Solar Activity

By: Gabriella Kendralla

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The Sun's Layers

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The Core

The core of the Sun is considered to extend from the center to about 0.2 to 0.25 solar radius. The core is the hottest part of the sun and the solar system. It has a temperature of about 15 million degrees Celsius. It has a density of 150 g/cm³ (150 times the density of liquid water) at the center of the core.

https://en.wikipedia.org/wiki/Solar_core

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Radiative Zone

The Sun's radiative zone is the section of the solar interior between the innermost core and the outer convective zone. In the radiative zone, energy generated by nuclear fusion in the core moves outward as electromagnetic radiation. In other words, the energy is conveyed by photons.

Matter in a radiation zone is so dense that photons can travel only a short distance before they are absorbed or scattered by another particle, gradually shifting to longer wavelength as they do so. For this reason, it takes an average of 171,000 years for gamma rays from the core of the Sun to leave the radiation zone. Over this range, the temperature of the plasma drops from 15 million K near the core down to 1.5 million K at the base of the convection zone.

https://en.wikipedia.org/wiki/Radiation_zone

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Convective Zone

The convection zone is the outer-most layer of the interior. It extends from a depth of 200,000 km up to the visible surface of the Sun. Energy is transported by convection in this region. The surface of the convection zone is where light (photons) is created. We can only see the surface of the convection zone. If we point our telescopes towards the Sun, what we see is a white ball with some dark patches.

https://www.cora.nwra.com/~werne/eos/text/convection_zone.html

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Photosphere

The photosphere is the visible surface of the Sun that we are most familiar with. Since the Sun is a ball of gas, this is not a solid surface but is actually a layer about 100 km thick (very, very, thin compared to the 700,000 km radius of the Sun). the solar disk near the limb.


A number of features can be observed in the photosphere with a simple telescope (along with a good filter to reduce the intensity of sunlight to safely observable levels). These features include the dark sunspots, the bright faculae, and granules. We can also measure the flow of material in the photosphere using the Doppler effect. These measurements reveal additional features such as supergranules as well as large scale flows and a pattern of waves and oscillations.

http://solarscience.msfc.nasa.gov/surface.shtml

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Chromosphere

The gases of the Sun extend far beyond the photosphere, which may be considered the lowest level of the solar atmosphere. The region immediately above the photosphere is called the chromosphere. The chromosphere is 2000-3000 km thick. It glows faintly relative to the photosphere and can only be seen easily in a total solar eclipse. When it can be seen it is reddish in color (because of strong Balmer H-alpha emission). This color is the origin of its name (chromos meaning ``color'').


The faint flow of the chromosphere is due to an emission spectrum from hot, low density gases emitting at discrete wavelengths. The discovery of helium noted earlier was from emission lines seen in the chromosphere during an eclipse in 1868. This new element was only found on the Earth in 1895. The chromosphere contains spikes of gas called spicules that rise through it.


http://csep10.phys.utk.edu/astr162/lect/sun/chromosphere.html

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Corona

A corona (Latin, 'crown') is an aura of plasma that surrounds the sun and other celestial bodies. The Sun's corona extends millions of kilometres into space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph. The word "corona" is a Latin word meaning "crown", from the Ancient Greek κορώνη (korōnē, “garland, wreath”).


The high temperature of the Sun's corona gives it unusual spectral features, which led some in the 19th century to suggest that it contained a previously unknown element, "coronium". Instead, these spectral features have since been explained by highly ionized iron (Fe-XIV). Bengt Edlén, following the work of Grotrian (1939), first identified the coronal lines in 1940 (observed since 1869) as transitions from low-lying metastable levels of the ground configuration of highly ionised metals (the green Fe-XIV line at 5303 Å, but also the red line Fe-X at 6374 Å). These high stages of ionisation indicate a plasma temperature in excess of 1,000,000 kelvin, much hotter than the surface of the sun.


