Using numerical simulations, we analyze the time evolution of the pitch-angle distribution of 500 MeV and 1 GeV solar protons, released impulsively near the Sun, at 1 au. The numerical model solves the equations of motion of an ensemble of particles that move in both the average Parker spiral field and a large-scale turbulent interplanetary magnetic field (IMF). Our model also includes the heliospheric current sheet (HCS). The focus of this study is to determine the effect of the large-scale turbulent IMF on the pitch-angle distribution of GV-rigidity protons and its time variations in terms of understanding variations in ground-level enhancement (GLE) events. Our particular interest is to explain the two distinct opposite-directed fluxes of the unusual event on 1989 October 22 (GLE#44). The results show that by adding the large-scale turbulence to the average Parker IMF, the pitch-angle distribution at 1 au depends strongly on the observer’s location relative to the release location of the particles at the Sun. Even a 0.2° displacement in latitude or longitude leads to a significant change in the observed distribution and/or its variation in time. We find that there are some observer locations for which the distinct sunward and antisunward fluxes coexist at certain times of the events. We also find that the HCS has an important effect. For instance, even in locations of poor magnetic connection with the release location at the Sun, but near the HCS, there can be two fluxes moving in different directions at the same time.
Prevalence of magnetic reconnection in the near-Sun heliospheric current sheet
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.
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- Award ID(s):
- 1805829
- NSF-PAR ID:
- 10293728
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 650
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A13
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Wind spacecraft measurements are analyzed to obtain a current sheet (CS) normal width
d csdistribution of 3374 confirmed magnetic reconnection exhausts in the ecliptic plane of the solar wind at 1 au. Thed csdistribution displays a nearly exponential decay from a peak atd cs= 25d i to a median atd cs= 85d i and a 95th percentile atd cs= 905d i with a maximum exhaust width atd cs= 8077d i . A magnetic fieldθ -rotation angle distribution increases linearly from a relatively few high-shear events toward a broad peak at 35° <θ < 65°. The azimuthalϕ angles of the CS normal directions of 430 thickd cs≥ 500d i exhausts are consistent with a dominant Parker-spiral magnetic field and a CS normal along the ortho-Parker direction. The CS normal orientations of 370 kinetic-scaled cs< 25d i exhausts are isotropic in contrast, and likely associated with Alfvénic solar wind turbulence. We propose that the alignment of exhaust normal directions from narrowd cs∼ 15–25d i widths to well beyondd cs∼ 500d i with an ortho-Parker azimuthal direction of a large-scale heliospheric current sheet (HCS) is a consequence of CS bifurcation and turbulence within the HCS exhaust that may trigger reconnection of the adjacent pair of bifurcated CSs. The proposed HCS-avalanche scenario suggests that the underlying large-scale parent HCS closer to the Sun evolves with heliocentric distance to fracture into many, more or less aligned, secondary CSs due to reconnection. A few wide exhaust-associated HCS-like CSs could represent a population of HCSs that failed to reconnect as frequently between the Sun and 1 au as other HCSs. -
Aims. We analyse particle, radio, and X-ray observations during the first relativistic proton event of solar cycle 25 detected on Earth. The aim is to gain insight into the relationship between relativistic solar particles detected in space and the processes of acceleration and propagation in solar eruptive events. Methods. To this end, we used ground-based neutron monitor measurements of relativistic nucleons and space-borne measurements of electrons with similar speed to determine the arrival times of the first particles at 1 AU and to infer their solar release times. We compared the release times with the time histories of non-thermal electrons in the solar atmosphere and their escape to interplanetary space, as traced by radio spectra and X-ray light curves and images. Results. Non-thermal electrons in the corona are found to be accelerated in different regions. Some are confined in closed magnetic structures expanding during the course of the event. Three episodes of electron escape to the interplanetary space are revealed by groups of decametric-to-kilometric type III bursts. The first group appears on the low-frequency side of a type II burst produced by a coronal shock wave. The two latter groups are accompanied at higher frequencies by bursts with rapid drifts to both lower and higher frequencies (forward- or reverse-drifting bursts). They are produced by electron beams that propagate both sunward and anti-sunward. The first relativistic electrons and nucleons observed near Earth are released with the third group of type III bursts, more than ten minutes after the first signatures of non-thermal electrons and of the formation of the shock wave in the corona. Although the eruptive active region is near the central meridian, several tens of degrees east of the footpoint of the nominal Parker spiral to the Earth, the kilometric spectrum of the type III bursts and the in situ detection of Langmuir waves demonstrate a direct magnetic connection between the L1 Lagrange point and the field lines onto which the electron beams are released at the Sun. Conclusions. We interpret the forward- and reverse-drifting radio bursts as evidence of reconnection between the closed expanding magnetic structures of an erupting flux rope and ambient open magnetic field lines. We discuss the origin of relativistic particles near the Earth across two scenarios: (1) acceleration at the CME-driven shock as it intercepts interplanetary magnetic field lines rooted in the western solar hemisphere and (2) an alternative where the relativistic particles are initially confined in the erupting magnetic fields and get access to the open field lines to the Earth through these reconnection events.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1051/0004-6361/202039863
