Annales Geophysicae最新文献

{"title":"How does auroral electron precipitation near the open–closed field line boundary compare to that within the auroral oval during substorm onset?","authors":"Maxime Grandin, Noora Partamies, Ilkka I. Virtanen","doi":"10.5194/egusphere-2024-483","DOIUrl":"https://doi.org/10.5194/egusphere-2024-483","url":null,"abstract":"<strong>Abstract.</strong> Auroral electron precipitation during a substorm exhibits complex spatiotemporal variations which are still not fully understood, especially during the very dynamic phase immediately following the onset. Since during disturbed times the auroral oval typically extends across several hundreds of kilometers in the latitudinal direction, one may expect that precipitating electron spectra differ at locations close to the open–closed field line boundary (OCB) compared to the central part of the auroral oval. We carry out a statistical study based on 57 auroral breakups associated with substorm onsets observed above Tromsø (66.7° N geomagnetic latitude, i.e. central oval) and 25 onsets occurring above Svalbard (75.4° N geomagnetic latitude, i.e. poleward boundary) between 2015 and 2022. The events were selected based on the availability of both optical observations and field-aligned incoherent scatter radar measurements. Those are two sets of different substorms; hence we compare solar wind driving conditions and geomagnetic indices for the two event lists in the statistical sense. Using the ELSPEC method (based on the inversion of the electron density profile) on the radar data, we retrieve precipitating electron fluxes within 1–100 keV around each onset time, and we apply the superposed epoch analysis method on the electron spectra at each location. We compare the statistical precipitation characteristics above both sites in terms of peak differential flux, energy of the peak, integrated energy flux, and their time evolution during the minutes following the onset. We find that the integrated energy flux associated with events occurring in the central part of the auroral oval (Tromsø) exhibit a sharp peak up to 25 mW m^-2 in the first two minutes following the auroral breakup, before decreasing and reaching stable values around 7 mW m^-2 for at least 20 min. In turn, no initial peak is seen near the open–closed field line boundary (Svalbard), and values remain low throughout (1–2 mW m^-2). A comparison of the median spectra indicates that the precipitating flux of &gt; 10 keV electrons is lower above Svalbard than above Tromsø by a factor of at least 10, which may partly explain the differences. However, it proves difficult to conclude whether the differences originate from the latitude at which the auroral breakup takes place or from the fact that the breakups seen from Svalbard occur equatorward from the radar beam, which only sees expansion-phase precipitation after a few minutes.","PeriodicalId":50777,"journal":{"name":"Annales Geophysicae","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2024-02-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139927632","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Observations of polar mesospheric summer echoes resembling kilometer-scale varicose-mode flows","authors":"Jennifer Hartisch, Jorge L. Chau, Ralph Latteck, Toralf Renkwitz, Marius Zecha","doi":"10.5194/angeo-42-29-2024","DOIUrl":"https://doi.org/10.5194/angeo-42-29-2024","url":null,"abstract":"Abstract. The mesosphere and lower thermosphere (MLT) region represents a captivating yet challenging field of research. Remote sensing techniques, such as radar, have proven invaluable for investigating this domain. The Middle Atmosphere Alomar Radar System (MAARSY), located in northern Norway (69∘ N, 16∘ E), uses polar mesospheric summer echoes (PMSEs) as tracers to study MLT dynamics across multiple scales. Chau et al. (2021) recently discovered a spatiotemporally highly localized event showing a varicose mode (simultaneous upward and downward movements), which is characterized by extreme vertical velocities (|w|≥3σ) of up to 60 m s−1 in the vertical drafts. Motivated by this finding, our objective is to identify and quantify similar extreme events or comparable varicose structures, i.e., defined by quasi-simultaneous updrafts and downdrafts, that may have been previously overlooked or filtered out. To achieve this, we conducted a thorough manual search through a MAARSY dataset, considering the PMSE months (i.e., May, June, July, August) spanning from 2015 to 2021. This search has revealed that these structures do indeed occur relatively frequently with an occurrence rate of up to 2.5 % per month. Over the 7-year period, we observed and recorded more than 700 varicose-mode events with a total duration of about 265 h and documented their vertical extent, vertical velocity characteristics, duration, and their occurrence behavior. Remarkably, these events manifest throughout the entire PMSE season with pronounced occurrence rates in June and July, while the probability of their occurrence decreases towards the beginning and end of the PMSE seasons. Furthermore, their diurnal variability aligns with that of PMSEs. On average, the observed events persisted for 20 min, while the varicose mode caused an average expansion of the PMSE layer by a factor of 1.5, with a maximum vertical expansion averaging around 8 km. Notably, a careful examination of the vertical velocities associated with these events confirmed that approximately 17 % surpassed the 3σ threshold, highlighting their non-Gaussian velocity distribution and extreme