EDP Sciences logo
Subscriber Authentication Point
Search
Menu
Home All issues Volume 683 (March 2024) A&A, 683 (2024) A126 Full HTML
Open Access
Issue
A&A
Volume 683, March 2024
Article Number A126
Number of page(s) 11
Section The Sun and the Heliosphere
DOI https://doi.org/10.1051/0004-6361/202347785
Published online 13 March 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.

1. Introduction

Coronal streamers are large-scale quasi-static structures rising above filaments and/or filament channels, which extend from the solar surface out into interplanetary space for many solar radii and are one of the most magnificent features seen during a solar eclipse (e.g., Saito & Tandberg-Hanssen 1973; Ambrož et al. 2009; Pasachoff et al. 2011, 2015; Boe et al. 2020; Liang et al. 2023). They are believed to be the results of a complex interaction between the surrounding solar wind and large-scale magnetic fields (Wang & Sheeley 1992). They can root at almost all latitudes during a sunspot maximum, and concentrate around the equator during a sunspot minimum (Spadaro et al. 2007; Ventura et al. 2005). The formation of a current system with a slowly dissipating magnetic field is thought to be responsible for such a stable structure being maintained over many solar radii and several months (Koutchmy & Livshits 1992). There are two classes of coronal streamers, helmet streamers and pseudo-streamers (Wang et al. 2007). Helmet streamers consist of systems of magnetic loops and open magnetic field lines with opposite magnetic polarities separated by the loops (Sturrock & Smith 1968; Romoli et al. 2021). The plasma escaping along these open magnetic field lines forms the helmet streamer rays (Eselevich & Eselevich 1999; Poirier et al. 2020). The polarity inversion line or coronal neutral line that forms near the top of helmet streamers is the coronal origin of the heliospheric current sheet (HCS; Crooker et al. 1993; Suess et al. 2009). Helmet streamer rays are thought to engulf the HCS and be the source of the heliospheric plasma sheet (HPS) typically measured in situ while crossing the HCS (Winterhalter et al. 1994; Grappin et al. 2000; Rouillard et al. 2020).

Coronal streamers are dynamic and small-scale transients such as blobs frequently occur in them (Sheeley et al. 1997, 2009; Wang et al. 1998; Song et al. 2009, 2012; Lyu et al. 2023). Such dynamics in coronal streamers might be related to the formation of slow solar wind detected in the interplanetary space, including that near the Earth (Lotova et al. 2000; Romoli et al. 2021; Ventura et al. 2023). By comparing the measured outflow velocities and abundances of similar elements with the slow solar wind measured in situ, many previous studies have concluded that the legs and stalks of the coronal helmet streamer may be the source regions of the slow solar wind (Raymond et al. 1997; Strachan et al. 2002; Uzzo et al. 2003; Zhao et al. 2021; Chen et al. 2021).

Sheeley et al. (1997) studied white-light coronal images taken by SOHO/LASCO and they traced small-scale nonuniform structures in a coronal helmet streamer with a length of about 1 R⊙ (solar radius) and a width of about 0.1 R⊙. These structures were found to move radially outward to a distance of 3−4 R⊙ at least, and referred to as “blobs” due to their density inhomogeneity. Using data from the twin satellites of STEREO, Sheeley et al. (2009) analysed streamer blobs observed simultaneously with edge-on and face-on views and found that the blobs have a concave outward structure in the edge-on views and an arch topology in the face-on views. This suggests that streamer blobs have a flux rope nature, as has been proposed in simulations (Thernisien & Howard 2006), and is consistent with observations of gradually accelerating coronal mass ejections related to streamer blobs (Wang & Sheeley 2006; Sheeley & Wang 2007). Crooker et al. (2004) concluded that the materials in blobs and coronal streamers are confined to narrow sheets of plasma, and blobs may be the counterpart of the heliospheric plasma sheet with a high β value. By tracing them in coronal images taken by SOHO/LASCO, these blobs are found to be accelerated while they are propagating away from the sun, and to be in a range below 150 km s−1 at a distance of 3−4 R⊙ and about 400 km s−1 at a distance of about 25 R⊙ (Sheeley et al. 1997, 1999; Wang et al. 1998). The latter one is characteristic of slow solar wind speed, and thus these blobs are candidates for the sources of slow solar wind (Sheeley et al. 1997, 1999; Wang et al. 1998). Such a proposal is also demonstrated by a recent work by Rouillard et al. (2020), in which they trace a series of coronal streamer blobs from about 4 R⊙ to about 100 R⊙ in the observations of STEREO-A/COR2/HI1. They find that those streamer blobs correspond to enhanced density structures lasting from tens of minutes to tens of hours at about 40 R⊙ in the in situ observations from PSP.

The origin of streamer blobs is thought to be crucial because that might directly link to the origin of the solar wind. Wang et al. (1998) proposed a scenario that interchange reconnection between loops and open field lines could release materials trapped in the close fields into open fields and then form the observed blobs in the coronal streamers (see their Fig. 8). Simulations of interchange reconnection dynamics in a solar pseudo-streamer has revealed that coronal plasma with a closed field can be injected into interplanetary space (Pellegrin-Frachon et al. 2023). Alternatively, the simulation carried out by Chen et al. (2009) suggests that the formation of blobs could be the result of an intrinsic instability in coronal streamers around the cusp due to the dynamic equilibrium between the weak magnetic field constraint and the high temperature expansion of the plasma. In an MHD simulation, Higginson & Lynch (2018) find that the flux rope structures of a blob in a coronal helmet streamer could be generated by pinching-off reconnections as the streamer top stretches out.

To understand what really is responsible for the formation of a streamer blob, observations of its origins are crucial. Recently, Lee et al. (2021) used coordinated observations from the ground-based coronagraph K-Cor and LASCO C2 to study the dynamics of streamers in great detail. They find that blobs are generated along the centers and legs of helmet streamers and pseudo-streamers before finally merging into their stalks. They also find that blobs formed along the centers of the helmet streamers originate at an average height of about 2.6 R⊙, which is below the top of the streamer cores, while blobs formed along the legs of helmet streamers and pseudo-streamers might be generated at or below 2 R⊙. Unfortunately, in their study, streamer blobs were not seen below 2 R⊙ by K-Cor, possibly due to the limitation of ground-based observations, and thus the forming processes of streamer blobs in the very beginning remain unclear.

