http://www.ann-geophys.net/ Annales Geophysicae Latest Articlesenhttp://www.ann-geophys.net/31/871/2013/<b>Discontinuities and Alfv&eacute;nic fluctuations in the solar wind</b><br /><br />Annales Geophysicae, 31, 871-887, 2013<br /><br />Author(s): G. Paschmann, S. Haaland, B. Sonnerup, and T. Knetter<br /><br />We examine the Alfvénicity of a set of 188 solar wind directional discontinuities (DDs) identified in the Cluster data from 2003 by Knetter (2005), with the objective of separating rotational discontinuities (RDs) from tangential ones (TDs). The DDs occurred over the full range of solar wind velocities and magnetic shear angles. By performing the Wal&eacute;n test in the de Hoffmann–Teller (HT) frame, we show that 77 of the 127 crossings for which a good HT frame was found had plasma flow speeds exceeding 80% of the Alfv&eacute;n speed at an average angular deviation of 7.7&deg;; 33 cases had speeds exceeding 90% of the Alfv&eacute;n speed at an average angle of 6.4&deg;. We show that the angular deviation between flow velocity (in the HT frame) and the Alfv&eacute;n velocity can be obtained from a reduced form of the Wal&eacute;n correlation coefficient. The corresponding results from the Wal&eacute;n test expressed in terms of jumps in flow speed and corresponding jumps in Alfv&eacute;n speed are similar: 66 of the same 127 cases had velocity jumps exceeding 80% with average angular deviation of 5.8&deg;, and 22 exceeding 90% of the jump in Alfv&eacute;n speed, with average angular deviation 6.2&deg;. We conclude that a substantial fraction of the 127 events can be identified as RDs. We present further evidence for coupling across the DDs by showing that, for most of the 127 crossings, the HT frame velocities, evaluated separately on the two sides of the DD, are nearly the same – a result required for RDs but not for TDs. We also show that the degree of Alfvénicity is nearly the same for the DDs and fluctuations in which the DDs are embedded. Whatever process causes deviations from ideal Alfvénicity appears to operate equally for the DDs as for the surrounding fluctuations. Finally, our study has established a unique relation between the strahl electron pitch angle and the sign of the Wal&eacute;n slope, implying antisunward propagation in the plasma frame for all 127 cases.Thu, 23 May 2013 00:00:00 +0200http://www.ann-geophys.net/31/859/2013/<b>High-precision measurement of satellite range and velocity using the EISCAT radar</b><br /><br />Annales Geophysicae, 31, 859-870, 2013<br /><br />Author(s): J. Markkanen, T. Nygrén, M. Markkanen, M. Voiculescu, and A. Aikio<br /><br />This paper is a continuation of an earlier work by Nygrén et al. (2012), where the velocity of a hard target was determined from a set of echo pulses reflected by the target flying through the radar beam. Here the method is extended to include the determination of range at a high accuracy. The method is as follows. First, the flight time of the pulse from the transmitter to the target is determined at an accuracy essentially better than the accuracy given by the sampling interval. This method makes use of the fact that the receiver filtering creates slopes at the phase flips of the phase modulated echo pulse. A precise flight time is found by investigating the echo amplitude within this slope. A value of velocity is calculated from each echo pulse as explained in the earlier paper. Next, the ranges together with velocities from a single beam pass are combined to a measurement vector for a linear inversion problem. The solution of the inversion problem gives the time-dependent range and velocity from the time interval of satellite flight through the radar beam. The method is demonstrated using the EISCAT (European Incoherent Scatter) UHF radar and radio pulses reflected by a satellite. The achieved standard deviations of range are about 5–50 cm and those of velocity are about 3–25 mm s⁻¹.Wed, 22 May 2013 00:00:00 +0200http://www.ann-geophys.net/31/845/2013/<b>Studies of gravity wave propagation in the mesosphere observed by MU radar</b><br /><br />Annales Geophysicae, 31, 845-858, 2013<br /><br />Author(s): H. Y. Lue, F. S. Kuo, S. Fukao, and T. Nakamura<br /><br />Mesospheric data were analyzed by a composite