Coronal Hole Boundaries Research Articles

Observations of ICMEs and ICME-like Solar Wind Structures from 2007 – 2010 Using Near-Earth and STEREO Observations

The generally low interplanetary magnetic field magnitude around the minimum between Solar Cycles 23 and 24 (SC 23/24 minimum) allows us to identify weak and small solar wind structures. We use observations from near-Earth and twin STEREO spacecraft to study solar wind conditions from January 2007 through December 2010. In addition to 84 clear interplanetary coronal mass ejections (ICMEs), we identified 58 ICME-like transients, which exhibit some classical ICME signatures but have weak magnetic fields (<7 nT) and/or short durations (<10 hours). The number of ICME-like transients peaked during the SC 23/24 minimum, while the ICME rate increased with increasing solar activity. The magnetic structures of flux rope type ICMEs and transients show similar solar cycle variation trends, suggesting that ICMEs and transients originate from similar polarity regions at the Sun. We observed a gradual transition from ICME-like structures to ICMEs. The identified events display continuous distributions in duration and magnetic field magnitude ranging from a few hours to several days and from a few nanoteslas to a few tens of nanoteslas, respectively. Our ICME-like transient rate (less than one event/month) is considerably smaller than that suggested by solar observations of narrow CMEs. This implies that the majority of small coronal ejections are merged as a part of the solar wind by the time they reach 1 AU. We found that ICME-like transients generally occur closer to stream interaction regions (SIRs) than ICMEs, and the majority of the events we identified in declining parts of fast solar wind streams were ICME-like structures. This suggests that ICME-like transients tend to arise close to coronal hole boundaries and thus may have an important role in coronal hole dynamics. Diverse solar wind transients presumably manifest the variation of solar eruptions from small-scale blobs to wide CMEs.

Relation between current sheets and vortex sheets in stationary incompressible MHD

Abstract. Magnetohydrodynamic configurations with strong localized current concentrations and vortices play an important role in the dissipation of energy in space and astrophysical plasma. Within this work we investigate the relation between current sheets and vortex sheets in incompressible, stationary equilibria. For this approach it is helpful that the similar mathematical structure of magnetohydrostatics and stationary incompressible hydrodynamics allows us to transform static equilibria into stationary ones. The main control function for such a transformation is the profile of the Alfvén-Mach number MA, which is always constant along magnetic field lines, but can change from one field line to another. In the case of a global constant MA, vortices and electric current concentrations are parallel. More interesting is the nonlinear case, where MA varies perpendicular to the field lines. This is a typical situation at boundary layers like the magnetopause, heliopause, the solar wind flowing around helmet streamers and at the boundary of solar coronal holes. The corresponding current and vortex sheets show in some cases also an alignment, but not in every case. For special density distributions in 2-D, it is possible to have current but no vortex sheets. In 2-D, vortex sheets of field aligned-flows can also exist without strong current sheets, taking the limit of small Alfvén Mach numbers into account. The current sheet can vanish if the Alfvén Mach number is (almost) constant and the density gradient is large across some boundary layer. It should be emphasized that the used theory is not only valid for small Alfvén Mach numbers MA

Short term topological changes of coronal holes associated with prominence eruptions and subsequent CMEs

We study the short-term topological changes of equatorial and polar coronal hole (CH) boundaries, such as a variation of their area and disintegration, associated to reconnection with nearby (within 15° distance) quiescent prominence magnetic fields leading to eruptions and subsequent Coronal Mass Ejections (CMEs). The examples presented here correspond to the recent solar minimum years 2008 and 2009. We consider a temporal window of one day between the CH topological changes and the start and end times of prominence eruptions and onset of CMEs. To establish this association we took into account observational conditions related to the instability of prominence/filaments, the occurrence of a CME, as well as the subsequent evolution after the CME. We found an association between short-term local topological changes in CH boundaries and the formation/disappearance of bright points near them, as well as, between short-term topological changes within the whole CH and eruptions of nearby quiescent prominences followed by the appearance of one or more CMEs.

