:Issued: 2010 Sep 28 1349 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 508 from 2010 Sep 20
SOLAR ACTIVITY
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According to the Catania observatory, four sunspot groups were observed on the Sun during the week. Catania sunspot groups 38 and 41 (NOAA ARs 1106 and 1108 respectively) were observed since the beginning of the week. They produced only a weak flaring activity (below the C-level). Catania sunspot group 38 disappeared behind the west solar limb on September 22.
On September 22, Catania sunspot groups 42 and 43 (attributed a single NOAA AR number 1109) appeared from behind the east limb. This active region (returning NOAA AR 1105) produced a significant flaring activity (four C-class flares) on September 20-21, while still behind the limb. The strongest flare (C2.1) peaked on September 20 at 19:45 UT. Further on, the flaring activity of this group decreased, with only B-class flares detected by GOES.
A low-latitude coronal hole in the northern hemisphere (elongated in the east-west direction) passed through the solar central meridian on September 20-22. Another low-latitude coronal hole in the northern hemisphere passed the central meridian on September 24-25.
GEOMAGNETIC ACTIVITY
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In the beginning of the week, the Earth was situated inside the slow solar wind flow, and the geomagnetic conditions were quiet. A faster flow (probably originating from the elongated low-latitude coronal hole in the northern hemisphere) arrived on September 21. The solar wind speed reached only 450 km/s, and the interplanetary magnetic field (IMF) magnitude did not reach values above 10 nT, so the geomagnetic conditions remained quiet. Another slow wind interval without significant geomagnetic consequences followed on September 22.
On September 23 the solar wind speed started to grow indicating the arrival of the interaction region between the slow and fast solar wind streams. The IMF magnitude in the interaction region reached 13 nT, but the speed was still relatively low for a geomagnetic disturbance to occur. The fast flow (probably originating from the second low-latitude coronal hole in the northern hemisphere) arrived on September 24, and the solar wind speed reached its peak values (around 650 km/s) on September 25. Due to weak to average values of the IMF magnitude (3-4 nT) in the fast stream, the geomagnetic conditions remained quiet (peak values of K = 3). On September 26 the solar wind speed decreased to 450 km/s, still with the quiet level of the geomagnetic conditions.
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DAILY INDICES
DATE RC EISN 10CM Ak BKG M X
2010 Sep 20 031 023 83 004 B1.5 0 0
2010 Sep 21 030 020 85 005 B2.0 0 0
2010 Sep 22 036 018 85 003 B1.5 0 0
2010 Sep 23 045 022 84 007 B1.5 0 0
2010 Sep 24 /// 023 83 019 B1.3 0 0
2010 Sep 25 056 025 83 009 B1.3 0 0
2010 Sep 26 /// 027 84 010 B1.2 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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