Correlation of auroral power with the polar cap index

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. A3, 1108, doi: /2002ja009556, 2003 Correlation of auroral power with the polar cap index K. Liou, J. F. Carbary, P. T. Newell, and C.-I. Meng Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA O. Rasmussen Solar-Terrestrial Physics Division, Danish Meteorological Institute, Copenhagen, Denmark Received 24 June 2002; revised 20 December 2002; accepted 3 January 2003; published 12 March [1] We compare auroral power (AP) as derived from a global imager with the polar cap (PC) index for the periods of December 1996 through January 1997 (winter) and June 1997 through July 1997 (summer). The auroral power is inferred from auroral luminosity in the N 2 Lyman-Birge-Hopfield bands (l nm) acquired from the ultraviolet imager on board the ISTP/GGS program s Polar satellite in the Northern Hemisphere. The PC index (1-min time resolution) is based on the horizontal component of the magnetometer recordings from a single near-geomagnetic polar station at Thule (85.4 corrected geomagnetic latitude). A fair correlation exists between the hemispheric auroral power and PC (r = 0.70), with higher correlation for winter (r = 0.77) compared to summer (r = 0.69). Also, PC correlates with nightside auroral power much better than with dayside auroral power. The best AP-PC correlation took place in the pre-dawn and postdusk sectors. The seasonal and diurnal effects are a result of competition between the ionospheric convection currents and field-aligned currents manifested by seasonal and diurnal variations in ionospheric conductivity. Finally, a comparison of the AP-PC correlation between substorm and non-substorm times indicates that substorm electrojets contribute little, if not at all, to the PC index. INDEX TERMS: 2407 Ionosphere: Auroral ionosphere (2704); 2475 Ionosphere: Polar cap ionosphere; 2455 Ionosphere: Particle precipitation; 2409 Ionosphere: Current systems (2708); KEYWORDS: polar cap index, auroral power, substorm and non-substorm Citation: Liou, K., J. F. Carbary, P. T. Newell, C.-I. Meng, and O. Rasmussen, Correlation of auroral power with the polar cap index, J. Geophys. Res., 108(A3), 1108, doi: /2002ja009556, Introduction [2] The polar cap (PC) index was first proposed by Troshichev et al. [1988] as a measure of geomagnetic effects in the polar cap caused by convection-driven Hall currents. Later studies indicated the importance of distant fieldaligned currents associated with auroral particle precipitation, particularly on the poleward edge of the oval [Vennerstrøm et al., 1991]. The dimensionless PC index (normalized by solar wind electric fields) is based on the horizontal component of the magnetic disturbances at a single near geomagnetic polar station. Currently, two stations near the magnetic pole are used to derive PC: Thule (now Qaanaaq) from the Arctic (85.4 in CGM) and Vostok in Antarctica ( 83.4 in CGM). PCN refers to the northern PC index from Thule, and PCS refers to the southern PC index from Vostok. [3] Although derived from only a single station, the PC index correlates surprisingly well with the variations of auroral electrojets. Vennerstrøm et al. [1991] used 7 years worth of 15-min averaged PC indices to study the relation between the PC and auroral electrojet (AE) indices and found that the PC index and the AE index in the Northern Copyright 2003 by the American Geophysical Union /03/2002JA Hemisphere are linearly correlated with higher correlation coefficients (r) in winter (r ) and lower coefficients in summer (r ). The seasonal dependence occurs because in winter field-aligned currents, often closely related to the auroral electrojet, become the dominant source over the convection-driven currents in the low ionospheric conductivity polar cap, while in summer the ionospheric conductivity is high and the convection-driven currents become dominant. These are directly controlled by the interplanetary magnetic field through dayside reconnection [Vennerstrøm et al., 1991]. In terms of current systems, magnetic disturbances in the polar cap are affected by auroral electrojet activity of the DP-2 type, which is associated with convection, and of the DP-1 type, which is associated with substorms, in the post-midnight sector. However, the substorm electrojet in the pre-midnight section seems to have less effect on PC [Vennerstrøm et al., 1991]. The high PC-AE correlation suggests that the auroral electrojet is highly coupled with the current system in the polar cap. [4] Because the PC index is available in near-real time and because of the good PC-AE relationship, the index has been used to predict the intensity of auroral electrojets [e.g., Vassiliadis et al., 1996; Takalo and Timonen, 1998]. Techniques proposed from these studies can generally predict the AE time series several minutes ahead of time with correla- SIA 3-1

