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Register a new userEnhanced Energy Transfer Rate in Solar Wind Turbulence Observed near the Sun from Parker Solar Probe
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Added by
Riddhi Bandyopadhyay
Publication date
2019
fields
Physics
and research's language is
English
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Direct evidence of an inertial-range turbulent energy cascade has been provided by spacecraft observations in heliospheric plasmas. In the solar wind, the average value of the derived heating rate near 1 au is $sim 10^{3}, mathrm{J,kg^{-1},s^{-1}}$, an amount sufficient to account for observed departures from adiabatic expansion. Parker Solar Probe (PSP), even during its first solar encounter, offers the first opportunity to compute, in a similar fashion, a fluid-scale energy decay rate, much closer to the solar corona than any prior in-situ observations. Using the Politano-Pouquet third-order law and the von Karman decay law, we estimate the fluid-range energy transfer rate in the inner heliosphere, at heliocentric distance $R$ ranging from $54,R_{odot}$ (0.25 au) to $36,R_{odot}$ (0.17 au). The energy transfer rate obtained near the first perihelion is about 100 times higher than the average value at 1 au. This dramatic increase in the heating rate is unprecedented in previous solar wind observations, including those from Helios, and the values are close to those obtained in the shocked plasma inside the terrestrial magnetosheath.
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One of the discoveries made by Parker Solar Probe during first encounters with the Sun is the ubiquitous presence of relatively small-scale structures standing out as sudden deflections of the magnetic field. They were called switchbacks as some of them show up the full reversal of the radial component of the magnetic field and then return to regular conditions. Analyzing the magnetic field and plasma perturbations associated with switchbacks we identify three types of structures with slightly different characteristics: 1. Alfvenic structures, where the variations of the magnetic field components take place while the magnitude of the field remains constant; 2. Compressional, the field magnitude varies together with changes of the components; 3. Structures manifesting full reversal of the magnetic field (extremal class of Alfvenic structures). Processing of structures boundaries and plasma bulk velocity perturbations lead to the conclusion that they represent localized magnetic field tubes with enhanced parallel plasma velocity and ion beta moving together with respect to surrounding plasma. The magnetic field deflections before and after the switchbacks reveal the existence of total axial current. The electric currents are concentrated on the relatively narrow boundary layers on the surface of the tubes and determine the magnetic field perturbations inside the tube. These currents are closed on the structure surface, and typically have comparable azimuthal and the axial components. The surface of the structure may also accommodate an electromagnetic wave, that assists to particles in carrying currents. We suggest that the two types of structures we analyzed here may represent the local manifestations of the tube deformations corresponding to a saturated stage of the Firehose instability development.
We present a statistical analysis for the characteristics and radial evolution of linear magnetic holes (LMHs) in the solar wind from 0.166 to 0.82 AU using Parker Solar Probe observations of the first two orbits. It is found that the LMHs mainly have a duration less than 25 s and the depth is in the range from 0.25 to 0.7. The durations slightly increase and the depths become slightly deeper with the increasing heliocentric distance. Both the plasma temperature and the density for about 50% of all events inside the holes are higher than the ones surrounding the holes. The average occurrence rate is 8.7 events/day, much higher than that of the previous observations. The occurrence rate of the LMHs has no clear variation with the heliocentric distance (only a slight decreasing trend with the increasing heliocentric distance), and has several enhancements around ~0.525 AU and ~0.775 AU, implying that there may be new locally generated LMHs. All events are segmented into three parts (i.e., 0.27, 0.49 and 0.71 AU) to investigate the geometry evolution of the linear magnetic holes. The results show that the geometry of LMHs are prolonged both across and along the magnetic field direction from the Sun to the Earth, while the scales across the field extend a little faster than along the field. The present study could help us to understand the evolution and formation mechanism of the LMHs in the solar wind.
