Solar wind velocity measurements near the sun using Ulysses radio amplitude correlations at two frequencies

Measurements of solar wind velocity have been derived from simultaneous coronal sounding observations of radio amplitude scintillations at both S-band and X-band during the solar conjunction of the Ulysses spacecraft in August 1991. The signal amplitude was recorded with an averaging time of 1 s. A cross-correlation analysis between S- and X-band amplitude fluctuations shows that the fluctuation signature at S-band appears to be shifted to earlier times with respect to the X-band recording. The time difference is proportional to the coronal separation of the ray paths and inversely proportional to the apparent velocity of plasma inhomogeneities across the ray paths. Preliminary estimates of solar wind speed obtained using model calculations of the differential refraction are found to lie near the expected transition from subsonic to supersonic velocities at solar offset distances of the order of 6–8 R. As a byproduct of the investigation, we find that the transition from weak to saturated scintillation occurs at about 16 R for S-band and 7 R for X-band.     

Sharp boundaries of solar wind plasma structures and their relationship to solar wind turbulence

Sharp (<10 min) and large (>20%) solar wind ion flux changes are common phenomena in turbulent solar wind plasma. These changes are the boundaries of small- and middle-scale solar wind plasma structures which can have a significant influence on Earth’s magnetosphere. These solar wind ion flux changes are typically accompanied by only a small change in the bulk solar wind velocity, hence, the flux changes are driven mainly by plasma density variations. We show that these events occur more frequently in high-density solar wind. A characteristic of solar wind turbulence, intermittency, is determined for time periods with and without these flux changes. The probability distribution functions (PDF) of solar wind ion flux variations for different time scales are calculated for each of these periods and compared. For large time scales, the PDFs are Gaussian for both data sets. For small time scales, the PDFs from both data set are more flat than Gaussian, but the degree of flatness is much larger for the data near the sharp flux change boundaries.     

Large geomagnetic storms of extreme solar event periods in solar cycle 23

During extreme solar events such as big flares or/and energetic coronal mass ejections (CMEs) high energy particles are accelerated by the shocks formed in front of fast interplanetary coronal mass ejections (ICMEs). The ICMEs (and their sheaths) also give rise to large geomagnetic storms which have significant effects on the Earth’s environment and human life. Around 14 solar cosmic ray ground level enhancement (GLE) events in solar cycle 23 we examined the cosmic ray variation, solar wind speed, ions density, interplanetary magnetic field, and geomagnetic disturbance storm time index (D_st). We found that all but one of GLEs are always followed by a geomagnetic storm with D_st  −50 nT within 1–5 days later. Most(10/14) geomagnetic storms have D_st index  −100  nT therefore generally belong to strong geomagnetic storms. This suggests that GLE event prediction of geomagnetic storms is 93% for moderate storms and 71% for large storms when geomagnetic storms preceded by GLEs. All D_st depressions are associated with cosmic ray decreases which occur nearly simultaneously with geomagnetic storms. We also investigated the interplanetary plasma features. Most geomagnetic storm correspond significant periods of southward B_z and in close to 80% of the cases that the B_z was first northward then turning southward after storm sudden commencement (SSC). Plasma flow speed, ion number density and interplanetary plasma temperature near 1 AU also have a peak at interplanetary shock arrival. Solar cause and energetic particle signatures of large geomagnetic storms and a possible prediction scheme are discussed.     

The characteristics of sharp (small-scale) boundaries of solar wind plasma and magnetic field structures

We investigate properties of large (>20%) and sharp (<10 min) solar wind ion flux changes using INTERBALL-1 and WIND plasma and magnetic field measurements from 1996 to 1999. These ion flux changes are the boundaries of small-scale and middle-scale solar wind structures. We describe the behavior of the solar wind velocity, temperature and interplanetary magnetic field (IMF) during these sudden flux changes. Many of the largest ion flux changes occur during periods when the solar wind velocity is nearly constant, so these are mainly plasma density changes. The IMF magnitude and direction changes at these events can be either large or small. For about 55% of the ion flux changes, the sum of the thermal and magnetic pressure are in balance across the boundary. In many of the other cases, the thermal pressure change is significantly more than the magnetic pressure change. We also attempted to classify the types of discontinuities observed.     

