179 research outputs found
Determining the size of lightning-induced electron precipitation patches
No full text[1] We analyze Trimpi signatures during 23 and 24 April 1994 at four sites on or near the Antarctic Peninsula (Palmer, Faraday, Rothera, and Halley) on subionospheric VLF signals received from four U. S. naval transmitters (NAA, NSS, NLK, and NPM). Electron precipitation patches are found to be large, i.e., similar to1500 km x 600 km, with the longer axis orientated east-west. Calculations using a three-dimensional Born scattering model, where patch densities are 1.5 electrons cm(-3) above ambient at the center at similar to84 km altitude, provides results that are consistent with this picture. A high proportion (38%) of the Trimpi events were associated with strong lightning flashes in eastern United States. When lightning discharges had currents >65 kA (positive or negative), there was a >80% chance of seeing an associated Trimpi event. The chance of seeing any Trimpi events fell to near zero for discharges of <45 kA. The largest Trimpi perturbations occur when the center of the precipitation patch is 700-800 km from the receivers. This result is consistent with the modeling calculations for large patches. The equatorward edge of the precipitation patch was estimated to be at &SIM;60&DEG;S, close to the magnetic conjugate of the lightning. The close association of the equatorward edge of the precipitation patch with the conjugate location of the causative lightning is consistent with a quasi-ducted whistler-induced precipitation mechanism. Nonducted whistler-induced precipitation mechanisms would predict a 5&DEG;-10&DEG; latitudinal gap between the lightning and the equatorward edge of the patch. However, the lack of observed whistlers at the time of the Trimpi events is consistent with the nonducted whistler mechanism and is not consistent with the quasi-ducted mechanism, although the distances from duct exit point to receiver may have been too large (&SIM;700-1000 km) for the signals to be detectable. Using the significantly larger patch dimensions determined in this study, it is estimated that lightning may well be 10-100 times more effective at depleting the radiation belts than hiss
Energetic electron precipitation during substorm injection events: High-latitude fluxes and an unexpected midlatitude signature
No full textGeosynchronous Los Alamos National Laboratory (LANL-97A) satellite particle data, riometer data, and radio wave data recorded at high geomagnetic latitudes in the region south of Australia and New Zealand are used to perform the first complete modeling study of the effect of substorm electron precipitation fluxes on low-frequency radio wave propagation conditions associated with dispersionless substorm injection events. We find that the precipitated electron energy spectrum is consistent with an e-folding energy of 50 keV for energies <400 keV but also contains higher fluxes of electrons from 400 to 2000 keV. To reproduce the peak subionospheric radio wave absorption signatures seen at Casey (Australian Antarctic Division), and the peak riometer absorption observed at Macquarie Island, requires the precipitation of 50–90% of the peak fluxes observed by LANL-97A. Additionally, there is a concurrent and previously unreported substorm signature at L < 2.8, observed as a substorm-associated phase advance on radio waves propagating between Australia and New Zealand. Two mechanisms are discussed to explain the phase advances. We find that the most likely mechanism is the triggering of wave-induced electron precipitation caused by waves enhanced in the plasmasphere during the substorm and that either plasmaspheric hiss waves or electromagnetic ion cyclotron waves are a potential source capable of precipitating the type of high-energy electron spectrum required. However, the presence of these waves at such low L shells has not been confirmed in this study
Correction to 'Radiation belt electron precipitation into the atmosphere: recovery from a geomagnetic storm'
No full textInternational audienceCorrection to “Radiation belt electron precipitation intothe atmosphere: Recovery from a geomagnetic storm”Craig J. Rodger, Mark A. Clilverd, Neil R. Thomson, Rory J. Gamble, Annika Seppälä,Esa Turunen, Nigel P. Meredith, Michel Parrot, Jean‐André Sauvaud,and Jean‐Jacques BerthelierReceived 17 August 2010; accepted 19 August 2010; published 25 September 2010.Citation: Rodger, C. J., M. A. Clilverd, N. R. Thomson, R. J. Gamble, A. Seppälä, E. Turunen, N. P. Meredith, M. Parrot, J.‐A.Sauvaud, and J.‐J. Berthelier (2010), Correction to “Radiation belt electron precipitation into the atmosphere: Recovery from ageomagnetic storm,” J. Geophys. Res., 115, A09324, doi:10.1029/2010JA016038
