Yan SongSenior Scientist, School of Physics and Astronomy
Ph.D., University of Minnesota, 1988
B.S., Beijing University, 1970
Chair, American Geophysical Union SPA Scarf Award Committee, 2006-2008
Theoretical space, solar, and cosmic plasma physics. Auroral particle acceleration at Earth and Jupiter; substorm physics and substorm auroral formation; radiation belt particle acceleration; the physical nature of magnetic reconnection; ULF wave and wave packet dynamics; formation of the Alfvenic EM plasma structure; collective behavior in quasi-neutral plasmas and in dynamical active plasmas.
About my work
Much of my research has focused on the investigation and development of comprehensive dynamical theories related to important high energy plasma processes occurring in the solar-terrestrial and cosmic plasma environments. These processes include the acceleration of charged particles to high energy and the associated emission of electromagnetic (EM) radiation. They often occur in active plasma regions, such as in the Earth’s and planetary auroral acceleration regions, in radiation belts, at interplanetary shocks, in solar flares and corona heating regions. During these processes, a large amount of stored free magnetic energy can be rapidly converted into the kinetic energy and thermal energy of charged particles, resulting in explosive energy release and high energy particle acceleration.
Most existing theories of plasma physics emphasize plasma behavior in a quasi-neutral state, which intrinsically exclude the generation of strong electric fields and is unable to explain the acceleration of high energy particles in active plasmas. To understand the particle acceleration mechanism, we need to develop a new dynamical theory of plasma states in active plasmas that can be applied to the generation of long lasting strong electric fields.
Development of dynamical theory of the generation of parallel electrostatic fields
Among other mechanisms for particle acceleration, parallel electrostatic fields related to charge separation are the most powerful means to directly and efficiently accelerate particles. It has been commonly believed that the parallel componence of the electron inertia, anomalous resistivity, and the electron pressure gradient can give rise to E||ES, causing auroral particle acceleration. We have pointed out (Song and Lysak, 2001, 2006) that the three non-ideal terms cannot generate electric fields. We have developed a dynamical theory for the generation of parallel electric fields (Song and Lysak, 2006), and pointed out that the E||-generation is described by the displacement current term in the parallel component of Ampere’s law. The generation of these fields is favored by a low plasma density, enhanced magnetic stress and strong Alfvenic interaction. The displacement current term provides a new mechanism of the generation of powerful high energy electrostatic fields, i.e., charge separation. The discovery of the role of the displacement current in particle acceleration may open the door to understand the mechanism of high energy particle acceleration in cosmic plasmas.
The formation of Alfvenic EM plasma structures which act as powerful high energy electric accelerators
We realize that once the parallel electric fields are produced, they will disappear right away due to the high mobility of charged particles, unless the electric fields can be continuously generated and sustained for a fairly long time. We further developed the dynamical theory of long-lasting high energy particle acceleration, and proposed that the Alfvenic EM electrostatic plasma structures, such as Alfvenic double layers (DLs) and charge holes (CHs), can serve as effective high energy particle accelerators. To produce powerful high energy particle accelerator in cosmic plasmas, a continuous energy supply and the generation of sustainable strong electric fields are necessary. These requirements can be accomplished through the formation of dynamical Alfvenic electrostatic plasma structures which act as powerful high energy accelerators. For example, in the formation of auroras, the Poynting flux carried by Alfven waves is necessary to provide continuous energy supply to the auroral acceleration region, supporting Alfvenic DLs, leading to strong long-lasting electrostatic fields for the formation of quasi-static and Alfvenic auroras covering small to large scales.
Exploring the dynamical space plasma theory with plasma’s characteristics beyond quasi-neutrality.
More than 99 percent of visible matter in the universe exist in the plasma state. The plasma is often defined as a “quasi-neutral gas of charged and neutral particles which exhibits collective behaviors” (e.g., Chen, 1974). In such a quasi-neutral state, a fundamental characteristic of the plasma is its ability to wipe out electric potentials or fields that are applied to it. We will develop a new dynamical theory of plasma states in active plasmas that can be applied to the generation of strong electric fields.
Exploring the physical nature of magnetic reconnection
Magnetic reconnection occurs when oppositely directed magnetic field lines in a plasma are broken and rejoined (Dungey, 1953). In the process the magnetic energy stored in the current sheet is converted into heat and kinetic energy in the form of plasma flows and accelerated charged particles. Magnetic reconnection has been considered as the universal importance, and has been widely accepted as a fundamental physical process which can cause numerous phenomena throughout the universe. To understand the physical nature of reconnection has been one of the major and urgent unsolved problems in space plasma physics. One of my ongoing research is to investigate the physical nature of magnetic reconnection based on the fundamental physical laws.
