Enter Zoom Meeting


Earth Rotation: Theoretical aspects, observation of temporal variations and physical interpretation

Accurate modeling and prediction of Earth rotation is of paramount importance in many disciplines, e.g., geodesy, astronomy and navigation. Over the past years geodetic observation systems have made significant advances in monitoring Earth rotational motion and its variability. That progress must be accompanied by an enhancement of the theories, conventional models, etc., as was recently recognized in Resolution 5 adopted at last IAG GA (Montreal, 2019).
In this session, we are interested in the progress in the theory of Earth rotation. We seek contributions that are consistent internally with the accurate observations at the mm-level, to meet the requirements of Global Geodetic Observing System (GGOS) and respond to IAG Res. 5. We invite presentations within the scope of the IAU/IAG JWG Improving Theories and Models of the Earth’s Rotation.
We welcome contributions that highlight new determinations and analyses of Earth Orientation Parameters (EOP) series, including combinations of different geodetic and astrometric observational techniques. We welcome discussions of EOP solutions in conjunction with a consistent determination of terrestrial and celestial frames, as tackled in the IAG/IAU/IERS JWG Consistent Realization of TRF, CRF and EOP.
We invite contributions of both the dynamical basis for links between Earth rotation, geophysical fluids, and other geodetic quantities, such as the Earth gravity field or surface deformation, and of investigations leading to detailed explanations for the physical excitations of Earth rotation. Besides tidal influences from outside the Earth, the principal causes for variable EOP appear to be related to angular momentum exchange from variable motions and mass redistribution of the fluid portions of the planet. Observations of the geophysical fluids (e.g., atmosphere and oceans) have achieved a new maturity in recent years. Independent observations include the results of recent gravity missions like GRACE.
We welcome contributions about the relationship between EOP variability and current or potential variability in fluids due to climate variation or global change signals. Forecasts of these quantities are important especially for the operational determination of Earth orientation and its application, e.g., spacecraft navigation; the effort to improve predictions currently is a topic of strong interest. We welcome input on the modeling, variability, and excitations of the rotation of other celestial bodies.

Convener: Alberto Escapa | Co-conveners: Sigrid Böhm, Maria Karbon, David Salstein, Florian Seitz
Attention: the start of this vPICO session is in a shared 90-minute time block where the previous session uses the first 45 minutes and this session the second 45 minutes. Both session share the same Zoom Meeting. If you enter this Zoom Meeting before your session starts, you will join a running session.
Welcome to this vPICO session. All conveners, speakers, and attendees join the Zoom Meeting for the live presentations through the green button to the top right. On this page, you will find a list of presentations, their abstracts linked, and you can use the handshake to start spontaneous chats with others.

Activation of the text chat sets a cookie in your browser that is automatically deleted at the end of the conference.

A chat user is typing ...
SHIFT+ENTER for line break
We are sorry but we encountered a problem while running the chat G3.3 . Please reload this browser window. In case this message is shown again after reloading, please contact us at: egu21@copernicus.org. We are sorry for this inconvenience.

Wed, 28 Apr, 11:45–12:30

Chairpersons: Alberto Escapa, Sigrid Böhm, Florian Seitz

Nicholas Stamatakos et al.

The accuracy of near real-time estimates and short-term predictions of Earth orientation parameters (EOPs) can be enhanced by using Atmospheric Angular Momentum (AAM) accounting for the global conservation of angular momentum in the Earth system. The US Navy analysis and forecast model provided by the Navy Global Environmental Model (NAVGEM) is being improved continually, and the resultant motion and mass fields are potentially useful for operational Earth orientation applicationsSimilarly, the AAM data available from the National Oceanic and Atmospheric Administration (NOAA) provide another possibly important contribution to the prediction of UT1-UTC. For operational use of these data, the systematic errors in scaling and bias must be considered. Statistical models accounting for these issues were developed for both data sources to provide forecast excess length of day (LOD) estimates for up to seven days in the future. This information was then integrated in time to provide independent predictions of UT1-UTC that can be compared with past predictions of the International Earth Rotation and Reference Systems (IERS) Rapid Service/Prediction Product Center. Three years of AAM data were analyzed for these comparisons.

