ALBERT

Description: During solar minimum the near-Venusian magnetotail exhibits a hemispheric asymmetry in the cross tail field distribution [Zhang et al., 2010]. It implies that the magnetic field lines in the –E hemisphere are wrapped more tightly around Venus than in the + E hemisphere. Therefore, a strong field reversal region occurs in the magnetotail, which is prone to the magnetic reconnection [Zhang et al., 2012]. Since the Venus magnetotail is formed due to the solar wind interaction with the ionosphere and the ionosphere is modulated by the solar activity, it is interesting to study the solar cycle dependence of the induced magnetosphere. Here we statistically examine the Venus Express magnetotail data during solar maximum. We find that the magnetic field configuration asymmetry in Venus magnetotail is very much similar to that during solar minimum. The hemispheric asymmetry in the magnetotail persists through the whole solar cycle and magnetic reconnection in the near-Venusian magnetotail might occur during solar minimum as well as solar maximum conditions.

Description: Electromagnetic ion cyclotron (EMIC) waves have been proposed to cause efficient losses of highly relativistic (〉1 MeV) electrons via gyroresonant interactions. Simultaneous observations of EMIC waves and equatorial electron pitch angle distributions, which can be used to directly quantify the EMIC wave scattering effect, are still very limited, however. In the present study, we evaluate the effect of EMIC waves on pitch angle scattering of ultra-relativistic (〉1 MeV) electrons during the main phase of a geomagnetic storm, when intense EMIC wave activity was observed in situ (in the plasma plume region with high plasma density) on both Van Allen Probes. EMIC waves captured by THEMIS probes and on the ground across the Canadian Array for Real-time Investigations of Magnetic Activity (CARISMA) are also used to infer their MLT coverage. From the observed EMIC wave spectra and local plasma parameters, we compute wave diffusion rates and model the evolution of electron pitch angle distributions. By comparing model results with local observations of pitch angle distributions, we show direct, quantitative evidence of EMIC wave-driven relativistic electron losses in the Earth's outer radiation belt.

Description: Abstract In this study, we report 20 years of data from three ponderosa pine plantations in northern California. Our sites span a natural gradient of forest productivity where climate variability and edaphic conditions delineate marked differences in baseline productivity (approximately threefold). Experimental herbicide application and fertilization significantly reduced competition and improved tree growth by 1.4‐ to 2.2‐fold across sites. At the site of lowest productivity, where soils are poorly developed and water limiting, tree growth increased strongly in response to understory suppression. Small but significant improvements in tree growth were observed in response to understory suppression at the moderate‐productivity site. At the site of highest productivity, where climate is favorable and soils well developed, fertilization increased productivity to a greater extent than did understory suppression. In most cases, the effect of understory suppression and fertilization caused an unexpected growth release, exceeding the anticipated maximum productivity by 〉5 m of additional height and 60–100% more basal area. At the site of highest productivity, however, understory suppression caused a weak increase on late‐season growth compared to fertilization alone, suggesting a beneficial effect of understory vegetation on long‐term growth at that site. Tree ring cellulose carbon isotopes indicate a negative relationship between intrinsic water use efficiency (iWUE) and tree growth in control stands, which shifted to a positive relationship as both iWUE and tree growth increased in response to management. Cellulose oxygen isotope ratios (δ18O) were positively correlated with iWUE and negatively correlated with vapor pressure deficit across sites, but δ18O was not a strong predictor of tree growth.

Description: Abstract Electron scale magnetic cavities are electron vortex structures formed in turbulent plasma, while the evolution and electron dynamics of these structures have not been fully understood. Recently, high‐energy, angular, and temporal electron measurements from Magnetospheric Multiscale have enabled the application of an energetic particle sounding technique to these structures. This study analyzes an electron scale magnetic cavity observed by Magnetospheric Multiscale on 7 May 2015 in the plasma sheet. A comprehensive sounding technique is applied to obtain the geometry and propagation velocities of the boundaries. The result shows that the scale size of the structure is ∼90 km, and the leading and trailing boundaries are moving in the same direction but with different speeds (∼11.5 ± 2.2 and ∼18.1 ± 3.4 km/s, respectively). The speed difference suggests a shrinking of the structure that may play a significant role in magnetic energy dissipation and electron energization of electron scale magnetic cavities.

