Solar wind driving and magnetospheric preconditioning during omega band substorm events

Omega bands are mesoscale structures in the auroral oval that appear during periods of enhanced geomagnetic activity. Omega band formation has previously been associated with highly localized drivers in the magnetotail and global compression regions in the solar wind. However, the identifying characteristics and magnetospheric preconditioning of substorms containing omega bands have never been quantified. We compare the solar wind driving, inner magnetospheric current systems, and magnetospheric preconditioning during 324 omega band substorms from 2005 to 2015 to the same parameters measured during 324 random substorms of similar magnitude. We find that omega band substorms are driven by solar wind characterized by enhanced interplanetary magnetic field and high proton density. In the inner magnetosphere, we find that omega band structures typically occur inside or on the eastward edge of the substorm current wedge. Nontrivial differences between the magnetospheric preconditioning for omega band substorms and typical substorms suggest that omega band substorms are preceded by activation of an inner magnetosphere current system during increased compression of the dayside magnetopause. Our results indicate the importance of large-scale solar wind drivers and magnetospheric preconditioning to the study of mesoscale auroral forms.

Substorms are well known to be an essential mechanism in processing the energy transferred through the coupled solar wind-magnetosphere-ionosphere system. In the traditional substorm picture, the magnetosphere and ionosphere are highly coupled through field aligned currents (FACs) and the convection electric field (Birkeland, 1913). Both systems are driven by the interaction of the solar wind with the magnetosphere through dayside and nightside reconnection. The time variability of dayside and nightside reconnection drives quasi-cyclic behavior in the inner magnetosphere and ionosphere (McPherron et al., 1973; Akasofu, 1981), where energy is stored in the magnetosphere in the substorm growth phase and released rapidly at onset and throughout the expansion and recovery phases (Akasofu and Kimball, 1964).

Auroral substorms are associated with the formation and enhancement of large-scale current systems in the inner magnetosphere (Baker et al., 1996). At substorm onset, increased magnetospheric convection increases plasma flux into the magnetotail and compresses the cross-tail current. The cross-tail current is diverted into the ionosphere along a Region 1-sense current loop, the substorm current wedge (SCW) (McPherron et al., 1973). The enhanced solar wind driving during substorm activity leads to an activation of the symmetric and asymmetric contributions of the ring current. The asymmetric contribution closes along Region 2 currents in the ionosphere flowing upward post-midnight and downward pre-midnight, and is referred to as the partial ring current (Cummings, 1966; Crooker and McPherron, 1972; Le et al., 2004).

The auroral signatures of substorm activity have been well documented (Akasofu and Kimball, 1964; Akasofu, 1974). At substorm onset, an arc brightens in the midnight sector of the auroral oval (Donovan et al., 2008). In the expansion phase, the arc expands both azimuthally and poleward, creating an auroral bulge (Akasofu, 1974). The expansion of the bulge coincides with the expansion of the SCW in the magnetosphere (Pulkkinen et al., 1995; Kepko et al., 2015). The propagation of the westward edge of the auroral bulge forms the westward traveling surge (Akasofu et al., 1966). The substorm expansion phase is followed by the recovery phase, during which the intensity of the ionospheric currents decrease and the bulge contracts (Akasofu, 1974). Evening sector and expansion phase aurora are dominated by discrete arcs, while morning sector and recovery phase aurora are dominated by diffuse forms (Akasofu et al., 1966; Partamies et al., 2015).

Omega bands are mesoscale auroral structures (Kepko et al., 2015) that appear as quasi-periodic eastward drifting protrusions in the poleward edge of the diffuse aurora in the post-midnight sector (Akasofu, 1974; Opgenoorth et al., 1983). The protrusion sizes are typically 400–1000 km, and they drift at speeds of 0.4–2 km/s (Opgenoorth et al., 1994). When omega bands pass overhead, Ps6 magnetic pulsations with periods of 4–40 min are measured on the ground (Saito, 1978) and rapidly varying magnetic fields can be observed (Schillings et al., 2022). This magnetic variability can drive geomagnetically induced currents (GICs) (Apatenkov et al., 2020) which in this frequency range can pose a threat to power system infrastructure (Pirjola and Boteler, 2002; Schrijver et al., 2015), making the study of these structures highly relevant to space weather forecasting.

