Small-scale Magnetic Structures Research Articles

Anomalous Transport of Energetic Particles Interacting with Dynamic Small-Scale Magnetic Flux Rope Structures

le Roux and Zank [25] showed previously how one can derive from first principles a pitch-angle dependent fractional diffusion-advection kinetic equation to model the anomalous diffusion of energetic particles interacting with small-scale magnetic flux ropes (SMFRs) in the inner heliosphere on the basis of the standard focused transport equation. This equation has the following limitations: (1) The asymptotic power law of a Lévy distribution was specified to model the non-Gaussian statistics of the disturbed energetic particle trajectories generated during energetic particle interaction with numerous SMFRs. The second moment (variance) and higher moments of the Lévy distribution are infinite, indicating over-efficient non-local transport that is scale-free. (2) The theory does not naturally allow for a transition of anomalous transport to normal diffusion, or to a different anomalous diffusion state. An outline of a derivation is presented in which an exponentially truncated Lévy distribution was specified instead, resulting in a tempered fractional diffusion-advection kinetic equation that addresses these two concerns.

The universality of power law slopes in the solar photosphere and transition region observed with HMI and IRIS

We compare the size distributions of self-organized criticality (SOC) systems in the solar photosphere and the transition region, using magnetogram data from Helioseismic and Magnetic Imager (HMI) and Interface Region Imaging Spectrograph (IRIS) data. For each dataset we fit a combination of a Gaussian and a power law size distribution function, which yields information on four different physical processes: (i) Gaussian random noise in IRIS data; (ii) spicular events in the plages of the transition region (described by power law size distribution in IRIS data); (iii) salt-and-pepper small-scale magnetic structures (described by the random noise in HMI magnetograms); and (iv) magnetic reconnection processes in flares and nanoflares (described by power law size distributions in HMI data). We find a high correlation (CCC = 0.90) between IRIS and HMI data. Datasets with magnetic flux balance are generally found to match the SOC-predicted power law slope αF = 1.80 (for mean fluxes F), but exceptions occur due to arbitrary choices of the HMI field-of-view. The matching cases confirm the universality of SOC-inferred flux size distributions, and agree with the results of Parnell et al. (ApJ, 2009, 698, 75–82), αF = 1.85 ± 0.14.

The Sun at millimeter wavelengths

Aims. We used solar observations of a plage-enhanced network with the Atacama Large Millimeter/sub-millimeter Array (ALMA) in Band 3 and Band 6, together with synthetic continuum maps from numerical simulations with Bifrost in the same bands, to carry out a detailed study of bright small-scale magnetic features. Methods. We made use of an algorithm to automatically identify and trace bright features within the field of view (FoV) of the ALMA observations and the simulation. In particular, the algorithm recovers information of the time evolution of the shape, motion of the centre of gravity, temperature, and size for each feature. These quantities are used to determine the oscillatory properties of each feature utilising wavelets analysis. Results. We found 193 and 293 features in the Bands 3 and 6 observations, respectively. In the degraded simulation, the total number of features were 24 for Band 3 and 204 for Band 6. In the original simulation, the total number of features were 36 for Band 3 and 392 for Band 6. Based on the simulation, we confirm the magnetic nature of the features. We have obtained average oscillation periods of 30–99 s for the temperature, 37–92 s for size, and 37–78 s for horizontal velocity. There are indications for the possible presence of transverse (kink) waves with average amplitude velocities of 2.1–5.0 km s−1. We find a predominant anti-phase behaviour between temperature and size oscillations suggesting that the variations of the bright features are caused by compressible fast-sausage magnetohydrodynamics (MHD) modes. For the first time to our knowledge, we estimated the flux of energy of the fast-sausage waves at the chromospheric heights sampled by ALMA as 453–1838 W m−2 for Band 3 and 3640–5485 W m−2 for Band 6. Conclusions. We have identified MHD waves, both transverse (kink) and compressible sausage modes, in small-scale (magnetic) structures, independently, in both ALMA Band 3 and Band 6 observations, along with their corresponding synthetic images from simulations. The decrease of wave energy-flux with height (from Band 6 to Band 3) could possibly suggest energy dissipation at chromospheric heights, namely, wave heating, with the assumptions that the identified small-scale waves are typical at each band and they propagate upward through the chromosphere.

