Onset Of Coronal Mass Ejections Research Articles

Coronal dimming and the coronal mass ejection onset

A set of five observations of extreme-ultraviolet (EUV) coronal dimming associated with coronal mass ejection (CME) activity is examined. Using spectroscopic data, plasma characteristics across a broad range of temperatures from 20 000 K to 2 million K are determined. The dimming events are found to coincide in time, and to coincide spatially, with the projected onset times and locations of the associated CMEs. The spectral data confirm that the dimming is due to mass- loss, and not temperature variations. The actual mass-loss calculated from the degree of dimming, using two dierent methods, shows that the extracted mass in each case, is of the same order as the mass of the associated CME. In some cases, the EUV observations are limited to relatively small regions under the CME events and it is expected that we do not witness the mass- loss associated with the entire event, for these. However, we believe that this analysis has provided a method for locating the source region of the trigger for a CME eruption, and that the dimming characteristics can be used to distinguish between onset processes of the CME. In particular, the gradual nature of the dimming process, which takes place over several hours, suggests that either the CME has a continuous driver rather than a sudden impulsive onset, or the low coronal response to a CME extends over a long period.

SOHO observations relating to the associationbetween flares and coronal mass ejections

Abstract Campaigns to investigate the solar coronal mass ejection (CME) onset have been run using the Solar andHeliospheric Observatory (SOHO) since 1996. These have included coronagraph and extreme-ultraviolet (EUV) disc imaging, along with magnetic mapping of the photosphere, in concert with EUV and UV spectroscopic observations. These campaigns have included co-ordination with ground-based observatories, and with other spacecraft, especially Yohkoh and the Transition Region and Corona Explorer (TRACE). This multi-instrument, multi-spacecraft effort has provided many rewards, with some spectacular observations of countless eruptions. It has included the discovery of unexpected phenomena such as EUV waves and groundbreaking work on coronal dimming, and the development of sigmoidal shaped structures. Much has been learnt about the CME onset yet the most basic questions still remain. We have an unprecedented view of CME eruptions, yet we are still unable to identify clearly the onset process and we do not fully understand the CME-flare relationship. With all of the campaigns producing excellent multi-wavelength observations of CMEs, how far have we progressed in the understanding of the CME onset and, in particular, the CME-flare relationship? Can we identify lines of research using the SOHO data, which will provide the answers we seek — or do we need fundamentally different observation scenarios? It is the author's opinion that we actually have the observational tools required to understand much about the onset process and the CME/flare links, and the emphasis should be on understanding the limitations of our instrumentation and on removing any preconceived ideas from our interpretations.

The Timing of Flares Associated with the Two Dynamical Types of Solar Coronal Mass Ejections

In this Letter, we consider a sample of Transition Region and Coronal Explorer flare-associated solar coronal mass ejections (CMEs) and study the timing behavior of the flares associated with fast or slow CMEs. We find that flares associated with fast CMEs tend to happen within half an hour of the CME onsets, while the timing of flares associated with slow CMEs is only loosely related to the CME onsets. This suggests that the occurrence of flares may be integral to the early development of fast CMEs but is not crucial for slow CMEs. This observational result supports a recent qualitative theory of the initiation and expulsion mechanism of the two dynamical types of CMEs.

The magnetic topology of a sigmoid

Recent surveys of solar features have linked the “sigmoid-to-arcade” scenario observed in the soft X-ray corona to coronal mass ejection (CME) onset (Geophys. Res. Lett. 26 (1999) 627, Geophys. Res. Lett. 14 (1998) 2481). Further to these observations, incorporation of extreme-ultraviolet, white light and H-alpha data into such a survey (Geophys. Res. Lett. 27 (2000) 2161) has illustrated the need for a quantitative definition of the term “sigmoid” and further understanding of such features if they are to be used as a means by which to predict CME onset. We analyse two sample active regions in detail, each appearing both sigmoidal and eruptive in Yohkoh soft X-ray telescope (SXT) full-disk data. Both regions were observed during October 1997 and each produced a flare displaying eruptive characteristics. In each case, formation of a flare-arcade was observed by both SXT and the extreme ultraviolet imaging telescope (EIT) following the event. EUV dimming and coronal EIT waves were also observed in each case. We have studied each active region both before and after eruption using soft X-ray, EUV and H-alpha data. A linear force-free field extrapolation has also been applied as a means by which to determine the active region field deviation from potential in each case. Each active region was observed to erupt by means of a different mechanism and while both events show signatures of eruption and consequently, mass ejection, only one produced a CME large enough to be observed by the SoHO large angle spectroscopic coronagraph. The implications of these observations in terms of CME prediction are discussed.

