A direct blast from AR9393 could have triggered widespread radio blackouts, disrupted satellite communications and even collapsed power grids. When AR9393 rotated over the Sun's western limb on April 3rd, most Sun watchers assumed it was gone for good. Indeed, the extraordinary spot remained whole throughout its two week journey across the back side of the Sun and it reappeared on April 19th. They knew AR9393 was returning because they never lost sight of it. These new techniques are still a work in progress, but already we can predict the appearance of large sunspots days before they rotate into direct view.
Sunspots are like high-energy lighthouses.
As the Sun turns, beams of UV radiation sweep through space and illuminate the interplanetary medium, a thin haze of gas and dust between the planets. The MDI team's approach to peering through the Sun is more subtle. The Sun is a hummimg ball of sound waves launched by turbulent convective motions inside our star. By monitoring the Sun's vibrating surface, helioseismologists can probe the stellar interior in much the same way that geologists use seismic waves from earthquakes to probe the inside of our planet. MDI and SWAN are complementary in their efforts to see through the Sun.