19Th Annual International Astrophysics Conference Oral Abstracts
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19TH ANNUAL INTERNATIONAL ASTROPHYSICS CONFERENCE ORAL ABSTRACTS Adhikari, Laxman Evolution of Entropy and Mediation of the Solar Wind by Turbulence Laxman Adhikari, CSPAR/UAH, USA Gary P. Zank, UAH/CSPAR, USA Lingling Zhao, CSPAR/UAH, USA Gary M. Webb, CSPAR/UAH, USA We study the evolution of solar wind entropy based on a conservative formulation of solar wind and turbulence transport model equations, and compare the model results to Voyager 2 measurements. For a polytropic index of γ = 5/3 (> 1), entropy increases with distance due to the dissipation of turbulence, being about 12.84% higher at 75 au than at 1 au. However, if the polytropic index satisfies γ < 1, entropy decreases. We show that not only the creation of pickup ions, but also stream‐shear leads to a decrease of the solar wind speed. We show that the sum of the solar wind flow energy (kinetic plus enthalpy) and turbulent (magnetic) energy is constant, indicating that kinetic solar wind energy is transferred into turbulent energy via stream‐shear and pickup ion isotropization, which then in turn heats the solar wind via the dissipation of turbulence. We compare the theoretical solutions of the solar wind entropy, the solar wind density, the thermal gas pressure, the solar wind proton temperature, and the fluctuating magnetic energy with those measured by Voyager 2. The results show that the theoretical results are in good agreement with the observed results. Asgari‐Targhi, An Observational Study of the Role of Flux Emergence, Flux Cancellation, and Non‐Potential Fields in the Heating of Active‐Region Loops Mahboubeh Mahboubeh Asgari‐Targhi, Harvard‐Smithsonian Center for Astrophysics, USA We study the high‐temperature (T>4 MK) emissions from mostly non‐flaring active regions using extreme ultraviolet images from the Atmospheric Imaging Assembly on the Solar Dynamic Observatory (SDO). We selected 48 active regions for detailed study. For each region, we measure the peak emission intensity in the Fe XVIII 94 A. This line is formed at a temperature of 7 MK, well above the peak of the differential emission measure distribution, so it represents high‐temperature emission produced by nanoflares. We also measure the total magnetic flux using the corresponding magnetograms from the Helioseismic and Magnetic Imager on SDO. We correlate the emission with other active‐region parameters such as the presence or absence of sunspots, non‐potential magnetic fields, and the emergence or cancellation of magnetic flux. We conclude that energy may be injected into the corona as a result of the dynamics of magnetic fields associated with sunspots and/ or emerging flux. Baker, Daniel Seven Years of Van Allen Probes Observations: Exploring the Sun‐Earth Connection D.N. Baker, University of Colorado Boulder, USA The Radiation Belt Storm Probes (RBSP) mission of NASA – later renamed the Van Allen Probes mission – came to its operational end in 2019 with the turning off of the RBSP‐A spacecraft in October 2019. From the time of launch in 2012, the more than seven years of observations from the RBSP sensors revealed a wealth of fascinating new features in the Earth’s radiation environs driven by solar and solar wind forcing events. In this presentation we examine the long run of energetic electron and proton data acquired by the dual RBSP sensor systems and we relate these results to the well‐observed features on the Sun that caused the near‐Earth responses. In this way, we demonstrate the immense progress that has resulted in our understanding of the “Sun‐Earth Connection” as a result of long‐ term synoptic observations. The measurements from the RBSP scientific sensors – a key part of NASA’s Living With a Star Program – reveal the immense benefits that would accrue from future continual monitoring with an operational version of the Van Allen Probes Mission. 19TH ANNUAL INTERNATIONAL ASTROPHYSICS CONFERENCE ORAL ABSTRACTS Bellan, Paul How Solar and Astrophysical Eruptions Generate Energetic Particles, X‐rays, and Waves Paul Bellan, Caltech, USA Models for solar and astrophysical eruptions are based on MHD and always start with a twisted magnetic field, i.e., an electric current flowing along a magnetic flux tube. Unfortunately, MHD cannot explain the most outstanding feature of eruptions, namely bursts of energetic particles, X‐rays, and waves. Nevertheless, MHD is obviously relevant, because MHD explains very well what happens before these bursts. Caltech lab experiments relevant to solar and astrophysical eruptions similarly demonstrate MHD dynamics followed by non‐MHD bursts of X‐rays and waves. Detailed examination of these experiments and consideration of the incompressible nature of ideal MHD instabilities identifies a clear path from MHD dynamics to the non‐MHD physics responsible for particle energization, X‐rays, and wave bursts. In this path, a multi‐step sequence of MHD instabilities leads to reduction of the cross‐section of a twisted flux tube; this choking results either from Rayleigh‐Taylor rippling or from