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Solar Plasma
The sun is made of very hot gas, so hot that atoms have their electrons removed to make positive ions and negatively charged electrons. A gas made of electrical charges is called plasma.
Solar plasma is mostly hydrogen with some helium and small amounts of bigger atoms. Bigger ions, like iron, are not common on the sun but are important for STEREO and SOHO space-craft images, by making ultraviolet light. High energy ultraviolet is emitted when iron and other ions are even hotter than white hot.
- Plasma is matter and all matter attracts other matter - called gravity.
- Gravity compresses the plasma into a ball we call the sun
- Compression of a gas or plasma makes it hot
- Very high temperature makes electrons leave their atomic nuclei, producing electrically charged ions.
- As solar plasma moves, its charged particles make an electric current.
- A moving electric current produces magnetism.
- The core of the plasma ball is so hot and compressed that thermonuclear reactions occur, making it hotter still.
- The plasma ball is rotating in a complex way, which has profound effects on the sun's properties
Stars are made of plasma. Studying the Sun tells us how other stars of a similar mass and age will behave.
Differential rotation
- Because the sun is plasma, it can and does rotate at different speeds in different parts. Not like a rigid golf ball at all.
- The core and radiation zone, which are the two inner layers, do rotate much like a ball.
- At the radiation / convection zone boundary, differential rotation sets in.
- The solar equator revolves faster than the poles.
- The layer where rotation changes from integrated to differential is called the tachocline.
- In the convection zone, charged plasma motion is turbulent and the distorted electric currents shear against the evenly rotating, inner, charged layers, to make a magnetic generator at the tachocline.
- The generated solar magnetic field gets wound up and twisted by the differential rotation, with further distortion as plasma moves up and down in the convection zone.
- Helicity is the name given to energy stored in the sun's twisted magnetic fields. Unlike sunspots, the magnetic polarity of helicity does not vary with the 11 year solar cycle. Helicity is being generated constantly, because the sun rotates differentially, but always in the same direction. The energy in helicity reaches a maximum at solar sun-spot maximum.
- The sun has to rid itself of helicity once the magnetic tension gets too great. (Similar to twisted rubber bands, which mount up in a complex way, only to untwist suddenly if given the chance).
- Helicity is ejected into space by solar flares and coronal mass ejections (CME's).
- We see twisted CME's and flares in 3D on this web site as the STEREO mission unfolds. CME's are less frequent at solar minimum (now) than at maximum. NASA expects enough CME's will be analysed, in the predicted 2 year duration of the mission, to provide significant information.
Visible sun and coronal heating
The core of the sun is exceedingly hot, but away from the centre the gravitational pressure gets less, the temperature drops, and thermonuclear fusion stops at the core / radiation zone boundary.
Cooling continues until the visible, bright, white "surface" of the sun is reached. The surface is not solid, but is plasma (ionised gas). Here the temperature has dropped to a little under 6,000 degrees Kelvin. The visible layer is called the photosphere. The photosphere is opaque and we cannot see through it to deeper parts of the sun.
Above the photosphere the plasma cools for a short distance and its pressure keeps dropping as the gravitational force on it decreases. It is even possible for atoms and carbon monoxide molecules to exist in this thin layer. A little higher and the lower pressure plasma heats up again to become translucent.
Just why the corona gets rapidly hotter in the transition zone and more gradually hotter higher up is not fully explained yet. Contributions to coronal heating include magnetic reconnection events, X-ray jets and Alfven waves. X-ray jets were shown by the Hinode space craft. Coming out of polar coronal holes, some are as broad as polar plumes, which are seen in ultraviolet (but with a bright base and transient). Others look like the smoke stacks seen in H-alpha and 304Å ultraviolet. Like stacks, X-ray jets are not confined to coronal holes. They may even turn out to be the x-ray equivalent of stacks. Their relation to the fast solar wind (which mainly comes from coronal holes) is not clear yet.
Just above the photosphere lies the chromosphere. Coloured red from H-alpha emission, the corona is the layer seen on H-alpha images made from earth by professional and amateur solar astronomers.
The temperature rapidly rises in the transition zone.
Higher and hotter still is the corona, so hot it is only seen by ultraviolet light (plus a little particle scatter of photospheric light, called the K corona, which is only seen during eclipses and with coronographs).
We can see down through the transparent corona and chromosphere to the photosphere. We cannot see deeper than the photosphere because it is emitting and absorbing white light from quite a thin plasma layer (where negative hydrogen ions are found). The photosphere is optically dense.
The images from STEREO and SOHO satellites are made in ultraviolet light, which we humans cannot see. There is no point in humans seeing ultraviolet. The earth's atmosphere absorbs nearly all of it and our vision has evolved to work in the abundant, visible sun light, which easily gets down through the air to our level on Earth's surface. The colours of UV images are artificial, since invisible radiation has no colour.
Wavelengths of STEREO and SOHO ultraviolet images
The hotter the plasma is, the shorter the wavelength of ultraviolet light it puts out. Ultraviolet images at shorter and shorter wavelengths let us see higher layers of the corona. As the corona gets hotter and hotter, it lies further above the photosphere and its pressure drops.
Solar wind
The corona blows out of the sun as the solar wind all the way out to the heliopause, far beyond the Oort cloud of comets. At earth, sun's corona gas pressure has dropped so low, it is nearly a perfect vacuum. Although the solar wind is exceedingly hot and fast, it would not even ruffle Mum's washing, because there are just not enough particles left in the solar plasma here.
(Temperature is defined by the average velocity of the individual gas particles and not by what a thermometer might record. Thermometer temperature in space varies, depending on the instrument being in sun-light, or shaded behind a space-craft, or planet. The lowest temperature in space is set by micro-wave background radiation, produced by the big bang and red shifted since then, by expansion of the universe, to just above absolute zero).
Life on our planet is only possible, so close to a searingly hot star, because of earth's atmosphere and magnetic field. During solar Coronal Mass Ejections and solar flares, charged particles (protons, electrons, alpha particles) may potentially injure astronauts, but not hurt us because they are deflected by earth's magnetism. Charged particles cause Aurora rings above the magnetic poles, as they crash into air molecules, ionising them and making them glow. Huge electric currents in earth's magnetic field can blow power transformers during a solar magnetic storm, but have no effect on our bodies. Satellite TV, GPS and old fashioned short wave radio may malfunction. Heating the upper atmosphere makes it expand and satellites in low orbits may slow down as the air rises to meet them, ultimately causing premature descent and burn-up in the atmosphere.
Distance on the sun:
Sun diameter: 1.4 million km
Earth's Diameter at the Equator: 12,756.1 km.
Earth's Diameter at the Poles: 12,713.5 km
Sun/Earth Diameter = 441
Measure the sun diameter on your computer screen = Dmm
Now measure any interesting sun object = Smm
Length of a sun object in the plane of the screen = 1.4 S/D million km
For example: sun diameter, D = 148mm on your screen
Sun object on your screen, S = 1mm
S actual size = 1.4 x 1 / 148 = 6.8 thousand km
The equation is good for measuring the height of prominences at the disc margin.
Because the spherical shape of the sun foreshortens objects on its surface, the equation only works for measuring sunspots in the middle of the solar disc. Otherwise you have to correct for solar latitude and longitude.
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