Subionospheric very low-frequency (VLF) radio signals are reflected by free electrons in the ionospheric D-region at about 60–90 km altitude and can propagate over long distances, which makes them useful for monitoring the state of the D-region or perturbations due to solar flares. At the D-region height, the ionosphere is mainly ionized by solar Lyman-α radiation. The reflection characteristics of VLF signals depend on the state and dynamics of the D-region, which is highly influenced by Lyman-α radiation. Although the amplitude of the received terrestrial VLF signal changes as a function of solar zenith angle over the course of the year, the VLF amplitude shows a distinctive sharp decrease around October, which is hence called the “October effect”. This study investigates the occurrence of the October effect and its dependencies on latitude and longitude. We developed a method to detect the occurrence of the October effect in the long-term VLF data and derive key parameters characterizing (start and end date, intensity) the sudden decrease in the signal amplitude. This investigation using a network of VLF stations distributed over low-, middle-, and high-latitude regions shows that the occurrence of the October effect has a clear latitudinal dependency, occurring earlier in high-latitude regions than at midlatitudes. No low-latitude signature is found.

The characteristics of very low frequency (VLF) radio wave propagation in the Earth‐ionosphere waveguide are determined particularly through dawn and dusk using phase and amplitude measurements of man‐made signals propagating below the ionospheric D region. For the first time variations of “Wait” height and sharpness parameters, H' and β, have been determined for dawn and dusk conditions. These measurements provide observational data to constrain D region modeling efforts, extending the capabilities of VLF propagation monitoring for geophysical phenomena such as lightning, solar flares, and energetic particle precipitation. At mid‐latitudes, H' varied from ∼85 km at night, then, starting from solar zenith angle (SZA) ∼ −97.5°, rapidly down to ∼73 km at dawn (SZA = −90°), then back up to ∼78 km at SZA ∼ −75° and then down to the appropriate noon value for the latitude (and season). In contrast, from noon through dusk to night, H' varied essentially monotonically from ∼70 to 75 km through ∼80 to ∼85 km. At low latitudes no dawn minimum in H' was observed, due to the reduced effect of galactic cosmic rays (GCR). Sharpness, β , varied from its nighttime value of ∼0.6 km −1 down to a minimum of ∼0.25 km −1 at SZA ∼85° near dusk or ∼75° near dawn, rising again to (SZA‐dependent) noon values of ∼0.35–0.5 km −1 . The results are interpreted through the geophysical effects controlling D region electrons, including the daytime dominant role of solar Lyman‐α from low to mid‐latitudes, and the greater role of GCR at increasingly higher mid‐latitudes.

Previously the geomagnetic Ap index has been used as a proxy to produce empirical energetic electron precipitation (EEP) forcing representations suitable for incorporation into coupled‐climate model runs. The long‐running Ap index has the advantage that it allows descriptions of EEP to be made for periods outside the current satellite era, but its suitability has not been checked against other reasonable proxies. In this study three different satellite electron precipitation data sets (DEMETER, POES, and SAMPEX) are used to examine the suitability of a variety of geomagnetic and solar wind proxies to represent EEP flux in different energy ranges. Analysis was undertaken using indices at their fundamental timescales (typically minutes or hours). For medium energy electron precipitation (i.e., >100 keV), the best proxy is found to be either Ap or Dst. For relativistic energy electron precipitation (i.e., >700 keV), the best proxy is Kp or AE, the latter suggesting a connection to substorm activity. The identification of the Ap index as one of the best proxies for medium energy EEP supports the approach taken by van de Kamp et al. (2016), https://doi.org/10.1002/2015jd024212 . An EEP forcing capability based on Ap was developed by those authors for inclusion as a solar forcing factor in the Coupled Model Intercomparison Project Phase 6 of the World Climate Research Program.

