Mark Wilber - Academia.edu (original) (raw)
Papers by Mark Wilber
We report on frequent observation by the CLUSTER CIS experiment of gyrating backstreaming ions di... more We report on frequent observation by the CLUSTER CIS experiment of gyrating backstreaming ions displaying gyrophase-bunching around the local magnetic field upstream of the Earth's bow shock. These ions are always associated with quasi-monochromatic low-frequency waves. The analysis of the 3D angular distributions indicates that field-aligned beam ions are observed at the onset of each event. Later on, the angular distribution is gyrophase-bunched and peaked at a nearly constant pitch-angle value during the interval of wave occurrence. The waves are left-handed (in the spacecraft frame) and propagate nearly along the ambient magnetic field; we have shown that they are fast mode waves in cyclotron resonance with (and likely generated by) the field-aligned beam observed just upstream. We solve the linear Maxwell-Vlasov dispersion relation with the observed parameters as input and confirm that the field-aligned beam drives an ion-ion instability at the observed frequency and wavenumber. From a statistical study, two types of gyrophase-bunched ion events can be observed. For the first type, the pitch-angle distribution is peaked at alpha0 ˜ thetaBN. This is consistent with specular reflection at the shock and the observed particle flux-modulation is interpreted in term of thetaBN changes at the shock due to the low frequency waves. For the second type, the gyrophase bunching is due to the nonlinear coherent disruption of the field-aligned beam by the low frequency waves through cyclotron resonant wave-particle interaction: the pitch-angle alpha0 of the ion distribution is explained by a wave-trapping process and is directly dependent on the amplitude of the waves. We discuss the impact of these results on the solar wind - magnetosphere coupling.
Frontiers in Ecology and the Environment, 2015
AIP Conference Proceedings, 2005
The foreshock region is the first signature of the interaction of the solar wind with a planet's ... more The foreshock region is the first signature of the interaction of the solar wind with a planet's plasma environment when approaching its collisionless bow shock. Part of its structure and dynamic is determined by instabilities, which are created by the interaction of the solar wind with backstreaming ion populations. The interaction of the reflected ions with the solar wind drives ion/ion beam instabilities, which generate waves that are then convected towards the shock by the solar wind. Subsequently they may mediate the shock structure and its reflection properties. The most well-know examples are the field aligned ion beams (FABs), produced by reflection processes in the quasi-perpendicular and oblique regions of the shock. Other prominent examples are the gyrating ions with well-defined pitch-angle and gyrophase organization around the local magnetic field observed downstream of the FABs region. These gyrophase-bunched ions are always associated with large amplitude quasi-monochromatic righthand mode low-frequency waves. Different mechanisms have been put forward to explain these ion features. This paper will discuss recent advances on this topic from multi-spacecraft observations (Cluster) as well as theoretical considerations.
Different types of backstreaming ion distributions have been reported in the region upstream from... more Different types of backstreaming ion distributions have been reported in the region upstream from the Earth's bow shock and magnetically connected to it (ion foreshock): field-aligned beams (FABs), gyrating ion and diffuse ion distributions. Contrary to the first type, the two others are always associated with ULF waves. Among them, gyrating ions with well-defined pitch-angle and gyrophase organization around the local magnetic field have been frequently observed in association with large amplitude quasi-monochromatic right-hand mode waves. These waves reveal the existence of coherent wave-particle interaction which is an efficient process to dissipate the energy of the particles reflected at the collisionless bow shock. It has been shown recently from a large data set from multi-spacecraft observations by Cluster that the gyrophase-bunched ion distributions are mainly produced by such a process from cyclotron-resonant FABs observed just both at the edge of the gyrating ions re...
When they cross the terrestrial magnetosheath (MS), the 4 Cluster spacecraft regularly observe ma... more When they cross the terrestrial magnetosheath (MS), the 4 Cluster spacecraft regularly observe magnetospheric (Msph) plasma incursions surrounded by a boundary layer (BL). This kind of region is known to be composed of an inner and an outer regions. Contrary to previous studies which distinguished these latter from the presence/absence of Msph particles, we show here that both of these regions contain not only an accelerated and/or heated MS plasma but also a diffusive high energy Msph population whose pitch angle distribution depends on the region of the BL crossed. Moreover, in the case presented here, a third population characterized by a lower energy, coming from the ionosphere, was also detected but on the inside edge of the Msph boundary. Its particular structure as a function of pitch angle suggests a heating mechanism different from the one existing in the boundary layer yet located nearby. From the electron, E and B fields and wave data, we show how the plasma is structured...
