Showing papers on "Solar transition region published in 2003"

Dynamics of solar coronal loops I. Condensation in cool loops and its effect on transition region lines

Abstract: We report numerical calculations of the condensation of plasma in short coronal loops, which have several interesting physical consequences. Firstly, we propose a connection between small, cool loops (T < 10 6 K), which constitute one of the basic components of the solar transition region, and prominences, in the sense that the same physical mechanism governs their dynamics: Namely the onset of instability and runaway cooling due to strong radiative losses. Secondly, we show that the temporal evolution of these loop models exhibits a cyclic pattern of chromospheric evaporation, condensation, motion of the condensation region to either side of the loop, and finally loop reheating with a period of 4000-8000 s for a loop of 10 Mm length. Thirdly, we have synthesized transition region lines from these calculations which show strong periodic intensity variations, making condensation in loops a candidate to account for observed transient brightenings of solar transition region lines. Remarkably, all these dynamic processes take place for a heating function which is constant in time and has a simple exponential height dependence.

MHD shocks and the origin of the solar transition region.

Abstract: Simultaneous observations of the solar atmosphere from its surface to the corona obtained with the Solar and Heliospheric Observatory (SOHO) and Transition Region and Coronal Explorer (TRACE) show a ubiquitous sequence of events that start from cancellation of photospheric magnetic fields, pass through shock formation, and result in transition region supersonic jets and microflares. These results support a novel view of the energy buildup in the solar atmosphere associated with a cascade of shock waves produced by interacting network magnetic elements in the photosphere and provide insight into the origin of the solar transition region. The findings account for the general mechanisms of energy production, transfer, and release throughout the Sun's and stellar atmospheres.

Extreme ultraviolet emission lines of Ar xiv in solar active region and flare spectra

Abstract: New R-matrix calculations of electron impact excitation rates for transitions among the 2s 2 2p, 2s2p 2 and 2p 3 levels of Ar XIV are presented. These data are subsequently used to derive the theoretical electron density diagnostic emission-line intensity ratios R 1 = I(187.95 A)/I(194.41 A) and R 2 = I(257.40 A)/I(243.78 A) for a range of densities (N e = 10 9 -10 1 3 cm - 3 ) and electron temperatures (T e = 10 6 . 3 -10 6 . 7 K) appropriate to solar transition region and coronal plasmas. A comparison of these diagnostics with observational data for solar active regions and flares, obtained with the Naval Research Laboratory's S082A spectrograph on board Skylab, reveals that the electron densities determined from R 1 are in good agreement with those estimated from line ratios in Fe XIV or Fe xv, which are formed at similar electron temperatures to Ar XIV. However, there are large discrepancies between densities inferred from the R 2 ratio and those from Fe xiv or Fe xv, confirming that the Ar XIV 243.78- and 257.40-A lines are badly blended with Fe xv 243.79 A and Fe XIV 257.38 A, respectively. Hence, R 2 cannot be employed as a density diagnostic, in contrast to R 1 , which does provide reliable N e estimates.

Temperature Measurements in the Solar Transition Region Using N III Line Intensity Ratios

Abstract: UV emission from B-like N and O ions a rather rare opportunity for recording spectral lines in a narrow wavelength range that can potentially be used to derive temperatures relevant to the solar transition region. In these ions, the line intensity ratios of the type (2s2p(sup 2) - 2p(sup 3)) / (2s(sup 2)2p - 2s2p(sup 2)) are very sensitive to the electron temperature. Additionally, the lines involving the ratios fall within a range of only - 12 A; in N III the lines fall in the 980 - 992 A range and in O IV in the 780 - 791 A range. In this work, we explore the use of these atomic systems, primarily in N III, for temperature diagnostics of the transition region by analyzing UV spectra obtained by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer flown on the Solar and Heliospheric Observatory (SOHO). The N III temperature-sensitive line ratios are measured in more than 60 observations. Most of the measured ratios correspond to temperatures in the range 5.7x10(exp 4) - 6.7x10(exp 4) K. This range is considerably lower than the calculated temperature of maximum abundance of N III, which is approx. 7.6x10(exp 4) K. Detailed analysis of the spectra further indicates that the measured ratios are probably somewhat overestimated due to resonant scattering effects in the 2s(sup 2)2p - 2s2p(sup 2) lines and small blends in the 2s2p(sup 2) - 2p3 lines. Actual lower ratios would only increase the disagreement between the ionization balance calculations and present temperature measurements based on a collisional excitation model. In the case of the O IV spectra, we determined that due to the close proximity in wavelength of the weak line (2s2p(sup 2)-2p3 transitions) to a strong Ne VIII line, sufficiently accurate ratio measurements cannot be obtained. Subject headings: atomic data --- atomic processes --- Sun: transition region --- Sun: U V radiation --- techniques: spectroscopic

Collisional filtration model in the solar transition region

Abstract: In this work we will try to show the physical reasons for the steep temperature increase in the solar transition region, trying to look for a solution of the Boltzmann equation, which seems to be the most suitable tool for describing the plasma of the solar transition region. To solve the Boltzmann equation, we have used a Monte Carlo method; as a solution of this equation we have obtained a temperature enhancement and a density drop; besides, we have computed an electron heat flux towards the innersphere, showing a departure from the classical transport theory.

On the possible source of the magnetic field dissipation in the solar plasma

Abstract: The oblique interaction of the solar chromospheric rotational or Alfven discontinuity with the transition region, described as a contact discontinuity, is studied. The appearance of the refracted magnetohydrodynamic shock wave going through the solar corona is shown. This wave makes the dissipation of the magnetic field energy inside the coronal plasma possible and may cause explosive events.