Abstract
Context.On 2020 November 29, the first widespread solar energetic particle (SEP) event of solar cycle 25 was observed at four widely separated locations in the inner (≲1 AU) heliosphere. Relativistic electrons as well as protons with energies > 50 MeV were observed by Solar Orbiter (SolO), Parker Solar Probe, the Solar Terrestrial Relations Observatory (STEREO)-A and multiple near-Earth spacecraft. The SEP event was associated with an M4.4 class X-ray flare and accompanied by a coronal mass ejection and an extreme ultraviolet (EUV) wave as well as a type II radio burst and multiple type III radio bursts.Aims.We present multi-spacecraft particle observations and place them in context with source observations from remote sensing instruments and discuss how such observations may further our understanding of particle acceleration and transport in this widespread event.Methods.Velocity dispersion analysis (VDA) and time shift analysis (TSA) were used to infer the particle release times at the Sun. Solar wind plasma and magnetic field measurements were examined to identify structures that influence the properties of the energetic particles such as their intensity. Pitch angle distributions and first-order anisotropies were analyzed in order to characterize the particle propagation in the interplanetary medium.Results.We find that during the 2020 November 29 SEP event, particles spread over more than 230° in longitude close to 1 AU. The particle onset delays observed at the different spacecraft are larger as the flare–footpoint angle increases and are consistent with those from previous STEREO observations. Comparing the timing when the EUV wave intersects the estimated magnetic footpoints of each spacecraft with particle release times from TSA and VDA, we conclude that a simple scenario where the particle release is only determined by the EUV wave propagation is unlikely for this event. Observations of anisotropic particle distributions at SolO, Wind, and STEREO-A do not rule out that particles are injected over a wide longitudinal range close to the Sun. However, the low values of the first-order anisotropy observed by near-Earth spacecraft suggest that diffusive propagation processes are likely involved.
Highlights
Solar energetic particle (SEP) events observed by widely separated spacecraft have led to fundamental questions about the underlying processes responsible for the wide spread of energetic particles in the heliosphere
Previous studies have proposed that widespread solar energetic particle (SEP) events are associated with extreme ultraviolet (EUV) waves propagating far from the parent active region, or by extended CMEdriven shocks, which inject particles onto field lines that are poorly connected to the active region
On 2020 November 29, a solar energetic particle event was observed by spacecraft near Earth (SOHO, Advanced Composition Explorer (ACE), and Wind) and by Solar Terrestrial Relations Observatory (STEREO)-A, PSP, and Solar Orbiter (SolO) at widely separated locations spanning ∼230◦ in heliolongitude and 0.81−0.99 AU in heliocentric distance, providing the first opportunity to study a widespread SEP event using such a constellation of four observers
Summary
Solar energetic particle (SEP) events observed by widely separated spacecraft have led to fundamental questions about the underlying processes responsible for the wide spread of energetic particles in the heliosphere. Cliver et al (1995) and Torsti et al (1999) proposed that coronal and interplanetary shocks driven by coronal mass ejections (CMEs) could accelerate and inject particles over large regions in the heliosphere ( see Kouloumvakos et al.2019) Transport processes are another candidate suggesting that particles originating from even a narrow source could propagate to distant heliolongitude either in the corona or in interplanetary space (e.g., Reinhard & Wibberenz 1974; Jokipii 1966). As examples of studies of such widespread events Dresing et al (2012) concluded, for a widespread SEP event observed on 2010 January 17, that large delays of the observed particle onsets and weak particle anisotropies are consequences of strong perpendicular diffusion in the interplanetary medium (Dröge et al 2010).
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