AIP Conference Proceedings, 2005
ABSTRACT For more than two decades the Earth’s bow shock and traveling interplanetary shocks have... more ABSTRACT For more than two decades the Earth’s bow shock and traveling interplanetary shocks have attracted much attention as researchers have attempted to understand the collisionless mechanisms that thermalize transmitted particles and accelerate those that are observed propagating away from the shock into the upstream. We are concerned here with the class of particles emerging from the shock that are field‐aligned and have energies of a few to several keV, and base our results on observations primarily from the Earth’s foreshock. While the basic empirical picture has been known for some time, fundamental questions about the underlying mechanisms producing them have resisted a comprehensive explanation. This review talk will begin with an overview of the observational framework, along with selected new results. The latter include recent refinements in the characterizations of upstream field‐aligned beams as a function of the shock geometry parameter θBn. Other observations from the Cluster spacecraft have shown the occurence of a very sharp boundary separating FABs and gyrating ion populations in the foreshock. The Wind spacecraft has seen FABs at distances in excess of ∼100 RE from the Earth, indicating lifetimes greater than expected from linear theory of the ion‐ion streaming instability. These observations prompt new questions. Some analytic calculations will be reviewed briefly. Models based upon the guiding center approximation and those which introduce diffusion as a means of enhancing the fluxes of upstream beams fail to produce the properties observed. © 2005 American Institute of Physics
Various nonlinear structures have been observed in the upstream region of Earth's bow shock. ... more Various nonlinear structures have been observed in the upstream region of Earth's bow shock. The structures often develop into shock-like structures, which may illustrate the early phase of collisionless shock development. In this study we investigate dynamics of particles associated with the development of the shock-like structures using Cluster multi-spacecraft measurements. Cluster has observed that compressional pulses formed at the upstream edge of density holes grow and steepen into shock-like structures. Examination of phase space distributions of ions shows that at the shock-like structures with amplified magnetic field solar wind ions are just compressed without significant thermalization, which is different from the dynamics of ions observed at Earth's bow shock. Electrons also behave differently from those observed at Earth's bow shock. While the phase space distribution of electrons at Earth's bow shock becomes isotropic, flat-top shape, only a field-alig...
It has been reported that sometimes the plasma sheet in the magnetotail consists of cold, dense i... more It has been reported that sometimes the plasma sheet in the magnetotail consists of cold, dense ions, especially during northward interplanetary magnetic field (IMF) periods. However, the mechanism explaining the formation of the cold, dense plasma sheet is not yet understood. In this study we will present multi-spacecraft observations showing transition of plasma sheet from cold, dense to hot, tenuous status in the near-Earth magnetotail. The transition tends to occur very rapidly, in less than 1 min. Before the transition, the ion density is typically ~1 cm-3, and the temperature ~1 keV. After the transition, the ion density drops to ~0.3 cm-3, while the temperature increases to ~2.5 keV. The transition boundary can be quite far from the flank magnetopause and low latitude boundary layer (LLBL). For example, transition can be observed in the vicinity of the central plasma sheet (|Bx| < 10 nT) at ~(-17.8, -7.06, 1.71 RE GSM). One interesting feature is that the transition is usu...
It has been reported that nonlinear steepening of waves and fluctuations frequently occurs in the... more It has been reported that nonlinear steepening of waves and fluctuations frequently occurs in the solar wind upstream of Earth's bow shock. The observation of nonlinear steepening provides important clues how shocks form in collisionless space plasmas. Previously, using Cluster multi-spacecraft measurements we reported that a compressional pulse formed at the edge of a density hole can steepen and develop into a shock-like structure. In that study we found that ion thermalization is insignificant during the nonlinear steepening and suggested that electrons may play an important role in the steepening. In this study we will focus on the interaction between electrons and waves. Investigation of electron phase space distributions shows that electron distributions in the downstream region of the steepened shock-like edge still have similar shape with the upstream solar wind distribution instead of becoming flat-top as observed in the downstream region of Earth's bow shock. On th...
