Solar Physics最新文献

Most solar hard X-ray (HXR) imagers in the past and current solar missions obtain X-ray images via Fourier transform imaging technology, which requires proper imaging algorithms to reconstruct images from spatially-modulated or temporally-modulated signals. A variety of algorithms have been developed during the last 50 years for the characteristics of respective instruments. In this work, we present a new imaging algorithm developed based on deep learning for the Hard X-ray Imager (HXI) onboard the Advanced Space-based Solar Observatory (ASO-S) and the preliminary test results of the algorithm with both simulated data and observations. We first created a training dataset by obtaining modulation data from simulated HXR images of single, double and loop-shaped sources, respectively, and the patterns of HXI sub-collimators. Then, we introduced machine-learning algorithm to develop a pattern-based deep learning network model: HXI_DLA, which can directly produce an image from modulation counts. After training the model with simple sources, we tested DLA for simple sources, extended sources, and double sources for imaging dynamic range. Finally, we compared CLEAN and DLA images reconstructed from HXI observations of three flares. Overall, these imaging tests revealed that the current HXI_DLA method produces comparable image result to those from the widely used imaging method CLEAN. In some cases, DLA images are even slightly better. Besides, HXI_DLA is super fast for imaging and parameter-free. Although this is only the first step towards a fully developed and practical DLA method, the tests have shown the potential of deep learning in the field of solar hard X-ray imaging.

Sunspot-area measurements using digital images captured by two telescopes at the Mitaka campus of the National Astronomical Observatory of Japan are conducted using automated sunspot detection. A comparison between sunspot areas derived from Mitaka and those from the reference data by Mandal et al. (Astron. Astrophys. 640, A78, 2020), who compiled a crosscalibrated daily sunspot-area catalog, revealed that the correlation coefficients between them are high (0.96 – 0.97), whereas the areas in the Mitaka data are 70 – 83% of those of Mandal et al. The correlation is limited by the differences in observation times and detection capabilities of spots near the limb, with discrepancies in areas arising from different definitions of spot outlines. Given the high correlation and the ease of calibrating area discrepancies with a correction factor, automated sunspot detection appears promising for future sunspot-area measurements. Furthermore, we addressed the measurements of brightness deficit caused by sunspots.

We present the ground calibration and on-orbit performance of the Full-disk vector MagnetoGraph (FMG) payload on board the Advanced Space-Based Solar Observatory (ASO-S), which is China’s first spaceborne magnetograph. FMG has the ability to acquire the full-disk Stokes I, Q/I, U/I, and V/I maps with a spatial resolution of about 1.5 arcsec. The Lyot filter for the flight model has a full width at half maximum of 0.01 nm. Using two calibration lenses, we measure the non-uniform wavelength drift across the entire field of view, with a maximum value of 0.003 nm. The on-orbit polarization sensitivity is approximately 0.00039 and 0.0009 for the deep integration and routine modes, corresponding to a cadence of 18 and 2 minutes, respectively. The corresponding sensitivity of the longitudinal magnetic field is 8.5 G and 20 G with the current linear calibration coefficient of 21,913. Since 1 April 2023, FMG has released Level 2 filtergram and longitudinal magnetic field data products for active regions. Furthermore, line-of-sight Carrington synoptic magnetograms spanning a 27-day solar rotation can be generated, which have been released to the public since February 2024. The longitudinal magnetic field obtained by FMG is consistent with that of the Helioseismic and Magnetic Imager on board the Solar Dynamic Observatory and the Solar Magnetism and Activity Telescope at Huairou Solar Observing Station for the regions without magnetic saturation.

I will review the history of the helioseismic observations since the beginning of the field. I will explain how each instrument was designed based upon the required observables, and to which modes these instruments are sensitive. The impact of these sensitivities on the rotation and structure inversion will be developed. I will conclude with what remains to be done in this field for the future of detection.

Interplanetary-scintillation (IPS) observations provide useful information on large-scale solar-wind disturbances, such as interplanetary coronal mass ejections (ICMEs) and stream interaction regions (SIRs), which impact the Earth and drive space weather. In the present study, we derived the (G_{mathrm{ave}})-index, which represents daily variations in the density-fluctuation level of the inner heliosphere, based on IPS observations at the Institute for Space-Earth Environmental Research of Nagoya University between 1997 and 2019, and investigated the response of (G_{mathrm{ave}}) to ICME and SIR events. A clear difference was observed in the temporal profile of (G_{mathrm{ave}}) obtained from the superposed-epoch analysis between ICME and SIR events. The (G_{mathrm{ave}})-values for the east and west sides of the sky plane for ICME events increased simultaneously and peaked at the ICME start time, which is consistent with the analysis of ICMEs directed toward the Earth. In contrast, the analysis of SIR events showed an asymmetric response between eastern and western (G_{mathrm{ave}}), with a distinct increase in (G_{mathrm{ave}}) observed on the west side after the SIR start time and higher (G_{mathrm{ave}})-values observed on the east side before the start time. These findings were explained by the effect of the spiral-shaped structure of the SIR. Significant positive correlations were found between (G_{mathrm{ave}}) and solar-wind density and speed, which also showed east–west asymmetry. These phenomena were ascribed to the effect of SIR events, while the occurrence of peak correlations between (G_{mathrm{ave}}) and density at zero delay time for Cycle 23 was ascribed to the effect of ICMEs. The difference in correlations between Cycles 23 and 24 was ascribed to the weakening of activity in Cycle 24. The occurrence of a correlation peak for a positive delay time suggests that eastern and western (G_{mathrm{ave}}) data are useful for predicting the arrival of the solar wind with increased density and speed, respectively, although the correlation magnitudes were weak.

