Interplume Regions Research Articles

Contribution of polar plumes to fast solar wind

Context. Several physical properties of solar polar plumes have been identified by different published studies, however such studies are rare and sometimes in disagreement. Aims. The purpose of the present work is to analyze a set of SOHO/UVCS data dedicated to the observation of plumes and to obtain a picture of the physical properties of plumes in the intermediate solar corona through a self-consistent analysis. Methods. We applied the Doppler Dimming technique to data acquired by SOHO/UVCS in April 1996, which was during the very early phases of the mission. From this we derived outflow speeds and electron densities. We used SOHO/LASCO images as context data in order to better identify plume and interplume regions in the UVCS field of view. Results. The results we obtain demonstrate that in three cases out of four plumes expand with outflow speeds comparable to those of interplumes, and in a single case with lower speeds. We estimate that the contribution of plumes to the wind coming from the solar poles is about 20%, and that different plumes provide a different contribution, possibly according to different stages of their evolution. Conclusions. We conclude that plumes are not static structures, and that they contribute significantly to the wind coming from the solar poles.

Density Fluctuations in a Polar Coronal Hole

Abstract We have measured the root-mean-square (rms) amplitude of intensity fluctuations, ΔI, in plume and interplume regions of a polar coronal hole. These intensity fluctuations correspond to density fluctuations. Using data from the Sun Watcher using the Active Pixel System detector and Image Processing on the Project for Onboard Autonomy (Proba2), our results extend up to a height of about 1.35 R ⊙. One advantage of the rms analysis is that it does not rely on a detailed evaluation of the power spectrum, which is limited by noise levels to low heights in the corona. The rms approach can be performed up to larger heights where the noise level is greater, provided that the noise itself can be quantified. At low heights, both the absolute ΔI, and the amplitude relative to the mean intensity, ΔI/I, decrease with height. However, starting at about 1.2 R ⊙, ΔI/I increases, reaching 20%–40% by 1.35 R ⊙. This corresponds to density fluctuations of Δn e/n e ≈ 10%–20%. The increasing relative amplitude implies that the density fluctuations are generated in the corona itself. One possibility is that the density fluctuations are generated by an instability of Alfvén waves. This generation mechanism is consistent with some theoretical models and with observations of Alfvén wave amplitudes in coronal holes. Although we find that the energy of the observed density fluctuations is small, these fluctuations are likely to play an important indirect role in coronal heating by promoting the reflection of Alfvén waves and driving turbulence.

A Statistical Study on the Frequency-dependent Damping of the Slow-mode Waves in Polar Plumes and Interplumes

Abstract We perform a statistical study on the frequency-dependent damping of slow waves propagating along polar plumes and interplumes in the solar corona. Analysis of a large sample of extreme ultraviolet imaging data with high spatial and temporal resolutions obtained from Atmospheric Imaging Assembly (AIA)/Solar Dynamics Observatory suggests an inverse power-law dependence of the damping length on the periodicity of slow waves (i.e., the shorter-period oscillations exhibit longer damping lengths), in agreement with the previous case studies. Similar behavior is observed in both plume and interplume regions studied in AIA 171 Å and AIA 193 Å passbands. It is found that the short-period (2–6 minutes) waves are relatively more abundant than their long-period (7–30 minutes) counterparts, in contrast to the general belief that the polar regions are dominated by the longer-period slow waves. We also derived the slope of the power spectra (α, the power-law index) statistically to better understand the characteristics of turbulence present in the region. It is found that the α values and their distributions are similar in both plume and interplume structures across the two AIA passbands. At the same time, the spread of these distributions also indicates the complexity of the underlying turbulence mechanism.

