bluuebunny/crossref_metadata_2025 huggingface dataset:
Auroral morphology of the midday oval
Authors: Meng C.‐I. , Lundin R.
Published: February 2008

Auroral displays in the noon sector were examined by using hundreds of Defense Meteorological Satellite Program auroral images, taken over the southern polar region in austral winters, in order to determine the morphology. The auroral displays in the midday part of the auroral oval can be grouped into five characteristic types, depending on the geomagnetic activity and the Bz component of the interplanetary magnetic field. An important characteristic is the clear disconnection in appearance between the noonside and the nightside discrete auroras. Also, there is the lack of correlation between the concurrent nightside substorm activity and midday discrete auroral activity. Thus, the occurrence of bright, discrete auroras in the midday oval may be caused by the local injection of the magnetosheath plasma into the dayside boundary layer. The observed discontinuity of discrete auroras between the dayside and the nightside ovals is consistent with the existence of two separated major injection regions along the auroral oval: the dayside cusp and the nightside plasma sheet.
Magnetospheric origin of energetic (E ≥ 50 keV) ions upstream of the bow shock: The October 31, 1977, event
Authors: Anagnostopoulos G. C. , Sarris E. T. , Krimigis S. M.
Published: February 2008

Simultaneous observations of energetic ions (≳50 keV) and electrons (≳220 keV) by the IMP‐7 and IMP‐8 spacecraft carrying identical instruments and located within the distant (∼37 Re) magnetotail and upstream of the bow shock, respectively, are reported during the period 0000–1100 UT on October 31, 1977. During this period, ISEE‐1 detectors had detected an upstream ion event [Ipavich et al., 1979]. The IMP observations, including measurements of the interplanetary magnetic field (IMF), reveal the following: (1) energetic ions and electrons were present inside the plasma sheet ≳ 1 hour prior to onset of the event upstream at IMP‐8. (2) Ion (50 ≤ E ≲ 200 keV) intensities inside the plasma sheet were always higher than those upstream by factors of ∼2 to ∼8. (3) The ion energy spectrum upstream of the shock can be fit well by a power law, dj/dE ∝ E−4.1, from ∼50 keV to ∼2 MeV; observations at higher energies were precluded by high fluxes of ambient solar energetic particles. (4) Dawn‐dusk anisotropies of upstream ions as large as 5∶1 are interpreted as due to density gradients; anisotropies in the plasma sheet were, at times, both earthward and tailward. (5) The presence of ions upstream was controlled by the direction of the IMF: connection to the bow shock was necessary, but not sufficient, for the appearance of ions. These observations show that ions observed upstream of the shock on October 31, 1978, most likely originated from within the earth's plasma sheet; a phenomenological model is suggested for such particle “leakage.” The observations are inconsistent with most major predictions of the Fermi acceleration model for such events, including the energy extent, the shape of the ion spectrum, the particle anisotropies, and the IMF connection time.
Viscosity in the solar wind
Authors: Holzer Thomas E. , Leer Egil , Zhao Xue‐Pu
Published: February 2008

The effects of viscosity on a steady, radial, spherically symmetric solar wind with an embedded, non‐radial magnetic field are reconsidered. The correct expression for the classical viscosity in the presence of a non‐radial magnetic field is shown to be different from that used in the past, and a means of describing non‐classical viscosity is presented. A physical interpretation of the classical and nonclassical descriptions of viscosity is provided, and observational inferences are used in discussing the nature and degree of viscous effects in the solar wind.
The importance of being earnest about shock fitting
Authors: Hsieh K. C. , Richter A. K.
Published: February 2008

To identify observed plasma and magnetic field discontinuities in interplanetary space as magnetohydrodynamic shocks requires fitting the plasma and magnetic field data to Rankine‐Hugoniot conditions. By statistics and specific samples we show that unless the fluctuations in the observed parameters in the preshock and postshock regions are seriously taken into consideration, careful fitting may still lead to erroneous identification and description of a shock. An approach to the unbiased choice of the preshock and postshock values is to systematically consider all possible combinations of preshock and postshock parameters averaged over different time intervals measured away from the shock.
Major flattening of the distant geomagnetic tail
Authors: Sibeck D. G. , Siscoe G. L. , Slavin J. A. , Lepping R. P.
Published: February 2008

We quantify the Michel‐Dessler magnetotail cross section model, which predicts that the magnetotail flattens due to the anisotropic pressure of draped magnetosheath field lines. We assume steady, typical, solar wind and magnetotail conditions and allow flux to pass through both the current sheet and magnetopause. With these assumptions, our numerical model predicts that the semimajor axis of the elliptical magnetotail cross section at x = −200 RE is 37.8 RE long and lies in the solar ecliptic plane. The semiminor axis (perpendicular to the ecliptic plane) should be no greater than 18.5 RE at this distance. By x = −500 RE, the semimajor and minor axes should be 60 and 11.7 RE long, respectively. The ratio of the major to minor axis lengths depends on the strength of the interplanetary magnetic field (IMF) in the plane transverse to the solar wind flow and is greater for a strong IMF. The reaction time of the distant magnetotail to an impressed IMF variation is on the order of 20 minutes for typical solar wind conditions. The locations of ISEE 3 magnetppause crossings and magnetotail (including boundary layers) observations beyond x = −180 RE are consistent with the model cross section. Our definition of the magnetotail requires
or Te > 5×105 K. Furthermore, magnetotail magnetopause boundary normals point nearly parallel to the semiminor axis of the ellipse and therefore indicate strong flattening. We look for, but do not find, evidence of flattening in prior observations of the near‐earth magnetotail cross sectional dimensions. Some Pioneer 7 and 8 observations of the deep tail (x < −500 RE) are consistent with magnetotail flattening.
Energetic particle beams in the plasma sheet boundary layer following substorm expansion: Simultaneous near‐Earth and distant tail observations
Authors: Scholer M. , Baker D. N. , Gloeckler G. , Klecker B. , Ipavich F. M. , Terasawa T. , Tsurutani B. T. , Galvin A. B.
Published: February 2008

