Atmospheric loss from the dayside open polar region and its dependence on geomagnetic activity: implications for atmospheric escape on evolutionary timescales

Abstract

Abstract. We have investigated the total O + escape rate from the dayside open polar region and its dependence on geomagnetic activity, specifically Kp. Two different escape routes of magnetospheric plasma into the solar wind, the plasma mantle, and the high-latitude dayside magnetosheath have been investigated separately. The flux of O + in the plasma mantle is sufficiently fast to subsequently escape further down the magnetotail passing the neutral point, and it is nearly 3 times larger than that in the dayside magnetosheath. The contribution from the plasma mantle route is estimated as ∼ 3.9×10 24 exp(0.45 Kp) [s −1 ] with a 1 to 2 order of magnitude range for a given geomagnetic activity condition. The extrapolation of this result, including escape via the dayside magnetosheath, indicates an average O + escape of 3 × 10 26 s −1 for the most extreme geomagnetic storms. Assuming that the range is mainly caused by the solar EUV level, which was also larger in the past, the average O + escape could have reached 10 27-28 s −1 a few billion years ago. Integration over time suggests a total oxygen escape from ancient times until the present roughly equal to the atmospheric oxygen content today.

Relative outflow enhancements during major geomagnetic storms – Cluster observations

“…This result is consistent with Yau et al (1988), who showed a Kp dependence on the O + flux at lower altitudes, and with Slapak et al (2017), who studied the oxygen ion escape from the plasma mantle and cusp and its dependence on the geomagnetic activity. Slapak et al (2017) found that in the plasma mantle and the dayside magnetosheath, the scaled O + outflow increases exponentially as exp(0.45Kp). In the plasma mantle, they observed an increase of 1.5 orders of magnitude for the scaled oxygen ion outflow between average conditions (Kp ≈ 3) and highest geomagnetic activities.…”

“…In the plasma mantle, they observed an increase of 1.5 orders of magnitude for the scaled oxygen ion outflow between average conditions (Kp ≈ 3) and highest geomagnetic activities. In comparison with Slapak et al (2017, their Fig. 4), who show the distribution of O + observations over Kp for the plasma mantle and the dayside magnetosheath, we estimate a lower scaled O + outflow, which is reasonable because our region of observations is in the polar cap.…”

“…In addition, the Halloween storm occurred near the solar maximum where increased oxygen flux has been observed in the past (Yau and Andre, 1997). This result is consistent with Yau et al (1988), who showed a Kp dependence on the O + flux at lower altitudes, and with Slapak et al (2017), who studied the oxygen ion escape from the plasma mantle and cusp and its dependence on the geomagnetic activity. Slapak et al (2017) found that in the plasma mantle and the dayside magnetosheath, the scaled O + outflow increases exponentially as exp(0.45Kp).…”

“…Indeed, the ion outflow under current major geomagnetic storms could have been the normal rate of ion outflow in the past (young sun), so ion outflow during storms may be especially important to understand atmospheric evolution on a geological timescale. Slapak et al (2017) roughly estimated the escape rate in the past. The authors extrapolated their result on the O + escape rate in the plasma mantle and dayside magnetosheath to the past and obtained a total O + loss of about 40 % of today's total oxygen mass in the atmosphere.…”

“…What the fate of these outflowing ions will be and where they will end up is not clear because of the relatively low altitude of these observations. Slapak et al (2017) carried out a similar study of Kp dependence based on high-altitude Cluster spacecraft data, trying to estimate the total atmospheric escape by looking at the plasma mantle and the magnetosheath. The authors estimate the contribution from the plasma mantle as 3.9 × 10 24 exp(0.45 Kp) s −1 .…”