The Heliospheric Plasma Sheet Observed in situ by Three Spacecraft over Four Solar Rotations (original) (raw)
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The review presents analysis and physical interpretation of available statistical data about solar wind plasma and interplanetary magnetic field (IMF) properties as measured in-situ at 1 A.U. by numerous space experiments during time period from 1964 to 2007. The experimental information have been collected in the OMNI Web/NSSDC data set of hourly averaged heliospheric parameters for last four solar cycles from 20 th to 23 rd . We studied statistical characteristics of such key heliospheric parameters as solar wind proton number density, temperature, bulk velocity, and IMF vector as well as dimensionless parameters. From harmonic analysis of the variations of key parameters we found basic periods of 13.5 days, 27 days, 1 year, and ~11 years, which correspond to rotation of the Sun, Earth and to the solar cycle. We also revealed other periodicities such as specific five-year plasma density and temperature variations, which origin is a subject of discussion. We have found that the distribution of solar wind proton density, temperature and IMF is very close to a log-normal function, while the solar wind velocity is characterized by a very broad statistical distribution. Detailed study of the variability of statistical distributions with solar activity was performed using a method of running histograms. In general, the distributions of heliospheric parameters are wider during maximum and declining phase of the solar cycle. More complicated behavior was revealed for the solar wind velocity and temperature, which distribution is characterized by two-or even tree-peak structure in dependence on the phase of solar cycle. Our findings support the concepts of solar wind sources in the open, closed and intermittent magnetic regions on the Sun. Citation: Dmitriev, A.V., A.V. Suvorova, I.S. Veselovsky, Statistical Characteristics of the Heliospheric Plasma and
Organization of solar wind plasma properties in a tilted, heliomagnetic coordinate system
Journal of Geophysical Research, 1981
We have used a superposed epoch analysis to examine the variation in solar wind properties observed in 1974 in a 'heliomagnetic' coordinate system tilted with respect to the solar equator. A tilt of 30 ø ___ 10 ø was found to produce the best 'organization' of these properties in such a coordinate system. The solar wind speed increased with heliomagnetic latitude, while the proton density and the proton flux density decreased. These variations are qualitatively consistent with those inferred from coronal hole and other interplanetary observations. 1. INTRODUCTION The three-dimensional structure of interplanetary space has been a topic of interest since Cortie [1912] suggested a strong latitude dependence in solar particle emission as the explanation for semiannual variations in geomagnetic activity. Searches for such a dependence in the solar wind speed have been carried out using in situ observations [Hundhausen et al., 1971; Bame et al., 1977], radio scintillations [Dennison and Hewish, 1967; Hewish and Symonds, 1969; Sime, 1976], and comet observations [Bertaux et al., 1973; Brantit et al., 1975]. The limited nature of these observations and inconsistencies among the findings of these studies have left some doubt as to the existence of such an effect. Recent studies of the role of c9ronal holes--vast regions of low density associated with rapidly diverging, open magnetic field lines--have also stimulated interest in the global structure of the solar corona and solar wind. Coronal holes have been identified as the sources of high-speed streams of solar wind and related to the interplanetary magnetic polarity (or sector) structure by numerous authors (see the references in the works by Zirker [1977] and Hundhausen [1979]). If coronal holes represent most of the magnetically open portion of the corona, their global pattern should be directly related to the spatial structure of both the interplanetary magnetic field and the flow of solar wind.
Astronomy & Astrophysics
Context. We investigated the plasma and magnetic field characteristics of the upstream regions of interplanetary coronal mass ejections (ICMEs) and their evolution as function of distance to the Sun in the inner heliosphere. Results are related both to the development of interplanetary shocks, sheath regions, and compressed solar wind plasma ahead of the magnetic ejecta (ME). Aims. From a sample of 45 ICMEs observed by Helios 1/2 and the Parker Solar Probe, we aim to identify four main density structures; namely shock, sheath, leading edge, and ME itself. We compared characteristic parameters (proton particle density, plasma-beta, temperature, magnetic field strength, proton bulk speed, and duration) to the upstream solar wind in order to investigate the interrelation between the different density structures. Methods. For the statistical investigation, we used plasma and magnetic field measurements from 40 well-observed Helios 1/2 events from 1974-1981. Helios data cover the distance range from 0.3-1 au. For comparison, we added a sample of five ICMEs observed with the Parker Solar Probe from 2019-2021 over the distance range of 0.32-0.75 au. Results. It is found that the sheath structure consists of compressed plasma as a consequence of the turbulent solar wind material following the shock and lies ahead of a region of compressed ambient solar wind. The region of compressed solar wind plasma is typically found directly in front of the magnetic driver and seems to match the bright leading edge commonly observed in remote sensing observations of CMEs. From the statistically derived density evolution over distance, we find the CME sheath becomes denser than the ambient solar wind at about 0.06 au. From 0.09-0.28 au, the sheath structure density starts to dominate over the density within the ME. The ME density seems to fall below the ambient solar wind density over 0.45-1.18 au. Besides the well-known expansion of the ME, the sheath size shows a weak positive correlation with distance, while the leading edge seems not to expand with distance from the Sun. We further find a moderate anti-correlation between sheath density and local solar wind plasma speed upstream of the ICME shock. An empirical relation is derived connecting the ambient solar wind speed with sheath and leading edge density. We provide constraints to these results in this paper. Conclusions. The average starting distance for actual sheath formation could be as close as 0.06 au. The early strong ME expansion quickly ceases with distance from the Sun and might lead to a dominance in the sheath density between 0.09 and 0.28 au. The leading edge can be understood as a separate structure of compressed ambient solar wind directly ahead of the ME and is likely the bright leading edge of CMEs often seen in coronagraph images. The results allow for better interpretation of ICME evolution and possibly the observed mass increase due to enlargement of the sheath material. The empirical relation between sheath and leading edge density and ambient solar wind speed can be used for more detailed modeling of ICME evolution in the inner heliosphere.
