arXiv subjects

N. Raouafi

Publications and source records attributed to N. Raouafi.

11 recordsLinked to original sources

arXiv2023

Properties of an interplanetary shock observed at 0.07 and 0.7 Astronomical Units by Parker Solar Probe and Solar Orbiter

The Parker Solar Probe (PSP) and Solar Orbiter (SolO) missions opened a new observational window in the inner heliosphere, which is finally accessible to direct measurements. On September 05, 2022, a coronal mass ejection (CME)-driven interplanetary (IP) shock has been observed as close as 0.07 au by PSP. The CME then reached SolO, which was well radially-aligned at 0.7 au, thus providing us with the opportunity to study the shock properties at so different heliocentric distances. We characterize the shock, investigate its typical parameters and compare its small-scale features at both locations. Using the PSP observations, we investigate how magnetic switchbacks and ion cyclotron waves are processed upon shock crossing. We find that switchbacks preserve their V--B correlation while compressed upon the shock passage, and that the signature of ion cyclotron waves disappears downstream of the shock. By contrast, the SolO observations reveal a very structured shock transition, with a population of shock-accelerated protons of up to about 2 MeV, showing irregularities in the shock downstream, which we correlate with solar wind structures propagating across the shock. At SolO, we also report the presence of low-energy ($\sim$ 100 eV) electrons scattering due to upstream shocklets. This study elucidates how the local features of IP shocks and their environments can be very different as they propagate through the heliosphere.

astro-ph.SR↗

arXiv2023

Constraints on the Alfv\'enicity of Switchbacks

Switchbacks (SBs) are localized structures in the solar wind containing deflections of the magnetic field direction relative to the background solar wind magnetic field. The amplitudes of the magnetic field deflection angles for different SBs vary from ~40 to ~160-170 degrees. Alignment of the perturbations of the magnetic field and the bulk solar wind velocity is observed inside SBs and causes spiky enhancements of the radial bulk velocity inside SBs. We have investigated the deviations of SB perturbations from Alfv\'enicity by evaluating the distribution of the parameter defined as the ratio of the parallel to $\Delta\vec{B}$ component of $\Delta\vec{V}$ to $\Delta\vec{V}_A = \Delta\vec{B}/4\pi n_i m_i$ inside SBs, i.e. $\alpha=V_{||}/|\Delta\vec{V}_A|$ ( $\alpha=\Delta\vec{V}/|\Delta\vec{V}_A|$ when $\Delta\vec{V}~\Delta\vec{B}$), which quantifies the deviation of the perturbation from an Alfv\'enic one. Based on Parker Solar Probe (PSP) observations, we show that $\alpha$ inside SBs has systematically lower values than it has in the pristine solar wind: $\alpha$ inside SBs observed during PSP Encounter 1 were distributed in a range of 0.2-0.9. The upper limit on $\alpha$ is constrained by the requirement that the jump in velocity across the switchback boundary be less than the local Alfv\'en speed. This prevents the onset of shear flow instabilities. The consequence is that the perturbation of the proton bulk velocity in SBs with deflection greater than 60 degrees cannot reach $\alpha=1$ (the Alfv\'enicity condition) and the highest possible $\alpha$ for a SB with the full reversal of B is 0.5. These results have consequences for the interpretation of switchbacks as large amplitude Alfv\'en waves.

astro-ph.SR↗

arXiv2023

Focused Space Weather Strategy for Securing Earth, and Human Exploration of the Moon and Mars

This white paper recognizes gaps in observations that will, when addressed, much improve solar radiation hazard and geomagnetic storm forecasting. Radiation forecasting depends on observations of the entire "Solar Radiation Hemisphere" that we will define. Mars exploration needs strategic placement of radiation-relevant observations. We also suggest an orbital solution that will improve geomagnetic storm forecasting through improved in situ and solar/heliospheric remote sensing.

astro-ph.IM↗

arXiv2020

Enhanced proton parallel temperature inside patches of switchbacks in the inner heliosphere

Switchbacks are discrete angular deflections in the solar wind magnetic field that have been observed throughout the heliosphere. Recent observations by Parker Solar Probe (PSP) have revealed the presence of patches of switchbacks on the scale of hours to days, separated by 'quieter' radial fields. 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. We fitted 3D bi-Maxwellian functions to the core of proton velocity distributions measured by the SPAN-Ai instrument onboard PSP to obtain the proton parallel, $T_{p,\|}$, and perpendicular, $T_{p,\perp}$, temperature. 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 $T_{p,\|}$, while $T_{p,\perp}$ remains relatively constant. Patches sometimes exhibit small proton and electron density enhancements. We interpret that patches are not simply a group of switchbacks, but rather switchbacks are embedded within a larger-scale structure identified by enhanced $T_{p,\|}$ that is distinct from the surrounding solar wind. We suggest that these observations are consistent with formation by reconnection-associated mechanisms in the corona.

