Observation of convective collapse and upward-moving shocks in the quiet Sun

We present spectropolarimetric evidence of convective collapse and destruct ion of magnetic flux by upward-moving fronts in the quiet Sun. The observat ional material consists of time series of the full Stokes vector of two inf rared spectral lines emerging from regions associated with Ca II K network points. The amplitude of the circular polarization profiles of a particular spatial point is seen to increase while the profiles are redshifted. It th en decreases during a much shorter phase characterized by large blueshifts. Inspection of the data indicates that the blueshift occurs because of the sudden appearance of a new, strongly displaced Stokes V profile of the same polarity. The amplification of the magnetic signal takes place in a time i nterval of about 13 minutes, while blueshifts and the concomitant decreasin g Stokes V amplitudes last for only 2 minutes. An inversion code based on t he thin flux-tube scenario has been applied to the data in order to derive the thermal, magnetic, and dynamic structures of the atmosphere. According to our results, the field strength undergoes a moderate increase from 400 t o 600 G at z = 0 km during the phase in which redshifts are present. The ob served redshifts are produced by internal downflows of up to 6 km s(-1) at z = 0 km. After similar to 13 minutes, the material falling down inside the tube appears to bounce off in the deeper layers, originating an upward-pro pagating front whose manifestation on the Stokes V profiles is a large blue shift. The front moves with a speed of 2.3 km s(-1) and has a downflow-to-u pflow velocity difference of about 7 km s(-1) initially and some 4 km s(-1) after 2 minutes. It strongly weakens the magnetic field strength and may b e responsible for the complete destruction of the magnetic feature. The obs erved behavior is in general agreement with theoretical predictions of flux expulsion, convective collapse, and development of shocks within magnetic flux tubes.