T. V. Zaqarashvili

Oscillations and Waves in Solar Spicules

Since their discovery, spicules have attracted increased attention as energy/mass bridges between the dense and dynamic photosphere and the tenuous hot solar corona. Mechanical energy of photospheric random and coherent motions can be guided by magnetic field lines, spanning from the interior to the upper parts of the solar atmosphere, in the form of waves and oscillations. Since spicules are one of the most pronounced features of the chromosphere, the energy transport they participate in can be traced by the observations of their oscillatory motions. Oscillations in spicules have been observed for a long time. However the recent high-resolution and high-cadence space and ground based facilities with superb spatial, temporal and spectral capacities brought new aspects in the research of spicule dynamics. Here we review the progress made in imaging and spectroscopic observations of waves and oscillations in spicules. The observations are accompanied by a discussion on theoretical modelling and interpretations of these oscillations. Finally, we embark on the recent developments made on the presence and role of Alfvén and kink waves in spicules. We also address the extensive debate made on the Alfvén versus kink waves in the context of the explanation of the observed transverse oscillations of spicule axes.

Observation of Kink Instability During Small B5.0 Solar Flare on 2007 June 4

Using multi-wavelength observations of SOHO/MDI, SOT-Hinode/blue-continuum (4504 ?), G band (4305 ?), Ca II H (3968 ?), and TRACE 171 ?, we present the observational signature of a highly twisted magnetic loop in AR?10960 during the period 04:43?UT-04:52?UT on 2007 June 4. SOT-Hinode/blue-continuum (4504 ?) observations show that penumbral filaments of positive polarity sunspot have counterclockwise twist, which may be caused by the clockwise rotation of the spot umbrae. The coronal loop, whose one footpoint is anchored in this sunspot, shows strong right-handed twist in chromospheric SOT-Hinode/Ca II H (3968 ?) and coronal TRACE 171 ? images. The length and the radius of the loop are L ~ 80?Mm and a ~ 4.0?Mm, respectively. The distance between neighboring turns of magnetic field lines (i.e., pitch) is estimated as 10?Mm. The total twist angle, ? ~ 12? (estimated for the homogeneous distribution of the twist along the loop), is much larger than the Kruskal-Shafranov instability criterion. We detected clear double structure of the loop top during 04:47?UT-04:51?UT on TRACE 171 ? images, which is consistent with simulated kink instability in curved coronal loops. We suggest that the kink instability of this twisted magnetic loop triggered a B5.0 class solar flare, which occurred between 04:40?UT and 04:51?UT in this active region.

Doppler-shift oscillations in solar spicules

Aims. We analysed the consecutive height series of Hα spectra in solar limb spicules taken on the 53 cm coronagraph of Abastumani Astrophysical Observatory at the heights of 3800−8700 km above the photosphere. Our aim is to observe oscillatory phenomena in spicules and consequently to trace wave propagations through the chromosphere. Methods. We use the Discrete Fourier Transform analysis of Hα Doppler shift time series constructed from the observed spectra at each height. Results. Doppler velocities of solar limb spicules show oscillations with periods of 20−55 and 75−110 s. There is also clear evidence of 3-min oscillations at the observed heights. Conclusions. The oscillations can be caused by wave propagations in thin magnetic flux tubes anchored in the photosphere. We suggest the granulation as a possible source of the wave excitation. Observed waves can be used as a tool for spicule seismology ,a nd the magnetic field strength in spicules at the height of ∼6000 km above the photosphere is estimated as 12−15 G.

