Cosmic Radio Noise Absorption Research Articles

Longitudinal Development of Cosmic Noise Absorption Based on Multipoint Observations at Subauroral Latitudes During Storm‐Time Substorms on 25–28 August 2018

AbstractEnhancements in electron density in the D‐region ionosphere attributed to the precipitation of high‐energy electrons, have previously been inferred from increases in cosmic radio noise absorption (CNA) using ground‐based riometers. However, there have been few studies of CNA observations at multi‐point stations distributed in longitudes. Thus, the spatio‐temporal development of the global distribution of CNA is not well understood. In this study, we investigated the longitudinal extent of CNA using simultaneous riometer observations at six stations at subauroral latitudes in Canada, Alaska, Russia, and Iceland. These stations are located encircling the earth at ∼60° north magnetic latitudes. We have conducted simultaneous observations of CNA at these stations since October 2017. Here we focus on seven substorms during a geomagnetic storm 25–28 August 2018 and study the spatio‐temporal development of the global distribution of CNA during these substorms. For all seven substorms, some stations observed CNA enhancements after the substorm onsets. In five cases, the CNA enhancements started around midnight and expanded eastward. The other two cases show westward and anti‐sunward development of CNA. The eastward expansion of CNA indicates the eastward drift of high‐energy electrons, which is the source of the CNA, due to gradient and curvature drift in the geomagnetic field. The westward expansion of CNA may correspond to westward expansion of the substorm injection region due to dawn‐to‐dusk electric fields. These results indicate that spatio‐temporal development of CNA at subauroral latitudes corresponds to high energy electron drift in the inner magnetosphere.

On the determination of ionospheric electron density profiles using multi-frequency riometry

Abstract. Radio wave absorption in the ionosphere is a function of electron density, collision frequency, radio wave polarisation, magnetic field and radio wave frequency. Several studies have used multi-frequency measurements of cosmic radio noise absorption to determine electron density profiles. Using the framework of statistical inverse problems, we investigated if an electron density altitude profile can be determined by using multi-frequency, dual-polarisation measurements. It was found that the altitude profile cannot be uniquely determined from a “complete” measurement of radio wave absorption for all frequencies and two polarisation modes. This implies that accurate electron density profile measurements cannot be ascertained using multi-frequency riometer data alone and that the reconstruction requires a strong additional a priori assumption of the electron density profile, such as a parameterised model for the ionisation source. Nevertheless, the spectral index of the absorption could be used to determine if there is a significant component of hard precipitation that ionises the lower part of the D region, but it is not possible to infer the altitude distribution uniquely with this technique alone.

Cosmic radio noise absorption in the high‐latitude ionosphere during solar wind high‐speed streams

AbstractThe effect of solar wind high‐speed streams (HSSs) on energetic particle precipitation at auroral and subauroral latitudes (L = 3.8–5.7) is studied by using cosmic noise absorption (CNA) data measured by the Finnish riometer chain during 95 HSS events occurring between 2006 and 2008. The data are divided into “long” and “short” HSS events, depending on whether the maximum solar wind speed is reached within 24 h or later after the arrival of the corotating interaction region (CIR) at the bow shock. We find that CNA is more frequent during long events and extends to subauroral latitudes, unlike during short events. CNA is observed for at least 4 days after the CIR arrival during long events, and only 3 days during short events. In addition, CNA is divided into three categories, depending on whether it is associated with local substorm activity, with ultralow frequency (ULF) wave activity, or neither of these. Substorm‐type CNA dominates in the 21–06 magnetic local time (MLT) sector, while ULF‐type CNA dominates from morning to afternoon and follows the daily SYM‐H index variations. This indicates the importance of having energetic (E > 30 keV) electrons available for interacting with the very low frequency (VLF) chorus waves, which may be modulated by ULF waves. Finally, we find a correlation between ULF‐type CNA on the dayside and substorm activity on the nightside with a 30 min delay, suggesting that substorm injections energize electrons, which then drift to the morning and dayside, where they precipitate into the ionosphere.

