A Case Study of Pc1 Waves Observed at the Polar Cap Associated with Proton Precipitation at Subauroral Latitudes

A Case Study of Pc1 Waves Observed at the Polar Cap Associated with Proton Precipitation at Subauroral Latitudes

1. Introduction

Geomagnetic Pc1 pulsations in the frequency range ~0.2–5.0 Hz represent the ground signatures of ElectroMagnetic Ion-Cyclotron (EMIC) waves. EMIC waves are generated in the equatorial magnetosphere by the cyclotron instability of energetic ions with distributions in the velocity space marked by thermal anisotropy T > T (in field-aligned coordinates-perpendicular and parallel direction taken w.r.t. local magnetic field). This mechanism is also found to occur during storm recovery phases [1] due to hot (a few dozen to a few hundred keV) ring current ions overlapping cold plasmaspheric plasma in regions near the plasmapause [2,3] and in plasmaspheric plumes [4].
EMIC waves are transmitted in the form of left-hand polarized Alfvén waves along magnetic field lines into the high-latitude ionosphere, where they experience mode conversion to compressional waves, undergoing gradual polarization rotation (left-hand to linear to right-hand) as they propagate far from the injection region [5]. Their propagation in the ionospheric waveguide—approximately centered at the altitude of the electron density maximum (~350 km)—is horizontal, equidirectional, and induced by ionospheric Hall currents [6]. During propagation, these waves are subject to attenuation due to absorption and leakages [7,8]. Wave attenuation is predicted to be larger in daytime than in nighttime, allowing propagation over distances of hundreds and thousands of km, respectively [8]. Leakages of the wave energy through the lower wall of the duct make possible their detection on the ground, even at large distances from the injection region [9,10,11].
In the upper ionosphere, Pc1 waves are detected by Low Earth Orbit (LEO) satellites. Recent statistical studies, based on Swarm data, have revealed that Pc1 waves at LEO mostly occur at subauroral/auroral latitudes with a dominant linear polarization and an oblique propagation with respect to the local magnetic field. Such waves are more frequent during the late recovery phase of geomagnetic storms [12,13]. Moreover, several studies have observed Pc1 waves at ground in the form of long-lasting, large-scale events [14,15,16]. Most recently, Liu et al. [17] statistically estimated the longitudinal extent of Pc1 pulsations using seven PWING ground stations at subauroral latitudes, and they found that the peak of the probability distribution of their longitudinal extent is ~82.5°, with a half maximum of ~114°.
On the ground, Pc1 pulsations are commonly observed from subauroral to polar latitudes [10,18,19]. Under very quiet geomagnetic conditions (Kp ≤ 1), the occurrence peak is usually in the prenoon sector, but it shifts to afternoon for increasing geomagnetic activity [19,20]. Kim et al. [10] found that the wave propagation in the ionospheric waveguide was generally poleward from an injection region, at a lowest magnetic latitude of ~62°; yet, for about 15% of these events, there was no clear poleward propagation, and the highest spectral power appeared elsewhere than the lowest latitude, thus implying either a higher-latitude or an off-meridional wave injection. In the polar cap, Francia et al. [21] observed simultaneous, highly coherent Pc1 waves at two Antarctic stations, Mario Zucchelli (MZS) and Concordia (DMC), located at a distance of about one thousand kilometers from each other and at approximately the same geographic latitude, but at different longitudes. These last observations suggested a wave propagation in the ionospheric waveguide from the same injection region at a lower latitude, toward the two polar cap stations, possibly along different paths.
EMIC waves produce the precipitation into the atmosphere of ring current protons from a few dozen to a few hundred keV, scattered by resonance [22,23]. Proton precipitation is the cause of Isolated Proton Auroras (IPAs) at subauroral latitudes (55°–65°), observed in association with ground Pc1 pulsations [24,25,26,27]. In a statistical study, Sakaguchi et al. [25] found that IPAs tend to occur across the late recovery phase of a storm in both post- and pre-midnight sectors. Also, those IPAs were found to move equatorward (poleward) with an increasing (decreasing) frequency of simultaneous Pc1 waves (and of the He+-band EMIC waves at the equatorial plane connected to the observed isolated arcs), suggesting that, since the ion cyclotron frequency depends on the magnetic field’s intensity, auroras are magnetically connected to the magnetospheric regions associated with EMIC generation.
Finally, it is worth noting that energetic precipitating particles might have important effects on the chemistry and electrical properties of the atmosphere at mesospheric and stratospheric altitudes [28].

In the present work, we present a case study of an EMIC/Pc1 event simultaneously observed at DMC in Antarctica and by the LEO CSES-01 spacecraft at L = 6.7 on 30 March 2021. The event was also observed in the northern hemisphere at the highest-latitude station in the Finnish pulsation magnetometer network. These data suggest a possible source region for the pulsations in the evening/L~6.7 magnetosphere, and provide further evidence of the wave propagation in the ionospheric waveguide up to the polar cap. In addition, the availability of measurements of precipitating particles and auroral radiance observations also allows to observe proton precipitation upon the wave event, accompanied by the corresponding occurrence of an IPA.

