Analyzing the Variations in the Spectral Energy Distribution of the BL Lac Object Mrk 421

The spectral energy distributions (SEDs) datas of Mrk 421 are collected, totally 73 states as research samples. The single-region lepton model under the electron energy spectrum of the steady-state transition power law spectrum is applied. Then the physical properties of Mrk 421 jets during the light transition is studied. Our main results are as follows:(1) There is an anticorrelation between magnetic B and Doppler δ in the sample, indicating that magnetic B and Doppler δ are interdependent in the Thomson region. (2) Electron spectral index p₁ supports the interpretation of shock wave or magnetic reconnection during light variation of Mrk 421. (3) There is a positive correlation between the synchronous peak frequency logν_syn^pk and the peak luminosity logν_syn^pkL_syn^pk, which means that there is an anti-flare variant sequence in the Mrk 421 during the light change. (4) According to the equal partition parameter ε=U_e/U_B (the ratio of relativistic electron energy density to magnetic field energy density), the electron energy and magnetic field energy of 26% of the states are nearly equal partition, the electron energy of 63% of the states is an order of magnitude larger than the magnetic field energy, the electron energy of 11% of the states is much larger than the magnetic field energy. This indicates that Mrk 421 is more likely to have the phenomenon that the electron energy is one order of magnitude larger than the magnetic field energy during the light change. It also means that the jets in the Mrk 421 are dominated by particles; From the relationship of P_e>P_B, it is further shown that the Mrk 421 jets are particle-dominated, P_B≲P_r, meaning that the Poynting flux cannot account for the radiated power; The ranges of P_r/P_e from 0.01 to 0.51, show that a small fraction of the relativistic electron energy may be used for the observed radiation. (5) Some ranges of the physical parameters of 3C 279 and Mrk 421 may be relatively similar, but the physical processes inside the jets of these two sources may not be the same.