Hidden in the Noise
Yoshifumi Futaana (Swedish Institute of Space Physics) shows how asking unusual questions can lead to ground-breaking science.
Read article in the fully formatted PDF of the Europlanet Magazine.
My scientific philosophy is that I try to answer the questions that people have never even thought about.
Noise in data is usually considered as junk or garbage. People don’t take it seriously. In line with my philosophy, we decided to try and look at noise data from instruments that we built at the Swedish Institute of Space Physics for the Mars Express and Venus Express missions. We developed an algorithm to extract information on cosmic rays in noise data from the ASPERA plasma sensors. We were successful and managed to extract valuable scientific information that we have now published in a paper in the Astrophysical Journal.1
Galactic cosmic rays are particles, travelling at almost the speed of light, that originate outside our Solar System. They are important because they can cause instrument errors in space systems, as well as destroying DNA in humans. So, galactic cosmic rays are a threat for future exploration in space and for all human activities in space.
The European Space Agency’s Mars Express mission was launched in 2003 and remains in service around the Red Planet. Its sister mission, Venus Express, operated from 2006 until 2014. We took the 17-year dataset from Mars and 8-year dataset from Venus, together with Earth-based cosmic ray measurements from the Thule neutron monitor in Greenland, and compared them with the number of sunspots visible on the surface of the Sun. We used the median value of cosmic ray counts over 3-month periods to minimise the influence of sporadic solar activity, such as flares or coronal mass ejections. We were able to show how cosmic ray counts are suppressed during peaks of activity in the 11-year solar cycle.
All the datasets showed a decrease in the number of cosmic ray detections as the peak in activity for Solar Cycle 24 was reached. In particular, 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 the Earth and at 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.
As well as the decadal-long relationship with the solar cycle, we looked at how cosmic ray detections varied over the short timescales of an orbit. To our surprise, we found that the area protected from cosmic rays behind Mars is more than 100 kilometres wider than the planet’s actual radius. To understand why this blocked area should be so large will need further study.
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.
- Futaana et al, 2022. DOI: doi.org/10.3847/1538-4357/ac9a49