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Register a new userIon Scale Electromagnetic Waves in the Inner Heliosphere
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Understanding the physical processes in the solar wind and corona which actively contribute to heating, acceleration, and dissipation is a primary objective of NASAs Parker Solar Probe (PSP) mission. Observations of coherent electromagnetic waves at ion scales suggests that linear cyclotron resonance and non-linear processes are dynamically relevant in the inner heliosphere. A wavelet-based statistical study of coherent waves in the first perihelion encounter of PSP demonstrates the presence of transverse electromagnetic waves at ion resonant scales which are observed in 30-50% of radial field intervals. Average wave amplitudes of approximately 4 nT are measured, while the mean duration of wave events is of order 20 seconds; however long duration wave events can exist without interruption on hour-long timescales. Though ion scale waves are preferentially observed during intervals with a radial mean magnetic field, we show that measurement constraints, associated with single spacecraft sampling of quasi-parallel waves superposed with anisotropic turbulence, render the measured quasi-parallel ion-wave spectrum unobservable when the mean magnetic field is oblique to the solar wind flow; these results imply that the occurrence of coherent ion-scale waves is not limited to a radial field configuration. The lack of strong radial scaling of characteristic wave amplitudes and duration suggests that the waves are generated {em{in-situ}} through plasma instabilities. Additionally, observations of proton distribution functions indicate that temperature anisotropy may drive the observed ion-scale waves.
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We perform a statistical study of the turbulent power spectrum at inertial and kinetic scales observed during the first perihelion encounter of Parker Solar Probe. We find that often there is an extremely steep scaling range of the power spectrum just above the ion-kinetic scales, similar to prior observations at 1 AU, with a power-law index of around $-4$. Based on our measurements, we demonstrate that either a significant ($>50%$) fraction of the total turbulent energy flux is dissipated in this range of scales, or the characteristic nonlinear interaction time of the turbulence decreases dramatically from the expectation based solely on the dispersive nature of nonlinearly interacting kinetic Alfven waves.
We analyze the evolution of the interplanetary magnetic field spatial structure by examining the inner heliospheric autocorrelation function, using Helios 1 and Helios 2 in situ observations. We focus on the evolution of the integral length scale (lambda) anisotropy associated with the turbulent magnetic fluctuations, with respect to the aging of fluid parcels traveling away from the Sun, and according to whether the measured lambda is principally parallel (lambda_parallel) or perpendicular (lambda_perp) to the direction of a suitably defined local ensemble average magnetic field B0. We analyze a set of 1065 24-hour long intervals (covering full missions). For each interval, we compute the magnetic autocorrelation function, using classical single-spacecraft techniques, and estimate lambda with help of two different proxies for both Helios datasets. We find that close to the Sun, lambda_parallel < lambda_perp. This supports a slab-like spectral model, where the population of fluctuations having wavevector k parallel to B0 is much larger than the one with k-vector perpendicular. A population favoring perpendicular k-vectors would be considered quasi-two dimensional (2D). Moving towards 1 AU, we find a progressive isotropization of lambda and a trend to reach an inverted abundance, consistent with the well-known result at 1 AU that lambda_parallel > lambda_perp, usually interpreted as a dominant quasi-2D picture over the slab picture. Thus, our results are consistent with driving modes having wavevectors parallel to B0 near Sun, and a progressive dynamical spectral transfer of energy to modes with perpendicular wavevectors as the solar wind parcels age while moving from the Sun to 1 AU.
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.
(Abridged) Aim: We attempt to determine robust estimates of the heliospheric magnetic flux ($Phi_H$) using Parker Solar Probe (PSP) data, analyze how susceptible this is to overestimation compared to the true open flux ($Phi_{open}$), assess its dependence on time and space, and compare it to simple estimates from Potential Field Source Surface (PFSS) models. Methods: We compare different methods of computation using data from PSP, STEREO A and Wind. The effects of fluctuations and large scale structure on the estimate are probed by using measured radial trends to produce synthetic data. Best estimates are computed as a function of time and space, and compared to estimates from PFSS models. Results: Radially-varying fluctuations of the HMF vector and variation of the Parker spiral angle cause the standard metrics of the mean and mode to evolve with radius independent of the central value about which the vector fluctuates. This is best mitigated by projecting the vector into the background Parker spiral direction. Nevertheless, we find a small enhancement in flux close to 1AU. The fraction of locally inverted field lines grows with radial distance from the Sun which remains a possible physical reason for this excess, but is negligible at PSP`s perihelia. Similarly, the impact of fluctuations in general is much reduced at PSP`s perihelia. The overall best estimate is ~2.5 nT AU2 . No strong dependence on latitude or longitude is apparent. The PFSS models predict lower values from 1.2 to 1.8 nT AU2. Conclusions: The heliospheric flux is robustly estimated relative to a mean Parker spiral direction at PSP`s perihelia where the decay of fluctuations and weakening importance of local flux
We present a statistical analysis for the characteristics and spatial evolution of the interplanetary discontinuities (IDs) in the solar wind, from 0.13 to 0.9 au, by using the Parker Solar Probe measurements on Orbits 4 and 5. 3948 IDs have been collected, including 2511 rotational discontinuities (RDs) and 557 tangential discontinuities (TDs), with the remnant unidentified. The statistical results show that (1) the ID occurrence rate decreases from 200 events/day at 0.13 au to 1 events/day at 0.9 au, following a spatial scaling r-2.00, (2) the RD to TD ratio decreases quickly with the heliocentric distance, from 8 at r<0.3 au to 1 at r>0.4 au, (3) the magnetic field tends to rotate across the IDs, 45{deg} for TDs and 30{deg} for RDs in the pristine solar wind within 0.3 au, (4) a special subgroup of RDs exist within 0.3 au, characterized by small field rotation angles and parallel or antiparallel propagations to the background magnetic fields, (5) the TD thicknesses normalized by local ion inertial lengths (di) show no clear spatial scaling and generally range from 5 to 35 di, and the normalized RD thicknesses follow r-1.09 spatial scaling, (6) the outward (anti-sunward) propagating RDs predominate in all RDs, with the propagation speeds in the plasma rest frame proportional to r-1.03. This work could improve our understandings for the ID characteristics and evolutions and shed light on the study of the turbulent environment in the pristine solar wind.
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