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The sun has emitted many coronal mass ejections in the week leading up to Oct. 31, 2013, which merged together over time to pass Mars. This is exactly the kind of event NASA's MAVEN mission is designed to observe. But MAVEN is sitting this one out in a hangar in Florida. The Mars-bound mission doesn't launch until November.

"I am so disappointed!" said Bruce Jakosky, MAVEN principal investigator at the University of Colorado Boulder. MAVEN hasn't missed its only chance, he added. These events happen frequently, more so in the sun's current solar activity phase, which goes through an approximately 11-year cycle. Once in orbit around Mars, MAVEN will be positioned to observe multiple future mass ejections and measure how they erode Mars' atmosphere over time.

But Jakosky still wishes MAVEN could have seen this Halloween flurry. "It's like standing in line to get into a football game when you hear the crowd cheer for a touchdown."

Coronal mass ejections, or CMEs, are a natural part of the solar atmosphere's variability. Magnetic fields emerge through the sun's surface, generated by internal currents of gas known as the solar dynamo. These fields control the structure of the corona, the sun's atmosphere, which can be seen as a bright halo surrounding the sun during a solar eclipse. A major disturbance in these fields may release a hot, fast plasma bubble as the corona adjusts itself – a CME. Spacecraft such as the joint European Space Agency and NASA Solar and Heliospheric Observatory and NASA's Solar Terrestrial Relations Observatory observe CMEs moving like compression waves – such as those preceding a fast speedboat plowing across a lake – trailed by a chaotic cloud of coronal plasma ejecta.

"It's more than a heavy-duty gust of solar wind," said Janet Luhmann, MAVEN's deputy principal investigator at the University of California Berkeley. "It's an actual piece of the corona." The coronal plasma is comprised of charged hydrogen gas, broken down into protons and electrons. When these plasma particles rocket out of the sun as a CME, they can travel at speeds of 600 miles per second (1,000 km/s) or even faster, compared to a speed of about 250 miles per second (400 km/s) for the normal solar wind.

But a CME can impel other particles to travel even faster. The leading compression wave, traveling through the solar wind, can steepen into an interplanetary shock wave. That shock can accelerate solar wind particles to up to one-third the speed of light.  These particles are called, appropriately, solar energetic particles and can be hazardous to both equipment and humans in space.

During periods of peak solar activity, the sun can fire off as many as three CMEs a day. MAVEN's active mission will come at a point in the 11-year solar activity cycle where solar activity is waning, yet solar storms are still frequent. CMEs aimed at Mars present the best scientific opportunities for MAVEN, but the spacecraft may still be able to detect effects of other events, including plasma, energetic particles and magnetic fields, Luhmann said.

Any spacecraft in the path of a CME – as MAVEN hopes to be – will first see a surge of solar energetic particles, which rush ahead of the leading shock wave. The particles may arrive at Mars as soon as 20 minutes after the event.

Once the shock wave arrives (within two or three days) the spacecraft will detect a sudden increase in magnetic field strength and in the density of solar wind particles, which are piled up by the compression wave like snow before a plow. Solar activity can interfere with spacecraft electronic systems and instruments, but it's unlikely that MAVEN will suffer permanent damage from the radiative onslaught.

"We've planned for that," Luhmann said. "Many other planetary spacecraft have survived space weather like this." At worst, she said, a particularly strong storm may tax the spacecraft's sensors. MAVEN is designed to continue science operations even during the strongest storms.

The atmosphere of Mars, on the other hand, may not emerge unscathed. MAVEN's mission at Mars, in part, is to observe how solar particles affect, and may deplete, Mars' atmosphere. Solar radiation normally produces an electrical charge in gas atoms, creating ions in a high-altitude layer of the atmosphere called the ionosphere. Some of these atmospheric ions, such as oxygen and carbon ions, are picked up by the magnetic fields in the passing solar wind and can be carried away into space. Some may also be carried back into the atmosphere where they can shed their energy, heating the atoms around them. This heating could also lead to atmospheric escape. CMEs enhance the energy of these ions, and may accelerate atmospheric loss.

Earth's magnetic field protects our atmosphere from such a direct solar assault. Mars enjoys no such planet-wide shield: the Red Planet boasts only a weak magnetic field that is present only in limited areas.

MAVEN's scientific team will study how CMEs affect the rate of atmospheric loss at Mars, compared with the loss due to the solar wind. Everyday atmospheric loss, occurring one molecule at a time, can be compared to sand-grain-by-sand-grain coastal erosion on Earth, said Jared Espley, space scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "One way of looking at coastal erosion is to see how much erodes every time the tide comes in," he said. "Or, you could also look to see how much erosion occurs every time a tsunami comes through."

Recent Mars missions discovered conclusive evidence that water flowed over the surface of Mars billions of years ago. "Something catastrophic happened to change it into the place it is today," Espley said. MAVEN hopes to explore the role atmospheric loss to space played in that catastrophic climate change.

Luhmann isn't concerned that MAVEN missed this solar event. On the contrary, she said, the Halloween CMEs reassure the team that MAVEN will likely see some solar fireworks. "We'd like to see how Mars responds to large space weather events, and so this actually is not bad at all," she said. "This is good."

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