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EDUCATIONAL ACTIVITY Calculating the Height of Formation of the Northern Lights

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EDUCATIONAL ACTIVITY Calculating the Height of Formation of the Northern Lights EDUCATIONAL ACTIVITY Calculating the height of formation of the Northern Lights. by Mr. Juan Carlos Casado. Astrophotographer tierrayestrellas.com, Barcelona. Dr. Miquel Serra-Ricart. Astronomer Instituto de Astrofísica de Canarias, Tenerife. Mr. Miguel Ángel Pio, Astronomer Instituto de Astrofísica de Canarias, Tenerife. Dr. Lorraine Hanlon. Astronomer University College Dublin, Irland. Dr. Luciano Nicastro. Astronomer Istituto Nazionale di Astrofisica, IASF Bologna. 1. Activity objectives Through this activity we will learn how to calculate the height of formation of the Northern Lights from digital photos. The objectives that we want to achieved are: 1. Implement a methodology for the calculation of a physical parameter (height) from an observable (digital images) as a technique for teaching applications, documentaries and research. Apply knowledge of trigonometry and basic atomic physics. 2. Understand and apply basic statistical techniques (error calculations). 3. Understand and apply basic analytical techniques of images (angular scale, height of stars, ...). 4. Work cooperatively as a team, valuing individual contributions and expressing democratic attitudes. 5. Contributing to scientific knowledge of the Aurora and Solar Activity. 2. Instrumentation The practice or activity will take place from digital images obtained in Greenland (Denmark) in August 2013. 3. Phenomenon Northern Lights 1 The Northern Lights are one of the greatest natural spectacles that can be observed from Earth. In the activity we will see what they are, how they are produced and where it can be observed. Also we will show two methods to calculate or estimate the height at which they form. 3.1. What are Northern Lights The Northern Lights or Aurora is a phenomenon in the form of glitter or glow in the night sky visible in areas of high latitudes (Arctic and Antarctic), but occasionally it may appear at lower latitudes for short periods of time. Figura 1. Aurora Borealis photographed from a Tasiusaq farm located south of Greenland (J.C. Casado- starryearth.com). In the Northern Hemisphere (most populated) is known as Aurora Borealis (term due to the French philosopher and scientist Pierre Gassendi in 1621) or popularly "Northern Lights". In the southern hemisphere Aurora Australis occurs, which simultaneously follows the same patterns of activity as the Northern Lights 2 Northern Lights. The Aurora Australis is visible especially in Antarctica (Fig. 1), although it can be seen from the southern areas of Australia and South America. Auroras are not a phenomenon unique to Earth. Other planets like Jupiter and Saturn, with strong magnetic fields, show similar phenomena. 3.2. What is the origin of the Northern Lights The Sun is continuously emitting high-energy particles, as well as all types of electromagnetic radiation, including visible light. This flow of particles is the so-called solar wind (hot gas or plasma), which is composed mainly of positive ions and electrons. There are very energetic phenomena such as flares or coronal mass ejections (CME stands for Coronal Mass Ejection in English) that increase the intensity of the solar wind. The solar wind particles traveling at speeds from 300 km/s (slow solar wind) to 1,000 km/s (fast solar wind), so that cross the Earth-Sun distance in about two or three days. In the vicinity of the Earth, the solar wind is deflected into space by Earth's magnetic field or magnetosphere. The solar wind pushes the magnetosphere and deform it, so that instead of a uniform beam of magnetic field lines as those that show an imaginary magnet placed in a north-south inside the Earth, what it is produced is an elongated structure with a long tail with the shape of a comet, in the opposite direction to the sun (Fig. 2). Northern Lights 3 Figure 2. Artistic representation of the sun emitting the solar wind and a coronal mass ejection that moves through space. When he reaches the Earth, most of the particles are deflected by Earth's magnetic field, which takes the form of a comet tail. A few particles are driven into the atmosphere of our planet canalized towards the magnetic poles along the lines of terrestrial magnetic field strength, which are displayed in the figure as green lines. A small part of the solar wind particles penetrate into the atmosphere following the earth's magnetic field lines, so that they are driven along the path that they mark. Trapped particles in the magnetosphere collide with neutral atoms and molecules in Earth's upper atmosphere, typically atomic oxygen (O) and molecular nitrogen (N2) found in the neutral state and in its lowest energy level, called fundamental level. The energy contribution provided by the particles from the Sun carries those atoms and molecules to the so-called excited states and they will return to their fundamental level emitting energy in form of light (Fig. 3). That light is what we see from the ground and we called auroras. Northern Lights 4 The Northern Lights are emitted typically between 100 and 400 km because at this altitude the atmosphere, though thin, is still dense enough that collisions with solar particles occur significantly. Figure 3. When an electron from the solar wind collides with an atom of oxygen (O) or a nitrogen molecule (N2) of the upper atmosphere, it transfers energy which get the atom to an excited state. Upon return to the ground state, the atom emits energy as light with a characteristic wavelength, corresponding to a specific color, as shown in this figure. 