Solar ALMA Observations: Constraining the Chromosphere above Sunspots

Loukitcheva, M.; Iwai, Kazumasa; Solanki, S. K.; White, S. M.; Shimojo, Masataka

doi:10.3847/1538-4357/aa91cc

Cited by 27 publications

(22 citation statements)

References 37 publications

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“…Whereas the 3 mm holes in the QS seem to be unrelated to the photospheric magnetic field, the 1.25 mm hole presented here is coincidentally hovering above a ∼1.3−1.8 kG magnetic concentration with a field inclination of ∼160−170 • as measured by Hinode. The range in the HMI magnetogram is approximately [−840, 230] G. Interestingly, its brightness temperatures are even lower than the ∼5300 K emission above a sunspot umbra with a magnetic field reaching over ∼2.5 kG (Loukitcheva et al 2017).…”

Section: Overviewmentioning

confidence: 92%

“…The Atacama Large Millimeter/Submillimeter Array (ALMA, Wootten & Thompson 2009) has overcome the spatial resolution limitations that observing at such long wavelengths entails. ALMA has shown that the landscape of the solar surface at 1 mm is marked by relatively cooler regions such as the QS and sunspot umbras, and hotter features such as plage regions (Loukitcheva et al 2017;Brajša et al 2018), and that the brightness of the mm continuum correlates with that of the Mg ii lines (Bastian et al 2017(Bastian et al , 2018Jafarzadeh et al 2019). However, the large scatter and offset between the two diagnostics is evidence for NLTE effects in the Mg ii lines that make them only partially sensitive to the chromospheric temperatures (e.g., Leenaarts et al 2013a), although there may also be systematic differences in their formation heights (e.g., da Silva Santos et al 2018).…”

Section: Introductionmentioning

confidence: 99%

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The multi-thermal chromosphere

Santos

Rodríguez

Leenaarts

et al. 2020

A&A

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Context. Numerical simulations of the solar chromosphere predict a diverse thermal structure with both hot and cool regions. Observations of plage regions in particular typically feature broader and brighter chromospheric lines, which suggests that they are formed in hotter and denser conditions than in the quiet Sun, but also implies a nonthermal component whose source is unclear. Aims. We revisit the problem of the stratification of temperature and microturbulence in plage and the quiet Sun, now adding millimeter (mm) continuum observations provided by the Atacama Large Millimiter Array (ALMA) to inversions of near-ultraviolet Interface Region Imaging Spectrograph (IRIS) spectra as a powerful new diagnostic to disentangle the two parameters. We fit cool chromospheric holes and track the fast evolution of compact mm brightenings in the plage region. Methods. We use the STiC nonlocal thermodynamic equilibrium (NLTE) inversion code to simultaneously fit real ultraviolet and mm spectra in order to infer the thermodynamic parameters of the plasma. Results. We confirm the anticipated constraining potential of ALMA in NLTE inversions of the solar chromosphere. We find significant differences between the inversion results of IRIS data alone compared to the results of a combination with the mm data: the IRIS+ALMA inversions have increased contrast and temperature range, and tend to favor lower values of microturbulence (∼3−6 km s−1 in plage compared to ∼4−7 km s−1 from IRIS alone) in the chromosphere. The average brightness temperature of the plage region at 1.25 mm is 8500 K, but the ALMA maps also show much cooler (∼3000 K) and hotter (∼11 000 K) evolving features partially seen in other diagnostics. To explain the former, the inversions require the existence of localized low-temperature regions in the chromosphere where molecules such as CO could form. The hot features could sustain such high temperatures due to non-equilibrium hydrogen ionization effects in a shocked chromosphere – a scenario that is supported by low-frequency shock wave patterns found in the Mg II lines probed by IRIS.