nature.","PeriodicalId":50777,"journal":{"name":"Annales Geophysicae","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2024-02-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139927726","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Characteristic analysis of the differences between total electron content (TEC) values in global ionosphere map (GIM) grids","authors":"Qisheng Wang, Jiaru Zhu, Genxin Yang","doi":"10.5194/angeo-42-45-2024","DOIUrl":"https://doi.org/10.5194/angeo-42-45-2024","url":null,"abstract":"Abstract. Using total electron content (TEC) from a global ionosphere map (GIM) for ionospheric delay correction is a common method of eliminating ionospheric errors in satellite navigation and positioning. On this basis, the TEC of a puncture point can be obtained by GIM grid TEC interpolation. However, in terms of grid, only few studies have analyzed the TEC value size characteristics of its four grid points, that is, the TEC difference characteristics among them. In view of this, by utilizing the GIM data from high solar-activity years (2014) and low solar-activity years (2021) provided by CODE (Center for Orbit Determination in Europe), this paper proposes the grid TEC difference as a way of analyzing TEC variation characteristics within the grid, which is conducive to exploring and analyzing the variation characteristics of the ionosphere TEC in the single-station area. The value is larger in high solar-activity years and generally small in low solar-activity years, and the value of high-latitude areas is always smaller than that of low-latitude areas. Specifically, in high solar-activity years, most of the GIM grid TEC internal differences are within 4 TECu (1 TECu = 1016 electrons m−2) in high-latitude and midlatitude regions, while only 78.17 % are in low-latitude regions. In low solar-activity years, the TEC difference values within a GIM grid are mostly less than 2 TECu, and most of them in the high and middle latitudes are within 1 TECu. The main finding of this analysis is that the grid TEC differences are small for most GIM grids, especially in the midlatitudes to high latitudes of low solar years. This means that relevant extraction methods and processes can be simplified when TEC within these GIM grids is needed.","PeriodicalId":50777,"journal":{"name":"Annales Geophysicae","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2024-02-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139927729","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Estimating gradients of physical fields in space","authors":"Yufei Zhou, Chao Shen","doi":"10.5194/angeo-42-17-2024","DOIUrl":"https://doi.org/10.5194/angeo-42-17-2024","url":null,"abstract":"Abstract. This study focuses on the development of a multi-point technique for future constellation missions, aiming to measure gradients at various orders, in particular the linear and quadratic gradients, of a general field. It is well established that, in order to estimate linear gradients, the spacecraft must not lie on a plane. Through analytical exploration within the framework of least squares, it is demonstrated that at least 10 spacecraft that do not lie on any quadric surface are required to estimate both linear and quadratic gradients. The spatial arrangement of the spacecraft can be characterized by a set of quality factors. In cases where there is poor temporal synchronization among the spacecraft leading to non-simultaneous measurements, temporal gradients must be included. If the spacecraft have multiple velocities, by incorporating temporal gradients it is possible to reduce the number of required spacecraft. Furthermore, it is proved that the accuracy of the linear gradient is of second order and that of the quadratic gradient is of first order. Additionally, a method for estimating errors in the calculation is also illustrated.","PeriodicalId":50777,"journal":{"name":"Annales Geophysicae","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2024-02-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139772744","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"High-time-resolution analysis of meridional tides in the upper mesosphere and lower thermosphere at mid-latitudes measured by the Falkland Islands SuperDARN radar","authors":"Gareth Chisham, Andrew J. Kavanagh, Neil Cobbett, Paul Breen, Tim Barnes","doi":"10.5194/angeo-42-1-2024","DOIUrl":"https://doi.org/10.5194/angeo-42-1-2024","url":null,"abstract":"Abstract. Solar tides play a major role in the dynamics of the upper mesosphere and lower thermosphere (MLT). Hence, a comprehensive understanding of these tides is important for successful modelling of the MLT region. Most ground-based observations of tidal variations in the MLT have been from meteor radar measurements with a temporal resolution of 1 h. Here, we take a different perspective on these tidal variations using high-resolution 1 min neutral-wind measurements from the Falkland Islands SuperDARN (Super Dual Auroral Radar Network) radar. This analysis shows that these higher-resolution data can be used to identify higher frequency tidal components than are typically observed by meteor radars (up to a heptadiurnal component). It also shows evidence of significant power in these higher frequency components, particularly in the quaddiurnal component, which may be particularly suitable for a global analysis using high-resolution SuperDARN neutral-wind measurements. The high-resolution analysis also shows evidence of fluctuations with a frequency of 1.5 cycles per day, as well as higher-frequency