bluuebunny/crossref_metadata_2025 huggingface dataset:
Sunlit Io atmospheric [O I] 6300 Å emission and the plasma torus
Authors: Oliversen Ronald J. , Scherb Frank , Smyth William H. , Freed Melanie E. , Woodward R. Carey , Marconi Max L. , Retherford Kurt D. , Lupie Olivia L. , Morgenthaler Jeffrey P.
Published: January 2004

A large database of sunlit Io [O I] 6300 Å emission, acquired over the period 1990–1999, with extensive coverage of Io orbital phase angle ϕ and System III longitude λIII, exhibits significant long‐term and short‐term variations in [O I] 6300 Å emission intensities. The long‐term average intensity shows a clear dependence on λIII, which establishes conclusively that the emission is produced by the interaction between Io's atmosphere and the plasma torus. Two prominent average intensity maxima, 70° to 90° wide, are centered at λIII ≈ 130° and λIII ≈ 295°. A comparison of data from October 1998 with a three‐dimensional plasma torus model, based upon electron impact excitation of atomic oxygen, suggests a basis for study of the torus interaction with Io's atmosphere. The observed short‐term, erratic [O I] 6300 Å intensity variations fluctuate ∼20–50% on a timescale of tens of minutes with less frequent fluctuations of a factor of ∼2. The most likely candidate to produce these fluctuations is a time‐variable energy flux of field‐aligned nonthermal electrons identified recently in Galileo plasma science data. If true, the short‐term [O I] intensity fluctuations may be related to variable field‐aligned currents driven by inward and outward torus plasma transport and/or transient high‐latitude, field‐aligned potential drops. A correlation between the intensity and emission line width indicates molecular dissociation may contribute significantly to the [O I] 6300 Å emission. The nonthermal electron energy flux may produce O(1D) by electron impact dissociation of SO2 and SO, with the excess energy going into excitation of O and its kinetic energy. The [O I] 6300 Å emission database establishes Io as a valuable probe of the torus, responding to local conditions at Io's position.
Understanding Io's space environment interaction: Recent energetic electron measurements from Galileo
Authors: Mauk B. H. , Williams D. J. , Eviatar Aharon
Published: January 2004

We present energetic (>20 keV) electron measurements from the Energetic Particle Detector on the Galileo spacecraft during recent (orbits I24 and I27) near encounters with Jupiter's satellite Io in the upstream (with respect to the plasma flow) and anti‐Jupiter flank regions. No magnetic field‐aligned electron beams were observed comparable to those observed during Galileo's J0 encounter within Io's plasma wake; only a hint of a weak field‐aligned intensity enhancement was observed during the I27 flank encounter. Motivated by these new findings, the J0 wake observations are reexamined by combining high‐energy (>20 keV) and recently published low‐energy (<10 keV) electron beam components. Comparisons with observations of Earth auroral processes suggest that the Io electron beams are created at low Jovian altitudes in a downward (with respect to Jupiter) electric current region. We thus suggest that the observed electron beams are not responsible for the spot‐like auroral emissions from Jupiter at the magnetic foot point of Io, but rather constitute part of the return current of the electric circuit that connects Io to Jupiter. During I27 Galileo entered a region depleted of energetic electrons that well matches modeled expectations of Io shadowing arising from combined electric and magnetic drifts. Also, within the shadow region Galileo moved onto field lines that are connected directly to Io or its atmosphere. Detailed angle distributions reveal the magnetic field strengths (BIo) where the particles were absorbed at Io along Io's flanks. BIo values are greater than the values at Galileo's location.
Charge neutrality and ion conic distributions at the equatorward electron edge of the midaltitude cusp
Authors: Topliss S. , Johnstone A. , Coates A. , Peterson W. K. , Kletzing C. A. , Russell C. T.
Published: January 2004

We have investigated the physical processes occurring in order to maintain charge neutrality at the equatorward edge of the cusp by examining data from 200 Polar cusp crossings. The significant differences in magnetosheath ion and electron velocities could potentially create a region at the equatorward edge of the cusp where only solar wind electrons have access. Our calculations suggest that Polar should encounter this region at least 3 min before the first solar wind ions are observed. Only six Polar cusp crossings were identified, however, where a clear separate electron edge was observed. We then used state‐of‐the‐art plasma instruments on Polar to examine particle features at the separate electron and ion edges. We find evidence that in the electron only region, electrons have been retarded by an electric potential above the spacecraft. We discuss the validity of examining electron spectra for evidence of retarding potentials, and we conclude that quantitative values cannot be obtained from single spacecraft measurements. We also find evidence of intermittent transverse ion acceleration in the electron only region. There are distinct differences in the ion conic distributions observed within the different regions at the equatorward edge of the cusp in terms of energy, pitch angle, and ion species. O+ conies are observed on closed field lines equatorward of the cusp, while H+ and He+ conies are seen at higher latitudes. In the electron only region, H+ and He+ conies have pitch angles between 90° and 120°. Further into the cusp, after the first solar wind ions are detected, higher‐energy H+ and He+ conies with pitch angles extending to 180° are measured. We suggest that processes occurring to maintain charge neutrality at the equatorward edge of the cusp may play a role in the generation of the ion conies observed.
Pc1 pearls revisited: Structured electromagnetic ion cyclotron waves on Polar satellite and on ground
Authors: Mursula K. , Bräysy T. , Niskala K. , Russell C. T.
Published: January 2004