The solar wind and magnetospheric drivers of omega bands have been previously investigated. Cribb et al. (2024) performed a statistical study of 28 omega band events from 2006 to 2013 and identified an association between omega band formation and compression regions in the solar wind typically associated with stream interaction regions (SIRs). In the magnetotail, omega bands have been mapped to distances of 5–13 R E (e.g., Pulkkinen et al., 1991), where two primary local magnetospheric sources of omega bands have been proposed. Henderson et al. (2002) suggests that auroral streamers driven by bursty bulk flows (BBFs) in the tail evolve into omega bands. Other authors propose omega bands are driven by a Kelvin-Helmholtz (KH) instability in the near-Earth plasma sheet mapping inward to the auroral oval (e.g., Rostoker and Samson, 1984; Janhunen and Huuskonen, 1993). Weygand et al. (2022) suggest fast Earthward flows that are diverted dawnward may create KH-associated omega bands that are biased towards later magnetic local time (MLT) sectors. There is no current consensus as to whether either mechanism, or a combination of the two, is the primary driver of omega band structures.

Omega bands have historically been studied in the context of auroral substorms (Pellinen, 1992; Henderson, 2012). While omega bands have previously been associated with the recovery phase of substorms (Akasofu, 1974; Opgenoorth et al., 1994), Cribb et al. (2024) found that omega bands occur on average near the peak of the expansion phase. Omega bands have been associated with a double oval configuration, which forms during geomagnetic storms or intense substorms (Elphinstone et al., 1995; Ohtani et al., 2012). Cribb et al. (2025) found an association between omega band formation and the activation of a current system in the inner magnetosphere consistent with either the partial ring current or SCW, both of which are enhanced during substorm periods. Despite these connections, the identifying characteristics of substorms containing omega bands have never been studied, especially in a statistical sense.

In this study, we analyze the characteristic parameters of the substorms featuring omega bands and the locations where omega bands appear relative to the large-scale current systems that form in the inner magnetosphere during these substorms. In Section 2, we describe the datasets and methodology used to perform this analysis. In Section 3.1, we identify the solar wind parameters measured during the omega band events used in this study in the context of the characteristic solar wind parameters of previously studied omega band event lists. In Section 3.2, we examine the solar wind and magnetosphere-ionosphere parameters measured during omega band substorms compared to typical substorm activity. In Section 3.3, we locate the omega band structures used in this study relative to the SCW. In Section 3.4, we study the global magnetospheric preconditioning during omega band substorms relative to typical substorm activity. The results of these analyses are presented in Section 4 and summarized in the conclusions.

500 omega band events occurring between 2005 and 2016 were identified from select Time History of Events and Macroscale Interactions during Substorms (THEMIS) All Sky Imager (ASI) stations by eye. The stations used were Athabasca (ATHA), Fort Simpson (FSIM), Fort Smith (FSMI), Goose Bay (GBAY), Gillam (GILL), Kuujjuac (KUUJ), Sanikiluaq (SNKQ), The Pas (TPAS), and Whitehorse (WHIT), with magnetic latitudes from 60.46 ° to 67.29 ° N (Donovan et al., 2006; Mende et al., 2009). The criteria for omega band selection are described by Weygand et al. (2022) and require that: (a) each auroral omega is required to appear in more than one image and last for a period longer than 2 min, (b) each omega has to resemble the Greek letter Ω over that time range, (c) each auroral omega must propagate eastward, (d) each auroral omega needs to appear taller than wider for part of its lifetime, and (e) every omega is required to fully fit within the ASI field of view for part of its lifetime, thus setting the scale size of the omega bands to be below 1000 km in diameter.

Substorms were identified using the list identified by McPherron and Chu (2018). The authors generate a substorm onset list using the SuperMAG westward/lower electrojet index (SML). Further details can be found in McPherron and Chu (2018). Since the McPherron and Chu substorm list extends from 1980 to 2015, we only considered the omega band events occurring between 2005 and 2015, resulting in a total of 468 events. It is important to note that while the formation time of an omega band is not necessarily the same as the detection time, omega band events typically have lifetimes of around tens of minutes, which is negligible when performing statistical analyses over hours or days.

