Remote sensing of solar magnetic fields

Abstract

Abstract. New techniques for remote sensing of solar magnetic fields now provide measures of the magnetic field vector within the solar atmosphere with high angular resolution and high precision. These measurements have enabled a much improved physical understanding of magnetic processes and phenomena in the solar atmosphere, processes that drive the variability of the Sun's radiative and particulate output. The new techniques are reviewed here in the context of the scientific advances they have fostered. Emphasis is given to techniques for inferring the field vector. The quantitative nature of the information needed to explore the solar phenomena sharply constrains the needed precision and angular resolution of the observations. These requirements are reviewed here, along with an assessment of how future improvements in observing capabilities will address these requirements. One may also attribute much of the recent advance in our understanding of solar magnetic fields to ongoing progress in techniques for analysis of the polarization measurements that underlie solar magnetometry. The status and prospects of analysis techniques are also reviewed. The Large-Scale Solar Magnetic Field and the Solar DynamoThe 11-year cycle of solar activity is certainly the most widely recognized mode of solar variability. In fact, it represents a 22-year magnetic cycle: Two l 1-year cycles are required to restore the dominant magnetic polarity near the poles of the Sun. This cycle is dramatically revealed in the synoptic "magnetic butterfly diagram" of Figure 3, which shows the evolution of the large-scale solar magnetic field during the past two sunspot cycles. This map is assembled from individual solar "magnetograms" of the full solar disk similar to that shown in Figure 4. The behavior of the solar magnetic field at these global scales reflects the working of the solar dynamo [Roberts, 1994]. The dynamo, which is believed to be operative near the base of the solar convective layer (or about one third the solar radius below the surface), results from the combined action of solar rotation and convective flows within the highly conducting solar plasma. The solar dynamo problem represents one of the major challenges of astrophysics: Dynamos are responsible for magnetic activity in a range of stellar types, and some stars sustain magnetic activity cycles that are much more energetic than that of the Sun. Figure 3 contains a wealth of information on the nature of the dynamo as reflected in the behavior of the magnetic fields as they penetrate the solar surface, for example, the latitude and phase in the cycle of the emergence of fields and the motions of large-scale unipolar

Magnetic Helicity Estimations in Models and Observations of the Solar Magnetic Field. Part I: Finite Volume Methods

“…In the particular case of applications to the solar atmosphere, one additional complication is that the magnetic field cannot be measured in the solar corona with the resolution necessary for the computation of magnetic helicity. The magnetic field is instead inferred by inverting spectropolarimetric measurements of emission from lower, higher-density layers of the atmosphere, yielding basically two-dimensional maps of the field vector mostly at photospheric heights, see e.g., Lites (2000). Therefore, in order to compute the helicity, it is first necessary to introduce a model of the solar corona based on the observed photospheric field values.…”

ON HELIUM 1083 nm LINE POLARIZATION DURING THE IMPULSIVE PHASE OF AN X1 FLARE

“…These profiles arise from the Zeeman effect (e.g. Jefferies et al 1989;Lites 2000). Unlike the Si I line, the He I impulsive phase spectra show signatures of strong atomic polarization, with Stokes Q and U profiles giving a polarization P/I which is several percent of the intensity, I, during the flare.…”

Extrapolation of a nonlinear force-free field containing a highly twisted magnetic loop

“…The comparison of the extrapolated fields at different resolutions permits investigating the influence of the resolution on the quality of the extrapolation. This is valuable information because the field on the surface of the Sun always possesses small-scale flux concentrations that cannot be resolved by present-day instruments (see, e.g., Lites 2000). With this in mind, we consider also resolutions (3∆ and 4∆) that exceed the length scale L B .…”

“…The derivation of the photospheric magnetic field (vector magnetogram) from the Stokes vector of polarized light measured from Earth or from a spacecraft requires the treatment of complex physical models of line formation and radiative transport (see, e.g., Lites 2000). Several sources of uncertainty and even error exist in such measurements.…”