FORWARD: A Toolset for Multiwavelength Coronal Magnetometry

Gibson, S. E.; Kucera, T. A.; White, S. M.; Dove, James B.; Fan, Yuhong; Forland, Blake Christopher; Rachmeler, Laurel A.; Downs, Cooper; Reeves, Katharine K.

doi:10.3389/fspas.2016.00008

Cited by 105 publications

(85 citation statements)

References 75 publications

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“…Leakage may also occur from Stokes U into V , but this is not a concern here because linear polarizations from the corona are negligible at our frequencies and observing bandwidths (e.g. Gibson et al, 2016). Of potential concern, however, is possible leakage from V into I.…”

Section: An Algorithm To Mitigate the Leakage Of Stokes I Into Vmentioning

confidence: 94%

“…where T e is the electron temperature, f is the emission frequency, f B is the electron gyrofrequency [f B = 2.8 × 10 6 B gauss Hz], and θ is the angle between the line of sight and the magnetic field direction (Dulk, 1985;Gelfreikh, 2004;Gibson et al, 2016). The plus sign refers to the o-mode, the minus sign refers to the x-mode, and the difference between the two modes produces the net circular polarization.…”

Section: Introductionmentioning

confidence: 99%

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The Low-Frequency Solar Corona in Circular Polarization

et al. 2019

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We present spectropolarimetric imaging observations of the solar corona at low frequencies (80 -240 MHz) using the Murchison Widefield Array (MWA). These images are the first of their kind, and we introduce an algorithm to mitigate an instrumental artefact by which the total intensity signal contaminates the polarimetric images due to calibration errors. We then survey the range of circular polarization (Stokes V ) features detected in over 100 observing runs near solar maximum during quiescent periods. First, we detect around 700 compact polarized sources across our dataset with polarization fractions ranging from less than 0.5 % to nearly 100 %. These sources exhibit a positive correlation between polarization fraction and total intensity, and we interpret them as a continuum of plasma emission noise storm (Type I burst) continua sources associated with active regions. Second, we report a characteristic "bullseye" structure observed for many low-latitude coronal holes in which a central polarized component is surrounded by a ring of the opposite sense. The central component does not match the sign expected from thermal bremsstrahlung emission, and we speculate that propagation effects or an alternative emission mechanism may be responsible. Third, we show that the large-scale polarimetric structure at our lowest frequencies is reasonably well-correlated with the line-of-sight (LOS) magnetic field component inferred from a global potential field source surface (PFSS) model. The boundaries between opposite circular polarization signs are generally aligned with polarity inversion lines in the model at a height roughly corresponding to that of the radio limb. This is not true at our highest frequencies, however, where the LOS magnetic field direction and polarization sign are often not straightforwardly correlated.

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Section: An Algorithm To Mitigate the Leakage Of Stokes I Into Vmentioning

confidence: 94%

Section: Introductionmentioning

confidence: 99%

The Low-Frequency Solar Corona in Circular Polarization

et al. 2019

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“…2 and 3 present another example of a pseudostreamer, and directly compare CoMP data to PFSS model predictions. Synthetic data are generated using the 'pfss' and 'FORWARD' (Gibson et al 2016) packages of SolarSoft IDL (Freeland & Handy 1998). Plasma weighting along the LOS is achieved by defining density radial profiles consistent with hydrostatic equilibrium: closed-field regions are assigned profiles that match coronal streamer observations , while open-field regions are given profiles to match coronal hole observations (Guhathakurta et al 1999).…”

Section: Coronal Magnetic Nulls From Linear-polarization Magnitudementioning

confidence: 99%

Magnetic Nulls and Super-radial Expansion in the Solar Corona

Gibson

Dalmasse

Rachmeler

et al. 2017

ApJL

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Magnetic fields in the sun's outer atmosphere -the corona -control both solar-wind acceleration and the dynamics of solar eruptions. We present the first clear observational evidence of coronal magnetic nulls in off-limb linearly polarized observations of pseudostreamers, taken by the Coronal Multichannel Polarimeter (CoMP) telescope. These nulls represent regions where magnetic reconnection is likely to act as a catalyst for solar activity. CoMP linearpolarization observations also provide an independent, coronal proxy for magnetic expansion into the solar wind, a quantity often used to parameterize and predict the solar wind speed at Earth. We introduce a new method for explicitly calculating expansion factors from CoMP coronal linear-polarization observations, which does not require photospheric extrapolations. We conclude that linearly-polarized light is a powerful new diagnostic of critical coronal magnetic topologies and the expanding magnetic flux tubes that channel the solar wind.

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“…For instance, both the red line and green line images revealed a well‐defined coronal cavity (e.g., Su YN and van Ballegooijen, 2012; Jibben et al, 2016) near the equator above the west limb of the Sun (Figure 2). The team are currently performing forward calculation (Gibson et al, 2016) of the line intensities and polarization states based on several constructed magnetic field models. The synthesized images will be compared with the eclipse observations, and the best fitted model will potentially yield the true magnetic structures associated with the cavity.…”

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confidence: 99%