Coronal Hole Properties Observed with Sumer

We analyze SUMER spectra of 14 lines belonging to 12 ions, obtained on both sides of the boundary of polar coronal holes as well as at other locations along the limb. We compare line intensities, shifts and widths in coronal holes with values obtained in the quiet Sun. We find that with increasing formation temperature, spectral lines show an increasingly stronger blueshift in coronal holes relative to the quiet Sun at an equal heliospheric angle. The width of the lines is generally larger (by a few km/s) inside the coronal hole. Intensity measurements show the presence of the coronal hole in Ne VIII lines as well as in Fe XII, with evidence for a slightly enhanced emission in polar coronal holes for lines formed below 10⁵ K. This revised version was published online in June 2006 with corrections to the Cover Date.     

A New Morphology of Solar Activity and Recurrent Geomagnetic Disturbances: The Late-Declining Phase of the Sunspot Cycle

Certain aspects of the Sun and resulting geomagnetic disturbances can be studied better on the source surface, an imaginary spherical surface of 3.5 solar radii, than on the photospheric surface. This paper presents evidence that the Sun exhibits one of the most fundamental aspects of activities most clearly during the late-declining phase of the sunspot cycle. It is the period when 27-day average values of the solar wind speed and of geomagnetic disturbances tend to be highest during the sunspot cycle. Important findings of this study on the late-declining phase of the sunspot cycle are the following:

1. By introducing a new coordinate system, modifying the Carrington coordinates, it is shown that various solar activity phenomena, solar flares, the brightest coronal regions, and also the lowest solar wind speed region, tend to concentrate in two quadrants, one around 90^° in longitude in the northern hemisphere (NE) and the other around 270^° in longitude in the southern hemisphere (SW). For this reason, the new coordinate system is referred to as the NESW coordinate system.
2. It is shown that the above results are closely related to the fact that the neutral line exhibits a single wave (sinusoidal or rectangular) in both the Carrington coordinates and the NESW coordinate system during the late-declining phase. The shift of the neutral line configuration during successive solar rotations during the late-declining phase causes longitudinal scatter of the location of solar flares with respect to the neutral line in a statistical study. The NESW coordinate system is designed to suppress the shift, so that the single wave location is fixed and thus a ‘nest’ of solar flares emerges in the NE and SW quadrants.
3. It is also shown that the single wave is the source of the double peak of the solar wind speed and two series of recurrent geomagnetic disturbances in each solar rotation, making the 27-day average solar wind and geomagnetic disturbances highest during the sunspot cycle. The double peak is a basic feature during the late-declining phase, but is obscured by several complexities which we identified in this paper; see item 8.
4. The single wave of the neutral line configuration can be approximated by three dipole fields, one which can be represented by a central dipole (parallel or anti-parallel to the rotation axis) and two hypothetical dipoles on the photosphere. This configuration is referred to as the triple dipole model.
5. The location of the two hypothetical photospheric dipoles coincide with the two active regions (solar flares, the brightest coronal region) and also the lowest solar wind speed region in the NESW coordinate system; the lowest solar wind regions are the cause of the valleys of the double peak of the solar wind speed.
6. The two hypothetical dipole fields actually do exist at the location of the two active regions in a coarse magnetic map (5 × 5^°). The two dipoles follow the Hale–Nicholson polarity law. Thus, they are real physical entities.
7. The apparent meridional rotation of the dipolar field on the source surface during the sunspot cycle results from combined changes of both the central dipole field and of the two photospheric dipoles, although the central dipole remains axially parallel or anti-parallel. Thus, the Sun has a general field that can be represented by an axially aligned dipole located at the center of the Sun throughout the sunspot cycle, except for the sunspot maximum period when the polarization reversal occurs.
8. The complexity of recurrent geomagnetic disturbances can also be understood by having the NESW coordinate system for various solar phenomena and the relative location of the earth with respect to the solar equatorial plane.
9. As the intensity of the two dipoles decreases toward the end of the sunspot cycle, the amplitude of the single wave decreases, and the neutral line tends to align with the heliographic equator.
10. The neutral line shows a double wave structure during certain epochs of the sunspot cycle. In such a situation, it can be considered that two NESW coordinate systems are present in one Carrington coordinate, resulting in four active regions.
11. The so-called classical “sector boundary” arises when the peaks (top and bottom) of the single wave reached 90^° in latitude in both hemispheres.
12. In summary: A study of the late-declining period of the sunspot cycle is very important compared with the sunspot maximum period. In the late-declining period, the Sun shows its activities in the simplest form. It is suggested that some of the basic features of solar activities and recurrent geomagnetic disturbances that have been studied by many researchers in the past can be synthesized in a simplest way by introducing the NESW coordinate system and the triple dipole model. There is a possibility that the basic results we learned during the late phase of the sunspot cycle can be applicable to the rest of the sunspot cycle.

