Coronal Funnels Research Articles

Diagnosing a Solar Flaring Core with Bidirectional Quasi-periodic Fast Propagating Magnetoacoustic Waves

Abstract Quasi-periodic fast propagating (QFP) waves are often excited by solar flares, and could be trapped in the coronal structure with low Alfvén speed, so they could be used as a tool for diagnosing both the flaring core and magnetic waveguide. As the periodicity of a QFP wave could originate from a periodic source or be dispersively waveguided, it is a key parameter for diagnosing the flaring core and waveguide. In this paper, we study two QFP waves excited by a Geostationary Operational Environmental Satellite-class C1.3 solar flare occurring at active region NOAA 12734 on 2019 March 8. Two QFP waves were guided by two oppositely oriented coronal funnels. The periods of two QFP waves were identical and were roughly equal to the period of the oscillatory signal in the X-ray and 17 GHz radio emission released by the flaring core. It is very likely that the two QFP waves could be periodically excited by the flaring core. Many features of this QFP wave event are consistent with the magnetic tuning fork model. We also investigated the seismological application with QFP waves, and found that the magnetic field inferred with magnetohydrodynamic seismology was consistent with that obtained in the magnetic extrapolation model. Our study suggests that the QFP wave is a good tool for diagnosing both the flaring core and the magnetic waveguide.

Diagnostics of a Coronal Hole and the Adjacent Quiet Sun by The Hinode/EUV Imaging Spectrometer (EIS)

A comparison between a Coronal Hole (CH) and the adjacent Quiet-Sun (QS) has been performed using spectroscopic diagnostics of Hinode/ the EUV Imaging Spectrometer (EIS). Coronal funnels play an important role in the formation and propagation of the nascent fast solar wind. Applying Gaussian fitting procedures to the observed line profiles, Doppler velocity, intensity, line width (FWHM) and electron density have been estimated over CH and adjacent QS region of a North Polar Coronal Hole (NPCH). The aim of this study is to identify the coronal funnels based on spectral signatures. Excess width regions (excess FWHM above a threshold level) have been identified in QS and CH. The plasma flow inversion (average red-shifts changing to blue-shifts at a specific height) in CH and excess width regions of QS take place at ~ 5.01$\times$10$^{5}$ K. Furthermore, high density concentration in excess width regions of QS provides an indication that these regions are the footprints of coronal funnels. We have also found that non-thermal velocities of CH are higher in comparison to QS confirming that the CHs are the source regions of fast solar wind. Doppler and non-thermal velocities as recorded by different temperature lines have been also compared with previously published results. As we go from lower to upper solar atmosphere, down-flows are dominated in lower atmosphere while coronal lines are dominated by up-flows with a maximum value of ~ 10-12 km s$^{-1}$ in QS. Non-thermal velocity increases first but after Log T$_{e}$ = 5.47 it decreases further in QS. This trend can be interpreted as a signature of the dissipation of Alfv\'en waves, while increasing trend as reported earlier may attribute to the signature of the growth of Alfv\'en waves at lower heights. Predominance of occurrence of nano-flares around O {\sc vi} formation temperature could also explain non-thermal velocity trend.

Estimating the contribution of Alfvén waves to the process of heating the quiet solar corona

We solve numerically the ideal MHD equations with an external gravitational field in 2D in order to study the effects of impulsively generated linear and non-linear Alfv\'en waves into isolated solar arcades and coronal funnels. We analyze the region containing the interface between the photosphere and the corona. The main interest is to study the possibility that Alfv\'en waves triggers the energy flux transfer toward the quiet solar corona and heat it, including the case that two consecutive waves can occur. We find that in the case of arcades, short or large, the transferred fluxes by Alfv\'en waves are sufficient to heat the quiet corona only during a small lapse of time and in a certain region. In the case of funnels the threshold is achieved only when the wave is faster than 10 km/s, which is extremely high. We conclude from our analysis, that Alfv\'en waves, even in the optimistic scenario of having two consecutive Alfv\'en wave pulses, cannot transport enough energy as to heat the quiet corona.

