Hida Observatory Research Articles

Sun-as-a-star Analysis of the X1.6 Flare on 2023 August 5: Dynamics of Postflare Loops in Spatially Integrated Observational Data

Postflare loops are loop-like plasmas observed during the decay phase of solar flares, and they are expected to exist for stellar flares. However, it is unclear how postflare loops are observed in stellar flares’ cases. To clarify behaviors of postflare loops in spatially integrated data, we performed the Sun-as-a-star analysis of the X1.6 flare that occurred on 2023 August 5, using GOES X-ray flux (∼107 K), extreme ultraviolet (EUV) images taken by Atmospheric Imaging Assembly on board the Solar Dynamic Observatory (≥104.9 K), and Hα data taken by Solar Dynamics Doppler Imager on board the Solar Magnetic Activity Research Telescope at Hida Observatory, Kyoto University (∼104 K). As a result, we found that this flare showed signatures corresponding to the important dynamics of the postflare loops even in the spatially integrated data: (1) The Hα light curve showed two distinct peaks corresponding to the flare ribbons and the postflare loops. The plasma cooling in the postflare loops generated different peak times in soft X-rays, EUV, and Hα light curves. (2) Downflows were confirmed as simultaneous redshifted/blueshifted absorptions in the Hα spectra. (3) The apparent rise of postflare loops was recognized as a slowing of the decay for the Hα light curve. These results are the key to investigating stellar postflare loops with spatially integrated data. We also discuss the dependence of our results on flare locations and their possible applications to stellar observations.

Multiwavelength Sun-as-a-star Analysis of the M8.7 Flare on 2022 October 2 Using Hα and EUV Spectra Taken by SMART/SDDI and SDO/EVE

This paper presents a multiwavelength Sun-as-a-star analysis of the M8.7 flare on 2022 October 2, which was associated with a filament eruption and the subsequent coronal mass ejection. The Sun-as-a-star analysis was performed using Hα data taken by Solar Dynamics Doppler Imager on board the Solar Magnetic Activity Research Telescope at Hida Observatory, Kyoto University, and full-disk integrated extreme ultraviolet (EUV) spectra taken by the Extreme ultraviolet Variability Experiment (EVE) on board the Solar Dynamics Observatory. The Sun-as-a-star Hα spectra showed blueshifted absorption corresponding to the filament eruption. Furthermore, the EVE O v 629.7 Å spectra showed blueshifted brightening, which can also be attributed to the filament eruption. Even when the blueshifted absorption became almost invisible in the Sun-as-a-star Hα spectra, the O v blueshifted brightening up to −400 km s−1 was still clearly visible. This result indicates that even when the shifted components—which are expected to originate from stellar eruptions—become almost invisible in the spatially integrated stellar Hα spectra, the erupting materials may still be present and observable in EUV spectra. Additionally, the Sun-as-a-star Hα and O v spectra exhibited redshifted absorption and brightening, respectively, during the decay phase of the flare. These components probably originate from the post-flare loops, providing clues to the multitemperature nature of the post-flare loops in the spatially integrated observation. Our Sun-as-a-star results suggest that the combination of Hα and EUV lines allows the investigation of the multitemperature structure and temporal development of stellar active phenomena even in spatially integrated spectra.

Plasma diagnostics and Alfvén wave heating of solar prominences by multiwavelength observations

Abstract Solar prominences are cool and dense plasma supported by the magnetic field in the solar corona. They are composed of fine-scale structures called threads. Prominences are heated by the incident radiation from the solar atmosphere, but previous studies have shown that additional heating is necessary to maintain the temperature of prominences of about 104 K. This study aims to investigate quantitatively the mechanical heating of the prominences from observations. We performed spectroscopic observations of prominences in Hα (6563 Å), Hβ (4861 Å), and Ca ii IR (8542 Å) with the Domeless Solar Telescope at Hida observatory. The plasma parameters of the prominences are estimated by fittings of line profiles using the single-slab model and by solving the statistical equilibrium equation. By assuming that the turbulent motion is the manifestation of propagating Alfvén waves, net radiative losses and Alfvén wave energy flux in the prominences are estimated from these parameters. As a result, it was found that the energy flux by Alfvén waves is sufficient to compensate for the net radiative losses in most regions of the central part of the prominences. On the other hand, Alfvén wave energy flux tends to be insufficient at the outer part of prominences. In such regions, another heating mechanism such as enthalpy flux of mixing with coronal plasma needs to be considered.