We used 1 min resolution data from the OMNI dataset (Papitashvili and King, 2020) to characterize the solar wind parameters during omega band events and omega band substorms. We used data from ground magnetometers compiled in the SuperMAG database (Gjerloev, 2012) to measure the ring current and auroral electrojets during omega band substorms. The SuperMAG upper (SMU) and lower electrojet indices are calculated from the auroral zone magnetometers that measure the largest and smallest northward magnetic field components, respectively. The local SMU and SML indices are calculated from the largest and smallest northward magnetic field components measured by magnetometers in the 3 h window centered on each local time (LT) sector (Newell and Gjerloev, 2014). The electrojet index (SME) is calculated by taking the difference between SMU and SML (Newell and Gjerloev, 2011). The SuperMAG ring current (SMR) index is calculated from the northward magnetic field component measured by the available subset of low-to mid-latitude magnetometers. Regional SMR indices for 0, 6, 12, and 18 LT are calculated by taking the average of the northward magnetic field components measured by magnetometers located in the 6 h window centered on each LT reference point. The average of these four LT indices is taken to find the total SMR index to ensure equal weighting of each time sector (Newell and Gjerloev, 2012).

For the solar wind analysis described in Section 3.1, we only considered omega band events that occurred at least 20 h apart in order to prevent the inclusion of multiple omega band events occurring during the same geomagnetic activity event. If multiple events occurred within the same 20 h interval, only the event with the earliest detection time was included. This process resulted in 296 omega band events. For the substorm comparison analysis described in Section 3.2, we only considered omega band events that occurred at least 3 h apart in order to prevent the inclusion of multiple omega band events occurring during the same substorm event. Similarly, if multiple events occurred within the same 3 h interval, only the event with the earliest detection time was included. This process resulted in 324 omega band events. To identify the omega band substorm events, we identified the closest preceding substorm onset in the McPherron and Chu (2018) list to each omega band event, resulting in 324 substorm onsets. The time offset between the substorm onset and the omega band detection time is shown in Figure 1.

We used data from the Geostationary Operational Environmental Satellites (GOES) located at 6.6 R E to characterize the inner magnetosphere during omega band events. The GOES satellites are placed above the eastern and western parts of the US, with GOES E at 75.2 ° W, and GOES W at 137.2 ° W (Singer et al., 1996). Each GOES satellite measures the local magnetic field with two fluxgate magnetometers (FGMs). For GOES E, measurements from GOES 12 were used from 2005–01-01 to 2009-09-16, and measurements from GOES 13 were used from 2009-09-16 to 2016-01-01. For GOES W, measurements from GOES 10 were used from 2005-01-01 to 2005-06-01, measurements from GOES 11 were used from 2005-06-01 to 2010-04-13, and measurements from GOES 15 were used from 2010-04-13 to 2016-01-01. The magnetic field data were resampled from 512-m to 1-min cadence.

Characteristic solar wind driving during omega band events

A superposed epoch analysis of the solar wind drivers during the 205 omega band events studied in Cribb et al. (2025) is shown in Figure 2a, while a superposed epoch analysis of the solar wind drivers during the 296 omega band events considered here is shown in Figure 2b. The events studied in Cribb et al. (2025) were identified from the Magnetometers - Ionospheric Radars - All sky Cameras Large Experiment (MIRACLE) network in Scandinavia and Finland and occurred between 1997 and 2007.

The two event lists yield similar results. Namely, both event lists are characterized by SIR-like solar wind driving featuring an increase in interplanetary magnetic field (IMF) magnitude, enhancement in southward IMF B z, and enhancements in solar wind flow speed and density beginning 1 day before the epoch time and maximizing at the time of omega band detection. These features are also similar to the results of the superposed epoch analysis performed in Cribb et al. (2024) (not shown).

We also calculated the instantaneous energy input from the solar wind into the magnetosphere using the formula defined in Wang et al. (2014) and given in Equation 1, which is derived from analysis of magnetohydrodynamic simulations:

Here, n s w is the solar wind density, V s w is the solar wind flow speed, B T is the transverse magnetic field, and θ is the solar wind clock angle. The advantage of Wang’s formulation is that it is in units of watts (directly related to energy input), and that it contains solar wind density as a driving parameter. As previous studies and the results in Figure 2b indicate the importance of elevated density for omega band occurrence, we wanted to use a coupling function that accounted for changes in density.

