Evidence suggests that gamma-ray burst ( GRB ) ejecta are likely magnetized , although the degree of magnetization is unknown .
When such magnetized ejecta are decelerated by the ambient medium , the characteristics of the reverse shock emission are strongly influenced by the degree of magnetization .
We derive a rigorous analytical solution for the relativistic 90 ^ { o } shocks under the ideal MHD condition .
The solution is reduced to the Blandford-McKee hydrodynamical solution when the magnetization parameter \sigma approaches zero , and to the Kennel-Coroniti solution ( which depends on \sigma only ) when the shock upstream and downstream are ultra-relativistic with each other .
Our generalized solution can be used to treat the more general cases , e.g .
when the shock upstream and downstream are mildly relativistic with each other .
We find that the suppression factor of the shock in the strong magnetic field regime is only mild as long as the shock upstream is relativistic with respect to the downstream , and it saturates in the high- \sigma regime .
This indicates that generally strong relativistic shocks still exist in the high- \sigma limit .
This can effectively convert kinetic energy into heat .
The overall efficiency of converting ejecta energy into heat , however , decreases with increasing \sigma , mainly because the fraction of the kinetic energy in the total energy decreases .
We use the theory to study the reverse shock emission properties of arbitrarily magnetized ejecta in the GRB problem assuming a constant density of the circumburst medium .
We study the shell-medium interaction in detail and categorize various critical radii for shell evolution .
With typical GRB parameters , a reverse shock exists when \sigma is less than a few tens or a few hundreds .
The shell evolution can be still categorized into the thick and thin shell regimes , but the separation between the two regimes now depends on the \sigma parameter and the thick shell regime greatly shrinks at high- \sigma .
The thin shell regime can be also categorized into two sub-regions depending on whether the shell starts to spread during the first shock crossing .
The early optical afterglow lightcurves are calculated for GRBs with a wide range of \sigma value , with the main focus on the reverse shock component .
We find that as \sigma increases from below the reverse shock emission level increases steadily until reaching a peak at \sigma \lower 4.0 pt \hbox { $ \buildrel < \over { \sim } $ } 1 , then it decreases steadily when \sigma > 1 .
At large \sigma values , the reverse shock peak is broadened in the thin shell regime because of the separation of the shock crossing radius and the deceleration radius .
This novel feature can be regarded as a signature of high \sigma .
The early afterglow data of GRB 990123 and GRB 021211 could be understood within the theoretical framework developed in this paper , with the inferred \sigma value \lower 4.0 pt \hbox { $ \buildrel > \over { \sim } $ } 0.1 .
The case of GRB 021004 and GRB 030418 may be also interpreted with higher \sigma 
values , although more detailed modeling is needed .
Early tight optical upper limits could be interpreted as very high \sigma cases , in which a reverse shock does not exist or very weak .
Our model predictions could be further tested against future abundant early afterglow data collected by the Swift UV-optical telescope , so that the magnetic content of GRB fireballs can be diagnosed .