An Inductively Coupled Plasma System for Investigating Spectropolarimetric Responses of Solar Plasmas to Anisotropic Fields

High precision measurements and accurate modeling of atomic polarization under three-dimensional radiation transfer are crucial to understand the structures of magnetized solar plasmas. To develop and validate spectropolarimetric measurements and analyses, we set up an inductively coupled plasma (ICP) generator designed especially for ∼1-eV plasmas interacting with radiation and weak magnetic fields. The device was put in front of the focal plane of the Horizontal Spectrograph of the Domeless Solar Telescope at Hida Observatory of Kyoto University. In helium discharges, the typical electron temperature, electron density, and helium column density of the ICP are comparable values to those of solar prominences, and the direct comparison of spectra shows almost the same opacity at He I 1083 nm. Magnetic and radiation fields were introduced to the ICP, and the system successfully reproduced reasonable spectropolarimetric signals as compared with those from the solar prominences.

Magnetic field of solar dark filaments obtained from He i10830 Å spectropolarimetric observation

AbstractSolar filaments are dense, cool plasma clouds in the solar corona. They are supposed to be supported in a dip of the coronal magnetic field. However, the models still disagree between two types of field configuration; one is the normal polarity model proposed by Kippenhahn and Schlueter (1957, Z. Astrophys., 43, 36), and the other is the reverse polarity model proposed by Kuperus and Raadu (1974, A&A, 31, 189). To understand the mechanism that makes the filaments become unstable before eruption, it is critical to know the magnetic structure of solar filaments. We performed spectropolarimetric observation in the He i (10830 Å) line to investigate the magnetic field configuration of dark filaments. The observation was carried out with the Domeless Solar Telescope at Hida Observatory with a polarization sensitivity of 3.0 × 10−4. We obtained eight samples of filaments in a quiet region. As a result of the analysis of full Stokes profiles of filaments, we found that the field strengths were estimated as 8–35 G. By comparing the direction of the magnetic field in filaments and the global distribution of the photospheric magnetic field, we determined the magnetic field configuration of the filaments, and we concluded that one out of the eight samples has normal polarity configuration, and seven out of the eight have reverse polarity configuration.

Thermodynamic properties of small flares in the quiet Sun observed by Hα and EUV: plasma motion of the chromosphere and time evolution of temperature/emission measure

ABSTRACT Small flares frequently occur in the quiet Sun. Previous studies have noted that they share many common characteristics with typical solar flares in active regions. However, their similarities and differences are not fully understood, especially their thermal properties. In this study, we performed imaging spectroscopic observations in the Hα line taken with the Solar Dynamics Doppler Imager on the Solar Magnetic Activity Research Telescope (SMART/SDDI) at the Hida Observatory and imaging observations with the Atmospheric Imaging Assembly onboard Solar Dynamics Observatory (SDO/AIA). We analysed 25 cases of small flares in the quiet Sun over the thermal energy range of $10^{24}{\!-\!}10^{27}\, \mathrm{erg}$ , paying particular attention to their thermal properties. Our main results are as follows: (1) We observe a redshift together with line centre brightening in the Hα line associated with more than half of the small flares. (2) We employ differential emission measure analysis using AIA multitemperature (channel) observations to obtain the emission measure and temperature of the small flares. The results are consistent with the Shibata & Yokoyama (1999, 2002) scaling law. From the scaling law, we estimated the coronal magnetic field strength of small flares to be 5–15 G. (3) The temporal evolution of the temperature and the density shows that the temperature peaks precede the density peaks in more than half of the events. These results suggest that chromospheric evaporations/condensations play an essential role in the thermal properties of some of the small flares in the quiet Sun, as does for large flares.

