https://www.bbc.com/future/article/20240702-beyond-the-northern-lights-
how-solar-flares-spill-out-across-the-solar-system

The Sun is extremely active right now, blasting the Earth with the biggest
solar storms in 20 years. This is what it is doing to the rest of the
Solar System.

If you happened to look skywards on a few nights in May 2024, there was a
good chance of seeing something spectacular. For those at relatively low
latitudes, there was a rare chance to see the flickering red, pink, green
glow of our planet's aurorae.

A powerful solar storm had sent bursts of charged particles barrelling
towards Earth and, as they bounced around in our planet's atmosphere, they
unleashed spectacular displays of the Northern and Southern Lights. The
dazzling displays of aurora borealis were visible far further south than
they might normally be � and far further north in the case of aurora
australis thanks to the power of the geomagnetic storm, the strongest in
two decades.

Although some people experienced only a faint, eerie glow, others were
treated to a myriad of colour as far south as London in the UK and Ohio in
the US. Reports even came in from just to the north of San Francisco,
California.

But while this spike in activity from the Sun left many on Earth
transfixed by the light display it produced, it has also had a profound
effect elsewhere in the Solar System. As most of us wondered at the
colours dancing across the night's sky, astronomers have been peering far
beyond to see the strange ways such intense bursts of particles affect
other planets and the space between them.

"The Sun can fire material outwards in any direction like a garden
sprinkler," says Jim Wild, a professor of space physics at Lancaster
University in the UK. "The effects are felt throughout the Solar System."

Our Sun is currently heading towards, or has already reached, its solar
maximum � the point in an 11-year cycle where it is most active. This
means the Sun produces more bursts of radiation and particles from solar
flares and events known as coronal mass ejections (CMEs). If these are
sprayed in our direction, they can supercharge the Earth's magnetic field,
causing magnificent aurorae but also posing problems for satellites and
power grids.

"Things really seem to be picking up right now," says Mathew Owens, a
space physicist at the University of Reading in the UK. "I think we're
about at solar maximum now, so we may see more of these kinds of storms in
the next couple of years."

Around the Sun, multiple spacecraft are observing this increase in
activity up close. One of those, the European Space Agency's (Esa) Solar
Orbiter, has been studying the Sun since 2020 on an orbit that takes it
within the path of Mercury. Currently the spacecraft is "on the far side
of the Sun as seen from Earth", says Daniel M�ller, project scientist for
the Solar Orbiter mission at Esa in the Netherlands. "So we see everything
that Earth doesn't see."

The storm that hit Earth in May originated from an active region of solar
flares and sunspots, bursts of plasma and twisting magnetic fields on the
Sun's surface, known as its photosphere. Solar Orbiter was able to see
"several of the flares from this monster active region that rotated out of
Earth's view", says M�ller, bright flashes of light and darkened regions
called sunspots on the Sun's surface.

One of the goals of Solar Orbiter is "to connect what's happening on the
Sun to what's happening in the heliosphere," says M�ller. The heliosphere
is a vast bubble of plasma that envelops the Sun and the planets of the
Solar System as it travels through interstellar space. What M�ller and his
colleagues hope to learn more about is where the solar wind � the constant
stream of particles spilling out from the Sun across the Solar System �
"blows into the interstellar medium", he says. "So we are particularly
interested in anything energetic on the Sun that we can find back in the
turbulence of the solar wind."

This particular cycle, cycle 25, appears to be "significantly more active
than what people predicted", says M�ller, with the relative sunspot number
� an index used to measure the activity across the visible surface of the
Sun � eclipsing what was seen as the peak of the previous solar cycle. The
National Oceanic and Atmospheric Administration (Noaa) in the US had
predicted a maximum monthly average of 124 sunspots a day in May, but the
actual number was 170 on average, with one day exceeding 240, according to
M�ller.

But the exact cause of the Sun's 11-year-long cycle and its variabilities
remains a bit of a mystery.

