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December 7, 2019 at 11:25 pm #9969Anonymous
THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 504 2019 December 8
Here is the latest round-up of news from the Society for Popular Astronomy. The SPA is arguably Britain's liveliest astronomical society, with members all over the world. We accept subscription payments online at our secure site and can take credit and debit cards. You can join or renew via a secure server or just see how much we have to offer by visiting
NEXT SOLAR CYCLE WILL BE LOWEST IN 200 YEARS
The forecast for the next solar cycle (25) suggests that it will be the weakest of the last 200 years. The maximum of this next cycle — measured in terms of sunspot number, a standard measure of solar activity level — could be 30 to 50% lower than the most recent one. The agency's results show that the next cycle will start in 2020 and reach its maximum in 2025.
The new research combined observations from two NASA space missions — the Solar and Heliospheric Observatory and the Solar Dynamics Observatory — with data collected since 1976 from the ground-based National Solar Observatory. One challenge for researchers working to predict the Sun's activities is that scientists don't yet completely understand the inner workings of our star. Plus, some factors that play out deep inside the Sun cannot be measured directly. They have to be estimated from measurements of related phenomena on the solar surface, like sunspots. The method differs from other prediction tools in terms of the raw material for its forecast.
Previously, researchers used the number of sunspots to represent indirectly the activity of the solar magnetic field. The new approach takes advantage of direct observations of magnetic fields emerging on the surface of the Sun — data which have only existed for the last four solar cycles. Mathematically combining the data from the three sources of Sun observations with the estimates of its interior activity generated a forecast designed to be more reliable than from any of those sources alone. In 2008 the researchers used this method to make their prediction, which was then put to the test as the current solar cycle unfolded over the last decade. It has performed well, with the forecast strength and timing of the solar maximum aligning closely with reality.
NASA attempts to paint the upcoming solar shutdown as a window of opportunity for space missions. The improving ability to make such predictions about space weather are good news for mission planners who can schedule human exploration missions during periods of lower radiation. NASA is effectively forecasting a return to the Dalton Minimum (1790-1830) but gives no mention of the brutal cold, crop loss, famine, war and powerful volcanic eruptions associated with it. Like the deeper Maunder and Sporer Minima preceding it, the Dalton brought on a period of lower-than-average global temperatures. The Oberlach Station in Germany, for example, experienced a 2C decline over 20 years, which devastated the country's food production. The Year Without a Summer also occurred during the Dalton Minimum, in 1816. It was caused by a combination of already low temperatures plus the after-
effects of the second-largest volcanic eruption in 2000 years: Mount Tambora's, on 1815 April 10. And the story was the same across the world: the potato crop in Ireland rotted in the ground resulting in widespread starvation. In England, France and Germany wheat crops failed, leading to bread shortages and food riots and looting. Northern China was also hard
hit, with thousands of people starving to death, while in southern Asia, torrential rains triggered a cholera epidemic that killed many more. The year 1816 went on to earn another, rather more morbid nickname, “Eighteen Hundred and Froze to Death”. Solar Cycle 25 will likely be a mere stop-off on our descent into the next Grand Solar Minimum — a period of even further reduced temperatures and crop yields (research Maunder Minimum, 1645-1715). And there are other researchers still insisting there won't be a solar cycle 25 at all.
GLOBAL STORMS ON MARS
Dust storms are common on Mars. But every decade or so, something unpredictable happens: a series of runaway storms breaks out, covering the entire planet in a dusty haze. Last year, a fleet of NASA spacecraft got a detailed look at the life cycle of the 2018 global dust storm that ended the Opportunity rover's mission. And while scientists are still puzzling over the data, two papers recently shed new light on a phenomenon observed within the storm: dust towers, or concentrated clouds of dust that warm in sunlight and rise high into the air. Scientists think that dust-trapped water vapour may be riding them like a lift to space, where solar radiation breaks apart their molecules. That might help explain how Mars' water disappeared over billions of years. Dust towers are massive, churning clouds that are denser
and climb much higher than the normal background dust in the thin Martian atmosphere. While they also occur under normal conditions, the towers appear to form in greater numbers during global storms.
