The Aditya-L1 is the first space-based solar observatory of the Indian Space Research Organization (ISRO). The spacecraft will carry seven payloads providing uninterrupted observations of the Sun from the first Lagrangian point. Aditya-L1 comprises four remote sensing instruments, viz. a coronagraph observing in visible and infrared, a full disk imager in Near Ultra-Violet (NUV), and two full-sun integrated spectrometers in soft X-ray and hard X-ray. In addition, there are three instruments for in-situ measurements, including a magnetometer, to study the magnetic field variations during energetic events. Aditya-L1 is truly a mission for multi-messenger solar astronomy from space that will provide comprehensive observations of the Sun across the electromagnetic spectrum and in-situ measurements in a broad range of energy, including magnetic field measurements at L1.

Observations of the Sun at 2.2 and 3.75 μm have been made at the Pic-du-Midi and Crimean observatories. High resolution (~ 1″) records of the sunspot and facular contrasts at various heliocentric distances as well as those at the extreme solar limb are presented. We find substantial variations in the sunspot-core relative intensity caused by magnetic activity of the spot. The extreme-limb contrasts of faculae at 3.75 μm are strong thus support flux-tube models which result in an enhancement of the extreme-limb facular brightness.

An overview is given of the structure and the physics of magnetic features in solar plages, as derived from observations of near-infrared lines. First, the diagnostic potential of near-infrared lines is compared with that of lines in the visible and at 12 μm. Then, the results on the magnetic and velocity structure of magnetic features obtained from 1.5 μm lines are described, discussed and compared with results of observations in the visible and with theoretical predictions. Finally, the past and present achievements of near-infrared investigations of Zeeman-split lines are summarized.

We report on new improved infrared (IR) imaging observations (1.6 μm) carried out with the National Solar Observatory's Vacuum Tower Telescope at Sacramento Peak, New Mexico (NSO/SP). Examples of high spatial resolution (up to 0.4″ images are shown, and results of comparisons between infrared and continuum at 526 nm and between infrared and the locations of enhanced magnetic field (“fluxtube regions” as defined by Mg I b1 line imaging) are given and discussed. Our results indicate that, close to disk center, magnetic elements have a positive contrast at the opacity minimum. This is contrary to the findings of several other authors (e.g., Worden 1975, Foukal et al. 1990). We emphasize the necessity of multi-color, high spatial and temporal resolution observations. The potential of present “almost-on-a-routine-basis” IR observations utilizing a fast video acquisition system developed at NSO/SP is pointed out.

Infrared array detectors are a new and promising tool for investigating the properties of magnetic field concentrations in the solar photosphere. Array measurements provide large statistical samples of polarized line profiles and display the spatial organization of the magnetic field. The wavelength region near 1.6 μm has important advantages for magnetometry: spectral lines with magnetic sensitivites ranging from low to very high; low continuum opacity; high continuum flux; and the possibility of sub-arcsecond angular resolution with existing telescopes. Initial results have extended earlier work on the distribution of field strength and flux in plages and revealed new properties specifically connected with spatial structure. The quality and flexibility of near infrared magnetographs can be expected to improve rapidly.

An investigation of a solar plage based on observations of UV, visible, and IR spectral lines is presented. Using model calculations, we have established distributions of the main physical parameters (temperature, density, turbulent velocity) in both plage and quiet-Sun atmospheres. Details of the models are presented and the role of magnetic pressure is discussed.

Imaging of active regions in continuum around 1.6 μm shows that many facular regions are less bright than the photosphere when observed nearer to disk center than μ = cos θ ~ 0.75. The contrast of these dark faculae increases with magnetic flux above a threshold of approximately 2 × 1018 Mx. This explains why not all faculae are dark at 1.6 μm, since the magnetic flux density in many regions of bright Ca K plage emission falls below this threshold. After correction for blurring, the typical contrast value is about 4-5%, so the brightness temperature deficit is about 130 K. Faculae are brighter than the photosphere at 1.63 μm nearer to the limb than μ ~ 0.5. The negative contrast of dark faculae may arise from cooling of the surrounding photosphere, or from increased visibility of cool layers of the facular flux tube itself. Quantitative comparison of these IR data with MHD models awaits calculation of flux tube contrasts at realistic angular resolution.