  It is well known among the scientific community that solar flare activity
often begins well before the main impulsive energy release. Our aim is to
investigate the earliest phase of four distinct flares observed by Solar
Orbiter/STIX and determine the relationships of the newly heated plasma to
flare structure and dynamics. The analysis focuses on four events that were
observed from both Earth and Solar Orbiter, which allows for a comparison of
STIX observations with those of GOES/XRS and SDO/AIA. The early phases of the
events were studied using STIX and GOES spectroscopic analysis to investigate
the evolution of the physical parameters of the plasma, including the
isothermal temperature and emission measure. Furthermore, to determine the
location of the heated plasma, STIX observations were combined with AIA images.
The events with clear emission prior to the impulsive phase show elevated
temperatures ($> 10\,\mathrm{MK}$) from the very beginning, which indicates
that energy release started before any detection by STIX. Although the
temperature shows little variation during the initial phase, the emission
measure increases by about two orders of magnitude, implying a series of
incrementally greater energy releases. The spectral analysis of STIX and GOES
from the very first time bins suggests that the emission has a multi-thermal
nature, with a hot component of more than $10\,\mathrm{MK}$. This analysis
confirms the existence of "hot onsets," with STIX detecting the hot onset
pattern even earlier than GOES. These elevated temperatures imply that energy
release actually begins well before any detection by STIX. Therefore, hot
onsets may be significant in the initiation, early development, or even
prediction of solar flares.
