The Sun is the main source of energy for all lives on the earth as well as the main defining object of the solar system dynamics. Like all other stars, the Sun has both inner complex dynamics and influence on interplanetary space. In this chapter, for the sake of completeness and to explain the origin of thesetremendous dynamics, the inner structure of the Sun as well as some important heliospheric activities are briefly discussed.
Our star, the Sun, is a G2-V type star (Aschwanden, 2014). All stars are in a balance of opposing forces between outward radiation fueled by nuclear fusion and inward pressure dictated by gravitational force . The radiative force is the product of the fusion of hydrogen atoms, which constitutes about 73 %, into helium atoms, which constitute 25 % of the Sun (Basu and Antia, 2007). This process is a thermonuclear reaction via turning a proton into a neutron.
Structurally, the Sun has inner zones and outer zones . The former consists of the core, radiative zone, interface layer (tachocline), and convection zone . The latter consists of the photosphere, the chromosphere, the transition region, and the corona (NASA, 2017) as shown in Figure 3.1.
***Figure 3.1:*The layers of the Sun (Credit: NASA, 2017)
The core of the Sun
The core of the Sun is 25% of the solar radius (Garc´ıa et al., 2007), the density is about 150g/cm3 (Basu et al., 2009) contains 34% of the solar mass even though only 0.8% of the total volume , and it is the hottest part of the Sun that a temperature of 15 million Kelvins (Dr. David H. Hathaway, 2015). This is the zone where the thermonuclear reaction is going . For the radiative zone , it starts from the edge of the core to the interface layer (tachocline) , and much of the energy generated in the core is carried away by photons though photons take a million years to reach the next layer due to the dense material of the region (Chemin, 2022).
The very next layer, the interface layer (tachocline) is much of our interest because it is believed that the solar magnetic dynamo originates in this very thin layer (Tobias and Weiss, 2007) that magnetic field lines can become stronger due to the changes in the fluid flow velocities crossing this layer . However, recent radio observations of brown dwarf stars show that, despite not having a tachocline layer , they can have similar magnetic strength and activities just like the Sun , which indicates the convection zone may be solely responsible for the solar dynamo (Route, 2016).
Next, the convection zone is the outermost layer of the inner zones and it extends about 200,000 km from the depth and the temperature is approximately 2 million Kelvins (Christensen-Dalsgaard et al., 1991), which makes the zone relatively cooler for heavy ions to keep their electrons so that the zone is opaque to the radiation (Ortu˜no-Araujo et al., 2012), which in turn traps the heat and eventually the fluid becomes unstable and start convecting . As a result, this layer is very turbulent (Brummell et al., 1995), which in addition to the rotational motion creates electric currents and magnetic fields , and the gas pressure is much more dominant than the magnetic pressure in this region, and because of this the magnetic field is dragged and twisted by the fluid , which then propagates passing through the photosphere, chromosphere, the transition region up to the corona and creates a multitude of activities on the surface of the Sun called solar activity.
In the photosphere , visible darker areas called sunspots appear due to localized strong magnetic regions , which inhibit some of the heat from reaching the surface and this makes the areas cooler than the other parts of the Sun (Babcock and Babcock, 1955). Because of the mentioned strong magnetic nature , the magnetic field lines around the sunspot often twist and cross , causing a tension of energy that bursts in an explosion called a solar flare (Chakraborty and Basak, 2022).
The corona is the hot and ionized outermost layer of the Sun and it is millions of Kelvins greater than that of the surface of the Sun (Aschwanden, 2006). The solar magnetic field confines much of the coronal plasma , but some of the plasma spread with a supersonic speed into interplanetary space , which is solar wind . The solar magnetic field forms so-called flux tubes . These ropes are tensed by the differential rotation of the Sun . Therefore, store plenty of energy . If a certain configuration appears (X-shape) the magnetic structure reorganizes . Hence, a huge amount of energy is released . The name of this 20 million K-degree flash is a solar flare . It emits plenty of energized protons and other ions in the heliosphere and very often, but not always a huge amount of solar plasma ejects to the IP space . The process is called coronal mass ejections (CME) . However, CME could occur without a flare too.