All‐sky interferometric riometry

AbstractThe first implementation of a Fourier‐based interferometric riometry technique for measuring electron density induced ionospheric opacity at VHF radio frequencies is presented. Unlike multibeam riometers, which form discrete beams on the sky, the interferometric technique permits all‐sky sampling of incoming cosmic radio noise emissions resulting in a spatially continuous radiogram of the entire sky. The map of the received power at each time may then be compared to the equivalent map from a “quiet day,” allowing the morphology of ionospheric absorption of cosmic radio noise to be ascertained. In this work, the high‐latitude Kilpisjärvi Atmospheric Imaging Receiver Array was used to carry out the first interferometric riometry measurements in late 2013, producing all‐sky absorption maps of space‐weather‐related ionization in the D region.

The enhancement of cosmic radio noise absorption due to hiss‐driven energetic electron precipitation during substorms

AbstractThe Van Allen probes, low‐altitude NOAA satellite, MetOp satellite, and riometer are used to analyze variations of precipitating energetic electron fluxes and cosmic radio noise absorption (CNA) driven by plasmaspheric hiss with respect to geomagnetic activities. The hiss‐driven energetic electron precipitations (at L ~ 4.7–5.3, magnetic local time (MLT) ~ 8–9) are observed during geomagnetic quiet condition and substorms, respectively. We find that the CNA detected by riometers increased very little in the hiss‐driven event during quiet condition on 6 September 2012. The hiss‐driven enhancement of riometer was still little during the first substorm on 30 September 2012. However, the absorption detected by the riometer largely increased, while the energies of the injected electrons became higher during the second substorm on 30 September 2012. The enhancement of CNA (ΔCNA) observed by the riometer and calculated with precipitating energetic electrons is in agreement during the second substorm, implying that the precipitating energetic electrons increase CNA to an obviously detectable level of the riometer during the second substorm on 30 September 2012. The conclusion is consistent with Rodger et al. (2012), which suggest that the higher level of ΔCNA prefers to occur in the substorms, because substorms may produce more intense energetic electron precipitation associated with electron injection. Furthermore, the combination of the observations and theory calculations also suggests that higher‐energy electron (>55 keV) precipitation contributes more to the ΔCNA than the lower energy electron precipitation. In this paper, the higher‐energy electron precipitation is related to lower frequency hiss.

Ionospheric electron density profiles inverted from a spectral riometer measurement

AbstractThe first implementation of the so‐called spectral riometer technique for the ionospheric electron density profile estimation is presented. In contrast to the traditional riometer operating at a single frequency, this experiment monitors the cosmic radio noise at 244 frequencies, ranging between 10 and 80 MHz, by using the new Kilpisj rvi Atmospheric Imaging Receiver Array radio telescope. The received power at each time and frequency is compared to the corresponding quiet‐day value, resulting in the cosmic radio noise absorption spectrum as a measurement of ionization in the ionosphere. In this study, the observed absorption spectrum is used to invert the corresponding electron density profile by applying a simple parameterized electron precipitation model. By comparing the inverted electron density profiles to a simultaneous and nearly colocated European Incoherent Scatter VHF radar measurement on 13–14 November 2012, we show that the spectral riometry approach is capable of producing realistic electron density profiles under conditions of substorm‐related electron precipitation.