The Impact of PMSE on VLF Propagation
No full textABSTRACT PMSE or Polar Mesosphere Summer Echoes are a well known phenomenon in the summer northern polar regions, in which anomalous VHF/UHF radar echoes are returned from heights ~85 km. Associated phenomena at these altitudes are noctilucent clouds and electron density biteouts. The latter are electron density depletion layers of up to 90%, which may be several kms thick. Using the NOSC Modefndr code based on Wait's modal theory for subionospheric propagation, we calculate the shifts in received VLF amplitude and phase that occur as a result of electron density biteouts. The code assumes a homogeneous background ionosphere and a homogeneous biteout layer along the Great Circle Path (GCP) corridor, for transmitter receiver path lengths in the range 500-6000 km. For profiles during the 10hrs about midnight and under quiet geomagnetic conditions, where the electron density at 85 km would normally be less than 500 el/cc, it was found that received signal perturbations were significant, of the order of 1-4 dB and 5-40 degrees of phase. Perturbation amplitudes increase roughly as the square root of frequency. At short range perturbations are rather erratic, but more consistent at large ranges, readily interpretable in terms of the shifts in excitation factor, attenuation factor and v/c ratios for Wait's modes. Under these conditions such shifts should be detectable by a well constituted experiment involving multiple paths and multiple frequencies in the north polar region in summer. It is anticipated that VLF propagation could be a valuable diagnostic for biteout/PMSE when electron density at 85 km is under 500 el/cc, under which circumstances PMSE are not directly detectable by VHF/UHF radars. Keywords: 0689 Wave propagation, 2475 Polar ionosphere, 6934 Ionospheric propagation
High-latitude geomagnetically induced current events observed on very low frequency radio wave receiver systems
No full textNoise burst events observed at Sodankylä, Finland, in the frequency range 20–25 kHz during January–April 2005 last up to 4 s, occur more often at midnight, are associated with high geomagnetic activity, and exhibit a quasi-constant amplitude perturbation ∼15 dB above the background noise levels. We considered the possibility that the events could be caused by lightning noise breakthrough. The association of the noise burst events with local midnight and high geomagnetic activity argues against a lightning link, as well as the lack of close thunderstorm location relative to Sodankylä during noise periods. While energetic electron precipitation is also associated with high geomagnetic activity, we showed that they occur at different times and exhibit significantly different amplitude characteristics. Finally, we compared in detail the geomagnetic induced current (GIC) in the Scottish power system in southern Scotland, during a storm event that occurred on 15 May 2005, with the noise burst event rate at Sodankylä. We found that the onset time and variability of the Scottish GIC activity was well matched by the variability in the noise burst event rate, particularly the high-frequency component of the GIC fluctuations. The technique used in our study of observing at a narrow band of frequencies allows GIC measurements to be made in built-up areas where mains interference is a problem for other experiments, such as magnetometers
The impact of PMSE and NLC particles on VLF propagation
No full textPMSE or Polar Mesosphere Summer Echoes are a well-known phenomenon in the summer northern polar regions, in which anomalous VHF/UHF radar echoes are returned from heights similar to85 km. Noctilucent clouds and electron density biteouts are two phenomena that sometimes occur together with PMSE. Electron density biteouts are electron density depletion layers of up to 90%, which may be several kms thick. Using the NOSC Modefndr code based on Wait's modal theory for subionospheric propagation, we calculate the shifts in received VLF amplitude and phase that occur as a result of electron density biteouts. The code assumes a homogeneous background ionosphere and a homo-geneous biteout layer along the Great Circle Path (GCP) corridor, for transmitter receiver path lengths in the range of 500-6000 km.
