The Solar Wind Electrons Alphas and Protons experiment on the Parker Solar Probe (PSP) mission measures the three-dimensional electron velocity distribution function. We derive the parameters of the ...core, halo, and strahl populations utilizing a combination of fitting to model distributions and numerical integration for ∼100,000 electron distributions measured near the Sun on the first two PSP orbits, which reached heliocentric distances as small as ∼0.17 au. As expected, the electron core density and temperature increase with decreasing heliocentric distance, while the ratio of electron thermal pressure to magnetic pressure (βe) decreases. These quantities have radial scaling consistent with previous observations farther from the Sun, with superposed variations associated with different solar wind streams. The density in the strahl also increases; however, the density of the halo plateaus and even decreases at perihelion, leading to a large strahl/halo ratio near the Sun. As at greater heliocentric distances, the core has a sunward drift relative to the proton frame, which balances the current carried by the strahl, satisfying the zero-current condition necessary to maintain quasi-neutrality. Many characteristics of the electron distributions near perihelion have trends with solar wind flow speed, βe, and/or collisional age. Near the Sun, some trends not clearly seen at 1 au become apparent, including anticorrelations between wind speed and both electron temperature and heat flux. These trends help us understand the mechanisms that shape the solar wind electron distributions at an early stage of their evolution.
The Axial Double Probe (ADP) instrument measures the DC to ∼100 kHz electric field along the spin axis of the Magnetospheric Multiscale (MMS) spacecraft (Burch et al., Space Sci. Rev.,
2014, this ...issue
), completing the vector electric field when combined with the spin plane double probes (SDP) (Torbert et al., Space Sci. Rev.,
2014, this issue
, Lindqvist et al., Space Sci. Rev.,
2014, this issue
). Two cylindrical sensors are separated by over 30 m tip-to-tip, the longest baseline on an axial DC electric field ever attempted in space. The ADP on each of the spacecraft consists of two identical, 12.67 m graphite coilable booms with second, smaller 2.25 m booms mounted on their ends. A significant effort was carried out to assure that the potential field of the MMS spacecraft acts equally on the two sensors and that photo- and secondary electron currents do not vary over the spacecraft spin. The ADP on MMS is expected to measure DC electric field with a precision of ∼1 mV/m, a resolution of ∼25 μV/m, and a range of ∼±1 V/m in most of the plasma environments MMS will encounter. The Digital Signal Processing (DSP) units on the MMS spacecraft are designed to perform analog conditioning, analog-to-digital (A/D) conversion, and digital processing on the ADP, SDP, and search coil magnetometer (SCM) (Le Contel et al., Space Sci. Rev.,
2014, this issue
) signals. The DSP units include digital filters, spectral processing, a high-speed burst memory, a solitary structure detector, and data compression. The DSP uses precision analog processing with, in most cases, >100 dB in dynamic range, better that −80 dB common mode rejection in electric field (
E
) signal processing, and better that −80 dB cross talk between the
E
and SCM (
B
) signals. The A/D conversion is at 16 bits with ∼1/4 LSB accuracy and ∼1 LSB noise. The digital signal processing is powerful and highly flexible allowing for maximum scientific return under a limited telemetry volume. The ADP and DSP are described in this article.
At solar minimum, the solar wind is observed at high solar latitudes as a predominantly fast (> 500 km/s), highly Alfvenic, rarefied stream of plasma originating deep within coronal holes, while near ...the ecliptic plane it is interspersed with a more variable slow (< 500 kms) wind. The precise origins of the slow wind streams are less certain, with theories and observations supporting sources from the tips of helmet streamers, interchange reconnection near coronal hole boundaries, and origins within coronal holes with highly diverging magnetic fields. The heating mechanism required to drive the solar wind is also an open question and candidate mechanisms include Alfven wave turbulence, heating by reconnection in nanoflares, ion cyclotron wave heating and acceleration by thermal gradients1. At 1 au, the wind is mixed and evolved and much of the diagnostic structure of these sources and processes has been lost. Here we present new measurements from Parker Solar Probe at 36 to 54 solar radii that show clear evidence of slow, Alfvenic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals associated with jets of plasma and enhanced Poynting flux and interspersed in a smoother and less turbulent flow with near-radial magnetic field. Furthermore, plasma wave measurements suggest electron and ion velocity-space micro-instabilities that have been identified with plasma heating and thermalization processes. Our measurements suggest an impulsive mechanism associated with solar wind energization and a heating role for micro-instabilities and provide strong evidence for low latitude coronal holes as a significant contribution to the source of the slow solar wind.
