Investigating energetic electron precipitation through combining ground-based and balloon observations

A detailed comparison is undertaken of the energetic electron spectra and fluxes of two precipitation events that were observed in 18/19 January 2013. A novel but powerful technique of combining simultaneous ground-based subionospheric radio wave data and riometer absorption measurements with X-ray fluxes from a Balloon Array for Relativistic Radiation-belt Electron Losses (BARREL) balloon is used for the first time as an example of the analysis procedure. The two precipitation events are observed by all three instruments, and the relative timing is used to provide information/insight into the spatial extent and evolution of the precipitation regions. The two regions were found to be moving westward with drift periods of 5–11 h and with longitudinal dimensions of ~20° and ~70° (1.5–3.5 h of magnetic local time). The electron precipitation spectra during the events can be best represented by a peaked energy spectrum, with the peak in flux occurring at ~1–1.2 MeV. This suggests that the radiation belt loss mechanism occurring is an energy-selective process, rather than one that precipitates the ambient trapped population. The motion, size, and energy spectra of the patches are consistent with electromagnetic ion cyclotron-induced electron precipitation driven by injected 10–100 keV protons. Radio wave modeling calculations applying the balloon-based fluxes were used for the first time and successfully reproduced the ground-based subionospheric radio wave and riometer observations, thus finding strong agreement between the observations and the BARREL measurements

A case study of electron precipitation fluxes due to plasmaspheric hiss

We find that during a large geomagnetic storm in October 2011 the trapped fluxes of >30, >100, and >300 keV outer radiation belt electrons were enhanced at L=3-4 during the storm main phase. A gradual decay of the trapped fluxes was observed over the following 5–7 days, even though no significant precipitation fluxes could be observed in the Polar Orbiting Environmental Satellite (POES) electron precipitation detectors. We use the Antarctic-Arctic Radiation-belt (Dynamic) Deposition - VLF Atmospheric Research Konsortium (AARDDVARK) receiver network to investigate the characteristics of the electron precipitation throughout the storm period. Weak electron precipitation was observed on the dayside for 5–7 days, consistent with being driven by plasmaspheric hiss. Using a previously published plasmaspheric hiss-induced electron energy e-folding spectrum of E0=365 keV, the observed radiowave perturbation levels at L=3-4 were found to be caused by >30 keV electron precipitation with flux ~100 el. cm−2 s−1 sr−1. The low levels of precipitation explain the lack of response of the POES telescopes to the flux, because of the effect of the POES lower sensitivity limit and ability to measure weak diffusion-driven precipitation. The detection of dayside, inner plasmasphere electron precipitation during the recovery phase of the storm is consistent with plasmaspheric hiss wave-particle interactions, and shows that the waves can be a significant influence on the evolution of the outer radiation belt trapped flux that resides inside the plasmapause

Geomagnetically Induced Currents and Harmonic Distortion: High time Resolution Case Studies

High time resolution (1‐5 s) magnetometer, geomagnetically induced current (GIC), and mains harmonic distortion data from the Halfway Bush substation in Dunedin, New Zealand are analyzed. A recently developed technique using VLF radio wave data provides high resolution measurements of mains harmonic distortion levels. Three case studies are investigated, each involving high rates of change of local geomagnetic field, but with different timescales of magnetospheric driver mechanisms, and different substation transformer configurations. Two cases of enhanced GIC during substorm events are analyzed, and one case of a storm sudden commencement. Time delays between magnetic field fluctuations and induced transformer currents are found to be ~100 s for substorm events, but only ~20 s for the storm sudden commencement containing higher frequency variations. Boxcar averaging of the magnetic field fluctuations using running windows of ± 2 minutes leads to spectral power profiles similar to those of GIC profiles, with reduced power at frequencies >0.003 Hz (periods 5 minutes). This low frequency component of the magnetic field power spectrum appears necessary for mains harmonic distortion to occur

