Element Fractionation by the Ponderomotive Force

Abstract

We describe a qualitatively new effect recently identified in solar physics and astrophysics, that of ion-neutral separation in the chromosphere caused by the ponderomotive force associated with Alfvén and fast mode waves. This provides a comprehensive explanation of coronal abundance anomalies observed in various regions of the solar corona and wind, and also in the coronae of late-type stars. Since the fractionation is connected to the initial energization of the solar atmosphere, rather being an endpoint of the thermalization of the energy input, as would be the case for quasi-thermal means of fractionation such as the various forms of diffusion, we argue that these abundance anomalies are a promising route to understanding many facets of solar activity and plasma energization.Exploiting these insights will require renewed emphasis on instrumentation and methods to measure element abundances, both remotely sensed spectroscopy and in situ detections, and theoretical work to interpret these data. We recommend: (1) High-throughput UV-EUV spectroscopy to study the time variation of the FIP fractionation on and off-disk; (2) routine deployment of solar wind mass and mass/charge spectrometers for on-satellite in situ measurements. Such an instrument is badly missed from Parker Solar Probe; and (3) significant funding for associated theory and modeling to support the data analysis. Profound questions are rarely answered with data alone.Direct observations of (Alfvénic) waves in various parts of the solar atmosphere are also becoming, if not routine, at least more commonly performed. From the lower chromosphere through the corona and out into the solar wind, waves are observed remotely and in situ by instrumentation such as IBIS on the Richard Dunn Solar Telescope, Coronal Multichannel Polarimeter (CoMP) at the Mauna Loa Solar Observatory, and the Fields suite on Parker Solar Probe, to name a few, and coronal abundance anomalies represent a direct consequence of these waves.White papers on related science issues have been submitted by A. Vourlidas, Y.-K. Ko, and Y. J. Rivera. DISTRIBUTION A. Approved for public release: Distribution unlimited.