MHD simulations of inward shocks in Cassiopeia A

Cassiopeia A, the brightest radio supernova remnant(SNR) in the sky, has several unique characteristics in comparison to its peers. Besides its radio brightness and prominent soft-concave radio spectrum,its γ-ray spectrum appears to have a low-energy cutoff near 2 GeV, and it is the only SNR with prominent hard X-ray emission. While the unusual radio properties may be attributed to strong emission from reverse shocks, the hard X-ray emission has been associated with high-speed inward shocks induced by high density gases. Then, the low-energy γ-ray spectral cutoff could be attributed to slow penetration of lower energy particles accelerated near the inward shocks into high-density emission zone. In this paper,we carry out magneto-hydrodynamic(MHD) simulations of shocks in Cassiopeia A and demonstrate that its inward shock structure can indeed be reproduced via shock interactions with clumps of gases with a density of ~ 20 cm~(-3).     

Forecast daily indices of solar activity, F10.7, using support vector regression method

The 10.7 cm solar radio flux （F10.7）, the value of the solar radio emission flux density at a wavelength of 10.7 cm, is a useful index of solar activity as a proxy for solar extreme ultraviolet radiation. It is meaningful and important to predict F10.7 values accurately for both long-term （months-years） and short-term （days） forecasting, which are often used as inputs in space weather models. This study applies a novel neural network technique, support vector regression （SVR）, to forecasting daily values of F10.7. The aim of this study is to examine the feasibility of SVR in short-term F10.7 forecasting. The approach, based on SVR, reduces the dimension of feature space in the training process by using a kernel-based learning algorithm. Thus, the complexity of the calculation becomes lower and a small amount of training data will be sufficient. The time series of F10.7 from 2002 to 2006 are employed as the data sets. The performance of the approach is estimated by calculating the norm mean square error and mean absolute percentage error. It is shown that our approach can perform well by using fewer training data points than the traditional neural network.     

The first detection of the solar U+Ⅲ association with an antenna prototype for the future lunar observatory

We report about observations of solar U+Ⅲ bursts on 2020 June 5 by means of a new active antenna designed to receive radiation in 4–70 MHz. This instrument can serve as a prototype of the ultralong-wavelength radio telescope for observations on the farside of the Moon. Our analysis of experimental data is based on simultaneous records obtained with the antenna arrays GURT and NDA in high frequency and time resolution, e-Callisto network as well as by using the space-based observatories STEREO and WIND. The results from this observational study confirm the model of Reid and Kontar.     

Resonant Heating of Ions by Parallel Propagating Alfvén Waves in Solar Coronal Holes

Resonant heating of H, O+5, and Mg+9 by parallel propagating ion cyclotron Alfven waves in solar coronal holes at a heliocentric distance is studied using the heating rate derived from the quasilinear theory. It is shown that the particle-AlfVen-wave interaction is a significant microscopic process. The temperatures of the ions are rapidly increased up to the observed order in only microseconds, which implies that simply inserting the quasilinear heating rate into the fluid/MHD energy equation to calculate the radial dependence of ion temperatures may cause errors as the time scales do not match. Different species ions are heated by Alfven waves with a power law spectrum in approximately a mass order. To heat O+5 over Mg+9 as measured by the Ultraviolet Coronagraph Spectrometer (UVCS) in the solar coronal hole at a region ≥1.9.R, the energy density of Alfven waves with a frequency close to the O+5-cyclotron frequency must be at least double of that at the Mg+9-cyclotron frequency. With an appropriate wa     

Observational Features of Large-Scale Structures as Revealed by the Catastrophe Model of Solar Eruptions

Large-scale magnetic structures are the main carrier of major eruptions in the solar atmosphere. These structures are rooted in the photosphere and are driven by the unceas-ing motion of the photospheric material through a series of equilibrium configurations. The motion brings energy into the coronal magnetic field until the system ceases to be in equilib-rium. The catastrophe theory for solar eruptions indicates that loss of mechanical equilibrium constitutes the main trigger mechanism of major eruptions, usually shown up as solar flares, eruptive prominences, and coronal mass ejections (CMEs). Magnetic reconnection which takes place at the very beginning of the eruption as a result of plasma instabilities/turbulence inside the current sheet, converts magnetic energy into heating and kinetic energy that are responsible for solar flares, and for accelerating both plasma ejecta (flows and CMEs) and energetic particles. Various manifestations are thus related to one another, and the physics behind these relationships is catastrophe and magnetic reconnection. This work reports on re- cent progress in both theoretical research and observations on eruptive phenomena showing the above manifestations. We start by displaying the properties of large-scale structures in the corona and the related magnetic fields prior to an eruption, and show various morphological features of the disrupting magnetic fields. Then, in the framework of the catastrophe theory, we look into the physics behind those features investigated in a succession of previous works, and discuss the approaches they used.     

