Imaging Riometer For Ionospheric Studies Research Articles

Empirical modelling of auroral absorption during disturbed periods of interplanetary coronal mass ejection events

Energetic charged particle precipitation associated with solar wind perturbations causes enhanced high-frequency radiowave absorption in the high-latitude ionosphere. This study models 38.2 MHz cosmic noise absorption (CNA) by utilising measurements from the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjärvi, Finland obtained during solar cycle 23 (1996–2009) associated with the passage of interplanetary coronal mass ejections (ICMEs) past Earth; ICMEs are a major driver of enhanced geomagnetic activity. Superposed epoch analysis suggests that the r6absorption vs. time profile depends on whether ICME arrival occurs in the day-time (10–14 MLT) or night-time (22-02 MLT) for IRIS, with peak absorption occurring ~2–3 h ahead of ICME arrival or ~4 h after ICME arrival, respectively. We determine which combinations of solar wind and IMF parameters show the best correlation with the absorption associated with day-time or night-time arriving ICMEs using superposed epoch analysis and the least squares estimation method. Various combinations of solar wind parameters (including bulk velocity v, density n, and the interplanetary magnetic field north and south components Bz and the SYMH geomagnetic index), have been ranked to obtain the best coupling function for the absorption associated with day- and night-time arriving ICMEs. The absorption for day-time events is found to correlate closely with the solar wind dynamic pressure, SYMH, and the northward direction of the Bz while the absorption for night-time events is most closely related to the direction of the Bz and SYMH. The coupling functions are found to model the observed absorption successfully, with correlation coefficients of ~0.7–0.8 between the observed and modelled absorption.

Solar flare induced cosmic noise absorption

Solar flare events are a major observing emphasis for space weather because they affect the ionosphere and can eject high-energy particles that can adversely affect Earth's technologies. In this study we model 38.2 MHz cosmic noise absorption (CNA) by utilising measurements from the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjärvi, Finland obtained during solar cycle 23 (1996–2009). We utilised X-ray archive for the same period from the Geostationary Operational Environmental Satellite (GOES) to study solar flare induced cosmic noise absorption. We identified the threshold of flare (M4 class) that could bear significant influence on CNA. Through epoch analysis, we show the magnitude of absorption that each class of flare could produce. Using the parameters of flare and absorption we present a model that could provide the basis for nowcast of CNA induced by M and X-class solar flares.

Extracting gravity wave parameters during the September 2002 Southern Hemisphere major sudden stratospheric warming using a SANAE imaging riometer

Abstract. Using absorption data measured by imaging riometer for ionospheric studies (IRIS) located at the South Africa National Antarctic Expedition (SANAE), Antarctica (72° S, 3° W), we extracted the parameters of gravity waves (GW) of periods between 40 and 50 min during late winter/spring of the year 2002, a period of the unprecedented major sudden stratospheric warming (SSW) in the Southern Hemisphere middle atmosphere. During this period, an unprecedented substantial increase of temperature by about 25–30 K throughout the stratosphere was observed. During the period of the occurrence of the major stratospheric warming, there was a reduction of both the GW horizontal phase speeds and the horizontal wavelengths at 90 km. The GW phase speeds and horizontal wavelengths were observed to reach minimum values of about 7 m s−1 and 19 km, respectively, while during the quiet period the average value of the phase speed and horizontal wavelength was approximately 23 m s−1 and 62 km, respectively. The observed event is discussed in terms of momentum flux and also a potential interaction of gravity waves, planetary waves and mean circulation.

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.

A comparison of two different techniques for deriving the quiet day curve from SARINET riometer data

Abstract. In this work, an upgrade of the technique for estimating the Quiet Day Curve (QDC) as proposed by Tanaka et al. (2007) is suggested. To validate our approach, the QDC is estimated from data acquired by the Imaging Riometer for Ionospheric Studies (IRIS) installed at the Southern Space Observatory (SSO/CRS/CCR/INPE – MCT, 29°4´ S, 53°8´ W, 480 m a.s.l.), São Martinho da Serra – Brazil. The evaluation was performed by comparing the difference between the QDCs derived using our upgrade technique with the one proposed by Tanaka et al. (2007). The results are discussed in terms of the seasonal variability and the level of magnetic disturbance. Also, the cosmic noise absorption (CNA) images for IRIS data operated at SSO was built using both the techniques aiming to check the implications of the changes in the methods of QDC determination on the CNA that resulted from it.

