Reconstruction of the solar EUV irradiance from 1996 to 2010 based on SOHO/EIT images
In , Velli et al. Velli et al. Taking into account that plumes are immersed in a background medium and that plumes and the ambient corona must be in pressure equilibrium, Velli et al. The outcome of their model predicts a plume to background speed that increases with distance and it is larger than observed at 0. Wang , on the contrary, focussed on solutions for the CH and plume flows at lower heliocentric distances and neglected the interaction between the plume and ambient medium. Wang solved the one-fluid mass, momentum and energy conservation equations assuming an exponential heating of the plume with two components, a global and a base heating, with different damping lengths.
The high pressure at the base of plumes can be achieved only via a base heating term, which, as anticipated in Section 5. Different choices of the free parameters that include, among others, plume temperature, base heating, conductive flux and damping lengths yield different solutions, which we do not individually illustrate here.
But is interesting to notice that asymptotic solutions for the plasma temperature and speed predict lower values in plumes than in the background regions. The models of Velli et al. Del Zanna et al. At higher altitudes, the authors use a 1-D model where plumes expand quasi-radially and are in pressure equilibrium with the background medium.
In the low corona, magnetic forces predominate over all other forces and temperature is constant; in the higher corona, the temperature profile is given a priori. These models pointed out that temperature plays a crucial role in determining the behavior of density and flow speed: in particular, the ratio between the temperature along the plume axis and the background temperature, heavily affects the position of the sonic point and the difference between plume and background wind. This issue was further explored by Casalbuoni et al. Casalbuoni et al. Also, they show that microstreams characteristic lower or higher local velocities, can be easily reproduced by varying the wave energy flux, at the coronal base, in the two regions: for equal temperature profiles, plumes are slower than interplumes if the wave flux is the same, but are faster if the wave flux is larger than that in the interplume ambient.
In conclusion, theoretical models are unable to give clear indications about what can be expected at large heliocentric distances, as a different choice of the parameters may lead to opposite predictions and observational constraints are not stringent enough to provide modelers with unambiguous information on their values. Left panel: Representative solutions for the temperature and flow speed of a plume vs. Right panel: solutions for the temperature and flow speed of a plume vs. The initial state for the plume decay is given by the plume solution; F p0 is then suddenly set to zero. Image reproduced with permission from Pinto et al.
Pinto et al. The opposite behavior occurs at the end of the life of the plume with the ambient coronal speed slowly recovering both the formation and the decay of the plume occur over a time interval of the order of one day. Going back to Section 3. That plumes survive the BP was also predicted by Wang and Muglach , who suggest, from an estimate of the radiative cooling time, that plumes typically linger a few hours, after the BP disappears, before fading away.
There are still too few observations of the initial and final stages of the life of a plume and of the BP to check how realistic are these suggestions. A time-dependent generalization of the work of Casalbuoni et al. The profiles of the heating functions of the ambient CH and of the four plumes are given a priori and are analogous to those given by Wang 1-D models.
At 1 AU, it turns out that the CH ambient plasma flows faster than plumes, whose speed is inversely dependent on their temperature: the hottest plumes in the low corona flowing at the slowest rate in the distant wind. Hence, the lack of observational evidence of in situ structures clearly associated with plumes keeps being unexplained. An interesting possibility has been advanced by Landi et al. Landi et al. According to Landi et al. On the other hand, Velli et al. Comparison of representative radial inversion features observed by Ulysses in left panels with Landi et al.
Image reproduced with permission from Landi et al. The effect of the ignition and termination of plumes on solar wind, as shown by Velli et al. Roughly, plumes start being observable at 1 AU a few tens of hours after their heating is switched on and gradually disappear after they are turned off. Also, plumes tend to overexpand in late stages of their lifetime. This behavior obviously does not consider the possibility that plumes become unobservable because of some kind of instability. An interesting effect that however has not been observed, yet, has been recently predicted by Pinto et al.
