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We performed numerical simulations to study mechanisms of solar prominence formation triggered by a single heating event. In the widely accepted ``chromospheric-evaporation condensation" model, localized heating at footpoints of a coronal loop drives plasma evaporation and eventually triggers condensation. The occurrence of condensation is strongly influenced by the characteristics of the heating.Various theoretical studies have been conducted along one-dimensional field lines with quasi-steady localized heating. The quasi-steady heating is regarded as the collection of multiple heating events among multiple strands constituting a coronal loop. However, it is reasonable to consider a single heating event along a single field line as an elemental unit.We investigated the condensation phenomenon triggered by a single heating event using 1.5-dimensional magnetohydrodynamic simulations. By varying the magnitude of the localized heating rate, we explored the conditions necessary for condensation. We found that when a heating rate approximately $\sim 10^{4}$ times greater than that of steady heating was applied, condensation occurred. Condensation was observed when the thermal conduction efficiency in the loop became lower than the cooling efficiency, with the cooling rate significantly exceeding the heating rate. Using the loop length $L$ and the Field length $\lambda_{\mathrm{F}}$, the condition for condensation is expressed as $\lambda_{\mathrm{F}} \lesssim L/2$ under conditions where cooling exceeds heating. We extended the analytically derived condition for thermal non-equilibrium to a formulation based on heating amount., Comment: 16 pages, 10 figures, Accepted for publication in ApJ

Magnetic tornadoes, characterized as impulsive Alfven waves initiated by photospheric vortices in intergranular lanes, are considered efficient energy channels to the corona. Despite their acknowledged importance for solar coronal heating, their observational counterparts from the corona have not been well understood. To address this issue, we use a radiative MHD simulation of a coronal loop with footpoints rooted in the upper convection zone, and synthesize the chromospheric and coronal emissions corresponding to a magnetic tornado. Considering SDO/AIA 171 A and Solar Orbiter/EUI 174 A channels, our synthesis reveals that the coronal response to magnetic tornadoes can be observed as an EUV brightening of which width is ~2 Mm. This brightening is located above the synthesized chromospheric swirl observed in Ca II 8542 A, Ca II K, and Mg II k lines, which can be detected by instruments such as SST/CRISP, GST/FISS, and IRIS. Considering the height correspondence of the synthesized brightening, magnetic tornadoes can be an alternative mechanism for the small-scale EUV brightenings such as the solar "campfires''. Our findings indicate that coordinated observations encompassing the chromosphere to the corona are indispensable for comprehending the origin of coronal EUV brightenings., Comment: Accepted for publication in ApJL

In solving the solar coronal heating problem, it is crucial to comprehend the mechanisms by which energy is conveyed from the photosphere to the corona. Recently, magnetic tornadoes, characterized as coherent, rotating magnetic field structures extending from the photosphere to the corona, have drawn growing interest as a possible means of efficient energy transfer. Despite its acknowledged importance, the underlying physics of magnetic tornadoes remains still elusive. In this study, we conduct a three-dimensional radiative magnetohydrodynamic simulation that encompasses the upper convective layer and extends into the corona, with a view to investigating how magnetic tornadoes are generated and efficiently transfer energy into the corona. We find that a single event of magnetic flux concentration merger on the photosphere gives rise to the formation of a single magnetic tornado. The Poynting flux transferred into the corona is found to be four times greater in the presence of the magnetic tornado, as compared to its absence. This increase is attributed to a reduction in energy loss in the chromosphere, resulting from the weakened magnetic energy cascade. Based on an evaluation of the fraction of the merging events, our results suggest that magnetic tornadoes contribute approximately 50% of the Poynting flux into the corona in regions where the coronal magnetic field strength is 10 G. Potentially, the contribution could be even greater in areas with a stronger coronal magnetic field., Comment: submitted to the Astrophysical Journal. first referee report received. comments welcome

Solar magnetic fields comprise an 11-year activity cycle, represented by the number of sunspots. The maintenance of such a solar magnetic field can be attributed to fluid motion in the convection zone, i.e. a dynamo. This study conducts the mean-field analyses of the global solar dynamo simulation presented by Hotta et al. (2016). Although the study succeeds in producing coherent large-scale magnetic fields at high Reynolds numbers, the detailed physics of the maintenance of this field have not been fully understood. This study extracts the alpha-tensor and the turbulent magnetic diffusivity tensor through mean-field analyses. The turbulent magnetic diffusivity exhibits a significant decrease towards high Reynolds numbers. The decrease in the turbulent magnetic diffusivity suppresses the energy conversion of large-scale field to small-scale field. This implies that the decrease in the turbulent magnetic diffusivity contributes to the maintenance of a large-scale magnetic field at high Reynolds numbers. A significant downward turbulent pumping is observed; it is enhanced in the weak phase of the large-scale field. This study proposes a cyclic reversal process of a large-scale field which is dominantly driven by the alpha-effect and is possibly triggered by downward pumping., Comment: 17 pages, 20 figures, Accepted for publication in ApJ

Magnetohydrodynamic (MHD) waves are candidates for heating the solar chromosphere, although it is still unclear which mode of the wave is dominant in heating. We perform two-dimensional radiative MHD simulation to investigate the propagation of MHD waves in the quiet region of the solar chromosphere. We identify the mode of the shock waves by using the relationship between gas pressure and magnetic pressure across the shock front and calculate their corresponding heating rate through the entropy jump to obtain a quantitative understanding of the wave heating process in the chromosphere. Our result shows that the fast magnetic wave is significant in heating the low-beta chromosphere. The low-beta fast magnetic waves are generated from high-beta fast acoustic waves via mode conversion crossing the equipartition layer. Efficient mode conversion is achieved by large attacking angles between the propagation direction of the shock waves and the chromospheric magnetic field., Comment: Accepted by ApJL

We perform magnetohydrodynamic (MHD) simulations to investigate the propagation of Alfven wave in magnetic chromosphere. We use the 1.5-dimensional expanding flux tube geometry setting and transverse perturbation at the bottom to generate the Alfven wave. Compared with previous studies, our expansion is that we include the radiative loss term introduced by Carlsson & Leenaarts (2012). We find that when an observation based transverse wave generator is applied, the spatial distribution of the time-averaged radiative loss profile in our simulation is consistent with that in the classic atmospheric model. In addition, the energy flux in the corona is larger than the required value for coronal heating in the quiet region. Our study shows that the Alfven wave driven model has the potential to explain chromospheric heating and transport enough energy to the corona simultaneously., Comment: Accepted by ApJ

