bluuebunny/crossref_metadata_2025 huggingface dataset:
Introduction to Special Section: Global Picture of Solar Eruptive Events
Authors: Gopalswamy Nat
Published: January 2004

This introduction highlights some of the scientific results reported in this special section on solar eruptive events and provides a brief description of issues related to the new results. Most of these papers grew out of the coordinated data analysis workshop held at the Goddard Space Flight Center during April 27–30, 1999, and the subsequent International Conference on Solar Eruptive Events held at the Catholic University of America, Washington, D. C. during March 6–9, 2000.
A stable auroral red arc observed over Japan after an interval of very weak solar wind
Authors: Shiokawa K. , Ogawa T. , Oya H. , Rich F. J. , Yumoto K.
Published: January 2004

May 10–11, 1999, is known as an interval of very weak solar wind, with an interplanetary plasma density of <1 cm−3. On May 12–13 the density increased to more than 10 cm−3 and a weak magnetic storm occurred. During the early recovery phase of the storm, we observed a stable auroral red (SAR) arc at Rikubetsu, Japan (43.5°N, 143.8°E, magnetic latitude = 34.7°N), at 1500–1740 UT (0000–0240 LT) on May 13. It is unusual to observe a SAR arc at such a low‐latitude station during a weak storm, which had a minimum Dst index of only −49 nT (1500 UT on May 13). The maximum SAR arc intensity at a wavelength of 630 nm reached ∼700 R in the northern sky of Rikubetsu. A weak emission (∼7 R) at 427.8 nm (N2+(1NG)) was also observed. Ground magnetic field data indicate a clear substorm starting at 1522 UT. The plasma density data obtained by the Akebono satellite (2000–8000 km altitude) show a steep plasmapause at ∼40° magnetic latitude during the recovery phase of the storm. The electron temperature data obtained by the Defense Meteorological Satellite Program F14 satellite (800 km altitude) show enhancement of the temperature at subauroral latitudes during the storm. Using these data, we discuss the generation of the SAR arc during the weak storm.
Investigating the possible association between thunderclouds and plasmaspheric ducts
Authors: Clilverd Mark A. , Rodger Craig J. , Thomson Neil R. , Yearby Keith H.
Published: January 2004

It has been suggested that quasi‐electrostatic electric fields from thunderclouds might produce whistler ducts by driving plasma interchange of geomagnetic flux tubes. Using lightning activity as a proxy for the presence of electrified clouds, the relation between thunderclouds and duct number was investigated through the study of two periods of 10 days, that is from June 13–22, 1993, and August 6–15, 1993. Typical duct numbers observed using NAA transmissions (24.0 kHz, Cutler, Maine) received at Faraday during the nights of June and August 1993 were ∼ 7, while the maximum duct numbers were 9–12. Duct numbers dropped down to zero or near zero during the day. Times of increased duct number could not be clearly associated with increased thunderstorm activity in the duct footprint region. Duct numbers observed on the path from NLK (24.8 kHz, Seattle, Washington) to Dunedin, New Zealand, were also studied, where very low thunderstorm activity levels imply that thundercloud‐driven duct formation should not be significant. The typical nighttime peak was ∼ 5 at about 1000 UT (23 LT). The difference between duct numbers in areas with high thunderstorm activity and quiet areas might be thought to be due to thundercloud duct formation. However, the F region absorption on the NLK footprints at 1000UT (23 LT) in June was found to be equivalent to 0000 UT (20 LT) in June on the NAA duct footprints. The typical number of ducts observable on the NAA path at 0000 UT in June is ≈ 5, very similar to the number observed on NLK at 0930 UT. The results found in this study are consistent with the idea that ionospheric absorption is a significant influence on the number of ducts observed. It does not appear that quasi‐electronstatic fields from thunderclouds play a significant role in the formation of plasmaspheric whistler ducts. The similarity in quiet time midlatitude duct nuumbers at these two different locations suggest a globally consistent duct formation mechanism, such as the interchange of magnetospheric tubes of plasma induced by spatially varying electric fields in the E region.
Alfvén wave propagation and ion cyclotron interactions in the expanding solar wind: One‐dimensional hybrid simulations
Authors: Liewer Paulett C. , Velli Marco , Goldstein Bruce E.
Published: January 2004

