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Abstract We report observations of direct evidence of energetic protons being accelerated above ∼400 keV within the reconnection exhaust of a heliospheric current sheet (HCS) crossing by NASA’s Parker Solar Probe (PSP) at a distance of ∼16.25 solar radii (Rs) from the Sun. Inside the exhaust, both the reconnection-generated plasma jet and the accelerated protons up to ∼400 keV propagated toward the Sun, unambiguously establishing their origin from HCS reconnection sites located antisunward of PSP. Within the core of the exhaust, PSP detected stably trapped energetic protons up to ∼400 keV, which is ≈1000 times greater than the available magnetic energy per particle. The differential energy spectrum of the accelerated protons behaved as a pure power law with spectral index of ∼−5. Supporting simulations using thekglobalmodel suggest that the trapping and acceleration of protons up to ∼400 keV in the reconnection exhaust are likely facilitated by merging magnetic islands with a guide field between ∼0.2 and 0.3 of the reconnecting magnetic field, consistent with the observations. These new results, enabled by PSP’s proximity to the Sun, demonstrate that magnetic reconnection in the HCS is a significant new source of energetic particles in the near-Sun solar wind. Our findings of in situ particle acceleration via magnetic reconnection at the HCS provide valuable insights into this fundamental process, which frequently converts the large magnetic field energy density in the near-Sun plasma environment and may be responsible for heating the Sun’s atmosphere, accelerating the solar wind, and energizing charged particles to extremely high energies in solar flares.
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The near-Sun streamer belt solar wind: turbulence and solar wind acceleration
The fourth orbit of Parker Solar Probe (PSP) reached heliocentric distances down to 27.9 R ⊙ , allowing solar wind turbulence and acceleration mechanisms to be studied in situ closer to the Sun than previously possible. The turbulence properties were found to be significantly different in the inbound and outbound portions of PSP’s fourth solar encounter, which was likely due to the proximity to the heliospheric current sheet (HCS) in the outbound period. Near the HCS, in the streamer belt wind, the turbulence was found to have lower amplitudes, higher magnetic compressibility, a steeper magnetic field spectrum (with a spectral index close to –5/3 rather than –3/2), a lower Alfvénicity, and a ‘1∕ f ’ break at much lower frequencies. These are also features of slow wind at 1 au, suggesting the near-Sun streamer belt wind to be the prototypical slow solar wind. The transition in properties occurs at a predicted angular distance of ≈4° from the HCS, suggesting ≈8° as the full-width of the streamer belt wind at these distances. While the majority of the Alfvénic turbulence energy fluxes measured by PSP are consistent with those required for reflection-driven turbulence models of solar wind acceleration, the fluxes in the streamer belt are significantly lower than the model predictions, suggesting that additional mechanisms are necessary to explain the acceleration of the streamer belt solar wind.
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- Award ID(s):
- 1752827
- PAR ID:
- 10319705
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 650
- ISSN:
- 0004-6361
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)During three of its first five orbits around the Sun, Parker Solar Probe (PSP) crossed the large-scale heliospheric current sheet (HCS) multiple times and provided unprecedented detailed plasma and field observations of the near-Sun HCS. We report the common detections by PSP of reconnection exhaust signatures in the HCS at heliocentric distances of 29.5–107 solar radii during encounters 1, 4, and 5. Both sunward and antisunward-directed reconnection exhausts were observed. In the sunward reconnection exhausts, PSP detected counterstreaming strahl electrons, indicating that HCS reconnection resulted in the formation of closed magnetic field lines with both ends connected to the Sun. In the antisunward exhausts, PSP observed dropouts of strahl electrons, consistent with the reconnected HCS field lines being disconnected from the Sun. The common detection of reconnection in the HCS suggests that reconnection is almost always active in the HCS near the Sun. Furthermore, the occurrence of multiple long-duration partial crossings of the HCS suggests that HCS reconnection could produce chains of large bulges with spatial dimensions of up to several solar radii. The finding of the prevalence of reconnection in the HCS is somewhat surprising since PSP has revealed that the HCS is much thicker than the kinetic scales required for reconnection onset. The observations are also in stark contrast with the apparent absence of reconnection in most of the small-scale and much more intense current sheets encountered near perihelia, many of which are associated with “switchbacks”. Thus, the PSP findings suggest that large-scale dynamics, either locally in the solar wind or within the coronal source of the HCS (at the tip of helmet streamers), plays a critical role in triggering reconnection onset.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1051/0004-6361/202039872
