Transition Zone Lines Research Articles

Complexity of the Upper Solar Atmosphere Revealed from Spectropolarimetry during a Solar Eclipse

We analyze linear polarimetric spectrum data of solar emission lines with different formation temperatures in a visible light band from 516.3–532.6 nm, obtained during the 2013 Gabon solar eclipse using the prototype Fiber Arrayed Solar Optical Telescope. Complexities are found from the chromosphere through the transition zone to the corona at the spatial resolution limit of 2″ and temporal resolution of seconds. The observations show irregular spatial and spectral variations in linear polarization amplitudes, directions, and profile shapes. Within the observational band, spectral lines with different formation temperatures can have comparable polarization amplitudes in one spatial volume but one order difference in another, and at the same spatial volume, the amplitudes can differ by one order at different lines. The polarization amplitudes do not consistently increase with elongation in local regions. The variation in the direction of the polarization along the elongation is found from the green coronal line and the transition zone line more frequently than from the chromospheric lines. Such a variation in orientation is not synchronous for the different lines. Finally, Stokes Q/I profiles of the broad lines, such as the magnesium triplet and the green coronal line, show very diverse and complicated patterns. After pixel binning, we show that some of the complexity may be caused by the integration over different polarization sources at subresolution scales and/or along the line of sight in the optically thin layers with complex geometric corrugations.

Transition zone flows observed in a coronal hole on the solar disk

view Abstract Citations (51) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Transition Zone Flows Observed in a Coronal Hole on the Solar Disk Dere, K. P. ; Bartoe, J. -D. F. ; Brueckner, G. E. ; Recely, F. Abstract A rocket flight of the NRL High Resolution Telescope and Spectrograph (HRTS) was performed to obtain ultraviolet spectra of a well-defined coronal hole on the solar system. Observed emission lines included strong transition zone lines of C IV at 1548 and 1550 A, strong chromospheric lines of C I near 1560 A, and chromospheric lines of Fe (II) at 1563 A. Continuum radiation formed in the temperature minimum was also recorded. The cumulative histogram of the C IV velocities in the coronal hole shows an average Doppler shift of 2 km/sec to the red (downflow) with 26 percent of the profiles having blueshifts (outflows) compared to 7 percent in the quiet sun. The present observations have the advantages of higher spatial resolution and a nearly absolute wavelength scale determined from narrow chromospheric lines of Si I with an accuracy of about 1 km/sec. The nonthermal velocities, derived from the C IV line widths, are only marginally larger inside the coronal hole than those in the quiet sun, thus indicating that the magnitude of small-scale motions, which may represent energy available for coronal heating and solar wind acceleration, is equal in the two regions. Publication: The Astrophysical Journal Pub Date: October 1989 DOI: 10.1086/185561 Bibcode: 1989ApJ...345L..95D Keywords: Coronal Holes; Solar Spectra; Solar Wind; Doppler Effect; Spectroheliographs; Ultraviolet Spectra; Solar Physics; SUN: CORONA; SUN: SPECTRA; SUN: TRANSITION REGION; SUN: SOLAR WIND full text sources ADS |

Evidence for extended chromospheres and transition zones in the uv spectra of FK comae stars

The chromospheres and transition zones of the fast rotating giants of the FK Comae type can be studied by analysing their ultraviolet emission line spectra. From relative line intensities, electron densities of the order of 1010to 1011cm−1 are found for the region where the Si IV emission arises. The sizes of the chromospheres and transition regions can be inferred from the emission measure distribution, and a temperature-height relation can be found on the assumption that hydrostatic equilibrium holds. We find the atmospheres of these stars to be clearly more extended than those of normal giant stars, and the flux in the higher excitation chromospheric and transition zone lines (e.g. C II, C IV, Si IV) is significantly stronger than in other stars of similar spectral type. Indeed, the location of these stars in the standard rotation-activity-correlation diagrams places them close to or even above the saturation limit for main sequence stars.

Discrete subresolution structures in the solar transition zone

During operations on the Spacelab-2 Shuttle mission, the NRL High Resolution Telescope and Spectrograph (HRTS) recorded spectra of a variety of solar features in the 1200–1700 A wavelength region which contains spectral lines and continua well suited for investigating the temperature minimum, the chromosphere and transition zone. These data show that, at the highest spatial resolution, the transition zone spectra are broken up from a continuous intensity distribution along the slit into discrete emission elements. The average dimensions of these discrete transition zone structures is 2400 km along the slit, but an analysis of their emission measures and densities shows that the dimensions of the actual emitting volume is conciderably less. If these structures are modelled as an ensemble of subresolution filaments, we find that these filaments have typical radii of from 3 to 30 km and that the cross-sectional fill factor is in the range from 10−5 to 10−2. The transport of mass and energy through these transition zone structures is reduced by this same factor of 10−5 to 10−2 which has significant consequences for our understanding of the dynamics of the solar atmosphere. Because the HRTS transition zone line profiles are not broadened by resolved large-spatial-scale solar velocity fields, the line widths of the Civ lines have been analyzed. The average line width is 0.195 A (FWHM) and requires an average nonthermal velocity of 16 km s−1 (most-probable) or 19 km s−1 (root-mean-square) which is lower than previously observed values.