We present the first statistical analysis with continuous data coverage and nonaveraged amplitudes of the prevalence and distribution of high‐amplitude (>5 mV/m) whistler mode waves in the outer ...radiation belt using 5 years of Van Allen Probes data. These waves are most common above L = 3.5 and between magnetic local time of 0–7 where they are present 1–4% of the time. During high geomagnetic activity, high‐amplitude whistler mode wave occurrence rises above 30% in some regions. During these active times the plasmasphere erodes to lower L and high‐amplitude waves are observed at all L outside of it, with the highest occurrence at low L (3.5–4) in the predawn sector. These results have important implications for modeling radiation belt particle interactions with chorus, as large‐amplitude waves interact nonlinearly with electrons. Results also may provide clues regarding the mechanisms which result in growth to large amplitudes.
Plain Language Summary
Our Earth is surrounded by a ring of high‐energy electrons, known as the outer radiation belt, which can cause damage to satellites in orbit. These electrons gain such high energy because of a type of electromagnetic waves called “whistler waves,” which exist in the space around Earth. Satellites have recently discovered whistler waves that are tens or hundreds of times as large as the average waves. Such large whistler waves can energize electrons very quickly and also cause electrons to be knocked into our atmosphere, creating aurora. Because these large waves are hard to measure, scientists have not been able to say how often they occur, where they occur, or even how exactly they form. This study uses a unique data set gathered by the Van Allen Probes to find out when and where these very large whistler waves occur. We found that these waves appear mostly in the nightside and morning side of the Earth, and they tend to appear much closer to Earth than smaller whistler waves do. This information offers us clues about how these monster waves form and what impact they might have on the radiation belt and the Earth.
Key Points
High‐amplitude whistler mode waves are most prevalent between midnight and dawn and above L of 3.5
During active times, high‐amplitude waves are present over 30% of the time in some regions
Waves greater than 20 mV/m preferentially occur at low L shell in the predawn sector
We report observations from the Magnetospheric Multiscale (MMS) satellites of a large guide field magnetic reconnection event. The observations suggest that two of the four MMS spacecraft sampled the ...electron diffusion region, whereas the other two spacecraft detected the exhaust jet from the event. The guide magnetic field amplitude is approximately 4 times that of the reconnecting field. The event is accompanied by a significant parallel electric field (E(sub parallel lines) that is larger than predicted by simulations. The high-speed (approximately 300 km/s) crossing of the electron diffusion region limited the data set to one complete electron distribution inside of the electron diffusion region, which shows significant parallel heating. The data suggest that E(sub parallel lines) is balanced by a combination of electron inertia and a parallel gradient of the gyrotropic electron pressure.
Context.
Parker Solar Probe (PSP) measures the magnetic field and plasma parameters of the solar wind at unprecedentedly close distances to the Sun. These data provide great opportunities to study ...the early-stage evolution of magnetohydrodynamic (MHD) turbulence in the solar wind.
Aims.
In this study, we make use of the PSP data to explore the nature of solar wind turbulence focusing on the Alfvénic character and power spectra of the fluctuations and their dependence on the distance and context (i.e., large-scale solar wind properties), aiming to understand the role that different effects such as source properties, solar wind expansion, and stream interaction might play in determining the turbulent state.
Methods.
We carried out a statistical survey of the data from the first five orbits of PSP with a focus on how the fluctuation properties at the large MHD scales vary with different solar wind streams and the distance from the Sun. A more in-depth analysis from several selected periods is also presented.
Results.
Our results show that as fluctuations are transported outward by the solar wind, the magnetic field spectrum steepens while the shape of the velocity spectrum remains unchanged. The steepening process is controlled by the “age” of the turbulence, which is determined by the wind speed together with the radial distance. Statistically, faster solar wind has higher “Alfvénicity,” with a more dominant outward propagating wave component and more balanced magnetic and kinetic energies. The outward wave dominance gradually weakens with radial distance, while the excess of magnetic energy is found to be stronger as we move closer toward the Sun. We show that the turbulence properties can significantly vary from stream to stream even if these streams are of a similar speed, indicating very different origins of these streams. Especially, the slow wind that originates near the polar coronal holes has much lower Alfvénicity compared with the slow wind that originates from the active regions and pseudostreamers. We show that structures such as heliospheric current sheets and velocity shears can play an important role in modifying the properties of the turbulence.
