Melvyn Goldstein - Academia.edu (original) (raw)

Papers by Melvyn Goldstein

The Astrophysical Journal, 2014

Pickup ions (PUIs) in the outer heliosphere and the local interstellar medium are created by char... more Pickup ions (PUIs) in the outer heliosphere and the local interstellar medium are created by charge exchange between protons and hydrogen (H) atoms, forming a thermodynamically dominant component. In the supersonic solar wind beyond >10 AU, in the inner heliosheath (IHS), and in the very local interstellar medium (VLISM), PUIs do not equilibrate collisionally with the background plasma. Using a collisionless form of Chapman-Enskog expansion, we derive a closed system of multi-fluid equations for a plasma comprised of thermal protons and electrons, and suprathermal PUIs. The PUIs contribute an isotropic scalar pressure to leading order, a collisionless heat flux at the next order, and a collisionless stress tensor at the second-order. The collisionless heat conduction and viscosity in the multi-fluid description results from a non-isotropic PUI distribution. A simpler one-fluid MHD-like system of equations with distinct equations of state for both the background plasma and the PUIs is derived. We investigate linear wave properties in a PUI-mediated three-fluid plasma model for parameters appropriate to the VLISM, the IHS, and the solar wind in the outer heliosphere. Five distinct wave modes are possible: Alfvén waves, thermal fast and slow magnetoacoustic waves, PUI fast and slow magnetoacoustic waves, and an entropy mode. The thermal and PUI acoustic modes propagate at approximately the combined thermal magnetoacoustic speed and the PUI sound speed respectively. All wave modes experience damping by the PUIs through the collisionless PUI heat flux. The PUI-mediated plasma model yields wave properties, including Alfvén waves, distinctly different from those of the standard two-fluid model.

Astrophysics and Space Science Proceedings

Alfvénic fluctuations are a ubiquitous component of the solar wind. Evidence from many spacecraft... more Alfvénic fluctuations are a ubiquitous component of the solar wind. Evidence from many spacecrafts indicate that the fluctuations are convected out of the solar corona with relatively flat power spectra and constitute a source of free energy for a turbulent cascade of magnetic and kinetic energy to high wave numbers. Observations and simulations support the conclusion that the cascade evolves most rapidly in the vicinity of velocity shears and current sheets. Numerical solutions of the magnetohydrodynamic equations have elucidated the role of expansion on the evolution of the turbulence. Such studies are clarifying not only how a turbulent cascade develops, but also the nature of the symmetries of the turbulence.

AIP Conference Proceedings, 2003

A global MHD model is developed to reproduce Ulysses observations during its fast latitude transi... more A global MHD model is developed to reproduce Ulysses observations during its fast latitude transition in 1994-1995. The governing polytropic single-fluid MHD equations are solved for a steady coronal outflow. The model includes Alfvén wave momentum and energy addition into open field regions. We combine a solution for a tilted dipole magnetic field in the inner computational region (1-20 R) with a three-dimensional solution in the outer region which extends to 1 AU. The inner region solution is essentially the same as in [1], but obtained with a different numerical algorithm and rotated to match the inclination inferred for the solar dipole from observations during the Ulysses transversal. The steady solution in the outer region is constructed by a marching-along-radius method and accounts for solar rotation. We show that the simulated variations of plasma and magnetic field parameters and in particular the extension of slow wind belt agree fairly well with the Ulysses observations.

