Soft X-ray Telescope Research Articles

Observations of a solar flare and filament eruption in Lyman $\mathsf{\alpha}$ and X-rays

Context. Lα is a strong chromospheric emission line, which has been relatively rarely observed in flares. The Transition Region and Coronal Explorer (TRACE) has a broad “Lyman α ” channel centered at 1216 A used primarily at the beginning of the mission. A small number of flares were observed in this channel.Aims. We aim to characterise the appearance and behaviour of a flare and filament ejection which occurred on 8th September 1999 and was observed by TRACE in Lα , as well as by the Yohkoh Soft and Hard X-ray telescopes. We explore the flare energetics and its spatial and temporal evolution. We have in mind the fact that the Lα line is a target for the Extreme Ultraviolet Imaging telescope (EUI) which has been selected for the Solar Orbiter mission, as well as the LYOT telescope on the proposed SMESE mission.Methods. We use imaging data from the TRACE 1216 A, 1600 A and 171 A channels, and the Yohkoh hard and soft X-ray telescopes. A correction is applied to the TRACE data to obtain a better estimate of the pure Lα signature. The Lα power is obtained from a knowledge of the TRACE response function, and the flare electron energy budget is estimated by interpreting Yohkoh /HXT emission in the context of the collisional thick target model. Results. We find that the Lα flare is characterised by strong, compact footpoints (smaller than the UV ribbons) which correlate well with HXR footpoints. The Lα power radiated by the flare footpoints can be estimated, and is found to be on the order of 1026 erg s-1 at the peak. This is less than 10% of the power inferred for the electrons which generate the co-spatial HXR emission, and can thus readily be provided by them. The early stages of the filament eruption that accompany the flare are also visible, and show a diffuse, roughly circular spreading sheet-like morphology, with embedded denser blobs.Conclusions. On the basis of this observation, we conclude that flare and filament observations in the Lα line with the planned EUI and LYOT telescopes will provide valuable insight into solar flare evolution and energetics, especially when accompanied by HXR imaging and spectroscopy.

DOES A POLAR CORONAL HOLE'S FLUX EMERGENCE FOLLOW A HALE-LIKE LAW?

Recent increases in spatial and temporal resolution for solar telescopes sensitive to EUV and X-ray radiation have revealed the prevalence of transient jet events in polar coronal holes. Using data collected by the X-Ray Telescope on Hinode, Savcheva et al. confirmed the observation, made first by the Soft X-ray Telescope on Yohkoh, that some jets exhibit a motion transverse to the jet outflow direction. The velocity of this transverse motion is, on average, 10 km s{sup -1}. The direction of the transverse motion, in combination with the standard reconnection model for jet production (e.g., Shibata et al.), reflects the magnetic polarity orientation of the ephemeral active region at the base of the jet. From this signature, we find that during the present minimum phase of the solar cycle the jet-base ephemeral active regions in the polar coronal holes had a preferred east-west direction, and that this direction reversed during the cycle's progression through minimum. In late 2006 and early 2007, the preferred direction was that of the active regions of the coming sunspot cycle (cycle 24), but in late 2008 and early 2009 the preferred direction has been that of the active regions of sunspot cycle 25. These findings aremore » consistent with the observations of Wilson et al. suggesting that each cycle of solar activity begins at polar latitudes soon after the onset of the previous cycle.« less

Thermal Considerations for Meeting 20°C and Stringent Temperature Gradient Requirements of IXO SXT Mirror Modules

The Soft X-Ray Telescope (SXT) is an instrument on the International X-Ray Observatory (IXO). Its flight mirror assembly (FMA) has a single mirror configuration that includes a 3.3 m diameter and 0.93 m tall mirror assembly. It consists of 24 outer modules, 24 middle modules and 12 inner modules. Each module includes more than 200 mirror segments. There are a total of nearly 14,000 mirror segments. The operating temperature requirement of the SXT FMA is 20 C. The spatial temperature gradient requirement between the FMA modules is 1 C or smaller. The spatial temperature gradient requirement within a module is 0.5 C. This paper presents thermal design considerations to meet these stringent thermal requirements.

