An international team of researchers has used cosmic rays hidden in spacecraft data to showcase the influence of the Sun’s activity across our inner Solar System.
Data from ESA’s long-serving twin missions, Mars Express and Venus Express, has revealed that cosmic ray counts are suppressed during peaks of activity in the 11-year solar cycle. The ASPERA plasma sensor, which was carried by both spacecraft, has shown that the number of sunspots visible on the surface of the Sun highlights the relationship between cosmic ray counts and the solar cycles at Mars and Venus.
The study, ‘Galactic Cosmic Rays at Mars and Venus: Temporal Variations from Hours to Decades Measured as the Background Signal of Onboard Microchannel Plates,’ has been published today in the Astrophysical Journal.
What are cosmic rays and what can they tell us about the solar cycle?
Cosmic rays are particles that originate outside our Solar System, travelling at almost the speed of light. They are a dangerous form of high-energy radiation that can cause electronic failures in spacecraft and damage the DNA of humans in space.
The researchers analysed the relationship between cosmic rays and the solar cycle and also looked at how cosmic ray detections varied over the short timescales of an orbit. The team discovered that the area protected from cosmic rays behind Mars is more than 100km wider than the planet’s actual radius. It is not yet clear why this blocked area is so large.
“The study shows the range of valuable insights that can be derived from what is actually background count information collected by the ASPERA instruments. 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,” said Dr Yoshifumi Futaana of the Swedish Institute of Space Physics.
Using data from Mars Express and Venus Express
The researchers compared data from Mars Express, which launched in 2003 and remains in service, and from Venus Express, which operated from 2006 to 2014, to discover more about the relationship between cosmic rays and the solar cycle. This data was compared with Earth-based cosmic ray measurements from the Thule neutron monitor in Greenland.
The scientists took the median value of cosmic ray counts over three-month periods to minimise the influence of sporadic solar activity, such as flares or coronal mass ejections. The databases of background radiation counts extracted for the study have been published and can be accessed through the Europlanet SPIDER planetary space weather service.
All the datasets revealed a decrease in the number of cosmic ray detections as the peak in activity for Solar Cycle 24 was reached. The Mars Express data and the observations from Earth showed very similar features; however, there was an apparent lag of around nine months between the maximum number of sunspots and the minimum in cosmic ray detections at Mars.
“Previous studies have suggested that there is a delay of several months between solar activity and the behaviour of cosmic rays at Earth and Mars. 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,” said Dr Futaana.
Challenges to comparing the data from the twin missions
Analysing the data of Venus Express has been complicated by changes in the way onboard processing was carried out from 2010 and beyond. As well as this, even though the ASPERA instruments carried by Mars Express and Venus Express were based on a common design, they were each tailored to the very different planetary environments in which they operated. Therefore, a direct comparison of cosmic ray fluxes at Mars and Venus is not possible using available datasets.
“The use of background counts to study the interaction of cosmic rays and high energy particles with planetary missions is relatively new. However, obtaining this information shows potential as a powerful tool, for example, in protecting the upcoming JUpiter Icy moon Explorer (JUICE) mission of the European Space Agency, which will explore the harsh environment around Jupiter’s icy moons,” said Nicolas Andre of the Institut de Recherche en Astrophysique et Planétologie (IRAP) in Toulouse, France, coordinator of the Europlanet SPIDER service and co-author of this study.