Formation, shape, and evolution of magnetic structures in CIRs at 1 AU

We have surveyed the properties of 153 co‐rotating interaction regions (CIRs) observed at 1 AU from January, 1995 through December, 2008. We identified that 74 of the 153 CIRs contain planar magnetic structures (PMSs). For planar and non‐planar CIRs, we compared distributions of the bulk plasma and magnetic field parameters. Our identification of CIRs and their features yields the following results: (1) The different pressures within CIRs are strongly correlated. (2) There is no statistical difference between planar and non‐planar CIRs in the distributions and correlations between bulk plasma and magnetic field parameters. (3) The mean observed CIR azimuthal tilt is within 1 σ of the predicted Parker spiral at 1 AU, while the mean meridional tilt is about 20°. (4) The meridional tilt of CIRs changes from one solar rotation to the next, with no relationship between successive reoccurrences. (5) The meridional tilt of CIRs in the ecliptic is not ordered by the magnetic field polarity of the parent coronal hole. (6) Although solar wind deflection is a function of CIR shape and speed, the relationship is not in agreement with that predicted by Lee (2000). We conclude the following: (1) PMSs in CIRs are not caused by a unique characteristic in the local plasma or magnetic field. (2) The lack of relationship between CIR tilt and its parent coronal hole suggests that coronal hole boundaries may be more complex than currently observed. (3) In general, further theoretical work is necessary to explain the observations of CIR tilt.

Evolution of coronal hole boundaries observed by SDO

With the observations from the Solar Dynamics Observatory , we investigate the boundary evolution of a coronal hole (CH). Small-scale solar activity ( e.g. , coronal jet) can lead to gradual change of CH boundaries (CHBs). Besides gradual change of CHBs, a quiet Sun region can change into a CH region on a large scale when magnetic fields in the photosphere evolve from mixed-polarities into one-polarity dominance. CH regions can also evolve into quiet regions on a large scale when the underlying photosphere becomes mixed-polarity fields from one-polarity dominated fields.

Improvement to the global distribution of coronal plasma and magnetic field on the source surface using expansion factor fs and angular distance θb

In this paper, we have developed an improved model to build a self-consistent global structure on the source surface of 2.5 Rs covering four different phases of solar activity. This model involves the topological effect of the magnetic field expansion factor fs and the minimum angular distance θb(at the photosphere) between an open field foot point and its nearest coronal hole boundary. The purpose of this effect is to separate the open field area and the close field area more effectively. The model uses as input for 136 Carrington Rotations (CRs) covering four different phases of solar activity: (1) an empirical model of the magnetic field topology on the source surface using Line-of-sight (los) photospheric field (Blos) measurements by Wilcox Solar Observatory (WSO); and (2) an empirically derived global coronal density distribution using K coronal polarized brightness (pB) by MKIII in High Altitude Observator (HAO). The solar wind speed on the source surface is specified by the function of both fs and θb, which are obtained by the magnetic field data. Then the coronal mass outputs are analyzed and the self-consistent global distribution on the source surface is numerically studied for the four different phases. Finally, the model estimates the solar wind speed at 1AU as a simple function of the speed on the source surface. Our numerical results indicate reasonable semi-quantitative agreement with observations at different phases of solar activity.

PLASMOID RELEASES IN THE HELIOSPHERIC CURRENT SHEET AND ASSOCIATED CORONAL HOLE BOUNDARY LAYER EVOLUTION

As the heliospheric current sheet (HCS) is corotating past STEREO-B, near-Earth spacecraft ACE, Wind and Cluster, and STEREO-A over more than three days between 2008 January 10 and 14, we observe various sections of (near-pressure-balanced) flux-rope- and magnetic-island-type plasmoids in the associated heliospheric plasma sheet (HPS). The plasmoids can qualify as slow interplanetary coronal mass ejections and are relatively low proton beta (<0.5) structures, with small length scales (an order of magnitude lower than typical magnetic cloud values) and low magnetic field strengths (2-8 nT). One of them, in particular, detected at STEREO-B, corresponds to the first reported evidence of a detached plasmoid in the HPS. The in situ signatures near Earth are associated with a long-decay X-ray flare and a slow small-scale streamer ejecta, observed remotely with white-light coronagraphs aboard STEREO-B and SOHO and tracked by triangulation. Before the arrival of the HPS, a coronal hole boundary layer (CHBL) is detected in situ. The multi-spacecraft observations indicate a CHBL stream corotating with the HCS but with a decreasing speed distribution suggestive of a localized or transient nature. While we may reasonably assume that an interaction between ejecta and CHBL provides the source of momentum for the slow ejecta's acceleration, the outstanding composition properties of the CHBL near Earth provide here circumstantial evidence that this interaction or possibly an earlier one, taking place during streamer swelling when the ejecta rises slowly, results in additional mixing processes.

