The current generation of space and ground-based experiments in solar physics produces many megab... more The current generation of space and ground-based experiments in solar physics produces many megabyte-sized image data arrays. Optical disk technology is the leading candidate for convenient analysis, distribution, and archiving of these data. The authors have been developing data analysis procedures which use both analog and digital optical disks for the study of solar phenomena.
... 80 000, 3508 TA Utrecht, The Netherlands GORAN SCHARMER Stockholm Observatory, 5-13336 Saltsj... more ... 80 000, 3508 TA Utrecht, The Netherlands GORAN SCHARMER Stockholm Observatory, 5-13336 Saltsjdbaden, Sweden Received 1991 February 15; accepted 1992 January21 At the Swedish Solar Observatory on Roque de los Muchachos in La Palma, Canary Islands we ...
Approved for public release; distribution unlimitec 9-9 8 %epresent init~a!: resuits5 on soia7 7r... more Approved for public release; distribution unlimitec 9-9 8 %epresent init~a!: resuits5 on soia7 7ranulation. oores and sunsoots from the white-light 9 ~ms obtained o~the ý'oiar Oplical L 7-' rsa: Parmter (SOUP! instrumnent on S.Daceiab 2. SOL-P contains a 30-cm Ca-eg_ 4 ,n teie'!COoý aLn 3Ci.;% ýecOn.-dr\ rmir.ror for 'z:ka,.e-'ablizatlon. and a white-!ight Optical systemñ n ri i rn n ir nd icp,) rame-s-Outputs .-7 ii. t me gijidance servo orovided-engineering data on-he perfor-manre 0? Ile E'z A lnc:.-jrnent P" 7nung-výzerr ;lP~i. SeVerai hours of movies-were taken a. var~ou5 diisK and3 !limb oosi-:ons :n c Ouiet anc ac::ve regions The !mages are diffiraction-limited at 0.5 arc se. onc es:,nan.o arp o' course, free ,-atm-ofohcrrc seeing and d:szortion. Properties of the granulation .n magne!:c a7.:-.on-7naenetic rer:cns are com * areo and are round to differ significantly in size. rate of tntzens~t. var~a:.on. ant; .ite-'rne. In quiet sun, on the order of fiftv percent of the area has at ;eastr one-e.\Podina, gmarultocc:mr:ng :rn it Cd.:ing a 25 minute period. Lo'cal cor-elation tracking has detected Seklrai LN opt; of ',rans~erse io%&s. inclurding systematic outflow from the penumbral boun'darv of a spot. motion of penurnoral ~iiarnents. and ceii-.ýar flow patterns of supergranular and mesogranular size. Feature tracking has sno~n that in quiet sun :,he average granule fragment has a velocity of about one kilometer per second.
<p>This poster was presented at the 224th AAS/SPD conference during 2014 in Boston, MA.<... more <p>This poster was presented at the 224th AAS/SPD conference during 2014 in Boston, MA.</p> <p>Abstract: Over the past few decades there have been a number of papers investigating the connection between flares occurring in succession. Statistically, any connection that affects the timing of successive flares that exists is found to be weak. However, the majority of previous investigations has been limited by only considering the causal connection between soft X-ray flares. More recent case studies have shown convincing evidence that large eruptions cause a global reorganization of overlying magnetic fields that can result in the eruption of both flares and filaments at large distances from the original event. In this work, the connection between GOES X-ray flares (C-, M-, and X-class) and filament eruptions occurring in succession in two different active regions is considered statistically. The filament eruptions are recorded in the Heliophysics Events Knowledgebase by observers using SDO/AIA data. A significant causal connection is found between the two event types, such that large flares are followed by filament eruptions within 24 hours much more often than they are preceded by filament eruptions. This stipulates that the flares either cause the filaments to erupt or affect the eruption timing such that the filament eruptions follow the flares more closely in time.</p> <p></p
Publications of the Astronomical Society of Japan, 2003
