High cadence imaging of the Sun
Principal Investigator: Mathias Maercker, Sven Wedemeyer
Institute: OSO, Sweden & University of Oslo, Norway
The continuum radiation of the Sun emerges from its chromosphere — an atmospheric layer sandwiched between the photosphere and the corona. Despite its fundamental importance for the energy transport in the solar atmosphere, observations of the chromosphere are hampered by the small number of available diagnostics and the difficulties with interpreting them. The radiation continuum at millimeter wavelengths offers diagnostic possibilities for the study of our Sun that are complementary to commonly used diagnostics but which only now can be exploited thanks to ALMA’s high spatial and temporal resolution. In particular, ALMA’s ability to serve as an essentially linear thermometer of the atmospheric gas at unprecedented spatial and temporal resolution in the millimeter wavelength range has great scientific potential. Consequently, solar ALMA observations will contribute significantly to answering long-standing questions about the structure, dynamics and energy balance of the outer layers of the solar atmosphere and thus promise high-impact results (see Wedemeyer et al. 2016; Bastian et al. 2018, and references therein).
In contrast to many other astronomical sources, the Sun does fill and actually exceeds the primary beam of ALMA with a complex emission pattern that covers a large range of spatial scales and evolves on extremely short time scales of only seconds and even below. Consequently, the majority of solar science cases require imaging at high cadence, i.e. essentially snapshot imaging, which limits the visibility data available for imaging at any given time. Exploiting the time domain to its full extent within the instrumental limitations is therefore crucial for reliable imaging of solar ALMA data. The currently offered cadence of 1 s is already remarkable but an even higher cadence is technically possible.
The challenges with processing solar ALMA data during the past years have revealed the need for a thorough and systematic review and the further development of the current solar observing mode and the processing procedures of the resulting data, which differ significantly from the standard processing of other ALMA data. Please see Shimojo et al. (2017) and White et al. (2017) for a technical introduction to solar observing with ALMA. The experimental study presented here explores the potential of increasing the cadence of solar observing even further towards sub-second time resolution in order to increase the reliability of the resulting image time series.