A First Comparison of Millimeter Continuum and Mg II Ultraviolet Line Emission from the Solar Chromosphere

Abstract

We present joint observations of the Sun by the Atacama Large Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging Spectrograph (IRIS). The observations were made of a solar active region on 2015 December 18 as part of the ALMA science verification effort. A map of the Sun's continuum emission of size $2.4^{\prime }\times 2.3^{\prime }$ was obtained by ALMA at a wavelength of 1.25 mm (239 GHz) using mosaicing techniques. A contemporaneous map of size $1.9^{\prime }\times 2.9^{\prime }$ was obtained in the Mg II h doublet line at 2803.5Å by IRIS. Both mm/submm$-\lambda$ continuum emission and ultraviolet (UV) line emission are believed to originate from the solar chromosphere and both have the potential to serve as powerful and complementary diagnostics of physical conditions in this poorly understood layer of the solar atmosphere. While a clear correlation between mm-$\lambda$ brightness temperature $T_B$ and the Mg II h line radiation temperature $T_{rad}$ is observed the slope is $<1$, perhaps as a result of the fact that these diagnostics are sensitive to different parts of the chromosphere and/or the Mg II h line source function includes a scattering component. There is a significant offset between the mean $T_B$(1.25 mm) and mean $T_{rad}$(Mg II), the former being $\approx 35\%$ greater than the latter. Partitioning the maps into "sunspot", "quiet regions", and "plage regions" we find that the slope of the scatter plots between the IRIS Mg II h line $T_{rad}$ and the ALMA brightness temperature $T_B$ is 0.4 (sunspot), 0.56 (quiet regions), and 0.66 (plage regions). We suggest that this change may be caused by the regional dependence of the formation heights of the IRIS and ALMA diagnostics, and/or the increased degree of coupling between the UV source function and the local gas temperature in the hotter, denser gas in plage regions.

arXiv comment8 pages, 2 figures

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• Observations of solar chromospheric oscillations at 3 mm with ALMA
• ALMA Observations of the Solar Chromosphere on the Polar Limb
• Transient brightenings in the quiet Sun detected by ALMA at 3 mm
• ALMA and IRIS Observations of the Solar Chromosphere I: an On-Disk Type II Spicule
• First Spectral Analysis of a Solar Plasma Eruption Using ALMA
• An overall view of temperature oscillations in the solar chromosphere with ALMA
• ALMA and IRIS Observations of the Solar Chromosphere II: Structure and Dynamics of Chromospheric Plage
• ALMA observations of transient heating in a solar active region
• ALMA observations of the variability of the quiet Sun at millimeter wavelengths
• First solar observations with ALMA
• A study of sunspot 3 minute oscillations using ALMA and GST
• A first look at the submillimeter Sun with ALMA
• Evaluating Non-LTE Spectral Inversions with ALMA and IBIS
• Flares detected in ALMA single-dish images of the Sun
• Power distribution of oscillations in the atmosphere of a plage region: Joint observations with ALMA, IRIS and SDO
• Incoherent Solar Radio Emission
• The multi-thermal chromosphere: inversions of ALMA and IRIS data
• The Solar ALMA Science Archive (SALSA)