Abstract: The nearly absence of water in the atmosphere of Venus is a major difference to the situation at Earth. The actual content of hydrogen in the exosphere is still an open issue, since no in situ measurements are available yet. A different method uses the presence of proton cyclotron waves as an early tracer of ionized planetary hydrogen picked-up by the solar wind, especially in the region upstream of the bow shock. Here, we report long-term observations over two full Venus-years of upstream proton cyclotron waves by the magnetometer on the Venus Express spacecraft, which indicate permanent ionization and pick-up of hydrogen by the solar wind upstream of the planetary bow shock up to high altitudes. The pick-up protons are shown to be of planetary origin, whereas other sources of neutral hydrogen have only negligible contribution. Therefore, the observation of upstream proton cyclotron waves in the solar wind is a clear indication for the existence of an extended neutral hydrogen corona at Venus, with significant local number densities up to an altitude of eight planetary radii. Recent observations of the exospheric Lyman-alpha emission also indicate hot neutral hydrogen densities which are higher than expected. Citation: Delva, M., M. Volwerk, C. Mazelle, J. Y. Chaufray, J. L. Bertaux, T. L. Zhang, and Z. Voros (2009), Hydrogen in the extended Venus exosphere, Geophys. Res. Lett., 36, L01203, doi: 10.1029/2008GL036164.
Abstract: Recent spacecraft observations of magnetic turbulence in the ion foreshock, in the magnetosheath, in the polar cusp regions, and in the magnetotail will be reviewed. Turbulence features like the fluctuation level, the spectral power law index, the turbulence anisotropy and intermittency, and the turbulence driver will be addressed.
Abstract: The magnetotail of the induced magnetosphere of Venus is investigated through the magnetic field and plasma data of Venus Express. A comparison is made between two neutral sheet crossings. One crossing shows the magnetic field is rather quiet and the plasma instrument indicates a change from energetic (few 100 eV) to low energy (few 10 eV) ions. The other crossing shows more dynamics in the magnetic field, including signatures that are interpreted as characteristic of a reconnection site, and the plasma instrument indicates ions that are energized to 1500 to 2000 eV, in the same magnetospheric region where in the first crossing only low energy ions showed up.
Abstract: At the end of August 2007, Venus, Earth and Ulysses were aligned within a few degrees. This unusual event gives the opportunity to attempt a coordinated study on the radial evolution of solar wind turbulence and coronal transients like CMEs between 0.7 and 1.4 AU. Interplanetary magnetic field data and moments of proton velocity distribution function such as density, speed and temperature are required for this programme and will be provided by ACE at Earth, Venus Express at Venus and Ulysses at 1.4 AU. This project has been recently proposed as a Coordinated Investigation Programme (CIP35) for the International Heliophysical Year.
Abstract: We investigate the structure of mirror modes in the solar wind at 0.72 AU using Venus Express magnetic field measurements. The mirror mode structure is identified as the presence of magnetic depression or magnetic "holes'' in the solar wind with little or no directional change across them. We determine the characteristic size and shape of these structures by examining their durations as a function of the orientation of the magnetic field to the solar wind flow. The mirror mode structure is best fitted with an ellipsoid of revolution, and the resultant shape of the mirror mode structure is a prolate spheroid, or in other words, a rotational ellipsoid. We introduce two parameters, namely the width across the field and the eccentricity to give a full description of the size and shape of the structures. We find that the mirror mode structures in the solar wind are two-dimensional and are more elongated along the magnetic field direction.
Abstract: We study the plasma turbulence, at scales larger than the ion inertial length scale, downstream of a quasiparallel bow shock using Cluster multispacecraft measurements. We show that turbulence is intermittent and well described by the extended structure function model, which takes into account the spatial inhomogeneity of the cascade rate. For the first time we use multispacecraft observations to characterize the evolution of magnetosheath turbulence, particularly its intermittency, as a function of the distance from the bow shock. The intermittency significantly changes over the distance of the order of 100 ion inertial lengths, being increasingly stronger and anisotropic away from the bow shock.
Abstract: We examined two consecutive plasma sheet oscillation and dipolarization events observed by Cluster in the magnetotail, which are associated with a pseudo-breakup and a small substorm monitored by the IMAGE spacecraft. Energy input from the solar wind and all associated enhancement of the cross-tail current lead to current sheet thinning and plasma sheet oscillations of 3-5 min periods, while the pseudo-breakups occur during the loading phase within a spatially limited area, accompanied by a localized dipolarization observed by DSP TC1 or GOES 12. That is, the so-called "growth phase" is a preferable condition for both pseudo-breakup and plasma sheet oscillations in the near-Earth magnetotail. One of the plasma sheet oscillation events occurs before the pseudo-breakup, whereas the other takes place after pseudo-breakup. Thus there is no causal relationship between the plasma sheet oscillation events and pseudo-breakup. As for the contribution to the subsequent small substorm, the onset of the small substorm took place where the preceding plasma sheet oscillations can reach the region. (c) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.
Abstract: A detailed analysis of successive tailward flow bursts in the near-Earth magnetotail (X similar to- 19 R-E) plasma sheet is performed on the basis of in-situ multi-point observations by the Cluster spacecraft on 15 September 2001. The tailward flows were detected during a northward IMF interval, 2.5h after a substorm expansion. Each flow burst (V-x <300 km/s) was associated with local auroral activation. Enhancements of the parallel and anti-parallel similar to 1 keV electron flux were detected during the flows. The spacecraft configuration enables to monitor the neutral sheet (B-x approximate to 0) and the level of B-x approximate to 10-15 nT simultaneously, giving a possibility to distinguish between closed plasmoid-like structures and open NFTE-like surges. The data analysis shows NFTE-like structures and localized current filaments embedded into the tailward plasma flow. 3-D shapes of the structures were reconstructed using the four-point magnetic filed measurements and the particle data.
