  We analyze the behavior of unstable modes in the glasma produced in high
energy heavy ion collisions, using a simple model with effective homogeneous
longitudinal color electric and magnetic fields. The unstable modes are
approximately described as Nielsen-Olesen unstable modes under the homogeneous
longitudinal gauge fields. We find that the Nielsen-Olesen unstable modes show
properties very similar to those of the exponentially increasing unstable modes
in the glasma recently shown by Romatschke and Venugopalan. Although initial
gauge fields in the glasma are much stronger than the ones in our model, they
decay with the production of the Nielsen-Olesen unstable modes. We discuss why
we can reproduce the features of the glasma by using the effective homogeneous
weak magnetic fields. Our analysis supports an idea that the decay of the gauge
fields in the glasma is caused by Nielsen-Olesen instability.
