The Solar Orbiter spacecraft crossed Comet Leonard’s ion tail on 2021 December 17 near 1 au. In and near the ion tail, significant amounts of singly charged oxygen (O ^+ ) ions were detected by the Heavy Ion Sensor on board the spacecraft. These ions are likely the result of outgassed water molecules from the comet that became ionized and disassociated into protons and O ^+ ions and that were then picked up by the interplanetary magnetic field and advected outward with the solar wind. At this time, the spacecraft was also located amidst the denser parts of the interstellar helium (He) neutrals that are focused here by their gravitational interaction with the Sun. Pickup He ^+ ions in greater numbers are also found in this region and form when neutrals interact with solar photons. Newly ionized ions can generate waves that propagate mainly along the background magnetic field before the waves scatter the ions toward isotropy. Spectral magnetic field analyses are employed to find mainly elliptically polarized waves associated with O ^+ and He ^+ from ring-beam instabilities. Waves associated with He ^+ are identified, but O ^+ waves are not seen. Visibility is concluded to be affected by the relative amplitude of the waves to turbulence, and the visibility increases when the sampling direction is more aligned with the background magnetic field.

Several data analysis studies have shown that the magnetospheric activity of the Earth increases as the amplitude of the magnetic-field fluctuation in the upstream solar wind increases, even during times when the solar-wind magnetic field is strictly northward and dayside reconnection is expected to be minimal. This is known as the “upstream turbulence effect,” for which an analogous effect is observed in Navier–Stokes fluids in wind tunnel experiments. Data analysis studies have also shown that the Alfvénic solar wind produces an enhanced magnetospheric reaction to upstream magnetic-field fluctuations compared to the non-Alfvénic solar wind. In the present study, we corroborate the Alfvénicity effect on the upstream turbulence effect. All prior data analysis studies were focused on the relationship between the solar-wind magnetic-field fluctuations and increased magnetospheric activity. The present study explores the use of solar-wind velocity fluctuation measurements compared to the use of magnetic fluctuations for driving the magnetospheric activity levels. We found that using the amplitude of the vector velocity fluctuation was as good as using the amplitude of the vector magnetic-field fluctuation to describe the upstream turbulence effect. Furthermore, we found that it was better to use the amplitude of the angular fluctuation of the solar-wind velocity vector than that of the solar-wind magnetic-field vector.

To estimate the effects of magnetic holes on the magnitude of B magnetic power spectral density, a 2-day interval of fast coronal-hole-origin plasma at 1 AU is examined. The magnetic holes are identified and removed from the magnetic time series, and the power spectral density of the hole-free time series is compared with the power spectral density of the original time series. The magnetic holes were found to contribute about a factor of two to the amplitude of the power spectral density.

The Space Weather UnderGround (SWUG) program is an educational outreach initiative in which high school and undergraduate students learn heliophysics fundamentals and build low-cost fluxgate magnetometers. The program has three primary goals: (1) training the future Science, Technology, Engineering, and Mathematics (STEM) workforce, (2) developing a cost-effective magnetometer network across the United States, and (3) providing high-spatial-resolution geomagnetic field data for heliophysics research. The Maryland SWUG program, launched in 2022, deployed a student-built magnetometer in Los Alamos, NM, in early October 2023 and operated it for ∼1 month to study geomagnetic field disturbances during the annular solar eclipse on 14 October 2023. Due to time and resource constraints, standard magnetometer calibration procedures could not be completed prior to the campaign; therefore, the SWUG magnetometer data were analyzed qualitatively rather than quantitatively. On geomagnetically quiet days, the instrument recorded a typical diurnal variation in the northward geomagnetic field component (ΔX): steady at night, turning southward after sunrise until late afternoon, and then returning to nighttime values. This pattern indicates that solar ionization of the upper atmosphere enhances westward ionospheric currents above Los Alamos. On the eclipse day, however, this southward progression was disrupted, suggesting weakened westward currents caused by the temporary reduction in solar illumination. Global ionospheric current maps derived from worldwide research-grade magnetometer networks independently confirmed the weakened westward current above Los Alamos during the solar eclipse. These results demonstrate that, with standard magnetometer calibration procedures, low-cost SWUG magnetometers can provide reliable and scientifically valuable data for heliophysics research.