Specially designed observations from the Solar Ultraviolet Imager (SUVI, Seaton & Darnel 2018; Vasudevan et al. 2019) on the Geostationary Operational Environmental Satellite 17 (GOES-17) spacecraft have revealed very dynamic natures of the middle corona in a range of 1.5−3 R⊙, providing evidence that the solar wind structures, including those carried by streamer blobs, have origins in this region (Seaton et al. 2021; Chitta et al. 2023; West et al. 2023). Here, we make use of these specially designed data, in combination with the data from the C2 coronagraph of the Large Angle and Spectrometric Coronagraph (LASCO C2, Brueckner et al. 1995) on the Solar and Heliospheric Observatory (SOHO, Domingo et al. 1995) and the Atmospheric Imaging Assembly (AIA, Lemen et al. 2012) on the Solar Dynamics Observatory (SDO, Pesnell et al. 2012), in order to investigate in depth the formation processes of a streamer blob. We aim to find out where the streamer blob is formed from the very beginning and which activities in the lower corona are responsible. From that, we engage in a further discussion on the mechanism of its formation. In what follows, we give descriptions of the analysed data in Sect. 2, the results in Sect. 3, the discussion in Sect. 4, and our conclusions in Sect. 5.

2. Observations

The coordinated data analysed in the present study were taken by GOES-17/SUVI, SOHO/LASCO C2, and SDO/AIA. GOES-17/SUVI is an EUV imager equipped with a CCD with 1280 × 1280 pixels and a pixel scale of 2.5 arcsec per pixel. LASCO C2 is an externally occulted white-light coronagraph with a field of view (FOV) extending from 2 R⊙ to 6 R⊙, a pixel size of 11.4 arcsec, and a cadence of 45 s. SDO/AIA is a set of EUV imagers observing the full disk of the sun with a resolution of 0.6 arcsec per pixel and a cadence of 12 s.

We analyzed the LASCO C2 data taken in the period between 00:00 UT to 23:59 UT on August 22, 2018, during which we observed a streamer blob initiating from the location of 2 R⊙. The data were calibrated by standard procedures provided by the instrument team along with solarsoft.

The SUVI data were specially designed to obtain the EUV images of the Sun with an east–west rastering mode so that the FOV can extend to about 5 R⊙ in the horizontal direction. In this observing mode, it cannot provide simultaneous views of the disk while pointing to the side. For each pointing, the cadence of the observations is around 20 min but not regular. More details of these observations can be found in Seaton et al. (2021). The SUVI data were calibrated by the instrument team and no further calibration is required. In order to trace the evolution of the inner corona below 1.3 R⊙, we used observations from AIA 193 Å passband, in coordination with the SUVI images taken at the passband of 195 Å. These two passbands observe the solar structures with similar representative temperatures, and the streamer structures are best seen. The AIA data were calibrated by the standard procedures provided by the instrument team.

In order to see better the coronal structures in the images, several techniques were employed. For the LASCO C2 images, the time-independent F corona was removed by subtracting a time-averaged background and then running difference images that were produced to bring out the faint, moving blob in the streamer. For the SUVI images, we first applied a normalizing radial graded filter (NRGF, Morgan et al. 2006) and then a two-dimensional Savitzky-Golay filter with a 7 × 7 window, which enabled us to reveal more details of the streamer (Seaton et al. 2021). The data from the different instruments were co-aligned by cross-checking the connections of the global structures near the limb, and we can see a good correlation between the coronal structures in the low to middle corona. To trace the evolution of the activities in the streamer appearing on the eastern limb, the center part of a SUVI image was replaced by the AIA image at a time close to that of the eastward-pointing image.

3. Results

The image of the corona seen by LASCO C2 is shown in panel a of Fig. 1, while the potential field given by the PFSS model is shown in panel b. We can see that the streamer is highly flattened toward the heliographic equator because the year 2018 is near the solar minimum (Wang et al. 2000), which is consistent with the PFSS model. We concentrate on the streamer appearing on the eastern limb, which can be considered to be a helmet streamer since it separates open field lines with opposite polarities, as is shown in the PFSS model. It consists of a cluster of coronal loops with open field lines at both sides.

thumbnail Fig. 1.

(a): Polarized white-light image of the corona recorded with the LASCO C2 on August 22, 2018. The inner boundary of the FOV is at 2 R⊙ and the outer boundary is at about 6 R⊙. The white circle indicates the limb of the solar disk. (b): Potential magnetic field extrapolated with PFSS model overlying on a photospheric synoptic magnetogram. The purple and green lines are representative of open field lines and the white lines are closed loops.

To follow the evolution of the streamer, the running difference images from the LASCO C2 observations are produced (see Fig. 2). A small blob in the streamer in the LASCO C2 FOV is firstly seen at 02:20 UT on August 22, 2018 (see the structure denoted by red circles in the left and middle panels of Fig. 2). The blob has a size of about 0.4 R⊙, which is typical in these phenomena (Sheeley et al. 1997, 2009). The blob propagates slightly toward the southern flank of the streamer, where we also observe the expansion of a bright, narrow, elongated feature. The expansion of this feature coincides with the propagation of the blob. Their possible connection will be discussed further later. We then make a height-time (H-T) plot to follow the propagation of the blob, as is shown in the right panel of Fig. 2. From the H-T plot, we can obtain the propagation speeds of the blob from the slopes of the tangents of the trajectory. We can see that the blob is accelerated from about 14 km s−1 at 2 R⊙ to about 42 km s−1 at 4.5 R⊙, which agrees with previous studies (Wang et al. 2000).

thumbnail Fig. 2.

Left and middle panels: running difference images obtained from LASCO C2 observations at 10:24 UT and 12:36 UT on August 22, in which a small propagating blob is shown as a structure with dark center surrounded by bright features (marked by circles in dashed red lines). Right panel: a time-distance map tracking the propagation of the streamer blob in the running difference images. The time starts at 00:00 UT on August 22. The dashed green line denotes the propagation of the streamer blob, from which the tangent derives its propagating speeds. An associated animation is given online.