Origin of extreme solar eruptive activity from the active region NOAA 12673 and the largest flare of solar cycle 24

AbstractDuring 2017, when the Sun was moving toward the minimum phase of solar cycle 24, an exceptionally eruptive active region (AR) NOAA 12673 emerged on the Sun during August 28-September 10. During the highest activity level, the AR turned into a δ-type sunspot region, which manifests the most complex configuration of magnetic fields from the photosphere to the coronal heights. The AR 12673 produced four X-class and 27 M-class flares, along with numerous C-class flares, making it one of the most powerful ARs of solar cycle 24. Notably, it produced the largest flare of solar cycle 24, namely, the X9.3 event on 2017 September 6. In this work, we highlight the results of our comprehensive analysis involving multi-wavelength imaging and coronal magnetic field modeling to understand the evolution and eruptivity from AR 12673. We especially focus on the morphological, spectral and kinematical evolution of the two X-class flares on 6 September 2017. We explore various large- and small-scale magnetic field structures of the active region which are associated with the triggering and subsequent outbursts during the powerful solar transients.

Large scale coherent magnetohydrodynamic oscillations in a sunspot

Although theoretically predicted, the simultaneous excitation of several resonant modes in sunspots has not been observed. Like any harmonic oscillator, a solar magnetic flux tube can support a variety of resonances, which constitute the natural response of the system to external forcing. Apart from a few single low order eigenmodes in small scale magnetic structures, several simultaneous resonant modes were not found in extremely large sunspots. Here we report the detection of the largest-scale coherent oscillations observed in a sunspot, with a spectrum significantly different from the Sun’s global acoustic oscillations, incorporating a superposition of many resonant wave modes. Magnetohydrodynamic numerical modeling agrees with the observations. Our findings not only demonstrate the possible excitation of coherent oscillations over spatial scales as large as 30–40 Mm in extreme magnetic flux regions in the solar atmosphere, but also paves the way for their diagnostic applications in other astrophysical contexts.

CПЕКТРАЛЬНІ ДОСЛІДЖЕННЯ СОНЯЧНИХ МАГНІТНИХ ПОЛІВ В АСТРОНОМІЧНІЙ ОБСЕРВАТОРІЇ КИЇВСЬКОГО НАЦІОНАЛЬНОГО УНІВЕРСИТЕТУ ІМЕНІ ТАРАСА ШЕВЧЕНКА У 1981–1986 рр.

In the second five-year period of spectral studies of solar magnetic fields in Astronomical Observatory of the Taras Shevchenko National University of Kyiv (1981-1986), a number of important and interesting scientific results were obtained, which mainly concern magnetic fields in solar flares and beyond. In particular, based on the analysis of the Echelle Zeeman spectrograms obtained at the observatory during the international FBS/SERF program, it was shown that the effect of the “magnetic transient” according to the data of the BBSO observatory probably has a non-magnetic nature. Also, “kilogauss” magnetic fields (1–3 kG) were discovered and previously studied in solar flares, more precisely, in the region of intense emission in strong FeI lines, which is formed in the upper photosphere and the temperature minimum zone. For a section of the photosphere of the active region without solar flares, observational evidences of the discreteness of local magnetic field intensities were obtained, which turned out to be equal to ≈ 4, 7 and 13 kG. For the same area, indications of the existence of a moderately strong (up to ~ 1 kG) background magnetic field of mixed polarity were found. It was possible to explain these observational data theoretically within the framework of the linear force-free MHD model, according to which the central parts of small-scale magnetic structures with strong and discrete fields are surrounded by concentric regions with a sign-changing and weaker magnetic field. In a solar flare, a sign-changing height gradient of the longitudinal magnetic field was discovered and investigated, one of the reasons for which may be a significant deformation of the magnetic field of the shear type, which gradually weakened towards the end of the flare. In all the above-mentioned questions, the “last word” has not yet been said – they require thorough research in the future.