We report on frequent observation by the CLUSTER CIS experiment of gyrating backstreaming ions di... more We report on frequent observation by the CLUSTER CIS experiment of gyrating backstreaming ions displaying gyrophase-bunching around the local magnetic field upstream of the Earth's bow shock. These ions are always associated with quasi-monochromatic low-frequency waves. The analysis of the 3D angular distributions indicates that field-aligned beam ions are observed at the onset of each event. Later on, the angular distribution is gyrophase-bunched and peaked at a nearly constant pitch-angle value during the interval of wave occurrence. The waves are left-handed (in the spacecraft frame) and propagate nearly along the ambient magnetic field; we have shown that they are fast mode waves in cyclotron resonance with (and likely generated by) the field-aligned beam observed just upstream. We solve the linear Maxwell-Vlasov dispersion relation with the observed parameters as input and confirm that the field-aligned beam drives an ion-ion instability at the observed frequency and wavenumber. From a statistical study, two types of gyrophase-bunched ion events can be observed. For the first type, the pitch-angle distribution is peaked at alpha0 ˜ thetaBN. This is consistent with specular reflection at the shock and the observed particle flux-modulation is interpreted in term of thetaBN changes at the shock due to the low frequency waves. For the second type, the gyrophase bunching is due to the nonlinear coherent disruption of the field-aligned beam by the low frequency waves through cyclotron resonant wave-particle interaction: the pitch-angle alpha0 of the ion distribution is explained by a wave-trapping process and is directly dependent on the amplitude of the waves. We discuss the impact of these results on the solar wind - magnetosphere coupling.
Frontiers in Ecology and the Environment, 2015
AIP Conference Proceedings, 2005
The foreshock region is the first signature of the interaction of the solar wind with a planet's ... more The foreshock region is the first signature of the interaction of the solar wind with a planet's plasma environment when approaching its collisionless bow shock. Part of its structure and dynamic is determined by instabilities, which are created by the interaction of the solar wind with backstreaming ion populations. The interaction of the reflected ions with the solar wind drives ion/ion beam instabilities, which generate waves that are then convected towards the shock by the solar wind. Subsequently they may mediate the shock structure and its reflection properties. The most well-know examples are the field aligned ion beams (FABs), produced by reflection processes in the quasi-perpendicular and oblique regions of the shock. Other prominent examples are the gyrating ions with well-defined pitch-angle and gyrophase organization around the local magnetic field observed downstream of the FABs region. These gyrophase-bunched ions are always associated with large amplitude quasi-monochromatic righthand mode low-frequency waves. Different mechanisms have been put forward to explain these ion features. This paper will discuss recent advances on this topic from multi-spacecraft observations (Cluster) as well as theoretical considerations.
Different types of backstreaming ion distributions have been reported in the region upstream from... more Different types of backstreaming ion distributions have been reported in the region upstream from the Earth's bow shock and magnetically connected to it (ion foreshock): field-aligned beams (FABs), gyrating ion and diffuse ion distributions. Contrary to the first type, the two others are always associated with ULF waves. Among them, gyrating ions with well-defined pitch-angle and gyrophase organization around the local magnetic field have been frequently observed in association with large amplitude quasi-monochromatic right-hand mode waves. These waves reveal the existence of coherent wave-particle interaction which is an efficient process to dissipate the energy of the particles reflected at the collisionless bow shock. It has been shown recently from a large data set from multi-spacecraft observations by Cluster that the gyrophase-bunched ion distributions are mainly produced by such a process from cyclotron-resonant FABs observed just both at the edge of the gyrating ions re...
When they cross the terrestrial magnetosheath (MS), the 4 Cluster spacecraft regularly observe ma... more When they cross the terrestrial magnetosheath (MS), the 4 Cluster spacecraft regularly observe magnetospheric (Msph) plasma incursions surrounded by a boundary layer (BL). This kind of region is known to be composed of an inner and an outer regions. Contrary to previous studies which distinguished these latter from the presence/absence of Msph particles, we show here that both of these regions contain not only an accelerated and/or heated MS plasma but also a diffusive high energy Msph population whose pitch angle distribution depends on the region of the BL crossed. Moreover, in the case presented here, a third population characterized by a lower energy, coming from the ionosphere, was also detected but on the inside edge of the Msph boundary. Its particular structure as a function of pitch angle suggests a heating mechanism different from the one existing in the boundary layer yet located nearby. From the electron, E and B fields and wave data, we show how the plasma is structured...
