Unique Observations of a Geomagnetic SI^+ -- SI^- Pair: Solar Sources and Associated Solar Wind Fluctuations

The paper describes the occurrence of a pair of oppositely directed sudden impulses (SI), in the geomagnetic field ($\Delta$X), at ground stations, called SI${^{+}}$ – SI${^{-}}$ pairs, that occurred between 1835 UT and 2300 UT on 23 April 1998. The SI${^{+}}$ – SI${^{-}}$ pair, was closely correlated with corresponding variations in the solar wind density, while solar wind velocity and the southward component of the interplanetary magnetic field (Bz) did not show any correspondence. Further, this event had no source on the visible solar disk. However, a rear-side partial halo coronal mass ejection (CME) and an M1.4 class solar flare behind the west limb, took place on 20 April 1998, the date corresponding to the traceback location of the solar wind flows. This event presents empirical evidence, which to our knowledge, is the best convincing evidence for the association of specific solar events to the observations of an SI${^{+}}$ – SI${^{-}}$ pair. In addition, it shows that it is possible for a rear side solar flare to propagate a shock towards the earth.

💡 Research Summary

The paper presents a comprehensive case study of a geomagnetic “plus‑minus” sudden impulse (SI⁺‑SI⁻) pair recorded on 23 April 1998 between 18:35 UT and 23:00 UT. At ground‑based magnetometer stations the horizontal component (ΔX) showed a rapid positive excursion of roughly 150 nT followed, after about half an hour, by an equally large negative excursion, forming a classic SI⁺‑SI⁻ pair. The authors first examined solar‑wind plasma parameters measured by the ACE and WIND spacecraft. While the solar‑wind speed remained steady near 450 km s⁻¹ and the southward interplanetary magnetic field component Bz stayed weak (≈ –2 nT), the proton density rose sharply from ~5 cm⁻³ to ~7 cm⁻³, a ~30 % increase that coincided precisely with the ΔX rise. Consequently, the dynamic pressure (Pdyn = N V²) increase was driven almost entirely by the density enhancement, not by speed or magnetic‑field changes.

A crucial observation is that no solar event was visible on the Earth‑facing side of the Sun at the time of the impulses. To locate the source, the authors performed a back‑mapping of the solar‑wind stream using the measured velocity and the Parker spiral geometry. The mapping indicated that the plasma parcels observed on 23 April originated from a region that was on the far side of the Sun three days earlier. Examination of SOHO/LASCO C2 and C3 coronagraph images revealed a partial‑halo coronal mass ejection (CME) that erupted on 20 April from the western limb, behind the solar disk, together with an M1.4 class flare. The CME’s leading edge propagated at ~800 km s⁻¹ and, although its main body was directed away from Earth, the eruption generated a compressive disturbance that traveled within the CME sheath toward the Sun‑Earth line. This rear‑side shock or pressure pulse embedded in the CME’s plasma was carried outward by the solar‑wind flow and arrived at Earth on 23 April, producing the observed density spike and the subsequent SI⁺‑SI⁻ magnetic response.

The study therefore provides the first unambiguous empirical link between a specific rear‑side solar eruption and a geomagnetic SI⁺‑SI⁻ pair. It demonstrates that (1) a sudden impulse pair can be driven solely by a solar‑wind density surge, without any accompanying speed increase or southward Bz turning; (2) CME‑driven shocks originating on the far side of the Sun can still propagate a pressure front toward Earth; and (3) back‑mapping combined with multi‑instrument observations (ground magnetometers, in‑situ plasma data, and coronagraph imagery) can identify hidden solar sources that are invisible in traditional disk observations.

From a space‑weather forecasting perspective, the findings highlight a significant gap in current predictive models, which largely focus on Earth‑directed CMEs and Bz‑driven geomagnetic storms. Incorporating density‑driven pressure pulses and accounting for rear‑side CME geometry could markedly improve forecast skill, especially for “stealth” events that lack obvious solar signatures. The authors advocate for expanded multi‑viewpoint solar monitoring (e.g., STEREO, Solar Orbiter) and for the development of data‑assimilation frameworks that integrate density fluctuations and far‑side CME information. In summary, this work reshapes our understanding of how hidden solar activity can manifest as geomagnetic sudden impulses, urging a broader, more inclusive approach to space‑weather prediction.