bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical and case studies of radio emissions observed near 2ƒce and 3ƒce in the auroral zone
Authors: Weatherwax A. T. , LaBelle J. , Trimpi M. L. , Treumann R. A. , Minow J. , Deehr C.
Published: February 2004

Receivers located at Two Rivers, Alaska (64.9°N, 146.9°W), and Circle Hot Springs, Alaska (65.5°N, 144.7°W), have been used to monitor the spectrum between 0.05‐4.8 MHz for extended periods. Seasonal and diurnal effects of auroral roar, a weak narrow band radio emission near 2 and 3 times the ionospheric electron cyclotron frequency (ƒce), have been determined. Many individual auroral roar events correlate with magnetic activity, and superposed epoch analysis using planetary K indices shows a correlation between magnetic activity and auroral roar commencement. For a 3ƒce roar event on March 23, 1992, riometer, magnetometer, photometer, and all‐sky camera data are available and show that individual bursts of 3ƒce auroral roar are associated with intensifications of the aurora, as was known previously for 2ƒce roar. Finally, two possible generation mechanisms are investigated in detail: nonlinear interaction between lower hybrid and electrostatic upper hybrid waves producing electromagnetic waves, and the feasibility of a lower ionospheric decameter maser for direct generation of X mode waves at harmonics of ƒce.
A kinetic model of the formation of the hot oxygen geocorona: 2. Influence of O+ion precipitation
Authors: Bisikalo D. V. , Shematovich V. I. , Gérard J. ‐C.
Published: February 2004

A model of the oxygen geocorona near the exobase solving the nonlinear Boltzmann equation with a Monte Carlo method is used to calculate the distribution of the hot oxygen atoms during geomagnetically disturbed nighttime conditions. The precipitation of energetic O+ions and the subsequent enhancement of the hot O corona at high latitudes is simulated for the September 17, 1971, storm conditions. It is found that in such circumstances, the O+precipitation is a significant source of superthermal O atoms leading to important perturbations of the velocity distribution of the bulk oxygen population. The effective gas temperature near the exobase is similar to that in the undisturbed atmosphere, but the hot O density rises considerably over the quiet condition values.
Solar photoionization as a loss mechanism of neutral interstellar hydrogen in interplanetary space
Authors: Ogawa H. S. , Wu C. Y. Robert , Gangopadhyay P. , Judge D. L.
Published: February 2004

Two primary loss mechanisms of interstellar neutral hydrogen in interplanetary space are resonance charge exchange ionization with solar wind protons and photoionization by solar EUV radiation. The later process has often been neglected since the average photoionization rate has been estimated to be as much as 5 to 10 times smaller than the charge exchange rate. These factors are based on ionization rates from early measurements of solar EUV and solar wind fluxes. Using revised solar EUV and solar wind fluxes measured near the ecliptic plane we have reinvestigated the ionization rates of interplanetary hydrogen. The result of our analysis indicates that indeed the photoionization rate during solar minimum can be smaller than charge exchange by a factor of 5; however, during solar maximum conditions when solar EUV fluxes are high, and solar wind fluxes are low, photoionization can be over 60% of the charge exchange rate at Earth orbit. To obtain an accurate estimate of the importance of photoionization relative to charge exchange, we have included photoionization from both the ground and metastable states of hydrogen. We find, however, that the photoionization from the metastable state does not contribute significantly to the overall photoionization rate.
Observations of downward moving O+ in the polar topside ionosphere
Authors: Chandler M. O.
Published: February 2004

New investigations of the core ion motion within the high‐latitude topside ionosphere and near‐Earth magnetosphere, using data from the Dynamics Explorer retarding ion mass spectrometer (RIMS), reveal the existence of significant regions of downward moving O+. The occurrences of downgoing versus upgoing O+ are not clearly separable in terms of either solar zenith angle or Kp but are well distinguished by the direction of the z component of the interplanetary magnetic field (IMF). On average, downflow dominates when IMF Bz < 0, while upflow dominates for IMF Bz > 0. Combining cross‐field convection velocities derived from an empirical convection electric field model with the observed parallel velocities yields a two‐dimensional ion velocity field. This velocity field is consistent with a scenario which has O+ of cusp/cleft and auroral zone origin convecting into the polar cap and, because of the dominance of gravitational energy over the upward kinetic energy, falling back into the ionosphere. This provides additional confirmation of the results of studies of the cleft ion fountain. Estimates of the flux of O+ in the upflowing and downflowing regions for Λ ≥ 60° give a total upflow of ∼6 × 1025 ions s−1 for IMF Bz > 0 and total upflow and downflow of ∼4 × 1025 ions s−1 and 1 × 1025 ions s−1, respectively, for IMF Bz < 0. In all cases the magnitude of the dayside outflow is consistent with previous work on upwelling ions. While the magnitudes vary for high and low Kp the ratios of upward to downward flow are roughly the same at ∼1.7. The downflowing O+ shows a correlation with the magnitude of the outflow of light ions in the same region but the cause and effect of this relationship is not distinguishable.
Low‐frequency waves associated with Langmuir waves in solar wind
Authors: Thejappa G. , Wentzel Donat G. , Stone R. G.
Published: February 2004

