Solar Spicules Research Articles

Dependence of Spicule Properties on the Magnetic Field—Results from Magnetohydrodynamics Simulations

Solar spicules are plasma jets observed in the interface region between the visible solar surface and the corona. At any given time, there is a forest of spicules originating in the chromosphere of the Sun. While various models attempt to elucidate their origin and characteristics, here, we consider the one driven by the magnetoconvection undulations. The radiative magnetohydrodynamic (rMHD) equations are solved using Pencil Code with a spatial resolution of 16 km using various magnetic field strengths. The obtained rMHD simulation data are investigated to unveil the various trends in spicular properties as a function of the applied magnetic fields. The important outcome of this study is the finding of a consistent reduction in both the number density and the maximum height reached by spicules as magnetic field strength increases. We also use parabolic fitting on time–distance curves of spicules that are taller than the 75th percentile in the distribution, in order to find a relation between the deceleration of the spicule tip and the magnetic field strength. Our results offer insights into the response of solar spicules to magnetic field strength.

Modelling the connection between propagating disturbances and solar spicules

Modelling the connection between propagating disturbances and solar spicules

Generation and Life Cycle of Solar Spicules

The physical mechanism for the creation of solar spicules is proposed with three stages of their life cycle. It is assumed that at stage I the density hump is formed locally in the x-y plane in the lower chromosphere in the presence of temperature gradients of electrons and ions along the z-axis (the vertical direction). In this region, the density structure of quasi-neutral (n i ≃ n e = n) plasma after taking birth is accelerated in the vertical direction owing to the thermal force F th ∝ ∇n(x, y, t) × (∇T e + ∇T i ). The exact time-dependent analytical solution of two-fluid plasma equations is presented assuming that density is maximum at the center of the density structure and decays away from it gradually. The 2D density structure is created as a step function H(t) in time at the bottom of the chromosphere, and consequently, the vertical plasma velocity turns out to be the ramp function of time R(t) = tH(t), whereas the source term S(x, y, t) for the density follows the delta function δ(t) form. The upward acceleration a=a(x,y)zˆ produced in this density structure is greater than the downward constant solar acceleration − g ⊙ in the chromosphere. In the transition region, the temperature gradients are steeper; therefore, the upward acceleration increases in magnitude g ⊙ ≪ a and the density hump spends less time there. This is stage II of its life cycle. In stage III, the density structure enters into the corona, where the gradients of temperatures vanish and the structure decelerates to zero velocity under the action of the solar gravitational force.

Spicules in IRIS Mg ii Observations: Automated Identification

We have developed an algorithm to identify solar spicules in the first ever systematic survey of on-disk spicules exclusively using Mg ii spectral observations. Using this algorithm we identify 2021 events in three Interface Region Imaging Spectrograph (IRIS) data sets with unique solar feature targets spanning a total of 300 minutes: (1) active region, (2) decayed active region/active network, and (3) coronal hole. We present event statistics and relate occurrence rates to the underlying photospheric magnetic field strength. This method identifies spicule event densities and occurrence rates similar to previous studies performed using Hα and Ca ii observations of active regions. Additionally, this study identifies spicule-like events at very low rates at magnetic field intensities below 20 G, and increasing significantly between 100 and 200 G in active regions and above 20 G in coronal holes, which can be used to inform future observation campaigns. This information can be be used to help characterize spicules over their full lifetimes, and compliments existing Hα spectral capabilities and upcoming Lyα spectral observations with the Solar eruptioN Integral Field Spectrograph (SNIFS) sounding rocket. In total, this study presents a method for detecting solar spicules exclusively using Mg ii spectra, and provides statistics for spicule occurrences in the Mg ii h line with respect to the magnetic field strength for the purpose of predicting spicule occurrences.

Resonant axion radiation conversion in solar spicules

It has recently been observed that solar spicules covering almost all of solar surface have strong magnetic field [Formula: see text]. They are supposed to be plasma jets emitted from chromosphere and they arrive up to [Formula: see text]. Their electron number density is such that [Formula: see text]. Corresponding plasma frequency [Formula: see text] (electron mass [Formula: see text]) is nearly equal to axion mass [Formula: see text]. Thus, resonant radiation conversion of axion with the mass can arise in the spicules. We show that radiations converted from axion dark matter possess flux density [Formula: see text]. The radiations show line spectrum with frequency [Formula: see text]. Our estimation has fewer ambiguities in physical parameters than similar estimation in neutron stars because physical parameters like electron number density have been more unambiguously observed in the sun. But, much strong solar thermal radiations would preclude sensitive observations of such radiations from the axions.

