H. Zhao

National Tsing Hua University, Hsin-chu-hsien, Taiwan, Taiwan

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Publications (23)72.12 Total impact

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    ABSTRACT: With the unique database from Michelson Doppler Imager aboard the Solar and Heliospheric Observatory in an interval embodying solar cycle 23, the cyclic behavior of solar small-scale magnetic elements is studied. More than 13 million small-scale magnetic elements are selected, and the following results are unclosed. (1) The quiet regions dominated the Sun's magnetic flux for about 8 years in the 12.25 year duration of Cycle 23. They contributed (0.94 - 1.44) $\times 10^{23}$ Mx flux to the Sun from the solar minimum to maximum. The monthly average magnetic flux of the quiet regions is 1.12 times that of active regions in the cycle. (2) The ratio of quiet region flux to that of the total Sun equally characterizes the course of a solar cycle. The 6-month running-average flux ratio of quiet region had been larger than 90.0% for 28 continuous months from July 2007 to October 2009, which characterizes very well the grand solar minima of Cycles 23-24. (3) From the small to large end of the flux spectrum, the variations of numbers and total flux of the network elements show no-correlation, anti-correlation, and correlation with sunspots, respectively. The anti-correlated elements, covering the flux of (2.9 - 32.0)$\times 10^{18}$ Mx, occupies 77.2% of total element number and 37.4% of quiet Sun flux. These results provide insight into reason for anti-correlated variations of small-scale magnetic activity during the solar cycle.
    The Astrophysical Journal 02/2011; 731(1). DOI:10.1088/0004-637X/731/1/37 · 6.28 Impact Factor
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    ABSTRACT: Context. Magnetic reconnection is a central concept for understanding solar activity, including filament eruptions, flares, and coronal mass ejections (CMEs). The existence of transverse and vertical current sheets, sites where reconnection takes place in the solar atmosphere, is frequently proposed as a precondition for flare/CME models, but is rarely identified in observations. Aims: We aim at identifying a transverse current sheet that existed in the pre-CME structure and persisted from the CME solar source to interplanetary space. Methods: STEREO A/B provide us a unique opportunity to calculate the interplanetary current sheets for the magnetic cloud. We analyze such a structure related to the fast halo CME of 2006 December 13 with assembled observations. A current sheet at the front of the magnetic cloud is analyzed to its origin in a transverse current sheet in the CME solar source, which can be revealed in the magnetic field extrapolations, XRT, and LASCO observations. Results: An interplanetary current sheet is identified as coming from the CME solar source by carefully mapping and examining multiple observations from the Sun to interplanetary space, along with nonlinear force-free magnetic field extrapolations of the active region NOAA 10930. Conclusions: The structure identified in the pre-flare state is a global transverse current sheet, which plays a role in the CME initiation, and propagates from the corona to interplanetary space.
    Astronomy and Astrophysics 01/2011; 525. DOI:10.1051/0004-6361/201015726 · 4.48 Impact Factor
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    ABSTRACT: SERENA (`Search for Exospheric Refilling and Emitted Natural Abundances') is an instrument package that will fly on board the BepiColombo Mercury Planetary Orbiter (MPO) it will investigate the Mercury's complex particle environment that surrounds the planet. Such an environment is composed by thermal and directional neutral atoms (exosphere) originating via surface release and charge-exchange processes, and by ionized particles originated through photo-ionization and again by surface release processes. In order to accomplish the scientific goals, in-situ analysis of the environmental elements is necessary, and for such a purpose the SERENA instrument shall include four units: two Neutral Particle Analyzers (ELENA and STROFIO) and two Ion Spectrometers (MIPA and PICAM). The scientific merit of SERENA is presented, and the basic characteristics of the four units are described, with a focus on novel technological aspects.
    06/2009; DOI:10.1063/1.3169309
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    ABSTRACT: Identifying the magnetic structure in the region where the magnetic field lines break and how reconnection happens is crucial to improving our understanding of three-dimensional reconnection. Here we show the in situ observation of magnetic null structures in the diffusion region, the dynamics, and the associated waves. Possible spiral null pair has been identified near the diffusion region. There is a close relation among the null points, the bipolar signature of the Z component of the magnetic field, and enhancement of the flux of energetic electrons up to 100 keV. Near the null structures, whistler-mode waves were identified by both the polarity and the power law of the spectrum of electric and magnetic fields. It is found that the angle between the fans of the nulls is quite close to the theoretically estimated maximum value of the group-velocity cone angle for the whistler wave regime of reconnection.
