JAXA Supercomputer System Annual Report April 2016-March 2017

Report Number: R16E0116

• Responsible Representative: Masaki Fujimoto(Institute of Space and Astronautical Science, Department of Solar System Sciences)
• Contact Information: Masaki Fujimoto(fujimoto.masaki@jaxa.jp)
• Members: Masaki Fujimoto, Kenya Shimizu
• Subject Category: Space(Space science,Space philosophy)

Abstract

Magnetic reconnection, a process that explosively releases magnetic energy, is considered to be triggered when two oppositely directed field lines are strongly pushed against each other so that the distance between the two field lines become small. How small the distance has to be for magnetic reconnection to take-off? We show that the distance can be more than 30 times larger than what has been considered, implying that magnetic reconnection happens more easily that what one used to assume.

Goal

Theoretical investigation of space plasma processes in order tosupport the science output from STP missions led by ISAS/JAXA

Objective

Magnetic reconnection is one of the most important space plasma processes in which magnetic energy is explosively released. Here we study its triggering mechanism that has remained unexplored in the past.

References and Links

N/A

Use of the Supercomputer

In order to treat plasma kinetic effects properly, we perform particle simulations of magnetic reconnection.

Necessity of the Supercomputer

We need large computational resource in order to perform plasma particle simulations.

Achievements of the Year

Magnetic reconnection is a process in space plasmas that releases stored magnetic energy in an explosive manner and is regarded as one of the most important space plasma physical processes because it is considered to be the underlying mechanism of various explosive phenomena in plasma astrophysical situations. Magnetic reconnection is triggered when a pair of oppositely directed field lines, with each situated on the other side of a current sheet, is pushed to each othe such that the thickness of the current sheet becomes small. The conventional wisdom has been that the triggering sets-in when the thickness becomes as small as the relevant ion Larmor radius. This critical thickness, however, is very small and thus the question of whether the current sheet thickness has to come down to this extremely thin-ness has been a natural question. On the other hand, a question in the opposite direction, whether an ion-scale current sheet thin is enough for reconnection triggering, has also been a valid one according to previous in-depth investigations. The presence of the two questions indicates how little has been known about magnetic reconnection triggering. Here we focus on the effect of the temperature anisotropy inside the current sheet on reconnection triggering. It is shown that when the perpendicular temperature is more than 1.5 times the parallel one, magnetic reconnection triggering is possible in a current sheet whose thickness is as thick as 30 times ion Larmor radius. On the other hand, for isotropic plasma, even a current sheet of one ion Larmor radius thickness is too thick for reconnection to take-off. It also means that, when intensity of the oppositely directed magnetic fields outside a current sheet is compressed up to 1.5 times the initial value, reconnection sets-in even if the current sheet thickness is as large as 30 times ion Larmor radius. Meanwhile reconnection does not happen if the compression ratio falls short of 1.5. This critical behavior related to the temperature anisotropy is a new finding and suggest a new perspective from which reconnection triggering problem would be solved.

Publications

Peer-reviewed articles

1) Shimizu, K.; Fujimoto, M.; Shinohara, I., On temporal variation of reconnection rate and X line electric field structure, Journal of Geophysical Research: Space Physics, Volume 121, Issue 10, pp. 9956-9971, 2016.

Presentations

1) Kenya Shimizu Masaki Fujimoto Iku Shinohara, Triggering of explosive reconnection in a thick current sheet by temperature anisotropy boosted tearing mode, AGU ,December,2016

2) Iku Shinohara Kenya Shimizu Masaki Fujimoto, Triggering of explosive reconnection in a thick current sheet via current sheet compression: Less current sheet thinning, more temperature anisotropy, AGU ,December,2016

Computational Information

• Parallelization Methods: Hybrid Parallelization
• Process Parallelization Methods: MPI
• Thread Parallelization Methods: OpenMP
• Number of Processes: 144
• Number of Threads per Process: 32
• Number of Nodes Used: 20
• Elapsed Time per Case (Hours): 12
• Number of Cases: 20

Resources Used

Total Amount of Virtual Cost(Yen): 16,302,462

Breakdown List by Resources

Note: Virtual Cost=amount of cost, using the unit price list of JAXA Facility Utilization program(2016)

JAXA Supercomputer System Annual Report April 2016-March 2017