bluuebunny/crossref_metadata_2025 huggingface dataset:
Imaging the three‐dimensional solar wind
Authors: Gruntman Mike
Published: February 2004

Charge exchange collisions between the solar wind alpha particles and heliospheric atomic hydrogen produce a unique emission in the 30.4‐nm line. We demonstrate that an all‐sky image in this emission would reveal the three‐dimensional flow properties of the solar wind, including the flow properties in the regions over the Sun's poles and on the far side of the Sun. The boundary separating the fast (high‐latitude) and slow (low‐latitude) solar wind can be established remotely, from 1 AU. Most of the observed 30.4‐nm emission would originate in the region extending up to 10 AU from the Sun. Solar wind flow velocities can be obtained both from radiance directional dependence (images) and directly from the Doppler shifts. The fast and slow solar wind flows would produce emissions spectrally separated by ∼0.03 nm (∼0.3 Å). The feasibility of solar wind imaging depends on the spectral properties and relative strength of the background radiation. The main background sources include galactic diffuse and stellar radiation, glow of the interstellar plasma beyond the heliopause, and glow of the solar wind pickup ions. We show that the solar wind emissions are clearly spectrally separated from the background radiation. Metastable single‐charged helium ions would also be produced in the solar wind charge exchange collisions. Two‐photon decay of these ions would dominate in many directions the continuum extreme ultraviolet background at 1 AU in the 30–90 nm spectral range.
Strength and phase of the solar dynamo during the last 12 cycles
Authors: Ruzmaikin A. , Feynman J.
Published: February 2004

We use the aa index of geomagnetic activity recorded for 130 years to study the strength and phase properties of the solar dynamo. We split the monthly averaged aa into two parts: one part proportional to the sunspot number and the other part proportional to the residual. We argue that the first part is a proxy for the solar dynamo's large‐scale mean toroidal magnetic field. The residual has the same periodicity as the sunspot cycle and has closely related amplitude, but is shifted in phase. We relate this residual to the poloidal field generated from the toroidal field in the dynamo process. The changes in both components demonstrate that there was a long‐term trend in the strength and relative phases of the toroidal and poloidal components of the mean‐field dynamo. The changes in the relative phase of the toroidal and poloidal fields suggest that the distributions of the differential rotation and helicity within the Sun's convection zone have changed over timescales of 50 years.
Differences in the maximum intensities and the intensity‐time profiles of cosmic rays in alternate solar magnetic field polarities
Authors: Lockwood J. A. , Webber W. R. , Debrunner H.
Published: February 2004

We have found that there is a significant difference in the intensity‐time profiles of the galactic cosmic rays with energies from 100 MeV to 50 GeV around the solar activity minima in the five alternate solar magnetic field polarities from 1954 to 2000. This difference in the appearance of the two halves of the 22‐year solar magnetic cycle, peaked and flat‐topped, supports the role of drifts near the cosmic ray intensity maxima. The intensity of cosmic rays at the solar activity minima also depends on the solar magnetic field polarity, and it is less at neutron monitor energies when the solar magnetic field polarity is positive than when it is negative. At energies of ∼500 MeV and lower the opposite is the case: The values of the intensity maxima are less when the solar magnetic field polarity is negative. This intensity difference arises from the role of drifts in the cosmic ray propagation, together with possibly a change in the value of the diffusion coefficient in the two polarity cycles, that produces changes in the shape of the proton and helium energy spectra. The rigidity dependence of the ratios of the maximum intensities of both protons and helium nuclei below 1 and 2 GV, respectively, in alternate solar magnetic cycles (e.g., 1987/1977) is given by P(0.45±0.05), but the ratios are shifted by a factor of 2 in rigidity. This shift does not appear to be predicted by current modulation theories that include cosmic ray drift effects.
Numerical simulation studies on the broad upshifted maximum of ionospheric stimulated electromagnetic emission
Authors: Xi H. , Scales W. A.
Published: February 2004

Stimulated electromagnetic emission (SEE) produced by interactions of high‐power radio waves with the Earth's ionosphere is currently a topic of significant interest in ionospheric modification physics. SEE is believed to be produced by nonlinear wave‐wave interactions involving the electromagnetic and electrostatic plasma waves in the altitude region where the pump wave frequency is near the upper hybrid resonance frequency. The most prominent upshifted feature in the SEE spectrum is the broad upshifted maximum (BUM). From characteristics of this feature, a four‐wave parametric decay process has been proposed as a viable mechanism for its production. The object of this work is to (1) investigate the early time nonlinear development of the four‐wave decay instability by using theoretical and numerical simulation models, (2) study the variation of the four‐wave decay instability spectral features for a wide range of plasma and pump wave parameters, and (3) access its possible role in the production of the BUM spectral feature. Results of this investigation show that there is good agreement between predictions of the proposed theoretical model and the numerical simulation experiments. The simulation electric field power spectrum exhibits many of the important features of the experimental observations. The numerical simulation results show that consideration of the full nonlinear development of the four‐wave parametric instability is crucial in providing insight into the asymmetric nature of the wave frequency spectrum observed during the experiments.
Determination of field‐aligned currents using the Super Dual Auroral Radar Network and the UVI ultraviolet imager
Authors: Bristow W. A. , Lummerzheim D.
Published: February 2004