    Journal of Geophysical Research Atmospheres 01/2009; 114(A7). DOI:10.1029/2008JA013197 · 3.44 Impact Factor
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    ABSTRACT: This paper reports for the first time the identification of a magnetic structure around a magnetic null in a magnetic reconnection region in the magnetotail. Magnetic reconnection is one of the fundamental processes in astrophysical and solar-terrestrial plasmas. Though the concept of reconnection has been studied for many years, the process that really occurs has not been fully revealed by direct measurements. In particular, the lack of a description of three-dimensional (3-D) reconnecting magnetic field from observations makes the task more difficult. The Cluster spacecraft array provide an opportunity to reconstruct the 3-D magnetic reconnection structure based on magnetic field vectors simultaneously measured at four positions. The identification of this structure comes from a new method of analysis of in situ measurements proposed here. Applying a fitting model of 10 spherical harmonic functions and a Harris current sheet function, plus a constant field, we reconstruct a 3-D magnetic field configuration around the magnetic null in an reconnection event observed by Cluster in the geo-magnetotail.
    Journal of Geophysical Research Atmospheres 01/2008; 113. DOI:10.1029/2007JA012609 · 3.44 Impact Factor
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    ABSTRACT: Detection of a separator line that connects magnetic nulls and the determination of the dynamics and plasma environment of such a structure can improve our understanding of the three-dimensional (3D) magnetic reconnection process. However, this type of field and particle configuration has not been directly observed in space plasmas. Here we report the identification of a pair of nulls, the null-null line that connects them, and associated fans and spines in the magnetotail of Earth using data from the four Cluster spacecraft. With di and de designating the ion and electron inertial lengths, respectively, the separation between the nulls is found to be ~0.7di and an associated oscillation is identified as a lower hybrid wave with wavelength ~ de. This in situ evidence of the full 3D reconnection geometry and associated dynamics provides an important step toward to establishing an observational framework of 3D reconnection.
    Nature Physics 06/2007; DOI:10.1038/nphys650 · 20.60 Impact Factor
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    ABSTRACT: The Venus Express mission is scheduled for launch in 2005. Among many other instruments, it carries a magnetometer to investigate the Venus plasma environment. Although Venus has no intrinsic magnetic moment, magnetic field measurements are essential in studying the solar wind interaction with Venus. Our current understanding of the solar wind interaction with Venus is mainly from the long lasting Pioneer Venus Orbiter (PVO) observations. In this paper, we briefly describe the magnetic field experiment of the Venus Express mission. We compare Venus Express mission with PVO mission with respect to the solar wind interaction with Venus. Then we discuss what we will achieve with the upcoming Venus Express mission.
    Planetary and Space Science 11/2006; DOI:10.1016/j.pss.2006.04.018 · 1.63 Impact Factor
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    ABSTRACT: We use the Poincaré index of isolated null-points in vector field to strictly identify 3D null-points in coronal magnetic fields reconstructed from the observed vector magnetogram of several important active regions. Base on the null-points we identified, we reveal the essential topology skeletons of 3D coronal magnetic fields. Comparing these topology skeletons with images of H-alpha, EIT, TRACE and X-ray, we find 3D topology structures of coronal magnetic fields are closely associated with solar flares and CMEs. We also find indication of 3D magnetic reconnection by studying temporal series of 3D topology structures of solar magnetic fields. To thoroughly investigate 3D topology structures of coronal magnetic fields is a key to understand the physical mechanism of solar activity.
