Simultaneous multifrequency observations of an eruptive prominence at millimeter wavelengths

Radio images and spectra of an eruptive prominence were obtained from simultaneous multifrequency observations at 36 GHz, 89 GHz, and 110 GHz on May 28, 1991 with the 45-m radio telescope at Nobeyama Radio Observatory (NRO), the National Astronomical Observatory, Japan (NAOJ). The radio spectra indicated that the optical depth is rather thick at 36 GHz whereas it is thin at 89 and 110 GHz. The H data, taken at Norikura Solar Observatory, NAOJ, suggest that the eruption of an active region filament was triggered by an H flare. The shape and position of the radio prominence generally coincided with those of H images. The radio emission is explained with an isothermal cool thread model. A lower limit for the electron temperature of the cool threads is estimated to be 6100 K. The range of the surface filling factors of the cool threads is 0.3–1.0 after the H flare, and 0.2–0.5 in the descending phase of the eruptive prominence. The column emission measure and the electron number density are estimated to be of the order of 10²⁸ cm^–5 and 10¹⁰ cm^–3, respectively. The physical parameters of a quiescent prominence are also estimated from the observations. The filling factors of the eruptive prominence are smaller than those of the quiescent prominence, whereas the emission measures and the electron densities are similar. These facts imply that each cool thread of the prominence did not expand after the eruption, while the total volume of the prominence increased.     

Observational Study of the Three-Dimensional Magnetic Field Structure and Mass Motion in Active Regions

Spectro-polarimetric observations of active regions were carried out in the spectral lines of Sii 10827.1 Å and Hei 10830 Å to study the three-dimensional magnetic field structure and associated plasma flow properties. Comparison of Sii and Hei magnetograms with the potential field model shows that a large fraction of the magnetic field is consistent with the potential field structure, by assuming that the height difference between the origin of the two lines is about 1200 km. The slope of the scatter plot between Sii and Hei magnetograms is 0.5, 0.76 in an emerging flux and a larger active region, respectively. These values are lower than the scatter plot slopes obtained from Kitt Peak photospheric and chromospheric magnetograms, in which case the corresponding values are 0.83 and 0.9, respectively. Considering the height difference between these two sets of chromospheric magnetograms, this implies that the magnetic field spreads out faster near the transition region heights. Dopplergrams obtained by determining the centroid of the asymmetric line profiles show that, in case of emerging flux region, the chromospheric upflow regions are located in the magnetic neutral line areas.     

Stray-Light Effect on Magnetograph Observations

To examine the stray-light effect in magnetograph observations, we have determined the point spread functions of the vector magnetograph mounted on the Japanese Solar Flare Telescope based on two indirect methods, one analyzing the solar limb intensity profile, and the other using the Fourier power spectra of photospheric intensity distributions. Point spread functions consist of two parts: a blurring part which describes seeing and small-spread-angle stray light, and a scattering part which describes large-spread-angle stray light. The FWHM spatial resolution is typically 3.0, and the amount of scattered light is about 15% on clear days. We find that the blurring part is well described by a Moffat function whose Fourier transform is given by an exponential function. Our results indicate that polarization measurements of low-intensity magnetic elements like sunspots may be significantly underestimated due to the large-spread-angle stray light, and polarization measurements of magnetic elements which are smaller than 5–7 appear to be disturbed by small-spread-angle stray light.     

Temporal Evolution of a Rapidly-Moving Umbral Dot

We performed two-dimensional spectroscopic observations of the preceding sunspot of NOAA 10905 located off disk center (S8 E36, μ≈0.81) by using the Interferometric BI-dimensional Spectrometer (IBIS) operated at the Dunn Solar Telescope (DST) of the National Solar Observatory, New Mexico. The magnetically insensitive Fe I line at 709.04 nm was scanned in wavelength repetitively at an interval of 37 s to calculate sequences of maps of the line-wing and line-core intensity, and the line-of-sight Doppler velocity at different line depths (3% to 80%). Visual inspection of movies based on speckle reconstructions computed from simultaneous broadband data and the local continuum intensity at 709.04 nm revealed an umbral dot (UD) intruding rapidly from the umbral boundary to the center of the umbra. The apparent motion of this object was particularly fast (1.3 km s⁻¹) when compared to typical UDs. The lifetime and size of the UD was 8.7 min and 240 km, respectively. The rapid UD was visible even in the line-core intensity map of Fe I 709.04 nm and was accompanied by a persistent blueshift of about 0.06 km s⁻¹.     

Solar meridional motions derived from sunspot observations

Sunspot drawings obtained at the National Astronomical Observatory of Japan during the years 1954–1986 were used to determine meridional motions of the Sun. A meridional flow of a few ms^–1 was found, which is equatorward in the latitude range from -20° to +15° and is poleward at higher latitudes in both hemispheres. A northward flow of 0.01° day^–1 or 1.4 ms^–1 at mid-latitudes (between 10° and 20°) was also detected. From our limited data-set of three solar cycles, an indication of solar-cycle dependence of meridional motions was found.     

