bluuebunny/crossref_metadata_2025 huggingface dataset:
Relationship between the Geotail spacecraft potential and the magnetospheric electron number density including the distant tail regions
Authors: Ishisaka K. , Okada T. , Tsuruda K. , Hayakawa H. , Mukai T. , Matsumoto H.
Published: February 2004

The spacecraft potential has been used to derive the electron number density surrounding the spacecraft in the magnetosphere and solar wind. We have investigated the correlation between the spacecraft potential of the Geotail spacecraft and the electron number density derived from the plasma waves in the solar wind and almost all the regions of the magnetosphere, except for the high‐density plasmasphere, and obtained an empirical formula to show their relation. The new formula is effective in the range of spacecraft potential from a few volts up to 90 V, corresponding to the electron number density from 0.001 to 50 cm−3. We compared the electron number density obtained by the empirical formula with the density obtained by the plasma wave and plasma particle measurements. On occasions the density determined by plasma wave measurements in the lobe region is different from that calculated by the empirical formula. Using the difference in the densities measured by two methods, we discuss whether or not the lower cutoff frequency of the plasma waves, such as continuum radiation, indicates the local electron density near the spacecraft. Then we applied the new relation to the spacecraft potential measured by the Geotail spacecraft during the period from October 1993 to December 1995, and obtained the electron spatial distribution in the solar wind and magnetosphere, including the distant tail region. Higher electron number density is clearly observed on the dawnside than on the duskside of the magnetosphere in the distant tail beyond 100 RE.
Radial variation of solar wind electrons inside a magnetic cloud observed at 1 and 5 AU
Authors: Skoug R. M. , Feldman W. C. , Gosling J. T. , McComas D. J. , Reisenfeld D. B. , Smith C. W. , Lepping R. P. , Balogh A.
Published: February 2004

The fortuitous observations of a magnetic cloud during the ACE/Ulysses radial alignment in March 1998 provide one of the first opportunities to study the same magnetic cloud at two different radial distances. In this paper, we use data from both ACE and Ulysses to examine the expansion of the cloud as well as of the adjacent noncloud solar wind. In particular, we study the evolution of the electrons as the cloud expanded from 1 to 5.4 AU, including the core, halo, and total electron populations. We find that the core and total electron temperatures at either spacecraft are moderately anticorrelated with electron density. However, combined observations from the two spacecraft show that both temperature and density decreased as the cloud expanded and thus that single point measurements cannot be used to determine a polytropic index describing the expansion of solar wind electrons within the cloud.
Dayside enhancements of thermospheric O/N2 following magnetic storm onset
Authors: Immel Thomas J. , Crowley Geoff , Craven John D. , Roble Raymond G.
Published: February 2004

One frequently observed effect of thermospheric storms is the reduction of atomic oxygen relative to molecular nitrogen at high and middle latitudes. These composition changes lead to a decrease in thermospheric O I 130.4‐nm emissions in the sunlit hemisphere. Such decreases have been observed by various satellite‐based instruments including the Dynamics Explorer 1 (DE 1) Spin‐Scan Auroral Imager. In contrast, this paper focuses on enhancements of the terrestrial 130.4‐nm dayglow emission observed with DE 1. Following the onset of a geomagnetic storm at 1610 UT on February 5, 1983, an increase (>20%) in the O I 130.4‐nm emission was observed at middle latitudes in the morning sector of the Southern Hemisphere. The increased O I 130.4‐nm emission indicates an increase of atomic oxygen relative to molecular nitrogen. The brightness enhancement coincided with the passage of a large‐scale gravity wave that was observed in measurements from a global network of ionosondes. The global FUV images and ionosonde observations are complemented by a TIMEGCM simulation of the February 1–6 period, which provides a framework for combining the ionosonde and DE 1 observations. The primary mechanisms for the FUV variations considered here are increases in the relative abundance of atomic oxygen in the morning sector caused by (1) the large‐scale gravity wave launched by the onset of magnetic activity and (2) corotation of previously affected parcels onto the dayside. The investigation leads to a physical interpretation of the observed FUV features in terms of the gravity wave effects and a transient Hadley circulation cell. This work represents the first detection of a large‐scale gravity wave from an orbiting FUV imager.
An improved method of inferring interplanetary sector structure, 1905‐present
Authors: Vennerstroem S. , Zieger B. , Friis‐Christensen E.
Published: February 2004

A new method of estimating interplanetary sector polarity from geomagnetic activity is presented. The method is based on a linear multiregression between the By component of the interplanetary magnetic field and hourly values of the magnetic perturbation (ΔX, ΔY, ΔZ) at selected magnetic observatories. By simultaneous use of all three components, and by using all hours of local time, the method yields an improved accuracy compared to previous subjective assessments of the sector polarity. Using this method, it is possible to derive the interplanetary sector structure at least back to 1926, with an accuracy which is sufficient to follow the development of large‐scale unipolar structures in the interplanetary medium. Furthermore, we present results indicating that it may be possible to derive sector polarity even further back in time to 1905, but with less accuracy. This is accomplished by including the two subauroral stations Sitka and Sodankylä, which have not previously been used for polarity determination. A major problem with this early polarity determination is a strong asymmetry favoring toward sectors.
Heliospheric modulation of cosmic rays and solar activity during the Maunder minimum
Authors: Usoskin Ilya G. , Mursula Kalevi , Kovaltsov Gennady A.
Published: February 2004