We carry out one‐dimensional hybrid simulations of Alfvén waves propagating along the magnetic field in the presence of a mean radial spherically expanding plasma outflow, representing fast solar wind streams. The equations for particle ions of multiple species and fluid electrons are solved using the Expanding Box Model, a locally Cartesian representation of motion in spherical coordinates, in a frame moving with the local average wind speed. The model gives a minimally consistent description of the effects associated with such motion on particle dynamics, e.g., the flux‐conserving decrease of magnetic field intensity and consequent decrease of cyclotron frequency with increasing distance from the Sun. The cyclotron frequency decreases faster than Alfvén wave frequency, allowing fluctuations below the cyclotron frequency at smaller distance from the Sun to come into cyclotron resonance at greater distances. The hybrid treatment yields a fully self‐consistent description of the consequent cyclotron wave‐particle interaction in a multi‐ion plasma. We present results for cases of monochromatic circularly polarized Alfvén waves propagating radially outward and for initially well developed Alfvénic spectra with and without alpha particles. When both alpha particles and protons are present, the alpha particles, which come into resonance first as the wind expands, are observed to be preferentially heated and accelerated. For high beta (equal to ratio of ion pressure to magnetic field pressure) the amount of alpha particles acceleration and heating is limited by the available wave power. For low beta cases the amount of heating and acceleration is limited, not by the wave power, but by the depletion of the distribution function in the resonance region by pitch‐angle scattering. The implication of these results for solar wind models is discussed.
Fast Auroral Snapshot observations of cusp electron and ion structures
Authors: Su Yi‐Jiun , Ergun R. E. , Peterson W. K. , Onsager T. G. , Pfaff R. , Carlson C. W. , Strangeway R. J.
Published: January 2004

The high time resolution observations by the Fast Auroral Snapshot satellite resolve burst‐like electrons and nonmonotonic ion dispersion in the cusp region. Electric field data from a dawn‐dusk cusp crossing show that the DC electric field has small‐scale variations superimposed on a large‐scale pattern. The large‐scale DC electric field variations are well correlated with the large‐scale perturbation magnetic field, indicating a field‐aligned current system that reflects a relatively homogenous conductivity. Ion structures best correlate with the perturbed magnetic field and the large‐scale electric field. The burst‐like precipitating electrons are correlated with the small‐scale electric field variations, which are most easily explained as propagating Alfvén waves. We also found that the observed ion structures result from the spacecraft crossing the flow streamlines, each of which has its own individual history of dayside reconnection. The magnetosheath ion structures therefore contain information of both the spatial and temporal variations of dayside reconnection.
Corection of the Jicamarca electron‐ion temperature ratio problem: Verifying the effect of electron Coulomb collisions on the incoherent scatter spectrum
Authors: Aponte Néstor , Sulzer Michael P. , González Sixto A.
Published: January 2004

Ever since the first attempts to fit Jicamarca autocorrelation function (ACF) measurements in the 1970s using a full nonlinear least squares analysis, an apparent electron‐ion temperature ratio below unity has been deduced for a large portion of the F region data. The cause of this unexpected and geophysically unreasonable result has been a mystery until recently, when Sulzer and González [1999] (herein SG) explained how electron Coulomb collisions can distort, or narrow, the incoherent backscatter spectrum, and that for this narrowing to be observable two conditions must be met. First, the radar k vector must lie in a small range near perpendicular to the magnetic field, and second, the radar wavelength must be sufficiently long. Both of these conditions are true at Jicamarca. The accurate calculations from the SG theory are now available in a compact library, which we have incorporated into an incoherent scatter least squares fitting code. Using this code, we have reduced Jicamarca ACF data taken with the Faraday double‐pulse mode, and find that the SG theory correctly interprets the ACF data from Jicamarca, thereby solving the longstanding Te/Ti ratio problem and thus allowing accurate electron and ion temperature measurements.
A theory of field line resonance in a dipole‐like axisymmetric magnetosphere
Authors: Leonovich A. S.
Published: January 2004

A self‐consistent field line resonance problem in the model of an axisymmetric dipole‐like magnetosphere is solved. The spatial structure not only of resonance Alfven waves but also of the magnetosonic wave exciting them was obtained in this paper. The field structure of the magnetosonic wave has been determined both inside the magnetospheric cavity and in the solar wind region near the magnetosphere. This made it possible to compare the amplitudes of magnetosonic waves incident from the solar wind on the magnetosphere and the resonance Alfven oscillations which they excite in the magnetosphere. It is shown that as magnetosonic oscillations penetrate deep into the magnetosphere, their amplitude decreases several orders of magnitude. The amplitude of Alfven oscillations excited inside the magnetosphere is comparable with that of magnetosonic waves in the solar wind region. The amplitude of resonance Alfven oscillations on internal magnetic shells is significantly larger than that of resonance oscillations excited in the transition layer of the magnetosphere.
Observations of highly correlated near‐simultaneous magnetic field perturbations at contraposed ground stations
Authors: Jackel Brian J. , Eglitis Paul , Donovan Eric F. , Viljanen Ari T. , Wallis Don D. , Cogger Leroy L. , Opgenoorth Hermann J.
Published: January 2004

A merged data set from Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) and Magnetometers‐Ionospheric Radars‐All‐Sky Cameras Large Experiment (MIRACLE) magnetometer stations is used to study variations observed at widely separated locations. Three years of 10‐s data are analyzed using canonical correlation techniques to determine the temporal offset of similar fluctuations observed at pairs of sites. Nearly simultaneous (<2‐min delay) signatures are often found between sites separated by 12 hours magnetic local time, with no evidence of more slowly propagating events (> 10‐min delay). Correlated episodes occur more often (roughly 10% of the time) when sites are in the dawn/dusk sectors and less often (∼5%) for noon/midnight configurations.
Resonance frequency of stretched magnetic field lines based on a self‐consistent equilibrium magnetosphere model
Authors: Lui A. T. Y. , Cheng C. Z.
Published: January 2004