method combining phase and group velocity tracing technique and the spectra method of Stokes parameter analysis to obtain the propagation parameters of atmospheric gravity waves (AGW) in the height ranges between 63.6 and 99.3 km, observed using the MU radar at Shigaraki in Japan in the months of November and July in the years 1986, 1988 and 1989. The data of waves with downward phase velocity and the data of waves with upward phase velocity were independently treated. First, the vertical phase velocity and vertical group velocity as well as the characteristic wave period for each wave packet were obtained by phase and group velocity tracing technique. Then its horizontal wavelength, intrinsic wave period and horizontal group velocity were obtained by the dispersion relation. The intrinsic frequency and azimuth of wave vector of each wave packet were checked by Stokes parameters analysis. The results showed that the waves with intrinsic periods in the range 30 min–4.5 h had horizontal wavelength ranging from 25 to 240 km, vertical wavelength from 2.5 to 12 km, and horizontal group velocities from 15 to 60 m s⁻¹. Both upward moving wave packets and downward moving wave packets had horizontal group velocities mostly directed in the sector between directions NNE (north-north-east) and SEE in the month of November, and mostly in the sector between directions NW and SWS in the month of July. Comparing with mean wind directions, the gravity waves appeared to be more likely to propagate along with mean wind than against it. This apparent prevalence for downstream wave packets was found to be caused by a systematic filtering effect existing in the process of phase and group velocity tracing analysis: A significant portion of upstream wave packets might have been Doppler shifted out of the vertical range in phase and group velocity tracing analysis.Wed, 15 May 2013 00:00:00 +0200http://www.ann-geophys.net/31/835/2013/<b>THEMIS and ground-based observations of successive substorm onsets following a super-long growth phase</b><br /><br />Annales Geophysicae, 31, 835-843, 2013<br /><br />Author(s): J.-M. Liu, Y. Kamide, B.-C. Zhang, H.-Q. Hu, and H.-G. Yang<br /><br />We present four successive substorm events, which followed a super-long, as long as 9 h, growth phase on 5 December 2008, observed by the Time History of Events and Macroscale Interaction during Substorms (THEMIS) and the GOES 11 satellite with simultaneous coverage by the Alaska and THEMIS ground magnetometers. Several interesting and unique features were found for these cases. The interplanetary magnetic field was steadily southward and the solar wind speed was slow, less than 450 km s⁻¹, which are thought to drive the long growth phase for the following onsets. At least four substorm expansion onsets occurred, including a double-onset event, which appears to be a challenge to the reconnection hypothesis for double-onset substorm and favored an instability mechanism for the onsets and could not be explained by the two neutral line models. For the onsets at 09:32 UT and 09:42 UT, the dipolarization signature was observed by GOES 11, which was located earthward of THEMIS C and THEMIS B. THEMIS C satellite caught a delayed and much weaker signature 1–3 min after GOES 11. THEMIS B observed no relating signature. These observations provide us with direct evidence that these events initiated at the near-earth region. The observations of THEMIS C and THEMIS B around the onsets favor the near-earth instabilities model for substorm onset.Mon, 06 May 2013 00:00:00 +0200http://www.ann-geophys.net/31/817/2013/<b>Comparative magnetotail flapping: an overview of selected events at Earth, Jupiter and Saturn</b><br /><br />Annales Geophysicae, 31, 817-833, 2013<br /><br />Author(s): M. Volwerk, N. André, C. S. Arridge, C. M. Jackman, X. Jia, S. E. Milan, A. Radioti, M. F. Vogt, A. P. Walsh, R. Nakamura, A. Masters, and C. Forsyth<br /><br />A comparison of magnetotail flapping (the up-and-down wavy motion) between the Earth and the two giant planets Jupiter and Saturn has been performed through investigation of the current sheet normal of the magnetotail. Magnetotail flapping is commonly observed in the Earth's magnetotail. Due to single spacecraft