Modern models and direct cosmic ray experiments deal with heliospheric modulation of cosmic rays only during the recent times of rather high overall solar activity level. On the other hand, the question of cosmic ray modulation during the exceptional conditions of very quiet heliosphere is important. In the present paper we compare the variations of cosmic ray intensity with solar and auroral activity during the Maunder minimum (1645–1715) when the Sun was extremely quiet. We use the newly presented group sunspot number series as a measure of early solar activity, the auroral observations in central Europe as an indicator of transient phenomena in the inner heliosphere, and the radiocarbon data as a proxy of cosmic ray intensity. We find that both cosmic ray intensity and auroral activity closely follow the dominant 22‐year cyclicity with sunspot activity during the Maunder minimum. Moreover, the strict antiphase between the 22‐year variation of cosmic ray intensity and sunspot activity suggests that the 22‐year variation in cosmic ray intensity can be explained by the diffusion‐dominated terms of cosmic ray modulation without significant drift effects. We also discuss the possible origin of the behavior of the 10Be data which is different from all other parameters during the Maunder minimum.
Nature of the boundary between open and closed magnetic field line regions at the Sun revealed by composition data and numerical models
Authors: Posner Arik , Zurbuchen Thomas H. , Schwadron Nathan A. , Fisk Lennard A. , Gloeckler George , Linker Jon A. , Mikić Zoran , Riley Pete
Published: February 2004

Recently, Fisk et al. [1999] have presented a theory that describes a number of features of the large‐scale coronal and heliospheric magnetic field. This theory predicts large‐scale transport of magnetic flux across the boundaries of the polar coronal holes, which leads to reconnection processes of open field lines with preliminary closed magnetic structures. Reconnection processes reveal themselves in solar wind composition data: Plasma released out of previously closed magnetic field structures exhibits hotter charge state distributions and has a tendency to be enriched by elements with low first ionization potentials. The idea of reconnection at the boundaries of coronal holes is not new. For example, Wang and Sheeley [1993] and Luhmann et al. [1999] found evidence for that mechanism by comparison of observations of the rotation and evolution of coronal holes with potential field models of the solar corona. We use Ulysses Solar Wind Ion Composition Spectrometer composition measurements and sophisticated numerical models [Linker et al., 1999; Riley et al., 1999] to accurately map these observations back to the solar surface. We then constrain the thickness of the stream interface at the Sun and compare the location of the source region with SOHO observations of the low corona. The results are discussed in the context of the global structure of the heliospheric magnetic field.
Interplanetary magnetic field: Statistical properties and discrete modes
Authors: Thomson D. J. , Lanzerotti L. J. , Maclennan C. G.
Published: February 2004

We report here detailed statistical studies of power spectra of 1 hour average interplanetary magnetic field (IMF) data that were acquired by the magnetometer instrument on the Ulysses spacecraft in the ecliptic plane of the heliosphere during Ulysses' cruise phase from Earth to Jupiter (October 1990 through January 1992). We have pursued these studies in order to examine in a critical manner our previously published conclusions [Thomson et al., 1995] that energetic particle fluxes in the interplanetary medium carried signatures of discrete modes. We attributed these signatures to the existence of solar acoustic and gravitational modes (p and g modes, respectively) in the heliosphere. Here we first examine the statistical distributions of g modes (less than ∼200 μHz) that are predicted by a theoretical model of the solar interior. With this background information we formulate the time series and statistical analysis procedures that must be used in order to provide information on possible modal signals in the IMF, independent of the source. We show from the statistical analyses that the modal components that are present in the IMF fluctuations can amount to ∼20% to ∼70% of the IMF fluctuation power in the frequency range of ∼10–140 μHz. We discuss these findings in the context of previously published work on searches for optical signatures of solar g modes in the solar photosphere as well as published research papers that have called into question some of the conclusions of Thomson et al. [1995]. We conclude that the interplanetary medium is not a totally chaotic medium, but can sustain high power levels of discrete, low‐frequency modes, probably of solar origin, over a long interval of time.
The latitudinal transport of energetic particles associated with corotating interaction regions
Authors: Giacalone Joe
Published: February 2004

We investigate the effect of large‐scale magnetic turbulence in the solar wind on the transport of ∼1–10‐MeV protons associated with corotating interaction regions (CIRs). We develop a quantitative model of the heliospheric magnetic field which is determined by the motions of magnetic foot points near the solar photosphere. The trajectories of many test particles are numerically integrated in this field to examine the variation of the particle distribution function with latitude. The foot points of the magnetic field lines, which extend radially from the source surface, execute a random walk in latitude and longitude. We find that the resulting long‐wavelength magnetic fluctuations lead to significant latitudinal field line diffusion which simultaneously leads to significant latitudinal transport of CIR‐associated energetic particles. The results from our model are qualitatively consistent with recent Ulysses observations of recurrent energetic particle intensity enhancements at high heliographic latitudes.
On the origin of microscale magnetic holes in the solar wind
Authors: Zurbuchen T. H. , Hefti S. , Fisk L. A. , Gloeckler G. , Schwadron N. A. , Smith C. W. , Ness N. F. , Skoug R. M. , McComas D. J. , Burlaga L. F.
Published: February 2004

