1 INTRODUCTION

Since the first observations of coronal mass ejections (CME) on the Orbiting Solar Observatory 7 (OSO-7) in 1971 and the related phenomena, the flows of matter in the corona have been carefully studied. The SOHO and STEREO spacecraft with coronagraphs on board provided new observational data on flows of matter in the corona in white light. Observations of Transition Region and Coronal Explorer (TRACE) have also shown that the solar corona is much more variable and dynamic than previously expected. At the same time, new problems of interpretation and modeling of the dynamic and time-varying solar atmosphere arose. Emission lines in the transitional region of the quiet Sun systematically showed a redshift (Brekke, 1999). A typical value of average velocity of the inflow was 5–10 km s−1, the redshift increased with temperature and then sharply decreased. A sharp transition from redshift to blueshift was observed at temperatures above 5 × 105 K. Previously, Feldman (1983) arrived at the conclusion that most of the solar atmosphere radiation at the transition region temperatures comes from unresolved fine structures (UFS).

There were observed movements with velocities directed both towards the Sun and away from it: plasma blobs move along narrow trajectories, while some of them occupy a wide range of position angles. Different events apparently have been caused by different physical mechanisms. Most of the matter moves away from the Sun. However, many cases have been identified where some objects returned back to the Sun. Sometimes, part of the matter moved towards the Sun, and at the same time another part moved upward, from the same point in space.

The SOHO measurements of ultraviolet radiation revealed that the lines in the upper transition region and the lower corona have blueshifts, with a sharp transition from the redshift to blueshift at the temperature above 0.5 MK (Chae et al., 1998). The work (Peter and Judge, 1999) presents data on Doppler shifts on large spatial scales (>50"), which clearly show the redshift from the center to the limb in all lines of the transition region formed below T ∼ 0.5 MK.

Sheeley and Wang (2014) studied the sunward flows (inflows) during 1996–2014. The flows in cycle 23 of solar activity and in the ascending and maximum phases of cycle 24 were considered. The authors found that inflows arise in long-lived streamers, where a strong longitudinal field gradient was observed. The inflows arise at the boundaries of sectors between areas of opposite polarity. The inflows occur at 2–6 R☉. Sometimes, inflows arise as “in/out pairs” that are formed when a slowly expanding coronal structure breaks away from the Sun. Weak inflows occur in the region below 5.5 R☉ (which seems to be a point of no return for the solar wind). Slow streams (with downstream velocities within a range of 50–100 km s–1) often seem dark against the background of a denser corona at a distance of 2–3 R☉ from the center of the Sun.

De Pontieu et al. (2009), from the asymmetry of spectral lines, found faint upflows with velocities of 50–100 km s−1 at temperatures from 105 K to several million degrees. These upflows correlate with type II spicules and have the same upstream velocities. Upflows with velocities of about 30 km s−1 were detected in the coronal hole, which were interpreted as evidence of an emerging solar wind (SW) stream.

The purpose of this work was to study the flows of matter observed in the corona and the related processes. These results can provide a deeper insight into the corona dynamics and help in understanding the physical problems of the corona, namely: the heating of the solar corona and the acceleration of the solar wind.

2 OBSERVATIONAL DATA

In this work, the study of velocities in the corona was performed on the basis of published works using data on coronal brightness in white light that were obtained from the SOHO and STEREO coronagraphs. The SOHO (launched in 1995) and STEREO (launched in 2006) spacecraft with onboard coronagraphs provided new observational data on the flows of matter in the corona. The solar corona brightness in white light is mainly the result of Thompson scattering of the photospheric light on free electrons in the corona. Two Large Angle and Spectrometric Coronagraph (LASCO) C2 and C3 provide images of the corona in white light and information about the coronal plasma movements in upper layers of the corona at distances from 2 to 30 R☉. The coronagraph C2 acquires the coronal image in the range of 2–6 R☉, while the coronagraph C3, within 3.7–30 R☉.

The STEREO space observatory consists of two identical spacecraft: STEREO-B (Behind), which gradually trails behind the Earth, and STEREO-A (Ahead), which moves ahead of the Earth. This makes it possible to simultaneously observe the Sun from two different points. Having the images recorded simultaneously from two spatially separated space stations, it is possible to obtain more accurate values of plasma velocities. Each station (STEREO-A and STEREO-B) has two white-light coronagraphs COR1 and COR2 designed to observe the inner 1.4–4 R☉ and outer 2–15 R☉ solar coronas, respectively.

