Expected spectral filtering for IMaX+: FTS spectrum around 525.0 nm and 517.3 nm in green; transmission peaks of the Fabry-Pérot etalon (not to scale) in red; in blue the pre-filter transmission profile; final transmision profile is in thick-red.
MAIN SCIENTIFIC DRIVERS FOR IMaX+
Sunrise (and IMaX+ as part of the whole experiment) will study the solar magnetism, with the aim of helping to answer the following key questions:
- What are the origin and the properties of the intermittent magnetic structure?
- How does the magnetic field provide energy to the upper solar atmosphere?
- How is the magnetic field brought up and removed from the solar surface?
- How does the variable magnetic field modify the solar brightness?
IMaX+ will provide an excellent temporal resolving power: with a cadence of one minute, maps of the continuum intensity, of the vector magnetic field, and of the LOS velocity will be available from the polarized raw images. This high temporal cadence is needed to study phenomena occurring on the Sun at very short temporal scales (for instance, the formation of kG flux tubes, the emergence of flux through the solar atmosphere, or the propagation of MHD waves). Also important is the fact that the Sun will be uninterruptedly observed during 5 days approximately, under conditions which are far more stable than from the ground. Only after these long observational sequences, the natural temporal scales of magnetic field appearance and disappearance will become clear. This is something known after the first two flights of Sunrise.
IMaX+ will observe the solar photosphere and the low chromosphere. The first is a thin layer above which the plasma becomes optically thin, and, as a consequence, is where most of the observed radiation comes from. In it, the kinetic, thermal, and magnetic energies are of the same order of magnitude. The kinetic energy is continuously converted into thermal and magnetic energies through different physical mechanisms, and vice versa (e.g., magnetic field reconnection or wave dissipation). The thermal energy density decreases with height more rapidly than the magnetic energy up to a point where they are in equipartition. Around this interface layer, known as plasma-beta-equal-to-one layer and located within the chromosphere, is where most of the magnetic energy is supposed to be converted into thermal energy. However, it is not understood how the magnetic energy is dissipated.
At the photosphere, the interaction between convection, radiation and magnetic field in a highly conducting medium produces intense magnetic field concentrations (1-2 kG) at spatial scales of 100 km or smaller. These structures are usually called flux tubes. The exact mechanism giving rise to their formation is not well understood yet, although our knowledge is improving with the advent of new instrumentation and, in particular, Sunrise. IMaX+ will have the opportunity to study the solar magnetic field with a spatial resolution better than 100 km and with a high temporal resolution not only in the photosphere as in former editions of the mission but also in the low chromosphere, the layer immediately on top of it. There, the magnetic field dominates the flow of gas. IMaX+ will be able to follow the dynamics of these magnetic structures, not only in the horizontal domain, but also with height in the solar atmosphere. The monitoring of how different magnetic structures, such as magnetic loops and waves, evolve with height is key for understanding the energy transfer processes.
IMaX+, hence, will provide unprecedented movies of the behavior of flux tubes and other magnetic structures in these two layers. This is relevant because these structures connect (couple) the different layers of the solar atmosphere, from the photosphere to the corona with a temperature of several million Kelvin. They can be considered as channels that transmit energy from the deeper layers to the most external ones and can play a crucial role in the very well-known problem of coronal heating. This energy propagation may be done with the help of MHD waves, producing shock waves at chromospheric and coronal heights, and releasing a large amount of energy that could heat the plasma. After our spatial resolution of flux tubes, direct detection of MHD waves is more likely than ever.
Several science questions that we formulated for the first IMaX have already been shed light with the first two flights. Specifically, the evolution of flux tubes and groups of flux tubes as single entities and their interaction with convection. The interaction of internetwork and network magnetic fields and the transfer of flux between them have been seen to occur in a similar way at the smaller scale of mesogranulation. The key ingredients have been a combination of spatial resolution, spectral resolution, polarimetric sensitivity, and temporal stability. Unfortunately, the longest span of the observations was around 20 min. With the new technology gondola for this third flight, we expect the time series to extend for hours and days, hence bringing an unprecedented view of the magnetic evolution of the solar atmosphere.
Besides the capabilities of the IMaX+ instrument working alone, the possibilities for the combined science with the other two instruments make Sunrise iii unique. Having almost simultaneous spectropolarimetric information from the IR to the visible and UV with a seeing-free 1 m telescope is a singular feature, hardly beaten by any other existing instrument including those with the largest apertures on ground. Last but not least, the three instruments aboard Sunrise iii will observe chromospheric spectral lines that are sensitive to atomic polarization and to the Hanle effect. This opens a new discovery window: the detection and interpretation of scattering signals in the IR Ca ii and K i lines, in the Mg i b2 spectral line, and in the UV.