太阳风离子尺度湍流与太阳风加热

太阳风源自太阳大气,在行星际空间传播过程中被持续加热,然而究竟是何种能量加热了太阳风至今未研究清楚.太阳风普遍处于湍动状态,其湍动能量被认为是加热太阳风的重要能源.然而,太阳风湍流通过何种载体、基于何种微观物理机制加热了太阳风尚不明确,这是相关研究的关键问题.将回顾人类对太阳风加热问题的研究历史,着重介绍近年来我国学者在太阳风离子尺度湍流与加热方面取得的研究进展,展望未来在太阳风加热研究中有待解决的科学问题和可能的研究方向.     

Reflection Driven MHD Turbulence in the Solar Atmosphere and Wind

Alfvénic turbulence is usually invoked and used in many solar wind models (Isenberg and Hollweg, 1982, J. Geophys. Res. 87:5023; Tu et al. 1984, J. Geophys. Res. 89:9695; Hu et  al. 2000, J. Geophys. Res. 105:5093; Li 2003, Astron. Astrphys. 406:345; Isenberg 2004, J. Geophys. Res. 109:3101) as a process responsible for the transfer of energy, released at large scale in the photosphere, towards small scale in the corona, where it is dissipated. Usually an initial spectrum is prescribed since the closest constraint to the spectrum is given by Helios measurements at 0.3 AU. With this work we intend to study the efficiency of the reflection as a driver for the nonlinear interactions of Alfvén waves, the development of a turbulent spectrum and its evolution in the highly stratified solar atmosphere inside coronal holes. Our main finding is that the perpendicular spectral slope changes substantially at the transition region because of the steep density gradient. As a result a strong turbulent heating occurs, just above the transition region, as requested by current solar wind models.     

The Search for Solar Gravity-Mode Oscillations in the Solar Wind Using it Ulysses Plasma Data

A recent report that energetic particles measured in the solar wind may be influenced by solar gravity-mode ( -mode) oscillations motivated the search for -mode signatures in the Ulysses solar wind plasma data. Ulysses solar wind plasma data from 1 March 1992 through the 12 April 1996 were examined in this study for signs of possible solar oscillations. The multi-taper method for spectral analysis was used to look for significant spectral peaks in the entire four-year data set, as well as in the smaller, more heliographically homogenous data set over the solar poles. Several frequencies satisfying certain significance requirements were found in the -mode frequency range in both data sets that also agree with the previously published findings. However, these identifications are shown to be false detections, and hence the frequencies found cannot be identified as solar modes.     

Multi-spacecraft Observations of CIR-Associated Ion Increases During the Ulysses 2007 Ecliptic Crossing

The period between 21 June and 8 October, 2007 (Carrington rotations 2058 to 2061), comprising the Ulysses ecliptic plane crossing, was characterized by low solar activity. Excluding the small solar energetic particle events observed during July, the ion increases observed in the inner heliosphere between 100?keV/n and 10?MeV/n were associated with Corotating Interaction Regions (CIRs). In this work, we investigate CIR-related ion increases using multipoint observations from Ulysses, ACE, and the twin STEREO spacecraft. The ballistic backmapping technique has been used to correlate in situ observations of these spacecraft and remote-sensing observations of coronal holes. Although the radial, longitudinal and latitudinal separation of the spacecraft (except Ulysses) are relatively small, we find discrepancies when a detailed comparison of narrow structures like stream interfaces and CIR-associated shocks is performed. Therefore we concentrate on the two CIR events from day 5 to day 10 of August 2007 and from day 25 to day 31 of August 2007, which lend themselves to a more undisturbed comparison. Using the multi-spacecraft measurements we could determine a radial gradient of 230±30% AU^?1, which is consistent with previous results by van Hollebeke et al. (J. Geophys. Res. 83, 4723, 1978) of ～?350% AU^?1 using Helios and Pioneer data.

     

High-Speed Solar Wind Streams and Geomagnetic Storms During Solar Cycle 24

An updated catalog is created of 303 well-defined high-speed solar wind streams that occurred in the time period 2009?–?2016. These streams are identified from solar and interplanetary measurements obtained from the OMNIWeb database as well as from the Solar and Heliospheric Observatory (SOHO) database. This time interval covers the deep minimum observed between the last two Solar Cycles 23 and 24, as well as the ascending, the maximum, and part of the descending phases of the current Solar Cycle 24. The main properties of solar-wind high-speed streams, such as their maximum velocity, their duration, and their possible sources are analyzed in detail. We discuss the relative importance of all those parameters of high-speed solar wind streams and especially of their sources in terms of the different phases of the current cycle. We carry out a comparison between the characteristic parameters of high-speed solar wind streams in the present solar cycle with those of previous solar cycles to understand the dependence of their long-term variation on the cycle phase. Moreover, the present study investigates the varied phenomenology related to the magnetic interactions between these streams and the Earth’s magnetosphere. These interactions can initiate geomagnetic disturbances resulting in geomagnetic storms at Earth that may have impact on technology and endanger human activity and health.     