We report X-ray spectroscopic results for four giant solar flares occurred on 2005 September 7 (X17.0), 2005 September 8 (X5.4), 2005 September 9 (X6.2), and 2006 December 5 (X9.0), obtained from Earth albedo data with the X-ray imaging spectrometer (XIS) onboard Suzaku. The good energy resolution of the XIS (FWHM$\sim$100 eV) enables us to separate a number of line-like features and detect the underlying continuum emission. These features include Si He$\alpha$, Si Ly$\alpha$, S He$\alpha$, S Ly$\alpha$, Ar He$\alpha$, and Ca He$\alpha$ originating from solar flares as well as fluorescent Ar K$\alpha$ and Ar K$\beta$ from the Earth atmosphere. Absolute elemental abundances (X/H) averaged over the four flares are obtained to be $\sim$2.0 (Ca), $\sim$0.7 (Si), $\sim$0.3 (S), and $\sim$0.9 (Ar) at around flare peaks. This abundance pattern is similar to those of active stars' coronae showing inverse first ionization potential (i-FIP) effects, i.e., elemental abundances decrease with decreasing FIP with a turnover at the low end of FIP. The abundances are almost constant during the flares, with an exception of Si which increases by a factor of $\sim$2 in the decay phase. The evolution of the Si abundance is consistent with a picture that the i-FIP plasma originates from the chromosphere evaporation and then mixes with the surrounding low-FIP biased materials. Flare-to-flare abundance varied by a factor of 2, agreeing with past observations of solar flares. Finally, we emphasize that Earth albedo data acquired by X-ray astronomy satellites like Suzaku and XRISM can significantly contribute to studies of solar physics., Comment: 19 pages, 10 figures, accepted for publication in The Astrophysical Journal

We succeeded in observing two large spicules simultaneously with the Atacama Large Millimeter/submillimeter Array (ALMA), the Interface Region Imaging Spectrograph (IRIS), and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory. One is a spicule seen in the IRIS Mg II slit-jaw images and AIA 304\AA\ images (MgII/304A spicule). The other one is a spicule seen in the 100GHz images obtained with ALMA (100GHz spicule). Although the 100GHz spicule overlapped with the MgII/304A spicule in the early phase, it did not show any corresponding structures in the IRIS Mg II and AIA 304A images after the early phase. It suggests that the spicules are individual events and do not have a physical relationship. To obtain the physical parameters of the 100GHz spicule, we estimate the optical depths as a function of temperature and density using two different methods. One is using the observed brightness temperature by assuming a filling factor, and the other is using an emission model for the optical depth. As a result of comparing them, the kinetic temperature of the plasma and the number density of ionized hydrogens in the 100GHz spicule are ~6800 K and 2.2 x 10^10 cm^-3. The estimated values can explain the absorbing structure in the 193A image, which appear as a counterpart of the 100GHz spicule. These results suggest that the 100GHz spicule presented in this paper is classified to a macrospicule without a hot sheath in former terminology., Comment: 8 pages, 4 figures, Accepted to ApJL

The understanding of magnetic reconnection in three-dimension (3D) is far shallower than its counterpart in two-dimension (2D) due to its potential complication, not to mention the evolving of the spontaneously growing turbulence. We investigate the reason for reconnection acceleration on the characters and development of diffusion regions and sheared 3D energy modes (energy modes that are not parallel to the anti-parallel magnetic fields) during the turbulence building stage. We found that multiple reconnection layers emerge due to the growth of 3D sheared tearing instability. Diffusion regions on adjacent reconnection layers form an inflow-outflow coupling that enhances the local reconnection. Further coupling of the existing energy modes breeds new energy modes near the current sheet edge. As reconnection layers span and interact with each other across the whole current sheet, global magnetic energy consumption accelerates. The significant contribution of 3D energy modes and their interaction to the reconnection rate enhancement seems to be independent on magnetic diffusivity. On the other hand, the global guide field changes the layout of 3D reconnection layer thus determines whether the system is fast-reconnection-preferable., Comment: 13 pages, 14 figures

Using a three-dimensional compressible magnetohydrodynamic (MHD) simulation, we have reproduced the fast solar wind in a direct and self-consistent manner, based on the wave/turbulence driven scenario. As a natural consequence of Alfvenic perturbations at its base, highly compressional and turbulent fluctuations are generated, leading to heating and acceleration of the solar wind. The analysis of power spectra and structure functions reveals that the turbulence is characterized by its imbalanced (in the sense of outward Alfvenic fluctuations) and anisotropic nature. The density fluctuation originates from the parametric decay instability of outwardly propagating Alfven waves and plays a significant role in the Alfven wave reflection that triggers turbulence. Our conclusion is that the fast solar wind is heated and accelerated by compressible MHD turbulence driven by parametric decay instability and resultant Alfven wave reflection., Comment: published from The Astrophysical Journal Letters

The structure of the photospheric vector magnetic field below a dark filament on the Sun is studied using the observations of the Spectro-Polarimeter attached to the Solar Optical Telescope onboard Hinode. Special attention is paid to discriminate the two suggested models, a flux rope or a bent arcade. "Inverse-polarity" orientation is possible below the filament in a flux rope, whereas "normal-polarity" can appear in both models. We study a filament in active region NOAA 10930, which appeared on the solar disk during 2006 December. The transverse field perpendicular to the line of sight has a direction almost parallel to the filament spine with a shear angle of 30 deg, whose orientation includes the 180-degree ambiguity. To know whether it is in the normal orientation or in the inverse one, the center-to-limb variation is used for the solution under the assumption that the filament does not drastically change its magnetic structure during the passage. When the filament is near the east limb, we found that the line-of-site magnetic component below is positive, while it is negative near the west limb. This change of sign indicates that the horizontal photospheric field perpendicular to the polarity inversion line beneath the filament has an "inverse-polarity", which indicates a flux-rope structure of the filament supporting field., Comment: 21 pages, 8 figures, accepted for publication in PASJ

