The multifaceted sun
The sun has many faces, and the human eye can only see one side of it in visible light (do not observe the sun directly with the naked eye!). ）。 But in addition to visible light, the sun constantly radiates other forms of light outward: from radio waves with the longest wavelength to gamma rays with the shortest wavelength.
electromagnetic spectrum.
In the 17th century, when the telescope was first invented, it, like our naked eye, could only collect visible light. Nowadays, with the rapid development of technology, telescopes that can detect light in different wavelengths continue to appear, giving astronomers the opportunity to write a complete story of the sun.
Today, we will focus on the X-rays emitted by the sun.
The energy of X-rays ranges from about 0.1 keV to hundreds of keV, and it is also divided into hard X-rays and soft X-rays. The peak flow of soft X-rays is also used as a standard to measure the level of solar flare bursts, and hard X-rays are what we are looking at today.
X-ray astronomy
At the end of the 19th century, when Wilhelm Roentgen accidentally discovered X-rays, he certainly did not anticipate how great an impact X-rays would have on our lives 100 years from now. In the fields of medicine, airport security, environmental monitoring, materials processing, semiconductor and microchip manufacturing, we can see a wide range of X-ray applications.
Amazingly, X-rays not only change our lives, but also give us a whole new window into the universe. In the universe, regions with high temperatures and large numbers of energetic particles, such as around black holes, supernova remnants, and solar flares, emit large amounts of X-rays. However, unlike visible light, most X-rays from outer space cannot directly penetrate the atmosphere to reach the ground, and they are blocked by the atmosphere. This makes it extremely difficult to observe the universe with X-rays.
It was not until 1957 that Sputnik 1 was successfully launched, becoming the first artificial satellite to enter space. This marked the official opening of the space age and the beginning of X-ray astronomy. X-ray Observatory Chandra, XMM Newton (XMM-Newton), Nuclear Spectroscopic Telescope Array (NuSTAR), etc. are all very famous X-ray space missions.
Due to the absorption of X-rays by the Earth's atmosphere, X-ray detection cannot be carried out on the ground and must be observed in space through high-altitude balloons, sounding rockets or satellites. The satellite in the X-ray band is ASO-S.
In order to conduct X-ray detection of the sun, so far, many solar exploration satellites have been launched internationally, such as the Sun Extreme Messenger (SMM), the Firebird (HINOTORI), the Sunshine (Yohkoh), the Solar High Energy Spectral Imaging Probe (RHESSI) and so on.
The protagonist of our discussion today is one of the payloads of the Advanced Space-based Solar Observatory (ASO-S), the "Hard X-ray Imager (HXI)", whose main scientific goal is to make spectroscopic and imaging observations of the solar flare's high-energy radiation in the energy band of about 30keV-200keV.
Solar flares with hard X-rays
Flares are a sudden, rapid brightening phenomenon on the surface of the Sun, in which the flow of X-rays can be enhanced by more than 5 orders of magnitude. Driven by flares, free electrons in the solar plasma heat up and accelerate to become high-energy electrons. Among them, high-temperature plasmas produce thermal radiation, and high-energy electrons move at high speeds along the magnetic field lines of the flare ring, even close to the speed of light. These high-energy, high-speed electrons carry enormous amounts of energy and also generate radiation during transmission motion, which is important for understanding the flare process.
However, in the process of studying these high-energy electrons, a series of problems to be solved have arisen:
How is the huge amount of energy in the flare released?
How many electrons in the flare have been accelerated? Where do they come from?
How do they accelerate, propagate, and radiate?
Where do they eventually disappear?
To solve these problems, we need to look for clues in the hard X-rays produced by the flare.
We know that radiation is generated when the speed of charged particles changes (accelerates or decelerates). Both electrons and ions radiate, but because ions are much more massive than electrons, it is much more difficult to change speed, so the radiation they produce is negligible. In solar flares, high-energy electrons interact with surrounding thermal ions (mainly protons) to lose energy, thereby being slowed down. During the change in speed, so-called radiative radiation (or brake radiation) is generated. In flares, those radiation produced by non-thermal electrons are called non-thermal radiation, usually produced by the relativistic electrons far from the thermal equilibrium through the radiation. This radiation is also found in high-energy radiation from auroras, lightning, and X-ray machines used to produce laboratory X-rays.
