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Huw Morgan

Publications and source records attributed to Huw Morgan.

At least 19 recordsLinked to original sources

Multi-Thermal CME Detection with ALMANAC

Reliable identification of low-coronal CME origins remains a key limitation in space weather forecasting with coronagraphs not directly resolving low-coronal signatures. We present a re-engineered multi-thermal implementation of the ALMANAC algorithm, designed to detect eruptive signatures in EUV observations. The framework extends the method to a multi-wavelength system, improving robustness against projection effects, instrumental artifacts, and wavelength-dependent ambiguities via complementary temperature responses. A spatiotemporal clustering scheme merges detections across channels, reducing event bifurcation and improving coherence while maintaining NRT performance through parallel computing. Benchmarking against 20 halo CMEs from CDAW shows improved interpretability and operational usability, with clearer separation of eruptions and more consistent onset localisation relative to coronagraph estimates. The main benefits arise from improved event discrimination, reduced fragmentation, and more interpretable source region identification. ALMANAC shows sensitivity to precursor low-coronal activity not always captured in white-light catalogues, highlighting its advantages for early warning detection. When coupled with the ARTop framework, it enables co-analysis of coronal intensity variability and photospheric magnetic evolution. In this context, kurtosis time-series from multi-wavelength EUV data exhibit recurrent pre-eruptive spikes that frequently align with enhancements in magnetic winding and helicity injection. Across multiple regions, these signatures often precede solar activity, including potential discrimination of X-class flares, while remaining suppressed during magnetically quiet intervals. Overall, integrating coronal diagnostics with photospheric topology offers a pathway toward improved eruption forecasting and space weather prediction.

astro-ph.SR

UK White Paper on Space-based total solar eclipse observations: structure and dynamics of the solar atmosphere

Our Sun is uniquely placed to enable a detailed study of astrophysical plasmas and how they are governed by the magnetic fields that thread through them. On the one hand, magnetic fields confine plasma and determine plasma heating, flows, and energisation. On the other hand, magnetic fields and their evolution give rise to the most violent eruptions in the Solar System. Understanding the details of how energy is built up and released, and the impact of these physical processes on the plasma, remain key open questions that directly map to UKRI's science strategy through the STFC Solar System Advisory Panel's roadmap for Solar System research goals: What are the causes, consequences and predictability of solar magnetic variability and the solar cycle? What are the structures, dynamics and energetics of the Sun? What are the underlying processes that drive Sun-planet connections? And what are the fundamental processes at work in the Solar System? As laid out in this White Paper, the Moon-Enabled Sun Occultation Mission (MESOM) directly addresses these questions and in doing so delivers several Pillars of the National Space Strategy.

astro-ph.IM

A multi-viewpoint comparison of the velocity field of coronal propagating disturbances

Small-scale propagating disturbances (PD) are ubiquitous in the solar corona. Time-Normalised Optical Flow (TNOF) is a method developed for mapping PD velocity fields in time series of Extreme-Ultraviolet (EUV) images. We show PD velocity fields of a quiet Sun (QS) region containing a small coronal hole (CH) and filament channel (FC) jointly observed by Extreme Ultraviolet Imager (EUI) aboard the Solar Orbiter and Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). The QS observations acquired on 28 October 2023 in 174A channel of High Resolution EUV Imager (HRIEUV) of EUI and 171A channel of AIA are used. During the time of the observations, the separation angle between Solar Orbiter and SDO was approximately 26\de. A novel image alignment analysis shows that the dominant formation heights are 11.4Mm for HRIEUV and 4Mm for AIA. Despite this height difference, the PD velocity fields obtained from the observations from the two instruments are in good agreement across the region. In the QS the median PD speed is around 6.7 and 7.4\kms\ for HRIEUV and AIA respectively, with maximum speeds of around 40\kms. The small equatorial CH is a region dominated by a low temperature of $\approx$0.8MK and is host to high PD speeds, with a median speed of 17\kms. The velocity field bridges coherently across the CH from neighbouring QS regions from east to west, thus the CH must be overlaid by a system of long, low-lying closed magnetic loops. This unexpected configuration is supported by a potential field (PF) magnetic model and may be due to the longevity of the CH, allowing time for interchange reconnection with neighbouring closed-field regions. The FC is observed to be multithermal, with a narrow central strip of high emission at both low (0.8MK) and high (2.5MK) temperatures and low emission at warm (1.2MK) temperature. The FC has PD speeds similar to those of the QS.

