Intense gravitational forces keep the sun in a roughly spherical shape, despite it being largely a liquid—well, technically, ionised plasma. It is not surprising that anything trying to escape has to be bold and dramatic. But material does escape in the form of solar flares, solar prominences, coronal loops and coronal mass ejections. Magnetism drives all of the sun’s outbursts. Plasma is a soup of positively charged particles (ions) and negatively charged particles (electrons), which rotates about the sun at different speeds, at different depths and at different latitudes. This violent swirling creates all sorts of competing magnetic fields so that the sun is not so much a dipole (having a north and a south pole) but experiencing many poles constantly twisting and warping. This generates the coronal loops and prominences, and coronal mass ejections all of which in turn cause various solar storms. Together they generate the solar wind that bathes Earth (and indeed all the planets and moons, etc) in ionized particles.
Sunspot
Sunspots are dark markings (plasma craters) on the sun’s photosphere. They range in size from a few hundred kilometres in diameter too several times the size of Earth. They are observed because their temperature is something like 2 000°C less than the surrounding photosphere. They tend to present in cycles of about 11 years, between intense activity with a large number of spots to minimum activity, even with weeks of complete absence. There is a major flare up every 90 years or so
. They are associated with strong local magnetic activity, and often present as pairs with opposite polarity. This polarity reverses from one solar cycle to the next. The process is associated with strong magnetic fields.
This image of the sun’s corona was captured on the 31st December 2013 by NASA’s Solar Dynamics Observatory showing features created by magnetic fields. Image Credit: NASA/SDO – AIA instrument.
Solar flare
A solar flare is a sudden intense brightening of a small part of the sun’s chromosphere in the vicinity of a plage (or flocculi). A plage is a mass of gases that is hotter or cooler than the surrounding region. They occur very rapidly and last for no more than 20 minutes. Most of the energy consists of ultraviolet radiation, x-rays, cosmic rays and other particles. The intense short-wave radiation can sometimes interfere with radio communications on Earth. Cosmic rays are extremely high energy particles, usually protons.
Solar prominence
A solar prominence is a huge gas eruption from the sun’s chromosphere into the corona, reaching altitudes of several hundreds of thousands of kilometres, before plunging back to the surface. They are often associated with sunspot activity.
Coronal loop and mass ejection
Coronal loops arc above the sun’s surface. They comprise electrified plasma that flows along curving lines of powerful magnetic fields. They vary in size. The largest rise many thousands of kilometres into the solar corona. They can have temperatures above a million degrees centigrade.
A coronal mass ejection is a huge burst of plasma that moves along the sun’s magnetic field. They usually occur independently but can sometimes be associated with solar flares.
Solar wind and energy from the sun
The solar wind is a steady stream of ionised atoms (electrons and protons) ejecting from the chromosphere at speeds in excess of 800 kilometres a second. It is very dangerous. It is estimated that the sun loses between 1.5 to 1.86 million tonnes of material every second to solar wind. This is in addition to the 4.4 million tonnes of mass lost in the fusion conversion.
Heliosphere
The heliosphere is a bubble in space produced by the solar wind. It contains the solar magnetic field and charged particles of electrons and protons. Its shape is influenced by three forces: the solar wind, the motion of the sun itself, and the presence of interstellar materials. The heliosphere extends way beyond the solar system to the limits of the sun’s gravitational reach. The solar wind travels at speeds of several hundred kilometres a second in all directions reaching a boundary determined by the greater force of the interstellar medium.
In front of the heliosphere is a shockwave in the direction of the sun’s orbit. Image the bow wave of a boat travelling through water. Instead of water, ionised streams of particles are compressed, and behind there is a long tale pulling the heliosphere out of shape like the wake of a ship.
| The extent of the sun's influence | |
|---|---|
| AU* | |
| Sun | 0 |
| Heliosphere | 75 - 95 |
| Heliosheath | 75 - 100 |
| Heliopause | 100 |
| Sedna | 76 - 989 |
| *AU (Astronomical Unit) equals the average distance of Earth from the sun, about 149.6 million kilometres. | |
The termination shock is the region where the solar wind is finally rebuffed by the pressure from the interstellar medium, and its speed drops dramatically to subsonic levels. This causes a sudden compression (shock) and the solar wind quickly loses further momentum in a transition area called the heliosheath, until the charged particles of the solar wind turn back and flow down the tail of the heliosphere. This reversal of the solar wind marks the region of heliopause. This is the boundary between the sun’s influence and the start of interstellar space. Beyond the heliopause the gravitational pull of the sun is, to all intents and purposes, zero.
What little data we have about the heliosphere is compiled from the Nasa missions of Voyager (recorded 2007-2008) and Pioneer. The termination shock is thought to be between 75AU and 95AU within the heliosphere. The heliosheath probably extends to no more than 100AU, ending at the heliopause. The satellites also detected a new phenomenon—the magnetic field bubble. As Voyager travelled through the heliosphere it recorded dramatic dips in the number of electrons it encountered, suggesting a kind of “foam zone” of magnetic bubbles around the perimeter of the heliosphere. Scientists suggest these bubbles act as traps for the electrons of the solar wind, creating dips in recordings. As the sun spins, its magnetic field lines are bunched into folds, creating bubbles as big as 160 million kilometres in diameter.
In this region, incidentally, we find the most distance dwarf planet of the solar system so far discovered Sedna, travelling in an extreme elliptical orbit between 76AU at perihelion and 989AU at aphelion.
An “AU” is an astronomical unit defined as the average distance of the sun from the earth, equivalent to 149 597 900 kilometres.
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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.
Last updated: Saturday, 27th February 2021