Field line resonance (FLR) has the potential to account for magnetic and electric oscillations observed near the equatorial region of the magnetosphere and on the ground associated with auroral arcs. Previous theoretical works based on dipolar field lines give frequencies too high to match with the observed frequencies when typical mass density is used. Inclusion of magnetic field stretching from empirical magnetic field models brings good agreement, but these models have no assurance of specifying an equilibrium magnetosphere. In this paper we present, for the first time, quantitative results of the resonance frequencies for the shear Alfvén and slow mode branches of FLR threading the auroral zone for field lines stretched to various distances in the nightside based on a self‐consistent equilibrium magnetosphere. The calculation is done in two steps. For a given degree of field line stretching we first obtain an MHD equilibrium magnetosphere with a finite temperature through a self‐consistent solution of the Grad‐Shafranov equation. We then solve for FLR frequencies of stretched magnetic field lines in the equilibrium magnetosphere according to the MHD theory of FLR. The results show that field line stretching can lead to low frequencies of FLR as observed without invoking unrealistically high mass density. Dependent on the wave mode and the amount of field line stretching, the FLR frequencies found in this calculation differ from those for a simple magnetic dipole of the Earth by varying degrees, from agreement within a factor of 2 to a disparity of greater than a factor of 10.
Substorm convection patterns observed by the Super Dual Auroral Radar Network
Authors: Bristow W. A. , Otto A. , Lummerzheim D.
Published: January 2004

A study of convection patterns during substorms observed by the Super Dual Auroral Radar Network (SuperDARN) radar network is presented. Specific attention is given to the growth phase and the time immediately following expansion onset. It is found that the growth phase is characterized by the enhancement of the velocity shear near midnight and its extension to low latitudes and to local times across the midnight meridian. The velocity shear was observed to diminish at expansion onset. In addition, the ionospheric flow velocity magnitude was observed to be enhanced for some period of time prior to expansion onset and to decrease at expansion onset. Possible explanations for the establishment of the midnight velocity shear during growth phase and its subsequent disappearance at expansion onset are presented. Also, arguments are presented regarding the decrease of ionospheric flow velocity at expansion onset.
Quasiperiodic modulation of the Pc1 geomagnetic pulsations: An unsettled problem
Authors: Guglielmi A. , Kangas J. , Potapov A.
Published: January 2004

Recently, it has been suggested that the standard model for interpretation of the Pc1 (pearl) pulsations, which is based on the concept of wave packet oscillating along the geomagnetic field lines by reflection from the ionosphere in the opposite hemispheres, is to be revised. The necessity to modify the standard model is clearly demonstrated by observations. However, it seems premature to reject this model completely, since the excitation of Pc1 pulsations is impossible without reflection of the waves from the ionosphere or from some turning points in the magnetosphere. We are inclined to believe that such turning points exist in the magnetosphere. Our hypothesis is that the Pc1 quasiperiodic wave packets observed over the Earth's surface are the result of radiation from the magnetosphere‐borne open resonator in which the oscillating ion cyclotron wave packets are generated owing to instability of the multicomponent magnetospheric plasma. We emphasize that much remains to be learned about the modulation of the Pc1 waves, and it should be realized that a number of unresolved questions refers equally to alternative approaches to this problem.
Foreword [to Special Section on Plasma Interaction With Io's Atmosphere]
Authors: Bagenal Fran
Published: January 2004

The Jovian moon Io resides in one of the harshest space environments yet visited by spacecraft. Indeed, the energetic particles that make Jupiter's inner magnetosphere interesting to the space physicist give space engineers nightmares. After the initial orbit‐insertion passage in December 1995, the Galileo spacecraft was deliberately kept well outside the orbit of Io and the plasma torus. After 20 plus successful orbits exploring the outer 3 Galilean moons, the spacecraft's perijove was reduced to allow 3 close flybys of Io in late 1999 to early 2000.
Direct experimental test of Joule heating damping of a Pc5 pulsation using the Scandinavian Twin Auroral Radar Experiment (STARE) radar system
Authors: Nielsen E.
Published: January 2004

An interplanetary shock interacted with the Earth on February 18, 1999, and excited an intense transient Pc5 pulsation. Analysis of the spatial variations and of the time variations of the electron drifts induced by the pulsation in the high‐latitude ionosphere allows a deduction of the latitudinal variation of the height‐integrated Pedersen conductivity. It is concluded that ionospheric Joule heating losses are sufficient to account for the attenuation of the pulsation. It is also shown that for a pulsation event of this type use of magnetometer observations lead to an overestimate of the attenuation, or, equivalently, to an underestimate of the ionospheric Pedersen conductivity.
Interpretation of the He II (304 Å) EUV image of the inner magnetosphere by using empirical models
Authors: Yoshikawa I. , Yamazaki A. , Shiomi K. , Yamashita K. , Takizawa Y. , Nakamura M.
Published: January 2004

