Exploring Magnetic Loops and Serpentine Fields in the Quiet Sun with the GRIS-IFU

Abstract

Synthetic observations produced from radiative magnetohydrodynamic simulations have predicted that higher polarization fractions in the quiet solar photosphere would be revealed by increasing the total integration time of observations at GREGOR resolutions. We present recently acquired disk center observations of the Fe i 15648.5 Å line obtained with the GREGOR telescope equipped with the GRIS-IFU during excellent seeing conditions, showing exceptionally high polarization fractions. Our observations reveal an internetwork region with a majority (>60%) of magnetized pixels displaying a clear transverse component of the magnetic field. This result is in stark contrast to previous disk center GRIS-IFU observations in this spectral line, which had predominantly vertical magnetic fields in the deep photosphere. At the same time, the median magnetic field strength is weaker than previous GRIS-IFU observations, indicating that the larger fraction of polarization signals cannot be explained by a more active target. We use the Stokes Inversion based on Response functions (SIR) code to analyze the data, performing over 45 million inversions, and interrogate the impact of two conflicting approaches to the treatment of noise on the retrieval of the magnetic inclination and azimuth. We present several case studies of the zoo of magnetic features present in these data, including small-scale magnetic loops that seem to be embedded in a sea of magnetism, and serpentine fields, focusing on regions where full-vector spectropolarimetry has been achieved. We also present a new open-source Python 3 analysis tool, SIR Explorer, which we use to examine the dynamics of these small-scale magnetic features.

DKIST Unveils the Serpentine Topology of Quiet Sun Magnetism in the Photosphere

“…We have presented the first spectropolarimetric observations of the quiet Sun with DKIST, the first 4 m class and largest solar optical telescope ever built. With an estimated spatial resolution of 0 08, these observations represent the highest spatial resolution full-vector spectropolarimetric observations ever obtained of the quiet Sun; previous high-resolution observations were achieved by the Swedish Solar Telescope (0 16; e.g., Schnerr & Spruit 2011), the Sunrise balloon-borne experiment (0 14-0 16; e.g., Lagg et al 2010;Wiegelmann et al 2013), the GREGOR telescope (0 3-0 4; e.g., Lagg et al 2016;Campbell et al 2023), and the Hinode spacecraft (0 32; e.g., Lites et al 2008;Bellot Rubio & Orozco Suárez 2012). After locating at least 53 magnetic elements of interest, we curated three case studies and examined their physical properties in detail to showcase how the small-scale magnetic fields are spatially organized at this resolution.…”

“…For 1C, the α values of models 1 and 2 are 0.17 and 0.83, respectively; the f value is 22°; and the v mac is 0.94 km s −1 for both models. Campbell et al 2023). As in Asensio Ramos (2011), writing the expressions for Δλ B and Δλ d allows us to say that a line is in the weak field regime when the magnetic field strength fulfills…”

“…We find that the field strengths at these locations differ by 2 orders of magnitude, and at the high end of these extremes, B exceeds 2 kG. We are motivated to sanity-check these values with the weak and strong field approximations (WFA and SFA) in order to ensure that the inversions are well calibrated (Campbell et al 2021a(Campbell et al , 2023 by estimating the longitudinal magnetic flux density with the WFA and magnetic field strength with the SFA.…”

“…In the current work, we focus on the weaker, smallerscale magnetic structures and curate a few case studies. We used the SIR Explorer (SIRE) tool (Campbell 2023;Campbell et al 2023) to locate and analyze these case studies in the fullscan inversions. We define a magnetic loop as two patches of opposite-polarity circular polarization in close contact (i.e., within only a few pixels of each other) with linear polarization connecting them.…”

“…Near-infrared spectropolarimetric observations have a demonstrated ability to measure linear polarization more effectively compared to photospheric diagnostics in the visible (Martínez González et al 2008a;Lagg et al 2016;Campbell et al 2021a). Recently, Campbell et al (2023) revealed an internetwork region with a relatively large fraction of linear and circular polarization (23% versus 60%) such that a majority of the magnetized pixels displayed a clear transverse component of the magnetic field, despite having set Stokes profiles with a signal of <5σ n to zero before the inversion, where σ n is the noise level as determined by the standard deviation in the continuum. Observations of subgranular, small-scale magnetic loops with lifetimes of a few minutes, sizes of 1″-2″, and unambiguous transverse components at the PIL were made possible with the use of an integral field unit such that the spatiotemporal resolution was not compromised.…”