Hasil untuk "Geomagnetism"

Ali Abdalsalam, Elhamy A.Tarabees, Kamal Abdelrahman et al.

This study integrates Very Low Frequency Electromagnetic (VLF-EM) and Time-Domain Electromagnetic (TDEM) methods to characterise subsurface geological structures and aquifer conditions in the New Nubariya-2 City, Egypt. VLF-EM analysis, enhanced by Fraser filtering and Karous-Hjelt pseudo-sections, identified nineteen conductive linear anomalies (F1–F19) interpreted as fracture zones. TDEM 1D inversion validated several of these anomalies and delineated four distinct geoelectric layers: a heterogeneous surface layer composed of dry sands, silts, clay, or gravel (5–216 Ω·m), a low-resistivity clay layer (1.2–2.4 Ω·m) with thicknesses of 28–71 m, a water-saturated sand aquifer (13–33 Ω·m) with depths increasing from 34 m in the east to approximately 104 m in the northwest, and a basal clayey sand formation (3.7–9.6 Ω·m). The southeastern and northwestern regions were identified as optimal for well drilling due to greater aquifer thickness, higher resistivity, and the presence of deep water-bearing fracture zones. These findings enhance the understanding of groundwater resources in the West Nile Delta and provide valuable insights for urban planning and water resource management in the New Nubariya-2 City.

Richard Douglas Elmore, Ramon Egli, Steven P. Lund

Helen Mavromichalaki, Maria-Christina Papailiou, Maria Livada et al.

As the current solar cycle 25 progresses and moves towards solar maxima, solar activity is increasing and extreme space weather events are taking place. Two severe geomagnetic storms accompanied by two large Forbush decreases in galactic cosmic ray intensity were recorded in March and May, 2024. More precisely, on 24 March 2024, a G4 (according to the NOAA Space Weather Scale for Geomagnetic Storms) geomagnetic storm was registered, with the corresponding geomagnetic indices Kp and Dst equal to 8 and −130 nT, respectively. On the same day, the majority of ground-based neutron monitor stations recorded an unusual Forbush decrease. This event stands out from a typical Forbush decrease because of its high amplitude decrease phase and rapid recovery phase, i.e., 15% decrease and an extremely rapid recovery of 10% within 1.5 h, as recorded at the Oulu neutron monitor station. Furthermore, on 10–13 May 2024, an unusual G5 geomagnetic storm (geomagnetic indices Kp = 9 and Dst = −412 nT) was registered (the last G5 storm had been observed in 2003). In addition, the polar neutron monitor stations recorded a Ground Level Enhancement (GLE74) during the recovery phase of a large Forbush decrease of 15%, which started on 10 May 2024. In this study, a detailed analysis of these two severe events in regard to the accompanying solar activity, interplanetary conditions and solar energetic particle events is provided. Moreover, the results of the NKUA “GLE Alert++ system”, the NKUA/IZMIRAN “FD Precursory Signals” method and the NKUA “ap Prediction tool” concerning these events are presented.

Prasanna Mahavarkar, S. Sriram, Bhagyashree Joshi et al.

Integrating spheres can be manufactured at sizes ranging from 1 mm to 3 m in diameter. It has been agreed widely by manufacturers that fitting a large-sized integrating sphere as per the customer requirement is a complex process. Thus, these spheres due to its large beam divergence from the exit port, have never been deployed for the calibration of aeronomy applications. In this article a optical system is described to integrate a large-sized integrating sphere with the Fabry–Perot etalon and thus devise a set-up to generate the concentric ring pattern and record it on the detector. The experimental set-up was verified by optical simulation using ZEMAX (OpticStudio) so as to validate the system’s performance in the laboratory. The important parameter finesse is calculated and its accuracy is found to be 80%.

Rajkumar Hajra, Bruce T. Tsurutani, Gurbax S. Lakhina et al.