Magnetic holes are sudden changes in the magnetic field intensity |B| from typical interplanetary values (∼10 nT) to less than 1 nT in a matter of seconds. The intensity then recovers within seconds or up to ∼30 min later. These |B| dropouts can be seen daily. Less often observed, but even more dramatic, are magnetic field depletions that last for up to several hours. We use selected periods of magnetic flux dropouts observed with various sensors of the Advanced Composition Explorer (ACE), which has a unique combination of magnetic field, plasma, and composition experiments, to establish the origin of these peculiar objects. We conclude that these microscale magnetic holes very likely develop in the heliosphere and are not of direct solar origin. We also suggest a possible formation mechanism associated with magnetic reconnection close to the Sun.
The heliospheric current sheet
Authors: Smith Edward J.
Published: February 2004

The heliospheric current sheet (HCS) is the boundary between open oppositely directed magnetic field lines which commonly originate as the outward extension of the solar magnetic dipole. The dipole tilt, the rotation of the Sun, and the outward propagation of the solar wind cause peaks and valleys in the current sheet which spiral outward. The HCS extends throughout the heliosphere to the greatest distances reached by Pioneer and Voyager. It serves as a magnetic equator, and solar wind parameters including speed, temperature, density, and composition vary with distance from the HCS. Extrapolated back to the Sun, especially near solar minimum, the HCS corresponds to the low‐latitude streamer belt. Both features are closely related to a neutral line obtained by extrapolating photospheric magnetic fields to a source surface at several solar radii. The current sheet and sector structure persist throughout the solar cycle including solar maximum. At 1 AU the width of the HCS is approximately 10,000 km while a surrounding plasma sheet is thicker by a factor of ∼30. The field inside the HCS does not simply decrease to a null and then reappear with the opposite sense. Instead, the field rotates at nearly constant magnitude from one polarity to the other. In spite of theoretical expectations that fields on opposite sides of the HCS will merge or reconnect, there is little evidence that such is occurring. Many scientific questions remain unanswered. What are the global properties of the HCS near solar maximum, and how faithfully are they reproduced by source surface models? Are multiple HCS crossings caused by waves on the current sheet or by multiple current sheets? What is the effect of coronal mass ejections on the HCS and vice versa?
Foreword [to Special Section on The Magnetic Structure of the Heliosphere]
Authors: Zurbuchen Thomas H. , Jokipii J. R.
Published: February 2004

Over the past 50 years our understanding of the Earth's environment in space has gone from essentially zero knowledge to the current picture. We now know that the Earth is immersed in a complex, magnetized plasma outflow from the Sun called the solar wind which extends out to at least 75 AU. We now also know that the interplanetary magnetic field, imbedded in this flow, is considerably different from the vacuum field that was initially postulated. This vast improvement in our knowledge has been a consequence of detailed in situ and remote observations and theoretical interpretation.
Change of energetic ion composition in the plasma sheet during substorms
Authors: Nosé M. , Ohtani S. , Lui A. T. Y. , Christon S. P. , McEntire R. W. , Williams D. J. , Mukai T. , Saito Y. , Yumoto K.
Published: February 2004

It has been reported by previous studies that the energetic particle flux of ions of ionospheric origin like O+ ions is more enhanced than that of H+ ions in the near‐Earth tail (X ∼ −6 to −16 RE) during substorms. To explain this strong O+ flux enhancement, some studies have surmised that thermal O+ ions in the plasma sheet boundary layer or the lobe are strongly accelerated at the magnetic reconnection region (X ∼ −20 to −30 RE), and are subsequently transported into the near‐Earth plasma sheet with earthward plasma flows. However, other studies have supposed that the strong O+ flux enhancement is caused by local magnetic field reconfiguration (local dipolarization). In the present study, we used Geotail/EPIC measurements of energetic (60 keV to 3.6 MeV) ion flux to test the above two scenarios. We investigated ion composition in the plasma sheet while earthward plasma flows and/or dipolarization signatures were observed. In terms of energy density ratio of oxygen ions to protons, the observational results can be summarized as follows: (1) earthward plasma flows without dipolarization signatures did not accompany large increases of the ratio in most cases; (2) when earthward plasma flows appeared with dipolarization signatures, they accompanied increases of the ratio; and (3) most of dipolarization events were associated with large increases of the ratio. These results suggest that the strong increase in the energetic oxygen constituent in the near‐Earth plasma sheet is due to acceleration of ions during dipolarization, consistent with the latter scenario.
Modeling the inner magnetosphere: The asymmetric ring current and Region 2 Birkeland currents revisited
Authors: Tsyganenko N. A.
Published: February 2004

