Frank Hill, Michael J. Thompson

Since all space weather originates in and on the Sun, forecasters need continuous, long-term, consistent, and reliable solar data as a foundation for useful predictions. An effective strategy to obtain nearly continuous solar data is the use of a ground-based network of identical observing instruments geographically distributed so that gaps from night time, weather and instrumental problems are reduced.

Vincent G.A. Böning, Markus Roth, Jason Jackiewicz, Shukur Kholikov

Accurate measurements of deep solar meridional flow are of vital interest for understanding the solar dynamo. In this paper, we validate a recently developed method for obtaining sensitivity functions (kernels) for travel-time measurements to solar interior flows using the Born approximation in spherical geometry, which is expected to be more accurate than the classical ray approximation. Furthermore, we develop a numerical approach to efficiently compute a large number of kernels based on the separability of the eigenfunctions into their horizontal and radial dependence. The validation is performed using a hydrodynamic simulation of linear wave propagation in the Sun, which includes a standard single-cell meridional flow profile. We show that, using the Born approximation, it is possible to accurately model observational quantities relevant for time-distance helioseismology such as the mean power spectrum, disc-averaged cross-covariance functions, and travel times in the presence of a flow field. In order to closely match the model to observations, we show that it is beneficial to use mode frequencies and damping rates which were extracted from the measured power spectrum. Furthermore, the contribution of the radial flow to the total travel time is found to reach 20% of the contribution of the horizontal flow at travel distances over 40^\circ. Using the Born kernels and a 2D SOLA inversion of travel times, we can recover most features of the input meridional flow profile. The Born approximation is thus a promising method for inferring large-scale solar interior flows.

Sanjay Gosain, Frank Hill, Joachim Staiger, Haosheng Lin, Wolfgang Zima, Dirk Soltau, Miroslav Klvana, Michal Sobotka

This document describes the technical concept developed under Solarnet WP80 “Synoptic Observations: Solar Physics Research Integrated Network Group (SPRING)” for the setup of a new ground-based network for continuous solar observations serving a large research community.

Sanjay Gosain, Frank Hill, Alexei Pevtsov, Valentin M. Pillet, Michael Thompson

Long-term synoptic observations of the Sun in different wavelength regions are essential to understand its secular behavior. Such observations have proven very important for discovery of 11 year solar activity cycle, 22 year magnetic cycle, polar eld reversals, Hale's polarity law, Joy's law, that helped Babcock and Leighton to propose famous solar dynamo model. In more recent decades, the societal impact of the secular changes in Sun's output has been felt in terms of solar inputs to terrestrial climate-change and space-weather hazards. Further, it has been realized that to better understand the activity phenomena such as flares and coronal mass ejections (CMEs) one needs synoptic observations in multiple spectral lines to enable tomographic inference of physical parameters. Currently, there are both space and ground based synoptic observatories. However, given the requirements for the long-term stability and reliability of such synoptic datasets, ground-based facilities are more preferable. Also, the ground based observatories are easy to maintain or upgrade while detailed and frequent calibrations are easily possible. The only ground-based facility that currently provides full-disk velocity and magnetic eld maps of the Sun around the clock and at good cadence, is the Global Oscillations Network Group (GONG) network of National Solar Observatory (NSO) which is operational since the mid 90s. Due to its aging instrumentation, operating for nearly three decades, and new requirements to obtain multiwavelength observations, a need is felt in the solar community to build a next generation synoptic observatory network. A group of international observatories have come together under the auspices of SOLARNET program, funded by European Union (EU), to carryout a preliminary design study of such a synoptic solar observing facility called "SPRING", which stands for Solar Physics Research Integrated Network Group. In this article we will present concept of SPRING and the optical design concept of its major instruments.