astro-ph.SR

Polarization Diagnostics Applied to Coronal Mass Ejections and the Background Solar Wind

The ratio of radially to tangentially polarized Thomson-scattered white light provides a powerful tool for locating the 3D position of compact structures in the solar corona and inner heliosphere, and the Polarimeter to Unify the Corona and Heliosphere (PUNCH) has been designed to take full advantage of this diagnostic capability. Interestingly, this same observable that establishes the position of transient blob-like structures becomes a local measure of the slope of the global falloff of density in the background solar wind. It is thus important to characterize the extent along the line of sight of structures being studied, in order to determine whether they are sufficiently compact for 3D positioning. In this paper, we build from analyses of individual lines of sight to three-dimensional models of coronal mass ejections (CMEs), allowing us to consider how accurately polarization properties of the transient and quiescent solar wind are diagnosed. In this way, we demonstrate the challenges and opportunities presented by PUNCH polarization data for various quantitative diagnostics.

astro-ph.SR

Linking Magnetic Field Diagnostics with 3D CME Speeds in Solar Active Regions

Understanding how active-region properties influence coronal mass ejection (CME) dynamics is essential for constraining eruption models and improving space-weather prediction. Magnetic diagnostics derived above polarity inversion lines (PILs), including the critical height ($h_{\rm crit}$) of torus instability onset, the overlying field strength ($B_{\rm t}$), and ribbon flux ($R_{\rm f}$), provide physically motivated measures of eruption onset. The two main aims of this work are to (i) show that $h_{\rm crit}$ and $B_{\rm t}$ can equally well predict CME speeds when evaluated over the region of interest (ROI) not directly above the PIL, and (ii) assess the value of $h_{\rm crit}$, $B_{\rm t}$ and $R_{\rm f}$ in predicting CME speed. Photospheric magnetograms are modeled with potential-field extrapolations to obtain decay index profiles. Critical heights above PILs correlate strongly with 3D CME speed ($r = 0.71$). Using ROIs of $\approx$ 1.8, 3.7, and 7.3 Mm), centered on the PIL, weighted $h_{\rm crit}$ from the 7.3x7.3 ROI provides the strongest correlation ($r = 0.73$), while mean $B_{\rm t}$ at 150 Mm is weaker ($r = 0.33$). Combining both offers little improvement ($r = 0.74$), confirming $h_{\rm crit}$ as the dominant predictor. CME speed correlates moderately with $B_{\rm t} \times R_{\rm f}$ ($r = 0.44$), and highest when combined with $h_{\rm crit}$ ($r = 0.76$). Thus, in potential field models, ROI-based critical heights are as predictive as those above the PIL, indicating that the broader active-region field structure is equally valid as a diagnostic. When all parameters are considered together, $h_{\rm crit}$ alone consistently shows the highest predictive power for CME speed.

astro-ph.SR

Polarimeter to Unify the Corona and Heliosphere (PUNCH)

The Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission is a NASA Small Explorer to determine the cross-scale processes that unify the solar corona and heliosphere. PUNCH has two science objectives: (1) understand how coronal structures become the ambient solar wind, and (2) understand the dynamic evolution of transient structures, such as coronal mass ejections, in the young solar wind. To address these objectives, PUNCH uses a constellation of four small spacecraft in Sun-synchronous low Earth orbit, to collect linearly polarized images of the K corona and young solar wind. The four spacecraft each carry one visible-light imager in a 1+3 configuration: a single Narrow Field Imager solar coronagraph captures images of the outer corona at all position angles, and at solar elongations from 1.5 degrees (6 R$_\odot$) to 8 degrees (32 R$_\odot$); and three separate Wide Field Imager heliospheric imagers together capture views of the entire inner solar system, at solar elongations from 3 degrees (12 R$_\odot$) to 45 degrees (180 R$_\odot$) from the Sun. PUNCH images include linear-polarization data, to enable inferring the three-dimensional structure of visible features without stereoscopy. The instruments are matched in wavelength passband, support overlapping instantaneous fields of view, and are operated synchronously, to act as a single ``virtual instrument'' with a 90 degree wide field of view, centered on the Sun. PUNCH launched in March of 2025 and began science operations in June of 2025. PUNCH has an open data policy with no proprietary period, and PUNCH Science Team Meetings are open to all.