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    ABSTRACT: Magnetic reconnection is one of the most important processes in astrophysical, space and laboratory plasmas. Identifying the structure around the point at which the magnetic field lines break and subsequently reform, known as the magnetic null point, is crucial to improving our understanding reconnection. But owing to the inherently three-dimensional nature of this process, magnetic nulls are only detectable through measurements obtained simultaneously from at least four points in space. Using data collected by the four spacecraft of the Cluster constellation as they traversed a diffusion region in the Earth's magnetotail on 15 September, 2001, we report here the first in situ evidence for the structure of an isolated magnetic null. The results indicate that it has a positive-spiral structure whose spatial extent is of the same order as the local ion inertial length scale, suggesting that the Hall effect could play an important role in 3D reconnection dynamics.
    Nature Physics 07/2006; DOI:10.1038/nphys342 · 20.60 Impact Factor
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    ABSTRACT: Magnetic reconnection is one of the most important processes in astrophysical, space and laboratory plasmas for explaining fast dynamic release of magnetic energy. Though reconnection is three-dimensional (3D) in nature, the fundamental properties of 3D reconnection are not clearly understood. Based on the theoretical models and numerical simulations, magnetic nulls are regarded the elementary structure in 3D reconnection, on which magnetic field lines are breaking and reconnecting. Due to its three dimensional geometric property, the null can only be detected by an at least 4-point measurement which is so far uniquely provided by Cluster mission. We report here a fortuitous event that as a tetrahedron of Cluster four-spacecraft traversing a diffusion region in the Earth's magnetotail on 15 September, 2001, an isolated magnetic null is identified. It is the first report of such a null ever observed with the in-situ spacecraft measurements. The magnetic null can also be sorted as Bs-type by analyzing the spiral structure of the magnetic field in the null's neighborhood with high resolution data. Analysis of spacecraft in-situ measurements also shows that the spine structure of the null has a scale of the local ion inertial length di, suggesting that the Hall effect may be important in 3D reconnection dynamics.
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    ABSTRACT: Based on the paper that the error estimates for the height measures grows rather fast in the first few solar radii and becomes reasonably flat above 5 solar radii We show here how to obtain reliable acceleration error estimates for the measures given in the catalog We show that for the fast CMEs the uncertainties in the accelerations are quite large when compared with the average variation within a particular velocity subgroup of CMEs and as such only events with very high acceleration can reliably be considered as accelerated For most events one can not decide if a particular event has positive or negative acceleration As a consequence and classification that separates CMEs according to acceleration criteria will find similar distributions for accelerated and decelerated CMEs but that only reflects the fact that the error in determining the acceleration is quit high On average we find that slower CMEs tend to have a positive acceleration about 1 m s 2 at heights above 5 solar radii while the 5 faster CMEs show an average negative acceleration about -2 5 m s 2 as they propagate from 5 to 30 solar radii
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    ABSTRACT: The spacecraft potential of Double Star TC-1 is positive in large parts of the orbits due to the photo-effect from solar EUV irradiation. These positive potentials typically disturb low energy plasma measurements on board. The potential can be reduced, and thereby the particle measurements improved, by emitting a positive ion beam. This method has successfully been applied on several other spacecraft and it has also been chosen for TC-1. The instrument TC-1/ASPOC is a derivative of the Cluster/ASPOC instruments, from which it has inherited many features. The paper describes the adaptations and further developments made for the ion emitters and the electronics. The instrument performs very well and can support higher beam currents than on Cluster. The expected significant improvement of the low energy particle measurements on board was indeed observed. The modifications of the electron distributions are analysed for a one-time interval when the spacecraft was located in the magnetosheath. The change in the potential due to the ion beam was determined, and first studies of the 3-D electron distributions in response to the spacecraft potential control have been performed, which indicate that the method works as expected.
    Annales Geophysicae 01/2005; DOI:10.5194/angeo-23-2813-2005 · 1.68 Impact Factor
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    ABSTRACT: Double Star Program (DSP) aims to investigate the trigger mechanism and physical models of geomagnetic storms and substorms. The DSP involves two satellites: the equatorial satellite of DSP (TC-1 in China) and the polar satellite of DSP (TC-2 in China). On board the two satellites of TC-1 and TC-2, there are four kinds of particle instruments developed by the Center for Space Science and Applied Research (CSSAR), namely: the High Energy Electron Detector (HEED, TC-1, 2), the High Energy Proton Detector (HEPD, TC-1, 2), the High Energy Heavy Ion Detector (HID, TC-1, 2) and the Low Energy Ion Detector (LEID, TC-2). HEED, HEPD and HID were developed and calibrated in China. The LEID was developed in China and calibrated in France. This paper introduces the scientific objectives and characteristics of each instrument, their status and some preliminary results.
