bluuebunny/crossref_metadata_2025 huggingface dataset:
A chlorophyll‐dependent semianalytical reflectance model derived from field measurements of absorption and backscattering coefficients within the Southern Ocean
Authors: Reynolds Rick A. , Stramski Dariusz , Mitchell B. Greg
Published: February 2004

A semianalytical model was developed for the prediction of spectral remote sensing reflectance (Rrs) as a function of fluorometric chlorophyll a concentration (Chl) for two regions within the Southern Ocean: the Ross Sea and the Antarctic Polar Front Zone (APFZ). The model is based upon Chl‐dependent parameterizations of the spectral absorption, a(λ), and backscattering, bb(λ), coefficients of seawater which were derived from field measurements. The relationships between a(λ) and Chl were similar in both regions, but for comparable Chl the particulate backscattering was on average 4 times greater in the APFZ. Measurements of particle size distributions suggest that particle assemblages in the APFZ were characterized by a greater predominance of smaller particles, consistent with the observed regional differences in backscattering properties. The model is used to examine the separate influences of absorption and backscattering on the blue to green ratio of reflectance, Rrs(490)/Rrs(555). Variability in the spectral absorption ratio, resulting principally from changes in the relative contribution of water to total absorption in each band, contributes >75% to changes in the Rrs(490)/Rrs(555) ratio as a function of Chl. However, variability in the spectral backscattering ratio appears to be the primary cause for the observed differentiation in the Rrs versus Chl relationships between the two regions.
Electrostatic particle simulations of solitary waves in the auroral region
Authors: Miyake T. , Omura Y. , Matsumoto H.
Published: February 2004

We study the formation mechanism of two‐dimensional solitary waves observed in the auroral region via two‐dimensional electrostatic particle simulations. The FAST satellite observed very strong “solitary waves” in the downward current regions of the mid‐altitude auroral zone, and the Polar satellite also succeeded in detecting solitary waves in the polar region. These “solitary waves” are reported to have isolated two‐dimensional structures. The present simulation study demonstrates that such isolated two‐dimensional “solitary waves” can be generated by a simple electron two‐stream instability. We performed two simulation runs; one was a run without ion dynamics, and the other was a run with ion dynamics. By comparing these two runs, we found isolated two‐dimensional potentials are generated due to the ion dynamics. In these simulations an initial electron beam instability first forms solitary potentials isolated in the parallel direction. When the ion dynamics are neglected, these potentials become coherent in the perpendicular direction, forming one‐dimensional structures, owing to the ion dynamics, however, quasi‐perpendicular lower hybrid waves are strongly excited through coupling with parallel drifting electron potentials. Potentials are divided in the perpendicular direction by these lower hybrid waves, forming isolated two‐dimensional potentials. In a long time evolution, these isolated two‐dimensional potentials are gradually aligned in the perpendicular direction, and the perpendicular potential energy decreases. Finally, they again become one‐dimensional potentials coherent in the direction perpendicular to the ambient magnetic field.
A review on the genesis of coronal mass ejections
Authors: Forbes T. G.
Published: February 2004

This paper provides a short review of some of the basic concepts related to the origin of coronal mass ejections (CMEs). The various ideas which have been put forward to explain the initiation of CMEs are categorized in terms of whether they are force‐free or non‐force‐free and ideal or nonideal. A few representative models of each category are examined to illustrate the principles involved. At the present time there is no model which is sufficiently developed to aid forecasters in their efforts to predict CMEs, but given the current pace of research, this situation could improve dramatically in the near future.
CRRES observations of density cavities inside the plasmasphere
Authors: Carpenter D. L. , Anderson R. R. , Calvert W. , Moldwin M. B.
Published: February 2004

Deep density troughs inside the plasmasphere in which electron density was a factor of from ∼2 to 10 below nearby plasmasphere levels were found in ∼13% of 1764 near‐equatorial electron density profiles derived from the sweep frequency receiver data acquired in 1990–1991 by the CRRES satellite. These “inner troughs” appeared in the aftermath of plasmasphere erosion episodes and are interpreted as the near‐equatorial manifestations of geomagnetic‐field‐aligned cavities. Inner troughs were found at all local times but were most common in the 1800–2400 magnetic local time (MLT) sector and least common between 0600 and 1200 MLT. Their inner boundaries, plasmapause‐like in form, were mostly at L < 3.5 but in ∼30% of the cases were at L < 2.5 under geomagnetic conditions that traditionally have been associated with plasmapause radii in the L = 3–3.5 range or beyond. The trough outer walls were exceptionally steep, in several cases exhibiting a factor of 4 or more density change within less than 100 km along the near‐equatorial satellite orbit. The extent of the troughs in L ranged from ΔL ∼ 0.5 to 2, and various forms of evidence, including earlier studies, suggest an extent of more than 20° in longitude. Such evidence includes plasma waves propagating in a free space mode within the inner trough while extending in frequency well above the upper limit of trapped continuum radiation detected beyond the plasmasphere. We suggest, as have previous authors, that the troughs are translated vestiges of plasma configurations established during preceding periods of plasmasphere erosion. In some such cases, dense plasma features lying beyond the troughs were probably connected to the main plasmasphere in a local time sector to the east of the observing longitude. However, in some of the cases of troughs with steep outer walls the dense plasma feature beyond that wall may have been shaped by a mechanism for detaching plasma from an originally larger outer plasmasphere, such as by shear flows in the premidnight sector associated with subauroral ion drifts.
Solar wind electron characteristics inside and outside coronal mass ejections
Authors: Skoug R. M. , Feldman W. C. , Gosling J. T. , McComas D. J. , Smith C. W.
Published: February 2004