The interiors of quiescent prominences are filled with turbulent flows. The evolution of upflow plumes, descending pillars, and vortex motions has been clearly detected in high-resolution observations. The Rayleigh-Taylor instability is thought to be a driver of such internal flows. Descending pillars are related to the mass budgets of prominences. There is a hypothesis of dynamic equilibrium where the mass drainage via descending pillars and the mass supply via radiative condensation are balanced to maintain the prominence mass; however, the background physics connecting the two different processes is poorly understood. In this study, we reproduced the dynamic interior of a prominence via radiative condensation and the mechanism similar to the Rayleigh-Taylor instability using a three-dimensional magnetohydrodynamic simulation including optically thin radiative cooling and nonlinear anisotropic thermal conduction. The process to prominence formation in the simulation follows the reconnection-condensation model, where topological change in the magnetic field caused by reconnection leads to radiative condensation. Reconnection is driven by converging motion at the footpoints of the coronal arcade fields. In contrast to the previous model, by randomly changing the speed of the footpoint motion along a polarity inversion line, the dynamic interior of prominence is successfully reproduced. We find that the mass condensation rate of the prominence is enhanced in the case with dynamic state. Our results support the observational hypothesis that the condensation rate is balanced with the mass drainage rate and suggest that a self-induced mass maintenance mechanism exists., Comment: Accepted for publication in ApJ, 26 pages, 12 figures

We report the results of the Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of the solar chromosphere on the southern polar limb. Coordinated observations with the Interface Region Imaging Spectrograph (IRIS) are also conducted. ALMA provided unprecedented high spatial resolution in the millimeter band ($\approx$ 2.0 arcsec) at 100 GHz frequency with a moderate cadence (20 s). The results are as follows: (1) The ALMA 100 GHz images show saw-tooth patterns on the limb, and a comparison with SDO/AIA 171\AA\ images shows a good correspondence of the limbs with each other. (2) The ALMA 100 GHz movie shows a dynamic thorn-like structure elongating from the saw-tooth patterns on the limb, with lengths reaching at least 8 arcsec, thus suggesting jet-like activity in the ALMA microwave range. These ALMA jets are in good correspondence with IRIS jet clusters. (3) A blob ejection event is observed. By comparing with the IRIS Mg II slit-jaw images, the trajectory of the blob is located along the spicular patterns., Comment: Accepted for publication in The Astrophysical Journal

We conduct a three-dimensional magnetohydrodynamic (MHD) simulation of the parametric decay instability of Alfv\'en waves and resultant compressible MHD turbulence, which is likely to develop in the solar wind acceleration region. Because of the presence of the mean magnetic field, the nonlinear stage is characterized by filament-like structuring and anisotropic cascading. By calculating the timescales of phase mixing and the evolution of Alfv\'en wave turbulence, we have found that the early nonlinear stage is dominated by phase mixing, while the later phase is dominated by imbalanced Alfv\'en wave turbulence. Our results indicate that the regions in the solar atmosphere with large density fluctuation, such as the coronal bottom and wind acceleration region, are heated by phase-mixed Alfv\'en waves, while the other regions are heated by Alfv\'en wave turbulence., Comment: Accepted for publication in The Astrophysical Journal Letters

Using numerical simulations, we investigate the onset and suppression of parametric decay instability (PDI) in the solar wind, focusing on the suppression effect by the wind acceleration and expansion. We also discuss the role of PDI in generating the density fluctuation in the solar wind and the cross-helicity evolution up to $1 {\rm \ au}$. Numerical simulations from the coronal base to 1 au are conducted in a self-consistent manner; we take into account the feedback of wave energy and pressure in the background. Monochromatic waves with various injection frequencies $f_0$ are injected to discuss the suppression of PDI, while broadband waves are applied to compare the numerical results with observation. We find, via the simulations with monochromatic waves, that high-frequency ($f_0 \gtrsim10^{-3} {\rm \ Hz}$) Alfv\'en waves are subject to PDI. Meanwhile, the maximum growth rate of the PDI of low-frequency ($f_0 \lesssim10^{-4} {\rm \ Hz}$) Alfv\'en waves becomes negative due to acceleration and expansion effects. Medium-frequency ($f_0 \approx 10^{-3.5} {\rm \ Hz}$) Alfv\'en waves have a positive growth rate but do not show the signature of PDI up to 1 au because the growth rate is too small. The medium-frequency waves experience neither PDI nor reflection so they propagate through the solar wind most efficiently. This is called the frequency-filtering mechanism of the solar wind. The simulations with broadband waves indicate that the observed trend of the density fluctuation is well explained by the evolution of PDI while the observed cross-helicity evolution is in agreement with low-frequency wave propagation., Comment: Accepted for publication in The Astrophysical Journal

Spicule oscillations involve high-frequency components with a typical period approximately corresponding to $40-50$ s. The typical time scale of the photospheric oscillation is a few minutes, and thus, the origin of this high-frequency component is not trivial. In this study, a one-dimensional numerical simulation is performed to demonstrate that the observed spicule oscillations originate from longitudinal-to-transverse mode conversion that occurs around the equipartition layer in the chromosphere. Calculations are conducted in a self-consistent manner with the exception of additional heating to maintain coronal temperature. The analyses indicate the following features: (1) mode conversion efficiently excites high-frequency transverse waves; (2) the typical period of the high-frequency waves corresponds to the sound-crossing time of the mode conversion region; and (3) simulated root-mean-square velocity of the high-frequency component is consistent with the observed value. These results indicate that the observation of spicule oscillation provides direct evidence of mode conversion in the chromosphere., Comment: accepted for publication in The Astrophysical Journal

We propose a novel one-dimensional model that includes both shock and turbulence heating and qualify how these processes contribute to heating the corona and driving the solar wind. Compressible MHD simulations allow us to automatically consider shock formation and dissipation, while turbulent dissipation is modeled via a one-point closure based on Alfv\'en wave turbulence. Numerical simulations were conducted with different photospheric perpendicular correlation lengths $\lambda_0$, which is a critical parameter of Alfv\'en wave turbulence, and different root-mean-square photospheric transverse-wave amplitudes $\delta v_0$. For the various $\lambda_0$, we obtain a low-temperature chromosphere, high-temperature corona, and supersonic solar wind. Our analysis shows that turbulence heating is always dominant when $\lambda_0 \lesssim 1{\rm \ Mm}$. This result does not mean that we can ignore the compressibility because the analysis indicates that the compressible waves and their associated density fluctuations enhance the Alfv\'en wave reflection and therefore the turbulence heating. The density fluctuation and the cross helicity are strongly affected by $\lambda_0$, while the coronal temperature and mass loss rate depend weakly on $\lambda_0$., Comment: Accepted for publicaiton in The Astrophysical Journal

We propose a reconnection-condensation model in which topological change in a coronal magnetic field via reconnection triggers radiative condensation, thereby resulting in prominence formation. Previous observational studies have suggested that reconnection at a polarity inversion line of a coronal arcade field creates a flux rope that can sustain a prominence; however, they did not explain the origin of cool dense plasmas of prominences. Using three-dimensional magnetohydrodynamic simulations including anisotropic nonlinear thermal conduction and optically thin radiative cooling, we demonstrate that reconnection can lead not only to flux rope formation but also to radiative condensation under a certain condition. In our model, this condition is described by the Field length, which is defined as the scale length for thermal balance between radiative cooling and thermal conduction. This critical condition depends weakly on the artificial background heating. The extreme ultraviolet emissions synthesized with our simulation results have good agreement with observational signatures reported in previous studies., Comment: 22 pages, 11 figures, accepted to ApJ