Field‐aligned currents (FACs) derived from the divergence of horizontal current estimates are presented. The technique for determining the currents is discussed and then example observations are presented. The horizontal currents were estimated from the product of the ionospheric conductance and the convection electric field. The ionospheric conductance was estimated from auroral ultraviolet luminosity observations from the ultraviolet imager instrument on the Polar satellite combined with an empirical model of the solar ultraviolet produced ionization. The convection electric field was estimated from plasma drift observations from the Super Dual Auroral Radar Network. The general features of the statistical FAC pattern [Iijima and Potemra, 1976] are found in the observations. In addition, transient currents in the vicinity of the magnetospheric cusp are observed and are found to be the most intense currents within the field of view. An example of FACs during the development of a magnetospheric substorm is presented. Conductance gradients are found to play an important role in determining the currents on both the dayside and nightside.
Ion distributions in large magnetic holes in the fast solar wind
Authors: Neugebauer M. , Goldstein B. E. , Winterhalter D. , Smith E. J. , MacDowall R. J. , Gary S. P.
Published: February 2004

Magnetic holes are isolated localized depressions in the magnitude of the interplanetary magnetic field. Six examples of long‐duration (9 to 30 min) magnetic holes observed by the Ulysses spacecraft at high heliographic latitudes near 3 AU are studied in detail. Each of the holes was in pressure balance with its surroundings. The proton‐proton and alpha‐proton differential streaming typically observed in the fast solar wind dropped to very low levels within the holes. The ion temperatures perpendicular to the magnetic field also increased, leading to marginal mirror‐mode stability. Possible causes of these high‐latitude magnetic holes are discussed. A new method of deconvolving the three‐dimensional angular distributions of the protons and alpha particles from the ion instrument's angular response is described in an appendix.
Relationship between X‐Ray, ultraviolet, and kilometric radiation in the auroral region
Authors: Imhof W. L. , Walt M. , Anderson R. R. , Hawley J. D. , Brittnacher M. J. , Petrinec S. M. , Matsumoto H.
Published: February 2004

An investigation is made of the simultaneous emission of X‐ray and UV photons as a function of time and location in the auroral oval, and comparisons are made of their time variations with the intensity variations of Auroral Kilometric Radiation (AKR). The X rays were measured with the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) instrument on the Polar satellite, and ultraviolet emissions were acquired with the UVI instrument also on the Polar satellite. AKR was measured using the Plasma Wave Instrument (PWI) carried on the Geotail satellite. For the events studied the time profiles of AKR were compared with those of electron precipitation into the auroral atmosphere as measured by the emission of X rays and ultraviolet radiation. The latter emissions exhibited fluctuations which on a scale of minutes to tens of minutes were coherent over several hours of magnetic local time (MLT). The polar region images of X‐ray and UV emissions were usually very similar. The cross‐correlation coefficients between X‐ray and ultraviolet emissions were as high as 0.8 over a wide range of MLT, indicating that the time variations of these different emissions were similar, although the UV observations were limited to the nightside of the Earth and to the field‐of‐view of the UVI. The correlation coefficient between AKR and precipitation as a function of the MLT of the precipitation often exceeded 0.8, usually near midnight, but was >0.5 over several hours of MLT. This widespread correspondence of AKR and precipitation fluctuations is traced to the large‐scale coherence of precipitation fluctuations around the auroral oval.
The phase structure of very low latitude ULF waves across dawn
Authors: Waters C. L. , Sciffer M. D. , Fraser B. J. , Brand K. , Foulkes K. , Menk F. W. , Saka O. , Yumoto K.
Published: February 2004

Ultralow frequency (ULF) waves at the geomagnetic equator are studied by using a small magnetometer array and a one‐dimensional electromagnetic wave model. Most ULF waves observed at low latitudes have been associated with shear Alfven mode hydromagnetic resonances in the plasmasphere. Near the equator it is difficult to excite these resonances, and the wave activity is attributed to fast mode waves. The phase difference data recorded at longitudinally spaced, equatorial magnetometers shows a marked change around dawn when one station is sunlit while the other is still in darkness. The phase structure with latitude also shows large phase shifts near the equator, in agreement with previous studies. A model of ULF wave propagation through the equatorial ionosphere is presented and used to compare with the observed amplitude and phase properties. It is shown that the observed frequencies are in the vicinity of the wave cutoff frequency where the phase structure becomes complicated. For frequencies above the cutoff, the phase structure across dawn is directly related to dawn‐associated changes in electron density in the ionosphere.
High‐resolution FUV spectroscopy of the terrestrial day airglow with the Far Ultraviolet Spectroscopic Explorer
Authors: Feldman Paul D. , Sahnow David J. , Kruk Jeffrey W. , Murphy Edward M. , Moos H. Warren
Published: February 2004

