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Physical understanding of total and spectral solar irradiance variation depends upon establishing a connection between the temporal variability of spatially resolved solar structures and spacecraft observations of irradiance. One difficulty in comparing models derived from different data sets is that the many ways for identifying solar features such as faculae, sunspots, quiet Sun, and various types of “network” are not necessarily consistent. To learn more about classification differences and how they affect irradiance models, feature “masks” are compared as derived from five current methods: multidimensional histogram analysis of NASA/National Solar Observatory/Kitt Peak spectromagnetograph data, statistical pattern recognition applied to SOHO/Michelson Doppler Imager photograms and magnetograms, threshold masks allowing for influence of spatial surroundings applied to NSO magnetograms, and “one-trigger” and “three-trigger” algorithms applied to California State University at Northridge Cartesian Full Disk Telescope intensity observations. In general all of the methods point to the same areas of the Sun for labeling sunspots and active-region faculae, and available time series of area measurements from the methods correlate well with each other and with solar irradiance. However, some methods include larger label sets, and there are important differences in detail, with measurements of sunspot area differing by as much as a factor of two. The methods differ substantially regarding inclusion of fine spatial scale in the feature definitions. The implications of these differences for modeling solar irradiance variation are discussed.

The National Solar Observatory/Kitt Peak synoptic rotation maps of the magnetic field and of the equivalent width of the He i 1083 nm line are used to identify and measure polar coronal holes from September 1989 to the present. This period covers the entire lifetime of the northern and southern polar holes present during cycles 22 and 23 and includes the disappearance of the previous southern polar coronal hole in 1990 and and formation of the new northern polar hole in 2001. From this sample of polar hole observations, we found that polar coronal holes evolve from high-latitude (∼ 60° ) isolated holes. The isolated pre-polar holes form in the follower of the remnants of old active region fields just before the polar magnetic fields complete their reversal during the maximum phase of a cycle, and expand to cover the poles within 3 solar rotations after the reversal of the polar fields. During the initial 1.2–1.4 years, the polar holes are asymmetric about the pole and frequently have lobes extending into the active region latitudes. During this period, the area and magnetic flux of the polar holes increase rapidly. The surface areas, and in one case the net magnetic flux, reach an initial brief maximum within a few months. Following this initial phase, the areas (and in one case magnetic flux) decrease and then increase more slowly reaching their maxima during the cycle minimum. Over much of the lifetime of the measured polar holes, the area of the southern polar hole was smaller than the northern hole and had a significantly higher magnetic flux density. Both polar holes had essentially the same amount of magnetic flux at the time of cycle minimum. The decline in area and magnetic flux begins with the first new cycle regions with the holes disappearing about 1.1–1.8 years before the polar fields complete their reversal. The lifetime of the two polar coronal holes observed in their entirety during cycles 22 and 23 was 8.7 years for the northern polar hole and 8.3 years for the southern polar hole.

The first passage of the Ulysses spacecraft to high latitudes was during the declining phase of solar cycle 22. The recent second passage to high latitudes was close to the maximum of solar cycle 23. The axis of the dipolar component of the Sun's magnetic field was close to 90° from the spin axis, and the coronal neutral line extended up to high latitudes. A variable but generally slow solar wind was observed all the way up to the highest latitudes reached by Ulysses, as was the sector structure of the magnetic field. The high-latitude heliosphere was populated with intensities of energetic particles with energies around 1 MeV several orders of magnitude above background. We show how the changes in the Sun's magnetic field, the coronal holes, and the configuration of the heliosphere could be responsible for the differences between particle observations in the two orbits.

We discuss some implications of the finite lifetime of a short-lived near-equatorial coronal hole for the structure of a corresponding transitory corotating stream and magnetic field in the heliosphere. The discussion is based on the illustrative example of a corotating stream that was observed by the Wind spacecraft in the ecliptic at 1 AU in early 1998 and the associated solar observations. The transitory corotating stream was associated with a near-equatorial coronal hole that formed, persisted for at least two solar rotations, and disappeared, all in response to the magnetic flux in evolving active regions. The corotating stream recurred for two solar rotations, and its emission time, based on the coronal hole observations, was ≈66 ± 6 days. The intersection of the resulting corotating stream with the ecliptic plane has the form of a spiral-shaped object with a radial extent on the order of 19 ± 2 AU. The magnetic field configurations in transitory corotating streams are not known. One possibility is that the transitory corotating stream originates from a localized near-equatorial coronal hole, possibly a member of a pair of coronal holes, joined by a flux tube, which evolve together. Such a transitory corotating stream could carry the flux tube through the corona and heliosphere. Another possibility is that the transitory corotating stream is the result of the latitudinal motion and deformation of the boundary of a polar coronal hole and the associated redistribution of a constant amount of open magnetic flux. Models and further observations of transitory corotating streams and their magnetic fields are needed in order to better understand them.

