bluuebunny/crossref_metadata_2025 huggingface dataset:
The dependence of winter aurora on interplanetary parameters
Authors: Baker J. B. , Ridley A. J. , Papitashvili V. O. , Clauer C. R.
Published: May 2003

The dependence of the northern winter aurora on interplanetary parameters is examined for images obtained from the Polar spacecraft Ultraviolet Imager (UVI) during the month of January 1997. Linear correlation analysis is used to relate auroral brightness to interplanetary parameters as a function of magnetic latitude (MLAT) and magnetic local time (MLT). Spatial maps of maximum magnitude correlation coefficient and optimal lag times are presented. In addition, the Method of Natural Orthogonal Components (MNOC) is applied to UVI images and an orthogonal set of eigenmodes of auroral behavior is produced. The dominant eigenmodes are determined to be (1) the main auroral oval brightness, (2) polar cap contraction/expansion, (3) dawn/dusk aurora, and (4) midnight sector brightening/quenching. The temporal coefficients for the various eigenmodes are correlated separately with solar wind parameters and interplanetary magnetic field (IMF) components. The dominant influence of the negative Bz component is confirmed, but the timescale of its influence is found to vary widely. The MNOC procedure allows some of the more subtle relationships between the aurora and the interplanetary parameters to be isolated, particularly those associated with the solar wind density and IMF Bx and By.
Moon‐related nonthermal ions observed by Nozomi: Species, sources, and generation mechanisms
Authors: Futaana Yoshifumi , Machida Shinobu , Saito Yoshifumi , Matsuoka Ayako , Hayakawa Hajime
Published: May 2003

The 3‐D velocity distribution functions of nonthermal ions observed in the vicinity of the Moon have been analyzed to understand the interaction between the solar wind and the Moon. The observation was made during the lunar swing‐by of the Nozomi spacecraft, and the closest approach was about 1.6 RL from the Moon's surface. Both Nozomi and the Moon were located in the solar wind, and nonthermal ions were observed only when the spacecraft was very close to the Moon. The nonthermal ions were measured by a Particle Spectrum Analyzer/Ion Spectrum Analyzer (PSA/ISA) on board the Nozomi. After analyses and careful examination of the 3‐D velocity distribution of the nonthermal ions, the characteristics of the nonthermal ions were revealed as follows: The nonthermal ions have a partial ring structure in the phase space, they are protons, their source location is the dayside of the Moon, and they have large velocities when they are generated. We propose the following scenario of a solar wind interaction with the Moon: The electromagnetic field in the vicinity of the Moon must have a dynamic structure, possibly a miniature bow shock associated with a local magnetic anomaly, where some of the solar wind protons are deflected violating their first adiabatic invariant. After the deflection, they move under the force of the convection electric field and gyrate around the magnetic field in the solar wind. This motion forms a partial ring structure with large initial velocities in the velocity phase space.
On wave generation by perpendicular currents
Authors: Brinca A. L. , Romeiras F. J. , Gomberoff L.
Published: May 2003

Previous assessment of the stability of perpendicular particle drifts in cold magnetoplasmas showed that their free energy could stimulate wave activity under various circumstances that included nonoscillatory (aperiodic, purely growing) instabilities and growth with zero‐drift thresholds. Here, the influence of finite temperatures is studied. The theoretical model, as in the cold plasma case, uses gravity to bring about the perpendicular currents but is applicable to other drift‐generating mechanisms and arbitrary drift velocities, and is not concerned with the stability of the macroscopic plasma‐gravity system. The zero‐order medium is a homogeneous equatorial‐plane configuration with the gravity force perpendicular to the background magnetic field, and the validity of neglecting the magnetic field generated by the drift currents is discussed. The kinetic electromagnetic wave matrix and dispersion equations are analytically derived for arbitrary directions of propagation and magnetized (nonrelativistic) plasma populations without further restrictive assumptions, and algorithms for their numerical evaluation are provided. Allowance for finite temperature plasmas does, in general, mitigate the cold plasma growth rates and instability bandwidths albeit introducing new modes that are fed by the free energy of the perpendicular drifts. Application of the developed tools can yield insight to the phenomenology encountered in space environments whose perpendicular currents and particle drifts arise from alternative generating mechanisms, namely, to the properties of the magnetotail current sheet and the possible stimulation of cyclotron harmonic wave activity.
A unified model of the response of ionospheric convection to changes in the interplanetary magnetic field
Authors: Freeman M. P.
Published: May 2003

Two competing paradigms of the response of ionospheric convection to changes in the interplanetary magnetic field are synthesized into the common framework of the expanding‐contracting polar cap (ECPC) model in order to reconcile apparently contradictory observations reported in the literature over the last 15 years or more. This is done by deriving analytical solutions to the ECPC model for two postulated idealized cases that capture the primary mathematical distinction between the two paradigms, and then simulating how these two responses would appear to different ways of observing them. It is shown that a single type of response could account for most of the variety of observed convection responses. In particular, we show that real phase motions along the polar cap boundary, such as due to an expansion of the reconnection X‐line at the magnetopause, would be more readily identified by some observing methods than others, providing an explanation for why authors disagree on whether such phase motions actually exist.
Frequency‐time spectra of magnetospherically reflecting whistlers in the plasmasphere
Authors: Bortnik J. , Inan U. S. , Bell T. F.
Published: May 2003

