Yohkoh Soft X-ray Telescope Research Articles

Open-field Coronal Structures Neighbouring the Sunspot of AR 8535

We make use of the Potential Field Source Surface (PFSS) model and reveal new details in the strong fields of the dominant sunspot of the active region (AR) 8535. Due to its importance in determining the field line connectivity and topology with resulting consequences for both plasma confinement and dynamics we construct the global magnetic fields with the PFSS model. The coronal emission – observed in the soft X-ray images taken with the Yohkoh Soft X-ray Telescope and 284 A images taken with the Extreme-ultraviolet Imaging Telescope aboard the Solar and HeliosphericObservatory (SOHO/EIT) – is largely asymmetric owing to two dark coronal lanes, which are traceable across the sunspot. The modelled open-field structures overlap with the observed coronal dark lanes and include two narrow coronal corridors that persist for days in the large sunspot. Since the open-field structures overlap also with the plasma upflow observed in the sunspot as Doppler blue-shifts in a set of EUV emission lines (SOHO/Coronal Diagnostic Spectrometer) on 9 May and 13 May 1999, we examine the plasma outflow from the AR 8535. From near-Earth measurements aboard the Advanced Composition Explorer (ACE) spacecraft we estimate the solar-wind speed $V _{\mathrm{p}} \approx 400~\mbox{km}\,\mbox{s}^{-1}$ and the charge state ratio $\mbox{O}^{7+}/\mbox{O}^{6+} = 0.4$ (corresponding to coronal “freezing-in temperature” $T _{\mathrm{e}} = 1.7 \times 10^{6}~\mbox{K}$). However, the abnormally low proton temperature of solar wind, $T _{\mathrm{p}} = 3 \times 10^{4}~\mbox{K}$, implies some impact of the interplanetary coronal mass ejection (ICME) on the plasma flow. The evidence in favour of the outflow from AR 8535 is given by the semi-empirical Wang–Sheeley–Arge model, as it predicts the ACE measurements of solar wind and the relevant features in the tomographic velocity map resulted from the Solar-Terrestrial Environment Laboratory observations of the interplanetary scintillations. Finally, we propose constructing a model atmosphere of the sunspot with the narrow coronal corridors included in order to further elaborate the magnetic-field structure through its effect on the observed microwave radiation.

Solar Cycle Variation of Coronal Temperature, Emission Measure, and Soft X-Ray Irradiance Observed with Yohkoh Soft X-Ray Telescope

Abstract This paper presents the solar soft X-ray irradiance (0.3–3.0 nm) obtained from the mission-long full-Sun X-ray images observed with the Soft X-ray Telescope (SXT) of the Yohkoh mission (1991–2001). The irradiance was calculated using filter-ratio temperatures and emission measures (EMs) from the full-Sun integrated X-ray signals employing a coronal spectrum synthesized with CHIANTI atomic database version 8.0. Dependence of the results on the assumption of elemental abundances is investigated. The SXT irradiance for a shorter wavelength range was also calculated and compared with the low energy flux (0.1–0.8 nm) of the X-Ray Sensor on board the GOES satellite. To incorporate the spatial information of the observed corona, we tried the alternative method to derive irradiance by applying the filter-ratio method to the images of northern hemispheric corona instead of spatially integrated signals from there. The EM weighted average of the spatially resolved temperatures turned out to be generally 20%–30% lower than those derived from the integrated signals, while the total of spatially resolved EMs are accordingly 1.5–2 times higher. This trend is enhanced when solar activity is the lowest in early 1996. The irradiance obtained from the alternative method does not significantly vary from the result from the integrated signals. This provides some validation for the simpler full-Sun integrated method. The temporal variation of the EMs of cool (<1.5 MK), medium, and hot (>2.5 MK) components indicates that the ratio of the hot component relative to the medium component is higher in the ascending phase (mid 1998–2001) than in the descending phase (1992–1995) of the solar activity cycle.

DERIVING PLASMA DENSITIES AND ELEMENTAL ABUNDANCES FROM SERTS DIFFERENTIAL EMISSION MEASURE ANALYSIS

We use high-resolution spectral emission line data obtained by the SERTS instrument during three rocket flights to demonstrate a new approach for constraining electron densities of solar active region plasma.We apply differential emission measure (DEM) forward-fitting techniques to characterize the multithermal solar plasma producing the observed EUV spectra, with constraints on the high-temperature plasma from the Yohkoh Soft X-ray Telescope. In this iterative process, we compare line intensities predicted by an input source distribution to observed line intensities for multiple iron ion species, and search a broad range of densities to optimize chi-square simultaneously for the many available density-sensitive lines. This produces a weighted by the DEM, which appears to be useful for characterizing the bulk of the emitting plasma over a significant range of temperature. This density technique is complementary to the use of density-sensitive line ratios and less affected by uncertainties in atomic data and ionization fraction for any specific line. Once the DEM shape and the DEM-weighted have been established from the iron lines, the relative elemental abundances can be determined for other lines in the spectrum. We have also identified spectral lines in the SERTS wavelength range that may be problematic

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.