We present simultaneous observations of energetic particles by ISEE 1 in the near‐earth geomagnetic tail and by ISEE 3 in the distant geomagnetic tail following the onset of a major substorm as indicated by ground magnetograms and synchronous orbit particle data. At substorm onset, both spacecraft are in the tail lobes. About 8 min after onset, ISEE 1 enters the plasma sheet; at the plasma sheet boundary, earthward directed ion beams are observed. At the same time (within the 128‐s time resolution) tailward directed ion and electron beams are observed at ISEE 3. These beams persist intermittently for 25 min. The observations are explained in terms of the substorm neutral line model, wherein the near‐earth and distant plasma sheet are disconnected after reconnection has proceeded to the last closed field line within the plasma sheet. Electron plasma and magnetic field data on ISEE 3 show that during the whole time period, ISEE 3 stays in the distant lobe and does not enter the plasma sheet. The plasmoid, expected to be released after reconnection has proceeded to the lobe field, is sensed by ISEE 3 as a traveling compression region. The velocity dispersion effects during the appearance and disappearance of the beams indicate that these beams are not due to temporal effects but due to crossing of layers with particles of different velocity emitted from a steady source. From the observation of gradient anisotropy effects in the high‐energy protons, the spacing of the layers of particles with different velocity can be derived. This allows determination of the maximum electric potential along the new neutral line. The corresponding electric field is considerably smaller than the energy of the highest energy particles observed. It is argued that the plasma sheet close to the substorm‐generated neutral line contains high fluxes of high‐energy particles that may be sufficient to produce the high‐energy boundary layer around plasmoids. Acceleration of the high energy particles may mainly occur during substorm recovery.
Localization of sources of ELF/VLF hiss observed in the magnetosphere: Three‐dimensional ray tracing
Authors: Cairó L. , Lefeuvre F.
Published: February 2008

Numerical simulation based upon three‐dimensional ray‐tracing from a single source point located in the equatorial region is used to explain the wave distribution functions (WDF) found for ELF/VLF hiss detected on the GEOS satellite. Emissions at very oblique k vectors and in all azimuthal directions are shown to produce the GEOS‐1 multipeaked WDFs at the longitudes and L values close to the longitudes and L values of a chosen single source point. Inverse ray‐tracings, started with the parameters of the WDF's observed emissions, are used to locate the sources.
The tail lobe ion spectrometer
Authors: Horwitz J. L.
Published: February 2008

A two‐dimensional kinetic “tail lobe ion spectrometer” model for the transport of ionospheric ions from the polar cleft ionosphere into the tail lobes is developed to semiquantitatively simulate the behavior of the observed O+ ion streams in the magnetotail lobes. The consequences of the present model include (1) the increase in velocity of the O+ streams away from the tail midplane; (2) the existence of “tongues” of O+ density from the cleft ionosphere into the tail lobes whose distribution configuration depends on the convection electric field and the source thermal energies; and (3) very low parallel thermal energies and speeds in the lobes which are correlated with the parallel ion bulk velocities. These consequences are reasonably consistent with the trends in lobe streams observed by recent spacecraft.
Observations of ionospheric magnetospheric coupling: DE and Chatanika coincidences
Authors: Green J. L. , Waite J. H. , Chappell C. R. , Chandler M. O. , Doupnik J. R. , Richards P. G. , Heelis R. , Shawhan S. D. , Brace L. H.
Published: February 2008

Observations from several experiments on board the Dynamics Explorer 1 and 2 (DE 1 and 2) spacecraft and ground‐based radar measurements from the Chatanika radar are combined in order to examine the details of ionospheric/magnetospheric coupling in the local evening sector. DE 1 and DE 2 were in coplanar polar orbits that provided measurements almost simultaneously in time and magnetically coincident with the Chatanika radar from L = 3 to L = 17. The coupling processes are inferred from the density, temperature, composition, and angular distributions of the low‐energy plasma observed from the E region of the ionosphere to magnetospheric altitudes of 2.5 earth radii (RE). Plasma characteristics of the plasmasphere, main trough, auroral zone, and polar cap can be studied in this data set. In the comparison of the data, it is clear that the only ionospheric trace of the plasmapause is seen as an increase of the electron temperature. The minimum plasma density in the high‐latitude trough of the ionosphere is magnetically coincident with a region of observed low‐energy bidirectional conical distributions of H+ and He+ at the DE 1 altitude of 1.5 to 1.8 RE. Polarward of this region, DE 1 observes unidirectional conies consisting of H+ and He+ ions that are magnetically coincident with low‐altitude (150‐km) auroral ionization as observed by the Chatanika radar. Over the polar cap, only field‐aligned outward flows of H+, He+, and O+ are seen. These observations imply that as L increases, the dominant coupling mechanism between the ionosphere and magnetosphere in the measured energy range changes from equilibrium diffusion to perpendicular acceleration and, finally, to parallel acceleration.
A numerical model of magnetosphere‐ionosphere coupling: Preliminary results
Authors: Watanabe K. , Ashour‐Abdalla M. , Sato T.
Published: February 2008

