Richard A. M. Lee

Characterization of the OCO-2 instrument line shape functions using on-orbit solar measurements

Accurately characterizing the instrument line shape (ILS) of the Orbiting Carbon Observatory-2 (OCO-2) is challenging and highly important due to its high spectral resolution and requirement for retrieval accuracy (0.25%) compared to previous spaceborne grating spectrometers. Onorbit ILS functions for all three bands of the OCO-2 instrument have been derived using its frequent solar measurements and high-resolution solar reference spectra. The solar reference spectrum generated from the 2016 version of the Total Carbon Column Observing Network (TCCON) solar line list shows significant improvements in the fitting residual compared to the solar reference spectrum currently used in the version 7 Level 2 algorithm in the O2 A band. The analytical functions used to represent the ILS of previous grating spectrometers are found to be inadequate for the OCO-2 ILS. Particularly, the hybrid Gaussian and super-Gaussian functions may introduce spurious variations, up to 5% of the ILS width, depending on the spectral sampling position, when there is a spectral undersampling. Fitting a homogeneous stretch of the preflight ILS together with the relative widening of the wings of the ILS is insensitive to the sampling grid position and accurately captures the variation of ILS in the O2 A band between decontamination events. These temporal changes of ILS may explain the spurious signals observed in the solar-induced fluorescence retrieval in barren areas.

Preflight Spectral Calibration of the Orbiting Carbon Observatory 2

This paper describes the preflight spectral calibration methods and results for the Orbiting Carbon Observatory 2 (OCO-2), following the approach developed for the first OCO. The instrument line shape (ILS) function and dispersion parameters were determined through laser-based spectroscopic measurements, and then further optimized by comparing solar spectra recorded simultaneously on the ground by the OCO-2 flight instrument and a collocated high-resolution Fourier transform spectrometer (FTS). The resulting ILS profiles and dispersion parameters, when applied to the FTS solar data, showed agreement between the spectra recorded by the spectrometers and FTS to approximately 0.2% RMS, satisfying the preflight spectral calibration accuracy requirement of <0.25% RMS. Specific changes to the OCO-2 instrument and calibration process, compared to the original OCO, include stray-light protection; improved laser setup; increased spectral sampling; enhanced data screening, and incremental improvements in the ILS, dispersion, and FTS optimization analyses.

Distributed antenna-coupled transition edge sensors

We describe progress toward realizing a new architecture for focal plane arrays for the Submillimeter and Far-Infrared (FIR) bands. This architecture is based on a detector design utilizing distributed hot-electron transition edge sensors (TES) coupled to slot antenna elements. Arrays utilizing this type of detector can be considerably easier to manufacture than membrane-isolated TES arrays, because the need for micro-machining is eliminated. We present background and rationale for this new array architecture and details of a new antenna design for an imaging polarimeter, which yields greater bandwidth than past designs. In addition, we describe a cryogenic facility for testing these arrays.

Measurement of the SOC State Specific Heat in 4He

When a heat flux Q is applied downward through a sample of liquid 4He near the lambda transition, the helium self organizes such that the gradient in temperature matches the gravity induced gradient in Tlambda. All the helium in the sample is then at the same reduced temperature tSOC = ((T[sub SOC] - T[sub lambda])/T[sub lambda]) and the helium is said to be in the Self-Organized Critical (SOC) state. We have made preliminary measurements of the 4He SOC state specific heat, C[del]T(T(Q)). Despite having a cell height of 2.54 cm, our results show no difference between C[del]T and the zero-gravity 4He specific heat results of the Lambda Point Experiment (LPE) [J.A. Lipa et al., Phys. Rev. B, 68, 174518 (2003)] over the range 250 to 450 nK below the transition. There is no gravity rounding because the entire sample is at the same reduced temperature tSOC(Q). Closer to Tlambda the SOC specific heat falls slightly below LPE, reaching a maximum at approximately 50 nK below Tlambda, in agreement with theoretical predictions [R. Haussmann, Phys. Rev. B, 60, 12349 (1999)].

