Heating signatures from small-scale magnetic reconnection events in the solar atmosphere have proven to be difficult to detect through observations. Numerical models that reproduce flaring conditions are essential in the understanding of how nanoflares may act as a heating mechanism of the corona. We study the effects of non-thermal electrons in synthetic spectra from 1D hydrodynamic RADYN simulations of nanoflare heated loops to investigate the diagnostic potential of chromospheric emission from small-scale events. The Mg II h and k, Ca II H and K, Ca II 854.2 nm, H-alpha and H-beta chromospheric lines were synthesised from various RADYN models of coronal loops subject to electron beams of nanoflare energies. The contribution function to the line intensity was computed to better understand how the atmospheric response to the non-thermal electrons affects the formation of spectral lines and the detailed shape of their spectral profiles. The spectral line signatures arising from the electron beams highly depend on the density of the loop and the lower cutoff energy of the electrons. Low-energy (5 keV) electrons deposit their energy in the corona and transition region, producing strong plasma flows that cause both redshifts and blueshifts of the chromospheric spectra. Higher-energy (10 and 15 keV) electrons deposit their energy in the lower transition region and chromosphere, resulting in increased emission from local heating. Our results indicate that effects from small-scale events can be observed with ground-based telescopes, expanding the list of possible diagnostics for the presence and properties of nanoflares.
We present the results of 1D hydrodynamic simulations of coronal loops which are subject to nanoflares, caused by either in-situ thermal heating, or non-thermal electrons (NTE) beams. The synthesized intensity and Doppler shifts can be directly compared with IRIS and AIA observations of rapid variability in the transition region (TR) of coronal loops, associated with transient coronal heating. We find that NTE with high enough low-energy cutoff (E$_\textrm{C}$) deposit energy in the lower TR and chromosphere causing blueshifts (up to~$\sim$~20 km/s) in the \emph{IRIS} \siiv~lines, which therma…
The million degree plasma of the solar corona must be supplied by the underlying layers of the atmosphere. The mechanism and location of energy release, and the precise source of coronal plasma, remain unresolved. In earlier work we pursued the idea that warm plasma is supplied to the corona via direct heating of the chromosphere by nanoflares, contrary to the prevailing belief that the corona is heated in-situ and the chromosphere is subsequently energized and ablated by thermal conduction. We found that single (low-frequency) chromospheric nanoflares could not explain the observed intensitie…
Nanoflare models for heating the solar corona usually assume magnetic braiding and reconnection as the source of the energy. However, recent observations at record spatial resolution from the {\sc Sunrise} balloon mission suggest that photospheric magnetic flux cancellation is much more common than previously realised. We therefore examine the possibility of three-dimensional reconnection driven by flux cancellation as a cause of chromospheric and coronal heating. In particular, we estimate how the heights and amount of energy release produced by flux cancellation depend on flux size, flux can…
During a solar flare, it is believed that reconnection takes place in the corona followed by fast energy transport to the chromosphere. The resulting intense heating strongly disturbs the chromospheric structure, and induces complex radiation hydrodynamic effects. Interpreting the physics of the flaring solar atmosphere is one of the most challenging tasks in solar physics. Here we present a novel deep learning approach, an invertible neural network, to understanding the chromospheric physics of a flaring solar atmosphere via the inversion of observed solar line profiles in Hα and Ca II λ8542.…