For profiles during the 10 h about midnight and under quiet 0 Geornagnetic conditions, where the electron density at 85 km would normally be less than 500el/cc, it was found that received signal perturbations were significant, of the order of 1-4 dB and 5-40degrees of phase. Perturbation amplitudes increase roughly as the square root of frequency. At short range perturbations are rather erratic, but more consistent at large ranges, readily interpretable in terms of the shifts in excitation factor, attenuation factor and v/c ratios for Wait's modes. Under these conditions such shifts should be detectable by a well constituted experiment involving multiple paths and multiple frequencies in the north polar region in summer. It is anticipated that VLF propagation could be a valuable diagnostic for biteout/PMSE when electron density at 85 km is under 500 el/cc, under which circumstances PMSE are not directly detectable by VHF/UHF radars
The plasmasphere during a space weather event: first results from the PLASMON project
No full textThe results of the first 18 months of the PLASMON project are presented. We have extended our three, existing ground-based measuring networks, AWDANet (VLF/whistlers), EMMA/SANSA (ULF/FLRs), and AARDDVARK (VLF/perturbations on transmitters’ signal), by three, eight, and four new stations, respectively. The extended networks will allow us to achieve the four major scientific goals, the automatic retrieval of equatorial electron densities and density profiles of the plasmasphere by whistler inversion, the retrieval of equatorial plasma mass densities by EMMA and SANSA from FLRs, developing a new, data assimilative model of plasmasphere and validating the model predictions through comparison of modeled REP losses with measured data by AARDDVARK network. The first results on each of the four objectives are presented through a case study on a space weather event, a dual storm sudden commencement which occurred on August 3 and 4, 2010
Investigating radiation belt losses though numerical modelling of precipitating fluxes
No full textIt has been suggested that whistler-induced electron precipitation (WEP) may be the most significant inner radiation belt loss process for some electron energy ranges. One area of uncertainty lies in identifying a typical estimate of the precipitating fluxes from the examples given in the literature to date. Here we aim to solve this difficulty through modelling satellite and ground-based observations of onset and decay of the precipitation and its effects in the ionosphere by examining WEP-produced Trimpi perturbations in subionospheric VLF transmissions. In this study we find that typical Trimpi are well described by the effects of WEP spectra derived from the AE-5 inner radiation belt model for typical precipitating energy fluxes. This confirms the validity of the radiation belt lifetimes determined in previous studies using these flux parameters. We find that the large variation in observed Trimpi perturbation size occurring over time scales of minutes to hours is primarily due to differing precipitation flux levels rather than changing WEP spectra. Finally, we show that high-time resolution measurements during the onset of Trimpi perturbations should provide a useful signature for discriminating WEP Trimpi from non-WEP Trimpi, due to the pulsed nature of the WEP arrival
Energetic particle precipitation into the middle atmosphere triggered by a coronal mass ejection
No full textPrecipitation of relativistic electrons into the atmosphere has been suggested as the primary loss mechanism for radiation belt electrons during large geomagnetic storms. Here we investigate the geographical spread of precipitation as a result of the arrival of a coronal mass ejection (CME) on 21 January 2005. In contrast to previous statistical studies we provide one of the first attempts to describe the geographic and temporal variability of energetic particle precipitation on a global scale using an array of instruments. We combine data from subionospheric VLF radio wave receivers, the high-altitude Miniature Spectrometer (MINIS) balloons, riometers, and pulsation magnetometers during the first hour of the event. There were three distinct types of energetic electron precipitation observed, one globally, one on the dayside, and one on the nightside. The most extensively observed form of precipitation was a large burst starting when the CME arrived at the Earth, where electrons from the outer radiation belt were lost to the atmosphere over a large region of the Earth. On the dayside of the Earth (10–15 MLT) the CME produced a further series of precipitation bursts, while on the nightside dusk sector (∼20 MLT) a continuous precipitation event lasting ∼50 min was observed at 2.5 < L < 3.7 along with Pc 1–2 pulsations observed with a ground-based magnetometer. These observations suggest that the generation of energetic electron precipitation at the inner edge of the outer radiation belt from electromagnetic ion cyclotron (EMIC) wave scattering into the loss cone is the most direct evidence to date connecting EMIC activity and energetic precipitation
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