The Astrophysical Journal, 2014

The formation of the observed core-halo feature in the solar wind electron velocity distribution ... more The formation of the observed core-halo feature in the solar wind electron velocity distribution function is a long-time puzzle. In this letter based on the current knowledge of nanoflares we show that the nanoflare-accelerated electron beams are likely to trigger a strong electron two-stream instability that generates kinetic Alfvén wave and whistler wave turbulence, as we demonstrated in a previous paper. We further show that the core-halo feature produced during the origin of kinetic turbulence is likely to originate in the inner corona and can be preserved as the solar wind escapes to space along open field lines. We formulate a set of equations to describe the heating processes observed in the simulation and show that the core-halo temperature ratio of the solar wind is insensitive to the initial conditions in the corona and is related to the core-halo density ratio of the solar wind and to the quasi-saturation property of the two-stream instability at the time when the exponential decay ends. This relation can be extended to the more general core-halo-strahl feature in the solar wind. The temperature ratio between the core and hot components is nearly independent of the heliospheric distance to the sun. We show that the core-halo relative drift previously reported is a relic of the fully saturated two stream instability. Our theoretical results are consistent with the observations while new tests for this model are provided. Subject headings: sun: corona-Acceleration of particles-Instabilities-Turbulence-Scattering-Solar wind 1. introduction Even with decades of extensive studies, our understanding is still poor as to what physical processes produce the non-Maxwellian electron velocity distribution functions (EVDFs) in the solar wind (Feldman et al. 1975; Pilipp et al. 1987a). Observations of EVDFs at heliocentric distances from 0.3 to 1 AU (Pilipp et al. 1987b) show a prominent "break" or a sudden change of slope at a kinetic energy of a few tens of electron volts, suggestive of two electron populations: the "core" that dominates below the break, and the "halo" that dominates at higher energies. In addition,

In this work we make use of the high angular, energy and time resolution and three-dimensional da... more In this work we make use of the high angular, energy and time resolution and three-dimensional data of the Cluster/PEACE electron spectrometer to identify and analyze the strahl component in the ambient solar wind since the launch of Cluster. The excursion of Cluster into the solar wind provides the opportunity to analyze this component of the electron velocity distribution function in a wide range of solar wind velocities and densities. The moment density and fluid velocity have been computed by spherical harmonic spectral model method. The analysis shows a correlation of the strahl density with the solar wind velocity and the time variation of the strahl density with solar cycle. This result agrees with the postulate that coronal holes are the source of this population. These preliminary results have been extended to include half solar cycle data (e.g., from 2001) to provide continuous results from solar maximum to minimum.

On March 18, 2002, the Cluster satellites traveled from the earth's northern mantle into the ... more 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, as well as evidence for waves accompanying reconnection at different length scales. We propose that the x-line is globally stable during Cluster's passage through the magnetopause, owing to the presence of a plasma depletion layer. Ion and electron flows in the magnetopause boundary layer are consistent with this and with the presence of reconnection in the conjugate hemisphere as well.

The Astrophysical Journal, 2014

Based on global conservation principles, magnetohydrodynamic (MHD) relaxation theory predicts the... more Based on global conservation principles, magnetohydrodynamic (MHD) relaxation theory predicts the existence of several equilibria, such as the Taylor state or global dynamic alignment. These states are generally viewed as very long-time and large-scale equilibria, which emerge only after the termination of the turbulent cascade. As suggested by hydrodynamics and by recent MHD numerical simulations, relaxation processes can occur during the turbulent cascade that will manifest themselves as local patches of equilibrium-like configurations. Using multi-spacecraft analysis techniques in conjunction with Cluster data, we compute the current density and flow vorticity and for the first time demonstrate that these localized relaxation events are observed in the solar wind. Such events have important consequences for the statistics of plasma turbulence.

The Ulysses mission provides an opportunity to study the cvohrtion of magnetohydrodynarnic (MH D)... more The Ulysses mission provides an opportunity to study the cvohrtion of magnetohydrodynarnic (MH D) turbulence in pure high-speed solar wind streams. The absence at high heliocentric latitudes of the strong shears in solar wind velocity generally present near the heliocentric current sheet allows investigation of how fluctuations in the magnetic field and plasma relax and CVOIVC in the radially expanding solar wind. We rcpor~ here results of an analysis of the radial and latitudinal varialion of the turbulence properties of the fluctuations, especially various plasma-field correlations, in high latitude regions. The results constrain current theories of the evolution of MFi D turbulence in the solar wind. Compared to similar observations at 0.3 AU by Hclios, wc find speetra that we similar in having a large frequency band with an ~-1 power spectrum in the outward traveling component of the waves, followed at higher frequencies (larger wave numbers) by a steeper spectrum. Ulysses obser...

On March 18, 2002, the Cluster satellites traveled from the earth's northern mantle into the magn... more 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.