We present a statistical study of chromospheric evaporation in solar flares using simultaneous observations by the RHESSI X-ray telescope and the IRIS UV spectrograph. The results are compared with radiation hydrodynamic flare models from the F-CHROMA RADYN database. For each event, we study spatially-resolved Doppler shifts of spectral lines formed in the transition region (C\,II\,1334.5\,Å) and hot coronal plasma (Fe\,XXI\,1354.1\,Å) to investigate the dynamics of the solar atmosphere during the flare impulsive phase. We estimate the energy fluxes deposited by high-energy electrons using X-r…
Recent observations at high spatial resolution have shown that magnetic flux cancellation occurs on the solar surface much more frequently than previously thought, and so this led Priest et al 2018 to propose magnetic reconnection driven by photospheric flux cancellation as a mechanism for chromospheric and coronal heating. In particular, they estimated analytically the amount of energy released as heat and the height of the energy release during flux cancellation. In the present work, we take the next step in the theory by setting up a two-dimensional resistive MHD simulation of two canceling…
Inspired by recent observations suggesting that photospheric magnetic flux cancellation occurs much more frequently than previously thought, we analytically estimated the energy released from reconnection driven by photospheric flux cancellation, and proposed that it can act as a mechanism for chromospheric and coronal heating (Priest et al., 2018). Using two-dimensional simulations we validated the analytical estimates and studied the resulting atmospheric response (Syntelis et al. 2019). In the present work, we set up three-dimensional resistive MHD simulations of two cancelling polarities i…
Magnetic energy is required to heat the corona, the outer atmosphere of the Sun, to millions of degrees. We study the nature of the magnetic energy source that is probably responsible for the brightening of coronal loops driven by nanoflares in the cores of solar active regions. We consider observations of two active regions (ARs), 11890 and 12234, in which nanoflares have been detected. To this end, we use ultraviolet (UV) and extreme ultraviolet (EUV) images from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) for coronal loop diagnostics. These images are…
Nanoflares are regarded as one of the major mechanisms of magnetic energy release and coronal heating in the solar outer atmosphere. We conduct a statistical study on the response of the chromosphere and transition region to nanoflares, as observed by the Interface Region Imaging Spectrograph (IRIS), by using an algorithm for the automatic detection of these events. The initial atmospheric response to these small heating events is observed, with IRIS, as transient brightening at the footpoints of coronal loops heated to high temperatures (>4 MK). For four active regions, observed over 143 hour…
Ellerman bombs (EBs) are brightenings in the H$α$ line wings that are believed to be caused by magnetic reconnection in the lower atmosphere. To study the response and evolution of the chromospheric line profiles, we perform radiative hydrodynamic simulations of EBs using both non-thermal and thermal models. Overall, these models can generate line profiles that are similar to observations. However, in non-thermal models we find dimming in the H$α$ line wings and continuum when the heating begins, while for the thermal models dimming occurs only in the H$α$ line core, and with a longer lifetime…
Solar Flare Chromospheric Line Emission: Comparison Between IBIS High-resolution Observations and Radiative Hydrodynamic Simulations
太阳耀斑涉及脉冲能量释放，这导致在宽光谱范围和宽高度范围内的辐射增强。特别是，从色球层的线发射可以提供等离子体加热过程的关键诊断。因此，高分辨率光谱观测和先进的数值模拟结果之间的直接比较可能是非常有价值的，但尚未尝试。在本文中，我们提出了在本文中这样一个自洽调查的M3.0耀斑观测的邓恩太阳望远镜的干涉二维光谱仪（IBIS）在2011年9月24日，我们与辐射流体动力学代码RADYN建模。我们用IBIS在H $α$ 6563和Ca II 8542中获得了耀斑区的图像和光谱，并用X射线的Ramaty高能太阳光谱成像仪（RHESSI）获得了耀斑区的图像和光谱。后者的观测被用来推断非热电子的人口，这是通过RADYN模拟大气响应电子碰撞加热。然后，我们合成了谱线，并将它们的形状和强度与IBIS观察到的进行了比较，发现两者基本一致。特别是，合成的Ca II 8542钙离子谱与观察到的谱拟合良好，而合成的H $α$谱在岩心中比观察到的更暗。这表明H $α$发射比Ca II 8542发射对非热电子通量更敏感。我们认为，有必要改进能量输入和其他过程是必要的，以解决这种差异。
Halpha line profile asymmetries and the chromospheric flare velocity field
