SIDC Sunspot group 940 (NOAA 4507) was a rather large region that produced C-class flares while transiting the solar disk. On 21 August, while rounding the west limb, it produced an M1.6 flare peaking at 10:17 UTC. The flare lasted from 09:09 UTC until 11:49 UTC, making it a long-duration event. The SDO imagery underneath show extreme ultraviolet (EUV) images at two different temperatures laid over each other. Green is SDO/AIA 171 at temperatures around 700.000 degrees, and pink are SDO/AIA 131 images showing the Sun at temperatures of several million degrees. Note the images have been rotated by 90°, with west on top and north to the left.
Following the eruption, an arcade (series of coronal loops) started to develop, topped by supra-arcade downflows, i.e. the "tadpoles" flowing downwards towards the top of the coronal loops. More info on these features as well as a sketch can be found in this STCE newsitem. In this case, the coronal loops would eventually reach a height of about 200.000 km, slightly more than half the Earth-Moon distance. Particularly high coronal loops were observed during an eruption on 15-16 October 2014 (see this STCE newsitem), where the top of the loops was well outside SDO/AIA's field-of-view.
The eruption was also associated with some radio emission, in particular at very high frequencies (30-300 MHz). The SPADE instrument (Small Phased Array DEmonstrator) at the Humain Radioastronomy Station (HuRaS) was able to track the disturbance. In the plot underneath, the frequency is on the vertical axis (range between 25 and 75 MHz, with the lowest frequency on top), while time is on the horizontal axis (from 09:15 UTC until 10:00 UTC). Interesting is that all of the radio emission took place prior to the flare's maximum intensity in soft x-rays.
Associated with the M1.6 flare was a fast (1200 km/s) partial halo Coronal Mass Ejection (SIDC CME 713) observed in SOHO's LASCO/C2 coronagraph imagery starting from around 09:36 UTC on 21 August, directed mainly to the west and to the northwest. The SIDC space weather forecaster expected that the bulk of the CME was going to miss Earth, but a glancing blow arrival around 24 August could not be excluded. Over the next few days however, the solar wind parameters did not show any obvious signature of a passing CME. The image underneath shows GOES/SUVI 131 (EUV; green) solar imagery overlaid onto GOES/CCOR1 coronagraphic imagery (white light; bluish). Here, north is on top and west is to the right. The little white dot on the Sun's west limb ("right") is the actual eruption which the CME was associated with.
