bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of the far ultraviolet airglow by the Ultraviolet Limb Imaging Experiment on STS‐39
Authors: Budzien S. A. , Feldman P. D. , Conway R. R.
Published: February 2004

The Ultraviolet Limb Imaging (UVLIM) experiment flew on STS‐39 in the spring of 1991 to observe the Earth's thermospheric airglow and included a far ultraviolet (1080‐1800 Å) spectrometer. We present first results from this spectrometer, including a spectroscopic analysis at 6‐Å resolution of H, O, N, and N2 day glow emissions and modeling of the observed limb‐scan profiles of dayglow emissions. The observed N2 Lyman‐Birge‐Hopfield (LBH) emission reflects a vibrational population distribution in the a¹Πg state that differs significantly from those predicted for direct electron excitation and excitation with cascade from the a′ ¹Σu− and w ¹Δu states. The Vibrational population distribution and LBH brightnesses suggest a total cascade rate 45% that of direct excitation, in contrast to laboratory measurements. For the first time, pronounced limb brightening is observed in both the N I λ1135 and N I λ1200 limb emission profiles, as expected for emissions excited by N2 dissociation which produces kinetically fast N fragments; however, optically thick components of these features are also observed. Preliminary modeling of the O I λ1356, H I λ1216, and O I λ1304 and O I λ1641 emissions agrees to within roughly 10% of the observed limb‐scan profiles, but the models underestimate the N2 LBH profiles by a factor of 1.4‐1.6, consistent with the inferred cascade effect. Other findings include: an O I λ1152/λ1356 intensity ratio that is inconsistent with the large cascade contribution to O I λ1356 from np 5P states required by laboratory and nightglow observations; nightglow observations of the tropical ultraviolet arcs exhibit a wide range of O I λ1356/λ1304 intensity ratios and illustrate the complicated observing geometry and radiative transfer effects that must be modeled; and we find a 3‐σ upper limit of 8.5 R to the total LBH vehicle glow emission.
Ground‐based studies of ionospheric convection associated with substorm expansion
Authors: Kamide Y. , Richmond A. D. , Emery B. A. , Hutchins C. F. , Ahn B. ‐H. , de la Beaujardiere O. , Foster J. C. , Heelis R. A. , Kroehl H. W. , Rich F. J. , Slavin J. A.
Published: February 2004

The instantaneous patterns of electric fields and currents in the high‐latitude ionosphere are deduced by combining satellite and radar measurements of the ionospheric drift velocity, along with ground‐based magnetometer observations for October 25, 1981. For this purpose, an updated version of the assimilative mapping of ionospheric electrodynamics technique has been used. These global patterns are unobtainable from any single data set. The period under study was characterized by a relatively stable southward interplanetary magnetic field (IMF), so that the obtained electric field patterns do reflect, in general, the state of sustained and enhanced plasma convection in the magnetosphere. During one of the satellite passes, however, an intense westward electrojet caused by a substorm intruded into the satellite (DE 2) and radar (Chatanika, Alaska) field of view in the premidnight sector, providing a unique opportunity to differentiate the enhanced convection and substorm expansion fields. The distributions of the calculated electric potential for the expansion and maximum phases of the substorm show the first clear evidence of the coexistence of two physically different systems in the global convection pattern. The changes in the convection pattern during the substorm indicate that the large‐scale potential distributions are indeed of general two‐cell patterns representing the southward IMF status, but the night‐morning cell has two positive peaks, one in the midnight sector and the other in the late morning hours, corresponding to the substorm expansion and the convection enhancement, respectively.
Statistical studies of impulsive events at high latitudes
Authors: Lin Z. M. , Bering E. A. , Benbrook J. R. , Liao B. , Lanzerotti L. J. , Maclennan C. G. , Wolfe A. N. , Friis‐Christensen E.
Published: February 2004

A statistical study has been made of the high‐latitude impulsive events that were observed during the 1985–1986 South Pole Balloon Campaign. The events were selected by searching for unipolar pulses ≥ 10 nT above background in the vertical component of the magnetic field on the ground and/or pedestal or “W” shaped horizontal electric field perturbations ≥ 10 mV/m in amplitude and accompanied by perturbations in the vertical electric field at balloon altitude. A main event list comprising 112 events was compiled from the 468 hours of data available. Three aspects of the events were examined: the solar wind conditions prior to the event, local time of observation, and intrinsic properties of the events. The local time distribution was obtained from the 112 entry main event list and was found to be nearly uniform across the dayside, with no midday gap. The event rate found using our low‐amplitude selection criteria was 0.7 event/hr, comparable to expectations based on in situ studies of the magnetopause. A total of 42 events were found for which data were available from Interplanetary Monitoring Platform (IMP) 8. Of these events, 12 occurred when the ZGSM component (BZ) of the interplanetary magnetic field (IMF) was northward and 30 occurred when BZ was southward or fluctuating. Only three of the BZ northward cases and only five of the BZ southward cases were preceded by pressure pulses greater than 0.4 nPa in amplitude. Ten of the events were studied in detail by means of a model‐fitting method discussed elsewhere. This method infers values of several parameters, including the total current flowing in a coaxial or monopole system and a two‐dimensional dipole system. The intrinsic properties of the events showed that only ∼ 10% of the total current contributed to momentum transfer to the high‐latitude ionosphere, that the direction of the motion depended more on local time of observation than IMF By, and that events were usually several hundred kilometers in size. The observed Bz control found in the 42 event list and the prevalence of coaxial current dominated events are inconsistent with the predictions of the pressure pulse model.
Characteristics of the field‐aligned current system in the nighttime sector during auroral substorms
Authors: Hoffman R. A. , Fujii R. , Sugiura M.
Published: February 2004