在太阳耀斑谱线轮廓中观察到的不对称性可以为耀斑大气的性质和动力学提供重要的诊断。本文利用瑞典1米太阳望远镜对M1.1耀斑的高空间、时间和光谱分辨率地面观测资料研究了Halalpha和Ca II 8542耀斑谱线的演化。从耀斑内核的H α线轮廓的时间演变显示过量的排放在红翼（红色不对称）耀斑最大之前，和过量的蓝翼（蓝色不对称）最大之后。然而，Ca II 8542的红外线并不遵循相同的模式，在耀斑期间只显示出微弱的红色不对称性。用RADYN模拟方法合成了耀斑大气的谱线廓线，与观测结果符合较好。我们发现，在Halpha中观察到的红色不对称性不一定与等离子体向下流动，蓝色不对称性可能与等离子体向上流动无关。事实上，我们的结论是，陡峭的速度梯度在耀斑色球修改的波长的中心反转的H α线的配置文件。最大不透明度的波长向较短和较长波长的偏移分别产生红色和蓝色不对称性。
We present radiative hydrodynamic simulations of solar flares generated by the RADYN and RH codes to study the perturbations induced in photospheric Fe I lines by electron beam heating. We investigate how variations in the beam parameters result in discernible differences in the induced photospheric velocities. Line synthesis revealed a significant chromospheric contribution to the line profiles resulting in an apparent red asymmetry by as much as 40 m/s close to the time of maximum beam heating which was not reflective of the upflow velocities that arose from the radiative hydrodynamic simula…
We study the evolution of chromospheric line and continuum emission during the impulsive phase of the X-class SOL2014-09-10T17:45 solar flare. We extend previous analyses of this flare to multiple chromospheric lines of Fe I, Fe II, Mg II, C I, and Si II, observed with IRIS, combined with radiative-hydrodynamical (RHD) modeling. For multiple flaring kernels, the lines all show a rapidly evolving double-component structure: an enhanced, emission component at rest, and a broad, highly red-shifted component of comparable intensity. The red-shifted components migrate from 25-50 km s$^{-1}$ towards…
Red-shifted components of chromospheric emission lines in the hard X-ray impulsive phase of solar flares have recently been studied through their 30 s evolution with the high resolution of IRIS. Radiative-hydrodynamic flare models show that these redshifts are generally reproduced by electron-beam generated chromospheric condensations. The models produce large ambient electron densities, and the pressure broadening of hydrogen Balmer series should be readily detected in observations. To accurately interpret upcoming spectral data of flares with the DKIST, we incorporate non-ideal, non-adiabati…
The 2014 March 29 X1 solar flare (SOL20140329T17:48) produced bright continuum emission in the far- and near-ultraviolet (NUV) and highly asymmetric chromospheric emission lines, providing long-sought constraints on the heating mechanisms of the lower atmosphere in solar flares. We analyze the continuum and emission line data from the Interface Region Imaging Spectrograph (IRIS) of the brightest flaring magnetic footpoints in this flare. We compare the NUV spectra of the brightest pixels to new radiative-hydrodynamic predictions calculated with the RADYN code using constraints on a nonthermal …
We proposed that the use of an approximate "jump condition" at the solar transition region permits fast and accurate numerical solutions of the one dimensional hydrodynamic equations when the corona undergoes impulsive heating. In particular, it eliminates the need for the very short timesteps imposed by a highly resolved numerical grid. This paper presents further examples of the applicability of the method for cases of non-uniform heating, in particular, nanoflare trains (uniform in space but non-uniform in time) and spatially localised impulsive heating, including at the loop apex and base …
Context. Recent observations have shown that magnetic flux cancellation occurs at the photosphere more frequently than previously thought. Aims. In order to understand the energy release by reconnection driven by flux cancellation, we previously studied a simple model of two cancelling polarities of equal flux. Here, we further develop our analysis to achieve a more general setup where the two cancelling polarities have unequal magnetic fluxes and where many new features are revealed. Methods. We carried out an analytical study of the cancellation of two magnetic fragments of unequal and oppos…