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.

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.

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.

Yohkoh SXT Full-Resolution Observations of Sigmoids: Structure, Formation, and Eruption

We study the structure of 107 bright sigmoids using full-resolution (2.5'' pixels) images from the Yohkoh Soft X-Ray Telescope (SXT) obtained between 1991 December and 2001 December. We find that none of these sigmoids are made of single loops of S or inverse-S shape; all comprise a pattern of multiple loops. We also find that all S-shaped sigmoids are made of right-bearing loops and all inverse-S-shaped sigmoids of left-bearing loops, without exception. We co-align the SXT images with Kitt Peak magnetograms to determine the magnetic field directions in each sigmoid. We use a potential-field source surface model to determine the direction of the overlying magnetic field. We find that sigmoids for which the relative orientation of these two fields has a parallel component outnumber antiparallel ones by more than an order of magnitude. We find that the number of sigmoids per active region varies with the solar cycle in a manner that is consistent with this finding. Finally, those few sigmoids that are antiparallel erupt roughly twice as often as those that are parallel. We briefly discuss the implications of these results in terms of formation and eruption mechanisms of flux tubes and sigmoids.

The relative timing of supra-arcade downflows in solar flares

Context. Supra-arcade downflows (generally dark, sunward-propagating features located above the bright arcade of loops in some solar flares) have been reported mostly during the decay phase, although some have also been reported during the rise phase of solar flares. Aims. We investigate, from a statistical point of view, the timing of supra-arcade downflows during the solar flare process, and thus determine the possible relation of supra-arcade downflows to the primary or secondary energy release in a flare. Methods. Yohkoh Soft X-ray Telescope (SXT) imaging data are examined to produce a list of supra-arcade downflow candidates. In many of our events supra-arcade downflows are not directly observed. However, the events do show laterally moving (or “waving”) bright rays in the supra-arcade fan of coronal rays which we interpret as due to dark supra-arcade downflows. The events are analysed in detail to determine whether the supra-arcade downflows (or the proxy waving coronal rays) occur during a) the rise and/or decay phases of the soft X-ray flare and b) the flare hard X-ray bursts. It is also investigated whether the supra-arcade downflows events show prior eruptive signatures as seen in SXT, other space-based coronal data, or reported in ground-based Hα images. Results. A substantial majority of supra-arcade downflow events show downflows which start during the soft X-ray flare rise phase (73%), occur during hard X-ray bursts (90%), and have prior eruptive signatures associated with them (73%). However, we find a single event (2% of the total) which clearly and unambiguously showed supra-arcade downflows starting during the soft X-ray flare decay phase. Conclusions. Since the majority of supra-arcade downflows occur during the rise phase of the soft X-ray flare and the time of hard X-ray bursts, and have prior eruptive signatures this suggests that they are related to the main flare energy release process. Furthermore, the suggested association of supra-arcade downflows with recently reconnected magnetic field lines means they may indeed be considered as evidence for a magnetic reconnection process. The single supra-arcade downflow event which unambiguously started during the decay phase of the flare occurred during hard X-ray bursts and thus appears to be related to late energy release.

Helicity Patterns of the Active Regions Connected by Transequatorial Loops

Using photospheric vector magnetograms of the Huairou Solar Observing Station and coronal X-ray images from the Yohkoh Soft X-Ray Telescope, we calculate the helicity patterns of 43 pairs of active regions and the chirality of 50 pairs of opposite magnetic polarity regions that are connected by transequatorial loops (TLs). To make the results more convincing, two helicity proxies including the local current helicity hc and the force-free factor αbest are computed. The results, which are similar for both parameters, are as follows: (1) Current helicity of the active regions pairs connected by transequatorial loops have no obvious regularity: About 50% of the active region pairs carry the same current helicity sign and about 50% of them have the opposite. (2) If we consider the magnetic polarity pairs connected by the TLs, the result is almost the same as that for the active region pairs, with a little more than half of them showing the same chirality. We also make linear force- free extrapolations for 33 TLs and determine their force-free parameter α by comparing extrapolated field lines to X-ray images of the TLs. Out of the 19 cases when the footpoints of the TLs have the same current helicity sign, we find that the sign of α of the TLs is the same as the sign of the current helicity in the footpoints in 12 cases, whereas it is of opposite sign in 4 cases, and in 3 cases the TLs were found to be potential.

Photon Spectroscopy with Imaging X-Ray Instruments

Individual X-ray photons in the keV energy range produce hundreds of photoelectrons in a single pixel of a CCD array detector. The number of photoelectrons produced is a linear function of the photon energy, allowing the measurement of spectral information with an imaging detector system. Most solar X-ray telescopes, such as Yohkoh/SXT and Hinode/XRT, use CCD detectors in an integrating mode and are designed to make temperature estimates from multiband filter photometry. We show how such instruments can be used in a new way to perform a limited type of this photon spectroscopy. By measuring the variance in intensity of a series of repeated images through a single filter of an X-ray source, the mean energy per detected photon can be determined. This energy is related to the underlying coronal spectrum, and hence it can be used to deduce the mean plasma temperature. We apply this technique to data from the Yohkoh Soft X-Ray Telescope and compare the temperatures obtained with this technique with the temperatures derived using the standard filter ratio method for a postflare loop system. Given the large dynamic range of the soft X-ray flux observed from the Sun, we describe the requirements for a future instrument that would be better suited to performing photon spectroscopy.

