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.

ALMA observations of transient heating in a solar active region

“…We note that our simulation does not include the effects of ambipolar diffusion, which has been shown to play an important role in 2.5D flux emergence experiments (e.g., Leake & Arber 2006;Martínez-Sykora et al 2020b;Nóbrega-Siverio et al 2020), and it could affect the visibility of different heating events through changes in temperature and density. However, the 2.5D simulation of Martínez-Sykora et al (2020a) that included the effects of ambipolar diffusion and nonequilibrium ionization of helium also suggests that ALMA Band 3 will observe canopy fibrils that originate from strong concentrations of magnetic field, but it predicts low brightness temperatures (∼4500−5000 K) as consequence of expansion of cool dense plasma to much higher heights than in the 3D case (Loukitcheva et al 2015) constituting a cool canopy. The above is in contrast with our observations that show that the 3 mm canopy within the active region is consistently warmer (T b ∼ 8000−9000 K) than the QS (T b ∼ 7300 K) and occasionally shows signs of impulsive heating and rapid flows (∼40−340 km s −1 ) of T b 10 kK plasma (or heat fronts) along fibrils (see Fig.…”

“…Peter et al (2014) originally proposed that UVBs occur in the photosphere, while magnetic field extrapolations place the reconnection site in the low chromosphere ∼500−1000 km (Chitta et al 2017;Tian et al 2018). In these circumstances it is not clear whether UVBs would be as obscured as Ellerman bombs by the chromospheric canopy if the mm continuum in active regions is formed at much higher heights than previ-ously thought (Martínez-Sykora et al 2020a). In the simulation of Hansteen et al (2017) the UVBs occur at chromospheric densities and originate strong mm emission (Sect.…”

“…However, recent studies that used some of the first solar ALMA observations brought up the need to better understand the formation heights of the ALMA bands in active regions (Loukitcheva et al 2017;da Silva Santos et al 2020;Chintzoglou et al 2020) as they likely differ from the quiet Sun (QS) where the emission is dominated by acoustic shocks (e.g., White et al 2006;Wedemeyer-Böhm et al 2007;Patsourakos et al 2020). Moreover, the contribution functions of the mm continuum may also depend on nonequilibrium ionization/recombination effects in the chromosphere (e.g., Carlsson & Stein 2002;Leenaarts & Wedemeyer-Böhm 2006;Rutten 2017;Martínez-Sykora et al 2020a). In theory, a contribution from the overlying corona to the mm brightness is also expected but deemed to be small, at least in the QS (e.g., White & Kundu 1992).…”

A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS)

“…De Pontieu, Martens, and Hudson, 2001;Ballester et al, 2020), further enhanced by interaction between different species (e.g. Zaqarashvili, Khodachenko, and Rucker, 2011;Popescu Braileanu et al, 2019;Martínez-Sykora et al, 2020c). Detailed comparisons between numerical models and high-resolution observations with IRIS and DKIST of spicules, which are known to carry Alfvénic waves (De Pontieu et al, 2007a;Okamoto and De Pontieu, 2011;De Pontieu et al, 2014c), could provide evidence of ion-neutral damping of Alfvén waves and possible associated heating.…”

An overall view of temperature oscillations in the solar chromosphere with ALMA

“…Thus far, Band 3 (centred near 3 mm) and Band 6 (centred at ≈1.25 mm), with 1 s and 2 s sampling cadences, have been provided for observing the Sun, both of which supposedly sample the mid-to-high chromosphere [62][63][64][65][66]. Their precise formation heights are, however, not known to date and are predicted (from numerical simulations and solar models) to span a large range between the low chromosphere and the transition region [67]. Numerical simulations have predicted the importance of millimetre observations in identifying upper chromospheric dynamics [68][69][70] and in the detection of 3-min chromospheric oscillations at those wavelengths with, e.g.…”