Neptune's disappearing clouds linked to the solar cycle

Astronomers have uncovered a link between Neptune's shifting cloud abundance and the 11-year solar cycle, in which the waxing and waning of the Sun's entangled magnetic fields drives solar activity.

This discovery is based on three decades of Neptune observations captured by NASA's Hubble Space Telescope and the W. M. Keck Observatory in Hawaii, as well as data from the Lick Observatory in California.

The link between Neptune and solar activity is surprising to planetary scientists because Neptune is our solar system's farthest major planet and receives sunlight with about 0.1% of the intensity Earth receives. Yet Neptune's global cloudy weather seems to be driven by solar activity, and not the planet's four seasons, which each last approximately 40 years.

At present, the cloud coverage seen on Neptune is extremely low, with the exception of some clouds hovering over the giant planet's south pole. A University of California (UC) Berkeley-led team of astronomers discovered that the abundance of clouds normally seen at the icy giant's mid-latitudes started to fade in 2019.

"I was surprised by how quickly clouds disappeared on Neptune," said Imke de Pater, emeritus professor of astronomy at UC Berkeley and senior author of the study. "We essentially saw cloud activity drop within a few months," she said.

"Even now, four years later, the most recent images we took this past June still show the clouds haven't returned to their former levels," said Erandi Chavez, a graduate student at the Center for Astrophysics | Harvard-Smithsonian (CfA) in Cambridge, Massachusetts, who led the study when she was an undergraduate astronomy student at UC Berkeley. "This is extremely exciting and unexpected, especially since Neptune's previous period of low cloud activity was not nearly as dramatic and prolonged."

To monitor the evolution of Neptune's appearance, Chavez and her team analyzed Keck Observatory images taken from 2002 to 2022, the Hubble Space Telescope archival observations beginning in 1994, and data from the Lick Observatory in California from 2018 to 2019.

In recent years, the Keck observations have been complemented by images taken as part of the Twilight Zone program and by Hubble's Outer Planet Atmospheres Legacy (OPAL) program.

The images reveal an intriguing pattern between seasonal changes in Neptune's cloud cover and the solar cycle -- the period when the Sun's magnetic field flips every 11 years as it becomes more tangled like a ball of yarn. This is evident in the increasing number of sunspots and increasing solar flare activity. As the cycle progresses, the Sun's tempestuous behavior builds to a maximum, until the magnetic field beaks down and reverses polarity. Then the Sun settles back down to a minimum, only to start another cycle.

When it's stormy weather on the Sun, more intense ultraviolet (UV) radiation floods the solar system. The team found that two years after the solar cycle's peak, an increasing number of clouds appear on Neptune. The team further found a positive correlation between the number of clouds and the ice giant's brightness from the sunlight reflecting off it.

"These remarkable data give us the strongest evidence yet that Neptune's cloud cover correlates with the Sun's cycle," said de Pater. "Our findings support the theory that the Sun's UV rays, when strong enough, may be triggering a photochemical reaction that produces Neptune's clouds."

Scientists discovered the connection between the solar cycle and Neptune's cloudy weather pattern by looking at 2.5 cycles of cloud activity recorded over the 29-year span of Neptunian observations. During this time, the planet's reflectivity increased in 2002 then dimmed in 2007. Neptune became bright again in 2015, then darkened in 2020 to the lowest level ever observed, which is when most of the clouds went away.

The changes in Neptune's brightness caused by the Sun appear to go up and down relatively in sync with the coming and going of clouds on the planet. However there is a two-year time lag between the peak of the solar cycle and the abundance of clouds seen on Neptune. The chemical changes are caused by photochemistry, which happens high in Neptune's upper atmosphere and takes time to form clouds.

"It's fascinating to be able to use telescopes on Earth to study the climate of a world more than 2.5 billion miles away from us," said Carlos Alvarez, staff astronomer at Keck Observatory and co-author of the study. "Advances in technology and observations have enabled us to constrain Neptune's atmospheric models, which are key to understanding the correlation between the ice giant's climate and the solar cycle."

However, more work is necessary. For example, while an increase in UV sunlight could produce more clouds and haze, it could also darken them, thereby reducing Neptune's overall brightness. Storms on Neptune rising up from the deep atmosphere affect the cloud cover, but are not related to photochemically produced clouds, and hence may complicate correlation studies with the solar cycle. Continued observations of Neptune are also needed to see how long the current near-absence of clouds will last.

The research team continues to track Neptune's cloud activity. "We have seen more clouds in the most recent Keck images that were taken during the same time NASA's James Webb Space Telescope observed the planet; these clouds were in particular seen at northern latitudes and at high altitudes, as expected from the observed increase in the solar UV flux over the past approximately 2 years," said de Pater.

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Fast changes between the solar seasons resolved by new sun clock

Violent activity on our Sun leads to some of the most extreme space weather events on Earth, impacting systems such as satellites, communications systems, power distribution and aviation. The roughly 11 year cycle of solar activity has three 'seasons', each of which affects the space weather felt at Earth differently: (i) solar maximum, the sun is active and disordered, when space weather is stormy and events are irregular (ii) the declining phase, when the sun and solar wind becomes ordered, and space weather is more moderate and (iii) solar minimum, when activity is quiet.

