Definition of A Geomagnetic Storm
A geomagnetic storm is defined by changes in the DST (disturbance – storm time) index. The Dst index estimates the globally averaged change of the horizontal component of the Earth’s magnetic field at the magnetic equator based on measurements from a few magnetometer stations. Dst is computed once per hour and reported in near-real-time. During quiet times, Dst is between +20 and -20 nano-Tesla (nT).
A geomagnetic storm has three phases: an initial phase, a main phase and a recovery phase. The initial phase is characterized by Dst (or its one-minute component SYM-H) increasing by 20 to 50 nT in tens of minutes. The initial phase is also referred to as a storm sudden commencement (SSC). However, not all geomagnetic storms have an initial phase and not all sudden increases in Dst or SYM-H are followed by a geomagnetic storm. The main phase of a geomagnetic storm is defined by Dst decreasing to less than -50 nT. The selection of -50 nT to define a storm is somewhat arbitrary. The minimum value during a storm will be between -50 and approximately -600 nT. The duration of the main phase is typically between 2 and 8 hours. The recovery phase is the period when Dst changes from its minimum value to its quiet time value. The period of the recovery phase may be as short as 8 hours or as long as 7 days.
The size of a geomagnetic storm is classified as moderate (-50 nT > minimum of Dst > -100 nT), intense (-100 nT > minimum Dst > -250 nT) or super-storm ( minimum of Dst < -250 nT).
Read more about this topic: Geomagnetic Storm
Famous quotes containing the words definition of a, definition of, definition and/or storm:
“Definition of a classic: a book everyone is assumed to have read and often thinks they have.”
—Alan Bennett (b. 1934)
“Definition of a classic: a book everyone is assumed to have read and often thinks they have.”
—Alan Bennett (b. 1934)
“Although there is no universal agreement as to a definition of life, its biological manifestations are generally considered to be organization, metabolism, growth, irritability, adaptation, and reproduction.”
—The Columbia Encyclopedia, Fifth Edition, the first sentence of the article on “life” (based on wording in the First Edition, 1935)
“Many Americans imagine simpler times even as a storm of social change swirls about, blowing parents here and children there. Sure, the 1950s ideal world would be wonderful. But knock on the nation’s doors: Ozzie and Harriet are seldom at home.”
—Leslie Dreyfous (20th century)