# Chromospheric Peculiar Off-limb Dynamical Events from IRIS Observations

E. Tavabi, S. Koutchmy

► To cite this version:

E. Tavabi, S. Koutchmy. Chromospheric Peculiar Off-limb Dynamical Events from IRIS Observations. The Astrophysical Journal, 2019, 883, 10.3847/1538-4357/ab3730. insu-03747921

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# Chromospheric Peculiar Off-limb Dynamical Events from IRIS Observations

E. Tavabi¹ and S. Koutchmy²

¹ Physics Department, Payame Noor University (PNU), 19395-3697, Tehran, Iran; etavabi@gmail.com

² Institut d'Astrophysique de Paris, CNRS and Sorbonne Univ. UPMC, UMR 7095, 98 Bis Bd. Arago, F-75014 Paris, France

Received 2017 December 17; revised 2019 July 24; accepted 2019 July 24; published 2019 September 18

## Abstract

To study motions and oscillations in the solar chromosphere and at the transition region level we analyze some extreme Doppler shifts observed off-limb with the *Interface Region Imaging Spectrograph (IRIS)*. Raster scans and slit-jaw imaging observations performed in the near-ultraviolet channels were used. Large transverse oscillations are revealed by the far wings profiles after accurately removing the bulk average line profiles of each sequence. Different regions around the Sun are considered. Accordingly, the cool material of spicules is observed in Mg II lines rather dispersed up to coronal heights. In the quiet Sun and especially in a polar coronal hole, we study dynamical properties of the dispersed spicules-material off-limb using high spectral, temporal, and spatial resolutions *IRIS* observations. We suggest that numerous small-scale jet-like spicules show rapid twisting and swaying motions evidenced by the large distortion and dispersion of the line profiles, including impressive periodic Doppler shifts. Most of these events repeatedly appear in red and blueshifts above the limb throughout the whole interval of the observation data sets, with an average swaying speed of order ±35 km s⁻¹ reaching a maximum value of 50 km s⁻¹ in the polar coronal hole region, well above the 2.2 Mm heights. We identified for the first time waves with a short period of order of 100 s, and less and transverse amplitudes of order of ±20–30 km s⁻¹ with the definite signature of Alfvén waves. No correlation exists between brightness and Doppler shift variations; the phase speed of the wave is very large and cannot definitely be determined from the spectral features seen along the quasi-radial features. Even shorter periods waves are evidenced, although their contrast is greatly attenuated by the overlapping effects along the line of sight.

**Key words:** Sun: corona – Sun: transition region – Sun: chromosphere

**Supporting material:** animation

## 1. Introduction

The first analysis of the transition region (TR) and chromospheric spectral lines and structures crossing the solar limb were done several decades ago (e.g., Nikolskii 1965; Brueckner & Bartoe 1983; Zirin 1996) with photographic emulsion as a detector. More recently, using the *Transition Region and Coronal Explorer* mission data (Alissandrakis et al. 2005), the extreme-ultraviolet time sequences were analyzed. Looking near and above the solar limb has the advantage of straightforwardly providing the geometric heights, which is not warranted in disk observations. Phenomena over a longer integration along the line of sight are analyzed; they preferably show transverse motions when Doppler effects are measured. A great amount of data is made available from *Interface Region Imaging Spectrograph (IRIS)* higher resolution spectral and image observations (De Pontieu et al. 2004), so it is timely to again carry out new studies of the transverse and twisty motions of spicule-like features and of TR macrospicules using spectra. *IRIS* observations provide simultaneous filtergrams and a wide range of spectra made with an unprecedented high resolution 0/33 width slit. This allows us to investigate the velocities in great detail. Studies of such motions have been done in several recent papers (e.g., De Pontieu et al. 2014a; Tavabi 2014a; Tavabi et al. 2015b). Alissandrakis et al. (2018) performed an extended work to deduce the “average” properties of a static chromosphere from the *IRIS* mission far-ultraviolet (FUV) data. The motions of spicules were previously less well established from spectra; some evidence of spinning motions were however reported using the Solar Optical Telescope (SOT) onboard the *Hinode* in Ca II H line imaging observations with swaying velocities of 20–30 km s⁻¹ (Suematsu et al. 2008; Tavabi et al. 2011; Tavabi 2014a).

