Observing the evolution of the Sun's global coronal magnetic field over eight months

The Sun's magnetic field plays a crucial role in various solar phenomena, including solar eruptions, plasma heating, and the 11-year solar activity cycle. Understanding these processes requires detailed measurements of the evolving magnetic field throughout the Sun's atmosphere. While photospheric magnetic fields are routinely measured using the Zeeman effect, extending these measurements to the corona is challenging due to weaker emissions and smaller line splitting. The coronal magnetic field is a primary energy source for coronal heating and solar eruptions, yet routine coronal magnetic field measurements have been lacking. Previous attempts to measure the coronal magnetic field involved spectro-polarimetric measurements of infrared lines (for strong fields), radio spectral imaging (for flaring regions), and a proposed method using Fe X ultraviolet line intensity (which lacks the necessary precision). Coronal seismology, inferring field strengths from magnetohydrodynamic (MHD) waves, provides single-event measurements. These techniques haven't enabled routine monitoring of the Sun's global coronal magnetic field evolution. While the Coronal Multi-channel Polarimeter (CoMP) produced individual global coronal magnetograms using 2D coronal seismology, the limited signal-to-noise ratio restricted its use to only a few datasets. Routine monitoring requires approximately daily measurements, which was previously unattainable.

Existing methods for measuring coronal magnetic fields each have limitations. Spectro-polarimetric methods using infrared lines are limited to regions of very strong magnetic fields. Radio imaging techniques only work for flaring regions. A method based on the Fe X ultraviolet spectral line intensity shows promise, but currently lacks the required instrumental precision. Coronal seismology, which infers magnetic field strength from MHD waves, is often limited to single oscillation events, providing only a single value for the field strength. None of these methods provide routine, global measurements of the coronal magnetic field's evolution.

This study utilized data from the Upgraded Coronal Multi-channel Polarimeter (UCoMP), which offers improved capabilities over its predecessor, CoMP. UCoMP provides daily imaging spectral observations of the off-limb corona at all latitudes, with an expanded field of view (1.05 to 1.6 solar radii), higher spatial resolution (~6 arcseconds), enhanced sensitivity, and improved data quality stability. UCoMP targets the Fe XIII 1074.7 nm and 1079.8 nm near-infrared spectral lines, allowing for measurements of line intensity, Doppler velocity, and electron number density. The analysis focused on prevalent propagating disturbances (kink waves) observed in the Doppler velocity maps. A modified wave tracking technique was applied to determine the wave propagation direction and phase speed. Using these wave parameters and density diagnostics, 2D coronal seismology was employed to derive global maps of the coronal magnetic field. The magnetic field strength (B) was calculated using the relationship B = Uphρ/μo, where Uph is the wave phase speed, ρ is the plasma density, and μo is the magnetic permeability in a vacuum. The resulting magnetic field strength is a line-of-sight (LOS) weighted average. A total of 114 global maps of the coronal magnetic field were generated over a period of 253 days (approximately eight months), spanning more than nine solar rotations. The data were used to create Carrington maps representing the evolution of the coronal magnetic field over multiple rotations at various altitudes and latitudes. These Carrington maps were further reprojected onto a spherical coordinate system to extract magnetic field distributions at different radii from the solar center. For comparison, three-dimensional magnetohydrodynamic (MHD) models generated using the Magnetohydrodynamic Algorithm outside a Sphere (MAS) code were employed. These models used observed photospheric magnetic fields as boundary conditions. The model output was processed to compute LOS emissivity-weighted parameters for comparison with the observations. A potential field source surface (PFSS) model was also used for comparison purposes.

The UCoMP observations provided a significant advancement in coronal magnetic field measurements, offering an unprecedented dataset of 114 global magnetograms over an eight-month period with an average cadence of approximately once every two days. This represents a substantial improvement over previous studies using CoMP, which yielded only one or two usable maps per year. The study successfully extended the coverage to higher altitudes (up to ~1.6 solar radii) and to nearly all latitudes, including the polar regions. Analysis of the UCoMP data revealed a range of coronal magnetic field strengths, varying from less than 1 Gauss to approximately 20 Gauss within the observed range of 1.05-1.6 solar radii. A distinct signature of active longitudes was observed in the coronal magnetic field, indicating a recurring pattern of strong-field regions at similar longitudes over multiple solar rotations. This suggests that the active longitudes observed in the photosphere extend their influence into the corona. Comparison of the coronal magnetic field maps with simultaneous photospheric observations revealed a correlation between strong-field regions in both layers of the solar atmosphere. However, some strong photospheric field regions did not show corresponding strong fields in the corona, suggesting potential explanations such as flux tube expansion with height or the presence of low-lying closed magnetic structures. The study found that MHD models (specifically, the MAS model) generally agreed with the UCoMP observations, particularly in low- and mid-latitude regions. However, discrepancies were observed, especially at higher latitudes. These discrepancies were attributed to limitations in the models, including lower spatial resolution than the observations, the use of photospheric magnetograms averaged over 27 days rather than being specific to the days of the UCoMP observations, and less reliable photospheric magnetic field measurements at high latitudes. Comparisons with PFSS models also revealed large-scale similarities but numerous discrepancies, primarily due to the limitations of the PFSS method.

The findings of this research address the long-standing challenge of obtaining routine measurements of the Sun's global coronal magnetic field. The high-cadence, high-resolution data provided by UCoMP allow for a significantly improved understanding of coronal magnetic field evolution. The observation of active longitudes extending into the corona provides valuable insights into the relationship between the photospheric and coronal magnetic fields and their influence on solar activity. The general agreement between the observations and MHD models validates the methods used and provides support for the current understanding of coronal magnetic field generation and dynamics. The discrepancies highlighted in the study, particularly at higher latitudes, point to areas where further model refinement and improved observation techniques are needed.

This study demonstrates the successful routine measurement of the Sun's global coronal magnetic field over eight months, providing 114 global magnetograms. Carrington maps revealed field strength variations from <1 to ~20 Gauss and active longitude signatures. Comparisons with photospheric data and MHD models highlighted both agreement and discrepancies, suggesting future research directions should focus on improving model resolution, incorporating real-time photospheric data, and addressing high-latitude measurement challenges.

The study acknowledges several limitations. The analysis relied on LOS-integrated observations, which may introduce uncertainties in the derived magnetic field parameters. The models used for comparison (MAS and PFSS) have limitations in spatial resolution and the representation of complex magnetic structures, particularly in active regions. Also, the high latitude data is less reliable due to projection effects and data scarcity in polar regions. These limitations may have affected the comparison between observations and model predictions and should be considered when interpreting the results.