During orbital verification the Far Ultraviolet Spectroscopic Explorer obtained spectra of the terrestrial day airglow between 905 and 1184 Å from an altitude of 766 km. The spectrographs have three apertures that can simultaneously record the atmospheric emissions with limiting instrumental spectral resolutions of approximately 0.4, 0.05, and 0.03 Å. Seven orbits were obtained of observations of the sunlit Earth and disclose a wealth of emissions resulting from the electron impact excitation of N2 in addition to emissions of O I, N I, and N II produced by both photoelectron impact and by photodestructive excitation and ionization of thermospheric O and N2 by extreme ultraviolet solar radiation. The argon resonance transitions are unambiguously identified as are previously unreported transitions between highly excited energy levels of O+. These spectra have the highest spectral resolution and sensitivity in this spectral range to date and will provide valuable input to the interpretation of lower resolution spectra from current and future Earth remote sensing missions.
High‐temperature, high‐resolution photoabsorption cross sections of O2 in the 108.5‐nm region
Authors: Wu C. Y. Robert , Hung T. , Judge D. L. , Matsui Takashi
Published: February 2004

The high‐resolution absorption cross‐section measurements of O2 in the 108.0‐ to 108.7‐nm spectral region have been carried out with a resolution of 0.0008 nm and at temperatures of 295 and 535 K. The absorption features in this spectral region exhibit primarily a blue‐shaded broad band with a band head peaked at 108.52 nm. The center of gravity of the diffuse band is located at 108.452 nm. The present room temperature cross‐section data of the NII 108.5‐nm sextuplets agree well with the unpublished data of J.A.R. Samson (private communication, 1990). As the temperature increases from 295 to 535 K, we observe a decrease in cross‐section values by as much as 30% over the contour of the diffuse band. Similarly, the cross‐section values of the sharp features decrease as temperature increases. The NII 108.5‐nm multiplets are one of the prominent emissions in the Earth's airglow. The EUV‐UV day glow limb scans of the thermosphere and ionosphere have currently been gathered by satellite, e.g., ARGOS, at a tangent ray height of 200 km, above where the ambient absorption due to O2 and N2 is expected. The present temperature‐dependent cross sections of O2 provide the important database required for modeling the dayglow observations.
Preferred solar longitudes with signatures in the solar wind
Authors: Ruzmaikin A. , Feynman J. , Neugebauer M. , Smith E. J.
Published: February 2004

Using spacecraft data collected over three solar cycles, Neugebauer et al. [2000] found a persistent dependence of the solar wind speed and the radial component of the interplanetary magnetic field on solar longitude, defined in a coordinate system with a rotation period of 27.03 days. Here we investigate the solar source of this period using observations of the photospheric magnetic field. We study the lowest‐order nonaxisymmetric modes of the solar field extracted from synoptic charts of the Wilcox Solar Observatory. We find there is a robust magnetic structure on the Sun, which rotates with the period found by Neugebauer et al. [2000] in the solar wind. This rotation is more rapid than the solar equatorial and core rotations. We discuss the association of this nonaxisymmetric mode with the magnetic field generated by the solar mean field dynamo.
A test for the stationariness of the Sq current system
Authors: Patil A. R. , Rajaram R.
Published: February 2004

We have shown here that a single chain of geomagnetic observatories, located at a fixed longitude, can provide the longitudinal gradients in the strength of the current system. The empirical model of Onwumechili [1967] for low latitude Sq currents, which is based on the diurnal variation of the horizontal component of the geomagnetic field, has been augmented to reproduce D variations that are consistent with the curl‐free nature of the surface magnetic field variations. It is then possible to use variations in declination to obtain the longitudinal gradients in the strength of the current system using data from a single longitude zone. The model is applied to the data from the Indian zone. A clear linear relation between the longitudinal and temporal variations of the strength of the current system is brought out. The result appears to indicate that, at least locally, in the 75°E longitude sector, the current system is stationary and the time dependence can be used to replicate the longitudinal variations.
Predicted timing for the turn‐on of radiation in the outer heliosphere due to the Bastille Day shock
Authors: Zank G. P. , Rice W. K. M. , Cairns Iver H. , Bieber J. W. , Skoug R. M. , Smith C. W.
Published: February 2004

The propagation of the July 14, 2000 (Bastille Day), shock complex is modeled throughout the heliosphere, including its interaction with the solar wind termination shock and subsequent propagation into the inner heliosheath. The model includes pickup ions and the ionization cavity explicitly. The Bastille Day shock is used (1) to predict the time when the Voyager spacecraft can expect to observe 2‐ to 3‐kHz radiation and (2) to place constraints on the distance to the heliopause in the upwind or nose direction. On the basis of the most widely accepted model for the generation of the 2‐ to 3‐kHz radiation, we predict that the Bastille Day shock, were it to produce observable radiation in the outer heliosheath, would turn on in mid‐October 2001. The distance to the heliopause at the nose is then estimated to be < 120–130 AU, and the distance to the termination shock is estimated to be <90 AU. A further conclusion is that transmitted interplanetary shocks are very weak when they enter the outer heliosheath, the putative site at which interplanetary shocks radiate, thus requiring that the interstellar plasma in this region be primed with a preexisting energized electron population.
Extraction of solar and thermospheric information from the ionospheric electron density profile
Authors: Zhang Shun‐Rong , Oliver William L. , Fukao Shoichiro , Kawamura Seiji
Published: February 2004