The solar wind is the extension of the Sun’s hot and ionized corona, and it exists in a state of continuous expansion into interplanetary space. The radial distance at which the wind’s outflow speed exceeds the phase speed of Alfvénic and fast-mode magnetohydrodynamic (MHD) waves is called the Alfvén radius. In one-dimensional models, this is a singular point beyond which most fluctuations in the plasma and magnetic field cannot propagate back down to the Sun. In the multi-dimensional solar wind, this point can occur at different distances along an irregularly shaped “Alfvén surface.” In this article, we review the properties of this surface and discuss its importance in models of solar-wind acceleration, angular-momentum transport, MHD waves and turbulence, and the geometry of magnetically closed coronal loops. We also review the results of simulations and data-analysis techniques that aim to determine the location of the Alfvén surface. Combined with recent perihelia of Parker Solar Probe, these studies seem to indicate that the Alfvén surface spends most of its time at heliocentric distances between about 10 and 20 solar radii. It is becoming apparent that this region of the heliosphere is sufficiently turbulent that there often exist multiple (stochastic and time-dependent) crossings of the Alfvén surface along any radial ray. Thus, in many contexts, it is more appropriate to use the concept of a topologically complex “Alfvén zone” rather than one closed surface. This article also reviews how the Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission will measure the properties of the Alfvén surface and provide key constraints on theories of solar-wind acceleration.

We study kink and fluting waves in expanding and twisted magnetic flux tubes. We use the thin-tube and zero-beta plasma approximations. The equilibrium magnetic field is force free with a constant proportionality coefficient between the electrical current and the magnetic field. We derive the equation governing the kink and fluting waves in a tube. Using this equation we study the propagation of kink waves in a particular case of a magnetic tube homogeneous in the axial direction. We show that while there is only one propagating kink wave with the phase speed equal to the kink speed in an untwisted tube, in a twisted tube there are two wave modes, accelerated and decelerated. The phase speed of the accelerated wave exceeds the kink speed, while the phase speed of the decelerated wave is less than the kink speed. We also show that the standing modes are defined by the same eigenvalue problem as that in the case of an untwisted tube. Hence, the frequencies of the standing-wave modes are not affected by the twist. This implies that the seismological results based on the observation of the standing-wave mode frequencies remain valid when the twist is taken into account. The only effect of twist is the variation of the direction of polarisation of the coronal magnetic-loop displacement along the loop. As a result, an apparent node can be detected near the loop apex if only one component of the loop displacement is observed. This can lead to an incorrect conclusion that the observed coronal loop kink oscillation was the first overtone, while in fact it was the fundamental mode.

Solar radio observations provide a powerful diagnostic of the physical conditions of the solar atmosphere over a wide range of heights. In this paper, we report regular solar mapping made at the X-band (8.1 – 9.2 GHz) with the Arecibo 12-m radio telescope, covering a period between 13 December 2021 and 9 April 2023. This has demonstrated its potential for identifying active regions and tracking their brightness-temperature changes. The X-band results are discussed along with the near-simultaneous datasets available from space- and ground-based observations. A comparison of magnetic properties of active regions with their emission characteristics indicates that the X-band brightness temperature provides better information of the magnetic-field strength associated with the emission and a brightness temperature in excess of 13 000 K allows us to infer the possibility of intense flares (i.e., ≳ M1 class) and coronal mass ejections. The ‘latitude–time’ distribution of the brightness temperature reveals the three-dimensional evolution of quiet regions on the Sun, coronal holes, and eruptive sites, over many solar rotations in the ascending phase of the current Solar Cycle 25.

Digital images of sunspot drawings of the archives of Georg Christoph Eimmart stored at the National Library of Russia, St. Petersburg, are analyzed to obtain sunspot-group numbers and sunspot areas as well as heliographic positions. Overall, more than a hundred drawings were processed. The impact of drawing and reproduction uncertainties and the aims of historical observations are considered. The sunspot positions are compared to those reported by contemporary observers of the Maunder minimum. The restored sunspot-group numbers and latitudes are compared to those extracted by Hoyt and Schatten (Solar Phys. 179, 189, 1998) as well as Hayakawa et al. (Solar Phys. 296, 154, 2021b) and Hayakawa et al. (Astrophys. J. 909, 166, 2021d). The persistence of long-lived sunspots over several solar rotations is discussed.