Inter-plume aerodynamics for gasoline spray collapse

The collapse or merging of individual plumes of direct-injection gasoline injectors is of fundamental importance to engine performance because of its impact on fuel–air mixing. However, the mechanisms of spray collapse are not fully understood and are difficult to predict. The purpose of this work is to study the aerodynamics in the inter-spray region, which can potentially lead to plume collapse. High-speed (100 kHz) particle image velocimetry is applied along a plane between plumes to observe the full temporal evolution of plume interaction and potential collapse, resolved for individual injection events. Supporting information along a line of sight is obtained using simultaneous diffused back illumination and Mie-scatter techniques. Experiments are performed under simulated engine conditions using a symmetric eight-hole injector in a high-temperature, high-pressure vessel at the “Spray G” operating conditions of the engine combustion network. Indicators of plume interaction and collapse include changes in counter-flow recirculation of ambient gas toward the injector along the axis of the injector or in the inter-plume region between plumes. The effect of ambient temperature and gas density on the inter-plume aerodynamics and the subsequent plume collapse are assessed. Increasing ambient temperature or density, with enhanced vaporization and momentum exchange, accelerates the plume interaction. Plume direction progressively shifts toward the injector axis with time, demonstrating that the plume interaction and collapse are inherently transient.

Electron acceleration and small‐scale coherent structure formation by an Alfvén wave propagating in coronal interplume region

We use the 2.5‐D electromagnetic particle‐in‐cell simulation code to investigate the acceleration of electrons in solar coronal holes through the interaction of Alfvén waves with an interplume region. The interplume is modeled by cavity density gradients that are perpendicular to the background magnetic field. The aim is to help explain the observation of suprathermal electrons under a relatively quiet Sun. Simulations show that Alfvén waves interacting with the interplume region give rise to a strong local electric field that accelerates electrons in the direction parallel to the background magnetic field. Suprathermal electron beams and small‐scale coherent structures are observed within interplume of strong density gradients. These features result from the nonlinear evolution of the electron beam plasma instability.

INFERRING THE CORONAL DENSITY IRREGULARITY FROM EUV SPECTRA

ABSTRACT Understanding the density structure of the solar corona is important for modeling both coronal heating and the solar wind. Direct measurements are difficult because of line-of-sight integration and possible unresolved structures. We present a new method for quantifying such structures using density-sensitive extreme ultraviolet line intensities to derive a density irregularity parameter, a relative measure of the amount of structure along the line of sight. We also present a simple model to relate the inferred irregularities to physical quantities, such as the filling factor and density contrast. For quiet-Sun regions and interplume regions of coronal holes, we find a density contrast of at least a factor of 3–10 and corresponding filling factors of about 10%–20%. Our results are in rough agreement with other estimates of the density structures in these regions. The irregularity diagnostic provides a useful relative measure of unresolved structure in various regions of the corona.

Solar Coronal Plumes and the Fast Solar Wind

The spectral profiles of the coronal Ne viii line at 77 nm have different shapes in quiet-Sun regions and Coronal Holes (CHs). A single Gaussian fit of the line profile provides an adequate approximation in quiet-Sun areas, whereas, a strong shoulder on the long-wavelength side is a systematic feature in CHs. Although this has been noticed since 1999, no physical reason for the peculiar shape could be given. In an attempt to identify the cause of this peculiarity, we address three problems that could not be conclusively resolved, in a review article by a study team of the International Space Science Institute (ISSI) (Wilhelm et al. 2011): (1) The physical processes operating at the base and inside of plumes, as well as their interaction with the Solar Wind (SW). (2) The possible contribution of plume plasma to the fast SW streams. (3) The signature of the First-Ionization Potential (FIP) effect between plumes and inter-plume regions (IPRs). Before the spectroscopic peculiarities in IPRs and plumes in Polar Coronal Holes (PCHs) can be further investigated with the instrument Solar Ultraviolet Measurements of Emitted Radiation (SUMER) aboard the Solar and Heliospheric Observatory (SOHO), it is mandatory to summarize the results of the review to place the spectroscopic observations into context. Finally, a plume model is proposed that satisfactorily explains the plasma flows up and down the plume field lines and leads to the shape of the neon line in PCHs.

