Phys.org's Team identifies low-energy solar particles from beyond Earth near the Sun links to the open access ApJ article Properties of Suprathermal-through-energetic He Ions Associated with Stream Interaction Regions Observed over the Parker Solar Probe's First Two Orbits.

It quotes "Dr. Mihir Desai, a mission co-investigator on the Integrated Science Investigation of the Sun (IS☉IS) instrument suite, a multi-institutional project led by Principal Investigator Prof. Dave McComas of Princeton University.. IS☉IS consists of two instruments, Energetic Particle Instrument-High (EPI-Hi) and Energetic Particle Instrument-Low (EPI-Lo)" as saying:

With EPI-Lo, we were able to measure extremely low-energy particles unexpectedly close to the solar environment. We considered many explanations for their presence, but ultimately determined they are the smoking gun pointing to interactions between slow- and fast-moving regions of the solar wind that accelerate high-energy particles from beyond the orbit of Earth. Some of those travel back toward the Sun, slowing against the tide of the outpouring solar wind but still retaining surprisingly high energies.

I'm having trouble parsing this very long single sentence. Why are these "extremely low-energy particles" also "still retaining surprisingly high energies"?

The Phys.org article talks about Stream Interaction Regions (SIRs) and the formation of turbulent corotating interaction regions (CIRs) that produce shock waves and accelerated particles, not unlike those produced by CMEs.

Question: I think it is saying that particles from the Sun go beyond Earth and then are turned around by turbulence and move back into close proximity of the Sun, at which time they have "extremely low-energy" for some reason. Is that at least a rough understanding of what's happening?

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    $\begingroup$ Just a guess, but is the "surprisingly high" signalled by the fact that they got back that close to the Sun at all? $\endgroup$ Commented Feb 16, 2021 at 10:59
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    $\begingroup$ Mihir was just trying to say that they expected if any particles came from outside the Earth's orbit and managed to get back in, close to the Sun, that they would be energized in the process otherwise could not make it. $\endgroup$ Commented Aug 31, 2021 at 15:00
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    $\begingroup$ @honeste_vivere I love simple, clear answers! If you can add one, that would be great. Anything you can add about how they managed to get back in would be wonderful! $\endgroup$
    – uhoh
    Commented Aug 31, 2021 at 15:04

1 Answer 1


If I understand your question correctly, it is about EPI-Lo which a part of the Integrated Science Investigation of the Sun (ISʘIS) . The picture is also taken from there:

enter image description here

EPI-Lo stands for Energetic Particle Instrument-Low energy which indeed sounds confusing at first sight. The essential definition can be found in the instrument paper:

EPI-Lo measures ions and ion composition from ∼20 keV/nucleon–15 MeV total energy and electrons from ∼25–1000 keV. EPI-Hi measures ions from ∼1–200 MeV/nucleon and electrons from ∼0.5–6 MeV.

The paper also addresses the scientific background of the measurements of SEPs, which are defined to have the following energy range:

They consist of protons, electrons and HZE ions with energy ranging from a few tens of keV to many GeV.

In short: The EPI-Lo measures the lower part of the SEP events.

Concerning the newsworthyness, I can only guess. Why SEPs are worth studying is nicely stated at Wikipedia:

They are of particular interest and importance because they can endanger life in outer space (especially particles above 40 MeV).

Solar energetic particles can originate either from a solar-flare site or by shock waves associated with coronal mass ejections (CMEs). However, only about 1% of CMEs produce strong SEP events.

I hope that my remarks answer (at least partially) your questions.

  • $\begingroup$ This is helpful background information, thanks! It doesn't answer the question but it may get us a step closer. $\endgroup$
    – uhoh
    Commented Feb 16, 2021 at 11:10
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    $\begingroup$ @uhoh I added some lines about the news-worthyness, but that is just a guess. It does not explain what the smoking-gun-sentence was about. $\endgroup$
    – B--rian
    Commented Feb 16, 2021 at 11:14

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