Abstract

The behaviour of the ionospheric and field-aligned currents (FACs) during geomagnetic storms are poorly understood, partly because the main focus of research concerning the ionospheric currents have been on time-scales of substorms and because geomagnetic storms exhibit large variations from one storm to another. To address this issue, this thesis studies the temporal and spatial development of ionospheric horizontal currents and FACs during 73 geomagnetic storms (Dstmin≤ −50 nT) from 2010 to 2017 driven by high-speed streams and associated stream interaction regions (HSS/SIRs), or sheaths and magnetic clouds (MC) associated with interplanetary coronal mass ejections (ICME).

The development of the horizontal equivalent currents (Jeq) and FACs were studied using a superposed epoch analysis (SEA) of SuperMAG and AMPERE data, respectively. The zero epoch time, t0, was set to the main phase onset, defined as the time the SYM-H index in each storm crossed to less than −15 nT. Storms driven by HSS/SIR tend to begin during the dense SIR ahead of the HSS. Those storms have total FAC and Jeq that begins to slowly increase 3 hr before t0 and reach maximum at t0 + 40 min and t0 + 58 min, respectively, with total downward FAC of 8.1 MA. By separating the HSS/SIR storms into those with low and high solar wind dynamic pressure, pdyn, around t0, the total FAC, Jeq and number of substorm onsets are clearly higher for high pdyn compared to low pdyn storms for the first 6 hours after main phase onset. This situation reversed in the storm recovery phase, when after t0 + 2 days the currents and number of substorms for low pdyn storms decay slower and become larger than for high pdyn storms. This might be due to the higher solar wind velocity for low pdyn storms at this time.

The ICME storms were separated into those driven by sheaths and MCs. Sheath-driven storms have shorter main phase durations and larger currents for the first 4 hr of the storm main phase than MC-driven storms. During sheath-driven storms, the superposed currents increase rapidly during the passage of the dense sheath region and maximise at t0 + 50 min with a total downward FAC of 8.9 MA. On the contrary, the currents during MC-driven storms developed gradually as the interplanetary magnetic field (IMF) rotated southward and reached maximum at t0 + 11 hrs, close to the end of the storm main phase, with a total FAC of 8.4 MA. In general, the total FAC during the first 12 hrs of geomagnetic storms is larger for ICME-driven than HSS/SIR-driven storms, while in the recovery phase, the currents in sheath-driven storms diminishes first. Furthermore, the Russell-McPherron (RM) effect is seemingly more important for HSS/SIR than for ICME-driven storms. In total, 79% and 86% of the high and low pdyn HSS/SIR storms were affected by the RM effect, contrary to only 42% and 46% of the sheath and MC-driven storms.

This thesis also quantified the response time of the currents to solar wind driving at Earth’s magnetopause using cross-correlation between the total FAC and Newell coupling function (NCF). Using 10-min resolution data, the best correlation coefficients (CC) were found when the total FAC lagged the NCF by 40±10 min for HSS/SIR and sheath-driven storms, and 60 ± 10 min for MC-driven storms, with CCs of 0.71, 0.84 and 0.87, respectively. The best NCF integration time was also quantified. By integrating the NCF using the preceding 90 min for HSS/SIR, 80 min for sheaths and 140 min for MC storms the CCs with total FAC reached maximum values of 0.83, 0.89 and 0.91, respectively. This shows that considering a prolonged interval of solar wind coupling better describes the currents than simply using lags.