Institution(s): 1. Big Bear Solar Observatory, 2. Seoul National University
Even though it is well-known from observations of the Sun that three-minute period chromospheric oscillations persist in the internetwork quiet regions and sunspot penumbrae, until now their origin and persistence has defied clear explanation. Here we provide a clear and simple explanation for it with a demonstration of how such oscillations at the chromosphere's critical frequency naturally arise in a gravitationally-stratified medium when it is disturbed. The largest-wavenumber components of a chromospheric disturbance produce the highest-frequency wave packets, which propagate out of the disturbed region at the group speed, which is close to the sound speed. Meanwhile, its smallest-wavenumber components develop into wave packets of frequencies close to the critical frequency that propagate at a group velocity that is much lower than the sound speed. Because of their low propagation speed, these wave packets with the natural frequency linger around the disturbed region and its surrounding regions for a long time, and it is these that one would observationally identify as the persistent, chromospheric three-minute oscillations. As well, according to our explanation, the power of the persistent chromospheric oscillations comes from disturbances with length scales that are greater than twice the pressure scale height in the source region and their frequencies are determined by the sound speed and the gravitational acceleration of the region.