Material Outflows from Coronal Intensity “Dimming Regions” during Coronal Mass Ejection Onset

One signature of expulsion of coronal mass ejections (CMEs) from the solar corona is the appearance of transient intensity dimmings in coronal images. These dimmings have generally been assumed to be due to discharge of CME material from the corona, and thus the are thought of as an important signature of the sources of CMEs. We present spectral observations of two dimming regions at the time of expulsion of CMEs, using the Coronal Diagnostic Spectrometer (CDS) on the SOHO satellite. One of the dimming regions is at the solar limb and associated with a CME traveling in the plane of the sky, while the other region is on the solar disk and associated with an Earth-directed halo CME. From the limb event, we see Doppler signatures of approximately equal to 30 kilometers per second in coronal (Fe XVI and Mg Ix) emission lines, where the enhanced velocities coincide with the locations of coronal dimming. This provides direct evidence that the dimmings are associated with outflowing material. We also see larger (approximately equal to 100 kilometers per second). Doppler velocities in transition region (0 v and He I) emission lines, which are likely to be associated with motions of a prominence and loops at transition region temperatures. An EIT accompanies the disk event, and a dimming region behind the wave shows strong blue-shifted Doppler signatures of approximately equal to 100 kilometers per second in 0 v, suggesting that material from the dimming regions behind the wave may be feeding the CME.

EUV Sprays: Jet-like eruptive activity on the solar limb

Jet-like eruptive events have been detected in the extreme ultraviolet (EUV) observations from the Solar and Heliospheric Observatory (SOHO) spacecraft. Some jet-like events are shown and for one event, that of January 22 2000, we provide a detailed analysis of the EUV observations. The January 22 observation was part of an on-going campaign to observe the onset of coronal mass ejections (CMEs) using extreme ultraviolet spectroscopy. A high-temperature fan-like jet was first detected in emission lines from plasmas at one million K and over, and it appears to be the onset of a narrow mass ejection identified later in coronagraph data. This was followed by a dramatic jet or finger of cool ($\le$250 000 K), rotating plasma which extended vertically, high into the corona. It extended well beyond the field of view of the instrument and appears to relate to a small ejected cloud which is detected in subsequent coronagraph data. The spectroscopic characteristics are used to provide plasma diagnostic information for these two events, and this combined with images for a wide range of temperatures allows a thorough investigation of the processes leading to such eruptions. A simple accreting magnetic field model is used to descibe the basic features of the jet activity.

EIT Waves and Fast-Mode Propagation in the Solar Corona

During the onset of coronal mass ejections, a front of enhanced EUV emission is sometimes seen to propagate away from a flaring active region across the solar disk. We present model simulations to test the hypothesis that these transients (called "EIT waves") represent fast-mode MHD waves. The distribution of the magnetosonic velocity vf in the corona is determined using a current-free extrapolation of the measured photospheric field and a density scaling law for coronal loops. In agreement with observations, the waves are deflected away from active regions and coronal holes, where vf is large; they are also refracted upward as they propagate away from their initiation point, since vf falls off rapidly above active regions. The average surface-projected expansion speeds are only of order 200 km s-1, comparable to or somewhat smaller than those of EIT waves observed during 1997-1998. The model is unable to account for the velocities in excess of 600 km s-1 associated with Moreton waves and type II radio bursts unless it is assumed that the initial disturbance has the form of a strong, super-Alfvénic shock.

