Multilevel Observations of the Oscillations in the First Active Region of the New Cycle

Draft version June 11, 2020 Typeset using LATEX twocolumn style in AASTeX61

MULTILEVEL OBSERVATIONS OF THE OSCILLATIONS IN THE FIRST ACTIVE REGION OF THE NEW CYCLE

Andrei Chelpanov1 and Nikolai Kobanov1

1Institute of Solar-Terrestrial Physics of Siberian Branch of Russian Academy of Sciences, Irkutsk, Russia

ABSTRACT For the first time, a multi-wave research of oscillation dynamics in a solar facula from its birth to decay was carried out. We performed spectral observations of active region NOAA 12744 at Horizontal Solar Telescope of the Sayan Solar Observatory in the Hα, He i 10830 A,˚ and Si i 10827 A˚ lines. We used Solar Dynamics Observatory (SDO) line- of-sight magnetic field data and the 1600 A,˚ 304 A,˚ and 171 A˚ UV channels. At the early stages of the facula evolution, we observed low-frequency (1–2 mHz) oscillations concentrate in the central part of the facula. In the lower solar atmosphere, this is registered in the intensity, line-of-sight velocity, and magnetic field signals. These frequencies were also observed in the transition region and corona (304 A˚ and 171 A˚ channels). At the maximal development phase of the facula evolution, the low frequency oscillations closely reproduce the coronal loop structures forming above the active region. At the decay phase, the spatial distributions of the observed frequencies resemble those found in and above the undisturbed chromosphere network. Our results indicate a direct relation of the low frequency oscillations observed in the lower solar atmosphere with the oscillations in the coronal loops, which is probably implemented through the loop footpoints. arXiv:2006.05223v1 [astro-ph.SR] 9 Jun 2020

Corresponding author: A. Chelpanov [email protected] 2 A. Chelpanov and N. Kobanov

