bluuebunny/crossref_metadata_2025 huggingface dataset:
Counterstreaming electrons in the near vicinity of the Moon observed by plasma instruments on board NOZOMI
Authors: Futaana Yoshifumi , Machida Shinobu , Saito Yoshifumi , Matsuoka Ayako , Hayakawa Hajime
Published: February 2004

The NOZOMI spacecraft approached the Moon at an altitude of about 2800 km at 0734 UT on December 18, 1998. Around the time of closest approach, a plasma instrument Particle Spectrum Analyzer/Electron Spectrum Analyzer (PSA/ESA) detected non‐solar‐wind electrons in addition to the normal solar wind component. From the characteristics of the electron distribution function, we can categorize these events into two types: (1) backstreaming electrons exhibiting a velocity distribution similar to that of the solar wind electrons, but its phase space density ratio to the solar wind electrons decreases as a function of velocity; (2) backstreaming electrons that are thermalized and have a flux comparable to or dominating that of the solar wind electrons. We considered possible source locations as well as possible mechanisms that can produce these backstreaming electrons. After careful investigation of the velocity distribution function of the electrons and the magnetic field orientation, we concluded that their origins are (1) the lunar wake region, where the electrostatic potential drop associated with ambipolar plasma expansion reflects the solar wind electrons and produces backstreaming electrons of category 1, and (2) the terrestrial bow shock, where the electrons are thermalized downstream and escaping electrons are of category 2.
Rigidity dependence of cosmic ray proton latitudinal gradients measured by the Ulysses spacecraft: Implications for the diffusion tensor
Authors: Burger R. A. , Potgieter M. S. , Heber B.
Published: February 2004

The latitudinal gradient of cosmic ray protons observed by Ulysses during September 1994 to July 1995 is small, and it increases as function of rigidity up to ∼ 2 GV and then decreases. Although previous drift models could reproduce the observed small positive gradient for an A > 0 solar polarity cycle, they produced a maximum at a rigidity well below 1 GV, in contrast to the observations. After exploring various options, it turns out that changing the rigidity dependence of the perpendicular diffusion coefficient (DC) in the polar direction so that it differs from that of the parallel DC is the most effective way to obtain good agreement with data. Specifically, we find that this DC must have a flatter rigidity dependence than the parallel DC in order to reproduce the observed rigidity dependence of the latitudinal gradient of protons during an A > 0 solar polarity cycle. We argue that the present study, combined with studies by other authors, suggests that the perpendicular mean‐free path of particles with rigidity R ≳ 0.1 GV has at least two distinct components. One is independent of particle rigidity, and one is proportional to the square of the particle rigidity at low rigidity and flattens to become almost independent of it at higher rigidity. We also show an example of the rigidity dependence of this gradient that Ulysses might observe during an A < 0 cycle. We point out that the gradient is then more sensitive to modulation parameters than during an A > 0 cycle and could impose stricter constraints on the diffusion tensor.
Direct generation of O‐mode emission in a dense, warm plasma: Applications to interplanetary type II emissions and others in its class
Authors: Farrell W. M.
Published: February 2004

A theory is presented that describes the direct generation of electromagnetic O‐mode emission via mildly energetic electron beams in a highly dense and warm plasma. In previous theories, direct generation of O‐mode emission from mildly energetic electron beams (1–10 keV) was considered unlikely because the Cerenkov wave phase speeds are much greater that the electron beam velocity, making the waves and electrons nonresonant. However, it is demonstrated that when cyclotron harmonic and relativistic effects are considered, resonance between particles and waves can occur under the conditions that ω be very near ωp, and near (but not exactly at) mΩe, m being the cyclotron harmonic number. In this case, the calculated wave phase speeds can become very low (<2 × 107 m/s), thereby establishing resonance with the electrons. Such conditions do exist in regions where interplanetary type II emission are generated, in the terrestrial foreshock region, and in planetary magnetospheres.
Evolution of ionospheric multicell convection during northward interplanetary magnetic field with |Bz/By| > 1
Authors: Huang Chao‐Song , Sofko G. J. , Koustov A. V. , Andre D. A. , Ruohoniemi J. M. , Greenwald R. A. , Hairston M. R.
Published: February 2004

During northward interplanetary magnetic field (IMF), it is generally believed that ionospheric convection appears as a four‐cell structure for |Bz/By| > 1 and as a distorted two‐cell structure for |Bz/By| < 1, where By and Bz are the Y and Z components of the IMF. In this paper we present Super Dual Auroral Radar Network (SuperDARN) observations of ionospheric convection in the Northern Hemisphere during northward IMF for |Bz/By| > 1 on November 11, 1998. We show in detail the evolution of the convection patterns as |Bz/By| changes. Nearly symmetric four‐cell convection, with two reverse cells in the polar cap and two normal cells at lower latitudes, occurs for |Bz/By| ≈ 7. The ionospheric flow associated with the reverse cells is closed almost completely on the dayside. A shifted four‐cell convection pattern, with the reverse cells shifted toward earlier magnetic local time (MLT) for negative By and toward later MLT for positive By, is observed for |Bz/By| ≈ 2.3. When |Bz/By| decreases to ∼1.7, the convection appears as a three‐cell pattern, with a single reverse cell focused near noon and two normal cells. The normal morning and afternoon cells are focused at quite high magnetic latitudes (between 76° and 80°); the spatial extent of the normal cells is 10°–15° or 1000–1500 km in the latitudinal direction. We also present Defense Meteorological Satellite Program (DMSP) satellite data which show sunward convection over the polar cap in the Southern Hemisphere at the same time as the Northern Hemisphere radar observations. We propose a new model of convection patterns during northward IMF for |Bz/By| > 1 and By < 0 on the basis of the combined observations. In the model the convection appears as a symmetric four‐cell structure for |Bz/By| ≥ 3, a shifted four‐cell structure for |Bz/By| = 2–3, and a three‐cell structure for |Bz/By| = 1–2.
Simultaneous observations of thin ion layers and the ionospheric electric field over Sondrestrom
Authors: Bedey David F. , Watkins Brenton J.
Published: February 2004

