Volume 49, Issue 22 e2022GL099768
Research Letter
Free Access

Investigating the Role of Magnetosheath High-Speed Jets in Triggering Dayside Ground Magnetic Ultra-Low Frequency Waves

Boyi Wang

Corresponding Author

Boyi Wang

Institute of Space Science and Applied Technology, Harbin Institute of Technology (Shenzhen), Shenzhen, China

Correspondence to:

B. Wang,

[email protected]

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Yukitoshi Nishimura

Yukitoshi Nishimura

Department of Electrical and Computer Engineering and Center for Space Sciences, Boston University, Boston, MA, USA

Contribution: Formal analysis, Writing - review & editing

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Heli Hietala

Heli Hietala

Department of Physics and Astronomy, Queen Mary University of London, London, UK

Contribution: Formal analysis

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Vassilis Angelopoulos

Vassilis Angelopoulos

Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA, USA

Contribution: Data curation, Writing - review & editing

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First published: 07 November 2022
Citations: 2

Abstract

Ultra-low frequency (ULF) waves significantly contribute to transferring energy and transporting particles in the coupled magnetosphere-ionosphere system. Recent studies suggested magnetosheath high-speed jets (HSJs) can drive magnetospheric ULF waves, but whether they can be an important source of dayside ground magnetic ULF oscillations has not been examined thoroughly. Utilizing 5 years of observations by the Time History of Events and Macroscale Interactions During Substorms satellites and ground-based magnetometers, we found that 37% of all the observed magnetosheath HSJs can trigger ground magnetic field oscillations and were thus determined as geoeffective HSJs. The occurrence rate has positive relationship with increasing flow speed and dynamic pressure of magnetosheath HSJs. We propose that magnetosheath HSJs can be a source of dayside ground magnetic ULF oscillations. We also identified the different behaviors of isolated and recurrent magnetosheath HSJs in generating ground magnetic ULF oscillations. The recurrence time of recurrent magnetosheath HSJs likely determines the frequencies of the ULF oscillations.

Key Points

  • A total of 37% of the magnetosheath high-speed jets (HSJs) caused ground magnetic ultra-low frequency (ULF) wave enhancements, which indicates HSJs as a source of ground magnetic ULF waves

  • The magnetosheath HSJ-driven ground magnetic ULF wave intensifies with larger anti-sunward speed and larger dynamic pressure of these HSJs

  • The recurrence time of recurrent magnetosheath HSJs may have determined the frequencies of the driven ground magnetic ULF oscillations

Plain Language Summary

Ultra-low frequency (ULF) waves are common to observe in the Earth's magnetosphere and ionosphere. These waves significantly contribute to transferring energy and transporting particles, which potentially further disturb satellite communication signals and harm ground electricity systems. Recent studies suggested magnetosheath high-speed jets (HSJs), which are generated frequently in the magnetosheath mostly due to foreshock and shock processes, can drive magnetospheric compressions and ULF waves. However, whether they can be an important source of dayside ground magnetic ULF oscillations has not been examined thoroughly. The observations by the Time History of Events and Macroscale Interactions During Substorms satellites and ground-based magnetometers show that 37% of all magnetosheath HSJs are associated with ground magnetic ULF wave power enhancements. The occurrence rate of these waves increases with flow speeds, dynamic pressure of magnetosheath HSJs and the distance of the satellites from the bow shock. We propose that magnetosheath HSJs can be an important source of dayside ground magnetic ULF oscillations. We also identified the different behaviors of isolated and recurrent magnetosheath HSJs in generating ground magnetic ULF oscillations, and their recurrence time likely determines the frequencies of the ULF oscillations.

1 Introduction

The Pc5 (1.66–6.66 mHz, corresponding to periods of 150–600 s) ultra-low frequency (ULF) band of ground magnetic ULF waves is important for energy transfer in the coupled magnetospheric and ionospheric system, interacting and coupling with different plasma populations and other plasma wave modes (Hartinger, 2012; Villante et al., 2016; Zong, Hao, et al., 2009; Zong, Zhou, et al., 2009; Zong et al., 2017). Previous studies have shown that localized and fast disturbing ULF waves are embedded into global magnetospheric processes and may be the drivers of some dayside geomagnetically induced current bursts, which can potentially affect ground electricity power grids or pipelines (Belakhovsky et al., 2019; Boteler et al., 1998; Pirjola, 2000; Viljanen, 1998; Viljanen et al., 2001; Yagova et al., 2021). Thus, determining the sources of ground magnetic field ULF oscillations is important in enhancing magnetospheric and ionospheric models and space weather forecast.

