The Experts below are selected from a list of 264 Experts worldwide ranked by ideXlab platform
E. Spanswick - One of the best experts on this subject based on the ideXlab platform.
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GPS phase scintillation at high latitudes during geomagnetic storms of 7–17 March 2012 – Part 1: The North American sector
Copernicus Publications, 2015Co-Authors: E. Spanswick, P. Prikryl, R. Ghoddousi-fard, E. G. Thomas, J. M. Ruohoniemi, S. G. Shepherd, P. T. Jayachandran, D. W. Danskin, Y. ZhangAbstract:The interval of geomagnetic storms of 7–17 March 2012 was selected at the Climate and Weather of the Sun-Earth System (CAWSES) II Workshop for group study of space weather effects during the ascending phase of solar cycle 24 (Tsurutani et al., 2014). The high-latitude ionospheric response to a series of storms is studied using arrays of GPS receivers, HF radars, ionosondes, Riometers, magnetometers, and auroral imagers focusing on GPS phase scintillation. Four geomagnetic storms showed varied responses to solar wind conditions characterized by the interplanetary magnetic field (IMF) and solar wind dynamic pressure. As a function of magnetic latitude and magnetic local time, regions of enhanced scintillation are identified in the context of coupling processes between the solar wind and the magnetosphere–ionosphere system. Large southward IMF and high solar wind dynamic pressure resulted in the strongest scintillation in the nightside auroral oval. Scintillation occurrence was correlated with ground magnetic field perturbations and riometer absorption enhancements, and collocated with mapped auroral emission. During periods of southward IMF, scintillation was also collocated with ionospheric convection in the expanded dawn and dusk cells, with the antisunward convection in the polar cap and with a tongue of ionization fractured into patches. In contrast, large northward IMF combined with a strong solar wind dynamic pressure pulse was followed by scintillation caused by transpolar arcs in the polar cap
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dynamics of the correlation between polar cap radio absorption and solar energetic proton fluxes in the interplanetary medium
Journal of Geophysical Research, 2014Co-Authors: Alexei Kouznetsov, E. Donovan, D J Knudsen, E. SpanswickAbstract:During solar energetic particle (SEP) events, large fluxes of energetic particles spreading throughout the interplanetary medium (IPM) have access to the upper polar atmosphere where they play important roles in physical and chemical processes. We examine the dynamics of the relation between solar energetic proton (SEP) fluxes detected outside the magnetosphere by SOHO, at geosynchronous orbit by GOES 8, and in low earth orbit by NOAA 15 (POES) satellites on one hand, and ionospheric absorption measured by a NORSTAR riometer in the polar cap on the other. We attempt to isolate SEP-related signals by rejecting periods influenced by electron precipitation and solar radio bursts. Under these conditions we find R2of up to 0.9 between the logarithms of SEP flux and absorption for all satellites, but with the additional restriction to periods beginning 10 h after event onsets and within tens of hours following times of maximum flux. We find that the flux-absorption correlation is poor during the first few hours of an event, which we attribute to the fact that SEP angular distributions are often highly peaked along the interplanetary magnetic field during the early stages of an event. Such highly anisotropic distributions map to small regions inside the polar caps, making it unlikely that the corresponding absorption signal will be detected by a single riometer. These observations suggest that Riometers in combination with a suitable numerical tool can be used as a diagnostic of properties of energetic proton populations in the interplanetary medium.
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remote sensing magnetospheric dynamics with Riometers observation and theory
Journal of Geophysical Research, 2007Co-Authors: W. W. Liu, E. Spanswick, J. Liang, E. DonovanAbstract:[1] The importance of monitoring particle injection into the inner magnetosphere from the plasma sheet is exceeded perhaps only by its difficulty. The recent progress in using ground-based riometer data [e.g., Spanswick et al., 2007] to detect by proxy particle injection has raised much hope that this important aspect of substorms can be more consistently monitored. In this paper we develop a theoretical model for explaining the observed dispersionless injection events reported by Spanswick et al. The substorm event on 3 October 1998 is analyzed to give the empirical context for riometer responses during a typical substorm. Our simulation shows that riometer electrons produce a clean and strong signal that can be uniquely and easily related to magnetic field dipolarization that normally gives rise to injection.
