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Posted on: Jul 13, 2004
A fleet of spacecraft dispersed throughout the solar system gave the most comprehensive picture to date of how blast waves from solar storms propagate through the solar system and the radiation generated in their wake.
The "Halloween" solar storms in October-November 2003 launched billions of tons of electrified gas (plasma) that blasted by Earth within a day and past Mars hours later. The most recent reports come from the twin Voyager spacecraft at the fringe of the solar system near an unexplored region where the solar wind becomes turbulent as it crashes into the thin gas between stars.
Fast moving solar wind carves out a local cavity in the galaxy called the heliosphere. The material launched by the huge solar storms last fall blasted by Earth at five million miles per hour (eight million km/hr) and raced past spacecraft near Earth, Mars, Jupiter, and Saturn on its way to Voyager.
Slowing to an average speed of 1.5 million miles per hour (2.4 million km/hr) as it plowed into the outer heliosphere, the blast wave reached Voyager 2 at 7 billion miles (11 billion kilometers) from the Sun on April 28 and continued outward toward Voyager 1 at almost 9 billion miles (14.5 billion km) from the Sun.
The Halloween storms were the most powerful ever measured. The storms broke all-time records for X-ray intensity and for speed and temperature of the solar wind observed near Earth. About a third of the total particle radiation emitted by the Sun in the last decade in the deadly 30-50 MeV energy range came from these storms, even though the solar activity cycle was well past its maximum. (Refer to Item 2 for a movie of these solar storms.)
There are at least two kinds of solar storm effects: prompt radiation and shocks that accelerate electrically charged (ionized) atomic particles. The prompt radiation travels at nearly the speed of light, causes the most severe electrical effects on satellites, and has the greatest impact on the Earth's electrically charged upper atmosphere (ionosphere) and long-distance radio communications. The prompt radiation was detected in radio waves throughout the solar system in the moments (hours in the case of Cassini out near Saturn) after each storm. The shocks that accelerate particles to millions of miles per hour take a little longer to develop, but they pack the biggest wallop when it comes to the aurora, power grids, and energetic particles that become trapped in the Earth's Van Allen radiation belts. These storms created a new radiation belt near Earth that lasted for several weeks.
The storms' effects on Earth were severe enough to cause the rerouting of aircraft, affect satellite operations, and precipitate a power failure in Malmoe, Sweden. Long-distance radio communications were disrupted because of the effects on the ionosphere, and northern lights (aurora borealis) were seen as far south as Florida. Fortunately no NASA satellites near Earth were severely damaged by the storms a tribute to advance planning and engineering. The International Space Station astronauts curtailed some of their activities and took shelter in the Russian-supplied Service Module several times during the storm.
The Earth wasn't alone in feeling the effects -- the storms rocked the inner solar system from Mars to Saturn. The Mars Radiation Environment Experiment (MARIE) instrument on the Mars Odyssey spacecraft was disabled by radiation in Mars' orbit. The MARIE instrument successfully monitored space radiation to evaluate the risks to future Mars-bound astronauts before it stopped working during the period of intense solar activity on Oct. 28, 2003. The Ulysses spacecraft near Jupiter and the Cassini spacecraft near Saturn both detected radio waves from magnetic storms generated as the blast wave slammed into the vast magnetic fields around these giant planets.
"It's striking that this blast wave was powerful enough to generate a magnetic storm all the way out to Saturn, almost ten times farther from the Sun than Earth is," said Dr. Edward Stone of Caltech/JPL.
The shocks created by the storms in the inner solar system not only accelerated electrons and protons to high energy, they also trapped the particles in the inner heliosphere. This resulted in elevated radiation levels everywhere between Venus and Mars that decayed only gradually over a period of weeks. This kind of event will have significant implications for radiation protection requirements for explorers who venture outside of the Earth's protective magnetosphere (magnetic field).
"Over many decades, improvements in weather forecasting have saved lives and property. Space weather forecasting is still in development, but is needed to better protect our space infrastructure and future human and robotic explorers," said Carl Walz, Astronaut and Program Executive for Advanced Concepts and Project Prometheus at NASA Headquarters, Washington.
The widely dispersed fleet of "space buoys" are helping scientists piece together a more comprehensive picture of how disturbances propagate through the solar system. What determines the evolving shape and variable speed with which the shocks travel in different directions is not well understood. The differences in the speeds and arrival times at Mars and Earth suggest that the process is not simple. The Sun's magnetic field also affects how well connected different places in the solar system are. Understanding how particle-accelerating shocks travel through the solar system will help us understand and predict how radiation levels will change in different locations in space. These widely scattered spacecraft provide some of the first information about the tracks of storms in the interplanetary "ocean."
In the months ahead, the blast wave will crash into the heliopause - the tangible edge of the heliosphere where the material ejected by the Sun piles up against the interstellar wind from explosions of nearby stars. The collision may generate extremely low-frequency radio signals that will give us a much more accurate understanding of the size of the Sun's domain. The energy carried by the material will push the interstellar gas outward by as much as 400 million miles (640 million km), about 4 times the distance from the Sun to the Earth.