Thermodynamic Spectrum of Solar Flares Based on SDO/EVE Observations: Techniques and Statistical Results

TL;DR

This paper introduces a new method to analyze the thermodynamic processes of solar flares using SDO/EVE spectral data, revealing patterns in flare cooling, temperature drift, and late-phase behavior, with implications for understanding solar eruptions.

Contribution

The paper develops a novel thermodynamic spectrum (TDS) chart technique for solar flare analysis and provides comprehensive statistical results on flare cooling and temperature drift patterns.

Findings

EUV peaks lag behind SXR peaks, with faster cooling in stronger flares.

Two temperature drift patterns identified: Type I (quadrilateral) and Type II (triangular).

Strong late-phase flares are confined and linked to energy re-deposition.

Abstract

SDO/EVE provides rich information of the thermodynamic processes of solar activities, particularly of solar flares. Here, we develop a method to construct thermodynamic spectrum (TDS) charts based on the EVE spectral lines. This tool could be potentially useful to the EUV astronomy to learn the eruptive activities on the distant astronomical objects. Through several cases, we illustrate what we can learn from the TDS charts. Furthermore, we apply the TDS method to 74 flares equal to or greater than M5.0-class, and reach the following statistical results. First, EUV peaks are always behind the soft X-ray (SXR) peaks and stronger flares tend to have a faster cooling rate. There is a power law correlation between the peak delay times and the cooling rates, suggesting a coherent cooling process of flares from SXR to EUV emissions. Second, there are two distinct temperature drift patterns, called Type I and Type II. For Type I flares, the enhanced emission drifts from high to low temperature like a quadrilateral, whereas for Type II flares, the drift pattern looks like a triangle. Statistical analysis suggests that Type II flares are more impulsive than Type I flares. Third, for late-phase flares, the peak intensity ratio of the late phase to the main phase is roughly correlated with the flare class, and the flares with a strong late phase are all confined. We believe that the re-deposition of the energy carried by a flux rope, that unsuccessfully erupts out, into thermal emissions is responsible for the strong late phase found in a confined flare. Besides, with some cases we illustrate the signatures of the flare thermodynamic process in the chromosphere and transition region in TDS charts. These results provide new clues to advance our understanding of the thermodynamic processes of solar flares and associated solar eruptions, e.g., coronal mass ejections.