The Helios 1 (H1) and Helios 2 (H2) spacecraft measured the solar winds at a distance between ∼0.3 and 1.0 au from the Sun. With increasing heliocentric distance ( r _h ), the plasma speed is found to increase at ∼34–40 km s ^−1 au ^−1 and the density exhibits a sharper fall ( ${{r}_{{\rm{h}}}}^{-2}$ ) compared to the magnetic field magnitude ( ${{r}_{{\rm{h}}}}^{-1.5}$ ) and the temperature ( ${{r}_{{\rm{h}}}}^{-0.8}$ ). Using all available solar wind plasma and magnetic field measurements, we identified 68 and 39 fast interplanetary shocks encountered by H1 and H2, respectively. The overwhelming majority (85%) of the shocks are found to be driven by interplanetary coronal mass ejections (ICMEs). While the two spacecraft encountered more than 73 solar wind high-speed streams (HSSs), only ∼22% had shocks at the boundaries of corotating interaction regions (CIRs) formed by the HSSs. All of the ICME shocks were found to be fast forward (FF) shocks; only four of the CIR shocks were fast reverse shocks. Among all ICME FF shocks (CIR FF shocks), 60% (75%) are quasi-perpendicular with shock normal angles ( θ _Bn ) ≥ 45° relative to the upstream ambient magnetic field, and 40% (25%) are quasi-parallel ( θ _Bn < 45°). No radial dependences were found in FF shock normal angle and speed. The FF shock Mach number ( M _ms ), magnetic field, and plasma compression ratios are found to increase with increasing r _h at the rates of 0.72, 0.89, and 0.98 au ^−1 , respectively. On average, ICME FF shocks are found to be considerably faster (∼20%) and stronger (with ∼28% higher M _ms ) than CIR FF shocks.

P. Alken, A. Chulliat, M. Nair

Abstract The International Geomagnetic Reference Field (IGRF) is a set of parameters representing the large-scale internal part of Earth’s magnetic field. The 13th generation IGRF requested candidate models for a definitive main field for 2015.0, a provisional main field for 2020.0, and a predictive secular variation covering the period 2020.0–2025.0. The University of Colorado (CU) and the National Centers for Environmental Information (NCEI), part of the National Oceanic and Atmospheric Administration (NOAA), have produced these three candidate models for consideration in IGRF-13. In this paper, we present the methodology used to derive our candidate models. Our candidates were built primarily from Swarm satellite data, and also relied on geomagnetic indices derived from the ground observatory network. The ground observatories played a crucial role as independent data in validating our candidates. This paper also provides a retrospective assessment of the CU/NCEI candidate model to the previous IGRF (IGRF-12) and discusses the impact of differences between candidate and final IGRF models on global model errors.

Jeffrey J. Love, Greg M. Lucas, Benjamin S. Murphy et al.

Abstract An analysis is made of Earth‐surface geoelectric fields and voltages on electricity transmission power‐grids induced by a late‐phase E3 nuclear electromagnetic pulse (EMP). A hypothetical scenario is considered of an explosion of several hundred kilotons set several hundred kilometers above the eastern‐midcontinental United States. Ground‐level E3 geoelectric fields are estimated by convolving a standard parameterization of E3 geomagnetic field variation with magnetotelluric Earth‐surface impedance tensors derived from wideband measurements acquired across the study region during a recent survey. These impedance tensors are a function of subsurface three‐dimensional electrical conductivity structure. Results, presented as a movie‐map, demonstrate that localized differences in surface impedance strongly distort the amplitude, polarization, and variational phase of induced E3 geoelectric fields. Locations with a high degree of E3 geoelectric polarization tend to have high geoelectric amplitude. Uniform half‐space models and one‐dimensional, depth‐dependent models of Earth‐surface impedance, such as those widely used in government and industry reports informing power‐grid vulnerability assessment projects, do not provide accurate estimates of the E3 geoelectric hazard in complex geological settings. In particular, for the Eastern‐Midcontinent, half‐space models can lead to (order‐one) overestimates/underestimates of EMP‐induced geovoltages on parts of the power grid by as much as ±1,000 volts (a range of 2,000 volts)—comparable to the amplitudes of the geovoltages themselves.

Harikrishnan Aravindakshan, Amar Kakad, Bharati Kakad et al.

Ion holes refer to the phase-space structures where the trapped ion density is lower at the center than at the rim. These structures are commonly observed in collisionless plasmas, such as the Earth’s magnetosphere. This paper investigates the role of multiple parameters in the generation and structure of ion holes. We find that the ion-to-electron temperature ratio and the background plasma distribution function of the species play a pivotal role in determining the physical plausibility of ion holes. It is found that the range of width and amplitude that defines the existence of ion holes splits into two separate domains as the ion temperature exceeds that of the electrons. Additionally, the present study reveals that the ion holes formed in a plasma with ion temperature higher than that of the electrons have a hump at its center.