Sun-as-a-star Analyses of Various Solar Active Events Using Hα Spectral Images Taken by SMART/SDDI

Sun-as-a-star analyses in which observational data is spatially integrated are useful for interpreting stellar data. For future applications to stellar observations, we performed Sun-as-a-star analyses of Hα spectra for various active events on the Sun, not only for flares and filament eruptions/surges on the solar disk, but also for eruptions of off-limb prominences using Hα spectral images taken by the Solar Magnetic Activity Research Telescope/Solar Dynamics Doppler Imager at Hida Observatory, Kyoto University. All the analyzed events show emission relative to the pre-event state and the changes in their Hα equivalent widths are all on the orders of 10−4 Å. Sun-as-a-star Hα spectra exhibit different features depending on the causes of the emission: (i) flares show emission at the Hα line center, together with red asymmetry and line broadening, as reported in a previous study, (ii) filament eruptions with and without flares show emission near the Hα line center, accompanied by blueshifted/redshifted absorption; notably, the disappearance of dark filaments leads to the apparent enhancement of the Hα line center emission, and (iii) eruptions of off-limb prominences show blueshifted/redshifted emission. These spectral features enable us to identify the active phenomena on Sun-like stars. We have also found that even the filament eruptions showing redshifted absorptions in Sun-as-a-star Hα spectra lead to coronal mass ejections (CMEs). This result suggests that even if the falling components of stellar filament eruptions are detected as redshifted absorptions in Hα spectra, such stellar filament eruptions may also develop into CMEs.

Polarization calibration of the Tandem Etalon Magnetograph of the Solar Magnetic Activity Research Telescope at Hida Observatory

Abstract The Tandem Etalon Magnetograph (TEM) is one of the instruments of the Solar Magnetic Activity Research Telescope of Hida Observatory. The TEM is a partial disk (320″ × 240″) filter magnetograph which scans the wavelength around a Fe i line at 6303 Å and achieves polarimetric sensitivity of <5 × 10−4 for each wavelength. To obtain the polarimeter response matrix of the instrument, we have carried out end-to-end polarization calibrations of the instrument. We have also measured the polarization characteristics of the polarization beam splitter (PBS), which is a crucial component of the instrument. As a result of end-to-end calibration, we found significant spatial variation in the response matrix across the field of view. From a laboratory test, we found that $1\%$ of the magnitude of a circular diattenuation of the PBS was due to the retardation caused by the stress in the cube and the linear diattenuation of the film. Although the spatial variation across the field of view is more than 10 times larger, to achieve the polarimetric sensitivity of <5 × 10−4, this can be well explained by the polarization characteristic of the PBS and corrected by using the response matrix obtained in the end-to-end calibration. In addition, we also obtained the daily variation of the polarization property of the TEM. We found that the crosstalk from Stokes Q to V changes by an amount comparable to the tolerance through a day. In the present configuration, we require a pixel-by-pixel calibration every 100 min to meet the accuracy requirement.

Sun-as-a-star Analysis of Hα Spectra of a Solar Flare Observed by SMART/SDDI: Time Evolution of Red Asymmetry and Line Broadening

