Geomagnetic indices are routinely used to characterize space weather event intensity. The DST index is well resolved, but only available over 5 solar cycles. The aa index extends over 14 cycles but is highly discretized with poorly resolved extremes. We parameterize extreme aa activity by the annual averaged top few % of observed values, show these are exponentially distributed and they track annual DST index minima. This gives a 14 cycle average ~4% chance of at least one great (DST<-500nT) storm and ~28% chance of at least one severe (DST<-250nT) storm per year. At least one DST=-809[-663,-955]nT event in a given year would be a 1:151 year event. Carrington event estimate DST~-850nT is within the same distribution as other extreme activity seen in aa since 1868 so that its likelihood can be deduced from that of more moderate events. Events with DST<-1000nT are in a distinct class, requiring special conditions.
We perform a geomagnetic event simulation using a newly developed magnetohydrodynamic with adaptively embedded particle-in-cell (MHD-AEPIC) model. We have developed effective criteria to identify reconnection sites in the magnetotail and cover them with the PIC model. The MHD-AEPIC simulation results are compared with Hall MHD and ideal MHD simulations to study the impacts of kinetic reconnection at multiple physical scales. At the global scale, the three models produce very similar SYM-H and SuperMag Electrojet (SME) indexes, which indicates that the global magnetic field configurations from the three models are very close to each other. We also compare the ionospheric solver results and all three models generate similar polar cap potentials and field aligned currents. At the mesoscale we compare the simulations with in situ Geotail observations in the tail. All three models produce reasonable agreement with the Geotail observations. At the kinetic scales, the MHD-AEPIC simulation can produce a crescent shape distribution of the electron velocity space at the electron diffusion region which agrees very well with MMS observations near a tail reconnection site. These electron scale kinetic features are not available in either the Hall MHD or ideal MHD models. Overall, the MHD-AEPIC model compares well with observations at all scales, it works robustly, and the computational cost is acceptable due to the adaptive adjustment of the PIC domain. It remains to be determined whether kinetic physics can play a more significant role in other types of events, including but not limited to substorms.
The influence of shield wires on Geomagnetically Induced Currents (GICs) in power systems is considered. For the most simple power network, with one single transmission line and one shield wire connecting two substations, we derive the expressions for the voltage source and the resistance of the Thévenin equivalent circuits that, in parallel with the substations grounding resistances, produce the same effects on GICs as the full circuit. Our model extends results from previous studies that considered the effect of shield wires resistances by also including the induced geoelectric field.
Using fully kinetic Particle-In-Cell (PIC) modelling we investigate how magnetic reconnection responds to a varying guide field in one of the inflow regions. We find that the reconnection rate varies significantly when the orientation of the magnetic field changes between being strictly antiparallel and having a guide field. These variations are fairly consistent with the scaling relation for asymmetric reconnection developed by Cassak and Shay (2007). However, the rate is also found to be non-linearly modulated by changes in the ion inflow velocity. The spatio-temporal change in the inflow velocity arises as the magnetic forces reconfigure to regions of different magnetic field strengths. The variations in the inflow magnetic field configuration allow for different gradients in the magnetic field, leading to asymmetries in the magnetic tension force. By momentum conservation, this facilitates asymmetries in the inflow velocity, which in turn affects the flux transport into the reconnection site. The outflow is found to be less laminar when the inflow varies, and various signatures of the inflow variations are identified in the outflow.
Measurements in the heliosphere and high-resolution fluid simulations give clear indications for the anisotropy of plasma turbulence in the presence of magnetic fields. How this anisotropy affects transport processes like diffusion and dispersion remains an open question. The first efforts to characterize Lagrangian single-particle diffusion and two-particle dispersion in incompressible magnetohydrodynamic (MHD) turbulence were performed a decade ago. We revisit those pioneering results through updated simulations performed at higher Reynolds number. We present new investigations that use the dispersion of many Lagrangian tracer particles to examine the extremes of dispersion and the anisotropy in direct numerical simulations. We then point out directions in which Lagrangian statistics need to be developed to address the fundamental problem of anisotropic MHD turbulence and transport in solar and stellar winds.
Jupiter possesses the strongest magnetic field of all planets in the solar system. Unique information about the dynamo process acting at Jupiter can be inferred by modelling and interpreting its field. Using the fluxgate magnetometer measurements acquired during the four years of the Juno mission, we derive an internal and secular magnetic field model in spherical harmonics. The static part is derived to degree 16 with a secular time variation to degree 8. We use properties of the power spectrum of the static field to infer the upper boundary of the dynamo convective region at 0.830±0.022 Jupiter radius. This confirms the role of the transition layer in the field generation inside Jupiter. The secular variation timescales indicate that advective effects dominate the dynamo and the secular variation structures estimated at the dynamo radius suggest that the complex flow involves non-zonal features.
