Browsing by Author "Goldstein, Melvyn"

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    A mechanism for electrostatic solitary structures in the Earth's magnetosheath
    (AGU, 2009-09-24) Lakhina, G. S.; Singh, S. V.; Kakad, A. P.; Goldstein, Melvyn; Viñas,A. F.; Pickett, J. S.
    Electrostatic solitary waves (ESWs) have been observed in the Earth's magnetosheath region by Cluster. A mechanism for the generation of these structures in terms of electron-acoustic solitons and double layers is discussed. The model simulates the magnetosheath plasma by a four-component plasma system consisting of core electrons, two counterstreaming electron beams, and one type of ions. The analysis is based on the fluid equations and the Poisson equation, and employs the Sagdeev pseudopotential techniques to investigate the solitary waves. The electric field amplitudes, the time durations, and the propagation speeds of the solitary structures predicted by the model are in good agreement with the observed electric fields, pulse widths, and speeds of the electrostatic bipolar pulses.
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    Acceleration of charged particles in magnetic reconnection: Solar flares, the magnetosphere, and solar wind
    (AGU, 1986-03) Goldstein, Melvyn; Matthaeus, W. H.; Ambrosiano, J. J.
    A possible source of free energy available for accelerating charged particles is conversion of magnetic energy to particle energy in reconnecting magnetic fields. Recent simulations using test particles suggests that reconnection may efficiently accelerate particles to the maximum energies that are observed in several astrophysical contexts. A simple analytic formula is used in conjunction with the simulation results to predict the maximum energy achievable in a particular plasma environment with the result that in solar flares reconnection is capable of accelerating particles to several GeV. In magnetospheric substorms the predicted maximum can reach several hundred keV, and near magnetic sector crossings in the solar wind the maximum energy can approach 100 keV.
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    Adiabatic acceleration of suprathermal electrons associated with dipolarization fronts
    (AGU, 2012-12-19) Pan, Qingjiang; Ashour-Abdalla, Maha; El-Alaoui, Mostafa; Walker, Raymond J.; Goldstein, Melvyn
    Recent observations in the inner magnetotail have shown rapid and significant flux increases (usually an order of magnitude of increase within seconds) of suprathermal electrons (tens of keV to hundreds of keV) associated with earthward moving dipolarization fronts. To explain where and how these suprathermal electrons are produced during substorm intervals, two types of acceleration models have been suggested by previous studies: acceleration that localizes near the reconnection site and acceleration that occurs during earthward transport. We perform an analytical analysis of adiabatic acceleration to show that the slope of source differential fluxes is critical for understanding adiabatic flux enhancements during earthward transport. Observationally, two earthward propagating dipolarization fronts accompanied by energetic electron flux enhancements observed by the THEMIS spacecraft have been analyzed; in each event the properties of dipolarization fronts in the inner magnetosphere (XGₛₘ ≈ −10RE) were well correlated with those further down the tail (XGₛₘ ≈ −15RE or XGₛₘ ≈ −20RE). Coupled with theoretical analysis, this enables us to estimate the relative acceleration that occurred as the electrons propagated earthward between the two spacecraft. During the two events studied, the differential fluxes of supra thermal electrons had steep energy spectra with power law indices of −4 to −6.These spectra were much steeper than those at lower energy, as well as those of the supra thermal electrons observed before the fronts arrived. A compression factor of 1.5 as the electrons propagated earthward induced a flux increase of suprathermal electrons by a factor of 7 to 17. Provided these steep spectra, we demonstrate that adiabatic acceleration from the betatron and Fermi mechanisms simultaneously operating can account for these flux increases. Since both analytical analysis and data from the two events show that adiabatic acceleration during earthward transport does not significantly change the power law indices, the steep spectra were likely to be traced back to their source region, presumably near the reconnection site.
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    Alfvén wave phase mixing driven by velocity shear in two-dimensional open magnetic configurations
    (AGU, 1999-08-01) Ruderman, M. S.; Goldstein, Melvyn; Roberts, D. A.; Deane, A.; Ofman, L.
