Low Corona Research Articles

The Double-bubble Coronal Mass Ejection of the 2020 December 14 Total Solar Eclipse

Abstract Total solar eclipses (TSEs) continue to provide an invaluable platform for exploring the magnetic topology of the solar corona and for studying dynamic events such as coronal mass ejections (CMEs)—with a higher spatial resolution over a larger spatially continuous extent than is possible to achieve with any other method at present. In this Letter, we present observations of the full extent of a “double-bubble” CME structure from the solar surface out to over 5 solar radii, as captured during the 2020 December 14 TSE. Its evolution through the corona was recorded from two observing sites separated by 13 minutes in their times of totality. The eclipse observations are complemented by a plethora of space-based observations including: Extreme Ultraviolet observations of the solar disk and low corona from SDO/AIA and STEREO-A/EUVI, white-light coronagraph observations from SOHO/LASCO-C2, radio from STEREO-A/WAVES and WIND/WAVES, and X-ray from GOES-16. We also characterize the magnetic field with a potential field source surface model. This CME event itself is of particular interest, as it demonstrates interactions between a prominence channel and an active region that led to the double-bubble structure. Despite the plethora of space-based observations, only the eclipse data are able to provide the proper context to connect these observations and yield a detailed study of this unique CME.

Uncovering erosion effects on magnetic flux rope twist

Context.Magnetic clouds (MCs) are transient structures containing large-scale magnetic flux ropes from solar eruptions. The twist of magnetic field lines around the rope axis reveals information about flux rope formation processes and geoeffectivity. During propagation MC flux ropes may erode via reconnection with the ambient solar wind. Any erosion reduces the magnetic flux and helicity of the ropes, and changes their cross-sectional twist profiles.Aims.This study relates twist profiles in MC flux ropes observed at 1 AU to the amount of erosion undergone by the MCs in interplanetary space.Methods.The twist profiles of two clearly identified MC flux ropes associated with the clear appearance of post eruption arcades in the solar corona are analyzed. To infer the amount of erosion, the magnetic flux content of the ropes in the solar atmosphere is estimated, and compared to estimates at 1 AU.Results.The first MC shows a monotonically decreasing twist from the axis to the periphery, while the second displays high twist at the axis, rising twist near the edges, and lower twist in between. The first MC displays a larger reduction in magnetic flux between the Sun and 1 AU, suggesting more erosion than that seen in the second MC.Conclusions.In the second cloud the rising twist at the rope edges may have been due to an envelope of overlying coronal field lines with relatively high twist, formed by reconnection beneath the erupting flux rope in the low corona. This high-twist envelope remained almost intact from the Sun to 1 AU due to the low erosion levels. In contrast, the high-twist envelope of the first cloud may have been entirely peeled away via erosion by the time it reaches 1 AU.

Periodicities in an active region correlated with Type III radio bursts observed by Parker Solar Probe.

Context.Periodicities have frequently been reported across many wavelengths in the solar corona. Correlated periods of ~5 min, comparable to solar p-modes, are suggestive of coupling between the photosphere and the corona.Aims.Our study investigates whether there are correlations in the periodic behavior of Type III radio bursts which are indicative of nonthermal electron acceleration processes, and coronal extreme ultraviolet (EUV) emission used to assess heating and cooling in an active region when there are no large flares.Methods.We used coordinated observations of Type III radio bursts from the FIELDS instrument on Parker Solar Probe (PSP), of EUV emissions by the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) and white light observations by SDO Helioseismic and Magnetic Image (HMI), and of solar flare X-rays by Nuclear Spectroscopic Telescope Array (NuSTAR) on April 12, 2019. Several methods for assessing periodicities are utilized and compared to validate periods obtained.Results.Periodicities of ~5 min in the EUV in several areas of an active region are well correlated with the repetition rate of the Type III radio bursts observed on both PSP and Wind. Detrended 211 and 171 Å light curves show periodic profiles in multiple locations, with 171 Å peaks sometimes lagging those seen in 211 Å. This is suggestive of impulsive events that result in heating and then cooling in the lower corona. NuSTAR X-rays provide evidence for at least one microflare during the interval of Type III bursts, but there is not a one-to-one correspondence between the X-rays and the Type III bursts. Our study provides evidence for periodic acceleration of nonthermal electrons (required to generate Type III radio bursts) when there were no observable flares either in the X-ray data or the EUV. The acceleration process, therefore, must be associated with small impulsive events, perhaps nanoflares.

