Yohkoh Soft X-ray Telescope Research Articles

POINT SPREAD FUNCTION OF THE SOFT X-RAY TELESCOPE ABOARD YOHKOH

Pre-launch calibration data have been analyzed for evaluating the point spread function (PSF) of Yohkoh Soft X-ray Telescope (SXT). Especially, it is found crucial that the effect of undersampling should be treated properly. The best fit solution of the SXT PSF, which is modeled by an elliptical Moffat function, has been derived by the comparison with the ground experiment data. In order to examine the off-axis variation of the SXT PSF, we need to define in advance the location of the optical axis on the CCD. According to the previous studies, the off-axis variation of effective area (the vignetting function) may be approximated either by two non-concentric cones or by a cone with some flat distortions. There have been, however, no fully approved representations for the SXT vignetting effect. The effect of the shift of the optical axis from the geometrical center of the telescope is investigated by numerical simulation. It is revealed from our study that the full width at half maximum (FWHM) of the SXT PSF stays nearly constant within an error bound over the central area of the CCD where the solar disk is located.

The Long‐Term Evolution of AR 7978: Testing Coronal Heating Models

We derive the dependence of the mean coronal heating rate on the magnetic flux density. Our results are based on a previous study of the plasma parameters and the magnetic flux density () in the active region NOAA 7978 from its birth to its decay, throughout five solar rotations using the Solar and Heliospheric Observatory Michelson Doppler Imager, Yohkoh Soft X-Ray Telescope (SXT), and Yohkoh Bragg Crystal Spectrometer (BCS). We use the scaling laws of coronal loops in thermal equilibrium to derive four observational estimates of the scaling of the coronal heating with (two from SXT and two from BCS observations). These results are used to test the validity of coronal heating models. We find that models based on the dissipation of stressed, current-carrying magnetic fields are in better agreement with the observations than models that attribute coronal heating to the dissipation of MHD waves injected at the base of the corona. This confirms, with smaller error bars, previous results obtained for individual coronal loops, as well as for the global coronal emission of the Sun and cool stars. Taking into account that the photospheric field is concentrated in thin magnetic flux tubes, both SXT and BCS data are in best agreement with models invoking a stochastic buildup of energy, current layers, and MHD turbulence.

The Long‐Term Evolution of AR 7978: The Scalings of the Coronal Plasma Parameters with the Mean Photospheric Magnetic Field

We analyze the evolution of the fluxes observed in X-rays and correlate them with the magnetic flux density in active region (AR) NOAA 7978 from its birth throughout its decay, for five solar rotations. We use Solar and Heliospheric Observatory Michelson Doppler Imager (MDI) data, together with Yohkoh Soft X-Ray Telescope (SXT) and Yohkoh Bragg Crystal Spectrometer (BCS) data, to determine the global evolution of the temperature and the emission measure of the coronal plasma at times when no significant brightenings were observed. We show that the mean X-ray flux and derived parameters, temperature and emission measure ( together with other quantities deduced from them, such as the density and the pressure), of the plasma in the AR follow power-law relationships with the mean magnetic flux density ((B) over bar). The exponents (b) of these power-law functions (a (B) over bar (b)) are derived using two different statistical methods, a classical least-squares method in log-log plots and a nonparametric method, which takes into account the fact that errors in the data may not be normally distributed. Both methods give similar exponents, within error bars, for the mean temperature and for both instruments (SXT and BCS); in particular, b stays in the range [0.27, 0.31] and [0.24, 0.57] for full-resolution SXT images and BCS data, respectively. For the emission measure, the exponent b lies in the range [0.85, 1.35] and [0.45, 1.96] for SXT and BCS, respectively. The determination of such power-law relations, when combined with the results from coronal heating models, can provide us with powerful tools for determining the mechanism responsible for the existence of the high-temperature corona.

Precise determination of cooling times of post-flare loops from the detailed comparison between Hα and soft X-ray images

A detailed study of the evolution and cooling process of post-flare loops is presented for a large X9.2 solar flare of 2 November 1992 by using Hα images obtained with Domeless Solar Telescope at Hida Observatory and soft X-ray images of Yohkoh Soft X-ray Telescope (SXT). The detailed analysis with a new method allows us to determine more precise values of the cooling times from 107 K to 104 K plasma in the post-flare loops than in previous works. The subtraction of sequential images shows that soft X-ray dimming regions are well correlated to the Hα brightening loop structure. The cooling times between 107 K and 104 K are defined as the time difference between the start of soft X-ray intensity decrease and the end of Hα intensity increase at a selected point, where the causal relation between Hα brightening and soft X-ray dimming loops is confirmed. The obtained cooling times change with time; about 10 min at the initial stage and about 40 min at the later stage. The combined conductive and radiative cooling times are also calculated by using the temperature and density obtained from SXT data. Calculated cooling times are close to observed cooling times at the beginning of the flare and longer in the later stage.

