Olaf Scholten

The Radio Neutrino Observatory in Greenland (RNO-G) is a neutrino detector under construction at Summit Station, with 8 out of a planned 35 stations currently deployed. We have designed and deployed a new phased array (PA) trigger based on delay-and-sum beamforming and power integration. This trigger improves detector performance by suppressing thermal noise and better targeting neutrino-induced Askaryan signals. The new PA trigger has been deployed since the 2025 season. The trigger performance has been characterized using test pulses and calibration pulsers both in the lab and in-situ, and we find across these tests a 25% average reduction in the signal-to-noise ratio (SNR) needed to trigger on signals. Simulations are shown to be representative of the detector, and simulated trigger efficiencies are within 10% of measured data. Following the in-situ trigger validation, we use data-driven trigger performance to inform simulations of the detector effective volumes. The PA trigger increases our effective volume significantly, by over a factor of 2 below 0.1 EeV and at least a factor of 1.3 at the highest energies of 100 EeV.

Sparkles are the intermittent, small-scale lightning discharges that occur near the top of intense thunderstorms. To increase the understanding of mechanisms that lead to sparkles, we compare high resolution lightning data from the LOw Frequency ARray (LOFAR) to data from a meteorological radar. The study focuses on the thunderstorms that crossed the northeast of the Netherlands on 18 June 2021. We use a two-stage clustering approach to computationally distinguish sparkles, defined as small and isolated lightning structures, from other lightning activity. Subsequently, we compare the radar data near sparkles to radar data near the other lightning activity. From radar images, we conclude that the convective systems that produced sparkles exhibit supercellular characteristics. Consistent with previous studies, we find that sparkles were present at high altitudes when radar reflectivity values were relatively high. Such values are associated with strong updrafts, lofting of graupel, and overshooting cloud tops. We confirm with a fuzzy-logic hydrometeor classification algorithm that graupel is often present near sparkles. Near sparkles, radar data also shows enhanced spectral width values and heterogeneous patterns in the radial velocity. This likely represents enhanced turbulence. Our observations match hypotheses to explain the small extent of sparkles, namely folding of a charged screening layer, and fragmentation of existing charge pockets. Additionally, we hypothesize that inductive charging, enhanced by turbulence, could play a role in the formation of sparkles.

The LOw Frequency ARray (LOFAR) radio telescope is a phased array of thousands of VHF dipoles centered in the Netherlands but spread throughout Europe. While originally mostly designed to observe astronomical phenomena, it is perfectly suited to image terrestrial lightning. Our group has been using LOFAR to image lightning in full 3D and time with nanosecond and sub-meter precision, which has revealed a myriad of new lightning phenomena that gives us new insight into both lightning plasma physics as well as Dutch thunderstorm structure. For example, almost every single lightning flash we image starts with a downward-growing negative leader (plasma channel). This is opposite lightning observed elsewhere, where most lightning starts with an upward growing negative leader. The conclusion is that Dutch thunderstorms are “bottom-heavy” relative to more traditionally studied thunderstorms. That is, where most thunderstorms have a main positive charge layer at higher altitude than the negative layer; the main positive layer in Dutch thunderstorms is below the negative layer. We have also studied small sparks at the top of very intense thunderstorms. These imply that, while most are too weak, the strongest Dutch storms have strong very localized turbulence at their tops when they interact with the ceiling of the troposphere. This local intense turbulence produces small complex charge pockets at higher altitude than the normal large charge layers. In this talk we will discuss recent results from imaging lightning with LOFAR, including extremely high precision images of lightning initiation, the discovery of small sparks that occur at the top of thunderstorms and before lightning initiation, as well as the realization that Dutch thunderstorms have upside-down charge polarity relative to thunderstorms in other places.