How to cite: Stamatakos, N., McCarthy, D., and Salstein, D.: IERS Rapid Service Prediction Center Use of Atmospheric Angular Momentum for Earth Rotation Predictions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1917, https://doi.org/10.5194/egusphere-egu21-1917, 2021.

Sara Bruni et al.

The availability of highly accurate Earth Orientation Parameters (EOPs) in near real time is of major importance for any type of positioning and navigation applications on Earth, Sea, Air and also in Space. This is equally true for all ESA missions and the EU space programs Galileo, EGNOS and Copernicus.

To ensure operational capability, ESA’s Navigation Support Office developed independent EOP products and services.

The EOPs are estimated based on a rigorous combination of the ESA’s contributions to the International Association of Geodesy (IAG) that are used as an input for the generation of the International Earth Rotation Service (IERS) products. For the ESA/ESOC EOP products, the individual parameters are combined on normal equation level and propagated with the contribution of model-based predicted Effective Angular Momentum (EAM) functions.

The ESA/ESOC’s EOP product generation is currently running in pre-operational mode.

This presentation will provide a high-level overview of the methodology and the status of ESA’s EOP products and services. In this context, the accuracy achieved in the test operations and the roadmap for the publication of ESA’s EOP products and services will be outlined.

How to cite: Bruni, S., Schoenemann, E., Mayer, V., Otten, M., Springer, T., Dilssner, F., Enderle, W., and Zandbergen, R.: ESA's Earth Orientation Parameter product, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12989, https://doi.org/10.5194/egusphere-egu21-12989, 2021.

Michael Schindelegger

Non-tidal Earth rotation variations at intraseasonal periods are almost exclusively driven by mass redistributions within the atmosphere and ocean. Our capacity to model these signals has advanced over the past decades, but differences between the observed and modeled portions of the planetary angular momentum budget are still as large as 1–5 cm when expressed as axis displacement at the Earth's surface. A likely source for these significant errors is the ocean, poorly sampled with observations and thus not amenable to the sequential data assimilation machinery developed for the atmosphere. Moreover, the recent delineation of basin-wide ocean mass exchanges associated with the Madden-Julian Oscillation (MJO) in a high-resolution baroclinic model emphasizes that a revisit of standard forward modeling choices (e.g., grid spacings of ∼100 km) may be in order to better describe rapid, large-scale oceanic mass motions. In this contribution, I provide a brief overview of recent progress in the field and suggest that dynamically consistent model-data syntheses, as practiced by the consortium on Estimating the Circulation and Climate of the Ocean (ECCO), are a viable route to mitigate deficiencies in present oceanic angular momentum (OAM) series on intraseasonal (but also longer) time scales. As ocean state estimates continue to be refined by the central ECCO production, I assess the benefits of higher model resolution with OAM series from an eddy-permitting (1/6°) forward simulation, descending from the current ECCO release in its discrete setup. The resulting mass and motion terms indeed provide smaller Earth rotation residuals than other available OAM estimates, possibly due to the model resolving important topographic interactions and the dynamic response to MJO in the 30–80-day band. However, these improvements come at disproportionally large computational costs, and iteratively fitting an eddy-permitting general circulation model to oceanographic observations may still be prohibitive in the near future. Instead, efforts should be devoted to extending the present coarser-resolution ECCO framework to new data constraints and shorter adjustment intervals. Of particular interest in the context of Earth rotation are non-standard daily GRACE gravity field solutions, which contain realistic information on oceanic mass-field variability below the nominal GRACE Nyquist period of 60 days.

How to cite: Schindelegger, M.: Modeling rapid Earth rotation variations – where are we going next?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1616, https://doi.org/10.5194/egusphere-egu21-1616, 2021.

Lisa Kern et al.

One hour single baseline VLBI sessions, so-called Intensives, are routinely observed to derive UT1-UTC with a short latency. The selection of baselines for VLBI Intensive sessions and their application for the determination of UT1-UTC is a complex task. Thus far, it has been understood that long east-west extensions are critical for the accuracy of UT1-UTC. In this presentation, we show, that the answer is not as simple as that. 

