J. S. Zhao

TWIST ACCUMULATION AND TOPOLOGY STRUCTURE OF A SOLAR MAGNETIC FLUX ROPE

To study the buildup of a magnetic flux rope before a major flare and coronal mass ejection (CME), we compute the magnetic helicity injection, twist accumulation, and topology structure of the three-dimensional (3D) magnetic field, which is derived by the nonlinear force-free field model. The Extreme-ultraviolet Imaging Telescope on board the Solar and Heliospheric Observatory observed a series of confined flares without any CME before a major flare with a CME at 23:02 UT on 2005 January 15 in active region NOAA 10720. We derive the vector velocity at eight time points from 18:27 UT to 22:20 UT with the differential affine velocity estimator for vector magnetic fields, which were observed by the Digital Vector Magnetograph at Big Bear Solar Observatory. The injected magnetic helicity is computed with the vector magnetic and velocity fields. The helicity injection rate was (– 16.47 ± 3.52) × 1040 Mx2 hr–1. We find that only about 1.8% of the injected magnetic helicity became the internal helicity of the magnetic flux rope, whose twist increasing rate was –0.18 ± 0.08 Turns hr–1. The quasi-separatrix layers (QSLs) of the 3D magnetic field are computed by evaluating the squashing degree, Q. We find that the flux rope was wrapped by QSLs with large Q values, where the magnetic reconnection induced by the continuously injected magnetic helicity further produced the confined flares. We suggest that the flux rope was built up and heated by the magnetic reconnection in the QSLs.

Using digital cameras for comparative phenological monitoring in an evergreen broad-leaved forest and a seasonal rain forest

article i nfo Article history: Digital cameras have been used in phenological observations for their high accuracy and low labor cost. Most studies successfully use greenness indices derived from digital images for timing the events related to leaf de- velopment. However, when timing the leaf senescence events, wide discrepancies between actual and esti- mated dates are common. In this study, images of three species (two from an evergreen broad-leaved forest and one from a seasonal rain forest) were used to estimate three phenological events of leaf develop- ment and senescence. Other than the greenness index, a redness index was also employed. Different annual patterns in color indices developed among the species. The redness index was more accurate when estimat- ing leaf senescence, while the greenness index was more accurate for estimating leaf development events in Acer heptalobum and Machilus bombycina. The absolute differences in estimations of phenological events ran- ged from �3 to 1 day, which is more accurate than estimates based on the greenness index only (� 2t o 27 days). With the introduction of the redness index, this technique has been much improved and is possible to be applied to more species. Furthermore, variations of color indices during periods of phenological events were highly related to the climatic factors with a time lag of around 10 days. Because of the ease of use and efficiency (i.e., automatic daily data output), digital cameras are expected to be used in ecosystem process modeling, networks of phenology assessment and validation of the remote sensing results from satellites.

Kinetic Alfvén Waves Excited by Oblique Magnetohydrodynamic Alfvén Waves in Coronal Holes

Kinetic Alfven 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{sub perpendicular} and reaches maximum at {lambda}{sub e} k{sub perpendicular} {approx} 0.3, where {lambda}{sub e} 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 {lambda}{sub e} k{sub perpendicular} < 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 {lambda}{sub e} k{sub perpendicular} < 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.

A nonlocal wave-wave interaction among Alfven waves in an intermediate-beta plasma

A nonlocal coupling mechanism to directly transfer the energy from large-scale magnetohydrodynamic (MHD) Alfven waves to small-scale kinetic Alfven waves is presented. It is shown that the interaction between a MHD Alfven wave and a reversely propagating kinetic Alfven wave can generate another kinetic Alfven 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 Alfven waves in the solar wind and solar corona plasmas.A nonlocal coupling mechanism to directly transfer the energy from large-scale Magnetohydrodynamic(MHD) Alfvén waves to small-scale kinetic Alfvén waves is presented. It is shown that the interaction between a MHD Alfvén wave and a reversely propagating kinetic Alfvén wave can generate another kinetic Alfvén 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 Alfvén waves in the solar wind and solar corona plasmas.

