Abstract:-
The Sun is the closest star to the earth and yet incredibly magnificent. It is the reason for the existence of the Earth and the ultimate source for astronomers to study stellar dynamics. The article covers one of the aspects of this magnificent star i.e. the Sunspots and attempts to understand the basic terminologies associated with it. Sunspots can be understood as active regions on the photosphere of the sun. The Sun’s complex magnetic field makes the study more interesting and intriguing. The article covers the Structure and formation of these sunspots highlighting the human quests for the star. The article's main objective is to understand the basic concepts of the Sun and ignite the curiosity to research more about solar dynamics.
Keywords:- sunspots, butterfly graph, solar maximum, solar minimum, coronal mass ejections, solar flares.
Introduction:-
The Sun Surface Dynamics are one of the most mysterious concepts of the universe for astronomers to work on. From the complexity of the solar magnetic field to the phenomena created by them everything involves immense study individually. The Study of Sunspots is one such aspect of Solar Astronomy. The Sunspots are patches of the Sun's surface that seem dark. The reason is the temperature drop in these areas. Additionally, an intense magnetic field is also released. Most of the aspects of the Sun somewhere find their connections and roots in the Solar Magnetic Field. The question arises when it is stated that such a powerful magnetic field is released from the surface of the sun through sunspots how is this magnetic field created and how are these sunspots formed?
1.1 How things work on the sun’s surface
The Sun’s Surface is quite busy. The Solar Magnetic field keeps it dynamic and active. The magnetic field inside the sun after becoming entangled during the solar maximum creates an active region on the photosphere (the surface of the sun) which we acknowledge as sunspots. The Sun’s magnetic field generation involves solar dynamo and the roles of the convection zone and the differential rotation of the sun. The Sunspots are the results of the Magnetic Field complexity and the upward pressure from the photosphere. The understanding of the Sun’s surface can be better done by the solar cycle and the butterfly graph which develops the concept of solar minimum and solar maximum depending upon the magnetic field of the sun.
1.2 History of Sunspots
The assumptions of the first observation of sunspots always point towards Galileo Galilei but the mentions are quite historical. It is believed that the Chinese astronomers recorded the solar activity in 800BC and also the Korean texts have mentioned the red vapor that soared and filled the sky. Even the John of Worcester’s chronicle of 1128 has a drawing of the sunspots. However the question of observing the sunspots before the invention of the telescope still lingers.
Fig 1.2.1 John of Worcester Sunspot Drawing, 1128
There is controversy behind the first observation of a sunspot. But the first historical record of the observation was found on 8th December 1910 in the notebook of Thomas Harriot. Later in 1611, a pamphlet was published named “On the Spots Observed in the Sun and their apparent rotation with the sun” by David and Johannes Fabricius. Galileo claimed to have discovered sunspots in the summer of 1610, but there is no proof that he did so until April of 1612. Regarding the disputes surrounding Galileo's life and what he discovered or did not discover before other scientists and/or observers, there are a lot of myths and misconceptions.
Christoph Scheiner was a German Jesuit mathematician who wanted to preserve the Sun’s perfection. There is mention of an interesting conversation between Galileo and Scheiner regarding the sunspots via letters. Scheiner proposed that the sunspots were satellites around the sun while Galileo argued that sunspots were on or near the surface of the sun and that their size is not constant. Galileo prevailed in this dispute after exchanging letters, and Scheiner published "Rosa Ursina," a book in 1630 that agreed with Galileo.
Fig 1.2.2 Rosa Ursina [1930- Cristoph Scheiner]
Background:-
To understand the formation and the working of sunspots, a few concepts related to the sun must be understood. The structure of the Sun will give an idea about the convection zone and the differential rotation will be understood with the aid of solar magnetic fields.
2.1 Structure of the Sun
Fig 2.1.1 The Structure of the Sun
The Sun’s layers can be divided into two main divisions. The inner and the outer layers can further be divided into three subdivisions respectively. The Inner layer consists of the core, radiative zone, and convection zone while the outer layer includes the photosphere, chromosphere, and corona. The core is essentially the place where the process of nuclear fusion occurs. A huge amount of energy is generated in the core with a temperature of about 15 million° C. Nuclear fusion involves the combining of hydrogen nuclei to form helium. The released energy during this reaction ultimately leaves the surface as visible light. The next layer is the radiative zone which is made up of plasma. In this layer the mode of energy transfer is radiation and the temperature drops to 2 million degrees Celsius. The convection zone is occupied by the constant movement of ions. In the convection zone, the layer of plasma at the beginning of the zone is constantly replaced by the heavier layer nearer to the photosphere. The primary cause resulting in the movement is the temperature difference. As the plasma moves towards the outer edge of the convection zone, the temperature drop results in the formation of heavier ions which then sink to the bottom of the layer and the process repeats. The movements of the ions create changing electric currents which contribute to the generation of the magnetic field. The outer layers of the Sun include the photosphere, chromosphere, and the corona. The photosphere is the visible layer and the corona is the outer atmosphere of the Sun.
2.2 Solar Magnetic Field
The sun’s magnetic field is complex and it involves depth of physics to be covered. Rather than going into details the main objective to understand the sunspots must be addressed. Differential rotation plays a key role in the creation of a solar magnetic field along with the convection zone. The Sun revolves every 25 days at the equator and takes slower to rotate at higher latitudes—up to 35 days at the poles—than Earth, which rotates every 24 hours at all latitudes. This type of rotation is termed differential. Different layers of the sun have different speeds. The rubber band analogy can be helpful to understand this.