We present a numerical method of simulating at any location in the magnetosphere, the observed frequency versus time (f‐t) spectrogram resulting from a lightning strike at any given latitude on Earth. Using a two‐dimensional ray tracing code, we calculate the trajectories of 5330 whistler rays that effectively sample the lightning strike's frequency spectrum and latitudinal spread about the source and then use these so‐called “sample rays” to create ∼120 million interpolated rays, each weighted with a measure of energy according to its frequency and injection latitude. This energy is progressively attenuated along the ray's trajectory using a Landau damping calculation with realistic suprathermal electron fluxes. A detection area is defined in the plasmasphere, and rays that cross this area are used to construct the f‐t spectrogram representative of what would be observed on a satellite located in that region. We investigate the role that the lightning source latitude, observation location, and plasmaspheric electron density structures have on the appearance of the simulated f‐t spectrograms and show that all three parameters exhibit distinct and well‐defined effects. In particular, we focus on plasmaspheric electron density structures and explain the connection between these structures and the appearance of specific observed features in the spectrograms. Using this analysis, it may be possible to crudely infer certain features of the source and plasmasphere from observed magnetospherically reflecting whistler spectrograms.
Predicting interplanetary magnetic field (IMF) propagation delay times using the minimum variance technique
Authors: Weimer D. R. , Ober D. M. , Maynard N. C. , Collier M. R. , McComas D. J. , Ness N. F. , Smith C. W. , Watermann J.
Published: May 2003

It has been known that the fluctuations in the interplanetary magnetic field (IMF) may be oriented in approximately planar structures that are tilted with respect to the solar wind propagation direction along the Sun‐Earth line. This tilting causes the IMF propagating from a point of measurement to arrive at other locations with a timing that may be significantly different from what would be expected. The differences between expected and actual arrival times may exceed an hour, and the tilt angles and subsequent delays may have substantial changes in just a few minutes. A consequence of the tilting of phase planes is that predictions of the effects of the IMF at the Earth, on the basis of IMF measurements far upstream in the solar wind, will suffer from reduced accuracy in the timing of events. It has recently been shown how the tilt angles may be determined using multiple satellite measurements. However, since the multiple satellite technique cannot be used with real‐time data from a single sentry satellite, then an alternative method is required to derive the phase front angles, which can then be used for more accurate predictions. In this paper we show that the minimum variance analysis (MVA) technique can be used to adequately determine the variable tilt of the plane of propagation. The number of points that is required to compute the variance matrix has been found to be much higher than expected, corresponding to a time period in the range of 7 to 30 min. The optimal parameters for the MVA were determined by a comparison of simultaneous IMF measurements from four satellites. With use of the optimized parameters it is shown that the MVA method performs reasonably well for predicting the actual time lags in the propagation between multiple spacecraft, as well as to the Earth. Application of this technique can correct for errors, on the order of 30 min or more, in the timing of predictions of geomagnetic effects on the ground.
Metallic ion transport associated with midlatitude intermediate layer development
Authors: Bishop R. L. , Earle G. D.
Published: May 2003

Although intermediate layers are frequently observed by the Arecibo Incoherent Radar Observatory and by ionosondes around the world, many questions still remain regarding their formation, structure, and composition. In this paper, we explore the effect of metallic ions, specifically Fe+, on intermediate layer development and evolution. Several studies have demonstrated that layers can form from either molecular or metallic ions. This paper extends these earlier studies by quantifying the effect of metallic ions on intermediate layer morphology. We show that the efficiency of metallic ion transport depends significantly on the amplitude and wavelength of the imposed horizontal wind field. Specifically, larger amplitudes and longer wavelengths result in increased ion transport in the direction of the propagating neutral wind field.
Barium cloud striations revisited
Authors: Kelley Michael C. , Livingston Robert
Published: May 2003

Rocket‐based barium cloud striation observations from the 1970s are revisited based on a better knowledge of naturally occurring irregularities and on a previously unpublished analysis. A sharp drop in the spectral density of the plasma density fluctuation power is found near 25 m wavelength. This mimics equatorial spread F (ESF) spectra reported in the 1990s. Structure balance arguments show that the effective perpendicular ambipolar diffusion coefficient is 25 times the classical value, in good agreement with the natural case. We speculate that three‐wave coupling occurs to small scales where damped modes provide this anomalous diffusion effect, which is one‐tenth the value of the Bohm diffusion.
Phase‐bunching effects in triggered VLF emissions: Antenna effect
Authors: Trakhtengerts V. Y. , Demekhov A. G. , Hobara Y. , Hayakawa M.
Published: May 2003

