We study spectroscopic observations of chromospheric evaporation mass flows in comparison to the energy input by electron beams derived from hard X-ray data for the white-light M2.5 flare of 2006 July 6. The event was captured in high cadence spectroscopic observing mode by SOHO/CDS combined with high-cadence imaging at various wavelengths in the visible, EUV and X-ray domain during the joint observing campaign JOP171. During the flare peak, we observe downflows in the He\,{\sc i} and O\,{\sc v} lines formed in the chromosphere and transition region, respectively, and simultaneous upflows in the hot coronal Si~{\sc xii} line. The energy deposition rate by electron beams derived from RHESSI hard X-ray observations is suggestive of explosive chromospheric evaporation, consistent with the observed plasma motions. However, for a later distinct X-ray burst, where the site of the strongest energy deposition is exactly located on the CDS slit, the situation is intriguing. The O\,{\sc v} transition region line spectra show the evolution of double components, indicative of the superposition of a stationary plasma volume and upflowing plasma elements with high velocities (up to 280~km~s$^{-1}$) in single CDS pixels on the flare ribbon. However, the energy input by electrons during this period is too small to drive explosive chromospheric evaporation. These unexpected findings indicate that the flaring transition region is much more dynamic, complex, and fine-structured than is captured in single-loop hydrodynamic simulations.
We present different signatures of chromospheric evaporation in two solar flares observed by the Interface Region Imaging Spectrograph (IRIS). In the B1.6 flare on 2016 December 6 (SOL2016-12-06T10:40), the transition region Si IV line and the chromospheric C II and Mg II lines show blueshifts with low velocities up to 20 km/s at the flare loop footpoints in the rise phase, indicative of a gentle chromospheric evaporation. While in the C1.6 flare on 2015 December 19 (SOL2015-12-19T10:51), the Si IV, C II, and Mg II lines exhibit redshifts with velocities from several to tens of km/s at the foo…
In the standard model of solar flares, energy deposition by a beam of electrons drives strong chromospheric evaporation leading to a significantly denser corona and much brighter emission across the spectrum. Chromospheric evaporation was examined in great detail by Fisher, Canfield, & McClymont (1985a,b,c), who described a distinction between two different regimes, termed explosive and gentle evaporation. In this work, we examine the importance of electron energy and stopping depths on the two regimes and on the atmospheric response. We find that with explosive evaporation, the atmospheric re…
We have studied the chromospheric evaporation flow during the impulsive phase of the flare by using the Hinode/EIS observation and 1D hydrodynamic numerical simulation coupled to the time-dependent ionization. The observation clearly shows that the strong redshift can be observed at the base of the flaring loop only during the impulsive phase. We performed two different numerical simulations to reproduce the strong downflows in FeXII and FeXV during the impulsive phase. By changing the thermal conduction coefficient, we carried out the numerical calculation of chromospheric evaporation in the …
Chromospheric evaporation refers to dynamic mass motions in flare loops as a result of rapid energy deposition in the chromosphere. These have been observed as blueshifts in X-ray and extreme-ultraviolet (EUV) spectral lines corresponding to upward motions at a few tens to a few hundreds of km/s. Past spectroscopic observations have also revealed a dominant stationary component, in addition to the blueshifted component, in emission lines formed at high temperatures (~10 MK). This is contradictory to evaporation models predicting predominant blueshifts in hot lines. The recently launched Interf…
We explored simultaneous observations of chromospheric evaporation and condensation during the impulsive phase of a C6.7 flare on 9 May 2019. The solar flare was simultaneously observed by multiple instruments, i.e., the New Vacuum Solar Telescope (NVST), the Interface Region Imaging Spectrograph, the Atmospheric Imaging Assembly (AIA), the Fermi, the Mingantu Spectral Radioheliograph, and the Nobeyama Radio Polarimeters. Using the single Gaussian fitting and the moment analysis technique, redshifted velocities at slow speeds of 15-19 km/s are found in the cool lines of C II and Si IV at one f…
