Geomagnetic Field Disturbances Caused by Heliospheric Current Sheet Crossings

The heliospheric current sheet (HCS) is modified by the solar activity. HCS is highly inclined during solar maximum and almost confined with the solar equatorial plane during solar minimum. Close to the HCS solar wind parameters as proton temperature, flow speed, proton density, etc. differ compared to the region far from the HCS. The Earth’s magnetic dipole field crosses HCS several times each month. Considering interplanetary coronal mass ejections (ICME) and high speed solar wind streams (HSS) free periods an investigation of the HCS influence on the geomagnetic field disturbances is presented. The results show a drop of the Dst index and a rise of the AE index at the time of the HCS crossings and also that the behavior of these indices does not depend on the magnetic polarity.     

激波在磁盔－电流片位场中的传播：（Ⅰ）激波与磁盔间的相互作用

利用新近获得的子午面磁盔－电流片背景太阳风稳态解，对激波从盔底沿电流片方向往外传播时与磁盔间的相互作用进行了数值模拟研究，重要新结果是：１．磁盔的存在使受扰介质速度跃变中央出现下凹，随着激波传出磁盔区并沿电流片方向传播，速度下凹逐渐减弱以致消失；２．激波将磁盔拉长并把盔顶的环形（垂直赤道面）磁场带到行星际空间，成为行星际磁场南向分量的来源之一；３．５个太阳半径（Ｒ⊙）内的磁盔部分将出现精细结构，沿盔外边界形成两条高速带，以及马蹄形密度（亮）环形结构等．这些结果表明，太阳附近高速等离子体与磁盔间存在重要的动力学相互作用过程，对行星际空间的太阳风三维结构有重要影响．     

激波在磁盔－电流片位场中的传播：（Ⅰ）激波与磁盔间的相互作用

利用新近获得的子午面磁盔－电流片背景太阳风稳态解，对激波从盔底沿电流片方向往外传播时与磁盔间的相互作用进行了数值模拟研究，重要新结果是：１．磁盔的存在使受扰介质速度跃变中央出现下凹，随着激波传出磁盔区并沿电流片方向传播，速度下凹逐渐减弱以致消失；２．激波将磁盔拉长并把盔顶的环形（垂直赤道面）磁场带到行星际空间，成为行星际磁场南向分量的来源之一；３．５个太阳半径（Ｒ⊙）内的磁盔部分将出现精细结构，沿盔外边界形成两条高速带，以及马蹄形密度（亮）环形结构等．这些结果表明，太阳附近高速等离子体与磁盔间存在重要的动力学相互作用过程，对行星际空间的太阳风三维结构有重要影响．     

Modulation of galactic cosmic rays in solar cycles 22–24: Analysis and physical interpretation

This work represents a physical interpretation of cosmic ray modulation in the 22nd–24th solar cycles, including an interpretation of an unusual behavior of their intensity in the last minimum of the solar activity (2008–2010). In terms of the Parker modulation model, which deals with regularly measured heliospheric characteristics, it is shown that the determining factor of the increased intensity of the galactic cosmic rays in the minimum of the 24th solar cycle is an anomalous reduction of the heliospheric magnetic field strength during this time interval under the additional influence of the solar wind velocity and the tilt angle of the heliospheric current sheet. We have used in the calculations the dependence of the diffusion tensor on the rigidity in the form K _ij ∝ R ^2?μ with μ = 1.2 in the sector zones of the heliospheric magnetic field and with μ = 0.8 outside the sector zones, which leads to an additional amplification of the diffusion mechanism of cosmic ray modulation. The proposed approach allows us to describe quite satisfactorily the integral intensity of protons with an energy above 0.1 GeV and the energy spectra in the minima of the 22nd–24th solar cycles at the same value of the free parameter. The determining factor of the anomalously high level of the galactic cosmic ray intensity in the minimum of the 24th solar cycle is the significant reduction of the heliospheric magnetic field strength during this time interval. The forecast of the intensity level in the minimum of the 25th solar cycle is provided.     

