A 27‐month periodicity in the low latitude geomagnetic field and its connection to the stratospheric QBO

Abstract

Geomagnetic daily variations are produced at a height of about 100 to 130 km due to dynamo action of thermospheric winds in combination with the ionospheric conductivity. They are influenced by longterm variability of thermospheric dynamics and are used as tool for investigating a QBO in thermospheric winds. Power spectrum analysis has been applied to monthly means of the daily range of the geomagnetic field. In addition to an annual modulation (and subharmonics) there is clear evidence for a sharp peak at about 27 months in the H‐component at low‐latitude observatories (ΔH ≈ 0.5 nT). At single observatories, statistical significance is at the 99% confidence level, squared coherency between distant observatories is highly significant, too. However, there is no global modulation of the Sq current system because (1) the D‐component contain no 27‐month periodicity, and (2) the oscillation is not present at mid‐latitude observatories. In order to explain the observations, a 27‐month oscillation in the low‐latitude prevailing zonal winds with an amplitude of few m/s at a height of about 100‐130 km is postulated. Squared Coherency between geomagnetic variations and prevailing winds in the equatorial stratosphere is significant at the 95%‐level.

Geomagnetic lunar and solar daily variations during the last 100 years

“…Similarly, the quasi‐biennial oscillation (QBO) in the stratosphere and mesosphere has some influence on tidal propagation [ Hagan et al , ; Xu et al , ]. Indeed, Olsen [] and Jarvis [, ] reported small but significant QBO signatures in the S variation. The effect of ENSO and QBO on the lunar tide, however, has not been studied.…”

Planetary wave trends in the lower thermosphere—Evidence for 22-year solar modulation of the quasi 5-day wave

“…For instance, a signature of the quasi biennial oscillation exists in the amplitude of the semidiurnal geomagnetic variation at the equator (Olsen, 1994) and at a range of other latitudes (Jarvis, 1996(Jarvis, , 1997. Kohsiek et al (1995) reported 10-and 16-day oscillations, indicative of planetary wave action associated with natural resonances or Rossby normal modes of the atmosphere (Salby, 1984), in geomagnetic data from two mid-latitude stations in the northern hemisphere.…”

Thermospheric inter-annual variability and its potential connection to ENSO and stratospheric QBO

“…For instance, ionospheric foF2 at two stations was found to highly correlate with ENSO (Pedatella and Forbes 2009). The equatorial electrojet and ionospheric foF2, hmF2 were found to oscillate at periods around 26-27 months, which was attributed to the influence of the stratospheric QBO (Olsen 1994;Kane 1995). Since the thermosphere is closely coupled to the ionosphere, it is interesting to examine whether the thermosphere bears imprints of ENSO and QBO as well.…”