A three‐dimensional simulation model was developed to study magnetosphere‐ionosphere coupling in the auroral region. We adopted one‐fluid MHD equations to model the magnetosphere, and current density continuity equations were solved consistently to model the ionosphere. In the preliminary simulation runs described here, the electrodynamics of region 1 field‐aligned currents were modeled. Initially, the electric field is taken to be in the magnetospheric equatorial plane. Alfvén waves then propagate down to the ionosphere, accompanying a field‐aligned current and exciting the electrostatic potential in the ionosphere by electrodynamic coupling. The results of the preliminary runs directly correspond with the fundamental characteristics of global magnetosphere‐ionosphere coupling. These characteristics include an ionospheric electrostatic potential which varies in its development in time, depending upon the ratio of ionospheric resistance to magnetospheric impedance, the flowing of field‐aligned currents into the ionosphere on the dawnside and out on the duskside, and the distribution along the geomagnetic field of the amplitude of the field‐aligned current density in proportion to the field intensity.
A procedure for estimating the changes in cosmic ray cutoff rigidities and asymptotic directions at low and middle latitudes during periods of enhanced geomagnetic activity
Authors: Flückiger E. O. , Smart D. F. , Shea M. A.
Published: February 2008

A procedure is presented for estimating the changes in the cosmic ray cutoff rigidity and the asymptotic directions during geomagnetically active time periods at low‐ and mid‐latitude locations. At a specific location the change in the rigidity associated with the first discontinuity in asymptotic longitude, ΔR1, and the equivalent changes in the other cutoff rigidity parameters can be related to the weighted sum of the changes in the horizontal component of the equatorial surface magnetic field, ΔHeq, sampled at intervals of 1 hour in local time from 0000 to 0700 to the east of the specified location. A set of generalized weighting factors is given for eight cutoff rigidity values in the range from 3 to 13.5 GV. For a cosmic ray particle having a rigidity R the asymptotic direction of approach during perturbed conditions corresponds in good approximation to the asymptotic direction of a particle with rigidity R − ΔR1 propagating through the quiescent geomagnetic field. Asymptotic directions during geomagnetically active periods can therefore be derived from the asymptotic directions computed in a quiescent geomagnetic field model after the associated change in the cutoff rigidity has been deduced from ground‐based magnetometer data. The basic principles of the procedure are demonstrated for the cosmic ray station Jungfraujoch, Switzerland.
Altitude and latitude effects of atmospheric neutrons
Authors: Bhatt V. L.
Published: February 2008

Ilford G5 emulsions were exposed at six altitudes from 4 to 40 mbar at λ = 7.8°N geomagnetic latitude (NGL). Atmospheric neutrons of 10–200 MeV are studied using three independent parameters. Spectra at 0 mbar were obtained by extrapolation. Energy spectra for neutrons were studied for seven different altitudes from 4 to 40 mbar. Spectra at 0 g/cm² for total neutrons and the upward moving albedo are compared with the experimental and theoretical results of other authors. Atmospheric depth curves for total neutrons and albedo neutrons are given, and results are compared with theory and other experiments. A study of the latitude effect gives a variation by a factor ∼2.3 for λ = 7.8° to λ = 41°NGL for total neutrons and a little higher value of 2.5 for albedo neutrons of 10–100 MeV, in good agreement with theory and the experimental results of other workers.
Configurations of corotating shocks in the outer heliosphere
Authors: Burlaga L. F. , Klein L. W.
Published: February 2008

Configurations of corotating shocks in the outer heliosphere are computed on the assumption that the shocks move at constant speeds between collisions. The basic physical process is the interaction of a forward shock with a reverse shock. The points at which this interaction takes place are determined algebraically in terms of the initial shock positions and speeds. Between the points at which the shocks interact, the shocks have the form of Archimedian spirals. Shock configurations are determined for the case of two corotating shock pairs originating at 2.5 AU and for a single shock pair at 2.5 AU. In both cases the heliosphere is divided into several distinct regions distinguished by the number of times the particles have passed through a shock. Since the shock strengths decrease when a forward shock interacts with a reverse shock, corotating shocks are expected to be weaker at larger distances from the sun.
Comparison of magnetic field structures in quasi‐parallel interplanetary shocks: Observations versus simulations
Authors: Mandt M. E. , Kan J. R. , Russell C. T.
Published: February 2008

A first step in a comparative study is made between simulations and ISEE satellite observations of quasi‐parallel interplanetary shocks. In this study we compare the magnetic field structures for three shock cases. A hybrid code is used in the simulation which is found to reproduce reasonably well the overall structures of low Mach number quasi‐parallel shocks in the laminar regime. The frequency, wavelength, and wave amplitude deduced from simulations are in reasonable agreement with observations.
Simultaneous measurements of polar cap electron distributions in opposite hemispheres
Authors: Mizera Paul F. , Evans Dave S.
Published: February 2008

