27 April

2001: On 27 April 2001, scientists announced that the ESA/NASA SOHO spacecraft could now detect activity on the far side of the Sun.

Teams in France and the USA used SOHO’s SWAN instrument to see ultraviolet rays sweeping like a lighthouse beam across interplanetary gas beyond the Sun. Also, the MDI instrument could peer right through the Sun to locate hidden sunspots and their active regions. From 27 April 2001, both teams made their observations available routinely to everyone, including the forecasters of space weather.

The announcement marked the fifth anniversary of the commissioning of the European-built SOHO, in April 1996, and the formal start at that time of the observations with a dozen sets of solar instruments. European and US scientific teams contributed the instruments to this project of international co-operation between ESA and NASA.

The Sun takes roughly four weeks to turn completely around on its axis, but active regions can appear and grow in only a few days. Until SOHO, nobody had any way of telling when an active region might come ‘around the corner’ - perhaps blazing away with eruptions as soon as it appeared. If an active region can be detected in the middle of the far side it will appear on the eastern (left-hand) side of the visible disk about seven days later. The SWAN team announced the telltale ultraviolet observations in June 1999.

In March 2000 Charles Lindsey of Tucson, Arizona, and Doug Braun of Boulder, Colorado, reported that they had detected, with SOHO’s MDI, sound waves reflected from far-side sunspots. Speeded by the intense magnetic fields associated with sunspot regions, the sound waves arrived a few seconds early at the Sun’s near-side face, compared with sound waves from sunspot-free regions. Decoding MDI data from a million points on the Sun’s near side, to obtain an impression of the far side, uses a technique called helioseismic holography and requires a powerful computer.

Both discoveries were made retrospectively from SOHO’s archives. Since then teams have streamlined their data gathering and analyses to the point where they can offer routine long-range forecasts of intense solar activity based on far-side foresight. The techniques are complementary, with MDI seeing the sunspot regions and SWAN reporting how active they are.

Although conceived for scientific research, SOHO has proved invaluable as a watchdog for spotting sunstorms. Forecasters already rely heavily on SOHO’s round-the-clock observations of flares and mass ejections that can have harmful effects on satellites, power lines and other technological systems. The new long-range, far-side forecasts may be especially useful for scheduling manned space operations, during which astronauts might be exposed to dangerous particles from solar explosions.