The Astrophysical Journal, 1979

A theory of the to-independent decameter radiation is developed. The radiation results from excit... more A theory of the to-independent decameter radiation is developed. The radiation results from excitation of the electromagnetic loss-cone instability by keV electrons, stably trapped near L = b. The radiation is excited in Band III of the extraordinary mode. When the effects of refraction are estimated, it is shown that above 10 MHz radiation is beamed into the equatorial plane in a wide, but thin, conical sheet (* ^5 80°). When the instability analysis is coupled with one of the octupole models of the Jovian magnetic field, the maximum convective growth of the instability occurs in the directions of the non-Io A, B and C sources. The shape of the peak radio flux frequency spectrum is found to be a consequence of the loss cone shape of the electron distribution function.

Solar Physics, 1976

In situ satellite observations of type III burst exciters at 1 AU show that the beam does not evo... more In situ satellite observations of type III burst exciters at 1 AU show that the beam does not evolve into a plateau in velocity space, contrary to the prediction quasilinear theory. The observations can be explained by a theory that includes mode coupling effects due to excitation of the parametric oscillating two-stream instability and its saturation by anomalous resistivity. The time evolution of the beam velocity distribution is included in the analysis.

A detailed study of the magnetic field data from both Voyagers 1 and 2 has revealed several inter... more A detailed study of the magnetic field data from both Voyagers 1 and 2 has revealed several interesting properties of the near and distant Jovian magnetotail. During the first encounter, as Voyager 1 passed between 80 and 140 RJ from Jupiter in the near tail, the spacecraft was almost entirely in the northerm lobe magnetic field. The frequency spectrum of magnetic fluctuation in this region cannot be characterized by a power law and does not appear to be turbulent. The distant tail spectra from Voyager 2 are compared with similar spectra obtained from Voyager 1 when it was in near radial alignment with Voyager 2. Although the gross properties of the tail and solar wind fields in most respects differ considerably, the shape and power levels of the spectra of the magnetic fluctuations are very similar, especially between .0001 and .001 Hz. At lower frequencies (.00001 to .0001 Hz) the spectra of magnetic helicity do differ.

Physical Review Letters, 2013

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Physical Review Letters, 1970

The expansion of the solar wind is likely to cause low energy cosmic ray particles to lose a sign... more The expansion of the solar wind is likely to cause low energy cosmic ray particles to lose a significant fraction of their energy in the interplanetary medium. It is shown that because of this effect, most of the protons observed below-100 MeV and alpha particles, below-60 MeV/nucleon originate at higher

Physical Review Letters, 1970

We deduce the interstellar electron spectrum from the nonthermal radio background. From 200 MeV t... more We deduce the interstellar electron spectrum from the nonthermal radio background. From 200 MeV to a few GeV the spectral index is 1.8 and there is evidence for residual solar modulation. Above a.£ew GeV, the spectrum is steeper and the intensity is similar to that observed at earth. A consistent

Physical Review Letters, 2012

Recent observations of the solar wind have pointed out the existence of a cascade of magnetic ene... more Recent observations of the solar wind have pointed out the existence of a cascade of magnetic energy from the scale of the proton Larmor radius p down to the electron Larmor radius e scale. In this Letter we study the spatial properties of magnetic field fluctuations in the solar wind and find that at small scales the magnetic field does not resemble a sea of homogeneous fluctuations, but rather a two-dimensional plane containing thin current sheets and discontinuities with spatial sizes ranging from l * p down to e and below. These isolated structures may be manifestations of intermittency that localize sites of turbulent dissipation. Studying the relationship between turbulent dissipation, reconnection, and intermittency is crucial for understanding the dynamics of laboratory and astrophysical plasmas.

Physical Review Letters, 2005

We identify drift-kinetic Alfvén waves in the vicinity of a reconnection X line on the Earth's ma... more We identify drift-kinetic Alfvén waves in the vicinity of a reconnection X line on the Earth's magnetopause. The dispersive properties of these waves have been determined using wavelet interferometric techniques applied to multipoint observations from the Cluster spacecraft. Comparison of the observed wave dispersion with that expected for drift-kinetic Alfvén waves shows close agreement. The waves propagate outwards from the X line suggesting that reconnection is a kinetic Alfvén wave source. Energetic O ions observed in these waves indicate that reconnection is a driver of auroral ion outflow.