We present the results of a series of incoherent‐scatter radar experiments conducted at Sondrestrom, a high‐latitude station (invariant latitude, 73.9°) over the period August 1994 through August 1998. A novel radar mode was employed that allowed simultaneous observation of the structure of thin ion layers along the geomagnetic meridian and of the accompanying ionospheric electric field; however, only layers with peak densities exceeding ∼1 × 105 cm−3 could be detected. Layers were observed during 31 out of 85 experiments for a total of 43 distinct events. The overall objective of this investigation was to examine the limits of the current theory of layer formation. Several features are evident in the data: (1) individual layers tend to be “patchy” in both a spatial and a temporal sense; (2) the direction of the ionospheric electric field when layers were observed is consistent with theory, but for northwesterly fields the layers form ∼10 km lower than expected; (3) variation in a layer's altitude along the geomagnetic meridian can often be correlated to corresponding variation in the direction of the electric field; (4) layers tend to appear when the electric field suddenly changes in direction, magnitude, or both and then remain steady; and (5) layers often disappear from the field of view despite the continued existence of a favorable electric field. We suggest that the altitude discrepancy alluded to in item 2 may result from a systematic error in the model for ion‐neutral collision frequency widely employed in studies of thin metallic‐ion layers. We also suggest that horizontal drift of layer structures may play an important role in explaining items 4 and 5, which are otherwise difficult to reconcile with the prevailing theory.
On the use of photometer data to map dynamics of the magnetotail current sheet during substorm growth phase
Authors: Wanliss J. A. , Samson J. C. , Friedrich E.
Published: February 2004

The Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) is a large‐scale ground‐based instrument array of remote sensing equipment monitoring the high‐latitude ionosphere from the north central to the northwest portion of North America. It provides realtime coverage of auroral events such as magnetospheric substorms. In this paper the variation of magnetotail thickness is estimated during substorm growth phase by using a simple magnetic field model and data from the meridian scanning photometers in the CANOPUS array. An important parameter in the modeling process is the κ parameter, defined as the square root of the minimum curvature radius to maximum Larmor radius ratio. Through use of this parameter we are able to grossly determine the expected proton precipitation regions in the ionosphere. Two examples presented (February 9, 1995, and March 9, 1995) show that crosstail current sheet thinning from 2–0.1 RE can occur during the course of a growth phase. Within the framework of our model it is demonstrated that the magnetotail field line stretching, associated with thinning of the current sheet, is correlated with the equatorward motion of Hβ auroral emissions. Also demonstrated is the Earthward movement of the inner edge of the plasma sheet to within 6 RE.
Triangulation of sprites, associated halos and their possible relation to causative lightning and micrometeors
Authors: Wescott E. M. , Stenbaek‐Nielsen H. C. , Sentman D. D. , Heavner M. J. , Moudry D. R. , Sabbas F. T. São
Published: February 2004

Sprite halos were recently identified as an impulsive but spatially diffuse phenomenon that sometimes occurs just prior to, but distinct from, sprites. The lack of discernible spatial structure and the temporal development sequence in halos differs markedly from the highly structured bodies and tendrils and the complex development sequences of sprites. However, both phenomena are thought to result from an electric field due to charge moment changes usually associated with large positive cloud‐to‐ground (CG) lightning but also following negative CG flashes. Three‐dimensional triangulations of sprites and sprite halos were made between stations in South Dakota and Wyoming in August 1999 during the NASA Sprites99 balloon campaign. Halos were found to have a Gaussian 1/e diameter of ∼66 km and 1/e thickness of ∼4 km. Comparison with the location of the underlying lightning strokes, as recorded by the National Lightning Detection Network (NLDN), confirms that the horizontal position of sprites may be laterally offset by as much as 50 km from the underlying parent lightning discharge, as has been previously reported. The point of maximum apparent brightness for sprite halos occurs at an altitude of ∼78 km, similar to that of sprites. However, unlike sprites, this point tends to be centered directly above the underlying parent lightning discharge, 4.6 ± 2.7 km mean distance from the center of the halo to the NLDN location. This difference in spatial location relative to the underlying lightning suggests that the electrical breakdown associated with discrete sprites may require a random ionizing event such as a micrometeor. In contrast, sprite halos do not appear to require such a random component.
Further indications of a ∼140 day recurrence in energetic particle fluxes at 1 and 5 AU from the Sun
Authors: Dalla S. , Balogh A. , Heber B. , Lopate C.
Published: February 2004