Heliospheric current sheet inclinations at Venus and Earth

We investigate the inclinations of heliospheric current sheet at two sites in interplanetary space, which are generated from the same solar source. From the data of solar wind magnetic fields observed at Venus (0.72 AU) and Earth (1 AU) during December 1978–May 1982 including the solar maximum of 1981, 54 pairs of candidate sector boundary crossings are picked out, of which 16 pairs are identified as sector boundaries. Of the remainder, 12 pairs are transient structures both at Venus and Earth, and 14 pairs are sector boundaries at one site and have transient structures at the other site. It implies that transient structures were often ejected from the coronal streamer belt around the solar maximum. For the 16 pairs of selected sector boundaries, we determine their normals by using minimum variance analysis. It is found that most of the normal azimuthal angles are distributed between the radial direction and the direction perpendicular to the spiral direction both at Venus and Earth. The normal elevations tend to be smaller than ≈45° with respect to the solar equatorial plane, indicating high inclinations of the heliospheric current sheet, in particular at Earth. The larger scatter in the azimuth and elevation of normals at Venus than at Earth suggests stronger effects of the small-scale structures on the current sheet at 0.72 AU than at 1 AU. When the longitude difference between Venus and Earth is small (<40° longitudinally), similar or the same inclinations are generally observed, especially for the sector boundaries without small-scale structures. This implies that the heliospheric current sheet inclination tends to be maintained during propagation of the solar wind from 0.72 AU to 1 AU. Detailed case studies reveal that the dynamic nature of helmet streamers causes variations of the sector boundary structure.     

行星际速度增幅扰动的演化

采用二维理想MHD模型，分别在日球赤道面（二维二分量模型）和日球子午面（二维 三分量模型）内研究太阳风中纯速度增幅扰动的演化. 结果表明，该扰动在向外传播的过程 中逐渐演化为双重激波对，即由4个激波组成的激波系统. 该4个激波按离太阳由近及远依次 为后向快激波、后向慢激波、前向慢激波和前向快激波. 双重激波对在子午面内相对扰动源 中心法线基本对称，而在赤道面内则不对称：扰动源中心法线西侧双重激波对结构更为明显 ，所跨经度范围宽于东侧. 初步分析表明，行星际磁场的螺旋结构是产生日球赤道面内双重 激波对结构东西不对称性的主要原因.     

行星际扰动和地磁活动对GEO相对论电子影响

利用1988—2010年小时平均的GOES卫星数据,对地球同步轨道(GEO)相对论电子变化进行了统计分析,研究了相对论电子通量(Fe)增强事件的发展过程,探讨了利于相对论电子通量增强的太阳风和地磁活动条件.主要结论如下:(1)GEO相对论电子通量即使是峰值,也具有明显的地方时特性,最大电子通量出现在磁正午时.午/夜电子通量比率随着太阳风速度(Vsw)增加而增大;在Dst-50nT时相对论电子具有规则的地方时变化.在太阳活动下降相,电子通量与各参数的相关性较好,与其相关性最好的Vsw、Kp指数以及三次根号下的太阳风密度(N)分别出现在电子通量前39~57h、57~80h和12~24h.(2)强(日平均电子通量峰值Femax≥104 pfu)相对论电子事件,在距离太阳活动谷年前两年左右和春秋分期间发生率最高,较弱(104Femax≥103 pfu)事件无此特点;大部分强相对论电子事件中,电子通量在磁暴主相开始增加,较弱事件中则在恢复相开始回升.(3)太阳风密度变化对相对论电子事件的发展具有重要指示作用.电子通量在太阳风密度极大值后0~1天达到极小值,太阳风密度极小值后0~2天达到极大值.(4)90%以上相对论电子事件是在磁暴及高速太阳风的条件下发生的,与其伴随的行星际参数和地磁活动指数极值满足以下条件:Vswmax516km/s,Dstmin-31nT,Nmin2.8cm-3,Nmax14.1cm-3,Bzmin-2.9nT,AEmax698nT.(5)磁暴过程中,Dstmin后日平均电子通量大于103 pfu的发生概率为53%左右,强/弱相对论电子事件占总数比例分别为36%/64%左右,磁暴强度对其无影响.磁暴过程中的Vsw、N和AE指数大小对于能否引起相对论电子增强起着指示作用.     

Obtaining of information about the IMF evolution from an analysis of the geomagnetic field data

A comparison of the time variations in the geomagnetic field characteristics (the u and aa indices of geomagnetic activity) with the variation in the solar magnetic dipole inclination shows close agreement between these variations. The linear correlation coefficients between the u and aa indices, the u index and solar magnetic dipole inclination, and the aa index and solar magnetic dipole inclination are 0.93, 0.45, and 0.49, respectively. This makes it possible to extend studying the IMF evolution in the 11-year cycle of solar activity to the 170-year period beginning from 1835. It has been indicated that the time variation in the heliospheric current sheet (HCS) surface deviation from the solar magnetic equator plane, calculated based on the actual HCS configuration, is in good agreement with the time variation in the amplitude of the Fourier series second harmonics in a harmonic analysis of the series of daily data on the IMF sign in the vicinity of the Earth. The linear correlation coefficient is 0.9 in this case.     

