K. P. Dere

EIT: Extreme-UltraViolet Imaging Telescope for the SOHO Mission

The Extreme-ultraviolet Imaging Telescope (EIT) will provide wide-field images of the corona and transition region on the solar disc and up to 1.5 R⊙ above the solar limb. Its normal incidence multilayer-coated optics will select spectral emission lines from Fe IX (171 Å), Fe XII (195 Å), Fe XV (284 Å), and He II (304 Å) to provide sensitive temperature diagnostics in the range from 6 × 104 K to 3 × 106 K. The telescope has a 45 x 45 arcmin field of view and 2.6 arcsec pixels which will provide approximately 5-arcsec spatial resolution. The EIT will probe the coronal plasma on a global scale, as well as the underlying cooler and turbulent atmosphere, providing the basis for comparative analyses with observations from both the ground and other SOHO instruments. This paper presents details of the EIT instrumentation, its performance and operating modes.

The Large Angle Spectroscopic Coronagraph (LASCO)

The Large Angle Spectroscopic Coronagraph (LASCO) is a three coronagraph package which has been jointly developed for the Solar and Heliospheric Observatory (SOHO) mission by the Naval Research Laboratory (USA), the Laboratoire d’Astronomie Spatiale (France), the Max-Planck-Institut fur Aeronomie (Germany), and the University of Birmingham (UK). LASCO comprises three coronagraphs, C1, C2, and C3, that together image the solar corona from 1.1 to 30 R⊙ (C1: 1.1–3 R⊙, C2: 1.5–6 R⊙, and C3: 3.7 – 30 R⊙). The C1 coronagraph is a newly developed mirror version of the classic internally-occulted Lyot coronagraph, while the C2 and C3 coronagraphs are externally occulted instruments. High-resolution imaging spectroscopy of the corona from 1.1 to 3 R⊙ can be performed with the Fabry-Perot interferometer in C1. High-volume memories and a high-speed microprocessor enable extensive on-board image processing. Image compression by a factor of about 10 will result in the transmission of 10 full images per hour.

On the Temporal Relationship between Coronal Mass Ejections and Flares

The temporal relationship between coronal mass ejections (CMEs) and associated solar flares is of great importance to understanding the origin of CMEs, but it has been difficult to study owing to the nature of CME detection. In this paper, we investigate this issue using the Large Angle and Spectrometric Coronagraph and the EUV Imaging Telescope observations combined with GOES soft X-ray observations. We present four well-observed events whose source regions are close to the limb such that we are able to directly measure the CMEs initial evolution in the low corona (~ 1-3 R☉) without any extrapolation; this height range was not available in previous space-based coronagraph observations. The velocity-time profiles show that kinematic evolution of three of the four CMEs can be described in a three-phase scenario: the initiation phase, impulsive acceleration phase, and propagation phase. The initiation phase is characterized by a slow ascension with a speed less than 80 km s-1 for a period of tens of minutes. The initiation phase always occurs before the onset of the associated flare. Following the initiation phase, the CMEs display an impulsive acceleration phase that coincides very well with the flares rise phase lasting for a few to tens of minutes. The acceleration of CMEs ceases near the peak time of the soft X-ray flares. The CMEs then undergo a propagation phase, which is characterized by a constant speed or slowly decreasing in speed. The acceleration rates in the impulsive acceleration phase are in the range of 100-500 m s-2. One CME (on 1997 November 6, associated with an X9.4 flare) does not show an initiation phase. It has an extremely large acceleration rate of 7300 m s-2. The possible causes of CME initiation and acceleration in connection with flares are explored.

Measurements of Flow Speeds in the Corona Between 2 and 30 R

Time-lapse sequences of white-light images, obtained during sunspot minimum conditions in 1996 by the Large Angle Spectrometric Coronagraph on the Solar and Heliospheric Observatory, give the impression of a continuous outflow of material in the streamer belt, as if we were observing Thomson scattering from inhomogeneities in the solar wind. Pursuing this idea, we have tracked the birth and outflow of 50-100 of the most prominent moving coronal features and find that: 1. They originate about 3-4 R☉ from Sun center as radially elongated structures above the cusps of helmet streamers. Their initial sizes are about 1 R☉ in the radial direction and 0.1 R☉ in the transverse direction. 2. They move radially outward, maintaining constant angular spans and increasing their lengths in rough accord with their speeds, which typically double from 150 km s-1 near 5 R☉ to 300 km s-1 near 25 R☉. 3. Their individual speed profiles v(r) cluster around a nearly parabolic path characterized by a constant acceleration of about 4 m s-2 through most of the 30 R☉ field of view. This profile is consistent with an isothermal solar wind expansion at a temperature of about 1.1 MK and a sonic point near 5 R☉. Based on their relatively small initial sizes, low intensities, radial motions, slow but increasing speeds, and location in the streamer belt, we conclude that these moving features are passively tracing the outflow of the slow solar wind.