On January 16, 1983, the NOAA 6 and NOAA 7 satellites traversed polar cap auroral arcs in the northern hemisphere while the DMSP‐F6 satellite traversed similar polar cap arcs in the southern hemisphere. All of these encounters were in the morning local time sector and were made within 5 min in universal time of one another. The interplanetary magnetic field at this time had a strong northward Bz. The satellite observations of auroral electrons indicated that the precipitation exhibited a high degree of similarity, indicating a linkage between the two hemispheres. In particular, the source electron distributions, inferred from the satellite observations of precipitating electrons, were identical to within experimental error. For the comparison made when the satellites were closest to the same magnetic local time‐invariant magnetic latitude locations and nearly simultaneous in universal time, the electron spectrums in the northern and southern hemispheres displayed a peak near 1 keV, suggesting a common source and a similar acceleration process for these electrons.
A magnetospheric critical velocity experiment: Particle results
Authors: Torbert R. B. , Newell Patrick T.
Published: February 2008

In March of 1983, a barium injection sounding rocket experiment (the Star of Lima) was conducted to investigate Alfvén's critical ionization velocity (CIV) hypothesis in space. Included in the instrumented payload was a University of California, San Diego (UCSD), particle detection experiment consisting of five retarding potential analyzers. Despite conditions that appeared to be optimal for the critical velocity effect, the particle data, in agreement with optical observations, indicate that a fractional ionization of only ˜5×10−4 was observed, indicating that the conditions required for the effect to occur are still not well understood. However many of the required phenomena associated with the CIV effect were observed; in particular a superthermal electron population was formed at the expense of ion drift kinetic energy in the presence of intense electrostatic waves near the lower hybrid frequency. The amount of ionization produced is plausibly consistent with the observed electron flux, but could also be accounted for by residual solar UV at the injection point. It is shown based on the UCSD data set that one obvious explanation for the low ionization efficiency, namely that the ionizing superthermal electrons may rapidly escape along field lines, can be ruled out.
A statistical study of the subauroral electron temperature enhancement using Dynamics Explorer 2 Langmuir probe observations
Authors: Kozyra J. U. , Brace L. H. , Cravens T. E. , Nagy A. F.
Published: February 2008

A statistical study of the F region subauroral electron temperature enhancement was undertaken using data from the Langmuir probe experiment on the DE‐2 satellite for 488 passes through the plasmapause region during the periods September 16–30 and October 17–26, 1981. Satellite passes covered the altitude range 350–1000 km and two local solar time intervals: 8–10 and 21–23 hours. The subauroral electron temperature enhancement has an altitude dependent occurrence frequency with strong day‐night differences. The position (invariant latitude) of the subauroral electron temperature enhancement varies with magnetic activity in much the same manner as the equatorial plasmapause signatures discussed in the literature. The magnitude of the electron temperature peak increases statistically with increasing magnetic activity about 8 K per 1 γ decrease in Dst for altitudes below 600 km and 23 K per 1 γ decrease in Dst for altitudes between 600 and 1000 km on the nightside. There is a positive gradient of peak electron temperature with height on the nightside that increases statistically from 1.4 K/km (Dst > −40) to 3 K/km (Dst < −40) reflecting enhanced heat flow from high altitudes. The statistical response of the magnitude and location of the subauroral electron temperature peak to magnetic activity can account for the observed SAR arc occurrence statistics.
The local time, substorm, and seasonal dependence of electron precipitation at L ≈ 4 inferred from riometer measurements
Authors: Rosenberg T. J. , Dudeney J. R.
Published: February 2008

We have examined the variations of electron precipitation at L ≈ 4 as inferred from riometer measurements of cosmic radio noise absorption made during 1975 at Siple Station and Halley Bay, Antarctica. The results are presented in the form of annual and seasonal averages of ½‐hourly values for two geomagnetic activity subsets, AE > 140 nT (disturbed) and AE ≦ 140 nT (quiet). Monthly quiet day curves were used to remove the diurnal and seasonal variations in the background noise levels. Generally, the local time characteristics of the absorption were the same at both stations; the highest absorption occurred in the 0400–1600 MLT sector during disturbed conditions and in the 1200–2000 MLT sector during quiet conditions. For high AE, the highest correlation was obtained at a lag equal to the magnetic local time difference (1.5 hours) between the two stations. On the other hand, for low AE, the highest correlation occurred for a lag of 3.0 hours, nearer the local solar time difference (3.8 hours). Consistently higher absorption was measured at Halley on the average during both levels of magnetic disturbance and in all seasons. At both locations, and for both geomagnetic activity subsets, more absorption was observed in summer and equinox than in winter. This is in contrast to earlier studies for L ≧ 6, and suggests that a meridional reversal of seasonal behavior occurs between L = 4 and L = 6. In a further comparison with Siple ELF/VLF data, the 2‐ to 4‐kHz emission intensity was found to maximize in the morning hours during disturbed conditions, as was found for the absorption. For quiet conditions, the emission intensity was near minimum levels during the afternoon absorption peak. The VLF intensity is maximum in winter in contrast to the riometer result. This is consistent with increased attenuation in summer as a result of increased ionospheric opacity at that time. The local time characteristics of the absorption and wave data can be explained by a model of electron drift and precipitation under quiet and disturbed conditions that was first suggested by Thorne et al. (1974) to account for the local time variation of ELF emission intensity with substorm activity. Thus, the afternoon‐evening absorption peak during quiet conditions is attributed to cyclotron resonance interactions of drifting energetic electrons with plasmaspheric hiss, an emission largely unobservable from the ground. We associate the correlation time delay of ∼3 hours with the time difference between encounters of the Halley and Siple flux tubes with the plasmasphere bulge. The morning maximum of ionospheric absorption and 2‐ to 4‐kHz wave intensity during disturbed conditions is consistent with cyclotron resonance interactions between substorm‐injected electrons and chorus emissions occurring outside the plasmapause. For these conditions the absorption data suggest that maximum precipitation is ordered in MLT.
Boom potential of a rotating satellite in sunlight
Authors: Lai S. T. , Cohen H. A. , Aggson T. L. , McNeil W. J.
Published: February 2008