Abstract: In this study, magnetic field measurements obtained by the Venus Express spacecraft are used to determine the bow shock position at solar minimum. The best fit of bow shock location from solar zenith angle 20-120 degrees gives a terminator bow shock location of 2.14 R-V (1 R-V = 6052 km) which is 1600 km closer to Venus than the 2.40 R-V determined during solar maximum conditions, a clear indication of the solar cycle variation of the Venus bow shock location. The best fit to the subsolar bow shock is 1.32 R-V, with the bow shock completely detached. Finally, a global bow shock model at solar minimum is constructed based on our best-fit empirical bow shock in the sunlit hemisphere and an asymptotic limit of the distant bow shock which is a Mach cone under typical Mach number of 5.5 at solar minimum. We also describe our approach to making the measurements and processing the data in a challenging magnetic cleanliness environment. An initial evaluation of the accuracy of measurements shows that the data are of a quality comparable to magnetic field measurements made onboard magnetically clean spacecraft. (c) 2008 Elsevier Ltd. All rights reserved.
Abstract: The importance of space weather and its forecasting is growing as interest in studying geoeffective processes in the Sun - solar wind - magnetosphere -ionosphere coupled system is increasing. In this paper higher order statistical moments of interplanetary magnetic field and geomagnetic SYM-H index fluctuations are compared. The proper description of fluctuations in the solar wind can elucidate important aspects of the geoeffectivity of upstream turbulence and contribute to our understanding of space weather. Our results indicate that quasi-stationary intervals during both quiet and stormy periods have to be investigated in order to find correlations between upstream and geomagnetic conditions. We found that the fourth statistical moment (kurtosis), which was not considered in previous studies, appears to be a new geoeffective parameter. Intermittency of the magnetic turbulence in the solar wind can influence the efficiency of the solar wind - magnetosphere coupling through affecting magnetic reconnection at the Earth's magnetopause.
Abstract: The objective of the paper is to asses the specific spectral scaling properties of magnetic reconnection associated fluctuations/turbulence at the earthward and tailward outflow regions observed simultaneously by the Cluster and Double Star (TC-2) spacecraft on 26 September 2005. Systematic comparisons of spectral characteristics, including variance anisotropy and scale-dependent spectral anisotropy features in wave vector space were possible due to the well-documented reconnection events, occurring between the positions of Cluster (X = 014-16 R-e) and TC-2 (X = -6.6 R-e). Another factor of key importance is that the magnetometers on the spacecraft are similar. The comparisons provide further evidence for asymmetry of physical processes in earthward/tailward reconnection outflow regions. Variance anisotropy and spectral anisotropy angles estimated from the multiscale magnetic fluctuations in the tailward outflow region show features which are characteristic for magnetohydrodynamic cascading turbulence in the presence of a local mean magnetic field. The multiscale magnetic fluctuations in the earthward outflow region are not only exhibiting more power, lack of variance, and scale-dependent anisotropies but also are having larger anisotropy angles. In this region the magnetic field is more dipolar and the main processes driving turbulence are flow breaking/mixing, perhaps combined with turbulence ageing and noncascade-related multiscale energy sources.
Abstract: Recent research has shown that distinct physical regions in the Venusian induced magnetosphere are recognizable from the variations of strength and of wave/fluctuation activity of the magnetic field. In this paper the statistical properties of magnetic fluctuations are investigated in the Venusian magnetosheath, terminator, and wake regions. The latter two regions were not visited by previous missions. We found 1/f fluctuations in the magnetosheath, large-scale structures near the terminator and more developed turbulence further downstream in the wake. Location independent short-tailed non-Gaussian statistics was observed.
Abstract: The importance of space weather and its forecasting is growing as interest in studying geoeffective processes in the Sun - solar wind - magnetosphere - ionosphere coupled system is increasing. This paper introduces the proper selection criteria for solar wind magnetic turbulence events during duskward electric field and southward B-z driven geomagnetic storms. Two measures for the strength of solar wind fluctuations were investigated: the standard deviations of magnetic field components and a proxy for the so-called Shebalin anisotropy angles. These measures were compared to the strength of geomagnetic storms obtained from a SYM-H index time series. We found a weak correlation between standard deviation of interplanetary magnetic field GSM component B-z and SYM-H index, and a strong correlation between Shebalin anisotropy angle and the SYM-H index, which can be the result of an increase of probability of magnetic reconnection in fluctuating magnetic fields.
Abstract: The solar wind interaction with Venus creates an induced magnetosphere around the planet. It is shown that within the space bound by Venus' bow shock and ionopause, there is a rich occurrence of mirror-mode-like structures in the magnetic field data. The dayside magnetosheath and nightside magnetosheath/wake regions are investigated separately. It is shown that the probability to observe mirror mode structures is much higher at the dayside, where it is also strongly dependent on the angle between the solar wind magnetic field and the bow shock normal. In Venus' wake the chance to observe these structures is low, most likely because of the fully developed turbulence in this region, which will decrease temperature anisotropies. The results stand in contrast to the very low occurrence rate claimed from data taken by the Pioneer Venus Orbiter mission.
