  Radio observations of stars trace the plasma conditions and magnetic field
properties of stellar magnetospheres and coronae. Depending on the plasma
conditions at the emitter site, radio emission in the metre- and decimetre-wave
bands is generated via different mechanisms such as gyrosynchrotron, electron
cyclotron maser instability, and plasma radiation processes. The ongoing LOFAR
Two-metre Sky Survey (LoTSS) and VLA Sky Survey (VLASS) are currently the most
sensitive wide-field radio sky surveys ever conducted. Because these surveys
are untargeted, they provide an opportunity to study the statistical properties
of the radio-emitting stellar population in an unbiased manner. Here, we
perform an untargeted search for stellar radio sources down to sub-mJy level
using these radio surveys. We find that the population of radio-emitting
stellar systems is mainly composed of two distinct categories:
chromospherically active stellar (CAS) systems and M dwarfs. We also seek to
identify signatures of a gradual transition within the M-dwarf population from
chromospheric/coronal acceleration close to the stellar surface similar to that
observed on the Sun, to magnetospheric acceleration occurring far from the
stellar surface similar to that observed on Jupiter. We determine that radio
detectability evolves with spectral type, and we identify a transition in radio
detectability around spectral type M4, where stars become fully convective.
Furthermore, we compare the radio detectability vs spectra type with X-ray and
optical flare (observed by TESS) incidence statistics. We find that the radio
efficiency of X-ray/optical flares, which is the fraction of flare energy
channelled into radio-emitting charges, increases with spectral type. These
results motivate us to conjecture that the emergence of large-scale magnetic
fields in CAS systems and later M dwarfs leads to an increase in radio
efficiency.
