Oscillation of the Ionosphere at Planetary‐Wave Periods

Abstract

F‐region ionospheric oscillations at planetary‐wave (PW) periods (2–20 days) are investigated, with primary focus on those oscillations transmitted to the ionosphere by PW modulation of the vertically propagating tidal spectrum. Tidal effects are isolated by specifically designed numerical experiments performed with the National Center for Atmospheric Research thermosphere‐ionosphere‐electrodynamics general circulation model for October 2009, when familiar PW and tides are present in the model. Longitude versus day‐of‐month perturbations in topside F‐region electron density (Ne) of order ±30–50% at PW periods occur as a result of PW‐modulated tides. At a given height, these oscillations are mainly due to vertical oscillations in the F layer of order ±15–40 km. These vertical movements are diagnosed in terms of changes in the F2‐layer peak height, ΔhmF2, which are driven by the vertical projections of E × B drifts and field‐aligned in situ neutral winds. E × B drifts dominate at the magnetic equator, while the two sources play more equal roles between 20° and 40° magnetic latitudes in each hemisphere. The in situ neutral wind effect arises from vertical propagation of PW‐modulated tides, whereas the E × B drifts originate from dynamo‐generated electric fields produced by the E‐region component of the same wind field; the former represents a new coupling mechanism for production of ionospheric oscillations at PW periods. Roughly half the above‐mentioned variability in Ne and hmF2 is associated with zonally symmetric (S0) oscillations, which contribute at about half the level of low‐level magnetic activity during October 2009. The thermosphere‐ionosphere‐electrodynamics general circulation model simulates the S0 oscillations in Ne observed from the CHAMP satellite well during this period and reveals that S0 oscillations in E × B play a significant role in driving S0 oscillations in ΔhmF2, in addition to neutral winds.

Lunar Tide in the F Region Ionosphere

“…The trans‐equatorial winds are consistent with a solar‐driven summer‐to‐winter circulation, but may also be modified by field‐aligned drifts due to in situ winds associated with the lunar tide that propagates into the upper thermosphere (e.g., Forbes et al, ). At higher latitudes, the situation is reversed, with E × B drifts playing a secondary role to that of field‐aligned drifts (see Forbes et al, , for an analogous situation with solar tides). In particular, as shown in Forbes et al (), the appearance of maxima in one hemisphere and/or the other is dependent on the relative amplitudes of the symmetric and antisymmetric modes of M 2 propagating upward from the lower atmosphere, the latter being produced by interaction between the main symmetric mode characterizing the lunar forcing and the asymmetric mean wind field in the middle atmosphere.…”

“…However, we do note some differences in amplitude between 20 • S and 20 • N, and phases are substantially different between hemispheres at 45 • magnetic latitude during a given analysis interval, or between analysis intervals within a given hemisphere. Such differences are possible signatures of field-aligned neutral winds driving Ne variations, as noted by Stening et al (1999) and Pedatella and Maute (2015) for the lunar tide and Forbes et al (2018) for solar tides.…”

“…For reasons explained below, this 9-year interval is divided into two 5-year analysis intervals as shown in Figure 1 The in situ measurements provided by CHAMP hold an advantage over height-integrated quantities like TEC. As shown by Forbes et al (2018) for solar tides and Stening et al (1999) and Pedatella and Maute (2015) for lunar tides, a significant portion of the ionospheric variability in response to tides exists in the form of vertical excursions of the F layer due to tidal-driven field-aligned winds and E × B drifts. These effects are especially manifested at a fixed height above or below h m F 2 but significantly muted in TEC when the F layer shape does not undergo much change.…”

Whole Atmosphere Coupling on Intraseasonal and Interseasonal Time Scales: A Potential Source of Increased Predictive Capability

“…There are several possible mechanisms advanced in the literature through which PW‐induced oscillations reach the ionosphere (see, e.g., Forbes, , and Forbes et al, , and references therein); these mechanisms include (1) direct penetration into the E ‐region wind‐dynamo region where the PWs modulate the electric fields that drive plasma drifts in the F ‐region ionosphere resulting in the modulation of the equatorial ionization anomaly (EIA); (2) modulation of tides in the MLT region that reach the dynamo region and extend into the F region, inducing oscillations in the vertical plasma drifts as well as field‐aligned ion motion; (3) secondary PWs in the dynamo region due to the dissipation of PW‐modulated GWs; and (4) PW‐induced changes in thermospheric composition.…”

“…If planetary‐scale waves modulate the tides, then direct tidal effects on the F ‐region winds might also account for the observed planetary‐scale oscillations. Using TIME‐GCM with lower boundary forcing (approximately 30‐km altitude) provided by 3‐hourly output from the Modern‐Era Retrospective Analysis for Research and Applications (MERRA) and TIE‐GCM (the Thermoshere‐Ionosphere‐Electrodynamics General Circulation Model) Forbes et al () showed that both are important. This result is consistent with earlier work by England et al (), who showed that in addition to coupling through the E ‐region dynamo, tides couple to the ionosphere through F ‐region winds and changes in thermospheric neutral composition ([O]/[N 2 ]).…”