We study an event of structured electromagnetic ion cyclotron (EMIC) waves observed by the Polar satellite in good conjunction with the Finnish ground stations on April 25, 1997. Polar observed two EMIC wave bands around the plasmapause. He+ band waves consisted of repetitive bursts which were observed on ground as a classical Pcl pearl band. H+ band waves occurred over a large latitude range of more than 5° invariant latitude and were observed on ground as a broad, diffuse Pc1 pearl band with several subbands. We found the same repetition period of ∼100 s for the Polar He+ band bursts and ground Pc1 pearls, in conflict with the bouncing wave packet (BWP) model. Comparing the burst structure of He+ band waves in Polar and on ground, we found a transit time of ∼45 s and an average group velocity of ∼500 km s−1. Within the BWP model this velocity would lead to a pearl repetition period of more than 250 s, in dramatic contradiction with the observed repetition period. Moreover, the bursts of the two Polar bands were roughly simultaneous with no significant dispersion, contrary to the expectation of the BWP model. These results clearly reject the classical BWP model, i.e., that Pc1 pearls are generated by one wave packet bouncing from one hemisphere to another. Instead, we find that EMIC waves were accompanied by long‐period ULF waves which had a period very close to the repetition period of the simultaneous EMIC bursts. Interestingly, plasma density showed simultaneous fluctuations with roughly the same period. As an alternative to the BWP model, we discuss models where the EMIC wave packet structure and ULF waves are connected. We note that the suggested relation of EMIC wave packets and ULF waves offers a new explanation to the well‐known preference of Pc1 pearls for the plasmapause. We have also estimated the full Poynting flux of EMIC waves for the first time, using the three components of electric and magnetic fields. The magnitude of the total Poynting flux of the He+ band waves was ∼20–25 μW m−2 and strongly directed downward away from the equator.
Internal and external reconnection in a series of homologous solar flares
Authors: Sterling Alphonse C. , Moore Ronald L.
Published: January 2004

Using data from the extreme ultraviolet imaging telescope (EIT) on SOHO and the soft X‐ray telescope (SXT) on Yohkoh, we examine a series of morphologically homologous solar flares occurring in National Oceanic and Atmospheric Administration (NOAA) active region 8210 over May 1–2, 1998. An emerging flux region (EFR) impacted against a sunspot to the west and next to a coronal hole to the east is the source of the repeated flaring. An SXT sigmoid parallels the EFR's neutral line at the site of the initial flaring in soft X rays. In EIT each flaring episode begins with the formation of a crinkle pattern external to the EFR. These EIT crinkles move out from, and then in toward, the EFR with velocities ∼20 km s−1. A shrinking and expansion of the width of the coronal hole coincides with the crinkle activity, and generation and evolution of a postflare loop system begins near the time of crinkle formation. Using a schematic based on magnetograms of the region, we suggest that these observations are consistent with the standard reconnection‐based model for solar eruptions but are modified by the presence of the additional magnetic fields of the sunspot and coronal hole. In the schematic, internal reconnection begins inside of the EFR‐associated fields, unleashing a flare, postflare loops, and a coronal mass ejection (CME). External reconnection, first occurring between the escaping CME and the coronal hole field and second occurring between fields formed as a result of the first external reconnection, results in the EIT crinkles and changes in the coronal hole boundary. By the end of the second external reconnection, the initial setup is reinstated; thus the sequence can repeat, resulting in morphologically homologous eruptions. Our inferred magnetic topology is similar to that suggested in the “breakout model” of eruptions [Antiochos, 1998], although we cannot determine if our eruptions are released primarily by the breakout mechanism (external reconnection) or, alternatively, primarily by the internal reconnection.
A prospective method for predicting coronal mass ejections from vector magnetograms
Authors: Falconer D. A.
Published: January 2004

Two quantitative measures of the global nonpotentiality of predominantly bipolar active regions are explored for their usefulness as indicators of whether an active region is likely to produce coronal mass ejections (CMEs). The two measures, length of strong‐field strong‐shear main neutral line (Lss) and global net current (IN), are obtained directly from vector magnetograms of the regions. From measurements of Lss and IN in four active regions for which the global nonpotentiality and CME productivity are assessed independently of Lss and IN, it appears that Lss and IN are each a useful indicator of an active region's likely CME productivity. The results also suggest that Lss and IN may be more reliable predictors of the CME productivity of active regions than is the presence or absence of sigmoidal structure in coronal X‐ray images of the regions and that there might be threshold values of Lss and IN above which CME production is likely and below which CME production is unlikely. While these results need to be certified by an expansion of this pilot study to more active regions, they show the promise of Lss and IN as quantitative measures of global nonpotentiality for CME prediction.
Dynamics of solar coronal eruptions
Authors: Vršnak Bojan
Published: January 2004

