Measurements taken at Earth's neighbors, Mars and Venus, show the battle between high-energy cosmic rays from beyond the solar system and the influence of the sun in the inner solar system.
In the new research, scientists compared data collected by similar ASPERA plasma sensors on the Mars Express and Venus Express spacecraft with the number of sunspots visible on the surface of the sun. The results revealed that peaks of high activity in the 11-year solar cycle suppressed cosmic rays, which are high-energy particles that travel through space at nearly the speed of light. And understanding cosmic rays could be vital for future space exploration because these high-energy particles can damage spacecraft electronics and even the DNA of astronauts.
"The study shows the range of valuable insights that can be derived from what is actually background count information collected by the ASPERA instruments," Yoshifumi Futaana, a scientist at the Swedish Institute of Space Physics and lead author on the new research, said in a statement. "Understanding the various relationships between cosmic rays and the solar cycle, the atmospheres of planets and the performance of spacecraft instrumentation is very important for future robotic missions and human exploration."
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The European Space Agency (ESA) Mars Express spacecraft launched in 2003 and remains in orbit around the Red Planet, while the agency's Venus Express operated around the second planet from the sun between 2006 and 2014, giving the scientists 17 years of data from Mars and eight from Venus. The researchers also included measurements of cosmic rays taken on Earth by the Thule neutron monitor in Greenland.
The team then divided the data into three-month periods to analyze cosmic ray counts over these periods, reducing the influence of sporadic solar events such as flares and coronal mass ejections.
The data from all three planets showed that cosmic ray detections declined as the sun's activity in Solar Cycle 24 reached its peak, which occurred around April 2014. However, the Red Planet data showed a nine-month lag between the maximum number of sunspots and the minimum in cosmic ray detection around Mars.
"Previous studies have suggested that there is a delay of several months between solar activity and the behavior of cosmic rays at the Earth and at Mars," Futaana said. "Our results appear to confirm this and also provide further evidence that Solar Cycle 24 was a bit unusual, perhaps due to the long solar minimum between Cycle 23 and 24, or the relatively low activity during Cycle 24."
(Currently, Solar Cycle 25 is ramping up, with its peak expected to occur around 2025.)
The data collected by Venus Express has been more challenging to analyze than data from Earth or Mars because the way data was processed aboard the spacecraft changed in 2010.
In addition, although the Venus Express and the Mars Express both use the same instrument to count cosmic rays, the individual ASPERA plasma sensors are tailored to the very different environments of the solar system's second and fourth planets. This prevented a direct comparison between cosmic ray counts at the two planets.
Futaana and his team also examined how cosmic ray detections varied over just hours, as each spacecraft orbited its planet. This work determined that cosmic rays can't reach an area behind Mars that's larger than the planet itself, although the researchers aren't sure yet why.
The team's research is described in a paper published on Monday (Dec. 5) in the Astrophysical Journal.
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