We are currently orbiting a giant mass of plasma with a nuclear reactor at its core and an enor-mously complex structure. The Sun is a G-type main-sequence star, just past the middle age of its 10 billion-year life, and it has been the subject of our interest since the beginning of times.
The evolution of life is directly linked to the presence and characteristics of the Sun, since it serves as a source of energy that enables water on Earth to be in liquid form, it is the source of the winds that drive the dynamics of our atmosphere and it provides a gravitational "lock" for our planet to orbit.
Given its significance, the Sun has been observed and studied for a long time. Understanding the inner workings of our host star is one of the most important scientific goals of today.
Just like the Earth, the Sun also has a very dynamic atmosphere. The presence of high tempera-tures and strong magnetic fields is the rudder that guides the apparently chaotic behaviour of the Sun’s atmosphere. The solar atmosphere is usually divided in three main layers: the photosphere, the chromosphere and the corona.
1.2.1 The photosphere
The photosphere is innermost layer of the Sun, with an average thickness of 100 km and temper-atures around 5800 K. It is the layer we see with our eyes, as it produces most of the visible light that reaches us from the Sun (see Figure 1, left).
The photosphere is divided into small bright regions called granules (Figure 1, right), which are formed as a result of the upwelling of hot plasma bubbles from the interior of the Sun. What we are really looking at is the top of the outermost layer of the solar interior, the convection zone.
The granules pop in and out of existence in a few minutes, with a diameter of approximately 1000 km.
Figure 1: Left: Photosphere seen with the Solar And Heliospheric Observatory (SOHO) in 2003.
The dark spots around the equator and in the southern hemisphere are sunspots. Right:
Granulation of the photosphere as seen by the Swedish 1-m Solar Telescope (SST) in 2010.
In regions of strong magnetic activity, if the magnetic field from the Sun breaks through the surface into the atmosphere, an active region is formed. The active regions can be easily located on images from the photosphere as dark regions called sunspots (see Figure 1, left).The earliest plausible records of sunspot classification dates back to 800 BC found in the Book of Changes, a Chinese ancient book [17]. The amount of sunspots present in the solar photosphere is directly related to the magnetic cycle of the Sun.
Our understanding of the physical processes that come into action in the photosphere to produce the phenomena that we observe is much deeper than what we know about the upper solar at-mosphere. The reason is rather simple: many of the approximations that simplify the physical description of the solar atmosphere rendering the problem mathematically simpler are only valid in the photosphere, while in the solar chromosphere and corona they no longer hold.
1.2.2 The chromosphere
Above the photosphere lies the layer known as chromosphere. It is called “chromo” because of its reddish color which can be spotted in solar eclipses (see Figure2). The reddish color is due to the recombination of hydrogen present in the chromosphere, specifically the Hα transition.
The first reports of chromospheric phenomena were seen during solar eclipses, and date back to medieval times [19]. The first documented observation of what is unequivocally chromospheric emission is from the 18th century [44]:
“Captain Stannyan, in a report on the eclipse of 1706, observed by him at Berne, noticed that the emersion of the Sun was preceded by a blood-red streak of light, visible for six or seven seconds on
the western limb... This outer envelope... seems to be made up not of overlying strata of different density, but rather of flames, beams and streamers, as transient as those of our own aurora borealis.
It is divided into two portions... the outer portion... may almost, without exaggeration, be likened to ‘the stuff that dreams are made of ’, since it is chiefly due to the ‘corona’... At its base, and in contact with the photosphere, is what resembles a sheet of scarlet fire... This is the ‘chromosphere’, a name first proposed by Frankland and Lockyer in 1869... in allusion to the vivid redness of the stratum, caused by the predominance of hydrogen in these flames and clouds."
At its base, the chromosphere is slightly cooler than the top of the photosphere, but the temperature increases again with height before reaching a plateau halfway to the top of the layer.
The chromosphere is a very enigmatic and chaotic layer, since the plasma there transitions between two very different situations. At the lower atmosphere, the plasma motions are not constrained to the shape of the magnetic field, and the gas pressure is higher than the pressure caused by the presence of the magnetic field. The opposite happens at the top of the solar atmosphere, since the magnetic pressure is much higher than the gas pressure, and the plasma is forced to move along the magnetic field lines. The chromosphere, being about 2000 km thick, is a transition region between these two regimes, and the dynamics of the plasma present in this layer are very complicated to study and comprehend.
Figure 2: August 1999 total solar eclipse [27]. The solar chromosphere can be identified as the red ring. The outer “cloud” surrounding the Sun is the solar corona.
The combined effects of magnetic guidance and small-scale gas thermodynamics [19] lead to a vast amount of fine structures such as jets, spicules(see Figure 3), fibrils, mottles, etc. that make the chromosphere a very unique part of the solar atmosphere. In particular, chromospheric spicules are the subject of study of this work, so they rightfully deserve a section of their own.
Figure 3: Off-limb spicules viewed in a high-resolution image observed by the Solar Optical Telescope (SOT) [16].
1.2.3 The corona
The corona is the outermost layer of the solar atmosphere. It is 10−12 times less dense than the photosphere but it is much warmer, with temperatures reaching 106 K. This rather unusual and counter-intuitive configuration of the solar atmosphere still baffles solar physicists, and it is known as the coronal heating problem.
The corona is much fainter than the photosphere, and it is only visible to the naked eye during solar eclipses, when the solar disk is obscured by the Moon (see Figure 2).
As mentioned above, in the corona the magnetic field drives the motion of the plasma, which means that the images of the solar corona are closely related to the topology of the magnetic field that is present in the region. A representative example of such magnetic structures are coronal loops (see Figure 4). The plasma serves as a tracer to delimit the shape of the magnetic field.
In regions where the Sun’s magnetic field opens to the interplanetary space, a region known as coronal hole appears. These regions are usually seen as dark spots in X-ray and UV images of the Sun (see Figure5), and their density is lower than that of the rest of the corona. The configuration of the magnetic field allows the particles to escape, and coronal holes are thought to be the main source of high speed solar wind streams.