The kinematics of 87 solar eruptive events (flare sprays, eruptive prominences, and coronal transients) observed above the solar limb are studied. The data reveal a clear statistical trend for the highest measured value of the acceleration to be lower in the events taking place at a larger radial distance. The majority of events (84%) show a phase of exponential‐like growth of the velocity. The growth rate decreases with the height at which this regime sets in. A phase of constant acceleration was found only in 11% of cases. In the postacceleration phase a constant velocity regime was found in 57% of events. A considerable number of eruptions (32%) exposed a deceleration, most often showing an exponential‐like decay of the velocity. The related theoretical models are confronted with the observations, and the implications are discussed.
Solar flares and coronal shock waves
Authors: Vršnak Bojan
Published: January 2004

The ignition of coronal shock waves by solar flares is investigated confronting the results of a simple MHD model with the observations. Characteristics of type II bursts recorded in the metric and decametric wavelength range are compared with the properties of the microwave and soft X‐ray bursts of the associated flares. The time delay of the type II burst beginning after the onset of a flare is found to be shorter for shorter rise time of a flare burst and for a more powerful flare. Starting frequencies of type II bursts are higher for more impulsive flares. Instantaneous relative bandwidths of the type II bursts are larger for more powerful flares, indicating a higher Mach number of the shock. Consistent with this, a positive correlation between the flare importance and the inferred shock velocity is found. The results are fully consistent with the implications of the model describing the shock formation by a flare‐ignited blast wave.
Generation of highly energetic electrons by shock waves in the solar corona
Authors: Mann G. , Classen H.‐T. , Motschmann U.
Published: January 2004

In the solar corona, shock waves are generated by flares or coronal mass ejections. These shock waves are able to accelerate electrons up to high energies as observed by radio astronomical methods in terms of type II radio bursts and by extraterrestrial in situ measurements. A mechanism of electron acceleration acting at small spatial and temporal scales near the shock transition is presented and then compared with measurements of the Comprehensive Suprathermal and Energetic Particle Analyzer (COSTEP) instrument aboard the SOHO spacecraft during a particular solar event. A numerical hybrid simulation performed under typical coronal circumstances shows that strong magnetic field fluctuations appear immediately at the vicinity of the shock transition. Multiple encounters of electrons with such large‐amplitude fluctuations lead to considerable acceleration up to high energies of about 1 MeV.
Eruption and acceleration of flare‐associated coronal mass ejection loops in the low corona
Authors: Neupert W. M. , Thompson B. J. , Gurman J. B. , Plunkett S. P.
Published: January 2004

Observations made by the EUV imaging telescope (EIT) and the Large‐Angle Spectrometric Coronagraph (LASCO) on board the Solar Heliospheric Observatory (SOHO) have been used to characterize the eruption and acceleration of flare‐associated coronal mass ejections (CMEs) in the low corona. For three well‐observed limb events we tracked CME loops back to preexisting but faint EUV‐emitting loops at heights of 100–250 Mm that initially brightened slowly and possibly increased slowly in height, apparently in response to filament activity and eruption in the associated active regions. Subsequent CME acceleration coincided with a rapid rise of the soft X‐ray flux, occurred between 100 and 350 Mm above the surface, and may have been as high as 0.5 km s−1 s−1, consistent with an impulsive acceleration of the CME to the speeds observed in subsequent white‐light observations. The existence of a delay of up to 30 min observed between initial filament eruption in H alpha and subsequent high acceleration of the CME in one event implies that there may have been two separate phases of magnetic reconnection, with the initial filament activity acting as a trigger for subsequent CME and energetic particle acceleration in the impulsive stage of the flare. The presence or absence of this impulsive phase may provide a basis for the two types of CMEs that have been discussed in the literature.
Coronal mass ejections, magnetic flux ropes, and solar magnetism
Authors: Low B. C.
Published: January 2004

This review on coronal mass ejections (CMEs) treats hydromagnetic issues posed by observations, in order to relate CMEs to flares and prominence eruptions and to consider the roles these processes play in the evolution of the solar corona in the course of an 11‐year cycle. This global view of the corona, proposed in varying degrees of completeness by the author, physically connects the corona to the photosphere and the dynamo in the solar interior. This view is synthesized afresh starting with CME phenomenology, in order to include some new insights and to arrive at definite statements on the hydromagnetic nature of CMEs. The synthesis shows that each CME culminates a long, coherent physical process involving magnetic‐flux emergence; flares and magnetic reconnection; creation of long‐lived, large‐scale coronal structures; conservation of magnetic helicity; and failure of confinement of magnetic flux ropes in the open atmosphere. Each CME contributes a systematic permanent change to the coronal magnetic field. In this view the cumulative changes brought by all the CMEs in the course of a solar cycle have fundamental implications for the magnetic‐flux budgets of the photosphere and the corona.
A new paradigm for solar filament eruptions
Authors: Rust David M.
Published: January 2004

