Solar minimum is coming

June 28, 2017
X-rays stream off the sun in this image showing observations from by NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, overlaid on a picture taken by NASA's Solar Dynamics Observatory (SDO). Credit: NASA

High up in the clear blue noontime sky, the sun appears to be much the same day-in, day-out, year after year.

But astronomers have long known that this is not true. The sun does change. Properly-filtered telescopes reveal a fiery disk often speckled with dark sunspots. Sunspots are strongly magnetized, and they crackle with solar flares—magnetic explosions that illuminate Earth with flashes of X-rays and extreme ultraviolet radiation. The sun is a seething mass of activity.

Until it's not. Every 11 years or so, sunspots fade away, bringing a period of relative calm.

"This is called solar minimum," says Dean Pesnell of NASA's Goddard Space Flight Center in Greenbelt, MD. "And it's a regular part of the cycle."

The sun is heading toward solar minimum now. Sunspot counts were relatively high in 2014, and now they are sliding toward a low point expected in 2019-2020.

While intense activity such as sunspots and solar flares subside during solar minimum, that doesn't mean the sun becomes dull. Solar activity simply changes form.

For instance, says Pesnell, "during solar minimum we can see the development of long-lived coronal holes."

Coronal holes are vast regions in the sun's atmosphere where the sun's opens up and allows streams of solar particles to escape the sun as the fast solar wind.

Pesnell says "We see these holes throughout the solar cycle, but during solar minimum, they can last for a long time - six months or more." Streams of solar wind flowing from coronal holes can cause weather effects near Earth when they hit Earth's magnetic field. These effects can include temporary disturbances of the Earth's magnetosphere, called geomagnetic storms, auroras, and disruptions to communications and navigation systems.

During solar minimum, the effects of Earth's upper atmosphere on satellites in low Earth orbit changes too.

Normally Earth's upper atmosphere is heated and puffed up by ultraviolet radiation from the sun. Satellites in low Earth orbit experience friction as they skim through the outskirts of our atmosphere. This friction creates drag, causing satellites to lose speed over time and eventually fall back to Earth. Drag is a good thing, for space junk; natural and man-made particles floating in orbit around Earth. Drag helps keep low Earth orbit clear of debris.

But during solar minimum, this natural heating mechanism subsides. Earth's upper cools and, to some degree, can collapse. Without a normal amount of drag, space junk tends to hang around.

There are unique space weather effects that get stronger during solar minimum. For example, the number of that reach Earth's increases during solar minimum. Galactic cosmic rays are high energy particles accelerated toward the solar system by distant supernova explosions and other violent events in the galaxy.

Pesnell says that "During , the sun's magnetic field weakens and provides less shielding from these cosmic rays. This can pose an increased threat to astronauts traveling through space."

Solar minimum brings about many changes to our sun, but less solar activity doesn't make the sun and our space environment any less interesting.

Explore further: Image: Daily sun images of 2016

Related Stories

Image: Daily sun images of 2016

January 16, 2017

This montage of 366 images shows our sun through the eyes of ESA's Proba-2 satellite, as seen each day in 2016.

Images of the sun from the GOES-16 satellite

February 28, 2017

The first images from the Solar Ultraviolet Imager or SUVI instrument aboard NOAA's GOES-16 satellite have been successful, capturing a large coronal hole on Jan. 29, 2017.

Fast solar wind causes aurora light shows

October 12, 2015

On the night of Oct. 8, 2015, a photographer in Harstad, Norway captured this image of the dancing northern lights. Auroras are created when fast-moving, magnetic solar material strikes Earth's magnetic bubble, the magnetosphere. ...

The Sun Loses its Spots

July 24, 2007

While sidewalks crackle in the summer heat, NASA scientists are keeping a close eye on the sun. It is almost spotless, a sign that the Sun may have reached solar minimum. Scientists are now watching for the first spot of ...

Recommended for you

NASA telescope studies quirky comet 45P

November 22, 2017

When comet 45P zipped past Earth early in 2017, researchers observing from NASA's Infrared Telescope Facility, or IRTF, in Hawai'i gave the long-time trekker a thorough astronomical checkup. The results help fill in crucial ...

Uncovering the origins of galaxies' halos

November 21, 2017

Using the Subaru Telescope atop Maunakea, researchers have identified 11 dwarf galaxies and two star-containing halos in the outer region of a large spiral galaxy 25 million light-years away from Earth. The findings, published ...

Cassini image mosaic: A farewell to Saturn

November 21, 2017

In a fitting farewell to the planet that had been its home for over 13 years, the Cassini spacecraft took one last, lingering look at Saturn and its splendid rings during the final leg of its journey and snapped a series ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.