Possible signature of Alfvén wave dissipation in the localized magnetic funnels of the equatorial solar corona

Abstract We analyse the Hinode/EIS 2″-spectroscopic scan data containing the spectral line formed at typical inner coronal temperature. The strong Fe xii 195.120 Å line shows the existence of funnel-like expanding flux-tubes which exhibit the signature of blue-/red-shifted plasma motions in the off-limb equatorial corona. These coronal funnels expand in the form of open magnetic field channels up to inner coronal heights. They are most likely the parts of large-scale and closed magnetic fields (loops) which exist at higher heights in the diffused equatorial corona. We also find the signature of decreasing line-widths with altitude in observed coronal funnels (e.g., funnel 1), which is the lower part of a curved loop system. This provides the most likely signature of Alfvén wave dissipation in lower part of this loop system. We also examine the blue-shifted and diffused coronal loop boundary and interfaced region (funnel 3) which shows increasing Fe xii 195.120 Å line-width along it. Therefore, it exhibits the most likely signature of Alfvén wave growth in this region which is slightly curved and rising higher in the corona. Density measurements in these funnels show that it falls off with height, but more rapidly in the second funnel. We conjecture the almost constant line-width trend as a most likely signature of Alfvén wave dissipation in this density-stratified second coronal funnel, which is also the lower part of a large-scale closed loop system. Both dissipative and growing Alfvén waves can change the non-thermal component and thus the full width at half-maximum of the Fe xii 195.120 Å line. We find the clues of Alfvén wave dissipation along the expanding field lines of the coronal funnel (lower parts of the loop system) imparting its energy to the outflowing plasma and thereby contributing to the formation of the nascent solar wind in the inner corona.

On Thermal-Pulse-Driven Plasma Flows in Coronal Funnels as Observed by the Hinode/EUV Imaging Spectrometer (EIS)

Using one-arcsecond-slit scan observations from the Hinode/EUV Imaging Spectrometer (EIS) on 05 February 2007, we find the plasma outflows in the open and expanding coronal funnels at the eastern boundary of AR 10940. The Doppler velocity map of Fe XII 195.120 A shows that the diffuse close-loop system to be mostly red-shifted. The open arches (funnels) at the eastern boundary of AR exhibit blue-shifts with a maximum speed of about 10-15 km/s. This implies outflowing plasma through these magnetic structures. In support of these observations, we perform a 2D numerical simulation of the expanding coronal funnels by solving the set of ideal MHD equations in appropriate VAL-III C initial temperature conditions using the FLASH code. We implement a rarefied and hotter region at the footpoint of the model funnel, which results in the evolution of slow plasma perturbations propagating outward in the form of plasma flows. We conclude that the heating, which may result from magnetic reconnection, can trigger the observed plasma outflows in such coronal funnels. This can transport mass into the higher corona, giving rise to the formation of the nascent solar wind.

Impulsively Generated Linear and Non-Linear Alfvén Waves in the Coronal Funnels

in the Coronal Funnels P. Chmielewski, A.K. Srivastava, K. Murawski and Z.E. Musielak Group of Astrophysics, UMCS, E. Radziszewskiego 10, 20-031 Lublin, Poland Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital-263 129, India and Department of Physics, Indian Institute of Technology (IIT)-BHU, Varanasi-221005, India Department of Physics, University of Texas at Arlington, Arlington, TX 76019, USA Kiepenheuer-Institut fur Sonnenphysik, Schoneckstr. 6, 79104 Freiburg, Germany

CAN PLASMA EXPANSION EXPLAIN THE OBSERVED ACCELERATION OF Ne 7+ IONS IN A CORONAL MAGNETIC FUNNEL?

Tu et al. measured outflow of Ne7+ at a height of 20 Mm above the photosphere demonstrating that the solar wind originates in coronal funnels footed in the chromospheric magnetic networks. We suggest that when the bottom of a coronal flux tube is populated by the chromospheric plasma consisting of protons as the major ions, Ne7+ as minor ions, and heated electrons in the magnetic networks, large-scale plasma expansion could accelerate the Ne7+ ions to a velocity ~10 km s–1 at a height ~ 20 Mm as measured. Two scenarios are discussed here, one with the bulk heating of the electrons to a temperature T e > 64 eV and another with a small fraction of the electrons heated to high temperatures T eh > 158 eV, appearing as an energetic tail to the electrons' velocity distribution function. In the former scenario, the expansion produces weak ambipolar electric fields distributed along the entire length of the funnel. In the latter one the electric field is concentrated in a double layer. The electric fields accelerate the ions against the solar gravity. The required electron temperatures in the above scenarios are large enough to ionize neon atoms yielding Ne7+.