This research needs further work to better define the properties of plumes, but the possibility of bursty plumes, if confirmed, represents a so far unobserved behavior of these objects. Occasionally, throughout this review, we have been pointing to issues that need to be further explored. Here, we expand on topics, not adequately illustrated in previous sections, that are likely to be the focus of future studies. In Section 5.
Here, we expand on relationships that have been even less analyzed between plumes and small-scale features, mostly originating from the magnetic network, whose observations have been recently soaring. Among these, spicules have a primary role. An interaction between spicules and plumes had already been suggested by, e. The recent space missions provided data that increased our knowledge of spicules: De Pontieu et al. Type II spicules, most frequent in quiet Sun and CHs, have shorter lifetimes and move mainly upwards at a higher speed than type I spicules.
Because they fade in the Ca ii H passband, it has been speculated they undergo thermal evolution and may show up at higher transition region TR and coronal temperatures. Also, the X-ray jets discussed in Section 5. Sterling et al. Although not all Ca ii type II spicules should necessarily correspond to blowout jets, Sterling et al. These phenomena as well have been suggested to be related to type II spicules, although their speed is about twice higher than that of type II spicules. Some of the jets show evidence of acceleration and, although jets show up both in quiet Sun and in CH areas, a preliminary study Tate Arbacher et al.
Given the link between plumes and jets, we may ask whether there is analogously a link between plumes and type II spicules. Are plumes typically hosting type II spicules? Are perhaps spicule miniature type II jets? We are in a completely unexplored field where questions are unanswered and originate only from speculations. We come back to this point in Section 8. The north polar regions host a mixture of plumes and pseudostreamers. Top right: Formation of a jet-like feature giant plume in the model of Pinto et al. Note the analogy between the magnetic configuration shown in this panel and that of Figure Images reproduced with permission from [top left, bottom] Wang et al.
The 2. The work of Pinto et al. As in the previous work, plasma outflows are envisaged, but the models are still too crude to make accurate predictions. The inclusion of dissipative effects is necessary to reach more reliable results. We notice that blobs have been observed in helmet streamers Wang, , but not in pseudostreamers, and a study of waves in these structures is still missing.
There is a different means to predict the speed of plasma outflows from pseudostreamers. In the Wang et al. Because flux tubes at the boundary of holes of the same polarity have no chance of rapidly expanding, they are likely sources of fast wind. More recently, Wang et al.
Wang et al. Observationally, several authors e. How much of these studies can be applied to plumes is still unclear: possibly the wind originating in plumes may change, both in time and as a function of geometrical factors still not accounted for.
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We expect that further theoretical and observational studies will help us understand whether the analogies between these structures contribute to a better knowledge of their characteristics. There are two means by which plumes may contribute to solar wind mass and energy supply: either being privileged channels by which a high enough wave flux is transported upwards, or by hosting a high enough number of outflowing events, likely generated by reconnection, capable of contributing the right amount of mass and energy. A recent advancement in this area has been made by Thurgood et al. The work of Thurgood et al.
Further work in this area is being done by, e. A few authors e. Research in this area is still scanty: it is not clear whether upflows occur preferentially in plumes rather than in interplumes or whether the faint interplume ambient hinders their identification and it is difficult to guess how large can be their contribution to the wind mass. We expect this to be a major research area in the near future.
With respect to older works, such as those of, e. Among those, jets are the first candidates as contributors to solar wind. Given the properties of standard and blowout jets e. A crude estimate of the mass and energy fluxes possibly supplied to the wind by blowouts has been done by Poletto et al.
These estimates implicitly assume that an unidentified mechanism supplies additional energy to the jets, increasing their speed, generally lower than the escape speed, so that they may become part of the solar wind. In conclusion, it looks unlikely that blowouts can significantly contribute to the wind. Still, this estimate may change, if type II spicules were proved to accompany blowouts: it is well known see, e. Here we are on a still unknown territory. Analogously, is there any relationship between the IRIS small-scale jets originating in network lanes Tian et al.
