Small-scale solar surface fields M. J. Martínez González Instituto de Astrofísica de Canarias

The multi-scale magnetic Sun Active regions φ ≈ 10 3 G B ≈ 10 3 G D ≈ 10 5 km Network φ ≈ 10 2 G B ≈ 10 3 G D ≈ 10 4 km Very quiet Sun φ ≈ 10 G B ≤ 10 2 G D ≤ 10 3 km Quiet Sun

Which is the origin of very quiet Sun magnetic fields ?  Statistical physical properties  Study of the newly appeared magnetic fields into the very quiet photosphere

Statistical physical properties

The very quiet Sun is alike everywhere (magnetic field vector ≈ isotropic) Orozco Suárez & Katsukawa 2012, ApJ, 746, 182 Hinode data at 630 nm See also Lites et al. 2008, ApJ, 672, 1237 Martínez González et al. 2008, A&A, 479, 229 data at 1.5 μm

Field strengths of the order of equipartition or below Modeling polarimetric signals with a uniform field, it would occupy only 2% of the resolution element. Khomenko et al. (2003), Martínez González et al. (2008), Orozco Suárez et al. (2008)

Uniform magnetic field There is evidence of unresolved structures in the (3D) resolution element. Magnetic scales possibly continue below ~ 100 km

Observing the Sun at different spatial resolutions tells us about the organisation of the magnetic fields. (correlations between adjacent pixels). B long  A/A pixel =0 (B long )  (A/A pixel ) 1/2 “ flux tube ” “ isotropic random field ”

The smallest the polarimetric signal, the smallest the spatial coherence Martínez González et al. 2010, ApJ, 711, 57L See also Stenflo 2010, A&A, 517, 37 Sánchez Almeida & Martínez González 2011, ASP, 437 scales well below 200 km

The smallest the polarimetric signal, the smallest the spatial coherence Martínez González et al. 2010, ApJ, 711, 57L See also Stenflo 2010, A&A, 517, 37 Sánchez Almeida & Martínez González 2011, ASP, 437 scales well below 200 km

…and even down to ~100 km (best present spatial resolution) Extended tails are indicators of intermittency

Newly emerged flux in the quiet photosphere, appears in form of intermittent, small-scale Ω-loops. Martínez González, M. J. 2006, PhD thesis Martínez González et al. 2007, A&A, 469, L39 B linear pol. circular pol. (+, -) observer

Study of newly emerged flux  Topology, physical properties (study of individual phenomena)  Statistics  Spatial distribution

Study of newly emerged flux  Topology, physical properties (study of individual phenomena)  Statistics  Spatial distribution

Ishikawa et al. 2010, ApJ, 713, 1310 Flux-tube like structures

Turbulence is not large enough to remove the coherency but a “flux tube” model seems to be too simplistic for some of these loops. Ressemble the more extended loops of granular flux emergence in active regions Martínez González et al. 2010, ApJ, 714, 94 Ortiz et al. 2014, ApJ, 781, 126 See also Vargas Domínguez et al. 2012, SoPh, 278, 99

Quiet Sun loops are not low-lying, they ascend through the atmosphere Line formation region  t=30 s 1)the apex (linear pol.) 2)the footpoints (circular pol.) separate and intensificate (the field becomes more vertical) 2000 km Martínez González & Bellot Rubio 2009, ApJ, 700, 1391

Gömöry et al. 2010, A&A, 511, 14

1 h The magnetic loops appear in “emergence centres”. In general, they emerge in the granules and travel to the closest intergranular lane (they never cross another granule). Centeno et al. 2007, ApJ, 666, 137

23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II). t=0 s We first detect the linear polarisation above a granule, the footpoints being under the noise level. Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II)

t=60 s The footpoints appear very close to the linear polarisation patch. Now the loop is completely formed. Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II).

t=120 s Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) The footpoints separate. We see upflows in the magnetic LOS velocity maps. 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II).

t=150 s Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) Between t=150 and t=180 s, the linear polarisation disappears below the noise. 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II).

t=150 s Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) Between t=150 and t=180 s, the linear polarisation disappears below the noise. 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II). Similar to granular flux emergence in active regions Ortiz et al. 2014, ApJ, 781, 126

t=180 s Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) The positive footpoint has been drifted to an intergranular lane and is more concentrated whereas the negative one is still rooted in the granule and is more diffuse. 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II).

