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1. Periodicities in an active region correlated with Type III radio bursts observed by Parker Solar Probe

   https://doi.org/10.1051/0004-6361/202039510  

   Cattell, Cynthia ; Glesener, Lindsay ; Leiran, Benjamin ; Dombeck, John ; Goetz, Keith ; Martínez Oliveros, Juan Carlos ; Badman, Samuel T. ; Pulupa, Marc ; Bale, Stuart D. ( June 2021 , Astronomy & Astrophysics)

   Context. Periodicities have frequently been reported across many wavelengths in the solar corona. Correlated periods of ~5 min, comparable to solar p -modes, are suggestive of coupling between the photosphere and the corona. Aims. Our study investigates whether there are correlations in the periodic behavior of Type III radio bursts which are indicative of nonthermal electron acceleration processes, and coronal extreme ultraviolet (EUV) emission used to assess heating and cooling in an active region when there are no large flares. Methods. We used coordinated observations of Type III radio bursts from the FIELDS instrument on Parker Solar Probe (PSP), of EUV emissions by the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) and white light observations by SDO Helioseismic and Magnetic Image (HMI), and of solar flare X-rays by Nuclear Spectroscopic Telescope Array (NuSTAR) on April 12, 2019. Several methods for assessing periodicities are utilized and compared to validate periods obtained. Results. Periodicities of ~5 min in the EUV in several areas of an active region are well correlated with the repetition rate of the Type III radio bursts observed on both PSP and Wind. Detrended 211 and 171 Å light curves show periodic profiles in multiple locations, with 171more »Å peaks sometimes lagging those seen in 211 Å. This is suggestive of impulsive events that result in heating and then cooling in the lower corona. NuSTAR X-rays provide evidence for at least one microflare during the interval of Type III bursts, but there is not a one-to-one correspondence between the X-rays and the Type III bursts. Our study provides evidence for periodic acceleration of nonthermal electrons (required to generate Type III radio bursts) when there were no observable flares either in the X-ray data or the EUV. The acceleration process, therefore, must be associated with small impulsive events, perhaps nanoflares.« less
2. Magneto-acoustic oscillations observed in a solar plage region

   https://doi.org/10.1088/1674-4527/21/7/179  

   Ji, Haisheng ; Hashim, Parida ; Hong, Zhenxiang ; Xu, Zhe ; Shen, Jinhua ; Ji, Kaifan ; Cao, Wenda ( August 2021 , Research in Astronomy and Astrophysics)

   Abstract We gave an extensive study for the quasi-periodic perturbations on the time profiles of the line of sight (LOS) magnetic field in 10 × 10 sub-areas in a solar plage region (corresponds to a facula on the photosphere). The perturbations are found to be associated with the enhancement of He I 10830 Å absorption in a moss region, which is connected to loops with million-degree plasma. FFT analysis to the perturbations gives a kind of spectrum similar to that of Doppler velocity: a number of discrete periods around 5 minutes. The amplitudes of the magnetic perturbations are found to be proportional to magnetic field strength over these sub-areas. In addition, magnetic perturbations lag behind a quarter of the cycle in the phase with respect to the p-mode Doppler velocity. We show that the relationships can be well explained with an MHD solution for the magneto-acoustic oscillations in high- β plasma. Observational analysis also shows that, for the two regions with the stronger and weaker magnetic field, the perturbations are always anti-phased. All findings show that the magnetic perturbations are actually magneto-acoustic oscillations on the solar surface, the photosphere, powered by p-mode oscillations. The findings may provide a new diagnosticmore »tool for exploring the relationship between magneto-acoustic oscillations and the heating of the solar upper atmosphere, as well as their role in helioseismology.« less
3. Impulsive wave excitation by rapidly changing granules

   https://doi.org/10.1051/0004-6361/202038288  

   Kwak, Hannah ; Chae, Jongchul ; Madjarska, Maria S. ; Cho, Kyuhyoun ; Song, Donguk ( October 2020 , Astronomy & Astrophysics)

