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Presentation Mode : All
Conference Day : 04/08/2021
Time Slot : AM2 11:00 - 13:00
Sections : ST - Solar and Terrestrial Sciences










Solar and Terrestrial Sciences | Wed-04 Aug




ST16-A015 | Invited
Signatures of Magnetic Reconnection at the Footpoints of Fan-shaped Jets on a Light Bridge Driven by Photospheric Convective Motions

Xianyong BAI1#+, Hector SOCAS-NAVARRO2, Nóbrega-Siverio DANIEL3, Jiangtao SU1, Yuanyong DENG1, Dong LI4, Wenda CAO5, Kaifan JI1
1Chinese Academy of Sciences, China, 2Instituto de Astrofísica de Canarias, Spain, 3University of Oslo, Norway, 4Purple Mountain Observatory, Chinese Academy of Sciences, China, 5Big Bear Solar Observatory, New Jersey Institute of Technology, United States


Dynamical jets are generally found on light bridges (LBs), which are key to studying sunspot decay. So far, their formation mechanism is not fully understood. In this paper, we used state-of-the-art observations from the Goode Solar Telescope, the Interface Region Imaging Spectrograph, the Spectro-polarimeter on board Hinode, and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory to analyze the fan-shaped jets on LBs in detail. A continuous upward motion of the jets in the ascending phase is found from the Hα velocity that lasts for 12 minutes and is associated with the Hα line wing enhancements. Two mini jets appear on the bright fronts of the fan-shaped jets visible in the AIA 171 and 193 Å channels, with a time interval as short as 1 minute. Two kinds of small-scale convective motions are identified in the photospheric images, along with the Hα line wing enhancements. One seems to be associated with the formation of a new convection cell, and the other manifests as the motion of a dark lane passing through the convection cell. The finding of three-lobe Stokes V profiles and their inversion with the NICOLE code indicate that there are magnetic field lines with opposite polarities in LBs. From the Hα -0.8 Å images, we found ribbon-like brightenings propagating along the LBs, possibly indicating slipping reconnection. Our observation supports the idea that the fan-shaped jets under study are caused by magnetic reconnection, and photospheric convective motions play an important role in triggering the magnetic reconnection.

ST16-A018
Chromospheric Fan-shaped Surges and Photospheric Convections Observed by Gst

Yuzong ZHANG#+
National Astronomical Observatories, Chinese Academy of Sciences, China


On 25 June 2015, two kinds of chromospheric jets, one is a non-periodic fan-shaped surges, and the other is a fibril group, with an oscillation period of about 4.4 minutes, were observed in two light bridges (LBs) of the active region 12371. At the same time, the two LBs have significant but different photo- spheric convective patterns. In LB1, where the surges are intermittently occurring, there are mainly four convective patterns: 1) grains moving uni-directly along or off the axis of LB1; 2) shearing among grains; 3) being compressed central dark lane; 4) paroxysmal clusters of grains, which is first observed. In addi- tion, the appearance of the surges is highly consistent with the duration of the grain clusters. In contrast, the convection in LB2, where the fibril group oscillates, is relatively gentle and mainly consists of off-axis movement of grains. Where the grain off-axis movement is stronger, the corresponding fibril amplitude is higher. Therefore, it suggests that photospheric convection can indicate the type of chromospheric jets to a certain extent. As for the driving mechanisms of two kinds of jets, we think that there should be a twisted magnetic flux tube lying along LB1, a reconnection with the magnetic field of the core in the sunspot occurring when the grain cluster rushing into; and the lifting of the fibrils is driven by the leaking acoustic waves.

ST16-A002
Small-scale Bright Blobs Ejected from a Sunspot Light Bridge

Fuyu LI1+, Yajie CHEN2, Yijun HOU3, Hui TIAN4#, Xianyong BAI5, Yongliang SONG3
1Institute of Optics and Electronics, Chinese Academy of Sciences,, China, 2Max Planck Institute for Solar System Research, Germany, 3National Astronomical Observatories, Chinese Academy of Sciences, China, 4Peking University, China, 5Chinese Academy of Sciences, China