     

Tropospheric New Particle Formation and the Role of Ions

The Suprathermal Electron (STE) instrument, part of the IMPACT investigation on both spacecraft of NASA’s STEREO mission, is designed to measure electrons from ～2 to ～100 keV. This is the primary energy range for impulsive electron/³He-rich energetic particle events that are the most frequently occurring transient particle emissions from the Sun, for the electrons that generate solar type III radio emission, for the shock accelerated electrons that produce type II radio emission, and for the superhalo electrons (whose origin is unknown) that are present in the interplanetary medium even during the quietest times. These electrons are ideal for tracing heliospheric magnetic field lines back to their source regions on the Sun and for determining field line lengths, thus probing the structure of interplanetary coronal mass ejections (ICMEs) and of the ambient inner heliosphere. STE utilizes arrays of small, passively cooled thin window silicon semiconductor detectors, coupled to state-of-the-art pulse-reset front-end electronics, to detect electrons down to ～2 keV with about 2 orders of magnitude increase in sensitivity over previous sensors at energies below ～20 keV. STE provides energy resolution of ΔE/E～10–25% and the angular resolution of ～20° over two oppositely directed ～80°×80° fields of view centered on the nominal Parker spiral field direction.     

Observation of Transition Region Fine Structures with Soho/Sumer

We present a preliminary analysis of two quiet Sun transition region areas observed with the SOHO/SUMER spectrometer, using lines from oxygen, nitrogen and silicon. The average quiet Sun physical parameters are studied as a function of line intensity. Systematic variations of line position and width with increasing intensity are found. A large number of small-scale active points have been observed, and preliminary analysis of the physical properties and dynamics of these active features is presented. A jet-like structure, found in an active point, is also investigated and its velocity along the line of sight determined. This revised version was published online in June 2006 with corrections to the Cover Date.     

Acceleration of the High Speed Solar Wind in Coronal Holes

We outline a theory for the origin and acceleration of the fast solar wind as a consequence of network microflares releasing a spectrum of high frequency Alfvén waves which heat (by cyclotron absorption) the corona close to the Sun. The significant features of our model of the fast wind are that the acceleration is rapid with the sonic point at around two solar radii, the proton temperatures are high (~ 5 million degrees) and the minor ions are correspondingly hotter, roughly in proportion to their mass. Moreover we argue that since the energy flux needed to power the quiet corona in closed field regions is about the same as that needed to drive the fast solar wind, and also because at deeper levels (< 2 × 10⁵ K) there is no great difference in the properties of supergranules and network in closed and open field regions, the heating process (i.e., dissipation of high frequency waves) must be the same in both cases. This revised version was published online in June 2006 with corrections to the Cover Date.     

Origin of the Solar Wind: Theory

A theory is presented for the origin of the solar wind, which is based on the behavior of the magnetic field of the Sun. The magnetic field of the Sun can be considered as having two distinct components: Open magnetic flux in which the field lines remain attached to the Sun and are dragged outward into the heliosphere with the solar wind. Closed magnetic flux in which the field remains entirely attached to the Sun, and forms loops and active regions in the solar corona. It is argued that the total open flux should tend to be constant in time, since it can be destroyed only if open flux of opposite polarity reconnect, a process that may be unlikely since the open flux is ordered into large-scale regions of uniform polarity. The behavior of open flux is thus governed by its motion on the solar surface. The motion may be due primarily to a diffusive process that results from open field lines reconnecting with randomly oriented closed loops, and also due to the usual convective motions on the solar surface such as differential rotation. The diffusion process needs to be described by a diffusion equation appropriate for transport by an external medium, which is different from the usual diffusion coefficient used in energetic particle transport. The loops required for the diffusion have been identified in recent observations of the Sun, and have properties, both in size and composition, consistent with their use in the model. The diffusive process, in which reconnection occurs between open field lines and loops, is responsible for the input of mass and energy into the solar wind. This revised version was published online in August 2006 with corrections to the Cover Date.     

