The autoregressive method provides a univariate procedure to predict the future sunspot number (SSN) based on past record. The strength of this method lies in the possibility that from past data it yields the SSN in the future as a function of time. On the other hand, its major limitation comes from the intrinsic complexity of solar magnetic activity that may deviate from the linear stationary process assumption that is the basis of the autoregressive model. By analyzing the residual errors produced by the method, we have obtained the following conclusions: (1) the optimal duration of the past time for the forecast is found to be 8.5 years; (2) the standard error increases with prediction horizon and the errors are mostly systematic ones resulting from the incompleteness of the autoregressive model; (3) there is a tendency that the predicted value is underestimated in the activity rising phase, while it is overestimated in the declining phase; (5) the model prediction of a new Solar Cycle is fairly good when it is similar to the previous one, but is bad when the new cycle is much different from the previous one; (6) a reasonably good prediction of a new cycle can be made using the AR model 1.5 years after the start of the cycle. In addition, we predict the next cycle (Solar Cycle 25) will reach the peak in 2024 at the activity level similar to the current cycle.

Abstract
1. INTRODUCTION
2. DATA AND TRANSFORMATION
3. MODEL
4. ANALYSIS
    4.1. Parameters
    4.2. Dependence of Standard Error on PredictionHorizon
    4.3. Error in Peak SSN and Time
5. EXAMPLES OF LONG-TERM PREDICTION
6. SUMMARY AND DISCUSSION
REFERENCES

• Yeon Han Kim(Solar and Space Weather Group, Korea Astronomy and Space Science Institute)
• Jongchul Chae(Department of Physics and Astronomy, Seoul National University) Corresponding author