We present a study of 9 months of data from the solar wind plasma electron instrument on the ACE spacecraft. Electron pitch angle distributions were used to identify intervals of counterstreaming halo electrons, which were observed ∼16% of the time that the spacecraft was not magnetically connected to the Earth's bow shock. Counterstreaming electrons presumably indicate a closed magnetic field topology and thus indicate the passage of coronal mass ejections (CMEs) in the solar wind. In this study, we separately examine electron moments at times with and without counterstreaming electrons, including both magnetic clouds and noncloud CMEs. The properties of both the core and halo electron populations were nearly identical at times with and without counterstreaming electrons. Both low and high electron densities and temperatures were observed in either type of event. In contrast, magnetic clouds, on average, showed increased densities and reduced temperatures. The core/halo density ratio and the total electron density were anticorrelated in all cases, indicating that the halo contributes more to the total electron density at low density times, regardless of magnetic topology. Total electron temperature and density were anticorrelated in all types of events. This consistent anticorrelation implies that such single‐spacecraft measurements of temperature and density cannot be used to determine a polytropic index for solar wind electrons. Instead, the anticorrelation may be due to pressure balance of the solar wind plasma or may be a remnant of coronal conditions.
The origin and development of the May 1997 magnetic cloud
Authors: Webb D. F. , Lepping R. P. , Burlaga L. F. , DeForest C. E. , Larson D. E. , Martin S. F. , Plunkett S. P. , Rust D. M.
Published: February 2004

A complete halo coronal mass ejection (CME) was observed by the SOHO Large‐Angle and Spectrometric Coronagraph (LASCO) coronagraphs on May 12, 1997. It was associated with activity near Sun center, implying that it was aimed earthward. Three days later on May 15 an interplanetary shock and magnetic cloud/flux rope transient was detected at the Wind spacecraft 190REupstream of Earth. The long enduring southward magnetic fields associated with these structures triggered a geomagnetic storm. The CME was associated with a small coronal arcade that formed over a filament eruption with expanding double ribbons inHα emission. The flare was accompanied by a circular EUV wave, and the arcade was flanked by adjacent dimming regions. We surmise that these latter regions marked the feet of a flux rope that expanded earthward into the solar wind and was observed as the magnetic cloud at Wind. To test this hypothesis we determined key parameters of the solar structures on May 12 and compared them with the modeled flux rope parameters at Wind on May 15. The measurements are consistent with the flux rope originating in a large coronal structure linked to the erupting filament, with the opposite‐polarity feet of the rope terminating in the depleted regions. However, bidirectional electron streaming was not observed within the cloud itself, suggesting that there is not always a good correspondence between such flows and ejecta.
On cyclotron wave heating and acceleration of solar wind ions in the outer corona
Authors: Tu C.‐Y. , Marsch E.
Published: February 2004

The preferential heating and acceleration of O+5 ions, as observed by Ultraviolet Coronagraph Spectrometer (UVCS) on Solar and Heliospheric Observatory (SOHO) [Kohl et al., 1998] in the solar coronal holes have been interpreted and modeled by invoking wave‐particle cyclotron resonance [Cranmer et al., 1999a, 1999b]. However, in the model of Cranmer et al. [1999a, 1999b] and in other subsequent models the assumption of a rigid slope of the wave spectrum was made in calculating the wave energy absortion by the different ion species. In the present paper it is shown that a self‐consistent treatment of the wave damping and absorption is necessary and leads to substantially different results. On the basis of quasi‐linear theory, the interaction between the ions and the ion‐cyclotron waves [Marsch et al., 1982a; Marsch, 1998] is studied. The total energy conservation equation, including the kinetic energy of the resonant particles and the wave energy, is derived and discussed in detail. The spectral evolution equation for cyclotron waves, when being controlled by the wave growth/damping rate and WKB effects, is solved self‐consistently together with the full set of anisotropic multifluid equations for the ions including the cyclotron‐resonance wave heating and acceleration rates. From the numerical results we reach the following conclusions: (1) It is physically questionable to use a spectrum with a fixed spectral slope near the cyclotron resonance when one calculates the partition of wave energy among the different ionic species and the kinetic degrees of freedom parallel and perpendicular to the magnetic field. This assumption neglects the important effects of wave absorption and the concurrent reshaping of the wave spectrum, and thus leads in the dissipation domain to extremely low amplitudes of the waves and to difficulties in supplying enough energy to balance the wave absorption at the cyclotron resonances. (2) If the spectrum is allowed to evolve self‐consistently and concurrently with the particles' heating and acceleration through wave absorption, such a high perpendicular temperature and corresponding large temperature anisotropy as observed by UVCS do not occur or cannot be maintained. We conclude that the UVCS oxygen ion observations have not yet been explained satisfactorily by the cyclotron‐resonance theory.
Simultaneous observations of lower thermospheric composition change during moderate auroral activity from Kangerlussuaq and Narsarsuaq, Greenland
Authors: Hecht J. H. , McKenzie D. L. , Christensen A. B. , Strickland D. J. , Thayer J. P. , Watermann J.
Published: February 2004

