Current Fragmentation and Particle Acceleration in Solar Flares
Particle acceleration in solar flares remains an outstanding problem in plasma physics and space science. While the observed particle energies and timescales can perhaps be understood in terms of acceleration at a simple current sheet or turbulence site, the vast number of accelerated particles, and the fraction of flare energy in them, defies any simple explanation. The nature of energy storage and dissipation in the global coronal magnetic field is essential for understanding flare acceleration. Scenarios where the coronal field is stressed by complex photospheric motions lead to the formation of multiple current sheets, rather than the single monolithic current sheet proposed by some. The currents sheets in turn can fragment into multiple, smaller dissipation sites. MHD, kinetic and cellular automata models are used to demonstrate this feature. Particle acceleration in this environment thus involves interaction with many distributed accelerators. A series of examples demonstrate how acceleration works in such an environment. As required, acceleration is fast, and relativistic energies are readily attained. It is also shown that accelerated particles do indeed interact with multiple acceleration sites. Test particle models also demonstrate that a large number of particles can be accelerated, with a significant fraction of the flare energy associated with them. However, in the absence of feedback, and with limited numerical resolution, these results need to be viewed with caution. Particle in cell models can incorporate feedback and in one scenario suggest that acceleration can be limited by the energetic particles reaching the condition for firehose marginal stability. Contemporary issues such as footpoint particle acceleration are also discussed. It is also noted that the idea of a “standard flare model” is ill-conceived when the entire distribution of flare energies is considered.
We thank many collaborators who have contributed over the years to these ideas. JFD acknowledges support from NSF grant ATM-0903964. G.B. and Å.N. acknowledge support from the SOLAIRE Research Training Network of the European Commission (MRTN-CT-2006-035484). In addition, G.B. acknowledges support from the Niels Bohr International Academy and the John von Neumann Institute for Computing, and Å.N. acknowledges support from PRACE (Partnership for Advanced Computing in Europe) and the European Commission’s Seventh Framework Programme (FP7/2007-2013) under the grant agreement SWIFF (project No. 263340, www.swiff.eu).
- P. Bak, How Nature Works (Oxford University Press, Oxford, 1999) Google Scholar
- G. Baumann, K. Galsgaard, A. Nordlund, Solar Phys. (2012a, in press). arXiv:1203.1018v1
- G. Baumann, T. Haugbolle, A. Nordlund, Astrophys. J. (2012b, in press). arXiv:1204.4947v2
- L. Fletcher, B.R. Dennis, H.S. Hudson, S. Krucker, K. Phillips, A. Veronig, M. Battaglia, L. Bone, A. Caspi, Q. Chen, P. Gallagher, P.T. Grigis, H. Ji, W. Liu, R.O. Milligan, M. Temmer, Space Sci. Rev., 159, 19 (2011) Google Scholar
- K. Galsgaard, in SOLMAG 2002, ESA SP-505 (2002) Google Scholar
- A. Lazarian, this issue (2012) Google Scholar
- A. Nordlund, K. Galsgaard, Astrophys. J. (2012) Google Scholar
- V. Petrosian, this issue (2012). doi: 10.1007/s11214-012-9900-6
- J. Raymond, R.P. Lin, S. Krucker, V. Petrosian, this issue (2012). doi: 10.1007/s11214-012-9897-x
- K. Shibata, in Proc. Nobeyama Symp., vol. 381 (1998), NRO report 479 Google Scholar
- K. Shibata, T. Magara, Living Reviews. Sol. Phys. 8, 6 (2011) Google Scholar
- T.W. Speiser, J. Geophys. Res. 70, 4129 (1965) Google Scholar
- L. Vlahos, S. Krucker, P.J. Cargill, in Turbulence in Space Plasmas, ed. by L. Vlahos, P.J. Cargill (Springer, Berlin, 2009) Google Scholar