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1. The relativistic solar particle event on 28 October 2021: Evidence of particle acceleration within and escape from the solar corona

   https://doi.org/10.1051/0004-6361/202243903  

   Klein, Karl-Ludwig ; Musset, Sophie ; Vilmer, Nicole ; Briand, Carine ; Krucker, Säm ; Francesco Battaglia, Andrea ; Dresing, Nina ; Palmroos, Christian ; Gary, Dale E. ( July 2022 , Astronomy & Astrophysics)

   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 driftsmore »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.« less
2. Extended radio emission associated with a breakout eruption from the back side of the Sun

   https://doi.org/10.1051/0004-6361/201936878  

   Morosan, D. E. ; Palmerio, E. ; Lynch, B. J. ; Kilpua, E. K. ( January 2020 , Astronomy & Astrophysics)

   Context. Coronal mass ejections (CMEs) on the Sun are the largest explosions in the Solar System that can drive powerful plasma shocks. The eruptions, shocks, and other processes associated to CMEs are efficient particle accelerators and the accelerated electrons in particular can produce radio bursts through the plasma emission mechanism. Aims. Coronal mass ejections and associated radio bursts have been well studied in cases where the CME originates close to the solar limb or within the frontside disc. Here, we study the radio emission associated with a CME eruption on the back side of the Sun on 22 July 2012. Methods. Using radio imaging from the Nançay Radioheliograph, spectroscopic data from the Nançay Decametric Array, and extreme-ultraviolet observations from the Solar Dynamics Observatory and Solar Terrestrial Relations Observatory spacecraft, we determine the nature of the observed radio emission as well as the location and propagation of the CME. Results. We show that the observed low-intensity radio emission corresponds to a type II radio burst or a short-duration type IV radio burst associated with a CME eruption due to breakout reconnection on the back side of the Sun, as suggested by the pre-eruptive magnetic field configuration. The radio emission consists ofmore »a large, extended structure, initially located ahead of the CME, that corresponds to various electron acceleration locations. Conclusions. The observations presented here are consistent with the breakout model of CME eruptions. The extended radio emission coincides with the location of the current sheet and quasi-separatrix boundary of the CME flux and the overlying helmet streamer and also with that of a large shock expected to form ahead of the CME in this configuration.« less
3. Tracking the Source of Solar Type II Bursts through Comparisons of Simulations and Radio Data

   https://doi.org/10.3847/1538-4357/ac2361  

   Hegedus, Alexander M. ; Manchester, IV, Ward B. ; Kasper, Justin C. ( November 2021 , The Astrophysical Journal)

   The most intense solar energetic particle events are produced by coronal mass ejections (CMEs) accompanied by intense type II radio bursts below 15 MHz. Understanding where these type II bursts are generated relative to an erupting CME would reveal important details of particle acceleration near the Sun, but the emission cannot be imaged on Earth due to distortion from its ionosphere. Here, a technique is introduced to identify the likely source location of the emission by comparing the dynamic spectrum observed from a single spacecraft against synthetic spectra made from hypothesized emitting regions within a magnetohydrodynamic (MHD) numerical simulation of the recreated CME. The radio-loud 2005 May 13 CME was chosen as a test case, with Wind/WAVES radio data being used to frame the inverse problem of finding the most likely progression of burst locations. An MHD recreation is used to create synthetic spectra for various hypothesized burst locations. A framework is developed to score these synthetic spectra by their similarity to the type II frequency profile derived from the Wind/WAVES data. Simulated areas with 4× enhanced entropy and elevated de Hoffmann–Teller velocities are found to produce synthetic spectra similar to spacecraft observations. A geometrical analysis suggests the easternmore »edge of the entropy-derived shock around (−30°, 0°) was emitting in the first hour of the event before falling off, and the western/southwestern edge of the shock centered around (6°, −12°) was a dominant area of radio emission for the 2 hr of simulation data out to 20 solar radii.

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4. PSP/IS⊙IS Observation of a Solar Energetic Particle Event Associated with a Streamer Blowout Coronal Mass Ejection during Encounter 6

   https://doi.org/10.3847/1538-4357/ac408f  

   Getachew, T. ; McComas, D. J. ; Joyce, C. J. ; Palmerio, E. ; Christian, E. R. ; Cohen, C. M. ; Desai, M. I. ; Giacalone, J. ; Hill, M. E. ; Matthaeus, W. H. ; et al ( February 2022 , The Astrophysical Journal)

   Abstract In this paper we examine a low-energy solar energetic particle (SEP) event observed by IS⊙IS’s Energetic Particle Instrument-Low (EPI-Lo) inside 0.18 au on 2020 September 30. This small SEP event has a very interesting time profile and ion composition. Our results show that the maximum energy and peak in intensity are observed mainly along the open radial magnetic field. The event shows velocity dispersion, and strong particle anisotropies are observed throughout the event, showing that more particles are streaming outward from the Sun. We do not see a shock in the in situ plasma or magnetic field data throughout the event. Heavy ions, such as O and Fe, were detected in addition to protons and 4He, but without significant enhancements in 3He or energetic electrons. Our analysis shows that this event is associated with a slow streamer blowout coronal mass ejection (SBO-CME), and the signatures of this small CME event are consistent with those typical of larger CME events. The time–intensity profile of this event shows that the Parker Solar Probe encountered the western flank of the SBO-CME. The anisotropic and dispersive nature of this event in a shockless local plasma gives indications that these particles are most likelymore »accelerated remotely near the Sun by a weak shock or compression wave ahead of the SBO-CME. This event may represent direct observations of the source of the low-energy SEP seed particle population.« less
5. Radio afterglow of magnetars’ giant flares

   https://doi.org/10.1093/mnras/stab2151  

   Mehta, Riddhi ; Barkov, Maxim ; Lyutikov, Maxim ( July 2021 , Monthly Notices of the Royal Astronomical Society)

   Abstract We develop a model for the radio afterglow of the giant flare of SGR 1806-20 arising due to the interaction of magnetically-dominated cloud, an analogue of Solar Coronal Mass Ejections (CMEs), with the interstellar medium (ISM). The CME is modeled as a spheromak-like configuration. The CME is first advected with the magnetar’s wind and later interacts with the ISM, creating a strong forward shock and complicated backwards exhaust flow. Using three-dimensional magnetohydrodynamic simulations, we study various relative configurations of the magnetic field of the CME with respect to the ISM’s magnetic field. We show that the dynamics of the forward shock mostly follows the Sedov-Taylor blastwave, while the internal structure of the shocked medium is considerably modified by the back flow, creating a multiple shock configuration. We calculate synthetic synchrotron emissivity maps and light curves using two assumptions: (i) magnetic field compression; (ii) amplification of the magnetic field at the shock. We find that models with magnetic field amplification account better for the observed radio emission.