The Sun's energy output varies on a number of different time scales, with a peak in solar activity--solar maximum--occurring roughly every 11 years. Solar activity is tracked by various means, including monitoring the number of sunspots, observation of flares and coronal disturbances, measurement of the Sun's x-ray output, etc. A commonly used indicator of solar activity is the "F10.7 cm Solar Radio Flux": the intensity of the microwave radiation emitted by the Sun at a wavelength of 10.7 cm (or a frequency of 2800 MHz).

• Current Solar Images from SOHO, Yohkoh, and Ground-based Solar Observatories

• Daily Solar Activity Reports (from the Big Bear Solar Observatory)

• F10.7 cm Solar Radio Flux (from Northwest Research Associates, Space Weather Services, Inc.)

• Sunspot Number Indices (from Northwest Research Associates, Space Weather Services, Inc.)

• Preliminary Report and Forecast of Solar Geophysical Data by the NOAA SEC Choose "Daily Solar Data" from the menu. First read "About the Indices, Events, and Regions Data Menu."