Soft X-ray observation of a large-scale coronal wave and its exciter

Recent extreme ultraviolet (EUV) observations from SOHO have shown the common occurrence of flare-associated global coronal waves strongly correlated with metric type II bursts, and in some cases with chromospheric Moreton waves. Until now, however, few direct soft X-ray detections of related global coronal waves have been reported. We have studied Yohkoh Soft X-ray Telescope (SXT) imaging observations to understand this apparent discrepancy, and describe the problems in this paper. We have found good X-ray evidence for a large-scale coronal wave associated with a major flare on 6 May 1998. The earliest direct trace of the wave motion on 6 May consisted of an expanding volume within 20 Mm (projected) of the flare-core loops, as established by loop motions and a dimming signature. Wavefront analyses of the soft X-ray observations point to this region as the source of the wave, which began at the time of an early hard X-ray spike in the impulsive phase of the flare. The emission can be seen out to a large radial distance (some 220 Mm from the flare core) by SXT, and a similar structure at a still greater distance by EIT (the Extreme Ultraviolet Imaging Telescope) on SOHO. The radio dynamic spectra confirm that an associated disturbance started at a relatively high density, consistent with the X-ray observations, prior to the metric type II burst emission onset. The wavefront tilted away from the vertical as expected from refraction if the Alfven speed increases with height in the corona. From the X-ray observations we estimate that the electron temperature in the wave, at a distance of 120 Mm from the flare core, was on the order of 2–4 MK, consistent with a Mach number in the range 1.1–1.3.

Analysis of the Temperature and Emission Measure of Solar Coronal Arcades and Test of a Scaling Law of Flare/Arcade Loop Length

We analyze 17 arcades to study the relations between solar flares and arcades. Soft X-ray images taken with Yohkoh's soft X-ray telescope are used to derive T0, EM0, and temporal variation of Tarc and EMarc, where Tarc and T0 are the temperatures of an arcade and prearcade region and EMarc and EM0 are the volume emission measures of an arcade and prearcade region. It is found that T0 ~ 2 MK and Tarc ~ 4 MK. We also estimate prearcade coronal electron density n0 and arcade electron density narc to find that narc is comparable to n0 (narc ~ n0 ~ 108 cm-3). Using these observed EM, T, and n0, we calculate the theoretical loop length Ltheor based on the scaling law for solar and stellar flares derived by Shibata & Yokoyama and compare it with observed flare/arcade loop length Lobs. The result shows a good correlation between them (Ltheor ~ Lobs) and indicates the need of plasma β for the scaling law (Ltheor ∝ EM3/5T-8/5nβ-6/5). This supports the theory of the scaling law and is indirect evidence that flares and arcades are heated by the same magnetic reconnection mechanism.

Solar and Heliospheric ObservatoryUltraviolet Coronagraph Spectrometer andYohkohSoft X‐Ray Telescope Observations of the High‐Temperature Corona above an Active Region Complex

We present the results of Solar and Heliospheric Observatory Ultraviolet Coronagraph Spectrometer (SOHO/UVCS) and Yohkoh Soft X-Ray Telescope (SXT) observations above an active region complex (AR 8194, 8195, and 8198) at the southeast limb on 1998 April 6-7. This active region complex appears to be the base of a small streamer seen by the Large Angle and Spectrometric Coronagraph Experiment (LASCO/C2) at the southeast limb. The UVCS was offset-pointed to observe low in the corona from 1.22 up to 1.6 R☉ with normal pointing. High-temperature lines such as [Fe XVIII] λ974 and Ne IX λ1248 were present in this region, implying that the electron temperature is higher than that in the quiet-Sun corona. This region of the corona is also seen as particularly bright in the Yohkoh/SXT, SOHO EUV Imaging Telescope high-temperature filter (Fe XV λ284) and SOHO/LASCO C1. The electron temperature analysis indicates a two-temperature structure, one of ~1.5 × 106 K, which is similar to that observed in quiet-Sun streamers, and the other at a high temperature of ~3.0 × 106 K. This two-temperature region likely corresponds to two distinct coronal regions overlapping in the line of sight. We compare the electron temperature and emission measure results from the SOHO/UVCS data with those from the Yohkoh/SXT data. The absolute elemental abundances show a general first ionization potential effect and decrease with height for all the elements. This is consistent with the effect of gravitational settling, which, however, cannot totally account for the observed elemental abundances. Other mechanisms that are likely to affect the coronal elemental abundance are discussed.

