:Issued: 2011 May 23 1621 UTC
:Product: documentation at http://www.sidc.be/products/bul
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# SIDC Weekly bulletin on Solar and Geomagnetic activity #
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WEEK 542 from 2011 May 16
SOLAR ACTIVITY
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Six sunspot groups were observed on the Sun during the week: NOAA ARs 1208, 1214, 1216, 1217, 1218 (Catania numbers 79, 87, 89, 88, 90, respectively) and 1219 (no Catania number attributed due to a data gap). Two C-class flares were detected during the week, both on May 18. Catania sunspot group 90 (NOAA AR 1218) produced a C1.2 flare peaking at 12:59 UT. A C2.0 flare peaked at 18:30 UT in the Catania sunspot group 79 (NOAA AR 1208) at the west limb. The latter flare was associated with a partial halo CME first appearing above the west solar limb in the LASCO C2 field of view at 18:24 UT. It had angular width around 170 degrees and speed around 1000 km/s. Coronal dimmings and a post-eruption arcade associated with the event were detected by SDO/AIA. Due to the limb position of the CME source region, the interplanetary counterparts of the CME did not arrive at the Earth.
Another partial halo CME was detected by CACTus on May 20, first appearing in the LASCO C2 field of view at 21:48 UT. STEREO/SECCHI data demonstrated that this CME originated on the far side of the Sun.
GEOMAGNETIC ACTIVITY
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In the beginning of the week the Earth was situated inside the solar wind stream with intermediate speed, followed by the fast solar wind stream. Although the interplanetary magnetic field (IMF) magnitude was elevated (around 10 nT on May 16), the north-south IMF component was fluctuating around zero. This resulted in quiet geomagnetic conditions. The solar wind speed then started to decrease. On May 18 the Earth entered a slow solar wind flow, and stayed there until the end of the week. The geomagnetic conditions remained quiet.
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DAILY INDICES
DATE RC EISN 10CM Ak BKG M X
2011 May 16 061 039 92 017 B1.4 0 0
2011 May 17 059 035 92 013 B1.4 0 0
2011 May 18 066 028 91 008 B1.3 0 0
2011 May 19 039 025 84 005 A9.7 0 0
2011 May 20 036 024 84 003 A7.5 0 0
2011 May 21 /// 027 84 005 A6.8 0 0
2011 May 22 /// 036 85 006 A7.5 0 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)
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NOTICEABLE EVENTS
DAY BEGIN MAX END LOC XRAY OP TENCM TYPE Cat NOAA NOTE
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# Solar Influences Data analysis Center - RWC Belgium #
# Royal Observatory of Belgium #
# Fax : 32 (0) 2 373 0 224 #
# Tel.: 32 (0) 2 373 0 491 #
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