2 SIA 3-2 LIOU ET AL.: POLAR CAP INDEX AND AURORAL POWER tion coefficient of 0.9 for winter season. The PC index has also been used as a proxy for the hemispheric Joule heating rate [Chun et al., 1999, 2002]. Although the Joule heating is inferred from a statistical model, Assimilative Mapping of Ionospheric Electrodynamics [Richmond and Kamide, 1988], their results do suggest that the PC index can serve as well as the AE index in predicting hemispheric Joule heat. The PC index is also linearly correlated with the crosspolar cap diameter/voltage [Troshichev et al., 1996; Chun et al., 2002] and may be used to indicate those parameters in near-real time or to predict them. [5] The auroral electrojet AE index is not perfect because only a limited number of ground-based magnetometers are used, but it does provide a measure of overall horizontal ionospheric current strength in the auroral zone. It has been known for more than half a century that there is a close relationship between auroral displays and magnetic disturbances in the polar region [Harang, 1951; Heppner, 1954]. The auroral electrojet currents intensify during periods of high magnetic activity such as magnetospheric storms and substorms, and the aurora and its associated field-aligned currents are also greatly enhanced [cf., Akasofu 1968]. Therefore, a close relationship between the AE index and the auroral intensity should be expected. Indeed, a recent case study by Meng and Liou [2002] supports the view. They demonstrate a good correlation (r = 0.85) between auroral power (AP) derived from global auroral images and the auroral electrojet AE of 1-min resolution on 25 January We will show here that the AP-AE relationship with 2 more days of data further supports these results. Recognizing these facts, one might expect a close relationship between auroral power and the polar cap PC index because of the high PC-AE correlation. [6] This paper compares aurora power, derived from global auroral images, and the PC index. Instead of using auroral luminosity, we will correlate auroral power (i.e., area-integrated energy flux from auroral particle precipitation) with the PC index. A good correlation between AE and PC does not necessarily imply a good correlation between the PC index and field-aligned currents in the auroral zone, because enhancements in convection can enhance the auroral electrojets. For example, the so-called convection bays [Pytte et al., 1978; Sergeev et al., 1998] and an intense growth phase [Shue et al., 2000] have been reported to occur during an extended southward interplanetary magnetic field without much auroral activity, and their strength can exceed a typical substorm level of several hundreds nanoteslas. Therefore, our study may provide direct evidence for the importance of field-aligned currents on the PC index-associated magnetic disturbances. 2. Data Analysis [7] Auroras of large spatial scale can be measured from space-based auroral imagers on high-altitude satellites. In this paper we use auroral images acquired from the ultraviolet imager (UVI) [Torr et al., 1995] aboard the ISTP/ GGS program s Polar satellite to derive a time sequence of auroral power. UVI is a snapshot, CCD-intensified optical sensor capable of imaging the oval approximately every 37 s, though the actual frequency of selected data may be lower because of filter selection. The spatial resolution is designed to be about km at the Polar apogee of 9 R E in the Northern Hemisphere. Wobble of the imager s platform additionally smears the images by 10 times of the spatial resolution in one (the wobble) direction. The direction transverse to the wobble is unaffected. [8] AP can be inferred from auroral luminosity in the long-wavelength spectrum of the N 2 Lyman-Birge-Hopfield (LBH) band. This is because electron impact is a sole excitation mechanism for this emission band and because auroral emission in this band is not subject to significant loss from O 2 absorption [e.g., Strickland et al., 1983; Lummerzheim et al., 1991]. Validation of converting auroral emissions from the Polar UVI LBH-long band filter ( nm) images to precipitating electron energy flux can be found from Germany et al. [1998]. In this study, we use a reasonable and robust procedure to handle the UVI data processing [Liou, 2001a, and references therein]. This includes corrections for line-of-sight, flat-field, and dayglow. The auroral power for each local hour sector is calculated by integrating auroral energy flux between the poleward and the equatorward boundaries of the oval within each local hour sector. The oval boundaries are calculated based on algorithms developed by Carbary et al. [2003]. The individual UVI pixels are first mapped from imager coordinates to magnetic latitude (MLAT)-magnetic local time (MLT) coordinates and then averaged into bins that are 1 hour in MLT by 1 in magnetic latitude. For each MLT, the latitude profile is fit to function having the form of a quadratic plus a Gaussian. The fit results in a Gaussian peak location and a Gaussian width. The equatorward boundary is given by the latitude of the peak minus the Gaussian width, and the poleward boundary is given by the latitude of the peak plus the Gaussian width. (Note that the boundaries are