Abstract
Marked velocity excesses of ions relative to neutrals are obtained from two time series of the neighboring emission lines He
i
5015 Å and Fe
ii
5018 Å in a quiescent prominence. Their ...Doppler shifts show time variations of quasi-periodic character where the ions are faster than the neutrals, 1.0 ≤
V
macro
(Fe
ii
)/
V
macro
(He
i
) ≤ 1.35 in series A and ≤1.25 in series B. This “ratio excess” confirms our earlier findings of a 1.22 ion velocity excess, but the present study shows a restriction in space and time of typically 5 Mm and 5 minutes. The ratio excess is superposed by a time- and velocity-independent “difference excess” of −0.3 ≤
V
macro
(Fe
ii
)−
V
macro
(He
i
) ≤ +0.7 km s
−1
in series A (also indicated in series B). The high repetition rate of 3.9 s enables the detection of high-frequency oscillations with several damped 22 s periods in series A. These show a ratio excess with a maximum of 1.7. We confirm the absence of a significant phase delay of He neutrals with respect to the Fe ions.
Aims.
Our aim is to investigate the role of acoustic and magneto-acoustic waves in heating the solar chromosphere. Observations in strong chromospheric lines are analyzed by comparing the deposited ...acoustic-energy flux with the total integrated radiative losses.
Methods.
Quiet-Sun and weak-plage regions were observed in the Ca
II
854.2 nm and H
α
lines with the Fast Imaging Solar Spectrograph (FISS) at the 1.6-m Goode Solar Telescope on 2019 October 3 and in the H
α
and H
β
lines with the echelle spectrograph attached to the Vacuum Tower Telescope on 2018 December 11 and 2019 June 6. The deposited acoustic energy flux at frequencies up to 20 mHz was derived from Doppler velocities observed in line centers and wings. Radiative losses were computed by means of a set of scaled non-local thermodynamic equilibrium 1D hydrostatic semi-empirical models obtained by fitting synthetic to observed line profiles.
Results.
In the middle chromosphere (
h
= 1000–1400 km), the radiative losses can be fully balanced by the deposited acoustic energy flux in a quiet-Sun region. In the upper chromosphere (
h
> 1400 km), the deposited acoustic flux is small compared to the radiative losses in quiet as well as in plage regions. The crucial parameter determining the amount of deposited acoustic flux is the gas density at a given height.
Conclusions.
The acoustic energy flux is efficiently deposited in the middle chromosphere, where the density of gas is sufficiently high. About 90% of the available acoustic energy flux in the quiet-Sun region is deposited in these layers, and thus it is a major contributor to the radiative losses of the middle chromosphere. In the upper chromosphere, the deposited acoustic flux is too low, so that other heating mechanisms have to act to balance the radiative cooling.
We study the evolution of a minifilament eruption in a quiet region at the center of the solar disk and its impact on the ambient atmosphere. We used high spectral resolution imaging spectroscopy in ...H acquired by the echelle spectrograph of the Vacuum Tower Telescope, Tenerife, Spain; photospheric magnetic field observations from the Helioseismic Magnetic Imager; and UV/EUV imaging from the Atmospheric Imaging Assembly of the Solar Dynamics Observatory. The H line profiles were noise-stripped using principal component analysis and then inverted to produce physical and cloud model parameter maps. The minifilament formed between small-scale, opposite-polarity magnetic features through a series of small reconnection events, and it erupted within an hour after its appearance in H . Its development and eruption exhibited similarities to large-scale erupting filaments, indicating the action of common mechanisms. Its eruption took place in two phases, namely, a slow rise and a fast expansion, and it produced a coronal dimming, before the minifilament disappeared. During its eruption, we detected a complicated velocity pattern, indicative of a twisted, thread-like structure. Part of its material returned to the chromosphere, producing observable effects on nearby low-lying magnetic structures. Cloud model analysis showed that the minifilament was initially similar to other chromospheric fine structures, in terms of optical depth, source function, and Doppler width, but it resembled a large-scale filament on its course to eruption. High spectral resolution observations of the chromosphere can provide a wealth of information regarding the dynamics and properties of minifilaments and their interactions with the surrounding atmosphere.
Context. It has been empirically determined that the umbra-penumbra boundaries of stable sunspots are characterized by a constant value of the vertical magnetic field. Aims. We analyzed the evolution ...of the photospheric magnetic field properties of a decaying sunspot belonging to NOAA 11277 between August 28–September 3, 2011. The observations were acquired with the spectropolarimeter on-board of the Hinode satellite. We aim to prove the validity of the constant vertical magnetic-field boundary between the umbra and penumbra in decaying sunspots. Methods. A spectral-line inversion technique was used to infer the magnetic field vector from the full-Stokes profiles. In total, eight maps were inverted and the variation of the magnetic properties in time were quantified using linear or quadratic fits. Results. We find a linear decay of the umbral vertical magnetic field, magnetic flux, and area. The penumbra showed a linear increase of the vertical magnetic field and a sharp decay of the magnetic flux. In addition, the penumbral area quadratically decayed. The vertical component of the magnetic field is weaker on the umbra-penumbra boundary of the studied decaying sunspot compared to stable sunspots. Its value seem to be steadily decreasing during the decay phase. Moreover, at any time of the sunspot decay shown, the inner penumbra boundary does not match with a constant value of the vertical magnetic field, contrary to what is seen in stable sunspots. Conclusions. During the decaying phase of the studied sunspot, the umbra does not have a sufficiently strong vertical component of the magnetic field and is thus unstable and prone to be disintegrated by convection or magnetic diffusion. No constant value of the vertical magnetic field is found for the inner penumbral boundary.
