An Erupting Active Region Filament: Three-Dimensional Trajectory and Hydrogen Column Density

From 15:33 through 16:02 UT on 13 June 1998, observations of an erupting filament as it crossed solar disk center were obtained with the NSO/KPVT and SOHO/CDS instruments as part of the SOHO Joint Observing Program 70. Context observations show that this event was the eruption of the north-east section of a small active region filament associated with NOAA 8237, that the photospheric magnetic field was changing in this active region between 12–14 June 1998, and that a coronal Moreton-wave disk event occurred, as well as a white-light CME off the south-west solar limb. The NSO/KPVT imaging spectroscopy data covered 512 × 512 arc sec of the disk center and were spectrally centered at the Hei 1083 nm line and captured ±1.0 nm of surrounding solar spectrum. The Hei absorption line is seen blue-shifted to velocities of between 200 and 300 km s^–1. The true solar trajectory of the eruption is obtained by using the projected solar coordinates and by integrating the Doppler velocity. The filament travels with a total velocity of about 300 km s^–1 along a path inclined roughly 49 deg to the solar surface and rises to a height of just over 1.5 solar radii before it becomes too diffuse to follow. The filament also shows internal motions with multiple Doppler components shifted by ±25 km s^–1. Finally, the KPVT data show no Stokes V profiles in the Doppler-shifted Hei 1083.03 nm absorption to a limit of roughly 3×10^–3 times the continuum intensity. The SOHO/CDS scanned the center of the KPVT FOV using seven EUV lines; Doppler-shifted filament emission is seen in lines from Hei 58.4 nm, Heii 30.4 nm, Oiv 55.5 nm, Ov 63.0 nm, Nevi 56.3 nm, and Mgx 61.0 nm representing temperatures from about 2×10⁴K through 1×10⁶K. Bound-free continuum absorption from Hi, without confusion from foreground emission and line emission, is seen as the filament obscures underlying chromospheric emission. A fit to the wavelength dependence of the absorption from five lines between 55.5 to 63.0 nm yields a column density _H I =4.8±2.5×10¹⁷ cm^–2. Spatial maps show that this filament absorption is more confined than the regions which show emission.     

Infrared Spectroscopy from San Fernando Observatory: He I 1083 nm, O I 1316 nm, and Fe I 1565 nm

Imaging spectroscopy of the Sun was carried out at the California State University Northridge San Fernando Observatory using an InGaAs near-IR video camera. Using the Sii 1082.71 nm and Hei 1083.03 nm lines the Evershed effect is measured simultaneously in the photosphere and the chromosphere for three sunspots; the speed of the Evershed flow is measured to be between 3 to 8 times greater in the Hei line than in the Sii line, and the direction is radially inward in the chromosphere and outward in the photosphere. Telluric absorption lines prevented a meaningful measurement of Oi 1128.7 nm limb emission, but an upper limit of 20×10^–3 B is measured for chromospheric limb emission at Oi 1316.3 nm. Zeeman splitting in Fei 1564.9 nm was observed in six sunspot umbrae, and a linear relationship between magnetic field and umbral continuum intensity is confirmed.     

Dynamics of Blinkers

The relative Doppler and non-thermal velocities of quiet-Sun and active-region blinkers identified in Ov with CDS are calculated. Relative velocities for the corresponding chromospheric plasma below are also determined using the Hei line. Ov blinkers and the chromosphere directly below, have a preference to be more red-shifted than the normal transition region and chromospheric plasma. The ranges of these enhanced velocities, however, are no larger than the typical spread of Doppler velocities in these regions. The anticipated ranges of Doppler velocities of blinkers are 10–15 km s^–1 in the quiet Sun (10–20 km s^–1 in active regions) for Hei and 25–30 km s^–1 in the quiet Sun (20–40 km s^–1 in active regions) for Ov. Blinkers and the chromosphere below also have preferentially larger non-thermal velocities than the typical background chromosphere and transition region. Again the increase in magnitude of these non-thermal velocities is no greater than the typical ranges of non-thermal velocities. The ranges of non-thermal velocities of blinkers in both the quiet Sun and active regions are estimated to be 15–25 km s^–1 in Hei and 30–45 km s^–1 in Ov. There are more blinkers with larger Doppler and non-thermal velocities than would be expected in the whole of the chromosphere and transition region. The recently suggested mechanisms for blinkers are revisited and discussed further in light of the new results.     