We consider the effect of initial phase bunching of energetic electrons on the generation of triggered ELF/VLF emissions in the magnetosphere. The electrons interacted with the primary whistler wave packet form a phase‐bunched beam in the velocity space, which serves as a traveling‐wave antenna emitting secondary waves. We calculate this antenna field and study its spectral and amplitude characteristics. It is shown that under the typical magnetospheric conditions, the secondary wave field is by about an order of magnitude smaller than the primary wave field. Therefore this antenna field is significant as the seed wave which is further amplified by the same beam via the cyclotron resonant mechanism. The frequency band of the antenna effect is of the order of 100 Hz near the primary‐wave frequency. This is consistent with the observed frequency shift of triggered emissions.
Equilibrium and catastrophe of coronal flux ropes in axisymmetrical magnetic field
Authors: Hu Y. Q. , Li G. Q. , Xing X. Y.
Published: May 2003

Using a 2.5‐dimensional, time‐dependent ideal magnetohydrodynamic model in spherical coordinates, we present a numerical study of the property of magnetostatic equilibria associated with a coronal magnetic flux rope embedded in an axisymmetrical background magnetic field. The background field is potential (either closed or partly opened), a magnetic flux rope emerges out of the solar surface, and the resultant system is allowed to relax to equilibrium through numerical simulation. It is shown that the flux rope either sticks to the solar surface so that the whole magnetic configuration stays in equilibrium or escapes from the top of the computational domain, leading to the opening of the background field. Whether the rope remains attached to the solar surface or escapes to infinity depends on the magnetic energy of the system. The rope sticks to the solar surface when the magnetic energy of the system is less than a certain threshold, and it escapes otherwise. The threshold is slightly larger than the open limit, i.e., the magnetic energy of the corresponding fully opened field. The gravity, say, associated with the prominence mass, will raise the threshold by an amount that is approximately equal to the magnitude of the excess gravitational energy associated with the prominence. It implies that a catastrophe occurs when the magnetic energy of the system exceeds the threshold. The implication of such a catastrophe in coronal mass ejections is briefly discussed.
Reflection and absorption of Alfvénic power in the low‐altitude magnetosphere
Authors: Streltsov A. V. , Lotko W.
Published: May 2003

Results from a numerical study of the nonlinear interaction between large‐amplitude Alfvén waves and the low‐altitude auroral magnetosphere and ionosphere are presented. In the model the Alfvén wave encounters collisionless resistive dissipation when and where the effective parallel drift of electrons carrying the wave field‐aligned current density exceeds a critical threshold corresponding to onset of current‐driven microinstability. The resulting parallel electric field and parallel potential drop introduce frequency, amplitude, and perpendicular length‐scale dependence in the absorption of Alfvén wave power. Analysis of the power flow shows that (1) most of the power generated by a constant‐current source in the magnetosphere and carried by relatively small‐amplitude, high‐frequency, and short transverse wavelength Alfvén waves is reflected at low altitudes by the wave‐induced collisionless resistive layer (RL); (2) Alfvén wave power is absorbed primarily in the RL except when stimulated by a constant‐voltage generator, wherein ionospheric Joule heating dominates the absorption at large transverse length scales; (3) the absorption is high (≈0.8) at length scales of 10–20 km (ionospheric projection) for constant voltage source conditions; and (4) the relation between the wave‐induced parallel potential drop and wave field‐aligned current is linear with proportionality constant of order 109 Ω‐m2 over much of the range of interest. Correspondence between predicted properties and satellite observations is demonstrated.
High‐latitude ionospheric electric field variability and electric potential derived from DE‐2 plasma drift measurements: Dependence on IMF and dipole tilt
Authors: Matsuo Tomoko , Richmond Arthur D. , Hensel Kristine
Published: May 2003

In this study the characteristics of electric field variability are investigated by using the sample standard deviations estimated from plasma drift measurements obtained during the Dynamics Explorer 2 (DE‐2) mission. The spatial distribution of the standard deviation over the area poleward of 45° magnetic latitude and its climatological behavior with respect to the magnitude and orientation of the interplanetary magnetic field (IMF) and the dipole tilt angle (season) are examined. In comparison with past studies based on ground‐based measurements and with results from a data assimilation model, this study quantifies the electric field variability with more complete spatial coverage and with more extensive climatological information and therefore is of importance to the problem of the global Joule heating estimation in thermospheric general circulation modeling. In general, the magnitude of the standard deviation exceeds the strength of the mean electric field in most of the polar area, especially under northward IMF conditions. In contrast to the climatological electric field, whose magnitude tends to be most intense in the polar cap, the standard deviation generally intensifies in the vicinity of the convection reversal and the cusp. Under most IMF clock angles the area of the largest electric field variability lies near the cusp; under the southward BZ condition the area extends toward the potential maximum on the dawn side and toward the region of strong sunward convection in the afternoon, while under the positive BY condition the area extends poleward of the potential maximum on the dawn side. The analysis reveals that electric field variability varies with magnetic latitude, magnetic local time, IMF, and season in a manner distinct from that of the climatological electric field. This indicates that empirical models and data assimilation models designed to reproduce the average electric potential or the average electric fields correctly are not necessarily well‐suited to represent the squared electric fields or the electric field variability correctly.
Comparisons of thermospheric high‐latitude nitric oxide observations from SNOE and global auroral X‐ray bremsstrahlung observations from PIXIE
Authors: Petrinec S. M. , Imhof W. L. , Barth C. A. , Mankoff K. D. , Baker D. N. , Luhmann J. G.
Published: May 2003