fluctuations, accompanying a quasi-2 d (two day) wave. We discuss the limitations of this high-resolution analysis method and the new opportunities that it may provide. We conclude that higher-resolution SuperDARN neutral-wind measurements need to be better exploited in the future as they provide a complementary way of studying tides and waves in the MLT.","PeriodicalId":50777,"journal":{"name":"Annales Geophysicae","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2024-02-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139663928","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Investigation of the October effect in VLF signals","authors":"Marc Hansen, Daniela Banyś, Mark Clilverd, David Wenzel, M. Mainul Hoque","doi":"10.5194/angeo-2023-38","DOIUrl":"https://doi.org/10.5194/angeo-2023-38","url":null,"abstract":"<strong>Abstract.</strong> Subionospheric Very Low Frequency (VLF) radio signals are reflected by free electrons in the ionospheric D-region at about 60–90 km altitude and can propagate over long distances, which makes them useful for monitoring the state of the D-region or perturbations due to solar flares. At the D-region height, the ionosphere is mainly ionized by the solar Lyman-α radiation. The reflection characteristics of VLF signals depend on the state and dynamics of the D-region which is highly influenced by the Lyman-α radiation. Although the amplitude of the received terrestrial VLF signal changes as a function of solar zenith angle over the course of the year, the VLF amplitude shows a distinctive sharp decrease around October, which is hence called the “October effect”. This study investigates the occurrence of the October effect and its dependencies on latitude and longitude. We developed a method to detect the occurrence of the October effect in the long-term VLF data and derive key parameters characterizing (start and end date, intensity) the sudden decrease in the signal amplitude. This investigation using a network of VLF stations distributed over low, middle and high latitude regions shows that the occurrence of the October effect has a clear latitudinal dependency, occurring earlier in high-latitude regions than at mid-latitudes. No low latitude signature is found.","PeriodicalId":50777,"journal":{"name":"Annales Geophysicae","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2024-01-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139422917","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Origins of a Near-Ecliptic Merged Interaction Region as a Magnetic-Cloud like Structure Embedded in a Co-rotating Interaction Region","authors":"Megan L. Maunder, Claire Foullon, Robert Forsyth, David Barnes, Jackie Davies","doi":"10.5194/angeo-2023-39","DOIUrl":"https://doi.org/10.5194/angeo-2023-39","url":null,"abstract":"<strong>Abstract.</strong> Using remote-sensing and in-situ observations across multiple spacecraft with complimentary methods of analysis, we investigate a Magnetic Cloud Like-structure (MCL) observed in-situ on 3–4 July 2007 near the ecliptic at OMNI, STEREO-A and -B (all within 15° longitude of Earth). The MCL is entrained in a Corotating Interaction Region (CIR) originating in the Northern heliospheric sector, to create a Merged Interaction Region (MIR). This event allows the comparison of MIR observations at different longitudes showing differences in size, formation of sheath, presence of forward and reverse waves and small-scale structuring, demonstrating the progression of the interaction between the CIR and MCL from West to East. In order to explore its origins further, we compare the MIR with the (Interplanetary) Coronal Mass Ejection (ICME/CME) studied in Maunder et al. (2022) in the mid-latitudes at <em>Ulysses</em> containing a Magnetic Cloud (MC) and present a comprehensive discussion of the challenges posed by observing and relating transients not in alignment, across different latitudes and longitudes, and in different solar wind environments. As the CME propagates almost directly towards <em>Ulysses</em>, we find through fitting and modelling that its flanks could also potentially skim the near-ecliptic spacecraft. Length-scale analysis appears to be consistent with this configuration. However, local expansion velocities of the MCL/MC indicate compression near the ecliptic and expansion at <em>Ulysses</em> and the magnetic flux rope orientations and helicities at the different latitudes oppose each other. The CIR likely causes more compression and re-aligns the transient axis orientation near the ecliptic while a High Speed Stream (HSS) from the Southern sector propagates directly into the back of the ICME/MC near the mid-latitude. Opposing signs of helicity could provide indications of flux added in the first stages of CME evolution or magnetic reconnection with the Heliospheric Current Sheet (HCS). These observations and analyses demonstrate the continued challenge of modelling and fitting the propagation of transients embedded in complex solar wind environments. We note some of the caveats and limitations in the methods and highlight the use of multi-spacecraft analysis to disentangle the origin and formation of ICME substructures from the solar wind and other transients.","PeriodicalId":50777,"journal":{"name":"Annales Geophysicae","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2024-01-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139422912","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Ionospheric Upwelling and the Level of Associated Noise at Solar Minimum","authors":"Timothy Wemimo David, Chizurumoke Michael Michael, Darren M. Wright, Adetoro Temitope Talabi, Abayomi E. Ajetunmobi","doi":"10.5194/angeo-2023-37","DOIUrl":"https://doi.org/10.5194/angeo-2023-37","url":null,"abstract":"<strong>Abstract.