Fujii et al. (1994) obtained characteristics of the electrodynamic parameters, that is, field‐aligned currents, electric fields, and electron precipitation, which are associated with auroral substorm events in the nighttime sector, through a unique analysis that places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images. In this paper we investigate in considerably more detail the characteristics of the field‐aligned currents using data from the same set of passes as the previous study. We show for the first time that the net upward field‐aligned currents throughout the surge and surge horn are sufficient to account for most if not all of the converging currents of the auroral electrojets. Current densities are largest in the surge and surge horn. Current region continuity does not appear to exist across the substorm bulge region. Much of the auroral substorm field‐aligned current is composed of filamentary currents and finite current segments at large angles to each other. The westward electrojet may contain large gradients in intensity both in local time and latitude due to sets of localized field‐aligned currents. The net downward current for several hours to the west of the surge is insufficient to account for the eastward electrojet, consistent with the concept that this electrojet originates primarily on the dayside. Our pattern of field‐aligned currents associated with the surge has common features and also differs significantly from the patterns previously derived from data from radars and ground‐based magnetometer arrays. Our pattern is considerably more complex, probably due to the much higher resolution in latitude of the satellite data. It is also larger in area, since our average substorm is much larger than those pertaining to the previous patterns, giving a substorm wedge considerably wider than that obtained from the radar and array data.
Four large‐scale field‐aligned current systems in the dayside high‐latitude region
Authors: Ohtani S. , Potemra T. A. , Newell P. T. , Zanetti L. J. , Iijima T. , Watanabe M. , Blomberg L. G. , Elphinstone R. D. , Murphree J. S. , Yamauchi M. , Woch J. G.
Published: February 2004

A system of four current sheets of large‐scale field‐aligned currents (FACs) was discovered in the data set of simultaneous Viking and DMSP‐F7 crossings of the dayside high‐latitude region. This paper reports four examples of this system that were observed in the prenoon sector. The flow polarities of FACs are upward, downward, upward, and downward, from equatorward to poleward. The lowest‐latitude upward current is flowing mostly in the CPS precipitation region, often overlapping with the BPS at its poleward edge, and is interpreted as a region 2 current. The pair of downward and upward FACs in the middle of the structure are collocated with structured electron precipitation. The precipitation of high‐energy (>1 keV) electrons is more intense in the lower‐latitude downward current sheet. The highest‐latitude downward flowing current sheet is located in a weak, low‐energy particle precipitation region, suggesting that this current is flowing on open field lines. Simultaneous observations in the postnoon local time sector reveal the standard three‐sheet structure of FACs, sometimes described as region 2, region 1, and mantle (referred to the midday region 0) currents. A high correlation was found between the occurrence of the four FAC sheet structure and negative interplanetary magnetic field (IMF) By. We discuss the FAC structure in terms of three types of convection cells: the merging, viscous, and lobe cells. During strongly negative IMF By, two convection reversals exist in the prenoon sector; one is inside the viscous cell, and the other is between the viscous cell and the lobe cell. This structure of convection flow is supported by the Viking electric field and auroral UV image data. Based on the convection pattern, the four FAC sheet structure is interpreted as the latitudinal overlap of midday and morning FAC systems. We suggest that the four‐current sheet structure is common in a certain prenoon local time sector during strongly negative IMF By.
Giotto ion mass spectrometer measurements at comet P/Grigg‐Skjellerup
Authors: Goldstein R. , Altwegg K. , Fuselier S. A. , Goldstein B. E. , Balsiger H. , Jost F. , Coates A. , Neubauer F. M. , Rosenbauer H.
Published: February 2004

