Coronal Mass Ejections: An Introduction: 376 (Astrophysics and Space Science Library)

The vertical bars represent the angle of closest approach to the Sun of each line of sight. The ordinate shows a the total scattered radiance dB and b the excess polarized component of the scattered radiance d pB. From Figure 2 of Howard et al. This effectively annihilates any 3-D information from the white light images themselves, when observing total radiance. Figure 5b shows the same plot as in panel a, but for the scattered polarized radiance d pB. The Thomson plateau is absent here, enabling the possibility for retaining 3-D information from the white light images. This is discussed further in Section 5.

It has been known for centuries that the corona is optically thin. In ancient terms, this meant that comets could pass behind the corona and still be observed even though we did not know until the end of the 19th century that the corona had a solar origin see, e. In modern terms, this means that the particles comprising the plasma within the corona do not scatter light from their neighbors, resulting in it being diffuse at most wavelengths. This likewise corresponds to transients within the corona, such as CMEs. Optical thinness has the advantage that, assuming that all of the observed light is Thomson scattered Sect.

The disadvantage is that it reduces our ability to distinguish between plasma that is part of the CME and plasma that is elsewhere along the LOS. The measured radiance contains no information as to where along a LOS the scattering volume of electrons lies or the level to which multiple volumes contribute to the measured radiance. When observing a CME as it appears in a white light image, it is tempting to regard it as a two-dimensional plane or as a spherically symmetric feature. This may be suitable and even appropriate when considering small CMEs but not for larger ones.

Recall from Section 2. It is widely believed that these are manifestations of a magnetic flux rope e. The most basic form of flux rope is of a coil, perhaps resembling a spring in structure. While it is assumed that the magnetic infrastructure comprising a CME is substantially more complex than this, I use the basic example to illustrate a point about geometry and perspective. Consider the three scenarios depicted in Figure 6.

In the first case panel a , we have an ideal coil linearly aligned along the plane of the observer; in the case depicted in panel b it moves with one end leading; and in panel c we have the center leading the flanks. I show each scenario viewed from three coplanar, but separate locations.

An Introduction

Given that CMEs are optically thin Sect. We can see how vastly different 2-D configurations emerge as we vary the scenario and viewing location. Illustrations of the effects of geometry and perspective when measuring CMEs. The simple case of a symmetric circular coil, intended to represent an oversimplified flux rope, is presented in three scenarios: In the case of white light imagery, a fictional observer would observe these configurations in their entirety, and a section at the bottom of the third coil has been highlighted in blue to demonstrate the variation of its location in each scenario.

In a separate color, I have also highlighted a fixed feature on the ideal coil, which remains in the same place at all times and in all three locations. I have selected a location at the bottom of the third coil. Unless observing the coil edge-on, it is difficult to identify where in the 3-D structure of the spring the feature lies;. Under most conditions we observe this ideal coil as a series of loops at different locations, but generally part of an overall collective. This might be interpreted as multiple eruptions or multiple magnetic structures, but in this case they are part of the same ideal coil.

Figure 1a shows a strikingly similar comparison between one of the scenarios depicted in Figure 6 and a CME observed by a white light image.

We can continue to expand upon the level of complexity surrounding the scenarios. It is physically reasonable to allow for the coil loops to expand as they erupt and even for them to expand at different rates. Flux ropes can be twisted or kinked e. I discuss this study further in Section 5. Commonly used white light imagers are calibrated into units of solar brightness — I mentioned this in Section 2.

The accuracy of such calibration is crucial for mass measurements and likely affects other measurements such as CME location as well. Certainly, the contrast of a given image of a CME will affect where one decides upon the location of the leading edge, but this is more of a display issue than a calibration issue.

It has been assumed that the white light imagers have been adequately calibrated see, e. Consider the plots shown in Figure 7 , which follow the technique devised by C. They represent the mass of a particular CME calculated using the standard technique described in Section 2. Common to each plot is the Thomson plateau Sect. This demonstrates the robustness of mass measurements but at the cost of the annihilation of information about the 3-D location, as mentioned earlier.

There are several possible reasons for this discrepancy; the bulk of the CME mass may not be observable by one or more observers, or we are measuring different areas comprising the CME since its geometry appears different in each image; but the most likely explanation is that one, two, or even all three of the coronagraphs have not been calibrated correctly. It is possible that we could mitigate this discrepancy by adding a correction post-analysis, for example we could have the LASCO plot meet the crossover point between the two CORs by adjusting its radiance such that its mass is tripled, but we would need a substantial investigation to determine whether it is indeed LASCO that requires the correction and not one or both of the CORs.

