Alfvén Ionisation in the Photosphere as a Key Driver for the FIP Bias in the Solar Atmosphere

1. Astronomy & Astrophysics Group School of Physics and Astronomy Alfvén Ionisation in the Photosphere as a Key Driver for the FIP Bias in the Solar Atmosphere Procheta C.V. Mallik Hugh E. Potts, Lyndsay Fletcher, Declan A. Diver RHESSI Workshop, Annapolis, MD, USA, 1-5 August 2010

2. Presentation Outline • Certain elements are more abundant in the solar atmosphere than in the photosphere : 4-10 x photospheric levels in some cases. • Conventional models are complex, and only partially successful. Here we present: • An overview of the observations • Our proposal that Alfvén Ionisation (AI) as the likely mechanism • Possibility of -ray spectroscopic observations in support of AI RHESSI Workshop, Annapolis, MD, USA, 1-5 August 2010

3. FIP Bias: an overview Streamer near limb Polar coronal hole Showing abundance anomalies as function of first ionisation potential (from Feldman & Widing, Phys of Plasmas, 9, 629, 2002) RHESSI Workshop, Annapolis, MD, USA, 1-5 August 2010

4. Alfvén Ionisation: 1 • Surface photospheric flow, when impinging on a magnetised plasma, can ionise particular elements by AI • The flow provides the energy source and the ionisation mechanism • Certain elements are preferentially ionised; others are left neutral • This is determined by the Critical Ionisation Velocity (CIV) of the element Alfvén Ionisation (AI) is a mechanism in which the kinetic energy of neutral gas flow energises electrons to ionisation energies, via gas-plasma interactions. RHESSI Workshop, Annapolis, MD, USA, 1-5 August 2010

5. + - + + - - + + + - - - - + + + - - - + - + + - - - - + + + + - - - + + + - + - Alfvén Ionisation: 2 Ions displaced by neutrals, leaving excess electrons positive ions Neutral gas + Neutral gas impinges on magnetised plasma creating a charge imbalance after collisions. Resulting self-electric field excites electrons to new potential, thereby inducing Alfvén ionisation. Alfvén proposal: 1942 - + + - - + + + - - - - + + + - - - + + + - - - - + + - + - + - + + - - electrons

6. Alfvén Ionisation: 3 In 1961, Fahleson experimentally verified Alfven’s proposition. Constant ‘burning’ voltage regardless of current shows that there is a critical voltage (and hence relative gas-plasma speed) which cannot be exceeded until the gas is fully ionised. This burning voltage is different for different species, consistent with AI. Plasma discharge between two concentric cylinders is driven by ExB azimuthally around the cylinder, against the neutral gas (Fahleson, Phys Fluids 4 123, 1961)

7. Solar Context: 1 • Strong photospheric flows in active regions are ideal conditions for AI • Since AI is species-selective, could provide simple explanation of abundance anomalies • Preferential ionisation of low CIV species leads to trapping of ions in magnetic structure, followed by diffusion upwards (density gradient) and eventual release into solar atmosphere RHESSI Workshop, Annapolis, MD, USA, 1-5 August 2010

8. Solar Context: 2 Solar abundance data grouped by FIP shows most anomalies below 9eV, but still some outliers (notably Xe) Solar abundance data grouped by CIV shows much better correlation, and crucially, an underlying physical reason Diver, Fletcher & Potts, Solar Physics 227, 207 (2005).

9. Solar Context: 3 Grouping of elements is more correlated to their CIVs (right) than their FIPs (left)

10. AI Simulation: 1 Key step in understanding comes from simulating behaviour of pockets of unbalanced electrons produced by gas flow impacting on magnetised plasma • Electron ensemble is energised by self-electric field (mutual repulsion) in presence of B • Simulation of evolution performed under different magnetic field and density conditions. • electrons can be accelerated to impact ionisation energies in certain conditions. • In magnetically dominated cases: electron energy distribution shows that typically few % can exceed the initial electrostatic potential associated with the unbalanced ensemble of electrons. Magnetic domination governed by RHESSI Workshop, Annapolis, MD, USA, 1-5 August 2010

11. AI Simulation: 2 Initial Final Final electron energy distribution Electron density Electric potential 10-3 Initial maximum of potential Magnetic domination means expansion of electron cloud not spherical: perpendicular electrons constrained & parallel electrons reach higher energies. Average energy See MacLachlan, Diver & Potts, New Journal of Physics 11, 063001 (2009)

12. AI Simulation Summary • Simulation value of PE = 10-3 • ~0.8% of the electrons accelerated to energies above the initial potential • Assuming a magnetic field strength B of between 0.1 and 1 T, implies electron densities of1017 and 1019 m-3 • elements with FIPs up to 9.9 and 11.93 eV, respectively, will be preferentially ionised in this scenario, provided flow speeds are 9 km/s or more • Given that the H density is about 1023 m-3, and abundances of elements are between 10-5 and 10-10 of the H density, there are ample electrons energetic enough to fully ionise low-FIP species RHESSI Workshop, Annapolis, MD, USA, 1-5 August 2010

13. Gamma ray spectroscopy: 1 • Corroborating evidence may be provided by -ray spectroscopy ... • Nuclear de-excitation lines in chromosphere/upper photosphere • candidates: He, C, N, O, Ne, Mg, Si and Fe, the last 3 are low-FIP elements • Murphy 1997 et al, Gan 2002, Murphy 2007 etc conclude that the FIP-bias exists in the corona and chromosphere Derived ambient chromospheric abundances relative to the photosphere with the C ratio normalized to 1 (solid). Also shown are coronal abundances relative to the photosphere (dotted). Murphy 2007, SSRv, 130, 127

14. Gamma ray spectroscopy: 2 • This suggests that the ionisation mechanism occurs at lower altitudes • Data also suggest that abundances vary from flare to flare and more crucially, change during the course of a flare – possible influence of flow variation? • Low FIP to high FIP element ratio is typically enhanced at higher altitudes, but some high FIP elements, like Ne, also show enhancement .... • ...this could point to the fact that metastable states of noble gases, that have a low ionisation potential, could be the reason for this anomaly, • for example, Ne has metastable states at around 16.7eV, leaving <5 eV for ionisation RHESSI Workshop, Annapolis, MD, USA, 1-5 August 2010

15. Conclusions • Alfven ionisation can explain the over abundance of certain elements in the upper solar atmosphere, since flow speeds are quite often in the 10 km/s range • In a magnetically dominated plasma (PE < 1), electrons get accelerated to energies exceeding the ionisation threshold of low-FIP elements, thereby preferentially ionising these minority species present in neutral gas flows • If photospheric flows of a neutral gas exceed the CIV of an element, then the element is likely to get ionised by AI, resulting in its over-abundance in the upper solar atmosphere • Using spectroscopic data to deduce the abundance of certain low-CIV elements, it should be possible to determine the ionisation source location, and correlate with horizontal surface flow-speed at a given active region RHESSI Workshop, Annapolis, MD, USA, 1-5 August 2010