Ponderomotive Acceleration in Coronal Loops

Abstract

Ponderomotive acceleration has been asserted to be a cause of the First Ionization Potential (FIP) effect, the by now well known enhancement in abundance by a factor of 3-4 over photospheric values of elements in the solar corona with FIP less than about 10 eV. It is shown here by means of numerical simulations that ponderomotive acceleration occurs in solar coronal loops, with the appropriate magnitude and direction, as a "byproduct" of coronal heating. The numerical simulations are performed with the HYPERION code, which solves the fully compressible three-dimensional magnetohydrodynamic equations including nonlinear thermal conduction and optically thin radiation. Numerical simulations of a coronal loops with an axial magnetic field from 0.005 Teslas to 0.02 Teslas and lengths from 25000 km to 75000 km are presented. In the simulations the footpoints of the axial loop magnetic field are convected by random, large-scale motions. There is a continuous formation and dissipation of field-aligned current sheets which act to heat the loop. As a consequence of coronal magnetic reconnection, small scale, high speed jets form. The familiar vortex quadrupoles form at reconnection sites. Between the magnetic footpoints and the corona the reconnection flow merges with the boundary flow. It is in this region that the ponderomotive acceleration occurs. Mirroring the character of the coronal reconnection, the ponderomotive acceleration is also found to be intermittent.

The First Ionization Potential Effect from the Ponderomotive Force: On the Polarization and Coronal Origin of Alfvén Waves

“…This probable necessity of resonant waves implies a coronal origin for the waves, presumably as a result of nanoflares (e.g. Dahlburg et al 2016).…”

“…Ruderman & Roberts 2002) or nanoflares (e.g. Dahlburg et al 2016). In the former case torsional m = 1 Alfvén waves result from the mode conversion of a kink oscillation at the plasma layer within a loop where the Alfvén wave transit time from one loop footpoint to the other matches the kink oscillation period (e.g.…”

“…Based partly on the simulations in Dahlburg et al (2016;see also Kigure et al 2010), we argue that coronal nanoflares release Alfvén waves and heat. The Alfvén waves cause FIP fractionation at loop footpoints magnetic connected to the nanoflare site, followed by evaporation as the electrons conduct the heat down to the chromosphere.…”

“…Alternatively, waves generated in the corona will naturally be on resonance (e.g. Ruderman & Roberts 2002;Dahlburg et al 2016), at least until chromospheric evaporation or other activity changes the coronal Alfvén speed or loop length. Such waves reflect from the top of the chromosphere back into the corona.…”

Determining the Elemental and Isotopic Composition of the Pre-solar Nebula from Genesis Data Analysis: The Case of Oxygen

“…Although it is possible for waves ultimately deriving from convection within the solar envelope to enter coronal loops at footpoints and propagate into the corona, typically the periods of these waves (three or five minutes) are too long for resonance. Resonant waves are most plausibly excited within the coronal loop itself, most likely as a byproduct of the mechanism(s) that heat the corona (Dahlburg et al 2016). In open field regions, such a resonance does not exist, and only waves propagating up from footpoints are possible.…”