Magnetic field inference in active region coronal loops using coronal rain clumps

Aims. We aim to infer information about the magnetic field in the low solar corona from coronal rain clumps using high-resolution spectropolarimetric observations in the Ca II 8542 Å line obtained with the Swedish 1 m Solar Telescope. Methods. The weak-field approximation (WFA) provides a simple tool to obtain the line-of-sight component of the magnetic field from spectropolarimetric observations. We adapted a method developed in a previous paper in order to assess the different conditions that must be satisfied in order to properly use the WFA for the data at hand. We also made use of velocity measurements in order to estimate the plane-of-the-sky magnetic field component, so that the magnetic field vector could be inferred. Results. We have inferred the magnetic field vector from a data set totalling 100 spectral scans in the Ca II 8542 Å line, containing an off-limb view of the lower portion of catastrophically cooled coronal loops in an active region. Our results, albeit limited by the cadence and signal-to-noise ratio of the data, suggest that magnetic field strengths of hundreds of Gauss, even reaching up to 1000 G, are omnipresent at coronal heights below 9 Mm from the visible limb. Our results are also compatible with the presence of larger magnetic field values such as those reported by previous works. However, for large magnetic fields, the Doppler width from coronal rain is not that much larger than the Zeeman width, thwarting the application of the WFA. Furthermore, we have determined the temperature, T, and microturbulent velocity, ξ, of coronal rain clumps and off-limb spicules present in the same data set, and we have found that the former ones have narrower T and ξ distributions, their average temperature is similar, and coronal rain has microturbulent velocities smaller than those of spicules.

Numerical Simulation of the Flow and Heat Transfer Induced by Corona Discharge Coupling With Electrostatically Forced Vibration

Abstract A thin, flexible plate electrode was adopted to generate both ionic wind and vibration in our previous study. The design contains a metal inductor placed next to the plate electrode so that it is attracted to vibrate by the induced electrostatic force. The resulting flow was used to enhance heat transfer. In this study, a numerical methodology is developed to unveil the flow structure induced by the corona discharge and electrode vibration. The oscillatory movement of the electrode is modeled as a cantilever beam vibrating at its first resonant mode. The electric and flow fields are solved by the finite volume methods (FVMs). It is shown that a jet-like flow is generated by the electric discharge. The oscillatory movement of the jet results in flat temperature profile in comparison with the corona only system. Owing to the unsteady characteristic, the jet strength is less strong than that without vibration. The calculated results are qualitatively in line with the experiments, though some considerable differences exist. It is found that the oscillatory flow brings about lower overall heat transfer effectiveness than that without vibration regardless of the corona voltage. On the contrary, experiments showed that heat transfer is enhanced at low corona voltages where the ionic wind is not so overwhelming. The disagreement is mainly attributed to the two-dimensional (2D) assumption made in the simulation. The experimental arrangement, the corona discharge, and the vortex flows resulted all are three-dimensional (3D). Therefore, 3D calculations become necessary.

The Color and Brightness of the F-corona Inferred from the 2019 July 2 Total Solar Eclipse

Abstract Total solar eclipses (TSEs) provide a unique opportunity to quantify the properties of the K-corona (electrons), F-corona (dust), and E-corona (ions) continuously from the solar surface out to a few solar radii. We apply a novel inversion method to separate emission from the K- and F-corona continua using unpolarized total brightness (tB) observations from five 0.5 nm bandpasses acquired during the 2019 July 2 TSE between 529.5 and 788.4 nm. The wavelength dependence relative to the photosphere (i.e., color) of the F-corona itself is used to infer the tB of the K- and F-corona for each line of sight. We compare our K-corona emission results with the Mauna Loa Solar Observatory (MLSO) K-Cor polarized brightness (pB) observations from the day of the eclipse, and the forward modeled K-corona intensity from the Predictive Science Inc. (PSI) magnetohydrodynamic (MHD) model prediction. Our results are generally consistent with previous work and match both the MLSO data and PSI-MHD predictions quite well, supporting the validity of our approach and of the PSI-MHD model. However, we find that the tB of the F-corona is higher than expected in the low corona, perhaps indicating that the F-corona is slightly polarized—challenging the common assumption that the F-corona is entirely unpolarized.