In early May 2024 6 Coronal Mass Ejections were launched toward the Earth in short succession. This triggered the G5 “Gannon” geomagnetic storm lasting from 17 UT on 10 May 2024 to 9 UT on 12 May 2024. Auroral displays were seen around the world including at lower latitudes than previous geomagnetic disturbances this century. Magnetic field variations measured at multiple sites in New Zealand exceeded values observed over the past 30 years. In 2022, following many years of industry‐research collaboration, an updated and enhanced “All of New Zealand” Geomagnetically Induced Current (GIC) mitigation strategy was developed. This strategy involves targeted line disconnections to reconfigure the network. The goal of this approach is to reduce GIC magnitudes and durations at the most at risk transformers, while still maintaining the continuous supply of power. Following training of control room staff, this strategy was declared operational in mid‐2023. Once disturbance levels reached the G5 threshold during the Gannon storm Transpower control room staff followed the planned procedure and implemented the mitigation strategy for the first time. There was no impact to New Zealand's electrical supply from this storm. GIC was measured at more than 70 transformers throughout the New Zealand power grid. Peak GICs observed after the mitigation strategy was in place reached 113 A at a transformer in the city of Dunedin. Without mitigation, modeling shows that a peak GIC of 200 A through the neutral would have occurred, exacerbated by the tripping of a transformer at a neighboring substation.

The important question of how much of the variability in the high latitude ionosphere is driven by atmospheric processes as opposed to space weather impacts remains unanswered. The EISCAT-3D radar provides a unique opportunity to probe this variability across multiple spatial and temporal scales. One of the key aims of the DRIIVE project (DRivers and Impacts of Ionospheric Variability with EISCAT-3D) is to determine the balance of energy input to the lower ionosphere and quantify the variability under different atmospheric and geomagnetic conditions. Here we present a preliminary study of the variability using historic data from the EISCAT UHF radar taken over the course of several years in the winter months. We identify wave like signatures that occur simultaneously with Travelling Ionospheric Disturbances (TID) as seen in coherent radar data (SuperDARN), alongside enhancements due to energetic precipitation. The magnitude of the variations are compared for different years and different driving conditions. This study will allow us to optimize the design of future experiments for EISCAT-3D to study the variability while developing effective analysis techniques to maximise utility of the new radar system.

AbstractLarge geomagnetic storms are a space weather hazard to power transmission networks due to the effects of Geomagnetically Induced Currents (GICs). GIC can negatively impact power transmission systems through the generation of even‐order current and voltage harmonics due to half‐cycle transformer saturation. This study investigates a decade of even‐order voltage total harmonic distortion (hereon referred to as Even‐Order Total Harmonic Distortion (ETHD)) observations provided by Transpower New Zealand Ltd., the national system operator. We make use of ETHD measurements at 139 locations throughout New Zealand, monitored at 377 separate circuit breakers, focusing on 10 large geomagnetic disturbances during the period 2013–2023. Analysis identified 5 key substations, which appeared to act as sources of ETHD. The majority of these substations include single phase transformer banks, and evidence of significant GIC magnitudes. The ETHD from the source substations was found to propagate into the surrounding network, with the percentage distortion typically decaying away over distances of 150–200 km locally, that is, at a rate of −0.0043 %km−1. During the study period some significant changes occurred in the power network, that is, removal of the Halfway Bush (HWB) single phase bank transformer T4 in November 2017, and decommissioning of the New Plymouth substation in December 2019. Decommissioning of these two assets resulted in less ETHD occurring in the surrounding regions during subsequent geomagnetic storms. However, ETHD still increased at HWB with increasing levels of GIC, indicating that three phase transformer units were still susceptible to saturation, albeit with about 1/3 of the ETHD percentage exhibited by single phase transformers.

