Measuring X-ray anisotropy in solar flares. Prospective stereoscopic capabilities of STIX and MiSolFA

Abstract

Context. During a solar flare, a large percentage of the magnetic energy released goes into the kinetic energy of non-thermal particles, with X-ray observations providing a direct connection to keV flare-accelerated electrons. However, the electron angular distribution, a prime diagnostic tool of the acceleration mechanism and transport, is poorly known. Aims. During the next solar maximum, two upcoming space-borne X-ray missions, STIX on board Solar Orbiter and MiSolFA, will perform stereoscopic X-ray observations of solar flares at two different locations: STIX at 0.28 AU (at perihelion) and up to inclinations of ∼25• , and MiSolFA in a low-Earth orbit. The combined observations from these cross-calibrated detectors will allow us to infer the electron anisotropy of individual flares confidently for the first time. Methods. We simulated both instrumental and physical effects for STIX and MiSolFA including thermal shielding, background and X-ray Compton backscattering (albedo effect) in the solar photosphere. We predict the expected number of observable flares available for stereoscopic measurements during the next solar maximum. We also discuss the range of useful spacecraft observation angles for the challenging case of close-to-isotropic flare anisotropy.Results. The simulated results show that STIX and MiSolFA will be capable of detecting low levels of flare anisotropy, for M1-class or stronger flares, even with a relatively small spacecraft angular separation of 20-30• . Both instruments will directly measure the flare X-ray anisotropy of about 40 M-and X-class solar flares during the next solar maximum. Conclusions. Near-future stereoscopic observations with Solar Orbiter/STIX and MiSolFA will help distinguishing between competing flare-acceleration mechanisms, and provide essential constraints regarding collisional and non-collisional transport processes occurring in the flaring atmosphere for individual solar flares.

The high-energy Sun - probing the origins of particle acceleration on our nearest star

“…(e.g. [11,12]), but albedo mirror analysis of strong solar flares [13,14], and linear X-ray polarization measurements (e.g. [15][16][17]), are the only methods that can study anisotropy in a single flare using a single instrument.…”

Monitoring and research of high-energy solar flare emissions with GECAM

“…非热电子的角分布、传输过程、辐射过程等很 可能都是各向异性的 [69] , 因此, 在不同视角所看到 的能谱形状及成像也不相同 [70] . [72,73] ), 但其在行星际 传播过程中的演化仍是未知.…”

Solar Flare Energy Partitioning and Transport -- the Impulsive Phase (a Heliophysics 2050 White Paper)

“…Radio observations yield fundamental information about the the magnetic-field strength and the anisotropy of the electron distribution above 200 keV, which are necessary to understand both acceleration and propagation mechanisms. The anisotropy can also be evaluated from X-ray polarimetry with improved sensitivity [10]. In addition to the electron anisotropy, the directivity of the bremsstrahlung emission affects the observed photon spectrum and can potentially help determine the electron angular distribution [11].…”