To obtain information regarding the spatial scales involved in lower thermospheric composition changes in the auroral zone, simultaneous observations were carried out using photometers at Kangerlussuaq and Narsarsuaq, Greenland, two sites separated by ∼750 km in distance. At night, Kangerlussuaq, located at 67°N geographic is most often at the northern edge of the auroral oval, while Narsarsuaq, at the southern tip of Greenland, is more often in or at the southern edge of the auroral oval. On January 29–30, 1998, moderate auroral activity was recorded from both sites. Besides the photometer systems, observations were obtained during part or all of this period from Polar satellite instruments, from the Greenland magnetometer chain, from the CANOPUS chain of magnetometers and photometers, from DMSP, and from the Sondrestrom incoherent scatter radar. The analysis of these data show the necessity of considering both nonlocal and local sources in modeling aurorally induced changes in lower thermospheric composition.
Heating and acceleration of coronal ions interacting with plasma waves through cyclotron and Landau resonance
Authors: Marsch E. , Tu C.‐Y.
Published: February 2004

On the basis of quasi‐linear theory, the parallel and perpendicular wave heating and acceleration rates for gyrotropic particle velocity distribution functions are derived. These rates can be used in anisotropic multicomponent fluid equations, in order to describe the wave‐particle interactions of ions with, for examples, kinetic Alfvén and electromagnetic or electrostatic ion cyclotron, respectively, magnetosonic waves propagating along or obliquely to the mean magnetic field. The waves of coronal origin propagating away from the Sun into the interplanetary medium can resonantly heat the solar wind ions and accelerate minor ions preferentially with respect to the protons. Such processes are required in order to explain and understand the measured characteristics of ion velocity distributions in the solar wind and to interpret the recent spectroscopic evidence obtained from EUV emission line measurements made by the Solar and Heliospheric Observatory (SOHO) spacecraft, which indicate cyclotron‐resonance‐related line broadenings and shifts.
Nonlinear MHD wave propagation in the magnetosphere: A time‐dependent approach
Authors: Lee Dong‐Hun , Kim Kihong , Lysak Robert L.
Published: February 2004

Dynamics of nonlinear MHD wave propagation is theoretically studied in the magnetosphere. In order to examine the time‐dependent propagation of arbitrary initial impulses, we adopt the approach of simple waves, which is often used in ordinary fluids. We obtain an exact solution for the velocity profile in space and time when arbitrary initial perturbations are assumed in a simplified model. These formulae enable us to investigate the characteristics of nonlinear wave propagation and its steepening process while the shock is being formed. We describe how various nonlinear impulses differentially evolve and propagate in time by assuming different timescales for a driving source.
Alpha/proton magnetosonic instability in the solar wind
Authors: Gary S. Peter , Yin Lin , Winske Dan , Reisenfeld Daniel B.
Published: February 2004

The average relative flow velocity between protons and alpha particles in collisionless plasmas can excite several distinct alpha/proton instabilities. Here linear theory and two‐dimensional hybrid simulations in a homogeneous plasma model are used to study one such mode, the alpha/proton magnetosonic instability, using plasma parameters characteristic of the high‐speed solar wind observed by the Ulysses spacecraft at high heliospheric latitudes. Wave‐particle scattering by enhanced fluctuations from this mode reduces the alpha/proton relative speed and heats the alphas more strongly than the protons. Post‐saturation results for the alpha/proton flow in the simulations are approximately bounded by the linear theory threshold condition for onset of the magnetosonic instability, which is also a statistical upper bound on the alpha/proton relative speed at high βp for some of the Ulysses observations. Thus it is likely that this instability is a constraining agent for the alpha/proton relative speed in some domains of the solar wind.
Improved solar Lyman α irradiance modeling from 1947 through 1999 based on UARS observations
Authors: Woods Thomas N. , Tobiska W. Kent , Rottman Gary J. , Worden John R.
Published: February 2004

The solar Lyman α radiation is the brightest solar vacuum ultraviolet (VUV: λ < 200 nm) emission, and this radiation is deposited in Earth's atmosphere above 70 km. The Lyman α irradiance and its variability are therefore important for many studies of the Earth's upper atmosphere. A long‐term data set of the solar Lyman α irradiance from 1947 through 1999 is constructed using the measurements from the Atmospheric Explorer E (AE‐E), the Solar Mesospheric Explorer (SME), and the Upper Atmosphere Research Satellite (UARS) along with predictions from proxy models to fill in data gaps and to extrapolate back to 1947. The UARS measurement is used as the reference, and the AE‐E and SME measurements and the proxy models are adjusted to agree with the UARS values. The estimated 1‐σ uncertainty for this long‐term Lyman α time series is 10%. The average solar rotation (27‐day) variability in Lyman α is 9% from this composite times series, and the solar rotation variability averaged over 2 years at solar minimum and maximum conditions is 5 and 11%, respectively. The average solar cycle (11‐year) variability is a factor of 1.5 when the data are smoothed over 2 years, and the extreme Lyman α variability is a factor of 2.1. The Lyman α irradiances averaged over 2 years during solar minimum and maximum conditions are 3.7 and 5.6 × 1011 photons s−1 cm−2, respectively. The proxy models include three components to better fit the UARS measurements; nonetheless, there remain differences between the proxy models and the observed Lyman α irradiance, which are related to the source of the Lyman α radiation being different than that for the proxies. The available proxies are primarily chromospheric and coronal emissions, whereas the Lyman α variability is manifested more in the transition region. It is shown that emissions throughout the chromosphere, transition region, and corona vary differently mainly because their contrasts for active network and plage components are different. A transition region proxy is needed to improve the empirical proxy models of solar irradiance, and this composite Lyman α time series could serve as a proxy for other transition region emissions.
Detection of small comets with a ground‐based telescope
Authors: Frank L. A. , Sigwarth J. B.
Published: February 2004