The widths of the TR lines place an upper limit of about ±30 km s⁻¹ on velocities in the TR occurring on spatial scales below the usual resolution limit of about 0/33; the resolution of velocities with *IRIS* is often less than 1 km s⁻¹. This is in agreement with the relatively small turbulence motions of TR fine structures in network rosette, typically between 1 and 3 km s⁻¹ (De Pontieu et al. 2014b). Sit-and-stare spectra and time-Doppler maps also indicate a TR axisymmetric spinning behavior in the elongated jet-like structures similar to spicules, and these data sets, especially in the quiet Sun regions, indicate that the integrated along the line-of-sight Mg II k line tend to have a bulk broad symmetric shape with red and blue wings that mask the very small-scale properties. This fact suggests twist motions of about ±30 km s⁻¹ within the range of spicule diameter (Pasachoff et al. 2009; Zaqarashvili & Erdélyi 2009; Tavabi et al. 2011). Time-Doppler maps within polar coronal holes indicate an extension of the TR dynamical parts of spicules known to go higher and forcing the chromosphere to be oblate (Filippov & Koutchmy 2000; Filippov et al. 2007) when over equator regions, where magnetic-field lines are rather closed. A coronal hole (CH) supplies mass to the fast solar wind (Tsiropoula et al. 2012; De Pontieu et al. 2014a). Magnetic forces in action seem to be the most likely source of the jet-like bright phenomena (Golub et al. 1974, 1975, 1977; Lorrain & Koutchmy 1993). The prevalence of kink and of helical Alfvén and magnetoacoustic waves accompanying the guided along field lines spicules can heat the TR and the corona (Brueckner & Bartoe 1983; Tavabi 2014a, 2018).

It is known that the magnetic field plays the most significant role at low-β plasma regime, such as in TR and coronal plasma, where the charged materials are driven by magnetic forces and are confined toward the axis of flux tubes because the gas
pressure decreases drastically toward the outer atmosphere layers. For an example of micromechanism implying currents starting from the feet, see Lorrain & Koutchmy (1993, 1996, 1998). Observations have revealed that the magnetoacoustic floating modes with significant amplitudes are extremely remarkable and need to be discussed using spectral data (Martinez-Sykora et al. 2015). Variation of the parameters with the heights need to be measured.

Wave motions have been sought in the chromosphere and TR over a wide range of frequencies (Nikolskii 1965; Tavabi et al. 2015a; Khutsishvili et al. 2017). Fluctuations in Doppler velocity and intensity are also observed in the 10⁵ K plasma. It seems to be connected to the temperature minimum layer (Georgakilas et al. 1999; Christopoulou et al. 2001) and characteristic of the cut-off frequency expected near the temperature minimum height. The magnetosonic waves are reflected by this layer (Hollweg et al. 1982; Okamoto & De Pontieu 2011) and the mix of assumed reflected downward waves with similar periods and amplitudes of upward waves could produce the standing waves that are dominant at higher altitudes. Georgakilas et al. (1999) compared simultaneous sequences of Hα and He II 304 Å images near the solar limb. They proposed distinguishing polar surges and giant spicules (macrospicules) among the He II structures observed beyond the solar limb. Polar surges have a complex structure when observed in Hα and an eruptive nature reminiscent of normal surges at a small scale. They found that the physical properties of the material (temperature, density, velocity) play a crucial role and, as a result, some parts of the macrospicules emit strongly in He II, whereas other parts may be better visible in Hα of the neutral hydrogen. It appears that limb surges become visible in He II before they do in Hα and remain visible for a longer period during the decay phase.

Some of these jet-like explosive events show velocities of up to 70 km s⁻¹ deduced from the blueshift of line profile. These giant jets or macrospicules seem to be magnetically confined plasma as a result of interacting loops and explosive ejection in the x-null point (Yokoyama & Shibata 1995; Filippov et al. 2009, 2013). They appear to occur consecutively enough to contribute to the coronal mass supply and to the solar wind mass loss of the coronal hole regions.

