The solar wind: how the Sun makes its own space weather

What is the solar wind and how does it affect the solar system?
John Loeffler
Coronal mass ejection.ESA/NASA/Soho

When we think of the Sun, we typically picture a glowing orb in space burning away at thousands of degrees Fahrenheit and casting light on the rest of the solar system that orbits it. But it is much more active than even that, thanks to the solar wind.

The idea of solar wind might not be intuitive at first glance, but it is one of the most fascinating features of our Sun. But what is it, and what does it mean for us here on Earth?

Fortunately, the solar wind can't hurt us here on Earth (at least not directly), but it is responsible for one of the most famous natural phenomenons in the world, so you're probably more familiar with the solar wind than you even realize.

What is solar wind?

The solar wind is the constant stream of charged particles flowing outward from the Sun.

It is produced by the outward expansion of plasma from the Sun's surface caused by the extreme heat of the nuclear fusion reactions taking place inside the Sun.

This heat creates an enormous amount of outward pressure, which pushes against the inward pull of the Sun's gravity. This balance between expanding plasma and the force of contraction produces the Sun's atmosphere, known as the corona.

The solar wind is created when the heat and pressure from below are more powerful than gravity, and the charged plasma in the corona is able to escape into space along the lines of the Sun's magnetic field.

This process is occurring all along the surface of the Sun, and since the Sun rotates, this has the effect of winding the lines of its magnetic field above its poles. And since the charged particles escaping from the Sun travel along those magnetic field lines, they ride this spiral out into space in a constant stream.

Is solar wind actual wind?

The solar wind: how the Sun makes its own space weather
Source: NASA

The solar wind is composed of charged particles, so it's not a perfect analog to the kind of wind we have here on Earth, but there are some very striking similarities.

While solar wind is constantly being produced, its intensity is anything but constant, much like winds here on Earth.

During lulls in the 11-year solar cycle, when complex structures like coronal holes and Sunspots are less frequent, solar wind could be likened to a  breeze. During periods when the Sun is more active, the solar wind takes on a much more complex character.

While these could be considered analogs for seasonal shifts in Earth's climate — like hurricane season or snowy winters in higher latitudes — the comparison is a bit more complicated owing to the charges of the particles in the wind itself.

A more direct analog to Earth's wind though is how solar wind typically flows off the Sun above its polar regions nearly twice as fast as it does near its equator.

This difference in solar wind speeds at various solar latitudes produces high- and low-density regions and high-speed wind interacting with the lower-speed wind to create what are known as corotating interaction regions.

These regions have a much higher density and a strong magnetic field. If we're keeping with the analogies here, these corotating interaction regions are akin to storm fronts here on Earth.

They don't call it space weather for nothing.

What is solar wind made of?

The solar wind: how the Sun makes its own space weather
A white light image of the solar corona during the period of totality of a solar eclipse | Source: M. Druckmüller/NASA

The Sun is primarily hydrogen gas, but there are a number of other elements being produced through the nuclear fusion process, and they all makeup solar wind.

The vast majority of solar wind is made of ionized hydrogen (individual protons and electrons), followed by less than 10% ionized helium.

Helium is the primary byproduct of the Sun's nuclear reactions, given the abundance of hydrogen at this stage of the Sun's life, but that helium isn't just sitting around waiting for stuff to do.

The reactions that occur in the core of the Sun are complex, to say the least, but hydrogen and helium are fusing with other particles to produce lithium, beryllium, carbon, oxygen, nitrogen, silicon, and other heavy elements, up to and including iron.

All of these elements can be found in the solar wind, though heavier elements are a tiny fraction of its overall composition.

The only thing you won't find, though, are many atoms with an equal number of electrons to protons.

The temperature of the Sun simply strips electrons from any atomic nuclei to produce a charged plasma and a mass of free electrons that get pulled toward opposite magnetic field lines once they are radiated up through the Sun's inner layers and into the Sun's corona. So, although the solar wind is electrically balanced, it consists almost exclusively of charged particles.

The magnetic field lines that solar wind rides away from the Sun generally keep electrons and ionized particles separated as the wind passes out into the solar system.

What is the effect of solar wind?

The solar wind is a highly charged stream of particles traveling at speeds of hundreds of miles a second. Even though we're only talking about electrons and individual charged ions of mostly hydrogen, you wouldn't think that solar wind would have that big of an effect.

You need to remember though that we are talking about a lot of charged particles blowing off the Sun. If you've ever seen a total solar eclipse, when the bright ring of fire it produces becomes visible, you're looking at the Sun's corona.

Those flame-like fingers stretching out into space can extend for millions of miles before gradually transforming into the solar wind.

If we got hit by a single blast of solar wind here on Earth, at this distance, the effect would barely register (aside from some particulars that we'll get to in a moment). But the thing is, nothing in the solar system is getting hit with a single blast of particles, we're being sprayed by a firehose of protons and electrons, and those particles are going to be interacting with atoms up and down the line.

Over time, these interactions can be devastating. It's believed that Mars once had a very Earth-like atmosphere as recently as 3 billion years ago, but today little of it remains.

The current consensus is that the steady stream of charged particles effectively sandblasted Mars' atmosphere out into space. The lack of a protective atmosphere might then have evaporated Mars' primordial lakes and oceans, leaving behind a barren, lifeless world.  

Everything in the solar system is affected by the solar wind one way or the other, though some things—like Earth—hold up better than others.

What protects the Earth from solar wind?

Fortunately, Earth has the benefit of a solid inner core and a liquid outer core, both made mostly of iron. This acts as a dynamo that produces a strong magnetic field around the planet.

And since solar wind is composed of charged particles, it interacts very strongly with that magnetic field and rides the magnetic field lines around Earth, instead of blasting Earth head-on in a non-stop bombardment.