The Giotto ion mass spectrometer high‐intensity spectrometer (IMS‐HIS) measured fluxes of ions from about 260,000 km before (1008∶37 UT spacecraft time) to about 86,000 km after (1701∶33 UT spacecraft time) closest approach to comet P/Grigg‐Skjellerup during the encounter on July 10, 1992. Although the HIS sensor was not designed to measure protons, these ions were measured far from the comet. Close in to the comet, the ions observed were probably also protons, although heavier ions cannot be completely ruled out. Considerable temporal structure appears in the data, well‐correlated with the data of other instruments onboard, especially those of the magnetometer. In particular, the ion count rate correlates with the direction of the magnetic field. This strong modulation at the water group ion cyclotron period (∼90 s) inside the inbound bow wave indicates a very narrow ion pitch angle distribution. Hence at Grigg‐Skjellerup the ions appear to experience very little pitch angle scattering. This may result from strong compression in the rapidly increasing magnetic field.
Midtail plasma flows and the relationship to near‐Earth substorm activity: A case study
Authors: Lopez R. E. , Goodrich C. C. , Reeves G. D. , Belian R. D. , Taktakishvili A.
Published: February 2004

Recent simulations of magnetotail reconnection have pointed to a link between plasma flows, dipolarization, and the substorm current wedge. In particular, Hesse and Birn (1991) have proposed that earthward jetting of plasma from the reconnection region transports flux into the near‐Earth region. At the inner edge of the plasma sheet this flux piles up, producing a dipolarization of the magnetic field. The vorticity produced by the east‐west deflection of the flow at the inner edge of the plasma sheet gives rise to field‐aligned currents that have region 1 polarity. Thus in this scenario the earthward flow from the reconnection region produces the dipolarization and the current wedge in a self‐consistent fashion. In this study we examine observations made on April 8,1985 by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module (IRM), the geosynchronous satellites 1979‐053,1983‐019, and 1984‐037, and Syowa station, as well as AE. This event is unique because IRM was located near the neutral sheet in the midnight sector for an extended period of time. Ground data show that there was ongoing activity in the IRM local time sector for several hours, beginning at 1800 UT and reaching a crescendo at 2300 UT. This activity was also accompanied by energetic particle variations, including injections, at geosynchronous orbit in the nighttime sector. Significantly, there were no fast flows at the neutral sheet until the great intensification of activity at 2300 UT. At that time, IRM recorded fast earthward flow simultaneous with a dipolarization of the magnetic field. We conclude that while the aforementioned scenario for the creation of the current wedge encounters serious problems explaining the earlier activity, the observations at 2300 UT are consistent with the scenario of Hesse and Birn (1991). On that basis it is argued that the physics of substorms is not exclusively rooted in the development of a global tearing mode. Processes at the inner edge of the cross‐tail current that cause a disruption of the current and a consequent dipolarization and current wedge may be unrelated to the formation of a macroscale reconnection region. Thus the global evolution of a substorm is probably a complicated superposition of such processes operating on a very localized scale and a global macroscale process that allows for such things as releasing the energy stored in lobe flux and the creation of plasmoids.
Mirroring of fast solar flare electrons on a downstream corotating interaction region
Authors: Anderson K. A. , Sommers J. , Lin R. P. , Pick M. , Chaizy P. , Murphy N. , Smith E. J. , Phillips J. L.
Published: February 2004

We discuss an example of confinement of fast solar electrons by a discrete solar wind‐interplanetary magnetic field structure on February 22, 1991. The structure is about 190,000 km in width and is clearly defined by changes in the direction of the magnetic field at the Ulysses spacecraft. This structure carries electrons moving toward the Sun as well as away from the Sun. A loss cone in the angular distribution of the fast electrons shows that mirroring, presumably magnetic, takes place downstream from the spacecraft. Following passage of this narrow structure, the return flux vanishes for 21 min after which time the mirroring resumes and persists for several hours. We identify the enhanced magnetic field region lying downstream from the Ulysses spacecraft that is responsible for the mirroring to be a corotating stream interaction region. Backstreaming suprathermal electron measurements by the Los Alamos National Laboratory plasma experiment on the Ulysses spacecraft support this interpretation.
The spectrum of electron density fluctuations in the solar wind and its variations with solar wind speed
Authors: Manoharan P. K. , Kojima M. , Misawa H.
Published: February 2004

We describe simultaneous interplanetary scintillation (IPS) measurements for the solar distance range 70‐185 R⊙ made using the Ooty Radio Telescope and the Solar Terrestrial Environment Laboratory three‐antenna system during the first half of the current solar cycle 22. These measurements have been used to establish the spectral characteristics of the electron density fluctuations in the solar wind, and we study the relation of these to the plasma flow speed. At times of low activity, for high‐speed streams (Vsw ≈ 600 km s−1) the spectra (sensitive to the IPS temporal frequency range 1‐10 Hz) are steep, with a power law exponent α ≈ 3.8. During high levels of activity for high‐speed streams, the spectra are less steep with α ≈ 3.4. In the low‐speed wind (Vsw ≈ 400 km s−1) the spectra are clearly flatter, α ≈ 2.9, and nearly invariant with solar activity. The average shape of the spectrum obtained from this study is consistent with results obtained via other techniques. By combining the present estimates with earlier spacecraft data, it is possible to consider the shape of the density fluctuation spectrum in the frequency range 10−5‐10¹ Hz. It is shown that the spectrum has different slopes in different spectral domains and is well described by a three‐component model as proposed by Coles and Harmon [1989]. Moreover, the high‐frequency components of the spectra in the low‐ and high‐speed streams involve different spatial frequency ranges.
Ulysses field and plasma observations of magnetic holes in the solar wind and their relation to mirror‐mode structures
Authors: Winterhalter Daniel , Neugebauer Marcia , Goldstein Bruce E. , Smith Edward J. , Bame Samuel J. , Balogh André
Published: February 2004

