Atmospheric tides are global-scale waves whose periods are an integer fraction of a solar day (Chapman & Lindzen, 1970). The tides are forced primarily by radiative and latent heating effects in the lower atmosphere (Hagan, 1996), but obtain their largest amplitudes in the mesosphere-lower-thermosphere (MLT) region (80-120 km altitude). There they are expressed as pronounced oscillations in a broad range of atmospheric fields, such as density, pressure, and wind. The migrating tides are those tides which follow the apparent motion of the sun, having a longitudinal zonal wavenumber (S) and latitudinal spherical harmonic (Hough mode) structure. In the current work, the focus lies on the migrating semidiurnal (SW2; for Semidiurnal, Westward S = 2) tide. The SW2 tidal winds maximize in the mid-and high-latitude MLT (Manson et al., 2002;Wu et al., 2011), where they form a major source of day-to-day and inter-seasonal variability of the MLT-ionosphere system (Arras et al., 2009;G. Shepherd et al., 1998;Smith, 2012). The SW2 tide is recognized as an important vertical coupling mechanism (Forbes, 2009;Pedatella & Forbes, 2010), and as a contributing factor to the vertical mixing and energy budget of the upper atmosphere (Becker, 2017;Forbes et al., 1993).The numerical study of the SW2 tide has a long history (e.g., Forbes & Garrett, 1979). Nevertheless, open questions remain about the mechanisms governing the tide's seasonal and short-term variability (

Abstract. This study uses hourly meteor wind measurements from a longitudinal array of 10 high-latitude SuperDARN high-frequency (HF) radars to isolate the migrating diurnal, semidiurnal, and terdiurnal tides at mesosphere–lower-thermosphere (MLT) altitudes. The planetary-scale array of radars covers 180∘ of longitude, with 8 out of 10 radars being in near-continuous operation since the year 2000. Time series spanning 16 years of tidal amplitudes and phases in both zonal and meridional wind are presented, along with their respective annual climatologies. The method to isolate the migrating tides from SuperDARN meteor winds is validated using 2 years of winds from a high-altitude meteorological analysis system. The validation steps demonstrate that, given the geographical spread of the radar stations, the derived tidal modes are most closely representative of the migrating tides at 60∘ N. Some of the main characteristics of the observed migrating tides are that the semidiurnal tide shows sharp phase jumps around the equinoxes and peak amplitudes during early fall and that the terdiurnal tide shows a pronounced secondary amplitude peak around day of year (DOY) 265. In addition, the diurnal tide is found to show a bi-modal circular polarization phase relation between summer and winter.

The energetic particle precipitation (EPP) indirect effect (IE) refers to the downward transport of reactive odd nitrogen (NOx = NO + NO2) produced by EPP (EPP‐NOx) from the polar winter mesosphere and lower thermosphere to the stratosphere where it can destroy ozone. Previous studies of the EPP IE examined NOx descent averaged over the polar region, but the work presented here considers longitudinal variations. We report that the January 2009 split Arctic vortex in the stratosphere left an imprint on the distribution of NO near the mesopause, and that the magnitude of EPP‐NOx descent in the upper mesosphere depends strongly on the planetary wave (PW) phase. We focus on an 11‐day case study in late January immediately following the 2009 sudden stratospheric warming during which regional‐scale Lagrangian coherent structures (LCSs) formed atop the strengthening mesospheric vortex. The LCSs emerged over the north Atlantic in the vicinity of the trough of a 10‐day westward traveling planetary wave. Over the next week, the LCSs acted to confine NO‐rich air to polar latitudes, effectively prolonging its lifetime as it descended into the top of the polar vortex. Both a whole atmosphere data assimilation model and satellite observations show that the PW trough remained coincident in space and time with the NO‐rich air as both migrated westward over the Canadian Arctic. Estimates of descent rates indicate five times stronger descent inside the PW trough compared to other longitudes. This case serves to set the stage for future climatological analysis of NO transport via LCSs.