The solar magnetic flux mid-term periodicities and the solar dynamo

We investigate here the fluctuations in the total, open and closed solar magnetic flux (SMF) for the period 1971–1999 by means of the maximum entropy method in the frequency range 5×10⁻⁹–10⁻⁷ Hz (6 yr to 120 days). We use monthly data for the total, open and closed magnetic solar fluxes. Periodicities found in the series are similar showing that there is some relationship between the fluxes. The most important finding of this work is the existence of fluctuations at around 1.3 and 1.7 yr in the SMF with alternating importance during consecutive even and odd solar cycles. These fluctuations are directly related with variations present in cosmic rays, solar wind parameters and geomagnetic activity indexes. A quasi-triennial periodicity previously found in sunspots and other solar phenomena is also of importance. The SMF is generated by the action of the solar dynamo; therefore, it is through the magnetic flux that the solar dynamo influences several heliospheric phenomena.     

Selected solar wind parameters at 1 AU through two solar activity cycles

In situ measurements of the solar wind largely cover more than two solar magnetic activity cycles, namely 20 and 21. This is a very appealing opportunity to study the influence of the activity cycle on the behaviour of the solar wind parameters. As a matter of fact, many authors so far have studied this topic comparing the long-term magnetic field and plasma averages. However, when the average values are evaluated on a data sample whose duration is comparable with (or even longer than) the solar rotation period we lose information about the contribution due to the fast and the slow solar wind components. Thus, discriminating in velocity plays a key role in understanding solar cycle effects on the solar wind. Based on these considerations, we performed a separate analysis for fast and slow wind, respectively. In particular, we found that: (a) fast wind carries a slightly larger momentum flux density at 1 AU, probably due to dynamic stream-stream interaction; (b) proton number density in slow wind is more cycle dependent than in fast wind and decreases remarkably across solar maximum; (c) fast wind generally carries a magnetic field intensity stronger than that carried by the slow wind; (d) we found no evidence for a positive correlation between velocity and field intensity as predicted by some theories of solar wind acceleration; (e) our results would support an approximately constant divergence of field lines associated with corotating high-velocity streams.     

Latitudinal distribution of the solar wind properties in the low- and high-pressure regimes: Wind observations

The solar wind properties depend on , the heliomagnetic latitude with respect to the heliospheric current sheet (HCS), more than on the heliographic latitude. We analyse the wind properties observed by Wind at 1 AU during about 2.5 solar rotations in 1995, a period close to the last minimum of solar activity. To determine , we use a model of the HCS which we fit to the magnetic sector boundary crossings observed by Wind. We find that the solar wind properties mainly depend on the modulus ||. But they also depend on a local parameter, the total pressure (magnetic pressure plus electron and proton thermal pressure). Furthermore, whatever the total pressure, we observe that the plasma properties also depend on the time: the latitudinal gradients of the wind speed and of the proton temperature are not the same before and after the closest HCS crossing. This is a consequence of the dynamical stream interactions. In the low pressure wind, at low ||, we find a clear maximum of the density, a clear minimum of the wind speed and of the proton temperature, a weak minimum of the average magnetic field strength, a weak maximum of the average thermal pressure, and a weak maximum of the average factor. This overdense sheet is embedded in a density halo. The latitudinal thickness is about 5° for the overdense sheet, and 20° for the density halo. The HCS is thus wrapped in an overdense sheet surrounded by a halo, even in the non-compressed solar wind. In the high-pressure wind, the plasma properties are less well ordered as functions of the latitude than in the low-pressure wind; the minimum of the average speed is seen before the HCS crossing. The latitudinal thickness of the high-pressure region is about 20°. Our observations are qualitatively consistent with the numerical model of Pizzo for the deformation of the heliospheric current sheet and plasma sheet.     

Wavelet image of a heliospheric storm in cosmic rays

During the sign reversal of the global solar magnetic field, the variations in the ratio of the quadrupole component of the field to its dipole part manifest themselves in a change of the two-sector structure of the heliospheric current sheet (HCS) into the four-sector and, then, multisector structures. At that time, a soliton-like wave packet (soliton of the envelope), precisely which is responsible for a wavelet image of heliospheric storm in cosmic rays, is formed in HCS.     