Initiation of a coronal mass ejection

There are by now only one or two examples to show the initiation of coronal mass ejections (CMEs). In this paper we present a clear instance of a CME initiation. The enhanced disturbance was seen as the slow upward motion in segments of a prominence. Four hours later, the prominence was accelerated to about 60 km s−1. It greatly expanded, showing twisted structure, in later one hour. The CME onset was characterized as the disrupting and opening of the prominence. During the onset phase of the CME, a nearby small prominence erupted. It appeared that the global instability responsible for the CME was triggered by the initial mass ejection of the first prominence.

Magnetic Shear and Cross‐Field Currents: Roles in the Evolution of the Pre–Coronal Mass Ejection Corona

A widely used approximation in modeling the solar corona is to assume that the magnetic field is force free, in which case all electric currents flow parallel to the magnetic field. However, observations of the large-scale corona show clearly the presence of nonradial density variations and thus, presumably, nonradial pressure gradients. In equilibrium, such pressure gradients must be balanced by magnetic forces arising from cross-field currents. It is possible that these deviations from force-free conditions play a significant role in the evolution of the corona, particularly in the evolution leading to the large-scale eruptions known as coronal mass ejections (CMEs). In this work we explore the role of cross-field currents in the evolution of a model corona involving a simple magnetic arcade in magnetostatic equilibrium, neglecting the solar wind flow. Our formalism builds on the generating-function approach used in some force-free modeling but transcends the limitation to force-free conditions by introducing a second generating function representing a nonhydrostatic pressure distribution and associated cross-field electric currents. The results show that the presence of cross-field currents makes our model corona more prone to eruptive behavior than a strictly force-free corona. This effect may be relevant to the onset of CMEs, suggesting that the coronal conditions most likely to result in mass ejections are those in which both field-aligned and cross-field currents are present.

Relation between Flare-associated X-Ray Ejections and Coronal Mass Ejections.

In an attempt to identify the direct signatures of coronal mass ejections (CMEs) in soft X-ray wavelengths, we have searched for plasma ejections in Yohkoh soft X-ray telescope (SXT) images in a total of 17 limb flares, and compared the results with the Solar and Heliospheric Observatory LASCO data. A general correlation exists between the presence/absence of the X-ray ejection and the CME. Although the height versus time relation often indicates (under the assumption of constant speed) that the CME onset coincides with the X-ray ejection, the latter probably does not represent the CME front, because the CME speed must result from acceleration, which would put the estimated onset at an earlier time. In some cases, the estimated CME onset time comes well before the impulsive phase of the associated flare. Although the role of the flare-associated plasma ejection in a CME is still unclear, we propose that its occurrence depends on the presence of open field lines, which can be due to a preceding CME. Lastly, we present a rare example of SXT observations of what appeared to be the three-part structure of a CME, which was seen a few minutes before a major flare started.

The initiation of coronal mass ejections-what observations are needed now?

We have been observing solar Coronal Mass Ejections (CMEs) for over two decades. We have a good idea of the mass, size and speed distributions of these events; we have a wealth of topological information and have studied their initiation and their effects on the Earth-environment through the observation of related activity and through interdisciplinary observations. However, we still have difficulties in understanding the CME onset process. It is suggested that interdisciplinary analyses using the SOHO spacecraft and through multiple spacecraft observations under the Inter-Agency Consultative Group umbrella will provide significant steps forward. However, in addition, we identify new methods of observation for investigating the onset of CMEs, in particular through the observation of the corona in 3-D, and assess their importance in unravelling the CME onset mystery.