Network jets can be the transition region counterparts of type II spicules, and have been shown to be well capable to provide for the solar wind. The interrelationships of these episodic phenomena are still largely unexplored and the issue of their connection with coronal plumes remains to be examined. Connecting these transient events throughout the solar atmosphere and understanding their contribution to solar wind and coronal heating may lead to change our ideas on the source of the wind mass and energy and will likely be the focus of many future researches.
The progress done over the years, from the time plumes were observed in WL only at the time of eclipses by ground equipments, is impressive and the advent of space era acted, and still acts, as a trigger for new advances. Does this mean that eclipse ground campaigns have become obsolete and there is no point in pursuing that kind of observations? Certainly this is not the case, for more than one reason. If we compare ground and space-based coronagraphs, we realize they face different obstacles: briefly, from ground, we need to fight with stray light and sky luminosity and it becomes difficult to make observations at large distances beyond the limb; from space, it is hard to take data close enough to the limb of the Sun.
While the Mark IV has a plate scale of 5. These characteristics are dictated by the need of creating artificial eclipses, that are reproduced via internal or external occulters. Natural eclipses offer the ideal solution to decrease the sky brightness and to get the highest spatial resolution, on the order of 1 arcsec see, e. The problem with eclipses is, obviously, their limited duration. This also can be, at least partially, overcome, with, e. This eclipse, with a totality phase of 2 min and 40 s, will be visible throughout 14 US states and provides the opportunity of building, through the coordinated effort of numerous, differently located, observers, a movie showing dynamical events in the corona with an unprecedented high temporal resolution and over a large range of altitudes over the limb of the Sun.
With favorable weather conditions, the project will monitor changes in the low corona never ever sampled at such high cadence, providing information unavailable from space experiments. Natural eclipses still offer unique advantages over man-made instrumentation: the UV emission observed from space decreases steeply with altitude because, see Section 2. Hence, even in these days, it looks like coordinated efforts of the WL and XUV communities are the most productive means for advancements in this area.
Nowadays, this phenomenological result can be interpreted in terms of the poleward migration of the magnetic flux, induced by meridional surface flows, that makes polar fieldlines fan out more rapidly than predicted by a dipolar field see, e. It is a pleasure to thank the Editorial Board of Living Reviews in Solar Physics for inviting me to write this review. Comments by C. DeForest and a second unknown referee helped me improving the paper. Conversations with Y. Wang and G. Del Zanna are also gratefully acknowledged. I also like to thank F. Schulz for his precious work on my manuscript, which he turned into a better and easier readable paper.
Skip to main content Skip to sections. Advertisement Hide. Download PDF. Open Access. First Online: 01 December Going back to the configuration of the magnetic field inferred from eclipse observations of polar plumes, what did it imply for the global magnetic field of the Sun? An example of these first attempts to reproduce the plume orientation via simulations of the magnetic field of the Sun is shown in Figure 2. Open image in new window.
Figure 1: Ground image of the white-light solar corona at the July 11 eclipse, as seen from Atoll Hao, in the French Polynesia. Figure 2: Simulation of polar magnetic fields, supposedly traced by polar rays, during the eclipses of 30 June left and of 20 June right. The source of this discrepancy is not clear: however, if we look at Figure 3 , we realize that there is an individual factor in interpreting data. Also, because of the weak signal of plumes at large distances, only the brightest structures can be identified out to these high altitudes — and extra care needs to be taken in the observing and processing procedures.
The variability of plumes see Section 5 adds to the uncertainties. A method quite analogous to that used by Ahmad and Withbroe has been adopted by, e. Whenever spectra with a variety of lines were available, emission measure diagnostic methods have been adopted, analogously to what done by Ahmad and Withbroe , still assuming plasma to be isothermal.
A more sophisticated technique, that does not require this approximation, is the differential emission measure DEM analysis. It becomes crucial to check the behavior of the ambient plasma: has a FIP effect ever been observed in interplumes? Doschek et al. A decade later, Curdt et al. The spatial changes of the ratio outline the plume pattern and seem to confirm an over abundance of low FIP elements.
