Magnetic fields and the supply of low-frequency acoustic wave energy to the solar chromosphere

Abstract

The problem of solar chromospheric heating remains a challenging one with wider implications for stellar physics. Several studies in the recent past have shown that small-scale inclined magnetic field elements channel copious energetic low-frequency acoustic waves, which are normally trapped below the photosphere. These magnetoacoustic waves are expected to shock at chromospheric heights, contributing to chromospheric heating. In this work, exploiting simultaneous observations of photospheric vector magnetic field, Doppler, continuum, and line-core intensity (of Fe I 6173 Å) from the Helioseismic and Magnetic Imager and lower-atmospheric UV emission maps in the 1700 and 1600 Å channels of the Atmospheric Imaging Assembly, both on board the Solar Dynamics Observatory of NASA, we revisit the relationships between magnetic field properties (inclination and strength) and the propagation of acoustic waves (phase travel time). We find that the flux of acoustic energy, in the 2–5 mHz frequency range, between the upper photosphere and lower chromosphere is in the range of 2.25–2.6 kW m−2 , which is about twice the previous estimates. We identify that the relatively less inclined magnetic field elements in the quiet Sun channel a significant amount of waves of frequency lower than the theoretical minimum acoustic cutoff frequency due to magnetic inclination. We also derive indications that these waves steepen and start to dissipate within the height ranges probed, while those let out due to inclined magnetic fields pass through. We explore connections with existing theoretical and numerical results that could explain the origin of these waves.