Three years (March 1998 through March 2001) of nitric oxide (NO) observations in the Northern Hemispheric thermosphere (90–170 km) as made by the Student Nitric Oxide Explorer (SNOE) spacecraft are compared in a broad statistical analysis with the daily‐averaged northern auroral bremsstrahlung x‐ray observations, which are taken to be a good proxy for the population of precipitating energetic electrons. The latter are made by the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on board the NASA GGS Polar spacecraft. A modest correlation between these two data sets is found, indicating that about 20–40% of the variation in the number density of thermospheric nitric oxide at high latitudes is caused by variations in the precipitation of energetic particles from the magnetosphere into the auroral regions of the ionosphere. Time delays between the two data sets are examined in this study, as well as the altitude profile of the nitric oxide observations. Differences in the response and recovery times in the two data sets are also carefully considered along with hemispheric asymmetries as a function of season, leading to stronger correlations which are then discussed in terms of the properties of the data sets used.
Migrating and nonmigrating semidiurnal tides in the upper atmosphere excited by tropospheric latent heat release
Authors: Hagan M. E. , Forbes J. M.
Published: May 2003

We overview the results of a series of global‐scale wave model (GSWM) calculations we designed to quantify the mesospheric and lower thermospheric migrating and nonmigrating semidiurnal tidal response to large‐scale tropospheric latent heat release associated with deep tropical convection. Our semidiurnal tidal forcing is based on a 7‐year database of global cloud imagery that is also characterized herein. The combined responses of the migrating and nonmigrating components are sufficiently large to modulate the radiatively excited migrating semidiurnal tide in the upper atmosphere during every month of the year. This modulation produces longitudinal tidal variability that may be important to the interpretation of both ground‐based and satellite‐borne semidiurnal tidal diagnostics.
Latitudinal effect in semiannual variation of geomagnetic activity
Authors: Lyatsky W. , Tan A.
Published: May 2003

The study of the semiannual variation in both geomagnetic activity indices and raw geomagnetic field data at a large number of geomagnetic observatories showed a dramatic decrease in the relative magnitude of this variation from low to high latitudes. While the semiannual variation in the Dst index at low latitudes is about 3 times, for the midlatitude Ap and Am indices it is only about 20–30%, and the auroral electrojet AE index and polar cap PCN index show a very small or no semiannual variation. The study of auroral electrojet AU and AL indices showed that the semiannual variation is evident in AL index and not observed in AU index. Although relative magnitudes of the semiannual variation in geomagnetic activity indices are strongly reduced at high latitudes, absolute magnitudes of these variations in the Dst, Ap, Am, and AL indices are approximately the same. The semiannual variation in geomagnetic field H component consists of two equinoctial minima probably caused by the increasing westward ring current and substorm westward auroral electrojet in equinoctial months. The additional analysis of raw geomagnetic field data from geomagnetic observatories in two hemispheres confirmed the results inferred from the study of geomagnetic indices. Meanwhile in the polar caps, the semiannual variation in raw geomagnetic data showing a strong decrease in geomagnetic field H component reveals a strong increase in the Z component which also might be caused by increasing the ring current and westward auroral electrojet during enhanced geomagnetic activity occurring in equinoctial months. It was also found that the amplitude of the semiannual variation at all latitudes increased significantly with increasing geomagnetic activity.
Simulating plasmaspheric field‐aligned density profiles measured with IMAGE/RPI: Effects of plasmasphere refilling and ion heating
Authors: Tu J.‐N. , Horwitz J. L. , Song P. , Huang X.‐Q. , Reinisch B. W. , Richards P. G.
Published: May 2003

The radio plasma imager (RPI) on board the IMAGE satellite measures electron density profiles along magnetic lines through radio sounding techniques. We simulate three density profiles measured by the RPI on 8 June 2001 using the field line interhemispheric plasma (FLIP) model and compare the modeled density profiles with observed profiles. While careful consideration of depletion of flux tubes followed by refilling over several days allows fair agreement between simulated and measured densities around the equator, the simulated densities in the off‐equator regions remain significantly lower than those measured for all simulated flux tubes. It is shown that direct heating of ions in the plasmasphere during the plasmasphere refilling, in addition to heating through Coulomb collisions with hotter plasmaspheric electrons, can improve the agreement in the off‐equator regions. The enhanced ion pressure in the high altitude part of magnetic field line due to the ion heating impedes the ionospheric inflow from both hemispheres toward the equator, giving rise to decreased transport of ionospheric ions into the equatorial region and thus increased density gradient along the magnetic field line.
Field line resonances in quiet and disturbed time three‐dimensional magnetospheres
Authors: Cheng C. Z. , Zaharia S.
Published: May 2003

