Solar wind thermal electrons in the ecliptic plane between 1 and 4 AU - Preliminary results from the ULYSSES radio receiver (original) (raw)
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Solar Wind Electron Parameters from Ulysses /URAP Quasi-Thermal Noise Measurements at Solar Maximum
Solar Physics - SOL PHYS, 2004
We present the solar wind plasma parameters obtained from the Ulysses spacecraft during its second pole-to-pole fast latitude scan near the 2001 solar maximum. We study the solar wind properties from the electron density and core temperature measurements made by the radio receiver on Ulysses using the method of quasi-thermal noise spectroscopy. We analyze these parameters as functions of heliographic latitude and distance. We present their histograms normalized to 1 AU and find a bimodal distribution for the electron core temperature. The cooler population can be associated with the fast wind flow emanating from coronal holes present at various latitudes. We discuss a slight north/south asymmetry found for the electron density. Finally, we compare the present results to those obtained during the 1996 solar minimum and 1991 solar maximum.
Journal of Geophysical Research, 1998
We present new in situ solar wind plasma measurements obtained during Ulysses fast transit from the south solar pole to the north one, which took place 1 year before the 1996 sunspot minimum. The data were obtained with the radio receiver of the Unified Radio and Plasma Wave Experiment, using the method of quasi-thermal noise spectroscopy, which is relatively immune to spacecraft potential perturbations and whose density measurements are independent on gain calibrations. We analyze the electron density and the core electron temperature. We deduce their radial profiles in the steady state fast solar wind; southward of 40 ø latitude, between 1.52 and 2.31 AU, the total electron density varies as n• ocr (-9"øø3+ø'ø15), while the core temperature varies as Tc oc r(-ø'64+ø'ø3). This allows to estimate the interplanetary electrostatic field using a simplified fluid equation. We also study, poleward of 40 ø (where the variance of both parameters are very low), the histograms of the electron density and core temperature scaled to 1 AU, assuming the above determined radial variation. Each histogram shows a single class of flow with a roughly normal distribution. We find a mean electron density of 2.65 cm-3 in the southern hemisphere which is about 8% larger than in the northern one. The core temperature histogram is centered at a mean of 7.5 x 104 K in the south, and of 7 x 104 K in the north. This small asymmetry may be due to a genuine solar asymmetry between the two hemispheres and/or to a temporal variation since solar activity slightly decreased during the Ulysses exploration. 1. Introduction During its recent pole to pole transit, Ulysses explored heliographic latitudes from 80 ø S to 80 ø N and heliocentric distances from 1.3 to 2.3 AU, between September 1994 and August 1995, about 1 year before the 1996 solar activity minimum. This offers an opportunity to study the radial and latitudinal structure of the solar wind, with minimal variation in the phase of the solar activity cycle, during a period when the heliosphere is expected to be in its simplest state since the heliospheric current sheet is close to the equator. This paper presents and analyzes in situ solar wind measurements deduced from the observations of the plasma quasi-thermal noise (QTN) with the radio receiver on the Unified Radio and Plasma Wave (URAP) Experiment [Stone et al., 1992]. The QTN is due to the voltage induced on the electric antenna by the random motion of the ambient particles which excite plasma waves near the plasma frequency f•,. The theoretical interpretation of the QTN spectrum around f•, yields the density and temperature of the core and halo electron populations [Meyer-Vernet and Perche, 1989, and references therein]. As noted by Meyer-Vernet et al. [1997], one of the main advantages of thermal
Latitude dependence of Solar Wind plasma thermal noise: ULYSSES radio observations
Thermal noise spectroscopy was performed on the URAP radio receiver data for the first time out of the ecliptic during the Ulysses spacecraft transit from southern to northern heliolatitudes. We present the results on the solar wind electron density and bulk temperature obtained from −43 • S to +43 • N. They indicate a rough symmetry of the solar wind electron plasma about the solar equator. The latitudinal gradient of the electron bulk temperature was found to be approximately −830K/ • latitude southward and −1100K/ • northward. Within ±20 • latitude, large fluctuations in the 1-AU scaled electron density and temperature were observed and presumably linked to the corotating structures existing in the equatorial band. Effects of the solar wind speed on the thermal noise spectra were highest at high latitudes as predicted. They will be fully taken into account in future work.
