Solar radio emission very near the plasma frequency

Plasma-frequency radio emission from the solar corona has been observed with very short duration and very narrow bandwidth. These observations imply an upper corona with a structured field-aligned density distribution. I evaluate the propagation characteristics of fundamental plasma radio emission when the bandwidth is small compared with the electron gyrofrequency (Ω), which is small compared with the plasma frequency (ω/sub p/). The ''normal'' group velocity, c(Ω/ω/sub p/)1/sup //2, applies only to radiation emitted nearly parallel to the magnetic field. Most of the radiation at first travels with a much lower group velocity and is delayed relative to the radiation emitted parallel to the magnetic field. Radiation emitted at one instant of time over a finite range of angles leaves the corona with a time profile of finite duration. Very short observed signals of about 20 ms duration may be explained in two ways. (1) We observe only rays from a small part of the cone of emission. Then the observed bandwidth may be small compared with the inherent bandwidth of emission. (2) The density scale height is substantially less than 105 km, and the direction of the density gradient is well outside the cone of emission. The shortest signals from the upper corona may indicate density variations across magnetic flux tubes with scale heights as small as 104 km