The Near-Sun Streamer Belt Solar Wind: Turbulence and Solar Wind Acceleration

The fourth orbit of Parker Solar Probe (PSP) reached heliocentric distances down to 27.9 Rs, allowing solar wind turbulence and acceleration mechanisms to be studied in situ closer to the Sun than previously possible. The turbulence properties were found to be significantly different in the inbound and outbound portions of PSP's fourth solar encounter, likely due to the proximity to the heliospheric current sheet (HCS) in the outbound period. Near the HCS, in the streamer belt wind, the turbulence was found to have lower amplitudes, higher magnetic compressibility, a steeper magnetic field spectrum (with spectral index close to -5/3 rather than -3/2), a lower Alfv\'enicity, and a "1/f" break at much lower frequencies. These are also features of slow wind at 1 au, suggesting the near-Sun streamer belt wind to be the prototypical slow solar wind. The transition in properties occurs at a predicted angular distance of ~4{\deg} from the HCS, suggesting ~8{\deg} as the full-width of the streamer belt wind at these distances. While the majority of the Alfv\'enic turbulence energy fluxes measured by PSP are consistent with those required for reflection-driven turbulence models of solar wind acceleration, the fluxes in the streamer belt are significantly lower than the model predictions, suggesting that additional mechanisms are necessary to explain the acceleration of the streamer belt solar wind

Statistical analysis of orientation, shape, and size of solar wind switchbacks

One of the main discoveries from the first two orbits of Parker Solar Probe (PSP) was the presence of magnetic switchbacks, whose deflections dominated the magnetic field measurements. Determining their shape and size could provide evidence of their origin, which is still unclear. Previous work with a single solar wind stream has indicated that these are long, thin structures although the direction of their major axis could not be determined. We investigate if this long, thin nature extends to other solar wind streams, while determining the direction along which the switchbacks within a stream were aligned. We try to understand how the size and orientation of the switchbacks, along with the flow velocity and spacecraft trajectory, combine to produce the observed structure durations for past and future orbits. We searched for the alignment direction that produced a combination of a spacecraft cutting direction and switchback duration that was most consistent with long, thin structures. The expected form of a long, thin structure was fitted to the results of the best alignment direction, which determined the width and aspect ratio of the switchbacks for that stream. The switchbacks had a mean width of $50,000 \, \rm{km}$, with an aspect ratio of the order of $10$. We find that switchbacks are not aligned along the background flow direction, but instead aligned along the local Parker spiral, perhaps suggesting that they propagate along the magnetic field. Since the observed switchback duration depends on how the spacecraft cuts through the structure, the duration alone cannot be used to determine the size or influence of an individual event. For future PSP orbits, a larger spacecraft transverse component combined with more radially aligned switchbacks will lead to long duration switchbacks becoming less common

Magnetic connectivity of the ecliptic plane within 0.5 AU : PFSS modeling of the first PSP encounter

We compare magnetic field measurements taken by the FIELDS instrument on Parker Solar Probe (PSP) during its first solar encounter to predictions obtained by Potential Field Source Surface (PFSS) modeling. Ballistic propagation is used to connect the spacecraft to the source surface. Despite the simplicity of the model, our results show striking agreement with PSPs first observations of the heliospheric magnetic field from 0.5 AU (107.5 Rs) down to 0.16 AU (35.7 Rs). Further, we show the robustness of the agreement is improved both by allowing the photospheric input to the model to vary in time, and by advecting the field from PSP down to the PFSS model domain using in situ PSP/SWEAP measurements of the solar wind speed instead of assuming it to be constant with longitude and latitude. We also explore the source surface height parameter (RSS) to the PFSS model finding that an extraordinarily low source surface height (1.3-1.5Rs) predicts observed small scale polarity inversions which are otherwise washed out with regular modeling parameters. Finally, we extract field line traces from these models. By overlaying these on EUV images we observe magnetic connectivity to various equatorial and mid-latitude coronal holes indicating plausible magnetic footpoints and offering context for future discussions of sources of the solar wind measured by PSP

Are old-old patients with major depression more likely to relapse than young-old patients during continuation treatment with escitalopram?

