[1] Using GPS total electron content (TEC) measurements, we analyzed ionosphere response to the great Kurile earthquake of 4 October 1994. High spatial resolution of the Japanese dense array of GPS receivers (GEONET) provided us the unique opportunity to observe the evolution of coseismic ionospheric disturbances (CID), which propagated for more than 1800 km away from the epicenter. Plotting a traveltime diagram for the CID and using an ''array processing'' technique within the approximation of a spherical CID wavefront, we observed a phenomenon of CID separation into two modes and we found that characteristics of the CID depend on the distance from the epicenter. The maximum of the CID amplitude was observed at $500 km from the epicenter. Within the first 600-700 km, the CID propagation velocity was about 1 km/s, which is equal to the sound speed at the height of the ionospheric F-layer. Starting from $600 to 700 km out from the epicenter, the disturbance seems to divide into two separate perturbations, with each propagating at a different velocity, about 3 km/s for the one and about 600 m/s for the other. Apparently, the TEC response in the far-field of the CID source is a mixture of signals that further ''splits'' into two modes because of the difference in their velocities. Our observations are in good agreement with the results of space-time data processing in the approximation of a spherical wavefront of CID propagation.

[1] Recently, it has been shown that the ionosphere is capable of showing images of seismic fault shortly after an earthquake. This gives rise to the idea of retrieval of seismic information from ionospheric observations. As the first step toward such inversion, here we study distinctive features of ionospheric response to shallow earthquakes, both submarine and inland, of moment magnitudes Mw7.2-9.1. Using GPS measurements of the ionospheric total electron content, we show that: (1) the amplitude of coseismic total electron content variations in the near-field is larger after more powerful earthquakes, and (2) stronger earthquakes (M > 7.9) are in general characterized by a longer negative phase in coseismic perturbations. Citation: Astafyeva, E., S. Shalimov, E. Olshanskaya, and P. Lognonné (2013), Ionospheric response to earthquakes of different magnitudes: Larger quakes perturb the ionosphere stronger and longer, Geophys.

[1] There is growing experimental evidence to suggest that mesoscale spread F is linked to the occurrence of midlatitude coherent backscatter from patchy sporadic-E layers, which are unstable to the gradient-drift and Farley-Buneman plasma instabilities. To validate this suggestion, we have compared E-region backscatter and spread-F ionosonde recordings from about 100 days of joint operation during summer and found a one-to-one relation in the occurrence of both phenomena. Also, midlatitude backscatter studies over the last few years have shown the existence of enhanced electric fields inside patchy sporadic E. These are believed to be polarization fields set up locally by neutral winds that transport the plasma patches horizontally, and by the relatively large Hall-to-Pedersen conductivity ratios at E-region altitudes. Moreover, midlatitude echoes were found to be associated with mostly westward drifting sporadic-E patches with typical scale lengths from 10 to more than 100 km and perturbed eastward electric fields from a few to maybe more than 10 to 15 mV/m. We propose that the enhanced polarization fields set up inside unstable sporadic-E patches can easily map up the magnetic field lines to the F region and thus contribute to the formation of midlatitude spread F. This new mechanism for spread-F generation is basically an image process that can account for key observational properties of the phenomenon. These include the rapid plasma upwelling and the abrupt changes in height (uplifts) of the F layer, as well as the scale sizes involved and morphological characteristics.

Abstract. Recent 50 MHz E region coherent backscatter observations and in situ rocket measurementsestabhshed the existence of enhanced electric fields in the midlatitude ionosphere that can become at times sufficiently large to excite the Farley-Buneman instabihty. To understand the origin of these fields, we present a simple quantitative model that relates to a local polarization process acting inside spatially confined, nighttime sporadic E layers of dense ionization. By including the effects of field-aligned currents in the current continuity equation we estimate the necessary conditions on the relative horizontal E layer extent and the ratio of integrated Pedersen conductivities above and inside the layer for the generation of both zonal and meridionM polarization fields. We show that the polarization process can account for the elevated electric fields of several millivolts per meter, which are implied often from backscatter Doppler measurements during unstable E region conditions at midlatitude. The polarization process can become much more effective for dense and strongly elongated E, layers under the action of an enhanced ambient electric field. In this case, large polarization fields that may be capable of exciting Farley-Buneman plasma waves can be sustained. The stringent requirements for strongly elongated sporadic E layers with sharp boundaries, low ionospheric Pedersen conductivities above the layer in relation to those inside, and relatively large ambient electric fields would explain why type 1 echoes are so rare in midlatitude E region backscatter.

Analysis of subionospheric VLF transmissions, observed in relation with sprites, has led to the identification of a new category of VLF perturbations caused by the direct effects of tropospheric lightning on the overlying lower ionosphere. They constitute a large subset of the so‐called “early/fast” events where now the term “fast,” which implies rapid onset durations less than ∼20 ms, does not apply. In contrast with early/fast, the perturbations have a gradual growth and thus “slow” onset durations ranging from about 0.5 to 2.5 s; thus these events are labeled herein as “early/slow.” They are indicative of a new physical process at work which, following a sprite‐causative cloud‐to‐ground discharge, leads to a gradual buildup of conductivity changes in the lower ionosphere which must be responsible for the long onset durations of the observed perturbations. Analysis of broadband VLF sferic recordings, made with a two‐channel receiver near the sprite producing storms, shows that the growth phase of an early/slow event coincides with the occurrence of complex and dynamic lightning action. This is composed of a few sequential cloud‐to‐ground lightning strokes and clusters (bursts) of sferics which are attributable to intracloud lightning. We postulate that the long onset durations are due to secondary ionization buildup in the upper D region below the nighttime VLF reflection heights, caused mainly by the impact on sprite‐produced electrons of sequential electromagnetic pulses radiated upward from horizontal in‐cloud discharges.

Abstract. Recent findings suggested the possibility that planetary waves play a role in the occurrence of midlatitude sporadic E layers. To account for this, we propose here a new mechanism for large-scale accumulation of metallic ions in the midlatitude E region ionosphere driven by planetary waves in the lower thermosphere. In this process, the plasma is forced to converge horizontally and accumulate inside areas of positive vorticity set up by cyclonic neutral wind shears within a planetary wave. In its simplest form, the proposed model is similar to the well-known vertical wind shear mechanism of Es formation, but with the geometry "turned on its side." Because of the long times required for ambipolar diffusion, the new mechanism can lead to significant plasma accumulation, acting as complementary to the vertical wind shear process so that dense E, can form more efficiently and frequently. The present model provides a physical base for understanding the long-term periodicities in occurrence and also the seasonal dependence of strong sporadic E layers at rhidlatitude.

The aim of this paper is to analyze the potential resources of GPS monitoring during the recording of potential earthquake precursors using the Hector Mine earthquake that occurred in California, USA, in October 16, 1999. This event was chosen because at the time of this fairly large earthquake (M=7.1) a dense network of ground-based GPS stations was operating, thus providing a fairly high spatial resolution. This paper offers a detailed analysis of the total electron content (TEC) over a fairly long time interval including the time of the earthquake (October 13 to 18, 1999). Examined in this research is the potential manifestation in the TEC data of the well-known seismo-ionospheric effects: quasiregular changes in the ionospheric parameters and internal gravity wave generation. However, our analysis showed that the observed TEC variations seem to have been controlled by the local time and by fairly moderate geomagnetic activity instead of being associated with any expected processes that usually accompany the process of earthquake preparation. Also discussed in this paper are the prospects of detecting small-scale ionospheric heterogeneities that are supposed to arise in the course of earthquake preparation, as follows from our special measurements of the magnitude and phase flickering of GPS signals.

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.