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WST和SDI在轨辐射定标

In-flight Radiometric Calibrations of WST and SDI

  • 摘要: 莱曼阿尔法太阳望远镜(Ly\alpha Solar Telescope, LST)是先进天基太阳天文台(Advanced Space-based Solar Observatory, ASO-S, 中文名为“夸父一号”)卫星上的有效载荷之一, 它包括白光太阳望远镜(White-light Solar Telescope, WST)、莱曼阿尔法全日面成像仪(Solar Disk Imager, SDI)和日冕仪(Solar Corona Imager, SCI) 3台科学仪器. 其中WST工作在(360±2) nm (近白光)波段, SDI工作在(121.6±4.5) nm (紫外莱曼阿尔法)波段, 两者的观测视场均为1.2 R_\odot (R_\odot为太阳半径, 整个视场相当于38.4′). 通过WST和SDI的成像数据可以探索太阳爆发活动在低层大气(光球、色球及过渡区)中的触发和响应, 比如研究太阳耀斑的触发机制、白光耀斑的物理性质以及爆发暗条/日珥的形态演化和运动学, 并推导出太阳大气的物理参数等. 若要获得WST和SDI观测的太阳大气不同特征的物理参数, 如耀斑能量、日珥温度和密度等, 则需要把它们观测的计数值(Digital Number, DN)转化为物理单位(如erg·cm−2·s−1·sr−1), 这个过程即称为辐射定标. 辐射定标是WST和SDI科学数据生产过程中的必要步骤之一. 目前, WST和SDI在轨辐射定标均以太阳为参考源, 其中前者使用美国材料与测试协会(American Society for Testing and Materials, ASTM)于2020年发布的太阳光谱数据, 后者则使用地球同步环境系列卫星(Geostationary Operational Environmental Satellite R, GOES-R)上搭载的极紫外传感器(Extreme Ultraviolet Sensors, EUVS)观测的数据. 给出了WST和SDI在2023年8月到2024年2月正常观测期间的在轨辐射定标系数及其不确定度. 通过拟合WST在轨辐射定标系数日平均值得到其经验公式. 利用辐射定标后的数据, 能够计算太阳耀斑在白光和莱曼阿尔法波段辐射出的能量以及获得日珥密度等, 有利于实现WST和SDI的科学目标.

     

    Abstract: The Ly\alpha Solar Telescope (LST) is one of the payloads on board the Advanced Space-Based Solar Observatory (ASO-S, also known as Kuafu-1) mission, which consists of three scientific instruments: a White-light Solar Telescope (WST), a Solar Disk Imager (SDI), and a Solar Corona Imager (SCI). Both WST and SDI have a field of view of 1.2 R_\odot (R_\odot represents the solar radius and the full field of view spans 38.4′) and operate in the (360±2) nm (near white light) and (121.6±4.5) nm (i.e., Ly\alpha) wavebands, respectively. Using imaging data from WST and SDI, we can explore the dynamics and evolution of solar activities in the lower atmosphere (from the photosphere to transition region), for example, to study the triggering mechanisms of solar flares, the physical properties of white-light flares, and the morphology evolution and kinematics of erupting filaments/prominences, as well as to derive physical parameters of the solar atmosphere. To obtain the physical parameters of different features in the solar atmosphere observed by WST and SDI, such as energy of flares and temperature and density of prominences, it is essential to convert the digital number (DN) of their observations into physical units (e.g., erg·cm−2·s−1·sr−1) through a process known as radiometric calibration. Radiometric calibration is a necessary step in the production of scientific data from WST and SDI. Currently, we perform in-flight radiometric calibrations of WST and SDI using the Sun as a reference source. For WST, we utilize solar spectral data released by the American Society of Testing and Materials (ASTM) in 2020, while for SDI we use the Extreme Ultraviolet Sensors (EUVS) aboard the Geostationary Operational Environmental Satellite R (GOES-R). In this paper, we provide the in-flight radiometric calibration factors and their uncertainties for WST and SDI during the normal observation period from August 2023 to February 2024. Furthermore, an empirical expression for the WST in-flight radiometric calibration factors is derived by fitting the daily averages of these coefficients. By utilizing the radiometric calibrated data, we can calculate the energy radiated by solar flares in both the near-white-light and Ly\alpha wavebands and derive the density of prominences. These calculations are critical for achieving the scientific objectives of WST and SDI.

     

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