一个基于观测的太阳风背景模型

建立由太阳光球磁场和日冕偏振亮度等观测约束的单流体太阳风模型,包括日冕和太阳风的等离子体密度、速度和磁场,温度还有待于以后处理.这里采用高山观测台(HAO)MKⅢ的日冕偏振亮度(pB)在1.36Rs上的观测概图,根据Guhathakurta在1996年发展的日冕电子密度反演模型确定日冕的电子密度分布.同时采用Wilcox太阳观测台(WSO)的光球磁场视向分量的观测概图作为底部边界,根据Zhao等在1994年发展的水平电流-电流片(HCCS)模型得到全球磁场.Phillips在1995年及McComas在2003年分别用Ulysses第一次和第二次跨极飞行的观测发现,归一化到1 AU的太阳风动量流密度除了在10°～30°的纬度范围内略低以外几乎不变.根据这一结论,结合已经得到的密度数据,就可以得到日冕和太阳风的速度.将上面的模型应用于1918卡林顿自转周稳态太阳风的研究,结果与太阳活动极小期的观测基本相符,但是与观测相比较低速高密度区偏大,因此密度模型还有待改进.      

Solar wind dependence of the Pc1 wave activity

There are a host of factors influencing the excitation of Pc1 geomagnetic pulsations, which are ULF waves in the frequency range between 0.2 and 5 Hz. We have studied carefully the dependence of the pearl-type Pc1 activity at Sodankylä, Finland (L = 5.1) on the plasma density N in front of the magnetosphere, the bulk velocity V of the solar wind, and the intensity B of the IMF. The result is as follows: high values of N and reduced values of V are favorable to appearance of Pc1, whereas the dependence of Pc1 activity on B is practically absent. We also show that the probability of Pc1 occurrence decreases with the interplanetary electric field, and increases with solar wind impact pressure and with the plasma to magnetic pressure ratio “beta”.     

Interplanetary shocks and sudden impulses during solar maximum (2000) and solar minimum (1995–1996)

In this work a study is performed on the correlation between fast forward interplanetary shock parameters at 1 Astronomical Unit and sudden impulse (SI) amplitudes in the H-component of the geomagnetic field, for periods of solar activity maximum (year 2000) and minimum (year 1995–1996). Solar wind temperature, density and speed, and total magnetic field, were taken to calculate the static pressures (thermal and magnetic) both in the upstream and downstream sides of the shocks. The variations of the solar wind parameters and pressures were then correlated with SI amplitudes. The solar wind speed variations presented good correlations with sudden impulses, with correlation coefficients larger than 0.70 both in solar maximum and solar minimum, whereas the solar wind density presented very low correlation. The parameter better correlated with SI was the square root dynamic pressure variation, showing a larger correlation during solar maximum (r = 0.82) than during solar minimum (r = 0.77). The correlations of SI with square root thermal and magnetic pressure were smaller than with the dynamic pressure, but they also present a good correlation, with r > 0.70 during both solar maximum and minimum. Multiple linear correlation analysis of SI in terms of the three pressure terms have shown that 78% and 85% of the variance in SI during solar maximum and minimum, respectively, are explained by the three pressure variations. Average sudden impulse amplitude was 25 nT during solar maximum and 21 nT during solar minimum, while average square root dynamic pressure variation is 1.20 and 0.86 nPa^1/2 during solar maximum and minimum, respectively. Thus on average, fast forward interplanetary shocks are 33% stronger during solar maximum than during solar minimum, and the magnetospheric SI response has amplitude 20% higher during solar maximum than during solar minimum. A comparison with theoretical predictions (Tsyganenko’s model corrected by Earth’s induced currents) of the coefficient of sudden impulse change with solar wind dynamic pressure variation showed excellent agreement, with values around 17 nT/nPa^1/2.     