<p>Abstract</p> <p>Background</p> <p>Escitalopram has shown efficacy and tolerability in the prevention of relapse in elderly patients with major depressive disorder (MDD). This <it>post-hoc </it>analysis compared time to relapse for <it>young-old </it>patients (n = 197) to that for <it>old-old </it>patients (n = 108).</p> <p>Method</p> <p>Relapse prevention: after 12-weeks open-label treatment, remitters (MADRS ≤12) were randomised to double-blind treatment with escitalopram or placebo and followed over 24-weeks. Patients were outpatients with MDD from 46 European centers aged ≥75 years (<it>old-old</it>) or 65-74 years of age (<it>young-old</it>), treated with escitalopram 10-20mg/day. Efficacy was assessed using the Montgomery Åsberg Depression Rating Scale (MADRS).</p> <p>Results</p> <p>After open-label escitalopram treatment, a similar proportion of <it>young-old </it>patients (78%) and <it>old-old </it>patients (72%) achieved remission. In the analysis of time to relapse based on the Cox model (proportional hazards regression), with treatment and age group as covariates, the hazard ratio was 4.4 for placebo <it>versus </it>escitalopram (χ²-test, df = 1, χ²= 22.5, p < 0.001), whereas the effect of age was not significant, with a hazard ratio of 1.2 for <it>old-old </it>versus <it>young-old </it>(χ²-test, df = 1, χ²= 0.41, p = 0.520). Escitalopram was well tolerated in both age groups with adverse events reported by 53.1% of <it>young-old </it>patients and 58.3% of <it>old-old </it>patients. There was no significant difference in withdrawal rates due to AEs between age groups (χ²-test, χ²= 1.669, df = 1, p = 0.196).</p> <p>Conclusions</p> <p><it>Young-old </it>and <it>old-old </it>patients with MDD had comparable rates of remission after open-label escitalopram, and both age groups had much lower rates of relapse on escitalopram than on placebo.</p

Evaluation of statistical methods for normalization and differential expression in mRNA-Seq experiments

<p>Abstract</p> <p>Background</p> <p>High-throughput sequencing technologies, such as the Illumina Genome Analyzer, are powerful new tools for investigating a wide range of biological and medical questions. Statistical and computational methods are key for drawing meaningful and accurate conclusions from the massive and complex datasets generated by the sequencers. We provide a detailed evaluation of statistical methods for normalization and differential expression (DE) analysis of Illumina transcriptome sequencing (mRNA-Seq) data.</p> <p>Results</p> <p>We compare statistical methods for detecting genes that are significantly DE between two types of biological samples and find that there are substantial differences in how the test statistics handle low-count genes. We evaluate how DE results are affected by features of the sequencing platform, such as, varying gene lengths, base-calling calibration method (with and without phi X control lane), and flow-cell/library preparation effects. We investigate the impact of the read count normalization method on DE results and show that the standard approach of scaling by total lane counts (e.g., RPKM) can bias estimates of DE. We propose more general quantile-based normalization procedures and demonstrate an improvement in DE detection.</p> <p>Conclusions</p> <p>Our results have significant practical and methodological implications for the design and analysis of mRNA-Seq experiments. They highlight the importance of appropriate statistical methods for normalization and DE inference, to account for features of the sequencing platform that could impact the accuracy of results. They also reveal the need for further research in the development of statistical and computational methods for mRNA-Seq.</p

Switchbacks, microstreams, and broadband turbulence in the solar wind

Switchbacks are a striking phenomenon in near-Sun coronal hole flows, but their origins, evolution, and relation to the broadband fluctuations seen farther from the Sun are unclear. We use the near-radial lineup of Solar Orbiter and Parker Solar Probe during September 2020 when both spacecraft were in wind from the Sun's Southern polar coronal hole to investigate if switchback variability is related to large scale properties near 1 au. Using the measured solar wind speed, we map measurements from both spacecraft to the source surface and consider variations with source Carrington longitude. The patch modulation of switchback amplitudes at Parker at 20 solar radii was associated with speed variations similar to microstreams and corresponds to solar longitudinal scales of around 5°–10°. Near 1 au, this speed variation was absent, probably due to interactions between plasma at different speeds during their propagation. The alpha particle fraction, which has recently been shown to have spatial variability correlated with patches at 20 solar radii, varied on a similar scale at 1 au. The switchback modulation scale of 5°–10°, corresponding to a temporal scale of several hours at Orbiter, was present as a variation in the average deflection of the field from the Parker spiral. While limited to only one stream, these results suggest that in coronal hole flows, switchback patches are related to microstreams, perhaps associated with supergranular boundaries or plumes. Patches of switchbacks appear to evolve into large scale fluctuations, which might be one driver of the ubiquitous turbulent fluctuations in the solar wind

Whistler wave occurrence and the interaction with strahl electrons during the first encounter of Parker Solar Probe

Aims. We studied the properties and occurrence of narrowband whistler waves and their interaction with strahl electrons observed between 0.17 and 0.26 au during the first encounter of Parker Solar Probe. Methods. We used Digital Fields Board band-pass filtered (BPF) data from FIELDS to detect the signatures of whistler waves. Additionally parameters derived from the particle distribution functions measured by the Solar Wind Electrons Alphas and Protons (SWEAP) instrument suite were used to investigate the plasma properties, and FIELDS suite measurements were used to investigate the electromagnetic (EM) fields properties corresponding to the observed whistler signatures. Results. We observe that the occurrence of whistler waves is low, nearly ~1.5% and less than 0.5% in the analyzed peak and average BPF data, respectively. Whistlers occur highly intermittently and 80% of the whistlers appear continuously for less than 3 s. The spacecraft frequencies of the analyzed waves are less than 0.2 electron cyclotron frequency (fce). The occurrence rate of whistler waves was found to be anticorrelated with the solar wind bulk velocity. The study of the duration of the whistler intervals revealed an anticorrelation between the duration and the solar wind velocity, as well as between the duration and the normalized amplitude of magnetic field variations. The pitch-angle widths (PAWs) of the field-aligned electron population referred to as the strahl are broader by at least 12 degrees during the presence of large amplitude narrowband whistler waves. This observation points toward an EM wave electron interaction, resulting in pitch-angle scattering. PAWs of strahl electrons corresponding to the short duration whistlers are higher compared to the long duration whistlers, indicating short duration whistlers scatter the strahl electrons better than the long duration ones. Parallel cuts through the strahl electron velocity distribution function (VDF) observed during the whistler intervals appear to depart from the Maxwellian shape typically found in the near-Sun strahl VDFs. The relative decrease in the parallel electron temperature and the increase in PAW for the electrons in the strahl energy range suggests that the interaction with whistler waves results in a transfer of electron momentum from the parallel to the perpendicular direction