The result of analyses of an arcade flare on 1999 July 19, observed by the Satellite TRACE, is re... more The result of analyses of an arcade flare on 1999 July 19, observed by the Satellite TRACE, is reported, and the significance of the following findings is stressed. It is clearly seen that four magnetic regions of alternating polarities [named A(+), B(−), C(+), and D(−) from the East in the flaring region] are involved in this arcade flare event in an essential way. The high structure covering the triangular region between A and the northern part of D disappears when the dark filament lying between B and C erupts. The X-ray arcade is formed only between B and C. The overlying structure connecting regions A and the northern part of D disappears and the entire region is involved in the flare. Low-lying loops connect the inner sources to the outer sources. The field lines from regions B to A and C to D are not much affected by the eruption of the dark filament, itself, but the loops near the upper surfaces of the closed loop regions are clearly seen to move toward each other (from both sides to the central line) as the arcade flare progresses between the inner pair, B and C; some of them appear to be converted into a flare arcade. The observed behavior can best be understood in terms of the quadruple magnetic source model advocated by one of the authors (YU). The long-conceived "reclosing of once opened bipolar magnetic arcade" models by the rising dark filament (called CSHKP model) find it difficult to explain this.
(HMI) on SDO provides doppler data every 30 seconds, line-of-sight magnetograms every 45 seconds,... more (HMI) on SDO provides doppler data every 30 seconds, line-of-sight magnetograms every 45 seconds, and vector magnetograms every 5 minutes. With the SDO data set and observations from the pair of STEREO satellites it has become apparent that many flares, filament eruptions, and CME's have causal connections. These connections often span a hemisphere or more. New numerical simulations indicates that there are several mechanisms for triggering of remote events. Maps of the magnetic topology constructed from the LOS field and a PFSS model indicates both how regions that are connected and their boundaries. Magnetic evolution can change both the shapes of the topological boundaries and the topological structure. Because of the large dynamic range of the AIA images (105) it is possible to directly map the evolution of the magnetic fields on a global scale. Movies of everts and numerical simulations will be presented as well as topological mappings that indicated the zones of connectivity.
The properties of a previously unresolved extreme-ultraviolet (EUV) emission in solar active regi... more The properties of a previously unresolved extreme-ultraviolet (EUV) emission in solar active regions are examined using coordinated data sets from the Transition Region and Coronal Explorer (TRACE) satellite, the Michelson Doppler Imager on the Solar and Heliospheric Observatory satellite, the Soft X-Ray Telescope (SXT) on the Yohkoh satellite, and the ground-based Swedish Vacuum Solar Telescope (SVST) on La Palma. The emission appears most prominently in TRACE Fe ix/Fe x 171 A ˚images as a bright dynamic network surrounding dark inclusions on scales of 2-3 Mm, confined to layers approximately 1-3 Mm thick with base heights approximately 2-4 Mm above the photosphere. It is seen only above plage regions that underlie K 6 (3-5) # 10 coronal loops visible in SXT images. The bright EUV elements emit at temperatures of about 10 6 K. Fine-scale motions and brightness variations of the emission occur on timescales of 1 minute or less. The dark inclusions correspond to jets of chromospheric plasma seen in simultaneous SVST filtergrams in the wings of Ha. The combined characteristics imply that we are at least partially resolving the structure and dynamics of the conductively heated upper transition region between the solar chromosphere and corona.
Proceedings of the International Astronomical Union, 2010
Observations with the Solar Optical Telescope on Hinode indicate that the Quiet Sun magnetic fiel... more Observations with the Solar Optical Telescope on Hinode indicate that the Quiet Sun magnetic field occurs on every scale of convection including granulation. Data reported here show that, regardless of the position on the disk, the polarity in the inner network regions are balanced to 1 part in 72. This is consistent with both local dynamo processes or the creation of surface features by the granulation downflows.