A possible ∼140 day recurrence in the fluxes and anisotropies of interplanetary MeV energy protons has previously been identified in data from the Anisotropy Telescopes (ATs) on board Ulysses. This measurement was made during 1998–1999 at a distance from the Sun of ∼5 AU. Earlier reports of a 154‐day periodicity in the occurrence rate of solar flares and other solar activity indicators exist, mainly for cycle 21 data. In this paper we compare the ATs measurement, made for protons in the 1.3–2.2 MeV energy range, with fluxes of relativistic electrons (∼4–10 MeV) and protons in the range ∼30–100 MeV. We consider 5‐AU data from the Ulysses Kiel Electron Telescope (KET) instrument and 1‐AU measurements from the IMP 8 Cosmic Ray Nuclear Composition (CRNC) telescope. We find that at these higher energies significant fluxes at 5 AU are detected only in correspondence with a specific feature in the ATs recurrent pattern, thus supporting the hypothesis of recurrent behavior. We look for signatures of these events at 1 AU in the CRNC data and find for all of them an associated long‐duration particle event, regardless of the longitudinal separation between Ulysses and IMP 8. Possible solar sources of the events are discussed.
Disintegration and reformation of intermediate‐shock segments in three‐dimensional MHD bow shock flows
Authors: De Sterck H. , Poedts S.
Published: February 2004

Recently, it has been shown that for strong upstream magnetic field, stationary three‐dimensional magnetohydrodynamic (MHD) bow shock flows exhibit a complex double‐front shock topology with particular segments of the shock fronts being of the intermediate MHD shock type. The large‐scale stability of this new bow shock topology is investigated. Two types of numerical experiments are described in which the upstream flow is perturbed in a time‐dependent manner. It is found that large‐amplitude noncyclic localized perturbations may cause the disintegration of the intermediate shocks, which are indeed known to be unstable against perturbations with integrated amplitudes above critical values, but that in the driven bow shock problem there are always shock front segments where intermediate shocks are reformed dynamically, resulting in the reappearance of the new double‐front topology with intermediate‐shock segments after the perturbation has passed. These MHD results indicate a theoretical mechanism for the possible intermittent formation of shock segments of intermediate type in unsteady space physics bow shock flows when upstream magnetic fields are strong, for example, in the terrestrial bow shock during periods of strong interplanetary magnetic field, which are more common under solar maximum conditions, or in leading shock fronts induced by fast coronal mass ejections in the solar corona. It remains to be confirmed if intermediate‐shock segments would be formed when kinetic effects and realistic dissipation in real space plasmas are taken into account. The detailed interaction of realistic, wave‐like cyclic perturbations with the intermediate‐shock segments in bow shock flows may lead to unsteady structures composed of (time‐dependent) intermediate shocks, rotational discontinuities, and nonlinear wave trains, as in the scenarios proposed by Markovskii and Skorokhodov [2000]. The possible relevance of the new bow shock topology with intermediate shocks for space weather phenomena is discussed.
The response of the high‐latitude ionosphere to the coronal mass ejection event of April 6, 2000: A practical demonstration of space weather nowcasting with the Super Dual Auroral Radar Network HF radars
Authors: Ruohoniemi J. M. , Barnes R. J. , Greenwald R. A. , Shepherd S. G.
Published: February 2004

The ionosphere at high latitudes is the site of important effects in space weather. These include strong electrical currents that may disrupt power systems through induced currents and density irregularities that can degrade HF and satellite communication links. With the impetus provided by the National Space Weather Program, the radars of the Super Dual Auroral Radar Network have been applied to the real‐time specification (“nowcasting”) of conditions in the high‐latitude ionosphere. A map of the plasma convection in the northern high‐latitude ionosphere is continually generated at the Johns Hopkins University Applied Physics Laboratory (JHU/APL) SuperDARN web site using data downloaded in real time from the radars via Internet connections. Other nowcast items include information on the conditions of HF propagation, the spatial extent of auroral effects, and the total cross polar cap potential variation. Time series of various parameters and an animated replay of the last 2 hours of convection patterns are also available for review. By comparing with simultaneous measurements from an upstream satellite, it is possible to infer the effective delay from the detection of changes in the solar wind at the satellite to the arrival of related effects in the high‐latitude ionosphere. We discuss the space weather products available from the JHU/APL SuperDARN web site and their uses by simulating a nowcast of the ionosphere on April 6, 2000, during the arrival of a coronal mass ejection (CME)‐related shock. The nowcast convection pattern in particular satisfies a critical need for timely, comprehensive information on ionospheric electric fields.
Preliminary impulse of the geomagnetic storm sudden commencement of November 18, 1993
Authors: Sastri J. H. , Takeuchi T. , Araki T. , Yumoto K. , Tsunomura S. , Tachihara H. , Luehr H. , Watermann J.
Published: February 2004

