Showing papers by "Säm Krucker published in 2018"

Microwave and Hard X-Ray Observations of the 2017 September 10 Solar Limb Flare

Abstract: We report the first science results from the newly completed Expanded Owens Valley Solar Array (EOVSA), which obtained excellent microwave imaging spectroscopy observations of SOL2017-09-10, a classic partially-occulted solar limb flare associated with an erupting flux rope. This event is also well-covered by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in hard X-rays (HXRs). We present an overview of this event focusing on microwave and HXR data, both associated with high-energy nonthermal electrons, and discuss them within the context of the flare geometry and evolution revealed by extreme ultraviolet (EUV) observations from the Atmospheric Imaging Assembly aboard the Solar Dynamics Observatory (SDO/AIA). The EOVSA and RHESSI data reveal the evolving spatial and energy distribution of high-energy electrons throughout the entire flaring region. The results suggest that the microwave and HXR sources largely arise from a common nonthermal electron population, although the microwave imaging spectroscopy provides information over a much larger volume of the corona.

Microwave and Hard X-Ray Observations of the 2017 September 10 Solar Limb Flare

Abstract: We report the first science results from the newly completed Expanded Owens Valley Solar Array (EOVSA), which obtained excellent microwave imaging spectroscopy observations of SOL2017-09-10, a classic partially-occulted solar limb flare associated with an erupting flux rope. This event is also well-covered by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in hard X-rays (HXRs). We present an overview of this event focusing on microwave and HXR data, both associated with high-energy nonthermal electrons, and discuss them within the context of the flare geometry and evolution revealed by extreme ultraviolet (EUV) observations from the Atmospheric Imaging Assembly aboard the Solar Dynamics Observatory (SDO/AIA). The EOVSA and RHESSI data reveal the evolving spatial and energy distribution of high-energy electrons throughout the entire flaring region. The results suggest that the microwave and HXR sources largely arise from a common nonthermal electron population, although the microwave imaging spectroscopy provides information over a much larger volume of the corona.

Fermi-LAT observations of the 2017 September 10$^{th}$ solar flare

Abstract: The Fermi-Large Area Telescope (LAT) detection of the X8.2 GOES class solar flare of 2017 September 10 provides for the first time observations of a long duration high-energy gamma-ray flare associated with a Ground Level Enhancement (GLE). The >100 MeV emission from this flare lasted for more than 12 hours covering both the impulsive and extended phase. We present the localization of the gamma-ray emission and find that it is consistent with the active region (AR) from which the flare occurred over a period lasting more than 6 hours contrary to what was found for the 2012 March 7 flares. The temporal variation of the proton index inferred from the gamma-ray data seems to suggest two phases in acceleration of the proton population. Based on timing arguments we interpret the second phase to be tied to the acceleration mechanism powering the GLE, believed to be particle acceleration at a coronal shock driven by the CME.

Identifying Typical Mg ii Flare Spectra Using Machine Learning

Abstract: IRIS performs solar observations over a large range of atmospheric heights, including the chromosphere where the majority of flare energy is dissipated. The strong Mg II h&k spectral lines are capable of providing excellent atmospheric diagnostics, but have not been fully utilized for flaring atmospheres. We aim to investigate whether the physics of the chromosphere is identical for all flare observations by analyzing if there are certain spectra that occur in all flares. To achieve this, we automatically analyze hundreds of thousands of Mg II h&k line profiles from a set of 33 flares, and use a machine learning technique which we call supervised hierarchical k-means, to cluster all profile shapes. We identify a single peaked Mg II profile, in contrast to the double-peaked quiet Sun profiles, appearing in every flare. Additionally, we find extremely broad profiles with characteristic blue shifted central reversals appearing at the front of fast-moving flare ribbons. These profiles occur during the impulsive phase of the flare, and we present results of their temporal and spatial correlation with non-thermal hard X-ray signatures, suggesting that flare-accelerated electrons play an important role in the formation of these profiles. The ratio of the integrated Mg II h&k lines can also serve as an opacity diagnostic, and we find higher opacities during each flare maximum. Our study shows that machine learning is a powerful tool for large scale statistical solar analyses.