We run Monte-Carlo simulations for a global 10° grid of artificial station locations and discuss the suitability of the individual baselines for UT1-UTC estimation based on the formal error of dUT1. The antennas are located at latitudes of -80° to 80° and longitudes of 0° to 180° and are assumed to have the same properties than the WETTZ13S telescope. The nine stations at longitude 0° on the northern hemisphere are defined as reference stations. In total, 2898 possible baselines between the reference stations and other artificial stations are investigated over one year based on monthly schedules to minimize potential seasonal variations. Thus, with this study, it is possible to derive a complete picture of which baselines are most suitable for dUT1 estimates. 

In general, the findings show optimal global geometries concerning Intensives. For example, we can confirm that the IVS-INT1 baseline including the stations Kokee and Wettzell is among the best ones available. Furthermore, we show that north-south baselines are also sensitive to dUT1 as long as their orientations are not parallel to the Earth rotation axis. Moreover, we highlight that east-west baselines on the equator are not suitable for estimating dUT1 due to the lack of variety in right-ascension of the visible sources. Additionally, we highlight, that very long baselines are problematic due to the highly restricted mutual visibility.

How to cite: Kern, L., Schartner, M., Soja, B., Nothnagel, A., and Böhm, J.: On the geometry of baselines suitable for UT1 estimation with VLBI Intensive sessions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-917, https://doi.org/10.5194/egusphere-egu21-917, 2021.

Radosław Zajdel et al.

Variations in the Earth's rotation can be examined in the low-frequency and high-frequency temporal scales. The low-frequency variations are dominated by the annual and Chandler wobbles, while the high-frequency variations are primarily caused by tidal effects and mass redistributions within the system Earth. Depending on the purpose, the Earth Rotation Parameters (ERPs) can be estimated in different time resolutions using space-geodetic techniques, especially using GNSS. However, the residual signals between different space geodetic techniques or satellite constellations indicate system-specific differences, which have to be correctly identified.

This research provides the daily, and sub-daily series of Earth Rotation Parameters (ERPs) estimated using GPS, GLONASS, and Galileo observations. We test different sampling intervals of estimated ERPs from 1h to 24h. The GNSS-based sub-daily estimates have been compared with the external models of variations in ERPs induced by the ocean tides from the IERS 2010 Conventions, a new model by Desai-Sibois, and the VLBI-based model by Gipson.

Any system-specific ERPs are affected by the orbital and draconitic signals. The orbital signals are visible in all system-specific ERPs at the periods that arise from the resonance between the Earth's rotation and the satellite revolution period, e.g., 8.87h, 34.22h, 3.4 days, 10 days for Galileo; 7.66h, 21.29h, 3.9 days, 7.9 days for GLONASS; 7.98h (S3 tidal term), 11.97h (S2 tidal term), 23.93h (S1 tidal term) for GPS. In the Galileo and GLONASS solutions, the artificial non-tidal signals' amplitudes can reach up to 30 µas. The GPS-derived sub-daily ERPs suffer from the overlapping periods of the diurnal and semidiurnal tidal terms and the harmonics of the GPS revolution period. After recovery of 38 sub-daily tidal terms, the Galileo-based model is more consistent with the external models than the GPS-based model, especially in the prograde diurnal band. The results confirmed that the Desai–Sibois model is more consistent with GNSS observations than the currently recommended model by the IERS 2010 Conventions. Moreover, GPS-based length-of-day (LoD) is systematically biased with respect to the IERS-C04-14 values with a mean offset of −22.4 µs/day, because of the deep resonance 2:1 between the satellite revolution period and the Earth rotation. The Galileo-based and GLONASS-based solutions are almost entirely free of this issue. Against the individual system-specific solutions, the multi-GNSS solution is not affected by most of the system-specific artifacts. Thus, multi-GNSS solutions are clearly beneficial for the estimation of both daily and sub-daily ERPs.

How to cite: Zajdel, R., Sośnica, K., Bury, G., and Kazmierski, K.: Estimating variations in Earth rotation using GPS, GLONASS, and Galileo, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2646, https://doi.org/10.5194/egusphere-egu21-2646, 2021.

