X. S. Feng

Time‐dependent MHD modeling of the global solar corona for year 2007: Driven by daily‐updated magnetic field synoptic data

In this paper, we develop a time-dependent MHD model driven by the daily-updated synoptic magnetograms (MHD-DUSM) to study the dynamic evolution of the global corona with the help of the 3D Solar-Interplanetary (SIP) adaptive mesh refinement (AMR) space-time conservation element and solution element (CESE) MHD model (SIP-AMR-CESE MHD Model). To accommodate the observations, the tangential component of the electric field at the lower boundary is specified to allow the flux evolution to match the observed changes of magnetic field. Meanwhile, the time-dependent solar surface boundary conditions derived from the method of characteristics and the mass flux limit are incorporated to couple the observation and the 3D MHD model. The simulated evolution of the global coronal structure during 2007 is compared with solar observations and solar wind measurements from both Ulysses and spacecrafts near the Earth. The MHD-DUSM model is also validated by comparisons with the standard potential field source surface (PFSS) model, the newly improved Wang-Sheeley-Arge (WSA) empirical formula, and the MHD simulation with a monthly synoptic magnetogram (MHD-MSM). Comparisons show that the MHD-DUSM results have good overall agreement with coronal and interplanetary structures, including the sizes and distributions of coronal holes, the positions and shapes of the streamer belts, and the transitions of the solar wind speeds and magnetic field polarities. The MHD-DUSM results also display many features different from those of the PFSS, the WSA, and the MHD-MSM models.

Three‐dimensional MHD simulation of two coronal mass ejections' propagation and interaction using a successive magnetized plasma blobs model

A three-dimensional (3-D), time-dependent, numerical magnetohydrodynamic (MHD) model is used to investigate the evolution and interaction of two coronal mass ejections (CMEs) in the nonhomogeneous ambient solar wind. The background solar wind is constructed on the basis of the self-consistent source surface with observed line of sight of magnetic field and density from the source surface of 2.5 R(s) to Earths orbit (215 R(s)) and beyond. The two successive CMEs occurring on 28 March 2001 and forming a multiple magnetic cloud in interplanetary space are chosen as a test case, in which they are simulated by means of a two high-density, high-velocity, and high-temperature magnetized plasma blobs model, and are successively ejected into the nonhomogeneous background solar wind medium along different initial launch directions. The dynamical propagation and interaction of the two CMEs between 2.5 and 220 R(s) are investigated. Our simulation results show that, although the two CMEs are separated by 10 h, the second CME is able to overtake the first one and cause compound interactions and an obvious acceleration of the shock. At the L1 point near Earth the two resultant magnetic clouds in our simulation are consistent with the observations by ACE. In this validation study we find that this 3-D MHD model, with the self-consistent source surface as the initial boundary condition and the magnetized plasma blob as the CME model, is able to reproduce and explain some of the general characters of the multiple magnetic clouds observed by satellite.

Three-dimensional MHD simulation of the evolution of the April 2000 CME event and its induced shocks using a magnetized plasma blob model

A three-dimensional (3-D) time-dependent, numerical magnetohydrodynamic (MHD) model with asynchronous and parallel time-marching method is used to investigate the propagation of coronal mass ejections (CMEs) in the nonhomogenous background solar wind flow. The background solar wind is constructed based on the self-consistent source surface with observed line-of-sight of magnetic field and density from the source surface of 2.5 R-s to the Earths orbit (215 Rs) and beyond. The CMEs are simulated by means of a very simple flux rope model: a high-density, high-velocity, and high-temperature magnetized plasma blob is superimposed on a steady state background solar wind with an initial launch direction. The dynamical interaction of a CME with the background solar wind flow between 2.5 and 220 Rs is investigated. The evolution of the physical parameters at the cobpoint, which is located at the shock front region magnetically connected to ACE spacecraft, is also investigated. We have chosen the well-defined halo-CME event of 4-6 April 2000 as a test case. In this validation study we find that this 3-D MHD model, with the asynchronous and parallel time-marching method, the self-consistent source surface as initial boundary conditions, and the simple flux rope as CME model, provide a relatively satisfactory comparison with the ACE spacecraft observations at the L1 point.

Using a 3‐D spherical plasmoid to interpret the Sun‐to‐Earth propagation of the 4 November 1997 coronal mass ejection event

We present the time-dependent propagation of a Sun-Earth connection event that occurred on 4 November 1997 using a three-dimensional (3-D) numerical magnetohydrodynamics (MHD) simulation. A global steady state solar wind for this event is obtained by a 3-D SIP-CESE MHD model with Parkers 1-D solar wind solution and measured photospheric magnetic fields as the initial values. Then, superposed on the quiet background solar wind, a spherical plasmoid is used to mimic the 4 November 1997 coronal mass ejection (CME) event. The CME is assumed to arise from the evolution of a spheromak magnetic structure with high-speed, high-pressure, and high-plasma-density plasmoid near the Sun. Moreover, the axis of the initial simulated CME is put at S14W34 to conform to the observed location of this flare/ CME event. The result has provided us with a relatively satisfactory comparison with the Wind spacecraft observations, such as southward interplanetary magnetic field and large-scale smooth rotation of the magnetic field associated with the CME.