Abstract: Recent research has shown that distinct physical regions in the Venusian induced magnetosphere are recognizable from the variations of strength of the magnetic field and its wave/fluctuation activity. In this paper the statistical properties of magnetic fluctuations are investigated in the Venusian magnetosheath and wake regions. The main goal is to identify the characteristic scaling features of fluctuations along Venus Express ( VEX) trajectory and to understand the specific circumstances of the occurrence of different types of scalings. For the latter task we also use the results of measurements from the previous missions to Venus. Our main result is that the changing character of physical interactions between the solar wind and the planetary obstacle is leading to different types of spectral scaling in the near-Venusian space. Noisy fluctuations are observed in the magnetosheath, wavy structures near the terminator, and in the nightside near-planet wake. Multiscale turbulence is observed at the magnetosheath boundary layer and near the quasi-parallel bow shock. Magnetosheath boundary layer turbulence is associated with an average magnetic field which is nearly aligned with the Sun-Venus line. Noisy magnetic fluctuations are well described with the Gaussian statistics. Both magnetosheath boundary layer and near-shock turbulence statistics exhibit non-Gaussian features and intermittency over small spatiotemporal scales. The occurrence of turbulence near magnetosheath boundaries can be responsible for the local heating of plasma observed by previous missions.
Abstract: [1] Magnetometer data from two Venus years of the Venus Express mission in orbit are investigated for the occurrence of ion cyclotron waves. Proton cyclotron waves were recently detected in the upstream region of Venus by the spacecraft, indicating pickup of planetary protons from Venus's exosphere by the solar wind and loss of hydrogen to interplanetary space. A study of representative cases illustrates the waveform, spectrum, duration, and higher-order resonances of the transverse waves with left-hand circular polarization and propagation nearly along the magnetic field; their properties in the magnetic field principal axes system are determined. A statistical approach studies the wave properties as a function of the angle between the solar wind and magnetic field direction, as a function of their occurrence in space, and with respect to the motional solar wind electric field. Proton cyclotron waves are found up to 9 Venus radii from the planet, for a large range of angles between the solar wind and magnetic field direction, independent from foreshock geometry and independent from the direction of the motional electric field. This reveals that cyclotron wave generation from local pickup of neutral hydrogen is efficient over a large volume of space upstream of the planet and imposes the existence of an extended reservoir of planetary neutral hydrogen at Venus.
Abstract: Venus Express interplanetary magnetic field measurements have been examined for magnetic "holes,'' accompanied by magnetic field directional changes. We examine both the thickness of the current sheet and the depth of the magnetic field depression. We find the thickness of the current sheet is not correlated with the depth of the field depression. The depth of the magnetic holes is related to directional angle change. Since total pressure should balance across these discontinuities, there must be enhanced plasma pressure within the magnetic holes. The dependence of the depth of the hole (i.e., size of the plasma pressure enhancement) on the directional changes suggests that the heating of the plasma associated with the hole formation may be provided by annihilation of the magnetic energy in the current sheet, via slow reconnection. Citation: Zhang, T. L., et al. ( 2008), Behavior of current sheets at directional magnetic discontinuities in the solar wind at 0.72 AU, Geophys. Res. Lett., 35, L24102, doi: 10.1029/2008GL036120.
Abstract: A dipolarization and its associated current systems are studied using Cluster, Double Star TC1, and ground-based observations. The Cluster spacecraft are located approximately 16 RE downtail near 0030 LT. The Double Star TC1 spacecraft is located more earthward at approximately 7 RE just before local midnight. Auroral observations by the Wideband Imaging Camera on the Imager for Magnetopause-to-Aurora Global Exploration spacecraft are used to determine the onset times of substorms. It is shown that the magnetic phenomena at the earthward site of a magnetic reconfiguration region are governed by field-aligned currents, which in their turn generate auroral brightenings near the foot points of the spacecraft. It is also shown that the inward and outward motion of the dipolarization front near Cluster has a direct influence on the parallel plasma flow at TC1, indicating a piston mechanism. Just like a piston, the inward moving dipolarization at Cluster pushes in plasma along with the flux transport, which turns to parallel plasma flow at TC1. When the flow reverses at Cluster, i.e., outgoing flux transport, the plasma gets "sucked out'' again, which is directly reflected in the plasma data from TC1.
Abstract: Observations made by a unique constellation of Cluster (at 14-16 R-E), TC2, Goes10, and LANL spacecraft (near 6.6 R-E) have allowed us to study the details of three reconnection events in the middle of a thick plasma sheet with the reconnection X-line located unusually close to Earth (10-12 R-E). We use mapping along field lines with magnetospheric models adapted to magnetic field observations to confirm that the reconnection region mapped onto localized auroral brightenings. Using simultaneous observations in the inflow and outflow regions, we describe an encounter with a localized tailward Alfvenic jet produced by a short isolated reconnection pulse. A good correlation between intense E and ion [BV] indicates that the concurrent strong turbulence could not destroy the frozen-in ion behavior in the reconnection outflow. We find that a steady quadrupole-like distribution of the magnetic By component in the turbulent reconnection outflow extended far beyond the ion diffusion region and existed for several minutes. We demonstrate an apparent V-x flow reversal, formed owing to the reappearance (switch-on) of reconnection at another location, rather than to a continuous motion of the active X-line. Using the Liouville mapping technique, we show that the acceleration of outflow electrons, after the particles passed a potential drop of 180 V, is consistent with Fermi/betatron acceleration. We also suggest another interpretation of the energetic particle bursts at the onsets, to emphasize the role of seed population and explain the sudden burst as a consequence of changing magnetic topology.
Abstract: In this paper first time observations of mirror mode like structures in Venus' magnetosheath are presented. Using magnetometer data from the Venus Express spacecraft it is shown that in two regions in the Venusian magnetosheath strong compressional waves exist, which propagate nearly perpendicular to the ambient magnetic field. They are most likely mirror-mode waves. The waves have periods between 5 and 15 sec, depending on the location in the magnetosheath. These waves show up just behind the quasi-perpendicular bow shock, and near the magnetopause during compression of the magnetosheath due to increased solar wind pressure. The characteristics of the waves are similar to mirror mode waves found in the Earth's magnetosheath, however, they are down-scaled in duration and frequency by a factor of 10, comparable with the difference in size of Venus' and Earth's magnetosheath.
