Earth could react more strongly than expected to the most intense solar storms.
A study published in Nature challenges the existence of a 'cap' that seemed to limit the geomagnetic disturbances induced by the most intense storms. This result directly affects risk assessment for satellites, communications, and some power grids.
When the solar wind became very powerful, the electrical currents in the upper atmosphere paradoxically seemed to stop increasing. The magnetosphere thus appeared to absorb an additional portion of the received energy.
Image credit: ESA & NASA
Such a cap would have been rather reassuring. It would have meant that beyond a certain intensity, a stronger solar storm would not proportionally produce more effects near Earth, on the ground or in orbit. Several physical mechanisms had been proposed to explain this apparent protection, without consensus.
A team led by Nithin Sivadas, from NASA's Goddard Space Flight Center, now estimates that this cap may mainly come from measurement uncertainties. The solar wind is generally observed near the Lagrange point L1, located about 1.5 million kilometers upstream of Earth.
Between this point and the magnetosphere, the speed, direction, and intensity of the solar wind can change. Its travel time also remains uncertain. An extreme value measured at L1 can therefore be mistakenly associated with a terrestrial disturbance produced by a solar wind that is actually less intense near Earth.
The researchers corrected this statistical bias. They also studied more than a million measurements taken closer to Earth by several missions, including MMS and THEMIS. After correction, the geomagnetic response continues to increase with solar wind strength over the range covered by the data.
The absence of an apparent cap is therefore not good news for infrastructure protection. It indicates that the magnetosphere does not automatically reduce the consequences of extreme events. According to the study, the impact could reach about twice the estimates from previous models for certain extrapolated intensities.
This result does not mean, however, that effects can increase without any physical limit. The data become insufficient beyond a solar wind electric field of about 15 mV/m. A cap therefore remains possible during even more powerful events, but no statistical evidence currently establishes it.
Space weather models will need to better incorporate this uncertainty. Additional observations made near the magnetosphere during strong storms are needed. They will help size the protections for satellites, navigation systems, radio communications, and exposed power grids.