arXiv subjects

Leon Ofman

Publications and source records attributed to Leon Ofman.

At least 37 records · Page 2Linked to original sources

arXiv2021

Automated identification of transiting exoplanet candidates in NASA Transiting Exoplanets Survey Satellite (TESS) data with machine learning methods

A novel artificial intelligence (AI) technique that uses machine learning (ML) methodologies combines several algorithms, which were developed by ThetaRay, Inc., is applied to NASA's Transiting Exoplanets Survey Satellite (TESS) dataset to identify exoplanetary candidates. The AI/ML ThetaRay system is trained initially with Kepler exoplanetary data and validated with confirmed exoplanets before its application to TESS data. Existing and new features of the data, based on various observational parameters, are constructed and used in the AI/ML analysis by employing semi-supervised and unsupervised machine learning techniques. By the application of ThetaRay system to 10,803 light curves of threshold crossing events (TCEs) produced by the TESS mission, obtained from the Mikulski Archive for Space Telescopes, the algorithm yields about 50 targets for further analysis, and we uncover three new exoplanetary candidates by further manual vetting. This study demonstrates for the first time the successful application of the particular combined multiple AI/ML-based methodologies to a large astrophysical dataset for rapid automated classification of TCEs.

astro-ph.EP↗

arXiv2021

Slow-Mode Magnetoacoustic Waves in Coronal Loops

Rapidly decaying long-period oscillations often occur in hot coronal loops of active regions associated with small (or micro-) flares. This kind of wave activity was first discovered with the SOHO/SUMER spectrometer from Doppler velocity measurements of hot emission lines, thus also often called "SUMER" oscillations. They were mainly interpreted as global (or fundamental mode) standing slow magnetoacoustic waves. In addition, increasing evidence has suggested that the decaying harmonic type of pulsations detected in light curves of solar and stellar flares are likely caused by standing slow-mode waves. The study of slow magnetoacoustic waves in coronal loops has become a topic of particular interest in connection with coronal seismology. We review recent results from SDO/AIA and Hinode/XRT observations that have detected both standing and reflected intensity oscillations in hot flaring loops showing the physical properties (e.g., oscillation periods, decay times, and triggers) in accord with the SUMER oscillations. We also review recent advances in theory and numerical modeling of slow-mode waves focusing on the wave excitation and damping mechanisms. MHD simulations in 1D, 2D and 3D have been dedicated to understanding the physical conditions for the generation of a reflected propagating or a standing wave by impulsive heating. Various damping mechanisms and their analysis methods are summarized. Calculations based on linear theory suggest that the non-ideal MHD effects such as thermal conduction, compressive viscosity, and optically thin radiation may dominate in damping of slow-mode waves in coronal loops of different physical conditions. Finally, an overview is given of several important seismological applications such as determination of transport coefficients and heating function.

astro-ph.SR↗

arXiv2020

Coronal heating by MHD waves

The heating of the solar chromosphere and corona to the observed high temperatures, imply the presence of ongoing heating that balances the strong radiative and thermal conduction losses expected in the solar atmosphere. It has been theorized for decades that the required heating mechanisms of the chromospheric and coronal parts of the active regions, quiet-Sun, and coronal holes are associated with the solar magnetic fields. However, the exact physical process that transport and dissipate the magnetic energy which ultimately leads to the solar plasma heating are not yet fully understood. The current understanding of coronal heating relies on two main mechanism: reconnection and MHD waves that may have various degrees of importance in different coronal regions. In this review we focus on recent advances in our understanding of MHD wave heating mechanisms. First, we focus on giving an overview of observational results, where we show that different wave modes have been discovered in the corona in the last decade, many of which are associated with a significant energy flux, either generated in situ or pumped from the lower solar atmosphere. Afterwards, we summarise the recent findings of numerical modelling of waves, motivated by the observational results. Despite the advances, only 3D MHD models with Alfvén wave heating in an unstructured corona can explain the observed coronal temperatures compatible with the quiet Sun, while 3D MHD wave heating models including cross-field density structuring are not yet able to account for the heating of coronal loops in active regions to their observed temperature.