The term “magnetic hole” has been used to denote isolated intervals when the magnitude of the interplanetary magnetic field drops to a few tenths, or less, of its ambient value for a time that corresponds to a linear dimension of tens to a few hundreds of proton gyro‐radii. Data obtained by the Ulysses magnetometer and solar wind analyzer have been combined to study the properties of such magnetic holes in the solar wind between 1 AU and 5.4 AU and to 23° south latitude. In order to avoid confusion with decreases in field strength at interplanetary discontinuities, the study has focused on linear holes across which the field direction changed by less than 5°. The holes occurred preferentially, but not without exception, in the interaction regions on the leading edges of high‐speed solar wind streams. Although the plasma surrounding the holes was generally stable against the mirror instability, there are indications that the holes may have been remnants of mirror‐mode structures created upstream of the points of observation. Those indications include the following: (1) For the few holes for which proton or alpha‐particle pressure could be measured inside the hole, the ion thermal pressure was always greater than in the plasma adjacent to the holes. (2) The plasma surrounding many of the holes was marginally stable for the mirror mode, while the plasma environment of all the holes was significantly closer to mirror instability than was the average solar wind. (3) The plasma containing trains of closely spaced holes was closer to mirror instability than was the plasma containing isolated holes. (4) The near‐hole plasma had much higher ion β (ratio of thermal to magnetic pressure) than did the average solar wind. (5) Near the holes, T⊥/T∥ tended to be either >1 or larger than in the average wind. (6) The proton and alpha‐particle distribution functions measured inside the holes occasionally exhibited the flattened phase‐space‐density contours in ν⊥‐ν∥ space found in some numerical simulations of the mirror instability.
A model for the generation of obliquely propagating ULF waves near the magnetic equator
Authors: McClements K. G. , Dendy R. O. , Lashmore‐Davies C. N.
Published: February 2004

Several spacecraft, notably OGO 3, IMP 6, Hawkeye 1, and GEOS 1 and 2, have detected harmonically structured wave emissions in the ULF range at low magnetic latitudes between L values of 4 and 8. These waves propagate at large angles with respect to the magnetic field, and, in the case of GEOS 1, appear to be associated with ringlike proton distributions. It is demonstrated that the excitation of obliquely propagating fast Alfvén waves by such distributions can account for many of the observed features of harmonically structured ULF emissions. Linear instability can occur at, or slightly below, the proton cyclotron frequency and all its harmonics. The maximum growth rates of some harmonics, notably the fundamental, occur at propagation angles which are oblique rather than strictly perpendicular. Because of magnetic field gradients, waves which propagate approximately azimuthally and within a few degrees of the perpendicular direction are likely to be amplified to the highest levels. Certain features of the ULF data, such as the finite bandwidth of the harmonics, can be explained only by allowing the wave vector to have a finite parallel component.
Statistical study of ELF/VLF emissions recorded by a low‐altitude satellite during seismic events
Authors: Parrot Michel
Published: February 2004

This paper presents a statistical study of low‐altitude satellite data linked to seismic events. Data of the satellite AUREOL 3 are considered during 24‐hour windows around the time of 325 earthquakes with Ms > 5. Amplitudes at the output of different frequency filters in the ELF/VLF range, connected to electric or magnetic components, were stored in a file. This file is organized as functions of three parameters: Δt, the difference between the time of the earthquake and the time of the satellite data; ΔLong, the difference between the longitude of the earthquake and the longitude of the satellite; and ΔInv.Lat, the difference between the invariant latitude of the earthquake and the invariant latitude of the satellite. When all data are averaged over time, the amplitude is maximum in the interval ΔLong < 10° regardless of ΔInv.Lat. This indicates that, due to propagation, waves related to seismic processes can be observed all along the magnetic meridian passing over the epicenter of an earthquake. However, as the AUREOL 3 data were not continuously recorded, thus providing a limited number of events, it was impossible to carry out a complete statistical study.
Simultaneous prenoon and postnoon observations of three field‐aligned current systems from Viking and DMSP‐F7
Authors: Ohtani S. , Potemra T. A. , Newell P. T. , Zanetti L. J. , Iijima T. , Watanabe M. , Yamauchi M. , Elphinstone R. D. , de la Beaujardière O. , Blomberg L. G.
Published: February 2004