   It is not yet fully understood how magnetohydrodynamic waves in the interior and atmosphere of the Sun are excited. Traditionally, turbulent convection in the interior is considered to be the source of wave excitation in the quiet Sun. Over the last few decades, acoustic events observed in the intergranular lanes in the photosphere have emerged as a strong candidate for a wave excitation source. Here we report our observations of wave excitation by a new type of event: rapidly changing granules. Our observations were carried out with the Fast Imaging Solar Spectrograph in the H α and Ca  II 8542 Å lines and the TiO 7057 Å broadband filter imager of the 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory. We identify granules in the internetwork region that undergo rapid dynamic changes such as collapse (event 1), fragmentation (event 2), or submergence (event 3). In the photospheric images, these granules become significantly darker than neighboring granules. Following the granules’ rapid changes, transient oscillations are detected in the photospheric and chromospheric layers. In the case of event 1, the dominant period of the oscillations is close to 4.2 min in the photosphere and 3.8 min in the chromosphere.more »Moreover, in the Ca  II –0.5 Å raster image, we observe repetitive brightenings in the location of the rapidly changing granules that are considered the manifestation of shock waves. Based on our results, we suggest that dynamic changes of granules can generate upward-propagating acoustic waves in the quiet Sun that ultimately develop into shocks.« less
4. Quasiperiodic Microjets Driven by Granular Advection as Observed With High-resolution Imaging at He i 10830 Å

   https://doi.org/10.3847/1538-4357/ac590c  

   Hong, Zhenxiang ; Wang, Ya ; Ji, Haisheng ( April 2022 , The Astrophysical Journal)

   Abstract With high-resolution narrowband He i 10830 Å filtergrams from Goode Solar Telescope, we give an extensive analysis for four granule-sized microeruptions which appear as the gentle ejection of material in He i 10830 Å band. The analysis was aided with the EUV data from Atmospheric Imaging Assembly and line-of-sight magnetograms from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. The microeruptions are situated on magnetic polarity inversion lines (PILs), and their roots are accurately traced down to intergranular lanes. Their durations are different: two microeruptions are repetitive microjets, lasting ∼50 and 27 minutes respectively, while the other two events are singular, lasting ∼5 minutes. For the two microjets, they are continuous and recurrent in the He i 10830 Å band, and the recurrence is quasiperiodic with a period of ∼5 minutes. We found that only transient cospatial EUV brightenings are observed for the longer duration microjets and EUV brightenings are absent for the two singular microeruptions. What is essential to the longer duration microjets is that granules with the concentration of a positive magnetic field persistently transport the magnetic field to the PILs, canceling the opposite magnetic flux and making the base of the two microjetsmore »and the underlying granules migrate with the speed of ∼0.25 and 1.0 km s −1 . The observations support the scenario of magnetic reconnection for the quasiperiodic microjets and further show that the reconnection continuously generates multitemperature components, especially the cool component with chromospheric temperature. In addition, the ongoing reconnection is modulated by p-mode oscillations inside the Sun.« less
5. The Precursor Phase of an X-class Flare: Magnetic Reconnection, Powering and Non-thermal Electrons

   https://doi.org/10.1088/1674-4527/ac389b  

   Shen, Jinhua ; Ji, Haisheng ; Su, Yingna ( January 2022 , Research in Astronomy and Astrophysics)

   Abstract In this paper, we report three interesting phenomena that occurred during the precursor phase of the X1.6 class flare on 2014 September 10. (1) The magnetic reconnection initiating the flare occurs between one of the two J-shaped magnetic flux ropes that constitute a sigmoidal structure and the overlying sheared magnetic arcade that runs across the sigmoid over its middle part. The reconnection formed an erupting structure that ultimately leads to flare onset. Another J-shaped magnetic flux rope remains unaffected during the whole eruption. The phenomenon is revealed by the observation made by the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory (SDO) at 94 and 131 Å. (2) Being simultaneously with starting time of the precursor, photospheric vertical electric current (VEC) around the footpoint region of the overlying magnetic arcade underwent an obvious increase, as observed by the Helioseismic and Magnetic Imager (HMI) on board SDO. By only taking into account the VEC with current density over 3 σ value (1 σ : 10 mA m −2 ), we are able to pick out precursor-associated VEC increase starting from nearly the level of zero. We regard it as a kind of powering process for the magnetic reconnection betweenmore »the two magnetic loops. (3) With high-resolution narrow-band Helium 10830 Å images taken by Goode Solar Telescope at Big Bear Solar Observatory (BBSO), we observe a narrow absorption (dark) front that runs along the erupting magnetic structure (or the erupting hot channel) and moves in the direction of the eruption during the precursor phase. Assuming the excitation mechanism of Helium atoms along the absorption front by non-thermal electrons, the phenomenon shows that the interaction between the erupted hot channel and the overlying (or surrounding) magnetic field has yielded electron acceleration.« less