Thomas Ulich

Evidence for long-term cooling of the upper atmosphere in ionosonde data

While several model estimates predict cooling of the upper atmosphere as a result of increasing concentrations of greenhouse gases, direct observational evidence of such a trend is scarce and partly susceptible, because the relevant data series do not cover sufficiently long time periods. We study the long-term data set from the ionosonde station at Sodankyla (67.4°N, 26.7°E), which has been operated during almost 4 solar cycles. We find a close to linear decrease in the altitude of the F2 layer peak during the last 39 years, when the effect of solar cycle variations is removed from the data. This local trend is qualitatively consistent with the model predictions of a cooling of the lower thermosphere.

Energetic particle precipitation into the middle atmosphere triggered by a coronal mass ejection

Precipitation of relativistic electrons into the atmosphere has been suggested as the primary loss mechanism for radiation belt electrons during large geomagnetic storms. Here we investigate the geographical spread of precipitation as a result of the arrival of a coronal mass ejection (CME) on 21 January 2005. In contrast to previous statistical studies we provide one of the first attempts to describe the geographic and temporal variability of energetic particle precipitation on a global scale using an array of instruments. We combine data from subionospheric VLF radio wave receivers, the high-altitude Miniature Spectrometer (MINIS) balloons, riometers, and pulsation magnetometers during the first hour of the event. There were three distinct types of energetic electron precipitation observed, one globally, one on the dayside, and one on the nightside. The most extensively observed form of precipitation was a large burst starting when the CME arrived at the Earth, where electrons from the outer radiation belt were lost to the atmosphere over a large region of the Earth. On the dayside of the Earth (10–15 MLT) the CME produced a further series of precipitation bursts, while on the nightside dusk sector (∼20 MLT) a continuous precipitation event lasting ∼50 min was observed at 2.5 < L < 3.7 along with Pc 1–2 pulsations observed with a ground-based magnetometer. These observations suggest that the generation of energetic electron precipitation at the inner edge of the outer radiation belt from electromagnetic ion cyclotron (EMIC) wave scattering into the loss cone is the most direct evidence to date connecting EMIC activity and energetic precipitation.

The causes of long‐term change in the aa index

The aa index provides the longest geomagnetic data set that can be used in the analysis of magnetospheric and ionospheric phenomena. All phases of the solar cycle show increases in storm activity since the end of cycle 14 in 1915. The activity increase does not appear to be strongly associated with any instrumental, ionospheric or magnetospheric effects. Small effects have been confirmed in the long-term change in ionospheric Pedersen and Hall conductivities due to the changing dipole moment of the Earth but not due to increasing greenhouse gases. Three instrumental effects have been identified where significant changes in quiet time conditions can be seen, that is, 1938, 1980, and 1997. These do not account for the majority of the increase in aa. Noise levels for the aa index are now close to those seen at the beginning of the data set. The prime cause of the increase in storm activity is an increase in solar activity. The average aa in cycle 23 should be about 1 nT less than that predicted from previous cycles due to the reduction in baseline noise levels at the start of the cycle (1997).

Radiation belt electron precipitation due to geomagnetic storms: Significance to middle atmosphere ozone chemistry

[1] Geomagnetic storms triggered by coronal mass ejections and high‐speed solar wind streams can lead to enhanced losses of energetic electrons from the radiation belts into the atmosphere, both during the storm itself and also through the poststorm relaxation of enhanced radiation belt fluxes. In this study we have analyzed the impact of electron precipitation on atmospheric chemistry (30–90 km altitudes) as a result of a single geomagnetic storm. The study conditions were chosen such that there was no influence of solar proton precipitation, and thus we were able to determine the storm‐induced outer radiation belt electron precipitation fluxes. We use ground‐based subionospheric radio wave observations to infer the electron precipitation fluxes at L = 3.2 during a geomagnetic disturbance which occurred in September 2005. Through application of the Sodankyla Ion and Neutral Chemistry model, we examine the significance of this particular period of electron precipitation to neutral atmospheric chemistry. Building on an earlier study, we refine the quantification of the electron precipitation flux into the atmosphere by using a time‐varying energy spectrum determined from the DEMETER satellite. We show that the large increases in odd nitrogen (NO x) and odd hydrogen (HO x) caused by the electron precipitation do not lead to significant in situ ozone depletion in September in the Northern Hemisphere. However, had the same precipitation been deposited into the polar winter atmosphere, it would have led to >20% in situ decreases in O 3 at 65–80 km altitudes through catalytic HO x cycles, with possible additional stratospheric O 3 depletion from descending NO x beyond the model simulation period. Citation: Rodger, C. J., M. A. Clilverd, A. Seppala, N. R. Thomson, R. J. Gamble, M. Parrot, J.‐A. Sauvaud, and T. Ulich (2010), Radiation belt electron precipitation due to geomagnetic storms: Significance to middle atmosphere ozone chemistry,

Residual solar cycle influence on trends in ionospheric F2‐layer peak height

[1] The longest data sets available for estimating thermospheric temperature trends are those from ground-based ionosondes, which often begin during the International Geophysical Year of 1957, close to a solar activity maximum. It is important to investigate inconsistencies in trend estimates from these data sets so that trends can be clearly determined. Here we use selected ionosonde stations to show that one of the most significant factors affecting the trend estimates is the removal of the solar cycle. The stations show trend behavior that is close to the behavior of a theoretical model of damped harmonic oscillation. The ringing features are consistent with the presence of solar cycle residuals from the analysis with an amplitude of 2.5 km. Some stations do not show trend behavior that is close to either the average behavior of the stations studied here or the theoretical model of oscillation. Four European stations (Poitiers, Lannion, Juliusruh, and Slough), three of which are closely located in western Europe, were analyzed with the expectation that their trend should be similar. Only Poitiers and Juliusruh showed an evolution that was close to the average behavior of other stations, while the other two were significantly different. The primary cause of this appears to be changes in the M(3000)F2 parameter and demonstrates the importance of incorporating consistency checks between neighboring ionosondes into global thermospheric trend estimates.