The characteristics of the geomagnetic sudden commencement (SC) that occurred at 1211:30 UT on November 18, 1993, following an interval of prolonged geomagnetic quietness are studied using high time resolution data of several magnetometer networks. We present the first results concerning the behavior of the preliminary reverse impulse (PRI) of the H component of the SC near the dip equator from simultaneous observations in different longitude (local time) sectors. It is found that the preliminary reverse impulse appeared only in the prenoon (0912 LT) sector very close to the magnetic equator (dip 0.6°). At locations farther away (dip 6.0°–7.2°) but on the same meridian, the preliminary reverse impulse diminished in amplitude and led to a delayed onset of the main impulse (MI) of the SC. The preliminary reverse impulse is not apparent at locations close to the magnetic equator (dip 1.2°–2.7°) in either the afternoon (1300 LT) or the near‐dusk (1740 LT) sectors. What is seen instead here is an unambiguous reduction in the rate of increase of H component coincident with the preliminary reverse impulse in the forenoon sector. HF Doppler radar measurements of F layer vertical plasma drift close to the magnetic equator (dip 4.9°) near the dusk sector showed the SC‐related disturbance to be a decrease in ambient upward drift with considerable temporal structure, which indicates the imposition of a westward electric field. The preliminary reverse impulse is seen in the subauroral to polar region (MLAT 56.8°–76°) on the afternoon side simultaneous with that near the prenoon dip equator and with a conspicuous increase in amplitude and duration with latitude over MLAT range 66°–76°. Theoretical calculations suggest that the global current system set up by a pair of field‐aligned currents at 80° latitude and shifted to morningside (centers at 1300 and 0300 LT) could, in general, account for the observed behavior of the preliminary reverse impulse, except in the dip equatorial region near dusk.
On the spectrum of magnetosonic eigenoscillations of an axisymmetric magnetosphere
Authors: Leonovich A. S. , Mazur V. A.
Published: February 2004

The structure of magnetosonic eigenoscillations of an axisymmetric magnetosphere in the direction across magnetic shells was determined within the WKB approximation. The spectrum of their eigenfrequencies was investigated numerically. The model of the medium assumes a dipole magnetic field and a two‐dimensional Alfven velocity distribution in the meridional plane which takes into account abrupt changes of the Alfven velocity at the plasmapause and magnetopause. It is shown that eigenfrequencies are complex valued. The imaginary part of the frequency is a damping decrement of the eigenoscillations associated with the escape of some of their energy to the solar wind. It is shown that in a dipole‐like geomagnetic field in which field lines concentrated toward the origin of coordinates, the amplitude of magnetosonic oscillations increases from the magnetopause to the ionosphere. While the oscillations penetrating deep into the magnetosphere from the solar wind or generated at the magnetospheric boundary have near the magnetopause a relatively small amplitude, as the Earth is approached, their amplitude can reach a significant value for geometrical reasons.
Statistical analysis of extreme values in space science
Authors: Koons H. C.
Published: February 2004

The extreme values of many parameters measured by the space science community are important because they have a significant impact on human activities. For example, estimates of extreme radiation belt fluxes, plasma temperatures, and solar particle events must be considered when designing spacecraft, and extreme auroral currents must be considered when designing ground‐based electric power systems. The mathematical tools that have been developed to address such problems are known as the statistics of extreme values. Here these tools have been applied to three data sets in order to determine if extreme value parametric models can accurately describe space weather parameters. The three examples chosen are (1) the annual maxima of the magnetic index Ap for the years from 1932 to 1997, (2) the annual maxima of the daily average flux of >60 MeV protons measured by the Interplanetary Monitoring Platform spacecraft from 1973 to 1998, and (3) the exceedances of the flux over a threshold of 104 el cm−2 s−1 sr−1 of >2 MeV electrons at geosynchronous orbit measured by the GOES spacecraft from 1986 to 1999. The parameters for each model were determined using the maximum likelihood estimate. The validity of each model was tested using a quantile‐quantile (Q‐Q) plot. In each case an excellent fit to the extremes of the sample distribution function is given by an appropriate extreme value distribution function. These extreme value distribution functions were also used to estimate the 50‐ and 100‐year values for each data set. The results show that the extreme values observed to date are not unusual in that they are well fit by extreme value models. They also show that larger values than observed to date can be expected for each of the parameters during any 100‐year period.
Comprehensive computational model of Earth's ring current
Authors: Fok M.‐C. , Wolf R. A. , Spiro R. W. , Moore T. E.
Published: February 2004

A comprehensive ring current model (CRCM) has been developed that couples the Rice Convection Model (RCM) and the kinetic model of Fok and coworkers. The coupled model is able to simulate, for the first time using a self‐consistently calculated electric field, the evolution of an inner magnetosphere plasma distribution that conserves the first two adiabatic invariants. The traditional RCM calculates the ionospheric electric fields and currents consistent with a magnetospheric ion distribution that is assumed to be isotropic in pitch angle. The Fok model calculates the plasma distribution by solving the Boltzmann equation with specified electric and magnetic fields. To combine the RCM and the Fok model, the RCM Birkeland current algorithm has been generalized to arbitrary pitch angle distributions. Given a specification of height‐integrated ionospheric conductance, the RCM component of the CRCM computes the ionospheric electric field and currents. The Fok model then advances the ring current plasma distribution using the electric field computed by the RCM and at the same time calculates losses along particle drift paths. We present the logic of CRCM and the first validation results following the H+ distribution during the previously studied magnetic storm of May 2, 1986. The H+ fluxes calculated by the coupled model agree very well with observations by AMPTE/CCE. In particular, the coupled model is able to reproduce the high H+ flux seen on the dayside at L ∼ 2.3 that the previous simulation, which employed a Stern‐Volland convection model with shielding factor 2, failed to produce. Though the Stern‐Volland and CRCM electric fields differ in several respects, the most notable difference is that the CRCM predicts strong electric fields near Earth in the storm main phase, particularly in the dusk‐midnight quadrant. Thus the CRCM injects particles more deeply and more quickly.
Relationships between phase structure and energy flux in magnetohydrodynamic waves in the magnetosphere
Authors: Southwood David J. , Kivelson Margaret G.
Published: February 2004