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Pc5 modulation of high energy electron precipitation: particle interaction regions and scattering efficiency
Annales Geophysicae, 2005Co-Authors: E. Spanswick, E. Donovan, G. BakerAbstract:Using the NORSTAR riometer and CANOPUS magnetometer arrays we have investigated the modulation of high energy electron precipitation by ULF waves in the Pc5 frequency band. We conducted two separate studies of Pc5 activity in the Riometers. The first is an independent survey of three riometer stations in the Churchill line (one at each sub-auroral, auroral, and typical polar cap boundary latitudes) in which we identified all riometer Pc5-band pulsations over 11 years. All had a corresponding magnetometer pulsation implying that a magnetic pulsation, is a necessary condition for a riometer pulsation (in the Pc5 Band). We find seasonal and latitude dependencies in the occurrence of riometer pulsations. By a factor of two, there are more riometer pulsations occurring in the fall-winter than the spring-summer. At higher latitudes there is a tendency towards noon pulsations during the spring-summer, suggesting that the criteria for riometer pulsations is affected by the dipole tilt. Our second study was based on the previous magnetometer study of Baker et al. (2003). Using the database of Pc5 activity from that study we were able to select the riometer Pc5 pulsations which adhere to the strict Pc5 definition in the magnetometer. We find that roughly 95% of the riometer pulsations occurred in the morning sector compared to 70% in the magnetometer. Given a magnetometer pulsation at Gillam in the morning sector, there is a 70% chance of there being a corresponding riometer pulsation. The morning sector probabilities at Rankin (geomagnetic (PACE) latitude 74°) and Pinawa (61°) are 3% and 5%, respectively. These statistics suggest there is a localized region in the pre-noon magnetosphere where Pc5 band ULF activity can modulate high energy electron precipitation. We also find that riometer pulsations display a Kp selection towards mid (i.e. 3?4) activity levels which mimics the product of the Kp dependence of high-energy electron fluxes on the dawn side (from CRRES) and all magnetic Pc5 activity. A superposed epoch analysis revealed that the elevated electron flux needed to produce a riometer pulsation is most likely provided by substorm injections on the nightside. We also find that the amplitude of modulated precipitation correlates well with the product of the background absorption and the magnetic pulsation amplitude, again leading to the idea that a riometer pulsation needs both favorable magnetospheric electron flux conditions and large enough magnetic Pc5 wave activity. We further separate our pulsations into field line resonances (FLRs), and non-field line resonances (non-FLRs), as identified in the Baker et al. (2003) survey. We find that FLRs are more efficient at modulating particle precipitation, and non-FLRs display an amplitude cutoff below which they do not interact with the high energy electron population. We conclude that the high energy electron precipitation associated with Pc5 pulsations is caused by pitch angle scattering (diffusion) rather than parallel acceleration. We suggest two future studies that are natural extensions of this one. Keywords. Energetic Particles/Precipitating; Wave-Particle Interactions; Auroral Phenomena
Farideh Honary - One of the best experts on this subject based on the ideXlab platform.
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Real-time HF Radio Absorption Maps Incorporating Riometer and Satellite Measurements
2016Co-Authors: Neil C. Rogers, Farideh Honary, Michael E. Warrington, Alan Stocker, Donald W. DanskinAbstract:A real-time model of HF radio propagation conditions is being developed as a service for aircraft communications at high latitudes. An essential component of this is a real-time map of the absorption of HF (3-30 MHz) radio signals in the D-region ionosphere. Empirical, climatological Polar Cap Absorption (PCA) models in common usage cannot account for day-to-day variations in ionospheric composition and are inaccurate during the large changes in recombination rate at twilight. However, parameters of such models may be optimised using an age-weighted regression to absorption measurements from Riometers in Canada and Scandinavia. Such parameters include the day- and night-time sensitivity to proton flux as measured on a geostationary satellite (GOES). Modelling the twilight transition as a linear or Gauss error function over a range of solar-zenith angles (χl < χ < χu) is found to provide greater accuracy than ‘Earth shadow’ methods (as applied in the Sodankylä Ionospheric Chemistry (SIC) model, for example) due to a more gradual ionospheric response for χ < 90°. The fitted χl parameter is found to be most variable, with smaller values (as low as 60°) post-sunrise compared with pre-sunset. Correlation coefficients of model parameters between Riometers are presented and these provide a means of appropriately weighting individual riometer contributions in an assimilative PCA model. At times outside of PCA events, the probability of absorption in the auroral zones is related to the energetic electron flux inside the precipitation loss cone, as measured on the polar-orbiting POES satellites. This varies with magnetic local time, magnetic latitude and geomagnetic activity, and its relation to the real-time solar wind – magnetospheric coupling function [Newell et al., 2007] will be presented