Solar magnetism displays a host of variational timescales of which the enigmatic 11-year sunspot ... more Solar magnetism displays a host of variational timescales of which the enigmatic 11-year sunspot cycle is most prominent. Recent work has demonstrated that the sunspot cycle can be explained in terms of the intra- and extra-hemispheric interaction between the overlapping activity bands of the 22-year magnetic polarity cycle. Those activity bands appear to be driven by the rotation of the Sun's deep interior. Here we deduce that activity band interaction can qualitatively explain the 'Gnevyshev Gap'-a well-established feature of flare and sunspot occurrence. Strong quasi-annual variability in the number of flares, coronal mass ejections, the radiative and particulate environment of the heliosphere is also observed. We infer that this secondary variability is driven by surges of magnetism from the activity bands. Understanding the formation, interaction and instability of these activity bands will considerably improve forecast capability in space weather and solar activity...
Space Telescopes and Instrumentation 2008: Ultraviolet to Gamma Ray, 2008
The High-resolution Lightweight Telescope for the EUV (HiLiTE) is a Cassegrain telescope that wil... more The High-resolution Lightweight Telescope for the EUV (HiLiTE) is a Cassegrain telescope that will be made entirely of Silicon Carbide (SiC), optical substrates and metering structure alike. Using multilayer coatings, this instrument will be tuned to operate at the 465 Å Ne VII emission line, formed in solar transition region plasma at ~500,000 K. HiLiTE will have an aperture of 30 cm, angular resolution of ~0.2 arc seconds and operate at a cadence of ~5 seconds or less, having a mass that is about 1/4 that of one of the 20 cm aperture telescopes on the Atmospheric Imaging Assembly (AIA) instrument aboard NASA's Solar Dynamics Observatory (SDO). This new instrument technology thus serves as a path finder to a post-AIA, Explorer-class missions.
We find that bipolar active regions that emerge onto the Sun's surface are part of a smoothly dec... more We find that bipolar active regions that emerge onto the Sun's surface are part of a smoothly decreasing frequency distribution that spans almost 4 orders of magnitude in flux and 8 orders of magnitude in frequency. Distributions of emergence latitude and dipole orientation narrow from nearly uniform for the smallest observed ephemeral regions ($5 Â 10 18 Mx) up to narrowly distributed about the mean for the largest active regions (close to 10 22 Mx), while the emergence frequency increases smoothly and rapidly with decreasing flux. At the low end of the flux spectrum, the cycle variation in emergence frequency is at most a factor of 1.5, in antiphase with the cycle variation of close to an order of magnitude for the large active regions. We discuss a scenario in which the ephemeral regions with fluxes below $30 Â 10 18 Mx have their origin in a turbulent dynamo, largely independent of the global sunspot cycle. Our empirical findings are based on a combination of previously published work on active regions and large ephemeral regions, complemented here with an analysis of the photospheric magnetic field outside active regions, as observed in SOHO/MDI full-disk magnetograms taken from the most recent sunspot minimum in 1996 to about 1 yr after sunspot maximum in 2001. We find that the spectrum of the emerging bipoles with fluxes ð6 30Þ Â 10 18 Mx can be approximated throughout this period by a fixed exponential distribution with an e-folding scale of ð5:3 AE 0:1Þ Â 10 18 Mx. We confirm that the ephemeral regions are an important source of flux for the quiet magnetic network, in particular for the smallest scales; the larger scale patterns are dominated by flux dispersing from decaying active regions. As the variation of these two sources is nearly in antiphase, the flux contained in the quiet-Sun network shows little overall variation: the flux spectrum and the total absolute flux for network concentrations with fluxes d20 Â 10 18 Mx are essentially independent of cycle phase. For network concentrations with fluxes e30 Â 10 18 Mx, mostly found in regions populated substantially by decayed active regions, the network flux distribution approaches an exponential for which the e-folding scale increases with sunspot activity from $20 Â 10 18 Mx to $33 Â 10 18 Mx, as the total flux in this component varies in phase with the sunspot cycle. A comparison of the flux-emergence rate with the network flux implies an overall mean replacement time for flux in quiet Sun of 8-19 hr.