Magnetohydrodynamic waves in solar partially ionized plasmas: two-fluid approach

Context. Partially ionized plasma is usually described by a single-fluid approach, where the ion-neutral collision effects are expressed by Cowling conductivity in the induction equation. However, the single-fluid approach is not valid for time-scales less than ion-neutral collision time. For these time-scales the two-fluid description is the better approximation. Aims. We aim to derive the dynamics of magnetohydrodynamic (MHD) waves in two-fluid partially ionized plasmas and to compare the results with those obtained under single-fluid description. Methods. Two-fluid equations are used, where ion-electron plasma and neutral particles are considered as separate fluids. Dispersion relations of linear waves are derived for the simplest case of homogeneous medium. Frequencies and damping rates of waves are obtained for different parameters of background plasma. Results. We found that two- and single-fluid descriptions give similar results for low-frequency waves. However, the dynamics of MHD waves in the two-fluid approach is significantly changed when the wave frequency becomes comparable with or higher than the ion-neutral collision frequency. Alfven and fast magneto-acoustic waves attain their maximum damping rate at particular frequencies (for example, the peak frequency equals 2.5 times the ion-neutral collision frequency for 50% of neutral hydrogen) in the wave spectrum. The damping rates are reduced for the higher frequency waves. The new mode of slow magneto-acoustic wave appears for higher frequency branch, which is connected to neutral hydrogen fluid. Conclusions. The single-fluid approach perfectly deals with slow processes in partially ionized plasmas, but fails for time-scales shorter than ion-neutral collision time. Therefore, the two-fluid approximation should be used for the description of relatively fast processes. Some results of the single-fluid description should be revised in future such as the damping of high-frequency Alfven waves in the solar chromosphere due to ion-neutral collisions.

Observation of multiple sausage oscillations in cool post-flare loop

Using simultaneous high spatial (1.3 arcsec) and temporal (5 and 10 s) resolution Hα observations from the 15 cm Solar Tower Telescope at Aryabhatta Research Institute of Observational Sciences (ARIES), we study the oscillations in the relative intensity to explore the possibility of sausage oscillations in the chromospheric cool post-flare loop. We use the standard wavelet tool, and find the oscillation period of ≈587 s near the loop apex, and ≈349 s near the foot-point. We suggest that the oscillations represent the fundamental and the first harmonics of the fast-sausage waves in the cool post-flare loop. Based on the period ratio P 1 /P 2 ∼1.68, we estimate the density scaleheight in the loop as ∼ 17 Mm. This value is much higher than the equilibrium scaleheight corresponding to Ha temperature, which probably indicates that the cool post-flare loop is not in hydrostatic equilibrium. Seismologically estimated Alfven speed outside the loop is ∼300-330 km s -1 . The observation of multiple oscillations may play a crucial role in understanding the dynamics of lower solar atmosphere, complementing such oscillations already reported in the upper solar atmosphere (e.g. hot flaring loops).

Magnetic Rossby Waves in the Solar Tachocline and Rieger-Type Periodicities

Apart from the eleven-year solar cycle, another periodicity around 155-160 days was discovered during solar cycle 21 in high-energy solar flares, and its presence in sunspot areas and strong magnetic flux has been also reported. This periodicity has an elusive and enigmatic character, since it usually appears only near the maxima of solar cycles, and seems to be related with a periodic emergence of strong magnetic flux at the solar surface. Therefore, it is probably connected with the tachocline, a thin layer located near the base of the solar convection zone, where a strong dynamo magnetic field is stored. We study the dynamics of Rossby waves in the tachocline in the presence of a toroidal magnetic field and latitudinal differential rotation. Our analysis shows that the magnetic Rossby waves are generally unstable and that the growth rates are sensitive to the magnetic field strength and to the latitudinal differential rotation parameters. Variation of the differential rotation and the magnetic field strength throughout the solar cycle enhance the growth rate of a particular harmonic in the upper part of the tachocline around the maximum of the solar cycle. This harmonic is symmetric with respect to the equator and has a period of 155-160 days. A rapid increase of the wave amplitude could give rise to a magnetic flux emergence leading to observed periodicities in solar activity indicators related to magnetic flux.