The Ulysses spacecraft has detected several events of low‐frequency electromagnetic waves in association with Langmuir waves in the solar wind. The high time resolution observations show that the Langmuir waves are very intense and occur as broad peaks superposed by collapsing millisecond spikes. The low‐frequency waves are identified as electromagnetic lower hybrid waves. The observed energy densities of these waves often exceed the strong turbulence thresholds. It is shown that none of the parametric decay instabilities involving Langmuir and low‐frequency waves are energetically favorable to explain the present observations. The low‐frequency waves are proposed to arise from currents associated with gradients in the electron beam originating at sites where Langmuir waves scatter the beam electrons.
Localized tearing modes in the magnetotail driven by curvature effects
Authors: Sundaram A. K. , Fairfield D. H.
Published: February 2004

The stability of collisionless tearing modes is examined in the presence of curvature drift resonances and the trapped particle effects. A kinetic description for both electrons and ions is employed to investigate the stability of a two‐dimensional equilibrium model. The main features of the study are to treat the ion dynamics properly by incorporating effects associated with particle trajectories in the tail fields and to include the linear coupling of trapped particle modes. Generalized dispersion relations are derived in several parameter regimes by considering two important sublayers of the reconnecting region. For a typical choice of parameters appropriate to the current sheet region, we demonstrate that localized tearing modes driven by ion curvature drift resonance effects are excited in the current sheet region with growth time of the order of a few seconds. Also, we examine nonlocal characteristics of tearing modes driven by curvature effects and show that modes growing in a fraction of a second arise when mode widths are larger than the current sheet width. Further, we show that trapped particle effects, in an interesting frequency regime, significantly enhance the growth rate of the tearing mode. The relevance of this theory for substorm onset phase and other features of the substorms is briefly discussed.
An examination of the effect of dipole tilt angle and cusp regions on the shape of the dayside magnetopause
Authors: Petrinec S. M. , Russell C. T.
Published: February 2004

The shape of the dayside magnetopause has been studied from both a theoretical and an empirical perspective for several decades. Early theoretical studies of the magnetopause shape assumed an inviscid interaction and normal pressure balance along the entire boundary, with the interior magnetic field and magnetopause currents being solved self‐consistently and iteratively, using the Biot‐Savart Law. The derived shapes are complicated, due to asymmetries caused by the nature of the dipole field and the direction of flow of the solar wind. These models contain a weak field region or cusp through which the solar wind has direct access to the ionosphere. More recent MHD model results have indicated that the closed magnetic field lines of the dayside magnetosphere can be dragged tailward of the terminator plane, so that there is no direct access of the magnetosheath to the ionosphere. Most empirical studies have assumed that the magnetopause can be approximated by a simple conic section with a specified number of coefficients, which are determined by least squares fits to spacecraft crossing positions. Thus most empirical models resemble more the MHD models than the more complex shape of the Biot‐Savart models. In this work, we examine empirically the effect of the cusp regions on the shape of the dayside magnetopause, and we test the accuracy of these models. We find that during periods of northward IMF, crossings of the magnetopause that are close to one of the cusp regions are observed at distances closer to Earth than crossings in the equatorial plane. This result is consistent with the results of the inviscid Biot‐Savart models and suggests that the magnetopause is less viscous than is assumed in many MHD models.
Millstone Hill incoherent scatter F region observations during the disturbances of June 1991
Authors: Buonsanto Michael J.
Published: February 2004

The extreme geomagnetic disturbances of early June 1991 were accompanied by dramatic effects in the Earth's thermosphere and ionosphere which are the subject of coordinated analysis by the Coupling, Energetics, and Dynamics of Atmospheric Regions (CEDAR) Storm Study project. F region observations taken at Millstone Hill over the interval June 5‐11 using a nine‐position experiment allow estimation of spatial gradients in ionospheric parameters above the station. From these gradients the motion term in the F region continuity equation is obtained and combined with calculations of rates of production and loss of the major ions in a semiempirical model of the F region ionosphere above Millstone Hill. Electron densities calculated from this semiempirical model using MSIS‐86 are generally much smaller than the measurements. This discrepancy could be explained by the presence of vibrationally excited N2 combined with a decrease in the atomic oxygen to molecular composition ratio of ≈2 or more on the most disturbed days. Neutral winds calculated by using the incoherent scatter line‐of‐sight ion drifts show a strong equatorward surge during the day on June 5 and strong equatorward surges during all the disturbed nights. Synoptic observations taken from Millstone Hill by using elevation and azimuth scans during the nights of June 12, 13, and 14 illustrate the large‐scale structure of electron density and its relation to neutral winds and electric fields observed over much of eastern North America.
High‐latitude ionospheric convection pattern during steady northward interplanetary magnetic field
Authors: Cumnock J. A. , Heelis R. A. , Hairston M. R. , Newell P. T.
Published: February 2004

