About these netCDF files:

These files contain geomagnetic field interpolation model results. The interpolation uses the method of spherical elementary current systems first described by Amm (1997) (see also Amm & Viljanen, 1999 and Pulkkinen et al. 2003). The results consist of gridded magnetic field component perturbations (delta-B), dX (northward perturbation), dY (eastward perturbation), and dZ (vertically downward perturbation) in units of nanoTesla (nT). The gridded data is generated at a cadence of 60 seconds and the netCDF files contain all available time steps for one day. The date for the data is indicated by the filename convention: yyyymmdd_secs-db.nc

Users should note that the accuracy of the model is limited, especially in locations that are several hundreds of kilometers from an actual geomagnetic observatory. Assessment of the model accuracy is ongoing. 

The netCDF files have the following attributes which can be read from the file:
FILENAME, CADENCE, NLATITUDES, NLONGITUDES, NGRIDPTS

The netcdf file makes use of four dimensions: time, latitude, longitude, stationlist (index to an individual station in obslist, see below).

The variables contained in the file are the following: 
time - seconds since the start of the UT day (one entry per time step)
nobs - number of observatories used for the interpolation (one entry per time step)
obslist - string array lists the stations used in the interpolation (one station list per time step)
latitude - array of geographical latitudes for the grid points (one entry per latitude)
longitude - array of geographical longitudes for the grid points (one entry per longitude)
dX - 3D array of northward perturbations (one element per time step, latitude value, and longitude value)
dY - 3D array of eastward perturbations (one element per time step, latitude value, and longitude value) 
dZ - 3D array of downward perturbations (one element per time step, latitude value, and longitude value) 

The time variable has an attribute REFTIME which gives the time of reference to which its value is added 
to determine the time tag for a given time step. REFTIME is a string format time in CCSDS format (i.e. yyyy-mm-ddThh:mm:ss.mss).

Data for each time step is added progressively through the current UT day. Files for past days are therefore fixed and are
the basis for the archive repository for these model outputs.

For documentation on netCDF files, please refer to https://www.unidata.ucar.edu/software/netcdf/

Acknowledgements:

The algorithm using the Spherical Elementary Current Systems method was developed and made available courtesy of the Finnish Meteorological Institute, with additional assistance provided by Antti Pulkkinen at NASA/GSFC. The adaptation of the algorithm for operational usage was carried out by Josh Rigler at the U.S. Geological Survey Geomagnetism program.  

For additional information about the SECS algorithm, please consult the following references:

Amm, O. (1997). Ionospheric elementary current systems in
spherical coordinates and their application. Journal of
Geomagnetism and Geoelectricity, 49(7), 947–955, https://doi.
org/10.5636/jgg.49.947

Amm, O., & Viljanen, A. (1999). Ionospheric disturbance
magnetic field continuation from the ground to the ionosphere
using spherical elementary current systems. Earth Planets
Space, 51, 431–440, https://doi.org/10.1186/BF03352247

Pulkkinen, A., Amm, O., Viljanen, A., & BEAR Working
Group (2003). Separation of the geomagnetic variation field
on the ground into external and internal parts using the
spherical elementary current system method. Earth Planets
Space, 55, 117–129, https://doi.org/10.1186/BF03351739

The following publication may also be consulted:

Rigler, E.J., Fiori, R.A.D., Pulkkinen, A.A., Wiltberger, M., Balch, C. (2019).
Interpolating Geomagnetic Observations: Techniques and Comparisons, chapter 2 in 
Geomagnetically Induced Currents from the Sun to the Power Grid, 
Geophysical Monograph 244, First Edition, editors Gannon, J.L., Swidinsky, A.,
Xu, Zhonghua, American Geophysical Union, Published by John Wiley & Sons, Inc.
https://doi.org/10.1002/9781119434412.ch2