An interpretation is provided for the behavior of long boom potential measurements taken on the spinning P78‐2 (SCATHA) satellite at near geosynchronous altitudes. This study uses data taken during a quiet day, with the satellite in sunlight. The data show periodic variations with a maximum amplitude of 6 V. The theory explains why the variations correlate well with sun direction but not with the geomagnetic field. A current balance model, assuming a Maxwellian distribution of photoelectrons, is studied. The photoelectron temperature, the degrees of positive charging of the boom and of the satellite, and the ambient electron flux are calculated. Deviations from the model are discussed.
A storm time, Pc 5 event observed in the outer magnetosphere by ISEE 1 and 2: Wave properties
Authors: Greenstadt E. W. , McPherron R. L. , Anderson R. R. , Scarf F. L.
Published: February 2008

A classical, storm time Pc 5 event (T ∼ 160 s) was recorded by the satellite pair ISEE 1, 2 during an inbound, nearly equatorial pass in the dusk sector on August 21–22, 1978. Irregular and quasi‐periodic pulsations composed of several harmonics (f ∼ 2–10 × 10−3 Hz) were recorded from just inside the magnetopause at 11 RE to a distance of ∼8 RE, where the pulsations became nearly sinusoidal, and disappeared at ∼7 RE just outside the plasmapause. Comparison of signals from the two spacecraft throughout the pass shows remarkable similarity of waveform at the second spacecraft following a few hundred to a thousand kilometers (∼300 s) behind the first. This duplication of waveforms having wave periods commensurate with the spacecraft separation time suggests that the two satellites were sampling slightly different phases of the same wave cycle. The more distant, irregular pulsations were encountered by the two spacecraft with essentially no consistent delay between them, while the innermost, regular waves were always encountered first by the leading spacecraft. Cycle‐by‐cycle hodograms show nearly linear or highly elliptical polarization in the meridian plane everywhere on the inbound orbit, dominated by compressional waves. During the last few cycles the ellipses broadened and reversed phase at both satellites, just before the oscillations terminated, while the azimuthal amplitude went to zero at one satellite but not at the other. The Pc 5 pulsations occurred during an interval of strongly southward interplanetary magnetic field and substorm activity and were accompanied by Pc 1 waves at the spacecraft.
The maser synchrotron instability in an inhomogeneous medium: Determination of the spectral intensity of auroral kilometric radiation
Authors: Zarka P. , Le Quéau D. , Genova F.
Published: February 2008

The auroral kilometric radiation (AKR), which emanates from high‐altitude auroral regions of the earth, is mainly emitted in cold plasma depleted regions on the right‐handed X mode. The maser synchrotron instability (MSI) is likely to be responsible for the generation of this emission, accounting for the low density of the cold plasma at the source and the small instantaneous bandwidth of AKR. As the emitted frequency lies very close to the local X mode cutoff frequency, the existence of a gradient in the earth's magnetic field dramatically influences the wave propagation at the source, and thus the resonant wave‐particle process leading to the MSI. A fully inhomogeneous treatment is therefore required. In particular, the magnetic field inhomogeneity quenches the usual nonlinear saturation mechanisms and limits the spatial extension of the source region. It becomes then plausible to determine directly the AKR intensity through a linear transfer equation, with a source term linked to the spontaneous X mode emission. Using the evaluation of the gain carried out in a previous work, we solve this linear transfer equation and determine the AKR spectral intensity. The computed AKR fluxes are consistent with those observed, provided that nonthermal features of the electronic distribution function are present at sufficient energies in the source region. It is found that no nonlinear effect occurs for most AKR events, while the saturation level linked to the trapping of resonant electrons in the wave electric field corresponds well to the maximum observed AKR spectral intensity. Finally, a scenario is proposed for the generation of AKR X mode.
A method for determining corotating interaction regions in the equatorial plane of the heliosphere from solar wind velocity data
Authors: Olmsted C. , Akasofu S.‐I.
Published: February 2008