Correlation of the Sunspot Number and the Waiting-time Distribution of Solar Flares, Coronal Mass Ejections, and Solar Wind Switchback Events Observed with the Parker Solar Probe

Abstract Waiting-time distributions of solar flares and coronal mass ejections (CMEs) exhibit power-law-like distribution functions with slopes in the range of α τ ≈ 1.4–3.2, as observed in annual data sets during four solar cycles (1974–2012). We find a close correlation between the waiting-time power-law slope α τ and the sunspot number (SN), i.e., α τ = 1.38 + 0.01 × SN. The waiting-time distribution can be fitted with a Pareto-type function of the form N(τ) = N 0 , where the offset τ 0 depends on the instrumental sensitivity, the detection threshold of events, and pulse pileup effects. The time-dependent power-law slope α τ (t) of waiting-time distributions depends only on the global solar magnetic flux (quantified by the sunspot number) or flaring rate, which is not predicted by self-organized criticality or magnetohydrodynamic turbulence models. Power-law slopes of α τ ≈ 1.2–1.6 were also found in solar wind switchback events, as observed with the Parker Solar Probe during the solar minimum, while steeper slopes are predicted during the solar maximum. We find that the annual variability of switchback events in the heliospheric solar wind and solar flare and CME rates (originating in the photosphere and lower corona) are highly correlated.

Coupling a Global Heliospheric Magnetohydrodynamic Model to a Magnetofrictional Model of the Low Corona

Abstract Recent efforts coupling our Sun-to-Earth magnetohydrodynamics (MHD) model and lower-corona magnetofrictional (MF) model are described. Our Global Heliospheric MHD (GHM) model uses time-dependent three-component magnetic field data from the lower-corona MF model as time-dependent boundary values. The MF model uses data-assimilation techniques to introduce the vector magnetic field data from the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, hence as a whole this simulation coupling structure is driven with actual observations. The GHM model employs a newly developed interface boundary treatment that is based on the concept of characteristics, and it properly treats the interface boundary sphere set at a height of the sub-Alfvénic lower corona (1.15 R ⊙ in this work). The coupled model framework numerically produces twisted nonpotential magnetic features and consequent eruption events in the solar corona in response to the time-dependent boundary values. The combination of our two originally independently developed models presented here is a model framework toward achieving further capabilities of modeling the nonlinear time-dependent nature of magnetic field and plasma, from small-scale solar active regions to large-scale solar wind structures. This work is a part of the Coronal Global Evolutionary Model project for enhancing our understanding of Sun–Earth physics to help improve space weather capabilities.

Energetic Proton Propagation and Acceleration Simulated for the Bastille Day Event of 2000 July 14

Abstract This work presents results from simulations of the 2000 July 14 (“Bastille Day”) solar proton event. We used the Energetic Particle Radiation Environment Model (EPREM) and the CORona-HELiosphere (CORHEL) software suite within the SPE Threat Assessment Tool (STAT) framework to model proton acceleration to GeV energies due to the passage of a CME through the low solar corona, and we compared the model results to GOES-08 observations. The coupled simulation models particle acceleration from 1 to 20 R ⊙, after which it models only particle transport. The simulation roughly reproduces the peak event fluxes and the timing and spatial location of the energetic particle event. While peak fluxes and overall variation within the first few hours of the simulation agree well with observations, the modeled CME moves beyond the inner simulation boundary after several hours. The model therefore accurately describes the acceleration processes in the low corona and resolves the sites of most rapid acceleration close to the Sun. Plots of integral flux envelopes from multiple simulated observers near Earth further improve the comparison to observations and increase potential for predicting solar particle events. Broken power-law fits to fluence spectra agree with diffusive acceleration theory over the low energy range. Over the high energy range, they demonstrate the variability in acceleration rate and mirror the interevent variability observed in solar cycle 23 ground-level enhancements. We discuss ways to improve STAT predictions, including using corrected GOES energy bins and computing fits to the seed spectrum. This paper demonstrates a predictive tool for simulating low-coronal solar energetic particle acceleration.

