Radial Interplanetary Field Research Articles

The Radial Interplanetary Field Strength at Sunspot Minimum as Polar Field Proxy and Solar Cycle Predictor

The minimum value of the geomagnetic aa index has served as a remarkably successful predictor of solar cycle amplitude. This value is reached near or just after sunspot minimum, when both the near-Earth solar wind speed and interplanetary magnetic field (IMF) strength fall to their lowest values. At this time, the heliospheric current sheet is flattened toward the heliographic equator and the dominant source of the IMF is the Sun’s axial dipole moment, which, in turn, has its source in the polar fields. As recognized previously, the success of aamin as solar cycle precursor provides support for dynamo models in which the sunspots of a given cycle are produced by winding up the poloidal field built up during the previous cycle. Because they are highly concentrated toward the poles by the surface meridional flow, the polar fields are difficult to measure reliably. Here we point out that the observed value of the radial IMF strength at solar minimum can be used to constrain the polar field measurements, and that this parameter, which is directly proportional to the Sun’s axial dipole strength, may be an even better solar cycle predictor than geomagnetic activity.

Solar Polar Magnetic Fields: Comparing Full-disk and High-resolution Spectromagnetograph Data

This is the first systematic comparison between photospheric polar magnetic field data from a full-disk synoptic observing program, the National Solar Observatory’s Synoptic Optical Long-term Investigations of the Sun Vector Spectromagnetograph (SOLIS/VSM), and a high-resolution vector spectromagnetograph, the Hinode Solar Optical Telescope Spectropolarimeter (SOT/SP). Polar magnetic fluxes derived from longitudinal magnetic field measurements from both telescopes and from SOT/SP full-Stokes vector data are all compared in the form of polar synoptic maps. Measurements taken over 35 day periods with advantageous rotation axis tilt angle are used; observations extend to the poles, and no synthetic pole-filling is needed. Polar fluxes are derived from longitudinal data assuming an approximately radial field, whereas those derived from vector data are based on measured vector magnitude and direction. However, the full-vector measurements may have a detection problem: polar fields are observed as mostly transverse from (near) Earth, and Zeeman sensitivity to transverse fields is significantly lower than for longitudinal fields. Accordingly, the SOT/SP vector-based polar fluxes are lower than the longitudinal-based fluxes from both telescopes, a result driven by pixels without sufficient Q and U signals for the full-Stokes inversions to detect significant radial field but with good Stokes V signal implying a significant field. Furthermore, the SOT/SP longitudinal-based fluxes are significantly higher than their VSM counterparts because of superior seeing-free spatial resolution and longer observation time. The SOT/SP longitudinal-based polar fluxes appear large enough to account for radial interplanetary field measurements whereas the SOT/SP vector-based and the VSM ones are generally too low.

The dayside magnetopause location during radial interplanetary magnetic field periods: Cluster observation and model comparison

Abstract. The present work has investigated the midlatitudinal magnetopause locations under radial interplanetary field (RIMF) conditions. Among 262 (256) earthward (sunward) RIMF events from years of 2001 to 2009, Cluster satellites have crossed the magnetopause 22(12) times, with 10 (7) events occurring at midlatitudes. The observed midlatitudinal magnetopause positions are compared with two empirical magnetopause models (Shue et al., 1998; Boardsen et al., 2000) (hereafter referred to as the Shue98 model and the Boardsen00 model). The observation–model differences exhibit local time asymmetry. For earthward RIMF cases, the Shue98 model underestimates the magnetopause positions in the postnoon sector, while it overestimates the magnetopause positions in the dawn and dusk sectors. The Boardsen00 model generally underestimates the magnetopause after 6 MLT (magnetic local time), with larger deviations in the postnoon sector as compared to those in the prenoon. For sunward RIMF cases, the selected events are mainly clustered around the dawn and dusk sectors. The comparison with the Shue98 model indicates contractions in the dawn and expansions in the dusk sector, while the comparison with Boardsen00 indicates general expansions, with larger expansions in the later local time sectors. The local time variations in the differences between observations and the Shue98 and the Boardsen00 models indicate that the real magnetopause could be asymmetrically shaped during radial IMF periods, which should be considered by magnetopause models. The observation–model differences in the magnetopause positions (Δ RMP) during RIMF periods correlate well with the solar wind dynamic pressure, with larger Δ RMP for larger Pd. The southern magnetopause expands further outward relative to the model prediction when the dipole tilt angle is more negative (local summer in the Southern Hemisphere). For earthward RIMF cases, the generally good correlations between Δ RMP and the IMF cone angle are consistent with the previous hypothesis (Dušík et al., 2010) that, with more radial IMF, the subsolar magnetopause will expand further outward, owever, this is not the case for the comparison with Boardsen00 during sunward IMF periods, as it shows less dependence on the IMF cone angle.

