Coronal Non-Thermal and Doppler Plasma Flows Driven by Photospheric Flux in 28 Active Regions

TL;DR

This study tackles the question of whether photospheric magnetic flux controls coronal heating as reflected in non-thermal line broadening. By combining full-disk Hinode/EIS measurements of Fe XII 195.119 Å (log $T\sim6.2$) with SDO/HMI photospheric flux for 28 active regions, the authors derive non-thermal velocities $v_{nt}$ and explore their relationships to magnetic flux, AR age/evolution, Doppler flows, and FIP bias. They find a moderate correlation, $r \approx 0.48$, between log total unsigned flux and $v_{nt}$, suggesting that stronger photospheric fields inject more upward Poynting flux that heats plasma via braiding/nanoflare or MHD wave processes; however, no strong coupling to FIP bias and no clear age trend are observed, leaving the dominant heating mechanism unresolved with current instrumentation. The results underscore the value of full-disk, statistically robust observations and point to the need for higher-resolution missions (e.g., SOLAR-C/EUVST) to disentangle nanoflare/braiding heating from Alfvénic-wave heating in coronal plasmas.

Abstract

Magnetohydrodynamic (MHD) waves and/or the braiding of magnetic field lines are largely thought to be responsible for heating the solar corona, both being mechanisms which are driven by the Sun's photospheric magnetic field. Recent modelling work leads us to expect that such heating mechanisms would be seen in the excess broadening (non-thermal velocity) of coronal spectral emission lines and that larger magnitudes of photospheric magnetic flux would generate more heating, but a direct connection between magnetic flux and spectral line broadening has been difficult to establish. We combine measurements of the photospheric magnetic field from SDO/HMI and non-thermal velocity in log T~6.2 coronal plasma from Hinode/EIS for 28 active regions and find a moderate correlation between the two exists in quiescent active regions, consistent with the photospheric field injecting upward Poynting flux into the solar corona and causing coronal heating. We find that no strong correlation with coronal composition makes it difficult to distinguish between MHD wave heating and magnetic field braiding heating using these diagnostics with current instrumentation.

Figures (12)

• Figure 1: Full Sun maps for 16-18th January 2013 (top row), 1-3 April 2015 (middle row), 18-20 October 2015 (bottom row) showing from Hinode/EIS the Fe XII 195.119 intensity (far left column), Fe XII 195.119 radial velocity (centre left column), Fe XII 195.119 non-thermal velocity (centre right column), and from SDO/HMI the radial photospheric magnetic field component (far right column). The active regions considered in this study are highlighted with yellow contours and numbered in the right-hand column. Alt text: Four measurements of full-disk solar maps from three observing periods, each showing intensity, Doppler velocity, non-thermal velocity, and radial magnetic field. Active regions are outlined and numbered.
• Figure 2: Histograms of non-thermal velocity and radial velocity (derived from Doppler velocity) measurements in active regions 1 and 23. The 25th and 75th percentiles are shown with red arrows and the 5th and 95th percentiles with green arrows, on the top and bottom axes for their respective measurements. Alt text: Histograms for two active regions showing distributions of non-thermal velocity and radial velocity. Percentile markers indicate differences in spread.
• Figure 3: Total radial unsigned magnetic flux ($\sum{}|B_r|\cdot{}A$) against the median non-thermal velocity for each active region, where $A$ is the pixel area in cm$^2$. Alt text: Scatter plot comparing non-thermal velocity with total magnetic flux.
• Figure 4: The relationship between the non-thermal velocity and unsigned magnetic flux density of the leading and trailing polarities of the active regions, with the 25th and 75th percentiles shown with bars and median values with spots. Alt text: Scatter plot of non-thermal velocity versus unsigned magnetic-flux density for leading and trailing polarities. Median points and percentile ranges are shown, illustrating similar behaviour in both polarities and increasing non-thermal velocity with stronger fields.
• Figure 5: The relationship between non-thermal velocity and age of the active regions. The median values are shown with circles, and the spread of non-thermal velocities between the 25th and 75th percentiles is shown with bars. For ages, the bars indicate the ranges taken from mihailescu_what_2022. Alt text: Scatter plot of non-thermal velocity versus active-region age, with median markers and percentile ranges. Data show wide variability at all ages and no clear age-dependent trend.