A. Fludra

Transition region oscillations above sunspots

We present observations of three sunspots made in the EUV wavelength range with the Coronal Diagnostic Spectrometer (CDS) on SOHO. We examine time series of line intensities obtained with a 15-s cadence and calculate their Fourier power spectra and the wavelet transforms. We find oscillations in the chromosphere and transition region above sunspots in the temperature range 2 10 4 -4 10 5 K. Most of the spectral power is contained in the 5-8 mHz frequency range, and the dominant frequency is 5.9 mHz (170 s period). We find a relationship between the oscillations and sunspot plumes which are compact features located above sunspots and bright in the transition region lines emitted between 1.7 10 5 and 4 10 5 K. When the CDS slit crosses a sunspot plume, the bright O IV 554.5 Aand O V 629.7 Alines show clear 3 min oscillations. An extreme alternative interpretation that excludes the presence of oscillations in sunspot plumes carries the requirement that the adjacent low-intensity plasma oscillate with high relative amplitudes greater than 50 percent. We also observe oscillations in a fainter area above a sunspot, outside the sunspot plume. The oscillations seen at different temperatures are compared. Using the wavelet transform, we find variability of the oscillation period in the range 120-200 s.

The widths of vacuum-ultraviolet spectral lines in the equatorial solar corona observed with CDS and SUMER

Observations of the solar equatorial corona between heights of 36 Mm and 184 Mm above the limb obtained by the SOHO spectrometers CDS and SUMER in December 2003 are presented and discussed with special emphasis on the widths of the spectral lines Mg  at 62.50 nm, Al  at 55.00 nm and 56.82 nm, Ca  at 55.78 nm, and Si  at 58.09 nm. SUMER observed, in addition, the lines Mg  60.98 nm, Ca  57.40 nm, Fe  124.20 nm, Fe  115.31 nm, and Ca  113.37 nm. The Si  52.11 nm line was only observed by CDS. A different behaviour of the line width of Mg  62.50 nm as a function of height above the limb had been found in studies carried out independently with both instruments at different times. It is the aim of this joint investigation to (a) study instrumental effects on line-width results; and (b) provide a thorough analysis of line profiles with altitude for the new campaign.

Radiative and magnetic properties of solar active regions - I. Global magnetic field and EUV line intensities

Context. The relationships between the photospheric magnetic flux and either the X-ray or extreme ultraviolet emission from the solar atmosphere have been studied by several authors. Power-law relations have been found between the total magnetic flux and X-ray flux or intensities of the chromospheric, transition region, and coronal emission lines in solar active regions. These relations were then used to infer the mechanism of the coronal heating. Aims. We derive accurate power laws between EUV line intensities and the total magnetic flux in solar active regions and discuss their applications. We examine whether these global power laws are capable of providing the diagnostics of the coronal heating mechanism. Methods. This analysis is based on EUV lines recorded by the Coronal Diagnostic Spectrometer (CDS) on SOHO for 48 solar active regions, as they crossed the central meridian in years 1996–1998. Four spectral lines are used: He I 584.3 A (3 × 10 4 K), O V 629.7 A (2.2 × 10 5 K), Mg IX 368.06 A (9.5 × 10 5 K), and Fe XVI 360.76 A (2.0 × 10 6 K). In particular, the Fe XVI 360.76 A line, seen only in areas of enhanced heating in active regions or bright points, has not been used before for this analysis. Results. Empirical power laws are established between the total active region intensity in the lines listed above and the total magnetic flux. We demonstrate the usefulness of some spatially integrated EUV line intensities, IT, as a proxy for the total magnetic flux, Φ, in active regions. We point out the approximate, empirical nature of the IT − Φ relationships and discuss the interpretation of the global power index. Different power index values for transition region and coronal lines are explained by their different dependence on pressure under the assumption of hydrostatic loop models. However, the global power laws are dominated by the size of the active regions, and we demonstrate for the first time the difficulties in uniquely relating the power index in the global IT − Φ relationship to the power index for individual loops and comment on results obtained by other authors. We caution against using global power laws to infer the coronal heating mechanism without a detailed knowledge of the distributions of the magnetic flux densities and instrumental response as a function of temperature. Despite these uncertainties, we show that the intensities of the transition region lines in individual loops depend on the photospheric magnetic flux density, φ, through Itr ∝ φ δt , δt 1, and under the assumption of hydrostatic loops we can place a constraint on the coronal heating models, obtaining the volumetric heating rate, EH (erg cm −3 s −1 ), EH ∝ φ γ ,w here 0.6