ACCELERATING WAVES IN POLAR CORONAL HOLES AS SEEN BY EIS AND SUMER

We present EIS/Hinode and SUMER/SOHO observations of propagating disturbances detected in coronal lines in inter-plume and plume regions of a polar coronal hole. The observation was carried out on 2007 November 13 as part of the JOP196/HOP045 program. The SUMER spectroscopic observation gives information about fluctuations in radiance and on both resolved (Doppler shift) and unresolved (Doppler width) line-of-sight velocities, whereas EIS 40'' wide slot images detect fluctuations only in radiance but maximize the probability of overlapping field of view between the two instruments. From distance–time radiance maps, we detect the presence of propagating waves in a polar inter-plume region with a period of 15–20 minutes and a propagation speed increasing from 130 ± 14 km s−1 just above the limb to 330 ± 140 km s−1 around 160'' above the limb. These waves can be traced to originate from a bright region of the on-disk part of the coronal hole where the propagation speed is in the range of 25 ± 1.3 to 38 ± 4.5 km s−1, with the same periodicity. These on-disk bright regions can be visualized as the base of the coronal funnels. The adjacent plume region also shows the presence of propagating disturbances with the same range of periodicity but with propagation speeds in the range of 135 ± 18 to 165 ± 43 km s−1 only. A comparison between the distance–time radiance map of the two regions indicates that the waves within the plumes are not observable (may be getting dissipated) far off-limb, whereas this is not the case in the inter-plume region. A correlation analysis was also performed to find out the time delay between the oscillations at several heights in the off-limb region, finding results consistent with those from the analysis of the distance–time maps. To our knowledge, this result provides first spectroscopic evidence of the acceleration of propagating disturbances in the polar region close to the Sun (within 1.2 R/R☉), which provides clues to the understanding of the origin of these waves. We suggest that the waves are likely either Alfvénic or fast magnetoacoustic in the inter-plume region and slow magnetoacoustic in the plume region. This may lead to the conclusion that inter-plumes are a preferred channel for the acceleration of the fast solar wind.

Drift instabilities in the solar corona within the multi-fluid description

Context. Recent observations have revealed that the solar atmosphere is highly structured in density, temperature, and magnetic field. The presence of these gradients may lead to the appearance of currents in the plasma, which in the weakly collisional corona can constitute sources of free energy for driving micro-instabilities. Such instabilities are very important since they represent a possible source of ion-cyclotron waves that have been conjectured as playing a prominent role in coronal heating, but whose solar origin remains unclear. Aims. Considering a density stratification transverse to the magnetic field, this paper aims at studying the possible occurrence of gradient-induced plasma micro-instabilities under the conditions typical of coronal holes. Methods. Taking the WKB (Wentzel-Kramers-Brillouin) approximation into account, we performed the Fourier plane wave analysis using the collisionless multi-fluid model. By neglecting the electron inertia, this model allowed us to take into account ion-cyclotron wave effects that are absent from the one-fluid model of magnetohydrodynamics (MHD). Realistic models of density and temperature, as well as a 2D analytical magnetic-field model, have been used to define the background plasma in the open-field funnel in a polar coronal hole. The ray-tracing theory has been used to compute the ray path of the unstable waves, as well as the evolution of their growth rates during the propagation. Results. We demonstrate that in typical coronal hole conditions, and when assuming typical transverse density length scales taken from radio observations, the current generated by a relative electron-ion drift provides enough free energy for driving the mode unstable. This instability results from coupling between slow-mode waves propagating in opposite directions. However, the ray-tracing computation shows that the unstable waves propagate upward to only a short distance but then are reflected backward. The corresponding growth rate increases and decreases intermittently in the upward propagating phase, and the instability ceases while the wave is propagating downward. Conclusions. Drift currents caused by fine structures in the density distribution in the magnetically-open coronal funnels can provide enough energy to drive plasma micro-instabilities, which constitute a possible source of the ion-cyclotron waves that have been invoked for coronal heating.