Observations of omega bands using an imaging riometer

Abstract. We present a case study of an omega band current system from 11 May 1998 using data from the Imaging Riometer for Ionospheric Studies (IRIS) in Finland. For the first time, images of a substorm-related omega band in cosmic noise absorption are shown. The substorm in question was one of a string that occurred on that day; inspection of geostationary satellite data indicates that this was a sawtooth event. Using a previously established statistical relationship, the IRIS data is used to provide maps of Hall conductance and compared with previous estimates utilising both HF and VHF coherent-scatter radars. Discrepancies are discussed with reference to precipitation spectrum and the geometry of the experimental set-up. The imaging riometer data provides a higher spatial resolution than the combined magnetometer-radar pairing for determining the Hall conductance and can also be used to identify the extent of the precipitation in the absence of optical data.

On the relationship between auroral absorption, electrojet currents and plasma convection

Abstract. In this study, the relationship between auroral absorption, electrojet currents, and ionospheric plasma convection velocity is investigated using a series of new methods where temporal correlations are calculated and analysed for different events and MLT sectors. We employ cosmic noise absorption (CNA) observations obtained by the Imaging Riometer for Ionospheric Studies (IRIS) system in Kilpisjärvi, Finland, plasma convection measurements by the European Incoherent Scatter (EISCAT) radar, and estimates of the electrojet currents derived from the Tromsø magnetometer data. The IRIS absorption and EISCAT plasma convection measurements are used as a proxy for the particle precipitation component of the Hall conductance and ionospheric electric field, respectively. It is shown that the electrojet currents are affected by both enhanced conductance and electric field but with the relative importance of these two factors varying with magnetic local time (MLT). The correlation between the current and electric field (absorption) is the highest at 12:00–15:00 MLT (00:00–03:00 MLT). It is demonstrated that the electric-field-dominant region is asymmetric with respect to magnetic-noon-midnight meridian extending from 09:00 to 21:00 MLT. This may be related to the recently reported absence of mirror-symmetry between the effects of positive and negative IMF By on the high-latitude plasma convection pattern. The conductivity-dominant region is somewhat wider than previously thought extending from 21:00 to 09:00 MLT with correlation slowly declining from midnight towards the morning, which is interpreted as being in part due to high-energy electron clouds gradually depleting and drifting from midnight towards the morning sector. The conductivity-dominant region is further investigated using the extensive IRIS riometer and Tromsø magnetometer datasets with results showing a distinct seasonal dependence. The region of high current-absorption correlation extends from 21:00 to 06:00 MLT near both equinoxes, however, it is narrower and rotated towards the morning (02:00–07:00 MLT) in summer, while in winter the correlation shows much greater variability with MLT. During periods of high current-electric-field correlation, the relationship between electric field and absorption can be described as an inverse proportionality, which can be explained by limitation of the electrojet current by the magnetospheric generator. Possible cases of electron heating absorption are also investigated with absorption showing no obvious dependence on the ion velocity or electron temperature.

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.

