Embed Size (px)
Citation preview
Small scale magnetic energy release driven by supergranular flows
Hugh Potts, Joe Khan and Declan Diver
How to automatically detect and analyse supergranular cells from high resolution surface flow data…
Balltrack – Ultra efficient flow measurement
How strong is the influence of the supergranulation flow ?
Flow driven, small scale magnetic reconnection
Identify source points that belong to the same cell, and calculate region that their outflow covers (Not trivial!)
The most detailed picture of supergranulation ever seen!The surface flow is indicated by the black arrows, with the upflow regions shown by white crosses. The downwards flowing intergranular lanes are indicated by the dark lines
From this data we can characterise many as yet unknown properties of supergranulation
We track the motions of the smallest magnetic elements resolvable by MDI high resolution magnetograms, using a targeted, advective integration LCT algorithm.
This motion can then be compared with the motion of local fluid trackers (figure right), to gauge the relative strengths of the magnetic and hydrodynamic forces on the elements.
At areas of high magnetic stress we expect the magnetic forces to dominate. We are developing Bayesian methods to quantify the effect.
• Limited by magnetogram resolution – Solar-B FPP data much better!
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.
Comparison of the actual motions of the magnetic elements over 36 hours (red and blue) to the the motion expected if the photospheric flow dominates their motion. The fuzzy white lines repesent the average position of the supergranular lanes over the full time period.
Filter to extract granulation signal(barely resolved in MDI data)
How accurate can you be?
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!
Take a flow field… Find the discontinuities in the resulting surface
Look at the end position with respect to the start position
Run test particles backwards on it
Locate the flow sources (from the test particle sinks)
The divergence field. Not terribly illuminating
Comparison of soft X-ray images of bright points from Yohkoh with regions of high convergence of flow driven magnetic elements. The area is 4x5.5 arcmin, roughly at disk centre.
Make a surface from the data and allow ‘floating’ tracking particles to be randomly nudged by the granulation ‘bumps’
Take MDI continuum data…
V(xi,yi,t) : smoothed velocity
: spatial smoothing radius
t : time smoothing interval
rn,t : distance from (xi,yi) to ball
Average the velocities of trackers over space and time to obtain macroscopic flow field
15 20 25 30
14
16
18
20
22
24
26
28
30
32
34
Mm
Mm
• 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 both polarities are being 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)
Should work even better on the higher resolution Solar-B data!
Elements selected for tracking
High Resolution MDI magnetogram data. The useable resolution is considerably less than the pixel resolution, probably due to unresolved magnetic elements
• On the limit of SXT low energy resolution. Solar-B XRT goes to lower temperatures with better spatial and temporal resolution
