The Sun

Sun is our star. Sun is an ensemble of layers, as it is an active body

Sun Like a Structure

Sun Like an Active Body

Sun Like a Structure

Sun has a layered structure

• The Sun's core is the place where our star is generating its energy by the mean of fusion nuclear reactions which transform the hydrogen into helium. At the core, the temperature is 27 million degrees Farenheit (15 million degrees C), with a density of 150 g/cm³ (a strong one!). The core has a radius of 108,700 miles (175,000 kilometers); we are 323,200 miles (520,000 km) under the surface. The energy is now going to ascend through the diverse layers of the Sun as it will eventually leave it under the form of light
• The energy, through a first layer -the radiative zone, is leaving the core under the form of a radiation -in that case light. The radiative zone extends from minus 323,200 to minus 124,300 mi (520,000 to minus 200,000 km); the temperature drops to 3.6 million degrees F (2 million degrees C) and the density passes to 0.2 g/cm³
• The energy then is passing through an intermediate layer -the 'tachocline', which is thought to yield the magnetic field of the Sun. It reaches a second layer -124,300 mi (200,000 km) below the surface- where elements are beginning to re-combine and where radiation becomes matter. It traps the heat and by the mean of convection, it's heading to the Sun's surface. That's why the layer is called the 'convective zone'
• The visible surface of the Sun is called the 'photosphere'. It is 62 mi (100 km) thick. It is this layer which is mainly accessible to observation. The temperature is now 10,400° F (5,760° C) as the density is 0.0000002 g/cm³ -1/10,000th of the air's density. The photosphere is the realm of sunspots. Sunspots are now believed to be places where magnetic fields are preventing the incoming heat to reach the solar surface. Hence the plasma becomes cooler. Hence heavier, and it sinks (at 3,000 mph -4,800 km/h). The sunspots are the places of this event. Sunspots are darker because they are cooler than their surroundings, as the sinking process holds together the magnetic lines of same polarity at the same time (sunspots work in pairs, each sunspot has one polarity, either positive or negative; the magnetic fields of same polarity found in one sunspot should tear it apart). The sunspots activity is ruled by a 11-year cycle. The solar activity -and the number of sunspots- is reaching a maximum, then a minimum

  click to a view of the Sun structure in layers (values in miles). click here for the same diagram with the values in km. diagram site 'Amateur Astronomy'

  A NASA Marshall Center study presented in 2003 found that the Sun's 11 years cycle might be driven in part by a 'giant circulation system' 125,000 mi (201,100 km) below the solar surface. Compressed gases would start at the poles and move to the equator at a depth of 125,000 mi (201,100 km) and at 3 mph (4.8 km/h). They would then move back to the poles, in the upper layers this time, and at 20-40 mph (32-64 km/h). This current would transport a magnetic field. When the 11-year cycle is longer than usual, the flow is slow. When the cycle is shorter, it's fast. In this later case, some magnetic field is piling up at the poles as it's then transported to the equator by the depth current. It's moreover amplified, as it yields a cycle with a strong activity. The same study showed that granules of the Sun are pillow-shaped and that faculae are accumulation of magnetic fields against the walls of the granules. The solar magnetic field, on the other hand, is fliping each 11 years, with its poles fliping from one pole of the Sun to the other (it's currently inverted, with the magnetic south pole near the Sun's north pole, and reciprocally) as the scientists suspect that the temperatures at the southern pole of the Sun might be different from the ones at the north pole
• Beyond the photosphere is lying the 'chromosphere'. It is an irregular layer where the temperature passes from 10,400 to 36,000° F (5,760-20,000° C). This high temperature makes that the hydrogen turns red. Hence the name of the layer ('chromos', in Greek, means 'color'). It is this process which is called the 'H-alpha emission'. The chromosphere is the zone of the prominences, these solar eruptions which may be seen rising on the limb during eclipses or through an H-alpha filter. Such events are magnetic, arc-shaped tubes of hot gas. Important prominences are often associated with coronal mass ejections (CMEs; see below) as smaller ones are linked to the boundaries between the sunspot active regions. Prominences have a relative low temperature compared to the background. They appear bright when seen on the limb as they appear dark when they are seen on the disk. In this case they are termed 'filaments'. The chromosphere may be considered the atmosphere of the Sun or its gaseous enveloppe
• A second external region -the 'transition region'- is then found. It's a very thin region which is produced by the heat which is descending from the solar corona into the chromosphere. The region is quickly passing from 40,000 to 1,800,000° F (22,000-1 million degrees C). The hydrogen becomes ionized (it's difficult to see), and the zone mainly produces light in the ultra-violet. Such a light is coming from the ionized carbon (C IV), the ionized oxygen (O IV) and the ionized silicon (Si IV), as it's observable from space only
• At the farthest from the Sun's surface, at last, a last region is found. It's the 'corona', or 'solar corona'. The corona is a kind of external atmosphere to the Sun. During a total eclipse, the solar corona is visible like a radiating zone, stretching far from the occulted solar disk. The corona is an extremely hot region 1.8 million degrees F (1 million degrees C) or more, where most of the elements, the heaviest excepted (and they are just a few of them like the iron or the calcium) are ionized. The general aspect of the corona varies according to the 11-year cycle. To observe the corona outside the period of an eclipse, one uses a coronographs, which is a device which artificially occults the solar disk), or one observe from space, in the X-rays. Due to its high temperature, the corona is very visible in this range of the wavelengths. On the other hand, as the photosphere is barely emitting in this range, the corona, in the X-rays, is even visible through the solar disk! The heating of the corona is still ill explained, with a part of it provided due to turbulences occurring in the chromosphere and generating so-called 'Alfven waves'. The Alfven waves are created when convective motions and sound waves push magnetic fields around, or when dynamic processes create electrical currents that allow the magnetic fields to change shape or reconnect. The Alfven waves last about several minutes
• Once the energy of the Sun has journeyed through the corona, here begins the realm of the solar wind, and the interplanetary space