Long-term modulation of galactic cosmic rays at solar activity minimums

Based on observations of long-term variations in galactic cosmic rays (CRs) on Earth and in the near-Earth space, we have determined, using our own semiempirical model, modulation of galactic CRs during solar cycles 19–23. The modulation model relates CR variations to the characteristics of the solar magnetic field obtained for the surface of the solar wind source at distances of 2.50 and 3.25 solar radii. The main focus is CR behavior at the minimums of cycles 19–23 and specific features of CR modulation at a prolonged (as compared to previous cycles) minimum of cycle 23, which is still ongoing. CR modulation at minimums related to a change in the solar field dipole component during this period of the cycle has been considered. It is indicated that the long-term variations in CRs are better described if the last two years (2007 and 2008) of cycle 23 with anomalously low solar activity (SA) are included in the model. The role and value of the contribution of the cyclic variations in each index used in the proposed CR modulation model to the observed CR modulation have been estimated.     

Solar and heliospheric geoeffective disturbances

The past decade has brought advances in several areas of solar-terrestrial physics which, when combined, provide nearly all of the pieces necessary for predicting geomagnetic storms. Advances in techniques for observing the Sun in X-rays and white light allow identification of solar disturbances headed toward Earth. Advances in our understanding of how the resulting heliospheric disturbances reflect aspects of the Sun's magnetic field allow predictions of their magnetic topology and, hence, provide some measure of the geoeffective southward component which they carry. Advances in our understanding of the relationship between transient heliospheric disturbances and high-speed streams and how storm strength depends upon solar wind density and the magnetic polarity of streams allow substantial refinement for prediction schemes.     

太阳风流界面的成因

在日心距离１ＡＵ处的高速流的前沿部位，经常观测到厚度≈１０^４ｋｍ的流界面（ｓｔｒｅａｍｉｎｔｅｒｆａｃｅ）：跨过它密度陡降，温度陡增，风速上升，气压和磁场几乎连续．本文从日心距离０．３ＡＵ处一典型高速流的方位剖面出发，采用二维定态ＭＨＤ模型，研究它在日球赤道面内随日心距离的演化．结果表明，流界面系高速流前沿非线性演化的产物．它先于前、后向激波形成，在日心距离１ＡＵ处得到充分发展，且作为高速流前沿的特征结构之一，可一直延伸到１ＡＵ以远的外日球层．     

Streamer belt in the solar corona and the Earth’s orbit

It has been indicated that the cross section of the streamer belt in the solar corona and its extension in the heliosphere—heliospheric plasma sheet (HPS)—have the form of two radially oriented closely located (at a distance of d ≈ 2.0–2.5° in the heliocentric coordinate system) rays with increased and generally different densities. The angular dimensions of the rays are ≈d. The neutral line of the magnetic field in the corona and the related sector boundary in the Earth’s orbit are located between the peaks of densities of these two rays. In the events, during which the true sector boundary coincides with the heliospheric current sheet, the transverse structure of the streamer belt in the heliosphere (or the HPS structure) is quasistationary; i.e., this structure slightly changes when the solar wind moves from the Sun to the Earth in, at least, 50% of cases. A hypothesis that a slow solar wind, flowing in the rays with increased density of the streamer belt, is probably generated on the Sun’s surface rather than at the top of the helmet, as was assumed in [Wang et al., 2000], is put forward.     

The equatorial ionospheric anomaly in electron content from solar minimum to solar maximum for South East Asia