The DMSP F8 satellite's coverage of Earth's polar regions provides horizontal ion drift velocities along the dawn‐dusk meridian at approximately 835 km altitude in each hemisphere during the ∼100 min orbital period. We examine the ionospheric convection signatures observed by this spacecraft in the summer and winter hemispheres during periods when the interplanetary magnetic field (IMF) is directed northward for at least 45 min prior to the satellite entering the polar region and remains northward throughout the polar pass. These convection signatures can be readily categorized by the number of sunward and antisunward flow regions and by their potential distributions. Here we describe the most frequently identifiable and reproducible features of the convection pattern that exist during steady northward IMF conditions. In addition to IMF Bz, the influences on the convection pattern of the IMF Bz/|By| ratio, season, latitude, and solar wind velocity are all considered. The ratio Bz/|By| provides a first order organization of the signatures that occur on the dayside of the dawn‐dusk meridian. Sunward flow at highest latitudes on the dayside of the dawn‐dusk meridian is the dominant feature seen in the large‐scale convection signature during steady northward IMF; however, sunward flow at highest latitudes does not imply the existence of a particular number of convection cells.
Forced three‐dimensional magnetic reconnection due to linkage of magnetic flux tubes
Authors: Otto A.
Published: February 2004

During periods of southward interplanetary magnetic field (IMF) orientation the magnetic field geometry at the dayside magnetopause is susceptible to magnetic reconnection. It has been suggested that reconnection may occur in a localized manner at several patches on the magnetopause. A major problem with this picture is the interaction of magnetic flux ropes which are generated by different reconnection processes. An individual flux rope is bent elbowlike where it intersects the magnetopause and the magnetic field changes from magnetospheric to interplanetary magnetic field orientation. Multiple patches of reconnection can lead to the formation of interlinked magnetic flux tubes. Although the corresponding flux is connected to the IMF the northward and southward connected branches are hooked into each other and cannot develop independently. We have studied this problem in the framework of three‐dimensional magnetohydrodynamic simulations. The results indicate that a singular current sheet forms at the interface of two interlinked flux tubes if no resistivity is present in the simulation. This current sheet is strongly tilted compared to the original current sheet. In the presence of resistivity the interaction of the two flux tubes forces a fast reconnection process which generates helically twisted closed magnetospheric flux. This linkage induced reconnection generates a boundary layer with layers of open and closed magnetospheric flux and may account for the brightening of auroral arcs poleward of the boundary between open and closed magnetic flux.
Simulation study on stochastic reconnection at the magnetopause
Authors: Fu S. Y. , Pu Z. Y. , Liu Z. X. , Zong Q. G.
Published: February 2004

A two‐dimensional MHD simulation model is presented to study stochastic reconnection at the magnetopause, the simulation indicates that in an open system with strong velocity shear and large Reynolds and magnetic Reynolds number (R and Rm), stochastic reconnection can take place in the magnetopause current sheet when the interplanetary magnetic field has a southward component, leading to the appearance of irregular mesoscale and small‐scale structures (IMSSSs). The larger the R and Rm are, the more IMSSSs are produced. The stronger the flow shear intensity is, the faster IMSSSs occur. Mesoscale and small‐scale structures can coalesce to form large‐scale ones if R and Rm are midranged (10 to 200), while large and mesoscale structures can break into smaller ones if R and Rm increase. It is expected that stochastic reconnection plays an important role in the solar wind‐magnetosphere coupling.
Light transmission and reflection in perennially ice‐covered Lake Hoare, Antarctica
Authors: McKay C. P. , Clow G. D. , Andersen D. T. , Wharton R. A.
Published: February 2004

We have investigated the transmission and albedo of the perennial ice cover on Lake Hoare, Antarctica. Our database consists of year‐round measurements of the photosynthetically active radiation (400–700 nm) under the ice, measurements of the spatial variation of the under‐ice light in midsummer, and spectrally resolved measurements from 400 to 700 nm of the albedo and transmission of the ice cover in early (November) and in midsummer (January). Our results show that the transmission decreases in the first part of summer, dropping by a factor of ∼4 from November to January. We suggest that this is due to heating in the upper layers of the ice cover and the formation of Tyndall figures. Later in the summer when a significant liquid water fraction occurs within the ice cover, the transmission increases. In the fall when the ice cover freezes solid the transmission drops markedly. The spectrally resolved measurements from 400 to 700 nm show that ∼2–5% of the incident light in this spectral region penetrates the 3.5‐m thick ice cover. We have analyzed the spectral data using a two‐stream scattering solution to the radiative transfer equation with three vertical layers in the ice cover. A surficial glaze of scattering ice 1 cm thick overlies a layer of sandy, bubbly ice about a meter thick, and below this is a thick layer of sand‐free ice with bubbles. We find that the ice cover is virtually opaque at wavelengths longer than 800 nm. The net transmission of solar energy is ∼2%. Significant changes in the thickness of the ice cover have been reported at Lake Hoare. These are due primarily to changes in the thickness of the bottom layer only. Because this layer is relatively clear, the effect on the transmission through the ice cover from these changes is less than would be predicted assuming a homogeneous ice cover.
Probability of earthquake occurrence in Greece with special reference to the VAN predictions
Authors: Honkura Y. , Tanaka N.
Published: February 2004