Design and calibration of a high-sensitivity and high-accuracy polarimeter based on liquid crystal variable retarders

Polarimetry plays an important role in the measurement of solar magnetic fields. We developed a high-sensitivity and high-accuracy polarimeter(HHP) based on nematic liquid crystal variable retarders(LCVRs), which has a compact setup and no mechanical moving parts. The system design and calibration methods are discussed in detail. The azimuth error of the transmission axis of the polarizer as well as the fast axes of the two LCVRs and the quarter-wave plate were determined using dedicated procedures. Linearly and circularly polarized light were employed to evaluate the performance of the HHP. The experimental results indicate that a polarimetric sensitivity of better than 5.7 × 10~(-3)can be achieved by using a single short-exposure image, while an accuracy on the order of 10~(-5)can be reached by using a large number of short-exposure images. This makes the HHP a high-performance system that can be used with a ground-based solar telescope for high-precision solar magnetic field investigations.     

Lunar surface potential and electric field

The Moon has no significant atmosphere, thus its surface is exposed to solar ultraviolet radiation and the solar wind. Photoemission and collection of the solar wind electrons and ions may result in lunar surface charging. On the dayside, the surface potential is mainly determined by photoelectrons, modulated by the solar wind;while the nightside surface potential is a function of the plasma distribution in the lunar wake. Taking the plasma observations in the lunar environment as inputs, the global potential distribution is calculated according to the plasma sheath theory, assuming Maxwellian distributions for the surface emitted photoelectrons and the solar wind electrons. Results show that the lunar surface potential and sheath scale length change versus the solar zenith angle, which implies that the electric field has a horizontal component in addition to the vertical one. By differentiating the potential vertically and horizontally, we obtain the global electric field. It is found that the vertical electric field component is strongest at the subsolar point,which has a magnitude of 1 V m-1. The horizontal component is much weaker, and mainly appears near the terminator and on the nightside, with a magnitude of several mV m-1. The horizontal electric field component on the nightside is rotationally symmetric around the wake axis and is strongly determined by the plasma parameters in the lunar wake.     

ON THE ENERGY STORAGE IN A SOLAR ACTIVE REGION AR5395 BY ROTATION MOVEMENT (ABSTRACT)

In this paper based on the fact that there is rotation movement in a solar active re-gion AR5395 a twisted intergrowth magnetic flow tube model of the flares was persented.A complets set of MHD equations was used to study the energy storage in the active re-gion by rotation movement. By means of numerical method the couple effect between     

压强电离对太阳演化模型的影响

We investigated the uncertainties in the formalism for pressure ionization and their influences on solar opacity calculations. The study is based on the formalism of the atomic configuration probability and the occupation probability for interpreting pressure ionization of stellar gases. Bound-free absorption for heavy elements is found to be affected by the effect of pressure ionization which reduces the solar interior opoacity considerably. The opacity reduction in solar interiors due to the effect of pressure ionization can reach as much as tens percent. We also calculated solar evolution models using the opacity data computed from the different formalisms for pressure ionization. The results indicate sensitivities of the calculation of solar evolution models to the opacity data of different models. The resulting structures of the Sun at present age have much different chracteristics. The central temperature has an uncertainty from 15.24 106 K to 14.52 106 K. The depth of the convection zone also has     

Effects of resistivity on standing shocks in low angular momentum flows around black holes

