Observed redshifts of transition region/corona lines

Solar UV observations reveal a redshifted emission at transition region temperatures, commonly interpreted as a net downflow of plasma. In earlier investigations the magnitude of the redshift has been found to increase with temperature, reaching a maximum at T=10⁵ K, and then to decrease towards higher temperatures. These observations, mostly from Skylab, suggested no significant shift of the O V line at 1218 Å formed at 2.4×10⁵ K. The variation of the downflow velocity with temperature is, however, uncertain since there are few reliable observations of lines formed at higher temperatures.Using spectrograms from the High Resolution Telescope and Spectrograph — HRTS we find an average net redshift of the O V lines at 1218 Å and 1371 Å at all locations extending from disk center to solar limb. A discrepancy between the observed flow velocity in the two lines is probably caused by uncertainty in the available laboratory wavelength of the intercombination line at 1218 Å (2s^{2 1}S₀-2s2p³P₁).The observed shift in O V is compared with corresponding measurements of lines formed at other temperatures (Si IV, C IV, N IV, O IV, and Fe XII). Large variations in the shift are found along the instrument slit. Thus, blueshifts are also observed with the sites of the largest upflow located in the sunspot umbrae and in a quiet region close to an active region.     

The extension of explosive events from the transition region to the corona

We describe the properties of high velocity events in the corona and upper transition region and propose that they are the same phenomenon as the well studied explosive events seen in the lower transition region around T=10⁵ K. Furthermore, we discuss how the SOHO spectrometers, CDS and SUMER, may be used to check this conjecture. Magnetic reconnection has been considered a strong candidate for the physical mechanism causing explosive events. We present a phenomenological model showing how some of the observed properties of explosive events may be explained by reconnection occurring in small magnetic loops.     

Theory of fragmented energy release in the Sun

The magnetic energy released inside an active region is closely related to its formation and evolution. Following the evolution of a collection of flux tubes inside the convection zone and above the photosphere we can show that many nonlinear structures (current sheets, shock waves, double layers etc.) are formed. We propose in this review that coronal heating, flares and particle acceleration are due to the interaction of the plasma with these nonlinear structures. Approaching active regions as a driven complex dynamical system we can show that several coherent ensembles of the nonlinear structures will appear spontaneously. The statistical analysis of these structures is a major problem in solar physics. We can also show that many observed large scale structures are the result of the convolution of non-observable fragmentation in the energy release process.     

Transition region line-shifts in the rebound shock spicule model

Spectral emission lines created in the solar chromosphere — corona transition region show net red-shifts. It has been proposed that this may be the result of the return of spicular material. We simulate a spicule numerically using the rebound shock model and find that the resulting hydrodynamic evolution leads to a perceived up-flow in transition region spectral lines even though the average velocity in the line forming region is directed downward. The explanation for this apparent paradox is found in the correlation between density and velocity in the waves generated by the rebound shock spicule.     

Observations of the solar wind from coronal holes

The solar wind emanating from coronal holes (CH) constitutes a quasi-stationary flow whose properties change only slowly with the evolution of the hole itself. Some of the properties of the wind from coronal holes depend on whether the source is a large polar coronal hole or a small near-equatorial hole. The speed of polar CH flows is usually between 700 and 800 km/s, whereas the speed from the small equatorial CH flows is generally lower and can be <400 km/s. At 1 AU, the average particle and energy fluxes from polar CH are 2.5×10⁸ cm^–2 sec^–1 and 2.0 erg cm^–2 s^–1. This particle flux is significantly less than the 4×10⁸ cm^–2 sec^–1 observed in the slow, interstream wind, but the energy fluxes are approximately the same. Both the particle and energy fluxes from small equatorial holes are somewhat smaller than the fluxes from the large polar coronal holes.Many of the properties of the wind from coronal holes can be explained, at least qualitatively, as being the result of the effect of the large flux of outward-propagating Alfvén waves observed in CH flows. The different ion species have roughly equal thermal speeds which are also close to the Alfvén speed. The velocity of heavy ions exceeds the proton velocity by the Alfvén speed, as if the heavy ions were surfing on the waves carried by the proton fluid.The elemental composition of the CH wind is less fractionated, having a smaller enhancement of elements with low first-ionization potentials than the interstream wind, the wind from coronal mass ejections, or solar energetic particles. There is also evidence of fine-structure in the ratio of the gas and magnetic pressures which maps back to a scale size of roughly 1° at the Sun, similar to some of the fine structures in coronal holes such as plumes, macrospicules, and the supergranulation.     