The visualization of the inner magnetosphere is a long‐cherished goal in magnetospheric physics. This goal was recently achieved with the He II (304 Å) image of the inner magnetosphere by using the EUV scanner on board the Planet‐B spacecraft (Nakamura et al., 2000). However, our understanding of the inner magnetosphere in relation to these images has not been assessed. The two‐dimensional (2‐D) EUV image is difficult to interpret, as it is necessary to infer the three‐dimensional (3‐D) spatial information of the inner magnetosphere from the 2‐D remote sensing data. We examine our present knowledge of the inner magnetosphere and compare simulated 2‐D images with the real EUV image. We conclude that an empirical model that assumes conventional E × B flows and combines the diffusive equilibrium model of Chiu et al. (1979) and the Weimer (1995) electric potential and Tsyganenko (1989) magnetic field models is in good agreement with the EUV image only within the Earth‐dominated flow regime. Furthermore, the model gives a better fit to the EUV image data, by assuming the convection‐dominated flow regime to be filled with ionospheric He+ at a filling rate of 1.2·(6.6/L)−3.0 (He+ ions cm−3 d−1), instead of the well‐known L−4 density dependence on flux tube volume.
The elementary current method for calculating ionospheric current systems from multisatellite and ground magnetometer data
Authors: Amm O.
Published: January 2004

The recently launched Cluster II mission provides for the first time the possibility to instantaneously obtain spatially distributed measurements of field‐aligned currents from a fleet of satellites. We present the “elementary current method” that combines such measurements mapped to the ionosphere with two‐dimensional ground magnetic data, to calculate actual (not equivalent) ionospheric currents, without the need of further assumptions. If additional two‐dimensional measurements of the ionospheric electric field from coherent scatter radars are available, the ionospheric Hall and Pedersen conductances can also be inferred. The applicability of the method is demonstrated for a passage of the Cluster II spacecraft over the Multi‐Instrument Array for Ionosphere‐Magnetosphere Coupling Studies (MIRACLE) network of ground‐based instruments in northern Fennoscandia on February 1, 2001, using preliminary but realistic orbit parameters. The geophysical situation of ionospheric electrodynamic parameters that we model during this passage refers to a real event of a plasma vortex propagating eastward over the MIRACLE field of view, as studied by Kosch et al. [2000]. The application of the elementary current method to the simulated ground magnetometer and satellite measurements calculated from this model shows that the modeled ionospheric currents, as well as the Hall and Pedersen conductances, can be reconstructed by the method to good accuracy.
Satellite limb tomography applied to airglow of the 630 nm emission
Authors: Frey S. , Mende S. B. , Frey H. U.
Published: January 2004

Airglow intensity variations in the 90 km altitude range are generally interpreted as the optical signature of gravity waves propagating through the airglow layer. In contrast, modulations of the 630 nm emission in the F region are subject to atmospheric wave propagation and ionospheric electrodynamics, which makes it difficult to determine the dominating process. Spacecraft‐based imaging of the 630 nm airglow could produce global data of airglow modulations greatly needed for the understanding of thermospheric dynamics. Observations of the thick 630 nm airglow layer between 200 and 300 km altitude are used to demonstrate the ability to tomographically retrieve the modulated volume emission rates from limb observations from a spacecraft. An algebraic reconstruction technique is used to obtain vertical and horizontal distributions with average errors of 20%. Its effectiveness is demonstrated by simulations with numerical data. Forward limb observations in the 630 nm range strongly reduce the angular range and limit the measurement scale to 300 km. These results are obtained without any assumptions about the original distribution other than nonnegativity. The overall computational effort would allow the analysis of a large number of observations as provided by global survey experiments.
Aurora conjugacy during substorms: Coordinated Antarctic ground and Polar Ultraviolet observations
Authors: Vorobjev V. G. , Yagodkina O. I. , Sibeck D. , Liou K. , Meng C.‐I.
Published: January 2004

Simultaneous optical observations at the Antarctic station Mirny (Φ′= −77.4° Corrected geomagnetic latitude (CGL)) and Ultraviolet Imager images of the northern auroral oval from the POLAR satellite were used to determine the displacement of nightside aurorae in the Northern Hemisphere relative to those in the Southern Hemisphere during 10 (1.5 to 3 hour long) time intervals including auroral substorms. Displacements of the poleward edge of the auroral bulge can be significant, up to 50 in CGL. The sense and magnitude of displacements are not related to the dipole tilt angle or differing ionospheric conductivity in the two hemispheres but rather to interplanetary magnetic field (IMF) orientations in the ecliptic plane. Aurorae reach higher latitudes in the Northern Hemisphere when Bx<0 and By>0 but occur at higher latitudes in the Southern Hemisphere when Bx>0 and By<0. Perturbations in conjugate ground magnetometers display a similar asymmetry. By contrast, displacements of the aurora are small when the IMF strength is weak, the IMF orientation fluctuates, or the IMF has an orthospiral orientation. Even when the latitudes are similar, local bright auroral forms and transient intensifications often occur in only one hemisphere. Our results testify to the effective penetration of the equatorial component of the IMF into the magnetosphere.
The cause‐and‐effect relationship of solar cycle length and the Northern Hemisphere air surface temperature
Authors: Reichel Richard , Thejll Peter , Lassen Knud
Published: February 2004