Physical Review Letters, 2012

The Earth's bow shock is a collisionless shock wave but entropy has never been directly measured ... more The Earth's bow shock is a collisionless shock wave but entropy has never been directly measured across it. The plasma experiments on Cluster and Double Star measure 3D plasma distributions upstream and downstream of the bow shock allowing calculation of Boltzmann's entropy function H and his famous H-theorem, dH/dt ≤ 0. Collisionless Boltzmann (Vlasov) equation predicts the total entropy does not change if the distribution function across the shock becomes non-thermal but it allows changes in the entropy density. Here we present the first direct measurements of entropy density changes across Earth's bow shock and show the results generally support the model of the Vlasov analysis. These observations are a starting point for a more sophisticated analysis that includes 3D computer modeling of collisionless shocks with input from observed particles, waves and turbulences.

Physical Review Letters, 2014

The observed ion-kinetic scale turbulence spectrum in the solar wind raises the question of how t... more The observed ion-kinetic scale turbulence spectrum in the solar wind raises the question of how that turbulence originates. Observations of keV energetic electrons during solar quiet-time suggest them as possible source of free energy to drive kinetic turbulence. Using particle-in-cell simulations, we explore how the free energy released by an electron two-stream instability drives Weibel-like electromagnetic waves that excite wave-wave interactions. Consequently, both kinetic Alfvénic and whistler turbulence are excited that evolve through inverse and forward magnetic energy cascades.

Physical Review Letters, 1982

We present a technique for the measurement of magnetic helicity from values of the two point magn... more We present a technique for the measurement of magnetic helicity from values of the two point magnetic field correlation, matrix under the assumption of spatial homogeneity. Knowledge of a single scalar function of space, derivable from the correlation matrix, suffices to determine the magnetic helicity. We illustrate the technique by reporting the first measurement of the magnetic helicity of the solar wind.

The Astrophysical Journal, 2014

Pickup ions (PUIs) in the outer heliosphere and the local interstellar medium are created by char... more Pickup ions (PUIs) in the outer heliosphere and the local interstellar medium are created by charge exchange between protons and hydrogen (H) atoms, forming a thermodynamically dominant component. In the supersonic solar wind beyond >10 AU, in the inner heliosheath (IHS), and in the very local interstellar medium (VLISM), PUIs do not equilibrate collisionally with the background plasma. Using a collisionless form of Chapman-Enskog expansion, we derive a closed system of multi-fluid equations for a plasma comprised of thermal protons and electrons, and suprathermal PUIs. The PUIs contribute an isotropic scalar pressure to leading order, a collisionless heat flux at the next order, and a collisionless stress tensor at the second-order. The collisionless heat conduction and viscosity in the multi-fluid description results from a non-isotropic PUI distribution. A simpler one-fluid MHD-like system of equations with distinct equations of state for both the background plasma and the PUIs is derived. We investigate linear wave properties in a PUI-mediated three-fluid plasma model for parameters appropriate to the VLISM, the IHS, and the solar wind in the outer heliosphere. Five distinct wave modes are possible: Alfvén waves, thermal fast and slow magnetoacoustic waves, PUI fast and slow magnetoacoustic waves, and an entropy mode. The thermal and PUI acoustic modes propagate at approximately the combined thermal magnetoacoustic speed and the PUI sound speed respectively. All wave modes experience damping by the PUIs through the collisionless PUI heat flux. The PUI-mediated plasma model yields wave properties, including Alfvén waves, distinctly different from those of the standard two-fluid model.