On June 7th, 2021 the Juno spacecraft visited Ganymede and provided the first in situ observations since Galileo’s last flyby in 2000. The measurements obtained along a one-dimensional trajectory can be brought into global context with the help of three-dimensional magnetospheric models. Here we apply the magnetohydrodynamic model of Duling et al. (2014) to conditions during the Juno flyby. In addition to the global distribution of plasma variables we provide mapping of Juno’s position along magnetic field lines, Juno’s distance from closed field lines and detailed information about the magnetic field’s topology. We find that Juno did not enter the closed field line region and that the boundary between open and closed field lines on the surface matches the poleward edges of the observed auroral ovals. To estimate the sensitivity of the model results, we carry out a parameter study with different upstream plasma conditions and other model parameters.
Recent measurements collected by the Mars Curiosity Rover at the Gale Crater revealed an unexpectedly large seasonal cycle of molecular oxygen (O2). We use a 1-D photochemical model, including inorganic and organic chemistry, and its adjoint model to quantify the sensitivity of changes in O2 to changes in inorganic and organic compounds. We show that O2 changes are most sensitive to changes in organic compounds from the oxidation of methane. We find that an accelerated loss of atmospheric methane, achieved either by increasing the atmospheric loss or by imposing an additional surface loss, does not reconcile model and observed values of O2 but it helps to explain the O2 seasonal variation. The resulting changes in atmospheric composition are below the detection limits of orbiting instruments.
The Angstrom-Prescott (A-P) model is widely suggested for estimating solar radiation (Rs) in areas without measured or deficiency of data. The coefficients of this model must be locally calibrated, to calculate evapotranspiration (ET) correctly. The aim of this research was calibration and validation of the coefficients of the A-P model at six meteorological stations across arid and semi-arid regions of Iran. This model was improved by adding the air temperature and relative humidity terms. Besides, the coefficients (’a’ and ‘b’) of the A-P model and improved models was calibrated using some optimization algorithms including Harmony Search (HS) and Shuffled Complex Evolution (SCE). Performance indices, i.e., Root Mean Square Error (RMSE), Mean Bias Error (MBE), and coefficient of determination (R2) were used to analyze the models ability in estimating Rs. The results indicated that the performance of the A-P model had more precision and less error than improved models in all the stations. In addition, the best results were obtained for the A-P model with the SCE algorithm. The RMSE varies between 0.82 and 2.67 MJ m-2 day-1 for the A-P model with the SCE algorithm in the calibration phase. In the SCE algorithm, the values of RMSE had decreased about 4% and 7% for Mashhad and Kerman stations in the calibration phase compared to the HS algorithm, respectively. In other words, the highest decrease of RMSE is related to Kerman station.
Reconstructions for global temperature development show an upward oscillation for the period of the 1880s through 1980s. This oscillation is being associated with natural variability and the temperature rise between the 1910s and 1940s with increased solar activity. The temperature impact of the 11-year solar cycle [Schwabe cycle] and the physical mechanism involved are insufficiently understood. Here, for the 22-year magnetic solar cycle [Hale cycle] a seawater surface temperature [SST] impact is described of 0,215 °C (0,238 ± 0,05 °C per W/m2); the derived impact for the 11-year cycle is 0,122 °C (0,135 ± 0,03 °C per W/m2). Also, a parallel development is described for seawater surface temperature [HadSST3 dataset] and the minima of total solar irradiance [LISIRD dataset] after a correction based on the 22-year solar cycle polarity change. With the correction, the combination of the positive and negative minima shows for the period 1890-1985 a high SST solar sensitivity: 1,143 ± 0,23 °C per W/m2 (with 90,5% declared variance). This implies that the Sun has caused a warming of 1,07 °C between Maunder minimum (late 17th century) and the most recent solar minimum year 2017 - which is well over half of the intermediate temperature rise of approximately 1,5 °C. The results demonstrate that the 22-year cycle forms a crucial factor required for better understanding the Sun-temperature relation. Ignoring the 22-year cycle leads to significant underestimation of the Sun’s influence in climate change combined with an overestimation of the impact of anthropogenic factors and greenhouse gases such as CO2.
Plasma beta is an important parameter characterizing dynamics of various systems such as the solar wind, planetary magnetospheres, and accretion disks. Dependence of some plasma properties such as spectral break, relative proton-electron heating, and intermittency has been studied using observations as well as simulations [1,2,3]. In this study, we use particle-in-cell (PIC) simulations of turbulence to study various, yet unexplored, aspects of this beta variation. We analyze kinetic range electric fields, the variation of scale-to-scale energy transfer, and higher order statistics with respect to plasma beta. Systematic trends in the behavior of various quantities are discussed, and their implications for kinetic plasma dissipation are examined. Finally, we extend this approach to laminar reconnection, which shows turbulence like properties of magnetic spectrum and energy cascade [4,5]. References: Chen, et. al., Geophy. Res. Lett. 41.22 (2014); (https://doi.org/10.1002/2014GL062009) Franci, et. al., ApJ 833 91 (2016); (https://doi.org/10.3847/1538-4357/833/1/91) Parashar, et. al., ApJ Lett. 864 L21 (2018);(https://doi.org/10.3847/2041-8213/aadb8b ) Adhikari, et. al., Phys. of Plasmas 27,042305 (2020); (https://doi.org/10.1063/1.5128376) Adhikari, et. al.(2021); (arXiv:2104.12013)
The explosive eruption of the Hunga-Tonga volcano in the southwest Pacific at 0415UT on 15 January 2022 triggered gigantic atmospheric disturbances with surface air pressure wave propagating around the globe in Lamb mode. In space, concentric traveling ionosphere disturbances (CTIDs) are also observed as a manifestation of air pressure acoustic waves in New Zealand ~0500UT and Australia ~0630UT. As soon as the wave reached central Australia ~0800UT, CTIDs appeared simultaneously in the northern hemispheres through magnetic field line conjugate effect, which is much earlier than the arrival of the air pressure wave to Japan after 1100UT. Combining observations over Australia and Japan between 0800-1000UT, CTIDs with characteristics of phase velocities of 320-390 m/s are observed, matching with the dispersion relation of Lamb mode. The arrival of atmospheric Lamb wave to Japan later created in situ CTIDs showing the same Lamb mode characteristics as the earlier arriving CTIDs.