We study two-dimensional low angular momentum flow around a black hole using the resistive magnetohydrodynamic module of PLUTO code.Simulations have been performed for the flows with parameters of specific angular momentum,specific energy and magnetic field which may be expected for the flow around Sgr A~*.For flows with lower resistivity η=10~(-6) and 0.01,the luminosity and shock location on the equator vary quasi-periodically.The power density spectra of luminosity variation show peak frequencies which correspond to the periods of 5 × 10~5,1.4 × 10~5 and 5 × 10~4 s.These quasi-periodic oscillations(QPOs) occur due to interaction between the outer oscillating standing shock and the inner weak shocks occurring at the innermost hot blob.While for cases with higher resistivity η=0.1 and 1.0,the high resistivity considerably suppresses the magnetic activity such as MHD turbulence and the flows tend to be steady and symmetric with respect to the equator.The steady standing shock is formed more outward compared with the hydrodynamical flow.The low angular momentum flow model with the above flow parameters and with low resistivity has a possibility to explain long-term flares of Sgr A~* with frequencies～one per day and ～ 5-10 days in the latest observations by Chandra,Swift and XMM-Newton monitoring of Sgr A~*.     

Modeling the 2017 September 10 solar energetic particle event using the iPATH model

On 2017 September 10, a fast coronal mass ejection(CME) erupted from the active region(AR)12673, leading to a ground level enhancement(GLE) event at Earth. Using the 2D improved Particle Acceleration and Transport in the Heliosphere(iPATH) model, we model the large solar energetic particle(SEP) event of 2017 September 10 observed at Earth, Mars and STEREO-A. Based on observational evidence, we assume that the CME-driven shock experienced a large lateral expansion shortly after the eruption, which is modeled by a double Gaussian velocity profile in this simulation. We apply the in-situ shock arrival times and the observed CME speeds at multiple spacecraft near Earth and Mars as constraints to adjust the input model parameters. The modeled time intensity profiles and fluence for energetic protons are then compared with observations. Reasonable agreements with observations at Mars and STEREO-A are found. The simulated results at Earth differ from observations of GOES-15. However, the simulated results at a heliocentric longitude 20° west to Earth fit reasonably well with the GOES observation. This can be explained if the pre-event solar wind magnetic field at Earth is not described by a nominal Parker field.Our results suggest that a large lateral expansion of the CME-driven shock and a distorted interplanetary magnetic field due to previous events can be important in understanding this GLE event.     

Effect of solar wind plasma parameters on space weather

Today's challenge for space weather research is to quantitatively predict the dynamics of the magnetosphere from measured solar wind and interplanetary magnetic field(IMF) conditions. Correlative studies between geomagnetic storms(GMSs)and the various interplanetary(IP) field/plasma parameters have been performed to search for the causes of geomagnetic activity and develop models for predicting the occurrence of GMSs, which are important for space weather predictions. We find a possible relation between GMSs and solar wind and IMF parameters in three different situations and also derived the linear relation for all parameters in three situations.On the basis of the present statistical study, we develop an empirical model. With the help of this model, we can predict all categories of GMSs. This model is based on the following fact: the total IMF Btotalcan be used to trigger an alarm for GMSs, when sudden changes in total magnetic field Btotaloccur. This is the first alarm condition for a storm's arrival. It is observed in the present study that the southward Bzcomponent of the IMF is an important factor for describing GMSs. A result of the paper is that the magnitude of Bzis maximum neither during the initial phase(at the instant of the IP shock) nor during the main phase(at the instant of Disturbance storm time(Dst) minimum). It is seen in this study that there is a time delay between the maximum value of southward Bzand the Dst minimum, and this time delay can be used in the prediction of the intensity of a magnetic storm two-three hours before the main phase of a GMS. A linear relation has been derived between the maximum value of the southward component of Bzand the Dst, which is Dst =(-0.06) +(7.65)Bz+ t.Some auxiliary conditions should be fulfilled with this, for example the speed of the solar wind should, on average, be 350 km s-1 to 750 km s-1, plasma β should be low and, most importantly, plasma temperature should be low for intense storms. If the plasma temperature is less than 0.5 × 106 K then the Dst value will be greater than the predicted value of Dst or if temperature is greater than 0.5 × 106 K then the Dst value will be less(some nT).     