Phase motion in waves as indicated by a measured phase delay between two points in a particular field component does not in general imply that energy is being carried in the direction of the phase delay so determined. We investigate here the relation between phase variation and energy flow in hydromagnetic wave signals in a model magnetosphere and identify when phase measurements between separated sites in space can indicate unambiguously the direction of the wave energy flux. For energy transfer parallel to the field, the plasma velocity (or electric field) is the most reliable guide. Perpendicular to the field, measurement of phase delays in the compressional magnetic field in space is the best guide, the phenomenon of field line resonance substantially complicating the behavior of the transverse components. In this case, however, although commonly the compressional component phase delay will be aligned with the direction of energy flow, there are important circumstances where the energy flow is antiparallel to the phase gradient and thus to the direction of the phase propagation. The analysis presented here will be especially useful in interpreting measurements from multispacecraft missions such as Cluster 2, a joint mission of the European Space Agency and NASA.
Atomic oxygen 3P → 3S0 (λ1304Å) transition revisited: Cross section near threshold
Authors: Doering J. P. , Yang J.
Published: February 2004

The direct excitation cross section for the atomic oxygen 3P → 3S0 transition (λ1304Å) has been measured at four incident energies between 13.4 and 40 eV. These measurements were made to add sufficient detail to previous direct excitation measurements to allow the near‐threshold cross section to be determined. Previous optical measurements of the 1304Å emission cross section disagree by up to a factor of 2 in this important energy region. We have also compared the atomic hydrogen e + H(1s) → H(2s + 2p) cross section, used previously as a secondary standard for our direct excitation measurements of atomic oxygen cross sections, to recent measurements of the total e + H → Lyman alpha cross section and have found agreement at the ±15% level, making a revision of our previously reported atomic oxygen cross sections unnecessary. The new direct excitation atomic oxygen cross section results, combined with previous measurements, support the emission cross section measured by Stone and Zipf [1974] and revised by Zipf and Erdman [1985] over the measurement of Wang and McConkey [1992]. Except for the region just above threshold at 10–25 eV incident energy, all the cross section measurements now agree to within ±20%. The 100 eV value of the atomic oxygen 3P → 3S0 emission cross section from a combination of the emission and direct excitation cross section experiments is 8.6 ± 11% × 10−18 cm2.
Numerical studies on the propagation of sudden impulses in the dipole magnetosphere
Authors: Lee Dong‐Hun , Hudson Mary K.
Published: February 2004

This paper presents a time‐dependent numerical experiment when the dayside magnetopause is perturbed by sudden impulses in the solar wind. Magnetic and electric fields in a three‐dimensional dipole model are investigated. Arrival timing over various locations is examined in the linear wave approximation. It is found that the propagation of the leading wavefronts undergoes significant refraction owing to the inhomogeneity of dipolar geometry. Compressional and transverse waves become differentiated in travel paths and arrival times near the polar ionospheric region. Our results also suggest that wave energy density becomes larger in the plasmasphere than in the outer magnetosphere. We discuss in detail how the plasmasphere affects the propagation of initial impulses.
Spatial structure of ULF waves: Comparison of magnetometer and Super Dual Auroral Radar Network data
Authors: Ponomarenko P. V. , Waters C. L. , Sciffer M. D. , Fraser B. J. , Samson J. C.
Published: February 2004

The spatial structure of ultralow frequency (ULF) waves is usually, though not exclusively, estimated from ground‐based magnetometer measurements. This paper compares ULF wave spatial structure obtained from coincident ground magnetometer and HF radar measurements and addresses the interpretation of Pc5 azimuthal wave numbers. ULF spatial structures estimated from magnetometer and radar data were quite different for the October 23, 1994, event presented by Ziesolleck et al. [1998]. Azimuthal wave numbers (m) were 3–5 and 12 for the ground and ionosphere, respectively. We reexamine this event and attempt to explain why the spatial structure of the ULF wave in the ionosphere, seen by the Saskatoon Super Dual Auroral Radar Network (SuperDARN) radar, may differ from that deduced from the magnetometer data. The radar data are used to develop a two‐dimensional (2‐D) model of the spatial distribution of ULF amplitude and phase in the ionosphere. Our modeling shows that the differences between ground and ionosphere measurements may be explained by spatial integration. In general, m numbers deduced from ground measurements should be smaller than the ionospheric values, and they are strongly dependent on the ionospheric ULF amplitude and phase distribution in both latitude and longitude.
The Jovian auroral oval
Authors: Hill T. W.
Published: February 2004

