The Morphology of Flare Time Profiles

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Nugget
Number: 412
1st Author: Larisa KASHAPOVA et al.
2nd Author:
Published: 12 July 2021
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Contents

Introduction

The decay phase of solar flares does not attract as much attention as the impulsive one, despite the comparable contribution to the energy budget of this phenomenon. The aim of our study (Ref. [1]) was to provide an template for the analysis of cooling during the decay phase and to reveal cases related to any additional sources of energy release. Our study was inspired by the result obtained in Ref.[2] for flares on a single prolific M4 dwarf star, GJ 1243. Despite a database of over 800 stellar flare events, the averaged profile showed remarkably little scatter and was well described by two exponential decay phases, one each ideally showing where thermal conductivity losses or radiative cooling losses dominate. This study used observations from NASA's Kepler satellite, which observed in broad-band white light. Our aim was to recreate Davenport's analysis but for solar flares. However, white-light flares are rare phenomena on the Sun, especially via Sun-as-a-star observations of the whole solar disk. Instead, we have recreated the analysis for solar flares observed in several spectral bands by SDO/AIA.

Observations and data processing

In the first stage of our study, we selected flares with classical time profiles using the GOES flare catalogue. A classical time profile was defined as a fast rise followed by a slow decay without any flattening or additional peaks. The initial list consisted of 359 solar flares from B5 to X9.3 class (as defined by GOES). Then we obtained the total Sun-as-a-star flux using the images obtained by SDO/AIA for the 1600 Å, 1700 Å and 304 Å channels. So that the flare time profiles could be compared, each was normalised to the flux maximum. To get the same time scale for events of different duration, we normalised the time axis of each event to units defined by the time taken for the intensity to decrease to half the maximum for each time profile (t1/2). We again checked the obtained time profiles in each channel to ensure each flare had a classical time profile. The final list contains 104 events in total from the 1600 Å channel, 102 events form the 304 Å channel, and only 53 events in the 1700 Å channel. For each channel, we then determined the median flare profile and interquartile ranges (see the left panel in Figure 1). The averaged time profiles were fitted with analytical templates, using different time intervals, that consisted of a combination of two independent exponents (see example in the right panel of the Figure 1) or a broken power law.

Figure 1: Left panel: Time profile of solar flare emission at 1600 Å. Red data points are the individual data from the 104 flares in the sample, thick blue line shows the median values, and thin green lines show the interquartile ranges. Right panel: Right panel: Average (median) time profile of solar emission at 1600 Å with two exponential functions for different periods.

Comparison with results of simulations and with stellar flare time profiles

The averaged time profiles of solar low atmosphere emission demonstrate a similar behaviour to the time profile of the M4 dwarf (see the left panel in Figure 2). Although the solar profiles can also be fitted with two exponentials, a slightly wider time range is required to represent the second phase, as the decay there is slower in the solar case. The simulated time profile of Ref. [3] shows good agreement in the initial decay phase, as can be seen in Figure 2. The simulation profile includes a kick associated with chromospheric evaporation at around 1.5t1/2, which is also in good agreement with the observations. However, after this kick, the model decays substantially faster than the observations (the right panel in Figure 2). There are two possible reasons for this - chromospheric evaporation could be more powerful, or the flux decaying could be flatter.

Figure 2: Left panel: The resulting time profiles of solar flare emissions vs the time profile of the M4 dwarf of Ref. [1]. Right panel: Comparison of the resulting time profile of solar flare emission at 304 Å with the theoretical time profile from Ref. [3].

Conclusions

Even though we compare different wavebands, the solar and M4 dwarf flare profiles are similar. The accuracy of the empirical model depends on the choice of time ranges for fitting. The cooling processes in solar flares were best described by the two-component model, fitted over the intervals t1= [0, 0.5] x t1/2 and t2= [3, 10] x t1/2. The broken power-law model provided a good fit to the first decay phase, as it was able to account for the impact of chromospheric plasma evaporation, but it did not successfully fit the second decay phase. However, the mechanisms of plasma cooling should be analysed more carefully for better reproducing at least the averaged time profiles by simulations.

Acknowledgements

The co-authors of this Nugget, and of Ref. [1], are Anne-Marie Broomhall, Elena Kupriyanova, Alena Larionova and Ilya Motyk.

References

[1] "The morphology of average solar flare time profiles from observations of the Sun's lower atmosphere"

[2] "Kepler Flares. II. The Temporal Morphology of White-light Flares on GJ 1243"

[3] "Radiative Hydrodynamic Models of Optical and Ultraviolet Emission from M Dwarf Flares"

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