Typical external sources of magnetospheric Pc5 ULF waves are dynamic pressure variations in the pristine solar wind (Claudepierre et al., 2010; Kepko et al., 2002; Shi et al., 2013; Zong, Hao, et al., 2009; Zhang et al., 2010) and Kelvin–Helmholtz instability (KHI) at the flank magnetopause (Kavosi & Raeder, 2015; Kronberg et al., 2021; Miura, 1992; Pu & Kivelson, 1983), which are usually large-scale perturbations at the magnetopause. Recently, multi-spacecraft observations near the Earth's bow shock and hybrid/kinetic models reported that foreshock transients, which are localized transient structures in the foreshock region, can also drive magnetospheric Pc5 ULF waves (Archer et al., 2015; Hartinger et al., 2013; Shen et al., 2018; Wang, Nishimura, Hietala, Shen, et al., 2018; Wang et al., 201920202021). The foreshock transients are reported to drive intense Pc5 ULF waves in the magnetosphere which further propagate globally (Wang et al., 2020). Thus, in terms of driving magnetospheric Pc5 ULF waves, the contributions from localized transient structures should not be ignored.

Besides foreshock transients, recent studies show that shock and foreshock processes can trigger supermagnetosonic jets in the magnetosheath, which are named as magnetosheath “high-speed jets (HSJs)” (Escoubet et al., 2020; Hietala et al., 2009; Koller et al., 2022; Plaschke et al., 2018; Raptis et al., 2022; Suni et al., 2021; Wang et al., 2022). Although magnetosheath HSJs' median spatial scale of about 0.12 urn:x-wiley:00948276:media:grl65024:grl65024-math-0001 (Plaschke et al., 2020) is not as large as solar wind structures or foreshock transients, magnetosheath HSJs can also be geoeffective due to their significant dynamic pressure increases and frequent occurrence (Plaschke et al., 2016). Especially, previous reports show that magnetosheath HSJs can trigger diffuse auroral brightenings, which indicates the occurrences of magnetospheric compressions and magnetospheric ULF waves (Wang, Nishimura, Hietala, Lyons, et al., 2018). Because of the frequent occurrence of magnetosheath HSJs, they can potentially be an important source of dayside magnetospheric and ground magnetic field ULF waves other than the known sources. However, the influence of magnetosheath HSJs on dayside magnetospheric or ground magnetic field ULF waves has not been systematically investigated. This motivated us to examine the contribution of magnetosheath HSJs as a potential source of ULF oscillations in the coupled magnetospheric and ionospheric system and on the ground.

In this study, we utilized the coordinated observations by the Time History of Events and Macroscale Interactions During Substorms (THEMIS) satellites and ground-based magnetometers (GMAGs) from 2008 to 2011 and in 2016, to examine the role of magnetosheath HSJs in triggering ground magnetic field Pc5 ULF oscillations, and investigate the mechanisms associated with isolated and recurrent magnetosheath HSJs that trigger the ground magnetic field Pc5 ULF oscillations. The descriptions of instruments and data set can be found in Section 2. The results of our case and statistical studies are shown in Section 3. Discussion and conclusions appear in Section 4.

2 Instruments and Data Set

We determined magnetosheath HSJs based on the in-situ observations by five THEMIS probes (THEMIS A-E) from 2008 to 2011 and in 2016. The THEMIS probes measure magnetic field, velocity and particles by the Flux Gate Magnetometer (FGM; Auster et al., 2008), Electrostatic Analyzer (ESA; McFadden et al., 2008), and Solid State Telescope (Angelopoulos, 2009; Turner et al., 2012). Based on these data, we utilize the criteria of selecting subsolar magnetosheath HSJs within urn:x-wiley:00948276:media:grl65024:grl65024-math-0002 hr of noon in the ecliptic plane, as listed in Section 2 of Plaschke et al. (2013).

A total of 2,036 magnetosheath HSJs were identified from the 5-year THEMIS satellite data in year 2008–2011 and 2016. We grouped two magnetosheath HSJs into one HSJ event if the interval between them was less than 12 min, to ensure that at least one wave cycle is included in the interval, because of the long periods of the Pc5 ULF waves. The HSJ events that only have one magnetosheath HSJ are determined as isolated HSJ events; The HSJ events that contain at least three magnetosheath HSJs are determined as recurrent HSJ events. Here, the HSJ events with two magnetosheath HSJs are not considered in this categorization in order to clearly distinguish recurrent HSJ events from isolated HSJ events.