Jupiter's aurora shows, among other features, a persistent, continuous oval of luminosity that encircles each magnetic pole and maps along magnetic field lines to the middle magnetosphere (r ≈ 30 Rj). This auroral oval is interpreted here as the ionospheric footprint of the upward Birkeland (magnetic field aligned) current that enforces partial corotation of magnetospheric plasma moving outward from its source (the Io plasma torus) to its sink (the outer magnetosphere and ultimately the solar wind). A simplified model of this current system, based on the assumption of a constant, uniform plasma outflow in a spin‐aligned dipole magnetic field, has a maximum upward Birkeland current density at an ionospheric latitude that maps to the middle magnetosphere. Strong upward Birkeland currents are known, from terrestrial studies, to produce bright aurora.
Comparison of pitch angle diffusion by turbulent and monochromatic whistler waves
Authors: Albert J. M.
Published: February 2004

A recent Hamiltonian analysis of magnetized test particles acted on by a small monochromatic wave is compared to a quasi‐linear formulation of pitch angle diffusion by a turbulent spectrum of whistler waves. The quasi‐linear expression has previously been applied to anthropogenic VLF transmissions to conclude that they dominate pitch angle diffusion of inner radiation belt electrons. However, because these transmissions are very narrow in frequency range, the monochromatic approach might be more appropriate. It is shown analytically that the monochromatic limit of the quasi‐linear pitch angle diffusion coefficient reduces to the same expression as the diffusive regime of the Hamiltonian treatment. Agreement of the Hamiltonian pitch angle diffusion coefficient with the quasi‐linear values is verified numerically using realistic parameters for the spread in frequency and wave‐normal angle. As an aside, a simple, alternate derivation of quasi‐linear diffusion coefficients is given.
Drift mirror instability in space plasmas, 2, Nonzero electron temperature effects
Authors: Pokhotelov Oleg A. , Onishchenko Oleg G. , Balikhin M. A. , Treumann R. A. , Pavlenko Vladimir P.
Published: February 2004

A linear theory of drift mirror instability accounting for the nonzero electron temperature effects is developed. Generalizing our previous approach to the analysis of this instability by accounting for a nonvanishing parallel electric field, we have derived the expressions for the mode frequency and instability growth rate. The origin of the electric field is due to the electron pressure gradient which builds up in a plasma with nonzero electron temperature, because the electrons are dragged by mirror‐accelerated protons as they pass from regions of high magnetic flux into those of lower magnetic flux. The electrostatic force drag associated with the parallel electric field provides a substantial reduction of the wave phase velocity and increases the drift mirror instability threshold. It is shown that in a plasma with nonzero electron temperature the drift mirror mode is accompanied by the field‐aligned current which varies in phase with the compressional changes in the magnetic field. The transition to the cold electron temperature limit is discussed.
Helium energetics in the high‐latitude solar wind: Ulysses observations
Authors: Reisenfeld D. B. , Gary S. P. , Gosling J. T. , Steinberg J. T. , McComas D. J. , Goldstein B. E. , Neugebauer M.
Published: February 2004

We present a study of the interplanetary evolution of solar wind helium (alpha particle) energetics. The analysis of Ulysses observations of the fast high‐latitude solar wind concentrates on the radial evolution of the alpha‐proton differential streaming vαp, the alpha temperature, and the alpha temperature anisotropy. Ulysses observations show that the average vαp steadily decreases with radius, ranging from ∼40 km s−1 at 1.5 AU to ∼15 km s−1 at 4.2 AU. In addition, observations indicate that the alphas cool more slowly than what would be expected from adiabatic expansion. The radial increase in the nonadiabatic heat content of the alphas matches the free energy liberated as vαp decreases with distance, suggesting that the dissipated energy acts to heat the alpha particles. The alphas also exhibit a temperature anisotropy of T⊥α/T‖α = 0.87, which is essentially constant with distance. These and other observations reported here place stringent constraints on recent plasma microinstability models that attempt to explain the evolution of alpha‐proton differential streaming and ion heating in the heliosphere.
Dominant role of the asymmetric ring current in producing the stormtime Dst*
Authors: Liemohn M. W. , Kozyra J. U. , Thomsen M. F. , Roeder J. L. , Lu G. , Borovsky J. E. , Cayton T. E.
Published: February 2004

