Plasma Physics – the ubiquitous environment

1. + - + + - - + + + - - - - + + + + - - - + - + - - + + + + - - - - - + + + + + - - - - - + + - + - - - - - + + - + + + - + - + + - - + + - - - neutrals Magnetised plasma + + + - Electrons are excited to new potential by local field. If then ionisation occurs + - Resultant electric field grows until resists further ejection of ions Magnetic field Plasma Physics – the ubiquitous environment Hugh Potts, Craig Stark, Craig MacLachlan and Declan Diver Alfvén Ionization and FIP Dust growth in plasmas Small scale magnetic reconnection on the sun The chemical composition of the solar Atmosphere is different from that of solar surface. Low first ionisation potential (FIP) elements are more abundant in the atmosphere by factors ranging from 4-11. How does this happen? Grains of dust in a plasma accumulate negative charge. The electric field produced by this charge accelerates ions to the surface of the grain. Asymmetries in the shape of the grain affect the shape of the electric field, and hence the subsequent growth of the grain. At the photosphere of the sun large scale convective flows interact with the motions of the smallest magnetic structures, tangling them and making them unstable, leading to the release of energy Proposed Mechanism: Alfvén ionisation elliptical dust grain uniform electron-ion plasma. mobility of the electrons ensures grain negatively charged sheath forms, with non-radial electric field ion transport to grain surface affected by distorted field - a spherical shell of ions follow nearly radial paths until close to the elliptical surface, where the local inhomogeneous field will deflect their trajectories and distort the mass deposition. The surface flows advect small magnetic elements of both polarities into the sink points of the convective cells. At these points of convergence small scale energy release is observed as soft X-ray bright points. Neutral flow encounters magnetised plasma • Use BALLTRACK to get high resolution flow field from MDI continuum data • Process flow field as described above, to get the convection cell structure (black lines, RH figure) • Track the smallest magnetic elements from MDI high resolution magnetogram data (red/blue, RH figure) • Find the areas where large amounts of field of bopth polarities are beiong advected to the same place (green circles, RH figure) • Compare with the bright points observed in soft X-ray data from Yohkoh-SXT (black circles, LH figure) Neutral – ion impact results in some ions being ejected Electrons transport strongly constrained bymagnetic field • arc length around ellipse is given by the incomplete elliptic integral of the second kind - Pocket of unbalanced charge is created, persisting until impeded electron transport can neutralise Neutral flux impinges on charge-balanced magnetised plasma • ratio of the two arc lengths in the first quadrant created by the intersection of the ellipse and the 450 parabola is • As the eccentricity tends to zero, material at infinity is uniformly deposited over the surface since Pocket of charge imbalance Contour and surface plot of grain potential hence Comparison of soft X-ray images of bright points from Yohkoh with regions of high convergence of flow driven magnetic elements. The black lines are the lanes between the convection cells. The area is 4x5.5 armin, roughly at disk centre. • RESULTS • critical parameter: • optimal grain size for elliptical growth - grains of a:b less than 3:1 do not grow elliptically Balltrack – efficient flow measurement Neutral-ion collisions eject ions, leaving strongly magnetised electrons in place Alfvén ionization fits the data better than existing models: Take MDI continuum images of the photosphere Stark et al, 2005 Note the anomalies: Observed bias for large FIP elements….. Low FIP can’t be the only factor for bias Filter to extract granulation signal Electronegative Plasmas Make a surface from the data and allow ‘floating’ tracking particles to be randomly nudged by the granulation ‘bumps’ Electronegative plasmas are unstable to formation of negative ions, resulting in up to 40% in modulated light output. We have a fundamental explanation of the instability. electron density high: p>  Electric Field excluded Potts, Barrett, & Diver: 2004, A & A, 424 253-262 Elements sorted by first ionisation energy. Shaded rows indicate elements where FIP bias has been observed in the upper solar atmosphere or wind Electron energy drops: formation of negative ions negative ions broken up by electric field Average the velocities of trackers over space and time to obtain macroscopic flow field Diver, Fletcher & Potts: 2005 Solar Physics, 228, 207 Negative species now less mobile: p<  Electric Field penetrates Elements sorted by Alfvén critical speed. V(xi,yi,t): smoothed velocity s : spatial smoothing radius Dt : time smoothing interval rn,t : distance from (xi,yi) to ball Alfvén in the Lab: Rotating plasma Reactor Periodic (kHz) optical emission from plasma: flashing instability How accurate can you be? Voltage steady over large discharge current range, then sudden increase at critical Voltage Vc – total ionisation Vc different for each element! Potts, Barrett & Diver: 2003, Solar Physics, 217 69-78 Corr, Steen & Graham PSST 12 265 2003 Numerical simulations show periodic electronegative ion formation and destruction, after transients MacLachlan et al, 2005 Critical voltage scales with magnetic field – shows that critical speed – E/B drift – is being selected The errors are dominated by the random walk of the granules, inherent in the fluid motion, so results are nearly as good as you can get! We will build a rotating plasma reactor for laboratory and astrophysical plasma experiments