Magnetic Reconnection Null Points as the Origin of Semirelativistic Electron Beams in a Solar Jet

Abstract: Magnetic reconnection, the central engine that powers explosive phenomena throughout the universe, is also perceived to be one of the principal mechanisms for accelerating particles to high energies. Although various signatures of magnetic reconnection have been frequently reported, observational evidence that links particle acceleration directly to the reconnection site has been rare, especially for space plasma environments currently inaccessible to in situ measurements. Here we utilize broadband radio dynamic imaging spectroscopy available from the Karl G. Jansky Very Large Array to observe decimetric type III radio bursts in a solar jet with high angular (∼20″), spectral (∼1%), and temporal resolution (50 ms). These observations allow us to derive detailed trajectories of semirelativistic (tens of keV) electron beams in the low solar corona with unprecedentedly high angular precision (<0.″65). We found that each group of electron beams, which corresponds to a cluster of type III bursts with 1–2 s duration, diverges from an extremely compact region (∼600 km2) in the low solar corona. The beam-diverging sites are located behind the erupting jet spire and above the closed arcades, coinciding with the presumed location of magnetic reconnection in the jet eruption picture supported by extreme ultraviolet/X-ray data and magnetic modeling. We interpret each beam-diverging site as a reconnection null point where multitudes of magnetic flux tubes join and reconnect. Our data suggest that the null points likely consist of a high level of density inhomogeneities possibly down to 10 km scales. These results, at least in the present case, strongly favor a reconnection-driven electron-acceleration scenario.

Fermi-LAT Observations of the 2017 September 10 Solar Flare

Abstract: The Fermi-Large Area Telescope detection of the X8.2 GOES class solar flare of 2017 September 10 provides for the first time observations of a long-duration high-energy gamma-ray flare associated with a ground-level enhancement (GLE). The >100 MeV emission from this flare lasted for more than 12 hr covering both the impulsive and extended phases. We present the localization of the gamma-ray emission and find that it is consistent with the active region from which the flare occurred over a period lasting more than 6 hr. The temporal variation of the gamma-ray flux and of the proton index inferred from the gamma-ray data seems to suggest three phases in acceleration of the proton population. Based on timing arguments we interpret the last phase to be tied to the acceleration mechanism powering the powering the production of the GLE particles.

Magnetic Reconnection Null Points as the Origin of Semi-relativistic Electron Beams in a Solar Jet

Abstract: Magnetic reconnection, the central engine that powers explosive phenomena throughout the Universe, is also perceived as one of the principal mechanisms for accelerating particles to high energies. Although various signatures of magnetic reconnection have been frequently reported, observational evidence that links particle acceleration directly to the reconnection site has been rare, especially for space plasma environments currently inaccessible to $\textit{in situ}$ measurements. Here we utilize broadband radio dynamic imaging spectroscopy available from the Karl G. Jansky Very Large Array to observe decimetric type III radio bursts in a solar jet with high angular ($\sim$20$''$), spectral ($\sim$1 %), and temporal resolution (50 milliseconds). These observations allow us to derive detailed trajectories of semi-relativistic (tens of keV) electron beams in the low solar corona with unprecedentedly high angular precision ($<0''.65$). We found that each group of electron beams, which corresponds to a cluster of type III bursts with 1-2-second duration, diverges from an extremely compact region ($\sim$600 km$^2$) in the low solar corona. The beam-diverging sites are located behind the erupting jet spire and above the closed arcades, coinciding with the presumed location of magnetic reconnection in the jet eruption picture supported by extreme ultraviolet/X-ray data and magnetic modeling. We interpret each beam-diverging site as a reconnection null point where multitudes of magnetic flux tubes join and reconnect. Our data suggest that the null points likely consist of a high level of density inhomogeneities possibly down to 10-km scales. These results, at least in the present case, strongly favor a reconnection-driven electron acceleration scenario.

NuSTAR Detection of X-Ray Heating Events in the Quiet Sun

Abstract: The explanation of the coronal heating problem potentially lies in the existence of nanoflares, numerous small-scale heating events occurring across the whole solar disk. In this Letter, we present the first imaging spectroscopy X-ray observations of three quiet Sun flares during the Nuclear Spectroscopic Telescope ARray (NuSTAR) solar campaigns on 2016 July 26 and 2017 March 21, concurrent with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) observations. Two of the three events showed time lags of a few minutes between peak X-ray and extreme ultraviolet emissions. Isothermal fits with rather low temperatures in the range 3.2–4.1 MK and emission measures of (0.6–15) × 10^(44) cm^(−3) describe their spectra well, resulting in thermal energies in the range (2–6) × 10^(26) erg. NuSTAR spectra did not show any signs of a nonthermal or higher temperature component. However, as the estimated upper limits of (hidden) nonthermal energy are comparable to the thermal energy estimates, the lack of a nonthermal component in the observed spectra is not a constraining result. The estimated Geostationary Operational Environmental Satellite (GOES) classes from the fitted values of temperature and emission measure fall between 1/1000 and 1/100 A class level, making them eight orders of magnitude fainter in soft X-ray flux than the largest solar flares.