The Solar Magnetic Field Makes a Difference at the Solar Poles The ESA-NASA Ulysses spacecraft found again -like in 1995-1996- that there is a difference of a 7 to 8 percent in temperature between a cooler north pole and a warmer south one, with a difference in temperature of the order of 80,000° F(44,000° C). It seems related to the structure of the magnetic field as that difference has followed the 11-year cycle of the flip of both solar magnetic poles. That's of importance as the solar wind emanates from the solar poles
The Ulysses saw another tricky observation, as a solar storm which emerged from a sunspot region at the equator was funneled in such a way that the craft was hit as it was overflying the south pole, as the solar wind emanates from the Sun's poles. That flow from the poles in terms of the solar wind may have the latter spill down to the Sun's equator, or to be confined to the mid-latitudes only

Sun Like an Active Body

The Sun is an active body. It's expelling, through various, usual or unusual events, a flurry of elements and radiation into the solar system. The Sun is working on a 11-year cycle that is that each 5-6 years the activity of the Sun is reaching a peak, as 5-6 years later, it's reaching a low. A 2003 study is showing further that the most unusual and energetic solar events, the Coronal Mass Ejections (CMEs) are related to a

Flares, CMEs, Proton Storms The most usual mechanism of a solar event is that a flare explodes from the magnetic field lying over a region of sunspots, unleashing visible light and X-rays. This translates into a Coronal Mass Ejection (CME), with a large amount of solar material expelled into the interplanetary space. As the CME moves, taking some days to reach Earth, it plows through the 'usual' solar wind, accelerating protons to high energies and pushing them ahead. The proton storm then hits Earth, as the CME too, trigerring auroras and bringing potentially harmful radiation to spacecraft and astronauts. Mastering the 'solar weather' is of importance for the satellites in orbit around Earth as it will be more important still for the renewed presence of man at Moon, and further to Mars

cleansing of the Sun's magnetic field. CMEs are increasing at the moment of the peak of the 11-year cycle as does their speed (during last peak, there were more than a thousand CMEs at the Sun). CMEs do not peak however until 2 years after the solar peak. During about the same duration, the solar magnetic field is reverted, that is the North pole of the dipole become the South pole and reciprocally. During these two years, CMEs are polar, occurring at one pole, then at the other two years later

All the various sorts of the solar outflow are interacting with the planets of the solar system, especially with Earth. Planets are protected from the solar wind, particles, and radiation by a magnetosphere, which is a tear drop-shaped magnetic field extending on the far side of the planet. The most known interaction between the solar events and the planetary media are the auroras. Some solar particles are managing to reach the poles of the planets, where they are making the atoms in the upper atmosphere glow. Auroras, at the Earth, are usually occuring from 60 miles (96 km) to several hundred miles (several hundred km) high, as some may occur to more than 500 miles