Comparison of Prominences in Hα and He II 304 Å

In this letter, we bring attention to prominences which show different morphology in H and Heii 304 Å, as observed simultaneously by BBSO and EIT on board SOHO. Those two lines have been thought to represent similar chromospheric structures although they are formed at significantly different temperatures. We give two examples representing two kinds of anomaly: (1) prominences showing strong H emissions in the lower part and strong Heii emissions in the upper part, and (2) erupting prominences showing extensive Heii emission, but nothing in H. Our results indicate that a part or the whole of a prominence may be too hot to emit H radiation, possibly due to heating or thermal instability. Please note that these are not just two isolated cases, many other prominences show the similar differences in H and Heii 304 Å.     

A narrow-band tunable solar filter for the near-infrared spectral region

A narrow-band tunable solar filter was constructed for the near-infrared spectral region. It is a pre-monochromator consisting of a double monochromator with dispersion subtraction, while the final passband is formed by a scanning Fabry–Pérot interferometer. Such a filter can be realized in practice for any optical spectral region. The tuning range of the filter for the near-infrared is 9000–11000 Å, FWHM of the passband equals 0.24 Å at the Hei 10830 Å line. The angular field of view on the sky is 3.6 for a diameter of the telescope of 100 mm. Filtergrams of the active region NOAA 8076 in the Hei 10830 Å line were obtained on 28 August 1997, the profiles of this line in the selected points of the image, and radial velocity field are presented.     

Structure and Evolution of the Transition Region Network Observed in he ii λ304

The length scale and life time of the transition region network cells were studied using Heii 304 filtergrams. The temporal structure function was calculated from spatially aligned Heii 304 images. The estimated life time of the network cell was about 27 hr. We compared this life time with the life time of photospheric magnetic network and of the extrapolated magnetic network. The spatial structure function was calculated from the Heii 304 filtergrams. The calculated spatial structure function saturates at 25000 km. The transition region network elements are bigger in size than the photospheric magnetic network element. The magnetic network element equals the size of the Heii 304 network element when the photospheric magnetic field is extrapolated to a height of 3000 km above the photosphere where the magnetic fluxes are deployed. The derived value of the diffusion speed of the network elements was 0.098 km s^–1.     

Comparative morphology of EIT/SOHO images and He ii excitation

Morphological differences between coronal images on the one hand, and a Hei image on the other, are used to demonstrate the independence of Heii excitation from coronal radiation. The distribution of magnetic flux is found to be more important for Heii excitation. Collisional excitation by non-thermal electrons produced in nano-flare events is proposed as the mechanism for Heii excitation.     

CORONAL RADIATION AND HELIUM λ584 EMISSION IN ACTIVE REGIONS

Based on EUV observations of eleven sunspot regions obtained with the Coronal Diagnostic Spectrometer, CDS, on SOHO we have studied the spatial distribution, temporal variation and wavelength shift of the Hei 584 line. We find a relatively high spatial correlation between the coronal line Fexvi 360 and the Hei 584 line. This points to coronal back-radiation as an important contributor to the formation of the Hei line in active regions. However, contribution to the line formation from another source is suggested by the following two findings: First, the red-shifted line profiles of both Hei 584 and the transition region lines tend to be more intense than blue-shifted profiles. Second, the Hei 584 emission changes significantly faster than the coronal line emission.     

Similarities between chromospheric and photospheric quiet-Sun magnetograms

Simultaneous observations of chromospheric (H) and photospheric (Fei 5324.19 Å) magnetograms in quiet solar regions enable us to study the spatial configuration of the magnetic field in the solar atmosphere. With the typical spatial resolution of the Huairou magnetograph, the photospheric and chromospheric magnetic structures of the quiet Sun maintain a very similar pattern. Moreover, the vertical magnetic flux is almost the same from the photosphere to the chromosphere. As an intermediate step, we analyze the formation of the working lines used by the Huairou video magnetograph of the Beijing Astronomical Observatory. The Stokes V contribution function of H and Fei 5324.19 Å are calculated. It is found that our H magnetograms provide the distribution of the chromospheric magnetic field at a height some 1000–1500 km above the photosphere.     