missions at the giant planets, the normal is determined through minimum variance analysis of magnetometer data during multiple intervals when the spacecraft crossed through the current sheet. It is shown that indeed a case can be made that magnetotail flapping also occurs at Jupiter and Saturn. Calculations of the wave period using generic magnetotail models show that the observed periods are much shorter than their theoretical estimates, and that this discrepancy can be caused by unknown input parameters for the tail models (e.g., current sheet thickness) and by possible Doppler shifting of the waves in the spacecraft frame through the fast rotation of the giant planets.Mon, 06 May 2013 00:00:00 +0200http://www.ann-geophys.net/31/805/2013/<b>GPS phase scintillation and proxy index at high latitudes during a moderate geomagnetic storm</b><br /><br />Annales Geophysicae, 31, 805-816, 2013<br /><br />Author(s): P. Prikryl, R. Ghoddousi-Fard, B. S. R. Kunduri, E. G. Thomas, A. J. Coster, P. T. Jayachandran, E. Spanswick, and D. W. Danskin<br /><br />The amplitude and phase scintillation indices are customarily obtained by specialised GPS Ionospheric Scintillation and TEC Monitors (GISTMs) from L1 signal recorded at the rate of 50 Hz. The scintillation indices <I>S</I>₄ and &sigma;_&Phi; are stored in real time from an array of high-rate scintillation receivers of the Canadian High Arctic Ionospheric Network (CHAIN). Ionospheric phase scintillation was observed at high latitudes during a moderate geomagnetic storm (Dst = &minus;61 nT) that was caused by a moderate solar wind plasma stream compounded with the impact of two coronal mass ejections. The most intense phase scintillation (&sigma;_&Phi; ~ 1 rad) occurred in the cusp and the polar cap where it was co-located with a strong ionospheric convection, an extended tongue of ionisation and dense polar cap patches that were observed with ionosondes and HF radars. At sub-auroral latitudes, a sub-auroral polarisation stream that was observed by mid-latitude radars was associated with weak scintillation (defined arbitrarily as &sigma;_&Phi; < 0.5 rad). In the auroral zone, moderate scintillation coincided with auroral breakups observed by an all-sky imager, a riometer and a magnetometer in Yellowknife. To overcome the limited geographic coverage by GISTMs other GNSS data sampled at 1 Hz can be used to obtain scintillation proxy indices. In this study, a phase scintillation proxy index (delta phase rate, DPR) is obtained from 1-Hz data from CHAIN and other GPS receivers. The 50-Hz and 1-Hz phase scintillation indices are correlated. The percentage occurrences of &sigma;_&Phi; > 0.1 rad and DPR > 2 mm s⁻¹, both mapped as a function of magnetic latitude and magnetic local time, are very similar.Mon, 06 May 2013 00:00:00 +0200http://www.ann-geophys.net/31/795/2013/<b>Variability and trend of diurnal temperature range in China and their relationship to total cloud cover and sunshine duration</b><br /><br />Annales Geophysicae, 31, 795-804, 2013<br /><br />Author(s): X. Xia<br /><br />This study aims to investigate the effect of total cloud cover (TCC) and sunshine duration (SSD) in the variation of diurnal temperature range (DTR) in China during 1954–2009. As expected, the inter-annual variation of DTR was mainly determined by TCC. Analysis of trends of 30-year moving windows of DTR and TCC time series showed that TCC changes could account for that of DTR in some cases. However, TCC decreased during 1954–2009, which did not support DTR reduction across China. DTRs under sky conditions such as clear, cloudy and overcast showed nearly the same decreasing rate that completely accounted for the overall DTR reduction. Nevertheless, correlation between SSD and DTR was weak and not significant under clear sky conditions in which aerosol direct radiative effect should be dominant. Furthermore, 30–60% of DTR reduction was associated with DTR decrease under overcast conditions in south China. This implies that aerosol direct radiative effect appears not to be one of the main factors determining long-term changes in DTR in China.Fri, 03 May 2013 00:00:00 +0200http://www.ann-geophys.net/31/787/2013/<b>Comparison of the characteristics of ionospheric parameters