Why does the total pressure on the subsolar magnetopause differ from the solar wind dynamic pressure?

Based on analysis of MHD equations and the results of numerical simulation in the magneto-sheath it is demonstrated that the total pressure on the magnetopause differs from the solar wind dynamic pressure in the majority of cases. From the equation of motion it follows that the total pressure is reduced due to deflection from the Sun-Earth line. At the same time, it increases because of formation of a magnetic barrier. This result is consistent with experimentally observed expansion of the magnetosphere for the radial direction of the interplanetary magnetic field, when no magnetic barrier is formed. In this paper we compare the behavior of pressure along the Sun-Earth line for the northward and radial interplanetary field, using the results of numerical MHD simulation and observational data from THEMIS. In the isotropic MHD approximation, the difference between the total pressure on the subsolar magnetopause at northern and radial IMFs does not exceed 10–12 percent. However, in the anisotropic approximation this difference increases up to 15–20 percent. The results of anisotropic modeling well agree with observed averaged profiles of pressure components in the subsolar magnetosheath.

UNDERSTANDING THE GEOMAGNETIC PRECURSOR OF THE SOLAR CYCLE

Geomagnetic activity late in the sunspot cycle has been used successfully to forecast the amplitude of the following cycle. This success is somewhat surprising, however, because the recurrent high-speed wind streams that trigger the activity are not proxies of the Sun's polar fields, whose strength is a critical factor in many solar dynamo models. Instead, recurrent geomagnetic activity signals increases in the Sun's equatorial dipole moment, which decays on the ~1-2 yr timescale of the surface meridional flow and does not survive into the next cycle. In accordance with the original empirical method of Ohl, we therefore argue that solar cycle predictions should be based on the minimum level of geomagnetic activity, which is determined by the Sun's axial dipole strength, not on the peak activity during the declining phase of the cycle. On physical grounds, we suggest that an even better indicator would be the total open flux (or strength of the radial interplanetary field component) at sunspot minimum, which in turn can be derived from the historical aa index by removing the contribution of the solar wind speed. This predictor yields a peak yearly sunspot number R max = 97 ± 25 for solar cycle 24.

Coronal Holes and Open Magnetic Flux

Coronal holes are low-density regions of the corona which appear dark in X-rays and which contain “open” magnetic flux, along which plasma escapes into the heliosphere. Like the rest of the Sun’s large-scale field, the open flux originates in active regions but is subsequently redistributed over the solar surface by transport processes, eventually forming the polar coronal holes. The total open flux and radial interplanetary field component vary roughly as the Sun’s total dipole strength, which tends to peak a few years after sunspot maximum. An inverse correlation exists between the rate of flux-tube expansion in coronal holes and the solar wind speed at 1 AU. In the rapidly diverging fields present at the polar hole boundaries and near active regions, the bulk of the heating occurs at low heights, leading to an increase in the mass flux density at the Sun and a decrease in the asymptotic wind speed. The quasi-rigid rotation of coronal holes is maintained by continual footpoint exchanges between open and closed field lines, with the reconnection taking place at the streamer cusps. At much lower heights within the hole interiors, “interchange reconnection” between small bipoles and the overlying open flux also gives rise to coronal jets and polar plumes.