Electro-Mechanical Coupling Between the Photosphere and Transition Region

We study the response of the chromosphere and transition region to dynamic changes in the photospheric network magnetic fields. We present results from simultaneous measurements taken by TRACE in chromospheric and transition region (Civ) images, high-resolution magnetograms taken by MDI, and spectra of chromospheric (Cii) and transition region lines (Ovi) obtained with the SUMER instrument on SOHO. Enhanced emission in the Civ line is generally co-spatial with the magnetic pattern in the photosphere. We propose a mechanism of electro-mechanical coupling between the photosphere and upper layers of atmosphere based on hydrodynamic cumulation of energy produced by reconnecting flux tubes in the photosphere/chromosphere region (Tarbell et al., 1999). We believe that a basic process causing energetic events is the cascade of shock waves produced by colliding and reconnecting flux tubes. The continuous supply of flux tubes in the magnetic carpet ensures the ubiquitous nature of this process and its imprint on the upper atmosphere. The appearance of bright transients often, but not always, correlates with canceling mixed polarity magnetic elements in the photosphere. In other cases, transients occur in regions of unipolar flux tubes, suggesting reconnection of oblique components. Transients are also seen in regions with no fields detected with the MDI sensitivity; these may be reconnections of tiny features with diameters less than 100 km. Blinkers and other bright transients are often accompanied by two directional plasma jets. These may be generated by cylindrical self-focusing of shock fronts or by collision of shocks produced by neighboring reconnection processes. The observations suggest that stronger emissions correspond to lower velocity jets, and vice versa; this property is a natural consequence of the proposed mechanism. Plasma flows are always seen whenever the slit crosses strong magnetic flux tubes or vertices of converging flows in the supergranular network. The overall energy distribution between heating and plasma flows is an intrinsic feature of our mechanism.     

A Study of the Rotation of the Solar Corona

Synoptic photoelectric observations of the coronal Fexiv and Fex emission lines at 530.3 nm and 637.4 nm, respectively, are analyzed to study the rotational behavior of the solar corona as a function of latitude, height, time and temperature between 1976 (1983 for Fex) and 2001. An earlier similar analysis of the Fexiv data at 1.15 R over only one 11-year solar activity cycle (Sime, Fisher, and Altrock, 1989, Astrophys. J. 336, 454) found suggestions of solar-cycle variations in the differential (latitude-dependent) rotation. These results are tested over the longer epoch now available. In addition, the new Fexiv 1.15 R results are compared with those at 1.25 R and with results from the Fex line. I find that for long-term averages, both ions show a weakly-differential rotation period that may peak near 80° latitude and then decrease to the poles. However, this high-latitude peak may be due to sensing low-latitude streamers at higher latitudes. There is an indication that the Fexiv rotation period may increase with height between 40° and 70° latitude. There is also some indication that Fex may be rotating slower than Fexiv in the mid-latitude range. This could indicate that structures with lower temperatures rotate at a slower rate. As found in the earlier study, there is very good evidence for solar-cycle-related variation in the rotation of Fexiv. At latitudes up to about 60°, the rotation varies from essentially rigid (latitude-independent) near solar minimum to differential in the rising phase of the cycle at both 1.15 R and 1.25 R . At latitudes above 60°, the rotation at 1.15 R appears to be nearly rigid in the rising phase and strongly differential near solar minimum, almost exactly out of phase with the low-latitude variation.     

Investigation of He I 10 830 Å `Dark Points' at the Sayan Solar Observatory and the Baikal Astrophysical Observatory

A new two-bandpass birefringent filter has been produced at ISTP, Irkutsk for the investigation of the fine structure of the chromosphere. One filter passband is centered on the Hei 10830 Å line, the second one is centered on H. The FWHM of the Hei 10830 Å passband is 0.46 Å and of the H passband is 0.3 Å. A large number of filtergrams were obtained with the filter at the Sayan observatory. At the same time, spectral observations with high spatial and spectral resolution were carried out by the large solar vacuum telescope at the Baikal Observatory. We selected 29 `dark point' spectra with sizes from 2 to 13, as well as `dark points' on the filtergrams. Comparison of spectrograms and filtergrams has shown a good agreement of their size and intensity in relation with the surrounding chromosphere as well as the absence of primary line-of-sight velocities in both observation types. From spectral observations, the depth of 10830 Å is over 30% for some `dark points', and the FWHM is more than 1 Å. Hei 10830 Å line profiles in `dark points' are more deep and wide than in quiet regions. The optical depth of the chromosphere in `dark points' is estimated. Comparison with the unperturbed chromosphere showed that `dark points' in Hei 10830 Å are more optically thin than the nearby chromosphere.     