Sections 3 and 4 describe in detail the obstacles presented to us when attempting to extract meaningful measurements of CMEs. Many of these are insurmountable, creating a limit in our accuracy Sect. The arrival of new datasets via the STEREO mission has, however, enabled the development of a few tools that we can utilize to attempt to improve our extraction abilities.

In recent years these have focused on utilizing the stereoscopic capabilities of STEREO, but have been met with only mixed success. We have found that additional information is available in heliospheric image data that is not available in coronagraphs, as they effectively remove an important impediment imposed upon the latter. Coronagraphs and heliospheric imagers observe in two dimensions only; what is imaged are the radiances integrated along each line of sight LOS through the feature, projected into the plane of the sky of the imager i.

The distances and masses determined from these images are projected values and therefore represent a lower limit on the actual parameters. To derive a value that has been corrected for 3-D effects we need to obtain an angle from the sky plane of the measured feature i. By far, the most commonly used mode used for the white light images is the unpolarized mode, i. As described further in Section 4. This means that the location of a feature along the LOS can vary by a large amount before any noticeable change in its radiance is detected. We must therefore seek alternative means to extract 3-D information about the CME.

The most common means by which workers have attempted to extract 3-D properties is by investigating auxiliary datasets. That is, workers seek datasets that exhibit features known to be related to CMEs, but contain information about their location in 3-D. In-situ measurements provide a great deal of information about a CME passing through a single point a large distance from the Sun, but workers typically seek images of the solar disk to assist in 3-D location.

Physically it makes sense to seek these as evidence of the source of the CME eruption, as they arise from magnetically complex regions and exhibit signs of either magnetic reconnection or some other high-energy magnetic field reconfiguration, which probably provided the launch mechanism for the CME. Other phenomena observed on the solar disk appear to serve as a better proxy for CME location; these include coronal dimming McIntosh et al.

Unfortunately, these are associated with only a small proportion of the CMEs observed in white light and the features best observed on the solar surface those near disk center correspond with the white light CMEs that are most difficult to measure halos. STEREO consists of twin spacecraft, oriented into orbits that increase their angular distance from the Sun-Earth line, and each other, with time. This enables simultaneous views of CMEs from separately spaced viewpoints for the first time.

The earlier reconstruction techniques are reviewed by Mierla et al. The vast majority of publications on the 3-D reconstruction of CMEs from white light images have involved coronagraph data. While the levels of complexity and sophistication vary, the methods for coronagraph reconstruction can be divided into the following three categories: Geometric triangulation of common features observed by each spacecraft e. Volume fitting around the CME envelope, typically using a predetermined configuration and adjusting its geometrical parameters to match that enclosing the CME e.

Geometric triangulation is based on the principle that a feature that appears in multiple observers is at the same location in 3-D space. A worker would select their feature of interest, measure its projected location in each of the coronagraphs most papers have involved just the two STEREO spacecraft but a few e. Such a technique is subject to several pitfalls, including the assumptions that the measured feature is sufficiently spatially localized, that it is the same feature as that observed with the other imagers and that the same part of the feature is being measured, and whether the instruments are sufficiently calibrated to identify the same feature in the same location.

Volume fitting assumes that the CME lies within a particular volume and adjusts the parameters of that volume to fit the observed geometry of the CME. The selected volume ranges in complexity, including a simple polygon e. Such techniques are appropriate for simple identification of the general volume in which the CME lies, but do not contain measurements of high levels of accuracy. Polarimetry enables the utilization of a property of Thomson scattering not available through unpolarized light. As shown in Figure 5b , the Thomson plateau Sect. The problem of non-localization along the LOS is therefore somewhat mitigated; in other words, a feature cannot move as far from the plane of the sky before its observed radiance is diminished by a detectable quantity.

By comparing the unpolarized with the polarized radiance we can therefore identify the distance from the plane of the sky of the feature along any given LOS. This enables the construction of the 3-D CME, pixel-by-pixel. The scattered radiance distribution is symmetric about the sky plane in coronagraphs, and so this technique alone cannot distinguish whether the feature is ahead of or behind the sky plane.

