Larry Kepko

The Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) mission was designed to study the loss of radiation belt electrons to the upper atmosphere. Previous analysis of data from balloon pairs showed that electron precipitation events manifest coherent quasi‐periodic variations (with periodicity on the order of 10–60 min) often lacking an obvious trigger internal to the magnetosphere. This suggested that the loss mechanism was related to solar wind driving involving quasi‐static magnetospheric field modulation and/or triggering of ultra low frequency waves by periodic density structures (PDSs). In this study, we performed a statistical investigation of the occurrence of quasi‐periodic X‐ray (QPX) variations in the ≈0.5–3.4 mHz (≈4.9–36.2 min) range from the seven BARREL campaigns and their relation to solar wind PDSs. QPX variations occur around 43%–73% of the campaign time of which 26%–30% involve PDSs. The frequency distribution of the QPX variations showed preferential occurrence at ≈0.6–1.0 mHz, ≈1.6–2.3 mHz, and tentatively ≈1.3 mHz. The QPX events map to different regions in the space depending on the frequency of the QPX variations and the type of solar wind driving. Simultaneous observations of the event by multiple balloons show QPX events extending up to 7.3 and 6.5. Finally, PDSs are more likely related to the X‐ray modulation during quiet to moderate geomagnetic conditions following substorm activity and possibly magnetospheric compressions.

Identifying the primary physical mechanisms and associated secondary processes (e.g., different plasma wave modes, instabilities and magnetic reconnection), that are responsible for plasma heating at the planetary magnetospheres and astrophysical plasmas is often difficult with a single spacecraft or even with a constellation of spacecraft within a single‐plasma scale. Here, we present a statistical study using data from four Magnetospheric Multiscale (MMS) satellites to characterize the cross‐scale plasma‐heating distributions associated with Kelvin‐Helmholtz instability (KHI) and drift mirror instability (DMI). We find that characteristic heating frequency (CHF) distributions follow distinct tail‐scaling regimes: KHI events exhibit low‐ power‐law‐like tails, whereas DMI events occupy larger‐ regimes closer to quasi‐Gaussian behavior. Comparison of a KHI event with Particle‐in‐Cell (PIC) simulations yields consistent tail‐scaling, supporting the physical interpretation. We also analyze an event with periodic encounters of diamagnetic cavities (DMCs) at the Earth's dayside magnetopause, and find that the CHF scaling together with KH growth‐rate analysis is consistent with KHI modulation of reconnection. These results establish statistical discrimination between KHI‐ and DMI‐associated heating, and can be used in future works to establish an automated identification of primary mesoscale processes driving cross‐scale non‐adiabatic heating in collisionless plasmas.

The Nightside Transition Region (NTR)—the boundary between the outer and inner magnetosphere—plays a critical role in magnetospheric plasma dynamics. During quiet periods, it acts as a “magnetic wall,” while in active times, it becomes a site of intense plasma energization. Although the large-scale morphology of the NTR is fairly well characterized, the role of mesoscale structures within this region remains poorly understood.Giant undulations—auroral features located at the equatorward edge of the diffuse aurora—offer a unique opportunity to probe mesoscale dynamics in the NTR. Historically, their formation and evolution have been examined using global auroral imaging, which is limited in both spatial and temporal resolution.In this study, we leverage recent advancements in ground-based optical instrument arrays to analyze the fine-scale characteristics and temporal evolution of giant undulations. Our findings provide new insights into the generation mechanisms of these structures and their contribution to the overall dynamics of the auroral region, offering a fresh perspective on mesoscale processes in the NTR.

Omega bands are mesoscale auroral structures that appear as eastward-moving poleward protrusions in the auroral oval. They typically appear in the post-midnight sector during periods of enhanced geomagnetic activity. Omega bands have been associated with Ps6 pulsations and rapidly time-varying magnetic fields on the ground, making them highly relevant to space weather forecasting. However, while the solar wind and magnetospheric drivers of omega bands have been previously studied, the characteristic signatures of omega band substorm events have not been identified.In this work, we compare solar wind, magnetospheric, and geomagnetic parameters measured during general substorm activity to the same parameters measured during 205 omega band events from 1997 to 2007 identified using the MIRACLE network. Non-omega substorm events are identified using spline fitting techniques to locate positive bays in the SuperMAG lower auroral electrojet index, and filtered to match the omega band substorm events in intensity. We use OMNI data to determine the solar wind drivers of these events and data from GOES and SuperMAG to identify magnetospheric and geomagnetic signatures of these events. This analysis will allow us to identify the unique solar wind drivers and magnetospheric configurations associated with omega band formation and improve our understanding of magnetosphere-ionosphere coupling dynamics during periods of enhanced geomagnetic activity.

