Extreme-ultraviolet Imaging Telescope Images Research Articles

A Catalog of Solar Flare Events Observed by the SOHO/EIT

Abstract We have compiled a catalog of solar flares (SFs) as observed by the Extreme ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) spacecraft and the Geostationary Operational Environmental Satellites (GOES) spacecraft over a span from 1997 to 2010. During mid-1998, the cadence of EIT images was revised from two images per day to 12 minutes. However, the low temporal resolution causes significant data gaps in capturing much of the flaring phenomenon. Therefore, we monitor possible errors in flare detection by flare parameters such as temporal overlap, observational wavelength, and considering full field-of-view images. We consider the GOES flare catalog as the primary source. We describe the technique used to enhance the GOES detected flares using the extreme ultraviolet image captured by the EIT telescope. In order to detect brightenings, we subtract the images with a maximum cadence of 25 minutes. We have downloaded and analyzed the EIT data via the Virtual Solar Observatory. This flare data set from the SOHO/EIT period proves indispensable to the process of the SF predictions as the instrument has covered most of Solar Cycle 23.

A CATALOG OF CORONAL “EIT WAVE” TRANSIENTS

SOHO Extreme Ultraviolet Imaging Telescope (EIT) data have been visually searched for coronal wave transients over the period beginning 24 March 1997 extending through 24 June 1998. The dates covered start at the beginning of regular high-cadence (more than 1 image every 20 minutes) observations, ending at the 4-month interruption of SOHO observations in mid-1998. 176 events are included in this catalog. The observations range from candidate events, which were either weak or had insufficient data coverage, to events which were well-defined and were clearly distinguishable in the data. Included in the catalog are times of the EIT images in which the events are observed, diagrams indicating the observed locations of the wavefronts and associated active regions, and the speeds of the wavefronts. The measured speeds of the wavefronts varied from less than 50 to over 700 km/sec with typical speeds of 200-400 Msec.

Observations of Low-Latitude Coronal Plumes

Using Fe ix/x 17.1 nm observations from the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO), we have identified many coronal plumes inside low-latitude coronal holes as they transited the solar limb during the late declining phase of cycle 23. These diffuse, linear features appear to be completely analogous to the familiar polar plumes. By tracking them as they rotate from the limb onto the disk (or vice versa), we confirm that EUV plumes seen against the disk appear as faint, diffuse blobs of emission surrounding a brighter core. When the EIT images are compared with near-simultaneous magnetograms from the SOHO Michelson Doppler Imager (MDI), the low-latitude, on-disk plumes are found to overlie regions of mixed polarity, where small bipoles are in contact with unipolar flux concentrations inside the coronal hole. The birth and decay of the plumes are shown to be closely related to the emergence of ephemeral regions, their dispersal in the supergranular flow field, and the cancellation of the minority-polarity flux against the dominant-polarity network elements. In addition to the faint polar and nonpolar plumes associated with ephemeral regions, we note the existence of two topologically similar coronal structures: the giant plume-like features that occur above active regions inside coronal holes, and the even larger scale “pseudostreamers” that separate coronal holes of the same polarity. In all three cases, the basic structure consists of open field lines of a given polarity overlying a photospheric region of the opposite polarity; ongoing interchange reconnection at the X-point separating the open field domains from the underlying double-arcade system appears to result in the steady evaporation of material from the closed into the open region.

Automated Detection of EUV Prominences

We present methods to detect automatically off-limb prominences in the extreme ultraviolet (EUV), using synoptic images taken by the extreme-ultraviolet imaging telescope (EIT) on board SOHO. The 304 A line is essential for the detection of EUV prominences, but the optimal detection is achieved through a combined image processing of the four synoptic EIT images. In addition, the difference between consecutive 304 A images serves to identify erupted prominences. Representation maps of the quiescent EUV prominences for a given Carrington rotation are generated and used for further analysis of the detected structures. Longitudinal profiles of long-lived prominences are investigated for three examples at different latitudes, in conjunction with on-disk intensity profiles in the EUV. The observations coincide with theoretically predicted apparent longitudinal profiles, which can be distinguished from the profile of a prominence rising before eruption. The developed algorithms may be relevant to study the 3D geometry of features seen in the EUV and may facilitate the analysis of data from the future STEREO mission.

