http://www.ann-geophys.net/Annales Geophysicae Latest Articleshttp://www.ann-geophys.net/28/1615/2010/<b>Estimating the seismotelluric current required for observable electromagnetic ground signals</b><br /><br />Annales Geophysicae, 28, 1615-1624, 2010<br /><br />Author(s): J. Bortnik, T. E. Bleier, C. Dunson, and F. Freund<br /><br />We use a relatively simple model of an underground current source co-located with the earthquake hypocenter to estimate the magnitude of the seismotelluric current required to produce observable ground signatures. The Alum Rock earthquake of 31 October 2007, is used as an archetype of a typical California earthquake, and the effects of varying the ground conductivity and length of the current element are examined. Results show that for an observed 30 nT pulse at 1 Hz, the expected seismotelluric current magnitudes fall in the range ~10–100 kA. By setting the detectability threshold to 1 pT, we show that even when large values of ground conductivity are assumed, magnetic signals are readily detectable within a range of 30 km from the epicenter. When typical values of ground conductivity are assumed, the minimum current required to produce an observable signal within a 30 km range was found to be ~1 kA, which is a surprisingly low value. Furthermore, we show that deep nulls in the signal power develop in the non-cardinal directions relative to the orientation of the source current, indicating that a magnetometer station located in those regions may not observe a signal even though it is well within the detectable range. This result underscores the importance of using a network of magnetometers when searching for preseismic electromagnetic signals.2010-08-31T00:00:00+02:00http://www.ann-geophys.net/28/1589/2010/<b>Numerical considerations in simulating the global magnetosphere</b><br /><br />Annales Geophysicae, 28, 1589-1614, 2010<br /><br />Author(s): A. J. Ridley, T. I. Gombosi, I. V. Sokolov, G. Tóth, and D. T. Welling<br /><br />Magnetohydrodynamic (MHD) models of the global magnetosphere are very good research tools for investigating the topology and dynamics of the near-Earth space environment. While these models have obvious limitations in regions that are not well described by the MHD equations, they can typically be used (or are used) to investigate the majority of magnetosphere. Often, a secondary consideration is overlooked by researchers when utilizing global models – the effects of solving the MHD equations on a grid, instead of analytically. Any discretization unavoidably introduces numerical artifacts that affect the solution to various degrees. This paper investigates some of the consequences of the numerical schemes and grids that are used to solve the MHD equations in the global magnetosphere. Specifically, the University of Michigan's MHD code is used to investigate the role of grid resolution, numerical schemes, limiters, inner magnetospheric density boundary conditions, and the artificial lowering of the speed of light on the strength of the ionospheric cross polar cap potential and the build up of the ring current in the inner magnetosphere. It is concluded that even with a very good solver and the highest affordable grid resolution, the inner magnetosphere is not grid converged. Artificially reducing the speed of light reduces the numerical diffusion that helps to achieve better agreement with data. It is further concluded that many numerical effects work nonlinearly to complicate the interpretation of the physics within the magnetosphere, and so simulation results should be scrutinized very carefully before a physical interpretation of the results is made. Our conclusions are not limited to the Michigan MHD code, but apply to all MHD models due to the limitations of computational resources.2010-08-27T00:00:00+02:00http://www.ann-geophys.net/28/1581/2010/<b>How do fits of simulated magnetic clouds correspond to their real shapes in 3-D?</b><br /><br />Annales Geophysicae, 28, 1581-1588, 2010<br /><br />Author(s): M. Vandas, E. Romashets, and A. Geranios<br /><br />Magnetic clouds are important objects for space weather forecasters due to their impact on the Earth's magnetosphere and their consequences during geomagnetic storms. Being considered as cylindrical or toroidal flux ropes, their size, velocity, magnetic field strength, and axis orientation determine its impact on Earth. Above mentioned parameters are usually extracted from model fits using measurements from one-spacecraft crossings of these structures. In order to relate solar events with these spacecraft observations, the parameters are then compared to situation at the Sun around a most probable source region with a goal to correlate them with near-Sun observed quantities for prediction purposes. In the past we performed three-dimensional simulations of magnetic cloud propagation