The connection of the blob observed by SUVI and LASCO/C2 is shown in Fig. 3 and the associated animation. We can see the overall structures of the streamer seen in SUVI and white light are very consistent. In the animation, we can see that the blob formed at about 1.8 R⊙ and gradually moved radially outward. The blob apparently formed at the tip of the streamer loops, as is seen in the SUVI images. While the streamer loops expand, we observe that the leading edge of the blob first becomes sharp and thin, and then moves outward. It reaches 2.0 R⊙ at about 00:00 UT on August 22.

thumbnail Fig. 3.

Streamer observed in the compositional images of SUVI 195 Å (red) and LASCO C2 (green). The small red circles indicate the location of the blobs recognized in LASCO C2 running difference images (i.e., the left and middle panels of Fig. 2). An associated animation is given online.

In Fig. 4, we show time-distance diagrams of the blob based on the SUVI observations. We find that the speeds of the blob range from ∼8 km s−1 (at 1.9 R⊙) to ∼25 km s−1 (at 2.5 R⊙). Such dynamics of the blob viewed in SUVI are consistent with those seen in LASCO/C2. We can see that the curve of the distances (Fig. 4) can be well fitted by a second-order polynomial function. This suggests that the acceleration of the blob starts at 1.9 R⊙ or lower. Together with the white-light observations (Fig. 2), we can see that the streamer blob is continuously accelerated for several solar radii. The mechanism driving such an acceleration should be an important proxy for understanding the formation of the solar wind.

thumbnail Fig. 4.

Top: time-distance map following the expansion of the bright elongated feature in the SUVI images (the time coordinate starts from 21:17 UT on August 21). The blob presents as the tip of the expanding feature, which can be tracked by the dashed gold line. Bottom: variation in the height of the blob seen in SUVI images (diamonds with a dashed line), which was identified manually. The solid line is a second-order polynomial fit to the variation curve.

We then trace the origin of the blob back in time and in the lower corona, as is seen in the SUVI and AIA observations. In Fig. 5, we show the evolution of the solar corona in the combined SUVI and AIA observations. We find that the blob is formed via the convergence of three clumps of brightenings. The first two clumps are first seen as early as 20:02 UT on August 20 (see the features in the boxes denoted by “N1” and “N2” in Fig. 5a). Both N1 and N2 are elongated structures extending more or less radially, apparently located in the southern flank of the streamer. Due to their bright natures against the ambient corona, they are likely parts of the southern legs of the outer layer of the loop system (see the dashed red and yellow lines in Fig. 6), which is also consistent with the field extrapolation shown in Fig. 1.

thumbnail Fig. 5.

Evolution of the solar corona seen in combined observations of AIA 193 Å (red) and SUVI 195 Å (green). The regions enclosed by dotted lines denote three clumps of brightenings (N1, N2, and N3), and their merging processes can be followed.

N1 is about 0.1 R⊙ in length and 0.02 R⊙ in width, while N2 is about 0.12 R⊙ in length and 0.05 R⊙ in width. These values are not very accurate because they are faint and do not stand out much. N1 starts at about 1.7 R⊙ and N2 starts slightly lower, at about 1.6 R⊙. At about 23:32 UT on August 20, another clump (N3) appears to be pinching off at the top of the cluster of coronal loops (∼1.7 R⊙) in the streamer base and to the north of N1 and N2 (see Figs. 5b and c). These loops seem to be low-lying ones underneath those hosting N1 and N2. The newly formed N3 is apparently approaching N1, and we observe that N1 and N2 are moving outward and begin to merge at around 03:21 UT on August 21 (see Fig. 5d). The convergence of N1 and N2 occurs at a height of about 1.8 R⊙ (see Figs. 5d–f). Meanwhile, N3 keeps moving toward the top point of the convergence of N1 and N2, and they begin to merge at around 18:47 UT on August 21 (Fig. 5g). Over the next eight hours, the merged structure becomes more compact at a height of about 2 R⊙, and then starts moving outward and forms the blob seen in the LASCO C2 FOV (Figs. 5g–i).

We also tracked these convergence processes manually to identify the variations in the centers of the three clumps. In Fig. 7, we show variations in their coordinates on the heliographic projection plane. In the X direction, N1 and N2 are moving away from the limb unidirectionally, while in the Y direction they show some fluctuations. N3 shows back-and-forth motions along X, but along Y it moves southward most of the time. The effective speeds of N1, N2, and N3 before their convergence are 1.5 km s−1, 0.6 km s−1, and 0.7 km s−1 respectively. However, these merging processes do not take into account the projection effect, and the real speeds could be much larger.

Next, we carefully checked the origins of these clumps of brightenings. Although N1 and N2 appear gradually without any clear dynamic processes, we see that the threads where N1 and N2 extend show swaying motions in the south-north direction before the two clumps moving outward. Such a motion can be better seen in the online animation associated with Fig. 6, in which the locations of the swaying threads are indicated by red arrows and the final combination of the three clumps is denoted by yellow arrows. Such swaying motions can also be indicated in the fluctuations of their Y coordinates shown in Fig. 7. We further took three slices perpendicular to the threads at 1.2 R⊙, 1.3 R⊙, and 1.6 R⊙ (Fig. 6) to produce space-time maps (see Fig. 8). The swaying motions of the two threads at these heights do not seem to be in sync. We find that N3 is born at the apex of loop-like structures, likely in a pinching-off process (see the region marked in Fig. 9). At the beginning (see e.g., Fig. 9a), we see that loop-like structures in the region are sparse. However, it becomes more chaotic when N3 is born (see Fig. 9). During this period of time, we also observe that the coronal loops in the lower corona (AIA FOV) show a clear expansion from about 1.4 R⊙ (the dotted black line in Figs. 9a and b) to about 1.5 R⊙ just prior to the birth of N3 (the dotted golden line in Fig. 9b), which the solar rotation cannot compensate for. Such an evolution in the birth of “N3” can be followed in the online animation associated with Fig. 9. A time–distance map demonstrating the birth of N3 is shown in Fig. 10, from which we can see that the expansion of the loop system has an apparent speed of about 4 km s−1 (see the dashed yellow line in the figure). The detachment of N3 is also accompanied by a slight contraction of the loop system, with an apparent speed less than 1 km s−1 (see the dashed cyan line after in Fig. 10). Such dynamics are similar to those reported previously by Wang & Hess (2018), but the present case occurs at a much lower height (below 2 R⊙) and the speeds are also one to two magnitudes smaller. Although we cannot find any direct connection between this motion of loop expansion and the birth of N3 due to the low resolution of the data, we suspect that the expanding loops might trigger the dynamics at the overlying loops and result in instability and/or reconnection there, similar to that suggested by Wang & Hess (2018). The appearance of N3 and its motion toward the two threads of N1 and N2 seem to be the direct trigger to the convergence of the three clumps and then the formation of the streamer blob.

thumbnail Fig. 6.