Magnetohydrodynamic simulation of a streamer beside a realistic coronal hole

Existing models of coronal streamers establish their credibility and act as the initial state for transients. The models have produced satisfactory streamer simulations, but unsatisfactory coronal hole simulations. This is a consequence of the character of the models and the boundary conditions. The models all have higher densities in the magnetically open regions than occur in coronal holes (Noci,et al., 1993).     

Signature of coronal holes and streamers in the interplanetary space

The properties of different solar wind streams depend on the large scale structure of the coronal magnetic field. We present average values and distributions of bulk parameters (density, velocity, temperature, mass flux, momentum, and kinetic and thermal energy, ratio of thermal and magnetic pressure, as well as the helium abundance) as observed on board the Prognoz 7 satellite in different types of the solar wind streams. Maximum mass flux is recorded in the streams emanating from the coronal streamers while maximum thermal and kinetic energy fluxes are observed in the streams from the coronal holes. The momentum fluxes are equal in both types of streams. The maximum ratio of thermal and magnetic pressure is observed in heliospheric current sheet. The helium abundance in streams from coronal holes is higher than in streams from streamers, and its dependences on density and mass flux are different in different types of the streams. Also, the dynamics of -particle velocity and temperature relative to protons in streams from coronal holes and streamers is discussed.     

Broadening of Fe X (6374 Å) profiles above the limb in a coronal hole

Profiles of the visible Fe X (6374 Å) coronal emission line as a function of height above the limb were obtained out to 1.16 solar radii in a coronal hole using the NSO/Sacramento Peak Observatory Coronagraph, Universal Spectrograph and a CCD camera. These are the first coronal line profiles obtained as a function of height in a coronal hole from the ground. Analysis of the line widths suggests a large component of nonthermal broadening which increases with height ranging from 40 to 60 km/s, depending upon the assumed temperature or thermal component of the profile.     

Analysis of a redshifted plasma flow over a sunspot

The ultraviolet spectrum of a redshifted plasma flow appearing over a sunspot observed during the first flight of the High Resolution Telescope Spectrograph (HRTS I) is analysed, and interpreted as a radiatively cooling plasma. For most of the lines emitted from this plasma, the assumption of ionization equilibrium during the cooling is good. However for He II (and other ions with a single electron outside of closed shells), this is not the case. Integrating differential equations for the various ionization fractions of helium and the temperature allows an approximate determination of the abundance of helium relative to other elements whose lines appear in the spectrum of the plasma flow.     

High energy cosmic-ray nuclei results on Ulysses: 2. Effects of a recurrent high-speed stream from the southern polar coronal hole

Since June 1992 the Kiel Electron Telescope on board ULYSSES measures 26-day variations of the order of 6% in the fluxes of high energy H and He. In May 1993 ULYSSES entered into the unipolar region of the southern polar coronal hole, but continued to observe similar effects: increases in the MeV proton channels due to acceleration near the shocks of the corotating interaction region and decreases in the intensity of galactic nuclei associated with the same region. Amplitude variations are presented for different magnetic rigidities and the effects are discussed in view of corotating shock development in a 3-dimensional heliospheric structure.     

