Large Angle And Spectrometric Coronagraph Research Articles

Three‐dimensional Structure of the 2002 April 21 Coronal Mass Ejection

A three-dimensional reconstruction of the 2002 April 21 partial halo Coronal Mass Ejection (CME) has been made based on the O VI 1032 A and [Fe XVIII] 974 A lines observed by the Ultraviolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO). We use the Doppler velocities to derive the CME structure along the line of sight. UVCS observed the O VI line profiles split into strongly Doppler red- and blueshifted components, and the region of split profiles grew rapidly along the long spectrograph slit. The more localized [Fe XVIII] bright emission starts at the same time as the maximum Doppler redshift of O VI, indicating that it is inside the CME. In the view from the solar west, the O VI looks like halo CMEs seen by the Large Angle and Spectrometric Coronagraph (LASCO). The [Fe XVIII] bright emission appears as a barlike structure seen nearly end-on from the Earth, and side-on from the solar north and from solar west. The reconstructed [Fe XVIII] emission allows two interpretations, as ejection of preexisting hot plasma or as a current sheet. The evidence favors the current sheet interpretation, although we cannot rule out the alternatives.

Observations of Flux Rope Formation in the Outer Corona

In previous studies employing the Large Angle and Spectrometric Coronagraph (LASCO), we identified a class of white-light ejections that separate into incoming and outgoing components at distances of ~3-5 R☉ from Sun center. These events, of which up to several per month are observed during high solar activity, are generally preceded by a gradual outward expansion of faint loops over a period of a day or more. The expansion terminates when the streamer material, in the form of an elongated stalk or a sheetlike structure, suddenly tears apart. The collapsing material is sometimes recognizable as a collection of loops, while the ejected component is usually poorly resolved. Here we describe a streamer detachment observed on 2005 December 11, in which the outgoing component can be clearly identified as a cylindrical flux rope with its ends anchored in the Sun. Based on simple three-dimensional white-light reconstructions, we conclude that in/out pairs in general represent the pinching off of streamer loop arcades to form flux ropes, as seen from different viewing angles.

Different Power-Law Indices in the Frequency Distributions of Flares with and without Coronal Mass Ejections

We investigated the frequency distributions of flares with and without coronal mass ejections (CMEs) as a function of flare parameters (peak flux, fluence, and duration of soft X-ray flares). We used CMEs observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) mission and soft X-ray flares (C3.2 and above) observed by the GOES satellites during 1996 to 2005. We found that the distributions obey a power-law of the form: dN/dX~X^-alpha, where X is a flare parameter and dN is the number of events recorded within the interval [X, X+dX]. For the flares with (without) CMEs, we obtained the power-law index alpha=1.98+-0.05 (alpha=2.52+-0.03) for the peak flux, alpha=1.79+-0.05 (alpha=2.47+-0.11) for the fluence, and alpha=2.49+-0.11 (alpha=3.22+-0.15) for the duration. The power-law indices for flares without CMEs are steeper than those for flares with CMEs. The larger power-law index for flares without CMEs supports the possibility that nanoflares contribute to coronal heating.

Properties and geoeffectiveness of halo coronal mass ejections

Halo coronal mass ejections (HCMEs) originating from regions close to the center of the Sun are likely to be geoeffective. Assuming that the shape of HCMEs is a cone and that the HCMEs propagate with constant angular widths and velocities, at least in their early phase, we have developed a technique which allowed us to obtain the space speed, width, and source location. We apply this technique to obtain the parameters of all full HCMEs observed by the Solar and Heliospheric Observatory (SOHO) mission's Large Angle and Spectrometric Coronagraph (LASCO) experiment until the end of 2002. Using this data, we examine which parameters determine the geoeffectiveness of HCMEs. We show that in the considered period of time, only fast halo CMEs (with space velocities higher than ∼1000 km/s) and originating from the Western Hemisphere close to the solar center could cause intense geomagnetic storms. We illustrate how the HCME parameters can be used for space weather forecast. It is also demonstrated that the strength of a geomagnetic storm does not depend on the determined width of HCMEs. This means that HCMEs do not have to be very large to cause major geomagnetic storms.