Ambipolar Electric Field and Potential in the Solar Wind Estimated from Electron Velocity Distribution Functions

The solar wind escapes from the solar corona and is accelerated, over a short distance, to its terminal velocity. The energy balance associated with this acceleration remains poorly understood. To quantify the global electrostatic contribution to the solar wind dynamics, we empirically estimate the ambipolar electric field (E∥) and potential (Φr,∞). We analyze electron velocity distribution functions (VDFs) measured in the near-Sun solar wind between 20.3 RS and 85.3 RS by the Parker Solar Probe. We test the predictions of two different solar wind models. Close to the Sun, the VDFs exhibit a suprathermal electron deficit in the sunward, magnetic-field-aligned part of phase space. We argue that the sunward deficit is a remnant of the electron cutoff predicted by collisionless exospheric models. This cutoff energy is directly linked to Φr,∞. Competing effects of E∥ and Coulomb collisions in the solar wind are addressed by the Steady Electron Runaway Model (SERM). In this model, electron phase space is separated into collisionally overdamped and underdamped regions. We assume that this boundary velocity at small pitch angles coincides with the strahl break-point energy, which allows us to calculate E∥. The obtained Φr,∞ and E∥ agree well with theoretical expectations. They decrease with radial distance as power-law functions with indices αΦ = −0.66 and αE = −1.69. We finally estimate the velocity gained by protons from electrostatic acceleration, which equals 77% calculated from the exospheric models, and 44% from the SERM model

Cross Helicity Reversals in Magnetic Switchbacks

International audienceWe consider 2D joint distributions of normalized residual energy, sigma(r)(s, t), and cross helicity, sigma(c)(s, t), during one day of Parker Solar Probe's (PSP's) first encounter as a function of wavelet scale s. The broad features of the distributions are similar to previous observations made by Helios in slow solar wind, namely well-correlated and fairly Alfvenic wind, except for a population with negative cross helicity that is seen at shorter wavelet scales. We show that this population is due to the presence of magnetic switchbacks, or brief periods where the magnetic field polarity reverses. Such switchbacks have been observed before, both in Helios data and in Ulysses data in the polar solar wind. Their abundance and short timescales as seen by PSP in its first encounter is a new observation, and their precise origin is still unknown. By analyzing these MHD invariants as a function of the wavelet scale, we show that magnetohydrodynamic (MHD) waves do indeed follow the local mean magnetic field through switchbacks, with a net Elsasser flux propagating inward during the field reversal and that they, therefore, must be local kinks in the magnetic field and not due to small regions of opposite polarity on the surface of the Sun. Such observations are important to keep in mind as computing cross helicity without taking into account the effect of switchbacks may result in spurious underestimation of sigma(c) as PSP gets closer to the Sun in later orbits

The Radial Dependence of Proton-scale Magnetic Spectral Break in Slow Solar Wind during PSP Encounter 2

International audienceMagnetic field fluctuations in the solar wind are commonly observed to follow a power-law spectrum. Near proton-kinetic scales, a spectral break occurs that is commonly interpreted as a transition to kinetic turbulence. However, this transition is not yet entirely understood. By studying the scaling of the break with various plasma properties, it may be possible to constrain the processes leading to the onset of kinetic turbulence. Using data from the Parker Solar Probe, we measure the proton-scale break over a range of heliocentric distances, enabling a measurement of the transition from inertial to kinetic-scale turbulence under various plasma conditions. We find that the break frequency f(b) increases as the heliocentric distance r decreases in the slow solar wind following a power law of f(b) similar to r(-1.11). We also compare this to the characteristic plasma ion scales to relate the break to the possible physical mechanisms occurring at this scale. The ratio f(b)/f(c) (f(c) for Doppler-shifted ion cyclotron resonance scale) is close to unity and almost independent of plasma beta(p). While f(b)/f(p) (f(p) for Doppler-shifted proton thermal gyroradius) increases with beta(p) approaching to unity at larger beta(p), f(b)/f(d) (f(d) for Doppler-shifted proton inertial length) decreases with beta(p) from unity at small beta(p). Due to the large comparable Alfven and solar wind speeds, we analyze these results using both the standard and modified Taylor hypotheses, demonstrating the robust statistical results