Propagating brightenings in small loop-like structures in the quiet-Sun corona: Observations from Solar Orbiter/EUI

Brightenings observed in solar extreme-ultraviolet images are generally interpreted as signatures of micro- or nanoflares occurring in the transition region or at coronal temperatures. Recent observations with the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter have revealed the smallest of such brightenings (called campfires) in the quiet-Sun corona. Analyzing EUI 174 Å data obtained at a resolution of about 400 km on the Sun with a cadence of 5 s on 30 May 2020, we report here a number of cases in which these campfires exhibit propagating signatures along their apparently small (3–5 Mm) loop-like structures. The measured propagation speeds are generally between 25 km s−1and 60 km s−1. If the loop plasma is assumed to be at a million Kelvin, these apparent motions would be slower than the local sound speed. Furthermore, these brightenings exhibit nontrivial propagation characteristics such as bifurcation, merging, reflection, and repeated plasma ejections. We suggest that these features are manifestations of the internal dynamics of these small-scale magnetic structures and could provide important insights into the dynamic response (∼40 s) of the loop plasma to the heating events and also into the locations of the heating events themselves.

Observation of Small-Scale Magnetic and Current Structures During Prolonged Dipolarizations in the Near Geomagnetic Tail by MMS Mission

Observation of Small-Scale Magnetic and Current Structures During Prolonged Dipolarizations in the Near Geomagnetic Tail by MMS Mission

Rapid modification of neutron star surface magnetic field: a proposed mechanism for explaining radio emission state changes in pulsars

ABSTRACT The radio emission in many pulsars shows sudden changes, usually within a period, that cannot be related to the steady state processes within the inner acceleration region (IAR) above the polar cap. These changes are often quasi-periodic in nature, where regular transitions between two or more stable emission states are seen. The durations of these states show a wide variety ranging from several seconds to hours at a time. There are strong, small-scale magnetic field structures and huge temperature gradients present at the polar cap surface. We have considered several processes that can cause temporal modifications of the local magnetic field structure and strength at the surface of the polar cap. Using different magnetic field strengths and scales, and also assuming realistic scales of the temperature gradients, the evolutionary time-scales of different phenomena affecting the surface magnetic field were estimated. We find that the Hall drift results in faster changes in comparison to both Ohmic decay and thermoelectric effects. A mechanism based on the partially screened gap (PSG) model of the IAR has been proposed, where the Hall and thermoelectric oscillations perturb the polar cap magnetic field to alter the sparking process in the PSG. This is likely to affect the observed radio emission resulting in the observed state changes.

Single magnetic domain Terfenol-D microstructures with passivating oxide layer

Studies on giant magnetostrictive intermetallic compound Tb0.3Dy0.7 Fe1.92 (Terfenol-D) thin films are gaining increasing attention due to the interest in small scale magnetic structures. In this work, polycrystalline Terfenol-D thin films (60 nm) are produced using a Direct Current (DC) magnetron sputtering process and a hybrid thermal annealing approach, i.e. substrate heating and post-annealing. The crystallized Terfenol-D films are micropatterned (3 μm and 20 μm diameters) using a photolithographic method followed by Argon etching. The continuous and micropatterned thin films are characterized with XRD, SQUID and MOKE magnetometers showing properties similar to bulk polycrystalline Terfenol-D. Additional studies are conducted using X-ray absorption spectroscopy and the X-ray magnetic circular dichroism to investigate oxidation. Results show that while the top surface of these Terfenol-D structures are capped with Ta thin films, the side walls of the structures are exposed to ambient atmosphere raising the concern of oxidation issues. This work shows that a passivating oxide layer forms on the side walls of these small-scale Terfenol-D disks which is attributed to both the Ta capping layer as well as residual stresses preventing continued stress corrosion cracking caused by the oxidation process.

Study of the Dynamics of Convective Turbulence in the Solar Granulation by Spectral Line Broadening and Asymmetry

Abstract In quiet regions on the solar surface, turbulent convective motions of granulation play an important role in creating small-scale magnetic structures, as well as in energy injection into the upper atmosphere. The turbulent nature of granulation can be studied using spectral line profiles, especially line broadening, which contain information on the flow field smaller than the spatial resolution of an instrument. Moreover, the Doppler velocity gradient along a line of sight (LOS) causes line broadening as well. However, the quantitative relationship between velocity gradient and line broadening has not been understood well. In this study, we perform bisector analyses using the spectral profiles obtained using the spectropolarimeter of the Hinode/Solar Optical Telescope to investigate the relationship of line broadening and bisector velocities with the granulation flows. The results indicate that line broadening has a positive correlation with the Doppler velocity gradients along the LOS. We found excessive line broadening in fading granules, that cannot be explained only by the LOS velocity gradient, although the velocity gradient is enhanced in the process of fading. If this excessive line broadening is attributed to small-scale turbulent motions, the averaged turbulent velocity is obtained as 0.9 km s−1.