3 RESULTS

Observations of the solar corona from space have revealed many unexpected small-scale features, including plasma blobs that are constantly ejected from streamer cusps. A cusp is a loop-like structure in the upper part of the streamer, which has a pointed shape in the form of a tip. Plasma blobs can be interpreted as various manifestations of the reconnection of magnetic field lines, in which plasma and magnetic flux are exchanged between regions of open and closed magnetic fields of the corona. These observations provide new insight into a number of important issues, including the origin of streamers in the outer corona and sources of the slow SW.

The observed events differ in their form, speed, and direction of motion. There are many small jets and blobs moving outwards. However, small blobs are observed that move inward. They usually form at distances of 3–5 R☉, (Sheeley and Wang, 2002). Bidirectional events are also observed. The average altitude of the event onset is about 3.7 R☉. Events moving sunward start only at altitudes ranging from approximately 3.0 R☉ to 5.5 R☉. The starting points have never been above 6 R☉ and below 2.5 R☉. The escaping matter usually moves at a higher speed than descending matter.

The LASCO C2 coronagraph shows many examples of oppositely directed mass flows along the same radius vector. For example, in the event on November 6, 1999 (Simnett, 2004), the outflow velocities were higher than the inflow velocities. The difference in ascent/descent velocities was ∼112 km s–1, see Fig. 1. The main result of (Simnett, 2004) is the identification of bidirectional mass flows in the corona that move inward and outward along the same radius vector; 40 bidirectional flows, located at distances from 3.1 to 4.2 R☉ (the average distance is about 3.7 R☉) were considered. The typical speed of the solar wind (SW) at the site of the event origination was about 60 km s−1. The events have been interpreted as an explosive release of energy in outgoing SW in areas where magnetic reconnection occurs. If this interpretation is correct, then these events represent the first direct observation of magnetic reconnection in the corona.

Fig. 1.
figure 1

The scheme of the arrangement of plasma blobs on November 6, 1999 for three moments of time: (1) 1502:51 UT, (2) 1526:54 UT, and (3) 1555:42 UT (constructed according to Fig. 1 from (Simnett, 2004)). Coronal plasma blobs are indicated by arrows. The semicircle represents half of the eclipsing disk of the LASCO C2 coronagraph. North is upward and west is on the right.

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Different types of inflow and outflow in the upper corona (both velocities and accelerations) differ both in their values and in their physical nature. Some types of motions can be explained by the process of magnetic reconnection, while some types of motions can be explained by the combined action of gravitational forces, the magnetic field and the interaction with moving background matter.

Streamer observations have improved significantly since the 1970s with the launch of space coronagraphs. Streamers are the main large-scale structures of the solar corona, consisting of a series of closed loops, are covered on the outside with a sheath of open magnetic lines, and have a helmet shape (Fig. 2). Streamers appear along the polarity inversion line of magnetic fields and can be located in active regions, in the quiet Sun, or along the filament channel. During the solar minimum, when the general magnetic field on the source surface (2.4 R☉) is well described by a dipole field, the system of streamers is located near the solar equator, bordering the Sun and forming the so-called belt of streamers. The shape of this surface is rather flat. During the maximum, the magnetic structure of the corona is complex, streamers are visible all around the solar disk.

Fig. 2.
figure 2

A schematic representation of the streamer structure.

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Using streamer blobs it is possible to track the rate of the outer corona expansion. These plasma clouds are observed at a distance of ∼3–4 R☉ from the center of the Sun in the form of radially elongated structures above the cusps of helmet-like streamers. Their initial dimensions are about 1 R☉ in the radial direction and 0.1 R☉ in the transverse direction. Due to their increased density, they can be observed against the background of the surrounding corona of the Sun and may serve as SW tracers at distances close to the Sun (Sheeley et al., 1997). As the distance from the Sun increases, the size of the blobs increases reaching 4 R☉ at distances of ~12 R☉. In this case, the streamer or its cusp is not destroyed as a result of the matter outflow. A number of electron density distribution models have been developed based on high-resolution white-light corona observations at LASCO.

By analyzing 11 blobs from 1996 to 2009 (Song et al., 2012), it was found that the blobs have velocities between 200 and 845 km s–1. Most of the blobs (58%) accelerated, while 20% either slowed down or moved at a nearly constant speed (22%).

