Phase reversal of the diurnal cycle in the midlatitude ionosphere

Abstract

[1] The typical diurnal cycle of the midlatitude F region electron density consists of a midday maximum and a midnight minimum. However, a phase reversal of this diurnal cycle has been found to occur in three distinct regions on the globe. They are the East Asian (EA) region centered around (53°N, 150°E), the Northern Atlantic (NA) region centered around (45°N, 50°W) and the South Pacific (SP) region centered around (60°S, 110°W). The intensively reported Weddell Sea Anomaly falls inside the SP region. The phase reversal occurs during March-August in EA and NA regions, and between August and March in SP region, being most prominent in local summer. Furthermore, this diurnal anomaly is more pronounced at solar minimum than at solar maximum, and more pronounced in SP region than in NA and EA regions, in terms of larger diurnal magnitude and more months it lasts in a year. It is emphasized that the diurnal anomaly consists of not only a nighttime enhancement, but also a concurrent noontime depletion. Hence, midlatitude summer nighttime enhancements reported in previous studies are just part of this reversed diurnal cycle. The cause for the phase reversal involves several interplaying physical processes. Among these, the neutral wind combined with the geomagnetic field configuration plays a pivotal role. It generates a one-wave longitudinal pattern at southern middle latitudes and a two-wave pattern at northern middle latitudes, whose wave peaks correspond to the center of the SP, EA, and NA regions, respectively. The seasonal variation of neutral winds and downward motion of the ionization induced by thermal contraction of the ionosphere at sunset may largely control the occurring local time of the nighttime density enhancement and how long it persists in different months. The phase reversal occurs as a result of close ion-neutral coupling. It is further noted that winter anomaly in the EA, NA, and SP regions is very weak or missing.Citation: Liu, H., S. V. Thampi, and M. Yamamoto (2010), Phase reversal of the diurnal cycle in the midlatitude ionosphere,

Weakening of the mid-latitude summer nighttime anomaly during geomagnetic storms

“…The SP region in the southern hemisphere has long been recognized and referred to as the "Weddell Sea Anomaly" (WSA) since 1950s (Bellchambers and Piggott, 1958;Dungey, 1961;Horvath and Essex, 2003;Horvath, 2006;Jee et al, 2009). The EA and NA in the northern hemisphere are revealed by recent satellite and ground observations (Liu et al, 2007(Liu et al, , 2010Lin et al, 2009;Thampi et al, 2009). The midlatitude summer nighttime anomaly (MSNA) is now generally used to refer to this phenomenon in both northern and southern hemispheres.…”

“…To facilitate easy comparison with results under quiet conditions (K p ≤ 3) reported in Liu et al (2010), we make use of the same in-situ measurements of electron density N e from the Planar Langmiur Probe (PLP) on board CHAMP satellite for disturbed conditions (K p ≥ 4). Data period covers from January 1, 2001 to December 31, 2006.…”

“…Furthermore, a normalization of the data to a common altitude of 400 km has been applied to possibly exclude variations induced by the orbit decay. These data period and analysis procedure are identical to that for quiet conditions described in Liu et al (2010). CHAMP is in a near-circular orbit with an inclination of 87.3…”

“…• W) (Liu et al, 2010). The SP region in the southern hemisphere has long been recognized and referred to as the "Weddell Sea Anomaly" (WSA) since 1950s (Bellchambers and Piggott, 1958;Dungey, 1961;Horvath and Essex, 2003;Horvath, 2006;Jee et al, 2009).…”

“…Among these, the neutral wind component in the geomagnetic meridian, combined with the later sunset in summer, play dominant roles as comprehensively discussed in He et al (2009) and Liu et al (2010). At middle latitudes, this meridional component (in geomagnetic Copyright c The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.…”