STATISTICAL DETECTION OF SLOW-MODE WAVES IN SOLAR POLAR REGIONS WITHSDO/AIA

Observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory are utilized to statistically investigate the propagating quasi-periodic oscillations in the solar polar plume and inter-plume regions. On average, the periods are found to be nearly equal in the three coronal channels of AIA 171, 193 angstrom, and 211 angstrom, and the wavelengths increase with temperature from 171 angstrom, 193 angstrom, and 211 angstrom. The phase speeds may be inferred from the above parameters. Furthermore, the speed ratios of upsilon(193)/upsilon(171) and upsilon(211)/upsilon(171) are derived, e. g., 1.4 +/- 0.8 and 2.0 +/- 1.9 in the plume regions, respectively, which are equivalent to the theoretical ones for acoustic waves. We find that there are no significant differences for the detected parameters between the plume and inter-plume regions. To our knowledge, this is the first time that we have simultaneously obtained the phase speeds of slow-mode waves in the three channels in the open coronal magnetic structures due to the method adopted in the present work, which is able to minimize the influence of the jets or eruptions on wave signals.

BIRTH, LIFE, AND DEATH OF A SOLAR CORONAL PLUME

We analyze a solar polar-coronal-hole (CH) plume over its entire ≈40 hr lifetime, using high-resolution Solar Dynamic Observatory Atmospheric Imaging Assembly (AIA) data. We examine (1) the plume's relationship to a bright point (BP) that persists at its base, (2) plume outflows and their possible contribution to the solar wind mass supply, and (3) the physical properties of the plume. We find that the plume started ≈2 hr after the BP first appeared and became undetectable ≈1 hr after the BP disappeared. We detected radially moving radiance variations from both the plume and from interplume regions, corresponding to apparent outflow speeds ranging over ≈(30-300) km s{sup –1} with outflow velocities being higher in the 'cooler' AIA 171 A channel than in the 'hotter' 193 A and 211 A channels, which is inconsistent with wave motions; therefore, we conclude that the observed radiance variations represent material outflows. If they persist into the heliosphere and plumes cover ≈10% of a typical CH area, these flows could account for ≈50% of the solar wind mass. From a differential emission measure analysis of the AIA images, we find that the average electron temperature of the plume remained approximately constant over its lifetime, at Tmore » {sub e} ≈ 8.5 × 10{sup 5} K. Its density, however, decreased with the age of the plume, being about a factor of three lower when the plume faded compared to when it was born. We conclude that the plume died due to a density reduction rather than to a temperature decrease.« less

2D and 3D Polar Plume Analysis from the Three Vantage Positions of STEREO/EUVI A, B, and SOHO/EIT

Polar plumes are seen as elongated objects starting at the solar polar regions. Here, we analyze these objects from a sequence of images taken simultaneously by the three spacecraft telescopes STEREO/EUVI A and B, and SOHO/EIT. We establish a method capable of automatically identifying plumes in solar EUV images close to the limb at 1.01 - 1.39 R in order to study their temporal evolution. This plume-identification method is based on a multiscale Hough-wavelet analysis. Then two methods to determined their 3D localization and structure are discussed: First, tomography using the filtered back-projection and including the differential rotation of the Sun and, secondly, conventional stereoscopic triangulation. We show that tomography and stereoscopy are complementary to study polar plumes. We also show that this systematic 2D identification and the proposed methods of 3D reconstruction are well suited, on one hand, to identify plumes individually and on the other hand, to analyze the distribution of plumes and inter-plume regions. Finally, the results are discussed focusing on the plume position with their cross-section area.

OBSERVATION OF HIGH-SPEED OUTFLOW ON PLUME-LIKE STRUCTURES OF THE QUIET SUN AND CORONAL HOLES WITHSOLAR DYNAMICS OBSERVATORY/ATMOSPHERIC IMAGING ASSEMBLY

Observations from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) reveal ubiquitous episodic outflows (jets) with an average speed around 120 km s-1 at temperatures often exceeding a million degree in plume-like structures, rooted in magnetized regions of the quiet solar atmosphere. These outflows are not restricted to the well-known plumes visible in polar coronal holes, but are also present in plume-like structures originating from equatorial coronal holes and quiet-Sun regions. Outflows are also visible in the "interplume" regions throughout the atmosphere. Furthermore, the structures traced out by these flows in both plume and inter-plume regions continually exhibit transverse (Alfvenic) motion. Our finding suggests that high-speed outflows originate mainly from the magnetic network of the quiet Sun and coronal holes, and that the plume flows observed are highlighted by the denser plasma contained therein. These outflows might be an efficient means to provide heated mass into the corona and serve as an important source of mass supply to the solar wind. We demonstrate that the quiet-Sun plume flows can sometimes significantly contaminate the spectroscopic observations of the adjacent coronal holes - greatly affecting the Doppler shifts observed, thus potentially impacting significant investigations of such regions.