The problem of the abundance of elements is not settled, yet, especially if we consider that different estimates may be also related to when, over the plume lifetime, observations have been acquired. That the FIP effect may depend on the time elements have been confined within a structure has been suggested by, e. A recent paper, Guennou et al. Over hour observations, one of the plume analyzed by Guennou et al. Guennou et al. The topic is still open to discussion and can be the subject of future research.
Hassler et al. The right panel of Figure 7 gives the Ne viii Doppler velocity map of a CH area, with the chromospheric network superposed, to help visualize the close correspondence between network boundaries and blueshifts. The map in the left panel refers to a midlatitude non-CH region. It is obvious from the figure that most of the blueshifts occur in the CH and are concentrated along the boundaries of the network. Analogous results were obtained by several authors e. Mixed polarity areas were associated with smaller blueshifts.
However, plumes are not mentioned in the paper. Tu et al. These rapidly expanding flux tubes funnels are the sources of solar wind: flows are accelerated by waves originated by the reconnection episodes triggered by intranetwork loops being pushed towards the network by supergranular convection. Figure 8 illustrates the funnel scenario. Figure 8: An illustration of the funnel scenario. Figure 9 shows the profile of outflows in plumes, as inferred by different authors, with different techniques, some of which will be discussed later in this paper.
The results of Hassler et al. These conclusions should be supported by further studies as the authors warn readers about stray light effect possibly affecting their data they obtain a higher density in CH than in quiet sun, which is obviously unlikely. However, if confirmed, Fu et al. CH plasma. Figure 9: The profile of the outflow speed in plumes, inferred by different authors listed at left.
More complete analyses of UVCS data, aimed at identifying the profile of the wind speed vs. Results from these works are in a not complete agreement: Teriaca et al. Teriaca et al. On the opposite, the reconstruction of a static plume, embedded in this outflowing atmosphere, turns out to reproduce nicely its observed line radiance.
Hence, plumes turn out to be either static or flowing at a negligible rate. However, the widths of the lines provide further relevant information, pointing, whenever they exceed their thermal values, to the occurrence of unresolved plasma motions originating from waves or turbulence. This result has been confirmed by further studies, from either ground-based coronagraph data Raju et al. Figure 11 gives an example from Banerjee et al. Although widths are larger in interplumes than in plumes, the difference is minor.
Figure Profile of the non-thermal velocity vs. Krishna Prasad et al.
Results from these works are not in complete agreement with those of Gupta et al. The data point to a higher propagation speed in interplumes, but there is hardly any sign of acceleration over the altitude interval arcsec above the limb covered by the AIA FOV. The reason for this discrepancy may be at least in part ascribed to the way acceleration is calculated. The speeds of the propagating disturbances are evaluated from the inclination of the slanted lines, drawn in a subjective procedure over the x - t maps, in an attempt to follow the oscillations trajectories.
As seen in Figure 12 , personal judgment enters heavily in the choice of the trajectories. Also, oscillations in plumes persist up to higher altitudes than shown in Gupta et al. As to the damping of oscillations, Krishna Prasad et al. Figure 13 is a cartoon representative of plume formation. We point out that during the processes described above, the base field is being modified and brightenings generated by the formation of closed loops may be observed.
Also, the reconnection involving different fieldlines at a time, leads us to envisage a plume as a structure integrated over multiple fine-scale features, possibly reminding us of the suggestion of Llebaria et al. Figure A cartoon showing the formation of a plume, when a bipole is dragged towards the preexisting unipolar field and reconnection occurs.
Indeed, successive analyses, on the basis of Ulysses data, by McComas et al. A few years later, Reisenfeld et al. Figure 14 shows that this only occurs at high latitudes and that the correlation weakens with heliocentric distance. The authors point out that the He abundance is established near the base of the corona and that abundance anomalies have been detected in plumes see Section 3. Speculating on the reason why He is enriched at the base of plumes, Reisenfeld et al.
