Profiles in Science

HAO 2012 Profiles In Science: Dr. Boon Chye Low

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Dr. Boon Chye (BC) Low graduated in mathematics in 1968 from the University of London, UK, and received his Ph.D. in physics in 1972 from the University of Chicago. He works on the magnetohydrodynamics of the solar atmosphere, with particular interests in the theory of basic physical processes and in the solar corona as an integral natural system. He is also interested in the applied and computational mathematics of magnetohydrodynamic processes, collaborating with Natasha Flyer and Piotr Smolarkiewicz at the NCAR Institute for Mathematics Applied to Geosciences. BC joined NCAR in 1981 as Scientist I, became Senior Scientist in 1987, and served as Acting Director of HAO in 1989.

Summary of Achievements

Overview

BC Low and his collaborators have made sustained contributions to our physical understanding of the solar corona as a hydromagnetic atmosphere. They work on the corona's large-scale evolution in response to the Sun's eleven-year cyclical magnetic-field reversals. Their works focus on the hydromagnetic processes basic to the corona and its couplings downward to the dense chromosphere and photosphere, and upward to the heliosphere.

The study of the Solar-Terrestrial System is not entirely an applied science, for the basic science for application is not all in place. New phenomena from ground- and space-based observatories are a major drive of our research. The discovery and understanding of basic hydromagnetic processes go hand in hand with the application of that knowledge to describe and predict the behaviors of the Solar-Terrestrial System. This kind of work builds the scientific basis for developing the capability of predicting space-weather.

In one hydromagnetic mode the million-degree hot corona behaves as a perfect electrical conductor over most observable length scales. Its magnetic fields of about 10 Gauss or stronger tend to evolve as though they are frozen-into the coronal plasma with no change in their topologies. This property is the means of storing free magnetic energy in the corona; the preservation of field topology demands for a specific measure of irremovable electric currents in the corona. Just this property of an invariant field topology brings the fields to also behave in a different hydromagnetic mode, the intensification of electric current density to unlimited magnitudes in thin sheets so that resistive heating sets in despite the high coronal electrical conductivity. Such a heating process is described by E. N. Parker, University of Chicago, in his book Spontaneous Current Sheets in Magnetic Fields (1994, Oxford University Press), with a realm of challenging basic questions being addressed in current research (A. M. Janse, B. C. Low & E. N. Parker 2010, Phys. Plasmas 17, 092901). This theory is basic to understanding quiescent heating in the corona as well as flares, prominences, and coronal mass ejections, the three major phenomena of coronal activity.

Central to this area of work is the magnetic helicity as a measure of field topology. Although field topology changes during magnetic reconnection via current-sheet dissipation, helicity gets transferred among the sub-systems of magnetic flux such that the net helicity of a coronal structure is approximately conserved, a consequence of the high electrical conductivity. In the course of an eleven-year cycle, new magnetic flux emerges from the solar interior into the corona to eventually reverse the polarity of the global coronal magnetic field typically within about three years into the cycle. Magnetic-flux systems do bodily make their way into the corona creating large scale structures with much reconnection and heating over a broad range of temporal and spatial scales. The flux systems emerge with fresh helicity and, by their coalescence into a growing coronal structure, this emergence results in a build-up of helicity in that structure. The three major coronal phenomena can thus be understood in basic terms: (1) the flare being explosive plasma heating by helicity-conserving dissipation of spontaneous current sheets; (2) the quiescent prominence being the manifestation of a macroscopically stable large-scale, twisted magnetic flux-rope formed by the accumulation of helicity; and (3) the CME being the bodily ejection of such a coronal structure whose accumulated helicity has exceeded a forbidding theoretical threshold. This theory of the corona has withstood the observational tests of three solar cycles of space and ground-based observations (B. C. Low 1996, Solar Phys. 167, 217; B. C. Low 2001, J. Geophys. R. 106, 25141; M. Zhang & B. C. Low 2005, Ann. Rev. Astron. Astrophys. 43, 103).

Power-point Lecture Presentations

The power-point presentations of BC's lectures:

Lecture 1: Basic magnetohydrodynamics
Lecture 2: Coronal heating & spontaneous current-sheet formation
Lecture 3: The solar wind & related coronal phenomena

These lectures were given at the International Solar/Space Physics Summer School, July 2011 at University of Science and Technology of China, Hefei, China and can be downloaded from http://download.hao.ucar.edu/pub/low/USTC_Lectures/.