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Assimilation of real-time riometer measurements into models of 30 MHz polar cap absorption
Journal of Space Weather and Space Climate, 2015Co-Authors: Neil Rogers, Farideh HonaryAbstract:Space weather events may adversely affect high frequency (HF) radio propagation, hence the ability to provide nowcasting and forecasting of HF radio absorption is key for industries that rely on HF communications. This paper presents methods of assimilating 30 MHz radio absorption measurements into two types of ionospheric polar cap absorption (PCA) model to improve their performance as nowcasting tools. Type 1 models calculate absorption as m times the square root of the flux of solar protons above an energy threshold, E t . Measurements from 14 Riometers during 94 solar proton events (1995–2010) are assimilated by optimising the day and night values of m by linear regression. Further non-linear optimisations are demonstrated in which parameters such as E t are also optimised and additional terms characterise local time and seasonal variations. These optimisations reduce RMS errors by up to 36%. Type 2 models incorporate altitude profiles of electron and neutral densities and electron temperatures. Here the scale height of the effective recombination coefficient profile in the D-region is optimised by regression. Furthermore, two published models of the rigidity cut-off latitude (CL) are assessed by comparison with riometer measurements. A small improvement in performance is observed by introducing a 3-h lag in the geomagnetic index K p in the CL models. Assimilating data from a single riometer in the polar cap reduces RMS errors below 1 dB with less than 0.2 dB bias. However, many high-latitude Riometers now provide absorption measurements in near-real time and we demonstrate how these data may be assimilated by fitting a low-order spherical harmonic function to both the measurements and a PCA model with optimised parameters.
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Assimilation of real-time riometer measurements into models of 30 MHz polar cap absorption
2015Co-Authors: Neil Christopher Rogers, Farideh HonaryAbstract:Space weather events may adversely affect high frequency (HF) radio propagation, hence the ability to provide nowcasting and forecasting of HF radio absorption is key for industries that rely on HF communications. This paper presents methods of assim-ilating 30 MHz radio absorption measurements into two types of ionospheric polar cap absorption (PCA) model to improve their performance as nowcasting tools. Type 1 models calculate absorption as m times the square root of the flux of solar protons above an energy threshold, Et. Measurements from 14 Riometers during 94 solar proton events (1995–2010) are assimilated by optimis-ing the day and night values of m by linear regression. Further non-linear optimisations are demonstrated in which parameters such as Et are also optimised and additional terms characterise local time and seasonal variations. These optimisations reduce RMS errors by up to 36%. Type 2 models incorporate altitude profiles of electron and neutral densities and electron temperatures. Here the scale height of the effective recombination coefficient profile in the D-region is optimised by regression. Furthermore, two published models of the rigidity cut-off latitude (CL) are assessed by comparison with riometer measurements. A small improvement in performance is observed by introducing a 3-h lag in the geomagnetic index Kp in the CL models. Assimilating data from a single riometer in the polar cap reduces RMS errors below 1 dB with less than 0.2 dB bias. However, many high-latitude Riometers now provide absorption measurements in near-real time and we demonstrate how these data may be assimilated by fitting a low-order spherical harmonic function to both the measurements and a PCA model with optimised parameters. Key words. Ionosphere (polar) – Proton precipitation – SEP – Aeronomy – Radio Science
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D-region HF absorption models incorporating real-time riometer measurements
