Relativistic microburst storm characteristics: Combined satellite and ground-based observations

We report a comparison of Solar Anomalous Magnetospheric Particle Explorer detected relativistic electron microbursts and short-lived subionospheric VLF perturbations termed FAST events, observed at Sodankyl Geophysical Observatory, Finland, during 2005. We show that only strong geomagnetic disturbances can produce FAST events, which is consistent with the strong link between storms and relativistic microbursts. Further, the observed FAST event perturbation decay times were consistent with ionospheric recovery from bursts of relativistic electron precipitation. However, the one-to-one correlation in time between microbursts and FAST events was found to be very low (similar to 1%). We interpret this as confirmation that microbursts have small ionospheric footprints and estimate the individual precipitation events to be <4 km radius. In contrast, our study strongly suggests that the region over which microbursts occur during storm event periods can be at least similar to 90 degrees in longitude (similar to 6 h in magnetic local time). This confirms earlier estimates of microburst storm size, suggesting that microbursts could be a significant loss mechanism for radiation belt relativistic electrons during geomagnetic storms. Although microbursts are observed at a much higher rate than FAST events, the ground-based FAST event data can provide additional insight into the conditions required for microburst generation and the time variation of relativistic precipitation

Nature's Grand Experiment: Linkage between magnetospheric convection and the radiation belts

The solar minimum of 2007–2010 was unusually deep and long lived. In the later stages of this period the electron fluxes in the radiation belts dropped to extremely low levels. The flux of relativistic electrons (>1 MeV) was significantly diminished and at times was below instrument thresholds both for spacecraft located in geostationary orbits and also those in low-Earth orbit. This period has been described as a natural “Grand Experiment” allowing us to test our understanding of basic radiation belt physics and in particular the acceleration mechanisms which lead to enhancements in outer belt relativistic electron fluxes. Here we test the hypothesis that processes which initiate repetitive substorm onsets drive magnetospheric convection, which in turn triggers enhancement in whistler mode chorus that accelerates radiation belt electrons to relativistic energies. Conversely, individual substorms would not be associated with radiation belt acceleration. Contrasting observations from multiple satellites of energetic and relativistic electrons with substorm event lists, as well as chorus measurements, show that the data are consistent with the hypothesis. We show that repetitive substorms are associated with enhancements in the flux of energetic and relativistic electrons and enhanced whistler mode wave intensities. The enhancement in chorus wave power starts slightly before the repetitive substorm epoch onset. During the 2009/2010 period the only relativistic electron flux enhancements that occurred were preceded by repeated substorm onsets, consistent with enhanced magnetospheric convection as a trigger

POES satellite observations of EMIC-wave driven relativistic electron precipitation during 1998-2010

[1] Using six Polar Orbiting Environmental Satellites (POES) satellites that have carried the Space Environment Module-2 instrument package, a total of 436,422 individual half-orbits between 1998 and 2010 were inspected by an automatic detection algorithm searching for electromagnetic ion cyclotron (EMIC) driven relativistic electron precipitation (REP). The algorithm searched for one of the key characteristics of EMIC-driven REP, identified as the simultaneity between spikes in the P1 (52 keV differential proton flux channel) and P6 (>800 keV electron channel). In all, 2331 proton precipitation associated REP (PPAREP) events were identified. The majority of events were observed at L-values within the outer radiation belt (3 < L < 7) and were more common in the dusk and night sectors as determined by magnetic local time. The majority of events occurred outside the plasmasphere, at L-values ~1 Re greater than the plasmapause location determined from two different statistical models. The events make up a subset of EMIC-driven proton spikes investigated by Sandanger et al. (2009), and potentially reflect different overall characteristics compared with proton spikes, particularly when comparing their location to that of the plasmapause, i.e., EMIC-driven proton precipitation inside the plasmapause, and potentially EMIC-driven REP outside the plasmapause. There was no clear relationship between the location of plasmaspheric plumes and the locations of the PPAREP events detected. Analysis of the PPAREP event occurrence indicates that high solar wind speed and high geomagnetic activity levels increase the likelihood of an event being detected. The peak PPAREP event occurrence was during the declining phase of solar cycle 23, consistent with the 2003 maximum in the geomagnetic activity index, Ap

Very low latitude whistler‐mode signals: Observations at three widely spaced latitudes

VLF radio signals with travel times ~100 ms were observed continuously for up to ~11 hours at night on Rarotonga (Cook Islands, ~21°S) at 21.4 kHz from US Navy transmitter NPM, Hawaii (~21°N). These signals travelled in the whistler‐mode on well‐defined paths, though not field‐aligned ducts, through the ionospheric F region, and across the equator reaching altitudes ~700‐1400 km depending on time of night. These same signals were also observed simultaneously in Dunedin (46°S), New Zealand, with very nearly the same travel times but with somewhat lower amplitudes and occurrence rates, consistent with the whistler‐mode part of the propagation being at very low latitudes. Both sets of signals had similar Doppler shifts, typically tens of mHz, but sometimes up to a few hundred mHz, being positive during most of the night, while the whistler‐mode group delays decreased due to both the shortening of the path and the decay of the near equatorial ionosphere, but negative near dawn when the Sun's rays start ionizing the F region. The signals are not observable during the day, fading out during dawn, due to increasing attenuation from the increasing electron density, and hence increasing collisions, in both the D and F regions. Similar weaker NPM signals were also seen at Rothera (68°S). In addition, similar 24.8 kHz signals were seen from the more distant NLK (Seattle, ~48°N) at Rarotonga, though clearly weaker than from NPM, but not at Dunedin