DIFFERENTIAL ROTATION RATE OF X-RAY BRIGHT POINTS AND SOURCE REGION OF THEIR MAGNETIC FIELDS

The differential rotation rate of the solar corona has statistically been estimated from the motion of X-ray bright points (XBPs) that were observed with the Yohkoh soft X-ray telescope and the source region of magnetic fields is discussed from the evaluated rotation rate. The rotation rate of a pair of XBPs in a given latitude band is calculated from the position of an XBP in an X-ray image and that of an arbitrary-chosen XBP in a different X-ray image obtained with a time lag Δt. We have made the histogram of rotation rate from such samples in each latitude band for many X-ray images, and it shows a symmetric distribution that is well fitted by a Voigt function, not a Gaussian function, with a background. The modal rates at various longitudes in a given latitude band become the same value when a height is adopted as a characteristic height of XBPs, and we define the modal value against latitude as the differential rotation rate of XBPs in the present study. The differential rotation rate shows a similar trend as that of photospheric magnetic fields evaluated from the local correlation technique. We find that the differential rotation rate changes with a parameter Δt that is associated with the lifetime of XBPs, and that it becomes smaller in height for shorter Δt with the lower end that approaches to the rotation rate determined from the photospheric Doppler measurements. This trend suggests that magnetic fields associated with XBPs with a short lifetime are rooted just below the surface of the Sun at the top of the convection zone, and that they have a different origin from active regions.

Partially-erupting prominences: a comparison between observations and model-predicted observables

<i>Aims. <i/>We investigate several partially-erupting prominences to study their relationship with other CME-associated phenomena and compare these observations with observables predicted by a model of partially-expelled-flux-ropes (Gibson & Fan 2006a, ApJ, 637, L65; 2006b, J. Geophys. Res., 111, 12103).<i>Methods. <i/>We studied 6 selected events with partially-erupting prominences using multi-wavelength observations recorded by the Extreme-ultraviolet Imaging Telescope (EIT), Transition Region and Coronal Explorer (TRACE), Mauna Loa Solar Observatory (MLSO), Big Bear Solar Observatory (BBSO), and Soft X-ray Telescope (SXT). The observational features associated with partially-erupting prominences were then compared with the predicted observables from the model.<i>Results. <i/>The partially-expelled-flux-rope (PEFR) model can explain the partial eruption of these prominences, and in addition predicts a variety of other CME-related observables that provide evidence of internal reconnection during eruption. We find that all of the partially-erupting prominences studied in this paper exhibit indirect evidence of internal reconnection. Moreover, all cases showed evidence of at least one observable unique to the PEFR model, e.g., dimmings external to the source region and/or a soft X-ray cusp overlying a reformed sigmoid.<i>Conclusions. <i/>The PEFR model provides a plausible mechanism to explain the observed evolution of partially-erupting-prominence-associated CMEs in our study.

FPGA-based flexible CCD control system for X-ray astronomy payloads

We have developed a Field Programmable Gate Array (FPGA)-based Charge Coupled Device (CCD) data acquisition system, which will be used in the focal plane of Soft X-ray Telescope (SXT) on India's first multi-wavelength astronomy satellite—ASTROSAT. The use of FPGA allows a flexible system that can be used for driving almost all types of CCDs with very little modifications. It can generate clocks up to 1 MHz with adjustable phase. CCD biases and clock amplitudes can be generated and uploaded very easily. Electronic noise is reduced by reading the CCD when FPGA is at its minimum activity. CCD22, a MOS CCD developed in Europe, was biased and clocked by using this system. Each pixel is read at 6 μs intervals. The energy resolution of 55Fe at Kα peak has a full-width at half-maximum of ≈153 eV and the readout noise of our system is ≈8e— rms.