    Annales Geophysicae 01/2005; 23(8). DOI:10.5194/angeo-23-2775-2005 · 1.68 Impact Factor
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    ABSTRACT: An ion emitter instrument ASPOC (Active Spacecraft Potential Control) belongs to the payload of the Chinese-European Double Star mission (TC-1) launched in December 2003. The instrument is a further development to the ones flown in the Cluster mission. Its objective is a reduction of the spacecraft potential in order to minimise the perturbations to the plasma measurements on board. The operation of the scientific payload began after commissioning in February 2004. Comparisons to Cluster are being made based on data from the first half year of the Double Star mission. The enhanced capabilities of the instrument allow to achieve even lower potentials than on Cluster. Differences to Cluster can also be expected because of the plasma environment at the equatorial orbit of TC-1. The effects of spacecraft potential control on the electron measurements by the instrument PEACE as observed during the first months of science operations are discussed.
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    ABSTRACT: On June 14, 1998, a Pi 2 pulsation of unusually large amplitude was observed simultaneously by ground stations located in both of dayside and nightside local time sectors and with 145-degree latitude span (from 70.6°N to 74.7° S). The geomagnetic field observed by stations in nightside clearly showed substrom signatures at the event time and it appeared that the dayside Pi 2 signature is also associated with the substorm. We analyzed the polarization of the dayside Pi 2 wave in the horizontal plane and found that the longitudinal variation of the polarization agreed with that generated by a current wedge. This has previously been reported for nighttime Pi 2 but is unexpected for daytime events.
    Advances in Space Research 11/2002; 30(10):2339-2343. DOI:10.1016/S0273-1177(02)80266-6 · 1.24 Impact Factor
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    ABSTRACT: Electrostatic charging of a spacecraft modifies the distribution of electrons and ions before the particles enter the sensors mounted on the spacecraft body. The floating potential of magnetospheric satellites in sunlight very often reaches several tens of volts, making measurements of the cold (several eV) component of the ambient ions impossible. The plasma electron data become contaminated by large fluxes of photoelectrons attracted back into the sensors. The Cluster spacecraft are equipped with emitters of the liquid metal ion source type, producing indium ions at 5 to 9 keV energy at currents of some tens of microampere. This current shifts the equilibrium potential of the spacecraft to moderately positive values. The design and principles of the operation of the instrument for active spacecraft potential control (ASPOC) are presented in detail. Experience with spacecraft potential control from the commissioning phase and the first two months of the operational phase are now available. The instrument is operated with constant ion current for most of the time, but tests have been carried out with varying currents and a "feedback" mode with the instrument EFW, which measures the spacecraft potential . That has been reduced to values according to expectations. In addition, the low energy electron measurements show substantially reduced fluxes of photoelectrons as expected. The flux decrease in photoelectrons returning to the spacecraft, however, occurs at the expense of an enlarged sheath around the spacecraft which causes problems for boom-mounted probes.
    Annales Geophysicae 01/2001; DOI:10.5194/angeo-19-1289-2001 · 1.68 Impact Factor
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    01/2000; 449:371.
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    01/2000; 449:339.
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    ABSTRACT: MERIDIAN is a 120° east ground based multi-station and multi-instrument project proposed in China. IWF/ÖAW in Graz, Austria in cooperation with IGPP/UCLA in Los Angeles, USA and CSSAR/CAS in Beijing, China has developed variometers for magnetic field observations along this MERIDIAN chain. The variometer (CHIMAG) is a fluxgate magnetometer especially for Earth field variation and pulsation measurements. Three variometer stations have been installed in Beijing (40° N), Hainan (19° N) and Wuhan (30.5° N), respectively. Two more will be installed in 1999. In this paper we will present the design and the scientific objectives of the CHIMAG as well as the results of the first observations.
    Advances in Space Research 01/2000; 25(7):1353-1356. DOI:10.1016/S0273-1177(99)00644-4 · 1.24 Impact Factor