Synoptic Charts of the Line-of-Sight Component of the Photospheric Magnetic Field

Synoptic charts are maps of the entire Sun produced in Carrington coordinates. Synoptic maps are constructed by merging together solar observations taken over many days. Magnetic-field synoptic charts are produced using central meridian data from HMI full-disk magnetograms.

Synoptic maps are constructed from HMI 720s line-of-sight Magnetograms collected over a 27-day solar rotation. Near-central-meridian data from 20 magnetograms contribute to each point in the final map.

HMI 720s line-of-sight magnetograms are first converted to 'radial field magnetograms' by dividing by the cosine of the angle from disk center, i.e. for this purpose we assume that HMI measures the line-of-sight component of a purely radial magnetic field. Individual 'radial' magnetograms are then remapped and interpolated onto a very high-resolution Carrington coordinate grid. For each Carrington longitude the values from the 20 magnetograms obtained closest in time to the central meridian passage (CMP) of that longitude are averaged. By using a constant number of contributing magnetograms, the variation of the noise over the entire map is minimized. Generally all data are taken within about 2 degrees of CMP. The effective temporal width of the HMI synoptic map contribution is about three hours, i.e. 20 720-s magnetograms are obtained within about 90 minutes of central meridian passage. The final HMI synoptic maps have a size of 3600 x 1440, which means the resolution is lower than the disk-center resolution of a single HMI magnetogram. A two-dimensional Gaussian function is applied to high-resolution remapped data to reduce the spatial resolution before generating the 3600*1440 synoptic maps. The width of the Gaussian is 3 pixels. The upper limit of the noise is 2.3 Mx cm2.

太阳综合图（Synoptic charts）是采用卡林顿坐标系（Carrington coordinates）绘制的全日面太阳地图。这类综合图通过合并多日的太阳观测数据构建而成。磁场综合图则基于HMI全日面磁像图的中央子午线数据生成。
太阳综合图由27天太阳自转周期内采集的HMI 720秒视向磁像图数据构建而成。最终图像的每个像素点均由20幅磁像图的近中央子午线数据生成。
首先，将HMI 720秒视向磁像图数据除以日面中心夹角的余弦值，转换为“径向场磁像图数据”；在此过程中，我们假设HMI测量的是纯径向磁场的视向分量。随后，将单幅“径向”磁像图数据重映射并插值至高分辨率卡林顿坐标系网格中。针对每个卡林顿经度，选取与该经度中央子午线过境（central meridian passage, CMP）时刻最接近的20幅磁像图数据进行平均。通过固定参与计算的磁像图数量，可将整张图像的噪声波动降至最低。通常而言，所有选取的数据均处于CMP两侧约2度范围内。HMI太阳综合图的有效时间覆盖宽度约为3小时，即在中央子午线过境前后约90分钟内采集20幅720秒的磁像图数据。最终生成的HMI太阳综合图尺寸为3600×1440，其分辨率低于单幅HMI磁像图的日面中心分辨率。在生成3600×1440的综合图之前，需对高分辨率重映射数据应用二维高斯函数以降低空间分辨率，该高斯函数的宽度为3像素。噪声上限为2.3 Mx·cm²。