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Ultrasoft magnetorheological elastomers (MREs) offer convenient real-time magnetic field control of mechanical properties that provides a means to mimic mechanical cues and regulators of cells in vitro. Here, we systematically investigate the effect of polymer stiffness on magnetization reversal of MREs using a combination of magnetometry measurements and computational modeling. Poly-dimethylsiloxane-based MREs with Young’s moduli that range over two orders of magnitude were synthesized using commercial polymers Sylgard™ 527, Sylgard 184, and carbonyl iron powder. The magnetic hysteresis loops of the softer MREs exhibit a characteristic pinched loop shape with almost zero remanence and loop widening at intermediate fields that monotonically decreases with increasing polymer stiffness. A simple two-dipole model that incorporates magneto-mechanical coupling not only confirms that micrometer-scale particle motion along the applied magnetic field direction plays a defining role in the magnetic hysteresis of ultrasoft MREs but also reproduces the observed loop shapes and widening trends for MREs with varying polymer stiffnesses. 

Based on the observations from the balloon‐borne instrument High‐altitude Interferometer WIND experiment (HIWIND) and the simulations from the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM), the Grid Agnostic MHD Environment for Research Applications (GAMERA)‐TIEGCM (GT), and the GAMERA‐TIEGCM‐RCM (GTR), we investigate the variations of summer high‐latitude thermospheric winds and their physical mechanisms from 25 to 30 June, 2018. HIWIND observations show that the meridional winds were the largest at midnight and exhibited strong day‐to‐day variations during the 6‐day period, which were generally reproduced by those three models. The day‐to‐day variations of winds were mainly associated with the interplanetary magnetic field (IMF)perturbations, while the magnetic latitude variations also contributed to the large day‐to‐day variations of the winds seen in the observations. Meanwhile, the zonal winds were mostly westward during the daytime, and the wind speed became large, especially in the afternoon, which is related to the westward ion drift velocity. The observed meridional winds tend to turn equatorward during the daytime on some days, while the simulated winds blow mostly poleward except for simulations by the GTR model on 26 June. The GTR model revealed that the equatorward meridional winds on 26 June were associated with strong and negative IMFconditions, which tilts the convection pattern to the prenoon sector. The simulations also revealed that the ring current could contribute to affecting the neutral wind variations, especially under geomagnetically active conditions.

 

The storm‐time ionospheric‐thermospheric (IT) state is of great interest, since the IT dynamics change dramatically as energy is input and dissipated in the upper atmosphere. Lagrangian coherent structures (LCSs), which are objective ridges in time‐evolving flows that describe the tendency of neighboring fluid elements to separate, provides a unique opportunity to infer the dynamics in the IT system. In this work, we model IT flows with the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model and identify the LCSs. We compare the LCSs in the neutral winds and plasma drifts during quiet times versus during active times. We find that LCSs are largely aligned in the modeled IT flows, with a dawn‐dusk asymmetry in their latitudinal position. During a geomagnetic storm, the thermospheric LCSs (T‐LCSs) and ionospheric LCSs (I‐LCSs) shift equatorward, align more closely with each other, and maintain a dawn‐dusk asymmetry. The collocation of T‐LCSs and I‐LCSs and their analogous response to the geomagnetic storm provide evidence of energy input into the thermosphere and ionosphere simultaneously, and the ion drag is the dominant effect causing LCS alignment during a geomagnetic storm.

 

This paper presents a detailed model‐data comparative study of the 17 March 2015 geomagnetic storm using the high‐resolution version of the thermosphere‐ionosphere‐electrodynamic general circulation model and the total electron content observations from a dense global navigation satellite system network. Driven by time‐dependent high‐latitude ionospheric convection and auroral precipitation inputs, together with an empirically defined subauroral plasma stream (SAPS) field, our simulation reproduce many observed storm‐related ionospheric phenomena, including large‐scale traveling ionospheric disturbances over Europe, the effects of prompt penetration electric field over South and Central America, and the formation of a storm‐enhanced density (SED) plume across the continental United States. Our simulation results reaffirm a number of important characteristics concerning the SED plume: (1) enhanced background ionospheric density is a necessary but not sufficient condition, and enhanced ion drift is required to form the SED plume; (2) the SAPS flow channel does not directly transport the plasma from midnight to postnoon via dusk to form the SED plume, instead, the SED plume is formed at the equatorward and westward edge of the SAPS channel; and (3) the SED plume appears to subcorotate with respect to the Earth.