The importance of spectroscopy in the 80–800 å region for plasma diagnostics in the solar atmosphere

We discuss the importance of the spectral range from about 80 to 800 Å for determining physical conditions in different regions of the solar atmosphere. We give examples of line ratios that may be used to determine electron densities in quiet Sun regions, active regions, and flares. We discuss the possibility of determining electron temperatures from line ratios in the EUV.We show that profiles as well as intensities of spectral lines must be obtained for a proper interpretation of the spectra. We give approximate parameters for a solar grazing incidence spectrograph suitable for the study of the 80–800 Å wavelength region.Given as an invited review paper at the EGAS Meeting in Munich, Germany, 11–14 July 1978.     

Spectroscopic Measurement of Coronal Compositions

Although the elemental composition in all parts of the solar photosphere appears to be the same this is clearly not the case with the solar upper atmosphere (SUA). Spectroscopic studies show that in the corona elemental composition along solar equatorial regions is usually different from polar regions; composition in quiet Sun regions is often different from coronal hole and active region compositions and the transition region composition is frequently different from the coronal composition along the same line of sight. In the following two issues are discussed. The first involves abundance ratios between the high-FIP O and Ne and the low-FIP Mg and Fe that are important for meaningful comparisons between photospheric and SUA compositions and the second involves a review of composition and time variability of SUA plasmas at heights of 1.0≤h≤1.5R _⊙.     

Formation and Evolution of Corotating Interaction Regions and their Three Dimensional Structure

Corotating interaction regions are a consequence of spatial variability in the coronal expansion and solar rotation, which cause solar wind flows of different speeds to become radially aligned. Compressive interaction regions are produced where high-speed wind runs into slower plasma ahead. When the flow pattern emanating from the Sun is roughly time-stationary these compression regions form spirals in the solar equatorial plane that corotate with the Sun, hence the name corotating interaction regions, or CIRs. The leading edge of a CIR is a forward pressure wave that propagates into the slower plasma ahead, while the trailing edge is a reverse pressure wave that propagates back into the trailing high-speed flow. At large heliocentric distances the pressure waves bounding a CIR commonly steepen into forward and reverse shocks. Spatial variation in the solar wind outflow from the Sun is a consequence of the solar magnetic field, which modulates the coronal expansion. Because the magnetic equator of the Sun is commonly both warped and tilted with respect to the heliographic equator, CIRs commonly have substantial north-south tilts that are opposed in the northern and southern hemispheres. Thus, with increasing heliocentric distance the forward waves in both hemispheres propagate toward and eventually across the solar equatorial plane, while the reverse shocks propagate poleward to higher latitudes. This paper provides an overview of observations and numerical models that describe the physical origin and radial evolution of these complex three-dimensional (3-D) heliospheric structures. This revised version was published online in August 2006 with corrections to the Cover Date.     

The fundamental parameters of the central stars of eight WR ring nebulae

We describe work that has recently been completed on deriving the fundamental parameters of eight WR stars through the photoionization modelling of their surrounding nebulae using non-LTE WR flux distributions. The resulting effective temperatures range from 57 000–71 000 K for the WN4-5 stars and <30 000–42 000 K for the WN6-8 stars. The derived stellar parameters are compared with those obtained from stellar emission line modelling. We find good agreement for the hot early WN stars, indicating that the non-LTE WR flux distributions have essentially the correct shape in the crucial far-UV region. We find lower temperatures for the four cooler late WN stars, particularly for the two WN6 stars. For the nebulae surrounding these stars, we find that the model flux distributions produce too much nebular ionization. We suggest that these discrepancies arise because of the lack of line-blanketing in the WR atmospheres. For the WO1 central star of G2.4+1.4, with strong nebular He II 4686 A emission, we derive a temperature of 105 000 K, somewhat less than previous estimates. The positions of our eight WR stars on the H-R diagram are compared with the evolutionary tracks of Maeder (1990) for solar metallicity. In common with previous workers, we find that our derived luminosities are too low, giving an initial mass range of 25–40 M, below that expected for the majority of WR stars.     