Modeling the Cooling of Postflare Loops

We present a model for the cooling of postflare loops. In our model, we form an arcade that consists of hundreds of loops with offset formation times to simulate a rising reconnection site. An initial temperature and density is assumed in each loop, and then the scaling laws of Cargill, Mariska, & Antiochos are used to determine the evolution of the temperature and density in the loop. Once these quantities are found, they are passed through the instrument response functions for TRACE and the Yohkoh Soft X-Ray Telescope (SXT) to obtain intensities, which are integrated over the arcade to give a simulated light curve. This light curve is then compared to observed light curves from the 2000 July 14 X6 flare. We find that this multiloop, multithermal approach to simulating the flare cooling fits the observed data much better than a single-loop model. There are some discrepancies between our simulations and the observed data in the decay phase of the flare, however, which may be due to residual late-phase heating. We also find that the temperatures calculated by using SXT filter ratios are generally lower than the initial loop temperatures needed in the simulation to give a good fit to the observed data.

Boundary Structures and Changes in Long‐lived Coronal Holes

We report a first systematic morphological study of the boundaries of coronal holes (CHs) as viewed in soft X-ray images from the Yohkoh Soft X-Ray Telescope. The special emphasis is on long-lived (several rotations) CHs that extend from the solar polar regions to midlatitudes. As shown earlier, such equatorward extensions tend to show rigid, rather than differential, rotation. Magnetic reconnection must occur at the closing boundary, in such a case, to maintain the CH integrity. We find three kinds of CH boundaries in the soft X-ray observations. The majority are generally ragged and not sharply defined; we also find smooth boundaries to occur near a matching-polarity active region (AR), and loopy boundaries to occur near an opposite-polarity AR. In this latter case the loops clearly do not extend far enough to reach another CH but instead end in normal corona. The CH boundaries evolve slowly, and neither large-scale transient X-ray events nor coronal bright points appeared significant factors in long-term CH boundary development. No direct evidence for magnetic reconnection is seen. We compare these results with those expected from current models, derived largely from considerations of heliospheric conditions rather than the detailed appearance of CHs in the low corona.

Statistical Evidence for Sympathetic Flares

Sympathetic flares are a pair of flares that occur almost simultaneously in different active regions, not by chance, but because of some physical connection. In this paper statistical evidence for the existence of sympathetic flares is presented. From GOES X-ray flare data, we have collected 48 pairs of near simultaneous flares whose positional information and Yohkoh soft X-ray telescope images are available. To select the active regions that probably have sympathetic flares, we have estimated the ratio R of actual flaring overlap time to random-coincidence overlap time for 38 active region pairs. We have then compared the waiting-time distributions for the two different groups of active region pairs (R > 1 and R 1. This is the first time such strong statistical evidence has been found for the existence of sympathetic flares. To examine the role of interconnecting coronal loops, we have also conducted the same analysis for two subgroups of the R > 1 group: one with interconnecting X-ray loops and the other without. We do not find any statistical evidence that the subgroup with interconnecting coronal loops is more likely to produce sympathetic flares than the subgroup without. For the subgroup with loops, we find that sympathetic flares favor active region pairs with transequatorial loops.

Nanoflare Statistics from First Principles: Fractal Geometry and Temperature Synthesis