Hence, the technique is generally used with another that enables this determination. For example, one method involves combining the volume fitting technique of de Koning et al. Despite the large number of papers that have been published describing methods of 3-D reconstruction, no successful method has yet emerged to accurately reconstruct the CME from coronagraph images alone.

The reasons that these new datasets have had so little impact are because of the physical limitations inherent in the instrumental measuring process, which are discussed in Sections 3 and 4. Heliospheric imagery involves the same principles as coronagraphs: Likewise the 3-D reconstruction techniques applied to heliospheric imagers have been limited in their success. Geometrical triangulation is nonsense at the distances observed by heliospheric imagers as it is impossible to identify a sufficiently localized feature with which to perform triangulation that is in any way reliable Howard b , and polarimetry is not available as no contemporary heliospheric imager has a polarizing capability.

This leaves us with the volume fitting technique, but here the heliospheric imagers have an advantage over coronagraphs. Since they observe across a very large angular extent, we have the ability to extract 3-D geometrical information that is not available in coronagraphs. Howard b provides a review on the enhanced abilities of heliospheric imagers for 3-D information extraction.

Briefly, two important assumptions that are applied for the analysis of coronagraph images — that the angles of all the observed features from the Sun are small and that the features are close to the sky plane — break down at larger angles from the Sun. Also the geometry of the CME itself, which can be regarded as sufficiently small to be ineffective at small angles, governs its appearance e.

This necessitates a full geometrical reconstruction of the CME, similarly to the volume fitting technique used for coronagraphs, but taking advantage of the physics of Thomson scattering at large angles from the Sun. As with those developed for coronagraphs, techniques of varying complexity and sophistication have been developed.

Finally, I would like to comment on the potential of polarized heliospheric imagery. The author and colleagues have developed the theory demonstrating this ability DeForest et al. Additionally, we performed some preliminary studies using simulated polarized heliospheric imagers Howard et al. Further, we have investigated the feasibility of identifying CMEs in significantly degraded HI-2 images, to explore the quality of the data needed to extract meaningful measurements. These studies, combined with the geometric advantages of heliospheric imagers over coronagraphs, enable us to increase the accuracy of 3-D reconstruction beyond the capabilities of coronagraphs.

Hence the next generation of heliospheric imagers should be one capable of polarimetry. Such a capability will not be present in those heliospheric imagers currently under development for the Solar Probe Plus or Solar Orbiters, but are under consideration for a separate mission being proposed for placement into an L5 Lagrange point orbit.

Even then we need to be fortunate enough such that all of the instruments are observing in the right place at the right time and for the CME to be directed in such a way as to optimize their measurement capabilities. To understand the physics involved we also need modeling, both empirical and theoretical.

The bounds lie in the data, as our ability to extract the information we need is hampered here as we have explored exhaustively in this review. Consequently, very few serious efforts have been made to physically reconstruct the CMEs using data alone. This orientation enabled the viewing of the CME roughly tangential and normal to the flux rope axis, allowing the observation of its central structure as well as its cross-section.

By combining the white light data with the EUV solar disk images, as well as the in-situ measurements, we were able to reconstruct the CME flux rope and develop a narrative for its launch and departure from the corona. Figure 8 shows a simplified cartoon of the flux rope launch and its configuration near 1 AU.

We constructed a narrative describing an asymmetric launch not unlike the scenario shown in Figure 6b , where the western end launched first and the rest followed in a manner perhaps analogous to a strip of carpet being torn from a floor. We note that the single flux rope was subject to three different onset mechanisms tether cutting, following by mass draining, followed by kink instability and a possible bisection before completely departing from the Sun.

The narrative was constructed via careful observation and measurement of features observed with white light and EUV imagery and in-situ spacecraft. The CME as it appeared in coronagraphs consisted of multiple loops enclosed within an overlying bubble see Fig. Several hours later a completely separate structure was observed in the coronagraphs, appearing as little more than a puff of increased density see Fig. This was the signature of the departure of the final segment of the flux rope.

For us to produce this narrative we relied on sophisticated processing of the white light data, careful observation and tracking of features in the EUV images, utilization of the in-situ datasets see DeForest et al. It is this kind of effort that will be required for the development of future CME geometry and eruption narrative reconstruction. The purpose of this review is to highlight the difficulties associated with the detection and measurement of CMEs. I have focused primarily on white light coronagraph and heliospheric imager detection, as they can be regarded as the binding agents between the various solar observations and the in-situ measurements of CMEs.

