Northern lights in Czechia

The northern lights are light produced 80 to 500 kilometres above Earth1. Charged particles — the solar wind — flow continuously from the Sun. When this flow strengthens, it disturbs Earth's magnetic field, and accelerated electrons race along it into the upper atmosphere near the poles. There they collide with oxygen and nitrogen atoms, making them glow1: oxygen glows green and, at greater heights, red; nitrogen adds the purple lower edge2⁠.

This is why the aurora mainly occurs in a band around the poles. It is common over Norway, Iceland and Canada, but rare here.

The northern lights (aurora borealis) are upper-atmosphere emissions at heights of approximately 80–500 km1. They are excited mainly by electrons accelerated in the magnetosphere, which precipitate along magnetic field lines into the auroral oval and collide with oxygen and nitrogen atoms and molecules1⁠.

The green atomic oxygen line at 557.7 nm (¹S state) typically glows at 120–400 km. The red 630.0 nm line (¹D state) is restricted to heights above 300 km: the excited state lasts over 150 seconds, and collisions in denser air quench it before it can emit light210. Nitrogen produces the purple lower edge at 120–200 km2⁠.

The aurora glows in an oval around the magnetic pole. Czechia lies far to its south, roughly between 48.5° and 51° latitude. The oval expands towards us only during a strong geomagnetic storm: according to NOAA, during a G3 storm (Kp 7) the aurora typically appears around 50° geomagnetic latitude, and during G4 (Kp 8) around 45°3⁠.

According to long-term observations by astro.cz, a camera in Czechia can capture it low above the horizon from Kp 4–6; it tends to be visible by eye from Kp 7, and from brightly lit Prague only from 85. According to NOAA, G3 storms occur on roughly 130 days per eleven-year solar cycle3 — and they still need to coincide with a clear, dark night.

The oval's position follows geomagnetic latitude, rather than geographic latitude. As activity increases, the oval expands towards the equator: NOAA gives a typical visibility limit of 55° geomagnetic latitude for G2, 50° for G3, 45° for G4 and 40° for G53. Czechia (48.5–51° N) is therefore near the boundary, where visibility depends on whether the aurora is still above the horizon — from lower latitudes we see only the upper, red part of the distant oval (see the box below).

Empirical thresholds for Czechia according to astro.cz: photography at Kp 4–6, by eye from 7, from Prague from 85; the page conservatively uses 5 / 7 / 8. Frequency per cycle according to NOAA (≈ 11 years): G3 ≈ 130 days, G4 ≈ 60 days, G5 ≈ 4 days3⁠.

Kp is a number from 0 to 9 that describes how much Earth's magnetic field fluctuates. It is calculated from measurements at 13 observatories worldwide, for each three-hour period4. From 5 upwards, NOAA classifies it as a geomagnetic storm, from G1 (minor) to G5 (extreme)3⁠.

Above, we show two numbers: the highest value for tonight and the last three-hour period (“now”). Kp does not say exactly where the aurora will be — it is one number for the whole planet. We therefore add darkness and cloud cover by region. The NOAA outlook for later nights can change within hours.

Julius Bartels introduced the planetary Kp index in 1949; its record extends back to 1932. Today, GFZ Potsdam calculates it from 13 observatories as a three-hour value in thirds: 0o, 0+, 1−, … 9o4⁠.

The page uses the NOAA SWPC Planetary K-index Forecast product, which combines observed, estimated and forecast three-hour values; each number is labelled with its type. Page thresholds: photo ≥ 5.0, good photo ≥ 7.0, by eye ≥ 8.0. We do not yet display solar wind data from L1 (speed, density and the Bz component).