This article discusses the formation, magnetic structure, and eruption of solar filaments in terms of two contrasting paradigms. The standard paradigm is that filaments are formed by condensation of plasma on coronal magnetic fields that are twisted or dimpled as a result of photospheric motions. According to this paradigm, filaments erupt when photospheric motions shear the fields, increasing their energy and decreasing their stability. According to a new paradigm, subsurface motions generate toroidal magnetic flux ropes, and after these flux ropes emerge to form active regions, the most twisted parts migrate into the corona to form filaments. Filaments become unstable and are ejected after a sufficient accumulation of twist (i.e., magnetic helicity). Various proposed mechanisms for producing the needed helicity are reviewed, and several observational tests are proposed to differentiate among the possible mechanisms.
The eruptive events on September 30, 1998: 1. The jet
Authors: Bagalá L. G. , Stenborg G. , Schwenn R. , Haerendel G.
Published: January 2004

The jet on September 30, 1998, is part of a complex event that involved also other eruptive phenomena. Changes in the coronal magnetic field topology were observed during the ejection of the jet, as deduced from the Fe XIV green line emission. The whole event was well observed by both the H‐Alpha Solar Telescope for Argentina (HASTA) and the Mirror Coronagraph for Argentina (MICA), which are installed in the German‐Argentinean Solar Observatory at El Leoncito, Argentina. The Soft X‐ray Telescope (SXT) on board Yohkoh also observed the jet. In this work, observations of the first part of this complex event showing the evolution of the jet and release of blobs are presented. Certain features observed are interpreted as signatures of a magnetic reconnection process in the region. We conclude that existing theoretical and phenomenological models based on magnetic reconnection mechanisms could explain our observations, which span three temperature regimes far apart from each other.
Solar magnetic fields and geomagnetic events
Authors: Pevtsov Alexei A. , Canfield Richard C.
Published: January 2004

Some interplanetary studies lead one to expect that the toroidal fields of individual active regions are directly related to their heliospheric structure. Other studies conclude that the large‐scale solar dipolar field is more important. We have carried out two studies that bear on these apparently conflicting views. We first studied geomagnetic events temporally associated with the eruption of 18 individual coronal X‐ray sigmoids, which occurred while the large‐scale solar dipolar magnetic field pointed southward. We found that if a coronal flux rope model is used to interpret magnetic structure, eruptions with a southward leading magnetic field are associated with stronger geomagnetic storms, but those with a northward leading field are associated with more storms. We next studied three full magnetic cycles, solar cycles 17–22. We examined the temporal variation of the ratio of the geomagnetic Ap index to the sunspot number. We found no statistically compelling fluctuations of this quantity on solar cycle timescales that are in phase with the reversal of active region polarities. On the other hand, we found a weak tendency for fluctuations that are in phase with the reversal of the large‐scale solar dipole field. From these two studies we infer that the magnetic structure of individual active regions plays a role in geomagnetic events, but their geoeffectiveness is complicated by asymmetries in the leading and following magnetic field and density. We conclude that simple cycle‐dependent generalizations have only statistical significance, and cannot dependably be used to predict the geomagnetic effects of a given solar eruption.
Magnetohydrodynamic modeling of prominence formation within a helmet streamer
Authors: Linker J. A. , Lionello R. , Mikić Z. , Amari T.
Published: January 2004

We present a 2.5‐D axisymmetric MHD model to self‐consistently describe the formation of a stable prominence that supports cool, dense material in the lower corona. The upper chromosphere and transition region are included in the calculation. Reducing the magnetic flux along the neutral line of a sheared coronal arcade forms a magnetic field configuration with a flux rope topology. The prominence forms when dense chromospheric material is brought up and condenses in the corona. The prominence sits at the base of a helmet streamer structure. The dense material is supported against gravity in the dips of the magnetic field lines in the flux rope. Further reduction in magnetic flux leads to an eruption of the prominence, ejecting material into the solar wind.
Fragmented structure of interplanetary type II bursts
Authors: Thejappa G. , MacDowall R. J.
Published: January 2004

We present the Ulysses observations of a highly fragmented interplanetary type II burst and an associated pair of forward shocks. We argue that the coupling of Langmuir waves excited by one of these shocks with escaping radiation at the sharp density gradient associated with the companion shock is responsible for the dumpiness of the type II event. This linear coupling process is shown to cause large fluctuations in the efficiency of conversion of Langmuir waves into electromagnetic radiation at the fundamental of the electron plasma frequency ƒpe, leading to the fragmentary structure of the type II radio burst.
New insights on solar energetic particles from Wind and ACE
Authors: Tylka Allan J.
Published: January 2004