{Chromospheric evaporation is a key process in solar flares that has extensively been investigated using the spectroscopic observations. However, direct soft X-ray (SXR) imaging of the process is rare, especially in remote brightenings associated with the primary flares that have recently attracted dramatic attention.} {We intend to find the evidence for chromospheric evaporation and figure out the cause of the process in sympathetic coronal bright points (CBPs), i.e., remote brightenings induced by the primary CBP.} {We utilise the high-cadence and high-resolution SXR observations of CBPs fro…
We investigate the chromospheric evaporation in the flare of 2007 January 16 using line profiles observed by the EUV Imaging Spectrometer (EIS) onboard Hinode. Three points at flare ribbons of different magnetic polarities are analyzed in detail. We find that the three points show different patterns of upflows and downflows in the impulsive phase of the flare. The spectral lines at the first point are mostly blue shifted, with the hotter lines showing a dominant blue-shifted component over the stationary one. At the second point, however, only weak upflows are detected; in stead, notable downf…
We present the first observation of a solar filament formed by magnetic reconnection and associated chromospheric evaporation and subsequent coronal condensation. Driven by shearing motion during flux emergence, a sequential tether-cutting reconnection processes occurred and resulted in an M1.3 confined flare accompany with the formation of a sigmoid structure. It is found that the flare had a conjugate compact footpoint brightenings, which correspond to the footpoints of the sigmoid. Furthermore, observational evidence of explosive evaporation is well diagnosed at the conjugate footpoint brig…
We explore the temporal relationship between microwave/HXR emission and Doppler velocity during the impulsive phase of a solar flare on 2014 October 27 (SOL2014-10-27), which displays a pulse on the light curves in microwave (34 GHz) and hard X-ray (HXR, 25-50 keV) bands before the flare maximum. Imaging observation shows that this pulse mainly comes from one footpoint of a solar flare loop. The slit of Interface Region Imaging Spectrograph (IRIS) stays at this footpoint during this solar flare. The Doppler velocities of Fe XXI 1354.09 A and Si IV 1402.77 A are extracted from the Gaussian fitt…
Chromospheric evaporation is observed as Doppler blueshift during solar flares. It plays one of key roles in dynamics and energetics of solar flares, however, its mechanism is still unknown. In this paper we present a detailed analysis of spatially-resolved multi-wavelength observations of chromospheric evaporation during an M 1.0 class solar flare (SOL2014-06-12T21:12) using data from the NASA's IRIS (Interface Region Imaging Spectrograph) and HMI/SDO (Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory) telescopes, and VIS/NST (Visible Imaging Spectrometer at New Solar Telesc…
With the elementary energy release events introduced in a previous paper (Liu & Fletcher 2009) we model the chromospheric evaporation in flaring loops. The thick-target hard X-ray (HXR) emission produced by electrons escaping from the acceleration region dominates the impulsive phase and the thin-target emission from the acceleration region dominates the low-energy thermal component in the gradual phase, as observed in early impulsive flares. Quantitative details depend on properties of the thermal background, which leads to variations in the correlation between HXR flux and spectral index. Fo…
We present observations of distinct UV spectral properties at different locations during an atypical X-shaped flare (SOL2014-11-09T15:32) observed by the Interface Region Imaging Spectrograph (IRIS). In this flare, four chromospheric ribbons appear and converge at an X-point where a separator is anchored. Above the X-point, two sets of non-coplanar coronal loops approach laterally and reconnect at the separator. The IRIS slit was located close to the X-point, cutting across some of the flare ribbons and loops. Near the location of the separator, the Si IV 1402.77 A line exhibits significantly …
