Abstract
Abstract. 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.
Highlights
Since magnetic observatories first began operating around the world in the middle of the 19th century, one of their most important products has been hourly data values
Historical hourly observatory data are useful for a variety of applications (e.g. Parkinson, 1983; Courtillot and Le Mouel, 1988; Barraclough et al, 1992; Prolss, 2004), including studying geomagnetic secular variation originating in the core (e.g. Sabaka et al, 2004), exploring the electrical conductivity of the mantle (e.g. Egbert et al, 1992), mapping electric currents in the ionosphere (e.g. Campbell, 1989), measuring the intensity of magnetospheric storms (e.g. Karinen and Mursula, 2005), and estimating long-term solar-terrestrial interaction (e.g. Macmillan and Droujinina, 2007)
Natural to ask: can we measure differences in the properties of hourly spot and hourly average values so that informed decisions can be made about using them? How do spot and hourly averages compare with other hypothetical average-sample types that might be proposed in the future as candidate observatory products? In seeking answers to these and other related questions, we examine the properties of hourly observatory data, measuring their variance, relative proportionality, correlation, autocorrelation, spectral power, amplitude distortion, and aliasing
Summary
Since magnetic observatories first began operating around the world in the middle of the 19th century, one of their most important products has been hourly data values. Researchers often use a combination of spot and hourlyaverage observatory data, as if, together, they represent a reliable long-term record of magnetic-field variation It is, natural to ask: can we measure differences in the properties of hourly spot and hourly average values so that informed decisions can be made about using them? The 1-min data are definitive (processed and calibrated) magnetic-vector values collected at observatories (Jankowski and Sucksdorff, 1996; Love, 2008) that meet Intermagnet standards (Kerridge, 2001; Rasson, 2007); we obtained the 1-min data from the Intermagnet website (www.intermagnet.org) for observatories situated at high (Barrow BRW), medium (Chambon la Foret CLF), and low (Huancayo HUA) geomagnetic latitudes They record a variety of magnetic-field variation, with high (low) latitude variation dominated by active auroral (equatorial) electrojets The details of these adjustments do not significantly affect the results that follow
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