astro-ph.SR↗

arXiv2020

Fast Magnetosonic Waves and Flows in a Solar Prominence Foot: Observations and Modeling

We study recent observations of propagating fluctuations in a prominence foot with Hinode Solar Optical Telescope (SOT) high-resolution observations in Ca~II and H alpha emission which we identify as nonlinear fast magnetosnic waves. Here we analyze further the observations of propagating waves and flows with Interface Region Imaging Spectrograph (IRIS) Mg~II slit jaw images, in addition to Hinode/SOT Ca~II images. We find that the waves have typical periods in the range of 5 - 11 minutes and wavelengths in the plane of the sky (POS) of about 2000 km, while the flows in narrow threads have typical speed in the POS of ~16-46 km/s. We also detect apparent kink oscillations in the threads with flowing material, and apply coronal seismology to estimate the magnetic field strength in the range 5-17 G. Using 2.5D MHD we model the combined effects of nonlinear waves and flows on the observed dynamics of the prominence material, and reproduce the propagating and refracting fast magnetosonic waves, as well as standing kink-mode waves in flowing material along the magnetic field. The modeling results are in good qualitative agreements with the observations of the various waves and flows in the prominence foot, further confirming coronal seismology analysis and improving the understanding of the fine scale dynamics of the prominence material.

astro-ph.SR↗

arXiv2019

Determination of transport coefficients by coronal seismology of flare-induced slow-mode waves: Numerical parametric study of 1D loop model

Recent studies of a flaring loop oscillation event on 2013 December 28 observed by the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO) have revealed the suppression of thermal conduction and significant enhancement of compressive viscosity in hot ($\sim$10 MK) plasma. In this study we aim at developing a new coronal seismology method for determining the transport coefficients based on a parametric study of wave properties using a 1D nonlinear MHD loop model in combination with the linear theory. The simulations suggest a two-step scheme: we first determine the effective thermal conduction coefficient from the observed phase shift between temperature and density perturbations as this physical parameter is insensitive to the unknown viscosity; then from the loop model with the obtained thermal conduction coefficient, we determine the effective viscosity coefficient from the observed decay time using the parametric modeling. With this new seismology technique we are able to quantify the suppression of thermal conductivity by a factor of about 3 and the enhancement of viscosity coefficient by a factor of 10 in the studied flaring loop. Using the loop model with these refined transport coefficients, we study the excitation of slow magnetoacoustic waves by launching a flow pulse from one footpoint. The simulation can self-consistently produce the fundamental standing wave on a timescale in agreement with the observation.

astro-ph.SR↗

arXiv2019

Nonlinear Evolution of Ion Kinetic Instabilities in the Solar Wind

In-situ observations of the solar wind (SW) plasma from 0.29 to 1AU show that the protons and alpha particles are often non-Maxwellian, with evidence of kinetic instabilities, temperature anisotropies, differential ion streaming, and associated magnetic fluctuations spectra. The kinetic instabilities in the SW multi-ion plasma can lead to preferential heating of alpha particles and the dissipation of magnetic fluctuation energy, affecting the kinetic and global properties of the SW. Using for the first time a three-dimensional hybrid model, where ions are modeled as particle using the Particle-In-Cell (PIC) method and electrons are treated as fluid, we study the onset, nonlinear evolution and dissipation of ion kinetic instabilities. The Alfven/ion-cyclotron, and the ion drift instabilities are modeled in the region close to the Sun (~10R_s). Solar wind expansion is incorporated in the model. The model produces self-consistent non-Maxwellian velocity distribution functions (VDFs) of unstable ion populations, the associated temperature anisotropies, and wave spectra for several typical SW instability cases in the nonlinear growth and saturation stage of the instabilities. The 3D hybrid modeling of the multi-ion SW plasma could be used to study the SW acceleration region close to the Sun that will be explored by the Parker Solar Probe mission.

physics.space-ph↗

arXiv2018

A Truly Global Extreme Ultraviolet Wave from the SOL2017-09-10 X8.2+ Solar Flare-Coronal Mass Ejection

