The Sun blasted one of its largest flares in 25 years from the same region harboring the largest sunspot of the current solar cycle Monday evening.

The region, designated active region 9393, has continued to rotate with the Sun and is no longer in line with the Earth, so most of the flare's energy was directed away from our planet. However, radiation from the flare temporarily disrupted radio communications, and flare-related events generated a storm of high-velocity particles that, in greater numbers and energies, can affect sensitive electronic equipment in space.

A mass ejection recorded by the LASCO C2 instrument on SOHO on April 2. Photo: SOHO/LASCO (ESA & NASA)

"This explosion was estimated as an X-20 flare, and was as strong as the record X-20 flare on August 16, 1989, " said Dr. Paal Brekke, the European Space Agency Deputy Project Scientist for the Solar and Heliospheric Observatory (SOHO), one of a fleet of spacecraft monitoring solar activity and its effects on the Earth. "It was more powerful that the famous March 6, 1989 flare which was related to the disruption of the power grids in Canada."

Monday's flare and the August 1989 flare are the most powerful recorded since regular X-ray data became available in 1976.

Solar flares, among the solar system's mightiest eruptions, are tremendous explosions in the atmosphere of the Sun capable of releasing as much energy as a billion megatons of TNT. Caused by the sudden release of magnetic energy, in just a few seconds flares can accelerate solar particles to very high velocities, almost to the speed of light, and heat solar material to tens of millions of degrees.

The flare erupted at 4:51 p.m. EDT Monday, and produced an R4 radio blackout on the sunlit side of the Earth. An R4 blackout, rated by the NOAA SEC, is second to the most severe R5 classification. The classification measures the disruption in radio communications. X-ray and ultraviolet light from the flare changed the structure of the Earth's electrically charged upper atmosphere (ionosphere). This affected radio communication frequencies that either pass through the ionosphere to satellites or are reflected by it to traverse the globe.

The explosion, near the Sun's northwest limb (the upper right in SOHO images), was associated with an eruption of a cloud of electrified gas, called a coronal mass ejection, or CME, into space, but apparently not directed towards Earth.

The solar flare of April 2 observed by the EIT instrument on SOHO. Photo: SOHO/EIT (ESA & NASA)

"We are perhaps lucky that this event didn't occur over the weekend, when the resulting CME would almost certainly have been aimed towards Earth," said Brekke. "A smaller flare- related CME event in March 1989 caused major power failures in Canada, and subsequent smaller events have disrupted communication and navigation satellites."

Solar ejections are often associated with flares and sometimes occur shortly after the flare explosion. CMEs are clouds of electrified, magnetic gas weighing billions of tons ejected from the Sun and hurled into space with speeds ranging from 12 to 1,250 miles per second. Depending on the orientation of the magnetic fields carried by the ejection cloud, Earth-directed CMEs cause magnetic storms by interacting with the Earth's magnetic field, distorting its shape and accelerating electrically charged particles (electrons and atomic nuclei) trapped within.

Severe solar weather is often heralded by dramatic auroral displays, northern and southern lights, and magnetic storms that occasionally affect satellites, radio communications and power systems. The flare and solar ejection has also generated a storm of high-velocity particles, and the number of particles with ten million electron-volts of energy in the space near Earth is now 10,000 times greater than normal. The increase of particles at this energy level still poses no appreciable hazard to air travelers, astronauts or satellites, and the NOAA SEC rates this radiation storm as a moderate S2 to S3, on a scale that goes to S5.

The SOHO project is an international cooperative program between NASA and the European Space Agency for the International Solar Terrestrial Science Program.