Solar Wind Magnetic Fluctuations and Electron Non-Thermal Temperature Anisotropy: Survey of Wind-Swe-Veis Observations

Abstract

The solar wind electron velocity distribution function (eVDF) exhibits a variety of non-thermal features that deviate from thermal equilibrium. These deviations from equilibrium provide a local source for electromagnetic fluctuation emissions, including the commonly observed electron whistler-cyclotron and firehose instabilities. We present a systematic analysis of Wind-SWE-VEIS observations of solar wind electron plasma and associated Wind-MFI observed magnetic fluctuations. For the first time using the full solar wind electron distribution and its moments, without separation of the various electron components, we show clear evidence that the temperature anisotropy threshold of the parallel electron cyclotron anisotropic instability bounds solar wind electrons during slow solar wind periods. We also demonstrate that during periods of slow solar wind, collisions—while infrequent—are the dominant mechanism by which solar wind electrons are constrained, leading to isotropization. During fast solar wind periods, magnetic fluctuations and solar wind anisotropies are enhanced above the parallel whistler anisotropic threshold boundary and collisional effects are significantly reduced. Preliminary calculations further show that the oblique electron whistler mirror anisotropic instability bounds both the slow and fast solar wind. Regardless of speed, the solar wind electron thermal anisotropy appears globally bounded by the parallel electron firehose instability for anisotropies . Our results indicate that collisions, while infrequent, play a necessary role in regulating the solar wind eVDFs. In striking contrast to solar wind ions, solar wind electron plasma, when considered globally as a single eVDF, is only marginally stable with respect to parallel propagating instabilities.

Particle-in-cell Simulations of Firehose Instability Driven by Bi-Kappa Electrons

“…Another relevant factor is the free energy of the solar wind expanding along the interplanetary magnetic field, which is expected to accumulate in a temperature anisotropy of protons (ions) > T T , where Pand ⊥ denote directions relative to the magnetic field (Chew et al 1956). For electrons the situation is slightly different, although their temperature anisotropy shows the same tendency, i.e., > T T , with increasing the distance from the Sun, see Figure 8 in Štverák et al (2008; the average behavior of the temperature anisotropy can be found in Adrian et al 2016). However, the anisotropy reported by the observations at large enough heliocentric distances, e.g., 1 au, is much below these expectation (Adrian et al 2016;Chen et al 2016).…”

Electron temperature anisotropy regulation by whistler instability

“…Several spacecraft observations in the Earth's magnetosheath [ Anderson et al ., ; Fuselier et al ., ; Phan et al ., ; Tan et al ., ; Gary et al ., ; Xu and Chen , ] and in the solar wind [ Marsch , ; Marsch et al ., , , ; Gary et al ., , ; Kasper et al , ; Hellinger et al ., ; Matteini et al ., , ; Bale et al ., ; Maruca et al ., ; Bourouaine et al ., ; Stverak et al ., ; Adrian et al ., ] show that the inverse correlations between temperature anisotropy and parallel beta exist pervasively. In addition, various numerical analysis and computer simulations for homogeneous space plasma have been conducted in order to confirm the above theoretical phenomenon over the last two decades [ Devine and Chapman , ; Gary et al ., , ; Gary and Nishimura , ; Yoon and Seough , ; Seough and Yoon , ].…”

Kinetic instabilities in the solar wind driven by temperature anisotropies

“…The magnetosheath temperature anisotropy instability problem was first extended to the solar wind proper and solar corona, by Gary et al (2001a, b). Many subsequent works followed including those by , Kasper et al (2003Kasper et al ( , 2008, Marsch et al (2004Marsch et al ( , 2006, , , Matteini et al (2007Matteini et al ( , 2012, Bale et al (2009), Bourouaine et al (2010, Maruca et al (2011Maruca et al ( , 2012, Osman et al (2011Osman et al ( , 2012Osman et al ( , 2013, Marsch (2012), , Adrian et al (2016), and others that the present review might have missed. These works further investigated the effects of temperature anisotropy instabilities in the solar wind by analyzing data obtained from an armada of spacecraft including Helios, Ulysses, WIND, ACE, Stereo, etc.…”