Nicole Mein

A Study of Hydrogen Density in Emerging Flux Loops from a Coordinated Transition Region and Coronal Explorer and Canary Islands Observation Campaign

During an international ground-based campaign in the Canary Islands coordinated with space instruments (i.e., Transition Region and Coronal Explorer [TRACE]), we observed an active region on 1998 September 10 with high spatial and temporal resolution. New emerging flux in the central part of the active region was observed in magnetograms of the Swedish Vacuum Solar Telescope, La Palma. Emerging loops (arch-filament systems [AFSs]) are well developed in Hα and Ca II according to the observations made at the Vacuum Tower Telescope (VTT) and THEMIS telescope in Tenerife with the Multichannel Subtractive Double Pass (MSDP) spectrographs. The TRACE images obtained at 171 and 195 A show low-emission regions that are easily identified as the individual AFS. They are due to absorption by hydrogen and helium continua in the cool filament plasma. We compare two techniques of measuring the hydrogen density in the cool dense fibrils of AFSs. The first method based on TRACE observations derived the neutral hydrogen column density of the plasma absorbing coronal lines. The second one using Hα line profiles provided by the MSDP spectrographs is based on the cloud model. The results are consistent. We derive also electron density values using Hα lines that are in good agreement with those derived from the 8542 A Ca II line observed with THEMIS (Mein et al.). The three types of observations (TRACE, VTT, THEMIS) are well complementary: absorption of coronal lines giving a good approximation for the maximum value of the neutral hydrogen column density, the Hα line giving a good determination of ne, and the 8542 A Ca II line a good determination of the electronic temperature.

Velocity waves in the quiet solar chromosphere

Propagation of velocity waves are investigated in the solar chromosphere, with a special view to high frequencies (periods ≥ 60 s). Four line profiles have been observed during 27 mn with the Sacramento Peak vacuum telescope (Hα, 3933, 8498 and 8542 Ca ii). Three Fourier analysis are performed according to the location in the cells of the chromospheric network. Phase-shifts and amplitude ratios between the line Doppler shifts are computed as functions of frequency. The “pollution” of high frequency results by energetic low frequency oscillations is investigated.Hα Doppler shifts are probably affected by the large width of line formation layers (low “transfer function’). Using “formation altitudes for Doppler shifts” previously computed for the infra-red lines, we show that acoustic waves propagating upwards cannot account for the observations. In particular, the phase-shifts between oscillations in different chromospheric layers are much smaller than theoretical predictions. As a first attempt for a qualitative agreement, we suggest that most of the high frequency oscillations (10–15 mHz) are magnetoacoustic waves, travelling in layers where the gradient of the Alfvén-speed cannot be neglected, and reflected at the top of the chromosphere. The amplitudes of these waves are probably underestimated as derived from the observed Doppler shifts.

The hydrogen balmer lines and the structure of the quiet solar chromosphere

AbstractSpectra of Hα, Hβ and Hδ have been taken under good seeing conditions with the vacuum tower telescope of Sacramento Peak Observatory. Intensity curves are given at various wavelengths in these lines to permit further comparison with a theoretical model. Moreover, considering in each case the range of height in which the lines are almost optically thin and using a few approximations, the following results are derived: between 2000 and 6000 km above the limb the average thermal + turbulent velocity of the atoms is found to increase from 20 km s−1 to 30 km s−1 and the mean number of hydrogen atoms per cm3 in level 2 is given by

EVOLUTION IN SPACE AND TIME OF SUPERPENUMBRAL CHROMOSPHERIC FIBRILS

\log n_2 {\text{ = }}4.5{\text{ }} - {\text{ 0}}{\text{.00056(}}z - 2000)

Magnetic changes observed in the formation of two filaments in a complex active region: TRACE and MSDP observations

z being the altitude above the limb in km. For line profile computations a new interpolation formula is presented; it gives good profiles with a small number of scans, saving microphotometer time.

Understanding the Mg II and Hα spectra in a highly dynamical solar prominence

The small phase-lag between velocities observed at different chromospheric levels is interpreted as being due to acoustic waves reflected by the very hot atmospheric layers of the chromosphere-corona transition zone. We consider first an isothermal slab, then a realistic solar atmospheric model and calculate weighting functions for velocities in Ca ii lines. It is shown that taking into account these functions and integrating over horizontal wave numbers leads to a good agreement with previous observations (Mein, 1977) in the case of 8498 and 8542 Ca ii lines. For the K line, the less good agreement shows that magnetoacoustic waves become important in the upper chromospheric layers.

The Role of

We have studied the spatial structure and temporal evolution of the intensity and Doppler velocity of dark fibrils forming the superpenumbra of an isolated regular sunspot. The observations were obtained with the Multichannel Subtractive Double Pass (MSDP) spectrograph which operates in Hα and is installed at the focus of the Vacuum Tower Telescope (VTT) at Tenerife (Canary Islands). The fibril pattern shows a remarkable stability during the period of our observations (64 min). Moreover, almost all individual fibrils are identifiable in all frames, but they undergo continual changes in contrast, shape and size. Investigating the temporal evolution of intensity and velocity of individual fibrils, fluctuations were found which have a quasi periodic behavior. As mechanisms for these changes we may suggest (a) change of the Doppler shift due to a wave, (b) periodic changes of the density of the Hα absorbing material, (c) disappearance and reappearance of fibrils, in more or less the same magnetic flux tube, at regular intervals.

Dynamics of post-flare ejections and magnetic loop geometry

Ground-based optical observations coordinated with Yohkoh/SXT of an old, disintegrating bipolar active region AR NOAA 7493 (May 1, 1993) provided a multiwavelength (magnetic fields, Hα and X-ray) data base for the study of a flaring “active region” X-ray bright point (XBP) of about 16 hours lifetime, and of the activity related to it in different layers of the solar atmosphere (van Driel-Gesztelyi et al., 1995).