Thermal/Nonthermal with MinXSS and RHESSI

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Number:	431
1st Author:	Shunsaku NAGASAWA
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Published:	13 June 2022
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Contents 1 Introduction 2 Method: Spectral analysis combined with MinXSS and RHESSI 3 Discussion and Conclusion 4 Future work 5 References

Introduction

During solar flares, the energy released through magnetic reconnection is converted into other forms through processes such as the heating of coronal plasma, bulk flows within coronal mass ejections, and particle acceleration. In addition, accelerated particles also contribute to plasma heating through their collisions within the ambient plasma. In this way, the heating, cooling, and particle acceleration processes should be closely related to the magnetic reconnection and correlated with one another. Therefore, to resolve such a complicated energy-conversion system, it is crucial to separate and follow the time evolution of "thermal emission" from heated plasma and "non-thermal emission" from accelerated electrons as a first step.

The time evolution and relationships of thermal and non-thermal emissions in flares have been studied using continuum emission observed by RHESSI in hard X-rays (HXR: E > 10 keV). However, RHESSI is limited in its sensitivity to lower energies corresponding to the soft X-ray (SXR: E < 10 keV) band, which is generally dominated by thermal emission (free-free, bound-free, and lines), particularly from flare plasmas with temperatures of 5-20 MK. Consequently, it is difficult to resolve the multi-temperature structure. Therefore, solar SXR spectral observations with high energy resolution (<1 keV FWHM) and high time resolution (<10 s, comparable to the [https://en.wikipedia.org/wiki/Alfvén_wae Alfven time scale) are required for a precise characterization of such a multi-thermal structure and its relationship to non-thermal emission.

The MinXSS (Miniature X-ray Solar Spectrometer) CubeSat has now routinely archived solar flare spectral observations with high energy resolution (∼0.15 keV FWHM) and high time resolution (10 s time cadence) in the SXR band (Ref. [1)]. Here, we conduct a wide-band X-ray spectral analysis using combined MinXSS SXR and RHESSI HXR data for understanding the thermal and non-thermal emissions in a solar flare.

Method: Spectral analysis combined with MinXSS and RHESSI

We have analyzed the GOES M7.6 class flare SOL2016-07-23T05:00 (note the date coincidence with RHESSI's most-studied flare SOL2002-07-23]). We performed spectral fitting to MinXSS and RHESSI data simultaneously in the broad energy range of 1.5-100 keV. Since, at this time, the standard solar spectrum-fitting software OSPEX cannot do simultaneous joint fitting of data from more than one instrument, we utilize XSPEC (Ref. [2]) instead. This is the standard spectrum analysis tool in the field of high-energy astronomy. Figure 1 shows the results of spectral fitting for each time interval using MinXSS and RHESSI, and the time evolution of the parameters of each of the thermal and non-thermal components are summarized in Figure 2.

By conducting simultaneous fitting using MinXSS and RHESSI spectra, it becomes possible to clearly resolve a non-thermal power-law component and multiple thermal components (a cool plasma at T ~ 3 MK, a hot plasma at T ~ 15 MK), and a super-hot plasma at T ~ 30 MK), and to follow their time evolution with a cadence of 10 s. From the beginning of the flare, both the cool and hot thermal components are required to explain the observed spectra - a single isothermal is not sufficient. As the non-thermal spectrum increases and hardens, the emission measures of both the hot and cool thermal components drastically increase. After that, the non-thermal emission softens, and the super-hot thermal emission (T ~ 30 MK) gradually increases, while continuous microwave emission - optically-thin gyro-synchrotron emission from mildly relativistic non-thermal electrons - is observed simultaneously. Subsequently, the HXR flux peaks a second time and the non-thermal emission hardens to its minimum spectral index of 2.6. Finally, as the non-thermal emission fades, each thermal emission gradually cools.

Discussion and Conclusion

This detailed time evolution information is a key to understanding the origins each spectral component. The time evolution of the spectral components and an imaging DEM (differential emission measure) analysis suggest that the cool and hot thermal components both correspond to plasma filling the flaring loop associated with chromospheric evaporation. On the other hand, a correlation between the super-hot thermal time evolution and microwave emission from non-thermal electrons suggests that the super-hot component could be explained by thermalization of the non-thermal electrons trapped in the flaring loop; see the original paper (Ref [3]) for details.

Future work

Following the time evolution of the multi-temperature structure of the spectra using MinXSS and RHESSI provides new insights into the possible origins of these thermal emissions. However, it is necessary to follow the time evolution of the spectra in each emission region, with improved dynamic range in each spatial region, to elucidate the origin of the super-hot thermal component and its relationship with non-thermal emission. In the future, direct-focusing SXR and HXR observations such as from FOXSI-4 and PhoENiX (Ref. 4) will allow us to observe a wide dynamic range and track spectral evolution region by region. By accumulating such observations, we will be able to clarify the origin of each thermal and non-thermal emission component and their relationship, which will help us understand the particle acceleration and resolve the complex energy conversion system associated with magnetic reconnection

References

[1] "The Miniature X-ray Solar Spectrometer (MinXSS)..."

[2] "XSPEC: The First Ten Years"

[3] "Study of Time Evolution of Thermal and Non-Thermal Emission from an M-Class Solar Flare"

[4] "White Paper of the `Soft X-ray Imaging Spectrocopy' "