In order to examine the ground ULF magnetic field responses to these subsolar magnetosheath HSJ events, we use the ground magnetic field data obtained from the SuperMAG and THEMIS GMAG data sets. These ground magnetic fields are in NEZ coordinates, where urn:x-wiley:00948276:media:grl65024:grl65024-math-0003 is positive in the northward direction, urn:x-wiley:00948276:media:grl65024:grl65024-math-0004 is positive in the eastward direction, and urn:x-wiley:00948276:media:grl65024:grl65024-math-0005 is normal to the horizontal plane and positive in the downward direction. The time resolutions of these magnetic field data were at least 1 min, which is sufficient for our focus on waves with wave periods longer than 150 s (including Pc5 ULF band).

In order to make sure the ground-based magnetometers can capture the responses to magnetosheath HSJs, we require that at least 3 of ground-based magnetometers at high latitude (60°–75°N and 60°–75°S) and within urn:x-wiley:00948276:media:grl65024:grl65024-math-0010 hr MLT around the magnetosheath HSJ are available. As a result, 644 HSJ events are determined for this research.

3 Results

3.1 Two Categories: Isolated and Recurrent Magnetosheath HSJs

Two examples are shown in this section to examine the different behaviors between isolated HSJ events and recurrent HSJ events in triggering ground magnetic field ULF oscillations.

Figures 1a–1k show one example of isolated HSJ events on 28 June 2016, measured by THEMIS A. The background magnetic field had a magnitude of tens of nT (Figure 1a) and the background ion energy flux was dominated from tens of eV to tens of keV (Figure 1e), which are typical characteristics of Earth's magnetosheath. There was only one magnetosheath HSJ in this event. It was associated with a density increase from the background of about 12 urn:x-wiley:00948276:media:grl65024:grl65024-math-0011 to the peak of 20.5 urn:x-wiley:00948276:media:grl65024:grl65024-math-0012 (Figure 1b) and a speed increase from the background of about 80 km/s to the peak of about 258 km/s in the negative GSE-X direction (Marked as urn:x-wiley:00948276:media:grl65024:grl65024-math-0013 hereinafter; Figure 1c). The concurrent increases of density and flow speed resulted in an increase of dynamic pressure in GSE-X direction (marked as urn:x-wiley:00948276:media:grl65024:grl65024-math-0014 hereinafter) of about 2 nPa (Figure 1d), which would potentially drive magnetospheric perturbations.

Details are in the caption following the image

Panels (a–k) show (a) magnetic field in GSE coordinates, (b) ion density, (c) ion velocity, (d) X component of dynamic pressure in GSE coordinates, (e) ion energy flux, (f) detrended ground magnetic field at Vieze Island (VIZ), (g) Pc5 band-pass filtered ground magnetic field data at VIZ, (h) wavelet power spectrum of urn:x-wiley:00948276:media:grl65024:grl65024-math-0015 at VIZ, (i) urn:x-wiley:00948276:media:grl65024:grl65024-math-0016 at VIZ, (j) wavelet power spectrum of urn:x-wiley:00948276:media:grl65024:grl65024-math-0017 at VIZ, and (k) urn:x-wiley:00948276:media:grl65024:grl65024-math-0018 at VIZ, respectively, measured on 28 June 2016. Panels (l–v) are in the same format as panels (a–k), except that the ground magnetic field in panels (q–v) was measured at Ittoqqortoormiit (SCO) and all of the data shown in these panels were measured on 16 June 2008.

The isolated HSJ event occurred at about 11.7 hr MLT. Figure 1f shows the ground magnetic field measured at Vieze Island (VIZ) station (74.6° MLAT; 10.9 hr MLT), which was within urn:x-wiley:00948276:media:grl65024:grl65024-math-0020 hr MLT of the magnetosheath HSJ. About 3 min after THEMIS A observed the magnetosheath HSJ, VIZ station recorded strong geomagnetic oscillations with a magnitude change of about 140 nT in several minutes within the Pc5 ULF band (Figure 1g). The wavelet analysis of urn:x-wiley:00948276:media:grl65024:grl65024-math-0021 (Figure 1h) and urn:x-wiley:00948276:media:grl65024:grl65024-math-0022 (Figure 1j) show a narrowband Pc5 ULF wave power enhancement, centered at about 450–500 s, and the damped oscillations lasted for more than 20 min, which was in a similar manner to the ground magnetic field responses to interplanetary shocks, except that the wavelet power spectrum right after the occurrence of the HSJ was less broadband, likely because the compression they cause was not as abrupt as that due to interplanetary shocks (Pilipenko et al., 2018; Zong et al., 2017). The frequency of this long-lasting wave power enhancement was likely the local Alfven frequency due to local field line resonances (Claudepierre et al., 2010; Kivelson & Southwood, 1986; Rankin et al., 2005). These HSJ-triggered ground magnetic field oscillations is in good agreement with a recent report by Norenius et al. (2021).

