Entity

Time filter

Source Type


Chen A.Q.,CAS National Astronomical Observatories | Chen A.Q.,National Center for Space Weather | Wang J.X.,CAS National Astronomical Observatories
Astronomy and Astrophysics | Year: 2012

Context. The vector magnetic field characteristics of superactive regions (SARs) hold the key for understanding why SARs are extremely active and provide the guidance in space weather prediction. Aims.We aim to quantify the characteristics of SARs using the vector magnetograms taken by the Solar Magnetic Field Telescope at Huairou Solar Observatory Station. Methods. The vector magnetic field characteristics of 14 SARs in solar cycles 22 and 23 were analyzed using the following four parameters: 1) the magnetic flux imbalance between opposite polarities; 2) the total photospheric free magnetic energy; 3) the length of the magnetic neutral line with its steep horizontal magnetic gradient; and 4) the area with strong magnetic shear. Furthermore, we selected another eight large and inactive active regions (ARs), which are called fallow ARs (FARs), to compare them with the SARs. Results.We found that most of the SARs have a net magnetic flux higher than 7.0×10 21 Mx, a total photospheric free magnetic energy higher than 1.0×10 24 erg cm -1, a magnetic neutral line with a steep horizontal magnetic gradient (≥300 GMm -1) longer than 30 Mm, and an area with strong magnetic shear (shear angle ≥80°) greater than 100 Mm 2. In contrast, the values of these parameters for the FARs are mostly very low. The Pearson χ 2 test was used to examine the significance of the difference between the SARs and FARs, and the results indicate that these two types of ARs can be fairly distinguished by each of these parameters. The significance levels are 99.55%, 99.98%, 99.98%, and 99.96%, respectively. However, no single parameter can distinguish them perfectly. Therefore we propose a composite index based on these parameters, and find that the distinction between the two types of ARs is also significant with a significance level of 99.96%. These results are useful for a better physical understanding of the SAR and FAR. © 2012 ESO. Source


Zhao J.S.,Chinese Academy of Sciences | Zhao J.S.,University of Chinese Academy of Sciences | Wu D.J.,Chinese Academy of Sciences | Lu J.Y.,National Center for Space Weather
Journal of Geophysical Research: Space Physics | Year: 2010

This paper considers the nonlinear decay of the kinetic Alfvén waves (KAW) in the space plasmas. By using a two-fluid model, we obtain a nonlinear equation to investigate the resonant interaction among three kinetic Alfvén waves. It is shown that the parametric instability of the kinetic Alfvén wave becomes important when its perpendicular wavelength is the order of the ion acoustic gyroradius or the electron inertial length. We give a detailed discussion for the KAW decay in the plasma inertial range and show that (1) the reverse decay of the kinetic Alfvén wave is stronger than its parallel decay for the arbitrary wavelength range; (2) the reverse decay is lager than the parallel decay for small angles of two perpendicular wave vectors of the decay waves, and these two decays are zero for large angles; (3) both growth rates depend on the choice of the wave number range of the decay waves; and (4) there exists two forbidden regions for the KAW decay. In this paper, we also discuss the nonlinear decay of the kinetic Alfvén waves in the auroral zone and show that the parametric instability can occur there and may play an important role in forming two reverse electron streaming fluxes in the electron acceleration region. Copyright 2010 by the American Geophysical Union. Source


Zhao J.S.,Chinese Academy of Sciences | Zhao J.S.,University of Chinese Academy of Sciences | Wu D.J.,Chinese Academy of Sciences | Lu J.Y.,National Center for Space Weather
Astrophysical Journal | Year: 2011

Kinetic Alfvén waves (KAWs) are small-scale dispersive AWs that can play an important role in particle heating and acceleration of space and solar plasmas. An excitation mechanism for KAWs created by the coupling between large-scale oblique AWs and small-scale KAWs is presented in this paper. Taking into account both the collisional and Landau damping dissipations, the results show that the net growth rate of the excited KAWs increases with their perpendicular wavenumber k ⊥ and reaches maximum at λe k ⊥ 0.3, where λe is the electron inertial length. However, for KAWs with shorter perpendicular wavelengths, the net growth rate decreases rapidly due to dissipative effects. The evaluation of the threshold amplitude of the AW implies that for KAWs with λe k ⊥ < 0.3, the relative threshold amplitude is well below 10%, which is easy to satisfy. In particular, when applying this mechanism to the case of a solar coronal hole containing a dense plume structure, our results show that KAWs with λe k ⊥ < 0.3 can be not only efficiently excited in the interplume region but also strongly dissipated in the dense plume due to the Landau damping. © 2011. The American Astronomical Society. All rights reserved. Source


Zhao J.S.,Chinese Academy of Sciences | Zhao J.S.,CAS National Astronomical Observatories | Wu D.J.,Chinese Academy of Sciences | Lu J.Y.,Nanjing University of Information Science and Technology | Lu J.Y.,National Center for Space Weather
Astrophysical Journal | Year: 2013

This study investigates the spectral structure of the kinetic Alfvén turbulence in the low-beta plasmas. We consider a strong turbulence resulting from collisions between counterpropagating wavepackets with equal energy. Our results show that (1) the spectra of the magnetic and electric field fluctuations display a transition at the electron inertial length scale, (2) the turbulence cascades mainly toward the magnetic field direction as the cascade scale is smaller than the electron inertial length, and (3) the parallel electric field increases as the turbulent scale decreases. We also show that the parallel electric field in the solar flare loops can be 102-10 4 times the Dreicer field as the turbulence reaches the electron inertial length scale. © 2013. The American Astronomical Society. All rights reserved.. Source


Zhao J.S.,Chinese Academy of Sciences | Zhao J.S.,University of Chinese Academy of Sciences | Wu D.J.,Chinese Academy of Sciences | Lu J.Y.,National Center for Space Weather
Physics of Plasmas | Year: 2011

A nonlocal coupling mechanism to directly transfer the energy from large-scale magnetohydrodynamic (MHD) Alfv́n waves to small-scale kinetic Alfv́n waves is presented. It is shown that the interaction between a MHD Alfv́n wave and a reversely propagating kinetic Alfv́n wave can generate another kinetic Alfv́n wave, and this interaction exists in the plasmas where the thermal to magnetic pressure ratio is larger than the electron to ion mass ratio. The proposed nonlocal interaction may have a potential application to account for the observed electron scale kinetic Alfv́n waves in the solar wind and solar corona plasmas. © 2011 American Institute of Physics. Source

Discover hidden collaborations