Lower atmospheric lines show peculiar profiles at the leading edge of ribbons during solar flares. In particular, increased absorption of the BBSO/GST \hei~10830~Å line \citep[e.g.][]{Xu2016}, as well as broad and centrally reversed profiles in the spectra of the \mgii~and \cii~lines observed by the \iris~satellite \citep[e.g.][]{Panos2018,Panos2021a} have been reported. In this work, we aim to understand the physical origin of the \iris\ ribbon front line profiles, which seem to be common of many, if not all, flares. To achieve this, we quantify the spectral properties of the \iris~\mgii~ribb…
We observed rapid variability ($\lesssim 60$ s) at the footpoints of transient hot ($\sim 8-10$ MK) coronal loops in active region cores, with the Interface Region Imaging Spectrograph (IRIS). The high spatial ($\sim 0.33$ arcsec) and temporal ($\lesssim 5$-10 s) resolution is often crucial for the detection of this variability. We show how, in combination with 1D RADYN loop modeling, these IRIS spectral observations of the transition region (TR) and chromosphere provide powerful diagnostics of the properties of coronal heating and energy transport (thermal conduction and/or non-thermal electr…
Solar flares show highly unusual spectra, in which the thermodynamic conditions of the solar atmosphere are encoded. Current models are unable to fully reproduce the spectroscopic flare observations, especially the single-peaked spectral profiles of the MgII h and k lines. We aim at understanding the formation of the chromospheric and optically thick MgII h and k lines in flares through radiative transfer calculations. We take a flare atmosphere obtained from a simulation with the radiative hydrodynamic code RADYN as input for a radiative transfer modeling with the RH code. By iteratively chan…
The He I 10830 Å triplet is a very informative indicator of chromospheric activities as the helium is the second most abundant element in the solar atmosphere. Taking advantage of the high resolution of the 1.6 m Goode Solar Telescope (GST) at Big Bear Solar Observatory (BBSO), previous observations have shown clear evidence of the enhanced absorption, instead of typically-observed emission, for two M-class flares. In this study, we analyze the evolution of the He I 10830 10830 Å emission in numerical models and compare it with observations. The models represent the RADYN simulation results ob…
The goal of this paper is to study the smallest brightening events observed in the EUV quiet Sun. We use commissioning data taken by the EUI instrument onboard the recently launched Solar Orbiter mission. On 2020 May 30, EUI was situated at 0.556AU from the Sun. Its HRIEUV telescope 17.4nm passband reached an exceptionally high two-pixel spatial resolution of 400km. The size and duration of small-scale structures is determined in the HRIEUV data, while their height is estimated from triangulation with the simultaneous SDO/AIA data. This is the first stereoscopy of small scale brightenings at h…
First time delay observation between two mid-infrared channels in solar flare footpoints
School of Physics & Astronomy, University of Glasgow（格拉斯哥大学物理与天文学学院）；School of Physics, Costa Rica Institute of Technology（哥斯达黎加理工学院物理学院）；Rosseland Centre for Solar Physics, University of Oslo（奥斯陆大学罗斯兰太阳物理中心）；Space Sciences Laboratory, University of California, Berkeley（加州大学伯克利分校空间科学实验室）；Center for Radio Astronomy and Astrophysics Mackenzie (CRAAM), Engineering School, Mackenzie Presbyterian University（麦肯锡长老会大学工程学院麦肯锡射电天体物理学中心）
Solar Vortices as Conduits for Magnetoacoustic Waves: Multi-Layer Coupling and Their Role in Atmospheric Heating
University of Sheffield（谢菲尔德大学）；National Observatory of Athens（雅典国家天文台）；Northumbria University（诺森比亚大学）；Queen’s University Belfast（贝尔法斯特女王大学）；University of Oslo（奥斯陆大学）
Height-dependent thermal properties of extreme-ultraviolet campfires in the quiet Sun
Leibniz Institute for Astrophysics Potsdam（波茨坦莱布尼茨天体物理研究所）；Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center（达沃斯物理气象观测站，世界辐射中心）；ETH-Zürich（苏黎世联邦理工学院）；Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium（比利时皇家天文台太阳-地球卓越中心SIDC）；Johns Hopkins University Applied Physics Laboratory（约翰斯·霍普金斯大学应用物理实验室）；Research Center for Astronomy and Applied Mathematics (RCAAM), Academy of Athens（雅典科学院天文学与应用数学研究中心）；University of Graz, Institute of Physics（格拉茨大学物理研究所）；European Space Agency (ESA/ESTEC)（欧洲空间局）；Columbia Astrophysics Laboratory, Columbia University（哥伦比亚大学天体物理学实验室）