The Iowa Robotic Observatory (IRO) located in the Sonoran desert near Sonoita, Arizona, was used for an optical search for small comets in the vicinity of Earth during the period October 1998 through May 1999. The previous reports of detection of the small comets with an optical telescope were based on the search with the Space watch Telescope during November 1987, January 1988, and April 1988. The searches with both of these telescopes required that their fields of view be moved in a special manner across the celestial sphere in order to maximize the dwell times of the comet images on a small set of pixels of the telescopes' array detectors. There were sightings of nine small comets in the set of 1500 usable images which were gained with the IRO. The possibility that these events were spurious owing to random fluctuations of the responses in the sensors was eliminated in two different ways. The first method was the use of a shutter to provide either two or three trails for the same small comet. In the two‐trail mode, no events were seen with three trails, and for the three‐trail mode, no events were seen with two trails. The second assurance that the trails were not due to noise was provided by a rigorous determination of the signals S and signal‐to‐noise ratios (S/N) in the trails due to random statistical fluctuations, or “random trails,” and the subsequent comparison with these parameters for the small‐comet sightings. The diameter of the primary mirror of the IRO is significantly smaller than that of the Spacewatch Telescope, and thus the uncertainties in the number densities of the small comets are greater with the IRO. However, within a factor of 2 or 3, the average number densities of small comets detected with the IRO are similar to those observed previously with the Spacewatch Telescope, that is, about 10−18/m3.
Counterstreaming electrons in magnetic clouds
Authors: Shodhan S. , Crooker N. U. , Kahler S. W. , Fitzenreiter R. J. , Larson D. E. , Lepping R. P. , Siscoe G. L. , Gosling J. T.
Published: February 2004

Two widely used signatures of interplanetary coronal mass ejections are counterstreaming suprathermal electrons, implying magnetic structures connected to the Sun at both ends, and magnetic clouds, characterized by large‐scale field rotations, low temperature, and high field strength. In order to determine to what extent these signatures coincide, electron heat flux data were examined for 14 magnetic clouds detected by ISEE 3 and IMP 8 near solar maximum and 34 clouds detected by Wind near solar minimum. The percentage of time during each cloud passage that counterstreaming electrons were detected varied widely, from 6 clouds with essentially no counterstreaming to 8 clouds with nearly 100% counterstreaming. All of the former but less than half of the latter occurred near solar minimum, suggesting a possible solar cycle dependence on the degree of magnetic openness. The counterstreaming intervals were distributed randomly throughout the clouds, with a median length of 2.5 hours. A plot of counterstreaming percentages against cloud diameter for 33 clouds modeled as cylindrical flux ropes indicates a linear dependence of the percentage of closed flux on cloud size, with the largest clouds being the most closed. Overall the results are consistent with the view that although magnetic field lines within a magnetic cloud can form a large‐scale, coherent structure, reconnection in remote regions of the structure, presumably near the Sun, sporadically alters its topology from closed to open until the cloud assimilates into the ambient solar wind.
Electron kinetic firehose instability
Authors: Li Xing , Habbal Shadia Rifai
Published: February 2004

The linear dispersion equation describing electromagnetic waves propagating in a homogeneous electron‐proton plasma along arbitrary directions relative to the direction of the background magnetic field is solved numerically for bi‐Maxwellian particle distributions. It is found that in the presence of an electron temperature anisotropy T⊥ < T‖ and a sufficiently warm plasma (βe > 2), several purely growing modes (zero real frequency) and a quasi‐parallel electron firehose instability develop. While the quasi‐parallel mode is unstable for both parallel and oblique propagation, the zero frequency modes are unstable only for oblique propagation. Comparison of these modes further shows that the propagation angle for maximum growth rate and the maximum growth rate are larger for the purely growing modes than the quasi‐parallel electron firehose while the threshold is lower. Potential application of the kinetic electron firehose instability to the slow solar wind is briefly discussed.
Precursors of geomagnetic storms observed by the muon detector network
Authors: Munakata Kazuoki , Bieber John W. , Yasue Shin‐ichi , Kato Chihiro , Koyama Morikazu , Akahane Shigenobu , Fujimoto Kazuhiko , Fujii Zenjiro , Humble John E. , Duldig Marcus L.
Published: February 2004

