:Issued: 2026 May 20 0800 UTC :Product: documentation at http://www.sidc.be/products/bul #--------------------------------------------------------------------# # SIDC Weekly bulletin on Solar and Geomagnetic activity # #--------------------------------------------------------------------# WEEK 1324 from 2026 May 11 Solar Active Regions (ARs) and flares --------------- Solar flaring activity was at low to moderate levels throughout the week, with forty-three C-class flares and five M-class flares observed. The strongest flare was an M5.7 flare (SIDC Flare 7671), peaking at 13:39 UTC on May 10, which was associated with the returning SIDC Sunspot Group 825 (NOAA Active Region 4436, magnetic type beta). There was a total of twelve active regions on the visible disk during the week. SIDC Sunspot Group 860 (NOAA Active Region 4432) was the most complex active region early in the week, with magnetic type beta-gamma, before rotating across the west limb. The remaining active regions were classified as magnetic type beta or alpha. No X-class flares were recorded during the week. Coronal mass ejections --------------------- Several Coronal Mass Ejections (CMEs) were observed during the week. A partial halo Coronal Mass Ejection (SIDC CME 650) was observed in LASCO/C2 and C3 coronagraph imagery starting from around 13:48 UTC on May 10. The CME was associated with the M5.7 flare from the returning SIDC Sunspot Group 825 (NOAA Active Region 4436). While the bulk of the ejecta appeared to be directed eastward, analysis indicated a possible glancing blow arrival at Earth around May 13. A wide and fast Coronal Mass Ejection (SIDC CME 656) was observed in LASCO/C2 and C3 coronagraph imagery starting from around 16:36 UTC on May 16, directed mainly northwestward. The CME was associated with an M1.9 flare from SIDC Sunspot Group 825 (NOAA Active Region 4436). Although the bulk of the ejecta appeared to be directed mainly northwestward, Although the bulk of the ejecta appeared to be directed mainly northwestward, analysis indicated a glancing blow arrival at Earth around May 19. The other observed CMEs were mostly events near the west limb, or backsided events, and were not expected to impact Earth. Coronal Holes --------------------- A small, positive-polarity coronal hole (SIDC Coronal Hole 159) crossed the central meridian early in the week. A high-speed stream may have arrived at Earth around May 13, possibly combined with the glancing blow arrival of SIDC CME 650. A large, negative-polarity transequatorial coronal hole (returning SIDC Coronal Hole 147) crossed the central meridian around May 13. The associated high-speed stream arrived at Earth late on May 15 and was responsible for enhanced solar wind conditions and increased geomagnetic activity during the second half of the week. Proton flux levels --------------------- The greater than 10 MeV proton flux was below the 10 pfu threshold throughout the week. Electron fluxes at GEO --------------------- The greater than 2 MeV electron flux as measured by GOES 18 and GOES 19 was mostly below the 1000 pfu threshold during the first part of the week and rose above the threshold from May 16 onward, following the arrival of the high-speed stream. The 24-hour electron fluence was initially at normal levels and reached moderate levels during the second part of the week. Solar wind --------------------- The solar wind parameters (ACE) reflected mostly slow solar wind conditions at the beginning of the week. A first enhancement was observed around May 13, most likely associated with the glancing blow arrival of the Coronal Mass Ejection (SIDC CME 650) observed at 13:48 UTC on May 10, possibly combined with a high-speed stream influence from the small positive polarity coronal hole (SIDC Coronal Hole 159). A stronger enhancement followed later in the week due to the arrival of the high-speed stream from the large, negative polarity transequatorial coronal hole (returning SIDC Coronal Hole 147). Speed values increased up to around 810 km/s. The interplanetary magnetic field reached values up to about 17 nT, while the Bz component reached values down to about -14 nT. During the final part of the week, solar wind conditions remained enhanced but gradually started returning towards a slower solar wind regime under the waning influence of the high-speed stream. Geomagnetism --------------------- Geomagnetic conditions globally were mostly at quiet to unsettled levels during the first part of the week. An interval of unsettled conditions was observed around May 13, likely related to the glancing blow arrival of SIDC CME 650, possibly combined with a high-speed stream influence from SIDC Coronal Hole 159. A stronger geomagnetic response was observed later in the week, following the arrival of the high-speed stream from the large, negative polarity transequatorial coronal hole (returning SIDC Coronal Hole 147). Global geomagnetic conditions reached moderate storm levels between 21:00 UTC on May 15 and 00:00 UTC on May 16, with NOAA Kp reaching 6+. Locally, several intervals of minor storm conditions were observed, with K BEL reaching 5. During the final part of the week, geomagnetic conditions gradually returned to mostly quiet levels globally and locally. --------------------------------------------------------------------------- DAILY INDICES DATE RC EISN 10CM Ak BKG M X 2026 May 11 073 083 111 006 B5.8 0 0 2026 May 12 066 059 111 005 B5.8 0 0 2026 May 13 091 066 103 013 B5.1 0 0 2026 May 14 063 052 106 006 B6.0 0 0 2026 May 15 /// 048 101 029 B3.9 0 0 2026 May 16 /// 057 109 031 B4.1 3 0 2026 May 17 /// 091 104 012 B4.7 1 0 --------------------------------------------------------------------------- # RC : Sunspot index (Wolf Number) from Catania Observatory (Italy) # EISN : Estimated International Sunspot Number # 10cm : 10.7 cm radioflux (DRAO, Canada) # Ak : Ak Index Wingst (Germany) # BKG : Background GOES X-ray level (NOAA, USA) # M,X : Number of X-ray flares in M and X class, see below (NOAA, USA) --------------------------------------------------------------------------- NOTICEABLE EVENTS SUMMARY DAY BEGIN MAX END LOC XRAY OP 10CM Catania/NOAA RADIO_BURST_TYPES 16 1550 1612 1622 N20W16 M1.9 2 07/4436 IV/2III/2 16 1622 1629 1634 N20W16 M1.3 2N 07/4436 VI/1 16 1739 1742 1744 N21W67 M1.9 S 02/4435 17 0329 0339 0342 ////// M1.4 02/4435 #--------------------------------------------------------------------# # Solar Influences Data analysis Center - RWC Belgium # # Royal Observatory of Belgium # # # # Website http://www.sidc.be. # # E-mail sidc-support@oma.be # # To unsubscribe http://www.sidc.be/registration/unsub.php # # # # Legal notices: # # - Intellectual Property Rights: # # http://www.astro.oma.be/common/internet/en/data-policy-en.pdf # # - Liability Disclaimer: # # http://www.astro.oma.be/common/internet/en/disclaimer-en.pdf # # - Use and processing of your personal information: # # http://www.astro.oma.be/common/internet/en/privacy-policy-en.pdf # #--------------------------------------------------------------------#