We present high spatial resolution observations of chromospheric evaporation in the flare SOL2014-03-29T17:48. Interface Region Imaging Spectrograph (IRIS) observations of the FeXXI 1354.1 A line indicate evaporating plasma at a temperature of 10 MK along the flare ribbon during the flare peak and several minutes into the decay phase with upflow velocities between 30 km s$^{-1}$ and 200 km s$^{-1}$. Hard X-ray (HXR) footpoints were observed by RHESSI for two minutes during the peak of the flare. Their locations coincided with the locations of the upflows in parts of the southern flare ribbon b…
Magnetic energy released in the corona by solar flares reaches the chromosphere where it drives characteristic upflows and downflows known as evaporation and condensation. These flows are studied here for the case where energy is transported to the chromosphere by thermal conduction. An analytic model is used to develop relations by which the density and velocity of each flow can be predicted from coronal parameters including the flare's energy flux $F$. These relations are explored and refined using a series of numerical investigations in which the transition region is represented by a simpli…
In this paper, we report our multiwavelength imaging observations of chromospheric evaporation in a C5.5 circular-ribbon flare (CRF) on 2014 August 24. The flare was observed by the Atmospheric Imaging Assembly (AIA) on board the \textit{Solar Dynamics Observatory} (\textit{SDO}), X-ray Telescope (XRT) on board the \textit{Hinode} spacecraft, and ground-based Nobeyama Radioheliograph (NoRH). The CRF consisted of a discrete circular ribbon with a diameter of $\sim$1$\arcmin$ and a short inner ribbon observed in ultraviolet (UV), extreme-ultraviolet (EUV), soft X-ray (SXR), and especially in 17 …
Phase mixing of Alfvén waves has been studied extensively as a possible coronal heating mechanism but without the full thermodynamic consequences considered self-consistently. It has been argued that in some cases, the thermodynamic feedback of the heating could substantially affect the transverse density gradient and even inhibit the phase mixing process. In this paper, we use MHD simulations with the appropriate thermodynamical terms included to quantify the evaporation following heating by phase mixing of Alfvén waves in a coronal loop and the effect of this evaporation on the transverse de…
Dynamics of hot chromospheric plasma of solar flares is a key to understanding of mechanisms of flare energy release and particle acceleration. A moderate M1.0 class flare of 12 June, 2014 (SOL2014-06-12T21:12) was simultaneously observed by NASA's Interface Region Imaging Spectrograph (IRIS), other spacecraft, and also by New Solar Telescope (NST) at the BBSO. This paper presents the first part of our investigation focused on analysis of the IRIS data. Our analysis of the IRIS data in different spectral lines reveals strong redshifted jet-like flow with the speed of ~100 km/s of the chromosph…
We discuss observations of chromospheric evaporation for a complex flare that occurred on 9 March 2012 near 03:30 UT obtained from the Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode spacecraft. This was a multiple event with a strong energy input that reached the M1.8 class when observed by EIS. EIS obtained a full-CCD spectrum of the flare. Chromospheric evaporation characterized by 150-200 km/s upflows was observed in multiple locations in multi-million degree spectral lines of flare ions such as Fe XXII, Fe XXIII, and Fe XXIV, with simultaneous 20-60 km/s upflows in million de…
Magnetic reconnection in the corona results in contracting flare loops, releasing energy into plasma heating and shocks. The hydrodynamic shocks so produced drive thermal conduction fronts (TCFs) which transport energy into the chromosphere and drive upflows (evaporation) and downflows (condensation) in the cooler, denser footpoint plasma. Observations have revealed that certain properties of the transition point between evaporation and condensation (the "flow reversal point" or FRP), such as temperature and velocity-temperature derivative at the FRP, vary between different flares. These prope…
We investigate the occurrence of slipping magnetic reconnection, chromospheric evaporation, and coronal loop dynamics in the 2014 September 10 X-class flare. The slipping reconnection is found to be present throughout the flare from its early phase. Flare loops are seen to slip in opposite directions towards both ends of the ribbons. Velocities of 20--40 km\,s$^{-1}$ are found within time windows where the slipping is well resolved. The warm coronal loops exhibit expanding and contracting motions that are interpreted as displacements due to the growing flux rope that subsequently erupts. This …
