April 17, 2021, was a day like any other day on the Sun, until a brilliant flash erupted and an enormous cloud of solar material billowed away from our star. Such outbursts from the Sun are not unusual, but this one was unusually widespread, hurling high-speed protons and electrons at velocities nearing the speed of light and striking several spacecraft across the inner solar system.
In fact, it was the first time such high-speed protons and electrons –
called solar energetic particles (SEPs) – were observed by spacecraft at five
different, well-separated locations between the Sun and Earth as well as by
spacecraft orbiting Mars. And now these diverse perspectives on the solar storm
are revealing that different types of potentially dangerous SEPs can be blasted
into space by different solar phenomena and in different directions, causing
them to become widespread.
On April 17, 2021, one of the Solar Terrestrial Relations Observatory (STEREO) spacecraft captured this view of a coronal mass ejection billowing away from the Sun (which is covered by the black disk at center to better see features around it). Learn more. NASA/STEREO-A/COR2
“SEPs can harm our technology, such as satellites, and
disrupt GPS,” said Nina Dresing of the Department of Physics and Astronomy,
University of Turku in Finland. “Also, humans in space or even on airplanes on
polar routes can suffer harmful radiation during strong SEP events.”
Scientists like Dresing are eager to find out where these particles come
from exactly – and what propels them to such high speeds – to better learn how
to protect people and technology in harm’s way. Dresing led a team of
scientists that analyzed what kinds of particles struck each spacecraft and
when. The team published its results in the journal Astronomy & Astrophysics.
Currently on its way to Mercury, the BepiColombo spacecraft, a joint
mission of ESA (the European Space Agency) and JAXA (Japan Aerospace
Exploration Agency), was closest to the blast’s direct firing line and was
pounded with the most intense particles. At the same time, NASA’s Parker Solar
Probe and ESA’s Solar Orbiter were on opposite sides of the flare, but Parker
Solar Probe was closer to the Sun, so it took a harder hit than Solar Orbiter
did. Next in line was one of NASA’s two Solar Terrestrial Relations Observatory
(STEREO) spacecraft, STEREO-A, followed by the NASA/ESA Solar and Heliospheric
Observatory (SOHO) and NASA’s Wind spacecraft, which were closer to Earth and
well away from the blast. Orbiting Mars, NASA’s MAVEN and ESA’s Mars Express
spacecraft were the last to sense particles from the event.
This diagram shows the positions of individual spacecraft, as well as Earth and Mars, during the solar outburst on April 17, 2021. The Sun is at the center. The black arrow shows the direction of the initial solar flare. Several spacecraft detected solar energetic particles (SEPs) over 210 degrees around the Sun (blue shaded area). Solar-MACH
Altogether, the particles were detected over 210
longitudinal degrees of space (almost two-thirds of the way around the Sun) –
which is a much wider angle than typically covered by solar outbursts. Plus,
each spacecraft recorded a different flood of electrons and protons at its
location. The differences in the arrival and characteristics of the particles
recorded by the various spacecraft helped the scientists piece together when
and under what conditions the SEPs were ejected into space.
These clues suggested to Dresing’s team that the SEPs were not blasted
out by a single source all at once but propelled in different directions and at
different times potentially by different types of solar eruptions.
“Multiple sources are likely contributing to this event, explaining its
wide distribution,” said team member Georgia de Nolfo, a heliophysics research
scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Also,
it appears that, for this event, protons and electrons may come from different
sources.”
The team concluded that the electrons were likely driven into space
quickly by the initial flash of light – a solar flare – while the protons were
pushed along more slowly, likely by a shock wave from the cloud of solar
material, or coronal mass ejection.
“This is not the first time that people have conjectured that electrons
and protons have had different sources for their acceleration,” de Nolfo said.
“This measurement was unique in that the multiple perspectives enabled
scientists to separate the different processes better, to confirm that
electrons and protons may originate from different processes.”
In addition to the flare and coronal mass ejection, spacecraft recorded
four groups of radio bursts from the Sun during the event, which could have
been accompanied by four different particle blasts in different directions.
This observation could help explain how the particles became so widespread.
“We had different distinct particle injection episodes – which went into
significantly different directions – all contributing together to the
widespread nature of the event,” Dressing said.
“This event was able to show how important multiple perspectives are in
untangling the complexity of the event,” de Nolfo said.
These results show the promise of future NASA heliophysics missions that
will use multiple spacecraft to study widespread phenomena, such as the Geospace Dynamics Constellation (GDC), SunRISE, PUNCH,
and HelioSwarm. While single spacecraft can reveal conditions locally, multiple
spacecraft orbiting in different locations provide deeper scientific insight
and offer a more complete picture of what’s happening in space and around our
home planet.
It also previews the work that will be done by future missions such as MUSE, IMAP,
and ESCAPADE, which will study explosive solar events and the acceleration of
particles into the solar system.
by Vanessa ThomasNASA’s Goddard Space Flight Center, Greenbelt, Md.
Source: Multiple Spacecraft Tell the Story of One Giant Solar Storm - NASA Science
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