On the Expansion of Coronal Funnels in the Quiet Sun

通过对观测的光球纵向磁场进行势场外推, 重构了一个太阳宁静区光球以上的磁场结构. 结果显示, 在20 Mm 以下, 开放磁力线呈现多个明显的小磁漏斗结构, 这些小磁漏斗结构随高度扩展, 并且在20 Mm 左右融合成大的漏斗结构. 通过系统地研究磁漏斗结构横截面积随高度的变化趋势, 发现太阳宁静区磁漏斗结构的截面积随高度近似线性扩展, 磁漏斗结构在较低高度上(<20Mm) 扩展的速度比在较高高度上 (>20Mm) 扩展的速度要快. 这一结果对太阳风起源和磁环中物质流动的二维数值模拟具有重要的意义. 同时还发现, 闭合磁力线的数目随高度以指数函数的形式减少.

Radiance and Doppler shift distributions across the network of the quiet Sun

Aims. The radiance and Doppler-shift distributions across the solar network provide observational constraints of two-dimensional modeling of transition-region emission and flows in coronal funnels. These distributions have not, however, been studied in detail and we attempt an investigation for a quiet Sun region. Methods. Two different methods, dispersion plots and average-profile studies, were applied to investigate these distributions for three EUV lines. In the dispersion plots, we divided the entire quiet Sun region scanned by SUMER into a bright and a dark part according to an image of Fe xii taken by EIT during the scanning; we plotted intensities and Doppler shifts in each bin as determined according to a filtered intensity of Si ii. We also studied the difference in height variations of the magnetic field as extrapolated from the MDI magnetogram, in and outside network, in the two parts. For the average-profile study, we selected 74 individual cases and derived the average profiles of intensities and Doppler shifts across the network. Cases with large values of blue shift of Ne viii were further studied. Results. The dispersion plots reveal that the intensities of Si ii and C iv increase from network boundary to network center in both the bright and dark parts. However, the intensity of Ne viii shows different trends, namely increasing in the bright part and decreasing in the dark part. In both parts, the Doppler shift of C iv increases steadily from internetwork to network center. The height variations in the magnetic field imply a more homogeneous magnetic structure at greater heights and clearly reflect the different magnetic structures in the two regions. The average-profile study reveals that the intensities of the three lines all decline from the network center to internetwork region. The binned intensities of Si ii and Ne viii have a good correlation. We also find that the large blue shift of Ne viii does not coincide with large red shift of C iv. Conclusions. Our results suggest that the network structure is still prominent at the layer where Ne viii is formed in the quiet Sun, and that the magnetic structures expand more strongly in the dark part than in the bright part of this quiet Sun region. Our results might also hint for a scenario of magnetic reconnection between open funnels and side loops.

Signature of mass supply to quiet coronal loops

Aims. The physical implication of large blue shift of Ne viii in the quiet Sun region is investigated in this paper. Methods. We compare the significant Ne viii blue shifts, which are visible as large blue patches on the Doppler-shift map of a middlelatitude quiet-Sun region observed by SUMER, with the coronal magnetic-field structures as reconstructed from a simultaneous photospheric magnetogram by means of a force-free-field extrapolation. Results. We show for the first time that coronal funnels also exist in the quiet Sun. The region studied contains several small funnels that originate from network lanes, expand with height and finally merge into a single wide open-field region. However, the large blue shifts of the Ne viii line are not generally associated with funnels. A comparison between the projections of coronal loops onto the solar x-y-plane and the Ne viii dopplergram indicates that there are some loops that reveal large Ne viii blue shifts in both legs, and some loops with upflow in one and downflow in the other leg. Conclusions. Our results suggest that strong plasma outflow, which can be traced by large Ne viii blue shift, is not necessarily associated with the solar wind originating in coronal funnels but appears to be a signature of mass supply to coronal loops. Under the assumption that the measured Doppler shift of the Ne viii line represents the real outflow velocity of the neon ions being markers of the proton flow, we estimate the mass supply rate to coronal loops to be about 10\^{34} s\^{-1}.

