Giuliana de Toma

Diagnostics of Polar Field Reversal in Solar Cycle 23 Using a Flux Transport Dynamo Model

Motivated by observed anomalous features in cycle 23, as inferred from records of photospheric magnetic flux, we develop a flux transport dynamo-based scheme in order to investigate the physical cause of such anomalies. In this first study we focus on understanding anomalies occurring in the polar field evolutionary pattern in cycle 23, namely, why the polar reversal in cycle 23 was slow, why after reversal the buildup of the polar field was slow, and why the south pole reversed approximately a year after the north pole did. We construct a calibrated flux transport dynamo model that operates with dynamo ingredients such as differential rotation, meridional circulation, and large-scale poloidal field source derived from observations. A few other dynamo ingredients, such as diffusivity and quenching pattern, for which direct observations are not possible, are fixed by using theoretical guidance. By showing that this calibrated model can reproduce major longitude-averaged solar cycle features, we initialize the model at the beginning of cycle 22 and operate by incorporating the observed variations in meridional circulation and large-scale surface magnetic field sources to simulate the polar field evolution in cycle 23. We show that a 10%-20% weakening in photospheric magnetic flux in cycle 23 with respect to that in cycle 22 is the primary reason for a ~1 yr slowdown in polar reversal in cycle 23. Weakening in this flux is also the reason for slow buildup of polar field after reversal, whereas the observed north-south asymmetry in meridional circulation in the form of a larger decrease in flow speed in the northern hemisphere than that in the southern hemisphere during 1996-2002 and the appearance of a reverse, high-latitude flow cell in the northern hemisphere during 1998-2001 caused the north polar field to reverse before the south polar field.

SOLAR CYCLE 23: AN ANOMALOUS CYCLE?

The latest SOHO VIRGO total solar irradiance (TSI) time series is analyzed using new solar variability measures obtained from full-disk solar images made at the San Fernando Observatory and the Mg II 280 nm index. We discuss the importance of solar cycle 23 as a magnetically simpler cycle and a variant from recent cycles. Our results show the continuing improvement in TSI measurements and surrogates containing information necessary to account for irradiance variability. Use of the best surrogate for irradiance variability due to photospheric features (sunspots and faculae) and chromospheric features (plages and bright network) allows fitting the TSI record to within an rms difference of 130 ppm for the period 1986 to the present. Observations show that the strength of the TSI cycle did not change significantly despite the decrease in sunspot activity in cycle 23 relative to cycle 22. This points to the difficulty of modeling TSI back to times when only sunspot observations were available.

A PICTURE OF SOLAR MINIMUM AND THE ONSET OF SOLAR CYCLE 23. I. GLOBAL MAGNETIC FIELD EVOLUTION

NSO/Kitt Peak synoptic charts of magnetic flux in the period from 1996 to 1998 are analyzed together with time series for the 10.7 cm radio flux, sunspot number, and Mg II chromospheric index to determine the origin of the two times of minimum activity in 1996 and to study their relationships in the ascending phase of solar cycle 23. The solar activity minima in February-April and September-November of 1996 are found to correspond to periods of low magnetic activity in the southern and northern solar hemispheres, respectively. The new solar cycle becomes dominant in early 1997, but it is only in the summer of 1997 that a significant increase can be detected in the magnetic fields observations as well as in irradiance data, and by the end of 1998, the activity level has increased to a value comparable to the one observed in 1993. Using the magnetic synoptic charts, we determine the number of persistent longitude bands of active nests during this rising phase of solar cycle 23. We find an increase in the number of active nests from zero in 1996 to three by 1998. We speculate that these persistent bands of flux emergence correspond to a pattern of low-order modes of instability of the type found in the theoretical work of Gilman, Fox, and Dikpati on joint instability of latitudinal differential rotation and toroidal magnetic fields at the base of the convection zone. We argue that the observed increase in the number of active nests is consistent with the increase in the longitudinal wavenumber of magnetic instabilities in a concentrated toroidal field in the tachocline discussed in 1999 by Gilman and Dikpati.