In (Decraemer et al., 2019), the three-dimensional structure of a coronal streamer observed simultaneously by coronagraphs from two spacecraft (SOHO/LASCO and STEREO/COR2), was studied for the first time. Observation by two spacecraft located at a distance of 90° made it possible to study the 3D-structure of the streamer. Figure 3 shows the simulated views of the streamer in comparison with the observed ones. The COR2 image shows a narrow streamer stalk at –39° latitude; the streamer is seen edge-on. On LASCO, the streamer has a fan-like structure with radially expanded areas of higher and lower brightness. The density along the azimuthal direction is not uniform. These studies can clarify the role of the fine structure of streamers in the global structure of the corona and their connection with forming the slow solar wind.

Fig. 3.
figure 3

3D-calculations of the streamer structure (on the left) in comparison to the observed images (on the right). The images are obtained at the COR2 A (at the top) and LASCO C2 (at the bottom) coronagraphs according to Fig. 16 from (Decraemer et al., 2019).

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We systematized the data on plasma velocities in streamers and compared the dynamics of the streamer belt during two minima; the results are shown in Fig. 4. According to observations made in 1996–1997, the spread of blob velocities was within 150 km s–1, and according to observations in 2006–2008, the spread was ~250 km/s. A large velocity spread in 2007 compared to the spread in 1996–1997 indicates the increased solar activity at a minimum between cycles 23 and 24. The streamer belt in 1996 was rather flat. During the solar minimum in 2007–2009, the belt had a wavy shape and a large extent along the heliographic latitude. Observations in white light with SOHO/LASCO coronagraphs showed that streamers are much more dynamic formations than previously thought (Porfirieva et al., 2010).

Fig. 4.
figure 4

The dependence of the average velocities V of mass ejections from streamers on the heliocentric distance r/R☉ for: (1) 80 events in 1997, LASCO/SOHO (Wang et al., 2000); (2) spectral observations in 1997, SOHO UVCS (Strachan et al., 2002), (3) streamer release in 2006, EIT, SOHO LASCO (Sheeley et al., 2007), (4) 106 events in 2007, SOHO LASCO (Song et al., 2009), (5) three events in 2008, STEREO (Sheeley et al., 2009). Dependences (1), (3–5) show white light observations, (2) corresponds to O VI line observations.

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It is known that the inner corona is best observed during total solar eclipses. Corona observations during eclipses can provide data that is not available to modern satellites. In (Psachoff et al., 2009), the white-light corona photographs are given, which were obtained during the observation of the 2008 eclipse in Novosibirsk and Mongolia (Altai). Figure 5 presents images of the corona at the east limb and above the south pole, which show changes in the fine structure of the corona over 19 min (time difference between two observation points). The ejection velocity over the south pole was estimated to be about 600 km s−1. Figure 9 in (Pasachoff et al., 2009) shows a composite image: the coronal image obtained in Mongolia is inserted into an area inaccessible to the LASCO C2 coronagraph.

Fig. 5.
figure 5

An illustration of corona dynamics on the eastern limb (at the top) and above the south pole (at the bottom), based on Figs. 7 and 8 from (Pasachoff et al., 2009). The images were obtained in Novosibirsk and Altai with a time difference of 19 min. Changes in the fine structure of the corona are visible. The new polar plume is marked with an arrow. The estimated ejection velocity is ≈ 600 km s−1.

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The line-of-sight velocities in the inner solar corona are calculated from the profiles of the coronal green and red emission lines obtained as a result of interferometric observations of the solar corona with a Fabry–Perot etalon. Starting from 1965, at the Sternberg Astronomical Institute the motions in the solar corona have been studied using interferograms of the red and green coronal lines Fe X 6374 Å and Fe XIV 5303 Å during total solar eclipses. Doppler velocities up to 100 km s–1 were obtained in small volumes of the corona, which caused some criticism, since, according to the observations of other authors, the velocities did not exceed tens of km s–1. The profiles (Delone and Makarova, 1969, 1975; Delone et al., 1988) had a complex shape, which was interpreted as evidence of the existence of many small moving objects. The velocities obtained as a result of the shift of the entire profile turned out to be lower than the velocities found from the shifted components of the line profiles.

