Lidong Xia

Coronal plasma flows and magnetic fields in solar active regions: Combined observations from SOHO and NSO/Kitt Peak

During the early days of the SOHO mission, SUMER observed a few active regions (ARs) connected with sunspots on the Sun and took their images and spectra in various EUV emission lines. In addition to these spectroscopic data magne- tograms of the photospheric footpoint regions of the AR loops were available from the MDI on SOHO and the National Solar Observatory/Kitt Peak (NSO/KP), data which here are used to construct the coronal magnetic field of the ARs by force-free- field extrapolation. The combined data set is analysed with respect to the large-scale circulation of coronal matter, which means that the Dopplershifts of various lines used as tracers of the plasma flow are investigated in close connection with the ambient magnetic field, which is found to be either closed or open in the coronal volume considered. The Dopplershift pattern is found to be clearly linked with the field topology, and several regions of strong velocity shear are identified. We also estimate the coronal currents. We discuss the results of this mainly phenomenological correlative study with the perspective to understand coronal heating and mass supply to the extended corona, and with respect to the role played by the field in guiding and constraining plasma flows.

What can We Learn about Solar Coronal Mass Ejections, Coronal Dimmings, and Extreme-ultraviolet Jets through Spectroscopic Observations?

Solar eruptions, particularly coronal mass ejections (CMEs) and extreme-ultraviolet (EUV) jets, have rarely been investigated with spectroscopic observations. We analyze several data sets obtained by the EUV Imaging Spectrometer on board Hinode and find various types of flows during CMEs and jet eruptions. CME-induced dimming regions are found to be characterized by significant blueshift and enhanced line width by using a single Gaussian fit, while a red-blue (RB) asymmetry analysis and an RB-guided double Gaussian fit of the coronal line profiles indicate that these are likely caused by the superposition of a strong background emission component and a relatively weak (~10%), high-speed (~100 km s–1) upflow component. This finding suggests that the outflow velocity in the dimming region is probably of the order of 100 km s–1, not ~20 km s–1 as reported previously. These weak, high-speed outflows may provide a significant amount of mass to refill the corona after the eruption of CMEs, and part of them may experience further acceleration and eventually become solar wind streams that can serve as an additional momentum source of the associated CMEs. Density and temperature diagnostics of the dimming region suggest that dimming is primarily an effect of density decrease rather than temperature change. The mass losses in dimming regions as estimated from different methods are roughly consistent with each other, and they are 20%-60% of the masses of the associated CMEs. With the guide of RB asymmetry analysis, we also find several temperature-dependent outflows (speed increases with temperature) immediately outside the (deepest) dimming region. These outflows may be evaporation flows that are caused by the enhanced thermal conduction or nonthermal electron beams along reconnecting field lines, or induced by the interaction between the opened field lines in the dimming region and the closed loops in the surrounding plage region. In an erupted CME loop and an EUV jet, profiles of emission lines formed at coronal and transition region temperatures are found to exhibit two well-separated components, an almost stationary component accounting for the background emission and a highly blueshifted (~200 km s–1) component representing emission from the erupting material. The two components can easily be decomposed through a double Gaussian fit, and we can diagnose the electron density, temperature, and mass of the ejecta. Combining the speed of the blueshifted component and the projected speed of the erupting material derived from simultaneous imaging observations, we can calculate the real speed of the ejecta.

Spatial structures of magnetic depression in the Earth's high-altitude cusp: Cluster multipoint observations

Magnetic depression structures (magnetic holes) of short time duration from seconds to minutes have been studied using Cluster data in the high-latitude cusp. Our multispacecraft analysis revealed that the magnetic depressions are spatial structures traveling across the spacecraft, and this result was further strengthened by the calculation of the boundary normal directions and velocities using various methods. In this article, we show that multiple properties of the magnetic depressions are consistent with those of mirror structures observed in the magnetosheath or solar wind. The plasma in the cusp is rarely unstable with respect to mirror instability. However, as has been shown by previous studies, once a large magnetic hole is created by mirror instability, it becomes relatively stable and can survive for extended periods of time even if surrounding plasma conditions drop well below the mirror threshold. Although local generation of these structures cannot be completely ruled out in some cases, we propose an interpretation of the magnetic depressions observed in the cusp as mirror structures generated upstream and convected to the cusp by plasma flow. Specifically, the magnetic holes could be generated in the magnetosheath and enter the cusp due to the open geometry of the cusp magnetic field.

