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

Procheta C.V. MallikHugh E. Potts, Lyndsay Fletcher, Declan A.

Diver

Astronomy & Astrophysics GroupSchool of Physics and Astronomy

RHESSI Workshop, Annapolis, MD, USA, 1-5 August 2010

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

Presentation Outline

RHESSI Workshop, Annapolis, MD, USA, 1-5 August 2010

FIP Bias: an overview

Showing abundance anomalies as function of first ionisation potential

(from Feldman & Widing, Phys of Plasmas, 9, 629, 2002)

Polar coronal hole Streamer near limb

RHESSI Workshop, Annapolis, MD, USA, 1-5 August 2010

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

Alfvén Ionisation: 2

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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

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Neutral gaspositive ions

electrons

Ions displaced by neutrals, leaving excess electrons

Alfvén Ionisation: 3

In 1961, Fahleson experimentally verified Alfven’s proposition.

Plasma discharge between two concentric cylinders is driven by ExB azimuthally around the cylinder, against the neutral gas (Fahleson, Phys Fluids 4 123, 1961)

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.

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

Solar Context: 1

RHESSI Workshop, Annapolis, MD, USA, 1-5 August 2010

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).

Solar Context: 2

Symbol At No. rel abun ϕi (eV) vc (km/s)

K 19 1.35E-07 4.34 4.63

Na 11 2.06E-06 5.14 6.57

Li 3 2.05E-09 5.39 12.24

Al 13 3.04E-06 5.99 6.54

Ca 20 2.19E-06 6.11 5.42

Sc 21 1.23E-09 6.56 5.31

V 23 1.05E-08 6.75 5.06

Cr 24 4.83E-07 6.77 5.01

Ti 22 8.60E-08 6.83 5.25

Mn 25 3.42E-07 7.43 5.11

Ni 28 1.77E-06 7.64 5.01

Mg 12 3.85E-05 7.65 7.79

Co 27 8.07E-08 7.88 5.08

Fe 26 3.22E-05 7.9 5.22

Si 14 3.58E-05 8.15 7.48

B 5 7.60E-10 8.3 12.17

Be 4 2.61E-11 9.32 14.13

S 16 1.84E-05 10.4 7.91

P 15 3.72E-07 10.5 8.09

C 6 3.62E-04 11.26 13.45

Xe 54 1.86E-10 12.13 4.22

Cl 17 1.88E-07 13 8.41

O 8 8.53E-04 13.6 12.81

H 1 1.00E+00 13.6 51.02

N 7 1.12E-04 14.5 14.13

Ar 18 3.62E-06 15.8 8.74

F 9 3.02E-08 17.4 13.29

Ne 10 1.23E-04 21.6 14.37

He 2 9.75E-02 24.6 34.43

Symbol At No. rel abun ϕi (eV) vc (km/s)

Xe 54 1.86E-10 12.13 4.22

K 19 1.35E-07 4.34 4.63

Ni 28 1.77E-06 7.64 5.01

Cr 24 4.83E-07 6.77 5.01

V 23 1.05E-08 6.75 5.06

Co 27 8.07E-08 7.88 5.08

Mn 25 3.42E-07 7.43 5.11

Fe 26 3.22E-05 7.9 5.22

Ti 22 8.60E-08 6.83 5.25

Sc 21 1.23E-09 6.56 5.31

Ca 20 2.19E-06 6.11 5.42

Al 13 3.04E-06 5.99 6.54

Na 11 2.06E-06 5.14 6.57

Si 14 3.58E-05 8.15 7.48

Mg 12 3.85E-05 7.65 7.79

S 16 1.84E-05 10.4 7.91

P 15 3.72E-07 10.5 8.09

Cl 17 1.88E-07 13 8.41

Ar 18 3.62E-06 15.8 8.74

B 5 7.60E-10 8.3 12.17

Li 3 2.05E-09 5.39 12.24

O 8 8.53E-04 13.6 12.81

F 9 3.02E-08 17.4 13.29

C 6 3.62E-04 11.26 13.45

Be 4 2.61E-11 9.32 14.13

N 7 1.12E-04 14.5 14.13

Ne 10 1.23E-04 21.6 14.37

He 2 9.75E-02 24.6 34.43

H 1 1.00E+00 13.6 51.02

Grouping of elements ismore correlated to their CIVs (right) than their FIPs (left)

Solar Context: 3

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

AI Simulation: 2

Initial Final

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Final electron energy distribution

Initial maximum of potential

Magnetic domination means expansion of electron cloud not spherical: perpendicular electrons constrained & parallel electrons reach higher energies.

10-3

See MacLachlan, Diver & Potts, New Journal of Physics 11, 063001 (2009)

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

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 chromosphereDerived 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

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

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