The PDFI_SS Electric Field Inversion Software

We describe the PDFI_SS software library, which is designed to find the electric field at the Suns photosphere from a sequence of vector magnetogram and Doppler velocity measurements, and estimates of horizontal velocities obtained from local correlation tracking using the recently upgraded FLCT code. The library, a collection of Fortran subroutines, uses the PDFI technique described by Kazachenko et al. (2014), but modified for use in spherical, Plate-Carree geometry on a staggered grid. The domain over which solutions are found is a subset of the global spherical surface, defined by user-specified limits of colatitude and longitude. Our staggered-grid approach, based on that of Yee (1966), is more conservative and self-consistent compared to the centered, Cartesian grid used by Kazachenko et al. (2014). The library can be used to compute an end-to-end solution for electric fields from data taken by the HMI instrument aboard NASAs SDO Mission. This capability has been incorporated into the HMI pipeline processing system operating at SDOs JSOC. The library is written in a general and modular way so that the calculations can be customized to modify or delete electric field contributions, or used with other data sets. Other applications include nudging numerical models of the solar atmosphere to facilitate assimilative simulations. The library includes an ability to compute global (whole-Sun) electric field solutions. The library also includes an ability to compute Potential Magnetic Field solutions in spherical coordinates. This distribution includes a number of test programs which allow the user to test the software.

Comparative Study of Data-driven Solar Coronal Field Models Using a Flux Emergence Simulation as a Ground-truth Data Set

For a better understanding of magnetic field in the solar corona and dynamic activities such as flares and coronal mass ejections, it is crucial to measure the time-evolving coronal field and accurately estimate the magnetic energy. Recently, a new modeling technique called the data-driven coronal field model, in which the time evolution of magnetic field is driven by a sequence of photospheric magnetic and velocity field maps, has been developed and revealed the dynamics of flare-productive active regions. Here we report on the first qualitative and quantitative assessment of different data-driven models using a magnetic flux emergence simulation as a ground-truth (GT) data set. We compare the GT field with those reconstructed from the GT photospheric field by four data-driven algorithms. It is found that, at least, the flux rope structure is reproduced in all coronal field models. Quantitatively, however, the results show a certain degree of model dependence. In most cases, the magnetic energies and relative magnetic helicity are comparable to or at most twice of the GT values. The reproduced flux ropes have a sigmoidal shape (consistent with GT) of various sizes, a vertically-standing magnetic torus, or a packed structure. The observed discrepancies can be attributed to the highly non-force-free input photospheric field, from which the coronal field is reconstructed, and to the modeling constraints such as the treatment of background atmosphere, the bottom boundary setting, and the spatial resolution.

Spontaneous Generation of delta-sunspots in Convective Magnetohydrodynamic Simulation of Magnetic Flux Emergence

Observations reveal that strong solar flares and coronal mass ejections tend to occur in complex active regions characterized by delta-sunspots, spot rotation, sheared polarity inversion lines (PILs), and magnetic flux ropes. Here we report on the first modeling of spontaneous delta-spot generation as a result of flux emergence from the turbulent convection zone. Utilizing state-of-the-art radiative magnetohydrodynamics code R2D2, we simulate the emergence of a force-free flux tube in the convection zone that stretches down to -140 Mm. Elevated by large-scale convective upflows, the tube appears on the photosphere as two emerging bipoles. The opposite polarities collide against each other due to the subsurface connectivity, and they develop into a pair of closely-packed delta-spots. The Lorentz force drives the spot rotation and a strong counter-streaming flow of 10 km/s at the PIL in delta-spots, which, in tandem with local convection, strengthens the horizontal field to 4 kG and builds up a highly-sheared PIL. In the atmosphere above the PIL, a flux rope structure is created. All these processes follow the multi-buoyant segment theory of the delta-spot formation, and they occur as a natural consequence of interaction between magnetic flux and turbulent convection, suggesting that the generation of delta-spots and the resultant flare eruptions may be a stochastically determined process.

Magnetic Flux Emergence in a Coronal Hole

A joint campaign of various space-borne and ground-based observatories, comprising the Japanese Hinode mission (HOP~338, 20,--,30~September 2017), the GREGOR solar telescope, and the textit{Vacuum Tower Telescope} (VTT), investigated numerous targets such as pores, sunspots, and coronal holes. In this study, we focus on the coronal hole region target. On 24~September 2017, a very extended non-polar coronal hole developed patches of flux emergence, which contributed to the decrease of the overall area of the coronal hole. These flux emergence patches erode the coronal hole and transform the area into a more quiet-Sun-like area, whereby bipolar magnetic structures play an important role. Conversely, flux cancellation leads to the reduction of opposite-polarity magnetic fields and to an increase in the area of the coronal hole. Other global coronal hole characteristics, including the evolution of the associated magnetic flux and the aforementioned area evolution in the EUV, are studied using data of the textit{Helioseismic and Magnetic Imager} (HMI) and textit{Atmospheric Imaging Assembly} (AIA) onboard the textit{Solar Dynamics Observatory} (SDO). The interplanetary medium parameters of the solar wind display parameters compatible with the presence of the coronal hole. Furthermore, a particular transient is found in those parameters.

Emergence of small-scale magnetic flux in the quiet Sun

We study the evolution of a small-scale emerging flux region (EFR) in the quiet Sun, from its emergence to its decay. We track processes and phenomena across all atmospheric layers, explore their interrelations and compare our findings with recent numerical modelling studies. We used imaging, spectral and spectropolarimetric observations from space-borne and ground-based instruments. The EFR appears next to the chromospheric network and shows all characteristics predicted by numerical simulations. The total magnetic flux of the EFR exhibits distinct evolutionary phases, namely an initial subtle increase, a fast increase and expansion of the region area, a more gradual increase, and a slow decay. During the initial stages, bright points coalesce, forming clusters of positive- and negative-polarity in a largely bipolar configuration. During the fast expansion, flux tubes make their way to the chromosphere, producing pressure-driven absorption fronts, visible as blueshifted chromospheric features. The connectivity of the quiet-Sun network gradually changes and part of the existing network forms new connections with the EFR. A few minutes after the bipole has reached its maximum magnetic flux, it brightens in soft X-rays forming a coronal bright point, exhibiting episodic brightenings on top of a long smooth increase. These coronal brightenings are also associated with surge-like chromospheric features, which can be attributed to reconnection with adjacent small-scale magnetic fields and the ambient magnetic field. The emergence of magnetic flux even at the smallest scales can be the driver of a series of energetic phenomena visible at various atmospheric heights and temperature regimes. Multi-wavelength observations reveal a wealth of mechanisms which produce diverse observable effects during the different evolutionary stages of these small-scale structures.