AIP Conference Proceedings, 2005
ABSTRACT For more than two decades the Earth’s bow shock and traveling interplanetary shocks have... more ABSTRACT For more than two decades the Earth’s bow shock and traveling interplanetary shocks have attracted much attention as researchers have attempted to understand the collisionless mechanisms that thermalize transmitted particles and accelerate those that are observed propagating away from the shock into the upstream. We are concerned here with the class of particles emerging from the shock that are field‐aligned and have energies of a few to several keV, and base our results on observations primarily from the Earth’s foreshock. While the basic empirical picture has been known for some time, fundamental questions about the underlying mechanisms producing them have resisted a comprehensive explanation. This review talk will begin with an overview of the observational framework, along with selected new results. The latter include recent refinements in the characterizations of upstream field‐aligned beams as a function of the shock geometry parameter θBn. Other observations from the Cluster spacecraft have shown the occurence of a very sharp boundary separating FABs and gyrating ion populations in the foreshock. The Wind spacecraft has seen FABs at distances in excess of ∼100 RE from the Earth, indicating lifetimes greater than expected from linear theory of the ion‐ion streaming instability. These observations prompt new questions. Some analytic calculations will be reviewed briefly. Models based upon the guiding center approximation and those which introduce diffusion as a means of enhancing the fluxes of upstream beams fail to produce the properties observed. © 2005 American Institute of Physics
Various nonlinear structures have been observed in the upstream region of Earth's bow shock. ... more Various nonlinear structures have been observed in the upstream region of Earth's bow shock. The structures often develop into shock-like structures, which may illustrate the early phase of collisionless shock development. In this study we investigate dynamics of particles associated with the development of the shock-like structures using Cluster multi-spacecraft measurements. Cluster has observed that compressional pulses formed at the upstream edge of density holes grow and steepen into shock-like structures. Examination of phase space distributions of ions shows that at the shock-like structures with amplified magnetic field solar wind ions are just compressed without significant thermalization, which is different from the dynamics of ions observed at Earth's bow shock. Electrons also behave differently from those observed at Earth's bow shock. While the phase space distribution of electrons at Earth's bow shock becomes isotropic, flat-top shape, only a field-alig...
It has been reported that sometimes the plasma sheet in the magnetotail consists of cold, dense i... more It has been reported that sometimes the plasma sheet in the magnetotail consists of cold, dense ions, especially during northward interplanetary magnetic field (IMF) periods. However, the mechanism explaining the formation of the cold, dense plasma sheet is not yet understood. In this study we will present multi-spacecraft observations showing transition of plasma sheet from cold, dense to hot, tenuous status in the near-Earth magnetotail. The transition tends to occur very rapidly, in less than 1 min. Before the transition, the ion density is typically ~1 cm-3, and the temperature ~1 keV. After the transition, the ion density drops to ~0.3 cm-3, while the temperature increases to ~2.5 keV. The transition boundary can be quite far from the flank magnetopause and low latitude boundary layer (LLBL). For example, transition can be observed in the vicinity of the central plasma sheet (|Bx| < 10 nT) at ~(-17.8, -7.06, 1.71 RE GSM). One interesting feature is that the transition is usu...
It has been reported that nonlinear steepening of waves and fluctuations frequently occurs in the... more It has been reported that nonlinear steepening of waves and fluctuations frequently occurs in the solar wind upstream of Earth's bow shock. The observation of nonlinear steepening provides important clues how shocks form in collisionless space plasmas. Previously, using Cluster multi-spacecraft measurements we reported that a compressional pulse formed at the edge of a density hole can steepen and develop into a shock-like structure. In that study we found that ion thermalization is insignificant during the nonlinear steepening and suggested that electrons may play an important role in the steepening. In this study we will focus on the interaction between electrons and waves. Investigation of electron phase space distributions shows that electron distributions in the downstream region of the steepened shock-like edge still have similar shape with the upstream solar wind distribution instead of becoming flat-top as observed in the downstream region of Earth's bow shock. On th...