Solar radio emission as a disturbance of aeronautical radionavigation

Abstract: On November 4th, 2015 secondary air traffic control radar was strongly disturbed in Sweden and some other European countries. The disturbances occurred when the radar antennas were pointing at the Sun. In this paper, we show that the disturbances coincided with the time of peaks of an exceptionally strong (∼105 Solar Flux Units) solar radio burst in a relatively narrow frequency range around 1 GHz. This indicates that this radio burst is the most probable space weather candidate for explaining the radar disturbances. The dynamic radio spectrum shows that the high flux densities are not due to synchrotron emission of energetic electrons, but to coherent emission processes, which produce a large variety of rapidly varying short bursts (such as pulsations, fiber bursts, and zebra patterns). The radio burst occurs outside the impulsive phase of the associated flare, about 30 min after the soft X-ray peak, and it is temporarily associated with fast evolving activity occurring in strong solar magnetic fields. While the relationship with strong magnetic fields and the coherent spectral nature of the radio burst provide hints towards the physical processes which generate such disturbances, we have so far no means to forecast them. Well-calibrated monitoring instruments of whole Sun radio fluxes covering the UHF band could at least provide a real-time identification of the origin of such disturbances, which reports in the literature show to also affect GPS signal reception.

NuSTAR Detection of X-Ray Heating Events in the Quiet Sun

Abstract: The explanation of the coronal heating problem potentially lies in the existence of nanoflares, numerous small-scale heating events occuring across the whole solar disk. In this paper, we present the first imaging spectroscopy X-ray observations of three quiet Sun flares during the NuSTAR solar campaigns on 2016 July 26 and 2017 March 21, concurrent with SDO/AIA observations. Two of the three events showed time lags of a few minutes between peak X-ray and extreme ultraviolet (EUV) emissions. Isothermal fits with rather low temperatures in the range $3.2-4.1$ MK and emission measures of $(0.6-15)\times10^{44} \textrm{ cm}^{-3}$ describe their spectra well, resulting in thermal energies in the range $(2-6)\times10^{26}\textrm{ ergs}$. NuSTAR spectra did not show any signs of a nonthermal or higher temperature component. However, since the estimated upper limits of (hidden) nonthermal energy are comparable to the thermal energy estimates, the lack of a nonthermal component in the observed spectra is not a constraining result. The estimated GOES classes from the fitted values of temperature and emission measure fall between $1/1000 \textrm{ and } 1/100$ A class level, making them 8 orders of magnitude fainter in soft X-ray flux than the largest solar flares.

Hard X-Ray Constraints on Small-Scale Coronal Heating Events

Abstract: Author(s): Marsh, AJ; Smith, DM; Glesener, L; Klimchuk, JA; Bradshaw, SJ; Vievering, J; Hannah, IG; Christe, S; Ishikawa, SN; Krucker, S | Abstract: Much evidence suggests that the solar corona is heated impulsively, meaning that nanoflares may be ubiquitous in quiet and active regions (ARs). Hard X-ray (HXR) observations with unprecedented sensitivity g3 keV are now enabled by focusing instruments. We analyzed data from the Focusing Optics X-ray Solar Imager (FOXSI) rocket and the Nuclear Spectroscopic Telescope Array (NuSTAR) spacecraft to constrain properties of AR nanoflares simulated by the EBTEL field-line-averaged hydrodynamics code. We generated model X-ray spectra by computing differential emission measures for homogeneous nanoflare sequences with heating amplitudes H 0, durations τ, delay times between events t N, and filling factors f. The single quiescent AR observed by FOXSI-2 on 2014 December 11 is well fit by nanoflare sequences with heating amplitudes 0.02 erg cm-3 s-1 99% confidence for all regions observed by either instrument.

Identifying typical Mg II flare spectra using machine learning

Abstract: IRIS performs solar observations over a large range of atmospheric heights, including the chromosphere where the majority of flare energy is dissipated. The strong Mg II h&k spectral lines are capable of providing excellent atmospheric diagnostics, but have not been fully utilized for flaring atmospheres. We aim to investigate whether the physics of the chromosphere is identical for all flare observations by analyzing if there are certain spectra that occur in all flares. To achieve this, we automatically analyze hundreds of thousands of Mg II h&k line profiles from a set of 33 flares, and use a machine learning technique which we call supervised hierarchical k-means, to cluster all profile shapes. We identify a single peaked Mg II profile, in contrast to the double-peaked quiet Sun profiles, appearing in every flare. Additionally, we find extremely broad profiles with characteristic blue shifted central reversals appearing at the front of fast-moving flare ribbons. These profiles occur during the impulsive phase of the flare, and we present results of their temporal and spatial correlation with non-thermal hard X-ray signatures, suggesting that flare-accelerated electrons play an important role in the formation of these profiles. The ratio of the integrated Mg II h&k lines can also serve as an opacity diagnostic, and we find higher opacities during each flare maximum. Our study shows that machine learning is a powerful tool for large scale statistical solar analyses.

