Helium isotopes precipitation in the Earth’s upper atmosphere

Understanding the helium budget of the Earth’s atmosphere is a longstanding challenge in atmospheric science. In the Earth's atmosphere, the abundance and isotopic composition of helium are shaped by interactions with both the solid Earth and outer space. A recent observation of a temporary excess of ³He in the polar atmosphere has been attributed to solar flares. The solar wind, which has an average helium content of ~5% He⁺⁺ and a ³He/⁴He ratio of ~2350—much higher than that of the atmosphere—precipitates mainly in the auroral zone, an oval-shaped region located at high latitudes.

The Earth is a magnetized planet surrounded by a magnetosphere, which acts as an interface between the solar wind and the Earth’s atmosphere. The dayside auroral zone, connected to the magnetospheric cusp, provides a direct path for solar wind precipitation. However, in the remainder of the auroral zone, ion precipitation consists of a mixture of solar wind ions and ionospheric ions that are returned to the Earth's atmosphere.

We use 11 years of ion precipitation measurements from the DMSP satellites, combined with in-situ He measurements in the solar wind (from the OMNI database) and empirical formulas derived from satellite observations of the Earth's magnetosphere, to estimate the ³He and ⁴He precipitation rates in the Earth's upper atmosphere. We analyze yearly averages and peak fluxes, considering separately the contributions from the dayside auroral zone and the rest of the auroral zone. Additionally, we discuss the locations of He precipitation regions and the effects of solar and geomagnetic activity on the precipitating He flux.

Our results show that auroral precipitation is a significant source of atmospheric ³He, comparable to outgassing from the Earth’s core. However, they suggest that solar-flare-associated ³He precipitation alone is likely insufficient to explain the observed polar excess.