Equatorial Plasma Bubbles (EPBs) are known by many names, including ionospheric plumes and Equatorial Spread F (ESF) (Kelley et al., 2011). The varying nomenclature is associated with the different observational techniques used to study them, since the 4-dimensional morphology of the bubble structures is never fully revealed by any single measurement approach. Different techniques yield different insights into the structures, because each views only a small part of the phenomenon. For example, when observed with airglow imaging from high altitudes the depleted plasma structures are C-shaped wedges (Kil et al., 2009), but airglow images of the depletions in the plane perpendicular to the geomagnetic field show multiple tilted branches extending from a plume-like base (Makela and Otsuka, 2011). These structures can extend hundreds of km in longitude and thousands of km in latitude. At low altitudes, the bubbles are initiated at the bottom side of the ionosphere by mesoscale undulations with horizontal wavelengths of several hundred kilometers. The appearance of these waves near 300 km altitude is a precursor to the formation of bubbles (Hysell, 2000).

Bubble structure observation is further complicated due to the range of scale-sizes of density variations associated with EPBs, such as scintillation structures (e.g., (Magdaleno et al., 2017)). Scintillation has often been used to determine the presence or boundaries of EPBs (Cervera and Thomas, 2006), but the relationship between scintillation intensity and EPBs is not understood. Scintillation observations offer unique challenges. While in-situ observations provide very detailed and accurate information of small-scale structures, observations utilizing remote sensing are highly dependent of the orientation of the signal path to the scintillation region, thus limiting their ability to fully ascertain the scintillated structure size and intensity. Further, intense scintillation can result in loss-of-lock of signals, limiting observations of strong scintillation regions.

The problem of plasma bubble prediction has been outstanding for over 80 years (Booker and Wells, 1938) for several reasons:

• Global daily measurements of the existence/non-existence of bubbles is lacking.

In this framework, we will separate the drivers in terms of both mesoscale (100 s of km) and large-scale (1,000 s of km) features. In general, plasma bubbles form when mesoscale waves (such as Gravity Waves) create a perturbation in the bottomside ionosphere when the ionosphere is sufficiently unstable. The definition of “sufficiently” here depends largely on the amplitude of this seed wave (Retterer and Roddy, 2014). A mesoscale seed with a sufficiently large amplitude could amplify the instability of the ionosphere in a region. The mesoscale waves as a seeding mechanism is strongly supported by observations of bubbles with a periodic spacing (e.g., Aa et al., 2020) The variability of Gravity Waves and their contributions to ionospheric structure is reviewed in Zawdie et al. (2022).

If the Rayleigh-Taylor Instability growth rate is large and positive, the perturbation will grow into a large plume of depleted plasma that grows into the topside region (Ossakow, 1981; Sultan, 1996). Likewise, if the growth rate is large and negative, bubbles can be suppressed. The growth rate is dependent on field-line integrated quantities, meaning that growth is not solely dependent on the local ionospheric conditions. Multiple paths and sources of energy conspire together to enhance or suppress the growth of plasma bubbles.

Due to the scale size of the bubbles themselves, the smaller scale size of density structures associated with bubbles (i.e., scintillation), the dynamics across altitudes, and temporal range of the drivers, single point measurements are unlikely to capture some events, and miss important dynamics, spatial structures, and the evolution of plasma bubbles. Multiple missions and ground-based observatories are often used to better capture the influence of the various contributions to bubble formation. This lack of adequate data coverage has limited the field’s ability to make substantial progress in determine the drivers and their relative contributions to bubble formation and evolution. While single satellite studies provide important insights, they inherently miss many events. In addition, capturing low-altitude measurements where bubbles are first formed increases satellite drag, which limits the lifetime of the spacecraft. In fact, by capturing these limited glimpses of plasma bubbles, misinterpretations of their characteristics, and thus their relationship to the different drivers are expected.

In order to accurately validate models and understand the drivers of plasma bubbles, constellation missions are necessary. Opportunistic studies and events which can make use of ad hoc constellations can help us push forward on this compelling and long unanswered science question. In addition to thinking about a constellation flying at a single altitude, satellites and/or remote sensing instruments that can probe other altitudes is necessary. As plasma bubble dynamics change significantly with altitude, it is important that we ensure missions can capture the 3-D spatial and temporal structure of these dynamics across a wide variety of scale sizes.

The orbital geometry of a single spacecraft limits in situ observations of bubbles from space. Single point measurements increase the likelihood of missing events, and orbital precession changes where events can be observed. This has led many space-based studies to focus on climatology rather than day-to-day variability.

However, there are key gaps in this existing roadmap when looking at the system science approach discussed here. Identifying the nightly existence/non-existence of bubbles globally requires a spacecraft approach, but this could take an in situ or remote approach. The GOLD spacecraft images the American and Atlantic sector nightly from a geostationary orbit at a relatively high sample rate, allowing for the identification of bubble onset times (Martinis et al., 2021). The images could be extended to multiple longitudes either through a different orbit (sacrificing revisit time for coverage) or by additional geostationary platforms.

From a satellite perspective, a very low inclination orbit ( ∼ 1 5 o ) would be desired. This allows for full coverage of the geomagnetic equator region. Pairing situ measurements with remote measurements would fill in the holes and provide information on the large-scale forcing that can enhance or suppress the growth of bubbles. A concept that implements this approach is the Geospace Observing System (listed as “FMT-4” in the LWSAC final report), which would provide two equatorial elliptical spacecraft with in situ instrumentation with a third spacecraft focused on remote measurements (Cohen et al., 2022). By pairing two elliptical orbits such that the apogee and perigee are in sync, the total satellite drag is reduced while low-altitude measurements are not compromised. This combination would provide comprehensive measurements of bubbles and their drivers.

In particular, low altitude in situ measurements of ions and neutrals would capture the bottomside formation of bubbles. Future mission concepts such as EN-LoTIS (ESA/NASA Lower Thermosphere-Ionosphere Science) could fill in the gap.

Where large missions are not planned to make required measurements, small satellites can also be used to fill in the gaps (Verkhoglyadova et al., 2021). The reduced development time could mean that the community should be planning these missions now to provide maximum impact alongside their larger counterparts.

As mentioned earlier, scintillation observations by remote sensing alone such as GNSS radio occultation (RO) may not provide accurate or spatially complete information regarding these structures. It is important that the number and orientation of RO line-of-sights through a given scintillation region be maximized. While the existing roadmap takes advantage of RO for EPB system science, the planned RO combined with in-situ observations will not adequately address the scintillation aspect of the system. However, taking advantage of commercial data buys for ionospheric RO data, small satellite hosting RO sensors, and RO sensors utilizing multiple GNSS constellations (e.g., GLONASS, Galileo), will enable EPB system science to include the generation and coupling of the smallest density structures associated with the phenomenon.

Numerical models of equatorial plasma bubbles have advanced significantly in the last 40 years (Yokoyama, 2017). One current divide is the scale sizes in the model required to capture global forcing vs. a high resolution localized model to capture bubbles development. Modeling techniques that incorporate growth rate analysis from a global perspective can be used to determine the relative importance of each energy path that modifies the equatorial regions (e.g., Carter et al., 2014b; Hysell et al., 2022; Smith and Klenzing, 2022). Another path forward is to incorporate variable grid scale ionosphere in a global model (Huba and Liu, 2020).