The Whole Heliosphere and Planetary Interactions (WHPI) is an international initiative to study the most recent solar minimum and its impact on the interconnected solar-heliospheric-planetary system by facilitating and encouraging interdisciplinary activities. Particular WHPI science foci include the global connected structure of the heliosphere and planetary space environments/atmospheres, the origins and impacts of high-speed solar wind streams, coronal mass ejections (CMEs) from Sun-to-Heliopause, and comparative solar minima. This is achieved through a series of coordinated observing campaigns, including Parker Solar Probe perihelia, and scientific virtual interactions including a dedicated workshop where observers and modelers gathered to discuss, compare, and combine research results. This introduction sets the scene for the WHPI interval, placing it into the context of prior initiatives and describing the overall evolution of the system between 2018-2020. Along with the accompanying articles, it presents a selection of key scientific results on the interconnected solar-heliospheric-planetary system at solar minimum.

The responses of Earth’s and Mars’ thermospheric densities to quasi-periodic solar rotation variations in flux were measured contemporaneously by the MAVEN, GOES, and Swarm-C satellites. While large solar rotation variability is found in both planetary thermospheres, correlation analyses performed on 6+ years of data reveal that, independently of flux level, Earth's daytime density response is about 10-50% larger than Mars' at a similar density level. Important altitude dependencies in the sensitivities are found in the Martian thermosphere, while the terrestrial thermosphere is shown to exhibit only small (±5%) day/night and latitude variations in the response. Detailed analyses focused on correlative periods in 2015-2016 and 2020 indicate important solar cycle effects in the sensitivities of both planetary thermospheres, with increased slopes with low solar flux. These results provide important new insights into processes relevant to the interpretation of the sources of short-term density variability in Mars’ and Earth's thermospheres associated with solar drivers and point to the need for targeted modeling efforts along with dedicated data analyses to help resolve current unknowns in thermal balance processes.

Using ICON and GOLD satellite observations, the response of the thermospheric daytime horizontal winds and neutral temperature to the 2020/2021 major sudden stratospheric warming (SSW) is studied at low- to middle latitudes (0° - 40°N). Comparison with observations during the non-SSW winter of 2019/2020 and the pre-SSW period (December 2020) clearly demonstrates the SSW-induced changes. The northward and westward thermospheric winds are enhanced during the warming event, while temperature around 150 km drops by up about 50 K compared to the pre-SSW phase. Changes in the horizontal circulation during the SSW can generate upwelling at low-latitudes, which can contribute to the adiabatic cooling of the low-latitude thermosphere. The observed changes during the major SSW are a manifestation of long-range vertical coupling in the atmosphere.

A key element of successful aerobraking operations at Mars is accurate thermospheric density predictions. Evidence suggests that much of the longitude variability in Mars’ aerobraking region is associated with atmospheric tides, and the day-to-day variability is connected with tidal modulation by longer-period global-scale waves. Specifically, ultra-fast Kelvin waves (UFKWs) and their modulation of the tidal spectrum play a key role in coupling Mars’ lower ($<$$\sim$80 km) and middle ($\sim$80-100 km) atmosphere with the aerobraking region above. In this study, over 5 years of Mars Atmosphere and Volatile Evolution (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS) CO$_2$ density observations are employed to reveal prominent, frequent, and persistent 2.5- to 4.5-day UFKW packets in the whole Martian middle and upper thermosphere (ca. 150-200 km), and large secondary waves arising from their nonlinear interactions with the tidal spectrum. Detailed analyses focusing on a prominent $\sim$2.5-day UFKW event in late 2015 demonstrate primary and secondary wave amplitudes growing twofold with altitude from $\sim$7-14\% near 150 km to $\sim$12-25\% near 200 km and their combined effects to account for $\sim$60-80\% of the altitude-longitudinal variability of Mars’ thermospheric density. Concurrent temperature measurements from Mars Reconnaissance Orbiter (MRO) Mars Climate Sounder (MCS) reveal consistent wave signatures near 80 km altitude suggesting propagation of both primary and secondary waves from the lower atmosphere. This study demonstrates that UFKWs and secondary waves from UFKW-tide interactions are sources of significant altitude-longitude variability in the Mars’ aerobraking region that should be accounted for when analyzing satellite observations and nonlinear models.

A document by Federico Gasperini. Click on the document to view its contents.

Sudden stratospheric warmings (SSWs) are large-scale phenomena characterized by dramatic dynamic disruptions in the stratospheric winter polar regions. Previous studies, especially those employing whole atmosphere models, indicate that SSWs have strong impacts on the circulation of the mesosphere lower thermosphere (MLT) and drive a reversal in the mean meridional circulation (MMC) near 90-125 km altitude. However, the robustness of these effects and the roles of SSW-induced changes in global-scale wave activity to drive the reversal have been difficult to observe simultaneously. This work employs horizontal lower thermospheric (~93-106 km altitude) winds near 10S-40N latitude from the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument onboard the Ionospheric Connection Explorer (ICON) to present observational evidence of a prominent MLT MMC reversal associated with the January 2021 major SSW event and to demonstrate connections to semidiurnal tidal activity and possible associations with a ~3-day ultra-fast Kevin wave (UFKW).

It is now well established that waves generated in the lower atmosphere can propagate upward and significantly impact the dynamics and mean state of the ionosphere-thermosphere (IT, 100-600 km) system. Given the geometry of magnetic field lines near the equator, a significant fraction of this IT coupling occurs at low latitudes and is driven by global-scale waves of tropical tropospheric origin, such as the diurnal eastward-propagating tide with zonal wavenumber 3 (DE3) and the ultra-fast Kelvin wave (UFKW). Despite recent progress, lack of coincident global observations has thus far precluded full characterization of the sources of day-today variability of these waves, including nonlinear interactions, and impacts on the low-latitude IT. In this work, in-situ ion densities from Ionospheric Connection Explorer (ICON) and Constellation Observing System for Meteorology, Ionosphere and Climate 2 (COSMIC-2) Ion Velocity Meter (IVM) along with remotely-sensed zonal winds from ICON Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) are used to reveal a rich spectrum of waves coupling the lower (∼90-105 km) and middle (∼200-270 km) thermosphere with the upper F-region (∼540 and ∼590 km) ionosphere. Spectral analyses for a 40-day period of similar local time demonstrate prominent IT coupling via DE3, a 3-day UFKW, and the two ∼1.43-day and ∼0.77-day secondary waves from their nonlinear interactions. While all these waves are found to dominate the F-region spectra, only the UFKW and the 1.43-day secondary wave can propagate to ∼270 km suggesting E-region wind dynamo processes as major contributors to their observed ionospheric signatures.