Antti Kero

Generation of artificial magnetic pulsations in the Pc1 frequency range by periodic heating of the Earth's ionosphere: indications of Alfvén resonator effects

Abstract A series of six experiments with the EISCAT HF heater device assisted by the EISCAT (European-Incoherent-Scatter) radar were carried out with the purpose of producing artificial magnetic pulsations in the 0.1–3 Hz frequency range. In only 3 of the 30 h of experiment time under a variety of ionospheric conditions was an artificial magnetic signal detected by ground-based magnetometers. A numerical model was used to explain the sporadic nature of the artificial signal in terms of ionospheric parameters. For several experiments the EISCAT radar provided an in situ electric field and/or electron density values; otherwise standard neutral atmosphere and ionosphere models were used. The PGI model was only partially successful. It could produce the right order of magnitude for the artificial signal when it was observed and it could demonstrate the different efficiencies when using either the O- or the X-mode of the HF wave, but it could not explain why the artificial signal was observed at a particular time and not at others. This is only partially due to the uncertainty in one or more input parameters. When the artificial signal was observed its spectrum usually exhibited spectral resonance structures of the Ionospheric Alfven Resonator (IAR), indicating that a “DC approach” is insufficient and that the generation of oscillating field-aligned currents, and thereby shear Alfven waves, has to be taken into account. We believe, however, that even with the introduction of the IAR into the model it will not be possible to resolve the sporadic character of the heating-induced artificial magnetic signal entirely. A more realistic way of D-region modeling will without doubt also be an important factor in resolving the puzzle.

KAIRA: The Kilpisjärvi Atmospheric Imaging Receiver Array—System Overview and First Results

The Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) is a dual array of omnidirectional VHF radio antennas located near Kilpisjärvi, Finland. It is operated by the Sodankylä Geophysical Observatory. It makes extensive use of the proven LOFAR antenna and digital signal-processing hardware, and can act as a stand-alone passive receiver, as a receiver for the European Incoherent Scatter (EISCAT) very high frequency (VHF) incoherent scatter radar in Tromsø, or for use in conjunction with other Fenno-Scandinavian VHF experiments. In addition to being a powerful observing instrument in its own right, KAIRA will act as a pathfinder for technologies to be used in the planned EISCAT_3-D phased-array incoherent scatter radar system and participate in very long baseline interferometry experiments. This paper gives an overview of KAIRA, its principal hardware and software components, and its main science objectives. We demonstrate the applicability of the radio astronomy technology to our geoscience application. Furthermore, we present a selection of results from the commissioning phase of this new radio observatory.

Ionospheric electron density profiles inverted from a spectral riometer measurement

The 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 Kilpisja 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.

Seeking sprite-induced signatures in remotely sensed middle atmosphere NO2: latitude and time variations

Recent research on sprites shows these and other transient luminous events can exert a local impact on atmospheric chemistry, although with minor effects at global scales. In particular, both modelling and remote sensing work suggest perturbations to the background NOx up to a few tens of per cent can occur above active sprite-producing thunderstorms. In this study we present a detailed investigation of MIPAS/ENVISAT satellite measurements of middle atmospheric NO2 in regions of high likelihood of sprite occurrence during the period August to December 2003. As a proxy of sprite activity we used ground based WWLLN detections of large tropospheric thunderstorms. By investigating the sensitivity of the analysis to the characteristics of the adopted strategy, we confirm the indication of sprite-induced NO2 enhancements of about 10% at 52 km height and tens of per cent at 60 km height immediately after thunderstorm activity, as previously reported by Arnone et al (2008b Geophys. Res. Lett. 35 5807). A further analysis showed the enhancement to be dominated by the contribution from regions north of the Equator (5 ◦ Nt o 20 ◦ N) during the first 30 to 40 days of the sample (i.e. the tail of Northern Hemisphere summer) and in coincidence with low background winds. (Some figures in this article are in colour only in the electronic version)

Effects of meteoric smoke particles on the D-region ion-chemistry

This study focuses on meteor smoke particle (MSP) induced effects on the D region ion chemistry. Hereby, MSPs, represented with an 11 bin size distribution, have been included as an active component into the Sodankya Ion and Neutral Chemistry model. By doing that, we model the diurnal variation of the negatively and positively charged MSPs as well as ions and the electron density under quiet ionospheric conditions. Two distinct points in time are studied in more detail, i.e., one for sunlit conditions (Solar zenith angle is 72∘) and one for dark conditions (Solar zenith angle is 103∘). We find nightly decrease of free electrons and negative ions, the positive ion density is enhanced at altitudes above 80 km and reduced below. During sunlit conditions the electron density is enhanced between 60 and 70 km altitude, while there is a reduction in negative and positive ions densities.

WACCM climate chemistry sensitivity to sprite perturbations

Transient luminous events affect Earths atmosphere between thunderstorm tops and the lower ionosphere through ion-neutral chemistry reactions. Particular emphasis has been given to sprites, with models and observations suggesting a capability of perturbing atmospheric nitrogen oxides at a local level, as it is known to occur for tropospheric lightning and laboratory air discharges. However, it is as yet unknown whether sprites can be a relevant source of nitrogen oxides for the upper atmosphere. In this paper, we study the sensitivity of the Whole Atmosphere Community Climate Model (WACCM) to sprite-like nitrogen oxide perturbations. We take a top-down approach to estimate what magnitude sprite perturbations should have to become significant as compared to other relevant atmospheric processes and study the sensitivity of the model response within the given uncertainties. We show that, based on current predictions by sprite streamer chemistry models, sprites can perturb Tropical NOx at 70 km altitude between 0.015 ppbv (buried in the background variability) and 0.15 ppbv (about 20%), adopting a local NOx production per sprite of 1.5·1023 and 1.5·1024 molecules respectively at this altitude. Below the lowest of the adopted values, sprites are irrelevant at global scales. Sprite NOx may build up to significantly larger amounts locally above active thunderstorms, further aided by other transient luminous events and possibly terrestrial gamma ray flashes. We also use model results to interpret the available observational studies and give recommendations for future campaigns.

