I. INTRODUCTION

II. MODEL OF H $\mathbf{H}$ OSCILLATION  
A. Model description  
B. Formula for $\gamma$-ray emissivity  

III. CALCULATING THE EMISSIVITIES  
A. Phases of the ISM  
B. Emissivities of the CNM, WNM and WIM  

IV. DERIVING BOUND ON $|\delta|$ USING FERMI LAT DATA  
A. Review of relevant $\gamma$-ray data  
B. Bounds on $|\delta|$  

V. DISCUSSION AND CONCLUSIONS