Fulvia Pucci

Bio: Fulvia Pucci is an academic researcher from National Institutes of Natural Sciences, Japan. The author has contributed to research in topics: Magnetic reconnection & Current sheet. The author has an hindex of 17, co-authored 43 publications receiving 958 citations. Previous affiliations of Fulvia Pucci include Princeton University & University of Rome Tor Vergata.

Reconnection of Quasi-singular Current Sheets: The "Ideal" Tearing Mode

Abstract: A strong indication that fast reconnection regimes exist within resistive magnetohydrodynamics was given by the proof that the Sweet-Parker current sheet, maintained by a flow field with an appropriate inflow-outflow structure, could be unstable to a reconnecting instability which grows without bound as the Lundquist number, S, tends to infinity. The requirement of a minimum value for S in order for the plasmoid instability to kick in does little to resolve the paradoxical nature of the result. Here we argue against the realizability of Sweet-Parker current sheets in astrophysical plasmas with very large S by showing that an ''ideal'' tearing mode takes over before current sheets reach such a thickness. While the Sweet-Parker current sheet thickness scales as ∼S {sup –1/2}, the tearing mode becomes effectively ideal when a current sheet collapses to a thickness of the order of ∼S {sup –1/3}, up to 100 times thicker than S {sup –1/2}, when (as happens in many astrophysical environments) S is as large as 10{sup 12}. Such a sheet, while still diffusing over a very long time, is unstable to a tearing mode with multiple x-points: here we detail the characteristics of the instability and discuss how it may help solve themore » flare trigger problem and effectively initiate the turbulent disruption of the sheet.« less

Explosive Magnetotail Activity

Abstract: Modes and manifestations of the explosive activity in the Earth’s magnetotail, as well as its onset mechanisms and key pre-onset conditions are reviewed. Two mechanisms for the generation of the pre-onset current sheet are discussed, namely magnetic flux addition to the tail lobes, or other high-latitude perturbations, and magnetic flux evacuation from the near-Earth tail associated with dayside reconnection. Reconnection onset may require stretching and thinning of the sheet down to electron scales. It may also start in thicker sheets in regions with a tailward gradient of the equatorial magnetic field $B_{z}$ ; in this case it begins as an ideal-MHD instability followed by the generation of bursty bulk flows and dipolarization fronts. Indeed, remote sensing and global MHD modeling show the formation of tail regions with increased $B_{z}$ , prone to magnetic reconnection, ballooning/interchange and flapping instabilities. While interchange instability may also develop in such thicker sheets, it may grow more slowly compared to tearing and cause secondary reconnection locally in the dawn-dusk direction. Post-onset transients include bursty flows and dipolarization fronts, micro-instabilities of lower-hybrid-drift and whistler waves, as well as damped global flux tube oscillations in the near-Earth region. They convert the stretched tail magnetic field energy into bulk plasma acceleration and collisionless heating, excitation of a broad spectrum of plasma waves, and collisional dissipation in the ionosphere. Collisionless heating involves ion reflection from fronts, Fermi, betatron as well as other, non-adiabatic, mechanisms. Ionospheric manifestations of some of these magnetotail phenomena are discussed. Explosive plasma phenomena observed in the laboratory, the solar corona and solar wind are also discussed.

Resistive Magnetohydrodynamics Simulations of the Ideal Tearing Mode

Abstract: We study the linear and nonlinear evolution of the tearing instability on thin current sheets by means of two-dimensional numerical simulations, within the framework of compressible, resistive magnetohydrodynamics. In particular we analyze the behavior of current sheets whose inverse aspect ratio scales with the Lundquist number S as S 1=3 . This scaling has been recently recognized to yield the threshold separating fast, ideal reconnection, with an evolution and growth which are independent of S provided this is high enough, as it should be natural having the ideal case as a limit for S! 1. Our simulations conrm that the tearing instability growth rate can be as fast as 0:6 A 1 , where A is the ideal Alfv enic time set by the macroscopic scales, for our least diusive case with S = 10 7 . The expected instability dispersion relation and eigenmodes are also retrieved in the linear regime, for the values of S explored here. Moreover, in the nonlinear stage of the simulations we observe secondary events obeying the same critical scaling with S, here calculated on the local, much smaller lengths, leading to increasingly faster reconnection. These ndings strongly support the idea that in a fully dynamic regime, as soon as current sheets develop, thin and reach this critical threshold in their aspect ratio, the tearing mode is able to trigger plasmoid formation and reconnection on the local (ideal) Alfv enic timescales, as required to explain the explosive aring activity often observed in solar and astrophysical plasmas. Subject headings: plasmas { MHD { methods: numerical.

