Stellar magnetic activity is important for formulating the evolution of the star. To represent the stellar magnetic activity, the S index is defined using the Ca II H+K flux measure from the Mount Wilson Observatory. MgII lines are generated in a manner similar to the formation of Ca II lines, which are more sensitive to weak chromospheric activity. MgII flux data are available from the International Ultraviolet Explorer (IUE). Thus, the main purpose of this study was to analyze the magnetic activity of stars. We used 343 high-resolution IUE spectra of 14 main-sequence G stars to obtain the MgII continuum surface flux and MgII line-core flux around 2,800 ˚A. We calculated S index using the IUE spectra and compared it with the conventional Mount Wilson S index. We found a color (B − V ) dependent association between the S index and the MgII emission line-core flux. Furthermore, we attempted to obtain the magnetic activity cycles of these stars based on the new S index. Unfortunately, this was not successful because the IUE observation interval of approximately 17 years is too short to estimate the magnetic activity cycles of G-type stars, whose cycles may be longer than the 11 year mean activity cycle of the sun.

To examine relations between stellar activity and rotation we estimated parameters of stellar activity such as R′ H K , R′ M g I I , R′ C I I , R′ C I V and R′ X − r a y from the published data which measure the activity levels of stellar chromospheres, transition regions and coronae. In the present study we considered only the main sequence stars in an attempt to minimize the influence of other stellar parameters such as radius, age and stellar convection on stellar activity since they are also known to affect the magnetic field generation. In the present analysis we selected only those stars that satisfy the following conditions: (1) flux measurements are available together with Ca II fluxes and (2) rotation periods are determined by Ca II observations. We derived relations between the ¯Rossby number Ro and stellar activity R′ H K , R′ M g I I , R′ C I I , R′ C I V and R′ X − r a y and assessed the relations by plotting R′ H K , R′ M g I I and R′ X − r a y against rotation period P rot for comparison with observations. From the comparison it is found that as far as the rotation-activity relation is concerned, (1) normalized surface flux R′ H K is better than the surface flux F′ H K , in the sense that R′ H K differentiates the color dependence better and (2) R′ H K defined by Rutten (1984) describes the observations notably better than R′ H K of Noyes et al. (1984).

Making use of our extended version of ¨ O p i k ′ s convection theory, we have calculated magnetic cycle periods of the sun and late type stars by using Parker's dynamo theory, where we have included the non-linear effect. We presented a relationship between the computed cycle period and spectral type to analyze observed magnetic activities of the late type stars and long-term luminosity variations. It is found that (1) the stellar magentic-cycle period increases towards the later spectral type, (2) the rapid rotation facilitates the activity-related luminosity variation of stars later than about K5, (3) differential rotation plays a critical role in determining the magnetic activity-cycle period, and (4) the non-local effect should be taken into account in order to understand the observed long-term luminosity variations.

A generalization of the original ¨ O p i k ′ s cellular convection theory has been made to accomodate a rotating convective medium. With the use of the formulation, a set of rotating model envelopes of the sun and late type main sequence stars have been constructed under three different rotation periods. Their thermal structures are presented and characteristics of their convection are discussed in the context of stellar dynamo. In the present study it is noted that the rotational angular velocity increases in wards with depth, and its increase turns out to be about 6% at the bottom of the solar convection zone.

New empirical relations between stellar CaII emission and rotation or age are derived by analyzing Wilson's CaII flux measurements (1968, 1978) of lower main sequence stars, and then we correlate them with their age and rotation rate. It is found that stellar chromospheric emission decays smoothly with age as a star slows down rotationally, establishing that both the emission level and rotation rate decrease with the square root of age.