LOFAR Observations of a Jet-driven Piston Shock in the Low Solar Corona

Abstract The Sun produces highly dynamic and eruptive events that can drive shocks through the corona. These shocks can accelerate electrons, which result in plasma emission in the form of a type II radio burst. Despite the large number of type II radio burst observations, the precise origin of coronal shocks is still subject to investigation. Here, we present a well-observed solar eruptive event that occurred on 2015 October 16, focusing on a jet observed in the extreme ultraviolet by the Atmospheric Imaging Assembly (SDO/AIA), a streamer observed in white light by the Large Angle and Spectrometric Coronagraph (SOHO/LASCO), and a metric type II radio burst observed by the LOw Frequency Array (LOFAR). LOFAR interferometrically imaged the fundamental and harmonic sources of a type II radio burst and revealed that the sources did not appear to be cospatial, as would be expected from the plasma emission mechanism. We correct for the separation between the fundamental and harmonic using a model that accounts for scattering of radio waves by electron density fluctuations in a turbulent plasma. This allows us to show the type II radio sources were located ∼0.5R ⊙ above the jet and propagated at a speed of ∼1000 km s−1, which was significantly faster than the jet speed of ∼200 km s−1. This suggests that the type II burst was generated by a piston shock driven by the jet in the low corona.

Connecting Atmospheric Properties and Synthetic Emission of Shock Waves Using 3D RMHD Simulations of the Quiet Sun

Abstract We analyze the evolution of shock waves in high-resolution 3D radiative MHD simulations of the quiet Sun and their synthetic emission characteristics. The simulations model the dynamics of a 12.8 × 12.8 × 15.2 Mm quiet-Sun region (including a 5.2 Mm layer of the upper convection zone and a 10 Mm atmosphere from the photosphere to corona) with an initially uniform vertical magnetic field of 10 G, naturally driven by convective flows. We synthesize the Mg II and C II spectral lines observed by the Interface Region Imaging Spectrograph (IRIS) satellite and extreme ultraviolet emission observed by the Solar Dynamics Observatory (SDO)/AIA telescope. Synthetic observations are obtained using the RH1.5D radiative transfer code and temperature response functions at both the numerical and instrumental resolutions. We found that the Doppler velocity jumps of the C II 1334.5 Å IRIS line and a relative enhancement of the emission in the 335 Å SDO/AIA channel are the best proxies for the enthalpy deposited by shock waves into the corona (with Kendall’s τ correlation coefficients of 0.59 and 0.38, respectively). The synthetic emission of the lines and the extreme ultraviolet passbands are correlated with each other during the shock-wave propagation. All studied shocks are mostly hydrodynamic (i.e., the magnetic energy carried by horizontal fields is ≤2.6% of the enthalpy for all events) and have Mach numbers >1.0–1.2 in the low corona. The study reveals the possibility of diagnosing energy transport by shock waves into the solar corona, as well as their other properties, by using IRIS and SDO/AIA sensing observations.

Radio Astronomical Tools for the Study of Solar Energetic Particles I. Correlations and Diagnostics of Impulsive Acceleration and Particle Propagation

Solar energetic particles (SEPs) are sporadically ejected from the Sun during flares and coronal mass ejections. They are of major astrophysical interest, because the proximity of the Sun allows for detailed multi-messenger studies. They affect space weather due to interactions with electronics, with the Earth’s atmosphere, and with humans if they leave the protective shield of the magnetosphere of the Earth. Since early studies in the 1950s, starting with particle detectors on the ground, SEP events have been related to radio bursts. Two subjects are addressed in this chapter: attempts to establish quantitative correlations between SEPs and microwave bursts produced by gyro synchrotron radiation of mildly relativistic electrons, and the information derived from type III radio bursts on impulsive processes of particle acceleration and the coronal and interplanetary propagation. Type III radio bursts produced by electron beams on open magnetic field lines have a wide range of applications, including the identification of acceleration regions, the identification of confined particle acceleration with coronal signatures, but no SEPs, and the paths that the electrons, and energetic charged particles in general, take to travel from the low corona to the Heliosphere in case they escape. Simple scenarios of coronal particle acceleration are confirmed in relatively simple and short events. But the comparison with particle transport models shows that longer and delayed acceleration episodes exist especially in large SEP events. They will be discussed in a companion chapter.

