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It is believed that type II radio bursts are generated by shock waves. Inorder to understand the generation conditions of type II radio bursts, in thispaper, we analyze the physical parameters of a shock front. The type II radioburst we selected was observed by Siberian Solar Radio Telescope (SSRT) andLearmonth radio station and was associated with a limb CME occurring on 2014January 8 observed by the Atmospheric Imaging Assembly (AIA) on board the SolarDynamics Observatory (SDO). The evolution of the CME in the inner coronapresents a double-layered structure that propagates outward. We fit the outerlayer of the structure with a partial circle and divide it into 7 directionsfrom -45$^\circ$ to 45$^\circ$ with an angular separation of 15$^\circ$. Wemeasure the outer layer speed along the 7 directions, and find that the speedin the direction of -15$^\circ$ with respect to the central direction is thefastest. We use the differential emission measure (DEM) method to calculate thephysical parameters at the outer layer at the moment when the type II radioburst was initiated, including the temperature ($T$), emission measure ($EM$),temperature ratio ($T_{d}/T_{u}$), compression ratio ($X$), and Alfv\'en Machnumber ($M_{A}$). We compare the quantities $X$ and $M_{A}$ to that obtainedfrom band-splitting in the radio spectrum, and find that this type II radioburst is generated at a small region of the outer layer that is located at thesector in 45$^\circ$ direction. The results suggest that the generation of typeII radio bursts (shock) requires larger values of $X$ and $M_{A}$ rather thansimply a higher speed of the disturbance.

Investigating the Conditions of the Formation of a Type II Radio Burst on 2014 January 8