magnetic annual variation
magnetic annual variation[mag′ned·ik ′an·yə·wəl ‚ver·ē′ā·shən]
The small, systematic temporal variation in the earth's magnetic field which occurs after the trend for secular change has been removed from the average monthly values. Also known as annual magnetic variation.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.