N. R. Sheeley

Modeling the Sun’s Magnetic Field and Irradiance since 1713

We use a flux transport model to simulate the evolution of the Suns total and open magnetic flux over the last 26 solar cycles (1713-1996). Polar field reversals are maintained by varying the meridional flow speed between 11 and 20 m s-1, with the poleward-directed surface flow being slower during low-amplitude cycles. If the strengths of the active regions are fixed but their numbers are taken to be proportional to the cycle amplitude, the open flux is found to scale approximately as the square root of the cycle amplitude. However, the scaling becomes linear if the number of active regions per cycle is fixed but their average strength is taken to be proportional to the cycle amplitude. Even with the inclusion of a secularly varying ephemeral region background, the increase in the total photospheric flux between the Maunder minimum and the end of solar cycle 21 is at most ~one-third of its minimum-to-maximum variation during the latter cycle. The simulations are compared with geomagnetic activity and cosmogenic isotope records and are used to derive a new reconstruction of total solar irradiance (TSI). The increase in cycle-averaged TSI since the Maunder minimum is estimated to be ~1 W m-2. Because the diffusive decay rate accelerates as the average spacing between active regions decreases, the photospheric magnetic flux and facular brightness grow more slowly than the sunspot number and TSI saturates during the highest amplitude cycles.

Properties of coronal mass ejections: SOHO LASCO observations from January 1996 to June 1998

We report the properties of all the 841 coronal mass ejections (CMEs) observed by the Solar and Heliospheric Observatory (SOHO) Large Angle Spectroscopic Coronagraph (LASCO) C2 and C3 white-light coronagraphs from January 1996 through June 1998, and we compare those properties to previous observations by other similar instruments. Both the CME rate and the distribution of apparent locations of CMEs varied during this period as expected based on previous solar cycles. The distribution of apparent speeds and the fraction of CMEs showing acceleration were also in agreement with earlier reports. The pointing stability provided by an L-1 orbit and the use of CCD detectors have resulted in superior brightness sensitivity for LASCO over earlier coronagraphs; however, we have not detected a significant population of fainter (i.e., low mass) CMEs. The general shape of the distribution of apparent sizes for LASCO CMEs is similar to those of earlier reports, but the average (median) apparent size of 72° (50°) is significantly larger. The larger average apparent size is predominantly the result of the detection of a population of partial and complete halo CMEs, at least some of which appear to be events with a significant longitudinal component directed along the Sun-Earth line, either toward or away from the Earth. Using full disk solar images obtained by the Extreme ultraviolet Imaging Telescope (EIT) on SOHO, we found that 40 out of 92 of these events might have been directed toward the Earth, and we compared the timing of those with the Kp geomagnetic storm index in the days following the CME. Although the “false alarm” rate was high, we found that 15 out of 21 (71%) of the Kp ≥ 6 storms could be accounted for as SOHO LASCO/EIT frontside halo CMEs. If we eliminate three Kp storms that occurred following LASCO/EIT data gaps, then the possible association rate was 15 out of 18 (83%).

Measurements of Flow Speeds in the Corona Between 2 and 30 R

Time-lapse sequences of white-light images, obtained during sunspot minimum conditions in 1996 by the Large Angle Spectrometric Coronagraph on the Solar and Heliospheric Observatory, give the impression of a continuous outflow of material in the streamer belt, as if we were observing Thomson scattering from inhomogeneities in the solar wind. Pursuing this idea, we have tracked the birth and outflow of 50-100 of the most prominent moving coronal features and find that: 1. They originate about 3-4 R☉ from Sun center as radially elongated structures above the cusps of helmet streamers. Their initial sizes are about 1 R☉ in the radial direction and 0.1 R☉ in the transverse direction. 2. They move radially outward, maintaining constant angular spans and increasing their lengths in rough accord with their speeds, which typically double from 150 km s-1 near 5 R☉ to 300 km s-1 near 25 R☉. 3. Their individual speed profiles v(r) cluster around a nearly parabolic path characterized by a constant acceleration of about 4 m s-2 through most of the 30 R☉ field of view. This profile is consistent with an isothermal solar wind expansion at a temperature of about 1.1 MK and a sonic point near 5 R☉. Based on their relatively small initial sizes, low intensities, radial motions, slow but increasing speeds, and location in the streamer belt, we conclude that these moving features are passively tracing the outflow of the slow solar wind.

Continuous tracking of coronal outflows: Two kinds of coronal mass ejections

We have developed a new technique for tracking white-light coronal intensity features and have used this technique to construct continuous height/time maps of coronal ejecta as they move outward through the 2–30 Rs field of view of the Large-Angle Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) spacecraft. Displayed as gray-scale images, these height/time maps provide continuous histories of the motions along selected radial paths in the corona and reveal a variety of accelerating and decelerating features, including two principal types of coronal mass ejections (CMEs): (1) Gradual CMEs, apparently formed when prominences and their cavities rise up from below coronal streamers: When seen broadside, these events acquire balloon-like shapes containing central cores, and their leading edges accelerate gradually to speeds in the range 400–600 km/s before leaving the 2–30 Rs field of view. The cores fall behind with speeds in the range 300–400 km/s. Seen along the line of sight, these events appear as smooth halos around the occulting disk, consistent with head-on views of optically thin bubbles stretched out from the Sun. At the relatively larger radial distances seen from this “head-on” perspective, gradually accelerating CMEs fade out sooner and seem to reach a constant speed more quickly than when seen broadside. Some suitably directed gradual CMEs are associated with interplanetary shocks and geomagnetic storms. (2) Impulsive CMEs, often associated with flares and Moreton waves on the visible disk: When seen broadside, these CMEs move uniformly across the 2–30 Rs field of view with speeds typically in excess of 750 km/s. At the relatively larger radial distances seen from a head-on perspective, impulsive events tend to have a more ragged structure than the gradual CMEs and show clear evidence of deceleration, sometimes reducing their speeds from 1000 to 500 km/s in 1 hour. Such decelerations are too large to represent ballistic motions in the Suns gravitational field but might be caused by shock waves, sweeping up material far from the Sun.