Observations of Heating along Intermittent Structures in the Inner Heliosphere from PSP Data

Abstract The solar wind proton temperature at 1 au has been found to be correlated with small-scale intermittent magnetic structures, i.e., regions with enhanced temperature are associated with coherent structures, such as current sheets. Using Parker Solar Probe data from the first encounter, we study this association using measurements of the radial proton temperature, employing the partial variance of increments (PVI) technique to identify intermittent magnetic structures. We observe that the probability density functions of high PVI events have higher median temperatures than those with lower PVI. The regions in space where PVI peaks were also locations that had enhanced temperatures when compared with similar regions, suggesting a heating mechanism in the young solar wind that is associated with intermittency developed by a nonlinear turbulent cascade in the immediate vicinity.

Identification of Magnetic Flux Ropes from Parker Solar Probe Observations during the First Encounter

Abstract The Parker Solar Probe (PSP) observed an interplanetary coronal mass ejection (ICME) event during its first orbit around the Sun, among many other events. This event is analyzed by applying a wavelet analysis technique to obtain the reduced magnetic helicity, cross helicity, and residual energy, the first two of which are magnetohydrodynamics (MHD) invariants. Our results show that the ICME, as a large-scale magnetic flux rope, possesses high magnetic helicity, very low cross helicity, and highly negative residual energy, thus pointing to a magnetic fluctuation dominated structure. Using the same technique, we also search for small-scale coherent magnetic flux rope structures during the period from 2018 October 22 to November 21, which are intrinsic to quasi-two-dimensional MHD turbulence in the solar wind. Multiple structures with durations between 8 and 300 minutes are identified from PSP in situ spacecraft measurements. The location and scales of these structures are characterized by wavelet spectrograms of the normalized reduced magnetic helicity, normalized cross helicity, and normalized residual energy. Transport theory suggests that these small-scale magnetic flux ropes may contribute to the acceleration of charged particles through magnetic reconnection processes, and the dissipation of these structures may be important for understanding the coronal heating processes.

Long quasi-periodic oscillations of the faculae and pores

Aims. The main goal of this work is to analyze the structural and temporal evolution of small-scale magnetic structures (SSMSs) observed in the solar atmosphere, such as solitary faculae and pores, and reveal long quasi-periodic oscillations of these structures. Methods. The statistical method of regression analysis and the wavelet transform were used to obtain the periods of oscillations and dependences between the parameters of magnetic structures and periods of oscillations. Results. Long-period oscillations with periods in the interval of 18−260 min are found for the structurally stable phase of SSMSs at the level of the solar photosphere. These long-period oscillations were interpreted as natural oscillations of the structurally stable long-lived magnetic structures around their equilibrium position. These oscillations, which are of similar nature, are observed in the chromospheric bright formations associated with photospheric SSMSs. Dependences between the magnetic field and the continuum intensity of the facula elements were found. It is shown that the continuum intensity of a SSMS decreases when its magnetic field increases.

Квазипериодические колебания мелкомасштабных магнитных структур и специальный метод измерения дифференциального вращения Солнца

The SDO/HMI data with an angular resolution of 1 arcsec have been used to explore the differential rotation on the Sun, using an original “p2p” effect on the basis of the movement of small-scale magnetic structures in the photosphere of the Sun. It is shown that a stable p2p artifact inherent in the SDO/HMI data can be an effective tool for measuring the speed of various tracers on the Sun. In particular, in combination with the Fourier analysis, it allows us to investigate the differential rotation of the Sun at various latitudes. The differential rotation curve obtained from the SDO/HMI magnetograms by this method is in good agreement with the curves obtained earlier from ground-based observations.

Eigen oscillations of facular knots

Solitary small-scale and long-lived magnetic structures observed on the solar photosphere, so-called facular knots, are investigated on the base of Solar Dynamics Observatory (SDO) data. The long-term variations of the magnetic field and the emission in UV-lines with periods in the range of 25–250 minutes are revealed in the facular knots. The statistically significant oscillatory modes for each object under the study are found by the Empirical Mode Decomposition (EMD) method. It is noted that, although the object keeps its structural identity, during all its lifetime, their parameters, such as the strength of magnetic field, area on the magnetograms and on the intensity maps, the intensities in white light and other quantities can vary significantly. These variations change the effective rigidity of the system (the response of the system to the external disturbances), and it change radically the character of oscillations. Quasi-periodic variations of the magnetic field of facular knots with periods from one to several hours can be interpreted as an specific eigen oscillations of the system as a whole, having a time-varying rigidity. A simple analytical model is proposed to describe the observed oscillations of the facular knots.