Solar energetic particle (SEP) events are occasional bursts of high‐energy particles from the Sun. Because of their episodic, highly variable, and (presently) unpredictable nature, these events are one of the primary constraints on the technological utilization of space and in the design and operation of space systems. New instruments on Wind and ACE, with geometry factors that are ∼100 times larger than those of previous solar cycles, have yielded unprecedented observations of temporal evolution in composition and spectra over a wide range of energies and species. These new results address some of the key physics that governs SEP production. Of particular importance in very large SEP events are transport effects due to proton‐generated AlfVén waves and the evolving nature of the coronal mass ejection driven shock.
SAMPEX observations of precipitation bursts in the outer radiation belt
Authors: Nakamura R. , Isowa M. , Kamide Y. , Baker D. N. , Blake J. B. , Looper M.
Published: February 2004

The occurrence frequency of precipitation bursts of > 1 MeV electrons in the outer radiation belt is examined using data from the SAMPEX satellite. Electron burst characteristics shown in this paper include the dependence of the precipitation on magnetic local time, radial distance and geomagnetic activity. Precipitation bursts with timescales <1 s, i.e., microbursts, are studied in detail, including their dependence on the phases of geomagnetic storms. It is found that precipitation bursts occur typically in the region between L = 4 and L = 6. Microbursts tend to occur at L lower than the bursts with timescales of several tens of seconds. The number of observed microbursts significantly increases during storms, appearing mainly in the morning sector early in the recovery phase of storms. These findings suggest that the microbursts may be due to interactions with electron whistler waves, which take place near the dawnside plasmapause in the density irregularities that are perhaps created in the “recovering” plasmasphere. The prevalence of bursty precipitation indicates that this enhanced loss component of the relativistic electron flux should be taken into account in any quantitative model of relativistic electron acceleration processes.
On the solar wind flow control of the polar cusp
Authors: Lundin R. , Aparicio B. , Yamauchi M.
Published: February 2004

The position of the polar cusp in the dayside magnetosphere is known to be controlled by the solar wind dynamics and the interplanetary magnetic field. Using plasma data from the Viking and IMP 8 satellites, we have done a statistical analysis of the influence of the solar wind azimuth flow component on the cusp position in magnetic local time (MLT). We show that the position of the cusp in MLT is strongly coupled to the azimuthal (east‐west) flow component of the solar wind. For instance, the cusp in the northern hemisphere moves toward prenoon when the unaberrated solar wind appears to come from the west of the Sun (as compared to the radial flow), and vice versa. This suggests, first of all, a strong dynamical coupling of the cusp to the solar wind flow. Second, the dawn‐dusk cusp displacement with respect to the azimuthal flow component indicates a transfer of angular momentum from the Sun, potentially by a J × B force in the solar wind as suggested in the “hetegonic nebulae” theory by Alfven and Arrhenius [1976]. This lateral deflection force plays, on the average, a more important role for the cusp location than the aberration effect of the Earth's prograde motion across the expanding solar wind. In previous studies similar displacements of the cusp have been mainly related to the IMF By component; for example, a positive By leads to a cusp displacement to the postnoon side and vice versa. We compare these results and discuss them in context with the dynamical solar wind flow control of the cusp.
Development of a compact EUV photometer for imaging the planetary magnetosphere
Authors: Yoshikawa I. , Yamazaki A. , Shiomi K. , Nakamura M. , Yamashita K. , Saito Y. , Hirahara M. , Takizawa Y. , Miyake W. , Matsuura S.
Published: January 2004

We have developed a lightweight, compact, and highly sensitive Extreme ultraviolet (EUV) photometer for a space‐borne instrument. The photometer is onboard Japan's Mars orbiter, Planet‐B, which was successfully launched on July 4, 1998, and stayed in the parking orbits around the Earth for 6 months. The new photometer is designed for studying the spatial distribution of helium ions and atoms by detecting He II (304 Å) and He I (584 Å), which are major emission lines detectable around a planet. The photometer is a type of normal‐incidence telescope that consists of a mirror, filters, and a detector. The mirror employs a new technology of a molybdenum/silicon multilayer coating and is designed to have peak reflectivity at 304 Å. The photometer is capable of taking two‐dimensional images by using the spin and orbital motions of the spacecraft. There are open questions concerning the Earth's plasmasphere and plasma sheet that cannot be resolved by in situ observations alone (e.g., global shape of the plasmasphere and cold ions in the plasma sheet). Our observations with the EUV photometer over a reasonable number of orbits will answer them. The photometer also shows an outstanding performance of measuring the Martian helium.
Polar cusp and vicinity under strongly northward interplanetary magnetic field on April 11, 1997: Observations and MHD simulations
Authors: Le G. , Raeder J. , Russell C. T. , Lu G. , Petrinec S. M. , Mozer F. S.
Published: January 2004