• The Solar Wind. The solar wind is originating in the solar corona, this outermost part of the Sun. The corona has such a high temperature that the Sun's gravity is letting the solar wind escape. The solar wind is

  Another Feature of Sun? A study of the Sun by the Japanese craft Hinode has spotted that powerful X-ray jets are expelled from the solar surface hundreds times a day, propeling American continent-sized blobs of hot gas at speeds of 2 million miles per hour (3.2 million km/h). Such jets were already known, as the new craft allowed to see that they are occurring at an important pace of about 240 daily. They are not linked to any place of the surface, occurring about anywhere there. It looks like they are triggered by reconnection events similar, albeit much smaller, than those which power the solar flares, at about a thousand times less than a typical M-class flare. Those X-ray jets might well represent up to between 10 and 25 percent of the input of the Sun into the solar wind. The X-ray jets might too provide for the heating of the corona which remained unclear until now, by throwing there 'Alfven waves' which crack there like a whip and heat the gas

  composed of ionized particles which are moving at speeds of between 250-500 miles per second (400-800 km/s). The solar wind is mostly made of hydrogen. It's measured by its speed and by its proton density. With a speed ranging 0.9-1.8 million mph (1.5-2.9 million km/h) the solar wind is reaching Earth in 4 to 8 days. Such a flow is the most usual product of the Sun. It's the usual entrance of the solar wind into the Earth's magnetosphere which is producing the auroras. It seems now well established that the solar storms and the dangerous radiation are yielded by fast CMEs (see below) which are plowing into a slow solar wind ahead of them. This builds a shock front which is accelerating the solar wind particles. A crude way to estimate the speed at which the solar wind is leaving the Sun was, until now, to ask whether it was leaving the Sun from a zone where the magnetic loops were closed, or from a zone where they were opened (like in the coronal holes found in permanence at the poles, or sometimes elsewhere at the surface), bringing to a slow -but dense, or a fast -but thin- solar wind respectively. A study in 2005 is showing that characterizing the chromosphere under from where the solar wind is originating allows to predict both the speed and the isotopic composition of it, providing a better way to predict solar storms and radiation. The chromosphere has been found shallow, dense, and compressed beneath area of closed magnetic loops, as it was found thicker and less dense under the coronal holes. In this later case, the thicker the chromospheric layer is, the more it is being allowed to expand by open magnetic fields and the faster the solar wind will blow. This new method is more precise than the old 'fast or slow' estimate. Such coronal holes appear like dark regions in the SOHO EIT 195 pictures. The fast solar wind, on the other hand, has been further accurately located in a source region lying between 3,100 and 12,400 mi (5,000-20,000 km) above the surface. The closed magnetic loops there are swept by convection to funnel regions where they reconnect with existing open lines. This reconnection allows the plasma trapped into the closed lines to escape, and to be accelerated!
• The Interplanetary Magnetic Field (IMF). The Interplanetary Magnetic Field (IMF) is the other usual component of the solar flow. Most of the time, the Sun's magnetic field is similar to the one of the Earth. It's magnet-shaped, that is a dipole with a North-South polarity. Great closed loops exist at the equator's level as opened lines pour from the poles. The IMF is interlaced with the solar wind. Due to the fact that the Sun is rotating in 27 days, the IMF is spiral-shaped (this spiral is called the 'Parker spiral'). The IMF is interacting too with the Earth's magnetosphere. When it's oriented South, that is at the opposite of the Earth's magnetosphere, the two fields are reconnecting together, which yields a crack in the magnetosphere. This is what is called the phenomenon of the 'reconnection'. Such a puncture may occur at 38,000 mi (61,000 km) above the Earth, as it's as large as twice the Earth. The solar wind is entering these cracks down to the upper atmosphere, where it produces auroras. Recent findings showed that the cracks may last up to 9 hours as the auroras they produce are visible more South than usual due to the unusual solar magnetic field's strength
• Coronal Holes. Coronal holes are dark areas in the solar corona as seen in the ultraviolet. These holes are often sources of solar wind gusts which may take away solar particles into space. Such solar wind streams then reach Earth where they may yield geomagnetic storms. Coronal holes are zones of weak solar magnetic field. This allows high speed solar wind particles to escape. These gusts take 2-3 days to reach Earth. Solar wind escaping from large coronal holes are of high speed. Usually lines of the Sun's magnetic field are falling back to the Sun's surface, but, as far as coronal holes are concerned, lines stretch into the solar wind. Coronal holes are usually located near Sun's poles but they may occur elsewhere too. Coronal holes are visually assessed and whether they are or not linked to a solar wind stream reaching Earth
• Solar Sunspots. Solar sunspots are places of intense magnetic loops which are partly controlling the outflow of the solar plasma. The temperature is so hot in the Sun's outer atmosphere that atoms are colliding there stripping each other from their electrons. Hence the atoms part into positively charged ions on one side and negatively charged electrons on the other. This is known as a plasma. This on the other hand is trapping this material along the lines of Sun's magnetic field. As the solar plasma is mostly the steady outflow of the solar wind, sunspots are at the origin of more unusual events. When the pressure of the solar material builds up, the magnetic lines eventually break, leading either to a