The spatial structure of dayside large‐scale field‐aligned current (FAC) systems is examined by using Viking and DMSP‐F7 data. We focus on four events in which the satellites simultaneously observed postnoon and prenoon three FAC systems: the region 2, the region 1, and the mantle (referred to as midday region 0) systems, from equatorward to poleward. These events provide the most solid evidence to date that the midday region 0 system is a separate and unique FAC system, and is not an extension of the region 1 system from other local times. The events are examined comprehensively by making use of a multi‐instrument data set, which includes magnetic field, particle flux, electric field, auroral UV image data from the satellites, and the Sondrestrom convection data. The results are summarized as follows: (1) Region 2 currents flow mostly in the CPS precipitation region, often overlapping with the BPS at their poleward edge. (2) The region 1 system is located in the core part of the auroral oval and is confined in a relatively narrow range in latitude which includes the convection reversal. The low‐latitude boundary layer, possibly including the outer part of the plasma sheet, and the external cusp are the major source regions of dayside region 1 currents. (3) Midday region 0 currents flow on open field lines and are collocated with the shear of antisunward convection flows with velocities decreasing poleward. On the basis of these results we support the view that both prenoon and postnoon current systems consist of the three‐sheet structure when the distortion of the convection pattern associated with interplanetary magnetic field (IMF) By is small and both morningside and eveningside convection cells are crescent‐shaped. We also propose that the midday region 0 and a part of the region 1 systems are closely coupled to the same source.
Structure of the magnetopause for low Mach number and strongly northward interplanetary magnetic field
Authors: Le G. , Russell C. T. , Gosling J. T.
Published: February 2004

We use ISEE magnetic field and plasma data to examine dayside magnetopause crossings under conditions of low Mach number and strongly northward interplanetary magnetic field (IMF). When the solar wind Mach number is low, the IMF strength and magnetosheath field strength are large, and we expect the effects of magnetic reconnection to be the strongest. When the IMF is strongly northward, we find that the location of the magnetopause boundary layer is very stationary in the space, and we observe many features that are common for both typical and low Mach numbers. However, under low Mach number conditions, we have observed some features that would be expected for cusp reconnection. The boundary layer near the subsolar region contains heated magnetosheath plasma with little hot magnetospheric component that has clearly entered the magnetosphere elsewhere. At least some of the structures present in the boundary layer are impulsive. Inside the boundary layer there is also clear evidence of accelerated flow from the cusp region for strongly northward IMF at low Mach number. Reconnection beyond the cusp can explain the observed field, plasma, and flow signatures. Therefore at low Mach number, reconnection is important in the formation of the boundary layer for northward IMF.
Comparisons of modeled N+, O+, H+, and He+ in the midlatitude ionosphere with mean densities and temperatures from Atmosphere Explorer
Authors: Craven P. D. , Comfort R. H. , Richards P. G. , Grebowsky J. M.
Published: February 2004

In this study, Atmosphere Explorer data and model results for the ion and electron temperature and the density of N+, O+, H+, and He+ between 120 and 1400 km altitude are compared for two midlatitude ranges (L=2 and L=4), noon and midnight local time, winter and summer, at solar minimum. The data for the heavy atomic ions (O+ and N+) show that their densities are greater at noon than at midnight for a given season and greater in summer than winter for a given local time. There is only a weak latitudinal variation in the density of these ions. The data show that the light ion (H+ and He+) densities are greater at midnight than at noon and are generally greater in winter than summer. There is a strong latitudinal variation of the light ion densities, with the densities decreasing with increasing latitude. The model densities are in good agreement with the AE densities for N+, O+, and H+. Model He+ densities are lower, by a factor of 2 or more, than the measured densities. Model ion and electron temperatures agree well with the measured temperatures with only a modest increase in plasmaspheric heating.
Cusp latitude magnetic impulse events: 3. Associated low‐latitude signatures
Authors: Konik R. M. , Lanzerotti L. J. , Maclennan C. G. , Wolfe A. , Venkatesan D.
Published: February 2004

Low‐latitude magnetic perturbations at San Juan, Puerto Rico, occurring simultaneously with the set of South Pole magnetic impulse events (MIEs) discussed by Lanzerotti et al. (1991) and Konik et al. (1994) are examined. The amplitudes of the magnetic deflections at the times of the MIEs at South Pole and at San Juan are compared to the magnetic deflections seen at these two stations during storm sudden commencements (SSCs). At the South Pole, we find the mean deflections in the magnetic H and V components to be larger for the MIEs than for the SSCs. At San Juan, we find the opposite to be true; the mean amplitudes of the deflections in the H and V components of the field are much smaller during the MIEs than during the SSCs. The interplanetary plasma conditions prevailing at the times of the MIEs are also examined and contrasted to those found during SSCs. We find that while the MIEs are characterized by changes in the solar wind dynamic pressure at the noise level, the SSCs are concurrent with large changes in the dynamic pressure. Examining the correlations between MIE amplitudes and dynamic pressure changes, we find no correlation for the MIE deflections at the South Pole, but a strong correlation for the concurrent deflections seen at San Juan. In contrast, we find strong correlations between SSC amplitudes and dynamic pressure changes at both the South Pole and San Juan. We conclude that the MIE signatures at the South Pole are not predominantly generated by dynamic pressure fluctuations, and that the concurrent ground magnetic signatures at San Juan are unrelated to those at the South Pole. We also examine our data in order to deduce any amplitude dependence upon local time and in particular whether larger ground signatures are found in local morning than in local afternoon. We find that for both the sets of MIEs and SSCs, the amplitudes of deflections at the South Pole exhibit no significant local time pattern.
How the magnetosphere is driven into the substorm
Authors: Zhu Xiaoming
Published: February 2004