Raju et al. (2011) have shown that the green coronal line profiles are of a complex shape and contain components with the excessive blueshift, which depend on solar activity. It is found that the value of Doppler velocity of the shifted components is in the range of 20–40 km s–1 and shows an increase towards the poles. Possible explanations for the multicomponent profiles are the type II spicules, which have recently been recognized as important for coronal heating or for the incipient solar wind flow, but the cause of the blue asymmetry in the coronal lines above the limb remains unclear.

During the total solar eclipse on June 21, 2001, observation was carried out in the green coronal line (Prabhakar et al., 2019). It was found that 59% of the line profiles are blueshifted, 34% are single components, and only 7% are redshifted. The spatial behavior of the green coronal line profiles was studied, and changes in intensity, line width, Doppler velocity, and line asymmetry were obtained. The studies revealed excessive blue shifts in the green line profiles, which were explained by the existence of type II spicules and emerging SW flows.

A brief review of turbulent and ordered plasma motions in the solar corona was given in (Yakunina et al., 2004). Velocities were analyzed in quiet regions of the corona, coronal loops, coronal holes, and a cavity around quiet prominences. Nonthermal velocities in coronal holes turned out to be higher than in the quiet corona. In the quiet corona, they are higher than in the coronal cavity around the prominence. It was concluded that a difference in the turbulence in various structural formations of the corona is associated with the magnetic field variability. The velocities of directed motions for various structures in the corona were also studied. The turbulent velocities are comparable in magnitude to Doppler velocities, but at the same time they are somewhat smaller. The relationship of nonthermal velocities with the magnetic field characteristics has been studied.

In 2018, the Parker Solar Probe (PSP) was launched, and in 2020, the Solar Orbiter (SolO) was put into orbit. The purpose of these probes is to study the inner heliosphere in order to understand the solar corona heating and the solar wind origin. One of the early results of the PSP is the detection of rapid changes in the radial orientation of the magnetic field (“switchbacks”). The rapid changes were observed at intervals ranging from seconds to tens of minutes, possibly hours. At the same time, field deviations sometimes reach full reversal with respect to the Parker spiral (Sterling and Moore, 2020; Rouillard et al., 2020). These reverse fields are accompanied by a rapid increase in the radial wind velocity, and therefore they are often referred to as “velocity spikes.” Probably, switchbacks play a decisive role in the solar corona heating. These switchbacks of the magnetic field direction increase the SW velocity (Sterling and Moore, 2020; Rouillard et al., 2020). Their origin remains unclear. The Parker probe provides an unprecedented opportunity to study sources of the slow SW.

The Metis coronagraph (SolO) simultaneously observes the solar corona in the visible light band (580 and 640 nm) and in the Lyman-α (121.6 nm) UV hydrogen line (Antonucci E. et al., 2020). Metis will be able to observe the solar corona at a close distance of 0.28 AU. The SolO data will make it possible to understand the origin of the slow wind and the formation of coronal plasma blobs from helmet-shaped streamers, which are the sources of SW flows in the corona.

One new result of the SolO spacecraft is that a slow SW flows in the coronal plasma sheet, which is a continuation of the equatorial streamer, at an almost constant speed (Romoli et al., 2021). The slowest wind is limited by a narrow layer of high-density plasma 20° wide, propagating up to 6 R☉ into the solar corona. The SW plasma must undergo a further significant acceleration processes beyond 6 R☉ to reach its interplanetary velocity of 300 km s–1.

The next decade of research using the PSP and SolO spacecraft promises to be rich in new discoveries in the field of streamer flows and sources of slow SW.

4 CONCLUSIONS

We have considered the observational data obtained at the SOHO/LASCO and STEREO coronagraphs, as well as the results of observations from the PSP and SolO space probes. Numerous structures (blobs and loops) that move from the Sun and towards the Sun imply a constant exchange of plasma and magnetic flux between the regions of closed and open fields. The atmosphere of the quiet Sun turned out to be extremely dynamic. These results may provide a greater understanding of the corona dynamics, as well as the origin and acceleration of SW. Studying these phenomena could lead to a better understanding of the magnetic reconnection and its role in the corona and solar wind.

The observed details of the corona in white light and in various spectral lines reflect a structure of the magnetic field, which, eventually, generates all phenomena of solar activity. The problem of heating has been and remains one of the main problems in the solar corona physics. New high-quality observations must be combined with new and sophisticated theoretical models to explain the dynamic nature of the corona.