Multiple cameras probe sunís secrets
(appeared in Oct 2015)

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Imaging the sun from a pair of vantage points has uncovered new truths about our mother star, says S.Ananthanarayanan.

The sun sustains the seasons and all life on earth with its bounty of sunshine. Sunshine is the light emitted by searing hot gases just near the sunís surface, by virtue of being hot. But the heat of the sun comes from feverish movement and energetic interactions which lead to emissions other than light, like alpha particles, which are Helium atoms whose electrons have been stripped, or protons, which are bare hydrogen atoms and also light of very high energy, like X Rays or gamma rays.

The reason for this variety of emissions is that the mass of the sun has compressed its core to very high pressures, setting off high energy nuclear reactions. This vaporises all matter, so that the sun is actually a ball of hot gas, gas so hot that collisions of atoms have separated the negatively charged electrons and the positive nuclei of atoms, and the hot gas is a maelstrom of charged particles. Add to this that the sun also rotates, like the earth, and there are currents and tides and waves in the seething mass of moving charged particles.

The sun is also difficult to observe, as the glare of the visible light blocks the possibility of any detail. It is only by indirect observation of images of the sun that many features, like sunspots, which are regions which get cooler than the surroundings for a short time and hence appear dark, or solar flares which are flashes of light as a result of eruptions of hot gases, can be detected. Solar flares are eruptions accompanied by massive emission of matter, a part of which has speeds enough to escape the sunís gravity, leading to sprays of charged particles striking the earth, which causes disruption of radio communication or power supply, for example. And even apart from matter emitted in flares, there is a continuous stream, a form of Ďevaporationí that emerges from the sun, the so called solar wind.

The features of the sun, and the sources of the emissions, known as Coronal Mass Emissions, or solar Energetic Particle (SEP) events, are of great interest, as they could reveal the activity in the interior of the sun, which is one of the billions of stars where similar processes are going on. The distance of the earth from the sun and the fact that the sun is rotating complicate the interpretation of what is observed. The sun goes round once in about twenty five days. Considering that the diameter of the sun is more than a hundred times that of the earth, this is a very fast speed for the sun. The matter that is emitted thus describes an arc in its approach to the earth, like the jet of water from a lawn sprinkler. Making sense out of what is seen could hence be easier if the observer were not on the earth, but somewhere well removed, and in communication with the earth. The origin of a stream of matter striking the earth is not where it appears to come from, but towards the right edge of the sun. With the help of satellites STEREO and ACE, an arrangement that enables observation both directly as well as from the side, two teams of researchers one led by Nariaki Nitta from the Lockheed Martin Advanced Technology Center in the USA in June 2015 and another led by Radoslav BucŪk from the Max Planck Institute for Solar System Research (MPS) in Germany, in Oct 2015, have independently reported in the Astrophysical Journal a possible mechanism behind one kind of emission from the sun.

The emission that was studied is of atoms of helium-3, a form of helium atoms whose nucleus has only one neutron, along with two protons, in place of the usual two neutrons. Helium-3 (denoted as 3H) is produced in high energy nuclear reactions and the emission from the sun was found to occur at the same time as massive waves, named blast waves, in the sunís atmosphere. Blast waves occur in fluids when large energy is pushed into a small volume, not unlike what causes the Tsunami. The waves, which were found to extend over half a million kilometers and moved at three hundred kilometers a second, were detected by observing the sunís atmosphere in extreme ultraviolet light. It is considered that the energy of the huge waves was able to accelerate the 3H particles towards the earth.

STEREO and ACE

Unlike observation stations placed in orbit around the earth, mainly to escape distortion by the earthís atmosphere, STEREO and ACE are satellites that have been placed in orbit around the sun, so that they can detect what the sun looks like from well outside the atmosphere and from a different angle than the view from the earth. STEREO, the acronym for Solar TErrestrial RElations Observatory - is a pair of satellites, one ahead of the earth and the other trailing. Observation from separated points helps scientists in the same way as the fact that we have a pair of eyes in place of only one is useful for us. Thanks to our pair of eyes, separated by a few inches, each eye sees a slightly different image, with the help of which the brain is able to make out a sense of depth, or the real distance of an object from the observer. In the same way, observations from the two satellites, which have been named STEREO A and STEREO B because one is Ahead and the other Behind, are able to create views of the sun which reveal much more that can be made out by observation from only one place.

ACE is the acronym for Advanced Composition Explorer, another NASA satellite in orbit around the sun in a region along the line from the earth and the sun, where the gravity of one balances the gravity due to the other. One of the services that ACE provides is that it gives radio operators on the earth advance warning of solar storms that may cause disturbance to communications

In early 2010, the satellite STEREO A was positioned so that it was looking at the right limb of sun, while ACE was observing the stream of matter emitted towards the earth. While ACE detected the particle stream coming towards the earth, STEREO A was able to observe the place where the stream came from, and thus the blast waves. Helium-3 ejection from the sun has been observed before and the trace of helium 3 on the earth is believed to have arisen from the sun in this way. But the conjunction of 3H emission with the blast wave had not been well observed before.

It was only in early 2010 that the position of STEREO A was just right. This position, which enables observing both the blast wave and the helium-3 burst, would not happen again till the year 2025, says a news release from the Solar Research Team in Germany. At the present time, STEREO B is out of radio communication and it is only STEREO A that is in action, the news release says.

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