The quiet Sun average Doppler shift of coronal lines up to 2 MK (original) (raw)
Related papers
SERTS‐95 Measurements of Wavelength Shifts in Coronal Emission Lines Across a Solar Active Region
The Astrophysical Journal, 1999
We used slit spectra from the 1995 Ñight of Goddard Space Flight CenterÏs Solar EUV Rocket Telescope and Spectrograph (SERTS-95) to measure wavelength shifts of coronal emission lines in the core of NOAA active region 7870 relative to its immediate surroundings (its "" edge ÏÏ). This method circumvents the unavailability of reliable laboratory rest wavelengths for the observed lines by using wavelengths from the edge spectrum as references. We derived the SERTS-95 wavelength calibration from measurements of a post-Ñight laboratory spectrum containing 28 He II and Ne II EUV standard wavelengths known to high accuracy. Wavelength measurements for lines of He I, Ne III, and additional lines of Ne II in the laboratory calibration spectrum provide more accurate values than were previously available, enabling these lines also to serve as future calibration standards. Six solar lines were chosen for this study, namely, He II at 303.78 Fe XII at 193.51 Fe XIII at 202.05 Fe XIV at 211.33 Fe XV at A , A , A , A , 284.15 and Fe XVI at 335.41 Because these lines are free from known blends in the SERTS-95 A , A. spectra and are either intrinsically strong or near the SERTS-95 peak sensitivity, they are our most reliable lines for measuring relative wavelength shifts in the spatially resolved active-region core spectra. The iron ions are the hottest ions ever used for this type of analysis. All six lines reveal statistically signiÐcant spatial variations in their measured relative wavelength shifts in the active-region core, including mixtures of blueshifts and redshifts (each with maximum values corresponding to relative Doppler velocities D15 km s~1), indicating a dynamic, turbulent corona. For each of these lines we calculated weightedaverage relative Doppler velocities from the wavelength shifts in the spatially resolved core spectra by weighting the shifts in the individual spatial pixels with their respective measurement uncertainties. This yields velocities of 3.3^1.1 km s~1 for He II, 5.2^1.6 km s~1 for Fe XII, 0.7^1.5 km s~1 for Fe XIII, [2.1^1.4 km s~1 for Fe XIV, 1.0^1.1 km s~1 for Fe XV, and [ 1.1^0.8 km s~1 for Fe XVI. We also calculated intensity-weighted relative Doppler velocities from the wavelength shifts in the spatially averaged core spectrum, obtaining corresponding values of 5.8^0.6 km s~1, 5.7^0.9 km s~1, 0.4^0.7 km s~1, [ 2.1^1.0 km s~1, 0.8^0.8 km s~1, and [ 1.1^0.5 km s~1. Combining the above six lines with several additional ones that are strong enough in both the edge and average core spectra to provide reliable centroid measurements, we Ðnd statistically signiÐcant net relative redshifts for lines of He II, Fe X, Fe XI, and Fe XII ; lines of Fe XIII and Fe XV show no signiÐcant shift while lines of Si XI, Fe XIV, and Fe XVI show a small net relative blueshift. Where multiple lines are available for a given ion, the directions (red or blue) and magnitudes (except for Fe XI) of the relative shifts are mutually consistent. The net relative blueshift observed in the hottest active-region coronal lines in our sample, combined with the net relative redshift observed in the cooler active-region coronal lines, suggests a net upÑow of heated material cospatially and cotemporally with a net downÑow of cooler material.
