Electron temperature in the ambient solar wind: Typical properties and a lower bound at 1 AU

Abstract

Abstract. Our understanding of what controls the solar wind electron temperature is far from complete. Previous studies from the Vela and IMP spacecraft have suggested that twice the proton temperature or an assumed average of • 150,000 K are reasonable estimations of total electron temperature at 1 AU. Eighteen months of continuous ISEE 3 solar wind data are analyzed in this paper and are found to have a mean electron temperature of 141,000 4-38,000 K, in good agreement with past measurements. No correlation is found between electron temperature and other solar wind parameters, including proton temperature. However, a very distinct lower bound on the electron temperature is found; this bound increases with proton temperature and is observed by both ISEE 3 and Ulysses spacecraft. The bound is also found to vary with bulk solar speed of the solar wind and with distance from the Sun. Solar wind plasma observed following stream interactions are often associated with temperatures near this bound, and enhanced electromagnetic wave activity in the 18-100 Hz range is coincident with intervals where this apparent temperature coupling is observed, suggesting the possible presence of wave-particle interactions. Possible explanations for the existence of this electron temperature bound are explored, but no definitive answer has been found at this time.

The Statistical Properties of Solar Wind Temperature Parameters Near 1 au

“…The last long-duration (i.e., more than 1 year) statistical study comparing electrons with protons was published 20 years ago (i.e., Newbury et al 1998) and relied upon only 18 months of ∼5 minute averaged data (i.e., ∼160,000 measurements). This study uses nearly 10 years of data spanning from the end of solar cycle 22 through much of solar cycle 23 (i.e., >1,000,000 measurements) with time periods separated into four categories (see Section 2 for definitions): all times (Constraints 1 and 2), all times excluding interplanetary (IP) shocks (Constraints 1-3), only times within MOs (e.g., Nieves-Chinchilla et al 2016 (Constraints 1, 2, and 4), and all times excluding MOs (Constraints 1, 2, and 5).…”

“…Most previous studies were case studies or limited to one parameter without summarizing tables providing quantities for future reference, and were independent of comparison to other parameters. For instance, the study by Newbury et al (1998) is one of the only studies that directly compared the electron and proton temperatures for a long-duration (i.e., more than 1 yr), statistically significant data set (they used 18 months of ISEE 3 data). It is also one of the most often cited works 7 for the average values of the ratio (T e /T p ) tot in the solar wind.…”

Absence of the strahl during times of slow wind

“…Averages of the quantities in each plot are shown either next to the right or left y axis. PDFs of the speed and anisotropy data when the strahl is present are basically what is observed in the majority of pure solar wind events (see, e.g., Newbury et al, 1998). The spread in the temperature anisotropy is a consequence of the presence of the field-aligned strahl, which is often observed to have temperature ratios greater than 1 and as high as 3 (Viñas et al, 2010).…”

Effects of suprathermal electrons on the proton temperature anisotropy in space plasmas: Electromagnetic ion-cyclotron instability

“…3 and 4 we keep constant the electron anisotropy A e = 1.5 but vary the electron/proton temperature ratio Θ = 1, 2, 4. All these values are chosen according to the observations in the solar wind (Štverák et al 2008;Newbury et al 1998).…”