J. Arnaud

On the Predominance of the Radial Component of the Magnetic Field in the Solar Corona

Polarimetric measurements of the corona out to 2 R☉ in the Fe XIII 10747 A line, the strongest of the iron forbidden lines, are placed for the first time in the context of spatially resolved images of coronal density structures. These measurements, which are the only tool currently available to yield the direction of the magnetic field, date to 1980, the only year when they were available with polarized brightness images of the corona. Through this comparison, the observed predominance of the radial component of the coronal magnetic field, discovered over three decades ago from eclipse observations and established systematically by Arnaud, is shown to point to the coexistence of two magnetic field components in the corona: a nonradial field associated with the large-scale structures known as streamers and a more pervasive radial magnetic field. This finding suggests that these two components are the coronal counterparts of the strong- and weak-field components recently observed in the quiet-Sun photospheric field and supported by recent theoretical investigations of the solar dynamo.

On the Detection of the Signature of Silicon Nanoparticle Dust Grains in Coronal Holes

We report on polarization measurements in the Fe XIII line at 1074.7 nm made during the total solar eclipse of 2001 June 21, which yielded the first signature of interplanetary dust in the inner corona. In the first-ever images at this wavelength, the signature of dust appears as a tangentially polarized emission in the radial extension of the low-temperature and low-density coronal holes as opposed to a predominantly radial polarization direction in the rest of the corona. The observed emission and polarization are attributed to fluorescence from silicon nanoparticle dust grains in the inner corona.