Investigation of Small-Scale Energy Releases in Hard X-rays with ​FOXSI

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Introduction

Investigation of small-scale energy releases in the solar corona with enhanced sensitivity and imaging dynamic range can be achieved by using focusing hard X-ray optics, which offer a narrow point spread function (PSF) and high signal-to-noise ratio due to the reduction in detector area. In 2012, the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment first demonstrated the power of direct focusing optics for imaging solar HXRs, observing over the energy range 4-15 keV and achieving a sensitivity ~10 times greater than that of RHESSI (Ref. [1]). To date, FOXSI has had three successful flights and has been funded for a fourth launch, scheduled for 2024.

FOXSI-2 microflare observations

The second FOXSI flight (FOXSI-2), with upgraded instrumentation, was successfully launched on 11 December 2014 from the White Sands Missile Range, New Mexico. During its flight, FOXSI-2 observed two sub-A class microflares from two active regions, AR 12230 and AR 12234. This flight was coordinated with several other instruments including the X-ray Telescope (XRT) onboard Hinode, the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), which observes the full Sun most of the time.

Multi-thermal Differential Emission Measure

This Nugget describes a combined analysis of the microflares observed with the FOXSI-2 flight. This analysis centers on the differential emission measure (DEM), which characterizes the distribution of plasma emission as a function of temperature. Here we make use of the imaging spectroscopy from FOXSI-2 5-6 keV, 6-7 keV, 7-8 keV channels, selected XRT channels (the Be-Med, Be-thin, Al-Med, C-poly, Ti-poly), and AIA (94, 131, 171, 193, 211Å) - three independent and complementary views of the flaring plasmas. These taken together provide unprecedented temperature coverage and compose a good overlap in temperature sensitivity. The resulting microflare DEMs peak around 3 MK and exhibit significant emission above 5 MK.

A well-constrained emission measure (EM) for the microflare from AR 12230 (Figure 1) along with EM loci curves show that FOXSI-2 can better constrain the high temperature emission than using SDO/AIA and Hinode/XRT alone. At log T > 6.7, only the FOXSI-2 flux actually defines the distribution. The contribution above 5 MK is more than two orders of magnitude lower than that of the main component, and at 10 MK is more than three to four orders lower. The other two telescopes (AIA and XRT) simply cannot observe these contributions.

Figure 1: The emission measure of the microflaring active region AR 12230. Black line represents the best EM solution, and the orange dashed line shows the selected EMs obtained during different runs of the Monte Carlo-based inversion algorithm. The colored solid lines are "EM-loci" curves, which provide upper limits for the emission at different isothermal temperatures. This clearly demonstrates that for log T > 6.7, FOXSI loci curves tightly constrain the emission, which otherwise cannot be measured using AIA and XRT alone.

Comparison of the DEM during the quiescent and flaring phases for AR 12234 (Figure 2) indicates a change in the high-temperature slope of the distribution. The decrease of the EM slope from T^-12 to T^-4 (logT = 6.6 to 7.0) during flare times clearly shows the contribution of hot plasma emission above 5 MK. Using these well-determined DEMs, we calculated the comprehensive thermal energy estimates for the microflares, which are found to be ~10²⁸ erg, around the energy at which RHESSI's sensitivity starts to limit the fraction of observed events.

Figure 2: Comparison of the DEM for AR 12234 during flare and non-flare times. The decrease in slope from quiet (∝T¹²) to flare (∝T⁴) phase shows the presence of excess hot plasma > 4 MK contributed from the microflare.

HXR Imaging Spectroscopy

In addition to the multi-thermal DEM analysis described in Ref. [2], we harness the enhanced capabilities of FOXSI-2 to perform the first direct imaging spectroscopy of solar HXRs with an angular resolution (~9 arc sec) at size scales relevant for microflares (Ref. [3]).

Through spectral analyses of the two FOXSI-2 microflares using an optically thin isothermal plasma model, we find evidence for plasma heated to temperatures (T) of ~10 MK and emission measures (EM) down to ~10⁴⁴ cm^-3. Comparing these results to microflares observed by other HXR instruments (Figure 3), we note that one of the microflares observed by FOXSI-2 is roughly an order of magnitude fainter than the faintest events studied with RHESSI. Additionally, FOXSI-2's sensitivity to higher temperature plasmas complements NuSTAR's sensitivity to lower temperature plasmas.

Figure 3: Comparison of microflares observed by FOXSI-2, RHESSI, and NuSTAR, using an isothermal model. This comparison highlights how direct HXR spectroscopic imagers are opening up a novel parameter space for microflare studies.

Examining FOXSI-2 images of the first microflare (AR 12230) in two energy bands during the third flight target (Target C), we find the centroid of the higher-energy emission (6-15 keV) to be east of the low-energy centroid (4-5.5 keV), suggesting higher plasma temperatures toward the eastern part of this flare. With the deconvolved FOXSI-2 images shown in Figure 4, we see that this higher-energy emission extends out towards a small eastern feature that is brightening in AIA 94 during this time interval, whereas the lower-energy emission is concentrated on the dominant middle feature. This combined data set (additional time intervals shown in Ref. [3]) provides compelling evidence for multiple energy releases during this sub-A class flare, indicating that it more closely resembles the complex structure and dynamics of a large flare rather than the single energy release of a nanoflare.

Figure 4: Deconvolved FOXSI-2 image contours of the first microflare (AR 12230) overlaid on contemporaneous AIA 94Å and IRIS 1330Å images. With FOXSI-2, we can clearly identify changes in HXR morphology during this small-scale event, such as the extension of higher-energy emission toward the eastern feature observed in AIA. This higher-energy emission additionally overlaps with the slit in IRIS (at x ~ 10").

Conclusions

As the first solar-dedicated direct HXR spectroscopic imager, FOXSI provides unprecedented capabilities for studying small-scale solar events. The main take-aways from the FOXSI-2 microflare analyses include:

1. The coordinated FOXSI-2 microflare observations produce one of the few definitive measurements of the distribution and the amount of plasma above 5 MK in two sub A-class microflares, showing that these small-scale energy releases have significant emission above 5 MK.

2. Direct imaging spectroscopy of FOXSI-2 microflare data reveals evidence of complex energy release for events down to this small size, indicating that they are more similar in evolution to large flares than to nanoflares.

References

[1] "First Images from the Focusing Optics X-ray Solar Imager"

[2] "FOXSI-2 Solar Microflares. I. Multi-instrument Differential Emission Measure Analysis and Thermal Energies"

[3] "FOXSI-2 Solar Microflares. II. Hard X-ray Imaging Spectroscopy and Flare Energetics"