The Current-Free Electric Double Layer in a Coronal Magnetic Funnel

Current-free double layers (CFDLs) have been recently discovered in a number of laboratory devices, when a low collisional plasma is forced to expand from a high magnetic field source region to a low magnetic field diffusion region. This experimental setup bears a striking resemblance to the natural conditions prevailing in the magnetic funnels of the solar corona. It was commonly thought that magnetic-field-aligned potential disruptions were driven by electron currents, although the theoretical possibility of a CDFL has been known of for some time. Given its recent experimental verification, we make here a contribution to solar plasma physics by investigating the possibility of CFDLs in coronal funnels, which have much in common with the laboratory experiments. Therefore, CFDLs may play an important role in supplying and accelerating plasma in coronal funnels.

Forward modeling of coronal funnels

We propose a forward modeling approach of coronal funnels to investigate the outer layers of the solar atmosphere with respect to their thermodynamical properties and resulting emission line spectra. We investigate the plasma flow out of funnels with a new 2D MHD time dependent model including the solar atmosphere all the way from the chromosphere to the corona. The plasma in the funnel is treated in the single-fluid MHD approximation including radiative losses, anisotropic thermal conduction, and two different parameterized heating functions. We obtain plasma properties (e.g. density, temperature and flow speed) within the funnel for each heating function. From the results of the MHD calculation we derive spectral profiles of a low corona emission line (Ne , 770 A). This allows us e.g. to study the Doppler shifts across the funnel. These results indicate a systematic variation of the Doppler shifts in lines formed in the low corona depending on the heating function used. The line shift above the magnetic field concentration in the network is stronger than in the inter-network in both cases. However, for one of the heating functions, the maximum blue-shift (outflow) is not to be found in the very center of the funnel but in the vicinity of the center. This is not the case of the second heating function where the maximum is well aligned with the centre of the funnel. This model directly relates for the first time the form of the heating function to the thermodynamic and spectral properties of the plasma in a funnel.

Solar Wind from Coronal Funnels and Transition Region Lyα

Using a newly developed gyrotropic solar wind model that extends continuously from the mid-chromosphere to 1 AU and that accounts for radiative losses in the transition region, we have studied the difference between the fast solar wind emanating from a funnel geometry and a traditional rapidly expanding wind. The main aim is to determine whether or not observations of the Lyα intensity in the low transition region can be reconciled with solar wind models. In a rapidly expanding geometry, we are not able to produce a Lyα intensity much higher than 1/10 of the observed values without creating a large pressure in the transition region and, as a result, a mass flux much higher than observed. In a funnel, on the other hand, we can easily obtain the observed Lyα intensity, while still having a wind solution in agreement with observations. The main reason for this is that the fast flow in the funnel causes hydrogen to be very far from ionization equilibrium, with the Lyα intensity coming from temperatures of about 5 × 104 K. At these elevated temperatures, the radiative loss is much more efficient. The results of this Letter support the idea that the solar wind originates from small coronal funnels.

Solar Wind Origin in Coronal Funnels

The origin of the solar wind in solar coronal holes has long been unclear. We establish that the solar wind starts flowing out of the corona at heights above the photosphere between 5 megameters and 20 megameters in magnetic funnels. This result is obtained by a correlation of the Doppler-velocity and radiance maps of spectral lines emitted by various ions with the force-free magnetic field as extrapolated from photospheric magnetograms to different altitudes. Specifically, we find that Ne7+ ions mostly radiate around 20 megameters, where they have outflow speeds of about 10 kilometers per second, whereas C3+ ions with no average flow speed mainly radiate around 5 megameters. Based on these results, a model for understanding the solar wind origin is suggested.

Electron cyclotron maser emission from solar coronal funnels?

The sun is covered by a network of supergranular cells. The convective motion of these cells leads to the formation of strong magnetic fields at the cell boundaries. At larger heights in the solar transition region and low corona, this magnetic field geometry expands rapidly within a short distance, and forms the magnetic structure of funnels. This field line geometry represents a magnetic mirror, and since the plasma density strongly increases with depth in the transition region, the electron velocity distribution function (VDF) can develop a loss cone. Within such a coronal funnel, the plasma frequency can have smaller values than the electron cyclotron frequency, ωp < Ωe. These are the necessary conditions for the generation of X-mode waves through the electron cyclotron maser mechanism. Since there is some observational evidence for radio emission from the supergranular network, it is of interest to investigate the possibility of this plasma wave generation in a quiet stellar atmosphere in detail. In this paper, a kinetic model is used to calculate the electron VDF in a coronal funnel. A method is derived to determine wave growth rates from the electron VDF. Its application on the coronal funnel VDF indeed results in X-mode wave growth. However, it is also found that wave absorption by higher-order resonances at larger heights in the atmosphere plays an important role.

