Solar Influences Data Center (SIDC) / RWC Belgium Royal Observatory of Belgium E-mail: Pierre.Cugnon@oma.be http://sidc.oma.be SIDC WEEKLY BULLETIN -------------------- WEEK 173 from 2004 Apr 19 SOLAR ACTIVITY: --------------- Four M-flares were collectively produced by the 3 active regions that one after the other dominated the space weather during the period. The beginning of the period was dominated by Catania sunspot group 08 (NOAA AR 596) culminating with an M1.2 flare on Thursday, April 22. After that Catania sunspot group 12 (NOAA AR 597), already at the West limb at that time, started an impressive firework of more than 10 C- flares, an M1.5 flare (11h50 UT) and an M1.1 flare (21h12 UT) both on Saterday, April 24. By the time Catania sunspot group 12 rotated over the west limb, a new active region (NOAA AR 599) took the lead with an M2.2 flare on Sunday, April 25 (05h37 UT). There were no proton events nor halo CMEs during the period. SOHO entered Emergency-Sun-Reacquisition (safe mode) on April 21, and again on April 23. This spurious events were tiggered by the \'Fine Sun Pointing Attitude Anomaly Detector\'. No EIT, LASCO or MDI data were available for the remaining part of the period. GEOMAGNETISM: ------------- The K index as measured in Wingst (G) indicate quiet geomagnetic conditions (Kp=2) for most of the period. The only time with a persistent deviation was on April 23 with K_Wingst=3-4 and K_p up to 5. The solar wind speed increased over the period from 350 km/s on Monday, April 19 till 550 km/s on Sunday, April 25. A coronal hole signature was observed on April 20 late (near midnight). --------------------------------------------------------------------------- DAILY INDICES DATE RC 10CM Ak BKG M X 2004 Apr 19 108 113 008 B1.7 0 0 2004 Apr 20 088 111 006 B1.4 0 0 2004 Apr 21 088 113 008 B1.2 0 0 2004 Apr 22 104 117 008 B2.0 1 0 2004 Apr 23 078 115 018 B4.4 2 0 2004 Apr 24 /// 112 010 B4.4 0 0 2004 Apr 25 /// 107 013 B2.0 1 0 # RC : Sunspot index from Catania Observatory (Italy) # 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 DAY BEGIN MAX END LOC XRAY OP 10CM TYPE Cat NOAA NOTE 22 0203 0219 0224 S09E19 M1.2 SF 08 0596 24 1141 1150 1152 S08W90 M1.5 12 0597 24 2102 2112 2114 S08W83 M1.1 SF 150 12 0597 25 0502 0537 0542 N15E36 M2.2 0599