  UV views of magnetic loops above sunspots as seen at the solar limb. picture SOHO

  solar flare when the burst occurs near the surface, or to a Coronal Mass Ejection (CME) when it occurs in the solar corona. The magnetic field of the sunspots is hundred times stronger than the usual solar magnetic field. Hence they add to the latter making the magnetic field at the surface of the Sun tangled and complicated. Magnetic loops seen above the sunspots are tubes of magnetic flux where a plasma of ions and electrons are magnetically trapped. Due to heat, atoms are colliding, stripping off their electrons. Sunspots are estimated by the sunspot number (see the tutorial 'Observing the Sun') and are assessed as being or not sources of solar flares
• Solar Flares. Solar flares are explosions happening when the energy stored in magnetic fields looping above sunspots is suddenly released. It's a quick, large scale change in the solar magnetic field, and a radiation event. The explosion occurs at the top of the loop, as it heats the solar atmosphere and accelerates electrons, which runs down each leg of the broken loop! Release expels radiations ranging from radio waves to X- and gamma-rays. A late study through NASA's RHESSI showed that solar flares are a reconnection of magnetic field when the latter is stretched away from the Sun

  Flares and CMEs A flare is a high-power energy release per unit time, a mostly radiative energy release event , and the most powerful storm in our solar system, as A CME is a lower power event occurring over a much longer time period, although carrying lots of mechanical (kinetic) and magnetic (potential) energy. CMEs blast out over a billion of tons of particles at millions of miles per hour. Post coronal loops can be seen coiled over the area where a flare occurred, lingering during about one Earth's day

  under the pressure of ejected solar plasma. Reconnection heats the Sun's atmosphere to tens of millions of degrees as the solar plasma is accelerated to almost the speed of light. It's why solar flares are shining in the X-rays. It was found lately that emerging magnetic fields from the solar interior which are not in alignment with the pre-existing field at some place, and interacting with it, are generating strong electrical current which, building on several hours, leads with a strong probability to a flaring activity. Solar flares were thought as potential culprits for the heat of the solar corona, but a 2003 study showed that microflares (a million times smaller than flares and far more frequent) might as a whole account for that. Important flares, on the other hand, have their powerful explosions yielding influence unto the slow oscillations which usually occur and exist in the convective, outermost part of the Sun's interior. Solar flares are sorted according to their brightness in the X-rays range and to their strength. There are 3 categories (X, M, C), and they are subdivided into nine subdivisions (C1-C9, M1-M9, X1-X9); there is too a B category. X class flares are major events which may yield radio blackouts for the entire Earth and long duration, intense radiation storms. Class M flares are medium events yielding short duration radio blackouts mainly in polar regions as C class flares are minor events without noteworthy consequences for Earth. Most powerful flares generate too bright auroras. A pecular form of flares have been observed in 2008, showing that those ones are getting hotter as they just focus on heating the solar atmosphere instead of dividing their energy between that and accelerating particles, thus becoming 9 million of degrees hotter
• Coronal Mass Ejections (CME). CMEs are large gas bubbles mixed with lines of magnetic field and expelled from the Sun on a duration of several hours. CMEs take place when Sun's magnetic field lines over the sunspots are stretched up into the solar corona, and that an increasing pressure leads them to a point that they snap. Billion tons of solar plasma are ejected in the solar system, travelling at speed over a million mph. Complex and distorted magnetic fields, bearing ionized particles, are travelling along with a CME, perturbating the solar wind flow. CMEs are eventually reaching Earth after a journey of two to four days (fastest in about 40 hours), triggering an intense activity at Earth's magnetosphere. CMEs is carrying a magnetic field, with a polarity, away from Sun. Like what occurs with the Interplanetary Magnetic Field, when the CME's magnetic field is South, it's interacting with the Earth's magnetosphere. When North the Earth is shielded by its own field. CMEs polarity may not be determined however before they reach monitoring satellites in orbit, that is about 15 minutes before reaching Earth. Such an activity may harm satellites and power grids, and threaten astronauts with high radiation doses. CMEs are increasing the aurora activity too, bringing auroras much more South (or North) than usual. When a CME is reaching Earth, its size is about 30 million miles (48 million km), that is about the orbit of Mercury! At the moment of the cycle maximum, there are 2-3 CMEs a day; at the moment of the cycle minimum, there are 1 CME a week. Some CMEs are this important and directed to Earth that, in perspective, they form an halo around the Sun (they are called 'halos CMEs'). CMEs are often associated with solar flares