In order to quantitatively describe the ground magnetic field responses to HSJ events, we defined two parameters urn:x-wiley:00948276:media:grl65024:grl65024-math-0023 and urn:x-wiley:00948276:media:grl65024:grl65024-math-0024. Here urn:x-wiley:00948276:media:grl65024:grl65024-math-0025, where SAWP is the scale-averaged wavelet power of urn:x-wiley:00948276:media:grl65024:grl65024-math-0026 over the wave periods from 150 to 700 s, M is the total number of magnetometers used in this event and i is an index ranging from 1 to M. Here, we use Equation 24 from Torrence and Compo (1998) to define SAWP as urn:x-wiley:00948276:media:grl65024:grl65024-math-0027, where urn:x-wiley:00948276:media:grl65024:grl65024-math-0028 is the scale step; urn:x-wiley:00948276:media:grl65024:grl65024-math-0029 is the time step; urn:x-wiley:00948276:media:grl65024:grl65024-math-0030 is the reconstruction factor of the Morlet wavelet function; urn:x-wiley:00948276:media:grl65024:grl65024-math-0031 is the wavelet scale; and urn:x-wiley:00948276:media:grl65024:grl65024-math-0032 is the wavelet spectrum of the scale urn:x-wiley:00948276:media:grl65024:grl65024-math-0033. We choose 700 s instead of 600 s as the upper limit of the wave period range that we focus on, because the ground magnetic field oscillates at periods extending above 600 s in many events, but usually not exceeding 700 s, similar to the wavelet power spectrum of urn:x-wiley:00948276:media:grl65024:grl65024-math-0034 in this first event (Figure 1j). Many other events also show that the ground magnetic field oscillated with some wave periods around or more than 600 s but usually not exceeding 700 s. It is also because previous observations and models have shown that Alfvenic waves at large L shells can be beyond the Pc5 band (Rankin et al., 2005). urn:x-wiley:00948276:media:grl65024:grl65024-math-0035 has the same definition except that the SAWP was calculated based on urn:x-wiley:00948276:media:grl65024:grl65024-math-0036. It is shown that these two parameters describe the magnetic field responses well (Figures 1i and 1k).

The recurrent magnetosheath HSJ events contain several magnetosheath HSJs occurring successively. We show one example on 16 June 2008, which contained 6 magnetosheath HSJs (highlighted by the magenta dashed lines in Figures 1l–1o and marked as HSJ1 to HSJ6) with dynamic pressure peaking at about 13:34:55, 13:41:50, 13:51:40, 13:57:05, 14:05:30, and 14:06:55 UT, respectively. The time intervals between two consecutive dynamic pressure peaks were 415, 590, 325, 505 and 85 s, respectively. These magnetosheath HSJs were associated with: (a) strong magnetic field oscillations (Figure 1l); (b) ion density increases to about 27, 32, 19, 25, 21, and 19 urn:x-wiley:00948276:media:grl65024:grl65024-math-0037, respectively, from a background of about 12 urn:x-wiley:00948276:media:grl65024:grl65024-math-0038 (Figure 1m); (c) ion velocity increases to about 357, 268, 236, 252, 232, and 244 km/s, respectively, from a background of 120–140 km/s (Figure 1n); (d) urn:x-wiley:00948276:media:grl65024:grl65024-math-0039 increases to about 4.8, 3.8, 1.5, 2.0, 1.6, and 1.4 nPa, respectively, from a background of 0.1–0.4 nPa (Figure 1o).

This recurrent HSJ event occurred at about 12.7 hr MLT and there was a conjunction observation at the Ittoqqortoormiit (SCO) station, which was at about 13.1 hr MLT and 71.6° MLAT during this event. Figure 1q shows that the ground magnetic field started to oscillate about 4 min after HSJ1 occurred.

The wavelet spectrum of urn:x-wiley:00948276:media:grl65024:grl65024-math-0041 shows that the wave had a period of about 400 s at first and then became intensified at about 500 s (Figure 1s). These two periods are comparable with the time interval between HSJ1 and HSJ2 and the time interval between HSJ4 and HSJ5, respectively. The wavelet spectrum of urn:x-wiley:00948276:media:grl65024:grl65024-math-0042 shows that the wave period was about 400 s at first and then became about 320, 600, and 500 s (Figure 1u). The 400 and 500-s wave periods were the same as those in the wavelet spectrum of urn:x-wiley:00948276:media:grl65024:grl65024-math-0043. The 600-s wave period was consistent with the time interval between HSJ2 and HSJ3. The 320-s wave period was likely related to the time interval between the dynamic pressure increase at 13:29 UT (highlighted by the gray dashed line in Figures 1l–1o) and HSJ1. These indicate that unlike the isolated magnetosheath HSJ event, the recurrence times of the magnetosheath HSJs during this event may have decided the main frequency of magnetospheric oscillations.

