solar eclipse, , Wendy Carlos and John Kern Structure of solar coronal loops: from miniature to large-scale Hardi Peter Max Planck Institute for Solar System Research Germany S. Bingert, J. A. Klimchuk, C. de Forest, J. W. Cirtain, L. Golub, A. R. Winebarger, K. Kobayashi, K. E. Korreck – miniature loops – sub-structure of loops A&A in press / arXiv

AIA 193 Å image during Hi-C flight Hi-C FOV Cirtain et al. (2013) Nat. 493, 501

Corona and chromosphere in Hi-C FOV → very complex active region: group with may spots → no “normal” nice large active region loops…

miniature coronal loops miniature coronal loops

Small coronal structures ► Hi-C sees structures down to resolution limit ► are these “miniature coronal loops” ? 100x shorter than “normal” loops → length: 1.5”– 2” → width: 0.3”– 0.4” 0.2” x 0.2” → Hi-C resolution:

SDO context Nature of the small coronal structures – footpoints of hot loops ? probably not: where is the hot loop?! – really small loops ? HMI shows “monopolar” region… but opposite polarities might be hidden… 15” x 15”

Magnetic field on small scales: the photosphere –1000 vertical magnetic field [G] – l.o.s magnetic field [G] HMI / SDO (0.5”/pxl)IMaX/S UNRISE (0.05”/pxl) 40” x 40” Wiegelmann et al (2010) ApJ 723, L185 comparison of HMI and IMaX, not showing the same structures… ≈0.6” S UNRISE data: opposite polarities with footpoint distance ≈ 1” within “unipolar” regions 4.5” x 4.5” 15” x 15” miniature loop

Miniature coronal loops ?! ^ ^ corona chromosphere & photosphere interior ~ 2 Mm ≈ 2” ► there is observational evidence small magnetic loops that reach “into corona” e.g. Ishikawa et al (2010) ApJ 713, 1310 ► can such loops exist ?!

sub-structure of coronal loops sub-structure of coronal loops

Corona and chromosphere in Hi-C FOV → very complex active region: group with may spots → no “normal” nice large active region loops…

Comparison of Hi-C and AIA → clear difference in plage region (upper left) → similar appearance in loop region Hi-C 193 Å

Comparison of Hi-C and AIA → clear difference in plage region (upper left) → similar appearance in loop region AIA 193 Å

No sub-structure of loops ► Hi-C has more noise (higher resolution / less photons) ► no substructure visible in loops ! (within noise level) 0.2” x 0.2” → Hi-C resolution:

Getting a lower limit for the strand diameter diameter of individual strands number of strands in whole loop: Either strands have to be smaller than 15 km or the loop is monolithic !

Visualization of strands and cross-loop profile Hi-C AIA integrate along Y – apply PSF – bin to AIA/Hi-C pixel size

Morphological comparison to model observations show: (a) constant cross-section loops in expanding envelope (b) thin individual loops (c) thick non-expanding structures 3D MHD models show similar features → more work is needed for quantitative comparison… See also talk by Feng Chen on Wed morning: “A coupled model for the formation of active region corona”

Conclusions

Structure of solar coronal loops: from miniature to large-scale ► tiny coronal loop-like structures exist (loop length below 2 Mm) → are these miniature coronal loops? → how are they connected to the photosphere ? → how are they sustained ? ► long coronal loops appear to have no substructure in Hi-C observations (0.2” spatial resolution) → either smooth structures → or very thin strands (much smaller that 100 km)

High-resolution Coronal Imager (Hi-C) – single rocket flight on 11 Jul 2012 (NASA/MSFC/CfA/LMSAL) – imaging in 193 Å band identical to AIA/SDO – 6x higher resolution compared to AIA – 5 minutes of data from one active region – first results and description: Cirtain et al (2013) Nat. 493, 501 Hi-C AIA/SDO pixel size0.1” 0.6” FOV~400” full disk channel193 Å 193 Å and many more [ Fe XII ~1.5 MK ] time cadence5.5 s 12 s obs. time:5 min … years

SDO context Nature of the smallest coronal structures – footpoints of hot loops ? probably not: where is the hot loop?! – really small loops ? HMI shows “monopolar” region… but opposite polarities might be hidden… 15” x 15” 45” x 45”

^ Observation of small “photospheric loops 4” x 2.3” circular polarization linear polarization integrated light time →  t = 0 s 130 s 260 s Hinode/SOT Ishikawa et al (2010) ApJ 713, 1310 emerging flux in / around a granule 1- signal in lin. pol. 2- circ. pol. at sides 3- lin.pol. vanishes ^ corona photosphere interior consistent with flux tube breaking through photosphere

^ Observation of small “photospheric loops 4” x 2.3” circular polarization linear polarization integrated light time →  t = 0 s 130 s 260 s Hinode/SOT Ishikawa et al (2010) ApJ 713, 1310 emerging flux in / around a granule 1- signal in lin. pol. 2- circ. pol. at sides 3- lin.pol. vanishes ^ Ishikawa et al (2010) reconstruction of B at  t = 130 s: is this a “photospheric loop” ?

Size of structures across the loop

Dere et al. (1987) Solar Phys. 114, 223 corduroy trousers scenario Multi-stranded loops ? classical multi-stranded loop scenario r ~200 km D r ≲ D → few strands hp 2013 © r < D → very many strands < very fine strands Hi-C pixel size ~2 Mm