Bright skies in dark places: how space weather affects the night

Image credit: Joshua Strang/USAF​
1657 words / 7-minute read

Summary: The brightness of the natural night sky, free of artificial skyglow, changes dynamically in response to space weather. Understanding this relationship is important for defining what a "dark" sky truly is. This post explains the origin of space weather, how it affects the Earth-space environment, and why it's important for protecting natural nighttime darkness.

The Sun is the nearest star, a slow-spinning globe of plasma that powers almost all life on Earth. But it's not only a passive radiator of energy that warms our planet. Rather, it's an active and dynamic system that reaches all the way out to us across almost 150 million kilometers. As its substance reaches the Earth, complex interactions yield various effects on the planetary environment. Among those is an influence on the brightness and appearance of the night sky we're still struggling to understand. The emerging view is that "space weather" determines what the night sky looks like in the absence of light pollution. And that turns out to be important in defining what a "dark sky" is. As we approach the peak of the current solar cycle, here we dig into the ways the Sun changes our experience of night.

A lively and changing system

Nuclear reactions taking place deep in its core power the Sun. Protons fuse to make helium nuclei at temperatures approaching 15 millions of degrees. These fusion reactions release light, which takes about a million years to work its way up to the thin edges of the Sun's atmosphere. Along the way there are a lot of electrically charged particles running around. The rotation of the Sun drags the particles along. In turn, that generates a magnetic field. And that's where things get interesting.

A simple view of this magnetic field is like a simple bar magnet, with a "north pole" and a "south pole". On large scales, the field is weak; a typical refrigerator magnet is about ten times more intense. If that were true everywhere on the Sun, not much in the way of interesting phenomena would ever happen. Yet the local strength of the magnetic field can be much higher.

​The lines of the global magnetic field twist as they wrap around the interior of the Sun. Much like winding up a rubber band, the field lines strain under the tension and begin to kink. Some of these contortions emerge from the visible surface of the Sun. We see these protuberances as sunspots. Eventually the contortions burst under pressure, producing what we see as solar "flares". Coronal mass ejections (CMEs) sometimes follow flares. These events release incredible amounts of energy and hot, charged particles into space.

Sunspot numbers since the early 17th century from a mixture of observations and proxy measurements. Source: Robert A. Rohde / Global Warming Art project (CC BY-SA 3.0) 

A few years after this process starts up, it takes about as long to quiet down again. Strong magnetic fields almost disappear at the surface, sunspots disappear, and flares end. The Sun remains quiet for some time. Then the process starts all over again. A complete cycle takes about 11 years to repeat, and we have seen it repeat with near-perfect reliability for at least four centuries.

We are now near the maximum of this cycle, the 25th such event since astronomers began counting in the 1700s. This cycle has an intensity like those over much of the past 250 years. Notable outbursts associated with CMEs have occurred in recent months. In May 2024, millions of people around the world saw auroral displays during a strong solar "storm".  Such events are likely through at least 2025.

'Space weather' and the night sky

The aurora lights up Earth's skies with dramatic, colorful displays, but such events are usually only seen near the poles. Less intense events happen with more frequency. Their effects are more subtle. In places far from city lights, these effects determine what the night sky looks like.
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In 2022, we wrote here that 'the natural night sky is alive with its own light'. The Sun accounts for much of that liveliness. Our planet's own magnetic field shapes the flow of incoming material from the Sun during its outbursts. That can trap significant numbers of charged particles in our magnetic environment. In turn, very large amounts of electrical energy are temporarily stored in space near the Earth.

It's fortunate that the Earth has a strong magnetic field. In fact, it's possible that there would be no life on Earth without its shielding effect. Still, very big solar radiation events can overwhelm this defense. Certain very intense storms, like the Carrington Event of 1859, can actually damage electrical equipment on the ground and in space.

Solar storms cause displays of the aurora, mostly at higher latitudes. Solar flares can ionize the upper atmosphere, triggering intense airglow. This light is much brighter than the background of stars and other sources of light in the nighttime sky. Even at solar minimum, the brightness of the night sky correlates with solar activity levels.

