51 research outputs found
Local proton heating at magnetic discontinuities in Alfvenic and non-Alfvenic solar wind
We investigate the local proton energization at magnetic
discontinuities/intermittent structures and the corresponding kinetic
signatures in velocity phase space in Alfv\'enic and non-Alfv\'enic wind
streams observed by Parker Solar Probe. By means of the Partial Variance of
Increments method, we find that the hottest proton populations are localized
around compressible, kinetic-scale magnetic structures in both types of wind.
Furthermore, the Alfv\'enic wind shows preferential enhancements of
as smaller scale structures are considered, whereas the
non-Alfvenic wind shows preferential enhancements. Although proton
beams are present in both types of wind, the proton velocity distribution
function displays distinct features. Hot beams, i.e., beams with beam-to-core
perpendicular temperature up to three times larger than the total distribution
anisotropy, are found in the non-Alfv\'enic wind, whereas colder beams in the
Alfv\'enic wind. Our data analysis is complemented by 2.5D hybrid simulations
in different geometrical setups, which support the idea that proton beams in
Alfv\'enic and non-Alfv\'enic wind have different kinetic properties and
origins. The development of a perpendicular nonlinear cascade, favored in
balanced turbulence, allows a preferential relative enhancement of the
perpendicular plasma temperature and the formation of hot beams. Cold
field-aligned beams are instead favored by Alfv\'en wave steepening.
Non-Maxwellian distribution functions are found near discontinuities and
intermittent structures, pointing to the fact that the nonlinear formation of
small-scale structures is intrinsically related to the development of highly
non-thermal features in collisionless plasmas
Quantifying the Energy Budget in the Solar Wind from 13.3-100 Solar Radii
A variety of energy sources, ranging from dynamic processes like magnetic
reconnection and waves to quasi-steady terms like the plasma pressure, may
contribute to the acceleration of the solar wind. We utilize a combination of
charged particle and magnetic field observations from the Parker Solar Probe
(PSP) to attempt to quantify the steady-state contribution of the proton
pressure, the electric potential, and the wave energy to the solar wind proton
acceleration observed by PSP between 13.3 and ~100 solar radii (RS). The proton
pressure provides a natural kinematic driver of the outflow. The ambipolar
electric potential acts to couple the electron pressure to the protons,
providing another definite proton acceleration term. Fluctuations and waves,
while inherently dynamic, can act as an additional effective steady-state
pressure term. To analyze the contributions of these terms, we utilize radial
binning of single-point PSP measurements, as well as repeated crossings of the
same stream at different distances on individual PSP orbits (i.e. "fast radial
scans"). In agreement with previous work, we find that the electric potential
contains sufficient energy to fully explain the acceleration of the slower wind
streams. On the other hand, we find that the wave pressure plays an
increasingly important role in the faster wind streams. The combination of
these terms can explain the continuing acceleration of both slow and fast wind
streams beyond 13.3 RS
Enhanced proton parallel temperature inside patches of switchbacks in the inner heliosphere
Context. Switchbacks are discrete angular deflections in the solar wind magnetic field that have been observed throughout the helio-sphere. Recent observations by Parker Solar Probe(PSP) have revealed the presence of patches of switchbacks on the scale of hours to days, separated by âquieterâ radial fields. Aims. We aim to further diagnose the origin of these patches using measurements of proton temperature anisotropy that can illuminate possible links to formation processes in the solar corona. Methods. We fit 3D bi-Maxwellian functions to the core of proton velocity distributions measured by the SPAN-Ai instrument onboard PSP to obtain the proton parallel, Tp,â, and perpendicular, Tp,â„, temperature. Results. We show that the presence of patches is highlighted by a transverse deflection in the flow and magnetic field away from the radial direction. These deflections are correlated with enhancements in Tp,â, while Tp,â„remains relatively constant. Patches sometimes exhibit small proton and electron density enhancements. Conclusions. We interpret that patches are not simply a group of switchbacks, but rather switchbacks are embedded within a larger-scale structure identified by enhanced Tp,âthat is distinct from the surrounding solar wind. We suggest that these observations are consistent with formation by reconnection-associated mechanisms in the corona
Parker Solar Probe observations of proton beams simultaneous with ion-scale waves
Parker Solar Probe (PSP), NASA's latest and closest mission to the Sun, is on
a journey to investigate fundamental enigmas of the inner heliosphere. This
paper reports initial observations made by the Solar Probe Analyzer for Ions
(SPAN-I), one of the instruments in the Solar Wind Electrons Alphas and Protons
(SWEAP) instrument suite. We address the presence of secondary proton beams in
concert with ion-scale waves observed by FIELDS, the electromagnetic fields
instrument suite. We show two events from PSP's 2nd orbit that demonstrate
signatures consistent with wave-particle interactions. We showcase 3D velocity
distribution functions (VDFs) measured by SPAN-I during times of strong wave
power at ion-scales. From an initial instability analysis, we infer that the
VDFs departed far enough away from local thermodynamic equilibrium (LTE) to
provide sufficient free energy to locally generate waves. These events
exemplify the types of instabilities that may be present and, as such, may
guide future data analysis characterizing and distinguishing between different
wave-particle interactions.Comment: 24 pages, 9 figures, 2 table
The Temperature, Electron, and Pressure Characteristics of Switchbacks: Parker Solar Probe Observations
Parker Solar Probe (PSP) observes unexpectedly prevalent switchbacks, which
