#### Re: What does X mean?

The scale measures power per unit area of solar X-rays, measured by earth satellites. It starts at A1, which is 10^-8 W/m^2, and increases linearly up to A9 (9x10^-8). Then instead of A10 for we have B1 (10^-7), and similarly up to B9, then C1 (10^-6) up to C9, then M1 (10^-5) up to M9, and then X1 (10^-4). There's no scale beyond X, so after X9 we just continue with X10.

An X4.9 flare is large but not exceptional. The largest recorded was at least X28 (during the previous solar maximum, in 2003), but it went off the scale of the recording instruments. The Carrington event of 1859 may have been even bigger. The current solar maximum is a weak one and we haven't had anything bigger than X7.

Look at http://www.swpc.noaa.gov/rt_plots/xray_1m.html for a graph labelled in both A/B/C/M/X scales and plain W/m^2.