kinking stretching the flux tube length. Choking the flux tube cross‐section greatly increases the axial electric current density so the electron drift velocity constituting the electric current becomes so large as to drive a kinetic instability which then greatly increases the local resistivity. The experiments and theoretical analysis both show that kinetic instability happens when the flux tube cross‐section is reduced to be of the order of the ion skin depth. The high local resistivity resulting from the kinetic instability acts as an opening switch that interrupts the electric current and thereby causes a large inductive voltage drop V= L dI/dt that accelerates charged particles to high energy. The ion skin depth is the scale at which MHD fails because Hall and electron inertia terms become important. Since the ion skin depth is of the order of 10’s of meters in the solar corona, it is improbable that a nominal megameter solar flux tube could be squeezed to such a small scale. It is proposed that instead, a megameter solar flux tube (or equivalent astrophysical structure) is in reality composed of a rope‐like braid of successively smaller filamentary current‐carrying flux tubes having a fractal scaling, and that the smallest scale of these filaments become squeezed by MHD instabilities to the ion skin depth. The effect is like a rope breaking because its fine strands break. Boldyrev, Electron Temperature of the Solar Wind Stanislav Stanislav Boldyrev, University of Wisconsin‐Madison, USA As the solar wind plasma expands form the hot solar corona, its temperature drops. However, the radial temperature decline is much slower than that required by adiabatic cooling. We will discuss the results recently obtained in [1] and [2] on the electron distribution function and the electron temperature in the solar wind. First, we will characterize the electrons beam (the strahl) streaming from the corona along the magnetic field lined. Second, we will show that due to weak Coulomb collisions, the electrons are slowly scattered from the beam and heat the background plasma. We show that in the inner heliosphere, this process leads to a universal scaling law of the electron temperature with the distance, T(r) ~ r^‐2/5. [1] Stanislav Boldyrev & Konstantinos Horaites, Kinetic theory of the electron strahl in the solar wind, Monthly Notices of the Royal Astronomical Society, 489 (2019) 3412. [2] Stanislav Boldyrev, Cary Forest, & Jan Egedal, On the temperature of the solar wind, submitted (2019); arXiv:2001.05125. 19TH ANNUAL INTERNATIONAL ASTROPHYSICS CONFERENCE ORAL ABSTRACTS Bower, Jonathan Compression and Shock Driven Variations of the Suprathermal He+ Pickup Ion Tail J. S. Bower, University of New Hampshire, USA E. Moebius, University of New Hampshire, USA A. Aly, University of New Hampshire, USA L. Berger, Christian Albrechts University of Kiel, Germany B. Klecker, Max‐Planck‐Institute for Extraterrestrial Physics, Germany M. Lee, University of New Hampshire, USA N. Schwadron, University of New Hampshire, USA We present a systematic analysis to determine variations in the suprathermal He+ pickup ion spectra, in solar wind compression regions, and around interplanetary shocks, shedding light on the acceleration mechanisms that generate suprathermal particle populations in the heliosphere. Suprathermal tails of solar wind and pickup ion populations have been widely observed throughout the heliosphere, surprisingly, even during quiet times when no other solar energetic particles are present. The spatial and temporal ubiquity of these tails, coupled with a characteristic power law spectra with v‐5 dependence in the solar wind frame, have led to a number of competing explanations as to their origin, ranging from the introduction of a new acceleration mechanism, driven by compressive fluctuations of the solar wind, to the superposition of power law, exponential, and Gaussian spectra from localized acceleration sources. Using STEREO‐A PLASTIC He+ observations, from 2007 to 2014, we expand on a superposed epoch analysis of the pickup ion distribution evolution across solar wind compressions to address variations of the suprathermal tail across compression regions and shocks. We find that the tail spectra and flux vary systematically across the compression region, and according to compression strength. The highest flux is observed inside the interaction region, with a magnitude that increases according to the solar wind speed gradient across the compression. In the compressed slow wind, these high fluxes are accompanied by strong tail spectra, close to v‐5, with generally softer spectra occurring in the fast wind. We find the lowest suprathermal flux in the center of the rarefaction region. While further verification is required, these observations appear largely consistent with theories of suprathermal tail generation and subsequent transport from compression and shock regions. Burlaga, Leonard Voyager 1 & 2 Observations of the Conversion of the Magnetic Fluctuations in the VLISM from Compressive Fluctuations near the Heliopause to Transverse Fluctuations Beyond L. Burlaga, NASA/GSFC, USA N.