John C. Dorelli

The Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) mission observes electron energy‐latitude dispersion at the equatorward edge of the magnetospheric cusp, and high‐cadence Analyzer for Cusp Electrons (ACE) measurements resolve the dispersed edge. The inverse velocity dispersion (low energy before high energy) encountered by TRACERS as it travels southward through the northern cusp rules out pure energy‐time dispersion from nearby injections or Alfvén wave‐driven acceleration. TRACERS observes electron dispersion at the equatorward edge of the northern cusp roughly half of the time for southward interplanetary magnetic field (IMF), and almost never for northward IMF. The TRACERS measurements therefore provide strong observational support for the hypothesis that the observed electron dispersion results from dayside magnetic reconnection and plasma convection, much like the ion dispersion that commonly extends across the cusp. Observations of multiple electron dispersions and electron steps suggest fine‐scale spatial and/or temporal variability in magnetic reconnection.

The primary scientific objective of the TRACERS (Tandem Reconnection and Cusp Reconnaissance Satellites) mission is to understand the spatial structure and temporal evolution of dayside magnetopause reconnection. TRACERS will accomplish this objective by making high time resolution observations with two closely separated (by about 10–120 seconds along track) satellites sun-synchronously orbiting at an altitude of about 590 km. The TRACERS team has defined a dayside “Region of Interest” (ROI) that maximizes the probable number of cusp crossings during its 1 year prime mission. While the TRACERS instruments will be operating at high cadence throughout the orbit, only data collected during the Region of Interest (about 8 minutes of the 90 minute orbit) will be stored at this high time resolution on board and downlinked for analysis on the ground. Outside of the Region of Interest, in the “Back Orbit”, the high time resolution data will be averaged or decimated by the Central Data Processing Unit before being stored for future downlink. Thus, with additional ground contacts, it may be possible to downlink high resolution data collected in the Back Orbit. Further, while TRACERS has been designed to be favorably magnetically aligned during the northern summer hemisphere cusp ROI, its spin axis will also be aligned with the magnetic field in the southern winter hemisphere nightside auroral zone. This raises the possibility that TRACERS could encounter thousands of nightside auroral zone precipitation events associated with magnetotail reconnection. In this paper, we review some of the outstanding science questions that the Heliophysics community could address using TRACERS data in the nightside auroral zone.

This study demonstrates the performance of the Comprehensive Inner Magnetosphere‐Ionosphere (CIMI) model in predicting auroral diffuse electron precipitation during the geomagnetically disturbed and quiet periods. The performance is evaluated with respect to Defense Meteorological Satellite Program (DMSP) precipitation data along the satellite orbits. CIMI underestimated DMSP particle fluxes at lower energies (0.5–1.4 keV) and performed better at higher energy ranges (14–30 keV). Improved performance was observed when CIMI was further upgraded with multiple reflections from the secondary low‐energy electrons. Performance metrics such as accuracy (Root Mean Square Error, RMSE), precision ( P _{σ ,ratio} ), and bias were improved for most cases after the incorporation of multiple reflections. The performance of CIMI was more accurate and precise during the geomagnetically quieter period than during the geomagnetically disturbed period. The improvement of the performance of CIMI with respect to DMSP observations at lower energy ranges after the inclusion of multiple reflections also indicates the possible presence of secondary electrons in the DMSP observations. This is consistent with the previous studies.

The overarching science goal of the Tandem Reconnection And Cusp Electrodynamics Reconnaissance Satellites (TRACERS) mission is to connect the cusp to the magnetosphere by discovering how spatial or temporal variations in magnetic reconnection drive cusp dynamics. This goal will be achieved with a simple mission design comprising two identical small spacecraft in identical low-Earth orbits in a follow-the-leader configuration. TRACERS will make repeated measurements in the cusp for a twelve-month primary mission using plasma and field instruments. These data will be analyzed using established dual-spacecraft techniques and supported by modeling that ensures science closure on the objectives. The TRACERS team leverages hardware collaborations from the University of Iowa, Southwest Research Institute, University of California Los Angeles, University of California Berkeley, and Millennium Space Systems. The larger science team consists of experts in reconnection, cusp physics, and modeling. TRACERS is dedicated to its proposer, and original Principal Investigator, Professor Craig Kletzing.

Using data from NASA's Magnetospheric Multiscale mission captured in a reconnection inflow on the magnetospheric side of Earth's dayside magnetopause, we find a region where the heat flux density gradient term balances the parallel compression term in the electron parallel temperature equation. Combining these observations with analysis of the generalized fluid equations indicates that such a behavior represents a quasi‐isothermal region, where cold magnetosheath beams that have transported across the magnetopause introduce non‐zero gradients in parallel heat flux density. This region should prevail near dayside reconnection X‐lines in inflows on the magnetospheric side due to the formation of mixed electron distributions and increased parallel temperatures that arise from three‐dimensional boundary dynamics.

In situ spacecraft observations are critical to our study and understanding of the various phenomena that couple mass, momentum, and energy throughout near‐Earth space and beyond. However, on‐orbit telemetry constraints can severely limit the capability of spacecraft to transmit high‐cadence data, and missions are often only able to telemeter a small percentage of their captured data at full rate. This presents a programmatic need to prioritize intervals with the highest probability of enabling the mission's science goals. Larger missions such as the Magnetospheric Multiscale mission (MMS) aim to solve this problem with a Scientist‐In‐The‐Loop (SITL), where a domain expert flags intervals of time with potentially interesting data for high‐cadence data downlink and subsequent study. Although suitable for some missions, the SITL solution is not always feasible, especially for low‐cost missions such as CubeSats and NanoSats. This manuscript presents a generalizable method for the detection of anomalous data points in spacecraft observations, enabling rapid data prioritization without substantial computational overhead or the need for additional infrastructure on the ground. Specifically, Principal Components Analysis and One‐Class Support Vector Machines are used to generate an alternative representation of the data and provide an indication, for each point, of the data's potential for scientific utility. The technique's performance and generalizability is demonstrated through application to intervals of observations, including magnetic field data and plasma moments, from the CASSIOPE e‐POP/Swarm‐Echo and MMS missions.

In-situ spacecraft observations are critical to our study and understanding of the various phenomena that couple mass, momentum, and energy throughout near-Earth space and beyond. However, on-orbit telemetry constraints can severely limit the capability of spacecraft to transmit high-cadence data, and missions are often only able to telemeter a small percentage of their captured data at full rate. This presents a programmatic need to prioritize intervals with the highest probability of enabling the mission’s science goals. Larger missions such as the Magnetospheric Multiscale mission (MMS) aim to solve this problem with a Scientist-In-The-Loop (SITL), where a domain expert flags intervals of time with potentially interesting data for high-cadence data downlink and subsequent study. Although suitable for some missions, the SITL solution is not always feasible, especially for low-cost missions such as CubeSats and NanoSats. This manuscript presents a generalizable method for the detection of anomalous data points in spacecraft observations, enabling rapid data prioritization without substantial computational overhead or the need for additional infrastructure on the ground. Specifically, Principal Components Analysis and One-Class Support Vector Machines are used to generate an alternative representation of the data and provide an indication, for each point, of the data’s potential for scientific utility. The technique’s performance and generalizability is demonstrated through application to intervals of observations, including magnetic field data and plasma moments, from the CASSIOPE e-POP/Swarm-Echo and MMS missions.