A signal astrophysicists once dismissed as contamination of X-ray observations could actually improve forecasts of dangerous space weather that threatens Earth.
Charged particles within the solar wind give off so-called soft X-rays when they collide with the magnetic field that shrouds Earth. The soft X-rays have longer wavelengths and lower frequencies than their hard X-ray cousins.
This signal was once dismissed as local cosmic noise that interfered with space observatory surveys of hot, distant objects such as supernovas, until some scientists realized its significance for Earth.
Measuring the soft X-ray emissions could allow scientists to build a real-time picture of what's happening with the planet's magnetic field, also known as the magnetosphere, which protects Earth against solar storms. Past observations show that soft X-ray data changes almost immediately in response to changes in the solar wind.
"It's not a question of advanced warning in this case, but rather getting the global view — that is, knowing what is happening everywhere at once," said Michael Collier, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md.
Having a complete, ever-changing view of the magnetosphere could not only boost space weather forecasts, but might also solve scientific controversies such as how magnetic reconnection works. That phenomenon takes place when tangled magnetic field lines merge and release energy in the form of heat and kinetic energy among charged particles.
Collier and his colleagues detailed their proposal in the June 15 issue of Eos, the weekly newsletter of the American Geophysical Union.
Such a use of soft X-rays seemed unthinkable two decades ago, because only very hot objects such as supernova remnants or stars typically produce X-rays. That view changed when Germany's Roentgen satellite spotted soft X-ray emissions coming from the comet Hyakutake in the 1990s.
Scientists found that ionized versions of carbon, oxygen and hydrogen atoms in the solar wind ended up colliding with neutral atoms close to the comet and stealing an electron from the atoms. That made the ions super-excited for a short period, before they relaxed and emitted the energy as soft X-rays.
That same phenomenon also takes place when the solar wind runs into the Earth's magnetosphere, Collier and his colleagues said. They point to observations made by the European Space Agency's X-ray Multi-Mirror (XMM) Newton spacecraft that revealed how the charged particles of the solar wind emit soft X-rays.
NASA's Chandra X-ray Observatory also took advantage of soft X-ray data to build a picture of how the solar wind flows around Mars and Venus.
But even if the existing soft X-ray telescopes have demonstrated the "proof of concept" for how to image Earth's magnetosphere, Collier said that a new, dedicated mission would be necessary to make full-time observations.
"A true magnetosheath imager, while relying on the same observational techniques applied in astrophysics telescopes, would require a redesign to achieve the wide field-of-view," Collier told Space.com.
The solar wind slows dramatically when it runs into the bow shock of Earth's magnetic field. Some particles from the weakened solar wind still penetrate the middle region, called the magnetosheath, but most do not get beyond the next boundary called the magnetopause.
A new space weather vane
Having the X-ray imager in the right spot could provide real-time views of changes in the magnetosphere almost around the clock. Placing the imager at a special spot called the L1 Lagrange point between Earth and the sun would allow the spacecraft to stay in a relatively fixed position, held in place by the balancing gravitational pulls of the two large bodies.
Even a spacecraft in Earth orbit might allow for almost 24/7 coverage, aside from a small part of the orbit where the imager ends up inside the magnetosphere looking out, Collier explained.
That coverage matters especially for space weather prediction models that try to forecast disruptive sun events. One solar flare sent out a wave of charged particles that knocked out the power supply in Quebec and disrupted power and communications across North America during a March 1989 solar storm.
But almost any vantage point would still provide for groundbreaking scientific observations, Collier noted. An X-ray imager could help pinpoint the precise locations of the magnetopause and bow shock, as well as uncover how solar wind energy flows around and through the magnetosphere.
"It really is like the blind men and the elephant: Existing heliophysics missions provide us very good observations of the trunk or the tail, so to speak," Collier said. "An imager will provide us the entire elephant."