In a new study led by the University of Warwick and published in The Astrophysical Journal, scientists found that the change from solar maximum to the declining phase is fast, happening within a few (27 day) solar rotations. They also showed that the declining phase is twice as long in even-numbered solar cycles as it is in odd-numbered cycles.

No two solar cycles are the same in amplitude or duration. To study the solar seasons, the scientists built a sun clock from the daily sunspot number record available since 1818. This maps the irregular solar cycles onto a regular clock. The magnetic polarity of the sun reverses after each roughly 11 year solar cycle giving a roughly 22 year magnetic cycle (named after George Ellery Hale) and to explore this, a 22 year clock was constructed. The effect on space weather at earth can be tracked back using the longest continuous records of geomagnetic activity over the past 150 years, and once the clock is constructed, it can be used to study multiple observations of seasonal solar activity which affect the earth.

With the greater detail afforded by the sun clock, the scientists could see that the switch from solar maximum to the declining phase is fast, occurring within a few (27 day) solar rotations. There was also a clear difference in the duration of the declining phase when the sun's magnetic polarity is 'up' compared to 'down': in even-numbered cycles it is around twice as long as odd-numbered cycles. As we are about to enter cycle 25, the scientists anticipate that the next declining phase will be short.

Lead author Professor Sandra Chapman of the University of Warwick Department of Physics said: "By combining well known methods in a new way, our clock resolves changes in the Sun's climate to within a few solar rotations. Then you find the changes between some phases can be really sharp.

"If you know you've had a long cycle, you know the next one's going to be short, we can estimate how long it's going to last. Knowing the timing of the climate seasons helps to plan for space weather. Operationally it is useful to know when conditions will be active or quiet, for satellites, power grids, communications."

The results also provide a clue to understanding how the Sun reverses polarity after every cycle.

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The sun's clock

Not only the very concise 11-year cycle, but also all other periodic solar activity fluctuations can be clocked by planetary attractive forces. This is the conclusion drawn by Dr. Frank Stefani and his colleagues from the Institute of Fluid Dynamics at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and from the Institute of Continuous Media Mechanics in Perm, Russia. With new model calculations, they are proposing a comprehensive explanation of all important known sun cycles for the first time. They also reveal the longest fluctuations in activity over thousands of years as a chaotic process. Despite the planetary timing of short and medium cycles, long-term forecasts of solar activity thus become impossible, as the researchers in the scientific journal Solar Physics assert.

Solar physicists around the world have long been searching for satisfactory explanations for the sun's many cyclical, overlapping activity fluctuations. In addition to the most famous, approximately 11-year "Schwabe cycle," the sun also exhibits longer fluctuations, ranging from hundreds to thousands of years. It follows, for example, the "Gleissberg cycle" (about 85 years), the "Suess-de Vries cycle" (about 200 years) and the quasi-cycle of "Bond events" (about 1500 years), each named after their discoverers. It is undisputed that the solar magnetic field controls these activity fluctuations.

Explanations and models in expert circles partly diverge widely as to why the magnetic field changes at all. Is the sun controlled externally or does the reason for the many cycles lie in special peculiarities of the solar dynamo itself? HZDR researcher Frank Stefani and his colleagues have been searching for answers for years -- mainly to the very controversial question as to whether the planets play a role in solar activity.

Rosette-shaped movement of the sun can produce a 193-year cycle

The researchers have most recently taken a closer look at the sun's orbital movement. The sun does not remain fixed at the center of the solar system: It performs a kind of dance in the common gravitational field with the massive planets Jupiter and Saturn -- at a rate of 19.86 years. We know from the Earth that spinning around in its orbit triggers small motions in the Earth's liquid core. Something similar also occurs within the sun, but this has so far been neglected with regard to its magnetic field.

The researchers came up with the idea that part of the sun's angular orbital momentum could be transferred to its rotation and thus affect the internal dynamo process that produces the solar magnetic field. Such coupling would be sufficient to change the extremely sensitive magnetic storage capacity of the tachocline, a transition region between different types of energy transport in the sun's interior. "The coiled magnetic fields could then more easily snap to the sun's surface," says Stefani.

The researchers integrated one such rhythmic perturbation of the tachocline into their previous model calculations of a typical solar dynamo, and they were thus able to reproduce several cyclical phenomena that were known from observations. What was most remarkable was that, in addition to the 11.07-year Schwabe cycle they had already modeled in previous work, the strength of the magnetic field now also changed at a rate of 193 years -- this could be the sun's Suess-de Vries cycle, which from observations has been reported to be 180 to 230 years. Mathematically, the 193 years arise as what is known as a beat period between the 19.86-year cycle and the twofold Schwabe cycle, also called the Hale cycle. The Suess-de Vries cycle would thus be the result of a combination of two external "clocks": the planets' tidal forces and the sun's own movement in the solar system's gravitational field.