1. INTRODUCTION of the activity cycle and to establish which frequencies Faculae are the most frequent expression of the solar dominate in the transition region and corona at different activity. They manifest themselves as large bright areas phases of the active region evolution. that harbor increased magnetic field flux. They are al- ways observed in active regions (AR), both containing 2. INSTRUMENTS AND DATA sunspots and those not related to sunspots. Faculae pre- We used data from the the Solar Dynamics Obser- cede the emergence of sunspots and remain during the vatory (SDO, Pesnell et al. 2012). The Atmospheric whole lifespan of ARs up to the sunspot decay and even Imaging Assembly (AIA) onboard SDO obtains full-disk after it. Due to their surface area and abundance, fac- images of the Sun in a number of ultraviolet spectral ulae affect the energy balance between the solar atmo- channels with a 0.600 spatial resolution and 12 to 24 s sphere layers, and their influence includes that through time cadence. For the analysis, we chose the 1600 A,˚ the wave mechanisms. Studying oscillation characteris- 304 A,˚ and 171 A˚ channels, which represent the chromo- tics in faculae is a relevant problem of solar physics. sphere, transition region, and corona, respectively. We To date, numerous studies have addressed this prob- also used the Helioseismic and Magnetic Imager (HMI) lem (Orrall 1965; Sheeley & Bhatnagar 1971; Khomenko measurements of the line-of-sight magnetic field. et al. 2008; Kobanov & Pulyaev 2011; Kolotkov et al. Ground-based observations at the Horizontal Solar 2017). Note that studying the spatial distributions of Telescope of the Sayan Solar Observatory complement oscillations in faculae is more difficult than in sunspots the SDO data. The working aperture diameter of the due to a more complicated magnetic field topology: fac- main mirror is 80 cm; the Earth’s atmosphere limits the ulae often contain several separated magnetic knots of spatial resolution to 1–1.5 arcsec, and the spectral res- different polarities. The evidence on the strength and di- olution varies between 4 and 8 mA˚ depending on the rection of the magnetic field is highly diverse. Narayan wavelength. A detailed description of the telescope can & Scharmer(2010) analysed polarimetric observations be found in Kobanov(2001); Kobanov et al.(2013). The of a facula with a high spatial resolution and found a lengths of the series varied between 135 and 210 min, and number of magnetic features: micropores, bright points, the cadence was 3.2 s. In the observations, we used three ribbons, flowers, and strings. Solov’ev & Kirichek(2019) spectral lines: the chromospheric He i 10830 A˚ and Hα identified three main magnetic structures: small-scale lines and the photospheric Si i 10827 A.˚ flux tubes, knots, and pores. Rabin(1992); Mart´ınez Pillet et al.(1997); Guo et al.(2010) claim that vertical magnetic fields dominate faculae, while Ishikawa et 3. MORPHOLOGY AND EVOLUTION OF THE al.(2008) found that small horizontal magnetic struc- ACTIVE REGION tures prevail in them. Blanco Rodr´ıguez& Kneer(2010) The first AR of solar activity cycle 25 emerged at the based on the polar facula observations in the Fe i 1.5 µm disk on 6 July 2019 at the southern hemisphere close line found two magnetic structure types: one with a to the eastern limb (S27E55). During the first six hours strength of 900–1500 G and the other of strengths lower after the first appearance of the magnetic flux, it rapidly than 900 G. While the structures of the first type are ver- developed into a quite big facula (approximately 5000 in tical, the second type shows arbitrary angles. The situa- diameter based on the images in the 1600 A˚ channel). tion becomes even more complicated when one considers After that, pores appeared in the AR. The new AR was the changes in the characteristics following the evolution given number NOAA 12744. of a facula. The first indications of a new solar activity On 8 July, the facula started to expand. It separated cycle can be observed in the emerging faculae. into head and tail parts of negative and positive mag- While the processes accompanying the evolution of netic polarities, correspondingly. On this day, the pores AR have been discussed quite thoroughly in the contem- disappeared, and the facula started to divide into several porary publications (van Driel-Gesztelyi & Green 2015, cores. and references therein), studies of oscillation spectra dy- The 171 A˚ channel images showed a blurred picture namics during the facular AR evolution are almost non- of forming coronal loops. The region continued to grow existent. With this study we are trying to fill this gap. on 10 July 2019 in the 1600 A˚ and 304 A˚ images, while This work is dedicated to multiwave studying of oscil- a clear structure of closed coronal loops formed in the lations in a solar facula registered as NOAA 12744—the 171 A˚ channel. first AR of the new solar activity Cycle 25. The goal Starting from 11 July, the loop structures in the 171 A˚ of this work is to reveal the most characteristic features images grew more blurred and elongated. During the of the oscillation processes in a facula at an early stage next two days, the loop structure continued to disinte- Oscillations in the first AO of Cycle 25 3 grate, and by the end of this period (01:00 on 14 July), the facula in the Si i 10827 A˚ (upper photosphere), He i they disappeared completely. 10830 A˚ and Hα (chromosphere) lines. On the eighth day after the facula appeared, it de- We analysed the spectra in the 1–8 mHz range. The cayed: it almost merged into the surrounding chromo- restriction at the lower end was imposed by the fast spheric network in the 1600 A˚ channel. evolution rate of the active region, which may lead to It must be noted that on 14 July at 01:00 UT after significant changes in its structure during the time series. the decay, a new magnetic reconnection occurred fol- The frequencies over 8 mHz showed insignificant power. lowed by a local brightening in the coronal lines and in Numerous earlier works have noted the dominance the upper chromosphere channels. A new loop structure of the five-minute oscillations in faculae photosphere developed above the facula rapidly within an hour. The and chromosphere (Deubner 1974; Teske 1974; Balthasar newly activated region lived for two more days, during 1990; De Pontieu et al. 2005). In our case, however, the which the decay process repeated concluded by a com- emergence phase showed the oscillation power concen- plete disappearance of the active region. By this time tration in the lower frequency range (1–2 mHz) in the the region neared the western limb. central part of the facula. This clearly shows in the os- We divided the evolution of the facula into five cillation spatial distribution in the HMI magnetic field phases based on the SDO/AIA images. The first phase signals and in the 1600 A˚ (Figure2). The lower frequen- (emergence) is characterized by the appearance of cies also dominate in the spectra of our ground-based small brightenings at an area of increased magnetic LOS-velocity observations in the Hα and He i 10830 A˚ field concentration. The second phase (growth) is dis- lines over the central part of the facula at its first evolu- tinguished by a rapid expansion of the bright elements. tion phase (Figure3). Concentrations of elements with At the third phase (maximal development phase), the lower-frequency oscillations are seen in the 304 A˚ a stable coronal loop structure is observed in the 171 A˚ and 171 A˚ as well. channel; the surface area and brightness of the facula Appearance of the lower frequencies in the chromo- are at their maxima. The fourth phase (decline) is sphere above faculae are mainly attributed to the de- characterized by a decrease in the brightness and the crease in the cut-off frequency in the areas of the inclined number of the coronal loops. At the fifth phase (decay), magnetic field (Michalitsanos 1973; Bel & Leroy 1977). the coronal loop structure completely disappears. However, Khomenko et al.(2008); Khomenko & Calvo We carried out the observations at the ground-based Santamaria(2013); Centeno et al.(2009) indicate that telescope on July 7th, 8th, 10th, and 11th, which cor- the cut-off frequency may decrease even in the areas of responded to the emergence, growth, maximal develop- the vertical magnetic field due to changes in the radia- ment, and decline phases of the facula evolution. We tive relaxation time (Roberts 1983). We believe that lack ground-based observation for the fifth phase—the both effects could be working in facula atmospheres. decay. In the 1600 A˚ dominant frequency distribution of the emergence phase, a ring of higher frequency (up to 4. RESULTS 8 mHz) interspersings surround the core of the facula. 4.1. Oscillation dynamics in the LOS velocity and We observe the same pattern in the ground-based spec- 00 intensity tra of the chromospheric lines: at a distance of 10 to 1500 from the central part of the facula, strong peaks The AIA telescope onboard SDO provides two- manifest in the 6–8 mHz range (Figure4). dimension images in UV and EUV channels, which we One should note that in this case we see a pattern used to construct spatial distributions of the dominant opposite to that observed in sunspots, where high fre- oscillation frequencies (Figure2). These distributions quencies reside in the umbra’s centre, and the dominant show which frequencies dominated at each point of the frequency gradually decreases closer to the outer edges faculae during each of the five phases of the facula de- of a sunspot; the lowest frequencies are found outside of velopment. We based the frequency distributions on the penumbra (Kobanov 2000; Kolobov et al. 2016). In the Fourier spectra. The dominant frequency ν at each d sunspots, such a frequency distribution is attributed to spatial point was determined as the frequency, for which the geometrical shape of the magnetic field—vertical at the integrated oscillation power in the ν±0.5 mHz range the centre and more and more inclined to the edges— shows the maximal value. To clear the images of the and to the cut-off frequency related to it. noise, we filled with white the spatial points, whose In the facula, however, the shift to the higher frequen- oscillation power did not reach the 3σ2 level. cies lacks the gradual transition through the middle- For the four out of five facula evolution phases that we range frequencies. The spectra show only two fre- specified, we have ground-based spectral observations of 4 A. Chelpanov and N. Kobanov