Three storms are examined to determine the contribution to the Dst* index from the symmetric and asymmetric (partial) components of the ring current. The storms (September 24–25, 1998, October 18–19, 1998, and May 14–15, 1997) all have a similar solar wind trigger (an initial shock followed by a coronal mass ejection with southward interplanetary magnetic field) and placement in the solar cycle (rising phase). The near‐Earth ion distribution function is simulated for each storm using a kinetic transport model. The use of a McIlwain magnetospheric electric field description improves the simulation results over the Volland‐Stern field used previously. It is found that most of the main phase magnetic field depression is due to the asymmetric component of the ring current (≥80% at the Dst* minimum for the three storms). Note that this is a minimum asymmetric ring current contribution, because the closed‐trajectory ions may also be spatially asymmetric. Ions in the partial ring current make one pass through the inner magnetosphere on open drift paths that intersect the dayside magnetopause. Changes in the density of the inner plasma sheet are transmitted directly along these open drift paths. For a steady convection field, an increase in the source population produces a decrease (more intense perturbation) in Dst*, while a decrease produces a Dst* recovery. As the storm recovery proceeds, a decrease in the electric field results in a conversion of open to closed drift paths, forming a trapped, symmetric ring current that dominates Dst*. The mostly H+ composition of the ring current for all three storms rules out the possibility of differential charge exchange being the cause of the fast and slow decay timescales, confirming that outflow is the main loss of ring current‐generated Dst* during the early phase decay. The slow decay timescale in the late recovery, however, is dominated by charge exchange with the hydrogen geocorona. The symmetric‐asymmetric ring current is also placed in the context of the solar wind and plasma sheet drivers.
Variability of a composite chromospheric irradiance index during the 11‐year activity cycle and over longer time periods
Authors: Lean J. L. , White O. R. , Livingston W. C. , Picone J. M.
Published: February 2004

Chromospheric emission lines are the dominant energy input to the Earth's upper atmosphere, where they create its embedded ionosphere. Knowledge of the Sun's chromospheric radiation is therefore essential for understanding variability in this region, which contributes significantly to space weather. With the goal of obtaining a suitable surrogate of extreme ultraviolet irradiance variability for atmospheric studies, we construct a composite chromospheric index by comparing and cross calibrating available ground‐ and space‐based indices. Since ground‐based flux observations of the chromosphere exist only since 1974, we construct 11‐year activity cycles of the index since 1950 by using a parameterization of the daily plage index and the 100‐day smoothed 10.7‐cm flux, and prior to that by an analogous parameterization of the daily arid 100‐day smoothed sunspot group numbers. Comparisons of solar and stellar chromospheric indices suggest that long‐term changes in the Sun's chromospheric emission since the Maunder minimum may exceed recent solar cycle amplitudes by as much as a factor of 2. We simulate this by adding to the 11‐year activity cycles a speculated varying background component derived from 15‐year smoothed sunspot group numbers that increases from 1900 to 1950 and remains approximately level in the decades since then.
Migrating thermospheric tides
Authors: Hagan M. E. , Roble R. G. , Hackney J.
Published: February 2004

The capabilities of the global‐scale wave model (GSWM) [Hagan et al., 1995, 1999] are extended to include migrating thermospheric solar tides. The GSWM thermospheric tidal forcing parameterization is based on neutral gas heating calculated from first principles in the National Center for Atmospheric Research (NCAR) thermosphere/ionosphere electrodynamics general circulation model (TIE‐GCM). This is the first time that a physics‐based thermospheric forcing scheme has been used in a model like GSWM. Previous two‐dimensional steady state linear tidal models used exospheric temperature measurements to calibrate upper atmospheric tidal forcing. New GSWM results illustrate thermospheric tidal responses that are largely consistent with tides in the TIE‐GCM. Diurnal temperature amplitudes increase with increasing solar activity, but there is no analogous diurnal wind response. The thermospheric semidiurnal tide is much weaker than the diurnal tide. Semidiurnal temperature perturbations peak in the lower thermosphere where the semidiurnal forcing maximizes. The new in situ results must be combined with the GSWM upward propagating tide in the lower thermosphere, because the upward propagating components dominate the semidiurnal response throughout the region and the diurnal response below ∼130 km. In situ forcing accounts for most of the diurnal response aloft. Our preliminary evaluation of the GSWM thermospheric predictions is inconclusive. More extensive evaluations are necessary to make a firm assessment of whether the model captures the salient features of the seasonal and solar cycle variability of thermospheric tides.
Observed trends in auroral zone ion mode solitary wave structure characteristics using data from Polar
Authors: Dombeck J. , Cattell C. , Crumley J. , Peterson W. K. , Collin H. L. , Kletzing C.
Published: February 2004

High‐resolution (8000 sample s−1) data from the Polar Electric Field Instrument are analyzed for a study of ion mode solitary waves in upward current regions of the auroral zone. The primary focus of this study is the relations between velocity, maximum potential amplitude, and parallel structure width of these solitary waves (SWs). The observed SW velocities consistently lie, within error bars, between those of the H+ and O+ beams observed simultaneously by the Toroidal Imaging Mass‐Angle Spectrograph (TIMAS) instrument. In addition, there is a trend that SW amplitudes are smaller when SW velocities are near the O+ beam velocity and larger when SW velocities are near the H+ beam velocity. These results are consistent with the observed ion mode SWs being a mechanism for the transfer of energy from the H+ beam to the O+ beam. A clear trend is also observed indicating larger amplitude with larger parallel spatial width. The results suggest that the observed solitary waves are a rarefactive ion mode associated with the ion two‐stream instability.
Microscopic signatures of the bipartite auroral oval: FAST measurements
Authors: Peria W. J. , Brittnacher M. J. , Parks G. K. , Carlson C. W.
Published: February 2004

