arXiv subjects

Benjamin L. Alterman

Publications and source records attributed to Benjamin L. Alterman.

7 recordsLinked to original sources

arXiv2026

How the Oblique Drift Instability Alters Solar Wind Heating and Constrains the Distribution of Solar Wind Observations

Ion-driven plasma instability thresholds, derived from linear theory, constrain the distribution of solar observations in parameter space, defining boundaries of stable plasma parameters. Excursions beyond these thresholds result in the emission of energy, transferred from particles to coherent electromagnetic waves, acting to adjust the system toward a more stable configuration. In this work, we use linear Vlasov--Maxwell theory to define parametric limits for a low-$β$ plasma that contains a drifting proton beam or helium ($α$-particle) population. A sufficiently fast and dense drifting population triggers an Oblique Drift Instability (ODI). This instability decreases the velocity drift between the thermal core proton and secondary populations and prevents the ratio of core thermal to magnetic pressure $β_c$ from decreasing below a minimum value by increasing the temperatures - i.e. heating - of both the core and drifting populations. Our theoretical results are of interest for Parker Solar Probe observations, as they provide an additional mechanism for perpendicular heating of ions active in the sub-\Alfvenic solar wind. The ODI may explain the discrepancy between long-standing expectations of measurements of very low-$β$ plasmas with very large ion temperature anisotropies in the near-Sun environment and in situ observations, where $β$ is consistently measured above a few percent and the secondary ion populations drift faster than the bulk of proton population by no more than approximately the local Alfven speed.

astro-ph.SR↗

arXiv2025

Observational Constraints on the Radial Evolution of O$^{6+}$ Temperature and Differential Flow in the Inner Heliosphere

Over decades of solar wind observations, heavy ions have been observed to have a higher temperature and flow faster than protons in the solar corona and heliosphere. Remote observations have largely been limited to the low corona ($< 4R_{\odot}$), while in situ observations for heavy ions ($Z>2$) have only been sampled at 1 au and beyond. As a result, theories that address heavy ion heating and acceleration remain largely unconstrained. With the launch of Solar Orbiter, heavy ion kinetics can be probed closer to the Sun, as close as the orbit of Mercury ($65R_{\odot}$), to examine their radial behavior. Through a statistical analysis of O$^{6+}$, this work provides a comprehensive analysis of the velocity and temperature of O$^{6+}$ from 0.3 au to 1 au. The study finds that the O$^{6+}$ relative drift, normalized to the local Alfvén speed, and its temperature compared to protons, both decrease with distance from the Sun and show some speed dependence. The O$^{6+}$ temperature is well fit by a single temperature adiabatic profile across all wind speeds, suggesting there is no significant heating at these heliocentric distances. This is in contrast to what is observed for protons and He$^{2+}$. Alfvénic fluctuations, with full 180$^{\circ}$ field rotation, create momentary negative differential streaming where the speed of O$^{6+}$ trails the protons. The amount of negative differential streaming gradually increases at larger distances. These results provide critical constraints to the proposed mechanisms seeking to describe ion heating and acceleration in the solar wind.

astro-ph.SR↗

arXiv2023

Defining the Middle Corona

The middle corona, the region roughly spanning heliocentric altitudes from $1.5$ to $6\,R_\odot$, encompasses almost all of the influential physical transitions and processes that govern the behavior of coronal outflow into the heliosphere. Eruptions that could disrupt the near-Earth environment propagate through it. Importantly, it modulates inflow from above that can drive dynamic changes at lower heights in the inner corona. Consequently, this region is essential for comprehensively connecting the corona to the heliosphere and for developing corresponding global models. Nonetheless, because it is challenging to observe, the middle corona has been poorly studied by major solar remote sensing missions and instruments, extending back to the Solar and Heliospheric Observatory (SoHO) era. Thanks to recent advances in instrumentation, observational processing techniques, and a realization of the importance of the region, interest in the middle corona has increased. Although the region cannot be intrinsically separated from other regions of the solar atmosphere, there has emerged a need to define the region in terms of its location and extension in the solar atmosphere, its composition, the physical transitions it covers, and the underlying physics believed to be encapsulated by the region. This paper aims to define the middle corona and give an overview of the processes that occur there.

