Context. It is well known that the polarity of the Sun's magnetic field reverses or flips around the maximum of each 11 year solar cycle. This is commonly known as polar field reversal and plays a key role in deciding the polar field strength at the end of a cycle, which is crucial for the prediction of the upcoming cycle. Aims. To investigate solar polar fields during cycle 24, using measurements of solar magnetic fields in the latitude range 55 • -90 • and 78 • -90 • , to report a prolonged and unusual hemispheric asymmetry in the polar field reversal pattern in solar cycle 24. Methods. This study was carried out using medium resolution line-of-sight synoptic magnetograms from the magnetic database of the National Solar Observatory at Kitt Peak (NSO/KP), USA for the period between February 1975 and October 2017, covering solar cycles 21 -24 and high-resolution line-of-sight synoptic magnetograms from the Michaelson Doppler Imager instrument onboard the Solar Heliospheric Observatory. Synoptic magnetograms using radial measurements from the Heliospheric Magnetic Imager instrument onboard the Solar Dynamics Observatory, covering solar cycle 23 and 24, were also used. Results. We show that the Southern solar hemisphere unambiguously reversed polarity in mid-2013 while the reversal in the field in the Northern solar hemisphere started as early as June 2012, was followed by a sustained period of near-zero field strength lasting until the end of 2014, after which the field began to show a clear rise from its near-zero value. While this study compliments a similar study carried out using microwave brightness measurements (Gopalswamy et al. 2016) which claimed that the field reversal process in cycle 24 was completed by the end of 2015, our results show that the field reversal in cycle 24 was completed earlier i.e. in late 2014. Signatures of this unusual field reversal pattern were also clearly identifiable in the solar wind, using our observations of interplanetary scintillation at 327 MHz which supported our magnetic field observations and confirmed that the field reversal process was completed at the end of 2014.

On 7 January 2005 (Ap=40) prompt penetration electric field perturbations of opposite polarities were observed over Thumba and Jicamarca on a few occasions during 13:45–16:30 UT. However, the electric field was found to be eastward during 14:45–15:30 UT over both Thumba and Jicamarca contrary to the general expectation wherein opposite polarities are expected at nearly antipodal points. On closer scrutiny, three important observational features are noticed during 14:10–15:15 UT. First, during 14:10–14:45 UT, despite increasing southward interplanetary magnetic field (IMF) Bz condition, the already westward electric field over Thumba weakened (less westward) while the eastward electric field over Jicamarca intensified (more eastward). Second, the electric field not only became anomalously eastward over Thumba but also got intensified further during 14:45–15:00 UT similar to Jicamarca. Third, during 15:00–15:15 UT, despite IMF Bz remaining steadily southward, the eastward electric field continued to intensify over Thumba but weakened over Jicamarca. It is suggested that the changes in IMF By component under southward IMF Bz condition are responsible for skewing the ionospheric equipotential patterns over the dip equator in such a way that Thumba came into the same DP2 cell as that of Jicamarca leading to anomalous electric field variations. Magnetic field measurements along the Indian and Jicamarca longitude sectors and changes in high‐latitude ionospheric convection patterns provide credence to this proposition. Thus, the present investigation shows that the variations in IMF By are fundamentally important to understand the prompt penetration effects over low latitudes.

This investigation shows that the significant electric field disturbances in the dip‐equatorial ionosphere during the geomagnetic storm of 6–8 September 2017 are due to the passage of two consecutive interplanetary coronal mass ejections (ICMEs). During the passage of the first ICME sheath, a long duration (∼10 hr) prompt penetration (PP) event is operational in which 60‐min periodic component is found to be present in vertical drift as well as in equatorial electrojet, but the 45‐min periodicity, though present, is not significant in equatorial electrojet. On 8 September, the shock associated with the second ICME enhances the F region vertical plasma drift to ∼150 m/s in the evening hours which is one of the highest vertical drift ever measured over Jicamarca. The same PP electric field causes unusually large enhancement of the equatorial electrojet strength to ∼135 nT in the early morning hours over the Philippine sector. The disturbance dynamo (DD) that follows the storm causes an upward vertical drift of ∼55 m/s during postmidnight hours over Jicamarca which is one of the highest observed. These unusually large electric field perturbations cause significant changes in the F region plasma fountain. It is shown that these electric field perturbations cannot be accounted by PP/DD electric field associated with the geomagnetic storm only and significant contribution from substorm is conspicuous. Therefore, the present investigation highlights the need to evaluate the role of substorm in unusually large electric field perturbations over equatorial ionosphere.