Journal of Geophysical Research, 1995
Polar coronal holes are relatively stable structures persisting over many solar rotations. The appearance of coronal holes in remote observations, however, can change on a daily basis due to variations of the denser and hotter plasma surrounding them. We explore the effect of these denser and hotter surrounding regions on coronal hole observations, using daily intensity measurements at 1.15 Rs of the green Fe XIV 5303 .l and red Fe X 6374 .l spectral lines. The observations, which were carried out at the National Solar Observatory at Sacramento Peak, New Mexico, cover a time period of about four solar rotations. We show that the "coronal hole" temperatures derived using the line ratio technique, vary by more than 0.8 x 10 • K over the time interval considered here. We also provide a short discussion of the expected accuracy of the atomic data for these two iron spectral lines. Using intensity measurements as a function of distance from the Sun, we briefly discuss how the regions surrounding the coronal holes might influence the inference of the temperature gradient in the coronal holes. The line of sight effect on the temperature gradient should be explored in more detail in the future using daily observations of the line intensities as a function of distance. These observations could be provided by ground-based coronagraphs and by instruments on board SOHO. ory predicts that the proton flux density increases almost exponentially with the mean temperature in the inner corona for a fixed density at the coronal base. The velocity on the other hand is much less sensitive to the coronal temperature. The observed values of the solar wind proton flux limits the range of possible temperatures in the inner corona to 0.8 < T < 2 x 106 K for most models if the density at the base of the corona is assumed to vary in the range 107 to 10 ø cm -a [e.g., Leer and Holzer, 1979; Esser et al., 1986]. If the variation Copyright 1995 by the American Geophysical Union. Paper number 95JA00594. 0148-0227/95/95 JA-00594505.00
On the ultraviolet signatures of small scale heating in coronal loops
Astronomy and Astrophysics, 2008
Aims. Studying the statistical properties of solar ultraviolet emission lines could provide information about the nature of small scale coronal heating. We expand on previous work to investigate these properties. We study whether the predicted statistical distribution of ion emission line intensities produced by a specified heating function is affected by the isoelectronic sequence to which the ion belongs, as well as the characteristic temperature at which it was formed (as found previously). Particular emphasis is placed on the strong resonance lines belonging to the lithium isoelectronic sequence. Predictions for emission lines observed by existing space-based UV spectrometers are given. The effects on the statistics of a line when observed with a wide-band imaging instrument rather than a spectrometer are also investigated. Methods. We use a hydrodynamic model to simulate the UV emission of a loop system heated by nanoflares on small, spatially unresolved scales. We select lines emitted at similar temperatures but belonging to different isoelectronic groups: Fe ix and Ne viii, Fe xii and Mg x, Fe xviii, Fe xix and Fe xxiv. Results. Our simulations confirm previous results that almost all lines have an intensity distribution that follows a power-law, in a similar way to the heating function. However, only the high temperature lines best preserve the heating function's power law index (Fe xix being the best ion in the case presented here). The Li isoelectronic lines have different statistical properties with respect to the lines from other sequences, due to the extended high temperature tail of their contribution functions. However, this is not the case for Fe xxiv which may be used as a diagnostic of the coronal heating function. We also show that the power-law index of the heating function is effectively preserved when a line is observed by a wide-band imaging instrument rather than a spectromenter.
The Astrophysical Journal, 1998
Recent observations of the spectral line profiles and intensity ratio of the O VI 1032Å and 1037.6Å doublet by the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO), made in coronal holes below 3.5 R s , provide evidence for Doppler dimming of the O VI 1037.6Å line and pumping by the chromospheric C II 1037.0182Å line. Evidence for a significant kinetic temperature anisotropy of O 5+ ions was also derived from these observations. We show in this Letter how the component of the kinetic temperature in the direction perpendicular to the magnetic field, for both isotropic and anisotropic temperature distributions, affects both the amount of Doppler dimming and pumping. Taking this component into account, we further show that the observation that the O VI doublet intensity ratio is less than unity can be accounted for only if pumping by C II 1036.3367Å in addition to C II 1037.0182Å is in effect. The inclusion of the C II 1036.3367Å pumping implies that the speed of the O 5+ ions can reach 400 km/s around 3 R s which is significantly higher than the reported UVCS values for atomic hydrogen in polar coronal holes. These results imply that oxygen ions flow much faster than protons at that heliocentric distance.
