Acoustic imaging of solar active regions - (Invited review)

Acoustic imaging is a new method to construct the acoustic signal at a poin t on the solar surface or in the solar interior with the signals measured a t the solar surface. The constructed signals contain both intensity informa tion and phase information. The intensity is computed by summing the square d amplitude of the constructed signal over time. The phase of constructed s ignals can be studied by the cross-correlation function between the time se ries constructed with ingoing waves and outgoing waves. The location of the envelope peak of the cross-correlation function and the phase of the cross -correlation function contain different information on the physical conditi ons of the plasma along the wave path. From the constructed signals, one ca n form the two-dimensional outgoing intensity map, absorption map, phase-sh ift map, and envelope-shift map of a target region at different focal depth s. The perturbed physical conditions caused by the magnetic fields of activ e regions manifest in these maps. The outgoing intensity is lower in magnet ic regions than the quiet Sun. The group travel time and phase travel time are smaller in magnetic regions than in the quiet Sun. In this paper, we re view the studies of active regions, including emerging flux regions, with a coustic imaging.