This exploratory study of ionospheric electron density Ne profile assimilation concerns the extraction of information on exospheric temperature Tex, atmospheric composition [O] and [N2], meridional thermospheric winds, and solar EUV flux. These parameters are used as input variables to an ionospheric model and adjusted, one, two, or three at a time, to bring the model Ne profile into best agreement with an experimental Ne profile. The important ambiguity/uniqueness issue in the multiple‐variable fit, which has yet to be addressed in the literature, is discussed. We found the following: (1) In a single‐variable fit, any variable tested may be adjusted successfully to improve the data‐model fit, but the validity of the derived variable generally depends on the validity of the values chosen for the fixed parameters. (2) The best fit model profile from a single‐variable fit sometimes shows clear differences from the measured profile, indicating that multiple‐variable fitting is warranted. The multiple‐variable fit often greatly reduces the model‐data profile differences and improves the error bars on the derived parameters. (3) Multiple‐variable fits often show large variable correlations, indicating that variable determination is often not unique. (4) The meridional wind often shows weak correlation with other variables, suggesting that the meridional wind and any other parameter can be reliably extracted, provided that the other atmospheric variables are known. (5) [N2] often shows strong correlation with other parameters (except with the wind). The EUV flux and [O] also show large correlation. (6) The Tex‐[O] and Tex‐EUV pairs show medium to high correlation. (7) The three‐variable fits nearly always show large ambiguities in the derived parameters.
Helicity dissipation in planar periodic geometries
Authors: Watson P. G. , Craig I. J. D.
Published: February 2004

Topological problems associated with magnetic merging in periodic geometries are considered. It is pointed out that although magnetic reconnection at a single site in a periodic ensemble of null points appears physically well defined, there is some difficulty in interpreting magnetic helicity. We investigate numerically the thought experiment of Berger [1997], in which he claims a periodic assemblage of flux tubes may be turned inside out in weakly resistive plasmas, thereby violating Taylor's conjecture that the helicity (or handedness) of the tubes should be approximately invariant. A concrete demonstration confirms the reversal of the flux tubes, and Berger's suggested periodic helicity measure also changes sign. We conclude that helicity is not approximately conserved in weakly resistive periodic plasmas and therefore that topological fidelity cannot be guaranteed. We go on to suggest that unlike their doubly periodic counterparts, magnetic merging experiments in singly periodic geometries may admit well‐defined topological invariants.
A general kinetic mirror instability criterion for space applications
Authors: Leubner M. P. , Schupfer N.
Published: February 2004

Suprathermal particle populations and loss‐cone structures are the most common characteristics of ion and electron velocity distributions observed in space plasmas. We introduce a significant generalization of the family of kappa distributions to a suprathermal loss‐cone distribution, applicable to a variety of space plasma modeling. On the basis of this concept a general mirror instability threshold criterion is derived from an energy principle for collisionless plasmas, covering the full range from two‐temperature Maxwellians to complex loss‐cone conditions occupied by high‐energy ion species. We demonstrate that nonthermal plasma characteristics are the source of significant mirror instability threshold reduction where the specific structure of the velocity‐space distribution dominates as a regulating mechanism for wave mode excitation.
Auroral precipitation power during substorms: A Polar UV Imager‐based superposed epoch analysis
Authors: Newell Patrick T. , Liou Kan , Sotirelis Thomas , Meng Ching‐I.
Published: February 2004

The Polar UV Imager (UVI) is useful both for determining substorm onset times with precision on the order of l min and for making quantitative estimates of global power associated with auroral precipitation. Combining these capabilities, we studied 390 substorms, not necessarily isolated, using the superposed epoch analysis technique. The results quantify the phenomenology of auroral power during substorms. Precipitating auroral power rises with three distinct timescales based on distance from the average onset position. The most dramatic results are seen in the location of the auroral bulge, 2100–0000 magnetic local time (MLT). Integrated over this premidnight sector, auroral power decreases by ∼10% in the few minutes before onset. After onset, premidnight auroral power increases by a factor of 3.4 during the first 9 min. The largest increase occurs within 3 min after onset. After peaking at an average of ∼9 GW, premidnight auroral power declines at a steady 0.045 GW min−1 over ∼100 min. Less dramatic increases with longer risetimes are seen elsewhere on the nightside. Outside the LT of the bulge origin, but close enough that the bulge is likely to eventually reach it, the delay from onset to peak auroral power is ∼15–18 min, and the increase is typically in the range of a factor of 2 or 3. Finally, several hours away from the bulge, auroral power increases by less than a factor of 2, with a time delay of ∼36–45 min. Predawn, this represents the travel time of hot eastward drifting electrons in the diffuse aurora. Postdusk (1800–2100 MLT), a fairly weak (between 1.5 and 2.0 fold) increase is seen for unclear reasons. Altogether, a substorm averages 59% greater auroral power dissipation over the entire nightside in the 120 min following onset than in the preceding 120 min (152 and 96 TJ, respectively). By contrast, dayside auroral power is virtually invariant during substorms. Uncertainty in defining onset time using Polar UVI images was less than the 3‐min bin period adopted. Auroral onset is thus demonstrated to be well specified.
Charge transfer of keV O+ ions with atomic oxygen
Authors: Lindsay B. G. , Sieglaff D. R. , Smith K. A. , Stebbings R. F.
Published: February 2004