We report the first systematic survey of cosmic ray precursors of geomagnetic storms. Our data set comprises the 14 “major” geomagnetic storms (peak Kp ≥ 8−) identified by Gosling et al. [1990] together with 25 large storms (peak Kp ≥ 7−) observed from 1992 through 1998. After eliminating events for which the muon detector network had poor coverage of the sunward interplanetary magnetic field (IMF) direction, we determined that 15 of the remaining 22 events (68%) had identifiable cosmic ray precursors with typical lead times ranging from 6 to 9 hours prior to the storm sudden commencement (SSC). Of the 15 precursors, 10 were of the “loss cone” (LC) type which is characterized by an intensity deficit confined to a narrow pitch angle region around the sunward IMF direction. Cosmic rays in the loss cone presumably originate in the cosmic‐ray‐depleted region downstream of the approaching shock. The remaining five precursors were of the “enhanced variance” (EV) type which is characterized by intensity increases or decreases that do not systematically align with the IMF direction. The incidence of precursors increases with storm size; for instance, 89% of storms with peak Kp greater than or equal to 8.0 had precursors. Our results show that the muon detector network can be a useful tool in space weather forecasting. However, new detector(s) installed to fill major gaps in the present network are urgently required for better understanding the nature of precursors and for reliable space weather forecasting.
Three‐dimensional MHD simulations of the steady state magnetosphere with northward interplanetary magnetic field
Authors: Guzdar P. N. , Shao X. , Goodrich C. C. , Papadopoulos K. , Wiltberger M. J. , Lyon J. G.
Published: February 2004

In the recent past, three‐dimensional global MHD codes have successfully demonstrated that they are invaluable tools for simulating global magnetospheric phenomena. In this paper, using the Lyon‐Fedder‐Mobarry code, we present results on the spatial structure of the steady state of Earth's magnetosphere for northward interplanetary magnetic field (IMF). We have shown that the steady state tail length of the Earth's magnetosphere for northward IMF is short and that the three dimensional structure of the tail lobe is of concave shape. This is in contradiction to the results presented by Fedder and Lyon [1995]. Explanation for this discrepancy is given in this paper. By varying the simple time‐independent solar wind velocity and interplanetary magnetic field, we present the parametric (power law) dependence of the geometrical aspects of the tail lobe on solar wind condition. In general, with higher solar wind velocity the radius of the tail lobe cross section is smaller and the tail length is longer. With higher northward IMF the whole tail lobe is smaller. These numerical studies have guided us in building analytical models based on global conservation constraints and force balance. We conclude that the lobe size is mainly determined by the solar wind velocity and the reconnection process in the cusp region which is affected by the magnitude of the IMF. We also show that the ionospheric effects are not significant in determining the lobe size. The evolution history of the tail length for magnetotail to reach steady state is also presented in this paper. Emphasis is placed on the fact that the steady state is a dynamical steady state.
Identification of sprites and elves with intensified video and broadband array photometry
Authors: Barrington‐Leigh Christopher P. , Inan Umran S. , Stanley Mark
Published: February 2004

Confusion in the interpretation of standard‐speed video observations of optical flashes above intense cloud‐to‐ground lightning discharges has persisted for a number of years. New high‐speed (3000 frames per second) image‐intensified video recordings are used along with theoretical modeling to elucidate the optical signatures of elves and sprites. In particular, a brief diffuse flash sometimes observed to accompany or precede more structured sprites in standard‐speed video is shown to be a normal component of sprite electrical breakdown and to be due entirely to the quasi‐electrostatic thundercloud field (sprites), rather than the lightning electromagnetic pulse (elves). These “sprite halos” are expected to be produced by large charge moment changes occurring over relatively short timescales (∼1 ms), in accordance with their altitude extent of ∼70 to 85 km. The relatively short duration of this upper, diffuse component of sprites makes it difficult to detect and to discriminate from elves and Rayleigh‐scattered light using normal‐speed video systems. Modeled photometric array signatures of elves and sprites are contrasted and shown to be consistent with observations. Ionization in the diffuse portion of sprites may be a cause of VLF scattering phenomena known as early/fast VLF events.
Two‐dimensional MHD simulation of the solar wind interaction with magnetic field anomalies on the surface of the Moon
Authors: Harnett Erika M. , Winglee Robert
Published: February 2004

Two‐dimensional magnetohydrodynamic simulations of the solar wind interaction with the magnetized regions on the surface of the Moon suggest “mini‐magnetospheres” can form around the regions on the Moon when the magnetic anomaly field strength is above 10 nT at 100 km above the surface (for a surface field strength of 290 nT) and when the solar wind ion density is below 40 cm−3, with typical observations placing anomalous magnetic field strengths around 2 nT at 100 km above the surface. The results suggest that not only can a bow shock and magnetopause form around the small anomalies, but their position and shape can change dramatically with changes in the solar wind conditions. A switch from southward to northward interplanetary magnetic field (IMF) causes the size of the mini‐magnetosphere to increase by 90% and the magnetic field at various positions inside the bow shock to increase by a factor of 10. In addition to affecting the stand‐off distance, changes in the IMF can also cause the mini‐magnetosphere to go from very round to flat and elongated. The scale size of the mini‐magnetospheres is 100 km for the range of typical solar wind conditions and the surface magnetic field strengths measured by Lunar Prospector. A stagnation point inside the shock region also exists for several solar wind conditions.
The transport of interstellar pickup ions
Authors: Lu J. Y. , Zank G. P.
Published: February 2004