We describe the evolution of a solar filament channel marked by extremes: a length near one solar radius, and a duration of a year. Its genesis centers on an episode of flux emergence so powerful that it launched a surge of photospheric magnetic flux almost to the northern polar cap. This extraordinary injection of new flux at the solar surface occurred in midterm of the longest lived activity complex of cycle 21 (~20 rotations). The new flux emerged just north of the equator as a pair of adjacent activity complexes—a "supercluster" of sunspots—remote from other active regions in a longitude band spanning ~90°. Channels quickly formed along separate polarity inversion lines in this large-scale quadrupolar configuration. None of the initial channels survived more than two solar rotations; none merged to form a greater whole. As individual bipoles within and between the activity complexes expanded, fragmented, and cancelled, only flux at the outermost edges of the adjacent complexes survived, thanks to the remoteness of other strong concentrations of magnetic flux. The result, after three solar rotations, was a simplified bipolar pattern of poleward-streaming flux subject to global processes of flux transport that sustained and extended it for up to a year. The long and long-lived filament channel formed in the shape of a "switchback" along the polarity inversion between the converging streams of opposite polarity flux, continuing along the polarity inversion between the migrating flux and the flux in the polar cap. Our observations reveal large-scale swirled patterns of chromospheric fibrils from which we infer that substantial negative helicity was built up across both adjacent activity complexes during their emergence. The patterns were still detectable in the migrating flux after the source regions had disappeared. Convergence of opposite polarity fluxes with negative helicity leads naturally to dextral filaments and filament channels, consistent with the chirality rule for the northern hemisphere found by Martin, Bilimoria, & Tracadas. We measured the chiralities of 10 filament channels associated with the initial massive emergence of magnetic flux and its subsequent surge poleward. Implications of our findings on models for forming filaments and filament channels are discussed.

We present observations of a long-lived solar filament cavity with soft X-ray sources along its axis. This structure appeared above the southern polar crown polarity-inversion line for approximately three rotations during 1997 June-August, centered at a west-limb passage on approximately July 3. At the limb, the Yohkoh soft X-ray data showed a bright region situated above and around the projected filament location but near the axis of the cavity. We describe measurements of the geometry of the cavity, which we interpret as a flux rope that is partially embedded in the photosphere, and use the Yohkoh data to describe the physical parameters of the structure. We find that the core consists of an unresolved mass of filamentary substructures, with a volume filling factor significantly less than unity for the soft X-ray telescope (SXT) resolution. The core has a higher temperature than the cavity surrounding it, ruling out explanations in terms of a transition region supported by thermal conduction. Transient activity occurred in the polar crown region, but no detectable destabilization or eruption of the cavity structure resulted from it. We suggest that the bright structure at the core of the cavity corresponds to higher altitude coronal segments of the field lines that support the filament material.

Data obtained by instruments on the Ulysses spacecraft during its rapid sweep through >90° of solar latitude, crossing the solar equator in early 1995, were combined with data obtained near Earth by the Wind spacecraft to study the spatial structure of the solar wind and to compare to different models of the interplanetary magnetic field derived from solar observations. Several different source-surface models matched the double sinusoidal structure of the heliospheric current sheet (HCS) but with differences in latitude as great as 21°. The source-surface model that included an interplanetary current sheet gave poorer agreement with observed current-sheet crossings during this period than did the other source-surface models or an MHD model. The differences between the calculated and observed locations of the HCS were minimized when 22° of solar rotation was added to the constant-velocity travel time from the source surface to the spacecraft. The photospheric footpoints of the open field lines calculated from the models generally agreed with observations in the He 10,830 A line of the locations of coronal holes with the exceptions that (1) in some places, open field lines originated outside the coronal hole boundaries and (2) the models show apparently closed-field regions just inside some coronal hole boundaries. The patterns of mismatches between coronal hole boundaries and the envelopes of open field lines persisted over at least three solar rotations. The highest-speed wind came from the polar coronal holes, with the wind originating deeper within the hole being faster than the wind coming from near the hole boundary. Intermediate and slow streams originated in smaller coronal holes at low latitudes and from open field regions just outside coronal hole boundaries. Although the HCS threaded regions of low speed, low helium abundance, high ionization temperature, and a high ratio of magnesium to oxygen densities (a surplus of an element with low first-ionization potential), there was a great deal of variation in these parameters from one place to another along the HCS. The gradient of speed with latitude varied from 14 to 28 km s−1 deg−1.