MHD waves permeate the solar atmosphere and constitute potential coronal heating agents. Yet, the waves detected so far may be but a small subset of the true existing wave power. Detection is limited by instrumental constraints, but also by wave processes that localise the wave power in undetectable spatial scales. In this study we conduct 3D MHD simulations and forward modelling of standing transverse MHD waves in coronal loops with uniform and non-uniform temperature variation in the perpendicular cross-section. The observed signatures are largely dominated by the combination of the Kelvin-Helmholtz instability (KHI), resonant absorption and phase mixing. In the presence of a cross-loop temperature gradient we find that emission lines sensitive to the loop core catch different signatures than those more sensitive to the loop boundary and the surrounding corona, leading to an out-of-phase intensity modulation produced by the KHI mixing. Common signatures to all considered models include an intensity and loop width modulation at half the kink period, fine strand-like structure, a characteristic arrow-shaped structure in the Doppler maps, overall line broadening in time but particularly at the loop edges. For our model, most of these features can be captured with a spatial resolution of $0.33\arcsec$ and spectral resolution of 25~km~s$^{-1}$, although severe over-estimation of the line width is obtained. Resonant absorption leads to a significant decrease of the observed kinetic energy from Doppler motions over time, which is not recovered by a corresponding increase in the line width from phase mixing and the KHI motions. We estimate this hidden wave energy to be a factor of $5-10$ of the observed value., Comment: 24 pages, 19 figures. Accepted for publication in ApJ. In this ArXiv version, some of the figures have reduced quality due to size limits

The main object of the paper is to present the condition of the non-diffusive part of the Reynolds stress for driving the double-cell structure of the solar meridional circulation, which has been revealed by recent helioseismic observations. By conducting a set of mean-field hydrodynamic simulations, we confirm for the first time that the double-cell meridional circulation can be achieved along with the solar-like differential rotation when the Reynolds stress transports the angular momentum upward in the lower part and downward in the upper part of the convection zone. It is concluded that, in a stationary state, the accumulated angular momentum via the Reynolds stress in the middle layer is advected to both the upper and lower parts of the convection zone by each of the two meridional circulation cells, respectively., Comment: 16 pages, 9 figures, accepted for publication in ApJ

In the highly structured solar corona, resonant absorption is an unavoidable mechanism of energy transfer from global transverse MHD waves to local azimuthal Alfv\'en waves. Due to its localised nature, a direct detection of this mechanism is extremely difficult. Yet, it is the leading theory explaining the observed fast damping of the global transverse waves. However, at odds with this theoretical prediction, recent observations indicate that in the low amplitude regime such transverse MHD waves can also appear decay-less, a yet unsolved phenomenon. Recent numerical work has shown that Kelvin-Helmholtz instabilities (KHI) often accompany transverse MHD waves. In this work, we combine 3D MHD simulations and forward modelling to show that for currently achieved spatial resolution and observed small amplitudes, an apparent decay-less oscillation is obtained. This effect results from the combination of periodic brightenings produced by the KHI and the coherent motion of the KHI vortices amplified by resonant absorption. Such effect is especially clear in emission lines forming at temperatures that capture the boundary dynamics rather than the core, and reflects the low damping character of the local azimuthal Alfv\'en waves resonantly coupled to the kink mode. Due to phase mixing, the detected period can vary depending on the emission line, with those sensitive to the boundary having shorter periods than those sensitive to the loop core. This allows to estimate the density contrast at the boundary., Comment: 8 pages, 4 figures; Accepted for publication in The Astrophysical Journal Letters

Using one-dimensional numerical simulations, we study the elementary process of Alfv\'{e}n wave reflection in a uniform medium, including nonlinear effects. In the linear regime, Alfv\'{e}n wave reflection is triggered only by the inhomogeneity of the medium, whereas in the nonlinear regime, it can occur via nonlinear wave-wave interactions. Such nonlinear reflection (backscattering) is typified by decay instability. In most studies of decay instabilities, the initial condition has been a circularly polarized Alfv\'{e}n wave. In this study we consider a linearly polarized Alfv\'en wave, which drives density fluctuations by its magnetic pressure force. For generality, we also assume a broadband wave with a red-noise spectrum. In the data analysis, we decompose the fluctuations into characteristic variables using local eigenvectors, thus revealing the behaviors of the individual modes. Different from circular-polarization case, we find that the wave steepening produces a new energy channel from the parent Alfv\'en wave to the backscattered one. Such nonlinear reflection explains the observed increasing energy ratio of the sunward to the anti-sunward Alfv\'{e}nic fluctuations in the solar wind with distance against the dynamical alignment effect., Comment: 31 pages, 12 figures, accepted by The Astrophysical Journal

Using a three-dimensional magnetohydrodynamic model, we simulate the magnetic reconnection in a single current sheet. We assume a finite guide field, a random perturbation on the velocity field and uniform resistivity. Our model enhances the reconnection rate relative to the classical Sweet-Parker model in the same configuration. The efficiency of magnetic energy conversion is increased by interactions between the multiple tearing layers coexisting in the global current sheet. This interaction, which forms a positive-feedback system, arises from coupling of the inflow and outflow regions in different layers across the current sheet. The coupling accelerates the elementary reconnection events, thereby enhancing the global reconnection rate. The reconnection establishes flux tubes along each tearing layer. Slow-mode shocks gradually form along the outer boundaries of these tubes, further accelerating the magnetic energy conversion.Such positive-feedback system is absent in two-dimensional simulation, three-dimensional reconnection without a guide field and a reconnection under a single perturbation mode. We refer to our model as the "shock-evoking positive-feedback" model., Comment: 36 pages, 18 figures, accepted for publication in ApJ