Coronal holes, solar wind streams, and recurrent geomagnetic disturbances: 1973–1976

Observations of coronal holes, solar wind streams, and geomagnetic disturbances during 1973–1976 are compared in a 27-day pictorial format which shows their long-term evolution. The results leave little doubt that coronal holes are related to the high-speed streams and their associated recurrent geomagnetic disturbances. In particular, these observations strongly support the hypothesis that coronal holes are the solar origin of the high-speed streams observed in the solar wind near the ecliptic plane.

Associations between coronal mass ejections and soft X-ray events

The association between white light coronal mass ejections (CMEs) and full-disk X ray events have been examined as a function of X ray duration during the recent years of high sunspot activity (1979-1981). On a time scale of hours, no duration interval has been found that separates X ray events into two distinct classes depending on whether or not they have associated CMEs. Rather, the tendency for long-duration X ray events to have associated CMEs reflects the fact that, as X ray duration increases, the differential distribution of events without CMEs falls off faster than the distriution of X ray events with CMEs.

ON THE WEAKENING OF THE POLAR MAGNETIC FIELDS DURING SOLAR CYCLE 23

The Suns polar fields are currently ~40% weaker than they were during the previous three sunspot minima. This weakening has been accompanied by a corresponding decrease in the interplanetary magnetic field (IMF) strength, by a ~20% shrinkage in the polar coronal-hole areas, and by a reduction in the solar-wind mass flux over the poles. It has also been reflected in coronal streamer structure and the heliospheric current sheet, which only showed the expected flattening into the equatorial plane after sunspot numbers fell to unusually low values in mid-2008. From latitude-time plots of the photospheric field, it has long been apparent that the polar fields are formed through the transport of trailing-polarity flux from the sunspot latitudes to the poles. To address the question of why the polar fields are now so weak, we simulate the evolution of the photospheric field and radial IMF strength from 1965 to the present, employing a surface transport model that includes the effects of active region emergence, differential rotation, supergranular convection, and a poleward bulk flow. We find that the observed evolution can be reproduced if the amplitude of the surface meridional flow is varied by as little as 15% (between 14.5 and 17 m s?1), with the higher average speeds being required during the long cycles 20 and 23.

Evolution of the sun's polar fields during sunspot cycle 21 - Poleward surges and long-term behavior

Longitudinally averaged observations of the photospheric field during 1976-1986 are analyzed using a flux transport model. The way in which source eruptions, supergranular diffusion, and meridional flow collaborate to produce strong, highly concentrated polar fields near sunspot minimum is clarified as follows: (1) widespread eruptions of individual bipolar magnetic regions, with their leading polarity flux equatorward of their trailing polarity flux, collectively establish a large-scale separation of polarities in latitude; (2) the low-latitude, leading polarity flux diffuses across the equator and merges with its opposite hemisphere counterpart; and (3) meridional flow carries the resulting surplus of trailing polarity flux to the poles, and concentrates it there against the spreading effect of diffusion. Episodic surges of flux to the poles are induced by fluctuations in the source eruption rate. Simulations indicate that relatively weak, trailing polarity surges may occur even in a steady flow field. However, in order to account for the giant surges of alternating polarity and the resulting oscillations in the polar field strength observed during 1980-1982, both accelerated flow and enhanced eruption rates are required. 24 refs.

The dynamical nature of coronal streamers

Recent high-sensitivity imaging of the Suns white-light corona from space has revealed a variety of unexpected small-scale phenomena, including plasma blobs that are ejected continually from the cusplike bases of streamers, fine raylike structures pervading the outer streamer belt, and inflows that occur mainly during times of high solar activity. These phenomena can be interpreted as different manifestations of magnetic field line reconnection, in which plasma and magnetic flux are exchanged between closed and open field regions of the corona. The observations provide new insights into a number of long-standing questions, including the origin of the streamer material in the outer corona, the sources of the slow solar wind, and the mechanisms that regulate the interplanetary magnetic field strength.

The long‐term variation of the Sun's open magnetic flux

The interplanetary magnetic field (IMF) has its origin in open magnetic regions of the Sun (coronal holes). The location of these regions and their total open flux Φ open can be inferred from current-free extrapolations of the observed photospheric field. We derive the long-term variation of Φ open during 1971-1998 and discuss its causes. Near sunspot minimum, the open flux originates mainly from the large polar coronal holes, whereas at sunspot maximum it is rooted in small, lower-latitude holes characterized by very high field strengths; the total amount of open flux thus remains roughly constant between sunspot minimum and maximum. Through most of the cycle, the variation of Φ open closely follows that of the Suns total dipole strength, showing much less dependence on the total photospheric flux or the sunspot number. However, episodic increases in large-scale sunspot activity lead to strengthenings of the equatorial dipole component, and hence to enhancements in Φ open and the IMF strength lasting typically ∼1 yr.