Robert Roy Britt writes in Space.com:
Space weather forecasters have it even tougher than regular weather forecasters. In trying to predict long-range solar activity, they have to rely on a picture of just the half of the Sun they can see. Storms brewing on the backside are hidden from view until they rotate to the front.
Jupiter to the rescue. The giant gas planet reflects solar activity, scientists have learned. And when Jupiter is on the other side of the solar system, it can act as a mirror for flare-ups from the back side of the Sun.
Scientists had previously measured X-rays emanating from the Jovian atmosphere. Those coming from the equator were theorized to be related to solar activity.
More here.