Comment on “The solar flare myth” by J. T. Gosling

Journal of Geophysical Research: Space PhysicsVolume 100, Issue A5 p. 7917-7919 Papers on Solar and Heliospheric Physics CommentariesFree Access Comment on “The solar flare myth” by J. T. Gosling M. I. Pudovkin, M. I. PudovkinSearch for more papers by this author M. I. Pudovkin, M. I. PudovkinSearch for more papers by this author First published: 1 May 1995 https://doi.org/10.1029/94JA03033Citations: 7AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References Akasofu, S.-I., S. Yoshida, The structure of the solar plasma flow generated by solar flares, Planet. Space Sci., 15, 39, 1967. Cane, H. V., The evolution of interplanetary shocks, J. Geophys. Res., 90, 191, 1985. Gold, T., Plasma and magnetic fields in the solar system, J. Geophys. Res., 64, 1665, 1959. Gosling, J. T., The solar flare myth, J. Geophys. Res., 98, 18,937, 1993. Harrison, R. A., E. Hildner, A. J. Hundhausen, D. G. Sime, G. M. Simnett, The launch of solar coronal mass ejection: Results from the coronal mass ejection onset program, J. Geophys. Res., 95, 917, 1990. Ivanov, K. G., N. V. Mikerina, Ground level geomagnetic effects of interplanetary shocks fronts from powerful isolated flares of the nearby zone (in Russian), Geomagn. Aeron., 20, 508– 514, 1980. Ivanov, K. G., N. V. Mikerina, Powerful isolated flares of the near zone and the connection of the magnetic field of bipolar groups with the field of interplanetary flare throws and with geomagnetic disturbances (in Russian), Geomagn. Aeron., 21, 34– 38, 1981. King, J. H., The Interplanetary Medium Data Book, National Space Science Data Center, Greenbelt, Md., 1977. Piddington, J. H., Cosmic electrodynamics, 87, John Willey, New York, 1969. Pudovkin, M. I., A. D. Chertkov, Magnetic field of the solar wind, Sol. Phys., 50, 213– 229, 1976. Pudovkin, M. I., S. A. Zaitseva, I. P. Oleferenko, Magnetic field of the flare stream, in Russian, Izv. Acad. Nauk SSSR Ser. phys., 41, 242– 251, 1977. Pudovkin, M. I., S. A. Zaitseva, A. D. Chertkov, E. M. Fomina, The structure and geoefficiency of high velocity solar wind streams, Solar-Terrestrial Predictions Proceedings, , 3 F. Donnelly, B56– 65, U.S. Department of Commerce, Washington, 1980. Sheeley, N. R., R. A. Howard, M. J. Koomen, D. J. Michels, R. Schwenn, K. H. Mühlhäuser, H. Rosenbauer, Coronal mass ejections and interplanetary shocks, J. Geophys. Res., 90, 163, 1985. Citing Literature Volume100, IssueA51 May 1995Pages 7917-7919 ReferencesRelatedInformation

Solar microwave precursors and coronal mass ejection: Possible connection

There are many results concerning the phenomena of preflare activity and coronal mass ejections (CME) formation. The situation is possible that the CME is the flare response or on the contrary the CME onset initiates the flare. In a number of recent papers R.A. Harrison and his coworkers have shown that CME initiation is associated with weak soft x-ray emission. We have studied a possible connection between the solar microwave precursors and CME. Following this goal, during the period of 1989, 36.5 hour observational samples preceding solar flares were investigated. There were 8 events among them which preceded proton flares. Data have been obtained at four wavelengths (3.2 cm, 10 cm, 30 cm, 46 cm) at the Zimenki Station near Nizhny Novgorod (Russia). To investigate the precursors we used the classification accepted for the description of radio emission bursts in Solar Geophysical Data.

Structure of a fast coronal mass ejection from radio observations

We investigate the thermal structure of a coronal mass ejection (CME) in meter-dekameter wavelengths and compare it with the optical evidence. The multifrequency observations enable us to infer the three dimensional structure of the CME. We estimated the mass of the CME and found to be in reasonable agreement with the range of values obtained from white light observations. Nonthermal emission was present in the form of type III and continuum emissions several minutes after the CME onset. We use this information to study the relative timing between flares and CMEs. Finally we develop a schematic model of the event and point out that particle acceleration high in the corona is possible.

Thermal and nonthermal emissions during a coronal mass ejection

We report on the thermal and nonthermal radio emissions from a coronal mass ejection (CME) observed at meter-decameter wavelengths using the Clark Lake multifrequency radioheliograph. From white-light observations of the Solar Maximum Mission Coronagraph/Polarimeter instrument the CME was found to have a speed of ∼ 450 km s−1. Since there was no nonthermal radio emission in the beginning of the event and the one which occurred later was quite weak, we were able to observe the thermal structure of the CME in radio. Type III bursts and a nonthermal continuum started several minutes after the CME onset. We use the radio and optical observations to show that the CME was not driven by the flare. We investigate the thermal structure and geometry of the mass ejection in radio and compare it with the optical evidence. Finally we develop a schematic model of the event and point out that particle acceleration high in the corona is possible.