Polymeric jets throw light on the origin and nature of the forest of solar spicules

Spicules are plasma jets, observed in the dynamic interface region between the visible solar surface and the hot corona. At any given time, it is estimated that about 3 million spicules are present on the Sun. We find an intriguing parallel between the simulated spicular forest in a solar-like atmosphere and the numerous jets of polymeric fluids when both are subjected to harmonic forcing. In a radiative magnetohydrodynamic numerical simulation with sub-surface convection, solar global surface oscillations are excited similarly to those harmonic vibrations. The jets thus produced match remarkably well with the forests of spicules detected in observations of the Sun. Taken together, the numerical simulations of the Sun and the laboratory fluid dynamics experiments provide insights into the mechanism underlying the ubiquity of jets: the nonlinear focusing of quasi-periodic waves in anisotropic media of magnetized plasma as well as polymeric fluids under gravity is sufficient to generate a forest of spicules on the Sun.

Fast magnetic reconnection induced by resistivity gradients in 2D magnetohydrodynamics

Using two-dimensional (2D) magnetohydrodynamics simulations, we show that Petschek-type magnetic reconnection can be induced using a simple resistivity gradient in the reconnection outflow direction, revealing the key ingredient of steady fast reconnection in the collisional limit. We find that the diffusion region self-adjusts its half-length to fit the given gradient scale of resistivity. The induced reconnection x-line and flow stagnation point always reside within the resistivity transition region closer to the higher resistivity end. The opening of one exhaust by this resistivity gradient will lead to the opening of the other exhaust located on the other side of the x-line, within the region of uniform resistivity. Potential applications of this setup to reconnection-based thrusters and solar spicules are discussed. In a separate set of numerical experiments, we explore the maximum plausible reconnection rate using a large and spatially localized resistivity right at the x-line. Interestingly, the resulting current density at the x-line drops significantly so that the normalized reconnection rate remains bounded by the value ≃0.2, consistent with the theoretical prediction.

Exact solution of partial differential equations for the creation of jet-like flows in plasmas and neutral fluids

An exact solution of two fluid ideal classical plasma equations is presented which shows that the one-dimensional jet-like axial outflow and two-dimensional magnetic field are generated simultaneously by the density and temperature gradients of both electrons and ions in cylindrical geometry. Particular profiles of density function ψ=ln n¯ (where n¯ is normalized by some constant density N0) and temperatures Tj (for j=e,i) have to be chosen to obtain an exact solution of the complicated nonlinear partial differential equations. But this is a natural analytical exact solution of the ideal two fluid plasma equations. The basic mechanism presented here explains the creation of plasma jet-like flows along with magnetic fields in astrophysical environments such as young stellar objects, active galactic nuclei, solar spicules, flares, and coronal loops. The theoretical model is also applicable to laser induced plasma where magnetic field and plasma ablation are produced simultaneously. An exact analytical solution of ideal neutral fluid equations is also presented which shows that the jet-like outflows can be generated by the density and temperature gradients in such systems.

Semi-empirical Models of Spicule from Inversion of Ca ii 8542 Å Line

Abstract We study a solar spicule observed off-limb using high-resolution imaging spectroscopy in the Ca ii 8542 Å line obtained with the CRisp Imaging SpectroPolarimeter (CRISP) on the Swedish 1 m Solar Telescope. Using a new version of the non-LTE code NICOLE specifically developed for this problem we invert the spicule single- and double-peak line profiles. This new version considers off-limb geometry and computes atomic populations by solving the 1D radiative transfer assuming a vertical stratification. The inversion proceeds by fitting the observed spectral profiles at 14 different heights with synthetic profiles computed in the model by solving the radiative transfer problem along its length. Motivated by the appearance of double-peak Ca ii 8542 Å spicule profiles, which exhibit two distinct emission features well separated in wavelength, we adopt a double-component scenario. We start from the ansatz that the spicule parameters are practically constant along the spicule axis for each component, except for a density drop. Our results support this ansatz by attaining very good fits to the entire set of 14 × 4 profiles (14 heights and 4 times). We show that the double-component model with uniform temperature of 9560 K, exponential decrease of density with a height scale of 1000–2000 km, and the counter-oriented line-of-sight velocities of components reproduce the double-peak line profiles at all spicule segments well. Analyses of the numerical response function reveals the necessity of the inversions of spectra at multiple height positions to obtain height-dependent, degeneracy-free reliable models with a limited number of free parameters.