The VAN earthquake predictions were made on the basis of seismic electric signals (SES), but the debate seems to be directed toward the statistical significance of the predictions from seismic data only. Accordingly, applying a logistic regression model to seismicity, we present our estimation of the probability of earthquake occurrence in Greece. The main purpose of our study is to examine whether or not we can find a specific seismicity pattern that can be used to considerably increase probability estimates. Our estimation of the probability of occurrence of an earthquake of Ms ≥ 5.0 is less than 0.25 in all the cases that we have examined. If we lower the threshold magnitude from 5.0 to 4.3, we can find cases in which the probability becomes as high as 0.75, comparable to the success rate of the VAN method estimated by Hamada [1993]. In these cases, however, such a high probability is due mostly to aftershocks, and if aftershocks are removed from the data set, the probability falls below 0.5.
Electrodynamic model for the formation of auroral ionospheric cavities
Authors: Doe R. A. , Vickrey J. F. , Mendillo M.
Published: February 2004

Auroral ionospheric cavities (AICs) are latitudinally narrow, field‐aligned density depletions of the wintertime polar F region ionosphere. AICs have been detected in incoherent scatter radar data from the Sondrestrom radar facility in a 2‐year sample and during two coordinated multi‐instrument campaigns (Doe et al., 1993, 1994). These data suggest the possibility that AICs are created by the ionospheric closure of field‐aligned currents (FACs) in the polar ionosphere. In this scenario, the cavity forms in a region where ionospheric electrons are evacuated upward as charge carriers for a downward FAC. In order to model this process, a two‐dimensional (altitude versus latitude) simulation has been constructed that imposes an oppositely directed FAC pair at the top of a polar ionosphere that is subject to chemical loss and diffusive transport; the pertinent equations are solved for the resultant system of closure currents and localized plasma loss. Electrodynamic evacuation is modeled by solving Ohm's law and ▽ · j = 0 on the ionospheric grid with an imposed constant topside potential. The sensitivity of the modeled ionosphere to modification from chemistry and diffusion alone is evaluated by removing the topside potential, and imposing a region of enhanced ion temperature ion‐neutral (slip) velocity at F region altitudes. This confirms our earlier conclusion that perturbed thermospheric temperatures and velocities alone cannot create AICs on observed time scales. Modeling results from field‐aligned current closure, on the other hand, indicate that FACs are very efficient at modifying the polar ionosphere: modest currents of 0.2 to 0.02 µA m−2 can create cavitylike structure on time scales from 30 to 64 s, respectively.
Spectral width of type 2 coherent echoes at large magnetic aspect angles
Authors: Kustov A. V. , Hussey G. C. , Koehler J. A. , Sofko G. J. , Mu J.
Published: February 2004

The spectral characteristics of smoothly varying type 2 signals were studied using two 50‐MHz CW radio links. The echoes observed came from two closely located scattering volumes where the geometrical aspect angles of observations were about 10° for both links but the azimuthal angles differed by 88°. The events reported here were selected when the spectra on both links were wide, with a comparable spectral width, but the mean Doppler shift was almost zero on one link and very high, sometimes in excess of the ion‐acoustic velocity, on the other link. Highly shifted Doppler spectra were observed to be more skewed than the spectra with low shifts. The skewness was negative for negative Doppler shifts and positive for positive Doppler shifts in accordance with previous radar observations at large magnetic aspect angles. The variation of spectral width with the radial velocity or with the magnetic disturbance level exhibited two regions of linear dependence, with a larger slope at small values and a much smaller slope at higher ones. These different slopes likely reflect the change in the rate of the irregularity amplitude increase with the electric field.
Damping and spectral formation of upstream whistlers
Authors: Orlowski D. S. , Russell C. T. , Krauss‐Varban D. , Omidi N. , Thomsen M. F.
Published: February 2004

Previous studies have indicated that damping rates of upstream whistlers strongly depend on the details of the electron distribution function. Moreover, detailed analysis of Doppler shift and the whistler dispersion relation indicate that upstream whistlers propagate obliquely in a finite band of frequencies. In this paper we present results of a kinetic calculation of damping lengths of wideband whistlers using the sum of seven drifting bi‐Maxwellian electron distributions as a best fit to the ISEE 1 electron data. For two cases, when upstream whistlers are observed, convective damping lengths derived from ISEE magnetic field and ephemeris data are compared with theoretical results. We find that the calculated convective damping lengths are consistent with the data and that upstream whistlers remain marginally stable. We also show that the slope of plasma frame spectra of upstream whistlers, obtained by direct fitting of the observed spectra, is between 5 and 7. The overall spectral, wave, and particle characteristics, proximity to the shock, as well as propagation and damping properties indicate that these waves cannot be generated locally. Instead, the observed upstream whistlers arise in the shock ramp, most likely by a variety of cross‐field drift and/or anisotropy driven instabilities.
Venus nightside ionospheric irregularities and their relationship to VLF bursts
Authors: Ho C. M. , Strangeway R. J. , Russell C. T.
Published: February 2004

