Solar Images (All ftp)

Solar Images -- The Mount Wilson Observatory, located in Mt. Wilson, California, has conducted daily chromospheric spectroheliograms of the sun in the K-line of calcium II (Ca II) since 1915. This massive archive of data (over 16,000 images) is one of the earliest collections of solar data photos in existence. The negatives of these photographs are kept and maintained at the Pasadena California offices of the Observatories of Carnegie Institution of Washington. Several years ago these photos were digitized and reduced by Dr. Peter Foukal of Cambridge Research and Instrumentation, Inc. (CRI) and his staff. The images were scanned to 512x512 resolution. Software calculated the mean and root mean square (rms) of the pixel intensities. The daily reduced Calcium plage area data for 1915-1984 are available online. A detailed description of the data, reduction procedures, and errors, along with the results, are given in "The Behavior of Solar Magnetic Plages Measured from Mt. Wilson Observations between 1915-1984" by P. Foukal, in Geophysical Research Letters (GRL), Vol. 23, p. 2169, 1996. These data are valuable for use in modeling the past total, ultraviolet (UV) and extreme ultraviolet (EUV) solar irradiances. They extend reliable information on the behavior of solar magnetic plage areas back to 1915.

While the Ca K photos obtained from Mt. Wilson compose one of the most complete solar data sets available to the public, the images in this archive are of varying quality. Problems include photos that are completely grey, overexposed, or underexposed. There can be large gaps between dates. Some photos have as well been blurred or obscured by dark stripes or large black spaces. Some are damaged by water, scratched, or have cracked with age. A student spent some time at NGDC reviewing the images and checking for internal consistency. The results of this effort are a more standardized set of images, useful for projects that do not need high resolution data. They can also serve as a complementary guide for other solar image collections and as a quick-look resource of solar activity over many years.

Currently Mt. Wilson is making high resolution scans of the archive. For detailed studies, we refer the user to their archive.

• Archive by decade, 1915-1984 images;
• Zipped file of all these low resolution images;
• Documentation about the image database;
• Some images with the wrong dates. Motivated users are invited to try to determine the correct dates for these images.