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Further details can be found in the journal article U.S. Geological Survey Geomagnetic Variometer Data: Capitalizing on Seismic Infrastructure by A. Ringler et. al. (2025) at https://doi.org/10.1785/0220250185  This dataset contains variometer magnetic field variation data, here transformed from calibrated geographic vector coordinates into local magnetic HEZ vector components.   The USGS provides both \"observatory\" grade magnetic field data and \"variometer\" grade magnetometer data. The former are generally considered to be more accurate, precise, and complete. Calibrated observatory archives can be obtained from the USGS Geomagnetism Program and INTERMAGNET, while uncalibrated USGS variometer data archives are maintained by IRIS/Earthscope.   Relative variations are considered reliable; however, static offsets can be substantial, will vary site-by-site, and may even change following events such as routine maintenance.  USGS variometer data is mirrored at THEMIS in both the 1 second and 0.1 second time cadence (1 Hz and 10 Hz sampling rate). The Time Resolution global attribute indicates the time cadence of this file."}]},{"Spase_datasetresourceid":[{"0":" "}]},{"Doi":[{"0":" "}]},{"LINK_TEXT":[{"0":"USGS source data can be found at the "},{"1":"USGS variometer data can be retrieved using the "},{"2":"The THEMIS data archive for this station can be found at the "}]},{"Mission_group":[{"0":"THEMIS"}]},{"Instrument_type":[{"0":"Ground-Based Magnetometers, Riometers, Sounders"}]},{"Acknowledgement":[{"0":"Original data provided by the USGS Geomagnetism Program."}]},{"Discipline":[{"0":"Space Physics>Magnetospheric Science"},{"1":"Space Physics>Ionospheric Science"}]},{"LINK_TITLE":[{"0":"USGS GeomagPlots page"},{"1":"Geomagnetism Data Web Service (under /algorithms/filter/)"},{"2":"THEMIS ftp site"}]},{"PI_affiliation":[{"0":"United States Geological Survey, 1711 Illinois St, Golden, CO 80401-1865, USA"}]},{"PI_name":[{"0":"The USGS Geomagnetism Program"}]},{"Rules_of_use":[{"0":"Access to preliminary data is provided as-is. We request that you acknowledge the USGS Geomagnetism Program in all published documents which make use of USGS data."}]},{"Descriptor":[{"0":"P57A>Homestead Farm, Martinsburg, WV, USA"}]},{"Logical_source_description":[{"0":"THEMIS GMAG Level 2 Ground-based Vector Magnetic Field at Homestead Farm, Martinsburg, WV, USA"}]},{"HTTP_LINK":[{"0":"https://geomag.usgs.gov/plots/variometer?stations=P57A"},{"1":"https://themis.ssl.berkeley.edu/data/themis/thg/l2/variometers/p57a/"},{"2":"https://geomag.usgs.gov/ws/docs#/default/Filtered_Algorithm_algorithms_filter__get"}]},{"MODS":[{"0":"Rev-2026-02-12 (dcar): CDF template created."},{"1":"Rev-2026-03-05 (dcar): Changed logical source for 10 Hz data to thg_l2_mag_p57a_100ms"}]}],"CDFVariables":[{"thg_mag_p57a_100ms":[{"VarDescription":[{"DataType":"CDF_REAL4"},{"NumElements":1},{"NumDims":1},{"DimSizes":[3]},{"RecVariance":"VARY"},{"DimVariances":["VARY"]},{"PadValue":-1.0E+30}]},{"VarAttributes":[{"CATDESC":"Magnetic field vector, HEZ vector components; depends on epoch, compno."},{"DEPEND_0":"thg_mag_p57a_100ms_epoch"},{"DEPEND_1":"thg_mag_p57a_100ms_compno"},{"DISPLAY_TYPE":"time_series"},{"FIELDNAM":"B in HEZ Components"},{"FILLVAL":"NaN"},{"FORMAT":"f9.2"},{"LABL_PTR_1":"thg_mag_p57a_100ms_labl"},{"UNIT_PTR":"thg_mag_p57a_100ms_unit"},{"VALIDMIN":[-60000.0,-60000.0,-60000.0]},{"VALIDMAX":[60000.0,60000.0,60000.0]},{"VAR_TYPE":"data"},{"SCALETYP":"linear"},{"DEPEND_TIME":"thg_mag_p57a_100ms_time"},{"DEPEND_EPOCH0":"thg_mag_p57a_100ms_epoch0"},{"COORDINATE_SYSTEM":"HEZ"},{"STATION_LATITUDE":"39.4835"},{"STATION_LONGITUDE":"281.9874"},{"STATION_ELEVATION":"191.0"},{"STATION_DECLINATION":"0.0"}]}]},{"thg_mag_p57a_100ms_unit":[{"VarDescription":[{"DataType":"CDF_CHAR"},{"NumElements":2},{"NumDims":1},{"DimSizes":[3]},{"RecVariance":"NOVARY"},{"DimVariances":["VARY"]},{"NumWrittenRecords":1},{"PadValue":" "},{"BlockingFactor":1}]},{"VarAttributes":[{"CATDESC":"Units for each of the B field vector components."},{"FIELDNAM":"P57A Units"},{"FORMAT":"a2"},{"VAR_TYPE":"metadata"},{"DICT_KEY":"label"}]},{"VarData":["nT","nT","nT"]}]},{"thg_mag_p57a_100ms_compno":[{"VarDescription":[{"DataType":"CDF_INT4"},{"NumElements":1},{"NumDims":1},{"DimSizes":[3]},{"RecVariance":"NOVARY"},{"DimVariances":["VARY"]},{"NumWrittenRecords":1},{"PadValue":-2147483647},{"BlockingFactor":1}]},{"VarAttributes":[{"CATDESC":"Index for the HEZ component number."},{"FIELDNAM":"P57A Component Number"},{"FILLVAL":-2147483648},{"FORMAT":"i2"},{"LABL_PTR_1":"thg_mag_p57a_100ms_labl"},{"VALIDMIN":[1,1,1]},{"VALIDMAX":[3,3,3]},{"VAR_TYPE":"support_data"},{"SCALETYP":"linear"},{"DICT_KEY":"number"},{"UNITS":" "}]},{"VarData":[1,2,3]}]},{"thg_mag_p57a_100ms_time":[{"VarDescription":[{"DataType":"CDF_REAL8"},{"NumElements":1},{"NumDims":0},{"RecVariance":"VARY"},{"PadValue":-1.0E+30}]},{"VarAttributes":[{"CATDESC":"UTC Time, in seconds since 01-Jan-1970 00:00:00. 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