Spicule-like giant features, macrospicules, and surges own dramatic signatures in TR spectra made with *IRIS* (e.g., Tavabi et al. 2015b) and further in Figures 5 and 6. Larger velocities are often apparently seen in the so-called type II spicule as ubiquitous narrower and short lifetime features that were reported from Ca H II *Hinode* /SOT mission broad filtergrams (De Pontieu et al. 2007). A distinguishing character of spicules is that they produce large red and blueshifts in emission lines formed at the chromospheric and at TR levels. Their size and comparatively short lifetime requires high spatial, temporal, and spectral resolution, so *IRIS* rasters and slit-jaw images (SJIs) have the adequate resolution to unambiguously identify spicules and macrospicules components and are suitable to follow the time evolution of the line-of-sight velocity of limb spicules. If plasma is swaying perpendicular to the spicule axis, one may expect to see the shifts in red and blue wings frequently at the same location, and one needs to consider the plausible mechanisms causing the periodic Doppler shifts. For example, Alfvén or other magnetoacoustic floating modes keep propagating along the spicules several times. They need to be more definitely identified as Alfvén or as kink waves to progress on the heating question of the TR and of the corona.

Furthermore, it is suggested that the magnetic reconnection “produce” kinetic energy that are clues to understand the origin of explosive ejection at the magnetic null points (Yokoyama & Shibata 1995). *IRIS* data of very high resolution should be suitable to study their signature.

Within this work we use a unique data set from *IRIS* high spatial resolution spectra to ascertain the nature of wave propagations. The present work analyzes the wave propagation at various chromospheric heights above the limb sampled by the *IRIS* observations at near-ultraviolet (NUV) wavelengths. The comparison between the different heights are performed using the time sequence spectra and different methods of analysis with emphasis on the novel aspect of short periodic signatures of the Alfvén wave.

## 2. Observation and Method of Reduction

*IRIS* sit-and-stare large raster scans, permitting building up a map of the time evolution of the line-of-sight velocity of limb fine features, are used. Accordingly, *IRIS* rasters and SJIs in the FUV 1389–1407 Å and NUV 2782–2835 Å with the Mg II h and k resonance lines, quite similarly to the well-known H and K lines Ca II bands, are used (see Figure 1).

The raster scans are made by scanning the slit across the solar limb with a 0"/33 step. Velocity resolution in *IRIS* spectra is 1 km s⁻¹. The *IRIS* NUV spectra correspond to a 0"/4 resolution along the slit (Table 1) and the exposure time is 8 s, whereas the raster step size and the Nyquist criterion, assuming an excellent signal-to-noise ratio (S/N), determine the spatial resolution across the slit, which means twice that of step size (De Pontieu et al. 2014b; Pereira et al. 2014). The Solar Software/Interactive Data Language program was used to stabilize the jitter effect, the value of the jitter being very small in polar regions (<1 pixel) and higher in equatorial regions. We used level 2 data with all types of corrections. Integrated intensity maps of the Mg II k and Si IV from the very large dense raster observations are constructed from each sequence (Figure 2). Note that the resonance Mg II lines are due to a low first ionization potential (FIP) element typical of the low temperature plasma produced near the minimum temperature at the 500 km heights above τ₅₀₀ = 1; the Mg II lines' FIP is 7.65 eV.

The difficulty related to the search of Doppler velocities is known due to the Mg II k line profile having double-peaks, therefore lines are actually optically thick. Being a resonance line, the classical explanation in the frame of a 1D model is to say that the outer layers of the structure of the source function drops below the Planck function due to non-local thermodynamic equilibrium effects, which makes it critical to interpret the details of the profiles. Indeed we deal with the whole extension along the line of sight of a chromospheric layer mixed with (i) erupting spicular material, (ii) inter-spicular material falling down, (iii) a static layer of unknown origin, and (iv) TR material of coronal origin, including beams of electrons from reconnections that penetrates inside this layer to a depth not yet defined and producing some evaporation in the under layers. The integration is as long as it is limited just by the curvature of the layer making a shell around the Sun. We naively assume that the component(s) we study give a signal outside the average-in-time profile, in the far wings and finally at distances outside the limb (*H* > 3") and outside active regions (CH region) where the optical thickness is reduced (see Figure 3); note that the Alissandrakis et al. (2018) analysis shows the bulk average profile the Mg II h line is double peaked
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Negatively selected frames extracted from the sequences of 2013 October 16 (top), 2014 February 19 (middle panel), and taken 2014 January 27 (bottom) shown in negative to improve the visibility. Some unsharp masking filtering is used for the CH image in the middle panel. The slit is seen as a vertical long thin line perpendicular to the limb on the second data set taken on 2014 February 19.</figcaption></figure>

