A comet‘s tail changes from day-to-day depending on how much material the comet is losing and how strong the solar wind it’s facing is. This image sequence shows Comet 12P/Pons-Brooks over nine days in 2024 from March 6th (top) through March 14th (bottom). The variations in the comet’s appearance are striking; some days show nearly no tail while others have long plumes with swirls of turbulence. It’s a reminder that, even if they appear unchanging in the moment you see one, a comet is in constant flux. (Image credit: Shengyu Li & Shaining; via APOD)
Tag: solar wind
The Unusual Auroras of Mars
Earth, Saturn, and Jupiter have auroras at their poles, generated by the interaction of their global magnetic fields with the solar wind. Mars has no global magnetic field, only remnants of one frozen into areas of its crust; yet it, too, has auroras. Mars’s auroras are rarer and discrete. They occur most often over the southern hemisphere, and researchers now think they know why.
Four billion years ago, we think Mars had a global magnetic field, much like Earth does. But somehow the planet lost that field. The traces that remain are caught in the minerals of its crust, much like the ancient magnetic fields recorded in areas of the Earth’s sea floor. These magnetized regions of Mars’s crust, shown above as contours in pink and blue, are where the discrete auroras occur.
Using data from NASA’s MAVEN spacecraft, which orbits Mars, the team discovered a pattern. They found that auroras occur most often when the magnetic lines of the incoming solar wind run antiparallel to the magnetic field lines of the crust. This suggests that the auroras happen as a result of magnetic reconnection, a process where antiparallel magnetic field lines rearrange themselves, releasing energy as a result. Reconnection events provide an opportunity for electrons from the solar wind to accelerate into Mars’s atmosphere, exciting molecules there and generating the auroras. So far we’ve only caught the auroras in UV light, but hopefully one day we’ll see them in visible light as well. (Image credit: R. Lillis et al.; research credit: C. Bowers et al. and B. Johnston et al.; via APS Physics)
Escaping the Sun
One enduring mystery of the solar wind — a stream of high-energy particles expelled from the sun — is how the particles get accelerated in the first place. The sun frequently belches out spurts of plasma, but without further momentum, that material simply falls back to the sun’s surface under the star’s gravity. Mechanisms like shock waves can further accelerate particles that are already moving quickly, but they cannot explain how the particles get going in the first place.
A recent study used supercomputers to tackle this challenging problem in turbulent plasma physics. Each simulation tracked nearly 200 billion particles, requiring tens of thousands of processors. The results showed that turbulence itself provides the necessary initial acceleration and serves as the first step to getting particles moving fast enough to escape the sun. (Image credit: NASA SDO; research credit: L. Comisso and L. Sironi; via Physics World)
A Comet’s Tail Swept Away
On Christmas Day 2021, Comet Leonard put on a show in our skies. Though the comet was a pale streak to the naked eye, photographer Gerald Rhemann caught a striking event: the moment part of the comet’s tail disconnected from its body. The solar wind swept the comet’s gas and dust away. Though I’ve talked about the fluid dynamics of comets before, this image is the most stunning example I’ve seen. It’s no wonder that it won the top prize at the Astronomy Photographer of the Year competition. (Image credit: G. Rhemann; via Colossal; see also APOTY)
Brilliant Auroras
Glowing auroras billow across Canada in this satellite image from a recent geomagnetic storm. As our sun enters a more active part of its solar cycle, we can expect more space weather as the high-energy particles of the solar wind interact with our planet’s magnetic field. The auroras themselves are light released by energetically excited atoms of oxygen and nitrogen high in the upper atmosphere.
Earth is not the only place in the solar system to experience these light shows. With their strong magnetic fields, Jupiter and Saturn have auroras that make Earth’s look paltry in comparison. (Image credit: J. Stevens; via NASA Earth Observatory)
Space Hurricanes
Researchers have observed their first “space hurricane” – a 1,000-km-wide vortex of plasma – in Earth’s upper atmosphere. Like conventional hurricanes, this storm featured precipitation (of electrons rather than rain), a calm eye at its center, and several spiral arms. Based on the group’s model, interactions between the solar wind and Earth’s magnetic fields drive the storm. Interestingly, the storm they observed occurred during a period of low solar and geomagnetic activity, which suggests that such space hurricanes could be frequent, both on Earth and in the upper atmospheres of other planets. (Image credit: Q. Zhang; research credit: Q. Zhang et al.; via Physics World)
Jovian Auroras
Like Earth, Jupiter is home to polar auroras that light the sky as charged particles interact with the planet’s magnetosphere. A recent paper identifies interesting features in the aurora that appear similar to expanding vortex rings (see inset below). Although the researchers cannot yet identify the origin of the rings, they hypothesize that the process begins at the far edges of Jupiter’s magnetosphere where it interacts with the incoming solar wind. One theory posits that shear flows and Kelvin-Helmholtz instabilities where the magnetosphere and solar wind meet drive the phenomenon. (Image credit: Jupiter – NASA, ESA, and J. Nichols, aurora features – NASA/SWRI/JPL-Caltech/SwRI/V. Hue/G. R. Gladstone/B. Bonfond; research credit: V. Hue et al.; via Gizmodo)
Grayscale Aurora
This swirling grayscale image shows a spring aurora over the Hudson Bay, as seen by the Suomi NPP satellite. As energetic particles from the sun zip past Earth, they interact with our magnetosphere, which tends to channel particles toward the poles. At these higher latitudes, some of the particles get trapped along Earth’s magnetic field lines and crash into the upper atmosphere where they excite oxygen and nitrogen molecules. It’s this molecular bombardment that creates the distinctive colors of the aurora. (Image credit: J. Stevens; via NASA Earth Observatory)
Shock Waves in the Solar Wind
The empty space of our solar system is not truly empty, as we’ve discussed previously. For one, there’s a fast-moving flux of charged particles – the solar wind – that flows constantly from the Sun. Sometimes these solar wind particles encounter their interstellar equivalents – charged ions from outside our solar system – and exchange energy.
One predicted mechanism for this energy swap is a solar wind shock wave, which occurs when a faster-moving clump of charged particles plows into a slower-moving one. Scientists hypothesized in the mid 1990s that far from the Sun, solar wind shock waves would lose their energy by passing it to these interstellar ions, in a process known as pickup. Data from the New Horizons spacecraft has finally provided evidence for this theory.
In October 2015, instruments on the spacecraft recorded a shock wave when the speed of solar wind ions nearby jumped from 380 km/s to 440 km/s. Comparing the energies of solar and interstellar ions before and after the event, researchers found that interstellar pickup ions became 30% more energetic while solar ions lost 85% of their energy. It’s an important confirmation of theoretical predictions and should help us better understand high-energy particle physics at the edges of our solar system. (Image credit: NASA; research credit: E. Zirnstein et al., via J. Ouellette)
Where Does the Sun End?
How do you define the edge of our sun? There’s a distinct surface to it, but our star is also surrounded by the corona, an even hotter region of plasma twisted by magnetic fields. The corona is sort of like the sun’s atmosphere. Farther out in the solar system, we receive a constant barrage of charged particles, known as the solar wind, that streams out from the sun. So where does the corona end and the solar wind begin?
Scientists have been studying the flow structure of the solar wind in search of an answer to this question, and they’ve found that there’s a clear transition point about 32 million kilometers from the sun. At this distance, the sun’s magnetic field weakens to the point where it no longer exerts the same hold on the solar particles and they begin to move turbulently, behaving more like a gas than a plasma. With special measurements and image processing, scientists were able to actually see this flow change in the solar wind! (Video/image credit: NASA; research credit: C. DeForest et al.; via FlowViz)