The separation of the nightside auroral oval into two distinct regions, sometimes referred to as a “double oval,” is often observed during substorm recovery in global images from the Ultraviolet Imager (UVI) experiment. The two regions of auroral emission are both extended for a few hours of magnetic local time and are separated from one another by a few degrees in magnetic latitude. High‐resolution, low‐altitude data from the FAST spacecraft reveal that the two regions (1) represent distinct types of electron precipitation, (2) contain distinct types of ion distributions, and (3) are accompanied by a regular and repeatable field‐aligned current pattern. A region of corotating plasma is often observed in conjunction with the equatorward region. The low‐altitude particle measurements indicate that the equatorward emission is caused by the diffusion into the loss cone of ∼2 keV electrons from the plasma sheet, while the poleward emission is caused by the electrostatic acceleration of electrons at much lower altitudes. The two regions represent electron energy fluxes of roughly the same magnitude (although the poleward flux is larger), but the poleward flux carries a substantial field‐aligned current, while the equatorward flux does not. This low‐altitude current pattern, interpreted in terms of ionospheric stress applied by convective flow in the tail, indicates that the equatorward emission is associated with very weak and uniform high‐altitude flow, while the poleward emission is at the footpoint of strongly sheared duskward flow.
Experimental investigations on photoelectric and triboelectric charging of dust
Authors: Sickafoose A. A. , Colwell J. E. , Horányi M. , Robertson S.
Published: February 2004

Experiments are performed pertaining to the charging of single dust particles in space due to three effects: (1) photoemission, (2) the collection of electrons from a photoemissive surface, and (3) triboelectric charging. The particles tested are 90–106 μm in diameter and include JSC‐1 (lunar regolith simulant) and JSC‐Mars‐1 (Martian regolith simulant). Isolated conducting grains (Zn, Cu, and graphite) illuminated by ultraviolet light reach a positive equilibrium floating potential (a few volts) that depends upon the work function of the particle. Conducting grains dropped past a photoemitting surface attain a negative floating potential for which the sum of the emitted and collected currents is zero. Nonconducting grains (glass, SiC, and the regolith simulants) have a large initial triboelectric charging potential (up to ± 15 V) with a distribution approximately centered on zero. The nonconducting grains are weak photoemitters, and they attain a negative floating potential when dropped past a photoemitting surface. Our experimental results show that for silicate planetary regolith analogs, triboelectric charging may be the dominant charging process and will therefore play an important role in the subsequent behavior of dust grains released from planetary surfaces.
Impact of a major meteor storm on Earth's ionosphere: A modeling study
Authors: McNeil William J. , Dressler Rainer A. , Murad Edmond
Published: February 2004

A comprehensive model of the effect of a major meteor storm on Earth's ionosphere is presented. The model includes meteor stream mass distributions based on visual magnitude observations, a differential ablation model of major meteoric metals, Fe and Mg, and state‐of‐the‐art modeling of the chemistry and transport of meteoric metal atoms and ions subsequent to deposition. Particular attention is paid to the possibility of direct ionic deposition of metallic species. The model is validated by calculating the effect of annual meteor showers on the background metal atom and ion abundances. A metallic ion density increase of up to 1 order of magnitude is observed, in agreement with in situ measurements during showers. The model is exercised for a hypothetical Leonid meteor storm of the magnitude reported in 1966. The model predicts the formation of a layer of metal ions in the ionosphericEregion that reaches peak densities of around 1 × 105cm−3, corresponding to a 2 order of magnitude increase of the quiescent nighttimeEregion density. Although sporadicElayers reaching or exceeding this density are relatively common, the effect is different in that it persists on the order of days and would be observed over nearly one‐half the globe. The model predictions are consistent with the available 1966 Leonid storm data. In particular, the observation of enhanced, predawn sporadicEactivity points to efficient collisional ionization of meteoric metals, as assumed in the model.
Ion acoustic damping effects on parametric decays of Alfvén waves: Right‐hand polarization
Authors: Gomberoff L. , Gomberoff K. , Brinca A. L.
Published: February 2004

We study ion acoustic damping effects on parametric decays of right‐hand‐polarized electromagnetic waves. We do this because ion beams have been observed in a variety of space environments, and consequently, these waves can exist in those places. Damping effects are incorporated into the model by adding to the longitudinal component of the equation of motion a collision‐like term. Like for left‐hand‐polarized waves, the effect of damping is twofold. On the one hand, damping decreases the maximum growth rate of the existing instabilities while increasing the instability range, and on the other hand, it destabilizes regions that are stable in the absence of damping. Thus, for low‐frequency pump waves, ω0 ≪ ωci, and for low β = vt/vA (ωci is the ion gyrofrequency, and vt and vA are the thermal and the Alfvén velocities, respectively), where the only parametric instability is a decay instability, damping destabilizes the frequency range between ω = 0 and the threshold of the decay instability. As β increases, two new parametric instabilities develop: One of them is a modulational instability, and above some threshold value of the pump wave amplitude, there is also a beat wave instability. The decay instability is also possible for pump wave amplitude above some threshold. For even larger β the decay instability is no longer possible. In all cases, damping effects reduce the growth rate of the existing instabilities and destabilize regions which are stable in the absence of damping. These results are in agreement with those obtained by Vasquez [1995]. It is also shown that for large‐frequency pump waves, electron/ion whistler waves, the decay instability reappears even for large β and has very large growth rates.