I have explored the progress in CME measurement over the decades and described in detail those factors that impede our ability to extend our measuring capabilities further. Despite the ongoing efforts by many workers, we may need to conclude that the accuracy of CME measurements cannot be improved upon with the current generation of white light imagers.

This is not a spatial resolution, cadence, or even sensitivity issue, it is simply that there is likely no more information available in these images. The factors explored in this review optical thinness, geometry, Thomson scattering leave us with the ability to reconstruct the CME as only a hazy mass, given the current generation of white light imagers.

This is probably sufficient for operational efforts such as space weather forecasting, as the important factors needed there leaving aside magnetic field information that is not available in white light imagers are whether the mass is directed toward Earth and when we can expect it to arrive there.

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However, for measurements of accuracy required to advance our scientific knowledge of CMEs we need to extend beyond the current generation of imagers. Polarimetry is not yet a fully explored area of white light imaging and it does offer some promise for advancement, particularly in heliospheric imaging, and these efforts need to continue to be developed.

Those CMEs that are of most interest to space weather are typically the large energetic ones. These are often observed by white light imagers in the classic three-part configuration. What we observe in white light is a 2-D manifestation of the entire 3-D configuration, combining the integrated lines of sight, all folded into an image on the sky plane.

Three-dimensional information is not directly available here, but we can employ auxiliary datasets and models to help us reconstruct the physics involved in the eruption of the CME structure. We must all get into the mindset of regarding CMEs as two-dimensional projections of a collective of lines of sight through a 3-D object. We must regard projection and simultaneous activity in the foreground and background with each observation.

Often, what appears to be a revealing new physical property of CMEs may simply be a projection effect. Following such notes of caution one may wonder what the best course of action for advancing our ability is to extract scientific information about CMEs. We have exhausted the information we can extract using unpolarized white light imagery, but further information is possibly still available using polarimetry.

The design of the next generation of white light imagers should include a polarizing ability and any novel means by which such information can be optimally made available. Beyond this we must coordinate our efforts, combining multiple observations of the Sun, corona, and heliosphere with modeling and carefully reconstruct the physical narrative of the lifetime of the CME. We must regard CMEs as extended 3-D objects and that the importance of perspective cannot be understated.

Finally, we must be careful in the language we use to describe CMEs and their related features. Given how some of the terms we use are poorly defined to begin with, it is not a bad idea to include a section in our papers defining the terms we will be using throughout. The editor thanks two anonymous referees for their assistance in evaluating this paper.

Cite this article as: Regarding the detectability and measurement of coronal mass ejections. Space Weather Space Clim. Data correspond to usage on the plateform after The current usage metrics is available hours after online publication and is updated daily on week days. All issues Volume 5 J.

Statistical Challenges in Solar Information Processing. Problems with CME measurement 6. Discussion Acknowledgments References List of tables List of figures. A model for solar coronal mass ejections. Analysis of the prominence associated with the coronal mass ejection of August Four coronal mass ejections and their associated surface activity observed on 26 October Petersburg, Russia, —, , DOI: The structure and origin of magnetic clouds in the solar wind.

Propagation of an Earth-directed coronal mass ejection in three dimensions. Automatic detection and tracking of coronal mass ejections, II. Multiscale filtering of coronagraph images. Interplanetary shocks preceded by solar filament eruptions.

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First determination of the true mass of coronal mass ejections: Solar and heliospheric geoeffective disturbances. A bubblelike coronal mass ejection flux rope in the solar wind. Monograph , 58 , —, Stereoscopic imaging of an Earth-impacting solar coronal mass ejection: Feasibility of heliospheric imaging from near Earth. Disconnecting open solar magnetic flux. Tracking coronal features from the low corona to Earth: Interplanetary shock waves generated by solar flares. The emergence of a twisted magnetic flux tube into a preexisting coronal arcade. Numerical simulations of three-dimensional coronal magnetic fields resulting from the emergence of twisted magnetic flux tubes.

Interplanetary coronal mass ejection and ambient interplanetary field correlations during the Sun-Earth connection events of October—November Morphological evolution of a three-dimensional coronal mass ejection cloud reconstructed from three viewpoints. ICMEs in the inner heliosphere: Report of working group G.

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