The sunspot cycle and “auroral” F layer irregularities

Abstract

The use of the word “aurora” for many different observations at high latitudes has limited the concepts involved; this is particularly true for F region irregularities. Observations setting the position of the auroral oval (Starkov and Fel'dshtein, 1970) were made using primarily the 555.7‐nm green line, which is emitted predominantly at E layer heights. These observations have shown that the change in position of the auroral oval for low values of Kp as a function of sunspot cycle is of the order of 1° to 2° between sunspot maximum and sunspot minimum. However, irregularities in the F region show much larger solar cycle variations in the locations of the equatorward boundary, typically 10°. A review of scintillation data indicates that at a given auroral latitude, the scintillation activity increases with sunspot number. In addition, for a constant scintillation intensity, the equatorward boundary moves to lower latitudes as sunspot maximum is approached. We review existing spread F studies and show that for quiet geomagnetic conditions, there is lower occurrence during years of low sunspot numbers than during years of high sunspot numbers. However, the spread F index, related to Δ ƒ/ƒ0F2, is higher during years of low sunspot number than during years of high sunspot number. We demonstrate that this apparent dichotomy can be reconciled by using a new method of normalizing the spread F index by the maximum electron concentration of the F layer. We briefly discuss the possible explanations for the observed solar cycle variations of irregularity occurrence in terms of the absolute values and gradients of electron concentration and the E region conductivity.

GPS phase scintillation observed over a high-latitude Antarctic station during solar minimum

“…The cases cited above may be evidence of the absence of F layer irregularities. In their studies, Aarons et al (1995) and found that during quiet magnetic conditions and particularly for years of low solar flux, F layer irregularities were more dominant than E layer irregularities in the production of scintillation. In anycase, such events emphasize the need for external energy injection into the ionosphere in order to create unstable conditions which induce the development of irregularities responsible for the production of scintillation in the auroral ionosphere.…”

“…Scintillation at high-latitude regions has been widely investigated at the GPS frequencies (Erukhimov et al, 1994;Aarons, 1997;Basu et al, 1998;Doherty et al, 2000;Smith et al, 2008). During solar maximum, highlatitude scintillation events are frequently observed in association with increased disturbed magnetic conditions, but are rarely observed during solar minimum conditions when there is low magnetic activity (Basu et al, 1988;Aarons et al, 1995;Basu et al, 1993). The auroral oval extends both equatorwards and polewards under disturbed geomagnetic conditions and contracts under quiet geomagnetic conditions.…”

Plasma irregularities in the high‐latitude ionospheric F‐region and their diamagnetic signatures as observed by CHAMP

“…Note that our observations and those of Prikryl et al [2011] correspond to different altitude ranges. The GPS scintillation can be affected by both the E‐ and F‐layer irregularities, especially during enhanced geomagnetic activity [e.g., Aarons et al , 1995; Aarons , 1997], whereas the CHAMP/FGM detects plasma pressure irregularities at the orbit altitude (generally above the F‐region peak). Prikryl et al [2011] suggested that scintillations at nightside auroral latitudes originate from particle precipitations.…”

Monitoring of global ionospheric irregularities using the Worldwide GPS Network

“…The projected irregularity locations under such conditions only give a relative estimation in the frame of globe. Figure 6a shows a ROTI map for the period of 0000-0015 UT on January 10, 1997, prior to the storm sudden commencement ( These nighttime and daytime ionospheric irregularities may be related to the plasma density "blobs" and "patches" associated with auroral oval expansion, and dynamical as well as plasma instability processes in the cusp/cleft and polar cap regions [e.g., Basu et al, 1994;Kersley et al, 1995;Pryse et al, 1996]. It is also noticed that even though the event occurred in a year around a solar minimum, ROTI values in some regions still exceeded a level of 3.…”