obtained from FORMOSAT-3 and digisonde over Ascension Island</b><br /><br />Annales Geophysicae, 31, 787-794, 2013<br /><br />Author(s): Y. J. Chuo, C. C. Lee, W. S. Chen, and B. W. Reinisch<br /><br />Electron density profile data obtained from the FORMOSAT-3 radio occultation (RO) measurements over Ascension Island are used to study the bottomside thickness parameter <I>B0</I> in the International Reference Ionosphere (IRI) model, scale height around the F region peak height, and other F2 region parameters. The RO data were collected when the radio occultation occurred at Ascension Island (345.6° E, 8.0° S) during the solar minimum activity period from May 2006 to April 2008. Results show that the <I>B0</I> values are in moderate agreement with the ground-based observations in the equinox period (correlation coefficient <I>r</I> = 0.682) and winter (<I>r</I> = 0.570), with a strong correlation in summer (<I>r</I> = 0.750). The seasonal and diurnal variations in <I>B0</I> over Ascension Island show peak values during the daytime and in winter. In addition, the <I>B0</I> values were underestimated and overestimated in the RO measurements during the daytime and nighttime, respectively. Moreover, the comparison of scale heights shows that scale heights obtained from the retrieved data and digisonde observations are weakly correlation in all three seasons. Furthermore, although the effective scale height (<I>H</I>_T) values were reverse of those obtained from the RO measurements and are higher during the nighttime than in the daytime, they are in good agreement with those from ground-based observations. This paper also provides a comprehensive discussion of the effect of the asymmetric ionospheric electron density profiles on RO measurements.Fri, 03 May 2013 00:00:00 +0200http://www.ann-geophys.net/31/775/2013/<b>Simultaneous observations of a Mesospheric Inversion Layer and turbulence during the ECOMA-2010 rocket campaign</b><br /><br />Annales Geophysicae, 31, 775-785, 2013<br /><br />Author(s): A. Szewczyk, B. Strelnikov, M. Rapp, I. Strelnikova, G. Baumgarten, N. Kaifler, T. Dunker, and U.-P. Hoppe<br /><br />From 19 November to 19 December 2010 the fourth and final ECOMA rocket campaign was conducted at Andøya Rocket Range (69° N, 16° E) in northern Norway. We present and discuss measurement results obtained during the last rocket launch labelled ECOMA09 when simultaneous and true common volume in situ measurements of temperature and turbulence supported by ground-based lidar observations reveal two Mesospheric Inversion Layers (MIL) at heights between 71 and 73 km and between 86 and 89 km. Strong turbulence was measured in the region of the upper inversion layer, with the turbulent energy dissipation rates maximising at 2 W kg⁻¹. This upper MIL was observed by the ALOMAR Weber Na lidar over the period of several hours. The spatial extension of this MIL as observed by the MLS instrument onboard AURA satellite was found to be more than two thousand kilometres. Our analysis suggests that both observed MILs could possibly have been produced by neutral air turbulence.Fri, 03 May 2013 00:00:00 +0200http://www.ann-geophys.net/31/765/2013/<b>Modeling solar flare induced lower ionosphere changes using VLF/LF transmitter amplitude and phase observations at a midlatitude site</b><br /><br />Annales Geophysicae, 31, 765-773, 2013<br /><br />Author(s): E. D. Schmitter<br /><br />Remote sensing of the ionosphere bottom using long wave radio signal propagation is a still going strong and inexpensive method for continuous monitoring purposes. We present a propagation model describing the time development of solar flare effects. Based on monitored amplitude and phase data from VLF/LF transmitters gained at a mid-latitude site during the currently increasing solar cycle no. 24 a parameterized electron density profile is calculated as a function of time and fed into propagation calculations using the LWPC (Long Wave Propagation Capability). The model allows to include lower ionosphere recombination and attachment coefficients, as well as to identify the relevant forcing X-ray wavelength band, and is intended to be a small step forward to a better understanding of the solar–lower ionosphere interaction mechanisms