The 1.5 meter solar telescope GREGOR Schmidt, W.; von der Lühe, O.; Volkmer, R. ...
Astronomische Nachrichten,
11/2012, Letnik:
333, Številka:
9
Journal Article
Aims.
In this work, we explore the spectral window containing Fraunhofer lines formed in the solar photosphere, around the magnetically sensitive Cr
I
lines at 5780.9, 5781.1, 5781.7, 5783.0, and ...5783.8 Å, with Landé
g
-factors between 1.6 and 2.5. The goal is to simultaneously analyze 15 spectral lines, comprising Cr
I
, Cu
I
, Fe
I
, Mn
I
, and Si
I
lines, without the use of polarimetry, to infer the thermodynamic and magnetic properties in strongly magnetized plasmas using an inversion code.
Methods.
Our study is based on a new setup at the Vacuum Tower Telescope (VTT, Tenerife), which includes fast spectroscopic scans in the wavelength range around the Cr
I
5781.75 Å line. The oscillator strengths log(
gf
) of all spectral lines, as well as their response functions to temperature, magnetic field, and Doppler velocity, were determined using the Stokes Inversion based on Response functions (SIR) code. Snapshot 385 of the enhanced network simulation from the Bifrost code serves to synthesize all the lines, which are, in turn, inverted simultaneously with SIR to establish the best inversion strategy. We applied this strategy to VTT observations of a sunspot belonging to NOAA 12723 on 2018 September 30 and compared the results to full-disk vector field data obtained with the Helioseismic and Magnetic Imager (HMI).
Results.
The 15 simultaneously inverted intensity profiles (Stokes
I
) delivered accurate temperatures and Doppler velocities when compared with the simulations. The derived magnetic fields and inclinations achieve the best level of accuracy when the fields are oriented along the line-of-sight (LOS) and less accurate when the fields are transverse to the LOS. In general, the results appear similar to what is reported in the HMI vector-field data, although some discrepancies exist.
Conclusions.
The analyzed spectral range has the potential to deliver thermal, dynamic, and magnetic information for strongly magnetized features on the Sun, such as pores and sunspots, even without the use of polarimetry. The highest sensitivity of the lines is found in the lower photosphere, on average, around log
τ
= −1. The multiple-line inversions provide smooth results across the whole field of view (FOV). The presented spectral range and inversion strategy will be used for future VTT observing campaigns.
Context.The search for active longitudes on the Sun has a long history, and many controversial results have been published. Recently the question became more important when active longitudes were ...found on other stars. Aims.The aim of this paper is to investigate an integral measure of solar activity available for a long time interval and which allows enough frequency resolution for the investigation of active longitudes. Such a measure is given by the daily sunspot relative numbers. Methods.A search for periodicities is performed with a classical Fast Fourier Transform (FFT), with a wavelet analysis and with the tool of superimposed epochs. Results.The FFT yields a hump of power peaks near the synodic rotation period of 27 days, but only a very weak and insignificant enhancement around 13.5 days, indicating that the mean rotational variation of the sunspot numbers typically has one maximum and one minimum (overlaid by minor fluctuations). The wavelet analysis shows that spectral power for single periods varies for certain time intervals. A systematic drift during the solar activity cycle is not detected. Similar results are obtained from the superimposed epochs. Periodic “flip-flops” with time scales of a few years as for some stars are not found for the Sun in this investigation. Conclusions.Sunspots are not distributed equally over the longitudes; there is a more active and a less active hemisphere. The rotation period derived from the pattern varies over long time scales. The results found in this work are not in favor of an explanation of the variations due to a differential rotation law. The rotation of the sunspot distribution pattern might reflect the internal rotation of the Sun, but it better fits the range of highest rotation rates in the upper convection zone than the rotation near the tachocline.