separated by twice the usual Gaussian width.) The boundary selection is subject to constraints on the size and width of the Gaussian peak, its strength relative to the quadratic background, and the lower limit of the equatorward boundary. If the fit does not conform to these criteria, or if the fit is of low quality as revealed by its sigma, then the boundaries are not used. [9] The database consists of 4 months of Polar UVI image data of LBH-long band from 4 December 1996 to 31 January 1997 for winter and from 1 June 1997 to 31 July 1997 for summer. A total of 26,506 auroral images were analyzed. These images were taken when the northern oval, not necessarily the entire oval, is within the field-of-view of the UVI. Because of the small field-of-view of the UVI, only those taken near the Polar apogee will have the entire oval coverage. To accommodate the Polar UVI image database, we use the northern polar cap index (PCN) at 1-min time resolution. The general term PC will be used to refer northern PC throughout the paper for simplicity. The northern PC index uses the magnetometer data from Thule (now Qaanaaq) in Greenland at 85.4 corrected geomagnetic latitude; these data are available from the web site of the World Data Center C1 for Geomagnetism. The PC index is then re-sampled to the center time of each UVI image using linear interpolation. Although the UVI integration time is 37 s, the time resolution for the present analysis should be 1 min, the longer time resolution between PC and AP. [10] Figure 1 is a scatterplot that shows the general relationship of the global auroral power (in gigawatts) with

3 LIOU ET AL.: POLAR CAP INDEX AND AURORAL POWER SIA 3-3 Figure 1. Relationship between the total hemispheric auroral power (in gigawatts) and the polar cap index for the entire 4-month database. The total hemispheric auroral power is derived from UVI images with full oval coverage. the polar cap index for the entire database of 4 months. Although large scatter exists, the two variables seem to follow a linear trend. The correlation coefficient is fair (r = 0.70). The number of events used in Figure 1 is 1048, which is far less than the actual number of the UVI images (26,506) used in the present study because the entire oval is not usually covered by UVI. The global auroral power shown in Figure 1 was derived from UVI images with a full oval coverage. [11] To show the detailed relationship of the global auroral power (in gigawatts) with the polar cap index we have plotted one day s worth of the two parameters along with the 1-min quick-look version of the auroral electrojet index AE QL in Figure 2. In agreement with the previous report by Vennerstrøm et al. [1991], the correlation between the PC and AE indices is quite good. The inferred AP is also closely related to both the AE QL and PC indices. For example, the three enhancements in PC commencing separately at 0300, 0930, and 1730 UT are clearly identified in AE QL and AP. Obviously, however, fine structures differ among the three parameters. [12] The relationship between the three parameters is quantified by calculating the Pearson s correlation coefficients. For this particular winter day in Figure 2 (N = 365) the correlation coefficient is r = 0.89 for AP-PC and r = 0.77 for AP-AE QL. Note that there are data gaps in the auroral power owing to a nonoptimal viewing angle for UVI. One thing needs to be clarified here, however. Since the nightside oval is almost always covered by UVI, the derived auroral power shown in Figure 2 represents the total hemispheric power when the entire oval is covered; however, when it is not covered, it represents at least the nightside auroral power. In spite of this, the AP-PC correlation for this particular day is much higher than the AP-PC correlation coefficient for the entire database (r = 0.70). Part of the reason is because PC is not correlated with dayside auroral activity, which will be shown later. Figure 2. (a) The PC index, (b) the AE index, and (c) auroral power on 7 January Both the PC and AE indices are at 1-min frequency; the auroral power is at 3 min-frequency. [13] Since the strength of currents flowing in the ionosphere strongly depends on the polar cap ionospheric conductivity, which is seasonally dependent, we also show these parameters for a summer day on 12 June 1997 in Figure 3 for comparison. There is a noticeable difference in the general shape of the three indices. On this particular day there were three substorms, as clearly indicated by AE QL and AP at 00, 10, and 14 UT. The PC index responded well to the first one but missed the other two substorms. The correlation between AP and PC for the entire day (N = 277) is poor (r = 0.31) and correlation between AP and AE QL is only moderate (r = 0.67) on this summer day. In comparison with the winter case, the weak response of the PC index to substorm activity on this summer day suggests a seasonally dependent source for magnetic variations in the high-latitude polar cap region. Although the response varies Figure 3. Same as Figure 2 but for 12 June 1997.