The Astrophysical Journal, 2011
We report on the first multi-wavelength coronal observations, taken simultaneously in white light, Hα 656.3 nm, Fe ix 435.9 nm, Fe x 637.4 nm, Fe xi 789.2 nm, Fe xiii 1074.7 nm, Fe xiv 530.3 nm, and Ni xv 670.2 nm, during the total solar eclipse of 2010 July 11 from the atoll of Tatakoto in French Polynesia. The data enabled temperature differentiations as low as 0.2 × 10 6 K. The first-ever images of the corona in Fe ix and Ni xv showed that there was very little plasma below 5 × 10 5 K and above 2.5 × 10 6 K. The suite of multi-wavelength observations also showed that open field lines have an electron temperature near 1×10 6 K, while the hottest, 2×10 6 K, plasma resides in intricate loops forming the bulges of streamers, also known as cavities, as discovered in our previous eclipse observations. The eclipse images also revealed unusual coronal structures, in the form of ripples and streaks, produced by the passage of coronal mass ejections and eruptive prominences prior to totality, which could be identified with distinct temperatures for the first time. These trails were most prominent at 10 6 K. Simultaneous Fe x 17.4 nm observations from Proba2/SWAP provided the first opportunity to compare Fe x emission at 637.4 nm with its extreme-ultraviolet (EUV) counterpart. This comparison demonstrated the unique diagnostic capabilities of the coronal forbidden lines for exploring the evolution of the coronal magnetic field and the thermodynamics of the coronal plasma, in comparison with their EUV counterparts in the distance range of 1-3 R . These diagnostics are currently missing from present space-borne and ground-based observatories.
Journal of Astrophysics and Astronomy, 2006
We have obtained spectroscopic observations in coronal emission lines by choosing two lines simultaneously, one [Fe x] 6374 Å and the other [Fe xi] 7892 Å or [Fe xiii] 10747 Å or [Fe xiv] 5303 Å. We found that in 95 per cent of the coronal loops observed in 6374 Å, the FWHM of the emission line increases with height above the limb irrespective of the size, shape and orientation of the loop and that in case of 5303 Å line decreases with height in about 89 per cent of the coronal loops. The FWHM of 7892 Å and 10747 Å emission lines show intermediate behavior. The increase in the FWHM of 6374 Å line with height is the steepest among these four lines. We have also studied the intensity ratio and ratio of FWHM of these lines with respect to those of 6374 Å as a function height above the limb. We found that the intensity ratio of 7892 Å and 10747 Å lines with respect to 6374 Å line increases with height and that of 5303 Å to 6374 Å decreases with height above the limb. This implies that temperature in coronal loops will appear to increase with height in the intensity ratio plots of 7892 Å and 6374 Å; and 10747 Å and 6374 Å whereas it will appear to decrease with height in intensity ratio of 5303 Å to 6374 Å line versus height plot. These findings are up to a height of about 200 arcsec above the limb. The varying ratios with height indicate that relatively hotter and colder plasma in coronal loops interact with each other. Therefore, the observed increase in FWHM with height above the limb of coronal emission lines associated with plasma at about 1 MK may not be due to increase in non-thermal motions caused by coronal waves but due to interaction with the relatively hotter plasma. These findings also do not support the existing coronal loop models, which predict an increase in temperature of the loop with height above the limb.
SUMER observations of Doppler shift in the quiet Sun and in an active region
Astronomy and Astrophysics, 1999
The UV spectral lines formed at transition region temperatures in the solar atmosphere, show a prevailing redshifted emission. Using the Solar Ultraviolet Measurements of Emitted Radiation spectrometer flown on the Solar and Heliospheric Observatory spacecraft, we measure the amount of line shift as a function of the temperature for several spectral lines formed in the range between 10 4 and 10 6 K. We analyze spectrograms relative to the quiet Sun and to the active region NOAA 7946. The velocities derived are increasing from a redshift of ∼ 0 km s −1 at ∼ 20000 K to 10 km s −1 at 1.9 10 5 K for the quiet Sun, and to ∼ 15 km s −1 at 10 5 K for the active region. At higher temperature an opposite behaviour is observed. In the quiet Sun a blueshift of ∼ −2 km s −1 is observed at the Ne viii formation temperature (6.3 10 5 K), while in the active region, a blue-shifted value around −8 km s −1 is observed for the same spectral line. The finding of blueshift in Ne viii is due to the adoption of a new rest wavelength of 770.428Å. By 10 6 K the blueshift is ∼ −10 km s −1 in the active region as measured by Fe xii 1242.
