After more than half a year of exploring the sun, what secrets did the Xihe satellite bring back?

"After the launch of 'Xihe', it has obtained for the first time in space the fine structure and spectral imaging of the sun's full-disk Hα spectrum, Si I spectrum and Fe I spectrum, as well as data on dozens of solar flares." On July 19, at the Ministry of Education's "Ten Years of Education" "1+1" series of press conferences on the achievements of colleges and universities in scientific and technological innovation reform and development, Fang Cheng, an academician of the Chinese Academy of Sciences, chief scientific adviser of "Xihe" and professor of Nanjing University, happily revealed this latest progress.

The full name of "Xihe" is "Solar Hα Spectral Detection and Dual Super Platform Scientific and Technological Experimental Satellite", which was launched on October 14, 2021. The launch of "Xihe" broke the history of my country's lack of dedicated satellites for solar exploration.

"We have now begun planning for three-dimensional exploration of the sun and looking at it from different angles. For example, we have started preliminary research on the Xihe-2 satellite and plan to launch it to the Lagrange point L5 in five years, where the gravitational pull of the sun is equivalent to that of the earth, and the satellite can observe the sun from the side. In addition, the domestic solar physics community also plans to launch the Xihe-3 satellite in less than five years to observe the sun's north and south poles," said Fang Cheng.

The complete profile of the Si I spectral line was observed in space for the first time

Fang Cheng introduced that the high spectral resolution and high temporal resolution observations of Xihe can perform in-depth "CT" scans of the solar photosphere and chromosphere. As the main scientific payload of the satellite, the Hα spectroscopic imager is the exclusive "photographer" of the sun. It can complete more than 4,600 steps of full-disk scanning in 46 seconds, obtain spectral information of more than 16 million points, and obtain solar images of more than 300 wavelength points. These spectra contain atmospheric information of different levels of the solar photosphere and chromosphere.

At 00:52 on October 24, 2021, "Xihe" officially "opened its eyes" and achieved its first light observation.

"It is worth mentioning that the pixel spectral resolution of the Hα spectroscopic imager reaches 0.024 angstroms, which is about 10 times higher than the resolution of the ground filter and can obtain more accurate spectral line information." Li Chuan, chief designer of the "Xihe" science and application system and professor at the School of Astronomy and Space Science of Nanjing University, told Science and Technology Daily.

During the observation, "Xihe" obtained the spectral profiles of the solar Hα spectrum line, Si I spectrum line and Fe I spectrum line for the first time.

Fang Cheng introduced that the solar Hα spectrum, Si I spectrum and Fe I spectrum are produced at different altitudes in the solar atmosphere. Among them, the Hα spectrum is the spectrum with the strongest response to solar activity in the solar lower atmosphere. The center of the Hα spectrum is formed at about 1,300 to 1,500 kilometers from the bottom of the photosphere, which can reflect the information of the solar chromosphere, and the line wings reflect the information of the solar photosphere; the Si I spectrum is formed at about 50 to 60 kilometers from the bottom of the solar photosphere; the Fe I spectrum is formed at 200 to 300 kilometers from the bottom of the photosphere.

"Analyzing the information of the three spectral lines can reveal the distribution of physical quantities in the solar atmosphere at different altitudes," said Fang Cheng.

What is different is that in the past, when observing from the ground, the Si I spectrum was obscured by the spectrum of water molecules in the Earth's atmosphere, so it was difficult to capture. However, the space spectrum information is not affected by the Earth's atmosphere. The "Xihe" achieved the first direct observation of the complete spectrum profile of Si I in the world.

"After obtaining the Si I spectral line, we can combine it with other spectral lines to inversely calculate the temperature and density of the solar atmosphere, which helps us study the atmospheric structure of the sun," said Fang Cheng.

The full-disk Doppler velocity field has high accuracy, which is equivalent to using a magnifying glass to observe the solar activity process.

In addition to full-disk spectral imaging, Xihe can also perform local solar disk spectral imaging and fixed-point spectral observations based on solar activity.

Fang Cheng introduced that the Xihe can obtain the fine profile of the Hα spectrum at any point on the solar disk, and thus can invert the high-precision Doppler velocity field of the entire solar disk, which has never been achieved in previous observations. At the same time, the Xihe also obtained a high-precision Doppler velocity field of the entire solar disk. Through the pixel spectral resolution and spectral line profile fitting analysis, the velocity field accuracy is better than 0.06 km/s.

What is the significance of this number? Li Chuan explained that the impact of the sun on humans is particularly manifested in the occasional solar flares and coronal mass ejections. A solar flare is an explosive activity in which a part of the sun suddenly brightens. A medium-intensity flare can emit powerful radiation from gamma rays to radio waves, with a total energy equivalent to about 1 billion to 10 billion atomic bomb explosions. A coronal mass ejection is a large-scale explosion of matter on the sun, and one explosion can eject about 100 million to 1 billion tons of matter into interplanetary space.

"The matter ejected by solar explosions moves very fast in space, often at hundreds to one or two thousand kilometers per second, and the speed also reflects the kinetic energy and force conditions of the matter. The accuracy of 0.06 kilometers per second is equivalent to looking at the process of solar activity through a magnifying glass, and then understanding the causes of the triggering of these explosive activities and the process of their propagation," said Li Chuan.

At present, "Xihe" generates about 1.2Tb of raw data every day. After preliminary analysis, the scientific research team will generate it into scientific data that can be directly used by scientists.

Original title: After more than half a year of chasing the sun, what secrets have been brought back? The Xihe spacecraft has obtained three solar spectral profiles for the first time

Science and Technology Daily reporter Jin Feng

Source: Science and Technology Daily