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Indices that Quantify Solar ActivityMeasuring Solar Activity Using Various Devised Indicators
Aug 14, 2010 © Harry P. Schlanger
Solar indices are indicators that quantify the Sun's activity. They highly correlate to each other and to the dominant 11-year solar cycle.
An index number (with its plural: indices) has a wide range of meanings in different subjects. In science
or engineering, including in a topic like the Sun, an index can simply be an "indicator".
According to a Princeton University
definition, an
index may be a number or ratio, a value on a scale of measurement, which is derived from a series of
observed facts. This index may indicate, or reveal relative changes as a function of time.
Concept of Solar Activity
The Sun is far from being in a quiet state but instead goes through various dynamic processes. Such
non-equilibrium and also, frequent eruptive processes can broadly be regarded as "solar activity".
Solar activity is governed by the sun's magnetic field, which is cyclic in nature and the driver for the
Solar Cycle.
Variety of Solar Indices
The astronomer, Ilya G. Usoskin (2008) stated: "a variety of indices quantifying solar activity have been
proposed in order to represent different observables and caused effects. Most of the indices are
highly correlated to each other due to the dominant 11-year cycle, but may differ in fine details and/or
long-term trends". Some of these are (Usoskin 2008, Hathaway 2010):
- Sunspot numbers and groups
- Radio flux
- Total solar irradiance
- Solar magnetic field
- Solar flares and coronal mass ejections
- Geomagnetic activity
- Galactic cosmic ray fluxes
- Radioisotopes in tree rings and ice cores that vary in association with sunspots
The above solar indices can be divided into two groups:
- Physical: Physical indices have clear physical meaning. They quantify directly measurable
values of a real physical observable, such as the radio flux.
- Synthetic: Synthetic indices are calculated, or synthesised, from observed data or
phenomena using a special algorithm. Sunspot number is an example.
Relative Importance of Solar Indices
The most commonly used index of solar activity is based on sunspot number (SSN) representing the
weighted number of individual sunspots and/or sunspot groups, calculated in a prescribed manner from
simple visual solar observations (Usoskin, 2008).
The
10.7 cm radio flux
refers to the emissions from the Sun at the radio wavelength
of 10.7 cm (2800 MHz). This measure (abbreviated F10.7) of solar activity has advantages over sunspot
numbers and areas in that it is completely objective and can be made under virtually all weather
conditions (Hathaway, 2010).
A number of
indices
of Earth's geomagnetic activity have been devised, most measuring rapid
changes in the strength and direction of the Earth’s magnetic field. The Planetary index, Ap,
measures in three-hourly intervals for the range of variability in the geomagnetic field.
NASA has an interesting student tutorial to measure the logarithmically related index, Kp.
Storminess of the Solar Index Numbers
The Australian Space Weather Agency provides tables (below) to indicate the storminess of solar activity
as the values of Sunspot Number and Ap change. There is also a table of conversions to compare the above indices.
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)
- "Student-Derived Kp index - Activity 18" in "Earth's Magnetic Personality - Magnetic Data
in the Classroom", National Aeronautics and Space Administration.
(pdf)
- "IPS Radio and Space Services: Space Weather - Indices", The Australian Space Weather Agency (pdf)
The copyright of the article Indices that Quantify Solar Activity: Measuring Solar Activity Using Various Devised Indicators 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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