Stellar flares sometimes show red/blue asymmetries of the Hα line, which can indicate chromospheric dynamics and prominence activations. However, the origin of asymmetries is not completely understood. For a deeper understanding of stellar data, we performed a Sun-as-a-star analysis of Hα line profiles of an M4.2-class solar flare showing dominant emissions from flare ribbons by using the data of the Solar Dynamics Doppler Imager on board the Solar Magnetic Activity Research Telescope at the Hida Observatory. Sun-as-a-star Hα spectra of the flare show red asymmetry of up to ∼95 km s−1 and line broadening of up to ∼7.5 Å. The Sun-as-a-star Hα profiles are consistent with spectra from flare regions with weak intensity, but they take smaller redshift velocities and line widths by a factor of ∼2 than those with strong intensity. The redshift velocities, as well as line widths, peak out and decay more rapidly than the Hα equivalent widths, which is consistent with the chromospheric condensation model and spatially resolved flare spectra. This suggests that as a result of superposition, the nature of chromospheric condensation is observable even from stellar flare spectra. The time evolution of redshift velocities is found to be similar to that of luminosities of near-ultraviolet rays (1600 Å), while the time evolution of line broadening is similar to that of optical white lights. These Hα spectral behaviors in Sun-as-a-star view could be helpful to distinguish whether the origin of Hα red asymmetry of stellar flares is a flare ribbon or other phenomena.

Temperature analysis of solar prominences by multi-wavelength observations

Abstract The aim of this paper is to determine the kinetic temperature of solar prominences from widths of emission lines with a careful evaluation of its validity, and to investigate the spatial distribution of the non-thermal velocity and the kinetic temperature in prominences. Spectroscopic observations of prominences were carried out using the Domeless Solar Telescope at the Hida observatory in four emission lines, i.e., H$\alpha$$656.3\:$nm, H$\beta$$486.1\:$nm, Ca$^{+}$IR $854.2\:$nm, and Ca$^{+}$K $393.4\:$nm. We found systematic differences in temperature and its spatial distributions in prominences obtained from different pairs of spectral lines. The reason for the difference is that the intrinsic widths of emission lines determined by a single-slab model are different for optically thick and thin lines, and can be attributed to the assumption of a Gaussian profile of the non-thermal broadening in the single-slab model. With numerical experiments, we found that the observed differences among line pairs can be reproduced by a multi-slab model, in which each slab has different non-thermal broadening and line-of-sight velocity. The most accurate evaluation of the temperature is obtained with the pair of H$\beta$ and Ca$^{+}$IR in our selection. Based on these results, we found the typical kinetic temperature of prominences to be 8000–$12000\:$K, which is significantly higher than the temperature determined by radiative balance and suggests the presence of mechanical heating in prominences. Also suggested is the presence of positive spatial correlation between non-thermal velocity and temperature. We revisit past observational results on the prominence temperature and discuss the reason for discrepancies among them.

Measurement of vector magnetic field in a flare kernel with a spectropolarimetric observation in He i 10830 Å

Abstract A flare kernel associated with a C4 class flare was observed in a spectral window including the He i triplet 10830 Å and Si i 10827 Å with a spectropolarimeter on the Domeless Solar Telescope at Hida Observatory on 2015 August 9. The observed Stokes profiles of the He i triplet in the flare kernel in its post-maximum phase are well reproduced through inversions considering the Zeeman and the Paschen–Back effects with a three-slab model of the flare kernel, in which two slabs which have upward and downward velocities produce emissions and one slab produces an absorption. The magnetic field strength inferred from the emission components of the He i line is 1400 G, which is significantly stronger than 690 G that is observed at the same location in the same line 6.5 hr before the flare. In addition, the photospheric magnetic field vector derived from the Si i10827 Å is similar to that of the flare kernel. To explain this result, we suggest that the emission in the He i triplet during the flare is produced in the deep layer, around which bombardment of non-thermal electrons leads to the formation of a coronal temperature plasma. Assuming a hydrogen column density at the location where the He i emissions are formed, and a power-law index of non-thermal electron energy distribution, we derived the low-energy cutoff of the non-thermal electron as 20–30 keV, which is consistent with that inferred from hard X-ray data obtained by RHESSI.