From a list of X-ray jets made by Shimojo et al. (1996), we selected events for which there were magnetic field data from NSO/Kitt Peak. Using co-aligned SXT and magnetograms, we examined the magnetic field properties of X-ray jets. We found that 8% of the jets studied occurred at a single pole (SP), 12% at a bipole (BP), 24% in a mixed polarity (MP) and 48% in a satellite polarity (ST). If the satellite polarity region is the same as the mixed polarity region, 72% of the jets occurred at the (general) mixed polarity region.

During the past five years at least six manifestations of a global organization of solar magnetic fields have been recognized. The magnetic chirality (handedness) of the following features shows a hemispheric preference: filament channels, quiescent filaments, sunspot whorls, superpenumbral fibrils, coronal arcades, and interplanetary clouds associated with CMEs. Although the patterns are clear in the data, their interpretation and their possible connection to the dynamo is open to question. This paper reviews the observations of the patterns, corrects some misinterpretations, and offers a scenario for the origin of the most marked pattern, the chirality of filaments. We suggest the pattern arises from the reconnection of coronal loops, under the influence of supergranulation and differential rotation. Unlike alternative scenarios, ours relies only on observable surface motions and fields.

The ensemble of bipolar regions and the magnetic network both contain a substantial and strongly variable part of the photospheric magnetic flux at any phase in the solar cycle. The time-dependent distribution of the magnetic flux over and within these components reflects the action of the dynamo operating in the solar interior. We perform a quantitative comparison of the flux emerging in the ensemble of magnetic bipoles with the observed flux content of the solar photosphere. We discuss the photospheric flux budget in terms of flux appearance and disappearance, and argue that a nonlinear dependence exists between the flux present in the photosphere and the rate of flux appearance and disappearance. In this context, we discuss the problem of making quantitative statements about dynamos in cool stars other than the Sun.

Patterns in the properties of bipolar active regions are determined throughout Cycle 21. Active regions that emerged on the visible hemisphere were identified on NSO/KP full-disk magnetograms during 29 solar rotations selected from 1975 through 1986. The bipolar active regions are included only once in this sampling; their properties are derived at the time of maximum development. In order to study an unbiased sample over the entire range of areas larger than 2.5 square degrees (or 373 Mm2), their counts are corrected for size-dependent effects that reduce the chance of their identifications. The size distribution of bipolar active regions is a well-defined function that decreases with increasing size. Except for the smallest regions, the shape of the size distribution is independent of the phase of a cycle, only the scaling factor varies. The shape of the size distribution function for bipoles emerging within existing sunspot regions is virtually the same as that for bipoles emerging outside existing regions. Over the cycle, at least 44% of the regions larger than 3.5 square degrees emerge within existing sunspot regions. Hence, the rate at which new regions emerge is much higher within the boundaries of existing sunspot regions than it is in the activity belts outside existing regions. For regions emerging outside of existing sunspot regions, this rate increases by a factor of 3.5 from cycle minimum to maximum, while for new bipoles within existing active regions, the emergence rate varies with a significantly lower magnitude. Through the cycle, for regions in all size bins, the emergence frequencies appear to vary in phase. The frequencies increase by a factor of more than 8 from minimum to maximum for regions larger than 3.5 square degrees, but no more than 4.7 for the smaller regions. Short-term variations in the emergence frequency of regions do not necessarily occur simultaneously for regions of all sizes, implying that the size distribution is variable on time scales of less than six rotations.

X-ray imaging from Skylab and various sounding rockets has established the existence of and begun the characterization of coronal X-ray bright points (XBPs). With the launch of Yohkoh, the Soft X-ray Telescope (SXT) provides a new opportunity to observe these small-scale structures with higher temporal resolution, improved dynamic range, and greater sensitivity. We present the results from the analysis of SXT full-disk images showing the location and detailed evolution of XBPs. We derive correlations of XBPs with magnetic bipoles, He I 10830-A dark points, and other coronal features. From the evolution of 518 XBPs presented in this initial study, we derive a mean lifetime of about 12 hours, with some XBPs lasting as long as 5 days and others less than 10 minutes. A comprehensive study of the relationship between XBP lifetime, X-ray output, size, location, and variability may well lead to a re-evaluation of the nature and definition of XBPs.

