Relative outflow enhancements during major geomagnetic storms – Cluster observations

Abstract

Abstract. The rate of ion outflow from the polar ionosphere is known to vary by orders of magnitude, depending on the geomagnetic activity. However, the upper limit of the outflow rate during the largest geomagnetic storms is not well constrained due to poor spatial coverage during storm events. In this paper, we analyse six major geomagnetic storms between 2001 and 2004 using Cluster data. The six major storms fulfil the criteria of Dst < −100 nT or Kp > 7+. Since the shape of the magnetospheric regions (plasma mantle, lobe and inner magnetosphere) are distorted during large magnetic storms, we use both plasma beta (β) and ion characteristics to define a spatial box where the upward O + flux scaled to an ionospheric reference altitude for the extreme event is observed. The relative enhancement of the scaled outflow in the spatial boxes as compared to the data from the full year when the storm occurred is estimated. Only O + data were used because H + may have a solar wind origin. The storm time data for most cases showed up as a clearly distinguishable separate peak in the distribution toward the largest fluxes observed. The relative enhancement in the outflow region during storm time is 1 to 2 orders of magnitude higher compared to less disturbed time. The largest relative scaled outflow enhancement is 83 (7 November 2004) and the highest scaled O + outflow observed is 2 × 10 14 m −2 s −1 (29 October 2003).

O+Escape During the Extreme Space Weather Event of 4–10 September 2017

“…Similarly, Slapak et al () made a statistical study using high‐altitude plasma mantle and magnetosheath Cluster data in order to quantify the total O + escape as a function of Kp. Complementary to that study, Schillings et al () presented that the escape rate during extreme geomagnetic storms could be higher than what a linear extrapolation of the results of Slapak et al () would predict. As the whole magnetosphere is affected by disturbed magnetospheric conditions, the O + outflow that does not escape into the solar wind is transported to the lobes through the polar cap.…”

“…Figure a shows that the local magnetic field varied slowly and was weaker in the regions marked as polar cap (PC1 and PC2), while it was stronger and variable in the cusp (Cusp 1 and Cusp 2). Figures b–e correspond to O + and H + energy and PA spectrograms, respectively (for more details see Schillings et al, ). The color bar represents counts per second, which is proportional to the particle differential energy flux, and the dashed (pink) rectangles distinguish the polar caps from the cusps.…”

“…These extreme events caused a strong response in the ionosphere and magnetosphere, for example, variations in the total electron content and enhanced magnetospheric convection. An additional effect is the strongly enhanced ion outflow from the polar ionosphere (Schillings et al, ).…”

Energy conversion through mass loading of escaping ionospheric ions for different Kp values

“…4b with the red dashed line. Schillings et al (2017) found such an effect for Kp ≥ 8+, while future studies will be needed to establish the geomagnetic activity level (including Kp) when such a nonlinearity appears. The second possible factor to add a nonlinear effect in Eq.…”

“…(7). Since the Kp-escape relation and the solar wind-Kp relation is reliable only up to Kp = 5 − 6, the estimation for Kp > 6 is given in the dashed line, where we have ignored the effect of the nonlinearly enhanced ion flux for extreme high Kp (≥ 8+) as reported by Schillings et al (2017). The red dashed line illustrates possible enhancement after adding such an effect for extreme high Kp.…”