Statistical Investigation on Galactic Cosmic Rays and Solar Wind Variation Based on ACE Observations

Based on the Galactic Cosmic Rays (GCRs) and plasma observations from ACE spacecraft, the relation between GCR counts and solar wind parameters during the two periods of solar minimums (the years of 2007.0-2009.0 and 2016.5-2019.0) was analyzed by means of the Superposed Epoch Analysis (SEA) method. The results indicate that GCRs are strongly modulated by Co-rotating Interaction Regions (CIRs) in solar wind, the Stream Interfaces (SIs) sandwiched between fast and slow solar wind are closely related with the depression of GCR counts. The mechanism of the GCR variation was investigated through the empirical diffusion coefficients. The so-called “snow-plough” effect of GCR variation prior to the SI crossing appears during the first period, then the GCR counts decrease after the crossing, which corresponds to the sudden drop of diffusion coefficient at the SI. However, this effect is not observed for the second period, the decrease of GCR counts may be caused by the enhancement of the diffusion coefficient after the SI crossing. Moreover, Heliospheric Current Sheet (HCS) correlates with GCR counts well, the GCRs drift along the current sheet, and then accumulate to a pileup structure. The interplay between drift and diffusion determines the GCR distribution and variation at a heliocentric distance of 1 AU.     

Initial Fe/O Enhancements in Large, Gradual, Solar Energetic Particle Events: Observations from Wind and Ulysses

Shocks driven by fast coronal mass ejections (CMEs) are the dominant particle accelerators in large, “gradual” solar energetic particle (SEP) events. In these events, the event-integrated value of the iron-to-oxygen ratio (Fe/O) is typically ～?0.1, at least at energies of a few MeV/nucleon. However, at the start of some gradual events, when intensities are low and growing, initially Fe/O is ～?1. This value is also characteristic of small, “impulsive” SEP events, in which particle acceleration is due to magnetic reconnection. These observations suggested that SEPs in gradual events also include a direct contribution from the flare that accompanied the CME launch. If correct, this interpretation is of critical importance: it indicates a clear path to interplanetary space for particles from the reconnection region beneath the CME. A key issue for the flare origin is “magnetic connectedness”, i.e., proximity of the flare site to the solar footpoint of the observer’s magnetic field line. We present two large gradual events observed in 2001 by Wind at L1 and by Ulysses, when it was located at >?60^° heliolatitude and beyond 1.6 AU. In these events, transient Fe/O enhancements at 5?–?10 MeV/nucleon were seen at both spacecraft, even though one or both is not “well-connected” to the flare. These observations demonstrate that an initial Fe/O enhancement cannot be cited as evidence for a direct flare component. Instead, initial Fe/O enhancements are better understood as a transport effect, driven by the different mass-to-charge ratios of Fe and O. We further demonstrate that the time-constant of the roughly exponential decay of the Fe/O ratio scales as R ², where R is the observer’s radial distance from the Sun. This behavior is consistent with radial diffusion. These observations thus also provide a potential constraint on models in which SEPs reach high heliolatitudes by cross-field diffusion.     

Electron Temperature in the Solar Wind From a Kinetic Collisionless Model: Application to High-Latitude Ulysses Observations

We use a kinetic collisionless model of the solar wind to calculate the radial variation of the electron temperature and obtain analytical expressions at large radial distances. In order to be compared with Ulysses observations, the model, which initially assumed a radial magnetic field, has been generalized to a spiral magnetic field. We present a preliminary comparison with Ulysses observations in the fast solar wind at high heliospheric latitudes. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Study of Correlation between Solar Wind Data Observed at Two Points in Interplanetary Space During the Recent Solar Maximum

Identifying co-rotating structures in solar wind enables us to predict solar wind variation at the Earth and, hence, geomagnetic disturbances. However, co-rotating structures during solar maximum are sometimes difficult to see. We correlated solar wind data obtained by two spacecraft, Nozomi heading towards Mars and ACE at the L1 point, from late 1999 through early 2002. There were intervals when the solar wind showed specific co-rotating structures even in the midst of the solar maximum, whereas no correlation was found during the other intervals. The coefficient was generally higher between Nozomi and ACE than for the 27-day recurrence at ACE, while there was some correlation, especially when the difference in longitude between the two spacecraft was less than 120°. Although frequency of occurrence of CMEs is partly responsible for the correlation, the results can be interpreted in terms of rapid changes in co-rotating high-speed streams from near-equatorial coronal holes at the solar maximum.     