Motivated by knowledge that directional discontinuities (strong current sheets) in the solar-wind magnetic time series have dominating effects on the magnetic power spectral density of the solar wind, the question is raised as to what are the effects of the magnetic fluctuations that reside in between those discontinuities. A methodology is developed to remove the effects of discontinuities in the magnetic time series and the resulting modified time series is Fourier examined. At 1 AU, an interval of fast Alfvénic coronal-hole-origin wind and an interval of slow non-Alfvénic streamer-belt-origin wind are analyzed. For both types of solar wind, the amplitude of the trace-B power spectral density in the inertial range is about an order of magnitude smaller for the time series of fluctuations between the discontinuities than it is for the raw (with discontinuities) time series, and the spectral index is shallower when the discontinuities are removed. For future global MHD computer simulations of the “upstream turbulence effect” on the solar-wind-driven Earth’s magnetosphere, we have a methodology to determine the effects of discontinuities on the Earth’s magnetosphere versus the effects of between-the-discontinuity magnetic fluctuations.

We report the first simultaneous observations of wave activity and pickup ions (PUIs) in the pristine solar wind upstream of Earth's bow shock (i.e., at 1 au) from the Magnetospheric Multiscale (MMS) mission. Low‐frequency electromagnetic waves induced by newborn interstellar PUIs have been confirmed as the dominant energy source that drives turbulence and thermal ion heating in the solar wind beyond 1 au. However, only a few observations of PUI‐generated waves exist near 1 au. Near 1 au, these waves are relatively weak and are best observed in the pristine solar wind, absent of any other wave activity or energized ion populations. In this work, we analyze a ∼3 min interval during which Earth was within the He focusing cone and MMS was in the pristine solar wind. We identify H + and He + PUI ring distributions provided by the Hot Plasma Composition Analyzer and compare their velocity space characteristics, which reveals that these PUIs were likely born from different neutral source populations (e.g., geocornal hydrogen and interstellar helium). We also identify potential signatures of distinct helium and hydrogen wave modes in the magnetic field power spectrum and perform a linear instability analysis which identifies the distinct wave growth rates. The peak growth rates coincide with enhancements in the magnetic power spectrum, suggesting that these waves could be generated by the observed H + and He + PUIs. These observations motivate the need for a systematic study of PUI‐generated waves near 1 au, which can be achieved using MMS data.

An interplanetary shock driven by a coronal mass ejection (CME) containing an interval of sub-Alfvénic flow impacted Earth on April 23, 2024. In this article, we analyze the turbulence in the sheath region between the shock and CME to determine how it evolves from L1 (as observed by Wind) to Earth (as observed by MMS, upstream of the bow shock). Wind and MMS were separated by 55 R_E in the dawn-dusk direction, but the shock normals differ by only 2.8^∘ and the Pearson correlation coefficient between time-shifted magnetic field components is ρ=0.93. We observe a shift in the break point of the magnetic power spectral density between inertial and ion kinetic scales toward the ion inertial length and a steepening of the spectral slope, indicating more active energy cascade closer to Earth. The distribution of increments becomes more non-Gaussian near Earth, particularly at ion kinetic scales, indicating the turbulence becomes more intermittent. Finally, the correlation length at Earth is 25% longer than at L1, indicating that the turbulence is smoothing out the magnetic field. The results present an example of substantial evolution of CME sheath turbulence from L1 to Earth.

Strong Thermal Emission Velocity Enhancement (STEVE) is a latitudinally narrow, purple-band emission observed at subauroral latitudes. Stable Auroral Red (SAR) arcs characterized by major red emission, and red/green arcs with both red and green emissions also occur at subauroral latitudes. Characteristics of magnetospheric source plasma and electromagnetic fields of these three types of arcs have not been fully understood because of the limited conjugate observations between magnetosphere and the ground. In this study, we report 11 conjugate observations (2 STEVEs, 7 SAR arcs, and 2 red/green arcs), using all-sky images obtained at seven ground stations over more than four years from January 2017 to April 2021 and magnetospheric satellites (Arase and Van Allen Probes). We found that, in the inner magnetosphere, the source region of STEVEs and red/green arcs were located outside the plasmasphere, and that of the SAR arc was in the region of spatial overlap between the plasmasphere and ring current region. Electromagnetic waves at frequencies below 1 Hz were observed for STEVEs and red/green arcs. SuperDARN radar data showed a strong westward plasma flow in the ionosphere, especially during STEVE events, whereas the plasma flows associated with SAR arcs and red/green arcs were generally weaker and variable. The STEVE and SAR arc can appear simultaneously at slightly different latitudes and STEVEs and red/green arcs can transform into SAR arcs. These first comprehensive ground-satellite measurements of three types of subauroral-latitude auroras increase our understanding on similarlity, differences, and coupling of these auroras in the ionosphere and the magnetosphere.

