**2. Connection from the inner core to the surface**

The central part of the Sun is composed of a core (about a fourth of its radius) where thermonuclear reactions generate energy. While its average density is about 10 times that of lead, its temperature is about 15 106 K. The core yields to the radiative zone of the Sun, which is about one-third of the Sun's radius. Both the core and the radiative zone transfer energy by radiative forces of photons following a random walk and seem to act as an apparent solid body [2].

The photosphere is 300–500Km deep, it is the part of the Sun from which the light is being emitted, before the plasma becomes opaque. The effective temperature of the Sun comes from the photosphere, at about 5.8 103 K, plasma convection is visible there under the form of granules of sizes measured in Mm (103 m). As any convective cell, granules [3, 4] have central heat upwelling and peripherial cool downwelling (**Figure 1**). Their life span is short, 8–20 minutes. Within the granules area, additionally to pores and magnetic flux tubes, sunspots may rise within mid-latitudes and converge to subequatorial latitudes. A sunspot is a cooler region and is measured in tens of Mm. It is composed of a central darkest part, the umbra, and surrounded by a less dark area called the penumbra made of radial acicularity (**Figure 1**). Its existence is inherently due to the Sun's magnetism.

Further parts outside of the Sun are referred to as part of its "atmosphere." They are the chromosphere and the corona (in that order).

The chromosphere is 2000 km deep, it is the Sun's eclipse "red ring of fire." It is characterized by a steep drop in material density, and an initial temperature drop from 5.8 103 K to 3.5 103 K to eventually reach 35 103 K. The chromosphere and its transition zone to the next zone are the subject of study of the Interface Region Imaging Spectrograph (http://www.nasa.gov/iris), especially the chromospheric jets associated with coronal heating (de Pontieu, 2011 @SETI Talks).

The corona is a very large volume above the chromosphere, vastly warmer too, made of ionized plasma of about 1 106 K, with a majority of emission coming from Fe-XIV and Fe-X. It is the origin of the *solar winds*. Some areas with open magnetic fields (**Figure 2**) yield faster *solar winds* (about 0.7 106 m/s). The Hinode mission

#### **Figure 2.**

*High-resolution image of the sun from solar orbiter, showing magnetically bound plasma. Credit: ESA & NASA/ solar orbiter/EUI team; data processing: E. Kraaikamp (ROB) [5].*

recorded the most sensitive information about the magnetism of the Sun (http:// www.nasa.gov/hinode/).

The interplay between convection and magnetic fields drives all the heating in the solar atmosphere and the space weather. The magneto-convective energy heats the corona, drives the *solar wind*, causes flares and coronal mass ejections (de Pontieu, 2011 @SETI Talks).

Thompson et al. [6] used continuous observations from the Global Oscillation Network Group (GONG) and inverse modeling (harmonics frequency splitting, inverting rotation kernels) to confirm that differential rotation on the surface is carrying through most of the convection zone, until the tachocline at 0.713 of the Sun's atmosphere radius (R), a zone (**Figure 3**) of strong shear where disassociation happens with the deeper part of the Sun [2], and where variations of rotations have been linked with the presumed depth of the solar dynamo. Howe [2] further found that temporal variations in the tachocline region extend in the radiative zone as far as 0.63R (blue overlay on **Figure 3**), which suggests complex physics properties at the shear zone converting from (apparent) nearly solid state to convective.

Thompson et al. [6] also observed a shear layer just below surface at lower latitudes. In the equatorial regions, as depth increases, the rotation rate first increases (orange overlay on **Figure 3**) and then decreases. This particularity is reducing with latitude (away from equator). Sunspots are forced by the differential rotation to "stretch" from their roots in the convection zone and below [7] following the equatorial convergence until their "elasticity" is reaching limits. At this point, they "snap" to release as a filament.

A filament eruption (http://science.nasa.gov/missions/trace/) is a magnetic line disconnecting from the underlying magnetic field after a too large disturbance, the Alfvén waves survive the Corona transfer and are depositing energy in the *solar wind* [8].

**Figure 3.** *Time-averaged rotation rates (Ω/2π) vs. partial sun radius (r/R) at different latitudes, redrawn from Howe [2].*