until 6"–8" above the limb. The main reason for selecting this line is its apparent “sensitivity” to large Doppler effects, which results from its excellent efficiency to reveal for the low plasma densities of highly dynamical structures met along the line of sight, including structures situated before the main part of the chromosphere. In short, the line gives an excellent S/N even

for low density small features. Regarding the bulk profile, we tested to see the corresponding to the net shifts for the Si IV line and the result is that the maximum intensity of Mg II h & k lines remain around 0 km s⁻¹. It convinced us to use this Mg II resonance spectrum line to compute the decidedly large amplitude line of sight velocities to analyze the highly
<table><caption>**Table 1**
Data Set Properties</caption><thead><tr><th>Date, OBSID</th><th>Time (UT)</th><th>Target X, Y (arcsec)</th><th>Slit Length</th><th>Spectral Cadence, Exp. Time (s)</th><th>SJI, (FOV)</th><th>SJI, Cadence (s)</th><th>SJI Bands (Å)</th></tr></thead><tbody><tr><td>2013 Oct 16, 3820009402</td><td>21:40–22:28</td><td>[688 698]</td><td>60"</td><td>9, 7.9</td><td>60" × 65"</td><td>18</td><td>1400 and 2796</td></tr><tr><td>2014 Jan 27, 3800009253</td><td>11:59–14:48</td><td>[737 629]</td><td>119"</td><td>10, 7.9</td><td>119" × 119"</td><td>19</td><td>1330 and 1400</td></tr><tr><td>2014 Feb 19, 3800258253</td><td>16:27–17:23</td><td>[6–980]</td><td>119"</td><td>9, 7.9</td><td>119" × 119"</td><td>23 and 19</td><td>1330 and 1400 and 2832</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Time-intensity maps of the sit-and-stare large rasters according to the summarized properties of observations in Table 1 and also shown in Figure 4, the integrated wavelength range is in order of 1 Å between 2795.9 and 2796.9 Å.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Examples of a redshift of Mg II k lines (dotted red curve) resulting from the subtraction of time-averaged profile (in dashed green curve) from the original profile (solid black curve) for a selected frame of 2014 February 19 at 17:30:53 UT (top). The bottom panel is showing a typical blueshift for this data set at 16:32:39 UT, at the position about 3"/3 above the limb for the redshift and 3"/7 for the blueshift.</figcaption></figure>

dynamical structures by systematically subtracting the average spectrum profile in time spectral profiles, due to the “non-dynamical” part of the chromospheric shell (Figure 3). This is similar to the general method used for looking at the departure details from the bulk part of a phenomenon that shows significant dispersion when looking at the details.

The C II line is from a rather high FIP element with a potential excitation corresponding to a higher temperature. The Si IV lines are produced at much higher temperatures with a potential of ionization of $Si^{+2}$ of 33.5 eV. In addition, SJIs with $175 \times 175$ arcsec$^2$ field of view reflected off the slit/prism assembly through a filter wheel with four different filters are made available during observations. To discuss the most sensitive parameters relevant to the motion and waves analyzed

off-limb, we use the best S/N spectra and we follow the usual procedure of extracting peculiar largely Doppler shifted features at successive heights (Vial et al. 2016). The Mg II k line profiles are now considered in detail, avoiding the optically thick double peaked part by subtracting the average-in-time profiles for each height from the whole set of original profiles. Above the limb, the maximum intensity of the Mg II k line remains around $0 \text{ km s}^{-1}$ (Alissandrakis et al. 2017). The method gives good results well above the limb, which means in the far wings, with presumably small optical thicknesses even for heights under 8".

Note that under 8", the continuity apparent in the behavior of the deduced profiles (see Figures 5 and 6) gives hope that we are not far from the real behavior of the “individual” analyzed structure met along the line of sight when a well coherent signature appears. The resulting subtracted spectra are further called “d-spectra” to make it simpler (d means difference).

### 3. Results and Interpretations

Doppler maps (Figure 4), intensity maps (Figure 2) and time-spectra diagrams (Figures 5 and 6) in the Mg II k line of IRIS observations consistently illustrate the occurrence of 3–4 minute oscillations. Time integrated spectral profiles show an explosive event where a macrospicule appears at the beginning of the time series. The maximum value of Doppler velocity on this macrospicule is reached in the blue wing (note the left-hand side in Figures 2 and 3 of the right panel).