Substorm induced energetic electron precipitation:morphology and prediction

The injection, and subsequent precipitation, of 20 to 300 keV electrons during substorms is modeled using parameters of a typical substorm found in the literature. When combined with onset timing from, for example, the SuperMAG substorm database, or the Minimal Substorm Model, it may be used to calculate substorm contributions to energetic electron precipitation in atmospheric chemistry and climate models. Here the results are compared to ground-based data from the Imaging Riometer for Ionospheric Studies riometer in Kilpisjärvi, Finland, and the narrowband subionospheric VLF receiver at Sodankylä, Finland. Qualitatively, the model reproduces the observations well when only onset timing from the SuperMAG network of magnetometers is used as an input and is capable of reproducing all four categories of substorm associated riometer spike events. The results suggest that the different types of spike event are the same phenomena observed at different locations, with each type emerging from the model results at a different local time, relative to the center of the injection region. The model's ability to reproduce the morphology of spike events more accurately than previous models is attributed to the injection of energetic electrons being concentrated specifically in the regions undergoing dipolarization, instead of uniformly across a single-injection region

Techniques to determine the quiet day curve for a long period of subionospheric VLF observations

Very low frequency (VLF) transmissions propagating between the conducting Earth's surface and lower edge of the ionosphere have been used for decades to study the effect of space weather events on the upper atmosphere. The VLF response to these events can only be quantified by comparison of the observed signal to the estimated quiet time or undisturbed signal levels, known as the quiet day curve (QDC). A common QDC calculation approach for periods of investigation of up to several weeks is to use observations made on quiet days close to the days of interest. This approach is invalid when conditions are not quiet around the days of interest. Longer-term QDCs have also been created from specifically identified quiet days within the period and knowledge of propagation characteristics. This approach is time consuming and can be subjective. We present three algorithmic techniques, which are based on either (1) a mean of previous days' observations, (2) principal component analysis, or (3) the fast Fourier transform (FFT), to calculate the QDC for a long-period VLF data set without identification of specific quiet days as a basis. We demonstrate the effectiveness of the techniques at identifying the true QDCs of synthetic data sets created to mimic patterns seen in actual VLF data including responses to space weather events. We find that the most successful technique is to use a smoothing method, developed within the study, on the data set and then use the developed FFT algorithm. This technique is then applied to multiyear data sets of actual VLF observations

The effects and correction of the geometric factor for the POES/MEPED electron flux instrument using a multisatellite comparison

Measurements from the Polar-Orbiting Environmental Satellite (POES) Medium Energy Proton and Electron Detector (MEPED) instrument are widely used in studies into radiation belt dynamics and atmospheric coupling. However, this instrument has been shown to have a complex energy-dependent response to incident particle fluxes, with the additional possibility of low-energy protons contaminating the electron fluxes. We test the recent Monte Carlo theoretical simulation of the instrument by comparing the responses against observations from an independent experimental data set. Our study examines the reported geometric factors for the MEPED electron flux instrument against the high-energy resolution Instrument for Detecting Particles (IDPs) on the Detection of Electromagnetic Emissions Transmitted from Earthquake Regions satellite when they are located at similar locations and times, thereby viewing the same quasi-trapped population of electrons. We find that the new Monte Carlo-produced geometric factors accurately describe the response of the POES MEPED instrument. We go on to develop a set of equations such that integral electron fluxes of a higher accuracy are obtained from the existing MEPED observations. These new MEPED integral fluxes correlated very well with those from the IDP instrument (>99.9% confidence level). As part of this study we have also tested a commonly used algorithm for removing proton contamination from MEPED instrument observations. We show that the algorithm is effective, providing confirmation that previous work using this correction method is valid

Evidence of sub-MeV EMIC-driven electron precipitation

Electromagnetic ion cyclotron (EMIC) waves are potentially important drivers of the loss of energetic electrons from the radiation belts. Numerous theoretical calculations exist with conflicting predictions of one of the key parameters: the minimum resonance energy of electrons precipitated into the atmosphere by EMIC waves. In this study we initially analyze an EMIC electron precipitation event using data from two different spacecraft instruments to investigate the energies involved. Combining observations from these satellites, we find that the electron precipitation has a peak flux at ∼250 keV. Extending the analysis technique to a previously published database of similar scattering events, we find that the peak electron precipitation flux occurs predominantly around 300 keV, with only ∼11% of events peaking in the 1–4 MeV range. Such a significant population of low-energy EMIC-driven electron precipitation events highlights the possibility for EMIC waves to be significant drivers of radiation belt electron losses

Observed Loss of Polar Mesospheric Ozone Following Substorm-Driven Electron Precipitation

Several drivers cause precipitation of energetic electrons into the atmosphere. While some of these drivers are accounted for in proxies of energetic electron precipitation (EEP) used in atmosphere and climate models, it is unclear to what extent the proxies capture substorm‐induced EEP. The energies of these electrons allow them to reach altitudes between 55 and 95 km. EEP‐driven enhanced ionization is known to result in production of HOx and NOx, which catalytically destroy ozone. Substorm‐driven ozone loss has previously been simulated, but has not been observed before. We use mesospheric ozone observations from the Microwave Limb Sounder and Global Ozone Monitoring by Occultation of Stars instruments, to investigate the loss of ozone during substorms. Following substorm onset, we find reductions of polar mesospheric (∼76 km) ozone by up to 21% on average. This is the first observational evidence demonstrating the importance of substorms on the ozone balance within the polar atmosphere