X- and XUV-radiation of the solar corona

First a survey of the ionization states and emission lines of the ions existing in the corona is given. Then instruments for taking pictures of the Sun in the X- and the XUV-region as well as for measuring spectra emitted in interesting locations on the Sun are presented. Methods of plasma diagnostics, in particular for the determination of the mean temperature and the differential emission measure are described.In the following review of observations, which are related to the topic of the workshop, types of coronal structures especially coronal holes, active regions and large scale structures are described. Their relations to the photospheric magnetic fields are dealt with; methods to calculate coronal magnetic fields are briefly discussed. As for temporal variations results of the analysis of expanding X-ray arches and of structures becoming visible in the outer corona in white light are mentioned. Finally, plasma diagnostics by means of high-resolution spectra are dealt with, in particular methods for the determination of the particle density by lines of He-like ions and of the local temperature by Li-like satellites lines. Thus non-thermal random velocities and outward moving plasma can be inferred during flares.Paper presented at the IX-th Lindau Workshop The Source Region of the Solar Wind.     

Interstellar Matter and the Boundary Conditions of the Heliosphere

The interstellar cloud surrounding the solar system regulates the galactic environment of the Sun, and determines the boundary conditions of the heliosphere. Both the Sun and interstellar clouds move through space, so these boundary conditions change with time. Data and theoretical models now support densities in the cloud surrounding the solar system of n(H0)=0.22±0.06 cm−3, and n(e−)∼0.1 cm−3, with larger values allowed for n(H0) by radiative transfer considerations. Ulysses and Extreme Ultraviolet Explorer satellite He0 data yield a cloud temperature of 6400 K. Nearby interstellar gas appears to be structured and inhomogeneous. The interstellar gas in the Local Fluff cloud complex exhibits elemental abundance patterns in which refractory elements are enhanced over the depleted abundances found in cold disk gas. Within a few parsecs of the Sun, inconclusive evidence for factors of 2–5 variation in Mg+ and Fe+ gas phase abundances is found, providing evidence for variable grain destruction. In principle, photoionization calculations for the surrounding cloud can be compared with elemental abundances found in the pickup ion and anomalous cosmic-ray populations to model cloud properties, including ionization, reference abundances, and radiation field. Observations of the hydrogen pile up at the nose of the heliosphere are consistent with a barely subsonic motion of the heliosphere with respect to the surrounding interstellar cloud. Uncertainties on the velocity vector of the cloud that surrounds the solar system indicate that it is uncertain as to whether the Sun and α Cen are or are not immersed in the same interstellar cloud. This revised version was published online in June 2006 with corrections to the Cover Date.     

Coronal Hole Boundaries and their Interactions with Adjacent Regions

Coronal hole boundaries are the interfaces between regions where the coronal magnetic field contains a significant component which is open into the heliosphere and regions where the field is primarily closed. It is pointed out that there are constraints on the magnetic field which opens into the heliosphere that must be satisfied in the corona: it must come into pressure equilibrium in the high corona, and the component of the field which connects to the polar regions of the Sun must differentially rotate. A model is presented in which satisfying these constraints determines which field lines are open and which are closed, and thus where the polar coronal hole boundaries occur. Some of the consequences of this model are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Doppler scintillation measurements of the heliospheric current sheet and coronal streamers close to the Sun

Prominent enhancements in Doppler scintillation lasting a fraction of a day (solar source several degrees wide) and overlying the neutral line represent the signature of the heliospheric current sheet and the apparent interplanetary manifestation of coronal streamers near the Sun. This first detection of coronal streamers in radio scintillation measurements provides the link betweenin situ measurements of the spatial wavenumber spectrum of electron density fluctuations beyond 0.3 AU and earlier measurements deduced from radio scintillation and scattering observations inside 0.3 AU. Significant differences between the density spectra of fast streams and slow solar wind associated with the heliospheric current sheet near the Sun reinforce the emerging picture that high- and low-speed flows are organized by the large-scale solar magnetic field, and that while the contrast between solar wind properties of the two flows is highest near the Sun, it undergoes substantial erosion in the ecliptic plane as the solar wind expands.     

