We present the CHAOS-7 model of the time-dependent near-Earth geomagnetic field between 1999 and 2020 based on magnetic field observations collected by the low-Earth orbit satellites Swarm , CryoSat-2, CHAMP, SAC-C and Ørsted, and on annual differences of monthly means of ground observatory measurements. The CHAOS-7 model consists of a time-dependent internal field up to spherical harmonic degree 20, a static internal field which merges to the LCS-1 lithospheric field model above degree 25, a model of the magnetospheric field and its induced counterpart, estimates of Euler angles describing the alignment of satellite vector magnetometers, and magnetometer calibration parameters for CryoSat-2. Only data from dark regions satisfying strict geomagnetic quiet-time criteria (including conditions on IMF $$B_z$$ B z and $$B_y$$ B y at all latitudes) were used in the field estimation. Model parameters were estimated using an iteratively reweighted regularized least-squares procedure; regularization of the time-dependent internal field was relaxed at high spherical harmonic degree compared with previous versions of the CHAOS model. We use CHAOS-7 to investigate recent changes in the geomagnetic field, studying the evolution of the South Atlantic weak field anomaly and rapid field changes in the Pacific region since 2014. At Earth’s surface a secondary minimum of the South Atlantic Anomaly is now evident to the south west of Africa. Green’s functions relating the core–mantle boundary radial field to the surface intensity show this feature is connected with the movement and evolution of a reversed flux feature under South Africa. The continuing growth in size and weakening of the main anomaly is linked to the westward motion and gathering of reversed flux under South America. In the Pacific region at Earth’s surface between 2015 and 2018 a sign change has occurred in the second time derivative (acceleration) of the radial component of the field. This acceleration change took the form of a localized, east–west oriented, dipole. It was clearly recorded on ground, for example at the magnetic observatory at Honolulu, and was seen in Swarm observations over an extended region in the central and western Pacific. Downward continuing to the core–mantle boundary, we find this event originated in field acceleration changes at low latitudes beneath the central and western Pacific in 2017.

中文翻译：

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CHAOS-7 地磁场模型和观测到的南大西洋异常变化

我们根据近地轨道卫星 Swarm、CryoSat-2、CHAMP、SAC-C 和 Ørsted 等收集的磁场观测资料，提出了 1999 年至 2020 年间随时间变化的近地地磁场的 CHAOS-7 模型。地面观测站测量月平均值的年度差异。CHAOS-7 模型包括一个高达 20 度球谐函数的时间相关内场，一个与 LCS-1 25 度以上岩石圈场模型合并的静态内场，一个磁层场模型及其感应对应物，估计描述卫星矢量磁力计对齐的欧拉角，以及 CryoSat-2 的磁力计校准参数。在场估计中只使用了来自满足严格地磁静默时间标准（包括 IMF $$B_z$$ B z 和 $$B_y$$ B y 在所有纬度的条件）的暗区数据。使用迭代重新加权的正则化最小二乘程序估计模型参数；与先前版本的 CHAOS 模型相比，时间相关内场的正则化在高球谐度数下得到放松。我们使用 CHAOS-7 来研究地磁场的近期变化，研究南大西洋弱场异常的演变和自 2014 年以来太平洋地区的快速场变化。在地球表面，南大西洋异常的次要最小值现在很明显非洲西南部。格林函数将地核-地幔边界径向场与地表强度相关联，表明该特征与南非下反向通量特征的运动和演化有关。主要异常的规模持续增长和减弱与南美洲向西运动和反向通量的聚集有关。2015 年至 2018 年间，在地球表面的太平洋地区，场的径向分量的二阶时间导数（加速度）发生了符号变化。这种加速度变化以局部的、东西向的偶极子的形式出现。它清楚地记录在地面上，例如在檀香山的磁力观测站，并在中太平洋和西太平洋扩展区域的 Swarm 观测中看到。向下继续到地核-地幔边界，