Furthermore, Figure 1t shows that urn:x-wiley:00948276:media:grl65024:grl65024-math-0044 increased twice and peaked after the occurrence of the third magnetosheath HSJ. This indicates that unlike isolated HSJ events, successive occurrences of magnetosheath HSJs kept transferring ULF wave energy to the coupled magnetospheric and ionospheric system, which further made the ground magnetic field oscillations last longer.

3.2 The Role of Magnetosheath HSJs in Generating Ground Waves

Based on the 644 conjunction events in years 2008–2011 and 2016, we perform a statistical study to investigate the role of magnetosheath HSJs in generating ground magnetic field ULF waves.

We use urn:x-wiley:00948276:media:grl65024:grl65024-math-0045, urn:x-wiley:00948276:media:grl65024:grl65024-math-0046, and urn:x-wiley:00948276:media:grl65024:grl65024-math-0047 to describe the ground wave power increases in N, E, and Z directions, respectively, after the occurrence of HSJ events. Here, urn:x-wiley:00948276:media:grl65024:grl65024-math-0048 is defined as urn:x-wiley:00948276:media:grl65024:grl65024-math-0049, where urn:x-wiley:00948276:media:grl65024:grl65024-math-0050 is the median value of the urn:x-wiley:00948276:media:grl65024:grl65024-math-0051 from the occurrence of the HSJ event to 15 min after the disappearance of the HSJ event; urn:x-wiley:00948276:media:grl65024:grl65024-math-0052 is the median value of the urn:x-wiley:00948276:media:grl65024:grl65024-math-0053 during 1 hr before the appearance of the HSJ event. urn:x-wiley:00948276:media:grl65024:grl65024-math-0054 and urn:x-wiley:00948276:media:grl65024:grl65024-math-0055 have the same definitions except that the SAWP was calculated based on urn:x-wiley:00948276:media:grl65024:grl65024-math-0056 and urn:x-wiley:00948276:media:grl65024:grl65024-math-0057, respectively.

Figure 2a shows the distribution of the number of HSJ events as a function of urn:x-wiley:00948276:media:grl65024:grl65024-math-0058 based on all HSJ events. It is clear that more events (411 out of 644 events; about 64%) are associated with positive urn:x-wiley:00948276:media:grl65024:grl65024-math-0059, which indicates that magnetosheath HSJs indeed contribute to the intensification of ground magnetic field Pc5 ULF waves. But it should be noted that many events are clustered symmetrically around urn:x-wiley:00948276:media:grl65024:grl65024-math-0060, which is likely due to the random distribution of non-geoeffective HSJ events. Since most of the symmetrically distributed events are associated with urn:x-wiley:00948276:media:grl65024:grl65024-math-0061, we consider that HSJ events with urn:x-wiley:00948276:media:grl65024:grl65024-math-0062 drive significant Pc5 ULF wave activity and thus are geoeffective (see the zoomed-in distribution in Figure S1a in Supporting Information S1). As a result, there were 37% of HSJ events (237 out of 644 events) determined as geoeffective HSJ events.

Details are in the caption following the image

Panel (a) shows the distribution of the number of events of urn:x-wiley:00948276:media:grl65024:grl65024-math-0063 of all high-speed jet (HSJ) events. Panel (b) shows the distribution of the number of events of urn:x-wiley:00948276:media:grl65024:grl65024-math-0064 of the HSJ events with peak urn:x-wiley:00948276:media:grl65024:grl65024-math-0065 larger than 2 nPa. Panel (c) shows the superposed epoch analysis of the wave periods of urn:x-wiley:00948276:media:grl65024:grl65024-math-0066 of the isolated HSJ events. Panel (d) shows the superposed epoch analysis of the wave periods of urn:x-wiley:00948276:media:grl65024:grl65024-math-0067 of the recurrent HSJ events. Panels (e, f and i, j) are in the same format of panels (a and b), except for urn:x-wiley:00948276:media:grl65024:grl65024-math-0068 and urn:x-wiley:00948276:media:grl65024:grl65024-math-0069, respectively. Panels (g, h and k, l) are in the same format of panels (c and d), except for urn:x-wiley:00948276:media:grl65024:grl65024-math-0070 and urn:x-wiley:00948276:media:grl65024:grl65024-math-0071, respectively.

Here, it is noted that there is a small portion of magnetosheath HSJ events with values of urn:x-wiley:00948276:media:grl65024:grl65024-math-0072, which occurred with some large ULF waves that were likely generated by previous strong magnetosheath HSJs or other sources (i.e., solar wind, foreshock transients and etc.).