4 SIA 3-4 LIOU ET AL.: POLAR CAP INDEX AND AURORAL POWER Figure 4. Correlation coefficients (r) and linear fits (s = slope) for, from left to right, 0600, 1200, 1800, and 2400 magnetic local hour sectors for (a) winter and (b) summer seasons. from case to case, this seasonal effect is common. We will show statistical results later. [14] In addition to the seasonal effect shown in Figures 2 and 3, ionospheric conductivity has a diurnal variation too, and it may be revealed in the correlation between AP and PC. To show both effects, we calculate correlation coefficients for each season and for each local time sector by correlating auroral sector power with PC. Selected results of the correlation and the linear fit for 0600, 1200, 1800, and 2400 magnetic local hour sectors are shown in Figure 4. In general, AP tends to increase with PC. However, the AP-PC relation seems not to be linear as is evident by the line fit on each panel; a quadratic term may exist. The correlation coefficients for the 0600, 1800, and 2400 local hour sectors are about similar for both seasons, with the summer season slightly smaller. Large scatter in the data is evident, consistent with the correlation coefficients. Interestingly, there is no correlation (r 0.1) between PC and AP in the noon (1200 MLT) sector for either seasons. [15] We also calculated correlation coefficients for the rest of the hour sectors; the results are shown in Figure 5. A large number of samples ranging from 3,000 for the noon sector to 13,000 for the night sector were used to derive the coefficients (see the upper panel of Figure 5). As we mentioned earlier, the smaller samples in the dayside sector indicate that the UVI was mostly pointed at the nightside oval. A clear diurnal effect is present. The PC index correlated much better with the nightside auroral power than with the dayside auroral power. This diurnal effect is present for both seasons. Actually, the AP-PC correlation Figure 5. Correlation coefficients for each hour sector auroral power with the PC index for both winter (open squares) and summer (closed squares) seasons. Number of samples used to derive the coefficients are given on the upper panel.

5 LIOU ET AL.: POLAR CAP INDEX AND AURORAL POWER SIA 3-5 coefficients sharply drop below 0.6 at 0800 and 1600 MLT and reach minimum (r ) at MLT. Two peaks in the correlation coefficients appear: one at 0400 MLT for both seasons and the other at 2000 MLT for the winter and 1800 MLT for the summer season, forming a local minimum at 2300 MLT for the summer and 2400 MLT for the winter season. A clear seasonal effect is also present. The AP-PC correlation coefficient is higher in winter than in summer for all local times except for the 0700 and MLT local time sectors. The correlation of PC with the Northern Hemispheric AP is r = 0.77 (N = 315) for the winter and r =0.69(N = 730) for the summer database (Figure not shown). The larger AP-PC correlation coefficients for winter than for summer, particularly in the night sector, may indicate that auroras produced in the dark hemisphere contribute more to the polar cap magnetic disturbances than those produced in the sunlit hemisphere. 3. Discussion [16] There is a close relationship among magnetic disturbances, field-aligned currents, and auroras that results from coupling between the solar wind and the magnetosphere-ionosphere system. These phenomena, however, are limited mainly to the auroral oval regions for most of the time. The PC index is derived with magnetograms from a single near-geomagnetic pole station. The remote source of field-aligned currents associated with aurora is expected to be too small to have major effects on the magnetic disturbances at the center of the polar cap. Nonetheless, the present study, along with previous results from Vennerstrøm et al. [1991] and Troshichev et al. [1996], establishes the close relationship between the PC index and auroral activity. The statistical study performed by Vennerstrøm et al. [1991] found a close relationship between the polar cap index and the auroral electrojet index. They also found the PC-AE correlation is generally better in winter (r ) than in summer (r ). On the basis of 4 months of auroral power data inferred from the Polar UVI images, we find similar results for a AP-PC correlation. The AP-PC correlation is higher in winter for all local times. [17] The PC index measures horizontal components of geomagnetic disturbances in the polar cap caused by ionospheric Hall currents induced by cross-polar cap geomagnetic field line convection and field-aligned currents associated with particle precipitation in the