This paper presents the results of an optical performance monitoring campaign for the primary mirrors installed in fluorescence telescopes at the Coihueco and Loma Amarilla sites at the Pierre Auger Observatory in Argentina. Since the end of 2003, this effort has developed into a unique long-term dataset that addresses the performance of the primary mirrors. We have focused on the scattering characteristics and specular reflectance of mirror segments, as well as their evolution over several years of operation. Despite being housed in climate-controlled buildings, dust accumulation, impurities, and natural aging — such as oxidation or other chemical or physical degradation processes — gradually deteriorate the specular reflectivity of the mirror segments. In this paper, we summarize data collected over eight years of fluorescence telescope operation since the last major cleaning in autumn 2016. During this period, periodic in situ measurements were performed of scattering characteristics using two techniques: (1) specular and diffuse reflectance measurements at a wavelength of 670 nm using a portable scatterometer, and (2) spectral reflectance measurements in the range from 300 nm to 700 nm using an integrating sphere and a spectrometer. This study builds upon a previous monitoring campaign conducted between 2012 and 2017. The measurements show that the primary mirrors remain in acceptable condition after more than 20 years of operation.

Decades of observations inside thunderclouds have failed to measure electric fields large enough to spontaneously initiate a spark. The initiation process of lightning is thus an open question. We present comprehensive observations of the breakdown processes occurring at initiation for several lightning flashes using very high frequency (VHF) radio observations made using the LOFAR radio telescope. We confirm earlier observations that lightning often initiates with a rapidly propagating positive discharge, followed by the formation of a negative leader. The observations show that while the propagation speeds of the positive initiating event (PIE) are nearly constant at about 5×10^6 m/s, both the e-folding rates in VHF-intensity and peak intensities differ significantly and show no correlation with altitude. Additionally, these structures are extremely narrow, with diameters under 0.8 meters, and maintain this confinement over propagation distances exceeding 100 meters. By tracing the tracks of subsequent dart leaders we show the PIE did not form a hot conductive structure.

We used the LOw‐Frequency ARray (LOFAR) to identify two new types of lightning discharges occurring along the tracks of earlier positive leaders. The first is a kind of intracloud dart leader that we call a “backward dart” whose behavior appears counterintuitive given the positive polarity of the channel, as they are negative dart leaders that initiate from the positive branches and propagate toward inactive positive leader tips, which is backward to the direction of normal negative dart leaders. In addition, we identify a second phenomenon consisting of bursts of spatially scattered VHF sources that lack directional propagation and only appear after dart leaders on the same positive leader channels, which we call “scattered discharges after darts.” Both of these phenomena are different from normal negative dart leaders, and thus at first seem counter‐intuitive and difficult to explain. We propose a possible explanation involving local current cut‐off and negative charge redistribution.

Sparkles are defined as intermittent, small-scale lightning discharges near the top of thunderstorms. To increase the understanding of mechanisms that lead to sparkles, we compare high resolution lighting data from the LOw Frequency ARray (LOFAR) to data from a meteorological radar. The study focuses on the thunderstorms that crossed the northeast of the Netherlands on June 18, 2021. We used a two-stage clustering approach to computationally distinguish sparkles from other lightning structures. Subsequently, we compare the radar data near sparkles to radar data near other lightning structures. The two convective systems that produced sparkles resemble, respectively, a supercell and a squall line. Consistent with previous studies, we find that sparkles were present at high altitudes when radar reflectivity values were relatively high. Such values are associated with strong updrafts, lofting of graupel, and overshooting cloud tops. We confirm with a fuzzy-logic hydrometeor classification algorithm that graupel is often present near sparkles. Given the altitude of the radar data, the findings support the hypothesis that sparkles are caused by large charged hydrometeors that get lofted to relatively high altitudes and near a stratospheric charged screening layer. Near sparkles, radar data also shows enhanced spectral width values and heterogeneous patterns in the radial velocity. This likely represents enhanced turbulence. Our observations match hypotheses to explain the small extent of sparkles, namely folding of a charged screening layer, and fragmentation of existing charge pockets. Additionally, we hypothesize that inductive charging, enhanced by turbulence, could play a role in the formation of sparkles.

