We investigate the 21 cm signals from the UV ionizing sources in the reionization epoch .
The formation and evolution of 21 cm emission and absorption regions depend essentially on the kinetics of photons in the physical and frequency spaces .
To solve the radiative transfer equation , we use the WENO algorithm , which is effective to capture the sharp ionization profile and the cut-off at the front of light ( r = ct ) and to handle the small fraction of neutral hydrogen and helium in the ionized sphere .
We show that a spherical shell of 21 cm emission and absorption will develop around a point source once the speed of the ionization front ( I-front ) is significantly lower than the speed of light .
The 21 cm shell extends from the I-front to the front of light ; its inner part is the emission region and its outer part is the absorption region .
The 21 cm emission region depends strongly on the intensity , frequency-spectrum and life-time of the UV ionizing source .
At redshift 1 + z = 20 , for a UV ionizing source with an intensity \dot { E } \simeq 10 ^ { 45 } { ergs } ^ { -1 } and a power law spectrum \nu ^ { - \alpha } with \alpha = 2 , the emission region has a comoving size of 1 - 3 Mpc at time \simeq 2 Myr .
Nevertheless , the emission regions are very small , and would be erased by thermal broadening if the intensity is less than \dot { E } \simeq 10 ^ { 43 } { ergs } ^ { -1 } , the frequency spectrum is thermal at temperature T \simeq 10 ^ { 5 } K , or the frequency spectrum is a power law with \alpha \geq 3 .
On the other hand , the 21 cm absorption regions are developed in all these cases .
For a source of short life-time , no 21 cm emission region can be formed if the source dies out before the I-front speed is significantly lower than the speed of light .
Yet , a 21 cm absorption region can form and develop even after the emission of the source ceases .