observations of mini coronal dimmings caused by small-scale eruptions in the quiet sun

(Solar Orbiter nugget #19 by R. Wang¹, Y. D. Liu^1,2, X. Zhao^3,4, & H. Hu¹ ​)

Introduction

Solar eruptive activities, including flares, eruptive filaments, CMEs, and coronal jets, are widely distributed in energy scales. The observed solar eruptions span a range of at least a factor of 10⁸ in energy [1]. Large, highly energetic “X-class” eruptions can release energy substantially exceeding ∼ 10³³ ergs. Small-scale eruptions could play an important role in coronal heating, generation of SEPs, and mass source of the solar wind. However, they are poorly observed and their characteristics, distributions, and origins remain unclear.

Here, a mini coronal dimming was captured by the recently launched Solar Orbiter spacecraft. Coronal dimmings are considered to be an important characteristic of solar eruptions, which can provide mass information of eruptions.

Aims and Methods

We identify the characteristic structures associated with a small-scale eruption with greater certainty, owing to the 17.4 nm EUV HRI (HRIEUV ) of the EUI [2] on board Solar Orbiter. Solar Dynamics Observatory allows us to carry out a joint observation of this small event (see Figure 1).

Figure 1. Joint observations of Solar Orbiter and Solar Dynamics Observatory on 20 May 2020. (a) Heliospheric positions of planets and spacecraft. (b) Location of the mini coronal dimming (red arrow) in HRIEUV 174 A. (c) and (d) Enlarged views of the dimming at different scales (see animation). (e) Post-eruption arcades (traced by red dotted curves) in HRIEUV 174A, with location shown by the green arrow in (c). (f) Flare ribbons in SDO/AIA 1600A. (g) SDO/HMI LOS magnetic fields.

Accompanying video for Figure 1

We use a differential emission measure (DEM; [3]) analysis with the AIA [4] EUV images at six passbands to examine the temperature and density properties of the coronal dimming (see Figure 2), and further to determine the CME mass from the dimming [5]. We aim to understand small-scale eruptions and their contributions to SEPs, the solar wind, and space weather.

Figure 2. DEM analysis of the coronal dimming. (a-d) Time evolution of AIA 0.7 MK DEMs at four typical　times. (e-f) EUV light curves of the normalized integrated intensities of the are region (black contour) and　the dimming region within the heart-shaped area (red contour). (g) Averaged EM (black) and EM-weighted　median temperature (red) within the dimming region as a function of time. The green vertical dashed lines　mark the corresponding times in (a-d).

To determine the total evacuated mass M of the eruption, we use the following equation:

where n is the total number of pixels within the heart-shaped area in Fig.2, µ is the mean molecular weight of the hydrogen ion (µ = 1.27), m_p is the proton mass, A_s is the area of the pixel, the integral of N(z) the total number of coronal plasma at each pixel in the dimming region along the LOS, and lambda_p the pressure scale height defined as: 

where k_B is the Boltzmann constant, T_e the electron temperature at each pixel calculated from the DEM analysis, and g the gravitational acceleration at the solar surface.

We obtain the value of the basal electron density N₀ with the help of the EM measurement (for a detailed derivation, we refer to the previous implementation about CME mass determination in [5]): the EM is the square density averaged along the line of sight, and it is also based on an isothermal hydrostatic equilibrium assumption that the density of the coronal plasma in the dimming region is radially stratified [6]. Therefore: 

where R_O is the solar radius, and I the integral quantity defined as:

We have set L = 5 lambda_p in order to include as much mass as possible along the LOS.

The total evacuated mass is determined by the difference between the mass M_before before the eruption and the mass M_after after the eruption, i.e.

Conclusions

The observations indicate that magnetic cancellation occurred before the eruption and likely resulted in a mini filament eruption. Then, the eruption results in the dimming and takes away approxmately 1.65 +/- 0.54 x 10¹³ g of mass, which also exhibits similar features as the sources of SEP events [7]. Our results fit the energy-mass relation [8], i.e., an eruption of Delta E_free ≈ 10²⁷ ergs carries the mass in the order of 10¹³ g. The results suggest that weak constraining force makes the flux rope associated with the mini filament easily enter a torus-unstable domain and propagate slowly along a radial direction in the lower corona (see Figure 3). We discuss that weak magnetic constraints from low-altitude background fields may be a general condition for the quiet-Sun eruptions, which provides a possible mechanism for the transport of coronal material and energy from the lower to the middle or even higher corona. We have also made a survey of these small dimming events by Solar Orbiter for the first time (see the Table 1 in the published paper).

Figure 3. Magnetic topological structure of the small-scale filament (prominence) and its ascending process in the STEREO-A/EUVI view (see animation). (a) Reconstructed flux rope structure with the EUV background at 304A. The positive (white) in [20, 50] G and negative (black) fluxes in [-20, -50] G are overplotted. The rising part of the mini filament is contoured in cyan dashed lines in (b). (c) Side view from STEREO-A/EUVI of the ascending filament (red arrows) along a red radial line which shows the slit for the time-distance map in (d). The red cross marks the approximate position of the eruption source.

Accompanying video for Figure 3

This study has been published in Rui Wang, et al. 2023, ApJL, 952, L29, doi:https://doi.org/10.3847/2041-8213/ace437.

Acknowledgements:

The research was supported by National Key R&D Program of China No. 2022YFF0503800 and No.2021YFA0718600, NSFC under grants 12073032, 42274201, 42004145 and 42150105, the Specialized

Research Fund for State Key Laboratories of China. We acknowledge the use of data from Solar Orbiter and SDO.

Affiliations:

¹State Key Laboratory of Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing 100190, People's Republic of China

²University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China

³Key Laboratory of Space Weather, National Satellite Meteorological Center (National Center for Space Weather), China Meteorological Administration, Beijing 100081, People's Republic of China

⁴School of Earth and Space Sciences, Peking University, Beijing 100871, People's Republic of China

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