Sparkles are defined as intermittent, small-scale lightning discharges near the top of thunderstorms. To increase the understanding of mechanisms that lead to sparkles, we compare high resolution lighting data from the LOw Frequency ARray (LOFAR) to data from a meteorological radar. The study focuses on the thunderstorms that crossed the northeast of the Netherlands on June 18, 2021. We used a two-stage clustering approach to computationally distinguish sparkles from other lightning structures. Subsequently, we compare the radar data near sparkles to radar data near other lightning structures. The two convective systems that produced sparkles resemble, respectively, a supercell and a squall line. Consistent with previous studies, we find that sparkles were present at high altitudes when radar reflectivity values were relatively high. Such values are associated with strong updrafts, lofting of graupel, and overshooting cloud tops. We confirm with a fuzzy-logic hydrometeor classification algorithm that graupel is often present near sparkles. Given the altitude of the radar data, the findings support the hypothesis that sparkles are caused by large charged hydrometeors that get lofted to relatively high altitudes and near a stratospheric charged screening layer. Near sparkles, radar data also shows enhanced spectral width values and heterogeneous patterns in the radial velocity. This likely represents enhanced turbulence. Our observations match hypotheses to explain the small extent of sparkles, namely folding of a charged screening layer, and fragmentation of existing charge pockets. Additionally, we hypothesize that inductive charging, enhanced by turbulence, could play a role in the formation of sparkles.

We present observations of small-scale discharges, or sparks, within thunderstorms before the initiation of a lightning leader. Using LOFAR A-TRID, which provides exceptional precision and accuracy through near-field beamforming with hundreds of antennas [1], we detected multiple spark-like discharges with varying characteristics. Some show a collective propagation direction, with some speeds as low as 1 x 106 m/s (similar to ultra-slow propagation), and some as fast as 1 x 107 m/s [2, 3]. As these sparks occur in a kilometer sized region adjacent to the initiation region, they could be used to map the extent of high-field regions within thunderstorms. These results suggest that failed initiation events may be infrequent and difficult to detect as they occur in sparse clusters on short spatiotemporal scales. This work will provide an overview of the physical properties of the spark discharges and implications for lightning initiation.1: Olaf Scholten, Steven A. Cummer, Joseph R Dwyer, et al. A Comprehensive analysis of High Resolution VHF Observations with LOFAR of the Positive Initiating Event for Several Lightning Flashes. ESS Open Archive . December 12, 2025.2: Sterpka, C., Dwyer, J., Liu, N., Demers, N., Hare, B. M., Scholten, O., & ter Veen, S. (2022). Ultra-slow discharges that precede lightning initiation. Geophysical Research Letters, 49, e2022GL101597. https://doi.org/10.1029/2022GL1015973: terpka, C., Dwyer, J., Liu, N., Hare, B. M., Scholten, O., Buitink, S., et al. (2021). The spontaneous nature of lightning initiation revealed. Geophysical Research Letters, 48, e2021GL095511. https://doi.org/10.1029/2021GL095511

In this work, we use lightning observations obtained by the LOFAR radio telescope to investigate the propagation dynamics of High Altitude Negative Leaders (HANLs), which have altitudes above 7 km. Operating in the very high frequency (VHF) range, LOFAR can probe lightning processesoccurring deep within the cloud at high altitudes with sub-meter precision and 100 ns integration times [2].HANLs exhibit step lengths exceeding 100 m, an order of magnitude larger than those of negative leaders observed at lower altitudes [1]. The plasma processes underlying these HANL steps remain unknown, and it is unclear whether HANLs propagate through the same mechanism as lower altitude negative leaders. To study these structures with enhanced precision and sensitivity, we apply ATRI-D, a near-field interferometric beamforming algorithm, to LOFAR data.Our observations reveal that the dynamics of HANL steps is increasingly complex at smaller scales. At large scales (kilometers and tens of milliseconds), HANL propagation appears as a sequence of discrete corona flashes. In contrast, on smaller scales (tens of meters and milliseconds), these “corona flashes” resolve into several branched networks of filaments that initiate at different times and locations. In addition, we find that each branched network begins with an intense VHF pulse occurring within 10 m of a previously formed filament. We will discuss some of the potential physics implications of these results.[1] O. Scholten et al.; Distinguishing features of high altitude negative leaders as observed with LOFAR. Atmospheric Research, 260:105688, October 2021. ISSN 0169-8095. doi: 10.1016/j.atmosres.2021.105688.[2] O. Scholten, M. Lourens et al.; Measuring location and properties of very high frequency sources emitted from an aircraft flying through high clouds. Nature Communications, 16(1), November 2025. ISSN 2041-1723. doi: 10.1038/s41467-025-65667-2.