These phenomena are the main drivers of space weather and the conditions of the terrestrial cosmic environment (Facsk´o et al., 2022). The solar activity is cyclic , which is called the solar cycle , and it is approximately 11 years (Center/SDO, 2015), see Figure 3.2 . Each cycle the magnetic field of the Sun flips , and the periods, in which, the sunspots are the greater in number are called solar maximum and the lower in number are called solar minimum.
***Figure 3.2:*The sunspot cycle over the last several decades (Figure is from Pasachoff et al., 2014; Figure 8)
▍ 3.2 The Solar Wind and the Interplanetary Magnetic Field**▍**
The Sun ejects highly energized and ionized charged particles continuously in all directions, which is the solar wind. The solar wind is a collisionless plasma that consists of equal amounts of protons and electrons with the addition of negligible 3 to 6% helium (Neugebauer, 1981). As a result, the solar wind is a quasi-neutral plasma whose characteristics can be defined by magnetohydrodynamics (MHD) . In MHD, Alfvén theorem states that in a fluid with high electric conductivity the magnetic field line is frozen in it and moves along with it (Alfvén, 1942). Since the solar wind is one such fluid the Sun's magnetic field lines are frozen in the flow that they are forced to propagate with the solar wind (Roberts, 2007), forming the interplanetary magnetic field (IMF).
Depending on the origination point whether in the coronal holes or the equatorial belt of the Sun, the solar wind is classified as the fast solar wind with a velocity of 750 km/s and the slow solar wind with a velocity of 400 km/s respectively (Feldman et al., 2005). The coronal holes are areas that appear dark in X-ray images because these areas are much cooler and less dense than other regions (Cranmer, 2009) and the magnetic field around these areas does not loop back down but extends into the interplanetary space (Parker, 1959), so the plasma can easily flow out, creating the fast solar wind . The holes can appear anywhere on the coronal areas during the solar maximum while they usually appear on northern or southern poles of the sun during the solar minimum (McComas et al., 2003).
***Figure 3.3:*Combined X-ray image of the Sun's active regions observed from several telescopes. High-energy X-rays from NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) are shown in blue, low-energy X-rays from Japan's Hinode spacecraft are green, and extreme ultraviolet light from NASA's Solar Dynamics Observatory (SDO) is yellow and red (Credit: NASA/JPL-Caltech/GSFC/JAXA, 2015)
The expansion of the solar wind is a radial propagation away from the Sun that it is diluted and cools down on its journey . As seen from some observations the density of the solar wind decreases approximately as r^(-2) while the temperature decrease is less significant about a factor of 20 (Baumjohann and Treumann, 2012). During the expansion of the radial flow, the Sun rotates, which is 27 days on average , and because the magnetic field lines are anchored in the sun , the radial flow looks like an Archimedean spiral called the Parker Spiral . At 1 AU the spiral makes 45° to the Earth-Sun line , see Figure 3.4.
***Figure 3.4:*Schematic representation of the Archimedes spiral structure of the interplan etary magnetic field (Figure is from Bittencourt, 2004; Figure 3)
The solar wind travels throughout the solar system and defines the heliosphere the region in space whose frontier is impeded by the interstellar medium. Consequently, the heliosphere is a giant bubble whose center is the Sun and protects the solar system from interstellar radiations and cosmic rays . The size of the heliosphere is about 121 AU (Cowen, 2013). The solar wind decelerates when it flows outward through the Solar System . At a certain point, the flow becomes subsonic and forms a shock . Its name is termination shock (Jokipii, 2013). Then the flow continues its journey outward and interacts with the interstellar material . It is under debate whether the speed of the Solar System is super- or sub-sonic . A bow shock or bow wave forms before the heliosphere , respectively. Furthermore, due to the interaction, the solar wind becomes turbulent and this region is called heliosheath which is in between the termination shock and the heliopause (Burlaga, 2015).
The average interplanetary magnetic field strength is around 6 nT at 1 AU (Lowrie and Fichtner, 2020) and considering the average speed of the solar wind, the solar wind is both supersonic and super-Alfvénic , which means it is super-magnetosonic. As a result of this nature of the solar wind, when it collides with celestial objects such as planets, moons, asteroids, and comets or its slow-flowing part, shocks are formed. The main characteristic of shocks that can be found in interplanetary space to interstellar regions is a denser state in contrast to the medium in which they propagate due to the shock formation. Furthermore, if the celestial objects are magnetized, then the shock formations create interesting interaction regions, and their physics looks very exciting.