Description: This paper investigates a possible physical mechanism of the observed dayside high-latitude upper thermospheric wind using numerical simulations from the coupled magnetosphere-ionosphere-thermosphere (CMIT) model. Results show that the CMIT model is capable of reproducing the unexpected afternoon equatorward winds in the upper thermosphere observed by the HIWIND balloon. Models that lack adequate coupling produce poleward winds. The modeling study suggests that ion-drag driven by magnetospheric lobe-cell convection is another possible mechanism for turning the climatologically-expected dayside poleward winds to the observed equatorward direction. The simulation results are validated by HIWIND, EISCAT and DMSP. The results suggest a strong momentum coupling between high-latitude ionospheric plasma circulation and thermospheric neutral winds in the summer hemisphere during positive IMF B z periods, through the formation of magnetospheric lobe-cell convection driven by persistent positive IMF B y . The CMIT simulation adds important insight into the role of dayside coupling during intervals of otherwise quiet geomagnetic activity

Description: Based on concurrent observations of the ACE and Geotail satellites from 1998 to 2005, we statistically analyzed and compared the earthward bursty bulk flows (BBFs) with local positive Bz under different interplanetary magnetic field (IMF) conditions. Four different magnetospheric activity levels (MALs), including quiet times and substorm growth/expansion/recovery phases are considered. The properties of the BBFs, including their ion temperature (T), Vx component, X component of the energy flux density (Qx), and the solar wind dawn-dusk electric field Ey (observed at ~1 AU) are analyzed. Main observations include that: 1) BBF tends to have less penetration distance for northward IMF (NW-IMF) than for southward IMF (SW-IMF). Inward of 15 R E the BBFs for SW-IMF are dominant. Few BBFs for NW-IMF occur within 15 R E ; 2) the occurrence probability of the BBFs at each MAL depend highly on the orientations of the IMF. During quiet times, the BBFs for NW-IMF are dominant. Reversely, during the growth and expansion phases of a substorm, the BBFs for SW-IMF are dominant; 3) the strengths of the BBF have significant evolution with substorm development. For SW-IMF condition, the strengths of the BBFs are the lowest for quiet times. The strength of the BBFs tends to increase during the growth phase, and reaches to the strongest value during the expansion phase, then, decays during the recovery phase. For NW-IMF condition, the strengths of the BBFs evolve with the substorm development in a similar way as for SW-IMF condition; 4) For SW-IMF, the solar wind Ey evolves with the substorm development in a similar way to the strength of the BBFs. However, no clear evolution is found for NW-IMF; 5) The strengths of the BBF Qx and solar wind Ey are closely related. Both tend to be stronger for growth phase than for quite time, reach the strongest for expansion phase, then decay for recovery phase. It appears that to trigger a substorm, the strength of the BBFs should achieve energy thresholds with values different for NW-IMF and SW-IMF.

Description: We describe a coupled geospace model that includes causally regulated ion outflow from a physics-based ionosphere/polar wind model. The model two-way couples the multi-fluid Lyon-Fedder-Mobarry (MFLFM) magnetohydrodynamics (MHD) model to the ionosphere/polar wind model (IPWM). IPWM includes the H  +  and O  +  polar wind as well as a phenomenological treatment of energetic O  +  accelerated by wave-particle interactions (WPI). Alfvénic Poynting flux from the MHD simulation causally regulates the ion acceleration. The WPI model has been tuned and validated with comparisons to particle-in-cell (PIC) simulations and empirical relationships derived from Fast Auroral SnapshoT (FAST) satellite data. IPWM captures many aspects of the ion outflow that empirical relationships miss. First, the entire coupled model conserves mass between the ionospheric and magnetospheric portions, meaning the amount of outflow produced is limited by realistic photochemistry in the ionosphere. Second, under intense driving conditions, the outflow becomes flux limited by what the ionosphere is capable of providing. Furthermore, the outflows produced exhibit realistic temporal and spatial delays relative to the magnetospheric energy inputs. The coupled model provides a flexible way to explore the impacts of dynamic heavy ion outflow on the coupled geospace system. Some of the example simulations presented exhibit internally-driven sawtooth oscillations associated with the outflow, and the properties of these oscillations are analyzed further in a companion paper.