Observations at 1 au have confirmed that enhancements in measured energetic particle fluxes are statistically associated with rough magnetic fields, i.e., fields having atypically large spatial derivatives or increments, as measured by the Partial Variance of Increments (PVI) method. One way to interpret this observation is as an association of the energetic particles with trapping or channeling within magnetic flux tubes, possibly near their boundaries. However, it remains unclear whether this association is a transport or local effect; i.e., the particles might have been energized at a distant location, perhaps by shocks or reconnection, or they might experience local energization or re-acceleration. The Parker Solar Probe (PSP), even in its first two orbits, offers a unique opportunity to study this statistical correlation closer to the corona. As a first step, we analyze the separate correlation properties of the energetic particles measured by the isois instruments during the first solar encounter. The distribution of time intervals between a specific type of event, i.e., the waiting time, can indicate the nature of the underlying process. We find that the isois observations show a power-law distribution of waiting times, indicating a correlated (non-Poisson) distribution. Analysis of low-energy isois data suggests that the results are consistent with the 1 au studies, although we find hints of some unexpected behavior. A more complete understanding of these statistical distributions will provide valuable insights into the origin and propagation of solar energetic particles, a picture that should become clear with future PSP orbits.
The slow solar wind is typically characterized as having low Alfvenicity. However, Parker Solar Probe (PSP) observed predominately Alfvenic slow solar wind during several of its initial encounters. From its first encounter observations, about 55.3% of the slow solar wind inside 0.25 au is highly Alfvenic ($|sigma_C| > 0.7$) at current solar minimum, which is much higher than the fraction of quiet-Sun-associated highly Alfvenic slow wind observed at solar maximum at 1 au. Intervals of slow solar wind with different Alfvenicities seem to show similar plasma characteristics and temperature anisotropy distributions. Some low Alfvenicity slow wind intervals even show high temperature anisotropies, because the slow wind may experience perpendicular heating as fast wind does when close to the Sun. This signature is confirmed by Wind spacecraft measurements as we track PSP observations to 1 au. Further, with nearly 15 years of Wind measurements, we find that the distributions of plasma characteristics, temperature anisotropy and helium abundance ratio ($N_alpha/N_p$) are similar in slow winds with different Alfvenicities, but the distributions are different from those in the fast solar wind. Highly Alfvenic slow solar wind contains both helium-rich ($N_alpha/N_psim0.045$) and helium-poor ($N_alpha/N_psim0.015$) populations, implying it may originate from multiple source regions. These results suggest that highly Alfvenic slow solar wind shares similar temperature anisotropy and helium abundance properties with regular slow solar winds, and they thus should have multiple origins.
We use emph{Parker Solar Probe} (emph{PSP}) in-situ measurements to analyze the characteristics of solar wind turbulence during the first solar encounter covering radial distances between $35.7R_odot$ and $41.7R_odot$. In our analysis we isolate so-called switchback (SB) intervals (folded magnetic field lines) from non-switchback (NSB) intervals, which mainly follow the Parker spiral field. Using a technique based on conditioned correlation functions, we estimate the power spectra of Elsasser, magnetic and bulk velocity fields separately in the SB and NSB intervals. In comparing the turbulent energy spectra of the two types of intervals, we find the following characteristics: 1) The decorrelation length of the backward-propagating Elsasser field $z^-$ is larger in the NSB intervals than the one in the SB intervals; 2) the magnetic power spectrum in SB intervals is steeper, with spectral index close to -5/3, than in NSB intervals, which have a spectral index close to -3/2; 3) both SB and NSB turbulence are imbalanced with NSB having the largest cross-helicity, 4) the residual energy is larger in the SB intervals than in NSB, and 5) the analyzed fluctuations are dominated by Alfvenic fluctuations that are propagating in the emph{sunward} (emph{anti-sunward}) direction for the SB (NSB) turbulence. These observed features provide further evidence that the switchbacks observed by emph{PSP} are associated with folded magnetic field lines giving insight into their turbulence nature.
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