Dingkun Zhong

THREE-DIMENSIONAL SOLAR WIND MODELING FROM THE SUN TO EARTH BY A SIP-CESE MHD MODEL WITH A SIX-COMPONENT GRID

The objective of this paper is to explore the application of a six-component overset grid to solar wind simulation with a three-dimensional (3D) Solar-InterPlanetary Conservation Element/Solution Element MHD model. The essential focus of our numerical model is devoted to dealing with: (1) the singularity and mesh convergence near the poles via the use of the six-component grid system, (2) the ∇ · B constraint error via an easy-to-use cleaning procedure by a fast multigrid Poisson solver, (3) the Courant-Friedrichs-Levy number disparity via the Courant-number insensitive method, (4) the time integration by multiple time stepping, and (5) the time-dependent boundary condition at the subsonic region by limiting the mass flux escaping through the solar surface. In order to produce fast and slow plasma streams of the solar wind, we include the volumetric heating source terms and momentum addition by involving the topological effect of the magnetic field expansion factor fS and the minimum angular distance θ b (at the photosphere) between an open field foot point and its nearest coronal hole boundary. These considerations can help us easily code the existing program, conveniently carry out the parallel implementation, efficiently shorten the computation time, greatly enhance the accuracy of the numerical solution, and reasonably produce the structured solar wind. The numerical study for the 3D steady-state background solar wind during Carrington rotation 1911 from the Sun to Earth is chosen to show the above-mentioned merits. Our numerical results have demonstrated overall good agreements in the solar corona with the Large Angle and Spectrometric Coronagraph on board the Solar and Heliospheric Observatory satellite and at 1 AU with WIND observations.

Daytime E region field‐aligned irregularities observed during a solar eclipse

The driving mechanism of the midlatitude field-aligned irregularities (FAIs) has been in dispute for many years. The experimental observations were carried out during the solar eclipse of 22 July 2009 in Wuhan, China, to study the possibility of the wave-driven irregularities. A high-frequency coherent scatter radar was used to detect the E region irregularities. An ionosonde was applied to record the trace of gravity waves in ionosphere. The E region FAIs occurred at the end of the solar eclipse with fluctuant Doppler. At the same time, the oscillations on the fEs (maximum reflecting frequency of Es) curve and in the Doppler velocity of the echoes from the Es layer were also recorded. The data analysis and comparison show that the gravity waves and the FAIs occurred at the same time with the in-phase variations in amplitude and phase. Thus, the solar eclipse and the gravity waves may play important roles in the occurrence of the irregularities. A schematic diagram of one-period gravity wave is used to explain the possible gravity wave-driven mechanism and the Doppler fluctuation in the irregularities. The daytime FAI in midlatitude is a rare phenomenon, even in the condition of solar eclipse. There are only four cases of the E region FAIs observed during a solar eclipse, including our observations. The unique feature of our observations is the synchronized oscillations in the irregularities and in the Es layer, which will help address the outstanding question of the source of the midlatitude E region FAIs.

MLT and seasonal dependence of auroral electrojets: IMAGE magnetometer network observations

Total eastward and westward electrojet currents (EEJ and WEJ) and their central latitudes derived from the International Monitor for Auroral Geomagnetic Effects (IMAGE) network magnetic measurements are analyzed for the combined MLT (magnetic local time) and seasonal dependence during the period 1995-2009. EEJ shows a strong MLT variation with significant dependence on season. During summer months the maxima occur around 1600-1800 MLT, whereas during winter months the maxima occur around 1800-2000 MLT. Moreover, the summer maxima are much larger than the winter maxima and appear at higher latitudes. The summer maxima are mainly associated with the solar EUV conductivity effect, while the winter maxima are mainly due to the contribution of northward convective electric field. EEJ exhibits a dominant annual variation with maximum in summer and minimum in winter. WEJ also exhibits a strong MLT variation with significant dependence on season. The maxima occur around 0200-0400 MLT during summer months, around 0000-0200 MLT during winter months, and around 0000-0400 MLT during equinoctial months. Moreover, the equinoctial maxima are much larger than the summer and winter maxima and appear at relatively lower latitudes. The seasonal variations in WEJ are the combinations of annual variations and semiannual variations. Both annual and semiannual variations show significant dependence on MLT. These results increase our knowledge on what factors contribute to the auroral electrojets as well as their magnetic signatures and hence help us better understand the limitations of global auroral electrojet indices, such as the AE and SME indices.

