Strong Southward Research Articles

The Nighttime Horizontal Neutral Winds at Mohe Station in Response to the Temporal Oscillations of Interplanetary Magnetic Field Bz

Temporal oscillations in the IMF Bz associated with Alfvén waves occur frequently in solar wind, with a duration ranging from minutes to hours. Using Swarm observations, Fabry–Pérot interferometer measurements at Mohe station, and Thermosphere–Ionosphere–Electrodynamic General Circulation Model simulations, the perturbations of zonal (ΔUN) and meridional (ΔVN) winds due to temporal oscillations in the IMF Bz on 23–24 April 2023 are explored in the following work. ΔUN is strong westward with a speed of greater than 100 m/s at pre-midnight on 23–24 April. This phenomenon is primarily driven by the pressure gradient, offsetting by the ion drag and Coriolis force. On 23 April, ΔVN is weak northward at the pre-midnight and strong southward at a speed of ~200 m/s at pre-dawn. On 24 April, ΔVN is strong (weak) northward at pre-midnight (pre-dawn). It is mainly controlled by a balance between the pressure gradient, ion drag, and Coriolis force.

Wind influence on residual circulation in Patagonian channels and fjords

Circulation in estuaries is typically considered to be driven by horizontal density gradients, called gravitational circulation, but can be modified by other forcing such as the wind. Studies have shown that wind influence can either augment or counteract density-driven residual flows in estuaries, but the temporal and spatial extent of this influence remains unknown, particularly in deep and highly stratified fjords. This study aims to fill this gap by investigating the seasonal variability in residual circulation, wind forcing, and stratification in an extensive fjord and channel system in Chilean Patagonia. Observations of current velocity and wind are supplemented by atmospheric and hydrodynamic circulation models in the Moraleda Channel, a system that connects the fjord system of Northern Patagonia to the Pacific Ocean. Results show a marked seasonality, given that the surface outflow is reversed by wind forcing during the winter, a season characterized by strong southward (up-channel) winds. This produces a near-surface flow directed up-channel in the upper ∼40 m of the water column, in opposition to the horizontal pressure gradient. Cross-channel transects revealed that the vertical structure of the along-channel residual velocity was three-layer during the winter. The near-surface (<50 m) along-channel current on the eastern side of the channel followed the wind direction (southward). Between 50 and 100 m depth the flow was out-channel and below 100 m depth the flow was again in-channel. On the western side, the along-channel residual flow was two-layer, with near surface flow directed northward (against the wind) with a return flow below (deeper than 50 m). During the spring, northward (out-channel) winds dominated and the vertical structure of the along-channel residual velocity was two-layer, augmenting the horizontal pressure gradient, which stabilized the water column, enhancing stratification, inhibiting mixing and elevated gravitational circulation.

Three‐Dimensional Temperature Responses to Northward‐Moving Typhoons in the Shallow Stratified Yellow Sea in Summer

AbstractCharacterized by a sharp thermocline and large cold‐water mass below, the Yellow Sea (YS) in summer is a shelf sea that is one of the most vulnerable to typhoons in the world. With observations and high‐resolution numerical simulations, we investigated the three‐dimensional temperature changes and underlying dynamics during the passage of Typhoon Bavi in August 2020, which was representative of northward‐moving typhoons in the eastern YS. Bavi's cyclonic strong winds caused intense turbulent mixing and Ekman divergence. The intense vertical mixing generated spatially coherent temperature cooling in the surface layer and partly caused temperature warming in the subsurface layer on both sides of the typhoon track. The largest surface cooling and subsurface warming both occurred on the right side of the typhoon track. Wind‐induced Ekman divergence generated two overturning circulation systems zonally across the YS, consisting of upper layer shoreward currents, coastal downwelling, lower layer seaward currents and upwelling on the typhoon track. Downwelling and seaward currents caused temperature warming below the thermocline, especially in the bottom layer. Upwelling shoaled the thermocline, resulting in subsurface cooling on the typhoon track. Additionally, Ekman divergence produced sea surface height minima on the typhoon track, causing strong southward barotropic currents to the left, which subsequently drove strong temperature cooling above the southern bottom slope of the South YS and fast southward movement of the YS Cold Water Mass. Further numerical experiments indicated that the magnitudes of the above changes increased with increasing maximum wind speeds and/or decreasing moving speeds of typhoons.

