Solar Activity

The solar activity follows a cycle of about 11 years that has been discovered by the german amateur astronomer Heinrich Schwabe. The solar cycles are numbered since the maximum of 1761. The cycle 23 has reached its maximum in 2001 and ends in 2007/2008.

During a solar cycle, the following characteristics evolve:

• the number of sunspots
• the solar irradiance, of about 0.1%
• the occurrence frequency of flares, coronal mass ejections and consequently geomagnetic storms
• the flux of short-wavelength solar radiation, UV, EUV and X-Ray, as well as the radio emissions
• the flux of high-energy galactic cosmic rays in the inner solar system

The variation of the sunspot numbers is measured through the sunspot index. These data are not reproduced here but are available on the website of the Solar Influences Data Analysis Center (SIDC) located in Belgium:

Unless otherwize stated, the data presented here below are from the Space Weather Prediction Center (SWPC):

X-ray Background Flux in the range 1 to 8 Å

The X-ray background flux is a daily average of the X-ray flux measured in the 1 to 8 Å range by the GOES satellites. This averaging is centered on the midday and intends to reduce the effects of flares.

This plot shows the evolution of the X-ray background flux since 1994:

• the red dots represent the daily value
• the red line represent the monthly average

X-ray Solar Flares

The X-ray solar flares are classified according to their X-ray brightness in the wavelength range 1 to 8 Å. This brightness is measured as the peak intensity of the burst by the GOES satellites.

• X-class flares have intensities greater than or equal to 10^-4 W/m². These events that can trigger planet-wide radio blackouts and long-lasting radiation storms.
• M-class flares have intensities greater than or equal to 10^-5 W/m² and lower than 10^-4 W/m². They can cause brief radio blackouts especially in the Earth's polar regions.
• C-class flares have intensities greater than or equal to 10^-6 W/m² and lower than 10^-5 W/m². They have few noticeable consequences on Earth.

This plot shows the monthly count of X-ray flares since 1995:

• the yellow bar represent the monthly count of C-class flares
• the orange bar represent the monthly count of M-class flares
• the red bar represent the monthly count of X-class flares
• the blue line represent the monthly number of SID events detected by this station (since 2006)

Optical Solar Flares

The optical solar flares are classified according to their solid angle at the time of maximum brightness in H-alpha (at the wavelength of 656.3 nm).

• a class 4 flare has a solid angle greater than or equal to 24.8 square degrees
• a class 3 flare has a solid angle in the range 12.5 to 24.7 square degrees
• a class 2 flare has a solid angle in the range 5.2 to 12.4 square degrees
• a class 1 flare has a solid angle in the range 2.1 to 5.1 square degrees
• a class 0 flare has a solid angle lower than or equal to 2.0 square degrees

One square degree corresponds to an area of 1.214·10⁴ km² at the Sun's surface. That corresponds to 48.5 millionths of the visible solar hemisphere.

The class is usually followed by a brightness qualifier F (Faint), N (Normal), or B (Brilliant).

This plot shows the monthly count of optical flares since 1995:

• the yellow bar represent the monthly count of class 0 flares
• the orange bar represent the monthly count of class 1 flares
• the dark orange bar represent the monthly count of class 2 flares
• the red bar represent the monthly count of class 3 flares