Introductory Chapter: The Sun and Its Phenomenal Material Flux

1. Introduction

The Sun is the closest (by far) of the stellar objects we can study. Yet, we remotely sense it only, by means of recording electromagnetic spectra emitted from its activity. Remote sensing studies permit to analyze the Sun in many different perspectives, according to the types of spectra the instrument focus is. Thermodynamics, hydrodynamics applied to plasma with magnetic fields are all needed to study the radiative, convective, and exo-atmospheric conditions of the Sun energy transport. Stellar objects of different characteristics have been observed for ages by astronomers, and many physical theories have been developed relating observations and life cycles (i.e., HR diagram and equations of stellar structure, respectively). Stellar oscillations [1], spherical harmonics, and resonance patterns analysis belong to geophysics and are now in common use to study and classify stars.

More recently, scientists have been able to evolve geophysics techniques to reach within layers of the Sun. Seismology is now having a branch dedicated to the Sun: helioseismology. Magneto-convective analysis relies on remote sensing and physics, modeling, and inverse modeling. The Sun’s core is in the process of nuclear fusion, where quantum physics applies. Atomic physics permits us to understand the life cycle of stars and their creation of atoms of different Z number depending on the type of star evolution as well as hydrostatic equilibrium conditions and environments. The Coulomb barrier and the Gamow peak involve both atomic physics and quantum statistics. Random walks taking 1 million years for a photon to leave the core to the radiative and arrive at the convective zone of the Sun are found from combined fields of physics and stochastic/Markovian modeling science.

Technological sciences such as supercomputing, applied experimental physics, engineering are also necessary to tackle complex modeling, simulation, and experiments. Astronomical engineering is particularly of importance in this case, on Earth and in orbit (space science/engineering) to gather remote sensing data. Lastly, theoretical mathematicians, physicists, and scientists have all had parts in developing the sciences used in the Sun analysis today, as the most fundamental of the sciences reverberate in the different bits of understanding found in the group of sciences that evolved from simple timing of and logging of visible events, to more complex instrumentation/analysis as observation sciences became more technological. A final question that comes to mind when analyzing electromagnetic interaction of magnetism such as the Zeeman effect is where is the electric part in the research on the Sun’s electromagnetism. Is there magnetism alone only in the Sun?

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 10⁶K. 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 10³K, plasma convection is visible there under the form of granules of sizes measured in Mm (10³m). 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.

Figure 1.

Granule convection cells around sunspots. Credit: ESA & NASA/solar orbiter/EUI team.

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 10³K to 3.5 10³K to eventually reach 35 10³K. 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 10⁶K, 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 10⁶m/s). The Hinode mission recorded the most sensitive information about the magnetism of the Sun (http://www.nasa.gov/hinode/).

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].

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.

Figure 3.

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

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].

3. The Sun convection zone and the processes of upwelling matter

Nandy et al. [7] studying the mass conservation of the Sun Convection Zone (SCZ) looked into modeling of flow transfer within meridional latitudes and inner tachocline structures. They found that the mass conservation must include toroidal fields in opposite direction at higher latitudes, to compensate the sunspot nursery at lower latitudes. What they also found is that the poloidal fields within the SCZ also drive mass transfer from equator to high latitudes. Their simulation also demonstrated that the mass transfer actually happens through the tachocline. It seems that the toroidal field is somehow passing through the tachocline at low latitudes (creating sunspots), but not its opposite direction equivalent at high altitudes (absence of sunspots). Is the tachocline generating a toroidal field constraint larger at high latitudes? It seems (Figure 4) that the combination of poloidal and toroidal field simulation adds explanations about the sunspots appearance in the lower latitudes, as the poloidal field/mass movement is outward at low latitude, and inward at high latitude [7]. In that condition, the threshold for sunspot generation used by Nandy et al. [7] is 10⁵ G above the base of the SCZ.

Figure 4.

Flow directions in toroidal fields (blue/red), poloidal fields (black lines), redrawn after [7].

and tachocline (gray) redrawn from Nandy et al. [7].