Modeling the Sun’s Large‐Scale Magnetic Field during the Maunder Minimum

We use a flux transport model to simulate the evolution of the Sun’s magnetic dipole moment, polar fields, and open flux under Maunder minimum conditions. Even when the rate of active region emergence is taken to be a factor of � 30 smaller than in recent solar cycles, regular polarity oscillations of the axial dipole and polar fields can be maintained if the speed of the poleward surface flow is reduced from � 20 to � 10 m s � 1 and the source flux emerges at very low latitudes (� 10 � ). The axial dipole is then found to have an amplitude of the order of 0.5 G, as compared with � 4 G during solar cycle 21. The strength of the radial interplanetary field component at Earth is estimated to be in the range � 0.3–0.7 nT, about a factor of 7 lower than contemporary values. We discuss the implications of these weak fields for our understanding of geomagnetic activity and cosmic-ray modulation during the Maunder minimum. Subject headings: interplanetary medium — solar-terrestrial relations — Sun: activity — Sun: corona — Sun: magnetic fields — Sun: photosphere In a previous paper (Wang, Lean, & Sheeley 2002a), we simulated the evolution of the Sun’s large-scale magnetic field during solar cycles 13–22 (1888–1997). The model included the effect of emerging flux and its subsequent transport over the solar surface by differential rotation, supergranular convection, and a poleward bulk flow. When the flux eruption rates were scaled according to the observed sunspot numbers and the meridional flow speed was fixed, we found that the polar fields built up during the more active cycles were too strong to be reversed during the less active ones. Regular polarity reversals were obtained only if the flow rate was assumed to be positively correlated with the cycle amplitude. The inferred poleward velocities varied from � 15 m s � 1 for cycles 14, 16, and 20 to � 27.5 m s � 1 for cycle 19; the corresponding maximum yearly sunspot numbers for these cycles range from � 60 to � 200. During the period of the Maunder minimum (1645– 1715), sunspot numbers peaked at values even lower than those recorded during present-day activity minima (see Eddy 1976, 1983; Ribes & Nesme-Ribes 1993; Hoyt & Schatten 1996; Beer, Tobias, & Weiss 1998; Cliver, Boriakoff, & Bounar 1998; Letfus 2000). We now ask whether, by suitably adjusting the meridional flow speed, we can maintain the polarity reversals even with such a dramatic reduction in the source flux. If so, how strong are the derived polar and interplanetary fields, and how do they vary with time? How much open and closed magnetic flux remains during intervals when no activity is present at all? In a related investigation, Mackay (2003) has presented simulations of idealized ‘‘ grand minima ’’ using a flux transport model. Mackay’s approach differs substantially from ours, however, in that he fixes the flow amplitude and does not require that the polar fields reverse their sign during every cycle. The organization of this paper is as follows. We begin with a brief discussion of the sunspot data used to constrain our simulations of the Maunder minimum (x 2). After outlining our basic model and procedure (x 3), we show how the evolution of the Sun’s axial dipole and polar fields depends on the meridional flow velocity and on the source latitudes (x 4) and then illustrate how the large-scale magnetic field might have evolved during the Maunder minimum (x 5). Our results and their implications are discussed in x 6.

Understanding the evolution of the Sun's open magnetic flux

The large‐scale magnetic field of the Sun, including the open flux that extends into the interplanetary medium, originates in active regions but is redistributed over the photosphere by differential rotation, supergranular convection, and poleward meridional flow. We use simulations to clarify the role of the surface transport processes in the evolution of the total open flux, Φopen, which determines the strength of the radial interplanetary field component. Representing the initial photospheric field configuration by one or more bipolar magnetic regions (BMRs), we show that Φopen varies approximately as the net dipole strength, determined by vectorially summing the dipole moments of the individual BMRs. As meridional flow carries the BMR flux to higher latitudes, the equatorial dipole component is annihilated on a timescale ∼1 yr by the combined effect of rotational shearing and supergranular diffusion. The remaining flux becomes concentrated around the poles, and Φopen approaches a limiting value that depends on the axisymmetric dipole strength of the original active regions. We discuss the implications of these results for the solar cycle evolution of Φopen.

Solar Implications of [ITAL]Ulysses[/ITAL] Interplanetary Field Measurements

Recent observations by the Ulysses magnetometer team have shown that the strength of the radial interplanetary field component, |B|, is essentially independent of latitude, a result which implies that the heliospheric currents are confined entirely to thin sheets. Using such a current sheet model, we extrapolate the observed photospheric field to 1 AU and compare the predicted magnitude and sign of Br with spacecraft measurements during 1970-1993. Approximate agreement can be obtained if the solar magnetograph measurements in the Fe I λ5250 line are scaled upward by a latitude-dependent factor, similar to that derived by Ulrich from a study of magnetic saturation effects. The correction factor implies sharply peaked polar fields near sunspot minimum, with each polar coronal hole having a mean field strength of 10 G.