Chromospheric impact of an exploding solar granule

Movies of solar granulation frequently show bright granules that expand rapidly and reach sizes larger than an average granule. 

Left to right: panel a): Hinode apparent longitudinal magnetic flux density map. The contours are from absolute 20 Mx/cm2 to 60 Mx/cm2. The apparent horizontal flows are indicated with colored arrows. The two vertical white lines delimit the position of the IRIS slit in use. Two dashed red horizontal lines and a solid red line mark a region of interest. Panels b) and c) are time-space images derived from spectra recorded by the IRIS slit. Panel b): line-core velocity of the high photospheric Fe II 2799.9 Å line (±1.9 km s−1 , where down flows are positive). Panel c): the intensity of the chromospheric Mg II k2v peak (in data units) is plotted. The red horizontal lines indicate the same area as in panel a). The short vertical lines in cyan indicate the time of the map in panel a). The right panel demonstrates the temporal evolution of the IRIS slit spectra at the slit location marked by the solid red horizontal line in panels a) to c) with the time of panel a) indicated again with lines in cyan. 

Using co-observations from the  Hinode and IRIS satellites we follow the evolution of an exploding granule and study its interaction with the surrounding pre-existing magnetic field. During the rapid expansion of the granule the magnetic elements in the bordering intergranular lanes are being squeezed and elongated by the opposing horizontal flows from the exploding granule as well as the neighboring granules. In the longitudinal magnetic flux density map in Fig.1 (left side) such an elongated magnetic structure can be seen. The observations in IRIS (middle and right panels) reveal oscillations in the intensity and velocity of the co-observed high photospheric and chromospheric spectra. We find that the upward propagating shock front dissipates in the chromosphere.

These kind of multi-wavelength observations are crucial for our understanding of the physical processes that couple the different regimes in the solar atmosphere.


The results were published in a recent Letter in the Astronomy & Astrophysics journal ( C.E.Fischer, N.Bello González, and R.Rezaei 2017, A&A, 602, L12).