Flare activity of the Sun and variations in its UV emission during cyrcle 24

TL;DR

This study analyzes solar flare activity and UV emission variations during cycle 24, highlighting differences from previous cycles, flare propagation directions, hemispheric asymmetries, and proton event delays.

Contribution

It provides new insights into the timing, distribution, and characteristics of solar flares and UV emission during cycle 24, including hemispheric asymmetries and propagation patterns.

Findings

Most powerful flares in cycle 24 occurred during rising and maximum phases.

Regression fits of UV indices differ significantly from previous cycles.

Delay of about 2 hours observed between flare onset and proton events.

Abstract

The flare activity and the ultraviolet emission of the sun during its 24-th cycle are analysed. As compared to cycles 21-23, where the most powerful flares were observed during the decay phase, in cycle 24 the greatest number of powerful flares (>X2.7) occurred in the rising phase and at the maximum with the exception of the two largest flares of cycle 24 X9.3 and X8.2 in September 2017. We showed that regression fits of solar UV indices to the overall radiation level from the sun are substantially different for cycle 24 compared to cycles 21-23. It is found that for the flare of August 9, 2011 (SDO and GOES-15 observations), the flare propagates in a direction from the upper corona to the transition region and to the chromosphere. A study of the N-S asymmetry in the distribution of the flares in cycle 24 reveals a strong predominance of flares in the N-hemisphere in 2011 and in the S-hemisphere in 2014. It is also found that during cycles 23 and 24, the delays in the onset of proton events relative to the onset of the flares that cause them have a distribution with a distinct maximum corresponding to a delay of 2 hours for protons with energies >10 MeV, as well as for those with energies >100 MeV.