SDO OBSERVATIONS OF MAGNETIC RECONNECTION AT CORONAL HOLE BOUNDARIES

With the observations from the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) aboard the \emph{Solar Dynamics Observatory}, we investigate the coronal hole boundaries (CHBs) of an equatorial extension of polar coronal hole. At the CHBs, lots of extreme-ultraviolet (EUV) jets, which appear to be the signatures of magnetic reconnection, are observed in the 193 {\AA} images, and some jets occur repetitively at the same sites. The evolution of the jets is associated with the emergence and cancelation of magnetic fields. We notice that both the east and the west CHBs shift westward, and the shift velocities are close to the velocities of rigid rotation compared with those of the photospheric differential rotation. This indicates that magnetic reconnection at CHBs results in the evolution of CHBs and maintains the rigid rotation of coronal holes.

MAGNETIC TOPOLOGY OF CORONAL HOLE LINKAGES

In recent work, Antiochos and coworkers argued that the boundary between the open and closed field regions on the Sun can be extremely complex with narrow corridors of open flux connecting seemingly disconnected coronal holes from the main polar holes, and that these corridors may be the sources of the slow solar wind. We examine, in detail, the topology of such magnetic configurations using an analytical source surface model that allows for analysis of the field with arbitrary resolution. Our analysis reveals three important new results: First, a coronal hole boundary can join stably to the separatrix boundary of a parasitic polarity region. Second, a single parasitic polarity region can produce multiple null points in the corona and, more important, separator lines connecting these points. It is known that such topologies are extremely favorable for magnetic reconnection, because they allow this process to occur over the entire length of the separators rather than being confined to a small region around the nulls. Finally, the coronal holes are not connected by an open-field corridor of finite width, but instead are linked by a singular line that coincides with the separatrix footprint of the parasitic polarity. We investigate how the topological features described above evolve in response to the motion of the parasitic polarity region. The implications of our results for the sources of the slow solar wind and for coronal and heliospheric observations are discussed.

SHORT-TERM EVOLUTION OF CORONAL HOLE BOUNDARIES

The interaction of open and closed field lines at coronal hole boundaries is widely accepted to be due to interchange magnetic reconnection. To date, it is unclear how the boundaries vary on short timescales and at what velocity this occurs. Here, we describe an automated boundary tracking method used to determine coronal hole boundary displacements on short timescales. The bound- ary displacements were found to be isotropic and to have typical expansion/contraction speeds of \leq2 km s^-1, which indicate magnetic reconnection rates of \leq 3 \times 10^-3. The observed displacements were used in conjunction with the interchange reconnection model to derive typical diffusion coeffi- cients of \leq 3 \times 10^13 cm^2 s^-1. These results are consistent with an interchange reconnection process in the low corona driven by the random granular motion of open and closed fields in the photosphere.

Interchange Reconnection: Remote Sensing of Solar Signature and Role in Heliospheric Magnetic Flux Budget

Interchange reconnection at the Sun, that is, reconnection between a doubly-connected field loop and singly-connected or open field line that extends to infinity, has important implications for the heliospheric magnetic flux budget. Recent work on the topic is reviewed, with emphasis on two aspects. The first is a possible heliospheric signature of interchange reconnection at the coronal hole boundary, where open fields meet closed loops. The second aspect concerns the means by which the heliospheric magnetic field strength reached record-lows during the recent solar minimum period. A new implication of this work is that interchange reconnection may be responsible for the puzzling, occasional coincidence of the heliospheric current sheet and the interface between fast and slow flow in the solar wind.