HNCO: a molecule that traces low-velocity shocks

Using data from Millimetre Astronomy Legacy Team Survey at 90 GHz(MALT90), we present a molecular line study of a sample of APEX Telescope Large Area Survey of the Galaxy(ATLASGAL) clumps. Twelve emission lines have been detected in all. We found that in most sources,emissions of HC_3 N, HN~(13) C, CH_3 CN, HNCO and Si O show more compact distributions than those of HCO~+, HNC, HCN and N_2 H~+. By comparing with other molecular lines, we found that the abundance of HNCO(χ(HNCO)) correlates well with other species such as HC_3N, HNC, C_2H, H~(13) CO~+and N_2 H~+. Previous studies indicate the HNCO abundance could be enhanced by shocks. However, in this study, we find the abundance of HNCO does not correlate well with that of Si O, which is also a good tracer of shocks. We suggest this may be because HNCO and Si O trace different parts of shocks.Our analysis indicates that the velocity of a shock traced by HNCO tends to be lower than that traced by Si O. In the low-velocity shocks traced by HNCO, the HNCO abundance increases faster than that of Si O. While in the relatively high-velocity shocks traced by Si O, the Si O abundance increases faster than that of HNCO. We suggest that in the infrared dark cloud MSXDC G331.71~+00.59, high-velocity shocks are destroying the molecule HNCO.     

Solar Energetic Particle Event of 2005 January 20： Release Times and Possible Sources

Based on cosmic ray data obtained by neutron monitors at the Earth's surface, and data on near-relativistic electrons measured by the WIND satellite, as well as on solar X-ray and radio burst data, the solar energetic particle (SEP) event of 2005 January 20 is studied. The results show that this event is a mixed event where the flare is dominant in the acceleration of the SEPs, the interplanetary shock accelerates mainly solar protons with energies below 130 MeV, while the relativistic protons are only accelerated by the solar flare. The interplanetary shock had an obvious acceleration effect on relativistic electrons with energies greater than 2 MeV. It was found that the solar release time for the relativistic protons was about 06:41 UT, while that for the near-relativistic electrons was about 06:39 UT. The latter turned out to be about 2 min later than the onset time of the interplanetary type III burst.     

Research on the principle of space high-precision temperature control system of liquid crystals based Stokes polarimeter

The magnetic field is one of the most important parameters in solar physics,and a polarimeter is the key device to measure the solar magnetic field.Liquid crystals based Stokes polarimeter is a novel technology,and will be applied for magnetic field measurement in the first space-based solar observatory satellite developed by China,Advanced Space-based Solar Observatory.However,the liquid crystals based Stokes polarimeter in space is not a mature technology.Therefore,it is of great scientific significance to study the control method and characteristics of the device.The retardation produced by a liquid crystal variable retarder is sensitive to the temperature,and the retardation changes 0.09°per 0.10℃.The error in polarization measurement caused by this change is 0.016,which affects the accuracy of magnetic field measurement.In order to ensure the stability of its performance,this paper proposes a high-precision temperature control system for liquid crystals based Stokes polarimeter in space.In order to optimize the structure design and temperature control system,the temperature field of liquid crystals based Stokes polarimeter is analyzed by the finite element method,and the influence of light on the temperature field of the liquid crystal variable retarder is analyzed theoretically.By analyzing the principle of highprecision temperature measurement in space,a high-precision temperature measurement circuit based on integrated operational amplifier,programmable amplifier and 12 bit A/D is designed,and a high-precision space temperature control system is developed by applying the integral separation PI temperature control algorithm and PWM driving heating films.The experimental results show that the effect of temperature control is accurate and stable,whenever the liquid crystals based Stokes polarimeter is either in the air or vacuum.The temperature stability is within±0.0150℃,which demonstrates greatly improved stability for the liquid crystals based Stokes polarimeter.     

Editorial: solar radiophysics—recent results on observations and theories

Solar radiophysics is a rapidly developing branch of solar physics and plasma astrophysics. Solar radiophysics has the goal of analyzing observations of radio emissions from the Sun and understanding basic physical processes operating in quiet and active regions of the solar corona. In the near future, the commissioning of a new generation of solar radio observational facilities, which include the Chinese Spectral Radio Heliograph（CSRH） and the upgrade of the Siberian Solar Radio Telescope（SSRT）, and the beginning of solar observations with the Atacama Large Millimeter/submillimeter Array（ALMA）, is expected to bring us new breakthrough results of a transformative nature. The Marie-Curie International Research Staff Exchange（MC IRSES） “RadioSun” international network aims to create a solid foundation for the successful exploitation of upcoming solar radio observational facilities, as well as intensive use of the existing observational tools, advanced theoretical modeling of relevant physical processes and observables, and training a new generation of solar radio physicists. The RadioSun network links research teams from China,Czech Republic, Poland, Russia and the UK. This mini-volume presents research papers based on invited reviews and contributed talks at the 1st RadioSun workshop in China. These papers cover a broad range of research topics and include recent observational and theoretical advances in solar radiophysics, MHD seismology of the solar corona, physics of solar flares, generation of radio emission, numerical modeling of MHD and plasma physics processes, charged-particle acceleration and novel instrumentation.     

