Nick Murphy's Research Page

While my current work focuses on research software engineering for PlasmaPy, I have also had research interests in several areas in solar physics, plasma astrophysics, and basic plasma science. I have been particularly interested in the physics of magnetic reconnection, plasma heating in coronal mass ejections, and connections between laboratory and astrophysical plasmas. I have used a combination of numerical simulations, observations, non-equilibrium ionization modeling, and analytic theory. The primary questions that have motivated my research are:

How does magnetic reconnection occur in partially ionized plasmas?

Magnetic reconnection in the lower solar atmosphere is thought to be responsible for dynamical phenomena such as Ellerman bursts, UV brightenings, jets, and possibly Type II spicules. Most simulations and theories of magnetic reconnection assume that the plasma is fully ionized. However, the ionization fraction of hydrogen in the solar chromosphere is often less than 1%. Models of chromospheric reconnection must account for ion-neutral drag, modifications to the resistivity due to collisions involving neutrals, neutral thermal conduction, enhancement of the Hall effect, and non-equilibrium ionization. I have performed numerical simulations of magnetic reconnection in partially ionized plasmas using the plasma-neutral module of HiFi. My investigations have focused on asymmetry, the Hall effect, and the development of the plasmoid instability.

References:

Nicholas A. Murphy and Vyacheslav S. Lukin, "Asymmetric Magnetic Reconnection in Weakly Ionized Chromospheric Plasmas," Astrophysical Journal, 805, 134 (2015) (article, journal link, ADS)

How does magnetic reconnection happen in three dimensions?

Magnetic reconnection is an intrinsically three-dimensional process. Theoretical investigations of magnetic topology offer the possibility of exact and widely applicable mathematical results that can provide greater understanding of 3D reconnection. The magnetic skeleton of a plasma consists of the features such as null points, spine field lines, separatrix surfaces, and separators that divide the plasma into distinct topological domains. Each of these boundaries is a preferred location for reconnection. Magnetic skeletons evolve both geometrically (as features move around in space) and topologically (as the overall structure changes due to bifurcations). My previous work has focused on the evolution of magnetic null points.

References:

Nicholas A. Murphy, Clare E. Parnell, and Andrew L. Haynes, "The appearance, motion, and disappearance of three-dimensional magnetic null points," Physics of Plasmas, 22, 102117 (2015) (article, journal link)

How does asymmetry affect the magnetic reconnection process?

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 used a combination of numerical simulations and analytic theory to investigate the role asymmetry has in the reconnection process.

References:

Nicholas A. Murphy, Aleida K. Young, Chengcai Shen, Jun Lin, and Lei Ni, "The plasmoid instability during asymmetric inflow magnetic reconnection," Physics of Plasmas, 20, 061211 (2013) (article, journal link, ADS)

N. A. Murphy, M. P. Miralles, C. L. Pope, J. C. Raymond, H. D. Winter, K. K. Reeves, D. B. Seaton, A. A. van Ballegooijen, and J. Lin, "Asymmetric Magnetic Reconnection in Solar Flare and Coronal Mass Ejection Current Sheets," Astrophysical Journal, 751, 56 (2012) (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)

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 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)

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 have used 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)

Acknowledgements

My past and present research has been supported by NASA grants NNX09AB17G, NNX11AB61G, NNX12AB25G, and NNX15AF43G; NASA contract NNM07AB07C; NSF SHINE grants AGS-1156076 and AGS-1358342; DOE grant DE-SX00163063; NSF CSSI award 1931388, and subcontract S014981-F from the Princeton Plasma Physics Laboratory to the Smithsonian Astrophysical Observatory.