Furthermore, if we only consider the 293 HSJ events that were associated with substantial dynamic pressure increases, which means that the peak dynamic pressure of each HSJ event in the negative GSE-X direction (marked as urn:x-wiley:00948276:media:grl65024:grl65024-math-0073 hereinafter) was larger than 2 nPa, the percentage of geoeffective HSJ events increases to 46% (135 events). These results indicate that the geoeffective magnetosheath HSJs are very common, which is consistent with the prediction by Plaschke et al. (2016).

Figures 2e and 2i show similar distributions of urn:x-wiley:00948276:media:grl65024:grl65024-math-0074 and urn:x-wiley:00948276:media:grl65024:grl65024-math-0075. Most of the symmetrically distributed events were clustered at smaller urn:x-wiley:00948276:media:grl65024:grl65024-math-0076 and smaller urn:x-wiley:00948276:media:grl65024:grl65024-math-0077 (as shown in Figures S1b and S1c in Supporting Information S1). There were: 214 HSJ events (33%) out of all HSJ events, and 119 HSJ events (41%) out of the events with substantial dynamic pressure increases were associated with urn:x-wiley:00948276:media:grl65024:grl65024-math-0078; 181 HSJ events (28%) out of all HSJ events and 112 HSJ events (38%) out of the events with substantial dynamic pressure increases were associated with urn:x-wiley:00948276:media:grl65024:grl65024-math-0079.

The superposed epoch analysis of the ground urn:x-wiley:00948276:media:grl65024:grl65024-math-0080 shows the average wavelet power spectra of the 121 isolated geoeffective HSJ events (Figure 2c) and the 65 recurrent geoeffective HSJ events that contains at least 3 magnetosheath HSJs (Figure 2d). Here, the wavelet power spectrum of each HSJ event is obtained by averaging all the wavelet power spectra that were measured at all the available stations in this event. The results show that the HSJ-related oscillations are not only within the Pc5 ULF band, but extend continuously toward about 700 s. So, it is reasonable to choose 700 s as the upper limit to calculate urn:x-wiley:00948276:media:grl65024:grl65024-math-0081. Here, it is noted that the continuous spectrum in this superposed epoch analysis does not mean that each station can observe a continuous spectrum after the occurrence of a geoeffective HSJ. It is most likely the result of the superposition of all the narrowband signatures from the different events, associated with the different recurrence times of magnetosheath HSJs (e.g., Figures 1s and 1u) or the slightly different frequencies of the waves that were induced by the field line resonances because of the different positions of the stations in MLT and MLAT and the different magnetospheric conditions during the events (e.g., Figures 1h and 1j). The wave power of waves with the frequencies below 300 s was less visible, which was consistent with that the magnetopause can filter away pressure variations shorter than a few minutes as a low-pass filter (Archer et al., 2013).

We also found that the wave powers were stronger with larger wave periods, which is likely because larger Alfvenic wave periods are generated at higher latitudes (Rankin et al., 2005) while magnetosheath HSJs are an external source of magnetospheric and ground ULF waves. For recurrent HSJ events, the wave power at larger wave periods peaked later than for isolated jets, between 10 and 20 min after the first HSJ, corresponding approximately to the second wave cycle. This is reasonable because the strong oscillations at high latitudes can continue for more than one cycle and the wave power can last increasing with new occurrences of magnetosheath HSJs (the second HSJ event can be an example).

The superposed epoch analysis of the urn:x-wiley:00948276:media:grl65024:grl65024-math-0082 and urn:x-wiley:00948276:media:grl65024:grl65024-math-0083 (Figures 2g, 2h, 2k, 2l) show similar results as those obtained from urn:x-wiley:00948276:media:grl65024:grl65024-math-0084, except that the wave power of urn:x-wiley:00948276:media:grl65024:grl65024-math-0085 oscillations in the geoeffective HSJ events were overall strongest, while the wave power of urn:x-wiley:00948276:media:grl65024:grl65024-math-0086 oscillations in the geoeffective HSJ events were much smaller than those of the oscillations in two horizontal components. This is likely because field line resonances generate toroidal and poloidal waves with large wave amplitudes, which correspond to the ground magnetic field oscillations dominantly in urn:x-wiley:00948276:media:grl65024:grl65024-math-0087 and urn:x-wiley:00948276:media:grl65024:grl65024-math-0088 components, respectively (Hughes, 19741994), while urn:x-wiley:00948276:media:grl65024:grl65024-math-0089 component mainly represented the compressional component. These are consistent with the result that less HSJ events were associated with urn:x-wiley:00948276:media:grl65024:grl65024-math-0090 than the HSJ events were associated with urn:x-wiley:00948276:media:grl65024:grl65024-math-0091 and urn:x-wiley:00948276:media:grl65024:grl65024-math-0092.