Figure 2 shows that magnetospheric energy input begins to increase ∼ 1 day before the epoch time and is similarly maximized at the omega band detection time. The energy input for the events considered here peaks at around 1.75 TW, while the energy input for the events considered in Cribb et al. (2025) peaks at around 2 TW.

We identified the omega band events considered in this study that occur concurrently to documented SIRs catalogued by Grandin et al. (2019). We find that 145 of the 296 events, or 49% of events, occur during SIRs. This is slightly larger than the share of SIRs in the event list considered in Cribb et al. (2025), where 86 out of 205 events, or 42% of events, occur during SIRs.

Comparison between substorms with and without omega bands

To identify the unique characteristics of omega band substorms, we compare the 324 omega substorm onsets to 324 randomly selected substorm onsets from the same time interval as the omega bands (2005–2015) and using a subset of the McPherron and Chu substorm list limited to substorms that had a minimum SML below −300 nT. The distributions in minimum SML for both substorm lists used here are similar, with peaks around −500 nT for both. The substorm onsets were selected to have the same monthly and yearly distribution as the omega band substorm onsets in order to remove biases introduced by seasonal variation and the solar cycle.

The results of a comparative solar wind analysis between the 324 omega band substorms and the 324 random substorms is shown in Figure 3. Southward IMF B z and magnetospheric energy input are slightly larger at onset time during omega band substorms relative to random substorms, while the solar wind flow speed during omega band substorms peaks ∼ 12 h later relative to random substorms but reaches a similar maximum value.

The most notable differences between the solar wind drivers of omega band substorms and random substorms are seen in the behavior of the IMF magnitude and the solar wind density. The IMF magnitude during random substorms is maximized ∼ 1.5 days before the epoch time and decreases through substorm onset, while the IMF magnitude during omega band substorms is maximized at substorm onset and is approximately 1 nT greater than the IMF magnitude of random substorms at onset time. The density during random substorms is enhanced up to 2 days before substorm onset and begins to decrease 1 day before onset. Conversely, the density during omega band substorms is maximized at the substorm onset and is approximately 1 cm − 3 greater than the density during random substorms at onset time. The density drops well below pre-onset levels over the day following the substorm onset for both omega band and random substorms.

The results of a comparative analysis of the geomagnetic parameters between the omega band substorms and random substorms is shown in Figure 4a. SMU and SML for both omega band substorms and random substorms peak within an hour of substorm onset. On average, the peak values of SMU and SML during omega band substorms are slightly larger than the peak values of SMU and SML during random substorms. Omega band substorms also feature sustained activity in SMU and SML over the 12 h following substorm onset relative to random substorms.

We performed a further analysis of the SMU and SML indices for each LT sector. The results are shown in Figure 4b. Substorm onset is followed by distinct enhancements in SMU and SML for both categories of substorms. The omega band substorms feature enhanced activity in SML near midnight before substorm onset and sustained activity in SML from 23 to 4 MLT following substorm onset relative to the random substorms. While sustained activity in SMU can be seen for both omega band substorms and random substorms, the enhancement in SMU during omega band substorms is shifted towards later MLT sectors relative to random substorms, especially before and immediately after the substorm onset time.

The results of a comparative analysis of the ring current index for omega band substorms and random substorms is shown in Figure 4a. The total ring current index is enhanced during omega band substorms relative to random substorms. At its peak ∼ 1 h after substorm onset, SMR is approximately 3 nT larger during omega band substorms. SMR remains enhanced relative to random substorms over the 4 days following the substorm onset (not shown).

A further analysis of the SMR LT indices for both categories of substorms is shown in Figure 5. While the value of SMR06 is similar for omega band substorms and random substorms, SMR12 and SMR00 are slightly more negative during omega band substorms. SMR18 is ∼ 5 nT more negative during omega band substorms than random substorms and remains depressed relative to the random substorms for multiple days following substorm onset (not shown), indicating a highly asymmetric ring current.

Inner magnetospheric current systems relative to omega band structures

To study the relation between omega band structures and the SCW, we extracted the magnetic field components measured by GOES E and GOES W, as well as the SML measured in each MLT sector, during each omega band event. We identified the location of the ground station where each omega band interval was observed relative to the locations of the footpoints of the GOES satellites in the northern hemisphere. A plot of these parameters during a representative omega band event is shown in Figure 6a.

