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Space Weather Alert - 30th September 2021

What Has Happened?

Difference image of the Sun's corona showing the CME. The Sun is behind the occulting disc. (SOHO NASA/ESA).


A waning sunspot, AR12781, currently at the southwestern region of the solar disk erupted a long duration C1.6 X-ray solar flare on the 28th September. This sunspot was also responsible for two M-class solar flares five days earlier. Associated with the low-level C-class flare was a coronal mass ejection (CME). This was an asymmetric partial-halo event. Some of the CME is Earth-directed and modelling estimates predict an arrival during the latter half of the 30th September.

In response to the arrival of the CME, geomagnetic activity is expected to become enhanced. Activity up to a STORM G1 or even STORM G2 level is possible on arrival at Earth depending on the configuration of the magnetic field embedded in the CME.

Assuming clear dark skies, and that the geomagnetic field is suitably disturbed, there is an increased chance of seeing the aurora. In the UK, those in Scotland, northern England and Northern Ireland may have the best opportunities.


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The British Geological Survey is part of UK Research and Iinnovation (UKRI) and is one of the Natural Environment Research Council's Research Centres.

CME or Coronal Mass Ejection
The eruption of a portion of the outer atmosphere of the Sun into space, caused by rapid changes in its magnetic field. Often occurs along with a solar flare.

Solar Flare
Energy released by the explosive reorganisation of magnetic fields within the Sun's atmosphere.

High Speed Stream
A fast moving stream of solar wind, responsible for magnetic storms.

Solar Wind
The ever-present expansion of the Sun’s hot outer atmosphere into the solar system, which carries space weather within it.

Sunspot/Active Region
A region of intense magnetic field in the Sun's visible outer atmosphere often associated with flares and CMEs.