are rapid magnetic field reversals that last from seconds to hours, in the
inner heliosphere, posing new challenges to understanding their nature, origin,
and evolution. In this work, we investigate the thermal states, electron pitch
angle distributions, and pressure signatures of both inside and outside
switchbacks, separating a switchback into spike, transition region (TR), and
quiet period (QP). Based on our analysis, we find that the proton temperature
anisotropies in TRs seem to show an intermediate state between spike and QP
plasmas. The proton temperatures are more enhanced in spike than in TR and QP,
but the alpha temperatures and alpha-to-proton temperature ratios show the
opposite trends, implying that the preferential heating mechanisms of protons
and alphas are competing in different regions of switchbacks. Moreover, our
results suggest that the electron integrated intensities are almost the same
across the switchbacks but the electron pitch angle distributions are more
isotropic inside than outside switchbacks, implying switchbacks are intact
structures but strong scattering of electrons happens inside switchbacks. In
addition, the examination of pressures reveals that the total pressures are
comparable through a switchback, confirming switchbacks are pressure-balanced
structures. These characteristics could further our understanding of ion
heating, electron scattering, and the structure of switchbacks.Comment: submitted to Ap
The Structure and Origin of Switchbacks: Parker Solar Probe Observations
Switchbacks are rapid magnetic field reversals that last from seconds to
hours. Current Parker Solar Probe (PSP) observations pose many open questions
in regards to the nature of switchbacks. For example, are they stable as they
propagate through the inner heliosphere, and how are they formed? In this work,
we aim to investigate the structure and origin of switchbacks. In order to
study the stability of switchbacks, we suppose the small scale current sheets
therein may work to braid and stabilize the switchbacks. Thus, we use the
partial variance of increments method to identify the small scale current
sheets, and then compare their distributions in switchbacks. With more than one
thousand switchbacks identified with PSP observations in seven encounters, we
find many more current sheets inside than outside switchbacks, indicating that
these micro-structures should work to stabilize the S-shape structures of
switchbacks. Additionally, with the helium measurements, we study the
variations of helium abundance ratios and alpha-proton differential speeds to
trace switchbacks to their origins. We find both helium-rich and helium-poor
populations in switchbacks, implying the switchbacks could originate from both
closed and open magnetic field regions in the Sun. Moreover, we observe that
the alpha-proton differential speeds also show complex variations as compared
to the local Alfv\'en speed. The joint distributions of both parameters show
that low helium abundance together with low differential speed is the dominant
state in switchbacks. The presence of small scale current sheets in switchbacks
along with the helium features are in line with the hypothesis that switchbacks
could originate from the Sun via interchange reconnection process. However,
other formation mechanisms are not excluded
Parker Solar Probe Observations of High Plasma Beta Solar Wind from Streamer Belt
In general, slow solar wind from the streamer belt forms a high plasma beta
equatorial plasma sheet around the heliospheric current sheet (HCS) crossing,
namely the heliospheric plasma sheet (HPS). Current Parker Solar Probe (PSP)
observations show that the HCS crossings near the Sun could be full or partial
current sheet crossing (PCS), and they share some common features but also have
different properties. In this work, using the PSP observations from encounters
4 to 10, we identify streamer belt solar wind from enhancements in plasma beta,
and we further use electron pitch angle distributions to separate it into HPS
solar wind that around the full HCS crossings and PCS solar wind that in the
vicinity of PCS crossings. Based on our analysis, we find that the PCS solar
wind has different characteristics as compared with HPS solar wind: a) PCS
solar wind could be non-pressure-balanced structures rather than magnetic
holes, and the total pressure enhancement mainly results from the less reduced
magnetic pressure; b) some of the PCS solar wind are mirror unstable; c) PCS
solar wind is dominated by very low helium abundance but varied alpha-proton
differential speed. We suggest the PCS solar wind could originate from coronal
loops deep inside the streamer belt, and it is pristine solar wind that still
actively interacts with ambient solar wind, thus it is valuable for further
investigations on the heating and acceleration of slow solar wind
Parker solar probe: four years of discoveries at solar cycle minimum
Launched on 12 Aug. 2018, NASAâs Parker Solar Probe had completed 13 of its scheduled 24 orbits around the Sun by Nov. 2022. The missionâs primary science goal is to determine the structure and dynamics of the Sunâs coronal magnetic field, understand how the solar corona and wind are heated and accelerated, and determine what processes accelerate energetic particles. Parker Solar Probe returned a treasure trove of science data that far exceeded quality, significance, and quantity expectations, leading to a significant number of discoveries reported in nearly 700 peer-reviewed publications. The first four years of the 7-year primary mission duration have been mostly during solar minimum conditions with few major solar events. Starting with orbit 8 (i.e., 28 Apr. 2021), Parker flew through the magnetically dominated corona, i.e., sub-AlfvĂ©nic solar wind, which is one of the missionâs primary objectives. In this paper, we present an overview of the scientific advances made mainly during the first four years of the Parker Solar Probe mission, which go well beyond the three science objectives that are: (1) Trace the flow of energy that heats and accelerates the solar corona and solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles
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