My past research also includes the following topics:
(1) Proposed a dynamical theory of the generation of the field-aligned current.
The field-aligned current (FAC) is often related to the Poynting flux carried by the Alfven waves. Therefore, to understand the physics of the generation of the FAC in auroral current system is important in understanding auroral particle acceleration. However, it has been widely believed that the FAC generation is caused by the force balance. We pointed out for the first time that the FAC generation cannot be obtained from the current continuity condition and the MHD momentum equation. We derived the equation of the generation of the total FAC including the displacement current by using the whole set of dynamical equation (see, Song and Lysak, 2000, 2001, 2006).
(2) Propose MHD wave packet or Alfvenon, which can be used to deal with strong nonlinear interactions of MHD waves. (Song and Lysak, 1994, 2000,2001, Song, 1998)
We suggested that MHD waves, such as the shear Alfvén wave, are often generated in a discrete form, so called Alfven wave packets or Alfvénons. The concept of MHD wave packet not only emphasizes the macro-particle aspect of MHD wave, but also reveals their topological properties. It has been noted that MHD wave packet dynamics plays an important role for phenomena occurring in active plasma regions. The topological properties of the Alfven wave packets can be used to deal with strong nonlinear interaction of incident and reflected Alfven waves packets in the auroral acceleration region.
(3) Evaluation of Twist Helicity at the Earth’s Dayside Magnetopause (Song and Lysak, 1989, 2001)
Solar wind constantly impinges Earth’s magnetopause, often leading to a phenomenon called the flux transfer event (FTE), which is viewed as the result of time-dependent localized reconnection at the dayside magnetopause. FTEs are characterized by a magnetic signature which indicates the presence of an isolated magnetic flux tube which carries a field aligned current, accompanying with a twisting in the flux tube. We have evaluated the twist magnetic helicity during the interaction between the solar wind magnetic flux tube and the magnetic flux tube at the magnetopause. The reconnection can remove topological constraints restricting reorganization of the magnetic field so that twist helicity can be obtained from the mutual linkage helicity or internal kink helicity. The degree of twist of the FTE can be evaluated given the topology of the initial configuration, independent of the details of the reconnection process. We are one of two groups to use topological approach to evaluate the twist helicity of the FTEs in the earth’s magnetosphere.
(4) Proposed Global Alfvenic Interaction scenario in exploring the mechanism of magnetospheric substorm onset. (Song, 1998; 2003; Song and Lysak, 2000, 2001, 2008; Lin et al., 2009)
The magnetospheric substorm is a brief disturbance in the Earth's magnetosphere that causes energy to be explosively released from the magnetotail of the magnetosphere and injected into the high latitude ionosphere. In most previous studies, substorm onset has been considered to be caused by some localized dissipative processes, such as a localized magnetic reconnection and current disruption, while the disturbances in other locations are considered as the consequence of the triggering process as the perturbations propagate. We have proposed an alternative mechanism emphasizing that the substorm onset is a result of Alfvénic interactions in the global current system including the tail and magnetopause current sheets as well as the auroral field-aligned current region in M-I coupling system. The theoretical foundation of the global Alfvenic interaction in the substorm process is the basic dynamic theories which include the generation of field-aligned currents and parallel electric fields, and the nonlinear Alfvénic interaction. Our research has provided a new physical insight and a new way of research into the substorm study which have not been considered before.
My work also benefits from collaboration with Professor Bob Lysak.
