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Polar Field Correction for HMI Line-of-Sight Synoptic Data  

Xudong Sun   Submitted: 2018-01-14 20:30

This document provides some technical notes on the polar field correction scheme for the HMI synoptic maps and daily updated synchronic frames. It is intended as a reference for the new data products and for some minor updates on our previous scheme for MDI (Sun et al. 2011).

Authors: Xudong Sun, for the HMI Team
Projects: SDO-HMI

Publication Status: arXiv only
Last Modified: 2018-01-16 11:09
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Super-Flaring Active Region 12673 Has One of the Fastest Magnetic Flux Emergence Ever Observed  

Xudong Sun   Submitted: 2017-11-22 18:26

The flux emergence rate of AR 12673 is greater than any values reported in the literature of which we are aware.

Authors: Xudong Sun, Aimee Norton
Projects: SDO-HMI

Publication Status: Accepted to Research Notes of the AAS
Last Modified: 2017-11-25 10:03
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Investigating the Magnetic Imprints of Major Solar Eruptions with SDO/HMI High-Cadence Vector Magnetograms  

Xudong Sun   Submitted: 2017-03-27 01:41

The solar active region photospheric magnetic field evolves rapidly during major eruptive events, suggesting appreciable feedback from the corona. Previous studies of these "magnetic imprints" are mostly based on line-of-sight only or lower-cadence vector observations; a temporally resolved depiction of the vector field evolution is hitherto lacking. Here, we introduce the high-cadence (90 s or 135 s) vector magnetogram dataset from the Helioseismic and Magnetic Imager (HMI) that is well suited for investigating the phenomenon. These observations allow quantitative characterization of the permanent, step-like changes that are most pronounced in the horizontal field component (Bh). A highly structured pattern emerges from analysis of an archetypical event, SOL2011-02-15T01:56, where Bh near the main polarity inversion line increases significantly during the earlier phase of the associated flare with a time scale of several minutes, while Bh in the periphery decreases at later times with smaller magnitudes and a slightly longer time scale. The dataset also allows effective identification of the "magnetic transient" artifact, where enhanced flare emission alters the Stokes profiles and the inferred magnetic field becomes unreliable. Our results provide insights on the momentum processes in solar eruptions. The dataset may also be useful to the study of sunquakes and data-driven modeling of the corona.

Authors: Xudong Sun, J. Todd Hoeksema, Yang Liu, Maria Kazachenko, Ruizhu Chen
Projects: SDO-HMI

Publication Status: ApJ (in press)
Last Modified: 2017-03-29 13:49
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Why Is the Great Solar Active Region 12192 Flare-Rich But CME-Poor?  

Xudong Sun   Submitted: 2015-04-20 13:23

Solar active region (AR) 12192 of October 2014 hosts the largest sunspot group in 24 years. It is the most prolific flaring site of Cycle 24 so far, but surprisingly produced no coronal mass ejection (CME) from the core region during its disk passage. Here, we study the magnetic conditions that prevented eruption and the consequences that ensued. We find AR 12192 to be "big but mild"; its core region exhibits weaker non-potentiality, stronger overlying field, and smaller flare-related field changes compared to two other major flare-CME-productive ARs (11429 and 11158). These differences are present in the intensive-type indices (e.g., means) but generally not the extensive ones (e.g., totals). AR 12192's large amount of magnetic free energy does not translate into CME productivity. The unexpected behavior suggests that AR eruptiveness is limited by some relative measure of magnetic non-potentiality over the restriction of background field, and that confined flares may leave weaker photospheric and coronal imprints compared to their eruptive counterparts.