Light bridges (LBs) are bright lanes that divide an umbra into multiple parts in some sunspots. Persistent oscillatory bright fronts at a temperature of 10^5 K are commonly observed above LBs in the 1400/1330 A passbands of the Interface Region Imaging Spectrograph (IRIS). Based on IRIS observations, we report small-scale bright blobs from the oscillating bright front above a light bridge. Some of these blobs reveal a clear acceleration, whereas the others do not. The average speed of these blobs projected onto the plane of sky is 71.7 ±14.7 km/s, with an initial acceleration of 1.9 ±1.3 km/s^2. These blobs normally reach a projected distance of 3-7 Mm from their origin sites. From the transition region images we nd an average projected area of 0.57 ± 0.37 Mm^2 for the blobs. The blobs were also detected in multi-passbands of the Solar Dynamics Observatory, but not in the H images. These blobs are likely to be plasma ejections, and we investigate their kinematics and energetics. Through emission measure analyses, the typical temperature and electron density of these blobs are found to be around 10^5.47 K and 10^9.7 /cm^3, respectively. The estimated kinetic and thermal energies are on the order of 10^22.8 erg and 10^23.3 erg, respectively. These small-scale blobs appear to show three dierent types of formation process. They are possibly triggered by induced reconnection or release of enhanced magnetic tension due to interaction of adjacent shocks, local magnetic reconnection between emerging magnetic bipoles on the light bridge and surrounding unipolar umbral elds, and plasma acceleration or instability caused by upward shocks, respectively.

ST16-A006 | Invited
Generation of Solar Spicules and Subsequent Atmospheric Heating

Tanmoy SAMANTA#+
National Aeronautics and Space Administration, Marshall Space Flight Center, United States


Rapidly evolving fine-scale jets known as spicules are the most prominent and dynamical phenomena observed in the solar chromosphere. At any given instant, around a few million of these spicules shoot plasma material out from the Sun’s surface. It is highly likely that these spicules play a crucial role in key solar physics mysteries, such as chromospheric and coronal heating and mass supply to the solar wind. Despite intensive delving in the past decades, still, there is no clear consensus on how these small-jets of magnetized plasma originate from the solar surface, nor we understand how exactly they transfer energy into and possibly heat the solar atmosphere. The exact source of these small-scale jets is hard to observe due to the resolution limitations of earlier telescopes. Therefore, they remain poorly understood. Using an unprecedented multi-wavelength and high-sensitive magnetic field observations from the 1.6-m Goode Solar Telescope at the Big Bear Solar Observatory, we strive to reach conclusions on the possible scenario among the many proposed hypotheses of spicule’s origin. We found that the dynamical interaction of magnetic fields in the partially ionized lower solar atmosphere is the precursor of these high-speed jets which subsequently energizes the upper solar atmosphere.

ST16-A011
Exploring The Nature Of Type II Spicules Through GST Data

Vasyl YURCHYSHYN#+
New Jersey Institute of Technology, United States


Large- and small-scale jets and upflows observed in the lower atmosphere of quiet Sun (QS) areas  are considered to play an important role in the transfer of mass and energy from the dense chromosphere into the corona. However, their origin and connection to the dynamics of the magnetic fields are not yet well understood and explored.
Type II spicules are a subset of these small-scale phenomena discovered in off-limb Hinode data. They have on-disk counterparts identified with Ca II “straws” and rapid blueshifted excursions (RBEs).The formation process of type II spicules is thought to affect the corona by generating shocks, flows, waves, and currents, which can be linked to other phenomena such as the red–blue asymmetries observed in UV data as well as propagating coronal disturbances. Their detailed physical cause and role in providing mass and energy to the corona remain largely unknown.
The related difficulties in the interpretation of solar data mainly arise from the limited spatial resolution and complexity of the chromosphere. Although appear in regions of seemingly unipolar magnetic fields, recent high resolution data suggest that they may be a product of reconnection. Here we will present recent progress in studying type II spicules facilitated by data from Goode Solar Telescope. Various data sets and approaches to analysis all seem to indicate that these events result from magnetic reconenction driven by rapidly varying small-scale magnetic fields present in highly turbulent solar photosphere.

ST16-A010 | Invited
Umbral Oscillations and Magnetoconvection Inside Sunspots

Kyuhyoun CHO#+
Seoul National University, Korea, South


The umbral oscillations are regarded as an observational phenomenon of the slow MHD waves. Recent observational studies reported that the umbral oscillations were spatially and temporally associated with umbral dots. It is direct observational evidence for the connection between wave generation and small-scale magnetoconvection. In addition, horizontal propagation of the umbral oscillations gives information about the depth of the wave sources. It allows us to conjecture the vertical size of the convection cells. Therefore, the umbral oscillations can be a powerful tool for the investigation of the magnetoconvection inside sunspots, and furthermore, the substructure of sunspots.