Cluster observations of the entry layer equatorward of the cusp under northward interplanetary magnetic field

[1] Various boundary crossings in the vicinity of the high-altitude cusp region were experienced by the Cluster spacecraft when the interplanetary magnetic field (IMF) was northward. In contrast to the southward IMF cases, in which a turbulent and diffusive entry layer is present equatorward of the cusp, a transition layer (without significant turbulence and diffusive properties) that shows clear differences in plasma parameters (sometimes step-like profile) compared to the adjacent regions was observed. We suggest that this transition layer, which contains both magnetosheath and magnetospheric populations, is the entry layer during northward IMF conditions. This transition layer is possibly formed by dual-lobe reconnection when the IMF is northward. The plasma property and the closed field line geometry of this layer indicate that it is possibly linked to the low-latitude boundary layer. The width of this layer varies from 480 to 2200 km. The results support the notion that high-latitude dual-lobe reconnection is a potential mechanism of the transport of solar wind into the magnetosphere during northward IMF through the formation of a high-altitude entry layer. The observations of different sublayers with evident density and temperature differences are consistent with the view that the reconnection process at the magnetopause is not steady.

On the network structures in solar equatorial coronal holes Observations of SUMER and MDI on SOHO

By combining observations of the Sun made by SUMER and MDI aboard SOHO, the network structures in equatorial coronal holes have been studied, in particular the relationship between the ultraviolet emission-line parameters (line radiance, Doppler shift and line width) and the underlying magnetic field. The bases of coronal holes seen in chromospheric spectral lines with relatively low formation temperatures generally have similar properties as normal quiet-Sun regions, i.e., small bright patches with a size of about 2 �� to 10 �� are the dominant features in the network as well as in cell interiors. With the increase of the formation temperature, these features become more diffuse, and have an enlarged size. Loop-like structures are the most prominent features in the transition region. In coronal holes, we found that many of such structures seem to have one footpoint rooted in the intra-network and to extend into the cell interiors. Some of them appear as star-shape clusters. In Dopplergrams of the O  line at 1032 A, there are also fine structures with apparent blue shifts, although, on average, they are red shifted. Structures with blue shifts have usually also broader line widths. They seem to represent plasma above large concentrations of unipolar magnetic field, without obvious bipolar photospheric magnetic features nearby.

STREAMER WAVES DRIVEN BY CORONAL MASS EJECTIONS

Between July 5th and July 7th 2004, two intriguing fast coronal mass ejection(CME)-streamer interaction events were recorded by the Large Angle and Spectrometric Coronagraph (LASCO). At the beginning of the events, the streamer was pushed aside from their equilibrium position upon the impact of the rapidly outgoing and expanding ejecta; then, the streamer structure, mainly the bright streamer belt, exhibited elegant large scale sinusoidal wavelike motions. The motions were apparently driven by the restoring magnetic forces resulting from the CME impingement, suggestive of magnetohydrodynamic kink mode propagating outwards along the plasma sheet of the streamer. The mode is supported collectively by the streamer-plasma sheet structure and is therefore named “ streamer wave” in the present study. With the white light coronagraph data, we show that the streamer wave has a period of about 1 hour, a wavelength varying from 2 to 4 solar radii, an amplitude of about a few tens of solar radii, and a propagating phase speed in the range 300 to 500 km s −1 . We also find that there is a tendancy for the phase speed to decline with increasing heliocentric distance. These observations provide good examples of large scale wave phenomena carried by coronal structures, and have significance in developing seismological techniques for diagnosing plasma and magnetic parameters in the outer corona. Subject headings: waves MHD Sun: corona coronal mass ejection

Links between magnetic fields and plasma flows in a coronal hole

We compare the small-scale features visible in the Ne VIII Doppler-shift map of an equatorial coronal hole (CH) as observed by SUMER with the small-scale structures of the magnetic field as constructed from a simultaneous photospheric magnetogram by a potential magnetic-field extrapolation. The combined data set is analysed with respect to the small-scale flows of coronal matter, which means that the Ne VIII Doppler-shift used as tracer of the plasma flow is investigated in close connection with the ambient magnetic field. Some small closed-field regions in this largely open CH are also found in the coronal volume considered. The Doppler-shift patterns are found to be clearly linked with the field topology.