The launch of solar coronal mass ejections: Results from the coronal mass ejection onset program

We describe a set of solar coronal mass ejection (CME) events where coincident data sets from both X ray and white light instruments have been made available through deliberate planning. Using these we have been able to put tight limits on possible descriptions of the typical sequence of events, and these we relate to interpretations of models involving flares and CMEs. Our findings confirm recent suggestions that CME onsets precede any related flare activity and that the associated flaring commonly lies to one side of the CME span. The CME launch appears to be associated with minor X ray (flare precursor) activity. Although this scenario has been previously discussed (see Harrison, 1986, and references therein), the abundance of flare and CME models which are not compatible with this picture demands that confirmation be sought using programs such as this.

The launch of coronal mass ejections: White light and X ray observations in the low corona

As part of an ongoing program to determine the processes which lead to the launch of solar coronal mass ejections (CMEs) we present a thorough examination of periods where there are observations from the hard X ray imaging spectrometer on the Solar Maximum Mission which are complementary to observations made by the MkIII K‐coronameter on Hawaii. At the time of the observations no attempt was made to produce compatible data sets so the analysis is hampered by unfortunate timing and pointing. Nevertheless, with the analysis of six events we are able to draw some useful conclusions which agree with the findings of our previous work but dispute the scenarios presented by most theoretical models of the CME launch and the CME/flare relationship. To summarize briefly our results: (1) None of the CME launches occur in coincidence with a flare onset, (2) flares which do occur in the vicinity of a CME launch site occur both after the CME onset and in one foot of the CME structure, (3) there is weak X ray activity also associated with the footpoints of the CME structures and these indicate the presence of large coronal structures which underlie the CME structures, (4) there are X ray enhancements after all of the CME onsets; in some cases these are bright enough to be classed as flares, and in others they are lesser brightenings which we refer to as subflares.

The onset of coronal mass ejections

This study addresses the onset of coronal mass ejections. From examination of sensitive X-ray images from the Solar Maximum Mission around the projected onset time of coronal mass ejections we identify two important new features: (1) there is usually a weak, soft X-ray enhancement 15–30 min prior to the linearly extrapolated chromospheric departure time of the ejection; (2) this activity is generally from two widely separated (≥ 105 km) parts of the Sun. Possible physical mechanisms for these phenomena are examined and it is concluded that a plausible explanation is that the initial energy release is converted first into kinetic energy of suprathermal protons, 102–103 keV. The protons are trapped in a large magnetic loop which later breaks open as the mass ejection; Coulomb losses are the destabilizing agent but the mass ejection is probably magnetically driven. Protons that escape into the loss cone will impact the loop footpoints to heat the upper chromospheric material to a sufficiently high temperature to generate the weak soft X-ray emission. There will also be an Hα signature, and this is observed in a number of events. There is in general no radio emission or hard X-ray emission accompanying the soft X-ray precursor. When the coronal mass ejection is followed by a flare, then this is generally from a point close to, but not identical to, one of the points with the earlier soft X-ray enhancement.

The relationship between coronal mass ejections and solar flares

X-ray observations show that at a time consistent with a coronal mass ejection onset there is a small, soft X-ray burst (precursor). Generally this is followed some 20–30m later by a more significant flare. At the onset time there is frequently simultaneous activity from widely separated points on the Sun (>10 5 km). We present a model which accounts for the relationship between the coronal mass ejection and the precursor using 10 2–10 3 keV protons as the energy transfer agent. The protons (1) heat the high coronal loop. Inferred from the simultaneous activity, destabilizing the pressure balance to produce the ejection and (2) are guided by the magnetic field to below the transition region where they heat the chromospheric plasma to produce the precursor X-rays. High correlation between these events and a subsequent flare suggests that there may be a feedback mechanism operating from the coronal mass ejection.