We present a correlative case study of the solar wind interaction with the magnetosphere using in situ observations of the polar cusp and surrounding regions, ground‐based and low‐altitude spacecraft polar cap observations, and global MHD simulations during an extended period of strongly northward interplanetary magnetic field (IMF) on April 11, 1997. Within this extended period of strongly northward IMF, the Polar spacecraft entered a region with magnetosheath‐like plasma and field lines above the polar cusp. Data from multiple instruments on Polar showed that the observed signatures can be interpreted as Polar entering the reconnection layer and crossing the current layer associated with it. The magnetosheath‐like field lines encountered by Polar have just reconnected poleward of the polar cusp in the north. The reversed ionospheric convection patterns derived from the assimilative mapping of ionospheric electrodynamics (AME) technique show the reversed convection over the polar cap due to high‐latitude reconnection throughout the whole interval. MHD simulations were performed using solar wind parameters observed by Wind, supporting the cusp reconnection interpretation.
Structure of the flank magnetopause for horizontal IMF: INTERBALL 1 observations
Authors: Fedorov A. , Dubinin E. , Song P. , Skalsky A. , Budnik E.
Published: January 2004

A case study is presented of a flank duskside magnetopause crossing by the INTERBALL 1 spacecraft during an interval of duskward horizontal interplanetary magnetic field. There is strong evidence that reconnection takes place in the flank magnetopause, where the draped sheath field has a large shear relative to the magnetospheric field. In our case the satellite was on the duskside above the equator. The assumed reconnection site is on the duskside of the northern flank slightly sunward at higher latitudes with respect to the satellite. The observations show energetic particles streaming along the field in the magnetosheath from the reconnection site, as expected. The deHoffman‐Teller frame analysis shows that propagation of the magnetopause perturbations originates from the derived reconnection site. Observations are, in general, consistent with reconnection models that are based on the one‐dimensional assumption. However, there is some significant deviation from the model prediction. We attribute the deviation to the presence of sheared plasma flow. The magnetopause in this case could not be described as a simple one‐dimensional rotational discontinuity.
Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Observations at low (≤10 keV) energies
Authors: Brandt P. C:son , Barabash S. , Roelof E. C. , Chase C. J.
Published: January 2004

Intense (≤106 cm−2 sr−1 s−1) fluxes of upflowing energetic neutral atoms (ENAs) from the polar cap have been observed in the energy range ∼0.1–10 keV (hydrogen assumed) by the ENA imager PIPPI (Prelude In Planetary Particle Imaging) onboard the Astrid satellite at 1000 km altitude. If a source altitude of 400 km is assumed, the ENA emissions maps to magnetic latitudes 70°–85°extending from dusk over nightside. There were no measurements in the midnight sector below approximately 80°. It is shown that the emissions cannot originate from an isotropically emitting source, which rules out any optical emissions. Local charged particle detection is ruled out on the basis of no correlation with local electron measurements and local magnetic field configuration. The ENA emissions can be explained by a conically pitch angle distributed ion source emitting ENAs in the 110°–140°pitch angle interval (source altitude 400 km). It is suggested, with support from low altitude rocket observations that the ENAs are produced by low altitude (400–600 km) ion conies of several hundreds eV charge exchanging with the upper atmosphere.
Geomagnetic sudden impulses at low latitude during northward interplanetary magnetic field conditions
Authors: Francia P. , Lepidi S. , Di Giuseppe P. , Villante U.
Published: January 2004

A statistical analysis of geomagnetic sudden impulses has been performed for data acquired at a low‐latitude station during northward interplanetary magnetic field conditions. The local time dependence of the ground response, characterized by very low values in the morning with respect to the afternoon and night sectors and by a clear maximum just after local noon, is very similar to that observed at auroral latitudes, suggesting that signatures of auroral ionospheric currents might be observed also at low latitude. The sense of polarization of the sudden impulses shows a clear reversal from counterclockwise to clockwise a few hours after local noon, consistent with the results obtained for low‐frequency geomagnetic fluctuations at the same station. In some cases a transient response (overshoot) is clearly observed; its amplitude is greater when the corrected Dst index is more negative, that is, when the ring current intensity is stronger; moreover, it tends to occur within 1–2 hours after the closest interplanetary magnetic field northward turning. These results seem to indicate that at our station overshoots characterize sudden impulses occurring a short time after periods with high magnetospheric activity. Since the local field line is embedded in the plasmasphere, the overshoot might tentatively be interpreted in terms of an overcompression of the plasmasphere which, after periods with high magnetospheric activity, is less dense and more elastic.
Global flows of energetic ions in Jupiter's equatorial plane: First‐order approximation
Authors: Krupp N. , Lagg A. , Livi S. , Wilken B. , Woch J. , Roelof E. C. , Williams D. J.
Published: January 2004