The wave motions are assumed to be present in only two directions of propagation: in azimuthal and in longitudinal directions of the spicule axis. The azimuthal motions are interpreted as Alfvénic waves. These waves could be included in all the transverse displacements such as kink ($m=1$), swaying motions etc. In the first approximation, the lowest modes of these waves consist of similar patterns.

The TR time-Doppler and time-spectra diagrams for very large sit-and-stare rasters are remarkable in showing the presumably axisymmetric rapid red and successive blueshifts, which indicate general torsional motions that propagate along the spicule axis at an average speed of $\pm 40 \text{ km s}^{-1}$ (De Pontieu et al. 2014a; Tavabi et al. 2015a). The amplitudes of the rotational flows are comparable to the spicule swaying motions, so it is likely that the Alfvénic waves propagate along the spicules, as reported in the literature (Kitiashvili et al. 2013;
<figure><img src="image_6.png" /></figure>

**Figure 4.** Time-Doppler maps formed by subtracting blue and redshifted intensities in the wings of Mg II k line (2796.32 Å core) of sit-and-stare large and very large dense rasters. The color scale is saturated at [-30 +30] km s⁻¹ corresponding to ±0.3 Å. Horizontal dotted lines show the boundaries of layers with respect to the limb shown. The elongated structures, which are significantly shifted in both wings (right panel), are parallel and adjacent to each other, and are repeated over the whole duration of the observations. Limbs are shown by an arrow and the inserted arrows showing the macrospicules for the first and the last time sequence. The positions of the limb are shown and the arrows point to the macrospicule events observed in two instances.

<figure><img src="image_7.png" /></figure>

**Figure 5.** Time sequence of d-spectra in Mg II k line of 2013 October 16, in adjacent layers above the limb on the quiet Sun region at the equator. The height difference between each plot is ~1′′5. Waves with periods near 100 s are well detected near 21:52 to 21:56 UT.

<figure><img src="image_8.png" /></figure>

**Figure 6.** Time sequence of d-spectra in the Mg II k line of 2014 February 19 in adjacent layers above the limb at the polar CH. The height difference between each plot is ~2′′5. 100 s period waves are seen at the beginning of the sequence and also around 17:23 UT. An animation is available. The video starts at height ≈−3′′52 ± 0′′64 and ends at height ≈25′′92 ± 0′′64. The dashed boxes shown in the last panel of this figure at height = 15′′5 are shown as examples 1 (≈15 minutes) and 2 (≈8 minutes) in the video. The video runtime is 14 s.

(An animation of this figure is available.)
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Wavelet power spectra of the oscillations in Mg II k in d-spectra for three layers above the limb (from bottom to top they correspond to the lowest level toward the highest level), with 4" difference distance for 2013 October 16 observations in the left panels corresponding to the near equatorial region. In the coronal hole region data set (2014 February 19), the distance separation is 6" (right panels). The contours determine the 95% confidence level, which was calculated by assuming a white noise background spectrum. Wavelet transform suffers from the wraparound errors at both edges of a time series (of finite length). The regions in which these effects are important are defined by the cone of influence. A horizontal colorbar below shows the power of wavelet spectrum, the redmost part corresponding to the largest amplitudes.</figcaption></figure>

De Pontieu et al. 2014a; Tavabi 2014a, 2014b; Gonzalez-Aviles et al. 2017). Sometimes, this has been interpreted as spinning motions in doublet threads or multi-component spicules (Suematsu et al. 2008; Tavabi et al. 2011). The wavelet analysis (see Figure 7) often shows periodic oscillations of the low-frequency of magnetosonic waves at 3–4 minutes and evidence of a much high-frequency oscillatory behavior in red and blueshifts with periods as low as 30 s.

The behavior along the slit needs a special discussion (see Figures 8, 9 and the animation associated with Figure 6). The concept of multi-component flows is another interesting aspect seen in the *IRIS* time–Doppler maps, which makes the discussion

rather difficult until more work is done on the subject. The heating mechanisms presumably operate in the TR and the corona (De Pontieu et al. 2004; Rouppe van der Voort et al. 2015).