The magnetic field in the solar photosphere frequently carries strong electric currents, even tho... more The magnetic field in the solar photosphere frequently carries strong electric currents, even though the global coronal configuration often resembles a potential field ringed by the heliospheric current sheet. To understand this, we compare TRACE EUV images of active-region coronae and potential-field source-surface extrapolations based on SOHO MDI magnetograms for 95 active regions. We conclude that significant nonpotentiality of the overall active-region coronal field occurs (1) when new flux has emerged within or very near a region within the last $30 hr, resulting in complex polarity separation lines, or (2) when rapidly evolving, opposite-polarity concentrations are in contact at 4 00 resolution. If these criteria are met by more than 15% of the region's flux, they correctly identify the (non) potentiality of active-region coronae in 88% of the cases. Flares are found to occur 2.4 times more frequently in active regions with nonpotential coronae than in near-potential regions, while their average X-ray peak flare brightness is 3.3 times higher. We suggest that the currents associated with coronal nonpotentiality have a characteristic growth and decay timescale of $10-30 hr. We find that shear flows drive enhanced flaring or coronal nonpotentiality only if associated with complex and dynamic flux emergence within the above timescale. We discuss the implications of this finding for the modeling of the coronal-heliospheric coupling.
Blue continuum images of active regions at ∼ 60 • from the center of the solar disk obtained with... more Blue continuum images of active regions at ∼ 60 • from the center of the solar disk obtained with the new Swedish 1-m Solar Telescope reveal heretofore unreported structure of the magnetized solar atmosphere. Perhaps the most striking aspect of these images is that, at an angular resolution of 0.12 , they show clearly the three-dimensional structure of the photosphere. In particular, the Wilson depression of the dark floors of pores is readily apparent. Conversely, the segmented structure of light bridges running through sunspots and pores reveal that light bridges are raised above the dark surroundings. The geometry of light bridges permits estimates of the height of their central (slightly darker) ridge: typically in the range 200-450 km. These images also clearly show that facular brightenings outside of sunspots and pores occur on the disk-center side of those granules just limbward of intergranular lanes that presumably harbor the associated plage magnetic flux. In many cases the brightening extends 0.5 or more over those granules. Furthermore, a very thin, darker lane is often found just centerward of the facular brightening. We speculate that this feature is the signature of cool down flows that surround flux tubes in dynamical models. These newly recognized observational aspects of photospheric magnetic fields should provide valuable constraints for MHD models of the magnetized photosphere, and examination of those models as viewed from oblique angles is encouraged.
Publications of the Astronomical Society of Japan, 2007
The Solar Optical Telescope (SOT) on board the Hinode satellite observed an X3.4 class flare on 2... more The Solar Optical Telescope (SOT) on board the Hinode satellite observed an X3.4 class flare on 2006 December 13. A typical two-ribbon structure was observed, not only in the chromospheric Ca II H line, but also in the G-band and Fe I 6302Å line. The high-resolution, seeing-free images achieved by SOT revealed, for the first time, sub-arcsec fine structures of the "white light" flare. The G-band flare ribbons on sunspot umbrae showed a sharp leading edge, followed by a diffuse inside, as well as a previously known core-halo structure. The underlying structures, such as umbral dots, penumbral filaments, and granules, were visible in the flare ribbons. Assuming that the sharp leading edge was directly heated by a particle beam and the diffuse parts were heated by radiative back-warming, we estimated the depth of the diffuse flare emission using an intensity profile of the flare ribbon. We found that the depth of the diffuse emission was about 100 km or less from the height of the source of radiative back-warming. The flare ribbons were also visible in the Stokes-V images of Fe I 6302Å, as a transient polarity reversal. This is probably related to a "magnetic transient" reported in the literature. The intensity increase in Stokes-I images indicates that the Fe I 6302Å line was significantly deformed by the flare, which may cause such a magnetic transient.