Long-lasting geomagnetically induced currents and harmonic distortion observed in New Zealand during the 07-08 September 2017 Disturbed Period

Several periods of Geomagnetically Induced Currents (GIC) were detected in the Halfway Bush substation in Dunedin, South Island, New Zealand, as a result of intense geomagnetic storm activity during 06 to 09 September 2017. Unprecedented data coverage from a unique combination of instrumentation is analyzed, i.e., measurements of GIC on the single phase bank transformer T4 located within the substation, nearby magnetic field perturbation measurements, very low frequency (VLF) wideband measurements detecting the presence of power system harmonics, and high‐voltage harmonic distortion measurements. Two solar wind shocks occurred within 25 hours, generating four distinct periods of GIC. Two of the GIC events were associated with the arrival of the shocks themselves. These generated large but short‐lived GIC effects that resulted in no observable harmonic generation. Nearby and more distant magnetometers showed good agreement in measuring these global‐scale magnetic field perturbations. However, two subsequent longer‐lasting GIC periods, up to 30 minutes in duration, generated harmonics detected by the VLF receiver systems, when GIC levels continuously exceeded 15 A in T4. Nearby and more distant magnetometers showed differences in their measurements of the magnetic field perturbations at these times, suggesting the influence of small‐scale ionospheric current structures close to Dunedin. VLF receiver systems picked up harmonics from the substation, up to the 30th harmonic, consistent with observed high‐voltage increases in even harmonic distortion, along with small decreases in odd harmonic distortion

Geomagnetically induced currents during the 07-08 September 2017 disturbed period: a global perspective

Measurements from six longitudinally separated magnetic observatories, all located close to the 53⁰ mid-latitude contour, are analysed. We focus on the large geomagnetic 16 disturbance that occurred during 7 and 8 September 2017. Combined with available geomagnetically induced current (GIC) data from two substations, each located near to a 18 magnetic observatory, we investigate the magnetospheric drivers of the largest events. We analyse solar wind parameters combined with auroral electrojet indices to investigate the driving mechanisms. Six magnetic field disturbance events were observed at mid-latitudes with dH/dt >60 nT/min. Co-located GIC measurements identified transformer currents >15 A during three of the events. The initial event was caused by a solar wind pressure pulse causing largest effects on the dayside, consistent with the rapid compression of the dayside geomagnetic field. Four of the events were caused by substorms. Variations in the Magnetic Local Time of the maximum effect of each substorm-driven event were apparent with magnetic midnight, morning-side, and dusk-side events all occurring. The six events 27 occurred over a period of almost 24 hours, during which the solar wind remained elevated at >700 km s -1, indicating an extended time scale for potential GIC problems in electrical power networks following a sudden storm commencement. This work demonstrates the challenge of understanding the causes of ground-level magnetic field changes (and hence GIC magnitudes) for the global power industry. It also demonstrates the importance of 32 magnetic local time and differing inner magnetospheric processes when considering the global hazard posed by GIC to power grids

High-resolution in situ observations of electron precipitation-causing EMIC waves

Electromagnetic ion cyclotron (EMIC) waves are thought to be important drivers of energetic electron losses from the outer radiation belt through precipitation into the atmosphere. While the theoretical possibility of pitch angle scattering-driven losses from these waves has been recognized for more than four decades, there have been limited experimental precipitation observations to support this concept. We have combined satellite-based observations of the characteristics of EMIC waves, with satellite and ground-based observations of the EMIC-induced electron precipitation. In a detailed case study, supplemented by an additional four examples, we are able to constrain for the first time the location, size, and energy range of EMIC-induced electron precipitation inferred from coincident precipitation data and relate them to the EMIC wave frequency, wave power, and ion band of the wave as measured in situ by the Van Allen Probes. These observations will better constrain modeling into the importance of EMIC wave-particle interactions

Developing a Nowcasting Capability for X-Class Solar Flares Using VLF Radiowave Propagation Changes

