D. J. Michels

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.

Properties of coronal mass ejections: SOHO LASCO observations from January 1996 to June 1998

We report the properties of all the 841 coronal mass ejections (CMEs) observed by the Solar and Heliospheric Observatory (SOHO) Large Angle Spectroscopic Coronagraph (LASCO) C2 and C3 white-light coronagraphs from January 1996 through June 1998, and we compare those properties to previous observations by other similar instruments. Both the CME rate and the distribution of apparent locations of CMEs varied during this period as expected based on previous solar cycles. The distribution of apparent speeds and the fraction of CMEs showing acceleration were also in agreement with earlier reports. The pointing stability provided by an L-1 orbit and the use of CCD detectors have resulted in superior brightness sensitivity for LASCO over earlier coronagraphs; however, we have not detected a significant population of fainter (i.e., low mass) CMEs. The general shape of the distribution of apparent sizes for LASCO CMEs is similar to those of earlier reports, but the average (median) apparent size of 72° (50°) is significantly larger. The larger average apparent size is predominantly the result of the detection of a population of partial and complete halo CMEs, at least some of which appear to be events with a significant longitudinal component directed along the Sun-Earth line, either toward or away from the Earth. Using full disk solar images obtained by the Extreme ultraviolet Imaging Telescope (EIT) on SOHO, we found that 40 out of 92 of these events might have been directed toward the Earth, and we compared the timing of those with the Kp geomagnetic storm index in the days following the CME. Although the “false alarm” rate was high, we found that 15 out of 21 (71%) of the Kp ≥ 6 storms could be accounted for as SOHO LASCO/EIT frontside halo CMEs. If we eliminate three Kp storms that occurred following LASCO/EIT data gaps, then the possible association rate was 15 out of 18 (83%).

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 an Earth‐directed coronal mass ejection on May 12, 1997

An earth-directed coronal mass ejection (CME) was observed on May 12, 1997 by the SOHO Extreme ultraviolet Imaging Telescope (EIT). The CME, originating north of the central solar meridian, was later observed by the Large Angle Spectrometric Coronagraph (LASCO) as a “halo” CME: a bright expanding ring centered about the occulting disk. Beginning at about 04:35 UT, EIT recorded several CME signatures, including dimming regions close to the eruption, post-eruption arcade formation, and a bright wavefront propagating quasi-radially from the source region. Each of these phenomena appear to be associated with the same eruption, and the onset time of these features corresponds with the estimated onset time observed in LASCO. We discuss the correspondence of these features as observed by EIT with the structure of the CME in the LASCO data.

A magnetic cloud containing prominence material: January 1997

This work discusses the relations among (1) an interplanetary force-free magnetic cloud containing a plug of cold high-density material with unusual composition, (2) a coronal mass ejection (CME), (3) an eruptive prominence, and (4) a model of prominence material supported by a force-free magnetic flux rope in a coronal streamer. The magnetic cloud moved past the Wind spacecraft located in the solar wind upstream of Earth on January 10 and 11, 1997. The magnetic field configuration in the magnetic cloud was approximately a constant-α, force-free flux rope. The 4He++/H+ abundance in the most of the magnetic cloud was similar to that of the streamer belt material, suggesting an association between the magnetic cloud and a helmet streamer. A very cold region of exceptionally high density was detected at the rear of the magnetic cloud. This dense region had an unusual composition, including (1) a relatively high (10%) 4He++/He+ abundance (indicating a source near the photosphere), and (2) 4He+, with an abundance relative to 4He++ of ∼1%, and the unusual charge states of O5+ and Fe5+ (indicating a freezing-in temperature of (1.6–4.0) × 105 °K, which is unusually low, but consistent with that expected for prominence material). Thus we suggest that the high-density region might be prominence material. The CME was seen in the solar corona on January 6, 1997, by the large angle and spectrometric coronagraph (LASCO) instrument on SOHO shortly after an eruptive prominence. A helmet streamer was observed near the latitude of the eruptive prominence a quarter of a solar rotation before and after the eruptive prominence. These observations are consistent with recent models, including the conceptual model of Low and Hundhausen [1995] for a quasi-static helmet streamer containing a force-free flux rope which supports prominence material and the dynamical model of Wu et al. [1997] for CMEs produced by the disruption of such a configuration.