Combined image of SUVI 195 Å image and AIA 193 Å image. The dashed red and yellow lines mark the loops associated with N1 and N2, respectively. Three slits at 1.2 R⊙, 1.3 R⊙, and 1.6 R⊙ where we obtained distance-time maps (shown in Fig. 8) are marked as black arrows (0 is given at the tail end). These slits are arranged roughly in the same opening angle. We note that the elongated structures are not perfectly aligned in the radial direction. An associated animation is given online.

thumbnail Fig. 7.

Variations in the centers (on the projected X − Y plane) of the three clumps of brightenings. The squares, triangles, and crosses are the locations of “N1”, “N2”, and “N3”, respectively. The change in time is shown as a change in color. The convergence of the three clumps can be followed in this diagram while they are moving toward the same position at the end.

thumbnail Fig. 8.

Time-distance maps obtained from the three slits shown in Fig. 6 based on AIA observations (1.2 R⊙) and SUVI observations (1.3 R⊙ and 1.6 R⊙). The swaying motion of the extended structures (in which “N1” and “N2” are sitting) at 1.2, 1.3, and 1.6 R⊙ can be tracked with the dashed lines.

thumbnail Fig. 9.

Birth of “N3” (enclosed by the dotted red line in panel b) and expansion of low-lying loops seen in the combined SUVI and AIA images. The dotted black lines denote the location of the loop seen in the AIA image at 21:27 UT on August 20 and the dotted yellow line marks that at 02:15 UT on August 21. An associated animation is given online.

thumbnail Fig. 10.

Time–distance map obtained along the expanding direction of the loop system and the propagation of “N3”. The blue arrow indicates the time and location when and where “N3” is pinching off. The dashed yellow line shows the expansion of the loop system and the fall of “N3”. The dashed cyan line indicates the retraction of the loop system.

4. Discussion

Based on these observations, we confirm that the formation of a streamer blob can be a complex process. Here, we suggest the streamer blob observed possibly forms via the processes shown in Fig. 11. A possible magnetic system of the streamer is shown in panel a of Fig. 11. It consists of a loop system, together with open fields rooted in both of its sides. The clumps of “N1” and “N2” are coupled with the southern legs of the outer loops. As the low-lying loops expand, they might force the outer loops that host “N1” and “N2” to converge and the two clumps might merge or simply be arranged at the same location on the projected plane (panel b). Meanwhile, the expanding motion from below of the loop system might lead to density inhomogeneities and/or intrinsic instability due to misalignment of the field lines, and thus uneven forces at different locations near the loop top, including for example a tearing mode instability (Karpen et al. 1998; Einaudi et al. 1999), pinching-off reconnection (Higginson & Lynch 2018), or simply magnetic diffusion (Chen et al. 2009). This results in “N3” breaking down (Figs. 11b and c). However, none of these processes can be demonstrated with the present data due to the limitation of spatial and temporal resolutions. Since streamers normally consist of filaments in their bases and streamer blobs are mostly consistent with flux rope geometries (Sheeley et al. 2009), “N3” likely consists of a flux-rope-type magnetic field. While “N3” moves southward, it could interact with the outer loops hosting “N1” and “N2” and might result in magnetic reconnection between the loop systems and the flux ropes (Fig. 11c). This can be a kind of component reconnection that allows the flux rope to escape from the close field and to be released into the open field system (Fig. 11d). This motion of a flux rope could also push the overlying loops to expand, as is observed in the SUVI images (see the animation associated with Fig. 3). Such a process might also provide the flux rope with extra momentum toward the flank of the open field system and that might drive interchange reconnection between the tangential flux of the flux rope and the open field. This interchange magnetic reconnection could also explain the expansion of the brightening at the southern flank of the streamer. After such an interchange reconnection, the open field should undergo a restoring process that forces the blob to move outward. While photospheric motions can drag the bases of the loops at the bottom of a streamer, leading to the expansion of the loop system, they result in shearing of the streamer (Linker & Mikic 1995), and finally the instabilities and/or magnetic reconnection develop and a streamer blob is formed and ejected into interplanetary space. We would like to mention that many details of the scenario proposed here require observations with higher temporal and spatial resolutions in order to be proved, which cannot be done with the data available at present.

thumbnail Fig. 11.

Schematic cartoon model of the formation of the streamer blob observed here. (a): The general magnetic topology of the streamer. The bright features of “N1” and “N2” are located in the southern legs of the overlying loops. The horizontal red arrows indicate the upward expansion of the loop system and that results in the squeezing of the overlying loop system (the vertical red arrows). (b): The geometry of the streamer while “N1” and “N2” have merged (or simply been arranged along the same line of sight) and pinching-off processes take place near the apex of the lower-lying loops. (c): The geometry of the streamer when a flux-rope-like feature (“N3”) has fallen off due to the pinching-off processes. Magnetic reconnection might take place while the flux rope moves toward the overlying loop system, allowing it to escape from the close field. (d): The geometry of the streamer after the flux rope is released in the open field system. The distortion of the field line resulting from magnetic reconnection could provide additional magnetic tension to drive the blob to move outward. The flux rope might also continuously interact with the open field and interchange magnetic reconnection might occur intermittently all the way out until it is fully dissolved.