The 17–22 October (1999) solar‐interplanetary‐geomagnetic event: Very intense geomagnetic storm associated with a pressure balance between interplanetary coronal mass ejection and a high‐speed stream

Using observations from the Advanced Composition Explorer (ACE) magnetic field and plasma experiments, we investigate the magnetic, thermal, and dynamic pressure balance in the border of a high‐speed stream (HSS) and an average‐speed interplanetary coronal mass ejection (ICME) observed on 21–22 October (1999). We believe that the HSS compressed the ICME and intensified its internal southward magnetic field, resulting in a very intense geomagnetic storm, with peak Dst of −237 nT. In solar cycle 23 this was the only event, out of 18 very intense geomagnetic storms, i.e., peak Dst < −200 nT, which was caused by such a mechanism. We also address the solar origin of this very intense geomagnetic storm, using combined solar data from three different sources: a ground‐based source, coronal hole maps from the National Solar Observatory (NSO) at Kitt Peak, and two satellite‐based sources, eruption activity from the Extreme Ultraviolet Imaging Telescope (EIT) and coronal mass ejection observations from the Large Angle and Spectrometric Coronagraph (LASCO), both aboard the Solar and Heliospheric Observatory (SOHO). A coronal hole is evident just beside (to the east of) the active region from which an average‐speed coronal mass ejection lifted off on 17–18 October (1999). This was the only possible solar origin of the 21–22 October interplanetary geomagnetic event.

The Filament Eruption of 1999 March 21 and Its Associated Coronal Dimmings and CME

We report a filament eruption near the center of the solar disk on 1999 March 21, in multi-wavelength observations by the Yohkoh Soft X-Ray Telescope (SXT), the Extreme-ultraviolet Images Telescope (EIT) and the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). The eruption involved in the disappearance of an H alpha filament can be clearly identified in EIT 195 angstrom difference images. Two flare-like EUV ribbons and two obvious coronal dimming regions were formed. The two dimming regions had a similar appearance in lines formed in temperature range 6 x 10(4) K to several 10(6) K. They were located in regions of opposite magnetic polarities near the two ends of the eruptive filament. No significant X-ray or H alpha flare was recorded associated with the eruption and no obvious photospheric magnetic activity was detected around the eruptive region, and particularly below the coronal dimming regions. The above surface activities were closely associated with a partial halo-type coronal mass ejection (CME) observed by the Large Angle and Spectrometric Coronagraphs (LASCO) on the SOHO. In terms of the magnetic flux rope model of CMEs, we explained these multiple observations as an integral process of large-scale rearrangement of coronal magnetic field initiated by the filament eruption, in which the dimming regions marked the evacuated feet of the flux rope.

Preliminary three‐dimensional analysis of the heliospheric response to the 28 October 2003 CME using SMEI white‐light observations

The Solar Mass Ejection Imager (SMEI) has recorded the inner heliospheric response in white‐light Thomson scattering to the 28 October 2003 coronal mass ejection (CME). This preliminary report shows the evolution of this particular event in SMEI observations, as we track it from a first measurement at approximately 20° elongation (angular distance) from the solar disk until it fades in the antisolar hemisphere in the SMEI 180° field of view. The large angle and spectrometric coronagraph (LASCO) images show a CME and an underlying bright ejection of coronal material that is associated with an erupting prominence. Both of these are seen by SMEI in the interplanetary medium. We employ a three‐dimensional (3‐D) reconstruction technique that derives its perspective views from outward flowing solar wind to reveal the shape and extent of the CME. This is accomplished by iteratively fitting the parameters of a kinematic solar wind density model to both SMEI white‐light observations and Solar‐Terrestrial Environment Laboratory (STELab), interplanetary scintillation (IPS) velocity data. This modeling technique separates the true heliospheric signal in SMEI observations from background noise and reconstructs the 3‐D heliospheric structure as a function of time. These reconstructions allow separation of the 28 October CME from other nearby heliospheric structure and a determination of its mass. The present results are the first utilizing this type of 3‐D reconstruction with the SMEI data. We determine an excess‐over‐ambient mass for the southward moving ejecta associated with the prominence material of 7.1 × 1016 g and a total mass of 8.9 × 1016 g. Preliminary SMEI white‐light calibration indicates that the total mass of this CME including possible associated nearby structures may have been as much as ∼2.0 × 1017 g spread over much of the earthward facing hemisphere.