    Phase mixing of torsional Alfvén waves in axisymmetric equilibrium magnetic configurations with purely poloidal magnetic field and stationary flow along the field lines in resistive viscous plasmas is studied. The characteristic wavelength along the magnetic field lines is assumed to be much smaller than the characteristic scale of inhomogeneity in the magnetic field direction, and the WKB method is used to obtain an analytic solution describing phase mixing. The general solution is applied to a particular configuration with the radial magnetic field and flow under the assumptions that the magnetic field and density are independent of the polar angle in the spherical coordinates and the flow velocity is independent of the radial coordinate. The only source of phase mixing in this configuration is velocity shear. The analytical solution is compared with a numerical simulation of the fully nonlinear resistive MHD equations. The numerical and analytical results are in good agreement. Consequences for wave energy deposition into the solar corona and solar wind and for the evolution of the Alfvén wave energy spectrum are discussed.
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    Alfvénic Turbulence Simulation in a Realistic Solar Wind
    (Astronomical Society of the Pacific, 2010) Usmanov, A. V.; Goldstein, Melvyn
    We present initial results from a new numerical model to simulate magnetohydrodynamic (MHD) turbulence in the solar wind above the Alfvénic critical point. Previously, we had defined a “virtual” heliosphere that contained a tilted rotating current sheet, microstreams, as well as Alfvén waves (Goldstein et al. 1999a). In this new restructured approach, we use the global, time-stationary, WKB Alfvén wave-driven solar wind model (Usmanov & Goldstein 2003a) to define the initial state of the system. Consequently, current sheets, and fast and slow streams are computed self-consistently from an inner photospheric boundary. To this steady-state configuration, we add fluctuations close to, but above, the surface where the flow becomes super-Alfvénic. The time-dependent MHD equations are then solved using a semi-discrete third-order Central Weighted Essentially Non-Oscillatory (CWENO) numerical scheme in the frame of reference corotating with the Sun. The computational domain now includes the entire sphere; the geometrical singularity at the poles is removed using the multiple grid approach described in Usmanov (1996). Wave packets are introduced at the inner boundary such as to satisfy Faraday’s Law (Yeh & Dryer 1985) and their nonlinear evolution is followed in time.
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    A beaming model of the Io-independent Jovian decameter radiation based on multipole models of the Jovian magnetic field
    (AAS, 1979-05) Goldstein, Melvyn; Eviatar, A.; Thieman, J. R.
    A geometrical model is presented in which the apparent source locations of the Io-independent decameter radiation are computed. The calculations assume that the radiation is produced by stably trapped electrons radiating near the local electron gyrofrequency and that the emission is then beamed onto a conical surface. The maximum occurrence probability of noise storms is associated with regions in the Jovian magnetosphere where the axis of the emission cone is most inclined toward the Jovian equatorial plane. The calculations utilize and compare two of the octopole spherical harmonic expansions of the Jovian magnetic field constructed from data accumulated by the fluxgate and vector helium magnetometers on board Pioneer 11.
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    Characteristics of the Taylor microscale in the solar wind/foreshock: magnetic field and electron velocity measurements
    (EGU, 2013-11-22) Gurgiolo, C.; Goldstein, Melvyn; Matthaeus, W. H.; Viñas, A.; Fazakerley, A. N.
    The Taylor microscale is one of the fundamental turbulence scales. Not easily estimated in the interplanetary medium employing single spacecraft data, it has generally been studied through two point correlations. In this paper we present an alternative, albeit mathematically equivalent, method for estimating the Taylor microscale (λT). We make two independent determinations employing multi-spacecraft data sets from the Cluster mission, one using magnetic field data and a second using electron velocity data. Our results using the magnetic field data set yields a scale length of 1538 ± 550 km, slightly less than, but within the same range as, values found in previous magnetic-field-based studies. During time periods where both magnetic field and electron velocity data can be used, the two values can be compared. Relative comparisons show λT computed from the velocity is often significantly smaller than that from the magnetic field data. Due to a lack of events where both measurements are available, the absolute λT based on the electron fluid velocity is not able to be determined.
