SPMG Kitt Peak Vacuum Telescope Corrected Magnetograms

Citation and Acknowledgements Please cite this database (see above), as well as adding the following acknowledgement: "SPMG Kitt Peak Vacuum Telescope Corrected Magnetograms were downloaded from the solar dynamo dataverse (https://dataverse.harvard.edu/dataverse/solardynamo), maintained by Andrés Muñoz-Jaramillo." Main Limitations The main limitations of these data are: Cadence is one image per day, but it is common to have missing days Some magnetograms may display reading errors (they look like repeated vertical rows/patterns). Some magnetograms may have slight variations due to cloud cover. Overall, the data is in great shape, especially compared to the original state. We have used them to create a catalog of active regions https://doi.org/10.7910/DVN/QEMSZ2t However, it is worth visually inspecting the magnetograms before using them. File Structure The database is comprised of one tar.gz file per month, each containing all available magnetograms for that given month. A month without data will not have an associated tar.gz file. Each magnetogram is stored using fits format with header information that includes a new set of parameters related to the correction. They are relatively large files because we have kept the original data untouched. The fits table has 5 layers (across the 3rd dimension): Layer 1: Original untouched data (Mx/cm^2) Layer 2: 868.8nm wing intensity (intensity units; gives a sense of cloud coverage) Layer 3: Fully corrected magnetogram (Mx/cm^2) Layer 4: New X pixel position to transform original into corrected geometry Layer 5: New Y pixel position to transform original into corrected geometry Description The SPMG instrument operated by doing four scanning passes, each of a quarter if the solar disk, a process that would take a total of ~48 minutes (~12 minutes per scan). These would happen as the Sun moved in the sky. In order to assemble the 4 scans into a single full disk image, it is necessary to fit the angle inclination of the slit with respect to the scan, atmospheric refraction, as well as taking limb information into consideration to deal with offsets in each of the scans. Jack Harvey and I (Andrés Muñoz-Jaramillo) worked in 2016 on improve on this process as well as making the identification of the zero point of the magnetograph much more stable. This has resulted in a significantly improvement on the quality of the majority of the SPMG magnetograms. While we never got around to make a publication documenting these changes, we feel that it is important for us to ensure that these magnetograms are open and available to the public so we are releasing them as they are. If possible, we will document better the process of improvement and will update this description with more information. Current corrections to the data include: 1. Remove the slit curvature. 2. Correct for atmospheric refraction. 3. Find the boundaries of the first two isophotes (circles of constant 868.8nm wing intensity) and find the center of each swath and its offset with respect of a perfectly centered Sun. During this step several things are done to try to remove pathologic deviations from an almost circular Sun. 4. Find the optimum values that minimize the difference between both isophotes and a circular sun with the size of the ephemeris radius that is perfectly centered in the image array. 5. Create the corrected image by remaping it using natural neighbor interpolation. 6. Remove pixels outside the ephemeris radius and assign them a NaN value. 7. Intensity adjustment based on previous and next observation (to fix drifting amplitudes on days of strong cloud coverage).

引用与致谢
请引用本数据库（见前文），并添加如下致谢语句："SPMG基特峰真空望远镜校正磁图下载自太阳发电机Dataverse数据库（https://dataverse.harvard.edu/dataverse/solardynamo），该库由Andrés Muñoz-Jaramillo维护。"

主要局限性
本数据集的主要局限性如下：
1. 数据采样频度为每日一张图像，但时常存在日期缺失的情况；
2. 部分磁图可能存在读取错误，表现为重复的垂直行或图案；
3. 部分磁图因云遮影响存在轻微畸变。
整体而言，该数据集质量优异，相较原始数据尤甚。我们已利用该数据集构建了活动区目录，相关链接为：https://doi.org/10.7910/DVN/QEMSZ2t。不过在使用前，建议对磁图进行目视检查。

文件结构
本数据集按月份打包为tar.gz压缩文件，每个压缩包包含对应月份的全部可用磁图。无数据的月份不会生成对应的tar.gz压缩文件。每张磁图以fits格式（Flexible Image Transport System）存储，文件头包含与校正流程相关的一组新参数。由于保留了原始数据未做修改，单文件体积相对较大。该fits文件包含5个数据层（沿第三维度分布）：
层1：未作修改的原始数据（单位：Mx/cm²）；
层2：868.8nm翼带强度（强度单位，可用于表征云遮情况）；
层3：完全校正后的磁图（单位：Mx/cm²）；
层4：将原始图像校正至标准几何位置所需的X像素坐标；
层5：将原始图像校正至标准几何位置所需的Y像素坐标。

数据集说明
SPMG设备通过四次扫描完成观测，每次覆盖太阳盘面的四分之一，单次观测总耗时约48分钟（单次扫描耗时约12分钟），该流程随太阳在天空中的运行同步进行。为将四次扫描结果拼接为完整的全日面图像，需要拟合扫描狭缝的倾角、大气折射效应，并结合日面边缘信息校正各次扫描的偏移量。
2016年，Jack Harvey与本人（Andrés Muñoz-Jaramillo）对该拼接流程进行了优化，并大幅提升了磁强计零点识别的稳定性。上述优化显著提升了绝大多数SPMG磁图的质量。尽管我们尚未完成相关变更的正式发表工作，但我们认为确保该批磁图向公众开放共享至关重要，因此决定直接发布原始校正后的数据。若条件允许，我们将完善校正流程的文档记录，并更新本说明文档。

当前已完成的数据校正流程包括：
1. 移除狭缝曲率畸变；
2. 校正大气折射效应；
3. 识别前两条等照度线（868.8nm翼带强度恒定的圆形区域）的边界，确定每条扫描带的中心及其相对于完美对日太阳的偏移量；此步骤中还将执行多项操作，以消除近乎圆形日面出现的异常偏差；
4. 寻找最优参数，使得两条等照度线与以图像阵列中心为圆心、星历表半径为大小的圆形太阳之间的差异最小化；
5. 采用自然邻域插值法对图像进行重映射，生成校正后的图像；
6. 移除星历表半径以外的像素，并将其赋值为NaN（非数值）；
7. 基于相邻观测数据进行强度校正（用于修正强云遮日期间的信号幅度漂移）。

搜集汇总

背景与挑战

背景概述

该数据集包含经过校正的Kitt Peak真空望远镜SPMG仪器采集的太阳全盘磁图数据，发布于2023年。数据以每日一张图像为频率（可能存在缺失），采用包含5个图层的FITS格式存储，涵盖原始数据、强度信息和几何校正参数。数据集经过多项处理（如狭缝曲率消除、大气折射校正等），显著提升了数据质量，但存在读取误差和云层覆盖影响等局限性，适用于太阳活动区域研究。