Hathaway and Rightmire [9] studied the 1996–2009 period’s magnetic maps made every 96 minutes from the MDI sensor (discontinued in 2011, follow-up by hmi.stanford.edu) on-board SOHO (soho.nascom.nasa.gov). MDI imaged the line-of-sight magnetic field by measuring the difference of polarization on both side of a Nickel absorption line in the Sun’s atmosphere. They found that the surface meridional (following N-S meridians) flow velocities away from the equator are in the range of 0–15 m/s (Figure 5), negligible near to differential rotation (~170 m/s), granulation (~300 m/s), and supergranulation (~3000 m/s). The velocity is however responsible for the rate of polar magnetism reversion, thus the Sun cycle life span and its activity overall. Zhao and Kosovichev [10] studied the interior of a sunspot region using time-distance helioseismology [11] on a dataset of 512 uninterrupted dopplergrams at 1-minute cadence on August 7 and 8, 2000, from MDI, following the data preparation from Giles [12]. Vortical flows in the subphotospheric zone have been estimated through inverse modeling, leading to suggest that kinetic and magnetic helicity extends from surface to depth. Such connectivity could be a source of great energy buildup and enters into the making of solar flares.

Figure 5.

Meridional flow velocity, approximate drawing from Hathaway and Rightmire [9].

4. Emerging flux characterization

Ilonidis et al. [13] found (from MDI data) some strong acoustic anomalies of 12–16 seconds as deep as 65 Mm, which became sunspots 1–2 days after detection with deep focus time-distance helioseismology (Figure 6). They ensured that measurements were made when magnetism was <300 G, otherwise masked out. They also found an emergence velocity of about 60 Mm in about 2 days, consistent with previously modeled velocities.

Figure 6.

Acoustic ray paths crossing an emerging flux from Ilonidis et al. [13].

Shibata et al. [14] used the Solar Optical Telescope (SOT: hinode.nao.ac.jp/sot_e/) on-board Hinode [15] in the Ca II H band (396.85 nm) of the Broadband Filter Imager (BFI) corresponding to chromospheric heating. They studied upper chromosphere and lower corona magnetic reconnection producing high-speed jets being involved in lower coronal X-ray jets and Hα surges. They found that solar nanoflares (spicule jets) events happen and propagate further than gravity-bound expectations by slow-mode magneto-acoustic shocks or fast-mode nonlinear Alfvèn waves shocks. They argue that their actual findings are one order of magnitude less than the necessary energy to heat the corona. To add some energy, they suggest a multiscale presence of nanoflares in the upper atmosphere, which are not all visible with Hinode instruments.

Karoff and Kjeldsen [16] found that the background noise from granulation correlates with flare activity, and that increase in the background noise transfers more power into high-frequency modes via stochastic excitation, as observed in their experiment.

5. Penumbral characterization and co-processes

Ichimoto et al. [17] observed the dynamical processes of strongly magnetized plasma through the twisting motions of penumbral filaments. When magnetic reconnection happens, Katsukawa et al. [18] observed penumbral microjets in chromospheric layers above the penumbra (Figure 7). With less than 0.4 Mm width and a life span less than 1 minute, they are by all means small features around sunspots. They are similar to limb’s spicules, dynamic fibrils in active regions and quiet Sun’s mottles. They might be involved in the thermal source of the sunspot’s coronal connectivity.

Figure 7.

Penumbral microjet and dark filaments [18].

Scharmer et al. [19] used the Crisp Imaging SpectroPolarimeter (CRISP) on the Swedish 1 m Solar Telescope in La Palma, Spain, to study the neutral carbon line (C I) of downward Doppler velocities in the penumbral area of a sunspot. Evidence directs to (dark) downflows that can reach 8 km/s in the blueshift, with an estimated horizontal magnetic direction, while (bright) outward flows have about 50 degrees angle with the surface.

Howe et al. [20] compared Doppler surface measurements with global and local helioseismology (MDI, GONG). They show that subsurface shear fine-tuning just below the surface is hard to reconcile across methods in high accuracy levels while quite agreeing at equator for good accuracy levels. Zonal flow patterns agree largely across methods about rotation-rate residuals allowing for multiscale sources generating wave signals discrepancies.

6. Concluding remarks

Chromospheric and coronal events are strongly connected to magneto-convective dynamics in the photosphere, tachocline, and radiative layers below to some extents. There is a lot of unknown still on the particulars of the dynamics and interconnectivity across the Sun layers. Helioseismology and acoustic sciences have helped greatly in the recent years of discoveries.

The actual Solar Orbiter mission is not only returning remote sensing imagery of very fine resolution, as seen in the first part of this nonexhaustive chapter, but its dedicated sensors payload now starts generating significant amount of research on solar wind processes (i.e., [21] and the related special issue of A&A).