Ignacio Ugarte-Urra

The spongy Fe ix 17.1 nm fine structure known as “moss” was first detected inside active region (AR) plages and has been widely interpreted as representing the transition region of the overlying hot coronal loops. However, 17.1 nm images show that similar reticulated structure is present at lower intensity levels everywhere on the Sun outside of coronal holes, although its presence in weak-field and internetwork regions is often masked by the overlying fibrils. In the quiet Sun, the moss tends to be clustered around the bases of Fe xii 19.3 nm loops, with continual jet-like ejections being observed along their stalks. Depending in part on the viewing angle, the curved structures surrounding the “holes” in the moss often resemble small, highly dynamic loops with sizes of ∼2–5 Mm. We discuss two different physical interpretations of the moss. First, in line with the traditional view, it may represent the transition region of coronal loops both inside and outside ARs, with heat being conducted downward into the moss from greater heights and the dark holes representing the spaces between the heated footpoints. An alternative interpretation is that the moss consists mainly of small loops that continually undergo magnetic reconnection with the overlying coronal field, with the associated heating rate depending on the local field strength. This scenario is supported by the tendency for the moss to spill over into weak internetwork regions and by the continual jetting activity observed from network flux concentrations.

Surface flux transport (SFT) models are routinely used to model the Sun’s photospheric magnetic field and provide inner boundary conditions for coronal and heliospheric models, yet they remain fundamentally limited by the lack of direct information about flux emergence on the farside of the Sun. To address this, we develop a framework for incorporating helioseismic images of far-side active regions (HIFAR) into the advective flux transport (AFT) model. Using near-simultaneous magnetic flux maps (HIFARM) inferred from HIFAR, and Solar Terrestrial Relations Observatory (STEREO)/Extreme Ultraviolet Imager (EUVI) 304 Å observations from 2010 May 13 to 2014 August 18, we develop a logistic regression model to estimate the probability that a helioseismic detection corresponds to an active region (AR) based on descriptors of magnetic flux, field strength, and location. The model achieves a precision of 0.93 for AR detections at a probability threshold of 0.73, selected to reject 90% of “ghost” ARs. We also derive an empirical scaling relationship between HIFARM and AFT flux to place HIFARM on the AFT flux scale, and demonstrate that the framework reproduces the flux evolution of ARs consistent with near-side observations over multiple solar rotations. At the global scale, we find that conventional nearside-driven AFT simulations underestimate the total unsigned magnetic flux by ≈10%−20% of the Sun’s total magnetic flux budget. The probabilistic AR detection model and the complete framework developed in this work provide a practical pathway for incorporating helioseismic far-side ARs into SFT models, a step toward realistic modeling of the full-Sun magnetic field.

It is commonly recognized that a coronal hole (CH) boundary region exists where the slow solar wind (SW) is formed by more than one physical process. The physical properties at the CH boundary region play an important role in dictating the slow SW properties by providing conditions for the relevant processes to work upon. However, how the CH boundary region is defined is still a work under debate. We present investigations on two large CHs that crossed the central meridian during 2015 August 17–21. We use imaging, spectroscopic and photospheric magnetic field observations by Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly, Hinode/Extreme-ultraviolet Imaging Spectrometer, and SDO/Helioseismic and Magnetic Imager to investigate the spatial and temporal variation in coronal emission, electron temperature, and the photospheric magnetic field across the CH and its vicinity to search for how a CH boundary region can be defined. The collective evidence from this study suggests that the CH/open field area is significantly larger than what is commonly identified by CH identification schemes. We find that the coronal line intensity at the peak of its intensity distribution within an area enclosing the CH and nearby quiet Sun/active region structures is a good indicator for the outer CH boundary where the mostly open field area meets with the quiet Sun/active region dominated by close loops. We discuss the role of this CH boundary region in the slow SW formation. The results also provide a solution for alleviating the open flux problem and serve as an empirical constraint for refining global coronal field models.

Extreme-ultraviolet (EUV) images from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory and the EUV Imager on the Solar Terrestrial Relations Observatory show that coronal hole boundaries often change from one day to the next on spatial scales up to several supergranules. Such changes may occur even in the absence of nearby sunspots or transient activity. We attribute the fluctuations to the action of supergranular convection, which continually rearranges the photospheric flux distribution both near and far from the hole boundaries. The boundary displacements may exceed a supergranular diameter because, in addition to simple advection, the open magnetic flux may undergo interchange reconnection with the long closed loops rooted just outside the boundary. This injects streamer material into the heliospheric plasma sheet but does not lead to a mixing of open and closed flux, whose interface remains clearly defined in EUV images and qualitatively consistent with current-free extrapolations of the (instantaneous) photospheric field. However, the boundary fluctuations are likely to be a major cause of the well-known variability of the slow solar wind, with the footpoint locations of the wind intercepted by a given spacecraft continually changing relative to the hole boundary on timescales of a day or less. This variability reflects the steep increase in the rate of flux-tube divergence toward the boundary, which leads to rapid changes in the measured wind speeds and densities. We also describe an unusual case in which a long-lived coronal hole forms suddenly without any nearby flux emergence, apparently as a result of transient-driven interchange reconnection with the north polar hole.