We have explored the relationship between hard X-ray (HXR) emissions and Doppler velocities caused by the chromospheric evaporation in two X1.6 class solar flares on 2014 September 10 and October 22, respectively. Both events display double ribbons and Interface Region Imaging Spectrograph (IRIS) slit is fixed on one of their ribbons from the flare onset. The explosive evaporations are detected in these two flares. The coronal line of Fe XXI 1354.09 A shows blue shifts, but chromospheric line of C I 1354.29 A shows red shifts during the impulsive phase. The chromospheric evaporation tends to a…
With observations from the Interface Region Imaging Spectrograph (IRIS), we track the complete evolution of $\sim$11 MK evaporation flows in an M1.1 flare on 2014 September 6 and an X1.6 flare on 2014 September 10. These hot flows, as indicated by the blueshifted Fe~{\sc{xxi}}~1354.08Å~line, evolve smoothly with a velocity decreasing exponentially from $\sim$200~km~s$^{-1}$ to almost stationary within a few minutes. We find a good correlation between the flow velocity and energy deposition rate as represented by the hard X-Ray flux observed with the Reuven Ramaty High Energy Solar Spectroscopi…
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…
Using the observations recorded by Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO), the Interface Region Imaging Spectrograph (IRIS) and the Extreme-ultraviolet Imaging Spectrometer (EIS) and X-Ray Telescope (XRT) both on-board Hinode, we present the evidence of chromospheric evaporation in a coronal loop after the occurrence of two active region transient brightenings (ARTBs) at the two footpoints. The chromospheric evaporation started nearly simultaneously in all the three hot channels of AIA such as 131~Å, 94~Å and 335~Å, which was observed to be temperature…
We studied an X1.6 solar flare produced by NOAA AR 12602 on 2014 October 22. The entirety of this event was covered by RHESSI, IRIS, and Hinode/EIS, allowing analysis of the chromospheric response to a nonthermal electron driver. We derived the energy contained in nonthermal electrons via RHESSI spectral fitting, and linked the time-dependent parameters of this call to the response in Doppler velocity, density, and nonthermal width across a broad temperature range. The total energy injected was $4.8\times10^{30}$ erg, and lasted $352$ seconds. This energy drove explosive chromospheric evaporat…
In this paper, we report our multiwavelength observations of the C4.2 circular-ribbon flare in active region (AR) 12434 on 2015 October 16. The short-lived flare was associated with positive magnetic polarities and a negative polarity inside, as revealed by the photospheric line-of-sight magnetograms. Such magnetic pattern is strongly indicative of a magnetic null point and spine-fan configuration in the corona. The flare was triggered by the eruption of a mini-filament residing in the AR, which produced the inner flare ribbon (IFR) and the southern part of a closed circular flare ribbon (CFR)…
We present a detailed study of explosive chromospheric evaporation during a microflare which occurred on 2007 December 7 as observed with the EUV Imaging Spectrometer (EIS) onboard Hinode. We find temperature-dependent upflows for lines formed from 1.0 to 2.5 MK and downflows for lines formed from 0.05 to 0.63 MK in the impulsive phase of the flare. Both the line intensity and the nonthermal line width appear enhanced in most of the lines and are temporally correlated with the time when significant evaporation was observed. Our results are consistent with the numerical simulations of flare mod…
Magnetic reconnection is believed to be the dominant energy release mechanism in solar flares. The standard flare model predicts both downward and upward outflow plasmas with speeds close to the coronal Alfvén speed. Yet, spectroscopic observations of such outflows, especially the downflows, are extremely rare. With observations of the newly launched Interface Region Imaging Spectrograph (IRIS), we report the detection of greatly redshifted ($\sim$125 km s$^{-1}$ along line of sight) Fe {\sc{xxi}} 1354.08Å emission line with a $\sim$100 km s$^{-1}$ nonthermal width at the reconnection site of …
