AbstractThe Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite has been making observations of the mesosphere and lower thermosphere (MLT) region for two decades. The TIMED Doppler Interferometer (TIDI) measures the neutral winds using four orthogonal telescopes. In this study, the line of sight (LOS) winds from individual telescopes are compared to the measurements from the Ionospheric Connection Explorer's (ICON's) Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) instrument from 90 to 100 km altitude during 2020. With the MIGHTI vector winds projected onto the LOS direction of each TIDI telescope, coincidences of the two data sets are found. The four telescopes perform differently and the performance depends on the satellite configuration and local solar zenith angle. Measurements from the coldside telescopes, Telescope 1 (Tel1) and Telescope 2 (Tel2), are better correlated with the MIGHTI winds in general with Tel2 having higher correlation coefficients across all conditions. The performance of Tel1 is comparable to that of Tel2 during backward flight while showing systematic errors larger than the average wind speeds during forward flight. The warmside LOS winds from Telescope 3 (Tel3) and Telescope 4 (Tel4) vary widely in magnitude, especially on the nightside. Compared with MIGHTI winds, the Tel4 measurements have the weakest correlation, while the Tel3 performance is comparable to that of the coldside telescopes during the ascending phase but deteriorates during the descending phase. Based on the TIDI/MIGHTI comparisons, figures of merit are generated to quantify the quality of measurements from individual telescopes in different configurations.
AbstractThe Cluster spacecraft, launched in the year 2000, were pioneers using multi‐point measurements to explore the dynamic space plasma environment of Earth. This capability offered a new, powerful approach to study in situ fundamental space plasma processes of the solar‐terrestrial interaction. Although not a major science objective of the mission, the aurora and associated processes proved to be a very fruitful target for exploration by the Cluster fleet. Here, results are presented from a selection of Cluster auroral studies using the multi‐point measurements capability to further our understanding on various open issues on the aurora. These include the nature and properties of the auroral acceleration region, in particular the altitude distribution of the quasi‐static parallel electric field and potential, the auroral density cavity and its relation to the acceleration region, the role of and relative contribution by quasi‐static and Alfvénic acceleration processes in producing aurora. A related topic addressed by Cluster is acceleration processes and dynamics of the downward current region and its relation to black aurora. To get a broader perspective on these phenomena, event and statistical studies were used, supported by, in a few cases, numerical simulation studies.
AbstractTotal electron count (TEC) enhancements due to space weather are a threat to communications and global positioning systems. It is known that TEC enhancements occur during magnetic storms and can cover large areas for many hours, but it is also not uncommon for TEC enhancements on the order of a few TEC units to occur during substorms. Although much is known about storm‐associated TECs, the temporal and spatial characteristics of substorm‐associated TECs are not well established. By combining two‐dimensional maps of TECs over North America and Greenland and with maps of ionospheric currents derived with the spherical elementary current method [Weygand et al., 2011], we investigate the temporal and spatial changes of TEC enhancement events for both a single substorm and for multiple substorms combined using a two‐dimensional superposed‐epoch analysis. Both the single event analysis and the statistical analysis show an increase of TECs during the expansion phase. Substorm values of the TEC enhancements peak within 10 min after auroral onset and recover to nominal levels after about 40 min. TEC enhancements occur mainly within the nightside region 1 current system and cover millions of square kilometers. Furthermore, these enhancements appear to be associated with enhanced precipitating electron fluxes. These results address one of goals of the Space Weather Action Plan, which are to establish benchmarks for space weather events and improve modeling and prediction of their impacts on infrastructure.
AbstractThe ring current of Saturn is obtained by taking the curl of the observed magnetic field. An analytical model of this ring current can then be obtained by fitting it to a lognormal in ρ and a Gaussian in z. Local time dependence is included as a harmonic expansion in ϕ for coefficients of the lognormal; radial dependence is included as a quadratic expansion in ρ of the width of the Gaussian. In this model, the ring current peaks at ~0.25 MA/RS2 at ~10 RS on the nightside near ~1 hr. The ring current strength minimizes at ~0.17 MA/RS2 on the dayside near ~16 hr. The cross‐sectional shape of the ring current is a teardrop extending from ~5 to ~20 RS or more.
AbstractThe magnetometer data for the Cassini mission (2004–2016) are used to derive a model of Saturn's meridional magnetic field lines. Using a bin map of the Bρ and Bz field components, the field lines can be traced from the equator to high latitudes in the inner magnetosphere. The traces reveal a magnetic field greatly compressed on the dayside and highly elongated on the nightside, which are presumably the effects of solar wind compression and viscous flow around the magnetosphere. A model of the traced field lines can accommodate this day‐night asymmetry and offer a way to connect various places in the magnetosphere. This model can be adjusted for magnetodisk warping by using solar latitude. The field line traces can be mapped down to the ionosphere using an offset dipole. The dayside magnetopause (L = 21) is mapped to colatitudes of ~13° in the north and ~16° in the south, while for L > 25 on the nightside, the mapped field lines approach asymptotic limits of ~16° in the north and ~18° in the south.
Lysosomes labeled by uptake of extracellular horseradish peroxidase display remarkable changes in shape and cellular distribution when cytoplasmic pH is experimentally altered. Normally, lysosomes in macrophages and fibroblasts cluster around the cell center. However, when the cytoplasmic pH is lowered to approximately pH 6.5 by applying acetate or by various other means, lysosomes promptly move outward and accumulate in tight clusters at the very edge of the cell, particularly in regions that are actively ruffling before acidification but become quiescent. This movement follows the distribution of microtubules in these cells, and does not occur if microtubules are depolymerized with nocodazole before acidification. Subsequent removal of acetate or the other stimuli to acidification results in prompt resumption of ruffling activity and return of lysosomes into a tight cluster at the cell center. This is correlated with a rebound alkalinization of the cytoplasm. Correspondingly, direct application of weak bases also causes hyperruffling and unusually complete withdrawal of lysosomes to the cell center. Thus, lysosomes appear to be acted upon by microtubule- based motors of both the anterograde (kinesin) type as well as the retrograde (dynein) type, or else they possess bidirectional motors that are reversed by changes in cytoplasmic pH. During the outward movements induced by acidification, lysosomes also appear to be smaller and more predominantly vesicular than normal, while during inward movements they appear to be more confluent and elongated than normal, often becoming even more tubular than in phorbol-treated macrophages (Phaire-Washington, L., S. C. Silverstein, and E. Wang. 1980. J. Cell Biol. 86:641-655). These size and shape changes suggest that cytoplasmic pH also affects the fusion/fission properties of lysosomes. Combined with pH effects on their movement, the net result during recovery from acidification is a stretching of lysosomes into tubular forms along microtubules.