within a consistent framework.Fri, 26 Apr 2013 00:00:00 +0200http://www.ann-geophys.net/31/755/2013/<b>A 20-day period standing oscillation in the northern winter stratosphere</b><br /><br />Annales Geophysicae, 31, 755-764, 2013<br /><br />Author(s): K. Hocke, S. Studer, O. Martius, D. Scheiben, and N. Kämpfer<br /><br />Observations of the ozone profile by a ground-based microwave radiometer in Switzerland indicate a dominant 20-day oscillation in stratospheric ozone, possibly related to oscillations of the polar vortex edge during winter. For further understanding of the nature of the 20-day oscillation, the ozone data set of ERA Interim meteorological reanalysis is analyzed at the latitude belt of 47.5° N and in the time from 1979 to 2010. Spectral analysis of ozone time series at 7 hPa indicates that the 20-day oscillation is maximal at two locations: 7.5° E, 47.5° N and 60° E, 47.5° N. Composites of the stream function are derived for different phases of the 20-day oscillation of stratospheric ozone at 7 hPa in the Northern Hemisphere. The streamline at &Psi; = &minus;2 &times; 10⁷ m² s⁻¹ is in the vicinity of the polar vortex edge. The other streamline at &Psi; = 4 &times; 10⁷ m² s¹ surrounds the Aleutian anticyclone and goes to the subtropics. The composites show 20-day period standing oscillations at the polar vortex edge and in the subtropics above Northern Africa, India, and China. The 20-day period standing oscillation above Aral Sea and India is correlated to the strength of the Aleutian anticyclone.Thu, 25 Apr 2013 00:00:00 +0200http://www.ann-geophys.net/31/745/2013/<b>Comparison of temporal fluctuations in the total electron content estimates from EISCAT and GPS along the same line of sight</b><br /><br />Annales Geophysicae, 31, 745-753, 2013<br /><br />Author(s): B. Forte, N. D. Smith, C. N. Mitchell, F. Da Dalt, T. Panicciari, A. T. Chartier, D. Stevanovic, M. Vuckovic, J. Kinrade, J. R. Tong, I. Häggström, and E. Turunen<br /><br />The impact of space weather events on satellite-based technologies (e.g. satellite navigation and precise positioning) is typically quantified on the basis of the total electron content (TEC) and temporal fluctuations associated with it. GNSS (global navigation satellite systems) TEC measurements are integrated over a long distance and thus may include contributions from different regions of the ionised atmosphere which may prevent the resolution of the mechanisms ultimately responsible for given observations. The purpose of the experiment presented here was to compare TEC estimates from EISCAT and GPS measurements. The EISCAT measurements were obtained along the same line of sight of a given GPS satellite observed from Tromsø. The present analyses focussed on the comparison of temporal fluctuations in the TEC between aligned GPS and EISCAT measurements. A reasonably good agreement was found between temporal fluctuations in TEC observed by EISCAT and those observed by a co-located GPS ionospheric monitor along the same line of sight, indicating a contribution from structures at E and F altitudes mainly to the total TEC in the presence of ionisation enhancements possibly caused by particle precipitation in the nighttime sector. The experiment suggests the great potential in the measurements to be performed by the future EISCAT_3D system, limited only in the localised geographic region to be covered.Tue, 23 Apr 2013 00:00:00 +0200http://www.ann-geophys.net/31/725/2013/<b>Spectral structure of Pc3–4 pulsations: possible signatures of cavity modes</b><br /><br />Annales Geophysicae, 31, 725-743, 2013<br /><br />Author(s): P. R. Sutcliffe, B. Heilig, and S. Lotz<br /><br />In this study we investigate the spectral structure of Pc3–4 pulsations observed at low and midlatitudes. For this purpose, ground-based magnetometer data recorded at the MM100 stations in Europe and at two low latitude stations in South Africa were used. In addition, fluxgate magnetometer data from the CHAMP (CHAllenging Minisatellite Payload) low Earth orbit satellite were used. The results of our analysis suggest that at least three mechanisms contribute to the spectral content of Pc3–4 pulsations typically observed at these latitudes. We confirm that a typical Pc3–4 