Effect of Inhomogeneous Heat Flow on the Enhancement of Heat Capacity in Helium‐II by Counterflow near Tλ

In 2000 Harter et al. reported the first measurements of the enhancement of the heat capacity ΔCQ[equivalent]C(Q)-C(Q=0) of helium-II transporting a heat flux density Q near Tλ. Surprisingly, their measured ΔCQ was ~7–12 times larger than predicted, depending on which theory was assumed. In this report we present a candidate explanation for this discrepancy: unintended heat flux inhomogeneity. Because C(Q) should diverge at a critical heat flux density Qc, homogeneous heat flow is required for an accurate measurement. We present results from numerical analysis of the heat flow in the Harter et al. cell indicating that substantial inhomogeneity occurred. We determine the effect of the inhomogeneity on ΔCQ and find rough agreement with the observed disparity between prediction and measurement.

New propagating mode near the superfluid transition in 4He

Abstract We have observed a new temperature-entropy wave that propagates opposite to the direction of a steady heat flux Q when the helium column is heated from above. Counter-intuitively this new mode, which resembles second sound, propagates on the normal fluid side of the transition, but it exists only when the column of helium is heated from above. Such a new mode had been predicted to exist on the self-organized heat transport state for Q less than about 100 nW / cm 2 . We confirm that this mode exists in this regime, however we also observe that it propagates even when the helium is held away from the self-organized heat transport state.

The CQ experiment: Enhanced heat capacity of superfluid helium in a heat flux

CQ will exploit the superfluid transition of pure liquid /sup 4/He, in a microgravity environment, in order to study a critical point phase transition under non-equilibrium conditions. It will be conducted in conjunction with the DYNAMX experiment (critical dynamics in microgravity) on board the ISS, using the same hardware and electronics, and on the same mission. We call the combined mission DX/CQ.

Quest to observe the bulk superfluid breakdown in a heat flux

A recent theory predicts that bulk superflow breaks down in a heat flux, Q, through an instability, where the fluctuations of the counterflow velocity diverge. In order to observe this interesting effect a number of obstacles must be overcome. First, it was recently suggested that in an ordinary thermal conductivity cell, the breakdown of superfluidity occurs initially at the hot end plate metal/liquid boundary, and not in the bulk superfluid. Second, the sample can become nonuniform due to a temperature gradient in the superfluid caused by vortices. Third, the sample may become non-uniform due to a pressure gradient induced by gravity. We will present analyses of these adverse effects in the Q-T plane and discuss ways to overcome them. The possibility of using the International Space Station to remove the gravity rounding effect will be elaborated. Introduction Due to the admirable success of the Renormalization Group theory and its extension, the Dynamic Renormalization Group theory, much of static and dynamical phase transition phenomena have been explained to some degree of satisfaction. It is therefore quite surprising that under an applied heat current, the properties of He on the superfluid side of the transition are found to be quite different from what is expected. For example, recent measurements of the heat capacity CQ of He in a constant applied heat flux Q showed that the change in the heat capacity due to Q is much larger than predicted. Aside from this discrepancy, it has been predicted that near the bulk superfluid breakdown temperature Tc (Q), CQ is expected to diverge with a very different exponent. Copyright

Heat capacity measurements of 4He at constant heat flux near Tλ

The heat capacity, C_Q, of superfluid ^4He in the presence of a constant heat flux, Q, is expected to diverge at a depressed transition temperature, T_c(Q). We have taken preliminary measurements of C_Q at various heat flux values in the range 1 µW/cm^2 ≤ Q ≤ 4 µW/cm^2. We observe that at sufficiently small reduced temperatures, C_Q is enhanced as a function of Q, and that the enhancement is larger than theoretical predictions by Chui et al. and Hausmann and Dohm (Phys. Rev. Lett 77 (1996) 980, 1793; preprint).