Fine-scale 3-D Dynamics of
Critical Plasma Regions:
Necessity of Multipoint
Measurements
R. Lundin1, I. Sandahl1, M. Yamauchi1,
U. Brändström1, and A. Vaivads2
1. Swedish Institute of Space Physics, Kiruna
2. Swedish Institute of Space Physics, Uppsala
*.***@irf.se
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
Electrons
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?
Ions
FAST orbit 1626 / Altitude ~ 4100 km
jup
jdown
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
En~JxB/en
seems to play an
important role.
Critical issue: Impulsive plasma injection
s/c1
s/c1
s/c3
s/c3
Pressure-pulse induced injection
Critical issue: Vortices
Cluster observation of vortices at the
magnetopause (Hasegawa et al. 2004)
Past missions up to Cluster
revealed that vortices exist
everywhere.
• Implications of a vortex?
• Connection to the
ionosphere?
Critical issue: Non-MHD behavior
ur
ur  ur
B
d
  
   E emf  d S
dt
 t

d / dt  0
(e/m forcing)
d / dt  0
B / t     v  B
(induction => e/m
energy created
from particle
energy)
Technological challenge
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.
(From Schwartz et al., 1998)
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.