The 3–4 minute magnetoacoustic waves are also presented in the time–altitude variations of the propagating waves demonstrated using time sequence of spectra of several adjacent layers (see Figures 5, 6 and 8 and animation associated with Figure 6). Wavelet power spectra of Figure 7 indicate a drastic decrease in the 3–4 minute oscillation powers at higher altitudes and an increase in the power of high-frequency oscillations (down to 20–30 s periods) in upper layers. For the first time, the well identified 100 s period presumably Alfvén waves at the height of TR are seen in the Mg II
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Time sequence of Mg II k spectra for the region that was in Figure 7 (the first dashed box at bottom) for 2014 February 19. The time lap of each panel is ~15 minutes between 16:35 and 16:50 UT, and the height distance between successive panels is three pixels or 1" (pixel's size is ~0"33) in space. The green arrows show a peculiar feature. The approximate heights are inserted in some frames with an error of about ±0"64 arcsec (see the animation associated with Figure 6 for higher resolution in space). Note between 14" and 18" the evidence of waves with very short periods (<1 minute). Between 5" and 10" the period of the main wave is around 3 minutes.</figcaption></figure>

k spectral line: see in Figures 5–7 and 9. The limb positions for the sequence of 2014 February 19 are particular as it is in the South Pole, so the solar rotation motion produces a negligible effect. In this data set, the slit is located perpendicular to the limb at the South Pole (Figure 1, bottom snapshots).

By comparing the spectrally resolved time sequence of spectra of Mg II k at various heights above the limb, a rough estimate of the periods, amplitudes, and position in the higher layers of TR are estimated. For this, we use a time sequence of d-spectra from the near- to off-limb altitudes, where we still have enough S/N to make plots for the dynamical components (Figures 5, 6 and 8, and see the animation associated with Figure 6). Apart from this, the panels (the limb corresponds to the top of the panel) reveal that all initial oscillatory features appear with a decreased intensity at higher layers. This is possible evidence of ions being dragged by an initial upward wave reaching all the way up to the TR with the same periods and similar amplitudes in our case.

The overlapping effects are more important for smaller features that are in larger number and for shorter periods, that we would rather avoid discussing, as with Figure 8.

## 4. Discussion

We note a dominant peak of power spectrum around 3–4 minutes. This peak seems to correspond to magnetosonic waves mainly appearing near the limb, and disappearing at higher levels. Spicules in the polar coronal hole region are more or less vertical and they are more inclined in the equator regions where the magnetic-field lines are closed. The inclination could help the mechanical acoustic waves to penetrate and propagate the volume of the corona more easily (De Pontieu et al. 2004).

High-frequency atypical oscillations are seen in Figures 5, 6, 8, and 9 with periods around 100 s, which were discovered using the SOT (Hinode) Ca II H filtergram series (Tavabi et al. 2011). Evidence of pulse-like displacements appears to suggest

rather violent processing well above the limb, as illustrated in Figure 8.

They need special attention because we believe they could explain the heating of the corona higher up into the corona. We identify these waves with Alfvén waves when no correlation exists between the large Doppler shifts we measured and the brightness variations that would reveal a possible variation of the pressure as it would be the case for a kink wave. In addition, such a high-frequency wave propagates better in the hot corona where the sound speed is also higher. We tried to measure some phase velocity using the analysis of Figure 7, without success but an evaluation of the lower value points to >500 km s⁻¹. Accordingly, the period of the best observed fast waves is of order of 100 s and they dominate at higher altitudes. This period could be related to the spinning motions of spicules.

The time-Doppler maps show dominating spinning motion patterns with an average helical velocity in the range of ±30 km s⁻¹. With the assumption that there is not any discontinuity, except for the spatial and temporal scales both of which are less than the spicule diameter, scale height, and lifetime, it certainly indicates that these twisty motions are attached to the individual structures. It means that bilateral velocity components originate at the same location without any time, space, and spectral discontinuities in time-Doppler plots. One may interpret these components as the components of a rotating spicule.