This study investigates energy dissipation in small‐scale solar wind turbulence using a novel approach to autocorrelation function analysis leveraging high‐resolution data from the Magnetospheric Multiscale Mission (MMS). We analyze magnetic field fluctuations at ion dissipation scales, focusing on short 20‐s intervals to isolate dissipation‐scale dynamics. Using MMS's fluxgate magnetometer, fast plasma investigation, and energetic particle detector, we compute normalized correlation functions as functions of the interplanetary magnetic field cone angle. Our results show that turbulence is correlated over short spatial scales (100–1,000 km), with shorter correlation lengths in the compressional component compared to the transverse components. We find that the transverse correlation lengths differ most when the interplanetary magnetic field is nearly perpendicular to the solar wind flow (cone angle ), and that they vary approximately as the inverse sine of the cone angle. The characteristics of the correlation length suggest that the turbulence observed is primarily two‐dimensional. These findings highlight the anisotropic nature of dissipation‐scale turbulence and its dependence on solar wind conditions, providing insights into energy transfer in space plasmas.

In S. J. Hollick et al., we surveyed the Voyager magnetic field data from launch through 1990 where the Voyagers 1 and 2 spacecraft reach 43.5 au and 33.6 au, respectively. We identified 637 intervals of wave activity that could be attributed to either interstellar pickup He ^+ , H ^+ , or both. In our quest to identify and study low-frequency magnetic waves arising from interstellar pickup H ^+ , we found 19 intervals, 16 with thermal ion data, within ∼3 au. We compared the growth rate of the waves with the rate of background turbulence they must overcome to reach observable levels. Ionization of interstellar neutral H is highly efficient, resulting in a factor of 10 reduction in density by 1 au relative to values at R > 10 au. At the same time, solar wind turbulence increases with decreasing distance to the Sun. This makes it unlikely that interstellar neutral hydrogen can penetrate within ∼3 au in sufficient number to explain the wave observations seen by the Voyager magnetometers. We consider the possibility that the so-called “inner source” for pickup H ^+ arising from the interaction of solar wind protons with dust grains may account for the density of newborn pickup ions required for the growth of the observed waves. Although the Voyagers lack the instrumentation required to measure pickup ions, we do conclude that the inner-source theories provide a compelling explanation for the majority of the observations while it is possible that a few could be due to either interstellar ions or shocks.

We have examined Ulysses magnetic field measurements for the years 1993 through 1996 as the spacecraft moved sunward from 5 au at high southern latitudes, passing through perihelion during the first fast-latitude scan to achieve high northern latitudes, and finally returning to 5 au. These years represent near-solar-minimum activity, providing a clear measure of high-latitude solar-wind turbulence. We apply a series of tests to the data, examining both the magnetic variance anisotropy and the underlying wavevector anisotropy, finding them to be consistent with past 1 au observations. The variance anisotropy depends upon both the thermal proton temperature parameter and the amplitude of the magnetic power spectrum, while the underlying wavevector anisotropy is dominated by the component perpendicular to the mean magnetic field. We also examine the amplitude of the magnetic power spectrum as well as the associated turbulent transport of energy to small scales that results in the heating of the thermal plasma. The measured turbulence is found to be stronger than that seen at low latitudes by the Voyager spacecraft as it traverses the distance from 1 to 5 au during the years approaching solar maximum. If the high- and low-latitude sources are comparable, this would indicate that while the heating processes are active in both regions, the turbulence has had less decay time in the transport of energy to small scales. Alternatively, it may also be that the high-latitude source is stronger.