A new approach to the time‐dependent anisotropic propagation of interstellar pickup ions in the interplanetary medium is presented. The model includes the effects of adiabatic focusing in a radial magnetic field, adiabatic deceleration, anisotropic pitch angle scattering, convection in the solar wind, and the continual injection of newly ionized particles. It is assumed that pickup ions experience difficulty in scattering through 90°. A two‐timescale scattering operator is introduced together with a generalized hemispherical model for the transport of pickup ions. The approach described here significantly extends the previous studies by Isenberg (1997) and Schwadron (1998) in that the pitch angle dependence of the pickup ions is not assumed to be of the form ƒ(r, v, t, μ) = ƒ_(r, v, t)H(μ) + ƒ+(r, v, t)H(−μ) (H(μ) is the Heaviside step function) from the outset. Specifically, (1) a higher‐order truncation of the underlying Boltzmann equation is used here, thus allowing a more careful analysis of the evolving pickup ion distribution; (2) we include a finite scattering rate for particles within each hemisphere and therefore present a more accurate treatment of pitch angle evolution; and (3) we do not assume instantaneous “isotropization” of the newborn pickup ion distribution within the sunward hemisphere but instead allow it to evolve into a scattered distribution on a timescale
, thus preserving the pitch angle characteristics of the ring beam. The anisotropic pitch angle scattering is found to result in the sunward accumulation of pickup ions, and particles moving sunward suffer more efficient cooling than those moving antisunward. Compared with the steepness at 90° pitch angle, the pitch angle dependence is not important within each hemisphere for moderately anisotropic scattering. However, for highly anisotropic scattering, the particle distribution is dominated by particles moving sunward, adiabatic cooling is more efficient, and the deviation of sunward moving particle distribution from a homogeneous hemisphere may be large at high velocities.
Observations of electromagnetic ion cyclotron waves during geomagnetic storms: Wave occurrence and pitch angle scattering
Authors: Erlandson R. E. , Ukhorskiy A. J.
Published: February 2004

This paper addresses the occurrence of low‐L (3.5 < L < 5) electromagnetic ion cyclotron (EMIC) waves during magnetic storms. Using ∼10 years of data, a statistical analysis indicates that EMIC waves in the equatorial magnetosphere occur 5 times more often during magnetic storms than during quiet times. EMIC wave occurrence during magnetic storms is important because EMIC waves pitch angle scatter energetic protons and thus represent a ring current loss mechanism. Observations in the equatorial magnetosphere, acquired by the Dynamic Explorer 1 (DE 1) satellite, are used to show the simultaneous occurrence of EMIC waves and enhanced proton fluxes in the loss cone. The study involves three cases selected only on the basis of observed EMIC wave activity and the availability of energetic ion data. The observations show that EMIC waves indeed scatter protons into the loss cone. Model calculations of the loss cone heat‐flux density compare favorably to the observed proton flux in the loss cone. These calculations are then used to estimate the effect of EMIC waves on ring current energy loss, assuming an effective area of wave activity and the duration of the wave activity. It is found that large‐amplitude, long‐duration EMIC waves can produce the observed recovery in the Dst index.
Interhemispheric propagation of VLF transmissions in the presence of ionospheric HF heating
Authors: Starks M. J. , Lee M. C. , Jastrzebski P.
Published: February 2004

Recent experiments carried out at the Arecibo Observatory in Puerto Rico and the magnetically conjugate location in Argentina suggest a connection between the prevalence and intensity of the interhemispheric propagation of VLF transmissions and HF heating of the ionosphere. During several nights of the study the O‐mode CW heater appeared to enhance the coupling of VLF transmissions into the affected region of plasma. These signals presumably then entered plasmaspheric ducts more efficiently and led to conjugate measurements of relatively high amplitude. No such effects were seen with pulsed or X‐mode heating. Short time resolution measurements of fading transmissions were marked by transit delays and amplitudes that remained nearly constant as the interhemispheric VLF signals diminished, consistent with the model of a plasmaspheric duct complemented by favorable plasma structures extending well into the F region but producing the majority of the total transit delay at plasmaspheric altitudes. It is also suggested that weak long‐delay receptions often observed in these data are most probably prolongitudinal whistlers that are not ducted but which represent a potential contaminant in ducted VLF experiments of all types.
Auroral plasma turbulence and the cause of auroral kilometric radiation fine structure
Authors: Pottelette Raymond , Treumann Rudolf A. , Berthomier Matthieu
Published: February 2004

