer hemisphere and oppose this circulation in the winter hemisphere. The changes in wind and temperature caused by the high‐latitude heat sources increase the relative concentration of N2 and O2 in the high‐latitude upper thermosphere and decrease the O concentration in the high‐latitude lower thermosphere. For prolonged moderate levels of geomagnetic activity the peak atomic oxygen density in the polar regions can decrease by factors of 2–3 from geomagnetic quiet conditions.
The global distribution of thermospheric odd nitrogen for solstice conditions during solar cycle minimum
Authors: Gérard J.‐C. , Roble R. G. , Rusch D. W. , Stewart A. I.
Published: February 2008

A two‐dimensional model of the minor neutral constituents N(²D), N(4S), and NO from 70 to 360 km is formulated and used to investigate the global distributions of these species for December solstice conditions during solar cycle minimum. For background properties the zonally averaged circulation, temperature, and compositional structure of major neutral constituents in the thermosphere are obtained from the coupled dynamical chemical model calculations of Roble and Kasting (this issue). Transport of the minor neutral constituents by both the meridional and vertical circulation and vertical molecular and eddy diffusion are considered in the model. The calculated distributions of the minor neutral constituents for solar ionization only and for solar plus auroral ionization are considered. The results show that the summer‐to‐winter pole meridional circulation transports both NO and N(4S) across the solar terminator into the polar night region where there is a downward vertical transport toward the mesosphere. The calculated latitudinal distribution of NO in the lower thermosphere for the solar‐ionization‐only case does not agree with the latitudinal NO distribution usually measured by the Atmosphere Explorer satellites. To obtain agreement between the calculated and observed structure, it is necessary to include ionization and dissociation sources due to auroral particle precipitation. The temperature and compositional structure and the circulation changes caused by high‐latitude heating and the particle‐induced production of N(²D) and N(4S) all combine to bring the calculated NO structure into better agreement with observations. The choice of the eddy diffusion coefficient critically controls the downward flow of thermospheric nitric oxide into the mesosphere. The model shows that the odd nitrogen densities at high winter latitudes are entirely controlled by particle precipitation and transport processes.
Patterns of potential magnetic field merging sites on the dayside magnetopause
Authors: Luhmann J. G. , Walker R. J. , Russell C. T. , Crooker N. U. , Spreiter J. R. , Stahara S. S.
Published: February 2008

Models of the magnetospheric and magnetosheath magnetic fields are used to determine the relative orientations of these fields at the dayside magnetopause in order to locate potential merging sites. Areas on the magnetopause with different fractional antiparallel components are displayed by contour diagrams for a variety of interplanetary field orientations. For interplanetary fields oriented perpendicular to the solar wind velocity the areas of nearly antiparallel field agree with those obtained by Crooker using simplified representations for the magnetic field geometry. Here, the application of more realistic models gives the locations of areas where any antiparallel component occurs. Potential merging sites for interplanetary fields with radial components are also illustrated. The results suggest that the topology of the magnetosheath and magnetospheric fields provides antiparallel components over a substantial fraction of the magnetopause for most interplanetary field orientations.