Semiempirical Models of the Slow and Fast Solar Wind

Coronal holes can produce several types of solar wind with a variety of compositional properties, depending on the location and strength of the heating along their open magnetic field lines. High-speed wind is associated with (relatively) slowly diverging flux tubes rooted in the interiors of large holes with weak, uniform footpoint fields; heating is spread over a large radial distance, so that most of the energy is conducted outward and goes into accelerating the wind rather than increasing the mass flux. In the rapidly diverging open fields present at coronal hole boundaries and around active regions, the heating is concentrated at low heights and the temperature maximum is located near the coronal base, resulting in high oxygen freezing-in temperatures and low asymptotic wind speeds. Polar plumes have a strong additional source of heating at their bases, which generates a large downward conductive flux, raising the densities and enhancing the radiative losses. The relative constancy of the solar wind mass flux at Earth reflects the tendency for the heating rate in coronal holes to increase monotonically with the footpoint field strength, with very high mass fluxes at the Sun offsetting the enormous flux-tube expansion in active region holes. Although coronal holes are its main source, slow wind is also released continually from helmet streamer loops by reconnection processes, giving rise to plasma blobs (small flux ropes) and the heliospheric plasma sheet.

Large-scale Bright Fronts in the Solar Corona: A Review of “EIT waves”

``EIT waves" are large-scale coronal bright fronts (CBFs) that were first observed in 195 \AA\ images obtained using the Extreme-ultraviolet Imaging Telescope (EIT) onboard the \emph{Solar and Heliospheric Observatory (SOHO)}. Commonly called ``EIT waves", CBFs typically appear as diffuse fronts that propagate pseudo-radially across the solar disk at velocities of 100--700 km s$^{-1}$ with front widths of 50-100 Mm. As their speed is greater than the quiet coronal sound speed ($c_s\leq$200 km s$^{-1}$) and comparable to the local Alfv\'{e}n speed ($v_A\leq$1000 km s$^{-1}$), they were initially interpreted as fast-mode magnetoacoustic waves ($v_{f}=(c_s^2 + v_A^2)^{1/2}$). Their propagation is now known to be modified by regions where the magnetosonic sound speed varies, such as active regions and coronal holes, but there is also evidence for stationary CBFs at coronal hole boundaries. The latter has led to the suggestion that they may be a manifestation of a processes such as Joule heating or magnetic reconnection, rather than a wave-related phenomena. While the general morphological and kinematic properties of CBFs and their association with coronal mass ejections have now been well described, there are many questions regarding their excitation and propagation. In particular, the theoretical interpretation of these enigmatic events as magnetohydrodynamic waves or due to changes in magnetic topology remains the topic of much debate.

Suprathermal electron flux peaks at stream interfaces: Signature of solar wind dynamics or tracer for open magnetic flux transport on the Sun?

The high variability of the intensity of suprathermal electron flux in the solar wind is usually ascribed to the high variability of sources on the Sun. Here we demonstrate that a substantial amount of the variability arises from peaks in stream interaction regions, where fast wind runs into slow wind and creates a pressure ridge at the interface. Superposed epoch analysis centered on stream interfaces in 26 interaction regions previously identified in Wind data reveal a twofold increase in 250 eV flux (integrated over pitch angle). Whether the peaks result from the compression there or are solar signatures of the coronal hole boundary, to which interfaces may map, is an open question. Suggestive of the latter, some cases show a displacement between the electron and magnetic field peaks at the interface. Since solar information is transmitted to 1 AU much more quickly by suprathermal electrons compared to convected plasma signatures, the displacement may imply a shift in the coronal hole boundary through transport of open magnetic flux via interchange reconnection. If so, however, the fact that displacements occur in both directions and that the electron and field peaks in the superposed epoch analysis are nearly coincident indicate that any systematic transport expected from differential solar rotation is overwhelmed by a random pattern, possibly owing to transport across a ragged coronal hole boundary.

