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Solar cycle 23
Solar cycle 23 was the 23rd solar cycle since 1755, when extensive recording of solar sunspot activity began.〔Kane, R.P. (2002). "(Some Implications Using the Group Sunspot Number Reconstruction )". ''Solar Physics'' 205(2), 383-401.〕〔(【引用サイトリンク】 url=http://www.spacetoday.org/SolSys/Sun/Sunspots.html )〕 The solar cycle lasted 12.6 years, beginning in May 1996 and ending in January 2008. The maximum smoothed sunspot number (monthly number of sunspots averaged over a twelve-month period) observed during the solar cycle was 120.8 (March 2000), and the minimum was 1.7. A total of 805 days had no sunspots during this cycle.
== History ==
One of the first major aurora displays of solar cycle 23 occurred on 6 April 2000, with bright red auroras visible as far south as Florida and South Europe. On 14 July 2000, the Coronal Mass Ejection (CME) hurled by a X5.7 solar flare provoked an extreme (G5 level) geomagnetic storm the next day. Known as the Bastille day event, this storm caused damage to GPS systems and some power companies. Auroras were visible as far south as Texas. Another major aurora display was observed on 1 April 2001, due to a coronal mass ejection hitting the Earth's magnetosphere. Auroras were observed as far south as Mexico and South Europe. A large solar flare (the second-most powerful ever recorded) occurred on 2 April 2001, an X20-class, but the blast was directed away from Earth.
In late October 2003, a series of large solar flares occurred. A X17.2-class flare ejected on 28 October 2003 produced auroras visible as far south as Florida and Texas. A G5 level geomagnetic storm blasted the Earth's magnetosphere the next two days.〔(【引用サイトリンク】title=Hotshot )〕 A few days later, the largest solar flare ever measured with instruments occurred on 4 November; initially measured at X28, it was later upgraded to an X45-class.〔(【引用サイトリンク】title=Hotshot )〕 This flare was not Earth-oriented and thus only resulted in high-latitude auroras. The whole sequence of events occurred from 28 October to 4 November is known as the Halloween Solar Storm. Other large solar flares occurred on 7 September 2005 (X17), 15 April 2001 (X14.4) and 29 October 2003 (X10), with auroras visible in mid-latitudes.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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