“Stealth storms are back”: the Sun just hit the Earth by surprise

On November 20, the Earth suffered a solar storm whose arrival no one had anticipated. These phenomena, described as “stealthy” by physicists specializing in space weather, occur without the usual light signatures that normally allow them to be identified.

When the Sun strikes on the sly

Unlike classic coronal mass ejections, which manifest themselves with blazing eruptions and cloud structures visible in coronagraphs, these furtive storms emerge without any particular flare. They progress slowly, remain tenuous and are only revealed through their impact on the terrestrial solar wind. That’s precisely what happened last Wednesday, when NOAA forecasters noted that a “integrated transient” disrupted solar wind conditions.

The magnetic field carried by this stream of particles, usually between four and six nanoteslas (nT), briefly climbed to 18 nT, while the velocity of the solar wind oscillated between 400 and 500 km/s. “Stealthy solar storms are backcommented Tamitha Skov, specialist physicist. They are stealthy because they have no signature in coronagraphic imaging. We only detect them when they arrive on Earth.”

These northern lights observed at night, from Maine to Denmarkwould probably result from this ejection combined with the effects of a rapid flow emanating from a coronal hole. The slightly amplified geomagnetic activity allowed the auroras to extend beyond their usual polar perimeter.

A study published in 2021 highlights that these furtive eruptions “pose many problems for space weather forecasting”their detection requiring multi-wavelength and multi-angle observations. These phenomena, originating from quiet solar regions with weak magnetic fields, paradoxically become more frequent during the waning phase of the one-decadal solar cycle.