Prompt effects of solar wind variations on the inner magnetosphere and midlatitude ionosphere

It is well known that the solar wind can significantly affect high-latitude ionospheric dynamics. However, the effects of the solar wind on the middle- and low-latitude ionosphere are much less studied. In this paper, we report observations that large perturbations in the middle- and low-latitude ionosphere are well correlated with solar wind variations. In one event, a significant (20–30%) decrease of the midlatitude ionospheric electron density over a large latitudinal range was related to a sudden drop in the solar wind pressure and a northward turning of the interplanetary magnetic field, and the density decrease became larger at lower latitudes. In another event, periodic perturbations in the dayside equatorial ionospheric E × B drift and electrojet were closely associated with variations in the interplanetary electric field. Since the solar wind is always changing with time, it can be a very important and common source of ionospheric perturbations at middle- and low-latitudes. The relationship between solar wind variations and significant ionospheric perturbations has important applications in space weather.     

Plasma flow variations and energetic protons upstream of the earth’s bow shock: A statistical study

Foreshock is a special region located upstream of the Earth’s bow shock characterized by the presence of various plasma waves and fluctuations caused by the interaction of the solar wind plasma with particles reflected from the bow shock or escaping from the magnetosphere. On the other hand, foreshock fluctuations may modify the bow shock structure and, being carried through the magnetosheath, influence the magnetopause. During the years 1995–2000, the INTERBALL-1 satellite made over 10,000 hours of plasma and energetic particles measurements in the solar wind upstream of the Earth’s bow shock. We have sorted intervals according to the level of solar wind ion flux fluctuations and/or according to the flux of back-streaming energetic protons. An analysis of connection between a level of ion flux fluctuations and fluxes of high-energy protons and their relation to the IMF orientation is presented.     

Coronal radio-sounding detection of a CME during the 1997 Galileo solar conjunction

Frequency fluctuations of the Galileo S-band radio signal were recorded nearly continuously during the spacecraft’s solar conjunction from December 1996 to February 1997. A strong propagating disturbance, most probably associated with a coronal mass ejection (CME), was detected on 7 February when the radio ray path proximate point was on the west solar limb at about 54 solar radii from the Sun. The CME passage through the line of sight is characterized by a significant increase in the fluctuation intensity of the recorded frequency and by an increase in the plasma speed from about 234 km s⁻¹ up to about 755 km s⁻¹. These velocity estimates are obtained from a correlation analysis of frequency fluctuations recorded simultaneously at two widely-separated ground stations. The density turbulence power spectrum is found to steepen behind the CME front. The Galileo radio-sounding data are compared with SOHO/LASCO observations of the CME in the corona and with WIND spacecraft data near the Earth’s orbit.     

Interplanetary shocks and geomagnetic activity during solar maximum (2000) and solar minimum (1995–1996)

Plasma and magnetic field parameter variations through fast forward interplanetary shocks were correlated with the peak geomagnetic activity index Dst in a period from 0 to 3 days after the shock, during solar maximum (2000) and solar minimum (1995–1996). Solar wind speed (V) and total magnetic field (B_t) were the parameters with higher correlations with peak Dst index. The correlation coefficients were higher during solar minimum (r² = 56% for V and 39% for B_t) than during solar maximum (r² = 15% for V and 12% for B_t). A statistical distribution of geomagnetic activity levels following interplanetary shocks was obtained. It was observed that during solar maximum, 36% and 28% of interplanetary shocks were followed by intense (Dst  −100 nT) and moderate (−50  Dst < −100 nT) geomagnetic activity, whereas during solar minimum 13% and 33% of the shocks were followed by intense and moderate geomagnetic activity. It can be concluded that the upstream/downstream variations of V and B_t through the shocks were the parameters better correlated with geomagnetic activity level, and during solar maximum a higher relative number of interplanetary shocks can be followed by intense geomagnetic activity than during solar minimum. One can extrapolate, for forecasting goals, that during a whole solar cycle a shock has a probability of around 50% to be followed by intense/moderate geomagnetic activity.     

Dynamic processes in the outer heliosphere: voyager observations and models

We review recent Voyager 2 observations in the vicinity of 70 AU. The character of the solar wind plasma data between 2002 and 2003 changed to a regime in which the speed, density and magnetic field magnitude are positively correlated. The average speed of the solar wind at Voyager 2 increased between early 2003 and mid-2004, which we attribute to a return of fast coronal hole flow. We use solar wind data at Earth as input to numerical models which include the effect of pickup ions to model the radial evolution of the solar wind. The model reproduces the basic features of the observations. As a specific example, we investigate the propagation of the Halloween (Oct.–Nov.), 2003 storms in the outer heliosphere. The model predictions are in reasonable agreement with Voyager 2 observations.     