2. Materials and Methods

In this study, we have examined Pc1 waves observed on 30 March 2021 by ground search-coil magnetometers located in Antarctica at Mario Zucchelli (MZS) and Concordia (DMC) stations, respectively, whose coordinates are shown in Table 1, together with their locations and station identifiers (IDs).

Both magnetometers provide variations in the geomagnetic H (northward), D (eastward), and Z (vertically downward) field components at a 5 Hz sampling rate.

For an interhemispheric comparison, we also have used geomagnetic observations from the Finnish Pulsation Magnetometer Network (LT = UT + 2), run by the Sodankylä Geophysical Observatory. Specifically, we selected search-coil magnetometers at a sampling rate of 250 Hz with a cut-off frequency of 35 Hz, located at the stations listed in Table 2.

It is worth noting that, in the present investigation, we have resampled geomagnetic signals at 5 Hz, in order to have the same sampling frequency as for southern observatories.

We have applied Welch’s method [29] to compute the power spectral density (PSD) over 120 s time intervals using the Hamming window and averages of 30 s sub-intervals, with a 50% overlapping and a frequency smoothing over three frequency bands, which results in about 24 degrees of freedom and a frequency resolution of 33.3 mHz.
We also have estimated the polarization parameters, applying the technique for partially polarized waves proposed by Fowler et al. [30]. In particular, we have estimated the polarization ratio R (i.e., the ratio of polarized to total intensity of the horizontal signal) and the ellipticity ε (i.e., the ratio of minor to major axis of the polarization ellipse in the horizontal plane). A positive (negative) value of the ellipticity is a mark of right-handed (left-handed) polarized waves; when the ellipticity is close to zero (generally |ε| 10,31].
In order to investigate in-situ ionospheric electric and magnetic fields, we have used data from the first China Seismo-Electromagnetic Satellite (CSES-01, [32]), which has a sun-synchronous orbit at an altitude of about 500 km, with an inclination angle of 97.4°. The local time of the descending (ascending) node is 14 LT (02 LT). The CSES-01 satellite is equipped with a search coil magnetometer (SCM) with a sampling rate of 1024 Hz [33], and an electric field detector (EFD) with a sampling rate of 5 kHz [34]. Although the satellite is usually turned off at geographic latitudes greater than 65° in both hemispheres, it is able to observe auroral and polar regions within specific geomagnetic configurations, as in the case of the present work.
Earth’s magnetic field data have also been acquired by the Iridium constellation. Iridium satellites provide voice and data coverage for satellite phones, pagers, and integrated transceivers over the Earth’s entire surface [35]. Magnetometer data from this constellation (of 66 active satellites in LEO (~780 km) that ensure a global coverage of the Earth) provide observations of the inner magnetospheric field. Such data are sent to the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) Science Data Center, where they are processed to extract the perturbation signatures associated with the Field Aligned Current (FAC) systems that connect the ionosphere to the magnetosphere [36]. AMPERE data are considered here to better characterize the energy transfer from the magnetosphere to the ionosphere.
To understand whether the observed EMIC/Pc1 event may have caused the precipitation of particles to the high-latitude ionosphere on 30 March 2021, we have analyzed and interpreted data acquired by the National Oceanic and Atmospheric Administration (NOAA, The National Oceanic and Atmospheric Administration)-operated Defence Meteorological Satellite Program (DMSP) Polar-orbiting Operational Environmental Satellites (POES), and the EUMETSAT-operated Meteorological Operational (MetOp) satellites. These are all LEO-polar satellites orbiting at about 850 km from the Earth’s surface. Specifically, the Special Sensor Ultraviolet Spectrographic Imagers (SSUSI [37]) on board DMSP satellites F17–F19 provide global auroral radiance observations at five wavelengths in the ultraviolet range (115–180 nm), with high spatial resolution (7–9 km at nadir), using 15-s scans across the satellite track [38]. In this study, the emission in the hydrogen line HI (121.6 nm) is used to detect proton precipitation [39].
Any POES and MetOp satellite is equipped with the Total Energy Detector (TED, [40]), which consists of two sets of subdetectors capable of monitoring the influx of either energetic ions or electrons under 20 keV to the atmosphere. TED proton total atmospheric integral energy flux at 120 km has been used in this work to track 40]), which also includes two couples of 30°-wide, high-energy proton/electron telescopes. The approximately zenith-pointing (0°) proton telescope operates in the range from 30 to 6900 keV over five differential energy channels, basically observing—at very high latitudes—fluxes of radiation-belt populations inside the bounce loss cone, i.e., precipitating beneath the spacecraft [41]. Channels P1 (30–80 keV) and P2 (80–240 keV) are best suited to monitor energies typical of the magnetospheric ring current [42].

Auroral activity has been additionally monitored from the ground, exploiting the all-sky images taken by the white-light cameras at the Antarctic Syowa Station, which is the mother station of the Japanese Antarctic Research Expedition (JARE), established in 1957.