Numerical solutions for field line resonances (FLRs) in the magnetosphere are presented for three‐dimensional equilibrium magnetic fields represented by two Euler potentials as B = ∇ψ × ∇α, where ψ is the poloidal flux and α is a toroidal angle‐like variable. The linearized ideal MHD equations for FLR harmonics of shear Alfvén waves and slow magnetosonic modes are solved for plasmas with the pressure assumed to be isotropic and constant along a field line. The coupling between the shear Alfvén waves and the slow magnetosonic waves is via the combined effects of geodesic magnetic field curvature and plasma pressure. Numerical solutions of the FLR equations are obtained for a quiet time magnetosphere as well as a disturbed time magnetosphere with a thin current sheet in the near‐Earth region. The FLR frequency spectra in the equatorial plane as well as in the auroral latitude are presented. The field line length, magnetic field intensity, plasma beta, geodesic curvature, and pressure gradient in the poloidal flux surface are important in determining the FLR frequencies. In general, the computed shear Alfvén FLR frequency based on the full MHD model is larger than that based on the commonly adopted cold plasma model in the βeq > 1 region. For the quiet time magnetosphere, the shear Alfvén resonance frequency decreases monotonically with the equatorial field line distance, which reasonably explains the harmonically structured continuous spectrum of the azimuthal magnetic field oscillations as a function of the dipole L shell in the L ≤ 8.8 RE region. However, the FLR frequency spectrum for the disturbed time magnetosphere with a near‐Earth thin current sheet is substantially different from that for the quiet time magnetosphere for R > 6 RE, mainly due to shorter field line length due to magnetic field compression by solar wind, reduced magnetic field intensity in the high β current sheet region, azimuthal pressure gradient and geodesic magnetic field curvature.
Identifying the plasmapause in IMAGE EUV data using IMAGE RPI in situ steep density gradients
Authors: Goldstein J. , Spasojević M. , Reiff P. H. , Sandel B. R. , Forrester W. T. , Gallagher D. L. , Reinisch B. W.
Published: May 2003

Remote sensing of the entire plasmasphere is routinely accomplished by the Extreme Ultraviolet (EUV) imager on the IMAGE satellite. EUV observes the helium distribution in the plasmasphere by detecting resonantly scattered solar 30.4‐nm ultraviolet radiation. In EUV images the plasmapause is assumed to be the “ He+ edge,” i.e., the outermost sharp edge where the brightness of 30.4‐nm He+ emissions drops abruptly. This assumption is verified by comparing the L‐shell of steep electron density gradients, extracted from passive mode dynamic spectrograms recorded by the IMAGE Radio Plasma Imager (RPI) when the satellite is at low magnetic latitude, with the L‐shell of EUV He+ edges obtained when the satellite is outside the plasmasphere near apogee. A statistical study of all inbound (dawn sector) plasmapause crossings was performed for the month of June 2001. When the plasmapause location observed by RPI is compared to the location of the He+ edge extracted from the closest‐in‐time EUV image, a correlation coefficient of 0.83 is obtained. When the EUV He+ edge location is taken as the average of two EUV measurements (one before and one after the RPI measurement), the correlation coefficient increases to 0.87. The high degree of correlation justifies the assumption that the He+ edge coincides with the plasmapause. For eighteen cases in which the plasmasphere has no sharp outer boundary the intensity of the uncalibrated EUV images is compared with the electron number density extracted from the RPI data, and the lower sensitivity threshold of the EUV instrument is estimated to be 40 ± 10 electrons cm−3.
Energetic particle propagation in the inner heliosphere as deduced from low‐frequency (<100 kHz) observations of type III radio bursts
Authors: Cane H. V. , Erickson W. C.
Published: May 2003

Solar energetic particle (SEP) events are well‐associated with solar flares. It is observed that the delay between the time of the flare and the first‐arriving particles at a spacecraft increases with increasing difference between the flare longitude and the foot point of the field line on which the spacecraft is located. This difference we call the “connection angle” and can be as large as ∼120°. Recently it has been found that all SEP events are preceded by type III radio bursts. These bursts are plasma emission caused by the propagation of 2–50 keV flare electrons through the solar corona and into the solar wind. The drift of these type III radio bursts to lower and lower frequencies enables the propagation of the flare electrons to be traced from the Sun to about 1 AU. We have made an extensive analysis of the type III bursts associated with >20 MeV proton events and find that, in most cases, the radio emission extends to the local plasma frequency when the energetic particles arrive within a few hours of the flare. We conclude that this emission at the lowest possible frequency is generated close to the spacecraft. We then use the time from when the burst started at the Sun to when it reached the local plasma frequency to infer the time it took the radio producing electrons to travel to the spacecraft. We find that these delay times are organized by the connection angle and correlate with the proton delay times. We also find that the differences between the radio delays at Wind and Ulysses are matched by differences in the relative arrival times of the energetic particles at the two spacecraft. The consistent timing between the relative arrival times of energetic electrons and protons and the start of the lowest frequency radio emissions suggests that the first arriving particles of both species are accelerated as part of the flare process and that they propagate to the spacecraft along trajectories similar to those of the lower‐energy flare electrons. To be detected by observers at locations distant from the nominal field lines originating in the flaring regions the particles must undergo lateral transport. The continuity of the radio bursts suggests that the cross‐field transport occurs in the interplanetary medium.
Relativistic electron pitch‐angle scattering by electromagnetic ion cyclotron waves during geomagnetic storms
Authors: Summers Danny , Thorne Richard M.
Published: May 2003

