Wednesday, January 31, 2024

SPACE

The hottest catalog of the year: the most comprehensive list of slow-building solar flares yet


Peer-Reviewed Publication

UNIVERSITY OF CALIFORNIA - SAN DIEGO

sun with solar flares 

IMAGE: 

THIS IMAGE, TAKEN ON AUG. 5, 2023, SHOWS A BLEND OF EXTREME ULTRAVIOLET LIGHT THAT HIGHLIGHTS THE INTENSELY HOT MATERIAL IN FLARES AND WHICH IS COLORIZED IN RED AND ORANGE.

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CREDIT: (CR: NASA/GSFC/SDO)




Solar flares occur when magnetic energy builds up in the Sun’s atmosphere and is released as electromagnetic radiation. Lasting anywhere from a few minutes to a few hours, flares usually reach temperatures around 10 million degrees Kelvin. Because of their intense electromagnetic energy, solar flares can cause disruptions in radio communications, Earth-orbiting satellites and even result in blackouts.

Although flares have been classified based on the amount of energy they emit at their peak, there has not been significant study into differentiating flares based on the speed of energy build-up since slow-building flares were first discovered in the 1980s. In a new paper in Solar Physics, a team, led by UC San Diego astrophysics graduate student Aravind Bharathi Valluvan, has shown that there is a significant amount of slower-type flares worthy of further investigation.

The width-to-decay ratio of a flare is the time it takes to reach maximum intensity to the time it takes to dissipate its energy. Most commonly, flares spend more time dissipating than rising. In a 5-minute flare, it may take 1 minute to rise and 4 minutes to dissipate for a ratio of 1:4. In slow-building flares, that ratio may be 1:1, with 2.5 minutes to rise and 2.5 minutes to dissipate.

Valluvan was a student at the Indian Institute of Technology Bombay (IITB) when this work was conducted. Exploiting the increased capabilities of the Chandrayaan-2 solar orbiter, IITB researchers used the first three years of observed data to catalog nearly 1400 slow-rising flares — a dramatic increase over the roughly 100 that had been previously observed over the past four decades. 

It was thought that solar flares were like the snap of a whip — quickly injecting energy before slowly dissipating. Now seeing slow-building flares in such high quantities may change that thinking.

“There is thrilling work to be done here,” stated Valluvan who now works in UC San Diego Professor of Astronomy and Astrophysics Steven Boggs’ group. “We’ve identified two different types of flares, but there may be more. And where do the processes differ? What makes them rise and fall at different rates? This is something we need to understand.”

Lopsided galaxies shed light on the speed of dark matter


Peer-Reviewed Publication

ESTONIAN RESEARCH COUNCIL

Dynamical friction illustration 

IMAGE: 

DYNAMICAL FRICTION. THE PANELS DEPICT SPARSE AREAS OF THE UNIVERSE WITH DARK COLOUR AND DENSE AREAS WITH LIGHT COLOUR. THE UPPER PANELS SHOW THE DENSITY AROUND A GALAXY IF THE GALAXY'S GRAVITY BENDS (LEFT) OR DOES NOT BEND (RIGHT) THE TRAJECTORIES OF DARK MATTER PARTICLES. THE LOWER PANEL SHOWS THE DIFFERENCE BETWEEN THEM, OR HOW THE GALAXY AFFECTS THE DISTRIBUTION OF DARK MATTER. THE ARROWS REPRESENT THE ACCELERATION CAUSED BY THE OVERDENSITY BEHIND THE GALAXY, FROM WHICH THE FRICTION ON THE CENTRE OF THE GALAXY IS DEDUCTED. SINCE THE ARROWS HAVE DIFFERENT DIRECTIONS AND STRENGTHS IN DIFFERENT AREAS, THE TIDAL FORCES ARE ABLE TO CHANGE THE SHAPE OF A GALAXY.

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CREDIT: RAIN KIPPER




So how can the speed of dark matter be measured? The prerequisite is to find a galaxy in the universe that moves relative to dark matter. Since everything in the universe is in motion and there is a great deal of dark matter, it is not difficult to find such galaxies.

Heavy objects, like galaxies, attract all types of matter, whether it is dark matter or visible matter that we encounter on a daily basis. As dark matter moves past a galaxy, the galaxy begins to pull the dark matter particles towards it. However, the change of speed direction of the particles takes time. Before their trajectory curves towards the galaxy, they already manage to pass the galaxy.

Thus, dark matter particles do not enter the galaxy, but instead move behind the galaxy (see video). Behind the galaxy, therefore, the density of matter increases, and this leads to a slowdown of the galaxy – a phenomenon called dynamical friction. The strength of dynamical friction, in turn, depends on how quickly dark matter particles pass the galaxy, that is, how long the galaxy has time to change the trajectory of the dark matter particles. When particles pass slowly, the density of matter increases closer to the galaxy, causing it to slow down more.

The green dot represents a galaxy, and the upper panels show the movement of dark matter particles past the galaxy (if a galaxy exists in the corresponding panel). The lower panels show the shape of all the trajectories, demonstrating that the gravity field of a galaxy affects the particles of matter, creating an overdensity behind the galaxy. Overdensity again slows down the galaxy and distorts its shape.

Let us assume that the galaxy causing the dynamical friction is not tiny, but large. In this case, the overdensity behind it generates friction of different strengths at different points in the galaxy, as seen in Figure 1. The difference in friction makes the shape of the galaxy more lopsided. We experience a similar change in shape on Earth as tidal cycles – high tides and low tides caused by the gravity of the moon.
It is irrelevant how big the dark matter particles eventually turn out to be – their orbit still curves behind the galaxy. The method might not produce accurate results if the particles were comparable in size to the galaxies themselves. However, these dark matter models are already excluded.

Finding the lopsided galaxies themselves is not difficult, because they make up about 30 percent of all galaxies in outer space. Of course, a lot depends on how far to look in the outer parts of a galaxy and what level of lopsidedness deems a galaxy lopsided.

Also, the lopsided shape of a galaxy may not be caused only by dynamical friction. There are a number of other reasons for that. For example, galaxies that were formed after the collision of several galaxies may be asymmetric. In this case, however, we should be able to detect somewhere inside the galaxy the nucleus of another galaxy or a larger stellar halo. Galactic lopsidedness can also be caused by a constant inflow of gas. In such situations, the shape of the galaxy will take a few billion years to recover.

Thus, to measure the velocities of dark matter, we need a lopsided galaxy that is as isolated from other galaxies as possible. In this case, it is more certain that nothing has happened to it other than the passage of dark matter.

In this research, we have figured out how to precisely calculate the forces that affect galaxies in tidal cycles. The next stage is to find galaxies sufficiently lopsided in the universe to study the velocity of dark matter relative to the galaxies.

Cosmology is an important test polygon of theoretical physics. Calculating the speed of dark matter can be important for testing new dark matter models and lifting the veil of secrecy over the nature of dark matter.

Dynamical friction video [VIDEO] |


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