A quantitative model is developed of the inner magnetospheric magnetic field, combining the effects of the azimuthally asymmetric ring current with those of field‐aligned currents, due to azimuthal variation of the plasma pressure. The axisymmetric part of the model ring current was derived from average observed radial profiles of the particle pressure and anisotropy and was analytically represented by a vector potential, fitted by least squares to one derived by the Biot‐Savart integral. The contribution of the asymmetric part, including the field of Birkeland currents, closed via the ionosphere, was represented as a divergenceless combination of expansions for the components of the B vector, and was also fitted by least squares to the corresponding Biot‐Savart field. The total field‐aligned current, associated with the noon‐midnight pressure asymmetry of the outer ring current, was found to be significantly smaller than the total Region 2 current, observed at low altitudes. It is therefore concluded that most of that current is generated in the near‐Earth plasma sheet. The goal of this work is a realistic and computationally efficient description of the asymmetric ring current, to be included in an advanced model of the external geomagnetic field.
The dynamical nature of coronal streamers
Authors: Wang Y.‐M. , Sheeley N. R. , Socker D. G. , Howard R. A. , Rich N. B.
Published: February 2004

Recent high‐sensitivity imaging of the Sun's white‐light corona from space has revealed a variety of unexpected small‐scale phenomena, including plasma blobs that are ejected continually from the cusplike bases of streamers, fine raylike structures pervading the outer streamer belt, and inflows that occur mainly during times of high solar activity. These phenomena can be interpreted as different manifestations of magnetic field line reconnection, in which plasma and magnetic flux are exchanged between closed and open field regions of the corona. The observations provide new insights into a number of long‐standing questions, including the origin of the streamer material in the outer corona, the sources of the slow solar wind, and the mechanisms that regulate the interplanetary magnetic field strength.
Flow bursts, braking, and Pi2 pulsations
Authors: Kepko L. , Kivelson M. G. , Yumoto K.
Published: February 2004

We examine six near‐Earth dipolarization events during which rapid flows were observed by Geotail at distances between 8 and 15 RE in the magnetotail. Each flow event was associated with local dipolarization, auroral arc brightening, and Pi2 pulsations (periods of 40–150 s). Variations in earthward flow velocity delayed by 60–90 s match the Pi2 waveforms on the ground at low latitudes on the flank. We conclude that low‐latitude Pi2 pulsations are directly driven by compressional pulses associated with braking of oscillatory earthward flows. In addition, we identify a new type of nightside Pi2, which is related to the oscillatory braking current that modulates the current in the substorm current wedge. For one event we estimate the magnitude of the braking current from the amplitude of ground perturbations and find I∼2–3×104 A. An independent calculation of the inertial current using the flow measurements yields 6×104 A. We separate Pi2 into three distinct physical types: low‐latitude directly driven, nightside transient response, and nightside inertial current. We propose a phenomenological model linking flow bursts, the substorm current wedge, and the three types of Pi2 pulsations.
Simulations of storm time diffuse aurora with plasmasheet electrons in strong pitch angle diffusion
Authors: Chen Margaret W. , Schulz Michael
Published: February 2004

Using a guiding‐center simulation of plasmasheet electrons postulated to be in strong pitch angle diffusion, we compute drift trajectories from a Hamiltonian formulation and map phase space densities from the nightside neutral line in Dungey's model magnetosphere (dipole field plus uniform southwardBz) according to Liouville's theorem modified for exponential loss implicit in the strong diffusion hypothesis. From the resulting phase space distributions we compute the precipitating energy flux into the auroral ionosphere as functions of magnetic latitude and magnetic local time (MLT) so as to simulate numerically the spatial and spectral structure of diffuse auroral electron precipitation for comparison with observational data. Storm‐associated impulses (by which we enhance the convection electric field) can typically transport plasmasheet electrons from the nightside neutral line to the near‐midnight region of maximum precipitating energy flux (latitude ≈ 65°) in ∼20–30 min, which is roughly the strong diffusion lifetime of 4‐keV electrons at the correspondingLvalue (≈5.7). The maximum precipitating electron energy flux in our simulation of the model storm is thus modulated by random variations in the mean cross‐magnetospheric electric potential drop over the 20–30 min before the time of interest. Our results also show a consistent lack of precipitating electron energy flux in the afternoon quadrant, essentially because this is the last quadrant to be visited by plasmasheet electrons (and therefore features the most strongly attenuated phase space densities) as they drift through the magnetosphere on open trajectories. The result agrees qualitatively with the typically observed “darkness” of X‐ray images of the diffuse aurora in that sector (1200–1800 MLT). While our simulation results locate the region of maximum energy flux slightly postmidnight during both prestorm and stormtime, in good agreement with previously published statistical compilations of auroral electron precipitation. Our simulations do not explain other large intensifications of electron precipitation (near dawn and in the morning sector) that are also found in both statistical and storm event studies. This suggests the need for a model in which pitch angle diffusion is less than everywhere strong throughout the plasma sheet.
An interpretation of banded magnetospheric radio emissions
Authors: Benson R. F. , Osherovich V. A. , Fainberg J. , F.‐Vinas A. , Ruppert D. R.
Published: February 2004

