基于波束形成和波叠加法的复合声全息技术

近场声全息是一种准确的噪声源识别技术.然而,由于存在测量要求严格、测点数多等缺点,一定程度上限制了它在工业现场的广泛应用.基于此,提出一种基于波束形成和波叠加法的复合声全息技术.首先利用传声器阵列获取声压场信息;再通过波束形成对声源进行定位;然后在这些声源位置附近配置等效源;最后应用波叠加法进行声场重构.该技术的优点是只需要少量传声器,在中、远距离测量,可以方便快速地实现声场重构.采用两个脉动球的声源模型进行了数值仿真,并在半消声室内采用音箱模拟噪声源进行了实验,都准确地重构了模型的外部声场.这表明,该技术可以准确地对辐射体进行声场重构,为其在工业现场中的应用奠定了基础.     

Pitch Angle Distribution Evolution of Energetic Electrons by Whistler-Mode Chorus

We develop a two-dimensional momentum and pitch angle code to solve the typical Fokker-Planck equation which governs wave-particle interaction in space plasmas. We carry out detailed calculations of momentum and pitch angle diffusion coefficients, and temporal evolution of pitch angle distribution for a band of chorus frequency distributed over a standard Gaussian spectrum particularly in the heart of the Earth＇s radiation belt L = 4.5, where peaks of the electron phase space density are observed. We find that the Whistler-mode chorus can produce significant acceleration of electrons at large pitch angles, and can enhance the phase space density for energies of 0.5 - 1 MeV by a factor of 10 or above after about 24h. This result can account for observation of significant enhancement in flux of energetic electrons during the recovery phase of a geomagnetic storm.