Magnetic flux emergence
Magnetic flux emergence corresponds to the mechanism leading to the establishment of magnetized structures in the solar atmosphere. The magnetic flux emergence is directly traced on the solar surface (in visible-white light) by the presence of dark, mainly round-shaped areas, called sunspots, surrounded by brighter regions called plages. Measurements of magnetic fields at the solar surface shows that sunspots tend to be grouped in pairs, one with positive and one with negative magnetic polarity. Such a group of sunspots forms what is called an active region. The occurrence of large-scale magnetic flux emergence follows the solar cycle periodicity and is governed by the solar dynamo process.
The solar surface is also covered by the so-called magnetic carpet with small magnetic bipoles scattered everywhere over the solar surface. These bipoles are due to the recycling of magnetic field by convection (granules and supergranules). This process is beyond the scope of the present discussion of magnetic flux emergence. This article deals with observable properties of magnetic field emergence in emerging flux regions (EFR) in the solar atmosphere and is complementary with the article on numerical simulations of magnetic field emergence.
Magnetic flux emergence is responsible for the formation of sunspots and active regions.
Observable properties of emerging flux regions
The birth of an active region is manifested by the appearance of small magnetic bipoles at the limit of the spatial resolution of present-day solar telescopes. The two opposite magnetic polarities of each bipole move apart at a relatively large speed (~5 km/s) in the initial phase and then slow down. New flux emerges continuously in the central part between the main polarities. The newly emerged magnetic polarities separate and reach the already emerged main polarities with high velocities: larger spots form by the coalescence of smaller magnetic elements. The growing phase of the EFR lasts about 3-5 days. The flux in an active region, for sunspots to be observed, is usually larger than 1021 Mx in each polarity.
During this growing phase, the distribution of the emerging magnetic polarities tends to adopt an ellipsoidal-shaped pattern (as AR 10488 in Figure 1). Progressively the leading sunspot and the trailing sunspot of opposite polarities grow and take on structure of a mature sunspot--a dark, circular umbra surrounded by a lighter penumbra with radial spokes. The line separating the two opposite polarities is called the magnetic field inversion line and has commonly a North-South direction, perpendicular to the generally East/West axis linking the center of the opposite magnetic polarities (for example AR 10487 in Figure 1). In some cases the magnetic inversion line is tilted and no longer perpendicular to the axis of the active region. In addition, some sunspots may rotate with an angular speed that can reach more than 1 degree per hour. Such sunspots display a characteristic pattern called tongues, with elongated magnetic areas oriented East-West, similar to an horizontal Taijitu (yin-yang) symbol: ☯ (see for example the active region of Figure 3). This pattern is due to the asymmetry of the azimuthal component of the magnetic field and traces the presence of twisted magnetic fields (see also Sect. 2.2. Such a configuration is favorable for major solar activity events, such as flares (central part of AR 10488 in Figure 1).
The distribution of sizes of active regions ranges from extended energetic active regions to the smallest ones called Ephemeral Active Regions that consist of tiny bipoles. All these regions can lead to powerful energy release that can contribute to coronal heating either by their strength or by their number.
The two following movies present examples of observed EFR.
- AR 8844 (gif; size: 7.3Mo)): Flux emergence observed with Flare Genesis Experiment  in January 2000.
- EFR within AR 10926 (avi; size: 8 Mo)): Flux emergence observed with Hinode/SOT in December 2006.
Emergence and solar activity
Origin of emergence
How does an active region appear on the solar surface? This question has intrigued many astronomers since the first telescopic observations of sunspots by Galileo in the early 17 th century. The generation of the solar atmospheric magnetic field lies deep below the solar surface between the convection zone and the radiative zone in a shell called the tachocline (located at about 0.7 solar radius). There, the differential rotation of the sun transforms part of the global kinetic energy of the solar rotation into magnetic energy by means of solar dynamo processes. Dynamo theory predicts that the intensity of the magnetic field in that region can reach 105 Gauss (~10 Tesla). Solar rotation stretches the magnetic field so that it adopts the shape of a torus in the tropical latitudes (royal zones) with stressed and twisted magnetic flux tubes. The triggering mechanism allowing the flux tubes to rise toward the solar surface is still unclear (buoyant instabilities may be involved, see review by Fan 2004) but it is generally accepted that the emergence process is driven by buoyancy. Buoyancy appears since the magnetic flux tubes, being in mechanical equilibrium with their surrounding environment but having a larger magnetic pressure, have a lower plasma density. This model is referred to as the Omega–shaped (concave) loop model of emergence (Zwaan et al. 1985). The sunspots are cross-sections of the emerging flux tubes with the solar surface (see Figure 1 and Figure 2).