Optical Characteristics of the Soft X-Ray Telescope Aboard Yohkoh

We have studied the point spread function (PSF) of the Yohkoh Soft X-ray Telescope (SXT) using the pre-launch calibration data. It is revealed from our study that both a careful consideration on the undersampling effect and a proper choice of statistics are indispensable for determining the best fit analytical function for the SXT PSF. We present the results of numerical simulations supporting our approach toward proper handling of the undersampled PSF. Examples of the derived SXT PSF using a limited number of pre-launch calibration data are shown as a case study.

Forward Modeling of Active Region Coronal Emissions. II. Implications for Coronal Heating

In Paper I, we introduced and tested a method for predicting solar active region coronal emissions using magnetic field measurements and a chosen heating relationship. Here, we apply this forward-modeling technique to 10 active regions observed with the Mees Solar Observatory Imaging Vector Magnetograph and the Yohkoh Soft X-ray Telescope. We produce synthetic images of each region using four parameterized heating relationships depending on magnetic field strength and geometry. We find a volumetric coronal heating rate (dEH/dV, not to be confused with dEH/dA quoted by some authors) proportional to magnetic field and inversely proportional to field-line loop length (BL−1) best matches observed coronal emission morphologies. This parameterization is most similar to the steady-state scaling of two proposed heating mechanisms: van Ballegooijen's "current layers" theory, taken in the AC limit, and Parker's "critical angle" mechanism, in the case where the angle of misalignment is a twist angle. Although this parameterization best matches the observations, it does not match well enough to make a definitive statement as to the nature of coronal heating. Instead, we conclude that (1) the technique requires better magnetic field measurement and extrapolation techniques than currently available, and (2) forward-modeling methods that incorporate properties of transiently heated loops are necessary to make a more conclusive statement about coronal heating mechanisms.

Optical Characteristics of the Soft X-Ray Telescope Aboard Yohkoh

We have studied the point spread function (PSF) of the Yohkoh Soft X-ray Telescope (SXT) using the pre-launch calibration data. It is revealed from our study that both a careful consideration on the undersampling effect and a proper choice of statistics are indispensable for determining the best fit analytical function for the SXT PSF. We present the results of numerical simulations supporting our approach toward proper handling of the undersampled PSF. Examples of the derived SXT PSF using a limited number of pre-launch calibration data are shown as a case study.

Formation Mechanism of Soft X-Ray Transient Trans-Equatorial Loop System

The Soft X-ray Telescope (SXT) onboard Yohkoh often observed large-scale coronal loops connecting two active regions situated in opposite hemispheres. These are the trans-equatorial loop systems (TLSs). The formation mechanism of TLSs is not yet known. We analyzed a TLS observed simultaneously with Yohkoh/SXT and a coronagraph (SOHO/LASCO-C1). SOHO/LASCO-C1 observed loop expansion and eruption at the west solar limb. Yohkoh/SXT observed a rising motion (chromospheric evaporation) of hot and dense plasmas from the active regions located at the footpoints of the loop. Important results of our analyses are that (1) the loop eruption and the rising motion of the plasmas were simultaneous, (2) the TLS had a cusp-like appearance, and (3) the highest temperature region of the TLS was located above the bright loop seen in soft X rays. These observational results (loop expansion, eruption, and chromospheric evaporation) suggest that this bright (high-density) TLS was created by the same mechanism by which a solar flare occurs, namely, magnetic reconnection. In this paper, we propose a formation mechanism of the TLS that forms between two independent active regions.

