A team from the Institute of Optics of the Higher Council for Scientific Research has developed the transmittance filters integrated into the Chinese satellite Fengyun-3E which, since July 5, monitors the climate, oceans, ice and space weather through 11 meteorological instruments. Among the devices is the Ionosphere Photometer (IPM) which, thanks to transmittance filters, will detect sunrise and sunset data in the desired band to improve observations and climate predictions. Transmittance filters are designed to let light in the desired spectral band pass through for information while absorbing other areas of the spectrum, especially visible light.

The Fengyun-3E meteorological satellite took off on July 5 from the Jiuquan Satellite Launch Center (China), becoming the first meteorological satellite in morning orbit for civil use. Its applications include the analysis of the wind on the surface of the ocean or the three-dimensional atmosphere to improve knowledge about the climate, the atmospheric environment and space weather. For this, it has 11 instruments, among which the Ionosphere Photometer stands out, in which transmittance filters play a key role, that is, filters that let the light emitted at a certain wavelength pass through.

"The filters created by the Thin Film Optics Group (GOLD) consist of a coating tuned to transmit in the desired far ultraviolet band and have demanding requirements for the rejection of visible light", highlights Juan Ignacio Larruquert, researcher at the Optics Institute that has led the development of the filters.

The objective of the Ionosphere Photometer device is to observe the atmosphere both in the 135 nanometer wavelength spectral line and in a band of the spectral band called Lyman-Birge-Hopfield (LBH), whose wavelength varies between 140 and 180 nanometers. “The data provided by the IPM are part of the daily cycle that one of the layers of the atmosphere undergoes: the ionosphere. The concentration of oxygen ions increases strongly during the day and decreases again at night. IPM obtains a measurement of the amount of oxygen ions in the ionosphere through the intensity relationship between the measurement it makes at 135 nm and the measurement in the LBH band ”, explains Larruquert.

The transmittance filters incorporated into the Ionosphere Photometer are based on a succession of sheets, in which sheets of metal and a dielectric material alternate, to be tuned either on the 135 nanometer line or on the LBH band. “Each sheet must have the appropriate thickness so that the interferences of the light through the multilayer give like result a sufficient transmission in the band of interest of the far ultraviolet and, simultaneously, produce a great rejection of the visible light. This requirement is necessary to prevent the light from the far ultraviolet band, which is very low intensity, from being masked by the much more intense visible light ”, he adds.
Satellites to observe the Earth's atmosphere

Since the 1960s, these observations for the physics of the atmosphere and for astrophysics have been made from space, by launching satellites, because the atmosphere absorbs the far ultraviolet. The opacity of the atmosphere thus extends through different regions of the electromagnetic spectrum, such as X-rays, which also have to be observed from the space field. The filters developed by the CSIC will make it possible to collect information in the band known as far ultraviolet, whose wavelength is between 100 and 200 nanometers.

The data obtained in the morning and evening orbits will increase the precision of the numerical models of meteorological prediction to improve the global Earth observation system. The results can be applied in the field of meteorological research and space weather monitoring in order to be better prepared for possible meteorological disasters.

Alejandro Parrilla / CSIC Communication