Associations between coronal mass ejections and soft X-ray events

The association between white light coronal mass ejections (CMEs) and full-disk X ray events have been examined as a function of X ray duration during the recent years of high sunspot activity (1979-1981). On a time scale of hours, no duration interval has been found that separates X ray events into two distinct classes depending on whether or not they have associated CMEs. Rather, the tendency for long-duration X ray events to have associated CMEs reflects the fact that, as X ray duration increases, the differential distribution of events without CMEs falls off faster than the distriution of X ray events with CMEs.

MHD description of the dynamical relationships between a flux rope, streamer, coronal mass ejection, and magnetic cloud : An analysis of the January 1997 Sun-Earth connection event

We investigate the dynamical relationships between a coronal flux rope, a streamer, a coronal mass ejection (CME), and a magnetic cloud by using observations from the satellites of the International Solar-Terrestrial Physics observatories together with a streamer and flux rope interaction model [Wu and Guo, 1997a]. This is the first physical description of the evolution of a CME related to a flux rope in a streamer near the Sun to a magnetic cloud at 1 AU. The distinctive physical configuration of the model is based on a theoretical suggestion [Low, 1994] and observations [Hundhausen, 1993] that the magnetic structure of a streamer with an embedded cavity provides favorable condition for launch of a CME. We explore this physical scenario by identifying a flux rope as the cavity and using a fully self-consistent numerical simulation to illustrate the dynamical process of evolution of the flux rope/CME into a magnetic cloud. The simulation results are then compared to solar and interplanetary data from the well-observed Sun-Earth connection event of January 6–12, 1997. The data used for this analysis were collected chiefly by the Solar and Heliospheric Observatory (SOHO) Large-Angle and Spectrometric Coronagraph Experiment coronagraph and the solar wind particle and field sensors on the Wind spacecraft, but ground-based solar data were used as well. Because we have detailed observations of the same disturbance both at the Sun (SOHO) and at 1 AU (Wind), this event gives us an unusual opportunity to test the magnetohydrodynamic methodology and to learn about the physical processes of the Sun-Earth connection. In this study we show that when the flux rope rises (owing to increasing axial current, as assumed here, or to some other mechanism), it disrupts the streamer-flux rope system, thus launching a coronal mass ejection. The flux rope then escapes from the streamer and evolves to become a magnetic cloud, as expected, in interplanetary space. The CME is a visible feature moving ahead of the flux rope. The model also predicts a fast-mode shock in front of the magnetic cloud, as observed.

Observations of Correlated White-Light and Extreme-Ultraviolet Jets from Polar Coronal Holes

Time-lapse sequences of white-light images recorded with the Large Angle Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) frequently show long, narrow structures moving outward over the Suns polar regions at high apparent speeds. By comparing the LASCO observations with Fe XII λ195 spectroheliograms made with the Extreme-ultraviolet Imaging Telescope (EIT) on SOHO between 1997 April and 1998 February, we have identified 27 correlated white-light and extreme-ultraviolet (EUV) jet events. In each case, the EUV jet was observed near the limb of the polar coronal hole 20-60 minutes before the corresponding white-light jet was registered in the coronagraphs 2-6 R☉ field of view. The jets originate near flaring EUV bright points and are presumably triggered by field line reconnection between magnetic bipoles and neighboring unipolar flux. The leading edges of the white-light jets propagate outward at speeds of 400-1100 km s-1, whereas the bulk of their material travels at much lower velocities averaging around 250 km s-1 at heliocentric distances of 2.9-3.7 R☉. These lower velocities may reflect the actual outflow speeds of the background polar wind.

Associations between coronal mass ejections and metric type II bursts

A statistical comparison of metric type II bursts and coronal mass ejections (CMEs) during 1979--1982 revealed the following principal results: 1. Type II bursts without CMEs were associated with short-lived (0.5 hr) soft X-ray events, but not with interplanetary shocks at the Helios 1 spacecraft. 2. Type II bursts with CMEs were associated with longer-lived X-ray events (3 hr on the average) and interplanetary shocks, and the CMEs had speeds greater than 400 km s/sup -1/. 3. CMEs without metric type II bursts were divided equally into groups faster and slower than 455 km s/sup -1/. The faster CMEs were associated with interplantary shocks, some of which originated on the visible disk where metric type II bursts should have been observed if they had occurred.