astro-ph.SR↗

arXiv2021

Ion-Driven Instabilities in the Inner Heliosphere I: Statistical Trends

Instabilities described by linear theory characterize an important form of wave-particle interaction in the solar wind. We diagnose unstable behavior of solar wind plasma between 0.3 and 1 au via the Nyquist criterion, applying it to fits of $\sim1.5$M proton and $α$ particle Velocity Distribution Functions (VDFs) observed by \emph{Helios I} and \emph{II}. The variation of the fraction of unstable intervals with radial distance from the Sun is linear, signaling a gradual decline in the activity of unstable modes. When calculated as functions of the solar wind velocity and Coulomb number, we obtain more extreme, exponential trends in the regions where collisions appear to have a notable influence on the VDF. Instability growth rates demonstrate similar behavior, and significantly decrease with Coulomb number. We find that, for a non-negligible fraction of observations, the proton beam or secondary component might not be detected due to instrument resolution limitations, and demonstrate that the impact of this issue does not affect the main conclusions of this work.

astro-ph.SR↗

arXiv2020

Proton Core Behaviour Inside Magnetic Field Switchbacks

During Parker Solar Probe's first two orbits there are widespread observations of rapid magnetic field reversals known as switchbacks. These switchbacks are extensively found in the near-Sun solar wind, appear to occur in patches, and have possible links to various phenomena such as magnetic reconnection near the solar surface. As switchbacks are associated with faster plasma flows, we questioned whether they are hotter than the background plasma and whether the microphysics inside a switchback is different to its surroundings. We have studied the reduced distribution functions from the Solar Probe Cup instrument and considered time periods with markedly large angular deflections, to compare parallel temperatures inside and outside switchbacks. We have shown that the reduced distribution functions inside switchbacks are consistent with a rigid phase space rotation of the background plasma. As such, we conclude that the proton core parallel temperature is the same inside and outside of switchbacks, implying that a T-V relationship does not hold for the proton core parallel temperature inside magnetic field switchbacks. We further conclude that switchbacks are consistent with Alfvénic pulses travelling along open magnetic field lines. The origin of these pulses, however, remains unknown. We also found that there is no obvious link between radial Poynting flux and kinetic energy enhancements suggesting that the radial Poynting flux is not important for the dynamics of switchbacks.

physics.space-ph↗

arXiv2020

Proton Temperature Anisotropy Variations in Inner Heliosphere Estimated with First Parker Solar Probe Observations

We report proton temperature anisotropy variations in the inner heliosphere with Parker Solar Probe (PSP) observations. Using a linear fitting method, we derive proton temperature anisotropy with temperatures measured by the Solar Probe Cup (SPC) from the SWEAP instrument suite and magnetic field observations from the FIELDS instrument suite. The observed radial dependence of temperature variations in the fast solar wind implies stronger perpendicular heating and parallel cooling than previous results from Helios measurements made at larger radial distances. The anti-correlation between proton temperature anisotropy and parallel plasma beta is retained in fast solar wind. However, the temperature anisotropies of the slow solar wind seem to be well constrained by the mirror and parallel firehose instabilities. The perpendicular heating of the slow solar wind inside 0.24 AU may contribute to its same trend up against mirror instability thresholds as fast solar wind. These results suggest that we may see stronger anisotropy heating than expected in inner heliosphere.

physics.space-ph↗

arXiv2019

Parallel-propagating Fluctuations at Proton-kinetic Scales in the Solar Wind are Dominated by Kinetic Instabilities

We use magnetic helicity to characterise solar wind fluctuations at proton-kinetic scales from Wind observations. For the first time, we separate the contributions to helicity from fluctuations propagating at angles quasi-parallel and oblique to the local mean magnetic field, $\mathbf{B}_0$. We find that the helicity of quasi-parallel fluctuations is consistent with Alfvén-ion cyclotron and fast magnetosonic-whistler modes driven by proton temperature anisotropy instabilities and the presence of a relative drift between $α$-particles and protons. We also find that the helicity of oblique fluctuations has little dependence on proton temperature anisotropy and is consistent with fluctuations from the anisotropic turbulent cascade. Our results show that parallel-propagating fluctuations at proton-kinetic scales in the solar wind are dominated by proton temperature anisotropy instabilities and not the turbulent cascade. We also provide evidence that the behaviour of fluctuations at these scales is independent of the origin and macroscopic properties of the solar wind.

physics.space-ph↗