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

The Sun’s magnetic field underpins coronal heating, transient eruptions, and the 11-year solar cycle. While photospheric fields are routinely measured via the Zeeman effect, the much weaker coronal emission and smaller line splitting make direct coronal field measurements difficult. Prior coronal field estimates have been limited to isolated regions or events and have not enabled routine, global monitoring. The study addresses the need for sustained, large-scale measurements of the coronal magnetic field’s strength and direction, aiming to capture its evolution across altitudes and latitudes over multiple solar rotations. By exploiting ubiquitous propagating transverse MHD (kink) waves and density diagnostics in coronal infrared lines, the authors seek to produce regular global magnetograms and track the coronal field’s temporal evolution, assessing connections to photospheric fields and benchmarking against coronal MHD models.

Previous coronal field measurements include infrared spectro-polarimetry in strong-field locales, radio spectral imaging during flares, and proposed UV intensity–field relationships that currently lack instrumental precision. Coronal seismology has inferred magnetic fields from individual oscillation events, yielding single-point values. Propagating transverse (kink) waves, pervasively observed by CoMP, enabled the first plane-of-sky (POS) coronal magnetograms, but earlier efforts were limited by signal-to-noise and cadence, producing only a few usable maps per year in a narrow height and latitude range. Modeling approaches (PFSS and global MHD with MAS) have been used to predict coronal fields from photospheric magnetograms, but their validation against routine LOS-integrated coronal observations has remained limited.

Observations: The Upgraded Coronal Multi-channel Polarimeter (UCoMP) provided near-daily off-limb imaging spectroscopy from 19 February to 29 October 2022, yielding 114 usable datasets spanning ~253 days (>9 Carrington rotations). The field of view typically covered ~1.05–1.6 solar radii with ~6″ spatial resolution. Spectral coverage targeted Fe XIII 1074.7 nm and 1079.8 nm. Data processing and plasma diagnostics: At each pixel, Fe XIII profiles were fit with Gaussians to derive line intensity and Doppler velocity. The intensity ratio of Fe XIII 1074.7/1079.8 nm provided electron density (and thus mass density) via CHIANTI atomic data. Quality control restricted analysis to regions with sufficient signal-to-noise. Wave tracking and seismology: Propagating transverse kink waves evident in Doppler velocity time series (~3.5 mHz) were analyzed using a modified coherence-based tracking method. For each pixel, the local propagation direction was derived from coherence patterns within 41×41 pixel neighborhoods via weighted linear fits. Phase speeds were computed by cross-correlating the central time series along each derived path with neighboring series and applying weighted linear regression of position versus lag. The outward-propagating component was isolated to mitigate reflections. The method assumes unresolved thin flux tubes and low-beta conditions, allowing the kink phase speed and density to yield the POS magnetic field strength via a seismology relation linking B to the local density and phase speed; uncertainties arise from phase-speed fitting and density diagnostics. Global magnetograms and Carrington maps: For each day, POS magnetic field strength and direction maps were produced, masked to reliable S/N. From 114 daily maps, the team constructed Carrington maps (latitude vs. Carrington longitude) of B, intensity, and density at multiple height shells by combining west/east limb measurements and interpolating along longitude. Spherical re-projections yielded magnetic field distributions in shells at selected radii. Model comparisons: Three MAS global thermodynamic MHD models (CR2254, CR2258, CR2260) driven by HMI synoptic radial photospheric magnetograms (scaled by 1.4 for consistency with prior MAS usage) were used to compute LOS emissivity-weighted POS magnetic field strength (B_POS) and direction (Ψ_POS) for direct comparison to UCoMP maps. Emissivities for Fe XIII 1074.7 nm were synthesized with FORWARD/CHIANTI, and LOS-weighted quantities were calculated along ±1 R_sun around the POS. Additional comparisons to a PFSS model (for 21 Feb 2022) provided context on potential-field assumptions. Uncertainty estimates combined phase-speed fit errors and line-ratio diagnostic uncertainties; typical B uncertainties were <20%, and direction uncertainties ~<2° in most regions.

