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Enhanced ionization in the polar ionosphere associated with geomagnetic storms

1960; Elsevier BV; Volume: 18; Issue: 2-3 Linguagem: Inglês

10.1016/0021-9169(60)90082-9

1878-593X

T. Obayashi, Y. Hakura,

Geomagnetism and Paleomagnetism Studies

New evidence indicating the existence of energetic solar particles associated with solar flares has been found from the analysis of world-wide data ofƒmin in the ionograms. An outstanding enhancement of ionization in the lower ionosphere occurs in the polar region with the delay of several hours after a large solar flare accompanied with major radio outbursts of type IV. Radio blackouts due to enhanced ionizations develop, spreading inside the whole polar cap without inducing any marked geomagnetic activity. With the onset of a geomagnetic storm, however, the blackout region spreads from the polar cap up to the so-called auroral zone. As the Dst field of geomagnetic storm develops, it comes down to considerably low latitudes forming a spiral-shaped ionization pattern. The radio blackouts, which precede the onset of geomagnetic storms, may be termed the polar cap blackouts because they are confined within the geomagnetic latitude of about 60 ~ 65°, while those related to geomagnetic storms and auroral displays are the auroral zone blackouts. It is shown that the polar cap blackouts are caused by the invasion of energetic particles of the order of 10 ~ 100 MeV, generated in an agitated solar coronal plasma bearing magnetic fields, and that those particles precipitate on the polar ionosphere following nearly the Störmer orbits. For the auroral zone blackouts, an improved version of the Störmer theory is applied, in which an interesting behaviour of a charged particle in the distorted geomagnetic field is revealed. The remarkable equatorward shifting of the auroral zone and some peculiar increases of cosmic ray intensity during the main phase of magnetic storms are explained consistently by the decrease of geomagnetic cut-off for incoming particles owing to the contraction of the geomagnetic cavity.