Abstract: Venus has no significant internal magnetic field(1), which allows the solar wind to interact directly with its atmosphere(2,3). A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum(4-7).) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere(8,9). Here we report magnetic field measurements from the Venus Express spacecraft(4) in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere(10). Therefore little solar wind enters the Venus ionosphere even at solar minimum.
Abstract: Using Cluster multipoint magnetic and plasma measurements we analyze the spatial structure of plasma flow-associated turbulence on 26 September 2005. The fortunate relative configuration of the spacecraft and the plasma flow allowed for the first time to compare the scale evolution of statistical moments both at the boundary and in the central part of the flow at the same time. The simultaneous increase of skewness and kurtosis at the boundary of the plasma flow over the time scale of seconds provides evidence for the existence of nonlocal coupling in flow-associated turbulence in the Earth's plasma sheet.
Abstract: Bursty bulk flow associated magnetic fluctuations exhibit at least three spectral scaling ranges in the Earth's plasma sheet. Two of the three scaling ranges can be associated with multi-scale magnetohydrodynamic turbulence between the spatial scales from similar to 100 km to several R-E (R-E is the Earth's radius). These scales include the inertial range and below similar to 0.5 R-E a steepened scaling range, theoretically not fully understood yet. It is shown that, in the near-Earth plasma sheet, the inertial range can be robustly identified only if multi-scale quasi stationary (MSQS) data intervals are selected. Multiple bursty flow associated magnetic fluctuations, however, exhibit 1/f type scaling indicating that large-scale fluctuations are controlled by multiple uncorrelated driving sources of the bulk flows (e.g. magnetic reconnection, instabilities).
Abstract: During the last several years significant progress has been made in understanding MHD turbulence in the Earth's plasma sheet. Due to the statistically transitory properties of fluctuations, finite size and boundary effects, however, issues of fundamental importance remain unresolved. Here we concentrate on such intrinsic features of plasma sheet turbulence as its origin and dynamical nature. In particular, we investigate bursty bulk flow driven multi-scale transfer of energy towards the dissipation scale, and provide evidence for the presence of non-linear interactions. We show that, in contrast with previous results, Alfvenic fluctuations together with 2D eddy interactions may appear as important constituents of turbulence in the plasma sheet.
Abstract: Thirty rapid crossings of the magnetotail current sheet by the Cluster spacecraft during July-October 2001 at a geocentric distance of 19 RE are examined in detail to address the structure of the current sheet. We use four-point magnetic field measurements to estimate electric current density; the current sheet spatial scale is estimated by integration of the translation velocity calculated from the magnetic field temporal and spatial derivatives. The local normal-related coordinate system for each case is defined by the combining Minimum Variance Analysis (MVA) and the curlometer technique. Numerical parameters characterizing the plasma sheet conditions for these crossings are provided to facilitate future comparisons with theoretical models. Three types of current sheet distributions are distinguished: center-peaked (type I), bifurcated (type II) and asymmetric (type III) sheets. Comparison to plasma parameter distributions show that practically all cases display non-Harris-type behavior, i.e. interior current peaks are embedded into a thicker plasma sheet. The asymmetric sheets with an off-equatorial current density peak most likely have a transient nature. The ion contribution to the electric current rarely agrees with the current computed using the curlometer technique, indicating that either the electron contribution to the current is strong and variable, or the current density is spatially or temporally structured.
Abstract: Using Cluster magnetometer data during summer 2001 and 2002, when the spacecraft traversed the magnetotail, we have performed a survey of current sheet and its current. We applied a superposed epoch analysis to simple, monotonous current sheet crossings and obtained a statistical profile of the current sheet. We find that the averaged structure of the current sheet fits well with the Harris current sheet description. By using the four spacecraft measurements, the current is calculated and a statistical current density profile of the current sheet is produced. (C) 2006 COSPAR. Published by Elsevier Ltd. All rights reserved,
Abstract: [1] Several approaches exist to address the problem of intermittency in solar wind turbulence. The traditional explanation based on spectral transfer of energy between separated scales through sequential steps within a cascade can elucidate scalings and intermittent emergence of small-scale dissipative structures. A recent point of view connects intermittency mainly with the occurrence of discontinuities and interaction of flux tube-like coherent structures. In this paper we analyze the interrelationship between skewness and kurtosis near interplanetary shocks. On the basis of a comparison of our results with the behavior of skewness and kurtosis in laboratory experiments and numerical simulations we show that large-scale boundaries and/or scalar gradient fields imposed on solar wind turbulence can lead to a local increase of small-scale intermittency via direct coupling between separated scales. As each scenario is supported by experiments, they represent a complementary way of looking at intermittency.
Abstract: Electromagnetic low-frequency waves in the magnetotail lobe close to the PSBL (Plasma Sheet Boundary Layer) are studied using the Cluster spacecraft. The lobe waves show Alfvenic properties and transport their wave energy (Poynting flux) on average toward the Earth along magnetic field lines. Most of the wave events are rich with oxygen (O+) ion plasma. The rich O+ plasma can serve to enhance the magnetic field fluctuations, resulting in a greater likelihood of observation, but it does not appear to be necessary for the generation of the waves. Taking into account the fact that all events are associated with auroral electrojet enhancements, the source of the lobe waves might be a substorm-associated instability, i.e. some instability near the reconnection site, or an ion beam-related instability in the PSBL.
Abstract: In general, if turbulence is present in MHD plasmas, it cannot be ignored. There is also evidence that the Sun is, the main driver of space weather. It has already been demonstrated in our recent study that the non-Gaussian characteristics of magnetic turbulence in the solar wind and the occurrence of intermittent magnetic turbulence in the Earth's plasma sheet can be interconnected. In this respect a comparative analysis of the solar wind magnetic and plasma parameters with the time evolution of the geomagnetic indices is insufficient. Therefore, a wider statistical study which includes the consideration of intermittency parameters during several coupling events during the period of 1996 - 2002 was performed as well.