Recently published Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) banded magnetospheric emissions, commonly referred to as “(n + 1/2)fce” emissions, where fce is the electron gyrofrequency, are analyzed by treating them as analogous to sounder‐stimulated ionospheric emissions. We show that both individual spectra of magnetospheric banded emissions and a statistically derived spectra observed over the 2‐year lifetime of the mission can be interpreted in a self‐consistent manner. The analysis, which predicts all spectral peaks within 4% of the observed peaks, interprets the higher‐frequency emissions as due to low group velocity Bernstein‐mode waves and interprets the lower‐frequency emissions as eigenmodes of cylindrical‐electromagnetic plasma oscillations. The demarcation between these two classes of emissions is the electron plasma frequency fpe where an emission is often observed. This fpe emission is not necessarily the strongest. None of the observed banded emissions were attributed to the upper hybrid frequency. We present Alouette 2 and ISIS 1 plasma resonance data and model electron temperature (Te) values to support the argument that the frequency spectrum of ionospheric sounder‐stimulated emissions is not strongly temperature dependent and thus that the interpretation of these emissions in the ionosphere is relevant to other plasmas (such as the magnetosphere) where Ne and Te can be quite different but where the ratio fpe/fce is identical. The Ne values deduced from the spectral interpretation do not agree with the values determined from the AMPTE/IRM three‐dimensional plasma instrument. The latter, which represent a lower bound, are found to be higher than the former by a factor of 3.2 – 3.5. All values were less than 1 cm−3, a domain known for measurement difficulties. One possible explanation is that the wave and plasma techniques respond to different components of a non‐Maxwellian magnetospheric electron distribution.
Observations of 50‐ and 12‐MHz auroral coherent echoes at the Antarctic Syowa station
Authors: Koustov A. V. , Igarashi K. , André D. , Ohtaka K. , Sato N. , Yamagishi H. , Yukimatu A.
Published: February 2004

Data on 50‐ and 12‐MHz E region coherent echoes from approximately the same directions and in a broad range of azimuths are available for 1995–1997 radar observations at the Antarctic Syowa station. One event of such observations is considered to compare power and Doppler velocity of echoes at these significantly different frequencies. For the considered event, Doppler velocities of more than 600 ms−1 (as recorded at both radar frequencies) were observed. We show that even though the 50‐MHz echoes exhibit strong flow angle variation of the power, the 12‐MHz echoes do not. Apparent aspect angle power attenuation is found to be 10 and 2.5 dBdeg−1 at 50 and 12 MHz, respectively. Measured Doppler velocities along specific radar directions are found to be comparable in spite of the significant difference in radar frequencies, with the velocity ratio V50MHz/V12MHz being in between 1.6 and 0.9 for high velocities of more than 400 ms−1. The velocity ratio varies with slant range and azimuth of observations. Ionospheric propagation effects for radio waves and plasma physical effects for irregularities are considered in an attempt to explain several observed features, for example, the echo power and velocity ratio variations with slant range.
Electrostatic potential patterns in the high‐latitude ionosphere constrained by SuperDARN measurements
Authors: Shepherd S. G. , Ruohoniemi J. M.
Published: February 2004

The recent addition of two radars to the existing network of six Super Dual Auroral Radar Network (SuperDARN) HF radars in the Northern Hemisphere has significantly extended the area in the high latitude where measurements of convecting ionospheric plasma are made. We show that the distribution of the electrostatic potential Φ associated with the “E × B” drift of ionospheric plasma can be reliably mapped on global scales when velocity measurments provide sufficient coverage. The global convection maps, or the equivalent electrostatic potential maps, are solved using an established technique of fitting velocity data to an expansion of Φ in terms of spherical harmonic functions. When the measurements are extensive, and especially when they span the region between the extrema in the potential distribution, the solution for the global pattern becomes insensitive to the choice of statistical model data used to constrain the fitting. That is, the statistical model data then only guide the solution in regions where no measurements are available, and the details of the model data have little effect on the gross features of the large‐scale convection patterns. The resulting total potential variation across the polar cap, ΦPC, is virtually independent of the statistical model. The ability to accurately determine ΦPC and the global potential Φ on the basis of direct measurements is an important step in understanding solar wind‐magnetosphere‐ionosphere coupling.
Lunar tide in the equatorial F region vertical ion drift velocity
Authors: Stening Robert J. , Fejer Bela G.
Published: February 2004

Vertical ion drift velocity data from Jicamarca have been analyzed for a lunar semidiurnal tide using a least squares fitting method. Amplitudes of up to 6 m s−1 are obtained with phases in agreement with lunar tidal determinations of other associated physical parameters. Variations between season, solar activity, and day to night are also examined. Generally, amplitudes are larger in the southern summer. Much of the phase variation with season is very similar for solar maximum and minimum years. There is a summer to winter phase change that is most distinct at solar maximum nighttime. A day‐to‐night phase reversal can also been seen in some seasons. Hints of this are also found in the lunar tide in the F region height and in the magnetic variations at Huancayo.
The interaction between the magnetosphere of Saturn and Titan's ionosphere
Authors: Nagy Andrew F. , Liu Yifan , Hansen Kenneth C. , Kabin Konstantin , Gombosi Tamas I. , Combi Michael R. , DeZeeuw Darren L. , Powell Kenneth G. , Kliore Arvydas J.
Published: February 2004