2014 XXXIth URSI General Assembly and Scientific Symposium (URSI GASS), 2014Co-Authors: Neil Rogers, Farideh HonaryAbstract:Absorption of HF (3–30 MHz) radio waves is largely determined by the electron density in the ionospheric D region (50–90 km altitude). During solar proton events (SPE), when the flux of >10 MeV solar protons exceeds 10 cm−2 s−1 sr−1, the D region ionization may be significantly enhanced at high latitudes where geomagnetic shielding is weaker. This results in polar cap absorption (PCA) events which can cause HF communications outages lasting several days. Models of PCA events are being improved to provide accurate real-time and short-term forecast models of HF absorption for use by HF radio users such as aircraft operating on trans-polar routes. The models are based on the D-region Absorption Prediction model (D-RAP) from the US Space Weather Prediction Service [1, 2] which predicts absorption from real-time measurements of solar X-ray and integral proton flux at one of the Geostationary Operational Environmental Satellites (GOES). Protons with energy below a cut-off energy E c at a given invariant latitude — a function of geomagnetic indices K p and D st [3] — lack the rigidity (momentum per unit charge) required to overcome geomagnetic shielding, whilst protons with energy less than thresholds E tn and E td for night and daytime ionospheres respectively, fail to penetrate down to the D-region. Coefficients of the D-RAP model were based on physical modelling and absorption measurements from a single riometer in Thule, Greenland [3] which measures cosmic noise absorption at 30 MHz. The accuracy of the model was validated for 11 SPEs at Thule by Sauer and Wilkinson [2] and for five further Riometers in Canada and Finland by Akmaev et al. [4] who suggested possible errors in the location of the rigidity cut-off at high geomagnetic latitudes. In this paper we extend validation of D-RAP to measurements from 13 Riometers in the Canadian NORSTAR array and a riometer in Kilpisjarvi, Finland for 93 solar proton events (SPE) spanning the whole of solar cycle 23 (1996–2008). To improve model performance, coefficients are optimized using a non-linear least-squares fit to minimize the root-mean-squared error (RMSE) of the absorption estimate. Using optimized coefficients the RMSE reduces from 0.78 dB to 0.72 dB (using all 14 Riometers) or from 0.82 dB to 0.53 dB taking only the single highest latitude riometer, Taloyoak (64.5°N, 93.6°W). By introducing linear terms characterizing the Magnetic Local Time (MLT) dependence, the RMSE may be further reduced to 0.66 dB (all Riometers). The inclusion of further linear terms proportional to the hardness of the proton energy spectrum and on solar-zenith angle yielded no significant improvement to the RMSE. The benefits of two modifications to D-RAP suggested by Neal et al. [5] based on Polar Operational Environmental Satellite (POES) measurements — a 1–2° correction in the rigidity cut-off invariant latitude and a 3-hour time lag in the K p index used in its determination — will also be presented. A short-term forecast capability may be implemented by measuring proton flux at the Advanced Composition Explorer satellite (ACE) located at the L1 libration point which provides 25–70 minute forewarning of proton flux changes (depending on solar wind velocity). An optimized model using ACE integral proton flux measurements (time-shifted to Earth's location) instead of D-RAP reduces the RMSE from 0.57 dB to 0.47 dB (Taloyoak riometer) and from 0.69 dB to 0.64 dB (all Riometers). The nowcast accuracy of the PCA model may be improved by finding model parameters coefficients using a weighted least-squares fit, with higher weights assigned in the most recent 30-minute period of riometer measurements. An example of this technique is presented for the 6-day SPE following the particularly intense solar flare of 14 July 2000, known as the “Bastille-day event”.
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Digital beam-forming imaging riometer systems
2011Co-Authors: Farideh Honary, Steve R. Marple, Keith Barratt, P. J. Chapman, Martin Grill, Erling NielsenAbstract:The design and operation of a new generation of digital imaging riometer systems developed by Lancaster University are presented. In the heart of the digital imaging riometer is a field-programmable gate array (FPGA), which is used for the digital signal processing and digital beam forming, completely replacing the analog Butler matrices which have been used in previous designs. The reconfigurable nature of the FPGA has been exploited to produce tools for remote system testing and diagnosis which have proven extremely useful for operation in remote locations such as the Arctic and Antarctic. Different FPGA programs enable different instrument configurations, including a 4 × 4 antenna filled array (producing 4 × 4 beams), an 8 × 8 antenna filled array (producing 7 × 7 beams), and a Mills cross system utilizing 63 antennas producing 556 usable beams. The concept of using a Mills cross antenna array for riometry has been successfully demonstrated for the first time. The digital beam forming has been validated by comparing the received signal power from cosmic radio sources with results predicted from the theoretical beam radiation pattern. The performances of four digital imaging riometer systems are compared against each other and a traditional imaging riometer utilizing analog Butler matrices. The comparison shows that digital imaging riometer systems, with independent receivers for each antenna, can obtain much better measurement precision for filled arrays or much higher spatial resolution for the Mills cross configuration when compared to existing imaging riometer systems.