pulsation contains a field line resonance (FLR) contribution, with latitude dependent frequency, and an upstream wave (UW) contribution, with frequency proportional to the IMF (interplanetary magnetic field) magnitude <I>B</I>_IMF. Besides the FLR and UW contributions, the Pc3–4 pulsations consistently contain signals at other frequencies that are independent of latitude and <I>B</I>_IMF. We suggest that the most likely explanation for these additional frequency contributions is that they are fast mode resonances (FMRs) related to cavity, waveguide, or virtual modes. Although the above contributions to the pulsation spectral structure have been reported previously, we believe that this is the first time where evidence is presented showing that they are all present simultaneously in both ground-based and satellite data.Tue, 23 Apr 2013 00:00:00 +0200http://www.ann-geophys.net/31/713/2013/<b>Double cusp encounter by Cluster: double cusp or motion of the cusp?</b><br /><br />Annales Geophysicae, 31, 713-723, 2013<br /><br />Author(s): C. P. Escoubet, J. Berchem, K. J. Trattner, F. Pitout, R. Richard, M. G. G. T. Taylor, J. Soucek, B. Grison, H. Laakso, A. Masson, M. Dunlop, I. Dandouras, H. Reme, A. Fazakerley, and P. Daly<br /><br />Modelling plasma entry in the polar cusp has been successful in reproducing ion dispersions observed in the cusp at low and mid-altitudes. The use of a realistic convection pattern, when the IMF-<I>B</I>_y is large and stable, allowed Wing et al. (2001) to predict double cusp signatures that were subsequently observed by the DMSP spacecraft. In this paper we present a cusp crossing where two cusp populations are observed, separated by a gap around 1° Invariant Latitude (ILAT) wide. Cluster 1 (C1) and Cluster 2 (C2) observed these two cusp populations with a time delay of 3 min, and about 15 and 42 min later Cluster 4 (C4) and Cluster 3 (C3) observed, respectively, a single cusp population. A peculiarity of this event is the fact that the second cusp population seen on C1 and C2 was observed at the same time as the first cusp population on C4. This would tend to suggest that the two cusp populations had spatial features similar to the double cusp. Due to the nested crossing of C1 and C2 through the gap between the two cusp populations, C2 being first to leave the cusp and last to re-enter it, these observations are difficult to be explained by two distinct cusps with a gap in between. However, since we observe the cusp in a narrow area of local time post-noon, a second cusp may have been present in the pre-noon sector but could not be observed. On the other hand, these observations are in agreement with a motion of the cusp first dawnward and then back duskward due to the effect of the IMF-<I>B</I>_y component.Fri, 19 Apr 2013 00:00:00 +0200http://www.ann-geophys.net/31/697/2013/<b>Variability of ionospheric scintillation near the equatorial anomaly crest of the Indian zone</b><br /><br />Annales Geophysicae, 31, 697-711, 2013<br /><br />Author(s): S. Chatterjee and S. K. Chakraborty<br /><br />Multistation observations of ionosphere scintillation at VHF (250 MHz) and GNSS L1 frequency from three locations – (i) Bokkhali (BOK) (geographic 21.6° N, 88.2° E, dip 31.48&deg;, (ii) Raja Peary Mohan College Centre (RPMC) (geographic 22.66° N, 88.4° E, dip 33.5°) and (iii) Krishnath College Centre (KNC), Berhampore (geographic 24.1° N, 88.3° E, dip 35.9°) – at ~ 1&deg; latitudinal separations near the northern crest of the equatorial ionization anomaly (EIA) of the Indian longitude sector are investigated in conjunction with total electron content (TEC) data and available ionosonde data near the magnetic equator to study fine structure in spatial and temporal variability patterns of scintillation occurrences. The observations are carried out in the autumnal equinoctial months of a high solar activity year (2011). In spite of smaller latitudinal/spatial separation among the observing stations, conspicuous differences are reflected in the onset time, duration, fade rate and fade depth of VHF scintillations as well as in spectral features. Scintillations are mostly associated with depletion in TEC around the anomaly crest and occurrence of ESF near the magnetic equator at an