3.3 The Occurrence Rate of Geoeffective HSJ Events

In this section, we investigate the contributions of the peak velocity of HSJ events in the negative GSE-X direction (marked as urn:x-wiley:00948276:media:grl65024:grl65024-math-0093 hereinafter; Figure 3a), and of the peak urn:x-wiley:00948276:media:grl65024:grl65024-math-0094 (Figure 3b) to the occurrence rate of geoeffective HSJ events.

Details are in the caption following the image

Panel (a) shows the distributions of the number of events of all high-speed jet (HSJ) events (gray bins) and the HSJ events with urn:x-wiley:00948276:media:grl65024:grl65024-math-0095 (cyan bins), respectively, as a function of peak urn:x-wiley:00948276:media:grl65024:grl65024-math-0096. The red and light red lines show the distributions of the occurrence rates of the geoeffective HSJ events for the bins with the event numbers urn:x-wiley:00948276:media:grl65024:grl65024-math-009720 and urn:x-wiley:00948276:media:grl65024:grl65024-math-009820, respectively. Panels (b and c) are in the same format of panel (a) except that the distributions are as functions of peak urn:x-wiley:00948276:media:grl65024:grl65024-math-0099 and F value, respectively. Panels (d and e) are in the relationships between urn:x-wiley:00948276:media:grl65024:grl65024-math-0100 and F value based on all HSJ events and geoeffective HSJ events, respectively. The squares and error bars indicate the medians and upper/lower quartiles every 0.2 F value.

The gray bins in Figures 3a and 3b show the distributions of all the HSJ events as functions of peak urn:x-wiley:00948276:media:grl65024:grl65024-math-0101 and peak urn:x-wiley:00948276:media:grl65024:grl65024-math-0102, respectively. The corresponding distributions of the geoeffective HSJ events and the corresponding occurrence rates are shown by cyan bins and red lines, respectively. The red line in Figure 3a shows that the occurrence rate of geoeffective HSJ events positively correlates to the increasing peak urn:x-wiley:00948276:media:grl65024:grl65024-math-0103. Similarly, the occurrence rate of geoeffective HSJ events increases with peak urn:x-wiley:00948276:media:grl65024:grl65024-math-0104. This is consistent with that when we only consider the HSJ events with substantial urn:x-wiley:00948276:media:grl65024:grl65024-math-0105 increases, the percentage of geoeffective HSJ events was larger, as shown in Section 3.2. This indicates that the ground magnetic field ULF wave energy originates in the dynamic pressure of magnetosheath HSJs.

In addition, it is interesting to see that the occurrence rate of geoeffective HSJ events have a negative correlation with increasing F values (Figure 3c). Here, we define the F value as urn:x-wiley:00948276:media:grl65024:grl65024-math-0106, where r is the radial distance of the magnetosheath HSJ from the Earth, and urn:x-wiley:00948276:media:grl65024:grl65024-math-0107 and urn:x-wiley:00948276:media:grl65024:grl65024-math-0108 are the distances of the magnetopause and the bow shock, respectively, from the Earth along the same line, according to the models of the magnetopause (Shue et al., 1998) and the bow shock (Wu et al., 2000). The F value ranges between 0 and 1 and indicates a normalized distance of the satellite observing the magnetosheath HSJ from the magnetopause to the bow shock. This result means that when HSJ events were observed closer to the magnetopause, the occurrence rate of geoeffective ones increased.

Figure 3c shows that 25% of the HSJ events observed near the bow shock (urn:x-wiley:00948276:media:grl65024:grl65024-math-0109) triggered ground magnetic field ULF oscillations (8 out of 32 events). Here, not many events were identified near the bow shock and the magnetopause because these regions are usually too complicated to differentiate magnetosheath HSJs and boundary oscillations. Although the identified HSJ events are not many, 25% can be a rough estimation of the occurrence rate of all the geoeffective magnetosheath HSJs that formed near the bow shock. A total of 50% of the HSJ events observed near the magnetopause (urn:x-wiley:00948276:media:grl65024:grl65024-math-0110) triggered ground magnetic field ULF oscillations (53 out of 105 events). Although the occurrence rate of geoeffective HSJ events kept increasing when they were observed closer to the magnetopause, it seems that there were still about half of HSJ events near the magnetopause which have no or weak geoeffectiveness. This deserves to be investigated in multi-satellite observations and kinetic/hybrid simulations in the future.