Thin Current Sheet Formation and Reconnection at X ∼ −10 RE During the Main Phase of a Magnetic Storm

AbstractThe main question addressed in this study is whether particles with relativistic energy can be injected directly into the inner magnetosphere by very near‐Earth reconnection during magnetic storms. We study a sequence of events observed in the solar wind, in the magnetotail, at geosynchronous orbit (GEO), in the inner magnetosphere, and in the ionosphere during the main phase of a geomagnetic storm on 16 June 2012–17 June 2012. The storm was caused by a magnetic cloud with a high dynamic pressure and a strong southward IMF lasting about 10 hr. These conditions caused an extreme compression of the magnetosphere (SymH reached ∼150 nT) and an enhancement of the lobe magnetic field strength to ∼90 nT at R ∼ 10 Earth radii (RE). We focus on an hour‐long interval between 1050 and 1150 UT on 17 June 2012 when the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites were at apogee at geocentric distances R ≈ 12 RE near midnight. At that time a thin current sheet formed between cis‐GEO distances and THEMIS. This thin current sheet reconnected between GEO and THEMIS. A strong lobe magnetic field enabled ion and electron energization to energies E ≈ 600 keV. Fluxes of high energy (up to relativistic) particles at GEO increased within ≈20 s after reconnection onset detected by THEMIS. Fluxes of relativistic electrons at L ∼ 4 in the morning sector increased within about 600 s after reconnection onset. We interpret these observations as signatures of direct injection of reconnected particles into the inner magnetosphere.

Synoptic Flow Variability in a River-Influenced Inner Shelf off Central Chile

The response of inner shelf circulation and bottom temperature variability to synoptic wind forcing and freshwater outflow is evaluated in an area with a wide continental shelf off central Chile. This forced circulation, with a strong seasonal evolution from upwelling- to downwelling-favorable conditions, is a key process modulating the exchange of water properties in a coastal zone characterized by multiple river outflows and high biological productivity. Ocean currents on the inner shelf (34 m depth) off the Itata River mouth were obtained and analyzed for a nine-month record (December 2008 to September 2009). The synoptic wind and current variability was defined between 2 and 16 days (0.02 to 0.0026 cph). The subinertial coastal circulation within the inner shelf off Itata River was dominated by the along-shelf flow, while cross-shelf flows driven by along-shelf winds were substantially reduced. The alongshore synoptic currents showed two distinct modal structures: (i) A mean two-layer flow field during upwelling-favorable wind stress with northward (southward) flow at the surface (bottom), and (ii) a mean southward flow through the entire water column during downwelling-favorable wind stress periods, which were intensified during peak river discharge events in winter. Calculations of the wind index clarified the relative importance of wind versus buoyancy forcing on the alongshore flow. The wind forcing dominated during summer when the river discharge was minimum, whereas the relative importance of the freshwater outflow from the Itata River became dominant in winter (May-August) when the buoyancy forcing, in conjunction with southward wind events, modulated strong barotropic southward flows. The change in the long-term regime of coastal winds and river discharges, with a dominance of upwelling winds and reduced river outflows in the last decade, is discussed in the context of observations of thinner river plumes under frequent upwelling conditions off central Chile.