The solar wind forms the largest wind tunnel for plasma and magnetofluid turbulence that is accessible to Earth. It evolves from what is thought to be a turbulent source that continues to drive nonlinear turbulent dynamics as it expands outward via large-scale, energy-containing wind shear and shocks. In the outer heliosphere, once the gradients in the flow have coalesced and they no longer provide an adequate source for the turbulence, the excitation of wave energy by the injection of interstellar pickup ions becomes the dominant source of energy that continues to drive the turbulence. While there are established formalisms for the determination of the strength of the turbulence and the evolution of the turbulent spectra is well-established, the actual nonlinear dynamics that are responsible for its formation and evolution remain unresolved and the subject of considerable debate. We examine the evidence and attempt to illuminate the various theories while demonstrating what is needed to resolve the debates and bring the subject of plasma turbulence into a new level of understanding.

Based on decades of single-spacecraft measurements near 1 au as well as data from heliospheric and planetary missions, multi-spacecraft simultaneous measurements in the inner heliosphere on separations of 0.05–0.2 au are required to close existing gaps in our knowledge of solar wind structures, transients, and energetic particles, especially coronal mass ejections (CMEs), stream interaction regions (SIRs), high speed solar wind streams (HSS), and energetic storm particle (ESP) events. The Mission to Investigate Interplanetary Structures and Transients (MIIST) is a concept for a small multi-spacecraft mission to explore the near-Earth heliosphere on these critical scales. It is designed to advance two goals: (a) to determine the spatiotemporal variations and the variability of solar wind structures, transients, and energetic particle fluxes in near-Earth interplanetary (IP) space, and (b) to advance our fundamental knowledge necessary to improve space weather forecasting from in situ data. We present the scientific rationale for this proposed mission, the science requirements, payload, implementation, and concept of mission operation that address a key gap in our knowledge of IP structures and transients within the cost, launch, and schedule limitations of the NASA Heliophysics Small Explorers program.

Heliospheric missions operating in the outer heliosphere and beyond are necessary for understanding the heliosphere and its interstellar neighborhoodInterstellar neutrals (ISNs), pickup ions (PUIs; ionized ISNs picked up by the local plasma fields), and energetic neutral atoms (ENAs; neutralized PUIs and solar wind ions) provide information about the interstellar medium, the global heliosphere, and processes at the transition region between solar and interstellar environments.These heliospheric data are usually collected from the vicinity of the Earth.Thus, they require careful and thoughtful interpretation of the observed signals and implementation of proper corrections for modulation by the solar environment.Interpretation is complicated because the solar wind and solar extreme ultraviolet (EUV) radiation ionize the incoming flux of interstellar particles measured near the Earth's orbit.The ionization rates vary depending on species and phase of solar activity.Hydrogen, the most abundant element, is the most prone to the modulation by the solar environment and undergoes the greatest losses inside the heliosphere.High ionization rates and radiation pressure create a density depletion region for ISN H at a few astronomical units from the Sun.The last decades provided strong arguments for measurements at least outside this cavity (beyond Jupiter's orbit) to mitigate the adverse solar environment losses for the fluxes of interstellar-born particles.In-situ heliospheric measurements far beyond 1 au should be a priority for the heliospheric community in the coming decades to make progress in data interpretation, improve quality of the observations, and advance our understanding of the heliosphere and its interstellar neighborhood.

EDITORIAL article Front. Astron. Space Sci., 22 February 2023Sec. Space Physics Volume 10 - 2023 | https://doi.org/10.3389/fspas.2023.1149649

In Fourier time-frequency power spectrograms of satellite magnetic field data, electromagnetic ion cyclotron (EMIC) waves may feature discrete, rising tone structures that rapidly increase in frequency. Using data from the Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) fluxgate magnetometer, we conducted a statistical study of EMIC waves from September 2012 through June 2016. We compared the occurrence rates and spatial distributions for all EMIC waves with those for rising tone EMIC waves as a function of magnetic local time (MLT) and L shell, as well as a function of R XY and Z in solar-magnetic (SM) coordinates. Overall, EMIC waves occurred during 2.4% of the time period considered, but rising tone EMIC waves were only found during 0.2% of the time period considered. About 7%-8% of the minutes of orbital coverage with H+ or He+ band EMIC waves had rising tones. The regions of peak occurrence rates for H+ and He+ band waves, as well as waves with rising tones, were found in the noon and dusk sectors for 4 < L < 6. The preferred regions for H+ waves as a function of R XY and Z SM suggest an association with magnetospheric compressions near noon and interactions between plumes and the ring current near dusk. Peak occurrence rates for O+ band waves were found between 2 < L < 4 at all MLT, and over a wide range of L shells near dusk. No rising tones were found in the O+ band.

