A relatively strong, C-class solar flare was observed on Sunday, July 17th by NASA’s Solar Dynamics Observatory. The flare is noteworthy in that the sun has been relatively quiescent, on its way to solar minimum. The image and the accompanying video below are composites, compiled from hard Ultraviolet and X-ray images obtained by the SDO, which has been continuously observing the sun in nine separate wavebands since 2010. Each “waveband” is indicative of a different layer within the sun’s upper convective zone, a layer bounded by the solar photosphere, the outer shell from which light and energy is radiated.
The Sun and all stars are dynamic, self-regulating systems, each powered by a huge nuclear fusion reactor in their cores. These cores of the sun and other stars in a similar evolutionary state currently produce helium and a tremendous amount of energy. From the 15 million-degree core plasma produced by these nuclear fusion reactors flows a stream of protons, electrons and helium nuclei, collectively referred to as the solar wind.
So why do these images and video appear mostly dark? Normally, we’re used to seeing the sun as a brilliant yellow-white disk. That’s because the sun’s “surface”, the photosphere has an effective temperature of 5,780 degrees Kelvin (9,945 F) and any object with that temperature will radiate most of its energy in the yellow-green part of the visible spectrum. This is why our eyes are most sensitive to that color range. As an aside, there is no “surface” to the sun, just the boundary between the photosphere and the chromosphere, followed by the Corona, the sun’s outer, tenuous, million-degree atmosphere. The “dark” areas and regions of the sun in the composites are much hotter than the paltry 5,800 Kelvin temperature of the photosphere. To radiate in the hard Ultraviolet or long X-ray bands of the spectrum, a gas has to be at least 200,000 Kelvin and, in the case of flares or outbursts from more interior regions of the sun, the temperature could approach one million Kelvin. These are the regions we’re seeing; we’re seeing “down into” the sun, especially in the case of a solar flare or “outburst”. The telescopes and cameras of the SDO are fitted with critically calibrated filters which are sensitive to these regions of the spectrum, regions only accessible from space.
Our sun has an 11-year activity cycle whose magnitude is roughly correlated with the average number of sunspots counted during a given period – the higher the number of sunspots, the more internally active the sun is. Currently, solar activity is waning and is at the “halfway” point to the next predicted minimum in late 2019. Considering the sun’s nature as a giant, roiling, super-heated, self-regulating ball of plasma that is modeled as a fluid, we expect certain irregularities in the sun’s behavior and activity, aspects of solar dynamics that give rise to solar storms and outbursts, phenomenae whose frequency of occurrence are directly linked to the solar cycle.
Understanding the sun and its cycle and how it affects our climate and our technology has a direct impact on our lives and how we craft and establish national and international climate policy. It is thus in our best interests to keep a watchful eye on our home star.