Abstract: Using Cluster magnetometer data during summer 2001 and 2002, when the spacecraft traversed the magnetotail, we have performed a survey of current sheet and its current. We applied a superposed epoch analysis to simple, monotonous current sheet crossings and obtained a statistical profile of the current sheet. We find that the averaged structure of the current sheet fits well with the Harris current sheet description. By using the four spacecraft measurements, the current is calculated and a statistical current density profile of the current sheet is produced. (c) 2006 COSPAR. Published by Elsevier Ltd. All rights reserved.
Abstract: The Venus Express mission is scheduled for launch in 2005. Among many other instruments, it carries a magnetometer to investigate the Venus plasma environment. Although Venus has no intrinsic magnetic moment, magnetic field measurements are essential in studying the solar wind interaction with Venus. Our current understanding of the solar wind interaction with Venus is mainly from the long lasting Pioneer Venus Orbiter (PVO) observations. In this paper, we briefly describe the magnetic field experiment of the Venus Express mission. We compare Venus Express mission with PVO mission with respect to the solar wind interaction with Venus. Then we discuss what we will achieve with the upcoming Venus Express mission. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: In this paper we analyze the vertical structure of the magnetotail current sheet for a few intervals during which Cluster repeatedly crosses the neutral sheet due to fast flapping motion. They include fast flows with episodes of flow reversals. The curl, gradient and divergence of the magnetic field are estimated using linear gradient/curl estimator techniques. The reconstruction of the electric current and magnetic field profiles are performed by integration of the estimated gradient scales. We found examples of thin current sheets, (half-thickness of 1000 km) with the current density maximum at the magnetic equator (B-x = 0), as well as examples of off-center or bifurcated current sheets. (C) 2004 Elsevier Ltd. All rights reserved.
Abstract: Using multi-satellite Cluster magnetic field data, we statistically examined how often an atypical, i.e., non-Harris type, current sheet is formed. We found that a cross-tail current sheet with an off-equatorial current density maximum, indicating bifurcation, is frequently formed in thin current sheets with a half thickness of similar to1500 km. The occurrence of off-equatorial current density maxima is correlated with the occurrence of fast plasma flows, namely if fast plasma flows occur nearby, one is more likely to observe off-equatorial current density maxima. We also found that many of the center-peaked current sheets exhibit a feature different from a normal Harris sheet: namely a very intense current concentrated near the equatorial plane, embedded in a broader current sheet with lower current density.
Abstract: The probability distribution functions (PDFs) of the differences of any physical variable in the intermittent, turbulent interplanetary medium are scale dependent. Strong non-Gaussianity of solar wind fluctuations applies for short time lag spacecraft observations, corresponding to small-scale spatial separations, whereas for large scales the differences turn into a Gaussian normal distribution. These characteristics were hitherto described in the context of the lognormal, the Castaing distribution, or the shell model. On the other hand, a possible explanation for non-locality in turbulence is offered within the context of nonextensive entropy generalization by a recently introduced bi-kappa distribution, generating through a convolution of a negative-kappa core and positive-kappa halo pronounced non-Gaussian structures. The PDFs of solar wind scalar field differences are computed from Wind and ACE data for different time lags and compared with the characteristics of the theoretical bi-kappa functional, well representing the overall scale dependence of the spatial solar wind intermittency. The observed PDF characteristics for increased spatial scales are manifest in the theoretical distribution functional by enhancing the only tuning parameter kappa, measuring the degree of nonextensivity where the large-scale Gaussian is approached for kappa --> infinity. The nonextensive approach assures for experimental studies of solar wind intermittency independence from influence of a priori model assumptions. It is argued that the intermittency of the turbulent fluctuations should be related physically to the nonextensive character of the interplanetary medium counting for nonlocal interactions via the entropy generalization.
Abstract: The observed scale dependence of the probability distributions of the differences of characteristic solar wind variables is analyzed. Intermittency of the turbulent fluctuations at small-scale spatial separations is accompanied by strongly non-Gaussian distributions that turn into a normal distribution for large-scale separation. Conventional theoretical models are subject to insufficient physical justification since nonlocality in turbulence should be based on long-range interactions, provided recently by the bi-kappa distribution in the context of nonextensive thermostatistics. Observed WIND and ACE probability distributions are accurately reproduced for different time lags by the one-parameter bi-kappa functional, a core-halo convolution, where kappa measures the degree of nonlocality or nonextensivity in the system. Gradual decoupling is obtained by enhancing the spatial separation scale corresponding to increasing kappa values, where a Gaussian is approached for infinite kappa. Consequently, long-range interactions introduced on the fundamental level of entropy generalization, are able to provide physically the source of the observed scale dependence of the turbulent fluctuations in the intermittent interplanetary medium.
Abstract: We use high resolution magnetic field data from the Cluster spacecraft to analyse the occurrence of specific anisotropy characteristics of magnetic fluctuations at two adjacent scales near the dissipation scale, during non-flow and bursty bulk flow (BBF) associated periods. The obtained anisotropy patterns show signatures of scale dependent anisotropy and can be explained by different physical processes. During non-flow periods the main source of the magnetic anisotropy are anisotropic ion populations within the plasma sheet boundary layer (PSBL). BBF-associated periods are characterized mainly by strong interaction of the plasma flow with the magnetic field. In both cases the local mean background magnetic field has a decisive role in the development of anisotropy in magnetic fluctuations.