Argo-ybj Observation of the Large-scale Cosmic ray Anisotropy During the Solar Minimum Between Cycles 23 and 24

This paper reports on the measurement of the large-scale anisotropy in the distribution of cosmic-ray arrival directions using the data collected by the air shower detector ARGO-YBJ from 2008 January to 2009 December, during the minimum of solar activity between cycles 23 and 24. In this period, more than 2 × 10 11 showers were recorded with energies between ∼1 and 30 TeV. The observed two-dimensional distribution of cosmic rays is characterized by two wide regions of excess and deficit, respectively, both of relative intensity ∼10 −3 with respect to a uniform flux, superimposed on smaller size structures. The harmonic analysis shows that the large-scale cosmic-ray relative intensity as a function of R.A. can be described by the first and second terms of a Fouries series. The high event statistics allow the study of the energy dependence of the anistropy, showing that the amplitude increases with energy, with a maximum intensity at ∼10 TeV, and then decreases while the phase slowly shifts toward lower values of R.A. with increasing energy. The ARGO-YBJ data provide accurate observations over more than a decade of energy around this feature of the anisotropy spectrum.

Kinetic Alfvén Turbulence and Parallel Electric Fields in Flare Loops

This study investigates the spectral structure of the kinetic Alfven 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 10(2)-10(4) times the Dreicer field as the turbulence reaches the electron inertial length scale.

On nonlinear decay of kinetic Alfvén waves and application to some processes in space plasmas

This paper considers the nonlinear decay of the kinetic Alfven 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 Alfven waves. It is shown that the parametric instability of the kinetic Alfven 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 Alfven 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 Alfven 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.

Convective cell generation by kinetic Alfven wave turbulence in the auroral ionosphere

Modulation of convective cells by kinetic Alfven wave (KAW) turbulence is investigated. The interaction is governed by a nonlinear dispersion relation for the convective cells. It is shown that KAW turbulence is disrupted by excitation of the large-scale convective motion through a resonant instability. Application of the results to the auroral ionosphere shows that cross-scale coupling of the KAW turbulence and convective cells plays an important role in the evolution of ionospheric plasma turbulence.

Interannual and seasonal variability of water use efficiency in a tropical rainforest: Results from a 9 year eddy flux time series

We used a continuous 9 year (2003-2011) eddy flux time series with 30min resolution to examine water use efficiency in a tropical rainforest and determine its environmental controls. The multiyear mean water use efficiency (W-ue) of this rainforest was 3.160.33 gC per kg H2O, which is close to that of boreal forests, but higher than subtropical forests, and lower than temperate forests. The water vapor deficit (V-PD) had a strong impact on instantaneous W-ue, in the manner predicted by stomatal optimization theory. At the seasonal scale, temperature was the dominant controller of W-ue. The negative correlation between temperature and W-ue was probably caused by high continuous photosynthesis during low-temperature periods. The V-PD did not correlate with W-ue at the interannual scale. No interannual trend was detected in W-ue or inherent water use efficiency (W-ei), either annually or seasonally. The fact that no increasing trend of W-ei was found in the studied tropical rainforest, along with other evidence of CO2 stimulation in tropical rainforests, requires special attention and data validation. There was no significant difference between W-ue during a drought and the 9 year mean values in the forest we studied, but we found that dry season transpiration (T-r) was consistently lower during the drought compared to the mean values. Finally, whether W-ue increases or decreases during a drought is determined by the drought sensitivity of gross primary production (G(PP)).

Parametric Instability of Whistler Waves in the Electron Magnetohydrodynamics

Using an electron magnetohydrodynamic model, we investigate the parametric decay among three whistler waves. A nonlinear equation to describe both linear and nonlinear properties of whistler waves is derived. Then we discuss the growth rate of the parametric decay of whistler waves in the long-wavelength region and show that the growth rate for two reverse decay waves is larger than that for two decay waves in the same direction. The nonlinear interaction among the long-wavelength and short-wavelength waves is also studied in this paper. This wave-wave interaction implies that long-wavelength waves can be decayed to short-wavelength waves and then dissipate their energy in the short-wavelength region. The possibility of applying our results to account for the generation of sunward propagating whistler waves is also discussed.