MULTISPECTRAL EMISSION OF THE SUN DURING THE FIRST WHOLE SUN MONTH: MAGNETOHYDRODYNAMIC SIMULATIONS

We demonstrate that a three-dimensional magnetohydrodynamic (MHD) simulation of the corona can model its global plasma density and temperature structure with sufficient accuracy to reproduce many of the multispectral properties of the corona observed in extreme ultraviolet (EW) and X-ray emission. The key ingredient to this new type of global MHD model is the inclusion of energy transport processes (coronal heating, anisotropic thermal conduction, and radiative losses) in the energy equation. The calculation of these processes has previously been confined to one-dimensional loop models, idealized two-dimensional computations, and three-dimensional active region models. We refer to this as the thermodynamic MHD model, and we apply it to the time period of Carrington rotation 1913 (1996 August 22 to September 18). The form of the coronal heating term strongly affects the plasma density and temperature of the solutions. We perform our calculation for three different empirical heating models: (1) a heating function exponentially decreasing in radius; (2) the model of Schrijver et al.; and (3) a model reproducing the heating properties of the quiet Sun and active regions. We produce synthetic emission images from the density and temperature calculated with these three heating functions and quantitatively compare them with observations from E W Imaging Telescope on the Solar and Heliospheric Observatory and the soft X-ray telescope on Yohkoh. Although none of the heating models provide a perfect match, heating models 2 and 3 provide a reasonable match to the observations.

Large‐Scale Coronal Density and Abundance Structures and Their Association with Magnetic Field Structure

We construct subsynoptic maps of the ultraviolet-line fluxes, electron density, and elemental abundances for an east limb corona at 1.63 solar radii from 2000 September 20 to October 1. The data, covering position angles of 85°-126°, were taken from the Ultraviolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO), and the maps are based on the variation of these parameters along the field of view of the UVCS slit. Combining these maps with the limb synoptic maps made from SOHO Extreme Ultraviolet Imaging Telescope, SOHO Large Angle and Spectrometric Coronagraph, and Yohkoh Soft X-ray Telescope observations provides a large-scale, distinct view of the contrast between different coronal structures in different physical properties and their relation to the underlying disk and magnetic field structures. During this time period, the east limb corona mainly consisted of three streamers and two dark areas that exhibited very different plasma properties. We construct a three-dimensional MHD coronal model that incorporates energy transport processes, and compare the large-scale coronal properties of the model with those of the observation. The comparison investigates areas of different abundances and densities, and their possible association with open and closed magnetic field structures. We find a good indication that the open field regions, which we believe to be the slow-wind source regions in this case, have lower coronal density and higher abundance values than the closed field regions. This is true for absolute abundance, and probably also for the FIP bias. Therefore, such synoptic maps can be very useful for identifying solar wind source regions.

Preflare Eruption Triggered by a Tether‐cutting Process

We have examined the preflare activity of an M1.2 flare that occurred in NOAA active region 8440 on 1999 January 16, using images from the Soft X-Ray Telescope (SXT) on board Yohkoh, 1600 A UV images from the Transition Region and Coronal Explorer (TRACE), X-ray flux data from the GOES satellite, and magnetograms from Big Bear Solar Observatory (BBSO). During the preflare phase, we note a weak GOES X-ray flux enhancement just 4 minutes before the main flare begins. The SXT images show that this enhancement occurs at one footpoint of a soft X-ray loop bundle, which exactly coincides with the kernel of the major flare. The series of TRACE images provides the following pieces of evidence for small-scale magnetic reconnections associated with the preflare activity. (1) A small-scale UV sigmoid is seen at the X-ray loop footpoint before the preflare activity, and it is located along the polarity inversion line. (2) The brightest among the UV brightenings is exactly coincident and cospatial with the soft X-ray brightening observed by the Yohkoh SXT and GOES. (3) There were several interactions and brightenings among small UV loops. After these brightenings, the connectivity of the UV loops was apparently changed. As a result, a large rising loop structure was formed, with a maximum rising speed of about 40 km s−1. (4) The main flare occurred in this structure. In the aspects of the overall configuration and morphological change of UV loops, the preflare activity is quite consistent with the tether-cutting model with a single-bipole magnetic explosion. We suggest that the preflare activity and the main flare in this event not only have similar physical mechanisms, but also have a causal relation.