earlier time. Not only the strength of EIA, but also the locations of observing stations with respect to the post-sunset resurgence peak of EIA seem to play dominant role in dictating the severity of scintillation activity. A secondary enhancement in diurnal TEC in the post-sunset period seems to accentuate the irregularity activities near the anomaly crest, and a threshold value of the same may fruitfully be utilized for the prediction of scintillation around the locations. An idea regarding latitudinal extent of scintillation is developed by considering observations at L1 frequency from the GPS and GLONASS constellation of satellites. A critical value of <I>h'F</I> near the magnetic equator for the occurrence of simultaneous scintillation at the three centres is suggested. The observations are discussed considering electrodynamical aspect of equatorial irregularities.Fri, 19 Apr 2013 00:00:00 +0200http://www.ann-geophys.net/31/689/2013/<b>The mechanism of mid-latitude Pi2 waves in the upper ionosphere as revealed by combined Doppler and magnetometer observations</b><br /><br />Annales Geophysicae, 31, 689-695, 2013<br /><br />Author(s): V. A. Pilipenko, E. N. Fedorov, M. Teramoto, and K. Yumoto<br /><br />The interpretation of simultaneous ionospheric Doppler sounding and ground magnetometer observations of low-latitude Pi2 waves is revised. We compare the theoretical estimates of the ionospheric Doppler velocity for the same amplitude of the ground magnetic disturbances produced by a large-scale compressional mode and an Alfvén mode. The plasma vertical displacement caused by the wave electric field is shown to be the dominating effect. Taking into account the correction of the previous paper, the observations of low-latitude Pi2 in the F layer ionosphere by Doppler sounding and SuperDARN (Super Dual Auroral Radar Network) radars give consistent results. We suggest that the Doppler response to Pi2 waves is produced by the Alfvén wave component, but not the fast-mode component, whereas the ground magnetic signal is composed from both Alfvén and fast magnetosonic modes.Wed, 17 Apr 2013 00:00:00 +0200http://www.ann-geophys.net/31/675/2013/<b>Observation and simulation of wave breaking in the southern hemispheric stratosphere during VORCORE</b><br /><br />Annales Geophysicae, 31, 675-687, 2013<br /><br />Author(s): M. Moustaoui, H. Teitelbaum, and A. Mahalov<br /><br />An interesting occurrence of a Rossby wave breaking event observed during the VORCORE experiment is presented and explained. Twenty-seven balloons were launched inside the Antarctic polar vortex. Almost all of these balloons evolved in the stratosphere around 500K within the vortex, except the one launched on 28 October 2005. In this case, the balloon was caught within a tongue of high potential vorticity (PV), and was ejected from the polar vortex. The evolution of this event is studied for the period between 19 and 25 November 2005. It is found that at the beginning of this period, the polar vortex experienced distortions due to the presence of Rossby waves. Then, these waves break and a tongue of high PV develops. On 25 November, the tongue became separated from the vortex and the balloon was ejected into the surf zone. Lagrangian simulations demonstrate that the air masses surrounding the balloon after its ejection were originating from the vortex edge. The wave breaking and the development of the tongue are confined within a region where a planetary Quasi-Stationary Wave 1 (QSW1) induces wind speeds with weaker values. The QSW1 causes asymmetry in the wind speed and the horizontal PV gradient along the edge of the polar vortex, resulting in a localized jet. Rossby waves with smaller scales propagating on top of this jet amplify as they enter the jet exit region and then break. The role of the QSW1 on the formation of the weak flow conditions that caused the non-linear wave breaking observed near the vortex edge is confirmed by three-dimensional numerical simulations using forcing with and without the contribution of the QSW1.Tue, 16 Apr 2013 00:00:00 +0200http://www.ann-geophys.net/31/665/2013/<b>Ray tracing of whistler-mode chorus elements: implications for generation mechanisms of rising and falling tone emissions</b><br /><br />Annales Geophysicae, 31, 665-673, 2013<br /><br />Author(s): K. Yamaguchi, T. Matsumuro, Y. Omura, and D. Nunn<br /><br />Using a well-established magnetospheric very-low-frequency (VLF) ray tracing method, in this work we trace the propagation of individual rising- and falling-frequency elements of VLF chorus from their generation point in the equatorial region of the magnetosphere through to at least one reflection at the lower-hybrid resonance point. Unlike recent work by Bortnik and co-workers, whose emphasis was on demonstrating that magnetospheric hiss has its origins in chorus, we here track the motion in the equatorial plane of the whole chorus element, paying particular regard to movement across field lines, rotation, and compression or expansion of the wave pulse. With a generation point for rising chorus at the equator, it was found the element wave pulse remained largely field aligned in the generation region. However, for a falling tone generation point at 4000 km upstream from the equator, by the time the pulse crosses the equator the wavefield had substantial obliquity, displacement, and compression, which has substantial implications for the theory of falling chorus generation.Fri, 12 Apr 2013 00:00:00 +0200http://www.ann-geophys.net/31/647/2013/<b>Investigation of radiative effects of the optically thick dust layer over the Indian tropical region</b><br /><br />Annales Geophysicae, 31, 647-663, 2013<br /><br />Author(s): S. K. Das, J.-P. Chen, M. Venkat Ratnam, and A. Jayaraman<br /><br />Optical and physical properties of aerosols derived from multi-satellite observations (MODIS-Aqua, OMI-Aura, MISR-Terra, CALIOP-CALIPSO) have been used to estimate radiative effects of the dust layer over southern India. The vertical distribution of aerosol radiative forcing and heating rates are calculated with 100 m resolution in the lower atmosphere, using temperature and relative humidity data from balloon-borne radiosonde observations. The present study investigates the optically thick dust layer of optical thickness 0.18 &plusmn; 0.06 at an altitude of 2.5 &plusmn; 0.7 km over Gadanki, transported from the Thar Desert, producing radiative forcing and heating rate of 11.5 &plusmn; 3.3 W m⁻² and 0.6 &plusmn; 0.26 K day⁻¹, respectively, with a forcing efficiency of 43 W m⁻² and an effective heating rate of 4 K day⁻¹ per unit dust optical depth. Presence of the dust layer increases radiative forcing by 60% and heating rate by 60 times at that altitude compared to non-dusty cloud-free days. Calculation shows that the radiative effects of the dust layer strongly depend on the boundary layer aerosol type and mass loading. An increase of 25% of heating by the dust layer is found over relatively cleaner regions than urban regions in southern India and further 15% of heating increases over the marine region. Such heating differences in free troposphere may have significant consequences in the atmospheric circulation and hydrological cycle over the tropical Indian region.Thu, 11 Apr 2013 00:00:00 +0200http://www.ann-geophys.net/31/639/2013/<b>Non-adiabatic electron behaviour due to short-scale electric field structures at collisionless shock waves</b><br /><br />Annales Geophysicae, 31, 639-646, 2013<br /><br />Author(s): V. See, R. F. Cameron, and S. J. Schwartz<br /><br />Under sufficiently high electric field gradients, electron behaviour within exactly perpendicular shocks is unstable to the so-called trajectory instability. We extend previous work paying special attention to short-scale, high-amplitude structures as observed within the electric field profile. Via test particle simulations, we show that such structures can cause the electron distribution to heat in a manner that violates conservation of the first adiabatic invariant. This is the case even if the overall shock width is larger than the upstream electron gyroradius. The spatial distance over which these structures occur therefore constitutes a new scale length relevant to the shock heating problem. Furthermore, we find that the spatial location of the short-scale structure is important in determining the total effect of non-adiabatic behaviour – a result that has not been previously noted.Thu, 11 Apr 2013 00:00:00 +0200