P. R. Shreedevi, R. K. Choudhary, Smitha V. Thampi et al.

Abstract The occurrence of St. Patrick's Day (17 March) geomagnetic storms during two different years (2013 and 2015) with similar solar flux levels but varying storm intensity provided an opportunity to compare and contrast the responses of the ionosphere‐thermosphere (IT) system to different levels of geomagnetic activity. The evolution of positive ionospheric storms at the southern polar stations Bharati (76.6°S MLAT) and Davis (76.2°S MLAT) and its causative connection to the solar wind driving mechanisms during these storms has been investigated in this paper. During the main phase of both the storms, significant enhancements in TEC and phase scintillation were observed in the magnetic noon/ midnight period at Bharati and Davis. The TEC in the midnight sector on 17 March 2015 was significantly higher compared to that on 17 March 2013, in line with the storm intensity. The TEC enhancements during both the storm events are associated with the formation of the storm‐enhanced densities (SEDs)/tongue of ionization (TOI). The strong and sustained magnetopause erosion led to the prevalence of stronger storm time electric fields (prompt penetration electric field (PPEF)/subauroral polarization streams (SAPS)) for long duration on 17 March 2015. This combined with the action of neutral winds at midlatitudes favored the formation of higher plasma densities in the regions of SED formation on this day. The same was weaker during the 17 March 2013 storm due to the fast fluctuating nature of interplanetary magnetic field (IMF) Bz. This study shows that the duration and extent of magnetopause erosion play an important role in the spatiotemporal evolution of the plasma density distribution in the high‐midlatitude ionosphere.

B. Zolesi, M. Pezzopane, C. Bianchi et al.

Abstract On 25 May 1928 the airship “Dirigibile Italia” during its return trip to the base in NyAlesund, after overflying the North Pole, shipwrecked on the ice‐pack in a region at about 400 km northeast of Svalbard Islands. Survivors, by using a portable high frequency (HF) radio transmitter, tried unsuccessfully to send SOS messages and to establish a radio link with the ship “Città di Milano” of the Italian Navy, closely anchored at King's Bay. Only after 9 days of repeated radio‐distress transmissions, a Russian radio amateur close to the town of Arkhangelsk about 1,900 km away was able to receive the messages launched by the survivors and raise the alarm. This paper aims at giving a retrospective analysis of the ionospheric and geomagnetic conditions of that epoch in order to explain the HF radio communications problems encountered by the survivors. The International Reference Ionosphere model has been applied, and early geomagnetic measurements have been evaluated, to come up with theories explaining the events. We assert the HF transmission difficulties were associated with the “radio silent” or “dead zones” associated with F‐region propagation. These may have been exacerbated by solar and geomagnetically disturbed conditions of the days immediately following the airship wreck.

Denny M. Oliveira, Hisashi Hayakawa, Ankush Bhaskar et al.

Abstract Being footprints of major magnetic storms and hence major solar eruptions, mid- to low-latitude aurorae have been one of the pathways to understand solar–terrestrial environments. However, it has been reported that aurorae are also occasionally observed at low latitudes under low or even quiet magnetic conditions. Such phenomena are known as “sporadic aurorae”. We report on a historical event observed by a scientist of the Brazilian Empire in Rio de Janeiro on 15 February 1875. We analyze this event on the basis of its spectroscopic observations, along with its visual structure and coloration, to suggest this event was a possible case of sporadic aurorae. Given the absence of worldwide aurora observations on that day as a consequence of low magnetic activity recorded on the days preceding the observation, in addition to a detailed description, the event observed can most likely be classified as a sporadic aurora. We discuss the geographic and magnetic conditions of that event. Thus, we add a possible case of sporadic aurora in the South American sector.

Greig A. Paterson, Christopher J. Davies, Ron Shaar

Joan Lluis Pijoan, David Altadill, Joan Miquel Torta et al.

The geophysical observatory in the Antarctic Spanish Station, Juan Carlos I (ASJI), on Livingston Island, has been monitoring the magnetic field in the Antarctic region for more than fifteen years. In 2004, a vertical incidence ionospheric sounder completed the observatory, which brings a significant added value in a region with low density of geophysical data. Although the ASJI is only operative during the austral summer, the geomagnetic station records the data throughout the year. A High Frequency (HF) transmission system was installed in 2004 in order to have the geomagnetic data available during the whole year. As the power supply is very limited when the station is not operative, we had to design a low-power HF transceiver with a very simple antenna, due to environmental aspects. Moreover, the flow of information was unidirectional, so the modulation had to be extremely robust since there is no retransmission in case of error. This led us to study the main parameters of the ionospheric channel and to design new modulations specially adapted to very low signal to noise scenarios with high levels of interference. In this paper, a review of the results of our remote geophysical observatory and associated transmission system in Antarctica during the last decade is presented.

