Here we present three-dimensional MHD models for the Parker instability in a thick magnetized disk , including the presence of a spiral arm .
The B -field is assumed parallel to the arm , and the model results are applied to the optical segment of the Carina-Sagittarius arm .
The characteristic features of the undular and interchange modes are clearly apparent in the simulations .
The interchange mode appears first and generates small interstellar structures in the inter-arm regions , but its development inside the arm is hampered by the acceleration of the spiral wave .
In contrast , the undular mode follows its normal evolution inside the spiral wave , creating large gas concentrations distributed along the arm .
This results in a clear arm/inter-arm difference : the instability triggers the formation of large interstellar clouds ( with masses in the range of 10 ^ { 6 } to 10 ^ { 7 } M _ { \odot } ) inside the arms , but generates only small structures with slight density enhancements in the inter-arm regions .
The resulting clouds are distributed in an antisymmetric way with respect to the midplane , and their masses are similar to those inferred for HI superclouds in our Galaxy .
Such a cloud distribution results in an azimuthal corrugation along the arm and , for conditions similar to those of the optical segment of the Carina-Sagittarius arm , it has a wavelength of about 2.4 kpc .
This structuring , then , can explain the origin of both HI superclouds and the azimuthal corrugations in spiral arms .
In particular , the wavelength of the fastest growing undular mode matches the corrugation length derived with the young stellar groups located in the optical segment of the Carina-Sagittarius arm .