ILCT: Recovering Photospheric Velocities from Magnetograms by Combining the Induction Equation with Local Correlation Tracking

Welsch, B. T.; Fisher, G. H.; Abbett, W. P.; Régnier, Stéphane

doi:10.1086/421767

Cited by 188 publications

(204 citation statements)

References 25 publications

(25 reference statements)

Supporting

Mentioning

199

Contrasting

Unclassified

Order By: Relevance

“…The velocity field is extracted from the magnetograms using the Fourier local correlation tracking (FLCT) method (Welsch et al 2004;Fisher & Welsch 2008). The method has a number of parameters: optimum values have been determined by an autocorrelation analysis, aiming to maximise frame-to-frame correlations and ensure robustness in the velocity estimate (see Welsch et al 2011).…”

Section: Photospheric Velocity Datamentioning

confidence: 99%

Lagrangian coherent structures in photospheric flows and their implications for coronal magnetic structure

Yeates

Hornig

Welsch

2012

A&A

Self Cite

View full text Add to dashboard Cite

Aims. We show how the build-up of magnetic gradients in the Sun's corona may be inferred directly from photospheric velocity data. This enables computation of magnetic connectivity measures such as the squashing factor without recourse to magnetic field extrapolation. Methods. Assuming an ideal evolution in the corona, and an initially uniform magnetic field, the subsequent field line mapping is computed by integrating trajectories of the (time-dependent) horizontal photospheric velocity field. The method is applied to a 12 h high-resolution sequence of photospheric flows derived from Hinode/SOT magnetograms. Results. We find the generation of a network of quasi-separatrix layers in the magnetic field, which correspond to Lagrangian coherent structures in the photospheric velocity. The visual pattern of these structures arises primarily from the diverging part of the photospheric flow, hiding the effect of the rotational flow component: this is demonstrated by a simple analytical model of photospheric convection. We separate the diverging and rotational components from the observed flow and show qualitative agreement with purely diverging and rotational models respectively. Increasing the flow speeds in the model suggests that our observational results are likely to give a lower bound for the rate at which magnetic gradients are built up by real photospheric flows. Finally, we construct a hypothetical magnetic field with the inferred topology, that can be used for future investigations of reconnection and energy release.

show abstract

Section: Photospheric Velocity Datamentioning

confidence: 99%

Lagrangian coherent structures in photospheric flows and their implications for coronal magnetic structure

Yeates

Hornig

Welsch

2012

A&A

Self Cite

View full text Add to dashboard Cite

show abstract

“…The SOT/NB data were analysed using the YAFTA feature tracking algorithm (Welsch et al 2004), which was set to detect and track magnetic features with a minimum magnetic flux density of 250 G in each pixel, and a minimum size of 20 pixels. Ten or more strong magnetic features were detected in all frames.…”

Section: D Magnetic Field Structurementioning

confidence: 99%

Hinode observations and 3D magnetic structure of an X-ray bright point

Alexander

Zanna

Maclean

2011

A&A

View full text Add to dashboard Cite

Aims. We present complete Hinode Solar Optical Telescope (SOT), X-Ray Telescope (XRT)and EUV Imaging Spectrometer (EIS) observations of an X-ray bright point (XBP) observed on the 10, 11 of October 2007 over its entire lifetime (∼12 h). We aim to show how the measured plasma parameters of the XBP change over time and also what kind of similarities the X-ray emission has to a potential magnetic field model. Methods. Information from all three instruments on-board Hinode was used to study its entire evolution. XRT data was used to investigate the structure of the bright point and to measure the X-ray emission. The EIS instrument was used to measure various plasma parameters over the entire lifetime of the XBP. Lastly, the SOT was used to measure the magnetic field strength and provide a basis for potential field extrapolations of the photospheric fields to be made. These were performed and then compared to the observed coronal features. Results. The XBP measured ∼15 in size and was found to be formed directly above an area of merging and cancelling magnetic flux on the photosphere. A good correlation between the rate of X-ray emission and decrease in total magnetic flux was found. The magnetic fragments of the XBP were found to vary on very short timescales (minutes), however the global quasi-bipolar structure remained throughout the lifetime of the XBP. The potential field extrapolations were a good visual fit to the observed coronal loops in most cases, meaning that the magnetic field was not too far from a potential state. Electron density measurements were obtained using a line ratio of Fe xii and the average density was found to be 4.95 × 10 9 cm −3 with the volumetric plasma filling factor calculated to have an average value of 0.04. Emission measure loci plots were then used to infer a steady temperature of log T e [K] ∼ 6.1. The calculated Fe xii Doppler shifts show velocity changes in and around the bright point of ±15 km s −1 which are observed to change on a timescale of less than 30 min.

show abstract

“…Further, the plasma flows in the filament channel are studied by computing horizontal flow velocities using local correlation technique (LCT) (Welsch et al, 2004), . We use GONG magnetograms, separated half-anhour apart, for computing these LCT maps during the entire period 20, 21 and 22 May.…”

Section: Horizontal Flows : Local Correlation Trackingmentioning

confidence: 99%

3D Evolution of a Filament Disappearance Event Observed by STEREO

Gosain

Schmieder²,

Venkatakrishnan³

et al. 2009

Sol Phys

View full text Add to dashboard Cite

A filament disappearance event was observed on 22 May 2008 during our recent campaign JOP 178. The filament, situated in the southern hemisphere, showed sinistral chirality consistent with the hemispheric rule. The event was well observed by several observatories in particular by THEMIS. One day before the disappearance, Hα observations showed up and down flows in adjacent locations along the filament, which suggest plasma motions along twisted flux rope. THEMIS and GONG observations show shearing photospheric motions leading to magnetic flux canceling around barbs. STEREO A, B spacecraft with separation angle 52.4 degrees, showed quite different views of this untwisting flux rope in He II 304Å images. Here, we reconstruct the 3D geometry of the filament during its eruption phase using STEREO EUV He II 304Å images and find that the filament was highly inclined to the solar normal. The He II 304Å movies show individual threads, which oscillate and rise to an altitude of about 120 Mm with apparent velocities of about 100 km s −1 , during the rapid evolution phase. Finally, as the flux rope expands into the corona, the filament disappears by becoming optically thin to undetectable levels. No CME was detected by STEREO, only a faint CME was recorded by LASCO at the beginning of the disappearance phase at 02:00 UT, which could be due to partial filament eruption. Further, STEREO Fe XII 195Å images showed bright loops beneath the filament prior to the disappearance phase, suggesting magnetic reconnection below the flux rope.

show abstract

ILCT: Recovering Photospheric Velocities from Magnetograms by Combining the Induction Equation with Local Correlation Tracking

Cited by 188 publications

References 25 publications

Lagrangian coherent structures in photospheric flows and their implications for coronal magnetic structure

Lagrangian coherent structures in photospheric flows and their implications for coronal magnetic structure

Hinode observations and 3D magnetic structure of an X-ray bright point

3D Evolution of a Filament Disappearance Event Observed by STEREO

Contact Info

Product

Resources

About