- Routine global coronal magnetograms: 114 POS magnetic field maps over ~8 months, averaging about one map every two days, covering ~1.05–1.6 solar radii and nearly all latitudes, including polar regions (where signals permitted). - Field strengths: Typical coronal POS field strengths within the UCoMP FOV were ~0.5–4 G; in low altitudes above active regions values reached ~20 G. Overall range across 1.05–1.6 R_sun was <1–20 G. Estimated uncertainties generally <20% for B and <~2° for direction in most areas. - Vertical and horizontal connectivity: Strong-field regions in the photosphere often corresponded to strong-field regions in the low corona at matched longitudes/latitudes, indicating cross-layer magnetic connectivity. However, some strong photospheric patches did not map to strong coronal fields at higher altitudes, consistent with flux tube expansion and/or low-lying closed structures. - Temporal evolution and active longitudes: Carrington maps revealed recurrent strong coronal field at similar longitudes across rotations, consistent with photospheric active longitudes and emergence of new active regions near previous sites. - Model agreement and discrepancies: MAS LOS emissivity–weighted predictions reproduced many observed features, especially at low/mid latitudes (Pearson r≈0.64 for B in |latitude|<50°). Discrepancies were larger at high latitudes (r≈0.34 for |latitude|>50°) and in active regions (underprediction of strong fields), attributed to model resolution, empirical heating, LOS weighting approximations, and limitations of synoptic boundary conditions (27-day construction, lack of polar data and time variability). PFSS comparisons showed broad similarities but numerous differences, particularly in magnitude and polar regions, reflecting model assumptions and lack of thermodynamic weighting. - Coverage and cadence: The dataset spanned more than nine Carrington rotations, enabling multi-rotation tracking of the coronal magnetic field.

The study demonstrates that 2D coronal seismology using UCoMP can routinely map the global coronal magnetic field’s POS component and its evolution over rotations. This addresses the longstanding gap between photospheric magnetogram ubiquity and sparse coronal field measurements, enabling assessment of how photospheric magnetic structures project into the corona. The coherence between photospheric and coronal strong-field regions supports magnetic connectivity across layers, while mismatches highlight effects of expansion and closed low-lying structures. The detection of recurrent strong coronal fields at similar longitudes extends the concept of active longitudes into the corona, with implications for understanding flux emergence patterns and possibly forecasting activity. Comparisons to LOS emissivity-weighted MAS outputs substantiate the interpretation of UCoMP measurements as LOS-weighted POS quantities and validate broad features of thermodynamic MHD coronal models; remaining differences, especially at high latitudes and in complex active regions, point to needs for improved boundary conditions, resolution, and thermodynamics. PFSS comparisons reveal limitations of potential-field assumptions for detailed coronal field magnitude and structure, particularly without emissivity weighting or plasma information.

This work establishes routine, global, and evolutionary measurements of the coronal magnetic field over eight months, producing 114 magnetograms with roughly two-day cadence. The results quantify coronal field strengths from <1 to ~20 G between 1.05–1.6 R_sun, demonstrate cross-layer connectivity and the presence of active longitudes in the corona, and show general agreement with LOS emissivity-weighted MHD model predictions while identifying regional discrepancies. The approach provides a framework for sustained monitoring of the coronal magnetic field, essential for understanding coronal heating, dynamics, and solar activity. Future work should enhance polar magnetograms and time-dependent boundary conditions, increase spatial and temporal resolution, extend observing duration for full-cycle context, incorporate multi-wavelength constraints (e.g., DKIST/Cryo-NIRSP), and refine modeling of LOS integration and thermodynamics to improve agreement in active and high-latitude regions.

- Observations provide the POS component and LOS emissivity-weighted averages, not full vector fields; LOS integration can bias magnitude and direction. - Phase speeds are POS projections; geometry may lead to underestimates of true speeds and thus B. - S/N limitations, especially at higher altitudes and in polar regions, increase uncertainties and mask some areas. - Assumptions in coronal seismology (thin, unresolved flux tubes; low-beta; density diagnostics dominated by photo-excitation) introduce systematic uncertainties; multi-structure superposition can affect tracking. - Cadence and data gaps (weather, operations) limit complete daily coverage. - Model limitations: MAS uses synoptic magnetograms compiled over ~27 days without real-time evolution or polar coverage, empirical heating, and lower resolution than observations; PFSS lacks currents and thermodynamics, precluding emissivity weighting. - B-angle not corrected in coronal Carrington maps; occulter obscuration and POS 180° direction ambiguity remain.