The Slowly Varying Corona. II. The Components of F 10.7 and Their Use in EUV Proxies
The Astrophysical Journal
Using four years of full-disk-integrated coronal differential emission measures calculated in Schonfeld et al. (2017) we investigate the relative contribution of bremsstrahlung and gyroresonance emission in observations of F 10.7 , the 10.7 cm (2.8 GHz) solar microwave spectral flux density and commonly used activity proxy. We determine that the majority of coronal F 10.7 is produced by the bremsstrahlung mechanism, but the variability observed over individual solar rotations is often driven by gyroresonance sources rotating across the disk. Our analysis suggests that the chromosphere may contribute significantly to F 10.7 variability and that coronal bremsstrahlung emission accounts for 14.2 ± 2.1 sfu (∼ 20%) of the observed solar minimum level. The bremsstrahlung emission has a power-law relationship to the total F 10.7 at high activity levels, and this combined with the observed linearity during low activity yields a continuously differentiable piecewise fit for the bremsstrahlung component as a function of F 10.7. We find that the bremsstrahlung component fit, along with the Mg II index, correlates better with the observed 5-37 nm spectrum than the common 81 day averaged F 10.7 proxy. The bremsstrahlung component of F 10.7 is also well approximated by the moderate-strength photospheric magnetic field parameterization from Henney et al. (2012), suggesting that it could be forecast for use in both atmospheric research and operational models.
The Sun-Spectroscopic Observations of the Coronal Emission Lines and ADITYA-1 Mission
Journal of Mountain Research, 2019
The sun and the solar atmosphere, known as solar corona have been studied in detail for centuries but still lot need to be understood about the sun. In the late 19th and earlier 20th century, the information gathered about the solar corona was from the brief moments available during the total solar eclipses. In the later part space observations provided a large body of information about the solar corona. We obtained systematic high resolution spectroscopic observations in [Fe x] 637.4 [Fe xi] 789.2, [Fe xiii] 1074.7, and [Fe xiv] 530.3 nm emission lines during the period of 1997-2007 with the 25-cm coronagraph at Norikura, Japan. The variation in line widths with height above the solar limb implies that one pair of lines indicate that top of coronal loops are hotter where as other pair of lines shows that loop top is cooler. To explain these results, we proposed an empirical model as all these results are difficult to explain using the existing models. With this background we planne...
Comments on the observability of coronal variations
Solar Physics, 1989
We discuss the observable variability of spectral lines in the soft X-ray and XUV region. Rapid variability of coronal emission, both in flaring and non-flaring structures has been reported and is particularly prominent when high spatial resolution is available. Examination of the ionization and recombination timescales for the formation and removal of ions with prominent solar emission lines shows that, even though ionization equilibrium generally prevails, the observable variability timescales are often limited by these atomic processes, independent of the physical process which is causing the change in the solar atmosphere. Rapid heating can lead to an initial freezing-in of abundances of some ions; observations of at least one low-and one high-excitation line from such an ion would permit studies of the time evolution of the emission measure and temperature. In a very limited number of cases, rapid cooling leads to freezing-in of the abundance of an ion and observations of a low,excitation line of this ion will not yield accurate information about the thermal evolution. Thus, future observations of Mgx 609 ,~ should be augmented by simultaneous observation at another wavelength, such as 63 ,~. In addition, with the ability to produce images in isolated spectral lines it becomes possible to select those for which rapid variability is observable, such as OvII, rather than lines which were selected on the basis of previous hardware constraints, such as OVIII.