Figure 15 gives representative solutions for plume models of Walker Jr et al. Both authors predict outflows in plumes; although Allen et al. Hence plumes may, in case, be sources of slow, rather than of fast, wind. Figure Top panel: Representative solutions for the temperature and flow speed of a plume vs. More recent models have been trying to reproduce the initial and final stages of the life of plumes. This requires a time-dependent simulation: Pinto et al.
Figure Left panel: Representative solutions for the temperature and flow speed of a plume vs. Figure 17 illustrates Ulysses data of two switchbacks observed at distances, respectively, of 2. However, the sheared magnetic field fluctuations back-reaction on the plume plasma, may eventually lead to the mixing of plume and background plasma and not all plumes will be traceable in the far solar wind. We cite in passing that recently Matteini et al.
Figure Comparison of representative radial inversion features observed by Ulysses in left panels with Landi et al. We may ask whether there are larger scale features, analogous to plumes and more easily accessible to observations, that might help us reach a better understanding of the physics of plumes. The quasi-steady behavior of pseudostreamers was thus suggested to correspond to the behavior of plumes: above pseudostreamers, outflows have indeed been observed see Figure Acknowledgements It is a pleasure to thank the Editorial Board of Living Reviews in Solar Physics for inviting me to write this review.
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Cited on pages 25 and To gather more information, Yang et al. The extra measurements revealed evolution in the magnetic field, as well as heightened emission in both the UV and EUV measurements near the locations of the cannonballs. This suggested that the solar magnetic field was intimately involved in the creation of these structures. The authors propose that a solar cannonball forms when magnetic reconnection occurs within the chromosphere Figure 4. In this process, small-scale magnetic field loops emerge and detach from stronger large-scale loops. The small loops rise up toward the large loops and reconnect, flinging chromospheric plasma along the large loops.
The centre-to-limb variation in line radiance is interesting per se, allowing for instance distinguishing between optically thin and thick lines but is also relevant for estimates of the disk-integrated line emission, which is proportional to the line irradiance in turn, aside from a constant and neglecting off-limb emission a good approximation for all but the hottest lines, as shown in Paper II. For an empirical estimate of the centre-to-limb variation in the quiet Sun component, we employed the same technique described in Andretta et al.
The corresponding bi-dimensional histograms vs. Curves derived from plane-parallel and spherical optically thin models are shown as dashed and continuous orange lines, respectively, in both right-hand panels. For the calculation of the I vs. The line-radiance maps are shown in the left-hand panels with circles indicating some representative heliocentric distances.
This function is shown in Fig. It is sometimes assumed e. However, as shown by Andretta et al. It, however, still serves as a warning against interpreting the empirical centre-to-limb variation in that line only in terms of optical thickness effects. The quadratic solid lines and power-law dashed lines fits to the USUN data are also shown. The resulting fitting curves are shown in the right-hand panels of Fig. In the case of Eq. The averages were done separately for dates corresponding to solar minimum and solar maximum. More specifically, we considered the following time intervals for the minima and maximum of solar activity: — minimum:: SYNOP mosaicsbetween 2 April and17 May noUSUN studies in that time interval ;.
In the upper-left panel , the centre-disk intensity, I 1 , from the power-law fit are in black, while results from quadratic fits are in grey. The mean empirical profiles are then again fitted with the same functions described above. The resulting fitting function are also shown in Fig.
We therefore use SYNOP data to measure the centre-to-limb variation in that line along the full solar cycle from the — to the — minima. No significant variation in the centre-to-limb variation is apparent in these data.
On the other hand, the variation in the radiance from centre to limb does not depend on the radiometric calibration. Therefore, its invariance between the two solar minima suggests that the structure of the transition region did not change. Note also that parameters of the centre-to-limb functions given in Table 1 do not change between solar miminum and maximum except for the hotter lines. The invariance of the centre-to-limb function is best seen by examining the factors f cl 1 , which are consistent with constant values in all cases within the stated errors, usually within one or two standard deviations.
However, Wilhelm et al. While we can state that our results are consistent with the results of Wilhelm et al. The majority of lines in the CDS wavelength range can still be assumed to be effectively optically thin, since their limb brightening can be modelled very well in many cases by an exponentially stratified emissivity in a spherical geometry, as shown in Fig.