Magnetic fields play a crucial role in heating the outer atmospheres of the Sun and Sun-like stars, but the mechanisms by which magnetic energy in the photosphere is converted to thermal energy in the corona remain unclear. Observations show that magnetic fields emerge onto the solar surface as bipolar regions with a broad range of length scales. On large scales, the bipolar regions survive for months before dispersing diffusively1,2,3. On the smaller scales, individual bipolar regions disappear within days but are continuously replenished by new small flux concentrations, resulting in a sustained state of mixed polarity4. Here we determine the rate of emergence of these small bipolar regions and we argue that the frequent magnetic reconnections associated with these regions (an unavoidable consequence of continued flux replacement) will heat the solar atmosphere. The model that describes the details of these mixed-polarity regions4 is complementary to the traditional diffusion model for large-scale flux dispersal1,2,3 and a combination of the two should lead to a more complete understanding of the role of magnetic fields in stellar atmospheres.

Soft X‐ray images of the solar corona sometimes show narrow dark features not obviously present in HE I 10830A images. We term these “narrow coronal holes” (NCHs). A prototype for this type of structure crossed solar central meridian on October 29, 2001. Standard source‐surface models showed open magnetic field lines in this feature, tending to confirm its identification as a coronal hole. The magnetic field in this example is relatively strong (above 100 G in the low‐resolution Kitt Peak magnetograms), and the boundaries of the open‐field domain fall within the unipolar area as expected. We have surveyed the Yohkoh SXT data for other examples of this phenomenon, and have found several candidates. From observations of the associated solar wind, and from modeling, we find these regions to be sources of slow solar wind.

The Solar Ultraviolet Radiation and Correlative Emissions (SOURCE) mission is intended to advance our ability to specify the spectral irradiance of the Sun in the extreme ultraviolet (EUV) wavelength range through simultaneous, radiometrically accurate measurements of the solar EUV spectral irradiance and measurements, including EUV and visible images, of solar parameters that are correlated with the EUV flux. The data will be used in combination with empirical modelling to develop and validate a more accurate system of proxy, or surrogate, indices for the solar EUV flux.

Simultaneous measurements of the magnetic fields in the photosphere and chromosphere were used to investigate if magnetic flux is submerging at sites between adjacent opposite polarity magnetic network elements in which the flux is observed to decrease or ‘cancel’. These data were compared with chromospheric and coronal intensity images to establish the timing of the emission structures associated with these magnetic structures as a function of height. We found that most of the cancelation sites show either that the bipole is observed longer in the photosphere than in the chromosphere and corona (44%) or that the timing difference of the disappearance of the bipole between these levels of the atmosphere is unresolved. The magnetic axis lengths of the structures associated with the cancelation sites are on average slightly smaller in the chromosphere than the photosphere. These observations suggest that magnetic flux is retracting below the surface for most, if not all, of the cancelation sites studied.

We have examined full-disk magnetograms and He I 1083 nm images from the National Solar Observatory/Kitt Peak to follow the development of coronal holes associated with a major active-region complex that emerged in 1996. This complex involved the emergence of six major active regions, the largest of which (NOAA 7978) had sunspot area exceeding 400 millionths. One additional region, important to the development of the coronal holes, had new-cycle magnetic polarity and emerged north of this complex. We follow the development of numerous new connections among these active regions and the polar coronal holes.

Two extremely similar interplanetary forward/reverse shock events, with bidirectional electron streaming, were detected by Ulysses in 1994 [Gosling et al., 1994]. Both events resulted in geomagnetic storms and presumably were associated with coronal mass ejections. In this paper we use the Yohkoh soft X-ray observations to characterize the conditions in the lower corona at the times appropriate for the launching of these two events. We find two strikingly different solar events to be the likeliest candidates: an LDE flare on 20 Feb. 1994, and a extremely large-scale arcade event on 14 April 1994.

We report results from an extension of a previous investigation of the magnetic roots of high-arching bright coronal loops (Porter et al 1994, in Proceedings of Kofu Symposium “New Look at the Sun”, ed. S. Enome, NRO Report No. 360, p. 65). In the previous work, the magnetic locations and magnetic structure of the brightest coronal features in a selected active region were determined by registering Yohkoh SXT images with a MSFC vector magnetogram via registration of the sunspots. The active region (AR 6982 on 26 Dec 91) was selected for study because it had a large delta sunspot with a core of strong magnetic shear along the polarity inversion; it was expected that such extemely nonpotential magnetic fields would foster exceptionally strong coronal heating and hence be exceptionally bright in coronal images. It was found that the coronal heating in this active region indeed was markedly more intense in the low-lying sheared core field than in the bulk of the field that arched over the sheared core and spanned the whole bipolar region. In addition, the coronal images showed something that was not anticipated in the selection of this region: a section of the high-arching envelope field was much brighter than the rest, showing that it received much more coronal heating than the rest of the envelope field. These enhanced high coronal loops stemmed from around an embedded island of opposite-polarity flux that was the site of microflaring and enhanced coronal heating. It was therefore surmised that the high bright loops somehow received their enhanced coronal heating from this foot-point activity. In the present work, by registering a full-disk Kitt Peak magnetogram with full-disk Yohkoh SXT images, we have found many more examples of large enhanced coronal loops having one foot rooted at a site of mixed polarity within an active region.

