Although most people think of the sun as a flawless, round, and smooth ball of fire with no flaws, a closer examination shows that it has some dark spots known as sunspots. These sunspots usually occur in pairs or groups, and they emerge from the sun’s interiors. They are noteworthy for being visible for a few weeks before disappearing into the solar interiors. Scholars and researchers have studied how and why sunspots form over the years. More specifically, the analysis of sunspots dates back to 1609 when Galileo claimed that, by observing these solar characteristics, he had known that the sun took approximately thirty days to rotate. Increased studies have identified that sunspots are dark patches on the sun, which are cooler than the immediate atmosphere. Apparently, a major characteristic of these spots is that they have a strong concentration of magnetic fields of up to 4000 gauss (dhdhdhd). While there is plenty of information about sunspots today than there was centuries ago, their formation remains an unresolved issue in physics. This paper scratches on the much needed research of current theories on sunspots formation and prediction methods.
Descriptions of Sunspots
Before making an attempt to understand sunspots formation and predictions clues, it is imperative to fathom of their composition. Admittedly, an understanding of what makes up these solar features provides a framework of establishing logical theories and arguments on formation processes. Although the specific origin of sunspots remains a mystery, the contemporary understanding of their nature is more enhanced than that of Sir Robert Hooke, who claimed that the dark patches were soot in the solar fire (fhfhfhf). The ancient researcher knew about sunspots because some of these blemishes have diameters as long as 50,000 kilometers. Noting that these marks may be as wide as the earth, they are visible to the naked eye. Furthermore, sunspots may exist in singles, pairs, or groups of up to 100 blemishes. Every day, the NOAA Space Weather Prediction Center examines the Active Region (sunspot group) facing the earth from the solar disk for their eruptive threat. Later, the experts assign each sunspot group with a number depending on the specific threats. Notably, large sunspots, particularly those with sophisticated magnetic layouts are believed to trigger natural disasters synonymous as solar flares including forests fires.
A group of sunspots: some are larger than earth
Over the years, numerous scholars have asserted that sunspots are relatively cooler patches that appear as black marks on the face of the sun. With a temperature of about 4,500 K, sunspots are about 1,500 chillier than the sun’s photosphere. Scientists and critics alike describe a sunspot as a thermal phenomenon by appearance. Nonetheless, although sunspots are said to be cooler than the sun, their temperature is ten times as high as that of boiling water. Also, these features radiate high amounts of light despite being darker than their immediate surroundings. Sunspots are cooler than their surroundings because of their strong magnetic fields that hinder the continuous transmission of heat. In tandem with the characteristic, many scholars regard sunspots as magnetic phenomena by origin. Overall, a typical sunspot has two parts: the umbra, which is the darkest part, and the penumbra, which is the lighter part that surrounds the umbra.
Parts of a sunspot
Theories Of Sunspots Formation
To date, the specific origins of sunspots remain a mystery, a factor that increases scholars’ interests in studying the sun. Over the years, different theorists have developed models that try understanding this unresolved problem of solar physics. A vast fraction of the developed theories have been disapproved by continued research and those that still exist do so tentatively.
The Rising Flux Tubes Theory
While modeling of the sun is challenging, this model is perhaps the most common the most common assumption of sunspots formation. In 1955, Parker developed the rising flux tubes theory based on magnetic buoyancy. The theory, which is also popular as the cluster model of sunspots formation, holds that after a magnetic flux tube forms the magnetic and gas pressure increases in tubes because of increased magnetic field. For horizontal pressure to balance, the density of the tube must decrease. As the tubes become lighter than their surroundings, the flux tubes tend to rise as shown in the figure below.
A sketch of an appearing magnetic flux tubes
Proponents of this theory opine that a dynamo triggers the flux tubes in an area of close proximity to the base of the convection region. In 1979, Parker proposed that these tubes have twists that help them to rise coherently despite of the disorderly convention zone (Jabbari 17). The assumption was affirmed by Brun and Jouve who, in 2007, used 3D numerical MHD simulations. Eventually, Parker concluded that sunspots occur when numerous flux tubes in the conventional zone rise via magnetic buoyancy and create single huge flux rube once they reach at the surface.
Other researchers advanced the flux tubes theory to explain why and how, sunspots remained visible for weeks but later disappeared into the solar interior. While using Parker’s suggestions, in 1979, Spruit attempted a description of the convective fall of the little flux tubes. The scholar held that the magnetic field represses convection when the former is bigger than the critical value. Naturally, the critical value at the solar surface is 1270G. Whenever the field strength is lesser than the critical value, a state of instability arises and results to downward flow. At the same time, temperatures fall and triggers high magnetic field concentrations in the upper layers. According to Spruit (1979), these processes are the convective collapse of flux tubes. On the flip side, when the field strength is greater than the critical value, the tubes acquire a new equilibrium with lower energy. Nonetheless, if the newly established magnetic field has a concentration lower than the critical value, the tube continues to sink and soon vanishes from the surface of the sun.
