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  • 538.7  (10)
  • English  (10)
  • 2020-2022  (10)
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  • English  (10)
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  • 1
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    Evolution of the Geomagnetic Daily Variation at Tatuoca, Brazil, From 1957 to 2019: A Transition From Sq to EEJ (2021)
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    Publication Date: 2021-10-28
    Description: The magnetic equator in the Brazilian region has moved over 1,100 km northward since 1957, passing the geomagnetic observatory Tatuoca (TTB), in northern Brazil, around 2013. We recovered and processed TTB hourly mean values of the geomagnetic field horizontal (H) component from 1957 until 2019, allowing the investigation of long-term changes in the daily variation due to the influence of secular variation, solar activity, season, and lunar phase. The H day-to-day variability and the occurrence of the counter electrojet at TTB were also investigated. Until the 1990s, ionospheric solar quiet currents dominated the quiet-time daily variation at TTB. After 2000, the magnitude of the daily variation became appreciably greater due to the equatorial electrojet (EEJ) contribution. The H seasonal and day-to-day variability increased as the magnetic equator approached, but their amplitudes normalized to the average daily variation remained at similar levels. Meanwhile, the amplitude of the lunar variation, normalized in the same way, increased from 5% to 12%. Within the EEJ region, the occurrence rate of the morning counter electrojet (MCEJ) increased with proximity to the magnetic equator, while the afternoon counter electrojet (ACEJ) did not. EEJ currents derived from CHAMP and Swarm satellite data revealed that the MCEJ rate varies with magnetic latitude within the EEJ region while the ACEJ rate is largely constant. Simulations with the Thermosphere-Ionosphere-Electrodynamics General Circulation Model based on different geomagnetic main field configurations suggest that long-term changes in the geomagnetic daily variation at TTB can be attributed to the main field secular variation.
    Keywords: 538.7 ; geomagnetism ; space physics ; geomagnetic daily variation ; solar quiet currents ; equatorial electrojet ; equatorial ionosphere
    Language: English
    Type: map
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    CITATION GEO-LEO
  • 2
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    The Norwegian-Greenland Sea, the Laschamps, and the Mono Lake Excursions Recorded in a Black Sea Sedimentary Sequence Spanning From 68.9 to 14.5 ka (2021)
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    Publication Date: 2021-10-15
    Description: A full-vector paleomagnetic record, comprising directional data and relative paleointensity (rPI), was derived from 16 sediment cores recovered from the southeastern Black Sea. The obtained data were used to create a stack covering the time window between 68.9 and 14.5 ka. Age models are based on radiocarbon dating and correlations of warming/cooling cycles monitored by high-resolution X-ray fluorescence (XRF) elementary ratios and by ice-rafted debris (IRD) in Black Sea sediments to the sequence of “Dansgaard-Oeschger” (D-O) events defined from the Greenland ice core oxygen isotope stratigraphy. The reconstructed prominent lows in paleointensity at about 64.5, 41.2, and 34.5 ka are coeval with the Norwegian-Greenland Sea, the Laschamps, and the Mono Lake excursions, respectively. For a further analysis, the stacked Black Sea paleomagnetic record was converted into one component being parallel to the direction expected from a geocentric axial dipole (GAD) and two components perpendicular to it (EW, inclined NS), representing definitely only non-GAD components of the geomagnetic field. Discussions of the field configurations at the Black Sea site are focused on the three excursional events. The Norwegian-Greenland Sea excursion was dominated by a decaying axial dipole and persisting weak nondipole field, with directional variations still within the range of normal secular variations. The Laschamps excursion comprises two full polarity transitions and a short stable interval of reversed polarity in between. The Mono Lake excursion was mostly dominated by a nondipole field, though with a less pronounced weakening of the axial dipole component.
