Gunter Stober

Real-time particle tracking at 10,000 fps using optical fiber illumination.

We introduce optical fiber illumination for real-time tracking of optically trapped micrometer-sized particles with microsecond time resolution. Our light source is a high-radiance mercury arc lamp and a 600 μm optical fiber for short-distance illumination of the sample cell. Particle tracking is carried out with a software implemented cross-correlation algorithm following image acquisition from a CMOS camera. Our image data reveals that fiber illumination results in a signal-to-noise ratio usually one order of magnitude higher compared to standard Köhler illumination. We demonstrate position determination of a single optically trapped colloid with up to 10,000 frames per second over hours. We calibrate our optical tweezers and compare the results with quadrant photo diode measurements. Finally, we determine the positional accuracy of our setup to 2 nm by calculating the Allan variance. Our results show that neither illumination nor software algorithms limit the speed of real-time particle tracking with CMOS technology.

Upper mesospheric lunar tides over middle and high latitudes during sudden stratospheric warming events

In recent years there have been a series of reported ground- and satellite-based observations of lunar tide signatures in the equatorial and low latitude ionosphere/thermosphere around sudden stratospheric warming (SSW) events. This lower atmosphere/ionosphere coupling has been suggested to be via the E region dynamo. In this work we present the results of analyzing 6 years of hourly upper mesospheric winds from specular meteor radars over a midlatitude (54°N) station and a high latitude (69°N) station. Instead of correlating our results with typical definitions of SSWs, we use the definition of polar vortex weaking (PVW) used by Zhang and Forbes (2014). This definition provides a better representation of the strength in middle atmospheric dynamics that should be responsible for the waves propagating to the E region. We have performed a wave decomposition on hourly wind data in 21 day segments, shifted by 1 day. In addition to the radar wind data, the analysis has been applied to simulations from Whole Atmosphere Community Climate Model Extended version and the thermosphere-ionosphere-mesosphere electrodynamics general circulation model. Our results indicate that the semidiurnal lunar tide (M2) enhances in northern hemispheric winter months, over both middle and high latitudes. The time and magnitude of M2 are highly correlated with the time and associated zonal wind of PVW. At middle/high latitudes, M2 in the upper mesosphere occurs after/before the PVW. At both latitudes, the maximum amplitude of M2 is directly proportional to the strength of PVW westward wind. We have found that M2 amplitudes could be comparable to semidiurnal solar tide amplitudes, particularly around PVW and equinoxes. Besides these general results, we have also found peculiarities in some events, particularly at high latitudes. These peculiarities point to the need of considering the longitudinal features of the polar stratosphere and the upper mesosphere and lower thermosphere regions. For example, during SSW 2009, we found that M2 enhances many days before PVW which is not in agreement with most of our results.

Quantifying gravity wave momentum fluxes with Mesosphere Temperature Mappers and correlative instrumentation

An Advanced Mesosphere Temperature Mapper and other instruments at the Arctic Lidar Observatory for Middle Atmosphere Research in Norway (69.3°N) and at Logan and Bear Lake Observatory in Utah (42°N) are used to demonstrate a new method for quantifying gravity wave (GW) pseudo-momentum fluxes accompanying spatially and temporally localized GW packets. The method improves on previous airglow techniques by employing direct characterization of the GW temperature perturbations averaged over the OH airglow layer and correlative wind and temperature measurements to define the intrinsic GW properties with high confidence. These methods are applied to two events, each of which involves superpositions of GWs having various scales and character. In each case, small-scale GWs were found to achieve transient, but very large, momentum fluxes with magnitudes varying from ~60 to 940 m2 s−2, which are ~1–2 decades larger than mean values. Quantification of the spatial and temporal variations of GW amplitudes and pseudo-momentum fluxes may also enable assessments of the total pseudo-momentum accompanying individual GW packets and of the potential for secondary GW generation that arises from GW localization. We expect that the use of this method will yield key insights into the statistical forcing of the mesosphere and lower thermosphere by GWs, the importance of infrequent large-amplitude events, and their effects on GW spectral evolution with altitude.

Modeling of colloidal transport in capillaries

We dynamically model the full ionic current signature of micron-sized colloids passing through microcapillaries in silico for the first time. Our novel computer simulation allows free adjustment of all relevant experimental parameters such as the geometry of the used orifice, noise sources, external applied pressure or voltage, and the charge of the particles passing through the channel. We demonstrate that our algorithm correctly describes the experimentally observed signals in our recently introduced microcapillary based Coulter counters. Finally, we quantitatively investigate the influence of DNA-functionalized particles on the signal amplitude as a function of salt concentration and particle size.