Shock waves are common in the heliosphere and beyond. The collisionless nature of most astrophysical plasmas allows for the energy processed by shocks to be partitioned amongst particle sub-populations and electromagnetic fields via physical mechanisms that are not well understood. The electrostatic potential across such shocks is frame dependent. In a frame where the incident bulk velocity is parallel to the magnetic field, the deHoffmann-Teller frame, the potential is linked directly to the ambipolar electric field established by the electron pressure gradient. Thus measuring and understanding this potential solves the electron partition problem, and gives insight into other competing shock processes. Integrating measured electric fields is space is problematic since the measurements can have offsets that change with plasma conditions. The offsets, once integrated, can be as large or larger than the shock potential. Here we exploit the high-quality field and plasma measurements from NASA's Magnetospheric Multiscale mission to attempt this calculation. We investigate recent adaptations of the deHoffmann-Teller frame transformation to include time variability, and conclude that in practise these face difficulties inherent in the 3D time-dependent nature of real shocks by comparison to 1D simulations. Potential estimates based on electron fluid and kinetic analyses provide the most robust measures of the deHoffmann-Teller potential, but with some care direct integration of the electric fields can be made to agree. These results suggest that it will be difficult to independently assess the role of other processes, such as scattering by shock turbulence, in accounting for the electron heating.
Solar wind magnetic holes are localized depressions of the magnetic field strength, on time scales of seconds to minutes. We use Cluster multipoint measurements to identify 26 magnetic holes which are observed just upstream of the bow shock and, a short time later, downstream in the magnetosheath, thus showing that they can penetrate the bow shock and enter the magnetosheath. For two magnetic holes we show that the relation between upstream and downstream properties of the magnetic holes are well described by the MHD Rankine-Hugoniot jump conditions. We also present a small statistic investigation of the correlation between upstream and downstream observations of some properties of the magnetic holes. The temporal scale size, and magnetic field rotation across the magnetic holes are very similar for the upstream and downstream observations, while the depth of the magnetic holes varies more. The results are consistent with the interpretation that magnetic holes in Earth's and Mercury's magnetosheath are of solar wind origin, as has earlierly been suggested. Since the solar wind magnetic holes can enter the magnetosheath, they may also interact with the magnetopause, representing a new type of localised solar wind-magnetosphere interaction.
It is now well-established through multiple event and statistical studies that the solar wind at 1 AU contains contains periodic, mesoscale (L~100-1000Mm) structures in the proton density. Composition variations observed within these structures and remote sensing observations of similar structures in the young solar wind indicate that at least some of these periodic structures originate in the solar atmosphere as a part of solar wind formation. Viall et al.  analyzed 11 years of data from the Wind spacecraft near L1 and demonstrated a recurrence to the observed length scales of periodic structures in the solar wind proton density. In the time since that study, Wind has collected 14 additional years of solar wind data, new moment analysis of the Wind SWE data is available, and new methods for spectral background approximation have been developed. In this study, we analyze 25 years of Wind data collected near L1 and produce occurrence distributions of statistically significant periodic length scales in proton density. The results significantly expand upon the Viall et al.  study, and further shows a possible relation of the length scales to solar “termination” events.
Although gender parity has been reached at the graduate level in the geosciences, women remain a minority in top-level positions. First authorship of peer-reviewed scholarship is a measure of academic success and is often used to project potential in the hiring process. Given the importance of first author publications for hiring and advancement, we sought to quantify whether women are underrepresented as first authors relative to their representation in the field. We compiled first author names across 13 leading geoscience journals from January 2013 to April 2019 (n = 35,183). Using a database of 216,286 names from 79 countries, across 89 languages, we classified the likely gender associated with each author’s given (first) name. We also estimated the gender distribution of authors who publish using only initials, which may itself be a strategy employed by some women to preempt perceived (and actual) gender bias in the publication process. Female-author names represent 13-30% of all first authors in our database, and are significantly underrepresented relative to the proportion of women in early career positions (30-50%). The proportion of female-name first authors varies significantly by subfield, reflecting variation in representation of women across subdisciplines. In geoscience, the quantification of this first authorship gender gap supports the hypothesis that the publication process; namely, achievement or allocation of first authorship is biased by social factors, which may modulate career success of women in the sciences.