It has previously been demonstrated that the mean land air temperature of the Northern Hemisphere could adequately be associated with a long‐term variation of solar activity as given by the length of the approximately 11‐year solar cycle. In this paper it is shown that the right cause‐and‐effect ordering, in the sense of Granger causality, is present between the smoothed solar cycle length and the cycle mean of Northern Hemisphere land air temperature for the twentieth century, at the 99% significance level. This indicates the existence of a physical mechanism linking solar activity to climate variations.
The Io mass‐loading disk: Constraints provided by ion cyclotron wave observations
Authors: Russell C. T. , Wang Y. L. , Blanco‐Cano X. , Strangeway R. J.
Published: January 2004

Galileo measurements on the first two encounters with Io have revealed that ion cyclotron waves associated with SO2+ and SO+ can extend to at least 20 Io radii (RIo) on the anti‐Jupiter side of Io but to only ∼7 RIo on the Jupiter facing side. In the flow direction the waves extend downstream at least 10 RIo, but not upstream of Io. Earlier Voyager observations, 11 RIo below Io, also show no evidence for the generation of ion cyclotron waves. Quite unlike expectations from growth rate calculations in a uniform infinite plasma, the waves are frequently found to be propagating at an oblique angle to the magnetic field. The absence of waves upstream of Io and the wide dispersal of the ions creating the waves inward and outward from Io are consistent with a mass‐loading disk created by a short period of acceleration of the ions by the Jovian corotation electric field followed by reneutralization, producing a spray of SO, SO2, S, and possibly H2S across the magnetic field similar to the spray proposed for the neutral sodium cloud by Wilson and Schneider [1999]. When these fast neutrals are reionized farther from Io, they lead to wave growth.
Determination of accurate solar wind electron parameters using particle detectors and radio wave receivers
Authors: Salem C. , Bosqued J.‐M. , Larson D. E. , Mangeney A. , Maksimovic M. , Perche C. , Lin R. P. , Bougeret J.‐L.
Published: January 2004

We present a new, simple, and semiempirical method for determining accurate solar wind electron macroscopic parameters from the raw electron moments obtained from measured electron distribution functions. In the solar wind these measurements are affected by (1) photoelectrons produced by the spacecraft illumination, (2) spacecraft charging, and (3) the incomplete sampling of the electron distribution due to a nonzero low‐energy threshold of the energy sweeping in the electron spectrometer. Correcting fully for these effects is difficult, especially without the help of data from other experiments that can be taken as a reference. We take here advantage of the fact that high‐resolution solar wind electron parameters are obtained on board Wind using two different instruments: the electron electrostatic analyzer of the three‐dimensional Plasma experiment (3DP), which provides 3‐D electron velocity distribution functions every 99 s as well as 3‐s resolution computed onboard moments, and the thermal noise receiver (TNR), which yields unbiased electron density and temperature every 4.5 s from the spectroscopy of the quasi‐thermal noise around the electron plasma frequency. The present correction method is based on a simplified model evaluating the electron density and temperature as measured by the electron spectrometer, by taking into account both the spacecraft charging and the low‐energy cutoff effects: approximating the solar wind electron distributions by an isotropic Maxwellian, we derive simple analytical relations for the measured electron moments as functions of the real ones. These relations reproduce the qualitative behavior of the variation of the raw 3DP electron density and temperature as a function of the TNR ones. In order to set up a precise “scalar correction” of the raw 3DP electron moments, we use the TNR densities and temperatures as good estimates of the real ones; the coefficients appearing in the analytical relations are obtained by a best fit to the data from both instruments during a limited period of time, chosen as a reference. This set of coefficients is then used as long as the mode of operation of the electron spectrometer is unchanged. We show that this simple scalar correction of the electron density and temperature is reliable and can be applied routinely to the high‐resolution 3DP low‐order moments. As a by‐product, an estimate of the spacecraft potential is obtained. The odd‐order moments of the distribution function (electron bulk speed and heat flux) cannot be corrected by the model since the distribution is assumed to be an isotropic Maxwellian. We show, however, that a better estimate of the electron heat flux can be obtained by replacing the electron velocity by the proton velocity.
Geotail observations of the Hall current system: Evidence of magnetic reconnection in the magnetotail
Authors: Nagai T. , Shinohara I. , Fujimoto M. , Hoshino M. , Saito Y. , Machida S. , Mukai T.
Published: January 2004

