[en] From the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) catalog we select events which have approximately the same GOES class (high C - low M or 500–1200 counts s"−"1 within the RHESSI 6–12 keV energy band), but with different maximal energies of detected hard X-rays. The selected events are subdivided into two groups: (1) flares with X-ray emissions observed by RHESSI up to only 50 keV and (2) flares with hard X-ray emission observed also above 50 keV. The main task is to understand observational peculiarities of these two flare groups. We use RHESSI X-ray data to obtain spectral and spatial information in order to find differences between selected groups. Spectra and images are analyzed in detail for six events (case study). For a larger number of samples (85 and 28 flares in the low-energy and high-energy groups respectively) we only make some generalizations. In spectral analysis we use the thick-target model for hard X-ray emission and one temperature assumption for thermal soft X-ray emission. RHESSI X-ray images are used for determination of flare region sizes. Although thermal and spatial properties of these two groups of flares are not easily distinguishable, power law indices of hard X-rays show significant differences. Events from the high-energy group generally have a harder spectrum. Therefore, the efficiency of chromospheric evaporation is not sensitive to the hardness of nonthermal electron spectra but rather depends on the total energy flux of nonthermal electrons. (paper)

[en] The power-law frequency distributions of the peak flux of solar flare X-ray emission have been studied extensively and attributed to a system having self-organized criticality (SOC). In this paper, we first show that, so long as the shape of the normalized light curve is not correlated with the peak flux, the flux histogram of solar flares also follows a power-law distribution with the same spectral index as the power-law frequency distribution of the peak flux, which may partially explain why power-law distributions are ubiquitous in the Universe. We then show that the spectral indexes of the histograms of soft X-ray fluxes observed by GOES satellites in two different energy channels are different: the higher energy channel has a harder distribution than the lower energy channel, which challenges the universal power-law distribution predicted by SOC models and implies a very soft distribution of thermal energy content of plasmas probed by the GOES satellites. The temperature ( T ) distribution, on the other hand, approaches a power-law distribution with an index of 2 for high values of T. Hence the application of SOC models to the statistical properties of solar flares needs to be revisited. (paper)

[en] This paper discusses the potential of future joint hard X-ray solar flare observations between the Hard X-ray Imager (HXI) onboard the Advanced Space-based Solar Observatory (ASO-S) mission and the Spectrometer/Telescope for Imaging X-rays (STIX) on Solar Orbiter. The different viewing perspectives of the two telescopes relative to the Sun will allow us for the first time to systematically study non-thermal hard X-ray emissions stereoscopically. During the 4-years of the nominal mission of ASO-S, we expect to jointly observe about 160 flares above GOES M1 class to systematically study hard X-ray directivity. For about 16 partially limb-occulted STIX flares, we will have observations of the entire flare by HXI. Such observations will enable us to simultaneously study the all-important coronal hard X-ray sources, which are generally lost in the instrument’s individual imaging dynamic range, in combination with the chromospheric footpoint emissions. The two different detector systems used in the two telescopes make the relative calibration between the two instruments a key task that needs to be addressed before creditable science results can be published. If an accurate inter-calibration can be achieved using jointly observed flares on the disk, observations with HXI and STIX will provide new and essential key diagnostics for solar flare physics. (paper)

[en] We investigate the 2005 August 22 flare event (00:54 UT) exploiting hard X-ray (HXR) observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and microwave (MW) observations from the Nobeyama Solar Radio Observatory. The HXR time profile exposes well-damped quasi-periodic pulsations with four sequential peaks, and the MW time profile follows the corresponding peaks. Based on this feature, we derive the time relationship of HXRs and MWs with multi-frequency data from the Nobeyama Radio Polarimeter, and the spatially resolvable data from RHESSI and the Nobeyama Radioheliograph. We find that both frequency dependent delays in MWs and energy dependent delays in HXRs are significant. Furthermore, MW emissions from the south source are delayed with respect to those from the north source at both 17 GHz and 34 GHz, but no significant delays are found in HXR emissions from the different sources at the same energies. To better understand all these long time delays, we derive the electron fluxes of different energies by fitting the observed HXR spectra with a single power-law thick-target model, and speculate that these delays might be related to an extended acceleration process. We further compare the time profile of a MW spectral index derived from 17 and 34 GHz fluxes with the flux densities, and find that the spectral index shows a strong anti-correlation with the HXR fluxes. (research papers)

[en] The frequency distribution for several characteristics of a solar flare obeys a power law only above a certain threshold, below which there is an apparent loss of small scale events presumably caused by limited instrumental sensitivity and the corresponding event selection bias. It is also possible that this deviation in the power law can have a physical origin in the source. We propose two fitting models incorporating a power law distribution with a low count rate cutoff plus a noise component for the frequency distribution of the hard X-ray peak count rate of all solar flare samples obtained with HXRBS/SMM and BATSE/CGRO observations. Our new fitting method produces the same power-law index as previously developed methods, a low cutoff of the power-law function and its corresponding noise level, which is consistent with measurements of the actual noise level of the hard X-ray count rate. We found that the fitted low cutoff appears to be related to the noise level, i.e., flares are only recognized when their peak count rate is 3σ greater than noise. Therefore, the fitted low cutoff, which is smaller than the aforementioned threshold, might be attributed to selection bias, and probably not to the actual count rate cutoff in flares at smaller scales. Whether or not the actual low cutoff physically exists needs to be checked by future observations with increased sensitivities