Abstract: Using four-point magnetic field measurements by the Cluster spacecraft, we statistically analyze the magnetic field and electric current configurations during rapid crossings of the current sheet observed in July-October 2001 at geocentric distances of 19 RE. The database includes 78 crossings, specially selected to apply multi-point data analysis techniques to calculate vector derivatives. Observed bipolar variations of j(z), often with vertical bar j(z)vertical bar > j(y) indicate that the electric currents follow kinks of the current sheet. The current density varies between 5-25 nA/m(2). The half-thickness of the current sheet during flapping varies over a wide range, from 1 to 20 ion thermal gyroradii (L,, calculated from average temperature and lobe magnetic field for each crossing). We found no relationship between the tilt angle of the current sheet normal and the half-thickness. In 68 cases the magnetic field curvature vector has a positive (earthward) X-component. Ten cases with a negative (tailward) curvature, associated with reconnection, were detected within 0 < Y-GSM < 7 R-E. The minimum curvature radii vary mainly between 1 and 10 L,P, and the adiabaticity parameter kappa <= 1 for 73 % of the events. The electric current density during flapping is often off-central, i.e. the main current density is shifted from the neutral sheet (vertical bar B(x)vertical bar < 5 nT) to the Northern or Southern Hemisphere. This is most likely a temporal effect related to the flapping. The analysis shows that the flapping motion of the current sheet is associated with kink-like waves on the sheet surface. The kink fronts, tilted in the Y-Z plane, moved toward dawn in the morning half and toward dusk in the evening half of the magnetotail.
Abstract: The structure of the high-speed flows in the midtail plasma sheet is discussed based on results from multi-point observation of Cluster spacecraft in summer 2001 and 2002. The average rate of change in flow speed along the "dawn-dusk" direction (perpendicular to the main flow and in the plane of the tail current sheet) and along the "north-south" direction suggests that the full width of the flow channel is 12,000-17,000 km in the "dawn-dusk" direction and about 9000-12,000 km in the "north-south" direction. This is consistent with the results obtained from the analysis of the dipolarization front of the flows. The profile of the gradient of the flow in the north-south direction and the shape of the dipolarization front suggest that the BBF has the shape of a localized flux tube. (c) 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.
Abstract: One of the important parameters in magnetotail studies is the thickness of the current sheet. The determination of this thickness is subject to many assumptions of the geometry, such as the Current sheet normal direction. The minimum variance analysis (MVA) has been widely used in studies of the current sheet in the tail or at the magnetopause with single-satellite magnetometer data. With four Cluster spacecraft, we are able to determine the current sheet normal direction by using the crossing timings. In this study, we perform MVA on selected Cluster neutral sheet crossing cases to determine the normal directions. We compare these MVA normal estimates with the timing-derived normals. We find that both MVA and four spacecraft timing analysis give useful data. Published by Elsevier Ltd on behalf of COSPAR.
Abstract: In order to estimate the dissipation time-scale in magnetic turbulence in the plasma sheet a novel method is introduced for classification of velocity dependent patterns of two-point probability density functions' shapes near their maxima. For the first time, we provide evidence for Reynolds number (velocity) dependent widening of the inertial range in the plasma sheet. Since spectral widening of the inertial range is a generic feature of many turbulent flows, its examination can facilitate the recognition of intermittent turbulence in the plasma sheet.
Abstract: Recent studies provide evidence for the multiscale nature of magnetic turbulence in the plasma sheet. Wavelet methods represent modern time series analysis techniques suitable for the description of statistical characteristics of multiscale turbulence. Cluster FGM (fluxgate magnetometer) magnetic field high-resolution (67 Hz) measurements are studied during an interval in which the spacecraft are in the plasma sheet. As Cluster passes through different plasma regions, physical processes exhibit nonsteady properties on magnetohydrodynamic (MHD) and small, possibly kinetic scales. As a consequence, the implementation of wavelet-based techniques becomes complicated due to the statistically transitory properties of magnetic fluctuations and finite size effects. Using a supervised multiscale technique which allows existence test of moments, the robustness of higher-order statistics is investigated. On this basis the properties of magnetic turbulence are investigated for changing thickness of the plasma sheet. (C) 2004 American Institute of Physics.
Abstract: Compressional waves in the Earth's current sheet, driven by the high-speed plasma flows connected to substorms. are investigated using the Cluster magnetometer and 9 plasma instrument. During the time that Cluster had its apogee in the magnetotail (July through October 2001), we have studied 5 events in detail. We find compressional waves in the 30-60 mHz band, at a spectral power density that is dependent on when and where the event is observed. There is a difference of two orders of magnitude in power density between waves at substorm onset and waves during quiet times. Strong plasma flows are the driver of the wave power. The spacecraft location in the current sheet is also important for the spectral power density. Having four spacecraft available we can discern spatial from temporal variations. We have determined the propagation direction of the waves in the 3060 mHz band and found that in the Cluster rest frame they propagate in the same direction as the plasma flow at an angle 30degrees < Deltaphi < 40degrees with respect to the plasma flow direction in the spacecraft' rest frame.
Abstract: Ion density, velocity and temperature, measured by Cluster spacecraft in the plasma sheet during an isolated substorm at 10:20-11:15 UT, 29 August 2001 are discussed. The Custer/CODIF data for subinterval 10:55-11:03 UT, during which a bifurcation of the current sheet was detected, are studied in particular. It is shown that ion density and temperature have a plateau-like profile in the central part of the bifurcated current sheet. An enhanced proton flow directed in positive Y-GSM was detected during the selected subinterval. Other components of proton bulk velocity are found to be negligibly small, i.e. the structure is likely not directly associated with magnetic reconnection. Alternative models, including ion anisotropy and localized magnetic turbulence, which result in a two-peak profile of the current density, are discussed briefly.