Studies on the imaging characteristics of Yohkoh Soft X-ray Telescope: Optical aberration and scattering

In order to better understand the characteristics of Yohkoh Soft X-ray Telescope (SXT) mirror, we have analyzed the in-flight overexposed image (the starburst image) obtained during the solar flare observation. It has been revealed from our study that the intensity distribution inside the shadows shown in the scattering difference image contains little of the scattered component of the PSF and matches almost correctly the extension of the PSF core profile. Also it is found that the scattering wing of the SXT PSF is connected smoothly to the PSF core within the distance of about 100–200 arcsec from the peak. With numerical simulations we have shown that an increase in energy affects not only the level of scattering wing, but also both the shape and the absolute level of the PSF core. The results have revealed, however, that the energy dependence for the SXT PSF cannot be easily estimated with the data obtained from one filter alone, which implies that the data analysis using multiple filters will enable us to determine the absolute amount of scattered component as well as the energy dependence of the SXT PSF. Details on the analysis of starburst image and the results from numerical simulations will be introduced and discussed thoroughly.

The solar wind disappearance event of 11 May 1999: source region evolution

Context. A recent, detailed study of the well-known “ solar wind disappearance event ” of 11 May 1999 traced its origin to a coronal hole (CH) lying adjacent to a large active region (AR), AR8525 in Carrington rotation 1949. The AR was located at central meridian on 05 May 1999 when the flows responsible for this event began. We examine the evolution of the AR-CH complex during 5-6 May 1999 to study the changes that apparently played a key role in causing this disappearance event. Aims. To study the evolution of the solar source region of the disappearance event of 11 May 1999. Methods. Using images from the Soft X-ray Telescope (SXT), the Extreme-ultraviolet Imaging Telescope (EIT) and the Michelson Doppler Imager (MDI) to examine the evolution of the CH and AR complex at the source region of the disappearance event. Results. We find a dynamic evolution taking place in the CH-AR boundary at the source region of the disappearance event of 11 May 1999. This evolution, which is found to reduce the area of the CH, is accompanied by the formation of new loops in EUV images that are spatially-and-temporally correlated with emerging flux regions as seen in MDI data. Conclusions. In the period leading up to the disappearance event of 11 May 1999, our observations, during quiet solar conditions and in the absence of CMEs, provide the first clear evidence for Sun-Earth connection originating from an evolving AR-CH region located at central meridian. With the exception of corotating interacting regions (CIR), these observations provide the first link between the Sun and space weather effects at 1 AU, arising from non-explosive solar events.

Solar coronal rotation determined by X-ray bright points in Hinode/XRT and Yohkoh/SXT full-disc images

Aims. Our aim is to identify and trace the X-ray bright points (XBPs) over the disc and use them as tracers to determine the coronal rotation. This investigation will help to clarify and understand several issues: (i) whether the corona rotates differentially; (ii) whether the rotation depends on the sizes of the XBPs; and (iii) whether there is a dependence on phases of the solar magnetic cycle.

The X-ray transient 080109 in NGC 2770: an X-ray flash associated with a normal core-collapse supernova