Signatures of Cross-sectional Width Modulation in Solar Spicules due to Field-aligned Flows

Abstract We report the first observational detection of frequency modulation in the cross-sectional width of spicule structures due to field-aligned plasma flows. Cross-sectional width variations were estimated for the least superimposed off-limb spicules observed in high-resolution Hα imaging spectroscopy data. Analysis of estimated cross-sectional widths suggest periodic oscillations, concurrent with 2D numerical modeling for a jet structure in a stratified solar atmosphere. Spectral analysis for both observed and simulated cross-sectional widths indicate frequency modulation as noticeable shifts in estimated periodicities during rise and fall phases of field-aligned plasma flows in the jet structure. Furthermore, the presence of the first overtone in a dynamic/spicular waveguide is also evident in both the observed and the simulated jet structures. These harmonics can be an important tool for future chromospheric magnetoseismology investigations and applications to dynamic waveguides (like spicules).

Ubiquitous hundred-Gauss magnetic fields in solar spicules

Aims. We aim to study the magnetic field in solar spicules using high-resolution spectropolarimetric observations in the Ca II 8542 Å line obtained with the Swedish 1-m Solar Telescope. Methods. The equations that result from the application of the weak field approximation (WFA) to the radiative transfer equations were used to infer the line-of-sight (LOS) component of the magnetic field (BLOS). Two restrictive conditions were imposed on the Stokes I and V profiles at each pixel before they could be used in a Bayesian inversion to compute its BLOS. Results. The LOS magnetic field component was inferred in six data sets totalling 448 spectral scans in the Ca II 8542 Å line and containing both active region and quiet Sun areas, with values of hundreds of Gauss being abundantly inferred. There seems to be no difference, from a statistical point of view, between the magnetic field strength of spicules in the quiet Sun or near an active region. On the other hand, the BLOS distributions present smaller values on the disc than off-limb, a fact that can be explained by the effect of superposition on the chromosphere of on-disc structures. We show that on-disc pixels in which the BLOS is determined are possibly associated with spicular structures because these pixels are co-spatial with the magnetic field concentrations at the network boundaries and the sign of their BLOS agrees with that of the underlying photosphere. We find that spicules in the vicinity of a sunspot have a magnetic field polarity (i.e. north or south) equal to that of the sunspot. This paper also contains an analysis of the effect of off-limb overlapping structures on the observed Stokes I and V parameters and the BLOS obtained from the WFA. It is found that this value is equal to or smaller than the largest LOS magnetic field components of the two structures. In addition, using random BLOS, Doppler velocities, and line intensities of these two structures leads in ≃50% of the cases to Stokes I and V parameters that are unsuitable to be used with the WFA. Conclusions. Our results present a scarcity of LOS magnetic field components smaller than some 50 G, which must not be taken as evidence against the existence of these magnetic field strengths in spicules. This fact possibly arises as the consequence of signal superposition and noise in the data. We also suggest that the failure of previous works to infer the strong magnetic fields in spicules detected here is their coarser spatial and/or temporal resolution.

Possible Evolution of Minifilament-Eruption-Produced Solar Coronal Jets, Jetlets, and Spicules, into Magnetic-Twist-Wave “Switchbacks” Observed by the Parker Solar Probe (PSP)