During magnetic storms, relativistic electrons execute nearly circular orbits about the Earth and traverse a spatially confined zone within the duskside plasmapause where electromagnetic ion cyclotron (EMIC) waves are preferentially excited. We examine the mechanism of electron pitch‐angle diffusion by gyroresonant interaction with EMIC waves as a cause of relativistic electron precipitation loss from the outer radiation belt. Detailed calculations are carried out of electron cyclotron resonant pitch‐angle diffusion coefficients Dαα for EMIC waves in a multi‐ion (H+, He+, O+) plasma. A simple functional form for Dαα is used, based on quasi‐linear theory that is valid for parallel‐propagating, small‐amplitude electromagnetic waves of general spectral density. For typical observed EMIC wave amplitudes (1–10nT), the rates of resonant pitch‐angle diffusion are close to the limit of “strong” diffusion, leading to intense electron precipitation. In order for gyroresonance to take place, electrons must possess a minimum kinetic energy Emin which depends on the value of the ratio (electron plasma frequency/electron gyrofrequency); Emin also depends on the properties of the EMIC wave spectrum and the ion composition. Geophysically interesting scattering, with Emin comparable to 1 MeV, can only occur in regions where (electron plasma frequency/electron gyrofrequency) ≥10, which typically occurs within the duskside plasmapause. Under such conditions, electrons with energy ≥1 MeV can be removed from the outer radiation belt by EMIC wave scattering during a magnetic storm over a time‐scale of several hours to a day.
Statistical study of nighttime medium‐scale traveling ionospheric disturbances using midlatitude airglow images
Authors: Shiokawa K. , Ihara C. , Otsuka Y. , Ogawa T.
Published: May 2003

We have investigated statistical characteristics of the nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) observed in 630‐nm airglow images at two stations, Rikubetsu (43.5°N, 34.8°MLAT) and Shigaraki (34.9°N, 25.4°MLAT), in Japan for 1998–2000 near the solar maximum period. Most of the observed MSTIDs propagate southwestward in the images. The typical wavelength, velocity, period, and amplitude are 100–300 km, 50–100 m/s, 0.5–1.5 h, and 5–15%, respectively. Seasonal variations in these parameters are not clear. The occurrence rate has a major peak (50–60%) in summer that appears ∼2 months earlier at lower latitudes and a minor peak in winter. Similar occurrence characteristics are obtained from midlatitude spread‐F signatures using multipoint ionosonde data in Japan, though the coincidence of the spread‐F and the MSTIDs in airglow images is only 10–15%.
A multidiagnostic investigation of the mesospheric bore phenomenon
Authors: Smith Steven M. , Taylor Michael J. , Swenson Gary R. , She Chiao‐Yao , Hocking Wayne , Baumgardner Jeffrey , Mendillo Michael
Published: May 2003

Imaging measurements of a bright wave event in the nighttime mesosphere were made on 14 November 1999 at two sites separated by over 500 km in the southwestern United States. The event was characterized by a sharp onset of a series of extensive wavefronts that propagated across the entire sky. The waves were easily visible to the naked eye, and the entire event was observed for at least 5 hours. The event was observed using three wide‐angle imaging systems located at the Boston University field station at McDonald Observatory (MDO), Fort Davis, Texas, and the Starfire Optical Range (SOR), Albuquerque, New Mexico. The spaced imaging measurements provided a unique opportunity to estimate the physical extent and time history of the disturbance. Simultaneous radar neutral wind measurements in the 82 to 98 km altitude region were also made at the SOR which indicated that a strong vertical wind shear of 19.5 ms−1km−1 occurred between 80 and 95 km just prior to the appearance of the disturbance. Simultaneous lidar temperature and density measurements made at Fort Collins, Colorado, ∼1100 km north of MDO, show the presence of a large (∼50 K) temperature inversion layer at the time of the wave event. The observations indicated that the event was most probably due to an undular mesospheric bore, a relatively uncommon disturbance which has only recently been reported [Taylor et al., 1995a]. Evidence is also shown to suggest that a large east‐west tropospheric frontal system lying over the northern United States was the origin of the disturbance.
Theoretically predicted properties of type II radio emission from an interplanetary foreshock
Authors: Knock S. A. , Cairns Iver H. , Robinson P. A. , Kuncic Z.
Published: May 2003