Transverse magnetohydrodynamic (MHD) waves are ubiquitous in the solar atmosphere and may be responsible for generating the Sun's million-degree outer atmosphere. However, direct evidence of the dissipation process and heating from these waves remains elusive. Through advanced numerical simulations combined with appropriate forward modeling of a prominence flux tube, we provide the observational signatures of transverse MHD waves in prominence plasmas. We show that these signatures are characterized by thread-like substructure, strong transverse dynamical coherence, an out-of-phase difference between plane-of-the-sky motions and LOS velocities, and enhanced line broadening and heating around most of the flux tube. A complex combination between resonant absorption and Kelvin-Helmholtz instabilities (KHI) takes place in which the KHI extracts the energy from the resonant layer and dissipates it through vortices and current sheets, which rapidly degenerate into turbulence. An inward enlargement of the boundary is produced in which the turbulent flows conserve the characteristic dynamics from the resonance, therefore guaranteeing detectability of the resonance imprints. We show that the features described in the accompanying paper (Okamoto et al. 2015) through coordinated Hinode and IRIS observations match well the numerical results., Comment: This is part 2 of a series of 2 papers. Part 1 corresponds to Okamoto et al. (2015, accepted). 36 Pages (single column), 10 figures. Accepted for publication in The Astrophysical Journal

Transverse magnetohydrodynamic (MHD) waves have been shown to be ubiquitous in the solar atmosphere and can in principle carry sufficient energy to generate and maintain the Sun's million-degree outer atmosphere or corona. However, direct evidence of the dissipation process of these waves and subsequent heating has not yet been directly observed. Here we report on high spatial, temporal, and spectral resolution observations of a solar prominence that show a compelling signature of so-called resonant absorption, a long hypothesized mechanism to efficiently convert and dissipate transverse wave energy into heat. Aside from coherence in the transverse direction, our observations show telltale phase differences around 180 degrees between transverse motions in the plane-of-sky and line-of-sight velocities of the oscillating fine structures or threads, and also suggest significant heating from chromospheric to higher temperatures. Comparison with advanced numerical simulations support a scenario in which transverse oscillations trigger a Kelvin-Helmholtz instability (KHI) at the boundaries of oscillating threads via resonant absorption. This instability leads to numerous thin current sheets in which wave energy is dissipated and plasma is heated. Our results provide direct evidence for wave-related heating in action, one of the candidate coronal heating mechanisms., Comment: 28 pages, 9 figures, accepted for publication in ApJ. Part II by Patrick Antolin et al. will appear soon

We propose an in-situ formation model for inverse-polarity solar prominence and demonstrate it using self-consistent 2.5-dimensional magnetohydrodynamics simulations, including thermal conduction along magnetic fields and optically thin radiative cooling. The model enables us to form cool dense plasma clouds inside a flux rope by radiative condensation, which is regarded as an inverse-polarity prominence. Radiative condensation is triggered by changes in the magnetic topology, i.e., formation of the flux rope from the sheared arcade field, and by thermal imbalance due to the dense plasma trapped inside the flux rope. The flux rope is created by imposing converging and shearing motion on the arcade field. Either when the footpoint motion is in the anti-shearing direction or when heating is proportional to local density, the thermal state inside the flux rope becomes cooling-dominant, leading to radiative condensation. By controlling the temperature of condensation, we investigate the relationship between the temperature and density of prominence and derive a scaling formula for this relationship. This formula suggests that the proposed model reproduce the observed density of prominence, which is 10-100 times larger than the coronal density. Moreover, the time evolution of the extreme ultraviolet emission synthesized by combining our simulation results with the response function of the Solar Dynamics Observatory Atmospheric Imaging Assembly filters agrees with the observed temporal and spatial intensity shift among multi-wavelength during in-situ condensation., Comment: 31 pages, 12 figures; accepted for publicaion in The Astrophysical Journal

In order to better understand the nature of active region outflows, the electron density was measured by using a density-sensitive line pair Fe xiv 264.78A/274.20A.Since coronal line profiles of the outflow region are composed of a major component with a Doppler shift of < 10 km s^-1 and a minor component (enhanced blue wing: EBW) blueshifted by up to 100 km s^-1, we extracted EBW from the line profiles through double-Gaussian fitting. We tried applying the simultaneous fitting to those two Fe xiv lines with several physical restrictions. Electron density for both components (n_Major and n_EBW, respectively) was calculated by referring to the theoretical intensity ratio as a function of electron density as per the CHIANTI database. We studied six locations in the outflow regions around NOAA AR10978. The average electron density was n_Major = 10^(9.16 +- 0.16) cm^-3 and n_EBW = 10^(8.74 +- 0.29) cm^-3. The magnitude relationship between n_Major and n_EBW was opposite in the eastern and western outflow regions. The column depth was also calculated for each component, which leads to the result that the outflows possess only a small fraction (~ 0.1) in the eastern region, while they dominate over the major component in the line profiles by a factor of five in the western region. When taking into account the extending coronal structures, the western region can be thought to represent the mass leakage. In contrast, we suggest a possibility that the eastern region actually contributes to the mass supply to coronal loops., Comment: 31 pages, 15 figures

Solar active regions (ARs) are thought to be formed by magnetic fields from the convection zone. Our flux emergence simulations revealed that a strong horizontal divergent flow (HDF) of unmagnetized plasma appears at the photosphere before the flux begins to emerge. In our earlier study, we analyzed HMI data for a single AR and confirmed presence of this precursor plasma flow in the actual Sun. In this paper, as an extension of our earlier study, we conducted a statistical analysis of the HDFs to further investigate their characteristics and better determine the properties. From SDO/HMI data, we picked up 23 flux emergence events over a period of 14 months, the total flux of which ranges from 10^{20} to 10^{22} Mx. Out of 23 selected events, 6 clear HDFs were detected by the method we developed in our earlier study, and 7 HDFs detected by visual inspection were added to this statistic analysis. We found that the duration of the HDF is on average 61 minutes and the maximum HDF speed is on average 3.1 km s^{-1}. We also estimated the rising speed of the subsurface magnetic flux to be 0.6-1.4 km s^{-1}. These values are highly consistent with our previous one-event analysis as well as our simulation results. The observation results lead us to the conclusion that the HDF is rather a common feature in the earliest phase of AR emergence. Moreover, our HDF analysis has capability of determining the subsurface properties of emerging fields that cannot be directly measured., Comment: 40 pages, 8 figures, 3 tables, accepted for publication in ApJ. A high-quality version can be found at http://www-space.eps.s.u-tokyo.ac.jp/~toriumi/toriumi2014apj.pdf