Justyna Śliwińska et al.

Assessing the impact of continental hydrosphere and cryosphere on polar motion (PM) variations is one of the crucial tasks in contemporary geodesy. The pole coordinates, as one of the Earth Orientation Parameters, are needed to define the relationship between the celestial and terrestrial reference frames. Therefore, the variations in PM should be monitored and interpreted in order to assess the role of geophysical processes in this phenomenon.

The role of hydrological and cryospheric signals in PM is usually examined by computing hydrological excitation (hydrological angular momentum, HAM) and cryospheric excitation (cryospheric angular momentum, CAM) of  PM, commonly treated together as HAM/CAM.

The Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) missions deliver temporal variations of the gravity field resulting from changes in global mass redistribution. The so-called GRACE/GRACE-FO Level-3 (L3) data delivers changes in terrestrial water storage (TWS) that can be used for computation of HAM/CAM.

For best possible representation of TWS, a number of corrections are introduced in the L3 data by computing centres. Such corrections are, among others, glacial isostatic adjustment (GIA) correction, geocenter correction and C20 coefficient correction.

The main goal of this study is to examine the impact of corrections included in GRACE/GRACE-FO data on HAM/CAM determined. More specifically, we test their influence on HAM/CAM trends, seasonal changes and non-seasonal variations. We also examine the change in compliance between HAM/CAM and hydrological plus cryospheric signal in geodetically observed excitation when the corrections are used. To achieve our goals, we use GRACE and GRACE-FO L3 datasets provided by Jet Propulsion Laboratory (JPL), Center for Space Research (CSR), and Goddard Space Flight Center (GSFC).

How to cite: Śliwińska, J., Wińska, M., and Nastula, J.: Assessing the role of corrections included in the GRACE/GRACE-FO data in determination of hydrological and cryospheric signal in polar motion excitation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5426, https://doi.org/10.5194/egusphere-egu21-5426, 2021.

Adam Scaife et al.

Angular momentum is fundamental to the structure and variability of the atmosphere and hence regional weather and climate. Total atmospheric angular momentum (AAM) is also directly related to the rotation rate of the Earth and hence the length of day. However, the long-range predictability of fluctuations in the length of day, atmospheric angular momentum and the implications for climate prediction are unknown. Here we show that fluctuations in AAM and the length of day are predictable out to more than a year ahead and that this provides an atmospheric source of long-range predictability of surface climate. Using ensemble forecasts from a dynamical climate model we demonstrate predictable signals in the atmospheric angular momentum field that propagate slowly and coherently polewards into the northern and southern hemisphere due to wave-mean flow interaction within the atmosphere. These predictable signals are also shown to precede changes in extratropical surface climate via the North Atlantic Oscillation. These results provide a novel source of long-range predictability of climate from within the atmosphere, greatly extend the lead time for length of day predictions and link geodesy with climate variability.

How to cite: Scaife, A., Hermanson, L., van Niekerk, A., Baldwin, M., Belcher, S., Bett, P., Comer, R., Dunstone, N., Geen, R., Hardiman, S., Ineson, S., Knight, J., Nie, Y., Ren, H., and Doug, S.: Long-Range Predictability of the Length of Day and Extratropical Climate., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-191, https://doi.org/10.5194/egusphere-egu21-191, 2020.

Franziska Göttl et al.

Increasing ice loss of the Antarctic and Greenland Ice Sheets (AIS, GrIS) due to global climate change affects the orientation of the Earth’s spin axis with respect to an Earth-fixed reference system (polar motion). Ice mass changes in Antarctica and Greenland are observed by the Gravity Recovery and Climate Experiment (GRACE) in terms of time variable gravity field changes and derived from surface elevation changes measured by satellite radar and laser altimeter missions such as ENVISAT, CryoSat-2 and ICESat. Beside the limited spatial resolution, the accuracy of GRACE ice mass change estimates is limited by signal noise (meridional error stripes), leakage effects and uncertainties of the glacial isostatic adjustment (GIA) models, whereas the accuracy of satellite altimetry derived ice mass changes is limited by waveform retracking, slope related relocation errors, firn compaction and the density assumption used in the volume-to-mass conversion.