In a two‐fluid picture of magnetic reconnection, inflow electrons flow with the magnetic field line to the diffusion region, whereas inflow ions cannot reach the diffusion region and rest around a distance of the ion inertial length. The relative motion of electrons and ions results in electric currents, that is, the Hall currents. The Hall current system produces a quadrupole structure in the cross‐tail component of the magnetic field near the magnetic reconnection region. Furthermore, this relative motion forms the electric field, whose direction is toward the equatorial plane (midplane). We have investigated the plasma and magnetic field structure near the magnetic reconnection region in the magnetotail with the Geotail spacecraft. We commonly observed inflowing low‐energy (less than 5 keV) electrons in the outermost layer of the plasma sheet in magnetic reconnection events, where accelerated ions and electrons flow away from the magnetic reconnection region. These electrons can carry currents to form part of the Hall current system. The observed east‐west variations in the magnetic field are consistent with the quadrupole structure produced by the Hall current system. We also noted that inflowing ions have consistently a dawnward motion, almost perpendicular to the magnetic field. These ions indicate the presence of the electric field toward the equatorial plane. The present observations demonstrate the ion‐electron decoupling processes for magnetic reconnection in the magnetotail.
Transients at the dusk side magnetospheric boundary: Surface waves or isolated plasma blobs?
Authors: De Keyser J. , Darrouzet R. , Roth M. , Vaisberg O. L. , Rybjeva N. , Smirnov V. , Avanov L. , Nemecek Z. , Safrankova J.
Published: January 2004

We revisit Interball‐Tail and Magion‐4 observations of the dusk side magnetospheric boundary on February 15–16, 1996. The observed transient behavior of the boundary can be interpreted in terms of surface waves or as the manifestation of isolated magnetosheath plasma entities embedded in the magnetosphere. We examine the arguments for each of these interpretations with high time resolution magnetic field and plasma data and by exploiting the dual‐satellite nature of the observations. We find strong evidence for magnetic field and flow vortices near the magnetospheric boundary and hence for the existence of flux tubes with helicoidal field lines; such structures can be associated with both interpretations. The cross‐correlation between the dual satellite observations and the apparent periodicity strongly suggest a Kelvin‐Helmholtz surface wave, although other interpretations are not impossible. In any case, the observations for this particular event allow us to derive constraints on surface wave generation mechanisms and on scenarios that could account for the presence of isolated plasma elements in the magnetosphere.
Auroral electrojet predictions with dynamic neural networks
Authors: Gleisner Hans , Lundstedt Henrik
Published: January 2004

Neural networks with internal feedback from the hidden nodes to the input [Elman, 1990] are developed for prediction of the auroral electrojet index AE from solar wind data. Unlike linear and nonlinear autoregressive moving‐aver age (ARMA) models, such networks are free to develop their own internal representation of the recurrent state variables. Further, they do not incorporate an explicit memory for past states; the memory is implicitly given by the feedback structure of the networks. It is shown that an Elman recurrent network can predict around 70% of the observed AE variance using a single sample of solar wind density, velocity, and magnetic field as input. A neural network with identical solar wind input, but without a feedback mechanism, only predicts around 45% of the AE variance. It is also shown that four recurrent state variables are optimal: the use of more than four hidden nodes does not improve the predictions, but with less than that the prediction accuracy drops. This provides an indication that the global‐scale auroral electrojet dynamics can be characterized by a small number of degrees of freedom.
Diurnal, seasonal, and geomagnetic variations of large field‐aligned ion upflows in the high‐latitude ionospheric F region
Authors: Liu Huixin , Ma S.‐Y. , Schlegel Kristian
Published: January 2004

A statistical analysis is carried out by using both the European Incoherent Scatter mainland radar (called EISCAT in the following) and the EISCAT Svalbard radar (ESR) data to investigate the morphology of large field‐aligned ion upflows in the high‐latitude F region ionosphere, emphasizing its dependence on geomagnetic activity and latitude. By binning data with respect to geomagnetic conditions, significant differences are found between quiet and disturbed periods in the morphology of field‐aligned upflows obtained by EISCAT around solar maximum: (1) the occurrence frequency on the nightside is more than 3 times higher during disturbed times than during quiet times; (2) the starting altitude of upflows lowers from 350–400 km under quiet conditions to 200–250 km under disturbed conditions; (3) the magnetic local time (MLT) distribution of the occurrence frequency of upflows exhibits a dawn‐dusk asymmetry above 400 km altitude, and it favors the dusk sector under quiet conditions but favors the dawn sector under disturbed conditions; (4) the disturbed‐time upflow occurrence frequency exhibits a semiannual variation above 400 km, with higher values around equinoxes. ESR observations during the rising phase of the solar cycle reveal some preliminary features of upflows in comparison to EISCAT observations: (1) the occurrence frequency of upflows on the dayside becomes significantly noticeable at about 200 km altitude, which is lower than that at EISCAT; (2) the upflow occurrence frequency above 400 km exhibits an obvious MLT distribution, with higher values during 0400‐1500 MLT, in contrast to 1800–0200 MLT at EISCAT; (3) the upflow occurrence frequency above ESR is higher than that above EISCAT during 0700–1500 MLT but is lower during 2000–0200 MLT. Possible clues on the driving mechanisms of upflows are drawn from these observations.
Field line diffusion in solar wind magnetic turbulence and energetic particle propagation across heliographic latitudes
Authors: Pommois P. , Veltri P. , Zimbardo G.
Published: January 2004