The plasma density and magnetic field are highly spatially and temporally variable in the Venus nightside ionosphere. This variability may affect the propagation and the generation of plasma waves. We determine how these irregularities are related to plasma wave activity. Previously, we have found two types of signals in the lower ionosphere, 100‐Hz narrow‐band signals and high‐frequency wideband bursts. We investigate how these two different wave phenomena depend on the structure of the ionosphere. We calculate the ratios of the instantaneous values of electron density, magnetic field, and electron temperature relative to their average (1 min) values, and also their gradient scale lengths. The 5.4‐kHz burst activity is related to density depressions and obvious electron temperature enhancements, but varies little with magnetic field strength. The 100‐Hz narrow‐band signals are often associated with significantly enhanced magnetic field and depressions of electron density. Both higher field strength and lower density result in a lower β (ratio of thermal pressure to magnetic pressure) plasma, which favors the propagation of whistler waves. We find that neither type of signal is associated with gradients of the magnetic field, but the 5.4‐kHz wideband signals have higher occurrence rates at higher electron density and temperature gradients, while the 100‐Hz narrow‐band burst rate increases only slightly for steeper gradients. A local gradient‐driven instability cannot explain most of the 100‐Hz narrow‐band signals, but some 5.4‐kHz signals may be associated with pressure gradients.
Wave structures in lower thermosphere density from satellite electrostatic triaxial accelerometer measurements
Authors: Forbes J. M. , Marcos F. A. , Kamalabadi F.
Published: February 2004

High‐resolution density data between 170 and 220 km from the SETA (Satellite Electrostatic Triaxial Accelerometer) Experiment during the summers of 1982 and 1983 are analyzed spectrally to investigate the nature of horizontal density structures ranging in scale from about 150 to 2500 km. The lower thermosphere is found to be frequently characterized by wavelike structures corresponding to peak‐to‐peak density variations of order 10–40%. Orbit‐by‐orbit spectral analyses of densities normalized to 200 km of altitude are performed, and these are combined to construct average periodograms for magnetically quiet and active periods. Integration of these curves provides the relative percentage of total density variance attributable to various horizontal scales; for instance, 10% for λH < 750 km and 70% for 1500 < λH < 2500 km. As geomagnetic activity increases from quiet (Kp < 2) to active (Kp > 4) levels, the spectral energy increases by a factor of 2 for wavelengths in the range 150 to 500 km and by a factor of 4 at longer wavelengths (1000–2500 km). In part the spectral energy enhancement at the longer wavelengths reflects mesoscale density variations unrelated to wave activity, but “direct” waves generated by auroral activity, which are known to travel long distances, probably also make important contributions at these wavelengths. The magnetic activity enhancement at shorter scales is also significant, especially at low latitudes, since waves with these horizontal wavelengths are not expected on theoretical grounds to propagate very far within the thermosphere away from an auroral source. Moreover, the shorter scale waves regularly occur during geomagnetically quiet times when an auroral source mechanism is unlikely.
Observations of ELF fields near the low‐altitude CRRES chemical releases
Authors: Koons H. C. , Roeder J. L.
Published: February 2004

The Combined Release and Radiation Effects Satellite (CRRES) performed a series of seven low‐altitude chemical releases between September 10, 1990, and August 12, 1991. Immediately following each chemical release, electric and magnetic fields were detected by the extremely low frequency wave analyzer sensors of the Low Altitude Satellite Studies of Ionospheric Irregularities (LASSII) experiment on the spacecraft. The time series and spectra of the two field components are quite similar for each of the releases but vary in detail from release to release. The index of refraction estimated from the ratio of the magnetic field to the electric field is too small by about 2 orders of magnitude for either the right‐hand wave or the extraordinary wave modes which are the only propagating electromagnetic modes in the detected band above the O+ ion gyrofrequency (∼30 Hz). ELF hiss observed at higher altitudes is found to be propagating in the extraordinary wave mode with the correct index of refraction. This confirms that the intensity measurements are being made correctly by the instrument and that an alternative explanation is required for the signals detected following the chemical releases. We show that the waves are primarily electrostatic and that the magnitude of the wave magnetic field is consistent with the transverse magnetic field component of ion acoustic waves.
Characteristics of quasi‐2‐day oscillations in the ƒoF2 at northern middle latitudes
Authors: Apostolov E. M. , Altadill D. , Alberca L.
Published: February 2004

The temporal variations of the characteristics of ƒoF2 quasi‐2‐day oscillations at middle latitudes in the northern hemisphere are investigated. The hourly data for the period 1976–1986 for four European ionospheric stations, Kiev (50.5°N, 30.5°E), Kaliningrad (54.7°N, 20.62°E), Lannion (48.75°N, 3.45°W) and Slough (51.48°N, 0.57°W), are used for analysis. Periodogram and complex demodulation analysis are used to evaluate the period and amplitude time variations of this oscillation. It is found that quasi‐2‐day oscillations in ƒoF2 are present almost continuously during the year. The amplitude is larger in the equinoxes and lower in the solstices. The amplitude variations are modulated by the long‐term geomagnetic variations during the solar cycle. There is a clear expressed seasonal variation of the oscillation period with minimum in the winter, November–February (42–47 hours), and maximum near summer solstice (48–55 hours). The observations suggest that there are three types of ƒoF2 quasi‐2‐day oscillations with behavior of (1) westward traveling planetary wave with zonal wave number one, predominantly during summer half year, with greatest occurrence in August, (2) stationary planetary wave with a maximum occurrence in the summer solstice with unknown wave number, and (3) independent oscillations in separate locations on relatively long distances. The vertical plasma drift variations can explain the annual and long‐term modulation effects of ƒoF2 quasi‐2‐day amplitude variations but encounter significant difficulties in explaining the discrepancy between zonal wave numbers in the mesosphere (three) and upper ionosphere (one). Another possibility is that the arising of quasi‐2‐day oscillation in the upper ionosphere may be induced by the forcing of such oscillation in the mesosphere with further independent development in the F region.
Observations of the neutral atmosphere between 100 and 200 km using ARIA rocket‐borne and ground‐based instruments
Authors: Hecht J. H. , Christensen A. B. , Gutierrez D. J. , Kayser D. C. , Sharp W. E. , Sharber J. R. , Winningham J. D. , Frahm R. A. , Strickland D. J. , McEwen D. J.
Published: February 2004