An Euler‐Lagrange coordinate transformation is applied to a computed solar wind velocity data set to obtain fluid streamlines which are also magnetic field lines. Plotting these lines for high‐resolution simulation data provides detailed information about the time variation of corotating interaction regions in the equatorial plane of the heliosphere.
The prereversal enhancement of the zonal electric field in the equatorial ionosphere
Authors: Farley D. T. , Bonelli E. , Fejer B. G. , Larsen M. F.
Published: February 2008

The electric fields in the ionospheric E and F regions near the magnetic equator often show a strong eastward enhancement shortly after sunset and before the eastward (normally) daytime field reverses to westward. Several theoretical models of the low‐latitude fields suggest that this enhancement is caused mainly or entirely by F region winds (the F region dynamo), but some authors have suggested that it could be produced solely by E region tidal winds. We give here additional calculations and arguments in support of the F region source. The enhancement of the eastward field for an eastward F region wind turns out to be a simple direct consequence of the fact that after sunset the ionospheric conductivity decreases far more rapidly in the E region than in the F region.
Analytical approach to backscattering of low‐energy electrons
Authors: Haider S. A. , Singhal R. P.
Published: February 2008

A new approach based on spatial yield spectra for calculating backscattered electron flux when a low‐energy monoenergetic electron is incident in a gas at any pitch angle is presented. Good agreement is obtained with the more elaborate calculations performed by Mantas and Walker (1976).
Magnetospheric interchange instability
Authors: Southwood David J. , Kivelson Margaret G.
Published: February 2008

The interchange instability is reviewed with particular emphasis on its relevance to planetary magnetospheric processes. Results of earlier work are reviewed and drawn together by the derivation of a general stability condition, based on a small perturbation approach, for a plasma with isotropic pressure contained in a curved magnetic field in the presence of gravity. The results are not restricted to low pressure (low β) or special field geometry, and the straight field result is recovered in the appropriate limit. We use a small perturbation approach for the instability analysis. Conditions under which an effective gravity may be introduced to simulate rotational effects are discussed. Recent criticism of the Jovian ring current impoundment hypothesis which was based on the straight field line results is shown to be ill founded.
Average electric field behavior in the ionosphere above Arecibo
Authors: Ganguly Suman , Behnke Richard A. , Emery Barbara A.
Published: February 2008

Plasma drift measurements taken at Arecibo during the solar minimum period of 1974–1977 are examined to determine their average behavior in the E, F1, and F regions. The drifts are generally diurnal in the E region and semidiurnal in the F1 region. These lower thermospheric drifts are set up by polarization fields generated by propagating and in situ atmospheric tides. In the F region the diurnal component is more pronounced, especially in the zonal direction. The magnitude of the drifts is of the order of 25–30 m/s (or 1 mV/m). Enhanced geomagnetic activity appears to increase the westward component of the drift in agreement with the theory of the ionospheric disturbance dynamo (Blanc and Richmond, 1980). Nighttime drifts appear to be at least partly explained in terms of polarization fields.
A magnetospheric substorm observed at Sanae, Antarctica
Authors: Gledhill J. A. , Dore I. S. , Goertz C. K. , Haggard R. , Hughes W. J. , Scourfield M. W. J. , Smits D. P. , Stoker P. H. , Sutcliffe P. R. , Wakerley P. A. , Walker A. D. M.
Published: February 2008

A magnetospheric substorm that occurred at Sanae, Antarctica, on July 27, 1979, was observed by a variety of techniques. A synthesis of the observations is presented, and an attempt made to deduce details of the behavior of the magnetosphere‐ionosphere system during the event. While there was some evidence of a growth phase, it was inconclusive. At the onset there was a rapid change in the tail field, which assumed a more dipolar form, accompanied by Pi 2 oscillations and the precipitation of 6‐keV electrons, with brightening of the auroral arc, auroral‐type sporadic E ionization, and riometer absorption. A positive spike was observed in the D magnetic component, instead of the expected negative one. There was no evidence of the usual westward traveling surge at the beginning of the expansion phase during which the precipitation region, auroral arc, and electrojet moved rapidly poleward, though it may have occurred outside the field of view from Sanae. The Hβ emission increased by a factor of less than 2, whereas the oxygen and nitrogen emissions monitored increased by 3–4. During the recovery phase, phenomena were consistent with a return of the tail field to an elongated form; a very high ratio of 557.7‐nm/630‐nm emissions, exceeding 10, was observed; and the electrojet lagged noticeably behind the photon emission regions.
Polar mesopause gravity wave activity in the sodium and hydroxyl night airglow
Authors: Myrabø H. K. , Deehr C. S. , Viereck R. , Henriksen K.
Published: February 2008

The OH (6–2) band and the Na D lines in the night airglow have been observed from Spitsbergen (78°N latitude) during a 3‐day period in the end of November 1983. Regular and quasi‐regular variations in temperature and the OH and sodium intensity were observed and are here interpreted in terms of gravity wave theory. In particular, waves with periods in the range 90–120 min having overlying ripple structure, with periods of 5–15 min, were observed. From the phase difference between the waves in the OH and sodium layer a height difference between the centroid of the two emissions of less than 1 km could be deduced. A gravity wave induced vertical eddy diffusion coefficient in the range 8 × 106 to 8 × 107 cm²/s was estimated from OH‐enhanced intensities. No significant net heating or cooling was observed during gravity wave activity. The gravity wave activity was not associated with geomagnetic activity. Mean OH (6–2) band and sodium night airglow intensities were 1.9 kR and 75 R, respectively.
Angular dependence of high Mach number plasma interactions
Authors: Thomas V. A. , Brecht Stephen H.
Published: February 2008

