In the paper we investigate three novel rising submillimeter ( THz ) bursts occurred sequentially in a super-Active Region NOAA 10486 .
The average rising rate of the flux density above 200 GHz is only 20 sfu/GHz ( corresponding spectral index \alpha of 1.6 ) for the THz spectral components of 2003 October 28 and November 4 bursts , while it can attain values of 235 sfu/GHz ( \alpha =4.8 ) for 2003 November 2 burst .
The steeply rising THz spectrum can be produced by a population of high relativistic electrons with a low-energy cutoff of 1 MeV , while it only requires a low-energy cutoff of 30 keV for the two slowly rising THz bursts , via gyrosynchrotron ( GS ) radiation based on our numerical simulations of burst spectra in the magnetic dipole field case .
The electron density variation is much larger in the THz source than that in microwave ( MW ) one .
It is interesting that the THz source radius decreased by 20–50 \% during the decay phase for the three events , but the MW one increased by 28 \% for the 2003 November 2 event .
In the paper we will present a calculation formula of energy released by ultrarelativistic electrons , accounting the relativistic correction for the first time .
We find that the energy released by energetic electrons in the THz source exceeds that in microwave one due to the strong GS radiation loss at THz range , although the modeled THz source area is 3–4 orders smaller than the modeled MW one .
The total energies released by energetic electrons via the GS radiation in radio sources are estimated , respectively , to be 5.2 \times 10 ^ { 33 } , 3.9 \times 10 ^ { 33 } and 3.7 \times 10 ^ { 32 } erg for the October 28 , November 2 and 4 bursts , which are 131 , 76 and 4 times as large as the thermal energies of 2.9 \times 10 ^ { 31 } , 2.1 \times 10 ^ { 31 } and 5.2 \times 10 ^ { 31 } erg estimated from the soft x-ray GOES observations .