S11-2

Cross Scale Coupling in Magnetospheric Plasma☚Ẵᅥࣈࣚࢫ࣏࡞࠽ࡄࡾࢪࢢ࣭ࣜ㛣⤎ྙ
Yoshifumi Saito, Masaki Fujimoto*, Iku Shinohara, Hirotsugu Kojima**㰳⸠⩇ᩝ, ⸠ᮇḿᶖ*, ⠓ཋ⫩, ᑚᔪᾀႵ**Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency
3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, JapanᏫᏼ⯗✭◂✪㛜Ⓠᶭᵋ࣬Ꮻᏼ⛁Ꮥ◂✪ᮇ㒂㸡ࠚ229-8510 ♼ዄᕖ┬┞ᶅཋᕰ⏜㔕ྋ㸨Ѹ㸦Ѹ㸦Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, JAPAN᮶ாᕝᴏኬᏕ, ࠚ152-8551 ᮶ா㒌┘㯦༇ኬᒱᒜ㸧Ѹ㸦㸧Ѹ㸦
Research Institute for Sustainable Humanosphere, Gokasho, Uji, Kyoto 611-0011, JAPANா㒌ኬᏕ⏍Ꮛᅥ◂✪ᡜ, ࠚ611-0011 ா㒌ᗋᏫ἖ᕰ஫ࣧᗁ
The time evolution of dynamic phenomena observed in magnetospheric plasma is determined by the crossscale coupling between global-scale dynamics and ion/electron scale dynamics in some key regions. Inorder to understand the phenomena in a true sense, understanding the cross scale coupling that exists overseveral different scales is indispensable. Currently, theoretical studies are in progress. In addition to these
efforts, a formation flying satellite mission “SCOPE” is under study for the future empirical study of crossscale coupling.

1. Cross Scale Coupling in Magnetosphere
1.1 Introduction
Magnetospheric plasma is collisionless. Thismeans that, unlike in the gas that surrounds us, thethermo-dynamical relaxation time scale is muchlarger than the time scale of interest. The
non-Maxwellian distribution function is everywhere.
Non-thermal particles are not necessarily minor andare not just test particles that only react to given
electro-magnetic fields but can play a significant
role in determining the time evolution of the system.
Energetic particles fill the magnetosphere because
they don’t lose their energies by colliding with theless-energetic thermal component. Explosive
phenomena such as substorms occur because usualrelaxation processes are absent and the dissipation
processes that trigger them show anomalous
behaviors. In other words, the dissipation sets inonly when certain thresholds for certain parameters
are crossed. Some if not most of the fascinating
aspects of magnetospheric dynamics have theirroots in this collisionless-ness of the plasma thatfills the space.
Fascinating aspects of magnetospheric processes
root in the collisionless-ness of the plasma. In otherwords, the true understanding of what we are
attracted to comes only when the collisionlesseffects (kinetic effects or particle effects) are fully
appreciated. MHD system in the present form treats
sub-MHD scale dynamics only in an ad-hoc manner.
On the other hand, previous studies on kinetic
effects mostly dealt only with sub-MHD scales. The
dynamical coupling among the scales, from thebottom of electron-kinetic up to MHD scales, hasbeen recognized but has not been studied
intensively. The traditional style has been tosummarize the sub-MHD scale effects into atransport coefficient (viscosity, resistivity,…) andadd the term to the MHD equations. This form does
not necessarily fully appreciate the wonder of thecollisionless plasma and that we should search for astyle that features the cross-scaling coupling natureof the magnetospheric dynamics.
1.2 Example: magnetic reconnection triggering
problem
Magnetic reconnection is one of the mostimportant energy conversion processes incollisionless plasmas that releases the energy stored
in the form of magnetic field. In the magnetosphericenvironment, a number of in-situ observations haveclarified not only its existence but also its
importance in regulating the flow of mass andenergy into the Earth’s magnetosphere from thesolar wind. The current sheet in the magnetotail isthe site for explosive magnetic reconnection, in
which various magnetospheric activities are
considered to originate.
One of the candidates to initiate reconnection is
the tearing instability. In contrast to the previous
studies that focused mostly on electron scale current
sheets, we have recently focused on thick current
sheets and have inspected if vigorous growth out ofquiet initialization is possible in them. We should
consider how an ion-scale current sheet would
become subject to reconnection triggering, beacusecurrent sheet thickness observed in themagnetosphere is of this order or even thicker. We
have found that there is a critical thickness above
which significant effects of the instability are notavailable.
While the above critical thickness severely limits
the ability of the two-dimensional tearing mode fortriggering quick reconnection, we have also
recently shown that the tearing mode survives as a
viable process by coupling to a three-dimensional
effect. Only in three dimensional full particle
simulations are the effects of the current-driveninstabilities taken into account. For an anti-parallel
magnetic field geometry and with the mass ratio of
400, we have shown that, even if the current sheet
thickness is of ion-scale, large scale reconnection istriggered very quickly. The definitive contrast
between 2- and 3-D results comes from thedifference if the lower-hybrid-drift instability(LHDI) is allowed or not. Only in the 3-D case thefreedom in the current direction is included and
LHDI is allowed to develop around the outer edgesof the current sheet. LHDI reduces the currentdensity locally at the outer edges, which induces
non-local redistribution of current density within
the current sheet. We have revealed that the current
density enhancement at the neutral sheet associatedwith this redistribution leads to the quickreconnection triggering (QMRT). The triggering is
attained as soon as LHDI is saturated and is very
quick even for an ion-scale current sheet. It is alsoemphasized that QMRT is obtained via non-local,
cross-scale coupling with LHDI and is not
something that can be modeled as a current driven
anomalous resistivity.
2. SCOPE mission
The Solar Terrestrial Physics (STP) group in Japan
has organized ‘New Magnetospheric Mission
Working Group’ in order to consider a newmagnetospheric satellite mission. The name of themission is ‘SCOPE’ that stands for ‘cross Scale
COupling in Plasma universE’. Fig. 1 shows aconceptual diagram of SCOPE mission. The main
purpose of this mission is to investigate thedynamic behaviors of plasma in the Terrestrial
magnetosphere that range over various time andspatial scales. The basic idea of the SCOPE mission
is to distinguish temporal and spatial variations ofphysical processes by putting five formation flying
spacecraft into the key region of the Terrestrial
magnetosphere. The formation consists of one large
mother satellite and four small daughter satellites.
High spatial resolution is realized by thecoordinated observations among the mother andfour daughter satellites. Three of the four daughter
satellites
surroundthemothersatellite
3-dimensionally maintaining the mutual distancethat ranges between 100km and several thousand
km (variable). The fourth daughter satellite stays
near the mother satellite with the distance of about
10km. By this configuration, we can obtain both themacro-scale and micro-scale information about theplasma disturbances at the same time. Highly
resource demanding high time resolution
measurements are realized by concentrating
available resources to the mother satellite. By doing
so, we can realize the time resolution of electron
measurement that is about 1000 times faster thanthat of our previous magnetospheric mission
GEOTAIL. It is essential for the daughter satellites
to be as small, and as light-weight as possible while
maintaining the high performance. Fig. 2 showsmodel science instruments onboard SCOPE satellite.
High performance Plasma/Particle sensors, DC/AC
Magnetic and Electric field sensors, and Wave
Particle Correlator are planned to be onboard.

Fig. 1. Conceptual Diagram of SCOPE (cross Scale
Coupling in Plasma universe) mission.

Fig. 2. Model Science Instruments onboard SCOPE
satellites.