Galileo, as the first orbiting spacecraft in an outer planet's magnetosphere, provides the opportunity to study global energetic ion distributions in Jupiter's magnetosphere. We present directional anisotropies of energetic ion distributions measured by the Galileo Energetic Particles Detector (EPD). The EPD measurements of proton (80–1050 keV), oxygen (26–562 keV/nucleon), and sulfur (16–310 keV/nucleon) distributions cover a wide energy range. Spatially, the data set includes measurements from 6 to 142 Jovian radii (RJ) and covers all local times inside the Jovian magnetosphere. For each species a single detector head scans almost the entire sky (≈ 4π sr), producing the three‐dimensional angular distributions from which the anisotropies are derived. Consequently, the resulting anisotropy estimates are both global and robust. Such anisotropies, generally produced by convective flow, ion intensity gradients, and field‐aligned components, have long been used to estimate flow velocities and to locate spatial boundaries within magnetospheres. They can therefore provide vital information on magnetospheric circulation and dynamics. We find that the EPD measured anisotropies in the Jovian magnetosphere are dominated by a component in the corotational direction punctuated by episodic radial components, both inward and outward. Under the assumption that anisotropies are produced predominantly by convective flow, we derive flow velocities of protons, oxygen ions, and sulfur ions. The validity of that approach is supported by the fact that these three independently derived flow velocities agree, to a large extent, in this approximation. Thus, for the first time, we are able to derive the global flow pattern in a magnetosphere of an outer planet. In a comparison between the first‐order EPD flow velocities and those predicted by a magnetohydrodynamic (MHD) simulation of the Jovian magnetosphere, we find that qualitatively the directions appear similar, although no firm evidence of steady outflow of ions has been observed at distances covered by Galileo. A first rough comparison indicates that the measured first‐order flow velocities are higher by at least a factor of 1.5 than the MHD simulation results.
Anisotropic MHD model of the dayside magnetosheath downstream of the oblique bow shock
Authors: Samsonov A. A. , Pudovkin M. I. , Gary S. Peter , Hubert Daniel
Published: January 2004

Large‐scale flow in the dayside magnetosheath is calculated by using a three‐dimensional anisotropic MHD model for the case when the angle between the interplanetary magnetic field and the solar wind velocity is 45°. The behavior of plasma and magnetic field parameters downstream of the quasi‐perpendicular and quasi‐parallel bow shocks is compared in the results from a single calculation. The model includes a limit on the proton temperature anisotropy based upon thresholds for onset of the ion cyclotron and mirror instabilities. Results are presented for three different values of the isotropization rate. The model shows the existence of the plasma depletion layer, corresponding to an increase of the magnetic field intensity and a decrease of the plasma density near the magnetopause, for all angles of the bow shock normal relative to the interplanetary magnetic field. There is a thin layer downstream of the quasi‐parallel shock where T‖p > T⊥p. The magnetosheath regions are shown where the threshold conditions for ion cyclotron and mirror instabilities are satisfied.
Magnetic field and electric current density distribution in the geomagnetic tail, based on Geotail data
Authors: Israelevich P. L. , Ershkovich A. I. , Tsyganenko N. A.
Published: January 2004

Analysis of Geotail magnetic data enables us to reveal the antisunward electric current flowing along the tail axis. To our knowledge, observation of this current has not been reported yet. Distributions of the magnetic field and electric current density (along with their dependencies on the tilt angle of the Earth's dipole and components of the interplanetary magnetic field) have been derived directly from Geotail data, without any ad hoc assumptions. Analysis of the electric current density distribution shows that, in addition to currents associated with the geomagnetic tail flaring, there is a current (tentatively identified as the Hall current) flowing antisunward along the tail axis. The total strength of this current is of the order of 1 MA. It closes through the midnight sector of the auroral zone resulting in field‐aligned currents in the region of the Harang discontinuity.
Cusp ion steps, field‐aligned currents and poleward moving auroral forms
Authors: Lockwood M. , Milan S. E. , Onsager T. , Perry C. H. , Scudder J. A. , Russell C. T. , Brittnacher M.
Published: January 2004

We predict the field‐aligned currents around cusp ion steps produced by pulsed reconnection between the geomagnetic field and an interplanetary magnetic field (IMF) with a BY component that is large in magnitude. For BY > 0, patches of newly opened flux move westward and eastward in the Northern and Southern Hemispheres, respectively, under the influence of the magnetic curvature force. These flow directions are reversed for BY < 0. The speed of this longitudinal motion initially grows with elapsed time since reconnection, but then decays as the newly opened field lines straighten. We predict sheets of field‐aligned current on the boundaries between the patches produced by successive reconnection pulses, associated with the difference in the speeds of their longitudinal motion. For low elapsed times since reconnection, near the equatorward edge of the cusp region where the field lines are accelerating, the field‐aligned current sheets will be downward or upward in both hemispheres for positive or negative IMF BY, respectively. At larger elapsed times since reconnection, as events slow and evolve from the cusp into the mantle region, these field‐aligned current directions will be reversed. Observations by the Polar spacecraft on August 26, 1998, show the predicted upward current sheets at steps seen in the mantle region for IMF BY > 0. Mapped into the ionosphere, the steps coincide with poleward moving events seen by the CUTLASS HF radar. The mapped location of the largest step also coincides with a poleward moving arc seen by the UVI imager on Polar. We show that the arc is consistent with a region of upward field‐aligned current that has become unstable, such that a potential drop of about 1 kV formed below the spacecraft. The importance of these observations is that they confirm that the poleward moving events, as seen by the HF radar and the UV imager, are due to pulsed magnetopause reconnection. Milan et al. [2000] noted that the great longitudinal extent of these events means that the required reconnection pulses would have contributed almost all the voltage placed across the magnetosphere at this time. The observations also show that auroral arcs can form on open field lines in response to the pulsed application of voltage at the magnetopause.
A closer look at remanence‐dominated aeromagnetic anomalies: Rock magnetic properties and magnetic mineralogy of the Russell Belt microcline‐sillimanite gneiss, northwest Adirondack Mountains, New York
Authors: McEnroe Suzanne A. , Brown Laurie L.
Published: February 2004

