Paper on energetic electron precipitation and auroral morphology accepted in JGR

A recent collaborative study between ISEE /Nagoya University and SGO/University of Oulu (+ other institutes) has been accepted to be published in Journal of Geophysical Research: Space Physics, please have a look:

http://onlinelibrary.wiley.com/doi/10.1002/2016JA023484/full

This paper shows a linkage between different auroral morphological structures and the energy spectra of the associated electron precipitation. The study is based on a selected set of case studies combining the EISCAT radar and the KAIRA spectral riometer measurements with the auroral camera observations.

Effects of solar wind high-speed streams on the high-latitude ionosphere: Superposed epoch study

Our study of the effects of solar wind high-speed streams on the high-latitude ionosphere, based on data from the SGO ionosonde during the years 2006–2008, has now been accepted for publication. An early-access version is available here, and the reference is:

Grandin, M., A. T. Aikio, A. Kozlovsky, T. Ulich and T. Raita (2015), Effects of solar wind high-speed streams on the high-latitude ionosphere: Superposed epoch study, J. Geophys. Res. Space Physics, 120, doi:10.1002/2015JA021785.

Here is the abstract:

Solar wind high-speed streams (HSSs) are the most important source of geomagnetic disturbances during the declining phase of the solar cycle. Their ionospheric response, especially at high latitudes, is not fully understood yet. We carried out a phase-locked superposed epoch analysis to study the effects of HSSs on the high-latitude ionospheric F region, using data from the Sodankylä ionosonde (L = 5.25) during 2006–2008. We found that the F layer critical frequency f_oF₂ decreases between 12 and 23 magnetic local time (MLT) in summer and around equinoxes for several days. Our interpretation, supported by numerical estimations, is that increased electric fields in the evening sector of the auroral and subauroral regions create ion-neutral frictional heating. Frictional heating will increase the loss rate of O⁺ due to two reasons. The first one is neutral heating producing thermal expansion of the atmosphere and enhancing N₂ and O₂ contents at the F region peak. The second one is ion heating which may occur under strong enough electric fields (about 50–60 mV/m), leading to enhancement of the reaction coefficients. An increase in f_oF₂ is observed in two different MLT sectors. First, a short-lived f_oF₂ increase is visible during all seasons near noon on the first day after the arrival of the HSS, possibly triggered by the compressed solar wind plasma pressure pulse, which may produce particle precipitation from the dayside central plasma sheet. Second, f_oF₂ is enhanced for several days in the morning sector during equinoxes and in winter. We suggest that this is caused by the low-energy tail of particle precipitation.

Variations of foF2 and max(foE, foEs) values compared to background values, by magnetic local time and day number relative to zero epoch. ©2015. American Geophysical Union.

Falling sky over Sodankylä

Increased levels of carbon dioxide and methane are well known to warm the atmosphere. However, this phenomenon is predicted to cool the thermosphere, i.e. the upper atmosphere. In order to study this phenomenon, we have considered Sodankylä hmF2 data (obtained from ionosonde data analysis) with dynamic linear models and tried to estimate the long-term trend of the so-called F2-layer peak. 

In our new method, we use an additive model composing of a slowly varying background level (i.e. the long-term trend), seasonal variations and solar effects (F10.7 used as a solar proxy). The nice thing about dynamic linear models is that the analysis output gives us estimates for all the model components with error bars. Also the seasonal variations can be seen to be modulated by solar activity. We conclude that we see an almost 30 km decrease of the F2-layer peak during 1957-2014. We also note that the trend is not linear. However, as this is a 'point measurement', we do not conclude anything about global trends - these need to be studied in subsequent papers!

Our study has been accepted for publication in Journal of Geophysical Research - Space Physics. Early access version can be downloaded here! Reference is:

L. Roininen, M. Laine and T. Ulich, Time-varying ionosonde trend: Case study of Sodankylä  hmF2 data 1957-2014, Journal of Geophysical Research - Space Physics, (2015) doi:10.1002/2015JA021176.

This paper is part of a special issue titled "Long-term Changes and Trends in the Stratosphere, Mesosphere, Thermosphere, and Ionosphere,  JGR-Atmospheres/Space Physics, 2014".

... and here is the title and abstract:

Time-varying ionosonde trend: Case study of Sodankylä  hmF2 data 1957-2014

We discuss trend analysis of non-stationary ionosonde hmF2 time-series measured at  Sodankylä Geophysical Observatory  (67.4$^\circ$N, 26.7$^\circ$E), Finland, 1957-2014. We model the hmF2 with a dynamic regression time-series model with the following components: a slowly varying background level, seasonal variations and solar effects. We analyze the time-series with a dynamic linear state-space model. Such an approach allows model components to vary in time, allowing us to study the dynamic stochastic nature of the underlying long-term trend. This feature is lacking in most time-series models used in atmospheric and environmental long-term trend analyses. Our objective is to understand the long-term hmF2 trend with respect to increased levels of carbon dioxide and methane in the atmosphere. Based on model estimates, this phenomenon is predicted to cool the thermosphere, and, leads to decrease of the altitude of the so-called F2-layer peak. After accounting for the effects of solar activity variations on the data, we see that the estimated trend shows an almost 30 km decrease of the F2-layer peak during the observation period. The decrease of the peak during 1990-2010 is significantly greater than during earlier observation period.