High energy resolution DEMETER satellite observations from the Instrument for the Detection of Particle (IDP) are analyzed during an electromagnetic ion cyclotron (EMIC)-induced electron precipitation event. Analysis of an Interval Pulsation with Diminishing Periods (IPDP)-type EMIC wave event, using combined satellite observations to correct for incident proton contamination, detected an energy precipitation spectrum ranging from similar to 150 keV to similar to 1.5 MeV. While inconsistent with many theoretical predictions of >1 MeV EMIC-induced electron precipitation, the finding is consistent with an increasing number of experimentally observed events detected using lower resolution integral channel measurements on the POES, FIREBIRD, and ELFIN satellites. Revised and improved DEMETER differential energy fluxes, after correction for incident proton contamination shows that they agree to within 40% in peak flux magnitude, and 85 keV (within 40%) for the energy at which the peak occurred as calculated from POES integral channel electron precipitation measurements. This work shows that a subset of EMIC waves found close to the plasmapause, that is, IPDP-type rising tone events, can produce electron precipitation with peak energies substantially below 1 MeV. The rising tone features of IPDP EMIC waves, along with the association with the high cold plasma density regime, and the rapidly varying electron density gradients of the plasmapause may be an important factor in the generation of such low energy precipitation, co-incident with a high energy tail. Our work highlights the importance of undertaking proton contamination correction when using the high-resolution DEMETER particle measurements to investigate EMIC-driven electron precipitation.

AbstractThe October effect is known as a rapid and strong decrease in the signal amplitude of radio waves with very low frequency (VLF), reflected at the lowest edge of the ionosphere. This strong decrease can be observed only during the daytime. Although the October effect is long known, it is hardly investigated and its mechanism is still unknown. To get closer to a mechanism, we answer why the October effect does not occur during nighttime. Therefore, average characteristics of the October effect are obtained from different VLF transmitter‐receiver combinations. The occurrence of the October effect is then compared with characteristics of the neutral atmosphere temperature at VLF reflection heights as it seems to act as a proxy for the unknown mechanism. The temperature shows an asymmetric seasonal behavior at daytime VLF reflection heights poleward of 50°N but not during the nighttime, resulting in the October effect.

Abstract. Subionospheric Very Low Frequency (VLF) radio signals are reflected by free electrons in the ionospheric D-region at about 60–90 km altitude and can propagate over long distances, which makes them useful for monitoring the state of the D-region or perturbations due to solar flares. At the D-region height, the ionosphere is mainly ionized by the solar Lyman-α radiation. The reflection characteristics of VLF signals depend on the state and dynamics of the D-region which is highly influenced by the Lyman-α radiation. Although the amplitude of the received terrestrial VLF signal changes as a function of solar zenith angle over the course of the year, the VLF amplitude shows a distinctive sharp decrease around October, which is hence called the “October effect”. This study investigates the occurrence of the October effect and its dependencies on latitude and longitude. We developed a method to detect the occurrence of the October effect in the long-term VLF data and derive key parameters characterizing (start and end date, intensity) the sudden decrease in the signal amplitude. This investigation using a network of VLF stations distributed over low, middle and high latitude regions shows that the occurrence of the October effect has a clear latitudinal dependency, occurring earlier in high-latitude regions than at mid-latitudes. No low latitude signature is found.

Graphical Abstract

High energy electron precipitation from the Earth's radiation belts is important for loss from the radiation belts and atmospheric chemistry. We follow up investigations presented in Reidy et al. (2021, ) where precipitating flux is calculated inside the field of view of the POES T0 detector using quasi-linear theory and pitch angle diffusion coefficients (D-alpha alpha) from the British Antarctic Survey (BAS). These results showed good agreements at >30 keV for L* >5 on the dawnside but the flux were too low at higher energies. We have investigated the effect of changing parameters in the calculation of the precipitating flux to improve the results for the higher energies using comparisons of in situ flux and cold plasma measurements from GOES-15 and RBSP. We find that the strength of the diffusion coefficients rather than the shape of the source spectrum has the biggest effect on the calculated precipitation. In particular we find decreasing the cold plasma density used in the calculation of D-alpha alpha increases the diffusion and hence the precipitation at the loss cone for the higher energies, improving our results. The method of calculating D-alpha alpha is also examined, comparing co-located rather than averaged RBSP measurements. We find that the method itself has minimal effect but using RBSP derived D-alpha alpha improved our results over using D-alpha alpha calculated using the entire BAS wave data base; this is potentially due to better measurements of the cold plasma density from RBSP than the other spacecraft included in the BAS wave data base (e.g., THEMIS).

