IMF effect on sporadic-E layers at two northern polar cap sites: Part II – Electric field

Abstract

Abstract. This paper is the second in a series on a study of the link between IMF and sporadic-E layers within the polar cap. In Paper I (Voiculescu et al., 2006), an analysis of the sporadic-E data from Thule and Longyearbyen was presented. Here we concentrate on the electric field mechanism of sporadic-E generation. By means of model calculations we show that the mechanism is effective even at Thule, where the direction of the geomagnetic field departs from vertical only by 4 • . The model calculations also lead to a revision of the electric field theory. Previously, a thin layer was assumed to grow at a convergent null in the vertical ion velocity, which is formed when the electric field points in the NW sector. Our calculations indicate that in the dynamic process of vertical plasma compression, a layer is generated at altitudes of high vertical convergence rather than at a null. Consequently, the layer generation is less sensitive than previously assumed to fluctuations of the electric field direction within the NW sector. The observed diurnal variations of sporadic-E occurrence at Longyearbyen and Thule are compared with the diurnal variations of the electric field, calculated using a representative range of IMF values by means of the statistical APL model. The results indicate that the main features of Es occurrence can be explained by the convection pattern controlled by the IMF. Electric fields calculated from the IMF observations are also used for producing distributions of sporadic-E occurrence as a function of electric field direction at the two sites. A marked difference between the distributions at Thule and Longyearbyen is found. A model estimate of the occurrence probability as a function of electric field direction is developed and a reasonable agreement between the model and the experimental occurrence is found. The calculation explains the differences between the distributions at the two sites in terms of the polar cap convection pattern. The conclusion is that the electric field is the maCorrespondence to: T. Nygrén (tuomo.nygren@oulu.fi) jor cause for sporadic-E generation and, consequently, IMF has a clear control on the occurrence of sporadic E within the polar cap.

Magnetic signatures of medium‐scale traveling ionospheric disturbances as observed by CHAMP

“…In order to form a closed current loop MMF‐related FACs need a conducting region in the conjugate ionospheric E region. A sporadic E (Es) layer, where the plasma density is enhanced because of the vertical ion compression [ Nygrén et al , 2006], can act as a conducting path for the FAC closure. According to the global Es climatology of Arras et al [2008], it chiefly occurs at the midlatitude E region.…”

Localized mesosphere-stratosphere-troposphere radar echoes from theEregion at 69°N: Properties and physical mechanisms

“…While this wind shear mechanism was originally proposed to explain the occurrence of sporadic E layers at midlatitudes where the Earth's magnetic field has a large horizontal component [ Whitehead , 1960], it has recently been shown by Nygrén et al [2008] that even a small horizontal component of the magnetic field can be sufficient for the wind shear mechanism to be active. This is also the case for instance at polar latitudes, where it is generally thought that electric fields are mainly responsible for sporadic E generation [e.g., Nygrén et al , 1984; Kirkwood and Nilsson , 2000; Voiculescu et al , 2006; Nygrén et al , 2006]. Using a combination of observations with the EISCAT Svalbard Radar and numerical modeling, Nygrén et al [2008] were able to demonstrate that depending on the specific situation, sporadic E generation in the polar cap can be mainly attributed to the wind shear mechanism, to the electric field mechanism, or to the combined effect of both.…”

Simultaneous Observations of a Polar Cap Sporadic‐E Layer by Twin Incoherent Scatter Radars at Resolute

“…In literature, several models have been proposed to explain the Es layer at high latitudes (e.g., Bristow & Watkins, 1991; Kirkwood & Collis, 1989; MacDougall & Jayachandran, 2005; MacDougall et al., 2000b; Nygrén et al., 2006), here some of which will be discussed in detail. First, the high‐frequency appearance of the Es layer over a “cusp latitude” station is reasonably explained by the two‐step mechanisms, including the horizontal convergence of ionization by the electric field of convection reversal at first, and then vertical convergence of these ionizations by the electric field in the polar cap (e.g., Chen et al., 2021; MacDougall & Jayachandran, 2005).…”