A three‐dimensional (3‐D) multi‐species magnetohydrodynamic model was used to study the interaction of Titan's ionosphere and Saturn's magnetosphere. The three generic species which were considered are light (e.g., H+, H2+, and H3+), medium (e.g., N+ and CH5+), and heavy (e.g., N2+ and HCNH+) ion species. The effects of exospheric mass loading, major chemical reactions, and ion‐neutral collisions were considered. The upstream parameters were selected to be the nominal values for the case when Titan is in the magnetosphere of Saturn. The simulation results are compared with Voyager measurements as well as related model calculations. The 3‐D three‐species model results reproduce reasonably well the global features such as magnetic barrier, magnetotail, and the distributions of the major ionospheric species. The outward escape flux of the major ionospheric species (i.e., the heavy ion species) from the tail is calculated to be approximately 6.5 × 1024 s−1.
Observations of persistent dayside F region electron temperature enhancements associated with soft magnetosheathlike precipitation
Authors: Doe R. A. , Kelly J. D. , Sánchez E. R.
Published: February 2004

A series of experiments with the Sondrestrom incoherent scatter (IS) radar (66.99°N latitude, 50.95°W longitude) were designed to examine F region structure in the dayside auroral oval in order to search for plasma signatures from magnetospheric regions such as the cusp, boundary plasma sheet, low‐latitude boundary layer, and mantle. This IS radar mode, optimized to search for ionospheric features which remain fixed in local time, was coordinated with overflights of the DMSP F‐10 satellite on 2 days in September 1992. For both study days, IS radar data show persistent (∼ 7 hour), enhanced Te regions at 300 to 500 km. These enhanced Te regions evolve during periods in which relatively unstructured, laminar Ne densities are observed and thus are not merely the result of a structured suppression of electron cooling. The cores of these Te enhancements were observed at latitudes and magnetic local times corresponding to DMSP satellite measurement of soft (< 100 eV) cusplike precipitation. These Te hotspots move systematically equatorward with increasing geomagnetic activity and display a sharp field‐aligned equatorward edge at the location of satellite cusp detection. Unlike prior IS radar/satellite cusp investigations, no significant Ne enhancements were measured coincident with Te hotspots. A simple ionospheric model is invoked to confirm that such soft cusplike precipitation does not significantly alter the magnitude of the ambient plasma density, and we argue that cusp detection based on collocated Ne and Te enhancements is seldom possible. The local time persistence of the Te enhancements, beyond the typical cusp widths, suggests an association with additional dayside magnetospheric regimes such as the low‐latitude boundary layer. Both latitudinal and vertical Te gradients maximize at the location of satellite cusp detection, suggesting that the heat source is a divergence of magnetospheric heat flux on freshly reconnected geomagnetic field lines.
Structure of the reconnection layer and the associated slow shocks: Two‐dimensional simulations of a Riemann problem
Authors: Cremer Michael , Scholer Manfred
Published: February 2004

The kinetic structure of the reconnection layer in the magnetotail is investigated by two‐dimensional hybrid simulations. As a proxy, the solution of the Riemann problem of the collapse of a current sheet with a normal magnetic field component is considered for two cases of the plasma beta (particle to magnetic field pressure): β = 0.02 and β = 0.002. The collapse results in an expanding layer of compressed and heated plasma, which is accelerated up to the Alfvén speed vA. The boundary layer separating this hot reconnection like layer from the cold lobe plasma is characterized by a beam of back‐streaming ions with a field‐aligned bulk speed of ≃ 2vA relative to the cold lobe ion population at rest. As a consequence, obliquely propagating waves are excited via the electromagnetic ion/ion cyclotron instability, which led to perpendicular heating of the ions in the boundary layer as well as further outside the layer in the lobe. In both regions, waves are found which propagate almost parallel to the magnetic field and which are identified as Alfvén ion cyclotron (AIC) waves. These waves are excited by the temperature anisotropy instability. The temperature anisotropy increases with decreasing plasma beta. Thus the anisotropy threshold of the instability is exceeded even in the case of a rather small beta value. The AIC waves, when convected downstream of what can be defined as the the slow shock, make an important contribution to the ion thermalization process. More detailed information on the dissipation process in the slow shocks is gained by analyzing individual ion trajectories.
Space weather observations using the SOHO CELIAS complement of instruments
Authors: Judge D. L. , McMullin D. R. , Gangopadhyay P. , Ogawa H. S. , Ipavich F. M. , Galvin A. B. , Möbius E. , Bochsler P. , Wurz P. , Hilchenbach M. , Grünwaldt H. , Hovestadt D. , Klecker B. , Gliem F.
Published: February 2004