Magnetic reconnection: recursive current sheet collapse triggered by “ideal” tearing

Abstract: We study, by means of MHD simulations, the onset and evolution of fast reconnection via the "ideal" tearing mode within a collapsing current sheet at high Lundquist numbers (). We first confirm that as the collapse proceeds, fast reconnection is triggered well before a Sweet?Parker-type configuration can form: during the linear stage, plasmoids rapidly grow in a few Alfv?n times when the predicted "ideal" tearing threshold S?1/3 is approached from above; after the linear phase of the initial instability, X-points collapse and reform nonlinearly. We show that these give rise to a hierarchy of tearing events repeating faster and faster on current sheets at ever smaller scales, corresponding to the triggering of "ideal" tearing at the renormalized Lundquist number. In resistive MHD, this process should end with the formation of sub-critical (S???104) Sweet?Parker sheets at microscopic scales. We present a simple model describing the nonlinear recursive evolution that explains the timescale of the disruption of the initial sheet.

Magnetic reconnection: Self-similar current sheet collapse triggered by "ideal" tearing

Abstract: We study, by means of MHD simulations, the onset and evolution of fast reconnection via the "ideal" tearing mode within a collapsing current sheet at high Lundquist numbers ($S\gg10^4$). We first confirm that as the collapse proceeds, fast reconnection is triggered well before a Sweet-Parker type configuration can form: during the linear stage plasmoids rapidly grow in a few Alfv\'en times when the predicted "ideal" tearing threshold $S^{-1/3}$ is approached from above; after the linear phase of the initial instability, X-points collapse and reform nonlinearly. We show that these give rise to a hierarchy of tearing events repeating faster and faster on current sheets at ever smaller scales, corresponding to the triggering of "ideal" tearing at the renormalized Lundquist number. In resistive MHD this process should end with the formation of sub-critical ($S \leq10^4$) Sweet Parker sheets at microscopic scales. We present a simple model describing the nonlinear recursive evolution which explains the timescale of the disruption of the initial sheet.

Alfven Waves in the Solar Corona

Abstract: Alfven waves, transverse incompressible magnetic oscillations, have been proposed as a possible mechanism to heat the Sun's corona to millions of degrees by transporting convective energy from the photosphere into the diffuse corona. We report the detection of Alfven waves in intensity, line-of-sight velocity, and linear polarization images of the solar corona taken using the FeXIII 1074.7-nanometer coronal emission line with the Coronal Multi-Channel Polarimeter (CoMP) instrument at the National Solar Observatory, New Mexico. Ubiquitous upward propagating waves were seen, with phase speeds of 1 to 4 megameters per second and trajectories consistent with the direction of the magnetic field inferred from the linear polarization measurements. An estimate of the energy carried by the waves that we spatially resolved indicates that they are too weak to heat the solar corona; however, unresolved Alfven waves may carry sufficient energy.

北京大学濮祖荫教授被推选为国际著名期刊Journal of Geophysical Research——Space Physics的主编

Abstract: 北京大学地球与空间科学学院濮祖荫教授被美国地球物理学会任命为Journal of Geophysical Research（Space Physics）的新一届亚洲与太平洋区域主编，任期为4年（2006-2009）．

Asymmetric Magnetic Reconnection: General Theory and Collisional Simulations

Abstract: A Sweet-Parker-type scaling analysis for asymmetric antiparallel reconnection (in which the reconnecting magnetic field strengths and plasma densities are different on opposite sides of the dissipation region) is performed. Scaling laws for the reconnection rate, outflow speed, the density of the outflow, and the structure of the dissipation region are derived from first principles. These results are independent of the dissipation mechanism. It is shown that a generic feature of asymmetric reconnection is that the X-line and stagnation point are not colocated, leading to a bulk flow of plasma across the X-line. The scaling laws are verified using two-dimensional resistive magnetohydrodynamics numerical simulations for the special case of asymmetric magnetic fields with symmetric density. Observational signatures and applications to reconnection in the magnetosphere are discussed.