We present a new computational approach that addresses the difficulty of obtaining the correct interaction between the solar corona and the transition region in response to rapid heating events. In the coupled corona, transition region and chromosphere system, an enhanced downward conductive flux results in an upflow (chromospheric evaporation). However, obtaining the correct upflow generally requires high spatial resolution in order to resolve the transition region. With an unresolved transition region, artificially low coronal densities are obtained because the downward heat flux jumps acros…
We analyzed high-cadence sit-and-stare observations acquired with the Hinode/EIS spectrometer and HXR measurements acquired with RHESSI during an M-class flare. During the flare impulsive phase, we observe no significant flows in the cooler Fe XIII line but strong upflows, up to 80-150 km/s, in the hotter Fe XVI line. The largest Doppler shifts observed in the Fe XVI line were co-temporal with the sharp intensity peak. The electron density obtained from a Fe XIII line pair ratio exhibited fast increase (within two minutes) from the pre-flare level of 5.01x10^(9) cm^(-3) to 3.16x10^(10) cm^(-3)…
Despite helium abundance (AHe = nH/nHe) is ~ 8 % at the solar photospheric/chromospheric heights, AHe can be found to exceed 8% in interplanetary coronal mass ejections (ICMEs) on many occasions. Although various factors like interplanetary shocks, chromospheric evaporation and "sludge removal" have been separately invoked in the past to address the AHe enhancements in ICMEs, none of these processes could explain the variability of AHe in ICMEs comprehensively. Based on extensive analysis of 275 ICME events, we show that there is a solar activity variation of ICME averaged AHe values. The inve…
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 …
With observations of IRIS, we study chromospheric heating and evaporation during an M1.6 flare SOL2015-03-12T11:50. At the flare ribbons, the Mg II 2791.59 line shows quasi-periodic short-duration red-wing enhancement, which is likely related to repetitive chromospheric condensation as a result of episodic heating. On the contrary, the Si IV 1402.77 line reveals a persistent red-wing asymmetry in both the impulsive and decay phases, suggesting that this line responds to both cooling downflows and chromospheric condensation. The first two episodes of red-wing enhancement occurred around 11:42 U…
The interaction between emerging magnetic flux and the pre-existing ambient field has become a "hot" topic for both numerical simulations and high-resolution observations of the solar atmosphere. The appearance of brightenings and surges during episodes of flux emergence is believed to be a signature of magnetic reconnection processes. We present an analysis of a small-scale flux emergence event in NOAA 10971, observed simultaneously with the Swedish 1-m Solar Telescope on La Palma and the \emph{Hinode} satellite during a joint campaign in September 2007. Extremely high-resolution G-band, H$α$…
We have studied the relationship between the velocity and temperature of a solar EUV jet. The highly accelerated jet occurred in the active region NOAA 10960 on 2007 June 5. Multi-wavelength spectral observations with EIS/Hinode allow us to investigate Doppler velocities at the wide temperature range. We analyzed the three-dimensional angle of the jet from the stereoscopic analysis with STEREO. Using this angle and Doppler velocity, we derived the true velocity of the jet. As a result, we found that the cool jet observed with \ion{He}{2} 256 Å$\log_{10}T_e[\rm{K}] = 4.9$ is accelerated to arou…
Starting from a realistically sheared magnetic arcade connecting chromospheric, transition region to coronal plasma, we simulate the in-situ formation and sustained growth of a quiescent prominence in the solar corona. Contrary to previous works, our model captures all phases of the prominence formation, including the loss of thermal equilibrium, its successive growth in height and width to macroscopic dimensions, and the gradual bending of the arched loops into dipped loops, as a result of the mass accumulation. Our 2.5-dimensional, fully thermodynamically and magnetohydrodynamically consiste…
