X-ray Log Letters

Nugget
Number:	526
1st Author:	Hugh HUDSON and
2nd Author:	Ed CLIVER
Published:	May 18, 2026
Next Nugget:	TBD
Previous Nugget:	How Extreme Can Solar Flares Get? A Statistical View

Introduction

As all students of solar activity know, the naming scheme for solar flares involves the letters ABCMX. These date from the dawn of the space age, when the first SOLRAD satellites began to monitor solar X-ray emission. Flares of a wide range of brightnesses were immediately apparent, and NOAA ionospheric physicist Don Baker (Ref. [1]) came up with a classification that matched decades of peak flux to letters: Cnn would mean nn x 10^-4, and the basic flux level 1.0 x 10^-6 W/m² would be class C1.0. Initially it was just CMX ("Common", "Medium", "Extreme") Each flare has a peak flux in the standard 1-8 Å band, and the range of magnitudes of flare peak fluxes is so great that it's most convenient to record the logarithm of the flux. This convention moved directly into the GOES satellite era, and from 1974 the standardized flux measurements have come from this series of spacecraft (Ref. [2]).

In standard units a C-class flare has a peak flux greater than 10^-7 W/m², with M-class and X-class greater by powers of 10. Conventionally a flare 10% brighter than the threshold, with peak flux 1.1 x 10^-7 W/m², would then become a C1.1 flare. The letter thus represents the truncated logarithm of the peak flux. The magnitudes are not rounded off and represent the solar X-ray flux conventionally in a 1-min integration interval. A background quiescent solar emission level is not subtracted; these routine observations show the total solar X-ray flux in "Sun-as-a-star" mode. Weaker events near the background level, which can indeed exceed C1 during active times, will thus not be quite as energetic as their name implies.

Low background solar X-ray fluxes during solar minima allowed the GOES detector technology to detect fainter events, and so two new decades were added to the letter string: A and B, with no particular descriptive interpretation. Thus we arrived at the current letter string ABCMX.

The SphinX breakthrough (to the bottom)

The first routine flare monitoring with sufficient sensitivity to detect the solar quiescent X-ray flux appeared with the SphinX instrument from the Sylwester group at Wrocław (Ref. [3]). For 1-8 Å this turned out to be about two decades below the A level. Hence, two more letters: Q "quiet" and S "small", leading to the more complete string QSABCMX. Figure 1 shows SphinX data from the 2009 solar minimum.

Figure 1: SphinX observations from solar minimum 2009. The regions marked with blue show the detection of the corona, at levels far below the original CMX range (Ref. [3]). Excesses above this level are due to weak flares and quiescent active regions. Translated to the GOES calibration, the weakest events are measured in units of 10^-10 W/m².

The greatest events (to the top)

At the top end of the GOES soft X-ray distribution, a handful of the most extreme events had saturated the GOES 1-8 Å readout. Ref. [4] describes an evenhanded patching of these items, together with a correction for a systematic offset (Ref. [2]). This produced a list of 37 >X10 events after background subtraction. This basic manipulation departs from tradition, and so we provide this [greatest flares] catalog using the letter system "QSabcmxyz". To translate, the new "y" class is the old "X10" class, sometimes now called "S" (Ref. 5]), and the new catalog lists 37 of them. The new "z" class has no examples yet (but we're hopeful), although the Carrington event might have been in that category.

References

[1] "Sentinels of the sun: forecasting space weather," B. Poppe and K. Jorden, 2006

[2] "GOES-R Series Solar X-ray and Ultraviolet Irradiance"

[3] "Soft X-ray variability over the present minimum of solar activity as observed by SphinX"

[4] "The Greatest GOES Soft X-ray Flares"

[5] "The Occurrence of Powerful Flares Stronger than X10 Class in Solar Cycles"