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Averaging and sampling for magnetic-observatory hourly data

Love, J. J. and Tsai, V. C. and Gannon, J. L. (2010) Averaging and sampling for magnetic-observatory hourly data. Annales Geophysicae, 28 (11). pp. 2079-2096. ISSN 0992-7689.

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A time and frequency-domain analysis is made of the effects of averaging and sampling methods used for constructing magnetic-observatory hourly data values. Using 1-min data as a proxy for continuous, geomagnetic variation, we construct synthetic hourly values of two standard types: instantaneous "spot" measurements and simple 1-h "boxcar" averages. We compare these average-sample types with others: 2-h average, Gaussian, and "brick-wall" low-frequency-pass. Hourly spot measurements provide a statistically unbiased representation of the amplitude range of geomagnetic-field variation, but as a representation of continuous field variation over time, they are significantly affected by aliasing, especially at high latitudes. The 1-h, 2-h, and Gaussian average-samples are affected by a combination of amplitude distortion and aliasing. Brick-wall values are not affected by either amplitude distortion or aliasing, but constructing them is, in an operational setting, relatively more difficult than it is for other average-sample types. It is noteworthy that 1-h average-samples, the present standard for observatory hourly data, have properties similar to Gaussian average-samples that have been optimized for a minimum residual sum of amplitude distortion and aliasing. For 1-h average-samples from medium and low-latitude observatories, the average of the combination of amplitude distortion and aliasing is less than the 5.0 nT accuracy standard established by Intermagnet for modern 1-min data. For medium and low-latitude observatories, average differences between monthly means constructed from 1-min data and monthly means constructed from any of the hourly average-sample types considered here are less than the 1.0 nT resolution of standard databases. We recommend that observatories and World Data Centers continue the standard practice of reporting simple 1-h-average hourly values.

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Tsai, V. C.0000-0003-1809-6672
Additional Information:© 2010 Author(s). This work is distributed under the Creative Commons Attribution 3.0 License. Received: 21 May 2010 – Revised: 25 October 2010 – Accepted: 5 November 2010 – Published: 12 November 2010. The results presented here rely on data collected at the Barrow, Chambon la Forêt, Eskdalemuir, and Huancayo observatories. The Barrow observatory is operated by the USGS Geomagnetism Program. We thank the Institut de Physique du Globe de Paris (France), the British Geological Survey (Great Britain), and the Instituto Geofisico (Perú) for continuing to support the operation of their observatories. We acknowledge Intermagnet ( for facilitating the dissemination of magnetic observatory data and for promoting high standards of magnetic observatory practice. We thank the Edinburgh and Kyoto World Data Centers for archiving observatory data and making them readily available to the scientific community. We thank the Boulder World Data Center (NGDC) for use of their observatory yearbooks. We thank P. S. Earle, J. W. Kirchner, K. Mursula, and L. Svalgaard for useful conversation/communication. We thank A. Chulliat, C. A. Finn, V. Lesur, and S. Macmillan, for reviewing a draft manuscript. Topical Editor I. A. Daglis thanks S. Macmillan, A. Chulliat, and another anonymous referee for their help in evaluating this paper.
Group:Seismological Laboratory
Subject Keywords:Electromagnetics (Measurement and standards)
Record Number:CaltechAUTHORS:20140611-145630940
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Official Citation:Love, J. J., Tsai, V. C., and Gannon, J. L.: Averaging and sampling for magnetic-observatory hourly data, Ann. Geophys., 28, 2079-2096, doi:10.5194/angeo-28-2079-2010, 2010
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:46218
Deposited By: Tony Diaz
Deposited On:11 Jun 2014 22:09
Last Modified:23 Nov 2016 20:00

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