astro-ph.SR

The evolution of chromospheric quiet-sun bright points during their lifetime

Bright points (BPs) are ubiquitous, small-scale energetic events with multithermal signatures, typically observed in the chromosphere and linked to both photospheric structure and coronal composition. Their evolution is influenced by various physical processes, including plasma dynamics and magnetic interactions. This paper examines BP evolution using a large statistical sample, focusing on when they reach maximum values for key attributes, and explores differences between BPs in the "Active Quiet Sun" (AQS, above the network) and the "True Quiet Sun" (TQS, above the internetwork). Observed attributes include maximum brightness (total and intrinsic), plane-of-sky (POS) speed, travel distance, acceleration, and apparent size (POS area). BPs can reach maximum brightness and size at almost any time during their lifetime, likely due to complex chromospheric interactions. AQS and TQS BPs show similar behaviour overall and tend to reach maximum POS speed around the midpoint of their lifetimes. Positive acceleration usually occurs early, while negative acceleration is more common near the end. Preliminary results suggest two distinct BP regimes with differing relationships between intrinsic brightness and area. We interpret these trends as evidence that BPs, whether due to plasma motion or a heating event, follow arched paths -- likely along short magnetic loops. In this model, the midpoint of a BP's life corresponds to the crest of the arch, producing the greatest POS speeds. Reconnection may occur at a footpoint, with the BP moving along the loop before returning to the photosphere. Deviations from this expected evolution may result from complex chromospheric interactions or BPs with highly non-linear POS motions.

astro-ph.SR

Linking Critical Heights in Solar Active Regions with 3D CME Speeds: Insights from Automated and Manual PIL Detection Methods

In a space weather context, the most geoeffective coronal mass ejections (CMEs) are fast CMEs from Earth-facing solar active regions. These CMEs are difficult to characterize in coronagraph data due to their high speed (fewer observations), faintness, Earthward orientation (halo CMEs), and disruptions from associated high-energy particle storms. Any diagnostic aiding in early CME speed identification is valuable. This study investigates whether the 3D speeds of 37 CMEs are correlated with the critical heights of their source regions, to test the hypothesis that if the critical height is located at a higher altitude in the corona, the weaker magnetic field environment will enable a faster CME to be produced. Critical heights near CME onset are calculated by identifying polarity inversion lines (PIL) in magnetogram data using automated and manual methods. 3D speeds are determined by fitting a Graduated Cylindrical Shell (GCS) model to multi-viewpoint coronagraph images. For the automated method, we find a high correlation of 71% +- 8% between CME speed and critical height, dropping to 48% +- 12% when using CME plane-of-sky speeds, on which most previous similar studies are based. An attempt to improve the critical height diagnostic through manual PIL selection yields a lower correlation of 58% +/- 13%. The higher correlation from the automated method suggests that encompassing the full PIL structure is a better measure of the magnetic conditions that influence CME dynamics. Our results highlight the potential for critical height as a continuously computable diagnostic for forecasting the 3D speeds of Earth-directed CMEs.

astro-ph.SR

An in-depth analysis of quiet-Sun IRIS Brightenings

Small-scale brightenigs are ubiquitous, dynamic and energetic phenomena found in the chromopshere. An advanced filter-detection algorithm applied to high-resolution observations from the Interface Region Imaging Spectrograph enables the detection of these brightenings close to the noise level. This algorithm also tracks the movement of these brightenings and extracts their characteristics. This work outlines the results of an in-depth analysis of a quiet-Sun dataset including a comparison of a brighter domain - associated with a super-granular boundary - to the quiescent inter-network domains. Several characteristics of brightenings from both domains are extracted and analysed, providing a range of sizes, durations, brightness values, travel distances, and speeds. The ``Active" quiet-Sun events tend to travel shorter distances and at slower speeds along the plane-of-sky than their ``True" quiet-Sun counterparts. These results are consistent with the magnetic field model of super-granular photospheric structures and the magnetic canopy model of the chromosphere above. Spectroscopic analyses reveal that BPs demonstrate blue-shift (as well as some bi-directionality) and that they may rise from the chromosphere into the TR. We believe these bright points to be magnetic in nature, are likely the result of magnetic reconnection, and follow current sheets between magnetic field gradients, rather than along magnetic field lines themselves.

astro-ph.SR

Connecting the Low to High Corona: Propagating Disturbances as Tracers of the Near-Sun Solar Wind