Periodic density structures (PDSs) are advecting quasi‐periodic (periodicities between several minutes and several hours) density enhancements observed in the solar wind and corona. There is strong evidence that PDSs are formed at the Sun. We report here an analysis of Ulysses data in which we identify PDSs using the multi‐taper method, representing the first of its kind in solar wind emanating from polar coronal holes (PCH) at high heliolatitude. Probability distributions of identified frequencies are very similar in both PCH wind and non‐PCH wind; they resemble the distribution of frequencies derived from recent analysis of Wind data (Kepko et al., 2024, https://doi.org/10.1029/2023ja031403 ). All resemble the distribution of transverse fluctuations in the corona. These results demonstrate that the source of PDSs is globally distributed. The large number of alpha‐to‐proton PDSs found indicate that PDSs originate in the low corona, where the alpha‐to‐proton ratio is set. Analyzing PDSs as a function of distance from the Sun, we found that their occurrence rate is generally decreasing, which supports the hypothesis that PDSs have a solar source and a processing en route gradually diminishes their coherence. In the ratio of the frequency distributions of protons and the alpha‐to‐proton density ratio, we found enhancements for non‐PCH wind at 0.3 mHz and for PCH wind at 0.4 mHz. Similarly, we calculated the occurrence distribution of instantaneous radial length scales and found that the slow wind (550 km ) peaked at 800 Mm, while the fast wind peaked at 1,000 Mm, suggesting some characteristics of PDSs depend on the magnetic field at their solar source.

During a special MMS Turbulence campaign, almost 80 min of continuous high resolution burst measurements were downlinked from a traversal across Earth's magnetosheath behind mixed quasi‐parallel/perpendicular bow shock conditions. Throughout the magnetosheath, we observed magnetic holes containing intense whistler waves known as “Lion Roars” (LRs). We compared the observed properties of LRs to determine why the whistler waves included higher order harmonics in some magnetic holes but not in others. We refer to the LR events with and without harmonics as “Higher Order Harmonic LRs (HOHLRs)” and “nonharmonic LRs”, respectively. From our observations of the LR events, we found that each wave train featured an electron beam moving at the electron Alfvén speed and parallel to the background magnetic field. We also observed that this electron beam had a stronger phase space density for the LRs near the bow shock. For the HOHLR events, we observed that they exhibited a strong antiparallel Poynting flux and high counts of solitary waves surrounding their corresponding magnetic holes, with some detected solitary waves at the edges of the magnetic hole. Additionally, the fundamental frequencies for the HOHLRs were observed around 0.18–0.23 of the electron cyclotron frequency, . For the nonharmonic LR events, we observed a strong presence of solitary waves within the confines of their magnetic hole, leading to a population of trapped electrons with a parallel/antiparallel temperature anisotropy, strong fluctuations, and Poynting flux flowing in an oblique direction with respect to the background magnetic field. Additionally, the fundamental frequencies of nonharmonic LRs were lower, ranging from 0.11 to 0.18 .

The adaptive Multitaper Method (aMTM) is a Fourier analysis‐based technique that produces a power spectral density (PSD) with a user‐defined frequency resolution, reduces variance associated with standard digital Fourier Transform (FT) algorithms, and minimizes spectral power leakage. This technique is the backbone of our periodic signal detection procedure, which models the background PSD and determines if a discrete enhancement in the PSD is statistically significant. Due to the utility of this Fourier analysis tool, we created an open‐source Python version and compared its functionality to the IDL procedure that is already well‐tested and widely used. Differences from the IDL code emerged due to how Python estimated the aMTM PSD and modeled the background spectrum. In computing the relative error estimates between the Python and IDL‐produced aMTM PSD, we observed discrepancies of less than 2% at each frequency. In generating 100 artificial red noise time series from a defined PSD ground truth model, we observed that Python reproduced the PSD with a better accuracy than IDL, and that both saw an improvement in precision as the length of the time series increased. As such, we consider the Python version to be an improvement in performing spectral analysis of identifying discrete periodic signal(s) with continued support to make the code more user‐friendly. Finally, we demonstrate an application of the routine on high resolution magnetic field data from NASA's Magnetospheric Multiscale (MMS) mission and demonstrate that our new Python code has the computational efficiency to analyze data intervals much longer than IDL is capable of.

Europe hosts a large and highly active community of scientists working in the broad domain of Heliophysics. This broad discipline addresses plasmas in the regions of space and atmosphere influenced by the Sun and solar wind. However, this community has historically been fragmented, both geographically and thematically, which has limited the potential for strategic coordination, collaboration, and growth. This has recently prompted a grass-roots community-building effort to foster communication and interactions within the European Heliophysics Community (EHC). This white paper outlines the motivation, priorities, and initial steps towards establishing the EHC, and presents a vision for the future of Heliophysics in Europe. As a crucial first step of this endeavour, a dedicated EHC website is now available: https://www.heliophysics.eu/ (last access: November 2025).

Omega bands are mesoscale auroral structures that appear as quasi‐periodic eastward drifting protrusions of the poleward edge of the diffuse aurora. Cribb et al. (2024), https://doi.org/10.1029/2024gl109756 demonstrated that the appearance of omega bands is associated with solar wind driving consistent with stream interaction regions. However, the formation of omega bands through physical processes in the magnetosphere and ionosphere is not well understood. We perform superposed epoch analyses of the solar wind, magnetospheric, and geomagnetic parameters during 205 omega band events from 1997 to 2007. We find that the large majority of omega bands are associated with CME‐ or SIR‐like solar wind driving. In the ionosphere, omega bands are correlated with sustained periods of activity as defined by the auroral electrojets. In the inner magnetosphere, the appearance of omega bands is associated with an activation of a global current system enhanced relative to typical substorm activity. Our findings suggest omega band formation is associated with an enhanced partial ring current or substorm current wedge.