Magnetic Field Gradient Across the Flank Magnetopause

Magnetic pressure inside the magnetopause is usually balanced with a sum of thermal plasma and magnetic pressures on the magnetosheath side. However, observations reveal that the magnetosheath magnetic field can be frequently larger than that in the magnetosphere (inverse magnetic field gradient across the magnetopause), and thus, the enhanced pressure from the magnetosheath side seems to be uncompensated. Such events are rare in the subsolar region, but their occurrence rate increases toward flanks. The analysis, based on statistical processing of about 35,000 THEMIS magnetopause crossings collected in the course of the years 2007–2017, shows that these events are more frequently observed under enhanced geomagnetic activity that is connected with a strong southward IMF. Case studies reveal that such a state of the magnetopause boundary layers can persist for several hours. This study discusses conditions and mechanisms keeping the pressure balance across the magnetopause under these conditions.

Bursty magnetic reconnection at the Earth's magnetopause triggered by high-speed jets

The impact of high-speed jets—dynamic pressure enhancements in the magnetosheath—on the Earth's magnetopause has been observed to trigger local magnetic reconnection. We perform a three-dimensional hybrid simulation to study the magnetosheath and magnetopause under turbulent conditions using a quasi-radial southward interplanetary magnetic field (IMF). In contrast to quasi-steady reconnection with a strong southward IMF, we show that after the impact of a jet on the magnetopause, the magnetopause moves inward, the current sheet is compressed and intensified and signatures of local magnetic reconnection are observed, showing similarities to spacecraft measurements.

Middle Miocene tropical oligotrophic lagoon deposit sheds light on the origin of the Western Australian coral reef province

The Western Australian margin is a unique coral reef province, with modern coral reef development occurring at latitudes as far south as 29°S. The genesis of this coral reef province may go back to the Oligo-Miocene, since geological features ~30 million-year-old and younger interpreted as coral reefs are known from offshore seismic surveys. The nature of these seismic reefs is, however, uncertain, as they are only sparsely sampled, and as time-equivalent outcrops are only present in a few remote and understudied locations. This study investigates middle Miocene shallow-water limestones with tropical fauna formed along the North West Shelf (NWS) between ~13 Ma and ~15 Ma, when the southward extension of the seismic reefs was at its maximum. The outcrops and cores investigated are dominantly composed of peloidal packstones and micritic floatstones containing larger benthic foraminifera, Halimeda sp. and scleractinian corals, including reef-building genera, that accumulated in a protected and oligotrophic, warm-water lagoonal environment. Climate was therefore warm during the middle Miocene acme of seismic reefs development, despite the NWS being located ~7° further south than its present position at that time. Results of this investigation also support the existence of a strong southward flowing Leeuwin-current-style oceanic circulation during the middle Miocene, which could have transported fauna from south-east Asia along the Western Australian margin. Development of the lagoon and seismic reefs may have also been promoted by middle Miocene aridification of the coast bordering the NWS, and by repeated eustatic-driven exposures of the NWS.

Effects of solar flares and coronal mass ejections on Earth’s horizontal magnetic field and solar wind parameters during the minimum solar cycle 24

ABSTRACT Previous studies have reported that coronal mass ejections (CMEs) and solar flares lead to the development of huge storms and high-speed streams. Our aim in this paper is to investigate the response of the geomagnetic index SYM/H to the solar wind parameters, such as solar wind speed V, dynamic pressure P, input energy IE and the interplanetary magnetic field (IMF) Bz component, associated with solar flares and CME events. The response of the ground geomagnetic field (H-component) to the solar wind parameters and the IMF Bz component at three low-latitude stations has also been analysed. Our findings show that the delay of the solar wind changes in the Earth’s magnetosphere in response to the weak geomagnetic storm (SYM/H = −30 nT) at the beginning of 2014 December 21. A weak storm of SYM/H = −30 nT in the middle of 2014 November 5 is suggested by a low magnetic reconnection process in the magnetosphere. In addition, the strong southward IMF Bz component and high solar wind changes in the magnetosphere system, which were a result of the X2.0 solar flare event and the CMEs on 2014 October 27, responded to the moderate storm (SYM/H = −60 nT) at the beginning of 2014 October 28. This dynamic physical process in the magnetosphere caused by solar wind variation is seen to excite the Earth’s H-component through ultra low frequency at the ground-based magnetometers at the BCL (Vietnam), TIR (India) and SCN (Indonesia) stations during the geomagnetic storm. This study relates to seismic investigations and geomagnetic-induced current on the ground.

