Type III bursts can occur alone, in small groups, or in chains referred to as Type III storms that may last many minutes. They are often subdivided into two types, coronal and interplanetary Type III bursts. Coronal refers to the case for which an electron beam is traveling in the corona within a few solar radii of the photosphere. They typically start at frequencies in the hundreds of MHz and drift down to tens of MHz over a few seconds. The electron beams that excite radiation travel along specific magnetic field lines that may be closed or open to interplanetary space. Electron beams that escape into interplanetary space may excite Langmuir waves in the solar wind plasma to produce interplanetary Type III bursts that can extend down to 20 kHz and below for beams that reach 1 Astronomical Unit and beyond. The very low frequencies of interplanetary bursts are below the ionospheric cutoff ( ≈ ≈ 10 MHz), meaning they are blocked by Earth's ionosphere and are observable only from space.