We revisit a quiet 14-day period of solar minimum during January 2008 and track sub-streamer propagating disturbances (PDs) from low heights in STEREO/EUVI to the extended corona through STEREO/COR1 and into STEREO/COR2 along nonradial paths that trace the structure of the underlying streamers. Using our recently developed method for generating nonradial Height-Time profiles of outward PDs (OPDs) and inward PDs (IPDs), we obtained their velocities along the radial and position angle directions. Our analysis of 417 unique OPDs revealed two classes: slow and fast OPDs. Slow OPDs form preferentially at $\approx$1.6 $R_\odot$ closer to the streamer boundaries, with asymmetric occurrence rates, and show speeds of $16.4_{-8.4}^{+26.6}km/s$ at 1.5 $R_\odot$ and accelerate up to $200.1_{-57.9}^{+71.1}km/s$ at 7.5 $R_\odot$. Fast OPDs form preferentially at $\approx$ 1.6 $R_\odot$ and at $\approx$3.0 $R_\odot$ both at the streamer boundaries and slightly more often within them. They show speeds of $87.8_{-24.8}^{+59.1}km/s$ at 1.5 $R_\odot$ up to $197.8_{-46.7}^{+61.8}km/s$ at 7.5 $R_\odot$. IPDs are observed forming at $\approx$1.8 $R_\odot$ with speeds of tens of $km/s$, mostly concentrated in the aftermath of a CME eruption. We present an example in which we show that periodic brightness variations related to OPDs remained in the range of 98 to 128 min, down to $\approx$2.0 $R_\odot$, well within the field of view of COR1. The velocity profiles of slow OPDs for heliocentric height below 3.0 $R_\odot$ show good agreement with speeds more closely related to the bulk solar wind obtained via interplanetary scintillation.

astro-ph.SR

Correcting Projection Effects in CMEs using GCS-based Large Statistics of Multi-viewpoint Observations

This study addresses the limitations of single-viewpoint observations of Coronal Mass Ejections (CMEs) by presenting results from a 3D catalog of 360 CMEs during solar cycle 24, fitted using the GCS model. The dataset combines 326 previously analyzed CMEs and 34 newly examined events, categorized by their source regions into active region (AR) eruptions, active prominence (AP) eruptions, and prominence eruptions (PE). Estimates of errors are made using a bootstrapping approach. The findings highlight that the average 3D speed of CMEs is $\sim$1.3 times greater than the 2D speed. PE CMEs tend to be slow, with an average speed of 432 km $s^{-1}$. AR and AP speeds are higher, at 723 km $s^{-1}$ and 813 km $s^{-1}$, respectively, with the latter having fewer slow CMEs. The distinctive behavior of AP CMEs is attributed to factors like overlying magnetic field distribution or geometric complexities leading to less accurate GCS fits. A linear fit of projected speed to width gives a gradient of 2 km $s^{-1}deg^{-1}$, which increases to 5 km $s^{-1}deg^{-1}$ when the GCS-fitted `true' parameters are used. Notably, AR CMEs exhibit a high gradient of 7 km $s^{-1}deg^{-1}$, while AP CMEs show a gradient of 4 km $s^{-1}deg^{-1}$. PE CMEs, however, lack a significant speed-width relationship. We show that fitting multi-viewpoint CME images to a geometrical model such as GCS is important to study the statistical properties of CMEs, and can lead to a deeper insight into CME behavior that is essential for improving future space weather forecasting.

astro-ph.SR

Derived electron densities from linear polarization observations of the visible-light corona during the 14 December 2020 total solar eclipse

A new instrument was designed to take visible-light (VL) polarized brightness ($pB$) observations of the solar corona during the 14 December 2020 total solar eclipse. The instrument, called the Coronal Imaging Polarizer (CIP), consisted of a 16 MP CMOS detector, a linear polarizer housed within a piezoelectric rotation mount, and an f-5.6, 200 mm DSLR lens. Observations were successfully obtained, despite poor weather conditions, for five different exposure times (0.001 s, 0.01 s, 0.1 s, 1 s, and 3 s) at six different orientation angles of the linear polarizer (0\de, 30\de, 60\de, 90\de, 120\de, and 150\de). The images were manually aligned using the drift of background stars in the sky and images of different exposure times were combined using a simple signal-to-noise ratio cut. The polarization and brightness of the local sky is also estimated and the observations were subsequently corrected. The $pB$ of the K-corona was determined using least squares fitting and radiometric calibration was done relative to the Mauna Loa Solar Observatory (MLSO) K-Cor $pB$ observations from the day of the eclipse. The $pB$ data was then inverted to acquire the coronal electron density, $n_e$, for an equatorial streamer and a polar coronal hole, which agreed very well with previous studies. The effect of changing the number of polarizer angles used to compute the $pB$ is also discussed and it is found that the results vary by up to $\sim$ 13\% when using all six polarizer angles versus only a select three angles.