Developments of a multi-wavelength spectro-polarimeter on the Domeless Solar Telescope at Hida Observatory

Abstract To obtain full Stokes spectra in multi-wavelength windows simultaneously, we developed a new spectro-polarimeter on the Domeless Solar Telescope at Hida Observatory. The new polarimeter consists of a 60 cm aperture vacuum telescope on an altazimuth mounting, an image rotator, a high-dispersion spectrograph, and a polarization modulator and an analyzer composed of a continuously rotating waveplate with a retardation that is nearly constant at around 127° in 500–1100 nm. There are also a polarizing beam splitter located close behind the focus of the telescope, fast and large format CMOS cameras, and an infrared camera. A slit spectrograph allows us to obtain spectra in as many wavelength windows as the number of cameras. We characterized the instrumental polarization of the entire system and established a polarization calibration procedure. The cross-talks among the Stokes Q, U, and V have been evaluated to be about 0.06%–1.2%, depending on the degree of the intrinsic polarizations. In a typical observing setup, a sensitivity of 0.03% can be achieved in 20–60 seconds for 500–1100 nm. The new polarimeter is expected to provide a powerful tool for diagnosing the 3D magnetic field and other vector physical quantities in the solar atmosphere.

Blue-wing enhancement of the chromospheric Mg ii h and k lines in a solar flare

Abstract We performed coordinated observations of AR 12205, which showed a C-class flare on 2014 November 11, with the Interface Region Imaging Spectrograph (IRIS) and the Domeless Solar Telescope (DST) at Hida Observatory. Using spectral data in the Si iv 1403 Å, C ii 1335 Å, and Mg ii h and k lines from IRIS and the Ca ii K, Ca ii 8542 Å, and Hα lines from DST, we investigated a moving flare kernel during the flare. In the Mg ii h line, the leading edge of the flare kernel showed an intensity enhancement in the blue wing and a smaller intensity of the blue-side peak (h2v) than that of the red-side one (h2r). The blueshift lasted for 9–48 s with a typical speed of 10.1 ± 2.6 km s−1, which was followed by a high intensity and a large redshift with a speed of up to 51 km s−1 detected in the Mg ii h line. The large redshift was a common property for all six lines, but the blueshift prior to it was found only in the Mg ii lines. Cloud modeling of the Mg ii h line suggests that the blue-wing enhancement with such a peak difference could have been caused by a chromospheric-temperature (cool) upflow. We discuss a scenario in which an upflow of cool plasma is lifted up by expanding hot plasma owing to the deep penetration of non-thermal electrons into the chromosphere. Furthermore, we found that the blueshift persisted without any subsequent redshift in the leading edge of the flare kernel during its decaying phase. The cause of such a long-lasting blueshift is also discussed.

Observational study on the fine structure and dynamics of a solar jet. II. Energy release process revealed by spectral analysis

Abstract We report on a solar jet phenomenon associated with the C5.4 class flare on 2014 November 11. The data of the jet was provided by the Solar Dynamics Observatory, the X-Ray Telescope (XRT) aboard Hinode, and the Interface Region Imaging Spectrograph and Domeless Solar Telescope (DST) at Hida Observatory, Kyoto University. These plentiful data enabled us to present this series of papers to discuss all the processes of the observed phenomena, including energy storage, event trigger, and energy release. In this paper, we focus on the energy release process of the observed jet, and mainly describe our spectral analysis on the Hα data of DST to investigate the internal structure of the Hα jet and its temporal evolution. This analysis reveals that in the physical quantity distributions of the Hα jet, such as line-of-sight velocity and optical thickness, there is a significant gradient in the direction crossing the jet. We interpret this internal structure as the consequence of the migration of the energy release site, based on the idea of ubiquitous reconnection. Moreover, by measuring the horizontal flow of the fine structures in the jet, we succeeded in deriving the three-dimensional velocity field and the line-of-sight acceleration field of the Hα jet. The analysis result indicates that part of the ejecta in the Hα jet experienced additional acceleration after it had been ejected from the lower atmosphere. This secondary acceleration was found to occur in the vicinity of the intersection between the trajectories of the Hα jet and the X-ray jet observed by Hinode/XRT. We propose that a fundamental cause of this phenomenon is magnetic reconnection involving the plasmoid in the observed jet.