Mg II core-to-wing index: Comparison of SBUV2 and SOLSTICE time series

The Mg II core-to-wing index is a ratio of the Mg II chromospheric emission at 280 nm to the photospheric radiation in the line wings and is used as an indicator of solar activity. Since October 1991, the Solar-Stellar Irradiance Comparison Experiment (SOLSTICE) has made daily irradiance measurements in the range 119–420 nm from the Upper Atmosphere Research Satellite (UARS). A new Mg II index, based on the SOLSTICE observations at a spectral resolution of 0.24 nm, is presented and compared to previous measurements. Spectral irradiance measurements of the Mg II doublet at low spectral resolution (≈ 1 nm) have been made by the Solar Backscatter UltraViolet (SBUV) instrument on Nimbus 7 since November 1978 and subsequently by the SBUV2 instruments on NOAA 9 and NOAA 11 satellites. We compare the SOLSTICE data with the Mg II time series derived from SBUV2 data by the groups at the National Oceanic and Atmospheric Administration (NOAA) and at the Goddard Space Flight Center (GSFC). SOLSTICE data are convolved to the lower SBUV2 resolution, and the NOAA and GSFC algorithms are then applied to this data set. The SOLSTICE Mg II indices constructed in this manner simulate the SBUV2 indices and can be used to validate the SBUV2 time series and identify data problems. From our analysis, we conclude that the NOAA Mg II time series is the most consistent during the period 1978–1994. The new GSFC Mg II time series has comparable accuracy for the period starting in 1989. We also derive the linear transformation equations required to put the high-and low-resolution time series onto common scales.

Impact of changes in the Sun's conveyor-belt on recent solar cycles

Plasma flowing poleward at the solar surface and returning equatorward near the base of the convection zone, called the meridional circulation, constitutes the Suns conveyor-belt. Just as the Earths great oceanic conveyor-belt carries thermal signatures that determine El Nino events, the Suns conveyor-belt determines timing, amplitude and shape of a solar cycle in flux-transport type dynamos. In cycle 23, the Suns surface poleward meridional flow extended all the way to the pole, while in cycle 22 it switched to equatorward near 60°. Simulations from a flux-transport dynamo model including these observed differences in meridional circulation show that the transport of dynamo-generated magnetic flux via the longer conveyor-belt, with slower return-flow in cycle 23 compared to that in cycle 22, may have caused the longer duration of cycle 23.

Differences in the Sun's radiative output in cycles 22 and 23

Analysis of the current solar cycle 23 shows a greater increase in total solar irradiance (TSI) for the early phase of this cycle than expected from measurements of the total magnetic flux and traditional solar activity indices, which indicate that cycle 23 is weaker than cycle 22. In contrast, space observations of TSI from the Solar and Heliospheric Observatory/VIRGO and the Upper Atmospheric Research Satellite/ACRIMII show an increase in TSI of about 0.8-1.0 W m-2 from solar minimum in 1996 to the end of 1999. This is comparable to the TSI increase measured by Nimbus 7/ERB from 1986 to 1989 during the previous cycle. Thus, solar radiative output near the maximum of the 11 yr cycle has been relatively constant despite a factor of 2 smaller amplitude increase for cycle 23 in sunspot and facular areas determined from ground-based observations. As a result, empirical models of TSI based on sunspot deficit and facular/network excess in cycle 22 underestimate the TSI measurements in 1999. This suggests either a problem in the observations or a change in the sources of radiative variability on the Sun.

