The third seminar (2015) of Center for Astro-geodynamics Research was held at SHAO on May 4th, which is also a part of serial seminars co-held by CAR(Center for Astro-geodynamics Research) and KLPS(Key Laboratory of Planetary Sciences, CAR). Dozens of researchers, including CAS Member Ye Shuhua, attended the seminar and made effective communications as well. 

Chen Jianxia and Tian Wei, from the research team of “Earth Rotation Variation” gave us impressive presentations on their own fields respectively. 

The title of Chen Jianxia’s presentation is “THE NATURE OF CME-FLARE ASSOCIATED CORONAL DIMMING”. Coronal mass ejections (CMEs) are often accompanied by coronal dimming evident in extreme ultraviolet (EUV) and soft X-ray observations. The locations of dimming are sometimes considered to map footpoints of the erupting flux rope. As emitting material expands in the corona, the decreased plasma density leads to reduced emission observed in spectral and irradiance measurements. Therefore, signatures of dimming may be used to diagnose initiation of CMEs. We analyze three events of flare, CME, and coronal dimming. Data from the Solar Dynamics Observatories Atmospheric Imaging Assembly (AIA) and EUV variability Experiment (EVE) are used for observations of the dimming, and the Solar Terrestrial Relations Observatories EUVI, COR1 and COR2 are used to obtain velocity for the associated CMEs. We also calculate the magnetic reconnection rate from the Helioseismic and Magnetic (HMI) combined with AIA 1600. The magnetic reconnection fluxes are correlated well with CME height profiles while the reconnection rate peaks at the CME acceleration maximum. In two events, the dimming light curve also shows good correlation with the CME height evolution. We model the dimming evolution based on several different assumptions of CME expansion: isothermal or adiabatic, self-similar or one-dimensional. The observed dimming light curves agree with the calculations based on one dimensional, isothermal CME expansion model. Dimming in the third event cannot be described by the above CME expansion models, and we speculate that the nature of dimming associated with the third CME event is different from the other two. 

While Tian Wei’s report, titled “Comparison of Different Multi Space Geodetic Techniques Combination Strategies Applied for the Computation of Earth Rotation Parameters” first introduces the current status and main trend of combination strategy of various space geodetic techniques applied for the computation of Earth rotation parameters. Then, starting from the general adjustment model, it analyzes the current combination strategies on three different levels: based on original observations, based on normal equations and solutions. The analysis concludes with that, in principle, combination based on  normal equations is equivalent with combination based on the original measurements, and that combination based on solutions has significant differences with the first combination strategies (imposing different constrains and imposing constrains in different order). The presenter  also previews the realization of the ITRF2008, and discusses several key procedures in the combination, which are reference frame definition and datum selection, relative weighting of different techniques and introduction of local ties. At the end, combination strategies of two terrestrial reference frames ITRF and DTRF, which are realized by the Institut Geographique National (IGN, France) and Deutsches Geodätisches Forschungsinstitut (DGFI, Germany), are generally compared. 

After that, the attendees (researchers and students), according to their own research area, questioned the presenters on aspects they are interested in, exchanged views, and furthermore, gave practical suggestion on future research.