This paper puts together several aspects of the substorm phenomena into a scenario subject to quantitative analysis, based on Ampere's law, magnetic stresses, and the effect of the parallel electric field. As the magnetospheric convection continuously shears the magnetic field, the parallel electric field suddenly switches on when the shear reaches a critical value. The parallel electric field not only accelerates the charged particles to cause the auroral display but also partially decouples the ionosphere from the magnetosphere so that the magnetospheric convection is enhanced significantly, causing the substorm expansion phase. Pi 2 pulsations are excited by the change of the magnetosphere‐ionosphere coupling. We show a direct relationship between the parallel potential drop, the enhanced convection rate, and the amplitude of the Pi 2 pulsations. With no adjustable parameter, the relationship shows quantitative agreement with the diverse observations.
Upstream wave activity at comet P/Grigg‐Skjellerup
Authors: Shevchenko V. I. , Shapiro V. D. , Ride S. K. , Baine M.
Published: February 2004

Comet P/Grigg‐Skjellerup is a comet with a moderate gas production rate. Quasi‐linear theory, which has successfully described waves upstream of the bow shock of comet P/Halley (a comet with a much higher gas production rate), is not appropriate in this case. In this paper, we develop the nonlinear theory of laminar MHD wave activity upstream of the bow shock of P/Grigg‐Skjellerup using a combination of analytical and numerical methods. The nonlinear saturation mechanism is the trapping of cometary ions, resulting in their bounce oscillations over pitch angle. For the specific case of the Giotto encounter with P/Grigg‐Skjellerup, during which the solar wind propagation was almost transverse to the magnetic field, the excitation of left‐hand circularly polarized waves propagating toward the comet is initially dominant. Later on, as a result of pitch angle scattering of resonant particles, we find that waves propagating toward the Sun are also excited. The numerical solutions show that under the action of waves propagating in both directions the cometary ions acquire a spread in energy. The results of this theoretical analysis are compared with measurements made during the Giotto encounter with P/Grigg‐Skjellerup and are found to be in good qualitative agreement.
Statistical studies of flux transfer events
Authors: Kuo H. , Russell C. T. , Le G.
Published: February 2004

Previous studies have revealed that flux transfer events (FTEs) have a clear dependence on the interplanetary magnetic field Bz component. Herein we examine other solar wind parameters, beta, dynamic pressure, and Mach number that possibly control the formation of FTEs. None of these other parameters appear to exercise strong control of the rate of FTE occurrence. Hence we conclude that the occurrence of FTEs is probably controlled by some intrinsic property of the magnetospheric system itself rather than by these solar wind parameters. In order to examine whether the bipolar signatures observed in the magnetosheath or magnetosphere are the same phenomenon, magnetosheath FTEs and magnetospheric FTEs are studied separately. The similar results for the two types of FTEs indicate that they do belong to the same statistical population.
A model for the three‐dimensional magnetic field correlation spectra of low‐frequency solar wind fluctuations during Alfvénic periods
Authors: Carbone Vincenzo , Malara Francesco , Veltri Pierluigi
Published: February 2004

Using statistical homogeneity, magnetic field fluctuations in the solar wind during Alfvénic periods are analyzed in terms of the two independent polarizations allowed for each wave vector k. It is shown that the energy spectra of the two polarizations can be related both to the correlation tensor and to the variance matrix, which is generally used to characterize the anisotropy of the turbulence. Assuming simple anisotropic power law models, the parameters defining the spectra of the two polarizations are determined by fitting the eigenvalues of the variance matrix on the corresponding eigenvalues evaluated by Bavassano et al. (1982) for Helios 2 data. Then the corresponding form of the correlation tensor is obtained. In particular, we found that the spectrum of polarization [1] fluctuations (corresponding, in a weak turbulence theory approach, to the Alfvén mode) is steeper than the polarization [2] spectrum (corresponding to the magnetosonic modes). While the former spectrum is dominated by wave vectors parallel to B0, the latter is strongly flattened on the plane containing the radial and the mean magnetic field B0 directions. A discussion of these results in connection with other observational and theoretical issues is outlined.
Ground satellite observations of Pc 1 magnetic pulsations in the plasma trough
Authors: Hansen H. J. , Fraser B. J. , Menk F. W. , Erlandson R. E.
Published: February 2004

