Wave generation in the terrestrial magnetosphere

The Hard X-ray Imaging Spectrometer aboard the SMM detected gigantic arches in the corona which are formed or, if preexisting, become excited after major two-ribbon flares. They are seen in 3.5–8 keV X-rays and extend along the H _∥ = 0 line to altitudes between 10⁵ and 2 × 10⁵ km. These arches are stationary and form the base of a stationary type I radio noise storm initiated by the flare. They are visible in X-rays for ten hours or more and may be revived, in temperature, density, and brightness, if another two-ribbon flare appears below them. We suggest that they are built-up through reconnection process during the flare from the upper reconnected loops in the Kopp and Pneuman model. These loops become interconnected along the H _∥ = 0 line in consequence of great shear of the reconnecting loops. Obviously, the coronal transient associated with such flares must be either accomplished prior to the formation of the arch, or it must be formed through a process different from the Anzer-Kopp-Pneuman mechanism. Striking brightness variations occur quasi-periodically in the corona below and above the arch a few hours after the flare. These variations are seen at about the same time in soft X-rays, hard X-rays, and on centimeter microwaves in the low corona, as well as at metric waves in the type I noise-storm region. In spite of their flare-like intensity, however, the variations have little response in the transition layer (O v line) and no response at all in the chromosphere (Hα). We suggest that these semi-periodic brightenings are due to repetitive acceleration processes in plasmoids that encircle the arch perpendicular to the H _∥ = 0 line from the low corona through the noise storm region, being completely detached from the lower atmospheric layers.