Heating the magnetically open ambient background corona of the Sun by Alfvén waves

Observations of UVCS/SOHO of very high emission line widths in the outer corona suggest that the open field regions there are heated by ion-cyclotron resonance absorption of Alfven waves resulting in the ions being much hotter than the electrons. In the lower corona it is usually assumed that the eective Coulomb-interactions ensure equal temperatures of ions and electrons. However, recent models have shown that in regions of strong magnetic field gradients the ion-cyclotron absorption can be so ecient that the ion temperature overcomes the electron temperature. In this paper we will present new observational results from SUMER/SOHO showing that the lines of Ov and Svi have very large line widths just above the limb. The peak line width occurs at about 10 00 above the limb and corresponds to ion temperatures of more than 3 10 6 K. We compare these observational results to new models in which plasma in coronal funnels is heated and accelerated by means of ion-cyclotron absorption of high-frequency waves. As our model is in good qualitative agreement with the observations we come to the conclusion that the open corona in coronal funnels could well be heated by an ion-cyclotron absorption mechanism, even close to the Sun in the low corona.

The Solar Chromosphere/Corona Interface. I. Far‐Ultraviolet to Extreme‐Ultraviolet Observations and Modeling of Unresolved Coronal Funnels

A coronal funnel model, developed by D. Rabin, was tested against a calibrated spectroheliogram recorded in the 170-1 75 A bandpass. This image was recorded on board a sounding-rocket experiment flown on 1994 November 3, called the Multi-Spectral Solar Telescope Array II (MSSTA II). MSSTA, a joint project of Stanford University, the NASA Marshall Space Flight Center, and the Lawrence Livermore National Laboratory' is an observing platform composed of a set of normal-incidence, multilayer-coated optics designed to obtain narrow-bandpass, high-resolution images (1 sec.- 3 sec.) at selected far-ultraviolet (FUV), extreme-ultraviolet (EUV), and soft X-ray wavelengths (44-1550 A). Using full disk images centered at 1550 A (C IV) and 173 A (Fe IX/X), the funnel model, which is based on coronal back-heating, was tested against the data incorporating observed constraints on global coverage and measured flux. Found was a class of funnel models that could account for the quiescent, globally diffuse and unresolved emission seen in the 171-175 A bandpass, where the funnels are assumed to be rooted in the C IV supergranular network. These models, when incorporated with the CHIANTI spectral code, suggest that this emission is mostly of upper transition region origin and primarily composed of Fe IX plasma. The funnels are found to have constrictions, Gamma approx. 6-20, which is in good agreement with the observations. Further, the fitted models simultaneously satisfy global areal constraints seen in both images; namely,that a global network of funnels must cover approx. 700 - 95% of the total solar surface area seen in the 171-175 A image, and a 5% of the disk area seen in the 1550 A bandpass. These findings support the configuration of the EUV magnetic network as suggested by Reeves et al. and put forth in more detail by Gabriel. Furthermore, the models are in good agreement with differential emission measure estimates made of the transition region by J. C. Raymond & J. G. Doyle for temperatures 250,000 K less than or = T less than or = 650,000 K, based on full-disk observations made on board Skylab.

On the outflow in an equatorial coronal hole

We report new observations concerning the source of the fast solar wind by directly comparing Doppler-shift maps of Ne 7 + with charts of the photospheric magnetic field in an equatorial coronal hole, which was observed by SUMER/SOHO and NSO/Kitt Peak on November 5, 1999. The relationship between the velocity field, line intensity and magnetic network is discussed. Our data show that there are both dark and bright regions in this coronal hole as seen in the Ne VIII line. The larger blue shifts of the Ne VIII line are associated mainly with the darker region, where the strong magnetic flux with a single polarity is concentrated. Conversely, the smaller blue shifts are measured mainly in the brighter region, with an underlying mixed-polarity magnetic structure. These observational results are in agreement with the model prediction that the fast solar wind is initially accelerated in the coronal funnels, which are regions with globally open coronal fields rooted in the magnetic network.