Study of energy release in flares

Recent advances in the study of energy release in Flares are reviewed. Progress has been made in modelling coronal X-ray emission and the chromospheric response to energy imput. These advances are based on theoretical studies and on the comparison of complementary data obtained from spacecraft and ground-based observatories. We first review the modelling of the coronal flare derived from radio, X-ray and XUV observations. Then we summarize results on the chromospheric response to various energy imput. Observations of X-ray continuum intensity and polarization, transition zone lines and chromospheric lines do not show evidence of particle trapping by a turbulent front. Although they might be in agreement with trapping and partial precipitation. White light flares appear to result from energy deposited above the photosphere. They are probably due to electron bombardment. The implication of these results on the primary energy release process are discussed and prospects for new research are presented.

Ultraviolet spectroscopy of the chromosphere and transition zone at high spatial and temporal resolution

The third rocket flight of the NRL High Resolution Telescope and Spectrograph (HRTS) recorded UV spectra of the quiet Sun in a 10″ × 800″ region. By rastering the slit in 2′' increments, a time series with 20s resolution of two-dimensional spectra with 1′' spatial resolution was obtained. The spectrum includes strong chromospheric diagnostic lines of C I, Si I, and Fe II, transition zone lines of C IV and the continuum which is produced in the temperature minimum. Images of the network show that Fe II emission is well-correlated with dark Hα mottles, that the transition zone is produced in extended structures which are apparently continuations of the mottles, and that the strongest continuum emission is from near the base of the mottles. Time series of C I profiles show little variation in most features aside from the chromospheric jets which develop rapidly over timescales of 30s. The C IV profiles also show only gradual variations over most of the slit except for the numerous small explosive events.

Chromospheric jets - Possible extreme-ultraviolet observations of spicules

Stigmatic EUV spectra of chromospheric lines obtained by the Naval Research Laboratory High Resolution Telescope and Spectrograph (HRTS) show small arc second regions with strong (10-20 km s/sup -1/) Doppler shifts which we call ''chromospheric jets''. Because of the strong resemblance of their physical and statistical properties, such as temperature, density, velocity, size, and birthrate, to those of visible light spicules, it is highly likely that they are different manifestations of the same phenomena, although a one-to-one identification has not yet been made. The main difference is the 40 s lifetime of the chromophseric jets which is shorter than the 5 minute lifetime of visible light spicules. The chromospheric jets are found in the supergranular cell interiors where chromospheric and transition zone line intensities are weak. Data H..cap alpha.. mottles found in the cell boundaries are well correlated with intense, symmetric chromospheric EUV line profiles and are only weakly related to the chromospheric jets.

Mass motions in optically thin solar transition zone lines

Mass motions in optically thin solar transition zone lines

Transition-zone observations of rapid flare events as observed by OSO-8

The rapid dissipation of flare energy has been observed in the transition-zone line of C iv at 1548.2 A using the University of Colorado spectrometer aboard OSO-8. Impulsive brightenings have been resolved with characteristic risetimes as low as 3.5 s. One event is analyzed in detail, in which it is inferred that the electron density is greater than 2 × 1011 cm−3 at T = 60 000 K, and that the flare energy is deposited at a rate of 2 ergs cm−3 s−1 or greater. The temporal behavior of the intensity at the center of the C iv line is consistent with a non-equilibrium ionization of C iii through C v. If this event is a result of the multiple tearing mode instability as the primary energy release mechanism, then the observations indicate a pre-flare magnetic field of about 175 G.

Steady flows in the chromosphere and transition-zone above active regions as observed by OSO-8

Two years of data from the University of Colorado ultraviolet spectrometer aboard OSO-8 were searched for steady line-of-sight flows in the chromosphere and transition-zone above active regions. The most conspicuous pattern that emerges from this data set is that many sunspots show persistent blueshifts of transition-zone lines indicating velocities of about 20 km s−1 with respect to the surrounding plage areas. The data show much smaller shifts in ultraviolet emission lines arising from the chromosphere: the shifts are frequently to the blue, but sometimes redshifts do occur. Plage areas often show a redshift of the transition-zone lines relative to the surrounding quiet areas, and a strong gradient of the vertical component of the velocity is evident in many plages. One area of persistent blueshift was observed in the transition-zone above an active region filament. The energy requirement of these steady flows over sunspots is discussed.