In this paper a 2‐1/2‐dimensional hybrid code is used to examine the collisionless large spatial scale (kc/ωpi ∼ 1) low‐frequency (ω ∼ ωci) interaction initiated by a plasma shell of finite width traveling at high Alfven Mach number relative to a uniform background plasma. Particular attention is given to the angle of the relative velocity relative to the ambient magnetic field for the range of angles 0 < θ < π/2. An attempt is made to parametrize some of the important physics including the Alfven ion cyclotron instability, the field‐aligned electromagnetic ion counter streaming instability, mixing of the plasma shell with the background ions, and structuring of the interaction region. These results are applicable to various astrophysical interactions such as bow shocks and interplanetary shocks.
The equatorward boundary of auroral ion precipitation
Authors: Gussenhoven M. S. , Hardy D. A. , Heinemann N.
Published: February 2008

The data from the SSJ/4 detector on the Defense Meteorological Satellite Program (DMSP) F6 satellite are used to study the difference in the location of the equatorward boundaries of auroral ion and electron precipitation, and the variation in the difference with magnetic local time and activity. Large geometric factors of the ion sensors of the SSJ/4 detector make identification of the ion boundary unambiguous in most cases. In this study, approximately 900 boundaries each for electrons and ions were determined from all DMSP F6 auroral passes in January of 1983. The boundaries occur over local times from 0400 to 0700 on the morningside of the oval and from 1700 to 2100 on the eveningside. The ion and electron boundaries both move systematically to lower latitudes with increasing magnetic activity, as measured by Kp. Over the evening sector sampled, the ion boundary is on average 1.4° equatorward of the electron boundary, with the difference commonly ranging up to 3°. For the morning sector sampled, the ion boundary is on average 2.6° poleward of the electron boundary with a significant number of cases with differences above 5.0°. The separation between the electron and ion boundaries is not dependent on Kp but does increase with MLT from midnight toward noon on both the morningside and the eveningside of the oval. The separation in boundaries can be explained by motion in a large‐scale, quasi‐static convection electric field if the time for development of the ion boundary is explicitly taken into account and if ion pitch angle diffusion is highly energy dependent.
Seasonal, latitudinal and diurnal distributions of whistler‐induced electron precipitation events
Authors: Carpenter D. L. , Inan U. S.
Published: February 2008

The seasonal, latitudinal, and diurnal distributions of whistler‐induced electron precipitation events, detected as subionospheric signal perturbations (Trimpi events), have been studied by means of data sets acquired in 1982‐1983 at Palmer and Siple stations, Antarctica. The data sets, substantially larger than any previously examined, confirm previous indications of a broad (∼ 4 hour) maximum in hourly event occurrence rates. The peak was centered ∼ 1‐2 hours after local midnight at Palmer for the roughly north‐south Argentina Omega 12.9‐kHz path, but was shifted several hours later for the 23.4‐kHz NPM path, which has a westerly arrival bearing at Palmer. The previously reported seasonal variation, with peaks at the equinoxes, was confirmed; activity persisted in the austral winter, particularly following a magnetic storm, but in general occurred on fewer days and during shorter daily periods. Comparisons of occurrence data at Siple (L ≃ 4.3) and Palmer (L ≃ 2.4) for particular signal sources showed the number of days of activity to be larger at Palmer by a factor of ∼ 2 or more. Comparisons of five months of simultaneous Palmer and Siple receptions of 21.4‐kHz NSS signals on north‐south paths suggest that most of the events observed at Siple occurred as the result of ionospheric perturbations relatively close to the L shell of Palmer, that is, in the range L = 2 – 3. This result is consistent with the expected variation with latitude of the energy of electrons precipitated due to equatorial gyroresonance at typical whistler frequencies.
Excitation of compressional waves and the formation of shocklets in the Earth's foreshock
Authors: Hada T. , Kennel C. F. , Terasawa T.
Published: February 2008