Authors: Xudong Sun, Monica Bobra, Todd Hoeksema, Yang Liu, Yan Li, Chenglong Shen, Sebastien Couvidat, Aimee Norton, George Fisher
Projects: SDO-HMI

Publication Status: ApJL, in press
Last Modified: 2015-04-21 10:46
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On Polar Magnetic Field Reversal and Surface Flux Transport During Solar Cycle 24  

Xudong Sun   Submitted: 2014-11-03 14:53

As each solar cycle progresses, remnant magnetic flux from active regions (ARs) migrates poleward to cancel the old-cycle polar field. We describe this polarity reversal process during Cycle 24 using four years (2010.33-2014.33) of line-of-sight magnetic field measurements from the Helioseismic and Magnetic Imager. The total flux associated with ARs reached maximum in the north in 2011, more than two years earlier than the south; the maximum is significantly weaker than Cycle 23. The process of polar field reversal is relatively slow, north-south asymmetric, and episodic. We estimate that the global axial dipole changed sign in October 2013; the northern and southern polar fields (mean above 60 degrees latitude) reversed in November 2012 and March 2014, respectively, about 16 months apart. Notably, the poleward surges of flux in each hemisphere alternated in polarity, giving rise to multiple reversals in the north. We show that the surges of the trailing sunspot polarity tend to correspond to normal mean AR tilt, higher total AR flux, or slower mid-latitude near-surface meridional flow, while exceptions occur during low magnetic activity. In particular, the AR flux and the mid-latitude poleward flow speed exhibit a clear anti-correlation. We discuss how these features can be explained in a surface flux transport process that includes a field-dependent converging flow toward the ARs, a characteristic that may contribute to solar cycle variability.

Authors: Xudong Sun, J. Todd Hoeksema, Yang Liu, Junwei Zhao
Projects: SDO-HMI

Publication Status: ApJ (in press)
Last Modified: 2014-11-05 11:49
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Hot Spine Loops and the Nature of a Late-Phase Solar Flare  

Xudong Sun   Submitted: 2013-10-08 09:00

The fan-spine magnetic topology is believed to be responsible for many curious features in solar explosive events. A spine field line links distinct flux domains, but direct observation of such feature has been rare. Here we report a unique event observed by the Solar Dynamic Observatory where a set of hot coronal loops (over 10 MK) connected to a quasi-circular chromospheric ribbon at one end and a remote brightening at the other. Magnetic field extrapolation suggests these loops are partly tracer of the evolving spine field line. Continuous slipping- and null-point-type reconnections were likely at work, energizing the loop plasma and transferring magnetic flux within and across the fan quasi-separatrix layer. We argue that the initial reconnection is of the ''breakout'' type, which then transitioned to a more violent flare reconnection with an eruption from the fan dome. Significant magnetic field changes are expected and indeed ensued. This event also features an extreme-ultraviolet (EUV) late phase, i.e. a delayed secondary emission peak in warm EUV lines (about 2-7 MK). We show that this peak comes from the cooling of large post-reconnection loops beside and above the compact fan, a direct product of eruption in such topological settings. The long cooling time of the large arcades contributes to the long delay; additional heating may also be required. Our result demonstrates the critical nature of cross-scale magnetic coupling - topological change in a sub-system may lead to explosions on a much larger scale.

Authors: Xudong Sun, J. Todd Hoeksema, Yang Liu, Guillaume Aulanier, Yingna Su, Iain G. Hannah, Rachel A. Hock
Projects: SDO-AIA,SDO-HMI,SDO-EVE

Publication Status: Accepted by ApJ
Last Modified: 2013-10-09 12:17
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A Non-radial Eruption in a Quadrupolar Magnetic Configuration with a Coronal Null  

