The chromosphere of the Sun photographed during the total solar eclipse. It is called the chromosphere because of the red and pink light emitted by hydrogen atoms.
For two decades since scientists learned of the existence of a chromosphere beneath the Sun’s corona, the only Time the Sun’s chromosphere could be seen was during total eclipses, occasionally in the silhouette of the Moon, as a bright red halo.
The chromosphere, as its name implies, is the second of the three main layers of the Sun’s atmosphere, about 2,000 kilometers thick, sandwiched between the corona and the photosphere. The low density of the photosphere layer, combined with the high brightness of the photosphere, means that this layer is generally invisible without special equipment. Therefore, more than 100 years after the discovery of the existence of this layer, scientists still have a limited understanding of this layer.
Why detect the chromosphere
While people are used to using terrestrial weather forecasts to organize various Life and production activities, scientists are working to expand the development of “space weather forecasts”, which focus on the activity of the Sun. The activity of solar surface ejecta and eruptions has a considerable impact on human activities on Earth, as well as on space projects.
Among the factors that influence solar activity, the Sun’s magnetic field is one of the important driving factors behind it. The Sun’s magnetic lines of force extend from the Sun’s surface to a farther range of space than the Earth, but they are not visible, and scientists can only detect them indirectly, for example, through light emitted by plasma, high-temperature gases, which illuminate their lines like headlights on a highway. The distribution characteristics of the magnetic lines – scattered and straight or tight and tangled – have a big impact on the activity on the surface of the Sun.
“The sun is both beautiful and mysterious, and its magnetic lines of force control its activity at every moment,” says Ryohko Ishikawa, a physicist at the National Astronomical Observatory of Japan who is the lead author of the new study.
Ideally, scientists believe that magnetic line data could be obtained from the coronal layer, where all of the Sun’s eruptive activity occurs, but this layer is too sparsely distributed for accurate readings. The density of the coronal layer is less than one billionth of the density of the Earth’s sea-level atmosphere.
So scientists later switched to measuring the much denser photospheric layer and then used mathematical models to extrapolate data from the coronal layer. However, scientists again found that the magnetic lines of force in the chromosphere are too variable, so skipping the chromosphere and using the model to infer the data is too inaccurate.
The chromosphere is a hot, messy layer,” said Laurel Rachmeler, a former NASA engineer. We simplified to make separate estimates for the photospheric and coronal layers, but those estimates failed because it wasn’t clear what was going on in the chromosphere.”
So, the chromosphere is a layer that cannot be ignored to understand solar activity.
This study breaks through that barrier for the first time, as scientists from the United States, Japan, Spain and France collaborated to invent a completely new way to measure the magnetic lines of force in the chromosphere.
A detector that falls off when you look at it
They modified the Chromospheric Spectrophotometer (CLASP) used in 2015 and launched the Chromospheric Spectrophotometer 2 (CLASP2) to measure the Sun’s magnetic lines of force. the CLASP series of projects is a measurement rocket that allows the measurement equipment to be launched into the air on board the rocket and fall back to the ground within minutes with the help of a parachute. This method of detection is less expensive and has a shorter planning time than launching a satellite probe, and is a good way to test new technologies.
In this study, CLASP2 was launched to an altitude of 274 kilometers above the ground, quickly completing measurements of the sun and then falling back to earth.
Three project collaborations
The project team arranged to have NASA’s Interface Region Imaging Spectrometer (IRIS), the Hinode solar exploration satellite, also take measurements at different altitudes in the same location on the Sun at the same time as the CLASP2 measurements.
Through the collaboration of these three projects, NASA has for the first time ever measured magnetic line data at four altitude levels at the same location in and around the Sun’s chromosphere.
The most striking feature seen in the data is that the magnetic lines of force in the solar chromosphere vary so much, the researchers said. The team hopes that the next use of this new measurement to map the magnetic field of the entire chromosphere will provide important information for space weather forecasting.
The results were published Feb. 19 in the journal Science Advances.
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