Intrinsic instability of coronal streamers

Plasma blobs are observed to be weak density enhancements as radially stretched structures emerging from the cusps of quiescent coronal streamers. In this paper, it is suggested that the formation of blobs is a consequence of an intrinsic instability of coronal streamers occurring at a very localized region around the cusp. The evolutionary process of the instability, as revealed in our calculations, can be described as follows: (1) through the localized cusp region where the field is too weak to sustain the confinement, plasmas expand and stretch the closed field lines radially outward as a result of the freezing-in effect of plasma-magnetic field coupling; the expansion brings a strong velocity gradient into the slow wind regime providing the free energy necessary for the onset of a subsequent magnetohydrodynamic instability; (2) the instability manifests itself mainly as mixed streaming sausage-kink modes, the former results in pinches of elongated magnetic loops to provoke reconnections at one or many locations to form blobs. Then, the streamer system returns to the configuration with a lower cusp point, subject to another cycle of streamer instability. Although the instability is intrinsic, it does not lead to the loss of the closed magnetic flux, neither does it affect the overall feature of a streamer. The main properties of the modeled blobs, including their size, velocity profiles, density contrasts, and even their daily occurrence rate, are in line with available observations.

Quasi-Periodic Releases of Streamer Blobs and Velocity Variability of the Slow Solar Wind near the Sun

We search for persistent and quasi-periodic release events of streamer blobs during 2007 with the Large Angle Spectrometric Coronagraph on the Solar and Heliospheric Observatory and assess the velocity of the slow solar wind along the plasma sheet above the corresponding streamer by measuring the dynamic parameters of blobs. We find ten quasi-periodic release events of streamer blobs lasting for three to four days. In each day of these events, we observe three – five blobs. The results are in line with previous studies using data observed near the last solar minimum. Using the measured blob velocity as a proxy for that of the mean flow, we suggest that the velocity of the background slow solar wind near the Sun can vary significantly within a few hours. This provides an observational manifestation of the large velocity variability of the slow solar wind near the Sun.

Sizes of transition-region structures in coronal holes and in the quiet Sun

Aims. We study the height variations of the sizes of chromospheric and transition-region features in a small coronal hole and the adjacent quiet Sun, considering images of the intensity, Doppler shift, and non-thermal motion of ultraviolet emission lines as measured by SUMER (Solar Ultraviolet Measurements by Emitted Radiation), together with the magnetic field as obtained by extrapolation from photospheric magnetograms. Methods. In order to estimate the characteristic sizes of the different features present in the chromosphere and transition region, we have calculated the autocorrelation function for the images as well as the corresponding extrapolated magnetic field at different heights. The Half Width at Half Maximum (HWHM) of the autocorrelation function is considered to be the characteristic size of the feature shown in the corresponding image. Results. Our results indicate that, in both the coronal hole and quiet Sun, the HWHM of the intensity image is larger than that of the images of Doppler-shift and non-thermal width at any given altitude. The HWHM of the intensity image is smaller in the chromosphere than in the transition region, where the sizes of intensity features of lines at different temperatures are almost the same. But in the upper part of the transition region, the intensity size increases more strongly with temperature in the coronal hole than in the quiet Sun. We also studied the height variations of the HWHM of the magnetic field magnitude B and its component |Bz|, and found they are equal to each other at a certain height below 40 Mm in the coronal hole. The height variations of the HWHM of |Bz/B| seem to be consistent with the temperature variations of the intensity size. Conclusions. Our results suggest that coronal loops are much lower, and magnetic structures expand through the upper transition region and lower corona much more strongly with height in the coronal hole than in the quiet Sun.