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    Cluster After 20 Years of Operations: Science Highlights and Technical Challenges
    (AGU, 2021-07-20) Escoubet, C. P.; Masson, A.; Laakso, H.; Goldstein, Melvyn; Dimbylow, T.; Bogdanova, Y. V.; Hapgood, M.; Sousa, B.; Sieg, D.; Taylor, M. G. G. T.
    The Cluster mission was the first constellation using four identical spacecraft to study Sun-Earth connection plasma processes. Using four spacecraft in a tetrahedron shape, it could measure, for the first time, 3D quantities such as electrical currents, plasma gradients or divergence of the electron pressure tensor and 3D structures such as boundaries, surface waves or vortices. Launched in pairs in July and August 2000, on two Soyuz rockets from Baikonur, the four spacecraft have been collecting data continuously for more than 20 years. The mission faced many challenges during the years of operations as some spacecraft subsystems had a lifetime of a few years beyond the initial two-year mission. The major one was to operate without functioning batteries and to successfully pass short and long eclipses, up to 3 h long, without damaging the on-board computers and transmitters and without freezing the fuel. More than 1,000 eclipses have been successfully passed since 2010 using a specially made procedure which switches off the complete spacecraft before entering into eclipse and switches it on when the Sun is again illuminating the solar panels. During 20 years, many discoveries and science results have been published in more than 2,700 scientific papers. A few highlights are presented here, focusing on how varying the spacecraft separation was essential to achieve the science goals of the mission. The Cluster Science Data System and the Cluster archive allows public access to all science data as well as spacecraft ancillary data.
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    Cluster electric current density measurements within a magnetic flux rope in the plasma sheet
    (AGU, 2003-04-02) Slavin, J. A.; Lepping, R. P.; Gjerloev, J.; Goldstein, Melvyn; Fairfield, D. H.; Acuna, M. H.; Balogh, A.; Dunlop, M.; Kivelson, M. G.; Khurana, K.; Fazakerley, A.; Owen, C. J.; Reme, H.; Bosqued, J. M.
    [1] On August 22, 2001 all 4 Cluster spacecraft nearly simultaneously penetrated a magnetic flux rope in the tail. The flux rope encounter took place in the central plasma sheet, βᵢ ∼ 1–2, near the leading edge of a bursty bulk flow. The “time-of-flight” of the flux rope across the 4 spacecraft yielded Vₓ ∼ 700 km/s and a diameter of ∼1 Rₑ. The speed at which the flux rope moved over the spacecraft is in close agreement with the Cluster plasma measurements. The magnetic field profiles measured at each spacecraft were first modeled separately using the Lepping-Burlaga force-free flux rope model. The results indicated that the center of the flux rope passed northward (above) s/c 3, but southward (below) of s/c 1, 2 and 4. The peak electric currents along the central axis of the flux rope predicted by these single-s/c models were ∼15–19 nA/m². The 4-spacecraft Cluster magnetic field measurements provide a second means to determine the electric current density without any assumption regarding flux rope structure. The current profile determined using the curlometer technique was qualitatively similar to those determined by modeling the individual spacecraft magnetic field observations and yielded a peak current density of 17 nA/m² near the central axis of the rope. However, the curlometer results also showed that the flux rope was not force-free with the component of the current density perpendicular to the magnetic field exceeding the parallel component over the forward half of the rope, perhaps due to the pressure gradients generated by the collision of the BBF with the inner magnetosphere. Hence, while the single-spacecraft models are very successful in fitting flux rope magnetic field and current variations, they do not provide a stringent test of the force-free condition.
