Solar Cycle 24 Group C
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Tue III: Global fields and magnetogram calibrations
Aimee Norton:
Emergence of sunspots early in a solar cycle might be tipped, evidence for Cycle 23, not known for earlier cycles (or for Cycle 24). Norton & Gilman (2005), and Norton, Raouafi & Petrie (2008).
Evidence is that dipole is always tipped for previous minimum, from streamer locations from LASCO. Generally agrees with PFSS-style models.
Is there evidence for off-axis polar cap? Hard to tell from magnetograms due to incomplete coverage of polar fields (especially for the one pole that tilted away from earth). From PFSS models, tilt ranged form 2 to 6 degrees. Was the azimuthal angle stable? For isolated periods (e.g., CR1915-1919), yes.
Todd: New cycle fields emerge with a pattern that suggests a systematic asymmetry, and it might not be related to the polar cap arrangement of flux, and so the dipole tilt angles shown here results from a combination of both effects.
Magnetogram calibration discussion:
MDI: Saturation in sunspots (MDI underestimates LOS field in sunspots)
MDI: Zero-point issues, maybe related to secular trends in monopole component of surface field configuration.
Hinode is showing us that fields are complicated! HMI line selection paper gives some idea of uncertainties in, say, MDI measurements of LOS field. In many cases, there are many field components in even a Hinode pixel.
Polar fields are particularly troublesome, given their steep viewing angle, and particularly important given that they determine the heliospheric flux during solar minimum.
A study comparing MDI and Hinode B_LOS should probably be done. Recent MDI recalibration was due to multiple line studies by Roger Ulrich at UCLA, and resulted in slightly weaker fields at poles.
Tue IV (with group I): Coronal loops and the slow solar wind
See group I write-up here.
Wed I (with group J): Filaments and field models
See group J write-up here.
Thu I (with group B): Ephemeral region emergence
Valentyna Abramenko
Talk based on Abramenko, Fisk & Yurchyshyn (2006). Ephemeral region emergence rate (for quiet-sun regions) is half as great inside of coronal holes than outside coronal holes. This observation supports diffusion hypothesis for migration of open-field lines across the photospheric surface as presented by Fisk (2005).
Mandy Hagenaar
Talk based on Hagenaar, DeRosa & Schrijver (2008). Ephemeral region emergence rate (for quiet-sun regions) depends on the degree of unipolarity into which the ephemeral region emerges. More unipolar regions tend to have lower ER emergence rates than regions with mixed polarity. Becuase coronal holes tend to be more unipolar, this result is in agreement with the Abramenko, Fisk & Yurchyshyn (2006), but Hagenaar, DeRosa & Schrijver (2008) find that this is not the primary discriminator. That is, a coronal-hole region and a non-coronal-hole region, both having the same degree of unipolarity, will tend to have the same ER emergence rate.
Thu III (with group H): Chromospheric field measurements
Marc DeRosa
Nonlinear force-free models of the corona are problematic at best, and impossible at worst. Major stumbling block is that vector magnetograms currently in use are photospheric, which is not force-free and thus violates the assumptions built into the model. Attempts to "preprocess" these magnetograms to make them more force-free improve things, but only marginally. It is likely that a greater understanding of the fields and currents in the chromosphere is needed to improve the models.
Bill Abbett
Fields in models photosphere/chromosphere regions are a tangled mess. We can only imagine what they are like on the sun.
Rebecca Centeno
Difficult but possible to get vector magnetograms from He I 10830. It has no influence from the photosphere and is straightforward to interpret its polarization signals in many cases.
Na Deng
Working on getting simultaneous vector measurements from Fe I 6302 pair (photosphere) and Mg I b2 517 lines (low chromosphere). Preliminary results indicate that B_LOS look similar at these heights, but B_t looks weaker in chromospheric line. Difficulties include getting sufficient polarimetric signal in the chromosphere, as well as calibrating and inverting them.