Exchange of water and contaminants between the Strait of Istanbul and the Golden Horn

The Golden Horn (Estuary) is a hotspot for marine pollution because of its close proximity to the Strait of Istanbul (Strait) with dense maritime traffic and high pollution risk of oil spill. Transport and fate of water and contaminants in Strait and Estuary are analyzed by three-dimensional numerical models. Eddies forming at the Golden Horn junction (Junction), either on a single layer or distributed over the entire water column, block the water exchange partly or entirely across the Junction. In case of an oil spill in adjacent waters, the size and distribution of eddies at Junction control the transport and fate of contaminants inside Estuary. Using twenty test cases, the exchange flows at Junction are classified in four types based on sea level difference between north and south boundaries of Strait and Reynolds number of surface flow. Eddies may prevent the natural water exchange between Strait and Estuary but also block the contaminants penetrating to Estuary. Under predominant conditions of mild and moderate southward net flow in Strait, surface waters enter Estuary. During very strong southward flow, Junction is blocked to Black Sea water and surface contaminants. Strait hydrodynamics control water exchange with Estuary through eddy formation. When horizontal eddies are formed at Junction, their size and distribution control the transport and fate of contaminants in branching inlets and estuaries by blocking the water exchange either entirely or partly in specific layers.

Study of the Quasi 10‐Day Waves in the MLT Region During the 2018 February SSW by a Meteor Radar Chain

AbstractWe present a study of the quasi 10‐day waves (Q10DWs) in the mesosphere and lower thermosphere (MLT) during a sudden stratospheric warming (SSW) event in February 2018 based on three meteor radars located at Mohe (MH, 53.5°N, 122.3°E), Beijing (BJ, 40.3°N, 116.2°E), and Wuhan (WH, 30.5°N, 116.6°E). The enhanced Q10DWs are observed in the meridional component at MH and in the zonal component at BJ and WH. In the meridional component, the monthly mean Q10DW amplitudes in February 2018 are greater than 10 m/s at MH but weaker than ∼7 m/s at BJ and WH. In the zonal component, monthly mean Q10DWs amplitudes are ∼15 m/s at BJ and WH but only 9 m/s at MH. These latitudinal differences of the Q10DWs in the MLT region are very likely due to the Q10DW activity in the lower atmosphere and the mean background winds. The strong southward winds might bring the wave energy to MH in the meridional component during the occurrence of SSW, while the enhanced Q10DWs in the zonal component at BJ and WH are likely associated with the strong Q10DWs in the lower atmosphere and weakening eastward winds after the central date of the SSW. The significance test indicates that the enhanced Q10DWs in the meridional wind of MH and in the zonal wind of WH are very likely affected by the SSW. In addition, nonlinear interactions between the Q10DWs and other planetary waves also modulate the variations in the MLT region at BJ during the SSW.

Causes and consequences of asymmetric lateral plume flow during South Atlantic rifting

Volcanic rifted margins are typically associated with a thick magmatic layer of seaward dipping reflectors and anomalous regional uplift. This is conventionally interpreted as due to melting of an arriving mantle plume head at the onset of rifting. However, seaward dipping reflectors and uplift are sometimes asymmetrically distributed with respect to the subsequent plume track. Here we investigate if these asymmetries are induced by preexisting lateral variations in the thickness of continental lithosphere and/or lithospheric stretching rates, variations that promote lateral sublithospheric flow of plume material below only one arm of the extending rift. Using three-dimensional numerical experiments, we find that South Atlantic rifting is predicted to develop a strong southward asymmetry in its distribution of seaward dipping reflectors and associated anomalous relief with respect to the Tristan Plume that "drove" this volcanic rifted margin, and that the region where plume material drains into the rift should experience long-lived uplift during rifting-both as observed. We conclude that a mantle plume is still needed to source the anomalously hot sublithospheric material that generates a volcanic rifted margin, but lateral along-rift flow from this plume, not a broad starting plume head, is what controls when and where a volcanic rifted margin will form.