We investigate the causes of variations in interplanetary type II radio bursts using an analytic model which predicts the emission generated by electron beams in the foreshock regions upstream of an interplanetary shock wave. Trends in source region characteristics and remotely observed radio fluxes are calculated as a function of a number of solar wind and shock parameters. Calculations are performed for a single three‐dimensional ripple on the global shock surface. Radio‐loud ripples are predicted to have larger shock speeds relative to the solar wind speed, higher levels of nonthermal electrons, larger radii of curvature, and be moving through higher density regions than radio‐quiet ripples. These predictions are qualitatively consistent with available observations. The predicted emission depends most sensitively on the speed of the shock relative to the solar wind. Strong correlations are found between the intensity of fundamental emission and the level of nonthermal electrons present in the tail of the incident solar wind electron distribution. Harmonic emission is found to be most sensitive to variations in the electron temperature Te of the incident solar wind. These results indicate that the bursty nature of typical type II observations can be accounted for by a shock propagating through an inhomogeneous solar wind.
Interplanetary magnetic field polarities inferred from the north–south cosmic ray anisotropy
Authors: Laurenza M. , Storini M. , Moreno G. , Fujii Z.
Published: May 2003

The interplanetary magnetic field sector polarity at the Earth's location can be inferred from ground measurements of the cosmic ray north‐south anisotropy. We present here the results of a systematic comparison between inferred and directly measured polarities in the period from January 1971 through December 1997. The overall success rate of the prediction method is 72%, in rather good agreement with previous findings. On the other hand, our analysis shows for the first time the limits of the method and calls for warning in its use. In fact, it turns out that the inferred polarities are affected by a bias, which becomes more relevant around the sunspot minimum. Even though the bias cannot be removed, it is possible to select conditions, occurring for 30% of the time, under which the inferred polarities are very reliable (the success rate being 95%).
Computer simulation of polarization electric fields as a source of midlatitude field‐aligned irregularities
Authors: Yokoyama T. , Yamamoto M. , Fukao S.
Published: May 2003

The generation of quasi‐periodic (QP) echoes and the associated polarization electric fields in the midlatitude E‐region are considered by using a computer simulation. It is shown that the horizontal structure of a sporadic‐E (Es) layer plays an important role in the generation of polarization electric fields. Where rod‐like enhancements of plasma density exist in an Es layer, a large polarization electric field can be generated by an ambient zonal electric field or meridional neutral wind. The generated polarization electric fields map along the geomagnetic field up to the higher E‐region and form a field‐aligned plasma density structure. The growth rate of the gradient‐drift instability becomes positive up to an altitude of 120 km. Meridional neutral winds can also produce striated QP echoes as a trace of the field‐aligned structure. Even if the Es layer is uniform, a polarization electric field is generated by atmospheric gravity waves, especially those propagating northward, which support the resemblance of a period and a wavelength between the gravity waves and the QP echoes. The large polarization electric fields associated with the Es layer are thought to be a common phenomenon in the midlatitude and play a key role in the generation of the QP echoes.
Interplanetary scintillation observations of the solar wind disappearance event of May 1999
Authors: Balasubramanian V. , Janardhan P. , Srinivasan S. , Ananthakrishnan S.
Published: May 2003

We report extensive interplanetary scintillation (IPS) observations of the solar wind during the period centered around 11 May 1999, when the Earth was engulfed by a region of solar wind with unusually low densities (<1 cm−3) and velocities (<350 km s−1). IPS observations with the Ooty Radio Telescope (ORT), operating at 327 MHz, probe the inner heliosphere along a large number of closely spaced lines of sight over the heliocentric distance range of 0.2 to 1.0 AU, daily, at the rate of about 150 sources in the course of 10 hours. Using these observations, the southern half of the inner heliosphere was monitored everyday between 3 and 16 May 1999. The data show that the solar wind densities were globally subdued, having started subsiding from 3 May 1999, itself at different parts of the heliosphere. This was accompanied by a noticeable decrease in the velocity of the general solar wind also. The IPS data also reveal a small subset of closely spaced sources lying to the west of the Sun that show a steep drop in inferred densities to immeasurably low amounts on and around the 11th May. The actual morphology of the subsidence event, from our observations, is shown to be that of a “void‐within‐a‐void”. The role of the polar magnetic field reversal that was taking place between Carrington rotations 1947–1954, which may have played a key role in generating the subsidence event, is also examined. To the best of our knowledge this is the first time that sustained levels of low densities and velocities have been recorded for such a long period over a large part of the inner heliosphere. Some possible signatures of high energy “strahl” electrons were also recorded in this period.
Seasonal and solar cycle variations of the vertical distribution of the occurrence probability of auroral kilometric radiation sources and of upflowing ion events
Authors: Kumamoto A. , Ono T. , Iizima M. , Oya H.
Published: May 2003