    Keywords: 538.7 ; Black Sea ; Norwegian-Greenland Sea excursion ; Laschamps excursion ; Mono Lake excursion ; paleosecular variations
    Language: English
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  • 3
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    Estimating Shortwave Clear-Sky Fluxes From Hourly Global Radiation Records by Quantile Regression (2021)
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    Publication Date: 2021-10-13
    Description: Estimates of radiative fluxes under cloud-free conditions (“clear-sky”) are required in many fields, from climatic analyses of solar transmission to estimates of solar energy potential for electricity generation. Ideally, these fluxes can be obtained directly from measurements of solar fluxes at the surface. However, common standard methods to identify clear-sky conditions require observations of both the total and the diffuse radiative fluxes at very high temporal resolution of minutes, which restricts these methods to a few, well-equipped sites. Here we propose a simple method to estimate clear-sky fluxes only from typically available global radiation measurements (Rsd) at (half-)hourly resolution. Plotting a monthly sample of observed Rsd against the corresponding incoming solar radiation at the top of atmosphere (potential solar radiation) reveals a typical triangle shape with clear-sky conditions forming a distinct, linear slope in the upper range of observations. This upper slope can be understood as the fractional transmission of solar radiation representative for cloud-free conditions of the sample period. We estimate this upper slope through quantile regression. We employ data of 42 stations of the worldwide Baseline Surface Radiation Network to compare our monthly estimates with the standard clear-sky identification method developed by Long and Ackerman (2000, https://doi.org/10.1029/2000JD900077). We find very good agreement of the derived fractional solar transmission (R2 = 0.73) across sites. These results thus provide confidence in applying the proposed method to the larger set of global radiation measurements to obtain further observational constraints on clear-sky fluxes and cloud radiative effects.
    Keywords: 538.7 ; cloud-free ; clear-sky ; global radiation ; transmission ; quantile regression
    Language: English
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  • 4
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    Recordings of Fast Paleomagnetic Reversals in a 1.2 Ma Greigite-Rich Sediment Archive From Lake Ohrid, Balkans (2021)
    Details
    Publication Date: 2021-10-11
    Description: State-of-the-art paleoclimate research strongly depends on the availability of time-equivalent markers as chronological control to disentangle interrelationships in the climate system from regional to global scale. Geomagnetic reversals are regarded as excellent age constraints because they are global events and independent from climatic conditions. However, spatial variations of timing and internal dynamics of reversals may limit their precision. Our 1.2 Ma high-resolution (~25 cm/kyr) sediment record from Lake Ohrid is promising to precisely depict the Matuyama-Brunhes (MB) reversal and the Jaramillo subchron. Two generations of diagenetic ferrimagnetic minerals are present in glacial intervals of the Lake Ohrid record. Early diagenetic greigite acquired a quasi synsedimentary chemical magnetization, while a late diagenetic greigite formation, triggered by the upward diffusion of H2S-rich waters, obscures the polarity record at the top of the Jaramillo. Interglacial intervals are unaffected by greigite formation, likely due to low iron concentrations. Based on an orbitally tuned age model with tephrostratigraphic markers, the base of Jaramillo can be precisely dated to 1072.4 ka, and the MB reversal to 778.5 ka. Both polarity reversals occurred very rapidly in our record, lasting 2.3 and 1 kyr, respectively. Our results reveal that the dipole component of the Earth's magnetic field fell below the nondipole components only for a short duration in the Mediterranean region. The comparison of the timing of the MB boundary across different archives implies that the onset of the reversal provides a more synchronous age marker compared to often used midpoint ages.
    Keywords: 538.7 ; magnetostratigraphy ; diagenesis ; rock magnetism ; greigite ; fast magnetic reversals ; Matuyama-Brunhes
    Language: English
    Type: map
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  • 5
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    Nowcasting and Predicting the Kp Index Using Historical Values and Real-Time Observations (2021)
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    Publication Date: 2021-10-11
    Description: Current algorithms for the real-time prediction of the Kp index use a combination of models empirically driven by solar wind measurements at the L1 Lagrange point and historical values of the index. In this study, we explore the limitations of this approach, examining the forecast for short and long lead times using measurements at L1 and Kp time series as input to artificial neural networks. We explore the relative efficiency of the solar wind-based predictions, predictions based on recurrence, and predictions based on persistence. Our modeling results show that for short-term forecasts of approximately half a day, the addition of the historical values of Kp to the measured solar wind values provides a barely noticeable improvement. For a longer-term forecast of more than 2 days, predictions can be made using recurrence only, while solar wind measurements provide very little improvement for a forecast with long horizon times. We also examine predictions for disturbed and quiet geomagnetic activity conditions. Our results show that the paucity of historical measurements of the solar wind for high Kp results in a lower accuracy of predictions during disturbed conditions. Rebalancing of input data can help tailor the predictions for more disturbed conditions.