Seasonal Variation of the Deep Limb of the Pacific Meridional Overturning Circulation at Yap‐Mariana Junction

AbstractThis study reveals the seasonal variability of the lower and upper deep branches of the Pacific Meridional Overturning Circulation (L‐PMOC and U‐PMOC) in the Yap‐Mariana Junction (YMJ) channel, a major gateway for deep flow into the western Pacific. On the western side of the YMJ channel, mooring observations in 2017 and in 1997 show the seasonal phase of the L‐PMOC at depths of 3,800–4,400 m: strong northward flow with speed exceeding 20 cm s−1 and lasting from December to next May and weak flow during the following 6 months. On the eastern side of the channel, mooring observations during 2014–2017 show two southward deep flows with broadly seasonal phases, one being the return flow of L‐PMOC below ~4,000 m and with the same phase of L‐PMOC but reduced magnitude. The second, shallower, southward deep flow corresponds to the U‐PMOC observed within 3,000–3,800 m and with opposite phase of L‐PMOC, that is, strong (weak) southward flow appearing during June–November (December–May). Seasonal variations of the L‐PMOC and U‐PMOC are accompanied by the seasonal intrusions of the Lower and Upper Circumpolar Waters (LCPW and UCPW) in lower and upper deep layers, which change the isopycnal structure and the deep currents in a way consistent with geostrophic balance.

Numerical simulation of sedimentation and erosion caused by the 2011 Tohoku Tsunami in Oarai Port, Japan

The 2011 Tohoku Tsunami induced sediment deposition of 334,000 m3 mainly in the basin and 9 m erosion at the tips of a breakwater and a groin in Oarai Port, Japan. The tsunami-induced bathymetry change was simulated using a numerical model and the simulated bathymetry change was compared with the measured one. The comparison showed that the simulated bathymetry change quantitatively agreed with the measured one including the sediment deposition and the erosion mentioned above although the erosion measured along the breakwater was not well reproduced by the model. The time series of simulated bathymetry change, water level, current and suspended sediment concentration showed that the sediment deposition in the center of the basin was mostly induced by the sediments that moved into the basin through its entrance between the breakwater and the groin and were redistributed by counterclockwise vortices. The severe erosion at the tip of the breakwater was mainly induced by the strong southward currents.

Two-step Dropouts of Radiation Belt Electron Phase Space Density Induced by a Magnetic Cloud Event

Abstract We report a two-step dropout event of radiation belt electron phase space density (PSD) induced by a typical magnetic cloud (MC) that drove an intense geomagnetic storm. The first and second steps of PSD dropout occurred, respectively, in the initial and main phases of the storm with a short-time partial recovery between the two dropouts. In this event, the initial phase after the sudden commencement lasted for near 21 hr, which gives an ideal opportunity to investigate the nature of the radiation belt electron dropout by isolating the main phase from any losses occurring during the initial phase. Detailed analysis shows that the first step of the dropout in the initial phase is likely associated with the magnetopause shadowing effect in combination with ultra-low frequency wave-induced outward transport caused by sustaining enhanced dynamic pressure activity before the MC. Comparably, the prolonged strong southward interplanetary magnetic field inside the MC that resulted in the storm main phase is supposed to play an important role in the second step of significant electron losses to the interplanetary space. Additionally, the partial recovery of electron PSD between the two steps of the dropout is possibly due to the acceleration processes via wave-particle interactions with whistler-mode chorus waves. Our study demonstrated that persistently enhanced solar wind dynamic pressure, which is frequently observed inside interplanetary coronal mass ejections and corotating interaction regions, can play an important role in modulating the radiation belt electron dynamics before the storm main phase driven by these solar wind disturbances.