ST16-A016
Contribution of Microturbulence to Spectral Line Broadening in Granular Convection Studied with Hinode SP

Ryohtaroh ISHIKAWA1#+, Yukio KATSUKAWA1, Takayoshi OBA2, David OROZCO SUÁREZ3, Masahito KUBO1, Yoshinori SUEMATSU1
1National Astronomical Observatory of Japan, Japan, 2Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Japan, 3Institute of Astrophysics of Andalusia, Spanish National Research Council, Spain


In the quiet regions on the solar surface, turbulent convective motions of the granulation play an important role in creating small-scale magnetic structures, as well as in injecting energy into the upper atmosphere. The turbulent nature of granulation can be studied using spectral line profiles, especially line broadening, which contains information on the flow field smaller than the spatial resolution of an instrument. In this study, we analyzed the spectral profiles obtained using the Spectro-Polarimeter of Hinode, and newly found significant line broadening in fading phase of granules. To investigate the mechanism of the line broadening, we performed spectral line inversion with the Stokes Inversion based on Response-function (SIR), comparing two inversion conditions: with and without microturbulence term. There are two possible scenarios to explain the observed spectral line broadening: one is microturbulence of about 1 km/s and the other is complicated gradients of Doppler velocity along the LOS. Although the distributions of temperature and vertical velocity estimated with and without microturbulence term are largely different in fading granules, it is difficult to distinguish them only with Fe I lines. Future multi-line observations with a ground-based telescope such as DKIST can determine the velocity distribution and enable us to resolve the scenarios.

ST16-A017
Impulsive Wave Excitation in the Quiet Sun by Rapidly Changing Granules

Hannah KWAK1#+, Jongchul CHAE2, Maria MADJARSKA3, Kyuhyoun CHO2, Donguk SONG4
1Korea Astronomy and Space Science Institute (KASI), Korea, South, 2Seoul National University, Korea, South, 3Max Planck Institute for Solar System Research, Germany, 4National Astronomical Observatory of Japan, Japan


Understanding how magnetohydrodynamic waves are excited in the interior and atmosphere of the Sun is still limited. 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. We report our observations of wave excitation by a new type of event: rapidly changing granules. Our observations were carried out by using the Fast Imaging Solar Spectrograph and the TiO 7057Å broadband filter imager of the 1.6m 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. After the granules' rapid changes, transient oscillations are detected in both of 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. In addition, in the Ca II - 0.5Å raster images, 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.



ST11-A007 | Invited
Dayside Transient Phenomena and Their Impact on the Global Magnetosphere

Hui ZHANG#+
SHANDONG UNIVERSITY (Weihai), China


Dayside transient phenomena are frequently observed upstream from the bow shock (such as Hot Flow Anomalies, foreshock cavities, and foreshock bubbles) and at the magnetopause (such as flux transfer events and surface waves). They play a significant role in the mass, energy and momentum transport from the solar wind into the magnetosphere and impact the global magnetosphere. They are universal phenomena that have been observed at Earth and other planets. The pressure variations associated dayside transient phenomena perturb the magnetopause, transmit compressional waves into the magnetosphere that can excite resonant ULF waves and cause particles to scatter into the loss cone and precipitate into the ionosphere, generate field-aligned currents in the magnetosphere that drive magnetic impulse events in the high-latitude ionosphere, and trigger transient auroral brightenings. This presentation will discuss the great progress made recently toward answering some specific outstanding science questions. Some outstanding questions are listed below. What are the physical differences and relationships between different transient phenomena at the bow shock? What are the formation conditions for the dayside transient phenomena? How does the magnetosphere respond to dayside transient phenomena? How do transient phenomena evolve with time?

ST11-A002
Energy Dissipation Via Magnetic Reconnection within the Coherent Structures of the Magnetosheath Turbulence

Shimou WANG#+, Rongsheng WANG, Quanming LU
University of Science and Technology of China, China


A series of intermittent coherent structures was observed in magnetosheath turbulence in the form of magnetic peaks. These magnetic peaks are always accompanied with enhancement of local current density, and three of them are studied in detail because of their intense current density. Based on the magnetic field signals, magnetic curvatures, and the toroidal magnetic field lines, three peaks are identified as magnetic flux ropes. In each trailing part of these three peaks, an extremely thin electron current layer was embedded within a much broader ion-scale current layer. The energy dissipation is evident within the peaks and direct evidence of magnetic reconnection was found within the thinnest electron current layer. The electrons were heated mainly in two regions of magnetic peaks, i.e. the reconnecting current layer by parallel electric field and the trailing edges by Fermi and betatron mechanisms. These results suggest that the ion-scale magnetic peaks are coherent structures associated with energy dissipation and electron heating in the magnetosheath. Thin current layers can be formed in magnetic peaks, and magnetic reconnection can play a significant role for the energy dissipation in magnetic peaks.  

ST11-A005 | Invited
Onset of Collisionless Magnetic Reconnection

Quanming LU#+, San LU, Dongkuan LIU, Rongsheng WANG
University of Science and Technology of China, China


Using particle-in-cell simulations, we investigate onset of magnetic reconnection from a quiescent current sheet in collisionless plasmas. After the current sheet is destabilized by the collisionless tearing mode instability, it proceeds to onset of reconnection, which manifests spontaneous thinning of current sheet and pileup of upstream magnetic flux. Once the current sheet thins to a critical thickness, about two electron inertial lengths, reconnection begins to grow explosively in this electron current sheet. In this stage, the process is governed by the electron dynamics, and only electrons are obviously accelerated. Then, magnetic reconnection expands from the electron scale to the ion scale, and ions are accelerated to about one Alfven speed in the outflow direction.

