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Introduction to the Solar CycleSunspots Growing and Retreating in Regular Cycles of Activity
Aug 8, 2010 © Harry P. Schlanger
The 11-year cycle of solar activity is characterised by the rise and fall in the number and surface area of sunspots. These are one of many solar activity indicators.
Solar activity is governed by the sun's magnetic field, and one of the
unsolved problems
in astronomy is the origin of the regular changes in the magnetic field that drive the activity cycle.
Discovery of the Solar Cycle
The German astronomer, Samuel Heinrich Schwabe (1789-1875) first discovered the solar cycle in 1843,
culminating from years of working with observations of sunspots.
Following this work, Rudolf Wolf (1816-1893), a Swiss astronomer,
carried out a historical reconstruction of solar activity going back to the seventeenth century. He also
originated the acquisition of daily solar observations and defined a useful standard index. This
sunspot index is called the International Sunspot Number and continues to be measured daily today.
Other Solar Activity Indicators
A previous article
explained that there are many solar indicators, or indices, used to describe solar activity.
According to Ilya G. Usoskin (2008), most of the indices are highly correlated
to each other due to the dominant 11-year cycle, but may differ in measured detail or long-term trend.
A few of these are (Usoskin 2008, Hathaway 2010):
- Radio flux
- Total solar irradiance
- Solar magnetic field
- Solar flares and coronal mass ejections
- Geomagnetic activity
Historical Sunspot Cycles
The first
record of sunspots
dates to around 800 BC in China and the oldest surviving drawing of a sunspot
dates to 1128. In 1610, astronomers began using the telescope to make observations of sunspots and their
motions.
The graph of Figure 1 shows the sunspot cycles over the last 400 years. Cycle periods as short as 9
years and as long as 14 years have been observed. Significant variations in amplitude also occur.
Fig 1. Four Hundred years of Sunspot Observations (Wikipedia).
Red data points represent sporadic observations since 1610.
The period between 1645 and 1715, a time during which very few sunspots were observed, is a real feature,
as opposed to an artefact due to missing data. This epoch is now known as the Maunder minimum, after
Edward Walter Maunder, who extensively researched this peculiar event. Another epoch of low solar
activity around the early 1800s and spanning three cycles is called the Dalton minimum.
Numbering Sunspot Cycles
Following the numbering scheme established by Wolf, the 1755–1766 cycle that occurred some time after
the Maunder minimum is traditionally numbered '1'.
Each next cycle number is incremented by one as shown in Figure 2 for the complete set of sunspot cycles.
The current cycle in progress is Cycle 24.
Fig 2. Numbering the sunspot cycles starting with the 1755–1766 cycle (Hathaway, 2010).
It is well recognised that the sunspot numbers were less reliable before Wolf's time due to observations
missing on
certain days of the month. They are shown as coloured dots on these graphs.
The graphs illustrate that solar cycles vary in amplitude, shape, and length. A detailed
list of solar cycles
tracked since 1755 has a mean of 11.1 years, or 131 ± 14 months.
Safe Sunwatching for Sunspots
To watch sunspots, the first concern should always be eye safety. One should never view the Sun directly
with the naked eye or with any unfiltered optical device, such as binoculars or a telescope. A safe way
is to
project an image of the
Sun through a telescope or binoculars onto a white screen.
References:
- "
A History of Solar Activity over Millennia," Ilya G. Usoskin. Living Rev. Solar Phys., 5, (2008), 3
(pdf)
- "
The Solar Cycle
," David H. Hathaway. Living Rev. Solar Phys., 7, (2010), 1
(pdf)
- "
Solar cycle", Wikipedia.
The copyright of the article Introduction to the Solar Cycle: Sunspots Growing and Retreating in Regular Cycles of Activity is owned by Harry P. Schlanger. Permission to republish in print or online must be granted by the author in writing.
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