What Determines the X-Ray Intensity and Duration of a Solar Flare?

Abstract

Solar flares are observed and classified according to their intensity measured with the GOES X-ray Sensors. We show that the duration of a flare, as measured by the full width at half maximum (FWHM) in GOES is not related to the size of the flare as measured by GOES intensity. The durations of X-class flares range from a few minutes to a few hours, and the same is true of M-and C-class flares. In this work, we therefore examine the statistical relationships between the basic properties of flares -temperature, emission measure, energy, etc. -in comparison to both their size and duration. We find that the size of the flare is directly related to all of these basic properties, as previously found by many authors. The duration is not so clear. When examining the whole data set, the duration appears to be independent of all of these properties. In larger flares, however, there are direct correlations between the GOES FWHM and magnetic reconnection flux and ribbon area. We discuss the possible explanations, finding that this discrepancy may be due to large uncertainties in small flares, though we cannot rule out the possibility that the driving physical processes are different in smaller flares than larger ones. We discuss the implications of this result and how it relates to the magnetic reconnection process that releases energy in flares.

The Neupert Effect of Flare UltraViolet and Soft X-ray Emissions

“…Therefore, the UV emission signatures at the foot-points of flare loops well capture heating events during the flare regardless of heating mechanisms. The flare heating energy E tot is also shown to scale with the total reconnection flux Φ rec and the median length of the flare half-loops L by E tot ∼ Φ 1.1±0.2 rec L 0.6±0.1 ; the dependence of the heating energy on the magnetic field is similar to scaling laws found in some studies, though with various contexts (Schrijver et al 2004;Zhu et al 2018;Reep & Knizhnik 2019), but different from some other studies such as by Aschwanden (2020a,c, and references therein). On the other hand, we do not find a strong dependence of the heating energy on the magnetic field (flux) and/or the loop length for individual loop events down to the pixel scale (∼ 0.6 ′′ ).…”

“…5 Here L is the median length of the loop events in units of Mm. The energy dependence on Φ rec is very close to that found by Reep & Knizhnik (2019) and Zhu et al (2018). Reep & Knizhnik (2019) analyzed a few thousand flares, and the energy in the scaling law refers to the flare thermal energy at the time of peak temperature, deduced from GOES SXR observations.…”

“…The energy dependence on Φ rec is very close to that found by Reep & Knizhnik (2019) and Zhu et al (2018). Reep & Knizhnik (2019) analyzed a few thousand flares, and the energy in the scaling law refers to the flare thermal energy at the time of peak temperature, deduced from GOES SXR observations. Zhu et al (2018) analyzed only one event SOL2011-12-16 (# 3) and the flux-energy patches are grouped into a few tens magnetic cells to construct the scaling law; Zhu et al (2018) did not reveal a dependence on the loop length, which does not vary significantly during this flare.…”

“…for 14 flares (excluding #14 and #15 that are not well modeled); again, the fluxenergy dependence is closest to the WM16 scaling law of the bolometric energy. The difference from the other scaling laws by, e.g., Warren & Antiochos (2004); Reep et al (2013); Reep & Knizhnik (2019); Aschwanden (2020b) may be due to the fact that flare heating takes place over an extended time period beyond the impulsive phase, and is not provided only by non-thermal electrons.…”

“…One-dimensional loop simulations by Reep et al (2013) yielded a similar scaling law F sxr ∼ E 1.7 tot . However, analyzing a few thousand flares using the database by Kazachenko et al (2017), Reep & Knizhnik (2019) found sub-linear scaling laws F sxr ∼ E 0.85 th , and F sxr ∼ E 0.85 tot , the former referring to the thermal energy of the flare (at the time of peak temperature) derived from the GOES SXR analysis, and the latter referring to the flare heating energy deduced from the traditional Neupert effect, i.e., E tot being the non-thermal electron energy. Similarly, Aschwanden (2020b) found F sxr ∼ E 0.7…”