Enhancement and HF Doppler observations of sporadic‐E during the solar eclipse of 22 July 2009

Abstract

[1] The sporadic E (Es) frequently emerging in midlatitude during summer is a very special layer in the ionosphere, and its formation mechanism is different to from that of other layers. The total solar eclipse of 22 July 2009 provided a very unique opportunity to study the relationship of Es and solar radiation. During the solar eclipse day and the days before and after, the vertical incidence ionosonde was located in Wuhan to record the ionograms for this event. Two oblique incidence high-frequency radio systems were used to record the waves from Wuhan to Suzhou and from Wuhan to Huaian. The enhancement of Es during the eclipse period was observed in the vertical and oblique incidence ionograms. The quasi-periodic fluctuations in the critical frequency and Doppler frequency shift curves indicated the possible existence of the gravity waves, which may be responsible for the Es enhancement. However, we find that the enhancement occurred earlier than the appearance of gravity waves and consider that there may be other mechanisms which contribute to the observed enhancement in the ionosphere. A hypothesis is put forward that the cooling effect of the lunar shadow induced powerful airflow from the northern and southern limits of the shadow toward its center, which accelerated the irregularities in Es to produce the large-scale Doppler shift in the reflected waves and form the meridional windshear. Both the windshear and the gravity waves may affect the Es layer and increase the electron concentration. Many observed phenomena are in accordance with this.

20 March 2015 solar eclipse influence on sporadic E layer

“…In particular, the solar eclipse did not affect at all the Es layer in terms of its maximum intensity, as it was already reported by other authors (e.g. Datta, 1973;Chen et al, 2010); in fact, the ftEs values do not show any magnitude difference if compared with those of the previous and next day (Fig. 1).…”

“…A decrease of the top frequency of Es (ftEs) was observed by Minnis (1955) and Stoffregen (1955), while an increase was reported by Datta (1973) and Chen et al (2010). Overall, it seems that the decreasing solar radiation characterizing a solar eclipse has a little direct impact on Es, even though the indirect effects of the eclipse, like the thermal gradient and the GWs, cannot be ignored (Chen et al, 2011).…”

Gravity waves and spreadEsobserved during the solar eclipse of 22 July 2009

“…First, there is a sudden increase in fEs at 09:43 LT, shortly after the third eclipse contact. The enhancement of the maximum electron concentration in Es during the solar eclipse has been explained by the supposition of eclipse‐enhanced wind shear [ Chen et al , 2010]. The subsequent wave‐like fluctuations on the fEs curve, which included five complete cycles from 10:30 to 12:45 LT of the eclipse day, is paid close attention to in this paper.…”

“…Generally, the ionospheric response to the solar eclipse is manifested as a decrease in total electron content [ Salah et al , 1986; Jakowski et al , 2008]. When the lunar shadow travels through the atmosphere during a solar eclipse, many other phenomena also have taken place, for instance, the electron density enhancement of ionospheric F 2 layer during the solar eclipse of July 1963 [ Evans , 1965]; the increasing critical frequency of sporadic E layer on the solar eclipse day of June 1955 and July 2009, respectively [ Datta , 1973; Chen et al , 2010]; a short or long time delay of the decline of foF2 responding to the solar eclipse [ Cheng et al , 1992; Adeniyi et al , 2007]; the nighttime‐like quasiperiodic (QP) echoes recorded by the middle and upper atmosphere radar (MU radar) at Shigaraki during a partial solar eclipse [ Thampi et al , 2010]; the periodic modulation of the ionospheric electron density and Doppler echoes reflected from ionosphere [ Hanuise et al , 1982; Altadill and Solé , 2001; Jones et al , 2004; Jakowski et al , 2008]. The gravity waves produced by a solar eclipse may be responsible for most of these phenomena.…”

Formation of sporadic-E (Es) layers under the influence of AGWs evolving in a horizontal shear flow

“…It was noted that the AGWs can produce horizontal convergence of ions, however for the eventual formation of the Es the existence of the horizontal wind with vertical shear is a necessary condition, which is similar to the above mentioned "windshear mechanism". Lately, various observed phenomena such as multilayered structure of Es (Wakabayashi and Ono, 2005;Yokoyama et al, 2005;Brahmanandam et al, 2013) and a quasi-periodic echoes Kelley et al, 1995;Hysell et al, 2013) were associated with a possibile influence of AGWs on the formation and modulation of Es Yokoyama et al, 2004;Chen et al, 2010). Didebulidze and Lomidze (2008; proposed a sporadic-E formation by the vortical perturbations excited in the horizontal shear flows.…”