The Heliospheric Heii30.4 nm Solar Flux During Cycle 23

Because of the orbit characteristics of the vast majority of spacecraft, the solar flux has predominantly been measured at Earth or at least in the plane of the ecliptic. Therefore, the existing data do not directly demonstrate the fact that the latitudinal distribution of the extreme-ultraviolet (EUV) solar flux is largely anisotropic. Indeed, in the EUV the nonuniform distribution of very contrasted bright features (i.e., active regions) and dark features (i.e., coronal holes) at the surface of the Sun produces both the obvious rotational (or longitudinal) modulation of the flux and also a strong latitudinal anisotropy. Although largely ignored up to now, the latitudinal anisotropy affects the physical conditions in the corona and heliosphere and should therefore be taken into account in several solar and heliospheric physics applications. We describe in this paper a technique for computing the He II 30.4 nm flux at an arbitrary position in the heliosphere from Solar and Heliospheric Observatory (SOHO) EUV Imaging Telescope (EIT) images. This procedure was used to produce daily all-sky maps of the 30.4 nm flux from 1996 January to 2003 August, covering the first 8 yr of solar cycle 23. As could be expected from the examination of the EIT images, the 30.4 nm flux was found to be strongly anisotropic. The anisotropy Ipol/Ieq between the fluxes computed for viewpoints located above the solar poles and within the solar equatorial plane ranges from 0.9 at solar minimum to 0.6 at solar maximum. A 20% difference was also discovered between the north and south polar fluxes. The generalization of this technique to other lines of the EUV and far-ultraviolet (FUV) spectrum is discussed.

Wavelet Spectrum Analysis Of Eit/Soho Images

The extreme ultraviolet imaging telescope (EIT) of SOHO offers a unique record of the solar atmosphere for its sampling in temperature, field of view, resolution, duration, and cadence. To investigate globally and locally its topology and evolution during the solar cycle, we consider a multi-scale approach, and more precisely we use the wavelet spectrum. We present three results among the applications of such a procedure. First, we estimate the typical dimension of the supergranules as seen in the 30.4 nm passband, and we show that the evolution of the characteristic network scale is almost in phase with the solar cycle. Second, we build pertinent time series that give the evolution of the signal energy present in the corona at different scales. We propose a method that detects eruptions and post-flaring activity in EUV image sequences. Third, we introduce a new way to extract active regions in EIT images, with perspectives in, e.g., long-term irradiance analysis.

Nine Years Of Euv Bright Points

We discuss early results derived from an algorithm that automates the detection, cataloging, and analysis of extreme-ultraviolet (EUV) “bright points” (BP) from 9 years of data acquired by the Extreme-ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO). The algorithm relies upon the computation of a map of “intensity significance”; this then contains the location of the EUV BPs. By mapping the location of BPs in each image and linking them through long sequences of EIT images we can describe the temporal and spatial variations of the 1.3× 108 EUV BPs observed by SOHO to date. We suggest that there is a considerable amount of physical information about the solar coronal plasma that can be readily gleamed from the BP detection database. In this paper we discuss only a small portion of the possible correlations, but we point to the possibility of BP lifetime distributions that are well described by modified power-laws; the components of which vary with the different temperature filters and with time over the present solar cycle.

New insight into the blinker phenomenon and the dynamics of the solar transition region

We present, for the first time, blinker phenomena being associated with brightenings in pre-existing coronal loops registered by the Extreme-ultraviolet Imaging Telescope (EIT) in Fe xii 195 Å. The brightenings occur during the emergence of new magnetic flux as registered by the Big Bear Solar Observatory (BBSO) magnetograph. The blinkers were identified using simultaneous observations obtained with the Coronal Diagnostic Spectrometer (CDS) and Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrograph. In light of the new observational results, we present one possible theoretical interpretation of the blinker phenomenon. We suggest that the blinker activity we observe is triggered by interchange reconnection, serving to provide topological connectivity between newly emerging flux and pre-existing flux. The EIT images show the existence of loop structures prior to the onset of the blinker activity. Based on the available spatial resolution the blinker occurs within, or nearby, an existing coronal loop. The temperature interfaces created in the reconnection process between the cool plasma of the newly emerging loop and the hot plasma of the existing loop are what we suggest to causes the observed activity seen in both the SUMER and CDS data. As the temperature interfaces propagate with the characteristic speed of a conduction front, they heat up the cool chromospheric plasma to coronal temperatures, an increasing volume of which brightens at transition region temperatures. We believe this new interpretation gives further qualitative understanding about the evolution of newly emerging flux on the Sun. This also provides new insight into the dynamic nature of the solar transition region.