Abstract: Spatial gradients of high-speed flows in the midtail plasma sheet are determined using multipoint observations from the Cluster spacecraft along the "dawn-dusk'' direction (perpendicular to the main flow and in the plane of the tail current sheet) and along the north-south direction. If we take the average or median of the spatial gradients and assume that the flow channel has a linear gradient, these values suggest that the full width of the flow channel is 2-3 R-E in the "dawn-dusk'' direction and 1.5-2 R-E in the north-south direction. The velocity gradient at the duskward edge of a flow tends to be sharper than that at the dawnward edge, possibly reflecting an asymmetry in the magnetosphere-ionosphere coupling process associated with the flow.
Abstract: A multi-scale analysis of magnetotail turbulence in the Earth's tail current sheet is presented based on Cluster magnetometer observations. Both Fourier and wavelet analysis is used to describe the spectral index and scaling indices of the turbulence for different frequency regions. Flows in the tail are very important for driving the observed turbulence. There is a strong correlation between the maximum perpendicular flow velocity and the turbulence power for maximum velocities 150 less than or equal to v(perpendicular to),max less than or equal to 400 km/s. At higher maximum flow velocities the turbulence power levels out, showing a saturation of the generation mechanism. The suspected presence of breaks in the slope of the spectrum at two frequencies (f(1) and f(2)) can be confirmed for f(1) approximate to 0.08 Hz, but based on the data analysis the second break at f(2) is expected at a frequency higher than 12.5 Hz, where the data cannot significantly be evaluated. A schematic turbulence power spectrum is presented based on the Cluster magnetic field measurements. Dependent on the presence of BBFs the spectral index or scaling index varies significantly.
Abstract: [1] Small-scale magnetic turbulence observed by the Cluster spacecraft in the plasma sheet is investigated by means of a wavelet estimator suitable for detecting distinct scaling characteristics even in noisy measurements. The spectral estimators used for this purpose are affected by a frequency-dependent bias. The variances of the wavelet coefficients, however, match the power-law shaped spectra, which makes the wavelet estimator essentially unbiased. These scaling characteristics of the magnetic field data appear to be essentially nonsteady and intermittent. The scaling properties of bursty bulk flow (BBF) and non-BBF associated magnetic fluctuations are analyzed with the aim of understanding processes of energy transfer between scales. Small-scale (similar to 0.08 - 0.3 s) magnetic fluctuations having the same scaling index alphasimilar to 2.6 as the large-scale ( similar to 0.7 - 5 s) magnetic fluctuations occur during BBF-associated periods. During non-BBF associated periods the energy transfer to small scales is absent, and the large-scale scaling index alpha similar to1.7 is closer to Kraichnan or Iroshnikov-Kraichnan scalings. The anisotropy characteristics of magnetic fluctuations show both scale-dependent and scale-independent behavior. The former can be partly explained in terms of the Goldreich-Sridhar model of MHD turbulence, which leads to the picture of Alfvenic turbulence parallel and of eddy turbulence perpendicular to the mean magnetic field direction. Nonetheless, other physical mechanisms, such as transverse magnetic structures, velocity shears, or boundary effects can contribute to the anisotropy characteristics of plasma sheet turbulence. The scale-independent features are related to anisotropy characteristics which occur during a period of magnetic reconnection and fast tailward flow.
Abstract: A statistical study of the Cluster FGM data is performed on compressional waves for 68 selected 12 min intervals when the spacecraft are in or near the neutral sheet. The data are well distributed in local time between 2100 and 0300 LT. The compressional wave power is strongly dependent on the maximum plasma flow velocity (based on the CIS-CODIF data) during the 12-min interval. Up to a perpendicular flow velocity of similar to400 km/s the spectral power increases with velocity; at higher velocities the spectral power saturates. The turbulence measured in the frequency range between 0.08 and 1 Hz shows that on that scale the magnetotail has fully developed quasi two-dimensional turbulence. We relate the quasi two-dimensional (2-D) turbulence to the finite extension of the flow channel, with a streaming instability as its source. There is an asymmetry in wave power between the premidnight and midnight/postmidnight sectors. The plasma flow direction splits the data into two distinct categories, the Earth/duskward and the tail/dawnward flows.
Abstract: [1] During the interval 0947-0951 UT on 1 October 2001, when Cluster was located at X-GSM = -16.4 R-E near Z(GSM) = 0 in the pre-midnight magnetotail, the Cluster barycenter crosses the neutral sheet four times. High speed proton flow, with reversal from tailward to Earthward, was detected during the crossings. Using a linear gradient/curl estimator technique we estimate current density and magnetic field curvature within the crossings. These observations exhibit the tailward passage of an X-line over the Cluster tetrahedron. These current sheet has a bifurcated structure in the regions of tailward and earthward flows and a flat and/or slightly bifurcated thin current sheet in between. A distinct quadrupolar Hall magnetic field component was observed.
Abstract: This paper demonstrates that intermittent magnetic field fluctuations in the plasma sheet exhibit transitory, localized, and multi-scale features. We propose a multifractal-based algorithm, which quantifies intermittence on the basis of the statistical distribution of the "strength of burstiness", estimated within a sliding window. Interesting multi-scale phenomena observed by the Cluster spacecraft include large-scale motion of the current sheet and bursty bulk flow associated turbulence, interpreted as a cross-scale coupling (CSC) process.
Abstract: A multifractal-based algorithm is used to demonstrate that intermittent fluctuations estimated from daily sunspot number data exhibit transitory and multi-scale features. The main aim of this work is the analysis of the time-scale behaviour of intermittence together with multi-periodic features visible in sunspot data.
Abstract: Preliminary results are presented which suggest that scaling and singularity characteristics of solar wind and ground-based magnetic fluctuations appear to be a significant component in the solar wind-magnetosphere interaction processes. Of key importance is the intermittence of the "magnetic turbulence" as seen in ground-based and solar wind magnetic data. The methods used in this paper (estimation of flatness and multifractal spectra) are commonly used in the studies of fluid or MHD turbulence. The results show that single observatory characteristics of magnetic fluctuations are different from those of the multi-observatory AE-index. In both data sets, however, the influence of the solar wind fluctuations is recognizable. The correlation between the scaling/singularity features of solar wind magnetic fluctuations and the corresponding geomagnetic response is demonstrated in a number of cases. The results are also discussed in terms of patchy reconnection processes in the magnetopause and forced and/or self-oraganized criticality (F/SOC) of internal magnetosphere dynamics.