Comparison between POES energetic electron precipitation observations and riometer absorptions: Implications for determining true precipitation fluxes

AbstractEnergetic electron precipitation (EEP) impacts the chemistry of the middle atmosphere with growing evidence of coupling to surface temperatures at high latitudes. To better understand this link, it is essential to have realistic observations to properly characterize precipitation and which can be incorporated into chemistry‐climate models. The Polar‐orbiting Operational Environmental Satellite (POES) detectors measure precipitating particles but only integral fluxes and only in a fraction of the bounce loss cone. Ground‐based riometers respond to precipitation from the whole bounce loss cone; they measure the cosmic radio noise absorption (CNA), a qualitative proxy with scant direct information on the energy flux of EEP. POES observations should have a direct relationship with ΔCNA and comparing the two will clarify their utility in studies of atmospheric change. We determined ionospheric changes produced by the EEP measured by the POES spacecraft in ~250 overpasses of an imaging riometer in northern Finland. The ΔCNA modeled from the POES data is 10–15 times less than the observed ΔCNA when the >30 keV flux is reported as <106 cm−2 s−1 sr−1. Above this level, there is relatively good agreement between the space‐based and ground‐based measurements. The discrepancy occurs mostly during periods of low geomagnetic activity, and we contend that weak diffusion is dominating the pitch angle scattering into the bounce loss cone at these times. A correction to the calculation using measurements of the trapped flux considerably reduces the discrepancy and provides further support to our hypothesis that weak diffusion leads to underestimates of the EEP.

Difficulties in the study of cosmic radio noise absorption at 30 MHz using riometer at low latitude station, Kolhapur (Lat-16.8°N, Long-74.25°E)

A dual dipole antenna has been installed at low latitude station Kolhapur (Geographic 16.8°N, 74.25°E), Maharashtra, India for the study of cosmic radio noise absorption using Solid State Riometer (which operates at 30MHz) during pre phase of 24th solar maxima. The aim for this type of study over Kolhapur was to know the response of lower (D region) ionosphere over low latitude by cosmic radio noise absorption using riometer technique during quite period as well as sudden ionospheric disturbances (SID). The observations are being taken for 3years. Two different sites (∼40km away from each other) were used for the installation of riometer equipment assuming minimum local noise. It is found that solar noise to cosmic radio noise hence resulting in signal saturation. The night time signal is relatively free of interference but sometimes local noise is responsible for spike-like signatures. Hence it is concluded that Kolhapur (a low latitude station) is not suitable for the study of cosmic radio noise absorption on 30MHz with riometer and dual dipole antenna. Proper choice for operating frequency of riometer and antenna gain is suggested for low latitude use of this technique for ionospheric deviative and nondeviative absorption studies.

Multi-instrument observations of plasma features in the Arctic ionosphere during the main phase of a geomagnetic storm in December 2006

Arctic ionospheric variations during the main phase of a magnetic storm on 14–15 December, 2006 were investigated to characterize the high energy particle precipitation caused effects, based on multi-instrument observations. These include electron density observations provided by the Global Positioning System (GPS) total electron content (TEC) measurements, European Incoherent Scatter (EISCAT) radar, the radio occultation (RO) from both the CHAMP satellite and the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellite, as well as the ionospheric absorption of cosmic radio noise measured by the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjärvi in the northern Finland (69.05°N, 20.79°E). Significant increases in the electron density for these different observations were found in the Arctic ionosphere during the main phase of the magnetic storm. These increase occurred in Scandinavian, Northwest part of Russia and Svalbard (SNRS) region, primarily at an altitude of about 110km. These results are first reported for the SNRS region, and our study contributes to a more complete description of this space weather event during 14–15 December, 2006. Our observations also provide direct evidence that the stormtime E-layer electron density enhancement (e.g., the sporadic E) can form a nearly dominant portion in the observed TEC increase. These increases were accompanied by the ionospheric absorption enhancement at the altitude of about 90km. The Y-component of magnetic field to the south of SNRS decreased, indicating strong upward field aligned electric current in the Arctic ionosphere. These features are interpreted as the effect of the high energy electron precipitation during the magnetic storm, which is caused by the sub-storm reflected on AL index and the measurements of IMAGE (International Monitor for Auroral Geomagnetic Effects) chain. The average energy of the precipitation electrons reached to about 10keV and the boundary of the high energy electron precipitation was also found to move poleward with a speed of about 800m/s.

