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Solar Atmosphere

Above the convective zone of the solar interior is the solar atmosphere. As said above, this is made up of three main layers, the photosphere, the chromosphere and the corona. These are described below.

The Photosphere

The photosphere is the lowest of the three layers of the atmosphere. It is the optical `surface' of the sun, but it isn't actually a solid surface, it is a shallow shell of gas about 400km deep. The fact that you can only see 400km into the layer suggests it is surprisingly opaque to visible light despite it being such a thin gas. All visible light is emitted from the photosphere akin to a black body (i.e. peak of the black body spectrum is in the visible waveband). Therefore, the temperature is found to be about 5800K using Wien's displacement law. This, as well as the `limb darkening' around the edge of the Sun, also confirms that the temperature decreases as you go deeper into the photosphere (towards the chromosphere).

Granulation at the photosphere

Granulation at the photosphere.

Granulation occurs at the photosphere. It represents the top of convection currents operating just below the photosphere, transporting heat from below to the surface. This convection produces columns of rising gas (granules) to the upper levels, which then cools off and plunges back down into the interior. This is done at a rate of a few kilometres per second, so not very rapid, but noticeable. In the above figure, you can see the granulation taking place. The brighter, central regions show hotter gas rising upwards and the darker, narrower parts correspond to cooler gas sinking back down into the interior of the Sun.

Supergranules within the photosphere

Supergranules are enormous convective cells within the photosphere.

Supergranules are also seen when using the Doppler effect to measure the flow of material in the photosphere. These are enormous convective cells within the photosphere. The above figure shows the super granules using the Doppler effect. The red is light from material moving away from us and the blue is light from material moving towards us. Faculae are more easily seen near the limb of the Sun. They are bright areas of concentrated magnetic field, which tend to make the sun look brighter. During the sunspot maximum, they make the sun look 0.1% brighter.

Sunspots provide obvious evidence for solar activity. They occur where there are regions of intense magnetic field (up to 3000G) on the photosphere. They appear as very large dark irregular shaped regions moving across the surface of the Sun expanding and contracting as they move across the solar disk. They are sometimes isolated but are frequently found in clusters referred to as Sunspot Groups. Sunspots are not permanent features on the photosphere, but they last between a few hours and a few months. The brighter border of a sunspot is called the penumbra and the darker central region is called the umbra. Temperatures of sunspots are lower than the rest of the photosphere, hence why they appear darker. Using Wien's law again, it can be shown that the typical temperature of the umbra is 4300K and the typical temperature of the penumbra is 5000K. Sunspots are can be used to measure the rotation of the Sun by tracking them as they move across the solar disk. However, they also show how the Sun exhibits differential rotation. This takes place, as the Sun doesn't rotate as a rigid body. The rotational period of the sun increases with increasing latitude, showing the period is slower at the poles.

The number of sunspots visible on the photosphere periodically varies every 11 years. This is known as the sunspot cycle and was discovered in the early 19th century. It shows that the variation in magnetic activity on the Sun's surface reaches a maximum and a minimum. Tunnels of flux are created just below the photospheric surface and then rise upward and emerge through the surface, due to this magnetic field region being dragged around the sun faster at the equator than at the poles. This is where sunspots appear and are sites where solar flares are observed to emanate from (this is known as an Active Regions - regions of intense magnetic activity). At the start of the sunspot cycle, sunspots tend to appear at latitudes between 30° and 40°, a bipolar sunspot pair, one with a negative polarity and one with a positive polarity and one always leading the other. As the cycle progresses, the sunspots tend to appear closer and closer to the equator as magnetic flux streams towards the poles. Sunspot maximum marks the part of the cycle where there is the most magnetic activity, i.e. more sunspots, and sunspot minimum, when there is less activity. Currently, we have just passed sunspot maximum of sunspot cycle number 23. The Maunder Minimum was between 1645 and 1715 when the Sun went through a very inactive stage. There were very few sunspots seen. This sunspot cycle is only one half of a bigger cycle, known as the solar cycle. The solar cycle lasts for 22 years as it takes into account the fact that at the end of one sunspot cycle (11 years), the polarity of the Sun reverses. Therefore, sunspots during one sunspot cycle may have a positive polarity, but in the next cycle, they will have a negative polarity.

The Chromosphere


The chromosphere situated above the photosphere and is the middle layer of the solar atmosphere. It is a layer of gas a few thousand kilometres (2000-3000km) thick and is transparent to visible light, so cannot be seen against the brightness of the photosphere. However, it can be observed during a total solar eclipse, as in the above image. It is seen as a reddish-pink strip around the moon. The temperature increases with height and ranges from 4400K to 25000K at the top. To observe the chromosphere otherwise, we have to make use of a naturally occurring phenomenon. Instead of the spectrum being dominated by absorption lines (like the photosphere), it has an emission spectrum. An emission spectrum is created when electrons of a hot gas fall from higher energy states to lower energy states, and emit a photon. In the case of the chromosphere, this photon has a wavelength of 656.3nm - the wavelength of the H-alpha Balmer line and is a reddish pink colour. This occurs when an electron falls from the n = 3 state to the n = 2 state. The Balmer series occurs in the optical part of the spectrum and deals with electrons falling to the n = 2 state from higher states. Therefore, to see the chromosphere, the fact that the H-alpha line is dominant is exploited and a H-alpha filter is used, which is only transparent to wavelengths of 656.3nm.