Solar wind prediction algorithms and simulations of coronal events often employ photospheric field maps that are assembled over a 27 day solar rotation. This has stimulated efforts to update and better synchronize the maps by applying flux transport and including observations of the back side of the Sun. Here, using potential-field source-surface extrapolations, we address the question of how the emergence of a large active region (AR) on the Sun’s farside affects the coronal field and configuration of coronal holes on the Earth-facing side. We find that, if the new AR is located ∼135°–180° in longitude from Earth, the effect on the coronal field and solar wind near the central meridian will be almost negligible. This is because, when sunspot activity is relatively low, the outermost AR loops will become connected to the nearby polar fields; when sunspot activity is high, the newly emerged flux will connect to neighboring ARs. However, large ARs that emerge near the solar limb may sometimes have a significant effect on the field near the central meridian. In particular, a coronal hole having opposite polarity to that of the nearest sector of the AR may partially close down, resulting in slower wind; conversely, if the coronal hole has the same polarity as the facing AR sector, it will tend to increase in areal size, resulting in faster wind. In most cases, the main effect of a new AR will be to redistribute open flux between itself and neighboring coronal holes (including the polar holes) through interchange reconnection.

A significant challenge facing heliospheric models is the lack of full Sun observational coverage. The lack of information about the farside photospheric magnetic field necessitates the use of various techniques to approximate the structure and appearance of this field. However, a recently developed technique that uses He ii 304 Å emission observed by the Solar Terrestrial Relations Observatory (STEREO) enables developing a magnetic flux proxy by imaging of active regions on the far side of the Sun. Incorporating information about these active regions on the far side of the Sun may have the potential to drastically improve heliospheric models. In this work, we run multiple heliospheric models with and without farside information obtained from STEREO observations of He ii 304 Å emission and compare the predicted in situ measurements from the models with real in situ data from STEREO and Earth. We find that although there are noticeable quantitative differences between the in situ predictions from the two models, they are dwarfed by the overall disagreement between the heliospheric model and the actual in situ data. Nevertheless, our results indicate that active regions that significantly change the ratio of open-to-closed and open-to-total flux create the biggest change in the predicted in situ measurements.

Observations of the Sun’s photospheric magnetic field are often confined to the Sun–Earth line. Surface flux transport (SFT) models, such as the Advective Flux Transport (AFT) model, simulate the evolution of the photospheric magnetic field to produce magnetic maps over the entire surface of the Sun. While these models are able to evolve active regions that transit the near side of the Sun, new far-side side flux emergence is typically neglected. We demonstrate a new method for creating improved maps of the magnetic field over the Sun’s entire photosphere using data obtained by the Solar TErrestrial RElations Observatory (STEREO) mission. STEREO He ii 304 Å intensity images are used to infer the time, location, and total unsigned magnetic flux of far-side active regions. We have developed an automatic detection algorithm for finding and ingesting new far-side active region emergence into the AFT model. We conduct a series of simulations to investigate the impact of including active region emergence in AFT, both with and without data assimilation of magnetograms. We find that while He ii 304 Å can be used to improve surface flux models, care must taken to mitigate intensity surges from flaring events. We estimate that during Solar Cycle 24's maximum (2011–2015), 4–6 × 10²² Mx of flux is missing from SFT models that do not include far-side data. We find that while He ii 304 Å data alone can be used to create synchronic maps of photospheric magnetic field that resemble the observations, it is insufficient to produce a complete picture without direct magnetic observations from magnetographs.

The launches of Parker Solar Probe (Parker) and Solar Orbiter (SolO) are enabling a new era of solar wind studies that track the solar wind from its origin at the photosphere, through the corona, to multiple vantage points in the inner heliosphere. A key ingredient for these models is the input photospheric magnetic field map that provides the boundary condition for the coronal portion of many heliospheric models. In this paper, we perform steady-state, data-driven magnetohydrodynamic (MHD) simulations of the solar wind during Carrington rotation 2258 with the Grid GAMERA model. We use the ADAPT and AFT flux transport models and quantitatively assess how well each model matches in situ measurements from Parker, SolO, and Earth. We find that both models reproduce the magnetic field components at Parker quantitatively well. At SolO and Earth, the magnetic field is reproduced relatively well, though not as well as at Parker, and the density is reproduced extremely poorly. The velocity is overpredicted at Parker, but not at SolO or Earth, hinting that the Wang–Sheeley–Arge (WSA) relation, fine-tuned for Earth, misses the deceleration of the solar wind near the Sun. We conclude that AFT performs quantitatively similarly to ADAPT in all cases, and that both models are comparable to a purely WSA heliospheric treatment with no MHD component. Finally, we trace field lines from SolO back to an active region outflow that was observed by Hinode/EIS, and which shows evidence of elevated charge state ratios.