The current generation of space and ground-based experiments in solar physics produces many megab... more The current generation of space and ground-based experiments in solar physics produces many megabyte-sized image data arrays. Optical disk technology is the leading candidate for convenient analysis, distribution, and archiving of these data. The authors have been developing data analysis procedures which use both analog and digital optical disks for the study of solar phenomena.
... 80 000, 3508 TA Utrecht, The Netherlands GORAN SCHARMER Stockholm Observatory, 5-13336 Saltsj... more ... 80 000, 3508 TA Utrecht, The Netherlands GORAN SCHARMER Stockholm Observatory, 5-13336 Saltsjdbaden, Sweden Received 1991 February 15; accepted 1992 January21 At the Swedish Solar Observatory on Roque de los Muchachos in La Palma, Canary Islands we ...
Approved for public release; distribution unlimitec 9-9 8 %epresent init~a!: resuits5 on soia7 7r... more Approved for public release; distribution unlimitec 9-9 8 %epresent init~a!: resuits5 on soia7 7ranulation. oores and sunsoots from the white-light 9 ~ms obtained o~the ý'oiar Oplical L 7-' rsa: Parmter (SOUP! instrumnent on S.Daceiab 2. SOL-P contains a 30-cm Ca-eg_ 4 ,n teie'!COoý aLn 3Ci.;% ýecOn.-dr\ rmir.ror for 'z:ka,.e-'ablizatlon. and a white-!ight Optical systemñ n ri i rn n ir nd icp,) rame-s-Outputs .-7 ii. t me gijidance servo orovided-engineering data on-he perfor-manre 0? Ile E'z A lnc:.-jrnent P" 7nung-výzerr ;lP~i. SeVerai hours of movies-were taken a. var~ou5 diisK and3 !limb oosi-:ons :n c Ouiet anc ac::ve regions The !mages are diffiraction-limited at 0.5 arc se. onc es:,nan.o arp o' course, free ,-atm-ofohcrrc seeing and d:szortion. Properties of the granulation .n magne!:c a7.:-.on-7naenetic rer:cns are com * areo and are round to differ significantly in size. rate of tntzens~t. var~a:.on. ant; .ite-'rne. In quiet sun, on the order of fiftv percent of the area has at ;eastr one-e.\Podina, gmarultocc:mr:ng :rn it Cd.:ing a 25 minute period. Lo'cal cor-elation tracking has detected Seklrai LN opt; of ',rans~erse io%&s. inclurding systematic outflow from the penumbral boun'darv of a spot. motion of penurnoral ~iiarnents. and ceii-.ýar flow patterns of supergranular and mesogranular size. Feature tracking has sno~n that in quiet sun :,he average granule fragment has a velocity of about one kilometer per second.
<p>This poster was presented at the 224th AAS/SPD conference during 2014 in Boston, MA.<... more <p>This poster was presented at the 224th AAS/SPD conference during 2014 in Boston, MA.</p> <p>Abstract: Over the past few decades there have been a number of papers investigating the connection between flares occurring in succession. Statistically, any connection that affects the timing of successive flares that exists is found to be weak. However, the majority of previous investigations has been limited by only considering the causal connection between soft X-ray flares. More recent case studies have shown convincing evidence that large eruptions cause a global reorganization of overlying magnetic fields that can result in the eruption of both flares and filaments at large distances from the original event. In this work, the connection between GOES X-ray flares (C-, M-, and X-class) and filament eruptions occurring in succession in two different active regions is considered statistically. The filament eruptions are recorded in the Heliophysics Events Knowledgebase by observers using SDO/AIA data. A significant causal connection is found between the two event types, such that large flares are followed by filament eruptions within 24 hours much more often than they are preceded by filament eruptions. This stipulates that the flares either cause the filaments to erupt or affect the eruption timing such that the filament eruptions follow the flares more closely in time.</p> <p></p
Publications of the Astronomical Society of Japan, 2003