Robust-augmentation nonlinear dynamic inversion control of over-actuated coaxial high-speed helicopter in transition mode

As the elevator and rudder can be used actively for control,in addition to the rotors,Coaxial High-speed Helicopters(CHHs)have the problems of control redundancy and changing control authority in the transition mode.This paper presents a robust-augmentation transitioning flight control design for a CHH under the adverse conditions of parametric uncertainties and exter-nal disturbances.First,based on control characteristic analysis,an Adaptive Filtered Nonlinear Dynamic Inversion(AFNDI)controller is proposed for the angular rate to handle the effect of unknown unstructured uncertainties and external turbulence.Theoretical analysis proves that the presented angular rate controller can guarantee steady-state and transient performance.Further-more,the attitude angle and velocity controllers are also added.Then,an Incremental-based Non-linear Prioritizing Control Allocation(INPCA)method is designed to take into account control surface transition and changing control authority,which efficiently distributes the required moments between coaxial rotors and aero-surfaces,and avoids the control reversal problem of the yaw channel.In the proposed control architecture,the low-pass filter is introduced to alleviate the adverse influence of time delay and measurement noise.Finally,the effectiveness of the pro-posed controller is demonstrated through nonlinear numerical simulations,and is compared with existing methods.Simulation results show that the proposed control law can improve both capabil-ities of disturbance rejection and fast response,and works satisfactorily for the CHH transitioning control characteristic.     

Application of shear-sensitive liquid crystal coating to visualization of transition and reattachment in compressor cascade

The present paper aims at introducing Shear-Sensitive Liquid Crystal Coating (SSLCC) technology into compressor cascade measurement for the first time and serves as a basis for better understanding of the influence from the boundary layers. Optical path layout, which is the most significant difficulty in internal flow field measurement, will be solved in this paper by self-designed image acquisition device. Massive experiments with different Mach number and incidence are conducted at a continuous subsonic cascade wind tunnel to capture the boundary layer phenomenon. Image processing methods, such as Three-Dimensional (3-D) reconstruction and Hue conversion, are used to improve the accuracy for transition position detection. The analysis of the color-images indicates that complex flow phenomena including transition, flow separation, and reattachment are captured successfully, and the effect of Mach number and incidence on the boundary layer flow is also discussed. The results show that: the Mach number has a significant effect on transition position; the incidence has little effect on transition position, but it has a great impact on the transition distance and leading-edge separation; influenced by the end-walls, the reattachment occurs in advance under positive angle of attack conditions.     

Conjugate heat transfer investigations of turbine vane based on transition models

The accurate simulation of boundary layer transition process plays a very important role in the prediction of turbine blade temperature field. Based on the Abu-Ghannam and Shaw (AGS) and c-Re h transition models, a 3D conjugate heat transfer solver is developed, where the fluid domain is discretized by multi-block structured grids, and the solid domain is discretized by unstructured grids. At the unmatched fluid/solid interface, the shape function interpolation method is adopted to ensure the conservation of the interfacial heat flux. Then the shear stress transport (SST) model, SST & AGS model and SST & c-Re h model are used to investigate the flow and heat transfer characteristics of Mark II turbine vane. The results indicate that compared with the full turbulence model (SST model), the transition models could improve the prediction accuracy of temperature and heat transfer coefficient at the laminar zone near the blade leading edge. Compared with the AGS transition model, the c-Re h model could predict the transition onset location induced by shock/boundary layer interaction more accurately, and the prediction accuracy of temperature field could be greatly improved.     

The Expansion of Coronal Plumes in the Fast Solar Wind

Coronal plumes are believed to be essentially magnetic features: they are rooted in magnetic flux concentrations at the photosphere and are observed to extend nearly radially above coronal holes out to at least 15 solar radii, probably tracing the open field lines. The formation of plumes itself seems to be due to the presence of reconnecting magnetic field lines and this is probably the cause of the observed extremely low values of the Ne/Mg abundance ratio. In the inner corona, where the magnetic force is dominant, steady MHD models of coronal plumes deal essentially with quasi-potential magnetic fields but further out, where the gas pressure starts to be important, total pressure balance across the boundary of these dense structures must be considered. In this paper, the expansion of plumes into the fast polar wind is studied by using a thin flux tube model with two interacting components, plume and interplume. Preliminary results are compared with both remote sensing and solar wind in situ observations and the possible connection between coronal plumes with pressure-balance structures (PBS) and microstreams is discussed. This revised version was published online in June 2006 with corrections to the Cover Date.