Chapter 7 Magnetic Observatory Data and Metadata
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Chapter 7 Magnetic Observatory Data and Metadata: Types and Availability S Reay1, D Herzog2, S Alex3, E Kharin4, S McLean2, M Nosé5, N Sergeyeva4 1. British Geological Survey, Edinburgh, United Kingdom 2. National Geophysical Data Center, Boulder CO, USA 3. Indian Institute of Geomagnetism, Mumbai, India 4. Geophysical Center, Russian Academy of Sciences, Moscow, Russia 5. Graduate School of Science, Kyoto University, Kyoto, Japan This is an author created version of: Reay SJ, Herzog DC, Alex S, Kharin E, McLean S, Nosé M and Sergeyeva N (2011) Magnetic Observatory Data and Metadata: Types and Availability. In: Mandea M, Korte M (eds) Geomagnetic Observations and Models, IAGA Special Sopron Book Series 5, DOI 10.1007/978-90-481-9858-0_7 pp149-181 The original publication is available at http://www.springerlink.com Abstract The availability of magnetic observatory recordings, and the development of technological data has evolved rapidly with the transition of capabilities such as the Personal Computer (PC); CD- observatories from analogue photographic and DVD-ROMs for the inexpensive archiving of magnetograms to digital electronic recordings, and large quantities of data; and the internet for instant the advent of the internet for instant global access to global access to information of every sort. Metadata information of every sort. Metadata (information (information about the data) are undergoing their own about the data) is undergoing its own transformation transformation in order to accompany the rapid and in order to accompany the rapid and extensive extensive dissemination of these various data sets. dissemination of these data. This chapter describes Metadata describes the content, quality, originator, the types of data historically and currently produced and other characteristics of a data set, and supports by geomagnetic observatories and introduces new the proper interpretation and use of the data. data types such as one-second and quasi-absolute For more than a century, magnetic observatory data recently discussed at the 11th IAGA Scientific data were only available on printed materials as Assembly in Sopron, Hungary. We review the analogue traces on photographic paper known as availability of these data types from the World Data magnetograms, products derived from those traces, Centres, INTERMAGNET and other sources. such as temporal averages (hourly, daily, etc.) and Finally, we discuss developments in metadata magnetic activity indices. Metadata consisted of describing the current efforts in the geomagnetism yearbooks in which information about the community to gather, store and distribute this observatories was provided, and copies of these information about the data to better assist scientific various data products were reproduced in these discovery. books. The launching of the International Geophysical Year (IGY) in 1957 brought with it the beginning of the World Data Centres (WDC) 7.1 Introduction established as central repositories where scientists could go to obtain data from a number of locations Magnetic observatories continuously measure the worldwide. strength and direction of the Earth’s magnetic field In the late 1960’s, magnetic observatories began (Macmillan, 2007). The availability of data from converting to digital electronic equipment and a true these observatories has evolved greatly with the revolution in data types and availability was transition from the production of analogue underway. The individual traces could now be kept photographic magnetograms to digital electronic separate (traces often crossed each other and became 2 confused on magnetograms) and the standard in Sopron, Hungary in 2009, we hope to provide an observatory data product changed from hourly means account of where we are today regarding these topics. to minute means. Most significantly, these data could now be collected, manipulated, archived and 7.2 Data Types disseminated using computers. The emergence of the PC in the 1980’s, and the 7.2.1 Printed Media development of CD-ROMs for data storage saw the beginning of “small science” where individual From the earliest days of magnetic observatory scientists were able to have large quantities of data operations, data were collected on paper records of available to study for minimal cost. In 1990, for one sort or another. Variations in the magnetic field example, the U.S. Geological Survey (USGS) were recorded as continuous traces on photographic produced the first CD-ROM of one-minute magnetic paper. Observations of the absolute magnitudes of the observatory data for a 5-year period (1985-1989) field components were recorded and provided from the USGS network of (then) 13 stations (Herzog calibration reference data with which to convert the and Lupica, 1992), and served as the model for the analogue trace amplitudes into magnetic field values. production of a series of CD-ROMs by These observations could then be reduced to produce INTERMAGNET containing a global set of tables of results at various time-scales: hourly, daily, observatory data. monthly, annual. In time, range data started to be During the 1980’s and 1990’s, significant presented and then activity measures, such as the K- advances were also made in the timeliness of data index (1939) were developed. Magnetic records also delivery through the use of satellites. Data Collection included absolute measurements, information on the Platforms (DCPs), equipped with satellite baseline used and indications of the most quiet and transmission systems, began delivering data from the active days (from 1911). observatories to data processing centres, with delays These records were presented in the form of of the order of tens of minutes. An organisation yearbooks. These yearbooks and photographic called INTERMAGNET was formed which began analogue traces are archived and are available from transferring data from observatories worldwide using different sources and in a variety of formats. Many satellites, and which developed Geomagnetic magnetogram traces are stored on microfilm and Information Nodes (GINs) where users could obtain microfiche. In this digital age we are keen to preserve preliminary data from the participating stations these, often, fragile documents and allow greater quickly by means of email requests. access for analysis of these historical records. Some But without question the greatest advancement in have been scanned as images and some have been data availability has come about from the impact of entered directly to form digitised electronic files. the World Wide Web (WWW) as a data discovery and distribution tool for the internet. Beginning in 7.2.1.1 Eye-Observations 1992, when WWW access became more broadly The earliest records from magnetic observatories available to the general public, the growth in use of were from eye-observations made manually by the WWW for access to data has been astonishing. observers at set times during each day. For example Today virtually every institution, public and private, at Greenwich observatory observations were made at has a website with the capability of providing access 2-hourly intervals except on “term-days” when to their information and data. As magnetic measurements were every 5 minutes. At Colaba observatories have progressed in their abilities to observatory observations were made hourly except collect and process data faster and with higher during disturbed times when they were every 15 cadences (one-second data, for example), these data minutes or every 5 minutes during severely disturbed have also become ever more readily available. This conditions. Fig 7.1 shows the 5-minute eye- change has implications for traditional WDC roles of observations for Colaba during the September 1859 data discovery and delivery, but also for long-term “Carrington Storm”. The laborious nature of these data archive. manual observations encouraged the development of In this chapter we will look at some of these automatic recording devices. In 1847 Brooke developments and advances in data and metadata designed the automatic photographic magnetograph types and how they have been made available. Using that was to form the standard technique for an historical perspective, and with the help of results observatory operations for over a century (Brooke, presented at the 11th IAGA Scientific Assembly held 1847). Fig 7.1 Eye-observations made with Grubb’s magnetometer at Colaba Observatory for 1-2 September 1859. The observation frequency increases from 15-minute to 5-minute observations as the severity of the storm increases (see Tsurutani et al., 2003). (Credit: Indian Institute of Geomagnetism) 7.2.1.2 Magnetograms enhanced magnetic activity; and rapid-run magnetograms are recorded on drums that rotate For more than a century, the changes in the Earth’s more than once a day as the drum moved laterally, magnetic field have been recorded at observatories as providing greater time resolution. In addition to the analogue traces on sheets of photographic paper. mirrors mounted on the magnets in the variometer, Under darkroom conditions, mirrors attached to stationary mirrors are also used to reflect light onto magnets suspended from quartz filament fibres, the recording drum and produce a fixed (non- known as variometers, reflect light from a source varying) trace known as a baseline. A schematic through a horizontal lens and onto a sheet of illustration of this arrangement can be seen in Fig photographic paper generally mounted on a drum that 7.2a. An example of a normal magnetogram,