It has been suggested that streamer blobs are an important source of the slow solar wind and/or microstructures in the solar wind (Sheeley et al. 1997, 1999; Wang et al. 1998; Wang 2012). An important phenomenon in the solar wind, switchbacks, has attracted a great deal of attention in the community, since the Parker Solar Probe has unearthed many details in recent years (Bale et al. 2019). Simulations show that low-amplitude Alfvén waves can naturally evolve into a turbulent state with features similar to switchbacks (Squire et al. 2020). Some studies also suggest that switchbacks might link to activities originating in the low corona or even lower (e.g., Tenerani et al. 2020; Neugebauer & Sterling 2021; Mozer et al. 2021; Fargette et al. 2021; Upendran & Tripathi 2022; Hou et al. 2022, 2023; Raouafi et al. 2023; Baker et al. 2023). A plausible mechanism releasing switchbacks is interchange reconnection between close loops and open field lines in the low corona (Zank et al. 2020; Liang et al. 2021; Antonucci et al. 2023). Numerical experiments have shown that modified interchange reconnection between open funnels and closed loops can produce both jet flows and Alfvénic wave pulses that can account for the observations of switchbacks in the solar wind (He et al. 2021). Further study suggests that switchbacks are a combination effect of intermittent interchange reconnection and torsional Alfvénic waves in the inner heliosphere triggered by repeated ejections of plasmoid (Wyper et al. 2022). Alternatively, flux ropes formed by multi-x-line reconnection between open and closed flux in the corona could also convect and be observed as switchbacks in the inner heliosphere (Drake et al. 2021). Recent observations by the Solar Orbiter reveal a single large propagating S-shaped vortex at 2−3 R⊙ that appears to be manifestation of interchange reconnection between active region loops and a bounded open field system. Such a structure could be the counterpart of a switchback in the low corona (Telloni et al. 2022). Observations and simulations also suggest that switchbacks and slow solar wind streams can be treated as two manifestations of the same interchange reconnection (Telloni et al. 2022). Whether streamer blobs are associated with the formation of switchbacks is an interesting question. Our observations here indicate a complex formation process of a streamer blob below 2 R⊙, which possibly includes the ejection of a flux-rope-like feature along open field lines. While such a flux rope propagates along an open field system, it might intermittently interact with open field lines and result in interchange magnetic reconnection between the tangential field and the open field. These interchange magnetic reconnections could transfer tangential flux into the open field and provide acceleration to the blob. Such an interchange magnetic reconnection process might take place all the way as the streamer blob propagates outward until it is fully dissolved, which is consistent with observations of the prevalence of magnetic reconnection in the near-Sun heliosphere (Phan et al. 2021). The transfer of tangential flux in the open field system might drive Alfvénic wave pulses locally in the near-Sun space. Therefore, the propagation of such a streamer blob may be associated with a switchback or small flux ropes (Zhao et al. 2021) in the inner heliosphere.

5. Conclusions

In this paper, we study the coordinated observations taken by SOHO/LASCO, GOES-17/SUVI, and SDO/AIA, aiming to track the formation of a propagating streamer blob down to the low corona near the limb. The streamer consists of closed loop systems and open field systems at its southern and northern flanks. We find that the streamer blob in the LASCO/C2 FOV was formed via the convergence of three clumps (“N1”, “N2”, and “N3”) of bright features occurring at about 1.8 R⊙, as is seen in SUVI 195 Å passband. In the SUVI 195 Å observations, “N1” and “N2” show as elongated features at 1.6−1.7 R⊙ near the southern flank of the streamer and they appear about 30 h before the formation of the streamer blob in the LASCO C2 images. “N3” forms at the apex of the streamer loop system (∼1.7 R⊙) about 27 h before the formation of the streamer blob in the LASCO C2 images. The convergence of “N1” and “N2” takes place about 23 h before the appearance of the streamer blob and it is apparently driven by swaying motions of their field system. While “N3” falls from the apex of the loop system, it moves toward “N1” and “N2” with an effective speed of about 0.7 km s−1 and merges with them ∼8 h before the appearance of the streamer blob above 2 R⊙. The merged clump becomes brighter and more compact, moves outward from about 1.8 R⊙ to 2 R⊙ in about 8 h, and forms the propagating blob observed in the LASCO C2 FOV. The acceleration of the blob starts at 1.9 R⊙ or even lower. The lower-lying loops at the base of the streamer seen in the AIA FOV apparently expand from 1.4 R⊙ to 1.5 R⊙, leading to an expanding speed of about 4 km s−1 seen in the SUVI images. When the low-lying loops expand, the field lines hosting “N1” and “N2” sway and “N3” is born at the loop apex. We speculate that the expansion of the low-lying loops might force the loop systems nearby (“N1” and “N2”) to sway, interact, and merge, and the expanding loop system itself could lead to an inhomogeneity at its apex that further develops into a pinching-off “N3”. The observations here provide unique evidence of the connection between the formation of a streamer blob and activities in the corona below 2 R⊙.

Based on the observations, we demonstrate that the formation of a streamer blob is a complex process, and we propose a scenario for the formation of the observed streamer blob. In our scenario, a magnetic flux rope might be pinching off from the apex of the low-lying loops due to the expansion of the lower loops, which play a crucial role in the formation of the streamer blob. The fallen-off magnetic flux rope might interact and/or merge with nearby field systems and that allows it to be transferred across the close field systems and to be released into the open field. While a flux rope propagates along open field lines, it might result in intermittent interchange magnetic reconnections that allow the tangential flux to be transferred across the field lines and provide additional acceleration to the plasma blob. Our observations suggest that small-scale disturbances in the solar wind observed in the inner heliosphere could arise in the corona below 2 R⊙, but the acceleration processes might take place intermittently as it propagates outward. Our observations demonstrate that a streamer blob might consist of multiple substructures, and thus provide more complexities in the interplanetary space plasma. The data from the Full Sun Imager (FSI) channel of the Extreme Ultraviolet Imager (EUI, Rochus et al. 2020) aboard the Solar Orbiter (SO, Müller et al. 2020), which might cover the corona up to 7 R⊙ (Auchère et al. 2023), should be very valuable for further studies, revealing more details about the formation and dynamics of streamer blobs and their connections to the solar wind.