EUV Full-Sun Imaged Spectral Atlas Using the SOHO Coronal Diagnostic Spectrometer

The Coronal Diagnostic Spectrometer (CDS) aboard the Solar and Heliospheric Observatory (SOHO) carries out a regular program of measuring the full-disk irradiance using the Normal Incidence Spectrograph (NIS). The full-disk solar spectrum is returned in the wavelength bands 308–379 Å and 513–633 Å, with a spectral resolution between 0.3 and 0.6 Å. A recent modification to the CDS on-board software allows simultaneous moderate resolution monochromatic images to be made of the stronger lines in these wavelength ranges. We report on observations made 23 April 1998, 21 May 1998, and 22 June 1998. A total of 69 monochromatic full-Sun images are extracted from the spectral line data. For the first time, spectrally resolved images of the full Sun in Heii 303.8 Å and Sixi 303.3 Å are presented and compared. Velocity maps of the Sun in singly ionized helium are presented. Correlations of intensity to velocity over a wide range of transition region and coronal temperatures are shown. Lines from Hei to Fexiv show statistical red shifts of 1–7 km s^–1 between active regions and quiet Sun areas. Velocity maps of Mgix andx are presented, showing strong upflow and downflow regions associated with active regions, but not correlated with the brightest emission. Changes in line width are also presented in Hei, with discussion of similar features in other lines of comparable temperature. Corrections which need to be applied to CDS/NIS data to extract meaningful velocities and line widths are presented and discussed. The identifications of the lines in the CDS spectrum are examined. The spatial and spectral variation of the background component of the CDS spectrum is examined.     

SWRI/LASP Sounding Rocket Inter-Calibration With The Eit Instrument On Board SOHO

Two successful sounding rocket flights were launched on 15 May 1997 and 2 November 1998 with an objective of providing inter-calibration with several of the instruments on board SOHO and TRACE. We will discuss here the results of the inter-calibration between the SwRI/LASP rocket imaging instruments and the Extreme-ultraviolet Imaging Telescope (EIT) on SOHO. The Multiple XUV Imager (MXUVI) sounding rocket instrument is a multi-layer mirror telescope equipped with an internal occulter and light trap to provide full-disk images of Feix/x 17.1 nm and off-limb observations of Heii 30.4 nm. The SOHO/EIT instrument is also a full-disk multi-layer imager with four channels, Fe ix/x 17.1 nm, Fexii 19.5 nm, Fexv 28.4 nm and Heii 30.4 nm. By comparison with the EIT observations taken at the same time, we provide new flat-field determinations for EIT which help quantify the sensitivity degradation of the EIT detector, as well as provide a measure of the off-limb stray-light characteristics of the two instruments. We find that the EIT stray-light function is strongly asymmetric, with greater stray light in the 17.1 and 19.5 nm quadrants than the 30.4 and 28.4 nm quadrants. Two possible causes of this asymmetry are the polishing processes of the EIT mirrors and the asymmetric support grid pattern in the foil mesh at the EIT pupil.     

Multi-Wavelength Observation of the 3n/x3.3 Flare of 28 November 1998

The 3N/X3.3 flare of 28 November 1998 was observed in multiple wavelength simultaneously. The available data include H images, spectra in Hei 1083 nm and Caii 854.2 nm from Purple Mountain Observatory (PMO), soft X-ray (SXR) and hard X-ray (HXR) images and flux from Yohkoh. Morphological relationship investigation and spectral analysis of these data show: (1) The sudden brightening at loop top above the active region and the steep increase of SXR flux before flare onset suggest that the corona there had already been heated to some extent in the preflare phase. (2) The scales of the Caii 854.2 nm emission areas are very similar to those of the H line, but the emission profiles look like those of the Caii K line. Most of the Hei 1083 nm emissions exist in the bright H kernels and can last to the decay phase. (3) Flare spectra show that line shift and asymmetry are very common in this flare not only in the impulsive phase but also in the decay phase. The difference in the line shifts or asymmetry between Caii 854.2 nm and Hei 1083 nm, as well as the difference between the line center and wings of Caii 854.2 nm imply the existence of a velocity gradient in the line-of-sight direction. (4) Post-flare loops with very deep absorption (70%) and very-high-velocity red shifts (30–90 km s^–1) were observed in Hei 1083 nm during the decay phase. However, only a slight dip can be found in the Caii 854.2 nm profile.     

A comparison between photospheric and chromospheric quiet-Sun magnetograms

Photospheric (Fei 5324.19 Å line) and chromospheric (H line) magnetic fields in quiet-Sun regions have been observed in the solar disk center by using the vector video magnetograph at Huairou Solar Observing Station of Beijing Astronomical Observatory. Observational results show that the quiet-Sun magnetic elements in the solar photosphere and chromosphere present similar magnetic structures. Photospheric and chromospheric magnetograms show corresponding time variations. This suggests that the magnetic fields in quiet-Sun regions present different 3-D magnetic configurations compared to those in solar active regions.     