High-speed video-based tracking of optically trapped colloids

We have developed an optical tweezer setup, with high-speed and real-time position tracking, based on a CMOS camera technology. Our software encoded algorithm is cross-correlation based and implemented on a standard computer. By measuring the fluctuations of a confined colloid at 6000? frames?s ? 1, continuously for an hour, we show our technique is a viable alternative to quadrant photodiodes. The optical trap is calibrated by using power spectrum analysis and the Stokes method. The trap stiffness is independent of the camera frame rate and scales linearly with the applied laser power. The analysis of our data by Allan variance demonstrates single nanometer accuracy in position detection.

A multistatic and multifrequency novel approach for specular meteor radars to improve wind measurements in the MLT region

Specular meteor radars (SMRs) have become a widely used tool to observe horizontal winds at the mesosphere and lower thermosphere (MLT). Typically 30 to 120 min mean winds are obtained assuming horizontal homogeneity of the observed area (i.e., few hundreds of kilometer radius). The quality of the measured wind velocity vector depends on the number of detected meteors per altitude and time bins. In order to improve the wind measurements of typical SMRs, here we propose a multistatic and multifrequency approach that consists mainly on adding GPS synchronized receiving stations with interferometric capabilities to existing SMRs. Compared to typical SMRs operating in a monostatic mode, our new approach called MMARIA (Multistatic and Multifrequency Agile Radar for Investigations of the Atmosphere) allows us to (a) increase the number of meteors using the same transmitter (by more than 70%), (b) increase the altitudinal coverage by 5–10 km higher depending on the geometry used, and (c) derive the horizontal wind field in the observed volume by relaxing the assumption of homogeneity. The latter result is facilitated by having common volume observations from at least two different viewing angles. We show the feasibility of these three goals from measurements at two different frequencies using a MMARIA configuration between Juliusruh and Kuhlungsborn in northern Germany.

The extraordinarily strong and cold polar vortex in the early northern winter 2015/16

The Arctic polar vortex in the early winter 2015/16 was the strongest and coldest o f the la st 68 years. Using global reanalysis data, satellite observations, and mesospheric radar wind measurements over northern Scandinavia we investigate the characteristics of the early sta g e polar vortex and relate them to previous winters. We found a correlation between the planetary wave (PW) activity and the strength and temperature of the northern polar vor- tex in the stratosphere and mesosphere. In Nov/D ec 2015, a reduced PW generation in the troposphere and a stronger PW filtering in the troposphere and stratosphere, caused by stronger zonal winds in mid-latitudes, resulted in a stronger polar vortex. Thi s effect was strengthened by the equator ward shift of PWs due to the strong zonal wind in polar latitudes resulting in a southward shift of the Eliassen-Palm flux divergence and hence inducinga decreased deceleration of th e polar vortex by PWs.

Nonspecular meteor trails from non‐field‐aligned irregularities: Can they be explained by presence of charged meteor dust?

Nonspecular meteor echoes have been associated with field-aligned irregularities and have been observed at low-latitude and midlatitude sites. We present observations obtained at high latitudes with range-time features that resemble those at lower latitudes. However, these echoes cannot come from field-aligned irregularities, since the radar-pointing angles are almost parallel to the magnetic field. Using interferometry, we have been able to discriminate space and time features. Our echoes could be qualitatively explained by the presence of charged dust forming from the meteoric material immersed in a turbulent flow. This can lead to a high Schmidt number plasma that can sustain meter-scale turbulence just as it does for the polar mesospheric summer echoes. These rare events require relatively large meteoroids. The result emphasizes the importance of charged dust in understanding all long-duration nonspecular meteor echoes. This dust will extend their diffusion times and will affect temperature estimations from specular echoes.

Neutral density variation from specular meteor echo observations spanning one solar cycle

Specular meteor radars have provided essential information about the mesospheric/lower thermosphere (MLT) dynamics. The Canadian Meteor Orbit Radar has been conducting continuous meteor echo observations in a fixed, stable configuration since 2002. Here we present estimates of the neutral air density variations derived from observations of the meteor peak flux altitude. Using a simple model assuming a linear trend and a sinusoidal solar cycle we derived a trend of a decreasing neutral density of 5.8 ± 1.1% per decade at approximately 91 km altitude and an amplitude across the most recent solar cycle, the solar cycle of 2.4 ± 0.7% for solar cycle 23/24. The long-term trend of decreasing neutral air density in the MLT is in good agreement with the model results from Akmaev et al. (2006).

Characterization of a Double Mesospheric Bore Over Europe

Observations of a pair of mesospheric bore disturbances that propagated through the nighttime mesosphere over Europe are presented. The observations were made at the Padua Observatory, Asiago (45.9°N, 11.5°E) by the Boston University all-sky imager on 11 March 2013. The bores appeared over the north-west horizon, approximately 30 minutes apart, and propagated towards the south-east. Using additional satellite and radar data, we present evidence indicating the bores originated in the mesosphere from a single, larger-scale mesospheric disturbance propagating through the mesopause region. Furthermore, the large-scale mesospheric disturbance appeared to be associated with an intense weather disturbance that moved southeastwards over the United Kingdom and Western Europe during 10 and 11 March.