Electron holes excited in the auroral kilometric radiation (AKR) source region in presence of a very dilute cool electron background are interpreted as causing the observed fine structure in AKR radiation. Using high time and frequency resolution measurements of the FAST wave tracker, we demonstrate that a substantial part of the AKR emission consists of a large number of elementary radiation events that we interpret as traveling electron holes that may have resulted from the nonlinear evolution of electron acoustic waves and have the properties of Bernstein‐Green‐Kruskal modes. Estimates of the propagation velocity of these structures are in good agreement with theory. Power estimates show that each elementary radiation event may contribute ≃ 103–4 W of power to AKR implying that a moderately large number of elementary radiators is required to reproduce the total AKR emission. The elementary radiation structures are sometimes reflected from the acceleration potential or become trapped in larger structures like ion acoustic waves or ion holes. The observations indicate that the radiation efficiency is highest at the turning point where the velocity of the elementary radiators with respect to the reflector system vanishes. Monitoring the time variation of the frequency drift of the elementary radiators allows to qualitatively infer about the mesoscale motion of the AKR source region and the spatial extension of the mesoscale field‐aligned electric potential drops.
Determination of three‐dimensional structure of coronal streamers and relationship to the solar magnetic field
Authors: Liewer P. C. , Hall J. R. , De Jong M. , Socker D. G. , Howard R. A. , Crane P. C. , Reiser P. , Rich N. , Vourlidas A.
Published: February 2004

We have determined the location, in three dimensions, of eight quasi‐stable coronal “streamers” from an analysis of Solar and Heliospheric Observatory (SOHO) Large‐Angle and Spectrometric Coronagraph images acquired over approximately one solar rotation. We use the locations to attempt to determine the solar origin of the streamers. Comparison of the streamers' locations (longitude and latitude at the R ≈ 2.5 Rs source surface) with that of the current sheet computed from a potential source surface model show that all of the streamers lie in or near the heliospheric current sheet. We assume that the streamers coincide with magnetic field lines and use a potential source surface magnetic model to map the location of the streamers from the source surface (R ≈ 2.5 Rs) to the photosphere. We find that many of the streamers are associated with strong magnetic field active regions. When a streamer and its associated active region are visible simultaneously, the active region is seen to be bright in the SOHO extreme ultraviolet imaging telescope (EIT) EUV full disk images. This, and other evidence, leads us to conclude that many of the bright streamers are the result of scattering from regions of enhanced density associated with active region outflow, and not a result of line‐of‐sight viewing through folds in a warped current sheet with uniform density.
Spacecraft studies of the interplanetary magnetic field
Authors: Ness Norman F. , Burlaga Leonard F.
Published: February 2004

In situ studies of the magnetic fields in space by spacecraft (S/C) began 4 decades ago. The experiments were initially difficult to perform and interpret accurately because of contamination by S/C‐generated magnetic fields, partial measurements of the vector field, limited mass and power for the instrumentation, low‐data‐rate sampling, discontinuous or spotty data coverage, and sometimes unknown orientation or location of the S/C. As each of these challenges was overcome, an initial view of the macrostructure of the interplanetary magnetic field (IMF) emerged, substantiating the Archimedean spiral structure and solar origin of the IMF as proposed theoretically by E. N. Parker. An important mesoscale feature noted early on was the sectoring of the IMF into an ordered sequence of alternating polarities associated with solar sources. Subsequent studies catalogued the microscale magnetohydrodynamic structures: shock waves, and tangential and rotational discontinuities and fluctuations. Correlations with solar photospheric magnetic fields, coronal holes and in situ plasma, and energetic particles elaborated more completely on the structure and dynamics of the IMF. This paper will provide a brief historical summary of the study of the IMF or, as it is now more commonly referred to, the heliospheric magnetic field (HMF).
An empirically‐driven global MHD model of the solar corona and inner heliosphere
Authors: Riley Pete , Linker J. A. , Mikić Z.
Published: February 2004

In this study we describe a three‐dimensional MHD model of the solar corona and heliosphere. We split the modeling region into two distinct parts: the solar corona (1 solar radius, RS, to 30 RS) and the inner heliosphere (30 RS to 5 AU). This combined model is driven solely by the observed line‐of‐sight photospheric magnetic field and can thus provide a realistic global picture of the corona and heliosphere for specific time periods of interest. We use the model to illustrate heliospheric structure during three different phases of the solar cycle: (1) Carrington rotation (CR) 1913 (August 22, 1996, to September 18, 1996), which occurred near solar minimum and overlapped the “Whole Sun Month” campaign; (2) CR 1892 (January 27, 1995, to February 23, 1995), which occurred during the declining phase of cycle 22 and coincided with the so‐called “Ulysses rapid latitude scan” and (3) CR 1947 (March 7, 1999, to April 4, 1999), which occurred approximately 15 months before the predicted maximum of solar cycle 23. We compare Ulysses and Wind observations with the simulation for CR 1913 and compare Ulysses observations during its traversal from pole to pole with CR 1892. We find that the simulations reproduce the overall large‐scale features of the observations. We use the near‐solar‐maximum results to speculate on the structure of the high‐latitude solar wind that Ulysses will encounter during its traversal of the southern and northern solar poles in 2000 and 2001, respectively. In particular, the results suggest that because of the presence of equatorial coronal holes the ordered pattern of corotating interaction region tilts and their associated shocks, which was observed during Ulysses' initial southward excursion in 1992, will likely disappear completely as Ulysses moves toward the south pole. We anticipate that Ulysses will encounter fast streams but will not remain within them for more than a fraction of a solar rotaton. Finally, the simulations suggest that crossings of the heliospheric current sheet will persist up to at least ∼70° heliographic latitude.
Zonally symmetric oscillations observed in the airglow from South Pole Station
Authors: Walterscheid R. L. , Sivjee G. G.
Published: February 2004