  Here is an example of a series of CMEs as seen by the SOHO spacecraft in November 2003 as solar activity was particularly active. CMEs here illustrated unfolded on a real timespan of 11½ hours. First of them is probably an halo CME. SOHO is using a coronograph to block Sun's light; central white circle is Sun's real size. Right is seen a 3-D modelized CME following a new technique developped in 2004 (click for a larger picture). pictures courtesy Soho observing the Sun (left) and NASA's Goddard Space Flight Center (right)

  and large prominences (these filaments are huge clouds of relatively cool dense plasma, suspended in Sun's corona, at times erupting and escaping the Sun's atmosphere) but they may exist independently too. An accompanying phenomenon to a solar flare-CME event is a proton storn. Solar protons are accelerated to nearly the speed of light by the explosion and they are reaching Earth minutes only after the flare, or they may take longer, about two hours or more. This is what is called a 'proton storm'. It may last some days as such highly electrified protons can penetrate 4.3" of water and are delivering radiation doses to any astronauts above any protective planetary magnetosphere. Scientists are still debating whether the energetic particles from such events are accelerated by the flare itself, by the shock front of an associated coronal mass ejection, or both. Magnetic fields inside a CME are spiral-shaped and are traceable through the CME gas (a plasma), which is showing this shape. A 2004 study using SOHO LASCO images managed to visualize such a CME travelling in space in three dimensions. A CME is an expanding swarm of arcades of loops, rather than a bubble or rope-like structure. Loops are remaining connected to the source region longer than thought, at least as long as the CME is visible inside SOHO LASCO field. A study at the Solar Physics Division of the AAA, June 2003 Annual meeting (Laurel, Md) showed that CMEs work like solar flares. Stretched magnetic fields snap and re-combine releasing a strong energy, and a 2003 study showed that fastest and largest CMEs are really linked to solar flares -CMEs have their source into the burst of a flare, as smallest CMEs would arise from another kind of magnetic field, which do not snap -because not compressed enough, but only gently rise, rising the CME with it into the corona. CMEs occur when there is a quick and large scale change of the Sun's magnetic field. CME may carry away up to 10 billion tons of plasma to speeds reaching 3,200 miles per second (2,000 km per second) -millions of mph (or km/h); with records at 4650 miles/s -2,890 km per second). The geomagnetic storms they are generating at Earth are categorized into active, minor and severe
• Interplanetary Shock Waves. Interplanetary shockwaves are caused by halo CMEs. They are due to the fact that materials which are expelled from the Sun at high speed hit previouly ejected, and slower matter. Hence energy is stacking and increasing in density. The solar wind becomes denser. Interplanetary shock waves are carrying an associated magnetic field which has a North or a South component. When the component is South (i.e. opposite to Earth's magnetic field), shock waves may cancel magnetosphere at the point where they reach allowing the solar wind to pervade into the magnetosphere. This often triggers auroras. Another more dangerous consequence of a shock wave is that the CME is accelerating protons, producing a proton storm at Earth. Such high intensity protons may be harmful to spacecraft and astronauts