Movies

Movie 1 associated with Fig. 2 (Fig2_Lascoc2) Access here

Movie 2 associated with Fig. 3 (Fig3_Lasco_Suvi_Blob) Access here

Movie 3 associated with Fig. 6 (Fig6_suvi) Access here

Movie 4 associated with Fig. 9 (Fig9_suvi_aia) Access here

Acknowledgments

We are grateful to the anonymous referee for the constructive comments and suggestions. We thank Prof. Jiansen He for his careful reading to and helpful comments on the manuscript. We acknowledge the GOES-17/SUVI team for making the data publicly available. The AIA and HMI data are used by courtesy of NASA/SDO, the AIA and HMI teams and JSOC. SOHO is a project of international cooperation between ESA and NASA. The SOHO/LASCO data used here are produced by a consortium of the Naval Research Laboratory (USA), Max-Planck Institute for Sonnensystemforschung (Germany), Laboratoire d’Astrophysique de Marseille (France), and the University of Birmingham (UK). This research is supported by National Key R&D Program of China No. 2021YFA0718600 and National Natural Science Foundation of China (42174201, 42230203, 41974201, 42074208).

References

  1. Ambrož, P., Druckmüller, M., Galal, A. A., & Hamid, R. H. 2009, Sol. Phys., 258, 243 [CrossRef] [Google Scholar]
  2. Antonucci, E., Downs, C., Capuano, G. E., et al. 2023, Phys. Plasmas, 30, 022905 [Google Scholar]
  3. Auchère, F., Berghmans, D., Dumesnil, C., et al. 2023, A&A, 674, A127 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  4. Baker, D., Démoulin, P., Yardley, S. L., et al. 2023, ApJ, 950, 65 [CrossRef] [Google Scholar]
  5. Bale, S. D., Badman, S. T., Bonnell, J. W., et al. 2019, Nature, 576, 237 [NASA ADS] [CrossRef] [Google Scholar]
  6. Boe, B., Habbal, S., & Druckmüller, M. 2020, ApJ, 895, 123 [Google Scholar]
  7. Brueckner, G. E., Howard, R. A., Koomen, M. J., et al. 1995, Sol. Phys., 162, 357 [NASA ADS] [CrossRef] [Google Scholar]
  8. Chen, Y., Li, X., Song, H. Q., et al. 2009, ApJ, 691, 1936 [NASA ADS] [CrossRef] [Google Scholar]
  9. Chen, C. H. K., Chandran, B. D. G., Woodham, L. D., et al. 2021, A&A, 650, L3 [EDP Sciences] [Google Scholar]
  10. Chitta, L. P., Seaton, D. B., Downs, C., DeForest, C. E., & Higginson, A. K. 2023, Nat. Astron., 7, 133 [NASA ADS] [Google Scholar]
  11. Crooker, N. U., Siscoe, G. L., Shodhan, S., et al. 1993, J. Geophys. Res., 98, 9371 [NASA ADS] [CrossRef] [Google Scholar]
  12. Crooker, N. U., Huang, C. L., Lamassa, S. M., et al. 2004, J. Geophys. Res.: Space Phys., 109, A03107 [NASA ADS] [Google Scholar]
  13. Domingo, V., Fleck, B., & Poland, A. I. 1995, Sol. Phys., 162, 1 [Google Scholar]
  14. Drake, J. F., Agapitov, O., Swisdak, M., et al. 2021, A&A, 650, A2 [EDP Sciences] [Google Scholar]
  15. Einaudi, G., Boncinelli, P., Dahlburg, R. B., & Karpen, J. T. 1999, J. Geophys. Res., 104, 521 [NASA ADS] [CrossRef] [Google Scholar]
  16. Eselevich, V. G., & Eselevich, M. V. 1999, Sol. Phys., 188, 299 [NASA ADS] [CrossRef] [Google Scholar]
  17. Fargette, N., Lavraud, B., Rouillard, A. P., et al. 2021, ApJ, 919, 96 [NASA ADS] [CrossRef] [Google Scholar]
  18. Grappin, R., Léorat, J., & Buttighoffer, A. 2000, A&A, 362, 342 [NASA ADS] [Google Scholar]
  19. He, J., Zhu, X., Yang, L., et al. 2021, ApJ, 913, L14 [NASA ADS] [CrossRef] [Google Scholar]
  20. Higginson, A. K., & Lynch, B. J. 2018, ApJ, 859, 6 [Google Scholar]
  21. Hou, C., He, J., Duan, D., Li, H., & Chen, Y. 2022, EGU General Assembly Conference Abstracts, EGU General Assembly Conference Abstracts, EGU22–9673 [Google Scholar]
  22. Hou, C., Zhu, X., Zhuo, R., et al. 2023, ApJ, 950, 157 [NASA ADS] [CrossRef] [Google Scholar]
  23. Karpen, J. T., Antiochos, S. K., DeVore, C. R., & Golub, L. 1998, ApJ, 495, 491 [NASA ADS] [CrossRef] [Google Scholar]
  24. Koutchmy, S., & Livshits, M. 1992, Space Sci. Rev., 61, 393 [CrossRef] [Google Scholar]
  25. Lee, J.-O., Cho, K.-S., An, J., et al. 2021, ApJ, 920, L6 [NASA ADS] [CrossRef] [Google Scholar]
  26. Lemen, J. R., Title, A. M., Akin, D. J., et al. 2012, Sol. Phys., 275, 17 [Google Scholar]
  27. Liang, H., Zank, G. P., Nakanotani, M., & Zhao, L. L. 2021, ApJ, 917, 110 [NASA ADS] [CrossRef] [Google Scholar]
  28. Liang, Y., Qu, Z., Hao, L., Xu, Z., & Zhong, Y. 2023, MNRAS, 518, 1776 [Google Scholar]
  29. Linker, J. A., & Mikic, Z. 1995, ApJ, 438, L45 [NASA ADS] [CrossRef] [Google Scholar]
  30. Lotova, N. A., Obridko, V. N., & Vladimirskii, K. V. 2000, A&A, 357, 1051 [NASA ADS] [Google Scholar]
  31. Lyu, S., Wang, Y., Li, X., & Zhang, Q. 2023, A&A, 672, A100 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  32. Morgan, H., Habbal, S. R., & Woo, R. 2006, Sol. Phys., 236, 263 [Google Scholar]
  33. Mozer, F. S., Bale, S. D., Bonnell, J. W., et al. 2021, ApJ, 919, 60 [NASA ADS] [CrossRef] [Google Scholar]
  34. Müller, D., St. Cyr, O. C., Zouganelis, I., et al. 2020, A&A, 642, A1 [Google Scholar]
  35. Neugebauer, M., & Sterling, A. C. 2021, ApJ, 920, L31 [NASA ADS] [CrossRef] [Google Scholar]
  36. Pasachoff, J. M., Rušin, V., Druckmüllerová, H., et al. 2011, ApJ, 734, 114 [NASA ADS] [CrossRef] [Google Scholar]