Using these parameters, the location of the upward and downward ionospheric current closures of the SCW were estimated for each event. Current closures were identified by dipolarizations or other quasi-instantaneous changes in the magnetic field components measured by GOES (Chu et al., 2021), and/or by the LT sectors of local enhancements in SML. For instance, for the event shown in Figure 6, the rapid enhancements in B y and B z at GOES W and the location of the enhancement in SML indicate that the depicted omega band event occurred at the eastward edge of the SCW.

Out of the 468 omega band events, 26 occurred immediately prior to substorm activity. 58 events had missing data or were otherwise inconclusive. 141 events occurred well into the substorm recovery phase and 113 events occurred during sustained substorm activity featuring prolonged enhancements in SML and/or multiple consecutive onsets. The location of the SCW could not be visually determined for these 338 events. Of the remaining 130 events, 6 events (5%) occurred on the westward edge of the SCW, 40 events (31%) occurred inside the SCW, 61 events (47%) occurred on the eastward edge of the SCW, and 23 events (18%) occurred outside of the SCW.

This analysis was repeated using the inversion technique described in Chu et al. (2014) to identify the SCW location using ground magnetic field data. The technique applies the Levenberg-Marquardt algorithm to magnetic field measurements from 45 ground magnetometers at midlatitudes in order to determine parameters of inner magnetospheric current systems, accounting for the effects of GICs and the solar quiet day variation. Further details are included in Chu et al. (2014).

Using the inversion results, the upward and downward current closures of the SCW were located for 182 of the 468 omega band events. 65 events (36%) were located inside the SCW, 107 events (59%) were located outside of the SCW to the east, and 10 events (5%) were located outside of the SCW to the west. Of the 182 events classified using the inversion code, 15 disagreements (8%) were found with the visual classifications described above, indicating good general correspondence between the two independent assessments.

The distribution of the MLT offset between the ground station where the omega bands were observed and the eastward edge of the SCW is shown in Figure 6b. The majority of the omega band events were detected at or to the east of the downward current closure. It is important to note that omega band events occurring within the SCW will have negative MLT offsets in this histogram.

Magnetospheric preconditioning before omega band events

To study the magnetospheric preconditioning during omega band events, we identified the magnetic field and plasma parameters measured by the THEMIS satellites (Angelopoulos, 2008) prior to and during the 324 substorm onsets associated with the omega band intervals. We created plots of the magnetic field and plasma data from all events every 30 min for the 3 h interval prior to substorm onset. To eliminate instantaneous changes in the data, we used the THEMIS data from the 15 min interval centered on each time step. To eliminate data from the solar wind, we removed data where THEMIS was either on the dayside or at a radial distance of at least 10 R E from the Earth, and where the ion flow speed exceeded 200 km/s. We calculated the plasma beta for each time step and eliminated the data with β < 0.2 to restrict our analysis to the plasma sheet. We binned the remaining data into 2 R E by 2 R E bins in the x − y Geocentric Solar Magnetospheric (GSM) plane and took the median value of the data in each bin.

To quantify the magnetospheric preconditioning for typical substorms, we repeated this process for the 324 random substorms analyzed in Section 3.2. Plots of the transverse magnetic field (B T = B x 2 + B y 2) and the ion density in the inner magnetosphere 3 h before, 90 min before, and at omega band substorm onset with the typical substorm values subtracted are shown in Figure 7.

From Figure 7a, we see that the transverse magnetic field in the dawn sector from approximately 2–6 MLT is enhanced prior to and at omega band substorm onset relative to typical substorm values. The transverse magnetic field is ∼ 30 nT stronger before and during substorms associated with omega bands, and the signature persists for at least 3 h.

Figure 7b shows that omega band substorm onset is preceded by increased ion density at the dayside magnetopause relative to typical substorm activity. The ion density before and during omega band substorms is generally enhanced around 9 MLT, except for 90 and 150 (not shown) minutes prior to substorm onset, when the region of high ion density is centered at 12 MLT. The ion density at the dayside magnetopause is ∼ 25 cm − 3 higher than typical substorm activity prior to omega band substorms. The density elevation is variable in time but can be observed 3 h before substorm onset.