In this study we use different GRACE gravity field models (CSR RL06M, JPL RL06M, ITSG-Grace2018) and satellite altimetry data (from TU Dresden, University of Leeds, Alfred Wegener Institute) to assess the accuracy of the gravimetry and altimetry derived polar motion excitation functions. We show that due to the combination of individual solutions, systematic and random errors of the data processing can be reduced and the robustness of the geodetic derived AIS and GrIS polar motion excitation functions can be increased. Based on these investigations we found that AIS mass changes induce the pole position vector to drift along the 60° East meridian by 2 mas/yr during the study period 2003-2015, whereas GrIS mass changes cause the pole vector to drift along the 45° West meridian by 3 mas/yr.

How to cite: Göttl, F., Groh, A., Kappelsberger, M., Strößenreuther, U., Schröder, L., Helm, V., Schmidt, M., and Seitz, F.: The influence of Antarctic and Greenland ice loss on polar motion: an assessment based on GRACE and multi-mission satellite altimetry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2564, https://doi.org/10.5194/egusphere-egu21-2564, 2021.

Sigrid Böhm and David Salstein

One special component of the Coupled Model Intercomparison Project phase 6 (CMIP6) is the so-called ScenarioMIP. Different Earth system variables are provided within this project originating from numerous models and model runs operated by research centers around the globe. The models simulate future climate, based on alternative scenarios of future greenhouse gas emissions and land use changes linked to socioeconomic factors. The simulations, which cover the 21st century, use different forcings that are defined from a combination of possible future pathways of societal development, the Shared Socioeconomic Pathways (SSPs), and the Representative Concentration Pathways (RCPs), identified by what radiative forcing level might exist in 2100. In this study, we focus on the analysis of multi-model projections of zonal wind fields, stemming from historical simulations and from the four tier 1 alternate scenarios SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5. We investigate the integrated effect of variations in the atmosphere on the axial component of the Earth rotation vector, quantified as length of day (LOD) variations. Generally, larger emissions lead to some stronger zonal wind patterns in much of the upper atmosphere.  The long-term variability and trends in LOD are examined w.r.t. a multi-model-mean and compared with the respective variations in the projected global temperature to study the potential impact of global warming on the Earth rotation speed.

How to cite: Böhm, S. and Salstein, D.: Axial atmospheric Earth rotation excitation predicted from CMIP6 model simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5601, https://doi.org/10.5194/egusphere-egu21-5601, 2021.

Małgorzata Wińska et al.

Continental hydrological loading by land water, snow, and ice is a process that influences the Earth’s inertia tensor and is very important for full understanding of the excitation of polar motion. In this study, the hydrological contribution to decadal, inter-annual and multi-annual suppress polar motion derived from different GRACE (Gravity Recovery and Climate Experiment) solutions as well as from SLR (Satellite Laser Ranging) and some climate models from CMIP6 project data is discussed here.

The main aim of this study is to show the influence of different representations of hydrological angular momentum (HAM) coming from different GRACE (mas concentration solutions - mascons, Terrestrial Water Storage changes, and Stokes Coefficients), SLR, and climate models solutions on agreement between Geodetic Angular Momentum (GAM) and geophysical excitations of polar motion been a sum of Atmospheric, Oceanic and Hydrological Angular Momentum (AAM+OAM+HAM) in different spectral bands.

To do that, the geodetic and geophysical excitation functions are transformed into time-scale domain using the discrete wavelet transform based on the Complex Morlet wavelet functions. Next, the time series (GAM vs. geophysical ones) are compared in terms of semblance filtering, on the basis of their phase, as a function of frequency, and amplitude information of their cross-wavelet power.

Here, we would like to present the consistency between full polar motion excitations and geophysical fluids,  that are the sum of AAM  (pressure + wind), OAM  (bottom pressure +  currents), and HAM contributions. This analysis could let us indicate, which hydrological representation of different HAM solutions cause the biggest errors in the geodetic budget.