The seasonal and solar cycle variations of the vertical distribution of the occurrence probability of auroral kilometric radiation (AKR) sources and of upflowing ion (UFI) events have been investigated based on 13 years of plasma wave data and 8 years of energetic ion data observed by the Akebono satellite. In the summer polar region, the peak of the vertical distribution of the occurrence probability of AKR sources, identified by an intensity larger than −150 dB W/m2 Hz, is at an altitude higher than 5000 km with a value of 10% in the solar maximum period and at an altitude of 5000–6000 km with a value of 40% in the solar minimum period. In the winter polar region, the peak altitudes and values are 4000–5000 km and 50% in the solar maximum period, and 4000 km and 60% in the solar minimum period, respectively. Based on the analysis of wave polarization data, intense R‐X mode AKR is responsible for these variations. Furthermore, it has been also revealed that the vertical distribution of the occurrence probability of UFI events, identified by an ion velocity distribution function with a peak above 1 keV, also shows similar seasonal and solar cycle variations as the AKR sources. In both seasonal and solar cycle variations, the AKR and UFI occurrence probabilities decrease with increasing solar EUV ionization in the ionosphere. Therefore, ionospheric seasonal and solar cycle variations are inferred to be responsible for the AKR and UFI variations. As for the control mechanism, upwelling plasma from the ionosphere to the exosphere, up to an altitude of about 5000 km, are expected to change the temperature and density of the background plasma and control unstable conditions of plasma waves like ion acoustic waves and electrostatic ion cyclotron (EIC) waves, which generate the auroral particle acceleration regions.
Probability distributions of the induced electric field of the solar wind
Authors: Breech B. , Matthaeus W. H. , Milano L. J. , Smith C. W.
Published: May 2003

The low‐frequency induced electric field in the near‐Earth interplanetary medium is investigated observationally. These electric field fluctuations are associated with magnetohydrodynamic‐scale fluctuations of the magnetic field and plasma velocity. Results of the study, based upon hourly samples with variations computed relative to 4‐day means, indicate that the electric field distribution is roughly exponential. More specifically, the electric field distribution is very nearly the one that is obtained from a quasi‐normal turbulence model [Milano et al., 2002]. These results may be useful for particle scattering, acceleration, and turbulence.
Observations of backscatter autocorrelation functions from 1.07‐m ionospheric irregularities generated by the European Incoherent Scatter Heater Facility
Authors: Nielsen E. , Rietveld M. T.
Published: May 2003

During an ionospheric heating experiment using the European Incoherent Scatter Heater Facility on 10 July 1999, artificial backscatter was detected by the Scandinavian Twin Auroral Radar Experiment (STARE) 140 MHz coherent radar system. The STARE radar operations included for the first time multipulse observations. The autocorrelation function and power spectrum of the backscattered radar signal were observed to be characterized by a half‐power correlation time 13 ± 1 ms or by a corresponding spectral width 47 ± 4 Hz. This spectral width is comparable to the width observed at much lower frequencies, at 46.9 MHz and 21.4 MHz. The spectral width may, in part, result from the spatial extent of the heated region and from the spatial resolution of the radar in connection with spatial gradients in the background electric field. An upper value of the turbulent diffusion coefficient derived from the observations is 4.5 m2/s. It is up to a factor of 10 larger than the classical coefficient. Apart from spatial effects the increase may be caused by increased electron scattering in the plasma waves excited by the heater activity.
Interplanetary and solar surface properties of coronal holes observed during solar maximum
Authors: Zhang J. , Woch J. , Solanki S. K. , von Steiger R. , Forsyth R.
Published: May 2003

Data from the Solar Wind Ion Composition Spectrometer (SWICS) on board the Ulysses spacecraft and synoptic maps from Kitt Peak are used to analyze the relatively short‐lived coronal holes which exist during the maximum phase of the solar activity cycle 23. They are compared with the persistent polar coronal holes which prevail around solar minimum. A solar wind velocity increase coinciding with a shift of the ionic charge composition toward lower charge states serves as a robust criterion for identifying solar wind streams emanating from solar maximum holes. This allows an unambiguous association of every stream identified in interplanetary space with a coronal hole on the solar surface with consistent magnetic polarity. Solar wind streams emanating from the solar maximum holes generally show lower velocities of 400 to 600 km/s compared to the polar hole stream velocities of 700 to 800 km/s. However, the SWICS O7+/O6+ charge‐state ratios, which are a proxy for coronal temperatures, do not reveal a consistent difference. Though a number of solar maximum holes have a significantly, up to three times, higher temperature compared to the polar coronal holes, the majority of the investigated holes and specifically those with new cycle polarity have a coronal temperature within the range of polar hole temperatures. Likewise, the magnetic flux density in the solar maximum holes and in the polar coronal holes, as derived from the synoptic maps, is not strikingly different. Therefore any intrinsic difference between solar maximum holes and polar coronal holes is small. The striking discrepancy in their kinetic properties, namely the slower velocity of the solar wind streams emanating from solar maximum holes, may partly be attributed to deceleration of the solar wind during propagation to the spacecraft. The discrepancy may also be influenced by active regions in close proximity to the coronal holes, which presumably is more likely for smaller holes. There may, however, be a tendency for the faster wind streams to emanate from cooler holes.