Turbulent Coronal Heating Mechanisms: Coupling of Dynamics and Thermodynamics

Context. Photospheric motions shuffle the footpoints of the strong axial magnetic field that threads coronal loops giving rise to turbulent nonlinear dynamics characterized by the continuous formation and dissipation of field-aligned current sheets where energy is deposited at small-scales and the heating occurs. Previous studies show that current sheets thickness is orders of magnitude smaller than current state of the art observational resolution (~700 km). Aim. In order to understand coronal heating and interpret correctly observations it is crucial to study the thermodynamics of such a system where energy is deposited at unresolved small-scales. Methods. Fully compressible three-dimensional magnetohydrodynamic simulations are carried out to understand the thermodynamics of coronal heating in the magnetically confined solar corona. Results. We show that temperature is highly structured at scales below observational resolution and nonhomogeneously distributed so that only a fraction of the coronal mass and volume gets heated at each time. Conclusions. This is a multi-thermal system where hotter and cooler plasma strands are found one next to the other also at sub-resolution scales and exhibit a temporal dynamics.Comment: A&A Letter, in pres

Numerical simulation of the compressible Orszag-Tang vortex 2. Supersonic flow

The numerical investigation of the Orszag-Tang vortex system in compressible magnetofluids will consider initial conditions with embedded supersonic regions. The simulations have initial average Mach numbers 1.0 and 1.5 and beta 10/3 with Lundquist numbers 50, 100, or 200. The behavior of the system differs significantly from that found previously for the incompressible and subsonic analogs. Shocks form at the downstream boundaries of the embedded supersonic regions outside the central magnetic X-point and produce strong local current sheets which dissipate appreciable magnetic energy. Reconnection at the central X-point, which dominates the incompressible and subsonic systems, peaks later and has a smaller impact as M increases from 0.6 to 1.5. Similarly, correlation between the momentum and magnetic field begins significant growth later than in subsonic and incompressible flows. The shocks bound large compression regions, which dominate the wavenumber spectra of autocorrelations in mass density, velocity, and magnetic field

Turbulence, Energy Transfers and Reconnection in Compressible Coronal Heating Field-line Tangling Models

MHD turbulence has long been proposed as a mechanism for the heating of coronal loops in the framework of the Parker scenario for coronal heating. So far most of the studies have focused on its dynamical properties without considering its thermodynamical and radiative features, because of the very demanding computational requirements. In this paper we extend this previous research to the compressible regime, including an energy equation, by using HYPERION, a new parallelized, viscoresistive, three-dimensional compressible MHD code. HYPERION employs a Fourier collocation -- finite difference spatial discretization, and uses a third-order Runge-Kutta temporal discretization. We show that the implementation of a thermal conduction parallel to the DC magnetic field induces a radiative emission concentrated at the boundaries, with properties similar to the chromosphere--transition region--corona system.Comment: 4 pages, 4 figures, Solar Wind 12 proceedings (in press

Stability and transition of the driven magnetohydrodynamic sheet pinch

The stability and transition properties of a bounded, current carrying magnetofluid are explored, using the hydrodynamic theory developed for plane shear flows as a guide. A driven magnetohydrodynamic sheet pinch equilibrium is employed. A sixth order, complex eigenvalue equation which governs the normal modes of small oscillations is derived, and solved numerically by the Chebyshev tau method. Eigenfunctions are shown, as well as the curve of neutral stability. The locus of critical Lundquist numbers has the form of a hyperbola. The nonlinear stability of a primary disturbance of the system is considered. For regions in parameter space close to criticality, a nonlinear stability equation of the Landau type is derived. These regions are characterized by low values of the Lundquist numbers, in contrast with the inviscid, highly conducting limit considered by Rutherford (1973). Amplitude phase planes for these disturbances are exhibited. The full set of two dimensional magnetohydrodynamic equations is solved numerically by a semi-implicit, mixed Fourier pseudospectral-finite difference algorithm. Both linear and random perturbations of the system are followed numerically into the nonlinear regime. Current sheets and deflection currents are nonlinear structures found to be significant to the evolution of the system. A secondary instability mechanism, the dynamic rupturing of the current density sheet, is also observed

Turbulent disruptions from the Strauss equations

The subject of this thesis is an analysis of results from pseudospectral simulation of the Strauss equations of reduced three-dimensional magnetohydrodynamics. We have solved these equations in a rigid cylinder of square cross section, a cylinder with perfectly conducting side walls, and periodic ends. We assume that the uniform-density magnetofluid which fills the cylinder is resistive, but inviscid. Situations which we are considering are in several essential ways similar to a tokamak-like plasma; an external magnetic field is imposed, and the plasma carries a net current which produces a poloidal magnetic field of sufficient strength to induce current disruptions. These disruptions are characterized by helical m = 1, n = 1 current filaments which wrap themselves around the magnetic axis. An ordered, helical velocity field grows out of the broad-band, low amplitude noise with which we initialize the velocity field. Kinetic energy peaks near the time the helical current filament disappears, and the current column broadens and is flattens itself out. We find that this is a nonlinear, turbulent phenomenon, in which many Fourier modes participate. By raising the Lundquist number used in the simulation, we are able to generate situations in which multiple disruptions are induced. When an external electric field is imposed on the plasma, the initial disruption, from a quiescent, state, is found to be very similar to those observed in the undriven runs. After the lobed m = 1, n = 1 stream function pattern develops, however, a quasi-steady state with flow is maintained for tens of Alfven transit times. If viscous damping is included in the driven problem, the steady state may be avoided, and additional disruptions produced in a time less than a large-scale resistive decay time

