HMC Index

The HMC (Hourly Magnetospheric Currents) index measures the activity of large-scale magnetospheric currents on Earth's surface from 1900 to 2015. It resolves the absolute intensity of low-frequency variations, especially at periods relevant to the solar cycle, more robustly than existing geomagnetic indices. HMC is based on hourly means of vector magnetic field measurements from 34 mid latitude geomagnetic observatories obtained from WDC Edinburgh (http://www.wdc.bgs.ac.uk/catalog/master.html). This data has been manually revised to correct for spikes, jumps and drifts. A detailed description of the derivation method is given in Pick et al., 2018 to which these data are supplementary material.
This directory contains the HMC index (hmc1900phor.hor) and the modified observatory data that it is based on (data.zip). The index and the observatory data files are formatted in compliance with the IAGA-2002 ASCII exchange format (https://www.ngdc.noaa.gov/IAGA/vdat/IAGA2002/iaga2002format.html).
Individual file names are composed of:[IAGA code of observatory] + [first active year during 1900-2015] + [p(provisional)] + [hor(hourly)] + [_mod(modified)].hor
Also included is information on how the data modifications (list in modifications.pdf) were applied (readme.txt).

HMC（小时地磁层电流，Hourly Magnetospheric Currents）指数是用于衡量1900年至2015年间地球表面大规模地磁层电流活动的指标。相较于现有地磁指数，该指数能够更稳健地解析低频变化的绝对强度，尤其是在与太阳周期相关的时段内。
HMC指数的构建基于世界数据中心爱丁堡分部（WDC Edinburgh，http://www.wdc.bgs.ac.uk/catalog/master.html）提供的34个中纬度地磁观测站的矢量磁场逐小时平均值数据。该数据集已经过人工修正，以校正尖峰、跳变与漂移误差。其推导方法的详细说明参见Pick等人2018年的研究，本数据集为该研究的补充材料。
本目录包含HMC指数文件（hmc1900phor.hor）及其所用的经修正观测台站数据压缩包（data.zip）。指数文件与观测台站数据文件均遵循IAGA-2002 ASCII交换格式（IAGA-2002 ASCII exchange format，https://www.ngdc.noaa.gov/IAGA/vdat/IAGA2002/iaga2002format.html）进行格式化。
单个数据文件的命名规则为：[观测台站IAGA代码] + [1900-2015年间首个活跃年份] + [p（临时数据标识，provisional）] + [hor（逐小时数据标识，hourly）] + [_mod（修正数据标识，modified）].hor
本数据集还附带了数据修正操作的实施说明（详见modifications.pdf）以及数据使用指南（readme.txt）。