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 170 from 2004 Mar 29 SOLAR ACTIVITY: --------------- Solar activity was low during the period but intense C-flaring was observed starting from March 29 noon and lasting till March 31 noon. All this C-flare activity came from Catania Sunspot group 93 (NOAA 0582), a beta-gamma region at that time. The series of C-flares (24 in total) ended with a long duration C3.4-flare lasting 24 hours. After this, the group relaxed and become more quiet. Since 30 March, it decayed in area coverage, spot count, and magnetic complexity and ended the period as a simple alpha spot. The long duration C-flare was probably associated with a \'full\' halo CME observed by LASCO on 2004/03/31, already in progress. The event was first observed in C3 at 21:53 UT as a faint front already surrounding the C3 occultor, much fainter in the E hemisphere. The mean plane-of-sky speed for the event as obtained from C3 data was only ~220 km/s at PA 239, showing slight deceleration. No flares larger than C-class were observed in the period, nor any proton flux enhancement events. GEOMAGNETISM: ------------- The beginning of the week was characterised by a slowly decreasing solar wind speed and an interplanetary magnetic field close to zero. This resulted in quiet (Kp<4) geomagnetic conditions from March 29 till April 3, only interrupted by an episode of Kp=4 on March 30. All this changed drastically on Sunday April 4, with the arrival of a disturbance in the solar wind which increased stepwise in speed from 380 to 500 km/s and a solar wind density that increased by an order of magnitude. The z-compoment of the interplanetary magnetic field made southward excursions down till - 10 nT. This disturbance was possibly related to the long duration flare of March 31. As a consequence, geomagnetic storm conditions were recorded with K-values reaching K=5 in Dourbes (Belgium) and K=6 in Izmiran (Russia). --------------------------------------------------------------------------- DAILY INDICES DATE RC 10CM Ak BKG M X 2004 Mar 29 /// 129 012 B3.4 0 0 2004 Mar 30 /// 127 012 B3.1 0 0 2004 Mar 31 /// 121 012 B3.2 0 0 2004 Apr 01 090 113 004 B2.1 0 0 2004 Apr 02 085 108 004 B1.7 0 0 2004 Apr 03 075 107 035 B1.5 0 0 2004 Apr 04 087 109 018 B1.5 0 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