in the inner heliosphere. Simulated spacecraft measurements are fitted by models of magnetic clouds and resulting parameters are compared with real shapes of magnetic clouds which can be directly obtained from our simulations. The comparison shows that cloud parameters are determined quite reliably for spacecraft crossings near the cloud axis.2010-08-26T00:00:00+02:00http://www.ann-geophys.net/28/1571/2010/<b>Accuracy analysis of the GPS instrumental bias estimated from observations in middle and low latitudes</b><br /><br />Annales Geophysicae, 28, 1571-1580, 2010<br /><br />Author(s): D. H. Zhang, W. Zhang, Q. Li, L. Q. Shi, Y. Q. Hao, and Z. Xiao<br /><br />With one bias estimation method, the latitude-related error distribution of instrumental biases estimated from the GPS observations in Chinese middle and low latitude region in 2004 is analyzed statistically. It is found that the error of GPS instrumental biases estimated under the assumption of a quiet ionosphere has an increasing tendency with the latitude decreasing. Besides the asymmetrical distribution of the plasmaspheric electron content, the obvious spatial gradient of the ionospheric total electron content (TEC) along the meridional line that related to the Equatorial Ionospheric Anomaly (EIA) is also considered to be responsible for this error increasing. The RMS of satellite instrumental biases estimated from mid-latitude GPS observations in 2004 is around 1 TECU (1 TECU = 10¹⁶/m²), and the RMS of the receiver's is around 2 TECU. Nevertheless, the RMS of satellite instrumental biases estimated from GPS observations near the EIA region is around 2 TECU, and the RMS of the receiver's is around 3–4 TECU. The results demonstrate that the accuracy of the instrumental bias estimated using ionospheric condition is related to the receiver's latitude with which ionosphere behaves a little differently. For the study of ionospheric morphology using the TEC derived from GPS data, in particular for the study of the weak ionospheric disturbance during some special geo-related natural hazards, such as the earthquake and severe meteorological disasters, the difference in the TEC accuracy over different latitude regions should be paid much attention.2010-08-25T00:00:00+02:00http://www.ann-geophys.net/28/1559/2010/<b>IMF dependence of Saturn's auroras: modelling study of HST and Cassini data from 12–15 February 2008</b><br /><br />Annales Geophysicae, 28, 1559-1570, 2010<br /><br />Author(s): E. S. Belenkaya, I. I. Alexeev, M. S. Blokhina, E. J. Bunce, S. W. H. Cowley, J. D. Nichols, V. V. Kalegaev, V. G. Petrov, and G. Provan<br /><br />To gain better understanding of auroral processes in Saturn's magnetosphere, we compare ultraviolet (UV) auroral images obtained by the Hubble Space Telescope (HST) with the position of the open-closed field line boundary in the ionosphere calculated using a magnetic field model that employs Cassini measurements of the interplanetary magnetic field (IMF) as input. Following earlier related studies of pre-orbit insertion data from January 2004 when Cassini was located ~ 1300 Saturn radii away from the planet, here we investigate the interval 12–15 February 2008, when UV images of Saturn's southern dayside aurora were obtained by the HST while the Cassini spacecraft measured the IMF in the solar wind just upstream of the dayside bow shock. This configuration thus provides an opportunity, unique to date, to determine the IMF impinging on Saturn's magnetosphere during imaging observations, without the need to take account of extended and uncertain interplanetary propagation delays. The paraboloid model of Saturn's magnetosphere is then employed to calculate the magnetospheric magnetic field structure and ionospheric open-closed field line boundary for averaged IMF vectors that correspond, with appropriate response delays, to four HST images. We show that the IMF-dependent open field region calculated from the model agrees reasonably well with the area lying poleward of the UV emissions, thus supporting the view that the poleward boundary of Saturn's auroral oval in the dayside ionosphere lies adjacent to the open-closed field line boundary.2010-08-19T00:00:00+02:00http://www.ann-geophys.net/28/1553/2010/<b>Retrieval of dust aerosols during night: improved assessment of long wave dust radiative forcing over Afro-Asian regions</b><br /><br />Annales Geophysicae, 28, 1553-1557, 2010<br /><br />Author(s): S. Deepshikha and J. Srinivasan<br /><br />Several investigators in the past have used the radiance depression (with respect to clear-sky infrared radiance), resulting from the presence of mineral dust aerosols in the atmosphere, as an index of dust aerosol load in the atmosphere during local noon. Here, we have used a modified approach to retrieve dust index during night since assessment of diurnal average