Solar eclipse will reveal the roiling fog of plasma we call home

On Monday 21 August, the shadow of a solar eclipse will drape itself across the US. If the weather cooperates, scientists and civilians alike, including your Astrophile author, will stand in the darkness and stare up at what remains of the sun: a wispy ring of plasma and arcing magnetic fields called the corona.

And doing so will take us back home.

Symbolically, I mean. For millennia, we’ve been looking up at eclipses and seeing a sliver of plasma. But in recent decades, scientists have started thinking of this halo less as out there in space and more as just here.

That fuzzy ring of seething particles around the sun? We’re in it. It wafts outward, roils, thins, sloughs off as the solar wind and eventually envelops the planets.

“Without this even being hyperbole, we actually live in the outer atmosphere of the sun,” says Lika Guhathakurta, a solar physicist at NASA.

Crowning glory

For almost all of human history, glimpses of the corona came only during eclipses.  Among other accounts, the Greek philosopher Plutarch described a hazy light around the rim of a darkened sun as early as AD 71, and Johannes Kepler noticed something similar in 1605.

But it took until the eclipses of 1868 and 1869, when the spectral signatures of atoms in the corona were first discovered, for us to start understanding it as an actual astrophysical place instead of just a shining mirage.

Countless ground and space-based observations later, we know it is made up of hydrogen, helium and a smattering of other elements. Even though the layer underneath it is only a few thousand degrees, parts of the corona simmer at a million degrees or higher.

Magnetic field lines, erupting from a dynamo inside the sun, sculpt the corona into a menagerie of shapes with different names: loops, streamers, empty holes and mass ejections that burp outwards.

In the 1970s, an even bigger picture started to emerge. The space station Skylab found that the ever-changing corona bleeds off into the solar wind, a flow of charged particles that reaches out to Earth and beyond.

Down to Earth

Once here, the particles interact with our planet’s comparatively tiny magnetic field and our atmosphere to make auroras –­ and to disrupt communications satellites.

Trying to grasp this space weather without understanding the corona is like predicting hurricanes without factoring in the ocean, Guhathakurta says.

And at the root of all this tangled physics is the place where the corona starts, right above the sun’s surface – the faint ring made visible during an eclipse.

While space-based observatories can now peer into the sun whenever they want, an eclipse is still the best time to observe the corona to try and figure out how dense and hot it is and what it is made of.

Guhathakurta is lead scientist for this particular eclipse at NASA, and she is a major figure in the modern effort to link the sun, its corona, space weather and Earth. She has been on nine eclipse expeditions already.

Six were successful. “Each of them is glorious,” she says. Three failed. In Mongolia it snowed, South Africa was cloudy and it rained in Indonesia. But an eclipse occurs somewhere in the world every 18 months, so she’s got plenty more chances.

This is my first one. But provided I get to see that sliver of plasma around the sun, I know what I’ll be thinking: This is the corona’s solar system – we’re just living in it.