Magnetic Reconnection Flux and Coronal Mass Ejection Velocity

We explore the relationship between the total reconnection flux ψrec estimated from flare observations and the velocity VCME of coronal mass ejections (CMEs) observed with the Large Angle and Spectrometric Coronagraph (LASCO) Experiment. Our study includes 13 events with varying magnetic configurations in source regions. It is shown that VCME is proportional to ψrec, with a linear cross-correlation of 89% and confidence level greater than 99.5%. This result confirms the importance of magnetic flux transferred by magnetic reconnection in the early stage of fast CMEs. On the other hand, the CME velocity and kinematic energy are probably independent of magnetic configurations of source regions.

Observations of an Emerging Flux Region Surge: Implications for Coronal Mass Ejections Triggered by Emerging Flux

It is well known that coronal mass ejections (CMEs) are often associated with flares and filament eruptions. Previous studies of CMEs, however, have not established any association between CMEs and surges. In this paper, we present a detailed analysis of a large emerging flux region (EFR) surge and a jetlike CME, both observed on 1998 April 16. Our analysis shows a close temporal and spatial relationship between the two. Using observations from the Large Angle and Spectrometric Coronagraph (LASCO) and Hida Flare Monitoring Telescope (Hα, Hα ± 0.8 A), we found that the CME's onset time and central position angle were coincident with the surge features. Magnetograms and Hα filtergrams showed that the surge resulted from the successive emergence of a bipolar sunspot group, NOAA Active Region 8203, which was the only active region in the northern hemisphere. The surge was impulsively accelerated at around the peak time of the GOES SXR flux. The associated CME appeared in the field of view of LASCO C2 16 minutes after the surge disappeared. Importantly, observations from the EUV Imaging Telescope at λ195 A clearly demonstrate topological changes in the coronal field due to its interaction with the EFR. An initially closed EFR-loop system opened up during the surge. There was no filament involved in this surge-CME event. We propose that the onset of the CME resulted from the significant restructuring of the large-scale coronal magnetic field as a result of flux emergence in the active region. This surge-CME event strongly suggests that emerging flux may not only trigger a surge but also simultaneously trigger a CME by means of small-scale reconnection in the lower atmosphere.

Effect of a Converging Flow at the Streamer Cusp on the Genesis of the Slow Solar Wind

The observation by the Large Angle and Spectrometric Coronagraph (LASCO) instrument on the Solar and Heliospheric Observatory of blobs of plasma generated near the cusp region of the streamer belt has sparked the hope of gaining new insight into the genesis of the slow solar wind. The observations suggest that open and closed field lines reconnect near the cusp of the helmet streamer to form blobs of higher density plasmas that are ejected into the slow solar wind. Models of this process have been suggested, and simulation studies have been conducted. We take the recent simulation work of Einaudi et al. as our starting point to investigate the formation of the slow solar wind and of the blobs observed by LASCO. We report two main conclusions. First, our results confirm the overall scenario suggested in the above paper (Einaudi et al.), which is that reconnection indeed causes the formation of blobs and the acceleration of the slow solar wind. Second, by employing more realistic two-dimensional configurations including the cusp and bent field lines with fast solar wind flow, we find that reconnection giving rise to plasma blobs is driven by converging flows with reconnection rates largely insensitive to changes in resistivity.