Large‐amplitude waves with typical frequencies of 0.01‐0.05 Hz are often observed in the foreshocks of earth and other planets. Large‐amplitude waves in the earth's foreshock are sometimes (but not always) observed in a highly time‐developed form, either as steepened pulses or as discrete oscillatory wave packets of finite length. This implies that nonlinearities are strong enough to modify their waveforms before the solar wind carries them out the foreshock. The instabilities and steepening of upstream waves in the earth's foreshock caused by backstreaming ions are discussed in the first part of the paper. For typical foreshock “diffuse” ion distributions, right and left‐hand polarized (RHP and LHP) waves propagating parallel to the local magnetic field are preferentially excited. Such noncompressional waves neither steepen nor grow fast enough to account for the amplitude polarizations and waveforms observed in the diffuse ion foreshock. Oblique waves develop a density compression and their magnetic field polarization is elliptical. Although these characteristics match the observations of the steepened waves in the diffuse ion zone, the growth rates of those waves oblique enough to steepen are too small to account for the observed amplitudes. On the other hand, the parallel propagating waves excited by the “reflected” ion distribution at the leading edge of the foreshock do grow fast enough, but do not steepen. We suggest that parallel propagating waves grow to finite amplitude in the “reflected” and “intermediate” ion zones of the earth's foreshock and refract as they are carried by the solar wind into the “diffuse” ion region, so that they become increasingly oblique and compressional. The more compressional they become, the more rapidly they steepen. Some steepen to the point where finite ion inertia dispersion wave creates a nonlinear wave train—a shocklet. Because wave refraction is less important in the very large foreshocks of interplanetary shocks, it is less likely that oblique, compressive, steepened waves will be generated in them, in agreement with observation. In the second part of this paper, we will simulate the time evolution of oblique low‐frequency compressive waves using a one‐dimensional hybrid code in which main ions are treated as superparticles, diffuse ions as a double‐adiabatic fluid, and electrons as an isothermal fluid. Unlike conventional hybrid codes, parallel and perpendicular pressures of fluids are treated independently. This is necessary in order to well describe the compressional properties of obliquely propagating right‐hand polarized waves in a high β plasma. We initialize the simulations with an oblique sinusoidal low‐frequency wave of finite amplitude. Nonlinear steepening, formation of discrete dispersive wave packets, and subsequent ion cyclotron damping of the wave packets are found to occur.
Drift motions of small‐scale irregularities in the high‐latitude F region: An experimental comparison with plasma drift motions
Authors: Ruohoniemi J. M. , Greenwald R. A. , Baker K. B. , Villain J. P. , McCready M. A.
Published: February 2008

On the evening of January 6, 1986, coordinated observations were carried out with the Johns Hopkins University Applied Physics Laboratory HF coherent scatter radar at Goose Bay, Labrador, and the SRI International incoherent scatter radar at Sondre Stromfjord, Greenland. The common field of view comprised a section of high‐latitude F region ionosphere centered on the great circle plane between the radar sites. Over a 40‐min period, the HF radar observed strong backscatter from small‐scale (13.9 m) field‐aligned irregularities. The bulk line‐of‐sight drift velocity of the irregularities is deduced from the backscatter data. The returns collected simultaneously with the incoherent scatter radar are processed for estimates of the mean line‐of‐sight ion velocity. Approximately 100 distinct comparisons are possible between the two sets of velocity estimates. Reversals exceeding 1000 m/s are present in both. In this paper, we demonstrate a correspondence between the measured irregularity and ion drifts that is consistent with the supposition that the motion of the irregularities is dominated by convective drift of the ambient plasma. This indicates that the small‐scale irregularities detected by HF radars in the high‐latitude F region can serve as tracers of ionospheric convective drift.
Lidar studies of the nighttime sodium layer over Urbana, Illinois: 2. Gravity waves
Authors: Gardner C. S. , Voelz D. G.
Published: February 2008

On 34 nights between December 1980 and May 1986, 171 monochromatic gravity waves were observed in the mesospheric Na layer at Urbana, Illinois, using a lidar system. The characteristics of the waves are compared with radar measurements of gravity wave activity and theoretical models of wave saturation and dissipation phenomena. The measured vertical wavelengths (λz) range from 2 km to 17 km, the observed periods (Tob) range from 25 min to 800 min and the horizontal wavelengths (λx) range from 20 km to almost 3000 km. The most apparent seasonal characteristic of the waves is the absence in summer of periods greater than 200 min and horizontal wavelengths greater than 400 km. The wave kinetic energy distributions are approximately proportional to kz−3, kx−15/14 and ƒ−5/3. Because the average amplitude growth length for the data is only 19 km and linear saturation theory predicts a kz−2 dependence for the kinetic energy distribution of saturated waves, the observed waves are not strongly damped and appear to be influenced more by dissipation rather than by saturation effects. Both λz and λx show a strong tendency to increase with Tob. The measured power law relationships are approximately λz ∝ Tob5/9 and λx ∝ Tob14/9. The wave induced mean flow acceleration ranges from 0 to −200 m s−1 day−1 and the average value is −27.2 m s−1 day−1. The effective viscosity limiting wave amplitudes ranges from 0 to 140 m² s−1 and the average value is 37 m² s−1.
Possible calorimetric effects at mercury due to solar wind‐magnetosphere interactions
Authors: Baker D. N. , Borovsky J. E. , Burns J. O. , Gisler G. R. , Zeilik M.
Published: February 2008

As a result of the solar wind interaction with the magnetosphere of Mercury, we expect several effects which may be observable with ground‐based and/or near‐earth radio and infrared telescopes: (1) there may be direct interaction of the solar wind with the surface of Mercury which could produce measurable heating effects; (2) substormlike magnetotail processes within Mercury's magnetosphere may precipitate particles carrying from 1010 to 1012 W (or more) of power into narrow latitudinal bands on the cold (≲ 130 K) darkside of Mercury, producing surface‐heated “auroral” zones; and (3) the presence of Jovian electrons within the inner Hermean magnetosphere may produce transient, very low frequency synchrotron‐emitting radiation belts. Hence remote radio and infrared observations hold substantial promise for providing information about the intrinsic properties of Mercury's magnetic field, about the planetary surface, and about the dynamical interaction of the Hermean magnetosphere with the solar wind at ∼0.4 AU.