Characteristic solar wind driving during omega band events

While the event list studied here provides a much larger number of omega band events, the solar wind analysis in Section 3.1 supports the results of Cribb et al. (2024) and Cribb et al. (2025) and indicates an association between compression regions, or SIR-like driving, in the solar wind and omega band formation in the ionosphere.

The energy input for the event list considered in Cribb et al. (2025) is slightly larger than the average energy input for the event list considered in this study. The formula defined in Wang et al. (2014) takes the product of powers of the solar wind density, flow speed, and transverse IMF. Since the values of IMF B z, flow speed, and proton density are fairly similar for the two event lists at the epoch time, it is likely that this discrepancy in energy input is due to the larger average IMF magnitude during the event list in Cribb et al. (2025). This may be due to the lower geoeffectiveness of SIRs during solar cycle 24 as compared to solar cycle 23 (Grandin et al., 2019).

Comparison between substorms with and without omega bands

The results of our comparative solar wind analysis indicate that solar wind intervals with enhanced IMF magnitude and proton density are more likely to drive substorms during which omega bands will be observed. While the IMF magnitude and density are enhanced relative to quiet time prior to the random substorms, this typically occurs 1–3 days before substorm onset, and the values of both parameters decrease from their peaks before the onset time. Conversely, the IMF magnitude and density measured during omega band substorms reach comparatively larger peaks at the epoch time. This suggests that there may exist a density threshold that is necessary to create the conditions for omega band formation in the ionosphere. The concurrent enhancements in IMF magnitude and proton density are consistent with a magnetic pileup associated with a compression region in an SIR, further supporting the results of Cribb et al. (2024) and Cribb et al. (2025).

The results of our comparative magnetosphere-ionosphere analysis indicate that omega bands typically occur during substorms that are slightly more intense and feature sustained activity relative to typical substorms. This is consistent with the typical substorm activity expected from SIR driving, which has been found to be characterized by long-duration activity in the auroral electrojets (Tsurutani et al., 2004). Together with the solar wind analysis, these results are also representative of enhanced geomagnetic activity driven by the transport of dense solar wind material into the inner magnetosphere through the plasma sheet (Borovsky et al., 1998; Thomsen et al., 1998).

The sustained activity in SML prior to substorm onset may be explained by the limitations of using a substorm onset list (Hodnett et al., 2025). The process used by McPherron and Chu to identify substorm onsets requires a multiple onset substorm to be assigned a singular onset time. If this onset time does not correlate with the beginning of activity in SML, this would create the sustained activity in SML prior to the assigned substorm onset seen in Figure 4b. This may suggest that omega band formation is associated with multiple onset substorms, which would also be consistent with the substorm activity expected from SIR driving.

The increased value of SML relative to typical substorm activity is consistent with an enhanced SCW as predicted by Cribb et al. (2025). The increased value of SMU relative to typical substorm activity is similarly consistent with an enhanced partial ring current as predicted by Cribb et al. (2025). The shift in SMU activation towards later MLT sectors indicates that the partial ring current is closer to midnight. This could result in electron precipitation in the post-midnight sector, where omega bands are typically observed.

The results of the comparative analysis of the ring current index show that omega band substorms are characterized by a highly asymmetric ring current relative to random substorms. The value of the ring current index at dawn is similar for omega band substorms and random substorms, indicating that the relative enhancement in the asymmetric ring current is not associated with an enhancement in the total ring current. The enhancement in SMR18 and SMR00 is consistent with a shifted partial ring current with an ionospheric closure near or even post-midnight. The slight enhancement of SMR12 may be a result of increased dayside compression of the magnetosphere resulting from the enhanced solar wind density.

Inner magnetospheric current systems relative to omega band structures

The results from both the inversion code and visual classification indicate that the majority of omega bands occur inside or on the eastward edge of the SCW, which agrees with existing literature. The inversion code method allowed for structures close to the eastward or westward edge of the SCW to be more precisely classified than they could be by the visual method, causing the majority of disagreements between the two classification methods.