The Solar and Heliospheric Observatory (SOHO) spacecraft located at Ll is well outside the Earth's magnetosphere and has been observing the Sun continuously since December 1995, except for relatively brief periods due to spacecraft operational interruptions. While a variety of instruments on the SOHO spacecraft investigate the solar properties important to an improved understanding of the Sun and its effect on space weather, the present work is limited to the observations provided by the Charge, Element, and Isotope Analysis System (CELIAS) proton monitor (PM) and Solar Extreme Ultraviolet Monitor (SEM) instruments and their relationship to other space weather observations. The CELIAS observations consist of particle and EUV/soft X‐ray solar flux measurements. A brief description of the CELIAS instrumentation and examples of the precursor information signaling the possibility of coronal mass ejection events observed by the CELIAS/SEM are presented. In addition, the entire SEM database since commissioning is presented on both expanded and compressed timescales in order to provide both the long‐term weather trends and short‐term storm data. The SEM data presented are full‐disk observations and have a 15 s sampling rate.
Wedge‐like dispersion of sub‐keV ions in the dayside magnetosphere: Particle simulation and Viking observation
Authors: Ebihara Y. , Yamauchi M. , Nilsson H. , Lundin R. , Ejiri M.
Published: February 2004

Particle drift simulation of low‐energy ions (less than a few keV) with a simple geomagnetic field (dipole) and the Volland‐Stern type convection electric field is found to be capable of investigating dispersed sub‐keV ion events deep inside the dayside ring current region observed by Viking. Here we show three types of such events and simulation results. From the shape of the dispersion in the energy‐latitude diagrams, they are called wedge‐like dispersions: type 1 (characteristic energy monotonically increasing with latitude), type 2 (energy increasing with latitude and subsequently decreasing with latitude), and type 3 (energy monotonically decreasing with latitude). All these cases are understood as results of energy‐dependent drift motion of ions coming from the nightside near‐Earth tail if the distribution function in the source region varies in time and space. The results indicate that the wedge‐like ion dispersions even may be associated with a substorm injection and/or a localized plasma flow channel in the near‐Earth tail.
Radar studies of midlatitude ionospheric plasma drifts
Authors: Scherliess L. , Fejer B. G. , Holt J. , Goncharenko L. , Amory‐Mazaudier C. , Buonsanto M. J.
Published: February 2004

We use incoherent scatter radar measurements from Millstone Hill and Saint Santin to study the midlatitude F region electrodynamic plasma drifts during geomagnetically quiet and active periods. We present initially a local time, season, and solar flux dependent analytical model of the quiet time zonal and meridional E × B drifts over these stations. We discuss, for the first time, the Saint Santin drift patterns during solar maximum. We have used these quiet time models to extract the geomagnetic perturbation drifts which were modeled as a function of the time history of the auroral electrojet indices. Our results illustrate the evolution of the disturbance drifts driven by the combined effects of prompt penetration and longer lasting perturbation electric fields. The meridional electrodynamic disturbance drifts have largest amplitudes in the midnight‐noon sector. The zonal drifts are predominantly westward, with largest amplitudes in the dusk‐midnight sector and, following a decrease in the high‐latitude convection, they decay more slowly than the meridional drifts. The prompt penetration and steady state zonal disturbance drifts derived from radar measurements are in good agreement with results obtained from both the ion drift meter data on board the Dynamics Explorer 2 (DE 2) satellite and from the Rice Convection Model.
Excitation of Langmuir waves in interplanetary space
Authors: Yoon Peter H. , Ziebell L. F. , Wu C. S.
Published: February 2004

In this paper, the relation between the apparently stable or marginally stable electron reduced distribution and the excitation of Langmuir waves is discussed. It is found that the positive slope in the reduced distribution function is not always necessary for the excitation of Langmuir waves and that under certain circumstances, Langmuir waves propagating in oblique directions with respect to the ambient magnetic field can be excited by the electrons which may not have a bump in the reduced distribution.
A model for the diurnal universal time variation of the Dst index
Authors: Takalo J. , Mursula K.
Published: February 2004

Using a simple model for the asymmetric ring current, we show that the main reason for the diurnal universal time (UT) variation of Dst index is the uneven distribution of the Dst network stations. The model takes into account the four Dst stations and the strong disturbance due to the partial ring current in the 1800 LT sector. In agreement with the equinoctial theory the diurnal variation is found to be larger in local summer than winter for Hermanus (HER). However, while Hermanus is the only Southern Hemisphere Dst station, this leads to a larger UT variation in the Dst index in the Northern Hemisphere summer. Moreover, we find evidence that the diurnal LT minimum is shifted by a couple of hours closer to the midnight during local winter, supporting the Malin‐Isikara effect. We also note that the Russell‐McPherron (RMP) effect is in phase (out of phase) with the modeled diurnal variation of the Dst index around vernal (autumnal) equinox, thus increasing (decreasing) the diurnal variation in the Dst index in spring (fall) by 20–30%. However, the RMP effect is not responsible for the overall diurnal UT variation of the Dst index.
Mass and energy transport in the near and middistant magnetotail around substorm onsets: Geotail observations
Authors: Miyashita Y. , Machida S. , Mukai T. , Saito Y. , Sutcliffe P. R.
Published: February 2004

