Meaning of X-Ray Fluxes From the Sun
The solar X-ray flux arises from two factors. Firstly, there is flux coming from sunspot regions and other features - the background flux - and this varies slowly from day to day. Secondly, solar flares produce large amounts of X-ray flux, but this is concentrated to the duration of the flare which is usually from minutes to several hours.
Solar X-ray flux is described as follows:
|Level||Flux (watts/sq meter)||Description|
|A||less than 10-8||Very Low Background|
|A||between 10-8 and 10-7||Low Background|
|B||between 10-7 and 10-6||Moderate Background|
|C||between 10-6 and 10-5||High Background/Low Flare|
|M||between 10-5 and 10-4||Moderate Flare|
|X||between 10-4 and 10-3||High Flare|
|Y||greater than 10-3||Extreme Flare|
Within these levels, a number is used to specify the flux. Hence a value M3.2 indicates that the flux is 3.2 × 10 -5 watts/metre2.
The Y classification of flares is new; and these extremely large flares are often still classed as X flares with a qualifying number greater than 10. Hence a Y1.6 flare is exactly the same as an X16 one.
Material prepared by Richard Thompson