We derive universal scaling laws for the physical parameters of flarelike processes in a low-� plasma, quan- tified in terms of spatial length scales l, area A, volume V, electron density ne, electron temperature Te, total emission measure M, and thermal energy E. The relations are specified as functions of two independent input parameters, the power index a of the length distribution, Nðl Þ/ la , and the fractal Haussdorff dimension D between length scales l and flare areas, Aðl Þ/ l D. For values that are consistent with the data, i.e., a ¼ 2:5 � 0:2 and D ¼ 1:5 � 0:2, and assuming the RTV scaling law, we predict an energy distribution NðE Þ/ E � � with a power-law coefficient of � ¼ 1:54 � 0:11. As an observational test, we perform statistics of nanoflares in a quiet-Sun region covering a comprehensive temperature range of Te � 1 4M K. We detected nanoflare events in extreme-ultraviolet (EUV) with the 171 and 195 Afilters from the Transition Region and Coronal Explorer (TRACE), as well as in soft X-rays with the AlMg filter from the Yohkoh soft X-ray telescope (SXT), in a cospatial field of view and cotemporal time interval. The obtained frequency dis- tributions of thermal energies of nanoflares detected in each wave band separately were found to have power- law slopes of � � 1:86 � 0:07 at 171 A ˚ (Te � 0:7 1:1 MK), � � 1:81 � 0:10 at 195 A ˚ (Te � 1:0 1:5 MK), and � � 1:57 � 0:15 in the AlMg filter (Te � 1:8 4:0 MK), consistent with earlier studies in each wavelength. We synthesize the temperature-biased frequency distributions from each wavelength and find a corrected power- law slope of � � 1:54 � 0:03, consistent with our theoretical prediction derived from first principles. This analysis, supported by numerical simulations, clearly demonstrates that previously determined distributions of nanoflares detected in EUV bands produced a too steep power-law distribution of energies with slopes of � � 2:0 2:3 mainly because of this temperature bias. The temperature-synthesized distributions of thermal nanoflare energies are also found to be more consistent with distributions of nonthermal flare energies deter- mined in hard X-rays (� � 1:4 1:6) and with theoretical avalanche models (� � 1:4 1:5). Subject headings: Sun: corona — Sun: flares — Sun: UV radiation — Sun: X-rays, gamma rays

Large-Scale Solar Coronal Structures in Soft X-Rays and Their Relationship to the Magnetic Flux

We have investigated the relationship between magnetic activity and coronal structures using soft X-ray data from the Yohkoh soft X-ray telescope and magnetic field data from the Kitt Peak Solar Observatory for the period of 1991-2001 and EUV data from the Solar and Heliospheric Observatory EUV Imaging Telescope for 1996-2001. The data are reduced to Carrington synoptic maps, which reveal two types of migrating structures of coronal activity at low and high latitudes in the time-latitudinal distribution. The low-latitude coronal structures, migrating equatorward, correspond to photospheric sunspot activity, and the high-latitude structures migrating toward the poles reflect polar activity of the Sun. We present the following new results:

Temperature and Density Measurements in a Quiet Coronal Streamer

Many previous studies have used emission line or broadband filter ratios to infer the presence of temperature gradients in the quiet solar corona. Recently it has been suggested that these temperature gradients are not real, but result from the superposition of isothermal loops with different temperatures and density scale heights along the line of sight. A model describing this hydrostatic weighting bias has been developed by Aschwanden & Acton. In this paper we present the application of the Aschwanden & Acton differential emission measure model to Solar and Heliospheric Observatory Solar Ultraviolet Measurement of Emitted Radiation (SUMER) observations of a quiet coronal streamer. Simultaneous Yohkoh soft X-ray telescope (SXT) observations show increases in the filter ratios with height above the limb, indicating an increase in temperature. The application of the Aschwanden & Acton model to these SUMER data, however, show that the temperature is constant with height and that the distribution of temperatures in the corona is much too narrow for the hydrostatic weighting bias to have any effect on the SXT filter ratios. We consider the possibility that there is a tenuous hot component (~3 MK) that accounts for the SXT observations. We find that a hot plasma with an emission measure sufficient to reproduce the observed SXT fluxes would also produce significant count rates in the high-temperature emission lines in the SUMER wavelength range. These lines are not observed, and we conclude that the SUMER spectra are not consistent with the SXT filter ratio temperatures. Calculations from a hydrodynamic loop model suggest that nonuniform footpoint heating may be consistent with the temperatures and densities observed at most heights, consistent with the recent analysis of relatively cool (~1 MK) active region loops. We also find, however, that at the lowest heights the observed densities are smaller than those predicted by uniform or footpoint heating.