Magnetic Reconnection Rate and Flux‐Rope Acceleration of Two‐Ribbon Flares

Forbes & Lin derived simple equations to link the properties of magnetic reconnection in the corona to observed signatures of solar flares. We measured the photospheric magnetic fields and the flare ribbon separation speeds then applied these equations to derive two physical terms for the magnetic reconnection rates: the rate of magnetic flux change rec involved in magnetic reconnection in the low corona and the electric field Erec inside the reconnecting current sheet (RCS) that is generated during magnetic reconnection. The central interest in this work is to investigate and quantify the statistical correlation between the magnetic reconnection rate and the corresponding flux-rope acceleration. From a sample of 13 well-observed two-ribbon flares, which are associated with filament eruptions or coronal mass ejections (CMEs), the acceleration of erupting filaments is found mainly in the range of 0.05-0.4 km s-2, up to 3 km s-2. Correspondingly, the maximum Erec and rec mostly occur in the range of 0.2-5 V cm-1 and 0.5-6 × 1018 Mx s-1, respectively. A positive and strong correlation is found with a cross-correlation coefficient of 0.94-0.97 between the magnetic reconnection rate and the acceleration of erupting filaments that represents the early stages of flux-rope eruptions in the low corona. However, the inferred reconnection rate is not correlated to the acceleration of CME fronts measured by the Large Angle and Spectrometric Coronagraph (LASCO) observations in the range of 2-30 solar radii (the correlation coefficient is less than 0.2). A reasonable correlation is found between the reconnection rate and the velocity of CMEs, which indicates the cumulative acceleration of CMEs from the low corona to the LASCO C2 field of view. The temporal correlation between the magnetic reconnection rate and the flare nonthermal emissions has also been verified in this paper.

Three-dimensional Structure of Coronal Mass Ejections from LASCO Polarization Measurements

The three-dimensional structure of a coronal mass ejection (CME) is essential information for understanding the process by which they are ejected from the Sun. Coronagraphic measurements readily provide information on the structure of CMEs in the plane of the sky. It has been known for some time that polarization measurements can provide information on the position of coronal plasmas out of the plane of the sky. Recently, T. G. Moran & J. M. Davila have shown that polarimetric measurements with the Large Angle and Spectrometric Coronagraph (LASCO) can be used to determine the three-dimensional structure of a CME with considerable definition. We have examined a series of high-cadence (1 hr) LASCO polarization measurements obtained during 2002 and discuss the analysis of two particularly well observed events. One event indicates that the CME structure is that of a rising arcade of loops, while the other appears to consist of a flux-rope type of structure. Because we have examined a large, relatively high cadence set of LASCO observations, we have been able to select events that provide significantly greater definition of CME structure than previously possible.

On the Relation between Filament Eruptions, Flares, and Coronal Mass Ejections

We present a statistical study of 106 filament eruptions, which were automatically detected by a pattern recognition program implemented at Big Bear Solar Observatory using Hα full-disk data from 1999 to 2003. We compare these events with Geostationary Operational Environmental Satellite soft X-ray time profiles, solar-geophysical data (SGD) solar event reports, Michelson Doppler Imager magnetograms, and Large Angle and Spectrometric Coronagraph (LASCO) data to determine the relationship between filament eruptions and other phenomena of solar activity. (1) Excluding eight events with no corresponding LASCO data, 55% or 56% of 98 events were associated with coronal mass ejections (CMEs). (2) Active region filament eruptions have a considerably higher flare association rate of 95% compared to quiescent filament eruptions with 27%, but a comparable CME association rate, namely, 43% for active region filament eruptions and 54% for quiescent filament eruptions. (3) 54% or 68% of 80 disk events were associated with new flux emergence. In addition, we derived the sign of magnetic helicity and the orientation of the magnetic field associated with seven halo CMEs and demonstrated that the geoeffectiveness of a halo CME can be predicted by these two parameters.

Three-dimensional polarimetric imaging of coronal mass ejections.

We present three-dimensional reconstructions of coronal mass ejections (CMEs), which were obtained through polarization analysis of single-view images recorded with the use of the Large Angle and Spectrometric Coronagraph (LASCO) C2 coronagraph on board the Solar and Heliospheric Observatory (SOHO) spacecraft. Analysis of a loop-like CME shows a complex three-dimensional structure centered at 40 degrees from the plane of the sky, moving radially at 250 kilometers/second. Reconstruction of two halo CMEs suggests that these events are expanding loop arcades.

