Vlasov Code Simulation of Anomalous Resistivity

Otto

et al. 2010

A&A

Aims. We study the particle acceleration at a magnetic null point in the solar corona, considering self-consistent magnetic fields, plasma flows and the corresponding convective electric fields. Methods. We calculate the electromagnetic fields by 3-D magnetohydrodynamic (MHD) simulations and expose charged particles to these fields within a full-orbit relativistic test-particle approach. In the 3-D MHD simulation part, the initial magnetic field configuration is set to be a potential field obtained by extrapolation from an analytic quadrupolar photospheric magnetic field with a typically observed magnitude. The configuration is chosen so that the resulting coronal magnetic field contains a null. Driven by photospheric plasma motion, the MHD simulation reveals the coronal plasma motion and the self-consistent electric and magnetic fields. In a subsequent test particle experiment the particle energies and orbits (determined by the forces exerted by the convective electric field and the magnetic field around the null) are calculated in time.Results. Test particle calculations show that protons can be accelerated up to 30 keV near the null if the local plasma flow velocity is of the order of 1000 km s −1 (in solar active regions). The final parallel velocity is much higher than the perpendicular velocity so that accelerated particles escape from the null along the magnetic field lines. Stronger convection electric field during big flare explosions can accelerate protons up to 2 MeV and electrons to 3 keV. Higher initial velocities can help most protons to be strongly accelerated, but a few protons also run the risk to be decelerated. Conclusions. Through its convective electric field and due to magnetic nonuniform drifts and de-magnetization process, the 3-D null can act as an effective accelerator for protons but not for electrons. Protons are more easily de-magnetized and accelerated than electrons because of their larger Larmor radii. Notice that macroscopic MHD simulations are blind to microscopic magnetic structures where more non-adiabatic processes might be taking place. In the real solar corona, we expect that particles could have a higher probability to experience a de-magnetization process and get accelerated. To trigger a significant acceleration of electrons and even higher energetic protons, however, the existence of a resistive electric field mainly parallel to the magnetic field is required. A physically reasonable resistivity model included in resistive MHD simulations is direly needed for the further investigations of electron acceleration by parallel electric fields.

Section: The Field Structure -Mhd Simulation Resultsmentioning

confidence: 99%

Section: Summary and Discussionmentioning

confidence: 99%

Is the 3-D magnetic null point with a convective electric field an efficient particle accelerator?

Guo

Otto

et al. 2010

A&A

“…We have considered an anomalous plasma resistivity to describe the heating by current dissipation. This resistivity can be justified by the physical processes that explain current dissipation in low collisional plasma (Büchner & Elkina 2005, 2006. Our plasma resistivity is constant in time and uniform in the xy-plane.…”

Section: Methodsmentioning

confidence: 78%

Nonlocal heat flux effects on temperature evolution of the solar atmosphere

Silva

Santos

et al. 2018

A&A

Context. Heat flux is one of the main energy transport mechanisms in the weakly collisional plasma of the solar corona. There, rare binary collisions let hot electrons travel over long distances and influence other regions along magnetic field lines. Thus, the fully collisional heat flux models might not describe transport well enough since they consider only the local contribution of electrons. The heat flux in weakly collisional plasmas at high temperatures with large mean free paths has to consider the nonlocality of the energy transport in the frame of nonlocal models in order to treat energy balance in the solar atmosphere properly. Aims. We investigate the impact of nonlocal heat flux on the thermal evolution and dynamics of the solar atmosphere by implementing a nonlocal heat flux model in a 3D magnetohydrodynamic simulation of the solar corona. Methods. We simulate the evolution of solar coronal plasma and magnetic fields considering both a local collision dominated and a nonlocal heat flux model. The initial magnetic field is obtained by a potential extrapolation of the observed line-of-sight magnetic field of AR11226. The system is perturbed by moving the plasma at the photosphere. We compared the simulated evolution of the solar atmosphere in its dependence on the heat flux model. Results. The main differences for the average temperature profiles were found in the upper chromosphere/transition region. In the nonlocal heat transport model case, thermal energy is transported more efficiently to the upper chromosphere and lower transition region and leads to an earlier heating of the lower atmosphere. As a consequence, the structure of the solar atmosphere is affected with the nonlocal simulations producing on average a smoother temperature profile and the transition region placed about 500 km higher. Using a nonlocal heat flux also leads to two times higher temperatures in some of the regions in the lower corona. Conclusions. The results of our 3D MHD simulations considering nonlocal heat transport supports the previous results of simpler 1D two-fluid simulations. They demonstrated that it is important to consider a nonlocal formulation for the heat flux when there is a strong energy deposit, like the one observed during flares, in the solar corona.

“…The threshold for current and gradient drift instabilities may be formulated in terms of a critical drift velocity of the current carriers (Büchner & Elkina 2005). This can be taken into account within the simulation by comparing the current-carrier velocity v cc = j/ne obtained from the macroscopic MHD plasma parameters with a critical value v crit by applying additional anomalous (i.e., turbulent) resistivity when v cc exceeds this critical value according to the relation…”

Section: Group 2: Variation In the Coefficient Of Localized Turbulentmentioning

confidence: 99%

“…The appeal is often made, Current address: Embry-Riddle Aeronautical University, Daytona Beach, FL 32114, USA, e-mail: adamsone@erau.edu therefore, to a turbulence driven anomalous resistivity in order to provide the required amount of dissipation (see, e.g., Spangler 2009). Such turbulence, however, is only generated in regions of strong current concentration on kinetic scales (Büchner & Elkina 2005). Unfortunately, the typical scales down to which characteristics of the magnetized plasma in the corona are resolved (as well as the typical MHD simulation scales) are much larger than the scales at which such turbulence is generated.…”

Section: Introductionmentioning

confidence: 99%

On the role of current dissipation in the energization of coronal bright points

Adamson

Otto

2013

A&A

Aims. In the context of the contributions by DC dissipation to the energization of solar coronal X-ray and EUV bright points, we analyze the role of Joule heating in the formation of X-ray and EUV coronal bright points through current dissipation due to anomalous resistivity. Methods. We utilized the improved resistive MHD simulation code MPSCORONA3D, which allows for investigating resistive effects within the corona over a wide range of Reynolds numbers, starting from the nearly ideal Spitzer value of resistivity and including various models of current-carrier velocity dependent resistivity, arising from the consideration of turbulence in microscopic theory. Results. DC current dissipation contributes to the thermal energy increase in coronal bright point regions only under the assumption of an unrealistically low critical current-carrier velocity at which turbulent resistivity is switched on. For more realistic resistivity models, the corona is energized by plasma compression rather than by Joule heating due to the dissipation of direct currents. Conclusions. Joule heating within the solar corona depends critically on the magnitude of the resistivity.