Recently we have observed a rapid change in brightness of comet C/2021 A1 (Leonard). Magnitude of this comet should be strongly enhanced by forward scattering around 14. and 15. December 2021 and the days around this date. However observations indicating possible outburst near this date.
Artyom Novichonok reported for Dec 14.93 a magnitude 2.6 for comet Leonard. A problem with Earth based observations are that there was predicted maximum for so called forward scattering enhancement for same day. However existence of outburst was independently confirmed by two other reliable sources. Nicolas Biver reported a increase of water production as measured on Nancay radiotelescope from 1.8 tons/s (Dec 13.5) to 6 tons/s (Dec 15.5), this measure can't be affected by forward scattering by any way. Second independent confirmation came from Karl Battams who posted a video from SECCHI/HI-2 camera on STEREO-A, which clearly shows increase of activity on Dec 14. I note that forward scattering enhancement doesn't take place from viewing angle of STEREO-A spacecraft.
Forward scattering prediction
Forward scattering itself complicates the situation, as it must enhanced the magnitude of dust cloud from outburst. A 3.3 fold increase in water production rate reported by Biver could be translated to a 1.3 magnitude increase (0.6 magnitude increase in Dec 16.5). We can't clearly estimate if there was same effect on magnitude itself as we don't know the dust to gas ratio of comet, however it is clerly much smaller than the visually observed ~2.5 magnitude increase. If we analyse forward scattering itself, we can clearly see, that this comet was very dust poor before the maximum of magnitude. The clearly effect from forward scattering should be visible from Dec 7 already if the dust to gas ratio of this comet would be near 1. Apparently it was much smaller. On the graph bellow you can see three lines, first line is for normal magnitude of comet based on magnitude parameters fited for T-45 to T-25 time span. Second line is for magnitude enhanced by forward scattering with extremely dust poor 0.05 dust to gas ratio, which well fit the magnitude enhancement untill Dec 12. Third line is for magnitudes enhanced by forward scattering with dust to gas ratio 0.25 (5 fold increase) which fit the magnitudes after Dec 14. If forward scattering would take place without an outburst, to fir observations after Dec 14, the magnitude of comet must be already 4.5 on Dec 11, however observed magnitudes got mean magnitude 5.0. On Dec 12, enhanced magnitude should be already near value 3.8 while only observation gives comet magnitude still at 5.1 mag. To explain this gap we would already require the 5 fold increase in dust to gas ratio between Dec 12 and 14, so the event is evident even from this analysis.
The real size of outburst
Apparently the "outburst" of comet C/2021 A1 (Leonard) was very small, but the increased amount of dust produced from nucleus was heavily enhanced by forward scattering which almost give no addition to magnitude days before. We may estimate that without forward scattering the magnitude increase of comet would be only between 1-2 mag from pre-outburst values for the first day and most likely <1 mag in the days after.
Today, Thomas Lehmann reported another small outburst, while he measured a magnitude 4.5 on Dec 17.78 and 3.9 on Dec 18.78. Due changes in distances from Sun, Earth and forward scattering enhancement we would expect a magnitude to drop by 0.5 between these two days, so additionaly we have observed another ~1 mag outburst, similar size to previous event. Apparently the activity evolution of this comet is very chaotic and uncalm.