Comparison of SEP derived abundances (adapted from Ko et al. 2012) with models for closed and open magnetic field regions. Gradual SEPs from an active region footpoint (red triangles) agree very well with the closed field model (red crosses). Open field model and data are shown in blue.
Comparison of SEP derived abundances (adapted from Ko et al. 2012) with models for closed and open magnetic field regions. Gradual SEPs from an active region footpoint (red triangles) agree very well with the closed field model (red crosses). Open field model and data are shown in blue.

Objective
Advance the understanding of the variable elemental composition of the Sun’s corona; a vital step towards the forecasting of the solar Extreme Ultraviolet (EUV) irradiance. Solar EUV radiation, mainly from ions of iron (Fe), is absorbed in the thermosphere, where it heats the ambient gas causing its scale height to lengthen, thereby increasing the density and associated drag on satellites in the earth’s upper atmosphere. Since the potential exists for debris collisions with operating spacecraft producing further explosions, thereby increasing the debris field in a runaway fashion, mitigation strategies, including forecasting the Solar EUV irradiance, are key.

Approach
Solar coronal elemental fractionation was first detected in 1963 by Pottasch. A viable explanation only recently emerged (Laming 2004, 2009, 2012). The important features are:

  • Elements ionized in the chromosphere (e.g. Fe, Si, Mg, with first ionization potential less than about 10 eV) are enhanced in coronal abundance by about x 3-4. This is called the “First Ionization Potential (FIP) effect”
  • The FIP effect arises as Alfvén or fast mode waves interact with chromospheric ions (but not neutrals), through a combination of wave pressure and refraction forces, known collectively as the ponderomotive force.
  • If the dominant waves have a coronal origin—as opposed to photospheric—a number of other facets of the abundance anomaly can be theoretically explained (with resonant absorption or nanoflares as the mechanisms responsible for coronal heating)

Deliverable/Value/Accomplishment

  • Strong chromospheric magnetic field (detectable with helioseismology) suppresses FIP fractionation. Applied to the farside of the sun, 7-day forecasts of coronal element abundances in active regions and flares appearing on the east limb of the sun become possible
  • The variation of FIP effect in other coronal structures (quiet sun, coronal holes) and on other stars can be understood on the basis of this model
  • The solar wind depletion of He, another longstanding puzzle in heliophysics, is also solved with this model, with implications for the origin of the slow speed solar wind