SOHO/EIT Observations of the 1997 April 7 Coronal Transient: Possible Evidence of Coronal Moreton Waves

We report observations obtained with the Extreme ultraviolet Imaging Telescope (EIT) on board SOHO of a large-scale coronal transient propagating across the disk of the Sun at a speed of 250 km s-1, in apparent association with a flare and coronal mass ejection. The observations consist of a series of images taken in the Fe XII 195xa0A bandpass at an average cadence of 15 minutes. A visible increase in coronal emission propagates away from the erupting region, traveling across most of the solar disk in less than an hour. As the wave propagates through the ambient corona, its path is not homogeneous, and it is less observable near strong magnetic features such as active regions and magnetic neutral lines. The characteristics of this event appear to be representative of several other EIT waves, which we identify as strong candidates for the coronal manifestation of Moreton waves.

EIT OBSERVATIONS OF THE EXTREME ULTRAVIOLET SUN

The Extreme Ultraviolet Imaging Telescope (EIT) on board the SOHO spacecraft has been operational since 2 January 1996. EIT observes the Sun over a 45 x 45 arc min field of view in four emission line groups: Feix, x, Fexii, Fexv, and Heii. A post-launch determination of the instrument flatfield, the instrument scattering function, and the instrument aging were necessary for the reduction and analysis of the data. The observed structures and their evolution in each of the four EUV bandpasses are characteristic of the peak emission temperature of the line(s) chosen for that bandpass. Reports on the initial results of a variety of analysis projects demonstrate the range of investigations now underway: EIT provides new observations of the corona in the temperature range of 1 to 2 MK. Temperature studies of the large-scale coronal features extend previous coronagraph work with low-noise temperature maps. Temperatures of radial, extended, plume-like structures in both the polar coronal hole and in a low latitude decaying active region were found to be cooler than the surrounding material. Active region loops were investigated in detail and found to be isothermal for the low loops but hottest at the loop tops for the large loops.Variability of solar EUV structures, as observed in the EIT time sequences, is pervasive and leads to a re-evaluation of the meaning of the term ‘quiet Sun’. Intensity fluctuations in a high cadence sequence of coronal and chromospheric images correspond to a Kolmogorov turbulence spectrum. This can be interpreted in terms of a mixed stochastic or periodic driving of the transition region and the base of the corona. No signature of the photospheric and chromospheric waves is found in spatially averaged power spectra, indicating that these waves do not propagate to the upper atmosphere or are channeled through narrow local magnetic structures covering a small fraction of the solar surface. Polar coronal hole observing campaigns have identified an outflow process with the discovery of transient Fexii jets. Coronal mass ejection observing campaigns have identified the beginning of a CME in an Fexii sequence with a near simultaneous filament eruption (seen in absorption), formation of a coronal void and the initiation of a bright outward-moving shell as well as the coronal manifestation of a ‘Moreton wave’.

Large-Angle Spectrometric Coronagraph Measurements of the Energetics of Coronal Mass Ejections

We examine the energetics of coronal mass ejections (CMEs) with data from the large-angle spectro- metric coronagraphs (LASCO) on SOHO. The LASCO observations provide fairly direct measurements of the mass, velocity, and dimensions of CMEs. Using these basic measurements, we determine the potential and kinetic energies and their evolution for several CMEs that exhibit —ux-rope morphologies. Assuming —ux conservation, we use observations of the magnetic —ux in a variety of magnetic clouds near the Earth to determine the magnetic —ux and magnetic energy in CMEs near the Sun. We —nd that the potential and kinetic energies increase at the expense of the magnetic energy as the CME moves out, keeping the total energy roughly constant. This demonstrates that —ux-rope CMEs are magnetically driven. Furthermore, since their total energy is constant, the —ux-rope parts of the CMEs can be con- sidered a closed system above D2 R _ . Subject headings: solar-terrestrial relationsSun: activitySun: coronaSun: magnetic —elds

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.

LASCO and EIT Observations of Helical Structure in Coronal Mass Ejections

Observations of coronal mass ejections (CMEs) by the Large Angle Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) show a significant fraction with circular intensity patterns. In the past, these would have been called disconnection events, but we suggest that these are evidence of CMEs containing helical magnetic flux ropes that are often central to many theoretical models of CMEs and have been observed in magnetic clouds near 1 AU. Three examples are examined in detail with the LASCO and Extreme-Ultraviolet Imaging Telescope (EIT) data sets, which provide observations from their initiation through 30 R☉.

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.