Measurements and modeling of cosmic noise absorption changes due to radio heating of the D region ionosphere

[1] Powerful high-frequency radio waves can heat the electrons in the D region of the ionosphere. This heating increases the electron-neutral collision frequency which modifies the absorption of other radio waves propagating through the heated plasma. A high spatial resolution imaging riometer was used to observe changes in cosmic radio noise absorption (CNA) induced by heating from the European Incoherent Scatter (EISCAT) HF facility, and the results were compared to a theoretical model using observed electron densities as an input. The model is found to overestimate the observed effect by a factor close to 2, despite different background electron density profiles and heater powers. However, the model reproduced the spatial morphology of the change in CNA rather well, and the same absorption calculation used in the heating model also reproduced the changes in CNA due to electron precipitation in the absence of heating well. When the assumption of a perfectly conducting ground is replaced with a more realistic model in the calculation of the HF radiated power, the power is reduced to about 75% of its original value, and the model overestimate of the CNA change is reduced to a factor of about 1.3.

Auroral infrasound associated with ionospheric electron precipitation.

Periods of persistent, high‐trace velocity infrasound signals lasting several hours or more have been observed in the data from the CTBT/IMS I53 infrasound station in Fairbanks, AK. Early studies showed that the infrasound activity was associated with times of geomagnetic disturbance. The imaging riometer at the Poker Flat Research Range near Fairbanks collects cosmic noise absorption (CNA) data from an array of 16 × 16 beams that cover a region that is approximately 200 × 200 km2 in the overhead ionosphere. Also available from the PFRR site are magnetometer data and all‐sky camera videos showing auroral events when the sky is dark. The CNA event data give direct evidence of the precipitation of high energy electrons into the D‐region of the lower ionosphere that produce both auroral displays captured in all‐sky images and disturbances in the ground magnetometers. The simultaneous occurrence at I53US of high trace‐velocity infrasonic waves, geomagnetic perturbations, and cosmic radio noise absorption provides verification that the observed infrasound is generated in the lower ionosphere. Examples of the high‐trace velocity signals along with the CNA and magnetometer data will be shown.

Calibrating an imaging riometer: Determination of the true beam azimuth angle position

[1] An imaging riometer uses the absorption of cosmic radio noise as a means of measuring electron density enhancements in the D region of the ionosphere. It employs a ground-based phased antenna array to produce narrow beams which typically observe a region 200 by 200 km at 90 km altitude. Imaging riometers have traditionally been used for studying energetic electron precipitation into the D region and have also recently been used to characterize mesospheric gravity waves. Determining the location and shape of the riometer beam pattern projected onto the ionosphere is therefore important because it allows the spatial distribution of space weather effects to be accurately visualized and also because it would allow mesospheric gravity wave characteristics to be accurately quantified. Currently, the beam patterns of imaging riometers are determined theoretically, and attempts have been made to validate them by comparison with cosmic radio maps of low spatial resolution; alternatively, they are determined by calibration overflights using aircraft in a complex and expensive procedure. Here we demonstrate a novel wavelet-based approach to accurately determine the azimuth angle of each imaging riometer beam from the riometer data itself using quiet day observations alone and by way of example show how the actual beam azimuth angles (pointing directions) of the Halley (76°S, 27°W) 49-beam imaging riometer vary from those expected using theoretical calculations.

