The paper reviews recent advances in studies of electric discharges in the stratosphere and mesosphere above thunderstorms, and their effects on the atmosphere. The primary focus is on the sprite discharge occurring in the mesosphere, which is the most commonly observed high altitude discharge by imaging cameras from the ground, but effects on the upper atmosphere by electromagnetic radiation from lightning are also considered. During the past few years, co-ordinated observations over Southern Europe have been made of a wide range of parameters related to sprites and their causative thunderstorms. Observations have been complemented by the modelling of processes ranging from the electric discharge to perturbations of trace gas concentrations in the upper atmosphere. Observations point to significant energy deposition by sprites in the neutral atmosphere as observed by infrasound waves detected at up to 1000 km distance, whereas elves and lightning have been shown significantly to affect ionization and heating of the lower ionosphere/mesosphere. Studies of the thunderstorm systems powering high altitude discharges show the important role of intracloud (IC) lightning in sprite generation as seen by the first simultaneous observations of IC activity, sprite activity and broadband, electromagnetic radiation in the VLF range. Simulations of sprite ignition suggest that, under certain conditions, energetic electrons in the runaway regime are generated in streamer discharges. Such electrons may be the source of X-and Gamma-rays observed in lightning, thunderstorms and the so-called Terrestrial Gamma-ray Flashes (TGFs) observed from space over thunderstorm regions. Model estimates of sprite perturbations to the global atmospheric electric circuit, trace gas concentrations and atmospheric dynamics suggest significant local perturbations, and possibly significant meso-scale effects, but negligible global effects.

We present observations made in space and on the ground during the growth phase and the onset of a substorm on August 31, 1986. About 20 min after the ε parameter at the magnetopause had exceeded 1011 W, magnetic field dipolarization with an increase of energetic particle fluxes was observed by the AMPTE Charge Composition Explorer (CCE) spacecraft at the geocentric distance of 8.7 RE close to magnetic midnight. The event exhibited local signatures of a substorm onset at AMPTE CCE and a weak wedgelike current system in the midnight sector ionosphere. However, it did not lead to a full‐scale substorm expansion, as determined by several ground‐based instruments, nor did it produce large particle injections at geostationary orbit. Only after another 20 min of continued growth phase the entire magnetosphere‐ionosphere system could apparently allow the onset of a regular substorm expansion. The initial activation is interpreted in the present paper as a “pseudobreakup.” We examine the physical conditions in the near‐Earth plasma sheet using spacecraft observations and analyze the development in the ionosphere using ground‐based magnetometers and electric field observations from the STARE radar. We find that the main observable differences between pseudobreakups and ordinary breakups are the strength and consequences. Furthermore, it is shown that ionospheric activity at the time of a pseudobreakup is not necessarily as localized in longitude as generally believed.

[1] Data of the first half year of operation of a sensitive search coil magnetometer at a remote site in the island of Crete, Greece (35.15°N, 25.20°E), was used to investigate properties of the spectral resonance structure (SRS) of the ionospheric Alfvén resonator (IAR) at L = 1.3. Most of the properties known from earlier reports and a recent paper (A. G. Yahnin et al., Morphology of the spectral resonance structure of the electromagnetic background noise in the range of 0.1-4 Hz at L = 5.2, submitted to Annales Geophysicae, 2002) (hereinafter referred to as paper CP) at mid and high latitude (L = 2.65 and L = 5.2) could be verified as being valid also at L = 1.3, but several new features were also found. In contrast to mid and high latitudes, SRS signatures were detected every night but not at all during daytime. The average frequency difference Áf between two adjacent harmonics is very small (0.2 Hz) and does not exhibit a local time dependence from evening to night hours. The seasonal dependence is very weak though distinct. A large variability of Áf from night to night was found which increases when proceeding from summer to winter. This variability could not be accounted for by standard IAR models employing an International Reference Ionosphere (IRI). Moreover, the modeled Áf values typically exhibited a systematic offset to higher values as compared to observed ones. It is expected that calibration of the IRI model by local f o F 2 ionosonde measurements will improve the agreement between model and observation, but it cannot explain fully the variability of Áf and the systematic offset. Most likely the standard IAR model itself requires revision to be fully applicable in the low-latitude ionosphere.

Abstract. Resonance features of the Ionospheric AlfvénResonator (IAR) can be observed in pulsation magnetometer data from Sodankylä, Finland using dynamic spectra visualizations. IAR resonance features were identified on 13 of 30 days in October 1998, with resonance structures lasting for 3 or more hours over 10 intervals. The diurnal evolution of the harmonic features was quantified for these 10 intervals using a manual cursor-clicking technique. The resonance features displayed strong linear relationships between harmonic frequency and harmonic number for all of the time intervals studied, enabling a homogeneous cavity model for the IAR to be adopted to interpret the data. This enabled the diurnal variation of the effective size of the IAR to be obtained for each of the 10 time intervals. The average effective size was found to be 530 km, and to have an average variation of 32% over each time interval: small compared to the average variation in Alfvén velocity of 61%. Thus the diurnal variation of the harmonics is chiefly caused by the changing plasma density within the IAR due to changing insolation. This study confirms Odzimek (2004) that the dominating factor affecting the IAR eigenfrequencies is the variation in the Alfvén velocity at the F-layer ion-density peak, with the changing IAR size affecting the IAR eigenfrequencies to a smaller extent. Another IAR parameter was derived from the analysis of the IAR resonance features associated with the phase matching structure of the standing waves in the IAR. This parameter varied over the time intervals studied by 20% on average, possibly due to changing ionospheric conductivity.

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