Morphology and spatial distribution of XUV and X-ray emissions in an active region observed from Skylab

We studied the morphology and spatial distribution of loops in an active region, using coordinated observations obtained with both the S082A XUV spectroheliograph and the S056 grazingincidence X-ray telescope on Skylab. The active region loops in the temperature range 5 × 105 −3 × 106 K fall basically into two distinctive groups: the hot loops with temperatures 2–3 × 106 K as observed in coronal lines and X-rays, and the relatively cool loops with temperature 5 × 105 −1 × 106 K as observed in transition-zone lines (Ne vii, Mg ix). The brightest hot coronal loops in the active region are mostly low-lying, compact, closely-packed, and show greater stability than the transition-zone loops, which are fewer in number, large, and slender. The observed aspect ratio of the hot coronal loops is in the range of 0.1 and 0.2, which are almost two orders of magnitude larger than those for the Ne vii loops. Brief discussion of the MHD stability of the loops in terms of the aspect ratio is presented.

Densities and mass motions in transition-zone plasmas in solar flares observed from Skylab

We studied the EUV line spectra of three flare observed with the NRL slit spectrograph on Skylab. The electron densities in the flare transition-zone plasmas are determined from density-sensitive lines of Si iii and O iv. The electron densities in all three flares studied were greatest during the flare maximum with values of the order of 1012 cm−3. The density decreases by a factor of 2 to 3 in the decay phase of the flares. The intensities of EUV lines from the flare chromospheric and transition-zone plasmas all are greatly enhanced. In contrast to lines for Oi, Ci, Feii and other chromospheric ions, the lines of Oiv and Nv and other transition-zone lines are not only enhanced but also very much broadened.

Limb broadening of quiet sun transition zone lines from OSO-8 observations

Limb broadening of quiet sun transition zone lines from OSO-8 observations

Emission-line spectra of two active regions on the solar limb - 1175-1940 A

Skylab emission-line spectra (1175 to 1940 A) of two active regions on the solar limb are examined. Electron densities for the active regions are derived using the intensity ratios of selected intersystem and allowed lines. The intensity ratios in the active regions are compared with those observed previously in quiet-sun and coronal-hole regions. The behavior of coronal forbidden lines relative to transition-zone lines is discussed along with the solar continuum intensities near and above the limb. These continuum intensities are then compared with those at similar slit positions for the quiet sun and the previously studied coronal hole. The active-region data are found to be consistent with either (1) multithermal loops with the high-temperature plasma occupying the tops of the loops or (2) nearly isothermal loops with the hotter loops extending to higher altitudes.

CO fluorescence in the extreme-ultraviolet solar spectrum

Emission lines in the fourth positive system of CO have been identified in the extreme-ultraviolet solar spectrum 1540-1660 A. These lines are excited by the C IV transition-zone lines at 1548 and 1551 A. They are strong in the spectrum of a sunspot and in parts of the adjacent active region. Some of them appear as weak, broad emission features in the quiet sun.

Analysis of the emission line spectra of a solar flare observed from Skylab

The EUV emission spectra in the wavelength range 110–1900 A of the 5 September 1973 flare observed with the NRL slit spectrograph on Skylab are studied. The results are: (1) The chromospheric and transition-zone lines are greatly enhanced during the flare. In particular, the allowed lines are enhanced more than the intersystem lines. The Ni ii and P ii lines show the greatest enhancement with a factor of 800 increase in intensity. Other lines such as O i, C i, Si iii, S iii, S iv, O iv, O v, and N v show increases in intensity 10–100 times during the flare. (2) The chromospheric lines, although greatly enhanced during the flare, maintain their sharp and gaussian profiles and are not appreciably broadened. The transition zone lines, on the other hand, show a red-shifted component during the initial phase of the flare. The deduced downward velocity in the transition zone is 50 km s−1. In addition, there are large turbulent mass motions. The downward mass motion is probably caused by the pressure imbalance between the flare hot plasma at 13 × 106 K and the cooler plasma at 105 K. (3) The density of the 105 K flare plasma, as deduced from density-sensitive lines, is greater than 1012 cm-3. The depth of the 105 K plasma in the flare transition zone is only of the order of 0.1 km, giving a steep temperature gradient. Consideration of the energy balance between the conductive flux and the radiative energy losses shows that, indeed, the high density in the transition zone requires that its thickness be very small. This is a consequence of the maximum radiative efficiency at the temperature around 105 K in the solar plasma.

