The best of... 2021!

A compilation of the most memorable space weather moments of 2021 can be found underneath. Using the versatile Space Weather JHelioviewer (SWHV) software, a ***MOVIE*** was created (available at YouTube and also at the STCE (wmv - 617MB)) containing one or more clips of each event. Usually, SDO-images were used, occasionally supplemented with imagery from PROBA2, SOHO, STEREO, and SolO.

As this chronological list concerns mostly punctual events on the Sun, it does not contain clips from other noteworthy events in 2021, such as e.g. the solar eclipses on 10 June and 4 December, the introduction of the K_BEL index, the space junk crisis with Cosmos 1408 on 15 November (in Addendum) , or Solar Orbiter's Earth flyby on 27 November.

Happy viewing!


Event 1: 7 May - M3.9 flare in NOAA 2822

NOAA 2822 was a compact heap of sunspots that had just rounded the Sun's east limb when it produced an M3.9 flare on 7 May. EUV (extreme ultraviolet) imagery showed that the flare was accompanied by coronal dimming (a temporary darkening in the solar corona, also -confusingly- known as "transient coronal hole") and an obvious coronal wave (see this STCE news item for more info). Together with the observed radio bursts, these were all signs that a coronal mass ejection (CME) was associated with the flaring event. Further analysis indicated that this CME had no earth-directed component.

Ref: STCE news item of 10 May 2021

Event 2: 29 May - First proton event of the new SC25

The first proton event of solar cycle 25 (SC25) took place on 29 May following a long duration C9.4 flare that peaked at 23:13UT on 28 May. The proton flux reached the 10 pfu alert threshold on 29 May at 03:00UT, peaked at 03:20UT (15 pfu, with 1 pfu = 1 proton / cm2 s sr), and ended at 05:40UT. This was a weak event, reaching only S1 on the NOAA scale, i.e. a minor solar radiation storm. The effects were barely noticeable in e.g. coronagraphic imagery. Also, the fluxes of protons with higher energies (> 50 MeV , > 100 MeV) remained at their background values. A shock in the solar wind from the associated CME was observed on 2 June at 12:20UT. The speed jumped from 289 km/s to 325 km/s and the interplanetary magnetic field magnitude from 3 nT to 6 nT. Active geomagnetic conditions were observed for a short period (max K-Dourbes = 4, max NOAA-Kp = 3).

Ref: STCE news item of 31 May 2021

Event 3: 3 July - First X-class flare of the new SC25

A small but quickly developing region near the Sun's northwest limb produced an X1.5 flare on 3 July at 14:29UT. This was the first X-class flare of solar cycle 25 and the first one since the X8 event on 10 September 2017 (see e.g. this STCE news item), thus ending a stretch of 1391 days without X-class events. Aside the X-class flare, NOAA 2838 managed to produce also 5 C- and 3 M-class flares in just 2 days. All the related CMEs were directed away from the Earth.  

Ref: STCE news item of 3 July 2021

Event 4: 15 July - Spectacular farside eruption

Starting around 22UT on 15 July, SOHO/LASCO C3 coronagraphic imagery showed a full halo CME emanating from the Sun. With the earth-facing side of the Sun nearly spotless, it quickly became clear the eruption must have originated from the farside. At the time of the eruption, STEREO-A was trailing the Earth by 46 degrees and was able to provide a side-view of this spectacular CME, as the source of the CME was behind the Sun's east limb even for STEREO-A. Due to the location of the blast site, there was only a poor magnetic connection with the Earth. Nonetheless, the eruption was powerful enough to enhance particle flux levels recorded by satellites at the L1 point (SOHO, ACE) near Earth.

Ref: STCE news item of 20 July 2021

Event 5: 9 October - M1.6 flare in NOAA 2882

NOAA 2882 rounded the northeast solar limb on 3 October as a large but symmetric sunspot group. It was flare quiet, until a small bipolar region emerged nearby and to the north of the main spot on 7 October. The magnetic interaction increased flaring activity and resulted in an M1.6 flare on 9 October while NOAA 2882 was close to the central meridian. A cusp-structure became visible in high-temperature EUV filters, as well as some coronal dimming and a coronal wave. The associated full halo CME had a true speed of around 950 km/s and arrived near Earth early on 12 October. A moderate geomagnetic storm was recorded.