Hard X-Ray Constraints on Small-Scale Coronal Heating Events

Abstract: Much evidence suggests that the solar corona is heated impulsively, meaning that nanoflares may be ubiquitous in quiet and active regions (ARs). Hard X-ray (HXR) observations with unprecedented sensitivity $>$3~keV are now enabled by focusing instruments. We analyzed data from the \textit{Focusing Optics X-ray Solar Imager (FOXSI)} rocket and the \textit{Nuclear Spectroscopic Telescope Array (NuSTAR)} spacecraft to constrain properties of AR nanoflares simulated by the EBTEL field-line-averaged hydrodynamics code. We generated model X-ray spectra by computing differential emission measures for homogeneous nanoflare sequences with heating amplitudes $H_0$, durations $\tau$, delay times between events $t_N$, and filling factors $f$. The single quiescent AR observed by \textit{FOXSI-2} on 2014 December 11 is well fit by nanoflare sequences with heating amplitudes 0.02 erg cm$^{-3}$ s$^{-1}$ $ $99\% confidence for all regions observed by either instrument.

Joint X-ray, EUV and UV Observations of a Small Microflare

Abstract: We present the first joint observation of a small microflare in X-rays with the Nuclear Spectroscopic Telescope ARray (NuSTAR), UV with the Interface Region Imaging Spectrograph (IRIS) and EUV with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). These combined observations allows us to study the microflare's hot coronal and cooler chromospheric/transition region emission. This small microflare peaks from SOL2016-07-26T23:35 to 23:36UT, in both NuSTAR, SDO/AIA and IRIS. Spatially this corresponds to a small loop visible in the SDO/AIA Fe XVIII emission, which matches a similar structure lower in the solar atmosphere seen by IRIS in SJI1330{\AA} and 1400\AA. The NuSTAR emission in both 2.5-4 keV and 4-6 keV, is located in a small source at this loop location. The IRIS slit was over the microflaring loop, and fits show little change in Mg II but do show intensity increases, slight width enhancements and redshifts in Si IV andO IV, indicating that this microflare had most significance in and above the upper chromosphere. The NuSTAR microflare spectrum is well fitted by a thermal component of 5.8MK and $2.3\times10^{44}$ cm$^{-3}$, which corresponds to a thermal energy of $10^{26}$ erg, making it considerably smaller than previously studied X-ray microflares. No non-thermal emission was detected but this could be due to the limited effective exposure time of the observation. This observation shows that even ordinary features seen in UV and EUV, can remarkably have a higher energy component that is clear in X-rays.

DCT-Tensor-Net for Solar Flares Detection on IRIS Data

Abstract: Flares are an eruptive phenomenon observed on the sun, which are major protagonists in space weather and can cause adverse effects such as disruptions in communication, power grid failure and damage of satellites. Our method answers the importance of the time component in some scientific video observations, especially for flare detection and the study is based on NASA's Interface Region Imaging Spectrograph (IRIS) observations of the sun since 2013, which consists of a very asymmetrical and unlabeled big data. For detecting and analyzing flares in our IRIS solar video observation data, we created a discrete cosine transform tool DCT- Tensor-Net which uses an empirically handcrafted harmonic representation of our video data. This is one of the first tools for detecting flares based on IRIS images. Our method reduces the false detections of flares by taking into consideration their specific local spatial and temporal patterns.

Solar radio emission as a disturbance of aeronautical radionavigation.

Abstract: On November 4th 2015 secondary air traffic control radar was strongly disturbed in Sweden and some other European countries. The disturbances occurred when the radar antennas were pointing at the Sun. In this paper, we show that the disturbances coincided with the time of peaks of an exceptionally strong ($\sim 10^5$ Solar Flux Units) solar radio burst in a relatively narrow frequency range around 1~GHz. This indicates that this radio burst is the most probable space weather candidate for explaining the radar disturbances. The dynamic radio spectrum shows that the high flux densities are not due to synchrotron emission of energetic electrons, but to coherent emission processes, which produce a large variety of rapidly varying short bursts (such as pulsations, fiber bursts, and zebra patterns). The radio burst occurs outside the impulsive phase of the associated flare, about 30 minutes after the soft X-ray peak, and it is temporarily associated with fast evolving activity occurring in strong solar magnetic fields. While the relationship with strong magnetic fields and the coherent spectral nature of the radio burst provide hints towards the physical processes which generate such disturbances, we have so far no means to forecast them. Well-calibrated monitoring instruments of whole Sun radio fluxes covering the UHF band could at least provide a real-time identification of the origin of such disturbances, which reports in the literature show to also affect GPS signal reception.