Solar activity

How to read the solar flare chart?

The chart shows changes in the flux of X-ray radiation emitted by the Sun in the 0.1–0.8 nm band. Measurements are made by XRS sensors aboard GOES weather satellites. These data form the basis of the widely used classification of solar flares into classes A, B, C, M, and X.

The line on the chart represents the level of X-ray radiation reaching the vicinity of Earth. During quiet periods, it changes relatively little. When a flare occurs on the Sun, the radiation flux can rise sharply, creating a characteristic hump or sharp peak. The higher the maximum of such an increase, the stronger the flare was.

What do classes A, B, C, M, and X mean?

The chart’s vertical axis uses a logarithmic scale. This means that moving to the next main class corresponds to a tenfold increase in X-ray flux.

The classes correspond to the following levels:

• A - from approximately 10⁻⁸ W/m²

• B - from 10⁻⁷ W/m²

• C - from 10⁻⁶ W/m²

• M - from 10⁻⁵ W/m²

• X - from 10⁻⁴ W/m²

Therefore, an M-class flare is not just slightly stronger than a C-class flare. The beginning of the M range corresponds to a flux ten times greater than the beginning of the C range and one hundred times greater than the beginning of the B range.

A number is also added to the letter to indicate the flare’s more precise strength. For example, C5 means a flux of about 5 × 10⁻⁶ W/m², while M5 means about 5 × 10⁻⁵ W/m². For X-class flares, the scale can continue above X9, so an exceptionally strong flare may be labeled, for example, X10 or X20.

What exactly can be seen on the chart?

It is worth paying attention not only to the chart’s highest point, but also to its shape. The start of a rapid rise may indicate when a flare began, the highest point corresponds to its maximum intensity in the X-ray band, and the subsequent decline shows the phenomenon gradually fading.

However, not every small jump in the line is a separate, significant flare. The Sun emits X-ray radiation continuously, and its background level changes with solar activity. During periods of high activity, the chart may remain at the C-class level for an extended time, and successive flares may overlap with an already elevated background.

GOES measurements cover the entire Sun as seen from the vicinity of Earth. Therefore, the chart itself shows that the amount of X-ray radiation has increased, but it does not directly indicate where the flare originated. Additional observations are needed to identify the active region on the Sun’s surface.

Why are we particularly interested in M- and X-class flares?

A- and B-class flares generally indicate minor activity. C-class events are common during active solar periods and usually have little significance for conditions near Earth.

M-class flares, and especially X-class flares, receive greater attention. A strong increase in X-ray and extreme ultraviolet radiation can quickly affect the upper layers of Earth’s atmosphere. On the sunlit side of Earth, this can cause, among other effects, disruptions to radio-wave propagation.

However, a strong flare alone does not automatically mean that an aurora will appear. Auroras are primarily associated with the later interaction of matter and magnetic fields from the Sun with Earth’s magnetosphere. A flare may accompany a coronal mass ejection (CME), but not every flare produces an ejection directed toward our planet.

Live data and flare list

In SkyLep, the chart is created using minute-by-minute XRS measurements. If an individual measurement is temporarily unavailable from the source, the chart may use the last valid value until new data arrive. For this reason, short, perfectly flat sections of the chart do not always mean that the Sun’s activity actually remained completely unchanged.

Below the chart is a list of detected flares. For each event, it may include the start time, peak time, end time, and flare class, among other details. The availability of this information depends on the data provided by the source. A missing time or class does not mean a zero value; it only means that this information is not present in the currently available data.

This makes the chart a quick overview of the Sun’s current activity. It lets you see at a glance whether our star remains quiet, whether a flare is currently underway, and how its strength compares with activity over the past hours or days.