We report absolute differential cross sections for charge transfer scattering of 0.5–5 keV O+(4S) ground state and O+(2D, 2P) metastable ions by atomic oxygen at angles between 0.04° and 3° in the laboratory frame. Absolute total charge transfer cross sections, derived from these measurements, are compared with previously published data. The measurements were made using a flowing gas target, which consisted of a mixture of atomic and molecular oxygen produced by passing O2 through a microwave discharge. The atomic oxygen cross sections were obtained by appropriate subtraction of the signal due to molecular oxygen from that due to the mixture of O and O2. These data are relevant to model calculations of energetic O+ precipitation into the upper atmosphere and to the diffusion of low‐energy O+ ions in the ionosphere.
Shock drift acceleration of pickup protons at corotating interaction regions
Authors: Chalov S. V.
Published: February 2004

It has been demonstrated recently that the diffusive shock acceleration theory based on the assumption that the velocity distribution function of particles is close to isotropy is incapable of explaining observed spectra of ions accelerated at shocks in corotating interaction regions. In the present paper the shock drift acceleration of protons suffering multiple reflections at shock fronts is studied as applied to the forward and reverse shocks detected at Ulysses on day 20, 1993, and on day 22, 1993. The velocity distribution function is considered to be anisotropic near the shock fronts, and the magnetic moment of particles is supposed to be an adiabatic invariant which is valid in the case of weak scattering.
Radial evolution of outward and inward Alfvénic fluctuations in the solar wind: A comparison between equatorial and polar observations by Ulysses
Authors: Bavassano B. , Pietropaolo E. , Bruno R.
Published: February 2004

Ulysses measurements done during the ecliptic phase of the mission are used to investigate the radial evolution of outward and inward Alfvénic fluctuations at hourly scale in near‐equatorial solar wind. This analysis has been stimulated by a recent study on Alfvénic turbulence in polar wind, showing that at hourly scale different radial regimes develop at different distances. In the present analysis a total of 30 time intervals, characterized by highly Alfvénic fluctuations, are examined, for a total duration of 2558 hours. They are quite uniformly distributed from 1.2 to 5.2 AU along the ecliptic trajectory of Ulysses. The results clearly indicate that in the investigated radial range the energy per unit mass of the dominant outward propagating fluctuations declines, for increasing distance, with approximately the same rate observed for inward fluctuations. In other words, the ratio of inward to outward fluctuation energy roughly remains the same in the examined region. Moreover, the gradient does not vary appreciably with radial distance. These features indicate that between 1 and 5 AU the Alfvénic fluctuations have a quite different behavior in polar and in near‐equatorial solar wind. All this should imply a different role, in the two kinds of wind, of the mechanisms expected to be active in driving the Alfvénic turbulence evolution at hourly scale. Our results, combined with previous observations by Helios spacecraft, also suggest the likely presence of solar cycle effects.
Correlation of solar wind parameters between SOHO and Wind
Authors: Coplan M. A. , Ipavich F. , King J. , Ogilvie K. W. , Roberts D. A. , Lazarus A. J.
Published: February 2004

We describe and interpret new results obtained by correlating measurements of solar wind speed, density, and flux made by the plasma instruments on the SOHO and Wind spacecraft at separations between ±150 Earth radii (RE) in theY(GSE) direction and up to 280REin theX(GSE) direction. These measurements cover the period from 1996 to mid‐1998 and all of 1999, a total of 3.5 years. We have correlated observations spaced 96 s apart over 2‐hour intervals and observations spaced 300 s apart over 6‐hour intervals. Typical average correlation coefficients for solar wind flux are 0.6 ± 0.2 with 10% of the 2‐hour intervals and 22% of the 6‐hour intervals having correlation coefficients >0.8. Comparing the 2‐ and 6‐hour results, correlation coefficients are systematically smaller for the higher time resolution data. This is a result of a larger random data component and fewer solar wind plasma parameter changes in the 2‐hour data compared to the 6‐hour data. We interpret the results in terms of inclined planar fronts which provide an attractive and useful way of organizing the data. The front azimuth angles are, on average, in the same quadrant as the average Parker spiral angle. A limited three‐dimensional analysis of the fronts shows their normals to be inclined, on average, by a few tens of degrees to the ecliptic plane. Possible factors that could control front angles are examined, and long‐term correlations with solar activity are discussed. The effect of discontinuities in the interplanetary medium is also discussed.
Foreword [to Special Section on Proton Precipitation Into the Atmosphere]
Authors: Raeder J. , Maynard N. C.
Published: February 2004