White-Light Observations and Polarimetric Analysis of the Solar Corona During the Eclipse of 1 August 2008

In order to study the solar corona during eclipses, a new telescope was constructed. Three coronal images were obtained simultaneously through a single objective of the telescope as the coronal radiation passed through three polarizers (whose transmission directions were turned 0°, 60°, and 120° in the chosen direction); one image was obtained without a polarizer. The telescope was used to observe the solar corona during the eclipse of 1 August 2008. We obtained the distributions of polarization brightness, K-corona brightness, the degree of K-corona polarization and the total polarization degree; the polarization direction, depending on the latitude and radius in the plane of the sky, was also obtained. We calculated the radial distributions of electron density depending on the latitude. The properties of all these distributions were compared for different coronal structures. We determined the temperature of the coronal plasma in different coronal structures assuming hydrostatic equilibrium.     

Understanding the Behavior of the Heliospheric Magnetic Field and the Solar Wind During the Unusual Solar Minimum Between Cycles 23 and 24

The properties of the heliospheric magnetic field and the solar wind were substantially different in the unusual solar minimum between Cycles 23 and 24: the magnetic-field strength was substantially reduced, as were the flow properties of the solar wind, such as the mass flux. Explanations for these changes are offered that do not require any substantial reconsideration of the general understandings of the behavior of the heliospheric magnetic field and the solar wind that were developed in the minimum of Cycle 22?–?23. Solar-wind composition data are used to demonstrate that there are two distinct regions of solar wind: solar wind likely to originate from the stalk of the streamer belt (the highly elongated loops that underlie the heliospheric current sheet), and solar wind from outside this region. The region outside the streamer-stalk region is noticeably larger in the minimum of Cycle 23?–?24; however, the increased area can account for the reduction in the heliospheric magnetic-field strength in this minimum. Thus, the total magnetic flux contained in this region is the same in the two minima. Various correlations among the solar-wind mass flux and coronal electron temperature inferred from solar-wind charge states were developed for the Cycle 22?–?23 solar minimum. The data for the minimum of Cycle 23?–?24 suggest that the correlations still hold, and thus the basic acceleration mechanism is unchanged in this minimum.     

Solar corona during the total solar eclipse on August 1, 2008

Shape and structure of the solar corona during the August 1, 2008, total solar eclipse is reported. The August 1, 2008, corona is classified as of near-minimum type with well developed northern and southern polar ray systems over polar coronal holes and several streamers of different brightness at the middle and low heliographic latitude. The flattening index was found to be 0.21.     

Observational Study about the Influence of the Solar Wind on the Earth's Rotation

The data on the variation of the Earth's rotation from IERS and the parameters of the solar wind measured by IPM 8 from July 1992 to September 1995 were used for a regression analysis. The results show that there seems to be a weak linear correlation between the velocity of the solar wind near the Earth, V, and the variation of the length of the day, lod₁ calculated from UT1 and averaged over five days. The correlation coefficient is R=0.186. The preliminary conclusion is that the Earth's rotation could be influenced by the variation of the velocity of the solar wind with a short time scale of several days.     

Solar Rotational Periodicities and the Semiannual Variation in the Solar Wind, Radiation Belt, and Aurora