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The effects of these changes in solar activity, however, extend far across
the Solar System. Earth is not the only planet to be hit by solar storms
as they billow across interplanetary space. Mercury, the closest planet to
the Sun, has a much weaker magnetic field than Earth � about 100 times
less � and lacks a substantial atmosphere. But solar activity can cause
the surface of the planet to glow with X-rays as solar wind rains down.

Venus also lacks a substantial magnetic field, but the planet does still
create auroras as the solar wind interacts with the planet's ionosphere.

At Mars, the effect of solar activity is more obvious. Here, a Nasa
spacecraft called Maven (Mars Atmosphere and Volatile Evolution) has been
studying the planet's atmosphere from orbit since 2014. "We were on the
declining side of solar cycle 24 [then]," says Shannon Curry, a planetary
scientist at the University of Colorado, Boulder in the US and the lead on
the mission. "We are now coming up on the peak of cycle 25, and this
latest series of active regions has produced the strongest activity Maven
has ever seen."

Between 14 and 20 May the spacecraft detected exceptionally powerful solar
activity reaching Mars, including an X8.7 � solar flares are ranked B, C,
M, and X in order from weakest to strongest. Results from the event have
yet to be studied, but Curry noted that a previous X8.2 flare had resulted
in "a dozen papers" published in scientific journals. Another flare on 20
May, later estimated to be an even bigger X12, hurled X-rays and gamma
rays towards Mars before a subsequent coronal mass ejection launched a
barrage of charged particles in the same direction.

Images beamed back from Nasa's Curiosity Rover on Mars revealed just now
much energy struck the Martian surface. Streaks and dots caused by charged
particles hitting the camera's sensors caused the images to "dance with
snow", according to a press release from Nasa. Maven, meanwhile, captured
glowing aurora as the particles hit the Mars' atmosphere, engulfing the
entire planet in an ultraviolet glow.

The entire atmosphere expands dozens of kilometres � exciting for
scientists but detrimental for spacecraft

The flares can cause the temperature of the Martian atmosphere to
"dramatically increase," says Curry. "It can even double in the upper
atmosphere. The atmosphere itself inflates. The entire atmosphere expands
dozens of kilometres � exciting for scientists but detrimental for
spacecraft, because when the atmosphere expands there's more drag on the
spacecraft."

The expanding atmosphere can also cause degradation of the solar panels on
spacecraft orbiting Mars from the increase in radiation. "The last two
flares caused more degradation than what a third of a year would typically
do," says Curry.

Mars, while it has lost most of its magnetic field, still has "crustal
remnant magnetic fields, little bubbles all over the southern hemisphere",
says Curry. During a solar event, charged particles can light those up and
excite particles. "The entire day side lights up in what we call a diffuse
aurora," says Curry. "The entire sky glows. This would most likely be
visible to astronauts on the surface."

By the time solar storms reach further out into the solar system, they
tend to have dissipated but can still have an impact on the planets they
encounter. Jupiter, Saturn, Uranus, and Neptune all have aurorae that are
in part driven by charged particles from the Sun interacting with their
magnetic fields.

But one of the key effects of solar activity on interplanetary space that
astronomers are eager to study is something called "slow solar wind", a
more sluggish, but denser stream of charged particles and plasma from the
Sun. Steph Yardley, a solar astronomer at Northumbria University in the
UK, says solar wind is "generally classed about 500km/s (310 miles/s)",
but slow wind falls below this. It also has a lower temperature and tends
to be more volatile.

Recent work by Yardley and her colleagues, using data from Solar Orbiter,
suggests that the Sun's atmosphere, its corona, plays a role in the speed
of the solar wind. Regions where the magnetic field lines, the direction
of the field and charged particles are "open" � stretching out into space
without looping back � provide a highway for solar wind to reach high
speeds. Closed loops over some active regions � where the magnetic field
lines have no beginning and end � can occasionally snap, producing slow
solar wind. The variability in the slow solar wind seems to be driven by
the unpredictable flow of plasma inside the Sun, which makes the magnetic
field particularly chaotic.