In this Letter we present a seismological detection of a rising motion of magnetic flux in the shallow convection zone of the Sun, and show estimates of the emerging speed and its decelerating nature. In order to evaluate the speed of subsurface flux that creates an active region, we apply six Fourier filters to the Doppler data of NOAA AR 10488, observed with SOHO/MDI, to detect the reduction of acoustic power at six different depths from -15 to -2 Mm. All the filtered acoustic powers show reductions, up to 2 hours before the magnetic flux first appears at the visible surface. The start times of these reductions show a rising trend with a gradual deceleration. The obtained velocity is first several km s^-1 in a depth range of 15--10 Mm, then ~1.5 km s^-1 at 10-5 Mm, finally ~0.5 km s^-1 at 5-2 Mm. If we assume that the power reduction is actually caused by the magnetic field, the velocity of order of 1 km s^-1 is well in accordance with previous observations and numerical studies. Moreover, the gradual deceleration strongly supports the theoretical model that the emerging flux slows down in the uppermost convection zone before it expands into the atmosphere to build an active region., Comment: 19 pages, 4 figures, accepted for publication in ApJL

Solar active regions are formed through the emergence of magnetic flux from the deeper convection zone. Recent satellite observations have shown that a horizontal divergent flow (HDF) stretches out over the solar surface just before the magnetic flux appearance. The aims of this study are to investigate the driver of the HDF and to see the dependency of the HDF on the parameters of the magnetic flux in the convection zone. We conduct three-dimensional magnetohydrodynamic (3D MHD) numerical simulations of the magnetic flux emergence and vary the parameters in the initial conditions. An analytical approach is also taken to explain the dependency. The horizontal gas pressure gradient is found to be the main driver of the HDF. The maximum HDF speed shows positive correlations with the field strength and twist intensity. The HDF duration has a weak relation with the twist, while it shows negative dependency on the field strength only in the case of the stronger field regime. Parametric dependencies analyzed in this study may allow us to probe the structure of the subsurface magnetic flux by observing properties of the HDF., Comment: 7 pages, 5 figures, 1 table, accepted for publication in Astronomy & Astrophysics

We studied electron spectral indices of nonthermal emissions seen in hard X-rays (HXRs) and in microwaves. We analyzed 12 flares observed by the Hard X-ray Telescope aboard {\it Yohkoh}, Nobeyama Radio Polarimeters (NoRP), and the Nobeyama Radioheliograph (NoRH), and compared the spectral indices derived from total fluxes of hard X-rays and microwaves. Except for four events, which have very soft HXR spectra suffering from the thermal component, these flares show a gap $\Delta\delta$ between the electron spectral indices derived from hard X-rays $\delta_{X}$ and those from microwaves $\delta_{\mu}$ ($\Delta\delta = \delta_{X} - \delta_{\mu}$) of about 1.6. Furthermore, from the start to the peak times of the HXR bursts, the time profiles of the HXR spectral index $\delta_{X}$ evolve synchronously with those of the microwave spectral index $\delta_{\mu}$, keeping the constant gap. We also examined the spatially resolved distribution of the microwave spectral index by using NoRH data. The microwave spectral index $\delta_{\mu}$ tends to be larger, which means a softer spectrum, at HXR footpoint sources with stronger magnetic field than that at the loop tops. These results suggest that the electron spectra are bent at around several hundreds of keV, and become harder at the higher energy range that contributes the microwave gyrosynchrotron emission., Comment: 24 pages, 6 figures, accepted for publication in ApJ

We investigate differential rotation in rapidly rotating solar-type stars by means of an axisymmetric mean field model that was previously applied to the sun. This allows us to calculate the latitudinal entropy gradient with a rea- sonable physical basis. Our conclusions are as follows: (1) Differential rotation approaches the Taylor-Proudman state when stellar rotation is faster than so- lar rotation. (2) Entropy gradient generated by the attached subadiabatic layer beneath the convection zone becomes relatively small with a large stellar angu- lar velocity. (3) Turbulent viscosity and turbulent angular momentum transport determine the spatial difference of angular velocity $\Delta \Omega$. (4) The results of our mean field model can explain observations of stellar differential rotation., Comment: 36 pages, 10 figures, accepted to ApJ

We present the new results of the two-dimensional numerical experiments on the cross-sectional evolution of a twisted magnetic flux tube rising from the deeper solar convection zone (-20,000 km) to the corona through the surface. The initial depth is ten times deeper than most of previous calculations focusing on the flux emergence from the uppermost convection zone. We find that the evolution is illustrated by the two-step process described below: the initial tube rises due to its buoyancy, subject to aerodynamic drag due to the external flow. Because of the azimuthal component of the magnetic field, the tube maintains its coherency and does not deform to become a vortex roll pair. When the flux tube approaches the photosphere and expands sufficiently, the plasma on the rising tube accumulates to suppress the tube's emergence. Therefore, the flux decelerates and extends horizontally beneath the surface. This new finding owes to our large scale simulation calculating simultaneously the dynamics within the interior as well as above the surface. As the magnetic pressure gradient increases around the surface, magnetic buoyancy instability is triggered locally and, as a result, the flux rises further into the solar corona. We also find that the deceleration occurs at a higher altitude than in our previous experiment using magnetic flux sheets (Toriumi and Yokoyama). By conducting parametric studies, we investigate the conditions for the two-step emergence of the rising flux tube: field strength > 1.5x10^4 G and the twist > 5.0x10^-4 km^-1 at -20,000 km depth., Comment: 42 pages, 13 figures, 2 tables, accepted for publication in ApJ. High-resolution figures will appear in the published version

We present a series of numerical experiments that model the evolution of magnetic flux tubes with a different amount of initial twist. As a result of calculations, tightly twisted tubes reveal a rapid two-step emergence to the atmosphere with a slight slowdown at the surface, while weakly twisted tubes show a slow two-step emergence waiting longer the secondary instability to be triggered. This picture of the two-step emergence is highly consistent with recent observations. These tubes show multiple magnetic domes above the surface, indicating that the secondary emergence is caused by interchange mode of magnetic buoyancy instability. As for the weakest twist case, the tube exhibits an elongated photospheric structure and never rises into the corona. The formation of the photospheric structure is due to inward magnetic tension force of the azimuthal field component of the rising flux tube (i.e., tube's twist). When the twist is weak, azimuthal field cannot hold the tube's coherency, and the tube extends laterally at the subadiabatic surface. In addition, we newly find that the total magnetic energy measured above the surface depends on the initial twist. Strong twist tubes follow the initial relation between the twist and the magnetic energy, while weak twist tubes deviates from this relation, because these tubes store their magnetic energy in the photospheric structures., Comment: 20 pages, 8 figures, accepted for publication in PASJ (Publications of the Astronomical Society of Japan)