The behavior of a number of solar wind, radiation belt, auroral and geomagnetic parameters is examined during the recent extended solar minimum and previous solar cycles, covering the period from January 1972 to July 2010. This period includes most of the solar minimum between Cycles 23 and 24, which was more extended than recent solar minima, with historically low values of most of these parameters in 2009. Solar rotational periodicities from 5 to 27 days were found from daily averages over 81 days for the parameters. There were very strong 9-day periodicities in many variables in 2005?–?2008, triggered by recurring corotating high-speed streams (HSS). All rotational amplitudes were relatively large in the descending and early minimum phases of the solar cycle, when HSS are the predominant solar wind structures. There were minima in the amplitudes of all solar rotational periodicities near the end of each solar minimum, as well as at the start of the reversal of the solar magnetic field polarity at solar maximum (～?1980, ～?1990, and ～?2001) when the occurrence frequency of HSS is relatively low. Semiannual equinoctial periodicities, which were relatively strong in the 1995?–?1997 solar minimum, were found to be primarily the result of the changing amplitudes of the 13.5- and 27-day periodicities, where 13.5-day amplitudes were better correlated with heliospheric daily observations and 27-day amplitudes correlated better with Earth-based daily observations. The equinoctial rotational amplitudes of the Earth-based parameters were probably enhanced by a combination of the Russell-McPherron effect and a reduction in the solar wind-magnetosphere coupling efficiency during solstices. The rotational amplitudes were cross-correlated with each other, where the 27-day amplitudes showed some of the weakest cross-correlations. The rotational amplitudes of the >?2 MeV radiation belt electron number fluxes were progressively weaker from 27- to 5-day periods, showing that processes in the magnetosphere act as a low-pass filter between the solar wind and the radiation belt. The A _p/K _p magnetic currents observed at subauroral latitudes are sensitive to proton auroral precipitation, especially for 9-day and shorter periods, while the A _p/K _p currents are governed by electron auroral precipitation for 13.5- and 27-day periodicities.     

Comparative Study of MHD Modeling of the Background Solar Wind

Knowledge about the background solar wind plays a crucial role in the framework of space-weather forecasting. In-situ measurements of the background solar wind are only available for a few points in the heliosphere where spacecraft are located, therefore we have to rely on heliospheric models to derive the distribution of solar-wind parameters in interplanetary space. We test the performance of different solar-wind models, namely Magnetohydrodynamic Algorithm outside a Sphere/ENLIL (MAS/ENLIL), Wang–Sheeley–Arge/ENLIL (WSA/ENLIL), and MAS/MAS, by comparing model results with in-situ measurements from spacecraft located at 1 AU distance to the Sun (ACE, Wind). To exclude the influence of interplanetary coronal mass ejections (ICMEs), we chose the year 2007 as a time period with low solar activity for our comparison. We found that the general structure of the background solar wind is well reproduced by all models. The best model results were obtained for the parameter solar-wind speed. However, the predicted arrival times of high-speed solar-wind streams have typical uncertainties of the order of about one day. Comparison of model runs with synoptic magnetic maps from different observatories revealed that the choice of the synoptic map significantly affects the model performance.     

Simulated (STEREO) Views of the Solar Wind Disturbances Following the Coronal Mass Ejections of 1 August 2010

Images observed by the twin spacecraft Solar TErrestrial RElations Observatory (STEREO) A and B appear as complex structures for two coronal mass ejections (CMEs) on 1 August 2010. Therefore, a series of sky maps of Thomson-scattered white light by interplanetary coronal mass ejections (ICMEs) on 1 August 2010 are simulated using the Hakamada–Akasofu–Fry (HAF) three-dimensional solar-wind model. A comparison between the simulated images and observations of STEREO-A and -B clarifies the structure and evolution of ICMEs (including shocks) in the observed images. The results demonstrate that the simulated images from the HAF model are very useful in the interpretation of the observed images when the ICMEs overlap within the fields of view of the instruments onboard STEREO-A and -B.     

Core, Halo and Strahl Electrons in the Solar Wind

Electron velocity distribution functions (VDF) observed in the low speed solar wind flow are generally characterized by ‘core’ and ‘halo’ electrons. In the high speed solar wind, a third population of ‘strahl’ electrons is generally observed. New collisional models based on the solution of the Fokker-Planck equation can be used to determine the importance of the different electron populations as a function of the radial distance. Typical electron velocity distribution functions observed at 1 AU from the Sun are used as boundary conditions for the high speed solar wind and for the low speed solar wind. Taking into account the effects of external forces and Coulomb collisions with a background plasma, suprathermal tails are found to be present in the electron VDF at low altitudes in the corona when they exist at large radial distances. This revised version was published online in July 2006 with corrections to the Cover Date.     

Anisotropy and Dissipation of Turbulence and Their Effects on Solar Models

1 INTRODUCTIONThe maing-length theory (MLT) is the most commonly used approach to calculate convective energy transport in stars and other astrophysical situations. Based on the original idea ofPrandtl (1952) that turbulent parcels trallsfer heat in a similar way as molecules of gas do inthermal conduction, the MLT assumes that convection cells, drived by buoyancy, move thlougha ~ng length 1 and release the heat they carry when they merge with their environment. Themost widely adopted f…     

STEREO Observations of Energetic Ions in Corotating Interaction Regions During the May 2007 Solar Events

Solar Physics - Elemental composition and energy spectra of ～?0.1?–?1.0 MeV/n heavy ions were analyzed in two corotating interaction region (CIR) events...