The atmospheric response in the aurora (ARIA) rocket was launched at 1406 UT on March 3, 1992, from Poker Flat, Alaska, into a pulsating diffuse aurora; rocket‐borne instruments included an eight‐channel photometer, a far ultraviolet spectrometer, a 130.4‐nm atomic oxygen resonance lamp, and two particle spectrometers covering the energy range of 1–400 eV and 10 eV to 20 keV. The photometer channels were isolated using narrow‐band interference filters and included measurements of the strong permitted auroral emissions N2 (337.1 nm), N2+ (391.4 nm), and O I (844.6 nm). A ground‐based photometer measured the permitted N2+ (427.8 nm), the forbidden O I (630.0 nm), and the permitted O I (844.6 nm) emissions. The ground‐based instrument was pointed in the magnetic zenith. Also, the rocket payload was pointed in the magnetic zenith from 100 to 200 km on the upleg. The data were analyzed using the Strickland electron transport code, and the rocket and ground‐based results were found to be in good agreement regarding the inferred characteristic energy (E0 ≈ 3 keV) of the precipitating auroral flux and the composition of the neutral atmosphere during the rocket flight. In particular, it was found that the O/N2 density ratio in the neutral atmosphere diminished during the auroral substorm, which started about 2 hours before the ARIA rocket flight. The data showed that there was about a 10‐min delay between the onset of the substorm and the decrease of the O/N2 density ratio. At the time of the ARIA flight this ratio had nearly returned to its presubstorm value. However, the data also showed that the O/N2 density ratio did not recover to its presubstorm value until nearly 30 min after the particle and joule heating had subsided. Both the photometer and oxygen resonance lamp data showed the presence of structure in the atomic oxygen densities in the region above 130 km. The observed auroral brightness ratio B337.1/B391.4 equaled 0.29 and was in agreement with other recent measurements. This ratio was also consistent with the greater than expected flux of secondary electrons measured by the onboard particle spectrometer between 40 and 10 eV.
Middle and upper atmosphere radar observations of turbulence and movement of midlatitude sporadic E irregularities
Authors: Ogawa Tadahiko , Yamamoto Mamoru , Fukao Shoichiro
Published: February 2004

Midlatitude field‐aligned irregularities (FAI) associated with sporadic E layers were continuously observed on June 17–19, 1989, with the middle and upper atmosphere radar to investigate spatial and temporal behaviors of the 3.2‐m‐scale FAI. Using Doppler spectrum, mean Doppler velocity, and echo intensity data, we present some features of the plasma turbulence and irregularity movement pertinent to two types of radar echoes: “continuous echoes” appearing at 90‐ to 100‐km altitudes at night and in the morning, and “quasi‐periodic (QP) echoes” appearing at 95‐ to 125‐km altitudes at night. For both types of echoes, echo intensities and Doppler velocities were modulated with periods of 5–20 min due to gravity wave activities. Most of the enhanced echoes were located within the region where Doppler velocities tend to be close to zero. On many occasions there were velocity shears across the enhanced echo region. The Doppler velocities and echoing‐patch movements for the continuous echoes are compared with the existing F region plasma drift and neutral wind models, respectively, to find a partial consistency. General characteristics of the Doppler velocity and spectral width associated with the QP echoes are consistent with previous observational results. Unusual movement of the FAI sheet related to the QP echoes is discussed in the light of recent theory.
Nonlinear evolution of magnetic flux ropes: 2. Finite beta plasma
Authors: Osherovich V. A. , Farrugia C. J. , Burlaga L. F.
Published: February 2004