Fig 2.2.1 Differential Rotation of the Sun
Consider the sun as a ball and the magnetic field lines as rubber bands or ropes tied to the ball. After rotating this ball the rubber band or the ropes inside begin to get entangled. This is exactly the case with the magnetic field lines inside the Sun. However according to the properties of magnetic field lines, the magnetic field lines never intersect. Thus rather than getting entangled like the rubber bands, the magnetic field lines become more twisted.
Sunspots:-
3.1 Formation of Sunspots
The magnetic field lines of the Sun become twisted and stretched due to the differential rotation of the Sun and are accompanied by the continuous movement of ions in the convection zone. Due to the constant twisting and stretching of the magnetic field lines, an active region is created on the photosphere, followed by an immense burst of magnetic energy which is otherwise suppressed by the pressure of the photosphere. As the plasma in this region is compressed, the region becomes colder relative to the photosphere of the sun, and the sunspots are formed.
3.2 Structure of Sunspots
Fig 3.2 ( Structure of the Sunspot)
The above figure gives an idea about the structure of the sunspot. The sunspot consists of the umbra (the darker region) and the penumbra (the semi-darker region). The magnetic field lines ejected from the umbra of the sun are vertical to the surface of the sun while in penumbra the field lines become horizontal which are accompanied by the radial flow of gas. This is known as Evershed flow which was discovered in the Kodaikanal Solar Observatory in India.
3.3 The Butterfly Diagram
Fig 3.3 (The Butterfly Diagram)
The data gathered from the observation of these sunspots is indeed very helpful for understanding the sun and solar dynamics. After plotting the sunspots from the data available a repetitive pattern is observed and near the equator of the Sun both above and below. This graphical “butterfly” repeats itself every 11 years. This period of 11 years forms one solar cycle. The bell-shaped bar graph in the earlier figure also represents the low frequency of the sunspots on the surface of the sun. The period when the solar activity is the least is known as solar minimum and marks the starting or ending of one solar cycle. Every 11 years, the twisting of the magnetic field lines gives rise to immense solar activity on the sun's surface, represented by the graph's high frequency. This is the period of solar maximum where the solar surface becomes very busy with continuous solar activities. One may wonder if the magnetic field is affected by this continuous 11-year solar cycle. The answer to this question lies in the solar maximum. During Solar Maximum magnetic field lines get more and more complicated and at a certain point the Sun reorganizes and rearranges its magnetic field by changing polarity. The process happens every year accompanied by arranged magnetic field lines and then the solar minimum.
Phenomena:-
The extreme solar activity that is released by the sun during solar maximum has a huge effect on life on the Earth. Various incidences including the power grid offs and the formation of beautiful auroras on the earth. The interactions of ions of the sun and their interaction with the magnetosphere of the earth is the result of such incidences.
4.1 Coronal Mass Ejections (CMEs) and Solar Flares
Fig 4.1 (a. CMEs b. Solar Flares)
While understanding the effects of sunspots, the coronal mass ejections, and solar flares become one of the most important phenomena. Coronal Mass Ejections (CMEs) are the result of a magnetic field erupting from the sunspots and the clouds are formed of coronal mass which includes the plasma of the corona of the sun. The temperature of the corona is larger than the surface of the sun. Protons and electrons make up the majority of the particle radiation seen in a CME, which also contains high magnetic fields that are stronger than those found in the solar wind. The magnetically agitated corona, or upper atmosphere of the Sun, is where these explosions originate. CMEs leave the solar system in an outward direction. While many of them entirely ignore Earth, some are aimed toward it. The Coronal mass ejections have the potential to cause geomagnetic storms on Earth and the Spacecraft and astronauts may be in danger from the radiation storms that are a component of CMEs. Coronagraph is used to understand and capture the CMEs which are used in various satellites including the SOHO satellite and the Aditya L1. A coronagraph is an instrument that is used to cover the bright light of the star and create an artificial total solar eclipse in this case. Solar Flares are another solar activity that usually accompanies Coronal magnetic fields.
4.2 Auroras
Auroras
Strong CME collisions have the potential to disrupt Earth's magnetosphere and release a burst of particle radiation into the upper atmosphere. The auroras are spectacular light displays that are produced when radiation interacts with gas molecules in Earth's atmosphere, causing them to glow and release light. (the northern and southern lights). The solar wind is produced by charged particles that the sun is constantly expelling from its corona, or upper atmosphere. The aurora is created when wind strikes Earth's ionosphere or high atmosphere. The phenomenon is known as the northern lights (aurora borealis) in the northern hemisphere and the southern lights (aurora australis) in the southern hemisphere.
Conclusion:-
One of the main indicators of solar activity, Sunspots provide insight into the complex mechanisms underlying the Sun's magnetic field. Sunspots are a window into the Sun's activity and its effects on the solar system, including Earth. They arise as a result of twisted magnetic field lines, and they are usually accompanied by major solar phenomena like solar flares and coronal mass ejections (CMEs). More understanding about the Sun and preparedeness for any potential effects on our technological society increases through research of sunspots and associated solar activity. The study of sunspots is an important component of the fascinating effort to solve the mysteries surrounding the Sun, which is an ongoing adventure.
References
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3] Fig 2.1.1- The Structure of the Sun- (https://www.nasa.gov/image-article/sun/)
4] NASA/Marshall Solar Physics
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9] SDO:- https://sdo.gsfc.nasa.gov/gallery/potw/item/838
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