</strong> We have studied the ionospheric upwelling with magnitude above 10¹³ <em>m</em>⁻² <em>s</em>⁻¹ using the data during the IPY-ESR 2007 campaign, which coincides with solar minimum. The noise level in low, medium and high-flux upflows is investigated. We found that the noise level in high-flux upflow is about 93 % while the low and medium categories are 62 % and 80 %, respectively. This shows that robust and stringent filtering techniques must be ensured when analysing incoherent data in order not to bias the result. Analysis reveals that the frequency of the low-flux upflow events is about 8 and 73 times the medium and high-flux upflow events, respectively. Seasonal observation shows that the noise level in the upflow classes is predominantly during winter. The noise is minimal in summer, with a notable result indicating occurrence of actual data above noise in the low-flux class. Moreover, the percentage occurrence of the noise level in the data increases with increasing flux strength, irrespective of the season. Further analysis reveals that the noise level in the local time variation peaked around 17 – 18 LT and minimum around local noon.","PeriodicalId":50777,"journal":{"name":"Annales Geophysicae","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2024-01-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139092217","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"On the importance of middle atmosphere observations on ionospheric dynamics using WACCM-X and SAMI3","authors":"Fabrizio Sassi, Angeline Burrell, Sarah McDonald, Jennifer Tate, John McCormack","doi":"10.5194/egusphere-2023-3065","DOIUrl":"https://doi.org/10.5194/egusphere-2023-3065","url":null,"abstract":"<strong>Abstract.</strong> Recent advances in atmospheric observations and modelling have enabled the investigation of thermosphere-ionosphere interactions as a whole atmosphere problem. This study examines how dynamical variability in the middle atmosphere (MA) affects day-to-day changes in the thermosphere and ionosphere. Specifically, this study investigates ionosphere-thermosphere interactions during different time periods of January 2013 using the Specified Dynamics Whole Atmosphere Community Climate Model, eXtended version (WACCM-X) coupled to the Naval Research Laboratory (NRL) ionosphere of the Sami3 is Another Model of the Ionosphere (SAMI3) model. To represent the weather of the day, the coupled thermosphere-ionosphere system is nudged below 90 km toward the atmospheric specifications provided by the Navy Global Environmental Model for High-Altitude (NAVGEM-HA). Hindcast simulations during January 2013 are carried out with the full data set of observations normally assimilated by NAVGEM-HA, and with a degraded dataset where observations above 40 km are not assimilated. Ionospheric regions with statistically significant changes are identified using key ionospheric properties, including the electron density, peak electron density, and height of the peak electron density. Ionospheric changes show a spatial structure that illustrates the impact of two different types of coupling between the thermosphere and the ionosphere: wind-dynamo coupling through electric conductivity and ion-neutral interactions in the upper thermosphere. The two simulations presented in this study show that changing the state of the MA affects ionosphere-thermosphere coupling through changes in the behavior and amplitude of non-migrating tides, resulting in improved key ionospheric specifications.","PeriodicalId":50777,"journal":{"name":"Annales Geophysicae","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2024-01-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139092309","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"On mechanisms for high-frequency pump-enhanced optical emissions at 557.7 and 630.0 nm from atomic oxygen in the high-latitude F-region ionosphere","authors":"Thomas B. Leyser, Tima Sergienko, Urban Brändström, Björn Gustavsson, Michael T. Rietveld","doi":"10.5194/angeo-41-589-2023","DOIUrl":"https://doi.org/10.5194/angeo-41-589-2023","url":null,"abstract":"Abstract. The EISCAT (European Incoherent Scatter Scientific Association) Heating facility was used to transmit powerful high-frequency (HF) electromagnetic waves into the F-region ionosphere to enhance optical emissions at 557.7 and 630.0 nm from atomic oxygen. The emissions were imaged by several stations of ALIS (Auroral Large Imaging System) in northern Sweden, and the EISCAT UHF incoherent scatter radar was used to obtain plasma parameter values. The ratio of the 557.7 to 630.0 nm column emission rates changed from I5577/I6300≈0.2 for the HF pump frequency f0=6.200 MHz ≈4.6fe to I5577/I6300≈0.5 when f0=5.423 MHz ≲4fe, where fe is the ionospheric electron gyro frequency. The observations are interpreted in terms of decreased electron heating efficiency and thereby weaker enhancement at 630.0 nm for f0=5.423 MHz ≲ 4 fe. The emissions at 557.7 nm are attributed to electron acceleration by upper hybrid waves of metre-scale wavelengths that can be excited with f0=5.423 MHz ≲ 4 fe.","PeriodicalId":50777,"journal":{"name":"Annales Geophysicae","volume":null,"pages":null},"PeriodicalIF":1.9,"publicationDate":"2023-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138580262","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}