Ref: STCE news items of 13 October 2021

Event 6: 28 October - Second X-class flare of the year

NOAA 2887 was all-in-all not an impressive sunspot region, but produced a number of low-level M-class flares as well as an X1 flare on 28 October. This eruption was associated with a minor proton event, but also the greater than 100 MeV reached its alert threshold (1 pfu). As a result, a small Ground Level Enhancement (GLE) was recorded by ground-based neutron monitors. GLE73 was the first GLE since September 2017. The bulk of the CME was directed south of the Earth, and only a minor geomagnetic storm was recorded on 31 October.

Ref: STCE news item of 28 October 2021

Event 7: 4 November - Cannibal CME

Following eruptive activity in NOAA 2887 on 1 November and in NOAA 2891 early on 2 November, the fast earth-directed CME associated with the latter caught up with the much slower CMEs related to the NOAA 2887 activity. The resulting type of CME is called a "cannibal CME" as it overtakes the slower CMEs in front of it, thus creating more complex and enhanced magnetic fields which may result in stronger geomagnetic storms than one would expect. The interplanetary CME (ICME) arrived at 19:24UT on 3 November when a shock was observed in the solar wind. Solar wind speed went up well over 700 km/s with some periods of significant and sustained southward magnetic field orientation, Bz getting as low as -18 nT. This resulted in the strongest geomagnetic storm of 2021 (Kp=8-) with the Dst index reaching -105 nT and as such descending for the first time this solar cycle under -100 nT (Kyoto World Data Center). Aurora were photographed as far south as California and New Mexico. The magnetic shielding provided by this strong ICME resulted in a significant decrease in the harmful cosmic rays, as recorded by neutron monitors around the world (a so-called "Forbush decrease"). The neutron monitor in Oulu, Finland showed a brief but sharp decrease of 11% in the 1-hour count rates compared to undisturbed levels.

Ref: STCE news item of 17 November 2021

Event 8: 9 November - Long duration M2 flare and SADs

While it was already well behind the northwest solar limb, NOAA 2891 produced a long duration M2 flare which was associated with a fast non earth-directed CME (900 km/s). EUV imagery of the flaring event (SDO/AIA 094) reveals the presence of so-called supra-arcade downflows (SADs) which were for the first time observed on 20 January 1999 by the Soft X-ray Telescope (SXT) on Yohkoh. They are dark voids that usually flow downwards above post-flare loops during the decay phase of the flare. Hence their name: supra ("above") arcade (= series of post-flare loops) downflows. Because of their appearance, they are often also called "tadpoles".

Ref: STCE news item of 30 November 2021


Event 9: 9 November - High sunspot numbers

After a slow start with even a few spotless days during the first half of December, a number of sunspot regions appeared and drove the daily international sunspot number (ISN) well above 100 for most of the remainder of the month. The highest ISN was recorded on 22 December with a preliminary value of 152. A string of 3 sunspot groups in the southern hemisphere (NOAA 2907-09) was followed by 2 medium-sized regions, one on each hemisphere (NOAA 2918 north and NOAA 2916 south). Those groups were also responsible for most of the 8 low-level M-class flares that were observed. The elevated values of the last few months in 2021 may indicate a somewhat higher than predicted maximum for solar cycle 25 (SC25 - See the STCE's Tracking SC25 webpage).

Ref: STCE news items of 30 November 2021 and of 3 January 2022

Event 10: 24-25 December - Solar Orbiter images spectacular eruption

A spectacular eruption was observed by the STEREO-A (ST-A) and Solar Orbiter (SolO) spacecraft on 24 and 25 December. From the Earth's perspective, the eruption took place on the Sun's farside. SolO's Full Sun Imager (FSI) has a much wider field-of-view than STEREO-A's EUV Imager (EUVI) at the average Sun-Earth distance, and was able to follow the ejected material for much longer and much further away from the solar limb than ST-A could. The eruption was still visible in FSI when it became visible in SOHO's white light coronagraphs.

Ref: STCE news item of 25 January 2022




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