We report SDO/AIA observations of an extraordinary global extreme ultraviolet (EUV) wave triggered by the X8.2+ flare-CME eruption on 2017 September 10. This was one of the best EUV waves ever observed with modern instruments, yet it was likely the last one of such magnitudes of Solar Cycle 24 as the Sun heads toward the minimum. Its remarkable characteristics include the following. (1) The wave was observed, for the first time, to traverse the full-Sun corona over the entire visible solar disk and off-limb circumference, manifesting a truly global nature, owing to its exceptionally large amplitude, e.g., with EUV enhancements by up to 300% at 1.1 Rsun from the eruption. (2) This leads to strong transmissions (in addition to commonly observed reflections) in and out of both polar coronal holes, which are usually devoid of EUV waves. It has elevated wave speeds >2000 km/s within them, consistent with the expected higher fast-mode magnetosonic wave speeds. The coronal holes essentially serve as new "radiation centers" for the waves being refracted out of them, which then travel toward the equator and collide head-on, causing additional EUV enhancements. (3) The wave produces significant compressional heating to local plasma upon its impact, indicated by long-lasting EUV intensity changes and differential emission measure increases at higher temperatures (e.g., log T=6.2) accompanied by decreases at lower temperatures (e.g., log T=6.0). These characteristics signify the potential of such EUV waves for novel magnetic and thermal diagnostics of the solar corona {\it on global scales}.

astro-ph.SR↗

arXiv2018

Effect of transport coefficients on excitation of flare-induced standing slow-mode waves in coronal loops

Standing slow-mode waves have been recently observed in flaring loops by the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO). By means of the coronal seismology technique transport coefficients in hot ($\sim$10 MK) plasma were determined by Wang et al.(2015, Paper I), revealing that thermal conductivity is nearly suppressed and compressive viscosity is enhanced by more than an order of magnitude. In this study we use 1D nonlinear MHD simulations to validate the predicted results from the linear theory and investigate the standing slow-mode wave excitation mechanism. We first explore the wave trigger based on the magnetic field extrapolation and flare emission features. Using a flow pulse driven at one footpoint we simulate the wave excitation in two types of loop models: model 1 with the classical transport coefficients and model 2 with the seismology-determined transport coefficients. We find that model 2 can form the standing wave pattern (within about one period) from initial propagating disturbances much faster than model 1, in better agreement with the observations. Simulations of the harmonic waves and the Fourier decomposition analysis show that the scaling law between damping time ($τ$) and wave period ($P$) follows $τ\propto{P^2}$ in model 2, while $τ\propto{P}$ in model 1. This indicates that the largely enhanced viscosity efficiently increases the dissipation of higher harmonic components, favoring the quick formation of the fundamental standing mode. Our study suggests that observational constraints on the transport coefficients are important in understanding both, the wave excitation and damping mechanisms.

astro-ph.SR↗

arXiv2018

Quasi-periodic Counter-propagating Fast Magnetosonic Wave Trains from Neighboring Flares: SDO/AIA Observations and 3D MHD Modeling

Since their discovery by SDO/AIA in EUV, rapid (phase speeds of 1000 km/s), quasi-periodic, fast-mode propagating wave trains (QFPs) have been observed accompanying many solar flares. They typically propagate in funnel-like structures associated with the expanding magnetic field topology of the active regions (ARs). The waves provide information on the associated flare pulsations and the magnetic structure through coronal seismology. The reported waves usually originate from a single localized source associated with the flare. Here, we report the first detection of counter-propagating QFPs associated with two neighboring flares on 2013 May 22, apparently connected by large-scale, trans-equatorial coronal loops. We present the first results of 3D MHD model of counter-propagating QFPs an idealized bi-polar AR. We investigate the excitation, propagation, nonlinearity, and interaction of the counter-propagating waves for a range of key model parameters, such as the properties of the sources and the background magnetic structure. In addition to QFPs, we also find evidence of trapped fast (kink) and slow mode waves associated with the event. We apply coronal seismology to determine the magnetic field strength in an oscillating loop during the event. Our model results are in qualitative agreement with the AIA-observed counter propagating waves and are used to identify the various MHD wave modes associated with the observed event providing insights into their linear and nonlinear interactions. Our observations provide the first direct evidence of counter-propagating fast magnetosonic waves that can potentially lead to turbulent cascade and carry significant energy flux for coronal heating in low-corona magnetic structures.

