Earth's ionospheric outflow dominated by hidden cold plasma

The Earth constantly loses matter, mostly in the form of H C and O+ ions, through various outflow processes from the upper atmosphere and ionosphere. Most of these ions are cold (below 1 eV in thermal energy), but can still escape and travel farther out along the magnetic field lines into the magnetospheric tail lobes(1,2). The outflow has previously been measured close to the Earth(3-7). To understand what fraction does not return but instead escapes, the measurements should be conducted at larger geocentric distances. However, at high altitudes the cold ions are normally invisible to spacecraft measurements, because the potential of a sunlit spacecraft exceeds the equivalent energy of the ions(8,9). Here we show that cold ions dominate in both flux and density in the distant magnetotail lobes, using a new measurement technique on the Cluster spacecraft(10,11). The total loss of cold hydrogen ions from the planet is inferred to be of the order of 10(26) s(-1), which is larger than the previously observed more energetic outflow(12,13). Quantification and insight of the loss processes of the Earth's atmosphere and ionosphere are also important for understanding the evolution of atmospheres on other celestial bodies.