3.3. Where, when and how to observe the Northern Lights The Northern Lights occur in some areas of the earth called auroral ovals, which are located around the north and south magnetic poles, respectively (Fig. 4). Northern Lights 5 Figure 4. Northern auroral oval. You can see the areas of frequent occurrence of auroras, and the reduction of the width of the oval on the dayside facing areas road (bottom of the image). The colors indicate the probability of observing an aurora and the red line is the southern (lower latitude) from where you can see the auroras (see model Ref6 OVATION-NASA). Northern Lights 6 The more intense is the solar wind and more energetic the particles ejected from the Sun, greater are the ovals. Therefore, if solar activity is moderate to low, the ovals are thin and in the case of boreal boundaries move farther north. However, during the great storms, the northern oval widens and moves further south. Figure 5. Evolution of the Magnetic North Pole. Auroras are formed in an oval around the Earth's magnetic poles (see Fig. 4). If solar activity is very intense, sometimes oval extends through the southern United States and Europe. For a given level of solar activity, the thinnest part of the auroral oval is always on the dayside terrestrial (earth meridian noon), while the thickest part of the oval is located on the night side of Earth, and therefore more likely to see the aurora from local midnight. The zones of highest frequency at which one can observe the auroras correspond to a circle situated in the auroral ovals (Fig. 5). In the northern hemisphere this zone extends from Alaska, northern Canada, southern Greenland, Iceland, northern Scandinavia (Norway, Sweden, Finland) and northern Siberia. The zone of maximum occurrence of Aurora Australis is found in Antarctica. In these ovals, the frequency of auroras per year may exceed the 240 nights during periods of high solar activity (discrete auroras), decreasing both inwards and outwards of the oval (diffuse aurora). By contrast the inhabitants of the southern USA, Mexico, southern Europe, and its surrounding areas may experience Northern Lights 7 the aurora (diffuse type) only once in life. It is estimated that in Ecuador terrestrial aurora can be seen every 200 years. In southern Europe, you can see this phenomenon very rarely times; the probability is about one per year at France, decreasing to 0.2 per year in the south of Spain or Italy. Coinciding with the last maximum of solar activity, the aurora was seen in areas of the Mediterranean and the Spanish the 6 April 2000 (Fig. 6). And still is remembered the Northern Lights January 25, 1938, during the Spanish Civil War, which was observable from Andalusia. Figure 6. Aurora borealis (diffuse type) visible as an intense red lighting with structure, in the north of Figueres (Girona), on April 6, 2000. Photo of Pere Horst. Our star has cycles of activity. During peak periods the solar wind increases and therefore is easier to observe auroras. The main observable in solar activity is the amount of spots that has the sun on the surface. Sunspots are areas of the surface cooler than their surroundings so they appear as dark images. After several years of data, it has been discovered that the amount of spots on the surface of the sun rises every 11 years or so, so that the cycle of activity is for 11 years (known as "undecenal cycle"). The last peak occurred in late 2000 and according to the latest data is expected a new high in late 2013. Northern Lights 8 The auroral are phenomenon low luminous, so it can be observed only at night. The weak auroras have brightness similar to the Milky Way one, while the brightest can come to have luminosity similar to the full moon. Due to the fact that auroras are visible only in the circumpolar regions, they shall not be observable during the summer due to the phenomenon of the midnight sun. Auroras can be observed from August to May, being the best months to observe them which are close to the equinoxes (September to March) due to the better geometric disposition of the Earth's magnetic fields, which results in the appearance of Geomagnetic Storms that facilitate the entry of solar energetic particles at the poles.
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