Many solar coronal jets result from erupting miniature-filament (“minifilament”) magnetic flux ropes that reconnect with encountered surrounding far-reaching field. Many of those minifilament flux ropes are apparently built and triggered to erupt by magnetic flux cancelation. If that cancelation (or some other process) results in the flux rope’s field having twist, then the reconnection with the far-reaching field transfers much of that twist to that reconnected far-reaching field. In cases where that surrounding field is open, the twist can propagate to far distances from the Sun as a magnetic-twist Alfvénic pulse. We argue that such pulses from jets could be the kinked-magnetic-field structures known as “switchbacks,” detected in the solar wind during perihelion passages of the Parker Solar Probe (PSP). For typical coronal-jet-generated Alfvénic pulses, we expect that the switchbacks would flow past PSP with a duration of several tens of minutes; larger coronal jets might produce switchbacks with passage durations ∼1hr. Smaller-scale jet-like features on the Sun known as “jetlets” may be small-scale versions of coronal jets, produced in a similar manner as the coronal jets. We estimate that switchbacks from jetlets would flow past PSP with a duration of a few minutes. Chromospheric spicules are jet-like features that are even smaller than jetlets. If some portion of their population are indeed very-small-scale versions of coronal jets, then we speculate that the same processes could result in switchbacks that pass PSP with durations ranging from about ∼2 min down to tens of seconds.

Generation of solar spicules and subsequent atmospheric heating

Generation of solar spicules and subsequent atmospheric heating

Possible Production of Solar Spicules by Microfilament Eruptions

Abstract We examine Big Bear Solar Observatory (BBSO) Goode Solar Telescope (GST) high spatial resolution (0.″06), high-cadence (3.45 s), Hα-0.8 Å images of central-disk solar spicules, using data of Samanta et al. We compare with coronal-jet chromospheric-component observations of Sterling et al. Morphologically, bursts of spicules, referred to as “enhanced spicular activities” by Samanta et al., appear as scaled-down versions of the jet’s chromospheric component. Both the jet and the enhanced spicular activities appear as chromospheric-material strands, undergoing twisting-type motions of ∼20–50 km s−1 in the jet and ∼20–30 km s−1 in the enhanced spicular activities. Presumably, the jet resulted from a minifilament-carrying magnetic eruption. For two enhanced spicular activities that we examine in detail, we find tentative candidates for corresponding erupting microfilaments, but not the expected corresponding base brightenings. Nonetheless, the enhanced-spicular-activities’ interacting mixed-polarity base fields, frequent-apparent-twisting motions, and morphological similarities to the coronal jet’s chromospheric-temperature component, suggest that erupting microfilaments might drive the enhanced spicular activities but be hard to detect, perhaps due to Hα opacity. Degrading the BBSO/GST-image resolution with a 1.″0-FWHM smoothing function yields enhanced spicular activities resembling the “classical spicules” described by, e.g., Beckers. Thus, a microfilament eruption might be the fundamental driver of many spicules, just as a minifilament eruption is the fundamental driver of many coronal jets. Similarly, a 0.″5-FWHM smoothing renders some enhanced spicular activities to resemble previously reported “twinned” spicules, while the full-resolution features might account for spicules sometimes appearing as 2D-sheet-like structures.

Dynamic Kink Instability and Transverse Motions of Solar Spicules

Abstract Hydrodynamic jets are unstable to the kink instability (m = 1 mode in cylindrical geometry) owing to the centripetal force, which increases the transverse displacement of the jet. When the jet moves along a magnetic field, the Lorentz force tries to decrease the displacement and stabilize the instability of sub-Alfvénic flows. The threshold of the instability depends on the Alfvén Mach number (the ratio of Alfvén and jet speeds). We suggest that the dynamic kink instability may be important to explain observed transverse motions of type II spicules in the solar atmosphere. We show that the instability may begin for spicules that rise up at the peripheries of vertically expanding magnetic flux tubes because of the decrease of the Alfvén speed in both the vertical and the radial directions. Therefore, inclined spicules may be more unstable and have higher transverse speeds. Periods and growth times of unstable modes in the conditions of type II spicules have values of 30 s and 25–100 s, respectively, which are comparable to the lifetime of the structures. This may indicate an interconnection between high-speed flow and the rapid disappearance of type II spicules in chromospheric spectral lines.

Generation of solar spicules and subsequent atmospheric heating.

Spicules are rapidly evolving fine-scale jets of magnetized plasma in the solar chromosphere. It remains unclear how these prevalent jets originate from the solar surface and what role they play in heating the solar atmosphere. Using the Goode Solar Telescope at the Big Bear Solar Observatory, we observed spicules emerging within minutes of the appearance of opposite-polarity magnetic flux around dominant-polarity magnetic field concentrations. Data from the Solar Dynamics Observatory showed subsequent heating of the adjacent corona. The dynamic interaction of magnetic fields (likely due to magnetic reconnection) in the partially ionized lower solar atmosphere appears to generate these spicules and heat the upper solar atmosphere.