Hα Dimming Associated With the Eruption of a Coronal Sigmoid in the Quiet Sun

We report on the occurrence of Hα dimming associated with a sigmoid eruption in a quiet-sun region on 14 August 2001. The coronal sigmoid in soft X-ray images from the Yohkoh Soft X-ray Telescope was located over an Hα filament channel. Its eruption was accompanied by a flare of GOES X-ray class C2.3 and possibly associated with a halo coronal mass ejection (CME) observed with the Large Angle and Spectroscopic Coronagraphs (LASCO) on board the Solar and Heliospheric Observatory (SOHO). During the eruption, coronal bipolar double dimming took place at the regions with opposite magnetic polarities around the two sigmoid ends, but the underlying chromospheric channel did not show observable changes corresponding to the coronal eruption. Different from the erupting coronal sigmoid itself, however, the coronal dimming had a detectable chromosphere counterpart, i.e., Hα dimming. By regarding the sigmoid as a coronal sign for a flux rope, these observations are explained in the framework of the flux rope model of CMEs. The flux rope is possibly deeply rooted in the chromosphere, and the coronal and Hα dimming regions mark its evacuated feet, through which the material is possibly fed to the halo CME.

Some statistical properties of transequatorial loops

Yohkoh soft X-ray telescope brought plenty of high quality images, it provides a good chance to research coronal loops, especially for transequatorial loops (TLs). In this paper, we focus on the statistical results of TLs including static properties and dynamic properties. There are two types of classification about TLs: according to configuration and according to magnetic polarities of footpoints, respectively. The footpoints of TLs never root in sunspot, in a general way, they exist in moderately strong field. The mean separation value of TLs is close to 30° and the separation value varies with solar cycle. The helicity patterns of active regions connected by TLs are discussed, the mean twist value of TLs is close to zero. The formation of TLs is generally thought to be caused by magnetic reconnection, the relationship of TLs eruption with flare and CME is introduced.

Observations of the coronal dynamics associated with solar radio spike burst emission

We present the results of an analysis of multi-wavelength data for a solar radio spike burst event which was observed on 1997 November 14 by the Solar Radio Spectrometers of the Astrophysikalisches Institut Potsdam (AIP). Using radio imaging data from the Nancay Radioheliograph we are able to determine the location of the spike burst emission sites in relation to the associated flare and its coronal environment as seen in images from the Yohkoh Soft X-ray Telescope (SXT), Hard X-ray Telescope (HXT) and the Solar and Heliospheric Observatory (SOHO) Extreme ultraviolet Imaging Telescope (EIT). For this event we find that the spike burst emission originated high in the corona, ~122 Mm above the soft X-ray and hard X-ray emitting regions of the associated (near-limb) flaring region. Using imaging data from SXT, HXT, and EIT we are also able to show, for the first time, the location of a solar radio spike burst in relation to the dynamics of coronal structures. We find that as an earlier flare-associated coronal mass ejection (CME) in the vicinity expanded outwards it appeared to compress the magnetic field structures at its flanks (associated with highly inclined coronal loops at the periphery of the active region). The radio spike bursts appeared at the time and in the vicinity of the compression of the coronal field. The compression presumably led to reconnection of the coronal magnetic field structures, particle acceleration and the radio spike burst emission signatures. For the specific event we analyse we find that the spike burst emission is associated with secondary flare energy release induced by an earlier flare-associated CME. These observations also provide evidence for a new category of sympathetic flaring where a large flare-associated coronal mass ejection compresses adjacent magnetic structures leading to nearby, but remote, small-scale flaring.

YOHKOH/WBS Recalibration and a Comprehensive Catalogue of Solar Flares Observed by YOHKOH SXT, HXT and WBS Instruments

The flare catalogue of the Yohkoh mission is compiled and linked to this article as an electronic supplement. For showing flare characteristics over wide energy range concisely, we provide the images of Hard X-ray Telescope (HXT) and the Soft X-ray Telescope (SXT), and the spectra of Hard X-ray Spectrometer (HXS) and Gamma-Ray Spectrometer (GRS) with the Wide Band Spectrometer (WBS) time profiles. The energy versus pulse height (PH) data channels in HXS and GRS are re-calibrated by using the data of the whole mission period. Secular gain changes are recognized in HXS, and the characteristics of power-law flare spectra simultaneously observed by HXT and HXS confirms the trend. The GRS gains are different for the flare observations during the previous maximum and for the current maximum. The total of 33 γ -ray events are observed, and for 12 of them γ-ray flare spectra are obtained.