Coordinated digisonde and OI 630.0 nm airglow observations from Thumba (TVM), an Indian dip equatorial station, in conjunction with magnetic and geosynchronous particle flux measurements, reveal three different types of electric field disturbances in the equatorial ionosphere-thermosphere system (ITS) occurring in succession over a period of 6 h on a single night (22-23 January,2012; A p = 24). These include (1) westward electric field perturbations owing to a pseudo-breakup and a substorm event, each lasting for about 30 min; (2) eastward electric field perturbations continuing for about an hour, owing to the southward excursion of Z component of interplanetary magnetic field (B z ); and (3) DP2-type fluctuating (period ∼40 min) electric field perturbation sustaining for about 4 h. The pseudo-breakup and the fully grown substorm events are found to be longitudinally localized and different in terms of response in the westward auroral electrojet index (AL) as well as geosynchronous electron/proton injections. The polarity of the prompt penetration of interplanetary electric field that affects the equatorial ionosphere is observed to be eastward during 2100-2200 IST (Indian Standard Time) which is observationally sparse but consistent with modeling studies. Interestingly, on the same night, DP2-type electric field fluctuations with ∼40 min periodicity and occasional eastward polarity (akin to daytime) are also found to affect the equatorial ITS for about 4 h (2200-0200 IST). The case study, thus, brings out different processes that constitute a long duration prompt penetration event which, otherwise, would have been categorized as a single event.

A total of 43 Corotating Interaction Region (CIR)‐induced geomagnetic storms during the unusually deep solar minimum of solar cycle 23 (2006–2010) were identified using a superposed epoch analysis technique. Of these 43 events, detailed cross‐spectrum analyses, between the variations in the Z component of the interplanetary magnetic field (IMF Bz) and the equatorial electrojet (EEJ) strength, were performed for 22 events when the daytime EEJ strengths from Jicamarca were available. The analyses revealed that the ∼30 and ∼60 min periodic components in IMF Bz were causally related to the EEJ strength subject to the average solar wind flow being radial to within 6° at L1 during the interval for which EEJ strengths were considered. This investigation elicits the important role of average solar wind azimuthal flow angle in determining the geoeffectiveness of CIR events.

Before the onset of a geomagnetic storm on 22 January 2012 (Ap = 24), an enhancement in solar wind number density from 10/cm3 to 22/cm3 during 0440–0510 UT under northward interplanetary magnetic field (IMF Bz) condition is shown to have enhanced the high‐latitude ionospheric convection and also caused variations in the geomagnetic field globally. Conspicuous changes in ΔX are observed not only at longitudinally separated low‐latitude stations over Indian (prenoon), South American (midnight), Japanese (afternoon), Pacific (afternoon) and African (morning) sectors but also at latitudinally separated stations located over high and middle latitudes. The latitudinal variation of the amplitude of the ΔX during 0440–0510 UT is shown to be consistent with the characteristics of prompt penetration electric field disturbances. Most importantly, the density pulse event caused enhancements in the equatorial electrojet strength and the peak height of the F layer (hmF2) over the Indian dip equatorial sector. Further, the concomitant enhancements in electrojet current and F layer movement over the dip equator observed during this space weather event suggest a common driver of prompt electric field disturbance at this time. Such simultaneous variations are found to be absent during magnetically quiet days. In absence of significant change in solar wind velocity and magnetospheric substorm activity, these observations point toward perceptible prompt electric field disturbance over the dip equator driven by the overcompression of the magnetosphere by solar wind density enhancement.

The responses of two High-Intensity Long-Duration Continuous AE Activity (HILDCAA) events are investigated using solar wind observations at L1, magnetospheric measurements at geosynchronous orbit, and changes in the global ionosphere. This study provides evidence of the existence of quasi-periodic oscillations (1.5–2 h) in the ionospheric electric field over low latitudes, total electron content at high latitudes, the magnetic field over the globe, energetic electron flux and magnetic field at geosynchronous orbit, geomagnetic indices (SYM-H, AE, and PC) and the Y-component of the interplanetary electric field (IEFy) during the HILDCAA events at all local times. Based on detailed wavelet and cross-spectrum analyses, it is shown that the quasi-periodic oscillation of 1.5–2 h in IEFy is the most effective one that controls the solar wind–magnetosphere–ionosphere coupling process during the HILDCAA events for several days. Therefore, this investigation for the first time, shows that the HILDCAA event affects the global magnetosphere–ionosphere system with a “quasi-resonant” mode of oscillation. Graphical Abstract

On 13 April 2013, the ACE spacecraft detected arrival of an interplanetary shock at 2250 UT, which is followed by the passage of the sheath region of an interplanetary coronal mass ejection (ICME) for a prolonged (18-hr) period. The polarity of interplanetary magnetic field Bz was northward inside the magnetic cloud region of the ICME. The ring current (SYM-H) index did not go below −7 nT during this event suggesting the absence of a typical geomagnetic storm. The responses of the global ionospheric electric field associated with the passage of the ICME sheath region have been investigated using incoherent scatter radar measurements of Jicamarca and Arecibo (postmidnight sector) along with the variations of equatorial electrojet strength over India (day sector). It is found that westward and eastward prompt penetration (PP) electric fields affected ionosphere over Jicamarca/Arecibo and Indian sectors, respectively, during 0545-0800 UT. The polarities of the PP electric field perturbations over the day/night sectors are consistent with model predictions. In fact, DP2-type electric field perturbations with ∼40-min periodicity are found to affect the ionosphere over both the sectors for about 2.25 hr during the passage of the ICME sheath region. This result shows that SYM-H index may not capture the full geoeffectivenss of the ICME sheath-driven storms and suggests that the PP electric field perturbations should be evaluated for geoeffectiveness of ICME when the polarity of interplanetary magnetic field Bz is northward inside the magnetic cloud region of the ICME.