We investigate the relationship between the blue shifts of a hot emission line and the nonthermal emissions in microwave and hard X-ray (HXR) wavelengths during the precursor of a solar flare on 2014 October 27. The flare precursor is identified as a small but well-developed peak in soft X-ray and extreme-ultraviolet passbands before the GOES flare onset, which is accompanied by a pronounced burst in microwave 17 & 34 GHz and HXR 25-50 keV. The slit of Interface Region Imaging Spectrograph (IRIS) stays at one ribbon-like transient during the flare precursor, where shows visible nonthermal emis…
Alfvénic waves have been proposed as an important energy transport mechanism in coronal loops, capable of delivering energy to both the corona and chromosphere and giving rise to many observed features, of flaring and quiescent regions. In previous work, we established that resistive dissipation of waves (ambipolar diffusion) can drive strong chromospheric heating and evaporation, capable of producing flaring signatures. However, that model was based on a simplified assumption that the waves propagate instantly to the chromosphere, an assumption which the current work removes. Via a ray tracin…
Using high-resolution observations from the 1.6 m New Solar Telescope (NST) operating at the Big Bear Solar Observatory (BBSO), we report direct evidence of merging/reconnection of cool H$α$ loops in the chromosphere during two homologous flares (B- and C-class) caused by a shear motion at the footpoint of two loops. The reconnection between these loops caused the formation of an unstable flux rope which showed counterclockwise rotation. The flux rope could not reach the height of torus instability and failed to form a coronal mass ejection. The HMI magnetograms revealed rotation of the negati…
We report on observations of a solar jet propagating along coronal loops taken by the Solar Dynamics Observatory (SDO), the Interface Region Imaging Spectragraph (IRIS) and 1-m New Vacuum Solar Telescope (NVST). The ejecta of the jet consist of multi-thermal components and propagate with a speed greater than 100 km/s. Brightenings are found in the remote footpoints of the coronal loops having compact and round-shape in the Halpha images. The emission peak of the remote brightening in the Atmospheric Imaging Assembly (AIA) 94 Åpassband lags 60 s behind that in the jet base. The brightenings in …
The chromosphere is a thin layer of the solar atmosphere that bridges the relatively cool photosphere and the intensely heated transition region and corona. Compressible and incompressible waves propagating through the chromosphere can supply significant amounts of energy to the interface region and corona. In recent years an abundance of high-resolution observations from state-of-the-art facilities have provided new and exciting ways of disentangling the characteristics of oscillatory phenomena propagating through the dynamic chromosphere. Coupled with rapid advancements in magnetohydrodynami…
This paper presents a detailed study of chromospheric evaporation using the EUV Imaging Spectrometer (EIS) onboard Hinode in conjunction with HXR observations from RHESSI. The advanced capabilities of EIS were used to measure Doppler shifts in 15 emission lines covering the temperature range T=0.05-16 MK during the impulsive phase of a C-class flare on 2007 December 14. Blueshifts indicative of the evaporated material were observed in six emission lines from Fe XIV-XXIV (2-16 MK). Upflow velocity (v_up) was found to scale with temperature as v_up (km s^-1)~8-18 T (MK). Although the hottest emi…
Acceleration and transport of high-energy particles and fluid dynamics of atmospheric plasma are interrelated aspects of solar flares. We present here self-consistently combined Fokker-Planck modeling of particles and hydrodynamic simulation of flare plasma. Energetic electrons are modeled with the Stanford unified code of acceleration, transport, and radiation, while plasma is modeled with the NRL flux tube code. We calculated the collisional heating rate from the particle transport code, which is more accurate than those based on approximate analytical solutions. We used a realistic spectrum…
Aims. We use advanced 3D NLTE radiative magnetohydrodynamic simulations of the solar atmosphere to carry out detailed tests of chromospheric diagnostics at millimeter and submillimeter wavelengths. Methods. We focused on the diagnostics of the thermal structure of the chromosphere in the wavelength bands from 0.4 mm up to 9.6 mm that can be accessed with the Atacama Large Millimeter/Submillimeter Array (ALMA) and investigated how these diagnostics are affected by the instrumental resolution. Results. We find that the formation height range of the millimeter radiation depends on the location in…