Broadband meter‐wavelength observations of ionospheric scintillation

Intensity scintillations of cosmic radio sources are used to study astrophysical plasmas like the ionosphere, the solar wind, and the interstellar medium. Normally, these observations are relatively narrow band. With Low-Frequency Array (LOFAR) technology at the Kilpisjarvi Atmospheric Imaging Receiver Array (KAIRA) station in northern Finland we have observed scintillations over a three-octave bandwidth. “Parabolic arcs,” which were discovered in interstellar scintillations of pulsars, can provide precise estimates of the distance and velocity of the scattering plasma. Here we report the first observations of such arcs in the ionosphere and the first broadband observations of arcs anywhere, raising hopes that study of the phenomenon may similarly improve the analysis of ionospheric scintillations. These observations were made of the strong natural radio source Cygnus-A and covered the entire 30–250 MHz band of KAIRA. Well-defined parabolic arcs were seen early in the observations, before transit, and disappeared after transit although scintillations continued to be obvious during the entire observation. We show that this can be attributed to the structure of Cygnus-A. Initial results from modeling these scintillation arcs are consistent with simultaneous ionospheric soundings taken with other instruments and indicate that scattering is most likely to be associated more with the topside ionosphere than the F region peak altitude. Further modeling and possible extension to interferometric observations, using international LOFAR stations, are discussed.

Energetic electron precipitation and auroral morphology at the substorm recovery phase

It is well known that auroral patterns at the substorm recovery phase are characterized by diffuse or patch structures with intensity pulsation. According to satellite measurements and simulation studies, the precipitating electrons associated with these aurorae can reach or exceed energies of a few hundreds of keV through resonant wave-particle interactions in the magnetosphere. However, because of difficulty of simultaneous measurements, the dependency of energetic electron precipitation (EEP) on auroral morphological changes in the mesoscale has not been investigated to date. In order to study this dependency, we have analyzed data from the European Incoherent Scatter (EISCAT) radar, the Kilpisjarvi Atmospheric Imaging Receiver Array (KAIRA) riometer, collocated cameras, ground-based magnetometers, the Van Allen Probe satellites, Polar Operational Environmental Satellites (POES), and the Antarctic-Arctic Radiation-belt (Dynamic) Deposition-VLF Atmospheric Research Konsortium (AARDDVARK). Here we undertake a detailed examination of two case studies. The selected two events suggest that the highest energy of EEP on those days occurred with auroral patch formation from postmidnight to dawn, coinciding with the substorm onset at local midnight. Measurements of the EISCAT radar showed ionization as low as 65 km altitude, corresponding to EEP with energies of about 500 keV.

Characteristics of PMSE associated with the geomagnetic disturbance driven by corotating interaction region and high‐speed solar wind streams in the declining solar cycle 23

We report interannual variations of the correlation between the reflectivity of polar mesospheric summer echoes (PMSEs) and solar wind parameters (speed and dynamic pressure), and AE index as a proxy of geomagnetic disturbances, and cosmic noise absorption (CNA) in the declining phase (2001–2008) of solar cycle 23. PMSEs are observed by 52 MHz VHF radar measurements at Esrange (67.8°N, 20.4°E), Sweden. In approaching the solar minimum years, high-speed solar wind streams emanate from frequently emerging coronal holes, leading to 7, 9, and 13.5 day periodicities in their arrival at Earth. Periodicities of 7 and/or 9 days are found in PMSE reflectivity in 2005–2006 and 2008. Periodicity-resolved correlations at 7 and 9 days of both D region ionization observed by cosmic noise absorption (CNA) and PMSE with solar wind speed and AE index vary from year to year but generally increase as solar minimum is approached. PMSEs have a higher periodicity-resolved correlation with AE index than the solar wind speed. In addition, cross correlation of PMSE reflectivity with AE index is mostly higher than with CNA in solar minimum years (2005–2008). This can signify that high-speed solar wind stream-induced high-energy particles possibly have strong influence on CNA, but not as much as on PMSE, especially for the years of significant periodicities occurring.

Electron Energy Spectrum and Auroral Power Estimation from Incoherent Scatter Radar Measurements

Differential energy flux of electrons precipitating into the high-latitude ionosphere can be estimated from incoherent scatter radar observations of the ionospheric electron density profile. We present a method called ELSPEC for electron spectrum estimation from incoherent scatter radar measurements, which is based on integration of the electron continuity equation and spectrum model selection by means of the Akaike information criterion. This approach allows us to use data with almost arbitrary time resolutions, enables spectrum estimation with dense energy grids, avoids noise amplifications in numerical derivatives, and yields statistical error estimates for all the output parameters, including the number and energy fluxes and upward field-aligned currents carried by the precipitating electrons. The technique is targeted for auroral energies, 1–100 keV, which ionize the atmosphere mainly between 80 and 150 km altitudes. We validate the technique by means of a simulation study, which shows that Maxwellian, kappa, and mono-energetic spectra, as well as combinations of those, can be reproduced. Comparison study for two conjugate satellite measurements to the EISCAT UHF radar are shown, for Reimei and Swarm, showing an agreement with the results. Finally, an example of a 2-hr measurement by the EISCAT radar is shown, during which we observe a variety of precipitation characteristics, from soft background precipitation to mono-energetic spectra with peak energies up to 60 keV. The upward field-aligned current varies from 0 to 10 μAm−2 and the total energy flux from 0 to 250 mWm−2.