The summer forecast is for violent storms and gusty winds — on the surface of the Sun. What's going on 93 million miles from Earth and how will it affect life on our planet?

On July 14, one of the Sun's most violent explosions in the past twenty years erupted from a sunspot on the solar surface. The blast triggered a massive solar flare in the Sun's corona, or outer atmosphere, that exploded with the energy of 40 billion atomic bombs.

The explosion sent out a massive interplanetary shock wave that triggered a severe geomagnetic storm in Earth's atmosphere the next day and blacked out some radio communications. Spectacular auroras were visible in the sky across much of North America and Europe.

The storm was the strongest yet in a series of outbursts that have raged on the Sun since early in the year. What causes these storms? Why have they recently become more frequent and violent?

The July 14 blast came from an active sunspot region numbered 9077. It is one of more than 200 sunspots that have been visible on the Sun's surface in recent weeks. Sunspots are relatively cool areas that appear as dark spots on the solar surface.

Like clockwork, the Sun goes through a cycle of increased and decreased sunspot activity every 11 years. In 2000-2001, the cycle is at its peak.

Astronomers believe sunspots are caused by a distortion or "twisting" of the Sun's magnetic field lines. The reason why the Sun's magnetic field goes through an 11-year cycle remains one of the great mysteries of solar astronomy.

Geomagnetic storms in Earth's atmosphere occur when the particles, radiation, and solar wind created by sunspots compress Earth's magnetic field more than usual. In years when there are more sunspots, the number of geomagnetic storms increases. This year, the Sun has zapped Earth's magnetic field with a steady string of powerful storms, which are expected to continue into next year as well.

• The sunspot graph to the right shows the average daily number of sunspots over the past 20 years. Use the graph to answer the questions below:

1. When was the last peak of the sunspot cycle?



2. Predict when the next peak will occur.



3. In what years would you expect the number of sunspots to again be near zero?



4. What was the average daily number of sunspots in the year you were born? Was your birth year closer to a solar maximum or solar minimum?

The tearing and reconnecting of magnetic field lines in the area surrounding sunspots triggers the explosions known as solar flares. The explosions rip electrons from the hydrogen gas in the Sun's corona and create hot, electrified gas called plasma. The plasma rises into the solar atmosphere along the solar magnetic field lines, forming fiery loops and arches. Plasma in the flares is heated to many millions of degrees Celsius.

The solar flares send enormous amounts of radiation racing towards Earth's atmosphere at the speed of light. They release radiation across the entire electromagnetic spectrum, from radio waves at the long-wavelength end to X-rays and gamma-rays at the short end. The radiation can disrupt electromagnetic signals used for satellite, radio, TV, and other broadcasts.

A strong solar storm like the one on July 14 may also spit out a long stream of particles called a coronal mass ejection, or CME. The electrified gas particles burst into space from the Sun's corona in a fierce solar wind at speeds that can exceed 2 million miles (3.2 million kilometers) per hour. A CME can carry as much as 10 billion tons of solar material into space.

Down to Earth
The weather in space can greatly affect Earth. Geomagnetic storms can trigger power blackouts and satellite failures. During the last solar cycle peak in 1989, millions of people lost electricity across Canada and the Northeast. In 1998, 45 million people lost their telephone pager service after solar wind particles caused the failure of a communications satellite.

NASA scientists must carefully check the space weather forecast before scheduling astronaut space walks. An astronaut's radiation exposure is quadrupled during strong geomagnetic storms.

Air travelers flying at high altitudes in the polar regions, where Earth's magnetic fields are strongest, get a dose of radiation equal to that of a brief chest X-ray. Fortunately, the magnetic field acts as a shield to protect Earthlings on the surface from significant radiation exposure.

• The graph below shows how a solar explosion on June 6 affected the solar wind on June 8, as measured by a NASA satellite orbiting Earth. The solar wind speed is shown on the y-axis in kilometers per second, and time is shown on the x-axis. (The time is calculated on a 24-hour cycle, so 9 = 9am and 21 = 9pm.)