Final Report

ISSI International Team 290 Final Report

Title: Ion and Electron Bulk Heating by Magnetic Reconnection

Team members: Tai Phan (lead), Nicolas Aunai (Young Scientist), Paul Cassak, James Drake, Jonathan Eastwood, Masaki Fujimoto, Jack Gosling, Colby Haggerty (YS), Heli Hietala (YS), Benoit Lavraud, Goetz Paschmann, Michael Shay, Bengt Sonnerup

Magnetic reconnection is a universal plasma process that converts magnetic energy into kinetic energy (i.e., plasma jetting) and plasma heating. While plasma jetting is theoretically well established and has been confirmed by observations, until recently plasma heating by reconnection has been poorly understood both observationally and theoretically.

In terms of electron heating, past observations have produced conflicting findings on the presence or absence of electron thermal heating by reconnection: some magnetopause and magnetosheath reconnection exhausts showed heating, while the few reported solar wind events showed essentially none, suggesting that the degree and characteristics of electron heating depend strongly on plasma parameter regimes and/or boundary conditions. Similarly, the physics of ion heating and its relationship to electron heating and to the total available energy was not known.

Our ISSI Team used a combination of plasma simulations and in-situ spacecraft observations to systematically study thermal ion and electron heating. Specifically, we focused on the following key questions:

  • What plasma parameter regimes or inflow boundary conditions determine the degree of ion and electron thermal heating in reconnection exhausts?
  • What is the energy partition between kinetic energy, ion and electron thermal heating in reconnection and how does the energy partition change with inflow boundary conditions?
  • What are the characteristics (e.g., anisotropy) of the ion and electron thermal heating and how do they change with inflow boundary conditions?
  • What are the key physical processes that heat the ions and electrons under different inflow boundary conditions?

We now list the accomplishments made by our team on these questions:

  • What plasma parameter regimes or inflow boundary conditions determine the degree of ion and electron thermal heating in reconnection exhausts?

Observationally, our team performed two statistical studies using ~80 magnetopause reconnection exhausts detected by the THEMIS spacecraft to investigate how the amount of ion and electron bulk heating depends on the inflow boundary conditions. We found that for both ions and electrons, the heating, ΔT, is correlated with the asymmetric Alfvén speed, VAL,asym, based on the reconnecting magnetic field and the plasma number density measured in both inflow regions. A best fit to the data produced the empirical relation ΔTi = 0.13 miVAL,asym2 and ΔTe = 0.017 miVAL,asym2,where mi is the proton mass. These findings thus suggest that the heating is simply proportional to the available magnetic energy in the inflow region [Phan et al., 2014].

Concurrently, our team performed a series of kinetic simulations over a large range of the inflow Alfven speed values, and found a similar functional dependence of the heating on miVA2 [Shay et al., 2014]. However, the degree of heating was found to be slightly different from the observations: ΔTe = 0.033 miVA2 and ΔTi = 0.12 miVA2. Remarkably, the sum of the ion and electron heating ΔTi + ΔTe = 0.15 miVA2 is the same in the simulation as at the magnetopause [Haggerty et al., 2015].

  • What is the energy partition between kinetic energy, ion and electron thermal heating in reconnection and how does the energy partition change with inflow boundary conditions?

Although our observational study (Phan et al. [2014]) suggested that the ratio of ion to electron heating is ~8 on average, the Haggerty et al. [2015] study suggested that this ratio is not constant, but is instead dependent on the upstream electron temperature which in turn regulates a potential structure inside the exhaust that lowers ion heating and increases electron heating. The sum of the ion and electron heating, however, appears to be a constant fraction of the inflow magnetic energy (see above).

  • What are the characteristics (e.g., anisotropy) of the ion and electron thermal heating and how do they change with inflow boundary conditions?

Based on our study [Hietala et al., 2015] of ion heating across the magnetotail exhaust (for nearly symmetric, small guide field conditions), a consistent pattern was found in both the spacecraft data and the simulations: While the total temperature across the exhaust is rather constant, near the exhaust boundaries Ti,|| dominates. The plasma is well above the firehose threshold within patchy spatial regions at |BX|∈[0.1,0.5]B0, suggesting that the drive for the instability is strong and the instability is too weak to relax the anisotropy. At the midplane Ti,⊥>Ti,|| and ions undergo Speiser-like motion despite the large distance from the X line.

In the presence of a guide field, our studies found that the electron perpendicular heating is suppressed [Shay et al., 2014].

  • What are the key physical processes that heat the ions and electrons under different inflow boundary conditions?

