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Element Fractionation by the Ponderomotive Force

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If you have a question about this talk, please contact Callum William Fairbairn.

Since 1963, we have known, or at least suspected, that element abundances in the solar corona and wind are different to those in the photosphere. Elements that are predominantly ionized in the photosphere and chromosphere, e.g. Fe, Si, Mg, are observed to be enhanced in abundance in the corona and wind by a factor of typically 3-4. In the 1990’s when abundance measurements in stellar coronae became possible, similar effects were seen in solar-like stars. An inverse effect, the depletion of Fe, Si, Mg, etc was discovered in M dwarfs and more active binary stars. Due to the dependence on the ionization potential, these phenomena have been dubbed the First Ionization Potential (FIP) Effect, or the Inverse FIP (IFIP) Effect. A model capturing both the FIP and IFIP effects invokes ion-neutral separation by the ponderomotive force due to Alfven and magnetosonic waves propagating through the chromosphere. More recently, Inverse FIP effect has been identified in solar flares, both in small regions of otherwise FIP enhanced flare plasma, and in bulk spatially unresolved solar flares. The spatially resolved flare observations indicate a connection with sub-photospheric reconnection, which is presumably a source of waves that become fast modes in the solar chromosphere. These selectively deplete ions through the ponderomotive force. Alfven waves have also been detected in the chromosphere beneath an active region, connected by magnetic field lines to coronal regions of strong FIP enhancement. These observations support the ponderomotive force model of the fractionation. Fractionation by such means is new to solar physics and astrophysics, but is not new to science. Manipulation of atoms, molecules, biological samples, etc. by the forces due to refraction of photons from lasers, known as “optical tweezers”, has a rather long history in optical sciences, and won Nobel Prizes for Steven Chu (in 1997) and Arthur Ashkin (in 2018). Our model is a precise analog of this work, but with magnetohydrodynamic waves instead of optical photons. This connection leads to a better physical understanding of the mechanisms at work in FIP or Inverse FIP fractionated plasma. Work supported by NASA Heliophysics Guest Investigator (80HQTR19T0029) and Supporting Research Programs (80HQTR20T0076), and by Basic Research Funds of the Office of Naval Research.

This talk is part of the DAMTP Astro Mondays series.

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