astro-ph.SR↗

arXiv2017

Energy release in the solar atmosphere from a stream of infalling prominence debris

Recent high-resolution and high-cadence EUV imaging has revealed a new phenomenon, impacting prominence debris, where prominence material from failed or partial eruptions can impact the lower atmosphere, releasing energy. We report a clear example of energy release and EUV brightening due to infalling prominence debris that occurred on 2011 September 7-8. The initial eruption of material was associated with an X1.8-class flare from AR11283, occurring at 22:30 UT on 2011 September 7. Subsequently, a semi-continuous stream of this material returned to the solar surface with a velocity v > 150 km/s, impacting a region remote from the original active region between 00:20 - 00:40 UT on 2011 September 8. Using SDO/AIA, the differential emission measure of the plasma was estimated throughout this brightening event. We found that the radiated energy of the impacted plasma was L_rad ~10^27 ergs, while the thermal energy peaked at ~10^28 ergs. From this we were able to determine the mass content of the debris to be in the range 2x10^14 < m < 2x10^15 g. Given typical promimence masses, the likely debris mass is towards the lower end of this range. This clear example of a prominence debris event shows that significant energy release takes place during these events, and that such impacts may be used as a novel diagnostic tool for investigating prominence material properties.

astro-ph.SR↗

arXiv2017

Slow-Mode Oscillations of Hot Loops Excited at Flaring Footpoints

The analysis of a hot loop oscillation event using SOHO/SUMER, GOES/SXI, and RHESSI observations is presented. Damped Doppler shift oscillations were detected in the Fe XIX line by SUMER, and interpreted as a fundamental standing slow mode. The evolution of soft X-ray emission from GOES/SXI and hard X-ray sources from RHESSI suggests that the oscillations of a large loop are triggered by a small flare, which may be produced by interaction (local reconnection) of this large loop with a small loop at its footpoint. This study provides clear evidence supporting our early conjecture that the slow-mode standing waves in hot coronal loops are excited by impulsive heating (small or microflares) at the loop's footpoint.

astro-ph.SR↗

arXiv2017

Excitation of flare-induced waves in coronal loops and the effects of radiative cooling

EUV imaging observations from several space missions (SOHO/EIT, TRACE, and SDO/AIA) have revealed a presence of propagating intensity disturbances in solar coronal loops. These disturbances are typically interpreted as slow magnetoacoustic waves. Recent spectroscopic observations with Hinode/EIS of active region loops, however, revealed that the propagating intensity disturbances are associated with intermittent plasma upflows (or jets) at the footpoints which are presumably generated by magnetic reconnection. For this reason, whether these disturbances are waves or periodic flows is still being studied. This study is aimed at understanding the physical properties of observed disturbances by investigating the excitation of waves by hot plasma injections from below and the evolution of flows and wave propagation along the loop. We expand our previous studies based on isothermal 3D MHD models of active region to a more realistic model that includes full energy equation accounting for effects of radiative losses. Computations are initialized with an equilibrium state of a model active region using potential (dipole) magnetic field, gravitationally stratified density and temperature obtained from polytropic equation of state. We model an impulsive injection of hot plasma into the steady plasma outflow along the loops of different temperature, warm ($\sim$1 MK) and hot ($\sim$6 MK). The simulations show that hot jets launched at the coronal base excite slow magnetoacoustic waves that propagate along the loops to the high corona, while the injected hot flows decelerates rapidly with heights. The simulated results support that the observed coronal disturbances are mainly the wave features. We also find that the effect of radiative cooling on the damping of slow-mode waves in 1-6 MK coronal loops is small, in agreement with the previous conclusion based on 1D MHD models.

astro-ph.SR↗

arXiv2016

Inference of Magnetic Field in the Coronal Streamer Invoking Kink Wave Motions generated by Multiple EUV Waves