In this paper, we report our multiwavelength observations of the C3.1 circular-ribbon flare SOL2015-10-16T10:20 in AR 12434. The flare consisted of a circular flare ribbon (CFR), an inner flare ribbon (IFR) inside, and a pair of short parallel flare ribbons (PFRs). During the impulsive phase of the flare, "two-step" raster observations of \textit{IRIS} with a cadence of 6 s and an exposure time of 2 s show plasma downflow at the CFR in the Si {\sc iv} $λ$1402.77 line, suggesting chromospheric condensation. The downflow speeds first increased rapidly from a few km s$^{-1}$ to the peak values of…
Employing the high spatio-temporal Interface Region Imaging Spectrograph 1330 Å observations, we investigated the jet-like features that occurred during the X8.2 class flare in NOAA active region (AR) 12673 on 2017 September 10. These jet-like features were rooted in the flare ribbons. We examined 15 features, and the mean values of the lifetimes, projected widths, lengths and velocities of these features were 87 s, 890 km, 2.7 Mm and 70 km s$^{-1}$, respectively. We also observed many jet-like features which happened during the X1.0 class flare on 2014 October 25. We studied the spectra at th…
Hard X-ray (HXR) sources are frequently observed near the top of solar flare loops, and the emission is widely ascribed to bremsstrahlung. We here revisit an alternative scenario which stresses the importance of inverse Compton processes and the Kelvin- Helmholtz instability (KHI) proposed by Fang et al. (2016). This scenario adds a novel ingredient to the standard flare model, where evaporation flows from flare-impacted chromospheric foot-points interact with each other near the loop top and produce turbulence via KHI. The turbulence can act as a trapping region and as an efficient accelerato…
We use high spatial and temporal resolution observations from the Swedish Solar Telescope to study the chromospheric velocities of a C-class flare originating from active region NOAA 10969. A time-distance analysis is employed to estimate directional velocity components in H-alpha and Ca II K image sequences. Also, imaging spectroscopy has allowed us to determine flare-induced line-of-sight velocities. A wavelet analysis is used to analyse the periodic nature of associated flare bursts. Time-distance analysis reveals velocities as high as 64 km/s along the flare ribbon and 15 km/s perpendicula…
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…
Quasi-periodic oscillations of the hard X-ray (HXR) emission of the large flare of 2 November 1991 have been investigated using HXR light curves and soft X-ray and HXR images recorded by the {\sl Yohkoh} X-ray telescopes. The results of the analysis of these observations are the following: i) The observations confirm that electrons are accelerated in oscillating magnetic traps which are contained within the cusp magnetic structure. ii) The amplitude of the HXR pulses increase due to the increase in the amplitude of the magnetic trap oscillations and the increase in the density within the traps…
We present new results from the Interface Region Imaging Spectrograph showing the dynamic evolution of chromospheric evaporation and condensation in a flare ribbon, with the highest temporal and spatial resolution to date. IRIS observed the entire impulsive phase of the X-class flare SOL2014-09-10T17:45 using a 9.4 second cadence `sit-and-stare' mode. As the ribbon brightened successively at new positions along the slit, a unique impulsive phase evolution was observed for many tens of individual pixels in both coronal and chromospheric lines. Each activation of a new footpoint displays the sam…
The heating of the solar chromosphere remains one of the most important questions in solar physics. Our current understanding is that small-scale internetwork (IN) magnetic fields play an important role as a heating agent. Indeed, cancellations of IN magnetic elements in the photosphere can produce transient brightenings in the chromosphere and transition region. These bright structures might be the signature of energy release and plasma heating, probably driven by magnetic reconnection of IN field lines. Although single events are not expected to release large amounts of energy, their global …