The Ulysses polar passages are producing a unique set of observations of solar-wind disturbances at high heliographic latitudes. In this paper we use the Yohkoh soft X-ray telescope (SXT) to locate some of these events, as defined by the Ulysses/SWICS data, in the solar corona. Of 8 events, we identify two with flares, three with front-side large arcade events, two with far-side events, and one was not seen in the Ulysses data. The arcade events generally resemble long-duration flares seen in active regions, but are larger, slower, and cooler. We present Yohkoh images of each of these events. In the large arcade events (see Alexander et al., 1996, for a detailed look at one of them) the magnetic morphology at the location of the Yohkoh arcade is generally consistent with the development of a large system of loops. Some of the identifications are ambiguous, and we summarize the reasons for this. From the SWICS data we have obtained ionization temperatures for several events, and find that they have no obvious pattern in relation to the X-ray temperatures; this may be expected on the basis that the interplanetary plasma cloud is physically distinct from the plasma trapped in the corona. Soft X-ray observations of the solar corona show occasional occurrences of large-scale brightenings in the form of arcades of loops. Such structures have been known since Skylab (e.g., Sturrock, 1980), and have a clear relationship with coronal mass ejections (e.g., Kahler, 1977). We now may study this phenomenon statistically with the much more comprehensive Yohkoh observations; with Yohkoh movies we can also begin to extend our knowledge to the three-dimensional development of the structures. At the same time Ulysses has sampled the latitude dependence of the interplanetary effects. With this paper we introduce this subject and provide a preliminary listing of events from the passage of Ulysses through high heliographic latitudes. The starting point of the present survey is a list of interplanetary plasma clouds (IPC’s) derived from Ulysses/SWICS data. These are essentially the same as the events termed CMEs by Gosling et al. (1994a, 1994b). For this identification the presence of bidirectional streaming in the suprathermal electron distribution is one of the main criteria. We note that there are no direct coronagraph observations, however. The Yohkoh observations were examined at the apparent time of origin of each Ulysses event, resulting in some clear and some less-certain identifications. We also studied the ionization temperatures of the IPC material as a beginning step to give the identifications a physical basis. There has been little study thus far of the Yohkoh soft X-ray observations in relationship to CMEs, which we believe to be closely related to the interplanetary disturbances. Hiei et al. (1993) reported the only Yohkoh event yet studied in conjunction with white-light coronagraph observations. However Klimchuk et al. (1994) showed that X-ray eruptive phenomena with parameters similar to those of CMEs occur frequently at the limb, and there have been several studies of individual eruptive events (e.g., Watanabe et al., 1992). Presently there is no systematic knowledge of the X-ray coronal counterparts of CMEs, and the survey represented here is part of the effort to rectify this situation.

With soft X-ray imaging we can study the entire coronal volume, except for cold inclusions such as prominences, as a function of time. This should allow us to observe the origins of coronal mass ejections. We report here an initial survey of the Yohkoh/SXT observations at the times of reported or apparent mass ejections: three LDE flare events and two large-scale arcade formations. For each of the events we can easily detect sudden coronal dimming, which we interpret as the launch interval of a CME. In one of the flare events we have found a well-defined plasma cloud, apparently formed from a set of loop structures, which rises and disappears during the growth phase of the flare emission. Its mass amounted to some 4×1014 g with a density of 3×108 cm−3 and a temperature of 2.8 MK before its disappearance.

Studies of the solar cycle have revealed that the size distribution of active regions does not vary with the solar cycle. Size, rate of rise and lifetime of active regions are roughly proportional although a wide range of variation exists among these parameters. The polar field typically reverses about 2 years after solar maximum. The new solar cycle does not seem to start until after the reversal of the sign of the magnetic poles. However, the new solar cycle does overlap appreciably with the previous cycle and begins 3 or more years prior to the minimum in sunspot producing active regions. The solar cycle begins with ephemeral regions at high latitudes. From the analysis of active region and ephemeral region data over more than a whole solar cycle it is concluded that ephemeral regions are in all respects the small-scale end of the distribution of active regions. No rationale was found for excluding ephemeral regions as one of the effects of the solar dynamo. The search for the early ephemeral regions associated with solar cycle 23 at high latitudes yielded the tentative conclusion that it was detected during our observing runs during the spring of 1993 although confirming data is needed. Solar cycle, Small-scale magnetic fields, Large-scale velocity fields, Polarity inversion zones, Filaments.