A technique for analyzing very low frequency (VLF) radiowave signals is investigated in order to achieve rapid, real-time detection of large solar flares, through the monitoring of changes in VLF radio signal propagation conditions. The reliability of the use of VLF phase and amplitude perturbations to determine the X-ray fluxes involved during 10 large solar flare events (>X1) is examined. Linear regression analysis of signals from the NPM transmitter in Hawaii, received at Arrival Heights, Scott Base, Antarctica, over the years 2011-2015 shows that VLF phase perturbations during large solar flares have a 1.5-3 times lower mean square error when modeling the long wavelength X-ray fluxes than the equivalent short wavelength fluxes. The use of VLF amplitude observations to determine long or short wavelength X-ray flux levels have a 4-10 times higher mean square error than when using VLF phase. Normalized linear regression analysis identifies VLF phase as the most important parameter in the regression, followed by solar zenith angle at the midpoint of the propagation path, then the initial solar X-ray flux level (from 5 min before the impact of the solar flare), with F10.7 cm flux from the day beforehand providing the least important contribution. Transmitter phase measurements are more difficult to undertake than amplitude. However, networks of VLF receivers already exist which include the high quality phase capability required for such a nowcasting product. Such narrowband VLF data can be a redundant source of flare monitoring if satellite data is not available.Peer reviewe

Solar flare X‐ray impacts on long subionospheric VLF paths

Solar flares increase the electron number concentration in the day-time ionosphere, potentially affecting radiowave propagation over several frequency ranges. In this study, we use ionospheric observations to determine both peak magnitudes and time variations of solar flare X-rays without using the direct measurement from the flare.. Ground-based observations of VLF transmitter phase perturbations are compared against measured X-ray flux levels during solar flares. Flare fluxes derived here from VLF phases on a west-east subionospheric path are compared with those from a previously analyzed north-south path. Using a wider selection of solar flares, including M-class flares for the first time, the best fit equations and root mean square (RMS) errors are computed with improved standard deviation (SD) uncertainty estimates for the peak fluxes. Good agreement is found between peak long X-ray wavelength fluxes (XL, 0.1-0.8 nm) derived for M- and X- class flares and those measured by the GOES satellites. Linear regression analysis on the two paths shows the uncertainties increase in inverse proportion to the path length. Investigations were made with a limited set of ‘operational’ parameters that could be used to derive XL fluxes. No increases in RMS or SD uncertainty levels were introduced by the removal of satellite-based regression parameters such as, the XL flux level measured prior to the flare onset. As such, these techniques support the idea of nowcasting M- and X-class flares from entirely ground-based measurements

Geomagnetically Induced Currents and Harmonic Distortion: Storm‐time Observations from New Zealand

Large geomagnetic storms are a known space weather hazard to power transmission networks due to the effects of Geomagnetically Induced Currents (GICs). However, research in this area has been hampered by a lack of GIC observations. Previous studies have noted that New Zealand is unusually fortunate in having a comparatively dense, high quality, set of GIC measurements, spanning >60 transformers in >20 substations. However, due to operational reasons these observations are clustered in the mid and lower South Island. In this paper we analyze space weather‐induced GIC impact patterns over the entire country by using a different set of sensors that monitor levels of harmonic distortion, with even and odd harmonics measured separately. GICs lead to half cycle transformer saturation and is one of the few ways in which even harmonics are produced in a well run power transmission network. We make use of harmonic distortion measurements at 377 circuit breakers made at 126 separate locations. Focusing on the intense geomagnetic storm activity during 06 to 09 September 2017, we show how the even harmonic distortion observations provide a useful new picture of GIC‐stressed transformers. These observations demonstrate how GIC effects can be monitored by using even harmonic distortion in locations where no GIC measurements are present (for example, the most of the North Island). We understand harmonic distortion measurements are fairly common in electrical networks and could provide a new tool for Space Weather researchers