Observations of reconnection exhausts associated with large-scale current sheets within a complex ICME at 1 AU

During 26-27 November 2000 a complex interplanetary coronal mass ejection, composed of four flux ropes, was detected by Wind and ACE at 1 AU. We identify two Petschek-like exhaust events within the interiors of the second and third flux ropes, respectively. In the first event, Wind and ACE detected an exhaust at the same side from the reconnection site, which was associated with a large-scale bifurcated current sheet with a spatial width of similar to 10,000 ion inertial lengths and the magnetic shear was 155 degrees. In the second event, the two spacecraft observed the oppositely directed exhausts from a single reconnection X line. The exhausts were also related to a large-scale current sheet with a spatial width of similar to 3000 ion inertial lengths and a shear angle of about 135 degrees. The two exhaust events resulted from fast and quasi-stationary reconnection. The related current sheets were both flat on the scale of a few hundred Earth radii and located close to the centers of subflux ropes. The decrease of radial expansion speed of each flux rope might account for the formation of the two current sheets. Reconnections at the centers of flux ropes may change the entire topology of the flux ropes and may fragment them into smaller ones.

An operational method for shock arrival time prediction by one‐dimensional CESE‐HD solar wind model

[1] With the purpose of operational real-time forecasting for arrival times of flare/ coronal mass ejection associated shocks in the vicinity of the Earth, a one-dimensional hydrodynamic (HD) shock propagation model is established by a novel numerical scheme, the space-time conservation element and solution element (CESE) method. The required observational data inputs to this new one-dimensional CESE-HD model are the low coronal radio Type II drift speed, the duration estimation, and the background solar wind speed for a solar eruptive event. Applying this model to 137 solar events during the period of February 1997 to August 2002, it is found that our model could be practically equivalent to the STOA, ISPM, HAFv.2, and SPM models in forecasting the shock arrival time. The absolute error in the transit time from our model is not larger than those of the other four models for the same set of events. These results may demonstrate the potential capability of our model in terms of improving real-time forecasting because the CESE method can be extended to three-dimensional magnetohydrodynamics (3D-MHD) from the solar photosphere to any heliospheric position.

An Improved Method for Deriving Daily Evapotranspiration Estimates From Satellite Estimates on Cloud-Free Days

An improved method, based on the daily surface resistance, is proposed to extend satellite evapotranspiration (ET) on a clear day into ET for each and every day. Alternative climatic variables such as soil moisture, wind speed, and net radiation are explored for estimating daily surface resistance using a Penman-Monteith (P-M) formulation. The study was carried out for the Yingke (YK) oasis plains area (maize cropland) and the Arou (AR) alpine meadow area (grassland) located in the midstream and upstream, respectively, of the Heihe River Basin of northwestern China. Statistical results show that the proposed method performs well for estimating daily ET for both study areas, with results slightly superior in the midstream, cropland area where the coefficient of determination (R2) was 0.9249 and the index of agreement (d) was 0.978. In the upstream alpine meadow area, the coefficient of determination (R2) was 0.9074, and the index of agreement (d) was 0.961. The proposed method provides an enhanced approach for estimating daily ET in the ETWatch model. Future work will focus on scaling this improved method to the estimation of regional daily ET map.

An improved satellite‐based approach for estimating vapor pressure deficit from MODIS data

Vapor pressure deficit (VPD) is an important variable widely used in ecosystem and climate models. In this paper, an improved satellite-based approach to estimating VPD was presented that uses several remote sensing products coupled with field measured data. The proposed method exploits an optimized algorithm to derive near-surface actual vapor pressure (ea) from Moderate Resolution Imaging Spectroradiometer (MODIS) data and upgrades Smiths (1966) methodology for estimating ea. The proposed new algorithm for calculating ea was evaluated against in situ measurements at 119 validation sites in China for 2 months in 2013. The mean absolute error (MAE) and root-mean-square error (RMSE) were less than 0.25 kPa and 0.33 kPa, respectively. The near-surface air temperature (Ta), which is an important input data for calculating VPD, was estimated from satellite-retrieved land surface temperature, and had an RMSE of less than 2.5 K. The estimated VPD values were validated with ground observation data from the Heihe River Basin for 5 months in 2012 and for all of China for August 2013. A coefficient of determination (R2) of 0.912, MAE of 0.27 kPa, and RMSE value of 0.32 kPa were achieved for the 2012 test data, and corresponding values of 0.88, 0.278 kPa, and 0.367 kPa for the 2013 test data. These results are promising, especially considering the comparatively high spatial resolution (1 km) of the VPD map estimated from the satellite data. Potential applications include global evapotranspiration estimation, fire warning, and vegetation analysis.

EFFECTS OF PHYSICAL FEATURES IN THE SOLAR ATMOSPHERE ON THE CORONAL MASS EJECTION EVOLUTION

Based on time-dependent MHD simulation, we investigate how physical features in the solar atmosphere affect the evolution of coronal mass ejections (CMEs). It is found that temperature and density play a crucial role in CME initiation. We argue that lower temperature facilitates the catastrophes occurrence, and that the CMEs which initiate in low density could gain lower velocity. In our numerical experiment, by employing different values of beta, the resulting eruptions of either slow or fast events may be obtained.

ENERGY DISSIPATION PROCESSES IN SOLAR WIND TURBULENCE

Turbulence is a chaotic flow regime filled by irregular flows. The dissipation of turbulence is a fundamental problem in the realm of physics. Theoretically, dissipation cannot be ultimately achieved without collisions, and so how turbulent kinetic energy is dissipated in the nearly collisionless solar wind is a challenging problem. Wave particle interactions and magnetic reconnection are two possible dissipation mechanisms, but which mechanism dominates is still a controversial topic. Here we analyze the dissipation region scaling around a solar wind magnetic reconnection region. We find that the magnetic reconnection region shows a unique multifractal scaling in the dissipation range, while the ambient solar wind turbulence reveals a monofractal dissipation process for most of the time. These results provide the first observational evidences for the intermittent multifractal dissipation region scaling around a magnetic reconnection site, and they also have significant implications for the fundamental energy dissipation process.