T. J. Rosenberg - One of the best experts on this subject based on the ideXlab platform.
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Conjugate high-intensity energetic electron precipitation at high latitude
Copernicus Publications, 2003Co-Authors: T. J. Rosenberg, D. L. Detrick, T. Christensen, N. Østgaard, G. A. Germany, P. StauningAbstract:On 6 August 1998 an intense precipitation event occurring at high latitude in the evening sector was observed by X-ray and far-ultraviolet imagers on board the Polar satellite and by several ground-based instruments. The precipitation region was centred at approximately 19:00 MLT at 74° MLAT (at an L-shell of about 13). The event started at 22:59 UT and lasted about 10 minutes. It happened during the late expansion phase of a substorm after two hours of strongly southward IMF. Imaging Riometers at geomagnetically conjugate sites recorded strong absorption levels which exceeded 7 dB at 38 MHz in a transient and localized intensification occurring within a poleward moving arc-like feature. The temporal and spatial similarities between the recordings from the two conjugate regions are remarkable. The arc-like precipitation region progressed poleward with a velocity of 1.5 km/s. Ground magnetometers co-located with the imaging Riometers observed disturbances consistent with poleward moving westward currents. In X-ray and riometer images which are sensitive only to energetic electrons (above 5–10 keV) the event seems isolated, but in UV images the event is seen to occur on the poleward edge of the rapidly poleward expanding evening side aurora. The energy spectrum of precipitating electrons was subject to a temporary hardening which peaked at a mean energy of about 20 keV when the event was at its most intense at 23:02 UT. The event is likely to have been caused by an accelerating mechanism at some height above the ionosphere or by an earthward flow burst in the magnetotail, or possibly both.Key words. Magnetospheric physics (energetic particles, precipitating; storms and substorms; magnetosphere-ionosphere interactions
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Conjugate high-intensity energetic electron precipitation at high latitude
Annales Geophysicae, 2003Co-Authors: T. Christensen, T. J. Rosenberg, D. L. Detrick, N. Østgaard, G. A. Germany, P. StauningAbstract:On 6 August 1998 an intense precipitation event occurring at high latitude in the evening sector was observed by X-ray and far-ultraviolet imagers on board the Polar satellite and by several ground-based instruments. The precipitation region was centred at approximately 19:00 MLT at 74° MLAT (at an L-shell of about 13). The event started at 22:59 UT and lasted about 10 minutes. It happened during the late expansion phase of a substorm after two hours of strongly southward IMF. Imaging Riometers at geomagnetically conjugate sites recorded strong absorption levels which exceeded 7 dB at 38 MHz in a transient and localized intensification occurring within a poleward moving arc-like feature. The temporal and spatial similarities between the recordings from the two conjugate regions are remarkable. The arc-like precipitation region progressed poleward with a velocity of 1.5 km/s. Ground magnetometers co-located with the imaging Riometers observed disturbances consistent with poleward moving westward currents. In X-ray and riometer images which are sensitive only to energetic electrons (above 5?10 keV) the event seems isolated, but in UV images the event is seen to occur on the poleward edge of the rapidly poleward expanding evening side aurora. The energy spectrum of precipitating electrons was subject to a temporary hardening which peaked at a mean energy of about 20 keV when the event was at its most intense at 23:02 UT. The event is likely to have been caused by an accelerating mechanism at some height above the ionosphere or by an earthward flow burst in the magnetotail, or possibly both.
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A programmable riometer for Earth and Mars
Radio Science, 2000Co-Authors: C. D. Fry, T. J. Rosenberg, L. Lutz, D. L. Detrick, A. T. Weatherwax, E. Knouse, H. Breden, J. GigantiAbstract:We have developed a miniature radio receiver designed to operate as a relative ionospheric opacity meter (riometer). This project was funded by NASA as an enabling technology for future planetary radio science missions. We sought to reduce the instrument's size, mass, and power so that it would be practical for a Mars lander or rover mission. A recent study by our group indicates that a riometer might work well on Mars and offers a potentially rich science return. The technology also has immediate terrestrial applications. For example, the University of Maryland operates a chain of imaging Riometers at the Automatic Geophysical Observatories (AGOs) in Antarctica. Our riometer includes features that are desirable for extended autonomous operation such as those with AGOs: low power consumption, wide dynamic range and linearity, computer command and data interface, and the ability to be remotely reconfigured. The receiver design provides significant improvements over previous implementations used in Riometers. The high degree of system linearity, combined with a digital feedback loop (including a low-duty calibration cycle), allows more time for viewing the radio sky. We implemented several of the receiver subsystems in a field-programmable gate array, including the receiver detector, the control logic, and the data acquisition and processing blocks. Considerable efforts were made to eliminate or minimize RF noise and spurious emissions generated by the receiver's digital circuitry. Results of laboratory and field tests are presented and discussed.
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High Frequency Active Auroral Research Program imaging riometer diagnostic
Radio Science, 1999Co-Authors: T. J. Rosenberg, D. L. Detrick, A. T. Weatherwax, L. LutzAbstract:This paper describes the prototype 16-beam, 38.6-MHz riometer system developed by Advanced Power Technologies, Inc., and the University of Maryland for the High Frequency Active Auroral Research Program (HAARP). The prototype system is the forerunner for a full-scale imaging riometer diagnostic instrument for characterizing the ionospheric volume perturbed by controlled radio frequency (RF) heating experiments. The prototype system, installed at the HAARP site near Gakona, Alaska, consists of a 1 × 16 antenna array phased in one dimension (beam width of ∼6.7°) and oriented approximately along the magnetic meridian. The system responds sensitively to natural variations of auroral absorption, such as those caused by magnetospheric substorms, and provides clear evidence of its capability to discern spatial structure and motion. A newly observed feature seen near dusk are intense, short-duration absorption spikes accompanied by only weak magnetic signatures. Because the HAARP heating facility is still a work in progress, there have been only limited opportunities to conduct science investigations with it. Consistent with estimates, heater-induced effects in the ionosphere have not been observed with the riometer at the power levels currently available. It has been noted that the proximity of the prototype system to the HAARP RF heater can result in interference to the riometer signal for some frequencies and operating modes of the heater. The full-scale imaging capability of the proposed instrument, and a remote location, may be required to detect small-scale modifications of the ionosphere caused by the heater.