Xudong Sun   Submitted: 2012-08-06 18:14

We report one of several homologous non-radial eruptions from NOAA active region (AR) 11158 that are strongly modulated by the local magnetic field as observed with the Solar Dynamic Observatory (SDO). A small bipole emerged in the sunspot complex and subsequently created a quadrupolar flux system. Non-linear force-free field (NLFFF) extrapolation from vector magnetograms reveals its energetic nature: the fast-shearing bipole accumulated ~2e31 erg free energy (10% of AR total) over just one day despite its relatively small magnetic flux (5% of AR total). During the eruption, the ejected plasma followed a highly inclined trajectory, over 60 degrees with respect to the radial direction, forming a jet-like, inverted-Y shaped structure in its wake. Field extrapolation suggests complicated magnetic connectivity with a coronal null point, which is favorable of reconnection between different flux components in the quadrupolar system. Indeed, multiple pairs of flare ribbons brightened simultaneously, and coronal reconnection signatures appeared near the inferred null. Part of the magnetic setting resembles that of a blowout-type jet; the observed inverted-Y structure likely outlines the open field lines along the separatrix surface. Owing to the asymmetrical photospheric flux distribution, the confining magnetic pressure decreases much faster horizontally than upward. This special field geometry likely guided the non-radial eruption during its initial stage.

Authors: Xudong Sun, Todd Hoeksema, Yang Liu, Qingrong Chen, Keiji Hayashi
Projects: GOES X-rays ,Hinode/SOT,Hinode/XRT,RHESSI,SDO-AIA,SDO-HMI,STEREO

Publication Status: Accepted by ApJ
Last Modified: 2012-08-07 09:16
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Evolution of Magnetic Field and Energy in A Major Eruptive Active Region Based on SDO/HMI Observation  

Xudong Sun   Submitted: 2012-01-16 14:12

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Xudong Sun, J. Todd Hoeksema, Yang Liu, Thomas Wiegelmann, Keiji Hayashi, Qingrong Chen, Julia Thalmann
Projects: SDO-AIA,SDO-HMI

Publication Status: Accepted by ApJ
Last Modified: 2012-01-16 21:07
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A New Method for Polar Field Interpolation  

Xudong Sun   Submitted: 2011-03-19 14:57

The photospheric magnetic field in the Sun's polar region is not well observed compared to the low-latitude regions. Data are periodically missing due to the Sun's tilt angle, and the noise level is high due to the projection effect on the line-of-sight (LoS) measurement. However, the large-scale characteristics of the polar magnetic field data are known to be important for global modeling. This report describes a new method for interpolating the photospheric field in polar regions that has been tested on MDI synoptic maps (1996-2009). This technique, based on a two-dimensional spatial/temporal interpolation and a simple version of flux transport model, uses a multi-year series of well-observed, smoothed north (south) pole observation from each September (March) to interpolate for missing pixels at any time of interest. It is refined by using a spatial smoothing scheme to seamlessly incorporate this filled-in data into the original observation starting from lower latitudes. For recent observations, an extrapolated polar field correction is required. Scaling the average flux density from the prior observations of slightly lower latitudes is found to be a good proxy of the future polar field. This new method has several advantages over some existing methods. It is demonstrated to improve the results of global models such as the Wang-Sheeley-Arge (WSA) model and MHD simulation, especially during the sunspot minimum phase.

Authors: Xudong Sun, Yang Liu, Todd Hoeksema, Keiji Hayashi, Xuepu Zhao
Projects: SoHO-MDI

Publication Status: Accepted by Solar Physics
Last Modified: 2011-03-20 01:18
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Abstracts by Author
Polar Field Correction for HMI Line-of-Sight Synoptic Data
Super-Flaring Active Region 12673 Has One of the Fastest Magnetic Flux Emergence Ever Observed
Investigating the Magnetic Imprints of Major Solar Eruptions with SDO/HMI High-Cadence Vector Magnetograms
Why Is the Great Solar Active Region 12192 Flare-Rich But CME-Poor?
On Polar Magnetic Field Reversal and Surface Flux Transport During Solar Cycle 24
Hot Spine Loops and the Nature of a Late-Phase Solar Flare
A Non-radial Eruption in a Quadrupolar Magnetic Configuration with a Coronal Null
Evolution of Magnetic Field and Energy in A Major Eruptive Active Region Based on SDO/HMI Observation
A New Method for Polar Field Interpolation

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