The primary goal of the Geospace Environment Modeling (GEM) program of the National Science Foundation (NSF) is the development of one or more comprehensive, physical, first principles model(s) of Earth's geospace environment. There are many needs for such models, including the codification of our understanding of the physical processes, as a tool for the analysis of single (or a few) spacecraft observations, and in an operational setting for the forecast of space weather, to name a few. The analogy with atmospheric sciences shows that such model development takes decades and that the quality of models mostly progresses in small, incremental steps. In the course of model development it is imperative to test the models frequently in real world scenarios in order to document the progress and to expose shortcomings.
Proton transport model: A review
Authors: Basu B. , Decker D. T. , Jasperse J. R.
Published: February 2004

The linear transport model for the proton aurora developed at the Air Force Research Laboratory is reviewed. Emphasis is given on the discussion of the studies where the predictions of the model were compared with those of other theoretical methods and with the available data. The excellent agreement between the linear transport model and other theoretical methods gives credence to the accuracy of the numerical technique adopted in solving the transport equations, while good to excellent agreements between theory and observations indicate that the transport model adequately describes the important physics issues of the problem. It is concluded here that our ability to accurately model the proton aurora is limited only by the uncertainties in the input cross‐section data (for some collision processes) and by our inadequate information about the incident proton spectra. Some discussions in this regard are included.
Auroral particle transport using Monte Carlo and hybrid methods
Authors: Solomon Stanley C.
Published: February 2004

A numerical model of energetic electron transport in the thermosphere and middle atmosphere has been extended for application to proton aurora. The electron model employs a Monte Carlo method for the primary flux and either a full Monte Carlo or two‐stream method for secondary electrons. The hybrid Monte Carlo / two‐stream (MC2S) method has been adapted to the case of auroral proton / hydrogen atom / secondary electron fluxes. The methodology for implementation of these models is described, and selected results are compared to other algorithms. Good agreement is obtained with recent results using equation of transfer methods.
Atmospheric gamma rays from solar energetic particles and cosmic rays penetrating the magnetosphere
Authors: Share Gerald H. , Murphy Ronald J.
Published: February 2004

We detail observations of gamma rays produced by interactions of cosmic rays and solar‐energetic particles on the Earth's atmosphere. The Solar Maximum Mission (SMM) spectrometer accumulated the quiescent atmospheric spectrum over its full 9‐year lifetime and our analysis revealed 20 resolved line features. We compare this spectrum with one collected on October 20, 1989, when SMM observed gamma rays produced by shock‐accelerated protons that impacted the atmosphere in the polar region following an intense solar flare and coronal mass ejection. Observed nuclear‐line intensities increased by over a factor of 50 during this event. Because this event was on the horizon and subtended a limited solid angle as viewed from SMM, the local increase was several times higher. We compare the gamma ray line energies, widths, and intensities from the solar event and quiescent atmospheric spectra and discuss their identifications. From this comparison we confirm direct observations that the solar proton spectrum is considerably softer than the spectrum of cosmic rays. Extension of an existing code for calculating solar gamma ray lines will provide information on the spectra of the protons reaching the Earth's atmosphere and thus on their transport through the magnetosphere. Broad spectral features found in the spectra are likely to be caused by a multitude of unresolved lines excited by neutron capture in the instrument, spacecraft, and Earth's atmosphere.
Prediction of the hourly ionospheric parameter ƒ0F2 using a novel nonlinear interpolation technique to cope with missing data points
Authors: Francis N. M. , Brown A. G. , Cannon P. S. , Broomhead D. S.
Published: February 2004

A technique for neural network time series prediction using radial basis functions, where the input data contain a significant proportion of missing points, is developed. This technique is intended to model the data while simultaneously providing a means of minimizing the impact upon the model of the missing points that are typical of geophysical time series. The two issues are inextricably entwined because missing data points have a significant impact upon the performance of data‐derived models in terms of prediction availability and accuracy. The core of the technique is a nonlinear interpolation scheme that assigns values to gaps in the input time series. Each missing point is interpolated such that the error introduced into any specific predictive function is minimized. This interpolative technique has a general application in any instance where the effects of interpolation upon a given analysis process need to be minimized or a complete time series needs to be constructed from incomplete data. The technique has been applied to the prediction of ƒ0F2 from Slough, United Kingdom. The resultant model prediction root‐mean‐square (RMS) error is shown to be 2.3% better than using recurrence interpolation (in terms of overall model accuracy rather than relative to each other), 3.8% better than using persistence interpolation, and 34.3% better than not using any interpolation. Utilizing the interpolation algorithm lowers the RMS error by 26% when incomplete data, in addition to complete data, are used as an input to both the interpolated and the uninterpolated models.
Plasma sheet dynamics observed by the Polar spacecraft in association with substorm onsets
Authors: Toivanen P. K. , Baker D. N. , Peterson W. K. , Li X. , Donovan E. F. , Viljanen A. , Keiling A. , Wygant J. R. , Kletzing C. A.
Published: February 2004