  37. Pasachoff, J. M., Rušin, V., Saniga, M., et al. 2015, ApJ, 800, 90 [NASA ADS] [CrossRef] [Google Scholar]
  38. Pellegrin-Frachon, T., Masson, S., Pariat, É., Wyper, P. F., & DeVore, C. R. 2023, A&A, 675, A55 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  39. Pesnell, W. D., Thompson, B. J., & Chamberlin, P. C. 2012, Sol. Phys., 275, 3 [Google Scholar]
  40. Phan, T. D., Lavraud, B., Halekas, J. S., et al. 2021, A&A, 650, A13 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  41. Poirier, N., Kouloumvakos, A., Rouillard, A. P., et al. 2020, ApJS, 246, 60 [Google Scholar]
  42. Raouafi, N. E., Stenborg, G., Seaton, D. B., et al. 2023, ApJ, 945, 28 [NASA ADS] [CrossRef] [Google Scholar]
  43. Raymond, J. C., Kohl, J. L., Noci, G., et al. 1997, Sol. Phys., 175, 645 [Google Scholar]
  44. Rochus, P., Auchère, F., Berghmans, D., et al. 2020, A&A, 642, A8 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  45. Romoli, M., Antonucci, E., Andretta, V., et al. 2021, A&A, 656, A32 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  46. Rouillard, A. P., Kouloumvakos, A., Vourlidas, A., et al. 2020, ApJS, 246, 37 [Google Scholar]
  47. Saito, K., & Tandberg-Hanssen, E. 1973, Sol. Phys., 31, 105 [Google Scholar]
  48. Seaton, D. B., & Darnel, J. M. 2018, ApJ, 852, L9 [NASA ADS] [CrossRef] [Google Scholar]
  49. Seaton, D. B., Hughes, J. M., Tadikonda, S. K., et al. 2021, Nat. Astron., 5, 1029 [Google Scholar]
  50. Sheeley, N. R., Jr., & Wang, Y. M. 2007, ApJ, 655, 1142 [NASA ADS] [CrossRef] [Google Scholar]
  51. Sheeley, N. R., Wang, Y. M., Hawley, S. H., et al. 1997, ApJ, 484, 472 [Google Scholar]
  52. Sheeley, N. R., Walters, J. H., Wang, Y. M., & Howard, R. A. 1999, J. Geophys. Res., 104, 24739 [Google Scholar]
  53. Sheeley, N. R., Jr., Lee, D. D. H., Casto, K. P., Wang, Y. M., & Rich, N. B. 2009, ApJ, 694, 1471 [NASA ADS] [CrossRef] [Google Scholar]
  54. Song, H. Q., Chen, Y., Liu, K., Feng, S. W., & Xia, L. D. 2009, Sol. Phys., 258, 129 [NASA ADS] [CrossRef] [Google Scholar]
  55. Song, H. Q., Kong, X. L., Chen, Y., et al. 2012, Sol. Phys., 276, 261 [NASA ADS] [CrossRef] [Google Scholar]
  56. Spadaro, D., Susino, R., Ventura, R., Vourlidas, A., & Landi, E. 2007, A&A, 475, 707 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  57. Squire, J., Chandran, B. D. G., & Meyrand, R. 2020, ApJ, 891, L2 [Google Scholar]
  58. Strachan, L., Suleiman, R., Panasyuk, A. V., Biesecker, D. A., & Kohl, J. L. 2002, ApJ, 571, 1008 [Google Scholar]
  59. Sturrock, P. A., & Smith, S. M. 1968, Sol. Phys., 5, 87 [NASA ADS] [CrossRef] [Google Scholar]
  60. Suess, S. T., Ko, Y. K., von Steiger, R., & Moore, R. L. 2009, J. Geophys. Res.: Space Phys., 114, A04103 [NASA ADS] [CrossRef] [Google Scholar]
  61. Telloni, D., Zank, G. P., Stangalini, M., et al. 2022, ApJ, 936, L25 [NASA ADS] [CrossRef] [Google Scholar]
  62. Tenerani, A., Velli, M., Matteini, L., et al. 2020, ApJS, 246, 32 [Google Scholar]
  63. Thernisien, A. F., & Howard, R. A. 2006, ApJ, 642, 523 [Google Scholar]
  64. Upendran, V., & Tripathi, D. 2022, ApJ, 926, 138 [NASA ADS] [CrossRef] [Google Scholar]
  65. Uzzo, M., Ko, Y. K., Raymond, J. C., Wurz, P., & Ipavich, F. M. 2003, ApJ, 585, 1062 [NASA ADS] [CrossRef] [Google Scholar]
  66. Vasudevan, G., Shing, L., Mathur, D., et al. 2019, in International Conference on Space Optics; ICSO 2018, eds. Z. Sodnik, N. Karafolas, & B. Cugny, SPIE Conf. Ser., 11180, 111807P [Google Scholar]
  67. Ventura, R., Spadaro, D., Cimino, G., & Romoli, M. 2005, A&A, 430, 701 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  68. Ventura, R., Antonucci, E., Downs, C., et al. 2023, A&A, 675, A170 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  69. Wang, Y. M. 2012, Space Sci. Rev., 172, 123 [NASA ADS] [CrossRef] [Google Scholar]
  70. Wang, Y. M., & Hess, P. 2018, ApJ, 859, 135 [NASA ADS] [CrossRef] [Google Scholar]
  71. Wang, Y. M., & Sheeley, N. R., Jr. 1992, ApJ, 392, 310 [NASA ADS] [CrossRef] [Google Scholar]
  72. Wang, Y. M., & Sheeley, N. R., Jr. 2006, ApJ, 650, 1172 [NASA ADS] [CrossRef] [Google Scholar]
  73. Wang, Y. M., Sheeley, N. R., Jr., Walters, J. H., et al. 1998, ApJ, 498, L165 [Google Scholar]
  74. Wang, Y. M., Sheeley, N. R., Jr., & Rich, N. B. 2000, Geophys. Res. Lett., 27, 149 [NASA ADS] [CrossRef] [Google Scholar]
  75. Wang, Y. M., Sheeley, N. R., Jr., & Rich, N. B. 2007, ApJ, 658, 1340 [CrossRef] [Google Scholar]
  76. West, M. J., Seaton, D. B., Wexler, D. B., et al. 2023, Sol. Phys., 298, 78 [NASA ADS] [CrossRef] [Google Scholar]
  77. Winterhalter, D., Smith, E. J., Burton, M. E., Murphy, N., & McComas, D. J. 1994, J. Geophys. Res., 99, 6667 [NASA ADS] [CrossRef] [Google Scholar]
  78. Wyper, P. F., DeVore, C. R., Antiochos, S. K., et al. 2022, ApJ, 941, L29 [NASA ADS] [CrossRef] [Google Scholar]
  79. Zank, G. P., Nakanotani, M., Zhao, L. L., Adhikari, L., & Kasper, J. 2020, ApJ, 903, 1 [NASA ADS] [CrossRef] [Google Scholar]
  80. Zhao, L. L., Zank, G. P., Hu, Q., et al. 2021, A&A, 650, A12 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]