From a distance, we perceive the Sun as an unchanging source of light. While this is functionally true on a day-to-day human level, at a scientific level, the Sun has many complexities. It interacts with and 'forces' the Earth's system in multiple ways.

The amplitude of Very Low Frequency (VLF) transmissions propagating from transmitter to receiver between the Earth’s surface and the ionospheric D-region is a useful measurement to detect changes in the ionization within the D-region ranging from 60-90 km. The VLF signal amplitude is disturbed by geomagnetic, solar, and atmospheric phenomena. To be able to identify perturbations in the VLF signal amplitude, we determine its averaged seasonal variation under quiet solar and geomagnetic conditions. Here it is challenging, that long time series of the VLF signal amplitude show significant jumps and outliers, which are caused artificially by technical adjustments/maintenance work.This paper presents a new approach for processing long VLF data time series over multiple years resulting in level 2 data. The new level 2 data enables the consideration of time series with artificial jumps since the jumps are leveled. Moreover, the outliers are removed by a robust and systematic 2-step outlier filtering.The average seasonal and diurnal variation for different transmitter-receiver combinations can be computed with the new level 2 data by applying a composite analysis. A subsequently applied polynomial fit obtains the quiet time lines for daytime and nighttime, representing the typical seasonal variation under undisturbed conditions of the VLF signal amplitude for each considered link.The developed quiet time lines may serve as a tool to determine perturbations of the VLF signal amplitude with solar and geomagnetic as well as atmospheric origin. Also, they allow comparison of the VLF signal amplitude variation for different transmitter-receiver links.

Abstract Reducing the impact of Geomagnetically induced currents (GICs) on electrical power networks is an essential step to protect network assets and maintain reliable power transmission during and after storm events. In this study, multiple mitigation strategies are tested during worst‐case extreme storm scenarios in order to investigate their effectiveness for the New Zealand transmission network. By working directly with our industry partners, Transpower New Zealand Ltd, a mitigation strategy in the form of targeted line disconnections has been developed. This mitigation strategy proved more effective than previous strategies at reducing GIC magnitudes and durations at transformers at most risk to GIC while still maintaining the continuous supply of power throughout New Zealand. Under this mitigation plan, the average 60‐min mean GIC decreased for 27 of the top 30 at‐risk transformers, and the total network GIC was reduced by 16%. This updated mitigation has been adopted as an operational procedure in the New Zealand national control room to manage GIC. In addition, simulations show that the installation of 14 capacitor blocking devices at specific transformers reduces the total GIC sum in the network by an additional 16%. As a result of this study Transpower is considering further mitigation in the form of capacitor blockers. We strongly recommend collaborating with the relevant power network providers to develop effective mitigation strategies that reduce GIC and have a minimal impact on power distribution.

Radar waves with very low frequency (VLF) are reflected at the lower edge of the ionosphere, in the D-region. The D-region (60 - 90km) is influenced by the solar zenith angle and space weather from above as well as by dynamical and chemical processes in the mesosphere. During October there is a well-known sharp decrease of the daytime VLF amplitude between transmitter and receiver combinations whose great circle paths lie mainly in polar latitudes. Until now we do not know what causes the October effect. Space weather phenomena can be ruled out as a cause since their time scales are either too short or too long. The solar zenith angle, strongly influencing the seasonal variation of the VLF amplitude can also be ruled out as a similar behavior is not observed in spring. Thus, there is a strong assumption that neutral dynamical processes in the mesosphere play a major role. We assume and confirm that a regional warming in the lower mesosphere, occurring simultaneously and with similar characteristics as the October effect, plays a major role in the formation process of the October effect. The VLF reflection height is about 15km higher during nighttime than during daytime. This difference in combination with the location of the regional warming explains, why the October effect can not be observed during nighttime.