We present observations of the Polar spacecraft of magnetospheric substorm signatures in the plasma sheet midway along auroral field lines between the ionosphere and the equatorial plasma sheet. On October 17, 1997, Polar was located in the onset meridian in conjunction with the Scandinavian magnetometer chain (International Monitor for Auroral Geomagnetic Effects; IMAGE). In addition, a geostationary spacecraft, LANL‐97A, was located near the onset meridian. On August 29, 1997, Polar was magnetically conjugate to the Canadian magnetometer chain (Canadian Auroral Network for the OPEN Program Unified Study; CANOPUS) ∼ 5 hours east of the onset meridian. In both cases, substorm activity was manifested as strong magnetic (20 nT) and electric (40 mV m−1) field variations with bursts of parallel Poynting flux (∼ 1 erg cm−2 s−1), predominantly directed toward the ionosphere. In the first event Polar was located in the plasma sheet near the plasma sheet boundary, and the field variations were initiated at the ground onset. In the second event, Polar crossed the plasma sheet boundary to the tail lobes a few minutes prior to a local plasma sheet expansion. As Polar was engulfed by the plasma sheet, the field variations occurred in the previously quiet plasma sheet boundary. This coincided with the auroral bulge reaching the CANOPUS stations. We compare these two events and argue that the field variations were most probably signatures of the reconnection of open field lines and the subsequent enhanced earthward flows. Furthermore, weak flow bursts were observed at Polar in both events ∼ 9 min before the onset. In the first event, a gradual development toward a negative bay and a burst of Pi2 pulsations were associated with the flow bursts. We anticipate that these signatures, often described in terms of pseudobreakups, were a precursor of the substorm onset, the initiation of the reconnection of closed field lines.
O+ observations in the cusp: Implications for dayside magnetic field topology
Authors: Fuselier S. A. , Petrinec S. M. , Trattner K. J. , Peterson W. K.
Published: February 2004

Under conditions of sustained northward interplanetary magnetic field (IMF), magnetospheric magnetic field lines on the dayside may undergo multiple reconnection, producing closed flux tubes filled with solar wind plasma. Ionospheric O+ beams with energies of ∼1 keV/e that are propagating parallel to the magnetic field in the northern cusp region are potential evidence for this multiple reconnection process. Using observations from the Polar spacecraft, we present evidence of parallel propagating O+ in the northern cusp. A statistical study of these events indicates that they are very rare in the Polar data. Therefore, under the assumption that parallel propagating O+ beams in the northern cusp are evidence of multiple reconnection, the occurrence frequency of multiple reconnection on the dayside under northward IMF conditions appears to be small. Thus this process is insufficient to account for the nearly continuous population of solar wind plasma on closed field lines in the magnetosphere.
Polar observations of topside field‐aligned O+ flows and auroral forms
Authors: Stevenson B. A. , Horwitz J. L. , Germany G. , Moore T. E. , Giles B. L. , Craven P. D. , Chandler M. O. , Su Y.‐J. , Parks G. K.
Published: February 2004

Measurements of thermal O+ ion densities, field‐aligned velocities, and fluxes from the Thermal Ion Dynamics Experiment (TIDE) on Polar obtained near 5000 km altitude over the Southern Hemisphere are compared with auroral images from the Ultraviolet Imager (UVI). Three passes were selected for analysis in this paper based on data availability from the TIDE and UVI instruments. Results indicate upward O+ flows in the cleft region but downward O+ flows in the polar cap region. Also, the O+ ion density follows a decreasing trend from the poleward side of the cusp region into the nightside aurora region. The magnitude of the downward O+ parallel velocities increases from dayside to nightside across the polar cap boundary. The upflows tend to occur over or near auroral forms, while the downflows are seen in relatively dark regions, such as the polar cap. These results are consistent with a cleft ion fountain source for the polar cap O+ ions. In the nightside polar cap, the results indicate a transition from downward to upflowing field‐aligned O+ ions near boundaries of bright auroral arcs.
Cavitation of lower hybrid waves in the Earth's ionosphere: A model analysis
Authors: Høymork S. H. , Pécseli H. L. , Lybekk B. , Trulsen J. , Eriksson A.
Published: February 2004

Lower hybrid wave cavities detected by the Freja satellite are analyzed. On the basis of simple statistical arguments by use of signals from two density probes, it is possible to obtain rather general results concerning the individual shapes of these cavities. In particular, it is demonstrated that a cylindrically symmetric Gaussian density depletion seems to give a very good fit for a vast majority of cases. The validity of this model is shown to give a great simplification in the interpretation of the data. As a result, a number of well‐defined characteristics are obtained which have to be accommodated within an ultimate theory describing the observed wave phenomena.
Flare‐ and coronal mass ejection (CME)‐associated type II bursts and related radio emissions
Authors: Leblanc Yolande , Dulk George A. , Vourlidas Angelos , Bougeret Jean‐Louis
Published: February 2004

