Page 33 - EngineerIt February 2021

SPACE SCIENCES

magnetosphere. The strike would push the magnetopause
down until it is only two Earth-radii above our planet’s
surface. Satellites in Earth orbit would suddenly find
themselves exposed to a hail of energy-charged particles,
potentially short-circuiting sensitive electronics. A “super
fountain” of oxygen ions rising up from the top of Earth’s
atmosphere might literally drag satellites down, hastening
their demise. (Note: Welling’s group stopped short of
modelling the super fountain.)

Now for the good news: perfect CMEs are rare
Angelos Vourlidas of Johns Hopkins University has studied
the statistics of CMEs. He notes that SOHO has captured
only two CMEs with velocities greater than 3 000 km/s since
the start of operations in 1996. “This means we expect
roughly one CME ejected at speeds above 3000 km/s per
solar cycle,” he says. Speed isn’t the only factor, however.
To be “perfect,” a 3 000 km/s CME would need to follow
another CME, clearing its path, and both CMEs must be
aimed directly at Earth.
It all adds up to something that doesn’t happen every
day. But one day, it will happen. As Welling et al conclude in
their paper: “Further exploring and preparing for such
extreme activity is important to mitigate space-weather
related catastrophes.” n

SOHO launched on 2 December 1995. It is stationed
1.5 million kilometres closer to the Sun than the
Earth, from where it enjoys uninterrupted views of
our star. The mission was launched with three
scientific objectives in mind. The first was to study
the dynamics and structure of the solar interior. The
second was to study why the Sun's outer atmosphere,
known as the corona, is so much hotter than its
surface, and the third was to study where and how
the solar wind of particles is accelerated.
Almost 6000 papers have now appeared in
refereed journals based on SOHO data, many of
them representing significant progress in our
understanding of the original objectives. In addition
to investigating how the Sun works, SOHO is the most
prolific discoverer of comets in astronomical history,
having spotted more than 4000 of these icy mini-
worlds during the sunward leg of their journeys. n

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