infrared dust forcing essentially requires information on dust aerosols during night. For this purpose, we used infrared radiance (10.5–12.5 μm), acquired from the METEOSAT-5 satellite (~ 5 km resolution). We found that the "dust index" algorithm, valid for daytime, will no longer hold during the night because dust is then hotter than the theoretical dust-free reference. Hence we followed a "minimum reference" approach instead of a conventional "maximum reference" approach. A detailed analysis suggests that the maximum dust load occurs during the daytime. Over the desert regions of India and Africa, maximum change in dust load is as much as a factor of four between day and night and factor of two variations are commonly observed. By realizing the consequent impact on long wave dust forcing, sensitivity studies were carried out, which indicate that utilizing day time data for estimating the diurnally averaged long-wave dust radiative forcing results in significant errors (as much as 50 to 70%). Annually and regionally averaged long wave dust radiative forcing (which account for the diurnal variation of dust) at the top of the atmosphere over Afro-Asian region is 2.6 ± 1.8 W m⁻², which is 30 to 50% lower than those reported earlier. Our studies indicate that neglecting diurnal variation of dust while assessing its radiative impact leads to an overestimation of dust radiative forcing, which in turn result in underestimation of the radiative impact of anthropogenic aerosols.2010-08-18T00:00:00+02:00http://www.ann-geophys.net/28/1539/2010/<b>Selection effects in identifying magnetic clouds and the importance of the closest approach parameter</b><br /><br />Annales Geophysicae, 28, 1539-1552, 2010<br /><br />Author(s): R. P. Lepping and C.-C. Wu<br /><br />This study is motivated by the unusually low number of magnetic clouds (MCs) that are strictly identified within interplanetary coronal mass ejections (ICMEs), as observed at 1 AU; this is usually estimated to be around 30% or lower. But a looser definition of MCs may significantly increase this percentage. Another motivation is the unexpected shape of the occurrence distribution of the observers' "closest approach distances" (measured from a MC's axis, and called CA) which drops off somewhat rapidly as |CA| (in % of MC radius) approaches 100%, based on earlier studies. We suggest, for various geometrical and physical reasons, that the |CA|-distribution should be somewhere between a uniform one and the one actually observed, and therefore the 30% estimate should be higher. So we ask, When there is a failure to identify a MC within an ICME, is it occasionally due to a large |CA| passage, making MC identification more difficult, i.e., is it due to an event selection effect? In attempting to answer this question we examine WIND data to obtain an accurate distribution of the number of MCs vs. |CA| distance, whether the event is ICME-related or not, where initially a large number of cases (<I>N</I>=98) are considered. This gives a frequence distribution that is far from uniform, confirming earlier studies. This along with the fact that there are many ICME identification-parameters that do not depend on |CA| suggest that, indeed an <I>MC event selection effect</I> may explain at least part of the low ratio of (No. MCs)/(No. ICMEs). We also show that there is an acceptable geometrical and physical consistency in the relationships for both average "normalized" magnetic field intensity change and field direction change vs. |CA| within a MC, suggesting that our estimates of |CA|, <I>B_O</I> (magnetic field intensity on the axis), and choice of a proper "cloud coordinate" system (all needed in the analysis) are acceptably accurate. Therefore, the MC fitting model (Lepping et al., 1990) is adequate, on average, for our analysis. However, this selection effect is not likely to completely answer our original question, on the unexpected ratio of MCs to ICMEs, so we must look for other factors, such as peculiarities of CME birth conditions. As a by-product of this analysis, we determine that the first order structural effects within a MC due to its interaction with the solar wind, plus the MC's usual expansion at 1 AU (i.e., the non-force free components of the MC's field) are, on average, weakly dependent on radial distance from the MC's axis; that is, in the outer reaches of a typical MC the non-force free effects show up, but even there they are rather weak. Finally, we show that it is not likely that a MC's size distribution statistically controls the occurrence distribution of the estimated |CA|s.2010-08-18T00:00:00+02:00http://www.ann-geophys.net/28/1533/2010/<b>Giant disturbance in the ionospheric F2 region prior to the <i>M</i>8.0 Wenchuan earthquake on 12 May 2008</b><br /><br />Annales Geophysicae, 28, 1533-1538, 2010<br /><br />Author(s): T. Xu, Y. Hu, J. Wu, Z. Wu, Y. Suo, and J. Feng<br /><br />On 12 May 2008 at 14:28 LT great earthquake (<i>M</i>=8.0) occurred at Wenchuan (31.00° N, 103.40° E), China. The hourly values of <i>fo</i>F2 are analyzed over ten ionospheric observatories: Haikou (20.00° N, 110.33° E), Kunming (25.00° N, 102.70° E), Guangzhou (20.00° N, 113.70° E), Chongqing (29.50° N, 106.40° E), Lhasa (29.63° N, 91.17° E), Lanzhou (36.07° N, 103.87° E), Beijing (40.00° N, 116.30° E), Urumqi (43.75° N, 87.63° E), Chuangchun (43.83° N, 125.30° E) and Manzhouli (49.60° N, 117.45° E). With a new factor, effective sunspot number <i>R</i>_eff, the results show that there were giant positive disturbances of <i>fo</i>F2 around the epicentral zone on 9 May, 3 days prior to the earthquake. Our results indicate that the observed positive ionospheric disturbances were most possibly associated with the imminent earthquake and the new analytic method has good prospects in practice.2010-08-13T00:00:00+02:00http://www.ann-geophys.net/28/1523/2010/<b>Thin current sheets caused by plasma flow gradients in space and astrophysical plasma</b><br /><br />Annales Geophysicae, 28, 1523-1532, 2010<br /><br />Author(s): D. H. Nickeler and T. Wiegelmann<br /><br />Strong gradients in plasma flows play a major role in space and astrophysical plasmas. A typical situation is that a static plasma equilibrium is surrounded by a plasma flow, which can lead to strong plasma flow gradients at the separatrices between field lines with different magnetic topologies, e.g., planetary magnetospheres, helmet streamers in the solar corona, or at the boundary between the heliosphere and interstellar medium. Within this work we make a first step to understand the influence of these flows towards the occurrence of current sheets in a stationary state situation. We concentrate here on incompressible plasma flows and 2-D equilibria, which allow us to find analytic solutions of the stationary magnetohydrodynamics equations (SMHD). First we solve the magnetohydrostatic (MHS) equations with the help of a Grad-Shafranov equation and then we transform these static equilibria into a stationary state with plasma flow. We are in particular interested to study SMHD-equilibria with strong plasma flow gradients perpendicular to separatrices. We find that induced thin current sheets occur naturally in such situations. The strength of the induced currents depend on the Alfvén Mach number and its gradient, and on the magnetic field.2010-08-13T00:00:00+02:00http://www.ann-geophys.net/28/1511/2010/<b>Magnetic field structure of large-scale plasmoid generated by the fast reconnection mechanism in a sheared current sheet</b><br /><br />Annales Geophysicae, 28, 1511-1521, 2010<br /><br />Author(s): M. Ugai<br /><br />On the basis of the spontaneous fast reconnection model, three-dimensional magnetic field profiles associated with a large-scale plasmoid propagating along the antiparallel magnetic fields are studied in the general sheared current sheet system. The plasmoid is generated ahead of the fast reconnection jet as a result of distinct compression of the magnetized plasma. Inside the plasmoid, the sheared (east-west) field component has the peak value at the plasmoid center located at <I>x</I>=</I>X</I>_C, where the north-south field component changes its sign. The plasmoid center corresponds to the so-called contact discontinuity that bounds the reconnected field lines in <I>x</I>&lt;<I>X</I>_C and the field lines without reconnection in <I>x</I>&gt;<I>X</I>_C. Hence, contray to the conventional prediction, the reconnected sheared field lines in <I>x</I>&lt;<I>X</I>_C are not spiral or helical, since they cannot be topologically connected to the field lines in <I>x</I>&gt;<I>X</I>_C. It is demonstrated that the resulting profiles of magnetic field components inside the plasmoid are, in principle, consistent with satellite observations. In the ambient magnetic field region outside the plasmoid too, the magnetic field profiles are in good agreement with the well-known observations of traveling compression regions (TCRs).2010-08-11T00:00:00+02:00http://www.ann-geophys.net/28/1499/2010/<b>Intermediate-<I>m</I> ULF waves generated by substorm injection: a case study</b><br /><br />Annales Geophysicae, 28, 1499-1509, 2010<br /><br />Author(s): T. K. Yeoman, D. Yu. Klimushkin, and P. N. Mager<br /><br />A case study of SuperDARN observations of Pc5 Alfvén ULF wave activity generated in the immediate aftermath of a modest-intensity substorm expansion phase onset is presented. Observations from the Hankasalmi radar reveal that the wave had a period of 580 s and was characterized by an intermediate azimuthal wave number (<I>m</I>=13), with an eastwards phase propagation. It had a significant poloidal component and a rapid equatorward phase propagation (~62° per degree of latitude). The total