Solar Fine-Scale Structures. I. Spicules and Other Small-Scale, Jet-Like Events at the Chromospheric Level: Observations and Physical Parameters

Over the last two decades the uninterrupted, high resolution observations of the Sun, from the excellent range of telescopes aboard many spacecraft complemented with observations from sophisticated ground-based telescopes have opened up a new world producing significantly more complete information on the physical conditions of the solar atmosphere than before. The interface between the lower solar atmosphere where energy is generated by subsurface convection and the corona comprises the chromosphere, which is dominated by jet-like, dynamic structures, called mottles when found in quiet regions, fibrils when found in active regions and spicules when observed at the solar limb. Recently, space observations with Hinode have led to the suggestion that there should exist two different types of spicules called Type?I and Type?II which have different properties. Ground-based observations in the Ca?ii H and K filtergrams reveal the existence of long, thin emission features called straws in observations close to the limb, and a class of short-lived events called rapid blue-shifted excursions characterized by large Doppler shifts that appear only in the blue wing of the Ca?ii infrared line. It has been suggested that the key to understanding how the solar plasma is accelerated and heated may well be found in the studies of these jet-like, dynamic events. However, while these structures are observed and studied for more than 130 years in the visible, but also in the UV and EUV emission lines and continua, there are still many questions to be answered. Thus, despite their importance and a multitude of observations performed and theoretical models proposed, questions regarding their origin, how they are formed, their physical parameters, their association with the underlying photospheric magnetic field, how they appear in the different spectral lines, and the interrelationship between structures observed in quiet and active regions on the disk and at the limb, as well as their role in global processes has not yet received definitive answers. In addition, how they affect the coronal heating and solar wind need to be further explored. In this review we present observations and physical properties of small-scale jet-like chromospheric events observed in active and quiet regions, on the disk and at the limb and discuss their interrelationship.     

Origin of Sunspots

Sunspots, seen as cool regions on the surface of the Sun, are a thermal phenomenon. Sunspots are always associated with bipolar magnetic loops that break through the solar surface. Thus to explain the origin of sunspots we have to understand how the magnetic field originates inside the Sun and emerges at its surface. The field predicted by mean-field dynamo theories is too weak by itself to emerge at the surface of the Sun. However, because of the turbulent character of solar convection the fields generated by dynamo are intermittent – i.e., concentrated into ropes or sheets with large spaces in between. The intermittent fields are sufficiently strong to be able to emerge at the solar surface, in spite of the fact that their mean (average) value is weak. It is suggested here that magnetic fields emerge at the solar surface at those random times and places when the total magnetic field (mean field plus fluctuations) exceeds the threshold for buoyancy. The clustering of coherently emerged loops results in the formation of a sunspot. A non-axisymmetric enhancement of the underlying magnetic field causes in the clustering of sunspots forming sunspot groups, clusters of activity and active longitudes. The mean field, which is not directly observable, is also important, being responsible for the ensemble regularities of sunspots, such as Hale's law of sunspot polarities and the 11-year periodicity.     

Fields and plasmas in the outer solar system

The most significant information about fields and plasmas in the outer solar system, based on observations by Pioneer 10 and 11 investigations, is reviewed. The characteristic evolution of solar wind streams beyond 1 AU has been observed. The region within which the velocity increases continuously near 1 AU is replaced at larger distances by a thick interaction region with abrupt jumps in the solar wind speed at the leading and trailing edges. These abrupt increases, accompanied by corresponding jumps in the field magnitude and in the solar wind density and temperature, consist typically of a forward and a reverse shock. The existence of two distinct corotating regions, separated by sharp boundaries, is a characteristic feature of the interplanetary medium in the outer solar system. Within the interaction regions, compression effects are dominant and the field strength, plasma density, plasma temperature and the level of fluctuations are enhanced. Within the intervening quiet regions, rarefaction effects dominate and the field magnitude, solar wind density and fluctuation level are very low. These changes in the structure of interplanetary space have significant consequences for the many energetic particles propagating through the medium. The interaction regions control the access to the inner solar system of relativistic electrons from Jupiter's magnetosphere. The interaction regions and shocks appear to be associated with an acceleration of solar protons to MeV energies. Flare-generated shocks are observed to be propagating through the outer solar system with constant speed, implying that the previously recognized deceleration of flare shocks takes place principally near the Sun. Radial gradients in the solar wind and interplanetary field parameters have been determined. The solar wind speed is nearly constant between 1 and 5 AU with only a slight deceleration of 30 km s⁺¹ on the average. The proton flux follows an r ⁺² dependence reasonably well, however, the proton density shows a larger departure from this dependence. The proton temperature decreases steadily from 1 to 5 AU and the solar wind protons are slightly hotter than anticipated for an adiabatic expansion. The radial component of the interplanetary field falls off like r ⁺² and, on the average, the magnitude and spiral angle also agree reasonably well with theory. However, there is evidence, principally within quiet regions, of a significant departure of the azimuthal field component and the field magnitude from simple theoretical models. Pioneer 11 has obtained information up to heliographic latitudes of 16°. Observations of the interplanetary sector structure show that the polarity of the field becomes gradually more positive, corresponding to outward-directed fields at the Sun, and at the highest latitudes the sector structure disappears. These results confirm a prior suspicion that magnetic sectors are associated with an interplanetary current sheet surrounding the Sun which is inclined slightly to the solar equator.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