First Results from ISO Spectra of Supernova Remnants Heavily Interacting with the ISM

ISO spectra of the supernova remnant RCW103 are presented. This object is the prototype of a SNR shock heavily interacting with dense ISM (probably a molecular cloud). The spectra are dominated by prominent lines and show very little continuum at λ < 40 μm suggesting that the 12 and 25 μm IRAS emission from these types of remnant could be dominated by lines rather than continuum emission from warm dust heated by the shock as generally believed. The ISO data provide for the first time a simple and reliable estimate of the gas phase abundances of Si and Fe which are found to be close to solar relative to non refractory species such as Ne, S and Ar. This indicates that the shock is very effective in destroying the ISM dust and may therefore explain the absence of warm dust behind the shock. Like the optical Nickel lines, [NiII]6.63 μm yields Ni/Fe abundances a factor ≥ 10 above solar which we conclude results from a large underestimation of the computed Ni+ collision strengths. This revised version was published online in August 2006 with corrections to the Cover Date.     

Polarization Position Angle Swings caused by Relativistic Effects

Polarization position angle swings of - 180 ° observed in extragalactic radio sources are a regular behavior of variability in polarization. They should be due to some kind of physically regular process. We consider relativistic shocks which propagate through and 'illuminate' regular configurations of magnetic field, producing polarization angle swing events. Two magnetic field configurations (force-free field and homogeneous helical field) are considered to demonstrate the results. It is shown that the properties of polarization angle swings and the relationship between the swings and variations in total and polarized flux density are critically dependent on the configuration of magnetic field and the dynamical behavior of the shock. In particular, we find that in some cases polarization angle swings can occur when the total and polarized flux densities only vary by a very small amount. These results may be useful for understanding the polarization variability with both long and short timescales obser     

Statistical Properties of Alfvén Ion Cyclotron Waves and Kinetic Alfvén Waves in the Inner Heliosphere

Alfvén ion cyclotron waves(ACWs) and kinetic Alfvén waves(KAWs) are found to exist at <0.3 au observed by Parker Solar Probe in Alfvénic slow solar winds. To examine the statistical properties of the background parameters for ACWs and KAWs and related wave disturbances, both wave events observed by Parker Solar Probe are selected and analyzed. The results show that there are obvious differences in the background and disturbance parameters between ACWs and KAWs. ACW events have a relatively hi...     

Magnetic Energy of Force-Free Fields with Detached Field Lines

Using an axisymmetrical ideal MHD model in spherical coordinates, we present a numerical study of magnetic configurations characterized by a levitating flux rope embedded in a bipolar background field whose normal field at the solar surface is the same or very close to that of a central dipole. The characteristic plasma β (the ratio between gas pressure and magnetic pressure) is taken to be sosmall (β= 10^-4) that the magnetic field is close to being force-free. The system as a whole is then let evolve quasi-statically with a slow increase of either the annular magnetic flux or the axial magnetic flux of the rope, and the total magneticenergy of the system grows accordingly. It is found that there exists an energy threshold: the flux rope sticks to the solar surface in equilibrium if the magneticenergy of the system is below the threshold, whereas it loses equilibrium if the threshold is exceeded. The energy threshold is found to be larger than that of thecorresponding fully-open magnetic field by a factor of nearly 1.08 irrespective as towhether the background field is completely closed or partly open, or whether the magnetic energy is enhanced by an increase of annular or axial flux of the rope.This gives an example showing that a force-free magnetic field may have an energy larger than the corresponding open field energy if part of the field lines is allowed tobe detached from the solar surface. The implication of such a conclusion in coronal mass ejections is briefly discussed and some comments are made on the maximum energy of force-free magnetic fields.