Observational study on the fine structure and dynamics of a solar jet. I. Energy build-up process around a satellite spot

Abstract We report on a solar jet phenomenon associated with successive flares on 2014 November 10. These explosive events were involved with the satellite spots’ emergence around a δ-type sunspot in the decaying active region NOAA 12205. The data for this jet were provided by the Solar Dynamics Observatory, the X-Ray Telescope aboard Hinode, and the Interface Region Imaging Spectrograph and Domeless Solar Telescope at Hida Observatory, Kyoto University. These abundant data enabled us to present this series of papers to discuss the entire process of the observed phenomena, including the energy storage, event trigger, and energy release. In this paper, we focus on the energy build-up and trigger phases, by analyzing the photospheric horizontal flow field around the active region by an optical flow method. The analysis shows the following results: (1) The observed explosive phenomena involved three satellite spots, the magnetic fluxes of which successively reconnected with their pre-existing ambient fields; (2) All of these satellite spots emerged in the moat region of a pivotal δ-type sunspot, especially near its convergent boundary with the neighboring supergranules or moat regions of adjacent sunspots; (3) Around the jet ejection site, the positive polarities of the satellite spot and adjacent emerging flux encountered the global magnetic field with a negative polarity in the moat region of the pivotal δ-type sunspot, and thus the polarity inversion line was formed along the convergent boundary of the photospheric horizontal flow channels.

Increase in the Amplitude of Line-of-sight Velocities of the Small-scale Motions in a Solar Filament before Eruption

Abstract We present a study on the evolution of the small-scale velocity field in a solar filament as it approaches the eruption. The observation was carried out by the Solar Dynamics Doppler Imager (SDDI) that was newly installed on the Solar Magnetic Activity Research Telescope at Hida Observatory. The SDDI obtains a narrowband full-disk image of the Sun at 73 channels from Hα − 9.0 Å to Hα + 9.0 Å, allowing us to study the line-of-sight (LOS) velocity of the filament before and during the eruption. The observed filament is a quiescent filament that erupted on 2016 November 5. We derived the LOS velocity at each pixel in the filament using the Becker’s cloud model, and made the histograms of the LOS velocity at each time. The standard deviation of the LOS velocity distribution can be regarded as a measure for the amplitude of the small-scale motion in the filament. We found that the standard deviation on the previous day of the eruption was mostly constant around 2–3 km s−1, and it slightly increased to 3–4 km s−1 on the day of the eruption. It shows a further increase, with a rate of 1.1 m s−2, about three hours before eruption, and another increase, with a rate of 2.8 m s−2, about an hour before eruption. From this result we suggest that the increase in the amplitude of the small-scale motions in a filament can be regarded as a precursor of the eruption.

Differences between Doppler velocities of ions and neutral atoms in a solar prominence

In astrophysical systems with partially ionized plasma the motion of ions is governed by the magnetic field while the neutral particles can only feel the magnetic field's Lorentz force indirectly through collisions with ions. The drift in the velocity between ionized and neutral species plays a key role in modifying important physical processes like magnetic reconnection, damping of magnetohydrodynamic waves, transport of angular momentum in plasma through the magnetic field, and heating. This paper investigates the differences between Doppler velocities of calcium ions and neutral hydrogen in a solar prominence to look for velocity differences between the neutral and ionized species. We simultaneously observed spectra of a prominence over an active region in H I 397 nm, H I 434 nm, Ca II 397 nm, and Ca II 854 nm using a high dispersion spectrograph of the Domeless Solar Telescope at Hida observatory, and compared the Doppler velocities, derived from the shift of the peak of the spectral lines presumably emitted from optically-thin plasma. There are instances when the difference in velocities between neutral atoms and ions is significant, e.g. 1433 events (~ 3 % of sets of compared profiles) with a difference in velocity between neutral hydrogen atoms and calcium ions greater than 3sigma of the measurement error. However, we also found significant differences between the Doppler velocities of two spectral lines emitted from the same species, and the probability density functions of velocity difference between the same species is not significantly different from those between neutral atoms and ions. We interpreted the difference of Doppler velocities as a result of motions of different components in the prominence along the line of sight, rather than the decoupling of neutral atoms from plasma.

