Last Update: 03 Jul 2003
Images and data from the surface, concerning sunspots and magnetic fields,
are vital for interpreting what goes on in the overlying solar atmosphere.
The visible surface is also the source of the rays that light and warm our
planet. Changes in its brightness are one of the ways in which the Sun may
affect the Earth's climate.
Two instruments on SOHO report on the condition of the Sun's bright face:
- MDI (Michelson Doppler Imager)
- VIRGO (Variability of Solar Irradiance and Gravity Oscillations)
MDI (Michelson Doppler Imager)
gauges the brightness at a million points and monitors the dark sunspots
and bright faculae. It measures magnetic fields, too, across the visible
surface, producing the best maps and movies of solar magnetism ever made.
These show that the pattern of surface magnetism changes completely in less
than two days. MDI also sees shock waves radiating outwards from the scene
of a solar flare, like the ripples seen when a stone falls into a pond.
Above: A rapidly expanding "solar quake" on the Sun's surface caused by a solar flare, depicted by MDI
(Variability of Solar Irradiance and Gravity Oscillations) measures the Sun's
total brightness, or irradiance, and its intensity in red, green and blue
light. It records many small variations in sunlight from minute to minute
and day to day. After SOHO's launch in 1995, when the count of sunspots was
at a minimum, the average intensity increased towards the year 2000 and the
sunspot maximum. The brightening seen by VIRGO confirmed variations reported
by previous spacecraft. The change was less than 0.1 per cent, and no long-term
change in brightness is yet apparent in data from space.