Large-scale propagation properties of interplanetary disturbances revealed from IPS and spacecraft observations

Interplanetary scintillation (IPS) observations may be used to study large-scale propagation properties of transient interplanetary disturbances in a three-dimensional manner, although current IPS observations have several limitations, e.g., poor time resolution and line-of-sight integration. Comparative studies with spacecraft solar wind and white-light coronal mass ejection (CME) observations are quite helpful in interpretation of IPS observations. An interplanetary disturbance apparently in association with a disappearing solar filament, which took place near the central meridian of the Sun on late 22 April, 1979, is discussed to examine previous deductions from IPS observations. Three-station IPS observations of the flow speed and spacecraft observations suggest that a quasi-spherical interplanetary disturbance was formed around the Sun-Earth line, whereas the center of the disturbance derived from the distribution of enhanced IPS across the sky (g-maps) is located to the east of the Sun-Earth line.Permanently at Research Institute of Atmospherics, Nagoya University, Toyokawa 442, Japan.     

Cosac, The Cometary Sampling and Composition Experiment on Philae

Comets are thought to preserve the most pristine material currently present in the solar system, as they are formed by agglomeration of dust particles in the solar nebula, far from the Sun, and their interiors have remained cold. By approaching the Sun, volatile components and dust particles are released forming the cometary coma. During the phase of Heavy Bombardment, 3.8--4 billion years ago, cometary matter was delivered to the Early Earth. Precise knowledge on the physico-chemical composition of comets is crucial to understand the formation of the Solar System, the evolution of Earth and particularly the starting conditions for the origin of life on Earth. Here, we report on the COSAC instrument, part of the ESA cometary mission Rosetta, which is designed to characterize, identify, and quantify volatile cometary compounds, including larger organic molecules, by in situ measurements of surface and subsurface cometary samples. The technical concept of a multi-column enantio-selective gas chromatograph (GC) coupled to a linear reflectron time-of-flight mass-spectrometer instrument is presented together with its realisation under the scientific guidance of the Max-Planck-Institute for Solar System Research in Katlenburg-Lindau, Germany. The instrument's technical data are given; first measurements making use of standard samples are presented. The cometary science community is looking forward to receive fascinating data from COSAC cometary in situ measurements in 2014.     

铝合金搅拌摩擦焊缝和母材疲劳裂纹扩展行为

对7075-T6和LC4CS两种铝合金搅拌摩擦焊缝的疲劳裂纹扩展(da/d N)行为进行实验研究,并与7075-T6母材疲劳性能进行对比分析。结果表明:沿焊缝中心、前进和后退边方向,7075-T6搅拌摩擦焊疲劳裂纹扩展速率da/d N均低于LC4CS的da/d N;但沿垂直于焊缝方向两者da/d N基本一致;同种材料焊缝中心da/d N为最低,其次是后退边和前进边,而沿垂直焊缝方向da/d N为最高;7075-T6焊缝中心da/d N与其母材比较,低载荷ΔK下焊缝中心的da/d N较低、但高ΔK下母材的da/d N较低;在前进边、后退边和母材区域疲劳裂纹断口形貌明显不同,在母材区域可以观察到典型的疲劳条纹特征;但沿焊缝中心(焊核)的疲劳裂纹断口,没有任何疲劳条纹特征;7075-T6搅拌摩擦焊缝比LC4CS焊缝具有更好的抗疲劳裂纹扩展阻力,在高ΔK载荷下搅拌摩擦焊核区细小等轴晶粒不能提高疲劳裂纹扩展阻力。