Dayside cleft aurora and its ionospheric effects

Abstract

The existence of an opening on the dayside of the earth's magnetic field through which solar plasma could penetrate was noted by Chapman and Ferraro in 1931, and although observational evidence of its auroral and ionospheric effects accumulated steadily over the years, the dayside cleft (or cusp) was not accepted as a real magnetospheric feature until 1971. The history of this development is described, and some reasons for the long delay in recognizing this profound ionospheric influence are given. The entry of magnetosheath plasma to ionospheric levels provides an essentially permanent dayside energy source for the F region that generates a rather constant 1–2 kR of 6300‐Å atomic oxygen emission, a weak hydrogen emission, and some other emissions. It also produces ionospheric irregularities and enhanced electron temperatures that can be detected by a variety of methods, all of this in a region normally centered at noon and 78° in invariant coordinates, with a latitude extent of a few degrees and a longitude extent of a few hours. The cleft ionization is an important source for the polar cap ionosphere, and the detailed nature of its effect depends upon the interplanetary field and on magnetospheric influences on the polar cap convection pattern. Some questions for further study are given in conclusion.

Monitoring the global-scale winter anomaly of total electron contents using GPS data

“…Other researchers have also suggested that the ionospheric effects of the soft electron precipitation cause the increases in the electron concentration found in the F 2 -region (e.g. Shepherd, 1979;Namgaladze et al, 1997). At the same time, this effect of magnetospheric processes is affected intensively by the poleward wind produced by solar heating, which extends to relatively low geographic latitudes in the 'near-magnetic-pole' sectors of North America (Rishbeth, 1998).…”

Synoptic distribution of dayside aurora: Multiple-wavelength all-sky observation at Yellow River Station in Ny-Ålesund, Svalbard

“…These bright spots could be related to the KHI occurring within the magnetospheric flank boundary layers (Lui et al, 1989). The distribution of postnoon auroral occurrence deduced from ground visible observation (Yang et al, 2000), however, presents not only a peak near the ''hot spot'', but also a peak centered at 1330 MLT, which corresponds to a strong auroral emission at 630.0 nm (Shepherd, 1979). At the same time, the auroras display different properties within a post-noon double-peak region of auroral occurrence (Yang et al, 2000).…”

Variations of the O(1D) and O(1S) peak volume emission rates without direct solar effects

“…3 are seasonally averaged measurements, which permit only the climatological behavior of the aurorae regardless of the orientation of the interplanetary magnetic field and the strength of geomagnetic activities. The red aurorae are most sensitive to the precipitation of magnetosheath-origin electrons at energies below about 200 eV, which enter the earth's atmosphere in the so-called cusp region 0900-1500 MLT and 70-80°MLAT (Shepherd, 1979;Sandholt et al, 2002). From WINDII data, the red aurorae are significantly enhanced in the cusp regions and more so in the equinox seasons than in the solstice seasons with peaks at 80°M LAT or higher.…”