Inversion of the intensity-magnetic field relationship in solar active regions

We discuss the relationship between the EUV spectral line intensities and the photospheric magnetic flux in solar active regions. Since the histograms of the magnetic flux density in active region plages can be approximated by an exponential function, the equation describing how the observed total line intensity integrated over an active region area arises from the magnetic field, can be approximated by a Laplace integral. We use this property to solve an inverse problem and derive a function relating the line intensity from individual loops to the photospheric magnetic flux density at their footpoints. We propose a simple model in which the intensity of a coronal line Fe XVI 360.8 A in an individual coronal loop is proportional to the footpoint magnetic flux density to the power of δ and explore how well the value of δ is constrained by the observations. Using EUV spectra from the Coronal Diagnostic Spectrometer (CDS) on SOHO and magnetograms from SOHO Michelson Doppler Imager for 26 active regions without sunspots, we find that the value of δ depends on the magnetic flux density threshold used to define active region magnetic area. When even the weakest fields are included, we obtain δ = 1.3, where 1.0

Radiative and magnetic properties of solar active regions - II. Spatially resolved analysis of O V 62.97 nm transition region emission

Context. Global relationships between the photospheric magnetic flux and the extreme ultraviolet emission integrated over active region area have been studied in a previous paper by Fludra & Ireland (2008, A&A, 483, 609). Spatially integrated EUVline intensities are tightly correlated with the total unsigned magnetic flux, and yet these global power laws have been shown to be insufficient for accurately determining the coronal heating mechanism owing to the mathematical ill-conditioning of the inverse problem. Aims. Our aim is to establish a relationship between the EUV line intensities and the photospheric magnetic flux density on small spatial scales in active regions and investigate whether it provides a way of identifying the process that heats the coronal loops. Methods. We compare spatially resolved EUV transition region emission and the photospheric magnetic flux density. This analysis is based on the O V 62.97 nm line recorded by the SOHO Coronal Diagnostic Spectrometer (CDS) and SOHO MDI magnetograms for six solar active regions. The magnetic flux density φ is converted to a simulated O V intensity using a model relationship I(φ,L) = Cφ δ L λ , where the loop length L is obtained from potential magnetic field extrapolations. This simulated spatial distribution of O V intensities is convolved with the CDS instrument’s point spread function and compared pixel by pixel with the observed O V line intensity. Parameters δ and λ are derived to give the best fit for the observed and simulated intensities. Results. Spatially-resolved analysis of the transition region emission reveals the complex nature of the heating processes in active regions. In some active regions, particularly large, local intensity enhancements up to a factor of five are present. When areas with O V intensities above 3000 erg cm −2 s −1 sr −1 are ignored, a power law has been fitted to the relationship between the local O V line intensity and the photospheric magnetic flux density in each active region. The average power index δ from all regions is 0.4 ± 0.1 and λ = −0.15 ± 0.07. However, the scatter of intensities in all regions is significantly greater than ±3σ from the fitted model. We therefore determine for the first time an empirical lower boundary for the IOV − φ relationship that is the same for five active regions. We postulate that it represents a basal heating. Because this boundary is present in the spatially-resolved data, this is compelling proof that the magnetic field is one of the major factors contributing to the basal component of the heating of the coronal plasma. We discuss the implications for the diagnostics of the coronal heating mechanism.

Brightness Variations in the Solar Atmosphere as Seen by SOHO

We present preliminary results of a statistical analysis of the brightness variations of solar features at different levels in the solar atmosphere. We observed quiet Sun regions at disc centre using the Coronal Diagnostic Spectrometer (CDS) onboard the Solar and Heliospheric Observatory (SOHO).