    Keywords: 538.7 ; Kp index ; geomagnetic activity ; empirical prediction ; solar wind ; forecast ; AI
    Language: English
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  • 6
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    The Effect of Plasma Boundaries on the Dynamic Evolution of Relativistic Radiation Belt Electrons (2021)
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    Publication Date: 2021-10-01
    Description: Understanding the dynamic evolution of relativistic electrons in the Earth's radiation belts during both storm and nonstorm times is a challenging task. The U.S. National Science Foundation's Geospace Environment Modeling (GEM) focus group “Quantitative Assessment of Radiation Belt Modeling” has selected two storm time and two nonstorm time events that occurred during the second year of the Van Allen Probes mission for in-depth study. Here, we perform simulations for these GEM challenge events using the 3D Versatile Electron Radiation Belt code. We set up the outer L* boundary using data from the Geostationary Operational Environmental Satellites and validate the simulation results against satellite observations from both the Geostationary Operational Environmental Satellites and Van Allen Probe missions for 0.9-MeV electrons. Our results show that the position of the plasmapause plays a significant role in the dynamic evolution of relativistic electrons. The magnetopause shadowing effect is included by using last closed drift shell, and it is shown to significantly contribute to the dropouts of relativistic electrons at high L*. We perform simulations using four different empirical radial diffusion coefficient models for the GEM challenge events, and the results show that these simulations reproduce the general dynamic evolution of relativistic radiation belt electrons. However, in the events shown here, simulations using the radial diffusion coefficients from Brautigam and Albert (2000) produce the best agreement with satellite observations.
    Keywords: 538.7 ; radiation belt ; simulation ; relativistic electrons ; magnetopause shadowing ; wave-particle interaction ; plasmapause
    Language: English
    Type: map
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  • 7
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    A Statistical Classifier for Historical Geomagnetic Storm Drivers Derived Solely From Ground-Based Magnetic Field Measurements (2021)
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    Publication Date: 2021-07-26
    Description: Solar wind observations show that geomagnetic storms are mainly driven by interplanetary coronal mass ejections (ICMEs) and corotating or stream interaction regions (C/SIRs). We present a binary classifier that assigns one of these drivers to 7,546 storms between 1930 and 2015 using ground-based geomagnetic field observations only. The input data consists of the long-term stable Hourly Magnetospheric Currents index alongside the corresponding midlatitude geomagnetic observatory time series. This data set provides comprehensive information on the global storm time magnetic disturbance field, particularly its spatial variability, over eight solar cycles. For the first time, we use this information statistically with regard to an automated storm driver identification. Our supervised classification model significantly outperforms unskilled baseline models (78% accuracy with 26[19]% misidentified interplanetary coronal mass ejections [corotating or stream interaction regions]) and delivers plausible driver occurrences with regard to storm intensity and solar cycle phase. Our results can readily be used to advance related studies fundamental to space weather research, for example, studies connecting galactic cosmic ray modulation and geomagnetic disturbances. They are fully reproducible by means of the underlying open-source software (Pick, 2019, http://doi.org/10.5880/GFZ.2.3.2019.003)
    Keywords: 538.7 ; geomagnetic observatory data ; geomagnetic storm drivers ; historical geomagnetic storms ; supervised machine learning
    Language: English
    Type: article
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  • 8
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    Suprathermal Fe in the Earth's Plasma Environment: Cluster RAPID Observations (2021)
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    Publication Date: 2021-07-26
    Description: Baryonic matter in geospace is almost exclusively in a plasma state, with protons (H +) and to some extent ionized helium (He) and oxygen (O) being the dominant ion species. But also other heavier ion species and even molecular ions are present in geospace. The Research with Adaptive Particle Imaging Detectors (RAPID) on board the Cluster satellites can identify and characterize some of these ions by utilizing their measured time of flight and energy. Usually, the measurements are then assigned into three discrete species channels; protons (H +), helium (He), and a common channel for carbon, nitrogen, and oxygen (CNO), each with flux, energy, and angular information. But RAPID also has a Direct Event (DE) diagnostic mode in which the full time of flight and energy information for a limited number of incident particles are stored. With knowledge about energy losses in the various detector parts, it is then possible to derive the atomic mass of the incident particle. In this paper we report on results from a study of Cluster DE events during the years 2001–2018, with a particular emphasis of iron (Fe) ions. We show that suprathermal Fe ions can be found all over geospace covered by Cluster, and that the time variation is consistent with modulation by geomagnetic disturbances and solar activity. We do not find any clear correlations between detection of suprathermal Fe and meteor showers or sputtering off the moon.
