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The Solar chromosphere: old challenges, new diagnostics

João M. da Silva Santos
Institute for Solar Physics, Stockholm University, Sweden

While the solar photosphere has been scrutinized in great detail and it is now well characterized, the fine thermal structure and dynamics of the chromosphere - and its heating problem - have been matter of debate for a long time. This is due to the complicated modelling that it requires as well as technical limitations on the observational side. Both have seen tremendous progress over the past decade, and have provided the community with a large volume of synthetic and real data in the visible and ultraviolet. Even so, we are left with the drawback that most chromospheric diagnostics form under non-LTE conditions, therefore they are weakly coupled to the local conditions of the plasma. Observations in the millimeter range with ALMA recently opened a new window into the solar chromosphere. The advantage is that the mm free-free continuum forms in the chromosphere and its source function can be simply treated in LTE, therefore it can be used to diagnose chromospheric temperatures in a linear way at unprecedented spatial resolution.
In this talk we revisit old problems in the modelling of the solar chromosphere and highlight recent advancements of high-resolution ground-based and space-born instrumentation, with an emphasis on the first results of solar ALMA observations. We present a case study of the usefulness of ALMA in improving inversions of non-LTE lines of Mg II and Ca II, and we discuss how ALMA could help constraining the stratification of temperature and microturbulence in plage regions.

2019 September 25, 13:30

Centro de Astrofísica da Universidade do Porto (Auditorium)
Rua das Estrelas, 4150-762 Porto

Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia