Anomalously Strong Localized First Ionization Potential Effect Associated with a Solar Subflare

Abstract

Plasma composition in the solar corona commonly differs from that of the photosphere, with the enhancement of low--first-ionization-potential (FIP) elements referred to as the FIP effect. This phenomenon provides important diagnostics of energy and mass transport between different layers of the solar atmosphere. In this work, we analyze an anomalously strong, localized FIP effect observed in active region 13486 associated with a subflaring episode on 2023 November 17, using multiwavelength observations combining high energy-resolution soft X-ray disk-integrated spectra obtained by the Macao Science Satellite-1B with spatially resolved EUV/UV and H$α$ imaging from Hinode/EIS, SDO/AIA and HMI, and CHASE/HIS. By investigating the temporal evolution of plasma composition in response to changes in magnetic field orientation, we provide new insight into the physical processes linking magnetic reconnection, ponderomotive force fractionation, and coronal abundance anomalies. This work reveals that the anomalously strong enhancement of low-FIP elements is localized in regions with strongly inclined magnetic fields despite a subflare. We interpret these observations within the framework of the ponderomotive force fractionation model and propose that the inclined magnetic geometry enhances the transmission of upward-propagating magnetohydrodynamic waves by reducing reflection near the plasma-$β$$\simeq$1 layer, enhancing FIP fractionation associated with a consequential upward-directed ponderomotive force. In addition, sustained chromospheric heating associated with chromospheric reconnection and flux cancellation appears to maintain the enhanced FIP effect for tens of minutes following the event.

Figures (6)

• Figure 1: Overview of the targeted AR 13486 observed on 2023 November 17. Context images from SDO/AIA, with the field of view (FOV) spanning from the bottom-left corner at [750, 280] to the top-right corner at [1000, 70], are displayed in the upper panels (a)--(t). Each row corresponds to a representative time during the evolution of the micro and subflaring activities associated with the AR, while each column shows observations in different AIA channels: (a)--(d) 94 Å, (e)--(h) 131 Å, (i)--(l) 193 Å, (m)--(p) 304 Å, and (q)--(t) 1600 Å. The channels and selected observation times are indicated in the legends. The following horizontal panel (u) shows the normalized light curves from the defined FOV for the AIA 94 Å (dark green), 304 Å (bright green), and 1600 Å (blue), with the soft X-ray (SXR) flux from GOES 1--8 Å (black) and 0.5--4.0 Å (red) overlaid. The AIA curves are smoothed using a moving average with a boxcar width of 5, and the GOES curves are smoothed using a median filter. Orange and blue overlays highlight the time intervals corresponding to microflare (peaking at 06:45 UT in the 304 Å channel) and subflare (peaking at 07:26 UT in the 304 Å channel) activity, respectively. Dashed blue vertical lines in the middle panel indicate the time intervals covered by CHASE H$\alpha$ line-core observations, with the corresponding light curves from the same FOV plotted in the bottom panels: (v) 05:58--06:18 UT and (w) 07:32--07:53 UT.
• Figure 2: Fitted MSS-1B SXR spectra on 2023 November 17 for (a) 03:20--03:45 UT, (b) 07:27--07:33 UT, and (c) 07:33--08:05 UT, obtained using the 2-$T$ model (2vth_abun). The selected fitting time periods are described in the text (see Section \ref{['sec_mssFIP']}). In each panel, the upper row displays the model fit (red curve), background-subtracted spectrum (black histogram), and background spectrum (gray histogram), all in units of $\mathrm{counts~s^{-1}~keV^{-1}}$. The best-fit temperatures and emission measures, together with the FIP biases for Ca, S, and Ar (with 1$\sigma$ uncertainties), are indicated in the upper-right corner of each panel, along with the reduced chi-square ($\mathrm{\chi_r^2}$) values. The FIP bias is defined as the ratio of the measured elemental abundance to its photospheric one, with coronal abundances adopted from feldmanElementalAbundancesUpper1992 and photospheric abundances from asplundChemicalCompositionSun2009. The lower row shows the normalized residuals, defined as the differences between the observed and best-fit fluxes divided by the $1\sigma$ statistical uncertainties.
• Figure 3: Maps of (a) intensity and (b) Doppler velocity from Fe12 195.12 Å line, as measured by Hinode/EIS. Velocities are given in units of $\mathrm{km~s^{-1}}$, with blue/red colors in panel (b) indicating plasma motion (or emission) toward/away from the observer, respectively. To provide context for the photospheric magnetic field, the LOS magnetogram from SDO/HMI, selected at the time closest to the start of the EIS raster observation, is shown in panel (c). The middle row displays FIP bias maps derived from the line ratios: (d) Ca14 193.87 Å/Ar14 194.40 Å, (e) Fe16 262.98 Å/S13 256.69 Å, and (f) Si10 258.38 Å/S10 264.23 Å. The bottom row (g)--(i) shows the corresponding KDE distributions of pixel values within the ROIs: ROI-BC, ROI-TL, and ROI-TR, indicated by purple, green, and blue rectangles, respectively, on the FIP bias maps. For each ROI, the peak KDE value is denoted by a dashed horizontal line with the value labeled adjacent to the line.
• Figure 4: Periodicity of AIA and CHASE light curves extracted from selected ROIs. Each column corresponds to the same region of interest (FOV, defined in Figure \ref{['fig_event']}; ROI-BC, ROI-TL, and ROI-TR, as in Figure \ref{['fig_FIP_20231117_075319_KDE_overlaidBlos']}), while each row corresponds to the same wavelength: AIA 304 Å (a--d), 1600 Å (e--h), and 1700 Å (i--l), over the observation period from 05:15 to 09:50 UT on 2023 November 17. The two bottom panels show CHASE H$\alpha$ line-core observations in the FOV defined in Figure \ref{['fig_event']} from 05:58--06:18 UT (left) and 07:32--07:53 UT (right) on 2023 November 17. In each panel, the left column shows the Morlet wavelet power spectrum, whitened by the background model and scaled for display. The green dashed and yellow solid contours enclose regions where the wavelet power exceeds the 95% local and global confidence levels, respectively. The gray hatched region denotes the cone of influence (COI), where edge effects dominate and the period signals are not statistically significant. The right column presents the time-averaged wavelet (black curve) and Fourier (gray histogram) power spectra. The background model fitted to the Fourier spectrum is shown by the blue solid line, with its mean components indicated by the blue dashed lines. The corresponding fitted power-law index $s$ and the reduced chi-square ($\chi^2$) value are displayed in the bottom-right (or top-right) corner. The 95% local and global time-averaged confidence levels are indicated by the green dashed and yellow solid lines, respectively, while the 95% confidence level of the Fourier spectrum is shown by the gray solid line.
• Figure 5: Inclination angle distributions derived from SDO/HMI SHARP vector magnetograms for (a) 03:46 UT and (b) 07:46 UT on 2023 November 17. The three ROIs---ROI-BC, ROI-TL, and ROI-TR---are outlined by purple, green, and blue polygons, respectively. The overlaid contours show the LOS magnetic field at $\pm150$ G, with green and white indicating positive and negative polarities, respectively. The bottom panels show the corresponding KDE distributions for (c) ROI-BC, (d) ROI-TL, and (e) ROI-TR, with the 03:46 UT and 07:46 UT intervals denoted as the blue and red curves, respectively.