    Keywords: 538.7 ; composition ; ion outflow ; energetic particles
    Language: English
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  • 9
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    Residuals to the CHAOS-6 Geomagnetic Field Model Caused by Magnetospheric Currents During Enhanced Magnetic Activity (2021)
    Details
    Publication Date: 2021-07-26
    Description: Thanks to mapping missions, like Ørsted, CHAMP, and Swarm, we have gained a detailed understanding of the geomagnetic field. High-resolution models like POMME, GRIMM, or CHAOS are able to describe the main parts of the Earth's magnetic field reliably. These models represent well contributions from the core and crustal fields. But their validity of describing magnetospheric field effects is limited to low activity periods (Kp ~ 0–2). Here, we study the differences between CHAMP magnetic observations and the predictions from CHAOS-6-x9, a recent version, outside this validity range. Systematic residuals appear at times of elevated activity. Mean amplitudes at the equator grow up to 12 nT around 20 hr magnetic local time for magnetic activity around Kp = 4.7. Negative residuals are obtained in the evening to midnight sector and positive ones in the morning. A seasonal dependence of the magnetospheric currents causes more negative deflections of the residuals in the winter than in the summer hemisphere. This hemispheric asymmetry cannot be accounted for by a degree 1 spherical harmonics function. A surprising observation is that the residuals show a clear longitude dependent pattern, which changes with local time. The analysis reveals that this feature can be interpreted as a Universal Time dependence of the residuals with a peak-to-peak amplitude of about 8 nT and a period of 12 hr at an activity level of Kp = 4.7. All these results call for a better parameterization of the magnetospheric current effects in a geomagnetic field model that is reliable at least up to moderate activity levels.
    Keywords: 538.7 ; geomagnetic field ; geomagnetic field model ; magnetospheric currents ; ring current ; near-Earth magnetic effect
    Language: English
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  • 10
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    One Hundred Thousand Years of Geomagnetic Field Evolution (2021)
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    Publication Date: 2021-07-26
    Description: Paleomagnetic records from sediments, archeological artifacts, and lava flows provide the foundation for studying geomagnetic field changes over 0–100 ka. Late Quaternary time-varying spherical harmonic models for 0–100 ka produce a global view used to evaluate new data records, study the paleomagnetic secular variation on centennial to multimillennial timescales, and investigate extreme regional or global events such as the Laschamp geomagnetic excursion. Recent modeling results (GGF100k and LSMOD.2) are compared to previous studies based on regional or global stacks and averages of relative geomagnetic paleointensity variations. Time-averaged field structure is similar on Holocene, 100 ky, and million-year timescales. Paleosecular variation activity varies greatly over 0–100 ka, with large changes in field strength and significant morphological changes that are especially evident when field strength is low. GGF100k exhibits a factor of 4 variation in geomagnetic axial dipole moment, and higher-resolution models suggest that much larger changes are likely during global excursions. There is some suggestion of recurrent field states resembling the present-day South Atlantic Anomaly, but these are not linked to initiation or evolution of excursions. Several properties used to characterize numerical dynamo simulations as “Earth-like” are evaluated and, in future, improved models may yet reveal systematic changes linked to the onset of geomagnetic excursions. Modeling results are useful in applications ranging from ground truth and data assimilation in geodynamo simulations to providing geochronological constraints and modeling the influence of geomagnetic variations on cosmogenic isotope production rates.
    Keywords: 538.7 ; Holocene ; paleomagnetic records ; geomagnetic field variations
    Language: English
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