Tid og sted for prøveforelesning
Bedømmelseskomité
- Dr. Harry Warren, Space Science Division, Naval Research Laboratory, Washington, USA.
- Dr. Ineke De Moortel, School of Mathematics and Statistics, University of St. Andrews, Skottland.
- Professor Frode K. Hansen, Institutt for teoretisk astrofysikk, Universitetet i Oslo.
Leder av disputas
Professor Per B. Lilje
Veileder
Sammendrag av avhandlingen
According to tropical wisdom, when the atmosphere feels hot and dense we can expect rain. Such thinking may also apply to the Sun, as this thesis explains. The presented new high-resolution observations with the Solar Optical Telescope (SOT) of Hinode and the CRISP spectropolarimeter at the Swedish 1-m Solar Telescope (SST) show a picture of the Sun in which coronal rain seems to be a far more common phenomenon of active regions (the hot and dense regions in the solar atmosphere) than previously thought.
Coronal rain, a phenomenon of thermal instability in plasmas for the case of coronal loops, is composed of small cool and dense blobs observed in chromospheric lines such as Hα or Ca II H, rapidly forming and falling down from coronal heights along loop-like paths. Apart from suggesting its ubiquitous character, in this thesis the importance of coronal rain is highlighted in 3 different ways.
First, its potential as a marker for coronal heating mechanisms is shown. More specifically, through numerical simulations the effects of Alfvén waves (a strong coronal heating candidate) on the thermal stability of loops is treated. The results indicate that coronae heated through shock heating from mode conversion of Alfvén waves cannot exhibit coronal rain, thus suggesting that this mechanism may not be important for the heating of active region coronae.
Second, the role it plays in coronal seismology is shown. Transverse MHD oscillations in loops are put in evidence by coronal rain in observations with Hinode/SOT, thus offering a way to estimate the coronal magnetic field strength, one of the hardest physical quantities to measure accurately, yet lying at the root of most solar and heliospheric physics.
Third, due to the very small sizes of the blobs of which it is composed of, it also serves as a probe for the internal structure and local thermodynamic conditions in loops. In the obtained picture with CRISP of the SST coronal loops appear with constant area cross-sections along their lengths, multi-stranded and unbraided. Furthermore, a significant fraction of strands in the loops show a coherent thermodynamic evolution, thus imposing several constraints on coronal loop modeling. The mass flux raining down is shown to be significant, as compared to the estimated mass injected into the corona from spicules.
For mer informasjon
For mer informasjon kontakt professor Frode K. Hansen.