auroral zone [Troshichev et al., 1988; Vennerstrøm et al., 1991]. Since there is generally a close relationship between auroral particle precipitation and field-aligned currents, our findings may suggest that field-aligned currents are the dominant source for the PC index variations in the winter time. This is because, according to Vennerstrøm et al. [1991], the polar cap ionosphere is dark in winter, resulting in a low conductive ionosphere and consequently a small Hall current, so that field-aligned currents become a dominant source in this season. In summer the entire polar cap is exposed to sunlight. Enhancements of the Hall currents because of a large ionospheric conductance contribute significantly to the PC index. On the other hand, since magnetometers respond to changes in the ionospheric currents, a small change in the ionospheric conductivity caused, for example, by auroral particle precipitation is more effective in producing magnetic perturbations in a low than a high conductive ionosphere. [18] It has been shown that mono-energetic electron acceleration events that produce auroral arcs occur more frequently in a dark rather than in a sunlit hemisphere [Newell et al., 1996]. This finding is somewhat supported by global auroral observations by Polar UVI but only in the night sector [Liou et al., 1997, 2001a], though UVI cannot distinguish discrete auroras from the typical diffuse oval. Ionospheric conductivity has been considered as the main cause of this winter-summer asymmetry. In winter the ionospheric conductivity is low. The driving of magnetospheric currents requires the acceleration of electrons into the ionosphere to form a conducting path. The higher AP- PC correlation in winter than in summer indicates that both PC and AP share the same major source of field-aligned currents associated with discrete arcs. On the other hand, in summer the ionospheric conductivity is already high and acceleration of electrons is not frequent. Variations of PC then come mainly from the convection-associated Hall currents, while variations of AP still rely on field-aligned currents carried by precipitating electrons. [19] Diurnal variations in the AP-PC correlation coefficients found in the present study may also be related to the day-night variations of ionospheric conductivity due to solar EUV photo-ionization. Presumably the Hall currents are the dominant source for PC in a sunlit hemisphere, and distant field-aligned currents are the dominant source for PC in a dark hemisphere, so a smaller AP-PC correlation coefficient on dayside than on nightside would be expected. However, we point out that diurnal variations in the AP-PC correlation exist in both summer and winter seasons. In summer the entire polar cap is exposed to sunlight, which causes a nonuniform ionospheric conductivity in the polar cap, with a gradient pointing toward the noon. A weaker AP-PC correlation on dayside than on nightside is thus expected. In winter, as defined by our database, the entire polar cap is in darkness and the Hall currents are always small. Therefore, distant field-aligned currents should become an important source of the PC index in winter. The much higher correlation of PC with nightside than with dayside auroral power indicates that the current systems that flow in the polar cap ionosphere and the nightside field-aligned currents are highly coupled. [20] Studies of relationship between auroral electrojets and PC have shown that the PC index is less responsive to the westward electrojet (DP-1) in the pre-midnight sector than in the midnight and post-midnight sectors [Vennerstrøm et al., 1991]. This suggests that PC is better correlated with DP-1 disturbances during non-substorm times than during substorm times because substorms take place most frequently in the non-substorm time pre-midnight sector [Craven and Frank, 1991; Liou et al., 2001b]. Is there a similar relationship between PC and AP? To divide our database into substorm and non-substorm periods, we take a simple approach here by using AE as a substorm indicator. Substorm times are defined by periods of AE > 120 nt and non-substorm times by periods of AE < 80 nt. The result is given in Figure 6, which has the same format as Figure 5. For non-substorm times the shape of the MLT-correlation pattern is very similar to the one from the entire database but with slightly smaller correlation coefficients (the largest