We have studied mass and energy transport in the magnetotail around substorm onsets by analyzing GEOTAIL data. We selected substorm events from Pi2 pulsations and applied a superposed epoch analysis. We found that particles (ions) are gradually provided from the upstream lobe to the downstream plasma sheet before onset. Energy is transported from the lobe to the plasma sheet in the form of the Poynting flux, which is particularly large and has a large Y component around X ∼ −10 RE continuously before and after onset. The Poynting flux as well as the mass flux toward the plasma sheet starts to increase first around (X, Y) ∼ (−20, 7) RE about 0 ∼ 2 min before onset and then successively increases in the surrounding regions. In the downstream plasma sheet the dominant energy flux is the thermal flux. It is directed duskward before onset as well as the mass flux. However, very large tailward mass and thermal fluxes develop in the region of −23 > X > −30 RE of the premidnight tail after onset in association with the plasmoid. They have a large duskward component in the premidnight sector. Earthward of X ∼ −20 RE, there are not only earthward mass and thermal fluxes but also large duskward fluxes in the plasma sheet after onset. The kinetic energy flux is much smaller than the Poynting and thermal fluxes in any regions before and after onset. These observational results would provide us with significant clues to an understanding of substorm processes, such as the magnetic reconnection and the dipolarization, and their relationship.
A new perspective concerning the influence of the solar wind on the Jovian magnetosphere
Authors: Southwood D. J. , Kivelson M. G.
Published: February 2004

The solar wind exerts a strong influence on the Jovian magnetosphere in changing its volume, in energizing plasma, and in stimulating the aurora and a host of other associated effects. However, whereas at Earth the dominant solar terrestrial coupling process is magnetic reconnection, the dominant energy reservoir in Jupiter's magnetospheric plasma, continually present, is the kinetic energy of its rotating plasma disk. This “flywheel”’ produces effects with no terrestrial analogy, some of which we describe here. The most surprising prediction from the analysis of this paper is that remotely sensed symptoms of Jovian magnetospheric activity are likely to occur in conjunction with solar wind pressure decreases. Compressions of the magnetosphere produced by forward shocks and other solar wind pressure increases will heat the magnetospheric plasma but substantially reduce the ionosphere‐magnetosphere current systems. The intensity of dayside aurora and of radio wave emissions associated with increased ionospheric‐magnetospheric current systems will tend to anticorrelate with magnetospheric compressions and correlate with expansions. The link to the aurora is based on an argument that the auroral zone maps to the plasma disk of the middle magnetosphere and is thus linked to plasma sheet dynamics. The effect of expansion on the plasma sheet is to increase the parallel pressure, setting up conditions that can produce detached plasma “blobs” and enhance mass loss. The analysis is particularly apposite in light of the opportunities for observing solar wind‐Jovian interactions using data from both the Galileo and the Cassini spacecraft during the Cassini flyby of Jupiter in late 2000, ideally supplemented by auroral imaging with ground‐based and Hubble telescopes.
Cascade of the parametric decay instability in ionospheric heating experiments
Authors: Kuo S. P.
Published: February 2004

A cascade of Langmuir waves excited by the O‐mode radio waves in ionospheric heating experiments via the parametric decay instability (PDI) is studied. The threshold powers for each resonant and nonresonant cascade step are determined. The nonresonant cascade of mother Langmuir waves proceeds at the same location. However, each step of the resonant cascade occurs at a slightly different location so that the daughter wave can satisfy the local dispersion relation. In the resonance case the threshold power remains the same in each cascade. However, the mother Langmuir wave has to propagate downward to the resonant location of the daughter Langmuir wave for the next resonant cascade step. To compensate the propagation loss of the mother Langmuir wave, a large power ratio (∼10 dB) between two consecutive cascade lines in the observed HF wave enhanced plasma lines is required. The threshold powers for the nonresonant cascade steps are found to increase progressively as the cascade process proceeds. It is shown that the dependence of the threshold power on the step number N of the cascade for the nonresonant case is governed by ve/vi, where ve and vi, are the electron collision frequency and ion Landau damping rate, respectively. The threshold power has a N2 dependence for ve ≫ 2Nvi and a √N dependence for ve ≪ 2Nvi. It explains why the second cascade line hardly generates a third in early Tromso heating experiments and why more cascade steps can occur in Arecibo's heating experiments.
Energetic ion dynamics in Jupiter's plasma sheet
Authors: Selesnick R. S. , Cohen C. M. S. , Khurana K. K.
Published: February 2004

A systematic study of energetic ion trajectories in Jupiter's plasma sheet region predicts a significant role for both regular and stochastic types of motion and reveals several aspects of the ion dynamics that aid in the interpretation of Galileo observations. The motion is generally confined to the vicinity of the current sheet in a way that is proportional to the variable current sheet thickness as seen in the particle and field data. The radial extent of the trajectories increases with rigidity and initial radial distance from the planet, explaining the corresponding lack of high‐rigidity ions and decreasing radial gradients at lower rigidity. Ion intensity increases associated with changes in current sheet thickness suggest an acceleration region at ∼25 to 30 Rj. Energy dispersion in ion events at larger radial distances can be explained by such a source combined with elastic ion scattering.