Planets as a metronome

For the 11.07-year cycle, Stefani and his researchers had previously found strong statistical evidence that it must follow an external clock. They linked this "clock" to the tidal forces of the planets Venus, Earth and Jupiter. Their effect is greatest when the planets are aligned: a constellation that occurs every 11.07 years. As for the 193-year cycle, a sensitive physical effect was also decisive here in order to trigger a sufficient effect of the weak tidal forces of the planets on the solar dynamo.

After initial skepticism toward the planetary hypothesis, Stefani now assumes that these connections are not coincidental. "If the sun was playing a trick on us here, then it would be with incredible perfection. Or, in fact, we have a first inkling of a complete picture of the short and long solar activity cycles." In fact, the current results also retroactively reaffirm that the 11-year cycle must be a timed process. Otherwise, the occurrence of a beat period would be mathematically impossible.

Tipping into chaos: 1000-2000-year collapses are not more accurately predictable

In addition to the rather shorter activity cycles, the sun also exhibits long-term trends in the thousand-year range. These are characterized by prolonged drops in activity, known as "minima," such as the most recent "Maunder Minimum," which occurred between 1645 and 1715 during the "Little Ice Age." By statistically analyzing the observed minima, the researchers could show that these are not cyclical processes, but that their occurrence at intervals of approximately one to two thousand years follows a mathematical random process.

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Solar activity reconstructed over a millennium

 What goes on in the sun can only be observed indirectly. Sunspots, for instance, reveal the degree of solar activity -- the more sunspots are visible on the surface of the sun, the more active is our central star deep inside. Even though sunspots have been known since antiquity, they have only been documented in detail since the invention of the telescope around 400 years ago. Thanks to that, we now know that the number of spots varies in regular eleven-year cycles and that, moreover, there are long-lasting periods of strong and weak solar activity, which is also reflected in the climate on Earth.

However, how solar activity developed before the start of systematic records has so far been difficult to reconstruct. An international research team led by Hans-Arno Synal and Lukas Wacker of the Laboratory of Ion Beam Physics at ETH, which included the Max Planck Institute for Solar System Research in Göttingen and Lund University in Sweden, has now traced back the sun's eleven-year cycle all the way to the year 969 using measurements of the concentration of radioactive carbon in tree rings. At the same time, the researchers have thus created an important database for more precise age determination using the C14 method. Their results were recently published in the scientific journal Nature Geoscience.

Solar activity from tree rings

To reconstruct solar activity over a millennium with an extremely good time resolution of just one year, the researchers used tree-ring archives from England and Switzerland. In those tree rings, whose ages can be precisely determined by counting the rings, there is a tiny fraction of radioactive carbon C14, with only one out of every 1000 billion atoms being radioactive. From the known half-life of the C14 isotope -- around 5700 years -- one can then deduce the concentration of radioactive carbon present in the atmosphere when the growth ring was formed. As radioactive carbon is mainly produced by cosmic particles, which in turn are kept away from the Earth to a greater or lesser extent by the magnetic field of the sun -- the more active the sun, the better it shields the Earth -- it is possible to deduce solar activity from a change in the concentration of C14 in the atmosphere.

Better results through modern detection techniques

Precise measurements of a change in that already very small concentration, however, resemble the search for a grain of dust on a needle in a huge haystack. "The only measurements of that kind were made in the 80's and 90's," says Lukas Wacker, "but only for the last 400 years and using the extremely laborious counting method." In that method, radioactive decay events of C14 in a sample are directly counted using a Geiger counter, which requires a relatively large amount of material and, owing to the long half-life of C14, even more time. "Using modern accelerator mass spectrometry we were now able to measure the C14 concentration to within 0.1 percent in just a few hours with tree-ring samples that were a thousand times smaller," adds PhD student Nicolas Brehm, who was responsible for those analyses.

In accelerator mass spectrometry, C14 and C12 atoms (the "normal," non-radioactive carbon; C14, by contrast, contains two additional neutrons in its nucleus) of the tree material are first electrically charged and then accelerated by an electric potential of several thousand volts, after which they are sent through a magnetic field. In that magnetic field the two carbon isotopes, which have different masses, are deflected to different degrees and can thus be counted separately. To eventually obtain the desired information on solar activity from that raw data, the researchers have to perform some intricate statistical analysis on it and further process the results using computer models.

Regular eleven-year cycle over a millennium

This procedure enabled the researchers to seamlessly reconstruct solar activity from 969 to 1933. From that reconstruction they could confirm the regularity of the eleven-year cycle as well as the fact that the amplitude of that cycle (by how much the solar activity goes up and down) is also smaller during long-lasting solar minima. Such insights are important for a better understanding of the internal dynamics of the sun. The measurement results also allowed a confirmation of the solar energetic proton event of 993. In such an event, highly accelerated protons that reach the Earth during a solar flare cause a slight overproduction of C14. Moreover, the research team also found evidence of two further, as yet unknown events in 1052 and 1279. This could indicate that such events -- which can severely disturb electronic circuits on Earth and in satellites -- happen more frequently than previously thought.

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