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    Cluster four spacecraft measurements of small traveling compression regions in the near-tail
    (AGU, 2003-12-09) Slavin, J. A.; Owen, C. J.; Dunlop, M. W.; Borälv, E.; Moldwin, M. B.; Sibeck, D. G.; Tanskanen, E.; Goldstein, Melvyn; Fazakerley, A.; Balogh, A.; Lucek, E.; Richter, I.; Reme, H.; Bosqued, J. M.
    Cluster observations taken during a substorm on September 19, 2001 have revealed the presence of small traveling compression regions (TCRs) in the near tail. These measurements are used to determine directly the speed and direction of TCR propagation and the amplitude of the underlying bulge in the plasma sheet. The time-of-flight speeds derived from the arrival times of the magnetic perturbations at the different Cluster s/c yielded a mean speed of 413 km/s. For 2 of the TCRs s/c 1, 2 and 4 were located sufficiently close to the plasma sheet that they were immersed in the central plasma sheet plasma as the TCR swept over s/c 3. In this manner the Cluster measurements directly demonstrated that these small TCRs are caused by moving bulges in the plasma sheet-lobe interface. In summary, our analysis of the Cluster measurements has directly demonstrated the existence of moving bulges in the north-south thickness of the plasma sheet, most probably due to the formation of flux ropes, and their role in producing traveling compression regions.
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    The Cluster mission
    (EGU, 2001-09-30) Escoubet, C. P.; Fehringer, M.; Goldstein, Melvyn
    The Cluster mission, ESA’s first cornerstone project, together with the SOHO mission, dating back to the first proposals in 1982, was finally launched in the summer of 2000. On 16 July and 9 August, respectively, two Russian Soyuz rockets blasted off from the Russian cosmodrome in Baikonour to deliver two Cluster spacecraft, each into their proper orbit. By the end of August 2000, the four Cluster satellites had reached their final tetrahedral constellation. The commissioning of 44 instruments, both individually and as an ensemble of complementary tools, was completed five months later to ensure the optimal use of their combined observational potential. On 1 February 2001, the mission was declared operational.
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    Cluster observation of continuous reconnection at dayside magnetopause in the vicinity of cusp
    (EGU, 2005-09-15) Zheng, Y.; Le, G.; Slavin, J. A.; Goldstein, Melvyn; Cattell, C.; Balogh, A.; Lucek, E. A.; Rème, H.; Eastwood, J. P.; Wilber, M.; Parks, G.; Retinò, A.; Fazakerley, A.
    In this paper, we present a case study of continuous reconnection at the dayside magnetopause observed by the Cluster spacecraft. On 1 April 2003, the four Cluster spacecraft experienced multiple encounters with the Earth's dayside magnetopause under a fairly stable southwestward interplanetary magnetic field (IMF). Accelerated plasma flows, whose magnitude and direction are consistent with the predictions of the reconnection theory (the Walén relation), were observed at and around the magnetopause current layer for a prolonged interval of ~3 h at two types of magnetopause crossings, one with small magnetic shears and the other one with large magnetic shears. Reversals in the Y component of ion bulk flow between the magnetosheath and magnetopause current layer and acceleration of magnetosheath electrons were also observed. Kinetic signatures using electron and ion velocity distributions corroborate the interpretation of continuous magnetic reconnection. This event provides strong in-situ evidence that magnetic reconnection at the dayside magnetopause can be continuous for many hours. However, the reconnection process appeared to be very dynamic rather than steady, despite the steady nature of the IMF. Detailed analysis using multi-spacecraft magnetic field and plasma measurements shows that the dynamics and structure of the magnetopause current layer/boundary can be very complex. For example, highly variable magnetic and electric fields were observed in the magnetopause current layer. Minimum variance analysis shows that the magnetopause normal deviates from the model normal. Surface waves resulting from the reconnection process may be involved in the oscillation of the magnetopause.
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    Cluster Observation of Magnetic Structure and Electron Flows at a Northward Interplanetary Magnetic Field X-Line
    (European Space Agency, 2006-01) Wendel, D. E.; Reiff, P. H.; Han, T. H.; Goldstein, Melvyn; Lucek, E.; Fazakerley, A.