Our findings suggest that Fermi acceleration, modified by a field-aligned trapping potential, could account for key characteristics seen (in space and in simulations) of heating in the exhaust [Haggerty et al., 2015]. Our team also reported a rather unusual occurrence of a dayside magnetopause exhaust bounded by two switch-off shocks [Sonnerup et al., 2016]. Heating occurred across both shocks, albeit with a much larger temperature increase on the low beta side, in good quantitative agreement with the MHD shock jump conditions. Interestingly, the degree of heating observed in this event was in general agreement with our finding of the scaling with the inflow Alfven speed for regular (shock-less) magnetopause.

In addition to addressing the main goal of understanding plasma heating by reconnection, a number of other reconnection-related work also resulted directly from this collaboration at ISSI:

  • Observations of sequentially released tilted flux ropes in the Earth’s magnetotail [Hietala et al., 2014]
  • Experimental test of the rho(1-alpha) evolution for rotational discontinuities using cluster magnetopause observations [Blagau et al., 2015]
  • Simulation of symmetric magnetic reconnection with a flow shear and applications to the magnetopause [Doss et al., 2015]
  • Full PIC simulations of kinetic equilibria and the role of the initial current sheet on steady asymmetric magnetic reconnection [Dargent et al., 2016]
  • Strategies for the capture and transmission of diffusion region burst data by the Magnetospheric Multi-Scale (MMS) Mission [Phan et al., 2015]
  • The role of the MMS mission in magnetic reconnection research [Cassak, 2016]

Publications resulting from the team work including acknowledgment to ISSI:

Blagau, A.; Paschmann, G.; Klecker, B.; Marghitu, O., Experimental test of the rho(1-alpha) evolution for rotational discontinuities: cluster magnetopause observations (2015), Ann. Geophys., 33,1, doi: 10.5194/angeo-33-79-2015.

Cassak, P. A., Inside the Black Box – Magnetic Reconnection and the Magnetospheric Multiscale (MMS) Mission, submitted to Space Weather Journal, September, 2015.

Dargent, Jeremy, Aunai, Nicolas, Gerard Belmont, Nicolas Dorville, Benoit lavraud, Michael Hesse, Full PIC simulations of kinetic equilibria and role of the initial current sheet on steady asymmetric magnetic reconnection, Jeremy Dargent,. Submitted to Physics of Plasmas, 2016.

E. Doss, C. M. Komar, P. A. Cassak, F. D. Wilder, S. Eriksson, and J. F. Drake, Asymmetric magnetic reconnection with a flow shear and applications to the magnetopause (2015), J. Geophys. Res., 120, 7748.

Haggerty, C. C.; Shay, M. A.; Drake, J. F.; Phan, T. D.; McHugh, C. T. (2015), The competition of electron and ion heating during magnetic reconnection, Geophys. Res. Lett., 42, doi:10.1002/2015GL065961.

Hietala, H., J. P. Eastwood and A. Isavnin (2014), Sequentially released tilted flux ropes in the Earth’s magnetotail, Plasma Phys. Control. Fusion, 56 (2014) 064011. doi:10.1088/0741-3335/56/6/064011.

Hietala, H.; Drake, J. F.; Phan, T. D.; Eastwood, J. P.; McFadden, J. P. (2015), Ion temperature anisotropy across a magnetotail reconnection jet, Geophysical Research Letters, Volume 42, Issue 18, pp. 7239-7247.

Phan, T. D., Drake, J. F., Shay, M. A., Gosling, J. T., Paschmann, G., Eastwood, J. P., Oieroset, M., Fujimoto, M., Angelopoulos, V. (2014), Ion bulk heating in magnetic reconnection exhausts at Earth’s magnetopause: Dependence on the inflow Alfvén speed and magnetic shear angle, Geophys. Res. Lett., 41, 20, doi: 10.1002/2014GL061547.

Shay, M. A., C. C. Haggerty, T. D. Phan, J. F. Drake, P. A. Cassak, P. Wu, M. Oieroset, M. Swisdak, and K. Malakit (2014), Electron Heating During Magnetic Reconnection: A Simulation Scaling Study, Physics of Plasmas, 21, 12. doi: 10.1063/1.4904203.

Sonnerup, B. U. O., Goetz Paschmann, Stein Haaland, Tai Phan, and Stefan Eriksson, Reconnection layer bounded by switch-off shocks: Dayside magnetopause crossing by THEMIS D, Submitted to J. Geophys. Res., 2016.