Astrophysics and Space Science Proceedings

Alfvénic fluctuations are a ubiquitous component of the solar wind. Evidence from many spacecraft... more Alfvénic fluctuations are a ubiquitous component of the solar wind. Evidence from many spacecrafts indicate that the fluctuations are convected out of the solar corona with relatively flat power spectra and constitute a source of free energy for a turbulent cascade of magnetic and kinetic energy to high wave numbers. Observations and simulations support the conclusion that the cascade evolves most rapidly in the vicinity of velocity shears and current sheets. Numerical solutions of the magnetohydrodynamic equations have elucidated the role of expansion on the evolution of the turbulence. Such studies are clarifying not only how a turbulent cascade develops, but also the nature of the symmetries of the turbulence.

AIP Conference Proceedings, 2003

A global MHD model is developed to reproduce Ulysses observations during its fast latitude transi... more A global MHD model is developed to reproduce Ulysses observations during its fast latitude transition in 1994-1995. The governing polytropic single-fluid MHD equations are solved for a steady coronal outflow. The model includes Alfvén wave momentum and energy addition into open field regions. We combine a solution for a tilted dipole magnetic field in the inner computational region (1-20 R) with a three-dimensional solution in the outer region which extends to 1 AU. The inner region solution is essentially the same as in [1], but obtained with a different numerical algorithm and rotated to match the inclination inferred for the solar dipole from observations during the Ulysses transversal. The steady solution in the outer region is constructed by a marching-along-radius method and accounts for solar rotation. We show that the simulated variations of plasma and magnetic field parameters and in particular the extension of slow wind belt agree fairly well with the Ulysses observations.

The Astrophysical Journal, 2014

The formation of the observed core-halo feature in the solar wind electron velocity distribution ... more The formation of the observed core-halo feature in the solar wind electron velocity distribution function is a long-time puzzle. In this letter based on the current knowledge of nanoflares we show that the nanoflare-accelerated electron beams are likely to trigger a strong electron two-stream instability that generates kinetic Alfvén wave and whistler wave turbulence, as we demonstrated in a previous paper. We further show that the core-halo feature produced during the origin of kinetic turbulence is likely to originate in the inner corona and can be preserved as the solar wind escapes to space along open field lines. We formulate a set of equations to describe the heating processes observed in the simulation and show that the core-halo temperature ratio of the solar wind is insensitive to the initial conditions in the corona and is related to the core-halo density ratio of the solar wind and to the quasi-saturation property of the two-stream instability at the time when the exponential decay ends. This relation can be extended to the more general core-halo-strahl feature in the solar wind. The temperature ratio between the core and hot components is nearly independent of the heliospheric distance to the sun. We show that the core-halo relative drift previously reported is a relic of the fully saturated two stream instability. Our theoretical results are consistent with the observations while new tests for this model are provided. Subject headings: sun: corona-Acceleration of particles-Instabilities-Turbulence-Scattering-Solar wind 1. introduction Even with decades of extensive studies, our understanding is still poor as to what physical processes produce the non-Maxwellian electron velocity distribution functions (EVDFs) in the solar wind (Feldman et al. 1975; Pilipp et al. 1987a). Observations of EVDFs at heliocentric distances from 0.3 to 1 AU (Pilipp et al. 1987b) show a prominent "break" or a sudden change of slope at a kinetic energy of a few tens of electron volts, suggestive of two electron populations: the "core" that dominates below the break, and the "halo" that dominates at higher energies. In addition,

In this work we make use of the high angular, energy and time resolution and three-dimensional da... more In this work we make use of the high angular, energy and time resolution and three-dimensional data of the Cluster/PEACE electron spectrometer to identify and analyze the strahl component in the ambient solar wind since the launch of Cluster. The excursion of Cluster into the solar wind provides the opportunity to analyze this component of the electron velocity distribution function in a wide range of solar wind velocities and densities. The moment density and fluid velocity have been computed by spherical harmonic spectral model method. The analysis shows a correlation of the strahl density with the solar wind velocity and the time variation of the strahl density with solar cycle. This result agrees with the postulate that coronal holes are the source of this population. These preliminary results have been extended to include half solar cycle data (e.g., from 2001) to provide continuous results from solar maximum to minimum.

On March 18, 2002, the Cluster satellites traveled from the earth's northern mantle into the ... more 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, as well as evidence for waves accompanying reconnection at different length scales. We propose that the x-line is globally stable during Cluster's passage through the magnetopause, owing to the presence of a plasma depletion layer. Ion and electron flows in the magnetopause boundary layer are consistent with this and with the presence of reconnection in the conjugate hemisphere as well.