A. L. Morozova, P. Ribeiro, P. Ribeiro et al.

The Coimbra Magnetic Observatory (International Association of Geomagnetism and Aeronomy code COI) in Portugal has a long history of observation of the geomagnetic field, spanning almost 150 yr since the first geomagnetic measurements in 1866. These long instrumental geomagnetic records provide very important information about variability of geomagnetic elements and indices, their trends and cycles, and can be used to improve our knowledge on the sources that drive variations of the geomagnetic field: liquid core dynamics (internal) and solar forcing (external). <br><br> However, during the long life of the Coimbra Observatory, some inevitable changes in station location, instrument's park and electromagnetic environment have taken place. These changes affected the quality of the data collected at COI causing breaks and jumps in the series of geomagnetic field components and local K index. Clearly, these inhomogeneities, typically shift-like (step-like) or trend-like, have to be corrected or, at least, minimized in order for the data to be used in scientific studies or to be submitted to international databases. <br><br> In this study, the series of local K index and declination of the geomagnetic field are analysed: the former because it allows direct application of standard homogenization methods and the latter because it is the longest continuous series produced at COI. For the homogenization, visual and statistical tests (e.g. standard normal homogeneity test) have been applied directly to the local geomagnetic K index series (from 1951 to 2012). The homogenization of the monthly averages of declination (from 1867 to 2012) has been done using visual analysis and statistical tests applied to the time series of the first differences of declination values, as an approximation to the first time derivative. This allowed not only estimating the level of inhomogeneity of the studied series but also detecting the highly probable homogeneity break points. These points have been cross-checked with the metadata, and the COI series have been compared with reference series from the nearest geomagnetic stations and, in the case of declination series, from the recent geomagnetic field model COV-OBS to set up the required correction factors. As a result, the homogenized series measured in COI are considered to be essentially free of artificial shifts starting from the second half of the 20th century, and ready to be used by the scientific community.

M Doiphode, R Nimje, S Alex

The World Data Centre for Geomagnetism, Mumbai has functioned as a division of the Indian Institute of Geomagnetism, Navi Mumbai since its full fledged activities commenced in 1991 in coordination with the International Council of Scientific Unions (ICSU) Panel on World Data Centres. Responsibility for the compilation of final hourly absolute values from nine of the Indian magnetic observatories and deposition of this data to the World Data Centres is undertaken at the centre. We have utilized the full advantage of technology advancement in upgrading our data preservation and conservation policy at various levels. In recent years, the centre has prioritized its activities related to digital preservation to ensure digital archiving of magnetic data from the traditional media and also digital conservation of very old hand written/printed data volumes and magnetograms. In view of the scientific importance of data from the Colaba-Alibag Magnetic Observatory, old magnetograms and data volumes are being converted to digital images for long term preservation. In the digital preservation process, the creation of metadata has become an important component in storing information related to old and current scientific records for future use. The centre also hosts a database driven website to make datasets available online to the global scientific community.

Alexandre Fournier, Lars Nerger, Julien Aubert

We present the application of the ensemble Kalman filter to a three‐dimensional, convection‐driven model of the geodynamo. Our implementation rests on a suitably modified version of the parallel data assimilation framework of Nerger and Hiller (2013). We resort to closed‐loop experiments for validation purposes, using a dynamo model of intermediate resolution. Observations for these experiments consist of spectral coefficients describing the surface poloidal magnetic field, with arbitrary truncation. Our synthetic tests demonstrate the efficacy and adaptivity of the method, provided the ensemble comprises O(500) members, in which case the typical spin‐up time we find for our system is O(1000)years . In case of a poor resolution of the observations, we find that the knowledge of the full covariance matrix describing the uncertainty affecting the spectral coefficients (as opposed to its sole diagonal) results in a much better estimate of the internal structure of the dynamo.

Wu Lin

J. Tarduno

Yue Zhang, Chong Kang, H. Li et al.

R. Parker