Emission measures, electron densities, and nonthermal velocities from optically thin UV lines near a quiet solar limb

Calibrated intensities and line widths are presented for 45 optically thin chromospheric and transition-zone UV lines observed in a quiet area near the solar limb. The observations were made over the wavelength range from 1175 to 1915 A with the slit spectrograph on the Skylab Apollo Telescope Mount. Three nonthermal-velocity components are derived from the transition-zone line widths: a main component of 23 km/s and two high-velocity components of 45 and 75 km/s, respectively, which contribute 5% to 10% of the averaged intensity. Absolute intensities are determined as a function of height, emission measures are obtained as functions of height as well as temperature, and electron densities are estimated from density-sensitive line ratios. The densities estimated from different sets of lines are found not to agree with each other, but the values obtained from the Si III multiplet between 1297 and 1301 A are considered to be the most reliable. An atmospheric model with inhomogeneous structures extending high above the solar surface is suggested.

The emission-line spectrum above the limb of a solar coronal hole - 1175-1940 A

Emission-line spectra of a coronal hole that coincided with the north pole of the sun are discussed which were obtained with a slit spectrograph aboard Skylab at positions within and above the solar white-light limb in the wavelength range from 1175 to 1940 A. Relative line intensities, line profiles, and full widths at half-maximum are presented for selected chromospheric and transition-zone lines observed above the present polar coronal hole. Average mass motions in the transition zone are determined as a function of electron temperature from the widths of the optically thin lines by assuming ionization equilibrium. The line intensities and profiles are compared with corresponding results deduced from spectra obtained above a quiet solar region. The coronal-hole spectra are found to imply an angular dependence for the source function as well as a radial dependence such that the source function is the smallest at the south pole and increases with decreasing solar latitude.

A Discussion on the physics of the solar atmosphere - A.t.m. observations on the X u.v. emission from solar flares

Far ultraviolet spectroheliograms (190-465 A) and ultraviolet spectra (900-1900 A) of selected flares obtained on board Skylab by two instruments of the Naval Research Laboratory are presented. Time sequences of three events are discussed in detail; they are compared with simultaneous X-ray measurements from the N.R.L.’s Solrad 9 and from the OSO-7 X-ray spectrometer of the University of California, San Diego. In all events, where proper observational coverage was obtained, a small kernel seems to be the source of the prime energy release of a flare. The size of this kernel is determined by the spatial resolution of the spectroheliograph, which is approximately 2". Densities in this kernel exceed 3 x 1012 cm-3. Ion temperatures in a subflare kernel are determined to be larger than 20 MK. It is very likely, that these temperatures are much higher in the flare kernel of more energetic events. Hot clouds of coronal gas at 20 MK surround the kernel. These clouds are trapped below high magnetic structures, and subsequently seem to be the energy reservoir for heating the post flare loops. During the explosive phase, strong Doppler broadening can be seen, especially in transition zone lines of the brightened plages. These can be interpreted as impact of high energetic particles guided down to the solar surface along the loops. The observations suggest that high speed mass motions have their origin in the exploding flare kernel.

Doppler wavelength shifts of transition zone lines measured in SKYLAB solar spectra

view Abstract Citations (230) References (4) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Doppler wavelength shifts of transition zone lines measured in Skylab solar spectra. Doschek, G. A. ; Feldman, U. ; Bohlin, J. D. Abstract Wavelengths of lines of the transition-zone ions Si IV, C IV, O IV, N V, and O V are observed to be redshifted relative to the wavelengths of chromospheric lines in XUV spectra obtained from the normal-incidence spectrograph on Skylab. The spectra cover the wavelength range from 1200 to 1565 A and were obtained with the slit positioned over chromospheric network and cell regions, on coronal holes, and above the limb. The network-area and coronal-hole spectra were obtained near the disk center. Only some of the spectra show redshifted transition-zone lines. The observed shifts are between 0.03 and 0.08 A, implying velocities of 15 km/s or less. The amount of wavelength shift does not always appear to be the same for lines of different ions. The shifts imply that descending plasma in the solar atmosphere produces more emission than ascending plasma at temperatures between approximately 70,000 and 200,000 K. Publication: The Astrophysical Journal Pub Date: May 1976 DOI: 10.1086/182118 Bibcode: 1976ApJ...205L.177D Keywords: Doppler Effect; Line Spectra; Red Shift; Skylab Program; Solar Spectra; Frequency Shift; Line Shape; Solar Atmosphere; Solar Corona; Spaceborne Astronomy; Solar Physics full text sources ADS |