The increase in OH rotational temperature during an active aurora event

Abstract. OH rotational temperatures have been observed at the Syowa Station, Antarctica (69° S, 39° E), which is located in the middle of the auroral zone and has a high-sensitivity spectrometer for the spectral region of the OH 8-4 band. A dataset of 153 nights was acquired during the 2008 austral winter season. Of the 153 nights, the weather and aurora conditions were only suitable on 6 nights to study the relationship between auroral activity and OH airglow variation. Of these 6 nights, a significant increase in the rotational temperature and a decrease in the intensity related to an aurora activity were identified on the night of 27/28 March 2008, but no such variations were seen during the other nights. The horizontal magnetic field disturbance on the night of 27/28 March was the largest of that winter, while the cosmic radio noise absorption was also very strong. These facts indicate that, compared with the other nights, a large flux of high-energy auroral particles precipitated during the night. It is suggested that the observed variations in the OH rotational temperature and airglow intensity were caused by a lowering of the average airglow height as a result of OH depletion in the upper part of the layer where high-energy auroral particles can reach.

Comparison of methods to determine auroral ionospheric conductances using ground-based optical and riometer data

Abstract. Ground-based images of auroral optical emissions and cosmic radio noise absorption provide information on particle precipitation which enhances ionospheric conductances. Knowledge of this conductance field is important to understand the current systems associated with auroral features. Three methods of using ground-based optical and riometer data to estimate ionospheric conductances in the aurora are compared to conductances derived from incoherent scatter radar measurements. It is shown that a method using the 557.7 nm emission intensity alone gives the best results for the Pedersen conductance whilst a method using both this intensity and cosmic noise absorption is best for the Hall conductance. A method using cosmic noise absorption alone gives reasonable performance for the Hall conductance and the Hall/Pedersen conductance ratio, but performs poorly for the Pedersen conductance. It also appears to underestimate the Hall conductance significantly during times when softer precipitation is present, for example in discrete auroral arcs. There is some indication that the methods do not degrade noticeably for angles up to ~20° off magnetic zenith.

Upper-mesospheric temperatures measured during intense substorms in the declining phase of the January 2005 solar proton events

Abstract. Temperature measurements from the ALOMAR Weber Na lidar together with cosmic radio noise absorption measurements from IRIS and particle measurements from NOAA 15, 16 and 17 are used to study effects of geomagnetic activity on the polar winter upper-mesospheric temperature. On 21–22 January 2005 we have 14 h of continuous temperature measurement with the Na lidar coinciding with strong geomagnetic activity in the declining phase of one of the hardest and most energetic Solar Proton Event (SPE) of solar cycle 23. According to measurements by the imaging riometer IRIS in northern Finland, the temperature measurements coincide with two periods of increased cosmic radio noise absorption. Particle measurements from the three satellites, NOAA 15, 16 and 17 that pass through and near our region of interest confirm that the absorption events are probably due to particle precipitation and not due to changes in e.g. the electron recombination coefficient. The measured temperature variation at 85 and 90 km is dominated by a 7.6-h wave with downward phase propagation and a vertical wavelength of approximately 10 km. Assuming that the wave is due to a lower altitude source independent of the particle precipitation, we do not find any temperature modification that seems to be related to the absorption events. The average temperature is larger than expected above 90 km based on MSIS and the monthly mean from falling spheres, which could be due to particle precipitation and Joule heating prior to our measurement period. There is also a possibility that the identified wave phenomenon is an effect of the geomagnetic activity itself. Earlier studies have reported of similar wavelike structures in wind observations made by the EISCAT VHF radar during SPEs, and found it conceivable that the wave could be excited by the effect of energetic particles precipitating into the mesosphere.

Observing mesospheric gravity waves with an imaging riometer

Gravity waves play an important role in determining the structure and dynamics of the mesopause region. In this study the imaging riometer for ionospheric studies (IRIS) at Halley, Antarctica ( 76 ∘ S , 27 ∘ W ) is shown to be capable of detecting short period mesospheric gravity waves using fluctuations of the ionospheric absorption of cosmic radio noise as a tracer. An analysis technique for quantifying these signatures is presented. The extraction of the wave period, horizontal phase speed and horizontal wavelength is demonstrated by applying wavelet analysis to synthetic imaging riometer absorption data, which contain known wave features. A mechanism to overcome the limitation on resolvable wavelengths is presented. The effect of noise on the analysis results is also discussed. The application of this technique to extract gravity wave parameters from imaging riometer data is demonstrated by comparison with those derived from a co-located airglow imager. Extension of this technique could, in future, enable a climatology of year round mesospheric gravity wave properties over Antarctica to be derived. It will also enable further gravity wave climatological studies to be performed using other imaging riometer datasets around the world.