Climatic patterns over the European Alps during the LGM derived from inversion of the paleo-ice extent

Quaternary climate has been dominated by alternating glacial and interglacial periods. While the timing and extent of past ice caps are well documented, local variations in temperature and precipitation as a response to cyclic glaciations are not resolved. Resolving these issues is necessary for understanding regional and global climate circulation. In particular, the impact of the cold high-pressure zone above the Fennoscandian ice cap on the position of the jet stream in Europe, and a possible change in the direction and the source of moisture flow are still debated. Here we reconstruct climate conditions that led to the observed ice extent in the European Alps during the last glacial maximum (LGM). Using a new inverse method to reconstruct the spatially variable position of the equilibrium line altitude (ELA), we investigate whether the Alpine LGM ice cap dominantly received precipitation in the south due to a strong southward shift of the westerlies in midlatitudes. We report inversion results that enable us to estimate the role of climate, inversion method parameters, ice dynamics, flexure and topography in modulating the inferred equilibrium line altitude. Our main finding is the increase of the position of the ELA from west to east and from north to south of the mountain range, which suggest dominant moisture delivery over the Alps during the LGM relatively similar to today.

Strong southward and northward currents observed in the inner plasma sheet

Abstract. It is generally believed that field-aligned currents (FACs) and the ring current (RC) are two dominant parts of the inner magnetosphere. However, using the Cluster spacecraft crossing the pre-midnight inner plasma sheet in the latitudinal region between 10 and 30∘ N, it is found that, during intense geomagnetic storms, in addition to FACs and the RC, strong southward and northward currents also exist which should not be FACs because the magnetic field in these regions is mainly along the x–y plane. Detailed investigation shows that both magnetic-field lines (MFLs) and currents in these regions are highly dynamic. When the curvature of MFLs changes direction in the x–y plane, the current also alternatively switches between being southward and northward. To investigate the generation mechanism of the southward and northward current, we employed the analysis of energetic particle flux up to 1 MeV. For energetic particles below 40 keV, observations from Cluster CIS/CODIF (Cluster Ion Spectrometry COmposition and DIstribution Function analyzer) are used. However, for higher-energy particles, the flux is obtained by extrapolations of low-energy particle data through Kappa distribution. The result indicates that the most reasonable cause of these southward and northward currents is the curvature drift of energetic particles.

Makassar Strait Throughflow Seasonal and Interannual Variability: An Overview

AbstractThe Makassar Strait throughflow of ~12–13 Sv, representing ~77% of the total Indonesian Throughflow, displays fluctuations over a broad range of time scales, from intraseasonal to seasonal (monsoonal) and interannual scales. We now have 13.3 years of Makassar throughflow observations: November 1996 to early July 1998; January 2004 to August 2011; and August 2013 to August 2017. Strong southward transport is evident during boreal summer, modulated by an ENSO interannual signal, with weaker southward flow and a deeper subsurface velocity maximum during El Niño; stronger southward flow with a shallower velocity maximum during La Niña. Accordingly, the southward heat flux, a product of the along‐channel current and temperature profiles, is significantly larger in summer and slightly larger during La Niña. The southward flow relaxed in 2014 and more so in 2015/2016, similar though not as extreme as during the strong El Niño event of 1997. In 2017, the throughflow increased to ~20 Sv. Since 2016, the deep layer, 300‐ to 760‐m southward transport increases, almost doubling to ~7.5 Sv. From mid‐2016 into early 2017, the transports above 300 m and below 300 m are about equal, whereas previously, the ratio was about 2.7:1. Near zero or northward flow occurs in the upper 100 m during boreal winter, albeit with interannual variability. Particularly strong winter reversals were observed in 2014/2015 and 2016/2017, the latter being the strongest winter reversal revealed in the entire Makassar time series.

