arXiv subjects

Å. Nordlund

Publications and source records attributed to Å. Nordlund.

24 records · Page 2Linked to original sources

arXiv2004

Magnetic Field Generation and electron acceleration in Collisionless Shocks

Using a three dimensional relativistic particle-in-cell code we have performed numerical experiments of plasma shells colliding at relativistic velocities. Such scenarios are found in many astrophysical objects e.g. the relativistic outflow from gamma ray bursts, active galactic nuclei jets and supernova remnants. We show how a Weibel-like two-stream instability is capable of generating small-scale magnetic filaments with strength up to percents of equipartition. Such field topology is ideal for the generation of jitter radiation as opposed to synchrotron radiation. We also explain how the field generating mechanism involves acceleration of electrons to power law distributions (N(E)~E^(-p))through a non-Fermi acceleration mechanism. The results add to our understanding of collisionless shocks.

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arXiv2004

An Ab Initio Approach to the Solar Coronal Heating Problem

We present an ab initio approach to the solar coronal heating problem by modelling a small part of the solar corona in a computational box using a 3D MHD code including realistic physics. The observed solar granular velocity pattern and its amplitude and vorticity power spectra, as reproduced by a weighted Voronoi tessellation method, are used as a boundary condition that generates a Poynting flux in the presence of a magnetic field. The initial magnetic field is a potential extrapolation of a SOHO/MDI high resolution magnetogram, and a standard stratified atmosphere is used as a thermal initial condition. Except for the chromospheric temperature structure, which is kept fixed, the initial conditions are quickly forgotten because the included Spitzer conductivity and radiative cooling function have typical timescales much shorter than the time span of the simulation. After a short initial start up period, the magnetic field is able to dissipate 3-4 10^6 ergs cm^{-2} s^{-1} in a highly intermittent corona, maintaining an average temperature of $\sim 10^6$ K, at coronal density values for which emulated images of the Transition Region And Coronal Explorer(TRACE) 171 and 195 pass bands reproduce observed photon count rates.

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arXiv2004

An Ab Initio approach to Solar Coronal Loops

Data from recent numerical simulations of the solar corona and transition region are analysed and the magnetic field connection between the low corona and the photosphere is found to be close to that of a potential field. The fieldline to fieldline displacements follow a power law distribution with typical displacements of just a few Mm. Three loops visible in emulated Transition Region And Coronal Explorer (TRACE) filters are analysed in detail and found to have significantly different heating rates and distributions thereof, one of them showing a small scale heating event. The dynamical structure is complicated even though all the loops are visible in a single filter along most of their lengths. None of the loops are static, but are in the process of evolving into loops with very different characteristics. Differential Emission Measure (DEM) curves along one of the loops illustrate that DEM curves have to be treated carefully if physical characteristics are to be extracted.

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arXiv2000

Solar Oscillations and Convection: II. Excitation of Radial Oscillations

Solar p-mode oscillations are excited by the work of stochastic, non-adiabatic, pressure fluctuations on the compressive modes. We evaluate the expression for the radial mode excitation rate derived by Nordlund and Stein (Paper I) using numerical simulations of near surface solar convection. We first apply this expression to the three radial modes of the simulation and obtain good agreement between the predicted excitation rate and the actual mode damping rates as determined from their energies and the widths of their resolved spectral profiles. We then apply this expression for the mode excitation rate to the solar modes and obtain excellent agreement with the low l damping rates determined from GOLF data. Excitation occurs close to the surface, mainly in the intergranular lanes and near the boundaries of granules (where turbulence and radiative cooling are large). The non-adiabatic pressure fluctuations near the surface are produced by small instantaneous local imbalances between the divergence of the radiative and convective fluxes near the solar surface. Below the surface, the non-adiabatic pressure fluctuations are produced primarily by turbulent pressure fluctuations (Reynolds stresses). The frequency dependence of the mode excitation is due to effects of the mode structure and the pressure fluctuation spectrum. Excitation is small at low frequencies due to mode properties -- the mode compression decreases and the mode mass increases at low frequency. Excitation is small at high frequencies due to the pressure fluctuation spectrum -- pressure fluctuations become small at high frequencies because they are due to convection which is a long time scale phenomena compared to the dominant p-mode periods.

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arXiv2000

Solar Oscillations and Convection: I. Formalism for Radial Oscillations

We present a formalism for investigating the interaction between pmode oscillations and convection by analyzing realistic, three-dimensional simulations of the near-surface layers of the solar convection zone. By choosing suitable definitions for fluctuations and averages, we obtain a separation that retains exact equations. The equations for the horizontal averages contain one part that corresponds directly to the wave equations for a 1-D medium, plus additional terms that arise from the averaging and correspond to the turbulent pressure gradient in the momentum equation and the divergence of the convective and kinetic energy fluxes in the internal energy equation. These terms cannot be evaluated in closed form, but they may be measured in numerical simulations. The additional terms may cause the mode frequencies to shift, relative to what would be obtained if only the terms corresponding to a 1-D medium were retained---most straightforwardly by changing the mean stratification, and more subtly by changing the effective compressibility of the medium. In the presence of time dependent convection, the additional terms also have a stochastic time dependence, that acts as a source of random excitation of the coherent modes. In the present paper, we derive an expression for the excitation power and test it by applying it to a numerical experiment of sufficient duration for the excited modes to be spectrally resolved.

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arXiv1998

Test of Convective Frequency Effects with SOI/MDI High-Degree Data

Advances in hydrodynamical simulations have provided new insights into the effects of convection on the frequencies of solar oscillations. As more accurate observations become available, this may lead to an improved understanding of the dynamics of convection and the interaction between convection and pulsation (\cite{Rosenthal+99}). Recent high-resolution observations from the SOI/MDI instrument on the SOHO spacecraft have provided the so-far most-detailed observations of high-degree modes of solar oscillations, which are particularly sensitive to the near-surface properties of the Sun. Here we present preliminary results of a comparison between these observations and frequencies computed for models based on realistic simulations of near-surface convection. Such comparisons may be expected to help in identifying the causes of the remaining differences between the observed frequencies and those of solar models.

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