Friday, January 28th 2011, was an exciting day for researchers observing the Sun. Within a matter of hours two gargantuan explosions erupted on the surface of our nearest star, spewing over a billion tonnes of plasma into the Solar System. These streams of particles were jettisoned at such angles that their trajectories would put them nowhere near Earth, nonetheless they were spectacular to behold. A movie of the events, below, is courtesy of NASA’s Solar Dynamics Observatory, a satellite sent up on February 11, 2010, with the goal of studying Solar activity and its impact on our pale blue dot.
According to NASA, the upper right explosion was an M-1 class Solar Flare, causing a Coronal Mass Ejection, whereas the lower left explosion was caused by an unstable magnetic filament. M Class Solar Flares are the second largest such phenomena observed, peaking at wavelengths in the X-ray portion of the electromagnetic spectrum. The magnificent CME that followed was captured by NASA’s Solar and Heliospheric Observatory:
So why do these explosions occur and what is the driving force behind them? As it turns out, Solar Flares are closely connected to Sun spots, and the dynamics of the Solar magnetic field. Since the Sun is incredibly hot, the Hydrogen and Helium (and traces of other elements) it is made of are ionized, creating a superheated fluid called a plasma. Plasmas have magnetic fields embedded in them, and the field lines are pulled along by the charged particles in the plasma. Coupled with the fact that the Sun has a differential rotation rate, meaning that the equatorial region spins faster than the Polar regions, we find the mechanism that causes the magnetic field lines to twist together to form compact filaments:
The filaments created by these twisting magnetic field have inside them many field lines close to one another. Magnetic field lines have a pressure that attempts to push them apart, so to counteract this outward push there has to exist a force pushing inward to keep them from dissipating. This force is produced by a density gradient within the filament; the center of the filament has a lower density than the surrounding material. This lower density plasma gives rise to a second effect: the filament rises, like a bubble in a boiling pot of water (the difference being that bubbles are spherical, not cylindrical!). It moves upward towards the surface of the star, and eventually punctures it forming two sun spots.
The locations where the magnetic field punctures the surface of the Sun are also cooler in temperature than the surrounding plasma. Though one would still shine magnificently if we found ourselves on the Sun starring at it, in comparison to the hotter, and hence brighter, surroundings, they give the appearance of being dark blotches to observers from afar.
Once these filaments are exposed to the freedom outside of the Solar interior, they continue to expand outward, dragging with them vast amounts of Solar material. Eventually some of the magnetic field lines begin to reconnect, creating closed loops that then fly freely out into the cold dark depths of space. These loops continue carrying Solar material, and have been given the name Coronal Mass Ejections. The average mass of material that is blown away by these immense events is on the order of a trillion kilograms, which is the same order of magnitude as the mass of a large mountain her on Earth:
If a very large CME is directed towards Earth, then it can reach our planet within about 20 minutes. The charged particles cause the beautiful aurora seen at the Poles, but can also wreak havoc on satellites in orbit and power grids on the ground. Most importantly, they can deposit enough radiation into an astronaut to kill them, so it is very important for researchers on Earth to be on constant lookout for these potentially deadly events so as to warn our spacefarers and give them ample time to hide behind radiation shielding.
The Sun certainly appears like an immortal, unchanging disc in the sky, but upon close observation it is far from. Sol is the closest star to our home planet, and the best way that we can learn about the universe’s luminous inhabitants. It’s interior is filled with twisting magnetic fields, it’s surface punctured by them. It is the breeding ground for enormous explosions, sun spots, flares, and coronal mass ejections. It is a seething ball of plasma; enormous, ever-changing, violent, and beautiful. Remember that the next time you look up at it sitting there in the heavens; take a moment to feel the warm, life-giving light grace your skin, and ponder the wonder of its violent birthplace.