    On March 18, 2002, the Cluster satellites traveled from the earth’s northern mantle into the magnetosheath. During this time, the IMAGE spacecraft observed a long-lived proton emission northward of the auroral zone. The Cluster electron and magnetic field data suggest Cluster passed within 1 km of an active reconnection line, entering the ion diffusion region and the edge of the electron diffusion region. We present the current structure, velocity, orientation, and size of the reconnection line. The functional fit to the data also gives an estimate of 100 km for the thickness of the current sheet. We propose that the x-line, though wavering over the spacecraft, is globally stable during Cluster's passage through the magnetopause.
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    Cluster observations of electron holes in association with magnetotail reconnection and comparison to simulations
    (AGU, 2005-01-19) Cattell, C.; Dombeck, J.; Wygant, J.; Drake, J. F.; Swisdak, M.; Goldstein, Melvyn; Keith, W.; Fazakerley, A.; André, M.; Lucek, E.; Balogh, A.
    Large-amplitude (up to ∼50 mV/m) solitary waves, identified as electron holes, have been observed during waveform captures on two of the four Cluster satellites during several plasma sheet encounters that have been identified as the passage of a magnetotail reconnection x line. The electron holes were seen near the outer edge of the plasma sheet, within and on the edge of a density cavity, at distances on the order of a few ion inertial lengths from the center of the current sheet. The electron holes occur during intervals when there were narrow electron beams but not when the distributions were more isotropic or contained beams that were broad in pitch angle. The region containing the narrow beams (and therefore the electron holes) can extend over thousands of kilometers in the x and y directions, but is very narrow in the z direction. The association with electron beams and the density cavity and the location along the separatrices are consistent with simulations shown herein. The velocities and scale sizes of the electron holes are consistent with the predictions of Drake et al. [2003]. Particle simulations of magnetic reconnection reproduce the observed Cluster data only with the addition of a small (0.2 of the reversed field) ambient guide field. The results suggest that electron holes may sometimes be an intrinsic feature of magnetotail reconnection and that in such cases the traditional neglect of the guide field may not be justified. Very large amplitude lower hybrid waves (hundreds of millivolts per meter), as well as waves at frequencies up to the electron plasma frequency, were also observed during this interval.
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    Cluster observations of multiple dipolarization fronts
    (AGU, 2011-04-13) Hwang, Kyoung-Joo; Goldstein, Melvyn; Lee, E.; Pickett, J. S.
    We present Cluster observations of a series of dipolarization fronts (DF 1 to 6) at the central current sheet in Earth's magnetotail. The velocities of fast earthward flow following behind each DF 1–3 are comparable to the Alfvén velocity, indicating that the flow bursts might have been generated by bursty reconnection that occurred tailward of the spacecraft. Based on multispacecraft timing analysis, DF normals are found to propagate mainly earthward at 160–335 km/s with a thickness of 900–1500 km, which corresponds to the ion inertial length or gyroradius scale. Each DF is followed by significant fluctuations in the x and y components of the magnetic field whose peaks are found 1–2 min after the DF passage. These (Bₓ, Bᵧ) fluctuations propagate dawnward (mainly) and earthward. Strongly enhanced field-aligned beams are observed coincidently with (Bₓ, Bᵧ) fluctuations, while an enhancement of cross-tail currents is associated with the DFs. From the observed pressure imbalance and flux tube entropy changes between the two regions separated by the DF, we speculate that interchange instability destabilizes the DFs and causes the deformation of the midtail magnetic topology. This process generates significant field-aligned currents and might power the auroral brightening in the ionosphere. However, this event is associated with neither the main substorm auroral breakup nor the poleward expansion, which might indicate that the observed multiple DFs have been dissipated before they reach the inner plasma sheet boundary.