X-ray-bright point (XBPs) are known to show variability on a number of timescales, including impulsive X-ray brightenings. The relationship between these XBP 'flares' and normal solar flares is poorly known. A fundamental question is whether nonthermal acceleration of particles takes place in XBP flares. We address this issue by searching for nonthermal radio emission at metric wavelengths from flaring XBPs identified in Yohkoh soft x-ray telescope (SXT) data. Unequivocal evidence for type III-like radio bursts, usually attributed to beams of nonthermal electrons on open field lines, is found. This suggests that XBP flares are similar to normal flares and can indeed accelerate nonthermal populations of energetic particles.

We investigate a nest of sunspots in which three ordinary bipolar pairs of sunspots are aligned collinearly. The usual spreading action of the growing regions brings two spots of leading polarity together (p-p collision) and forces the leading and trailing spots of the two interior regions to overlap inot a single penumbra (p-f collision), thus forming a delta-spot. We examine digitally processed images from the Ottawa River Solar Observatory of two related events inside the delta-spot 5 days after the p-f collision begins: the violent disruption of the f-umbra, and the formation in less than a day of an hydrogen-alpha filament. The evolutionary changes in shape, area, relative motions, and brightness that we measure for each spot in the elongated nest are more compatible with Parker's (1979a) hypothesis of a sunspot as a cluster of flux tubes held together by downdrafts than with the notion of a sunspot as a monolithic plug of magnetic flux. From chromospheric developments over the delta-spot, we show that a shearing motion along a polarity inversion is more effective than convergence for creating a chromospheric filament. We invoke the release of an instability, triggered by a sequence of processes lasting 1 day or more, to explain the disruption of the f-umbra in this delta-spot. We show that the sequence is initiated when the colliding p-f umbrae reach a critical separation around 3200 +/- 200 km. We present a descriptive model in which the reconnected magnetic fields block vertical transport of convective heat flux just beneath the photosphere. We observe the formation of an unusual type of penumbra adjacent to the f-polarity portion of this delta-spot just before its disruption. A tangential penumbral band grows out of disordered matter connected to the f-umbra. We present this as evidence for the extrusion of umbral magnetic flux by thermal plumes rising through a loosely bound umbra.

The evolution of several observed solar active regions is simulated by solving a transport equation for magnetic flux at the photosphere. The rates of rotation, meridional flow, and diffusion of the flux are determined self-consistently in the calculations. The findings are in good quantitative agreement with previous measures of the rotation rate and diffusion constant associated with photospheric magnetic fields. Although the meridional velocities are consistent in direction and magnitude with recently reported poleward flows, relatively large uncertainties in the velocity determinations make this result inconclusive.

We have studied the relation of a flare-induced wave and the type II and III radio bursts associated with the 26 April 1969, 2258 UT flare. Our observations suggest the flare-wave and type II bursts were produced by a common source.

Photospheric magnetic fluxes and average field strengths have been measured beneath 33 coronal holes observed on 63 occasions during 1975–1980. The principal result is that low-latitude holes contained 3 times more flux near sunspot maximum than near minimum despite the fact that their sizes were essentially the same. Average magnetic field strengths ranged from 3–36 G near sunspot maximum compared to 1–7 G near minimum. Evidently the low-latitude coronal holes received a proportion of the extra flux that was available at low latitudes near sunspot maximum.

Ephemeral active regions (ER) identified on Kitt Peak daily full-disk magnetograms from April through November 1975 were analyzed and compared with larger active regions during the same interval. The 1975 ER were also compared with ER data from 1970, 1973, 1976, and 1977. ER were found to vary approximately with the sunspot cycle. However, a minimum in the number of ER occurred at least one year prior to sunspot minimum. All evidence to date indicates that the early ER minimum was due to the rise of solar cycle 21 primarily in the form of ER. ER were statistically identified as belonging to both outgoing solar cycle 20 and incoming cycle 21 by maxima in their distribution in latitude and by their statistically dominant orientation as a function of latitude. From the identification of ER with specific solar cycles and the persistent presence of high latitude ER maxima since 1970, it is suggested that the outgoing and incoming solar cycles may co-exist on the sun longer than the 0–3 year period of overlap between successive cycles already known from the properties of large sunspot-producing active regions.