The Astrophysical Journal, 2014

Based on global conservation principles, magnetohydrodynamic (MHD) relaxation theory predicts the... more Based on global conservation principles, magnetohydrodynamic (MHD) relaxation theory predicts the existence of several equilibria, such as the Taylor state or global dynamic alignment. These states are generally viewed as very long-time and large-scale equilibria, which emerge only after the termination of the turbulent cascade. As suggested by hydrodynamics and by recent MHD numerical simulations, relaxation processes can occur during the turbulent cascade that will manifest themselves as local patches of equilibrium-like configurations. Using multi-spacecraft analysis techniques in conjunction with Cluster data, we compute the current density and flow vorticity and for the first time demonstrate that these localized relaxation events are observed in the solar wind. Such events have important consequences for the statistics of plasma turbulence.

The Ulysses mission provides an opportunity to study the cvohrtion of magnetohydrodynarnic (MH D)... more The Ulysses mission provides an opportunity to study the cvohrtion of magnetohydrodynarnic (MH D) turbulence in pure high-speed solar wind streams. The absence at high heliocentric latitudes of the strong shears in solar wind velocity generally present near the heliocentric current sheet allows investigation of how fluctuations in the magnetic field and plasma relax and CVOIVC in the radially expanding solar wind. We rcpor~ here results of an analysis of the radial and latitudinal varialion of the turbulence properties of the fluctuations, especially various plasma-field correlations, in high latitude regions. The results constrain current theories of the evolution of MFi D turbulence in the solar wind. Compared to similar observations at 0.3 AU by Hclios, wc find speetra that we similar in having a large frequency band with an ~-1 power spectrum in the outward traveling component of the waves, followed at higher frequencies (larger wave numbers) by a steeper spectrum. Ulysses obser...

On March 18, 2002, the Cluster satellites traveled from the earth's northern mantle into the magn... more 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.

The Astrophysical Journal, 1979

A theory of the to-independent decameter radiation is developed. The radiation results from excit... more A theory of the to-independent decameter radiation is developed. The radiation results from excitation of the electromagnetic loss-cone instability by keV electrons, stably trapped near L = b. The radiation is excited in Band III of the extraordinary mode. When the effects of refraction are estimated, it is shown that above 10 MHz radiation is beamed into the equatorial plane in a wide, but thin, conical sheet (* ^5 80°). When the instability analysis is coupled with one of the octupole models of the Jovian magnetic field, the maximum convective growth of the instability occurs in the directions of the non-Io A, B and C sources. The shape of the peak radio flux frequency spectrum is found to be a consequence of the loss cone shape of the electron distribution function.

Solar Physics, 1976

In situ satellite observations of type III burst exciters at 1 AU show that the beam does not evo... more In situ satellite observations of type III burst exciters at 1 AU show that the beam does not evolve into a plateau in velocity space, contrary to the prediction quasilinear theory. The observations can be explained by a theory that includes mode coupling effects due to excitation of the parametric oscillating two-stream instability and its saturation by anomalous resistivity. The time evolution of the beam velocity distribution is included in the analysis.

A detailed study of the magnetic field data from both Voyagers 1 and 2 has revealed several inter... more A detailed study of the magnetic field data from both Voyagers 1 and 2 has revealed several interesting properties of the near and distant Jovian magnetotail. During the first encounter, as Voyager 1 passed between 80 and 140 RJ from Jupiter in the near tail, the spacecraft was almost entirely in the northerm lobe magnetic field. The frequency spectrum of magnetic fluctuation in this region cannot be characterized by a power law and does not appear to be turbulent. The distant tail spectra from Voyager 2 are compared with similar spectra obtained from Voyager 1 when it was in near radial alignment with Voyager 2. Although the gross properties of the tail and solar wind fields in most respects differ considerably, the shape and power levels of the spectra of the magnetic fluctuations are very similar, especially between .0001 and .001 Hz. At lower frequencies (.00001 to .0001 Hz) the spectra of magnetic helicity do differ.