KINETIC ALFVÉN WAVES EXCITED BY OBLIQUE MAGNETOHYDRODYNAMIC ALFVÉN WAVES IN CORONAL HOLES

Kinetic Alfven waves (KAWs) are small-scale dispersive AWs that can play an important role in particle heating and acceleration of space and solar plasmas. An excitation mechanism for KAWs created by the coupling between large-scale oblique AWs and small-scale KAWs is presented in this paper. Taking into account both the collisional and Landau damping dissipations, the results show that the net growth rate of the excited KAWs increases with their perpendicular wavenumber k{sub perpendicular} and reaches maximum at {lambda}{sub e} k{sub perpendicular} {approx} 0.3, where {lambda}{sub e} is the electron inertial length. However, for KAWs with shorter perpendicular wavelengths, the net growth rate decreases rapidly due to dissipative effects. The evaluation of the threshold amplitude of the AW implies that for KAWs with {lambda}{sub e} k{sub perpendicular} < 0.3, the relative threshold amplitude is well below 10%, which is easy to satisfy. In particular, when applying this mechanism to the case of a solar coronal hole containing a dense plume structure, our results show that KAWs with {lambda}{sub e} k{sub perpendicular} < 0.3 can be not only efficiently excited in the interplume region but also strongly dissipated in the dense plume due to the Landau damping.

Morphology, dynamics and plasma parameters of plumes and inter-plume regions in solar coronal holes

Coronal plumes, which extend from solar coronal holes (CH) into the high corona and - possibly - into the solar wind (SW), can now continuously be studied with modern telescopes and spectrometers on spacecraft, in addition to investigations from the ground, in particular, during total eclipses. Despite the large amount of data available on these prominent features and related phenomena, many questions remained unanswered as to their generation and relative contributions to the high-speed streams emanating from CHs. An understanding of the processes of plume formation and evolution requires a better knowledge of the physical conditions at the base of CHs, in plumes and in the surrounding inter-plume regions (IPR). More specifically, information is needed on the magnetic field configuration, the electron densities and temperatures, effective ion temperatures, non-thermal motions, plume cross-sections relative to the size of a CH, the plasma bulk speeds, as well as any plume signatures in the SW. In spring 2007, the authors proposed a study on "Structure and dynamics of coronal plumes and inter-plume regions in solar coronal holes" to the International Space Science Institute (ISSI) in Bern to clarify some of these aspects by considering relevant observations and the extensive literature. This review summarizes the results and conclusions of the study. Stereoscopic observations allowed us to include three-dimensional reconstructions of plumes. Multi-instrument investigations carried out during several campaigns led to progress in some areas, such as plasma densities, temperatures, plume structure and the relation to other solar phenomena, but not all questions could be answered concerning the details of plume generation process(es) and interaction with the SW.

Propagating intensity disturbances in polar corona as seen from AIA/SDO

Polar corona is often explored to find the energy source for the acceleration of the fast solar wind. Earlier observations show omni-presence of quasi-periodic disturbances, travelling outward, which is believed to be caused by the ubiquitous presence of outward propagating waves. These waves, mostly of compressional type, might provide the additional momentum and heat required for the fast solar wind acceleration. It has been conjectured that these disturbances are not due to waves but high speed plasma outflows, which are difficult to distinguish using the current available techniques. With the unprecedented high spatial and temporal resolution of AIA, we search for these quasi-periodic disturbances in both plume and interplume regions of the polar corona. We investigate their nature of propagation and search for a plausible interpretation. We also aim to study their multi-thermal nature by using three different coronal passbands of AIA. In almost all the locations chosen, in both plume and interplume regions we find the presence of propagating quasi-periodic disturbances, of periodicities ranging from 10-30 min. These are clearly seen in two channels and in a few cases out to very large distances (~250 arcsec) off-limb, almost to the edge of the AIA field of view. The propagation speeds are in the range of 100-170 km/s. The average speeds are different for different passbands and higher in interplume regions. Observed disturbances are insensitive to changes in slit width. This indicates that a coherent mechanism is involved. In addition, the observed propagation speed varies between the different passpands, implying that these quasi-periodic intensity disturbances are possibly due to magneto-acoustic waves. The propagation speeds in interplume region are higher than in the plume region.