That is, they derive a limb brightnening in the helium resonance lines from their calculations which is still too strong. For what concerns a comparison to other measurements, we mention the work by Mango et al. We thus do not regard a comparison with the results of our fits as very significant. The factor f cl can be compared with the factor that is given in Table 4 of Wilhelm et al.
The only lines in common between Table 4 of Wilhelm et al. For the latter line, the value 1. The value given by Wilhelm et al.
Images of the solar upper atmosphere from SUMER on SOHO - Ghent University Library
It should be noted that these factors allow an estimate of the disk irradiance due to QS emission only no active regions nor coronal holes are accounted for. For the remainder of this work in any case, we adopt the quadratic fits Eq. Using the characterisation of the centre-to-limb variation in line median radiances described in the previous section, we first demonstrate in Fig.
Such biases in peak position and width of the QS histograms are still clearly detectable when restricting the analysis to pixels within 0. In the latter case, however, the statistics are significantly worsened because both of a smaller number of pixels contributing to the histogram and a larger contribution from active region belts outside the epochs of minimal solar activity. The original radiance map from the USUN mosaic taken on December 15, is shown in the upper-left panel. The fit to the average centre-to-limb variation at the minimum of solar activity is shown in the upper-right panel as a solid line i.
The map of radiances corrected for that limb brightnening function is shown in the lower-left panel. The widths of the Gaussian fit to the core of the histograms are, respectively: 0. The horizontal lines mark representative dates: 23 April green , 30 October orange , 1 May red , and 15 December grey. The gap in is due to the loss of contact of SOHO. The lower panels show the corresponding radiance histograms with the same colour coding. All the histograms are normalised to the total number of pixels; those shown in the lower panels are then also normalised to the peak of the average histogram at the solar minimum of activity.
We chose therefore to consider all on-disk pixels but removed the average centre-to-limb variations. The resulting images were then analysed to determine the quiet-Sun component of the radiance histogram. In the upper panels of Fig. It is readily apparent that the quiet Sun contribution is usually still well approximated by a log-normal distribution, even around the maximum of the cycle. We therefore estimated the position of the peak and its width by fitting a Gaussian to histogram bins around the maximum of the distribution.
In case of multi-component distributions, we fitted the peak corresponding to the lowest mean radiance. In the lower panels of the same figure, we show examples of radiance histograms at different phases of the solar cycle for four representative dates from the rising phase of cycle 23 23 April green through its maximum 30 October orange and from the decaying phase 1 May red , to the deep minimum before cycle 24 15 December grey histogram. The data points used to compute the average values of Table 2 are marked in red.
A similarly small contribution is also seen in the April histogram too but is almost completely absent in the histogram corresponding to the maximum of solar activity October A more detailed characterisation of the variability in the distribution of QS radiances along the solar cycle is given in Fig. This is not easily disentangled from the QS component see again histograms of Fig. As shown in at least two of the examples of Fig. Hence, the variability shown in Fig. In principle, with careful fitting of multiple Gaussians, or masking out the ARs on the disk, it could be possible to recover the properties of the residual QS component.
Such an approach, however, is made difficult by the large area covered by AR-related emission, as discussed in Sect. For the purpose of this paper, the current analysis is sufficient. This is true for the QS or lowest-radiance component but is also true for the components due to the emission by active regions see again Fig. It is beyond the scope of this paper to analyse the properties of the histogram components due to ARs in detail, but it seems the value of these histogram widths are characteristic of each line from the data shown here.
Table 2 reports the average values at solar-minimum epochs, as defined in Sect. The data points used to compute the averages are highlighted in red in Fig. We recall that the position of the peak is in practice a measure of the median radiance; the mean radiance can be computed by using Eq. This is also clearly seen from the data in Fig.
As already mentioned in Sect. It is not clear why the values measured by Wilhelm et al. We were very careful in fitting only the core of the histograms, thus filtering out most other contributions. Moreover, the values measured by Wilhelm et al.