GPS scintillation over the European Arctic during the November 2004 storms

Small-scale irregularities in the background electron density of the ionosphere can cause rapid fluctuations in the amplitude and phase of radio signals passing through it. These rapid fluctuations are known as scintillation and can cause a Global Positioning System (GPS) receiver to lose lock on a signal. This could compromise the integrity of a safety of life system based on GPS, operating in auroral regions. In this paper, the relationship between the loss of lock on GPS signals and ionospheric scintillation in auroral regions is explored. The period from 8 to 14 November 2004 is selected for this study, as it includes both geomagnetically quiet and disturbed conditions. Phase and amplitude scintillation are measured by GPS receivers located at three sites in Northern Scandinavia, and correlated with losses of signal lock in receivers at varying distances from the scintillation receivers. Local multi-path effects are screened out by rejection of low-elevation data from the analysis. The results indicate that losses of lock are more closely related to rapid fluctuations in the phase rather than the amplitude of the received signal. This supports the idea, suggested by Humphreys et al. (2005) (performance of GPS carrier tracking loops during ionospheric scintillations. Proceedings Internationsl Ionospheric Effects Symposium 3–5 May 2005), that a wide loop bandwidth may be preferred for receivers operating at auroral latitudes. Evidence from the Imaging Riometer for Ionospheric Studies (IRIS) appears to suggest that, for this particular storm, precipitation of particles in the D/E regions may be the mechanism that drives the rapid phase fluctuations in the signal.

Characteristics of night-time absorption spike events

Abstract. Sudden increases in cosmic radio noise absorption, known as spike events, have been identified as signatures of substorms in the previous studies. Using data from the IRIS (Imaging Riometer for Ionospheric Studies) at Kilpisjärvi, Finland (L~6) more than 450 night-time spike events between 1994 and 2003 have been identified. Spike events fall into four distinct categories based on their structure and the background magnetic activity as indicated by a local westward electrojet (IL index) derived from the IMAGE (International Monitor for Auroral Geomagnetic Effects) magnetometer network as well as Pi2 magnetic pulsations from SAMNET (The UK Sub-Auroral Magnetometer Network). Classifying the types of absorption spikes allows for identification of phenomena such as multiple onsets and pseudobreakups from riometer data. In addition we have studied the statistical variation of absorption spikes and their sub-classes. This includes examining the magnetic local time (MLT) distribution and the seasonal and solar-cycle variation in spike occurrence. Those that seem to represent substorm onsets show a decidedly different MLT variation to those isolated spikes that represent pseudobreakups. The occurrence of spikes during different levels of geomagnetic activity is examined using the Kp index. Wavelet analysis has been used for studying the temporal structure of spikes; also the direction of motion of spike events and localisation of spikes are presented for all events and each sub-class and results are compared with previous studies. Statistical studies are supported with X-ray images of aurora from PIXIE (The Polar Ionospheric X-ray Imaging Experiment) when available.

The statistical dependence of auroral absorption on geomagnetic and solar wind parameters

Abstract. Data from the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjärvi, Finland, have been compiled to form statistics of auroral absorption based on seven years of observations. In a previous study a linear relationship between the logarithm of the absorption and the Kp index provided a link between the observations of precipitation with the level of geomagnetic activity. A better fit to the absorption data is found in the form of a quadratic in Kp for eight magnetic local time sectors. Past statistical investigations of absorption have hinted at the possibility of using the solar wind velocity as a proxy for the auroral absorption, although the lack of available satellite data made such an investigation difficult. Here we employ data from the solar wind monitors, WIND and ACE, and derive a linear relationship between the solar wind velocity and the cosmic noise absorption at IRIS for the same eight magnetic local time sectors. As far as the authors are aware this is the first time that in situ measurements of the solar wind velocity have been used to create a direct link with absorption on a statistical basis. The results are promising although, it is clear that some other factor is necessary in providing reliable absorption predictions. Due to the substorm related nature of auroral absorption, this is likely formed by the recent time history of the geomagnetic activity, or by some other indicator of the energy stored within the magnetotail. For example, a dependence on the southward IMF (interplanetary magnetic field) is demonstrated with absorption increasing with successive decreases in Bz; a northward IMF appears to have little effect and neither does the eastward component, By. Key words. Magnetospheric physics (energetic particles, precipitating; solar wind-magnetosphere interactions) – Ionosphere (modeling and forecasting)

High‐resolution maps of the characteristic energy of precipitating auroral particles