TWO-TEMPERATURE MODELS FOR POLAR PLUMES: COOLING BY MEANS OF STRONG BASE HEATING

In earlier one-fluid hydrodynamical calculations incorporating heat conduction and radiative losses, it was shown that the high densities in polar plumes could be reproduced by including a concentrated heat source near the plume base, in addition to the global heating required in both the plume and interplume regions of the coronal hole. The extra heating (attributed to interchange reconnection between the open flux and an underlying magnetic bipole) results in lower flow speeds and temperatures relative to the interplume gas, predictions that have since been confirmed by spectroscopic measurements. Here, the model is extended to the two-fluid case, in which ions and electrons are allowed to have different temperatures, coupling is via Coulomb collisions, and heat transport is mainly by electrons. Again, we find that depositing energy very close to the coronal base, in either the protons or electrons (or both), raises the densities and decreases the flow speeds everywhere along the flux tube. The higher densities in turn act to lower the ion temperatures by coupling the protons more closely to the energy-losing electrons. In addition, we find that energy must be deposited globally in both the electrons and the ions; without this direct heating, the electrons would end up cooler in the interplume region than in the plume, contrary to observations. Increasing the rate of flux-tube expansion has the effect of lowering the electron and ion temperatures and reducing the asymptotic flow speed, both in the plume and the interplume region; the observed densities and temperatures can be matched by taking the magnetic field to fall off with radius roughly as r –4.

SUMER Observations of Coronal-Hole Temperatures

Observations of emission lines in the vacuum-ultraviolet spectral range with calibrated instrumentation provide crucial information on the prevailing plasma temperatures in the solar atmosphere. Coronal-hole temperatures measured by the SUMER spectrometer on SOHO will be presented in this contribution. Electron temperatures can be estimated from the formation temperatures of the observed emission lines. Line-ratio and emission-measure analyses, however, offer higher accuracies. Typical electron temperatures at altitudes of H<200 Mm in coronal holes are below 1 MK in bright structures—the coronal plumes—with higher values in darker areas—the inter-plume regions. Line-width measurements yield effective ion temperatures, which are much higher than the electron temperatures. Observations of line profiles emitted from species with different masses allow a separation of the effective temperatures into ion temperatures and unresolved non-thermal motions along the line of sight.

THE TEMPERATURE-DEPENDENT NATURE OF CORONAL DIMMINGS

The opening-up of the magnetic field during solar eruptive events is often accompanied by a dimming of the local coronal emission. From observations of filament eruptions recorded with the Extreme-Ultraviolet Imager on STEREO during 2008-2009, it is evident that these dimmings are much more pronounced in 19.5 nm than in the lower-temperature line 17.1 nm, as viewed either on the disk or above the limb. We conclude that most of the cooler coronal plasma is not ejected but remains gravitationally bound when the loops open up. This result is consistent with Doppler measurements by Imada and coworkers, who found that the upflow speeds in a transient coronal hole increased dramatically above a temperature of 1 MK; it is also consistent with the quasistatic behavior of polar plumes, as compared with the hotter interplume regions that are the main source of the fast solar wind. When the open flux reconnects and closes down again, the trapped plasma is initially heated to such high temperatures that it is no longer visible at Fe IX 17.1 nm. Correspondingly, 17.1 nm images show a dark ribbon or ``heat wave'' propagating away from the polarity inversion line and coinciding with the brightened Fe XV 28.4 nm and Fe XII 19.5 nm post-eruptive loops and their footpoint areas. Such dark ribbons provide a clear example of dimmings that are not caused by a density depletion. The propagation of the ``heat wave'' is driven by the closing-down, not the opening-up, of flux and can be observed both off-limb and on-disk.