The transport of energetic particles in the heliosphere is strongly influenced by the magnetohydrodynamic turbulence found in the solar wind. This turbulence causes a magnetic field line random walk, which can explain the Ulysses observations at high heliographic latitudes of particles accelerated at corotating interaction regions (CIRs). A three‐dimensional model of magnetic turbulence allows us to evaluate a nonquasi‐linear magnetic field line diffusion coefficient, even in the case of anisotropic turbulence, as observed in the solar wind. The excursion in latitude of a magnetic field line starting from a CIR located at a large heliospheric distance r0 ∼ 8–10 AU, and at the limiting latitude for the direct observation of CIRs, ϑ ∼ 35°, is evaluated with a Monte Carlo simulation. In the calculations the random “force” terms are proportional to the square root of the diffusion coefficient in each direction. Considering that the correlation length in the latitudinal direction is larger than the one in the direction perpendicular to the mean magnetic field, we find that the magnetic field lines travel faster in the latitudinal direction. The implications for energetic particle transport are discussed and compared to the observation of energetic particle events at high solar latitudes by the Ulysses spacecraft. Both ion observations and electron observations can be explained by field line random walk.
Suprathermal electron acceleration in magnetic reconnection
Authors: Hoshino M. , Mukai T. , Terasawa T. , Shinohara I.
Published: January 2004

The suprathermal electrons of ≥20 keV that extend from the hot thermal electron with 2–3 keV temperature are sometimes observed in Earth's magnetosphere in association with reconnection. We study the origin of the hot and suprathermal electrons in terms of the kinetic magnetic reconnection process by using the two‐dimensional particle‐in‐cell simulation. We find that the hot and suprathermal electrons can be formed in the nonlinear evolution of a large‐scale magnetic reconnection. The electrons are, at the first stage, accelerated in the elongated, thin, X‐type current sheet. Next the preheated/accelerated electrons are transported to the stronger magnetic field region produced by piling up of magnetic field lines due to colliding of the fast reconnection outflow with the preexisting plasma. In this region they are further accelerated owing to the ∇B drift and the curvature drift. The mirror force of the reconnecting magnetic fields, the effective pitch angle scattering that occurs when the Larmor radius is comparable to the magnetic field line curvature radius, and the broadband waves excited by the Hall electric current are the other important agents to control the particle acceleration.
Fluid and kinetics signatures of reconnection at the dawn tail magnetopause: Wind observations
Authors: Phan Tai D. , Sonnerup Bengt U. Ö. , Lin Robert P.
Published: January 2004

We report an accelerated plasma flow event detected by Wind at the low‐latitude dawn tail magnetopause (xGSE≈−10 RE) when the local magnetic shear across the magnetopause was high (∼180°) and the GSM y component of the interplanetary magnetic field was positive and much larger than the z component. High time resolution (3 s) three‐dimensional ion and electron distributions were obtained for this event. We have performed rigorous tests of fluid and particle predictions of reconnection. Consistent with reconnection, we observed at the magnetopause (1) jetting of plasma, with a flow speed, measured in the deHoffmann‐Teller frame, at 98% of the Alfvén speed; (2) mixing of a nearly isotropic hot plasma sheet ion distribution with a field‐aligned magnetosheath distribution having a low‐energy cutoff at the predicted deHoffmann‐Teller velocity; (3) opposite streaming of magnetosheath and plasma sheet electrons consistent with open field topology; and (4) a finite inward (earthward) directed normal magnetic field (BN<0) at and an associated earthward plasma flow (at 94% of the normal Alfvén speed) across the magnetopause. The combined fluid and particle signatures provide a comprehensive set of evidence for reconnection at the magnetopause. These reconnection signatures were observed in the tail flank magnetopause, where the presence of fast plasma flow (at almost twice the local Alfvén speed) in the adjacent magnetosheath has been predicted to suppress reconnection. The sense of the flow enhancement, the direction of the electron heat flux, and the polarity of BN are all consistent with each other and with the spacecraft being located tailward and northward of the reconnection site. Our analysis places the reconnection site ∼7 Earth radii south of the magnetic equator. The dimensionless reconnection rate at this flank magnetopause is estimated to be in the range of 0.1 to 0.2, which is similar to values reported for the subsolar region. In essence, the present event provides unambiguous evidence for reconnection and shows that reconnection signatures at the tail flank magnetopause are not noticeably different from those predicted and observed in the subsolar region.
Currents at the subsolar low shear magnetopause
Authors: Belenkaya E. S.
Published: January 2004

A model of a current system at the subsolar low shear magnetopause is suggested. This model is valid under certain conditions of an overlap of magnetosheath and magnetospheric populations. The current system contains two current layers created by two different sorts of ions (magnetospheric and magnetosheath) at their boundary. For northward interplanetary magnetic field, the magnetopause azimuthal currents in these layers are antiparallel: a current carried by magnetosheath ions flows from noon to dusk, and a current created by magnetospheric ions is directed from noon to dawn. In this case, due to the rotation of magnetic field caused by two antiparallel currents, a negative spike in the northward component of the magnetic field Bz arises at the magnetopause. This specific feature allows us to define the magnetopause location in the case of a low magnetic shear, when the identification of the magnetopause is not obvious. A goal of the paper is to explain the main physical reason leading to the described current system. This reason is a change in all plasma parameters and magnetic field strength at the subsolar magnetopause. Model estimations allow us to compare the obtained results with observations and with existing theoretical models.
Interplanetary magnetic field By and auroral conductance effects on high‐latitude ionospheric convection patterns
Authors: Tanaka T.
Published: January 2004

