The white-light F-corona arises from light scattered by circumsolar dust. Using weekly models of the eastern side of the F-corona between 5° and 24° elongation, we analyzed the elongation and time dependence of the brightness of its photometric axis. The models were constructed from STEREO-A SECCHI/HI-1 images taken between 2007 December and 2014 March. We found that the brightness profiles can be approximated by power laws, with the coefficients of the models depending upon the observer’s ecliptic longitude. Their variation is not symmetric with respect to the orbital nodes of the dust plane, nor is the behavior similar in the two halves of the spacecraft orbit delimited by the line of nodes. The exponents range between −2.31 and −2.35, the former occurring when the observer is at the nodes. The asymmetry observed in the behavior of the proportionality constant is indicative of the projected center of the dust cloud being offset from the Sun’s center by ∼0.4 R ⊙. The coefficients exhibit a secular variation correlated with the location of the barycenter of the solar system. We also used the HI-1 frames obtained during STEREO-A calibration rolls to model the 360° F-corona. We found that (1) its flattening index ( f = R eq / R pol − 1 ) decreases from ∼0.66 to ∼0.46 with decreasing elongation and (2) the isophotes’ shape can be approximated by a series of superellipses, with the superellipse index n increasing (nonlinearly) with brightness ( ∼ 1.54 < n < ∼ 1.65 ). Cubic extrapolation of the results below 5° elongation points to a circular F-corona below 1° elongation.

To test a technique to be used on the white-light imager onboard the recently launched Parker Solar Probe mission, we performed a numerical differentiation of the brightness profiles along the photometric axis of the F-corona models that are derived from STEREO Ahead Sun Earth Connection Heliospheric Investigation observations recorded with the HI-1 instrument between 2007 December and 2014 March. We found a consistent pattern in the derivatives that can be observed from any S/C longitude between about 18° and 23° elongation with a maximum at about 21°. These findings indicate the presence of a circumsolar dust density enhancement that peaks at about 23° elongation. A straightforward integration of the excess signal in the derivative space indicates that the brightness increase over the background F-corona is on the order of 1.5%–2.5%, which implies an excess dust density of about 3%–5% at the center of the ring. This study has also revealed (1) a large-scale azimuthal modulation of the inner boundary of the pattern, which is in clear association with Mercury’s orbit; and (2) a localized modulation of the inner boundary that is attributable to the dust trail of Comet 2P/Encke, which occurs near ecliptic longitudes corresponding to the crossing of Encke’s and Mercury’s orbital paths. Moreover, evidence of dust near the S/C in two restricted ranges of ecliptic longitudes has also been revealed by this technique, which is attributable to the dust trails of (1) comet 73P/Schwassmann–Wachmann 3, and (2) 169P/NEAT.

We have developed a procedure for tracking sunspots observed by the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory and for making curvature-corrected space/time maps of the associated line-of-sight magnetic field and continuum intensity. We apply this procedure to 36 sunspots, each observed continuously for nine days around its central meridian passage time, and find that the proper motions separate into two distinct components depending on their speeds. Fast (∼3–5 km s−1) motions, comparable to Evershed flows, are produced by weak vertical fluctuations of the horizontal canopy field and recur on a timescale of 12–20 min. Slow (∼0.3–0.5 km s−1) motions diverge from a sunspot-centered ring whose location depends on the size of the sunspot, occurring in the mid-penumbra for large sunspots and at the outer edge of the penumbra for small sunspots. The slow ingoing features are contracting spokes of a quasi-vertical field of umbral polarity. These inflows disappear when the sunspot loses its penumbra, and may be related to inward-moving penumbral grain. The slow outgoing features may have either polarity depending on whether they originate from quasi-vertical fields of umbral polarity or from the outer edge of the canopy. When a sunspot decays, the penumbra and canopy disappear, and the moat becomes filled with slow outflows of umbral polarity. We apply our procedure to decaying sunspots, to long-lived sunspots, and to numerical simulations of a long-lived sunspot by Rempel.

Suspensions of superparamagnetic colloids that equilibrate in a toggled magnetic field undergo a Rayleigh-Plateau instability with a characteristic wavelength λ = 600 μm for the toggle frequency ν = 0.66 Hz. The instability is suppressed when the chamber length L in the field direction is less than 2λ. The final size of the magnetic domains perpendicular to the field, D, follows a power law relation of D ∼ L(0.71±0.07). These results demonstrate the structural differences of field-directed suspensions when confined to lengths scale set by the phase separation process and can potentially be used to create self-assembled colloidal crystals with well-defined size and shape.

We present observations of NOAA AR 11159, obtained on 2011 February 14 in the 4.7 μm band of carbon monoxide (CO) and coordinated with spectroscopic imaging of three atomic lines (Na i 5896 Å, Fe i 7090 Å, and Ca ii 8542 Å) which sample heights from the mid-photosphere to the chromosphere. Phase-difference spectra between the observed spectral lines instead indicate that the CO lines form at z ≈ 530−650 km in the quiet Sun. During the two hours of observations, seven long-lived cooling events (“cold bubbles”) were observed in CO in the region surrounding a large pore, but were not visible in the three atomic lines. These events show self-similar temporal evolution with time scales consistent with the chemical formation rate of CO at z ≈ 1000 km. Due to the lack of such features in the surrounding quiet Sun, we hypothesize that the magnetic canopy field surrounding the pore, which suppresses the upward propagation of acoustic waves into the chromosphere and the subsequent formation of shocks, depresses the rate of acoustic heating and allows CO to condense and cool the atmosphere at those heights. These “cold bubbles” may be a source of the chromospheric CO that produces the unexpectedly high (z ≈ 1000 km) limb extensions seen in the stronger CO lines, and may provide a unique opportunity to study this enigmatic component of the solar atmosphere in spatially resolved observations.