Team 81

The effect of ULF turbulence and flow chaotization

on plasma energy and mass transfer at the magnetopause

Abstract

One of the most important unresolved problems in the dilute collisionless plasmas almost everywhere in the universe is energy and mass transfer across boundaries. This is frequently attributed to magnetic reconnection. It is known by now that reconnection proceeds on "small" scales. It has, however, not been clarified yet, what these scales are, how they are generated and what is their precise role in the physics of energy and mass transfer. Ion scales, such as the ion inertial and gyroradius lengths are expected to strongly influence the conditions for reconnection, in particular its non stationary and localized character. Reconnection proceeds in sheared magnetic field current layers. The transverse scales of these current layers are of the order of the ion lengths. The parallel electric field in reconnection evolves on electron scales such as the electron inertial scale, gyroradius or Debye length. These scales are probably vital for reconnection. When a resistive scale can be defined it should play an important role as well. The historical models of reconnection refer to particular geometries, typically "X points", supposed to remain static or quasi-static.

The required short scales in reconnection can be provided by electromagnetic and/or electrostatic non-stationary fluctuations ("waves" in a broader sense). These fluctuations are the object of this study. The origin of small-scale fluctuations remains to be elucidated: Are they caused by local instabilities at the smallest scales, i.e. the electron inertial length, gyro radii, or do they result from a cascade down from large scale (possibly non local) ULF waves to smaller scales? In fact one may assume that most of the free energy is injected at the scales of the latter since high amplitude fluctuations are indeed observed in this range. A new phenomenon to address, which can substantially affect the small-scale generation is the filamentation and collapse of Alfvén waves. This had been predicted theoretically but so far not convincingly identified in the data and hardly in numerical models. Since Alfvén waves are a common feature also in astrophysical plasmas identification of their effects near the magnetopause has far reaching importance. This way of converting energy from ULF waves to particles can for instance be suspected to be a basic mechanism for explaining Jupiter auroras from the Io Alfvén wing, and emission of jets from instabilities of the accretion disks. Another efficient mechanism for creating the needed short scales, identified recently by Sahraoui et al. (2005), is related to the fragmentation of the mirror/magnetosonic structures (Stasiewicz, 2004). These strongly compressive structures are known to develop frequently near the astrophysical boundaries due to the compression of the solar wind for instance, where the condition of large b are frequently reported. The point that will be addressed, at least experimentally, is which and under what physical conditions the two possible scenarios for large-to-small scales energy transfers is dominant: shear Alfvén waves filamentation or compressive mirror/magnetosonic cascade.

The exact role of waves in reconnection also remains to be elucidated. Most of the classical scenarios just treat them in a statistical way so that their role can be reduced to some kind of “anomalous” transport, an “anomalous resistivity”, allowing for the use of resistive MHD models. Waves are then just considered as unknown “sub-grid” phenomena, injected in classical X point models. We intend to investigate how waves can play a central role via the introduction of short scales, i.e. replace, at least at electron scales, the specific geometries usually considered.

Another observation of high effect in (or instead of) reconnection is related to the chaotization of the magnetosheath flow leading to the formation of narrow jets out of the laminar large-scale flow. The cascade causing chaotization is still poorly understood. The nonlinear interactions of the magnetosheath eigenmodes with other structures are primary candidates for this cascading. The eigenmodes can interact with the magnetopause boundary itself, or with other waves, either eigenmodes of the boundary layer flow, or with waves reflected from the magnetopause. Observational case studies have reported the generation of near-sonic structured jets in the course of such an interaction. The extremely high dynamic pressure of the jets can, in principle, lead to further cascading and to driving reconnection at the jet-deformed magnetopause.

Plasma jetting, reconnection, and links with Alfvén and other waves, presents a crucial problem for astrophysical application. Testing the inferred patterns using the database gathered in the magnetosphere thanks to Cluster, Interball, Polar etc,. and elaborating on adequate theoretical and numerical models will be the main tasks of the proposed ISSI team.

It is intended to bring together specialists in the field of theory, simulation, and experiment in a team in order to discuss in detail the scale dependence of reconnection. The team will discuss in particular the non-linear evolution of the mirror/ magnetosonic modes since these modes are important in the magnetosheath, have been observed close to the magnetopause and may be of high potential importance in magnetopause reconnection, at least by the magnetic field decrease they can cause.