In order to characterize the interplanetary conditions and the geomagnetic activity during and around the wave event, we have used OMNI solar wind and Interplanetary Magnetic Field (IMF) data [43], as well as the geomagnetic index SYM-H [44], both with 1-min resolution.
Finally, the plasmapause location has been assessed using the Liu and Liu model [45], which is based on the experimental THEMIS-D satellite plasmapause crossing database. The model relies on the following equations:

L p p = a 1 1 + a M L T c o s φ 2 π a φ 24 × l o g 10 D s t + b 1 1 + b M L T c o s φ 2 π b φ 24

with MLT of THEMIS-D and other parameters as reported in Table 1 of [46].

4. Discussion

The importance of EMIC ultra-low-frequency (ULF) waves (and Pc1 counterparts) is connected to their critical role in triggering energetic particle precipitation from the magnetosphere to the conjugated ionosphere via pitch angle scattering. In particular, through ionization processes, precipitating particles can produce variations in the chemistry and electric properties of the atmosphere, possibly impacting climate modeling [49]. Statistical analyses have shown significant correlations between Pc1 activity and atmospheric properties at stratospheric and tropospheric altitudes in Antarctica [50,51,52]. A recent study has demonstrated the temporal correlation between an EMIC-driven IPA and a localized mesospheric ozone loss [53].
The localized bursts of protons caught by LEO satellites all over the ring current energy range (Figure 8, mid and bottom panels) in strict correspondence with the Pc1 waves per serepresent a strong mark of possible EMIC-driven precipitation [54]. Let us take a better look to their low-energy extension (TED data; Figure 8, upper panel) and concomitance with IPA occurrence (Figure 9), which can be considered a proxy of typical EMIC/particle interactions.
As shown in Figure 9 (panel a), proton precipitation occurred in an ionospheric region characterized by downward FACs (blue shaded areas) at the same time as Pc1 wave activity was recorded by the DMC ground magnetometer (Figure 2). Concurrently, DMSP/SSUSI captured proton precipitation in the same magnetic sector crossed by MetOp–01 (Figure 9b). The same phenomenon was observed from the ground by all-sky, white-light cameras at Syowa station: a related optical keogram in Figure 10 clearly shows the appearance of an isolated auroral arc in strict positional and temporal concomitance with MetOp–01 and DMSP/SSUSI observations (Figure 9).
Back to Figure 9, MZS location (panel b, blue triangle) provides a possible explanation for the intense low-frequency activity recorded, masking Pc1 signatures, since the magnetic observatory was crossing the cusp region [55] at the time when DMC observed the “necklace”. In addition, note how CSES–01 satellite’s track (red full curve, panel b) approaches the proton precipitation region (panel b), marked also by downward FACs (blue shaded area, panel a), suggesting that the same Pc1 wave activity captured at LEO corresponds to the one recorded on the ground. A further corroboration came from the presence of an electric field component of the Pc1 waves observed by CSES–01 (Figure 5), which is parallel to the local magnetic field.
Finally, Figure 11 provides a counterpart, in the northern hemisphere, to the left plot in Figure 9. The magnetic position of each of the four ground magnetometers listed in Figure 6 are also reported. Interestingly, unlike the other stations in the array, both KIL and KEV (black and blue triangle, respectively) appeared to be in an ionospheric sector associated with downward FACs (blue shaded area). This gives a potential explanation for the presence of Pc1 wave activity in a very narrow latitudinal band, as shown in Figure 6.
The wave/particle scenario described so far globally suggests that Pc1 waves at ~1 Hz observed at ground and in the top-side ionosphere probably have their source in a magnetospheric equatorial region at L ~6.6/6.7 in the evening/night sector, not far from the plasmapause (Figure 12). Original EMIC waves propagate along magnetic field lines toward the high-latitude ionosphere in both hemispheres. In the southern ionosphere, the waves are detected by CSES at L = 6.7, ~02 LT; they show the characteristic properties of incident waves on the ionosphere, in that they are transverse with almost linear polarization, suggesting that the satellite is located close to the injection region. These waves can propagate horizontally in the polar ionosphere with slow attenuation, so that they are observed at DMC station (polar cap) with a right-hand polarization, as expected for waves far from their injection region [5]. Simultaneously, in the northern hemisphere, linearly polarized Pc1-class waves are observed at KEV (L = 6.6, ~23:30 MLT), plausibly close to the injection region.

That said, we suggest that the EMIC waves propagated from their magnetospheric source along the magnetic field lines both southward and northward; after their injection into the northern ionosphere, they were transmitted to KEV ground station (probably close to the injection region); while in the southern ionosphere, they arrived at the CSES satellite, located near to the injection region, then propagating up to the field line with footprint at DMC polar station.

Wave propagation at such large distances from the source (~2000 km) in the dusk/night sector might be indicative of weak attenuation [8].

The timing of the Pc1 event, when compared to LEO satellite observations of sudden and localized proton precipitation, is considered a strong point in favor of an associated resonant wave–particle interaction.

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