Tag Archives: solar system

How the Sun lost its spots

It may look like a static yellow ball from here, but in reality the Sun is alive with activity. Right now it is becoming more active each day as we get closer to the next solar maximum, which is expected to peak in July 2013. However, a couple of years ago it was quieter than it had been for nearly a century. It had very few sunspots and radiated very little energy. This variation is normal — the Sun goes through regular cycles where its activity and number of sunspots go up and then down again. What was unusual was the depth of this solar minimum.

Dibyendu Nandy, from the Indian Institute of Science Education and Research in West Bengal, and colleagues Andres Munoz-Jaramillo and Petrus Martens, from Montana State University, think they might have found the reason for this almost unprecedented solar calm.

An image of the Sun taken in September 2008 — not a single sunspot in sight. Credit: SOHO/ESA/NASA

Each solar cycle lasts roughly 11 years. After this time, its magnetic field flips over. After two cycles the magnetic field has flipped twice and it ends up back where it started. During these cycles the amount of solar activity goes up and down too.

Sunspots are a good measure of the amount of activity going on in the Sun at any point, and the number of sunspots on the Sun follow the 11 year solar cycles; there are more sunspots at a solar maximum and less at a minimum. A sunspot’s magnetic field is very strong and stops the transfer of heat from the interior of the Sun to the surface. Sunspots look dark because this loss of heat makes them cooler than their surroundings. In fact the surrounding area is brighter than it would be without the sunspot. This means that, counterintuitively, the more sunspots there are on the Sun, the more energy radiates out of it — even though it looks darker than usual.

Spotless days in red, number of sunspots in blue. Only cycle 14 had a deeper minimum than the last one (cycle 23). Credit: Nandy et al, Nature, 3rd March 2011

The last solar minimum was unusual because there were a very high number of days — about 800 — without any sunspots at all. Nandy and colleagues created a computer model to try to work out why this happened.

They found that great loops of electrical current, which flow in the plasma that makes up the Sun, were interfering with the formation of new sunspots. In a plasma, the electrons have been stripped away from their atoms, leaving them free to move about and conduct such currents. The currents flow around the surface of the Sun, going down into the interior at the poles and resurfacing at the equator. Dying sunspots get dragged underneath the surface, where their magnetic field is given a boost. They are then sent back up to the top to form a new sunspot.

Close up picture of a sunspot taken in ultraviolet light by NASA's TRACE spacecraft. Credit: NASA

During a deep solar minimum, however, it doesn’t quite happen like this. In the first half of the solar cycle the plasma flows quickly, but in the second half it slows down. This fast movement at the start stops strong magnetic fields forming inside the Sun, so that it eventually runs out of steam and stops making sunspots during that cycle. The slow plasma flow afterwards means that the formation of the next lot of sunspots takes a bit longer to get going that usual.

This all adds up to long stretches of time without a single spot on the surface of the Sun.

The team’s simulation, which modelled this physics, reproduced what we saw during the last solar minimum, showing that very deep solar minima are generally linked to the Sun’s weakened magnetic field.

Being able to predict when solar minima like this are going to occur is a very useful thing. When the Sun’s magnetic field is weakened, so is the solar wind. The solar wind is a stream of charged particles that are ejected from the Sun’s atmosphere and into space, and is responsible for aurorae, geomagnetic storms and the tails of comets, amongst other things. It also stops lots of cosmic rays getting into the solar system. When the Sun’s magnetic field is weakened, the solar wind lets more cosmic rays through, making space a more dangerous place. This new model will hopefully mean we can predict hazardous changes in space weather and plan missions accordingly.

Reference
Nandy D, Muñoz-Jaramillo A, & Martens PC (2011). The unusual minimum of sunspot cycle 23 caused by meridional plasma flow variations. Nature, 471 (7336), 80-2 PMID: 21368827

Links

3D Sun iPhone app for photos and videos of the latest solar activity — very cool (hat tip to @Psycasm for telling me about this)

Sunspot plotter — find the number of sunspots on any day back to Jan 1st 1755

NASA animations of plasma flows and the sunspots they create

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Saturn’s rings get spontaneously shaken up

From far away Saturn’s rings look pretty solid – I’m sure I’m not the only person who, as a child, imagined it’d be possible to skate around the planet on them. In reality, though, they’re made up of millions and millions of bits of ice and dust, ranging in size from micrometres to metres. Until recently, scientists thought that the occasionally odd behaviour of the most massive ring, known as the B ring, was solely due to the pull of one of Saturn’s moons, Mimas. However, new research published in the December issue of the Astronomical Journal explains that Mimas is not the only reason for the variations that we see in this ring…

Saturn as seen by the Cassini Orbiter. Image: NASA

Joseph Spitale and Carolyn Porco from the Space Science Institute at Boulder, Colorado looked at four years worth of images of Saturn’s rings from the Cassini mission. They saw evidence of wave patterns in the B ring that seemed to have arisen spontaneously – without being forced by Mimas. The waves are thought to come about because of the high density of the B ring, and are given a boost by its sharp edge which reflects and amplifies the waves. Spitale and Porco also found small moons, known as “moonlets”, near the outer edge of the B ring.