Periodic Variations of Drag Coefficient for the ANDE Spherical Satellites During its Lifetime

A drag coefficient (C_D) inversion method is introduced to study the variations of the drag coefficient for orbital satellites with spherical geometry. Drag coefficients of the four micro satellites in the Atmospheric Neutral Density Experiment (ANDE) are compiled out with this new method. The Lomb-Scargle Periodgram (LSP) analysis of the four ANDE satellites' C_D series has shown that there are obvious 5, 7, 9, and 27 days' period in those data. Interesting results are found through comparing the LSP analysis with series of the daily solar radio flux at 10.7 cm (F_10.7 index), the Ap index, and the daily averaged solar wind speed at 1AU. All series in the same time interval have an obvious period of about 27 days, which has already been explained as the association with the 27 days' solar rotation. The oscillating periods less than 27 days are found in series of C_D, Ap and solar wind speed at 1AU, e.g., the 5, 7, 9 days period. However, these short periods disappeared in the time series of F_10.7 index. The same periodicities of 5, 7, 9 days in Ap and solar wind are presented at the same time interval during the declining phase of solar cycle 23. While in the ascending phase of solar cycle 24, these short oscillations are not so obvious as that in the declining phase of solar cycle 23. These results provide definite evidence that the C_D variations with period of 5, 7 and 9 days are produced by a combination of space weather effects caused by the solar wind and geomagnetic activity.      

Statistical properties of the IMF clock angle in the solar wind with northward and southward interplanetary magnetic field based on ACE observation from 1998 to 2009: Dependence on the temporal scale of the solar wind

Utilizing ACE satellite observations from 1998 to 2009, we performed the elaborate study on the properties of the clock angle θ_CA (arctan(By/Bz) (?90° to 90°) of the interplanetary magnetic field (IMF) in the solar wind at 1?AU. The solar wind with northward IMF (NW-IMF) and southward IMF (SW-IMF) are analyzed, independently. Statistical analysis shows that the solar wind with SW-IMF and NW-IMF has similar properties in general, including their durations, the IMF Bz and By components, and the IMF θ_CA. Then, the solar wind with NW-IMF (SW-IMF) is classified into five different temporal scales according to the duration of the NW-IMF (SW-IMF), i.e., very-short wind of 10–30?min, short-scale wind of 0.5–1?h, moderate-scale wind of 1–3?h, long-scale wind of 3–5?h, and super-long wind >5?h. Our analysis reveals that the IMF θ_CA has a distinct decrease with increase of the temporal scale of the solar wind. Next, the solar wind is classified into two groups, i.e., the high-speed solar wind (>450?km/s) and the low-speed solar wind (<450?km/s). Our analysis indicates that the IMF θ_CA depends highly on the solar wind speed. Statistically, high-speed solar wind tends to have larger IMF θ_CA than low-speed solar wind. The evolutions of the solar wind and IMF with the solar activity are further studied, revealing no clear solar variation of the IMF θ_CA. Finally, we analyze the monthly variation of the IMF θ_CA. Superposed epoch result strongly suggests the seasonal variation of the IMF θ_CA.     

Statistical characteristics of low latitude Pc4s: Influence of solar wind and IMF parameters

Pc4 signatures for the year 2013, extracted from geomagnetic north–south and east–west components of induction coil magnetometer (LEMI 30) from low latitude station Desalpar (DSP), operated by Institute of Seismological Research (ISR), India have been investigated vis-à-vis the prevalent interplanetary parameters (IMF) as well as the geomagnetic activity indices. A clear dominance of Pc4-5 (467 events) over Pc3 (17 events) is observed. Local time variation of Pc4 shows a peak in the noon sector in both X and Y components. Our investigations show that the dominant peak frequency is 10 mHz at low latitude region. Correlations with solar wind and IMF parameters illustrate highest occurrence of Pc4 for a solar wind speed of 300–400 km/s and average IMF B field of 3–6 nT. The amplitude of Pc4s at DSP shows an increase with increasing solar wind speed, plasma density, solar wind dynamic pressure and average B field which is also reflected in the trend of frequency variation of these pulsations. We report that IMF clock angle at low latitude does not have influence on Pc4 occurrence. Based on the characteristics of these events, detected in latitudinally distributed stations from low and mid-latitudes from northern and southern hemisphere, we infer that modes were compressional, which could be driven by K-H instability or solar wind dynamic pressure, as compressional modes can propagate to low latitude with little attenuation.     

