Solar flares are known to be prolific electron accelerators, yet determining the mechanism( s) for such efficient electron acceleration in magnetic reconnection occasions at the Sun (and comparable astrophysical settings) provides a major difficulty in Astrophysics (e.g. Holman et al 2011). Accelerated electrons with energies above $sim 20$ keV are exposed by hard X-ray (HXR) bremsstrahlung emission, while sped up electrons with even greater energies normally manifest themselves through radio gyrosynchrotron emission. $, respectively) to the total number density of background electrons in velocity area.
HXR observations offer crucial restrictions on the portion of electron acceleration above 20 keV. $$ It is interesting to keep in mind that the microwave spectrum analysis by Fleishman et al. (2022) utilizing 2″ pixels, which are smaller than EOVSA beam resolution of $( 45-5)”$ for the $( 2-18)$ GHz range, gives roughly hundred times bigger fraction of accelerated electrons in the very same region of flare.
Solar flares are understood to be respected electron accelerators, yet recognizing the system( s) for such effective electron velocity in magnetic reconnection occasions at the Sun (and comparable astrophysical settings) presents a major obstacle in Astrophysics (e.g. Holman et al 2011). Accelerated electrons with energies above $sim 20$ keV are revealed by tough X-ray (HXR) bremsstrahlung emission, while accelerated electrons with even higher energies normally manifest themselves through radio gyrosynchrotron emission. Of significant interest is the nature of the procedure that speeds up particles to high energies are the ratios of the number densities (cm$ ^ $) of thermal and nonthermal electrons ($ n _ nth $ and $n _ $, respectively) to the overall number density of background electrons in velocity region.
Recently Kontar et al, 2023 have actually integrated RHESSI HXR observations of a well-observed solar flare with coexisting EUV observations from the Solar Dynamics Observatory Atmospheric Imaging Assembly in order to much better constrain both the total variety of accelerated electrons and the all-important ratio $n _ nth/ n_p$ in 2017 September 10 solar flare (Figure 1) that exposed clear proof for a reconnection existing sheet located above the flare loop-top. The outcomes indicates that the ratio of nonthermal electrons to ambient electrons in ROI-1 at a time near the peak of the X-ray emission is $n _ nth/, n_p simeq 0.01– 0.02$.
The color-scale image represents thermal (10-12 keV) emission. The blue dashed lines are the shapes at 20, 70, and 90% of the peak 50-100 keV non-thermal emission. The area of interest identified ROI-0 is specified by the solid blue shape at the 50% level of the 50-100 keV image.
Conclusions
HXR observations offer essential constraints on the portion of electron velocity above 20 keV. RHESSI observations recommend that this portion of accelerated electrons stays less than a few present. Such numbers are consistent with the classical flare speeding up electrons ($>> 20$ keV) at the rate of $dot =10 ^ $ electrons per second, so the number density of electrons leaving velocity box through the area of $A= 10 ^ 18 $ cm$ ^ 2$ with speed $v(>> 20mbox keV )simeq 10 ^ 10 $ cm/sec.$$ N _ (>> 20 mbox )= frac A v(>> 20mbox keV) simeq 10 ^ 8;; mbox ^ -3 $$ It is interesting to note that the microwave spectrum analysis by Fleishman et al. (2022) using 2″ pixels, which are smaller sized than EOVSA beam resolution of $( 45-5)”$ for the $( 2-18)$ GHz range, provides around hundred times larger portion of sped up electrons in the very same region of flare.
Based on a current paper by Kontar, E. P., Emslie, A. G., Motorina, G. G., and Dennis, B.R., The Efficiency of Electron Acceleration during the Impulsive Phase of a Solar Flare, The Astrophysical Journal Letters, 947 L13, (2023 ), DOI:10.3847/ 2041-8213/ acc9b7.
References:.
Holman, G. D., Aschwanden, M. J., Aurass, H., et al. 2011, SSRv, 159.
Fleishman, G. D., Nita, G. M., Chen, B., Yu, S., & & Gary, D. E., 2022, Nature, 606, 674.
Hannah, I. G., & & Kontar, E. P. 2012, A&A, 539, A14.