Anti-phase oscillations of H α $\alpha$ line Doppler velocity and width in solar limb spicules

Spectroscopic observations of limb spicules were carried out during September 25 and October 17–19, 2012 in H $\alpha$ line using large 53-cm non-eclipsing coronagraph of Abastumani Astrophysical Observatory (Georgia). The spectrograph slit was located at the height of about 7500 km above the solar limb. Spectrograms in H $\alpha$ line were obtained in a second order of the spectrograph, where reversed dispersion equal to 0.96 A/mm. Doppler velocities and FWHM of 35 spicules were measured with the average cadence of 4.5 s and standard error equal to ±0.3 km/s and 0.03 A, respectively. Lifetimes of almost all measured spicules were 12–16 min, therefore they resemble type I spicules. We studied the temporal variations of Doppler shift and width of H $\alpha$ line using the Lomb periodogram algorithm for unevenly distributed time series. We found the oscillations in both, velocity and width, with periods of 4–9 min (240–540 s) at confidence levels of 95% and clear anti-phase relations (stronger Doppler velocity–weaker Doppler width and vice versa) between Doppler velocity and FWHM in all 35 spicules. The observed anti-phase oscillation with long periods can be explained by up and down motions of turbulent plasma in type I spicules, but the oscillations with shorter periods can be caused by helical motion of spicule axis formed after superposition of two linearly polarised magnetohydrodynamic kink waves.

Two-fluid Numerical Simulations of Solar Spicules

Abstract We aim to study the formation and evolution of solar spicules by means of numerical simulations of the solar atmosphere. With the use of newly developed JOANNA code, we numerically solve two-fluid (for ions + electrons and neutrals) equations in 2D Cartesian geometry. We follow the evolution of a spicule triggered by the time-dependent signal in ion and neutral components of gas pressure launched in the upper chromosphere. We use the potential magnetic field, which evolves self-consistently, but mainly plays a passive role in the dynamics. Our numerical results reveal that the signal is steepened into a shock that propagates upward into the corona. The chromospheric cold and dense plasma lags behind this shock and rises into the corona with a mean speed of 20–25 km s−1. The formed spicule exhibits the upflow/downfall of plasma during its total lifetime of around 3–4 minutes, and it follows the typical characteristics of a classical spicule, which is modeled by magnetohydrodynamics. The simulated spicule consists of a dense and cold core that is dominated by neutrals. The general dynamics of ion and neutral spicules are very similar to each other. Minor differences in those dynamics result in different widths of both spicules with increasing rarefaction of the ion spicule in time.

A Three-dimensional Magnetohydrodynamic Simulation of the Formation of Solar Chromospheric Jets with Twisted Magnetic Field Lines

Abstract This paper presents a three-dimensional simulation of chromospheric jets with twisted magnetic field lines. Detailed treatments of the photospheric radiative transfer and the equations of state allow us to model realistic thermal convection near the solar surface, which excites various MHD waves and produces chromospheric jets in the simulation. A tall chromospheric jet with a maximum height of 10–11 Mm and lifetime of 8–10 minutes is formed above a strong magnetic field concentration. The magnetic field lines are strongly entangled in the chromosphere, which helps the chromospheric jet to be driven by the Lorentz force. The jet exhibits oscillatory motion as a natural consequence of its generation mechanism. We also find that the produced chromospheric jet forms a cluster with a diameter of several Mm with finer strands. These results imply a close relationship between the simulated jet and solar spicules.

On the generation of solar spicules and Alfvénic waves.

In the lower solar atmosphere, the chromosphere is permeated by jets known as spicules, in which plasma is propelled at speeds of 50 to 150 kilometers per second into the corona. The origin of the spicules is poorly understood, although they are expected to play a role in heating the million-degree corona and are associated with Alfvénic waves that help drive the solar wind. We compare magnetohydrodynamic simulations of spicules with observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. Spicules are shown to occur when magnetic tension is amplified and transported upward through interactions between ions and neutrals or ambipolar diffusion. The tension is impulsively released to drive flows, heat plasma (through ambipolar diffusion), and generate Alfvénic waves.