Giant 'holes' dozens of times the size of our planet are appearing on the sun. Here's how they could affect Earth.

• Two huge coronal holes, dozen of times the size of the Earth, have appeared on the sun.
• These have been spewing fast solar winds toward Earth as fast as 1.8 million mph.

Two enormous dark 'holes' have been spotted on our sun this month, as our star nears the peak of its activity.

These coronal holes can spew solar winds at 1.8 million mph toward our planet, which can cause stunning auroras and disrupt satellites.

On their own, such holes aren't particularly dangerous. But if they happen at the same time as another big solar event, they can create a "perfect storm" capable of disrupting power grids and causing radio blackouts.

Here's what coronal holes are, how they form, and what they could do to Earth:

Coronal holes are made by magnetic fields on the sun

The sun is a big ball of plasma. Liquid, charged particles boil up from the center of the sun, gurgle around on the surface, and sink back down.

Just like you can get a magnetic field by charging a wire coil, as the plasma travels around, it creates magnetic fields on the surface of the sun.

When the sun is fairly quiet, these fields live in harmony. But when our star is reaching a peak of activity, like it is now, the magnetic fields crash into each other, merge, and split.

On occasion, these fields point straight up into space. That creates a coronal hole.

Coronal holes don't make the solar winds, they just set them free

The magnetic fields around the sun usually keep the plasma from escaping into space.

"Charged particles have to follow the magnetic field. They trace it out in space," said Mathew Owens, a professor of space physics at the University of Reading.

"When you look at one of these nice images and you see all these loops, what you're seeing is the charged particles."

But open field lines that are pointing straight out into space allow sun plasma to shoot outwards. This creates solar wind formed of plasma that is propelled into space at tremendous speeds.

Coronal winds travel at up to 1.8 million miles per hour

When coronal holes release solar winds, they can hurtle throughout space at remarkable speeds — up to 800 kilometers per second, or 1.8 million miles per hour.

Why these winds can travel faster than other solar winds is still a matter of debate among scientists, the experts said.

One theory is that the movement of the field lines themselves helps propel the winds forward.

"If you stick these open magnetic field lines into this bubbling surface plasma of the sun, it starts to shake the field lines. They move like waves upwards, and that can give extra energy to the solar wind," said Daniel Verscharen, an associate professor of space and climate physics at University College London.

Coronal holes can knock out satellites and trigger auroras

The solar winds don't tend to be that dangerous on their own, but they can cause some havoc.

"The biggest overall shows and the most hazardous space weather are all driven by coronal mass ejections (CMEs)," said Owens. "But the fast wind can have effects. It's just not quite as spectacular," he said.

The Earth is surrounded by its own layer of fast-traveling magnetically-charged particles, which is called the Van Allen radiation belt.

As particles come from the sun and crash into this belt, they disrupt the magnetic fields in the Van Allen belt. Solar winds from coronal aren't very strong, compared to clouds ejected by CMEs, for instance. But they last a lot longer.

"These wind streams can go on for days, and if you expose the Earth to a long-lasting stream of fast wind from a coronal hole, it actually can put a lot of energy into the Earth system," said Versharen.

The good news is that it can create more auroras. These won't be as spectacular as those we see when a coronal mass ejection lands on Earth, but they can still be enhanced.

The bad news is those particles can mess with satellites in orbit.

"If you're lucky, they just changed a few things in the electronics, they flip a bit over to another value. And so you get some weird disturbances that you can then correct. But in the worst case, they can destroy your satellite," said Verscharen.

The real danger is when solar winds combine with coronal mass ejections

Scientists are most concerned about coronal holes teaming up with CMEs.

CMEs happen when closed magnetic loops become stressed and snap in half, ejecting the plasma that was traveling on their magnetic field into space. If the CME is pointed toward Earth, that cloud of plasma can hit our planet with much more violence than solar winds.

"That's the stuff that scientists are nervous about," said Versharen.

If those CMEs happen near a coronal hole, that cloud can become much more powerful as it heads toward Earth.

"You tend not to get CMEs within coronal holes. But you can get a CME going off close to a corona hole and then if the fast wind comes behind, it can compress and accelerate that CME," Owens said.

This is what solar physicists call "a perfect storm," Alex Young, the associate director for science at NASA Goddard's Heliophysics Science Division, previously told Insider.

This is what happened on the week of March 24, when auroras were seen as far down as New Mexico.

Coronal mass ejections that burst right next to the coronal hole landed on Earth, turning what scientists had thought would be a category G2 storm into a category G4 storm on a scale that goes from 1 to 5.

Had the winds been any stronger, it could have created a G5 storm, which can take down power grids.

Coronal holes aren't actually holes in the sun

When the sun has a coronal hole, it looks like someone has gouged a divot in our star.

But what has happened is not visible to the naked eye. Coronal holes appear in UV imaging, designed to show very hot, magnetically active areas on the sun.

Because the region inside the hole is losing material, it is cooler and less active, which is why it seems darker.

"Because the field is open, it can escape into space and so there's just less material there," said Owens.

It is rare for coronal holes to line up with the Earth

Coronal holes don't usually line up with the Earth because at quiet times they tend to stay toward the poles of the sun.

"Normally these coronal holes sit at the north and south poles of the sun," said Owens.

At that point, the winds are being ejected into space and won't line up with the Earth.

But the sun is currently nearing its solar maximum, a period that rolls around about every 11 years when solar activity is ramping up, which means it is much more likely for these open magnetic field lines to appear around the equator, where they can be pointing straight at Earth.

"As solar activity ramps up as we get towards the solar maximum, we do start to see more of these coronal holes reaching down to the equator. That means it's easier for that fast wind to get to Earth," said Owens.

Coronal holes are different from sunspots

While coronal holes and sunspots may look similar to the untrained eye, they are very different.

For one, sunspots happen on the surface of the sun and can be seen in visible light. Coronal holes happen in the "corona," the atmosphere of the sun, and can only be seen in UV or X-ray light.

Sunspots happen at the two ends of a closed magnetic line which curves up and back down like a rainbow and is connected to the sun at both ends. Coronal holes happen when the magnetic lines point straight outwards.

Sunspots can create coronal mass ejections, coronal holes cannot.

We could see this month's coronal holes again next month

As the holes spin around the sun and go out of our sight, they could still be spewing solar winds away from the Earth.

But they can be pretty stable, which means we could see the coronal holes again as they rotate back into view in about a month — enough time for the sun to rotate on its axis.

However, they may disappear.

"These holes that reached out to the equator, they tend not to last as long as the ones up at the poles that will sit there for years at a time," said Owens.

"When they're at the equator, it's normally around solar maximum and everything on the sun is just changing rapidly," he said.

A mission will aim to see coronal holes a few days earlier

According to Verscharen, we are about forty years behind in space weather predictions compared to what we do for normal weather predictions.

After all, if you can't see the back of the sun, you can't know what's coming around the corner. But a mission from the European Space Agency called Vigil, planned to launch mid-2020s, is aiming to remedy that.

The idea is to place a satellite at a point where it can still communicate with Earth, but will also be able to see the sun from a different angle.

"So that means from there, when you look at the sun, you can see the surface already four and a half days before," said Verscharen.