GOES X-Ray Flux Explained

GOES X-ray flux is the amount of X-ray energy arriving from the Sun, measured by instruments on NOAA's GOES weather satellites in geostationary orbit. The number is a power per unit area, in watts per square metre, and the standard chart plots the 0.1 to 0.8 nanometre band on a logarithmic scale.

It updates every minute and it is the fastest solar indicator available, because X-rays travel at the speed of light. A flare on the Sun shows up on the chart about eight minutes later, which is simply how long light takes to cross the distance.

The logarithmic scale is why the chart looks the way it does. Each labelled step is ten times the one below it, so a flat-looking line near the bottom can jump two orders of magnitude and still fit on the same graph.

Flares are classified by peak flux in that 0.1 to 0.8 nm band. Each letter is ten times the previous one:

• A - below 10⁻⁷ W/m². Background. Happens constantly, means nothing.
• B - 10⁻⁷. Still background on an active Sun.
• C - 10⁻⁶. Common. Minor, rarely produces effects at Earth.
• M - 10⁻⁵. Moderate. Can cause brief radio blackouts on the sunlit side, and worth paying attention to.
• X - 10⁻⁴ and above. Major. Radio blackouts, radiation storms, and the class that makes the news.

Within a letter, the number is a linear multiplier: M5 is five times M1, and X2 is twice X1. The X class has no ceiling, so an X20 is possible and has been recorded.

The largest reliably measured flare, in November 2003, saturated the GOES detectors at X17 and was later estimated at around X28. The instrument ran out of scale before the Sun did.

This is the part that trips people up, and it is the main reason to understand the chart at all.

An X-ray flare is light. It arrives in eight minutes, it affects the ionosphere on the daylit side of Earth, and then it is over. It does not produce aurora on its own.

Aurora needs matter: a coronal mass ejection, a cloud of charged particles thrown off the Sun. CMEs often accompany large flares but not always, and they travel far slower, typically taking one to three days to reach us.

So the chain is: flare on the chart, then a check for whether a CME came with it, then whether that CME is aimed at Earth rather than off to one side, then a wait of one to three days, then the solar wind measurements at the L1 point about an hour out, then finally the Kp response. Plenty of X-class flares produce no aurora at all because they fired off the side of the Sun.

Treat the X-ray chart as an early warning that something might be coming, not as tonight's forecast. For tonight, the useful numbers are Bz and solar wind speed, which our tonight page covers, and the Kp index for how far south it reaches.

Watch for M and X class flares from an Earth-facing region. The position matters as much as the size. A flare near the centre of the solar disc as we see it can send a CME straight at us. The same flare on the limb sends it past us.

Then look for a CME. NOAA and NASA publish coronagraph imagery showing whether material actually left the Sun and in which direction. A "halo CME", which appears to expand in a ring around the whole disc, is the signature of something coming towards or away from us.

Then wait, and switch instruments. One to three days later the relevant data is solar wind speed and the Bz component from the DSCOVR and ACE spacecraft at L1. That is roughly an hour of warning, and it is the number that actually predicts whether tonight works.

For most people planning an aurora trip, none of this beats simply going during the equinox months at a high enough latitude. The X-ray chart is for the nights when something unusual is brewing and you want to know before it arrives.