The fourth major solar flare in less than a week erupted Friday, triggered by the largest sunspot scientists have seen in 20 years.

Peaking at 5:41 EDT, the solar flare burst from the enormous sunspot known as AR 12192. The spot has been active in the past week, erupting three previous times within the past few days. The flare was classified as an X3.1-class solar storm, which researchers consider the most powerful type on the Sun.

According to NASA spokesperson Karen Fox, “This is the fourth substantial flare from this active region since Oct. 19.”

The solar flare was powerful enough to cause a widespread radio blackout. While the radiation from flares cannot affect humans directly, they can interfere with communications and GPS signals in the atmosphere.

Solar storms are classified according to strength, with X-class flares being the most potent. M-class flares are a step below, and are usually strong enough to cause a vivid aurora display at night. Then there are weaker A, B, and C-class storms.

AR 12192 has fired off four major flares this week, ranked at X1.1, M8.7, X1.6, and X3.1. It’s the first major episode of solar flares since 2003, when a series erupted over Halloween.

The sunspot is roughly the size of Jupiter, measuring 2700 microhemispheres (MH) across. In contrast, the Earth measures 169 MH.

In his blog, astrophysicist C. Alex Young of the NASA Goddard Space Flight Center wrote that “AR 12192 is the largest in almost 24 years. It is the largest sunspot since AR6368, which measured 3080 MH on November 18, 1990.”