Latitudinal extent of the January 2005 solar proton event in the Northern Hemisphere from satellite observations of hydroxyl

Abstract. We utilise hydroxyl observations from the MLS/Aura satellite instrument to study the latitudinal extent of particle forcing in the northern polar region during the January 2005 solar proton event. MLS is the first satellite instrument to observe HOx changes during such an event. We also predict the hydroxyl changes with respect to the magnetic latitude by the Sodankylä Ion and Neutral Chemistry model, estimating the variable magnetic cutoff energies for protons using a parameterisation based on magnetosphere modelling and the planetary magnetic index Kp. In the middle and lower mesosphere, HOx species are good indicators of the changes in the atmosphere during solar proton events, because they respond rapidly to both increases and decreases in proton forcing. Also, atmospheric transport has a negligible effect on HOx because of its short chemical lifetime. The observations indicate the boundary of the proton forcing and a transition region, from none to the "full" effect, which ranges from about 57 to 64 degrees of magnetic latitude. When saturating the rigidity cutoff Kp at 6 in the model, as suggested by earlier studies using observations of cosmic radio noise absorption, the equatorward boundary of the transition region is offset by ≈2 degrees polewards compared with the data, thus the latitudinal extent of the proton forcing in the atmosphere is underestimated. However, the model predictions are in reasonable agreement with the MLS measurements when the Kp index is allowed to vary within its nominal range, i.e., from 1 to 9 in the cutoff calculation.

Statistical relationships between cosmic radio noise absorption and ionospheric electrical conductances in the auroral zone

Statistical models expressing the Hall and Pedersen conductances and their ratio as functions of cosmic noise absorption (CNA) are derived for five intervals of magnetic local time (MLT). The models are based on simultaneous measurements of electron densities from the European Incoherent Scatter UHF radar at Tromsø (69.6°N, 19.2°E) and absorption from the imaging riometer at Kilpisjärvi (69.1°N, 20.8°E). The Hall conductance and the conductance ratio are found to be rather strongly related to CNA, whereas the Pedersen conductance is less so. The Hall conductance‐CNA relationship is strongly dependent on MLT. These results are interpreted as being the consequence of the particular sensitivity of CNA to the typical energy of electron precipitation, the latter changing as a function of MLT as the electrons drift around the Earth. The models are compared to a previous study which did not use simultaneous measurements or take into account the MLT dependence. There is a significant difference between that study and the results presented here.

Energetic electron precipitation during sawtooth injections

Abstract. 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.

Statistical signature of active D-region HF heating in IRIS riometer data from 1994–2004

Abstract. In this paper we study the effect of artificial HF heating on cosmic radio noise absorption in the D-region ionosphere. The effect has earlier been studied theoretically in idealised cases and without experimental verification. Here we present a 3-dimensional modelling of the effect, taking into account the directivity patterns of the vertical beam of the EISCAT Heater at Tromsø, Norway, and the intersecting beam of the IRIS imaging riometer at Kilpisjärvi, Finland. The heater-induced enhancement of cosmic radio noise absorption at the IRIS frequency (38.2 MHz) is estimated to be between 0.02 dB and 0.05 dB in the most representative model cases. However, a statistical study of IRIS data from a selected set of heating experiments carried out during the years 1994–2004 shows that the median effect is between 0.002 dB and 0.004 dB, i.e. an order of magnitude less than theoretically predicted. This indicates that the actual HF heating effect at D-region altitudes is substantially overestimated by the present theory.