3 The Era of 0.1 – 0.5” Resolution: Small-Scale Magnetic Structures in Sunspots

In Section 2, we focused on sunspot’s global magnetic structure. To that end we studied the radial variation of the azimuthally averaged magnetic properties: three components of the magnetic field vector, plasma-β, potentiality, currents, etcetera. In this section, we will investigate the small-scale structure of the magnetic field. This will help us understand and identify some of the basic building blocks of the sunspot’s magnetic field, as well as the fundamental physical processes that occur in sunspots. Another difference with Section 2, where only the magnetic field structure was discussed, is that in this section we will also address the velocity field since they are both intimately linked at small scales (e.g., Evershed flow; Evershed, 1909Jump To The Next Citation Point).

In this section spectropolarimetric observations at the highest spatial resolution will be employed and, instead of discussing averaged quantities, we will focus mostly in their pixel-to-pixel variations. In the first part of this section we will address the fine structure of the umbral magnetic field, whereas the second will be devoted to the penumbral magnetic field. This division is somewhat artificial because the current paradigm points towards a clear relationship between the small-scale structure in these two different regions (Rimmele, 2008Jump To The Next Citation Point). However, there is one important difference between these two regions (umbra and penumbra), and it has to do with the mean inclination of the ambient magnetic field ζ: in the umbra the magnetic field is mostly vertical, whereas in the penumbra is highly inclined (see Figure 11View Image). This difference leads to a somewhat different interaction between the convective motions and the magnetic field.

View Image

Figure 20: Map of a sunspot (AR 11072) umbra and inner penumbra obtained with the Swedish 1-m Solar Telescope (SST). This sunspot was observed on May 23, 2010 at Θ = 15°. The image was taken with a 10 Å filter located between the Ca H and Ca K spectral lines. It was subsequently restored using Multi-Object Multi-Frame Blind Deconvolution (MOMFBD) technique. The red circles surround a local intensity enhancement in the umbral core: umbral dot (UD), and a portion of a light bridge (LB) (adapted from Henriques et al., 2011; in preparation).

As we saw in Section 2.5, typical temperatures at τ = 2∕3 in the umbra and penumbra are approximately 4500 K and 6000 K, respectively. Plasma heated up to this temperature loses energy in the form of radiation. If the brightness of the umbra and penumbra is to remain constant, the energy loses due to radiation must be compensated by some other transport mechanism that will bring energy from the convection zone into the photosphere. The mechanism usually invoked is convection. However, the strong magnetic field present in the sunspots (see Figure 11View Image) inhibits convective flows (Cowling, 1953). This inhibited convection is, therefore, the reason why umbra and penumbra posses a reduced brightness compared to the granulation. How do these convective movements take place? In the umbra the answer to this question is to be found in the so-called umbral dots, whereas in the penumbra convection occurs within the penumbral filaments.

 3.1 Sunspot umbra and umbral dots
  3.1.1 Central and peripheral umbral dots
  3.1.2 Thermal and magnetic structure of umbral dots
  3.1.3 Signatures of convection in umbral dots
  3.1.4 Light bridges
  3.1.5 Subsurface structure of sunspots: cluster vs. monolithic models
 3.2 Sunspot penumbra and penumbral filaments
  3.2.1 Spines and intraspines
  3.2.2 Relation between the sunspot magnetic structure and the Evershed flow
  3.2.3 The problem of penumbral heating
  3.2.4 Vertical motions in penumbra and signature of convection
  3.2.5 Inner structure of penumbral filaments
  3.2.6 The Net Circular Polarization in sunspots
  3.2.7 Unified picture and numerical simulations of the penumbra

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