Polar flux, cross-equatorial flux, and dynamo-generated tachocline toroidal flux as predictors of solar cycles

We evaluate the skill of three solar cycle predictors, namely, polar magnetic flux, flux crossing the equator, and tachocline toroidal flux, using both observations and a calibrated dynamo model. Polar flux measurements are available only for the past three sunspot cycles, implying poor statistics. However, the correlation between observed north and south polar flux peaks, and peaks of the next sunspot cycle is r = 0.785. We find that the correlation between the observed cross-equatorial flux and the observed peak of the next solar cycle is also high, close to that of Cameron & Schussler, and the statistics are more reliable. Thus, the cross-equatorial flux is a better predictor of the next cycle than is the polar flux. From the dynamo model, the correlations with observed cycle peaks for polar flux, cross-equatorial flux, and toroidal flux are 0.48, 0.76, and 0.96, respectively. All these correlations decline when the northern and southern hemispheres are simulated separately, as well as with shortening of the averaging length in input data. A very high correlation between the model polar flux at the end of a cycle and the observed peak of that cycle implies that, within a calibrated flux transport dynamo, the polar flux follows the sunspot cycle, rather than being a precursor to it. With short-term averaging of input data, the polar and cross-equatorial fluxes retain much more short-term variability than does the toroidal flux. This is because the long traversal time of the input poloidal fields to the bottom of the convection zone in a mean-field model smooths out the short-term variability in the toroidal flux. The observed slowdown in meridional flow during 1996-2004 leads to a weaker polar flux, but a stronger cross-equatorial flux compared to the case with steady meridional flow. We infer that it is unlikely that both the cross-equatorial and the polar fluxes can be good predictors of solar cycle peaks.

Effect of Spectral Resolution on the Mg II Index as a Measure of Solar Variability

The solar Mgii core-to-wing ratio is a useful index of UV variability throughout the solar cycle because it has been measured since 1978 in a series of successive satellite missions: Nimbus 7, Solar Mesosphere Explorer (SME), the NOAA 9–14 series, Upper Atmosphere Research Satellite (UARS), and ERS-2. Eventual construction of a single time series from 1978 to the present by combining these measurements will give a long record of almost daily UV variability to serve as a surrogate for estimating both UV and EUV solar radiation. Here we address the effect of spectral resolution on determination of both long-term and short-term solar variability from this index. We use UARS/SOLSTICE measurements of the Mgii line from October 1991 to December 1996 to study the effect of two spectral resolution regimes characteristic of existing measurements, 0.20 to 0.25 nm and 1.10 to 1.15 nm, on determination of the amplitude of 27-day rotational modulation and the more gradual change in chromospheric radiation in the declining phase of solar cycle 22. The two Mgii indices give solar variations that differ by a scaling factor of ≈ 2× for both the solar cycle change from 1992 to 1997 and the amplitude of 27-day modulation over the same period. Both types of measurements appear to yield solar signal equally well except at solar minimum when the solar changes become quite small.

Long-Term Variation of the Interplanetary H Lyα Glow: Voyager UVS Measurements and Implications for the Solar H Lyα Irradiance

In this paper we study interplanetary (IP) Lyα data taken with the Voyager 1 and Voyager 2 spacecraft from 1980 to 1995. The coverage in time is equal to about 156 and 220 points yr-1 for Voyager 1 and Voyager 2, respectively, with almost no gaps. The IP Lyα data are normalized for spatial changes in the emissivity, which arise from variations in observing geometry, by using a radiative transfer model. The normalized data show the variation of the solar H Lyα line-center flux during the solar cycle. We compare this variation with the solar H Lyα irradiance measurements of integrated flux from the Solar Mesosphere Explorer and the Upper Atmosphere Research Satellite/Solar-Stellar Irradiance Comparison Experiment (SOLSTICE), and, when direct solar measurements are not available, we use estimated irradiances from magnesium and helium indices. The comparison between Voyager IP data and solar data shows that the best agreement is found with the SOLSTICE set of measurements, when no differences in the variation of the line-center flux and the integrated flux are taken into account.

Solar Cycle 23: An Unusual Solar Minimum?

We are currently observing the minimum phase of Cycle 23. Magnetic activity during the years 2006–2009 has been very weak with sunspot numbers reaching the lowest values in about 100 years. This long and extended minimum is characterized by weak polar magnetic fields, small polar coronal holes, and a relatively complex coronal morphology. This magnetic configuration at the Sun is remarkably different from the one observed during the previous two solar minima. We review observations made at the Sun and in the solar wind during the recent solar minima and discuss the implications of the observed differences for the heliosphere and geospace.