Twisty motions have already been reported with a velocity of about 25 km s⁻¹ (De Pontieu et al. 2014a; Tavabi et al. 2015a) and they are predicted by magnetic vortex plasma simulation (Kitiashvili et al. 2013; Gonzalez-Aviles et al. 2017). An explosive event could eject plasma in all directions and produce equal red and blueshifts, as for bidirectional jets (Tavabi et al. 2015b). The asymmetric (with a tilt angle) profiles on the red and blue wing of the events could be explained by a rotational ejection. Spinning configurations are clearly suggested in the time-Doppler maps we produced. They
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Extracted time sequence of spectra [1"3, 12"1] in the Mg II k band with a possible reversal of Doppler velocities with heights for 2014 February 19; the time interval is the same as Figure 8. On the left, note the wave with a 100 s period at 3 minutes time that does not show a measurable phase shift with the heights in this interval.</figcaption></figure>

were also described in the literature as Alfvénic waves propagation (Brueckner & Bartoe 1983; Martinez-Sykora et al. 2015).

The raster spectral lines and SJI's observations suggest that giant spicules (or miniature surges) are also impulsive phenomena and much more numerous on a scale smaller than polar surges. They give the impression of jet-like features, similar to ordinary spicules except for their dimensions. Thus, they appear to be as a result of magnetic reconnection on a scale smaller than that of polar surges, revealing the fact that giant spicules protrude higher into the hot corona (Georgakilas et al. 1999). Hence, these observations suggest that there is a close association between polar surges with explosive events, supporting the hypothesis that magnetic reconnection triggered by emerging flux provides the acceleration mechanism for this polar region event. Furthermore, the equator region sequence (2013 October 16) shows a bilateral motion in symmetric schema in near- and off-limb positions. The Doppler velocities in Figures 7 and 8 are approximately equal in both wings, with typical peaks in $\pm 30 \text{ km s}^{-1}$; in these figures, the left panels show features that are often normal to the dispersion direction.

Finally, let us notice that the importance of high-frequency waves should not be underestimated in the frame of the heating problem of the corona where these waves, including pure acoustic waves, propagate much more easily than the popular 5 and/or 3 minutes oscillations. Even more significantly for evaluating observations, their amplitudes are indeed greatly underestimated because no account is made of the instrumental attenuation at high frequencies occurring for small-scale variations (under the spatial resolution, they are just removed) and even more, the superposition effects along the line of sight are becoming more important with the cross section of structures getting smaller and accordingly, the number of integrated features is getting larger but the resulting contrast drastically decreases. Just to mention one important typically geometric effect remember that along the line of sight we have a lot of overlapping of different optically thin and optically thick dynamical features impossible to properly take into account. With a large coronagraph looking at the profiles of the Fe XIV coronal line, high-frequency (with dominant periods near 85 and 43 s) transverse velocity oscillations were already observed in Doppler shifts in the low-$\beta$ corona, (see e.g., Koutchmy et al. 1983) above regions showing episodic small surges or micro-surge in $H\alpha$, suggesting that magnetic waves propagating toward the corona are produced as described here. Thanks to the high resolution provided by *IRIS* we were able to show that plenty of high-frequency waves are produced in the low corona, suggesting that the so-called theoretically suggested turbulent heating could be a reality. Eventually, the presented material confirms that Alfvén waves are produced at the feet of the field lines and they propagate far away into the corona. More work should be done on fast and small-scale variations inside the thick multi-component shell surrounding the Sun, including the analysis of hotter coronal lines.

We warmly acknowledge the work of our referee, who provided an extended detailed report and added many interesting suggestions and requests for greatly improving the paper. The authors gratefully acknowledge the use of data from the *IRIS* databases. *IRIS* is a NASA small explorer mission developed and operated by LMSAL with mission operations
executed at NASA ARC with the contribution of NSC (Norway). We thank B. Filippov (Pushkov Moscow Institute) and J-C. Vial (IAS Orsay) for meaningful discussions.

ORCID iDs

E. Tavabi https://orcid.org/0000-0003-3210-9716

References

Alissandrakis, C. E., Koukras, A., Patsourakos, S., & Nindos, A. 2017, A&A, 603, A95

Alissandrakis, C. E., Vial, J.-C., Koukras, A., Buchlin, E., & Chane-Yook, M. 2018, SoPh, 293, 20

Alissandrakis, C. E., Zachariadis, Th., & Gontikakis, C. 2005, in Proc. 11th European Solar Physics Meeting, The Dynamic Sun: Challenges for Theory and Observations (ESA SP-600), ed. D. Danesy et al. (Noordwijk: European Space Agency), 54.1