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Riometer and HF radar signatures of polar patches
Radio Science, 1999Co-Authors: Alan S. Rodger, T. J. RosenbergAbstract:Polar patch was the name given originally to a spatially limited enhancement of the 630-nm emission observed in the dark polar cap. More recently, the term has been applied to related phenomena observed by other experimental techniques. The interrelationship between patches observed by differing methods has yet to be determined fully. In this paper the signatures of a series of polar patches observed by Halley HF radar and the South Pole broad-beam and imaging Riometers are presented. Most frequently, the HF radar patch signature immediately precedes that of the riometer polar patch signature. The interpretation is that the leading edge of the electron concentration structure that forms the patch is steeper than its trailing edge. On a few occasions, HF radar polar patch signatures coincide with the trailing edge of riometer patches and sometimes are seen throughout the riometer patch. Halley digital ionosonde data are used to show that riometer patches are more common over South Pole when the maximum F region plasma concentration is considerably in excess of 5×1011 m−3, which often occurs in the afternoon plasma convection cell.
F. Honary - One of the best experts on this subject based on the ideXlab platform.
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Effects of D-region RF heating studied with the Sodankylä Ion Chemistry model
2012Co-Authors: -f. C. Enell, A. Kero, E. Turunen, Th. Ulich, P. T. Verronen, S. Marple, F. HonaryAbstract:Abstract. The upper mesosphere and lower thermosphere, or ionospheric D region, is an atmospheric layer which is difficult to access experimentally. A useful method that also has a large potential for further studies is artificial heating of electrons by means of powerful radio transmitters. Here we estimate the effect of D-region heating for a few typical cases of high electron density − daylight, typical auroral electron precipitation, and a solar proton event − by coupling a model of RF electron heating to the Sodankylä Ion Chemistry (SIC) model. The predicted effects are among others an increase in the ratio of the concentration of negative ions to that of free electrons, and an increase in the absorption of cosmic noise as measured by Riometers. For the model runs presented in this paper we have calculated the absorption for the frequency (38.2 MHz) of the IRIS imaging riometer in Kilpisjärvi, Finland, as observing the ionosphere above the EISCAT Heater in Tromsø, Norway. The predicted enhancements of the absorption are 0.2–0.8 dB, an effect which is clearly detectable
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Case study of the mesospheric and lower thermospheric effects of solar X-ray flares: coupled ion-neutral modelling and comparison with EISCAT and riometer measurements
Copernicus Publications, 2008Co-Authors: C.-f. Enell, F. Honary, A. Kero, P. T. Verronen, A. Seppälä, M. J. Beharrell, J. P. Vierinen, T. Ulich, E. TurunenAbstract:Two case studies of upper mesospheric and lower thermospheric (UMLT) high-latitude effects of solar X-ray flares are presented. Sodankylä Ion-neutral Chemistry Model (SIC) electron density profiles agree with D-region EISCAT and riometer observations, provided that the profiles of the most variable ionisable component, nitric oxide, are adjusted to compensate for NOx production during preceding geomagnetically active periods. For the M6-class flare of 27 April 2006, following a quiet period, the agreement with cosmic noise absorption observed by the Sodankylä Riometers was within reasonable limits without adjustment of the [NO] profile. For the major (X17-class) event of 28 October 2003, following high auroral activity and solar proton events, the NO concentration had to be increased up to on the order of 108 cm−3 at the D-region minimum. Thus [NO] can in principle be measured by combining SIC with observations, if the solar spectral irradiance and particle precipitation are adequately known. As the two case events were short and modelled for high latitudes, the resulting neutral chemical changes are insignificant. However, changes in the model ion chemistry occur, including enhancements of water cluster ions
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Energetic electron precipitation during sawtooth injections