Plasma flux and gravity waves in the midlatitude ionosphere during the solar eclipse of 20 May 2012

The solar eclipse effects on the ionosphere are very complex. Except for the ionization decay due to the decrease of the photochemical process, the couplings of matter and energy between the ionosphere and the regions above and below will introduce much more disturbances. Five ionosondes in the Northeast Asia were used to record the midlatitude ionospheric responses to the solar eclipse of 20 May 2012. The latitude dependence of the eclipse lag was studied first. The foF2 response to the eclipse became slower with increased latitude. The response of the ionosphere at the different latitudes with the same eclipse obscuration differed from each other greatly. The plasma flux from the protonsphere was possibly produced by the rapid temperature drop in the lunar shadow to make up the ionization loss. The greater downward plasma flux was generated at higher latitude with larger dip angle and delayed the ionospheric response later. The waves in the foEs and the plasma frequency at the fixed height in the F layer are studied by the time period analytic method. The gravity waves of 43–51 min center period during and after the solar eclipse were found over Jeju and I-Cheon. The northward group velocity component of the gravity waves was estimated as ~108.7 m/s. The vertical group velocities between 100 and 150 km height over the two stations were calculated as ~5 and ~4.3 m/s upward respectively, indicating that the eclipse-induced gravity waves propagated from below the ionosphere.

SIP-CESE MHD model of solar wind with adaptive mesh refinement of hexahedral meshes

Solar-interplanetary space involves many features, such as discontinuities and heliospheric current sheet, with spatial scales many orders of magnitude smaller than the system size. The scalable, massively parallel, block-based, adaptive-mesh refinement (AMR) promises to resolve different temporal and spatial scales on which solar-wind plasma occurs throughout the vast solar-interplanetary space with even less cells but can generate a good enough resolution. Here, we carry out the adaptive mesh refinement (AMR) implementation of our Solar-Interplanetary space-time conservation element and solution element (CESE) magnetohydrodynamic model (SIP-CESE MHD model) using a six-component grid system (Feng et al., 2007, 2010). The AMR realization of the SIP-CESE MHD model is naturalized directly in hexahedral meshes with the aid of the parallel AMR package PARAMESH available at http://sourceforge.net/projects/paramesh/. At the same time, the topology of the magnetic field expansion factor and the minimum angular separation (at the photosphere) between an open field foot point and its nearest coronal-hole boundary are merged into the model in order to determine the volumetric heating source terms. Our numerical results for the validation study of the solar-wind background of Carrington rotation 2060 show overall good agreements in the solar corona and in interplanetary space with the observations from the Solar and Heliospheric Observatory (SOHO) and spacecraft data from OMNI

Midlatitude ionospheric responses to the 2013 SSW under high solar activity

Ionospheric responses to sudden stratospheric warming (SSW) are not well understood, particularly in the midlatitudes and under high solar conditions. During the 2013 SSW, ionospheric disturbances were observed in eight locations on the meridional chain from 30.5 degrees N to 42.8 degrees N in northern China. The midlatitude ionosphere responded strongly to the SSW despite being under high solar flux. The F-2 layer maximum electric density increased by more than 80%, and the peak height was elevated more than 60km. Well-set and organized semidiurnal variations were recorded in early and middle January during the SSW in eight observation locations. The expected f(o)F(2) decrease in the afternoon hours was not clearly discernible; however, nighttime enhancements occurred frequently. The time-period spectra of the average f(o)F(2) and zonal winds and meridional winds at altitudes of 86-95km presented quasi-16day planetary wave-like oscillations during the warming event. The coupling between the atmosphere and ionosphere may be strengthened by the quasi-16day waves. The amplified diurnal, semidiurnal, and terdiurnal tides in f(o)F(2) were also recorded during the warming, in good agreement with earlier observations. Importantly, the variations in the semidiurnal tides included a 16day periodic component, indicating that the modulated semidiurnal tides may transmit these 16day planetary wave-like oscillations to the F region through wind dynamo. Although the PW-tide interaction theory is not novel, it is of significance in the midlatitude ionospheric response to SSW.