The solar coronal origin of a slowly drifting decimetric-metric pulsation structure

We report observations associated with a short duration, slowly drifting decimetric-metric pulsation structure seen by the Phoenix-2 Radio Spectrometer on 2000 August 25. The range of frequencies over which this drifting radio feature occurred included frequencies observed by the Nan cay Radioheliograph enabling the spatial location and development of such a radio source to be determined for the rst time. The radio feature was closely associated with a solar flare. This flare was observed by the Yohkoh Soft X-ray Telescope (SXT) allowing us to compare the radio locations with the development of coronal structures seen in soft X-rays. The Yohkoh SXT images reveal two main soft X-ray features: a small flaring kernel region consisting of one or more bright loops located low in the corona and much fainter soft X-ray ejecta observed above the flare kernel region. The radio sources of the drifting pulsation structure moved outward with the soft X-ray ejecta. Our results indicate that the drifting decimetric-metric burst for this event was closely associated with the soft X-ray ejecta.

A Nonthermal Collimated Ejection Observed with the Nobeyama Radioheliograph

We have observed with the Nobeyama Radioheliograph a microwave collimated ejection that moved up to 1.3 × 105 km above the solar limb at an average speed of ≈3000 km s-1. Although the Yohkoh soft X-ray telescope pointed to another active region and therefore no soft X-ray data was available, the apparent length and width of the ejection were consistent with those of typical soft X-ray jets, but its velocity was significantly higher than that of soft X-ray jets. The microwave ejector had a bipolar or helical, closed magnetic structure that seemed to be tied to the chromosphere by its double footpoints and was located above the intense main microwave flare source with which hard X-ray double footpoint sources were associated. Association of an intense meter-wave type III burst with the impulsive peak of the microwave flare suggests that magnetic fields were opened outside the microwave ejector composed of closed loops. Simultaneous observations at 17 and 34 GHz showed that the microwave emission from the collimated ejector was nonthermal. High-energy electrons in the ejector had the same origin as those in the main flare source. It was also accompanied by an Hα surge with a speed of 100-150 km s-1, and the surge moved up to roughly the same height as the microwave ejector. We suggest that the microwave collimated ejection was caused by an apparent motion driven by successive injections of high-energy electrons into preexisting, higher elongated loops.

X-ray observations of a large-scale solar coronal shock wave

We report observations of a propagating disturbance in the solar corona observed in emission in soft X-ray images from the Yohkoh Soft X-ray Telescope (SXT). The disturbance was associated with a flare which began at about 09:04 UT on 1997 November 03. This flare was associated with a type II radio burst observed at decimetric-dekametric wavelengths by the Astrophysikalisches Institut Potsdam Radio Spectrograph. H α data from Kanzelhohe Solar Observatory show that a Moreton wave was associated with this event. Moreover, Solar and Heliospheric Observatory Extreme Ultraviolet Imaging Telescope (EIT) 195 A data show an `EIT wave' associated with this event. Extrapolations of the leading edge of the propagating soft X-ray disturbance show a close association with both of these wave features. The soft X-ray disturbance is observed to travel with a speed of about 546 km s -1 . Using Nancay Radioheliograph data we directly determine the source locations of the type II radio burst. These are found to be located close to the soft X-ray disturbance and show motions consistent with the soft X-ray motions. These results lead us to conclude that the “SXT wave” is the coronal counterpart of a Moreton wave, analogous to EIT waves, i.e., it is the first confirmed direct observation of a solar coronal shock wave in X-rays.

Energy and Mass Supply in the Decay Phase of Long-Duration Solar Flare Events

A new mechanism is proposed to supply energy and mass in the decay phase of long-duration solar flare events. The long-duration event (LDE) flares are known to be caused by filament eruptions, and bright arcade structures continue for several hours or even a day. To overcome the short cooling time, continuous energy and mass supplies are required. Continuous magnetic reconnection in the current sheet extended above the arcade has been proposed to supply energy and mass. However, no direct observations of the current sheet have been reported. The new mechanism that we are proposing is based on the recent observations of downflows onto the top of arcades along the vertical spiky features observed by the Yohkoh soft X-ray telescope. We interpret these downflows as free-falling plasma. Plasma falling from a high altitude can convert its potential energy into thermal energy and heat itself with an average temperature of several million kelvins. Observations show that the downflows continue for several hours. Coronagraph observations by the Solar and Heliospheric Observatory Large Angle and Spectrometric Coronagraph also show many falling features after coronal mass ejections. Instead of magnetic reconnection, we propose that these falling plasmas are the source of the energy and mass supply for LDEs in the long-lasting decay phase.

