Solar Storm

 

This is a NASA image of a huge solar prominence on September 29, 2008, obtained as a part of the STEREO Project. The data was taken at a wavelength of 304 nm in the ultraviolet. The event rose over a period of several hours like a flag unfurling and then broke apart and headed into space. The material involved is ionized helium at about 60,000 Kelvin. The size of this prominence is many times the size of the Earth, as can be seen in a relative scale image of the Earth and Sun.

While sunspots are relatively cool and quiescent regions on the Sun, the photosphere around them sometimes erupts with outflows of high energy particles in active regions. Most often these eruptions are in the form of loops and sheets called prominences which remain under the control of the intense magnetic fields associated with solar storms. There are other events which in a matter of minutes can release enormous amounts of energy and eject material out into space. Such violent events are called solar flares.

Solar prominences are typically observed to be high loops of hot ionized gases which follow the geometry of the magnetic fields associated with pairs of sunspots. According to Chaisson & McMillan, the size of a typical solar prominence is on the order of 100,000 km or around 10 times the diameter of Earth. Larger ones can reach a half-million kilometers. Prominences can show surges in time scales of hours, but the quiescent ones can persist for days or weeks. The energy release of the largest prominences can be on the order of 1025 joules, compared to a total energy release of the Sun on the order of 1026 joules/sec.

Solar Flares

The most violent events on the surface of the Sun are sudden eruptions called solar flares. Flares typically last a few minutes and can release energies equivalent to millions of hydrogen bombs. Flares become frequent near sunspot maximum, when smaller flares can occur daily and large flares can occur about once a week. The adjacent image shows shows a flare eruption in H-alpha light at 15:00 UT on July 14th, 1996, recorded at the Big Bear Solar Observatory (Ref).

Coronal Mass Ejections

During a flare the material in the flare may be heated to temperatures of 10 million K; matter at these temperatures emits copious amounts of UV and X-Ray, as well as visible light. In addition, flares tend to eject matter, primarily in the form or protons and electrons, into space at velocities that can approach 1000 km/second. These events are called coronal mass ejections, and produce bursts in the solar windthat influence much of the rest of the Solar System, including the Earth (However, there is controversy within the astrophysics community about whether coronal mass ejections and flares should be classified together; see this discussion). Thus, the observation of a large flare on the surface of the Sun is usually a signal for increased aurorasand related activity several days hence when the ejected burst reaches Earth.

Here is a spectacular MPEG movie(489 kB) of a coronal ejection event captured by the LASCO instrument on SOHO (Ref). In the movie a comet first appears to encounter the Sun and the coronal mass ejection then occurs almost as if in response to the comet. However, this is just a coincidence; the two events are unrelated.

The Cause of Solar Flares

Although the cause of flares is not completely understood, they are known to be associated with the magnetic field of the Sun. One favored explanation is that they occur when magnetic fields in the Sun pointing in opposite directions interact strongly with each other. Such a situation can be brought about by the churning motion of solar material near the surface, and is more likely during periods of the active sun. Thus, there typically is a correlation between the frequency of flares and the number of sunspots.