Solar activity was quite low from January 2008 to February 2009, ever since the start of Solar Cycle 24. In 2008, the number of days in a year during which there were no sunspots was the fourth-largest since 1842. However, it is likely that the sunspot number would not have been able to reveal the solar inactivity in the beginning phase of cycle 24 because it hit the lower limit (zero) on more than two-thirds of the days of 2008. Geomagnetic data covers the longest span of time next to sunspot number, but its value has never reached that limit. The K-index, which is a geomagnetic index, from 2008 to the beginning of 2009, has been recorded at several observatories as being the smallest in the history of observation. The K
p-index, which characterizes the geomagnetic planetary activity, has also significantly decreased. Taking into account the relation between the K
p-index during the minimum phase of the solar cycle and sunspot numbers of the following maximum, it has been suggested that the peak solar activity in cycle 24 will be quite low. If solar activity in cycle 24 continues to be quiet, geomagnetic data are expected to be one of the key tools for monitoring space climate.
INTRODUCTIONIn the past, geomagnetic field observations were recorded in analog form on photographic paper (Jankowski & Sucksdorff, 1996) with a silver bromide emulsion, known as bromide paper. Up to now, these analog magnetograms have been read only by hand scaling with low time resolution. Many observatories have analog records covering long observation periods (Iyemori, Nose, McCreadie, Odagi, Takeda, Kamei et al., 2005). At Kakioka Magnetic Observatory (KMO) in Japan, records on photographic paper of geomagnetic observations go back to 1924. However, most of the numerical data available from these magnetograms are hourly values that were digitized by hand scaling. Conversion of these analog records to high-resolution numerical data would make them very useful in investigations of past geomagnetic activity. We therefore developed a method for converting the analog magnetograms into digital data with high time and amplitude resolutions and then examined the conversion accuracy.
OUTLINE OF THE CONVERSION METHODThe first step was to obtain high-resolution scans of the photographic paper records and to store them as graphic files. We then used an image processing program that we developed to distinguish lines and curves in the graphic files. The identified pixels were converted into numerical time and geomagnetic field data. Each step of the conversion is described in detail below.
Photographic paper record and scan specificationsEach record is a sheet of photographic paper about 510 mm long by 195 mm wide (Figure 1 In our digitization method, we scanned the photographic records at a relative resolution of 600 pixels per inch, which corresponds to about 4,600 by 12,000 pixels per sheet. This resolution is equivalent to a time resolution of about 7.5 s per pixel and an amplitude resolution of about 0.1 nT (H and Z) and 0.01 minutes (D) per pixel. The scanned photographic records are stored as 24-bit color bitmap images so that each pixel can be weighted according to its luminosity during image processing.
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