Interestingly, 127 of the events classified using the inversion code could not be located relative to the SCW using the visual method. Combining the results of the two methods and removing the disagreements yields 242 classified events in total, shown in Figure 8. 20 events (8%) were located on the westward edge of the SCW or outside of the SCW to the west, 150 events (62%) were located on the eastward edge of the SCW or outside of the SCW to the east, and 72 (30%) of events were located inside the SCW. This supports the earlier conclusion that the majority of omega bands occur inside or to the east of the SCW. Over half of the total events were located relative to the SCW, underlining the importance of the SCW to omega band formation as suggested in Cribb et al. (2025).

It is important to note that the omega band event times used for this analysis correspond to the times at which each omega band interval was observed by a ground station, which is not necessarily the time of omega band formation. The data used here is not sufficient to determine the location of omega band formation relative to the SCW. This work is left for a future study.

It is also important to note that the accuracy of the inversion technique described by Chu et al. (2014) is lower at substorm onset and well into the recovery phase. It is possible that the inversion technique does not determine the location of the SCW during the omega band intervals visually identified to fall into these two categories with complete accuracy. However, the agreement between the overall results of the inversion code and the visual classification indicates that the omega band intervals are correctly located with respect to the SCW.

Magnetospheric preconditioning before omega band events

The difference plots shown in Figure 7 indicate that there are nontrivial differences in the magnetospheric preconditioning for omega band substorms as compared to typical substorm activity. Specifically, the enhanced transverse magnetic field in the dawn sector prior to and during omega band substorms is consistent with a stretched magnetic field on the dawnside of an FAC in that sector (Chu et al., 2021). The location of the FAC is colocated with the expected location of the downward closure of the SCW in the ionosphere, which supports the analysis in Sections 4.2, 4.3 and the conclusions of Cribb et al. (2025). The persistence of the SCW signature 3 h prior to substorm onset supports an association between omega band formation and multiple onset substorms.

The increased ion density in the dayside magnetopause is consistent with magnetospheric compression associated with pressure pulses in the solar wind. This supports the association between omega band events and SIR-like driving characterized by compression regions in the solar wind discussed in Section 4.1. The dawnward bias of the ion density enhancement suggests that omega band substorms may be associated with asymmetrical compression of the magnetosphere. Further studies are needed to verify this association.

While the signature in the transverse magnetic field is consistent with the downward ionospheric closure of the SCW, we expect to see a similarly stretched magnetic field in the pre-midnight sector colocated with the upward ionospheric closure. Enhanced transverse magnetic field is seen in some of the time steps prior to substorm onset, but does not appear consistently. It is possible that the dawnside signature is stronger in this event list and the duskside signature does not appear clearly due to data scarcity, in which case this analysis should be repeated with a larger event list when available.

The variability of the increased ion density in the dayside magnetopause during omega band substorms indicates that these events may be associated with periodic pressure pulses in the solar wind that drive intervals of magnetospheric compression. These repeated pressure pulses would not appear clearly in our superposed epoch analysis and would require case studies of the solar wind driving of each omega band event. This is left to future work.

In this paper, we analyzed the solar wind and magnetospheric characteristic parameters of 324 omega band substorms relative to typical substorm activity and performed statistical analyses of in-situ data to determine the magnetospheric configuration and preconditioning of these events. The results are summarized as follows: Omega band substorms are caused by solar wind driving characterized by enhanced IMF magnitude and proton density relative to typical substorm levels. This supports an association between omega band formation and SIR-like driving in the solar wind.

Omega band events are characterized by sustained activity in the auroral electrojets and ring current relative to typical substorm activity and may be associated with multiple onset substorms.

Both visual classification and substorm inversion of 468 omega band events indicate that the large majority of omega band structures occur inside or on the eastward edge of the SCW.

An enhancement of the transverse magnetic field in the dawn sector prior to and at omega band substorm onset indicates the activation of a current system in the inner magnetosphere consistent with the SCW.

Increased ion density in the dayside magnetopause during omega band substorms indicates increased magnetospheric compression during these events that is consistent with pressure pulses in the solar wind. These pressure pulses may have a preferential dawnside inclination which should be verified with future work.

Our analysis points to the importance of considering global solar wind drivers and magnetospheric current systems in the study of mesoscale auroral forms. While future work is needed to identify the intermediate mechanisms by which global magnetospheric current systems drive the formation of localized auroral structures, considering the requisite large-scale drivers and magnetospheric preconditioning of these events is vital for developing effective space weather forecasting tools.