astro-ph.SR↗

arXiv2020

Solar physics in the 2020s: DKIST, parker solar probe, and solar orbiter as a multi-messenger constellation

The National Science Foundation (NSF) Daniel K. Inouye Solar Telescope (DKIST) is about to start operations at the summit of Haleakala (Hawaii). DKIST will join the early science phases of the NASA and ESA Parker Solar Probe and Solar Orbiter encounter missions. By combining in-situ measurements of the near-sun plasma environment and detail remote observations of multiple layers of the Sun, the three observatories form an unprecedented multi-messenger constellation to study the magnetic connectivity inside the solar system. This white paper outlines the synergistic science that this multi-messenger suite enables.

astro-ph.SR↗

arXiv2020

Localized magnetic field structures and their boundaries in the near-Sun solar wind from Parker Solar Probe measurements

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.

physics.space-ph↗

arXiv2019

Observations of Energetic-Particle Population Enhancements along Intermittent Structures near the Sun from Parker Solar Probe

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.

physics.space-ph↗

arXiv2019

Enhanced Energy Transfer Rate in Solar Wind Turbulence Observed near the Sun from Parker Solar Probe

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 Kármán 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.

physics.space-ph↗

arXiv2019

Measures of Scale Dependent Alfvénicity in the First PSP Solar Encounter

The solar wind shows periods of highly Alfvénic activity, where velocity fluctuations and magnetic fluctuations are aligned or anti-aligned with each other. It is generally agreed that solar wind plasma velocity and magnetic field fluctuations observed by Parker Solar Probe (PSP) during the first encounter are mostly highly Alfvénic. However, quantitative measures of Alfvénicity are needed to understand how the characterization of these fluctuations compares with standard measures from prior missions in the inner and outer heliosphere, in fast wind and slow wind, and at high and low latitudes. To investigate this issue, we employ several measures to quantify the extent of Alfvénicity -- the Alfvén ratio $r_A$, {normalized} cross helicity $σ_c$, {normalized} residual energy $σ_r$, and the cosine of angle between velocity and magnetic fluctuations $\cosθ_{vb}$. We show that despite the overall impression that the Alfvénicity is large in the solar wind sampled by PSP during the first encounter, during some intervals the cross helicity starts decreasing at very large scales. These length-scales (often $> 1000 d_i$) are well inside inertial range, and therefore, the suppression of cross helicity at these scales cannot be attributed to kinetic physics. This drop at large scales could potentially be explained by large-scale shears present in the inner heliosphere sampled by PSP. In some cases, despite the cross helicity being constant down to the noise floor, the residual energy decreases with scale in the inertial range. These results suggest that it is important to consider all these measures to quantify Alfvénicity.

physics.space-ph↗

arXiv2019

Observations of heating along intermittent structures in the inner heliosphere from PSP data

The solar wind proton temperature at 1-au has been found to be correlated with small-scale intermittent magnetic structures, i.e., regions with enhanced temperature are associated with coherent structures such as current sheets. Using Parker Solar Probe data from the first encounter, we study this association using measurements of radial proton temperature, employing the Partial Variance of Increments (PVI) technique to identify intermittent magnetic structures. We observe that the probability density functions of high-PVI events have higher median temperatures than those with lower PVI, The regions in space where PVI peaks were also locations that had enhanced temperatures when compared with similar regions suggesting a heating mechanism in the young solar wind that is associated with intermittency developed by a nonlinear turbulent cascade.n the immediate vicinity.

physics.space-ph↗

arXiv2019

Clustering of Intermittent Magnetic and Flow Structures near Parker Solar Probe's First Perihelion -- A Partial-Variance-of-Increments Analysis

During the Parker Solar Probe's (PSP) first perihelion pass, the spacecraft reached within a heliocentric distance of $\sim 37~R_\odot$ and observed numerous magnetic and flow structures characterized by sharp gradients. To better understand these intermittent structures in the young solar wind, an important property to examine is their degree of correlation in time and space. To this end, we use the well-tested Partial Variance of Increments (PVI) technique to identify intermittent events in FIELDS and SWEAP observations of magnetic and proton-velocity fields (respectively) during PSP's first solar encounter, when the spacecraft was within 0.25 au from the Sun. We then examine distributions of waiting times between events with varying separation and PVI thresholds. We find power-law distributions for waiting times shorter than a characteristic scale comparable to the correlation time, suggesting a high degree of correlation that may originate in a clustering process. Waiting times longer than this characteristic time are better described by an exponential, suggesting a random memory-less Poisson process at play. These findings are consistent with near-Earth observations of solar wind turbulence. The present study complements the one by Dudok de Wit et al. (2020, present volume), which focuses on waiting times between observed "switchbacks" in the radial magnetic field.

physics.space-ph↗