Solar wind thermal electrons from 1.15 to 5.34 AU: Ulysses observations
Advances in Space Research, 1993
Using unique 3-d velocity space measurements by the Ulysses solar wind plasma experiment from 1.15 to 5.34 AU, we assess the radial gradient in thermal electron temperature. Until 3.8 AU, the gradient was steeper than previously reported but flatter than adiabatic; after 3.8 AU the gradient flattened. Trends in the observed electron distribution shapes qualitatively support predictions for regulation by Coulomb collisions and by expansion in a spiral IMF.
Solar wind electron density and temperature over solar cycle 23: Thermal noise measurements on Wind
Advances in Space Research, 2005
We present the solar wind plasma parameters obtained from the Wind spacecraft during more than nine years, encompassing almost the whole solar cycle 23. Since its launch in November 1994 Wind has frequently observed the in-ecliptic solar wind upstream of the EarthÕs bow shock. The WIND/WAVES thermal noise receiver was specially designed to measure the in situ plasma thermal noise spectra, from which the electron density and temperature can be accurately determined. We present and discuss histograms of such measurements performed from 1994 to 2003. Using these large data sets, we study the density and core temperature variations with solar activity cycle and with different regimes of the solar wind. We confirm the anticorrelation of the electron density with the sunspot number, and obtain a positive correlation of the core temperature, with the sunspot number.
Quasi-thermal noise in a drifting plasma: Theory and application to solar wind diagnostic on Ulysses
1999
The present paper provides the basic principles and analytic expressions of the quasi-thermal noise spectroscopy extended to measure the plasma bulk speed, as a tool for in situ space plasma diagnostics. This method is based on the analysis of the electrostatic field spectrum produced by the quasi-thermal fluctuations of the electrons and by the Dopplershifted thermal fluctuations of the ions; it requires a sensitive radio receiver connected to an electric wire dipole antenna. Neglecting the plasma bulk speed, the technique has been routinely used in the low-speed solar wind, and it gives accurate measurements of the electron density and core temperature, in addition to estimates of parameters of the hot electron component. The present generalization of the method takes into account the plasma speed and thereby improves the thermal electron temperature diagnostic. The technique, which is relatively immune to spacecraft potential and photoelectron perturbations, is complementary to standard electrostatic analysers. Application to the radio receiver data from the Ulysses spacecraft yields an accurate plasma diagnostic. Comparisons of these results with those deduced from the particle analyser experiment on board Ulysses are presented and discussed. QTN spectrum around the plasma frequency f•, consists of a noise peak just above f•, produced by electron thermal fluctuations. Since the plasma density r,e is proportional to f•, this allows an accurate measurement of the electron density. In addition, the electrons passing within a Debye length from the antenna induce voltage pulses on it, producing in the spectrum a plateau just below f•,, and above f•,, a noise level which decreases as the observing frequency increases. The analysis of these spectrum regions gives the electron core temperature Tc [Meyer-Vernet and Perche, 1989]. As pointed out by Meyer-Vernet et al. [1998], one of the main
Physics of Space: Growth Points and Problems, 2001
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.
Journal of Geophysical Research, 2009
1] We have performed a statistical study of a substantial amount of solar wind electron velocity distribution functions (eVDFs). In our data set, we combine measurements acquired onboard three spacecrafts (Helios, Cluster II, and Ulysses) in the low ecliptic latitudes covering the heliocentric distance from 0.3 up to 4 AU. In this study, we focus on the nonthermal properties of the measured eVDFs in both the slow and the fast solar wind regimes. The aim of the present study is (1) to provide, for the first time, an analytical model to fit separately all three components of the solar wind eVDFs (i.e., the core, the halo, and the strahl) and to study the fractional densities of the three electron components and also the non-Maxwellian character of the high-energy eVDF tails as a function of the radial distance from the sun. Basically, our study is incremental to the previous studies of the fast solar wind and primarily extends their conclusions on a large number of slow wind observations in the ecliptic plane. We confirm that the halo and the strahl relative densities vary in an opposite way. The relative number of strahl electrons is decreasing with radial distance, whereas the relative number of halo electrons is increasing. The fractional density of the core population remains roughly constant. These findings suggest that there are mechanisms in the solar wind that scatter the strahl electrons into the halo. Also, we find that the relative importance of the nonthermal electrons in the fast solar wind is slightly higher compared to the slow wind.