In this second paper on the evolution of magnetic flux ropes we study the effects of gas pressure. We assume that the energy transport is described by a poly tropic relationship and reduce the set of ideal MHD equations to a single, second‐order, nonlinear, ordinary differential equation for the evolution function. For this conservative system we obtain a first integral of motion. To analyze the possible motions, we use a mechanical analogue—a one‐dimensional, nonlinear oscillator. We find that the effective potential for such an oscillator depends on two parameters: the polytropic index γ and a dimensionless quantity κ the latter being a function of the plasma beta, the strength of the azimuthal magnetic field relative to the axial field of the flux rope, and γ. Through a study of this effective potential we classify all possible modes of evolution of the system. In the main body of the paper, we focus on magnetic flux ropes whose field and gas pressure increase steadily towards the symmetry axis. In this case, for γ>1 and all values of κ, only oscillations are possible. For γ<1, however, both oscillations and expansion are allowed. For γ<1 and κ below a critical value, the energy of the nonlinear oscillator determines whether the flux rope will oscillate or expand to infinity. For γ<1 and κ above critical, however, only expansion occurs. Thus by increasing κ while keeping γ fixed (<1), a phase transition occurs at κ = κcritical and the oscillatory mode disappears. We illustrate the above theoretical considerations by the example of a flux rope of constant field line twist evolving self‐similarly. For this example, we present the full numerical MHD solution. In an appendix to the paper we catalogue all possible evolutions when (1) either the magnetic field or (2) the gas pressure decreases monotonically toward the axis. We find that in these cases critical conditions can occur for γ>1. While in most cases the flux rope collapses, there are notable exceptions when, for certain ranges of κ and γ, collapse may be averted.
Dynamic response of the cusp morphology to the interplanetary magnetic field changes: An example observed by Viking
Authors: Yamauchi M. , Lundin R. , Potemra T. A.
Published: February 2004

We present a unique Viking cusp observation during a period when the interplanetary magnetic field (IMF) observed by IMP 8 made a steplike change from southward to steady northward and subsequently back to steady southward again. The solar wind density and velocity were fairly steady during the entire period. Viking detected two independent plasma signatures. The first (equatorward part) is associated with southward IMF, and the second (poleward part) is associated with northward IMF. In the transition from the first signature to the second one, the ion data show an “overlapping injection” signature, i.e., two independent plasma populations exist on the same field lines and are well separated in the energy domain at any time and at any pitch angle. The second plasma population is found on the higher‐energy side of the first population. A good relation between keV ion morphology and the field‐aligned current pattern exists even though the particle cusp is undergoing a dynamic change. We discuss two possible scenarios for the present observation which involve the dynamic reaction of the cusp morphology to the stepwise change in the IMF condition.
Observations of electron beam propagation perpendicular to the Earth's magnetic field during the TSS 1 mission
Authors: Hardy David A. , Oberhardt Marilyn R. , Burke William J. , Thompson Donald C. , Raitt William J. , Gentile Louise C.
Published: February 2004

We report on measurements by the Shuttle Potential and Return Electron Experiment (SPREE), acquired during a period of the Electrodynamic Tethered Satellite mission when the fast pulsed electron generator (FPEG) injected a 1‐keV electron beam nearly perpendicular to the Earth's magnetic field. Using multiangular electrostatic analyzers mounted on rotary tables, SPREE was capable of determining the flux of electrons and ions in the energy range from 10 eV to 10 keV and over a solid angle of 2π sr. SPREE was located in the shuttle bay where it could observe beam electrons after they had completed ∼1 gyrocycle when fired nearly perpendicular to the local magnetic field. For the case presented here, the beam's intensity decreased from ∼100 mA cm−2 at FPEG's aperture to ∼0.18 nA cm−2 at the location of SPREE. The spectrum of the return electrons displays a sharp peak at the beam energy with an intensity at the peak of approximately 2×1010 electrons cm−2 s−1 sr−1. The distribution of the electrons around the peak has a half width of several hundred eV, with observed energies as high as 1850 eV. For energies between 10 and a few hundred eV, intense fluxes of electrons are seen at all look angles. For angles where the beam is observed the spectrum in this energy range has a power law shape. At angles away from the direction of beam return, the spectrum in this energy range can display a more thermal shape with a peak at energies up to 50 eV. In general, the flux intensity in the lower‐energy portion of the spectrum is isotropic with an average integral flux of 0.5 to 2×1012 electrons cm−2 s−1 sr−1. Integrating over energy and pitch angle gives number densities of ∼ 5×104 electrons cm−3. The return current density of 0.5 to 2µA cm−2 s−1 sr−1 carried by this isotropic component is sufficient to balance that emitted by FPEG and keep the shuttle at a low potential. We find that both scattering and spreading of the beam near FPEG are necessary for primary electrons to reach the locations of the SPREE detectors.
Structure of auroral precipitation during a theta aurora from multisatellite observations
Authors: Feldstein Y. I. , Newell P. T. , Sandahl I. , Woch J. , Leontjev S. V. , Vorobjev V. G.
Published: February 2004

A θ aurora previously discussed on the basis of Viking images (northern hemisphere) and DE 1 images (southern hemisphere) is reexamined in light of additional data, primarily the auroral plasma distribution as determined from the Viking, DMSP F6, and DMSP F7 satellites. This event, which occurred before a substorm expansion phase on August 3, 1986, appeared in the images to consist of a single arc along the morning side in the northern hemisphere and along the evening side in the southern hemisphere and was isolated from the auroral oval in both sets of images. On the basis of the auroral plasma distribution inferred from three satellites, the brightest arcs do occur at the locations indicated by the imagers, but the arcs are in fact connected to the main oval with continuous precipitation, and weaker secondary arcs (not observed by the imagers) occur in the opposite hemisphere magnetically conjugate to the bright arcs. These observations support the interpretation of the θ aurora as occurring on closed field lines as a result of the expansion of the morning and evening sector ovals into the polar cap. A careful examination of the characteristics of the observed auroral energy plasma suggests additional conclusions. It appears that the ionospheric manifestation of the recently discovered low‐energy electron layer can be identified with a complicated structure of soft precipitation at the poleward edge of the main precipitation region. Finally, unlike recent reports, the ions were not observed to have a cutoff in the polar cap that is any sharper than that of the electrons.
Observational determination of magnetic connectivity of the geosynchronous region of the magnetosphere to the auroral oval
Authors: Hones E. W. , Thomsen M. F. , Reeves G. D. , Weiss L. A. , McComas D. J. , Newell P. T.
Published: February 2004