太阳风质子束流密度的径向变化

利用Helios2飞船的数据,对太阳风速度分布中质子束流部分与整个质子的密度之比随日心距离的变化做了分析.为了排除碰撞因素的影响,有针对性地分析了太阳风高速流(600相似文献   

Solar wind structure in the outer heliosphere

A solar wind parcel evolves as it moves outward, interacting with the solar wind plasma ahead of and behind it and with the interstellar neutrals. This structure varies over a solar cycle as the latitudinal speed profile and current sheet tilt change. We model the evolution of the solar wind with distance, using inner heliosphere data to predict plasma parameters at Voyager. The shocks which pass Voyager 2 often have different structure than expected; changes in the plasma and/or magnetic field do not always occur simultaneously. We use the recent latitudinal alignment of Ulysses and Voyager 2 to determine the solar wind slowdown due to interstellar neutrals at 80 AU and estimate the interstellar neutral density. We use Voyager data to predict the termination shock motion and location as a function of time.     

Classification of solar wind structures and intense geomagnetic storm alarms with self-organizing maps

Intense geomagnetic storms (D_st < −100 nT) usually occur when a large interplanetary duskward-electric field (with Ey > 5 mV m⁻¹) lasts for more than 3 h. In this article, a self-organizing map (SOM) neural network is used to recognize different patterns in the temporal variation of hourly averaged Ey data and to predict intense storms. The input parameters of SOM are the hourly averaged Ey data over 3 h. The output layer of the SOM has a total of 400 neurons. The hourly Ey data are calculated from solar wind data, which are provided by NSSDC OMNIWeb and ACE spacecraft and contain information on 143 intense storms and a fair number of moderate storms, weak storms and quiet periods between September 3, 1966 and June 30, 2002. Our results show that SOM is able to classify solar wind structures and therefore to give timely intense storm alarms. In our SOM, 21 neurons out of 400 are identified to be closely associated with the intense storms and they successfully predict 134 intense storms out of the 143 ones selected. In particular, there are 14 neurons for which, if one or more of them are present, the occurrence probability of intense storms is about 90%. In addition, several of these 14 neurons can predict big magnetic storm (D_st  −180 nT). In summary, our method achieves high accuracy in predicting intense geomagnetic storms and could be applied in space environment prediction.     

Correlations between neutral and ionized solar wind

We report results of a statistical study correlating ionized solar wind (ISW) fluxes observed by ACE during late 2000 and throughout 2001 with neutral solar wind (NSW) fluxes observed by IMAGE/LENA over the same period. The average correlation coefficient between the neutral and ionized solar wind is 0.66 with correlations greater than 0.80 occurring about 29% of the time. Correlations appear to be driven by high solar wind flux variability, similar to results obtained by in situ multi-spacecraft correlation studies. In this study, however, IMAGE remains inside the magnetosphere on over 95% of its orbits. As a function of day of year, or equivalently ecliptic longitude, the slope of the relationship between the neutral solar wind flux and the ionized solar wind flux shows an enhancement near the upstream direction, but the symmetry point appears shifted toward higher ecliptic longitudes than the interstellar neutral (ISN) flow direction by about 20°. The estimated peak interstellar neutral upstream density inside of 1 AU is about 7 × 10⁻³ cm⁻³.     

二维磁流体动力学平衡日冕(Ⅱ)

基于解析和数值相结合的方法,进一步讨论了非均匀引力场中日冕的二维磁流体动力学平衡。对临界点进行了比较仔细的处理。得到了包含闭场区、中性片和开场区的大尺度日冕磁场位形,闭场区和中性片构成冕流结构。在高纬和低纬地区几个太阳半径之外,等离子体径向流动速度超过了局地声速和局地Alfvén速度。在1AU处,太阳风速度可达到400kms_-1以上