A Study of the Kinematic Evolution of Coronal Mass Ejections

We report the kinematic properties of a set of three coronal mass ejections (CMEs) observed with the LASCO (Large Angle and Spectrometric Coronagraph) on the Solar and Heliospheric Observatory (SOHO) spacecraft, which showed characteristics of impulsive, intermediate, and gradual acceleration, respectively. The first CME had a 30 minute long fast acceleration phase during which the average acceleration was about 308 m s-2; this acceleration took place over a distance of about 3.3 R☉ (from 1.3 to 4.6 R☉, height measured from disk center). The CME characterized by intermediate acceleration had a long acceleration phase of about 160 minutes during which the average acceleration was about 131 m s-2; the CME traveled a distance of at least 4.3 R☉, reaching a height of 7.0 R☉ at the end of the acceleration phase. The CME characterized by gradual acceleration had no fast acceleration phase. Instead, it displayed a persistent weak acceleration lasting more than 24 hr with an average acceleration of only 4.0 m s-2 throughout the LASCO field of view (from 1.1 to 30 R☉). This study demonstrates that the final velocity of a CME is determined by a combination of acceleration magnitude and acceleration duration, both of which can vary significantly from event to event. The first two CME events were associated with soft X-ray flares. We found that in the acceleration phase there was close temporal correlation both between the CME velocity and the soft X-ray flux of the flare and between the CME acceleration and derivative of the X-ray flux. These correlations indicate that the CME large-scale acceleration and the flare particle acceleration are strongly coupled physical phenomena occurring in the corona.

Empirically Determined Anisotropic Velocity Distributions and Outflows of O5+Ions in a Coronal Streamer at Solar Minimum

Empirical constraints on the O5+ velocity distributions and outflow speeds in a solar minimum equatorial streamer between 2.6 and 5.1 R☉ are determined using a spectral synthesis code that includes O VI Doppler dimming. These constraints follow directly from UV spectra taken on 1996 October 12 with the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) satellite and three-dimensional electron densities derived from tomography applied to a time series of polarized white-light images taken with the Large Angle and Spectrometric Coronagraph (LASCO) also on SOHO. Four conclusions result from this work: (1) our analysis shows O5+ velocity distribution anisotropy in the streamer legs and stalk and gives strong evidence that the microscopic velocity distribution (which excludes wave motions that equally affect all charged particles) is anisotropic, where the most probable speed perpendicular to the magnetic field direction exceeds that in the parallel direction; (2) there is preferential heating of the O5+ ions over the protons in the streamer stalk and legs; (3) there is no evidence for preferential O5+ heating in the core; and (4) the outflow velocity of the O5+ ions is determined at heights above 4.6 R☉. All results have a confidence level of at least 70%.

Sympathetic Coronal Mass Ejections

We address the question whether there exist sympathetic coronal mass ejections (CMEs), which take place almost simultaneously in different locations with a certain physical connection. For this study, the following three investigations are performed. First, we have examined the waiting-time distribution of the CMEs that were observed by the SOHO Large Angle and Spectrometric Coronagraph (LASCO) from 1999 February to 2001 December. The observed waiting-time distribution is found to be well approximated by a time-dependent Poisson distribution without any noticeable overabundance at short waiting times. Second, we have investigated the angular difference distribution of successive CME pairs to examine their spatial correlations. A remarkable overabundance relative to background levels is found within 10° of the position angle difference, which supports the existence of quasi-homologous CMEs that occur sequentially in the same active region. Both of the above results indicate that sympathetic (interdependent) CMEs are far less frequent than independent CMEs. Third, we have examined the EUV Imaging Telescope running difference images and the LASCO images of quasi-simultaneous CME pairs and found a candidate sympathetic CME pair, of which the second CME may be initiated by the eruption of the first CME. Possible mechanisms of the sympathetic CME triggering are discussed.

Identification of Solar Sources of Major Geomagnetic Storms between 1996 and 2000