Reconnection Rate in the Decay Phase of a Long Duration Event Flare on 1997 May 12

Recent analyses of long duration event (LDE) flares indicate successive occurrences of magnetic reconnection and resultant energy release in the decay phase. However, quantitative studies of the energy release rate and the reconnection rate have not yet been made. In this paper we focus on the decay phase of an LDE flare on 1997 May 12 and derive the energy release rate H and the reconnection rate MA = vin/vA, where vin is the inflow velocity and vA is the Alfven velocity. For this purpose, we utilize a method to determine vin and the coronal magnetic field Bcorona indirectly, using the following relations: where Ar, Bfoot, and vfoot are the area of the reconnection region, the magnetic field strength at the footpoints, and the separation velocity of the footpoints, respectively. Since H, Ar, vfoot, and Bfoot are obtained from the Yohkoh Soft X-Ray Telescope data and a photospheric magnetogram, vin and Bcorona can be determined from these equations. The results are as follows: H is ~1027 ergs s-1 in the decay phase. This is greater than 1/10th of the value found in the rise phase. MA is 0.001-0.01, which is about 1 order of magnitude smaller than found in previous studies. However, it can be made consistent with the previous studies under the reasonable assumption of a nonunity filling factor. Bcorona is found to be in the range of 5-9 G, which is consistent with both the potential extrapolation and microwave polarization observed with the Nobeyama Radioheliograph.

The magnetic topology of a sigmoid

Recent surveys of solar features have linked the “sigmoid-to-arcade” scenario observed in the soft X-ray corona to coronal mass ejection (CME) onset (Geophys. Res. Lett. 26 (1999) 627, Geophys. Res. Lett. 14 (1998) 2481). Further to these observations, incorporation of extreme-ultraviolet, white light and H-alpha data into such a survey (Geophys. Res. Lett. 27 (2000) 2161) has illustrated the need for a quantitative definition of the term “sigmoid” and further understanding of such features if they are to be used as a means by which to predict CME onset. We analyse two sample active regions in detail, each appearing both sigmoidal and eruptive in Yohkoh soft X-ray telescope (SXT) full-disk data. Both regions were observed during October 1997 and each produced a flare displaying eruptive characteristics. In each case, formation of a flare-arcade was observed by both SXT and the extreme ultraviolet imaging telescope (EIT) following the event. EUV dimming and coronal EIT waves were also observed in each case. We have studied each active region both before and after eruption using soft X-ray, EUV and H-alpha data. A linear force-free field extrapolation has also been applied as a means by which to determine the active region field deviation from potential in each case. Each active region was observed to erupt by means of a different mechanism and while both events show signatures of eruption and consequently, mass ejection, only one produced a CME large enough to be observed by the SoHO large angle spectroscopic coronagraph. The implications of these observations in terms of CME prediction are discussed.

X‐Ray Bright Points and Photospheric Bipoles during Cycles 22 and 23

We use the Yohkoh soft X-ray telescope (SXT) full-disk images made from 1993 to 2000 to manually identify X-ray bright points (XBPs). We also employ the National Solar Observatory (Kitt Peak) full-disk longitudinal magnetograms made between 1992 April and 2001 April and an automatic procedure to identify photospheric bipoles whose magnetic field strength is above 20 G, with a pole size (cross-section) between 5'' and 552, and with pole separation between 55 and 483. We use these data to study statistical properties of XBPs and photospheric bipoles during the declining phase of solar cycle 22 and the rising phase of cycle 23. The XBP number follows well-known anti-cycle variation. The average number of XBPs (~10 per disk image) remained approximately the same from 1993 to 1994. Beginning in 1995 it grew, reached a maximum around 1996 December (~50 XBPs per image), and then dropped back to pre-1995 levels in 1998. By contrast, the average number of photospheric bipoles remained approximately the same (~250 per disk image) between 1992 and 2001, despite sunspot activity changes from high (1992, cycle 22) to low (1996, solar minimum) and a return to high activity again in 2000 (solar maximum, cycle 23). Since we expect that a fraction of photospheric bipoles associated with X-ray bright points is independent of solar activity, we see this as a clear indication that the anti-cycle variation of XBP numbers is not real. Most likely, the variation in XBP numbers is the result of a change in the background brightness of the quiet-Sun corona, which is affected by the presence of active regions. On the other hand, annual latitudinal histograms of XBPs show an excess of coronal bright points at active region latitudes, contrary to the effect of changing background brightness. Photospheric bipoles show no enhancement of their distribution at active latitudes. We consider two alternative explanations for this inconsistency.