One such fascinating magnetized object is our planet, the Earth. The Earth has its magnetic field, known as the magnetosphere , which originated from its inner core via the dynamo effect (Gilbert, 2003). The Earth's magnetosphere extends 60000 kilometers in the sunward direction while a million kilometers in the anti-sunward direction (Lakhina et al., 2009). Such a big extension is a common characteristic of all the magnetized planets , and as a result of this extension, the cross-section of a planet is increased by a large factor . For example for the Earth, the factor is 150 (Baumjohann and Treumann, 2012).
The magnetic field frozen in the supersonic and super-Alfvénic solar wind plasma cannot enter the magnetosphere , the region where the terrestrial magnetosphere dominates . As a result, a special standing wave, the bow shock forms (Baumjohann and Nakamura, 2007). Consequence of the slowing down of the solar wind plasma, the kinetic energy of some of the particles is converted into thermal energy, which occurs behind the bow shock, and this region is called the magnetosheath. The boundary region between two magnetic field lines is called the magnetopause . Bow shock formation is common to all the planets and celestial objects with or without magnetospheres (Mazelle et al., 2004).
Figure 3.5: Illustration of the Earth's magnetosphere and its interaction with the solar wind. (Figure is from Kivelson and Bagenal, 2007; Figure 1)
***Figure 3.6:*Illustration of a CME event. (Credit: NASA, 2017)
In addition, there are several other shocks , the previously mentioned termination shock , coronal mass ejection (CME) driven shock , and a co-rotating interaction region (CIR) driven shock . When a CME event occurs , it moves faster than the background solar wind flow , resulting in a shock wave . On the created shock waves charged particles accelerate . So usually CMEs are one of the main causes of the solar energetic particles (SEPs) (Cane and Lario, 2006), see Figure 3.6.
Similarly to the shocks caused by CMEs , When the fast-moving solar wind flow catches the slow-moving solar wind flow , the so-called co-rotating interaction region is formed , and if the pressure gradient gets sufficiently large and the speed difference surpasses the local speed shocks can arise (Heber et al., 1999), Figure 3.7.
***Figure 3.7:*Co-rotating interaction region. (Figure is from David Burgess, 2017)
3.3.1 Classifications of Interplanetary (IP) shocks
Figure 3.8: Categorizations of IP shocks. N, T, B, and V denote number density, the proton temperature, the magnitude of the magnetic field, and speed respectively. (Figure is from WIND MFI Team Science, 2001) Armed with the above shock definitions and categorizations knowledge , now we can finally be able to classify IP shocks . The IP shocks can be classified based on their travel directions concerning the solar wind frame of reference . If the shock is moving away from its driver , here the Sun is referred but drivers can be detailed such as ICMEs, CIRs, etc , the shock is called forward shock (FS), and if it is moving toward its driver , it is called reverse shock (RS) . Adding the previous definitions of fast and slow shocks , IP shocks are usually categorized as fast forward (FF), fast reverse (FR), slow forward (SF) and slow reverse (SR) shocks (Berdichevsky et al., 2000), see Figure 3.8.
As you can see from Figure 3.8, the solar wind plasma parameters -- number density N, the proton temperature T, the magnitude of the magnetic field B, and the bulk speed V parameters increase dramatically from upstream (unshocked) to downstream (shocked) regions in fast forward (FF) shocks while the parameters except for the bulk speed decrease in fast reverse (FR) shocks [1] . In the case of slow forward (SF) shocks , the parameters except for the magnitude of the magnetic field increase from upstream to downstream whereas in the case of slow reverse (SR) shocks , the number density N and the proton temperature T decrease while the magnitude of magnetic field B and the bulk speed V increase [2].
Within 1 AU, the most frequent IP shocks are fast forward (FF) shocks (Richter et al., 1985). [1] In a sense of the reverse upstream to downstream in fast reverse (FR) shocks , the parameters except for the bulk speed increase from reversed upstream to downstream
[2] Again in the sense of the reversed upstream to downstream in slow reverse (SR) shocks , the number density N and the proton temperature T increase while the magnitude of magnetic field B and the bulk speed V decrease