 

In this work, the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model is used to investigate the responses of ionospheric electrodynamic processes to the solar flares at the flare peaks and the underlying physical mechanisms on September 6 and 10, 2017. Simulations show that solar flares increased global daytime currents and reduced the eastward electric fields during the daytime from the equator to middle latitudes. Furthermore, westward equatorial electric fields and equatorial counter electrojets occurred in the early morning. At the flare peak, these electrodynamic responses are predominantly related to the enhanced E‐region conductivity by flares, as the responses of neutral winds and F‐region conductivity to flares are negligible. Specifically, the Cowling conductance enhancement is not the major process causing the reduction of zonal electric fields. This electric field reduction is primarily associated with the decrease of the ratio between the field line‐integrated wind‐driven currents and the conductance. The flare‐induced conductivity enhancement is larger but the background wind speed is smaller in the E‐region than in the F‐region, as a result, the increase of total integrated wind‐driven currents is less than the conductance enhancement.

 

The high‐resolution thermosphere‐ionosphere‐electrodynamics general circulation model has been used to investigate the response ofF₂region electron density (Ne) at Millstone Hill (42.61°N, 71.48°W, maximum obscuration: 63%) to the Great American Solar Eclipse on 21 August 2017. Diagnostic analysis of model results shows that eclipse‐induced disturbance winds causeF₂region Ne changes directly by transporting plasma along field lines, indirectly by producing enhanced O/N₂ratio that contribute to the recovery of the ionosphere at and below theF₂peak after the maximum obscuration. Ambipolar diffusion reacts to plasma pressure gradient changes and modifies Ne profiles. Wind transport and ambipolar diffusion take effect from the early phase of the eclipse and show strong temporal and altitude variations. The recovery ofF₂region electron density above theF₂peak is dominated by the wind transport and ambipolar diffusion; both move the plasma to higher altitudes from below theF₂peak when more ions are produced in the lowerF₂region after the eclipse. As the moon shadow enters, maximizes, and leaves a particular observation site, the disturbance winds at the site change direction and their effects on theF₂region electron densities also vary, from pushing plasma downward during the eclipse to transporting it upward into the topside ionosphere after the eclipse. Chemical processes involving dimming solar radiation and changing composition, wind transport, and ambipolar diffusion together cause the time delay and asymmetric characteristic (fast decrease of Ne and slow recovery of the eclipse effects) of the topside ionospheric response seen in Millstone Hill incoherent scatter radar observations.

 

A total solar eclipse occurred in the Southern Hemisphere on 2 July 2019 from approximately 17 to 22 UT. Its effect in the thermosphere over South America was imaged from geostationary orbit by NASA's Global‐scale Observation of Limb and Disk (GOLD) instrument. GOLD observed a large brightness reduction (>80% around totality) in OI 135.6 nm and N₂LBH band emissions compared to baseline measurements made 2 days prior. In addition, a significant enhancement (with respect to the baseline) in the ΣO/N₂column density ratio (~80%) was observed within the eclipse's totality. This enhancement suggests that the eclipse induced compositional changes in the thermosphere. After the eclipse passed, a slight enhancement in ΣO/N₂column density ratio (~7%) was also seen around the totality path when compared to measurements before the eclipse. These observations are the first synoptic imaging measurements of an eclipse's thermospheric effects with the potential to drastically improve and test our understanding of how the thermosphere responds to rapid, localized changes in solar short wavelength radiation.

 

It is well‐known that solar eclipses can significantly impact the ionosphere and thermosphere, but how an eclipse influences the magnetosphere‐ionosphere system is still unknown. Using a coupled magnetosphere‐ionosphere‐thermosphere model, we examined the impact on geospace of the northern polar‐region eclipse that occurred on June 10, 2021. The simulations reveal that the eclipse‐induced reduction in polar ionospheric conductivity causes large changes in field‐aligned current, cross‐polar cap potential and auroral activity. While such effects are expected in the northern hemisphere where solar obscuration occurred, they also occurred in the southern hemisphere through electrodynamic coupling. Eclipse‐induced changes in monoenergetic auroral precipitation differ significantly between the northern hemisphere and southern hemisphere while diffuse auroral precipitation is interhemispherically symmetric. This study demonstrates that the geospace response to a polar‐region solar eclipse is not limited just to the eclipse region but has global implications.