Abstract
Cold plasma of ionospheric origin has recently been found to be a much larger contributor to the magnetosphere of Earth than expected1,2,3. Numerous competing mechanisms have been postulated to drive ion escape to space, including heating and acceleration by wave–particle interactions4 and a global electrostatic field between the ionosphere and space (called the ambipolar or polarization field)5,6. Observations of heated O+ ions in the magnetosphere are consistent with resonant wave–particle interactions7. By contrast, observations of cold supersonic H+ flowing out of the polar ionosphere8,9 (called the polar wind) suggest the presence of an electrostatic field. Here we report the existence of a +0.55 ± 0.09 V electric potential drop between 250 km and 768 km from a planetary electrostatic field (E∥⊕ = 1.09 ± 0.17 μV m−1) generated exclusively by the outward pressure of ionospheric electrons. We experimentally demonstrate that the ambipolar field of Earth controls the structure of the polar ionosphere, boosting the scale height by 271%. We infer that this increases the supply of cold O+ ions to the magnetosphere by more than 3,800%, in which other mechanisms such as wave–particle interactions can heat and further accelerate them to escape velocity. The electrostatic field of Earth is strong enough by itself to drive the polar wind9,10 and is probably the origin of the cold H+ ion population1 that dominates much of the magnetosphere2,3.
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Data availability
Endurance ephemeris data and all science data presented in this article are available at the Space Physics Data Facility of NASA (https://spdf.gsfc.nasa.gov/data_orbits.html) through the Coordinated Data Analysis Web (CDAWeb) tool (https://cdaweb.gsfc.nasa.gov/) by selecting ‘Sounding Rockets’ from the data sources.
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Acknowledgements
We thank the 100+ strong team of engineers, scientists and technicians who made the Endurance rocketship mission a success. We thank A. P. Collinson for the useful discussions in preparing and editing the paper. Endurance was funded through the NASA grant 80NSSC19K1206. EISCAT support was supported through the National Environment Research Council grant NE/R017000X/1. EISCAT is an international association supported by research organizations in China (CRIRP), Finland (SA), Japan (NIPR and ISEE), Norway (NFR), Sweden (VR) and the UK (UKRI).
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The Endurance mission and its overall methodology were conceived by G.A.C. and A. Glocer, who acquired the funding, administered the science team and drafted this paper. The instruments were developed by G.A.C., R.P., A. Barjatya, R. Clayton, A. Breneman, J.C., R.M., L.D., E.R., D.S., L.N., P.U., T.C., A. Ghalib, H.V., N.G. and S.D. Data analysis was performed by G.A.C., A. Glocer, R.P., A. Barjatya, R. Conway, A. Breneman, J.C., F.E., D.M., S.I., H.A., L.D., A.K., D.S., S.X. and J.M.
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Extended data figures and tables
Extended Data Fig. 1 Layout of the Endurance spacecraft showing scientific instruments used in this study.
View from above looking aft. Magnetic field into page on upleg and out of page on downleg.
Extended Data Fig. 2 Example spectra from the Photoelectron Spectrometer.
Data calibrated but uncorrected for spacecraft potential. a, PES Scan 72 showing standard resolution (black) and high resolution (red). b, PES Scan 72 zoomed in to the He-II photopeaks showing a gaussian fit (blue) to the primary N2 A2Πu dominated photopeak. c,d, The same for PES Scan 38.
Extended Data Fig. 3 Conversion from peak energy of photopeaks to planetary potential drop below Endurance.
Upleg, top panels; downleg, bottom panels. a,d, Peak energy of N2 A2Πu dominated photopeak as measured. b,e, Energy of photopeak after correction for S/C potential from SLP. c,f, Potential drop below Endurance (as Fig. 2a,b).
Extended Data Fig. 4 Measurements by the Swept Langmuir Probe.
Area denotes ±1σ error. a, Colour-coded timeline of Endurance mission (as per Fig. 1a, Fig. 2a). b, Altitude versus time. c, Total Electron density (cm−3). d, Electron temperature (K). e, Potential difference between Endurance and ambient plasma. The periodic (70 s) firing of the ACS thrusters (amber, panel a) temporarily perturbed the plasma environment around the spacecraft. The resulting erroneous measurements by SLP have been cut from the dataset.
Extended Data Fig. 6 Radar measurements from the EISCAT Radar in black compared to in situ measurements by the SLP instrument in gold.
a,b, Plasma density; c,d, Electron temperature. e,f, Ion temperature; g,h, Ion velocity. These plots were made by time-averaging measurements from the upleg and downleg portion of the flight. Error bars represent the standard deviation. EISCAT data were truncated above 500 km in Fig. 3 owing to the large error bars but are shown here in full. The good agreement between independent measurements of \({n}_{e}\) and \({t}_{e}\) by EISCAT and SLP give good confidence in our SLP data analysis.
Extended Data Fig. 7 Geomagnetic activity for the two days surrounding the launch of Endurance.
a, Planetary KP index; b, planetary AP index. Both indexes show low activity (G0).
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Collinson, G.A., Glocer, A., Pfaff, R. et al. Earth’s ambipolar electrostatic field and its role in ion escape to space. Nature 632, 1021–1025 (2024). https://doi.org/10.1038/s41586-024-07480-3
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DOI: https://doi.org/10.1038/s41586-024-07480-3
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