This is a report of a program to study the magnetic connectivity between the auroral region of the ionosphere and the equatorial geosynchronous region of the magnetosphere. The program uses plasma measurements made with polar orbiting Defense Meteorological Satellite Program (DMSP) satellites and several geosynchronous satellites, seeking time intervals when nearly identical plasma electron spectra (32 eV to 30 keV) indicate magnetic connectivity between a polar/geosynchronous satellite pair. When such signatures of connectivity are found, the locations of the relevant satellite pair are compared with the locations that would be predicted by a magnetospheric model. Here we report results from the initial application of this program that uses data from DMSP F‐8, F‐9 and F‐10 polar satellites and the synchronous satellites 1989‐046 and 1990‐095 acquired in the 6‐day interval March 7–12, 1991. The results are compared with predictions of the T89a model [Tsyganenko, 1989; Peredo et al., 1993]. Orbital calculations predicted 96 close conjunctions among these satellites during the interval. Of those we have made spectral comparisons for 47, finding 20 for which close spectral similarity occurred during few‐second intervals. Ionospheric footpoints of the satellite pairs calculated by the T89a model for these intervals revealed eight for which the discrepancy between the DMSP latitude at “best spectral match” and the model projection of the synchronous satellite was smaller than 1°. However, there were five intervals for which the discrepancy was greater than 6°. The latter were all found in the local time interval 0700–0900, perhaps indicating a particular inadequacy of the T89a model in that region. This initial work also supports the view that the auroral oval of discrete arcs is an ionospheric mapping of the full thickness of the plasma sheet (e.g., Feldstein and Galperin, 1985) and not just of the plasma sheet boundary layer (e.g., Eastman et. al., 1985). During just its first few years of concurrent operation this constellation of well‐instrumented synchronous and polar‐orbiting satellites has provided data for literally thousands of close conjunctions such as we analyze in this report. Thus an algorithm is developed enabling the verification and testing of proposed mappings between the ionosphere and magnetosphere (such as T89a).
New model of magnetospheric current‐voltage relationship
Authors: Pierrard V.
Published: February 2004

A new model based on the generalized Lorentzian velocity distribution function (VDF) is used to estimate the total current density Jtot along the magnetic field lines as a function of the field‐aligned electric potential difference V between the ionosphere and the magnetosphere. As in the earlier kinetic models based on the Maxwellian VDF, Jtot is a nonlinear function of V, except for values of V between 0.1 and 10 kV, where the current‐voltage relationship becomes linear. The effect of an enhanced population of suprathermal particles in the tail of the VDF is to increase the value of the applied potential for which the linear relationship is a valid approximation. The application of this current‐voltage relationship to the auroral precipitation region and return current region is discussed.
Effect of anomalous resistivity on MHD wave damping
Authors: Tikhonchuk V. T. , Bychenkov V. Y.
Published: February 2004

We investigate the effect of nonlinearity of the voltage current plasma characteristic on the damping rate of shear and compressional Alfvén waves in current‐carrying plasmas. It is shown that the shear Alfvén wave experiences much stronger turbulent damping than compressional wave due to its effect on the level of ion acoustic turbulence excited by an external current.
Charging of ice grains by low‐energy plasmas: Application to Saturn's E ring
Authors: Jurac S. , Baragiola R. A. , Johnson R. E. , Sittler E. C.
Published: February 2004

The charging of ice grains in planetary plasmas is studied, including the effects of secondary electron emission and backscattering of the incident electrons. It is shown that existing charging models can not be simply extrapolated to the low‐energy electron regime (below 30 eV) common in planetary magnetospheric plasmas. We derive expressions for the electrical potential of a grain immersed in a low‐energy plasma which more carefully account for electron reflection and the threshold for secondary electron emission. Using plasma parameters from Voyager PLS experiment, we calculate the potential of Saturn's E ring grains to vary from −5.5 V at 4 Rs to 5 V at 10 Rs.
Semiannual fluctuation depending on the polarity of the solar main magnetic dipole field
Authors: Baranyi T. , Ludmány A. , Terdik G.
Published: February 2004

We examined the recently found semiannual fluctuation in the Sun‐weather relations on a broader European region. The fluctuation means stronger solar effects on the weather in the spring and autumn periods than in summer or winter. The existence of this effect has been confirmed but only during the parallel orientations of the terrestrial and solar main magnetic dipole fields; this latter varies by a 22‐year period. The phenomenon probably contributes to the interconnection mechanisms between the solar and terrestrial magnetic fields and apparently, through a completely unknown mechanism, to certain tropospheric processes. Although the fluctuation is perceivable, it should be noted that the statistical evidence of the results is still far from striking, and the physical explanation is still missing.