Soho, the Solar Orbiting and Heliospheric Observatory, is positioned 1.4 million km (one million miles) closer to the Sun than the Earth.
The Earth will now have an extra week's warning of eruptions from the Sun's surface thanks to the Sun-monitoring Soho spacecraft.
Teams in France and the US will now provide predictions of what the Sun could have in store for us by using two different ways of detecting activity on the Sun's far side, before it swings into view of the Earth.
From Thursday, both teams are making their observations available routinely to everyone, including the space weather forecasters.
Sun-Earth day
The new service coincides with the celebration of Sun-Earth Day 2001, by various national space agencies. It also marks the fifth anniversary of the commissioning of the European-built Soho spacecraft.
Soho, the Solar Orbiting and Heliospheric Observatory, is positioned 1.4 million km (one million miles) closer to the Sun than the Earth.
"What started as unusual research has become an everyday tool," said Jean-Loup Bertaux of the French Service d'Aéronomie near Paris, who leads the French-Finnish team responsible for the Swan instrument.
"We should no longer be taken by surprise by highly active regions that suddenly come into view as the Sun rotates."
Important data
The data on the Sun's hidden side is important because the Sun takes roughly four weeks to turn on its axis, but active regions can appear and grow in only a few days.
Until Soho's preliminary observations two years ago, no-one had any way of telling when an active region might come "around the corner".
Based on their initial observations, the Swan team announced their findings in June 1999. Now the technique has been refined and is useful for predicting solar activity.
A year later, in March 2000, scientists reported they had detected, with Soho's MDI sensor, sound waves reflected from far-side sunspots.
Affected 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.
Astronaut protection
Both discoveries were made retrospectively from Soho's archive of observations. Since then, teams have streamlined their data gathering and analyses to the point where they can now offer routine long-range forecasts of intense solar activity based on far-side foresight data.
"When we started work with Soho five years ago, most experts thought it would be impossible to see right through the Sun," says Philip Scherrer of Stanford University, principal investigator for the MDI instrument.
"Now we do it regularly in real time. For practical purposes, we've made the Sun transparent."
Although conceived for scientific research, Soho has proved invaluable as a watchdog for spotting sunstorms. Forecasters now rely on Soho's 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 will prove especially useful for scheduling manned spaceflights during which astronauts might be exposed to dangerous radiation from solar flares.