Context. The various mechanisms of magneto-convective energy transport determine the structure of sunspots and active regions. Aims. We characterise the appearance of light bridges and other ...fine-structure details and elaborate on their magneto-convective nature. Methods. We present speckle-reconstructed images taken with the broad-band imager (BBI) at the 1.5 m GREGOR telescope in the 486 nm and 589 nm bands. We estimate the spatial resolution from the noise characteristics of the image bursts and obtain 0.08″ at 589 nm. We describe structure details in individual best images as well as the temporal evolution of selected features. Results. We find branched dark lanes extending along thin (≈1″) light bridges in sunspots at various heliocentric angles. In thick (≳ 2″) light bridges the branches are disconnected from the central lane and have a Y shape with a bright grain toward the umbra. The images reveal that light bridges exist on varying intensity levels and that their small-scale features evolve on timescales of minutes. Faint light bridges show dark lanes outlined by the surrounding bright features. Dark lanes are very common and are also found in the boundary of pores. They have a characteristic width of 0.1″ or smaller. Intergranular dark lanes of that width are seen in active region granulation. Conclusions. We interpret our images in the context of magneto-convective simulations and findings: while central dark lanes in thin light bridges are elevated and associated with a density increase above upflows, the dark lane branches correspond to locations of downflows and are depressed relative to the adjacent bright plasma. Thick light bridges with central dark lanes show no projection effect. They have a flat elevated plateau that falls off steeply at the umbral boundary. There, Y-shaped filaments form as they do in the inner penumbra. This indicates the presence of inclined magnetic fields, meaning that the umbral magnetic field is wrapped around the convective light bridge.
Aims.
The regular pattern of quiet-Sun magnetic fields was disturbed by newly emerging magnetic flux, which led a day later to two homologous surges after renewed flux emergence, affecting all ...atmospheric layers. Hence, simultaneous observations in different atmospheric heights are needed to understand the interaction of rising flux tubes with the surrounding plasma, in particular by exploiting the important diagnostic capabilities provided by the strong chromospheric H
α
line regarding morphology and energetic processes in active regions.
Methods.
A newly emerged active region NOAA 12722 was observed with the Vacuum Tower Telescope (VTT) at Observatorio del Teide, Tenerife, Spain, on 11 September 2018. High spectral resolution observations using the echelle spectrograph in the chromospheric H
α
λ
6562.8 Å line were obtained in the early growth phase. Noise-stripped H
α
line profiles yield maps of line-core and bisector velocities, which were contrasted with velocities inferred from Cloud Model inversions. A high-resolution imaging system recorded simultaneously broad- and narrowband H
α
context images. The Solar Dynamics Observatory provided additional continuum images, line-of-sight (LOS) magnetograms, and UV and extreme UV (EUV) images, which link the different solar atmospheric layers.
Results.
The active region started as a bipolar region with continuous flux emergence when a new flux system emerged in the leading part during the VTT observations, resulting in two homologous surges. While flux cancellation at the base of the surges provided the energy for ejecting the cool plasma, strong proper motions of the leading pores changed the magnetic field topology making the region susceptible to surging. Despite the surge activity in the leading part, an arch filament system in the trailing part of the old flux remained stable. Thus, stable and violently expelled mass-loaded ascending magnetic structures can coexist in close proximity. Investigating the height dependence of LOS velocities revealed the existence of neighboring strong up- and downflows. However, downflows occur with a time lag. The opacity of the ejected cool plasma decreases with distance from the base of the surge, while the speed of the ejecta increases. The location at which the surge becomes invisible in H
α
corresponds to the interface where the surge brightens in He
II
λ
304 Å. Broad-shouldered and dual-lobed H
α
profiles suggests accelerated or decelerated and highly structured LOS plasma flows. Significantly broadened H
α
profiles imply significant heating at the base of the surges, which is also supported by bright kernels in UV and EUV images uncovered by swaying motions of dark fibrils at the base of the surges.
Conclusions.
The interaction of newly emerging flux with pre-existing flux concentrations of a young, diffuse active region provided suitable conditions for two homologous surges. High-resolution spectroscopy revealed broadened and dual-lobed H
α
profiles tracing accelerated or decelerated flows of cool plasma along the multi-threaded structure of the surge.
Context. Active regions are the most prominent manifestations of solar magnetic fields; their generation and dissipation are fundamental problems in solar physics. Light bridges are commonly present ...during sunspot decay, but a comprehensive picture of their role in the removal of the photospheric magnetic field is still lacking. Aims. We study the three-dimensional configuration of a sunspot, and in particular, its light bridge, during one of the last stages of its decay. Methods. We present the magnetic and thermodynamical stratification inferred from full Stokes inversions of the photospheric Si i 10 827 Å and Ca i 10 839 Å lines obtained with the GREGOR Infrared Spectrograph of the GREGOR telescope at the Observatorio del Teide, Tenerife, Spain. The analysis is complemented by a study of continuum images covering the disk passage of the active region, which are provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. Results. The sunspot shows a light bridge with penumbral continuum intensity that separates the central umbra from a smaller umbra. We find that in this region the magnetic field lines form a canopy with lower magnetic field strength in the inner part. The photospheric light bridge is dominated by gas pressure (high-β), as opposed to the surrounding umbra, where the magnetic pressure is higher. A convective flow is observed in the light bridge. This flow is able to bend the magnetic field lines and to produce field reversals. The field lines merge above the light bridge and become as vertical and strong as in the surrounding umbra. We conclude that this occurs because two highly magnetized regions approach each other during the sunspot evolution.
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