2013/14 Aircraft Operations Counts for Select Airports

This technical report presents aircraft operations estimates for three non-towered airports. Eight acoustic counts were taken at each airport, starting in April 2013 and ending in March 2014; each sample covered seven continuous days. The samples were extrapolated over the entire year, and the results are presented in this report. Results are used by multiple sources to monitor aircraft activity levels and as a base for planning and forecasting documents

Numerical simulation of solar coronal magnetic fields

Many aspects of solar activity are believed to be due to the stressing of the coronal magnetic field by footpoint motions at the photosphere. The results are presented of a fully spectral numerical simulation which is the first 3-D time dependent simulation of footpoint stressing in a geometry appropriate for the corona. An arcade is considered that is initially current-free and impose a smooth footpoint motion that produces a twist in the field of approx 2 pi. The footprints were fixed and the evolution was followed until the field relaxes to another current-free state. No evidence was seen for any instability, either ideal or resistive and no evidence for current sheet formation. The most striking feature of the evolution is that in response to photospheric motions, the field expands rapidly upward to minimize the stress. The expansion has two important effects. First, it suppresses the development of dips in the field that could support dense, cool material. For the motions assumed, the magnetic field does not develop a geometry suitable for prominence formation. Second, the expansion inhibits ideal instabilities such as kinking. The results indicate that simple stearing of a single arcade is unlikely to lead to solar activity such as flares or prominences. Effects are discussed that might possibly lead to such activity

Viscous, resistive MHD stability computed by spectral techniques

Expansions in Chebyshev polynomials are used to study the linear stability of one dimensional magnetohydrodynamic (MHD) quasi-equilibria, in the presence of finite resistivity and viscosity. The method is modeled on the one used by Orszag in accurate computation of solutions of the Orr-Sommerfeld equation. Two Reynolds like numbers involving Alfven speeds, length scales, kinematic viscosity, and magnetic diffusivity govern the stability boundaries, which are determined by the geometric mean of the two Reynolds like numbers. Marginal stability curves, growth rates versus Reynolds like numbers, and growth rates versus parallel wave numbers are exhibited. A numerical result which appears general is that instability was found to be associated with inflection points in the current profile, though no general analytical proof has emerged. It is possible that nonlinear subcritical three dimensional instabilities may exist, similar to those in Poiseuille and Couette flow

Primary Amide Raman Vibrations as Environmental and Structural Markers of Glutamine and Asparagine Protein Side Chains

UV resonance Raman (UVRR) is a powerful spectroscopic technique for the study of protein conformation and dynamics. Excitation at ~200 nm selectively enhances secondary amide vibrations, which are sensitive to the peptide backbone secondary structure and local environment. Primary amide bands are also resonance enhanced in UVRR spectra and could be used to report on glutamine and asparagine side chains in biophysical studies of proteins and peptides. IR absorption, visible Raman, and UVRR were used to investigate the small primary amide molecule propanamide. Dramatic spectral changes in the primary amide vibrations were observed upon aqueous solvation. Aqueous solvation impacts the dielectric and hydrogen bonding environment of the primary amide group resonance structures. This leads to a decrease in C--O and increase in C--N bond order of the primary amide group and therefore alters the resonance enhancement and vibrational frequencies of the primary amide vibrations substantially. Due to this significant response, several primary amide bands can be used as sensitive environmental markers for the glutamine and asparagine side chains. Visible Raman and UVRR spectra of L-glutamine and five derivative molecules, D-glutamine, N-Acetyl-L-glutamine, L-glutamine t-butyl ester, Glycyl-L-glutamine, and L-seryl-L-asparagine, were collected and assigned in the 950-1200 cm-1 region. The OCCC dihedral angle of each was determined from X-ray crystal structures. An empirical relationship between the AmIIIP vibrational frequency and the OCCC dihedral angle was observed. This dependence can be explained by hyperconjugation that occurs between the Cbeta--Cgamma sigma orbital and the C=O pi* orbital of the primary amide group. This interaction induces an increase in the Cbeta--Cgamma bond length. As the Cbeta--Cgamma bond length increases, the stretching force constant decreases, downshifting the AmIIIP band. Due to this sensitivity, the AmIIIP can be used as a structural marker diagnostic of the OCCC dihedral angle, such as in the side chains glutamine and asparagine in peptide and protein conformational studies