t=270 s Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) We observe weak polarisation signals in the Mg magnetogram that are cospatial with the photospheric footpoints. We also see downflows in the Mg dopplergram (~-0.4 km/s) in the position of the positive footpoint (loop has to get rid of its matter to rise into a less dense medium). 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II).

t=360 s Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II).

t=450 s Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II).

t=570 s Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) The Mg signals are now much more intense and correspond to both footpoints rooted in intergranular lanes. 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II).

t=660 s Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II).

t=720 s Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) The footpoints are clear in the Mg magnetogram and dopplergram and we start seeing a brightening in the Ca II image corresponding to the positive footpoint. 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II).

t=780 s Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) We clearly see bright points in the Ca II intensity map, meaning that the loop has reached the chromosphere. Moreover we see bright points in the CN molecular band. 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II).

t=900 s Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II).

t=1020 s Photospheric Photospheric Minimum T Lower granulation magnetogram magnetogram chromosphere (CN) (630 nm) (Mg b I) (Ca II) The Ca/CN brightenings disappear. The positive footpoint will disappear but the polarisation features of the negative one will survive thanks to the interaction with a strong polarisation patch. 23 % of the detected small-scale loops cross the Mg b I formation region (minimum temperature region) and reach the lower chromosphere (brightenings in Ca II).

Still not constrained by observations. The loops transport at least erg cm -2 s -1 ( erg cm -2 s -1 rad. losses) (for simplicity, only one magnetic field line represented) Martínez González et al. 2010, ApJ, 714, 94

Gömöry et al. 2013, A&A, 556, 7 TRACE Ly alpha Imprints on the chromosphere

Gömöry et al. 2013, A&A, 556, 7 Imprints on the chromosphere

Study of newly emerged flux  Topology, physical properties (study of individual phenomena)  Statistics  Spatial distribution

Martínez González et al. 2012, ApJ, 755, % Ω-shaped loops rising through the atmosphere. Some few loops are complex (extended feet, sea-serpent like). Only 3% of the linear polarization appears after the loop had already disappeared. (a loop that emerges and then submerges in the photosphere, or a “magnetic bubble,” i.e., a circle of magnetic field lines).

Martínez González & Bellot Rubio 2009, ApJ, 700, 1391 Complicated wandering of their footpoints Do not follow Hale’s polarity law.

Manso Sainz et al. 2011, A&A, 531, 9 Within granules loops are passively advected by the plasma, which is roughly laminar with a characteristic mean velocity of 2 km/s. In intergranular lanes, they remain there and are buffeted by the random flows of neighbouring granules and turbulent intergranules, following random walks, and disperse across the solar surface with a diffusion constant of 195 km 2 /s. t a a = 1.7 t a a = 0.97

Quiet Sun loops are at the tail of the distribution of new photospheric flux Parnell et al. 2009, ApJ, 698, 75 −1.85 ± 0.14

Do they continue to smaller scales ? Loops as small as the spatial resolution limit

Orozco Suárez et al. 2008, A&A, 481, 33 Possibly YES

Study of newly emerged flux  Topology, physical properties (study of individual phenomena)  Statistics  Spatial distribution

Martínez González et al. 2012, ApJ, 755, 175

Distribution of voids (dead calm areas in the very quiet Sun)

Stangalini 2014, A&A, 561, 6 Dead calm areas possibly related to supergranulation

… quiet Sun, why should we care? … because it is most of the Sun, most of the time.

Daily emergence 7  – 6  Mx Emergence rate: 0.02 loops arcsec -2 h -1 Martínez González & Bellot Rubio (2009) Emergence rate: 0.2 loops arcsec -2 h -1 Martínez González et al. (2011) Quiet Sun loops Mx Mx, t~min-hours …as compared to.. Large active regions sunspots, >10 21 Mx, t~weeks-months Small active regions pores, Mx, t~days-weeks Ephemeral active regions – Mx Mx, t~hours-days – Mx

… quiet Sun, why should we care? Although fields are weak, its magnetic flux content is larger than that of active regions. It seems a rather stochastic magnetism but intermittent  -shaped loops connect the quiet photosphere with upper layers, carrying at least erg cm -2 s -1 to the base of the chromosphere. Small-scale loops are the lowest fluxes of the power law extending over five decades of flux (Parnell et al. 2009). They do not appear totally uniformly in the solar surface (voids and clumps). Are quiet Sun fields of the same origin than active regions? They have similarities but, e.g., the quiet Sun occupies the full Sun and does not follow the activity cycle. Could they be created by different mechanisms but all are dominated by surface processes ?

Thank you !