Investigation of the Neupert effect in solar flares. I. Statistical properties and the evaporation model

TL;DR

This study analyzes 1114 solar flares to investigate the Neupert effect, revealing that electron-beam-driven evaporation explains many events, but other energy transport mechanisms are also significant in a substantial fraction of flares.

Contribution

It provides a comprehensive statistical analysis of the Neupert effect in solar flares, highlighting the role of electron-beam-driven evaporation and identifying additional energy transport mechanisms.

Findings

About half of the flares are consistent with the Neupert effect timing.

A significant fraction shows deviations indicating other energy transport processes.

Strong correlation between SXR peak flux and HXR fluence in compatible events.

Abstract

Based on a sample of 1114 flares observed simultaneously in hard X-rays (HXR) by the BATSE instrument and in soft X-rays (SXR) by GOES, we studied several aspects of the Neupert effect and its interpretation in the frame of the electron-beam-driven evaporation model. In particular, we investigated the time differences ($\Delta t$) between the maximum of the SXR emission and the end of the HXR emission, which are expected to occur at almost the same time. Furthermore, we performed a detailed analysis of the SXR peak flux -- HXR fluence relationship for the complete set of events, as well as separately for subsets of events which are likely compatible/incompatibe with the timing expectations of the Neupert effect. The distribution of the time differences reveals a pronounced peak at $\Delta t = 0$. About half of the events show a timing behavior which can be considered to be consistent with the expectations from the Neupert effect. For these events, a high correlation between the SXR peak flux and the HXR fluence is obtained, indicative of electron-beam-driven evaporation. However, there is also a significant fraction of flares (about one fourth), which show strong deviations from $\Delta t = 0$, with a prolonged increase of the SXR emission distinctly beyond the end of the HXR emission. These results suggest that electron-beam-driven evaporation plays an important role in solar flares. Yet, in a significant fraction of events, there is also clear evidence for the presence of an additional energy transport mechanism other than the nonthermal electron beams, where the relative contribution is found to vary with the flare importance.