A cartoon of the Earth’s magnetic environment interacting with charged particles from the Sun.
​Source: NASA (public domain)

Some of this takes place continuously throughout the ebb and flow of the solar cycle. We know that the night sky on average tends to be brightest near the equinoxes and darkest near the solstices. This results from something called the Russell-McPherron effect. It has to do with the Earth's magnetic environment being sort of a 'gatekeeper'. Its strength is weakest when the direction of the interplanetary magnetic field points south. That allows more solar material to enter the space right around our planet.

Why this matters to dark-sky conservation

We see tremendous variation in the brightness of the night sky even in places far from cities. And in recent years we have come to better understand why that is. Even until today, many activists, conservationists and researchers have in mind a more quiet night sky. They talk about "pristine" skies as though one number alone characterizes their brightness. Isolated from all other influences, that would be true. But reality is a bit more messy.

​For almost 25 years, DarkSky International (formerly the International Dark-Sky Association) has run a program called International Dark Sky Places. It recognizes efforts around the world that "preserve and protect dark sites through responsible lighting policies and public education." Some of its designation categories include night-sky quality requirements. That, in turn, involves something of a value judgment that concerns what a "dark" sky is. It expressed the value as a series of tiers: Gold, Silver and Bronze. About a decade after the first designation under this system, DarkSky abandoned it. Real night-sky brightness data were too variable and inconsistent to make it workable.

To know what we're losing to light pollution, we need to understand the variation of the natural sky. We need to watch what the natural sky does over many years. That will tend to show both the regular cycles as well as unpredictable disruptions. What's clear already is that no one night, considered in isolation from others, is representative of any site. It takes some time to measure a place to figure out what is "normal". To then know the range over which the variation away from normal occurs takes longer still.

The title of our 2022 post here referenced researcher Al Grauer, who has said that “the natural night sky is not dark. It is alive with its own lights." We contacted him for this post and asked about the intensity of the effect seen in his own data. “Interactions between the Earth’s magnetosphere and the solar wind routinely cause the natural night sky to vary by a factor of two in brightness," Grauer says. While such observations can persuade scientists, it's harder to make the case to the public. That's especially true in the case of people who don't live in or near naturally dark places. Their experiences in those conditions tend to be limited, so they fail to notice the changes around them.

​It turns out, though, that conveying this sense of change may be critical to achieving conservation goals. Few people spend much time outside at night to begin with. They aren't aware of the extent to which the night is lost to light pollution around the world. Honest assessments about the degree of other kinds of pollution were essential in bringing them under control through legal means. There are reasons to think that is also true in this situation.

Bright green airglow waves light up the night sky over Loveland Pass, Colorado.
​Source: Bryce Bradford (CC BY-NC-ND 2.0)

Where we can go next

We wrote here about a recent academic conference at which researchers discussed the idea of "reference sites". The dark-sky movement is more often now leaning on policy makers to take actions to not only slow the advance of light pollution. They want authorities to take steps to restore the night where light pollution has harmed it.

An example is the recently enacted European Union "Nature Restoration Law". Its aim is ambitious: to restore "all ecosystems in need of restoration by 2050". And the law contains some language specific to light pollution: "with artificial light increasing, light pollution has become a pertinent issue." One example of 'restorative measures' in its annex is: "stop, reduce or remediate pollution from ... light in all ecosystems.”

At the same time, we know there is a variable amount of natural light in those ecosystems. Light from the sky relates to the flux of light on the ground, which is relevant to biology. Ecosystems evolved in conditions of variable natural nighttime light. That gives us a clue about the amount of artificial light at night they can tolerate.

We want to tie policy goals to measurable reductions in light pollution. In turn, that ties them to measurable reductions in skyglow and light falling in sensitive areas. But first we must establish these references and track them so we know if reductions are real or not. And that means we have to watch for some years — at least through a solar cycle.

​It's often the case that with every answer science provides more questions. When we interrogate one part of nature, the results may point out deficiency in some other area. This is the nature of discovery and integration of new colors and shades into our picture of nature. We have learned much about the ways space weather changes the night sky. We continue to learn as we gather more data over longer periods of time. This all contributes in meaningful ways to advancing the cause of caring for and protecting the night. The need to do so has never been as great as it is now.