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Section: Overviewmentioning

confidence: 92%

Section: Introductionmentioning

confidence: 99%

The multi-thermal chromosphere

Santos

Rodríguez

Leenaarts

et al. 2020

A&A

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“…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).…”

Section: Introductionmentioning

confidence: 99%

ALMA observations of transient heating in a solar active region

Santos

Rodríguez

White

et al. 2020

A&A

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Aims. We aim to investigate the temperature enhancements and formation heights of solar active-region brightenings such as Ellerman bombs (EBs), ultraviolet bursts (UVBs), and flaring active-region fibrils (FAFs) using interferometric observations in the millimeter (mm) continuum provided by the Atacama Large Millimeter/submillimeter Array (ALMA). Methods. We examined 3 mm signatures of heating events identified in Solar Dynamics Observatory observations of an active region and compared the results with synthetic spectra from a 3D radiative magnetohydrodynamic simulation. We estimated the contribution from the corona to the mm brightness using differential emission measure analysis. Results. We report the null detection of EBs in the 3 mm continuum at ∼1.2″ spatial resolution, which is evidence that they are sub-canopy events that do not significantly contribute to heating the upper chromosphere. In contrast, we find the active region to be populated with multiple compact, bright, flickering mm-bursts – reminiscent of UVBs. The high brightness temperatures of up to ∼14 200 K in some events have a contribution (up to ∼7%) from the corona. We also detect FAF-like events in the 3 mm continuum. These events show rapid motions of > 10 kK plasma launched with high plane-of-sky velocities (37 − 340 km s−1) from bright kernels. The mm FAFs are the brightest class of warm canopy fibrils that connect magnetic regions of opposite polarities. The simulation confirms that ALMA should be able to detect the mm counterparts of UVBs and small flares and thus provide a complementary diagnostic for localized heating in the solar chromosphere.

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“…The ALMA dataset we used was provided by the ALMA observatory as part of its Science Verification data release. Previous publications based on this ALMA data release are Bastian et al (2017), Iwai et al (2017), Loukitcheva et al (2017), andShimojo et al (2017). Only Shimojo et al (2017) used data from the same day (2015 December 17) as considered here, and that study was focused on a small brightening rather than the sunspot itself.…”

Section: Observationsmentioning

confidence: 99%

A study of sunspot 3 minute oscillations using ALMA and GST

Chai,

Gary,

Reardon

et al. 2021

Preprint

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Waves and oscillations are important solar phenomena, not only because they can propagate and dissipate energy in the chromosphere, but also because they carry information about the structure of the atmosphere in which they propagate. The nature of the three-minute oscillations observed in the umbral region of sunspots is considered to be an effect of propagation of magnetohydrodynamic (MHD) waves upward from below the photosphere. We present a study of sunspot oscillations and wave propagation in NOAA AR 12470 using an approximately one-hour long data set acquired on 2015 December 17 by the Atacama Large Millimeter/submillimeter Array (ALMA), the Goode Solar Telescope (GST) operating at the Big Bear Solar Observatory (BBSO), the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO), and the Interface Region Imaging Spectrograph (IRIS). The ALMA data are unique in providing a time-series of direct temperature measurements in the sunspot chromosphere. The two-second cadence of ALMA images allows us to well resolve the three-minute periods typical of sunspot oscillations in the chromosphere. Fourier analysis is applied to ALMA Band 3 (∼100 GHz, ∼3 mm) and GST Hα data sets to obtain power spectra as well as oscillation phase information. We analysed properties of the wave propagation by combining multiple wavelengths that probe physical parameters of solar atmosphere at different heights. We find that the ALMA temperature fluctuations are consistent with that expected for a propagating acoustic wave, with a slight asymmetry indicating non-linear steepening.

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Solar ALMA Observations: Constraining the Chromosphere above Sunspots

Cited by 27 publications

References 37 publications

The multi-thermal chromosphere

The multi-thermal chromosphere

ALMA observations of transient heating in a solar active region

A study of sunspot 3 minute oscillations using ALMA and GST

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