astro-ph.SR

Automated analysis of oscillations in coronal bright points

Coronal bright points (BPs) are numerous, bright, small-scale dynamical features found in the solar corona. Bright points have been observed to exhibit intensity oscillations across a wide range of periodicities and are likely an important signature of plasma heating and/or transport mechanisms. We present a novel and efficient wavelet-based method that automatically detects and tracks the intensity evolution of BPs using images from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) in the 193Å bandpass. Through the study of a large, statistically significant set of BPs, we attempt to place constraints on the underlying physical mechanisms. We used a continuous wavelet transform (CWT) in 2D to detect the BPs within images. One-dimensional CWTs were used to analyse the individual BP time series to detect significant periodicities. We find significant periodicity at 4, 8-10, 17, 28, and 65 minutes. Bright point lifetimes are shown to follow a power law with exponent $-1.13\pm0.07$. The relationship between the BP lifetime and maximum diameter similarly follows a power law with exponent $0.129\pm0.011$. Our wavelet-based method successfully detects and extracts BPs and analyses their intensity oscillations. Future work will expand upon these methods, using larger datasets and simultaneous multi-instrument observations.

astro-ph.SR

Tracking Non-Radial Outflows in Extreme Ultraviolet and White Light Solar Images

Understanding the solar corona requires knowledge of its dynamics through its various layers and subsequent connectivity to the heliosphere. This requires understanding the nature of the outflows and the physical transitions through the middle corona (~1.5-6.0 Rs). While this region is still inaccessible to in situ measurements, remote sensing observations are available, but their interpretation can be controversial due to line-of-sight effects and the non-radial motion of outflowing structures close to the Sun (<3.0 Rs). In this work, we describe a method to mitigate these challenges by generating non-radial Height-Time profiles of outflows by using advanced image processing techniques. The North and South boundaries of a large equatorial streamer during the 2008 solar minimum were identified in STEREO/SECCHI solar images, using two different methodologies based on thresholds of brightness and piece-wise polynomial function fitting. To address line-of-sight issues, we used tomographic reconstruction of the 3D distribution of the coronal electron density based on SECCHI/COR2 images. Spectral analysis of the time series of the position angle of the streamer boundary revealed its oscillatory nature at some heights at 36-48 hours and 10.5-14.6 hours. Dividing the distance between the North and South streamer boundaries in equal parts at each height, we obtained non-radial Height-Time paths from which we generated non-radial profiles of corona/solar wind plasma outflow. We tracked outflows as they moved uninterruptedly from the Sun in EUVI, through COR1 and into COR2. Finally, we discuss preliminary results of non-radial plane-of-sky velocities for a CME and two small-scale features.

astro-ph.SR

Automated detection of coronaL MAss ejecta origiNs for space weather AppliCations (ALMANAC)

Alerts of potentially hazardous coronal mass ejections (CME) are based on the detection of rapid changes in remote observations of the solar atmosphere. This paper presents a method that detects and estimates the central coordinates of CME eruptions in Extreme Ultraviolet (EUV) data, with the dual aim of providing an early alert, and giving an initial estimate of the CME direction of propagation to a CME geometrical model. In particular, we plan to link the ALMANAC method to the CME detection and characterisation module of the Space Weather Empirical Ensemble Package (SWEEP), which is a fully automated modular software package for operational space weather capability currently being developed for the UK Meteorological Office. In this work, ALMANAC is applied to observations by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO). As well as presenting the method, a proof of concept test is made on a limited set of data associated with twenty halo CMEs recorded by the Coordinated Data Analysis Workshop (CDAW) catalogue near the activity maximum of solar cycle 24. SDO/AIA data for each event is processed at 6 minute cadence to identify the on-disk location and time of each CME. The absolute mean deviance between the ALMANAC and CDAW source event coordinates are within 37.05 +- 29.71 minutes and 11.01 +- 10.39 degrees. These promising results give a solid foundation for future work, and will provide initial constraints to an automated CME alert and forecasting system.