A tower starts at the planet's surface as an area of rapidly lifted dust about 60 km wide. By the time a tower reaches a height of 80 kilometres, as seen during the 2018 global dust storm, it may be as wide as 220 km. As the tower decays, it can form a layer of dust 56 kilometres above the surface that can be wider than the continental United States. The recent findings on dust towers come courtesy of NASA's Mars Reconnaissance Orbiter (MRO), which is led by the agency's Jet Propulsion Laboratory in Pasadena, California. Though global dust storms cloak the planet's surface, MRO can use its heat-sensing Mars Climate Sounder instrument to see through the haze. The instrument is designed specifically for measuring dust levels. Its data, coupled with images from a camera aboard the orbiter called the Mars Context Imager (MARCI), enabled scientists to detect numerous swelling dust towers.
FIRST GLOBAL MAP OF TITAN COMPLETED
The first map showing the global 'geology' of Saturn's largest moon, Titan, has been completed and fully reveals a dynamic world of dunes, lakes, plains, craters and other terrains. Titan is the only planetary body in our solar system other than Earth known to have stable liquid on its surface. But instead of water raining down from clouds and filling lakes and seas as on Earth, on Titan what rains down is methane and ethane — hydrocarbons that we think of as gases but are liquids in Titan's frigid climate. Despite the different materials, temperatures and gravity fields between Earth and Titan, many surface features are similar between the two worlds
and can be interpreted as being products of the same geological processes. The map shows that the different geological terrains have a clear distribution with latitude, globally, and that some terrains cover far more area than others. Scientists used data from NASA's Cassini mission, which operated between 2004 and 2017 and did more than 120 flybys of the Mercury-size moon. Specifically, they used data from Cassini's radar imager to penetrate Titan's opaque atmosphere of nitrogen and methane. In addition, the team used data from Cassini's visible and infrared instruments, which were able to capture some of Titan's larger geological features through the methane haze. The Cassini mission revealed that Titan is a geologically active world, where hydrocarbons like methane and ethane take the role that
water has on Earth. The hydrocarbons rain down on the surface, flow in streams and rivers, accumulate in lakes and seas, and evaporate into the atmosphere. It's quite an astounding world!
CLOSER TO SIGNAL FROM COSMIC DAWN
Around 12 billion years ago, the Universe emerged from a great cosmic dark age as the first stars and galaxies lit up. With a new analysis of data collected by the Murchison Widefield Array (MWA) radio telescope, scientists are now closer than ever to detecting the ultra-faint signature of this turning point in cosmic history. The researchers configured the MWA
specifically to look for the signal of neutral hydrogen, the gas that dominated the Universe during the cosmic dark age. The analysis sets a new limit — the lowest limit yet — for the strength of the neutral hydrogen signal. Despite its importance in cosmic history, little is known about the period when the first stars formed, which is known as the Epoch of Reionization (EoR). The first atoms that formed after the Big Bang were positively charged hydrogen ions — atoms whose electrons were stripped away by the energy of the infant Universe. As the Universe cooled and expanded, hydrogen atoms reunited with their electrons to form neutral hydrogen. And that's just about all there was in the Universe until about 12 billion years
ago, when atoms started clumping together to form stars and galaxies. Light from those objects re-ionized the neutral hydrogen, causing it to largely disappear from interstellar space. The goal of projects like the one happening at MWA is to locate the signal of neutral hydrogen from the dark ages and measure how it changed as the EoR unfolded. Doing so could reveal
new and critical information about the first stars — the building blocks of the Universe we see today. But catching any glimpse of that 12-billion-year-old signal is a difficult task that requires instruments with very high sensitivity.