We report on two events that occurred within 6 hours of each other on November 3, 1997. They were observed with ground‐based spectrographs in the meter to decameter range and with the WAVES experiment on the Wind spacecraft at longer wavelengths. Complementary observations were made with Extreme Ultraviolet Imaging Telescope (EIT) and Large Angle and Spectrometric Coronagraph (LASCO) experiments on the Solar and Heliospheric Observatory (SOHO). The two events are very similar in many ways: both consist of type III bursts, type II shocks, shock‐accelerated type III bursts proceeding from the type II to low frequencies, and type IV continuum. Flares and coronal mass ejections (CMEs) are associated with the two events. We trace the history of the two events from the time of the impulsive phase of the flare, which is coincident with the start time of the radio bursts and 10 to 30 min after the CME liftoff from the Sun. We derive the height‐time progression of the type II shocks by using a coronal‐solar wind density model and compare it with the progression of the CME in the plane of the sky. The results show that the speeds of the type II shocks in the low corona were high, 900–950 km s−1. Then, at a height of ≈2 RS, the fast shocks decelerate and become slower shocks, ≲380 km s−1. We discuss (1) the relationship between these type II shocks, flares and CMEs, including the apparent deceleration of the type II shocks, (2) the hypotheses of blast wave versus piston‐driven shocks, (3) the acceleration of electrons at the shock front producing the shock‐accelerated type III bursts, and (4) the acceleration of electrons that become trapped in the expanding loops of the CME and emit type IV continuum.
Isotropization of the terrestrial low‐frequency radio bursts
Authors: Steinberg J.‐L. , Lacombe C. , Hoang S.
Published: February 2004

We analyze ISEE 3 radio data acquired in the solar wind. Using a new technique we measure the spectrum of the isotropic component (“tail” or isotropic terrestrial kilometric radiation (ITKR)) of low‐frequency (LF) bursts which extends from ≃ fp.sw (the interplanetary (IP) medium plasma frequency) to an upper limit fmax which can reach 5×fp.sw. The relative intensities of the auroral kilometric radiation (AKR) burst, often associated with LF bursts, and of the ITKR are highly variable when observed from the Lagrange point Ll most probably because of propagation effects. In the range of frequencies 2fp.sw < f < 5fp.sw the AKR source angular radius measured from Ll is a few degrees, while that of the ITKR source is close to 90°: the radiation from these two sources must propagate through very different regions of the magneto sphere. The isotropization of the radiation from a magnetospheric source at fmax requires the presence of large overdense structures in the IP medium. For each event, such structures were identified and their peak plasma frequency fp.bump measured using ISEE 3 plasma density data acquired in the ecliptic. The frequency fmax is always larger than fp.bump and there is only a weak correlation between these quantities. Thus fp.bump cannot be taken as a characteristic of the three‐dimensional IP structures needed to isotropize the radiation. These structures should be rough and capable of sending some radiation towards the Sun to reach IP regions where higher values of fp.sw will be encountered. The study of the isotropization of LF bursts requires a deeper knowledge of the dense structures of the IP medium.
Features of the broad upshifted structure in stimulated electromagnetic emission spectra
Authors: Frolov V. L. , Ermakova E. N. , Kagan L. M. , Komrakov G. P. , Sergeev E. N. , Stubbe P.
Published: February 2004

We present experimental results concerning features of the broad upshifted structure (BUS) in stimulated electromagnetic emission spectra, generated in the ionospheric F region plasma by a powerful electromagnetic wave. The BUS is observed for pump frequencies far from electron cyclotron harmonics. Modification experiments, using O‐mode HF pump waves, were conducted at the SURA heating facility (56°N, 44°E, Nizhny Novgorod, Russia). We investigated the BUS properties for pump frequencies ranging from the third to the sixth electron cyclotron harmonic, concentrating on the dependence of (1) the BUS spectral features on pump frequency and power and (2) the BUS temporal evolution under different ionospheric conditions with and without additional heating of the ionosphere by another pump wave. We find (1) that the frequency range of BUS generation depends on the gyroharmonic mode number, becoming narrower with increasing mode number, and (2) that the BUS is produced in the vicinity of the upper hybrid resonance layer, located typically a few kilometers below the pump wave reflection height. An important result is that small‐scale striations, being produced concurrently with the upper hybrid plasma waves due to the thermal parametric instability, do not play a crucial role for the BUS evolution. A possible relationship of the BUS to gyroharmonic features is discussed.
Ulysses observations of sector boundaries at aphelion
Authors: De Keyser J. , Roth M. , Forsyth R. , Reisenfeld D.
Published: February 2004

We study a sample of sector boundaries observed by Ulysses near its early 1998 aphelion at 5.4 AU. We relate these sector boundaries to solar wind structure seen by Wind at 1 AU, guided by a hydrodynamic simulation. For each Ulysses sector boundary we are able to identify a corresponding 1 AU sector boundary, except when strong transients are present. Sector boundaries appear embedded in complex plasma structures that generally are in a state of pressure balance. Minimum variance analysis confirms the tangential discontinuity nature of the heliospheric current sheet (HCS) and indicates that the current sheet tends to be inclined more steeply than at 1 AU (HCS normal closer to the equatorial plane and nearly radially outward). We discuss evidence for the nonplanarity of the current sheet. Magnetic field depressions are characteristic features of the sector boundaries. We attribute them to particles that are magnetically confined inside the current sheet and that carry the diamagnetic current responsible for the large magnetic field rotation.