Injection of magnetic energy and helicity
Associated with the injection of magnetic flux, the emerging process leads to the transfer of magnetic energy from the solar interior to the solar atmosphere. Indeed the emerging flux tube contains a significant amount of magnetic energy. This magnetic energy is the main source of energy in active regions and thus the main driver of solar activity. In a given emerging flux tube, the transported magnetic energy is all the more important as the magnetic field is far from its lowest energy state. An efficient way for magnetic flux tubes to carry a significant amount of energy is when the field lines are twisted or writhed, i.e. there is an important non-axial (mainly azimuthal) component of the magnetic flux. Every emerging flux tube forming an active region has a certain level of twist, and transports therefore magnetic energy. The level of twist/writhe in the emerging flux is quantified by the magnetic helicity. Magnetic helicity is transported from the solar interior into the solar atmosphere and is conserved during this process. Indirect signs of the twist of the emerging flux tubes can be observed as rotating sunspots and tongues. Actually, numerical simulations of magnetic field emergence have demonstrated that a minimum twist is necessary for flux tubes to conserve their integrity when rising through the convection zone.
Chromospheric and coronal heating
Coronal observations of active regions, in the UV or X-ray ranges, show the presence of loops joining the sunspots of opposite magnetic polarities, see Figure 2e and Figure 2f. These loops are bright in various wavelengths, from the extreme ultraviolet, corresponding to about 1 million K plasma ( Figure 2c), to the soft X-ray region (Figure 2f), which indicates the presence of four-million K plasma. They appear to have been formed independently (Schmieder et al. 2004). Many heating mechanisms have been invoked: turbulence, formation of current sheet above the new emerging flux, magnetic reconnection, etc., but there is no widely accepted model.
During the emergence, the evolution of the magnetic fields in the photosphere creates extremely complex patterns of strong vertical electric currents, see Figure 2b These electric current patterns clearly reveal the filamentary nature of the emerged magnetic fields, which induce a highly structured three-dimensional configuration of magnetic loops in the overlying solar corona.
The magnetic emerging flux brings a great amount of magnetic energy to the surface. Extremely twisted/writhed flux tubes carry the most magnetic energy and helicity, and tend to give birth to very complex active regions (in terms of photospheric magnetic field distribution), which are the more likely to produce large flares.
In addition, by itself, the interaction of the emerging flux with pre-existing magnetic system leads to violent solar activity. Magnetic flux emergence triggers flares, eruptions, surges, jets and coronal mass ejections (CMEs) (Shibata et al. 1989). The magnetic energy is transformed into kinetic, thermal and non-thermal energy. Large-scale twisted emerging flux forming complex active regions with mixed polarities leads to large flares, small-scale emerging flux may also trigger important activity but that depends strongly on the environment of the emerging flux; the topology of the surrounding fields is a crucial factor.
The amount of magnetic energy and helicity in the emerging flux tubes and the link between the emerging flux and the size of flares and eruptions are key problems in present solar physics. Obviously, observations of emerging photospheric magnetic fields are essential to our understanding of solar activity.
Plages, pores and photospheric transient darkenings
The first signature of the imminent birth of an active region is the appearance of plages. Plages consist of many compact and very bright features that can be observed in chromospheric spectral lines. In these lines, the area in and around the EFR appears brighter than the surrounding quiet-sun regions (see Figure 1, bottom panels, Hα and Ca lines). The plages tend to expand as the emergence proceeds and as the active region grows. Plages are the earlier emergence of magnetic tubes. Inside the magnetic elements, the magnetic pressure leads to a lower plasma density within the tube. When the flux tube are small (diameter < 600km), the deeper hotter layers are thus observed and the flux tubes therefore appear bright. When the flux tubes are getting larger and more intense (by coalescence of smaller flux tubes), convection is inhibited and the transport of heat gets less efficient as compared to the surroundings. These cooler structures appear as dark features in white light and are called pores. The subsequent coalescence of several pores leads to the formation of sunspots.
At the photospheric level, the granulation pattern tends to be modified compared to the quiet-sun. The granulation looks fuzzy and transient dark threads, corresponding to elongated intergranular lanes, can be observed in the continuum and in the core of photospheric lines, in the central part of the EFR (Strous et al. 1999). These transient darkenings have a lifetime of the order of 10 minutes. They are roughly aligned with the axis of the active region and are parallel to the direction of the arch filaments.