How to cite: Wińska, M., Śliwińska, J., and Nastula, J.: Comparison between polar motion excitation functions estimated from recent geophysical models and observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9490, https://doi.org/10.5194/egusphere-egu21-9490, 2021.

Sadegh Modiri et al.

The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) provides the geodetic infrastructure needed to monitor the Earth system.. The understanding of forced temporal variations of celestial pole motion (CPM) could bring us significantly closer to meeting the GGOS goals (i.e. 1 mm accuracy and 0.1 mm/year stability on global scales in terms of the ITRF defining parameters). Besides astronomical forcing, CPM excitation depends on the processes in the fluid core and the core-mantle boundary. The same processes are responsible for the variations of the geomagnetic field (GMF). This study investigates the interconnection between the celestial pole offset (CPO) and effective geophysical processes that contribute to the Earth's rotational variation. We use the CPO time series obtained from very long baseline interferometry (VLBI) observations together with the latest GMF data such as geomagnetic jerk and magnetic dipole moment, and a state-of-the-art geomagnetic field model to explore the correlation between CPM and GMF. 
Our results confirm the findings of previous studies, revealing that  substantial free core nutation (FCN) disturbance occurred at the epochs close to the GMJ events. The results also reveal some common features in the FCN and GMF variation, which show the potential to improve knowledge regarding the GMF's contribution to the Earth's rotation.

How to cite: Modiri, S., Heinkelmann, R., Belda, S., Hoseini, M., Korte, M., Malkin, Z., Ferrándiz, J. M., and Schuh, H.: A First Assessment of the interconnection between celestial pole offset and geomagnetic field variations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7235, https://doi.org/10.5194/egusphere-egu21-7235, 2021.

José M. Ferrándiz et al.

In 2020 new estimations of nutation amplitudes or precession parameters have been published or presented at main meetings. The derivation of corrections to improve the current precession-nutation models was encouraged by Resolution 5 of the 2019 General Assembly of the International Association of Geodesy (IAG). Besides, the GGOS/IERS Unified Analysis Workshop held in October 2019 recommended that effort to be prioritized among the tasks of the current IAU/IAG Joint Working Group on Improving Theories and Models of the Earth’s rotation (JWG ITMER).

This presentation is intended to present comparisons of some of those new semi-empirical and semi-theorical precession-nutation models developed by different authors from either VLBI solutions of individual analysis centers or combinations of them. The models recently introduced by the authors that were reported at the AGU 2020 Fall Meeting are included in this assessment.

How to cite: Ferrándiz, J. M., Juárez, M. A., Belda, S., Baenas, T., Modiri, S., Heinkelmann, R., Escapa, A., and Schuh, H.: Assessing recently improved precession-nutation models , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10180, https://doi.org/10.5194/egusphere-egu21-10180, 2021.

Alberto Escapa et al.

IAU2000 (Mathews et al. 2002) incorporates some second order terms in the sense of perturbation theories in its formulation. In particular, the second order Poisson amplitudes independent of the Earth structure. They are borrowed from the rigid Earth theory REN2000 by Souchay et al. (1999). Their inclusion, however, is inconsistent (Escapa et al. 2020) since they are convolved with the MHB2000 transfer function, rendering them Earth dependent.

In that IAU2000 scheme, second order contributions depending on the Earth structure are totally ignored, as it is the case in the rigid Earth theory (Souchay et al. 1999). That structure dependent terms affect both a part of Poisson second order amplitudes and all the Oppolzer ones. Getino et al. (2021) have shown that the numerical contribution of the ignored Poisson terms is not negligible. In addition, the dependence of the respective amplitudes on the fluid core present quite different features from those of first order terms.

These facts pose some significant problems in the application of IAU2000 transfer function and the estimation of basic Earth parameters when second order terms are included, which are discussed in this communication.

How to cite: Escapa, A., Getino, J., Ferrándiz, J. M., and Baenas, T.: Implications of second order nutation terms in IAU2000 framework, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14236, https://doi.org/10.5194/egusphere-egu21-14236, 2021.

Meet the authors in their breakout text chats

A chat user is typing ...