We have applied cross‐spectral techniques to airglow data obtained from South Pole Station at three different azimuths. The emissions originate from an OH‐Meinel airglow layer near the mesopause. The analysis has identified spectral features at frequencies that may be associated with tidal harmonics from 3 to 24 hours. The dominant tidal frequencies are 6 and 8 hours. Also, quasi 10‐ and 15‐hour spectral features are seen. Both are most likely normal modes. There is a strong tendency for the cross‐spectral phase of coherent features to be close to zero and distinct from migrating oscillations with zonal wavenumbers that are not a multiple of 3. This rules out migrating tides for the 6‐, 12‐, and 24‐hour tides. Migrating tides are possible for the 4‐ and 8‐hour tides, but theory indicates that these tides should have very small airglow brightness amplitudes at very high latitudes. The simplest interpretation of the data is that all of the observed oscillations are zonally symmetric.
Testing electric and magnetic field models of the storm‐time inner magnetosphere
Authors: Kistler Lynn M. , Larson Douglas J.
Published: February 2004

We calculate the equipotential contours, drift trajectories, and predicted proton energy spectra in the inner magnetosphere using combinations of dipole or Tsyganenko magnetic field models and Volland‐Stern or Weimer electric fields models. We compare the results with observations of proton energy spectra at L=4–5 over a range of local times from the Active Magnetospheric Particle Tracer Explorers/charge‐energy‐mass instrument. We find that the trajectories, and in particular the source location in the tail and the exit location on the dayside, depend significantly on both the electric and magnetic field models. However, the predicted energy spectra in the inner magnetosphere depended almost entirely on the choice of electric field. Using the Weimer electric field gives less loss along the trajectories, which is usually in better agreement with the observed energy spectra.
Assimilated observations of thermospheric winds, the aurora, and ionospheric currents over Alaska
Authors: Conde M. , Craven J. D. , Immel T. , Hoch E. , Stenbaek‐Nielsen H. , Hallinan T. , Smith R. W. , Olson J. , Sun Wei , Frank L. A. , Sigwarth J.
Published: February 2004

We present simultaneous measurements of thermospheric winds, auroral emissions, and ionospheric currents over Alaska, obtained from four separate instruments. Thermospheric (F region) wind maps were recorded by an all‐sky imaging Fabry‐Perot spectrometer located at Poker Flat and observing at λ630.0 nm. Auroral images at λ557.7 nm were obtained from the low‐resolution visible imager on board the Polar satellite. White‐light all‐sky auroral images were recorded by ground‐based all‐sky cameras located in Alaska at Poker Flat (65° 07′N, 212° 34′E) and at Kaktovik (70° 06′N, 217° 24′E). Finally, the east‐west component of the ionospheric F region plasma convection was inferred using the Alaskan meridian chain of magnetometers. Montage images of these four data sets are presented, projected onto a geographic map of the Alaskan region. We examine a 10‐hour period during the Alaskan local nighttime of February 10, 1997. These montages illustrate a close relationship between spatial structures occurring in the aurora, in the ionospheric plasma convection, and in the F region wind field. Latitudinal shear of the geomagnetic zonal wind, often observed in the premidnight time sector, was seen to be associated with both the equatorward and poleward boundaries of the discrete aurora. We focus particularly on a period commencing just after 0900 UT, when a strong shear in the zonal wind was observed to sweep southward across Alaska. After magnetic midnight the wind field was dominated by the emergence of the “cross‐polar jet” from the polar cap. This overwhelmed any wind features associated with local auroral processes.
Measurements of the topside ionosphere over Arecibo during the total solar eclipse of February 26, 1998
Authors: MacPherson B. , González S. A. , Sulzer M. P. , Bailey G. J. , Djuth F. , Rodriguez P.
Published: February 2004

The Arecibo incoherent scatter radar facility was operated on February 26, 1998, and was used to observe the total solar eclipse that occurred over the Caribbean. A maximum of 87% obscuration was observed over Arecibo at 1430 LT (1830 UT). The radar was operated using an experimental technique, which uses a 300 μs single/multi‐frequency pulse, to gather data from the altitude range 146–2412 km. The Sheffield University plasmasphere ionosphere model was used to interpret the measurements. The electron temperature was found to have decreased by 600 K at 400 km altitude, but the magnitude of the decrease becomes smaller with increasing altitude. This is shown to be the result of the lesser degree of obscuration of the solar disk at latitudes north of Arecibo. Conjugate point photoelectron heating effects are also shown to play a significant role in the electron energy balance during the eclipse. The H+ ion temperature exhibited a response to the eclipse, with temperatures being around 200 K lower than expected at the time of maximum obscuration. There was relatively little variation observed in the O+ temperature. The response of the topside ionosphere is characterized by a downward motion arising from the contraction of the plasma due to reduced plasma temperatures. This is most clearly seen in the O+‐H+ transition altitude which falls by 200 km. The transition altitude fully recovers within 2 hours after the eclipse. The location of the transition altitude acts to mitigate the effects of the eclipse on the topside electron densities.