Strong correlation between quasiperiodic echoes and plasma drift in the E region

Simultaneous observations of quasiperiodic (QP) echoes and plasma drift in the ionospheric E region were conducted in Fuke (19.5 degrees N, 109.1 degrees E), Hainan province, China, to investigate the QP striation tilts under varying plasma drift conditions. The E region field-aligned irregularities (FAIs), observed using the Hainan VHF radar, and the drift velocities of the plasma blobs in the E-s layer, recorded by the Hainan Digisonde operating in drift mode, are reported. The QP echoes and drift data recorded during the entire year of 2013 were analyzed and compared. A surprising consistency between the striation tilt of the QP echoes and the drift direction of the plasma blobs was discovered. A negative echo striation of the QP FAIs was recorded when the measured drift direction of the plasma blobs was southward, whereas a positive echo striation was observed during the northward drift. Furthermore, the echo trace was continuous, whereas the QP striation changed from negative to positive, and vice versa. Thus, it can be concluded that the morphology of the QP echoes may be controlled by the background wind fields in the E region. The northward/southward-drifting striated FAIs in the observation region of a coherent scatter radar might induce the positive/negative QP echo striation in the range-time-intensity plots.

Annual variations in westward auroral electrojet and substorm occurrence rate during solar cycle 23

The International Monitor for Auroral Geomagnetic Effects network magnetic measurements during the period 1995-2009 are used to characterize the annual variations in the westward electrojet. The results suggest that the annual variations in different local time sectors are quite different due to the different sources. In the MLT sector 2200-0100, the annual variations with maxima in winter suggest they are caused by the combined effects of the convective electric field and the conductivity associated with particle precipitation. Furthermore, the conductivity seems to play a more important role in the MLT sector similar to 2200-2320, while the convective electric field appears to be more important in the MLT sector similar to 2320-0100. In the MLT sector 0300-0600, the annual variations with maxima in summer suggest they are caused by solar EUV conductivity effect and the equinoctial effect. The solar EUV conductivity effect works by increasing ionospheric conductivity and enhancing the westward electrojet in summer, while the equinoctial effect works by decreasing solar wind-magnetosphere coupling efficiency and weakening the westward electrojet in winter. In the MLT sector 0100-0300, the annual variations are relatively weak and can be attributed to the combined effects of annual variations caused by all the previously mentioned effects. In addition, we find that a significant annual variation in substorm occurrence rate, mainly occurring in the premidnight region, is quite similar to that in the westward electrojet. We suggest that elevated solar wind driving during the winter months contributes to higher substorm occurrence in winter in the Northern Hemisphere.

Simulation of small-scale coronal explosives due to magnetic reconnections

The dynamics of small-scale explosive phenomena in the lower corona have been simulated by solving the compressible magnetohydrodynamic equations. Numerical results show that the magnetic reconnections in a long coronal current sheet consist of a series of discrete small reconnection events, coalescence of magnetic islands, and plasmoid ejections, corresponding to the explosive events occurring intermittently and as bursts in a mentioned observational case. The generation of magnetic islands via multiple-X-point reconnection and their coalescence processes, to some extent, are qualitatively similar to the sequence of brightenings in the active region NOAA 8668. The strong ejections are possibly related to the recorded extreme ultraviolet (EUV) emitting structures. Morphological comparison and quantitative check of the plasma parameters support this candidate mechanism, and the idea that explosive events that appear to last long may not be single events, but a succession of explosive events either resolved...

Low‐latitude daytime F region irregularities observed in two geomagnetically quiet days by the Hainan coherent scatter phased array radar (HCOPAR)

Hainan coherent scatter phased array radar (HCOPAR) located at low-latitude of China has recorded the extremely rare daytime F-region irregularities at noon of 22 July 2013 and 23 May 2016. The two field-aligned irregularities (FAIs) appeared in the topside F2-layer and presented small Doppler velocities and narrow spectral widths. The fan sector maps show that the FAIs moved northward with almost no zonal speed. The irregularities emerged in the geomagnetically quiet condition and were irrelevant to the storm induced eastward electric field as other daytime cases. More than two hours after the emergency of the daytime irregularities over Hainan, the Shaoyang digisonde situated ~870 km north to the HCOPAR recorded the spread-F in ionospheric F1-layer. According to the echo altitudes, the spread-F may connect the daytime bubbles via magnetic field line. The strong photo-ionization after sunrise made it difficult to generate the plasma bubbles in the sunlit ionosphere. Consequently, the two midday FAIs over Hainan may drift along the magnetic field lines from higher altitudes in the south and are most likely the remnant of previous nights bubbles.