Advances in Space Research, 2000
The periods 1975-77, 1985-87 and 1995-97 were selected between pivot points of the solar cycles to analyze available spacecraft data for three successive solar minima. The mean values of the solar wind velocity, dynamic pressure, and temperature are decreasing during each indicated time interval. All variations are of several 0.1. The interplanetary magnetic field is minimal at solar minima. There is no strong difference between solar wind and interplanetary magnetic field parameters as well as solar activity indices for these solar cycles. Nevertheless, the proton cosmic ray intensities and spectra during these especially quiet periods possibly show the remarkable 22-year variation with the minimal proton fluxes in 1985-87. 0 2000 COSPAR. Published by Elsevier Science Ltd.
2022
Context: We investigate the evolution of the sheath and leading edge (LE) structure of interplanetary coronal mass ejections (ICMEs) as function of distance in the inner heliosphere. Results are related both to the magnetic ejecta (ME) and to the ambient solar wind (SW). Aims: From a sample of 40 well-observed Helios 1/2 events, we derive the average density separately for sheath, LE, and ME. The results are placed into comparison with the upstream SW to investigate at which distance the sheath is formed. Methods: We use plasma and magnetic field measurements from Helios 1/2 data in the distance range 0.3-1 au from the ICME list by Bothmer and Schwenn (1998). For comparison, we add a sample of four ICMEs observed with PSP in 2019-2021 covering 0.32-0.62 au. Results. At the distance of ~13 Rs, the CME sheath becomes denser than the ambient SW density. At ~38 Rs the sheath structure density starts to dominate over the density within the ME. The ME density falls below the ambient SW de...
Solar System Research, 2001
The average values of the parameters of the solar wind and the interplanetary magnetic field at the Earth's orbit are calculated by using the results of direct measurements performed in the current and three previous solar cycles. Individual and general features of each cycle are analyzed by the method of superposition of epochs and hysteresis curves. The similarity in trends of solar cycles 23 and 20 at their growth phase is revealed. This gives additional reason to expect that the current solar cycle as a whole will be weaker than the two previous cycles.
Additional acceleration of solar-wind particles in current sheets of the heliosphere
Annales Geophysicae, 2015
Particles of fast solar wind in the vicinity of the heliospheric current sheet (HCS) or in a front of interplanetary coronal mass ejections (ICMEs) often reveal very peculiar energy or velocity profiles, density distributions with double or triple peaks, and well-defined streams of electrons occurring around or far away from these events. In order to interpret the parameters of energetic particles (both ions and electrons) measured by the WIND spacecraft during the HCS crossings, a comparison of the data was carried out with 3-D particle-in-cell (PIC) simulations for the relevant magnetic topology (Zharkova and Khabarova, 2012). The simulations showed that all the observed particle-energy distributions, densities, ion peak velocities, electron pitch angles and directivities can be fitted with the same model if the heliospheric current sheet is in a status of continuous magnetic reconnection. In this paper we present further observations of the solar-wind particles being accelerated ...
Heliospheric plasma sheet and coronal streamers
Geophysical Research Letters, 1997
Helios 2 measurements of solar wind plasma and magnetic field are used to investigate the structure of the hellospheric plasma sheet between 0.3 and 1 AU. In agreement with previous observations at 1 AU, the plasma sheet thickness is much larger than that of the embedded current sheet. The plasma sheet appears surrounded by a density halo, a region of slightly raised density. High-time resolution data show that decreases in relative helium abundance coincide with the plasma sheet boundaries, reinforcing the notion that the solar wind within the plasma sheet is of a different nature (with different solar origins) than that outside it. Although radio occultation measurements of the corona were not available at the time of the Helios data, a synthesis of recent results on coronal streamers shows that there is a remarkable similarity between their major features and those of plasma sheets, demonstrating that the coronal counterpart of the plasma sheet is the stalk of the coronal streamer. These measurements also suggest that the density halo seen in the Helios data is associated with the radial extension of the boundaries of the streamer observed in the extended corona before the streamer narrows to a stalk.