The result of analyses of an arcade flare on 1999 July 19, observed by the Satellite TRACE, is re... more The result of analyses of an arcade flare on 1999 July 19, observed by the Satellite TRACE, is reported, and the significance of the following findings is stressed. It is clearly seen that four magnetic regions of alternating polarities [named A(+), B(−), C(+), and D(−) from the East in the flaring region] are involved in this arcade flare event in an essential way. The high structure covering the triangular region between A and the northern part of D disappears when the dark filament lying between B and C erupts. The X-ray arcade is formed only between B and C. The overlying structure connecting regions A and the northern part of D disappears and the entire region is involved in the flare. Low-lying loops connect the inner sources to the outer sources. The field lines from regions B to A and C to D are not much affected by the eruption of the dark filament, itself, but the loops near the upper surfaces of the closed loop regions are clearly seen to move toward each other (from both sides to the central line) as the arcade flare progresses between the inner pair, B and C; some of them appear to be converted into a flare arcade. The observed behavior can best be understood in terms of the quadruple magnetic source model advocated by one of the authors (YU). The long-conceived "reclosing of once opened bipolar magnetic arcade" models by the rising dark filament (called CSHKP model) find it difficult to explain this.
(HMI) on SDO provides doppler data every 30 seconds, line-of-sight magnetograms every 45 seconds,... more (HMI) on SDO provides doppler data every 30 seconds, line-of-sight magnetograms every 45 seconds, and vector magnetograms every 5 minutes. With the SDO data set and observations from the pair of STEREO satellites it has become apparent that many flares, filament eruptions, and CME's have causal connections. These connections often span a hemisphere or more. New numerical simulations indicates that there are several mechanisms for triggering of remote events. Maps of the magnetic topology constructed from the LOS field and a PFSS model indicates both how regions that are connected and their boundaries. Magnetic evolution can change both the shapes of the topological boundaries and the topological structure. Because of the large dynamic range of the AIA images (105) it is possible to directly map the evolution of the magnetic fields on a global scale. Movies of everts and numerical simulations will be presented as well as topological mappings that indicated the zones of connectivity.
The properties of a previously unresolved extreme-ultraviolet (EUV) emission in solar active regi... more The properties of a previously unresolved extreme-ultraviolet (EUV) emission in solar active regions are examined using coordinated data sets from the Transition Region and Coronal Explorer (TRACE) satellite, the Michelson Doppler Imager on the Solar and Heliospheric Observatory satellite, the Soft X-Ray Telescope (SXT) on the Yohkoh satellite, and the ground-based Swedish Vacuum Solar Telescope (SVST) on La Palma. The emission appears most prominently in TRACE Fe ix/Fe x 171 A ˚images as a bright dynamic network surrounding dark inclusions on scales of 2-3 Mm, confined to layers approximately 1-3 Mm thick with base heights approximately 2-4 Mm above the photosphere. It is seen only above plage regions that underlie K 6 (3-5) # 10 coronal loops visible in SXT images. The bright EUV elements emit at temperatures of about 10 6 K. Fine-scale motions and brightness variations of the emission occur on timescales of 1 minute or less. The dark inclusions correspond to jets of chromospheric plasma seen in simultaneous SVST filtergrams in the wings of Ha. The combined characteristics imply that we are at least partially resolving the structure and dynamics of the conductively heated upper transition region between the solar chromosphere and corona.
Proceedings of the International Astronomical Union, 2010
Observations with the Solar Optical Telescope on Hinode indicate that the Quiet Sun magnetic fiel... more Observations with the Solar Optical Telescope on Hinode indicate that the Quiet Sun magnetic field occurs on every scale of convection including granulation. Data reported here show that, regardless of the position on the disk, the polarity in the inner network regions are balanced to 1 part in 72. This is consistent with both local dynamo processes or the creation of surface features by the granulation downflows.
