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Multiple NASA spacecraft tell the story of a giant solar storm that occurred on April 17, 2021.

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On April 17, 2021, the Solar Terrestrial Relations Observatory (STEREO) spacecraft provided a captivating glimpse of a coronal mass ejection, skillfully revealing solar features by obscuring the Sun with a black disk at the center of the image.

On April 17, 2021, the Sun unexpectedly unleashed a formidable display of solar activity, launching a massive cloud of solar material into space. While solar eruptions are not uncommon, this particular event stood out for its unprecedented scale, propelling solar energetic particles (SEPs) – high-speed protons and electrons – toward multiple spacecraft across the inner solar system. Remarkably, it marked the first instance of such SEPs being observed simultaneously by spacecraft at five distinct locations, including those positioned between the Sun and Earth and others orbiting Mars. These diverse perspectives are shedding light on the varied origins and trajectories of potentially hazardous SEPs, emphasizing the complexity of solar phenomena and their impact on space weather.

SEPs possess the potential to inflict damage on our technology, including satellites, and disrupt GPS systems,” emphasized Nina Dresing from the Department of Physics and Astronomy at the University of Turku in Finland. The repercussions extend beyond technology, as humans in space or on airplanes following polar routes can be exposed to harmful radiation during intense SEP events. Driven by the imperative to safeguard both individuals and technological assets, scientists, led by Dresing, are fervently investigating the precise origins of these particles and the mechanisms propelling them to high speeds.

The team meticulously analyzed the composition of particles that impacted various spacecraft and discerned the temporal patterns of these encounters. Their findings, elucidated in the journal Astronomy & Astrophysics, contribute crucial insights into mitigating the risks associated with space weather events.

As the BepiColombo spacecraft embarks on its journey to Mercury, it found itself in the direct firing line of a solar blast, experiencing the most intense particle bombardment. This joint mission by the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) provided a unique vantage point. Meanwhile, NASA’s Parker Solar Probe and ESA’s Solar Orbiter, positioned on opposite sides of the solar flare, faced varying degrees of impact, with Parker Solar Probe enduring a more formidable assault due to its closer proximity to the Sun.

NASA’s Solar Terrestrial Relations Observatory (STEREO-A) followed in line, followed by the NASA/ESA Solar and Heliospheric Observatory (SOHO) and NASA’s Wind spacecraft, strategically positioned farther from the solar eruption. At a greater distance, orbiting Mars, NASA’s MAVEN and ESA’s Mars Express spacecraft were the last to detect particles emanating from this celestial event, offering a comprehensive perspective on the spatial dynamics of the solar disturbance.

Individual spacecraft as well as Earth and Mars during a giant solar storm.


In this diagram illustrating the solar outburst on April 17, 2021, the positions of individual spacecraft, Earth, and Mars are depicted in relation to the Sun at the center. The black arrow indicates the direction of the initial solar flare. Notably, multiple spacecraft detected solar energetic particles (SEPs) within a vast expanse exceeding 210 degrees around the Sun, as indicated by the blue shaded area. This visualization offers a spatial understanding of the widespread impact of the solar event on the surrounding celestial bodies and spacecraft in our solar system.

The particle detection spanned over 210 longitudinal degrees of space, covering a substantial portion around the Sun. This wide angle exceeded the typical range of solar outbursts. Additionally, each spacecraft observed a distinct influx of electrons and protons at its specific location. By analyzing the variations in particle arrival and characteristics recorded by different spacecraft, scientists were able to reconstruct the timing and conditions of the solar energetic particle (SEP) ejections. These findings led Dresing’s team to infer that SEPs were not uniformly expelled from a single source but rather propelled in diverse directions and at distinct times, possibly originating from various types of solar eruptions.

Team member Georgia de Nolfo from NASA’s Goddard Space Flight Center proposed that multiple sources are likely contributing to the wide distribution of the event. The team’s analysis suggests that, for this particular event, protons and electrons may originate from different sources. The conclusion drawn is that the electrons were swiftly propelled into space by the initial solar flare, a flash of light. In contrast, the protons moved more slowly, likely influenced by a shock wave from the solar material cloud or coronal mass ejection.

Georgia de Nolfo emphasized that the idea of electrons and protons having distinct acceleration sources has been previously conjectured. However, this measurement stands out due to its uniqueness in providing multiple perspectives, allowing scientists to effectively distinguish between the acceleration processes for electrons and protons. Beyond the solar flare and coronal mass ejection, the spacecraft detected four groups of radio bursts from the Sun during the event. These radio bursts could potentially be associated with four separate particle blasts in different directions, shedding light on the mechanism behind the widespread distribution of particles.

Dresing highlighted the significance of distinct particle injection episodes, each traveling in significantly different directions, collectively contributing to the widespread nature of the event. Georgia de Nolfo emphasized the event’s role in showcasing the importance of multiple perspectives in unraveling its complexity. These findings underscore the potential of future NASA heliophysics missions, including the Geospace Dynamics Constellation (GDC), SunRISE, PUNCH, and HelioSwarm. While single spacecraft offer local insights, having multiple spacecraft in different orbits enhances scientific understanding, providing a comprehensive view of space and our home planet.

This sets the stage for upcoming missions like MUSE, IMAP, and ESCAPADE, designed to investigate explosive solar events and the acceleration of particles within the solar system.

Multiple NASA spacecraft tell the story of a giant solar storm that occurred on April 17, 2021.

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