Fine-scale 3-D Dynamics of Critical Plasma Regions: Necessity of Multipoint Measurements R. Lundin 1, I. Sandahl 1, M. Yamauchi 1, U. Brändström 1, and A. Vaivads 2 1. Swedish Institute of Space Physics, Kiruna 2. Swedish Institute of Space Physics, Uppsala

Abstract Both Cluster and SOHO revealed that the key plasma processes that are critical to large-scale energy conversion and dynamics (e.g, current disruption, acceleration/radiation processes, shock formation, initiating reconnection, non-reconnection plasma entry) lie in a small-scale physics with scale size comparable to the electron scale and electron-ion hybrid scales, which is beyond what Cluster has aimed for. To reveal these processes, we need a new generation of multi-spacecraft missions and continued solar and solar wind observations: * An Earth-orbiting multi-spacecraft system with at least 8 spacecraft, 4 of them placed within typical electron scale lengths (e.g. gyroradii) in critical regions, the other with typical meso-scale separation (several ion gyroradii). * Continued solar and solar wind monitoring upstream of the Earth.

Introduction: Post-Cluster science Energy conversion and transport in plasma, i.e., laboratory, space, and inside stars, are of common interest to fundamental physics, space physics, and astrophysics. While energy flow in the MHD has been rather well conceived and most predictions were confirmed in both laboratory and space-bone observation, those related to kinetic effects (ion and electron effects) need further investigation. The Cluster project, with its power of multipoint techniques, has reached the resolution of ion inertia scale, revealing for the first time the ion scale dynamics of key regions such as magnetopause, cusp, magnetotail, and auroral acceleration region. At the same time Cluster has revealed that the key plasma processes that are critical to the entire magnetospheric plasma dynamics lies in small-scale physics with scale size comparable to the electron scale and electron-ion hybrid scales. Similarly, the SOHO satellite also revealed that key regions with large energy release such as the coronal mass ejections (CME) and the solar flares are located in very confined regions, requiring us to focus more on the particle behaviour in these key phenomena.

What are the critical issues? Reconnection (diffusion region) Plasma Vortices (site and condition) Plasma filamentation (site and condition) Current sheet dynamics and instability (boundary and substorms) Acceleration and radiation (auroral and shock) Energy transfer (meso & micro scale) All the other non-MHD (E ≠ - v x B) phenomena Determination of curl B, curl E, dB/dt, and dE/dt in small scale All involve inertia & gyroradii scale physics

Critical issue: Acceleration & Radiation j up j down Auroral arc and dual (up & down) field-aligned current and electric field. We now see fine structure. However, What causes these structures and waves? How to separate temporal development from spatial structures? Electrons Ions FAST orbit 1626 / Altitude ~ 4100 km

Critical issue: Filamentation & current sheets Field-aligned current sheets (thickness, strength…) Striations Dynamics, temporal, spatial Data from ALIS Photo: Y. Ebihara

Critical issue: Reconnection Cluster resolved the ion inertia length during its crossing through the diffusion region. But this is not enough to understand the mechanism because the electron Hall effect E n ~JxB/en seems to play an important role.

Critical issue: Impulsive plasma injection s/c1 s/c3 s/c1 s/c3 Pressure-pulse induced injection

Critical issue: Vortices Past missions up to Cluster revealed that vortices exist everywhere. Implications of a vortex? Connection to the ionosphere? Cluster observation of vortices at the magnetopause (Hasegawa et al. 2004)

Critical issue: Non-MHD behavior (induction => e/m energy created from particle energy) (e/m forcing)

Technological challenge (From Schwartz et al., 1998) The next generation missions must cover different spatial scales (electron, electron-ion, and ion). This requires one step higher technology from Cluster in both the instrumentation and the spacecraft operation, i.e., multi-spacecraft systems using autonomy, and on-board/swarm intelligence. It is a good technological challenge as well as scientific challenge.

Conclusions We need a next generation mission after Cluster and SOHO which will resolve the 4-D (space and time) structure of the electron and electron-ion scales in key region of important space plasma phenomena. To achieve this we suggest the following strategy for ESA : 1. Launch an Earth-orbiting multi-spacecraft system with at least 8 spacecraft, 4 of them placed within typical electron scale lengths (e.g. gyro-radii), the other with meso-scale separation (several ion gyro radii). 2. Provide continued solar and solar wind monitoring upstream of the Earth. (Can be done in collaboration with other agencies.)

Why magnetospheric physics? The space and the solar atmosphere are the only place where these scales can be directly measured within next few decades. Furthermore, the magnetospheric physics is: required for understanding the solar-terrestrial coupling - on long term and short term basis only partially understood - a complex system crucial for understanding the evolution of the solar system (comparative planetology) crucial for understanding the acceleration of matter up to high velocities (plasma acceleration) crucial for understanding the evolution of stars and galaxies (plasma escape, mass loss)

Energization

summary Microphysics of the electron scale, electron-ion scale, and ion-scale (from electron Debye/inertia length up to ion inertia length and ion gyroradius) are important in the largest energy conversion and transport such as CME formation in solar physics, substorm onset in space physics, active galactic nuclea in astrophysics, and in dense plasma physics such as fusion and internal stars.