ВПЛИВ ЗМІН ПРИРОДНИХ УМОВ НА РОЗКВІТ І ЗАНЕПАД ЗЕМЛЕРОБСЬКОЇ САБАТИНІВСЬКОЇ КУЛЬТУРИ ПІЗНЬОЇ БРОНЗИ НА ПІВДНІ УКРАЇНИ

A pollen and soil study of deposits of the Novokyivka site sheds light on the environment of the Sabatynivka culture (Late Bronze Age, XIV-XIII cent. BC), which was located in the central part of the southern steppe of Ukraine: this record demonstrates that this time span was characterized by a sharp increase in humidity. This is reflected in the spread of mesophytic herbs in the area between the River Dnieper and the Sea of Azov. The dry treeless Artemisia-Poaceae steppe that existed in southern Ukraine at the threshold of the Late Bronze Age was replaced by an association of forbs and Poaceae. The area of the Oleshya woodland, in the low reaches of the River Dnieper, was larger than today, and mesophilic species (Carpinus betulus, Ulmus sp., Quercus robur, Tilia cordata, Corylus avellana) grew there. Earlier pollen and palaeopedological studies of the Late Bronze settlements in the forbs-Poaceae steppe of SE Ukraine also indicate a strong southward shift of meadow-steppe and forest-steppe zones. All of this evidence indicates a significantly higher humidity of the climate than today. The Sabatynivka culture demonstrates an unprecedented rise in the number of settlements and in arable agricultural development. The latter is confirmed by the presence of pollen grains of Cerealia and segetal weeds. The decline of the Sabatynivka culture was caused by another climatic shift: to aridification, that started in the XII cent. BC, and culminated in the X-VIII cent. BC. The population of the xeric steppe between the River Dnieper and the Sea of Azov was not able to continue the previous sedentary agriculture that had typified the Sabatynivka culture. Only temporary camps of cattle breeders occurred in the south-east of Ukraine. This crisis was not caused by the human-induced exhaustion of soil-biological resources, but by climatic factors, as it was not a local phenomenon: this strongest aridification at the end of the Late Bronze Age is revealed everywhere in areas of semi-arid and arid climate in the East- European steppe and the Mediterranean realm.

Forecasting Periods of Strong Southward Magnetic Field Following Interplanetary Shocks

AbstractLong periods of strong southward magnetic fields are known to be the primary cause of intense geomagnetic storms. The majority of such events are caused by the passage over Earth of a magnetic ejecta. Irrespective of the interplanetary cause, fast‐forward shocks often precede such strong southward Bz periods. Here we first look at all long periods of strong southward magnetic fields as well as fast‐forward shocks measured by the Wind spacecraft in a 22.4‐year span. We find that 76% of strong southward Bz periods are preceded within 48 hr by at least a fast‐forward shock, but only about 23% of all shocks are followed within 48 hr by strong southward Bz periods. Then, we devise a threshold‐based probabilistic forecasting method based on the shock properties and the pre‐shock near‐Earth solar wind plasma and interplanetary magnetic field characteristics adopting a superposed epoch analysis‐like approach. Our analysis shows that the solar wind conditions in the 30‐min interval around the arrival of fast‐forward shocks have a significant contribution to the prediction of long‐duration southward Bz periods. This probabilistic model may provide on average a 14‐hr warning time for an intense and long‐duration southward Bz period. Evaluating the forecast capability of the model through a statistical and skill score‐based approach reveals that it outperforms a coin‐flipping forecast. By using the information provided by the arrival of a fast‐forward shock at L1, this model represents a marked improvement over similar forecasting methods. We outline a number of future potential improvements.