Solar magnetism displays a host of variational timescales of which the enigmatic 11-year sunspot ... more Solar magnetism displays a host of variational timescales of which the enigmatic 11-year sunspot cycle is most prominent. Recent work has demonstrated that the sunspot cycle can be explained in terms of the intra- and extra-hemispheric interaction between the overlapping activity bands of the 22-year magnetic polarity cycle. Those activity bands appear to be driven by the rotation of the Sun's deep interior. Here we deduce that activity band interaction can qualitatively explain the 'Gnevyshev Gap'-a well-established feature of flare and sunspot occurrence. Strong quasi-annual variability in the number of flares, coronal mass ejections, the radiative and particulate environment of the heliosphere is also observed. We infer that this secondary variability is driven by surges of magnetism from the activity bands. Understanding the formation, interaction and instability of these activity bands will considerably improve forecast capability in space weather and solar activity...
Space Telescopes and Instrumentation 2008: Ultraviolet to Gamma Ray, 2008
The High-resolution Lightweight Telescope for the EUV (HiLiTE) is a Cassegrain telescope that wil... more The High-resolution Lightweight Telescope for the EUV (HiLiTE) is a Cassegrain telescope that will be made entirely of Silicon Carbide (SiC), optical substrates and metering structure alike. Using multilayer coatings, this instrument will be tuned to operate at the 465 Å Ne VII emission line, formed in solar transition region plasma at ~500,000 K. HiLiTE will have an aperture of 30 cm, angular resolution of ~0.2 arc seconds and operate at a cadence of ~5 seconds or less, having a mass that is about 1/4 that of one of the 20 cm aperture telescopes on the Atmospheric Imaging Assembly (AIA) instrument aboard NASA's Solar Dynamics Observatory (SDO). This new instrument technology thus serves as a path finder to a post-AIA, Explorer-class missions.
We find that bipolar active regions that emerge onto the Sun's surface are part of a smoothly dec... more We find that bipolar active regions that emerge onto the Sun's surface are part of a smoothly decreasing frequency distribution that spans almost 4 orders of magnitude in flux and 8 orders of magnitude in frequency. Distributions of emergence latitude and dipole orientation narrow from nearly uniform for the smallest observed ephemeral regions ($5 Â 10 18 Mx) up to narrowly distributed about the mean for the largest active regions (close to 10 22 Mx), while the emergence frequency increases smoothly and rapidly with decreasing flux. At the low end of the flux spectrum, the cycle variation in emergence frequency is at most a factor of 1.5, in antiphase with the cycle variation of close to an order of magnitude for the large active regions. We discuss a scenario in which the ephemeral regions with fluxes below $30 Â 10 18 Mx have their origin in a turbulent dynamo, largely independent of the global sunspot cycle. Our empirical findings are based on a combination of previously published work on active regions and large ephemeral regions, complemented here with an analysis of the photospheric magnetic field outside active regions, as observed in SOHO/MDI full-disk magnetograms taken from the most recent sunspot minimum in 1996 to about 1 yr after sunspot maximum in 2001. We find that the spectrum of the emerging bipoles with fluxes ð6 30Þ Â 10 18 Mx can be approximated throughout this period by a fixed exponential distribution with an e-folding scale of ð5:3 AE 0:1Þ Â 10 18 Mx. We confirm that the ephemeral regions are an important source of flux for the quiet magnetic network, in particular for the smallest scales; the larger scale patterns are dominated by flux dispersing from decaying active regions. As the variation of these two sources is nearly in antiphase, the flux contained in the quiet-Sun network shows little overall variation: the flux spectrum and the total absolute flux for network concentrations with fluxes d20 Â 10 18 Mx are essentially independent of cycle phase. For network concentrations with fluxes e30 Â 10 18 Mx, mostly found in regions populated substantially by decayed active regions, the network flux distribution approaches an exponential for which the e-folding scale increases with sunspot activity from $20 Â 10 18 Mx to $33 Â 10 18 Mx, as the total flux in this component varies in phase with the sunspot cycle. A comparison of the flux-emergence rate with the network flux implies an overall mean replacement time for flux in quiet Sun of 8-19 hr.