Context.
Parker Solar Probe’s first solar encounter has revealed the presence of sudden magnetic field deflections in the slow Alfvénic solar wind. These structures, which are often called ...switchbacks, are associated with proton velocity enhancements.
Aims.
We study their statistical properties with a special focus on their boundaries.
Methods.
Using data from SWEAP and FIELDS, we investigate particle and wavefield properties. The magnetic boundaries are analyzed with the minimum variance technique.
Results.
Switchbacks are found to be Alfvénic in 73% of cases and compressible in 27%. The correlations between magnetic field magnitude and density fluctuations reveal the existence of both positive and negative correlations, and the absence of perturbations in the magnetic field magnitude. Switchbacks do not lead to a magnetic shear in the ambient field. Their boundaries can be interpreted in terms of rotational or tangential discontinuities. The former are more frequent.
Conclusions.
Our findings provide constraints on the possible generation mechanisms of switchbacks, which have to be able to also account for structures that are not purely Alfvénic. One of the possible candidates, among others, manifesting the described characteristics is the firehose instability.
We discuss the solar wind electron temperatures Te as measured in the nascent solar wind by Parker Solar Probe during its first perihelion pass. The measurements have been obtained by fitting the ...high-frequency part of quasi-thermal noise spectra recorded by the Radio Frequency Spectrometer. In addition we compare these measurements with those obtained by the electrostatic analyzer discussed in Halekas et al. These first electron observations show an anticorrelation between Te and the wind bulk speed V: this anticorrelation is most likely the remnant of the well-known mapping observed at 1 au and beyond between the fast wind and its coronal hole sources, where electrons are observed to be cooler than in the quiet corona. We also revisit Helios electron temperature measurements and show, for the first time, that an in situ ( ) anticorrelation is well observed at 0.3 au but disappears as the wind expands, evolves, and mixes with different electron temperature gradients for different wind speeds.
Aims.
We survey the electron heat flux observed by the Parker Solar Probe (PSP) in the near-Sun environment at heliocentric distances of 0.125–0.25 AU.
Methods.
We utilized measurements from the ...Solar Wind Electrons Alphas and Protons and FIELDS experiments to compute the solar wind electron heat flux and its components and to place these in context.
Results.
The PSP observations reveal a number of trends in the electron heat flux signatures near the Sun. The magnitude of the heat flux is anticorrelated with solar wind speed, likely as a result of the lower saturation heat flux in the higher-speed wind. When divided by the saturation heat flux, the resulting normalized net heat flux is anticorrelated with plasma beta on all PSP orbits, which is consistent with the operation of collisionless heat flux regulation mechanisms. The net heat flux also decreases in very high beta regions in the vicinity of the heliospheric current sheet, but in most cases of this type the omnidirectional suprathermal electron flux remains at a comparable level or even increases, seemingly inconsistent with disconnection from the Sun. The measured heat flux values appear inconsistent with regulation primarily by collisional mechanisms near the Sun. Instead, the observed heat flux dependence on plasma beta and the distribution of suprathermal electron parameters are both consistent with theoretical instability thresholds associated with oblique whistler and magnetosonic modes.
The high temperatures and strong magnetic fields of the solar corona form streams of solar wind that expand through the Solar System into interstellar space. At 09:33 UT on 28 April 2021 Parker Solar ...Probe entered the magnetized atmosphere of the Sun 13 million km above the photosphere, crossing below the Alfvén critical surface for five hours into plasma in casual contact with the Sun with an Alfvén Mach number of 0.79 and magnetic pressure dominating both ion and electron pressure. The spectrum of turbulence below the Alfvén critical surface is reported. Magnetic mapping suggests the region was a steady flow emerging on rapidly expanding coronal magnetic field lines lying above a pseudostreamer. The sub-Alfvénic nature of the flow may be due to suppressed magnetic reconnection at the base of the pseudostreamer, as evidenced by unusually low densities in this region and the magnetic mapping.
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