Threaded-field-line Model for the Low Solar Corona Powered by the Alfvén Wave Turbulence

Abstract We present an updated global model of the solar corona, including the transition region. We simulate the realistic three-dimensional (3D) magnetic field using the data from the photospheric magnetic field measurements and assume the magnetohydrodynamic (MHD) Alfvén wave turbulence and its nonlinear dissipation to be the only source for heating the coronal plasma and driving the solar wind. In closed-field regions, the dissipation efficiency in a balanced turbulence is enhanced. In the coronal holes, we account for a reflection of the outward-propagating waves, which is accompanied by the generation of weaker counterpropagating waves. The nonlinear cascade rate degrades in strongly imbalanced turbulence, thus resulting in colder coronal holes. The distinctive feature of the presented model is the description of the low corona as almost-steady-state low-beta plasma motion and heat flux transfer along the magnetic field lines. We trace the magnetic field lines through each grid point of the lower boundary of the global corona model, chosen at some heliocentric distance, R = R b ∼ 1.1R ⊙, well above the transition region. One can readily solve the plasma parameters along the magnetic field line from 1D equations for the plasma motion and heat transport together with the Alfvén wave propagation, which adequately describe the physics within the heliocentric distance range R ⊙ < R < R b , in the low solar corona. By interfacing this threaded-field-line model with the full MHD global corona model at r = R b , we find the global solution and achieve a faster-than-real-time performance of the model on ∼200 cores.

Radio-loud and radio-quiet CMEs: solar cycle dependency, influence on cosmic ray intensity, and geo-effectiveness

Coronal mass ejections (CMEs) largely influence the space weather and cause geomagnetic perturbations. Hence, the statistical studies pertaining to the occurrence of CMEs over the solar cycles and their consequence at the near-Earth region are extremely important. For an in-depth understanding, such studies need to be carried out considering various observational aspects of CMEs. With this motivation, here we present a statistical study of radio-loud (RL) and radio-quiet (RQ) CMEs during solar cycles 23 and 24. We also assess their geo-effectiveness and analyze their influence on cosmic ray intensity (CRI). The RL and RQ CMEs constitute 40% and 60% cases, respectively, of the total population of CMEs that arrive the near-Earth region at 1 AU. The mean speed of RL CMEs (≈1170 km s−1) was found to be significantly higher (almost twice) than the mean speed of RQ CMEs (≈519 km s−1) in the low corona while their speed became comparable (≈536 km s−1 for RL and ≈452 km s−1 for RQ CMEs) at near-Earth region. The yearly-averaged speeds of Earth-reaching CMEs follow solar cycle variations. The CRI and geomagnetic Dst index are found to have good negative correlation with speed of Earth-reaching CMEs. RL CMEs were found to be more effective in producing CRI depressions and geomagnetic storms (GSs) in comparison to RQ CMEs; in about 70% cases RL CMEs produced CRI depression and GSs earlier than the RQ CMEs. Superposed epoch analysis suggests strongest depression in CRI occurs 2-5 days and 4-9 days after the onset of RL and RQ CMEs, respectively. Further, GS events show a time-lag of 1-5 days and 3-8 days, respectively, with respect to RL and RQ CMEs.