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Figure 1. Snapshots of the facula in the three AIA channels and HMI magnetograms for the five phases of the active region evolution. quency domains with strong peaks: the lower frequen- field strength in individual parts of a facula. They con- cies around 2 mHz and higher frequencies around 7 mHz. cluded that the dominant oscillation period in the photo- In the areas out of the facula’s core, the high-frequency sphere and low chromosphere increases in the areas with peaks grow, and the power in the central part of the the increased strength of the underlying magnetic field. spectra (3.5–5.5 mHz) remains quite low in all the ar- Chelpanov et al.(2016) found that the power of the eas of the facula (Figure4). To answer the question high-frequency oscillations increased in at the locations whether such behavior is typical of the early stages of a of strong magnetic field. The mismatch of these results solar cycle or it characterizes a birth of an active region could be explained by the fact that Kostik & Khomenko at any stage of a cycle will require numerous observa- (2013) used the full-vector data, while Chelpanov et al. tions of the oscillation dynamics in facular active regions (2016) used the LOS component. The lower frequencies throughout a whole cycle. are usually associated with inclined field. Besides, the The magnetic field topology dramatically influences observations in the two studies were carried out in dif- the distribution of the dominant frequencies in the facu- ferent sets of spectral lines and with different series time lar photosphere and chromosphere. Kostik & Khomenko lengths. We should also note that the facula observed in (2013) studied the oscillation period dependence on the the former study was at the last stage of its evolution: Oscillations in the first AO of Cycle 25 5