Annales Geophysicae, 2007Co-Authors: A. J. Kavanagh, F. Honary, E. F. Donovan, G. D. Reeves, J. Manninen, T. J. ImmelAbstract:We present simultaneous riometer observations of cosmic noise absorption in the nightside and dawn-noon sectors during sawtooth particle injections during 18 April 2002. Energetic electron precipitation (>30 keV) is a feature of magnetospheric substorms and cosmic radio noise absorption acts as a proxy for qualitatively measuring this precipitation. This event provides an opportunity to compare the absorption that accompanies periodic electron injections with the accepted paradigm of substorm-related absorption. We consider whether the absorption is consistent with the premise that these injections are quasi-periodic substorms and study the effects of sustained activity on the level of precipitation. Four consecutive electron injection events have been identified from the LANL (Los Alamos National Laboratory) geosynchronous data; the first two showing that additional activity can occur within the 2?4 h sawtooth periodicity. The first three events have accompanying absorption on the nightside that demonstrate good agreement with the expected pattern of substorm-absorption: discrete spike events with poleward motion at the onset followed by equatorward moving structures and more diffuse absorption, correlated with optical observations. Dayside absorption is linked to gradient-curvature drifting electrons observed at geostationary orbit and it is shown that low fluxes can lead to a lack of absorption as precipitation is suppressed; precipitation begins when the drifting electron flux surpasses some critical level following continuous injections of electrons from the magnetotail. In addition it is shown that the apparent motion of absorption determined from an azimuthal chain of Riometers exhibits an acceleration that may be indicative of an energisation of the drifting electron population.
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Effects of D-region RF heating studied with the Sodankylä Ion Chemistry model
Annales Geophysicae, 2005Co-Authors: C.-f. Enell, F. Honary, A. Kero, E. Turunen, Th. Ulich, P. T. Verronen, A. Seppälä, S. Marple, A. SeniorAbstract:The upper mesosphere and lower thermosphere, or ionospheric D region, is an atmospheric layer which is difficult to access experimentally. A useful method that also has a large potential for further studies is artificial heating of electrons by means of powerful radio transmitters. Here we estimate the effect of D-region heating for a few typical cases of high electron density ? daylight, typical auroral electron precipitation, and a solar proton event ? by coupling a model of RF electron heating to the Sodankylä Ion Chemistry (SIC) model. The predicted effects are among others an increase in the ratio of the concentration of negative ions to that of free electrons, and an increase in the absorption of cosmic noise as measured by Riometers. For the model runs presented in this paper we have calculated the absorption for the frequency (38.2MHz) of the IRIS imaging riometer in Kilpisjärvi, Finland, as observing the ionosphere above the EISCAT Heater in Tromsø, Norway. The predicted enhancements of the absorption are 0.2?0.8dB, an effect which is clearly detectable. Keywords. Ionosphere (Active experiments; Ion chemistry and composition; Wave propagation)
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Observations of the spatial structure of electron precipitation pulsations using an imaging riometer
Copernicus Publications, 2003Co-Authors: F. HonaryAbstract:Electron precipitation can be modulated by geomagnetic pulsation activity. This can be observed as pulsation of cosmic noise absorption as measured by Riometers. Observations of such pulsations exhibiting field-line resonance and particle-driven characteristics using an imaging riometer are presented and the capability of the instrument to map their spatial structure is demonstrated. It is shown that for the events studied, the spatial variation of pulsation phase as measured by the riometer agrees with that inferred from ground-based magnetometers, whereas the spatial variation of pulsation amplitude may show a different structure. It is suggested that this is consistent with the mechanism proposed by Coroniti and Kennel (1970) where one would expect a fixed phase relationship between magnetic and absorption pulsations, but where the amplitude of the absorption pulsation can depend on several factors other than the amplitude of the magnetic pulsation.Key words. Ionosphere (ionosphere–magnetosphere interactions; particle precipitation) – Magnetospheric physics (MHD waves and instabilities