THREE-DIMENSIONAL SOLARWINDMODELING FROM THE SUN TO EARTH BY A SIP-CESE MHD MODEL WITH A SIX-COMPONENT GRID

The objective of this paper is to explore the application of a six-component overset grid to solar wind simulation with a three-dimensional (3D) Solar-InterPlanetary Conservation Element/Solution Element MHD model. The essential focus of our numerical model is devoted to dealing with: (1) the singularity and mesh convergence near the poles via the use of the six-component grid system, (2) the ∇ · B constraint error via an easy-to-use cleaning procedure by a fast multigrid Poisson solver, (3) the Courant-Friedrichs-Levy number disparity via the Courant-number insensitive method, (4) the time integration by multiple time stepping, and (5) the time-dependent boundary condition at the subsonic region by limiting the mass flux escaping through the solar surface. In order to produce fast and slow plasma streams of the solar wind, we include the volumetric heating source terms and momentum addition by involving the topological effect of the magnetic field expansion factor fS and the minimum angular distance θ b (at the photosphere) between an open field foot point and its nearest coronal hole boundary. These considerations can help us easily code the existing program, conveniently carry out the parallel implementation, efficiently shorten the computation time, greatly enhance the accuracy of the numerical solution, and reasonably produce the structured solar wind. The numerical study for the 3D steady-state background solar wind during Carrington rotation 1911 from the Sun to Earth is chosen to show the above-mentioned merits. Our numerical results have demonstrated overall good agreements in the solar corona with the Large Angle and Spectrometric Coronagraph on board the Solar and Heliospheric Observatory satellite and at 1 AU with WIND observations.

Coronal hole boundaries evolution at small scales

We developed an automated procedure for the identification of transient brightenings in images from the X-ray telescope on-board Hinode taken with an Al Poly filter in the equatorial coronal holes, polar coronal holes, and the quiet Sun with and without transient coronal holes. We found that in comparison to the quiet Sun, the boundaries of coronal holes are abundant with brightening events including areas inside the coronal holes where closed magnetic field structures are present. The visual analysis of these brightenings revealed that around 70% of them in equatorial, polar and transient coronal holes and their boundaries show expanding loop structures and/or collimated outflows. In the quiet Sun only 30% of the brightenings show flows with most of them appearing to be contained in the solar corona by closed magnetic field lines. This strongly suggests that magnetic reconnection of co-spatial open and closed magnetic field lines creates the necessary conditions for plasma outflows to large distances. The ejected plasma always originates from pre-existing or newly emerging (at X-ray temperatures) bright points. The present study confirms our findings that the evolution of loop structures known as coronal bright points is associated with the small-scale changes of coronal hole boundaries. The loop structures show an expansion and eruption with the trapped plasma consequently escaping along the "quasi" open magnetic field lines. These ejections appear to be triggered by magnetic reconnection, e.g. the so-called interchange reconnection between the closed magnetic field lines (BPs) and the open magnetic field lines of the coronal holes. We suggest that these plasma outflows are possibly one of the sources of the slow solar wind.

ON THE “EXTENDED” SOLAR CYCLE IN CORONAL EMISSION

Butterfly diagrams (latitude-time plots) of coronal emission show a zone of enhanced brightness that appears near the poles just after solar maximum and migrates toward lower latitudes; a bifurcation seems to occur at sunspot minimum, with one branch continuing to migrate equatorward with the sunspots of the new cycle and the other branch heading back to the poles. The resulting patterns have been likened to those seen in torsional oscillations and have been taken as evidence for an extended solar cycle lasting over {approx}17 yr. In order to clarify the nature of the overlapping bands of coronal emission, we construct butterfly diagrams from green-line simulations covering the period 1967-2009 and from 19.5 nm and 30.4 nm observations taken with the Extreme-Ultraviolet Imaging Telescope during 1996-2009. As anticipated from earlier studies, we find that the high-latitude enhancements mark the footpoint areas of closed loops with one end rooted outside the evolving boundaries of the polar coronal holes. The strong underlying fields were built up over the declining phase of the cycle through the poleward transport of active-region flux by the surface meridional flow. Rather than being a precursor of the new-cycle sunspot activity zone, the high-latitude emission forms a physically distinct,more » U-shaped band that curves upward again as active-region fields emerge at midlatitudes and reconnect with the receding polar-hole boundaries. We conclude that the so-called extended cycle in coronal emission is a manifestation not of early new-cycle activity, but of the poleward concentration of old-cycle trailing-polarity flux by meridional flow.« less