In order to better understand the possibility of coronal heating by MHD waves, we analyze Fe xii 195.12{\AA} data observed with EUV Imaging Spectrometer (EIS) onboard Hinode. We performed a Fourier analysis of EUV intensity and Doppler velocity time series data in the active region corona. Notable intensity and Doppler velocity oscillations were found for two moss regions out of the five studied, while only small oscillations were found for five apexes of loops. The amplitudes of the oscillations were 0.4 - 5.7% for intensity and 0.2 - 1.2 kms-1 for Doppler velocity. In addition, oscillations of only Doppler velocity were seen relatively less often in the data. We compared the amplitudes of intensity and those of Doppler velocity in order to identify MHD wave modes, and calculated the phase delays between Fourier components of intensity and those of Doppler velocity. The results are interpreted in terms of MHD waves as follows: (1) few kink modes or torsional Alfv'en mode waves were seen in both moss regions and the apexes of loops; (2) upwardly propagating and standing slowmode waves were found inmoss regions; and (3) consistent with previous studies, estimated values of energy flux of the waves were several orders of magnitude lower than that required for heating active regions., Comment: 22 pages, 1 table, 5 figures

We perform two-dimensional MHD simulations on the solar flux emergence. We set the initial magnetic flux sheet at z=-20,000 km in the convection zone. The flux sheet rises through the convective layer due to the Parker instability, however, decelerates beneath the photosphere because the plasma on the flux sheet piles up owing to the convectively stable photosphere above. Meanwhile, the flux sheet becomes locally unstable to the Parker instability within the photosphere, and the further evolution to the corona occurs (two-step emergence model). We carry out a parameter survey to investigate the condition for this two-step model. We find that magnetic fluxes which form active regions are likely to have undergone the two-step emergence. The condition for the two-step emergence is 10^21 - 10^22 Mx with 10^4 G at z=-20,000 km in the convection zone., Comment: 41 pages, 15 figures, 1 table, Accepted for publication in ApJ

In this paper, results of 2.5-dimensional magnetohydrodynamical simulations are reported for the magnetic reconnection of non-perfectly antiparallel magnetic fields. The magnetic field has a component perpendicular to the computational plane, that is, guide field. The angle theta between magnetic field lines in two half regions is a key parameter in our simulations whereas the initial distribution of the plasma is assumed to be simple; density and pressure are uniform except for the current sheet region. Alfven waves are generated at the reconnection point and propagate along the reconnected field line. The energy fluxes of the Alfven waves and magneto-acoustic waves (slow mode and fast mode) generated by the magnetic reconnection are measured. Each flux shows the similar time evolution independent of theta. The percentage of the energies (time integral of energy fluxes) carried by the Alfven waves and magneto-acoustic waves to the released magnetic energy are calculated. The Alfven waves carry 38.9%, 36.0%, and 29.5% of the released magnetic energy at the maximum (theta=80^\circ) in the case of beta=0.1, 1, and 20 respectively, where beta is the plasma beta (the ratio of gas pressure to magnetic pressure). The magneto-acoustic waves carry 16.2% (theta=70^\circ), 25.9% (theta=60^\circ), and 75.0% (theta=180^\circ) of the energy at the maximum. Implications of these results for solar coronal heating and acceleration of high-speed solar wind are discussed., Comment: Accepted for publication in PASJ. 24 pages, 11 figures

The formation and evolution process and magnetic configuration of solar prominences remain unclear. In order to study the formation process of prominences, we examine continuous observations of a prominence in NOAA AR 10953 with the Solar Optical Telescope on the \emph{Hinode} satellite. As reported in our previous Letter, we find a signature suggesting that a helical flux rope emerges from below the photosphere under a pre-existing prominence. Here we investigate more detailed properties and photospheric indications of the emerging helical flux rope, and discuss their relationship to the formation of the prominence. Our main conclusions are: (1) A dark region with absence of strong vertical magnetic fields broadens and then narrows in Ca \textsc{ii} H-line filtergrams. This phenomenon is consistent with the emergence of the helical flux rope as photospheric counterparts. The size of the flux rope is roughly 30,000 km long and 10,000 km wide. The width is larger than that of the prominence. (2) No shear motion or converging flows are detected, but we find diverging flows such as mesogranules along the polarity inversion line. The presence of mesogranules may be related to the emergence of the helical flux rope. (3) The emerging helical flux rope reconnects with magnetic fields of the pre-existing prominence to stabilize the prominence for the next several days. We thus conjecture that prominence coronal magnetic fields emerge in the form of helical flux ropes that contribute to the formation and maintenance of the prominence., Comment: 26 pages, 12 figures. Accepted in ApJ

We present a detailed examination on the coronal nonthermal emissions during the preflare phase of the X4.8 flare that occurred on 2002 July 23. The microwave (17 GHz and 34 GHz) data obtained with Nobeyama Radioheliograph, at Nobeyama Solar Radio Observatory and the hard X-ray (HXR) data taken with {\it Reuven Ramaty High Energy Solar Spectroscopic Imager} obviously showed nonthermal sources that are located above the flare loops during the preflare phase. We performed imaging spectroscopic analyses on the nonthermal emission sources both in microwaves and in HXRs, and confirmed that electrons are accelerated from several tens of keV to more than 1 MeV even in this phase. If we assume the thin-target model for the HXR emission source, the derived electron spectral indices ($\sim 4.7$) is the same value as that from microwaves ($\sim 4.7$) within the observational uncertainties, which implies that the distribution of the accelerated electrons follows a single power-law. The number density of the microwave-emitting electrons is, however, larger than that of the HXR-emitting electrons, unless we assume low ambient plasma density of about $1.0 \times 10^9$ cm$^{-3}$ for the HXR-emitting region. If we adopt the thick-target model for the HXR emission source, on the other hand, the electron spectral index ($\sim 6.7$) is much different, while the gap of the number density of the accelerated electrons is somewhat reduced., Comment: 21 pages, 6 figures, ApJ accepted

Continuous observations were obtained of active region 10953 with the Solar Optical Telescope (SOT) on board the \emph{Hinode} satellite during 2007 April 28 to May 9. A prominence was located over the polarity inversion line (PIL) in the south-east of the main sunspot. These observations provided us with a time series of vector magnetic fields on the photosphere under the prominence. We found four features: (1) The abutting opposite-polarity regions on the two sides along the PIL first grew laterally in size and then narrowed. (2) These abutting regions contained vertically-weak, but horizontally-strong magnetic fields. (3) The orientations of the horizontal magnetic fields along the PIL on the photosphere gradually changed with time from a normal-polarity configuration to a inverse-polarity one. (4) The horizontal-magnetic field region was blueshifted. These indicate that helical flux rope was emerging from below the photosphere into the corona along the PIL under the pre-existing prominence. We suggest that this supply of a helical magnetic flux into the corona is associated with evolution and maintenance of active-region prominences., Comment: 10 pages, 2 figures, accepted for publication in ApJ Letters