Analysis of Oscillatory Modes of the Magnetic Field of Solar Facular Formations

Quasi-periodical variations in the magnetic fields of long-living, small-scale magnetic structures, solar facular formations (FFs), are analyzed. We used the method of Empirical Mode Decomposition (EMD) to reveal the empirical modes of time series of magnetic field variations in four FFs and tested them for the existence of coloured noise. We proposed a qualitative interpretation of statistically reliable oscillatory modes.

Effects of Coronal Magnetic Field Structures on the Transport of Solar Energetic Particles

Abstract This Letter presents a model calculation of solar energetic particle (SEP) transport to test the sensitivity of the distribution of escaped SEPs in interplanetary space and dependence upon the details of the magnetic field structure in the corona. It is applied to a circumsolar event on 2011 November 3, in which SEPs are observed promptly after the solar event eruption by three spacecraft (the twin Solar TErrestrial RElations Observatories (STEREO-A and STEREO-B) and ACE) separated by more than 100° in longitude from each other. The corona magnetic field reconstructed from photosphseric field measurements using the PFSS method changes substantially before and after the solar eruption, especially around the active region. The locations of open field regions, separatrix surfaces including the heliospheric current sheet, and footpoints of magnetic field lines connected to the spacecraft location have shifted substantially. We inject 100 keV energetic electrons on the open field lines at 1.5 R s within the size of observed coronal mass ejections (CMEs) and follow their propagation in the corona and the interplanetary space. We find that with a perpendicular diffusion due to field line random walk equal to 10% of the supergranular diffusion rate, the overall distribution of escaped SEPs does not change much even though the region of open field lines from SEPs has changed. The result suggests that detailed small-scale coronal magnetic field structures and the exact magnetic field connection are not crucially important for observing SEPs in the interplanetary space.

Observational Evidence for Self-generation of Small-scale Magnetic Flux Ropes from Intermittent Solar Wind Turbulence

Abstract We present unique and additional observational evidence for the self-generation of small-scale coherent magnetic flux rope structures in the solar wind. Such structures with durations between 9 and 361 minutes are identified from Wind in situ spacecraft measurements through the Grad–Shafranov (GS) reconstruction approach. The event occurrence counts are on the order of 3500 per year on average and have a clear solar-cycle dependence. We build a database of small-scale magnetic flux ropes from 20 yr worth of Wind spacecraft data. We show a power-law distribution of the wall-to-wall time corresponding well to the inertial range turbulence, which agrees with relevant observations and numerical simulation results. We also provide the axial current density distribution from the GS-based observational analysis, which yields a non-Gaussian probability density function consistent with numerical simulation results.

Precipitation and Release of Solar Energetic Particles from the Solar Coronal Magnetic Field

Abstract Most solar energetic particles (SEPs) are produced in the corona. They propagate through complex coronal magnetic fields subject to scattering and diffusion across the averaged field lines by turbulence. We examine the behaviors of particle transport using a stochastic 3D focused transport simulation in a potential field source surface model of coronal magnetic field. The model is applied to an SEP event on 2010 February 7. We study three scenarios of particle injection at (i) the compact solar flare site, (ii) the coronal mass ejection (CME) shock, and (iii) the EUV wave near the surface. The majority of particles injected on open field lines are able to escape the corona. We found that none of our models can explain the observations of wide longitudinal SEP spread without perpendicular diffusion. If the perpendicular diffusion is about 10% of what is derived from the random walk of field lines at the rate of supergranular diffusion, particles injected at the compact solar flare site can spread to a wide range of longitude and latitude, very similar to the behavior of particles injected at a large CME shock. Stronger pitch-angle scattering results in a little more lateral spread by holding the particles in the corona for longer periods of time. Some injected particles eventually end up precipitating onto the solar surface. Even with a very small perpendicular diffusion, the pattern of the particle precipitation can be quite complicated depending on the detailed small-scale coronal magnetic field structures, which could be seen with future sensitive gamma-ray telescopes.