[en] China’s first solar mission, the Advanced Space-based Solar Observatory (ASO-S), is now changing from Phase B to Phase C. Its main scientific objectives are summarized as ‘1M2B’, namely magnetic field and two types of bursts (solar flares and coronal mass ejections). Among the three scientific payloads, Hard X-ray Imager (HXI) observes images and spectra of X-ray bursts in solar flares. In this paper, we briefly report on the progresses made by the HXI science team (data and software team) during the design phase (till May 2019). These include simulations of HXI imaging, optimization of HXI grids, development of imaging algorithms, estimation of orbital background, as well as in-orbit calibration plan. These efforts provided guidance for the engineering, improved HXI’s imaging capability and reduced the cost of the instrument. (paper)

[en] The Full-disk vectorMagnetoGraph (FMG) instrument will carry out polarization observations at one wavelength position of the Fe I 5324.179 °A spectral line. This paper describes how to choose this single wavelength position, the relevant data-processing and the magnetic field calibrations based on the measured polarization signals at one single wavelength position. It is found that solar radial Doppler velocity, which can cause the spectral line to shift, is a disadvantageous factor for the linear calibration at one wavelength position. Observations at two symmetric wavelength positionsmay significantly reduce the wavelength shift effect (∼ 75%), but simulations show that such polarization signals located at the solar limbs (e.g., beyond the longitude range of ±30°) are not free from the effect completely. In future work, we plan to apply machine learning techniques to calibrate vector magnetic fields, or employ full Stokes parameter profile inversion techniques to obtain accurate vector magnetic fields, in order to complement the linear calibration at the single wavelength position. (paper)

[en] The Lyman-alpha Solar Telescope (LST) is one of the three payloads onboard the Advanced Space-based Solar Observatory (ASO-S) mission. It aims at imaging the Sun from the disk center up to 2.5 R_⊙ targeting solar eruptions, particularly coronal mass ejections (CMEs), solar flares, prominences/filaments and related phenomena, as well as the fast and slow solar wind. The most prominent speciality of LST is the simultaneous observation of the solar atmosphere in both Lyα and white light (WL) with high temporospatial resolution both on the solar disk and the inner corona. New observations in the Lyα line together with traditionalWL observations will provide us with many new insights into solar eruptions and solar wind. LST consists of a Solar Corona Imager (SCI) with a field of view (FOV) of 1.1 – 2.5R_⊙, a Solar Disk Imager (SDI) and a full-disk White-light Solar Telescope (WST) with an identical FOV up to 1.2R_⊙. SCI has a dual waveband in Lyα (121.6 ± 10 nm) and in WL (700 ± 40 nm), while SDI works in the Lyα waveband of 121.6 ± 7.5 nm and WST works in the violet narrow-band continuum of 360 ± 2.0 nm. To produce high quality science data, careful ground and in-flight calibrations are required. We present our methods for different calibrations including dark field correction, flat field correction, radiometry, instrumental polarization and optical geometry. Based on the data calibration, definitions of the data levels and processing procedures for the defined levels from raw data are described. Plasma physical diagnostics offer key ingredients to understand ejecta and plasma flows in the inner corona, as well as different features on the solar disk including flares, filaments, etc. Therefore, we are making efforts to develop various tools to detect the different features observed by LST, and then to derive their physical parameters, for example, the electron density and temperature of CMEs, the outflow velocity of the solar wind, and the hydrogen density and mass flows of prominences. Coordinated observations and data analyses with the coronagraphs onboard Solar Orbiter, PROBA-3, and Aditya are also briefly discussed. (paper)

[en] As one of the payloads for the Advanced Space-based Solar Observatory (ASO-S) mission, the Lyman-alpha (Lyα) Solar Telescope (LST) is aimed at imaging the Sun and the inner corona up to 2.5R_⊙ (mean solar radius) in both the Lyα (121.6 nm) and visible wavebands with high temporo-spatial resolution, mainly targeting solar flares, coronal mass ejections (CMEs) and filaments/prominences. LST observations allow us to trace solar eruptive phenomena from the disk center to the inner corona, to study the relationships between eruptive prominences/filaments, solar flares and CMEs, to explore the dynamical processes and evolution of solar eruptions, to diagnose solar winds, and to derive physical parameters of the solar atmosphere. LST is actually an instrument suite, which consists of a Solar Disk Imager (SDI), a Solar Corona Imager (SCI), aWhite-light Solar Telescope (WST) and two Guide Telescopes (GTs). This is the first paper in a series of LST-related papers. In this paper, we introduce the scientific objectives, present an overview of the LST payload and describe the planned observations. The detailed design and data along with potential diagnostics are described in the second (Paper II) and third (Paper III) papers, respectively, appearing in this issue. (paper)

[en] The Advanced Space-based Solar Observatory (ASO-S) is a mission proposed for the 25th solar maximum by the Chinese solar community. The scientific objectives are to study the relationships between the solar magnetic field, solar flares and coronal mass ejections (CMEs). Three payloads are deployed: the Full-disk vector MagnetoGraph (FMG), the Lyman-α Solar Telescope (LST) and the Hard X-ray Imager (HXI). ASO-S will perform the first simultaneous observations of the photospheric vector magnetic field, non-thermal imaging of solar flares, and the initiation and early propagation of CMEs on a single platform. ASO-S is scheduled to be launched into a 720 km Sun-synchronous orbit in 2022. This paper presents an overview of the mission till the end of Phase-B and the beginning of Phase-C. (paper)