ST11-A006 | Invited
Fast Magnetic Field Annihilation in Electron-scale Current Sheet in Earth’s Magnetotail

Hiroshi HASEGAWA1#+, Richard DENTON2, Takuma NAKAMURA3, Kevin GENESTRETI4, Tai PHAN5
1Institute of Space and Astronautical Science, JAXA, Japan, 2Dartmouth College, United States, 3Austrian Academy of Sciences, Austria, 4University of New Hampshire, United States, 5University of California, Berkeley, United States


Magnetic reconnection is the key to fast release of magnetic energy in many space and astrophysical plasma systems, such as during magnetospheric substorms, but little observational information is available to understand exactly how magnetic-to-electron energy conversion occurs in electron-scale diffusion regions (EDRs). It is generally believed that the EDR has an X-type magnetic field geometry around which the energy of anti-parallel magnetic fields is mostly converted to electron bulk-flow energy. We present multi-spacecraft observations in Earth’s magnetotail of an elongated EDR in which, contrary to the standard model of reconnection, the fast energy conversion was caused mostly by magnetic field annihilation, rather than magnetic topology change. The experimental discovery of the annihilation-dominated EDR reveals a new form of energy conversion in the collisionless reconnection process.

ST11-A001
Role of Electromagnetic Turbulence in the Reconnection Current Layer

Keizo FUJIMOTO1#+, Richard SYDORA2
1Beihang University, China, 2University of Alberta, Canada


Intense electromagnetic waves are often observed in the reconnection current layer in space and laboratory plasmas. The waves near the reconnection x-line potentially have an impact on the magnetic dissipation through anomalous momentum transfer, driving the reconnection process that extends to a large scale. Recent 3D kinetic simulations have also demonstrated intense activities of electromagnetic waves in the thin current layer formed around the x-line. Nevertheless of these evidences in observations and simulations, the generation mechanisms of the waves and their roles in reconnection were poorly understood yet. The present study has carried out a large-scale 3D PIC simulation for the anti-parallel and no guide field configuration. The simulation results suggest that the electron Kelvin-Helmholtz instability (eKHI) plays a primary role in driving intense electromagnetic turbulence in the reconnection current layer, leading to the dissipation and electron heating. The turbulence intensity is significantly enhanced due to the ejection of magnetic islands from the current layer. It is found that the ions hardly react to the turbulence, which indicates that the turbulence does not cause siginificant momentum exchange between electrons and ions resulting in electrical resistivity. It is demonstrated that the dissipation is mainly caused by viscosity associated with electron momentum transport across the current layer. The present results suggest a fundamental modification of the current MHD models using the resistivity to generate the dissipation. In this talk, we will show the generation mechanism of the elecromagnetic waves and their impacts on the magnetic dissipation.

ST11-A008
Higher-order Differential Magnetic Diffusion Effect in MHD Simulations of Petschek Reconnection Model

Tohru SHIMIZU1#+, Keizo FUJIMOTO2
1Ehime University, Japan, 2Beihang University, China


Petschek (PK) reconnection model is widely studied for 60 years and a strong candidate mechanism to explain the solar flares and substorms. In MHD (magnetohydrodynamic) theory, the reconnection process is driven by electric resistivity based on classical Ohm's law, which is generally described as second-order differential magnetic diffusion effect in Faraday's law. Some previous studies suggest that the uniform electric resistivity results in Sweet-Parker (SP) reconnection model rather than PK model, but this suggestion is still a controversial topic.  In this paper, with 2D MHD simulations, the forth-order differential magnetic diffusion effect is compared with the classical second-order differential effect. All the diffusion coefficient examined here is assumed to be uniform in time and space, for both of second-order and forth-order effects. In fact, such forth-order differential magnetic diffusion effect is predicted in the recent 3D plasma kinetic simulations of fast magnetic reconnection process. In another perspective, such higher-order differential magnetic diffusion effect is universally included in the numerical truncation errors of numerical MHD simulations, which prevents the numerical explosions of simulations, where such higher-order differential effect generally leads to stronger damping of shorter wave length. In this paper, it is shown that the forth-order effect tends to change the magnetic reconnection process from SP-like model to PK-like model, in contrast to the second-order effect. This work is important for the cross-scale coupling problem of fast magnetic reconnection process between 3D plasma kinetic simulations and MHD simulations. Also, it is important to consider the numerical error problems universally included in MHD simulation of the reconnection process in extremely-thin current sheet.