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Figure 2. Maps of the dominant frequencies in the faculae in the HMI LOS magnetic field signals and in the tree studied AIA channels at the five phases of its evolution. it had disintegrated; its surface area had decreased, and coronal 171 A˚ line distribution in large part is occupied the magnetic field strength had weakened compared to with the low-frequency oscillations (1–2 mHz) in, which its maximal development phase three days before. reproduces the loop structures that had formed by this Starting from the growth phase, the main oscillation time. This suggests that in the 171 A˚ line we see the power in the LOS velocity signals in the chromosphere most horizontal apexes of the loops, and that the low- shifted towards the five-minute range. A similar shift frequency oscillations prefer such structures. Note that is observed in the 304 A˚ and 171 A˚ channels (Figure2), the spatial distribution of the low frequencies represents where in the dominate frequency distributions, the five- a picture averaged over the time series, while the images minute oscillations occupy extensive areas in the facula. in Figure1 are one-moment snapshots, thus one should At the maximal development phase, the oscillation not expect a complete visual match. power in the 5–6 mHz range increases in the lower levels. At the last two evolution phases, the frequency dis- This is especially noticeable in the LOS velocity signals tribution in the lower atmosphere levels is close to that of the He i line (Figure3). In the maximal phase, the in the elements of the undisturbed chromosphere net- 6 A. Chelpanov and N. Kobanov

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Figure 3. Line-of-sight velocity oscillations in the central part of the facula based on the ground-based data.

Hα LOS velocity He I LOS velocity In the spectra of the ground-based intensity signals, 1 1 the main peaks are distributed in a wider frequency range. In the photosphere, the oscillation power concen- 0 0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 trates around the 3–4 mHz, and in the chromosphere, 1 1 the main peaks mostly seat in the 1–3 mHz range. In addition, the intensity spectra of the two chromospheric 0 0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 1 lines often diﬀer signiﬁcantly, as opposed to the velocity signals, where the oscillations of the He i and Hα lines

0 0 behave similarly. 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 normalized power normalized 1 1 4.2. Oscillations in the LOS magnetic ﬁeld signals

0 0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 1 The reality of the observed oscillations in the magnetic ﬁeld signals is often doubted (Lites et al. 1998;

0 0 Rüediet al. 1999). Observations of oscillations in the 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 frequency, mHz magnetic field strength signals in faculae are rare. Nev- ertheless, results of such observations in the 1–7 mHz Figure 4. Line-of-sight velocity oscillations in the chromo- range are given in (Muglach et al. 1995; Kobanov & sphere in the central part of the facula at its emergence phase Pulyaev 2007; Chelpanov et al. 2016). Strekalova et al. and at points farther away from it (from top to bottom). (2016); Solov’ev et al.(2019); Riehokainen et al.(2019) reported on longer periods of tens and even hundred work (Gontikakis et al. 2005; Gupta et al. 2013). The minutes. In our study, the changes in the magnetic field loop structure in the corona disappears, and the lower topology caused by the fast evolution of the active region frequency areas become fragmented with higher concen- restricted us to the 1–8 mHz range. tration in the positive magnetic polarity area. At the The main part of the facula area in the distribution of decay phase, the low frequency oscillations largely fade the magnetic field oscillations is occupied with the low at the chromosphere level, and the higher frequencies of frequencies at the early stages of the facula development. 5–7 mHz evolve around the areas of the magnetic field The locations of the low frequencies correspond to the concentrations. areas of the moderate magnetic field strength within the In the photospheric Si i line, oscillations with 3–4 mHz facula. By the time of the decay, the low and mid-range frequencies (five-minute oscillations) dominate at all the frequencies locate only close to the magnetic field con- phases of the facula development. centration knots, and all the other areas between them Oscillations in the first AO of Cycle 25 7

7 171 Å

are occupied with the multi-colored background, which 2 a) b) 6 corresponds to the signals of low oscillation power. A 5 4 similar spectral composition ﬁlls the areas of the quiet 3 2