Continuous observations of a flare productive active region 10930 were successfully carried out with the Solar Optical Telescope onboard the Hinode spacecraft during 2007 December 6 to 19. We focus on the evolution of photospheric magnetic fields in this active region, and magnetic field properties at the site of the X3.4 class flare, using a time series of vector field maps with high spatial resolution. The X3.4 class flare occurred on 2006 December 13 at the apparent collision site between the large, opposite polarity umbrae. Elongated magnetic structures with alternatingly positive and negative polarities resulting from flux emergence appeared one day before the flare in the collision site penumbra. Subsequently, the polarity inversion line at the collision site became very complicated. The number of bright loops in Ca II H increased during the formation of these elongated magnetic structures. The flare ribbons and bright loops evolved along the polarity inversion line and one footpoint of the bright loop was located in a region having a large departure of field azimuth angle with respect to its surroundings. The SOT observations with high spatial resolution and high polarization precision reveal temporal change in fine structure of magnetic fields at the flare site: some parts of the complicated polarity inversion line then disappeared, and in those regions the azimuth angle of photospheric magnetic field changed by about 90 degrees, becoming more spatially uniform within the collision site., Comment: 6 pages, 4 figures, accepted for publication in PASJ (Hinode Special Issue)

On 2005 September 13 a filament eruption accompanied by a halo CME occurred in the most flare-productive active region NOAA 10808 in Solar Cycle 23. Using multi-wavelength observations before the filament eruption on Sep. 13th, we investigate the processes leading to the catastrophic eruption. We find that the filament slowly ascended at a speed of 0.1km/s over two days before the eruption. During slow ascending, many small flares were observed close to the footpoints of the filament, where new magnetic elements were emerging. On the basis of the observational facts we discuss the triggering mechanism leading to the filament eruption. We suggest the process toward the eruption as follows: First, a series of small flares played a role in changing the topology of the loops overlying the filament. Second, the small flares gradually changed the equilibrium state of the filament and caused the filament to ascend slowly over two days. Finally, a C2.9 flare that occurred when the filament was close to the critical point for loss of equilibrium directly led to the catastrophic filament eruption right after itself., Comment: 31 pages, 15 figures, 2 tables, accepted for publication in ApJ

We consider MHD waves as a heating source of cool cores of galaxy clusters. In particular, we focus on transverse waves (Alfven waves), because they can propagate a longer distance than longitudinal waves (sound waves). Using MHD simulations, we found that the transverse waves can stably heat a cool core if the wave period is large enough (>~ 10^8 yr). Moreover, the longitudinal waves that are created as a by-product of the nonlinear evolution of the transverse waves could be observed as the 'ripples' found in cool cores., Comment: Accepted for publication in ApJL

It has been recognized that magnetic reconnection process is of great importance in high-energy astrophysics. We develop a new two-dimensional relativistic resistive magnetohydrodynamic (R$^2$MHD) code, and carry out numerical simulations of magnetic reconnection. We find that outflow velocity reaches Alfv\'{e}n velocity in the inflow region, and that higher Alfv\'{e}n velocity provides higher reconnection rate. We also find Lorentz contraction plays an important role in enhancement of reconnection rate., Comment: 11 pages, 4 figures, accepted by ApJL

We report a statistical study of flares observed with the Soft X-ray Telescope (SXT) onboard Yohkoh in the year of 2000. We measure physical parameters of 77 flares, such as the temporal scale, the size, and the magnetic flux density and find that the sizes of flares tend to be distributed more broadly as the GOES class becomes weaker and that there is a lower limit of magnetic flux density that depends on the GOES class. We also examine the relationship between these parameters and find weak correlation between temporal and spatial scales of flares. We estimate reconnection inflow velocity, coronal Alfven velocity, and reconnection rate using above observed values. The inflow velocities are distributed from a few km/s to several tens km/s and the Alfven velocities in the corona are in the range from 10^3 to 10^4 km/s. Hence the reconnection rate is 10^-3 - 10^-2. We find that the reconnection rate in a flare tends to decrease as the GOES class of the flare increases. This value is within one order of magnitude from the theoretical maximum value predicted by the Petschek model, although the dependence of the reconnection rate on the magnetic Reynolds number tends to be stronger than that in the Petschek model., Comment: 21 pages, 8 figures, accepted for publication in ApJ

On December 27, 2004, plasma particle detectors on the GEOTAIL spacecraft detected an extremely strong signal of hard X-ray photons from the giant flare of SGR1806-20, a magnetar candidate. While practically all gamma-ray detectors on any satellites were saturated during the first ~500 ms interval after the onset, one of the particle detectors on GEOTAIL was not saturated and provided unique measurements of the hard X-ray intensity and the profile for the first 600 ms interval with 5.48 ms time resolution. After ~50 ms from the initial rapid onset, the peak photon flux (integrated above ~50 keV) reached the order of 10^7 photons sec^{-1} cm^{-2}. Assuming a blackbody spectrum with kT=175 keV, we estimate the peak energy flux to be 21 erg sec^{-1} cm^{-2} and the fluence (for 0-600 ms) to be 2.4 erg cm^{-2}. The implied energy release comparable to the magnetic energy stored in a magnetar (~10^{47} erg) suggests an extremely efficient energy release mechanism., Comment: 6 pages, 2 color figures, submitted to Nature

We present the relation of the spatial distribution of H-alpha kernels with the distribution of hard X-ray (HXR) sources seen during the 2001 April 10 solar flare. This flare was observed in H-alpha with the (Sartorius) telescope at Kwasan Observatory, Kyoto University, and in hard X-rays (HXRs) with the Hard X-ray Telescope (HXT) onboard Yohkoh. We compared the spatial distribution of the HXR sources with that of the H-alpha kernels. While many H-alpha kernels are found to brighten successively during the evolution of the flare ribbons, only a few radiation sources are seen in the HXR images. We measured the photospheric magnetic field strengths at each radiation source in the H-alpha images, and found that the H-alpha kernels accompanied by HXR radiation have magnetic strengths about 3 times larger than those without HXR radiation. We also estimated the energy release rates based on the magnetic reconnection model. The release rates at the H-alpha kernels with accompanying HXR sources are 16-27 times larger than those without HXR sources. These values are sufficiently larger than the dynamic range of HXT, which is about 10, so that the difference between the spatial distributions of the H$\alpha$ kernels and the HXR sources can be explained., Comment: 10 pages, 3 figures