Helios Observations of Quasiperiodic Density Structures in the Slow Solar Wind at 0.3, 0.4, and 0.6 AU

On Differentiating Multiple Types of ULF Magnetospheric Waves in Response to Solar Wind Periodic Density Structures

Identifying the nature and source of ultra-low frequencies (ULF) waves (f ⪅ 4 mHz) at discrete frequencies in the Earth's magnetosphere is a complex task. The challenge comes from the simultaneous occurrence of externally and internally generated waves, and the ability to robustly identify such perturbations. Using a recently developed robust spectral analysis procedure, we study an interval that exhibited in magnetic field measurements at geosynchronous orbit and in-ground magnetic observatories both internally supported and externally generated ULF waves. The event occurred on 9 November 2002 during the interaction of the magnetosphere with two interplanetary shocks that were followed by a train of 90 min solar wind periodic density structures. Using the Wang-Sheeley-Arge model, we mapped the source of this solar wind stream to an active region and a mid-latitude coronal hole just prior to crossing the Heliospheric current sheet. In both the solar wind density and magnetospheric field fluctuations, we separated broad power increases from enhancements at specific frequencies. For the waves at discrete frequencies, we used the combination of satellite and ground magnetometer observations to identify differences in frequency, polarization, and observed magnetospheric locations. The magnetospheric response was characterized by: (a) forced breathing by periodic solar wind dynamic pressure variations below ≈1 mHz, (b) a combination of directly driven oscillations and wave modes triggered by additional mechanisms (e.g., shock and interplanetary magnetic field discontinuity impact, and substorm activity) between ≈1 and 4 mHz, and (c) largely triggered modes above ≈4 mHz.

Nine Outstanding Questions of Solar Wind Physics

In situ measurements of the solar wind have been available for almost 60 years, and in that time plasma physics simulation capabilities have commenced and ground-based solar observations have expanded into space-based solar observations. These observations and simulations have yielded an increasingly improved knowledge of fundamental physics and have delivered a remarkable understanding of the solar wind and its complexity. Yet there are longstanding major unsolved questions. Synthesizing inputs from the solar wind research community, nine outstanding questions of solar wind physics are developed and discussed in this commentary. These involve questions about the formation of the solar wind, about the inherent properties of the solar wind (and what the properties say about its formation), and about the evolution of the solar wind. The questions focus on (1) origin locations on the Sun, (2) plasma release, (3) acceleration, (4) heavy-ion abundances and charge states, (5) magnetic structure, (6) Alfven waves, (7) turbulence, (8) distribution-function evolution, and (9) energetic-particle transport. On these nine questions we offer suggestions for future progress, forward looking on what is likely to be accomplished in near future with data from Parker Solar Probe, from Solar Orbiter, from the Daniel K. Inouye Solar Telescope (DKIST), and from Polarimeter to Unify the Corona and Heliosphere (PUNCH). Calls are made for improved measurements, for higher-resolution simulations, and for advances in plasma physics theory.

Quasi-periodic ripples in high latitude electron content, the geomagnetic field, and the solar wind

In this study we present evidence for the existence of quasi-periodic ripples in the F-region electron content at high latitude, the magnetic flux density at geosynchronous orbit, and the solar wind dynamic pressure and IMF flux density at L1. The ripples are shown to be continually present during all 14 days of observations in different years and seasons, and at all local times, though they are generally small in magnitude relative to the background. The frequencies of all the parameters are, on average, remarkably similar, and the amplitudes of the selected peaks of some parameters show strong correlations for which regression formulae are given. These results support the proposition that the ripples propagate from the solar wind to the F-region, and that they are a related, persistent phenomenon.