Using near-simultaneous full disk solar X-ray images and Hei λ10830 spectroheliograms from three rocket flights, we compare dark points identified on the Hei maps with X-ray bright points identified on the X-ray images. We find that for the largest and most obvious features there is a strong correlation: most Hei dark points correspond to X-ray bright points. However, about two-thirds of the X-ray bright points were not identified on the basis of the helium data alone. Once an X-ray feature is identified it is almost always possible to find an underlying dark patch of enhanced Hei absorption which, however, would not a priori have been selected as a dark point. Therefore, the Hei dark points, using current selection criteria, cannot be used as a one-to-one proxy for the X-ray data. Hei dark points do, however, identify the locations of the stronger X-ray bright points.

A time sequence of magnetograms and velocity-grams in the Hα and Fe i 6569 A lines has been made at a rate of 12 h−1 of McMath Region 10385 from 26 to 29 October, 1969. The 14 flares observed during this period have been studied in relation to the configuration and changes in the magnetic and velocity fields. There was little correlation between flare position and the evolutionary changes in the photospheric magnetic and velocity field, except at large central meridian distances where the velocity observations suggested shearing taking place at flare locations. At central meridian distances > 30° we found that flares are located in areas of low line-of-sight photospheric velocity surrounded by higher velocity hills. The one exception to this was the only flare which produced a surge. Blue-shifted velocity changes in the photosphere of 0.3 to 1 km s−1 were observed in localized areas at the times of 8 of 14 flares studied.

A time sequence of longitudinal magnetograms of two active regions, McMath Regions 9281 and 9760, have indicated magnetic field changes occurring in localized areas with time scales of the order of hours. We believe the observed field changes are evolutionary in nature, rather than related to the occurrence of small flares. Three examples of evolutionary magnetic changes are discussed.

Ephemeral active regions attain maximum development within 1 day or less of their initial appearance and are typically observed for 1–2 days. They appear mostly as small bipolar regions having a typical dimension of about 30000 km and a maximum total flux of the order of 1020 Mx. The ephemeral regions generally do not produce sunspots and flares, though they are identified in Hα as small active centers.

The explosive phase of a flare can be defined by a simple photometric measurement of Hα film records of the flare development. Using the quantitative definition, improved correlations are found between the start of the explosive phase and the start of 10.7 cm radio bursts and Sudden Frequency Deviations compared to earlier correlations of the same data using visual estimates of the start of the explosive phase. Explosive development may be confined to only part of a flare.

The properties of small (< 2″) moving magnetic features near certain sunspots are studied with several time series of longitudinal magnetograms and Hα filtergrams. We find that the moving magnetic features: A model to help understand the observations is proposed.

The solar cycle has been defined in terms of a sequential periodic variation in sunspot numbers, the period being the interval between successive minima, currently averaging 11.2 years. But a number of observations have indicated that the activity cycle may begin at higher latitudes before the emergence of the first sunspots of the new cycle. Here we report results from sunspot cycle 21 concerning the ephemeral active regions, the coronal green-line emission and the torsional oscillation signal, which confirm the earlier suggestions. In particular, we report the appearance of a high-latitude population of ephemeral active regions in the declin-ing phase of sunspot cycle 21, with orientations that tend to favour those for cycle 22 rather than 21. Taken together, these data indicate that sunspot activity is simply the main phase of a more extended cycle that begins at high latitudes before the maximum of a given sunspot cycle and progresses towards the equator during the next 18–22 yr, merging with the conventional 'butterfly diagram' (the plot of the latitudes of emerging sunspots against time) as it enters sunspot latitudes. We suggest that this extended cycle may be understood in the perspective of a model of giant convective rolls that generate dynamo waves propagating from pole to equator.

The photospheric line-of-sight velocity field of 24 active regions was examined in relation to the flare activity of the regions. There is a good correlation between the complexity of the velocity field and the level of flare activity in the active region within plus or minus 12 hours. Specific velocity patterns could be recognized at 55 of 73 flare sites. Failure to detect patterns at all but one of the remaining flare sites can be understood as due to unfavorable geometry for the detection of the patterns. The specific velocity patterns found at flare sites consist of anomalous Evershed flow patterns in sunspot penumbras (12 of 55 sites), small-scale, mulitpolar velocity structures (15 of 55 sites), and apparent horizontal shears of the velocity field (48 of 55 sites). The shears are frequently but not necessarily associated with magnetic polarity reversal lines. There appears to be a better relation between flare activity and the spatial extent of a velocity shear line than with the strength of the shear. As an aid to predicting the level of flare activity, the velocity complexity of an active region appears to be a practical and valuable parameter.

