This work presents an analytical investigation for a Self-Organized Criticality abelian model that describes basic properties of rainfall phenomena. The knowledge of the exact solution for the probability that a site topples when mass is added to any other site of the lattice leads to a large number properties of the model, including the exponent of the power law that describes presence of the events as function of their magnitude. It is shown that the model belongs to the same universality class of a first model proposed by Dhar and Ramaswamy (DR). However, for finite size lattices, it is found that its exponent is larger than that one for the DR model.
I IntroductionAbelian sand pile models (ASM) [1] became quite important for the understanding of basic properties of Self-Organized Criticality (SOC) [2,3]. They satisfy the remarkable property that the final state of the model, after subsequent addition of grains to any two sites, is independent of the order in which the grains have been added.The first analyses by Dhar and Ramaswamy [4] considered a model (DR) based on a critical height criterion for toppling, i.e., a site becomes unstable and topples when its amount of mass exceeds a threshold value m th . In this model, a site topples to exactly d distinct neighbors. This is a directed and deterministic model as, i) the toppling rules breaks the isotropy of the lattice and avalanches develop along one direction; and ii) a deterministic rule indicates the fixed number of grains that each site receives when an unstable site topples. These two properties impose the condition that each site topples only once during an avalanche.Abelian models constitute a special set where analytical investigation has lead to exact results, what are still rare subjects in the SOC landscape. More recently, a number of works have focused on certain variants of abelian models: they include the random distribution of toppling grains onto a restricted number of neighboring sites [6][7][8], and the complete toppling of all grains from an unstable site [9][10][11]. The first modification does keep the models in the abelian class, and an exact solution for the probability distribution function of events (PDF) has been discussed. The properties of such models are distinct from those with deterministic toppling rules. On the other hand, the second modification breaks the commutativity of the models, that seem to be nonintegrable.Other SOC models, that were initially proposed based on the so-called gradient condition for toppling, as the Bak-Tang-Wiesenfeld (BTW) model [2], also belong to this class, provided the variables are conveniently re-interpreted. But, as far as they allow for multiple toppling for a single avalanche, they are not exactly integrable.The exact solution for the DR model is based on the evaluation of the probability distribution function P (s 1 ; s 0 ). It measures the probability that a site s 1 topples when one grain is added in a site s 0 , and takes into account all configurations of the lattice which satisfies th...