Spicules are seen when observing the chromosphere through a H-alpha filter. They are jet eruptions of gas that shoot upwards, ejecting material off of the surface and outwards into the Corona at speeds of about 20 -30 km/s. They are usually found on boundaries between supergranuales and form as a result of magnetic field disturbances in the solar atmosphere. They appear dark against the photosphere as they are cooler and have relatively short lifetimes (of order of a few minutes). Perhaps the most impressive of activity seen on the chromosphere (and indeed the sun itself) are solar prominences. They are dense clouds of incandescent ionised solar gas suspended above the Sun's surface by its magnetic field. They appear at the limb and can last between a few hours and a few months depending on their type. There are two main types of prominences: Quiescent and Active.

Quiescent Prominences are the more stable and long-lived prominences. They can retain their structure for periods up to a year before they evolve by breaking up, suddenly disappearing or occasionally violently erupting into space with a tremendous burst of energy, then dispersing. They mainly appear as long, thin vertical sheets of material, sometimes shaped like an arch, with average dimensions of length; 200,000km, height; 40,000km, thickness; 6000km. Quiescent prominences appear to form along neutral lines separating regions of opposite polarity within sunspot groups or in active regions in general. Active Prominences are the opposite of quiescent prominences. They tend to be much shorter lived and smaller. The term `active' arises from the fact that they tend to show dramatic changes in their form in a matter of minutes, so one can witness their flow of material easier. Their average length is 60,000km and can take the form of loops, in which the material flows along the magnetic field lines, or as puffs or sprays, where material is hurled upwards in a violent manner. These are the most violent of solar phenomena and are associated with solar flares. Solar flares occur in active regions where the chromospheric material is shot upwards into the Corona at speeds of around 100 - 200 km/s. They are sudden violent releases of energy, which heat the material up to millions of degrees in a matter of minutes, releasing millions of tons of energy in many different forms (electromagnetic, energetic particles and mass flow). They are characterised according to their brightness in the x-ray part of the spectrum. Filaments are the same type of phenomena as prominences except that they don't occur at the limb of the Sun. Prominences are seen in an emission spectrum as they are at relatively low temperatures and high densities. This makes them a compact source of Hydrogen atoms giving H-alpha emission. However, Filaments are seen in an absorption spectrum and appear as dark lines on the solar disk.

The Corona

Corona emission

The corona is the outermost layer of the solar atmosphere, seen as a white glow around the limb of the Sun in the above image. It extends from the top of the chromosphere out to a distance of several million kilometres, where it becomes the Solar Wind. The corona is not as bright as the photosphere, hence it can only be seen when the photosphere is blocked out in some way. E.g. during a total solar eclipse or by using a Coronagraph. Although the corona has a very high temperature (between 1 million and 5million Kelvin), it is not actually `hot' as that it contains very little thermal energy. This is because there are very few atoms per cubic metre. Although the high temperatures cause these atoms to move at very high speeds, because there are so few, there are few collisions. The corona shines very brightly in the x-ray part of the spectrum, despite being blocked out by the photosphere in the visible. This is because of the very high temperatures. The solar wind is a stream of high-speed photons and electrons constantly escaping the Sun at velocities as high as 400km/s. It occurs because the temperature of the corona is so high that the Sun's gravity can't counter it.

From early observations of the corona, you could see bright emission lines at wavelengths, which didn't correspond to any known elements at the time. The reason for this is the temperatures are so high that the gases are superheated, stripping the minor elements such as Hydrogen and Helium of their electrons. Therefore, only the heavier elements, such as Calcium and Iron, retain some of their electrons, producing these spectral lines. This is known as the Emission Line Corona.

This fact is used in coronagraphs, where a disk covers the solar disk and a filter is used to detect the Calcium and Iron spectral lines. Coronal Holes are associated with the `open' magnetic field lines found at the solar poles. These are places of weak magnetic field where the field lines don't connect regions on the solar surface. Solar winds are known to originate from coronal holes. They appear as dark regions, where the corona is very dark. Coronal Mass Ejections (CME's) are huge bubbles of hot gases entwined with magnetic field lines, ejected from the sun over several hours. They can carry as much as up to ten billion tons of solar material each time. As with most solar features, CME's vary with the sunspot cycle. They disrupt the flow of the solar wind and is they hit the Earth's magnetosphere, can have huge consequences such as geomagnetic storms which, in turn, disrupt electricity supplies and communications. Coronal loops are found in active regions, around sunspots. These are associated with the closed magnetic field lines connecting different regions on the surface. These loops last between a few days and a few weeks. However, loops visible for shorter periods are associated with solar flares.