HOP 0449

abstract of observational proposal

Main Objective:
To investigate plasma diagnostics and dynamics during flare precursor events, where hot emission is observed in X-rays.

Scientific Justification:
Despite the standard picture describing solar flares can be considered a satisfactory approximation of reality, more sophisticated interpretations are needed in order to explain the observations. For instance, it has been observed since SMM that preflare enhancements are seen in coronal emission lines and this is not yet fully understood.

In this regard, Hudson et al. (2021) reported an unexpected common temperature behavior during the earliest detectable stages of solar flares. By means of GOES/XRS diagnostics, they reported temperatures between 10 and 15 MK. Moreover, by analyzing SDO/AIA images, they suggested that these early hot onset sources are mainly footpoints and low-lying loops that subsequently become parts of the structure of the flare. However, on the one hand, the limited GOES/XRS response does not allow to detect any potential X-ray emission produced by non-thermal bremsstrahlung during these early stages. On the other hand, SDO/AIA alone, due to the broad temperature response of its passbands, does not allow to properly constrain the origin of these hot sources.

In order to have a better temporal description of both temperature and location of these hot sources, X-ray imaging and spectroscopy in a broader temperature range is needed. The X-ray telescope on board Solar Orbiter, STIX, diagnoses and gives imaging information of thermal and non-thermal emission at high cadence for plasma at temperatures from about 8 MK and above. Moreover, the observations provided by XRT will allow us to constrain the high-temperature end of the SDO/AIA DEM, which, in combination with the STIX measurements, allow to clearly disentangle these hot sources in the EUV images provided by AIA.

EIS and IRIS observations have been able to spatially resolve the locations of preflare enhancements seen in coronal emission lines from SMM. The locations of the enhancements were found to differ depending on whether the flare was eruptive or not (Harra et al., 2013). In this IHOP we want to explore the preflare behavior in EIS, IRIS, STIX and XRT and try to spatially and temporarily correlate these preflare enhancements seen at different wavelengths, in order to shed some light on this compelling matter.

other participating instruments

IRIS requests:
The IRIS request is similar to HOP 0446 :

PART A (observation repeated by first 80% of the observation time)
-high-data rate:
OBS 3622606133: Large coarse 8-step raster; tracker =ON
Large coarse 8-step raster 14x120 8s C II Si IV Mg II h/k Mg II w s Deep x 4 Spatial x 2, Spectral x 2 FUV spectrally rebinned x 4 SJI cadence 0.5x faster

-low-data rate:
OBSID: 3622606132: Large coarse 8-step raster; tracker =ON
Medium coarse 8-step raster 14x60 8s C II Si IV Mg II h/k Mg II w s Deep x 4 Spatial x 2, Spectral x 2 FUV spectrally rebinned x 4 SJI cadence 0.5x faster

PART B (observation repeated by last 20% of the observation time):
-high-data rate:
-OBSID: 3660259103, tracker =ON
Large sit-and-stare 0.3x120 1s C II Si IV Mg II h/k Deep x 8 FUV spectrally reb nned x 2

-low-data rate:
-OBSID: 3660259102, tracker =ON
Medium sit-and-stare 0.3x60 1s C II Si IV Mg II h/k Deep x 8 FUV spectrally rebinned x 2

Additional instrument coordination:
Solar Orbiter/STIX

remarks

Dates: ToO program in coordination with STIX, a day or two before and after the 28th of March 2023. During this period, Solar Orbiter will observe the same face of the Sun as Earth, but from a distance of about 0.4 AU. We do not have constraints on the minimum number of observation days during the campaign, but to increase the chance to observe flares, the maximum number of possible days will be highly appreciated. For the proposed observations, it is not necessary to be on consecutive days.

Time window: For this campaign, there are no time window constraints, since STIX is continuously observing the full-disk.

Target(s) of interest: Flares, therefore we would prioritize complex active regions with higher probability of producing large flares.

Previous HOPs:
HOP 361 - was run in September/October 2020 - a paper is just in the review process.