Without a strong magnetic field, Earth would probably look a lot like Mars today, and it's likely that life would never have evolved on Earth past the most basic archaea and single-celled organisms before the planet became inhospitable to life as we know it.

So what happens when solar wind hits Earth?

The solar wind: how the Sun makes its own space weather
The Northern Lights in the sky over Alaska in 2017 | Source: NASA/Terry Zaperach

While Earth's magnetic field is strong enough to deflect most of the solar winds, some of it still manages to make it through at points where the magnetic field lines that the particles are riding pass through the Earth's magnetic poles.

These particles interact with the atmosphere and produce a spectacular light show that we call auroras. Usually confined to the very northern and southern latitudes around the arctic and antarctic circles, when the solar wind is stronger than usual—such as during especially active periods of the Sun's cycle—these auroras can be seen at lower latitudes.

Now, none of this means that solar wind hasn't been a problem for us in recent years. Before widespread electrification and the invention of electronics, our relationship to solar wind really was just confined to the presence of auroras. Once we started using electricity to power more of our modern world, however, the solar wind has become much more of an issue.

Since solar wind definitionally carries a strong electromagnetic charge, it can severely damage electrical infrastructure and fry electronic equipment like satellites in low Earth orbit.

And even when it doesn't outright destroy something, the solar wind can still have unpredictable effects on our environment that we're only just beginning to understand. SpaceX lost 40 brand new Starlink satellites shortly after deployment when powerful solar winds interacted with Earth's atmosphere, causing it to expand outward.

This created an unexpected and sharp increase in atmospheric drag that the satellites experienced and prevented them from climbing to their planned orbital height. As a result, the satellites had to be abandoned to burn up in the atmosphere, although the solar wind didn't cause any direct damage to the satellites themselves.

Who discovered solar wind?

The solar wind: how the Sun makes its own space weather
Eugene Parker, who discovered solar wind in the 1950s, attends a NASA pre-briefing on the solar probe that bears his name in 2018 | Source: NASA/Kim Shiflett

Given that two of the most dramatic celestial sights—auroras and a total solar eclipse—are both intimately connected to solar winds, it's somewhat surprising that nobody made the connection until a US physicist named Eugene Parker in 1957.

Proposing a complex interaction between fusion, electromagnetism, and highly-charged solar plasma, Parker called the result "solar wind." When Parker submitted a paper on the idea to the Astrophysical Journal, the reviewers were scathing. However, the editor of the journal, the renowned astrophysicist Subrahmanyan Chandrasekhar, couldn’t find anything wrong with Parker’s math, so he overruled the reviewers and published the paper. 

Just three years later, NASA's Mariner II probe took readings on its journey to Venus that proved Parker correct and his theory soon became the scientific consensus.

Once humanity began putting satellites into orbit, we were able to confirm more detail about solar wind itself.

"Much of his pioneering work, which has been proven by subsequent spacecraft, defined a great deal of what we know about the how the Sun-Earth system interacts," NASA wrote of Parker.

"More than half a century later, the Parker Solar Probe mission now provides key observations on Parker’s groundbreaking theories and ideas, which have informed a generation of scientists about solar physics and the magnetic fields around stars." 

What is the difference between a solar flare and solar wind?

The solar wind: how the Sun makes its own space weather
A coronal mass ejection, which is sometimes preceded by solar flares, though we don't know quite how or even if they are related phenomena | Source: SOHO (ESA/NASA)

Solar flares and solar wind sound similar but there are some very key differences between the two since they are very different phenomena.

First, solar flares are the sudden, localized eruption of electromagnetic radiation in the Sun's atmosphere; they aren't an eruption of particles.

Second, they are produced by a different process than the solar wind. There is still debate on the matter, but it is broadly believed that solar flares occur in highly magnetically active regions of the solar atmosphere when charged particles are accelerated near the speed of light and interact with the Sun's plasma.

One possible cause for this might be something known as magnetic reconnection, a physical process where the strong magnetic field lines interact and rearrange themselves. This takes magnetic energy and converts it into kinetic and thermal energy, which can produce the sudden acceleration of charged particles like electrons.

One theory of how this happens in the Sun is when solar arcades, a tight series of coronal loops, form along magnetic field lines that then magnetically reconnect with another arcade and form a suddenly magnetically disconnected helix.

This disconnection would then release an enormous amount of energy across the electromagnetic spectrum, which we see as a solar flare.

The reason most people associate solar flares with the solar wind is what we can sometimes see after a solar flare. This burst of energy in the corona is sometimes followed by a coronal mass ejection, which is a sudden eruption of charged plasma out of the corona and into the solar wind.

In our space weather system of analogies, a coronal mass ejection is to solar wind what a hurricane is to a pleasant day at the beach. Most of our concerns about space weather taking down satellites, knocking out power grids, and even bringing down the internet is really just about coronal mass ejections.

The amount of plasma that can be ejected into space by a coronal mass ejection is orders of magnitude greater than an active solar wind event, and if the Earth were to be hit by a large coronal mass ejection today, it would overpower Earth's magnetic field and directly bombard the planet with highly charged particles.

To be clear, other than those with pacemakers or similar medical devices, this would not make all that much of a difference to life on the ground.

Few people would notice anything different—except for maybe staring at the sky over Equador in wonder as it suddenly fills with auroras.

In fact, scientists believe that the Earth has been hit with such coronal mass ejections countless times over the past 4.5 billion years, and we're all still here.

Still, we live in the modern world, and given their frequency and simple probability, we're going to get hit with another massive coronal mass ejection at some point in the future—probably sooner than most of us realize.

We should absolutely prepare for that inevitability, and an important part of that is studying space weather and the role that solar wind plays in shaping the solar system we live in.

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