Time: 15:00-16:00, May 27
Location: SIST 1A-200
Host: Dr. Meiling Wang
Abstract:
Maybe the concept of space weather is known to many of you, and maybe the fact that space weather comes from the solar wind, which arises from solar flares is known to some of you. Maybe some of you even know that solar flares are related to the conversion of stored magnetic energy to thermal energy in the solar corona. But probably, not known to many of you, is the fact that there is no easy way to measure the strength of the magnetic field of our suns crown (corona). We will introduce a proposed purely spectroscopic method for determining the field strength of the solar corona, based on what we call a magnetic sensitive line. Magnetic sensitive lines were first discovered in 2003 at the Livermore Electron Beam Ion Trap laboratory. However in that particular case (Ne-like Ar) an external field of several Tesla is needed to induce this very special transition. Through extensive studies we have discovered a magnetic induced transition that is sensitive to field strengths we expect in the solar corona, i.e. less than around 0.25 Tesla. The magnetic sensitive line in our studies is from the ion Fe X, i.e. Cl-like Fe. This charge state of Fe is abundant in the solar corona, known from the so-called very strong Corona red line. The magnetic sensitive line has already been observed in spectra of the solar corona suing a satellite called HINODE (or Solar-B). We will show how an Electron Beam Ion Trap is an ideal instrument for providing the atomic data needed to extract the coronal magnetic field strength from the spectroscopic data, In fact one of our Electron Beam Ions Traps should be re-named as a “Solar Flare Laboratory” as many other aspects of solar flare/corona phenomena can also be studied. Finally we will show that, as in many walks of life, it is better, (or at least as good), to be lucky than smart, as the Fe X magnetic induced transition is aided in its intensity by a near energy degeneracy with an allowed transition.
Bio:
Previously to moving to Fudan University in 2005 my career had been centered at the University of Lund in Sweden. Part of the physics department in Lund was the atomic spectroscopy group then headed by Professor Indrek Martinson who was a pioneer in the field of Beam-Foil spectroscopy. I followed in my professors footsteps and obtained my Phd in 1988 using beam-foil to investigate level lifetimes for highly charged ions. My major contribution as a Phd student was to show that all contemporary calculations for lifetimes in the relatively simple atomic system of Na-like ions were wrong. This was a very surprising result as the assumption at the time was that such calculations were trivial. It was true, the calculations were trivial, but they all missed the polarization of the 2p6 core by the outer electron. Nowadays no one would even imagine doing a calculation without including core-polarization. After my Phd., I went to the Lawrence Berkeley Laboratory as post doc and studied slow ion-atom collisions. Following post doc studies I went back to Lund and continued spectroscopy studies of highly charged ions using both the beam foil technique and also ECR ion sources. In 2005 I got the chance to join the EBIT laboratory at the Institute of Modern Physics at Fudan University. EBITs had always fascinated me as an instrument for spectroscopy studies of highly charged ions. In the years since then we have pioneered studied of: (a) visible transitions from highly charged tungsten ions (b) magnetic induced transitions (c) studies of Layzer quenched states and (d) using atomic structure to intensity calibrate soft x ray spectrometers.
Some detail of my current work:
(a) Spectral lines from highly charged tungsten ions in the visible spectral region would be of great importance to the diagnostics of the ITER fusion plasma. Visible light can be “removed” from the harsh environment of the fusion vessel using fiber optics to spectrometers sitting in nicer conditions.
(b) As the abstract to my talk tells. There are no current easy methods for measuring the strength of the solar corona magnetic field. This is despite the significance this field plays in the development of solar flares.
(c) Layzer quenched states are ones where first order electron correlation is very small. Therefore in atomic systems having such states we have proposed studies of multi-electron QED and the Breit interaction.
(d) In order to get quantitative information from spectra, the spectrometer recording the data needs to be calibrated for both wavelength and intensity. This is trivial in the spectral region above 2000 Å but very difficult below and extremely difficult in the soft x ray region. We are working on the idea of using atomic structure for such calibrations. In the end the spectrometer needing calibration will need an EBIT as a light source, but nowadays EBITs are very simple to operate and relatively in-expensive. We have already demonstrated a scheme that works in the vacuum ultra-violet and are working on a scheme for the soft x ray region.
SIST-Seminar 18161
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