Every 11 years, the Sun cycles through increasing and decreasing energetic activity. When activity is at a peak, the number of sunspots increases. Major explosions burst from the surface - some as powerful as a billion atomic bombs. If you were to observe the Sun using your sunglasses, though, you wouldn’t notice much of the change, even at the peak of activity. Why? Because these energetic changes aren’t happening within the visible band of the electromagnetic (EM) spectrum.

Our eyes are tuned to a narrow energy band of light, the visible band of the EM spectrum. The entire spectrum ranges from very long-wavelength, low energy light to short-wavelength, high-energy light. The bands of the spectrum, in order of increasing energy, are: radio, microwaves, infrared, visible, ultraviolet, X rays, and gamma rays. Most of the solar activity scientists want to observe isn’t detectable within the visible band of the spectrum. Instead, scientists want to “see” in the ultraviolet band. More specifically, they want to see in the EUV or “extreme ultraviolet” band.

You probably have heard of ultraviolet rays, or UV light, in the context of sunburns. Indeed, regular UV light can be damaging to living tissues. Extreme UV is even more energetic, and, lucky for us, the atmosphere blocks extreme UV rays. For scientists who want to observe changes in the Sun’s energy output, extreme UV is the perfect energy band in which to collect data.

NASA’s “EVE” sensor – short for “EUV Variability Experiment” observes the Sun from on board the Solar Dynamic Observatory satellite. During the Sun’s active phase, extreme UV emissions can change by many orders of magnitudes over just a few minutes, causing “space weather” that even Earth-bound satellites can experience. Extreme UV emissions can also shear atoms apart, creating free ions that can interrupt radio and other signals.

The EVE mission constantly monitors and track the Sun’s extreme UV emissions. Interestingly, it was launched during a time of solar calm, when activity is at a lull. This will provide a useful neutral point of comparison – or baseline - for later in the Sun’s cycle when activity will intensify. By comparing cycle-to-cycle maxima of energetic brightness, scientists have theorized that, overall, the Sun’s energy is dimming and its total irradiance (energy emitted) is slowly decreasing. The EVE project will help confirm or refute this theory by adding much more data to scientists’ view of the Sun.

Print Background Essay