Light from the corona comes from three primary sources, which are called by different names although all of them share the same volume of space. The K-corona (K for kontinuierlich, "continuous" in German) is created by sunlight scattering off free electrons; Doppler broadening of the reflected photospheric absorption lines completely obscures them, giving the spectral appearance of a continuum with no absorption lines. The F-corona (F for Fraunhofer) is created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the F-corona extends to very high elongation angles from the Sun, where it is called the zodiacal light. The E-corona (E for emission) is due to spectral emission lines produced by ions that are present in the coronal plasma; it may be observed in broad or forbidden or hot spectral emission lines and is the main source of information about the corona's composition.

https://en.wikipedia.org/wiki/Corona

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More About the Sun

Sunspots

Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They correspond to concentrations of magnetic field flux that inhibit convection and result in reduced surface temperature compared to the surrounding photosphere. Sunspots usually appear in pairs, with pair members of opposite magnetic polarity. The number of sunspots varies according to the approximately 11-year solar cycle.

https://en.wikipedia.org/wiki/Sunspot

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Prominences

A prominence is a large, bright, gaseous feature extending outward from the Sun's surface, often in a loop shape. Prominences are anchored to the Sun's surface in the photosphere, and extend outwards into the Sun's corona. While the corona consists of extremely hot ionized gases, known as plasma, which do not emit much visible light, prominences contain much cooler plasma, similar in composition to that of the chromosphere. The prominence plasma is typically a hundred times cooler and denser than the coronal plasma. A prominence forms over timescales of about a day, and prominences may persist in the corona for several weeks or months. Some prominences break apart and may then give rise to coronal mass ejections. Scientists are currently researching how and why prominences are formed.

https://en.wikipedia.org/wiki/Solar_prominence

Flare

A solar flare is a sudden flash of brightness observed near the Sun's surface. It involves a very broad spectrum of emissions, requiring an energy release of up to 6 × 1025 joules of energy (roughly the equivalent of 160,000,000,000 megatons of TNT, over 25,000 times more energy than released from the impact of Comet Shoemaker–Levy 9 with Jupiter). Flares are often, but not always, accompanied by a spectacular coronal mass ejection. The flare ejects clouds of electrons, ions, and atoms through the corona of the sun into space. These clouds typically reach Earth a day or two after the event.The term is also used to refer to similar phenomena in other stars, where the term stellar flare applies.


Solar flares affect all layers of the solar atmosphere (photosphere, chromosphere, and corona), when the plasma medium is heated to tens of millions of Kelvin, while the cosmic-ray-like electrons, protons, and heavier ions are accelerated to near the speed of light. They produce radiation across the electromagnetic spectrum at all wavelengths, from radio waves to gamma rays, although most of the energy is spread over frequencies outside the visual range and for this reason the majority of the flares are not visible to the naked eye and must be observed with special instruments. Flares occur in active regions around sunspots, where intense magnetic fields penetrate the photosphere to link the corona to the solar interior. Flares are powered by the sudden (timescales of minutes to tens of minutes) release of magnetic energy stored in the corona. The same energy releases may produce coronal mass ejections (CME), although the relation between CMEs and flares is still not well established.

https://en.wikipedia.org/wiki/Solar_flare

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Auroras

The dancing lights of the aurora provide spectacular views on the ground, but also capture the imagination of scientists who study incoming energy and particles from the sun. Aurora are one effect of such energetic particles, which can speed out from the sun both in a steady stream called the solar wind and due to giant eruptions known as coronal mass ejections or CMEs. After a trip toward Earth that can last two to three days, the solar particles and magnetic fields cause the release of particles already trapped near Earth, which in turn trigger reactions in the upper atmosphere in which oxygen and nitrogen molecules release photons of light. The result: the Northern and Southern lights.

http://www.nasa.gov/mission_pages/sunearth/news/gallery/aurora-index.html#.VmcKc9KrTZ4

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