Physical Review Letters, 2013

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Physical Review Letters, 1970

The expansion of the solar wind is likely to cause low energy cosmic ray particles to lose a sign... more The expansion of the solar wind is likely to cause low energy cosmic ray particles to lose a significant fraction of their energy in the interplanetary medium. It is shown that because of this effect, most of the protons observed below-100 MeV and alpha particles, below-60 MeV/nucleon originate at higher

Physical Review Letters, 1970

We deduce the interstellar electron spectrum from the nonthermal radio background. From 200 MeV t... more We deduce the interstellar electron spectrum from the nonthermal radio background. From 200 MeV to a few GeV the spectral index is 1.8 and there is evidence for residual solar modulation. Above a.£ew GeV, the spectrum is steeper and the intensity is similar to that observed at earth. A consistent

Physical Review Letters, 2012

Recent observations of the solar wind have pointed out the existence of a cascade of magnetic ene... more Recent observations of the solar wind have pointed out the existence of a cascade of magnetic energy from the scale of the proton Larmor radius p down to the electron Larmor radius e scale. In this Letter we study the spatial properties of magnetic field fluctuations in the solar wind and find that at small scales the magnetic field does not resemble a sea of homogeneous fluctuations, but rather a two-dimensional plane containing thin current sheets and discontinuities with spatial sizes ranging from l * p down to e and below. These isolated structures may be manifestations of intermittency that localize sites of turbulent dissipation. Studying the relationship between turbulent dissipation, reconnection, and intermittency is crucial for understanding the dynamics of laboratory and astrophysical plasmas.

Physical Review Letters, 2005

We identify drift-kinetic Alfvén waves in the vicinity of a reconnection X line on the Earth's ma... more We identify drift-kinetic Alfvén waves in the vicinity of a reconnection X line on the Earth's magnetopause. The dispersive properties of these waves have been determined using wavelet interferometric techniques applied to multipoint observations from the Cluster spacecraft. Comparison of the observed wave dispersion with that expected for drift-kinetic Alfvén waves shows close agreement. The waves propagate outwards from the X line suggesting that reconnection is a kinetic Alfvén wave source. Energetic O ions observed in these waves indicate that reconnection is a driver of auroral ion outflow.

Physical Review Letters, 2012

The Earth's bow shock is a collisionless shock wave but entropy has never been directly measured ... more The Earth's bow shock is a collisionless shock wave but entropy has never been directly measured across it. The plasma experiments on Cluster and Double Star measure 3D plasma distributions upstream and downstream of the bow shock allowing calculation of Boltzmann's entropy function H and his famous H-theorem, dH/dt ≤ 0. Collisionless Boltzmann (Vlasov) equation predicts the total entropy does not change if the distribution function across the shock becomes non-thermal but it allows changes in the entropy density. Here we present the first direct measurements of entropy density changes across Earth's bow shock and show the results generally support the model of the Vlasov analysis. These observations are a starting point for a more sophisticated analysis that includes 3D computer modeling of collisionless shocks with input from observed particles, waves and turbulences.

Physical Review Letters, 2014

The observed ion-kinetic scale turbulence spectrum in the solar wind raises the question of how t... more The observed ion-kinetic scale turbulence spectrum in the solar wind raises the question of how that turbulence originates. Observations of keV energetic electrons during solar quiet-time suggest them as possible source of free energy to drive kinetic turbulence. Using particle-in-cell simulations, we explore how the free energy released by an electron two-stream instability drives Weibel-like electromagnetic waves that excite wave-wave interactions. Consequently, both kinetic Alfvénic and whistler turbulence are excited that evolve through inverse and forward magnetic energy cascades.

Physical Review Letters, 1982

We present a technique for the measurement of magnetic helicity from values of the two point magn... more We present a technique for the measurement of magnetic helicity from values of the two point magnetic field correlation, matrix under the assumption of spatial homogeneity. Knowledge of a single scalar function of space, derivable from the correlation matrix, suffices to determine the magnetic helicity. We illustrate the technique by reporting the first measurement of the magnetic helicity of the solar wind.