Interstellar neutral atoms enter the heliosphere at a relatively slow speed corresponding to the motion of the Sun through the local interstellar medium, which is approximately 25 km s ^−1 . Neutral hydrogen atoms enter from the approximate location of the Voyager spacecraft and are eventually ionized primarily by collision with thermal solar wind ions. An earlier analysis by Hollick et al. examined low-frequency magnetic waves observed by the Voyager spacecraft from launch through 1990 that are thought to arise from the scattering of newborn interstellar pickup H ^+ and He ^+ . We report an analysis of Voyager 1 observations in 1991, which is the last year of high-resolution magnetic field data that are publicly available, and find 70 examples of low-frequency waves with the characteristics that suggest excitation by pickup H ^+ and 10 examples of waves consistent with excitation by pickup He ^+ . We find a particularly dense cluster of observations at the tail end of what is thought to be a Merged Interaction Region (MIR) that was previously studied by Burlaga & Ness using Voyager 2 observations. This is not unexpected if the MIR is followed by a large rarefaction region, as they tend to be regions of reduced turbulence levels that permit the growth of the waves over the long time periods that are generally required of this instability.

We revisit the question of how the unstable scattering of interstellar pickup ions (PUIs) may drive turbulence in the outer solar wind, and why the energy released into fluctuations by this scattering appears to be significantly less than the standard bispherical prediction. We suggest that energization of the newly picked-up ions by the ambient turbulence during the scattering process can result in a more spherical distribution of PUIs, and reduce the generated fluctuation energy to a level consistent with the observations of turbulent intensities and core solar wind heating. This scenario implies the operation of a self-regulation mechanism that maintains the observed conditions of turbulence and heating in the PUI-dominated solar wind.

HelioSwarm (HS) is a NASA Medium-Class Explorer mission of the Heliophysics Division designed to explore the dynamic three-dimensional mechanisms controlling the physics of plasma turbulence, a ubiquitous process occurring in the heliosphere and in plasmas throughout the universe. This will be accomplished by making simultaneous measurements at nine spacecraft with separations spanning magnetohydrodynamic and sub-ion spatial scales in a variety of near-Earth plasmas. In this paper, we describe the scientific background for the HS investigation, the mission goals and objectives, the observatory reference trajectory and instrumentation implementation before the start of Phase B. Through multipoint, multiscale measurements, HS promises to reveal how energy is transferred across scales and boundaries in plasmas throughout the universe.

Isolated proton auroras (IPAs) appearing at subauroral latitudes are generated by energetic protons precipitating from the magnetosphere through interaction with electromagnetic ion cyclotron (EMIC) waves. Thus, an IPA is the ionospheric projection of the spatial and temporal variation of wave‐particle interaction regions in the magnetosphere. In this study, we conducted unique multi‐event analysis of simultaneous observations of IPAs and their source regions on 22 April, 7 September, and 22 March 2018, using all‐sky imagers at subauroral latitudes and the Van Allen Probes. When the satellite footprint passed over the IPAs associated with ground Pc1 geomagnetic pulsations, locally generated He + ‐band EMIC waves with the same frequencies as the ground Pc1 pulsations were observed in all events. The IPAs and EMIC waves had comparable narrow widths in the latitudinal direction. The EMIC waves appeared during the rapid enhancement of the ring current proton flux at energy range of ∼10–50 keV, while they disappeared at the rapid decrease of the electron density. From these results, we conclude that the boundaries of the localized IPAs and EMIC waves were determined by the overlap region of energetic proton enhancement and the plasmasphere. This overlap of ring‐current protons and plasmasphere is a favorable condition for the pitch‐angle scattering of protons by the EMIC waves. Characteristic magnetic and electric field variations with the IPAs were not observed by the satellite, indicating that the IPAs were not accompanied by field‐aligned currents comparable to that of oval auroral arcs.