karen meyer university of st andrews scotland 1 st year phd student (3 months in)
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Karen Meyer
University of St Andrews
Scotland
1st year PhD student (3 months in)
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Magnetic Carpet• Magnetic Carpet
• Super-granular cells. On small scales they give the dominant flow pattern on the surface of the
Sun.
Diameters: 10-50,000km ( ~ 14,000km).
Flows : Up-flow at cell centre/down-flow at edge. Horizontal radial outflow in cell centres
Velocities ~ 0.5 km/s. Strong downflows where 2 or more cells meet.
• Supergranular cells play an integral part in the evolution of the magnetic carpet
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Magnetic Carpet Fluxes• Three main classifications: ephemeral regions, network fields, intra-network
fields.
• Ephemeral Regions (Harvey and Martin 1973). Cluster of newly emerging bipolar pairs. Hagenaar (2001) - 38,000 fragments. - flux ~ 2.6x1018 - 4.1x1020Mx ( 3x1019Mx, Harvey 1993). • Network Fields (Martin 1988): - found at sights of strong down-flows ( edges of 2 or more s/g cells). - produced from residuals of other flux concentrations - Properties : flux: 1x1018 ~ 1x1019 Mx. - 90% ~ephemeral regions, 10% ~ intra-network elements.
• Intra-network fields: smallest of concentrations- originate within s/g cells as emerging bipoles (flux: 1x1016 ~ 1x1018 Mx)
• Dynamics of three flux types important for many small scale solar
phenomena- X-ray Bright Points, Explosive Events.
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Evolution of Magnetic Carpet Fields• Four main processes: 1) Flux Emergence: new magnetic features of opposite polarity (equal
flux) appear as pairs.
2) Cancellation : distinct opposite polarity fragments come into contact and mutually loose flux (consider coronal consequences next).
3) Coalescence: joining of same polarity elements on contact.
4) Fragmentation: splitting up of large features into 2 or more elements (mechanism unclear ~ granulation or instability).
Emergence CoalescenceCoalescence Fragmentation Cancellation
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Magnetic Carpet Coronal Field.• The magnetic elements in the carpet extend up into the chromosphere and
lower corona where they are connected between one another.
• Emergence and surface motions may built up
significant amounts of energy : coronal heating,
X-ray bright points
• Recycle time for photospheric field ~14hr
(Hagenaar 2001).
• Close (2004) – remap time for the corona.
12hr, 240x240 Mm2, 286 fragments
Coronal remap time 1.4hr ~ quiet sun
corona is highly dynamic.
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NLFFF Models• Previous theoretical models have mostly considered only potential fields.
Aim : Construct a nlfff magnetic carpet model.
• Features.
Extended area of the Sun – 10x10 supergranular cells
Include flux emergence, cancellation, etc.
Model a nlfff corona based on surface motions.
• Topics to consider.
Complex non-potential small scale coronal field.
Magnetic Topology and Connectivity.
Location of free magnetic energy and electric current.
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The Model
• Two component model (van Ballegooijen, Mackay).
Coronal : magnetofrictional relaxation technique
Photosphere: Evolves Bz at the photosphere through an analytical boundary condition (van Ballegooijen)
- no numerical overshoot
- no numerical diffusion.
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Flux transport movies