  There are two goals in this study. One is to investigate how frequently
white-light flares (WLFs) occur in a flare-productive active region (NOAA
active region 11515). The other is to investigate the relationship between WLFs
and magnetic transients (MTs). We use the full-disk continuum filtergrams and
line-of-sight magnetograms taken by SDO/HMI to identify WLFs and MTs,
respectively. Images taken by SDO/AIA are also used to show the morphology of
the flares in the upper atmosphere. We found at least 20 WLFs out of a total of
70 flares above C class (28.6%) in NOAA active region 11515 during its passage
across the solar disk. Each of these WLFs occurred in a small region, with a
short duration of about 5 minutes. The enhancement of white-light continuum
intensity is usually small, with an average enhancement of 8.1%. The 20 WLFs
observed were found along an unusual configuration of the magnetic field
characterized by a narrow ribbon of negative field. Furthermore, the WLFs were
found to be accompanied by MTs, with radical changes in magnetic field strength
(or even a sign reversal) observed during the flare. In contrast, there is no
obvious signature of MTs in those 50 flares without white-light enhancements.
Our results suggest that WLFs occur much more frequently than what was
previously thought, with most WLFs being fairly weak enhancements. This may
explain why WLFs are not frequently reported. Our observations also suggest
that MTs and WLFs are closely related and appear co-spatial and co-temporal,
when considering HMI data. A larger enhancement of WL emission is often
accompanied by a larger change of the line-of-sight component of the unsigned
magnetic field. Considering the close relationship between MTs and WLFs, many
previously reported flares with MTs may be WLFs.

Solar and Stellar Astrophysics