Absolute electron density in the ionosphere at day time.
Sebastian Janke
CC BY-SA 2.5
own work, data bases on the book "Vogelsang, E., Wellenausbreitung in der Funktechnik" (~"Wave Propagation in Radio Applications"), <a href="//commons.wikimedia.org/wiki/Special:BookSources/3-486-23731-4" title="Special:BookSources/3-486-23731-4">ISBN 3-486-23731-4</a>