6 SIA 3-6 LIOU ET AL.: POLAR CAP INDEX AND AURORAL POWER used longer time (15 min) averages of the PC index, which generally result in a better correlation. The auroral power is probably a better proxy of field-aligned currents than AE and, therefore, the present study should provide a more reliable result for the relationship between auroral activity and magnetic disturbances in the polar cap. Note that the auroral power used in the present analysis does not include the much weaker polar cap particle precipitation. A better AP-PC correlation is expected if the polar cap aurora is included. This will be the subject of future studies. Figure 6. Same format as Figure 5, with squares for nonsubstorm (AE < 80 nt) times and circles for substorm (AE > 120 nt) times. r is 0.65). Furthermore, the seasonal difference in the AP- PC correlation seems to decrease. During substorm times, however, the AP-PC correlation coefficients drop substantially below 0.4. Note that the winter events are substantially fewer than the summer events because 1996 AE data are not available and 1-month AP data are used. In good agreement with the results from Vennerstrøm et al. [1991], this clear difference in the AP-PC correlation between substorm and non-substorm times suggests that substorm auroral electrojets contribute insignificantly to PC. [21] A minor but interesting finding from the present study is that the AP-PC correlation does not peak before midnight, where substorms occur most frequently [e.g., Liou et al., 2001b], but rather at dusk and dawn. The dawn peak (0400 MLT) corresponds approximately with the statistical center (0300 MLT) of the westward auroral electrojet [Kamide, 1991, and references therein]. Actually, AP-PC correlation showed a local nightside minimum at 2300 MLT. It is possible that both DP-1 and DP-2 currents are enhanced during substorms because of increased particle precipitation. The convection-related DP-2 currents should have greater effects on the PC index than the DP-1 currents. This is because DP-1 currents are closed locally and form a so-called substorm current wedge, and therefore do not flow across the polar cap to have significant effect to the PC index. [22] While similar to the AE-PC relation obtained previously [Vennerstrøm et al., 1991], the correlation between AP and PC is generally much weaker. The higher AE-PC correlation is because AE, which is a measure of auroral electrojets, is related not only to particle precipitations but also to the convection electric field, the same source as PC. Another reason is that previous studies [Vennerstrøm et al., 1991; Vassiliadis et al., 1996; Takalo and Timonen, 1998] 4. Summary and Conclusions [23] We have examined the statistical relationship between auroral power (AP) and the polar cap (PC) index at 1-min time resolution. A total of 26,506 auroral images that spanned two 2-month periods are analyzed. The correlation between AP and PC is generally fair (r = 0.70); and the AP-PC correlation is higher in the winter (r = 0.77) than in the summer hemisphere (r = 0.69). The AP-PC correlation is much higher on nightside than on dayside. In terms of local time effect, the peak AP-PC correlation occurs not at midnight but at pre-dawn near 0400 MLT and at dusk around 1900 MLT. The AP-PC correlation appears to be much better during non-substorm times than during substorm times. [24] In conclusion, 48% (summer) to 59% (winter) of the variance in the 1-min PC is predictable by hemispheric auroral power and is most likely produced by field-aligned currents associated with auroral particle precipitation. Our results also suggest that field-aligned currents associated with the non-substorm electrojets in the nightside sector play the major role in affecting the horizontal magnetic disturbances at Thule for both winter and summer seasons. [25] Acknowledgments. The Ultraviolet Imager (UVI) on board the Polar satellite was built by M. Torr. G. Parks is the principal investigator for the Polar UVI. The Polar cap index is provided by Oleg Rasmussen through World Data Center C1 for Geomagnetism. This work was supported by NASA grant NAG to The Johns Hopkins University Applied Physics Laboratory. [26] Arthur Richmond thanks Francis K. Chun and another reviewer for their assistance in evaluating manuscript 2002JA References Akasofu, S.-I., Polar and Magnetospheric Substorms, D. Reidel, Norwell, Mass., Carbary, J. F., T. Sotirelis, P. T. Newell, and C.-I. Meng, Auroral boundary correlations between UVI and DMSP, J. Geophys. Res., 108(A1), 1018, doi: /2002ja009378, Chun, F. K., D. J. Knipp, M. G. McHarg, G. Lu, B. A. Emery, and O. A. Troshichev, Polar cap index as a proxy for hemispheric Joule heating, Geophys. Res. Lett., 26, 1101, Chun, F. K., D. J. Knipp, M. G. McHarg, J. R. Lacey, G. Lu, and B. A. 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