The Formation Height of Millimeter-wavelength Emission in the Solar Chromosphere

Abstract

In the past few years, the ALMA radio telescope has become available for solar observations. ALMA diagnostics of the solar atmosphere are of high interest because of the theoretically expected linear relationship between the brightness temperature at mm wavelengths and the local gas temperature in the solar atmosphere. Key for the interpretation of solar ALMA observations is understanding where in the solar atmosphere the ALMA emission originates. Recent theoretical studies have suggested that ALMA bands at 1.2 (band 6) and 3 mm (band 3) form in the middle and upper chromosphere at significantly different heights. We study the formation of ALMA diagnostics using a 2.5D radiative MHD model that includes the effects of ion-neutral interactions (ambipolar diffusion) and non-equilibrium ionization of hydrogen and helium. Our results suggest that in active regions and network regions, observations at both wavelengths most often originate from similar heights in the upper chromosphere, contrary to previous results. Non-equilibrium ionization increases the opacity in the chromosphere so that ALMA mostly observe spicules and fibrils along the canopy fields. We combine these modeling results with observations from IRIS, SDO and ALMA to suggest a new interpretation for the recently reported "dark chromospheric holes", regions of very low temperatures in the chromosphere.

5 figures under review in ApJL

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Cited by in corpus (17)

• Ambipolar diffusion in the Bifrost code
• Heating of the solar chromosphere through current dissipation
• 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
• High-frequency wave power observed in the solar chromosphere with IBIS and ALMA
• ALMA and IRIS Observations of the Solar Chromosphere II: Structure and Dynamics of Chromospheric Plage
• An overall view of temperature oscillations in the solar chromosphere with ALMA
• ALMA observations of transient heating in a solar active region
• ALMA observations of the variability of the quiet Sun at millimeter wavelengths
• The Sun at millimeter wavelengths. III. Impact of the spatial resolution on solar ALMA observations
• Evaluating Non-LTE Spectral Inversions with ALMA and IBIS
• Power distribution of oscillations in the atmosphere of a plage region: Joint observations with ALMA, IRIS and SDO
• Where is the base of the Transition Region? Evidence from TRACE, SDO, IRIS and ALMA observations
• A new view of the solar interface region from the Interface Region Imaging Spectrograph (IRIS)
• EMISSA: Exploring millimetre indicators of solar-stellar activity III. Comparison of Ca II indices and millimetre continua in a 3D model atmosphere
• Utilizing the slope of the brightness temperature continuum as a diagnostic tool of solar ALMA observations
• The Solar ALMA Science Archive (SALSA)