Radio Measurements of the Magnetic Field in the Solar Chromosphere and the Corona

The structure of the upper solar atmosphere, on all observable scales, is intimately governed by the magnetic field. The same holds for a variety of solar phenomena that constitute solar activity, from tiny transient brightening to huge Coronal Mass Ejections. Due to inherent difficulties in measuring magnetic field effects on atoms (Zeeman and Hanle effects) in the corona, radio methods sensitive to electrons are of primary importance in obtaining quantitative information about its magnetic field. In this review we explore these methods and point out their advantages and limitations. After a brief presentation of the magneto-ionic theory of wave propagation in cold, collisionless plasmas, we discuss how the magnetic field affects the radio emission produced by incoherent emission mechanisms (free-free, gyroresonance, and gyrosynchrotron processes) and give examples of measurements of magnetic filed parameters in the quiet sun, active regions and radio CMEs. We proceed by discussing how the inversion of the sense of circular polarization can be used to measure the field above active regions. Subsequently we pass to coherent emission mechanisms and present results of measurements from fiber bursts, zebra patterns, and type II burst emission. We close this review with a discussion of the variation of the magnetic field, deduced by radio measurements, from the low corona up to ~ 10 solar radii and with some thoughts about future work.

ORFEES – a radio spectrograph for the study of solar radio bursts and space weather applications

Radio bursts are sensitive tracers of non-thermal electron populations in the solar corona. They are produced by electron beams and shock waves propagating through the corona and the heliosphere, and by trapped electron populations in coronal mass ejections (CMEs) and in quiescent active regions. Combining space-borne and ground-based radio spectrographs allows one to track disturbances between the low corona, near or at the sites of particle acceleration, and the spacecraft. Radio observations are, therefore, a significant tool in probing the solar origin of heliospheric disturbances, which is a central research topic as witnessed by the Parker Solar Probe and Solar Orbiter missions. The full scientific return of these projects needs vigorous ground-based support, which at radio wavelengths covers altitudes up to about a solar radius above the photosphere. Besides research in solar and heliospheric physics, monitoring solar radio bursts also supports space weather services. On occasion, radio bursts can themselves be a space weather hazard. The Nançay radio astronomy station in central France has a long tradition of monitoring radio emission at decimetre-to-meter wavelengths. This article describes the radio spectrograph ORFEES (Observations Radiospectrographiques pour FEDOME et l’Etude des Eruptions Solaires). It observes the whole-Sun flux density between 144 and 1004 MHz, pertaining to regions between the low corona and about half a solar radius above the photosphere. ORFEES results from a partnership between Observatoire de Paris and the French Air Force, which operates the experimental space weather service FEDOME. The primary use of the instrument at the Paris Observatory is astrophysical observation. Low-resolution data with rapid availability are presently produced for the French Air Force. Similar information can be made available to a broader range of space weather service providers. This article gives an overview of the instrument design and access to the data and shows a few illustrative observations.

Research on the peak energy spectrum of the solar cosmic ray ground level enhancement event (GLE72)

The ground level enhancement (GLE) event energy spectrum provides important information about the acceleration and propagation of cosmic ray. In this paper, we analyze the proton flux peak energy spectrum of recent GLE event (2017.09 GLE72) by using GOES15 satellite and neutron monitor experiment data. The method of adjacent averaging smoothing and weighted average are applied to study GOES15 satellite data, and obtain the flux peak and flux peak time. By fitting, the energy spectrum index of proton flux peak is 1.88 in the satellite observation energy range. Again, the energy spectrum index of the neutron monitor observation energy range, 4.86, is obtained by using the new neutron monitor yield function. It can be seen that the peak energy spectrum index given by satellites in the lower energy range (5–433 MeV) is much smaller than that given by the neutron monitor in the higher energy range (0.44–19 GeV). This means that, the energy spectrum in the lower energy range is harder than that in the higher energy range. Hence, the results of the energy spectrum could be explained qualitatively by the re-acceleration mechanism of high energy solar particles. In the low corona region, first, the particles released by the solar flare are accelerated, and the energy spectrum index of the high-energy range is twice that of the low-energy range. Then part of the solar high energy particles from the low corona enter into the CME, where they will be re-acceleration by the shock wave. The GLE72 event high energy range energy spectrum index given by the neutron monitor experiment is 4.86, so the energy spectrum index in low energy range should be 4.86/2 = 2.43. However, the low energy range energy spectrum index is 1.88 (lower than 2.43 in low energy range). The reason may be that the energy spectrum index is further reduced due to the re-acceleration effect in the shock wave generated by the CME. The observation of GLE event is one of the main research subjects of the Large High Altitude Air Shower Observatory (LHAASO). Also, the GLE72 proton peak energy spectrum results provide important information to observe solar high energy particles in the LHAASO experiment.

