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Probing the energetic particle environment near the Sun

Abstract

NASA’s Parker Solar Probe mission1 recently plunged through the inner heliosphere of the Sun to its perihelia, about 24 million kilometres from the Sun. Previous studies farther from the Sun (performed mostly at a distance of 1 astronomical unit) indicate that solar energetic particles are accelerated from a few kiloelectronvolts up to near-relativistic energies via at least two processes: ‘impulsive’ events, which are usually associated with magnetic reconnection in solar flares and are typically enriched in electrons, helium-3 and heavier ions2, and ‘gradual’ events3,4, which are typically associated with large coronal-mass-ejection-driven shocks and compressions moving through the corona and inner solar wind and are the dominant source of protons with energies between 1 and 10 megaelectronvolts. However, some events show aspects of both processes and the electron–proton ratio is not bimodally distributed, as would be expected if there were only two possible processes5. These processes have been very difficult to resolve from prior observations, owing to the various transport effects that affect the energetic particle population en route to more distant spacecraft6. Here we report observations of the near-Sun energetic particle radiation environment over the first two orbits of the probe. We find a variety of energetic particle events accelerated both locally and remotely including by corotating interaction regions, impulsive events driven by acceleration near the Sun, and an event related to a coronal mass ejection. We provide direct observations of the energetic particle radiation environment in the region just above the corona of the Sun and directly explore the physics of particle acceleration and transport.

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Fig. 1: Observations of energetic particles during orbits 1 and 2.
Fig. 2: Recurring corotating energetic particle events.
Fig. 3: CME-related low-energy event and subsequent high-energy event.
Fig. 4: Pair of impulsive events near second perihelion.

Data availability

All data used in this study is available to the public via NASA’s Space Physics Data Facility (SPDF) at https://spdf.gsfc.nasa.gov/.

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Acknowledgements

We are indebted to everyone who helped make the PSP mission possible. In particular, we thank all of the scientists, engineers, technicians, and administrative support people across all of the ISIS institutions that produced and supported the ISIS instrument suite and support its operations and the scientific analysis of its data. This work was supported as a part of the PSP mission under contract NNN06AA01C. S.D.B. acknowledges the support of the Leverhulme Trust Visiting Professorship programme and A.P.R. acknowledges financial support from the ANR project COROSHOCK ANR-17-CE31-0006-01 and from the ERC project SLOW_SOURCE – DLV-819189.

Author information

Authors and Affiliations

Authors

Contributions

D.J.M. is ISIS Principal Investigator (PI) and led the data analysis and writing of the study. E.R.C. is ISIS Deputy PI, helped develop EPI-Hi, and participated in the data analysis. C.M.S.C. helped develop EPI-Hi and participated in the data analysis. A.C.C. helped develop EPI-Hi and participated in the data analysis. A.J.D. helped develop EPI-Hi and participated in the data analysis. M.I.D. participated in the data analysis. J.G. participated in the data analysis. M.E.H. helped develop EPI-Lo and participated in the data analysis. C.J.J. produced Figs. 3, 4 and participated in the data analysis. S.M.K. participated in the data analysis. A.W.L. helped develop EPI-Hi and participated in the data analysis. R.A.L. helped develop EPI-Hi and participated in the data analysis. O.M. participated in the data analysis. W.H.M. participated in the data analysis. R.L.M. led the development of EPI-Lo and participated in the data analysis. R.A.M. helped develop EPI-Hi and participated in the data analysis. D.G.M. helped develop EPI-Lo and participated in the data analysis. A.P. participated in the data analysis. J.S.R. helped develop EPI-Hi and participated in the data analysis. E.C.R. participated in the data analysis. N.A.S. led the development of the ISIS Science Operations Center and participated in the data analysis. E.C.S. helped develop EPI-Hi and participated in the data analysis. J.R.S. led the development of the analysis tool, produced Figs. 1, 2, and participated in the data analysis. M.E.W. led the development of EPI-Hi and participated in the data analysis. S.D.B. is FIELDS PI and participated in the data analysis. J.C.K. is SWEAP PI and participated in the data analysis. A.W.C. helped develop SWEAP and participated in the data analysis. K.E.K. helped develop SWEAP and participated in the data analysis. R.J.M. helped develop FIELDS and participated in the data analysis. M.P. helped develop FIELDS and participated in the data analysis. M.L.S. helped develop SWEAP and participated in the data analysis. A.P.R. led the CME simulation work and participated in the data analysis.

Corresponding author

Correspondence to D. J. McComas.

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Competing interests

The authors declare no competing interests.

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Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Peer review information Nature thanks Hazel Bain and Monica Laurenza for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Fig. 1 Viewing geometry and observation of coronal mass ejection.

a, A view of the ecliptic plane from solar north at 14:00 10 November 2018 ut showing the relative positions of STEREO-A and PSP. The dashed curves, from innermost to outermost, represent the orbits of Mercury, Venus and Earth. The red area shows the field of view of the COR-2 instrument onboard STEREO-A. A CME off the east limb of the Sun as viewed from STEREO-A would be roughly propagating towards PSP. This CME very gradually entered the field of view of COR-2, part of the SECCHI suite of imaging instruments26 aboard the STEREO spacecraft. b, A running-difference image of the CME taken at 02:39 ut on 11 November 2018 by COR-2A (a visible-light coronagraph), extending in the plane of the sky from 2R to 15R, provided images during the entire acceleration phase of the CME. This CME entered COR-2A around 18:00 ut on 10 November 2018 and transited through the COR-2 field of view over about 12 h.

Extended Data Fig. 2 CME model and comparison to magnetic field data.

a, The same as in Extended Data Fig. 1b but with superposed fitted flux-rope shape of the CME at 02:39 11 November 2018 ut when the CME had passed halfway through the COR-2A field of view. The CME is very weak and no shock–sheath structure can be identified in these images. The typical aspect of the CME in the image results from the line-of-sight integration of plasma distribution on a bent toroid such that its major axis is located in a plane containing the observing spacecraft (see very similar events in refs. 27,28). b, The position (red) and speed (blue) of the apex of the flux-rope model was derived by iteratively comparing each synthetic image produced by the three-dimensional model with each available COR-2A image. A functional form (arctangent) was imposed for the flux rope’s varying speed. The fitted CME structure assumed here is a bent toroid with an exponential increase of its cross-sectional area from foot point to apex as in ref. 29. The speed was derived by fitting a hyperbolic tangent to the modelled CME position. The speed increases rapidly from under 100 km s−1 at 18:00 10 November 2018 ut to over 350 km s−1 when it exited the COR-2A field of view at around 6:00 ut on 11 November. c, An internal magnetic field structure was expressed analytically inside the envelope of the fitted CME (smooth curves) as in ref. 30, but here keeping a simple circular cross-section of the flux rope. By propagating this flux rope at a constant speed of 380 km s−1 from the time it exits the COR-2 field of view, we predict the CME reaches PSP on 12 November 2018. The predicted arrival time and the magnetic properties of the CME (thick smooth line) are in good agreement with those measured in situ by the FIELDS (magnetic field data; thin lines) and SWEAP instruments (not shown). We therefore conclude that the fitting procedure presented here provides a good description of the evolution of the CME from the upper corona to PSP.

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McComas, D.J., Christian, E.R., Cohen, C.M.S. et al. Probing the energetic particle environment near the Sun. Nature 576, 223–227 (2019). https://doi.org/10.1038/s41586-019-1811-1

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