Although it is generally thought that long-duration gamma-ray bursts (LGRBs) are associated with core-collapse supernovae (SNe), so far only four pairs of GRBs and SNe with firmly established connection have been found. All the four GRB-SNe belong to special class of Type Ic – called the broad-lined SNe indicative of a large explosion energy, suggesting that only a small fraction of SNe Ibc have GRBs associated with them. This scheme has been refreshed by the discovery of a bright X-ray transient in NGC 2770 on 2008 January 9, which was followed by a rather normal Type Ib SN 2008D. In this paper, I argue that the transient 080109 is an X-ray flash (XRF, the soft version of a GRB) because of the following evidence. (1) The transient cannot be interpreted as a SN shock breakout event. (2) The GRB X-ray flare interpretation is not supported by the high-energy observation. I then show that XRF 080109 satisfies the well-known relation between the isotropic-equivalent energy and the peak spectral energy for LGRBs, which highly strengthens the XRF interpretation. Finally, I point out that the peak spectral energy of XRF 080109 and the maximum bolometric luminosity of SN 2008D agree with the Eγ,peak–LSN,max relationship of Li, strengthening the validity of the relationship. I speculate that events like XRF 080109 may occur at a rate comparable to SNe Ibc, and a soft X-ray telescope devoted to surveying for nearby X-ray flares will be very fruitful in discovering them.

Characteristics of Anemone Active Regions Appearing in Coronal Holes Observed with theYohkohSoft X‐Ray Telescope

Coronal structure of active regions appearing in coronal holes is studied by using the data obtained with the Soft X-Ray Telescope (SXT) aboard {\it Yohkoh} from 1991 November to 1993 March. The following characteristics are found; Many of active regions appearing in coronal holes show a structure that looks like a ``sea-anemone''. Such active regions are called {\it anemone ARs}. About one-forth of all active regions that were observed with SXT from their births showed the anemone structure. For almost all the anemone ARs, the order of magnetic polarities is consistent with the Hale-Nicholson's polarity law. These anemone ARs also showed more or less east-west asymmetry in X-ray intensity distribution, such that the following (eastern) part of the ARs is brighter than its preceding (western) part. This, as well as the anemone shape itself, is consistent with the magnetic polarity distribution around the anemone ARs. These observations also suggest that an active region appearing in coronal holes has simpler (less sheared) and more preceding-spot-dominant magnetic structure than those appearing in other regions.

Detection of high-energy solar neutrons and protons by ground level detectors on April 15, 2001

In association with the large solar flare of April 15, 2001, the Chacaltaya neutron monitor observed a 3.6 σ enhancement of the counting rate between 13:51 and 14:15 UT. Since the enhancement was observed beginning 11 min before the GLE, solar neutrons must be involved in this enhancement. The integral energy spectrum of solar neutrons can be expressed by a simple power law in energy with the index γ = - 3.0 ± 1.0 . On the other hand, an integral energy spectrum of solar protons has been obtained in the energy range between 650 MeV and 12 GeV. The spectrum can also be expressed by a power law with the power index γ = - 2.75 ± 0.15 . The flux of solar protons observed at Chacaltaya (at ⩾12 GeV) was already one order less than the flux of the galactic cosmic rays. It may be the first simultaneous observation of the energy spectra of both high-energy protons and neutrons. Comparing the Yohkoh soft X-ray telescope images with the observed particle time profiles, an interesting picture of the particle acceleration mechanism has been deduced.

Hinode EUV Study of Jets in the Sun’s South Polar Corona

Abstract A number of coronal bright points and associated plasma jet features were seen in an observation of the South polar coronal hole during 2007 January. The 40$^{\prime\prime}$ wide slot was used at the focus of the Hinode EUV Imaging Spectrometer to provide spectral images for two of these events. Light curves are plotted for a number of emission lines that include He II 256 Å (0.079 MK) and cover the temperature interval from 0.4 MK to 5.0 MK. Jet speed measurements indicate values less than the escape velocity. The light curves show a post-jet enhancement in a number of the cooler coronal lines indicating that after a few minutes cooling, the plasma fell back to its original acceleration site. This behavior has not been previously observed by e.g., the Yohkoh Soft X-ray Telescope due to the comparatively high temperature cut-off in its response. The observations are consistent with the existing models that involve magnetic reconnection between emerging flux and the ambient open field lines in the polar coronal hole. However we do not have sufficient coverage of lines from lower temperature ion species to register the H$\alpha$-emitting surge material that is associated with some of these models.