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    CLUSTER – SCIENCE AND MISSION OVERVIEW
    (Springer, 1997-01) Escoubet, C.P.; Schmidt, R.; Goldstein, Melvyn
    The European Space Agency's Cluster programme is designed to study the small-scale spatial and temporal characteristics of the magnetospheric and near-Earth solar wind plasma. The programme is composed of four identical spacecraft which will be able to make physical measurements in three dimensions. The relative distance between the four spacecraft will be varied between 200 and 18000 km during the course of the mission. This paper provides a general overview of the scientific objectives, the configuration and the orbit of the four spacecraft and the relation of Cluster to other missions.
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    Cluster: New Measurements of Plasma Structures in 3D
    (Springer, 2005) Escoubet, C. P.; Laakso, H.; Goldstein, Melvyn
    After 2.5 years of operations, the Cluster mission is fulfilling successfully its scientific objectives. The mission, nominally for 2 years, has been extended 3 more years, up to December 2005. The main goal of the Cluster mission is to study in three dimensions the small-scale plasma structures in the key plasma regions in the Earth’s environment: solar wind and bow shock, magnetopause, polar cusps, magnetotail, and auroral zone. During the course of the mission, the relative distance between the four spacecraft will vary from 100 km up to a maximum of 18,000 km to study the physical processes occurring in the magnetosphere and its environment at different scales. The inter-satellites distances achieved so far are 600, 2000, 100, 5000 km and recently 250 km. The latest results, which include the derivation of electric currents and magnetic curvature, the analysis of surface waves, and the observation of reconnection in the tail and in the cusp will be presented. We will also present the description of the access to data through the Cluster science data system and several public web servers, and the future plans for a Cluster archive.
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    Clustering of Intermittent Magnetic and Flow Structures near Parker Solar Probe's First Perihelion—A Partial-variance-of-increments Analysis
    (AAS, 2020-02-03) Chhiber, Rohit; Goldstein, Melvyn; Maruca, B. A.; Chasapis, A.; et al
    During the Parker Solar Probe's (PSP) first perihelion pass, the spacecraft reached within a heliocentric distance of ∼37 R⊙ and observed numerous magnetic and flow structures characterized by sharp gradients. To better understand these intermittent structures in the young solar wind, an important property to examine is their degree of correlation in time and space. To this end, we use the well-tested partial variance of increments (PVI) technique to identify intermittent events in FIELDS and SWEAP observations of magnetic and proton-velocity fields (respectively) during PSP's first solar encounter, when the spacecraft was within 0.25 au from the Sun. We then examine distributions of waiting times (WT) between events with varying separation and PVI thresholds. We find power-law distributions for WT shorter than a characteristic scale comparable to the correlation time of the fluctuations, suggesting a high degree of correlation that may originate in a clustering process. WT longer than this characteristic time are better described by an exponential, suggesting a random memory-less Poisson process at play. These findings are consistent with near-Earth observations of solar wind turbulence. The present study complements the one by Dudok de Wit et al., which focuses on WT between observed "switchbacks" in the radial magnetic field.
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    A comparative study of four-field and fully compressible magnetohydrodynamic turbulence in the solar wind
    (AGU, 1999-11-01) Bhattacharjee, A.; Ng, C. S.; Ghosh, S.; Goldstein, Melvyn
    A four-field system of equations has been recently derived from the compressible magnetohydrodynamic (MHD) equations to describe turbulence in the solar wind. These equations apply to a plasma permeated by a spatially varying mean magnetic field when the plasma beta is of the order unity or less. In the presence of spatial inhomogeneities, the four-field equations predict pressure fluctuations of the order of the Mach number of the turbulence, as observed by Helios 1 and 2. It is shown that inhomogeneities that occur only in the form of pressure-balanced structures cannot account for density fluctuations as large as the Mach number of the turbulence. Numerical predictions from the four-field and compressible MHD equations are compared using the same initial condition. The comparison shows that the predictions of the four-field model are qualitatively consistent with the predictions of the compressible MHD model and suggests that the four-field model is a viable model of compressible turbulence in the solar wind.