Description: The magnetotail current sheet is active and often flaps back and forth. Knowledge about the flapping motion of current sheet is essential to explore the related magnetotail dynamic processes, e.g. plasma instabilities. Due to the inability of single-point measurements to separate the spatial-temporal variation of magnetic field, the moving velocity of flapping current sheets cannot be revealed generally until the multi-point measurements are available, e.g. the Cluster mission. Therefore, currently the flapping behaviors are hard to be resolved only relying on single-point magnetic field analysis. In this study, with minimum variance analysis, we develop a technique based on single-point magnetic field measurement to qualitatively diagnose the flapping properties including the flapping type and the travelling direction of kink-like flapping. The comparison with Cluster multipoint analysis via several cases studies demonstrates that this technique is applicable, it should, however, be used with caution especially when the local sheet surface is either quasi-horizontal, or quasi-vertical. This technique will be useful for the planetary magnetotail exploration where no multipoint observations are available.

Description: General methods for improving the specification of electron precipitation in global simulations are described and implemented in the Lyon-Fedder-Mobarry (LFM) global simulation model, and the quality of its predictions for precipitation is assessed. LFM's existing diffuse and monoenergetic electron precipitation models are improved, and new models are developed for lower energy, broadband and direct-entry cusp precipitation. The LFM simulation results for combined diffuse plus monoenergetic electron precipitation exhibit a quadratic increase in the hemispheric precipitation power as the intensity of solar wind driving increases, in contrast with the prediction from the OVATION Prime (OP) 2010 empirical precipitation model which increases linearly with driving intensity. Broadband precipitation power increases approximately linearly with driving intensity in both models. Comparisons of LFM and OP predictions with estimates of precipitating power derived from inversions of Polar satellite UVI images during a double substorm event (28-29 March 1998) show that the LFM peak precipitating power is 〉 4× larger when using the improved precipitation model and most closely tracks the larger of three different inversion estimates. The OP prediction most closely tracks the double peaks in the intermediate inversion estimate, but it overestimates the precipitating power between the two substorms by a factor 〉2 relative to all other estimates. LFMs polar pattern of precipitating energy flux tracks that of OP for broadband precipitation, exhibits good correlation with duskside region 1 currents for monoenergetic energy flux that OP misses, and fails to produce sufficient diffuse precipitation power in the prenoon quadrant that is present in OP. The prenoon deficiency is most likely due to the absence of drift kinetic physics in the LFM simulation.

Description: In this paper, we statistically analyzed and compared the earthward flow (EF) and the tailward flow (TF) in the plasma sheet. It is found that the properties of the EF/TF in the central plasma sheet (CPS) of β〉1 and the outer plasma sheet (OPS) of 0.1〈β〈1 are distinctly different. The main conclusions include that: 1) the EF occur in both the CPS and the OPS while the TF mainly occur in the OPS; 2) both flows are dominantly convective in the CPS, and parallel in the OPS; 3) in the CPS, the EF and the TF have similar characteristics, including their bulk velocities and ion densities and Ey components. Both flows tend to have isotropic temperatures; 4) in the OPS, the EF tend to have higher ion velocity, density and Ey than the TF. The EF tend to have anisotropic temperatures, while the TF tend to have more isotropic temperatures. As a whole, combined characteristics of the EF and the TF are consistent with (1) reflection at the “magnetic mirror point” near the Earth for parallel flows in the OPS, and (2) bouncing off/back from the dipolar field closer to the Earth for convective flows in the CPS.