Radiation-induced.
Since hard X-rays mainly come from the interaction of solar flare high-energy electrons with hot plasma during the transmission of the solar atmosphere, the detection of hard X-rays, especially imaging, can give us a better understanding of the production, transmission and release of high-energy electrons, thus helping to answer the scientific questions mentioned above.
Hard X-ray imaging
X-rays have a strong ability to penetrate matter and can pass through general mirrors and lenses, which makes X-ray imaging difficult to achieve focusing as easily as optical imaging. At present, the X-ray satellites that have been launched internationally mainly use two ways to achieve X-ray imaging, which are direct and indirect.
Direct imaging and indirect imaging (sliding view).
Our protagonist today, HXI on ASO-S, uses the spatial modulation Fourier transform imaging principle mentioned above, but it has a larger number of sub-collimators, a wider range of energy detection, and higher spatial resolution than Yohkoh.
The HXI consists of three stand-alone machines in a structure: collimator, energy measuring device, and electronic control box, which can be compared to the lens, CCD and control system in the camera.
HXI。
The spatial resolution of HXI can reach 3.1 arcseconds, which is about 1/1200 of a degree.
The relationship between arcseconds, angular fractions, and degrees.
HXI Imaging Principles
When X-rays illuminate the HXI collimator, a portion of the photons enter the detector through the slit between the metal tungsten grating (grid-like baffles made of materials with a higher absorption rate of X-rays), and some photons cannot be detected by the detector due to the blocking of the grating. As the direction of photon incidence changes, their transmittance on the detector will change and exhibit a periodic trigonometric wave function.
Schematic diagram of HXI imaging principles.
To obtain hard X-ray images of the solar source region, the HXI used up to 91 subcolligiators with different combinations of pitches and angles. We obtain images by comprehensively interpreting the counts of each detector and using the interrelationships between them.
The HXI detector is like a high-tech compound eye, in which the grating with different pitches and different placement angles is like many lens tubes of different sizes, each performing its own duties, some are good at looking at large outlines, some are good at looking at small details, and they are all integrated, which is a powerful observation array.
Real-shot image (swipe to view). HXI has processed more than 3400 tungsten gratings, each with thousands of slits on it.
The count of each pair of sub-collimators is a contribution from all positions in the sun plane. Then, in turn, by counting a pair of sine and cosine collimators, we can get a Fourier component, using the Fourier component obtained by 91 detectors on HXI, performing a Fourier inverse transformation, which will obtain the X-ray intensity distribution of the solar plane.
The result of simulated imaging of X-ray sources in the shape of "H, X, I" using HXI's raster parameters. As the number of light grids continues to increase, the inversion of the image is closer to the original appearance.
The direct restoration of the detector data results in a blurry image called a "dirty map" that contains more noise. After that, the dirty images need to be processed by various image reconstruction algorithms (such as CLEAN, MEM, FORWARD, etc.) to restore these blurry source images to a clear target.
Space weather
Dig deep into the principles of X-ray imaging, elaborate HXI, explore imaging reconstruction algorithms... Scientists are working hard toward scientific goals. In life, it is difficult for us to feel the undercurrent surging under the calm surface of the sun. However, when solar flares erupt, they are often accompanied by coronal mass ejections and high-energy particle events, which can have a serious impact on the Earth's magnetic field, space equipment, ground power grids, aerospace safety, and communication and navigation. Therefore, a better understanding of flare activities will help us prevent and reduce the harm and impact of these catastrophic space weather on human space activities.
We also expect that when ASO-S is finally launched, HXI will bring new and even unexpected information.
#创作团队:
Text: Chen Wei, Li Zhentong, Su Yang
#图片来源:
telescope:
Chandra：NASA/CXC/NGST
XMM-Newton：NASA
NuSTAR：NASA/JPL-Caltech
INTEGRAL：NASA
SWIFT：NASA E/PO, Sonoma State University/Aurore Simonnet
RHESSI：NASA
Yohkoh：NASA/MSFC