Spectroscopic Studies of Solar Corona VII. Formation of a Coronal Loop by Evaporation

We obtained time-sequence spectroscopic observations in (Fe x) 6374 Å and (Fe xiv) 5303 Å lines successively with the 25-cm coronagraph, and narrow-band and Doppler images in 5303 Å line by the 2-D 10-cm Doppler coronagraph NOGIS at the Norikura Solar Observatory, of a coronal region for about 7 h on 9 19–20, 2001. The raster scans were obtained with a quasi-periodicity of about 14 min and NOGIS obtained the images with an interval of about 1 min. The coronal region observed showed the formation of a coronal loop by a high-speed surge in the 6374 Å line rising from one of the footpoints of the loop. Off the limb spectroscopic observations in the 6374 Å line showed large velocities along the line of sight and vertical to the solar limb at the time of formation of the loop. The 5303 Å line observations showed negligible line-of-sight velocities and low vertical velocities when compared to those in the 6374 Å line. A hump in the intensity plots in 5303 Å with height appears to move up with respect to the solar limb with an average velocity of 4km s^–1. The FWHM of the 6374 Å showed a much smaller value of about 0.7 Å near the foot point as compared to a value of 1.2 Å at larger heights at the beginning of observations. Later as the loop developed, the FWHM of 6374 Å line showed a gradual decrease along the loop up to 70 from the limb, reached a minimum value of about 0.5 Å and then increased with height during the formation of the loop; this trend lasted for about 2 h. About 3 h after the beginning of the formation of the loop, the FWHM of 6374 Å emission line showed normal values and normal rate of increase with height with some fluctuations. The FWHM of the 5303 Å line did not show such variations along the loop and showed normal decrease in FWHM with height found earlier (Singh et al., 2003a). These observations suggest that a relatively cooler plasma at a temperature of about 0.7 MK or less (corresponding to minimum value of FWHM of 0.5 Å) was ejected from the transition region with a large velocity of about 48km s^–1, heated up in the corona by some process and formed a coronal loop with a height of about 200 above the limb that had lifetime greater than 4 h. It appears that the plasma moved from one of the footpoints and the loop was formed by evaporation of chromospheric plasma. No large-scale brightening and H flare were observed in this region during the observational period of 7 h.On leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

The Hinode Spectro-Polarimeter

The joint Japan/US/UK Hinode mission includes the first large-aperture visible-light solar telescope flown in space. One component of the Focal Plane Package of that telescope is a precision spectro-polarimeter designed to measure full Stokes spectra with the intent of using those spectra to infer the magnetic-field vector at high precision in the solar photosphere. This article describes the characteristics of the flight hardware of the Hinode Spectro-Polarimeter, and summarizes its in-flight performance.     

Polarization Calibration of the <Emphasis Type="Italic">Chromospheric Lyman-Alpha SpectroPolarimeter</Emphasis> for a 0.1% Polarization Sensitivity in the VUV Range. Part II: In-Flight Calibration

The Chromospheric Lyman-Alpha SpectroPolarimeter is a sounding rocket instrument designed to measure for the first time the linear polarization of the hydrogen Lyman-\({\upalpha }\) line (121.6 nm). The instrument was successfully launched on 3 September 2015 and observations were conducted at the solar disc center and close to the limb during the five-minutes flight. In this article, the disc center observations are used to provide an in-flight calibration of the instrument spurious polarization. The derived in-flight spurious polarization is consistent with the spurious polarization levels determined during the pre-flight calibration and a statistical analysis of the polarization fluctuations from solar origin is conducted to ensure a 0.014% precision on the spurious polarization. The combination of the pre-flight and the in-flight polarization calibrations provides a complete picture of the instrument response matrix, and a proper error transfer method is used to confirm the achieved polarization accuracy. As a result, the unprecedented 0.1% polarization accuracy of the instrument in the vacuum ultraviolet is ensured by the polarization calibration.     

Spectroscopic Studies of the Solar Corona – V. Physical Properties of Coronal Structures

Spectra around the 6374 Å [Fex] and 7892 Å [Fexi] emission lines were obtained simultaneously with the 25-cm coronagraph at Norikura Observatory covering an area of 200 ×500 of the solar corona. The line width, peak intensity and line-of-sight velocity for both the lines were computed using Gaussian fits to the observed line profiles at each location (4 ×4 ) of the observed coronal region. The line-width measurements show that in steady coronal structures the FWHM of the 6374 Å emission line increases with height above the limb with an average value of 1.02 mÅ arc sec^–1. The FWHM of the 7892 Å line also increases with height but at a smaller average value of 0.55 mÅ arc sec^–1. These observations agree well with our earlier results obtained from observations of the red, green, and infrared emission lines that variation of the FWHM of the coronal emission lines with height in steady coronal structures depends on plasma temperatures they represent. The FWHM gradient is negative for high-temperature emission lines, positive for relatively low-temperature lines and smaller for emission lines in the intermediate temperature range. Such a behaviour in the variation of the FWHM of coronal emission lines with height above the limb suggests that it may not always be possible to interpret an increase in the FWHM of emission line with height as an increase in the nonthermal velocity, and hence rules out the existence of waves in steady coronal structures.     

Stray-light correction in magnetograph observations using the maximum entropy method

We have developed a method of stray-light correction which is applicable to filter-based magnetograph observations. Stray-light-corrected Stokes images are obtained by performing the deconvolution of observed Stokes images by the point spread function which is determined from the Stokes I image. For image deconvolution, the maximum entropy principle is used to guarantee that intensity should be positive and polarization degrees should be less than unity. We present an iterative algorithm for the maximum entropy method, which seeks the solution in Fourier space and thus accomplishes fast convergence. We find that our method is effective in correcting stray light which has a spread angle greater than the full width at half maximum of the point spread function. We also discuss the effect of stray light on magnetograph calibration.