Impulsivity Parameter for Solar Flares

TL;DR

This paper introduces a quantifiable impulsivity parameter for solar flares, based on the duration of the impulsive phase, and proposes a new classification system for flare impulsivity levels.

Contribution

It defines and computes the impulsivity parameter for solar flares using three methods, and introduces a new classification system independent of calculation approach.

Findings

Impulsivity parameter effectively characterizes flare energy release speed.

Three methods provide accurate and fast impulsivity measurements.

Impulsivity classification correlates with different hard X-ray generation processes.

Abstract

Three phases are typically observed during solar flares: the preflare, impulsive, and decay phases. During the impulsive phase, it is believed that the electrons and other particles are accelerated after the stored energy in the magnetic field is released by reconnection. The impulsivity of a solar flare is a quantifiable property that shows how quickly this initial energy release occurs. It is measured via the impulsivity parameter, which we define as the inverse of the overall duration of the impulsive phase. We take the latter as the raw width of the most prominent nonthermal emission of the flare. We computed this observable over a work sample of 48 M-class events that occurred during the current Solar Cycle 24 by using three different methods. The first method takes into account all of the nonthermal flare emission and gives very accurate results, while the other two just cover fixed energy intervals (30-40 keV and 25-50 keV) and are useful for fast calculations. We propose an alternative way to classify solar flares according to their impulsivity parameter values, defining three different types of impulsivity, namely, high, medium, and low. This system of classification is independent of the manner used to calculated the impulsivity parameter. Lastly, we show the relevance of this tool as a discriminator of different HXR generation processes.