An analysis of concurrent Pc 1 pulsation activity (0.2–0.6 Hz) recorded on the ground at Mawson, Antarctica (70°S magnetic latitude (MLAT); 19°E magnetic longitude (MLONG); L=8), and on board the Viking polar‐orbiting satellite, when the spacecraft was located near apogee (13.5×10³ km) from 50–77° MLAT above the northern ionosphere, shows that simultaneous pulsations were observed only when Viking's conjugate point was within 60‐70°S MLAT and 30°E–33°W MLONG, an extent of about 1000 km in latitude and 1700 km in longitude. Spectral analysis confirms that the waves responsible for these pulsations are electromagnetic ion cyclotron (EMIC) waves that have propagated to the spacecraft and ground from the geomagnetic equator. For the intervals when the waves were on field lines above and equatorward of Mawson, convective EMIC wave modeling shows wave growth is dominant below the equatorial He+ gyrofrequency. Waves generated on field lines poleward of Mawson exhibit frequencies above the equatorial He+ gyrofrequency. These propagate to Mawson, most likely through a purely proton plasma rather than a multi‐ion environment. Frequency shifts in the Pc 1 activity at Mawson appear to be directly linked to solar wind velocity and flux variations measured by IMP 8. For certain intervals these linked observations are consistent with an interpretation based on the velocity and flux changes altering the radial position of the magnetopause through pressure balance considerations. This, in turn, changes the magnitude of the ambient magnetic field in the equatorial plasma trough and consequently the ion gyrofrequency and EMIC growth rate. Pc 3–5 activity was recorded at Mawson in conjunction with the Pc 1 frequency shifts and in association with rapid BIMF variations. The changes in the temperature anisotropy and energy of the resonant ions in the plasma trough, which are causing the Pc 1 emission changes, are associated with the entry of dayside Pc 3–5 pulsation energy into the dayside magnetosphere.
Horizontal distribution of hydrogen in the summer thermosphere from AE‐C satellite measurements
Authors: Sanatani S. , Breig E. L. , Coley W. R.
Published: February 2004

We describe satellite observations and analyses leading to the abundance of neutral atomic hydrogen near 300 km and its horizontal distribution with latitude and local time through the summer thermosphere. These H concentrations were derived from the charge‐exchange equilibrium relationship between hydrogen and oxygen atoms and ions, with basic in situ measurements accomplished in the F region with the AE‐C satellite around the June 1974 solstice near a solar minimum. Critical observational information concerning the low ion densities of H+ was provided by the magnetic ion mass spectrometer (MIMS) on board this spacecraft. A partitioning of the available database serves to illustrate the diurnal variability of thermospheric hydrogen in several prechosen zones of latitude to the 68° orbital inclination of AE‐C; an alternative approach accentuates the dependence on latitude in select local time intervals. Observed seasonal changes reflect a general decrease in hydrogen density from the equatorial region toward the summer pole, with the largest latitudinal gradients near predawn and the smallest during afternoon. We demonstrate reasonable consistency of the broad horizontal behavior with other available observations and the mass spectrometer/incoherent scatter 1986 atmospheric model, both for the extensively studied diurnal variation at low latitude and the lesser‐known latitudinal dependence. Of special significance are apparent effects on the thermospheric H abundance and distribution associated with several geomagnetic disturbances during the extended time period of data acquisition. These features are most discernible as perturbations in the recorded diurnal variation but also alter the latitudinal gradients. We include a brief discussion regarding the observed relationship between local H concentration and the temperature as they vary through the summer thermosphere, noting distinctly different power law behaviors in the hydrogen distribution for temperatures lesser and greater than ∼800°K.
Investigation of intrinsic variability in one‐dimensional parallel shocks using steady state hybrid simulations
Authors: Bennett Lee , Ellison Donald C.
Published: February 2004

We have developed a means of producing a steady state hybrid simulation of a collisionless shock. The shock is stopped in the simulation box by transforming into the shock frame and by modifying the downstream boundary conditions to allow the plasma to flow through the simulation box. Once the shock is stationary in the box frame, the simulation can be run for an arbitrary time with a fixed box size and a fixed number of simulation particles. Using this technique, we have shown that certain gross properties associated with the shock, such as the particle distribution function (including energetic particles produced by Fermi acceleration) and the flow speed profile, are constant (except for statistical variations) over hundreds of gyroperiods when averaged over times short compared to the average residence time of energetic particles. Our results imply that any microphysical processes responsible for particle heating and/or injection into the Fermi mechanism can be viewed as smooth and continuous on timescales longer than a few gyroperiods.
Penetration of the interplanetary magnetic field, By and magnetosheath plasma into the magnetosphere: Implications for the predominant magnetopause merging site
Authors: Newell Patrick T. , Sibeck David G. , Meng Ching‐I.
Published: February 2004