Using MHD seismology by observed kink waves, the magnetic field profile of a coronal streamer has been investigated. STEREO-B/EUVI temporal image data on 7 March 2012 shows an evolution of two consecutive EUV waves that interact with the footpoint of a coronal streamer evident in the co-spatial and co-temporal STEREO-B/COR-I observations. The evolution of EUV waves is clearly evident in STEREO-B/EUVI, and its energy exchange with coronal streamer generates kink oscillations. We estimate the phase velocities of the kink wave perturbations by tracking it at different heights of the coronal streamer. We also estimate the electron densities inside and outside the streamer using SSI of polarized brightness images in STEREO-B/COR-1 observations. Taking into account the MHD theory of kink waves in a cylindrical waveguide, their observed properties at various heights, and density contrast of the streamer, we estimate the radial profile of magnetic field within this magnetic structure. Both the kink waves diagnose the exponentially decaying radial profiles of the magnetic field in coronal steamer upto 3 solar radii. Within the limit of uncertainties in the measurements, it is indicated that coronal magnetic field of the streamer varies slowly in time at various heights, although its nature always remains exponentially decaying. It is seen that during the evolution of second kink motion in the streamer, it increases in brightness, and also in areal extent slightly, which may be associated with the decreased photospheric magnetic flux at its footpoint. As a result, the magnetic field profile produced by the second kink wave is reduced within the streamer compared to the one diagnosed by the first one. The precisely estimated magnetic field profiles with the uncertainty less than 10% match well with the empirical profile and various observational estimations of the outer coronal magnetic field.

astro-ph.SR↗

arXiv2015

Quasi-periodic Fast-mode Magnetosonic Wave Trains Within Coronal Waveguides Associated with Flares and CMEs

Quasi-periodic, fast-mode, propagating wave trains (QFPs) are a new observational phenomenon recently discovered in the solar corona by the Solar Dynamics Observatory with extreme ultraviolet (EUV) imaging observations. They originate from flares and propagate at speeds up to ~2000 km/s within funnel-shaped waveguides in the wakes of coronal mass ejections (CMEs). QFPs can carry sufficient energy fluxes required for coronal heating during their occurrences. They can provide new diagnostics for the solar corona and their associated flares. We present recent observations of QFPs focusing on their spatio-temporal properties, temperature dependence, and statistical correlation with flares and CMEs. Of particular interest is the 2010-Aug-01 C3.2 flare with correlated QFPs and drifting zebra and fiber radio bursts, which might be different manifestations of the same fast-mode wave trains. We also discuss the potential roles of QFPs in accelerating and/or modulating the solar wind.

astro-ph.SR↗

arXiv2015

Evidence of thermal conduction suppression in hot coronal loops: Supplementary results

Slow magnetoacoustic waves were first detected in hot ($>$6 MK) flare loops by the SOHO/SUMER spectrometer as Doppler shift oscillations in Fe XIX and Fe XXI lines. Recently, such longitudinal waves have been found by SDO/AIA in the 94 and 131 Å channels. Wang et al. (2015) reported the first AIA event revealing signatures in agreement with a fundamental standing slow-mode wave, and found quantitative evidence for thermal conduction suppression from the temperature and density perturbations in the hot loop plasma of $\gtrsim$ 9 MK. The present study extends the work of Wang et al. (2015) by using an alternative approach. We determine the polytropic index directly based on the polytropic assumption instead of invoking the linear approximation. The same results are obtained as in the linear approximation, indicating that the nonlinearity effect is negligible. We find that the flare loop cools slower (by a factor of 2-4) than expected from the classical Spitzer conductive cooling, approximately consistent with the result of conduction suppression obtained from the wave analysis. The modified Spitzer cooling timescales based on the nonlocal conduction approximation are consistent with the observed, suggesting that nonlocal conduction may account for the observed conduction suppression in this event. In addition, the conduction suppression mechanism predicts that larger flares may tend to be hotter than expected by the EM-$T$ relation derived by Shibata & Yokoyama (2002)