A computational model, based on diffusion, differential rotation, and meridional flow, has been developed to simulate the transport of magnetic flux on the Sun. Using Kitt Peak magnetograms as input, as have determined a best-fit diffusion constant by comparing the computed and observed fields at later times. This paper presents the initial results of a project to simulate the transport of solar magnetic flux using diffusion, differential rotation, and meridional flow. The study concerns the evolution of large-scale fields on a time scale of weeks of years, and ignores the rapid changes that accompany the emergence of new magnetic regions and the day-to-day changes of the supergranular network itself.

This paper presents some of the results that have been obtained from the Kitt Peak observations of coronal holes and the NRL observations of coronal transients during the recent years near sunspot maximum (1979–1981). On the average, low-latitude coronal holes of comparable size contained 3 times more flux near sunspot maximum than near the previous minimum. In the outer corona, transients occurred at the observed rate of at least 2 per day, and quiet conditions persisted during less than 15% of the observed days. We describe a sample of the more than 800 events that we have observed so far, including the observation of a comet apparently colliding with the Sun.

A study of ephemeral active regions (ER) identified on good quality full-disk magnetograms reveals: A comparison of parameters of ER and regular active regions suggests that ER are the small-scale end of a broad spectrum of active regions. The role of ER in the light of present theories of solar activity is investigated but is not yet clear. Heating of the chromosphere and corona may be significantly affected by ER.

General properties of ephemeral active regions were studied using Kitt Peak daily magnetograms from Apr.-Nov. 1975. Although this interval was prior to sunspot minimum, ephemeral regions related to incoming cycle 21 were already more numerous than ephemeral regions related to outgoing cycle 20. The transition between the old and new solar cycle was identified by a reversal of the statistically dominant orientation of regions and sometimes by a minimum in the latitude distribution where adjacent cycles overlapped. During this interval the transition between cycle 20 and 21 was at N18 deg and S24 deg. Comparing this 1975 data with previously studied data from 1970 and 1973, we find evidence that incoming cycle 21 was already present on the sun at middle and high latitudes in 1973 and 1970. Extrapolating backward and forward in time from these three periods, we find that it is conceivable that two solar cycles may be present on the sun at all times. It appears that further statistical studies of ephemeral active regions may yield long-term prognostic information on the future course of solar activity. The birth of 90 ephemeral regions was recorded. Prior to the birth of regions, existing network was found to disappear or show lateral displacement. The growth and decay rates of regions were comparable. In the decaying phase, some flux elements simply disappeared; some merged with network or other elements of regions of similar polarity; some collided and simultaneously disappeared with network or elements of other regions of opposite polarity. All clearly identifiable ephemeral regions dissipated by these processes while continuing to expand.

A time sequence of longitudinal magnetograms of two active regions, McMath Regions 9281 and 9760, have indicated magnetic field changes occurring in localized areas with time scales of the order of hours. We believe the observed field changes are evolutionary in nature, rather than related to the occurrence of small flares. Three examples of evolutionary magnetic changes are discussed.

Localized brightenings are found throughout the magnetic network in quiet sun image sequences obtained in the C IV 1548 A line by the SMM satellite's UV spectrometer and polarimeter. Some bright sites are short-lived, while others persist. Plots of the intensity fluctuations show that the enhancements at both short- and long-lived sites are the result of localized impulsive heating events that occur intermittently at the short-lived sites and in more rapid succession at the long-lived ones. The number of these events and their visibility in the wings of the C IV line are consistent with their identification as the explosive events seen in UV spectra.

Synoptic maps for the 1975-1984 period are used to determine the time-dependent distribution function of magnetic flux densities in the solar atmosphere. The distribution function depends only on the global level of magnetic activity, and it is used to study how relations between magnetic flux densities and radiative flux densities from different temperature regimes in the outer atmosphere (derived from spatially resolved solar observations) transform into relations between surface-averaged flux densities. It is found that the transformation to surface-averaged fluxes preserves the power-law character of relations between radiative and magnetic flux densities for spatially resolved data.

We have solved numerically a transport equation which describes the evolution of the large-scale magnetic field of the Sun. Data derived from solar magnetic observations are used to initialize the computations and to account for the emergence of new magnetic flux during the sunspot cycle. Our objective is to assess the ability of the model to reproduce the observed evolution of the field patterns. We discuss recent results from simulations of individual active regions over a few solar rotations and of the magnetic field of the Sun over sunspot cycle 21.