Publications

Instruments onboard the satellites Hinode and Solar Dynamics Observatory, respectively, have delivered coronal observations of unprecedented qualities in spatial, temporal and spectral resolutions. One of the major impacts of these new observations is in the understanding of the basic processes in solar quiescent prominences.

Tom Berger (formerly Lockheed-Martin, now NSO) led the discovery and study of the constant restless dynamical state in the prominence interior at scales of 300 km and minutes, with descending and rising motions at well below the free-fall speeds, leading to the proposal that the prominence and its cavity are a low-b magneto-thermal convective structure associated with a twisted magnetic flux-rope (Berger et al. 2011, Nature 472, 197). Mass and magnetic flux are cycled to maintain both the prominence and a gradual growth of the cavity with accumulation of helicity to the point of a CME eruption.

(1) Liu, W.; Berger, T. E.; & Low, B. C. 2012, First SDO/AIA Observation Of Solar Prominence Formation Following An Eruption: Magnetic Dips And Sustained Condensation And Drainage, ApJ 745, L21(doi:10.1088/2041-8205/745/2/L21).
Summary: Observations of this prominence formation show that a mass of the order of 1015 g, comparable to the mass of a CME, can drain down a quasi-static quiescent prominence over a 20 hour period, with the prominence total mass slowly varying around a mean value of 1014 g at any time. The result shows that a macroscopically quiescent prominence is microscopically dynamic, involving the passage of a significant mass through it, maintained by a continual mass supply against a comparable mass drainage, which bears important implications for CME initiation mechanisms in which mass unloading is important. This result also corroborates the Berger et al. proposal that the prominence-cavity is a magneto-convective structure.

(2) Berger, T. E.; Liu, W.; & Low, B. C. 2012, SDO/AIA Detection of Solar Prominence Formation within a Coronal Cavity, ApJ 758, L37 (doi:10.1088/2041-8205/758/2/L37).
Summary: Observations of this prominence disappearance and re-formation give the first detection of the condensation of the million-degree plasma in a cavity into the cool prominence at chromospheric temperatures, in support of the Berger et al. proposal of a magneto-thermal convection in coronal magnetic flux ropes.

(3) Low, B. C.; Berger, T. ; Casini, R.; & Liu, W. 2012, The Hydromagnetic Interior Of A Solar Quiescent Prominence. I. Coupling Between Force Balance And Steady Energy Transport, ApJ 755, 34 (doi:10.1088/0004-637X/755/1/34).
Summary: This series of theoretical works attributes the observed constant state of motions in the interior of a quiescent prominence to the resistive dissipation of extremely thin sheets of electric current. These sheets form under a high degree of frozen-in condition to such a thinness that this condition must breakdown. In this first paper, it is shown that current sheets develop readily because the balance of heating, radiative loss, and thermal conduction restricted along the magnetic field generally cannot avoid a thermal collapse into these sheets.

(4) Low, B. C. ; & Liu, W. ; Berger, T. ; & Casini, R. 2012, The Hydromagnetic Interior Of A Solar Quiescent Prominence. Ii. Magnetic Discontinuities And Cross-Field Mass Transport, ApJ 757, 21 (doi:10.1088/0004-637X/757/1/21).
Summary: In this second paper, it is shown that the thermal diffusion of heat across the magnetic field is suppressed to an even stronger degree than the small magnetic resistive diffusion in the corona and prominence. The thermal equilibrium along adjacent thin flux tubes thus produces uncorrelated thermal structures along the tubes. This leads readily to fluid-pressure discontinuities across flux tubes to be balanced by compensating magnetic-pressure discontinuities. The latter implies extremely-thin sheets of electric current that must dissipate resistively at the weak but non-zero electric resistivity of a real plasma. Papers 3 and 4 demonstrate special cases of the Parker spontaneous current sheets, forming readily in the prominence environment. Thus, the constant state of motions in prominence interior has a resistive origin.

Solutions of force-free magnetic fields in the unbounded space external to a unit sphere are generally intractable. The simplification of axisymmetry, combined with numerical methods, allows for constructing explicit solutions to investigate the elementary properties of these fields, among which is the storage of magnetic helicity.

(5) Zhang, M.; Flyer, N.; & Low, B. C. 2012, Magnetic Helicity Of Self-Similar Axisymmetric Force-Free Fields, ApJ 755, 78 (doi:10.1088/0004-637X/755/1/78).
Summary: This ongoing work investigates the amount of helicity that a force-free field in an open atmosphere may store. Its principal result is the demonstration that a radially self-similar force-free field among all the axisymmetric force-free fields that share with it the same normal-flux distribution at the unit sphere, is characterized with a maximum storage of helicity.