The magnetic field in the solar photosphere frequently carries strong electric currents, even tho... more The magnetic field in the solar photosphere frequently carries strong electric currents, even though the global coronal configuration often resembles a potential field ringed by the heliospheric current sheet. To understand this, we compare TRACE EUV images of active-region coronae and potential-field source-surface extrapolations based on SOHO MDI magnetograms for 95 active regions. We conclude that significant nonpotentiality of the overall active-region coronal field occurs (1) when new flux has emerged within or very near a region within the last $30 hr, resulting in complex polarity separation lines, or (2) when rapidly evolving, opposite-polarity concentrations are in contact at 4 00 resolution. If these criteria are met by more than 15% of the region's flux, they correctly identify the (non) potentiality of active-region coronae in 88% of the cases. Flares are found to occur 2.4 times more frequently in active regions with nonpotential coronae than in near-potential regions, while their average X-ray peak flare brightness is 3.3 times higher. We suggest that the currents associated with coronal nonpotentiality have a characteristic growth and decay timescale of $10-30 hr. We find that shear flows drive enhanced flaring or coronal nonpotentiality only if associated with complex and dynamic flux emergence within the above timescale. We discuss the implications of this finding for the modeling of the coronal-heliospheric coupling.
Blue continuum images of active regions at ∼ 60 • from the center of the solar disk obtained with... more Blue continuum images of active regions at ∼ 60 • from the center of the solar disk obtained with the new Swedish 1-m Solar Telescope reveal heretofore unreported structure of the magnetized solar atmosphere. Perhaps the most striking aspect of these images is that, at an angular resolution of 0.12 , they show clearly the three-dimensional structure of the photosphere. In particular, the Wilson depression of the dark floors of pores is readily apparent. Conversely, the segmented structure of light bridges running through sunspots and pores reveal that light bridges are raised above the dark surroundings. The geometry of light bridges permits estimates of the height of their central (slightly darker) ridge: typically in the range 200-450 km. These images also clearly show that facular brightenings outside of sunspots and pores occur on the disk-center side of those granules just limbward of intergranular lanes that presumably harbor the associated plage magnetic flux. In many cases the brightening extends 0.5 or more over those granules. Furthermore, a very thin, darker lane is often found just centerward of the facular brightening. We speculate that this feature is the signature of cool down flows that surround flux tubes in dynamical models. These newly recognized observational aspects of photospheric magnetic fields should provide valuable constraints for MHD models of the magnetized photosphere, and examination of those models as viewed from oblique angles is encouraged.
Publications of the Astronomical Society of Japan, 2007
The Solar Optical Telescope (SOT) on board the Hinode satellite observed an X3.4 class flare on 2... more The Solar Optical Telescope (SOT) on board the Hinode satellite observed an X3.4 class flare on 2006 December 13. A typical two-ribbon structure was observed, not only in the chromospheric Ca II H line, but also in the G-band and Fe I 6302Å line. The high-resolution, seeing-free images achieved by SOT revealed, for the first time, sub-arcsec fine structures of the "white light" flare. The G-band flare ribbons on sunspot umbrae showed a sharp leading edge, followed by a diffuse inside, as well as a previously known core-halo structure. The underlying structures, such as umbral dots, penumbral filaments, and granules, were visible in the flare ribbons. Assuming that the sharp leading edge was directly heated by a particle beam and the diffuse parts were heated by radiative back-warming, we estimated the depth of the diffuse flare emission using an intensity profile of the flare ribbon. We found that the depth of the diffuse emission was about 100 km or less from the height of the source of radiative back-warming. The flare ribbons were also visible in the Stokes-V images of Fe I 6302Å, as a transient polarity reversal. This is probably related to a "magnetic transient" reported in the literature. The intensity increase in Stokes-I images indicates that the Fe I 6302Å line was significantly deformed by the flare, which may cause such a magnetic transient.
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