Two‐Temperature Coronal Models fromSOHO/EIT Observations

We present a method for deriving a two-thermal-component approximation to the differential emission measure distribution of plasma in the Sun's corona in the temperature range to which the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) spacecraft is sensitive. EIT takes high-resolution full-disk coronal images in three of its four optimized channels by observing emission lines of highly ionized Fe whose formation temperatures overlap and cover the range from 0.7 to 2.8 MK. It is straightforward to show that the traditional single-temperature models based on the ratio of a pair of EIT images at different wavelengths are not able adequately to represent the plasma contributing to all three wavelength ranges. In this paper, we develop a modified image-ratio method that results in a two-thermal-component model for the plasma producing the coronal emission observed by EIT. The products of this method are two temperature and two emission measure full-disk maps of the Sun's corona, with the full resolution of the EIT telescope, in two temperature regimes: one from 0.8 to 1.6 MK and the other from 1.6 to 2.6 MK. The two-component solutions are tested using a series of model differential emission measures (DEMs) from the CHIANTI atomic database package. This method appears to produce realistic results in all regions of the Sun's atmosphere with the exception of coronal holes, where very cool Si VII/Mg VII lines (<0.7 MK) contribute more to the EIT 284 A image than the otherwise dominant hot Fe XV lines and result in unrealistically high temperatures for the hot component there. We demonstrate that while the raw EIT images are dominated by the spatial distribution of emission measure in the corona, the temperature maps often emphasize fine structure, which is less visible in the flux images. The emission measure of the hot component is always larger than that of the cool component. On the disk there appears to be a firm lower limit to the integrated column emission measure along any line of sight, including toward coronal holes. There is no overall correlation between temperature and emission measure.

Comparison of Hα and Heiiλ304 Macrospicules

Simultaneous observations of limb macrospicules were carried out in Hα by the Big Bear Solar Observatory (BBSO) and in He II λ304 by the Extreme-Ultraviolet Imaging Telescope (EIT) on board SOHO. For the first time, Hα and He II macrospicules are compared, with high spatial and temporal resolution and image enhancement. Data were obtained on 1996 October 6, 7, and 8. The target of the first and last day was the north pole; on October 7, it was the quiet west limb. BBSO uses a 12 bit digital camera to obtain high-resolution Hα filtergrams at -0.65 A, line center, and 0.65 A. The pixel resolution ranges between 017 and 033, and temporal resolution ranges between 30 and 90 s. EIT images have a fixed pixel resolution of 25 and temporal resolution between 1 and 7 minutes. We found the following: within the common field of view of BBSO and EIT, all 53 identified He II λ304 macrospicules have counterparts in Hα. However, morphologies of He II λ304 and Hα macrospicules are completely different. He II λ304 macrospicules are typically in the form of an elongated ejection, whereas Hα macrospicules are either looplike bright features or much shorter jets. In the polar region, 55 (over 50%) Hα macrospicules do not show any corresponding He II λ304 macrospicules. As expected, He II λ304 macrospicules occur much more frequently in the polar coronal hole area than in the equatorial region. However, Hα macrospicules occur at an equal rate in the pole and in the equatorial limb. Equatorial Hα macrospicules tend to be confined because of their looplike structure and the lack of jetlike structure. Based on common properties of different events, we propose that the disk counterparts of macrospicules might be Hα - 1.0 A jets or a subset of explosive events. If this assumption is true, macrospicules are the results of magnetic reconnections, likely due to the network-ephemeral region or network-intranetwork interactions. We propose that magnetic reconnection occurs at about the same rate in the polar coronal hole and in the quiet regions. Hα macrospicules are direct manifestation of magnetic reconnection. He II λ304 images detect substantially taller structures that are substantially hotter. Because of dominant vertical line configuration near the pole, reconnection tends to produce He II λ304 macrospicules; in the noncoronal hole regions, He II macrospicules become undetectable because of the inclined magnetic field configuration.

Imaging the solar corona in the EUV

The SOHO (SOlar and Heliospheric Observatory) satellite was launched on December 2nd 1995. After arriving at the Earth-Sun (L1) Lagrangian point on February 14th 1996, it began to continuously observe the Sun. As one of the instruments onboard SOHO, the EIT (Extreme ultraviolet Imaging Telescope) images the Sun's corona in 4 EUV wavelengths. The He II filter at 304 Å images the chromosphere and the base of the transition region at a temperature of 5 − 8 × 10 4 K; the Fe IX–X filter at 171 Å images the corona at a temperature of ∼ 1.3 × 10 6 K; the Fe XII filter at 195 Å images the quiet corona outside coronal holes at a temperature of ∼ 1.6 × 10 6 K; and the Fe XV filter at 284 Å images active regions with a temperature of ∼ 2.0 × 10 6 K. About 5000 images have been obtained up to the present. In this paper, we describe also some aspects of the telescope and the detector performance for application in the observations. Images and movies of all the wavelengths allow a look at different phenomena present in the Sun's corona, and in particular, magnetic field reconnection.