This paper presents identification of solar coronal mass ejection (CME) sources for 27 major geomagnetic storms (defined by disturbance storm time index ≤ -100 nT) occurring between 1996 and 2000. Observations of CMEs and their solar surface origins are obtained from the Large Angle and Spectrometric Coronagraph (LASCO) and the EUV Imaging Telescope (EIT) instruments on the SOHO spacecraft. Our identification has two steps. The first step is to select candidate front-side halo (FSH) CMEs using a fixed 120 hr time window. The second step is to use solar wind data to provide further constraints, e.g., an adaptive time window defined based on the solar wind speed of the corresponding interplanetary CMEs. We finally find that 16 of the 27 (59%) major geomagnetic storms are identified with unique FSH CMEs. Six of the 27 events (22%) are associated with multiple FSH CMEs. These six events show complex solar wind flows and complex geomagnetic activity, which are probably the result of multiple halo CMEs interacting in interplanetary space. A complex event occurs when multiple FSH CMEs are produced within a short period. Four of the 27 (15%) events are associated with partial-halo gradual CMEs emerging from the east limb. The surface origin of these events is not known because of a lack of any EIT signature. We believe that they are longitudinally extended CMEs having a component moving along the Sun-Earth connection line. One of the 27 major geomagnetic storms is caused by a corotating interaction region. We find an asymmetry in the longitudinal distribution of solar source region for the CMEs responsible for major geomagnetic storms. They are more likely to originate from the western hemisphere than from the eastern hemisphere. In terms of latitude, most geoeffective CMEs originate within a latitude strip of ±30°. The average transit time for a solar CME to arrive at the near-Earth space is found to be 64 hr, while it takes 78 hr on average to reach the peak of the geomagnetic storm. There is a correlation between CME transit time from the Sun to the near-Earth space (T, in hours) and the CME initial velocity (V, in unit of kilometers per second) at the Sun, which can be simply described as T = 96 - (V/21). We also find that while these geoeffective CMEs are either full-halo CMEs (67%) or partial-halo CMEs (30%), there is no preference for them to be fast CMEs or to be associated with major flares and erupting filaments.

Coronal White‐Light Jets near Sunspot Maximum

During the 1996-1997 activity minimum, the Large Angle and Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) recorded numerous jetlike ejections above the Sun's polar regions. In a previous study, we showed that these white-light ejections were the outward extensions of extreme-ultraviolet (EUV) jets, which in turn originated from flaring bright points inside the polar coronal holes. Here we investigate a number of jetlike events observed with LASCO during the current sunspot maximum. To identify the solar surface counterparts of these events, we again use Fe XII λ195 images obtained by the EUV Imaging Telescope on SOHO. The white-light jets in our sample have angular widths of ~3°-7° and velocities typically of order 600 km s-1; they tend to be brighter and wider than the polar jets observed near sunspot minimum and are distributed over a much greater range of latitudes. Many of the ejections are recurrent in nature and originate from active regions located inside or near the boundaries of nonpolar coronal holes. We deduce that the jet-producing regions consist of systems of closed magnetic loops partially surrounded by open fields; perturbations in the closed fields caused them to reconnect with the overlying open flux, releasing the trapped energy in the form of jetlike ejections. In some events, the core of the active region erupts, producing fast, collimated ejections with widths of up to ~15°.

Observations of Core Fallback during Coronal Mass Ejections

White-light observations made with the Large Angle and Spectrometric Coronagraph (LASCO) during the present solar maximum have revealed a multitude of faint, inward-moving features at heliocentric distances of r ~ 2-6 R☉. Most of these structures appear to originate above r ~ 3 R☉ and may be signatures of the closing-down of magnetic flux at the boundaries of coronal holes or in the aftermath of coronal mass ejections (CMEs). Here, we present observations of a different type of inflow, in which material within the bright core of a CME collapses back toward the Sun after rising to heights of r ~ 2.5-6 R☉. We have identified roughly 20 such fallback events during 1998-2001. The core structures, which have the form of loops or concave-outward flux ropes, ascend into the coronagraph field of view beyond 2 R☉ with speeds of ~100-400 km s-1 but return with speeds of only ~50-200 km s-1. The initial deceleration rates of ~20-100 m s-2 are comparable to the local gravitational deceleration GM☉/r2 but continually decrease with time. The associated CMEs tend to be impulsive but relatively slow, with the leading front moving outward at ~250-450 km s-1 and often showing some deceleration. It is thus not surprising that some fraction of the core material fails to reach escape speeds, remaining bound to the Sun by gravitational and magnetic tension forces. We suggest that the dynamical behavior of the core may be determined in part by momentum exchanges with the background medium, which consists of ongoing outflows of CME material, ambient solar wind, and inflow streams. In particular, we attribute the asymmetry of the up-down trajectories to the action of such drag forces, whose direction changes from inward to outward as the core decelerates.