astro-ph.SR

An inner boundary condition for solar wind models based on coronal density

Accurate forecasting of the solar wind has grown in importance as society becomes increasingly dependent on technology that is susceptible to space weather events. This work describes an inner boundary condition for ambient solar wind models based on tomography maps of the coronal plasma density gained from coronagraph observations, providing a novel alternative to magnetic extrapolations. The tomographical density maps provide a direct constraint of the coronal structure at heliocentric distances of 4 to 8Rs, thus avoiding the need to model the complex non-radial lower corona. An empirical inverse relationship converts densities to solar wind velocities which are used as an inner boundary condition by the Heliospheric Upwind Extrapolation (HUXt) model to give ambient solar wind velocity at Earth. The dynamic time warping (DTW) algorithm is used to quantify the agreement between tomography/HUXt output and in situ data. An exhaustive search method is then used to adjust the lower boundary velocity range in order to optimize the model. Early results show up to a 32% decrease in mean absolute error between the modelled and observed solar wind velocities compared to that of the coupled MAS/HUXt model. The use of density maps gained from tomography as an inner boundary constraint is thus a valid alternative to coronal magnetic models, and offers a significant advancement in the field given the availability of routine space-based coronagraph observations.

physics.space-ph

Tracing the magnetic field topology of the quiet corona using propagating disturbances

The motion of faint propagating disturbances (PD) in the solar corona reveals an intricate structure which must be defined by the magnetic field. Applied to quiet Sun observations by the Atmospheric Imaging Assembly (AIA)/Solar Dynamics Observatory (SDO), a novel method reveals a cellular network, with cells of typical diameters 50\arcsec\ in the cool 304Å channel, and 100\arcsec\ in the coronal 193Å channel. The 193Å cells can overlie several 304Å cells, although both channels share common source and sink regions. The sources are points, or narrow corridors, of divergence that occupy the centres of cells. They are significantly aligned with photospheric network features and enhanced magnetic elements. This shows that the bright network is important to the production of PDs, and confirms that the network is host to the source footpoint of quiet coronal loops. The other footpoint, or the sinks of the PDs, form the boundaries of the coronal cells. These are not significantly aligned with the photospheric network - they are generally situated above the dark internetwork photosphere. They form compact points or corridors, often without an obvious signature in the underlying photosphere. We argue that these sink points can either be concentrations of closed field footpoints associated with minor magnetic elements in the internetwork, or concentrations of upward-aligned open field. The link between the coronal velocity and magnetic fields is strengthened by a comparison with a magnetic extrapolation, which shows several general and specific similarities, thus the velocity maps offer a valuable additional constraint on models.

astro-ph.SR

An improved method for estimating the velocity field of coronal propagating disturbances

The solar corona is host to a continuous flow of propagating disturbances (PD). These are continuous and ubiquitous across broad regions of the corona, including the quiet Sun. The aim of this paper is to present an improved, efficient method to create velocity vector field maps, based on the direction and magnitude of the PD as observed in time series of extreme ultraviolet (EUV) images. The method is presented here for use with the Atmospheric Imaging Assembly (AIA)/Solar Dynamics Observatory (SDO) EUV channels, and takes as input \app2 hours of images at the highest 12s cadence. Data from a region near disk center is extracted, and a process called time normalization applied to the co-aligned data. Following noise reduction using \atrous\ decomposition, the PD are effectively revealed. A modified Lucas Kanade algorithm is then used to map the velocity field. The method described here runs comfortably on a desktop computer in a few minutes, and offers an order of magnitude improvement in efficiency compared to a previous implementation. Applied to a region of the quiet Sun, we find that the velocity field describes a mosaic of cells of coherent outwardly-diverging PD flows, of typical size 50 to 100\arcsec\ (36 to 72Mm). The flows originate from points and narrow corridors in the cell centres, and end in the narrow boundaries between cells. Visual comparison with ultraviolet AIA images shows that the flow sources are correlated with the bright photospheric supergranular network boundaries. Assuming that the PD follow the local magnetic field, the velocity flow field is a proxy for the plane-of-sky distribution of the coronal magnetic field, and therefore the maps offer a unique insight into the topology of the corona. These are particularly valuable for quiet Sun regions where the appearance of structures in EUV images is hard to interpret.

astro-ph.SR