Nick Murphy's Research Page

My research focuses on several areas in solar physics, plasma astrophysics, and basic plasma science. I am particularly interested in the physics of magnetic reconnection, the thermal energy content of CMEs, and connections between laboratory and astrophysical plasmas. I use a combination of numerical simulations, observations, non-equilibrium ionization modeling, and analytic theory. The primary questions that motivate my research are:

How does asymmetry affect the magnetic reconnection process?

Magnetic reconnection is the process whereby the connectivity of magnetic field lines is changed in a highly conducting plasma. Reconnection is responsible for energy release during solar flares, several aspects of space weather, and loss of confinement in laboratory plasma devices such as tokamaks. Most models of magnetic reconnection assume that the process is symmetric: that the reconnecting magnetic fields are of equal strength and that the outflow jets propagate into regions with similar properties. However, reconnection in space, laboratory, and astrophysical plasmas is in general asymmetric. For example, reconnection between the Earth's magnetic field and the solar wind involves asymmetric inflow; and reconnection outflow jets during solar flares propagate into plasmas with substantially different properties. Importantly, asymmetry in the outflow direction can drastically affect where the energy released by reconnection can go. I am using a combination of numerical simulations and analytic theory to investigate the role asymmetry has in the reconnection process.

References:

N. A. Murphy and C. R. Sovinec, "Global axisymmetric simulations of two-fluid reconnection in an experimentally relevant geometry," Physics of Plasmas 15, 042313 (2008) (article, journal link, ADS)

N. A. Murphy, C. R. Sovinec, and P. A. Cassak, "Magnetic Reconnection with Asymmetry in the Outflow Direction," Journal of Geophysical Research, 115, A09206, doi:10.1029/2009JA015183 (2010) (article, journal link, ADS)

N. A. Murphy, "Resistive magnetohydrodynamic simulations of X-line retreat during magnetic reconnection," Physics of Plasmas, 17, 112310 (2010) (article, journal link, ADS)

What heats coronal mass ejection plasma?

Coronal mass ejections (CMEs) are explosive events often associated with solar flares that expel huge amounts of plasma into the solar wind. Several recent observational results suggest that the cumulative heating energy during the eruption is comparable to or greater than the kinetic energy of the ejecta. We are using observations by SDO/AIA, Hinode/XRT, and SOHO/UVCS to provide constraints on plasma heating during these events. Because the ionization and recombination time scales are comparable to the expansion time scales, we use non-equilibrium ionization models to determine how much heating is necessary and where the heating occurs. The physical mechanisms responsible for the heating have not been unambiguously identified. However, candidate mechanisms include: (1) upflow from the current sheet that forms in the wake behind the rising plasmoid, (2) small-scale relaxation and reconnection during flux rope expansion and propagation, and (3) collisions between the thermal plasma and energetic particles.

References:

N. A. Murphy, J. C. Raymond, and K. E. Korreck, "Plasma Heating During a Coronal Mass Ejection Observed by the Solar and Heliospheric Observatory," Astrophysical Journal, 735, 17 (2011) (article, journal link, ADS)

How does magnetic reconnection occur in partially ionized plasmas?

Most simulations and theories of magnetic reconnection assume that the plasma is fully ionized. However, the ionization fraction of hydrogen in the solar photosphere and chromosphere is often less than 1%. Therefore, it is necessary to include effects such as ambipolar diffusion, enhancement of the Hall effect (Pandey & Wardle 2008, Malyshkin & Zweibel 2011), and time-dependent ionization in order to understand how reconnection happens in these parts of the lower solar atmosphere. In particular, chromospheric reconnection has been proposed to explain the origin of the first ionization potential (FIP) effect (Arge & Mullan 1998). We are developing simulations using the HiFi code to test this hypothesis (see also Leake et al. 2012).

References:

C. N. Arge and D. J. Mullan, "Modelling of Magnetic Interactions in Partially Ionized Gas: Application to the FIP Effect," Solar Physics, 182, 293 (1998) (ADS)

J. E. Leake, V. S. Lukin, M. G. Linton, and E. T. Meier, "Multi-Fluid Simulations of Chromospheric Magnetic Reconnection in a Weakly Ionized Reacting Plasma," Astrophysical Journal, 760, 109 (2012) (ADS)

L. M. Malyshkin and E. G. Zweibel, "Onset of Fast Magnetic Reconnection in Partially Ionized Gases," Astrophysical Journal, 739, 72, (2011) (ADS)

B. P. Pandey and M. Wardle, "Hall magnetohydrodynamics of partially ionized plasmas," Monthly Notices of the Royal Astronomical Society, 385, 2269 (2008) (ADS)

Acknowledgements

My research is supported by NASA grants NNX09AB17G, NNX11AB61G, NNX12AB25G, NASA contract NNM07AB07C, and NSF SHINE grant AGS-1156076 to the Smithsonian Astrophysical Observatory.