Cassini image of Saturn's B ring, taken in 2009. Image: NASA

The small chunks of ice and dust that make up Saturn’s rings may be left over from the formation of the planet itself, or could be all that is left of a moon that strayed too close to its parent and got broken up by Saturn’s gravity.* Either way, these new findings show that the rings are anything but the static bands of ice we sometimes imagine them to be, and that their motion doesn’t even always come from outside influences.

But these findings don’t just tell us about the behaviour of Saturn’s rings. They also offer insight into other systems in the universe that may have similar oscillations, such as spiral galaxies and protoplanetary disks. This is an example of one of the amazing things about physics. By observing something close to us, we can learn about the behaviour of systems on the other side of the universe.

*There’s something known as the Roche limit that dictates how close a moon can get to its planet before it’s broken up by tidal forces caused by the planet itself.

References:
Joseph N. Spitale, & Carolyn C. Porco (2010). Free Unstable Modes and Massive Bodies in Saturn’s Outer B Ring Astron.J.140:1747-1757,2010 arXiv: 0912.3489v2

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Solar system might be older than we thought…

Researchers from Arizona State University have found the oldest solar system object ever discovered. In fact, it’s so old that it formed up to two million years before the solar system did, according to current estimates. It might be time for a rethink of when and how our little place in the Universe came into existence…

Planets and dwarf planets in the solar system. Sizes to scale, distances (obviously) not. Image: NASA

Coming up with a successful model for the formation of the solar system is not an easy task. Such a model must explain everything we know about the solar system today, from the fact that all the planets revolve the same way around the Sun and in the same plane, to the composition of the planets themselves.

The most generally accepted model is the Solar Nebula Disk Model (SNDM), which is a modern variant of the Nebular Hypothesis originally put forward by Laplace and Swedenborg in the 16th Century. In the SNDM, stars form in huge rotating clouds of molecular hydrogen. Our own Sun started out its life as a proto-star in one of these clouds, and formed when a small part of the cloud underwent gravitational collapse. Most of the collapsing mass went into the formation of the proto-Sun, with the rest making the protoplanetary disk that surrounded it. Next came planetesimals, which are believed to be the starting point of planets. It is thought that they grow when bits of material in the disk stick together after collisions, and once they reach a certain size, around a kilometer across, gravity takes over and they attract more and more mass. Not all planetesimals become fully-fledged planets; only the largest are able to survive long enough to make it.

When our solar system was evolving, the planetesimals that didn’t get swept up to form planets likely became asteroids instead. It is these asteroids that large meteorites found on earth are believed to originate from. Because they were created at the birth of the solar system, meteorites can give us some clues about its formation and age.

Artists impression of a protoplanetary disk. Image: NASA

Audrey Bouvier and colleague Meenakshi Wadhwa looked at something known as the calcium-aluminium rich inclusions (CAIs) in a meteorite found in the Sahara desert. The CAIs range in size from a few centimeters down to sub-millimeter lengths, and are believed to have formed in the protoplanetary disk as the solar system was beginning to take shape.

Several different radioactive decays can be investigated to determine how old a piece of rock is. The half-life of the each decay is the key to finding out the rock’s age. Researchers can look at how much of an isotope is present in the sample and compare it with how much there is of whatever it decays into, and then use the decay’s half-life to find the age of the sample. By looking at several different decays and combining the age estimates found for each it’s possible to get an even more precise estimate.

Bouvier and Wadhwa did this for the CAIs in their meteorite and found that it was 4,568.2 million years old. That’s between 0.3 and 1.9 million years older than previous research suggests the solar system is.

During their research, Bouvier and Wadhwa also learnt about how the solar system started. By comparing the time of formation of the CAIs with the time of formation of small, round grains of rock known as chondrules, they were able to determine the concentration of an isotope of iron, Fe-60, at the beginning of the solar system. Pushing back the formation of the solar system means that the concentration of Fe-60 at its beginning was twice as much as estimated using previous knowledge. Fe-60 is only made in the end stages of a star’s life, and is then scattered into space when the star dies. The high concentration found makes it very likely that the source of the Fe-60 was the death of a nearby star: our solar system evolved out of the remnants of a supernova.

Reference:
Audrey Bouvier, & Meenakshi Wadhwa (2010). The age of the Solar System redefined by the oldest Pb–Pb age of a meteoritic inclusion Nature Geoscience : 10.1038/ngeo941

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