Praise and Criticism of the Rising Flux Tubes Theory
Over time, the rising flux tubes theory has received praise and criticism almost in equal measures. On the on hand, scientists claim that through this model, they can observe diverse sunspots’ properties such as their east-west orientation, bipolarity, and polarity inversion with latitude and time. Additionally, the rising flux tube theory of sunspot formation facilitates an understanding of sunspots’ positions in low latitudes as well as their diverse tilt angles. Nonetheless, despite these appraisals, the theory has faced some criticism. To form sunspots with such great magnetic fields, there must be flux tubes with greater magnetic fields. However, neither observations nor simulations by researchers such as Fan in 2009 for Guerrero and Kapyla in 2011 have proven such scenarios. These tests did not affirm the presence of magnetic fields with maximum strengths enough to push the flux tubes to the surface and establish sunspots. Overall, the rising flux tube theory remains one of the most notable explanations on formation of sunspots.
The Negative Effective Magnetic Pressure Instability (NEMPI)
Besides the Rising Flux Tube Theory, the NEMPI models is the other most celebrated approach of explaining sunspot formation. From 1989 to 1990, Kleeorin and others embarked on a research to an alternative explanation of the gigantic magnetic field concentration the sun’s turbulent plasma (sunspots). In tandem with their model, when large-scale magnetic fields suppresses the sum tumultuous pressure, there occurs a negative turbulent impact to the overall field magnetic pressure, which in turn triggers large scale instabilities. The NEMPI model has proven to be effective in explaining the origin of Active Regions on the surface of the sun. Given the established fact that it is solar dynamos that establish large scale magnetic fields in the sun, one strategy of having a realistic model that explains sunspots formation is by studying a system where Negative Effective Magnetic Pressure Instability (NEMPI) originates from a dynamo- generated magnetic field.
Over the years, numerous scholars have proved that Negative Effective Magnetic Pressure Instability (NEMPI) works in scales involving many turbulent eddies. In relation to isothermal layers, mean field simulations (MFS) and Direct Numerical Simulation (DNS) have shown that the onset of sunspots caused by NEMPI occur at the same depths. However, as the magnetic field increases, so does the depths. In long run, the maximum growth rate of the instability and the field strength are independent of each other. However, for this phenomenon to occur, the magnetic structures are supposed to be fully contained in this domain.
Since the Galileo discovered sunspots in the seventieth century, scientists embarked on a mission to not only understand the composition and formation of active regions but also a strategy of predicting their occurrences. The need to establish effective methods of predicting the formation of active regions in exacerbated by the fact that sunspots have a high tendency of causing natural disasters such as forest flares. Although some of the predicting methods have low levels of accuracy, they are still important.
The easiest and most observable feature of solar activity is the dark blemishes that occur on the surface of the sun. Since 1610, scientists have used telescopic objects to observe those marks in the solar disk. Ancient observers opined that the number of sunspots at any given time present an index of the overall solar magnetic actions. According to Eddy, Hoyt, and Schatten, the number of marks in an active region shows time changes in a unique cyclic manner. In solar-terrestrial physics, the sunspots number R is used as a proxy for the overall state of solar activity. Notably, “R” is more accurate after 1850 since daily averages are seen often. Although a solar dynamo approach to predicting solar activity would be the most ideal, scientists are yet to develop such a model. Currently, only two approaches are used to predict solar activities.
Strictly Numerical models
These methods rely on the identification of any amplitudes and periodicities that can generate past solar cycles. A primary advantage of the numerical-based prediction tactic is that it is possible to predict future solar cycles regardless of the exact time in future. Additionally, this strategy can be beneficial in the reconstruction of past solar activities that occurred before scientists took their measurements. Through the approach, scientists can tell with accuracy when a future solar activity will occur. However, a disadvantage of such models is that it does not use any physical information. The method assumes that the essential part of the phenomenon is periodic, therefore, critical periodicities have been observed in data.
These approaches rely on statistics and they aim at establishing a relationship between geographical parameters and sunspots numbers during solar maximum or any other point in the course of the cycle. Over the years, scientists such as Ohl and Kane have asserted that that the precursor methods are relatively successful than other approaches. Although scholars have widely employed these techniques, the precursor approaches have failed to predict with accuracy the solar cycle 23 maximum.
Charbonneau, Paul. “Solar dynamo theory.” Annual Review of Astronomy and Astrophysics 52 (2014): 251-290.
Jabbari, Sarah. Origin of solar surface activity and sunspots. Diss. Stockholm University, 2014.
Schlichenmaier, R., et al. “The role of emerging bipoles in the formation of a sunspot penumbra.” Astronomische Nachrichten331.6 (2010): 563-566.
Thomas, John H., and Nigel O. Weiss, eds. Sunspots: Theory and observations. Vol. 375. Springer Science & Business Media, 2012.