Three-dimensional Simulations of the Inhomogeneous Low Solar Wind

Abstract In the near future, the Parker Solar Probe will put theories about the dynamics and nature of the transition between the solar corona and the solar wind to stringent tests. The most popular mechanism aimed to explain the dynamics of the nascent solar wind, including its heating and acceleration, is magnetohydrodynamic (MHD) turbulence. Most of the previous models focused on nonlinear cascade induced by interactions of outgoing Alfvén waves and their reflections, ignoring effects that might be related to perpendicular structuring of the solar coronal plasma, despite overwhelming evidence for it. In this paper, for the first time, we analyze through 3D MHD numerical simulations the dynamics of the perpendicularly structured solar corona and solar wind, from the low corona to 15 R ⊙. We find that background structuring has a strong effect on the evolution of MHD turbulence, on much faster timescales than in the perpendicularly homogeneous case. On timescales shorter than nonlinear times, linear effects related to phase mixing result in a 1/f perpendicular energy spectrum. As the turbulent cascade develops, we observe a perpendicular (parallel) energy spectrum with a power-law index of −3/2 or −5/3 (−2), a steeper perpendicular magnetic field than velocity spectrum, and a strong build-up of negative residual energy. We conclude that the turbulence is most probably generated by the self-cascade of the driven transverse kink waves, referred to previously as “uniturbulence,” which might represent the dominant nonlinear energy cascade channel in the pristine solar wind.

Small-scale Flux Emergence, Coronal Hole Heating, and Flux-tube Expansion: A Hybrid Solar Wind Model

Abstract Extreme-ultraviolet images from the Solar Dynamics Observatory often show loop-like fine structure to be present where no minority-polarity flux is visible in magnetograms, suggesting that the rate of ephemeral region (ER) emergence inside “unipolar” regions has been underestimated. Assuming that this rate is the same inside coronal holes as in the quiet Sun, we show that interchange reconnection between ERs and open field lines gives rise to a solar wind energy flux that exceeds 105 erg cm−2 s−1 and that scales as the field strength at the coronal base, consistent with observations. In addition to providing ohmic heating in the low corona, these reconnection events may be a source of Alfvén waves with periods ranging from the granular timescale of ∼10 minutes to the supergranular/plume timescale of many hours, with some of the longer-period waves being reflected and dissipated in the outer corona. The asymptotic wind speed depends on the radial distribution of the heating, which is largely controlled by the rate of flux-tube expansion. Along the rapidly diverging flux tubes associated with slow wind, heating is concentrated well inside the sonic point (1) because the outward conductive heat-flux density and thus the outer coronal temperatures are reduced, and (2) because the net wave energy flux is dissipated at a rate proportional to the local Alfvén speed. In this “hybrid” solar wind model, reconnection heats the lower corona and drives the mass flux, whereas waves impart energy and momentum to the outflow at greater distances.

Radio Observations of Coronal Mass Ejection Initiation and Development in the Low Solar Corona

Coronal mass ejections (CMEs) are large eruptions of plasma and magnetic field from the low solar corona into the heliosphere. These eruptions are often associated with energetic electrons that produce various kinds of radio emission. However, there is ongoing investigation into exactly where, when, and how the electron acceleration occurs during flaring and eruption, and how the associated radio emission can be exploited as a diagnostic of both particle acceleration and CME eruptive physics. Here, we review past and present developments in radio observations of flaring and eruption, from the destabilization of flux ropes to the development of a CME and the eventual driving of shocks in the corona. We concentrate primarily on the progress made in CME radio physics in the past two decades and show how radio imaging spectroscopy provides the ability to diagnose the locations and kinds of electron acceleration during eruption, which provides insight into CME eruptive models in the early stages of their evolution (<10 R-circle dot). We finally discuss how new instrumentation in the radio domain will pave the way for a deeper understanding of CME physics in the near future.