Generalized Moore spectra and Hopkins' Picard groups for a smaller chromatic level

Let $\mathcal L_n$ for a positive integer $n$ denote the stable homotopy category of $v_n^{-1}BP$-local spectra at a prime number $p$. Then, M.~Hopkins defines the Picard group of $\mathcal L_n$ as a collection of isomorphism classes of invertible spectra, whose exotic summand Pic$^0(\mathcal L_n)$ is studied by several authors. In this paper, we study the summand for $n$ with $n^2\le 2p+2$. For $n^2\le 2p-2$, it consists of invertible spectra whose $K(n)$-localization is the $K(n)$-local sphere. In particular, $X$ is an exotic invertible spectrum of $\cL_n$ if and only if $X\wedge MJ$ is isomorphic to a $v_n^{-1}BP$-localization of the generalized Moore spectrum $MJ$ for an invarinat regular ideal $J$ of length $n$. For $n$ with $2p-2<n^2\le 2p+2$, we consider the cases for $(p,n)=(5,3)$ and $(7,4)$. In these cases, we characterize them by the Smith-Toda spectra $V(n-1)$. For this sake, we show that $L_3V(2)$ at the prime five and $L_4V(3)$ at the prime seven are ring spectra.