A large, distinct negative aeromagnetic anomaly of over 2000 nT associated with microcline‐sillimanite‐quartz gneisses in the Russell area, northwest Adirondack Mountains, was previously shown to be remanence‐dominated, although the carriers of remanence were not well documented. Russell Belt gneisses have a strong natural remanent magnetization with steep remanence directions, D = 263°, I = −58°, an average intensity of 3.6 A/m, and typical susceptibilities of 10−4 SI. The remanence is thermochemical in origin, acquired during cooling from peak metamorphic conditions of 650°–750°C during the Ottawan Orogen (1050–1080 Ma). The reversed polarity of remanence reflects a reversed paleofield, rather than self‐reversed, contrary to earlier suggestions. The gneisses contain up to 3% oxide, predominantly metamorphic titanohematite, which accounts for the low susceptibility values and highly stable remanence. Optical observations show titanohematite grains with multiple generations of ilmenite, pyrophanite, rutile, and spinel exsolution lamellae. Microprobe analyses confirm titanohematite compositions ranging from 72 to 97% Fe2O3 with hematite83 being most typical. In rare samples, inclusions of magnetite were identified. The ubiquitous presence of titanohematite, and the rare occurrence of magnetite, is supported by thermal and alternating field demagnetization studies, saturation magnetization measurements, hysteresis properties, temperature‐hysteresis studies, and low‐temperature remanence measurements. Numerous crustal granulites have titanohematite as part of the oxide assemblage, and this may contribute a strong rémanent component to what have previously been considered to be solely induced anomalies.
Gravity synclines
Authors: Carena Sara , Borgia Andrea , Pasquarè Giorgio , Battaglia Aldo , Ferraris Mauro , Martelli Luca , De Nardo M. Teresa
Published: February 2004

The Scurano‐Vetto‐Carpineti syncline in the northern Apennines (Italy) is a small, isolated, bowl‐shaped syncline made of arenaceous rocks resting on a clayey substratum. This syncline was thought to have formed during lower Pliocene. Our work shows instead that it is much younger and that gravitational tectonics is important in its formation. The syncline forms a mesa‐like relief, with the top surface shaped as a “bowl”. The outer slopes of this “mesa” are very steep and are cut by strata dipping inward. All around the mesa is a zone of maximum tectonic deformation in the clayey substratum. We rule out regional tectonics in the generation of the syncline because (1) the structures follow the mesa boundary instead of the regional trends; (2) there are tectonic contacts that prove the existence of substratum clay diapirs around the mesa; and (3) there are marine abrasion planes predating the deformation. Our deductions are supported by modeling: fluid−mechanical analysis shows that when there is a brittle plate overlying a ductile substratum, gravitational deformation is inevitable, leading to a synformal structure. In addition, a silicone‐and‐sand experiment shows that the deformation generates a complex syncline with a horst‐and‐graben structure in the sand plate and with diapirs in the silicon substratum. Thus our syncline forms in a local extensional tectonic regime dominated by gravitational processes, in contrast to “common” synclines, which are the result of regional compressive tectonics. From the order‐of‐magnitude strain‐rate of the syncline we estimate the viscosity of the ductile substratum to be 1018 Pa s.
Lava and ice interaction at stratovolcanoes: Use of characteristic features to determine past glacial extents and future volcanic hazards
Authors: Lescinsky David T. , Fink Jonathan H.
Published: February 2004

Structures resulting from lava and ice interaction are common at glaciated stratovolcanoes. During summit eruptions at stratovolcanoes, meltwater is produced and travels freely down steep slopes and thin permeable valley glaciers, eroding the ice and enlarging preexisting glacial drainages. As a result, eruptions in this environment have produced few catastrophic floods. Lava flowing into the open channels and voids in the glaciers becomes confined and grows thicker, filling the available space and producing steep‐sided bodies with smooth, bulbous contact surfaces. Quenching of lava against ice or by water forms small‐scale features such as tensional fractures and glass. As the amount of meltwater in contact with the lava increases, the type and abundance of smaller‐scale features become similar to those produced during subglacial eruptions into meltwater lakes. Identification of large‐ and small‐scale lava‐ice contact features in the field can be used to reconstruct paleoglacial extent and, combined with geochronology of lavas, to determine past paleoclimate. An understanding of lava‐ice interaction allows us to better assess the hazards posed by future eruptions at glaciated volcanoes.