equatorward phase variation over the wave signatures visible in the radar field-of-view exceeded the 180° associated with field line resonances. The wave activity is interpreted as being stimulated by recently-injected energetic particles. Specifically the wave is thought to arise from an eastward drifting cloud of energetic electrons in a similar fashion to recent theoretical suggestions (Mager and Klimushkin, 2008; Zolotukhina et al., 2008; Mager et al., 2009). The azimuthal wave number <I>m</I> is determined by the wave eigenfrequency and the drift velocity of the source particle population. To create such an intermediate-<I>m</I> wave, the injected particles must have rather high energies for a given L-shell, in comparison to previous observations of wave events with equatorward polarization. The wave period is somewhat longer than previous observations of equatorward-propagating events. This may well be a consequence of the wave occurring very shortly after the substorm expansion, on stretched near-midnight field lines characterised by longer eigenfrequencies than those involved in previous observations.2010-08-05T00:00:00+02:00http://www.ann-geophys.net/28/1483/2010/<b>Spectral characteristics of protons in the Earth's plasmasheet: statistical results from Cluster CIS and RAPID</b><br /><br />Annales Geophysicae, 28, 1483-1498, 2010<br /><br />Author(s): S. Haaland, E. A. Kronberg, P. W. Daly, M. Fränz, L. Degener, E. Georgescu, and I. Dandouras<br /><br />We present a study of the spectral characteristics of protons in the Earth's plasma sheet for various geomagnetic disturbance levels. The study is based on about 5400 h of data combined from the Cluster RAPID and CIS instruments obtained during the tail season (July–October). The overall proton spectral shape is generally that of a κ distribution, that is, resembling a Maxwellian at lower energies which smoothly merges into a power-law tail at higher energies. The actual spectral long-term slope depends on various magnetospheric driver parameters, but is on average around 3.5–4. During disturbed conditions, such as geomagnetic storm or substorm periods, a shift in the characteristic energy is observed. For two individual storms, we also found a hardening of the spectra. Unlike the electron spectra, we do not see any significant local time dependence in the spectral slope, but we find higher average ion fluxes in the dusk side. We also do not find any direct response in the energy spectra to changes in the interplanetary magnetic field or solar wind dynamic pressure. This suggests that energization of the ions are mainly due to internal processes in the plasma sheet.2010-08-05T00:00:00+02:00http://www.ann-geophys.net/28/1475/2010/<b>Turbulence for different background conditions using fuzzy logic and clustering</b><br /><br />Annales Geophysicae, 28, 1475-1481, 2010<br /><br />Author(s): K. Satheesan and S. Kirkwood<br /><br />Wind and turbulence estimated from MST radar observations in Kiruna, in Arctic Sweden are used to characterize turbulence in the free troposphere using data clustering and fuzzy logic. The root mean square velocity, &nu;_fca, a diagnostic of turbulence is clustered in terms of hourly wind speed, direction, vertical wind speed, and altitude of the radar observations, which are the predictors. The predictors are graded over an interval of zero to one through an input membership function. Subtractive data clustering has been applied to classify &nu;_fca depending on its homogeneity. Fuzzy rules are applied to the clustered dataset to establish a relationship between predictors and the predictant. The accuracy of the predicted turbulence shows that this method gives very good prediction of turbulence in the troposphere. Using this method, the behaviour of &nu;_fca for different wind conditions at different altitudes is studied.2010-08-03T00:00:00+02:00http://www.ann-geophys.net/28/1467/2010/<b>Ion dynamics during compression of Mercury's magnetosphere</b><br /><br />Annales Geophysicae, 28, 1467-1474, 2010<br /><br />Author(s): D. C. Delcourt, T. E. Moore, and M.-C. H. Fok<br /><br />Because of the small planetary magnetic field as well as proximity to the Sun that leads to enhanced solar wind pressure as compared to Earth, the magnetosphere of Mercury is very dynamical and at times subjected to prominent compression. We investigate the dynamics of magnetospheric ions during such compression events. Using three-dimensional single-particle simulations, we show that the electric field induced by the time varying magnetic field can lead to significant ion energization, up to several hundreds of eVs or a few keVs. This energization occurs in a nonadiabatic manner, being characterized by large enhancements of the ion magnetic moment and bunching in gyration phase. It is obtained when the ion cyclotron period is