The detection of the line-of-sight (LOS) velocity of coronal mass ejections (CMEs) is essential for understanding their origins and early propagation, and for predicting their arrival time at Earth. The true velocity and evolution of CMEs are crucial for studying the mechanisms of solar eruptions and space weather forecasting. However, the velocity obtained from imaging observations represents only the plane-of-sky (POS) component, not the true velocity vector. To obtain the true velocity, both the POS and LOS components are essential. Based on the Doppler effect, it is possible to measure the LOS velocity of CMEs using a Sun-as-a-star extreme ultraviolet (EUV) spectrograph with a spectral resolving power greater than 500. However, the spectral resolutions of existing instruments are insufficient to meet the detection requirements of CMEs LOS velocity. Therefore, to achieve a spectral resolving power of 500 for detecting the LOS velocity of CMEs, we propose a new detection scheme using a concave varied-line-spacing (VLS) grating and an sCMOS detector. We design and develop a prototype in the wavelength range of 18?30 nm, which achieves EUV spectra with spectral resolving powers greater than 700. Additionally, we propose a data processing method to obtain the one-dimensional spectrum by integrating the two-dimensional spectrum along the slit. This method can improve the signal-to-noise ratio and significantly reduce the downstream data volume, which is suitable for deep space exploration missions. Our investigation provides important support for the development of full-disk integrated spectrograph (FIS) for the solar polar-orbit observatory (SPO) and for detecting the LOS velocity of CMEs.

We first determine the requirements of the Sun-as-a-star EUV spectrograph based on scientific objectives. The field of view (FOV) of the spectrograph is configured to 34', covering most of the EUV radiance from both the solar disk. The wavelength range of 18?30 nm is selected to measure the LOS velocity of CMEs formed at typical temperatures. A spectral resolving power exceeding 500 is necessary to achieve an accurate measurement of the CMEs LOS velocity. These requirements are ensured in the optical design, structure design, and data processing. Second, we use a new detection scheme with a concave VLS grating and an sCMOS detector, which is designed as a grazing incidence optical structure referring to MEGS-A in SDO/EVE. We comprehensively consider the slit width, grating, and pixel resolution to ensure spectral resolution. Based on the meridian focusing condition of the concave grating and line dispersion, we select the grating with a larger curvature radius and variable line spacing to reduce aberration and improve spectral resolution. The detector, with a pixel size of 6.5 μm, is the sCMOS detector validated by the solar upper transition region imager (SUTRI). The slit width is set to be 20 μm according to the sampling theory and the magnification of the grating. We also use SHADOW VUI ray tracing software to calculate the spectral resolution and error range of the system. Then, we optimize the structure design and assembly methods. We rely on machining accuracy and high-precision turntables to ensure the accuracies of the distance from slit to grating and the incident angle in the air, and fine-tune the detector position through a flexible corrugated pipe in the vacuum environment. Using a narrow linewidth EUV light source, namely a hollow cathode lamp, helium gas can be ionized under high pressure to obtain EUV radiation. Finally, we propose a data processing method for the uneven and tilted spectra: 1) reducing the dark field of the superimposed images from multiple frames and performing spectral line identification; 2) identifying and correcting thermal and damaged pixels in the image using median filtering; 3) calculating the spectral tilt using cubic spline interpolation for sub-pixel translation; 4) integrating 2048 rows of spectra along the slit.

Based on the optical parameters of the grating and sCMOS detector (Table 2), we obtain the simulated spectral resolving powers of 772, 876, and 965 for 24.3, 25.63, and 30.37 nm, respectively (Fig. 2). We further design and develop a prototype for the Sun-as-a-star EUV spectrograph (Figs. 3 and 4), equipped with relevant gas injection systems, high-pressure molecular pumps, refrigeration equipment, etc., and finally obtain three ionization spectral lines of helium at He Ⅱ24.303 nm, He Ⅱ25.632 nm, and He Ⅱ30.378 nm (Fig. 5). The superimposed spectrum shows a significant improvement in the signal-to-noise ratio compared to the spectrum extracted from single row after correction by dark field (Fig. 6). We use Gaussian fitting on the corrected spectra and obtain spectral resolving powers of 745, 788, and 865 for the three spectral lines, respectively (Fig. 6, Table 3). These results indicate that the new detection scheme can achieve a high spectral resolving power of over 500, which meets the detection requirement for CMEs LOS velocity. Compared with existing Sun-as-a-star EUV spectrographs, we use a larger curvature radius, a longer slit, and a data processing method that improves the signal-to-noise ratio and reduces the amount of downstream data. It offers more comprehensive advantages in terms of luminous flux, spectral resolution, and data transmission (Table 4).

We propose a new scheme with high spectral resolution using concave VLS grating and sCMOS detector for the detection of CMEs LOS velocity. We complete simulation calculations, optics and structure design, actual spectral calibration, and the exploration of data processing method on orbit for the Sun-as-a-star EUV spectrograph. Using helium as the ionized gas, we obtain spectra of He Ⅱ24.303 nm, He Ⅱ25.632 nm, and He Ⅱ30.378 nm. We propose a data processing method for the measured spectra that involves reducing the dark field, identifying and correcting thermal and damaged pixels, and correcting spectral tilt. This method can integrate two-dimensional array data into one-dimensional spectral data, which can improve the signal-to-noise ratio without reducing spectral resolution and significantly decrease transferred data. Therefore, this method is highly suitable for data processing on orbit in deep space exploration missions. The spectral resolving powers of He Ⅱ24.303 nm, He Ⅱ25.632 nm, and He Ⅱ30.378 nm are 745, 788, and 865, respectively, which are three times higher than the similar instrument SDO/EVE. The research provides an important basis for the detection of CMEs LOS velocity and the development of the FIS equipped on the SPO in China.