On April 30, 2022, NASA’s Solar Dynamics Observatory obtained this image of a solar flare, which can be seen as a brilliant flash in the top right area of the image. credit:NASA/SDO
Solar flares are massive bursts of electromagnetic radiation from the Sun that can last anywhere from a few minutes to many hours in duration. Due to the fact that electromagnetic radiation travels at the speed of light, any influence on the sunlit side of the Earth’s exposed outer atmosphere happens simultaneously with the occurrence of the event itself. As a result of an increase in the amount of X-ray and extreme ultraviolet (EUV) radiation reaching the Earth’s surface, ionisation occurs in the ionosphere’s lowest layers on the Earth’s sunny side. By refraction through the higher layers of the ionosphere, high frequency radio waves (HF radio waves) are capable of supporting communication over vast distances under normal circumstances. It is possible that a powerful solar flare may cause ionisation to occur in the lower, more dense layers of the ionosphere (the D-layer), causing radio waves that contact with electrons in layers to lose energy because more collisions will occur in the higher density environment of the D-layer. This can result in the degradation or full absorption of high-frequency radio waves. As a result, there is a radio blackout — the absence of high-frequency transmission – affecting mostly the 3 to 30 MHz frequency range. The D-RAP (D-Region Absorption Prediction) product is a correlation between flare intensity and D-layer absorption strength and spread. It was developed by NASA.
In most cases, solar flares occur in active zones, which are locations on the Sun characterised by the presence of high magnetic fields and which are frequently connected with sunspot groups. As these magnetic fields grow, they may reach a threshold of instability, at which time they will begin to release energy in a variety of shapes and sizes. Among these is electromagnetic radiation, which is detected in the form of solar flares.
There is a wide variety of solar flare intensities, and they are classed based on their peak emission in the NOAA/GOES/XRS soft x-rays spectral band (0.1–0.8 nm), which is the most visible part of the sun’s spectrum. The X-ray flux levels begin with the “A” level (nominally starting at 10-8 W/m2) and progress through the levels. After that comes the “B” level (10-7 W/m2), which is ten times more powerful; next come “C” flares (10-6 W/m2), “M” flares (10-5 W/m2), and lastly “X” flares (10-4 W/m2), which are ten times more powerful yet.
Radio blackouts are graded on a five-level NOAA Space Weather Scale, with the highest level corresponding to the maximum peak in soft X-rays reached or predicted during the flare. As part of our 3-day forecast and forecast discussion products, SWPC presently forecasts the chance of C, M, and X-class flares and correlates this probability with the probability of R1-R2 and R3 or bigger events in the solar wind. When an M5 (R2) flare occurs, the SWPC will also send an alert.
Further Information: https://www.swpc.noaa.gov/phenomena/solar-flares-radio-blackouts