Arch Filament System
In the quasi-static phase, the emergence of flux in the chromosphere is characterized by the formation of low-lying dark filaments, called Arch Filament System (AFS), see Figure 2c and Figure 3. The solar plasma is compressed by the rising of new flux, and as it becomes denser, it flows down under gravity at both ends of the loops. Downflows are commonly observed at the footpoints of the AFS and up-flows around the apex of the loops (Malherbe et al. 1998, Strous et al. 1996). Typical physical parameters of AFS are the rising speed ~10 km/s, the plasma downflows ~20-50 km/s, the electron density 5-10x 1010 in cm-3, the gas pressure 0.15-0.25 dyne cm-2, the life time ~10-20 minutes and the magnetic field of the order of 50G.
Moving magnetic features
New emerging flux tubes are very fragmented with strong magnetic flux accumulations distributed to spatial scales at the limit of the spatial resolution of present-day instruments. The observations of the Flare Genesis Experiment’s (FGE) launched in 2000 show magnetic structures of the order of its spatial resolution (~0.5 arcsec or ~360 km) (Figure 2a). Hinode/SOT with its better spatial resolution (~120 km) shows even smaller magnetic structures (Otsuji et al. 2007). High resolution observations of magnetic flux emergence reveal many small-scale moving magnetic features, called MMFs, with closely joined pairs of opposite polarities moving coherently (Figure 4). Because of their dipole character they are also referred to as Moving Dipole Features (MDF). They emerge in the center of large convection cells and quickly move as single units towards the cells’ edges (~ 0.4 km/s) (Bernasconi et al. 2002). Furthermore, a close look at the direction of the magnetic field vectors clearly indicates that MMFs are U-shaped (convex) magnetic loops stitched into the upper photosphere and not the earlier observed Omega–shaped (concave) loops. MMFs look like little wiggling disturbances embedded in a sea of mostly horizontal magnetic field lines. And at the center of MMF poles, persistent flows of material are found downwards into the photosphere. All MMFs are remarkably similar in size, field strength and orientation of the vector magnetic field. This suggests they are formed by an instability in the fields that has a characteristic wavelength, in this case it is about 3000 km. These flows further distort the field lines, amplifying each concave depression in them. The bending of the field lines triggers further flow of material towards the center of each depression thus creating U-loops. Each newly formed MMF is swept by the horizontal outward flows within the convection cell towards the edge of the cell where finally the entrained material can slide down into the sunspot or the network.
Ellerman bombs (EBs) are typical features observed in emerging flux regions. They are observed as point-like brightenings in the lower chromosphere (Figure 2c, Figure 2d). They were discovered by Ellerman 1917, who called them solar hydrogen bombs at that time. They consist of brief emissions best observed in chromospheric lines (Hα, Ca II 8542 A ) (Fang et al. 2006, Pariat et al. 2007). The profiles are characterized by a deep absorption at the line center and strong emission in the wings, see Figure 5. The EBs were called “ moustaches” because of this peculiar appearance of spectra (Severny, 1958). Each bomb lasts about 10 minutes and may recur after half an hour or more. Detailed morphological and statistical analysis of the EBs show that they share the physics of small solar flares, obeying self-similar distribution functions in their total and peak released energies and their durations (Georgoulis et al. 2004). Typical energies of EBs were estimated in the range 1027-1028 ergs, which is comparable to that of microflares. Large numbers of EBs could contribute significantly to the heating of the chromosphere in emerging active regions. EBs are manifestations of stochastic, low-altitude, magnetic reconnection in the solar atmosphere.
Serpentine field lines
The prevailing, traditional viewpoint of a smooth emergence of convex active region magnetic loops (Omega–shaped loops) into the solar atmosphere is fundamentally incomplete. This Omega-shaped loop model is valid for large scale emergence and in the mid term of the emergence which has a typical life time of a day. At the solar surface, the observed Omega–shaped loops are formed by the resistive emergence of undulatory flux ropes (Pariat et al. 2004). The upper parts of the emerging flux ropes are significantly deformed and decelerated when reaching the photosphere thus creating a large number of MDFs and localized concave emerging magnetic structures, see Figure 6 For these U – loops to fully emerge into the solar atmosphere the action of magnetic reconnection (via resistive effects, i.e., energy dissipation in electric currents) is necessary. EBs are the signature of the resistive emergence of undulatory flux ropes in the solar atmosphere.
Magnetic flux emergence is an inherently complex process characterized by a significant fragmentation of the magnetic flux bundles. The filamentary nature of the emerged magnetic flux is the reason for this complexity. The signatures of emerging flux tubes are various and can be summarized in the sketch in Figure 7 presenting the evolution of an emerging tube from below the solar surface to the corona. It presents the relation between the different phenomena associated with flux emergence, such as plages, moving magnetic features (MMF), serpentine field lines, Ellerman bombs (EB) and arch filaments (AF). The several dispersed pieces of the flux emergence puzzle fit together into a coherent picture that reconciles flux emergence, magnetic reconnection, and heating in the solar atmosphere (Pariat et al. 2004).
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