comparable to the field variation time scale. This condition for nonadiabatic heating is realized in distinct regions of space for ions with different mass-to-charge ratios. During compression of Mercury's magnetosphere, heavy ions originating from the planetary exosphere may be subjected to such an abrupt energization, leading to loading of the magnetospheric lobes with energetic material.2010-08-03T00:00:00+02:00http://www.ann-geophys.net/28/1463/2010/<b>Size of the coming solar cycle 24 based on Ohl's Precursor Method, final estimate</b><br /><br />Annales Geophysicae, 28, 1463-1466, 2010<br /><br />Author(s): R. P. Kane<br /><br />In Ohl's Precursor Method (Ohl, 1966, 1976), the geomagnetic activity during the declining phase of a sunspot cycle is shown to be well correlated with the size (maximum sunspot number <I>Rz</I>(max)) of the next cycle. For solar cycle 24, Kane (2007a) used <I>aa</I>(min)=15.5 (12-month running mean), which occurred during March–May of 2006 and made a preliminary estimate <I>Rz</I>(max)=124±26 (12-month running mean). However, in the next few months, the <I>aa</I> index first increased and then decreased to a new low value of 14.8 in July 2007. With this new low value, the prediction was <I>Rz</I>(max)=117±26 (12-month running mean). However, even this proved a false signal. Since then, the aa values have decreased considerably and the last 12-monthly value is 8.7, centered at May 2009. For solar cycle 24, using <I>aa</I>(min)=8.7, the latest prediction is, <I>Rz</I>(max)=58.0±25.0.2010-07-16T00:00:00+02:00http://www.ann-geophys.net/28/1449/2010/<b>Ionospheric storms at geophysically-equivalent sites – Part 2: Local time storm patterns for sub-auroral ionospheres</b><br /><br />Annales Geophysicae, 28, 1449-1462, 2010<br /><br />Author(s): M. Mendillo and C. Narvaez<br /><br />The response of the mid-latitude ionosphere to geomagnetic storms depends upon several pre-storm conditions, the dominant ones being season and local time of the storm commencement (SC). The difference between a site's geographic and geomagnetic latitudes is also of major importance since it governs the blend of processes linked to solar production and magnetospheric input, respectively. Case studies of specific storms using ionospheric data from both hemispheres are inherently dominated by seasonal effects and the various local times versus longitude of the SCs. To explore inter-hemispheric consistency of ionospheric storms, we identify "geophysically-equivalent-sites" as locations where the geographic and geomagnetic latitudes have the same relationship to each other in both hemispheres. At the longitudes of the dipole tilt, the differences between geographic and geomagnetic latitudes are at their extremes, and thus these are optimal locations to see if pre-conditioning and/or storm-time input are the same or differ between the hemispheres. <br><br> In this study, we use ionosonde values of the F2-layer maximum electron density (<I>Nm</I>F2) to study geophysical equivalency at Wallops Island (VA) and Hobart (Tasmania), using statistical summaries of 206 events during solar cycle #20. We form average patterns of Δ<I>Nm</I>F2 (%) versus local time over 7-day storm periods that are constructed in ways that enhance the portrayal of the average characteristic features of the positive and negative phases of ionospheric storms. The results show a consistency between four local time characteristic patterns of storm-induced perturbations, and thus for the average magnitudes and time scales of the processes that cause them in each hemisphere. Subtle differences linked to small departures from pure geophysical equivalency point to a possible presence of hemispheric asymmetries governed by the non-mirror-image of geomagnetic morphology in each hemisphere.2010-07-15T00:00:00+02:00http://www.ann-geophys.net/28/1441/2010/<b>Influence of turbidity and clouds on satellite total ozone data over Madrid (Spain)</b><br /><br />Annales Geophysicae, 28, 1441-1448, 2010<br /><br />Author(s): J. L. Camacho, M. Antón, D. Loyola, and E. Hernandez<br /><br />This article focuses on the comparison of the total ozone column data from three satellite instruments; Total Ozone Mapping Spectrometers (TOMS) on board the Earth Probe (EP), Ozone Monitoring Instrument (OMI) on board AURA and Global Ozone Monitoring Experiment (GOME) on board ERS/2, with ground-based measurement recorded by a well calibrated Brewer spectrophotometer located in Madrid during the period 1996–2008. A cluster classification based on solar radiation (global, direct and diffuse), cloudiness and aerosol index allow selecting hazy, cloudy, very cloudy and clear days. Thus, the differences between Brewer and satellite total ozone data for each cluster have been analyzed. The accuracy of EP-TOMS total ozone data is affected by moderate cloudiness, showing