Measuring the solar extreme ultraviolet (EUV) irradiance (λ < 100 nm) is a high priority for space weather and upper atmospheric research. Although it is only a small fraction of the total solar irradiance, the EUV is absorbed at high altitudes in the Earth’s atmosphere, where it produces the ionosphere and drives the variation in the thermosphere. Intense solar EUV, often more than 10x the minimum level, can disturb the upper atmosphere, disrupting RF communication and navigation (e.g. GPS) and increasing the drag on satellites in low Earth orbits. RSI, teaming with the Naval Research Lab (NRL), has designed a sensor to measure the total solar irradiance in several EUV spectral bands.
Our team has fabricated a sensor for the 17.1-19.5 nm spectral band that contains numerous Fe emission lines and we are developing another similar sensor for the 30.4 nm He II spectral line. Each of these sensors uses a zone plate (ZP) to focus the in-band solar radiation onto a small pinhole in front of a detector. A zone plate focuses radiation by diffraction, with its focal length inversely proportional to wavelength, as illustrated in Figure 1.
Each sensor has two thin aluminum films to attenuate out-of-band radiation: the first, supported on a mesh, at the entrance aperture of the sensor and the second directly deposited on the front surface of the detector. These thin films are semi-transparent over a range of EUV wavelengths but are virtually opaque to radiation at longer wavelength. Each sensor also has a 4 mm diameter ZP with a 2 mm diameter central occulting disk to block 0-order (undiffracted) radiation from reaching the pinhole. The pinhole’s diameter is sufficient to admit the focused +1-order in-band radiation from the full solar disk and inner corona.
A ZP has a number of advantages over a diffraction grating. Because it focuses in-band radiation, a ZP allows the detector to be smaller than the aperture, reducing both the dark current and the out-of-band response. The circular symmetry of the ZP eliminates both the polarization sensitivity and the shift in the spectral band with field angle that are intrinsic to a linear grating. The optical assembly is contained in a very small volume. We are planning to fly these sensors as a secondary payload on one or more solar sounding rocket missions in the near future: NRL’s VERIS and/or NASA/GSFC’s EUNIS.