1 Sun around the facula. power,oscillation σ 0 1 2 3 4 5 6 7 Since we analyse the line-of-sight component of the frequencies, mHz c)1–2 mHz d) 2–3 mHz e) 3–4 mHz magnetic field strength, we should note that the angle 1.0 1.0 1.0 at which the facula was observed changed in a quite wide 0.5 0.5 0.5 0.0 0.0 0.0 range: the facula moved along the thirtieth parallel from -0.5 -0.5 -0.5 ◦ meridian 50 East to the central meridian, and then to -1.0 02:20 02:30 02:40 -1.0 02:20 02:30 02:40 -1.0 02:20 02:30 02:40 time, UT meridian 40◦ West. Thus, the angle between the line ◦ of sight and the surface normal varied between 30 and Figure 5. a) the 171 A˚ loop image with the reference points ◦ 65 . This could have influenced the observed shape of on the sides of the loop marked with crosses; b) oscillation the magnetic field frequency distribution. power spectrum of the 171 A˚ signal from a reference point; c-e) signals from the points on the sides of the loop wavelet- filtered in three frequency ranges. 4.3. Coronal loop oscillations ˚ In the spatial distributions of 171 A channel dominant ries to the periods when the loops existed stably at one frequencies, the most prominent features are the struc- location. In the following example, we used a contrast tures of low frequencies that outline the coronal loops. coronal loop that was lit up during 32 minutes (02:13 to This stands out the most at the maximal phase of the 02:45) at the maximal development phase. facula evolution. The presence of the low frequencies in To distinguish the oscillations related to the loop dis- the coronal loop spectra has been noted before (Wang placements, we analysed the signals from the points lo- et al. 2009; Aschwanden et al. 2002; Verwichte et al. cated on the sides of the loop central axis (Figure5). In 2009; Duckenfield et al. 2018). Kobanov et al.(2015); such signals, the oscillations caused by the loop move- Kobanov & Chelpanov(2014) studied spatial distribu- ments should be in anti-phase, while the oscillations of tions of oscillations above faculae; their analysis showed the loop brightness should be in phase. that the fan structures are clearly reproduced at the fre- The power spectra are dominated by the low frequen- quencies of 1–1.5 mHz. cies accompanied by weaker peaks from 2 to 5 mHz. We Yuan et al.(2011) found frequencies between 0.2 and used narrow-band wavelet filtration in order to separate 0.6 mHz in the corona, as well as in the chromosphere different frequency oscilations. The oscillations of the above a sunspot, which indicates a long-period connec- 2–3 mHz and 3–4 mHz ranges have the opposite phases. tivity between these two layers. They suggested that This implies that they result from the transverse dis- these waves might be incited in the sunspot magnetic placements of the loop. The phase difference in the 1– flux tubes by the g-modes. Restricted by the length 2 mHz band differs from 180◦, which indicates an input of the time-series, we observed shorter-period observa- from other oscillations sources. tions. However one cannot exclude the same excitation mechanism, although in our case it is even more difficult to estimate the viability of this scenario, since the sub-photosphere magnetic field structure in faculae has been much less studied compared to sunspots. 5. CONCLUSION Coronal loop spectra could be influenced, apart from For the first time, we carried out a multi-wave re- the intrinsic intensity variations of the loop material, by search of oscillation dynamics in a solar facula from its the apparent changes in brightness caused by the trans- birth to decay. At the emergence phase of the facula verse loop movements. Such movements are present in evolution, the low-frequency (1–2 mHz) oscillations con- the most active regions (Anfinogentov et al. 2015). Even centrated in the centre of the facula in the photospheric a sub-pixel displacement of a loop changes the brightness and chromospheric signals of the intensity, LOS veloc- of each separate pixel. Goddard et al.(2016) analysed 58 ity, and LOS magnetic field. This is the most apparent kink oscillation events and found that most often the 5- in the spatial dominant frequency distribution in the minute period oscillations occurred in them; they found chromosphere 1600 A˚ channel. The high-frequency (5– the 8–12-minute oscillations as well. 7 mHz) oscillations group at the edges of the emerging Besides, the observed low frequencies could be related active region. We suggest that the occurrence of the low to the short lifetime of the loops. To exclude this factor frequencies could be considered as a precursor for the from the analysis, we restricted the analyzed time se- coronal loop structure development in a growing facula. 8 A. Chelpanov and N. Kobanov

Later, at the more developed phases of the facula evo- quencies take space between the magnetic ﬁeld concen- lution, 5-minute oscillations dominate the photosphere tration areas. and chromosphere of the facula. Our results conﬁrm the version that the sources of The spatial distribution of the dominant frequencies in the low frequency oscillations in the coronal loops are the corona (171 A)˚ is closely related to the coronal loop located in the lower layers of the solar atmosphere. dynamics in the facula. During the maximal development of the coronal loop system these distributions are dominated by the low frequencies. At these phases, the low frequency distribution closely resembles the coronal The research was supported by the Russian founda- loops. In the transition region frequency distributions, tion for Basic Research under grant 20-32-70076 and the low-frequency areas look more patchy: they concen- Project No. II.16.3.2 of ISTP SB RAS. Spectral data trate around the footpoints of the coronal loops. were recorded at the Angara Multiaccess Center facili- At the last phase of the evolution, the low frequencies ties at ISTP SB RAS. We acknowledge the NASA/SDO in the chromosphere almost disappear, and the high fre- science teams for providing the data.

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