Median hourly, electron content-latitude profiles obtained in South East Asia under solar minimum and maximum conditions have been used to establish seasonal and solar differences in the diurnal variations of the ionospheric equatorial anomaly (EIA). The seasonal changes have been mainly accounted for from a consideration of the daytime meridional wind, affecting the EIA diffusion of ionization from the magnetic equator down the magnetic field lines towards the crests. Depending upon the seasonal location of the subsolar point in relation to the magnetic equator diffusion rates were increased or decreased. This led to crest asymmetries at the solstices with (1) the winter crest enhanced in the morning (increased diffusion rate) and (2) the same crest decaying most rapidly in the late afternoon (faster recombination rate at lower ionospheric levels). Such asymmetries were also observed, to a lesser extent, at the equinoxes since the magnetic equator (located at about 9○N lat) does not coincide with the geographic equator. Another factor affecting the magnitude of a particular electron content crest was the proximity of the subsolar point, since this increased the local ionization production rate. Enhancements of the EIA took place around sunset, mainly during the equinoxes and more frequently at solar maximum, and also there was evidence of apparent EIA crest resurgences around 0300 LST for all seasons at solar maximum. The latter are thought to be associated with the commonly observed, post-midnight, ionization enhancements at midlatitudes, ionization being transported to low latitudes by an equatorward wind. The ratio increases in crest peak electron contents from solar minimum to maximum of 2.7 at the equinoxes, 2.0 at the northern summer solstice and 1.7 at northern winter solstice can be explained, only partly, by increases in the magnitude of the eastward electric field E overhead the magnetic equator affecting the [E×B] vertical drifts. The most important factor is the corresponding increase in ionization production rate due to the increase in solar radiation flux. The EIA crest asymmetries observed at solar maximum were less significant, and this is probably due to the corresponding increase in ionization densities leading to an increase of the retarding effect of ion-drag on the daytime meridional winds.     

Galactic Cosmic Ray Intensity in the Upcoming Minimum of the Solar Activity Cycle

During the prolonged and deep minimum of solar activity between cycles 23 and 24, an unusual behavior of the heliospheric characteristics and increased intensity of galactic cosmic rays (GCRs) near the Earth’s orbit were observed. The maximum of the current solar cycle 24 is lower than the previous one, and the decline in solar and, therefore, heliospheric activity is expected to continue in the next cycle. In these conditions, it is important for an understanding of the process of GCR modulation in the heliosphere, as well as for applied purposes (evaluation of the radiation safety of planned space flights, etc.), to estimate quantitatively the possible GCR characteristics near the Earth in the upcoming solar minimum (~2019–2020). Our estimation is based on the prediction of the heliospheric characteristics that are important for cosmic ray modulation, as well as on numeric calculations of GCR intensity. Additionally, we consider the distribution of the intensity and other GCR characteristics in the heliosphere and discuss the intercycle variations in the GCR characteristics that are integral for the whole heliosphere (total energy, mean energy, and charge).     

MHD spiral vortex tubes of the solar wind in the outer heliosphere

Summary MHD vortex flow near the heliomagnetic equatorial plane in the outer heliosphere is studied. The possibility of spiral vortex tubes existing on both sides of the equatorial plane is shown. A structure of this type with the scale of the order of several astronomical units can be formed during the solar minimum when the solar wind velocity has a distinct latitudinal dependence with a minimum near the equatorial plane. Another cause of this vortex structure can be the presence of inhomogeneities of the solar wind in the inner heliosphere.     

Solar wind velocity distribution on the heliospheric current sheet during Carrington rotations 1787-1795

The solar wind velocity distribution in the heliosphere is best represented using a v-map, where velocity contours are plotted in heliographic latitude-longitude coordinates. It has already been established that low-speed regions of the solar wind on the source surface correspond to the maximum bright regions of the K-corona and the neutral line of the coronal magnetic field. In this analysis, v-maps on the source surface for Carrington rotations (CRs) 1787-1795, during 1987, have been prepared using the interplanetary scintillation measurements at Research Institute of Atmospherics (RIA), Nagoya Univ., Japan. These v-maps were then used to study the time evolution of the low-speed (\leq450 km s⁻¹) belt of the solar wind and to deduce the distribution of solar wind velocity on the heliospheric current sheet. The low-speed belt of the solar wind on the source surface was found to change from one CR to the next, implying a time evolution. Instead of a slow and systematic evolution, the pattern of distribution of solar wind changed dramatically at one particular solar rotation (CR 1792) and the distributions for the succeeding rotations were similar to this pattern. The low-speed region, in most cases, was found to be close to the solar equator and almost parallel to it. However, during some solar rotations, they were found to be organised in certain longitudes, leaving regions with longitudinal width greater than 30^○ free of low-speed solar wind, i.e. these regions were occupied by solar wind with velocities greater than 450 km s⁻¹. It is also noted from this study that the low-speed belt, in general, followed the neutral line of the coronal magnetic field, except in certain cases. The solar wind velocity on the heliospheric current sheet (HCS) varied in the range 300–585 km s⁻¹ during the period of study, and the pattern of velocity distribution varied from rotation to rotation.