For the first time we produce high‐resolution maps of the characteristic energy of precipitating electrons from ground‐based instrumentation in the auroral zone over northern Scandinavia. This is done by combining intensity‐calibrated optical data at 557.7 nm from the Digital All‐Sky Imager (DASI) with auroral absorption images from the Imaging Riometer for Ionospheric Studies (IRIS). Energy maps are produced with high temporal (10 s) and spatial (10 km) resolution within a common geographic area of 240 × 240 km. Both IRIS and DASI have the European Incoherent Scatter (EISCAT) radar within their common field of view. EISCAT is capable of making accurate measurements of the electron density height profile which, with the assistance of an atmospheric model, are inverted into equivalent energy spectra of the flux of precipitating electrons. However, incoherent scatter radars generally have a very small field of view (<1°), making studies of the energy spectrum of the precipitating particles over a wide field of view impractical. Since IRIS and DASI are sensitive to high‐ and medium‐energy electrons, respectively, EISCAT data are used to calibrate the characteristic energy of the precipitating particles for an assumed energy spectrum against a combination of IRIS and DASI data. This empirical calibration is then used throughout the common field of view of IRIS and DASI. An initial study illustrates the spatial relationship between the different energy ranges during a substorm onset and illustrates a new way to interpret auroral phenomena.

An imaging riometer for ionospheric studies

An imaging riometer for ionospheric studies (IRIS) is described. This phased-array system has been continuously operating in northern Finland since late 1994, and permits spatial observation of ionospheric absorption of radio waves over the surrounding area. The paper gives an introduction to the principles of riometry and ionospheric absorption; it describes the physical implementation of the system and presents some initial results. Consideration is also given to the effect of the absorption on radio links operating in the auroral regions, which can be severe.

A comparative study of imaging and broad‐beam riometer measurements: The effect of spatial structure on the frequency dependence of auroral absorption

Observations of the spatial structure and motion of auroral absorption regions obtained with the 38.2‐MHz imaging riometer for ionospheric studies (IRIS) at South Pole Station, Antarctica, have been used to simulate the absorption that should be measured by the 20.5‐, 30‐, 38.2‐, and 51.4‐MHz broad‐beam riometers also operating at the station. The simulation assumes that the absorption measured by each of the 49 narrow IRIS beams obeys the absorption‐frequency relationship, A(ƒ) ∼ ƒ−2, predicted by generalized magnetoionic theory to apply at altitudes above ∼70 km. Excellent agreement to quite fine detail is obtained between the simulated and measured broad‐beam absorption profiles. The power law exponents derived from absorption ratios, or from least squares fits to the absorption as a function of frequency, range between 1 and 2 and are essentially the same for simulated and measured broad‐beam data, to within the experimental and computational uncertainties. These results indicate that spatial gradients in the absorption regions, corresponding to similar features in the electron precipitation source, are the cause of deviations from the inverse‐square frequency dependence noted previously in broad‐beam riometer measurements and erroneously interpreted as signifying very high energy electron precipitation and enhanced ionization at much lower altitudes.

A phased‐array radiowave imager for studies of cosmic noise absorption

Most measurements of cosmic radio noise absorption in the polar regions have been made with riometers using broad‐beam (∼60 deg) antennas. Only limited information about the spatial structure and dynamics of the aurora can be obtained by this means. Recent trends in riometry have emphasized the use of multiple narrow‐beam antennas operated in a fixed beam or one‐dimensional scanning mode to examine smaller ionospheric regions. A further step in this direction has been the development of the imaging riometer for ionospheric studies (IRIS). This instrument provides a two‐dimensional image of regions of enhanced cosmic noise absorption at 38.2 MHz with a spatial resolution as small as 20 km and time resolution of 1 s. The IRIS antenna is a 64‐element dipole array, phased to produce 49 independent beams viewing an ionospheric area about 200 km square at 90 km altitude. One IRIS instrument has been operating at the South Pole station since January, 1988; a second instrument was recently installed at Sondre Stromfjord, Greenland. This paper presents a technical description of the IRIS system. The response of the IRIS to a sun‐aligned absorption arc is illustrated and compared with that of a broad‐beam riometer. An intense, localized structure that developed within the arc was observed by IRIS to propagate in the magnetic poleward direction with a speed of 1 km/s.