Instability of twisted magnetic tubes with axial mass flows

Context. Recent observations of various kinds of jets in the solar atmosphere motivate studying the influence of mass flow on the stability of solar magnetic structures. Aims. We study the influence of axial mass flows on the stability of twisted magnetic flux tubes. Methods. We use the incompressible magnetohydrodynamic equations to get the dispersion relation governing the behaviour of normal modes in uniformly twisted magnetic tubes with sub-Alfvenic flows. The dispersion relation is then solved analytically and numerically to find stability criteria for twisted tubes with flow. Results. Two main important results are found. First, the axial mass flow reduces the threshold of kink instability in twisted magnetic tubes. Second, the twist of magnetic tubes leads to the Kelvin-Helmholtz instability of sub-Alfvenic flows for the harmonics with a large enough azimuthal wave number -m. Conclusions. The observed mass flow may trigger the kink instability in magnetic configurations that are near their stability threshold, leading to solar flares and coronal mass ejections. The effect is more significant for photospheric magnetic tubes than for coronal ones. Sub-Alfvenic flows undergo the Kelvin-Helmholtz instability in slightly twisted magnetic tubes if the azimuthal wavenumber is big enough.

QUASI-BIENNIAL OSCILLATIONS IN THE SOLAR TACHOCLINE CAUSED BY MAGNETIC ROSSBY WAVE INSTABILITIES

Quasi-biennial oscillations (QBOs) are frequently observed in solar activity indices. However, no clear physical mechanism for the observed variations has been suggested so far. Here, we study the stability of magnetic Rossby waves in the solar tachocline using the shallow water magnetohydrodynamic approximation. Our analysis shows that the combination of typical differential rotation and a toroidal magnetic field with a strength of ?105?G triggers the instability of the m = 1 magnetic Rossby wave harmonic with a period of ~2?years. This harmonic is antisymmetric with respect to the equator and its period (and growth rate) depends on the differential rotation parameters and magnetic field strength. The oscillations may cause a periodic magnetic flux emergence at the solar surface and consequently may lead to the observed QBO in solar activity features. The period of QBOs may change throughout a cycle, and from cycle to cycle, due to variations of the mean magnetic field and differential rotation in the tachocline.

Numerical simulations of spicule formation in the solar atmosphere

Context. We study the upward propagation of a localized velocity pulse that is initially launched below the transition region within the solar atmosphere. The pulse quickly steepens into a shock, which may lead to the formation of spicules. Aims. We aim to explore the spicule formation scenario in the framework of the rebound shock model. Methods. We solve two-dimensional time-dependent magnetohydrodynamic equations numerically to find spatial and temporal dynamics of spicules. Results. The numerical simulations show that the strong initial pulse may lead to the quasi periodic rising of chromospheric material into the lower corona in the form of spicules. The periodicity results from the nonlinear wake that is formed behind the pulse in the stratified atmosphere. The superposition of raising and falling off plasma portions resembles the time sequence of single and double (sometimes even triple) spicules, which is consistent with observational findings. Conclusions. The two-dimensional rebound shock model may explain the observed speed, width, and heights of type I spicules, as well as observed multi-structural and bi-directional flows. The model also predicts the appearance of spicules with 3 5 min period due to the consecutive shocks.

Numerical simulations of solar macrospicules

Context. We consider a localized pulse in the component of velocity, parallel to the ambient magnetic field lines, that is initially launched in the solar chromosphere. Aims. We aim to generalize our recent numerical model of spicule formation by implementing a VAL-C model of solar temperature. Methods. With the use of the code FLASH we solve two-dimensional ideal magnetohydrodynamic equations numerically to simulate the solar macrospicules. Results. Our numerical results reveal that the pulse located below the transition region triggers plasma perturbations, which exhibit many features of macrospicules. We also present an observational (SDO/AIA 304 A) case study of the macrospicule that approximately mimics the numerical simulations. Conclusions. In the frame of the model we devised, the solar macrospicules can be triggered by velocity pulses launched from the chromosphere.