The dependence of the ionospheric electric potential (convection) on the interplanetary magnetic field (IMF) and the ionospheric conductivity is investigated to understand the generation of convection patterns in the framework of the solar wind‐magnetosphere‐ionosphere (S‐M‐I) coupling scheme and the merging concept. A numerical magnetohydrodynamic (MHD) simulation is adopted for the study of the present problem. To achieve a high resolution in the ionosphere, the MHD calculation employs the finite volume (FV) total‐variation diminishing (TVD) scheme with an unstructured grid system. The two‐cell convection patterns reproduced from simulation are shown for several cases under the southward IMF condition during the growth‐phase interval. In the investigation of these results, special attention is paid to the analysis of mirror symmetry in the convection patterns with respect to the IMF By. On the dayside in the Northern Hemisphere, IMF By− (By+) generates flow deflection on newly opened field lines toward the dusk (dawn) without a severe violation of the mirror symmetry. While the mirror symmetry of the convection pattern is maintained even on the nightside when the ionospheric conductivity is uniform, it is not maintained on the nightside when the ionospheric conductivity is nonuniform. A realistic ionospheric conductivity modifies the convection pattern in the Northern (Southern) Hemisphere so as to emphasize distinctive features seen for IMF By+ (By−) under a uniform conductivity, and the reproduced convection patterns coincide with the observation quite well including fine signatures on the nightside, both for IMF By− and By+. Because of the nonuniform conductivity, cell centers of convection are shifted to the earlier magnetic local times, and the antisunward flow in the northern polar cap is nearly aligned with noon‐midnight meridian for IMF By−, while the flow in the northern polar cap has a significant inclination from prenoon to premidnight for IMF By+. These convection patterns can be understood by considering the effect due to the Hall current closure of the region‐1 field‐aligned current. The analysis for the dependence of nightside convection on IMF By and ionospheric conductivity shows that the Harang discontinuity is attributed partially to the structure of magnetospheric driver but mainly to the effect of nonuniform auroral conductivity. As a consequence, it is more adequate to say that convection patterns are more or less caused by the synthesized effect of more than one process rather than a single elementary process. Reproduced convection patterns in this paper show a particular coincidence with satellite observations summarized by adopting the pattern‐recognition‐based approach.
The Io mass‐loading disk: Model calculations
Authors: Wang Yongli , Russell Christopher T. , Raeder Joachim
Published: January 2004

The observations of ion cyclotron waves up to 0.5 RJ beyond the orbit of Io are best explained by the presence of a thin disk of fast neutrals whose ionization and pickup provide the free energy for the waves. We extend the model of Wilson and Schneider [1999] in order to explain the observed properties of this mass‐loading region, especially the most recent Galileo observations near Io. In the extended model, some of the molecules of sulfur compounds in Io's exobase are first ionized by photoionization, impact ionization, and charge exchange. These charged particles are accelerated in the corotation electric field associated with the motion of the magnetized Io torus plasma that is moving through its exosphere. After a period of acceleration the heavy ions are neutralized by charge exchange with other exospheric neutral particles or combined with local electrons. These newly neutralized particles continue with high velocities similar to those of their former charged state, moving only under the influence of the gravity fields of Jupiter and Io, not affected by the electric and magnetic field. If they do not impact Io or its atmosphere, these neutral particles can propagate large distances across the magnetic field before they are reionized. Eventually, the reionized particles are lost by dissociation. Characteristic Io mass‐loading particle distributions, such as high torus plasma density outside Io's orbit and the lower density inside, and the directional feature of the mass‐loading neutral cloud, are qualitatively reproduced in the model. Meanwhile, the configuration of the mass‐loading region, in which the ion cyclotron waves are observed, is obtained, and the results are consistent with Galileo wave observations. Three parameters are found to control the structure of the neutral and ion loading disks: the characteristic lifetimes of the initially created ions, of the neutral molecules, and of the ions generated by neutral particle reionization. In addition, particle source geometry, gravity, and the near Io field configurations play important roles in the Io mass‐loading process.
On the pressure balance in the distant magnetotail
Authors: Matsumoto Y. , Mukai T. , Saito Y. , Kokubun S. , Hoshino M.
Published: January 2004

Pressure balance between the lobe and the plasma sheet in the distant magnetotail is statistically studied with data obtained by the Geotail spacecraft. We selected events where the spacecraft traversed from the lobe to the neutral sheet and vice versa. As a result, we have found that the total pressure variations can be categorized by three types. Type 1, in which the total pressure is kept almost constant during the crossings, contains about a half of all events. This type is regarded as an average static feature of the distant magnetotail. Type 2 and Type 3 events are characterized by enhancement and depression of the total pressure in the plasma sheet, respectively. While the excess total pressure in Type 2 events can be explained by the passage of a plasmoid during the crossing, characteristics of Type 3 events suggest that they are located near the reconnection region. Depression of the total pressure indicates that part of the total pressure (energy density) is converted into plasma kinetic energy across a slow‐mode shock boundary.