The bubble-like shape of the heliosphere observed by Voyager and Cassini

Abstract

For more than five decades, the shape and interactions of the heliosphere with the local interstellar medium have been discussed in the context of two competing models, posited in 19611: a magnetosphere-like heliotail and a more symmetric bubble shape. Although past models broadly assumed the magnetosphere-like concept, the accurate heliospheric configuration remained largely undetermined due to lack of measurements. In recent years, however, Voyagers 1 and 2 (V1 and V2) crossed the termination shock — the boundary where the solar wind drops — north and south of the ecliptic plane at 94 au2,3 and 84 au4 in 2004 and 2007, respectively, and discovered the reservoir of ions and electrons that constitute the heliosheath, while Cassini remotely imaged the heliosphere5 for the first time in 2003. Here we report 5.2–55 keV energetic neutral atom (ENA) global images of the heliosphere obtained with the Cassini/Ion and Neutral Camera (INCA). We compare them with 28–53 keV ions measured within the heliosheath by the low-energy charged particle (LECP) experiment onboard V1 and V2 over an 11-year period (2003–2014). We show that the heliosheath ions are the source of ENA. These observations also demonstrate that the heliosphere responds promptly, within ~2–3 years, to outward propagating solar wind changes in both the nose and tail directions. These results, together with the V1 measurement of a ~0.5 nT interstellar magnetic field6 and the enhanced ratio between particle pressure and magnetic pressure in the heliosheath7, strongly suggest a diamagnetic bubble-like heliosphere with few substantial tail-like features. Our results are consistent with recent modelling811.

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Figure 1: Conceptual models of the global heliosphere.
Figure 2: ENAs and ions in the heliosheath compared with SSN.
Figure 3: ‘Ground truth’ ENA-ion intensities in the heliosphere.

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Acknowledgements

This work was supported at JHU/APL by NASA under contract NAS5 97271 and NNX07AJ69G and by subcontract at the Office for Space Research and Technology. The authors are grateful to M. Kusterer for software development and assistance with the INCA data processing and Dogfish graphic designers (http://www.dogfish.gr/en/) for assistance with graphics in Fig. 1a.

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All authors were actively involved in all aspects of this manuscript. K.D. contributed most of the text and carried out most of the data analysis; S.M.K., D.G.M. and E.C.R. contributed to the text and provided most of the theory and interpretations; R.B.D. and S.M.K. performed the Voyager ion data analysis; D.G.M. oversaw the Cassini/INCA data.

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Correspondence to K. Dialynas.

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Supplementary Table 1, Supplementary Figures 1–3 and Supplementary References. (PDF 756 kb)

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Dialynas, K., Krimigis, S., Mitchell, D. et al. The bubble-like shape of the heliosphere observed by Voyager and Cassini. Nat Astron 1, 0115 (2017). https://doi.org/10.1038/s41550-017-0115

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