Brueckner, G. E., & Bartoe, J.-D. F. 1983, ApJ, 272, 329

Christopoulou, E. B., Georgakilas, A. A., & Koutchmy, S. 2001, SoPh, 199, 61

De Pontieu, B., Erdélyi, R., & James, S. P. 2004, Natur, 430, 536

De Pontieu, B., McIntosh, S., Hansteen, V. H., et al. 2007, PASJ, 59, S655

De Pontieu, B., Rouppe van der Voort, L., McIntosh, S. W., et al. 2014a, Sci, 346, 1255732

De Pontieu, B., Title, A. M., Lemen, J. R., et al. 2014b, SoPh, 289, 2733

Filippov, B., Golub, L., & Koutchmy, S. 2009, SoPh, 254, 259

Filippov, B., & Koutchmy, S. 2000, SoPh, 196, 311

Filippov, B., Koutchmy, S., & Tavabi, E. 2013, SoPh, 286, 143

Filippov, B., Koutchmy, S., & Vilinga, J. 2007, A&A, 464, 1119

Georgakilas, A. A., Koutchmy, S., & Alissandrakis, C. F. 1999, A&A, 341, 610

Golub, L., Krieger, A. S., Harvey, J. W., & Vaiana, G. S. 1977, SoPh, 53, 111

Golub, L., Krieger, A. S., Silk, J. K., Timothy, A. F., & Vaiana, G. S. 1974, ApJL, 189, L93

Golub, L., Krieger, A. S., & Vaiana, G. S. 1975, SoPh, 42, 131

Gonzalez-Aviles, J. J., Guzman, F. S., & Fedun, V. 2017, ApJ, 836, 24

Hollweg, J. V., Jackson, S., & Galloway, D. 1982, SoPh, 75, 35

Khutsishvili, D., Zaqarashvili, T. V., Khutsishvili, E., et al. 2017, A&SS, 362, 10

Kitiashvili, I. N., Kosovichev, A. G., Lele, S. K., Mansour, N. N., & Wray, A. A. 2013, ApJ, 770, 37

Koutchmy, S., Zhugzhda, I. D., & Locans, V. 1983, A&A, 120, 185

Lorrain, P., & Koutchmy, S. 1993, A&A, 269, 518

Lorrain, P., & Koutchmy, S. 1996, SoPh, 165, 115

Lorrain, P., & Koutchmy, S. 1998, SoPh, 178, 39

Martinez-Sykora, J., Rouppe van der Voort, L., Carlsson, M., et al. 2015, ApJ, 803, 44

Nikolskii, G. M. 1965, AZh, 42, 86

Okamoto, T. J., & De Pontieu, B. 2011, ApJL, 736, L24

Pasachoff, J. M., Jacobson, W. A., & Sterling, A. C. 2009, SoPh, 260, 59

Pereira, T. M. D., De Pontieu, B., Carlsson, M., et al. 2014, ApJL, 792, L15

Rouppe van der Voort, L., De Pontieu, B., Pereira, T. M. D., Carlsson, M., & Hansteen, V. 2015, ApJL, 799, L3

Suematsu, Y., Ichimoto, K., Katsukawa, Y., et al. 2008, in ASP Conf. Ser. 397, First Results From Hinode, ed. S. A. Matthews, J. M. Davis, & L. K. Harra (San Francisco, CA: ASP), 27

Tavabi, E. 2014a, Ap&SS, 350, 489

Tavabi, E. 2014b, Ap&SS, 352, 43

Tavabi, E. 2018, MNRAS, 486, 868

Tavabi, E., Koutchmy, S., et al. 2015a, A&A, 573, 7

Tavabi, E., Koutchmy, S., & Ajabshirzadeh, A. 2011, NewA, 16, 296

Tavabi, E., Koutchmy, S., & Golub, L. 2015b, SoPh, 290, 2871

Tsiropoula, G., Ztiotziou, K., Kontogiannis, I., et al. 2012, SSRv, 169, 181

Vial, J.-C., Pelouze, G., Heinzel, P., Kleint, L., & Anzer, U. 2016, SoPh, 291, 67

Yokoyama, T., & Shibata, K. 1995, Natur, 375, 42

Zaqarashvili, T. V., & Erdélyi, R. 2009, SSRv, 149, 355

Zirin, H. 1996, SoPh, 169, 313