ACCELERATING WAVES IN POLAR CORONAL HOLES AS SEEN BY EIS AND SUMER

We present EIS/Hinode and SUMER/SOHO observations of propagating disturbances detected in coronal lines in inter-plume and plume regions of a polar coronal hole. The observation was carried out on 2007 November 13 as part of the JOP196/HOP045 program. The SUMER spectroscopic observation gives information about fluctuations in radiance and on both resolved (Doppler shift) and unresolved (Doppler width) line-of-sight velocities, whereas EIS 40'' wide slot images detect fluctuations only in radiance but maximize the probability of overlapping field of view between the two instruments. From distance–time radiance maps, we detect the presence of propagating waves in a polar inter-plume region with a period of 15–20 minutes and a propagation speed increasing from 130 ± 14 km s−1 just above the limb to 330 ± 140 km s−1 around 160'' above the limb. These waves can be traced to originate from a bright region of the on-disk part of the coronal hole where the propagation speed is in the range of 25 ± 1.3 to 38 ± 4.5 km s−1, with the same periodicity. These on-disk bright regions can be visualized as the base of the coronal funnels. The adjacent plume region also shows the presence of propagating disturbances with the same range of periodicity but with propagation speeds in the range of 135 ± 18 to 165 ± 43 km s−1 only. A comparison between the distance–time radiance map of the two regions indicates that the waves within the plumes are not observable (may be getting dissipated) far off-limb, whereas this is not the case in the inter-plume region. A correlation analysis was also performed to find out the time delay between the oscillations at several heights in the off-limb region, finding results consistent with those from the analysis of the distance–time maps. To our knowledge, this result provides first spectroscopic evidence of the acceleration of propagating disturbances in the polar region close to the Sun (within 1.2 R/R☉), which provides clues to the understanding of the origin of these waves. We suggest that the waves are likely either Alfvénic or fast magnetoacoustic in the inter-plume region and slow magnetoacoustic in the plume region. This may lead to the conclusion that inter-plumes are a preferred channel for the acceleration of the fast solar wind.

Signatures of Alfvén waves in the polar coronal holes as seen by EIS/Hinode

Context. We diagnose the properties of the plume and interplume regions in a polar coronal hole and the role of waves in the acceleration of the solar wind. Aims. We attempt to detect whether Alfven waves are present in the polar coronal holes through variations in EUV line widths. Methods. Using spectral observations performed over a polar coronal hole region with the EIS spectrometer on Hinode, we study the variation in the line width and electron density as a function of height. We use the density sensitive line pairs of Fe xii 186.88 A & 195.119 A and Fe xiii 203.82 A & 202.04 A . Results. For the polar region, the line width data show that the nonthermal line-of-sight velocity increases from 26 km/s at 1000 above the limb to 42 km/s some 15000 (i.e. 110,000 km) above the limb. The electron density shows a decrease from 3:3 10^9 cm^-3 to 1:9 10^8 cm^-3 over the same distance. Conclusions. These results imply that the nonthermal velocity is inversely proportional to the quadratic root of the electron density, in excellent agreement with what is predicted for undamped radially propagating linear Alfven waves. Our data provide signatures of Alfven waves in the polar coronal hole regions, which could be important for the acceleration of the solar wind.

Understanding structures at the base of the solar corona – polar plumes

AbstractRecent work on coronal polar plumes (Gabriel et al. 2003, 2005) has aimed at determining the outflow velocity in plume and interplume regions, using the Doppler dimming technique on oxygen VI observations by SUMER and UVCS on SOHO. By comparing observations of SOHO/EIT with plume modelling, we show that the major part of plumes is the result of chance alignments along the line-of-sight of small enhancements in intensity. This confirms the so-called curtain model. These plumes can be attributed to reconnection activity along the boundaries of supergranule cells. A second population of plumes has a lower abundance and arises from surface bright points having a particular magnetic configuration. New observations using the Hinode/EIS spectrometer are in progress, with the aim of providing further insight for this model.