astro-ph.SR↗

arXiv2015

Nonlinear MHD waves in a Prominence Foot

We study nonlinear waves in a prominence foot using 2.5D MHD model motivated by recent high-resolution observations with Hinode/SOT in Ca~II emission of a prominence on October 10, 2012 showing highly dynamic small-scale motions in the prominence material. Observations of H$α$ intensities and of Doppler shifts show similar propagating fluctuations. However the optically thick nature of the emission lines inhibits unique quantitative interpretation in terms of density. Nevertheless, we find evidence of nonlinear wave activity in the prominence foot by examining the relative magnitude of the fluctuation intensity ($δI/I\sim δn/n$). The waves are evident as significant density fluctuations that vary with height, and apparently travel upward from the chromosphere into the prominence material with quasi-periodic fluctuations with typical period in the range of 5-11 minutes, and wavelengths $\sim <$2000 km. Recent Doppler shift observations show the transverse displacement of the propagating waves. The magnetic field was measured with THEMIS instrument and was found to be 5-14 G. For the typical prominence density the corresponding fast magnetosonic speed is $\sim$20 km s$^{-1}$, in qualitative agreement with the propagation speed of the detected waves. The 2.5D MHD numerical model is constrained with the typical parameters of the prominence waves seen in observations. Our numerical results reproduce the nonlinear fast magnetosonic waves and provide strong support for the presence of these waves in the prominence foot. We also explore gravitational MHD oscillations of the heavy prominence foot material supported by dipped magnetic field structure.

astro-ph.SR↗

arXiv2015

Evidence of thermal conduction suppression in a solar flaring loop by coronal seismology of slow-mode waves

Analysis of a longitudinal wave event observed by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) is presented. A time sequence of 131 A images reveals that a C-class flare occurred at one footpoint of a large loop and triggered an intensity disturbance (enhancement) propagating along it. The spatial features and temporal evolution suggest that a fundamental standing slow-mode wave could be set up quickly after meeting of two initial disturbances from the opposite footpoints. The oscillations have a period of ~12 min and a decay time of ~9 min. The measured phase speed of 500$\pm$50 km/s matches the sound speed in the heated loop of ~10 MK, confirming that the observed waves are of slow mode. We derive the time-dependent temperature and electron density wave signals from six AIA extreme-ultraviolet (EUV) channels, and find that they are nearly in phase.The measured polytropic index from the temperature and density perturbations is 1.64$\pm$0.08 close to the adiabatic index of 5/3 for an ideal monatomic gas. The interpretation based on a 1D linear MHD model suggests that the thermal conductivity is suppressed by at least a factor of 3 in the hot flare loop at 9 MK and above. The viscosity coefficient is determined by coronal seismology from the observed wave when only considering the compressive viscosity dissipation. We find that to interpret the rapid wave damping, the classical compressive viscosity coefficient needs to be enhanced by a factor of 15 as the upper limit.

astro-ph.SR↗

arXiv2015

Three dimensional MHD Modeling of Vertical Kink Oscillations in an Active Region Plasma Curtain

Observations on 2011 August 9 of an X6.9-class flare in active region (AR) 11263 by the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO), were followed by a rare detection of vertical kink oscillations in a large-scale coronal active region plasma curtain in EUV coronal lines. The damped oscillations with periods in the range 8.8-14.9 min were detected and analyzed recently. Our aim is to study the generation and propagation of the MHD oscillations in the plasma curtain taking into account realistic 3D magnetic and density structure of the curtain. We also aim at testing and improving coronal seismology for more accurate determination of the magnetic field than with standard method. We use the observed morphological and dynamical conditions, as well as plasma properties of the coronal curtain based on Differential Emission Measure (DEM) analysis to initialize a 3D MHD model of its vertical and transverse oscillations by implementing the impulsively excited velocity pulse mimicking the flare generated nonlinear fast magnetosonic propagating disturbance interacting with the curtain obliquely. The model is simplified by utilizing initial dipole magnetic field, isothermal energy equation, and gravitationally stratified density guided by observational parameters. Using the 3D MHD model, we are able to reproduce the details of the vertical oscillations and study the process of their excitation by nonlinear fast magnetosonic pulse, propagation, and damping, finding agreement with the observations. We estimate the accuracy of simplified slab-based coronal seismology by comparing the determined magnetic field strength to actual values from the 3D MHD modeling results and demonstrate the importance of taking into account more realistic magnetic geometry and density for improving coronal seismology.

astro-ph.SR↗