Magnetosheath plasma penetrates into the magnetosphere creating the particle cusp, and similarly the interplanetary magnetic field (IMF) By component penetrates the magnetopause. We reexamine the phenomenology of such penetration to investigate implications for the magnetopause merging site. Three models are popular: (1) the “antiparallel” model, in which merging occurs where the local magnetic shear is largest (usually high magnetic latitudes); (2) a tilted merging line passing through the subsolar point but extending to very high latitudes; or (3) a tilted merging line passing through the subsolar point in which most merging occurs within a few Earth radii of the equatorial plane and local noon (subsolar merging). It is difficult to distinguish between the first two models, but the third implies some very different predictions. We show that properties of the particle cusp imply that plasma injection into the magnetosphere occurs most often at high magnetic latitudes. In particular, we note the following: (1) The altitude of the merging site inferred from midaltitude cusp ion pitch angle dispersion is typically 8‐12 RE. (2) The highest ion energy observable when moving poleward through the cusp drops long before the bulk of the cusp plasma is reached, implying that ions are swimming upstream against the sheath flow shortly after merging. (3) Low‐energy ions are less able to enter the winter cusp than the summer cusp. (4) The local time behavior of the cusp as a function of By and Bz corroborates predictions of the high‐latitude merging models. We also reconsider the penetration of the IMF By component onto closed dayside field lines. Our approach, in which closed field lines move to fill in flux voids created by asymmetric magnetopause flux erosion, shows that strict subsolar merging cannot account for the observations.
A comparison of Viking UVI auroral observations and model calculations of camera responses
Authors: Steele D. P. , McEwen D. J. , Murphree J. S.
Published: February 2004

We have performed model calculations of far ultraviolet (FUV) auroral emissions, their radiative transfer, and the responses of the two cameras of the Viking UVI to these emissions for comparison with a small sample of Viking images with complementary simultaneous DMSP F7 particle data. We selected the images on the basis of good viewing of the DMSP F7 footprint and stable electron precipitation around the image time. We used current models to calculate the FUV auroral emissions resulting from the measured electron spectra, atmospheric absorption of LBH emissions, and the O I 130.4‐nm and 135.6‐nm auroral radiances seen by the Viking UVI. In five of six LBH camera events and one of two 1304 camera events, the model calculations agreed with the observations within experimental error, validating the cross sections and model atmospheres used in the model calculations and the UVI sensitivity at FUV wavelengths. We attribute the remaining discrepancies chiefly to errors in radiative transfer calculations caused by the failure of the approximation of horizontally uniform emission, in two events where the precipitating auroral electron flux had sharp gradients. The events treated here, occurring under quiet to moderately disturbed geomagnetic conditions, were modeled successfully without adjusting MSIS 86 species concentrations.
Characteristics of ionospheric convection and field‐aligned current in the dayside cusp region
Authors: Lu G. , Lyons L. R. , Reiff P. H. , Denig W. F. , de la Beaujardiére O. , Kroehl H. W. , Newell P. T. , Rich F. J. , Opgenoorth H. , Persson M. A. L. , Ruohoniemi J. M. , Friis‐Christensen E. , Tomlinson L. , Morris R. , Burns G. , McEwin A.
Published: February 2004

The assimilative mapping of ionospheric electrodynamics (AMIE) technique has been used to estimate global distributions of high‐latitude ionospheric convection and field‐aligned current by combining data obtained nearly simultaneously both from ground and from space. Therefore, unlike the statistical patterns, the “snapshot” distributions derived by AMIE allow us to examine in more detail the distinctions between field‐aligned current systems associated with separate magnetospheric processes, especially in the dayside cusp region. By comparing the field‐aligned current and ionospheric convection patterns with the corresponding spectrograms of precipitating particles, the following signatures have been identified: (1) For the three cases studied, which all had an IMF with negative y and z components, the cusp precipitation was encountered by the DMSP satellites in the postnoon sector in the northern hemisphere and in the prenoon sector in the southern hemisphere. The equatorward part of the cusp in both hemispheres is in the sunward flow region and marks the beginning of the flow rotation from sunward to antisunward. (2) The pair of field‐aligned currents near local noon, i.e., the cusp/mantle currents, are coincident with the cusp or mantle particle precipitation. In distinction, the field‐aligned currents on the dawnside and duskside, i.e., the normal region 1 currents, are usually associated with the plasma sheet particle precipitation. Thus the cusp/mantle currents are generated on open field lines and the region 1 currents mainly on closed field lines. (3) Topologically, the cusp/mantle currents appear as an expansion of the region 1 currents from the dawnside and duskside and they overlap near local noon. When By is negative, in the northern hemisphere the downward field‐aligned current is located poleward of the upward current; whereas in the southern hemisphere the upward current is located poleward of the downward current. (4) Under the assumption of quasi‐steady state reconnection, the location of the separatrix in the ionosphere is estimated and the reconnection velocity is calculated to be between 400 and 550 m/s. The dayside separatrix lies equatorward of the dayside convection throat in the two cases examined.