All Figures

thumbnail Fig. 1.

(a): Polarized white-light image of the corona recorded with the LASCO C2 on August 22, 2018. The inner boundary of the FOV is at 2 R⊙ and the outer boundary is at about 6 R⊙. The white circle indicates the limb of the solar disk. (b): Potential magnetic field extrapolated with PFSS model overlying on a photospheric synoptic magnetogram. The purple and green lines are representative of open field lines and the white lines are closed loops.
In the text
thumbnail Fig. 2.

Left and middle panels: running difference images obtained from LASCO C2 observations at 10:24 UT and 12:36 UT on August 22, in which a small propagating blob is shown as a structure with dark center surrounded by bright features (marked by circles in dashed red lines). Right panel: a time-distance map tracking the propagation of the streamer blob in the running difference images. The time starts at 00:00 UT on August 22. The dashed green line denotes the propagation of the streamer blob, from which the tangent derives its propagating speeds. An associated animation is given online.
In the text
thumbnail Fig. 3.

Streamer observed in the compositional images of SUVI 195 Å (red) and LASCO C2 (green). The small red circles indicate the location of the blobs recognized in LASCO C2 running difference images (i.e., the left and middle panels of Fig. 2). An associated animation is given online.
In the text
thumbnail Fig. 4.

Top: time-distance map following the expansion of the bright elongated feature in the SUVI images (the time coordinate starts from 21:17 UT on August 21). The blob presents as the tip of the expanding feature, which can be tracked by the dashed gold line. Bottom: variation in the height of the blob seen in SUVI images (diamonds with a dashed line), which was identified manually. The solid line is a second-order polynomial fit to the variation curve.
In the text
thumbnail Fig. 5.

Evolution of the solar corona seen in combined observations of AIA 193 Å (red) and SUVI 195 Å (green). The regions enclosed by dotted lines denote three clumps of brightenings (N1, N2, and N3), and their merging processes can be followed.
In the text
thumbnail Fig. 6.

Combined image of SUVI 195 Å image and AIA 193 Å image. The dashed red and yellow lines mark the loops associated with N1 and N2, respectively. Three slits at 1.2 R⊙, 1.3 R⊙, and 1.6 R⊙ where we obtained distance-time maps (shown in Fig. 8) are marked as black arrows (0 is given at the tail end). These slits are arranged roughly in the same opening angle. We note that the elongated structures are not perfectly aligned in the radial direction. An associated animation is given online.
In the text
thumbnail Fig. 7.

Variations in the centers (on the projected X − Y plane) of the three clumps of brightenings. The squares, triangles, and crosses are the locations of “N1”, “N2”, and “N3”, respectively. The change in time is shown as a change in color. The convergence of the three clumps can be followed in this diagram while they are moving toward the same position at the end.
In the text
thumbnail Fig. 8.

Time-distance maps obtained from the three slits shown in Fig. 6 based on AIA observations (1.2 R⊙) and SUVI observations (1.3 R⊙ and 1.6 R⊙). The swaying motion of the extended structures (in which “N1” and “N2” are sitting) at 1.2, 1.3, and 1.6 R⊙ can be tracked with the dashed lines.
In the text
thumbnail Fig. 9.

Birth of “N3” (enclosed by the dotted red line in panel b) and expansion of low-lying loops seen in the combined SUVI and AIA images. The dotted black lines denote the location of the loop seen in the AIA image at 21:27 UT on August 20 and the dotted yellow line marks that at 02:15 UT on August 21. An associated animation is given online.
In the text
thumbnail Fig. 10.

Time–distance map obtained along the expanding direction of the loop system and the propagation of “N3”. The blue arrow indicates the time and location when and where “N3” is pinching off. The dashed yellow line shows the expansion of the loop system and the fall of “N3”. The dashed cyan line indicates the retraction of the loop system.
In the text
thumbnail Fig. 11.

Schematic cartoon model of the formation of the streamer blob observed here. (a): The general magnetic topology of the streamer. The bright features of “N1” and “N2” are located in the southern legs of the overlying loops. The horizontal red arrows indicate the upward expansion of the loop system and that results in the squeezing of the overlying loop system (the vertical red arrows). (b): The geometry of the streamer while “N1” and “N2” have merged (or simply been arranged along the same line of sight) and pinching-off processes take place near the apex of the lower-lying loops. (c): The geometry of the streamer when a flux-rope-like feature (“N3”) has fallen off due to the pinching-off processes. Magnetic reconnection might take place while the flux rope moves toward the overlying loop system, allowing it to escape from the close field. (d): The geometry of the streamer after the flux rope is released in the open field system. The distortion of the field line resulting from magnetic reconnection could provide additional magnetic tension to drive the blob to move outward. The flux rope might also continuously interact with the open field and interchange magnetic reconnection might occur intermittently all the way out until it is fully dissolved.
In the text

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.