A solar flare is a sudden burst of light and X-rays from the Sun; a coronal mass ejection is a giant cloud of magnetised gas the Sun throws into space.
The Sun's magnetic fields are tangled, especially around sunspot groups. Sometimes twisted fields snap into a less tense shape and release huge energy at once. Two things can come out: a flash of radiation (a flare) and a cloud of material (a coronal mass ejection, or CME). They often happen together, but they reach Earth very differently.
A flare is a large burst of electromagnetic radiation that lasts from minutes to hours. Because it travels at the speed of light, its effects on the sunlit side of Earth begin at the moment the flare is seen — there is no warning. Its X-rays add extra charge to the lowest layer of the ionosphere, which then absorbs high-frequency (3–30 MHz) radio signals. The result is a radio blackout.
Flares are graded by X-ray brightness as A, B, C, M or X. Each letter is ten times stronger than the one before, so an X1 flare is ten times an M1.
A CME can carry billions of tonnes of the Sun's outer atmosphere, together with its own magnetic field. Speeds range from under 250 km/s to nearly 3,000 km/s. The fastest Earth-directed CMEs arrive in about 15–18 hours; slow ones take several days. If the CME's magnetic field points south for a long time when it reaches Earth, the geomagnetic storm is much stronger.
During the week of 8–15 May 2024, sunspot region AR13664 fired several X-class and M-class flares, with CMEs on 8 and 9 May. Aditya-L1's X-ray instruments, SoLEXS and HEL1OS, recorded the flares on 8–9 May. On 10–11 May its in-situ instruments, ASPEX and the magnetometer, detected the CME material sweeping past L1, and a major geomagnetic storm struck Earth on 11 May.
The flash arrived at once; the cloud took about two days. That gap is the difference between light and matter.
Look for the May 2024 entries: Aditya-L1 saw the flares on 8–9 May and felt the CME pass L1 on 10–11 May. Follow Aditya-L1
Travel times follow from distance ÷ speed. NASA's Earth Fact Sheet gives the mean Sun–Earth distance as 149.6 million km.
The flare classes form a logarithmic scale in peak soft X-ray flux (0.1–0.8 nm), measured by NOAA's GOES satellites: A from about 10⁻⁸ W/m², B 10⁻⁷, C 10⁻⁶, M 10⁻⁵, X 10⁻⁴. The number after the letter multiplies it, so M5 = 5 × 10⁻⁵ W/m² and X10 = 10⁻³ W/m². Since each letter is a factor of ten, an X1 flare is 100 times a C1 (two steps) and 1,000 times a B1 (three steps).
NOAA links radio blackout levels to flare classes: R1 ≈ M1, R2 ≈ M5, R3 ≈ X1, R4 ≈ X10, R5 ≈ X20. In an average solar cycle NOAA expects about 2,000 R1 events but fewer than one R5.
In the May 2024 example, flares on 8–9 May and in-situ detection at L1 on 10–11 May imply a CME transit of roughly two days — a speed of about 149,600,000 km / (2 × 86,400 s) ≈ 870 km/s, well inside NOAA's 250–3,000 km/s range.
The flare's X-rays travel at the speed of light, so they reach Earth at the same moment the flare is seen; nothing faster can carry a warning.
One hundred times. Each class letter is ten times the one before, and X is two steps above C (C → M → X).
The in-situ payloads: ASPEX and PAPA, which sample solar-wind particles, and the magnetometer, which measures the magnetic field.