When it began operating in 2013, the MWA was an array of 2,048 radio antennae arranged across the remote countryside of Western Australia. The antennae are bundled together into 128 'tiles', whose signals are combined by a supercomputer called the Correlator. In 2016, the number of tiles was doubled to 256, and their configuration across the landscape was altered to improve their sensitivity to the neutral hydrogen signal. The new paper is the first analysis of data from the expanded array. Neutral hydrogen emits radiation at a wavelength of 21 centimetres. As the Universe has expanded over the past 12 billion years, the signal from the EoR is now stretched to about 2 metres, and that's what MWA astronomers are looking for. The problem is there are myriad other sources that emit at the same wavelength — human-made sources like digital television as well as natural sources from within the Milky Way and from millions of other galaxies. The other sources are many orders of magnitude stronger than the signal we're trying to detect. Even an FM radio signal that's reflected off an aeroplane that
happens to be passing above the telescope is enough to contaminate the data. To home in on the signal, the researchers use a myriad of processing techniques to weed out those contaminants. At the same time, they account for the unique frequency responses of the telescope itself. Those data-analysis techniques, combined with the expanded capacity of the telescope itself, resulted in a new upper bound of the EoR signal strength. It's the second consecutive best-limit-to-date analysis to be released by MWA and raises hope that the experiment will one day detect the elusive EoR signal.
HOW BLACK HOLES AND NEUTRON STARS SHINE BRIGHT
For decades, scientists have speculated about the origin of the electromagnetic radiation emitted from celestial regions that host black holes and neutron stars — the most mysterious objects in the Universe. Astrophysicists believe that that high-energy radiation — which makes neutron stars and black holes shine bright — is generated by electrons that move at nearly the speed of light, but the process that accelerates these particles has remained a mystery. Now, researchers have employed massive super-computer simulations to calculate the mechanisms that accelerate those particles. They concluded that their energization is a result of the interaction between chaotic motion and reconnection of super-strong magnetic fields. Turbulence and magnetic reconnection — a process in which magnetic field lines tear and rapidly reconnect — conspire together to accelerate particles, boosting them to velocities that approach the speed of light. The region that hosts black holes and neutron stars is permeated by an extremely hot gas of charged particles, and the magnetic field lines dragged by the chaotic motions of the gas drive vigorous magnetic reconnection. It is thanks to the electric field induced by reconnection and turbulence that particles are accelerated to the most extreme energies, much higher than in the most powerful accelerators on Earth, such as the Large Hadron Collider at CERN. When studying turbulent gas, scientists cannot predict chaotic motion precisely. Dealing with the mathematics of turbulence is difficult, and it
constitutes one of the seven “Millennium Prize” mathematical problems. To tackle this challenge from an astrophysical point of view, researchers designed extensive super-computer simulations — among the world's largest ever done in this research area — to solve the equations that describe the turbulence in a gas of charged particles.
They used the most precise technique — the particle-in-cell method — for calculating the trajectories of hundreds of billions of charged particles that self-consistently dictate the electromagnetic fields. And it is this electromagnetic field that tells them how to move. The crucial point of the study was to identify the role that magnetic reconnection plays within the
turbulent environment. The simulations showed that reconnection is the key mechanism that selects the particles that will be subsequently accelerated by the turbulent magnetic fields up to the highest energies. The simulations also revealed that particles gained most of their energy by bouncing randomly at an extremely high speed off the turbulence fluctuations. When the magnetic field is strong, this acceleration mechanism is very rapid. But the strong fields also force the particles to travel in curved paths, and by doing so, they emit electromagnetic radiation. That is indeed the radiation emitted around black holes and neutron stars that make them shine, a phenomenon we can observe on Earth. The ultimate goal is to get to know what is really going on in the extreme environment surrounding black holes and neutron stars, which could shed additional light on fundamental physics and improve our understanding of how our Universe works. The researchers plan to connect their work even more firmly with observations, by comparing their predictions with the electromagnetic spectrum emitted from the Crab Nebula, the most intensely studied bright remnant of a supernova (a star that violently exploded in the year 1054). This will be a stringent test for their theoretical explanation.
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