Earth Tides and Lense–Thirring Effect

The general relativistic Lense—Thirring effect can be measured by inspecting a suitable combination of the orbital residuals of the nodes of LAGEOS and LAGEOS II and the perigee of LAGEOS II. The solid and ocean Earth tides affect the recovery of the parameter by means of which the gravitomagnetic signal is accounted for in the combined residuals. Thus an extensive analysis of the perturbations induced on these orbital elements by the solid and ocean Earth tides is carried out. It involves the l=2 terms for the solid tides and the l=2,3,4 terms for the ocean tides. The perigee of LAGEOS II turns out to be very sensitive to the l=3 part of the ocean tidal spectrum, contrary to the nodes of LAGEOS and LAGEOS II. The uncertainty in the solid tidal perturbations, mainly due to the Love number k ₂, ranges from 0.4% to 1.5%, while the ocean tides are uncertain at 5–15% level. The obtained results are used in order to check in a preliminary way which tidal constituents the Lense-Thirring shift is sensitive to. In particular it is tested if the semisecular 18.6-year zonal tide really does not affect the combined residuals. It turns out that, if modeled at the level of accuracy worked out in the paper, the l=2,4 m=0 and also, to a lesser extent, the l=3, m=0 tidal perturbations cancel out.This revised version was published online in October 2005 with corrections to the Cover Date.     

Wavelength Shifts in the Solar Photospheric Spectrum

Wavelength shifts converted to velocities between solar lines observed at disc center and laboratory wavelengths of Fei, Feii, Tii, Nii, and Fei lines in the near infrared are plotted as a function of the logarithm of their solar equivalent width in milliångstroms. The need for wavelengths based on the wavelength standards is stressed. A comparison of photographic Fei solar wavelength is shown to agree, on the average, with Fourier Transform Spectrometer solar wavelengths within less than 0.5 milliångstroms. Using Balthasar's limb effect tables we convert the disc center velocities to limb velocities and find, though the scatter is large, that there is little evidence for a super-gravitational red shift.     

Making sense of sunspot decay – II. Deviations from the Mean Law and Plage Effects

In a statistical analysis of Debrecen Photoheliographic Results sunspot area data we find that the logarithmic deviation (logD) of the area decay rate D from the parabolic mean decay law (derived in the first paper in this series) follows a Gaussian probability distribution. As a consequence, the actual decay rate D and the time-averaged decay rate are also characterized by approximately lognormal distributions, as found in an earlier work. The correlation time of (logD) is about 3 days. We find a significant physical anticorrelation between (logD) and the amount of plage magnetic flux of the same polarity in an annulus around the spot on Kitt Peak magnetograms. The anticorrelation is interpreted in terms of a generalization of the turbulent erosion model of sunspot decay to the case when the flux tube is embedded in a preexisting homogeneous plage field. The decay rate is found to depend inversely on the value of this plage field, the relation being very close to logarithmic, i.e., the plage field acts as multiplicative noise in the decay process. A Gaussian probability distribution of the field strength in the surrounding plage will then naturally lead to a lognormal distribution of the decay rates, as observed. It is thus suggested that, beside other multiplicative noise sources, the environmental effect of surrounding plage fields is a major factor in the origin of lognormally distributed large random deviations from the mean law in the sunspot decay rates.     

Infrared Imaging Solar Spectrograph At Purple Mountain Observatory

Since 1986, we have made some improvements to the multichannel solar spectrograph at Purple Mountain Observatory (PMO) step by step, and now we have developed and added to it a multichannel infrared imaging solar spectrograph. The original spectrograph can be used to observe simultaneously solar activity at 9 wave bands including Caii H and K line, Mgi b line, Hei D3 line and H through H. The newly developed infrared imaging spectrograph can work in three wavelengths, i.e., Hei 10830 Å, Caii 8542 Å, and H. We replaced plates in the original system with CCDs and placed an image reducer before each CCD in order to match the CCD pixel size. The dispersions for Hei 10830 Å, Caii 8542 Å, and H of the new imaging solar spectrograph are 0.0693 Å, 0.0767 Å, and 0.0754 Å per CCD pixel respectively, and each vertical CCD pixel represents 0.34 arc sec of solar disk. We can obtain the line-center and off-band intensities of the three lines and the intensities of continua adjacent to these lines through the new instrument. We can also acquire velocity maps and line profiles. Therefore, it is specially suitable for two-dimensional (2D) spectroscopic observations of solar flares and active regions. We carry out scanning observation by rotating the second mirror of the coelostat system. In this paper, we introduce the improvements we made and the new imaging solar spectrograph. Some observation results are also presented in this article.