Daily Double: Solar Wind

We were excited to see “what is the solar wind” featured recently as a Daily Double on JEOPARDY! While the contestant missed the answer (oops!) it raises a valid point: the solar wind is an often misunderstood thing, and can be challenging to communicate. In this blog post, we’ll pull together some resources so that when it next comes up, you’ll be the first with the answer!

A Jeopardy contestant is next to a screen that reads "The Sun's corona propels this stream of particles continuously at hundreds of miles per second."

Still from JEOPARDY! December 29, 2021. The solar wind’s typical speed of 400 km/s is equal to 249 miles per second, or 894,775 miles per hour. That’s about 50 times faster than a rocket!

According to NASA’s Heliopedia, the solar wind is a gusty stream of material that flows from the Sun in all directions, all the time, carrying the Sun’s magnetic field out into space. While it is much less dense than wind on Earth, it is much faster, typically blowing at speeds of one to two million miles per hour (about 447 to 894 km/s). The solar wind is made of charged particles — electrons and ionized atoms — that interact with one another and the Sun’s magnetic field. The extent of the solar wind creates the “heliosphere,” the Sun’s region of influence within interstellar space. (Pro tip: You might see the solar wind’s magnetic field called the “interplanetary magnetic field” or “IMF” for short.) 

Windlike material blows out of the Sun

Caption:  An artist’s animation of the solar wind. Credit:  NASA’s Goddard Space Flight Center Conceptual Image Lab/Adriana Manrique Gutierrez

Origin Story

The Heliopedia tells us that the Sun’s dynamic upper atmosphere is called the corona. It is filled with plasma, whose movements are governed by the tangle of magnetic fields emanating from the Sun. Temperatures in the corona can reach up to millions of degrees. 

In their book, Storms From the Sun: The Emerging Science of Space Weather, Michael Carlowicz and Ramon Lopez explain, “the electrified plasma of the solar wind flows out of the [Sun’s] corona like water gushing through cracks in a dam. The solar wind essentially seeps out through the edges of honeycomb-shaped patterns in the surface of the Sun, escaping around the edges of large convection cells bubbling up from the interior.” They give an example by Dr. Helen Mason: “If you think of these cells as paving stones in a patio, then the solar wind is breaking through like grass around the edges, concentrated in the corners where the paving stones meet.” (Carlowicz and Lopez, p. 82). 

The solar wind flows out from the Sun carrying particles and magnetic fields through the solar system. This combination is central to aurora formation not just on Earth, but on other planets as well. 

Earth Effects

The solar wind actively shapes the Earth’s magnetic field, an area of near-Earth space called the “magnetosphere”. It “flows past Earth like water past a cruising boat. Tenuous compared to air, the solar wind is still potent enough to confine Earth’s magnetic field, molding it into the shape of a comet or wind sock.” (Carlowicz and Lopez, p. 83). 

The solar wind is continuous, unlike the more dynamic aurora-driving space weather events you may hear about, such as coronal holes and coronal mass ejections. It is also why “when will the next aurora happen?” is a complicated question. On a global scale, auroras are happening all the time—we just can’t always see them. 

Studying the Solar Wind

A model called WSA-ENLIL helps scientists track the solar wind and predict space weather. It works in three steps. First, scientists at NOAA’s Space Weather Prediction Center (SWPC) use observations to make a map of the Sun, including where the solar wind is flowing out. Then, scientists plug this data into a fancier model that looks at where and how the solar wind flows into the area around the Sun and all the way out to Earth. From there, they can overlay satellite data of coronal mass ejections, or CMEs. A CME is a huge bubble of radiation and particles from the Sun that explodes into space at very high speed—like a solar sneeze. Since CMEs drive solar storms, scientists model how the CME may blow in the solar wind and whether it will hit Earth. ENLIL is usually two-dimensional, but recently the Space Weather Technology, Research and Education Center team at University of Colorado Boulder created a 3D version

Animated diagram of L points rotating around the Sun with the Earth

A diagram showing the Lagrangian points as they relate to the Sun (large yellow circle in the center) and the Earth (small blue circle along the gray orbit)

The key properties of the solar wind are speed, density, and magnetic field. At the moment, scientists don’t have a satellite that directly measures all the properties of the solar wind as it blows off the surface of the Sun. The magnetic field can only be measured near Earth, hence a lot of uncertainty in predicting aurora. Scientists have to wait until it blows past the Deep Space Climate Observatory (DSCOVR) and the Advanced Composition Explorer (ACE) satellites in order to get the crucial magnetic field data. It takes two to three days for the solar wind to reach DSCOVR and ACE; there, they orbit a special, “gravitationally stable” point called Lagrangian point 1 (L1 for short) that is always between Earth and the Sun. That way, it is always in a good place to intercept the solar wind about an hour before it reaches Earth.

When these satellites measure the solar wind, they send data on a number of properties back to Earth. Density, speed, and Bz are the most important solar wind quantities to keep tabs on for most aurora hunters. Density looks at how many particles are in a cubic centimeter (roughly the size of a sugar cube). Typical densities of the solar wind are usually around 1 – 10 microscopic solar particles per cubic centimeter.  This may not sound like very much, but the solar wind is always blowing a variable stream of these particles. 

Space Weather and Storms

Solar wind is the medium in which space weather happens, so it is the flowing plasma soup in which CMEs sail toward Earth. When the density of solar wind particles is high (like when a CME plows through and pushes the solar wind particles in front of it), it creates more pressure on Earth’s magnetosphere and the result is a stronger aurora. Similarly, higher solar wind speeds (like when a gusty CME blows through space) increase pressure and can more easily drive the processes that generate aurora. 

Although it doesn’t get as much attention as its flashier space weather siblings, the solar wind is a vital part of the relationship between the Sun and the Earth. We hope that next time it comes up on trivia night, you’ll enjoy impressing your friends with your space weather knowledge! 

Sources

Carlowicz, Michael J., and Ramon E. Lopez. Storms from the Sun. Joseph Henry Press, 2002.

The Heliopedia, NASA. https://www.nasa.gov/mission_pages/sunearth/the-heliopedia

McCloat, Sean. “What is the Solar Wind?” Aurorasaurus Blog. http://blog.aurorasaurus.org/?p=173

 

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