INTERCHANGE RECONNECTION AND CORONAL HOLE DYNAMICS

We investigate the effect of magnetic reconnection between open and closed field, (often referred to as reconnection), on the dynamics and topology of coronal hole boundaries. The most important and most prevalent 3D topology of the interchange process is that of a small-scale bipolar magnetic field interacting with a large-scale background field. We determine the evolution of such a magnetic topology by numerical solution of the fully 3D MHD equations in spherical coordinates. First, we calculate the evolution of a small-scale bipole that initially is completely inside an open field region and then is driven across a coronal hole boundary by photospheric motions. Next the reverse situation is calculated in which the bipole is initially inside the closed region and driven toward the coronal hole boundary. In both cases we find that the stress imparted by the photospheric motions results in deformation of the separatrix surface between the closed field of the bipole and the background field, leading to rapid current sheet formation and to efficient reconnection. When the bipole is inside the open field region, the reconnection is of the interchange type in that it exchanges open and closed field. We examine, in detail, the topology of the field as the bipole moves across the coronal hole boundary, and find that the field remains well-connected throughout this process. Our results imply that open flux cannot penetrate deeply into the closed field region below a helmet streamer and, hence, support the quasi-steady models in which open and closed flux remain topologically distinct. Our results also support the uniqueness hypothesis for open field regions as postulated by Antiochos et al. We discuss the implications of this work for coronal observations. Subject Headings: Sun: corona Sun: magnetic fields Sun: reconnection Sun: coronal hole

GLOBAL SIMULATION OF AN EXTREME ULTRAVIOLET IMAGING TELESCOPE WAVE

We use the observation of an Extreme Ultraviolet Imaging Telescope (EIT) wave in the lower solar corona, seen with the two Solar Terrestrial Relations Observatory (STEREO) spacecraft in extreme ultraviolet light on 2007 May 19, to model the same event with a three-dimensional (3D) time-depending magnetohydrodynamic (MHD) code that includes solar coronal magnetic fields derived with Wilcox Solar Observatory magnetogram data, and a solar wind outflow accelerated with empirical heating functions. The model includes a coronal mass ejection (CME) of Gibson and Low flux rope type above the reconstructed active region with parameters adapted from observations to excite the EIT wave. We trace the EIT wave running as circular velocity enhancement around the launching site of the CME in the direction tangential to the sphere produced by the wave front, and compute the phase velocities of the wave front. We find that the phase velocities are in good agreement with theoretical values for a fast magnetosonic wave, derived with the physical parameters of the model, and with observed phase speeds of an incident EIT wave reflected by a coronal hole and running at about the same location. We also produce in our 3D MHD model the observed reflection of the EIT wave at the coronal hole boundary, triggered by the magnetic pressure difference between the wave front hitting the hole and the boundary magnetic fields of the coronal hole, and the response of the coronal hole, which leads to the generation of secondary reflected EIT waves radiating away in different directions than the incident EIT wave. This is the first 3D MHD model of an EIT wave triggered by a CME that includes realistic solar magnetic field, with results comparing favorably to STEREO Extreme Ultraviolet Imager observations.

Trans-Equatorial Loop System Arising from Coronal Hole Boundaries through Interactions between Active Regions and Coronal Holes

It is not clear how trans-equatorial loop systems (TLSs) are formed, although they have been observed often with Yohkoh/SXT. We focus here on a TLS that appeared on 27 May 1998. Yokoyama and Masuda (Solar Phys.254, 285, 2009) proposed a new scenario for the formation mechanism of the TLS. In this scenario, they pointed out the importance of magnetic interaction between an active region and a coronal hole to make “strong-seed magnetic fields” before a transient (bright and short-lived) trans-equatorial loop was created. The main aims of this study are to verify the scenario and to make the TLS formation mechanism clear, based on observational data. Yohkoh/SXT images, SOHO/MDI magnetograph data, and Kitt Peak coronal-hole maps were mainly used for our analyses. We investigated the TLS in detail from the time that there were no signatures of the TLS to its clear appearance. The following results are obtained: i) an active region emerged in the vicinity of a coronal-hole boundary, ii) the coronal-hole boundary retreated during the period when the active region was developing, iii) temporal variations of soft X-ray intensities were roughly synchronized between the coronal-hole boundary and a trans-equatorial region, and iv) new closed loops were observed in soft X-rays clearly at the coronal-hole boundary. Since i), ii), iii), and iv) are just what we expect in the scenario of YM2009, the scenario found support. We conclude that the TLS was originating with large-scale magnetic fields of the coronal-hole boundary through magnetic reconnection between the active region and a coronal hole.