a mean absolute bias error (MABE) of 2.0%. In addition, the turbidity also has a significant influence on EP-TOMS total ozone data with a MABE ~1.6%. Those data are in contrast with clear days with MABE ~1.2%. The total ozone data derived from the OMI instrument show clear bias at clear and hazy days with small uncertainties (~0.8%). Finally, the total ozone observations obtained with the GOME instrument show a very smooth dependence with respect to clouds and turbidity, showing a robust retrieval algorithm over these conditions.2010-07-14T00:00:00+02:00http://www.ann-geophys.net/28/1431/2010/<b>Developing an ionospheric map for South Africa</b><br /><br />Annales Geophysicae, 28, 1431-1439, 2010<br /><br />Author(s): D. I. Okoh, L. A. McKinnell, and P. J. Cilliers<br /><br />The development of a map of the ionosphere over South Africa is presented in this paper. The International Reference Ionosphere (IRI) model, South African Bottomside Ionospheric Model (SABIM), and measurements from ionosondes in the South African Ionosonde Network, were combined within their own limitations to develop an accurate representation of the South African ionosphere. The map is essentially in the form of a computer program that shows spatial and temporal representations of the South African ionosphere for a given set of geophysical parameters. A validation of the map is attempted using a comparison of Total Electron Content (TEC) values derived from the map, from the IRI model, and from Global Positioning System (GPS) measurements. It is foreseen that the final South African ionospheric map will be implemented as a Space Weather product of the African Space Weather Regional Warning Centre.2010-07-12T00:00:00+02:00http://www.ann-geophys.net/28/1419/2010/<b>Some characteristics of large-scale travelling ionospheric disturbances and a relationship between the F₂ layer height rises of these disturbances and equatorial pre-sunrise events</b><br /><br />Annales Geophysicae, 28, 1419-1430, 2010<br /><br />Author(s): G. G. Bowman and I. K. Mortimer<br /><br />Initially some characteristics of large-scale travelling ionospheric disturbances (LS-TIDs) have been discussed briefly particularly as reported in the early literature. These discussions also involve the literature on the generation of LS-TIDs at times of geomagnetic bays. Secondly, the possibility that LS-TIDs may be responsible for the F₂ layer equatorial pre-sunrise height rises is investigated. Tabulations at hourly intervals of <I>h'F</I> at Huancayo and Washington for a Rz max period (1957–1960) have been used to identify height rises. For a three-hour interval at Huancayo <I>h'F</I> levels equal to or greater than 40 km of medians are used to identify the pre-sunrise height rises. Also height rises at Washington, which occurred earlier than those at Huancayo, have been considered for evidence of travelling disturbances. For 40 events analysed using geomagnetic bays and Washington height rises, a few hours before they occur at Huancayo, indicate the statistical significance of an association with LS-TIDs. Similar results of statistical significance have been obtained using Washington events and bays on average 34 h before 46 Huancayo events. These delays ranged from 29 h to 38 h. The results indicate that bays which occur the day before are responsible for LS-TIDs which encircle the earth.2010-07-12T00:00:00+02:00http://www.ann-geophys.net/28/1409/2010/<b>Calculation of signal spectrum by means of stochastic inversion</b><br /><br />Annales Geophysicae, 28, 1409-1418, 2010<br /><br />Author(s): T. Nygrén and Th. Ulich<br /><br />The standard method of calculating the spectrum of a digital signal is based on the Fourier transform, which gives the amplitude and phase spectra at a set of equidistant frequencies from signal samples taken at equal intervals. In this paper a different method based on stochastic inversion is introduced. It does not imply a fixed sampling rate, and therefore it is useful in analysing geophysical signals which may be unequally sampled or may have missing data points. This could not be done by means of Fourier transform without preliminary interpolation. Another feature of the inversion method is that it allows unequal frequency steps in the spectrum, although this property is not needed in practice. The method has a close relation to methods based on least-squares fitting of sinusoidal functions to the signal. However, the number of frequency bins is not limited by the number of signal samples. In Fourier transform this can be achieved by means of additional zero-valued samples, but no such extra samples are used in this method. Finally, if the standard deviation of the samples is known, the method is also able to give error limits to the spectrum. This helps in recognising signal peaks in noisy spectra.2010-07-06T00:00:00+02:00