Key features of >30 keV electron precipitation during high speed solar wind streams: A superposed epoch analysis

We present an epoch analysis of energetic (>30 keV) electron precipitation during 173 high speed solar wind streams (HSS) using riometer observations of cosmic noise absorption (CNA) as a proxy for the precipitation. The arrival of the co-rotating interaction region (CIR) prior to stream onset, elevates the precipitation which then peaks some 12 h after stream arrival. Precipitation continues for several days following the HSS arrival. The MLT distribution of CNA is generally consistent with the statistical pattern explained via the substorm process, though the statistical deep minimum of CNA/precipitation does change during the HSS suggesting increased precipitation in the 15–20 MLT sector. The level of precipitation is strongly controlled by the average state of the IMF BZ component on the day prior to the arrival of the stream interface. An average negative IMF BZ will produce higher CNA across all L-shells and MLT, up to 100% higher than an average positive IMF BZ.

Determining long-term change in the ionosphere

Contemporary studies of long-term changes in the ionosphere stem mostly from the suggestions by Roble and Dickinson [1989] that “global warming” in the lower atmosphere is accompanied by “global cooling” of the thermosphere, and subsequently, from the suggestion by Rishbeth [1990] that the resulting thermal contraction lowers the height of the ionospheric F2-peak hmF2. The subject is attractive to study because decades of ionosonde data exist from dozens of stations worldwide, the data are well organized in a consistent format, and the analysis requires no great computing power. However, the trends from different stations are far from consistent and often show interruptions or reversals. There are tantalizing details, such as opposite trends of rising hmF2 at places east of longitude 30°E, falling hmF2 west of 30°E [Bremer, 1998]. We have to ask: Are these real? Indeed, we should ask the more general question: “What is needed to make reported trends “believable”?

Long‐term determination of energetic electron precipitation into the atmosphere from AARDDVARK subionospheric VLF observations

We analyze observations of subionospherically propagating very low frequency (VLF) radio waves to determine outer radiation belt energetic electron precipitation (EEP) flux magnitudes. The radio wave receiver in Sodankyla, Finland (Sodankyla Geophysical Observatory) observes signals from the transmitter with call sign NAA (Cutler, Maine). The receiver is part of the Antarctic-Arctic Radiation-belt Dynamic Deposition VLF Atmospheric Research Konsortia (AARDDVARK). We use a near-continuous data set spanning November 2004 until December 2013 to determine the long time period EEP variations. We determine quiet day curves over the entire period and use these to identify propagation disturbances caused by EEP. Long Wave Propagation Code radio wave propagation modeling is used to estimate the precipitating electron flux magnitudes from the observed amplitude disturbances, allowing for solar cycle changes in the ambient D region and dynamic variations in the EEP energy spectra. Our method performs well during the summer months when the daylit ionosphere is most stable but fails during the winter. From the summer observations, we have obtained 693 days worth of hourly EEP flux magnitudes over the 2004–2013 period. These AARDDVARK-based fluxes agree well with independent satellite precipitation measurements during high-intensity events. However, our method of EEP detection is 10–50 times more sensitive to low flux levels than the satellite measurements. Our EEP variations also show good agreement with the variation in lower band chorus wave powers, providing some confidence that chorus is the primary driver for the outer belt precipitation we are monitoring.

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.

Thermal and dynamical perturbations in the winter polar mesosphere‐lower thermosphere region associated with sudden stratospheric warmings under conditions of low solar activity

The upper mesospheric neutral winds and temperatures have been derived from continuous meteor radar (MR) measurements over Sodankyla, Finland, in 2008–2014. Under conditions of low solar activity pronounced sudden mesospheric coolings linked to the major stratospheric warming (SSW) in 2009 and a medium SSW in 2010 are observed while there is no observed thermal signature of the major SSW in 2013 occurred during the solar maximum. Mesosphere-ionosphere anomalies observed simultaneously by the MR, the Aura satellite, and the rapid-run ionosonde during a period of major SSW include the following features. The mesospheric temperature minimum occurs 1 day ahead of the stratospheric maximum, and the mesospheric cooling is almost of the same value as the stratospheric warming (~50 K), the former decay faster than the latter. In the course of SSW, a strong mesospheric wind shear of ~70 m/s/km occurs. The wind turns clockwise (anticlockwise) from north-eastward (south-eastward) to south-westward (north-westward) above (below) 90 km. As the mesospheric temperature reaches its minimum, the gravity waves (GW) in the ionosphere with periods of 10–60 min decay abruptly while the GWs with longer periods are not affected. The effect is explained by selective filtering and/or increased turbulence near the mesopause.