A new model to predict the electron density and effective recombination coefficient of the lower ionosphere under solar flare conditions is presented. This model relies on space-borne solar irradiance measurements in coincidence with ground recorded active transmissions of Very Low Frequency (VLF), (<30 kHz) signals. Use is made of the irradiance measured by broad-band radiometers onboard the satellites: GOES, SDO, and PROBA2. Measurements are made over succeeding and partly overlapping wavelength intervals of the instrument band-pass ranges altogether covering the range 0.1-20 nm. The aim is to determine the effectiveness of the particular instrument bandpass in producing changes in the ionization of the lower ionosphere (D-region) during solar X-ray flares. Ionization efficiency is evaluated using modelled Solar Spectral Irradiance for each flare separately and for each instrument as a function of its bandpass.The new model is based on coupling of the continuity equation with the Appleton relation and uses the concept of time delay - the time lag of the extreme VLF amplitude and phase behind the flare irradiance maximum. The solution of the continuity equation predicts the electron density time -height profile for 55-100 km altitude.An analysis of M to X class flares shows the flare-enhanced electron densities due to a particular ionizing wavelength domain are in good agreement for the case where irradiance is taken over the bandpass of (1) either GOES (0.1-0.8 nm) or SDO/ESP (0.1-7 nm) for up to 90 km (2) either SDO/ESP or PROBA2/LYRA (1-2 +6-20 nm) at heights above 90 km. The results agree within 22% for heights up to 90 km, and differ by at most a factor of 2 for heights above 90 km. Remarkable agreement is shown between measured and evaluated time delay; discrepancies are generally less than 8%. The effective recombination coefficient is deduced from the model itself and is found to be consistent with other independent estimates.

Sudden Stratospheric Warmings (SSW) and Elevated Stratopause (ES) events are atmospheric wave driven winter phenomena, which lead to significant changes in atmospheric dynamics and temperatures. SSWs are characterized by a sudden warming in the stratosphere by up to 90K and a mesospheric cooling by up to 30K. At the same time the background wind decelerates and can even revers which modifies the vertical mass transport. Occasionally SSW are followed by an ES where the stratopause at 50-60 km vanishes and subsequently reforms in elevated altitude ranges of 70-85 km. This leads to a temperature increase of up to 50 K in mesospheric heights. The temperature increase during an ES is accompanied by strongly enhanced positive zonal winds and a downward directed mass transport, which leads to changes in neutral chemistry. Very low frequency (VLF) signals transmission, which is used for long distance communication, is generally conducted from a transmitter station to a receiver station within the so-called wave guide. This is the region between the Earth surface and the bottom side of the ionosphere (~60-90 km), which is behaving as a reflection boundary. Any changes in D-region ionization are able to modify the propagation of the VLF signal.The above described significant changes in wind, temperature and neutral composition during SSW/ES events occur within the VLF reflection heights and likely influence the VLF propagation.For the identification of SSW/ES induced perturbations of the VLF signal we need to remove the typical seasonal variation and outliers caused by noise, technical adjustments or solar events. For this purpose, a quiet time curve is required, which represents the seasonal VLF signal variation under undisturbed conditions, for each link respectively. We developed the quiet time winter curve with a polynomial fit of the wintertime composite. In preparation for the composite, the VLF data needed to be leveled due to artificial amplitude steps with technical origin in the timeseries. The leveling was done with help of the Pruned Exact Linear Time method. Additionally, outliers have been removed using the Median Absolute Deviation, a method from robust statistics. The developed quiet time winter curve allows us to determine VLF signal perturbations, which we analyze to examine the impact of SSW/ES events on the VLF signal. Furthermore, by studying different links in high latitudes, we want to investigate if there occur longitudinally differences in the VLF signal perturbation as the ES events vary strongly with longitude.