The galloping chromosphere

Oscillating velocity fields can be observed on H-alpha filtergrams as a shifting pattern of intensity fluctuations known as 'the galloping chromosphere'. The characteristics of this activity are those of horizontal running waves of typical period of about 300 sec and long wavelength (about 20,000 km) that can be interpreted as acoustic-gravity waves propagating in the acoustic domain. Periods are longer in dark, structured regions, and in fibrils, and the change is quantitatively consistent with the reduction of resonance frequency in a magnetic field of 1 to 10 gauss. These easily observed fluctuations thus offer a means of estimating magnetic-field strength at specific locations in the chromosphere. Phase velocities are high, ranging upward from typical values between 50 and 100 km per sec, and tending to be lower in active regions and toward the limb.