Lysak, R. L., Y. Song, M. D. Sciffer, and C. L. Waters (2015), Propagation of Pi2 pulsations in a dipole model of the magnetosphere, J. Geophys. Res. Space Physics, 120, doi:10.1002/2014JA020625, 2015
Lysak, R. L., and Y. Song, Development of parallel electric fields at the plasma sheet boundary layer, J. Geophys. Res., 116, A00K14, doi:10.1029/2010JA016424, 2011
Lysak, R. L., Y. Song, and T. W. Jones, Propagation of Alfvén waves in the magnetotail during substorms, Ann. Geophys., 27, 2237-2246, 2009
Lin, N., H. U. Frey, S. B. Mende, F. S. Mozer, R. L. Lysak, Y. Song, and V. Angelopoulos, Statistical study of the substorm timing sequence, J. Geophys. Res., 114, A12204, doi: 10.1029/2009JA14381, 2009
Lysak, R. L., and Y. Song, Propagation of kinetic Alfvén waves in the ionospheric Alfvén resonator in the presence of density cavities, Geophys. Res. Lett., 35, L20101, doi:10.1029/2008GL035728, 2008
Lysak, R. L., and Y. Song, Magnetosphere-ionosphere coupling by Alfvén waves: Beyond current continuity, Adv. Space Res.,38(8), 1713, 2006
Song, Y., and R. L. Lysak, The displacement current and the generation of parallel electric fields, Phys. Rev. Lett., 96, 145002, 2006
Lui, A.T.Y., C. Jacquey, G. S. Lakhina, R. Lundin, T. Nagai, T.-D. Phan, Z. Y. Pu, M. Roth, Y. Song, R. A. Treumann, M. Yamauchi, and L. M. Zelenyi, Critical Issues on Magnetic Reconnection, Space Sci. Rev., 116, 497, 2005
Lysak, R. L., and Y. Song, Non-local interactions between electrons and Alfvén waves on auroral field lines, J. Geophys. Res., 110, A10S06, doi:10.1029/2004JA010803, 2005
Lysak, R. L., and Y. Song, Kinetic theory of the Alfvén wave acceleration of auroral electrons, J. Geophys. Res., 108(A4), 8005, doi:10.1029/2002JA009406, 2003
Song, Y., Challenge to the magnetic reconnection hypothesis, Proceedings of the Magnetic Reconnection Meeting in Kiruna, Sweden, September 2002, R. Lundin and R. McGregor (eds). IRF Scientific Report 280, p. 25-35, 2003
Lysak, R. L., and Y. Song, Energetics of the ionospheric feedback interaction, J. Geophys. Res., 107(A8), 10.1029/2001JA000308, 2002
Lee, D.-H., M. K. Hudson, K. Kim, R. L. Lysak, and Y. Song, Compressional MHD wave transport in the magnetosphere, 1, Reflection and transmission across the plasmapause, J. Geophys. Res., 107(A10), 1307, doi:10.1029/2002JA009239, 2002
Lysak, R. L., and Y. Song, A three-dimensional model of the propagation of Alfvén waves through the auroral ionosphere: First results, Adv. Space Research, 28, 813, 2001
Song, Y., and R. L. Lysak, Towards a new paradigm: From a quasi-steady description to a dynamical description of the magnetosphere, Space Science Reviews, 95, 273, 2001
Song, Y., and R. L. Lysak, The physics in the auroral dynamo regions and auroral particle acceleration, Phys. Chem. Earth, 26, 33, 2001
Song, Y., and R. L. Lysak, Paradigm transition in cosmic plasma physics, magnetic reconnection and the generation of field-aligned current, in Magnetospheric Current Systems, S.-I. Ohtani et al. (eds.), AGU Monograph 118, American Geophysical Union, Washington, p. 11, 2000
Song, Y., Theoretical constraints on mechanisms for the substorm current wedge, in Substorms-4, S. Kokubun and Y. Kamide (eds.), Terra Scientific, Tokyo, p. 543, 1998
Song, Y., and R. L. Lysak, MHD mesoscale interactions at the magnetopause and driven reconnection, Physics of the Magnetopause, P. Song, B. U. Ö. Sonnerup, and M. Thomsen (eds.), AGU Monograph, American Geophysical Union, Washington, p. 349, 1995
Song, Y., and R. L. Lysak, Alfvénon, driven reconnection and the direct generation of field-aligned current, Geophys. Res. Lett., 21, 1755, 1994a
Song, Y., R. L. Lysak, and N. Lin, Control of the generation of field-aligned current and ULF waves by the magnetic helicity input, in Solar Wind Sources of Magnetospheric Ultra-Low-Frequency Pulsations, M. Engebretson, K. Takahashi and M. Scholer (eds.), Geophysical Monograph 81, American Geophysical Union, Washington, p. 223, 1994b
Song, Y. and R. L. Lysak, Solar-wind/magnetospheric dynamos: MHD scale collective entry of the solar wind energy, momentum and mass into the magnetosphere, Substorms I, European Space Agency, Paris, p. 149, Kiruna, Sweden, 1992
Song, Y., and R. L. Lysak, The current dynamo effect and its statistical description during 3-D time-dependent reconnection, Physics of Magnetic Flux Ropes, C. T. Russell, E. R. Priest, and L. C. Lee (eds.), American Geophysical Union, Washington, p. 533, 1990
Song, Y. and R. L. Lysak, Evaluation of twist helicity in FTE flux tubes, J. Geophys. Res., 94, 5273, 1989a
Song, Y. and R. L. Lysak, Current dynamo effect of 3-d time-dependent reconnection in the dayside magnetopause, Geophys. Res. Lett., 16, 911, 1989b