In order to investigate the reason why the occurrence rate negatively correlated to increasing F value, we use scatter plots to show the relationship between peak urn:x-wiley:00948276:media:grl65024:grl65024-math-0111 and F value, based on all the HSJ events (Figure 3d) and the geoeffective events (Figure 3e), respectively. It shows that the HSJs near the magnetopause were with smaller dynamic pressure than those away from the magnetopause, and the HSJs near the bow shock need larger urn:x-wiley:00948276:media:grl65024:grl65024-math-0112 to be geoeffective. This is likely because the dynamic pressure of HSJs is diffused during propagation toward the magnetopause and some HSJs may be stopped by secondary shocks in the magnetosheath (Hietala et al., 20092012).

Overall, as the dynamic pressure or flow speed of the magnetosheath HSJs increases, the probability of triggering ground magnetic field ULF oscillations also increases.

4 Discussion and Conclusions

Using the coordinated observations between the THEMIS satellites and ground-based magnetometers, we selected 644 magnetosheath HSJ events to determine the role of magnetosheath HSJs in triggering ground magnetic field ULF oscillations and investigate their mechanisms.

As shown in Section 1 and Figure 4, foreshock and shock processes can generate HSJs in the magnetosheath. These magnetosheath HSJs can propagate toward the magnetopause, hit the magnetopause and trigger magnetospheric and ground magnetic field ULF oscillations, although they were localized and transient.

Details are in the caption following the image

An illustration of the geoeffectiveness of isolated high-speed jet (HSJ) events and recurrent HSJ events, respectively.

Our statistical results show that 37% of all the observed HSJ events and 25% of the HSJ events near the bow shock can trigger ground magnetic field ULF oscillations. This indicates that they can be an important wave source of ground magnetic Pc5 ULF oscillations. It transforms our traditional knowledge about the external source of ground magnetic field ULF oscillations, which considers geomagnetic ULF oscillations originally come from the solar wind or KHI along the flanks. Although several studies have shown that fewer geomagnetically induced currents are observed in the dayside than those observed in other MLTs (Engebretson et al., 2019; Schillings et al., 2022), magnetosheath HSJs are likely one important dayside driver of geomagnetically induced currents.

We also found that the magnetosheath HSJs with larger flow speed and dynamic pressure have a higher probability to cause ground magnetic field Pc5 ULF oscillations. It is possibly because the magnetosheath HSJs with larger dynamic pressure are more likely to reach the magnetopause. Some previous studies have also proposed that the widths of HSJs and the solar wind conditions could affect the propagation of HSJs to the magnetopause (Plaschke et al., 2016; LaMoury et al., 2021), which is an interesting topic to be investigated with magnetospheric and ground observations in the future.

The two categories of the HSJ events described by two examples in Section 3.1 are illustrated in Figure 4. Isolated HSJ events behave in a similar manner as to interplanetary shocks and pressure pulses in the solar wind and likely increase the wave power at the eigenfrequencies of the coupled magnetospheric and ionospheric system, which can further propagate to the ground. In recurrent magnetosheath HSJ events, the ground magnetic field ULF wave power keeps increasing during each event due to the continuous occurrence of HSJs. It is also interesting to find that the recurrence time of magnetosheath HSJs can determine the frequencies of ground magnetic field oscillations.

In a nutshell, in this study, we show that magnetosheath HSJs can be an important source of high latitudinal magnetic field ULF oscillations and investigate the mechanisms associated with isolated and recurrent magnetosheath HSJs that trigger them. More details about the variations of the Pc5 ULF signatures observed at different magnetic latitudes and magnetic local times deserve to be investigated in the future.

Acknowledgments

This work was supported by NSFC Grant 42104154 and Grants JCYJ20210324124010027 and RCBS20210609103650048 from the Science, Technology and Innovation Commission of Shenzhen. The work of HH is supported by Royal Society awards URF\R1\180671 and RGF\EA\181090. We acknowledge NASA contract NAS5-02099 for use of data from the THEMIS Mission, C. W. Carlson and J. P. McFadden for use of ESA data, and K. H. Glassmeier, U. Auster, and W. Baumjohann for the use of FGM data provided under the lead of the Technical University of Braunschweig and with financial support through the German Ministry for Economy and Technology and the German Center for Aviation and Space (DLR) under contract 50 OC 0302. We acknowledge SuperMAG for providing ground magnetometer data.

    Data Availability Statement

    THEMIS data were obtained from http://themis.ssl.berkeley.edu/data/themis/ as daily CDF files. SuperMAG data were obtained from https://supermag.jhuapl.edu/mag/ as annual ASCII files. THEMIS data access and processing was done using SPEDAS V3.1; see Angelopoulos et al. (2019). Our data list of HSJ events is available at https://doi.org/10.6084/m9.figshare.20486223.