E. Donovan - One of the best experts on this subject based on the ideXlab platform.
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dynamics of the correlation between polar cap radio absorption and solar energetic proton fluxes in the interplanetary medium
Journal of Geophysical Research, 2014Co-Authors: Alexei Kouznetsov, E. Donovan, D J Knudsen, E. SpanswickAbstract:During solar energetic particle (SEP) events, large fluxes of energetic particles spreading throughout the interplanetary medium (IPM) have access to the upper polar atmosphere where they play important roles in physical and chemical processes. We examine the dynamics of the relation between solar energetic proton (SEP) fluxes detected outside the magnetosphere by SOHO, at geosynchronous orbit by GOES 8, and in low earth orbit by NOAA 15 (POES) satellites on one hand, and ionospheric absorption measured by a NORSTAR riometer in the polar cap on the other. We attempt to isolate SEP-related signals by rejecting periods influenced by electron precipitation and solar radio bursts. Under these conditions we find R2of up to 0.9 between the logarithms of SEP flux and absorption for all satellites, but with the additional restriction to periods beginning 10 h after event onsets and within tens of hours following times of maximum flux. We find that the flux-absorption correlation is poor during the first few hours of an event, which we attribute to the fact that SEP angular distributions are often highly peaked along the interplanetary magnetic field during the early stages of an event. Such highly anisotropic distributions map to small regions inside the polar caps, making it unlikely that the corresponding absorption signal will be detected by a single riometer. These observations suggest that Riometers in combination with a suitable numerical tool can be used as a diagnostic of properties of energetic proton populations in the interplanetary medium.
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remote sensing magnetospheric dynamics with Riometers observation and theory
Journal of Geophysical Research, 2007Co-Authors: W. W. Liu, E. Spanswick, J. Liang, E. DonovanAbstract:[1] The importance of monitoring particle injection into the inner magnetosphere from the plasma sheet is exceeded perhaps only by its difficulty. The recent progress in using ground-based riometer data [e.g., Spanswick et al., 2007] to detect by proxy particle injection has raised much hope that this important aspect of substorms can be more consistently monitored. In this paper we develop a theoretical model for explaining the observed dispersionless injection events reported by Spanswick et al. The substorm event on 3 October 1998 is analyzed to give the empirical context for riometer responses during a typical substorm. Our simulation shows that riometer electrons produce a clean and strong signal that can be uniquely and easily related to magnetic field dipolarization that normally gives rise to injection.
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Pc5 modulation of high energy electron precipitation: particle interaction regions and scattering efficiency
Annales Geophysicae, 2005Co-Authors: E. Spanswick, E. Donovan, G. BakerAbstract:Using the NORSTAR riometer and CANOPUS magnetometer arrays we have investigated the modulation of high energy electron precipitation by ULF waves in the Pc5 frequency band. We conducted two separate studies of Pc5 activity in the Riometers. The first is an independent survey of three riometer stations in the Churchill line (one at each sub-auroral, auroral, and typical polar cap boundary latitudes) in which we identified all riometer Pc5-band pulsations over 11 years. All had a corresponding magnetometer pulsation implying that a magnetic pulsation, is a necessary condition for a riometer pulsation (in the Pc5 Band). We find seasonal and latitude dependencies in the occurrence of riometer pulsations. By a factor of two, there are more riometer pulsations occurring in the fall-winter than the spring-summer. At higher latitudes there is a tendency towards noon pulsations during the spring-summer, suggesting that the criteria for riometer pulsations is affected by the dipole tilt. Our second study was based on the previous magnetometer study of Baker et al. (2003). Using the database of Pc5 activity from that study we were able to select the riometer Pc5 pulsations which adhere to the strict Pc5 definition in the magnetometer. We find that roughly 95% of the riometer pulsations occurred in the morning sector compared to 70% in the magnetometer. Given a magnetometer pulsation at Gillam in the morning sector, there is a 70% chance of there being a corresponding riometer pulsation. The morning sector probabilities at Rankin (geomagnetic (PACE) latitude 74°) and Pinawa (61°) are 3% and 5%, respectively. These statistics suggest there is a localized region in the pre-noon magnetosphere where Pc5 band ULF activity can modulate high energy electron precipitation. We also find that riometer pulsations display a Kp selection towards mid (i.e. 3?4) activity levels which mimics the product of the Kp dependence of high-energy electron fluxes on the dawn side (from CRRES) and all magnetic Pc5 activity. A superposed epoch analysis revealed that the elevated electron flux needed to produce a riometer pulsation is most likely provided by substorm injections on the nightside. We also find that the amplitude of modulated precipitation correlates well with the product of the background absorption and the magnetic pulsation amplitude, again leading to the idea that a riometer pulsation needs both favorable magnetospheric electron flux conditions and large enough magnetic Pc5 wave activity. We further separate our pulsations into field line resonances (FLRs), and non-field line resonances (non-FLRs), as identified in the Baker et al. (2003) survey. We find that FLRs are more efficient at modulating particle precipitation, and non-FLRs display an amplitude cutoff below which they do not interact with the high energy electron population. We conclude that the high energy electron precipitation associated with Pc5 pulsations is caused by pitch angle scattering (diffusion) rather than parallel acceleration. We suggest two future studies that are natural extensions of this one. Keywords. Energetic Particles/Precipitating; Wave-Particle Interactions; Auroral Phenomena