Abstract: The best known manifestations of the solar wind impact on the Earth's magnetosphere are the geomagnetic storms. At middle latitudes, the magnetic storm is best described in terms of the horizontal component of the geomagnetic field. We use the method of neural networks which is suitable for nonlinear dynamical systems modeling and for the nowcasting as well as forecasting of the magnetic storms. For constructing a neural network model, a multivariate method for determination of the inputs is applied first. This method enables us to reduce the original 17 input variables to two variables, the so-called principal components. The performance of the model is characterized by the correlation coefficient rho. Its mean value is 0.93 considering two principal components and time history 6 h, Our interest is also focused on the more than I h ahead forecasting of the geomagnetic index D-st. Another question investigated here is how the inclusion of the history of D-st index into the input matrix influences the predicting ability of the network. (C) 2002 Elsevier Science Ltd. All rights reserved.
Abstract: Local scaling and singularity properties of solar wind and geomagnetic time series were analysed using Holder exponents a. It was shown that in analysed cases due to the multifractality of fluctuations, a changes from point to point. We argued there exists a peculiar interplay between regularity/irregularity and amplitude characteristics of fluctuations which could be exploited for the improvement of predictions of geomagnetic activity. To this end, a layered back-propagation artificial neural network model with feedback connection was used for the study of the solar wind magnetosphere coupling and prediction of the geomagnetic D-st index. The solar wind input was taken from the principal component analysis of the interplanetary magnetic field, proton density and bulk velocity. Superior network performance was achieved in cases when the information on local Holder exponents was added to the input layer.
Abstract: Modern time series analysis methods are used to study the dynamical features of high- frequency flutuations present in daily sunspot number data. The results are discussed in terms of solar dynamo models incorporating turbulent gas motions within convective regions.
Abstract: A four-years-long minute-mean geomagnetic time-series recorded in a middle-latitude observatory was used to investigate the macroscopic dynamics of the magnetosphere-ionosphere complex system. Through probability density and structure function analyses it is shown that the investigated signal exhibits intermittent fluctuations in a certain temporal scale range. This property is regarded as being similar to the turbulent phenomena of magneto-hydro dynamical systems. In this work, discrete orthonormal wavelet transformation and filtering are carried out in order to identify and separate the intermittent parts of the signal from the homogeneous noise-like background. The empirical probability distributions of the laminar time between and the energy content of intermittent events are computed in the time-scale domain representation of the original signal provided by the wavelet transformation. It is shown that the results obtained are in contradiction with each other in the context of the phenomenology of the classical self-organized critical state model. We argue that 'near-SOC' or chaotic turbulence model can explain the observed features without ambiguity. (C) 2001 Elsevier Science Ltd. All rights reserved.
Abstract: In order to contribute to the understanding of solar wind-magnetosphere interactions the multifractal scaling properties of high-latitude geomagnetic fluctuations observed at the Thule observatory have been studied. Using the local observatory data and the present experimental knowledge only it seems hard to characterize directly the, presumably intermittent, mesoscale energy accumulation and dissipation processes taking place at the magnetotail, auroral region, etc. Instead a positive probability measure, describing the accumulated local geomagnetic signal energy content at the given time scales has been introduced and its scaling properties have been studied. There is evidence for the multifractal nature of the so defined intermittent field epsilon, a result obtained by using the recently introduced technique of large deviation multifractal spectra. This technique allows us to describe the geomagnetic fluctuations locally in time by means of singularity exponents alpha, which represent a generalization of the local degree of differentiability and characterize the power-law scaling dependence of the introduced measure on resolution. A global description of the geomagnetic fluctuations is insured by the spectrum of exponents f(alpha) which represents a rate function quantifying the deviations of the observed singularities alpha from the expected value. The results show that there exists a multifractal counterpart of the previously reported spectral break and different types of f(alpha) spectra describe the fluctuations in direct dissipation or loading-unloading regimes of the solar wind-magnetosphere interaction. On the time scale of substorms and storms the multifractal structure of the loading-unloading mode fluctuations seems to be analogous to the simple multiplicative P-model, while the f(alpha) spectra in direct dissipation regime are close but not equal to the features of a uniform distribution. Larger deviations from the multiplicative model are observed when the influence of the solar wind fluctuations is examined. On this basis it is expected that an extended multifractal analysis of the singularity structure of near-Earth plasma system fluctuations would lead to improved geomagnetic diagnosis of the magnetospheric dynamics.
Abstract: The notion of extended self-similarity (ESS) is applied here for the X - component time series of geomagnetic field fluctuations. Plotting nth order structure functions against the fourth order structure function we show that low-frequency geomagnetic fluctuations up to the order n = 10 follow the same scaling laws as MHD fluctuations in solar wind, however, for higher frequencies (f > 1/5[h]) a clear departure from the expected universality is observed for n > 6. ESS does not allow to make an unambiguous statement about the non triviality of scaling laws in "geomagnetic" turbulence. However, we suggest to use higher order moments as promising diagnostic tools for mapping the contributions of various remote magnetospheric sources to local observatory data.
Abstract: In order to study the nonlinear physical processes connected with substorm activity we analyse time series of local geomagnetic field variations. The concepts of deterministic chaos and magnetospheric chaotic attractors are examined. The general objective of this article is to detect low dimensional magnetosphere chaos and to properly interpret it as a consequence of magnetosphere - ionosphere informational - energetic coupling.
