Helical Motion and Magnetic Mirrors
When a charged particle moves along a magnetic field line into a region where the field becomes stronger, the particle experiences a force that reduces the component of velocity parallel to the field. This force slows the motion along the field line and here reverses it, forming a "magnetic mirror. "
Source
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"OpenStax College, College Physics. November 27, 2012."
http://cnx.org/content/m42375/latest/?collection=col11406/1.7
OpenStax CNX
CC BY 3.0.