A New Solar Imaging System for Observing High-Speed Eruptions: Solar Dynamics Doppler Imager (SDDI)

A new solar imaging system was installed at Hida Observatory to observe the dynamics of flares and filament eruptions. The system (Solar Dynamics Doppler Imager; SDDI) takes full-disk solar images with a field of view of \(2520~\mbox{arcsec} \times 2520~\mbox{arcsec}\) at multiple wavelengths around the \(\mathrm{H}\alpha\) line at 6562 A. Regular operation was started in May 2016, in which images at 73 wavelength positions spanning from \(\mathrm{H}\alpha -9~\mathring{\mathrm{A}}\) to \(\mathrm{H}\alpha +9~\mathring{\mathrm{A}}\) are obtained every 15 seconds. The large dynamic range of the line-of-sight velocity measurements (\({\pm}\,400~\mbox{km}\,\mbox{s}^{-1}\)) allows us to determine the real motions of erupting filaments in 3D space. It is expected that SDDI provides unprecedented datasets to study the relation between the kinematics of filament eruptions and coronal mass ejections (CME), and to contribute to the real-time prediction of the occurrence of CMEs that cause a significant impact on the space environment of the Earth.

DIAGNOSIS OF MAGNETIC AND ELECTRIC FIELDS OF CHROMOSPHERIC JETS THROUGH SPECTROPOLARIMETRIC OBSERVATIONS OF H I PASCHEN LINES

Magnetic fields govern the plasma dynamics in the outer layers of the solar atmosphere, and electric fields acting on neutral atoms that move across the magnetic field enable us to study the dynamical coupling between neutrals and ions in the plasma. In order to measure the magnetic and electric fields of chromospheric jets, the full Stokes spectra of the Paschen series of neutral hydrogen in a surge and in some active region jets that took place at the solar limb were observed on May 5, 2012, using the spectropolarimeter of the Domeless Solar Telescope at Hida observatory, Japan. First, we inverted the Stokes spectra taking into account only the effect of magnetic fields on the energy structure and polarization of the hydrogen levels. Having found no definitive evidence of the effects of electric fields in the observed Stokes profiles, we then estimated an upper bound for these fields by calculating the polarization degree under the magnetic field configuration derived in the first step, with the additional presence of a perpendicular (Lorentz type) electric field of varying strength. The inferred direction of the magnetic field on the plane of the sky (POS) approximately aligns to the active region jets and the surge, with magnetic field strengths in the range 10 G < B < 640 G for the surge. Using magnetic field strengths of 70, 200, and 600 G, we obtained upper limits for possible electric fields of 0.04, 0.3, and 0.8 V/cm, respectively. Because the velocity of neutral atoms of hydrogen moving across the magnetic field derived from these upper limits of the Lorentz electric field is far below the bulk velocity of the plasma perpendicular to the magnetic field as measured by the Doppler shift, we conclude that the neutral atoms must be highly frozen to the magnetic field in the surge.

Empirical investigation on the impact of hydrogen collisions for the formation of C i 1.07 μm lines based on the solar center-to-limb variation

Abstract With the aim of examining the validity of non-LTE line-formation calculations for the strong C i lines of multiplet 1 at 1.068–1.069 μm, especially in terms of the treatment of collisions with neutral hydrogen (H i) atoms, we computed theoretical equivalent widths (Wλ) of these lines corresponding to specific intensities of different angles (μ = cos θ) on the solar disk under various conditions, which were then compared with the empirical Wλ vs. μ relations obtained from our spectroscopic observations with the Domeless Solar Telescope at Hida Observatory. It turned out that our observational data are almost consistent with the theoretical simulations done with the H i collision rates computed with the classical formula, which suggests that the necessity of its significant revision (e.g., considerable reduction) is unlikely.